(305e) Hydrocarbon Electro-Oxidation On Pd-Ceria: A DFT Investigation of the Surface Structure and Catalytic Activity
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. Palladium supported on ceria is an effective catalytic material for three-way automotive catalysis, catalytic combustion, and solid-oxide fuel cell (SOFC) anodes. The morphology, oxidation state, and particle size of Pd on ceria affects catalytic activity, and are a function of experimental conditions. 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. Over specific oxygen partial pressure, cell potential, 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. 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. We show that temperature, oxygen pressure, and cell potential in an SOFC can influence the stable states of palladium supported on ceria surfaces, providing insight into structural stability during SOFC operation.
Palladium incorporation into the ceria surface lowers the vacancy formation energy, thus increasing the surface reducibility. Methane activation is more exothermic over the mixed Pd-ceria surface than over pure ceria, and methane activation energies correlate with surface reduction (oxygen vacancy) energies. We evaluate the thermodynamics and kinetics of methane oxidation over pure ceria, Pd-doped ceria, pure Pd metal, and PdO to identify rate limiting steps and stable intermediates on possible phases present in Pd-supported ceria. Our results show that the catalytic activity of ceria-based metal oxides for hydrocarbon oxidation is a function of surface reducibility, and that the rate of oxidation over Pd-doped ceria may be limited by the replenishment of surface oxygen. These results aid in both interpreting experimental behavior and guiding design of improved ceria-based anode electrocatalysts for direct utilization of hydrocarbons in SOFCs.