MICROKINETIC ANALYSIS OF THE WATER-GAS SHIFT REACTION UNDER INDUSTRIAL CONDITIONS

To form the basis for a microkinetic understanding of the low-temperat ure water-gas shift reaction over Cu-based catalysts as operated indus trially, the kinetics have been measured under a wide range of reactio n conditions. To elucidate possible support effects the reaction was s tudied over catalysts of Cu supported on Al2O3, SiO2, or mixed ZnO/Al2 O3. The proposed microkinetic model is based on a ''surface redox'' me chanism deduced from Cu single-crystal studies. All the input data for the elementary steps were taken from available Cu single-crystal stud ies and the total number of sites was the only free parameter in the m icrokinetic analysis. It was found to be important especially at high pressure to include in the mechanism the synthesis and hydrogenation o f formate. The different dependencies of the overall kinetics have als o 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 s pite of the severe constraints placed on the microkinetic model it cou ld account for many of the important kinetic dependencies of the indus trial water-gas shift reaction over the different Cu-based catalysts. Furthermore, the deduced number of active sites agrees well with the i nitial Cu surface area of the reduced catalysts determined separately by H-2-TPD suggesting that the model is also satisfactory for describi ng quantitatively the magnitude of the rates. Thus, a good starting po int in interpreting the water-gas shift kinetics is to consider the ca talysis occurring solely on the metallic Cu particles in the catalysts . The nature of the support may, however, have important secondary rol es. For example, dynamic restructuring of the Cu particles may take pl ace 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.