3504
J. Phys. Chem. 1984,88, 3504-3508
Adsorption of COP on Ti02 and Pt/TI02 Studled by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Katsumi Tanaka,* K. Miyahara, and I. Toyoshima Research Institute for Catalysis, Hokkaido University, Sapporo 060, Japan (Received: January 16, 1984)
The adsorption of CO, on TiOz and Pt/TiOz was studied by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The presence of six different species was recognized in the C 1s XP spectrum. These were assigned as graphitic carbon (binding energy (BE) = 284.1-284.6 eV), HC03- (BE = 285.0-286.1 eV), C 0 3 2 -(BE = 288.5-289.0 eV), adsorbed CO, (BE = 290.6-292.5 eV), TiC (BE = 281.2 eV) on TiO,, and adsorbed CO on Pt (BE = 287.4 eV). The observation of CO adsorbed on Pt suggests that the dissociation reaction of COz occurs on Pt/TiO, samples. On an Ar-ion-bombarded Pt/titania, dissociation of COzwas also observed, where the oxygen atoms produced react with adsorbed CO, molecules to form a C03,- species held on the titania support. No CO can adsorb on Pt due to the strong metal-support interaction. It was confirmed that the formation of carbonate species caused by the electron abstraction reaction is accompanied by an electron-transfer reaction through oxidation of Ti3+ to Ti4+ species.
Introduction Carbon dioxide is the most stable carbon compound and, consequently, the final product of the oxidation of hydrocarbons. Due to the huge bond dissociation energy (1 27 kcal/mol), activation of CO, has been through to be unimportant under mild conditions.' However, CO, plays an important role as one of the raw materials in catalytic chemical syntheses such as methanol synthesis with Hz on copper-derived catalysts and the water-gas shift reaction. Recently, direct conversion of a mixture of CO, and CH4into CO and hydrocarbons by microwave activation has been reported., Thus, it is obvious that, far from being inactive, CO, is an active participant in these catalytic syntheses. Several authors have focussed on the dissociation of COz on Rh catalyst3-' and Pt/TiO,.* On both oxidized and reduced Pt/TiO,, CO, is found to be dissociated at room temperature and the oxygen atoms produced change the surface symmetry on TiO, to make several different carbonate species, which are detected by FT-IR techniques.* In this paper we report further results of COS adsorption on TiO, and Pt/TiO,, as studied by XPS and AES techniques. Adsorption of CO, was also performed on an Ar-ion-bombarded Pt/titania, on which the titania surface is strongly reduced and the extent of reduction can be evaluated from the intensity ratio A/B in the fine structure of the Ti L M M Auger signal, as well as from the concentration of Ti3+ species estimated from the Ti 2p X P spectra. Experimental Section Titanium dioxide was prepared by the oxidation of @-titanic acid (H2Ti03)for 24 h in a constant flow of 0, at 600 O C . The @-titanicacid was obtained by the reaction of titanium tetrachloride with concentrated ammonium hydroxide. The resulting precipitate was washed with distilled water until no chlorine was d e t e ~ t e d . Oxidation ~ of H,Ti03 at 600 " C caused it to change into anatase, which was ascertained by the X-ray diffraction method using Mo K a radiation. The Ti0, was soaked in a (1) Y. Kitano, M. Ichikawa, T. Osa, S. Inoue, and K. Asada, "Chemistry of Carbon Dioxide", Kyoritsu, Tokyo, 1976, Chapters 2 and 3. (2) K. Tanaka, J. Okabe, and K. Aomura, J . Chem. SOC.,Chem. Commun., 921 (1982). (3) M. Primet, J . Chem. SOC.,Faraday Trans. 1 , 74, 2570 (1978). (4) D.G. Castner, B. A. Sexton, and G . A. Somorjai, Surf. Sci., 71, 519 (1978). (5) D. G. Castner and G. A. Somorjai, Surf. Sci., 83, 60 (1979). (6) F. Solymosi, A. Erdohelyi, and M. Kocsis, J . Catal., 65, 428 (1980). (7) T. Iizuka and Y . Tanaka, J. Catal., 70, 449 (1981). (8) Katsumi Tanaka and J. M. White, J . Phys. Chem., 86, 3977 (1982). (9) T. Watanabe, K. Tanaka, K. Miyahara, and K. Tanabe, Shokubai (Catalyst), 20, 255 (1978).
