J. Chem. SOC., Faraday Trans. I , 1984,80, 1357-1365 Tin Oxide Surfaces Part 13.-A Comparison of Tin(1v) Oxide, Tin@) Oxide-Palladium Oxide and Tin(rv) Oxide-Silica: an Infrared Study of the Adsorption of Carbon Dioxide BY PHILIP G. HARRISON* AND BARRY M. MAUNDERS Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Received 28th April, 1983 The adsorption of carbon dioxide onto tin(1v) oxide, tin(1v) oxide-palladium oxide and tin(1v) oxide-silica heat-treated in the temperature range 320-703 K has been studied by infrared spectroscopy. The surface species formed depend on the pretreatment temperature of the oxide. Bicarbonate and unidentate carbonate are formed at low pretreatment temperatures on all three oxides, but the higher Bronsted acidity of tin(rv) oxide-silica allows more bicarbonate formation than the other two oxides.At pretreatment temperatures 2 473 K the major adsorption products are unidentate carbonate and bidentate carbonate, together with small amounts of ‘organic’ or bridging carbonate. Complementary to the ammonia-adsorption study described in the previous paper1 is the investigation of the chemisorption of carbon dioxide onto the same three oxides, tin@) oxide, tin@) oxide-palladium oxide and tin(1v) oxide-silica. Ammonia adsorption indicated significant differences in the behaviour of these three oxides, in particular the higher Bronsted acidity of tin(1v) oxide-silica and the ability of surface palladium sites to induce dissociative chemisorption of ammonia, producing surface amido groups.Adsorption studies employing carbon dioxide should provide further information regarding the comparative nature of the surfaces of these oxides, especially ‘ acid-base ’ sites, where a reactive surface (hydr)oxide closely adjoins an ‘ exposed ’ or incompletely coordinated metal ion, which are also important in several cat a1 y tic processes. RESULTS The adsorption of carbon dioxide on a tin(1v) oxide disc pretreated at 320 K gave rise to only very weak bands centred at ca. 1575-1 580 and 1430-1440 cm-l, regardless of the pressure of carbon dioxide. Heat pretreatment at 390 K (fig. l), however, produced adsorption bands of increased intensity at 1575-1580 and 1430-1440 cm-l, along with a new, weak band at 1370-1375 cm-l and a weak shoulder at 1460 cm-l.Pumping off the carbon dioxide totally removed the 1575-1 580 cm-l band and greatly reduced the 1430-1440cm-1 band, leaving only a weak broad band at 1450cm-l, while the 1370-1 375 cm-l band was largely unaffected. The infrared spectra became more complex when the pretreatment temperature was raised to 473 K (fig. 2), with absorption maxima occurring at 1700-1720,159O-1595, 1430,1365-1 370, 1295-1 300 and 1220 cm-l, along with the shoulder at 1460 cm-l. The 1590-1595 cm-l band was sharp on the high-wavenumber side but broad on the low-wavenumber side, with a slight shoulder at ca. I550 cm-l. Again, on evacuating the cell only the 1365-1 370 cm-l 13571358 58 83 83 81 n g 5 Q) c.’ .g c 2 w 67 - 10 O/O - TIN OXIDE SURFACES I 1 1 I I I I I 1800 1600 1400 1200 wavenum ber/cm -1 Fig.1. Infrared spectra of carbon dioxide adsorbed at 320 K on tin(rv) oxide evacuated at 390 K. (1) Starting surface; carbon dioxide pressure: (2) 1.6, (3) 4.39 and (4) 11.04 kN m-*; (5) subsequent desorption (390 K, 1 h). band was unaffected, and a weak band at 1450 cm-l remained. Although the intensity of the absorption bands appears greatly reduced on a disc pretreated at 623 K compared with a 473 K pretreated disc, the actual difference is not that great. When the absorption bands at ca. 1590 cm-l were converted from percentage transmittance to