CO methanation on low-weight loading Ni/Al/sub 2/O/sub 3/: multiple reaction sites

Temperature-programmed reaction (TPR) with mass spectrometric detection was used to study the hydrogenation of adsorbed carbon monoxide on low-weight loading Ni/Al/sub 2/O/sub 3/ catalysts. These catalysts differ from high-weight loading nickel/alumina catalysts and two distinct pathways for carbon monoxide methanation were demonstrated. These pathways are attributed to two forms of adsorbed CO. The CO site which reacts at low temperature (446 K) may be attributed to carbon monoxide adsorbed on nickel atoms that are bonded to other nickel atoms. Activation energy for methanation is 51 kJ/mol. The CO site that reacts at high temperature (546 K) 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 steady-state kinetic measurements is demonstrated. 30 references, 3 figures, 8 tables.