0022-3654/84/2088-3504$01.50/0
chloroplantinic acid solution, then dried at 120 O C to obtain a Pt/Ti02 with 2 wt% Pt. Each sample was pressed into a thin pellet and fixed on the sample holder. It was then transferred to the preparation chamber in the XPS system. Every sample was treated in situ in the following order: oxidation in 400 torr of 0, for 12 h, reduction with several torr of Hz for 12 h, and finally evacuation for 1 h at the appropriate temperature. The notation shows the temperature of treatment mentioned above. One special procedure lies in the oxidation of the sample, that is, each pellet mounted on the sample holder was evacuated at 25 OC, exposed to 02, and heated to the appropriate temperature. This procedure has been used to clean TiO, surfaces for infrared experiments.1° X P spectra were recorded with a Vacuum Generators ESCA 3 photoelectron spectrometer employing Mg K a X-radiation (hv = 1253.6 eV). The background pressure'was always below 5 X torr. Analyzer energies of either 20 or 50 eV were used, depending on sample condition. Differences were observed between the two analyzer energies in absolute peak intensity and full width at half-maximum (fwhm). AE spectra were obtained with an X-ray source with an analyzer energy of 200 eV. The other experimental conditions were the same as for the XPS experiments. Binding energies by XPS were shifted to the higher-energy side (charge up) on both oxidized TiOz and Pt/TiO, samples. They were corrected with reference to the peak at 530 eV for 0 1s which is common in the spectra of metal With this method, the binding energy of Ti 2p5/, was determined to be 458.7 eV which is close to the value previously C 0 2 exposure was carried out at -196 "C, which was measured on the sample holder; thus, the surface temperature may be somewhat higher. CO, was dosed at a constant pressure between 1X and 1 X torr (the sticking probability is quite small), and no CO, associated species was detected after evacuation at the same temperature. Research-grade COz stored in a glass cylinder was used and the gas manifold was heated under evacuation to remove water before CO, introduction. The ion pump was cut off to prevent backdiffusion of CO when C 0 2 experiments were carried out. X P spectra were taken with gas-phase C 0 2 present. Results and Discussion Assignments of Carbon X P and Auger Spectra. The binding energies (BE) of C 1s observed on TiO, and Pt/TiO,, as well as Katsumi Tanaka and J. M. White, J. Phys. Chem., 86,4708 (1982). T. J. Barr, J . Phys. Chem., 82, 1801 (1978). M. Scrocco, Chem. Phys. Lett., 61, 453 (1979). C. C. Kao, S. C. Tsai, and Y. W. Chung, J . Catal., 73, 136 (1982). B. H. Chen and J. M. White, J . Phys. Chem., 86, 3534 (1982). S. C. Fung, J. Catal., 76, 225 (1982). J. A. Schreifels, D. N. Belton, J. M. White, and R. L. Hance, Chem. Phys. Lett., 90, 261 (1982). (10) (11) (12) (13) (14) (15) (16)
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3505
Adsorption of COz on T i 0 2 and Pt/TiO, TABLE I: Binding Energies (eV) of C 1s Peaks treatment CI
CIV
cv
286.1
288.5 288.7
292.5
285.4
288.2
291.5
Cll
Clll
CA,
