17862
J. Phys. Chem. 1996, 100, 17862-17865
Evidence for Weakly Bound Oxygen on Ceria Films E. S. Putna, J. M. Vohs, and R. J. Gorte* Department of Chemical Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104 ReceiVed: March 12, 1996; In Final Form: August 29, 1996X
Temperature-programmed desorption (TPD) has been used to compare the O2 desorption characteristics of a thin film of ceria prepared by vapor deposition on an R-Al2O3(0001) substrate with that of a CeO2(111) surface. On CeO2(111), there is no substantial desorption of O2 below ∼1300 K, while a low-temperature state, between 800 and 1250 K, was observed from the ceria film. Reexposure of the ceria film to O2 above 600 K repopulated this desorption feature, showing that adsorption-desorption cycles did not alter the morphology of the film. By adding Rh to the ceria film and then measuring the reaction of adsorbed CO on the Rh, it was demonstrated that this low-temperature feature is responsible for the oxidation of CO during TPD. The implications of these results for understanding the oxygen storage function of ceria-supported catalysts are discussed.
Introduction Ceria plays an important role in three-way, automotive catalysts, primarily as an oxygen-storage medium which can provide oxygen under fuel-rich conditions and remove oxygen under lean conditions. Since CeO2 is not that thermodynamically easy to reduce to Ce2O3, however, the mechanism by which ceria performs this function is not obvious. It appears that contact with a group VIII metal is required for sufficient rates of reduction under automotive exhaust conditions.1-3 Because loss of the oxygen-storage function with catalyst aging is a problem,4 significant effort has gone into understanding this phenomenon. It is known that catalyst aging is accompanied by an increase in ceria crystallite size and concurrent loss of surface area.1-3 Therefore, a widely accepted explanation for catalyst deactivation is that contact between the precious metal and the ceria is lost.5 However, an alternative explanation is that the reducibility of even surface ceria is affected by the crystallite size and the presence of defects associated with small crystallites.1,6 In studies of model catalysts in which Rh was vapor deposited onto various ceria samples so as to maintain the same interfacial contact between the Rh and the ceria, it was found that the ability of ceria to donate oxygen to Rh was strongly dependent on the ceria structure.6 In both temperature-programmed desorption (TPD) measurements of CO and steady-state CO oxidation rates, evidence for oxygen transfer from the ceria to the Rh was observed for ceria films prepared by calcination of Ce(NO3)3 in air at 600 K, but nearly absent for ceria single crystals or ceria films calcined at 1720 K. On the lowtemperature ceria films, a significant fraction of CO adsorbed on the Rh desorbed as CO2 in TPD measurements. In steadystate CO oxidation, a second mechanism, with a much lower activation energy, was found for Rh on the ceria films calcined at low temperatures.7 For ceria single crystals and ceria films calcined at high temperatures, CO TPD curves and steady-state oxidation rates were very similar to that obtained for Rh/alumina catalysts. The reducibility of ceria has, of course, been studied extensively by temperature-programmed reduction (TPR).1-3,8-10 TPR has provided a great deal of practical information on the effect of dopants and pretreatment conditions;1,2 however, the X
Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)00762-9 CCC: $12.00
