J. Chem. Soc., Faraday Trans. I, 1989, 85(4), 929-943 Carbon Monoxide and Carbon Dioxide Adsorption on Cerium Oxide studied by Fourier- transform Infrared Spectroscopy Part 1.-Formation of Carbonate Species on Dehydroxylated CeO, at Room Temperature Can Li,? Yoshihisa Sakata, Toru Arai, Kazunari Domen, Ken-ichi Maruya and Takaharu Onishi" Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan The adsorption of CO and CO, on cerium oxide has been studied by Fourier- transform infrared spectroscopy (F.t.i.r.). For CO adsorption at room temperature, in addition to linearly adsorbed CO (21 77 and 21 56 cm-l), two kinds of carbonate (unidentate: 854, 1062, 1348 and 1454 cm-* and bidentate: 854, 1028, 1286 and 1562 cm-') and inorganic carboxylate (1310 and l m cm-l) species were identified spectroscopically.As for CO, adsorption, apart from weak bands at 1728, 1396, 1219 and 11 32 cm-' attributed to bridged carbonate species, bands due to unidentaie carbonate, bidentate carbonate and inorganic carboxylate species, similar to those arising from CO adsorption, were observed. Except for the linearly adsorbed-CO, all species arising from CO and CO, are stable at room temperature in uacuo. The desorption of these species at elevated temperatures shows that the order of thermal stability is bridged carbonate < bidentate carbonate < inorganic carboxylate < unidentate carbonate species, and the residual of unidentate carbonate species can remain on the surface up to 773 K under evacuation.Forming carbonate and inorganic carboxylate species proved that the CeO, surface could be partially reduced by CO even at room temperature. No bands in the region 2300-800 cm-' were detected below 373 K for CO adsorption on hydroxylated CeO,. This indicates that CO adsorption depends on the degree of dehydroxylation of the surface. The mechanism of CO adsorption is also discussed. CO is a useful probe in the characterization of surface properties of both metals' and metal oxides2 via adsorption. In addition to surface investigations, the subject of CO adsorption is closely related to the studies of reactions involving CO, such as CO hydrogenation and CO oxidation. Therefore, studies on CO adsorption are an important field and will be continued in the future.Although CO adsorbed on metal oxides has been extensively studied by infrared spectroscopy,2. infrared-spectroscopic studies of CO adsorption on cerium oxide have not been carried out in detail until now. Some authors4* have studied CO adsorption on ceria-supported metal catalysts ; however, their main attention was focused on the metals instead of on ceria. Recently, Jin et aL6 reported the adsorption of CO and CO, on Pt/Ce02 and CeO, by infrared spectroscopy and other techniques, but they failed to find any peaks corresponding to CO on pure CeO, in the region 1000-3000 cm-'. For the above reasons, i.r. studies of CO adsorption on CeO, have been initiated in 7 Permanent address: Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 129 Street, Dalian, China.929930 CO and CO, Adsorption on CeO, our laboratory. We have found that CO not only is adsorbed in a linear form as on other oxides, but also interacts strongly with the CeO, surface, forming a considerable number of carbonate species even at room temperature. A similar phenomenon on oxides has been discussed previously. '-11 In this paper i.r. spectra of adsorbed CO on cerium oxide are displayed. In order to assign those bands arising from adsorbed CO, the adsorption of 13C0 and CO, on cerium oxide has also been studied by Fourier-transform i.r. spectroscopy. Furthermore, the thermal stability of the species obtained from adsorbed CO and CO, has been investigated by in situ F.t.i.r. Finally, the results of CO adsorption on hydroxylated CeO, and the mechanism of forming carbonate and carboxylate species are discussed.Experiment a1 Materials Cerium oxide (ceria or CeO,) used in this study was obtained by the thermal decomposition of cerium hydroxide gel at 773 K in air for 3 h. The cerium hydroxide was prepared through precipitation from an aqueous solution of cerium(Ir1) nitrate with NH,OH