TiO, 400-NO-400 CO,, 1 x io+ torr 500-NO-500 co2,1 x 1 0 - ~torr 500-500-500 co,, 1 x 10-~torr
284.4 284.4 284.1 284.1 284.3 284.4
400-NO-400 C02, 1 x 10" torr 400-200-400 co,, 1 x 10" torr 400-4 00-4 00 c o , , 1 x lo4 torr
284.6
Ar+, 90 min cO2, 1 x 10" torr evac at 25 "C
284.2 284.2 284.2
29 1.7
Pt/TiO, 285.0 284.2 284.2 284.5 284.1
289.0 288.8 288.7 288.7 288.8 288.8
287.5 287.4 287.4
291.0 291.0 291.0
Pt-Titania following Ar ion Bombardment 288.5
TABLE I1 Assignment of Carbon Species species BE, eV assignment CI 284.1-284.6 graphitic C on TiO, CII 285.5-286.1 HC0,-on TiO, CIII 287.4-287.5 CO on Pt ClV 288.5-289.0 CO,,- on TiO, CV 290.6-292.5 C 0 2 on Ti02 c.4, 281.2 T i c (Ar' bombardment)
those observed in the COz experiments, are listed in Table I, termed tentatively CArto Cv (from low BE to high BE). Our assignments of carbon species are listed in Table 11. Carbon 1s spectra recorded on an oxidized TiO, are shown in Figure 1,and they are reported in Table I for various treatments. The notation 500-NO-500 means that the sample has been oxidized at 500 OC followed by evacuation at the same temperature with no reduction. On a fresh sample, only one peak was observed at 284.1 eV (CI). As the pressure of COz to which TiO, was exposed was increased, three new peaks appeared at 285.4 (CII), 288.2 (CIv), and 291.5 eV (Cv). In addition, during exposure to 1 X torr of COz, a shoulder was observed at 532.2 eV on the edge of a huge peak centered at 530 eV. The adsorption behavior of C 0 2 was studied on oxidized T i 0 2 (400-NO-400). The CIv peak was apparently present at 288.5 eV on the fresh sample, as we report in Table I. On a fresh surface of a reduced TiOz (500-500-SOO), the CI peak had low intensity. When a reduced TiOz sample was exposed to CO,, a Cv peak was detected around 291.7 eV. It is noted her that the CII and CIv peaks were difficult to detect under COz experiments on reduced samples. Assignments of CI, CII, CIv, and Cv peaks on TiOZ are as follows. As shown in Table I, the Cv peak was observed in every system when COzwas present in the gas phase and it was easily removed by evacuation. Consequently, this peak can be assigned to adsorbed COz. Previous studies of the binding energy of C 1s associated with adsorbed CO, have reported them to be 291.8, 291.1, and 291.5 eV for COz on Cu, Pt,I7 and A g ( l l l ) , 1 8 accompanied by the 0 1s BE at 535.4, 534.5, and 534.4 eV, respectively. Carbon dioxide adsorbs molecularly on Ti02, Cu, Pt, a i d Ag( 11l), therefore binding energies of C 1s would be expected to be similar. We turn to a consideration of the CII and CIv peaks. Previous work has shown the existence of two kinds of carbonate species in the CO2-TiO2 system, bicarbonate (HC03-) and bidentate carbonate (C032-) speciesS8The former species are produced by the reaction between O H groups on anatase and COz molecules, which is determined by infrared. Adsorption temperatures used in the inf&red (room temperature) are quite different from those (17) R. P. Norton and R. L. Tapping, Chem. Phys. Lett., 38,207 (1976). (18) T. E. Felter, W. H. Weinberg, G . Ya. Lastushkina, A. I. Boronin, P. A. Zhdan, G . K. Boreskov, and J. Hrbek, Surf. Sci., 118, 369 (1982).
