Ethylene oxidation to acetic acid with Pd-V2O5 catalysts II. Kinetics of the catalytic reaction

The kinetics of the catalytic oxidation of ethylene into acetic acid over Pd-V2O5 was investigated. We studied, in a first stage, the enhancement of catalytic performance by the addition of palladium to V2O5. Then, the influence of reactant partial pressures on the initial catalytic reaction rates and on the final composition of the catalyst was established. The parallelism observed between enhancement of V2O5 reducibility and of catalytic performance, and the possibility of obtaining acetaldehyde and acetic acid from ethylene oxidation by lattice oxygen of V2O5, leads us to invoke a redox mechanism for the catalysis. Moreover, the simultaneous formation, at low conversions, of acetic acid and acetaldehyde is due to the occurrence of the consecutive oxidation process in the adsorbed phase, according to what has been called a rakelike scheme. A kinetic equation is proposed which fits the experimental results and permits the calculation of kinetic constants from these experimental results. An attempt has been made to determine, on the basis of the simplest redox mechanism, the theoretical steady state of bulk composition of the catalyst, as a function of reactant partial pressure. Comparison with experiment shows that there is a reasonable agreement if we consider the approximation which we have been obliged to make.