at pH 8-9, and the resulting precipitate was repeatedly washed with deionized water until NH; and NO, ions were eliminated, prior to drying and calcination. H, was purified via a Deoxo arrangement and then through a liquid-nitrogen trap. 0,, CO and 13C0 (99.7%) were refined through a liquid-nitrogen trap in a circulating system for 15 min. CO, was prepared from the decomposition of NaHCO,. Apparatus 1.r.spectra were recorded on a JEOL JIR-100 F.t.i.r. spectrometer with 256 scans at 4 cm-l resolution using a liquid-nitrogen-cooled HgCdTe detector. All the spectra shown in the paper are in absorbance, and their backgrounds were recorded before admitting the adsorbate gas under corresponding experimental conditions of the spectra. The same calcined at 773 K was pressed into a self-supporting disc, weight ca. 150 mg, with a diameter of 20 mm. The disc was placed in a conventional i.r. cell with NaCl windows and a furnace capable of heating the disc in situ up to 1100 K. The i.r. cell, connected to a vacuum system can be kept below lop4 Torr (1 Torr w 133.3 N m-,) by an oil-diffusion pump and a mechanical pump. Procedure In order to eliminate the species contaminants, adsorbed H,O and hydroxyl groups prior to the first experiment, a new disc in the cell was outgassed at 1000 K for 10 min and then treated in 0, at 873 K for 3 days (the time was shortened to 12 h for subsequent experiments).After pretreatment the sample was outgassed again at 1000 K for 30 min. The i.r. spectra showed that no peaks of contaminants, residual water or OH groups remained on the surface after pretreatment and outgassing. Therefore, unless noted otherwise, the sample used in this study is a dehydroxylated CeO,, symbolized as CeO,( 1000 K). Continuing the above processes, the disc was progressively cooled to the given temperatures for recording background spectra and finally to room temperature for CO or CO, adsorption. The thermal-desorption and CO-adsorption experiments at elevated temperatures wee performed in a stepwise way and the temperature rate of an interval was ca.25 K min-l. A given temperature was not changed until the spectra reached a steady state. A hydroxylated CeQ, surface was obtained by treating the sample which had undergone the above pretreatment in water vapour from room temperature to 673 K and at 673 K for 1 h, then outgassing for 1 h at the same temperature. The i.r. spectraC. Li, Y. Sakata, T. Arai, K. Domen, K-i. Maruya and T. Onishi 93 1 2300 2100 1900 2000 1600 1200 800 wavenumberfcm- ' Fig. 1. 1.r. spectra of CO adsorbed on CeO,(l000 K) at room temperature: (A) with decreasing CO pressure, (a) 110, (b) 87 and (c) 21 Torr and ( d ) outgassing for 1 min after (c); (B) after admission of CO for (a) 5, (6) 20 and (c) 124 min.of the hydroxylated sample showed that two sharp peaks at 3664(strong)cm-' and 3629(weak) cm-l, due to different terminal OH groups, and a broad peak centred at ca. 3427 cm-l, attributed to hydrogen-bonding OH groups, were reproduced, indicating that the CeO, surface was truly hydroxylated. Results CO Adsorption at Room Temperature Fig. 1 shows the i.r. spectra of CO adsorbed on CeO, at room temperature, and the results are described in two sections as follows. In the range 230&1900 cm-' in fig. 1 A, two bands at 2177 and 21 56 cm-', attributed to CO linearly adsorbed on Ce", were observed. The intensities of the two bands decrease simultaneously with a decrease in the CO pressure [fig. 1 A(a)-(c)] and both bands disappear quickly on evacuation at room temperature [fig.1 A(d)]. The dependence of the band intensities upon the pressure of CO agrees with a Langmuir relationship, and the CO pressure for saturated adsorption is ca. 100Torr at room temperature. The two bands were gradually weakened at elevated temperatures even in a CO atmosphere (1 10 Torr) and completely vanished at ca. 350 K. These results suggest that the two bands are associated with weakly adsorbed CO and are similar to those on other oxides. In the region 1800-800 cm-', as soon as CO was introduced into the i.r. cell, nine distinct peaks at 854, 1028, 1062, 1286, 1310, 1348, 1454, 