The chemisorption of carbon dioxide has been studied on a wide range of metal oxides and the identities of species in the present work have been proposed on the basis of the infra -red data obtained from previous studies. The fonuation of carboxylate type groups, unidentate carbonate, bidentate carbonate, organic type carbonate, bicarbonate, and physically adsorbed carbon dioxide have all been identified by various authors on the basis of infrared measurements Bands can be assigned by the comparison with infrared spectra of inorganic compounds whose structure have been unequivocally determined by other methods.
All the perovskites and metal oxide samples studied here exhibited weak bands attributable to carbonates, fonned possibly on the adsorption of carbon dioxide from the atmosphere prior to the study.
5.4.1: METAL OXIDES
Bands positioned at 1247, 1132, and 987 cm’^ and bands in the region ~ 600 cm'^ were observed in all the spectra.
In studies carried out on tin (IV) oxide by P.G Harrison et al a spectrum taken of the region 1750 - 1150 cm"’ recorded at the ambient temperature of the beam (329K) revealed principal regions of adsorption at ca. 1600 and 1440 cm'^ (broad band), sharp band at 1223 cm '\ a weak broad band at 1380 cm'^ and 1300 cm'^ in addition to a band at 1182 cm’\ All the bands were found to vary with the pretreatment temperatures with the exception of the band at 1182 cm '\ In our study, carbon dioxide adsorption on tin (IV) oxide gave bands at 1247, 1200, 1132, 1016, 985 cin’^ and finally the envelope of bands at 672, 6 6 8, and 649 cm '\
CHAPTER 5: F.T.I.R. STUDIES
No adsorption bands were located in the region 1600 - 1440 cm'^ due to noise in that region of the spectrum.
A feature of the tin (IV) oxide spectrum that bears similarities to the feature found in the study carried out by P.G Harrison et al. was the sharp band at 1247 cm’’ (observed in our study) with that detected at 1223 cm ’. The discrepancy in the band positions can be explained by the difference in the samples utilised (a tin (IV) oxide gel was used by others whereas in our study a tin (IV) oxide powder was used) and possibly the lack of thermal pretreatment of the sample. Therefore, following the assignment by Harrison et al and others this band is identified as the symmetric C = O stretch of a surface bidentate carbonate.
This band also falls close to the general range 1270 - 1260 cm ’ specified for the frequency of the symmetrical stretch of the carbon oxygen double bond of a bidentate type carbonate by Little thus providing further proof that this band might be associated with a bidentate carbonate.
Band growth and desorption characteristics studied by Harrison et al indicated that the band at 1223 cm’’ was associated with the band at ca. 1600 cm’’. Bands at 1540 and 1300 cm’’ were also found to belong to the same type of surface species. The bands at ca. 1600 and 1300 cm’’ arising from the adsorption of carbon dioxide on tin (IV) oxide were also observed by others and have been assigned to the antisymmetric and symmetric CO stretching modes of a surface bidentate carbonate.
Another feature of the spectrum in the present study for tin (IV) oxide that resembles that described by other authors was the weak band located at 1132 cm’’. P.G Harrison et al. observed a band of low intensity at 1182 cm’’ in the adsorption of carbon dioxide on tin (IV) oxide, which was present regardless of whether carbon dioxide or carbon monoxide was the adsorbing gas. This band was
CHAPTER 5: F.T.I.R. STUDIES
not removed by D%0 exchange thus eliminating the possibility that this band may have arisen due to an interaction with surface hydroxyl group to form a surface bicarbonate species. They attributed this band to the CO stretching mode of a bridging surface carbonate by comparison with studies of the system AI2O3 / CO2
(5, 6,11,12,19 - 22)
^
such a band of low intensity was found to be associatedwith a band at 1850 cm'^ also of low intensity. In their study it was concluded that the corresponding high frequency mode was the weak band at ca. 1700 cm '\ In our study the associated band was not detected at the high frequency end of the spectrum due to noise.
Alternatively the band at 1132 cm'^ may be attributed to a bidentate carbonate, as this band was in close agreement with the band located at 1134 cm'^ observed together with associated bands at 1324 and 1545 cm'^ by A.J Goodsel in spectra of CO2 adsorption on cobalt oxide (C03O4), and assigned by them to the symmetric stretching mode of a carbon - oxygen single bond of a bidentate carbonate.
