H Tunneling Effects on Sequential Dissociation of Methane over Ni(111) and the Overall Rate of Methane Reforming

Abstract

The rate constants and the activation energies for the C–H bond dissociation of CH3, CH2, and CH species adsorbed on the surface of Ni(111) are calculated in the framework of a previously developed methodology. It considers the dissociations as a result of quantum transitions between discrete vibrational energy levels of an initial C–H bond in methane and energy continuum of the dissociated final state while the system reaches the transition state along other classical degrees of freedom. These constants along with other ones calculated in our previous papers are used for calculating the overall rate using a microkinetic model of methane steam reforming (MSR) incorporating methane and water dissociation and CO formation either by C + O or by CH + O → CHO → CO + H. The analysis shows that the latter steps cannot be ignored. On this metal, where the rate of water dissociation is much larger than that of methane, the rate can be expressed explicitly. The dependencies of the MSR rate on partial pressures of methane, water, hydrogen, and carbon monoxide are studied and are compared with experiment. The isotope effect (IE) for overall rate of MSR is calculated using the rate constants of all steps including disruption of C–H, O–H, and H–H bonds. For certain applications, like MSR in a membrane reactor, rates are analyzed to show that hydrogen separation may suppress the forward reaction.