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Master thesis UHasselt - Niels Palmans - Adsorption_desorption_behaviour_of_CO2_and_H2O_on_ThO2_powders - 2019-2020.pdf

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Rémi Delville

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Confidentiality statement The information contained in this document is only for the information of the intended recipient. It may not be used, modified, published or redistributed without the prior explicit written consent of SCK CEN. In all circumstances, SCK CEN staff and external workers shall handle this document in accordance with the policies and security controls described in the 'information protection policy' (197-POL-001). Deze masterproef werd geschreven tijdens de COVID-19 crisis in 2020. Deze wereldwijde gezondheidscrisis heeft mogelijk een impact gehad op de opdracht, de onderzoekshandelingen en de onderzoeksresultaten. SCK CEN/39400143 Rev. 1.0

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II Acknowledgments Firstly, I would like to thank SCK CEN for allowing me to do research with such advanced measurement equipment. The team of scientists that was active in lab 46, where the largest majority of the practical work was performed, went out of their way to make sure I had always access to the necessary equipment, which was greatly appreciated.

Subsequently, many people made a significant contribution to this thesis, without whom this work would not have been possible. Most notably, I would like to thank my external promotor Dr. Beatriz Acevedo Muñoz, who provided the main guidance and was the most closely involved in the research subject. As time was of the essence, she provided a detailed day-by-day planning for me to structure the experimental work. Furthermore, she provided me with an abundance of literature sources to aid me in my literature review and get me worked in in the subject. She was always ready to assist me in any practical work and showed great patience when some clumsiness got the better of me. Even when the planning took a heavy blow after the Corona measures were set in place, she performed a multitude of experiments in my place, assuring experimental data for all samples. Without her, there would have been no experimental data of the 2-step alkali precipitated thoria powder. Additionally, she took the time to provide me with important advice and remarks concerning the dissertation which raised the overall quality to a higher standard. Being grateful would therefore be an understatement.

Furthermore, I am very grateful for the guidance Dr. Rémi Delville provided for me. He made sure regular reality checks were performed during my time at SCK CEN so that I did not get overwhelmed and stayed focussed on the right tasks. Additionally, his remarks on the dissertation during the writing process taught me an invaluable lesson about efficiency, focus, and relevancy.

I would also like to thank Prof. Dr. Sonja Schreurs, who provided important insight into the general structure of the thesis and valuable remarks on the subjects discussed. Her advice also allowed me to greatly improve the quality of the thesis by addressing some key issues in my early writing style.

Next, I would like to thank Dr. Marc Verwerft for helping me keeping a structured approach to my work amidst pandemic chaos. Weekly progress reports made sure I kept track of my progress while planning new tasks ahead of time and spreading the workload evenly.

I also greatly appreciate the assistance I received from Koen Vanaken and Peter Dries in the lab environment. When I had any questions or doubts during my time working in the nuclear labs, Koen and Peter would often be present to assist me whenever necessary. Koen also provided me with a detailed explanation about the practical safety guidelines within the lab so that I would always be working safely with the equipment.

In general, I would like to thank the other members of the FMA group for making my rather limited time at SCK CEN a very pleasant experience. The work environment was very welcoming and I felt at home quickly.

On a more personal note, I would like to thank my family for the perpetual moral support. I would also like to thank my closest friends, Marijn Roothooft, Vincenth Willems and Mattias Simons for their incredible support and encouragement during the COVID-19 quarantine (and subsequently the social isolation). Regular electronic meet-ups assured I stayed focussed and motivated throughout this difficult time. SCK CEN/39400143 Rev. 1.0

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IV Influence of COVID-19 pandemic The research described in this thesis work was performed during the 2020 COVID-19 pandemic. As measures imposed by the Belgian government were tightened, I was no longer legally allowed to continue performing experiments at SCK CEN myself due to the abrupt suspension of internships starting on the 16th of March. Therefore, the experimental planning was interrupted and some compromises had to be made. Most notably, as the access to the labs was limited, repetitions of experiments with some questionable results were difficult and the overall amount of experiments had to be scaled down. The latter mostly affected the ThO_TSA experiments as only three different adsorption times mere measured once. Furthermore, seeing how the cycle of reuse of a sample influenced the adsorption capacity, I would have liked to repeat a few experiments with a freshly calcined oxalate sample. However, this was not feasible knowing the time such an experiment takes. Additionally, as internships were suspended, my external promotor Beatriz Acevedo Muñoz performed some of the experimental work in my place. To keep the workload in check, the number of experiments was decreased. Additionally, the workflow was severely interrupted due to organisational difficulties following the sudden necessity of electronic media for communication. That being said, I firmly believe this research has a valid role to play in the study of adsorption of atmospheric gases to thoria powders. SCK CEN/39400143 Rev. 1.0

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VI Table of Contents List of Tables ...... IX List of figures ...... XI Abstract (EN) ...... XIII Abstract (NL) ...... XV 1. Introduction ...... 17 1.1 A brief history ...... 17 1.2 Benefits of thorium-based fuel ...... 18 1.3 Problem statement ...... 20 2. Objectives ...... 23 3. Theoretical background ...... 25 3.1 Thorium fuel cycle ...... 25 3.2 Thorium-based fuel manufacturing: the front-end fuel cycle ...... 26 3.2.1 Mining ...... 27 3.2.2 Purification ...... 27 3.2.3 Enrichment ...... 28 3.2.4 Pellet fabrication...... 28 3.2.5 Scrap recycling and bundle assembly ...... 29 3.3 Adsorption theory ...... 29 3.3.1 Fundamentals of adsorption theory ...... 30 3.3.2 Derivation of Henry’s and Freundlichs adsorption isotherm ...... 32 3.3.3 The Langmuir adsorption isotherm ...... 33 3.4 Applications for adsorption techniques...... 34 3.4.1 The use of adsorption isotherms ...... 34 3.4.2 The meaning of adsorption hysteresis ...... 35 3.4.3 The BET model...... 37 3.4.4 Calculation of pore volume ...... 39

3.5 State-of-the-art on H 2O and CO2 adsorption on metal oxide surfaces ...... 39 4. Methods and materials ...... 43 4.1 Materials ...... 43 4.1.1 Raw materials ...... 43 4.1.2 Measurement instruments ...... 43 4.2 Methodology ...... 46 4.2.1 Overview of experimental workflow ...... 46 4.2.2 Safety guidelines for working in a nuclear environment ...... 47 4.2.3 Calcination ...... 47 SCK CEN/39400143 Rev. 1.0

VII 4.2.4 Measuring N 2 adsorption isotherm, BET area and pore volume ...... 48 4.2.5 Sorption experiments ...... 49 5. Results and discussion ...... 51 5.1 Textural properties ...... 51

5.1.1 N2 adsorption isotherms ...... 51 5.1.2 Surface and pore analysis ...... 52 5.1.3 SEM/TEM images ...... 53 5.2 Oxalate precipitated thoria powder ...... 55

5.2.1 Influence of exposure time on adsorption of CO2 and H 2O ...... 55

5.2.2 Influence of H 2O sorption on CO2 sorption ...... 57 5.3 2-step alkali precipitated thoria powder ...... 59

5.3.1 Influence of exposure time on adsorption of CO2 and H 2O ...... 59

5.3.2 Influence of H 2O sorption on the sorption of CO2 ...... 61 5.4 Influence of precipitation strategy on sorption behaviour ...... 63 5.4.1 Influence of precipitation strategy on adsorption behaviour ...... 64 5.4.2 Influence precipitation strategy on adsorption capacity ...... 65 5.4.3 Proposition for possible adsorption mechanism on ThO_Ox and ThO_TSA ...... 66 6. Conclusion ...... 71 7. Outlook ...... 73 8. References ...... 75 Appendix I ...... 79 Appendix II...... 81 Appendix III...... 83

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VIII List of Tables Table 1: Critical and binding energy of multiple fissile and fertile isotopes...... 19 Table 2: Parameters for the fission neutron multiplicity equation...... 20 Table 3: Fission, capture and absorption cross-sections for different nuclides respectively...... 20 Table 4: Estimated World thorium resources [17] ...... 27 Table 5: Masses of samples used in TriStar II ...... 48 Table 6: Overview of all sorption experiments in chronological order. The colours are representative for the same source materials and thus the same samples...... 50 Table 7: Summary of the results of the surface analysis...... 52 Table 8: The total relative mass that was measured to desorb from both samples for different adsorption times...... 65 SCK CEN/39400143 Rev. 1.0

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X List of figures Figure 1: Illustration of the breeding process converting fertile 232Th into fissile 233U via neutron capture. Each fission reaction produces 2-3 new neutrons (indicated by the partial spheres) which can either fission a new uranium atom or convert another thorium atom [4]...... 25 Figure 2: (a) the unsaturated bonds at the surface and the saturated bonds in the bulk materials, (b) the a simplified diagram of the physisorption process, (c) a chemisorbed system [30]...... 30 Figure 3: Simplified diagram of monolayer adsorption (left) mostly associated with chemical adsorption and multilayer adsorption (right) mostly associated with physical adsorption [30]...... 31 Figure 4: Potential energy diagram for non-activated dissociative chemisorption [30]...... 31 Figure 5: Depiction of the specific binding sites as proposed by Langmuir...... 33 Figure 6: Classification of physisorption isotherms as proposed by the IUPAC technical report of 2015 [37] ...... 35 Figure 7: Classification of hysteresis loops as proposed by IUPAC technical report of 2015 [37] ...... 36 Figure 8: BET adsorption isotherm and determination of constants ...... 38 Figure 9: Surface structure of ideal face-centered cubic single crystals and their corresponding numbers [63]...... 40

Figure 10: Schematic representation of water layers adsorbed on the surface of TiO2 as proposed by Chung-Yi Wu et al. [52]...... 41

Figure 11: Schematic representation of surface species for the coadsorption of water and CO2 on oxide surfaces. In this example, M represented Ti or Ce [64] ...... 42 Figure 12: The mass spectrometer (red) connected to the TGA (grey-blue) via a tube that was heated up to 150°C to decrease condensation of gases during transport from TGA to MS. On the right side, the balance present in the top side of the TGA is exposed. The laser and balance are visible...... 44 Figure 13: Schematic representation of most important components of a generic volumetric physical adsorption analyser in its most elementary form [65]...... 45 Figure 14: Schematic representation of methodology ...... 46 Figure 15: Evolution of temperature with time during the calcination process...... 48 Figure 16: Picture of the Micromeritics Tristar II taken from the official website ...... 48 Figure 17: Evolution of temperature with time during the desorption process...... 50

Figure 18: N 2 isotherm for ThO_Ox and ThO_TSA sample supplemented with a magnification of the first part of the isotherm to reveal the inflection point...... 51 Figure 19: SEM pictures of ThO_Ox provided by FMA. The non-porous plate-like particles are clearly visible and clump together. Mesopore sized cavities are also visible in between the platelets...... 54 Figure 20: TEM images of uncalcined 2-step alkali precipitate powder...... 54

Figure 21: CO2 and H 2O desorption spectra (a and b), the relative mass loss (c) and the derivative of the relative mass loss (d) for different adsorption times for ThO_Ox...... 56 Figure 22: Comparison between 1 week natural – and forced adsorption experiments on ThO_Ox in

terms of MS desorption spectra for CO2 and H 2O (a and b). (c) presents the relative mass loss of the powders normalized by initial weight of the sample and (d) presents the derivative of the presented mass loss to highlight the mass loss behaviour...... 58

Figure 23: Results of sorption experiments on ThO_TSA for different adsorption times, CO2

desorption spectrum (a), H 2O desorption spectrum (b), the relative mass loss (c) and the derivative of the mass loss (d)...... 60 Figure 24: Comparison between 1 week natural – and forced adsorption experiments on ThO_TSA in

terms of MS desorption spectra for CO2 and H 2O (a and b). (c) presents the relative mass loss of the powders normalized by initial weight of the sample and (d) presents the derivative of the presented mass loss to highlight the mass loss behaviour...... 62 SCK CEN/39400143 Rev. 1.0

XI Figure 25: Comparison of 1 week natural adsorption between ThO_Ox and ThO_TSA in terms of CO2

desorption (a), H 2O desorption (b), relative mass loss (c) and the derivative of the mass loss (d)...... 63

Figure 26: A suggested CO2 multilayer adsorption mechanism for a pure (100) exposed crystal face on ThO_Ox. Red dotted lines present the hydrogen bonds. Note that each adsorbed watermolecule has 4 hydrogen bonds, restricting further water adsorption by means of hydrogen bonding. The partial charges are also shown...... 67 Figure 27: Schematic representation of the (111) (left) and (100) (right) faces of a face centered cubic crystal. A clear difference can be observed in atom density. A (111) surface is more closely packed togheter as compared to (100) [66]...... 68 Figure 28: (repetition of figure 6) Classification of physisorption isotherms as proposed by the IUPAC technical report of 2015 [37]...... 83

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XII Abstract (EN) Thorium dioxide has recently regained interest as an alternative to uranium-based fuels due to its less long-lived transuranium element production or its higher natural abundance, among others. Though,

ThO2 pellet production faces many challenges such as reaching high theoretical densities. This can be improved by sintering the pellets, but this treatment is altered by many factors. In fact, a release of adsorbed gases was observed which can negatively change the grain growth. This thesis presents a

systematic study on the adsorption of CO2 and H 2O on two types of ThO2 powders reproducing storage conditions, one derived from thorium oxalate (ThO_OX) and one from a novel 2-step alkali

precipitation route (ThO_TSA). Both materials were characterized by means of N 2 adsorption isotherm

at 77K, SEM and TEM. Moreover, CO2 and H 2O desorption was experimentally studied by TGA coupled to mass spectrometry. Surface analysis indicates that both materials are non-rigid aggregates with mesoporosity attributed to interparticular cavities which agrees with the SEM and TEM images. However, SEM/TEM also shows that the particle shapes differ significantly, being plate-like for

ThO_Ox, and sphere-like for ThO_TSA. The powders retained CO2 adsorbed well above 600°C, and it

seems the adsorbed water has a significant effect on the CO2 adsorption. ThO_TSA reached the saturation point earlier than ThO_Ox, suggesting a lower gas adsorption capacity. Therefore, the 2-

step alkali precipitation route is recommended for long ThO2 powder storage periods. SCK CEN/39400143 Rev. 1.0

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XIV Abstract (NL) Thoriumdioxide heeft onlangs weer interesse gewekt als alternatief voor reguliere splijtstof door minder lang levende transuranen productie of hogere nautuurlijke abundantie. Een uitdaging voor

ThO2 pellet productie is echter voldoende hoge theoretische dichtheden halen. Dit wordt o.a. bereikt m.b.v. pellet sintering wat beïnvloed wordt door vele factoren. Zo werd een vrijgave van geadsorbeerde gassen waargenomen, wat de korrelgroei negatief beïnvloedt. Dit proefschrift

presenteert een systematisch onderzoek naar de sorptie van atmosferische gassen, CO2 en H 2O, op

twee soorten gecalcineerde ThO2 poeders om opslag condities te simuleren, één afgeleid van thoriumoxalaat (ThO_OX) en één afgeleid van een nieuwe 2-staps alkalische precipitatie (ThO_TSA).

Beide poeders werden gekarakteriseerd m.b.v. N 2-adsorptie-isotherm bij 77K, SEM en TEM. Bovendien

werd CO2 en H 2O adsorptie experimenteel bestudeerd m.b.v TGA en MS. Oppervlakte-analyse toonde dat beide materialen niet-stijve aggregaten zijn met mesoporositeit door de interparticulaire holtes. Dit werd bevestigd door de SEM- en TEM-afbeeldingen. SEM/TEM toont echter ook dat de vorm van de deeltjes aanzienlijk verschilt: plaatachtig voor ThO_Ox en bolachtig voor ThO_TSA. Er werd

waargenomen dat ThO_Ox een fractie van het geadsorbeerde CO2 vasthield tot ruim boven 600°C. Verder bleek ThO_TSA eerder een saturatiepunt te bereiken dan ThO_Ox, waardoor ThO_Ox een groter adsorptievermogen heeft. Daarom wordt ThO_TSA aanbevolen voor lange opslagperioden om gasadsorptie te minimaliseren. SCK CEN/39400143 Rev. 1.0

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XVI 1. Introduction The SCK CEN is one of the largest research centres of Belgium and is situated in Mol. SCK CEN focuses on peaceful applications of nuclear technology and currently counts over 800 employees. Multiple projects and challenges are divided over a wide arrange of research groups present at SCK CEN. One of these groups is the Fuel Material group (FMA) which belongs to the Nuclear Materials Science institute. Their focus lies in the study and improvements of nuclear fuel materials. The same type of nuclear fuel, uranium dioxide, has been mainly used in nuclear reactors since the 1950s and research still aims at improving the fuel performance by studying how the chemical and/or physical compositions of the fuel pellets influence the fuel behaviour.

Recently, thorium-based fuels have regained interest due to the many advantages over regular fuel matrices [1]. However, thorium dioxide (also referred to as thoria) as an alternative for regular uranium-based fuels faces many challenging issues and therefore, it is an ongoing study. Thorium- based fuel material production processes still need to be improved in order to achieve the required specifications. This research aims to get a better understanding about the processes that happen during the last thermal treatment on the pellet production process called sintering, of the thorium oxide fuel material in order to improve it. In particular, this thesis addresses the unresolved question about the apparent differences in gas adsorption/desorption properties during the sintering of

thorium dioxide powders produced via different precipitation routes. Adsorbed CO2 and H 2O gases on thorium dioxide powders could negatively influence the success of the sintering phase by hampering the grain growth In this chapter, an overview of the most prevalent advantages of thorium-based fuels and a description of the current problems that thorium fuels face related to recent research aspects is presented.

This dissertation has been organized in different chapters. Chapter 1 shows an overview of the most prevalent advantages and a description of the current problems of thorium-based fuels. Chapter 2 describes the objectives of this study in more detail. Chapter 3 gives a theoretical background on relevant research topics for this study. This includes the thorium fuel cycle, which generally describes how thorium is used in the reactor environment and what the possibilities are in current reactor configurations. In particular, a closer look at the front-end fuel cycle of thorium-based fuels is taken. This describes the general manufacturing process of thorium-based fuels. The emphasis lies on some of the precipitation routes for the powder synthesis and the pelletization process as these steps are the most closely related to this thesis. Moreover, the fundamentals of adsorption mechanics are also discussed. A description of general adsorption theory and its foundation is presented, followed by some of its applications used during this research. Lastly, a review on the current state of knowledge concerning gas adsorption to calcined thorium powders is presented. Chapter 4 presents the materials and methods and is divided into two major parts. Firstly, a complete description of the materials and measurement instruments that were used during this thesis is given, followed by the methodology of the performed experiments. Chapter 5 presents the results of the experimental work and contains the discussion. Chapter 6 summarizes the major findings of this research project. The thesis ends with an outlook on future work in chapter 7.

