Kinetics of methane decomposition over Zr-Ni alloys

The decomposition of methane over Zr, Zr4Ni and Zr2Ni was investigated to develop highly active materials for capturing tritiated methane inevitably formed in tritium handling systems. The entire decomposition or absorption curves, however, could not been described by any simple kinetic equations appearing in the literature. The present paper describes verification of plausible kinetic equations reproducing the observed absorption curves, assuming a reaction mechanism consisting of a progressive removal of hydrogen atoms according to CH4(g)→CH3(α)→CH2(α)→CH(α)→C(α) and its modification including carbon segregation schemes by solving a set of kinetic equations by means of finite difference method, by taking into account of ab-initio calculations of potential energy surfaces by Gaussian 03. It was found that a step-by-step H-deletion model could not generate the experimentally observed absorption curves, but they could be reproduced quite well in the whole reaction range by a modified reaction scheme assuming coagulation of a carbon residue like CH2(α) or CH(α) to carbonaceous deposits, described as CH4(g)(k1 decay)CH3(α)(k2 decay)CH(α)(k3 decay)C-deposits. It was concluded that the final third step (with a rate constant of k3) governs the overall absorption reactivity (methane consumption beyond 90%) of the materials. (author)