28 1.2 281.2 28 1.2
290.9
CI
I
I
3 0 5r
.-cVI W c
c
H
284
288
292
Binding Energy ( e V )
Figure 1. Carbon 1s spectra in the C 0 2exposure experiment at -196 O C on oxidized TiO, (500-NO-500). (a) On the fresh surface at -196 OC. Constant dosed CO, pressures were (b) 1 X lo-' torr, (c) 1 X 10" torr, and (d) 1 X torr, respectively.
of the XPS technique (-196 "C); however, the CII peak could correspond to a bicarbonate species on TiOz since the CII peak is detected on the C0,-dosed sample containing a high concentration of O H groups, as ascertained by infrared spectroscopy. Previous authors report the binding energy of C 1s between 288 and 290 eV for carbonate species. No distinction is made between the monodentate and bidentate species. Hammond et al. reported binding energies of C 1s and 0 1s to be 288.2 and 530.4 eV for AgzC03and 289.1 and 531.2 eV for CdC03, respe~tively.'~They are reported to be 290 ad 531 eV for nickel carbonate 2o 289 and 530.3 eV for silver carbonate.I8 These binding energies are close to those of the CIv peak in our system so that an assignment of the CIv peak as C032-seems reliable. In our previous infrared (19) J. S. Hammond, S. W. Gaarenstroom, and N. Winograd, Anal. Chem., 47, 2193 (1975). (20) L. Sabattini, B. Morelli, P. Zambonin, and B. A. DeAngelis, J. Chem. Soc., Faraday Trans., 1, 75, 2628 (1979).
Tanaka et al.
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
3506
IC1
CI
I
a
0
> .-c In
c
+ w C
I
2
1
I
1
284
I
I
288
I
I I
292
Binding Energy ( e V )
Figure 2. Carbon 1s spectra in the COz exposure experiment on reduced Pt/Ti02 (400-200-400). (a) On the fresh surface at -196 "C. (b) C02 exposure with constant pressure of 1 X 10" torr at -196 "C. (c) Evacuation at 25 "C. (d) Evacuation at 400 "C.
results, the surface carbonate species is unstable and easily removed by evacuation at room temperature.* However, the CIv peak is quite stable up to 400 "C as shown in Table I. This discrepancy may arise from the experimental conditions of the infrared work, where adsorption of COzwas carried out on the sample treated at 400 OC. The 0 1s peak was observed at 532.2 eV in COz adsorption on an oxidized TiO,. On this system, COz, HC03-, and C032species are all produced. This makes assignment of the peak at 532.2 eV difficult. In every system, the CI peak is observed. This peak decreased after reduction a t 500 OC or Ar-ion bombardment (mentioned later); however, it is difficult to remove it entirely. Therefore we conclude that some fraction of the species corresponding to CI stay in the bulk. According to ref 21, graphitic and (CH,), species have binding energies around 284-285 eV. Norton et al. reported the C 1s binding energy of elemental carbon on Pt at 284.8 eV,22,23 while Bonzel et al. found 283.9 eV for the CH, species, 284.2 eV for carbidic carbon, and 284.7 eV for graphitic carbon on Fe(l1 l), where assignments of carbon species were performed by comparison with the carbon spectral features in A E S W These species are known to show different behavior in reaction with H2 on several kinds of metals and show different thermodynamics in the production of materials from synthesis gas.25 Assuming graphitic carbon would be most strongly held on the surface and less likely to be removed, we assign CI speecies tentatively as graphitic carbon. Carbon peaks observed on Pt/TiOz samples are listed in Table I. In Figure 2, spectra obtained in the adsorption of COz on Pt/TiO, (400-200-400) are shown. The binding energy of Pt 4f,f, was 70.8 eV with a fwhm of 2.1 eV, with 3.4 eV of peak-to-peak distance between Pt 4f7/2 and Pt4fsfz, which shows that Pt is zerovalent. The ratio of Pt/Ti estimated by Pt 4f and Ti 2p (21) C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder, and G. E. Mullenberg, "Handbook of X-ray Photoelectron Spectroscopy", Physical Electrics Industries, Inc., 1979. (22) P. R. Norton, Surf.Sci., 44, 624 (1974). (23) P. R. Norton and R. L. Tapping, Chem. Phys. Lett., 38,207 (1976). (24) H. P. Bonzel and H. J. Krebs, Surf. Sci., 91,499 (1980). (25) C. H. Bartholomew, Catal. Rev. Sci.-Eng., 24, 67 (1982).