1510 and 1562 cm-' were detected as shown in fig. 1 B(a). All the bands grew slowly with time (fig. 1 B) and it took ca.200 min to reach saturated adsorption at room temperature. Various CO pressures in the range 1&100 Torr made no apparent difference both to the spectra and to the time of saturated adsorption (thus hereafter pressures of 10-20 Torr of CO and 13C0 were932 CO and CO, Adsorption on CeO, 4 - A h - A - 0.95 0.85 rD 00 2 -4 3 0.75 0.65 0.5 5 10 60 110 160 tlmin Fig. 2. Variations of i.r. band intensities of CO adsorbed on CeO, (1000 K) at room temperature as a function of prolonged time: A, 1286; A, 1562; 0, 1348; @, 1454 and 0, 1510 cm-I. adopted). All the bands become stronger and more distinguishable with prolonged adsorption time, except that at 1310 cm-l, which was submerged by two intense neighbouring bands at 1286 and 1348 cm-l. Comparing the bands in the two regions, 2300-1900 and 1800-800 cm-', the former are rapidly formed in a CO atmosphere and easily outgassed by pumping; however, the latter are produced slowly and cannot be removed by evacuation at room temperature. This indicates that the surface species related to the bands in the two regions are appreciably different from each other in nature.With a view to distinguishing the species with i.r. bands in the region 1800-800 cm-l, the variations of five main bands with adsorption time after admission of CO are presented in fig. 2, in which the scale is the ratio of absorbance of the indicated band to that of the 1286 cm-l band. The bands at 1286 and 1562 cm-l grow at the same rate; hence their ratios are parallel. The two curves at 1348 and 1454 cm-' are nearly coincident; the two bands keep pace with each other and obviously increase the most quickly among the five bands.The curve of 1510 cm-' is different from those of other bands in slope. On the basis of a knowledge that the bands from the same species will exhibit the same tendency during a variation in the amount of species present, the five bands can be reasonably classified into three groups due to three kinds of surface species, here termed (A) 1286 and 1562 cm-l, (B) 1348 and 1454 cm-' and (C) 1510 cm-l. Isotopic-shift methods are often used to clarify and confirm problems in the interpretation of i.r. spectra. The i.r. spectra of adsorbed 13C0 on CeO, are basically the same as those of adsorbed CO apart from a reasonable isotopic shift.The bands of adsorbed 13C0 correspond to those of adsorbed CO, and the isotopic shifts and ratios, are listed in table 1.C. Li, Y . Sakata, T. Arai, K. Domen, K-i. Maruya and T. Onishi 933 Table 1. Vibrational frequencies (below 1800 cm-l) of l2C0 and 13C0 adsorbed on CeO, (crn-l). isotopic isotopic l2C0 13C0 shifts ratios 854 (m) 1028 (m) 1062(w, b) 1286 (s) 1 3 10 (w) 1348 (s) 1454 (s) 1510(s) 1562 (s) 827 (m) 1024 (m) very weak 1256 (s) 1279 (w) 1319 (s) 1410 (s) 1475 (s) 1518 (s) 27 4 30 31 29 44 35 44 - 1.033 1.004 1.024 1.024 1.022 1.03 1 1.024 1.029 - a w, weak; m, medium; s, strong; b, broad. ; I : I l l I I I I I I I 2000 1600 i200 800 wavenumber/cm-' Fig. 3. 1.r. spectra of CO adsorbed on CeO, (1000 K) at elevated temperatures in the presence of 10 Torr CO: (a) 300 K for 60 min; (b) 373 K for 35 min after (a); (c) 473 K for 20 min after (b) and (d) 573 K for 30 min after (c).CO Adsorption at Elevated Temperatures Fig. 3 displays a series of i.r. spectra of adsorbed CO formed at various temperatures. Spectra were recorded at elevated temperatures stepwise from room temperature to the indicated temperature in the presence of 20 Torr CO. At room temperature bands at 1286 and 1562 em-' appear at the beginning of CO adsorption and are dominant in the spectra. With increasing temperature the bands at 1348, 1454 and 15 10 cm-l become934 CO and CO, Adsorption on CeO, 0.7 0.6 0.5 0.4 8 e ' 0.3 0.2 011 0 I I I 273 373 473 573 673 TIK Fig. 4. Increase in i.r. band intensities of 1286 (O), 1348 (A) and 1510 (0) cm-' in the presence of 10 Torr CO at elevated temperatures (data from fig.3). significantly strong, such that the 1286 are 1562 cm-' bands are almost overwhelmed at 573 K. The weak bands at 