information is indirect as to how the labile oxygen is held in these materials. A much more direct measure would be oxygen desorption, but the complexity of the desorption process on porous materials makes these measurements difficult to interpret.11,12 Diffusion, coupled with readsorption, can shift desorption features by hundreds of degrees. Therefore, to avoid these complexities, we have studied oxygen desorption from two flat ceria catalysts: a ceria film capable of transferring oxygen to Rh and an inactive CeO2(111) crystal. We will show that there is a low-temperature (800-1200 K) desorption feature for oxygen on the ceria film, not present on the ceria crystal, which can be associated with the labile oxygen which reacts with CO adsorbed on supported metals. This provides additional evidence that changes in ceria structure are largely responsible for loss of the oxygen storage capacity in aged three-way catalysts. Experimental Section The equipment used in these experiments was the same as that described in previous publications.13 An ion-pumped vacuum chamber, which has a base pressure of ∼3 × 10-10 Torr and was used for the TPD experiments, is equipped with a CMA for Auger electron spectroscopy and a quadrupole mass spectrometer. To improve sensitivity for TPD, the quadrupole was placed in a stainless steel cone with an aperture, in front of which the sample could be placed. The oxide substrates, R-Al2O3(0001) and CeO2(111) single crystals, were smaller than that used in previous studies, having dimensions of 3 mm, so that higher temperatures could be achieved. The sapphire crystal was obtained from Crystal Systems Inc. and was used without further characterization. Characterization of the CeO2(111) crystal has been discussed in a previous publication.14 As in previous studies, both crystals were mounted on a Ta foil that could be resistively heated to 1400 K. A Pt-10% PtRh thermocouple, attached to the backside of the oxide substrates with a ceramic adhesive, was used to monitor the temperature. The adsorbates were introduced into the chamber through beam dosers in order to maintain a low base pressure. The pressure in front of the doser has been estimated to be approximately 20 times higher than the background pressure, and exposures quoted in this paper were determined using this factor. All TPD measurements were carried out with a linear heating rate of 12 K/s. © 1996 American Chemical Society
Weakly Bound Oxygen on Ceria Films
Figure 1. O2 TPD curves for (a) an R-Al2O3(0001), (b) a CeO2(111) single crystal, (c) a ceria film, 1016 CeO2/cm2, on R-Al2O3(0001), and (d) Rh particles (1 × 1015 Rh/cm2) on R-Al2O3(0001). The conditions used to adsorb O2 are discussed in the text.
After cleaning the substrates by Ar+ ion bombardment at room temperature, they were annealed in 5 × 10-8 Torr of O2 at 1273 K for 10 min and subsequently oxidized in 5 × 10-8 Torr of O2 at 773 K for 30 min. Ceria or Rh films were then deposited onto these substrates. Coverages of the films were determined using a quartz crystal film thickness monitor.15,16 Because preliminary investigations showed that the method used to prepare the ceria films could affect the results, all ceria films were prepared in the following manner: Metallic Ce (Johnson Matthey, 99.9%) was vaporized in the presence of 1 × 10-7 Torr of O2, while heating the substrate to 623 K; the films were then further oxidized in 1 × 10-7 Torr of O2 for 15 min at 673 K to ensure complete oxidation. For these conditions, the results were completely reproducible. Rh was also vapor deposited, but the substrates were always held at room temperature in vacuum. In this study, the density of the ceria films was 1 × 1016 CeO2/cm2, while the Rh films were always 1 × 1015 Rh/cm2. The Rh coverage corresponds to approximately one monolayer. After heating, the Rh always formed particles. On R-Al2O3(0001), the average Rh particle size was ∼30 Å, as estimated from dispersion measurements.15 Average particle sizes were similar for Rh when ceria was present. Results and Discussion Oxygen desorption curves for each of the substrates of interest, all on the same scale, are shown in Figure 1. With the exception of the curve for Rh/R-Al2O3(0001), each sample was oxidized in the manner described in the Experimental Section, i.e., in 1 × 10-7 Torr of O2 at 673 K for 15 min. Figure 1a was obtained on the clean R-Al2O3(0001) surface and provides a base line, since no oxygen is expected to desorb from this sample. Figure 1b is the oxygen TPD curve for the CeO2(111). Again, no oxygen desorbed below ∼1300 K, which is to be expected based on bulk thermodynamics. For the reaction 2CeO2 ) Ce2O3 + 0.5O2, the partial pressure of O2 should reach ∼10-12 Torr at 1400 K. Even assuming a sticking coefficient
