The Radiation Chemistry of Gases at the Interface with Ceramic Oxides

A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences

2015

Luke Jones

School of Chemistry Dalton Cumbrian Facility

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List of Contents

List of Contents ...... 1

List of Figures ...... 6

List of Tables ...... 14

Abbreviations and Acronyms ...... 16

Abstract ...... 17

Declaration ...... 18

Acknowledgements ...... 20

The Author ...... 22

Thesis Structure ...... 23

1 Introduction ...... 25

The Challenge ...... 25

1.1 Background ...... 26

1.2 Radiation Chemistry ...... 32

1.2.1 Sources of Radiation ...... 32

1.2.2 Radiation Interactions with Matter ...... 35

1.2.3 Radiolytic Track Formation ...... 44

2 Literature Review ...... 47

2.1 푃푢푂2 Storage Canisters ...... 47

2.2 Water Adsorption on 푃푢푂2 ...... 50

2.3 Radiolysis of Adsorbed Water ...... 51

2.4 Radiolysis of Gases in Contact with 푃푢푂2 ...... 52

2.5 Radiolysis of 퐻2 − 푂2 in Contact with Other Materials ...... 55

2.6 Radiolysis of Hydrogen and Oxygen ...... 56

2.7 Air Radiolysis ...... 58

Aims and Objectives ...... 60

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3 Experimental ...... 61

3.1 Materials ...... 61

3.1.1 Gases ...... 61

3.1.2 Chemicals ...... 61

3.2 Irradiation Sources ...... 62

3.2.1 Cobalt-60 Source ...... 62

3.2.2 Pelletron Ion Accelerator ...... 64

3.3 Analytical Techniques ...... 65

3.3.1 Gas Chromatography (GC) ...... 65

3.3.2 Ion Chromatography (IC) ...... 66

3.3.3 Surface Area Measurements...... 66

3.3.4 Thermogravimetric Analysis (TGA) ...... 67

3.3.5 Diffuse Reflectance Infra-red Spectroscopy (DRIFT) ...... 67

3.3.6 UV-Vis Spectroscopy ...... 68

3.3.7 Scanning Electron Microscopy (SEM)...... 68

3.4 Experimental ...... 69

3.4.1 Mixing of 퐻2 − 푂2 − 퐴푟 Samples ...... 69

3.4.2 Air Radiolysis ...... 71

3.4.3 Oxide Regeneration ...... 73

3.4.4 Accelerator Experiments ...... 73

4 Development of γ-Irradiation Reaction Vessel ...... 76

4.1 Initial Vessel Design ...... 76

4.1.1 GC Configuration and Calibration ...... 77

4.2 Reaction Vessel Mark II ...... 82

4.2.1 GC Calibration ...... 83

4.3 Reaction Vessel Mark III ...... 86

4.3.1 GC Configuration and Calibration ...... 86

4.4 Gas Mixing ...... 92

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5 Dosimetry ...... 97

5.1 Background ...... 97

5.2 Aqueous Dosimetry ...... 99

5.3 Calculation of Absorbed Dose using 60퐶표 Source ...... 103

5.3.1 Adsorbed Dose in Gaseous Systems ...... 105

5.3.2 Literature Review of Heterogeneous System Dosimetry ...... 107

5.4 Disadvantages of Fricke Dosimetry with Metal Vessels ...... 109

5.5 Gas Phase Dosimetry ...... 110

5.5.1 Gas Phase Dosimetry Literature...... 110

5.5.2 Ethylene Dosimetry Results ...... 113

5.6 Ion Accelerator Dosimetry ...... 114

6 Oxide Powder Characterisation ...... 118

6.1 Properties of 퐶푒푂2 ...... 118

6.1.1 As Received ...... 118

6.1.2 Regenerated 퐶푒푂2 Properties ...... 121

6.1.3 Comparison of ‘As Received’ and Regenerated 퐶푒푂2 ...... 125

6.2 Properties of 푍푟푂2 ...... 127

6.2.1 As Received ...... 127

6.2.2 Regenerated 푍푟푂2 Properties...... 130

6.2.3 Comparison of ‘As Received’ and Regenerated 푍푟푂2 ...... 135

6.3 Comparison of Regenerated 퐶푒푂2 and 푍푟푂2 ...... 137

7 푯2 – 푶2 Radiolysis Results and Discussion ...... 139

7.1 Discussion of 퐶2퐻4 Dosimetry in Comparison with Fricke Dosimetry ...... 139

7.2 Source of Errors in Ethylene Dosimetry ...... 142

7.3 Mechanism of Ethylene (퐶2퐻4) Radiolysis ...... 146

7.4 퐻2 Production from Adsorbed Water on Oxide Powders...... 149

7.5 Radiolysis of Ethylene in Contact with Oxides ...... 152

7.6 Gamma Radiolysis of 퐻2 − 푂2 − 퐴푟 Gas Mixtures ...... 162 3

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7.6.1 Discussion ...... 168

7.7 Gamma Radiolysis of 퐻2 − 푂2 − 퐴푟 in the Presence of an Oxide Surface ...... 172

7.8 Comparison of Homogeneous and Heterogeneous Radiolysis ...... 177

7.9 Discussion of Pelletron Dosimetry ...... 181

7.9.1 Source of Errors in Ethylene Analysis ...... 183

7.9.2 Source of Errors in Current Measurements ...... 184

7.10 퐻2 − 푂2 − 퐴푟 Radiolysis using an Ion Accelerator ...... 186

2+ 7.11 Comparison of γ and 퐻푒 Irradiation of Gaseous 퐻2 − 푂2 − 퐴푟 Samples ...... 187

8 Air Radiolysis Results and Discussion ...... 191

8.1 Ion Chromatogram Calibration ...... 191

8.2 Air Radiolysis ...... 193

8.3 Air Radiolysis in the Presence of an Oxide Surface ...... 195

8.3.1 Comparison of 퐶푒푂2 Data ...... 202

8.3.2 Discussion ...... 203

8.3.3 Comparison of 푍푟푂2 Data ...... 206

8.4 Explanation of Scatter in 50% and 90% (by volume) 푍푟푂2 Results ...... 208

8.5 Refinement of Experimental Data in the Presence of an Oxide Surface ...... 214

8.5.1 Compiled Data ...... 217

8.5.2 Discussion ...... 220

8.6 Oxalate ...... 224

8.6.1 Oxalate Discussion ...... 227

8.7 Synthetic Air ...... 232

8.8 Sintered 퐶푒푂2 ...... 234

8.8.1 Oxide Properties ...... 234

8.8.2 Nitrate Production over Sintered 퐶푒푂2 ...... 237

8.8.3 Comparison with Un-sintered 퐶푒푂2 Results ...... 238

8.8.4 Discussion ...... 239

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9 Final Conclusions ...... 244

9.1 퐻2 − 푂2 System ...... 244

9.2 Air Radiolysis System ...... 245

10 Future work ...... 247

10.1 퐻2 − 푂2 − 퐴푟 System ...... 247

10.2 Air Radiolysis System ...... 248

10.3 퐶2퐻4 System ...... 248

10.4 Generic Recommendations ...... 249

10.5 Future Work with Accelerated Ions ...... 251

11 Bibliography ...... 254

Word Count: 51,594

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List of Figures

Figure 1.1: 푃푢푂2 surface area as a function of calcination temperature ...... 28

Figure 1.2: SEM images of 푃푢푂2 stored at the Sellafield site ...... 29

Figure 1.3: Stainless steel three can system used to package THORP 푃푢푂2 ...... 30

Figure 1.4: Comparison of path length and penetration for a β particle ...... 36

Figure 1.5: Emission of Bremsstrahlung ...... 37

Figure 1.6: Positron annihilation ...... 38

Figure 1.7: Overview of Auger electron emission ...... 39

Figure 1.8: γ-ray interaction processes and their dependence on photon energy and Z of medium ... 40

Figure 1.9: The photoelectric effect at i) low photon energies and ii) high photon energies ...... 41

Figure 1.10: Compton scattering ...... 41

Figure 1.11: Pair production followed by positron annihilation ...... 43

Figure 1.12: Radiolytic track structure of i) α particle and ii) fast electron ...... 45

Figure 2.1: Postulated mechanism of water adsorption onto a 푃푢푂2 surface ...... 50

Figure 3.1: Foss Therapy Model 812 60퐶표 Irradiation source ...... 62

Figure 3.2: Decay scheme for cobalt-60 isotopes ...... 63

Figure 3.3: Schematic of 5 MV Ion accelerator located at DCF ...... 64

Figure 3.4: Bespoke gas mixing manifold system ...... 70

Figure 3.5: Picture of l-r 1 g, 50% oxide (by volume) and 90% oxide (by volume) for 푍푟푂2 samples ... 71

2+ Figure 3.6: Bespoke glassware for 퐻푒 ion radiolysis of 퐻2 − 푂2 − 퐴푟 gaseous mixtures ...... 74

Figure 3.7: Configuration of window assembly through which the beam travels before reaching the

sample ...... 75

Figure 4.1: Reaction vessel for gamma radiation studies of 퐻2 − 푂2 system ...... 76

Figure 4.2: Sample holder for gamma irradiation of 퐻2 − 푂2 system ...... 77

Figure 4.3: GC valve configuration for ‘in-line’ analysis i) ‘Load’ position ii) ‘Inject’ position ...... 78

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Figure 4.4: Calibration of the gas chromatograph using certified calibration standards in the range

0.1-4% 퐻2/퐴푟 ...... 79

Figure 4.5: Gas chromatograms of 2% 퐻2/퐴푟 and 0.5% 퐻2/퐴푟 calibration gases ...... 79

Figure 4.6: Overlain chromatograms of initial trials of 퐻2 − 푂2 radiolysis experiments ...... 81

Figure 4.7: Mechanical degradation of 푃푇퐹퐸 taps ...... 82

Figure 4.8: Stainless steel sampling cylinder for 퐻2 − 푂2 gamma irradiation experiments ...... 83

Figure 4.9: Calibration plot for pure 퐻2 using the direct injection methodology ...... 84

Figure 4.10: Overlay of gas chromatograms highlighting varying injection volumes of pure 퐻2 ...... 85

Figure 4.11: Final vessel iteration to investigate radiolysis of 퐻2 − 푂2 systems ...... 86

Figure 4.12: Final GC valve configuration i) ‘Load’ position ii) ‘Inject’ position ...... 87

Figure 4.13: Plot of sample loop pressure as a function of time for six repeat injections with vacuum

GC configuration ...... 88

Figure 4.14: GC calibration curve of hydrogen partial pressure as a function of peak area for vacuum

sampling system ...... 89

Figure 4.15: Plot of 퐻2 peak area as a function of sample loop pressure for a series of samples

containing 5:5:90 퐻2: 푂2: 퐴푟 ...... 91

Figure 4.16: Plots of 퐻2 peak area as a function of sample loop pressure of four different gas mixes i)

2% 퐻2/퐴푟 calibration gas, ii) pure hydrogen gas, iii) 10:90 퐻2: 퐴푟 gas mix from manifold

and iv) 5:5:90 퐻2: 푂2: 퐴푟 gas mix from manifold ...... 93

Figure 4.17: Manifold schematic with new mixing cylinder and 푃푇퐹퐸 stirrer bar addition ...... 95

Figure 4.18: Final iteration of manifold design ...... 95

Figure 4.19: Mixing efficiency of manifold with mixing cylinder for a three component gas mixture . 96

Figure 5.1: Fricke dosimetry results for the test tube rack array showing i) Unattenuated dose rate

ii) Fully attenuated dose rate (units – Gy min-1) ...... 102

Figure 5.2: γ-ray interaction processes and their dependence on photon energy and Z of medium . 105

Figure 5.3: Results of ethylene dosimetry (units – Gy min-1) ...... 114

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Figure 5.4: Plot of current as a function of time for a 15 min irradiation using the ion accelerator

showing the current measured on the 푇𝑖 window ...... 116

Figure 6.1: Scanning electron micrograph of 퐶푒푂2 (as received) ...... 118

Figure 6.2: EDS spectrum of 퐶푒푂2 (as received) ...... 119

Figure 6.3: BET adsorption (solid trace) - desorption (dashed trace) of 퐶푒푂2 (as received) ...... 119

Figure 6.4: DRIFT spectra of 퐶푒푂2 (as received) ...... 120

Figure 6.5: SEM images of regenerated 퐶푒푂2 illustrating the macrostructure of the powder (top) and

a large particle (bottom) ...... 122

Figure 6.6: BET adsorption (solid trace) – desorption (dashed trace) isotherm for regenerated 퐶푒푂2

...... 123

Figure 6.7: Thermogram of regenerated 퐶푒푂2 decomposed under 푁2 (blue) and static air (red).

Heating rate 10 °C min-1 ...... 124

Figure 6.8: DRIFT spectrum of regenerated 퐶푒푂2 ...... 125

Figure 6.9: DRIFT spectra of 퐶푒푂2 (as received) and regenerated 퐶푒푂2 upto five subsequent

regeneration cycles ...... 126

Figure 6.10: SEM image of 푍푟푂2 (as received) ...... 127

Figure 6.11: EDS spectrum of 푍푟푂2 (as received) ...... 128

Figure 6.12: BET adsorption (solid trace) – desorption (dashed trace) isotherm of 푍푟푂2 (as received)

...... 128

Figure 6.13: DRIFT spectrum of 푍푟푂2 (as received) ...... 129

Figure 6.14: SEM images of regenerated 푍푟푂2 illustrating large agglomerated particles ...... 131

Figure 6.15: EDS spectrum of regenerated 푍푟푂2 ...... 132

Figure 6.16: Thermogram of regenerated 푍푟푂2 decomposed under 푁2 (blue) and static air (red).

Heating rate 10 °C min-1 ...... 133

Figure 6.17: BET adsorption (solid trace)-desorption (dashed trace) isotherm of regenerated 푍푟푂2 134

Figure 6.18: DRIFT spectrum of regenerated 푍푟푂2 ...... 135

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Figure 6.19: DRIFT spectra of 푍푟푂2 (as received) and regenerated 푍푟푂2 upto four subsequent

regeneration cycles ...... 137

Figure 7.1: Comparison of dose rates obtained by different chemical dosimeters (units Gy min-1)

i) Fricke dosimetry and ii) ethylene dosimetry ...... 140

Figure 7.2: Plot of scatter in each sample of ethylene as a function of the peak area of 퐻2 in the first

injection ...... 142

Figure 7.3: Gas chromatogram overlay of three subsequent injections of post irradiated ethylene

highlighting the 퐻2 signal ...... 143

Figure 7.4: Gas chromatogram of two separate ethylene samples irradiated for i) 540 min and

ii) 5760 min ...... 144

Figure 7.5: Results of ethylene dosimetry at increased pressure (units – Gy min-1) ...... 146

Figure 7.6: Hydrogen production as a function of absorbed dose from water adsorbed to 푍푟푂2

(primary y-axis) and 퐶푒푂2 (secondary y-axis) ...... 150

Figure 7.7: Gas chromatograms showing a comparison of the 퐻2 signal of irradiated ethylene (퐶2퐻4)

(blue trace), 퐶푒푂2 in 퐴푟 atmosphere (green trace) and 퐶푒푂2 in ethylene (퐶2퐻4) (red trace)

irradiated for 9 h in identical radiation fields ...... 153

Figure 7.8: Gas chromatograms showing a comparison of the 퐻2 signal of irradiated ethylene (퐶2퐻4)

(blue trace – secondary y-axis), 푍푟푂2 in 퐴푟 atmosphere (green trace – secondary y-axis)

and 푍푟푂2 in ethylene (퐶2퐻4) (red trace – primary y-axis) irradiated for 9 h in identical

radiation fields ...... 154

Figure 7.9: Postulated schematic of ethylene interaction with an oxide surface ...... 156

Figure 7.10: DRIFT spectra of regenerated 퐶푒푂2 pre-irradiation (blue) and post-irradiation (red) in an

ethylene atmosphere ...... 158

Figure 7.11: DRIFT spectra of regenerated 푍푟푂2 pre-irradiation (blue) and post-irradiation (red) in an

ethylene atmosphere ...... 159

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Figure 7.12: DRIFT spectra of irradiated 퐶푒푂2 in an ethylene atmosphere analysed between

20 – 400 °C ...... 160

Figure 7.13: DRIFT spectra of irradiated 푍푟푂2 in an ethylene atmosphere analysed between

20 – 400 °C ...... 161

Figure 7.14: Results of gamma radiolysis of different ratios of 퐻2 − 푂2 − 퐴푟 illustrating 퐻2 depletion

as a function of absorbed dos ...... 163

Figure 7.15: Plot of G(-퐻2) as a function of absorbed dose for several different ratios of 퐻2 −

푂2 − 퐴푟 gas using gamma radiation using the data in Figure 7.14 ...... 165

Figure 7.16: 푂2 depletion as a function of absorbed dose using gamma radiation of different ratios of

퐻2 − 푂2 − 퐴푟 gas mixtures ...... 166

Figure 7.17: Plot of G(-푂2) as a function of absorbed dose for several different ratios of 퐻2 −

푂2 − 퐴푟 gas using gamma radiation using the data in Figure 7.16 ...... 167

Figure 7.18: 퐻2 consumption as a function of absorbed dose for various 퐻2 − 푂2 − 퐴푟 gas mixtures

in contact with 퐶푒푂2 ...... 173

Figure 7.19: Plot of G(-퐻2) as a function of absorbed dose for several different ratios of 퐻2 −

푂2 − 퐴푟 gas in contact with 퐶푒푂2 ...... 174

Figure 7.20: Plot of 퐻2 consumption as a function of absorbed dose for the five gaseous systems of

relevance in contact with 푍푟푂2 ...... 175

Figure 7.21: G(-퐻2) as a function of absorbed dose for five gaseous mixtures of 퐻2 − 푂2 − 퐴푟

irradiated in contact with 푍푟푂2 ...... 176

Figure 7.22: 퐻2 consumption as a function of absorbed dose in samples of 5: 5: 90 퐻2 − 푂2 − 퐴푟

concentration in contact with 퐶푒푂2 and 푍푟푂2 and in pure gas system only ...... 177

Figure 7.23: 퐻2 consumption as a function of absorbed dose in samples of 5: 2.5: 92.5 퐻2 − 푂2 − 퐴푟

concentration in contact with 퐶푒푂2 and 푍푟푂2 and in pure gas ...... 179

Figure 7.24: 퐻2 consumption as a function of absorbed dose in samples of 2.5: 5: 92.5 퐻2 − 푂2 − 퐴푟

concentration in contact with 퐶푒푂2 and 푍푟푂2 and in pure gas ...... 180

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Figure 7.25: Plot of 퐻2 production as a function of absorbed dose for ethylene experiments using an

ion accelerator ...... 182

Figure 7.26: Plot of pressure as a function of time for a sample of ethylene irradiated using 5.5 MeV

퐻푒2+ ions. Irradiation time 30 min, 10 nA current on sample ...... 183

Figure 7.27: 퐻2 depletion as a function of absorbed dose for three different mixtures of

2+ 퐻2 − 푂2 − 퐴푟 utilising 5.5 MeV 퐻푒 accelerated ions ...... 186

Figure 7.28: 퐻2 depletion as a function of absorbed dose for three various mixtures of 퐻2 − 푂2 − 퐴푟

utilising 60퐶표 γ-rays and 5.5 MeV 퐻푒2+ accelerated ions ...... 188

− Figure 8.1: Calibration plot of 푁푂3 peak area as a function of 푁푎푁푂3 concentration using ion

chromatography ...... 192

Figure 8.2: Effect of γ radiation dose on nitrate production from laboratory air ...... 193

Figure 8.3: Effect of γ radiation dose on the production of nitrate in water saturated and unsaturated

laboratory air. Volume of air = 11.9-12.0 cm3 at 35 °C ...... 194

Figure 8.4: Nitrate production as a function of dose for systems containing 1 g of either 퐶푒푂2 or 푍푟푂2

powder and water saturated air (no oxide) ...... 196

Figure 8.5: Nitrate production as a function of absorbed dose for systems containing 50% oxide (by

volume) and for water saturated air (no oxide) ...... 199

Figure 8.6: Nitrate production as a function of dose for systems containing 90% oxide (by volume)

and for water saturated air (no oxide) ...... 201

Figure 8.7: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume) 퐶푒푂2

systems and for water saturated air (no oxide) ...... 202

Figure 8.8: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume) 퐶푒푂2

systems and for water saturated air (no oxide) up to an absorbed dose of 2.0x1019 eV .. 203

Figure 8.9: i) Face-centred cubic crystal structure unit cell, and ii) atomic structure of each face in the

unit cell ...... 205

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Figure 8.10: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume)

푍푟푂2 systems and for water saturated air (no oxide) ...... 207

Figure 8.11: Three ion chromatograms of samples containing: i) 1 g, ii) 50% and iii) 90% 푍푟푂2 (by

volume) illustrating the emergence of a second signal at 6.6 min...... 209

Figure 8.12: Ion chromatogram of 50 μM oxalic acid, 0.1 mM sodium nitrate and a mixed solution of

both ...... 212

Figure 8.13: Chromatogram of 0.1 mM 푁푎푁푂3 and 50 μM oxalic acid mixed solution using eluent

concentration of 14 mM 퐾푂퐻 ...... 213

Figure 8.14: Nitrate production as a function of dose for samples containing 1 g of oxide powder and

for water saturated air (no oxide) ...... 214

Figure 8.15: Nitrate production as a function of dose for samples containing 50% (by volume) 퐶푒푂2

and 푍푟푂2 and from water saturated air (no oxide) ...... 215

Figure 8.16: Nitrate production as a function of dose for samples containing 90% (by volume) 퐶푒푂2

and 푍푟푂2 and from water saturated air (no oxide) ...... 216

Figure 8.17: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume)

퐶푒푂2 systems and for water saturated air (no oxide) ...... 217

Figure 8.18: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume)

푍푟푂2 systems and water saturated air (no oxide) ...... 219

− Figure 8.19: Pictorial representation of 푁푂3 bonding modes with metal centres depicting (l-r)

monodentate, bidentate and bridging adsorption modes ...... 222

2− Figure 8.20: Calibration plot of 퐶2푂4 peak area as a function of 퐻2퐶2푂4 concentration using ion

chromatography ...... 224

Figure 8.21: Plot of oxalate production as a function of absorbed dose for samples containing 1 g,

50% and 90% (by volume) of 퐶푒푂2 and from water saturated air (no oxide) ...... 225

Figure 8.22: Plot of oxalate production as a function of absorbed dose for samples containing 1 g,

50% and 90% (by volume) of 푍푟푂2 and from water saturated air (no oxide) ...... 226

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Figure 8.23: Thermogravimetric analysis of cerium oxalate under 푁2 (blue) and static air (red)

atmospheres. Heating rate 2 °C min-1 ...... 227

Figure 8.24: Plot of nitrate production as a function of absorbed dose for synthetic air, laboratory air

and water saturated laboratory air. Volume of air =11.9 - 12.0 cm3 at 35 °C ...... 233

Figure 8.25: BET adsorption (solid trace) – desorption (dashed trace) isotherm of 퐶푒푂2 sintered at

950 °C for 2 h...... 235

Figure 8.26: i) Scanning electron micrograph and ii) EDS spectra for sintered 퐶푒푂2 ...... 236

Figure 8.27: Nitrate production as a function of absorbed dose for samples containing 1 g, 50% and

90% (by volume) sintered 퐶푒푂2 and from water saturated air (no oxide) ...... 237

Figure 8.28: Compiled data plot of nitrate production as a function of dose for systems containing

1 g, 50% and 90% (by volume) regenerated 퐶푒푂2, 1 g, 50% and 90% (by volume) sintered

퐶푒푂2 and from water saturated air (no oxide) ...... 238

− Figure 8.29: Plot of G(푁푂3 ) as a function of surface area for the three oxide systems utilised in this

research and for reference, the water saturated air (no oxide) yield ...... 240

Figure 10.1: Sketch of possible reaction vessel to study heterogeneous systems using an ion

accelerator ...... 252

Figure 10.2: Second possible reaction vessel for heterogeneous system experiments using an ion

accelerator ...... 253

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List of Tables

Table 7-1: Calculated G(-퐻2) values for several different ratios of 퐻2 − 푂2 − 퐴푟 gas using gamma radiation ...... 164

Table 7-2: Calculated G(-푂2) values for several different ratios of 퐻2 − 푂2 − 퐴푟 gas using gamma radiation ...... 166 Table 7-3: Ionisation energy of the three gas molecules in the initial system ...... 168

Table 7-4: Calculated G(-퐻2) in samples containing 5: 5: 90 퐻2 − 푂2 − 퐴푟 (by volume) in the

presence of 퐶푒푂2, 푍푟푂2 and in pure gas phase ...... 178 2+ Table 7-5: Calculated G(-퐻2) values from experiments utilising 5.5 MeV 퐻푒 accelerated ions ..... 187

Table 7-6: Calculated G(-퐻2) values and associated errors for three different mixtures of 60 2+ 퐻2 − 푂2 − 퐴푟 utilising 퐶표 γ-rays and 5.5 MeV 퐻푒 accelerated ions ...... 188 − Table 8-1: Calculated G(푁푂3 ) values and standard deviation of the gradient for 1 g oxide systems and water saturated air (no oxide) ...... 196

Table 8-2: Calculated mass of water in systems containing 1 g 퐶푒푂2, 1 g 푍푟푂2 and water saturated air (no oxide) ...... 197

Table 8-3: Calculated mass of water in systems containing 50% 퐶푒푂2 and 50% 푍푟푂2 (by volume) and water saturated air (no oxide) ...... 198 − Table 8-4: Calculated G(푁푂3 ) values and standard deviation of the gradient for experiment with 50% oxide (by volume) and with water saturated air (no oxide) ...... 199

Table 8-5: Calculated mass of water in systems containing 90% 퐶푒푂2 and 90% 푍푟푂2 (by volume) and water saturated air (no oxide) ...... 200 − Table 8-6: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 90% oxide (by volume) and for water saturated air (no oxide) ...... 201 − Table 8-7: Calculated G(푁푂3 ) values for the 퐶푒푂2 containing systems and for water saturated air (no oxide) ...... 202 Table 8-8: Initial yield of nitrate pre-irradiation ...... 204 − Table 8-9: Calculated G(푁푂3 ) values for the system containing 푍푟푂2 and for water saturated air (no oxide) ...... 207 Table 8-10: Anions and corresponding retention times (in minutes) present in deionised water ...... 210 − Table 8-11: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 1 g oxide and water saturated air (no oxide) ...... 214 − Table 8-12: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 50% oxide (by volume) and water saturated air (no oxide) ...... 215

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− Table 8-13: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 90% oxide (by volume) and water saturated air (no oxide) ...... 216 − Table 8-14: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 1 g,

50% and 90% (by volume) 퐶푒푂2 and water saturated air (no oxide) ...... 218 − Table 8-15: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 1 g,

50% and 90% (by volume) 푍푟푂2 and water saturated air (no oxide) ...... 219 − Table 8-16: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems containing

1 g, 50% and 90% (by volume) sintered 퐶푒푂2 and water saturated air (no oxide) ...... 237 − Table 8-17: Comparison between calculated G(푁푂3 ) for samples containing either regenerated 퐶푒푂2

or sintered 퐶푒푂2 and water saturated air (no oxide) ...... 239

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Abbreviations and Acronyms

AGR - Advanced Gas-Cooled Reactor

B.E.T. - Brunauer – Emmett – Teller Theory

DCF - Dalton Cumbrian Facility

DRIFT - Diffuse Reflectance Infrared Fourier Transform Spectroscopy

ECD - Electrochemical Detector

EDS - Energy Dispersive X-ray Spectroscopy

G (±X) - yield of change (units: molecules 100 eV-1)

G.C. - Gas Chromatography

Gray - SI unit of ionising radiation dose (units: J kg-1)

I.C. - Ion Chromatography

LANL - Los Alamos National Laboratory

L.E.T. - Linear Energy Transfer

Magnox - Magnesium Non-Oxidising Fuel

MOX - Mixed Oxide Fuel

NNL - National Nuclear Laboratory

PUREX - Plutonium and Uranium Recovery by Extraction

SEM - Scanning Electron Microscopy

SSA - Specific Surface Area

TCD - Thermal Conductivity Detector

TGA - Thermogravimetric Analysis

THORP - Thermal Oxide Reprocessing Plant

TORVIS - Toroidal Volume Ion Source

UHV - Ultra-high vacuum

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Abstract

The University of Manchester Luke Jones

Thesis submitted for the degree of Doctor of Philosophy

The Radiation Chemistry of Gases at the Interface with Ceramic Oxides

October 2015

As of 2011, the UK had 112 tonnes (t) of plutonium dioxide (푃푢푂2) in interim storage at the Sellafield site and this is increasing by approximately 5 t per annum with the continued reprocessing of spent nuclear fuel. 푃푢푂2 is stored in small quantities in a sealed multi- canister system for security and ease of handling. During long term storage, radiolysis of the gas phase and adsorbed species could potentially lead to canister pressurisation and/or failure. It is of great importance to understand the mechanisms occurring in the gas phase and to understand the resulting gas phase composition after decades of storage.

This research investigates the radiation chemistry of two gas phase systems in the presence or absence of inactive 푃푢푂2 surrogate material (namely cerium dioxide (퐶푒푂2 ) and zirconium dioxide (푍푟푂2)).

The systems of interest are, firstly, radiolysis of hydrogen (퐻2), oxygen (푂2) and argon gas 60 2+ mixtures utilising both 퐶표 gamma rays and 퐻푒 accelerated ions. Depletion of 퐻2 and 푂2 has been investigated using gas chromatography. A bespoke manifold has been designed to mix these gases in various ratios, suitable reaction vessels and a subsequent sampling system has been developed to undertake this research. The rate of 퐻2 depletion is independent of initial 퐻2 concentration and radiation type. In the presence of an oxide surface, the rate of 퐻2 depletion is vastly increased when compared to homogeneous 60 studies using 퐶표 gamma rays. Depletion is greatest in the presence of 푍푟푂2. In all systems, depletion of 퐻2 is linear with increasing absorbed dose. The second system of interest is the radiolysis of moist air utilising 60퐶표 gamma rays. Formation of nitric acid (퐻푁푂3) has been investigated using ion chromatography to − determine nitrate (푁푂3 ) anion production. Nitrate production increases linearly with absorbed dose and is greater in the presence of an oxide powder. The rate of production 2− increases with increasing mass of oxide. Oxalate (퐶2푂4 ) was produced radiolytically from dimerisation of carbon dioxide and was greatest in the presence of 푍푟푂2. Reducing the specific surface area of 퐶푒푂2 reduced the concentration of nitrate formed when compared to higher surface area 퐶푒푂2.

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Declaration

No portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning

………………………………………..

Luke Jones November 2015

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The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the “Copyright”) and he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes.

Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made.

The ownership of certain Copyright, patents, designs, trademarks and other intellectual property (the “Intellectual Property”) and any reproductions of copyright works in the thesis, for example graphs and tables (“Reproductions”), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions.

Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=487), in any relevant Thesis restriction declarations deposited in the University Library, The University Library’s regulations (see http://www.manchester.ac.uk/library/aboutus/regulations) and in The University’s policy on Presentation of Theses

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Acknowledgements

There are many people I would like to thank for their input throughout the course of this research.

Firstly, I would like to thank Professor Simon M. Pimblott for selecting me to undertake this research. I am grateful for his supervision and input and advice with regards to literature components and experimental suggestions.

Thanks to the EPSRC and Sellafield Sites Ltd. for providing funding for this research.

My gratitude’s go to Martin Jennings from the School of Chemistry and Alastair Bewsher from the School of Earth, Atmospheric and Environmental Sciences for undertaking thermogravimetric analysis and ion chromatography troubleshooting respectively.

I would like to acknowledge Dr Sven Koehler for helping me with the design of the bespoke gas mixing manifold and other bespoke experimental equipment. His insight into surface and gas phase chemistry was much appreciated. Also for his non-related bicycle knowledge.

Thanks to Paul Cook and Jeff Hobbs (both of Sellafield Sites Ltd.) for their advice and being useful sources of reference, and for putting this research into context.

I am extremely grateful to both Howard E. Sims and Robin M. Orr (both of National Nuclear Laboratory) for their pseudo-supervision and source of encouragement throughout this research. They have both given an almighty amount of time and effort to enhance this project and ensured momentum was maintained. Howard, for his never-ending knowledge of radiation chemistry and his in-depth mechanistic discussions and Robin, for his steel trap-esque data analysis and his help with gas chromatography and experimental advice.

I would like to thank all the staff and researchers at the Dalton Cumbrian Facility for making it an enjoyable (and sometimes challenging) place to work. In particular I would like to thank fellow researchers Logan Barr, Gregory Horne and Rhiannon Monckton (the fellow trolls) for helping me maintain my sanity and providing an atmosphere for my sense of humour to flourish, to help stave off cabin fever during the first year of this project and finally, for their scientific discussions on anything radiation related.

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My largest thanks go to Jan, Dave and Adam (a.k.a. mum, dad and big brother) for their help in making the transition from Oldham to West Cumbria as smooth as possible, for always being at the end of the telephone to offer reassurance and advice and mostly just for being themselves.

Finally (and by no means least) to my partner Rebecca, for putting up with my occasional moods and sarcastic nature, for always being there and offering a sympathetic ear to my random nonsensical rants, for proof-reading all of my reports and for always being able to put a smile on my face.

Throughout the highs and lows of this research, many people have reassured me with the phrase ‘it builds character’ and in a way, they have all been right.

“Most people say that it is intellect which makes a great scientist. They are wrong: it is character.” - Albert Einstein 1879-1955 German-born theoretical physicist

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The Author

The author graduated from the University of Manchester in 2011 with an MChem. (Hons) degree in Chemistry with Industrial Experience. He spent the third year of his degree course as an industrial placement student at AMEC Power and Process Europe (name has subsequently changed) based at Birchwood, Warrington as part of the Waste Processing

Technology team. This placement included working on projects for several customers including Dounreay Site Restoration Limited (DSRL) and Sellafield Sites Ltd. He spent the final year of his degree undertaking a research project in the Centre for Nanoporous

Materials investigating the synthesis conditions on the crystal morphology of zeolite T.

In September 2011, he joined the Radiation Science research group under the supervision of

Professor Simon Pimblott at the brand new Dalton Cumbrian Facility as a PhD researcher to investigate the radiation chemistry of gaseous systems at the interface with ceramic oxides.

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Thesis Structure

Chapter One outlines the challenge this project hopes to address and gives background information to put this research into context. It concludes with a basic introduction to radiation chemistry and how ionising radiation interacts with matter.

Chapter Two reviews the literature relevant to this research. It highlights gaps in the knowledge base that this research hopes to contribute to. Finally, it outlines the aims and objectives of this project.

Chapter Three highlights the facilities and experimental equipment utilised to undertake this research. It details the materials necessary to carry out this project and details the experimental methods employed to execute this research project.

Chapter Four details the development undertaken in designing a reaction vessel suitable to implement part of this research. It also details the development in analysis techniques to complement the evolving reaction vessel.

Chapter Five introduces the concept of dosimetry and the challenges this provides with regards to this research. Relevant literature is reviewed to provide information on how these challenges are addressed in other systems and concludes with how dosimetry is undertaken in the different systems of interest to this research.

Chapter Six details the physical properties of the solid materials utilised throughout the course of this research.

Chapter Seven details the results pertaining to hydrogen-oxygen radiolysis in homogeneous and heterogeneous systems. These results are discussed and hypotheses given throughout the chapter. Finally preliminary results using accelerated ions are detailed and discussed.

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Chapter Eight details the results from air radiolysis experiments. These results are analysed and discussed throughout the chapter.

Chapter Nine pulls together the conclusions drawn from the results detailed in Chapters

Seven and Eight and places these results in the context of the research.

Chapter Ten recommends further experiments that can be undertaken to develop this research further and answer the questions that have been posed as a result of this research project.

Chapter Eleven is a bibliography of all the literature sources used throughout this thesis as a source of information and reference.

Chapter 1 Introduction 7131060

1 Introduction

This chapter gives context to this research, to outline the challenges this research hopes to address and the information it aims to contribute to. It also provides an introduction to radiation chemistry including sources of radiation and how radiation interacts with matter, forming radiolytic tracks.

The Challenge

There is currently ~120 t of plutonium dioxide (푃푢푂2) in storage at the Sellafield site inside sealed metal canisters. Some of this plutonium stockpile has been in storage for several decades, and will continue to be stored for several more. Whilst in storage, these canisters are dynamic systems, with the overlying gas phase undergoing irradiation from plutonium as it radioactively decays. The centre-line temperature of the majority of these canisters can be several hundred °C. During long term storage, it is possible that some of these canisters may pressurise and could potentially fail. Due to the quantity and contents of the canisters, it is difficult to monitor each canister for possible swelling and/ or failure. There is also minimal knowledge of the composition of the gas phase as a function of time during the long term storage of this plutonium stockpile.

This research aims to investigate the gas phase radiation chemistry of two gaseous systems of interest at the interface of a 푃푢푂2 surrogate material.

This research aims to deliver a better mechanistic understanding of the gas phase radiation chemistry in contact with an oxide powder.

It also hopes to aid the safety case for long term storage of 푃푢푂2.

Chapter 1 Introduction 7131060

1.1 Background

The UK currently has the largest stockpile of civil separated plutonium in the world.

Plutonium is a by-product of using uranium metal and uranium oxide (푈푂2), as fuel in nuclear reactors. The spent fuel from the reactor contains approximately 96% uranium and

1% plutonium by weight [1], whilst the remainder is made up of highly radioactive fission products and minor actinides. It is possible to reuse the spent uranium as new reactor fuel, however, it must be separated from the fission products and minor actinides first. This recycling is achieved by reprocessing the spent fuel. Recycling of the uranium allows more energy to be generated without the use of new supplies, leading to a more sustainable energy source. In addition, separating uranium from the spent fuel, reduces the volume of waste generated significantly. Separation of the highly radioactive fission products from the spent fuel allows for them to be treated separately before going to disposal. The plutonium generated in spent fuel can also be used as a reactor fuel by blending plutonium dioxide

(푃푢푂2) with 푈푂2 in ratios of 7:93 to create a mixed oxide fuel (MOX). This approach leads to further energy generation from a single batch of reactor fuel [1].

At present, most nuclear reactors that utilise uranium fuels are allowed a maximum of 33% loading of MOX fuel in the core due to safety concerns [2]. A different type of nuclear reactor (termed ‘fast’ reactor) can utilise plutonium on its own as a fuel type, however, there are no operational commercial ‘fast’ reactors in the world at this moment in time and construction is unlikely to begin until the 2040s. These facts, along with continued reprocessing of spent fuel leads to large inventories of 푃푢푂2 stored around the world.

As of 2011, the UK currently had 112 t of civil separated plutonium in storage [3], the majority of which is stored at the Sellafield site as 푃푢푂2 powder. There are two product

Chapter 1 Introduction 7131060

streams at Sellafield by which this powder is formed. Magnox 푃푢푂2 is obtained from spent fuel from Magnesium non-oxidising (Magnox) reactors which used uranium metal, while

푃푢푂2 product from the THermal Oxide Reprocessing Plant (THORP) is obtained from spent fuel from Advanced Gas-Cooled (AGR) reactors which use 푈푂2 as the fuel type.

To separate the uranium and plutonium from the fission products and minor actinides, the

Plutonium and Uranium Recovery by EXtraction (PUREX) process is employed. This process has been used globally for several decades as a means of reprocessing spent fuel. The spent fuel is first dissolved in nitric acid (퐻푁푂3) before using an organic ligand, tri-butyl phosphate

(푇퐵푃) and odourless kerosene (푂퐾) to separate the dissolved species based on their relative solubility in the organic and aqueous phases [4]. This PUREX process is highly specific for uranium and plutonium and allows over 99% recovery of these species. Control of the 퐻푁푂3 concentration allows separation of uranium from plutonium due to their differing redox chemistry. Once separated, each component is put through an individual purification cycle to generate an oxide material. There are several steps involved in each purification cycle which will not be covered in detail here. The main process involved in the plutonium purification cycle is the reaction of the dissolved plutonium species with oxalic acid (퐻2퐶2푂4) to form a plutonium oxalate (푃푢(퐶2푂4)2. 6퐻2푂) precipitate. The oxalate derivative is then washed and thermally decomposed in an oxygen (푂2) environment to produce the finished oxide product (Reaction 1.1):

3푃푢(퐶2푂4)2. 6퐻2푂 + 푂2 → 3푃푢푂2 + 8퐶푂2 + 4퐶푂 + 8퐻2푂 Reaction 1.1

Chapter 1 Introduction 7131060

After the oxalate decomposition step (Reaction 1.1), the oxide powder is calcined at higher temperatures to remove adsorbed volatiles and moisture. The moisture content is required to be less than 0.5 wt.% before the product can go into storage, in accordance with the

United States Department of Energy standard [5]. The temperature at which the 푃푢푂2 product is calcined can have an effect on the physical properties of the powder. One such property is the surface area of the finished product. Figure 1.1 shows how increasing the calcination temperature reduces the surface area as the powder starts to sinter and loses any porosity.

14.0

- g

12.0 2 10.0 8.0 6.0 4.0

2.0 B.E.T. Surface Area / m / Area Surface B.E.T. 0.0 400 600 800 1000 1200 Calcination temperature / oC

Figure 1.1: 푃푢푂2 surface area as a function of calcination temperature (derived from [6])

Higher calcination temperatures remove more of the adsorbed species and reduce the quantity of residual carbon from the oxalate. In the United States, 푃푢푂2 is calcined in an oxidising atmosphere at 950 °C for a minimum of two hours before being stored. The trade- off is that lower surface area reduces the efficiency of potential MOX fuel. There is little open source information on the calcination conditions used for 푃푢푂2 produced in the UK, however, the calcination temperature employed is thought to be nearer to 600 °C.

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The calcination temperature can also affect the morphology of the resulting 푃푢푂2 grains.

Machuron-Mandard and Madic [6] discovered two distinct morphologies of 푃푢푂2 grains.

One is a smooth grain shape corresponding to a truncated octahedra, which is consistent with a cubic crystal structure. The second structure is less defined with a rough surface.

With increasing calcination temperature, the quantity of the octahedral grains increases.

However it is dependent on the synthesis procedure. Figure 1.2 shows electron micrographs of 푃푢푂2 currently in storage in the UK [7, 8].

Figure 1.2: SEM images of 푃푢푂2 stored at the Sellafield site

The image on the left shows the truncated octahedra crystal morphology typical of the fluorite structured oxides. The image on the right shows a more agglomerated structure with flat platelet type crystals. The 푃푢푂2 products from the two streams at Sellafield have both types of crystal morphology.

