(661d) Sintering and Deactivation Mechanism of Platinum/Palladium Two-Phase Catalysts

Despite efforts to create novel catalysts with inexpensive materials, precious metal catalysts remain widespread in many applications due to their high activity. Maintaining the long-term activity of these catalysts by preventing degradation is therefore of substantial economic importance. One model system for studying deactivation is a well-controlled platinum/palladium catalyst for methane oxidation, in which platinum represents a necessary catalytic component but its sintering causes deactivation of the palladium catalyst toward fuel oxidation.^1-2 We seek to understand the mechanism by which this deactivation occurs, toward the goal of designing new catalysts with improved resistance to sintering.

In this presentation, we use density functional theory calculations to elucidate the mechanism by which an oxidized palladium catalyst (PdO) on an Al₂O support deactivates toward methane combustion in the presence of platinum. We show that gas-phase PtO₂ acts as the primary carrier of Pt to PdO, on which it nucleates in surface oxygen vacancies generated by the methane combustion cycle. The migration is thermodynamically driven, and leads to the formation of large Pt-Pd bimetallic particles with uniform particle size and severely reduced methane oxidation activity. Experimental evidence allows differentiation of this O₂-assisted gas-phase migration from whole-particle surface migration, as catalyst deactivation is severely reduced in an argon atmosphere (which cannot form PtO₂, in the absence of O₂). This work demonstrates the power of integrating theory and experiments on well-defined materials to understand and elucidate interaction mechanisms in complex catalysts composed of multiple phases.

¹C. F. Cullis, B. M. Willatt, J Catal 83, 267 (1983)
²R. Strobel, J.-D. Grunwaldt, A. Camenzind, S. E. Pratsinis, A. Baiker, Catal Lett 104, 9 (2005)