J. Phys. Chem. 1996, 100, 785-789
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Effect of Ceria Structure on Oxygen Migration for Rh/Ceria Catalysts H. Cordatos, T. Bunluesin, J. Stubenrauch, J. M. Vohs, and R. J. Gorte* Department of Chemical Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104 ReceiVed: July 21, 1995; In Final Form: October 9, 1995X
The adsorption and reaction properties of Rh particles supported on a CeO2(100) surface and on polycrystalline CeO2 films annealed at low (LT, 970 K) and high (HT, 1720 K) temperatures were studied using temperatureprogrammed desorption (TPD) and steady-state, CO oxidation kinetics. SEM and XRD of the polycrystalline samples showed significant growth of the ceria crystallite size and change in the surface morphology upon high-temperature annealing. In TPD, a substantial fraction of the CO adsorbed on Rh supported on the LT polycrystalline film was found to desorb as CO2, with the oxygen coming from the CeO2 support. In contrast, oxidation of CO to CO2 was not observed in significant amounts during TPD from Rh on the CeO2(100) sample or the HT film. In agreement with the adsorption measurements, steady-state CO oxidation exhibited a second, ceria-mediated process for Rh on the LT film which was not observed on the HT film. These results indicate that the structure of ceria plays an important role in the reactivity of Rh/ceria catalysts.
Introduction Ceria is an important additive in automotive, emissions control catalysts. While ceria may play a number of roles in enhancing the catalytic performance, its ability to store and release oxygen appears to be the most significant.1-5 This property results from the fact that both Ce3+ and Ce4+ are stable, allowing the oxide to shift between CeO2 and CeO2-x. The lattice oxygen released during ceria reduction can react with hydrocarbons and CO under rich conditions. Unfortunately, it has been found that the oxygen storage capacity deteriorates irreversibly during operation of a catalytic converter.6 This change has been attributed to the loss of surface area resulting from sintering of ceria particles; however, this has yet to be verified. Although surface area is undoubtedly important, it has been shown that performance is not improved by simply adding more ceria.7 This suggests that other factors are also important. Recent work on model catalysts, prepared by vapor deposition of the catalytic metal onto flat ceria supports, may provide insights into this question. Using Rh on polycrystalline ceria supports, Zafiris and Gorte showed that part of the catalytic cycle involving the oxygen storage properties of ceria could be duplicated on model catalysts.8 Following adsorption of CO onto freshly deposited Rh in ultrahigh vacuum, a substantial fraction of the CO, as much as 50% in some experiments, desorbed as CO2 in temperature-programmed desorption (TPD) measurements. The oxygen required to form CO2 almost certainly came from the reduction of ceria in contact with Rh. It is interesting to compare this result with that obtained by Stubenrauch and Vohs using a single crystal CeO2(111) substrate. In that study, only a small fraction of the CO, ∼2%, reacted to form CO2. These results suggest that the structure of the CeO2 is important for oxygen transport to Rh.9 In order to further understand the role of the ceria structure on the ability of this oxide to denote oxygen for catalytic reactions, we have now examined the adsorption properties of Rh deposited onto a CeO2(100) epitaxial film and a polycrystalline film that was annealed to 1720 K to form large crystallites. The CeO2(100) surface was examined to assess whether oxygen transport is dependent on the crystallographic X
Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-0785$12.00/0
orientation of the ceria. The high-temperature-annealed, polycrystalline ceria film allowed the effects of crystalline size and defect concentration to be examined. On both ceria surfaces, the TPD results from Rh particles were similar to those obtained on CeO2(111) in that only small amounts of CO2 were formed. Furthermore, steady-state measurements of CO oxidation on Rh films deposited onto films annealed at low temperature (970 K) and high temperature (1720 K) show significantly different dependencies on the CO partial pressure, with a ceria-mediated reaction mechanism observed only on the low-temperature film. The implications of these observations for automotive catalysis and oxygen storage will be discussed. Experimental Section Three ceria supports were examined using TPD: an epitaxial CeO2 film with (100) surface orientation on r-plane sapphire and two polycrystalline ceria films on R-Al2O3(0001). The epitaxial CeO2(100) film was provided by Los Alamos National Laboratory and was grown using 90° off-axis radio-frequency magnetron-sputtering with a gas phase consisting of 60% Ar and 40% O2 at a total pressure of 4 × 10-2 Torr. The average thickness of the film was ∼100 Å. The two polycrystalline samples were prepared by spray pyrolysis of an aqueous solution of Ce(NO3)3 (99% purity, Johnson Matthey) onto an R-Al2O3(0001) crystal which was held at ∼600 K.10 Both polycrystalline samples were annealed overnight at 970 K in air, but one sample was then annealed at 1720 K in air for an additional 4.5 h. The ceria films formed by this method were ∼10-20 µm thick. Hereafter, these polycrystalline samples will be designated LT (for low-temperature annealed) and HT (for hightemperature annealed). The polycrystalline samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and BET isotherms. Following both low- and high-temperature annealing, the XRD patterns were similar to the standard data for the powder diffraction of CeO2 (Powder Diffraction File, compiled by JCPDS International Center for Diffraction Data, Swarthmore, PA), as shown in Figure 1a,b. However, there is significant broadening of the lines, especially for the LT sample, due to the small crystalline size. Using Scherrer’s formula and the measured peak width at half-maximum, the average crystallite sizes were estimated to be between 90 and 120 Å © 1996 American Chemical Society
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Figure 1. X-ray diffraction patterns of the (a) LT and (b) HT ceria films before deposition of the metal.
