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J. Phys. Chem. B 2000, 104, 5414-5416
FTIR Study of CO Oxidation on Au/TiO2 at 90 K and Room Temperature. An Insight into the Nature of the Reaction Centers Flora Boccuzzi* and Anna Chiorino UniVersita` di Torino, Dipartimento di Chimica I.F.M., Via P. Giuria 7, 10125 Torino, Italy ReceiVed: February 25, 2000; In Final Form: April 13, 2000
An FTIR study of CO adsorption and oxidation on Au/TiO2 is reported. At 90 K different CO-18O2 interactions have been performed, with preadsorbed carbon monoxide or preadsorbed isotopically labeled oxygen, with dry or wet carbon monoxide. From the experimental results it can be inferred that (i) at 90 K carbon monoxide and oxygen are molecularly adsorbed on gold particles, (ii) the reaction occurs if carbon monoxide is preadsorbed (it is almost completely inhibited if oxygen is preadsorbed), and (iii) the reaction occurs more easily in the presence of water. It is proposed that dissociation of oxygen is needed in order to observe the oxidation reaction.
1. Introduction The discovery that some supported gold catalysts are able to oxidize carbon monoxide at temperatures well below room temperature1 produced a large number of papers, which were devoted to the different factors controlling the activity of gold catalysts and have been recently reviewed.2 From the complex of these studies it appears clear that the diameter and the thickness of the gold particles play a significant role.3-5 Also the effects of the size on the electronic structure of the clusters appear important in understanding their catalytic behavior.6 It has been proposed that dissociation of O2 is the first step in the reaction and it occurs when there is enough back-donation from the clusters into the antibonding π* oxygen orbital. The dissociation will be favored if the energy of the highest occupied levels in the metallic particle is close to the antibonding π* oxygen orbital. To understand more deeply the effect of the gold particle sizes on the CO chemisorption and CO oxidation, we have undertaken a systematic study of the reaction on Au/TiO2 catalysts prepared by the deposition-precipitation method followed by calcination at different temperatures and with different monodispersed gold particle sizes. The samples were prepared in the Haruta laboratory and a full paper will be presented.7 In this paper we will focus on a FTIR study of CO adsorption and oxidation on supported gold nanoparticles, of one of these samples in the temperature range 90-300 K. All the previous FTIR studies were made by us8,9 or by other authors10 at room temperature (RT). However, taking into account that on Au/ TiO2 the reaction occurs at temperatures well below RT, we have undertaken this study in order to get a deeper insight into the nature of the reaction centers. So far there is still uncertainty about where and in which form the two involved molecules, carbon monoxide and oxygen, interact. From the experimental data it appears that CO is weakly and molecularly adsorbed on the metallic phase and on the support. The oxygen involved in the elementary step can be a superoxidic species formed on the oxygen vacancies at the perimeter interface between gold * Tel: +39011 6707542. Fax: +39011 6707855. E-mail: boccuzzi@ ch.unito.it.