absorbance units, the peak heights were measured as 0.159 and 0.131 units for the 473 and 623 K pretreated discs, respectively, at a carbon dioxide pressure of 1.33 kN m-2. The spectra obtained with carbon dioxide adsorption on tin(1v) oxide-palladium oxide discs were similar to those formed on the pure tin(1v) oxide discs, with some slight differences in the band positions. The other main differences were that on the 320 K pretreated disc, the resulting spectra were more intense than on the similarly treated tin(1v) oxide disc, and resembled more the spectra of the tin(1v) oxide disc pretreated at 390 K, with a band being present at 1380 cm-l and the 1595 cm-l band being broader on the low-wavenumber side with a shoulder at ca.1555 cm-l. After evacuation of the cell only the 1380cm-l band and a weak band at 1450cm-l remained. Pretreatment of a disc at 393 K (fig. 3) had little effect on the number of absorption bands but increased the intensity of the 1440 cm-l band, with respect to the 1590 cm-l band, and the 1378 cm-l band.More intense absorption bands at 1440 and 1378 cm-l remained upon subsequent evacuation in comparison with the bandsP. G . HARRISON AND B. M. MAUNDERS 1359 V I I I I I I 1800 1600 1400 1200 w avenumberlcm -' Fig. 2. Infrared spectra of carbon dioxide adsorbed at 320 K on tin(1v) oxide evacuated at 473 K. (1) Starting surface; carbon dioxide pressure: (2) 1.33, (3) 4.39 and (4) 10.11 kNm-2; (5) subsequent desorption (473 K, 1.5 h). remaining on the 320 K pretreated disc. In a similar manner to the tin(rv) oxide disc, a tin@) oxide-palladium oxide disc pretreated at 483 K gave rise to new absorption bands at 1700-1725 cm-l (broad and weak), 1300 cm-l (weak) and 1222 cm-l. The 1595-1 600 cm-l band, shifted from 1590 cm-l, appeared sharper on the high- wavenumber side than before.The intensity of the 1222 cm-l band was significantly greater when the pretreatment temperature was raised to 593 K (fig. 4) and a slight shift in the 1700-1725 cm-l band to ca. 1740 cm-l was observed. Carbon dioxide adsorption on tin(rv) oxide-silica differed from the previous two oxides in so far as that on the discs pretreated at 320 and 383 K (fig. 5) only absorption bands at 1600-1590 and 1440-1448 cm-l were observed, with the slight shoulder on the 1440-1448 cm-l band being present as on the previous two oxides. No absorption band was observed in the 1370-1 380 cm-l region until the pretreatment temperature was raised to 473 K, when a weak band appeared at 1380 cm-l, along with very weak bands at 1700-1720 and 1350 cm-l and a band at 1222 cm-l, in a similar fashion to the bands formed on tin(1v) oxide and tin(1v) oxide-palladium oxide.Also, similarly to the previous two oxides, the 1590-1595 cm-l band now appeared sharper on the high-wavenumber side. Pretreatment at 593 K (fig. 6) increased the intensities of the 1700-1720 cm-l band, shifting it to 1760-1780 cm-l, and the 1370-1380 cm-l band. The 1350 cm-l band was effectively removed. Finally, unlike the other two oxides,1360 TIN OXIDE SURFACES 52 75 1800 1600 1400 1200 wavenum berlcm-' Fig. 3. Infrared spectra of carbon dioxide adsorbed at 320 K on tin(1v) oxide-palladium oxide evacuated at 393 K. (1) Starting surface; carbon dioxide pressure: (2) 1.73, (3) 5.19 and (4) 851 kN m-2; subsequent desorption at (5) 320 K, 10 min and (6) 393 K, 2.5 h.when the pretreatment temperature was raised to 703 K the 1370 cm-l band became quite strong and sharp in comparison with the 1600 cm-l band. The 1440 cm-l band was also replaced by a broad band centred at 1460 cm-l, the 1222 cm-l band was removed and the 1760-1780 cm-l band was