Experimental and theoretical investigation of oxidative methane activation on Pd-Pt catalysts.

ABSTRACT

Density functional theory (DFT) and measurements of rate are used to provide evidence for the rate determining step (RDS) and requirements of the active site for CH4 combustion on Pd-Pt bimetallic catalysts in five different distinct kinetic regimes. These five regimes exhibit different rate equations for methane combustion due to the reaction rate constants and diverse dominant adsorbed species for these different kinetically relevant steps. Oxygen chemical potential at the Pd-Pt surface was replaced by oxygen pressure, reflecting the kinetic coupling between C-H and O[double bond, length as m-dash]O bond cleavage steps. C-H bond cleavage occurs on different active sites in five of these kinetic regimes, evolving from vacancy-vacancy (*-*) to oxygen-vacancy (O*-*), oxygen-oxygen (O*-O*) site pairs, monolayer Pd-O, and ultimately to oxide bulk with Pd-O site pairs as the oxygen chemical potential increases. It is easier to form a metallic surface at low oxygen pressure, implying minimal O* coverage. The sole kinetically relevant step on uncovered Pd-Pt surfaces for methane combustion is O[double bond, length as m-dash]O bond cleavage. The supply of oxygen is obviously more important than the supply of methane in regime (I). As vacancies become less available on metallic surfaces, C-H bond cleavage occurs via O*-* paired sites, the energy barrier of which is much higher than that on uncovered Pd-Pt surfaces. In this regime (II), O[double bond, length as m-dash]O bond cleavage is still an irreversible process because O* will be consumed by the rapidly formed products of methane dissociation. For the oxygen saturated surfaces in regime (III), C-H bond cleavage occurs on two adjacent adsorbed oxygens that form OH and weak CH3-O bond interactions, resulting in a low activity for methane combustion. On the oxidation surfaces (IV and V), exposed metal atoms and their adjacent exposed lattice oxygen were the active sites, leading to a large decrease in C-H bond cleavage energy barrier, deduced from both experiment and theory. The increase of the metallic oxide thickness (increase of oxygen potential) increases the methane combustion turnover rates on Pd-Pt catalysts.