Modulation excitation pair distribution function experiments in catalysis to increase phase sensitivity

Abstract

Heterogeneous catalysts are often undergoing subtle structural changes in both the nanoparticles and the support phase under operating conditions. Catalytic cycling in in situ experiments between catalysis and dropout conditions allows for exploiting the power of modulation excitation (ME) experiments. While phase-sensitive detection (PSD) of ME experiments has been showcased for infrared and X-ray absorption spectroscopy in the past, here we present its first application to pair distribution function (PDF) analysis. We have performed in situ X-ray total scattering studies on two alumina-supported nickel (Ni@alumina) catalysts under methanation and hydrogen dropout conditions and showcased how PSD analysis of the modulation excitation pair distribution function (ME-PDF) data improves the signal-to-noise ratio in the phase-resolved PDF data, enabling us to detect otherwise hidden structural changes. We identify a metal–support interaction of the Ni nanoparticles with the γ-Al₂O₃ support when choosing the deposition–precipitation method for catalyst preparation. By way of contrast, an industrial catalyst with comparable catalytic performance and nanoparticle diameter showed dynamic surface oxidation of Ni nanoparticles during unstable H₂ supply.