further shifted to ca. 1800 cm-l. DISCUSSION Carbon dioxide may be absorbed on oxide surfaces in many ways: physically adsorbed CO,, a carboxylate-type group, uncoordinated carbonate, a unidentate or bidentate carbonate group, an ' organic-type ' carbonate group and a bicarbonate group have all been reported by various authors.2-10 The bands which occur at ca. 1580-1590 and 1430-1450 cm-l on all three oxides at low pretreatment temperatures can be assigned to the v4 and v, modes, respectively, of a surface bicarbonate species.Miller and Wilkinsll have reported bicarbonate bands at 1660-1632 and 1410-1300 cm-l for some inorganic bicarbonate species, whilst Parkynsgp lo has reported bicarbonate bands at ca. 1640 and 1480 cm-l for carbon dioxide adsorbed on alumina. This assignment is in conflict with our previousf2 assignments for carbon dioxide adsorption on tin@) oxide, in which we failed to assign the 1580-1585 cm-l band and incorrectly assigned the band at 1222 cm-l to the S(C0H) vibration of a surface bicarbonate, which we suggested did not form on oxideP. G. HARRISON AND B. M. MAUNDERS 1361 n 40 9 42 5 42 E 42 2 2 Y Y .- * - 10 % - 33 1800 1600 1400 1200 wavenumber/crn -' Fig.4. Infrared spectra of carbon dioxide adsorbed at 320 K on tin(1v) oxide-palladium oxide evacuated at 593 K. (1) Starting surface; carbon dioxide pressure: (2) 1.60, (3) 5.45 and (4) 8.91 kN m-2; (5) subsequent desorption (320 K, 0.5 h). surfaces pretreated below 508 K. Since bicarbonate formation might be expected to occur with greater facility on more highly hydroxylated surfaces, i.e. those pretreated at low temperatures, we consider that our previous suggestion is in error, and that bicarbonate is only formed in significant quantities on discs pretreated at low temperatures, and is only present in small amounts on high-temperature (2 473 K) pretreated tin(rv) oxide. The 1370-1380 cm-l band, observed on tin(1v) oxide and tin(1v) oxide-palladium oxide, can be assigned to the v,(A,) mode of a unidentate carbonate group [v(C-O~~)+V(C-O~)] for which the v5(B2) mode [v(C-OII)] is involved in the 1430-1450cm-l band.This assignment is in good agreement with the unidentate carbonate bands assigned by Nakamoto13 for some cobalt complexes, and agrees with previous similar assignments.12 The vz(A1) mode, reported by Nakamoto at 1070-1050cm-1, is not observed, and is presumably obscured by the bulk oxide absorptions. The size of the 1370-1 380 cm-l band, compared with the 1580-1 590 cm-l band, suggests that the unidentate carbonate is a minor product. The shoulder at 1555 cm-l is probably due to a surface carboxylate species, the corresponding symmetric stretching mode for which may be the weak band at 1300 cm-l.The shoulder at 1460 cm-l could be due to the v5 mode of a second unidentate carbonate species. A further complicating factor is that an uncoordinated carbonate ion is expected to exhibit an antisymmetric stretching vibration in the1362 59 63 73 72 h E Q, E Y * -g C E Y 63 - 10 Ole- - TIN OXIDE SURFACES 1 5J-J I I I I 1 I I 1800 1600 1400 1200 wavenumber/cm -1 Fig. 5. Infrared spectra of carbon dioxide adsorbed at 320 K on ti@) oxide-silica evacuated at 383 K. (1) Starting surface; carbon dioxide pressure: (2) 1.33, (3) 4.66 and (4) 7.71 kN m-*; (5) subsequent desorption (383 K, 3 h). 