)
I
I
I
284 288 Binding Energy (eV)
I
I
292
Figure 3. Carbon 1s spectra in the CO exposure experiment on reduced Pt/Ti02 (500-200-500). (a) On the fresh surface at -196 O C . (b) CO exposure with a pressure of 1 X torr at -196 "C. (c) Evacuation at -196 OC. (d) Evacuation at 25 OC.
spectral area was 0.071, which is compared to that (0.10) observed
on the starting material. N o change was observed for Pt 4f and Ti 2p spectra during CO, exposure experiments. On fresh Pt/TiOz (400-200-400), the CI and CIv peaks were seen at 284.2 and 288.7 eV, respectively (a in Figure 2). When COz was introduced at -196 O C at a constant pressure of 1 X lo4 torr, a broad peak appeared around 288.5 eV (spectrum b). The presence of CIIIand Cv was made clear by deconvolution, which were centered at 287.4 and 291.0 eV. The CIIIpeak, which was not produced on TiO,, is further identified in Figure 3. The Cv peak disappeared following evacuation at 25 OC (spectrum c). The intensity of the CIv peak was greater in spectrum c than in spectrum a of Figure 2. When this sample was evacuated at 400 O C , the intensity of the Cv peak decreased (spectrum d). The same behavior was observed on the COz-Pt/TiOz (400-400-400) system. The binding energy of C 1s was also studied on an oxidized Pt/TiOz (400-NO-400). As shown in Table I, a new broad peak was observed at 285 eV. When CO, was introduced to this sample at -196 "C, the CIIIand Cv peaks were distinguished by deconvolution around 287.5 and 291 .O eV, respectively. The peak at 285 eV is thought to be a superposition of the CI and CII peaks. In passing, the binding energy, fwhm of Pt 4f7/2,and the peak-to-peak distance between Pt 4f peaks were 72.5, 2.2, and 3.0 eV for the Pt/TiO, (400-NO-400) and 70.7, 1.7, and 3.4 eV for the 400-400-400 treated sample, respectively. On both oxidized and reduced Pt/TiO,, CO, dissociates to form adsorbed C O and oxygen species.* To determine whether the CIIl peak corresponds to CO on Pt, a series of CO exposure experiments was carried out on a reduced Pt/TiO, (500-200-500) sample at -196 OC, as shown in Figure 3. The CI peak was observed with a small CIv peak as shown in spectrum a and it was relatively easy to detect the CIIIpeak. When C O was introduced on this sample at a constant pressure of 1 X torr at -196 "C, a new peak grew at 287.4 eV (b in Figure 3). This peak corresponds to the C 1s spectrum attributed to adsorbed C O molecules. To see whether it adsorbs on Pt or on the TiO, support, the system was evacuated to 1 X torr at -196 OC. As shown in Figure 3c, the peak at 287.4 eV remained following evacuation. Consequently, assignment of the CIIrpeak as CO on Pt is thought to
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3507
Adsorption of C 0 2 on T i 0 2 and Pt/Ti02
T i 2P
1
I
I
I
A c Ar I
i
0
I
cv
.-+ In 0
4-
Ti3+ 40 8%
c
Y
I
I
456
1
I
4 60
I
I
464
Binding Energy ( e V )
I
2ao
I
I
284
I
I
288
I
I
500
400
Kinetic Energy (eV1
Figure 5. Titanium 2p spectra and Auger spectra of Ti LMM transition. (a) and (c) correspond to those in Figure 4.
