Evaluation of the Influence of Metal-Support Interactions On the Catalytic Activity of Pd-Ceria Using Density Functional Theory and Ab Initio Thermodynamics

Monday, November 8, 2010
Hall 1 (Salt Palace Convention Center)
Adam D. Mayernick, Department of Chemical Engineering, Pennsylvania State University, University Park, PA and Michael Janik, Department of Chemical Engineering, Pennsylvania State University-University Park, University Park, PA

Ceria (CeO2) offers unique properties as a heterogeneous catalyst or catalyst support due to its ability to store and release oxygen, or more generally to readily transition between oxidation states. For Pd/CeO2 systems, strong interactions between Pd and ceria influence the stable surface structure and catalytic activity. This work utilizes ab initio thermodynamics using density functional theory (DFT+U) methods to evaluate the stability of Pd atoms, PdOx species, and small Pd particles in varying configurations on CeO2 (111), (110), and (100) single crystal surfaces. The ceria support shifts the transition between formal Pd oxidation states (Pd0, Pd2+, Pd4+) relative to bulk palladium, and stabilizes certain oxidized palladium species on each surface. Over specific oxygen partial pressure and temperature ranges, palladium incorporation to form a mixed surface oxide is thermodynamically favorable versus other single Pd atom states, on each ceria surface. For example, Pd atoms may incorporate into Ce fluorite lattice positions in a Pd4+ oxidation state on the CeO2 (111) surface. Cationic palladium species provide unique catalytic activity for methane oxidation over Pd-ceria versus metallic Pd, however in situ characterization of active sites and identification of stable reaction intermediates during catalytic operation remain challenging. We evaluate the thermodynamics and kinetics of methane oxidation over pure CeO2(111), single supported Pd atoms on CeO2(111), single incorporated atoms in CeO2(111), the extended Pd(111) surface, and PdO(100) to identify rate limiting steps and stable intermediates on possible surface phases present in Pd-supported ceria. Methane activation is more exothermic over the mixed Pd-ceria surface than over pure ceria, Pd metal, PdO, or supported Pd atoms, and the apparent barrier for methane oxidation is also lowest over the mixed Pd-ceria surface. Our results show that the catalytic activity of ceria-based metal oxides for methane oxidation is a function of surface reducibility, and that the rate of oxidation is limited by C-H activation. These results aid in both interpreting experimental behavior and guiding design of improved ceria-based catalysts for hydrocarbon oxidation.

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