After calcination, the 푃푢푂2 product is packaged in multi-can containers. In the UK there are two designs. Magnox 푃푢푂2 is packaged in a screw top aluminium can, placed inside a polyethylene bag and welded into a stainless steel outer can. The atmosphere inside the can is a 50:50 mix of argon and air. 푃푢푂2 from the THORP product line is packaged in a stainless steel three can system with a pure argon atmosphere [7, 9]. Figure 1.3 illustrates the cans

Chapter 1 Introduction 7131060

used to package THORP 푃푢푂2 product. Only small quantities (< 1 kg) of 푃푢푂2 are packaged in each can for ease of handling and to avoid criticality.

Figure 1.3: Stainless steel three can system used to package THORP 푃푢푂2

There has been growing national and international pressure to determine a long term management strategy for the UK’s plutonium stockpile. At the highest level, there are three credible options [10]:

 long term storage in a time bound manner;

 immobilisation and disposal as waste; and

 re-use as reactor fuel followed by management of spent fuel and subsequent

disposal.

The default option is long term storage, with the Sellafield site plan suggesting the material will be stored until the site end point in 2120. The continued storage of plutonium materials will require new stores and infrastructure to be built at a substantial cost.

Prolonged storage of plutonium, results in the gradual conversion into different isotopes as the isotopes move along their natural radioactive decay sequence. Plutonium-241 (241푃푢), may be present in the stored material, which undergoes β-decay to americium-241 (241퐴푚),

Chapter 1 Introduction 7131060 which is an α-emitter and also emits γ-rays. The half-life of 241푃푢 is fourteen years, therefore prolonged storage leads to a significant in-growth of 241퐴푚. The presence of this isotope creates significant challenges in storage of this material and also leads to other options becoming harder to execute.

There are several options for immobilisation of the plutonium stockpile, these being: encapsulation in cement, vitrification in glass or immobilisation in a ceramic matrix. There are concerns, however, about the maturity of the technologies; about the possibility of extracting the plutonium material at a later date; and about the environmental impact of disposing of 120 t of plutonium.

Conversion of the stockpile into MOX fuel for use in thermal reactors or conversion to fuel to be utilised in fast reactors has many advantages, including power generation and good proliferation resistance. Considerable research and development into the use of plutonium fuels in next generation reactors such as fast and high temperature reactors [10] is being undertaken in France, however, this option comes with the substantial financial costs of building fabrication plants, new reactors, and waste remediation facilities and supporting infrastructure.

Whichever option is to be selected in the future by the UK government, all require opening of the multi-can system in Figure 1.3. Whilst in storage, the cans are a dynamic system.

푃푢푂2 and its radioactive daughters undergo decay and generate heat. This environment will potentially lead to chemistry occurring with the overlying headspace and any adsorbed species on the oxide surface. A number of processes may occur which could lead to pressurisation of the cans and possible can failure. Therefore an understanding of the radiation chemistry of the gas phase in the presence of an oxide surface is of importance.

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1.2 Radiation Chemistry

The following section outlines the sources of ionising radiation, how these interact with matter and, finally, how this interaction leads to radiation induced chemistry.

1.2.1 Sources of Radiation

Radiation is a process by which energetic particles or photons travel through a medium. It is generally split into two types: ionising and non-ionising. Non-ionising radiation has either no mechanism to transfer energy to the electrons of the material or insufficient energy to ionise matter and instead, on interaction with an atom or molecule, may cause excitation within it leading to processes such as luminescence, dissociation, etc... Examples of non- ionising radiation include visible light, infra-red and microwaves. This type of radiation is not utilised in this research. Ionising radiation is radiation that does have enough energy to ionise a given species. This is mostly done by ejection of an electron out of a valence shell to produce a positively charged ion and an energetic electron. Ionising radiation can also lead to excitation when interacting with matter.

Radiation chemistry is the study of chemical and physical effects that are produced when a material is exposed to high energy, ionising radiation. There are two sources of ionising radiation, firstly, natural or artificial radioactive isotopes and, secondly, those that rely on a form of particle accelerator.

Radioactive isotopes: These are unstable elements that undergo decay emitting particles and/or photons. Radioactive decay is a spontaneous nuclear transformation that is unaffected by pressure, temperature and chemical form of the decaying species. This allows radioactive decay to be characterized by the decay period and the mode and energy of the decay [11].

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Half-life: The decay period is expressed as a half-life (symbol 푡1). The half life is the time 2 required for half of the radioactive atoms in a sample to undergo decay. Half-lives can range from less than a second to 1x1019 years.

α- decay: An α particle is a helium atom that has been stripped of both electrons and is

4 2+ denoted by 2퐻푒 . Alpha decay is observed naturally for elements heavier than lead and certain lanthanides. Alpha particles are emitted by nuclei with discrete energies that are characteristic of the decaying nuclei, meaning different α emitters are easy to distinguish by alpha spectroscopy. Alpha particles usually have energies in the range of 4-9 MeV, with α- decay from 239푃푢 having energy of 5.593 MeV. The energy of α decay is split between the daughter nuclide and the α particle. The majority of which goes to the lighter α particle.

β- decay: Beta decay is any one of three processes: negatron emission, positron emission and electron capture. Beta particles are fast moving electrons or positrons that don’t have discrete energies. Their energy ranges from zero to a maximum energy, (denoted 퐸훽), that is characteristic of the decaying element. The value of 퐸훽 determines the greatest range the particle will have in a given medium.

3 Negatron decay is characteristic of lighter, neutron rich nuclei such as tritium (1퐻). At the atomic level, a neutron is transformed into a proton with the emission of a negatron

(electron).

Positron decay occurs for low and medium mass, neutron poor nuclei. At the atomic level, a proton is transformed into a neutron with the emission of a positron.

Along with positron and negatron emission, another particle is emitted; this is the neutrino

(symbol ν). It has zero mass and charge, but does have spin and energy. The neutrino is

Chapter 1 Introduction 7131060 emitted alongside positron decay and the anti-neutrino (symbol 휐) is emitted alongside negatron decay.

Another process of β decay that does not involve the emission of a negatron or positron is electron capture. In this intra-molecular transformation process, a proton and an atomic electron are transformed into a neutron.

γ- decay: Isotopes that undergo gamma decay emit electro-magnetic radiation with energies ranging between 40 keV to 4 MeV. The electro-magnetic radiation reflects the transition between energy levels of the same nucleus. Gamma rays either have monoenergetic energies or a small number of discrete energies that are characteristic of the decaying nucleus.

Gamma decay occurs alongside other types of radioactive decay (either α or β). When a parent nucleus undergoes α or β decay, the daughter nuclide may be in an excited state. It then loses this excess energy by emitting one or several γ-rays.

The second source of ionising radiation employs particle accelerators. These produce a focused beam of accelerated electrons or positive particles with energies usually ranging from keV to MeV. The most commonly used positive particles are protons, deuterons and helium ions, but heavier ions can be produced if needed. More detail of the ion accelerator at the Dalton Cumbrian Facility (DCF) will be given in Chapter Three.

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1.2.2 Radiation Interactions with Matter

α-particles: Alpha particles predominantly lose their energy by inelastic collisions with electrons of the medium in their path. Some of the kinetic energy of the α particle is transferred to the electron in an excitation event. However, due to the alpha particles heavier mass when compared to an electron, only a small fraction of its energy is transferred to the electron, thus only slight deflection occurs.

The maximum energy transferred from the alpha particle to the electron can be calculated using the conservation of both energy and momentum and is given by Equation 1.1 [12]:

4푚푀 푄 = [ ] 퐸 Equation 1.1 푚푎푥 (푚+푀)2

1 푀푉2 퐸 = 2 Equation 1.2 where 푀 and 푉 are the mass and velocity of the incident alpha particle and 푚 is the mass of the electron.

Alpha particles only have a small penetration in liquids and solids (a few µm) before losing the majority of their energy. In addition, as their mass is much larger than the electron there is little deflection of the alpha particle when it collides with electrons. Its track through a medium is fairly straight, creating a column of excited and ionised species.

β-particles: Like alpha particles, β particles lose their energy predominantly by inelastic collisions with electrons. As their mass is equivalent to the electrons of the material, a β particle can lose all of its energy in a single collision and can be deflected by a large angle. It is difficult to differentiate between an incoming and the ejected electron, so the maximum energy transfer in this case is half the kinetic energy of the incident electron. Deflection may

Chapter 1 Introduction 7131060 also occur when a β particle passes close to an atomic nucleus. As a result of these deflections, β particles with the same initial energy can have different ranges in a medium, however, there will be a maximum distance of penetration. As a consequence of these deflections, the path length travelled by the β particle will be far greater than the penetration into a medium (Figure 1.4):

Figure 1.4: Comparison of path length and penetration for a β particle

The relative importance of Bremsstrahlung, inelastic and elastic collisions depends heavily on the energy of the incident particle and the nature of the absorbing material.

Bremsstrahlung (breaking radiation) occurs when an electron passes close to an atomic nucleus and is decelerated, thus changing direction. With this change in velocity there is a decrease in energy of the particle. The lost energy is emitted as an X-ray (Figure 1.5):

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Figure 1.5: Emission of Bremsstrahlung

This process is favoured for high energy electrons and high atomic number, (Z), stopping materials. The emitted X-ray has an energy range from near zero to the energy of the incident electron and is equal to the energy lost by the particle as it is decelerated. The X-ray does not produce excitation or ionisation unless it subsequently interacts with the medium.

If the initial electron energy is below 100 keV, then Bremsstrahlung emission is negligible.

Lower energy electrons (such as negatrons) lose their energy predominantly by inelastic collisions with the medium and may also undergo elastic scattering, where they are deflected by the Coulomb field of the atomic nucleus. Eventually they will be absorbed by the medium.

The positron is an antiparticle of an electron and is short lived; undergoing positron annihilation (Figure 1.6). During annihilation, two electron masses are converted to electromagnetic radiation. The kinetic energy of the particles is near zero and so the total energy of the annihilation process is 1.02 MeV. This value is two times the value of the equivalent electron mass (511 keV):

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Figure 1.6: Positron annihilation

In order to conserve momentum, the photons are emitted 180 ° to each other.

During β and γ decay, and following collisions and ionisation of a core electron, electrons may be emitted that have distinct energies. These monoenergetic electrons are known as conversion electrons and Auger electrons. Conversion electrons may accompany gamma ray emission from the nucleus. The gamma ray can interact with atomic electrons and be absorbed which gives the electron enough energy to be ejected. The energy of a conversion electron is the energy of the gamma ray minus the binding energy of the electron in the atom.

Auger electrons originate from electron rearrangement. After electron capture or ionisation of a core electron occurs, there is a core electron vacancy that is immediately filled by an electron from an outer orbital. This electron rearrangement means there is excess energy in the system which can either be lost by X-ray emission or the emission of another electron from the atom (an Auger electron). If an Auger electron is emitted, a vacancy is created which is then filled by another electron from an outer orbital. This can lead to an electron cascade unless an X-ray is emitted (Figure 1.7). If an electron cascade does occur, this will

Chapter 1 Introduction 7131060 lead to a highly charged species which will eventually capture electrons from the surroundings to create a stable species.

Figure 1.7: Overview of Auger electron emission

Electromagnetic Radiation

There are 4 main processes by which γ-rays interact with matter:

 coherent (Rayleigh) scattering;

 photoelectric effect;

 Compton scattering; and

 pair production

The latter three are the most important in radiation chemistry and depend greatly on the photon energy and the atomic number of the stopping material as is shown in Figure 1.8:

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120

100

80 photoelectric pair 60 effect production 40 Z of medium of Z Compton 20 effect 0 0.01 0.1 1 10 100 hν / MeV

Figure 1.8: γ-ray interaction processes and their dependence on photon energy and Z of medium (replicated from [13])

Photons can scatter with little loss of energy when they are absorbed and re-emitted by atomic electrons; this is coherent scattering and is most likely to take place when the photons have low energies (< 0.1 MeV) and high atomic number materials. It occurs when the probability for the photoelectric effect to take place is large. It is often neglected because the small angle of scatter makes these photons hard to differentiate from the main beam of photons unless a narrow beam is used.

Photoelectric effect: This is predominant for low energy photons. The process involves the entire energy of an incident photon (퐸표) being transferred to a single atomic electron which is then ejected from the atom with energy (퐸퐸) equal to the difference between the incident energy and the binding energy of the electron in the atom (퐸푆).

At lower photon energies, the electron is ejected close to 90 ° from the incident beam. As photon energy increases, the ejection angle gets smaller and the electron is ejected in the same direction as the beam (Figure 1.9):

Chapter 1 Introduction 7131060

i) ii)

Figure 1.9: The photoelectric effect at i) low photon energies and ii) high photon energies

Energy and momentum are conserved by recoil of the remaining atom.

Compton scattering: This occurs when a photon is scattered by interaction with an electron.

This interaction leads to the acceleration of the electron and scattering of the photon with less energy. The energy and momentum of the incident photon is shared between the scattered photon and the recoiling electron (Figure 1.10):

Figure 1.10: Compton scattering

The more the photon is scattered (i.e. greater θ value), the larger the amount of energy transferred to the electron. The energy of the recoil electron is equal to the incident photon energy minus the energy of the scattered photon.

Chapter 1 Introduction 7131060

The energy of the recoiling electron has a range from zero to a maximum which can be calculated by assuming the photon scatter angle is 180 ° (i.e. photon rebounds off the electron). The most probable values for 퐸퐸 are those near zero or the maximum energies

(maximum energy is favoured if 퐸표 is high). The direction of the recoil electron follows the initial incident photon path when more of the energy is transferred from the photon to the electron.

Pair production: This occurs when a photon is completely absorbed in the vicinity of an atomic nucleus (or less frequently, an electron). This absorption produces two particles: an electron and a positron. The energy of the photon is shared between the kinetic energy of the electron and positron minus the rest energy of the two particles (both equivalent,

2 therefore both 푚푒푐 ). A small almost negligible amount of energy is transferred to the nucleus. The momentum is shared by the recoiling nucleus and the emitted particles. The positron is slowed down similar to an electron and eventually undergoes positron annihilation (Figure 1.6). The overall process is shown in Figure 1.11:

Chapter 1 Introduction 7131060

Figure 1.11: Pair production followed by positron annihilation

Pair production can only occur if the incident photon has greater energy than 1.02 MeV (the rest mass of the two particles).

Another process that occurs with photons is photonuclear reactions. This process requires very high energy photons which are not utilised as part of this research

Electromagnetic radiation can lose relatively large amounts of its energy in a few interactions with matter, however, only a fraction of γ-rays are absorbed by the medium with the rest being transmitted through with the same incident energy. The total linear attenuation coefficient is given by Equation 1.3:

휇 = 휏 + 𝜎 + 휅 Equation 1.3 where 휏, 𝜎 and 휅 are the linear attenuation coefficients (in cm-1) for the photoelectric,

Compton and pair production processes respectively. Their values are constant for a given photon energy and a given stopping material [12].

A further type of radiation that may occur in radioactive systems is neutron radiation. These are not utilised in this research, therefore are not detailed in this thesis.

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1.2.3 Radiolytic Track Formation

When a charged particle or photon interacts with matter, it gives rise to a track of ionised and excited species. The nature of the species will be identical for a given medium irrespective of the type of radiation utilised. Different types of radiation lose energy at different rates. The concentration of the ionised and excited species will differ and the length of the track will differ. This non-homogeneous concentration profile in turn leads to a difference in observed chemical effects for differing types of radiation, especially for liquid and solid media. For example, 퐻2 production from the radiolysis of an aqueous solution containing potassium nitrate (퐾푁푂3) and nitrous oxide (푁푂), is two and a half times greater using 12 MeV 퐻푒2+ ions compared to 60퐶표 γ-rays [14]. In gaseous media, where the density is much lower, the effect is less prominent. The formation of ethane (퐶2퐻6) from radiolysis of methane (퐶퐻4), is not greatly affected by using either γ-rays from spent fuel elements or

2.8 MeV electron beams [15, 16].

Along with the primary charged particle or photon, ejected electrons (termed secondary electrons) may also cause ionisation and excitation within the medium. If the secondary electron has less than 100 eV of energy, then its range will be short and lead to secondary ionisation events close to the primary track and give rise to a cluster or ‘spur’ of ionisation/excitation events. If the electrons have energies larger than 100 eV, however, then their range increases and can lead to extended secondary tracks of ionised/excited species. Figure 1.12 shows a comparison of track structure from an α particle and a fast electron. These tracks only persist for a short period of time before dissipating in the medium.

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ii)

Figure 1.12: Radiolytic track structure of i) α particle and ii) fast electron

The amount of chemical change a medium undergoes when interacting with ionising radiation depends both on the total energy of the radiation and the rate at which its energy is deposited. The first factor will determine the yield of reactive species and the second factor, their proximity. Linear Energy Transfer (LET) is a term used in radiation chemistry to parameterise these factors. It is defined as the rate of energy loss by an ionising particle or photon traversing a medium (Equation 1.4). The most common units are keV µm-1:

푑퐸 퐿퐸푇 = Equation 1.4 푑푙 where 푑퐸 is the change in particle energy and 푑푙 is the path length of the particle.

Equation 1.4 does not take into consideration the fact that the rate of energy loss decreases as the particle slows down and loses energy. Furthermore, it does not reflect the fact that secondary ionisation/excitation events may take place away from the primary track. For a given energy, photons and high energy electrons have the lowest LET due to the fact they have a high penetration (푑푙) but deposit only a small quantity of energy. In contrast, α

Chapter 1 Introduction 7131060 particles and fission fragments have the highest LET as they have low penetration and deposit the majority of their energy in a single collision.

To denote yields in radiation chemistry, the term G-value is used throughout journals and texts. It has the following symbol G(±푥) and represents the number of molecules of species

푥, either lost or produced in the system per 100 eV of energy deposited in the said system.

Chapter 2 Literature Review 7131060

2 Literature Review

This chapter gives a review of the work previously undertaken in this area of research. This review includes the limited data available with respect to plutonium storage canisters and possible mechanisms occurring inside during storage. Focus is given to one such mechanism; the radiolysis of adsorbed moisture and subsequent gas phase radiation chemistry. With respect to storage of plutonium-bearing materials in the UK, attention is given to the radiolysis of air in the presence of an oxide surface. Finally, this chapter outlines the aims and objectives this research hopes to achieve and how the knowledge base may be enhanced.

2.1 푷풖푶ퟐ Storage Canisters

A substantial amount of work has been undertaken by the Los Alamos National Laboratory

(LANL) with regards to hazards associated with long term storage of 푃푢푂2. Eller et al. have investigated the pressurisation of 푃푢푂2 storage canisters and have hypothesised four possible mechanisms that could lead to pressurisation [17], these are:

 thermal heating;

 helium accumulation from radioactive decay;

 radiolytic and chemical degradation; and

 radiolysis of adsorbed moisture

° The centre line temperature in 푃푢푂2 storage canisters can be several hundred C, this can lead to pressurisation of the gas phase. Eller et al. used a computer model to predict the temperature of the gas phase from this centre-line temperature. The model predicted that

Chapter 2 Literature Review 7131060

° the gas phase would increase in temperature from ambient to 211 C once 푃푢푂2 had been added to the canisters. This temperature increase would lead to an increase in pressure from 14 psi to 23 psi. This mechanism was discounted, as this increase in pressure would be visible after several weeks of storage.

The α-decay of plutonium could lead to helium accumulation inside the canisters. It was predicted that the pressure inside the canister would increase by 13 psi over a period of 50 years of storage for a canister containing 5 kg of 푃푢푂2. However, this value is an overestimation as helium would be trapped in the solid matrix [18].

Chapter One outlined the finishing process for 푃푢푂2, which involved calcination at high temperatures, this process would remove any organics from the product. No signs of corrosion in the canisters further highlight the absence of organics.

The final mechanism discussed was that of radiolysis of adsorbed moisture. Before going into storage the moisture content is kept below 0.5 wt.%. This quantity is insufficient to generate significant pressurisation, however, it could lead to the formation of 퐻2 and 푂2 gas atmospheres.

A fifth mechanism was postulated by Paffett and Bailey [19] which is a pressure pulse from the deflagration or detonation of a combustible atmosphere. This mechanism ties in with the radiolysis of moisture outlined by Eller et al.

Mason et al. carried out headspace gas analysis of ten sealed 푃푢푂2 storage canisters that had come from a range of sources with varying purities and had been stored for various timescales (1-18 y) [20]. From these analyses, several observations were noted. Even though the canisters were not airtight, eight of the canisters’ had an internal pressure that was less

Chapter 2 Literature Review 7131060 than the atmospheric pressure of the site where they had been stored. Analysis of the gas phase indicated that the main component was air, with only two samples having significant concentrations of 퐻2. The final observation stated that canisters where 푂2 was present had no 퐻2 present and vice-versa.

Almond et al. performed gas analysis on over thirty containers of plutonium bearing materials from the Hanford and Rocky Flats sites that had been stored for 4-6 y [21]. They found that seven of the containers had more than 0.1 mol% 퐻2 and very little 푂2 in the resulting headspace. The other twenty four containers did not have a sizeable amount of 퐻2 present although four of these containers had a moisture content of ≥ 0.1%, which is within the range for anticipated 퐻2 production.

From these references, it is clear that radiolysis of adsorbed moisture is the main mechanism of pressurisation inside 푃푢푂2 storage containers. Therefore knowledge of how this water adsorbs and the mechanisms leading to 퐻2 and 푂2 generation are important.

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2.2 Water Adsorption on 푷풖푶ퟐ

Stakebake has postulated that water adsorption to a 푃푢푂2 surface occurs in three steps

[22], which are shown pictorially in Figure 2.1.

Step 1

Step 2 + H2O

Step 3 + 2H2O

Figure 2.1: Postulated mechanism of water adsorption onto a 푃푢푂2 surface (replicated from [22])

The first step involves bonding between the plutonium cation and the oxygen from water.

This step allows hydrogen bonding between the hydrogen atoms from water and the oxygen

atoms neighbouring the plutonium cation. This interaction leads to dissociation of the water

molecule into hydroxyl groups that become chemisorbed to the surface. The second step

involves interaction between the chemisorbed hydroxyl groups and unbound water. This

interaction may be mono or bi-dentate and is termed quasi-chemisorption [23]. The final

step is the physisorption of unbound water to the quasi-chemisorbed water molecules. This

Chapter 2 Literature Review 7131060 final adsorption is very weak as there is no formal bond between the adsorbate and the surface. It is not known what effect (if any) the oxide surface plays in this final step.

During calcination, it is likely that only the chemisorbed water will remain attached to the surface, unless very high temperatures are used.

Haschke and Ricketts [23] have determined the mass of this chemisorbed monolayer of

-2 water on 푃푢푂2. They have determined values between 0.21-0.24 mg m depending on the crystal plane and assuming association of one water molecule to one plutonium atom.

Work by Alexandrov et al. showed that this mechanism was also true for thorium dioxide

(푇ℎ푂2) and cerium dioxide (퐶푒푂2), which are often used as surrogates for 푃푢푂2 and heavy actinide oxides [24].

From the mechanism outlined above, it is clear, that adsorbed species play a role in the adsorption of further water molecules. This mechanism follows the BET theory of the formation of multi-layers on a surface as opposed to the Langmuir theory that assumes once a monolayer of molecules has been adsorbed there will be no further adsorption to the surface as all the active sites have been filled.

2.3 Radiolysis of Adsorbed Water

Petrik et al. have investigated the γ-irradiation of adsorbed water on several metal oxides to determine the quantity of 퐻2 produced [25]. Their work classified oxides into three groups.

The first group had a lower yield (G-value) of 퐻2 than radiolysis of liquid water (G(퐻2) = 0.45

-1 molecules 100 eV [26]). The second group had G(퐻2) values close to bulk water radiolysis and the final group have a higher G(퐻2) value than bulk water. It is postulated that the

Chapter 2 Literature Review 7131060

increase in 퐻2 formation is due to an enhanced energy transfer between the oxide surface and the adsorbed water. They highlighted a resonance between the band gap of the oxide and the bond dissociation energy of water (5.15 eV [27]) The band gap is the energy needed by electrons to move from the highest occupied band (valence band) to the lowest unoccupied band (conduction band). Metal oxides that had a band gap or approximately

5 eV produced the greatest quantity of 퐻2.

Aleksandrov et al. highlighted that physisorbed water and surface hydroxyls had a negligible contribution to 퐻2 yield during radiolysis of adsorbed water [28]. They found that the energy adsorbed by the oxide was transferred entirely to the first layer of chemisorbed water. They determined that the active layer of the oxide involved in the heterogeneous system was no more than 100 nm thick.

LaVerne and Tandon investigated 퐻2 production from 퐶푒푂2 and 푍푟푂2 using cobalt-60 gamma rays [29] and determined that 퐻2 yield increased with decreasing numbers of water

-1 monolayers adsorbed to the surface. G(퐻2) values of 20 and 150 molecules 100 eV were determined for samples containing 퐶푒푂2 and 푍푟푂2, respectively.

2.4 Radiolysis of Gases in Contact with 푷풖푶ퟐ

An extensive amount of research has been undertaken on the radiolysis of adsorbed species

(e.g. water, nitrates) on 푃푢푂2 and other oxides. However, there are fewer publications on gas phase radiolysis or gases as part of a heterogeneous system. A lot of the emphasis is on post irradiation analysis of gases from either solutions or organics. However, some papers have tried to rectify this.

Chapter 2 Literature Review 7131060

Haschke et al. exposed 푃푢푂2 to a 2:1 퐻2 − 푂2 mixture and measured the pressure as a function of time [30]. They reported a rapid drop in pressure before eventually (after 72 d) staying constant. Mass spectrometer data from the gas phase found no water vapour present even though mixtures of 퐻2 − 푂2 are thermodynamically unstable relative to water.

The results suggested there were two reactions occurring simultaneously. These are the oxide catalysed recombination of 퐻2 and 푂2 (Reaction 2.1) and the subsequent water catalysed oxidation of 푃푢푂2 (Reaction 2.2) to form the super-stoichiometric plutonium dioxide, 푃푢푂2+푥:

2퐻2 + 푂2 + 푃푢푂2 → 2퐻2푂 + 푃푢푂2 Reaction 2.1

푃푢푂2 + 푂2 + 퐻2푂 → 푃푢푂2+푥 + (1 − 푥)푂2 + 퐻2푂 Reaction 2.2

The conclusion of this work was that 푃푢푂2 is a catalytically active metal that promotes the

° formation of 푃푢푂2+푥 below 200 C.

It has been reported by others that 퐻2 and 푂2 are not produced stoichiometrically from the radiolysis of absorbed water [31]. Headspace analysis from canisters that have been stored for over twenty years show that 푂2 levels are depleted whilst 퐻2 levels are enriched, although there is no overpressure. This is further evidence of the formation of the super- stoichiometric oxide.

Subsequent work by Haschke et al. surmised that the system pressure decreases as 푃푢푂2+푥 is formed at a constant rate by Reaction 2.2 [32]. Once 푂2 is depleted, formation of 퐻2 from the radiolysis of the adsorbed water causes the pressure to increase. Therefore pressurisation is possible over long periods if there is constant moisture adsorbed on the oxide surface.

Chapter 2 Literature Review 7131060

Morales et al. also concluded that 푃푢푂2 acted as a catalyst for 퐻2 and 푂2 recombination

[33]. Their results showed that stoichiometric amounts of 퐻2 and 푂2 were lost from the gas phase and the total pressure of the system decreased after the reactants had combined.

Like Haschke, water vapour was not observed in the final gas composition, suggesting that any water formed is adsorbed to the surface of the oxide. Initially, the active sites on the

푃푢푂2 are free and so can act as a catalyst in the reaction between 퐻2 and 푂2; this is supported by the initial drop in pressure of the system. Eventually, these sites become blocked by water molecules and 푂퐻 moieties and so the reaction rate decreases. At temperatures above 100 °C, there is enough thermal energy for these sites to be regenerated thus the rate increases. The implication of this mechanism is that the reaction is driven by surface catalysis and not by radiolytic formation of radicals. The establishment of a steady state pressure indicates that the rate of recombination of 퐻2 and 푂2 is equal to the rate of radiolysis of absorbed water.

Several papers have hypothesised about the production of the super-stoichiometric 푃푢푂2+푥 from the reaction of 푃푢푂2 with water and 퐻2 − 푂2 mixtures. The mechanisms for this are not fully understood and the fact that oxidation of 푃푢푂2 in 푂2 does not generate 푃푢푂2+푥, only with the presence of water does it occur, makes the mechanism more complex [34].

Korzhavyi et al. have proposed that the lack of observed oxygen by Haschke et al. [30, 32] is potentially due to the formation of a more stable product than 푃푢푂2+푥, possibly a hydroxide or peroxide compound of plutonium [34]. However more experimental data is needed.

Chapter 2 Literature Review 7131060

2.5 Radiolysis of 푯ퟐ − 푶ퟐ in Contact with Other Materials

Further papers have looked at the recombination rates of 퐻2 and 푂2 over different materials in the absence of radiation. Quigley looked at the recombination rates over several materials including stainless steel, strips of tin and 퐶푒푂2 [35]. This was to address the issue of over-pressurisation in stainless steel canisters relevant to nuclear waste storage.

° ° Experiments carried out at 200 C and 300 C highlighted that 퐶푒푂2 was by far the most efficient at removing 퐻2 and 푂2 from the gas phase. Results for stainless steel also showed that it acts as a catalyst for the recombination. The tin strips however became oxidised and so were not as efficient in the recombination process.

Katz studied the reaction of 퐻2 and 푂2 in the presence of concrete that incorporated simulated radioactive waste [36]. Their results revealed concrete that had been prepared at higher temperatures and pressures (cured for 24 hrs at 100-250 °C below 600 psi) had a higher recombination rate than the rate of gas production from radiolysis of water in the system.

From the work already carried out, it is clear that the presence of a surface increases the rate of 퐻2 − 푂2 recombination. However, different surfaces whether an oxide, cement or steel all have the same effect; therefore more work on understanding the mechanism of recombination is needed. Several papers also suggest the formation of the super- stoichiometric 푃푢푂2+푥, however, only in the presence of moisture. Work on other oxides such as 퐶푒푂2, should help to enhance the understanding of its formation.

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2.6 Radiolysis of Hydrogen and Oxygen

A significant quantity of work was undertaken by Lind and co-workers, who investigated the recombination of 퐻2 and 푂2 using radium powder as a radiation source [37]. Only pressure was measured and several assumptions were made, including that no steady state was reached, radium had no catalytic effect on the system and the only reaction occurring is

푀 Reaction 2.1 (푀 instead of 푃푢푂 ). In earlier work, the term was utilised instead of G- 2 푁 values, where 푀 is the number of molecules undergoing change (formed or reacted) and 푁 is the number of ion pairs formed. These values can be converted to G- values using

Equation 2.1.

푀 100 퐺 = Equation 2.1 푁 푊 where 푊, is the mean energy required to form an ion pair. A lot of early works use the value for air which is 32.5 eV, therefore:

3푀 퐺 = Equation 2.2 푁

-1 Lind et al. calculated G(퐻2푂) values of 11.76 molecules 100 eV . Further work investigating the effect of added water vapour hypothesised that at ambient temperatures, water vapour condenses on the vessel walls and plays no part in gas phase reactions, but at higher temperatures, contributes significantly to the ionisation and reaction rates [38]. Work replacing 퐻2 with deuterium, 퐷2, found that the recombination rate was approximately 25-

30% slower in comparison to 퐻2 − 푂2 recombination [39]. The discussion centred around a lower efficiency in one of the preliminary reaction steps.

Benjamin and Isbin investigated 퐻2 − 푂2 recombination at high temperatures, of interest to boiling water reactors [40]. They calculated G(-퐻2) values in the range 3-6 molecules

Chapter 2 Literature Review 7131060

100 eV-1 and the rate of recombination was independent of temperature up to 138 °C. They postulated that the reaction was first order in 퐻2 concentration, however, this statement needs further clarification as to whether the effect the concentration has on absorbed dose has been taken into account. Like Schifflett et al. [38], their data highlights the rate of recombination is independent of water vapour concentration.

Dautzenberg irradiated mixtures of 퐻2 − 푂2 using gamma radiation and investigated the effects of pressure, temperature and matrix gases on the reactivity of 퐻2 − 푂2 mixtures

[41]. He found that the G(-퐻2) value was affected by all these parameters and achieved values in the range of 5.5-160 molecules 100 eV-1, depending on reaction conditions. Unlike

Benjamin et al. [40], Dautzenberg determined that the rate of recombination was independent of 퐻2 − 푂2 ratio. Furthermore, he found that pre-treating the reaction vessels also had an effect. By heat treating the vessels at 500 °C for 24 hrs and then out-gassing

under vacuum at 200 °C for 30 min, he noted that the G(-퐻2) value decreased. He surmised that this was due to surface impurities helping the recombination of 퐻2 − 푂2 and so using a

‘clean’ surface decreased the reaction rate. Dautzenberg also proposed that the reaction occurred by a radical chain scheme.

Summary

From the literature reviewed in the previous sections, it is clear that radiolysis of adsorbed moisture is the primary mechanism by which 푃푢푂2 storage canisters can pressurise. A lot of work has been undertaken on this system, and with other materials, however, there is much debate as to a mechanism for the enhancement of 퐻2 yields.

Chapter 2 Literature Review 7131060

Early work has investigated the gas phase recombination of 퐻2 − 푂2, however there is a broad range of G(-퐻2) values quoted in the literature, there are also conflicting results as to the order of the recombination.

There has been very little published literature on the radiation chemistry of gaseous species in contact with an oxide surface (푃푢푂2 or otherwise). Hopefully this research will help to add to this area.

2.7 Air Radiolysis

Chapter One outlined the storage conditions for 푃푢푂2 in the UK. In one of the product streams, the fill gas is a 50:50 mixture of air and argon. Irradiation of air could lead to further chemistry occurring in the canisters, therefore an understanding of the radiation chemistry of air is needed.

Willis et al. have investigated the radiolysis of 푁2 − 푂2 mixtures utilising electron pulses

[42]. They found ozone, 푂3, to be the dominant product. With nitrogen dioxide, 푁푂2, being below the limits of detection.

Harteck and Dondes outlined the following reaction mechanism for radiolysis of air [43, 44]:

푁2 ⇝ 2푁 Reaction 2.3

푂2 ⇝ 2푂 Reaction 2.4

푁 + 푂2 → 푂 + 푁푂 Reaction 2.5

2푁푂 + 푂2 → 2푁푂2 Reaction 2.6

Chapter 2 Literature Review 7131060

푁 + 푁푂2 → 2푁푂 Reaction 2.7

푁 + 푁푂2 → 푂 + 푁2푂 Reaction 2.8

푁 + 푁푂 → 푂 + 푁2 Reaction 2.9

푂 + 푂2 + 푀 → 푂3 Reaction 2.10

From this mechanism, it is clear that the dominant products are 푂3, 푁2푂 and 푁푂2.

In the presence of moisture, the dominant product is nitric acid (퐻푁푂3), (Reactions 2.11 and 2.12) [45, 46].

2푁푂2 + 퐻2푂 → 퐻푁푂3 + 퐻푁푂2 Reaction 2.11

2퐻푁푂2 → 푁푂 + 푁푂2 + 퐻2푂 Reaction 2.12

-1 Kanda et al. measured G(퐻푁푂3 ) values of 1.46 molecules 100 eV by irradiating atmospheric air using a cyclotron radiation source [45]. Jones measured G(퐻푁푂3) values of

1.2 molecules 100 eV-1 and found that nitric acid disappeared on exhaustion of water vapour in the system leading to formation of stoichiometric amounts of 푁푂2 and water [46].

This reformed water did not lead to further nitric acid formation.

Summary

The radiolysis of air is a well understood system and the dominant products are well known.

In the presence of moisture, nitric acid is the dominant product. Previous sections in this chapter outlined that water would be present in the 푃푢푂2 storage canisters, therefore the formation of nitric acid is a concern.

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There has been no attention given to the formation of nitric acid from radiolysis of air in the presence of an oxide material. This research hopes to address this issue.

Aims and Objectives

In light of the findings of this literature review, the aim of this research project is to study the gas phase radiation chemistry of two gaseous systems in the presence or absence of a

푃푢푂2 surrogate material. These systems are firstly, mixtures of 퐻2 − 푂2 in an 퐴푟 matrix and secondly, moist air.

These aims were pursued via the following distinct objectives which are summarised below:

 Develop an adequate reaction vessel to investigate the recombination of 퐻2 and 푂2

in homogeneous and heterogeneous systems using a 60퐶표 γ source

 Determine a better mechanistic understanding of the 퐻2 − 푂2 system, and what role

an oxide surface may play in this system

 Investigate the radiolysis of air in the presence of a 푃푢푂2 surrogate material and

determine the identity and yield of species produced

 Determine the role an oxide surface plays (if any) in the radiation chemistry of air

 Investigate both gaseous systems utilising a beam of accelerated 5.5 MeV 퐻푒2+ ions

to simulate the α-decay of 238푃푢, which is the α emitter with the shortest half-life

(87.7 y) in the storage canisters

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3 Experimental

The following chapter outlines the materials necessary to carry out this research and the irradiation facilities used to perform the work. It also details the experimental techniques utilised to execute this work. Finally, it details the experimental methodologies employed during the course of this research.

3.1 Materials

3.1.1 Gases

All gases are supplied by BOC unless otherwise stated. Hydrogen (CP Grade 99.999%), oxygen (N5.0 99.999%) and argon (Zero grade 99.999%) are utilised for the preparation of known concentrations of gas mixtures used to carry out part of this research. Synthetic air

(supplied by BOC) is used during this research and is produced by mixing 20% 푂2 with 80%

푁2. The impurities are certified as < 1 vppm 퐶푂2, < 2 vppm 퐻2푂 and < 0.1 vppm 푁푂푥.

Calibration gases for the gas chromatograph are supplied by Scientific and Technical Gases in lecture bottle size. Mixtures ranging from 5% 퐻2⁄퐴푟 down to 0.1% 퐻2⁄퐴푟 are used for the calibration of the gas chromatograph. Each bottle has an accuracy of ±0.1%.

3.1.2 Chemicals

Cerium oxide (퐶푒푂2) is purchased from Alfa Aesar in the form of a 14 µm powder with

99.99% purity; the largest impurity is ≤ 0.1% silicon dioxide (푆𝑖푂2). Zirconium oxide (푍푟푂2) is also purchased from Alfa Aesar and has a more crystalline powder form than 퐶푒푂2. It is supplied as 99.978% pure and the largest impurity is certified as 29 ppm of tantalum and

9 ppm of hafnium. Cerium oxalate hydrate (퐶푒2(퐶2푂4)3. 푥퐻2푂), which is used for TGA

Chapter 3 Experimental 7131060 analysis, has a purity of 99.9% trace metals, and sodium hydroxide (reagent grade ≥ 98%)

(푁푎푂퐻) which was utilised in the post irradiation analysis of the air radiolysis experiments were both supplied by Sigma-Aldrich.

3.2 Irradiation Sources

Two sources of ionising radiation are used to induce a chemical change in the systems of interest; 60퐶표 γ – rays and 퐻푒2+ ions. The following section details how these sources are used.

3.2.1 Cobalt-60 Source

A Foss Therapy Model 812 self-shielded irradiation source is utilised to perform gamma irradiations. Figure 3.1 shows the 9 L irradiation chamber which is able to house a wide range of samples. The irradiator has three source rods located at the front of the chamber, however, only the two outer source rods have pellets of cobalt-60 inside. The irradiator is capable of delivering a dose rate between 400 and 4 Gy min-1 depending on attenuation and distance from the sources.

Figure 3.1: Foss Therapy Model 812 60퐶표 Irradiation source

Chapter 3 Experimental 7131060

The decay scheme for 60퐶표 is shown in Figure 3.2. There are two distinct types of radiation emitted from the source, these are β and γ, however, due to the stainless steel shielding, the β particles will not penetrate through the source rods. Only the two γ emissions of 1.17 and 1.33 MeV contribute to the radiation chemical affect.

Figure 3.2: Decay scheme for cobalt-60 isotopes

The irradiator is controlled by computer, which can set irradiations to last for a desired time or absorbed dose.

Due to the nature of the sample chamber and the location of the source rods, the dose rate within the chamber can differ substantially due to the inverse square law.

1 𝑖푛푡푒푛푠𝑖푡푦 ∝ Equation 3.1 푑푖푠푡푎푛푐푒2

For this reason, precise dosimetry is required to assess the dose rate of a given position within the chamber. This topic is discussed further in Chapter Five.

Chapter 3 Experimental 7131060

3.2.2 Pelletron Ion Accelerator

An NEC 5 MV tandem ion accelerator is utilised to perform ion irradiation studies on the gaseous systems relevant to this research. The accelerator is capable of delivering a beam of positive ions of light charged particles (퐻+ and 퐻푒2+) up to energies of 10 and 15 MeV respectively. It is also a source of heavy charged particles of many elements within the periodic table. Samples are placed at the end of one of the accelerator’s six beam lines where irradiations and/or analysis can be carried out in-situ. In this research, 퐻푒2+ ions will be used to simulate the α decay of 푃푢푂2 (specifically plutonium-238). Ions are generated by feeding 퐻푒 gas into a Toroidal Volume Ion Source (TORVIS). A current is placed over a filament to ionise the gas and form 퐻푒+ ions. These ions are passed through a rubidium vapour to add electrons forming 퐻푒− and 퐻푒2−. The negative ions are accelerated to ground and stripped of electrons to form positive ions which are accelerated from ground, down the appropriate beam line to the sample. Figure 3.3 shows a schematic layout of the accelerator. From left to right, the ions are generated at the source, accelerated into the tank, converted to positive ions and accelerated down one of the beam lines.

Figure 3.3: Schematic of 5 MV Ion accelerator located at DCF

Chapter 3 Experimental 7131060

3.3 Analytical Techniques

The two primary analytical techniques utilised in this research are gas chromatography (GC) and ion chromatography (IC). GC is used to determine the concentrations of 퐻2 and 푂2 in the gas phase throughout this research. IC is primarily used to determine the concentration

− of nitrate (푁푂3 ) anions in air radiolysis experiments, however, other anions can be detected in parallel. Chromatography is a separation technique reliant on both a stationary phase that is immobilised on a solid support and a mobile phase that pushes the analyte through the stationary phase before separating the components of a mixture using a particular property [47]. Other techniques are applied to study the solid phase during this research, however, focus will be given to the two primary techniques named above.

