Methanol steam reforming over Cu/ZnO/Al2O3 catalyst:: kinetics and effectiveness factor

Steam reforming of methanol over a commercial Cu/ZnO/Al2O3 catalyst was studied at atmospheric pressure and in a temperature range between 160 and 260 degreesC. The reaction rate depended upon methanol and hydrogen partial pressures, and was independent of the partial pressures of carbon oxides and water, which was in excess of the methanol partial pressure. Small amounts of carbon monoxide, less than 1% in the product gas, were formed at high temperatures; the amounts were well below the equilibrium amounts of reverse water-gas-shift reaction (RWGS). This was in support of the reaction sequence of methanol steam reforming followed by the RWGS. A power-law and a Langmuir-Hinselwood rate expression were developed for the reforming reaction by fitting the expressions to the experimental data. As the data were found to be affected by internal diffusion at high temperatures, the effectiveness factor of the catalyst particle was estimated in the fitting in order to obtain the intrinsic kinetics. Details of the estimation of the factor are elucidated. In order to predict a non-zero, finite rate in the absence of hydrogen, the hydrogen partial pressure term in the power-law expression was corrected by a fitted constant to avoid an infinite reaction rate, since the exponent of the hydrogen partial pressure was a negative number due to the hydrogen inhibition effect in the reforming; in the reaction mechanism for the Langmuir-Hinselwood expression, it was necessary to assume two different kinds of active sites on the catalyst: one for adsorbed methoxy and the other for adsorbed hydrogen. In addition, an excellent fitting of the data by the Langmuir-Hinselwood expression indicates that dehydrogenation of the adsorbed methoxy to the adsorbed oxymethylene is the rate-determining step (RDS), and that adsorption of all the species other than methoxy and hydrogen on the active sites is negligible. (C) 2004 Published by Elsevier B.V.