Methanol–steam reforming on Cu/ZnO/Al 2O 3 catalysts. Part 2. A comprehensive kinetic model

Abstract

Surface mechanisms for methanol–steam reforming on Cu/ZnO/Al 2O 3 catalysts are developed which account for all three of the possible overall reactions: methanol and steam reacting directly to form H 2 and CO 2, methanol decomposition to H 2 and CO and the water-gas shift reaction. The elementary surface reactions used in developing the mechanisms were chosen based on a review of the extensive literature concerning methanol synthesis on Cu/ZnO/Al 2O 3 catalysts and the more limited literature specifically dealing with methanol–steam reforming. The key features of the mechanism are: (i) that hydrogen adsorption does not compete for the active sites which the oxygen-containing species adsorb on, (ii) there are separate active sites for the decomposition reaction distinct from the active sites for the methanol–steam reaction and the water-gas shift reaction, (iii) the rate-determining step (RDS) for both the methanol–steam reaction and the methanol decomposition reaction is the dehydrogenation of adsorbed methoxy groups and (iv) the RDS for the water-gas shift reaction is the formation of an intermediate formate species. A kinetic model was developed based on an analysis of the surface mechanism. Rate data were collected for a large range of conditions using a fixed-bed differential reactor. Parameter estimates for the kinetic model were obtained using multi-response least squares non-linear regression. The resultant model was able to accurately predict both the rates of production of hydrogen, carbon dioxide and of carbon monoxide for a wide range of operating conditions including pressures as high as 33 bar.