1 Synthesis and characterisation of hydrotalcites of formula

2 Ca6Al2(CO3)(OH)16·4H2O 3 4 Ray L. Frost,  Sara J. Palmer and Frederick Theiss 5 6 1 Inorganic Materials Research Program, School of Physical and Chemical Sciences, 7 Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, 8 Australia. 9 10 ABSTRACT 11 12 Hydrotalcites containing calcium are a naturally occurring mineral which has been 13 successfully synthesised. Insight into the unique structure of hydrotalcites has been obtained 14 using a combination of X-ray diffraction, infrared and Raman spectroscopy. Calcium

15 containing hydrotalcites of formula Ca4Al2(CO3)(OH)12·4H2O (2:1 Ca-HT) to

16 Ca8Al2(CO3)(OH)18·4H2O (4:1 Ga-HT) have been successfully synthesised and characterised 17 by X-ray diffraction and Raman spectroscopy. The d(003) spacing varied from 7.83 Å for 18 the 2:1 hydrotalcite to 7.89 Å for the 3:1 calcium containing hydrotalcite. Raman 19 spectroscopy complimented with selected infrared data has been used to characterise the

20 synthesised calcium containing hydrotalcites of formula Ca6Al2(CO3)(OH)16·4H2O. Raman -1 21 bands observed at around 1083, 1085 and 1087cm were attributed to the ν1 symmetric 2- 2- 22 stretching modes of the (CO3 ) units. Multiple ν3 CO3 antisymmetric stretching modes are 23 found at around 1406 and 1473cm-1. The splitting of this mode by ~67 cm-1 indicates the 24 carbonate anion is in a strongly perturbed state. Raman bands observed at 711 and 712 cm-1 2- 25 assigned to the ν4 (CO3 ) modes support the concept of multiple carbonate species in the 26 interlayer. 27 28 29 Keywords: hydrotalcite synthesis, hydrocalumite, hydrocalumite, Raman spectroscopy, 30 calcium, thermal stability

 Author to whom correspondence should be addressed ([email protected]) 1

31 Introduction 32 33 Hydrotalcites or layered double hydroxides (LDHs), have been known for an 34 extended period of time. [1-3] Hydrotalcites, are fundamentally known as anionic clays [4]. 35 Hydrotalcites consist of stacked layers of metal cations (M2+ and M3+) similar to

36 (Mg(OH)2). The structure of hydrotalcite can be derived from a brucite structure (Mg(OH)2) 37 in which e.g. Al3+ or Fe3+ (pyroaurite-sjögrenite) substitutes a part of the Mg2+ [2, 5-7]. This 38 substitution creates a positive layer charge on the hydroxide layers, which is compensated by 39 interlayer anions or anionic complexes. In general any divalent cation including calcium 40 could substitute for the Mg in the brucite-like layer. Equally as well, any trivalent cation may 41 substitute for in the brucite layer. In hydrotalcites, a broad range of compositions 2+ 3+ 2+ 3+ 42 are possible of the type [M 1-xM x(OH)2] x/n.yH2O, where M and M are the di- and 43 trivalent cations in the octahedral positions within the hydroxide layers with x normally 44 between 0.17 and 0.33. An- is an exchangeable interlayer anion. [8] The positively charged 45 hydroxyl layers are neutralised through the intercalation and adsorption of anionic 46 species, therefore stabilising the structure. Anions that are intercalated between the 47 hydroxyl layers need to meet certain criteria, including having a high charge density and 48 small anionic radius. 49 50 To the best of the authors’ knowledge, no comprehensive studies of hydrotalcites with 51 the replacement of the by calcium have been reported. There is some evidence 52 that in bauxite, calcium is found as a minor impurity as calcium hydroxide [9-11]. The 53 reaction of red mud and seawater results in the formation of hydrotalcites based not only 54 upon magnesium but also calcium. This is the basis of the underlying reason why this 55 research is being undertaken. This study focusses upon the synthesis, and spectroscopic 56 characterisation of hydrotalcites with calcium substituting for magnesium in the brucite 57 layer. 58 59 Experimental 60 61 Synthesis of hydrotalcite samples 62 Co-precipitation is probably the best technique for the synthesis of hydrotalcites, as it 63 allows homogeneous precursors as starting materials. For co-precipitation it is necessary to 64 work under conditions of supersaturation mostly achieved by variation in pH [12, 13]. Two 2

