Comparison of mechanistic understanding and experiments for CO methanation over nickel

A reaction mechanism based on Density Functional Theory (DFT) calculated energies was built using the Ni(211) step as representative for the active sites in Ni-based methanation catalysts and an analytical kinetic model based on two rate determining steps was used to evaluate a comprehensive experimental dataset over a wide range of conditions. Micro-kinetic analysis by means of the degree of rate control showed that CO splitting through a COH intermediate is the rate determining step of the catalytic reaction at high temperatures where the CO coverage is low, while CO splitting competes with CH hydrogenation at lower temperatures and high CO coverage. The inhibiting effect of carbon monoxide at high CO pressures could be attributed to carbon buildup at the B5-site at Ni(211). The model predicts the experimental shift in the hydrogen reaction order from alpha(H2) approximate to 1/2, at low CO pressure to alpha(H2) approximate to 1 at high CO pressures. This behavior is explained by removal of CO induced carbon from the B5-site as the hydrogen pressure is raised. The effects of temperature and total pressure are also well-described by the new two-step model. (C) 2016 Elsevier Inc. All rights reserved.