First principles calculations and experimental insight into methane steam reforming over transition metal catalysts

This paper presents a detailed analysis of the steam reforming process from first-principles calculations, supported by insight from experimental investigations. In the present work we employ recently recognised scaling relationships for adsorption energies of simple molecules adsorbed at pure metal surfaces to develop an overview of the steam reforming process catalyzed by a range of transition metal surfaces. By combining scaling relationships with thermodynamic and kinetic analysis, we show that it is possible to determine the reactivity trends of the pure metals for methane steam reforming. The reaction is found to be kinetically controlled by a methane dissociation step and a CO formation step, where the latter step is found to be dominant at lower temperatures. The particle size of the metal catalysts particles have been determined by transmission electron microscopy (TEM) and the turn over frequency observed to be linearly dependent on the dispersion, supporting the theoretical notion that the active sites are most likely present as one dimensional edges. It has been found that determination of the correct particle size distribution of small (2-4 nm) Ru particles requires in situ TEM measurements under a hydrogen atmosphere. The overall agreement between theory and experiment (at 773 K, 1 bar pressure and 10% conversion) is found to be excellent with Ru and Rh being the most active pure transition metals for methane steam reforming, while Ni, Ir, Pt, and Pd are significantly less active at similar dispersion.