1.1 A brief history In 1828, thorium was chemically isolated for the first time by the Swedish chemist Jons Berzelius. The unknown element was named after the Norse god of thunder Thor. 70 years later, Marie Curie discovered that thorium was naturally radioactive. Subsequently, Ernest Rutherford researched the element and its decay products. He found that thorium was very stable and existed on earth for more than four billion years, effectively categorizing it as a primordial isotope [2]. SCK CEN/39400143 Rev. 1.0

17 In modern nuclear reactors, uranium-based fuel is globally the most common type of fuel. Originally, thorium-based reactors have been in development as early as the 1950s and were being developed in parallel with its uranium counterpart. Thorium was proven to be a valid candidate for future nuclear fuel and many experiments were successfully carried out over the next 20 years. During this time, multiple reactor configurations were tested. This includes molten salt reactors, pressurized water reactors and high-temperature gas reactors [2]. However, Moir and Teller [3] wrote that the competition came down to a liquid metal fast breeder reactor (LMFBR) which operated on the U-Pu cycle and a thermal reactor that operated on the Th-233U cycle. The latter is also known as the molten salt breeder reactor and is the most promising short-term application in which thorium-based fuel can easily be implemented. However, due to fast reactors having more neutrons per fission while losing fewer neutrons to capture interactions, the larger breeding rate of the LMFBR caused the U-Pu fuel cycle to come out ahead. Additionally, uranium would appear to be more abundant than originally thought and the electricity growth rate was smaller than first predicted [3]. This originally halted the development of the molten salt breeder reactors and accelerated the development of the modern nuclear reactor. The molten salt type reactors would also pose various challenges that prohibit its further development. The highly corrosive nature of the salt poses many challenges to material engineering. As Moir wrote in his 2008 recommendation on the restart for molten salt reactor development: “ Once a program has been killed there is a stigma attached that creates a legacy of its own”[1].

However, recent renewed interest in thorium technology has served as motivation for the restart of the molten salt reactor development. Although thorium can be used in multiple reactor types, the molten salt reactors reactor type is the most promising for short-term applications due to their inherent properties concerning safety [2]. The U.S. Department of Energy’s Nuclear Energy Research Advisory Committee identified the molten salt reactor as one of the six Generation IV reactor types [4]. This technology currently puts heavy strain on material engineering as the highly corrosive nature of the molten salt significantly shortens the lifespan of in reactor components. Furthermore, due to the presence of a liquid core as oppose to solid cores in other reactor types, the neutron kinetics inside the core are hard to model. Delayed neutrons can be born anywhere where the fluid core resides. Since delayed neutrons are the foundation for predicting and controlling reactor behaviour, this problem is not to be taken lightly. 1.2 Benefits of thorium-based fuel Although research on thorium-based fuel slowly decreased during the Cold War, it has recently gained new attention due to its many advantages in, among others, safety over uranium-based fuels. These advantages include a higher melting point than uranium, better chemical stability and improved radiation resistance. These factors result in a lower fission product release rate that is one order of

magnitude lower for ThO2-based fuels as compared to UO2-based fuels. Furthermore, thoria has better

thermal conductivity and a lower thermal expansion coefficient. Therefore, ThO2-based fuels are expected to have better in-pile performance. Additionally, due to the relatively inert properties of

ThO2, it does not oxidize to the same extent as UO2 which oxidizes easily toU 3O8 and UO3. This property

facilitates long term interim storage and permanent disposal in repositories for spent ThO2-based fuel [5].

Another important benefit to consider the use of thoria for reactor fuels is the fact that thorium is 3 to 4 times more abundant in the earth’s crust than uranium, causing natural reserves to last longer [2], [6]. The most popular source for thorium currently is monazite, a mixed thorium rare earth uranium phosphate that can be found in many countries in sand deposits around beaches and rivers. Until now, almost all thorium production is a direct by-product of rare-earth extraction from monazite sands. SCK CEN/39400143 Rev. 1.0

18 Mining and extracting thorium from these monazite sands is relatively easy and differs heavily from the mining process of uranium ores. The impact of the mining site on the direct environment is also much smaller than the mining activities necessary to extract uranium ore [5].

Another advantage stems directly from the nuclear properties of 232Th. Firstly, the isotope is fertile, not fissile. This means that it is not easily fissioned when struck by neutrons. Table 1 shows the critical energy and the binding energy for different isotopes commonly regarded as nuclear fuel. The critical energy is the minimum energy necessary to add to the nucleus in order to induce a fission reaction. The binding energy is the energy added when an external neutron with no additional kinetic energy is captured by the nucleus. If the binding energy is larger than the critical energy, an incoming neutron will always induce a fission reaction regardless of its kinetic energy. Conversely, if the binding energy is lower than the critical energy, the incoming neutron needs extra kinetic energy to induce fission. In the case of 232Th, a neutron would need at least 0.8 MeV kinetic energy to cause the thorium nucleus to fission. However, 0.8 MeV greatly exceeds the neutron energy present in the thermal neutron spectrum (~0.025 eV). This means thorium is not fissile in the thermal neutron spectrum.

Table 1: Critical and binding energy of multiple fissile and fertile isotopes.

Nucleus Z²/A Ecritical (MeV) EB (MeV) Th_232 34.9 5.9 5.1 U_238 35.6 5.9 4.9 U_235 36.0 5.8 6.4 U_233 36.4 5.5 6.6 Pu_239 37.0 5.5 6.4

Furthermore, Due to the increased cross-section for thermal neutrons of 232Th, thorium is a better fertile material than 238U in thermal neutron energy range and is subsequently more efficiently bred into fissile 233U as compared to the rather limited breeding of 238U into 239Pu in current fuel matrices. Hence, a higher conversion to 233U is possible with 232Th than with 238U to 239Pu. Appendix 1 shows the absorption cross-sections for some important isotopes in nuclear engineering [7]. The absorption cross-section for thermal neutrons of 232Th is 5.13 barns while the absorption cross-section of 238U is almost half that amount at 2.73 barns. Note that these values are given for thermal neutron energies.

Furthermore, 233U liberates more neutrons per absorbed neutron during fission over a wide range of the thermal neutron spectrum as compared to 235U and 239Pu. This means 233U has a higher fission neutron multiplicity ν which can be calculated by the following equation:

(1) ɋൌ ɋ଴ ൅൉ܧ

Table 2 shows the values of ν 0 and A for different nuclides. E represents the energy of the incoming neutron and A is a constant. Note that the fission neutron multiplicity does not take into account the delayed neutrons that might be produced later during the chain reaction. The multiplicity refers only to prompt neutron release.

The different types of cross-sections for the most common nuclides used in nuclear fuel are shown in table 3. The fact that the capture cross-section of 233U (48 barns) is much smaller than that of 235U (99 barns) and 239Pu (269 barns) for thermal neutrons facilitates recycling of 233U from a reactivity point of SCK CEN/39400143 Rev. 1.0

19 view as non-fissile absorptions that lead to higher isotopes (such as 234U, 236U and 240Pu) are much less likely using thorium [5].

Table 3: Parameters for the fission neutron multiplicity Table 2: Fission, capture and absorption cross-sections for equation. different nuclides respectively. -1 Isotope ν0 A (MeV ) Validity range Nucleus Vf (barns) Vc (barns) Va (barns) α (-) Th_232 1.87 0.164 all E U_233 531 48 579 0.09 2.48 0.075 E ≤ 1 MeV U_235 582 99 681 0.17 U_233 2.41 0.136 E > 1 MeV Pu_239 743 269 1012 0.362 2.43 0.065 E ≤ 1 MeV U_238 2.7 2.7 U_235 2.35 0.150 E > 1 MeV U_nat 4.2 3.4 7.6 0.81 U_238 2.3 0.160 all E 2.87 0.148 E ≤ 1 MeV Pu_239 2.91 0.133 E > 1 MeV

Another advantage of thorium is that due to the higher conversion to fissile 233U, the thorium fuel cycle produces less radiotoxic waste as compared to the uranium fuel cycle. In particular, less long-lived transuranium elements are formed causing a decrease in the production of high-risk radioactive waste [8]. This significantly decreases the burden on countless future generations to deal with radioactive waste produced during present reactor operation. An added benefit of thorium-based fuels is the intrinsic proliferation-resistance due to the formation of 232U, 233Pa and 233U [2], [9].

Only a few benefits of using thorium in nuclear reactors were highlighted, a more detailed discussion on the advantages of thorium goes beyond the scope of this paper. The review report “Thorium fuel cycle — Potential benefits and challenges” published by IAEA in 2005 a detailed overview of this subject [5]. 1.3 Problem statement The 2005 publication of IAEA “Thorium fuel cycle-Potential benefits and challenges” also presents an extensive list of the different challenges that the use of thorium faces as nuclear fuel faces [5]. One important challenge that concerns this thesis is the manufacturing difficulties of thorium-based fuel pellets. The properties of thoria that are considered to be advantageous in the reactor environment present difficulties during the manufacturing processes. In particular, the higher thermal stability of thoria increases operation safety margins. This in turn introduces additional costs in the pellet sintering phase as the sintering temperature necessary to achieve the required density exceeds the temperatures than can be reached by commercial furnaces [10].

As the front-end of the nuclear fuel cycle for uranium is similar to that of the thorium fuel cycle, the fabrication steps of uranium-based fuels can serve as a starting point from which the thorium fabrication can be considered. A look at the different steps in the this part of the fuel cycle allows to situate the research topic described in this dissertation in the overall fuel cycle. The front-end uranium fuel cycle can be divided into four discrete steps [11]:

1. Mining of raw materials;

2. Chemical purification and conversion to UF6; 3. Enrichment to increase the concentration of fissile uranium; 4. Fuel fabrication by physically transforming the uranium dioxide powders into pellets.

As this thesis handles the desorption of gases during sintering, the 4 th step is most closely related to this research. SCK CEN/39400143 Rev. 1.0

20 One major difference in the general structure of the fabrication process is that thorium does not need an enrichment process. The different steps will be discussed further in section 3. An important subject for current research concerning the manufacturing of thorium-based fuels is the pelletization process. The standards for the theoretical density (TD) of these pellets imposed by the nuclear industry amounts to 95-97% and is a measure for the porosity of the fuel pellet [12]. This can be reached by closing the porosity that remains from the calcination process and is done through sintering. The main reasons a high TD is desirable are the increased thermal conduction of the pellet, improved fission gas retention and the higher concentration of fissile material. Note that a few percents of porosity is desirable as the pores serve as sinks for fission gases and slow down fission gas release [13]. Thermal diffusion treatment parameters such as heating rate, temperature, sintering gas, sintering additives,

the size and shape of the agglomerates, porosity and surface area of ThO2 have already been studied to improve the sintering behaviour which in turn determines the success of the required density.

Currently, a TD of ~98% was reached [10]. ThO2 powder is known to have a high affinity for H 2O and

CO2 [14]. During the sintering process, the release of adsorbed gases at high temperatures has been observed. This can have a significant influence on the success of the sintering by hindering grain growth. Therefore, gas adsorption to the calcined thoria powders should be minimized. These gases originate from the initial adsorption of atmospheric gases during the storage period of the powder before the sintering process. There is currently a lack of knowledge regarding the exact influence of this phenomenon and more research was proposed [10]. Additionally, the sorption mechanisms of atmospheric gases are not yet well understood and these might proof useful to reduce adsorption on the calcined thoria powders. A more systematic approach to this problem in which thorium oxide

powders are exposed to CO2 and H 2O for a variable amount of time would allow more insight in this issue and can help to uncover the underlying sorption mechanisms. SCK CEN/39400143 Rev. 1.0

21 SCK CEN/39400143 Rev. 1.0

22 2. Objectives The main goal is to get a better understanding of the influence of the precipitation strategy on the

adsorption/desorption behaviour of atmospheric gases, particularly CO2 and H 2O, on thoria powders. The desorption of atmospheric gases during sintering can be an important parameter that influences the quality and success rate of the sintering process. In particular, the aim is to resolve the question about the apparent differences in gas adsorption/desorption properties during the sintering of thorium dioxide powders produced from thorium oxalate precipitation and 2-step alkali precipitation. In order to assess it, , the following sub-goals are proposed.

Firstly, surface area and porosity will be studied to provide more insight into the textural properties of the powders. Although both thoria powders are chemically identical, the material properties differ as a result of the precipitation strategy. The BET area, pore volume and powder morphology will be assessed during this first phase.

Secondly, to study the sorption behaviour of CO2 and H2O on thoria surfaces, we will assess the

influence of water adsorption of the adsorption of CO2.

Lastly, it is expected that longer storage periods, which in terms of experimental setup translates to higher adsorption times, correspond to higher quantities of adsorbed gases. A lower amount of adsorbed gases is preferable for the sintering phase. Therefore, the influence of the exposure time on the quantity of adsorbed gas is studied.

Utilizing these three sub-goals, the main goal is the following: Attempt to describe the general

adsorption/desorption mechanisms on both ThO2 powders. A recommendation is then made on which precipitation strategy produces thoria powder that is most satisfactory for minimizing atmospheric gas adsorption and is therefore most suitable for long term storage. SCK CEN/39400143 Rev. 1.0

23 SCK CEN/39400143 Rev. 1.0

24 3. Theoretical background This chapter includes a description of the general thorium fuel cycle, as well as an overview of the front-end of the thorium fuel cycle. As this thesis studies the adsorption characteristics of specific thoria powders, a description of general adsorption theory will be given followed by the most relevant applications of the adsorption phenomenon for this research. This includes the 2015 IUPAC classification of isotherm types and hysteresis shapes, the BET method and assessment of porosity of materials. Lastly, an overview of the current state of the knowledge concerning the adsorption of atmospheric gases to metal oxide surfaces (focusing on thorium dioxide) will be presented. 3.1 Thorium fuel cycle An important property of 232Th is that it is not fissile in the thermal spectrum like 238U or 235U. 232Th is a fertile material, meaning that as mentioned earlier, it first has to be converted into fissile 233U via neutron irradiation processes to be used for energy production. This breeding process is presented in eq. (2).

ଶଷଶ ଶଷଷ ଶଶ ௠௜௡ ଶଷଷ ଶ଺ǡଽ ௗ ଶଷଷ (2) ܷ ሺ݊ǡ Jሻ݄ܶ ሱۛۛۛሮ ܲܽ ሱۛۛሮ݄ܶ An illustration of the breeding process converting fertile 232Th into fissile 233U via neutron capture is depicted in figure 1. Each fission reaction produces 2-3 new neutrons (indicated by the partial spheres) which can either fission a new uranium atom or convert another thorium atom [4].This conversion process does require an external neutron source to initialize the reaction. This can be accomplished by adding fissile uranium or plutonium isotopes to the fuel matrix. The bred 233U of a previous cycle is also a valid option to initialise the reaction as non-fission thermal neutron captures are half as likely to

Figure 1: Illustration of the breeding process converting fertile 232Th into fissile 233U via neutron capture. Each fission reaction produces 2 -3 new neutrons (indicated by the partial spheres) which can either fission a new uranium atom or convert another thorium a tom [4].

occur. This can also be seen in the values for the capture cross section as depicted in table 3. Additionally, this significantly reduces the need for mined uranium [3], [10].

For thorium-based fuels to be applicable in the short term, it cannot depend on the development of new reactor types as this takes a significant amount of time. This means familiar and well-understood reactor concepts have to be used when introducing thorium in the fuel matrix [10]. The fissile 233U that was bred in the reactor through neutron interactions with 232Th can be used in two ways: an “open fuel cycle” or a “closed fuel cycle”. Both will be shortly discussed in the next section. SCK CEN/39400143 Rev. 1.0

25 An open fuel cycle focuses on the irradiation of 232Th and in situ fission of 233U. it ignores the chemical separation of 233U by avoiding the complications associated with reprocessing and refabrication of radiotoxic 233U-based fuels. The use of thorium in this once-through mode can be seen in the Radkowsky concept of the light water reactor where each fuel assembly consists of a central seed with fissile material like enriched uranium or plutonium, and a thorium blanket covering the central seed. Introducing thorium in nuclear reactors makes in situ utilization of 233U possible as to avoid having to handle any 233U outside the core. Another usage for thorium in once-through cycle is its ability to be used for the incineration of weapons-grade plutonium. Light-water reactors of type WWER-1000 are in combination with thorium able to incinerate 239Pu and not breed it. This reactor type can use a mixed

thorium plutonium ( a0.5% PuO2) oxide as driver fuel. Aside from degrading the contents of weapons- grade plutonium, this spent mixed thorium plutonium oxide fuel also becomes proliferation-resistant due to the formation of 232U (via (n,2n) reaction with 232Th). This could also significantly decrease the stock of civil plutonium using the same combination with thorium in WWER-1000 type reactors. Without any modifications to the core or reactor operation, mixed thorium plutonium oxide fuels can directly replace low enriched uranium fuels [5].

In the closed fuel cycle, reprocessing of irradiated thorium-based fuels and separation of 233U is required. As mentioned in the previous section, reactors of type WWER-1000 that use mixed thorium plutonium oxide fuel also produce small but significant amounts of 232U (around 3000 ppm for a standard burnup of 40 MWd/kg). This is an important factor in the recycling of 233U. Two main recycle options were proposed [5]:

x The use of 232Th-233U oxide fuels; x The use of depleted U-233U oxide or reprocessed U from WWER-233U oxide fuels.

