Hydrogenation of CO 2 on group VIII metals : II. Kinetics and mechanism of CO 2 hydrogenation on nickel

Abstract

The rate of CO 2 hydrogenation on Ni/SiO 2 was measured as a function of H 2 and CO 2 partial pressures at 500–600 K, 140 kPa, and 30,000–90,000 h −1. The data show that the rate of CO 2 hydrogenation is moderately dependent on CO 2 and H 2 concentrations at low partial pressures but essentially concentration independent at high partial pressures. Under most typical reaction conditions CO is observed as a product of the reaction at levels determined by quasi-equilibrium between surface and gas phase CO species. Addition of CO to the reactants above this equilibrium level causes a significant decrease in the rate of CO 2 hydrogenation apparently as a result of product inhibition. Reaction orders and the true activation energy are quite temperature dependent indicating that a simple power law rate expression provides an inadequate fit of the data. Indeed, the kinetic results are consistent with a complex Langmuir-Hinshelwood mechanism involving dissociative adsorption of CO 2 to CO and atomic oxygen followed by hydrogenation of CO via a carbon intermediate to methane.