854 and 1062 cm-l also grow, apparently at the same time. This implies that the two bands may be ascribed to species B or C instead of species A. The band at 1028 cm-l seems to be in step with the 1286 and 1562 cm-l bands during this process. Fig. 4 quantitatively illustrates the enhancement of the three main bands at 1286, 1348 and 1510 cm-', which are taken as representatives of the three species A, B and C, respectively. The bands at 1348 and 1510 cm-l grow steeply with increasing adsorption temperature, and their intensities vary more than tenfold in the range 300-573 K, while the 1286 cm-l band only doubles in intensity from room temperature to 473 K, and shows no obvious change above 473 K.These results clearly suggest that the activation energies for formation of species B and C are higher than that for species A, and that species A might reach saturated adsorption at 473 K. CO, Adsorption at Room Temperature In an attempt to assign the bands arising from CO adsorption, an i.r. study of CO, adsorption on CeO, was performed under the same conditions as in CO adsorption, and the spectra of CO, adsorbed at room temperature are presented in fig. 5. Adsorbed CO, gives more and stronger bands than adsorbed CO in the region 2000-800 cm-', but the main peaks in the spectra for CO, and CO adsorption are very similar, i.e. bands at 856, 101 1, 1045, 1286, 1354, 1454, 1506 and 1568 cm-l stemming from adsorbed CO, resemble in position and relative intensity corresponding bands at 854, 1028, 1062, 1286,C.Li, Y. Sakata, T. Arai, K. Domen, K-i. Maruya and T. Onishi 93 5 I I I I I 2000 1600 1200 800 wavenumberlcm-' Fig. 5. 1.r. spectra of CO, adsorbed on CeO, (1000 K) at room temperature after admission 1 Torr CO, for (a) 1 , (b) 3, (c) 5 and ( d ) 125 min. 1348, 1510 and 1562 em-' for adsorbed CO. In addition to the abovementioned bands, four weak bands at 1 132, 1219, 1396 and 1728 cm-l appear for CO, adsorption but are absent for CO adsorption. Another apparent feature is that all the bands increase in intensity rapidly with time of exposure of CO,, and reach a maximum within 10 min. Desorption of Adsorbed CO and CO, An experiment on the desorption of adsorbed CO and CO, was undertaken with a view to examining the thermal stability of surface species and to investigate the behaviour of different species during the course of desorption.Fig. 6 shows a series of spectra of adsorbed CO recorded at the temperatures indicated after achieving a steady state of desorption. As shown in fig. 6(a)-(c), bands at 1028, 1286 and 1562 cm-' are reduced remarkably in the same step by heating the sample from room temperature to 473 K, while in contrast bands at 1062, 1348 and 1454 cm-' grow slightly instead of decrease, and the 1510 cm-l band is almost unaffected. Above 473 K, all the bands decrease with further increasing temperature, and the bands at 1062, 1286, 1562 and 15 10 cm-' are almost removed at 673 K, while the bands at 854, 1348 and 1454 cm-' survive a thorough elimination even up to 773 K [fig.6(d)-(f)]. These results confirm the conclusion that the bands below 1800 cm-I are due to three kinds of surface species distinguished by their variations in behaviour when subjected to warming and degassing. The weak bands at 1028 and 1062 cm-l might belong to species A and B, because their behaviour is in accord with bands at 1286, 1562 and 1348 and 1454 cm-I, respectively.936 CO and CO, Adsorption on CeO, I I I I I 2 000 1600 1200 800 w avenumber/cm- ' Fig. 6. 1.r. spectra of CO adsorbed on CeO, (1000 K) in the course of desorption at elevated temperatures in uacuo: (a) 300 K outgassing for 25 min after 100 rnin contact with 20 Torr CO, (b) 373 K for 60 min after (a), (c) 473 K for 30 min after (b), (d) 573 K for 20 min after (c), (e) 673 K for 30 rnin after (d) and (f) 773 K for 20 min after (e).The band at 854 em-' is unique during desorption. In the range 300-473 K it becomes weaker, accompanying the decrease of species A (1028, 1286 and 1562 cm-l); how- ever, in the range 473-573 K its behaviour coincides with species B (1062, 1348 and 1454 cm-l). Therefore it seems reasonable