X-ray absorption fine structure studies on nickel phosphide catalysts for the non-oxidative coupling of methane reaction using a theoretical model

Abstract

SiO2-supported Ni phosphide catalysts are highly active toward the non-oxidative coupling of methane (NOCM) reaction, and their catalytic activity is strongly dependent on their Ni:P ratio. We analyzed Ni phosphide catalysts using X-ray absorption fine structure (XAFS) to elucidate the structure-catalytic activity relationship. Because only Ni2P was available as a reference material, we calculated the theoretical XAFS based on the reference crystal structures using the FEFF program and compared them with the experimental spectra of Ni phosphide catalysts. We demonstrated that catalysts with Ni to P ratios of 1:1, 2:1, and 3:1 consisted mainly of the Ni2P, Ni12P5, and Ni3P, respectively. We found that Ni2P exhibited the highest activity toward NOCM because of its optimum balance of C–H cleavage activity and coke formation. This analysis demonstrated that the theoretical XAFS simulation could be used to identify the structure of supported catalysts based on the crystal structure of reference compounds.