J. Phys. Chem., Vol. 100, No. 45, 1996 17863 of unity for O2 on Ce2O3, one should not expect to see a significant desorption rate from CeO2 at temperatures much below this. The oxygen which begins to desorb at ∼1300 K almost certainly results from partial reduction of the bulk. Figure 1c shows that the desorption of oxygen from the ceria film on R-Al2O3(0001) begins at a much lower temperature than that observed for CeO2(111). Again, onset of bulk reduction occurs at ∼1300 K, but now a new, low-temperature (LT) feature is observed between 800 and 1200 K. The formation of the LT O2 desorption feature is reversible. Repeated oxidation treatments in 1 × 10-7 Torr of O2 for 15 min at 673 K produced identical TPD results, suggesting that the film morphology was not irreversibly altered between successive oxidations and reductions. Exposure to O2 at temperatures below 400 K did not repopulate the LT O2 feature. Also, preliminary results from 18O2 exchange studies demonstrate that the oxygen associated with this desorption feature is dissociated. From the desorption temperature, one can estimate how strongly this oxygen is bound. For second-order desorption with a standard preexponential of 0.01 cm2/s, the activation energy for a desorption peak centered at 1000 K would be approximately 62 kcal/mol. The binding energy should be similar to the desorption activation energy. Compared to the enthalpy for removal of O2 from CeO2, -∆H ) 178 kcal/mol using the reaction 4CeO2 ) 2Ce2O3 + O2, the oxygen associated with bulk reduction is much more strongly bound. It is worth noting that the enthalpy for bulk reduction includes that for the transformation from the fluorite structure of CeO2 to the hexagonal structure of Ce2O3, while the fluorite structure is almost certainly retained in the initial reduction of CeO2. Additionally, the removal of oxygen from the bulk does not take into account the presence of the surface. Although these factors may account for some of the discrepancy between the experimental and calculated energies, it is apparent that reduction of the ceria film is quite different from the ceria crystal. Figure 1d is the TPD curve for oxygen from the one monolayer (1 × 1015/cm2) Rh film on R-Al2O3(0001), following a 10 langmuir (1 langmuir ) 10-6 Torr‚s ) 3.7 × 1014 molecules/cm2) exposure to O2 at room temperature. This TPD curve, which shows a prominent peak at 975 K, with additional features to higher temperatures, is almost identical to desorption curves reported for Rh(111)17 and similar to curves reported for Rh(100).18 For the Rh single crystals, desorption stopped abruptly at 1400 K, with a return of the oxygen pressure to the base line. In our experiments, we stopped the heating ramp at 1400 K so that the return to the base line does not show. However, exposure of the Rh to CO at room temperature following the TPD run showed that essentially all of the oxygen had been removed. The TPD result for Rh is interesting for several reasons. First, there does not appear to be a big difference in the desorption properties of small Rh particles (3040 Å in this case) from that of Rh single crystals for the desorption of oxygen. Furthermore, the fact that oxygen desorbs from Rh in a very similar temperature range to that for the ceria film suggests that oxygen is bound to both Rh and the ceria film with similar energetics. For Rh(111), the effective energy of desorption was ∼56 kcal/mol at low coverages,17 a number similar to that calculated for the ceria film. Finally, the areas under the desorption curves provide a means to estimate the amount of oxygen which desorbs from the ceria film. The area under the low-temperature feature between 800 and 1250 K for the ceria film is approximately 0.25 that of the area under the desorption curve for oxygen from Rh. Since the ceria film had at least 10 times the oxygen, only a relatively small, but