3.3.1 Gas Chromatography (GC)

The GC used in this research is an SRI Instruments Multiple gas analyser #1, equipped with a thermal conductivity detector (TCD) which can be used to detect molecular gaseous species such as 퐻2, 푂2 and 푁2. The detector uses a set of filaments in a Wheatstone bridge configuration to detect analytes which have a different thermal conductivity to the reference gas flow. The stationary phase is a 6 ft. long, 1/8 ” diameter packed column of molecular sieve 13X beads. This column is particularly useful as it acts as a trap for moisture which would subsequently interfere with the resulting chromatogram. The GC is also equipped with a 10-port sampling valve which allows for switching between loading and injecting positions and has a 50 μl sample loop attached. The mobile phase is high purity argon (99.999% pure), specifically chosen to increase the sensitivity of the instrument towards 퐻2 measurements due to the differing thermal conductivities of 퐴푟 and 퐻2. The instrument conditions are as follows: the carrier gas flow rate is set at 28 psi, thus allowing

Chapter 3 Experimental 7131060 for measurements to be acquired in several minutes; the valve temperature is 60 °C, the oven temperature which houses the column is set at 80 °C and the detector is at 130 °C.

These conditions allowed for good separation of analytes without shortening the lifespan of the instrument.

3.3.2 Ion Chromatography (IC)

The IC used in this research is a Thermo-Scientific ICS2100, consisting of an electrochemical detector (ECD) and a Dionex Ion Pac AS-18 column. The stationary phase is composed of an ion exchange resin in the form of 4 μm beads which are packed into the column. The mobile phase is a potassium hydroxide (퐾푂퐻) eluent which is made from an eluent generator to tailor the concentration of 푂퐻− to the application. This setup allows for the determination of inorganic anions and low molecular weight organic acids.

The instrument conditions are: column temperature 30 °C, 퐾푂퐻 eluent concentration

23 mM with a flow rate of 0.25 ml min-1. The injection volume for each sample is 0.25 μl.

These conditions allowed for analysis times of ten minutes for each sample. For large numbers of samples, an auto sampler is used to increase efficiency.

3.3.3 Surface Area Measurements

To determine the surface area of the oxide powders of interest, a Tri-Star II surface area and porosity analyser supplied by Micromeritics is used. It is important to know the specific surface area (SSA) of the oxide powders as this may help to elucidate any catalytic affect the surface may have on gas phase radiolysis. In all the experiments, 푁2 is used as the adsorbate gas. Surface area is calculated using the BET theory and the following equation:

푣푚NA푠 푆퐴퐵퐸푇 = ( ) Equation 3.2 푀푉

Chapter 3 Experimental 7131060

where 푣푚 is the volume of 1 monolayer of adsorbed gas, NA is Avogadro’s constant, 푠 is the

2 cross sectional area of the gas adsorbate (for 푁2 this is 0.162 nm ) and 푀푉 is the molar volume of the adsorbate gas [48]. This value is then divided by the sample mass to calculate the surface area in units of m2 g-1. Sample masses are in the range of 0.5-1 g before being de-gassed under vacuum at 250 °C overnight. The samples are re-weighed and analysis performed.

3.3.4 Thermogravimetric Analysis (TGA)

To determine whether there are any organic impurities absorbed on the oxide powders and to investigate their thermal stabilities, samples were submitted for thermogravimetric analysis. TGA involves heating a small amount of sample at a set heating rate and measuring the difference in mass over a required temperature range. To carry out TGA, a small mass of sample (< 10 mg) is placed inside an aluminium crucible and heated. For samples of 퐶푒푂2

° -1 ° and 푍푟푂2, the heating rate employed is 10 C min , heated up to 1000 C and held for

10 min. A sample of cerium oxalate was also analysed, however, due to the decomposition of the sample, a heating rate of 2 °C min-1 is employed and the sample is heated to 600 °C.

This temperature ensures the oxalate has decomposed fully. All samples are analysed in both static air and nitrogen to provide a comparison between oxidative and inert atmospheres.

3.3.5 Diffuse Reflectance Infra-red Spectroscopy (DRIFT)

A Vertex 70 FT-infra-red spectroscope supplied by Bruker is used to investigate the surface of the oxide powders with respect to adsorbed organic species. A Praying Mantis diffuse reflectance accessory is used to increase the spectral output due to the uneven nature of the sample surface. The accessory has two parabolic mirrors to collect all of the reflected

Chapter 3 Experimental 7131060 light off the sample and focus it into the detector. A mercury cadmium telluride (퐻𝑔퐶푑푇푒) detector, which is cooled with liquid nitrogen to improve signal/noise ratio is used. The resolution is set to 4 cm-1 and 250 scans of each sample are taken.

3.3.6 UV-Vis Spectroscopy

To perform Fricke dosimetry experiments, an Agilent Technologies Cary Series UV-Vis-NIR spectrophotometer is utilised. Disposable 1 cm path length cuvettes are used to analyse the solution and three repeats of each sample are taken. An average of these readings is used as the absorption value compared to a reference blank.

3.3.7 Scanning Electron Microscopy (SEM)

To determine the morphology of the oxide grains, a small quantity of powder is mounted onto a carbon stub and coated with a fine layer of carbon using a Cressington 208 high vacuum carbon coater. The samples are loaded into the chamber of a JEOL 6400 scanning electron microscope. The instrument is equipped with a Princeton Gamma Tech energy dispersive X-ray spectroscope (EDS) which allows for elemental analysis of the sample to be investigated. The beam energy used is 15 keV.

Chapter 3 Experimental 7131060

3.4 Experimental

The following section outlines how this research is carried out, detailing the equipment needed to mix various gases together and how the air radiolysis experiments are accomplished. Finally, details of experiments carried out using the ion accelerator are explained.

3.4.1 Mixing of 푯ퟐ − 푶ퟐ − 푨풓 Samples

A bespoke manifold system has been designed and commissioned to be able to mix 퐻2, 푂2 and argon gas in various concentrations. Figure 3.4 shows a visual image of the manifold and a schematic drawing showing its design features.

Chapter 3 Experimental 7131060

Figure 3.4: Bespoke gas mixing manifold system

The manifold is made from 316 stainless steel with Swagelok © fittings. It consists of a rotary vane vacuum pump and a Pirani vacuum gauge and is able to evacuate the entire system to a pressure of < 20 mTorr. Once this pressure is achieved, the vacuum pump is isolated from the system and the gases can then be mixed by using two independent pressure gauges to verify the ratios of each gas. Each gas cylinder is fitted with a one-way check valve to ensure flashbacks cannot occur. The manifold is fitted with ultra-torr © fittings which allow gases to be mixed in the reaction vessel directly. Throughout the course of these experiments, all irradiations will be investigated at atmospheric pressure (1 bar absolute), this is to simulate the conditions under which the majority of the 푃푢푂2 canisters are stored.

Chapter 3 Experimental 7131060

A specialist reaction vessel had to be developed to perform 퐻2 − 푂2 studies, which involved several iterations of design. This development will be discussed in Chapter Four.

3.4.2 Air Radiolysis

For this set of experiments, 12 ml sample vials with a butyl rubber crimp cap were employed. Three quantities of oxide loading were used during the experiments. These were

1 g, 50% oxide (by volume) and 90% oxide (by volume). This can be seen for 푍푟푂2 samples in

Figure 3.5. All masses were within ± 0.01 g of the required quantity.

Figure 3.5: Picture of l-r 1 g, 50% oxide (by volume) and 90% oxide (by volume) for 푍푟푂2 samples

The three loading choices give a wide range of gas to solid ratio. Samples that contain 1 g of oxide contain over 95% air (by volume) therefore it is expected that these samples will closely replicate the experiments without any oxide powder present. Samples with 50% oxide present represent a much larger mass of oxide than the 1 g samples. These samples should help to confirm if there is a catalytic effect on nitrate formation in the presence of an oxide surface as there is a lot more oxide surface than is present in 1 g samples for any gas phase species to adsorb to. Samples with 90% volume of oxide are analogous to the 푃푢푂2

Chapter 3 Experimental 7131060 storage cans where the cans are almost full of oxide powder to reduce the number of cans required.

To perform irradiations, samples are placed inside a test tube rack and irradiated with standard laboratory air atmospheres. Post irradiation, the samples are injected with 5/7 ml of 5 mM 푁푎푂퐻 solution to extract any gaseous nitric acid into solution. Samples containing

1 g of oxide are washed with 5 ml of the base solution and the 50% and 90% samples are washed with 7 ml of the base solution. These volumes are chosen to ensure firstly, that there is an excess of base in all the samples and secondly, that all the nitric acid can be extracted. Due to the ‘sticky’ nature of nitric acid [49] it is important to have enough solution to wash all of the surfaces in the system. Also there needs to be enough supernate above the oxide to be able to draw off the solution to be analysed. This is a similar analytical approach utilised by Kanda et al. [45]. After the samples are washed with the base solution, they are homogenised and left to settle. After a period of time where two separate layers have started to form, a portion of the solution is drawn off and filtered through a polyvinylidene fluoride (푃푉퐷퐹) syringe filter; this is to ensure any fine particulates do not plug the instrument. The filtered solution is placed inside an IC auto-sampler vial and analysed using the IC.

Throughout these experiments, disposable needles and syringes are used for transferring solutions between vials; this is to reduce the chance of contamination between samples.

The 12 ml glass sample vials are only used for one experiment so as to avoid contamination.

When adding the base solution to the irradiated samples, a gas tight 5/10 ml syringe is used with disposable needles in order to ensure the same volume of 푁푎푂퐻 solution is injected into each sample to increase repeatability across the experiment range. Finally, the IC auto-

Chapter 3 Experimental 7131060 sampler vials are also disposable to ensure there is no cross-contamination from any previous samples.

3.4.3 Oxide Regeneration

After analysis of the aqueous phase it is important to ‘regenerate’ the oxide powder to its unirradiated state for future experiments. This is achieved by filtering the oxide with an aliquot of 5 mM 푁푎푂퐻 solution, to wash the surface and remove any adsorbed species. It is then washed with copious volumes of deionised water to remove any excess sodium cations from the basic solution before baking the powder in a furnace at 400 °C for 6 h under a static air atmosphere. This temperature is high enough to remove any organic species that haven’t been removed by washing and physisorbed water but low enough that it will not change the crystallographic nature of the powder. The baked powder is then filtered through an appropriate sized sieve to break up any agglomerated powder.

3.4.4 Accelerator Experiments

Bespoke glassware has been developed to investigate heavy ion radiolysis of the 퐻2 − 푂2 −

퐴푟 system. The glassware has similar features to the glassware used by Schuler et al. [50] and is presented visually and schematically in Figure 3.6.

Chapter 3 Experimental 7131060

2+ Figure 3.6: Bespoke glassware for 퐻푒 ion radiolysis of 퐻2 − 푂2 − 퐴푟 gaseous mixtures

The vessel is cylindrical in shape to allow for the ion beam to penetrate into the sample and to be fully attenuated in the sample, not the glass itself. The vessel is approximately 30 cm in length to accommodate the greater penetration distance of heavy ions into gaseous targets. It has a platinum wire that is used to obtain current measurements in the sample during irradiation. It also contains two Rotaflo taps which allow the cell to be purged with the required gas mixture and to be connected to the GC for post irradiation analysis. As the ion beam is only irradiating the sample and the taps are not located inside the radiation field, radiation resistant materials are not required for their construction. There is a side port to which a pressure transducer is attached during irradiations to log the pressure change in the vessel. The end of the vessel is open ended with a ground glass flange, to which a thin sheet of mica is attached to the flange using an epoxy resin and secured in

Chapter 3 Experimental 7131060 place with the ground glass ring. Due to the fragility of the mica window, all experiments are investigated at atmospheric pressure (1 bar absolute).

All experiments are carried out with 퐻푒2+ ions to simulate α decay from 238푃푢. Before the ions hit the sample, the beam has to traverse a series of material windows (Figure 3.7), therefore a calculation is needed to account for the energy loss of the beam from the accelerator terminal to the sample. This is achieved by inputting the density of each window into a program called SRIM (Stopping Range of Ions in Matter) [51]. This software calculates the final ion energy on the sample from a machine starting energy or the initial ion energy needed from the machine to generate a certain ion energy on sample.

Calculations were done using the latter parameters. Due to the slight variation in the density of the mica window in the different reaction vessels, the exact ion energy on sample ranged from 5.52-5.96 MeV. The currents employed during the series of experiments were 10 and

20 nA and irradiation times ranged from 10-30 mins. This configuration allowed for equivalent doses as achieved using the 60퐶표 source in 64 h.

Figure 3.7: Configuration of window assembly through which the beam travels before reaching the sample

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

4 Development of γ-Irradiation Reaction Vessel

This chapter details the extensive development involved in designing a reaction vessel suitable to perform γ-irradiations of 퐻2 − 푂2 − 퐴푟 mixtures. The chapter also outlines the development in sampling and analysis techniques that coincide with the iteration of reaction vessel development to enhance the sensitivity of the gas chromatograph towards

퐻2 analysis.

4.1 Initial Vessel Design

60 The initial reaction vessel utilised to perform 퐻2 − 푂2 irradiations using the 퐶표 source is shown below (Figure 4.1):

Figure 4.1: Reaction vessel for gamma radiation studies of 퐻2 − 푂2 system

The vessel is made of borosilicate glass, with a ‘bulb’ volume of 10 cm3. It has two Rotaflo taps which are made from polytetrafluoroethylene (푃푇퐹퐸) and are chemically inert and gas tight. These taps are removable which allows for oxide powder to be installed into the vessel. The two outlets allow for the vessel to be attached directly to the manifold and to be attached to the GC for ‘in-line’ analysis.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

To perform irradiations, the glass vessels are mounted onto a rail system shown in Figure

4.2.

Figure 4.2: Sample holder for gamma irradiation of 퐻2 − 푂2 system

This allows for vessels to be mounted in a set position thus ensuring a constant dose rate during irradiation. The sample can be moved closer/further from the radiation sources so that a range of dose rates can be achieved.

4.1.1 GC Configuration and Calibration

To accommodate this experimental set-up, the sampling valve of the GC had to be reconfigured to allow the carrier gas to ‘force’ the irradiated gas mixture through to the detector. Figure 4.3 shows the GC valve set up in i) load position, where the sample is attached to the GC and ii) inject position as the carrier gas injects the sample into the GC.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

i) ii)

Figure 4.3: GC valve configuration for ‘in-line’ analysis i) ‘Load’ position ii) ‘Inject’ position

When a vessel is not attached to the GC, the two outlets are connected together and flushed with argon gas in order to minimise the volume of air within the system. When a vessel is attached as shown in Figure 4.3i there is a delay of a couple of seconds between disconnecting the bridge between the two outlets and attaching the vessel, allowing air to ingress into the sample loop. The total volume of the sample loop and connections in comparison with the 10 cm3 volume of the injected sample means that the air impurity will be less than 1% of the injected sample.

The GC is calibrated using the 퐻2 calibration gases detailed in Section 3.1.2. This is achieved by purging the glass bulb with the gas for 60 s before attaching the vessel to the GC for analysis. Several repeats of each calibration gas are done to increase accuracy (Figure 4.4).

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

3500

3000

2500 2000 1500

1000 Peak area / AU area Peak 500 0 0 0.1 0.2 0.3 0.4 0.5 3 H2 Volume / cm Figure 4.4: Calibration of the gas chromatograph using certified calibration standards in the range 0.1-4% 퐻2⁄퐴푟

At lower concentrations of hydrogen (< 1%) there is a linear relationship between the concentration and the peak area. Above this concentration however, the calibration is non- linear and there is a higher degree of scatter between repeat samples. The explanation for this scatter can be seen in Figure 4.5; this is an overlay of two chromatograms, one containing 2% 퐻2⁄퐴푟, and the second containing 0.5% 퐻2⁄퐴푟.

800

700

600 2% H2/Ar 500 0.5% H2/Ar 400 300

200 SIgnal Intensity /AU Intensity SIgnal 100 0 0 25 50 75 100 125 150 175 200 225 250 Retention time / s

Figure 4.5: Gas chromatograms of 2% 퐻2⁄퐴푟 and 0.5% 퐻2⁄퐴푟 calibration gases

The signal present at 10 s is a systematic ‘satellite’ caused by the pressure change in the system as the sampling valve rotates between ‘load’ and ‘inject’ positions.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

In the 0.5% 퐻2⁄퐴푟 chromatogram there are two distinct peaks, with retention times of 115 and 165 s, respectively. The first peak is from hydrogen and the second is from oxygen which is present in small quantities from the sample loop. In the 2% 퐻2⁄퐴푟 chromatogram there are three peaks, however, the first two at retention times of 90 and 135 s, respectively are overlapping. These two peaks are 퐻2 and 푂2. There is a four-fold difference in hydrogen concentration between the calibration gases, therefore it is easier to assign the relevant peak to this gas. The third peak in the 2% 퐻2⁄퐴푟 chromatogram is due to nitrogen, again present in small quantities in the sample loop. Nitrogen may also be present in the 0.5%

퐻2⁄퐴푟 chromatogram, however, the program ended before it could reach the detector. Due to the interference in the first two peaks it is impossible to integrate the peak and attain a true peak area.

This effect is still as prevalent in irradiated samples that contain higher concentrations of hydrogen. Figure 4.6 shows the resultant chromatograms from initial trials of 퐻2 − 푂2 radiolysis studies. The gas ratios in the samples are 3.75:2.65:90 퐻2: 푂2: 퐴푟. Samples are irradiated individually using the same position within the irradiator with doses received ranging between 1 and 5 kGy. By assuming the dose is absorbed by all of the gas, the absorbed doses have been calculated to be in the range of 7.07x1015 and 3.48x1016 eV.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

1400 7.07x10^15 eV

1200 1.41x10^16 eV H2 2.10x10^16 eV 1000 2.80x10^16 eV 3.48x10^16 eV 800

600 O2 N2 400

Signal Intensity / / AU Intensity Signal 200 0 0 50 100 150 200 250 300 Retention time / s

Figure 4.6: Overlain chromatograms of initial trials of 퐻2 − 푂2 radiolysis experiments

(n.b. - data set superimposed on - data set)

From Figure 4.6 two things become apparent. The first is the issue of separation between

퐻2 and 푂2 signals and the second is that in this absorbed dose range, there is very little chemistry occurring that is affecting the concentrations of either hydrogen or oxygen.

Before any attempts could be made to increase the separation between 퐻2 and 푂2 signals, it became apparent that irradiated samples and samples of pure argon had significant levels of 푂2 and 푁2 present that are similar to pure air. Further inspection of the vessels showed that the 푃푇퐹퐸 taps had mechanically degraded (Figure 4.7) which had led to the original sample being lost. This is a major issue as higher absorbed doses are needed to investigate the 퐻2 − 푂2 system, however, the current vessel design does not tolerate doses higher than several 100 kGy. This led to a redesign of the reaction vessel and subsequent sampling method.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

Figure 4.7: Mechanical degradation of 푃푇퐹퐸 taps

An initial idea was to replace the 푃푇퐹퐸 taps with borosilicate stopcocks, however, this would require a grease to lubricate the taps and ensure a good seal. Although radiation resistant greases are available these would eventually degrade and could be a source of hydrocarbons such as methane or 푂2 into the system.

4.2 Reaction Vessel Mark II

After extensive research, a commercially available metal sampling cylinder was chosen as the reaction vessel (Figure 4.8). This has the benefit of being pressure tested up to pressures of 60 bar as well as tolerating a greater temperature range. As the vessel is made of 316 stainless steel there should be no source of foreign gases into the system. Also, with a commercial product comes greater repeatability between sample vessels with regards to volumes and thickness. The volume of the metal cylinder is 10 cm3. Due to the geometry of the vessels, the initial sample holder seen in Figure 4.3 is obsolete. A metal test tube rack that is affixed to the base plate was developed to accommodate more samples than the previous design.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

Figure 4.8: Stainless steel sampling cylinder for 퐻2 − 푂2 gamma irradiation experiments

4.2.1 GC Calibration

Due to this vessel only having one inlet, the analysis method using the GC was also revised.

After samples have been irradiated, a rubber septa is placed over the outlet and purged with argon gas. A gas tight sample syringe is then driven through the septa and into the sample directly. The syringe is then pumped several times to ensure adequate mixing of the sample and its contents are then injected onto the GC column directly using the separate injection port.

Figure 4.9 shows the resulting calibration plot using this sampling method. From this figure, it is evident that there is less scatter in the data and there is a linear response of the

3 detector up to 1 cm of pure 퐻2.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

4000 3500

3000 y = 3634.4x 2500 R² = 0.999 2000 1500

Peak area / AU area Peak 1000 500 0 0 0.2 0.4 0.6 0.8 1 3 H2 volume / cm

Figure 4.9: Calibration plot for pure 퐻2 using the direct injection methodology

3 In Figure 4.4, a volume of 0.4 cm of 퐻2 produced a peak with an area of approximately

3000 AU. Compared to the injection method, this has decreased to approximately 1200 AU.

This is due to the decrease in the sample volume that is being analysed from 10 cm3 (volume of glass ‘bulb’) to 1 cm3 (volume of the gas syringe).

Although the volume has been decreased, the resultant chromatograms still show sharp distinguishable peaks with a Gaussian profile (Figure 4.10). It is noticeable that the quantity of 푂2 and 푁2 has decreased significantly with this sampling method. However only with the

1 ml injection volume are there signs of other components in the sample.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

1000 900 0.2 ml 0.4 ml

800 0.6 ml 700 0.8 ml 600 1.0 ml 500 400

300

Signal Intensity / / AU Intensity Signal

200 100 0 100 150 200 250 300 350 400 Retention Time / s

Figure 4.10: Overlay of gas chromatograms highlighting varying injection volumes of pure 퐻2

After irradiating these vessels in a pure argon atmosphere, it was discovered that the valves in Figure 4.8 had a ball and socket joint which was made of ultra-high molecular weight polyethylene (푈퐻푀푊푃퐸). Under irradiation conditions, this polymer decomposes and cross links [52] which leads to the evolution of 퐻2 gas and thus to interference with post irradiation analysis of the hydrogen yield.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

4.3 Reaction Vessel Mark III

The final iteration of the reaction vessel design is shown in Figure 4.11. This is a bellows sealed valve which is non-wetted and made entirely of various metal components.

Figure 4.11: Final vessel iteration to investigate radiolysis of 퐻2 − 푂2 systems

4.3.1 GC Configuration and Calibration

To eradicate any air from the system and for increased repeatability, a new analysis system was developed and commissioned. This system reverted back to using the sample loop and sampling valve highlighted in Section 4.1.1. The load and inject configurations of the sampling valve are shown in Figure 4.12.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

i) ii)

Figure 4.12: Final GC valve configuration i) ‘Load’ position ii) ‘Inject’ position

This sampling method has a vacuum pump attached to one end of the sample loop, with the reaction vessel attached to the other side. The pump can evacuate the loop directly to the valve on the reaction vessel, once a good vacuum has been achieved, the pump is isolated.

The valve on the sample vessel is opened to expand the gas volume into the sample loop, after which the sample vessel is isolated and the sampling valve rotates to inject the sample inside the loop onto the column and to the detector. The sample loop has a pressure transducer attached which can log the pressure throughout the analysis. It has a span of

2.5 bar absolute and an accuracy of ± 2.5 mbar.

This method allows for multiple injections of a single sample as a correlation can be made between the pressure of the sample in the loop and the resulting peak area, however, due to dead volume this is not a perfect correlation. Figure 4.13 is a plot of pressure where six injections are made. Between each injection the sample loop is evacuated to remove the remnants of the previous sample. At the end of each injection, the pressure increases in the sampling loop due to the GC valve rotating back to the ‘load’ position which contains carrier gas at a higher pressure than the sampling loop.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

1400 1

1200

1000 2

800 3 600 4 400 5

6 Pressure / mbar / Pressure 200 0 0 100 200 300 400 500 600 700 800 Time / s Figure 4.13: Plot of sample loop pressure as a function of time for six repeat injections with vacuum GC configuration

This configuration is calibrated by evacuating the sample vessels using the manifold and filling the vessels with a calibration gas of known 퐻2 concentration. Three injections of each sample are taken and the results shown in Figure 4.14. The partial pressure of hydrogen is calculated using Equation 4.1.

% 푝 푝 = 퐻2 푙표표푝 Equation 4.1 퐻2 100

where 푝퐻2 is the partial pressure of hydrogen, %퐻2 is the concentration of hydrogen in the calibration gas and 푝푙표표푝 is the pressure in the sample loop as the sample is injected into the

GC.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

50.0

45.0 H2 conc. in Ar 40.0 5% 35.0 2%

30.0 1%

25.0 0.50%

mbar 20.0 0.10% y = 0.142x - 0.027 15.0 R² = 0.999

10.0

partial pressure in sample loop / loop sample in pressure partial

2 5.0 H 0.0 0 50 100 150 200 250 300 350 Peak area / AU

Figure 4.14: GC calibration curve of hydrogen partial pressure as a function of peak area for vacuum sampling system

The standard error of the slope is ±0.001. This value is within tolerable limits and reduces the scatter from the initial sampling design significantly.

The following series of equations are used to retrospectively calculate the number of moles of hydrogen in the post irradiated sample from the partial pressure of hydrogen in the sample loop. This value will then be able to determine the number of moles of hydrogen lost/formed during irradiation by comparison with the initial number of moles in the sample. Equation 4.2 is used to calculate the partial pressure of hydrogen within the sample loop

" " 푝퐻2(푝푙표표푝) = 푘퐻2퐴퐻2(푝푙표표푝) + 푏퐻2 Equation 4.2

" " where 푝퐻2(푝푙표표푝) is the partial pressure of 퐻2 in the sample loop, 푘퐻2 and 푏퐻2 are

calibration constants (from the trend line equation in Figure 4.14) and 퐴퐻2(푝푙표표푝) is the peak area for 퐻2 from the sample at pressure 푝푙표표푝.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

Assuming ideal gas behaviour, the mole fraction of hydrogen will remain constant during the expansion of the sample from the vessel into the sample loop, giving the following expression:

푝 푝 (푝 ) 퐻2 = 퐻2 푙표표푝 Equation 4.3 푝푠푎푚푝푙푒 푝푙표표푝

where 푝푠푎푚푝푙푒 is the pressure of the initial sample. The partial pressure of 퐻2 in the

irradiated sample (푝퐻2) is calculated by substituting Equation 4.2 into Equation 4.3 to give the following:

푝푠푎푚푝푙푒 " " 푝퐻2 = [푘퐻 퐴퐻2(푝푙표표푝) + 푏퐻 ] Equation 4.4 푝푙표표푝 2 2 and the number of moles in the irradiated sample is calculated thus:

푝푠푎푚푝푙푒 푉푠푎푚푝푙푒 " " 푛퐻2 = [푘퐻 퐴퐻2(푝푙표표푝) + 푏퐻 ] Equation 4.5 푝푙표표푝 R푇 2 2

where 푉푠푎푚푝푙푒 is the volume of the vessel.

To ensure there is no reaction between the steel vessels and the gaseous sample, processed blanks are made for each set of experiments. These blanks have the same composition of gas as the irradiated samples, however, are left on the bench for the same time as the irradiated samples.

An initial indication of whether the radiation had any effect on the system was to plot the

퐻2 peak area against the pressure in the sample loop and to compare the translation in respect to the y-axis of the series with respect to the unirradiated blank. Figure 4.15 is shown as an example. The R2 values are very consistent, however, as stated previously, the dead volume leads to a slight offset in correlation.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

1600 Unirradiated R² = 0.9994 1400

Vessel 1 R² = 0.9998 1200 Vessel 2 R² = 0.9989 1000 Vessel 3 R² = 0.9999 Vessel 4 R² = 0.9996 800

600

Peak area / AU / area Peak

2 400 H 200 0 200 400 600 800 1000 1200 1400 Pressure / mbar

Figure 4.15: Plot of 퐻2 peak area as a function of sample loop pressure for a series of samples containing 5:5:90 퐻2: 푂2: 퐴푟

After the initial trials were completed, it became apparent that there was little correspondence between the absorbed dose the samples had received and the loss of hydrogen yield as determined from the GC. One hypothesis is the efficiency with which the manifold mixed the gases pre-irradiation, as this random error could lead to the observed differences in hydrogen yield in the GC results.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

4.4 Gas Mixing

To test this hypothesis, four experiments were trialled using different gas mixtures. A minimum of four vessels were attached to the manifold and evacuated to a pressure below

20 mTorr. The vacuum pump was isolated and the vessels were filled to identical pressures of the relevant gas mix. The samples were analysed consecutively on the GC. The four gas mixtures were as follows:

. A pre-mixed two component calibration gas of known hydrogen concentration;

. A single component gas (in this case hydrogen);

. A two component gas mixture of hydrogen and argon mixed within the vessels and

concentration determined by pressure; and

. A three component gas mixture of hydrogen, oxygen and argon mixed in the vessels

and concentration determined by pressure.

Figure 4.16 shows the resulting plots of hydrogen peak area as a function of sample loop pressure for the four gas mixtures.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

120

110 2%H2/Ar 100 calibration gas 90 80 Vessel 1 n.b. All four data sets 70 Vessel 2 are superimposed

Vessel 3

peak area / AU / area peak

2 60 Vessel 4 H 50 R.S.D. 0.18 40 350 450 550 650 750 i) Pressure / mbar 5000

Pure H 4500 2 4000 Vessel 1 n.b. All four data sets 3500 Vessel 2 Vessel 3 are superimposed 3000

peak area / AU / area peak Vessel 4

H 2500 R.S.D. 2.51 2000 350 450 550 650 750 ii) Pressure / mbar

2000

10:90 H2 - Ar 1500

1000 n.b. Vessel 1 and 3 data sets are superimposed

peak area AU / area peak Vessel 1 Vessel 2

500 2

H R.S.D. 16.45 Vessel 3 Vessel 4 0 350 450 550 650 750 iii) Pressure / mbar 1200

5:5:90 H -O -Ar 1000 2 2 800 600

400 peak area / AU / area peak

Vessel 1 Vessel 2 2

H 200 R.S.D. 29.62 Vessel 3 Vessel 4 Vessel 5 Vessel 6 0 350 450 550 650 750 iv) Pressure / mbar

Figure 4.16: Plots of 퐻2 peak area as a function of sample loop pressure of four different gas mixes i) 2% 퐻2/퐴푟 calibration gas, ii) pure hydrogen gas, iii) 10:90 퐻2: 퐴푟 gas mix from manifold and iv) 5:5:90 퐻2: 푂2: 퐴푟 gas mix from manifold

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

From this series of graphs, it is evident that as more components are mixed with the manifold system, the more scatter in the GC analysis. This can be seen with the values of the relative standard deviation (RSD), starting at 2.51 for a single component gas and rising to

29.62 for a three component gas mix. It is not a question of number of components as the calibration gas has the least scatter across a series of samples with a relative standard deviation of 0.18 despite being a two component mixture. As the gases are mixed in series, the manifold relies solely on diffusion of the components to mix homogeneously inside the vessels. The timescales employed in the mixing of the gases (minutes) is not long enough for displacement of the lighter gases by the denser gases assuming Brownian motion occurs, therefore an alternative mixing method is needed.

To solve this issue, the gases need to be pre-mixed before being expanded into the vessels

(such as the calibration gases). To achieve this, a large volume mixing cylinder was added to the manifold, shown schematically in Figure 4.17. The entire manifold is placed under vacuum and once a good vacuum is achieved (< 20 mTorr), the vessels (red circles) are isolated from the system. The gases are mixed in series inside the 1 L cylinder. To increase the mixing speed of the gas mixtures, a 푃푇퐹퐸 coated stirrer bar is placed inside the mixing cylinder and a magnetic stirrer placed under the cylinder. After approximately 30 mins, the valve separating the irradiation vessels and the mixing cylinder is opened and the difference in pressure leads the gas mixture to expand into the vessels. To achieve near atmospheric pressure in the vessels once the mixture has been expanded, the mixing cylinder has to be mixed at 1 bar above atmospheric pressure. As the entire manifold is made from 316 steel there is no issue with pressure build up or leakage.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

Figure 4.17: Manifold schematic with new mixing cylinder and 푃푇퐹퐸 stirrer bar addition

The initial pressure of each gas inside the mixing cylinder is used to calculate the concentration of each gas and their partial pressures. The pressure inside the reaction vessels (once the gas has been expanded) will be used as 푝푠푎푚푝푙푒. Equation 4.3 can be adapted to relate the partial pressure of hydrogen inside the mixing cylinder to the partial pressure of hydrogen inside the irradiation vessels (Equation 4.6)

푝 푝 (푝 ) 퐻2 = 퐻2 푠푎푚푝푙푒 Equation 4.6 푝푐푦푙푖푛푑푒푟 푝푠푎푚푝푙푒

Figure 4.18: Final iteration of manifold design

The final configuration of the manifold is illustrated in Figure 4.18.

Chapter 4 Development of γ-Irradiation Reaction Vessel 7131060

Figure 4.19 highlights a repeat of the experiment seen in Figure 4.16iv where three gaseous components are mixed and several samples are analysed to ascertain how much scatter there is between them. This time however, the gases are mixed in the mixing cylinder first and agitated for 30 mins before being expanded into the vessels.

350

300 5:5:90 H2:O2:Ar

250

200 Vessel 1

150 Vessel 2

peak area / AU / area peak

2 R.S.D. 1.02 Vessel 3 H 100 Vessel 4 50 200 300 400 500 600 700 800 900 Pressure / mbar

Figure 4.19: Mixing efficiency of manifold with mixing cylinder for a three component gas mixture (n.b. all four data sets superimposed)

The relative standard deviation has significantly decreased from 29.62 to 1.02; this gives significant confidence in the mixing ability of the manifold.

Summary

A significant quantity of time and effort has been expended to ensure that all aspects of this part of the research, from initial gas mixing to sampling and final analysis are as accurate and repeatable as possible.

Chapter 5 Dosimetry 7131060

5 Dosimetry

The following chapter outlines what dosimetry is and why it is an important part in radiation chemistry. It discusses the different types of dosimetry utilised in this research and the advantages and disadvantages of each one for a particular system of interest. It will also look at previous literature and how dosimetry is dealt with in similar systems and circumstances to this research.

5.1 Background

It is important to quantify the amount of energy that is transferred between a radiation field and an absorbing material and how this energy is distributed in the absorbing system. This quantification constitutes radiation dosimetry and is termed absorbed dose, which has units of amount of energy absorbed per unit mass of irradiated material. The SI derived unit of absorbed dose is the Gray (Gy) (Equation 5.1), however, older sources of literature use the rad. Conversion between Gray and rad is shown in Equation 5.2.

1 Gy = 1 J kg−1 Equation 5.1

1 Gy = 100 rad Equation 5.2

The amount of energy transferred to a material and its distribution throughout the material are dependent on many factors; these include the type of radiation (α particles transfer a larger quantity of energy during collisions than γ- rays and lead to a column-like structure of ionisation and excitation events), the energy of the emitted radiation (higher energy radiation will penetrate further into the material) and finally, the composition of the absorbing material (a solid will absorb more energy than a gaseous material due to the higher density and greater stopping power of the solid).

Chapter 5 Dosimetry 7131060

Dosimetry can be divided into two categories: absolute and secondary methods [12].

Absolute methods determine the absorbed dose by direct measurements of a physical property of the system. This can be either ionisation of a gas or charge carried by a beam of charged particles with a known energy. Secondary dosimeters are used in systems whose response to radiation is known from previous absolute dosimeter measurements. The type of system being irradiated will determine which dosimeter is of most use.

In radiation chemistry studies, the most common type of dosimeters employed are chemical dosimeters. These are secondary dosimeters where the radiation dose is determined from the chemical change produced by the radiation field in a particular system. To calculate the absorbed dose in the system, a G-value is required for the reaction or product of interest for a particular radiation type. This is found by comparison with absolute dosimetry measurements.

To generate a G-value it is important to understand the primary radiolysis chemical yields of the system composing the dosimeter. The information provided by a chemical dosimeter is an average quantity of energy absorbed across the entire dosimeter.

Chapter 5 Dosimetry 7131060

5.2 Aqueous Dosimetry

The most widely used chemical dosimeter is that of the Fricke dosimeter [12]. As a result, it is one of the most mechanistically understood systems. It concerns the redox chemistry of

2+ 3+ the ferrous ion (퐹푒 ) and its oxidation to the ferric ion (퐹푒 ) by the products of 퐻2푂 radiolysis. The dosimeter is composed of an aerated aqueous solution of iron (II) sulphate

(퐹푒푆푂4) and sulphuric acid (퐻2푆푂4). The sulphuric acid acts as a proton source which helps to convert reducing radicals to oxidising radicals and create an oxidising environment, in turn leading to the oxidation of 퐹푒2+ to 퐹푒3+. The reaction scheme for the radiolysis of the

Fricke dosimeter is outlined in the following reactions:

− − 푒푎푞 + 푂2 → 푂2 Reaction 5.1

+ − . 퐻 + 푂2 → 퐻푂2 Reaction 5.2

− + . 푒푎푞 + 퐻푎푞 → 퐻 Reaction 5.3

. . 퐻 + 푂2 → 퐻푂2 Reaction 5.4

2+ . 3+ − 퐹푒 + 퐻 + 퐻2푂 → 퐹푒 + 퐻2 + 푂퐻 Reaction 5.5

퐹푒2+ + 푂퐻. → 퐹푒3+ + 푂퐻− Reaction 5.6

2+ 3+ . − 퐹푒 + 퐻2푂2 → 퐹푒 + 푂퐻 + 푂퐻 Reaction 5.7

2+ . 3+ . − 퐹푒 + 퐻푂2 + 퐻2푂 → 퐹푒 + 푂퐻 + 푂퐻 Reaction 5.8

퐹푒3+ + 퐻. → 퐹푒2+ + 퐻+ Reaction 5.9

Once irradiated, the resulting solution is analysed using UV-Vis spectrophotometry, as the

퐹푒3+ ion exhibits a characteristic absorption at 304 nm. The absorption value at this wavelength is subtracted from an unirradiated ‘blank’ value to obtain the true absorption value. The dose received by the irradiated solution is then determined using Equation 5.3:

Chapter 5 Dosimetry 7131060

100 F 퐴푏푠 Absorbed Dose, 퐷 = Equation 5.3 퐷 휀 휌 퐺(퐹푒3+) 퐿 where 퐴푏푠 is the true absorption value at 304 nm for the irradiated Fricke solution, F is the

Faraday constant (9.648x104 C mol-1), ε is the molar extinction coefficient of the 퐹푒3+ ion

(2174 dm3 mol-1 cm-1), ρ is the density of the Fricke solution (1.024 g cm-3), 퐺(퐹푒3+) is the radiolytic yield of 퐹푒3+ (15.5 molecules 100 eV-1 for 60퐶표 gamma rays) and L is the path length of the light in the measured Fricke solution. Once the absorbed dose is calculated, the dose rate can be calculated by dividing the absorbed dose by the irradiation time.

The Fricke dosimeter has a linear absorption within the range of 0-400 Gy. Above this dose, the oxygen dissolved in the solution is exhausted which leads to a reduction in the oxidation of 퐹푒2+. This oxidation can also occur with light; therefore all solutions must be kept in the dark.

To make 500 ml of stock Fricke solution, 0.2 g 퐹푒푆푂4. 퐻2푂 (Fisher chemicals, laboratory reagent grade) and 11 ml concentrated 퐻2푆푂4 (Fluka Analytical, ≥ 97.5%) are combined with deionised water. This gives a concentration of 1 mM 퐹푒푆푂4 in 0.4 M 퐻2푆푂4 stock solution.

Irradiations are carried out by placing 3 ml of the stock solution into a vial that is representative of the reaction vessel to be used in the research. This is irradiated for a short period of time and the resulting solution is pipetted into a cuvette and analysed at 304 nm in the UV-Vis spectrophotometer. All experiments use a test tube rack configuration for irradiations, therefore dosimetry was carried out for each position within this rack. Both

60퐶표 source rods were used and no attenuation was in place due to the fact that this configuration gave the highest dose rates achievable. To ascertain the relative dose rate of each individual position in the rack, two different irradiation times and three repeats of

100

Chapter 5 Dosimetry 7131060 each time were carried out. This reduced the random error of transferring the stock solution between the irradiation vessel and the cuvette to a minimum. To replicate multiple samples being irradiated at once, empty vessels were placed in front of the irradiated solution to investigate the attenuation of the reaction vessels. Figure 5.1 shows the results for the 4x6 rack showing i) the unattenuated dose rate and ii) fully attenuated dose rates.

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Chapter 5 Dosimetry 7131060

ii) Figure 5.1: Fricke dosimetry results for the test tube rack array showing i) Unattenuated dose rate ii) Fully attenuated dose rate (units – Gy min-1) (correct as of 10th February 2015)

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Chapter 5 Dosimetry 7131060

From Figure 5.1 it is clear that, as expected, the dose rate decreases significantly as the samples are placed further away from the source rods (in accordance with Equation 3.1), however, there is also dose rate variation across the rack where samples are equidistance from the sources. As stated in Section 3.2.1, only sources A and C are filled with 60퐶표, therefore the central two columns of the rack are equal distance from the two sources, however, the outside two columns are closer to one source than the other leading to a reduction between 10-20% of the dose rate across the row. Figure 5.1ii shows that there is an approximate 10% decrease of dose rate throughout the rack when positions are attenuated by other samples. It is evident that source C contains more 60퐶표 than source A, as the dose rate is not symmetrical across the chamber. Samples on the right hand side of the chamber, closer to source C, have a higher dose rate than samples in the corresponding positions on the left hand side of the chamber.

Following the extensive dosimetry of the source, a dose decay series is set up using

Microsoft Excel. This dose rate decay calculator is then validated monthly via single point dosimetry to ensure the decay calculated dose rates are correct.

5.3 Calculation of Absorbed Dose using 60푪풐 Source

Now that a dose rate for each position has been calculated, it is important to determine the absorbed dose by real samples irradiated under the same conditions. This is easily calculated if the sample and dosimeter are both homogeneous and have the same density and size, as the absorbed dose will be the same. This is true with aqueous solutions and the

Fricke dosimeter. If the samples differ from the dosimeter then extra calculations are needed to determine the absorbed dose.