Using 232Th-233U oxide fuels will result in a build-up of 232U in 233U in subsequent fuel cycles. Using depleted uranium allows an easy transition to thorium fuel cycle with very little adjustments to the technology that handles the spent fuel. Although the latter seems more interesting, using depleted or reprocessed uranium in combination with 233U is no longer a pure thorium cycle as in this case, 235U is also being used alongside 233U with the inevitable build-up of 239Pu due to the presence of 238U. Furthermore, if 232U is used to recycle 233U, the main advantage of the thorium fuel cycle is not being utilized, that is, utilizing the full energy potential of thorium because of its efficient conversion into 233U in thermal reactors. Additionally, the relatively small build-up of minor actinides and plutonium to minimize radiotoxicity of the waste that is related to the usage of thorium-based fuels, is also not being utilized in that case. Besides, it is also possible to combine plutonium and 233U in the fuel composition in order to conform with the safety regulations with respect to the temperature coefficient of the reactivity. As the replacement of 235U by 233U in WWER-1000 reactors causes a shift in the water temperature coefficient of reactivity to the positive region, 239Pu can make up for this deficiency as it causes a reduction in the water temperature coefficient of reactivity, effectively neutralizing this effect. Additionally, self-sufficiency in 232Th-233U fuel cycle with a breeding ratio larger than 1.0 was shown to be possible in a BN-800 type molten salt LMFBR by theoretical calculations [15]. The same results were found in other types of reactors. However, these calculations showed the breeding ratio coming close to 1.0 but not exceeding it. Furthermore, as India possesses one of the largest thorium reserves in the world, a three stage indigenous nuclear power program was developed to link the closed fuel cycles of 3 different types of reactors [5]. 3.2 Thorium-based fuel manufacturing: the front-end fuel cycle This section will give an adequate summary of the most important steps in the nuclear fuel production process. SCK CEN/39400143 Rev. 1.0

26 As implementation of thorium heavily relies on it being a substitute for uranium when prices increase, it has to be competitive with the cost of uranium. Currently, thorium is often but a by-product of other rare-earth extractions. Dedicated infrastructure for the front-end of the fuel cycle for thorium is one of the major barriers holding it back from wide-spread use. Therefore, it is important to look for the possibility to use existing facilities and techniques that are normally used for the front-end of the uranium fuel cycle, for the front-end of the thorium fuel cycle. Generally speaking, the front-end of the uranium fuel cycle can be divided several discrete steps [10]:

Mining Æ purification Æ enrichment Æ powder synthesis Æ pelletization Æ scrap recycling Æ fuel rod loading and assembly construction

These steps are very similar to the ones that would be used for the front-end of the thorium fuel cycle aside from the enrichment step. Enrichment of thorium is not necessary due to its fertile properties. The different steps in the manufacturing process will shortly be discussed, with the emphasis put on the powder synthesis and pelletization processes, as these are the most closely related to the studied subject matter. 3.2.1 Mining The first step in the manufacturing process of nuclear fuel pellets is the mining of raw materials. On average, the soil contains 6 ppm thorium, which is 3 to 4 times more than uranium. Thorium can be found in several minerals, but the most common source is monazite, which is a rare-earth-thorium- phosphate that contains up to 12% thorium oxide [16]. As previously mentioned, thorium is currently often a by-product of the processing of heavy mineral sand deposits that contain titanium, zirconium or tin and most thorium is recovered from monazite coming from these mining sites. Table 4 shows the estimated world thorium resources [17]. A smaller source of thorium is also obtained from uranium

deposits, where it is found in the form of (Th,U)O2, an intermediate mineral between uranite and thorianite. Table 4: Estimated World thorium resources [17]

Country Tonnes x 1000 % Australie 452 17.6 USA 400 15.6 Turkey 344 13.4 India 319 12.4 Brazil 302 11.7 Venezuela 300 11.7 Norway 132 5.1 Egypt 100 3.9 Russia 75 2.9 Greenland 54 2.1 Canada 44 1.7 South Africa 18 0.7 Other countries 33 1.3 Total 2573 3.2.2 Purification Generally, in the case of uranium ore, it is first dissolved into nitric acid. Further purification is then often accomplished by extraction with tributyl phosphate before re-extracting from an organic phase to an aqueous phase [18]. Thorium has similar extraction processes with tributyl phosphate [19]. Often, thorium containing ores also contain uranium. Therefore, separation is necessary. As uranium SCK CEN/39400143 Rev. 1.0

27 extracts better into tributyl phosphate than thorium, separation is possible by varying the tributyl phosphate and nitric acid concentrations. Lower concentrations can be used to extract the uranium contents before extracting the thorium [10]. 3.2.3 Enrichment After the elements are extracted from their ores, enrichment would be the next step. Although this is not necessary for thorium, it is an important step for uranium and was added for the sake of completion. The goal of the enrichment process is to elevate the concentrationU 235. The basic principle 238 235 of enrichment is the following: A gaseous mixture of UF6 containing U and U is put in a centrifuge. As there is a slight mass difference between the two isotopes, the heaviest isotope will experience a stronger centrifugal force, causing it to accumulate as far from the centre as possible. The lighter U 235

will collect closer to the centre. As such, the UF6 closer to the middle will have a higher concentration of U235. Because the mass difference between these isotopes is very small, this process has to be repeated many times in order to reach high enough enriched uranium for reactor operation. For all purposes, this is a simplification of the centrifugal enrichment process and more techniques exist although centrifugal enrichment has been shown to be the most economic feasible. 3.2.4 Pellet fabrication After the enrichment process, powder synthesis and pelletization follows. For uranium, large scale fuel

fabrication exist to convert enriched uranium in the form of UF6 into UO2 pellets, an example for such

an organisation is Framatome-ANP [11]. This is, however, not the case for the manufacturing of ThO2 fuels as no such facilities currently exist. The goal of fuel fabrication and development is to develop a fabrication process that requires the fewest operations and simplest and most easily maintainable equipment. Therefore, it is important to develop powders that minimize difficulties during fabrication and sintering operations. Ideally, the thorium nitrate would be immediately converted into dust free and easily sinterable powder [20].

Precipitation processes are used to convert the thorium nitrate solution, which was produced during the purification step, into thorium oxide powder. Currently, the oxalate precipitation route is the best known and most studied production route for thorium oxide powders and is the only technique that has been practiced on larger scale for thoria production [10]. But multiple alternative production routes for thorium oxide are reported in literature. This includes but is not limited to other powder precipitation routes, gel routes microsphere pelletization and direct denitration. Generally, gel routes of sol-gel processes are used to form solid materials from small molecules, most often used for the production of metal oxides. Sol-gel methods are executed in multiple steps. These steps include hydrolysis, condensation and a drying process [21]. Direct denitration is less common and involves immediately converting thorium nitrate into thorium oxide by decomposition [10]. Concerning powder precipitation routes, multiple methods have been studied. This includes oxalate, hydroxide, carbonate and photolytic or radiolitic precipitation. As the oxalate precipitation route was used for the production of the first badge of thoria powders in this thesis, this method will be shortly summarized. The novel 2-step alkali precipitation route for thorium is, as of writing this, not yet published but most closely resembles the hydroxide precipitation route.

In the oxalate precipitation route, thorium nitrate coming from the purification step is mixed with an oxalic acid solution to precipitate thorium oxalate. Equation (3) shows this chemical reaction [22].

(3) ଷܱܰܪଶܱ൅Ͷܪଶܱସሻଶ ൉ݔܥଶܱ ՜݄ܶሺܪሻଶ ൅ݔܪܱܱܥሺܱܰଷሻସ ൅ʹሺ݄ܶ The exact value of x is depending on the drying conditions. The thorium oxalate is then filtered and

calcined to produce ThO2 powder. This thermal treatment will decompose the thorium oxalate into SCK CEN/39400143 Rev. 1.0

28 ThO2. The exact decomposition mechanisms are still being interpreted. T. Wangle [10] presented the following outline for the decomposition mechanism of thorium oxalate into thorium dioxide by combining multiple studies.

݄ܶሺܥଶܱସሻଶ ͸ܪଶܱ ՜݄ܶሺܥଶܱସሻଶ ʹܪଶܱ൅Ͷܪଶܱ

݄ܶሺܥଶܱସሻଶ ʹܪଶܱ ՜ ݄ܶሺܥଶܱସሻଶ ܪ ଶܱ൅ ܪଶܱ

݄ܶሺܥଶܱସሻଶ ܪ ଶܱ ՜ ݄ܶሺܥଶܱସሻଶ ൅ܪଶܱ

݄ܶሺܥଶܱସሻଶ ՜݄ܶሺܥܱଷሻଶ ൅ʹܥܱ

݄ܶሺܥܱଷሻଶ ՜݄ܱܶܥܱଷ ൅ ܥܱଶ

݄ܱܶܥܱଷ ՜݄ܱܶଶ ൅ ܥܱଶ Note that in this case, the hexahydrate (x=6) is the starting material and is converted into a dihydrate in the first step.

Once the thoria powder is produced, the powder is then pelletized. After a few optional pre-processes such as granulation or adding binder to improve pressing characteristics, the powder is pressed into a so-called green pellet. The same type of presses can be used both for thorium oxide and uranium oxide, although each material will need a dedicated press to prevent contamination [10]. Next, the green pellet is sintered into the final fuel pellet. Sintering thorium oxide presents a few challenges as was mentioned in section 1.3. 3.2.5 Scrap recycling and bundle assembly During the production process, so-called scrap is produced. Scrap recycling is therefore an important step during the fabrication process. For thorium dioxide fabrication, this scrap comes mostly in the form of grinding dust and pellets discarded after quality control. Process optimisation and reduction of losses of nuclear materials are crucial during nuclear fuel fabrication. Aside from the use of the standard thorium/uranium extraction dissolution process (THOREX) for recycling of nuclear fuel fabrication scrap, triflic acid was proposed for the recycling of thoria scrap [23].

Finally, the thorium pellets are loaded into the cladding which is then inserted int the fuel assembly. If

the dimensions of the ThO2 pellets are similar to those of the UO2 pellets, the same loading mechanism can be used. This removes the need for special infrastructure for thorium-based fuels in this step of the fabrication process [10]. 3.3 Adsorption theory Adsorption is a process that takes place when the surface of a solid is exposed to a gas or liquid. It is defined by F. Rouquerol et al. [24] as the enrichment of material or increase in the density of the fluid in the vicinity of an interface. It is of high importance in today’s technology and can be used as desiccants [25], catalyst and catalyst supports [26], separation or storage of gases [27], controlled drug delivery [28] and so on. Aside from the applications of adsorption phenomena in the industrial field, it has also a widespread use in measurement techniques used to characterize the surface properties as well as determine the surface area and porosity of materials. This section will describe some general terminology and aspects concerning adsorption theory, as well as outline some of the first attempts to derive adsorption isotherms to describe the adsorption process. In particular, Henry’s, Freundlich and Langmuir adsorption isotherms will be discussed as these early models served as the foundation for more advanced adsorption models such as the well-known Brunauer-Emmet-Teller (BET) method. SCK CEN/39400143 Rev. 1.0

29 3.3.1 Fundamentals of adsorption theory An adsorption system generally consists of an adsorbent, which is the solid material on which the adsorption occurs, and an adsorbate, which is the molecule adsorbing [24]. This process initially occurs due to a bond deficiency, meaning there is an unbalance between the forces amongst the atoms at the surface of the solid as compared to the balanced forces between the atoms at the bulk of the solid. Therefore, it is energetically favourable for the solid material to adsorb molecules. This unbalance between bonds is shown in figure 2(a). The difference between the unbalanced forces at the surface and the balanced forces in the bulk is called the surface energy and acts as the driving force for adsorption [29].

Generally, two types of adsorption can be distinguished: Physisorption (figure 2(b)) and chemisorption (figure 2(c)). Both are used in measurement techniques for different purposes [29]. Physisorption occurs due to the Van der Waal’s forces acting between the surface atoms and the adsorbate molecules. This force mainly consists but is not limited to dipole-dipole interactions. This is an exothermic reaction with a low enthalpy change and is also known as the heat of adsorption [30]. Physisorption occurs at lower temperatures and decreases with increasing temperatures. A physisorption process is reversible, meaning the adsorbed molecules are easily removed from the adsorbent which is an important property for surface characterisation of materials. This proces is not very specific as the adsorbate does not prefer specific binding sites on the adsorbent. Physisorption is driven by weak molecular forced between the molecules and acts the same for any combination of adsorbent-adsorbate system. It is generally also able to form multi-molecular layers, causing multiple layers of adsorbate to build up upon the suface of the solid [31]. A simplified presentation of a multilayer adsorption system is visible in figure 3.

Figure 2: (a) the unsaturated bonds at the surface and the saturated bonds in the bulk materials, (b) the a simplified diagram of the physisorption process, (c) a chemisorbed system [30]. SCK CEN/39400143 Rev. 1.0

30 On the other hand, chemisorption forms only adsorption monolayers and is highly specific in nature. This means that chemisorption will only occur with specific combinations of adsorbates and adsorbents. Figure 3 aslo shows a simplified diagram of a monolayer adsorption system. Chemisorption occurs at higher temperatures where the adsorbates undergo an electronic rearrangement with the surface atoms. This causes formation and seperation of chemical bonds at the surface of the solid. For this reason, the heat of adsorption is significantly larger for the chemisorption process as compared to the physisorption process. Chemisorption is subsequently mostly irreversible.

Figure 3: Simplified diagram of monolayer adsorption (left) mostly associated with chemical adsorption and multilayer adsorption (right) mostly associated with physical adsorption [30].

Another way to differentiate physisorption and chemisorption is based on the potential energy diagram shown in figure 4. This diagram depicts the variation of the potential energy of a molecular system when the adsorbate approaches the solid surface. Initially, when the adsorbate molecule approaches the surface, it is attracted by the weak attractive force attributed to the surface energy. This results in a flat potential minimum at larger distances from the surface and corresponds to non- dissociative physical adsorption. Depending on the interaction, the molecule can proceed to a non- dissociative chemisorbed state and potentially move to a stable dissociated state. When the interaction is less strong, the molecule will only physically adsorb on the surface of the adsorbent or may reside in a non-dissociative chemisorbed state. When the interaction is purely governed by Van der Waal’s forces, the adsorbate will only be in a physisorbed state. If the interaction is stronger involving electronic interactions, the process may directly go to a dissociative chemisorbed state. Generally, when the crossing points of the curve are below the line of zero potential energy, the process is non-activated. If the curve is above the line of zero potential energy, the process initially requires an activation [29], [30], [31], [32].

non-dissociative state

dissociative state

Figure 4: Potential energy diagram for non-activated dissociative chemisorption [30]. SCK CEN/39400143 Rev. 1.0

31 3.3.2 Derivation of Henry’s and Freundlichs adsorption isotherm Between the adsorbate and the adsorbent, a dynamic equilibrium is established. The net amount of adsorbed molecules is now constant and is permanently adsorbing and desorbing to maintain this equilibrium:

ݐ݁݉ݏݕݏ ݌ݐ݅݋݊ݎ݋ݏ݀ܣݐ֖ܾ݊݁ݎ݋ݏ݀ܣݐ݁൅ܾܽݎ݋ݏ݀ܣ Therefore, the equilibrium constant of adsorption can be defined as:

ݐܽݐ݁ݏ ܾ݀݁ݎ݋ݏ݀ܽ ݊݅ ݏ݋݈݁ܿݑ݈݁݉ ܭ௔ௗ௦ ൌ ݁ݏ݌݄ܽ ݏܽ݃ ݊݅ ݏݐ݁ ݉݋݈݁ܿݑ݈ܾ݁ܽݎ݋ݏ݀ܽ This adsorption equilibrium is established after considerable adsorption of gas on the absorbent surface, and can be represented as the following general equation [32], [33]:

݂ሺܽǡ݌ǡܶሻ ൌͲ ܽ݊݀ ܽൌ݂ሺ݌ǡܶሻ where a represents the quantity of gas adsorbed on the surface per gram of the adsorbent, p is the equilibrium pressure of the adsorbate in gas phase and T is the temperature. When studying adsorption equilibria, one parameter will remain constant, giving rise to three types of curves [32], [19]:

When T is constant and p is varied, the plot is called the “adsorption isotherm” in which case:

ݐܽ݊ݐݏൌ݂ሺ݌ሻ ܽ݊݀ ܶൌܿ݋݊ܽ When p is constant and T is varied, the plot is called the “adsorption isobar” in which case:

ݐܽ݊ݐݏൌ݂ሺܶሻ ܽ݊݀ ݌ൌܿ݋݊ܽ When a is kept constant and the variation of the equilibrium pressure is plotted with respect to the temperature, the plot is called the “adsorption isostere” in which case:

ݐܽ݊ݐݏ݌ൌ݂ሺܶሻ ܽ݊݀ ܽൌܿ݋݊ An analogy with Henry’s law for the solubility of gases in liquids can be made. It states that the amount adsorbed by an adsorbent should be proportional to the equilibrium pressure and can be presented by the following scheme [32]:

ݐܾ݊݁ݎ݋ݏ݀ܽ ݋݈݁ܿݑ݈݁ ݋݊ܯ֖ ݁ݏ݌݄ܽ ݏܽ݃ ݊݅ ݋݈݁ܿݑ݈݁ܯ Appendix II provides the derivation of Henry’s adsorption isotherm from which the result is shown as equation (3). Henry’s (linear) adsorption isotherm presented one of the first empirical equation that described the adsorption isotherm.

(3) ܽൌ ܭ௔ǡ௣݌ where a is the total amount of adsorbate (in mol/g). Henry’s adsorption isotherm could also be expressed in terms of surface coverage θ, which is shown in equation (4).

௦ ܿ ܽ ܭ ܭ௔ǡ௣ (4) ൌ ௦ ൌ ൌ ௦ ݌ൌ ݌ ܿ௠ ܽ௠ ܴܶ ܿ ௠ ܽ௠ Where the subindex m refers to the quantities corresponding to complete monolayer coverage, cs refers to the surface concentration of the total adsorbed amount a. The Freundlich isotherm, proposed SCK CEN/39400143 Rev. 1.0

32 (5) by Boedecker as a purely empirical equation for the adsorption isotherm but popularized by Freundlich, followed not long after Henry’s adsorption isotherm and presented a relation between the concentration of the adsorbate layer and the concentration of the gas the surface is in contact with. This relation is presented in equation (5).

ܽൌ݇݌ଵȀ௡ This model had, however, some important limitations. Namely, the equation did not exactly describe the isotherm in a wide pressure range. However, his work is today seen as one of the first quantitative attempts to use the adsorption phenomena for the characterisation of porous materials and serves as an important foundation to the adsorption isotherm theory [34]. For more information on the aforementioned isotherms, we refer to appendix II. 3.3.3 The Langmuir adsorption isotherm It was Langmuir who had a new take on the interpretation of the adsorption phenomena and presented what is today known as the Langmuir adsorption model. It is the most common one used to quantify the amount of adsorbate that is adsorbed on an adsorbent as function of partial pressure at a predetermined temperature [24]. While the Freundlich adsorption isotherm was purely empirical, Langmuir presented a semi-empirical model [31]. The Langmuir adsorption model was proposed by Irving Langmuir in the early 20th century where he made some important assumptions concerning this isotherm model [31], [32], [35], [36]. Firstly, he assumed that adsorption only occurs at specific binding sites that are localized on the adsorbents surface. Figure 5 shows a schematic representation of these discrete binding sites. Each empty box represents an unoccupied binding site and covers a specific area on the surface of the adsorbent. The adsorbed molecules (orange) are bound to some of these discrete binding sites. The blue spheres represent the adsorbate molecules in the gas phase. The amount of specific binding sites is proportional to the surface area. The adsorbed molecules were then assumed to be immobile.

Figure 5: Depiction of the specific binding sites as proposed by Langmuir.

The second assumption was that all adsorption sites on the surface are identical to one another. Therefore, the filling of these binding sites occurs randomly as there are no preferences. Thirdly, surface of the adsorbent is covered in a monolayer of adsorbed molecules, as depicted in figure 3. Note that the Langmuir model did not allow for multilayer adsorption.

