Synergism of cobalt and palladium in MFI zeolite of relevance to NO reduction with methane

Abstract

A series of MFI supported bimetallic catalysts with high Co/Pd ratios and high metal loading was prepared by first subliming CoBr₂ vapor onto H/MFI, followed by ion exchange of some Pd from an aqueous solution of Pd(NH₃)₄(NO₃)₂. They were tested for NO reduction with CH₄ in the presence of 2% O₂. Best performance was observed with Co/Pd = 14/1. In this catalyst Pd⁰ sites are assumed to activate methane. At higher Pd loading the catalytic selectivity is lower because large PdO particles are formed, which catalyze methane combustion. The reducibility of Co is significantly enhanced by small amounts of Pd. At 55 °C, the Pd and a comparable amount of Co are co-reduced. The particles consist of two phases, including a phase of almost pure Pd capable of forming Pd hydride. Metal reduction is complete at 550 °C. Many reduced particles are biphasic, with a Pd-rich kernel and Co-rich mantle. EXAFS reveals Pd–Co bonds at the interface. A substantial reorganization of the metal particles takes place at 800 °C, Pd-rich large alloy particles coexist with small particles of Co and Co-rich alloy. The alloy particles exhibit high and stable activity for NO reduction to N₂ with CH₄ at 300 °C, but lose this activity at high reaction temperature.