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Equation 5.3 shows how absorbed dose, 퐷퐷, is related to the radiochemical yield of product formed by irradiation (in this case 퐺(퐹푒3+) for Fricke dosimeter). The absorbed dose in a sample 퐷푆 is related to absorbed dose in the dosimeter by Equation 5.4:

-1 [푃]푆 휌퐷 퐷퐷 퐺(푃)푠 (molecules 100 eV ) = 퐺(푃′)퐷 ′ Equation 5.4 [푃 ]퐷 휌푆 퐷푆

where 퐺(푃′)퐷 is the radiochemical yield of product 푃′ formed in the dosimeter, [푃]푆 and

′ ′ [푃 ]퐷 are the measured yields of 푃 and 푃 (moles per unit volume) in the sample and dosimeter when exposed to the same radiation field, 𝜌퐷 and 𝜌푆 are the densities of the

퐷 sample and dosimeter and 퐷 is the ratio of absorbed dose per unit mass in the sample and 퐷푆 dosimeter. The absorbed dose in the sample and dosimeter are related by the following equation:

휇 ( 푒푛) 휌 푆 퐷푆 = 퐷퐷 휇 Equation 5.5 ( 푒푛) 휌 퐷

휇 휇 where ( 푒푛) and ( 푒푛) are the mass energy absorption coefficients for the sample and 휌 푆 휌 퐷 dosimeter respectively.

From the 60퐶표 decay scheme seen in Figure 3.2, the energies of photons emitted by the source are 1.17 and 1.33 MeV and from Figure 1.8 (replicated in Figure 5.2) it is seen that these energies lie in the region where Compton scattering is the dominant process of γ – ray absorption.

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Chapter 5 Dosimetry 7131060

120

100

80 Photoelectric Pair effect production 60

40 Z of medium of Z Compton 20 effect

0 0.01 0.1 1 10 100 hν / MeV

Figure 5.2: γ-ray interaction processes and their dependence on photon energy and Z of medium (replicated from [13])

Therefore Equation 5.5 can be simplified to the following:

(푍⁄퐴)푆 퐷푆 = 퐷퐷 Equation 5.6 (푍⁄퐴)퐷 where 푍⁄퐴 is the ratio of atomic number to atomic weight for an element.

This simplification is incorporated because Compton absorption is proportional to the number of electrons in the medium and not to the way in which they are bound in the atoms. As the number of electrons per unit mass of material is proportional to 푍⁄퐴, it does not require knowledge of mass energy absorption coefficients.

5.3.1 Adsorbed Dose in Gaseous Systems

For a mixture such as the systems of interest in this research, a mean value (푍̅̅̅⁄̅̅퐴̅ ), is calculated using Equation 5.7.

̅̅̅̅̅̅ 푍 푍⁄퐴 = 훴 [푤푖 ( ) ] Equation 5.7 퐴 푖

푤푖 is the weight fraction of element i in the medium. Equation 5.6 then becomes:

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Chapter 5 Dosimetry 7131060

(푍̅̅̅⁄̅̅퐴̅) 퐷 = 퐷 푆 Equation 5.8 푆 퐷 (푍̅̅̅⁄̅̅퐴̅) 퐷

The Fricke dosimeter has the following weight percentages: 10.84% 퐻2, 87.91% 푂2 and

̅̅̅̅̅̅̅̅ 1.25% sulphur, therefore (푍⁄퐴)퐷 is calculated thus:

2 16 16 (̅̅푍̅̅⁄̅̅퐴̅̅) = [0.108 x ( )] + [0.88 x ( )] + [0.012 x ( )] 퐷 2.016 31.998 32.06

= 0.5532

A gas mixture with partial pressures of 5:5:90 퐻2: 푂2: 퐴푟 has the following weight

3 percentages (in a 10 cm reaction vessel at 1 bar absolute), 0.27% 퐻2, 4.25% 푂2 and 95.48%

̅̅̅̅̅̅̅̅ 퐴푟, therefore (푍⁄퐴)푆 is calculated thus:

2 16 18 (̅̅푍̅̅⁄̅̅퐴̅̅) = [0.0027 x ( )] + [0.0425 x ( )] + [0.9548 x ( )] 푆 2.016 31.998 39.948

= 0.4541

Equation 5.8 then becomes:

퐷푆 = 퐷퐷 0.8209 Equation 5.9

Assuming that both sample and dosimeter are of the same size (volume), exposed to the same irradiation field and irradiated in the same vessels, then a gaseous mix of 5:5:90

퐻2: 푂2: 퐴푟 will receive 82% of the equivalent absorbed dose of the Fricke dosimeter.

For samples that contain an oxide powder, only the dose absorbed by the gas is considered.

This is because the species formed/reacted that are of interest are gaseous species that originate from the gas phase. The presence of an oxide powder may lead to catalytic or

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Chapter 5 Dosimetry 7131060 steric effects but it is unknown whether it plays a part in the radiation chemistry reaction mechanism.

5.3.2 Literature Review of Heterogeneous System Dosimetry

There are several different approaches when dealing with the dosimetry of heterogeneous systems in the literature. In a paper by Nilsson and Jonsson, investigating the γ – ray induced

− dissolution of 푈푂2 pellets in 10 mM hydrogen carbonate (퐻퐶푂3 ) solutions, the dose rate is just that determined by Fricke dosimetry without any subsequent correction [53].

Experiments carried out by Petrik et al. [25] which investigated the gamma radiolysis of water on the surface of a variety of metal oxide surfaces, stated that the absorbed dose rate is determined by the Fricke dosimeter only, without subsequent correction. In a paper by

Stone [54] investigating the radiolysis of cyclohexane in a xenon matrix, Fricke dosimetry was used to determine the dose rate of the gamma source, however, for the higher Z materials of interest, an ion chamber with walls composed of graphite was used to determine the energy absorbed by the cyclohexane and the results extrapolated to ascertain a weighting factor for the dose absorbed by the xenon. The same approach was used by Sagert and Robinson [55] when investigating nitrous oxide (푁2푂), adsorbed to silica gel and 푍푟푂2. The dose rate of the source was determined by Fricke dosimetry and an ion chamber with walls composed of 푍푟푂2 was utilised to investigate the equivalent dose absorbed by the 푍푟푂2. There was no reference made to how the energy absorbed by 푁2푂 was calculated, however it was found that 푍푟푂2 absorbs 10% more energy than the Fricke solution when exposed to the same radiation field. This value has been used in subsequent papers by LaVerne and Tandon [29, 56] which investigated the hydrogen formation from radiolysis of adsorbed water on several metal oxide powders. The energy absorbed by the

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Chapter 5 Dosimetry 7131060 adsorbed water was calculated directly from Fricke measurements. A 10% weighting factor was added to this energy to determine the energy absorbed by the metal oxides. They plotted their results as a function of dose absorbed by the water alone and also as a function of total energy absorbed by the entire system (oxide and water).

From these various references it is clear that there is not a clearly outlined methodology for determining the absorbed dose in a heterogeneous system. In most cases, the dose absorbed by the phase of interest (usually an adsorbed organic species) is calculated as that absorbed by the equivalent Fricke measurement whilst dose absorbed by the solid metal oxide phase is neglected. In other references however, a weighting factor is added to the absorbed dose calculated by the Fricke dosimeter for higher Z materials.

In this research, only the energy absorbed by the gas phase will be utilised to determine G- values for the systems of interest. The dose rate from the gamma irradiation source will be determined by Fricke dosimetry and these values corrected for the difference in sample density, using Equation 5.9 for the energy absorbed by 퐻2: 푂2: 퐴푟 gas mixtures and

Equation 5.10 to estimate the dose absorbed by air.

퐷푆 = 퐷퐷 0.9022 Equation 5.10

assuming air has the following weight percentage, 75.56% 푁2, 23.15% 푂2 and 1.29% 퐴푟 and

̅̅̅̅̅̅̅̅ thus (푍⁄퐴)푆 = 0.4992.

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Chapter 5 Dosimetry 7131060

5.4 Disadvantages of Fricke Dosimetry with Metal Vessels

As stated previously, dosimetry using a Fricke solution is investigated by filling an appropriate reaction vessel with an aliquot of the solution. In this research there are two distinct reaction vessels. These are the glass headspace vials shown in Figure 3.5 pertaining to the air radiolysis experiments and the metal sample cylinders seen in Figure 4.11 to investigate the radiolysis of 퐻2 − 푂2 gas mixtures. Borosilicate glass is very chemically resistant to dilute acidic solutions therefore no issues arise from irradiating Fricke solutions in these reaction vessels. However, sulphuric acid does react with most metals to liberate hydrogen gas and a salt of the metal, thus even at these low concentrations (0.4 M), there may be an etching effect on the steel. It is important to quantify the affect the steel has on attenuating the radiation field, therefore the dose rates attained from Fricke in glass vials cannot be extrapolated for steel vessels. A different type of chemical dosimeter will consequently be needed to investigate the dose absorbed by 퐻2 − 푂2 gas mixtures.

There are several other materials that are used for dosimetry in radiation chemistry.

Another commonly used chemical dosimeter is that of ceric sulphate which investigates the reduction of ceric ions (퐶푒4+) to cerrous ions (퐶푒3+). This suffers the same problem as

Fricke dosimetry in that it is a 0.4 M acid solution. There are also solid state dosimeters such as polymethylmethacrylate ( 푃푀푀퐴 ), which degrades under irradiation and forms chromophores which can be measured spectrophotometrically. This will not react with the steel vessels and is available from a commercial company which therefore increases repeatability across several measurements. A disadvantage of this type of dosimeter is that the 1 cm strips of 푃푀푀퐴 are ready made to be able to fit into a UV-Vis spectrophotometer.

These dimensions do not allow the strips to be placed in the sample cylinders without

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Chapter 5 Dosimetry 7131060 cutting them down to an appropriate size. This reduces the repeatability of each strip as the dimensions will change between samples and also the certified calibration for each batch may not be fit for purpose. Another disadvantage is that the density between 푃푀푀퐴 and gaseous samples is greater than that of gaseous samples and Fricke solutions, therefore more corrections are needed to ascertain the absorbed dose by the gas phase.

5.5 Gas Phase Dosimetry

It was decided to use a gas phase dosimeter in order to negate the need for any density corrections. The two most commonly used gas phase dosimeters are 푁2푂 and ethylene

(퐶2퐻4). Both systems are well investigated and understood and use gas chromatography to measure the yield of 푁2 and 퐻2 respectively and determine the energy absorbed by the system. The following section analyses the literature to review the work that has been undertaken for these gaseous dosimeter systems.

5.5.1 Gas Phase Dosimetry Literature

As stated in Figure 5.2, the main energy deposition process for solid and liquid samples of low Z numbers is Compton scattering where the energy absorbed can be calculated from a secondary dosimeter. In gaseous samples however, there is very little direct absorption of gamma radiation and most of the absorbed energy comes from secondary electrons generated by the gamma radiation interacting with the vessel walls [57]. Therefore, the sample vessel geometry and nature of the wall materials is of importance for gas phase dosimetry.

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Chapter 5 Dosimetry 7131060

60 Johnson [58] irradiated 푁2푂 with a 퐶표 gamma source and calculated a G(푁2) value of

12.8 ± 0.4 molecules 100 eV-1. They used two parallel methods to determine the energy absorbed in the system; these were ionisation current measurements and the Fricke dosimeter. Corrections were applied to these measurements to determine the dose absorbed by 푁2푂, before determining the dose rate from ionisation current measurements by applying the following equation (5.11):

퐷푅 = 퐽 푊푁2푂 Equation 5.11

where 퐽 is the number of ion pairs formed per minute and 푊푁2푂 is the mean energy required to produce an ion pair in eV.

Determination of the energy absorbed by 푁2푂 as a function of the Fricke dosimetry has been calculated using Equation 5.12.

푍푁2푂푆푁2푂 퐷푁2푂 = 퐷퐷 Equation 5.12 푍퐷푆푊

where 퐷퐷 is the dose absorbed by the Fricke dosimeter, 푍푁2푂 and 푍퐷 are the number of

-1 electrons cm for 푁2푂 gas and the Fricke dosimeter respectively and 푆푁2푂 and 푆푊 are the stopping powers per electron in 푁2푂 and the vessel wall respectively. This equation has several uncertainties however, and the author disregarded this calculation and relied solely on ionisation current measurements.

McLaren [59] replicated part of this work and attained G(푁2) values of 9.03 ± 0.1 molecules

100 eV-1 by using ionisation measurements to determine dose rates. They attribute the difference in yield determined by Johnson to leaks of air into the system during experiments. These errors limit the accuracy of the dosimeter to 10%.

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Jankovsky [60] compared the use of 푁2푂 and ethylene for gas phase dosimetry and found

-1 that the yield of G(푁2) from 푁2푂 value varied between 9.03 and 12.8 molecules 100 eV of energy absorbed using 60퐶표 as the radiation source. This data has a relative standard deviation of 0.1. This scatter is attributed to leaks in the system and also to the unavoidable

3 difficulties in determining small 푁2 yields from irradiation cells with ≤ 100 cm volume.

Studies using ethylene found yields of hydrogen varied between 1.2-1.36 molecules 100 eV-1 energy absorbed for 60퐶표 gamma rays. The relative standard deviation for this data is 0.04.

Therefore their conclusions are that ethylene is the better system to determine gas phase dosimetry when small reaction volumes are utilised.

Ethylene gas has also been used in mixed radiation fields to determine absorbed dose [61,

62]. Ikezoe et al. determined the yield of 퐻2 from ethylene radiolysis to be 1.3 molecules

100 eV-1 for both 60퐶표 gamma rays and reactor radiation which was a mix of neutrons and gamma rays [61]. A similar study by Srinivasan et al. determined the yield of 퐻2 to be 1.2 molecules 100 eV-1 in a reactor radiation field [62].

Summary

From this literature selection it is evident that ethylene is the more suitable option for a gas phase dosimeter. This is due to the amount of scatter in the G(푁2) yields which arises from air ingress into the system and also the limits of detection required for low volumes of sample analysis. As stated in Section 3.3.1, the current gas chromatogram configuration is optimised for the detection of 퐻2 down to very low concentrations (~100 ppm) which indicates that the configuration is more suited to detect the radiolysis products from

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ethylene than that of 푁2푂. Another factor is the vacuum pump employed in the GC analysis system does not evacuate the sample loop to absolute vacuum, only to approximately 1% of atmospheric pressure. This means it is almost impossible to remove 푁2 from the sample loop entirely without further considerations, so that yields of 푁2 from 푁2푂 radiolysis could not be determined accurately.

5.5.2 Ethylene Dosimetry Results

To ascertain the dose rate inside the steel cylinders, the cylinders are attached to the manifold outlined in Section 3.4.1 and evacuated to a pressure of < 20 mTorr. The ethylene calibration gas is fed into the cylinders until atmospheric pressure is achieved inside the vessels before the samples are placed inside the test tube rack array and irradiated for several hours. Once irradiated, the samples are analysed using the GC, following the same procedure as outlined in Section 4.3.3. Following analysis, the vessels are washed with methanol and baked at 250 °C for several hours in order to ensure any polymeric products that may condense onto the vessel walls are removed before the vessels are used again.

Due to the steric constraints inside the irradiation chamber and the high doses required, only the front row of the test tube rack has been tested (Figure 5.3). For the calculation of

-1 the absorbed dose, the value of G(퐻2) = 1.36 molecules 100 eV was utilised [59] and a value of 28.05 g mol-1 to represent the molecular weight of ethylene was used to calculate the mass of ethylene irradiated.

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Figure 5.3: Results of ethylene dosimetry (units – Gy min-1) (dose rates correct as of 17th February 2015)

These results in comparison to those attained with the Fricke dosimeter results will be discussed further in Chapter Seven.

5.6 Ion Accelerator Dosimetry

As seen in Section 5.3, dosimetry of a heterogeneous system using a gamma source is a very complex issue, one that still is not fully understood in the wider research community. In contrast, dosimetry of an ion source is far simpler. The previous sections have dealt with several secondary dosimeters, however, dosimetry of an ion source can be calculated using absolute measurements. As seen in Figure 3.7, the ion beam passes through a titanium window before entering the sample vessel. This configuration of windows acts as a Faraday cup [50, 63] which allows for the collection of charged particles hitting the cup. The resulting current can be measured and used to calculate the number of ions entering a sample.

Throughout this research, the beam consists of ions in a single charge state, therefore for a

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Chapter 5 Dosimetry 7131060 continuous beam, Equation 5.13 can be used to determine the number of ions 푁 penetrating the sample.

퐼.푡 푁 = Equation 5.13 e where 퐼 is the measured electric current (in amperes), 푡 is the irradiation time (in seconds) and e is the elementary charge (in Coulombs). This value is used to calculate the absorbed dose 퐷, in the sample using Equation 5.14:

푁퐸 퐷 = Equation 5.14 푛 where 퐸 is the incident energy of each ion (in MeV) and 푛 is the ion charge state. All experiments using the ion accelerator utilise a beam of 퐻푒2+ ions, therefore 푛 is 2.

This method is the most commonly used for ion accelerator dosimetry [12, 64].

The electric current is measured by a pico-ammeter that is connected at several points along the beam line.

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2.5E-08

2.0E-08

1.5E-08

1.0E-08

5.0E-09 Ti window current / Amps / current window Ti 0.0E+00

-5.0E-09 3 8 13 18 23 Time / min Figure 5.4: Plot of current as a function of time for a 15 min irradiation using the ion accelerator showing the current measured on the 푇𝑖 window

Figure 5.4 shows the resulting current profile of a 15 min irradiation using a current of

20 nA. The current put on the 푇𝑖 window is very stable and only drops once during the irradiation.

Summary

This chapter outlines the difficulties in calculating the absorbed dose in the systems of interest to this research. In previous literature sources, there are many methods to determine this dose, however, there is not a unified concept which is agreed upon. In this research there are three systems of interest:

 γ-radiolysis of air in contact with a metal oxide powder;

 γ-radiolysis of 퐻2: 푂2: 퐴푟 mixtures in contact with metal oxide powders; and

 radiolysis of 퐻2: 푂2: 퐴푟 mixtures using an accelerated ion beam

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Each system has different requirements with regards to how absorbed dose is calculated.

Dosimetry of the gamma source has been carried out with secondary chemical dosimeters, due to their simplicity and adaptability. In the first system of interest, only the dose absorbed by the air will be used to calculate G-values. This is achieved by the Fricke dosimeter multiplied by a weighting factor (Equation 5.10) due to the difference in sample density.

The dose absorbed by the 퐻2: 푂2: 퐴푟 mixtures will solely be used to determine loss of reactants in the second system. Due to the nature of the sample vessels, ethylene gas is used as the chemical dosimeter. The dose rates utilised are shown in Figure 5.3 which are corrected for decay of the 60퐶표 source.

Experiments that use an ion accelerator as the radiation source, absolute dosimetry from currents measurements collected during each experiment will be used to calculate the dose received by the sample.

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6 Oxide Powder Characterisation

This chapter outlines the physical properties of the two oxide powders utilised in this research. Each oxide is characterised as received and after undergoing the regeneration process outlined in Section 3.4.3. Finally, a comparison is made between the properties of

퐶푒푂2 and 푍푟푂2. If the oxide powders have any catalytic effects on the gas phase chemistry, then it is important to determine their physical properties.

6.1 Properties of 푪풆푶ퟐ

6.1.1 As Received

To ascertain the purity and morphology of the 퐶푒푂2 (as received), SEM and EDS analysis were undertaken.

Figure 6.1: Scanning electron micrograph of 퐶푒푂2 (as received)

Figure 6.1 illustrates the powder morphology of 퐶푒푂2 using an SEM. It is clear from this image that there is not a dominant morphology within the powder. There are well defined

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Chapter 6 Oxide Powder Characterisation 7131060 edges along with irregular shaped grains and the average particle has faces with surface area of approximately 3 µm2.

The EDS spectrum for the, as received 퐶푒푂2 is shown in Figure 6.2. The carbon signal arises from the carbon stub. Silicon is the largest impurity present, as stated in Section 3.1.2.

Figure 6.2: EDS spectrum of 퐶푒푂2 (as received)

The BET adsorption-desorption isotherm is shown in Figure 6.3.

25.0

3 20.0

15.0

10.0

5.0 Quantity adsorbed / cm/ adsorbed Quantity

0.0 0 0.2 0.4 0.6 0.8 1 Relative pressure / (P/P ) o

Figure 6.3: BET adsorption (solid trace) - desorption (dashed trace) of 퐶푒푂2 (as received)

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Chapter 6 Oxide Powder Characterisation 7131060

The isotherm in Figure 6.3 is a type three isotherm [65]. This indicates that adsorbate molecules have a higher affinity for themselves rather than the surface of the sample [48].

When the relative pressure nears 1, the adsorbate molecules finally adhere to the surface in large quantities. It is clear from the desorption isotherm (dashed trace) that there is hysteresis in the isotherm, indicating the sample has a mesoporous nature [48].

The calculated BET surface area is 6.33 ± 0.02 m2 g-1.

The DRIFT spectra of the 퐶푒푂2 (as received) is highlighted in Figure 6.4.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

Reflectance / % / Reflectance 0.8

0.6

0.4

0.2

0.0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber / cm

Figure 6.4: DRIFT spectra of 퐶푒푂2 (as received)

The broad band at approximately 3500 cm-1 is the stretching mode of 푂 − 퐻 and arises from the adsorbed water on the oxide surface [66]. The water is present as water of crystallisation or intra-molecular bound water. The sharp inverse band at 2300 cm-1 is due to the asymmetrical stretching band of 퐶푂2 present in the atmosphere and is used as a

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Chapter 6 Oxide Powder Characterisation 7131060 reference to align the module [67]. The bands present between 1700-1300 cm-1 are a mixture of 퐶 − 푂 and 푁 − 푂 vibrational modes. Nitrate and carbonate moieties exhibit similar vibrational bands when adsorbed to metal oxides [68]. When 퐶푂2 adsorbs to a

2− surface where water is present, it can have several forms; linear 퐶푂2, carbonate (퐶푂3 ),

− − bicarbonate (퐻퐶푂3 ) and carboxylate species (퐶푂2 ). These species have bands representing asymmetrical stretching (1415 cm-1), 푂 = 퐶 = 푂 bending (1700-1200 cm-1) and symmetrical and asymmetrical 푂 − 퐻 bending bands (1655-1370 cm-1) from interaction with surface hydroxyl groups [68]. 푁푂2 adsorbed onto metal oxides has three distinct bands in the

-1 region, at 1680, 1400 and 1320 cm , representing asymmetrical 푁푂2 stretching, 푂 − 퐻 bending and symmetrical 푁푂2 stretching respectively [69]. The broad signal present at 650 cm-1 is the stretching mode of 퐶푒 − 푂 − 퐶푒 [66]. This is the dominant signal as it is the dominant bond in the oxide powder.

It is evident from Figure 6.4 that there are several adsorbed species on 퐶푒푂2 (as received) surface in addition to water, namely 퐶푂2and 푁푂푥. High temperatures are required to remove water completely, however, this procedure may affect other properties of the oxide. It is not known how strongly the other species are adsorbed to the surface and whether they are removed when placed under vacuum on the manifold prior to investigation of 퐻2 − 푂2 − 퐴푟 experiments.

6.1.2 Regenerated 푪풆푶ퟐ Properties

Experiments investigating the radiolysis of air used regenerated oxide powders which had been washed and baked according to Section 3.4.3. Therefore, it was of importance to characterise the regenerated oxide and determine if this process had any effects on the oxide properties.

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Section 3.4.2 outlined the methodology employed to investigate the radiolysis of air over an oxide surface. To determine the quantity of oxide powder needed to produce the 50% and

90% (by volume) oxide samples, the bulk density of the powder was required. This parameter was determined by filling a known volume with oxide and recording its weight.

-3 The bulk density of regenerated 퐶푒푂2 was calculated as 1.424 ± 0.005 g cm . This value is

-3 much lower than the crystal density of 퐶푒푂2 which is 7.215 g cm , arising from the inability of the powder to pack as tightly as a single crystal.

Figure 6.5: SEM images of regenerated 퐶푒푂2 illustrating the macrostructure of the powder (top) and a large particle (bottom)

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Figure 6.5 illustrates two electron micrographs from the regenerated oxide. There is no dominant morphology within the sample with well-defined edges and irregular shaped grains being dominant.

Changes in the surface area of the powder are significant as this may affect the catalytic properties (if any) of the oxide in determining the gas phase radiation chemistry. Figure 6.6 illustrates the adsorption-desorption isotherm for regenerated 퐶푒푂2. The BET surface area

2 -1 for the regenerated 퐶푒푂2 is 7.42 ± 0.02 m g .

20.0

1 18.0

- g

3 3 16.0 14.0 12.0 10.0 8.0 6.0 4.0 Quantity adsorbed / cm/ adsorbed Quantity 2.0 0.0 0 0.2 0.4 0.6 0.8 1 Relative pressure / (P/P ) o Figure 6.6: BET adsorption (solid trace) – desorption (dashed trace) isotherm for regenerated 퐶푒푂2

The isotherm in Figure 6.6 is identical in shape to the isotherm generated from 퐶푒푂2 (as received).

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Figure 6.7 illustrates the results from thermogravimetric analysis of regenerated 퐶푒푂2

° decomposed under static air and 푁2 up to 1000 C:

102.0

101.5

101.0

100.5

100.0 Mass / % / Mass 99.5

99.0 N2 Air 98.5

98.0 0 100 200 300 400 500 600 700 800 900 1000 o Temperature / C

Figure 6.7: Thermogram of regenerated 퐶푒푂2 decomposed under 푁2 (blue) and static air (red). Heating rate 10 °C min-1

It is clear from this figure, that there are no impurities or vast quantity of adsorbed species on regenerated 퐶푒푂2, with the mass change being ± 0.5% from the initial mass.

Figure 6.8 shows the DRIFT spectra measured for regenerated 퐶푒푂2. It highlights the nature of any adsorbed species on the oxide surface:

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Chapter 6 Oxide Powder Characterisation 7131060

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6 Reflectance / % / Reflectance 0.4

0.2

0.0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber / cm

Figure 6.8: DRIFT spectrum of regenerated 퐶푒푂2

All of the absorption bands present in Figure 6.8 have previously been assigned in Figure

6.4.

6.1.3 Comparison of ‘As Received’ and Regenerated 푪풆푶ퟐ

It is noticeable when comparing SEM images in Figures 6.1 and 6.5 that larger agglomerated particles are present in the regenerated 퐶푒푂2 that were not observed in the oxide before washing and baking. The surface area of the faces has increased from 3 to 300 µm2. This increase is likely to be an effect of heating the oxide and causing grains to combine and agglomerate.

Although the particle size has increased in the regenerated oxide, the BET surface area remained within 10% of the 퐶푒푂2 (as received) after five subsequent regeneration cycles.

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The bulk density of the regenerated oxide is within 1% of the 퐶푒푂2 (as received), highlighting that the regeneration process has little effect on the oxide properties.

There was very little difference between the DRIFT spectra of 퐶푒푂2 (as received) and regenerated 퐶푒푂2 (Figures 6.4 and 6.8). Both spectra had identical absorption bands indicating that the regeneration cycle outlined in Section 3.4.3 has no effect on the concentration or identity of adsorbed species on 퐶푒푂2. Figure 6.9 highlights DRIFT spectra of an identical sample of 퐶푒푂2 (as received) that has undergone five subsequent regeneration cycles. A sample was taken after each cycle and anaylsed.

2.0

1.8 As received Regen. 1 Regen. 2 Regen. 3 1.6 Regen. 4 Regen. 5

1.4

1.2

1.0

0.8 Reflectance / % / Reflectance

0.6

0.4

0.2

0.0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber / cm

Figure 6.9: DRIFT spectra of 퐶푒푂2 (as received) and regenerated 퐶푒푂2 up to five subsequent regeneration cycles

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Chapter 6 Oxide Powder Characterisation 7131060

It is clear from Figure 6.9, that the regeneration process has no effect on the oxide powder as no new bands are detected. Absorption bands present in 퐶푒푂2 (as received) have not diminished after consecutive washing and baking.

6.2 Properties of 풁풓푶ퟐ

6.2.1 As Received

To ascertain the purity and morphology of the as received 푍푟푂2, SEM and EDS analysis were undertaken.

Figure 6.10: SEM image of 푍푟푂2 (as received)

An SEM image of ‘as received’ 푍푟푂2 is shown in Figure 6.10. The 푍푟푂2 particles have a defined rod like structure, and range in size from 10 to 40 µm in length.

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Chapter 6 Oxide Powder Characterisation 7131060

The EDS spectrum of 푍푟푂2 is shown in Figure 6.11. It indicates that there are no major impurities in the powder, with zirconium and oxygen being the only detectable elements in the sample, apart from the carbon stub.

Figure 6.11: EDS spectrum of 푍푟푂2 (as received)

Figure 6.12 highlights the adsorption-desorption isotherm for 푍푟푂2 (as received). The calculated BET surface area for this powder was 2.02 ± 0.04 m2 g-1.

7.0

- 6.0

3 5.0

4.0

3.0

2.0

Quantity adsorbed / cm/ adsorbed Quantity 1.0

0.0 0 0.2 0.4 0.6 0.8 1 Relative pressure / (P/Po)

Figure 6.12: BET adsorption (solid trace) – desorption (dashed trace) isotherm of 푍푟푂2 (as received)

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Chapter 6 Oxide Powder Characterisation 7131060

Like the adsorption isotherms highlighted for 퐶푒푂2 (Figures 6.3 and 6.6), this isotherm is also type three. The features of which, were discussed after Figure 6.3.

The DRIFT spectra of 푍푟푂2 (as received) is highlighted in Figure 6.13.

1.2

0.8

0.6 Reflectance / % / Reflectance 0.4

0.2

0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber / cm

Figure 6.13: DRIFT spectrum of 푍푟푂2 (as received)

The broad band at approximately 3500 cm-1 is the stretching mode of 푂 − 퐻 and arises from the adsorbed water on the oxide surface [66]. The water is present as water of crystallisation or intra-molecular bound water. The band present between 1700-1400 cm-1 is a mixture of 퐶 − 푂 and 푁 − 푂 vibrational modes. Nitrate and carbonate moieties exhibit similar vibrational bands when adsorbed to metal oxides [68]. When 퐶푂2 adsorbs to a

2− surface where water is present, it can have several forms; linear 퐶푂2, carbonate (퐶푂3 ),

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Chapter 6 Oxide Powder Characterisation 7131060

-1 region, at 1680, 1400 and 1320 cm , representing asymmetrical 푁푂2 stretching, 푂 − 퐻 bending and symmetrical 푁푂2 stretching respectively [69]. All of these bands are visible in

Figure 6.13, however, some are stronger than others.

6.2.2 Regenerated 풁풓푶ퟐ Properties

As with 퐶푒푂2, it is important to characterise the regenerated oxide powder and to determine if the regeneration process changes any of the oxide properties. The bulk density

-3 of regenerated 푍푟푂2 has been calculated as 2.059 ± 0.007 g cm , which is approximately

-3 35% of the crystal density (5.68 g cm ) for the reasons explained in Section 6.1.2 for 퐶푒푂2.

Figure 6.14 illustrates the morphologies of particles of regenerated 푍푟푂2:

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Chapter 6 Oxide Powder Characterisation 7131060

Figure 6.14: SEM images of regenerated 푍푟푂2 illustrating large agglomerated particles

The dominant morphology of regenerated 푍푟푂2 is large agglomerated particles with well- defined faces.

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Chapter 6 Oxide Powder Characterisation 7131060

Figure 6.15: EDS spectrum of regenerated 푍푟푂2

Figure 6.15 shows the EDS spectrum of the regenerated 푍푟푂2. The most distinguishing feature of Figure 6.15 is the presence of sodium in the regenerated 푍푟푂2. This impurity is attributable to the regeneration process, where the oxide is washed with an aliquot of

5 mM 푁푎푂퐻 to remove any adsorbed organics from the surface. The powder is washed with copious amounts of deionised water to remove any excess sodium, however, it is clear that 푍푟푂2 has a large affinity for sodium cations which are likely to adsorb onto the oxide surface.

The thermogravimetric analysis results of regenerated 푍푟푂2 decomposed under static air and 푁2 are displayed in Figure 6.16:

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Chapter 6 Oxide Powder Characterisation 7131060

101.0 100.8 100.6 100.4

100.2 100.0

Mass / % / Mass 99.8 N2 99.6 Air 99.4 99.2 99.0 0 200 400 600 800 1000 o Temperature / C

Figure 6.16: Thermogram of regenerated 푍푟푂2 decomposed under 푁2 (blue) and static air (red). Heating rate 10 °C min-1

As seen in Figure 6.7, there are no large impurities or adsorbed species in samples of regenerated 푍푟푂2. The mass difference is within ± 0.5%.

Figure 6.17 is the adsorption-desorption isotherm for regenerated 푍푟푂2. The resulting BET surface area was 2.24 ± 0.02 m2 g-1.

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Chapter 6 Oxide Powder Characterisation 7131060

9.0

1 8.0

- g 3 3 7.0 6.0 5.0 4.0 3.0 2.0

Quantity adsorbed / cm/ adsorbed Quantity 1.0 0.0 0 0.2 0.4 0.6 0.8 1 Relative pressure / (P/P ) o Figure 6.17: BET adsorption (solid trace)-desorption (dashed trace) isotherm of regenerated 푍푟푂2

The isotherm depicted in Figure 6.17 is of type three. There is a hysteresis loop in the isotherm, indicating the sample has a mesoporous nature.

The DRIFT spectra for regenerated 푍푟푂2 is depicted in Figure 6.18.

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Chapter 6 Oxide Powder Characterisation 7131060

1.2

1.0

0.8

0.6 Reflectance / % / Reflectance 0.4

0.2

0.0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber / cm

Figure 6.18: DRIFT spectrum of regenerated 푍푟푂2

The absorption bands present in Figure 6.18 have all been identified and assigned in Figure

6.13 for 푍푟푂2 (as received).

6.2.3 Comparison of ‘As Received’ and Regenerated 풁풓푶ퟐ

Comparing Figure 6.10 and Figure 6.14, the regenerated 푍푟푂2 has larger particle sizes than the fresh oxide powder. The rod-like morphology is still present in regenerated 푍푟푂2, however, larger agglomerated particles have been observed. These agglomerated particles have faces with surface areas of at least 400 µm2 which is an order of magnitude above the particles observed in Figure 6.10. Although the particles of the regenerated oxide are an order of magnitude larger than 푍푟푂2 (as received), they still have defined faces and appear to be agglomerations of smaller particles.

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Chapter 6 Oxide Powder Characterisation 7131060

The largest difference between the ‘as received’ and regenerated 푍푟푂2 is highlighted in the

EDS spectra (Figures 6.11 and 6.15). The regeneration process has led to sodium being retained by the oxide.

The bulk density value for the regenerated oxide is within 1% of the 푍푟푂2 (as received).

The surface area of 푍푟푂2 has not been altered too greatly by the regeneration process.

2 -1 푍푟푂2 (as received) had a surface area of 2.02 ± 0.04 m g , this had changed to 2.24 ± 0.02 m2 g-1 after four regeneration cycles. There was no direct increase or decrease in surface area after subsequent regeneration cycles indicating that the change in macro particle size and the uptake of sodium onto the oxide surface has no effect on these properties.

There is very little difference between the DRIFT spectra of 푍푟푂2 (as received) and regenerated 푍푟푂2 (Figures 6.13 and 6.18). To highlight the effect (if any) of the regeneration cycle has on the adsorbed species on 푍푟푂2, Figure 6.19 highlights the DRIFT spectra of a sample of 푍푟푂2 (as received) that has undergone four subsequent regeneration cycles, with a sample taken after each cycle and analysed.

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Chapter 6 Oxide Powder Characterisation 7131060

1.2

As received 1.0 Regen. 1 Regen. 2 Regen. 3 Regen. 4

0.8

0.6 Reflectance / % / Reflectance 0.4

0.2

0.0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber / cm-1

Figure 6.19: DRIFT spectra of 푍푟푂2 (as received) and regenerated 푍푟푂2 up to four subsequent regeneration cycles

This figure highlights that the regeneration process has no effect on the concentration or identity of adsorbed species on 푍푟푂2.

6.3 Comparison of Regenerated 푪풆푶ퟐ and 풁풓푶ퟐ

The major effect of the regeneration process outlined in Section 3.4.3 is the formation of large agglomerated particles. This occurs with both 퐶푒푂2 and 푍푟푂2 and leads to particles with well-defined faces of surface area 300 μm2 and 400 μm2, respectively. The regeneration process also leads to sodium uptake in 푍푟푂2 but not in 퐶푒푂2. This uptake does

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Chapter 6 Oxide Powder Characterisation 7131060

not affect the surface area of 푍푟푂2. The BET surface area is three times greater for 퐶푒푂2

2 -1 2 -1 3 -1 (7.42 m g ) than 푍푟푂2 (2.24 m g ) highlighted by the quantity of 푁2 adsorbed; 18 cm g

3 -1 for 퐶푒푂2 compared to 8 cm g with 푍푟푂2. This reflects the larger surface area and more porous nature of 퐶푒푂2.

-3 -3 푍푟푂2 has a slightly higher bulk density than 퐶푒푂2 (2.06 g cm compared to 1.42 g cm ) which means a larger mass of 푍푟푂2 is required for air radiolysis experiments containing 50 and 90% (by volume) of oxide.

The thermograms of both 퐶푒푂2 and 푍푟푂2 highlighted that both oxides were of high purity with little or no adsorbed organic species present.

The DRIFT spectra of regenerated 퐶푒푂2 and 푍푟푂2 highlight that 퐻2푂, 퐶푂2 and 푁푂푥 are present as adsorbed species on both oxides. These bands were also present in the ‘as received’ powders, therefore are not due to regenerating the oxide powders. There are more adsorption bands present in the regenerated 퐶푒푂2 spectrum (Figure 6.8) than in the regenerated 푍푟푂2 spectrum (Figure 6.18) highlighting the fact that the adsorption is more dominant for 퐶푒푂2 than 푍푟푂2.

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7 푯2 – 푶2 Radiolysis Results and Discussion

The following chapter details the results of radiolysis of 퐻2-푂2 gas mixtures in an argon matrix in the presence or absence of an oxide surface. The initial focus of this chapter is discussion of the radiolysis of ethylene results outlined in Chapter Five in comparison to those results attained using the Fricke dosimeter. The chapter discusses the sources of error in the experimental configuration and the mechanism of ethylene radiolysis. Finally the chapter outlines preliminary trials of 퐻2 - 푂2 gas radiolysis using an ion accelerator as the radiation source and discusses the use of ethylene as a chemical dosimeter compared to absolute current measurements for accelerator dosimetry. The relevance of these results are discussed and interpreted throughout the chapter with a final conclusion given at the end.

7.1 Discussion of 푪ퟐ푯ퟒ Dosimetry in Comparison with Fricke Dosimetry

Comparison of the ethylene dosimetry results in Figure 5.3 against those attained using the

Fricke dosimeter seen in Figure 5.1 (both reproduced in Figure 7.1) illustrates that there is a good agreement between the calculated dose rates of the two dosimeters.

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ii)

Figure 7.1: Comparison of dose rates obtained by different chemical dosimeters (units Gy min-1) i) Fricke dosimetry and ii) ethylene dosimetry (dose rates correct as of 17th February 2015)

It is clear from Figure 7.1 that there is very good agreement between the two dosimeter systems utilised. The small difference in dose rate can be attributed to the different vessels

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 used in each measurement. As stated previously, in gaseous samples, the majority of the absorbed dose comes from secondary electrons ejected by the vessel wall [57], therefore the difference in density between the glass vials used for Fricke dosimetry and the steel vessels used for ethylene dosimetry will have an effect on the secondary electron yield.

Another difference is the irradiated volume of the sample. In the Fricke dosimeter experiments, only 3 ml of solution was irradiated in the 12 ml glass vials, however, for the ethylene experiments, the entire gas volume is irradiated. As seen in Figure 7.1i, there is lateral dependence on dose rate inside the chamber as well as depth dependence.

Therefore it is entirely plausible that there will be a vertical dependency on dose rate as well.

The majority of the focus in Section 5.3 has been on determining the absorbed dose in a sample. The dose rate is determined by dividing the absorbed dose by the irradiation time, however, little attention has been given to errors associated with the time constituent. The program that controls the 60퐶표 irradiator can set irradiations to last a certain time required within a second of accuracy. The two source rods are raised into the chamber independently of each other and rely on an air compressor to do so. The speed of which the rods raise is dependent on the compressor pressure and can alter the speed by up to a second. Within this second, the sources are still emitting γ–rays which are being absorbed by the sample.

For short irradiations such as that employed by Fricke dosimetry where irradiations can last up to a minute, this time error becomes more significant than the time error associated with ethylene experiments, where irradiation times are at least one hour in length. The minimum error associated with the time coefficient of Fricke measurements is 3%. This could explain

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 why the dose rates did not unilaterally increase or decrease between the dosimeters as the source rods may have risen slower during one dosimeter experiment and not the other.

7.2 Source of Errors in Ethylene Dosimetry

After collating several results from ethylene radiolysis experiments, it became apparent that there was disagreement in several samples across the multiple injections with respect to 퐻2 concentration in the GC sample loop. Figure 7.2 is a plot of the scatter in each sample against the peak area of 퐻2 from the first injection of that sample for three different irradiation times.

14.0

12.0 540 min 10.0 3900 min 5760 min 8.0

6.0

4.0

RSD of results from run / / % run from results of RSD 2.0

0.0 0.00 10.00 20.00 30.00 40.00 50.00 60.00

H2 peak area from first injection / AU

Figure 7.2: Plot of scatter in each sample of ethylene as a function of the peak area of 퐻2 in the first injection

From this figure, two observations can be made. Firstly, after the first injection of the sample, if the 퐻2 peak area is less than 10 AU, there is a higher disagreement between the pressure in the sample loop and the peak area of 퐻2 for subsequent injections. The scatter

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varies between 1-13%. When the initial 퐻2 peak area is at least double this figure, the scatter is always less than 2%. This trend could infer that the lower detection limit of the GC detector is being reached as there is a larger error in smaller signals. Figure 7.3 is an overlay of three chromatograms from three subsequent injections of a post irradiated ethylene sample (irradiated for 540 min).

4000

3500

3000 1st injection 2nd injection 2500 3rd injection 2000

1500

1000

Signal Intensity / AU / Intensity Signal 500

-500

-1000 20 25 30 35 40 45 50 55 60 Retention time / s Figure 7.3: Gas chromatogram overlay of three subsequent injections of post irradiated ethylene highlighting the 퐻2 signal

With subsequent injections, the baseline becomes less stable as the signal to noise ratio increases. This effect is highlighted further in Figure 7.4, which shows the chromatogram profile of the 퐻2 signal for the first injection of two different samples:

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

3000

2500

2000

1500

1000

500 Signal Intensity / AU / Intensity Signal 0

-500 20 25 30 35 40 45 50 55 60 Retention time / s i) 17000

14000

11000

8000

5000 Signal Intensity /AU Intensity Signal

2000

-1000 20 25 30 35 40 45 50 55 60 Retention time / s ii)

Figure 7.4: Gas chromatogram of two separate ethylene samples irradiated for i) 540 min and ii) 5760 min

The red bands in Figure 7.4 are the bounding limits of the noise in the chromatogram. As can be seen in Figure 7.4i, a larger percentage of the 퐻2 signal is within this noise limit compared to the 퐻2 signal in 7.4ii. With subsequent injections, as the signal gets smaller, a greater portion will lie within the noise limit. The sample represented by Figure 7.4i had

12.4% scatter across all three injections as the signal to noise ratio decreased. The sample

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 represented by Figure 7.4ii had 0.7% scatter across all three injections as the signal outweighed the noise limits of the instruments.