65 frequently used techniques are coprecipitation at low [14-17] and at high supersaturation [18- 66 20]. In this study we used the latter route for the preparation of hydrotalcites with Ca2+ 3+ 2+ 3+ 2- 67 combined with Al in a molar ratio M /M of 6/2 and CO3 as charge balancing anion. 68 69 Hydrotalcites can be synthesised in the laboratory using analytical grade chemicals. 70 The hydrotalcites were synthesised by the co-precipitation method. Two solutions were 2+ 71 prepared, solution 1 contained 2M NaOH and 0.2 M Na2CO3, and solution 2 contained Ca 3+ 72 as CaCl2 at different concentrations, together with Al (AlCl3.6H2O). Solution 2 was added 73 at a steady rate to solution 1 drop wise, under vigorous stirring. A separating funnel was used 74 to deliver solution 2 to solution 1. The precipitated minerals were washed at ambient 75 temperatures thoroughly with ultra pure water to remove any residual salts and dried in an 76 oven (85 °C overnight. 77 2+ 78 M is based upon CaCl2 : 2:1 2.5:1 3:1 3.5:1 4:1

Concentration of CaCl2 0.67M 0.71M 0.75M 0.77M 0.80M

Masses of CaCl2 3.72g 3.94g 4.16g 4.27g 4.44g

Concentration of AlCl3.6H2O 0.33M 0.29M 0.25M 0.22M 0.20M

Masses of AlCl3.6H2O 3.98g 3.45g 3.02g 2.68g 2.41g 79 80 Table 1 Table of concentrations for the synthesis of calcium hydrotalcites 81 82 Raman spectroscopy 83 84 FT-Raman spectra were recorded on a Perkin-Elmer system 2000 FT-Raman spectrometer 85 (Perkin-Elmer, Beaconsfield, UK) equipped with a quartz beam splitter and an InGaAs 86 detector at ambient temperature. One hundred scans at a resolution of 4 cm-1 were 87 accumulated for each sample using a mirror velocity was 0.1 cms-1 and strong Beer–Norton 88 apodization. Laser excitation was provided by an Optomech continuous wave Nd:YAG laser 89 emitting at 1064 nm. Laser power was 200 mW.

90 91 Infrared spectra (over the 4000-525 cm-1 range) were obtained using a Nicolet Nexus 92 870 FTIR spectrometer with a smart endurance single bounce diamond ATR cell. Spectral

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93 manipulation such as baseline correction/adjustment and smoothing were performed using the 94 Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band 95 component analysis was undertaken using the Jandel ‘Peakfit’ software package that enabled 96 the type of fitting function to be selected and allows specific parameters to be fixed or varied 97 accordingly. Band fitting was done using a Lorentzian-Gaussian cross-product function with 98 the minimum number of component bands used for the fitting process. The Gaussian- 99 Lorentzian ratio was maintained at values greater than 0.7 and fitting was undertaken until 100 reproducible results were obtained with squared correlations of r2 greater than 0.995. 101 102 Results and discussion 103 104 X-ray diffraction 105 The XRD patterns of the synthesised hydrotalcites containing Ca with varying Ca/Al 106 ratios are displayed in Fig. 1. The (00l) reflections (003), (006) and (009) are easily 107 recognised, although the last one shows overlap with the (012) and (015) reflections resulting 108 in a broad signal between 40 and 45° 2θ. Furthermore, the two reflections of (110) and (113) 109 can be clearly distinguished between 70 and 75° 2θ. The (00l) reflections are characterised by 110 high intensities combined with broad line shapes indicating that the hydrotalcites are of 111 relatively high crystallinity but with very small crystallites. No other crystalline phases can be 112 detected in the XRD patterns indicating that all three syntheses were successful. The position 113 of the d(003) peak varies from 7.85Å for the 4:1 (Ca/Al) hydrotalcite to 7.89Å for the 3:1 114 HT. The width of the d(003) peak is constant; thus showing the crystallinity of the HT does 115 not vary with the divalent/trivalent ratio. Fig. 1 shows the possible impurities in the HTs.