Lastly, it was assumed that no (chemical) interactions occurred between the adsorbed molecules. The Langmuir adsorption model was formulated on the basis of a dynamic equilibrium between the SCK CEN/39400143 Rev. 1.0

33

(6) adsorbed phase and the gaseous phase. In a state of dynamic equilibrium, the rate of adsorption and rate of desorption are equal. Using this as a starting point and taking into account the assumption, the following equation can be derived (via the kinetic gas theory) and is appropriately named the Langmuir adsorption isotherm [24], [32].

ܾ݌ ൌ ͳ൅ܾ݌ Where θ represents the fraction of binding sites occupied, p is the equilibrium pressure and b is the adsorption coefficient. b is exponentially related to the positive value of the adsorption energy E and is defined as [24]:

ாൗ (7) ܾൌ ܭ݁ ோ் with K being the ratio between the adsorption and desorption coefficients. It is clear that for low θ, equation (6) is reduced to Henry’s law (equation (3)). At high θ, the fraction of coverage will approach 1 and represents the completion of the monolayer. It is common to apply equation (6) in its linear form [24]:

݌ ͳ ݌ ൌ ൅ (8) ݊ ݊௠ܾ ݊௠

where n is the specific amount of gas adsorbed at the equilibrium pressure p and nm is the is the monolayer capacity. Note that the Langmuir model did not allow for either porosity or physisorption (and thus multilayer adsorption). It did, however, build a foundation from which the BET method and other more finetuned physisorption isotherm equations could be deduced [24]. Section 3.4 will discuss the most common applications for the adsorption theory. 3.4 Applications for adsorption techniques The adsorption of inert gases can thus be used to characterize some of the properties of materials. These properties include specific surface area, porosity and heats of adsorption (change in enthalpy). The latter will not be discussed in this thesis as this property was not studied. Although adsorption knows many applications, as noted in the beginning of section 3.3, the goal of this section is to give a comprehensible description for the techniques used in this study. 3.4.1 The use of adsorption isotherms To assess surface area and porosity, it is common to experimentally measure the isotherm using an

inert adsorbate like N2. After all, directly measuring the surface area or porosity is not possible. Therefore, these techniques rely on measuring pressures of the gas in the sample cell under a constant temperature. From this pressure, the adsorbed quantity can be calculated using the temperature and the ideal gas law. The resulting isotherms can indicate some of the general properties of the materials that adsorbed the probing gas. In the 2015 IUPAC technical report, 8 types of physisorption isotherms were identified. Each type is shown to be closely related to particular pore structures [37]. Figure 6 shows the classification of physisorption isotherms. Each type is discussed very shortly and more detailed information on the different adsorption isotherms can be found appendix III.

The type I isotherm is typically associated with the Langmuir adsorption isotherm and therefore complies with the assumptions listed in section 3.3.3. Type I(b) is associated with materials containing a wider range of pore sizes.

Type II isotherms are related to the physisorption of gases on non-porous or macroporous materials and is characterized by the asymptotic behaviour towards a relative pressures. The inflection point SCK CEN/39400143 Rev. 1.0

34 indicates monolayer completion (point B). It should be noted that it is possible for a type II isotherm to show hysteresis behaviour. It is then referred to as type II(b) [24].

Type III isotherms closely resemble type II isotherms without the inflection point.

Type IV isotherms show clear hysteresis behaviour which is attributed to the presence of mesopores. The plateau at high relative pressures is an important feature of this type of isotherm.

Type V isotherms show stepwise adsorption which is attributed to the adsorption of gases on uniform non-porous materials.

Figure 6: Classification of physisorption isotherms as proposed by the IUPAC technical report of 2015 [37] 3.4.2 The meaning of adsorption hysteresis The shape of the hysteresis loop can also be an indication for certain material properties and is most frequently linked with mesoporous behaviour. Hysteresis occurs when the adsorption branch and desorption branch do not coincide due to different mechanisms being present in the adsorption phase and desorption phase. This is often the result of capillary condensation which occurs during the adsorption branch. As vapour molecules are forced to interact in the narrow capillary such as mesopores, the attractive Van der Waals forces become more prevalent between vapour molecules allowing the vapour to condensate at pressures lower than saturation pressure [38]. Capillary condensation is best described by the Kelvin equation:

SCK CEN/39400143 Rev. 1.0 (9)

35 J ܲ ʹ ܸ௟ Ž ଴ ൌ െ ܲ ݎ௄ܴܶ With

= equilibrium vapor pressure ܲ

଴ = saturation vapor pressure ܲ = mean radius of curvature of the meniscus ݎ௄ = liquid/vapor surface tension J = liquid molar volume ܸ௟ = ideal gas constant ܴ = temperature ܶ

All parameters aside from Pv and rK are constants. Therefore, rk can be calculated using the isotherm data. Other effects such as adsorption metastability [39] and possible network effects [40] also influence the hysteresis. Evaporation typically occurs during the desorption branch of the isotherm.

As the shape of the hysteresis is closely related to the pore structure and the adsorption mechanism, the IUPAC technical report of 2015 identified six shapes of hysteresis loops. These six types are shown in figure 7 and will be shortly discussed due to their relevance for this thesis.

Figure 7: Classification of hysteresis loops as proposed by IUPAC technical report of 2015 [37]

Type H1 hysteresis loops is most common for materials with a narrow range of uniform mesopores. The steepness of the loop indicates the occurrence of delayed condensation on the adsorption branch. Delayed condensation happens when the pore potential in a certain pore is decreased below the pore potential of a void of similar geometry but with solid walls. This occurs due to small channels branching away from this pore leaving holes in the walls [41].

Type H2 hysteresis loops are the result of more complex pore structures where network effects are considered to be important. Type H2(a) shows a particularly steep desorption branch which is SCK CEN/39400143 Rev. 1.0

36 attributed either to cavitation-induced evaporation or to pore-blocking. Pore-blocking occurs when adsorbate molecules obstruct the entrance to the pore, limiting further pore filling as a result [42]. Type H2(a) loops can be given by ordered mesoporous materials and are related to pore-blocking in a narrow range of neck widths. Type H2(b) loops are also associated with pore blocking, but the width distribution of the necks of the pores is now significantly larger as compared to type H2(a) loops.

Type H3 hysteresis loops are recognizable by two distinct features. The first is the fact that the adsorption branch significantly resembles a type II isotherm. Secondly, the lower limit of the desorption branch coincides with location of the cavitation-induced p/p0. This is typically the result of non-rigid aggregates consisting of plate-like particles. This type of hysteresis can also be attributed to materials with pore networks that consist of macropores that are not completely saturated with pore condensate.

Type H4 hysteresis loops lightly resemble type H3 loops. The adsorption branch is bears a resemblance to a hybrid of a type I and II isotherm. The sharper bend at low relative pressures is related to micropore filling.

Type H5 hysteresis loops are rare. They are very recognizable and are related to specific pore structures that contains both open and obstructed mesopores.

As clearly shown in the previous sections, N2 isotherms serve as good first indicators for certain material properties. However, to more accurately assess material characteristics, more refined techniques are required. Often, mathematical models are applied to calculate traits of a material. This includes the calculation of surface area by means of the well-known BET method, as well as the calculation of pore volumes. As both these methods were used during this research, they will be discussed in the following paragraphs from a more theoretical point of view.

An important aspect of nitrogen adsorption as a measuring technique is that it alone cannot be expected to provide anything more than a semi-quantitative evaluation of pore size distribution. In order to assess the available range of pore sizes in a material, multiple probe molecules of different sizes should be used [43]. 3.4.3 The BET model The Brunauer-Emmet-Teller method, or BET method for short, is one of the most widely used methods for calculating the surface area of porous and finely-divided materials such as powders [37]. The first article mentioning the BET technique, was published in 1938 by Stephen Brunauer, Paul Hugh Emmett, and Edward Teller in the Journal of the American Chemical Society [44]. As it became clear that physical adsorption of a vapour was not restricted to monomolecular surface coverage at increased relative pressures, as was proposed by the Langmuir model, Brunauer et al. extends the Langmuir model by allowing a layer of adsorbed molecules to serve as new adsorption sites, giving rise to a multilayer adsorption theory. They derived a multilayer adsorption isotherm, called the BET equation, which had a type II character. Emmet and Brunauer empirically deduced that the beginning of the near linear section of a type II isotherm (the so-called point B) most likely corresponds to monolayer completion [24]. Brunaer et al. made a number of simplifying assumptions [24], [29], [31]:

x Gas molecules behave ideally; x No interaction between adsorbate molecules; x Adsorbate molecules are immobile; x Multilayer adsorption; x First adsorption layer follows Langmuir adsorption; x Adsorption heat of first layer is larger than that of the second layer; SCK CEN/39400143 Rev. 1.0

37 x Adsorption heat of second layer is equal to adsorption heat of all other layers and is equal to the heat of condensation; x All adsorption sites on the surface are equal.

Using these assumptions and similar techniques as Langmuir, on the basis of kinetic considerations, the following equation for the surface coverage was derived:

݌ ܥ ݌଴ (10) ൌ ݌ ݌ ൬ͳെ ൰ሺͳ൅ ሺܥെͳሻ ሻ ݌଴ ݌଴

where C represents a constant that is related to the difference between enthalpy of the first layer ( Q1)

and the enthalpy of condensation ( Qc). C is then defined as [32]:

(11) ʹǤ͵Ž‘‰ܥൌ ܳଵ െ ܳ௖ Equation (10) can now be rewritten in linear form, presenting the BET isotherm:

݌ ଴ ݌ ͳ ܥെͳ ݌ (12) ଴ כ ݌ ൌ ൅ ܽሺͳെ ሻ ܽ௠ܥ ܽ௠ܥ ݌ ݌଴

where a presents the total amount adsorbed vapour and am the monolayer capacity. Plotting the adsorption isotherm enables us to deduce several parameters and calculate the specific surface area. 0 0 0 Using the coordinate system (p/p )/a(1-p/p ) and p/p , the constants am and C can be derived directly from the slope of a straight line and the point of intersection with the Y-axis. Figure 8 shows a plot of the BET isotherm and the determination of the constants.

݌ ଴ ݌ ݌ ܽሺͳെ ሻ ݌଴

ܥെͳ ݐ݃ ൌ ܽ௠ܥ

ͳ ܾൌ ܽ௠ܥ ݌ Figure 8: BET adsorption isotherm and determination of constants ݌଴ Knowing the value of the monolayer capacity, the specific surface area of the material can now be calculated using the following equation:

ܵൌ ܽ௠ܰ஺߱௠ (13)

whereω m is the surface occupied by one molecule in the adsorption layer. This parameter is dependant

of the adsorbate used. For the BET method, N2 is often used which has a ωm of 0.162 nm² [37]. Concerning the applicability of the BET method, there are a few limitations to be considered. The BET method can be applied to many type II and type IV isotherm, but experimental agreement for the BET SCK CEN/39400143 Rev. 1.0

38 theory only exist in a narrow range of relative pressures (0.05-0.3) [37]. typical deviations are the result of the BET technique anticipating a (too) low amount of adsorbed molecules at low pressures and a (too) high amount of adsorbed molecules at high pressures. Although modifications of the BET equation exists, they are not widely used. The main shortcoming of this theory, and for most earlier adsorption theories, is the negligence of lateral interactions between adsorbate molecules which can be significant in many systems [32]. 3.4.4 Calculation of pore volume Generally, allowing condensation of the adsorbate to happen within the pores allows one to evaluate the finer pore structure. As the pressure increases, the gas will condensate first in the smallest pores. By the time the pressure reaches saturation, all pores will be filled with liquid adsorbate. Once the pressure is decreasing, the condensed adsorbate will start to evaporate. By evaluating the adsorption and desorption branches and the hysteresis between them (typically type IV isotherms), information about the pore size and volume can be deduced [45].

If the adsorbent is considered to be meso- and microporous without containing macropores, the type IV isotherm reaches a horizontal plateau at high relative pressures. The pore volume can then be deduced from the amount of adsorbate that is adsorbed at a relative pressure close to 1 as per the Gurvitsh rule: at the saturation pressure, the liquid volume of different adsorbates, when measured on porous adsorbents, is essentially constant and is independent of the adsorbate. The amount of adsorbed molecules, the temperature of the adsorbate and the ideal gas law can then be used to directly calculate the total pore volume from the isotherm [46]. The pore volume calculated is then the mesopore volume in addition to the micropore volume as macropores are not considered. Indeed, Thommes et al. [37] mentions that when macropores are present, it is not possible to evaluate the total pore volume as these type II/type IV hybrid isotherms no longer present a plateau near p/p0 = 1.

Additionally, the Dubinin-Radushkevich (DR) adsorption isotherm [47] is often used to describe adsorption in microporous solids and was also used in this study for the assessment of the micropore volume. Dubinin and Radushkevich found that the characteristics of the measured adsorption isotherm were related to the porous structure of the adsorbent. The approach was based on the concept of expressing the physisorption isotherm data in the form of a temperature invariant “characteristic curve”. Assuming the micropore size distribution is Gaussian, the DR equation was proposed [24], [32], [48]:

బ (19) ௦ ௦ ି௕ሾோ் ୪୬ ሺ௣ Ȁ௣ሻሿ; ܸ ൌ ܸ଴ ݁ with ଶ where ଶ is the convergence coefficient, Vs represents the volume of the adsorbed ܾൌ݇Ȁ௔ ௔ s layer, V 0 represents the limiting volume at potential H = 0 which is approximately equal to the volume of the micropores.

3.5 State-of-the-art on H 2O and CO2 adsorption on metal oxide surfaces This section will give a brief overview of the current knowledge concerning adsorption of atmospheric gases on thorium powders. It is expected that if commercial thorium fuel takes off, significant periods of powder storage will occur regularly. There have been studies done on the adsorption phenomenon and the results will be shortly discussed.

Early research in 1958 on the adsorption of CO2 on thoria showed that thoria presumably has two different adsorption sites [49]. At temperatures below 200°C, adsorption was observed on both sites while at temperatures above 200°C, only one of the sites was involved. Charles H. Pitt et al. suggested

that CO2 tightly binds to the thoria surface (presumably chemisorbed), especially at lower surface coverages. Considering entropy, it was suggested that highly mobile carbonate ions form which was SCK CEN/39400143 Rev. 1.0

39 associated with one type of surface site. The CO2 that adsorbed on the second type of surface site at lower temperatures was found to be immobile.

In 1967, H.F. Holmes et al. [50] studied the adsorption of water on porous and non-porous samples. It was hypothesized that bare thorium oxide surfaces chemisorb water by dissociative adsorption to form hydroxyl groups on the surface. The adsorption capacity of the surfaces was observed to be reduced by the presence of irreversibly adsorbed water. In the case of the porous sample, this was attributed to chemisorbed water blocking part of the pore volume. Two models were proposed: a model of a fully hydrated (100) thoria surface (which refers to the Miller-indices concerning crystal structure of the thorium oxide) and a model for a fully hydrated (111) thoria surface. Note that thorium oxide was assumed to have a face-centered cubic crystal as has been done before [51], [52]. In the case of (100), the hydrogen bonding capacity of the water molecule was completely satisfied so that no additional adsorption, nor physical nor associative, can take place by means of hydrogen bonding. In the case of the hydrated (111) surface, the hydrogen bonding was not fully satisfied leaving sites open for potential adsorption by means of hydrogen bonding. Depending on the production route of the thoria powder, one model was preferred over the other. The oxalate precipitated sample was known to preferentially expose the (100) crystal face. For clarity, figure 9 shows these crystal faces.

Figure 9: Surface structure of ideal face-centered cubic single crystals and their corresponding numbers [63].

In 1971, Breysse et al. [14] studied the catalytic properties of thorium oxide (oxalate derived) in the

oxidation of carbon monoxide. O 2 was found to adsorb very weakly to thoria and passage from a weak adsorbed state to a strong adsorbed state can be observed if the adsorbent remains in the presence

of oxygen for an excess of 20 hours. CO2 was found to be able to adsorb in two different ways in the temperature range of (100)-500°C. The first type of adsorption would only occur below 200°C and was also considered to be immobile. The second type of adsorption occurred at higher temperatures and

was assumed to be relatively mobile. I.r. spectrometry of chemisorbed CO2 confirmed the presence of two types of adsorption. The stronger adsorption mode was related to monodentate carbonate SCK CEN/39400143 Rev. 1.0

40 formation and would only disappear when heated up to 500°C while the weaker one was related to

bidentate formation, which would already be eliminated at 180°C. This study also confirmed that CO2 inhibits CO oxidation on thoria because both molecules compete for the same adsorption sites. It was also observed that on a porous adsorbent, bidentate could desorb and readsorb in the form of a monodendate.

In 1969, Gammage et al. [51] studied the adsorption of water vapor on thorium oxide, produced via hydroxide and oxalate precipitation. They observed that adsorbed water at high relative pressures is taken up irreversibly and that it may change the nature of the surface. They mentioned that at a water- free surface, water can irreversibly adsorb in an amount equal to three chemisorbed layers. The first layer is chemisorbed in a dissociative way (figure 4) and forms two hydroxyl groups on the surface for each thorium ion. Next, this hydroxyl layer is hydrated with water molecules to form the additional two layers. These water molecules bind in an non-dissociative way (figure 4) and are rather immobile. It was mentioned that at least a temperature of 1000°C was necessary to provide the dry weight of the thorium oxide. In general, such a water-based adsorbate layer on a metal-oxide surface is but a few monolayers in thickness. Often, thicknesses of two or three monolayers are reported. Note that in this

case, only the adsorption of water is taken into consideration, no CO2 was involved in the process. A recent study (2017) proposed a schematic representation for the structure of water layers adsorbed

on the surface of TiO2 [52]. This structure is shown in figure 10 [52].

Figure 10: Schematic representation of water layers adsorbed on the surface of TiO2 as proposed by Chung-Yi Wu et al. [52].

As clearly shown in figure 10, hydrogen bonding seems to be the primary mechanism for multiple

adsorption layers of water. Similar structures are expected for ThO2. It should be noted that hydrogen bonding is the result of the dipole-dipole interaction between molecules, which is considered to be a Van der Waals force and is therefore classified as physisorption rather than chemisorption. The hydroxyl groups chemically bonded with the metal and are therefore classified as chemisorbed.

In 1974, Fuller et al. [53] studied the sorption of water, CO2 and nitrogen on sol-gel thorium oxide. It was mentioned that this gel material has a considerable affinity for water and carbon dioxide. Another important conclusion that was drawn, was that the amount of reactive surfaces are heavily influenced by past history effects. The removal and replacements of adsorbates seemed to significantly alter the amount of available surface. It was also observed that tightly bound chemisorbed molecules are able to exclude physisorption, which is an effect not unique to sol-gel thoria. It was concluded that the

adsorption of inert gases like N2, which weakly interact with the surface, are useful to study the alterations cause to the surface by stronger chemical and physical forces or adsorbed water.

