Carbon and carbon monoxide hydrogenation on nickel: Support effects

Abstract

Hydrogenation of carbon, deposited on nickel catalysts by CO disproportionation, was investigated by temperature-programmed surface reaction (TPSR) for four oxide supports, Al 2O 3, SiO 2, TiO 2, and SiO 2 · Al 2O 3. The rate of carbon monoxide hydrogenation was measured by temperature-programmed reaction (TPR) for comparison. The rate of carbon hydrogenation to methane was found to be independent of the support and an average activation energy of 42 kJ/mol was estimated. In contrast, the rate of carbon monoxide hydrogenation was very sensitive to the catalyst support. Nickel supported on TiO 2 exhibited the highest specific activity, and two distinct sites for methanation were observed on Ni TiO 2 and Ni Al 2O 3 . The lowest specific activities were observed for Ni SiO 2 and Ni SiO 2 · Al 2O 3 . For all catalysts, carbon hydrogenation occurred at a lower temperature than carbon monoxide hydrogenation. For both TPR and TPSR, small amounts of ethane were formed and at a lower temperature than methane. The amount of less-active, β-carbon observed in TPSR experiments was very small on all catalysts. These results indicate that at high coverages, carbon hydrogenation does not depend on the support, and thus it is not rate-determining for CO hydrogenation in excess hydrogen. The support is also shown to change the specific rate of carbon monoxide methanation; activity differences seen in steady-state experiments are not just due to differences in site densities.