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

Abstract

The nature and the reactivity of an intermediate hydrocarbon species, CH_{x, 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 H₂. 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 H₂ produced that two steps were responsible for H₂ production: the dissociative adsorption of CH₄ to produce (4 –x)/2 H₂ and CH_{x, ads} and the simultaneous production of x/2 H₂ together with CO from the CH_{x, ads} species. Separation of the amount of H₂ 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 CH_{x, ads} with CO₂(or O_ads) is the rate-determining step in CO₂-reforming.