1440-1460 cm-l region. The symmetric stretching frequency for this species occurs at too low a wavenumber to be observed, so no further evidence exists for an uncoordinated carbonate structure.With higher pretreatment temperatures the 1580-1 595 cm-I band shows a marked change, shifting slightly in position and becoming sharper on the high-wavenumber side; also a sharp band appears at 1220-1222 an-’ in each case. These two bands can be assigned to the v,(A,) [v(C-011) + v(C-O,)] and v5(B2) [v(C-0,) + S(0, * CO,,)] modes of a bidentate carbonate species, respectively. The band positions are in good agreement with assignments of bidentate carbonate bands by Nakamoto13 for cobalt complexes. The formation of surface bidentate carbonate at these pretreatment temperatures, but not below, can be justified on the grounds that these are the temperatures at which surface hydroxyl groups are condensing to eliminate water, and so leave surface oxide groups with adjacent bare metal cations.High pretreatment temperatures will also give rise to ‘ strained’ oxide linkages, which can also react with carbon dioxide to produce the bridging or ‘organic’ type of surface carbonate responsible for the weak absorption bands at 1700-1725 cm-l. On tin(1v) oxide-palladium oxide the 1700-1725 cm-l band shifts to 1740 cm-l on a disc pretreated at 593 K, while on the tin(xv) oxide-silica it shifts to 1800 cm-l for a 703 K pretreated disc. At the same time on the 473-483 K pretreated discs weak bands were observed at 1295-1 300 cm-l [tin@) oxide and tin@) oxide-palladiumFig. 6. at 593 P. G. HARRISON AND B. M. MAUNDERS 1363 1800 1600 1400 1200 wavenurn ber/crn Infrared spectra of carbon dioxide adsorbed at 320 K on tin(rv) oxidesilica evacuated K.(1) Starting surface; carbon dioxide pressure: (2) 1.46, (3) 5.73 and (4) 9.18 kN m-2; (5) subsequent desorption (320 K, 0.75 h). Table 1. Carbon dioxide adsorbed on tin@) oxide pretreatment band position/cm-l temperature/K 320 - - 1575-1580 (w, br) - 473 1700-1720 (w, br) 1590-1595 (vs) - 1460 (sh) 623 1700-1720 (w, br) 1585 (m) - 1460 (sh) 390 - - 1575-1580 (s) 1460 (sh) VIII I, VI I 11, I11 ~~ 320 1430-1440 (w, br) - - - 473 1430 (vs) 1365-1 370 (w) 1295-1300(~) 1220 623 1430 (m) 1365-1370(~) 1295-1300(~) 1220 - - 390 1430-1440 (m) 1370-1 375 (vw) IV V IX VII Assignments: I, vq bicarbonate; 11, v, bicarbonate; 111, uncoordinated carbonate ion?; IV, vg unidentate carbonate; V, v, unidentate carbonate; VI, v, bidentate carbonate; VII, v, bidentate carbonate; VIII, C=O stretch of ' organic-type' carbonate; IX, CO, asymmetric stretch of ' organic-type' carbonate.w o\ P Table 2.Carbon dioxide adsorbed on tin(1v) oxide-palladium oxide ~ ~~ pretreatment band position/cm-l temperature/K 320 - - 1595 (s) 1555 (sh) 1460 (sh) 1440 (m) 1380 (w) - - 393 - 1590 (s) 1560 (sh) 1460 (sh) 1440 (s) 1375-1380 (w) - - VIII I, VI I X 11, I11 IV V IX VII - 483 1700-1 720 (vw, br) 1600 (s) - 1560 (sh) 1460 (sh) 1440 (s) 1380 (w) 1300 (w) 1222 (m) - 1740 (w, br) 1590 (s) - 1560 (sh) 1460 (sh) 1430 (s) 1380-1385 (w) 1300 (w) 1222 (vs) - 4 2 A Assignments: I-IX, as table 1; X, CO, asymmetric stretch of carboxylate anion. 2 e Table 3. Carbon dioxide adsorbed on tin@) oxide-silica 9 $2 pretreatment band position/cm-l I/] temperature/K 320 - - 1595-1 600 (s) 1460 (sh) 1445-1450 (m) - - - - - 1460 (sh) 1440 (s) - - - 1590 (s) 383 473 1700-1720 (vw, br) 1592 (vs) - 1460 (sh) 1435-1440 (VS) 1380 (vw) 1350 (vw) 1222 (m) - 1222 (m) 593 1760-1780 (v, br) 1595 (vs) - 703 1800 (w, br) 1600 (s) - 1460 (m) 1430 (sh) 1370 (m) - - - VIII I, VI I 11, I11 IV V IX VII 1460 (sh) 1435-1440 (VS) 1380 (w) Assignments: as table 1.P. G.HARRISON AND B. M. MAUNDERS 1365 oxide] and 1350 cm-1 [tin(rv) oxide-silica] which can be assigned to the corresponding antisymmetric CO, stretch, Such shifts in the position of the v(C=O) stretching band can be rationalised in terms of increased covalency of the metal-oxygen bond of the