Kinetics of methane combustion on model supported Pd/LaxMnO3 natural gas vehicle catalysts: Sensitivity of La-stoichiometry on the catalytic properties of Pd

Abstract

The kinetics of the catalytic CH4/O2 reaction has been studied on 1 wt% Pd/LaxMnO3 with x = 0.7, 1.0 and 1.3. Steady-state kinetic measurements have been performed in lean conditions at 400 °C on pre-reduced and on aged catalysts after exposure to wet atmosphere at 750 °C. A single site reaction mechanism occurs preferentially on Pd/La1.3MnO3 exhibiting a La-rich surface. In contrast a dual-site reaction mechanism, combining Pd and surface reactive oxygen species from the support, is preferentially involved on Pd-La0.7MnO3 characterized by a Mn-rich surface. Pd/LaMnO3 exhibits a mixed regime but subsequent deterioration of the Pd-LaxMnO3 interface during thermal aging leads to increasing contribution of the single site reaction mechanism. High surface Pd/Mn ratio and high concentration of Pdn+ with n > 2 can be considered as good descriptors to probe the efficacy of Pd/LaxMnO3 catalysts. Remarkably, the high thermal stability of the Pd-La0.7MnO3 interface, preserving the Pd dispersion on Pd/La0.7MnO3, leads to lower sensitivity to deactivation.