Isotopic and kinetic assessment of the mechanism of reactions of CH4 with CO2 or H2O to form synthesis gas and carbon on nickel catalysts

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 CH4 with CO2 or H2O and for its stoichiometric decomposition on Ni/MgO catalysts. Turnover rates for forward reactions of CH4/CO2 and CH4/H2O mixtures were proportional to CH4 pressure (5-450 kPa) and independent of the partial pressure of the CO2 Or H2O coreactants (5-450 kPa). These turnover rates and their first-order rate constants and activation energies are also similar to those measured for CH4 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 CH4/CD4 kinetic isotope effects (k(H)/k(D) = 1.62-1.71) for reforming and decomposition reactions and by undetectable H2O/D2O isotopic effects. The kinetic relevance of C-H bond activation is consistent with the relative rates of chemical conversion and isotopic mixing in a CH4/CD4/CO2 mixture and with the isotopic evidence for the quasi-equilibrated nature of coreactant activation and H-2 and H2O desorption obtained from reactions of CH4/CO2/D-2 and (CH4)-C-12/(CO2)-C-12/(CO)-C-13 mixtures. These quasi-equilibrated steps lead to equilibrated water-gas-shift reactions during CH4 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-CH4 P-CO/P-CO2 (or P-CH4 P-H2/P-H2O) 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 CH4-reforming reactions, under conditions of strict kinetic control and relevant commercial practice and over a wide range of compositions and metal dispersions. (C) 2004 Elsevier Inc. All rights reserved.