carbonate group. The separation between the v(C-0,) and v(C-OII) stretching modes of carbonate species is sensitive to the mode of bonding and is larger for bidentate carbonate than for unidentate ~arb0nate.l~ With increasing covalency of the metal-oxygen bond the separation between the two bands increases further, so that with completely covalent bonding (as in dimethylcarbonate) the separation is ca.600 cm-l. The main effect of covalency is in the C-0,, bond, whilst the C-0, bond, despite being attached to the metal, is hardly perturbed. In the present case the Si-0 bond is significantly more covalent than the Sn-0 bond. Thus on tin@) oxide-silica pretreated below 703 K, the ‘organic-type’ carbonate can be presumed to bridge two tin ions. However, above 703 K the observed shift can be rationalised in terms of a bridged carbonate involving at least one C-0 bond attached to silicon, which has become available owing to dehydroxylation of SiOH groups at this temperature.The smaller shift to higher wavenumbers for this band, in the case of carbon dioxide adsorption on tin(rv) oxide-palladium oxide pretreated at 593 K, can be taken to indicate a smaller increase in the covalent character of the M-0 bond than occurs on tin(rv) oxide-silica. Our previous assignments of the sharp band at I222 cm-l to the d(C0H) deformation mode of a surface bicarbonate, made by analogy to the 1233 cm-l band observed after carbon dioxide adsorption on al~mina,~ is obviously erroneous. The revised assignment to the v, mode of a bidentate carbonate is preferred since the band does not appear until higher pretreatment temperatures.* As the pretreatment temperature is raised, the surface bidentate carbonate structure becomes more favoured. This is particularly clear in the case of adsorption on tin(rv) oxide-palladium oxide. The reason for this is uncertain, although it may be due to a steric effect, whereby increased dehydroxylation at higher temperatures permits more adsorption sites with less steric hindrance, allowing two oxygens to come in close approach of the surface. A second possibility is that a certain amount of weak hydrogen bonding may occur with a unidentate carbonate, which prevents the formation of bidentate carbonate. The observed bands for all three oxides are summarised in tables 1-3, with appropriate assignments. P. G. Harrison and B. M. Maunders, J. Chem. SOC., Faraday Trans. I , 1983, 80, 1341. J. H. Taylor and C. H. Amberg, Can. J. Chem., 1961, 39, 535. J. B. Peri, J. Phys. Chem., 1966, 70, 3168. 4 R. P. Eischens and W. A. Pliskin, Adu. Catai., 1957, 2, 662. 5 C. E. O’Neill and D. J. C. Yates, Spectrochim. Acta, 1961, 17, 953. ? Y . Fukuda and K. Tanabe, Bull. Chem. SOC. Jpn, 1973,46, 1616. * J. V. Evans and J. L. Whateley, Trans. Faraday SOC., 1967, 63, 2769. N. D. Parkyns, J. Chem. SOC. A, 1969,410. lo N. D. Parkyns, J. Phys. Chem., 1971, 75, 526. l1 F. A. Miller and C. H. Wilkins, Anal. Chem., 1952, 24, 1253. 12 E. W, Thornton and P. G. Harrison, J. Chem. SOC., Faraday Trans. I , 1975,71,461. l 3 K. Nakamoto, Injrared Spectra of Inorganic and Coordination Compounds (Wiley, London, 1970). l4 M. L. Hair, Infrared Spectroscopy in Surface Chemistry (Arnold, London, 1967). l5 A. Guest and P. G. Harrison, unpublished data. M. Courtois and S. J. Teichner, J. Catai., 1962, 1, 121. (PAPER 3/668) * Recent work on the adsorption onto tin(1v) oxide from synthetic air containing 30-50 ppm levels of CO has demonstrated unequivocally that the band at 1222 cm-’ is not due to a surface bicarbonate species since identical spectra are obtained from both hydroxylated and deuteroxylated surface^.'^