to attribute the 854 cm-l band to both species A and B. The variations of the three species A, B and C marked by the bands at 1286, 1348 and 15 10 cm-l, respectively, are illustrated more clearly in fig. 7. Species B is more stable than species A and C, and the order of their thermal stabilities will be B > C > A.The increase in species B is only due to the conversion of species A, since species C remains the same and there is no further addition of CO to the gas phase during the process. By combining the results shown in fig. 3 and 4, we concluded that the species A may be an intermediate of species B and C. As soon as CO was introduced onto CeO,, only species A was formed on the surface initially; it was then converted into species B and C slowly (fig. 1 B and 3). As a consequence, a high temperature should facilitate the conversion of species A into B and C. This must be the reason why there is an increase of B and no loss of species C on going from 300 to 473 K in vacuu (see fig. 7).C. Li, Y. Sakata, T. Arai, K. Domen, K-i. Maruya and T.Onishi 937 0.6 0.5 0.4 8 5 e 0.3 2 % a2 0.1 2 73 373 473 573 673 TIK Fig. 7. Variations of i.r. band intensities of 1286 (a), 1348 (A) and 1510 (0) cm-' in the course of desorption at elevated temperatures (data from fig. 6). The desorption of adsorbed CO, is very like that of adsorbed CO, except for four weak bands at 1132, 1219, 1396 and 1728 cm-l which are not present in CO adsorption and disappear simultaneously on heating to 373 K. This suggests that, in addition to the three species A, B and C which are the same as formed from CO adsorption, at least one other species is produced from CO, adsorption ; species (named D) may be generated via weakly adsorbed CO,, and differs from species A, B and C in its thermal stability. The same variation in the spectra of adsorbed CO and CO, strongly supports the view that the same species A, B and C could be formed from both CO and CO, adsorption on dehydroxylated CeO,. CO Adsorption on Hydroxylated CeO, As previously described, hydroxylated CeO, was prepared by heating CeO, in the presence of H,O vapour.The hydroxylated surface was exposed to 20Torr CO at 300 K for 120 min and then heated to 373 K for 20 min, while no peaks due to adsorbed CO were observed at the two temperatures, as shown in fig. 8(a) and (b). On progressively heating the sample above 473 K, besides very weak bands at 2939, 2848, 1576, 1558, 1369 and 1307 cm-l arising from formate species produced via the reaction of adsorbed CO and surface OH groups, similar bands to those in fig. 3 appeared, but all the band intensities were much weaker than those on dehydroxylated CeO,.This enabled us to suggest that the surface OH groups hinder the CO from forming adsorbed species, especially at low temperatures.938 CO and CO, Adsorption on CeO, 4000 3000 2000 1600 1200 800 wavenumber/cm-' Fig. 8. 1.r. spectra of CO adsorbed on CeO, (hydrated at 673 K) in the presence of 20 Torr CO at elevated temperatures: (a) 300 K for 120 min, (6) 373 K for 20 min after (a), (c) 473 K for 25 min after (b) ( d ) 573 K for 22 min after (c) and (e) 673 K for 1 min after (d). Discussion Linearly Adsorbed CO On most of the oxides the i.r. bands from CO adsorption appear in the region 230&2000 cm-l, and their wavenumbers are often substantially higher than that of gas- phase CO (2143 cm-').The explanation of this phenomenon is controversial, but two popular corollaries seem reasonable :' (1) these bands are attributed to CO linearly adsorbed on surface-exposed metal ions, e.g. Lewis-acid sites; (2) the more positive the state of the metal ions, the higher is vibrational frequency of adsorbed CO. In the light of these views, we assigned the band at 2177 cm-l to CO linearly adsorbed on Ce4+ and the band at 21 56 cm-l to Ce4+ in a more unsaturated coordination state created by severe degassing at 1000 K. No linearly adsorbed CO was formed on hydroxylated CeO, because the surface was almost covered with OH; consequently no metal ions were exposed to the surface for CO coordination. Assignment of the Bands below 1800 em-' Generally speaking, i.r.bands below 1800 cm-' arising from adsorbed CO, are due to carbonate-like species. Extensive i.r. studies on the assignment of the spectra in this region have been made for CO, and CO adsorption on transition-metal oxides, since the pioneer work on NiO by Eichens and Pliskin.12 Fujita et a1.13 calculated the presence of two kinds of carbonates in a Co"' complex, and their results have been considered as criteria for the assignment of carbonate species3. 