The mechanism and kinetics of methane formation by decomposition of methanol on a Ni SiO₂ catalyst

The rate of methane formation by decomposition of methanol is studied at temperatures between 473 and 583 K and methanol pressures between 0.2 and 10.4 kPa. Coverages of adsorbed species are measured using temperature-programmed hydrogenation (TPH), temperature-programmed desorption (TPD), and magnetic measurements. The reaction system can be adequately described assuming the decomposition of methanol to proceed in two steps, viz. a decomposition into CO and H₂ followed by methanation of CO. The former reaction is much more rapid than the latter. Assuming CH hydrogenation to be rate-determining, the rate of methane formation could be described by the LHHW rate equation TOF = 1.2 × 10⁵e- ^{-68 RT}p^1.5 (1 + 1.8 × 10^-1e ^{15 RT}p^0.5 + 3.1 × 10^-16 e ^{144 RT}p)² TOF is the turnover frequency in s^-1, p is the methanol pressure in kPa, T is the temperature in K, and R = 8.314 × 10^-3kJ mol^-1 K^-1. Coverages of adsorbed species predicted by this equation differ strongly from results obtained by measurements of total surface coverages using TPH, TPD, and magnetic measurements. This has been interpreted as an indication that the number of surface sites actively participating in the methanation reaction composes only a small fraction of the total number of sites.