T. Wangle et al. also shortly studied the desorption of carbon dioxide and water from thorium powders calcined at 500°C, 700°C and 1000°C that had been exposed to a natural atmosphere for 3 weeks, as SCK CEN/39400143 Rev. 1.0

41 to simulate the conditions within a storage silo [10]. The powder was produced via the oxalate precipitation route. They performed thermogravimetry up to 1400°C to study the nature of the weight loss. They found that 30% of the total mass loss occurred at room temperature in all cases. Upon

heating the sample, a larger release of H 2O was observed around 125°C which was accompanied by a

smaller CO2 release peak. After this initial peak, the H2O release was measured to have a tail that

extended to 300°C. On the other hand, the CO2 release occurred in 3 steps: a first release between 40- 150°C, next a sharper release peak at 200°C and finally second release peak at 400°C. The sample calcined at 500°C also showed a well-defined release peak at 600°C and was attributed to incomplete

calcination. Both samples calcined at 700°C and 1000°c showed strong tailing of CO2 until 1000°C, after which very little release was measured. The powder calcined at 1000°C, which had the smallest porosity, showed minimal capillary water release and the second release was attributed to the

hydration water of hydroxide. It was also observed that a fraction of CO2 remains bound until temperatures well above 600°C and that another important fraction is released at intermediate temperatures of around 400°C. Further research was proposed to elucidate the desorption mechanisms of carbonates form thorium oxide surfaces.

The presence of adsorbed water also appears to play an important role in the adsorption behaviour of

CO2 on oxide surfaces. Baltrusaitis et al. [54] studied the surface reactions of CO2 at the adsorbed water-oxide interface. It was concluded that in the absence of water, surface hydroxyl groups react

directly with CO2 forming bicarbonates. The following reaction mechanism was proposed:

ܪܱܥଶ െܱଶܯ ֖ ଶܱܥ൅ܪଶ െܱܯ Where M presents the metal element. The structure was via quantum chemical calculations determined to be a bidentate bicarbonate although the formation mechanism of this structure is far more complicated. Figure 11 presents an overview of the surface species expected to occur on oxide surfaces for the coadsorption of water and CO2. In the presence of adsorbed water, a reaction occurs between H 2O and CO2 within the adsorbed water layer to form carbonic acid. After deprotonation, adsorbed carbonate and protonated hydroxyl groups are formed. Such a adsorbate layer is expected to have a limited thickness of a few monolayers. It was also suggested that surface adsorbed water can block potential adsorption sites for the formation of bicarbonates, effectively decreasing the adsorption capacity of CO2 on the surface.

Figure 11: Schematic representation of surface species for the coadsorption of water and CO2 on oxide surfaces. In this example, M represented Ti or Ce [64]

However, figure 11 mostly involves chemisorption of CO2. In literature, various types of adsorption

configurations for the physisorption of CO2 have also been considered. These include the adsorption

of linear CO2 molecules parallel or upright on the surface. Less common species such as bent CO2 was also considered for physisorption although it was shown that pure physisorption was consistently

related to linear CO2 molecules as this configuration maximized the Van der Waals forces that are responsible for the physisorption process [55]. SCK CEN/39400143 Rev. 1.0

42 4. Methods and materials Section 4.1 describes the materials that were used as well as the measurement instruments. Section 4.2 describes the methodology and the experimental setup. 4.1 Materials 4.1.1 Raw materials The Fuel Material group (FMA) at SCK CEN provides two raw materials to perform this thesis i.e. thorium oxalate precipitated powder and a second thorium oxide precursor powder which was produced by means of a 2-step alkali precipitation route. In both cases, the powders were prepared

from 1.9M Th(NO3)4 and 1.6M HNO3 supplied by Solvay.

The thorium dioxide powder produced by means of calcination from the thorium oxalate powder is further referred to as ThO_Ox, and the one produced from the 2-step alkali precipitation route powder is referred to as ThO_STA. 4.1.2 Measurement instruments The measurement instruments used for this study were the SETARAM thermogravimetric analyzer SETSYS evolution (TGA), the OMNIStar gas analysis system of PFEIFFER for mass spectrometry (MS) and

the Tristar II 3020 from Micromeritics for measuring N2 isotherms. The latter included software to calculate the BET surface area and the pore volume. Concerning the software for the other instruments, Calisto was used on the TGA and Quadera was used to collect and process the data produced by the MS. Finally, SEM images of ThO_Ox and TEM images of uncalcined ThO_TSA were provided by FMA to assess powder morphology.

4.1.2.1 Thermogravimetric analyser (TGA) The TGA is used to record small mass differences in function of the temperature. This is used during the desorption measurement of a sample to accurately record the evolution of the mass in order to link specific mass releases to the desorption of gases at specific temperatures. Figure 12 shows the TGA on the right side. An important additional step that had to be carried out was a blank measurement for each experiment to remove the signal from the crucible. Therefore, an empty crucible was placed in the TGA and the mass loss of the crucible was recorded under conditions similar to the sorption experiments. This signal was subtracted later in the data processing software to yield the mass loss of the sample. Measuring this mass difference is achieved by an accurate mechanical balance present in the top part of the device. The sample powder is hung in a small crucible on one end of the balance while a counterweight on the other side is used for stabilization. The counterweights are manually adjusted by the operator for each sample to ensure the balance is optimally tuned. Small deviations in the mass of the sample are then measured by a laser also positioned in the top part of the TGA. This laser is able to accurately measure the movement of the balance. This delicate contraption is visible in figure 12. It is important for the operator to stabilize the balance in a correct manner to ensure that the mass difference is within the measuring range of the TGA. Therefore, the TGA has two settings for the thermogravimetric range. The large range amounts to ±200 mg, meaning the mass of the sample can increase or decrease by 200 mg before falling outside the measuring range. The small range amounts to ±20 mg. This option is more accurate but severely limits the range in which the mass can change. As it was initially unclear how the mass of the sample would evolve, we opted for the large TG range for all desorption experiments.

4.1.2.2 Mass spectrometry (MS) Additionally, to have a better understanding of which gases are desorbing from the sample, the MS was connected to the exhaust of the furnace of the TGA to detect any gas molecules desorbing from SCK CEN/39400143 Rev. 1.0

43 the sample. The MS is visible in figure 12 on the left side. Generally, a mass spectrometer works by the following principle. Firstly, the incoming molecules need to be ionized as the MS separates molecules according to their specific mass-to-charge ratio. Once ionized, the ions are separated based on the mass-to-charge ratio using electric and magnetic fields. Each mass-to-charge ratio is assigned to a

preconfigured channel. This way, a clear distinction can be made between CO2 desorbing from the

sample, which has a molecular mass of 44u and is mostly monovalent once ionized, and H 2O desorbing from the sample, which has a molecular mass of 18u and is also monovalent. Note that different isotopes will have different mass-to-charge ratios. For example, as oxygen has different isotopes that

occur naturally and thus end up adsorbed to the sample, the molecular weight of H 2O can theoretically be 16u, 17u or 18u depending on which oxygen isotope has bonded with hydrogen. However, even though the signals of these other isotopes were present in the data produced by the MS, the desorption spectrum these isotopes would show were not nearly as clear as the desorption spectrum of the most common isotope. Often, these less common isotopes were negligible compared to the most prevalent one. Although the data from all isotopic compositions was acquired, we focused on the

signal of the molecules consisting out of the most common isotopes. Therefore, for H 2O and CO2, we used the mass-to-charge ratio of 18 and 44 respectively. Figure 12 shows the full setup of the TGA- MS.

Figure 12: The mass spectrometer (red) connected to the TGA (grey-blue) via a tube that was heated up to 150°C to decrease condensation of gases during transport from TGA to MS. On the right side, the balance present in the top side of the TGA is exposed. The laser and balance are visible.

4.2.1.3 Gas analysis system A gas analysis system was used for the basic characterisation of the powders. Figure 13 presents a schematic representation of the inner workings of the gas analysis system that was used to measure

the N 2 isotherms. The critical components are indicated by the numbers (1)-(8):

1. The analysis manifold of which the volume and pressure is accurately known; 2. The vacuum system with a valve to the manifold;

3. Source of adsorbate gas (typically N 2) with valve to manifold; 4. Pressure transducer and temperature sensor; 5. Records signal and data from (4); 6. Sample tube of precisely known free or void-space; SCK CEN/39400143 Rev. 1.0

44 7. Connection of tube between manifold and sample tube;

8. Reduces the temperature of the sample tube when required, often achieved with liquid N 2.

Firstly, sample preparation is undertaken. The adsorptive gas supply valve (3) is closed and the vacuum (2) and sample (7) valves are open allowing the manifold and sample tube to be evacuated. The sample tube is not in the cold bath, so the sample is at ambient temperature. When the necessary vacuum is achieved, valves (2) and (7) close and sample is introduced to the cold bath, cooling the sample to the

analysis temperature. For N 2 isotherm measurements, this is at 77K and is reached using liquid N 2 as

coolant. Secondly, the manifold is charged to a pressure P m slightly above vacuum by opening valve (3). This prepares the instrument to release a dose of adsorbate onto the sample. The quantity of gas in the manifold can be determined using the universal gas law. Thirdly, valve (7) is opened allowing gas to enter in the sample tube. Some amount of gas will adsorb to the sample and is therefore removed from the gas phase. Once the pressure stabilizes, the adsorption has reached a point of equilibrium.

The equilibrium pressure P e is then recorded. The quantity of gas that remains in the manifold and the

sample tube volume (Vm + V s) can then be calculated by the universal gas law after which the quantity

of gas adsorbed to the sample is determined at P e. This establishes one point on the isotherm (Pe, n ads). Valve (7) is then closed and valve (3) opens. The manifold charges to a pressure that is slightly haigher

than Pe after which the dosing and equilibration processes are repeated. This cycle continues until pressures near the saturation pressure are reached. By this time, the adsorpiton isotherm is fully determined. The desorption isotherm is measured in a similar manner by reducing the pressure step- wise. At low pressures, most of the molecules that were initially physically adsorbed will have been removed from the surface. Once the isotherm is fully finished, the BET surface and pore volume are calculated using software.

Figure 13: Schematic representation of most important components of a generic volumetric physical adsorption analyser in its most elementary form [65].

4.2.1.4 SEM and TEM images Lastly, to assess powder morphology, SEM and TEM images were provided for ThO_Ox and uncalcined ThO_TSA respectively. We would have preferred to use similar imaging techniques for both powders and to have images of calcinded ThO_TSA rather than uncalcined ThO_TSA to facilitate the SCK CEN/39400143 Rev. 1.0

45 comparison. However, as we were unable to make the images ourselves during the research period (due to the COVID-19 situation), FMA provided these images from other research projects.

SEM (Scanning Electron Microscope) is most often used to characterize surface morphology. It is a type of electron microscope that takes pictures of a sample by scanning its surface using a focused beam of electrons rather than visible light. The shorter wavelenght of the electron then allows for far more detail in the images as compared to regural microscopic images. Additonally, it provides a three- dimensional image allowing for depth-perception. However, this technique is limited to the morphology of the sample and cannot be used to view nanosized particles.

TEM (Transmission Electron microscope) on the other hand, has a much higher resolution as compared to SEM and is able to analyze powders at nanoscale. It can be used to measure nano particles size, grain size, crystallite size and even the atomic arrangement in the material. However, the images are only two-dimensional as oppose to the three-dimensional images produced by SEM. For our purposes, these techniques will only be used to asses the morphology of the particles. 4.2 Methodology 4.2.1 Overview of experimental workflow The experimental workflow will be described in this section. Figure 14 shows a schematic overview of the experiments that were performed. Each step is described in detail in sections 4.2.1-4.2.4.

Figure 14: Schematic representation of methodology SCK CEN/39400143 Rev. 1.0

46 4.2.2 Safety guidelines for working in a nuclear environment Since working in a nuclear environment involves a certain set of risks, safety precautions were in place to assure these risks were minimalized. As there was no experience working in a nuclear laboratory prior to onset of my internship at SCK CEN, the following safety guidelines were put in place. Firstly, it was obligated to carry a film dosimeter at all times while being present on SCK CEN territory. Next, before entering the guarded area of the nuclear laboratory, in which little to no fissile material was studied and estimated dosage stayed below 6 mSv/yr, it was requested to carry an additional electronic dosimeter (EPD) to monitor radiation dosage in μSv in real time. This allowed for a continuous monitoring of the total received dosage when working in the nuclear lab. A lab coat to wear while crossing the guarded zone on the way to the controlled area was provide. When entering the controlled area, in which possible doses exceed 6 mSv/yr and fissile materials such as uranium, thorium and plutonium are studied, it was requested to change lab coats and wear plastic overshoes. Working within the controlled area, protective equipment such gloves and tape was provided to minimize contamination risk, both for myself and the lab environment. When handling the radioactive materials, which in my case was thorium dioxide powder, wearing gloves was obligated and the powder was put under a fume hood to extract any dust created while handling it. Regularly measuring the gloves made sure no thorium powder would stick to the gloves without being alarmed. Gloves were regularly changed and the lab table and equipment under the fume hood were frequently cleaned with ethanol to remove any radioactive particles sticking to the table surfaces. Any potentially contaminated waste was disposed of correctly following a set of rules imposed by nuclear waste regulators. For the first few weeks of the internship, near constant supervision and guidance of my external promotors as well as colleague scientists made sure safety was the priority at all times and that there was full compliance with the strict rules that apply in nuclear laboratories. Upon leaving the controlled area, the overshoes were removed and disposed of as nuclear waste and a hand-and-foot monitor was used to detect any potential contamination that would stick to hands or shoes. Finally, when leaving the guarded area, hands and shoes were measured one more time after which it was requested to wash hands to assure no radioactive particles would leave the guarded area. 4.2.3 Calcination Calcination was the first step to convert the thorium oxalate and 2-step alkali precipitates into thoria powder, referred to as ThO_Ox and ThO_STA respectively. The calcination process took mostly place inside the TGA for small quantities ( amg) of raw material. For the preparation of larger quantities ( ag) of thoria powder which would not fit inside the small furnace of the TGA, an alumina tube horizontal three zone furnace from Carbolite was used. The latter was mostly used for the preparation of the

thoria powder for the measurements of the N2 isotherms, BET and porosity as this type of measurement required larger amounts of sample. The exact calcination conditions were kept the same in both furnaces.

Firstly, about 160 mg of the precipitate was put into a 1300 μL Al2O3 crucible before being introduced in the furnace of the TGA. A larger crucible of the same type was used for the tubular furnace. There, the powder was put under a constant gas flow of 80 ml/min argon while the calcination procedure was being programmed and loaded into the software. The calcination conditions were as follows. A gas

flow of 60 ml/min 0.5% O 2/Ar was used for the entire duration of the procedure and was controlled via a gas panel. First, the furnace was heated up from ambient temperature to 40 °C where it was held for 30 min. The temperature was being measured constantly by a type S thermocouple. Next, the furnace would heat up from 40 °C up to 1000 °C with a heating rate of 10 °C/min. Once the furnace hit 1000 °C, it was held at that temperature for 1 hour before actively cooling down to 40 °C with a cooling rate of 10 °C/min. Finally, the sample was held at 40 °C for 1 hour before it was removed from the furnace to stabilize the signal. Figure 15 shows the evolution of the temperature with time during the SCK CEN/39400143 Rev. 1.0

47 calcination process. The only difference between the TGA and tubular furnace for the calcination, aside from the volume of the furnace and crucible, was the cooling rate. The tubular furnace did not have active cooling causing the sample to cool down from 1000°C to 40°C at a natural rate. After the calcination, the oxalate and 2-step alkali precipitates should be converted into chemically identical

ThO2 powder. This procedure would significantly reduce the mass of the sample. For the oxalate precipitated powder, the calcination yield was measured to be 51.2 ± 0.29%.

Figure 15: Evolution of temperature with time during the calcination process.

4.2.4 Measuring N 2 adsorption isotherm, BET area and pore volume

TheN 2 isotherm for the two samples was measured using the TriStar II 3020 from Micromeritics (figure 16). The powder calcined in the Carbolite furnace was directly placed in the vial and degassed in vacuum for 2 hours at 350 °C to remove the possible occluded gases. Then the sample containing vials were connected to the TriStar II analyzer. Table 5 shows the masses of each of the samples. The isotherm was determined in the range of p/p0 10-3 to 0.9922 at 77K. In total, four measurements were performed for each sample.

Table 5: Masses of samples used in TriStar II

Experiment ThO_Ox (g) ThO_TSA (g) 1 1.2596 1.5669 2 1.2566 1.1767 3 1.2408 1.7332 4 1.2693 1.6473

Figure 16: Picture of the Micromeritics Tristar II taken from the official website

The data treatment was carried out using the software TriStar II 3020 V1.03. The BET method was used to calculate the specific surface area of the powders and was applied in the relative pressure range 0.05-0.3, using a fitting for at least 5 datapoints and a minimum value for the correlation coefficient of SCK CEN/39400143 Rev. 1.0

48 0.9999 to ensure good function fitting. Next, the pore volume was calculated at p/p0 = 0.97 as at this relative pressure, the pore volume corresponds to the total volume of micropores and mesopores for this particular instrument. The pores are filled with pore condensate, allowing the estimation of the pore volume. The relative pressure to be considered varies from 0.96 to 0.98 depending on the instrument. If there is no macroporosity, the pore volume at p/p0 0.97 corresponds to the total pore volume of the sample. If the sample has macroporosity, this pore size cannot be characterized by means of this instrument, other techniques are required. The estimation of micropore volume was calculated applying the Dubinin-Radushkevich method. This method was applied in the relative pressure range 0.0001-0.015. The mesopore volume can be estimated by subtracting the micropore volume from the pore volume at p/p0 = 0.97.

4.2.5 Sorption experiments The sorption experiments consisted of two phases: the adsorption phase, in which the powder will be able to adsorb gas, and the desorption phase, in which the powder is heated up in an attempt to remove adsorbed gases. During the latter, the desorbing molecules and the evolution of the sample mass will be measured using the MS and TGA respectively. To assess the influence of water sorption

on the sorption of CO2, two types of adsorption were performed.

Natural adsorption allowed the powder to adsorb molecules present in the ambient environment.

These gases include CO2, CO, H 2O, O 2 and N 2. During this phase, the samples were put in a fume hood enclosed in a glass container. This made sure the sample was stored safely during the adsorption phase while still being able to adsorb atmospheric molecules.