The second observation that can be made from Figure 7.2 is that longer irradiation times lead to larger 퐻2 peak areas for the first injection. This is to be expected as the sample is in the radiation field for longer therefore more of the ethylene will undergo radiolysis.

As there is no back reaction to reform ethylene, a steady state will not be reached in the system.

To ascertain more accurate and reproducible results for the ethylene dosimeter, a larger quantity of hydrogen needs to be measured to reduce the interference from the signal/ noise ratio of the GC. This can be attained by two methods. The first is to irradiate the samples for longer periods of time to ensure a greater percentage of ethylene undergoes radiolysis. This is not feasible however, due to scheduling conflicts with the irradiation source and also the linear range of the dosimeter. As mentioned in Section 4.1, the Fricke dosimeter has a linear oxidation rate between 0-400 Gy; the ethylene dosimeter has a linear range of 퐻2 formation between 5-200 kGy [12]. For the central two positions in the rack, any irradiation time over 11 h leads to the dosimeter being out of range and the value of

G(퐻2) = 1.36 is obsolete. To circumvent the issue of shorter irradiation times to increase 퐻2 production a second method is employed. Previously, all samples have been irradiated at atmospheric pressure (1 bar); therefore by increasing the pressure inside the sample vessel there is a larger number of ethylene molecules that can undergo radiolysis. The results of experiments at higher pressures are shown in Figure 7.5:

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Figure 7.5: Results of ethylene dosimetry at increased pressure (units – Gy min-1) (dose rates correct as of 24th February 2015)

These dose rates are put into the same decay series spreadsheet as the Fricke results to determine the dose rate on a given date. These dose rates are used to determine the absorbed dose by a particular sample during the course of 퐻2 − 푂2 − 퐴푟 radiolysis experiments. This decay series is then validated once a month with single point dosimetry.

7.3 Mechanism of Ethylene (푪ퟐ푯ퟒ) Radiolysis

It is important to understand the mechanism of ethylene radiolysis and what factors may affect the yield of 퐻2 produced with it being used as dosimeter for 퐻2 - 푂2 radiolysis experiments.

Work has been carried out to investigate the decomposition products in the radiolysis of ethylene. It is understood that the primary products are 퐻2, acetylene (퐶2퐻2), and higher molecular weight polymeric products [61]. It is widely agreed that 퐻2 is formed from direct molecular elimination processes (Reaction 7.1 and 7.2):

퐶2퐻4 ⇝ 퐶2퐻2 + 퐻2 Reaction 7.1

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퐶2퐻4 ⇝ 퐶퐻2퐶 + 퐻2 Reaction 7.2

The hydrogen atoms can be eliminated from separate carbon atoms (Reaction 7.1) or both can come from a single carbon atom (Reaction 7.2) [13]. However, both hydrogen atoms are eliminated from a single ethylene molecule. This has been investigated with isotopic experiments where 50:50 mixtures of 퐶2퐻4 and 퐶2퐷4 have been irradiated [70]. Mass spectroscopic data showed that 95% of the decomposition product was either 퐻2 or 퐷2 with only 5.2% being 퐻퐷. The yield of 퐶퐻퐶퐷 was also very low, which suggested that the methylene species (퐶퐻2 or 퐶퐷2), was not involved in the production of acetylene. The product in Reaction 7.2 (퐶퐻2퐶) can rearrange to form acetylene 퐶2퐻2 [71] which accounts for roughly 10% of the ethylene reacted. The remaining 90% undergoes condensation reactions to form higher molecular weight products [70]. The formation of 퐻2 by molecular elimination has also been observed in the photolysis of ethylene gas using a xenon lamp source [72].

Alongside Reactions 7.1 and 7.2, the radiolysis of ethylene can produce a primary excitation process which leads to hydrogen atom formation (Reactions 7.3 and 7.4) [73]:

∗ ∙ 퐶2퐻4 ⇝ 퐶2퐻4 → 퐶2퐻2 + 2퐻 Reaction 7.3

∗ ∙ ∙ ⇝ 퐶2퐻4 → 퐶2퐻3 + 퐻 Reaction 7.4

The hydrogen atom yield has been calculated as G(퐻∙) = 6.8. This atomic species reacts very

. rapidly with ethylene to form the ethyl radical (퐶2퐻5), (Reaction 7.5) [62, 74, 75]:

∙ ∙ 퐻 + 퐶2퐻4 → 퐶2퐻5 Reaction 7.5

The ethyl radical can undergo recombination and disproportionation reactions with other

∙ ∙ radical species such as the vinyl (퐶2퐻3), and methyl (퐶퐻3), to form the higher molecular

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 weight polymeric and unsaturated hydrocarbon products. It can also react with the hydrogen atom to form ethane (Reaction 7.6):

∙ ∙ 퐻 + 퐶2퐻5 → 퐶2퐻6 Reaction 7.6

This reaction (7.6) has been shown to occur via radical recombination by Yang et al. [75] who added nitric oxide (푁푂), to the system to act as a radical scavenger. Their results showed firstly, that with addition of 푁푂, ethane was not formed and secondly, that the yields of 퐻2 and 퐶2퐻2 were not affected by the addition of 푁푂 to the system. This suggests that the recombination of hydrogen atoms to form 퐻2 does not occur.

Alongside excitation reactions, formation of fragmented ions can occur [74]:

+ ∙ − 퐶2퐻4 ⇝ 퐶2퐻2 + 2퐻 + 푒 Reaction 7.7

The ethyl cation can then react with ethylene as seen in Reaction 7.8.

+ + ∙ 퐶2퐻2 + 퐶2퐻4 → 퐶3퐻5 + 퐶퐻3 Reaction 7.8

These products can then go on to form higher molecular weight species.

From this reaction scheme it can be seen that ethylene radiolysis is well understood and the majority of products are gaseous hydrocarbon species. Due to their size, these products will not interfere with 퐻2 analysis as they will not get through the packed column of the GC.

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7.4 푯ퟐ Production from Adsorbed Water on Oxide Powders

Before experiments were undertaken investigating the radiation chemistry of 퐻2 − 푂2 − 퐴푟 gas mixtures in contact with an oxide surface, it was important to determine the quantity of

퐻2 produced from adsorbed water radiolysis and whether this concentration affects the measurements of 퐻2 initially added to the system.

The metal vessels in Figure 4.11 were filled with the relevant oxide to capacity using the bulk density calculated in Chapter Six. The vessels were assembled and attached to the gas mixing manifold (Figure 4.18). The samples were placed under vacuum to remove the headspace and any physisorbed species on the oxide surface. After approximately 30 min under vacuum, the vessels were filled with argon to atmospheric pressure (1 bar absolute) and irradiated for different periods of time. Only the dose absorbed by the adsorbed water was considered. To determine this value, the mass of water adsorbed onto the oxide powder needed to be determined. It is assumed that 1 monolayer of water will be adsorbed onto the oxide due to the relative humidity of the laboratory and the vacuum pump removing any physisorbed species. Haschke and Ricketts have determined the mass of one

-2 water monolayer as 0.21 mg m adsorbed onto 푃푢푂2 powder [23]. Due to the similarities in crystal structure, lattice parameters and surface area, this value is a good approximation for the mass of one water monolayer adsorbed onto 퐶푒푂2 and 푍푟푂2. Knowledge of the SSA and mass of oxide powder in each sample allows for the mass of water in each sample to be determined and therefore, the absorbed dose. The results of these experiments are shown in Figure 7.6.

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2500.0 50.0

ZrO2 45.0

2000.0 CeO2 40.0

35.0

) molec. ) molec. ) 16 1500.0 30.0 16 25.0 1000.0 20.0

15.0

production / (x10 / production (x10 / production

500.0 10.0 2

H H 5.0 0.0 0.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 Absorbed Dose / (x1016) 100 eV

Figure 7.6: Hydrogen production as a function of absorbed dose1 from water adsorbed to 푍푟푂2 (primary y-axis) and 퐶푒푂2 (secondary y-axis)

There are two distinct features from the results shown in Figure 7.6. Firstly, the yield of 퐻2 from water adsorbed onto 퐶푒푂2 is at least an order of magnitude below the yield from

푍푟푂2 and has to be plotted on a second axis. One possible hypothesis is; there is more water adsorbed onto 푍푟푂2 than 퐶푒푂2, therefore more 퐻2 can be produced by radiolysis.

Using the value of Haschke and Ricketts of 0.21 mg m-2 for the mass of one monolayer [23] and the surface area of each oxide determined in Chapter Six; the surface area present in

2 each sample is 90 and 45 m for 퐶푒푂2 and 푍푟푂2, respectively. From these values, the mass of water adsorbed can be determined and assuming 퐻2 formation from water radiolysis occurs via Reaction 7.9; then the maximum number of 퐻2 molecules that could be formed is

20 20 6.3x10 and 3.15x10 molecules in samples containing 퐶푒푂2 and 푍푟푂2, respectively.

2퐻2푂푎푑푠 ⇝ 2퐻2 (푔) + 푂2 (푔) Reaction 7.9

1 Throughout this Chapter, in all plots of species production/depletion as a function of absorbed dose, the absorbed dose axis has been plotted in units of 100 eV so that G-values for the species in question can be determined from the gradient of the trend line of the data.

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

From these values it is clear that there is sufficient water in 1 monolayer to produce the quantity of 퐻2 that has been measured with the reaction efficiency calculated as 0.03 and

6.35% for adsorbed water radiolysis on 퐶푒푂2 and 푍푟푂2, respectively.

The explanation for greater 퐻2 production from 푍푟푂2 than 퐶푒푂2 is due to an increase in energy transfer from the oxide to the adsorbed species. This has been investigated by

LaVerne and Tandon [29] who found 푍푟푂2 produced five times more 퐻2 than 퐶푒푂2 for the same quantity of adsorbed water using 60퐶표 γ-rays. Petrik et al. have attributed this phenomena partly to the resonance between the band gap of 푍푟푂2 (5.0 eV) [76] and the bond dissociation energy of water (퐻푂 − 퐻) (5.15 eV) [27]. The band of 퐶푒푂2 is in the range

3.1-3.5 eV [77]. Therefore, not enough energy can be transferred from 퐶푒푂2 to adsorbed water to enhance the formation of 퐻2. However, LaVerne et al. [29] state that other metal oxides with a band gap inside this range does not increase the yield of 퐻2. Chelnokov et al

[78] investigated the electron transfer at the interface between liquid water and several metal oxides with band gaps ranging between 3.3-9.0 eV under the influence of ionising radiation. They found that the interaction between the radiation field and the oxide nanoparticles produces secondary electrons with enough energy to transfer across to the water independent of oxide band gap. This electron transfer led to increased yields of

− solvated electrons (푒푎푞 ), across all oxides normalised to that of pure water. Other explanations such as exciton formation and water adsorption form have also been given. It is widely agreed that an energy transfer mechanism is responsible for these results, however, no conclusions as to the nature of this mechanism has received widespread acceptance.

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

The second feature of Figure 7.6 is the scattered nature of 퐻2 yields from water adsorbed to

푍푟푂2. There appears to be a positive correlation between absorbed dose and production of

퐻2 from 푍푟푂2, however, there is a large variance in the quantity of 퐻2. It is possible that there are differing quantities of 퐻2푂 adsorbed across all the samples. The oxides were not treated beforehand, therefore, removal of adsorbed water was reliant solely on the vacuum pump. This parameter may become important, however, for this research it is important to qualify the amount of 퐻2 that can be produced from adsorbed water radiolysis.

It is important to note that 푂2 is not detected (or below the limits of detection of the gas chromatograph) in samples of either 퐶푒푂2 or 푍푟푂2. This effect has been highlighted by other researchers investigating radiolysis of adsorbed water on metal oxide surfaces

[25, 29]. This has led to debate as to the location of 푂2. One hypothesis is that 푂2 oxidises the metal oxide, leading to the formation of the super-stoichiometric 푀푂2+푥 product [79]. A second hypothesis is the formation of interstitial hydroxyl groups leading to a product with chemical formula 푀푂2(푂퐻) [80]. This has not been studied further as the primary concern is 퐻2 formation.

7.5 Radiolysis of Ethylene in Contact with Oxides

To help elucidate a better understanding of dosimetry in heterogeneous systems, and the dose received by the overlying gas phase in contact with an oxide powder, ethylene gas was added to sample vessels which contained either 퐶푒푂2 or 푍푟푂2. The vessels were filled to capacity with the relevant oxide and sealed for the duration of experiments investigating ethylene radiolysis. The mass of each oxide was noted to determine the volume occupied by oxide. The volume of ethylene was calculated by subtracting the volume of oxide (using the

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 crystal density) from the reaction vessel volume. The corrected volume of ethylene was used to determine the mass of ethylene and the absorbed dose.

Figure 7.7 shows the resulting chromatograms for a sample containing pure ethylene

(퐶2퐻4), a vessel containing 퐶푒푂2 in an argon atmosphere and a vessel containing 퐶푒푂2 in an

퐶2퐻4 atmosphere. The vessel volume was identical in all three samples and the mass of

퐶푒푂2 in the two heterogeneous systems was the same. All three samples were irradiated at the same pressure for an identical time period in the same position in the test tube array.

25.0

20.0 CeO2 - C2H4 CeO2 - Ar 15.0 C2H4

10.0

Signal Intensity / / AU Intensity Signal 5.0

0.0 30 35 40 45 50 Retention time / seconds

Figure 7.7: Gas chromatograms showing a comparison of the 퐻2 signal of irradiated ethylene (퐶2퐻4) (blue trace), 퐶푒푂2 in 퐴푟 atmosphere (green trace) and 퐶푒푂2 in ethylene (퐶2퐻4) (red trace) irradiated for 9 h in identical radiation fields

In the figure above, the chromatograms are taken from the first injection of each sample, where the quantity of 퐻2 is at its greatest. Each chromatogram has been normalised to the same sample loop pressure (that of ethylene (blue trace)) so that the variables between samples remains at a minimum.

Section 7.4 highlighted the presence of adsorbed water on 퐶푒푂2 which can lead to small quantities of 퐻2 being produced. The efficiency of the reaction is not great and only a small

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 proportion of the adsorbed water is radiolysed. This is evident from Figure 7.7 where the yield of 퐻2 from adsorbed water is approximately half the yield of 퐻2 from pure ethylene radiolysis. Figure 7.7 also highlights that the 퐻2 generated from ethylene radiolysed in contact with 퐶푒푂2 (red trace) is greater than the sum of 퐻2 generated from pure ethylene radiolysis and adsorbed water radiolysis.

Figure 7.8 is the parallel study using 푍푟푂2 as the oxide surface.

2,400.0 50.0 45.0 ZrO2 - C2H4 2,000.0

40.0

ZrO2 - Ar 35.0 1,600.0 C2H4 30.0 1,200.0 25.0 20.0 800.0

15.0

Signal Intensity /AU Intensity Signal Signal Intensity / / AU Intensity Signal 10.0 400.0 5.0 0.0 0.0 30 35 40 45 50 55 Retention time / seconds

Figure 7.8: Gas chromatograms showing a comparison of the 퐻2 signal of irradiated ethylene (퐶2퐻4) (blue trace – secondary y-axis), 푍푟푂2 in 퐴푟 atmosphere (green trace – secondary y-axis) and 푍푟푂2 in ethylene (퐶2퐻4) (red trace – primary y-axis) irradiated for 9 h in identical radiation fields

The chromatograms in Figure 7.8 have been normalised to the same sample loop pressure

(that of ethylene (blue trace)) for the reason mentioned previously.

In Figures 7.7 and 7.8, the chromatogram from ethylene (blue trace) is the same sample.

This allows a comparison to be made between 퐶푒푂2 and 푍푟푂2 samples under identical conditions. As seen in Section 7.4, the yield of 퐻2 from adsorbed water on 푍푟푂2 is at least

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

five times greater than a parallel 퐶푒푂2 sample in Figure 7.7. The stand-out feature of Figure

7.8, is the chromatogram of 푍푟푂2 − 퐶2퐻4 (red trace), which is plotted on the primary vertical axis whereas the chromatograms of irradiated 푍푟푂2 in an argon atmosphere and

퐶2퐻4 gas only are plotted on the secondary vertical axis. From the scale of these axes, it is noted that the yield of 퐻2 from ethylene is almost 450 times greater in the presence of

푍푟푂2 than with no oxide present. In concurrence with 퐶푒푂2 results, this yield is greater than the sum of ethylene radiolysis and adsorbed water radiolysis.

The increase in 퐻2 production from adsorbed water on 푍푟푂2 in comparison to 퐶푒푂2 is partly attributed to the resonance between the band gap of 푍푟푂2 and the bond dissociation of water as discussed earlier. The bond dissociation energy of ethylene (퐶퐻2퐶퐻 − 퐻) is

4.81 eV [27]. This value is below the band gap of 푍푟푂2, therefore energy transfer is likely to occur. The bond dissociation energy of ethylene is greater than the band gap of 퐶푒푂2, therefore there may be another mechanism occurring alongside energy transfer which leads to the increase in ethylene decomposition in the presence of an oxide surface. This other mechanism may occur in adsorbed water radiolysis discussed previously [25, 29]; however, there is no chemical change to the inert gas phase that is measureable.

It is evident from Figure 7.6 that the oxide powders have water adsorbed onto the surface, however, it is not clear as to the extent of water coverage and also the extent of physisorbed vs. chemisorbed water. Any interaction between ethylene and the oxide surface is likely to be through physisorption (Figure 7.9).

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Figure 7.9: Postulated schematic of ethylene interaction with an oxide surface

This enhancement of ethylene decomposition may be due to a catalytic mechanism rather than radiolytic. Studies have shown that doped metal oxides (in particular 퐶푒푂2 and 푍푟푂2) are useful in many hydrocarbon reactions [81] including ethylene dimerisation (Reaction

7.10):

2 퐶2퐻4 → 퐶4퐻6 + 퐻2 Reaction 7.10

This reaction may lead to enhanced 퐻2 yields that are falsely attributed to radiolysis. This hypothesis was tested by filling several sample vessels with either 퐶푒푂2 or 푍푟푂2, evacuating the gas phase and replacing with ethylene gas. The samples were then left in a water bath at

50 °C for 10 h in order to simulate conditions inside the 60퐶표 irradiator. After this period, the samples were analysed using the GC. The resulting chromatograms had no 퐻2 present, with any quantities, below the limits of detection. Pressure measurements of the samples in the water bath showed no change in pressure other than to be expected for temperature fluctuation. This indicates that there is no significant adsorption of ethylene onto the oxide surface or catalytic reactions occurring (which would lead to a pressure decrease) and the

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gas chromatography results confirm there is no 퐻2 being produced. Therefore the effect seen in Figures 7.7 and 7.8 is a radiation driven process.

As the oxide is in powder form, there will be cavities inside the powder which the ethylene will occupy. This may lead to cavity ionisation. This is a theory postulated by Gray [82], where the ionisation in a gas cavity inside a medium is related to the energy absorbed by the surrounding medium. The Bragg-Gray cavity theory is expressed quantitatively in

Equation 7.1:

퐸휈 = 퐽휈 푊𝜌 Equation 7.1

where 퐸휈 is the energy absorbed by the medium, 퐽휈 is the ionisation produced in the cavity,

푊 is the average energy lost by secondary electrons per ion pair formed in the gas and 𝜌 is the ratio of the stopping powers of the medium and the gas. This theory neglects the existence of high energy secondary electrons which has led to discrepancies in experimental results of ionisation measurements taken in air filled cavities from that predicted by

Equation 7.1 [83, 84]. These studies also found that the discrepancy increased with increasing wall Z number. From this, it would be expected that the dose received by the gas inside a cavity within 퐶푒푂2 to be higher than the dose received by the same gas in a 푍푟푂2 cavity. The results, however, do not support this claim as the yield of 퐻2 is greater from

푍푟푂2 samples than 퐶푒푂2 samples.

It is evident in heterogeneous systems that there is energy transfer between the oxide powder and the gas phase, however, a definitive mechanism is not forthcoming and there is no definitive value of how much the oxide surface contributes to the absorbed dose by the gas phase.

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

Figure 7.10 highlights the DRIFT spectra of regenerated 퐶푒푂2 before and after irradiation under an ethylene atmosphere.

2.2

2.0

1.8 Pre-irradiation 1.6 Post-irradiation

1.4

1.2

1.0

0.8

Reflectance / % / Reflectance 0.6

0.4

0.2

0.0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber / cm

Figure 7.10: DRIFT spectra of regenerated 퐶푒푂2 pre-irradiation (blue) and post-irradiation (red) in an ethylene atmosphere

The main difference in the spectra highlighted in Figure 7.10 is the emergence of a group of bands between 3000-3800 cm-1. These bands have been assigned as 퐶 − 퐻 stretches [66].

The emergence of these absorption bands indicates adsorption of ethylene radiolysis products onto the oxide surface.

Figure 7.11 is the comparative spectra for 푍푟푂2.

158

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

1.2

1.0

Pre-irradiation 0.8

Post-irradiation

0.6

0.4 Reflectance / % / Reflectance

0.2

0.0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber /cm

Figure 7.11: DRIFT spectra of regenerated 푍푟푂2 pre-irradiation (blue) and post-irradiation (red) in an ethylene atmosphere

As seen in Figure 7.10, absorption bands associated with 퐶 − 퐻 stretches are present in the post irradiated sample indicating adsorption of ethylene radiolysis products.

At the conclusion of these experiments, the oxide powders were baked in a furnace at

400 °C under static air for 12 h. This temperature ensured the removal of any organic contaminants on the surface of each oxide without changing the physical properties of the oxide outlined in Chapter Six. Figures 7.12 and 7.13 depict DRIFT spectra up to 400 °C for the post irradiated 퐶푒푂2 and 푍푟푂2, respectively.

159

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

18.0

16.0 20 degC 60 degC 14.0 120 degC 250 degC

12.0 400 degC

10.0

8.0

Reflectance /% Reflectance 6.0

4.0

2.0

0.0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber / cm-1

Figure 7.12: DRIFT spectra of irradiated 퐶푒푂2 in an ethylene atmosphere analysed between 20 – 400 °C

160

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

25.0

20 degC 20.0 60 degC 120 degC

250 degC 15.0 400 degC

10.0 Reflectance / % / Reflectance

5.0

0.0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber / cm

Figure 7.13: DRIFT spectra of irradiated 푍푟푂2 in an ethylene atmosphere analysed between 20 – 400 °C

It is clear from the previous two figures (7.12 and 7.13), that the temperature employed is high enough to decompose and remove any organic species that are adsorbed onto the oxide surfaces. This is seen by the diminishing adsorption band between 3000-2800 cm-1. At higher temperatures, adsorbed water is driven off the surface as well; however, this will be re-adsorbed as the temperature decreases as the baking of the oxides is carried out under static air conditions. At 400 °C, the adsorption band identified as 퐶푂2 increases, this is more prominent for 푍푟푂2 samples. This is due to hydrocarbon species decomposing in air to form

퐶푂2 and water vapour.

161

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

After the oxides have been baked, the 퐶푂2 band is reduced to atmospheric levels. Before the oxides are re-used in further experiments, they are evacuated under vacuum on the manifold system, which will remove any physisorbed species.

7.6 Gamma Radiolysis of 푯ퟐ − 푶ퟐ − 푨풓 Gas Mixtures

The following section outlines the results from pure gas phase radiolysis of five different

60 mixtures of 퐻2 − 푂2 − 퐴푟, irradiated using 퐶표 γ-rays. In all of the samples, argon constitutes at least 90% (by volume) of the sample. The five ratios utilised in this project were:

 퐻2 rich atmospheres (stoichiometry 10 : 1 : 89 퐻2 − 푂2 − 퐴푟 (by volume));

 푂2 rich atmospheres (stoichiometry 1 : 10 : 89 퐻2 − 푂2 − 퐴푟 (by volume));

 Equal volumes of 퐻2 and 푂2 (stoichiometry 5 : 5 : 90 퐻2 − 푂2 − 퐴푟 (by

volume));

 Water stoichiometry of 퐻2 and 푂2 (stoichiometry 5 : 2.5 : 92.5 퐻2 − 푂2 −

퐴푟 (by volume)); and

 푂2 excess atmospheres (stoichiometry 2.5 : 5 : 92.5 퐻2 − 푂2 − 퐴푟 (by

volume))

Throughout this chapter each ratio has an assigned colour, which is maintained throughout the figures.

Figure 7.14 shows the results of 퐻2 depletion as a function of dose for the five systems outlined above.

162

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

0.0

10.0

20.0

) molec. ) 30.0 17

40.0

H2: O2 (%) 50.0

10:01 depletion / (x10 / depletion

05:02.5 2 H 60.0 05:05 2.5:5 70.0 01:10

80.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0

16 Absorbed Dose / (x10 ) 100 eV

2 Figure 7.14: Results of gamma radiolysis of different ratios of 퐻2 − 푂2 − 퐴푟 illustrating 퐻2 depletion as a function of absorbed dose (n.b. trend line for 01:10 data set obscured by 2.5:5 trend line)

The green data points circled in red are anomalous results due to complete depletion of 퐻2 from the gas phase. This most likely occurred at a lower dose, which would bring the points closer to the entire data set. These points have been omitted from the G(-퐻2) calculations.

Table 7-1 outlines the G(-퐻2) values calculated from each trend line of the relevant data set.

Error calculations are within one standard deviation.

2 For clarity, only the 퐻2 : 푂2 ratio is shown in the legend of each figure

163

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

System (푯ퟐ − 푶ퟐ − 푨풓 (by G(-푯ퟐ) / Error (흈풔풍풐풑풆) volume)) molecules 100 eV-1 10 : 1 : 89 3.56 ± 0.67

5 : 2.5 : 92.5 4.66 ± 0.37

5 : 5 : 90 4.68 ± 0.13

2.5 : 5 : 92.5 3.81 ± 0.12

1 : 10 : 89 3.60 ± 1.58

Table 7-1: Calculated G(-퐻2) values for several different ratios of 퐻2 − 푂2 − 퐴푟 gas using gamma radiation

The system that contains 10: 1: 89 퐻2 − 푂2 − 퐴푟 is the only system where complete depletion of 퐻2 cannot occur due to lack of available 푂2 to form water. The other four systems can be compared. The G(-퐻2) values in Table 7-1 increase with increasing 퐻2 concentration from 3.60 to 4.66 molecules 100 eV-1 which suggests a first order relationship.

However, this statement is difficult to clarify as the concentration of 푂2 changes in the system as well. Also the associated error with samples containing 1: 10: 89 퐻2 − 푂2 − 퐴푟 is large in comparison to the other errors in Table 7-1. Another hypothesis is that the system is zero order with 퐻2 depletion being independent of starting 퐻2 concentration.

The order of the rate of recombination of 퐻2 − 푂2 is still debated in literature sources, with both zero and first order kinetics highlighted and discussed [40, 41].

Figure 7.15 is a plot of the G(-퐻2) value for each individual point in Figure 7.14.

164

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

20.0

18.0

16.0 H2: O2 (%) 10:01 14.0 05:02.5

12.0 05:05

)

2.5:5

2 H

- 10.0

( 01:10 G 8.0

6.0

4.0

2.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0

16 Absorbed Dose / (x10 ) 100 eV

Figure 7.15: Plot of G(-퐻2) as a function of absorbed dose for several different ratios of 퐻2 − 푂2 − 퐴푟 gas using gamma radiation using the data in Figure 7.14

At low absorbed doses, the G(-퐻2) scatter is much larger due to the formula used to calculate G-values (molecules lost or formed per 100 eV of energy absorbed). At low doses, this calculation is the division of two numbers of similar magnitudes; if the rate of formation/ consumption is constant, then at higher doses, the denominator becomes a much larger number than the number of molecules formed/ consumed.

From Figure 7.15, it is clear that the G(-퐻2) values for all five gaseous systems lies in the range of 2-6 molecules of 퐻2 consumed per 100 eV of energy absorbed. This range is in good agreement with Dautzenberg’s earlier work [41].

The corresponding 푂2 depletion data is plotted in Figures 7.16 and 7.17. The G(-푂2) values for each system is tabulated in Table 7-2. As highlighted in Chapter Four, the GC

165

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

configuration utilised in this research is less sensitive to 푂2 than 퐻2, however, in the concentration range used, this effect should be nullified.

System (푯ퟐ − 푶ퟐ − 푨풓 (by G(-푶ퟐ) / Error (흈풔풍풐풑풆) volume)) molecules 100 eV-1 10 : 1 : 89 2.44 ± 0.19

5 : 2.5 : 92.5 3.89 ± 0.28

5 : 5 : 90 4.27 ± 0.65

2.5 : 5 : 92.5 1.87 ± 0.28

1 : 10 : 89 2.55 ± 0.09

Table 7-2: Calculated G(-푂2) values for several different ratios of 퐻2 − 푂2 − 퐴푟 gas using gamma radiation

-40.0

-20.0

0.0

) molec. ) 20.0 17

40.0

60.0 H2: O2 (%) depletion /(x10 depletion

10:01 2

O 80.0 05:02.5 05:05 100.0 2.5:5 01:10 120.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0

16 Absorbed Dose / (x10 ) 100 eV

Figure 7.16: 푂2 depletion as a function of absorbed dose using gamma radiation of different ratios of 퐻2 − 푂2 − 퐴푟 gas mixtures

166

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

Although 푂2 is present in percent concentrations, there is a lot more scatter in these results compared to the corresponding 퐻2 data in Figure 7.14 which in turn leads to larger associated errors. This is likely to be as a result of the dead volume inside the gas chromatograph and the inability of the vacuum pump to achieve a high vacuum. Despite this, 푂2 consumption appears to be zero order with respect to initial 푂2 concentration.

20.0

18.0 H : O (%) 16.0 2 2 10:01 14.0 05:02.5 05:05

12.0

) 2.5:5

O 01:10

- 10.0

( G 8.0

6.0

4.0

2.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0

16 Absorbed Dose / (x10 ) 100 eV

Figure 7.17: Plot of G(-푂2) as a function of absorbed dose for several different ratios of 퐻2 − 푂2 − 퐴푟 gas using gamma radiation using the data in Figure 7.16

Assuming that 퐻2 and 푂2 recombines to form water via Reaction 7.11, then it would be expected that the G(-퐻2) values would be twice the order of G(-푂2) values. The average ratio between G(-퐻2) values in Table 7-1 and the G(-푂2) values in Table 7-2 is 1.44. As this value is greater than 1, it can be assumed that hydrogen peroxide (퐻2푂2) is not the final stable product. Therefore, it is likely that there are other stable products that can be formed during irradiation alongside water.

167

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

2퐻2 + 푂2 → 2퐻2푂 Reaction 7.11

7.6.1 Discussion

To understand the reaction order of the results, a mechanistic understanding of the system is needed.

The initial step of this system is the ionisation and excitation of each component (Reactions

7.12-14).

퐴푟 ⇝ 퐴푟+ + 푒− → 퐴푟∗ Reaction 7.12

+ − ∗ 퐻2 ⇝ 퐻2 + 푒 → 퐻2 Reaction 7.13

+ − ∗ 푂2 ⇝ 푂2 + 푒 → 푂2 Reaction 7.14

Charge transfer between the initial ionised species is important to determine the concentration of radical species. Table 7-3 gives the ionisation energies of each ground- state gas molecule [85].

Gas Molecule Ionisation Energy / eV 퐴푟 15.76

퐻2 15.43

푂2 12.07

Table 7-3: Ionisation energy of the three gas molecules in the initial system

From this table it is clear that 퐴푟 can ionise both 퐻2 and 푂2 (Reactions 7.15 and 7.16), and

퐻2 can ionise 푂2 (Reaction 7.17).

168

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

+ + -15 2 퐴푟 + 퐻2 → 퐴푟 + 퐻2 σ = 1.13x10 cm [86] Reaction 7.15

+ + -15 2 퐴푟 + 푂2 → 퐴푟 + 푂2 σ = 8.0x10 cm [87] Reaction 7.16

+ + -16 2 퐻2 + 푂2 → 퐻2 + 푂2 σ = 3.43x10 cm [88] Reaction 7.17

+ The reaction cross-sections of Reactions 7.15 - 7.17 are highlighted above. This leads to 푂2 being the dominant species after initial ionisation. The ionised states of 퐻2 and 푂2 will

∗ undergo geminate recombination and dissociation (Reactions 7.18 and 7.19), 퐻2 will

∗ 1 3 dissociate into 퐻 atoms and 푂2 dissociates primarily into a singlet ( D) and a triplet ( P) state

푂 atoms [89] which have different reaction rates with different species.

∗ 퐻2 → 2퐻 Reaction 7.18

∗ 1 3 푂2 → 푂( D) + 푂( P) Reaction 7.19

The atomic species produced in Reactions 7.18 and 7.19 then react further and lead to

1 3 consumption of 퐻2 and 푂2. As 푂( D) and 푂( P) are the dominant products of the initial ionisation, their reaction rates with 퐻2 and 푂2 will be important to understanding the mechanism occurring in the gas phase. These reaction rates3 have been determined and are outlined in the following reactions:

1 -10 3 -1 -1 푂( D)+ 퐻2 → 푂퐻 + 퐻 k= 2.87x10 cm molecule sec [90] Reaction 7.20

3 -18 3 -1 -1 푂( P)+퐻2 → 푂퐻 + 퐻 k= 9.08x10 cm molecule sec [91] Reaction 7.21

-34 6 -2 -1 푂 + 푂2 + 푀 → 푂3 + 푀 k= 5.8x10 cm molecule sec [92] Reaction 7.22

Reactions 7.20 and 7.21 highlight the different reaction rates of the singlet and triplet states of 푂 and Reaction 7.22 highlights the competing reaction with 푂2 requires a third body

3 Throughout this chapter, reaction rate coefficients are given at 300K

169

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

19 (푀=푂2/푁2/퐴푟/퐻푒). If 퐴푟 is assumed to be the third body, the concentration is 2.188x10 molecules cm-3 (assuming 0.9 bar partial pressure and ambient temperature). This concentration can be used to turn the third order rate constant into a pseudo-second order rate constant with a value of k= 1.27x10-14 cm3 molecule-1 sec-1. Reaction 7.22 is faster in comparison to Reaction 7.21, but not as fast as the singlet oxygen reaction with 퐻2

(Reaction 7.20).

Hydrogen atoms formed in Reaction 7.18 will also react with 퐻2 and 푂2 in competition with

Reactions 7.20-22:

-13 3 -1 -1 퐻 + 푂2 → 푂 + 푂퐻 k= 4.25x10 cm molecule sec [93] Reaction 7.23

-32 3 -1 -1 퐻 + 퐻2 → 퐻2 + 퐻 k= 1.96x10 cm molecule sec [94] Reaction 7.24

Recombination of 푂 and 퐻 atoms to form 푂2 and 퐻2 molecules both require a third body.

The reaction rates are 4.82x10-33 cm6 molecule-2 sec-1 and 1.3x10-32 cm6 molecule-2 sec-1

[95, 96] respectively. The pseudo-second order reaction rates are k= 1.05x10-13 cm3 molecule-1 sec-1 and k= 2.84x10-13 cm3 molecule-1 sec-1, respectively.

The 푂퐻 formed in Reactions 7.20, 7.21 and 7.23 will then react with molecular and atomic species as outlined below:

-15 3 -1 -1 푂퐻 + 퐻2 → 퐻2푂 + 퐻 k= 6.96x10 cm molecule sec [97] Reaction 7.25

3 -11 3 -1 -1 푂퐻 + 푂( P)→ 푂2 + 퐻 k= 2.92x10 cm molecule sec [98] Reaction 7.26

Reactions of 푂퐻 with 푂2 and 퐻 require high temperatures to occur.

170

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

Reaction 7.23 can also form 퐻푂2 but requires a third body. The rate constant for this reaction is 2.2x10-32 cm6 molecule-2 sec-1 [99]. The pseudo-second order rate constant is very fast however, with a value of k= 4.81x10-13 cm3 molecule-1 sec-1.

퐻푂2 is a very stable species and can undergo recombination to form hydrogen peroxide

(Reaction 7.27). Hydrogen peroxide can then react with 푂퐻 to form water (Reaction 7.28).

-13 3 -1 -1 2퐻푂2 → 퐻2푂2 + 푂2 k= 6.89x10 cm molecule sec [100] Reaction 7.27

-12 3 -1 -1 퐻2푂2 + 푂퐻 → 퐻2푂 + 퐻푂2 k= 1.91x10 cm molecule sec [101] Reaction 7.28

The reaction mechanism outlined above leads to the formation of water, which is fairly unreactive to other species, as high temperatures are needed for reactions to occur with atomic hydrogen and oxygen, and reaction with 푂퐻 leads to hydrogen exchange (Reaction

7.29). Water will however, undergo radiolysis leading to 푂퐻 and 퐻 formation. These species can then feed back into the mechanism outlined above.

-16 3 -1 -1 푂퐻 + 퐻2푂 → 퐻2푂 + 푂퐻 k= 2.2x10 cm molecule sec [102] Reaction 7.29

The fastest reaction outlined above is the singlet oxygen atom with 퐻2 (Reaction 7.20), therefore in systems where 푂2 is in excess, consumption of 퐻2 should be at its greatest. This is not the case however, suggesting that other reactions may dominate. There are very few reactions that lead to 퐻2 being reformed, as 퐻 atom recombination requires a third body, therefore the probability of the reaction taking place is lower. It is possible that in excess 푂2 conditions, Reaction 7.22 is increased, as the concentration of 푀 will be at its greatest.

The mechanism outlined above is only at a primitive stage, ionic reactions have not been included and will play a role in the overall radiation chemistry of the system. Further

171

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 fundamental experiments are required to elucidate a better understanding of the mechanism.

It should also be noted, that in Figure 7.14, in the absorbed dose range investigated, 퐻2 continues to be consumed. There are no plateaus in any of the data sets which would lead to the suggestion of a steady state being reached between recombination and radiolysis

(Reaction 7.30). Therefore, in the system studied in this research, the rate of 퐻2 and 푂2 recombination is greater than the rate of water vapour radiolysis.

2퐻2 + 푂2 ⇌ 2퐻2푂 Reaction 7.30

7.7 Gamma Radiolysis of 푯ퟐ − 푶ퟐ − 푨풓 in the Presence of an Oxide Surface

The following section outlines results of 퐻2 − 푂2 − 퐴푟 radiolysis in the presence of 퐶푒푂2 and 푍푟푂2. In all experiments detailed herein, the metal reaction vessels are filled to capacity with the relevant oxide material. The mass and bulk density of the relevant oxide is used to determine the remaining gas volume and therefore, the starting concentrations of each gaseous component. Only the dose absorbed by the gas volume is used to calculate G(-퐻2) values.

Figure 7.18 highlights the consumption of 퐻2 in the five gaseous mixtures of interest in contact with 퐶푒푂2.

172

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

-10.0 0.0

10.0 20.0

) molec. ) 30.0 17 40.0 50.0

60.0 H2 : O2 (%)

depletion / (x10 / depletion 10:01

2 70.0

H 05:02.5 80.0 05:05 2.5:5 90.0 01:10 100.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 16 Absorbed Dose / (x10 ) 100 eV

Figure 7.18: 퐻2 consumption as a function of absorbed dose for various 퐻2 − 푂2 − 퐴푟 gas mixtures in contact with 퐶푒푂2

At low doses, Figure 7.18 highlights that there is more 퐻2 in the gas phase than was added initially. It is presumed that this ‘excess’ 퐻2 is from adsorbed water radiolysis as highlighted in Figure 7.6. The maximum yield of 퐻2 produced from radiolysis of adsorbed water on

19 퐶푒푂2 measured in Figure 7.6 was 0.33 µmol. This yield was attained after 7.0x10 eV had been absorbed by the water. In Figure 7.18, the maximum yield measured was 88.0 nmol after 1.21x1018 eV had been absorbed. These two values are approximately proportional with dose. However, as the absorbed dose increases, 퐻2 is not produced in any significant quantities, it is only consumed.

After initial excitation, any energy transfer from the oxide must interact with adsorbed species before gaseous species. Therefore the rate of water radiolysis is initially faster than the rate of recombination. As the adsorbed water is radiolysed, it increases the

173

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 concentrations of 푂퐻 and 퐻, therefore, the rates of Reactions 7.23, 7.35 and 7.26 will increase, leading to 퐻2 and 푂2 consumption.

The comparative plot of G(-퐻2) values for the data presented in Figure 7.18 is shown in

Figure 7.19.

300.0

250.0

H2 : O2 (%)

200.0 10:01 05:02.5

150.0 05:05

)

2 2.5:5

- ( 01:10 G 100.0

50.0

0.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0

-50.0 16 Absorbed Dose / (x10 ) 100 eV

Figure 7.19: Plot of G(-퐻2) as a function of absorbed dose for several different ratios of 퐻2 − 푂2 − 퐴푟 gas in contact with 퐶푒푂2

It is clear from this figure that the increased rate of 퐻2 consumption leads to much larger

G(-퐻2) values in this system than in pure gas phase radiolysis (Figure 7.15). However, like gas phase data presented in Figure 7.15, there is greater variation at lower absorbed doses.

Due to the scatter present in the 푂2 consumption data in Figure 7.16 from pure gas-phase experiments, the corresponding data for samples containing 퐶푒푂2 are not shown in this thesis.

174

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

Figure 7.20 illustrates the consumption of 퐻2 as a function of absorbed dose to the five gaseous systems of interest in contact with 푍푟푂2.

-20.0 -10.0

0.0 H2 : O2 (%)

10.0 10:01

20.0 05:02.5 ) molec. )

17 05:05 30.0 2.5:5 40.0 01:10 50.0

60.0

depletion / (x10 / depletion

2 70.0 H 80.0 90.0 100.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 Absorbed dose / (x1016) 100 eV

Figure 7.20: Plot of 퐻2 consumption as a function of absorbed dose for the five gaseous systems of relevance in contact with 푍푟푂2

As highlighted previously in Figure 7.18, at low absorbed doses, the concentration of 퐻2 initially increases in Figure 7.20, due to the radiolysis of adsorbed water on the oxide

18 powder. The maximum yield of 퐻2 measured in Figure 7.20 is 2.0 µmol at 2.0x10 eV absorbed dose. This yield is an order of magnitude above the corresponding yield measured for the same absorbed dose in Figure 7.6 (0.58 µmol). This may not be a ‘real’ effect, as these are two separate samples, with slight differences in 푍푟푂2 mass. This will lead to differences in calculation of the dose absorbed by each sample.