following the low-temperature annealing and between 300 and 350 Å following high-temperature annealing. The SEM results for the two samples are shown in Figure 2a,b. The LT sample shows flat regions separated by cracks at ∼1 µm intervals. Since the BET surface area for this film was 12 m2/g, a value close to that which would be calculated for 0.1 µm, dense, ceria spheres, the flat regions may be slightly porous. Hightemperature annealing changed the surface dramatically. The ceria formed particles with rounded surfaces which, upon close observation, reveal facets of (111) and (100) surfaces, as determined by their relative orientation. The TPD results were measured in an ultrahigh-vacuum chamber equipped with a CMA for Auger electron spectroscopy (AES), an evaporative Rh source, and a calibrated, quartz crystal, film thickness monitor.11 Following Rh deposition, the sample could be rotated in front of an aperture on a stainless steel cone, inside of which was a quadrupole mass spectrometer. Each TPD measurement was carried out with a constant heating rate of ∼7 K/s, and the temperature of the sample was measured with a chromel-alumel thermocouple attached to the back surface via a ceramic cement. The base pressure of the chamber was below 5 × 10-10 Torr. For the LT ceria and the CeO2(100) epitaxial film, it was necessary to lightly sputter the substrate, prior to Rh deposition, in order to achieve a carbon-free surface. On the HT ceria, cleaning the surface was unnecessary. Steady-state, CO oxidation rates were measured on two, polycrystalline films which were very similar to those used in the TPD measurements, except that the ceria was deposited onto either an alumina wafer which could be broken to fit into a 1/ -in. Pyrex reactor or an oxidized Al foil which could be rolled 4 to fit into the reactor. On both samples, a film consisting of 5 × 1015 Rh atoms/cm2 was deposited onto the external surface by vapor deposition. The reaction rates were negligible prior to the addition of Rh. The amount of catalyst in the reactor and the total flow rate could be varied so that conversions were always differential, usually well below 1% of the limiting reagent. The total pressure in the reactor was 1 atm, with the reactant composition being controlled by varying the flow rates of air, CO, and N2. The CO was research purity (99.99%) and was further purified by passing through both an activated-carbon
Figure 2. High-resolution scanning electron micrographs of the (a) LT and (b) HT ceria films, both shown to the same scale.
trap to remove carbonyls and NaOH pellets to eliminate residual CO2. The O2 was 99.9% purity (20%, during the course of the measurement, at temperatures between 350 and 450 K. Since oxygen was not introduced into the chamber prior to CO adsorption and since carbon was not deposited onto the sample during the TPD experiment, which would occur if the oxygen resulted from CO dissociation, the oxygen atoms required for CO2 formation must have originated from the ceria lattice. This would result in the reduction of a portion of the ceria. In agreement with this conclusion, consecutive CO adsorption-desorption cycles showed a continuous decrease in the amount of CO2 which formed, resulting from a depletion of oxygen from the surface.8 (It is important to notice that the extent of ceria reduction, a very important parameter in commercial catalysts, cannot be estimated on these model systems. Previous measurements have shown that only the ceria in the region near the Rh is reduced under our experimental conditions, leaving most of the thick ceria film unaffected.8) Near-surface ceria could be reoxidized either by exposure to
J. Phys. Chem., Vol. 100, No. 2, 1996 787 gas-phase O2 or by annealing to 900 K to bring oxygen from the bulk. After reoxidation, the amount of CO2 produced in a TPD experiment returned to its original value. As mentioned previously, only a small fraction (
