Low-Temperature Methane Combustion over Pd/H-ZSM-5: Active Pd Sites with Specific Electronic Properties Modulated by Acidic Sites of H-ZSM-5

Abstract

Pd/H-ZSM-5 catalysts could completely catalyze CH4 to CO2 at as low as 320 °C, while there is no detectable catalytic activity for pure H-ZSM-5 at 320 °C and only a conversion of 40% could be obtained at 500 °C over pure H-ZSM-5. Both the theoretical and experimental results prove that surface acidic sites could facilitate the formation of active metal species as the anchoring sites, which could further modify the electronic and coordination structure of metal species. PdOx interacting with the surface Brönsted acid sites of H-ZSM-5 could exhibit Lewis acidity and lower oxidation states, as proven by the XPS, XPS valence band, CO-DRIFTS, pyridine FT-IR, and NH3-TPD data. Density functional theory calculations suggest PdOx groups to be the active sites for methane combustion, in the form of [AlO2]Pd(OH)-ZSM-5. The stronger Lewis acidity of coordinatively unsaturated Pd and the stronger basicity of oxygen from anchored PdOx species are two key characteristics of the active sites ([AlO2]Pd(OH)-ZSM-5) for methane combustion. As a result, the PdOx species anchored by Brønsted acid sites of H-ZSM-5 exhibit high performance for catalytic combustion of CH4 over Pd/H-ZSM-5 catalysts.