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Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

The ‘excess’ 퐻2 measured in Figure 7.20, is over an order of magnitude greater than the

‘excess’ 퐻2 measured in Figure 7.18. This further highlights the increase in energy transfer from 푍푟푂2 to adsorbed water than is seen in the corresponding 퐶푒푂2 system.

After the initial increase in 퐻2, at higher absorbed doses, 퐻2 is then consumed in all five gaseous systems of interest.

Figure 7.21 highlights the G(-퐻2) yield for the five gaseous systems in contact with 푍푟푂2.

500.0

H2 : O2 (%) 400.0 10:01 05:02.5 300.0 05:05

2.5:5

)

2 01:10 H

- 200.0

(

100.0

0.0 0.0 5.0 10.0 15.0 20.0

-100.0 16 Absorbed dose / (x10 ) 100 eV

Figure 7.21: G(-퐻2) as a function of absorbed dose for five gaseous mixtures of 퐻2 − 푂2 − 퐴푟 irradiated in contact with 푍푟푂2

The G(-퐻2) values are two orders of magnitude greater than measured in the gaseous system. This wide range of G(-퐻2) values is unlikely to be an effect of the scatter that was highlighted in Figure 7.15, but is hypothesised to be evidence of a chain reaction.

176

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

7.8 Comparison of Homogeneous and Heterogeneous Radiolysis

In comparing the scale of the x-axes in Figures 7.14, 7.18 and 7.20, it is evident that, the rate of consumption of 퐻2 is vastly increased in the presence of an oxide surface. More data has been collected on systems containing the following 퐻2 − 푂2 − 퐴푟 concentrations: 5: 5: 90,

5: 2.5: 92.5 and 2.5: 5: 92.5. Therefore, these systems will be compared.

Figure 7.22 compares the consumption of 퐻2 for systems containing equal concentration of

퐻2 and 푂2 in pure gas and in contact with 푍푟푂2 and 퐶푒푂2.

0.0

10.0

20.0 Gas only

CeO2 30.0

ZrO2 ) molec. )

17 40.0

50.0

60.0 depletion / (x10 / depletion

70.0

2 H 80.0

90.0

100.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 16 Absorbed Dose / (x10 ) 100 eV

Figure 7.22: 퐻2 consumption as a function of absorbed dose in samples of 5: 5: 90 퐻2 − 푂2 − 퐴푟 concentration in contact with 퐶푒푂2 and 푍푟푂2 and in pure gas system only

The calculated G(-퐻2) values from this graph with associated errors are tabulated in Table

7-4:

177

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

System G(-푯ퟐ) / Error (흈풔풍풐풑풆) molecules 100 eV-1 Gas Only 4.68 ± 0.13

퐶푒푂2 35.07 ± 2.18

푍푟푂2 48.27 ± 6.48

Table 7-4: Calculated G(-퐻2) in samples containing 5: 5: 90 퐻2 − 푂2 − 퐴푟 (by volume) in the presence of 퐶푒푂2, 푍푟푂2 and in pure gas phase

The G(-퐻2) values in Table 7-4, emphasise the effect the presence of an oxide powder on the recombination of 퐻2 and 푂2.

A possible hypothesis is that radiolysis of adsorbed water on the oxide surface leads to an increase in 퐻2 in the gas phase and increases the ratio of 퐻2 to 푂2 closer to the favourable water stoichiometry (2:1). This stoichiometry leads to the favourable formation of water in the system (Reaction 7.31). Radiolysis of adsorbed water on 푍푟푂2 contributes more 퐻2 than adsorbed water on 퐶푒푂2, therefore, the rate of consumption of 퐻2 is greatly increased.

퐻2 + 푂2 + 푥퐻2 → (1 + 푥)퐻2푂 (푥 ≤ 1) Reaction 7.31

Figure 7.23 illustrates the rate of 퐻2 consumption in samples containing 5: 2.5: 92.5

퐻2 − 푂2 − 퐴푟 (by volume) in contact with 퐶푒푂2, 푍푟푂2 and in pure gas.

178

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

-20.0

-10.0 Gas only CeO2 0.0 ZrO2

10.0 ) molec. )

17 20.0

30.0

40.0

depletion / (x10 / depletion 50.0

2 H 60.0

70.0

80.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0

16 Absorbed Dose / (x10 ) 100 eV

Figure 7.23: 퐻2 consumption as a function of absorbed dose in samples of 5: 2.5: 92.5 퐻2 − 푂2 − 퐴푟 concentration in contact with 퐶푒푂2 and 푍푟푂2 and in pure gas

This figure further highlights how the presence of an oxide surface increases the rate of consumption of 퐻2. It is evident from this figure, that in both systems containing an oxide,

퐻2 is produced at low doses. This was not evident in Figure 7.22. Investigation of the gas stoichiometries may lead to an explanation. The initial gas concentration is set at water stoichiometry (2:1), therefore ‘excess’ 퐻2 from adsorbed water radiolysis, will lead to excess

퐻2 in the gas phase (Reaction 7.32), which is measured at low doses.

2퐻2 + 푂2 + 푥퐻2 → 2퐻2푂 + 푥퐻2 Reaction 7.32

At higher doses, the ‘excess’ 퐻2 may adsorb to the oxide surface (more likely as 퐻 than 퐻2) and react with hydroxyl groups on the surface, leading to adsorbed water formation.

179

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

The compiled results for 퐻2 depletion from samples with 2.5: 5: 92.5 퐻2 − 푂2 − 퐴푟 gas compositions are shown in Figure 7.24.

-10.0

0.0 Gas only CeO2 10.0 ZrO2

20.0

) molec. ) 17 30.0

40.0

50.0

depletion / (x10 / depletion

2 H 60.0

70.0

80.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 16 Absorbed Dose / (x10 ) 100 eV

Figure 7.24: 퐻2 consumption as a function of absorbed dose in samples of 2.5: 5: 92.5 퐻2 − 푂2 − 퐴푟 concentration in contact with 퐶푒푂2 and 푍푟푂2 and in pure gas

In this figure, ‘excess’ 퐻2 is only measured in samples that contains 푍푟푂2. The maximum yield of 퐻2 is approximately 0.8 µmol, in comparison to 2.1 µmol measured in the corresponding sample in Figure 7.23. In this system, 푂2 is initially in excess, therefore any

퐻2 formed from adsorbed water radiolysis, will enhance the rate of recombination. This is highlighted by the fact that, ‘excess’ 퐻2 is not measured in samples containing 퐶푒푂2 and the yield measured from 푍푟푂2 containing samples is an order of magnitude below what is expected. The overall reaction in this system is shown in Reaction 7.33.

퐻2 + 2푂2 + 푥퐻2 → (1 + 푥)퐻2푂 (푥 ≤ 3) Reaction 7.33

180

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

Summary

From the figures presented in Section 7.8, it is clear that the presence of an oxide powder, greatly increases the rate of 퐻2 consumption in mixtures of 퐻2 − 푂2 − 퐴푟 gas, in comparison to homogeneous studies. It is not yet clear however, if this is due to a catalytic effect from the oxide powder, whether the presence of adsorbed water enhances the gas phase radiation chemistry or whether other physical properties of the oxides have an effect.

7.9 Discussion of Pelletron Dosimetry

Section 5.6 outlined the methodology used to determine the dose absorbed by a sample using an accelerated ion source. To enhance the validity of the absolute current measurements, several experiments were carried out irradiating a chemical dosimeter in order to make direct comparison between the two systems utilised. The chemical dosimeters discussed in Sections 5.2 and 5.5 have been investigated using ion accelerators

[64, 103]. Due to the geometry of the reaction vessel and the nature of the radiation field, the Fricke dosimeter is an unsuitable candidate. The solution is not agitated during irradiation and the ion beam will only penetrate a few tens of μm, which will lead to significant errors in determining the dose by the entire solution. Only mixtures of

퐻2 − 푂2 − 퐴푟 gas will be studied using the ion accelerator therefore ethylene gas was chosen as the appropriate choice for a chemical dosimeter. The benefits of using ethylene gas instead of 푁2푂 have been discussed previously for use with gamma radiation. Another disadvantage of using 푁2푂 with accelerated ions is the potential to form activated nitrogen isotopes. The formation of 13푁, which is a β emitter with half-life of ten minutes may lead to

181

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 errors in analysis due to the elapsed time in post irradiation analysis whilst this isotope decays to safe background levels.

Ethylene radiolysis using a beam of accelerated ions was carried out following the method outlined in Section 3.4.4. The GC has been adapted to accept glass vessels instead of the steel sample vessels utilised in γ radiation studies. The glass vessels have a larger volume than the corresponding metal cylinders which leads to a smaller change in pressure during subsequent injections on the GC; this ensures the fragile mica window is not subjected to large pressure differences.

The results of the ethylene trials are shown in Figure 7.25.

60.0

50.0

40.0 ) molec. )

17 G(H2)= 1.39 ± 0.02

30.0

20.0

production / (x10 / production

2 H 10.0

0.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 Absorbed Dose / (x1017) 100 eV

Figure 7.25: Plot of 퐻2 production as a function of absorbed dose for ethylene experiments using an ion accelerator

In Figure 7.25, the absorbed dose has been calculated by integrating the current profile of

-1 each experiment. The calculated G-value for 퐻2 formation is 1.39 ± 0.02 molecules 100 eV .

182

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

This value is within five percent of the value quoted in several literature sources

(G(퐻2)=1.36) [12, 59]. Possible deviations of these values are discussed with respect to errors associated with ethylene analysis and current measurements.

7.9.1 Source of Errors in Ethylene Analysis

With respect to ethylene, the errors discussed in Section 7.3 are still valid; however, due to the fragility of the mica window assembly, irradiation at higher pressures is not possible.

Another source of error may be due to a leak in the system. The presence of 퐻2 in the post irradiated chromatogram is evidence of a major leak not being present.

Figure 7.26 is a plot of the sample pressure during irradiation of a sample of ethylene.

975 Irradiation Time

970

965

960

955 Pressure / mbar / Pressure

950

945

940 0 500 1000 1500 2000 2500 3000 3500 Time / s

Figure 7.26: Plot of pressure as a function of time for a sample of ethylene irradiated using 5.5 MeV 퐻푒2+ ions. Irradiation time 30 min, 10 nA current on sample

183

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

This figure shows pressure increases inside the sample vessel as the ion beam was accelerated into the sample. The pressure decreases by the same quantity when the beam was stopped. The pressure decreased at a rate of 0.93 mbar min-1 during the fifteen minutes the beam was irradiating the sample. This 2.89% decrease was attributed to radiolysis of the ethylene. Higher molecular weight polymeric products will condense onto the vessel walls leading to a decrease in the pressure as will the reaction of ethylene molecules. The stability of the sample pressure outside of the window of irradiation suggests there is not a leak in the vessel.

Anderson and Best [103] have studied the ethylene dosimeter using a cyclotron accelerator.

They irradiated samples at approximately 530 mbar and utilised substantially higher dose rates than in this research. They found the yield of 퐻2 produced was linear up to absorbed doses of 3x1020 eV with varying dose rates, ion energy and type.

7.9.2 Source of Errors in Current Measurements

A possible source of error in the current measurements is due to ‘crosstalk’. A small percentage of ions, when hitting the mica window may be rebounded back onto the 푇𝑖 window which leads to some ions being counted more than once. This phenomenon is a larger problem when more dense materials are placed after the 푇𝑖 window assembly. It has been shown that the current on the 푇𝑖 window is greater when a steel slide is placed after the window compared to a quartz slide. Mica is less dense than these materials and is placed further away from the 푇𝑖 window due to the glass collar. As the ions will have a lower energy after rebounding off the mica window and traversing across to the 푇𝑖 window, the dose enhancement will be negligible.

184

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

It is possible that electrons can be ejected from the mica window when the ion beam traverses the window. Any electrons that are ejected from the window towards the 푇𝑖 window will cancel the charge of the positive ions. The probability of this happening is very small as the majority of electrons will be ejected in the direction of the beam and into the sample.

Summary

Experiments utilising ethylene as a secondary dosimeter show there is excellent agreement between the absorbed dose calculated using ethylene dosimetry and absolute current measurements, with the calculated G-value being within 5% of the literature value. This has validated the use of the absolute current measurements to determine the absorbed dose in samples of 퐻2 − 푂2 − 퐴푟 gas.

185

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

7.10 푯ퟐ − 푶ퟐ − 푨풓 Radiolysis using an Ion Accelerator

Due to operational and time constraints, a revised experiment set was investigated using accelerated ions. As a result of the complexity of the source and the system of interest, only gaseous samples were irradiated and only three different compositions. These were:

 Equal volumes of 퐻2 and 푂2 (stoichiometry 5 : 5 : 90 퐻2 − 푂2 − 퐴푟 (by

volume));

 Water stoichiometry of 퐻2 and 푂2 (stoichiometry 5 : 2.5 : 92.5 퐻2 − 푂2 −

퐴푟 (by volume)); and

 푂2 excess atmospheres (stoichiometry 2.5 : 5 : 92.5 퐻2 − 푂2 − 퐴푟 (by

volume))

The results of these experiments are outlined in Figure 7.27 and Table 7-5.

0.0

0.5 H2 : O2 (%) 05:2.5

1.0 05:05

1.5 2.5:5

) molec. ) 19 2.0

2.5

3.0

depletion / (x10 / depletion

2 H 3.5

4.0

4.5 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 18 Absorbed Dose / (x10 ) 100 eV

Figure 7.27: 퐻2 depletion as a function of absorbed dose for three different mixtures of 2+ 퐻2 − 푂2 − 퐴푟 utilising 5.5 MeV 퐻푒 accelerated ions

186

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

System (푯ퟐ − 푶ퟐ − 푨풓 (by G(-푯ퟐ) / Error (흈풔풍풐풑풆) volume)) molecules 100 eV-1 5 : 2.5 : 92.5 5.04 ± 0.72

5 : 5 : 90 4.76 ± 0.61

2.5 : 5 : 92.5 5.38 ± 0.46

2+ Table 7-5: Calculated G(-퐻2) values from experiments utilising 5.5 MeV 퐻푒 accelerated ions

The errors associated with this data are larger than the corresponding errors for data collected with the 60퐶표 source. This is due to a smaller sample set.

In Figure 7.27, 퐻2 is being depleted linearly with absorbed dose. The G-values calculated for these data sets indicate the depletion of 퐻2 is independent of initial 퐻2 concentration.

ퟐ+ 7.11 Comparison of γ and 푯풆 Irradiation of Gaseous 푯ퟐ − 푶ퟐ − 푨풓 Samples

60 It is possible to compare the results of 퐻2 − 푂2 − 퐴푟 radiolysis using both 퐶표 gamma rays and 퐻푒2+ accelerated ions, as there should be no linear energy transfer (LET) effect in gaseous samples, due to the low density of the systems.

Figure 7.28 combines the results of 퐻2 depletion as a function of absorbed dose using gamma rays (Figure 7.14) and accelerated 퐻푒2+ ions (Figure 7.27) for the three gaseous mixtures investigated using both sources and the calculated G-values are shown in Table

7-6.

187

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

0.0 5:2.5 gamma 5:2.5 He2+ 50.0 5:5 gamma 5:5 He2+

100.0 2.5:5 gamma 2.5:5 He2+

150.0

) molec. ) 17 200.0

250.0

300.0

depletion / (x10 / depletion

H 350.0

400.0

450.0 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 16 Absorbed Dose / (x10 ) 100 eV

Figure 7.28: 퐻2 depletion as a function of absorbed dose for three various mixtures of 60 2+ 퐻2 − 푂2 − 퐴푟 utilising 퐶표 γ-rays and 5.5 MeV 퐻푒 accelerated ions

System 60푪풐 γ-rays 5.5MeV 푯풆ퟐ+ accelerated Ions

(푯ퟐ − 푶ퟐ − 푨풓 (by volume)) G(-푯ퟐ) / Error (흈풔풍풐풑풆) G(-푯ퟐ) / Error (흈풔풍풐풑풆) molecules 100 eV-1 molecules 100 eV-1

5 : 2.5 : 92.5 4.66 ± 0.37 5.04 ± 0.72

5 : 5 : 90 4.68 ± 0.13 4.76 ± 0.61

2.5 : 5 : 92.5 3.81 ± 0.12 5.38 ± 0.46

Table 7-6: Calculated G(-퐻2) values and associated errors for three different mixtures of 60 2+ 퐻2 − 푂2 − 퐴푟 utilising 퐶표 γ-rays and 5.5 MeV 퐻푒 accelerated ions

188

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060

The G-values in Table 7-6, are very close, however, the error associated with the accelerated ion data is significantly higher. Sections 7.3 and 7.9 discussed the potential sources of errors with the dosimetry systems used in each set of measurements and Chapter Four dealt with the errors associated with GC analysis.

This data confirms there is no radiation type effect and therefore no LET effect as to be expected in gaseous systems.

The primary radiation sources utilised (60퐶표 γ-rays and 5.5 MeV 퐻푒2+ accelerated ions) leads to different radiation fields in the samples. Inside the 60퐶표 source, the entire sample is within the radiation field, however, only part of the sample is in the primary radiation field when accelerated ions are used. This effect is negated by the secondary electron radiation field. In gaseous systems the majority of the absorbed dose is from interactions of the secondary electrons with the gas, not the primary radiation [57]. These secondary electrons can have ranges in the order of metres, therefore all of the sample can still be irradiated.

Conclusions

60푪풐 Studies

60 The use of ethylene (퐶2퐻4), as a gas-phase dosimeter for 퐶표 γ-rays provides results in good agreement with the more commonly used Fricke dosimeter. This allows for a good approximation of the dose absorbed by the gas phase studied in this chapter. The detection limits of the GC leads to higher irradiation pressures being required to attain repeatable results.

Adsorbed water on both 퐶푒푂2 and 푍푟푂2 leads to 퐻2 formation during irradiations. The quantity of 퐻2 produced is five times greater in the presence of 푍푟푂2 than 퐶푒푂2, in good agreement with literature [25, 29], an explanation of this, is the resonance of the oxide

189

Chapter 7 퐻2 - 푂2 Radiolysis Results and Discussion 7131060 band gap with the bond dissociation energy of water. The approximation of one monolayer of adsorbed water is valid, with the quantities of 퐻2 measured.

A similar effect is seen with radiolysis of ethylene in contact with an oxide surface. The yield of 퐻2 is greater than the sum of radiolysis of ethylene alone and 퐻2 produced from adsorbed water radiolysis. The bond dissociation energy of ethylene is 4.81 eV, which is low enough for energy transfer from 푍푟푂2 to enhance decomposition.

The gamma radiolysis of 퐻2 − 푂2 − 퐴푟 gas mixtures follows zero order kinetics with regards to 퐻2 depletion. This is in agreement with literature data [41], however, a complete explanation is not forthcoming. The presence of an oxide surface greatly enhances depletion of 퐻2 from the gas phase. At low doses, there is an ‘excess’ of 퐻2 due to the radiolysis of adsorbed water having a faster rate than recombination of 퐻2 and 푂2, at higher doses, however, recombination dominates. A steady state between the two processes is not reached.

푯풆ퟐ+ Studies

The use of ethylene gas as a dosimeter for ion accelerator experiments provides good validation of the absolute current measurements. The G(퐻2) value of 1.39 is within 5% of literature values.

Experiments utilising accelerated 퐻푒2+ ions confirm there is no LET effect in samples of

퐻2 − 푂2 − 퐴푟. These experiments also highlight that no steady state is reached between the recombination of 퐻2 − 푂2 and the radiolysis of water vapour at higher absorbed doses than achieved with 60퐶표 γ-rays.

190

Chapter 8 Air Radiolysis Results and Discussion 7131060

8 Air Radiolysis Results and Discussion

The following chapter details the results from radiolysis of air experiments in the presence/absence of a particular oxide surface. The experimental details were outlined in

Chapter Three (all experiments undertaken in 12.0 cm3 glass vials, with the temperature inside the 60퐶표 irradiator being approximately 35 °C). The results are interpreted and their relevance discussed throughout.

This chapter details the results from radiolysis of air measurements and their significance.

This is followed by initial results with samples containing an oxide surface. During these results, an interference from another species was identified and resolved. New data from air radiolysis experiments in the presence of an oxide surface are presented and discussed followed by results and discussion of the interfering species. A final conclusion of the results presented in this chapter is given at the end.

8.1 Ion Chromatogram Calibration

During the course of air radiolysis experiments, the primary product ion of interest is the

− nitrate anion (푁푂3 ). To determine the detection limit of the ion chromatograph and its linearity, calibration solutions of sodium nitrate (푁푎푁푂3) with known concentrations were injected into the ion chromatograph. Two injections of each standard were taken to ensure there were no instrument anomalies. The calibration results are shown in Figure 8.1:

191

Chapter 8 Air Radiolysis Results and Discussion 7131060

60.0

50.0 y = 49.279x R² = 0.9968 40.0

30.0

Peak area / AU area Peak 20.0

10.0

0.0 0 0.2 0.4 0.6 0.8 1 1.2 [NaNO3] / mM − Figure 8.1: Calibration plot of 푁푂3 peak area as a function of 푁푎푁푂3 concentration using ion chromatography

− -1 The ion chromatograph has a linear response to 푁푂3 in the range of 0.05-1 mmol L . The standard deviation of the slope (σ) is 0.99. To validate the performance of the detector, fortnightly calibrations using fresh 푁푎푁푂3 solutions were carried out.

192

Chapter 8 Air Radiolysis Results and Discussion 7131060

8.2 Air Radiolysis

To determine the baseline yield of nitrate without an oxide present, the glass vials shown in

Figure 3.5 were sealed and irradiated. The results are shown in Figure 8.2:

120.0

100.0 G(NO -)=1.39 ) molec. ) 3 16 80.0

60.0

40.0

20.0 Nitrate production / / (x10 production Nitrate 0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 Dose to air / (x1016) 100 eV 1

Figure 8.2: Effect of γ radiation dose4 on nitrate production from laboratory air Volume of sample = 12.0 cm3 at 35 °C

Initially there is a linear relationship between the dose absorbed by the gas and the

− production of 푁푂3 . This initial production has been plotted on the graph (black dashed line)

− -1 and gives an initial yield, G(푁푂3 ), value of 1.39 ± 0.04 ions 100 eV . As seen in Chapter Two, this yield is within the value quoted in the literature [45, 46]. Above an absorbed dose of

19 − 6x10 eV, the measured production of 푁푂3 drops off from the initial rate of formation and starts to plateau with increasing dose. This decreased production suggests that a steady

− state has been reached for this system, in which the amount of 푁푂3 is 1 µmol.

4 Throughout this Chapter, in all plots of species production as a function of absorbed dose, the absorbed dose axis has been plotted in units of 100 eV so that G-values for the species in question can be determined from the gradient of the trend line of the data.

193

Chapter 8 Air Radiolysis Results and Discussion 7131060

− A plausible explanation for the levelling of the 푁푂3 production is the depletion of a reactant that leads to the formation of nitric acid. This reactant is likely to be water vapour as outlined by Jones [46]. To assess this hypothesis, 100 µl of liquid water was added to the vials and irradiations were performed under the same conditions. Figure 8.3 shows the results:

120.0

100.0

- molec.

G(NO3 ) = 1.17

) ) 80.0 16

(x10 60.0

40.0 Lab. Air Air (H2O added)

20.0 Nitrate yield / yield Nitrate

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 16 Dose to air / (x10 ) 100 eV Figure 8.3: Effect of γ radiation dose on the production of nitrate in water saturated and unsaturated laboratory air. Volume of air = 11.9-12.0 cm3 at 35 °C

− When water is added to the system, the initial rate of 푁푂3 formation seen in Figure 8.2 is

− extended to higher doses and shows no sign of levelling off. The G-value for 푁푂3 formation in the water saturated system is 1.17 ± 0.06 ions 100 eV-1.

If the concentration of water is the factor limiting the production of nitric acid in air radiolysis, it is important to quantify the amount of water vapour present in the initial system. The relative humidity in the laboratory where the samples are prepared is approximately 40% with an average temperature of 25 °C. These conditions give a saturated

194

Chapter 8 Air Radiolysis Results and Discussion 7131060 vapour density of 23 g m-3 of water. Using Equation 8.1, the actual vapour density in the room is calculated as 9.2 g m-3:

푎푐푡푢푎푙 푣푎푝표푢푟 푑푒푛푠푖푡푦 푅. 퐻. = x 100% Equation 8.1 푠푎푡푢푟푎푡푒푑 푣푎푝표푢푟 푑푒푛푠푖푡푦

The glass vials in Figure 3.5 have a crimped volume of 12 cm3. Therefore the quantity of water in the initial vial is 3.69x1018 molecules. Assuming that each water molecule can react with 푁2 and 푂2 to form 2 molecules of nitric acid (퐻푁푂3) (Reaction 8.1), the maximum yield of nitrate theoretically attainable is 12.26 µmol.

2퐻2푂 + 푁2 + 2푂2 → 4퐻푁푂3 Reaction 8.1

This value exceeds the maximum yield achieved in Figure 8.2 (1 µmol) by approximately

90%, suggesting that there is another mechanism occurring that leads to the removal of water vapour from the system. Chapter Seven discussed this topic in more detail with the likely mechanism being water vapour radiolysis, forming 퐻2 and 푂2 gas.

In the subsequent data plots, the water saturated air results will be plotted to represent the system in the absence of an oxide. This is to show the trend of nitrate formation assuming an excess of water vapour was present.

8.3 Air Radiolysis in the Presence of an Oxide Surface

In the following section, results are given for the systems containing an oxide powder.

Dosimetry of this system was detailed in Section 5.3, with a weighting factor applied to

Fricke dosimeter results to correct for the difference in density of the dosimeter solution and air. The volume of gas irradiated is calculated by subtracting the volume of oxide (using

195

Chapter 8 Air Radiolysis Results and Discussion 7131060 the crystal density and the mass) from the total volume. The actual volume of gas inside the vial is used to determine the dose absorbed of the gas.

Figure 8.4 shows the results of the system containing 1 g of an oxide powder. In this system, the majority of the volume is still occupied by the gas phase (~95%) therefore the surface should not have a profound effect on the nitrate yield.

160.0

140.0

) molec. ) 120.0 16 100.0

80.0

60.0 No Oxide 40.0 CeO2

20.0 ZrO2 Nitrate production / (x10 / production Nitrate 0.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 Dose to air / (x1016) 100 eV

Figure 8.4: Nitrate production as a function of dose for systems containing 1 g of either 퐶푒푂2 or 푍푟푂2 powder and water saturated air (no oxide)

The G-values of nitrate formation calculated from the gradient of each data set is given in the following table:

− 5 System G(푵푶ퟑ ) / Error (흈풔풍풐풑풆) molecules 100 eV-1 No oxide 1.17 ± 0.06

1 g 퐶푒푂2 1.96 ± 0.03

1 g 푍푟푂2 2.10 ± 0.04 − Table 8-1: Calculated G(푁푂3 ) values and standard deviation of the gradient for 1 g oxide systems and water saturated air (no oxide)

5 Throughout this Chapter, errors are calculated within one standard deviation.

196

Chapter 8 Air Radiolysis Results and Discussion 7131060

From this table, it is clear that in the presence of an oxide surface, the yield of nitrate formation increases by 60 and 80% with 퐶푒푂2 and 푍푟푂2, respectively. Furthermore, this increased rate of formation remains constant up to absorbed doses of 7x1019 eV. This trend suggests that there is more water in the irradiated system in the presence of an oxide than is present in laboratory air alone. The water is most likely to be adsorbed to the oxide surface.

Table 8-2 highlights the water available in the system, either in the headspace or adsorbed onto the oxide powder. Several assumptions have been made in these calculations. Firstly, only one monolayer of water is adsorbed onto the oxide. The mass of this monolayer has

-2 been calculated as 0.21 mg m for 푃푢푂2 [23], this value should not differ greatly for 퐶푒푂2

-3 or 푍푟푂2. Secondly, the vapour density of the air is 9.2 g m with 40% relative humidity. The surface area of each oxide was determined in Chapter Six.

System Surface Area / Oxide Mass of water Mass of Total mass m2 Volume6 / on oxide / water in of water in cm3 mg headspace / system / µg mg Water - - - 110.40 0.18 Saturated air (0.1 mg of (no oxide) liquid water) 1 g 퐶푒푂2 7.42 0.14 1.56 109.11 1.67

1 g 푍푟푂2 2.24 0.18 0.47 108.74 0.58

Table 8-2: Calculated mass of water in systems containing 1 g 퐶푒푂2, 1 g 푍푟푂2 and water saturated air (no oxide)

Clearly the mass of water adsorbed on the oxide surface far outweighs the mass of water in the gas phase, and it is unlikely that water depletion will occur in experiments on samples containing the oxide powder.

6 -3 Using crystal density values of: 7.215 and 5.68 g cm for 퐶푒푂2and 푍푟푂2, respectively 197

Chapter 8 Air Radiolysis Results and Discussion 7131060

The following paragraph details the results from experiments on a system with 50% oxide

(by volume). As explained in Section 3.4.2 and determined in Chapter Six, the bulk density of each oxide was used to calculate the mass needed to fill the glass vials to approximately half the volume. The mass of oxide required was 6 and 11 g for 퐶푒푂2 and 푍푟푂2, respectively.

Table 8-3 highlights the calculated water available in the system for both oxides.

System Surface Area / Oxide Mass of water Mass of Total mass m2 Volume / on oxide / water in of water in cm3 mg headspace / system / µg mg Water - - - 110.40 0.18 Saturated air (0.1 mg of (no oxide) liquid water) 50% 퐶푒푂2 44.5 0.83 9.35 102.76 9.45 (by volume) (~6 g) 50% 푍푟푂2 24.6 1.94 5.17 92.55 5.26 (by volume) (~11 g) Table 8-3: Calculated mass of water in systems containing 50% 퐶푒푂2 and 50% 푍푟푂2 (by volume) and water saturated air (no oxide)

The results are shown in Figure 8.5 and Table 8-4:

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Chapter 8 Air Radiolysis Results and Discussion 7131060

450.0

400.0

350.0

300.0

) molec. ) 16 250.0

200.0 No Oxide 150.0 CeO2 (~6 g) ZrO2 (~11 g) Nitrate yield / (x10 / yield Nitrate 100.0 ZrO2 (2) (~11 g) 50.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 Dose to air / (x1016) 100 eV

Figure 8.5: Nitrate production as a function of absorbed dose for systems containing 50% oxide (by volume) and for water saturated air (no oxide)

− System G(푵푶ퟑ ) / Error (흈풔풍풐풑풆) molecules 100 eV-1 No oxide 1.17 ± 0.06 50% 퐶푒푂2 (~6 g) 2.96 ± 0.09

50% 푍푟푂2 (~11 g) 3.55 ± 0.11

50% 푍푟푂2 (2) (~11 g) 4.77 ± 1.30 − Table 8-4: Calculated G(푁푂3 ) values and standard deviation of the gradient for experiment with 50% oxide (by volume) and with water saturated air (no oxide)

The most prominent feature of Figure 8.5 is that at low absorbed doses (< 1x1019 eV) there is a discrepancy in the 푍푟푂2 system which leads to some samples (orange data) producing almost double the yield of nitrate. The orange data set represents a separate experiment run in comparison to the green data set. The data represented by the orange markers has a higher degree of scatter and standard deviation. The explanation for this will be discussed in the following section.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

As seen in the 1 g data set, the presence of an oxide surface greatly enhances the formation

− of nitric acid. The yield of 푁푂3 is two and a half times greater with 50% 퐶푒푂2 present and three times greater with 푍푟푂2 present. The yield is greater with 푍푟푂2 present than 퐶푒푂2, as seen with the 1 g data set. This difference is not simply due to an increase in surface area and a possible increase in catalytic sites on the powder, as there is three times more surface area in the 퐶푒푂2 system than in the 푍푟푂2 system.

The oxide masses used in experiments with 90% oxide (by volume) are 12 g of 퐶푒푂2 and 20 g of 푍푟푂2. Table 8-5 highlights the calculated water available in each system.

System Surface Area / Oxide Mass of water Mass of Total mass m2 Volume / on oxide / water in of water in cm3 mg headspace / system / µg mg Water - - - 110.40 0.18 Saturated air (0.1 mg of (no oxide) liquid water) 90% 퐶푒푂2 89.0 1.66 18.7 95.13 18.80 (by volume) (~12 g) 90% 푍푟푂2 44.8 3.52 9.41 78.02 9.49 (by volume) (~20 g) Table 8-5: Calculated mass of water in systems containing 90% 퐶푒푂2 and 90% 푍푟푂2 (by volume) and water saturated air (no oxide)

Figure 8.6 and Table 8-6 show the results from experiments with 90% oxide present (by volume).

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Chapter 8 Air Radiolysis Results and Discussion 7131060

300.0

No Oxide

250.0 CeO2 (~12 g) ZrO2 (~20 g)

200.0

) molec. ) 16

150.0

100.0 Nitrate yield / (x10 / yield Nitrate 50.0

0.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 Dose to air / (x1016) 100 eV

Figure 8.6: Nitrate production as a function of dose for systems containing 90% oxide (by volume) and for water saturated air (no oxide)

− System G(푵푶ퟑ ) / molecules Error (흈풔풍풐풑풆) 100 eV-1 No oxide 1.17 ± 0.06

90% 퐶푒푂2 (~12 g) 4.54 ± 0.72

90% 푍푟푂2 (~20 g) -20.55 ± 11.54

90% 푍푟푂2 (~20 g) 19.78 ± 3.57 manipulated (orange line) − Table 8-6: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 90% oxide (by volume) and for water saturated air (no oxide)

At low absorbed doses, there is a large amount of scatter in the 푍푟푂2 system. This scatter was also apparent in the 50% system (Figure 8.5). The size of the possible error in the yields obtained for the 퐶푒푂2 system has also increased from the 50% system. This increase will be discussed in the next section. The presence of the two 푍푟푂2 data points with yields of nitrate above 1.8x1016 molecules (3 µmol) (highlighted in red circles) complicates the data analysis and leads to apparently negative yield of nitrate calculated. By removing these

− -1 points, the G(푁푂3 ) yield is 19.78 molecules 100 eV and the standard deviation of the slope

201

Chapter 8 Air Radiolysis Results and Discussion 7131060 is 3.57. This slope is highlighted in orange in Figure 8.6. This data manipulation greatly reduces the scatter in the results; however, the G-value calculated is much larger than that measured in the previous 푍푟푂2 data sets. The large increase was not seen between the 1 g and 50% sample systems, therefore this may not be a real effect.

8.3.1 Comparison of 푪풆푶ퟐ Data

Figure 8.7 and Table 8-7 compares γ-radiation results for the 퐶푒푂2 system and displays how nitrate formation is affected by the presence of 퐶푒푂2.

450.0

400.0

350.0 Increasing oxide surface area

300.0

) molec. ) 16 250.0

200.0 No Oxide 150.0 1 g 50% (~6 g)

Nitrate yield / (x10 / yield Nitrate 100.0 90% (~12 g) 50.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 Dose to air / (x1016) 100 eV

Figure 8.7: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume) 퐶푒푂2 systems and for water saturated air (no oxide)

− System G(푵푶ퟑ ) / Error (흈풔풍풐풑풆) Surface Area / molecules 100 eV-1 m2 No oxide 1.17 ± 0.06 N/A

1 g 퐶푒푂2 1.96 ± 0.03 7.42

50% 퐶푒푂2 (~6 g) 2.96 ± 0.09 44.5

90% 퐶푒푂2 (~12 g) 4.54 ± 0.72 89.0 − Table 8-7: Calculated G(푁푂3 ) values for the 퐶푒푂2 containing systems and for water saturated air (no oxide)

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Chapter 8 Air Radiolysis Results and Discussion 7131060

It is clear from this table that the yield of nitrate is affected by the presence of 퐶푒푂2 in the system. The yield rises with increasing quantity of oxide, however, the rate of increase, does not correspond to the increase in volume or surface area. In the systems containing 50% (by volume) 퐶푒푂2, there is a six fold increase in mass and surface area compared to samples containing 1 g 퐶푒푂2, yet the yield of nitrate only increases by 50%; this trend is seen in systems with 50% and 90% (by volume) 퐶푒푂2 where the mass and surface area are doubled but the nitrate yield only increases by 50%. This trend will be discussed later in the chapter.

8.3.2 Discussion

An interesting feature of Figure 8.7 is that none of the trend lines for the yield of nitrate appear to go through zero. At low absorbed doses (< 1x1019 eV), there is an initial off-set between the yield of nitrate and the mass of oxide present. This is shown more clearly in

Figure 8.8:

100.0 No Oxide 90.0 1 g 80.0 50% (~6 g)

70.0 90% (~12 g) ) molec. )

16 60.0

50.0

40.0

30.0

Nitrate yield / (x10 / yield Nitrate 20.0

10.0

0.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 16 Dose to air / (x10 ) 100 eV Figure 8.8: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume) 퐶푒푂2 systems and for water saturated air (no oxide) up to an absorbed dose of 2.0x1019 eV

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Chapter 8 Air Radiolysis Results and Discussion 7131060

Extrapolation of the trend lines to zero absorbed dose generates the following nitrate yields

(Table 8-8):

System Inferred Quantity of Nitrate at zero absorbed dose / µmol No oxide 0.17 ± 0.04

1 g 퐶푒푂2 0.14 ± 0.01

50% 퐶푒푂2 (~6 g) 0.50 ± 0.06

90% 퐶푒푂2 (~12 g) 0.35 ± 0.23 Table 8-8: Initial yield of nitrate pre-irradiation

It is evident from the above data that there appears to be significantly more nitrate present in the higher loading systems than in the 1 g and the water saturated air system. This initial yield could be due to a fast reaction of some description that leads to the oxide surface becoming saturated with nitrate or another species. Once the surface is covered with a layer of nitrate, the rate of the reaction decreases and leads to the yields calculated in Table 8-8.

Another explanation is the presence of a contaminant, either in the oxide powder or the

푁푎푂퐻 solution used in the analysis of samples.

Cerium and zirconium dioxide both have fluorite crystal structures which are built around a face centred cube unit cell (Figure 8.9i) with 퐶푒4+/푍푟4+ located at the octahedrally coordinated lattice points and 푂2− located at the tetragonal interstitials.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

i) ii)

Figure 8.9: i) Face-centred cubic crystal structure unit cell7, and ii) atomic structure of each face in the unit cell [104]

The unit cell contains four atoms with each face having one atom. From the lattice parameter, the area of each surface atom can be determined (Figure 8.9ii). This allows for the determination of the total number of surface atoms in the solid which can act as adsorption sites for gas phase molecules. The lattice parameter of 퐶푒푂2 is 5.4 Å [105] and the lattice parameter of 푍푟푂2 is 5.27 Å [106]. Using this lattice parameter, the cross- sectional area of each atom is 1.14x10-19 m2. Assuming one gas phase molecule can adsorb to each surface atom, the surface coverage of one monolayer will be 8.78x1018 molecules

-2 18 -2 m . This value is reasonable as the corresponding value for 푇𝑖푂2 is 5.2x10 molecules m

[107].

As the surface area of each oxide is known, the number of molecules that can adsorb to the

19 -1 surface and form one monolayer will be approximately 1.61x10 molecules g for 퐶푒푂2 and

18 -1 5.0x10 molecules g for 푍푟푂2. If the gas phase molecule in question requires two surface atoms to adsorb to the surface (bridging mode), the calculated number of adsorbed molecules will be reduced by a factor of two.

7 Tetragonal interstitials not included for clarity

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Chapter 8 Air Radiolysis Results and Discussion 7131060

From Figure 8.8 and Table 8-8, it is clear that the initial yield of nitrate in the 50% (by volume) system is 0.5 µmol. This is equal to 3.0x1017 molecules, which is two orders of magnitude below the calculated value for the number of molecules needed to form one monolayer on 퐶푒푂2, assuming one nitrate molecule can adsorb to one surface atom.

Therefore, allowing for differences in the lattice parameter and the number of surface sites needed for a nitrate molecule to adsorb to, the off-set seen in Figure 8.8 is not due to surface saturation.

An alternative explanation is that a quantity of nitrate is already adsorbed onto the 퐶푒푂2 surface before experiments begin. As explained in Section 3.4.3, the oxide powders are regenerated after analysis by washing with water and baking for several hours. It is possible that these conditions do not fully remove adsorbed nitrate from the surface before further experiments are undertaken. DRIFT spectra of the regenerated oxides were shown in

Chapter Six, the spectra of both 퐶푒푂2 and 푍푟푂2 highlighted the presence of 푁푂푥 moieties on the oxide surface which could potentially oxidise and enhance the yield of nitrate. This will be discussed in more detail in the following section.

8.3.3 Comparison of 풁풓푶ퟐ Data

Figure 8.10 and Table 8-9 highlights the corresponding compiled data for 푍푟푂2.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

140.0

120.0

100.0 ) molec. ) 16 80.0

60.0

No Oxide 40.0 1 g Nitrate yield / (x10 / yield Nitrate 50% (~11 g) 20.0 50% (2) (~11 g) 90% (~20 g)

0.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 16 Dose to air / (x10 ) 100 eV Figure 8.10: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume) 푍푟푂2 systems and for water saturated air (no oxide)

− System G(푵푶ퟑ ) / Error (흈풔풍풐풑풆) Surface Area / molecules 100 eV-1 m2 No oxide 1.17 ± 0.06 N/A

1 g 푍푟푂2 2.10 ± 0.04 2.24

50% 푍푟푂2 (~11 g) 3.55 ± 0.11 24.6 (green data) 50% 푍푟푂2 (2) (~11 g) 4.77 ± 1.30 24.6 (orange data) 90% 푍푟푂2 (~20 g) 19.78 ± 3.57 44.8 (manipulated) − Table 8-9: Calculated G(푁푂3 ) values for the system containing 푍푟푂2 and for water saturated air (no oxide)

As seen with the 퐶푒푂2 data, the production of nitrate increases with increasing mass of

푍푟푂2. For qualitative interpretation, the increase in scatter at higher loadings of oxide is a concern and therefore only limited conclusions can be drawn from the data set given in

Table 8-9.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

8.4 Explanation of Scatter in 50% and 90% (by volume) 풁풓푶ퟐ Results

The concentration of nitrate is determined by the integrated peak area of the ion chromatogram signal relative to the calibration plot. Ascertaining the true peak area for the nitrate signal for the samples containing 50 and 90% (by volume) of 푍푟푂2 proved challenging. Figure 8.11 shows typical chromatograms measured with post irradiated analysis of samples containing various loadings of 푍푟푂2:

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Chapter 8 Air Radiolysis Results and Discussion 7131060

70.0

60.0 1 g

Nitrate 50.0

40.0

30.0

Conductivity / µS / Conductivity 20.0

10.0

0.0 3 4 5 6 7 8 9 i) Retention time / min 13.0

11.0 50% Nitrate 9.0

7.0

5.0

Conductivity / µS / Conductivity 3.0

1.0

-1.0 3 4 5 6 7 8 9 ii) Retention time / min 28.0

23.0 90% Nitrate

18.0

13.0

8.0 Conductivity / µS / Conductivity 3.0

-2.0 3 4 5 6 7 8 9 iii) Retention time / min Figure 8.11: Three ion chromatograms of samples containing: i) 1 g, ii) 50% and iii) 90% 푍푟푂2 (by volume) illustrating the emergence of a second signal at 6.6 min

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Chapter 8 Air Radiolysis Results and Discussion 7131060

In the chromatograms shown in Figure 8.11, the signal representing nitrate has been labelled. It has a retention time of approximately 7 min. The four signals present in the chromatograms that have retention less than 6 min have been assigned and tabulated in

Table 8-10.