116 Hydrocalumite Ca2Al(OH)6Cl·2H2O is apparently present in all of the synthesised samples, 117 even though it is in very low concentrations. 118 119 Raman and Infrared Spectroscopy 120 121 The calcium containing hydrotalcite (Ca-HT) of 3:1 ratio has a formula

122 Ca6Al2(CO3)(OH)16·4H2O. In the synthesis of the Ca-HT, the ratio of Ca to Al was designed

123 to vary from 4:1 to 2:1. This means the formula will vary from Ca8Al2(CO3)(OH)18·4H2O to

124 Ca4Al2(CO3)(OH)12·4H2O. Irrespective of the formula, each hydrotalcite contains vibrating 125 units of the carbonate anion, the surface hydroxyl units and water. The summation of the 126 Raman spectra of these vibrating units will be the Raman spectrum of the HT. In addition 4

127 metal-oxygen units will contribute to the Raman spectra. The complete Raman spectra of all 128 of the synthesised Ca-HTs are displayed in Figure 2. This spectrum shows the relative 129 intensity of the different bands. The Raman spectra in the 1000-1100 cm-1 region for the 130 synthesised hydrotalcites are displayed in Figure 3. The band centered upon 1085 cm-1 is not 131 symmetric and component bands may be resolved. The unperturbed carbonate is a planar

132 triangle with point symmetry D3h. Group theoretical analysis of the carbonate ion predicts - 133 four normal modes: the 1 symmetric stretch of A1` symmetry normally observed at 1063 cm 1 -1 134 , the anti-symmetric stretch of E` symmetry observed at 1415 cm , the 2 out of plane bend -1 -1 135 at 879 cm and the in-plane bend at 680 cm . For the unperturbed carbonate anion, the 1 136 mode is Raman active only. For the perturbed carbonate anion, all modes are Raman and

137 infrared active, except for the 2 mode, which is IR active only. For the 3:1 Ca-HT, Raman -1 138 bands are observed at 1084, 1086 and 1087 cm . These Raman bands are attributed to the ν1 2- 139 CO3 symmetric stretching mode. The observation of multiple symmetric stretching modes 140 provides evidence of carbonate anions involved in different environments. 141 142 The infrared spectra in the 1100 to 1700 cm-1 region are shown in Figure 4. 143 Variation in intensity is observed for the different Ca-HTs. A series of infrared bands are 144 observed at around 1406, 1475 and ~1640cm-1. The first two infrared bands are attributed to 2- 2- 145 the ν3 CO3 antisymmetric stretching modes. Splitting of the ν3 CO3 mode is observed 146 with values of around 67 cm-1. If the carbonate anion was unperturbed, then a single band at 147 around 1415 cm-1 would be expected. The fact that multiple bands are observed proves that 148 the carbonate anion is perturbed in the Ca-HT structure. The observation of multiple bands 149 also supports the concept of more than one species of carbonate anion in the HT interlayer. 150 151 The broad infrared band of low intensity centered upon ~1640 cm-1 is assigned to the 152 water bending mode. The band is of quite low intensity, which is not unexpected for the 153 Raman peak of the water bending mode. The position of the band is indicative of water 154 which is hydrogen bonded. The position of the band for non hydrogen bonded water as 155 occurs with water vapour and water in the tunnels of zeolites is around 1595 cm-1. Very 156 strongly hydrogen bonded water provides bands in positions higher than 1640 cm-1. Bands 157 may be found as high as 1675 cm-1. The band shifts to lower wavenumbers with increasing 158 Ca/Al ratios. 159

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160 There are two bending modes of the carbonate anion, one bending vibration is -1 -1 161 normally found at around 880 cm (ν2) and the second at around 680 cm (ν4). The ν2 162 bending mode is extremely weak in the Raman spectrum and is often not observed. However, 163 the band is observed in the infrared spectrum. The Raman spectrum of the 700 to 715 cm-1

