Reaction Pathway for Coke-Free Methane Steam Reforming on a Ni/CeO2 Catalyst: Active Sites and Role of Metal-Support Interactions

Abstract

Methane steam reforming (MSR) plays a key role in the production of\nsyngas and hydrogen from natural gas. The increasing interest in the use of\nhydrogen for fuel cell applications demands the development of catalysts with\nhigh activity at reduced operating temperatures. Ni-based catalysts are\npromising systems because of their high activity and low cost, but coke\nformation generally poses a severe problem. Studies of ambient-pressure X-ray\nphotoelectron spectroscopy (AP-XPS) indicate that CH4/H2O\ngas mixtures react with Ni/CeO2(111) surfaces to form OH, CHx\nand CHxO at 300 K. All these species are easy to form and\ndesorb at temperatures below 700 K when the rate of the MSR process\naccelerates. Density functional theory (DFT) modeling of the reaction over\nceria-supported small Ni nanoparticles predicts relatively low activation\nbarriers between 0.3–0.7 eV for the complete dehydrogenation of methane to\ncarbon and the barrierless activation of water at interfacial Ni sites. Hydroxyls\nresulting from water activation allow CO formation via a COH intermediate with\na barrier of about 0.9 eV, which is much lower than that through a pathway\ninvolving lattice oxygen from ceria. Neither methane nor water activation are\nrate-determining steps, and the OH-assisted CO formation through the COH\nintermediate constitutes a low-barrier pathway that prevents carbon accumulation.\nThe interaction between Ni and the ceria support and the low metal loading are\ncrucial for the reaction to proceed in a coke-free and efficient way. These\nresults could pave the way for further advances in the design of stable and highly\nactive Ni-based catalysts for hydrogen production.