Methanation of CO over Nickel: Mechanism and Kinetics at High H2/CO Ratios

The CO methanation reaction over nickel was studied at low CO concentrations and at hydrogen pressures slightly above ambient pressure. The kinetics of this reaction is well described by a first-order expression with CO dissociation at the nickel surface as the rate-determining step. At very low CO concentrations, adsorption of CO molecules and H atoms compete for the sites at the surface, whereas the coverage of CO is close to unity at higher CO pressures. The ratio of the equilibrium constants for CO and H atom adsorption, KCO/KH, was obtained from the rate of CO methanation at various CO concentrations. KH was determined independently from temperature programmed adsorption/desorption of hydrogen to be KH = 7.7 × 10-4 (bar-0.5) exp[43 (kJ/mol)/RT] and hence the equilibrium constants for adsorption of CO molecules may be calculated to be KCO = 3 × 10-7 (bar-1) exp[122 (kJ/mol)/RT]. Furthermore, the rate of dissociation of CO at the catalyst surface was determined to be 5 × 109 (s-1) exp[−96.7 (kJ/mol)/RT] assuming that 5% of the surface nickel atoms are active for CO dissociation. The results are compared to equilibrium and rate constants reported in the literature.