Anion Retention Time / min

2− Carbonate (퐶푂3 ) 3.55

Chloride (퐶푙−) 4.45

− Bicarbonate (퐻퐶푂3 ) 5.10

2− Sulphate (푆푂4 ) 5.95

Table 8-10: Anions and corresponding retention times (in minutes) present in deionised water

These anions are present in the deionised water used to prepare the 5 mM 푁푎푂퐻 solution.

Due to the basic nature of the solution, carbon dioxide (퐶푂2) from the atmosphere dissolves in the solution and hydrolyses to form both carbonate and bicarbonate anions. These two anions are indistinguishable using the current instrument configuration, therefore the signals with retention times of 3.55 and 5.10 min can be labelled as carbonate or bicarbonate. The concentration of these anions does not increase with absorbed dose in the system, therefore they are not a product of radiolysis. The chloride signal does increase, however, with increasing mass of oxide. Therefore it is likely that chloride is present as a contaminant counter ion in the oxide lattice and will not significantly affect the chemistry of the system.

In the chromatogram shown in Figure 8.11i, the signal produced by nitrate has an asymmetric non-Gaussian profile. The asymmetry could be due to the response of the

210

Chapter 8 Air Radiolysis Results and Discussion 7131060 detector or interference from another signal. Figure 8.11ii shows the emergence of a second signal at 6.6 min. The profile of the nitrate signal is now more symmetrical, however, the second signal appears as a ‘shoulder’ on the nitrate peak. In the final chromatogram

(Figure 8.11iii), the second signal has a much larger concentration and has started to split from the nitrate signal, however, the two signals are not fully resolved and still overlap. The close proximity of the two peaks makes integration of a single peak difficult and assumptions of true peak profiles will lead to errors in the subsequent nitrate yield calculation. Both of the signals discussed here, with retention between 6.5 and 7 min do not appear in unirradiated blanks, therefore both anions are formed from radiolytic processes.

The second 푍푟푂2 data set plotted in Figure 8.5 (orange data points) all had chromatogram profiles similar to that shown in Figure 8.11ii. The integrated peak area used in the subsequent nitrate calculation was the sum of the nitrate signal and the shoulder. The instrument software is unable to differentiate the presence of two signals. The consequence is an increased area which leads to overestimation of the nitrate concentration in the solution and the radiolytic yield of nitrate formation.

It is of importance to separate and identify this second signal to ensure that a true nitrate

3− yield can be calculated. Several inorganic anions such as phosphate (푃푂4 ) and simple

− organic anions based around carbon, nitrogen and oxygen moieties including nitrite (푁푂2 ) and cyanate (퐶푁푂−) have been analysed to determine the identity of this second signal.

2− After intensive investigation, the second signal was identified as oxalate (퐶2푂4 ). Figure

8.12 shows the ion chromatogram peak profile of 50 μM oxalic acid (퐻2퐶2푂4), 0.1 mM sodium nitrate and a mixed solution of both compounds.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

30.0 0.1 mM NaNO3

25.0 0.05 mM H2C2O4

0.1 mM NaNO3 / 0.05 mM H2C2O4

20.0

15.0

Conductivity / µS / Conductivity 10.0

5.0

0.0 5.5 5.7 5.9 6.1 6.3 6.5 6.7 6.9 7.1 7.3 7.5 Retention time / min Figure 8.12: Ion chromatogram of 50 μM oxalic acid, 0.1 mM sodium nitrate and a mixed solution of both

The signal profile of the sample containing both oxalate and nitrate anions is identical to the signal profile seen in Figure 8.11iii. It is clear from this figure that, with the current instrument configuration, the retention of oxalate and nitrate are very similar and lead to interference between the two signals.

To separate these anions, several of the instrument conditions were changed, including flow rate, column temperature and eluent concentration. Altering the flow rate and column temperature led to longer elution times, however, the peaks were still poorly separated.

Lowering the eluent concentration was found to have the desired effect of separating the oxalate and nitrate peaks whilst increasing the elution time of both. Figure 8.13 illustrates the chromatogram of the mixed solution containing both compounds seen in Figure 8.12, however, the 퐾푂퐻 eluent concentration has been lowered from 23 mM to 14 mM.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

25.0 0.1 mM NaNO3 / 0.05 mM H2C2O4

20.0 Nitrate

Oxalate

15.0

10.0 Conductivity / / µS Conductivity Sulphate

5.0

0.0 6 7 8 9 10 11 12 13 14 Retention Time / min

Figure 8.13: Chromatogram of 0.1 mM 푁푎푁푂3 and 50 μM oxalic acid mixed solution using eluent concentration of 14 mM 퐾푂퐻

The nitrate anion now elutes at approximately 8.9 min and oxalate elutes at 11.5 min. If the eluent concentration is decreased below 14 mM then the signal from sulphate starts to interfere with nitrate. The signal from sulphate can be seen in Figure 8.13 at a retention time of 7.95 min.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

8.5 Refinement of Experimental Data in the Presence of an Oxide Surface

The following section details new nitrate production data that has been collected after the identification of the oxalate interference had been resolved.

Figure 8.14 and Table 8-11 detail the refined data for the system containing 1 g of oxide:

300.0

250.0

molec.

) ) 16

200.0 (x10

150.0

100.0 No Oxide CeO2 ZrO2

50.0 Nitrate production / production Nitrate 0.0 0.0 50.0 100.0 150.0 200.0 16 Dose to air / (x10 ) 100 eV Figure 8.14: Nitrate production as a function of dose for samples containing 1 g of oxide powder and for water saturated air (no oxide)

− System G(푵푶ퟑ ) / Error (흈풔풍풐풑풆) molecules 100 eV-1 No oxide 1.17 ± 0.06

1 g 퐶푒푂2 2.07 ± 0.07 1 g 푍푟푂2 1.48 ± 0.04 − Table 8-11: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 1 g oxide and water saturated air (no oxide)

Figure 8.14 illustrates that the presence of the oxide still has an effect on the formation of nitrate in the radiolysis of air. The yield of nitrate in the system containing 퐶푒푂2 is almost double the yield from radiolysis of air alone, whilst the yield in the 푍푟푂2 system is 25% greater than in humid air.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

Figure 8.15 and Table 8-12 outline the results of nitrate formation yields for the systems containing 50% oxide (by volume). The masses and surface areas are the same as outlined in

Table 8-3.

250.0

No Oxide

molec. 200.0

) ) CeO2 16 ZrO2

(x10 150.0

100.0

50.0

Nitrate production / production Nitrate 0.0 0.0 20.0 40.0 60.0 80.0 100.0 16 Dose to air / (x10 ) 100 eV Figure 8.15: Nitrate production as a function of dose for samples containing 50% (by volume) 퐶푒푂2 and 푍푟푂2 and from water saturated air (no oxide)

− System G(푵푶ퟑ ) / Error (흈풔풍풐풑풆) molecules 100 eV-1 No oxide 1.17 ± 0.06

50% 퐶푒푂2 (~6 g) 2.48 ± 0.02

50% 푍푟푂2 (~11 g) 2.13 ± 0.11 − Table 8-12: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 50% oxide (by volume) and water saturated air (no oxide)

It is clear from Table 8-12 that the yields of nitrate in the presence of an oxide surface are greater than when no oxide is present. Now that the interference from oxalate has been removed, the nitrate yields are considerably reduced from the original apparent values shown in Table 8-4. This reduction is greater in 푍푟푂2 systems, with the yield decreasing by

40% compared to less than 20% for samples containing 퐶푒푂2 from the original value,

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Chapter 8 Air Radiolysis Results and Discussion 7131060

suggesting that the interference from oxalate was far greater in 푍푟푂2 containing samples than in 퐶푒푂2 samples.

Figure 8.16 and Table 8-13 are the new refined data sets for systems containing 90% (by volume) of oxide. The respective masses and surface areas are identical to values outlined in

Table 8-5.

400.0

350.0

molec. 300.0

) ) 16

250.0 (x10

200.0 No Oxide CeO2 150.0 ZrO2 100.0

Nitrate production / production Nitrate 50.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 16 Dose to air / (x10 ) 100 eV Figure 8.16: Nitrate production as a function of dose for samples containing 90% (by volume) 퐶푒푂2 and 푍푟푂2 and from water saturated air (no oxide)

− System G(푵푶ퟑ ) / Error (흈풔풍풐풑풆) molecules 100 eV-1 No oxide 1.17 ± 0.06

90% 퐶푒푂2 (~12 g) 3.19 ± 0.09

90% 푍푟푂2 (~20 g) 6.50 ± 0.19 − Table 8-13: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 90% oxide (by volume) and water saturated air (no oxide)

When the oxide volume is significantly larger than the volume of the gas phase, the yield of

− nitrate produced is vastly increased. The G(푁푂3 ) values calculated are greater than the

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Chapter 8 Air Radiolysis Results and Discussion 7131060

values calculated for the 50% oxide (by volume) system. In the 90% 푍푟푂2 system, the nitrate yield is six times greater than that measured for air radiolysis and is greater than the yield calculated for the corresponding 퐶푒푂2 system.

8.5.1 Compiled Data

To illustrate the yield of nitrate produced as a function of absorbed dose for the differing oxide quantities present in the samples, Figure 8.17 is the compiled data set for samples

− containing 퐶푒푂2 and water saturated air. Table 8-14 lists the calculated G(푁푂3 ) yields and the standard deviation of each set of data points.

400.0 Increasing oxide 350.0 No Oxide surface area 1 g 300.0 50% (~6 g)

90% (~12 g) molec.

) ) 250.0 16

(x10 200.0

150.0

Nitrate yield / yield Nitrate 100.0

50.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 16 Dose to Air / (x10 ) 100 eV Figure 8.17: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume) 퐶푒푂2 systems and for water saturated air (no oxide)

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Chapter 8 Air Radiolysis Results and Discussion 7131060

− System G(푵푶ퟑ ) / Error Surface Area / -1 2 molecules 100 eV (흈풔풍풐풑풆) m No oxide 1.17 ± 0.06 N/A

1 g 퐶푒푂2 2.07 ± 0.07 7.42

50% 퐶푒푂2 (~6 g) 2.48 ± 0.02 44.5

90% 퐶푒푂2 (~12 g) 3.19 ± 0.09 89.0 − Table 8-14: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 1 g, 50% and 90% (by volume) 퐶푒푂2 and water saturated air (no oxide)

− It is clear from Figure 8.17 that the initial ‘off-step’ in 푁푂3 concentration seen in Figure 8.8 has now been removed as a result of the oxalate interference being separated. It is also

− evident that the presence of an oxide surface increases the yield of 푁푂3 . The largest difference is seen with 1 g 퐶푒푂2 in comparison with saturated air results where the yield of

− 푁푂3 has almost doubled with the oxide present. This increase in nitrate production is due to the presence of the oxide, however, it is not simply a surface area effect as this increase

− in 푁푂3 yield is not linear with the higher loading of oxide data sets.

Figure 8.18 illustrates the compiled data for samples containing 푍푟푂2 and water saturated

− air. The calculated G(푁푂3 ) yields are detailed in Table 8-15.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

300.0

250.0 Increasing oxide

surface area

200.0

molec.

) ) 16

(x10 150.0

No Oxide 100.0 1 g

Nitrate yield / yield Nitrate 50% (~11 g) 90% (~20 g) 50.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 16 Dose to Air / (x10 ) 100 eV Figure 8.18: Nitrate production as a function of absorbed dose for 1 g, 50% and 90% (by volume) 푍푟푂2 systems and water saturated air (no oxide)

− System G(푵푶ퟑ ) / Error (흈풔풍풐풑풆) molecules 100 eV-1 No oxide 1.17 ± 0.06

1 g 푍푟푂2 1.48 ± 0.04

50% 푍푟푂2 (~11 g) 2.13 ± 0.11

90% 푍푟푂2 (~20 g) 6.50 ± 0.19 − Table 8-15: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems with 1 g, 50% and 90% (by volume) 푍푟푂2 and water saturated air (no oxide)

It is clear from Figure 8.18 and Table 8-15 that the scatter seen in previous data (Figure

8.10) has now been removed. It is also evident that the presence of 푍푟푂2 (especially 90%

− (by volume)) has a large effect on the yield of 푁푂3 produced. This increase is not linear with mass or surface area as there is an order of magnitude difference in the mass of 푍푟푂2 in 1 g

− and 50% (by volume) samples, however, the yield of 푁푂3 increases by 50%. Samples that

219

Chapter 8 Air Radiolysis Results and Discussion 7131060

− contain 1 g and 50% (by volume) of 푍푟푂2 have a lower yield of 푁푂3 than the corresponding

퐶푒푂2 samples.

8.5.2 Discussion

Section 2.7 outlined the mechanism for radiolysis of dry air and water saturated air. These systems are well understood, with the stable products of dry air radiolysis being ozone (푂3), nitrous oxide (푁2푂) and nitrogen dioxide (푁푂2) [108]. In the presence of water, the major stable product is nitric acid (퐻푁푂3). Nitric acid is formed from 푁푂2 reacting with water radiolysis products such as hydroxyl radicals (Reaction 8.2) and oxygen atoms (Reaction 8.3) as well as with water itself (Reaction 8.4) [109, 110].

. 푁푂2 + 퐻푂 → 퐻푁푂3 Reaction 8.2

. 푁푂2 + 푂 + 푀 → 푁푂3 + 푀 Reaction 8.3

2푁푂2 + 퐻2푂 → 퐻푁푂2 + 퐻푁푂3 Reaction 8.4

In Reaction 8.4, nitrous acid (퐻푁푂2) is also produced which will be discussed later. The nitrate radical formed in Reaction 8.3 can react with 푁푂2 to form dinitrogen pentoxide

(푁2푂5) (Reaction 8.5) [111] which can further react to form nitric acid (Reaction 8.6):

. 푁푂3 + 푁푂2 → 푁2푂5 Reaction 8.5

푁2푂5 + 퐻2푂 → 2퐻푁푂3 Reaction 8.6

The mechanism outlined above leads to the nitrate yield of 1.17 molecules 100 eV-1 of absorbed dose seen in the saturated air samples. In the presence of an oxide powder

(independent of quantity) this yield is much higher and is closer to the limiting yield of G(푁)

[42] which has a value of approximately 6 atoms 100 eV-1. The lower yield in saturated

220

Chapter 8 Air Radiolysis Results and Discussion 7131060

laboratory air radiolysis is due to the fast reaction of nitrogen atoms with 푁푂2 and 푁푂

(Reactions 8.7 and 8.8), which lead to the reformation of 푁2:

푁 + 푁푂2 → 2푁푂 Reaction 8.7

푁 + 푁푂 → 푂 + 푁2 Reaction 8.8

These reactions have rate constants of 5.9x10-12 cm3 molecule-1 s-1 and 2.2x10-11 cm3 molecule-1 s-1 respectively.

When an oxide is present, Reaction 8.7 is inhibited, allowing higher yields of nitrate to be produced. The most plausible explanation is that 푁푂2 is adsorbed to the oxide surface and undergoes reactions including Reactions 8.2-8.4 to produce nitric acid.

The oxidation of 푁푂2 on an oxide surface has been investigated by Rodriguez et al. [112] who investigated the reaction of 푁푂2 with stoichiometric 퐶푒푂2 and with partially reduced ceria (퐶푒푂2−푥 ) from an environmental catalysis perspective. At room temperature, adsorbed nitrate was the only major product found on pure 퐶푒푂2 with a mixture of 푁, 푁푂 and 푁푂3 co-existing on the surface of partially reduced ceria.

The phenomenon is also known to occur with other oxide materials. Nanayakkara et al. have studied the reactivity of several atmospheric gases with 푇𝑖푂2 and found that 96% of 푁푂2 reacts with surface hydroxyl groups to form bridged, mono and bidentate nitrate [113]. The overall reaction is given in Reaction 8.9 and the bridging modes are illustrated in Figure

8.19. This is confirmed in other work by Hadjiivanov et al. [114].

− − 3푁푂2 (푔) + 2푂퐻 → 2푁푂3 (푎푑푠) + 푁푂(푔) + 퐻2푂(푎푑푠) Reaction 8.9

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Chapter 8 Air Radiolysis Results and Discussion 7131060

− Figure 8.19: Pictorial representation of 푁푂3 bonding modes with metal centres depicting (l-r) monodentate, bidentate and bridging adsorption modes

− Haubrich et al. have proposed an alternative mechanism for 푁푂3 formation on a rutile

(푇𝑖푂2) surface, which involves the disproportionation of adsorbed 푁푂2 (Reaction 8.10)

[115]:

− 2푁푂2 (푎푑푠) → 푁푂3 (푎푑푠) + 푁푂(푔푎푠) Reaction 8.10

The 푁푂 species will initially be adsorbed to the surface but will quickly desorb to the gas phase due to the weak adsorption [116].

− Chromia (퐶푟2푂3) is another oxide where 푁푂2 adsorption leads to the formation of 푁푂3 on the surface [117].

All of these systems mentioned above used ultra-high vacuum (UHV) conditions and studied the adsorption and oxidation of 푁푂2 using infra-red spectroscopy. Radiation, in any form was not utilised as part of the research.

Harteck and Dondes studied the decomposition of 푁푂 and 푁푂2 using fission fragments from uranium-235 [43]. They postulated that 푁2, 푁2푂 and 푂2 were the stable species, however, analysis was carried out by pressure measurements alone. Nitrogen dioxide decomposition was an order of magnitude slower than 푁푂 decomposition and they did not see any surface effects. The main reason for this negative result is the small quantity of oxide powder utilised in the research (5 mg).

222

Chapter 8 Air Radiolysis Results and Discussion 7131060

− Another possible explanation for the increased yields of 푁푂3 in the presence of an oxide powder is that of energy transfer from the oxide to the gas phase. This phenomenon was discussed in detail in Section 2.3 with respect to water radiolysis on an oxide surface. One explanation for the enhancement of 퐻2 yield from adsorbed water radiolysis was due to the similarities in the band gap of certain metal oxides and the bond dissociation energy of water (5.15 eV) [25]. The band gaps of 푍푟푂2 and 퐶푒푂2 are 5.0 eV [76] and 3.1-3.5 eV

[77, 118], respectively. The ionisation potentials of 푁2 and 푂2 are 15.58 and 12.07 eV, respectively [85]. The bond dissociation energies of 푁 ≡ 푁 and 푂 = 푂 are 9.79 and 5.15 eV, respectively [27]. Therefore there is not enough ‘excitation energy’ available to be transferred from either metal oxide utilised in this research to directly ionise or dissociate

푁2. It is also unlikely that 푁2 will adsorb to the oxide surface.

From the explanation outlined above, initial ionisation of 푁2 and 푂2 occurs in the gas phase.

The surface will only have an effect on subsequently formed species, which will be molecular, radical or ionic in character.

− In all of the experiments investigated in this chapter, nitrite (푁푂2 ) is not present in any of the post-irradiated chromatograms. Reaction 8.4 outlined the formation of nitrous acid

(퐻푁푂2) in the reaction mechanism, however, this isn’t present in the final system. Work by

Saliba et al. have found that adsorbed nitrous acid undergoes decomposition on silica surfaces [119], as shown in Reaction 8.11:

2퐻푁푂2 (푎푑푠) → 푁푂 + 푁푂2 + 퐻2푂(푎푑푠) Reaction 8.11

Nitrite can also be oxidised on the surface by 푂퐻. to form nitric acid.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

8.6 Oxalate

2− It is clear from Section 8.4 that oxalate (퐶2푂4 ) is present in the post irradiated samples that contain 푍푟푂2. It is important to quantify the yield of oxalate and to determine whether it is a contaminant or a radiolytically produced species. Now that the oxalate signal has been identified and separated from the nitrate signal, it is possible to detect the yield of both anions in parallel for each sample. It is necessary to calibrate the response of the ion chromatograph detector to oxalate in order to determine an accurate yield. To achieve this, oxalic acid (퐻2퐶2푂4) was used as a source of oxalate anions and several solutions over a range of concentration similar to the concentrations used in calibrating the ion chromatograph with nitrate (Section 8.1) were utilised. The resulting calibration plot is shown in Figure 8.20:

70.0

60.0

50.0

40.0 y = 61.886x R² = 0.9986 30.0

Peakarea / A.U. 20.0

10.0

0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 [H2C2O4] / mM 2− Figure 8.20: Calibration plot of 퐶2푂4 peak area as a function of 퐻2퐶2푂4 concentration using ion chromatography

From this figure, it can be seen that the detector has a linear response to oxalate concentration in the range of 0.05-1 mM. In comparison with Figure 8.1, the sensitivity for both anions is very similar, with almost identical peak areas for the same concentration of anions.

224

Chapter 8 Air Radiolysis Results and Discussion 7131060

Figure 8.21 is the compiled data set for oxalate yields in the system containing 퐶푒푂2:

10.0

9.0

8.0 No Oxide 1 g 7.0 50% (~6 g)

90% (~12 g)

molec. ) )

16 6.0

(x10 5.0

4.0

3.0 Oxalate yield / yield Oxalate 2.0

1.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 16 Dose to Air / (x10 ) 100 eV Figure 8.21: Plot of oxalate production as a function of absorbed dose for samples containing 1 g, 50% and 90% (by volume) of 퐶푒푂2 and from water saturated air (no oxide)

From this plot, there appears to be a lot of scatter in the sample sets containing 50% and

90% 퐶푒푂2 when compared to samples of water saturated air and 1 g 퐶푒푂2. However, the maximum yield of oxalate detected is 66.4 nmol in the system containing 90% 퐶푒푂2; this yield is an order of magnitude below the yield of nitrate for the corresponding system.

Therefore the scatter in Figure 8.21 is an effect of the detection limits of the ion chromatograph. Samples without 퐶푒푂2 appear to have the least scatter. The G-value for this

2− -1 data set has been calculated as G(퐶2푂4 ) = 0.03 molecules 100 eV .

Figure 8.22 shows the calculated oxalate yields in samples containing 푍푟푂2 and pure air:

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Chapter 8 Air Radiolysis Results and Discussion 7131060

30.0

No Oxide 25.0 1 g

50% (~11 g)

90% (~20 g)

20.0

molec.

) ) 16

(x10 15.0

10.0 Oxalate yield / yield Oxalate

5.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 16 Dose to Air / (x10 ) 100 eV Figure 8.22: Plot of oxalate production as a function of absorbed dose for samples containing 1 g, 50% and 90% (by volume) of 푍푟푂2 and from water saturated air (no oxide)

It is clear from comparing Figures 8.21 and 8.22 that the yield of oxalate is much higher in samples containing 푍푟푂2 than in the corresponding 퐶푒푂2 samples. The yield is up to five times greater in 90% 푍푟푂2 (0.41 µmol) than in 90% 퐶푒푂2 (66.4 nmol). This large yield of oxalate led to the large scatter seen in 푍푟푂2 samples at low doses (Figure 8.10).

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Chapter 8 Air Radiolysis Results and Discussion 7131060

8.6.1 Oxalate Discussion

It is of importance to understand the source of the oxalate, whether it is an impurity from the oxide or from the gas phase. Once the source has been identified, the mechanism of its formation can be discussed.

Both 푍푟푂2 and 퐶푒푂2 are produced commercially by calcination of zirconium and cerium containing compounds. One such compound is the oxalate. This is also true of 푃푢푂2, as outlined in Chapter One. To determine whether oxalate is present in the starting oxide,

퐼퐼퐼 thermogravimetric analysis of cerium oxalate (퐶푒2 (퐶2푂4)3. 푥퐻2푂) was investigated to determine the thermal stability of the oxalate group. Figure 8.23 shows the thermograms of

퐼퐼퐼 퐶푒2 (퐶2푂4)3. 푥퐻2푂 decomposed under both air and 푁2:

100.0 95.0 Step 1 90.0 N2 85.0 Step 2

80.0 Air

75.0

70.0 Mass / % / Mass 65.0 Step 3 60.0 55.0 50.0 45.0 0.0 100.0 200.0 300.0 400.0 500.0 600.0 o Temperature / C

Figure 8.23: Thermogravimetric analysis of cerium oxalate under 푁2 (blue) and static air (red) atmospheres. Heating rate 2 °C min-1

This figure highlights that there are three distinct mass changing events as the oxalate is heated. The first step occurs between 35-130 °C, with this initial step being due to the water of crystallisation being driven off. This step occurs at the same temperature, independent of

227

Chapter 8 Air Radiolysis Results and Discussion 7131060 flow gas, thus showing that this dehydration step is thermally driven rather than reaction with the gas. This mass difference is approximately 15% therefore the number of water molecules bound to the compound can be calculated as 5.4. The second step occurs between 130-180 °C in both atmospheres and represents a mass change of approximately

3%. Due to the nature of the mass change, this step is assumed to be further dehydration of the solid with loss of strongly bound water. The final mass change event is dependent on

° ° flow gas used. In an air flow, this step occurs at 220 C and at 350 C under an 푁2 flow. This step corresponds to the decomposition of the oxalate group.

During the decomposition of cerium oxalate into cerium oxide, cerium is oxidised from 퐶푒퐼퐼퐼 to 퐶푒퐼푉; in an inert atmosphere there is nothing to oxidise the oxalate until it becomes unstable and undergoes self-decomposition, whereas in oxidising atmospheres such as 푂2 or air, this oxidation occurs much quicker. This can be seen by the difference in the temperature onset of the final step. The oxygen in the air starts to decompose the oxalate

° 130 C below the temperature at which this starts to occur under 푁2. The steps outlined above (dehydration followed by decomposition) have been investigated and confirmed by other groups [120, 121]. The decomposition of cerium oxalate occurs in two distinct steps in an air atmosphere, independent of heating rate employed [121, 122] suggesting that this step is thermodynamically controlled. However, in a recent paper, De Almeida et al. discovered under an argon flow, that an intermediate decomposition step occurred which led to an oxy-carbonate intermediate with the chemical formula 퐶푒2푂2퐶푂3 [120]. This step is not present however, in Figure 8.23 or at least not resolved.

After the heating program was completed, the sample decomposed in an air atmosphere had 50% of the initial mass. Under 푁2, this value was 55.4%. The difference in final mass is

228

Chapter 8 Air Radiolysis Results and Discussion 7131060 due to the incomplete decomposition of the oxalate group and residual carbon in the sample decomposed under 푁2.

In Section 3.4.3, the regeneration of the oxide powders between subsequent experiments was outlined. The powders are baked at 400 °C for six hours under a static air atmosphere.

From Figure 8.23 it is clear that this temperature is sufficient to ensure complete decomposition of any oxalate impurity. Therefore the source of the oxalate discovered in the ion chromatograms is not an impurity from the solid.

The only other source of carbon during the experiments is carbon dioxide (퐶푂2) in the air.

The DRIFT spectra in Chapter Six of the regenerated oxides (Figures 6.8 and 6.18) identified the presence of 퐶푂2 compounds adsorbed onto the surfaces of both oxide materials. These figures only gave qualitative information, therefore the concentrations of 퐶푂2 present in the pre-irradiated samples is unknown, however, a maximum concentration can be calculated in the gas phase. The concentration of 퐶푂2 in the atmosphere is approximately 400 ppm. In the 12 ml sample vials used in this experiment, that is equal to 0.11 mmol of 퐶푂2. Assuming that two 퐶푂2 molecules will dimerise to form one oxalate molecule, the maximum theoretical yield of oxalate is 54.5 µmol in pure air. Samples that contain 90% (by volume) of oxide could generate a maximum yield of 47.0 µmol for 퐶푒푂2 containing samples and 10.8

µmol for 푍푟푂2 containing samples. These maximum yields are far greater than the measured yields of oxalate in Figures 8.21 and 8.22. The efficiency of the reaction assuming simple dimerisation is 0.14% in the presence of 퐶푒푂2 and 3.84% in the presence of 푍푟푂2.

These figures are based on the gas phase concentration of 퐶푂2 and do not include any 퐶푂2 adsorbed onto the oxide surface.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

Carbon dioxide is very radiation stable with less than 0.1% undergoing decomposition in gas phase studies alone. Initially, it was postulated by Hirschfelder and Taylor that this was due to the back reaction with ozone [123] (Reaction 8.12)

퐶푂 + 푂3 → 퐶푂2 + 푂2 Reaction 8.12

This reaction is exothermic, however, it has a rate coefficient of k ≤ 4x10-25 cm3 molecule-1 sec-1 at ambient temperature [124], therefore would be very slow. Work by Harteck and

Dondes [125] outlined the following reaction mechanism for 퐶푂2 radiolysis:

Initial ionisation 퐶푂2 ⇝ 퐶푂 + 푂 Reaction 8.13

⇝ 퐶 + 2푂 Reaction 8.14

Reaction 8.13 has a G-value of 8 molecule 100 eV-1 and is the more favoured primary reaction of 퐶푂2 irradiation. The reaction continues thus:

퐶푂 + 퐶 + 푀 → 퐶2푂 + 푀 Reaction 8.15

퐶2푂 + 퐶푂 + 푀 → 퐶3푂2 + 푀 Reaction 8.16

푂 + 푂2 + 푀 → 푂3 + 푀 Reaction 8.17

퐶3푂2 + 푂 (or 푂3) → 퐶2푂 + 퐶푂2(+푂2) Reaction 8.18

퐶푂 + 푂 + 푀 → 퐶푂2 + 푀 Reaction 8.19

The 푂2 in Reaction 8.17 is formed by the recombination of oxygen atoms from Reaction

8.14. Dicarbon monoxide (퐶2푂), acts as a catalyst in Reaction 8.19 before eventually diffusing to the walls with carbon suboxide (퐶3푂2), and forming polymerised products.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

As 퐶푂2 is the only source of carbon in the experimental configuration, another mechanism must be occurring to hinder the reformation of 퐶푂2, as the yields of oxalate are greater than from 0.1% 퐶푂2 decomposition.

An explanation is given by Harteck and Dondes [125] where they found that a small quantity of 푁푂2 prevented the reformation of 퐶푂2 by reaction with radicals. As outlined in Section

8.5.1, 푁푂2 will be present in the system due to radiolysis of 푁2 and 푂2. The reactions of 푁푂2 in the system are shown below:

푂 + 푁푂2 → 푁푂 + 푂2 Reaction 8.20

퐶 + 푁푂2 → 푁푂 + 퐶푂 Reaction 8.21

2푁푂 + 푂2 → 2푁푂2 Reaction 8.22

Reaction with oxygen atoms (8.20) inhibits both Reactions 8.18 and 8.19, both of which lead to the reformation of 퐶푂2, and reaction with carbon atoms (8.21) leads to a lower yield of

퐶2푂 which is the catalyst in Reaction 8.19. With 푁푂2 present, G(−퐶푂2) = 8 molecules

100 eV-1.

The mechanism outlined above will be occurring in the saturated air experiments. The G-

2− -1 value for oxalate formation in this system is G(퐶2푂4 ) = 0.03 molecules 100 eV based on dose absorbed by the air. It is clear from Figures 8.21 and 8.22 that the yield of oxalate in samples with an oxide present are much greater, therefore a mechanism involving the surface that inhibits the reformation of 퐶푂2 must be occurring. It is postulated that 퐶푂2 adsorbs onto the oxide prior to irradiation and then is reduced on the oxide surface during

− irradiation forming 퐶푂2 . If there are two reduced molecules in close proximity, these will undergo dimerisation to form the oxalate (Reaction 8.23).

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Chapter 8 Air Radiolysis Results and Discussion 7131060

− 2− 2퐶푂2 + 푀 → 퐶2푂4 + 푀 Reaction 8.23

Carbon dioxide adsorption onto metal oxide surfaces has been investigated by

Baltrusaitis et al. for several nanoparticle surfaces [68]. They studied 퐶푂2 adsorption to iron

(III) oxide (퐹푒2푂3), aluminium oxide (퐴푙2푂3) and titanium dioxide (푇𝑖푂2) in the presence and absence of water vapour. They found that 퐶푂2 readily reacts with surface hydroxyls to form

− bicarbonate species (퐻퐶푂3 ). Carbon dioxide will also react to form monodentate and

2− − bidentate carbonate species (퐶푂3 ). Carboxylate (퐶푂2 ) is also present on the surface of

푇𝑖푂2. All of these species were formed under dry conditions. When co-adsorbed water was present, they found that the carbonate species became solvated and the surface hydroxyl groups became protonated. This occurs when no radiation is present, therefore in the presence of ionising radiation, reduction of 퐶푂2 is likely to be increased, leading to a higher

− concentration of 퐶푂2 on the oxide surface and therefore a higher concentration of oxalate formation.

8.7 Synthetic Air

In an effort to remove 퐶푂2 from the system, a series of samples were out-gassed with synthetic air. Synthetic air is produced by mixing 20% 푂2 with 80% 푁2, the impurities are certified as < 1 vppm 퐶푂2, < 2 vppm 퐻2푂 and < 1 vppm 푁푂푥. This was achieved by inserting a needle through the septa of the vial and flowing gas through the sample for several minutes. Figure 8.24 highlights the nitrate yield results of pure synthetic air radiolysis compared to laboratory air and water saturated lab air:

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Chapter 8 Air Radiolysis Results and Discussion 7131060

120.00

100.00

molec.

) 80.00 16

(x10 60.00

40.00 Lab. Air Air (H2O added) Synthetic air 20.00

Nitrate production / production Nitrate 0.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 16 Dose to air / (x10 ) 100 eV Figure 8.24: Plot of nitrate production as a function of absorbed dose for synthetic air, laboratory air and water saturated laboratory air. Volume of air =11.9 - 12.0 cm3 at 35 °C

In theory, there should be no nitrate produced in the radiolysis of synthetic air, because as previously discussed in Section 2.7, radiolysis of dry air, results in the steady state concentrations of ozone (푂3), nitrous oxide (푁2푂) and nitrogen dioxide (푁푂2) [108]. Figure

8.24 clearly shows that nitrate is produced and in the same yields as normal lab air, therefore it can be hypothesised that water is present in the system, either as water vapour that hasn’t been fully removed or as silanol groups on the glass surface of the vials.

As seen in previous results, there is a large quantity of water present in the systems containing an oxide powder as adsorbed water. Therefore, the yield of nitrate did not change when samples containing an oxide were irradiated in a synthetic air atmosphere.

The oxalate yield in samples containing 퐶푒푂2 that were irradiated in synthetic air did not differ from the yields seen in Figure 8.21. This yield was very small in laboratory air, therefore it is likely that a small quantity of 퐶푂2 remains adsorbed to the oxide surface even after out-gassing with synthetic air, which can be irradiated to form the oxalate species. It is

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Chapter 8 Air Radiolysis Results and Discussion 7131060 likely that the out gassing method used was not adequate enough to displace adsorbed species on the oxide.

Samples containing 푍푟푂2 that were irradiated in synthetic air had a markedly reduced yield of oxalate after subsequent analysis. The yields were similar to comparative samples of

퐶푒푂2. The explanation of this is likely to be inadequate removal of adsorbed species.

8.8 Sintered 푪풆푶ퟐ

In all previous experiments, the same batch of 퐶푒푂2 and 푍푟푂2 has been used. These oxides have a disparity in surface area of approximately three. To investigate the effect of surface

° area on the radiolysis of air, a sample of 퐶푒푂2 was sintered at 950 C under static air for two hours using a heating rate of 10 °C min-1. The following section outlines the sintered oxide properties and subsequent experimental results.

8.8.1 Oxide Properties

Figure 8.25 represents the BET adsorption isotherm for the sintered oxide product. The surface area was calculated as 2.20 m2 g-1. This is a decrease of 70% and is approximately the same surface area as the 푍푟푂2 samples used in previous experiments.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

8.0

- 7.0

g 3 3 6.0 5.0 4.0 3.0 2.0

Quantity adsorbed / cm/ adsorbed Quantity 1.0 0.0 0 0.2 0.4 0.6 0.8 1 Relative pressure / (P/Po)

Figure 8.25: BET adsorption (solid trace) – desorption (dashed trace) isotherm of 퐶푒푂2 sintered at 950 °C for 2 h

As seen in Figures 6.3 and 6.6, the adsorption-desorption isotherm for sintered 퐶푒푂2 is of type three [65]. The isotherm also has hysteresis, indicating that the oxide still has a porous structure, however, the size of this loop has decreased indicating the loss of porosity. The quantity of nitrogen adsorbed has reduced by 60% indicating the decrease in the BET surface area.

After sintering, the bulk density has increased from 1.44 to 2.23 g cm-3. This is an increase of approximately 55% and is to be expected as the porous structure becomes annealed at higher temperatures. The increase in bulk density leads to increase masses of oxide needed for 50% and 90% (by volume) samples. The mass of sintered 퐶푒푂2 required is 13.5 g for 50% volume samples and 24 g for 90% volume samples.

SEM and EDS data for sintered 퐶푒푂2 are shown in Figure 8.26:

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Chapter 8 Air Radiolysis Results and Discussion 7131060

ii)

Figure 8.26: i) Scanning electron micrograph and ii) EDS spectra for sintered 퐶푒푂2

From the SEM image, it is clear that there is no dominant crystal morphology, this was evident in the un-sintered 퐶푒푂2 (Figure 6.1). The average particle size has not changed drastically in Figure 8.26i, with an average area of 5 µm2. The EDS data shows that there is no change in the elemental make-up of the oxide, with cerium and oxygen, the major constituents, and silicon and carbon present as an impurity and from the carbon stub respectively.

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Chapter 8 Air Radiolysis Results and Discussion 7131060

8.8.2 Nitrate Production over Sintered 푪풆푶ퟐ

Figure 8.27 and Table 8-16 outlines the compiled results for nitrate yield as a function of absorbed dose for the sintered 퐶푒푂2 system.

120.0

100.0

80.0

molec

) ) 16

(x10 60.0 No Oxide 1 g 40.0 50% (~13.5 g)

90% (~24 g) Nitrate yield / yield Nitrate

20.0

0.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 16 Dose to Air / (x10 ) 100 eV Figure 8.27: Nitrate production as a function of absorbed dose for samples containing 1 g, 50% and 90% (by volume) sintered 퐶푒푂2 and from water saturated air (no oxide)

− System G(푵푶ퟑ ) / Error (흈풔풍풐풑풆) molecules 100 eV-1 No oxide 1.17 ± 0.06

1 g sintered 퐶푒푂2 1.21 ± 0.07

50% sintered 퐶푒푂2 2.35 ± 0.17 (~13.5 g)

90% sintered 퐶푒푂2 2.59 ± 0.22 (~24 g) − Table 8-16: Calculated G(푁푂3 ) values and standard deviation of the gradient for systems containing 1 g, 50% and 90% (by volume) sintered 퐶푒푂2 and water saturated air (no oxide)

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Chapter 8 Air Radiolysis Results and Discussion 7131060

It is clear from Table 8-16 that the yield of nitrate increases with growing mass of sintered

퐶푒푂2 which is in agreement with un-sintered 퐶푒푂2 (Table 8-14). However, this increase is not directly related to the mass of 퐶푒푂2 present in the sample. The mass of 퐶푒푂2 in the 50% and 90% samples is over double the quantity of 퐶푒푂2 present in the un-sintered samples, however, the yield has not doubled. This suggests that the formation of nitrate is not purely a mass driven process.

8.8.3 Comparison with Un-sintered 푪풆푶ퟐ Results

Figure 8.28 outlines the comparative results for samples containing either regenerated or sintered 퐶푒푂2 and Table 8-17 details the respective masses, surface areas and nitrate yields for the regenerated and sintered 퐶푒푂2 samples.

400.0

350.0 1 g 1 g sin 50% (~6 g) 50% sin (~13.5 g)

90% (~12 g) 90% sin (~24 g)

300.0 No Oxide molec

) ) 250.0 16

(x10 200.0

150.0

Nitrate yield / yield Nitrate 100.0

50.0

0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 16 Dose to Air / (x10 ) 100 eV Figure 8.28: Compiled data plot of nitrate production as a function of dose for systems containing 1 g, 50% and 90% (by volume) regenerated 퐶푒푂2, 1 g, 50% and 90% (by volume) sintered 퐶푒푂2 and from water saturated air (no oxide)

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Regenerated 푪풆푶ퟐ Sintered 푪풆푶ퟐ

− − System Mass Surface G(푵푶ퟑ ) / Error Mass Surface G(푵푶ퟑ ) / Error 2 2 / g Area / m molecules (흈풔풍풐풑풆) / g Area / m molecules (흈풔풍풐풑풆) 100 eV-1 100 eV-1

No - - 1.17 ± 0.06 - - 1.17 ± 0.06 Oxide

1 g 1 7.42 2.07 ± 0.07 1 2.2 1.21 ± 0.07

50% ~6 44.5 2.48 ± 0.02 ~13.5 29.7 2.35 ± 0.17

90% ~12 89.0 3.19 ± 0.09 ~24 52.8 2.59 ± 0.22

− Table 8-17: Comparison between calculated G(푁푂3 ) for samples containing either regenerated 퐶푒푂2 or sintered 퐶푒푂2 and water saturated air (no oxide)

An initial observation is that the yield of nitrate is greater in samples containing regenerated

퐶푒푂2 than the corresponding sintered 퐶푒푂2 samples, however, this observation does not account for the difference in surface area available in the system.

