Methane formation from methanol on Ni/SiO₂, Ni/Al₂O₃ and Ni/TiO₂ catalysts

Abstract

The kinetics of the formation of methane from methanol have been studied on a 50 wt% Ni/SiO₂, a 20 wt% Ni/SiO₂, a 10 wt% Ni/Al₂O₃ and a 5 wt% Ni/TiO₂ catalyst. Temperatures were typically between 475 and 585 K with methanol partial pressures between 0.2 and 10.4 kPa. The results were compared with data obtained using a CO/H₂ = 0.5-mixtures as the reactant gas. The apparent activation energy for methane formation was independent of the support. Specific rates (TOF's) increased in the order 20 wt%Ni/SiO₂, 50 wt% Ni/SiO₂, 10 wt% Ni/Al₂O₃, 5 wt% Ni/TiO₂. Both deactivation rates and selectivity towards higher hydrocarbons increased in the same order. Contrary to the results obtained on the other catalysts, methane formation rates are much larger on Ni/TiO₂ when CO/H₂ was used as the reactant gas instead of CH₃OH. The observations were explained assuming a mechanism in which methapnol first decomposes into CO and H₂ followed by the hydrogenation of CO. In case of silica and alumina supported catalysts the first reaction is much faster than the latter and the rate of methane formation from methanol equals the rate of methane formation from CO/H₂. Due to the presence of chemically altered sites on TiO₂-supported nickel the rate of the first reaction (a dehydrogenation) is decreased, whereas the rate of CO-hydrogenation is known to increase strongly. This causes the rate of methane formation from methanol to be lower than the rate of methane formation from CO/H₂. The higher selectivity towards higher hydrocarbons as well as the higher deactivation rates are also explained assuming the presence of chemically altered sites.