particles and the support, an activated molecular peroxo or superoxo species formed on the gold particles or it can be an atomic state, formed by an easy dissociation of the oxygen molecules on special gold sites on thin and small gold particles. Therefore, as for the location of the reaction, it may take place between chemisorbed carbon monoxide and oxygen on the metallic gold particles or at the gold-metal oxide interface. This last hypothesis is claimed by Liu et al.11 Probably different reaction channels are possible under different experimental conditions. By changing the temperature, we can get evidence concerning these different reaction channels. 2. Materials and Methods The sample has been prepared by the deposition precipitation method12 on a P-25 titania. The gold loading is 1wt %, the calcination temperature in air was 573 K, the mean diameter of the metallic particles, determined by TEM, is 3.1 ( 0.8 nm. Catalyst powder was pressed into a self-sustaining disk and put into a cell allowing to run spectra in controlled atmosphere at 90 and 300 K. The sample has been submitted to alternate treatments in vacuo and in oxygen from 298 up to 573 K and before the 90 K spectroscopic experiments shortly exposed to the laboratory atmosphere. The spectra have been taken on a 1760 Perkin-Elmer spectrometer. The experiments reported in this paper concern CO - 18O2 interactions performed in different ways, with preadsorbed CO, with preadsorbed oxygen at 90 K and at 300 K. Isotopically labeled 18O2 was used to discriminate between the CO2 production on the metallic particles from the possible catalytic oxidation involving oxygen atoms of the support. 3. Results and Discussion Figure 1 reports the FTIR spectra in the 2400-2000 cm-1 region of the sample interacting with 4 mbar of CO at 90 K (curve 1) and after outgassing CO at the same temperature (curve 2). Three bands are observed in curve 1, at 2175, 2153, and 2099 cm-1. When CO is outgassed at 90 K, the 2175 cm-1 band strongly reduces its intensity, the 2153 cm-1 one is almost completely depleted, while the 2099 cm-1 remains nearly not modified. In the OH stretching region (not shown for the sake
10.1021/jp000749w CCC: $19.00 © 2000 American Chemical Society Published on Web 05/20/2000
Letters
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5415
Figure 1. FTIR difference absorption spectra at 90 K of Au/TiO2 sample in the 2400-2000 cm-1 range produced by 4 mbar CO (curve 1) and by 0.5 mbar CO (curve 2).
of brevity) the bands, assigned to the surface Ti-OH free hydroxyls, strongly reduce their intensity by the interaction with 4 mbar of CO and new and stronger bands are observed at 3570-3450 cm-1, related to the OH groups interacting with the physisorbed hydrogen-bonded CO. The outgassing of CO almost completely restores the initial spectrum in the OHstretching region. On the basis of their different lability and of the additional data concerning the OH stretching region we can assign the three bands: the 2153 cm-1 band to a CO species physisorbed on hydroxyls of the surface, the 2175 cm-1 one to CO weakly chemisorbed on surface cations, possibly Ti4+ or Au3+ sites, according to the data reported in ref 2, Table 10, and the 2099 cm-1 band to CO adsorbed on gold metallic sites. In previous papers, concerning the FTIR spectra of CO adsorbed at RT on different Au/TiO2 samples, the band frequencies of CO on Au were found in the 2106-2118 cm-1 range.2,8-10 The lower frequency observed in this case can be due to the fact that at 90 K a higher surface coverage than at RT is realized. The effect of the size and of the morphology of gold particles can be hypothesized and it is being studied by comparing the spectra of samples with different gold sizes.7 In Figure 2, section a, the effect of the admission of 18O2 at 90 K (curve 2) on the spectrum of CO still remaining adsorbed on the sample after outgassing at 90 K (curve 1) in the 24002000 cm-1 range is shown. It appears evident that the 18O2 inlet produces on the carbonylic band an erosion from the lowfrequency side, a small blue shift of the band, from 2099 up to 2105 cm-1 and the growth of a shoulder at 2116 cm-1 (see inset). At the same time a band at 2323 cm-1, assigned to C16O18O solidlike phase grows up, accompanied by a very weak band at 2340 cm-1, assigned to C16O2 solidlike phase. As the reaction proceeds at 90 K, the maximum intensity of the CO absorption band remains almost constant. In Figure 2, section b, curve 1 shows the spectrum produced by the interaction of 0.5 mbar of CO on preadsorbed 18O2. Here, with respect to the spectrum of the interaction with CO alone, a strong decrease in the intensity of the band assigned to CO adsorbed on top of gold metallic sites is observed and a new broad band grows at 2124 cm-1. Moreover, unlike in the previous experiment with preadsorbed CO (curve 2, section a), the band assigned to C16O18O is weak, as the band at 2340 cm-1, assigned to the C16O2 solidlike phase. The decrease in the intensity of the band assigned to CO adsorbed on top of gold sites is strong evidence that oxygen is
Figure 2. FTIR difference absorption spectra of Au/TiO2 sample in the 2400-2000 cm-1 range. Section a: 0.5 mbar preadsorbed CO at 90 K (curve 1, the same as curve 2 of Figure 1), followed by admission of 1 mbar 18O2 at 90 K (curve 2); (inset) spectral difference between curve 2 and curve 1 in the 2150-2050 cm-1 range. Section b: interaction of 0.5 mbar of dry CO on preadsorbed 18O2 at 90 K (curve 1) and interaction of 0.5 mbar of wet CO on preadsorbed 18O2 at 90 K (curve 2). Section c: interaction of 10 mbar CO and 5 mbar 18O2 at RT.