The structure that is thought to give rise to the bands at 1132 cm'^ is shown below:
^ - O - C
o
It may be argued that in view of the fact that the study was carried out at room temperature (and that low temperature is considered to be < 400 K), it is probable that such conditions favour the formation of a bidentate carbonate surface species. Studies carried out at higher temperatures might favour the fonuation of a bridging type of surface carbonate; because high temperatures might give rise to strained oxide links which could react with the carbon dioxide to produce a bridging type of surface carbonate.
CHAPTER 5: F.T.I.R. STUDIES
A somewhat tentative proposal is that the band at 1132 cm'^ may belong to both a bidentate species and a bridged carbonate but such information can only be derived from desorption studies. In the absence of band growth and temperature desorption studies such a proposal cannot be verified.
Supplementary information gained from the results of the in-situ control study in which the samples were exposed to pure carbon dioxide, that was not apparent from 'non in-situ' studies in which the sample were exposed to carbon dioxide outside the FTIR chamber for a period of two hours, concerned the bands at ca. 1247 and 1142 cm'^ . This study revealed that both bands were fonned at the sample surface within the first three minutes indicating that activation energy for this process is small.
The possible presence of a bridged carbonate is also supported by the similarity of the bands seen in this study with that found in the system
C&O
2ICO
2(1728,1396,1219,1132 cm'^). If this is fact the correct assignment we would expect to locate a band ca. 1700 cm'^ and in the region 1200 cm '\ Bands were in fact observed in the 1200 cm"’ region, at 1200 cm'^ (Sn0 2/C0 2), 1202 cm'^ (C11O/CO2), and at 1205 cm'^ (BaC0 3/C0 2). In the perovskites similar bands were located at 1204 cm'^ (BaSn03/C0 2), and 1207 cm'^ (BaSno.99Sbo.01O3/CO2). Noise in the region 1800 - 1600 cm'^ in all spectra prevented the observation of adsorption bands at approximately 1700 cm '\
The band located at 986 cm’^ after carbon dioxide adsorption on the Sn0 2 and at 987 cm '\ in CuO and BaC0 3, may be assigned to the symmetric C - O stretching mode usually located in the region 1100 - 1000 cm '\ These band values appear to have shifted to a lower wavenumber and are consistent with those bands associated with a bidentate carbonate as expected from the results of Fujita et al
CHAPTER 5: F.T.I.R. STUDIES
The work of J.Saussey et al. on the system ZnO / CO2, showed that CO2
adsorption lead to the appearance of bands in the following spectral range: 1665 - 1580 , 1348 - 1303, 1007- 999, 852 - 841, and 680 - 677 cm"’ which corresponded to the regions in which vibrations of the bidentate ion might occur. On the basis of this general band assigmnent it can be assumed that the band at 1247 cm"’ (the stretching of a carbon oxygen double bond) displaced to a frequency below normal may be associated with the band at 987 cm"’ (stretching vibration of the carbon oxygen single bond).
The band in the present work occurring at 856 cm"’ can be identified it' the above spectral range given by J. Saussey et al. is applied, as this band falls near to the values attributed by them to the out of plane deformation of the C - O bond of a bidentate carbonate group. Further evidence, supporting the proposed identity of the band at 856 cm"’ comes from Fujita et al who found that a symmetrical carbonate (free carbonate ion) has a single band located in the I.R spectrum at 867 - 870 cm"’. The vibration arising from the symmetric C -O stretch is infra -red inactive. For a monodentate carbonate group bands are located at 1005 and 850 cm"’. There is a shift to lower frequencies of both bands on going from the carbonate ion to the unidentate to the bidentate group.
Usually bands located at ~ 600 cm"’ are attributed to the bending vibrations in the plane of the bidentate group; but in this study the band at -6 7 0 cm"’ was assigned to atmospheric carbon dioxide. This particular band was only observed in initial spectra, when the sample had just been put into the FTIR chamber on being removed from the carbon dioxide pressure chamber, that were collected in a static environment (as stated in section 5.2). This band was found to be removed when the nitrogen purging the chamber was switched on. Its appearance in spectra was associated with the introduction of carbon dioxide (resulting from the influx of air
CHAPTER 5: F.T.I.R. STUDIES
when the chamber was opened) into the environment of the spectrometer on transferral of the sample to the FTIR chamber. The subsequent disappearance of this band on purging the chamber with nitrogen occurred within a time period of one minute.