164 region is shown in Figure 5. This region is where the ν4 mode is observed. The band profile 165 is not symmetric and for the 4:1 Ca-HT two Raman bands are resolved at 711 and 713 cm-1. 166 For the 3:1 Ca-HT the bands are observed in similar positions. There appears to be a shift in 167 the band positions as the Ca:Al ratio is reduced. The observation of two bands in this 168 spectral region supports the concept that more than one carbonate anion is present in the HT 169 structure. One likely model is based upon water hydrogen bonded to the carbonate anion and 170 a free or weakly hydrogen bonded carbonate anion. The infrared spectrum of the Ca-HTs -1 171 (Figure 6) shows a single symmetric band at around 874 cm assigned to the ν2 bending 172 mode. In this set of infrared spectra, a low intensity band at around 712 cm-1 assigned to the

173 ν4 bending mode. The observation of this band provides evidence for the perturbation of the 174 carbonate anion and reduction in its symmetry. Such a band should be infrared inactive and 175 Raman active. Only when the carbonate anion is perturbed is such a band observed. The 176 broad band centered upon 787 cm-1 is thought to be a water librational mode. This band is not 177 observed in the Raman spectra. 178 179 The Raman spectra of the Ca-HTs in the 500 to 575 cm-1 region are shown in Figure 7. The 180 bands in this spectral region are the second most intense band in the Raman spectra. The 181 intensity of this band infers that it is due to a symmetric stretching vibration. One possibility 182 is that the band originates from the carbonate-water unit. The two hydrogens of the water 183 molecule hydrogen bond to the two oxygens of the carbonate. The 549 cm-1 band appears to 184 be unique to the Raman spectrum for the hydrotalcite structure. It is proposed that this band 185 is a result of Al-O-Al linkages in the HT structure. Such a concept is supported by the -1 186 infrared spectrum of the 3 region where splitting is observed with some 30 cm difference.

187 This means the symmetry of the carbonate has been reduced from D3h to C2v. The proposed -1 188 model is of C2v symmetry. The Ca3OH deformation mode is observed at 979 cm but is very -1 189 weak. The Al3OH deformation mode is around 935 cm but is masked by the intense CO 190 stretching mode of the carbonate. Bands in this position are observed in minerals such as 191 boehmite. 192

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193 In brucite type solids, there are tripod units of M3OH with metal such as M, M’, 194 M”. In hydrotalcites such as those based upon Ca and Al of formula

195 Ca6Al2(OH)16(CO3)·4H2O, a number of statistical permutations of the M3OH units are

196 involved. These are Ca3OH, Al3OH and combinations such as

197 Ca2 AlOH, Al2CaOH, and even CaAlOH. These types of units will be distributed according 198 to a probability distribution according to the composition. In this model, a number of 199 assumptions are made, namely that the molecular assembly is random and that no islands or 200 lakes of cations are formed. Such assembly is beyond the scope of this work but needs to be

201 thoroughly investigated. In the simplest case namely Ca6Al2(OH)16(CO3)·4H2O the types of

202 units would be Ca3OH, Ca2AlOH, CaAl2OH and Al3OH. In a somewhat oversimplified

203 model, for the Ca6Al2(OH)16(CO3)·4H2O hydrotalcite, the most intense bands would be due

204 to the Ca3OH and Al3OH bands. One of the difficulties of obtaining the infrared spectra of 205 hydrotalcites is the overlap of the water hydroxyl stretching bands with those of hydroxyls 206 bound to metal centres. The infrared absorption of water is so intense that the hydroxyl 207 absorption bands of the metal hydroxides are difficult to detect. Water is a very poor Raman 208 scatterer and so Raman microscopy is more useful for the measurement of the OH stretching

209 region of the M3OH units for the study of hydrotalcites. The Raman and infrared spectra in 210 the OH stretching region are reported in Figures 8 and 9. Raman bands are observed at 211 around 3370 and 3450 cm-1 for the 2:1 Ca-HT. Bands are observed in similar positions for 212 the other Ca-HTs. The problem with the Raman spectra of the OH stretching region is that 213 the intensity of the scattered radiation is so weak this far from the excitation line. This is why 214 the spectra are so noisy. One reason is that the detector droops really badly and is almost dead 215 at 3500 cm-1. The true peak maximum may not be at 3445 cm-1 in that first spectrum because 216 the apparent band profile on the high wavenumber side of the band may just be a measure of 217 the decreasing detector sensitivity. However, there is some residual detectivity out to 3800 218 cm-1. 219 220 In the infrared spectra, more complexity exists and more component bands are resolved. One 221 model for the assignment of the infrared bands depends on the statistical assessment above. -1 222 The bands at 3664, 3636 and 3593 cm are assigned to the Al3OH and Ca3OH stretching 223 vibrations. These bands are in similar positions for the 4:1, 3.5:1 and 3:1 Ca-HTs. The band -1 224 at 3642 cm is assigned to the mixed species such as CaAl2OH, Ca2AlOH. These bands are 225 not so clearly observed for the 2.5:1 and 2:1 Ca-HTs. The infrared bands at the lower 226 wavenumbers at 3056, 3363, 3490 cm-1 are attributed to water stretching bands. The band at 7

