Microkinetic analysis of the water-gas shift reaction under industrial conditions

To form the basis for a microkinetic understanding of the low-temperature water-gas shift reaction over Cu-based catalysts as operated industrially, the kinetics have been measured under a wide range of reaction conditions. To elucidate possible support effects the reaction was studied over catalysts of Cu supported on Al2O3, SiO2, or mixed ZnO/Al2O3. The proposed microkinetic model is based on a ''surface redox'' mechanism deduced from Cu single-crystal studies. All the input data for the elementary steps were taken from available Cu single-crystal studies and the total number of sites was the only free parameter in the microkinetic analysis. It was found to be important especially at high pressure to include in the mechanism the synthesis and hydrogenation of formate. The different dependencies of the overall kinetics have also been evaluated by a power law kinetic model, which was found to give an excellent representation of the kinetic data. It is seen that in spite of the severe constraints placed on the microkinetic model it could account for many of the important kinetic dependencies of the industrial water-gas shift reaction over the different Cu-based catalysts. Furthermore, the deduced number of active sites agrees well with the initial Cu surface area of the reduced catalysts determined separately by H-2-TPD suggesting that the model is also satisfactory for describing quantitatively the magnitude of the rates. Thus, a good starting point in interpreting the water-gas shift kinetics is to consider the catalysis occurring solely on the metallic Cu particles in the catalysts. The nature of the support may, however, have important secondary roles. For example, dynamic restructuring of the Cu particles may take place by changing the synthesis conditions and may depend on the nature of the support, as recently evidenced in separate EXAFS experiments. (C) 1996 Academic Press, Inc.