Steam and dry reforming of methane on Rh: Microkinetic analysis and hierarchy of kinetic models

CH4 steam reforming (SR) and dry reforming (DR) on Rh have been analyzed using a comprehensive, thermodynamically consistent microkinetic model. Our analysis pointed out mechanistic analogies between the two processes. In particular, regardless of the co-reactant, methane consumption proceeds via pyrolysis and carbon oxidation by OH* (CH4 -> C* -> CO*), and the role of the co-reactant (either CO2 or H2O) is to provide the main oxidizer, OH*. Moreover, in line with isotopic kinetic experiments reported in the literature, methane activation is predicted to be the rate-determining step, and all of the steps involving co-reactant turn out to be quasi-equilibrated. It also was found that under typical experimental conditions, SR and DR always occur with water-gas shift (WGS) reaction close to equilibrium. Adopting a systematic reduction methodology, we propose a hierarchy of models for SR and DR. In particular, first a reduced microkinetic model and then overall rate equations for the SR, DR, and WGS reactions are derived from the microkinetic models. Overall, our kinetic analysis is able to predict correctly the most important features found in experiments, namely that the overall reaction rate exhibits a first-order dependence on CH4 concentration and is independent of the co-reactant (H2O or CO2). Product inhibition, which becomes important at lower temperatures, also is predicted. (C) 2008 Elsevier Inc. All rights reserved.