292
Binding Energy ( e V )
Figure 4. Carbon 1s spectra in the COz exposure experiment on Pt-titania after Ar-ion bombardment at 300 OC for 9 0 min. (a) On the fresh surface at -196 O C . (b) C 0 2 exposure with a pressure of 1 X IOd torr at -196 O C . (c) Evacuation at 25 OC.
be reasonable. The temperature of the sample was raised slowly to 25 OC under evacuation. Spectrum d was recorded at 25 OC, on which the CO species on Pt was present in lower concentration than at -196 "C. The 0 1s peak due to adsorbed CO on Pt could not be distinguished from the 0 1s peak due to Ti02. N o C O adsorption was observed on T i 0 2 following evacuation at 25 OC, which is consistent with the infrared results.1° We assigned the CIIIpeak at 287.4 eV as C O adsorbed on Pt, which implies that C02dissociation occurs on Pt/Ti02. Norton et al. studied C O adsorption on polycrystalline Pt, where the binding energies of C 1s and of 0 1s are 286.6 and 532.7 eV, re~pectively.~~ They observed two sets of C 1s and 0 1s signals, called the PI and P2 states on the CO-Pt( 1 1 1) system,26 with binding energies reported to be 285.8, 531.1 eV for PI and 286.8, 532.6 eV for p2. They assigned the P1 and p2 states are bridged and linear C O species on Pt sites, respectively. Their resutls are consistent with an assignment done by Ibach et al. using LEED and HRELS.27 Infrared spectra of Pt/Ti02 samples show the presence of two kinds of linear CO species and one kind of bridged C O species at 2094, 2077, and 1854 cm-1.28 It is reasonable to suggest that the CII, peak in our system is a shake-up peak. Binding energies of C 1s for molecularly adsorbed CO are reported on Ni,29 Ag( 1 1 1),l8and Fe( 1 10)24330as 285.4-284.8, 285.6-284.9, and 285.3-285.9 eV, respectively. It seems generally that the C 1s binding energy for molecular CO on metals has a value between 285 and 287 eV. This general rule holds true for our system too. Exposure of C 0 2 to a PtlTitania following Ar-Ion-Bornbardment. It is likely that Ar-ion bombardment causes a reduction of metal oxides, as well as removing impurities from the substrate surface. To investigate the mechanisms of C 0 2 dissociation and the subsequent behavior of oxygen atoms produced, a C 0 2 exposure was carried out on an Ar-ion bombarded Pt/Titania sample. The conditions for the Ar-ion bombardment was 6 kV and 20 FA at 300 OC for 90 min, carried out on the sample following oxidation and reduction at 400 "C. The trivalent titanium signal was as high as 46.3% of the total intensity by deconvolution of the Ti 2p XPS spectrum. The binding energy and fwhm of Pt 4f7/2,peak-to-peak distance between Pt 4f signals, (26) P. R. Norton, J. W. Goodale, and E. B. Selkirk, Surf. Sci., 83, 189 ( 1979). (27) H. Hopster and H. Ibach, Surf. Sci., 77, 109 (1978). (28) Katsumi Tanaka and J. M. White, J . Catal., 79, 81 (1983). (29) P. R. Norton, R. L. Tapping, and J. W. Goodale, Chem. Phys. Lett., 41, 241 (1976).
(30) K. Kishi and M. W. Roberts, J . Chem. SOC.,Faraday Trans. 1.71. 1715 (1975).
