Steam and Dry Reforming and Partial Oxidation of Methane on Rh. A Comprehensive Kinetics Study

CH4 steam reforming (SR) and dry reforming (DR) as well as partial oxidation (POX) on Rh have been analysed using a comprehensive, thermodynamically consistent microkinetic model. Our analysis pointed out that, regardless of the co-reactant, methane consumption proceeds via pyrolysis and carbon oxidation by OH* (CH4=C*= CO*) and that 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 the steps involving co-reactant turn out to be quasi-equilibrated. It was also found that 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 expressions for the SR, DR, and WGS reactions are derived. Overall, our kinetic analysis is able to predict correctly the most important features found in experiments, namely that the overall reaction rate exhibits a 1st-order dependence on CH4 concentration but is independent of the co-reactant (H2O or CO2). Product inhibition, which becomes important at lower temperatures, is also predicted. Related analysis of the POX process reveals the importance of transport phenomena in dictating whether a direct or indirect path to syngas dominates.