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

Abstract

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, K_CO/K_H, was obtained from the rate of CO methanation at various CO concentrations. K_H was determined independently from temperature programmed adsorption/desorption of hydrogen to be K_H = 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 K_CO = 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.