8.8.4 Discussion

As discussed in Section 8.5.2, the higher yields of nitrate observed in oxide containing systems is hypothesised to be due to the adsorption of 푁푂2 which inhibits the back reaction with nitrogen atoms (Reactions 8.7 and 8.8). To determine whether a decrease in the area of surface available for 푁푂2 to adsorb to, reduces the yield of nitrate produced, Figure 8.29

− is a plot of G(푁푂3 ) as a function of surface area for the three types of oxide used in this research:

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Chapter 8 Air Radiolysis Results and Discussion 7131060

7.00 H2O sat. air 6.00 Regenerated CeO2 Sintered CeO2 5.00 ZrO2

4.00

)

- 3

(NO 3.00 G

2.00

1.00

0.00 0.0 20.0 40.0 60.0 80.0 100.0 Surface area / m2

− Figure 8.29: Plot of G(푁푂3 ) as a function of surface area for the three oxide systems utilised in this research and for reference, the water saturated air (no oxide) yield

− It is clear from this figure that the difference in G(푁푂3 ) values seen in Table 8-17 correlates with an increase in surface area present in the system. Figure 8.29 highlights that there is a linear relationship between surface area and yield of nitrate formed in samples containing

퐶푒푂2 of differing surface areas. When there is little surface available for adsorption of 푁푂2, as in 1 g samples of low SSA oxides, the yield of nitrate is comparable to that of water saturated air. In these systems, the oxide equates to less than 5% of the total volume.

Therefore, the majority of the chemistry will occur in the gas phase and the surface will only play a role when reactive species in the gas phase have diffused to the surface.

The mean free path can be used as an approximation for the distance travelled by a species before interacting with another species. Equation 8.2 can be used to determine this value:

퐿 = (𝜎푛)−1 Equation 8.2

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Chapter 8 Air Radiolysis Results and Discussion 7131060 where 퐿 is the mean free path (in metres), 𝜎 is the collision cross-section (in m2) and 푛 is the number of molecules (per m3).

-8 The mean free path of an 푁2 molecule is approximately 6.6x10 m in air at 1 atmosphere of pressure and 20 °C [126]. The collisional cross section value is very similar for 푂2 therefore the mean free path will not differ dramatically. In the samples containing 1 g of oxide, there is approximately 2.5 cm of headspace above the powder; from this mean free path, it is evident that the majority of species generated in the gas phase will have reacted before reaching the surface. Species within this distance from the surface will adsorb to the surface

− and inhibit the back reaction with other gas phase species. This is why the G(푁푂3 ) for 1 g systems are slightly above the value for saturated air samples. In samples with higher loading of oxides, the distance needed to travel by a gas phase species before encountering a surface is greatly reduced, therefore adsorption of 푁푂2 will be greater, leading to the higher yields of nitrate reported throughout this chapter.

푍푟푂2 initially follows the trend in Figure 8.29, however, samples containing 90% oxide have a much higher yield than would be expected. As explained in Section 8.5.2, the band gap of

푍푟푂2 is too low to directly ionise 푁2 or 푂2, however, it will greatly increase the radiolysis of adsorbed water and the species generated from this (as seen in Chapter Seven). This higher concentration of water radiolysis species (namely 푂퐻. and 퐻 atoms) will increase the yield of nitrate produced (Reaction 8.2).

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Conclusions

The radiolysis of pure air reaches a maximum nitrate yield of 83 μmol L-1 due to the exhaustion of water vapour in the system. The G-value for nitrate formation is 1.39 molecules 100 eV-1 absorbed energy.

In the presence of an oxide powder, the yield of nitrate greatly increases due to the surface acting as a sink for 푁푂2 which inhibits the back reaction with 푁 atoms to reform 푁2. The yield of nitrate increases proportionally to the quantity of surface area available in the sample. However, this trend is not followed by samples containing 90% (by volume) of

− 푍푟푂2, which produces significantly higher yields of nitrate with a G(푁푂3 ) = 6.5 molecules

100 eV-1. This value is close to the limiting yield of G(푁) = 6 atoms 100 eV-1. This large increase in nitrate formation is due to the higher concentration of 푂퐻∙ from the radiolysis of adsorbed water on the 푍푟푂2 surface. This species enhances nitrate formation in the following reaction:

. 푁푂2 + 퐻푂 → 퐻푁푂3 Reaction 8.2

2 -1 Reducing the surface area of 퐶푒푂2 from 7.42 to 2.2 m g in turn reduces the yield of nitrate formed due to the relationship between nitrate formation and the surface area present in the system.

Carbon dioxide (퐶푂2) in the atmosphere gets reduced on the oxide surface and undergoes

2− dimerisation to form oxalate (퐶2푂4 ). The yields are far greater in the presence of 푍푟푂2 than corresponding 퐶푒푂2 samples. The yield of oxalate, however, is an order of magnitude below the yield of nitrate formation in corresponding samples. There is very little oxalate formed in pure air samples due to the reaction outlined below:

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Chapter 8 Air Radiolysis Results and Discussion 7131060

퐶푂 + 푂 + 푀 → 퐶푂2 + 푀 Reaction 8.19

Irradiating samples in synthetic air atmospheres removes a large quantity of 퐶푂2 from the gas phase and the yields of oxalate in 푍푟푂2 samples are reduced to the level of 퐶푒푂2 samples. Due to 퐶푂2 adsorbed to the oxide surface prior to irradiation, there is still oxalate present in the post-irradiated samples.

Irradiating in synthetic air atmospheres had no effect on the nitrate yield as the water adsorbed onto the oxide powder far outweighs the quantity of water vapour in the headspace (Tables 8-2, 8-3 and 8-5) and, in pure air samples, the silanol (푆𝑖 − 푂 − 퐻) groups on the glass vials lead to nitric acid formation.

− In all the systems investigated in this chapter, nitrite (푁푂2 ) is absent from the post- irradiated chromatogram. This is due to the decomposition of nitrous acid on the surface as outlined below:

2퐻푁푂2 (푎푑푠) → 푁푂 + 푁푂2 + 퐻2푂(푎푑푠) Reaction 8.11

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9 Final Conclusions

This chapter pulls together the conclusions drawn from 퐻2 − 푂2 − 퐴푟 radiolysis studies in the presence or absence of an oxide surface using γ-rays and 퐻푒2+ accelerated ions that were discussed in Chapter Seven and the results of air radiolysis in the presence or absence of an oxide surface using γ-rays discussed in Chapter Eight.

9.1 푯ퟐ − 푶ퟐ System

The development of a suitable reaction vessel to investigate the radiation chemistry of

퐻2 − 푂2 − 퐴푟 gas mixtures has been achieved, after several iterations of design. With these iterations, development of the analysis technique was needed.

60푪풐 Studies

In homogeneous systems, the recombination of 퐻2 − 푂2 appears to be independent of

-1 concentration of either gas. The G(-퐻2) values in the range of 3.5-4.7 molecules 100 eV were calculated, which is in good agreement with literature. The corresponding G(-푂2)

-1 values are within the range of 1.8-4.2 molecules 100 eV . The average ratio of G(-퐻2) to

G(-푂2) is 1.44. As this value is greater than one, it suggests that hydrogen peroxide (퐻2푂2) is not the stable product in this system.

Addition of an oxide surface greatly increased the rate of recombination. At low doses, 퐻2 from radiolysis of adsorbed water is detected, however, this is soon depleted. It is not known at this time whether, the effect of the oxide surface is catalytic, radiolytic or simply as a surface for species to be adsorbed onto.

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In all of these systems, a steady state is not reached, therefore the rate of recombination is greater than the rate of water vapour radiolysis.

푯풆ퟐ+Studies

Development of an adequate reaction vessel to investigate the recombination of 퐻2 − 푂2 using accelerated ions as a radiation source has been completed and preliminary experiments have been undertaken.

Ethylene gas has been utilised as a secondary dosimeter to validate the absolute current measurements. The results are within 5% of literature values, instilling confidence in the absorbed dose calculations for this system.

In the results attained so far irradiating mixtures of 퐻2 − 푂2 − 퐴푟 gas, 퐻2 continues to be depleted with absorbed dose and no steady state is reached between 퐻2 − 푂2 recombination and radiolysis of water vapour. The G(-퐻2) values calculated for this data lie in the range of 4.7-5.4 molecules 100 eV-1, in good agreement with 60퐶표 γ-ray studies.

Preliminary results highlight the absence of a linear energy transfer (LET) effect between

60퐶표 γ-rays and accelerated 퐻푒2+ ions, as is to be expected with gaseous systems.

9.2 Air Radiolysis System

The presence of an oxide surface, greatly increases the yield of nitric acid in the radiolysis of

-1 air system. G(퐻푁푂3) values increase from 1.17 molecules 100 eV in moist air, to

-1 -1 3.19 molecules 100 eV in the presence of 퐶푒푂2 and 6.5 molecules 100 eV in the presence of 푍푟푂2. It is hypothesised, that this increase is due to the surface acting as a sink for 푁푂2 and thus preventing the back reaction to 푁2 (Reactions 8.7 and 8.8).

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Chapter 9 Final Conclusions 7131060

2− The presence of an oxide surface also leads to the formation of oxalate (퐶2푂4 ), from the dimerisation of 퐶푂2 in the air.

Decreasing the surface area of 퐶푒푂2 leads to a reduction of nitric acid formation in proportion to higher surface area 퐶푒푂2.

Conclusion

In canisters of 푃푢푂2, it is likely that both of these systems may be occurring simultaneously.

It is not known what effect, one system may have on the other. The presence of the 푃푢푂2 leads to a very complex system with redox chemistry occurring on the surface, as well as energy transfer from the solid to adsorbed species and gaseous species.

Hopefully this research has added to the knowledge gap that was outlined in Chapter Two, however, more work is needed to achieve a complete understanding of the mechanism occurring inside 푃푢푂2 storage canisters.

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10 Future work

While the research chapters of this thesis have gone some way in exploring the radiation chemistry of gaseous systems in contact with an oxide surface, there are still many areas in which this research could be developed further. These are discussed below in the context of specific experiments and general research directions.

10.1 푯ퟐ − 푶ퟐ − 푨풓 System

To further elucidate the reaction mechanism of 퐻2 − 푂2 recombination, a kinetic model of elementary reactions could be developed. Foy and Joyce have tried to model the chemistry occurring inside 푃푢푂2 canisters, however, this work is very preliminary and is not extensive

[127]. Modelling of a compiled reaction scheme taken from the extensive photochemistry literature may help to explain the zero order nature of the gas phase radiolysis results presented in Section 7.6. Once good agreement has been reached with this gas phase chemistry model, a surface can then be added to incorporate the chemistry occurring at an oxide surface such as in 푃푢푂2 canisters. Development of this model would require significant effort as rate constants for many of the important three-body reactions are yet to be determined.

Section 8.8 detailed results of air radiolysis experiments using sintered 퐶푒푂2. This study was performed to determine the effect of SSA on the radiation chemistry of moist air. A similar experiment was not performed to investigate 퐻2 − 푂2 recombination, and it is therefore, not clear if the difference in results for 퐶푒푂2 and 푍푟푂2, presented in Section 7.7, is due to

SSA effects or chemical differences. Experiments with particles of the same oxides of

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differing surface areas should be investigated to determine the effect of SSA on 퐻2 − 푂2 recombination.

10.2 Air Radiolysis System

− Figure 8.29 highlighted an increase in the radiolytic production of 푁푂3 , with an increase of surface area. In this research the surface area of the two oxides of interest were in the

2 -1 range of 2-8 m g , this surface area is analogous to 푃푢푂2 currently in storage. To

2 -1 determine the extent of this trend, 퐶푒푂2 and 푍푟푂2 with higher surface areas (100’s m g ) such as nano-particles should be investigated.

10.3 푪ퟐ푯ퟒ System

By chance, it has been shown in this research, that decomposition of ethylene is enhanced in the presence of an oxide surface with a 450 fold increase in 퐻2 production in the presence of 푍푟푂2 than in pure ethylene. Further research is required to elucidate a mechanism for this phenomenon and to what extent SSA, oxide band gap and quantity of adsorbed water may have on this system.

This experiment was initially performed to help determine the dose adsorbed by the gas phase in contact with an oxide surface, however, it has suggested another topic of research interest.

Chapter One outlined the packaging criteria for 푃푢푂2 from the two product streams at the

Sellafield site. Magnox 푃푢푂2 product is packaged in an aluminium can and placed inside a polyethylene ( 푃퐸 ) bag. During long term storage these bags have thermally and

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Chapter 10 Future Work 7131060

radiolytically degraded, leading to the formation of 퐻2, small gaseous hydrocarbons and unsaturated polymeric hydrocarbons. Other gaseous hydrocarbons, for example, ethane and butene should be investigated to assess the decomposition of hydrocarbons in the presence of an oxide surface and how this may contribute to 퐻2 concentration in the ullage space.

Alongside gaseous hydrocarbons, powdered 푃퐸 could be mixed in various ratios with an oxide powder and irradiated. This experiment would help to determine if the physical form of the hydrocarbon has an effect on decomposition.

10.4 Generic Recommendations

The following recommendations are not specific for either of the gaseous system investigated in this project.

In this research, surrogates of 푃푢푂2 have been used to investigate the gas phase radiation chemistry occurring in the presence of an oxide surface. Although 퐶푒푂2 and 푍푟푂2 have been used extensively as 푃푢푂2 surrogates, there may be better alternatives. Thorium dioxide (푇ℎ푂2) and uranium dioxide (푈푂2) both have the same fluorite structure as 푃푢푂2 and as actinide oxides may bear a closer resemblance to 푃푢푂2 than either 퐶푒푂2 or 푍푟푂2.

Both of these oxides are radioactive and like 푃푢푂2 are α-emitters. This would simulate

푃푢푂2 canisters better as they are internal radiation sources with alpha decay energies similar to plutonium (4-4.6 MeV for thorium and uranium isotopes in comparison to 5.2-5.6

MeV for plutonium isotopes).

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The final stage of this research programme would be to undertake studies with 푃푢푂2. Using surrogate oxides gives a good knowledge basis for potential reaction mechanisms inside the storage canisters, but this approach will never be able to fully simulate the real system. It is possible to replicate the physical properties of 푃푢푂2 (SSA, crystal structure, adsorbed species); however, it is impossible to simulate the damage (and impurities) induced by radioactive decay. 푃푢푂2 may have a large quantity of defects in the crystal lattice which may affect the gas phase chemistry which cannot be replicated in a facile way.

The use of the three actinide oxides detailed above requires specialist facilities with regards to handling and safeguards. All this work could not be undertaken at the DCF due to environmental licensing and current infrastructure. Some of this work would have to be undertaken at NNL’s Central Laboratory located at the Sellafield site.

Throughout this research, the effects of adsorbed species (in particular water) have been highlighted as having an effect on the gas phase radiation chemistry. To determine the full extent of this effect it is necessary to investigate a ‘clean’ surface where the identity and quantity of adsorbates is known. This may be achieved by utilising a single crystal or thin film of the relevant oxide. UHV conditions would be needed to undertake this research to maintain the surface properties of the oxide. Similar work has been undertaken with, for instance, titanium dioxide thin films [128] using low energy electrons as the radiation source. Experiments utilising energetic electrons and accelerated ions as the radiation sources are also feasible. In addition, it would also be possible to use an internal radiation source such as the actinide dioxides described above.

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10.5 Future Work with Accelerated Ions

Section 7.9 outlined experiments utilising ethylene gas as a validation method for the absolute current measurements. Good agreement was achieved with both systems for a range of currents and absorbed doses. The low density of ethylene should allow for several ion types to be investigated with reasonable penetration into the sample. This will help to determine if 퐻2 production from ethylene radiolysis is dependent on ion type.

As described in Section 7.10, investigations with the ion accelerator are at a preliminary stage, however, there is good agreement with corresponding 60퐶표 data to suggest there is no LET effect. More data and using different ion types should further cement this hypothesis.

In the research presented in this thesis; only gaseous systems have been investigated using the ion accelerator. Experiments of 퐻2 − 푂2 − 퐴푟 gas in the presence of an oxide surface using an ion accelerator will help to create a better simulation of the 푃푢푂2 canisters.

However, there are several difficulties to overcome with this experiment as the penetration of the beam will be greatly reduced from several centimetres to several microns. The majority of the beam will be absorbed by (the first few microns of) the oxide powder; this small portion of oxide will be heated by the beam as oxides are poor thermal conductors and this could lead to the oxide surface becoming annealed. Due to this fact, external cooling of the sample may be needed and/ or agitation of the sample may have to be considered.

The short range of the incoming ions in the solid may lead to very negligible energy being deposited into the gas phase, therefore, making comparison with gamma studies presented in this thesis may be difficult to attain.

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Chapter 10 Future Work 7131060

Any solutions to this issue would require the reaction vessel in Figure 3.6 to be redesigned as the ion beam would not penetrate very far into the vessel and radiation chemistry would only likely occur in the first few millimetres of the sample. One solution is highlighted in

Figure 10.1. In this figure, the ion beam is stopped entirely in the gas phase, therefore there would be no energy transferred from the oxide to the gas phase and the solid would act simply as a sink for gaseous species.

Figure 10.1: Sketch of possible reaction vessel to study heterogeneous systems using an ion accelerator

An issue with this design is in keeping the oxide powder in position during irradiations. As the powder would not play any part in the radiation chemistry mechanism, this design is a poor simulation of the 푃푢푂2 canister system.

A second reaction vessel design is illustrated in Figure 10.2. In this design, the ion beam is absorbed by both gas and solid phase allowing for potential energy transfer to take place.

This would simulate the 푃푢푂2 canister system better; however, the issue of oxide annealing would still occur at the interface with the mica window.

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Figure 10.2: Second possible reaction vessel for heterogeneous system experiments using an ion accelerator

This vessel would require secure placement and further beam diagnostics to help determine the energy partitioning between the two phases.

This reaction vessel could be utilised in investigations of oxide powders in air atmospheres as well as 퐻2 − 푂2 − 퐴푟 studies.

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11 Bibliography

1. Sharrad, C.A., L.M. Harwood, and F.R. Livens, Chapter 2 Nuclear Fuel Cycles: Interfaces with the Environment, in Nuclear Power and the Environment. 2011, The Royal Society of Chemistry. p. 40-56. 2. Various, Plutonium Separation in Nuclear Power Programs, 2015. 3. Management of the UK's plutonium stocks: a consultation on the long-term management of UK owned separated civil plutonium, 2011, Department of Energy and Climate Change. p. 12- 13. 4. Herbst, R.S., P. Baron, and M. Nilsson, 6 - Standard and advanced separation: PUREX processes for nuclear fuel reprocessing, in Advanced Separation Techniques for Nuclear Fuel Reprocessing and Radioactive Waste Treatment. 2011, Woodhead Publishing. p. 141-175. 5. Stabilization, Packaging, and Storage of Plutonium-Bearing Materials, D.o. Energy, Editor 2012. 6. Machuron-Mandard, X. and C. Madic, Plutonium dioxide particle properties as a function of calcination temperature. Journal of Alloys and Compounds, 1996. 235(2): p. 216-224. 7. Cook, P., H.E. Sims, and D. Woodhead, Safe and Secure Storage of Plutonium Dioxide in the United Kingdom. Actinide Research Quarterly, 2013(2): p. 20-25. 8. Orr, R.M., H.E. Sims, and R.J. Taylor, A review of plutonium oxalate decomposition reactions and effects of decomposition temperature on the surface area of the plutonium dioxide product. Journal of Nuclear Materials, 2015. 465: p. 756-773. 9. Sims, H.E., et al., Hydrogen yields from water on the surface of plutonium dioxide. Journal of Nuclear Materials, 2013. 437(1-3): p. 359-364. 10. Gilchrist, P., Plutonium: Credible Options Analysis (Gate A), N.D. Authority, Editor 2010. 11. Choppin, G.R., J.O. Liljenzin, and J. Rydberg, Chapter 4 - Unstable Nuclei and Radioactive Decay, in Radiochemistry and Nuclear Chemistry (Third Edition). 2002, Butterworth- Heinemann: Woburn. p. 58-93. 12. Spinks, J.W.T. and R.J. Woods, An Introduction to Radiation Chemistry. 3rd ed. 1964: John Wiley & Sons. 13. Choppin, G.R., J.O. Liljenzin, and J. Rydberg, Chapter 6 - Absorption of Nuclear Radiation, in Radiochemistry and Nuclear Chemistry (Third Edition). 2002, Butterworth-Heinemann: Woburn. p. 123-165. 14. Appleby, A. and H.A. Schwarz, Radical and molecular yields in water irradiated by gamma- rays and heavy ions. The Journal of Physical Chemistry, 1969. 73(6): p. 1937-1941.

254

Chapter 11 Bibliography 7131060

15. Yang, K. and P.J. Manno, The Role of Free Radical Processes in the γ-Radiolysis of Methane, Ethane and Propane. Journal of the American Chemical Society, 1959. 81(14): p. 3507-3510. 16. Hauser, W.P., Radiolysis of Methane-C14 1. The Journal of Physical Chemistry, 1964. 68(6): p. 1576-1579. 17. Eller, P.G., R.E. Mason, and D.R. Horrell, 1999, Los Alamos Technical Report. 18. Ronchi, C. and J.P. Hiernaut, Helium diffusion in uranium and plutonium oxides. Journal of Nuclear Materials, 2004. 325(1): p. 1-12. 19. Paffett, M. and G. Bailey, Gas Generation from Actinide Oxide Marterials, 1999: Los Alamos Technical Report. p. 3-9. 20. Mason, R., T. Allen, and L. Morales, Gas Pressurization from Calcined Plutonium Oxides, 1999. 21. Almond, P.M., R.R. Livingston, and L.E. Traver, Gas Analyses from Headspace of Plutonium Bearing Materials Containers, 2010: Savannah River National Laboratory. 22. Stakebake, J.L., Thermal desorption study of the surface interactions between water and plutonium dioxide. The Journal of Physical Chemistry, 1973. 77(5): p. 581-586. 23. Haschke, J.M. and T.E. Ricketts, Adsorption of water on plutonium dioxide. Journal of Alloys and Compounds, 1997. 252(1–2): p. 148-156. 24. Alexandrov, V., et al., Actinide Dioxides in Water: Interactions at the Interface. Journal of Physical Chemistry Letters, 2011. 2(24): p. 3130-3134. 25. Petrik, N.G., A.B. Alexandrov, and A.I. Vall, Interfacial energy transfer during gamma radiolysis of water on the surface of ZrO2 and some other oxides. Journal of Physical Chemistry B, 2001. 105(25): p. 5935-5944. 26. Pastina, B., J.A. LaVerne, and S.M. Pimblott, Dependence of molecular hydrogen formation in water on scavengers of the precursor to the hydrated electron. Journal of Physical Chemistry A, 1999. 103(29): p. 5841-5846. 27. Luo, Y.R., Bond Dissociation Energies, in Handbook of Chemistry and Physics. 2009, CRC Press. p. 9.64-9.69. 28. Aleksandrov, A.B., et al., Radiolysis of Adsorbed Substances on Oxide Surfaces. Zhurnal Fizicheskoi Khimii, 1991. 65(6): p. 1604-1608. 29. LaVerne, J.A. and L. Tandon, H2 Production in the Radiolysis of Water on CeO2 and ZrO2. The Journal of Physical Chemistry B, 2002. 106(2): p. 380-386. 30. Haschke, J.M., T.H. Allen, and L.A. Morales, Reactions of plutonium dioxide with water and hydrogen-oxygen mixtures: Mechanisms for corrosion of uranium and plutonium. Journal of Alloys and Compounds, 2001. 314(1-2): p. 78-91.

255

Chapter 11 Bibliography 7131060

31. Lloyd, J.A., Literature search on hydrogen/oxygen recombination and generation in plutonium storage environments, 1999. 32. Haschke, J.M. and T.H. Allen, Interactions of Plutonium Dioxide with Water and Oxygen- Hydrogen Mixtures, 1999. 33. Morales, L.A., Preliminary report on the recombination rates of hydrogen and oxygen over pure and impure plutonium oxides, 1998. 34. Korzhavyi, P.A., et al., Oxidation of plutonium dioxide. Nature Materials, 2004. 3(4): p. 225- 228. 35. Quigley, G.P., Hydrogen/Oxygen Recombination Rates in 3013-Type Environments: An Interim Report on the Rate of Loss of Hydrogen in Dry Air from Cell Containing Non-Radiolytic Samples, 1998. 36. Katz, S., The Reaction of Hydrogen and Oxygen in the Presence of Concrete Incorporating Simulated Radioactive Waste, 1979. 37. Lind, S.C., Chemical action produced by radium emanation I The combination of hydrogen and oxygen. Journal of the American Chemical Society, 1919. 41: p. 531-551. 38. Schiflett, C.H. and S.C. Lind, The temperature coefficient of the rate of combination of hydrogen and oxygen under alpha radiation. Journal of Physical Chemistry, 1934. 38(3): p. 327-337. 39. Lind, S.C. and C.H. Schiflett, Chemical action produced by alpha particles: The combination of deuterium and oxygen. Journal of the American Chemical Society, 1935. 57(1): p. 1051-1052. 40. Benjamin, B.M. and H.S. Isbin, Recombination of Hydrogen and Oxygen in Presence of Water Vapour under Influence of Radiation. Energia Nucleare, 1966. 13(4): p. 165-172. 41. Dautzenberg, D., Gamma-radiolysis of Hydrogen Oxygen Mixtures 1 Influences of Temperature, Vessel Wall, Pressure and Added Gases (N2, AR, H2) on the Reactivity of H2/O2-Mixtures. Radiation Physics and Chemistry, 1989. 33(1): p. 61-67. 42. Willis, C., A.W. Boyd, and M.J. Young, Radiolysis of air and nitrogen–oxygen mixtures with intense electron pulses: determination of a mechanism by comparison of measured and computed yields. Canadian Journal of Chemistry, 1970. 48(10): p. 1515-1525. 43. Harteck, P. and S. Dondes, Decomposition of Nitric Oxide and Nitrogen Dioxide by the Impact of Fission Fragments of Uranium-235. The Journal of Chemical Physics, 1957. 27(2): p. 546- 551. 44. Harteck, P. and S. Dondes, The Kinetic Radiation Equilibrium of Air. The Journal of Physical Chemistry, 1959. 63(6): p. 956-961.

256

Chapter 11 Bibliography 7131060

45. Kanda, Y., et al., Measurement of radiolytic yield of nitric acid in air. Radiation Physics and Chemistry, 2005. 74(5): p. 338-340. 46. Jones, A.R., Radiation Induced Reactions in the N2-O2-H2O System. Radiation Research, 1959. 10(6): p. 655-663. 47. Rouessac, F. and A. Rouessac, Chemical Analysis - Modern Instrumentation Methods and Techniques. 1994: John Wiley & Sons Ltd. 48. Webb, P.A. and C. Orr, Analytical Methods in Fine Particle Technology. 1997: Micromeritics Instrument Corporation. 49. Finlayson-Pitts, B.J. and J.N. Pitts Jr, Chapter 7 - Chemistry of Inorganic Nitrogen Compounds, in Chemistry of the Upper and Lower Atmosphere. 2000, Academic Press. p. 264-293. 50. Schuler, R.H. and A.O. Allen, Absolute Measurement of Cyclotron Beam Currents for Radiation-Chemical Studies. Review of Scientific Instruments, 1955. 26(12): p. 1128-1130. 51. Ziegler, J.F. SRIM (Stopping Range of Ions in Matter). Available from: www.srim.org. 52. Charlesby, A., The Effects of Ionising Radiation on Polymers, in Irradiation Effects on Polymers. 1991, Elsevier Science Publishers. p. 55-57. 53. Nilsson, S. and M. Jonsson, H2O2 and radiation induced dissolution of UO2 and SIMFUEL pellets. Journal of Nuclear Materials, 2011. 410(1–3): p. 89-93. 54. Stone, J.A., Radiolysis of cyclohexane in a xenon matrix at 77 °K. Canadian Journal of Chemistry, 1968. 46(8): p. 1267-1277. 55. Sagert, N.H. and R.W. Robinson, Radiolysis in the adsorbed state. II. N2O adsorbed on silica gel and zirconia. Canadian Journal of Chemistry, 1968. 46(12): p. 2075-2080. 56. LaVerne, J.A. and L. Tandon, H2 Production in the Radiolysis of Water on UO2 and Other Oxides. The Journal of Physical Chemistry B, 2003. 107(49): p. 13623-13628. 57. Armstrong, D.A., The Radiation Chemistry of Gases, in Radiation Chemistry Principles and Applications. 1987, VCH Publishers inc. p. 263-319. 58. Johnson, G.R.A., The nitrous oxide radiation dosimeter. Journal of Inorganic and Nuclear Chemistry, 1962. 24(5): p. 461-468. 59. McLaren, K.G., Gas phase dosimetry based on nitrogen yield from nitrous oxide. The International Journal of Applied Radiation and Isotopes, 1974. 25(2): p. 87-93. 60. Janovsky, I., Use of Nitrous-Oxide and Ethylene for Gas-phase Dosimetry. International Journal for Radiation Physics and Chemistry, 1976. 8(3): p. 396-397. 61. Ikezoe, Y., S. Sato, and A. Danno, Chemical Dosimeters for Mixed Radiations in a Nuclear Reactor. Journal of Nuclear Science and Technology, 1971. 8(7): p. 394-399.

257

Chapter 11 Bibliography 7131060

62. Srinivas.S and W.E. Smith, Ethylene as a Dosimeter at high Pressures, Temperatures and Dose Rates. International Journal of Applied Radiation and Isotopes, 1966. 17(11-1): p. 643- 647. 63. Leay, L., et al., Development of irradiation capabilities to address the challenges of the nuclear industry. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2015. 343(0): p. 62-69. 64. LaVerne, J.A. and R.H. Schuler, Radiation chemical studies with heavy ions: oxidation of ferrous ion in the Fricke dosimeter. The Journal of Physical Chemistry, 1987. 91(22): p. 5770- 5776. 65. Brunauer, S., et al., On a Theory of the van der Waals Adsorption of Gases. Journal of the American Chemical Society, 1940. 62(7): p. 1723-1732. 66. Socrates, G., Infrared and Raman Characteristic Group Frequencies. Third Edition ed. 2001: John Wiley and Sons Ltd. 67. Busca, G. and V. Lorenzelli, Infrared spectroscopic identification of species arising from reactive adsorption of carbon oxides on metal oxide surfaces. Materials Chemistry, 1982. 7(1): p. 89-126. 68. Baltrusaitis, J., et al., Carbon dioxide adsorption on oxide nanoparticle surfaces. Chemical Engineering Journal, 2011. 170(2–3): p. 471-481. 69. Goodman, A.L., E.T. Bernard, and V.H. Grassian, Spectroscopic study of nitric acid and water adsorption on oxide particles: Enhanced nitric acid uptake kinetics in the presence of adsorbed water. Journal of Physical Chemistry A, 2001. 105(26): p. 6443-6457. 70. Sauer, M.C. and L.M. Dorfman, The Radiolysis of Ethylene: Details of the Formation of Decomposition Products. The Journal of Physical Chemistry, 1962. 66(2): p. 322-325. 71. Ausloos, P. and R. Gorden, Hydrogen Formation in the γ Radiolysis of Ethylene. The Journal of Chemical Physics, 1962. 36(1): p. 5-9. 72. Sauer, M.C. and L.M. Dorfman, Molecular Detachment Processes in the Vacuum uv Photolysis of Gaseous Hydrocarbons. I. Ethylene. II. Butane. The Journal of Chemical Physics, 1961. 35(2): p. 497-502. 73. Meisels, G.G. and T.J. Sworski, Radiolysis of Ethylene. I. Yield of Hydrogen Atoms and Formation of Saturated Hydrocarbons1. The Journal of Physical Chemistry, 1965. 69(3): p. 815-821. 74. Meisels, G.G., Radiolysis of Ethylene. II. Primary Free Radicals and Their Reactions1. Journal of the American Chemical Society, 1965. 87(5): p. 950-957.

258

Chapter 11 Bibliography 7131060

75. Yang, K. and P.J. Manno, Gamma Radiolysis of Ethylene. Journal of Physical Chemistry, 1959. 63(5): p. 752-753. 76. Emeline, A., et al., Spectroscopic and photoluminescence studies of a wide band gap insulating material: Powdered and colloidal ZrO2 sols. Langmuir, 1998. 14(18): p. 5011-5022. 77. Ren, Y., et al., Elastic properties and electronic structure of transition metal atoms in CeO2 solid solution: First principle studies. Computational Materials Science, 2015. 98: p. 459-465. 78. Chelnokov, E., et al., Electron Transfer at Oxide/Water Interfaces Induced by Ionizing Radiation. Journal of Physical Chemistry C, 2014. 118(15): p. 7865-7873. 79. Haschke, J.M., T.H. Allen, and L.A. Morales, Reaction of plutonium dioxide with water: Formation and properties of PuO2+x. Science, 2000. 287(5451): p. 285-287. 80. Penneman, R.A. and M.T. Paffett, An alternative structure of Pu4O9 ("PuO2.25") incorporating interstitial hydroxyl rather than oxide. Journal of Solid State Chemistry, 2005. 178(2): p. 563-566. 81. Il Pae, Y., D.C. Shin, and J.R. Sohn, CeO2-Promoted highly active catalyst, NiSO4/CeO2-ZrO2 for ethylene dimerization. Bulletin of the Korean Chemical Society, 2006. 27(12): p. 1989- 1996. 82. Gray, L.H., An Ionization Method for the Absolute Measurement of Gamma Ray Energy. Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 1936. 156(889): p. 578-596. 83. Attix, F.H., L.D. Vergne, and V.H. Ritz, Cavity Ionisation as a Function of Wall Material. Journal of Research of the National Bureau of Standards, 1958. 60(3): p. 235-243. 84. Spencer, L.V. and F.H. Attix, A Theory of Cavity Ionization. Radiation Research, 1955. 3(3): p. 239-254. 85. Lias, S.G., Ionization Energies of Gas-Phase Molecules, in Handbook of Chemistry and Physics. 2009, CRC Press. p. 10.206-10.221. 86. Phelps, A.V., Collisions of H+, H2(+), H3(+), ARH+, H-, H, and H2 with AR and of AR+ and ARH+ with H2 for Energies from 0.1 EV to 10 KEV. Journal of Physical and Chemical Reference Data, 1992. 21(4): p. 883-897. 87. Kobayashi, N., Low Energy Ion-Netural Reactions. III. Ar++NO, Kr++NO, Ar++O2, Kr++O2 and Ar++CO. Journal of the Physical Society of Japan, 1974. 36(1): p. 259-266. 88. Hasan, A.T., Single electron capture in slow collisions of H2+ ions with H2, O2 and N2 diatomic molecules. International Journal of Mass Spectrometry, 2005. 247(1–3): p. 81-84. 89. Willis, C. and A.W. Boyd, Excitation in the radiation chemistry of inorganic gases. International Journal for Radiation Physics and Chemistry, 1976. 8(1–2): p. 71-111.

259

Chapter 11 Bibliography 7131060

90. Tully, J.C., Reactions of O(1D) with atmospheric molecules. The Journal of Chemical Physics, 1975. 62(5): p. 1893-1898. 91. Presser, N. and R.J. Gordon, The kinetic isotope effect in the reaction of O(3P) with H2, D2, and HD. The Journal of Chemical Physics, 1985. 82(3): p. 1291-1297. 92. Lin, C.L. and M.T. Leu, Temperature and third-body dependence of the rate constant for the reaction O + O2 + M → O3 + M. International Journal of Chemical Kinetics, 1982. 14(4): p. 417-434. 93. Kurzius, S.C. and M. Boudart, Kinetics of the branching step in the hydrogen-oxygen reaction. Combustion and Flame, 1968. 12(5): p. 477-491. 94. Schulz, W.R. and D.J. Le Roy, Kinetics of the Reaction H+pH2 = oH2+H. The Journal of Chemical Physics, 1965. 42(11): p. 3869-3873. 95. Campbell, I.M. and C.N. Gray, Rate constants for O(3P) recombination and association with N(4S). Chemical Physics Letters, 1973. 18(4): p. 607-609. 96. Larkin, F.S. and B.A. Thrush, Recombination of hydrogen atoms in the presence of atmospheric gases. Discussions of the Faraday Society, 1964. 37(0): p. 112-117. 97. Orkin, V.L., et al., Rate constant for the reaction of OH with H2 between 200 and 480 K. Journal of Physical Chemistry A, 2006. 110(21): p. 6978-6985. 98. Lewis, R.S. and R.T. Watson, Temperature dependence of the reaction O(3P) + OH(2II) = O2 + H. The Journal of Physical Chemistry, 1980. 84(26): p. 3495-3503. 99. Michael, J.V., et al., Rate Constants For H + O2 + M → HO2 + M in Seven Bath Gases. The Journal of Physical Chemistry A, 2002. 106(21): p. 5297-5313. 100. Dobis, O. and S.W. Benson, Reaction of the ethyl radical with oxygen at millitorr pressures at 243-368 K and a study of the Cl + HO2, ethyl + HO2, and HO2 + HO2 reactions. Journal of the American Chemical Society, 1993. 115(19): p. 8798-8809. 101. Vaghjiani, G.L., A.R. Ravishankara, and N. Cohen, Reactions of hydroxyl and hydroxyl-d with hydrogen peroxide and hydrogen peroxide-d2. The Journal of Physical Chemistry, 1989. 93(23): p. 7833-7837. 102. Dubey, M.K., et al., Isotope specific kinetics of hydroxyl radical (OH) with water (H2O): Testing models of reactivity and atmospheric fractionation. Journal of Physical Chemistry A, 1997. 101(8): p. 1494-1500. 103. Anderson, A.R. and J.V.F. Best, Use of Ethylene as a Gas Phase Dosimeter for Accelerator Radiation. Nature, 1967. 216(5115): p. 576-577. 104. Housecroft, C.E. and A.G. Sharpe, Structures and Energetics of Metallic and Ionic Solids, in Inorganic Chemistry. 2008, Pearson Education Limited. p. 164-171.

260

Chapter 11 Bibliography 7131060

105. Rodriguez, J.A., et al., Properties of CeO2 and Ce1-xZrxO2 Nanoparticles: X-ray Absorption Near-Edge Spectroscopy, Density Functional, and Time-Resolved X-ray Diffraction Studies. The Journal of Physical Chemistry B, 2003. 107(15): p. 3535-3543. 106. Teufer, G., The crystal structure of tetragonal ZrO2. Acta Crystallographica, 1962. 15(11): p. 1187. 107. Lane, C.D., et al., Site-dependent electron-stimulated reactions in water films on TiO2(110). The Journal of Chemical Physics, 2007. 127(22): p. 224706. 108. Reed, D.T. and R.A. Van Konynenburg, Effect of ionizing radiation on moist air systems, 1987. 109. Karasawa, H., et al., Radiation Induced Decomposition of Nitrogen. Radiation Physics and Chemistry, 1991. 37(2): p. 193-197. 110. Tokunaga, O., et al., Radiation Treatment of Exhaust Gases 4 Oxidation of NO in Moist Mixtures of O2 and N2. Radiation Physics and Chemistry, 1978. 11(3): p. 117-122. 111. Morris, E.D. and H. Niki, Reaction of dinitrogen pentoxide with water. The Journal of Physical Chemistry, 1973. 77(16): p. 1929-1932. 112. Rodriguez, J.A., et al., Chemistry of NO2 on CeO2 and MgO: Experimental and theoretical studies on the formation of NO3. Journal of Chemical Physics, 2000. 112(22): p. 9929-9939. 113. Nanayakkara, C.E., W.A. Larish, and V.H. Grassian, Titanium Dioxide Nanoparticle Surface Reactivity with Atmospheric Gases, CO2, SO2, and NO2: Roles of Surface Hydroxyl Groups and Adsorbed Water in the Formation and Stability of Adsorbed Products. The Journal of Physical Chemistry C, 2014. 118(40): p. 23011-23021. 114. Hadjiivanov, K., et al., Infrared spectroscopy study of the species arising during nitrogen dioxide adsorption on titania (anatase). Langmuir, 1994. 10(2): p. 464-471. 115. Haubrich, J., et al., In Situ Ambient Pressure Studies of the Chemistry of NO2 and Water on Rutile TiO2(110). Langmuir, 2010. 26(4): p. 2445-2451. 116. Sorescu, D.C., C.N. Rusu, and J.T. Yates, Adsorption of NO on the TiO2(110) surface: An experimental and theoretical study. Journal of Physical Chemistry B, 2000. 104(18): p. 4408- 4417. 117. Hadjiivanov, K.I., D.G. Klissurski, and V.P. Bushev, IR spectroscopic study of NO2 adsorption on chromia. Journal of the Chemical Society, Faraday Transactions, 1995. 91(1): p. 149-153. 118. Ansari, S.A., et al., Band gap engineering of CeO2 nanostructure using an electrochemically active biofilm for visible light applications. Rsc Advances, 2014. 4(32): p. 16782-16791. 119. Saliba, N.A., H. Yang, and B.J. Finlayson-Pitts, Reaction of Gaseous Nitric Oxide with Nitric Acid on Silica Surfaces in the Presence of Water at Room Temperature. The Journal of Physical Chemistry A, 2001. 105(45): p. 10339-10346.

261

Chapter 11 Bibliography 7131060

120. De Almeida, L., et al., Insights into the Thermal Decomposition of Lanthanide(III) and Actinide(III) Oxalates – from Neodymium and Cerium to Plutonium. European Journal of Inorganic Chemistry, 2012. 2012(31): p. 4986-4999. 121. Altaş, Y. and H. Tel, Structural and thermal investigations on cerium oxalate and derived oxide powders for the preparation of (Th,Ce)O2 pellets. Journal of Nuclear Materials, 2001. 298(3): p. 316-320. 122. Moosath, S.S., J. Abraham, and T.V. Swaminathan, Thermal Decomposition of Rare Earth Metal Oxalates 4 Oxalates of Cerium and Thorium. Zeitschrift Fur Anorganische Und Allgemeine Chemie, 1963. 324(1-2): p. 103-105. 123. Hirschfelder, J.O. and H.S. Taylor, The Alpha-Particle Reactions in Carbon Monoxide, Oxygen and Carbon Dioxide Systems. The Journal of Chemical Physics, 1938. 6(12): p. 783-790. 124. Arin, L.M. and P. Warneck, Reaction of ozone with carbon monoxide. The Journal of Physical Chemistry, 1972. 76(11): p. 1514-1516. 125. Harteck, P. and S. Dondes, Decomposition of Carbon Dioxide by Ionizing Radiation. Part I. The Journal of Chemical Physics, 1955. 23(5): p. 902-908. 126. Jennings, S.G., The mean free path in air. Journal of Aerosol Science, 1988. 19(2): p. 159-166. 127. Foy, B.R. and S.A. Joyce, Gas-phase Radiolysis in Plutonium Dioxide Powder, 2008, Los Alamos National Laboratory. 128. Lane, C.D., et al., Electron-stimulated oxidation of thin water films adsorbed on TiO2(110). Journal of Physical Chemistry C, 2007. 111(44): p. 16319-16329.

262