Isotopic and kinetic assessment of the mechanism of reactions of CH 4 with CO 2 or H 2O to form synthesis gas and carbon on nickel catalysts

Abstract

Kinetic and isotopic measurements for catalysts and conditions that rigorously excluded transport and thermodynamic artifacts led to a common sequence of elementary steps for reactions of CH 4 with CO 2 or H 2O and for its stoichiometric decomposition on Ni/MgO catalysts. Turnover rates for forward reactions of CH 4/CO 2 and CH 4/H 2O mixtures were proportional to CH 4 pressure (5–450 kPa) and independent of the partial pressure of the CO 2 or H 2O coreactants (5–450 kPa). These turnover rates and their first-order rate constants and activation energies are also similar to those measured for CH 4 decomposition, indicating that these reactions are mechanistically equivalent and that CH bond activation is the sole kinetically relevant step in all three reactions. These conclusions were confirmed by identical CH 4/CD 4 kinetic isotope effects ( k H/ k D=1.62–1.71) for reforming and decomposition reactions and by undetectable H 2O/D 2O isotopic effects. The kinetic relevance of CH bond activation is consistent with the relative rates of chemical conversion and isotopic mixing in a CH 4/CD 4/CO 2 mixture and with the isotopic evidence for the quasi-equilibrated nature of coreactant activation and H 2 and H 2O desorption obtained from reactions of CH 4/CO 2/D 2 and 12CH 4/ 12CO 2/ 13CO mixtures. These quasi-equilibrated steps lead to equilibrated water–gas-shift reactions during CH 4 reforming, a finding confirmed by measurements of the effluent composition. These elementary steps provide also a predictive model for carbon filament growth and identify a rigorous dependence of the carbon thermodynamic activity on various kinetic and thermodynamic properties of elementary steps and on the prevalent concentrations of reactants and products, specifically given by P CH 4 P CO/ P CO 2 (or P CH 4 P H 2 / P H 2O ) ratios. These mechanistic features on Ni surfaces resemble those previously established for supported noble metal catalysts (Rh, Pt, Ir, Ru). These direct measurements of CH bond activation turnover rates allowed the first direct and rigorous comparison of the reactivity of Ni and noble metal catalysts for CH 4-reforming reactions, under conditions of strict kinetic control and relevant commercial practice and over a wide range of compositions and metal dispersions.