14, l5 on the surface of metals and oxides. On CeO,, only Guenin" and Jin' have reported i.r. studies of CO, adsorption. Table 2 lists some assignments of carbonate-like species related to the present work. By comparing our results presented above with those displayed in table 2, one may assignC. Li, Y.Sakata, T. Arai, K. Domen, K-i. Maruya and T. Onishi 939 Table 2. Assignment of carbonate-like species (from the liter at we) a frequency/cm-l assignment ref. CO,/CeO, COJCeO, calculated results of CO(II1) carbonate complex generally 1670-1 695 (6) C 1310-1338 650-970 1590-1630 -'\ 1260-1270 0 1020-1030 ,- do T+ 0 -c : 1560 1410 1470-1530 /.S=O 1040-1080 T- 0 1300-1370 - 0-c: 1580-1290 bidentate carbonate ( 1 6) 1680- 1240 bidentate carbonate 1480- 1 370 uniden tate carbonate 1405 inorganic carboxylate inorganic carboxylate (symmetric) 1 570- 1 360 1483 CO; (as) / O \ , 9 ( 1 3 ) 1373 CO; (sy) co c .eM 1030 C=O \. 0 0 1595 C=O 1282 CO; (as) Co 1038 CO;(sy) \ / (3) -O\ c=o 1780 C=O 1260 CO; (as) 1020 co; (sy) - 0 / -~ bridged carbonate sy, symmetric vibration ; as, asymmetric vibration.species A to a bidentate form and species B to unidentate form, since the bands of species A and B in this study are in excellent agreement with those of bidentate and unidentate carbonates, respectively, in CoI" carbonate complexes. The band at ca. 854 cm-l has been detected in complexes and inorganic compound^,'^^^^ and was attributed to the out- of-plane vibration of the carbonate COi- group, but it is difficult to detect this species in adsorption experiments because of the low transmission of oxides below 1000 cm-'. Our postulation that the band at 854 cm-l is due to both species A and B is confirmed, since both the bidentate and unidentate carbonate species possess a C0:- group. Its isotopic shift of 27 cm-' on substitution by 13C given in table 1 is also in accord with the isotopic shift for Ca13C0, in comparison with Ca12C03.1s The bands at 1510 cm-l (1506 cm-l from adsorbed CO,), previously grouped as 32 F A R I940 CO and CO, Adsorption on CeO, Table 3.Assignment of i.r. bands (below 1800 cm-l) due to CO and CO, adsorbed on CeO, species frequency/cm-l assignment CO (CO,) A 854(856) 0 on CeO, 1028 (1011) Ce ' ' C E O B 854(856) / O \ do \ O / 1286 (1 286) 1562 (1 568) bidentate carbonate 1062 (1045) Ce C ' 1348 (1354) \. 1454 (1454) unidentate carbonate C 1310 (very weak) Ce-C 1510 (1506) +.o inorganic carboxylate CO, on D 1132 CeO, 1219 1396 0 1728 bridged carbonate species C, and 1310 cm-l (weak) may be attributed to inorganic carboxylate rather than carbonate species, since no band in the 1200-1000 cm-l region is in agreement with them.The recognition of inorganic carboxylate species on oxide surfaces is difficult because their bands are usually confused with those of carbonate species. Fortunately, on CeO, (as shown in fig. 1 B and 3) the band at 1510 cm-l(l506 cm-l in fig. 5 ) assigned to an asymmetric vibration is well resolved, and the band at 1310 cm-l ascribed to a symmetric vibration can be clearly identified at the beginning of CO adsorption, although it is very weak and is overwhelmed by neighbouring bands eventually. From the table summarized by Little,3 the inorganic carboxylate species was identified by two bands whose positions vary in the regions 1570-1 510 and 1410-1 310 cm-l, respectively, on different oxides; in addition, the band at 1410-1310 cm-' is weak or absent on some oxides.The two bands at 15 10 and 13 10 cm-' in our study fit well with the above features of inorganic carboxylate species. The assignment concerning carbonate-like species on CeO, may be further verified by the results (in table 1) for isotopic