______
and the Pt/Ti ratio measured by Pt 4f and Ti 2p spectral area were 70.7 eV, 1.6 eV, 3.3 eV, and 0.034, respectively. A set of C 1s spectra is shown in Figure 4. In spectrum a, which was recorded on a fresh surface, two carbon peaks were observed at 28 1.2 and 284.2 eV, termed CArand CI. No CIvpeak was observed at 288.5 eV. According to ref 21, carbon species with binding energy as low as 28 1.2 eV correspond to titanium carbide. The same species were observed on Ar-ion-bombarded titania. When C 0 2 was introduced to Ar-ion-bombarded Pt/ titania at -196 OC under a constant pressure of 1 X lo6 torr, the Cv peak appeared at 290.9 eV (b in Figure 4). The presence of the CIIIpeak was negligible. Spectrum c was recorded following evacuation at 25 OC, where the peak at 290.9 eV disappeared. The CIv peak was observed at 288.5 eV. This result implies that C02dissociates into CO and oxygen, after which the oxygen atoms react with adsorbed COz molecules to form C032- species. It might be interesting to consider the possibility of C 0 2 dissociation on Ar-ion-bombarded Pt/titania. Proving the formation of CO by C 0 2 dissociation is crucial, because heavily reduced titania-supported metal catalysts retain no C O adsorption ability due to the SMSI.31,32 In fact, no CIII peak could be observed on an Ar-ion-bombarded Pt/titania sample in the CO-exposure experiment a t -196 OC. To investigate the mechanism of electron transfer required for the production of C032-species on an Ar-ion-bombarded Pt/titania, we compared the Ti 2p X P spectra. As shown in Figure 5, Ti3+ species were observed as much as 46.3% of the total intensity. When C 0 2 was added to this sytem, no change was observed in the Ti 2p spectrum. After C 0 2 exposure at -196 OC and subsequent evacuation, while raising the temperature to 25 "C, the concentration of Ti3+decreased to 40.8% of the total intensity. The decrease in the Ti3+species concentration means the titania support was oxidized. The Auger spectra were simultaneously recorded. The intensity ratio, A/B, of the Ti LMM transition reflects the oxidation state of the titania support. Henrich et al. found an increase in this ratio reflected the extent of reduction of titanium oxides, T i 0 2 to TizO3. They find A / B values of 2.2 for Ti203and 0.5 for Ti02.33 Under our experimental conditions, a good relationship was obtained between the concentration of Ti3+ measured by XPS and the A/B intensity ratio in the Ti L M M Auger fine structure, as shown in Figure 6. In Figure 5, the intensity ratio decreased from 2.48 to 2.10 following C 0 2 exposure and subsequent evacuation while increasing the temperature to 25 OC, implying oxidation of the titania support. (31) S.J Tauster, S. C. Fung, and R. L. Garten, J . Am. Chem. SOC.,100, 170 (1978). (32) S . J. Tauster, S.C. Fung, R. T. Baker, and J. A. Horsley, Science, 211, 1121 (1981). (33) V. E. Henrich, G. Dresselhaus, and H. J. Zeiger, Phys. Rev. E , 17, 4908 (1978).
J. Phys. Chem. 1984, 88, 3508-3516
3508
A/B ratio upon water adsorption at -196 O C , followed by heating to room temperature. The overall raction scheme is as follows: on Pt
e c
0 L
C02----CO+O
G .-
on titania
2.0
0
LL L
c02
a
+ 2e-
+
02-
-
02-
(2)
co32-
(3)
2Ti3+ + 2Ti4+
I
f
1.0 u0 .-+
e
m \ U I
O1
10
I
I
I
I
40 50 Concentration of Ti+3 (%)
20
30
Figure 6. A relation between the concentration of Ti3+and the A/B intensity ratio of the Ti LMM Auger fine structure.
