CO methanation on low-weight loading [formula omitted]: Multiple reaction sites

Abstract

Temperature-programmed reaction (TPR) with mass spectrometric detection was used to study the hydrogenation of adsorbed carbon monoxide on low-weight loading Ni Al 2O 3 catalysts. These catalysts are shown to differ from high-weight loading nickel/alumina catalysts and are shown to have two distinct pathways for carbon monoxide methanation. These pathways are attributed to two forms of adsorbed CO. The CO site which reacts at low temperature (446 K) appears to be from carbon monoxide adsorbed on nickel atoms that are bonded to other nickel atoms. It has an activation energy for methanation of 51 kJ/mol. The CO site that reacts at high temperature (546 K) has an activation energy of 145 kJ/mol. The high-temperature site, which is sensitive to the pretreatment temperature, results from CO adsorbed on nickel atoms that are interacting strongly with an oxide phase of the catalyst. Interconversion of CO between the two sites occurs and is influenced by hydrogen. Neither of the pathways appears limited by direct hydrogenation of surface carbon; this step is shown to be slightly faster under TPR conditions. The ability of TPR to measure specific reaction rates and to separate reaction pathways that might be obscured in steadystate kinetic measurements is demonstrated.