17864 J. Phys. Chem., Vol. 100, No. 45, 1996 nontrivial, fraction of the oxygen from ceria is removed under these conditions. In order to determine whether the oxygen which desorbs at low temperatures from ceria can be associated with CO oxidation on supported metals, we next examined the adsorption properties of Rh overlayers on the ceria. As discussed earlier, a substantial fraction of CO adsorbed on Rh films deposited onto the ceria calcined at low temperatures reacts to CO2 during TPD.19 Here, we first prepared the ceria film on R-Al2O3(0001) as in Figure 1c and then heated the sample to either 800 to 1250 K. Then, one-monolayer Rh films were deposited onto the ceria at room temperature, the samples were again ramped to 800 K to form Rh particles, and the samples were exposed to 10 langmuirs of CO at room temperature. The TPD curves from these samples after this pretreatment are shown in Figure 2. To provide a basis of comparison, Figure 2a shows the TPD curves for CO and CO2 from Rh/R-Al2O3(0001) with no ceria. The curve was obtained following consecutive exposures to 10 langmuirs of O2 (an exposure equal to that for the O2 TPD in Figure 1d) and CO at room temperature. Figure 2a, which is similar to what would be observed for coadsorption of CO and O2 on bulk Rh,20 shows that CO desorbs with a peak temperature near 500 K, with a broad shoulder to lower temperatures. CO2 desorbs between 350 and 550 K; the areas under the CO (m/e ) 28) and CO2 (m/e ) 44) peaks are in a ratio of 1 to 0.8, which suggests that roughly half of the adsorbed CO reacts for this oxygen coverage. In Figure 2b, the results are shown for Rh deposited on the ceria which had been heated to only 800 K, a temperature lower than the oxygen desorption feature. The TPD curves are very similar to what is observed for Rh/R-Al2O3(0001) in Figure 2a, with the exception that only about one-fourth as much CO2 is formed (CO2/CO ∼ 30%). This agrees reasonably well with the fact that the area under the oxygen peak in Figure 1d is approximately quadruple the area under the low-temperature oxygen peak in Figure 1c. For the results in Figure 2c, O2 that desorbs at low temperature had been removed from the ceria by heating to 1250 K prior to deposition of Rh, but all other treatments were the same. A much smaller fraction of the CO reacts to CO2 (CO2/CO ∼ 5%) following this pretreatment. These results imply that there must be oxygen, derived from the reduction of specific sites on ceria, which is capable of reacting with CO adsorbed on Rh. Based on the similarity of the results for coadsorbed CO and O2 on Rh with those observed for CO adsorbed on Rh/CeO2, it appears that the oxygen actually migrates onto the Rh prior to CO adsorption. While ∆G ) +66 kcal/mol at 298 K for the reaction Rh + 2CeO2 ) Ce2O3 + RhO,19 suggesting that this reaction should be unfavorable, the O2 desorption results indicate that bulk thermodynamics should not be used to predict the chemistry associated with supported particles on ceria. The energetics for oxygen transfer from the special sites in ceria do not appear to be unfavorable, at least to any large degree. If one assumes that the oxygen on ceria is highly localized, while oxygen can move on Rh, there may be an entropic advantage to the transfer. An interesting question is what is the nature of the oxygen which migrates easily from ceria to the supported metal. One possible explanation is that there is a crystal plane dependence on the ability of CeO2 to undergo reduction. Calculations have predicted that oxygen can be removed more easily on certain low-index crystal planes,21 although similar results for oxygen transfer from ceria to Rh were observed for both CeO2(111) and CeO2(100).6,14 Crystallite size may also be a key factor. For example, the excess surface energies associated with smaller
Putna et al.
Figure 2. TPD curves following CO adsorption at room temperature on Rh particles formed by heating a film of 1 × 1015 Rh/cm2. In (a), the Rh particles were supported on R-Al2O3(0001) and exposed to 10 langmuirs of O2 at 300 K. In (b), the Rh particles were formed on the ceria film (1016 CeO2/cm2 on R-Al2O3(0001)) after heating to only 800 K. The curves in (c) were obtained on a sample prepared exactly as in (b), but after heating to 1250 K.