adsorbed on gold particles and inhibits the CO adsorption. Moreover, new modified adsorption sites, characterized by the absorption band at 2124 cm-1, where CO is more strongly bonded than on clean on top sites, are produced. Curve 2, section b, shows the spectrum produced by the interaction of 0.5 mbar of wet CO on preadsorbed 18O2. This experiment has been done after the previously illustrated one, simply after outgassing at RT the gas phase. It appears evident that in the presence of moisture the 2124 cm-1 band is nearly absent and the 2101 cm-1 one shows an intensity similar to that observed in section a, curve 2. Moreover, the band assigned to C16O18O shows an intensity similar to that of curve 2. Also in this case only C16O18O is formed in the reaction, without any participation of the oxygen of the support or of the water.
5416 J. Phys. Chem. B, Vol. 104, No. 23, 2000 These data suggest that the oxygen and the carbon monoxide involved in the reaction are activated on the gold particles. It can be proposed that, in the case of dry CO preadsorption the O2 molecules are dissociated on gold sites. Probably, an intermolecular interaction with CO adsorbed on adjacent sites may help the oxygen dissociation. The nascent atomic oxygen atoms can immediately react with CO, according to the following reaction:
CO(a) + 18O(a) w C16O18O It was postulated by Bondzie et al.4 that gold ultrathin islands, in contrast to bulk Au, can dissociate oxygen molecules, since they bind Oa much more strongly than bulk Au. In the case of oxygen preadsorption, at high oxygen coverages, it can be proposed that at T < 100 K, as observed on Pt13,14 by different techniques and proposed on Au,5,15 oxygen is mainly adsorbed on the metallic particles molecularly in a superoxo and/or in a peroxo state. The CO adsorption at 90 K on samples precovered by molecular oxygen is partly inhibited and partly strongly modified as a consequence of the electron transfer from the metal to the adsorbed oxygen molecules. Probably, as already observed on platinum, the oxygen decomposition is inhibited at high oxygen coverage. In this hypothesis we assign the 2124 cm-1 band to CO adsorbed on gold sites interacting with a superoxidic or peroxidic oxygen molecular species:
O2--Au+-CO
νCO ) 2124 cm-1
The bands observed previously in the same spectral region on Cu/ZnO and on Cu/TiO216 and on Cu/SiO217 may have the same assignment. On these catalysts, it was observed that at room temperature interactions with CO after interaction with molecules producing atomic oxygen, as N2O and NO, do not produce any band at 2120-2140 cm -1, which was found after molecular oxygen interaction. In the case of Pt18 it was shown by theoretical calculations that at high coverages CO coadsorption stabilizes the peroxo and superoxo-like adsorbed species over the dissociated atomic adsorbate. The lack of the band assigned to C16O18O in this experiment appears as an indication that by preadsorption of oxygen at 90 K mainly molecular forms of adsorbed oxygen are produced. In the presence of moisture the superoxo species reacts with the water, producing nascent oxygen and OH groups on the support:
O2 2- + H2O w O + 2OHallowing the occurrence of the reaction CO + 18O. From this experiment it appears evident that water actiVates the oxygen dissociation on the gold particles, so explaining the promoting effect of water on the reaction. In all the experiments so far illustrated at T < 100 K, no changes are observed in the carbonate-like region, indicating that the formation of carbonates on the support is thermally activated. By increasing the temperature, new bands appear in the carbonate-like region, first the bands assigned to side-on bent CO2- species19,20 and then carbonate and bicarbonate species, already assigned and discussed in previous papers (spectra not shown for the sake of brevity). By interaction at room temperature with CO and 18O2 (Figure 2, section c) a strong triplet of absorption bands assigned to C16O2, C16O18O, and C18O2 weakly adsorbed on titania is
