Structure of Pd/CeO x /Al 2 O 3 Catalysts for NO x Reduction Determined By in Situ X-ray Absorption Spectroscopy Joseph H. Holles and Robert J. Davis* Department of Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22904-4741 ReceiVed: April 7, 2000; In Final Form: August 8, 2000 In situ X-ray absorption spectroscopy at the Ce L III edge and Pd K edge was used to characterize the oxidation state and structural parameters of CeO x /Al 2 O 3 and Pd/CeO x /Al 2 O 3 catalysts during the reduction of NO by CO. The samples were exposed to oxidizing (5% NO in He), reducing (5% CO in He), and stoichiometric (5% NO/5% CO in He) gaseous environments. The Ce L III edge structure indicated that one-third of the cerium readily changed oxidation state between 3+ and 4+ upon exposure to various mixtures of NO and CO at 673 K. However, structural parameters derived from EXAFS analysis indicated that cerium remained in the CeO 2 crystal structure regardless of gaseous environment. The average oxidation state of Pd was also affected by gaseous environment with an average oxidation state between 0 and 2+ for a stoichiometric mixture of NO and CO. Exposure of Pd particles to NO resulted in the formation of chemisorbed oxygen and/or a surface oxide layer that can be completely removed by exposure to CO at 573 K. Introduction Automobile exhaust catalysts are designed to reduce emis- sions of carbon monoxide, nitrogen oxides, and uncombusted hydrocarbons. These catalysts typically contain noble metals such as Pt, Pd, and Rh with a ceria promoter supported on alumina. Traditionally, the principal function of the Rh is to control emissions of nitrogen oxides (NO x ) 1 by reaction with carbon monoxide. However, use of Pd to control NO x emissions has increased. As a result, the NO + CO reaction has been studied recently on high surface area Pd/Al 2 O 3 powder catalysts. 2-6 Although ceria has been shown to effectively promote Rh for the NO + CO reaction, 7 its use as a promoter for Pd has not been as widely studied. 6,8 Nevertheless, substantial improvements in reaction rates are observed when ceria is added to supported Pd catalysts. Investigations on the role of ceria in automobile exhaust catalysts have demonstrated its capacity to store and release oxygen during the catalytic reaction. 9 Whether this oxidation- reduction cycle results in CeO 2 conversion to Ce 2 O 3 or simply a disordered suboxide under reaction conditions is still unclear. X-ray absorption spectroscopy (XAS) provides a valuable tool for investigating ceria since it allows determination of both oxidation state and atomic structural parameters. The technique has been used previously to elucidate the valence of cerium in ceria-supported Pd and Rh catalysts. 8,10 Unfortunately, these studies were not performed in situ and the catalysts were reduced in H 2 and oxidized in air instead of CO and NO. A later paper also reported on the in situ reduction of ceria reduction by H 2 as measured by XAS. 11 Since the oxidation state of cerium in these catalysts is dependent on the operating conditions, in situ experiments using NO and CO are needed to derive conclusions that are relevant to automotive catalysis. The Pd component of NO x reduction catalysts can also be easily probed by XAS. For example, Matsumoto and Tanabe have shown Pd in zeolite Y consists of small clusters under operating conditions for the reaction of NO reduction by propane. 12 Additionally, Ali et al. have used XAS to show that Pd is transformed into dispersed Pd 2+ ions on acidic supports and to Pd oxide clusters on nonacidic supports after exposure to the reaction mixture of methane, nitrogen monoxide, and oxygen. 13 The present study uses XAS to investigate in situ the oxidation state and structural parameters of both Ce and Pd in catalysts that have demonstrated activity for NO reduction by CO. Typical three-way catalysts operate at temperatures around 900 K and do not promote NO x reduction except in a narrow temperature window in the vicinity of catalyst lightoff. 1 This is a concern during cold start since the catalyst has not yet warmed to operating temperature and a significant amount of total emissions is released during this period. Thus, the temperatures used in this work are less than 673 K to simulate the state of the catalyst relevant to cold-start conditions. Experimental Section Sample Preparation. A CeOx/Al 2 O 3 sample was prepared by stirring γ-Al 2 O 3 (Alfa Aesar, 99.97%) and cerium(III) acetylacetonate (acac) hydrate (Aldrich) in toluene for 2 h at 353 K, removing excess solvent under vacuum in a rotary evaporator, and drying for 24 h in air at 473 K. The catalyst was then calcined in flowing air (BOC gases) by heating to 673 K at 0.5 K min -1 and then remaining at 673 K for 4 h. A Pd/Al 2 O 3 sample was prepared similarly using palladium acetylacetonate (Aldrich, 99%) and then drying and calcining as described above. To prepare a ceria-promoted sample (Pd/ CeO x /Al 2 O 3 ), palladium was deposited on the CeO x /Al 2 O 3 catalyst using the above procedure. Subsequent reduction of the Pd catalysts occurred at 673 K for 2 h in flowing dihydrogen (99.999% from BOC gases, passed through a Matheson model 8371v purifier). * Author to whom correspondence should be addressed at Department of Chemical Engineering, Chemical Engineering Bldg., Room 117A, University of Virginia, 102 Engineers Way, Charlottesville, VA 22904- 4741. Phone: (804) 924-6284. Fax: (804) 982-2658. E-mail: rjd4f@ virginia.edu. 9653 J. Phys. Chem. B 2000, 104, 9653-9660 10.1021/jp001347r CCC: $19.00 © 2000 American Chemical Society Published on Web 09/22/2000