crystallites have been shown to affect the structure of Y2O3 and ZrO2.22 Finally, the relative ease of oxygen removal in ceria may be the result of the presence of grain boundaries and defects, which are also known to increase the rate of oxygen diffusion through ceria.23-25 Obviously, additional work will be needed to answer questions about the nature of the special oxygen sites in ceria. Conclusions We have shown that a weakly bound oxygen species exists on ceria films which is not present in CeO2(111) crystals. This
Weakly Bound Oxygen on Ceria Films weakly bound oxygen migrates easily to supported Rh particles and appears to cause the special oxidation activity of Rh/ceria catalysts. Acknowledgment. This work was supported by the DOE, Basic Energy Sciences, Grant DE-FG03-85-13350. Some facilities were provided by the NSF, MRL Program, Grant DMR 88-19885. References and Notes (1) Zhang, Y.; Andersson, S.; Muhammed, M. Appl. Catal. B 1995, 6, 325. (2) Nunan, J. G.; Robota, H. J.; Cohn, M. J.; Bradley, S. A. J. Catal. 1992, 133, 309. (3) Trovarelli, A.; de Leitenburg, C; Dolcetti, G.; LLorca, J. J. Catal. 1995, 151, 111. (4) Fisher, G. B.; Theis, J. R.; Casarella, M. V.; Mahan, S. T. SAE paper 931034, 1993. (5) Basic Research Needs for Vehicles of the Future, DOE and NSF Workshop Report, January 1995 (http://pmi.princeton.edu/conference/ futurevehicles/). (6) Cordatos, H.; Bunluesin, T.; Stubenrauch, J.; Vohs, J. M.; Gorte, R. J. J. Phys. Chem. 1996, 100, 785. (7) Bunluesin, T.; Cordatos, H.; Gorte, R. J. J. Catal. 1995, 157, 222. (8) Logan, A. D.; Shelef, M. J. Mater. Res. 1994, 9 (2), 468.
J. Phys. Chem., Vol. 100, No. 45, 1996 17865 (9) Shyu, J. Z.; Weber, W. H.; Gandhi, H. S. J. Phys. Chem. 1988, 92, 4964. (10) Laachir, A.; Perrichon, V.; Badri, A.; Lamotte, J.; Catherine, E.; Lavalley, J. C.; El Fallah, A.; Hilaire, L.; le Normand, F.; Quemere, E.; Sauvion, G.; Touret, O. J. Chem. Soc., Faraday. Trans. 1991, 87, 1601. (11) Gorte, R. J. J. Catal. 1982, 75, 164. (12) Demmin; Gorte, R. J. J. Catal. 1984, 90, 32. (13) Zafiris, G. S.; Gorte, R. J. J. Catal. 1991, 132, 275. (14) Stubenrauch, J.; Vohs, J. M. J. Catal. 1996, 159, 50. (15) Altman, E. I.; Gorte, R. J. J. Catal. 1988, 113, 185. (16) Roberts, S. I.; Gorte, R. J. J. Chem. Phys. 1990, 93, 5337. (17) Thiel, P. A.; Yates, Jr., J. T.; Weinberg, W. H. Surf. Sci. 1979, 82, 22. (18) Fisher, G. B.; Schmieg, S. J. J. Vac. Sci. Technol. A 1983, 1, 1064. (19) Zafiris, G. S.; Gorte, R. J. J. Catal. 1993, 139, 561. (20) Schwartz, S. B.; Schmidt, L. D.; Fisher, G. B. J. Phys. Chem. 1986, 90, 6194. (21) Sayle, T. X. T.; Parker, S. C.; Catlow, R. A. J. Chem. Soc., Chem. Commun. 1992, 977. (22) Scandan, G.; Foster, C. M.; Frase, H.; Ali, M. N.; Parker, J. C.; Hahn, H. Nanostruct. Mater. 1992, 1, 313. (23) Sanchez, M. G.; Gazquez, J. L. J. Catal. 1987, 104, 120. (24) Sayle, T. X. T.; Parker, S. C.; Catlow, C. R. A. Surf. Sci. 1994, 316, 329. (25) Jin, T.; Okuhara, T.; Mains, G. J.; White, J. M. J. Phys. Chem. 1987, 91, 3310.
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