shifts. Among the three bands of species A, i.e. the bidentate carbonate species, the biggest isotopic shift is 44 cm-l for the 15 18 cm-l band. This is close to that for the stretching frequency of the l3C_O molecule, which is estimated to be ca. 48 cm-', so the band arises from the C=O stretching vibration in bidentate carbonate species. The 1024 cm-l band is hardly shifted; this confirms that the carbon atom is unaffected in the vibration. Therefore, it should be ascribed to a symmetric vibration.The band at 1256 cm-', with an isotopic shift of 30 cm-I, is due to an asymmetric vibration, since the carbon atom participates in the vibration but is not involved to the same extent as in the C=O vibration. On the same principle, the isotopic shift of the asymmetric vibration is larger than the symmetric vibration in the carbonate; the two bands at 1410 and 1319 cm-' with isotopic shifts of 44 and 29 cm-l, respectively,C. Li, Y. Sakata, T. Arai, K. Dornen, K-i. Maruya and T. Onishi 94 1 are correspondingly attributed to the asymmetric and symmetric vibrations of unidentate carbonate species. For inorganic carboxylate species the isotopic shifts of 15 18 and 1279 cm-l are closed to each other, not as in carbonate species.As the inorganic carboxylate is a triatomic group the carbon atom is not at the centre as in the carbonate; therefore the carbon atom takes part in both the asymmetric and symmetric vibrations, as is the case for the oxygen atom in a water molecule. This is the reason why the asymmetric and symmetric vibrations show an isotopic shift to the same extent. Species D, with bands at 1728, 1396, 1219 and 1132 cm-', is ascribed to a bridged carbonate species by the characteristic band at 1728 cm-l, which is due to the C=O vibration of an organic carbonate. Furthermore, species D is weakly adsorbed on CeO, and can be eliminated upon heating to 373 K in vacuo; similar results have been reported for Cr203.19 By summarizing the above discussions, the assignment of the bands below 1800 cm-l arising from adsorbed CO and CO, on dehydroxylated CeO, are listed in table 3.1.r. spectroscopic studies of CO adsorption on MgO,' CaO and SrOlO well degassed at high temperatures have shown that adsorbed CO gives rise to a large number of bands in the range 2200-1000 cm-l. These bands were attributed to carbonate species and some unusual CO polymeric species, (C0):- (n = 2,3,4, . . ., x = 2,4, . ..). Although adsorbed CO on CeO, also exhibited rich bands in the same region, they were completely different from those of the CO polymeric species in terms of their band positions and relative intensities. Furthermore, the polymeric species were sensitive to oxygen, being easily destroyed by exposure thereto; however, no apparent change in the spectra of CO adsorbed on CeO, has been found after dosing 0, on the CeO, surface preadsorbed with CO.Apart from these differences, the increase in the intensities of i.r. bands due to CO adsorbed on CeO, at room temperature is a function of CO contact time but is independent of the CO pressure (10-100 Torr); nevertheless, the formation of the CO polymeric species is strongly dependent on the CO pressure. We therefore rule out the possibility of existing CO polymeric species on CeO, in the present study. Mechanism of CO Adsorption on Hydroxylated CeO, The formation of carbonate and inorganic carboxylate species from CO adsorption indicates that CO is oxidized by surface oxygen species; in other words, the surface of CeO, is partially reduced by CO. It is expected that CO reacts with some oxides to form carbonate species at high temperatures, but it seems impossible that the phenomenon occurs at temperature.The results on CeO, are not general for all oxides, and imply that the CeO, surface might possess either special adsorption sites or very active oxygen species, or both. Therefore a short discussion on the mechanism of CO adsorption is indispensable to the conclusions of our paper. CO oxidation on CeO, above 473 K has been investigated by Breysse et aZ.20*2r Their results confirmed the participation of lattice oxide ions during the reaction. A