The second type of adsorption is referred to as forced adsorption. The goal is to expose the sample to

an atmosphere containing only CO2 as to remove water from the adsorption atmposphere. The

influence of H2O can then be deduced by comparing these results with the results of the natural adsorption experiments. As forced adsorption required a specific atmosphere, the sample was introduced to the furnace of the TGA where the atmosphere and gas flow could be controlled via a gas panel. The following conditions were applied to all forced adsorption experiments. The sample was

subjected to a 60 ml/min gas flow of 5% CO2/Ar at 40°C for a variable amount of time.

To study the influence of the adsorption time, the following main exposure times were chosen: 1 hour, 24 hours and 1 week. However, initially other adsorption times were added to more accurately finetune the adsorption/desorption tests in order to ensure decent results. Table 6 presents an overview of all adsorption/desorption tests performed during this study in chronological order. Note that the cycle is also mentioned in table 6. Early tests revealed that the reuse of a sample for multiple adsorption/desorption cycles heavily influenced that samples adsorption capacity. Therefore, it is an important parameter to take into consideration when discussing the results. Cycle 2 means that the same sample has been adsorbed-desorbed once more and is now being tested for the second time.

After one of two types of adsorption were finished, the powder was desorbed. This took place in the TGA and the desorption conditions were kept the same for every experiment. After the sample was inserted in the furnace of the TGA, the desorption phase would start with a 2 hour purge, flushing the furnace with 60 ml/min argon at 40 °C to clean the furnace and connected pipelines of any impurities. Next, the furnace was held at 40 °C for 30 min and was put under a 25 ml/min argon gas flow for the remainder of the desorption phase. The furnace would heat up at a heating rate of 10 °C/min to 1000 °C where it was kept for 1 hour. Afterwards, the furnace would actively cool down at a cooling rate of 10 °C/min to 40 °C where it was kept for 1 hour to stabilize the signal. Figure 17 shows the temperature profile of the desorption phase. SCK CEN/39400143 Rev. 1.0

49 Since both the adsorption and desorption phase for the forced adsorption experiments took place in the TGA, both phases were carried out sequentially without interruption of the system. This results in a four-step experiment that starts with a 2 hour purge, followed by the forced adsorption phase, after

which another purge is performed to clean the setup of any CO2 left in the furnace and finally, a desorption phase. Table 6: Overview of all sorption experiments in chronological order. The colours are representative for the same source materials and thus the same samples.

Sample adsorption time Cycle 30 min 1 30 min 2 1 hour 3 Natural 1 week 4 24 hours 5 1 hour 1 1 week 6 ThO_Ox 30 min 1 1 hour 2 3 hours 3 Forced CO 2 5 hours 4 24 hours 5 3 hours 6 Natural 1 week 2

Forced CO2 24 hours 3 Natural 1 hour 1 Forced CO 1 hour 2 ThO_TSA 2 Natural 24 hours 1

Forced CO2 24 hours 2 ThO_Ox Natural 1 week 1 Natural 1 week 1 ThO_TSA Forced CO2 1 week 2 Natural 5 week 7 ThO_Ox Forced CO2 1 week 2

Figure 17: Evolution of temperature with time during the desorption process. SCK CEN/39400143 Rev. 1.0

50 5. Results and discussion Section 5.1 presents the results of the gas analysis measurements for both powders as well as some SEM and TEM images to asses powder morphology. Section 5.2 presents the description of the results for the oxalate derived thoria powder (ThO_Ox). First, the influence of the adsorption times is described, comparing the results of the TGA and MS for multiple different adsorption times.

Subsequently, the influence of adsorbed water on the adsorption behaviour of CO2 is described by comparing the results of the forced adsorption experiments with the results of the natural adsorption experiments. Section 5.3 mirrors section 5.2 for the 2-step alkali precipitated thoria sample (ThO_TSA).

In section 5.4, the discussion of the influence of the precipitation strategy on the CO2 and H2O

adsorption/desorption onto ThO2 is addressed, as well as potential explanations about the different behaviours of the powders. 5.1 Textural properties

5.1.1 N2 adsorption isotherms

The N 2 isotherms taken at 77K from both thoria powders are shown in figure 18. The shape of the two isotherms are very similar. Both isotherms resemble a type II isotherm [37] which is characterized by its inflection point at low relative pressures and the unrestricted monolayer-multilayer adsorption at high relative pressures as shown by the near asymptotic behaviour towards relative pressures near unity. The characteristic inflection point (“Point B”) is shown more clearly in the magnification of the isotherm at low p/p0 on the left side of figure 18. Additionally, hysteresis behaviour is observed in both isotherms suggesting that both materials are mesoporous. Although one could argue that the isotherms resemble a type IV(a) isotherm, the clear lack of a plateau or onset thereof at high relative pressures classifies this hysteresis type as a type H3 hysteresis loop. As mentioned in section 3.4.1, type II isotherms that show type H3 hysteresis behaviour are referred to as type II(b) isotherms and should not be classified as a type IV isotherm [43].

“Point B”

Figure 18: N 2 isotherm for ThO_Ox and ThO_TSA sample supplemented with a magnification of the first part of the isotherm to reveal the inflection point.

Type II(b) isotherms are typically the result of non-porous materials. However, the SEM and TEM images (figure 19 and 20) show that the particles formed aggloremates. Additionally, this type SCK CEN/39400143 Rev. 1.0

51 hysteresis behaviour suggests a mesoporous material and the presence of non-rigid aggregates of plate-like particles. However, this type of hysteresis could also be the result of pore networks consisting of macropores which are not completely filled with pore condensate [37]. Rouquerol et al. [24] suggested the narrow hysteresis loop to be the result of interparticular capillary condensation within non-rigid aggregates. Therefore, it is plausible to accept that although the particles are considered non-porous, the observed mesoporosity can be attributed to the aggregation of the particles, meaning that the interparticular cavities are of mesopore size. Therefore, the general powder effectively behaves as a mesoporous material. Capillary condensation then occurs in the space between the particles rather than in pores on the particle surface. These types of isotherms have been further associated with metastability of the multilayer and delayed capillary condensation which was attributed to a low degree of pore curvature, suggesting the presence of slit-like pores [56]. Although the general shape of the isotherm for both powders seems very similar, two distinct differences can be pointed out. Firstly, ThO_TSA seems to adsorb more N 2 as its oxalate counterpart. This may be an early indicator for ThO_TSA having a larger availabe adsorption surface, although this is highly dependent on the adsorbate used. Secondly, the hysteresis loop is significantly larger with ThO_Ox. The lower closure point of the hysteresis loop is estemated to be at relative pressure of a0.42. For ThO_TSA, the same point is estimated to be at a relative pressure of a0.83. This significant difference may suggest that the mesopores of ThO_Ox, which in our case refers to the size of the interparticular cavities, are smaller than the mesopores of ThO_TSA. However, further research is needed to confirm this suspicion. 5.1.2 Surface and pore analysis

The results of the surface analysis are summarized in table 7 and were directly obtained from the N 2 isotherms from the previous paragraph. For both samples, the BET surface area was calculated. ThO_Ox has a BET surface area of 5.2 ± 0.7 m²/g while ThO_TSA had a larger BET surface area of 6.6 ± 0.2 m²/g. Wangle et al. [10] also found for oxalate derived thoria samples calcined at 1000°C that the BET surface was around 5.8 m²/g, confirming our findings. H.F. Holmes et al. [50] reported similar surface areas of 5.50 m²/g for oxalate derived thorium oxide. Breysse et al. [14] also measured the BET surface of thoria powder from oxalate precipitation. However, the precipitates were calcined at 500°C rather than 1000°C, yielding a far larger BET area of 30 m²/g, which is in line with the expectation that the surface area decreases with higher calcination temperatures [10]. The BET surface of ThO_TSA has not yet been measured prior to this study but can best be compared to thoria powders from hydroxide precipitation. Early research pointed out that the surface area of thoria powder derived from thorium hydroxide is highly variable and dependent on the exact precipitation steps undertaken during the process. Values for the BET area could range from 0.3-50 m²/g depending on the exact precipitation conditions [57]. But in general, ThO_Ox seems to have a smaller BET area as compared to ThO_TSA, which confirms our expectations of the previous paragraph.

Table 7: Summary of the results of the surface analysis. ThO_Ox ThO_TSA BET area 5.2 ± 0.7 6.6 ± 0.2 m²/g Total pore volume (p/p0 = 0.97) 0.013 ± 0.001 0.022 ± 0.001 cm³/g Micropore volume (DR) 0.0017 ± 0.0003 0.0021 ± 0.0003 cm³/g Mesopore volume 0.011 ± 0.001 0.020 ± 0.001 cm³/g

Vmicro/Vtot 13.7 9.6 %

First, the total pore volume was calculated at a relative pressure of 0.97. Note, that since the particles are non-porous, the total pore volume in this case represents the sum of the micropore volume and SCK CEN/39400143 Rev. 1.0

52 mesopore volume. For ThO_Ox, the pore volume at p/p0 = 0.97 was measured to be 0.013 ± 0.001 cm³/g. The pore volume of ThO_TSA was measured to be almost double at 0.022 ± 0.001 cm³/g.

The Dubinin-radushkevich method was fitted in the relative pressure range 0.0001-0.015 to assess micropore volume. The micropore volume was found to be 0.0017 ± 0.0003 cm³/g and 0.0021 ± 0.0003 cm³/g for ThO_Ox and ThO_TSA respectively. Subtracting the micropore volume from the pore volume calculated at p/p0 = 0.97 yields a rough estimate for the mesopore volume of the samples. A mesopore volume of 0.011 ± 0.001 cm³/g and 0.020 ± 0.001 cm³/g are then obtained. It can therefore be concluded that ThO_TSA has significantly higher pore volume as compared to ThO_Ox in both

micropore and mesopore ranges. However, note that the relative micropore volume (Vmicro/Vtot) of ThO_Ox is larger.

More advanced mathematical models such as BJH or DFT methods for more complex mesopore size distributions were not performed because they are not reliable for type II isotherms with H3 hysteresis loops [43]. 5.1.3 SEM/TEM images The isotherms suggested the presence of aggregates of plate-like particles. However, it was found by means of SEM and TEM that the morphology of each material is different. FMA provided me with SEM images of ThO_Ox (figure 19) and TEM images of uncalcined 2-step alkali precipitate powder (figure 20). ThO_Ox clearly shows the plate-like particles that were expected on the basis of the isotherm. However, although TEM images lack the three-dimensional clarity of SEM images, it can be observed that the particles of ThO_TSA are not plate-like, but rather sphere-like. They were also observed to clump together forming aggregates, as was the case for ThO_Ox. The powder morphology differs significantly between the two samples. This can potentially influence how the aggregates are formed. This subsequently results in a different pore size distribution as was found in the section 5.1.2. It might also suggest a different preferential orientation for the exposed crystal face at the surface of the particles. Note, that the TEM images show the raw material powder, rather than its calcined form. However, it has been mentioned that the spherical shape of the particles was retained after calcination for a similar ceramic (TiO2) [58]. Therefore, we assume the observed spherical particles also retain their morphology after calcination. However, one should not suggest that the calcination process does not influence the morphology, as this has been proven before [56]. SCK CEN/39400143 Rev. 1.0

53 Figure 19: SEM pictures of ThO_Ox provided by FMA. The non-porous plate-like particles are clearly visible and clumptogether.Mesoporesizedcavitiesarealsovisibleinbetweentheplatelets.

Figure 20: TEM images of uncalcined 2-step alkali precipitate powder. SCK CEN/39400143 Rev. 1.0

54 5.2 Oxalate precipitated thoria powder

5.2.1 Influence of exposure time on adsorption of CO2 and H 2O The results of the sorption experiments for ThO_Ox for different exposure times is presented in figure 21. While the signal of the mass spectrometer presented in figure 21(a) and (b) is useful for studying the general sorption behaviour with temperature, it is unsatisfactory for a quantitative assessment. Therefore, the thermogravimetric (TG) results are presented in figure 21(c) and (d). Note, that the MS was not calibrated. Therefore, intensity is expressed in A.U. and one should interpret the results from a qualitative point of view. Often, inconsistent vertical offsets were measured when using the MS, causing a vertical shift when comparing desorption spectra of different experiments. A clear example of this can be seen in the water desorption spectrum. It is expected all mass releases would start at the same intensity. This is clearly not the case. Multiple desorption experiments showed that this vertical offset was not related to any controllable parameter and thus no reliable way was found to correct for this error. Therefore, the focus lies on the shape and characteristics that can be observed in the spectra.

To assess the influence of the exposure time on the sorption of CO2 and H 2O, only the results from the natural adsorption experiments were used for these results as it is meant to simulate the storage conditions.

The results from the MS, both in the desorption of CO2 and H 2O, show some clear differences when

comparing different adsorption times. The CO2 release spectrum shown in figure 21(a), presents an initial release at 130°C in the form of a shoulder followed by a large peak at 220°C. These releases seems to coincide with different adsorption times although the sharpness of the shoulder seems to be depend on the adsorption time. The main release at 220°C is very similar for all exposure times. However, starting from 300°C, major differences are observed, particularly in the desorption spectrum

of the 1 week adsorption experiment. For this experiment, significant releases of CO2 are observed at 400°C, 550°C and 800°C which is in stark contrast to the lower adsorption time experiments. One should take special notice of the distinct desorption peak observed at 400°C in the 1 week adsorption experiment. A soft shoulder is visible at 370°C, indicating this desorption peak might consist of two separate releases that occur in close proximity. Furthermore, the broad desorption peak observed around 800°C was also observed in the 24 hours experiment and thus seems to decrease in intensity for lower adsorption times.

For the desorption of water, a clear release is observed between (100)-220°C and seems to be present in all experiments. However, the shape of this release seems to change with adsorption time and is thought to consist of two separate release that lay in close proximity to one another. The shoulder

observed around 120°C, which coincides with the shoulder at 120°C that was measured in the CO2 release, seems to increase steadily in intensity with exposure time. The second release estimated to be at 180°C, seems to have a constant intensity independent of adsorption time. A continuous release of water is then observed until high temperatures. Note that the 1 week adsorption experiment poses an additional release at a380°C.

To assess the mass release from a quantitative point of view, the thermogravimetric results are shown in figure 21(c). Figure 21(d) presents the derivative from the mass loss to clarify small but important differences in the mass loss behaviour between samples. It is clear that a higher adsorption time corresponds to a higher mass losses. The 1 week experiment lost 0.40% of its original mass during the desorption phase, which conversely means the sample initially adsorbed about 0.40% of its original mass. This significantly larger than the mass loss observed in the other samples. Note that the 24 hour adsorption experiment was in its 5 th cycle, as is indicated by the number between brackets in the SCK CEN/39400143 Rev. 1.0

55 Figure 21: CO2 and H 2O desorption spectra (a and b), the relative mass loss (c) and the derivative of the relative mass loss (d) for different adsorption times for ThO_Ox.

legend. It is known that reusing samples for multiple adsorption/desorption cycle severely decreases the adsorption capacity [51]. The same was observed in our tests. It is expected that repeating this SCK CEN/39400143 Rev. 1.0

56 experiment with a newly calcined sample, the sample would adsorb about 0.35% of its total mass. Therefore, increasing the adsorption time generally increases adsorbed quantity.

The derivatives of the mass loss curves were calculated and portrayed in figure 21(d). From these curves, it can be clearly observed that for the experiments with a lower adsorption time, i.e. 30 minutes and 1 hour, the initial large release between (100)°C and 250°C is more narrow. While all peaks end on very similar temperatures, the onset of the peak differs significantly. Upon closer inspection, this major release between (100)° and 250°C consists of three separate releases in close proximity. The

first peak is estimated to be at 130°C and corresponds to the CO2 and H 2O release observed at similar temperatures. This release seems most influenced by the adsorption time as the peak, which starts as a shoulder at low exposure times, significantly increases in intensity when adsorption time rises. Therefore, this peak is thought to be related to a multilayer of physisorbed gases. As these are the most weakly bound, they are expected to desorb first. The second peak is observed at around 180°C and presumably corresponds to the main water release peak seen in figure 21(b). Lastly, the final peak

is estimated to be at 215°C and likely corresponds to the main CO2 release observed at similar temperatures. Note that the 1 week adsorption sample poses an additional release just under 400°C.

This release is presumably related to the release of CO2 and H 2O observed at similar temperatures, although it is believed this release is only related to the desorption of water as the shape of mass loss derivative closely resembles the shape of the water desorption spectrum at this temperature. As temperatures increase, the derivative becomes less clear and noise becomes more prevalent on the signal. Further averaging the signal to decrease noise removed the subtle differences observed at lower temperatures and was therefore not performed.

5.2.2 Influence of H 2O sorption on CO2 sorption

To assess the influence of water sorption on the sorption of CO2, the results of the forced adsorption experiments are compared to the results of the natural adsorption experiments. Figure 22 shows a set

of figures similar to those presented in figure 22. The CO2 desorption signal from the forced adsorption experiment was manually shifted down in order to facilitate comparison between natural and forced

adsorption because the general intensity of the CO2 release from the forced adsorption experiments was significantly higher.

Between (100)°C and 300°C, CO2 release seems to behave in a similar way. However, from 300°C until

1000°C, the CO2 release observed in the forced adsorption experiment seems to pose a slow but steady increase without any characteristic features. This continuous release is not related to the desorption

of CO2 from the sample itself as is shown by the blank measurement. It is expected that when the TGA

is subjected to a continuous stream of CO2 during the adsorption phase, some of this CO2 will also

adsorb on the inside of the furnace, causing a persistent release of CO2 towards high temperatures.

Therefore, the forced adsorption experiment CO2 release seems to flatten after 300°C, showing no signs of any distinct releases.

Furthermore, although the goal was to remove water from the adsorption atmosphere in the forced adsorption experiments, figure 22(b) clearly shows that a significant amount of water desorbed when heating the powder. Initially, water seems to desorb in a similar manner in both experiments, although the water that was adsorbed during forced adsorption shows far stronger tailing behaviour towards high temperatures.

The results of the TG show that the quantity absorbed during natural adsorption is significantly larger than the quantity absorbed during forced adsorption, respectively 0.40% and 0.26% of the total sample mass. This makes sense as we expect the amount of water adsorbed during forced adsorption to be far smaller than during natural adsorption. The derivatives in particular, show that the first release SCK CEN/39400143 Rev. 1.0

57 Figure 22: Comparison between 1 week natural – and forced adsorption experiments on ThO_Ox in terms of MS desorption spectra for CO2 and H 2O (a and b). (c) presents the relative mass loss of the powders normalized by initial weight of the sample and (d) presents the derivative of the presented mass loss to highlight the mass loss behaviour. peak that is observed in the natural adsorption around 130°C seems to be significantly smaller in the forced adsorption mass release, showing up as a mere shoulder rather than a distinct peak. The mass SCK CEN/39400143 Rev. 1.0

58 loss behaviour of the force adsorption seems to be of a similar shape as the mass release behaviour observed from low adsorption time natural adsorptions in figure 21. As this initial mass release at 120°C is barely visible after 1 week of forced adsorption, it is suggested that this release is related to

the release of water (presumably physisorbed), rather than the release of CO2.