227 around 3050 cm-1 is attributed to water which is strongly bonded to the carbonate anion in the 228 interlayer. 229 230 CONCLUSIONS 231 232 The synthesis of phase pure hydrotalcites containing various divalent metals like Ca is 233 relatively simple and results in the formation of good crystalline material. This material is 234 very suitable for detailed spectroscopic investigations resulting in a much more detailed 235 assignment of the bands in both the infrared and Raman spectra. This study has shown that 236 changes in the composition, by changing the ratio of the divalent to trivalent metal cations, 237 results in small but significant changes in band positions of the modes related to the hydroxyl 238 groups. This is expected as each hydroxyl group in the hydrotalcite structure is coordinated 239 to the two metal cations. This study has also shown that changing the divalent cation content 240 in the hydroxide layers has also a small but significant effect on the interlayer water 241 molecules and carbonate anions as evidenced by small shifts in band positions and the 242 occurrence of doublets, especially for the interlayer carbonate ions. 243

244 Calcium containing hydrotalcites of formula Ca4Al2(CO3)(OH)12·4H2O (2:1 Ca-HT)

245 to Ca8Al2(CO3)(OH)18·4H2O (4:1 Ca-HT) have been successfully synthesised and 246 characterised by X-ray diffraction. The XRD patterns proved that the Ca-HTs were 247 synthesised with high purity. Some hydrocalcumite was observed as an impurity. Raman 248 spectroscopy combined with selected infrared spectroscopy was used to determine the 249 molecular structure of the Ca-containing HTs. Raman bands characteristic of carbonate 250 anions, OH and water units were determined. 251 -1 252 Raman bands were observed at around 1085 cm and were attributed to the ν1 2- 253 symmetric stretching modes of the (CO3 ) units. Component bands were resolved. The 254 observation of multiple symmetric stretching modes were attributed to different types of 255 carbonate units in the interlayer of the hydrotalcite. Two types of carbonate anions were 256 proposed: a) one in which the water molecule is strongly hydrogen bonded to the carbonate 257 anion and (b) one in which the carbonate anion is free of water bonding. There is no 258 evidence of the carbonate anions bonded directly to the brucite-like surface, in which case the 259 Raman band would be predicted to be in the 1080 to 1120 cm-1 range. Two bands associated

260 with the ν4 bending modes supports the concept of different types of carbonate anions. The 8

2- 261 position of the suite of (CO3 ) bands associated with the carbonate ion indicates the 262 carbonate ion is perturbed and not bonded to the metal centres but is strongly hydrogen 263 bonded to interlayer water. 264 265 Insight into the unique structure of hydrotalcites has been obtained using Raman

266 spectroscopy. The hydroxyl-stretching units of Ca3OH and Al3OH are identified by unique 267 band positions. Water plays a unique role in the stabilisation of the hydrotalcite structure. 268 The position and intensity of the Raman bands in the hydroxyl-stretching region indicates 269 that the water is highly structured. The position of the bands in the hydroxyl deformation 270 region of the infrared spectrum supports the concept of structured water between the 271 hydrotalcite layers. Four types of water are identified (a) water hydrogen bonded to the 272 interlayer carbonate ion (b) interlamellar water (c) water hydrogen bonded to the hydroxyl

273 units if the hydroxyl surface and (d) water which bridges the carbonate anion and the M3OH 274 surface. 275 276 Acknowledgements 277 The financial and infra-structure support of the Queensland Research and 278 Development Centre (QRDC-RioTintoAlcan) and the Queensland University of Technology 279 Inorganic Materials Research Program of the School of Physical and Chemical Sciences is 280 gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the 281 instrumentation. 282 283

