Copper promotion of high temperature shift

A four-step, redox kinetic mechanism provides an accurate description of the rate of water-gas shift over ferrochrome catalysts. When the heats of formation of the surface intermediates in the model are decreased by only 10 kJ mol−1, as might be expected when copper cations substitute for iron cations in the oxide catalyst, the model predicts increased activity and decreased inhibition by CO2. Experimental measurements using copper-promoted ferrochrome catalysts confirm these predictions. Furthermore, with small changes in most of the thermodynamic parameters, the same four-step, redox kinetic mechanism accurately models the rate of water-gas shift over the promoted catalyst. A four-step mechanism involving a formate intermediate also fits the experimental data for ferrochrome and for copper-promoted ferrochrome. The changes in the thermodynamic parameters for the formate mechanism are greater than those for the redox mechanism, but still within a range that can be ascribed to substitution of copper cations for iron cations in the ferrochrome catalyst. The results show that the formation of metallic copper is not necessary to explain the effect of copper promotion for either redox or associative mechanisms.