Kinetics, intermediates and mechanism for the CO2-reforming of methane on supported nickel catalysts

The nature and the reactivity of an intermediate hydrocarbon species, CHx, ads, have been determined for the CO2-reforming of CH4 over supported Ni catalysts by using pulse surface reaction rate analysis (PSRA). When a small amount of CH4 pulse was injected into flowing gas mixture of CO2 and He, it was immediately adsorbed on the catalyst and then gradually reacted with CO2 to CO and H-2. Analysis of the dynamic behaviour of the produced CO enabled us to determine the first-order rate constant for the reaction between adsorbed CH4 and CO2. It was found from the dynamics of the H-2 produced that two steps were responsible for H-2 production: the dissociative adsorption of CH4 to produce (4 - x)/2 H-2 and CHx, ads and the simultaneous production of x/2 H-2 together with CO from the CHx, ads species. Separation of the amount of H-2 between these two steps led us to determine the average number of hydrogen atoms involved in the hydrogen-deficient adsorbed hydrocarbon species, i.e. 2.7 for Ni/MgO, 2.4 for Ni/Al2O3, 1.9 for Ni/TiO2 and 1.0 for Ni/SiO2. The reactivity of the intermediate species was not correlated with the number of hydrogen atoms involved. The highest activity on Ni/TiO2 and almost the same activity on Ni/MgO, Ni/Al2O3 and Ni/SiO2 were observed both for the pulse reaction and for the steady-state reaction, suggesting that the reaction of CHx, ads with CO2 (or O-ads) is the rate-determining step in CO2-reforming.