Furthermore, the release of CO2 at this temperature seems to always be at the same relative intensity

compared to the main CO2 release at 220°C, suggesting there is a limit to the CO2 adsorption that is related to this temperature region. Conversely, the adsorption of water in this temperature region appears to increase indefinitely as seen in figure 22(b).

The origin of the adsorbed water was not entirely clear. It could have been the result of a small leak in the TGA-MS system. However, a plausible theory is that since the forced adsorption sample was in its 2nd adsorption/desorption cycle, some water had already irreversibly adsorbed to the powder during the first sorption experiment, forming a layer of hydroxyl groups on the surface through dissociative adsorption. Although samples were stored in the desiccator before use, it is known that these hydroxylated surfaces are highly hydrophilic in character [12]. It is possible that while weighing and transporting the sample from desiccator to TGA, some water adsorbed during this process in the form of molecular water. It is possible that we underestimated the strength of this hydrophilic character, resulting in significant desorption of water during the forced adsorption experiments. We were thus unable to completely remove water from the adsorbate layer. Water is known to play a significant role

in the sorption behaviour of CO2 [54]. Therefore, we are unable to accurately assess the sorption

behaviour of CO2 on a bare ThO2 surface. This applies for all forced adsorption results. 5.3 2-step alkali precipitated thoria powder

5.3.1 Influence of exposure time on adsorption of CO2 and H 2O

Figure 23(a) and (b) present, analogue to section 5.2.1, the desorption spectra of CO2 and H2O in function of temperature for ThO_TSA. The same adsorption times were taken as with ThO_Ox to facilitate comparison discussed in section 5.4. However, the 30 minute adsorption was not performed on ThO_TSA. Furthermore, for ThO_TSA all experiments were performed with newly calcined thoria powders (first adsorption/desorption cycle) facilitating comparison between the different adsorption times.

The release of CO2 is clearly influenced by the adsorption time. Although all adsorption times show

similar desorption peaks, the intensity of the distinct CO2 releases appears to be highly dependent on

the exposure time. Higher exposure times appear to translate into more distinct and clearer CO2 releases. Note that the first two release peaks are now more easily distinguishable as compared to

ThO_Ox where the first CO2 release often appeared as a shoulder rather than a peak on its own.

Concerning the desorption of water, similar results are found to those of the ThO_Ox experiments. One major difference between the desorption of water from ThO_Ox and ThO_TSA is that the additional shoulder at a380°C now appears in all adsorption times rather than just in the 1 week natural adsorption as was the case with ThO_Ox. This release of moisture seems to perfectly coincide

with the release of CO2 at similar temperatures, indicating a possible relation between these desorptions. Tailing is observed mostly in the 1 week adsorption sample.

Interestingly, the adsorbed quantity appears not to be influenced by the adsorption time as all samples adsorbed a0.30% of their total mass. The derivatives coincide nearly perfectly above 200°C. However, a small difference is observed around 120°C as this initial mass release in the 1 hour adsorption experiment appears much smaller as compared to the other adsorption times. This difference is attributed to the different desorption behaviour of water at these temperatures. Although the first SCK CEN/39400143 Rev. 1.0

59 Figure 23: Results of sorption experiments on ThO_TSA for different adsorption times, CO 2 desorption spectrum (a), H2O desorption spectrum (b), the relative mass loss (c) and the derivative of the mass loss (d).

major mass release observed in ThO_Ox could be divided into three separate releases in close proximity, only two releases can now be distinguished. This is because the second major release of

water now occurs at the same temperature as the second major release of CO2. This explains why the release peak in figure 23(d) at 220°C is much sharper than the one observed in figure 21(d). In the SCK CEN/39400143 Rev. 1.0

60 latter, this mass release could only attributed to the release of CO2 because the second major release of water occurred at a lower temperature of 180°C, indicated by the second mass release in figure 23(b) at 180°C. For ThO_TSA, the mass release at 220°C appears to be a combination of the release of

CO2 and water. This marks a major difference between ThO_Ox and ThO_TSA.

5.3.2 Influence of H 2O sorption on the sorption of CO2 Figure 24 presents the comparison between the natural and forced adsorption on ThO_TSA.

Concerning the CO2 release, the results are very similar to those of ThO_Ox.

In the water release spectrum, it appears that the release of water in the forced adsorption experiment experienced a shift to higher temperatures. It is expected for the desorption of water from this powder to have two major consecutive water releases at 130°C and 220°C respectively. Although these are visible in the natural adsorption experiments, the forced adsorption experiments appear to be shifted to higher temperatures as the mass releases were recorded at 150°C and 250°C. Similar to the results of ThO_Ox, the distinct plateau observed in the natural adsorption experiment at 380°C is absent in the forced adsorption experiment and appears to be replaced by stronger tailing behaviour. In both cases, water was detected to desorb towards high temperatures. Therefore, as was the case for

ThO_Ox, we were unable to study the sorption behaviour of CO2 on a bare ThO2 surface to assess the

influence of water adsorption on the adsorption behaviour of CO2.

As expected, the results of the thermogravimetric analysis (figure 24(c) and (d)) show that during natural adsorption, the quantity of gas adsorbed is significantly larger than during forced adsorption. This discrepancy was earlier attributed to the notable decrease of water adsorption during the lather. During 1 week forced adsorption, the powder adsorbed about 0.19% of its total mass while 0.30% was adsorbed during 1 week natural adsorption. Furthermore, the mass loss curve occurs in a similar way as both derivatives present the same shape indicating similar sorption behaviour. The most notable difference can be observed in the mass release at 380°C in the natural adsorption experiment. This

mass release coincides with the desorption of water and CO2 at similar temperatures. These desorptions were absent in the forced adsorption experiment and are therefore not visible in the mass loss curve. SCK CEN/39400143 Rev. 1.0

61 Figure 24: Comparison between 1 week natural – and forced adsorption experiments on ThO_TSA in terms of MS desorption spectra for CO2 and H 2O (a and b). (c) presents the relative mass loss of the powders normalized by initial weight of the sample and (d) presents the derivative of the presented mass loss to highlight the mass loss behaviour.

SCK CEN/39400143 Rev. 1.0

62 5.4 Influence of precipitation strategy on sorption behaviour Comparing the results of the 1 week natural adsorption experiments performed on ThO_Ox and ThO_TSA allows one to study the influence of the precipitation strategy on the sorption behaviour.

Figure 25 presents a direct comparison in CO2 -,H 2O - and mass release between the two studied thoria powders.

Figure 25: Comparison of 1 week natural adsorption between ThO_Ox and ThO_TSA in terms of CO2 desorption (a), H 2O desorption (b), relative mass loss (c) and the derivative of the mass loss (d). SCK CEN/39400143 Rev. 1.0

63 5.4.1 Influence of precipitation strategy on adsorption behaviour One major difference that was mentioned earlier is the temperature at which the second major release

of water is observed. For ThO_Ox, this release occurs at 180°C, just before the main CO2 desorption peak at 220°C. This results in three separate mass releases in close proximity. The broad mass release peak in figure 25(d) can thus be separated into three distinct mass releases. The first one occurs at

130°C and is the result of a combined desorption of water and CO2 at these temperatures. The second peak, which also has the highest mass release rate, can be attributed to the release of water at 180°C.

The third peak is mainly the result of the desorption of CO2 at 220°C. For ThO_TSA, the second major

desorption of water occurs at a higher temperature of 220°C, coinciding with its main CO2 release peak at similar temperatures. Because these releases coincide, the major mass release between (100)°C and 300°C can now only be divided into two separate mass releases. The first is located at 130°C and the

second at 220°C, both involve the combined desorption of CO2 and H 2O.

The water releases below 200°C most likely result from the evaporation of capillary water that adsorbed in between the particles of the powder [10]. In section 5.1.1, it was suggested that ThO_Ox possibly had smaller mesopores (smaller interparticular cavities) than ThO_TSA. This could explain why the desorption of water from ThO_Ox occurs at a slightly lower temperature than for ThO_TSA. The smaller mesopore allow for less capillary condensation of water to occur. Therefore, it makes sense that this sample would desorb its capillary water at a lower temperature. The water desorbed at higher temperatures (>200°C and <300°C) is attributed to the desorption of the hydrated water that initially adsorbed on top of the hydroxyl surface groups [51]. However, one should notice that when comparing these temperatures to the temperatures mentioned in figure 10, these temperatures are significantly higher. This might be due to a more stable hydrated layer. This concept is explored further in section 5.4.2.

Above 300°C, the desorption behaviour of H 2O appears to be similar. Both powders show a desorption peak around 380°C and express tailing behaviour towards high temperatures. It has been reported that water is able to stay chemisorbed up to 1000°C [51]. It is fairly well established that the adsorption of water on thorium oxide surfaces occurs by first covering the surface with hydroxyl groups after which water molecules bind strongly to these surface groups [50],[51],[54]. Due to different binding strengths of water, thorium oxide is able to retain water to temperatures approaching 1000°C [10].

Below 550°C, CO2 is known to adsorb under two different forms: monodentate carbonates and bidentate carbonates [10],[14], [54]. The monodentate carbonates formed on the surface are known to be stronger adsorbed and were assumed to be relatively mobile [14]. It was assumed that these

specific CO2 species bonded with a superficial oxygen atom, possibly an oxygen atom from the ThO2 crystal lattice itself. The bidentate on the other hand was more weakly adsorbed and was considered to be immobile. The latter would already be removed at 200°C. Therefore, it is safe to assume that the

first CO2 desorption peak is related to the desorption of bidentate carbonate. The second major release

peak at 220°C and the following CO2 releases below 550°C can be assumed to be the desorption of monodentate carbonates.

It is also highly unusual for CO2 to desorb from an adsorbed state at temperatures above 600°C. At

these temperatures, bidentate or monodentate structures of CO2 are normally do not stay adsorbed as they are reported to be completely removed at 200°C and 550°C respectively [14]. It should be noted

that only on the ThO_Ox sample, CO2 has been observed to desorb at high temperatures ( a800°C). Therefore, an explanation for this behaviour might be found in the different morphological structure of the powder compared to ThO_TSA. It is expected that ThO_Ox, because of its plate-like shaped particles, has more slit-like pores (V-pores). ThO_TSA is expected to have more rounded pores with

less sharp edges. CO2, it being a linear molecule with a kinetic diameter of 0.33nm, is often reported SCK CEN/39400143 Rev. 1.0

64 to prefer adsorption in micropores [59]. It might be possible that CO2 initially adsorbs in the micropore sized cavities, after which the entrance is blocked with hydroxyl groups, adsorbed water and adsorbed

CO2 that bonded with the adsorbed water layer effectively trapping the CO2 in the pores. During

desorption, the entrance has to be cleared first before the CO2 can escape. That being said, water (in molecular form) has been reported to have a kinetic diameter of 0.265nm [60], which is smaller than

the kinetic diameter of CO2. Therefore, CO2 and H 2O might compete for the micropore filling. Due to the different pore geometries, it is possible that ThO_Ox traps the micropore content more effectively

than ThO_TSA. To desorb the possibly trapped CO2, the entrance needs to be cleared first. The adsorbate layer on ThO_Ox is thought to be more stable due to the saturated hydrogen bonding of the associative chemisorbed water molecules on the surface hydroxyl layer. This is explained in far more

detail in section 5.4.2. This might close off the micropores more strongly, trapping the CO2 in the micropores even at high temperatures.

It was also reported that CO2, being acidic in character, is expected to adsorb more strongly on basic sites [61]. In general, a metal oxide that has a low charge-to-radius ratio is more ionic and will have more basic sites [62]. This applies to our powders since thorium is a metal. Z. Yong et al. [62] mentioned that when metal oxides have basic surface characteristics, these metal oxide sorbents have good

adsorption capacity for CO2, even at higher temperatures. It could be that case that ThO_Ox has a more

basic surface character as compared to ThO_TSA, facilitating the adsorption of CO2 up to high temperatures. However, the surface chemistry of the powders was not determined during this research and is highly recommended for future studies on the subject. 5.4.2 Influence precipitation strategy on adsorption capacity Table 8 shows the total relative mass losses for different adsorption times of both thoria powders. Notice that since we try to simulate silo conditions for powder storage, these are the results for natural adsorption. Furthermore, the 30 minutes adsorption experiment was not performed for ThO_TSA. To facilitate the comparison, only the samples with a cycle of 1 are presented with the exception for the 24 hours adsorption experiment on ThO_Ox. No such experiment was performed with a lower cycle of reuse. Therefore, the adsorption capacity of this sample is expected to be significantly lower.

Table 8: The total relative mass that was measured to desorb from both samples for different adsorption times. Total mass percentage desorbed at 1000°C Adsorption time and cycle ThO_Ox (%) ThO_TSA (%) 30 min (1) 0.28 / 1 hour (1) 0.33 0.30 24 hours (5)/(1) 0.33 0.31 1week (1) 0.40 0.29

Table 8 clearly shows that in the case of ThO_Ox, a higher adsorption time steadily increases the adsorbed mass. We expected that when the measurement of the 24 hours adsorption would be repeated for ThO_Ox with a newly calcined sample, the adsorbed quantity is expected to be between 0.33 % and 0.40 %. On the other hand, the adsorption time seems to have little influence on the adsorbed quantity on the ThO_TSA. The relative mass release hovers around a constant value of 0.30 %, regardless of adsorption time. Therefore, it can be concluded that ThO_TSA reaches its saturation point much sooner than ThO_Ox. The latter has thus a slightly larger adsorption capacity as compared to ThO_TSA. A saturation point was not reached for ThO_Ox. It is uncertain whether ThO_Ox reached a saturation point after 1 week or if it is able to further increase its adsorbed mass past 1 week. This has to be verified in future research by extending the adsorption time of 1 week. SCK CEN/39400143 Rev. 1.0

65 Linking these conclusions to the earlier established surface properties, this is rather counterintuitive. As ThO_Ox had a smaller BET area, overall pore volume and has presumably smaller mesopores than ThO_TSA, it is expected that ThO_Ox would adsorb a smaller quantity of gas. At smaller adsorption times (< 1 hour), this seems to be the case. However, at larger adsorption times, it is clear that ThO_TSA did not absorb any more gas than with smaller adsorption times. Therefore, the BET area and pore volume appear to not be sufficient to find an explanation for the observed difference in adsorption capacity and the difference in influence of adsorption time on the adsorbed quantity.

An important parameter that is known to influence the sorption behaviour, in particular the adsorption of water molecules, is the preferential exposed face of the crystal lattice on the surface [50]. As mentioned in section 3.5, thorium dioxide is assumed to be a face-centered cubic crystal and therefore exposes either the (100) or the (111) crystal face, or a combination of both. If one accepts that the adsorption of water in large quantities is to be attributed to the formation of surface hydroxyl groups and the subsequent slow hydration of water molecules on the basis of one water molecule per surface hydroxyl group, the difference in preferential orientation greatly influences the sorption mechanics of

water on the surface. Since the presence of adsorbed water greatly influences the sorption of CO2 [54], it might proof useful to dive deeper in some of the models that were proposed for hydrated thoria surfaces exposing either the (100) or the (111) crystal face [50]. The most important feature of the

model proposed for a hydrated (100) ThO2 surface was that the hydrogen bonding capacity of the water molecules is fully saturated, making additional adsorption by means of hydrogen bonding not possible. This surface type effectively limits the amount of water that can be adsorbed as each water molecule is able to form a maximum of 4 hydrogen bonds. On a hydrated (100) crystal face, 2 bonds are formed with the underlying hydroxyl groups on the surface and 2 bonds are formed with adjacent water molecules. This saturated hydrated layer, visible in figure 26, might explain the stronger bonding of this layer to the underlying hydroxyl layer, making it more difficult to desorb and thus more efficient

in closing off micropores and trapping CO2.

The proposed model for exposing preferentially the (100) crystal face was preferred for a ThO2 powder derived from oxalate precipitation and calcined at 1000°C, which is similar to our ThO_Ox sample [50].

Conversely, the model for hydrated (111) ThO2 surface allowed for additional adsorption of water by means of hydrogen bonding due to the fact that although water molecule can make 4 hydrogen bonds, only three bonds can be established in this arrangement. Since hydrated surfaces are known to be hydrophilic in character [12], this model allows for additional water adsorption on the surface via

hydrogen bonding, possibly limiting CO2 adsorption. 5.4.3 Proposition for possible adsorption mechanism on ThO_Ox and ThO_TSA In what follows, a proposition is done for the sorption mechanisms on both the ThO_Ox surface and the ThO_TSA surface. It should be noted that an important assumption is made for the proposed mechanisms, i.e. that the surface is already mostly covered in hydroxyl groups and is (at least partially) hydrated. This assumption was made on the basis that surface hydroxyl groups form rather easily on

metal oxide surfaces as a consequence of the interaction of water with the bare ThO2 surface. Furthermore, the hydrophilic nature of the hydroxyl groups is expected to prefer water molecules

rather than CO2. Additionally, one should note that the data acquired during this research is not sufficient to propose a definitive adsorption mechanism. For this, the surface chemistry as well as XRD results are required data to obtain from both samples. Therefore, uncovering the underlying adsorption mechanisms is ambitious and a careful approach to the following model is recommended.