The bands in the region 1070 - 1050 cm'^ were observed in a study of cobalt complexes carried out by Nakomoto , but such bands were not observed by Harrison & Maunders in studies of CO2 adsorption on S11O2 . The latter authors presumed that the bands were masked by the vibration of the oxide lattice. The band envelopes at 1500 - 1422 cm'^ and 1401 - 1320 cm'^ (separation ca. 100 cm'’) were assigned by P.G. Harrison et al to the antisymmetric and symmetric CO stretching mode of a surface unidentate carbonate. Also T. Morimoto et al. have reported unidentate bands at 1500 and 1390 cm'’ in the system La203 / CO2, while others reported corresponding bands at ca. 1449 - 1488 cm ’ and 1351 - 1370 cm ’ for carbon dioxide adsorbed on cobalt complexes. Similar bands were not observed in this study due to the noise in that region.
5.4.2: PEROVSKITES
Similarities exist between the band positions (occurring in the spectral region 1360 - 500 cm'’) observed after the adsorption of carbon dioxide on metal oxides, Copper oxide and Sn0 2 with those that appeared on the perovskite, Barium stannate and samples of BaSnO] that have been doped with antimony, calcium and copper oxide (BaSno.99Sbo.01O3, Bao.5Cao.5Sno.97Sbo.03O3, and CuO-BaSn0 3) after the exposure to pure carbon dioxide under similar experimental conditions. Bands at 1247, 1132, 987 and in the region ~ 600 cm ’ for CO2 adsorbed on BaSn03
were also present in spectra of all the doped samples of BaSn0 3. Bands in the 600 cm ’ region of the spectrum were located in the doped perovskite sample at 669,
CHAPTER 5: F.T.I.R. STUDIES
652, and 618 cm '\ In addition to these bands, a band located at 679 cm'^ was observed in Bao.5Cao.5Sno.97Sbo.03O3 and CnO-BaSnOg only. The slight shifts that were observed in the band positions were attributed to the altered chemical nature of the sample brought about by doping.
The majority of the bands observed in the spectra of the perovskites were identical in intensity and position to those observed in spectra obtained for the metal oxides; in addition the behaviour of these bands on purging with nitrogen were similar. Therefore such bands were grouped and attributed to the saiue species as those assigned for Sn0 2 and the arguments that were used in the assignments of the bands for SnO% are applicable to BaSnO] and the other perovskites.
All the bands observed in this study are summarised in table 5.4.1 with relevant assignments.
5.4.3: CONCLUSION
No major differences were observed in the surface species that occurred at the surface of various sample following exposure to carbon dioxide apart from the variation in intensity of the band located at 1132 cm"\ seen on the BaSnOg - CuO composite. That the other bands did not change in intensity indicates some surface species peculiar to the composite and not found for either of the components individually. Apart from this observation, the present FTIR study has been unable to indicate the reason for the differing electrical response to CO2 exhibited by the different saiuples, the surface species present being essentially the saiue on all. Therefore no information was gained on how the presence of carbon dioxide in the environment can elicit varying responses in the samples.
Table 5. 4. 1. Assignment of bands observed in the Fourier Transform Infra-red spectra of the metal oxides and perovskites following the adsorption of pure CO2
at room temperature
Band frequency / cm"' Assignment / Structure Comments References
' 1246 (s) C = 0
(symmetrical stretching vibration).
Bidentate carbonate.
A change in the intensity of this band was not observed on purging with N; (g). 16, 17, 18. 1204 (w) Bridged organic carbonate. 0 C = 0 0 as above. 23.
1132 (m) (i) Bridging organic
carbonate. or
(ii) Bidentate carbonate
A reduction in the intensity of this band occurred only in the sample, CuO-BaSnO^ oil being purged with N2 (g). 5, 6, 11, 12, 16, 19 - 22, 23. 32. 987 (w) C - 0 (symmetric stretching mode). Bidentate carbonate. as above. 25. 856 C - 0 (out of plane deformation). as above. 24, 25. -670 C - 0 (bending
vibrations in the plane of the carbonate group). Atmospheric carbon dioxide. as above. 1.
CHAPTER 5: F.T.I.R. STUDIES
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th
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CHAPTER 5: F.T.I.R. STUDIES
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