Letters observed. The intensity of the first two components is similar, while the last one is weaker. The presence of the multiplet of bands clearly indicates that at room temperature, in contrast to low temperature, a quite extensive exchange reaction with the oxygen atoms of the support occurs. Moreover, at the same time, a weak, but definite band at 2060 cm-1, already assigned to the 18CO stretching vibration, is observed, as already shown previously.8,9 The fact that this feature appears only after interaction at room temperature, simultaneously to the extensive exchange reaction with the surface oxygen atoms revealed by the multiplet of bands in the CO2 region, can be considered as an indication that the scrambling reaction is a secondary reaction occurring via the intermolecular interaction of CO and atomic oxygen adsorbed on adjacent sites. The dissociation of the CO2 isotopomers on gold sites in close contact with titania is the reverse reaction of the CO2 isotopomers formation:
C16O18O w C18O + 16O 4. Conclusions Looking to the open questions, reported in the Introduction, concerning where and in which form the reactants, carbon monoxide and oxygen, are activated, it is suggested by the illustrated experiments that (i) CO and molecular oxygen are competitively adsorbed on the gold particles at 90 K, CO is adsorbed on top of gold sites, and oxygen is adsorbed most likely in a peroxo or superoxo form, (ii) the differences observed in the experiments performed in different ways, CO preadsorbed or oxygen preadsorbed, in the absence or in the presence of moisture suggest that the determining step in the reaction is the oxygen dissociation on free, reactive gold sites on thin gold particles, and (iii) at higher temperatures other reaction channels, at the gold-metal oxide interface, are also active. References and Notes (1) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (2) Bond, G. C.; Thompson, D. T. Catal. ReV. Sci. Eng. 1999, 41, 319 and references therein. (3) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (4) Bondzie, V. A.; Parker, S. C.; Campbell, C. T. Catal. Lett. 1999, 63, 143. (5) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W. D.; Hakkinen, H.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573. (6) Heinz, U.; Sanchez, A.; Abbet, S.; Schneider, W. D. J. Am. Chem. Soc. 1999, 121, 3214. (7) Boccuzzi, F.; Chiorino, A.; Haruta, M. To be published. (8) Boccuzzi, F.; Chiorino, A.; Tsubota, S.; Haruta, M. J. Phys. Chem. 1996, 100, 3625. (9) Boccuzzi, F.; Chiorino, A.; Tsubota, S.; Haruta, M. Catal. Lett. 1994, 29, 225. (10) Dekkers, M. A. P.; Lippits, M. J.; Nieuwenhuys, B. E. Catal. Lett. 1998, 56, 195. Grunwaldt, J. D.; Baiker, A. J. Phys. Chem. B 1999, 103, 1002. (11) Liu, H.; Kostov, A. I.; Kostova, A. P.; Shido, T.; Akakura, K.; Iwasawa, Y. J. Catal. 1999, 185, 252. (12) Sakurai, M.; Tsubota, S.; Haruta, M. Appl. Catal. A 1993, 102, 125. (13) Avery, N. R. Chem. Phys. Lett. 1983, 96, 371. (14) Eichler, A.; Hafner, J. Phys. ReV. Lett. 1997, 79, 4481. (15) Gottfried, J. M.; Schmidt, K. J.; Schroeder, S. L. M.; Christmann, K. Surf. Sci., in press. (16) Boccuzzi, F.; Chiorino, A. J. Phys. Chem. 1996, 100, 3617. (17) Boccuzzi, F.; Coluccia, S.; Martra, G.; Ravasio, N. J. Catal. 1999, 184, 316. (18) Eichler, A.; Hafner, J. Phys. ReV. B 1999, 59, 5960. (19) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Andreva, D.; Tabakova, T. J. Catal. 1999, 188, 176. (20) Ramis, G.; Busca, G.; Lorenzelli, V. Mater. Chem. Phys. 1991, 29, 425.