On the basis of the aforementioned models [50], a mechanism which might explain why ThO_Ox

adsorbs a higher quantity of gas is proposed. The model for the hydrated (100) ThO2 surface does not allow for additional water adsorption via hydrogen bonding which was pointed out earlier to be the SCK CEN/39400143 Rev. 1.0

66 (100) exposed crystal face

Figure 26: A suggested CO2 multilayer adsorption mechanism for a pure (100) exposed crystal face on ThO_Ox. Red dotted lines present the hydrogen bonds. Note that each adsorbed watermolecule has 4 hydrogen bonds, restricting further water adsorption by means of hydrogen bonding. The partial charges are also shown. primary mechanism for multiple adsorbed water layers. Therefore, additional molecular water is unable to adsorb on top of the first hydrated layer by means of hydrogen bonding. This allows for

easier access to the CO2 adsorption sites and more CO2 can be adsorbed. It is known that due to its

quadrupole moment, there is a strong quadrupole-quadrupole interaction between CO2 molecules

[59]. Therefore, it might be possible that CO2 multilayer adsorption occurs on top the hydrated surface, explaining the ever increasing adsorbed quantity with higher adsorption times. Multilayer adsorption

is directly related to physical adsorption rather than chemical and CO2 is thought to occur in its linear

form in the physisorption layer. Attractive lateral CO2-CO2 interactions [55] presumably help keeping

the multilayer together. Figure 26 presents a proposition for this multilayer CO2 adsorption

mechanism on top of the saturated hydrated hydroxyl surface layer. It has been reported that CO2 possibly physically adsorbs upright on the surface [55]. This arrangement seemed to make sense for the proposed multilayer model. It is assumed that when the molecular water adsorbs associative on the hydroxyl layer in the configuration shown in figure 26, the surface becomes “charged” due to the polar properties of the water molecule. The model proposes that due to the specific configuration in which the adsorbed water is found, all water molecules are oriented in the same direction, yielding a

surface that is slightly positively charged. CO2 molecules can subsequently adsorb on the surface as

they possess a similar partial charge. The attractive lateral CO2-CO2 interactions, that now occur vertically, allow the multilayer to thicken. This could explain how the adsorbed mass steadily increases with adsorption time. It is thought that the multilayer thickens with longer adsorption times. However, this is a simplification as no bidentate or monodentate carbonates were shown to form in the proposed

model for CO2 multilayer adsorption. In reality, the adsorbate structure is expected to be far more complex and would consist of numerous defects as opposed to the ideal adsorbate layer proposed in SCK CEN/39400143 Rev. 1.0

67 figure 26. Although ThO_Ox prefers exposing the (100) crystal face, other crystal surfaces like (111) are

assumed to be also present on the surface albeit to a lesser extent. These imperfections in the ThO2 surface structure as well as in the hydrated hydroxyl layer allow for bidentate and monodentate

structures to form through the direct interactions of CO2 molecules with the hydroxyl groups. Chemically, stronger bound bicarbonates are then formed [54]. This can occur frequently since the hydroxyl groups are only “shielded” behind a single layer of adsorbed water molecules which further facilitates access to the hydroxyl groups. Furthermore, it is also possible that the defects in the

adsorbed water layer allow CO2 to react with the associative chemisorbed water directly, forming carbonic acids [54].

Therefore, it is expected that ThO_Ox has a relatively high adsorption capacity for CO2 in the context

of non-porous low surface area materials, as almost all surface species of CO2 are theoretically possible

to occur on the surface. Furthermore, the physical adsorption of CO2 molecules in the multilayer, which

is governed by Van der Waal’s forces and lateral CO2-CO2 interaction, allows the continuous physical

adsorption of CO2. Although there is certainly a limit to the adsorbate layer thickness, this thickness seems to be rather dependant on the adsorption time and seems to increase even after a week of adsorption.

The preferential exposed crystal face of ThO_TSA is not known. However, a significantly different morphology was observed in the TEM. Faceting of materials depend on the characteristic surface energy which in turn is dependent on the amount of broken chemical bonds on the surface. In this regard, for a face-centered cubic crystal, (111) is energetically most favourable followed by (100). As the TEM images of ThO_TSA do not show specific preferential surfaces, it is assumed that (111) is more

prevalent on the surface than (100). The model for the hydrated (111) ThO2 surface suggests that the associative adsorbed water molecules are not saturated with hydrogen bonds allowing additional water adsorption to occur through hydrogen bonding. The hydrophilic nature of a hydroxyl surface layer attracts more water which forms a solid layer of adsorbed water. It should be noted that this layer is not expected to have the same structure as shown in figure 26, but rather looks like the water adsorbate layer shown in figure 10. Baltrusaitis et al. [54] suggested that surface adsorbed water in the form of water molecules may block adsorption sites for the formation of bicarbonates, effectively

decreasing CO2 adsorption capacity for this type of powders. As additional water can adsorb on the

hydrated layer, CO2 is effectively obstructed by multiple layers of adsorbed water rather than the single layer of adsorbed water formed on the surface of ThO_Ox. Furthermore, It has be shown that tightly bound chemisorbed molecules are able to exclude physisorption [53]. Since the surface of a (111) crystal face is more densely packed than the surface of a (100) crystal face (figure 27), the physisorption

of CO2 (and thus multilayer adsorption) is not possible on (111) exposing surfaces. Although further research should be performed to confirm the preferred surface exposed crystal face, the latter could explain why the adsorbed quantity on ThO_TSA seems to not be influenced by adsorption time. If

Figure 27: Schematic representation of the (111) (left) and (100) (right) faces of a face centered cubic crystal. A clear difference can be observed in atom density. A (111) surface is more closely packed togheter as compared to (100) [66]. SCK CEN/39400143 Rev. 1.0

68 ThO_TSA would expose predominantly the (111) crystal face, a layer of adsorbed molecular water is

formed after which further adsorption of CO2 is obstructed. It was suggested that in this case, an adsorbate layer of a few monolayers in thickness is formed by the reaction between the associative

adsorbed water layer and the CO2 [54]. In this adsorbate layer, carbonic acid is formed which deprotonates to yield adsorbed carbonates and protonated hydroxyl groups [54]. The limited width of

this adsorbate layer, which is in stark contrast to the suggested CO2 multilayer adsorption thought to be possible on ThO_Ox, could explain the independence of adsorption time for thoria powders that prefer to expose the (111) crystal surface. Additionally, the more closely packed associative chemisorbed water molecules are able to exclude physisorption processes.

Therefore, ThO_Ox has more sites for CO2 adsorption than ThO_TSA, which is confirmed by additional

CO2 desorption peaks as compared to ThO_TSA in figure 25(a). This suggests that ThO_TSA is more suitable for long term storage applications due to its lower sensitivity to adsorption time (and thus

storage period) as well as a lower quantity of adsorbed CO2. SCK CEN/39400143 Rev. 1.0

69 SCK CEN/39400143 Rev. 1.0

70 6. Conclusion The sorption behaviour of atmospheric gases on two thoria powders produced via a different precipitation routes was studied. The first sample group was thoria powder derived from oxalate precipitate and the second sample group was thoria powder derived from the novel 2-step alkali

precipitation route. SEM/TEM images,N 2 isotherm analysis, thermogravimetry and mass spectrometry

were used to study the sorption behaviour of CO2 and H2O on both thoria surfaces. The following conclusions were made.

Firstly, reusing the same thoria sample for multiple adsorption/desorption cycles heavily influences adsorption capacity due to the closure of the pores at sintering temperatures. This was reported in literature and was also observed during this research. Therefore, it should be noted that applications for thorium dioxide as a catalyst are limited due to the limited uses.

Secondly, from the type II(b), it was concluded that both powders behaved mesoporous-like due to the interparticular cavities of mesopore size. Pore analysis showed a larger BET area and pore volume for ThO_TSA. SEM and TEM images showed that the individual particles were non-porous and that the powder morphology differed significantly. ThO_Ox showed plate-like particles while ThO_TSA showed sphere-like particles. Both clumped together to form aggregates. Concerning the surface analysis, ThO_TSA had a larger BET area and pore volume.

Thirdly, the sorption experiments showed the desorption of CO2 at high temperatures. This was

particularly the case for ThO_Ox, which showed a broad CO2 desorption peak around 800°C. This same release was not observed in ThO_TSA. This was attributed to the difference in pore geometry and powder morphology.

Additionally, all forced adsorption experiments were performed with samples in the 2nd cycle. Therefore, water had presumably irreversibly adsorbed to the powders during the first cycle.

Therefore, it was not possible to study the sorption behaviour of CO2 on a bare ThO2 surface. After all,

water is known to react with CO2 molecules forming a multitude of CO2 surface species, such as

bidentate and monodentate carbonates and bicarbonates which form when CO2 interacts the surface

hydroxyl groups, and carbonic acids which form when CO2 interacts with associatively chemisorbed water molecules.

Lastly, a model was carefully proposed for the adsorption mechanism involving CO2 and H2O on thorium dioxide surfaces. Both powders showed very different sorption behaviours with respect to the adsorption time. This was attributed to the different powder morphology and more in particular, the different preferential exposed crystal face on the surface. ThO_Ox is known to predominantly expose the (100) crystal face on the surface while ThO_TSA was suggested to predominantly expose the (111) crystal face. The exposed crystal face most notably influences the adsorption behaviour of water on

the surface. The difference in the water adsorption mechanism then causes CO2 to interact in a

different way on each sample. It was concluded that ThO_Ox allowed for more CO2 adsorption in general. This could occur either via chemisorption on the dissociative adsorbed hydroxyl groups on the surface to the form of bicarbonate structures or on the associative adsorbed water molecules. Physisorption on top of the hydrated layer was also considered and was made possible through the surface charge induced by the polarity of the hydrated layer. For the latter, a proposition was done for

the structure of such an CO2 adsorbate multilayer in which CO2 vertically adsorbs to the hydrated layer. The thickness of the adsorbate layer is therefore dependent on the adsorption time. For ThO_TSA, the adsorbate layer is thought to have a very different and more complex structure. The more densely packed atoms on a (111) crystal face caused more tightly bound chemisorbed hydroxyl groups and

hydrated layer which excluded possible CO2 physisorption. CO2 can directly interact with the SCK CEN/39400143 Rev. 1.0

71 associative chemisorbed water molecules, yielding carbonates. A direct interaction with the hydroxyl layer is less common due to the thicker adsorbed water layer. Such an adsorbate layer is a few monolayers in thickness and does not allow for multilayer adsorption, causing it to reach its saturation point much earlier than ThO_Ox.

For this reason, we should recommend the use of the 2-step alkali precipitation route when long storage periods are expected. Not only is the adsorbed quantity less dependent on exposure time, the

amount of CO2 adsorbed is also expected to be less than with oxalate derived thoria powder. SCK CEN/39400143 Rev. 1.0

72 7. Outlook COVID-19 complicated some of the experimental work by limiting the possibility for repetitions. Therefore, some of the lessons we learned during early experiments could not be applied. For this reason, a few recommendations are made for future research.

When studying sorption behaviour, one should not use the same powders during multiple adsorption/desorption cycles. Repeated use severely decreases adsorption capacity and possibly changes adsorption behaviour. Freshly calcined samples are recommended for each new sorption experiment. The forced adsorption experiment were always performed in their second cycle of reuse.

This means that water was able to irreversibly chemisorb to the surface, limiting CO2 adsorption. To be increase certainty that no water is irreversibly chemisorbed to the surface, we recommend performing the calcination of the powder and the forced adsorption experiment sequentially without interruption. This way, the amount of water that can adsorb is minimized and one can study the

adsorption mechanisms of CO2 on bare ThO2 surfaces without the presence of chemisorbed water.

Furthermore, as the measured mass losses during desorption in the TGA for at 1000°C calcined powders are very small (<<20mg), we recommend setting the measurement range of the TGA to small rather than to large as this would provide more detail on the mass loss behaviour.

Accurately assessing the underlying sorption mechanisms is an ambitious task. Although a plausible explanation was given for the clear difference in adsorption capacity and influence of adsorption time between both thoria powders, no clear links were found between the location of certain desorption peaks and physical phenomena. Further research is recommended on the subject to uncover the intricacies behind the observed sorption mechanisms. SCK CEN/39400143 Rev. 1.0

73 SCK CEN/39400143 Rev. 1.0

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78 Appendix I

atomic Abundance V V Nuclide Half-life* a f number (a/0) (barns)** (barns)*** 0 n 12m 1 H_1 99.985 333mb H_2 0.015 0.53mb H_3 12.33y 3 Li_6 92.5 941 Li_7 7.42 45.7mb 5 B_10 19.6 3840 B_11 80.4 5.5mb 6 C_12 98.89 3.4mb C_13 1.11 1.37mb C_14 5736y 7 N_14 99.64 1.9 N_15 0.36 24μb 8 O_16 99.756 0.190mb O_17 0.039 0.239 O_18 0.204 0.16mb 53 I_135 6.7h 54 Xe_135 9.17h 2.65x106*** 61 Pm_143 53.1h 62 Sm_149 13.83 41000*** 90 Th_232 100 1.41x1010y5.13 Th_233 23.3m 1465 15 92 U_233 1.592x105y 575*** 529*** U_234 0.0055 2.46x105y 103.47 0.465 U_235 0.72 7.038x108y 687.0*** 587*** U_236 2.34x107y5.2 U_238 99.27 4.68x109y 2.73*** U_239 23.5m 36 14 94 Pu_239 24110y 1020*** 749*** Pu_240 6564y 289.5 0.064 Pu_241 14.35y 1378 1015 Pu_242 3.733x105y 10.3 <0.002 *m = minute, h = hour, y = year ** Cross secitons at 0.0253 eV or 2200 m/s. *** non-1/v absorber SCK CEN/39400143 Rev. 1.0

79

SCK CEN/39400143 Rev. 1.0

80 Appendix II When the surface is homogeneous, the concentration of the adsorbate in the surface layer is constant for the complete surface. The equilibrium constant for such a system can be written as:

௦ ௦ ݂ ܿ (1) ܭൌ ݂ܿ where cs and c are the concentrations of the adsorbate in the surface layer and gas phase respectively. Analogous aref s andf the activity coefficients in the surface layer and in the gas phase. The equilibrium constant K is only in function of the temperature and thus represents an adsorption isotherm as it relates cs to c. Equation (1) can then be rewritten to:

ܭ݂ ܿ ௦ ൌ ܿ (2) ݂௦ This adsorption isotherm is however, not linear as fs and f are dependent on the concentration. Assuming f = fs = 1 when dealing with low concentrations of adsorbate (pressures up to 105 N/m²) and rewriting the equation using the ideal gas law yields the following equation:

ܭ (3) ܿ௦ ൌ ݌ ܴܶ It is possible to calculate the total amount a of adsorbate (in mol/g) in the volume of the surface layer using the following equation:

௦ ௦ (4) ܽൌ ݏ ݈ ܿ where s represents the specific surface area of the adsorbent and l s the thickness of the surface layer. It applies now that (5) ௦ ௦ ܸ ൌݏ ݈ And thus Vs represents the volume of the surface layer. The following equation can now easily be derived from equation (4) and (5):

(6) ௦ ௦ ܭ ܽൌ ܸ ܭܿൌ ܸ ݌ ܴܶ For a given adsorption system where T is taken to be constant, V s and K are also constant. This yields the following expression and is known as Henry’s adsorption isotherm:

(7) ܽൌ ܭ௔ǡ௣݌ with the corresponding Henry’s constant. This adsorption law is formally identical to Henry’s law for gas absorption in liquids and therefore, carries the same name. However, instead of the amount adsorbed a or the surface concentration c s, the fractional coverage of the surface θ is often used. This quantity is defined as the number of occupied adsorption sites compared to the total number of available sites. Henry’s adsorption isotherm can now be expressed in terms of the surface coverage θ.

௦ ܿ ܽ ܭ ܭ௔ǡ௣ (8) ൌ ௦ ൌ ൌ ௦ ݌ൌ ݌ ܿ௠ ܽ௠ ܴܶ ܿ ௠ ܽ௠ From equation (8) can be deduced that the fractional coverage of the adsorbent surface in Henry’s region is proportional to the pressure of the adsorbate in gas phase. However, at the end of the 19th SCK CEN/39400143 Rev. 1.0 (9) 81 century, Boedecker proposed an empirical equation for the adsorption isotherm in the following form [31], [32]:

ଵȀ௡ (10) ܽൌ݇݌ where k and n are constants that depend on the adsorbent and gas at a given temperature. Equation (10) is also known as the Freundlich adsorption equation/isotherm as he popularized its application. Freundlich was one of the first who thought of the adsorption process as a form of surface condensation. SCK CEN/39400143 Rev. 1.0

82 Appendix III

Figure 28: (repetition of figure 6) Classification of physisorption isotherms as proposed by the IUPAC technical report of 2015 [37] Type I isotherms are the result of microporous (pores < 2 nm) solids with small external surfaces. It is characterized by the flattening of the isotherm towards higher relative pressures indicating a plateau. This plateau is a direct result of the micropore volume limiting the uptake of adsorbate as opposed to the internal surface area as limiting factor. A steep curve at low relative pressures results from enhanced interactions in narrow micropores between the adsorbate and the adsorbent. This results in

micropore filling at low relative pressures. When using N 2 or Ar at 77K, type I(a) isotherms are found by microporous materials with narrow micropores (referring to a width of less than 1 nm). Type I(b) isotherms result from materials with a broader range of pore sizes. This includes wider micropores and narrow mesopores (pore width 2-50 nm). Type I isotherms are reversible.

Type II isotherms are usually the result of physisorption of most gases to non-porous or macroporous (pore width > 50 nm) materials. Towards high relative pressures, the adsorbed quantity seems to increase dramatically and therefore indicates unrestricted monolayer-multilayer adsorption. The thickness of the adsorption layer can increase infinitely towards p/p0 = 1. The knee indicated by “B”, represents the completion of a monolayer. A sharp knee shows a clear completion of a monolayer. If the curve is smoother around this area, there is a significant overlap between the completion of a monolayer and the beginning of the multilayer adsorption. Type II isotherms are also reversible. SCK CEN/39400143 Rev. 1.0

83 However, it should be noted that hysteresis can occur on type II isotherms. In this case, it is referred to as a type II(b) isotherm [24].

Type III isotherms do not have the distinct knee (point B). This type of isotherm therefore does not show the completion of a specific monolayer. There are relatively weak adsorbate-adsorbate interactions present and molecules are clustered around the most desired adsorption sites. This type of isotherm also indicates a non-porous or macroporous solid. Note that the adsorbed quantity at a relative pressure equal to 1 is now limited as oppose to the unlimited uptake associated with type II isotherms.

Type IV isotherms are the result of mesoporous adsorbents. The adsorption behaviour in mesopores is governed by the interactions between the adsorbate and the adsorbent as well as interactions between molecules in a condensed state as capillary condensation occurs inside mesopores. In this case, the initial forming of a monolayer and subsequently a multilayer corresponds to the adsorption behaviour seen in type II isotherms and causing type IV isotherms to share the same shape for the first half of the isotherm after which pore condensation occurs. Adsorbed gas now condenses to a liquid- like phase inside pores at pressures lower than the saturation pressure p 0 of the bulk liquid. Typically, type IV isotherms end in a saturation plateau. This plateau can have different lengths and might be reduced to a mere inflection point when the plateau is very short. Type IV(a) show hysteresis behaviour. This occurs when the pore size exceeds a critical size. The critical width of a pore is a function of the temperature and the general adsorption system. If the pore width is below this critical value, the adsorption is completely reversible resulting in type IV(b) isotherms. Generally, type IV(b) isotherms are given by cone- and cylinder-shaped mesopores that are closed at the tapered end.

Type V isotherms resemble type III isotherms for low relative pressures. This is the result of the weak adsorbate-adsorbent interactions. At higher relative pressures, molecular clustering occurs which is followed by pore filling.

Type VI isotherms show clear discrete steps, representing an adsorption process that occurs layer-by- layer. This is typical for uniform surfaces on non-porous materials. The step height is an indication for the adsorption capacity of each individual layer. SCK CEN/39400143 Rev. 1.0

84