Nature and identity of the active site via structure-dependent microkinetic modeling: An application to WGS and reverse WGS reactions on Rh
Raffaele Cheula, Matteo Maestri
Abstract
We propose a methodology to perform a structure-dependent microkinetic analysis of a catalytic process. The methodology makes it possible to unveil the nature and identity of the active site in a self-consistent manner. The morphology of heterogeneous catalyst nanoparticles as a function of the gas chemical potential is determined using ab initio thermodynamics and Wulff-Kaishew construction methods. The reaction rates are calculated by integrating a microkinetic model which describes the catalytic activity of the crystal facets exposed by the catalyst under reaction conditions. The method is applied for the analysis of the direct and reverse water-gas shift (WGS) reacting systems on a 4% Rh/α-Al2O3 kinetic experiments from the literature. Our findings make it possible to rationalize that far from equilibrium the two different reacting systems not only follow different reaction pathways in agreement with the experimental evidence but also show that the dominant active sites are different for WGS and reverse WGS. Indeed, the WGS reaction occurs mainly on the Rh(111) facet, whereas reverse WGS proceeds on the active sites of Rh(100). As a whole, this methodology makes it possible a concomitant description of the nature of the catalyst material in reaction conditions and of its catalytic consequences in terms of reactivity. As such, it paves the way towards the use of first-principles methods for the interpretation of the experimental evidence in terms of structure-activity relationships.