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284 References 285 286 [1] R. Allmann, Acta Crystallographica, Section B: Structural Crystallography and 287 Crystal Chemistry 24 (1968) 972-977. 288 [2] L. Ingram, H.F.W. Taylor, Mineralogical Magazine and Journal of the Mineralogical 289 Society (1876-1968) 36 (1967) 465-479. 290 [3] H.F.W. Taylor, Mineralogical Magazine 39 (1973) 377-389. 291 [4] V. Rives, Editor, Layered Double Hydroxides: Present and Future, 2001. 292 [5] G. Brown, M.C. Van Oosterwyck-Gastuche, Clay Minerals 7 (1967) 193-201. 293 [6] H.F.W. Taylor, Mineralogical Magazine and Journal of the Mineralogical Society 294 (1876-1968) 37 (1969) 338-342. 295 [7] R.M. Taylor, Clay Minerals 17 (1982) 369-372. 296 [8] J.T. Kloprogge, D. Wharton, L. Hickey, R.L. Frost, American Mineralogist 87 (2002) 297 623-629. 298 [9] S.J. Palmer, R.L. Frost, Journal of Materials Science 44 (2009) 55-63. 299 [10] S.J. Palmer, R.L. Frost, T. Nguyen, 253 (2009) 250-267. 300 [11] S.J. Palmer, H.J. Spratt, R.L. Frost, Journal of Thermal Analysis and Calorimetry 95 301 (2009) 123-129. 302 [12] F. Cavani, M. Koutyrev, F. Trifirò, A. Bartolini, D. Ghisletti, R. Iezzi, A. Santucci, 303 G.D. Piero, J. Catal. 158 (1996) 236. 304 [13] F. Cavani, F. Trifirò, A. Vaccari, Catal. Today 11 (1991) 173-301. 305 [14] S. Miyata, Clays Clay Miner. 23 (1975) 369-375. 306 [15] S. Miyata, Clays Clay Miner. 28 (1980) 50-56. 307 [16] S. Miyata, Clays Clay Miner. 31 (1983) 305-311. 308 [17] S. Miyata, A. Okada, Clays Clay Miner. 25 (1977) 14-18. 309 [18] W.T. Reichle, J. Catal. 94 (1985) 547-557. 310 [19] W.T. Reichle, Solid State Ionics 22 (1986) 135-141. 311 [20] W.T. Reichle, S.Y. Kang, D.S. Everhardt, J. Catal. 101 (1986) 352-359. 312 313 314

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315 List of Figures 316 317 Figure 1 Powder X-ray diffraction patterns of the synthesised calcium hydrotalcites. 318 319 Figure 2 Raman spectra of the synthesised calcium hydrotalcites in the 200 to 3800 cm-1 320 region. 321 322 Figure 3 Raman spectra of the synthesised calcium hydrotalcites in the 1050 to 1125 cm-1 323 region. 324 325 Figure 4 Infrared spectra of the synthesised calcium containing hydrotalcites in the 1100 to 326 1700 cm-1 region. 327 328 Figure 5 Raman spectra of the synthesised calcium containing hydrotalcites in the 700 to 725 329 cm-1 region. 330 331 Figure 6 Infrared spectra of the synthesised calcium containing hydrotalcites in the 650 to 332 900 cm-1 region. 333 334 Figure 7 Raman spectra of the synthesised calcium containing hydrotalcites in the 500 to 335 575cm-1 region. 336 337 Figure 8 Raman spectra of the synthesised calcium containing hydrotalcites in the 3100 to 338 3700 cm-1 region. 339 340 Figure 9 Infrared spectra of the synthesised calcium hydrotalcites in the 2400 to 4000 cm-1 341 region. 342

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343 344 345 Figure 1

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346 347 348 Figure 2

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349 350 351 Figure 3 352 353

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354 355 356 Figure 4 INFRARED

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357 358 359 Figure 5 360 361

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362 363 364 Figure 6 INFRARED

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365 366 367 Figure 7 368 369

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370 371 Figure 8 372 373

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374 375 376 Figure 9

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