On Ar-ion-bombarded titania alone, where the intensity ratio was 1.75, no conversion of adsorbed C02into CO3-was observed and no A/B intensity ratio change was obtained. This result indicates that transformation of COz to CO?- species occurs only in the presence of Pt. In addition, no change was observed in the
(1)
+ 2e-
(4) The adsorbed carbon dioxide molecule dissociates on Pt to form a CO molecule and an oxygen atom. The oxygen species pick up electron supplied by the oxidation reaction of Ti3+to Ti4+. The 02-species reacts with adsorbed COz to deposit on the titania support as a C032- species. It is worth mentioning that the dissociation of C 0 2can be detected only in the presence of the titania support since this reaction has never been detected in unsupported Pt metal at low temperatures. CO formation by the water-gas shift reaction between CO, and adsorbed hydrogen does occur but not below room temperature. It is very interesting that COzdissociates and changes into CO and oxygen on TiOz-supported Pt catalysts; however, this reaction is unfortunately not catalytic, because the C032-species are strongly held on the support and can be removed by evacuation only at temperatures as high as 500 O C . If the oxygen species produced by C 0 2 dissociation could be consumed and/or removed from the surface faster than the subsequent reaction with adsorbed COz to make up C032-species, the reaction could then take place catalytically. Registry No. CO2, 124-38-9;Pt, 7440-06-4; TiO,, 13463-67-7.
Structures and Properties of Excited States of Benzene and Some Monosubstituted Benzenes E. J. Padma Malar and Karl Jug* Theoretische Chemie, Universitdt Hannover, 3000 Hannover 1, Federal Republic of Germany (Received: January 18, 1984)
Configuration interaction (CI) calculations using the semiempirical SINDO1 approach were carried out for the structural optimization of low-lying singlet and triplet excited states in benzene, fluorobenzene,phenol, aniline, toluene, and nitrobenzene, The equilibrium geometries, adiabatic excitation energies, charges, dipole moments, and degree of aromaticity are discussed for the various excited states. Planarity vs. nonplanarity of the ring and the substituents are investigated in connection with delocalized (ring) or localized (substituent) excitation. The calculated results compare well with the available experimental data.
Introduction Although considerable attention has been focused on the spectral analyses of benzene and a large number of its derivatives, the structures and the properties of these benzenoid systems in their excited states are not clearly understood.' Structural rearrangement accompanying electronic excitation in benzene was first detected by de Groot and van der Waals in 1963 when they noticed from the magnetic resonance spectrum (EPR) that the lowest triplet state of benzene is distorted from the regular hexagonal structure.2 Since then a few experimental studies3" on the (1) C. J. Seliskar, 0.S. Khalil, and S. P. McGlynn in "Excited States", Vol. 1, E. C. Lim, Ed., Academic Press, New York, 1974. (2) M. S.de Groot and J. H. van der Waals, Mol. Phys., 6, 545 (1 963). (3) H. D. Bist, J. C. D. Brand, and D. R. Williams, J . Mol. Spectrosc.,
24, 413 (1967).
(4) J. Christoffersen,J. M. Hollas, and G. H. Kirby, Mol. Phys., 16, 441
(1969). (5) K.-T. Huang and J. R. Lombardi, J . Chem. Phys., 52, 5613 (1970).
monosubstituted benzenes have revealed that the first excited singlet state (lB2) has appreciable quinoidal character in phenol and aniline. However, in fluorobenzene it is inferred from an examination of dipole moments that the 'B, state has less quinoidal character than in the ground state.5 Furthermore, experimental evidence also indicates that the first excited singlet states in some of the benzene derivatives are n ~ n p l a n a r . ~ @Although ~ these experimental results help in understanding the structures of excited states in benzenoid systems, they are at present not capable of providing complete structural details. Experimental techniques used are subjected to limitations like low resolution of rotational (6) E. D. Lipp and C. J. Seliskar, J . Mol. Spectrosc., 87, 242 (1981). (7) R. A. Coveleskie and C. S. Parmenter, J. Mol. Sperrrosr., 86, 86 (1981) --, \ - -
(8) J. C. D. Brand, D. R. Williams, and T.J. Cook, J . Mol. Spectrosc., 20, 359 (1966). (9) J . Christoffersen, J. M. Hollas, and G. H. Kirby, Mol. Phys., 18, 451 (1970).
0022-365418412088-3508$01.50/0 0 1984 American Chemical Society