mechanism was proposed involving cyclic reduction/oxidation of the surface to explain CO oxidation on CeO,. Jin2, and his colleagues concluded that the formation of CO, on Pt/CeO, from CO temperature-programmed description was due to lattice oxygen from the interface between Pt and CeO,.From a temperature-programmed reaction study by Yao et aZ.,23 two peaks at 770 and 1020 K were ascribed to the reduction of surface-capping oxygen ions and bulk oxygen ions, respectively; another t.p.r. peak was due to oxygen species which could be converted into the capping and bulk oxygen ions. The oxygen species, i.e. mononuclear and molecular oxygen anions, on CeO, have also been identified by e.s.r. spectro- In the present case, we are inclined to the view that the surface oxygen species and part of the capping oxygen ions (but not the lattice oxide ions) are responsible for the formation of carbonate and carboxylate species on CeO, at room temperature, because 32-2942 CO and CO, Adsorption on CeO, the former are more easily reduced than lattice oxide ions.It is not plausible to extract lattice oxygen, especially at room temperature. It may be possible that the lattice oxide ions migrate to the surface to compensate for the surface oxygen vacancies created by CO reduction with increasing temperature; as a result, the surface carbonate and carboxylate species are enhanced at elevated temperatures, as shown fig. 3 and 4. The surface oxygen species and capping oxygen ions are necessary for forming carbonate and inorganic carboxylate species, but the more important reason for CO oxidation at room temperature should be the presence of surface-active sites that can activate CO to be oxidized easily. The large difference in CO adsorption on dehydroxylated and hydroxylated CeO, (fig.1, 3 and 8) leads us to suggest that the coordinatively unsaturated surface sites may play a key role for activation of CO to form carbonate and inorganic carboxylate species, since the surface unsaturated sites can be generated via dehydroxylation at high temperature. E.s.r. have shown that oxygen defect sites could be formed on the CeO, surface by outgassing at high temperatures, and similar e.s.r. resultsz6 postulated that Ce3+ sites may be produced after pretreatment at 773 K in uacuo. The proposal that the surface-active sites could be produced through dehydration has also been made by Ze~china,~ on Cr,O,. For hydroxylated CeO,, the surface was saturated with OH groups and was hence inactive for both CO adsorption and oxidation at room temperature.Fig. 1 B(a) and fig. 3 ( a ) clearly show that bidentate carbonate species predominate on the surface during the first stage after the admission of CO. It is assumed that CO reacts directly with surface oxygen species and capping oxygen ions to form bidentate carbonate species, and then the bidentate carbonate species as an intermediate are converted into unidentate carbonate and inorganic carboxylate species, as shown in fig. 2, 4 and 7. Conclusions The following summarizations can be made from this work. (1) CO adsorption at room temperature on the dehydroxylated CeO, surface shows the formation of four adsorbed species, linearly adsorbed CO, unidentate carbonate, bidentate carbonate and inorganic carboxylate species.(2) The linearly adsorbed CO can be removed at room temperature by pumping, and its amount is dependent on the CO pressure. The other three species are stable at room temperature and their thermal stability is in the order bidentate < carbonate < unidentate carbonate < inorganic carboxylate species. (3) The bidentate carbonate species can be converted into unidentate carbonate and inorganic carboxylate species, especially at high temperature. (4) Adsorbed CO, produces two carbonate and inorganic carboxylate species which are the same as those from adsorbed CO and bridged carbonate species. (5) Dehydroxylated CeO, can be partially reduced at room temperature by CO. (6) Surface OH groups prohibit CO adsorption. This is interpreted as indicating that there are no active sites for CO coordination. 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