Time-Resolved XAS Investigation of the Local Environment and Evolution of Oxidation States of a Fischer–Tropsch Ru–Cs/C Catalyst

Abstract

Here we present a time-resolved in situ X-ray absorption spectroscopy investigation of the local chemical environment and the electronic structure of the active metal (Ru) and the promoter (Cs) present in catalysts supported over high-surface-area graphite. These catalysts were submitted to a reduction treatment under hydrogen gas and then to alternate atmospheres of Fischer–Tropsch reactants, CO/He, and H 2 /He atmospheres, respectively. This study reveals that Ru reduction is a complex process taking place through a two-step mechanism via an intermediate oxidation state and occurs in parallel to the Cs partial reduction. It also demonstrates a close association between Cs and Ru atoms as first neighbors. The evidence that the Ru–Cs particle morphology reverses (from flat 2D to a 3D shape with low coordination numbers) when the H 2 atmosphere is changed with CO proves the strong interaction of CO with the surface of the Ru–Cs nanoparticles, a point confirmed by microcalorimetry of CO adsorption. To our knowledge, this is the first time that this reversible particle reconstruction upon syngas reactant switching has been demonstrated.