Microkinetic modeling of ethylene oxidation over silver

In the present work a detailed microkinetic model based on the surface science of ethylene oxidation over silver has been developed. The active phase of the catalyst is suggested to be a surface oxide on which atomic oxygen and ethylene can adsorb and react to form an oxametallacycle. The oxide oxygen creates Agδ+ sites which promotes the adsorption of ethylene and is often termed nucleophilic or ionic oxygen in the literature. A less stable atomic oxygen often referred to as electrophilic or covalent in the literature is suggested to compete with ethylene for the Agδ+ sites. The oxametallacycle is a common intermediate in epoxidation, ethylene combustion, and ethylene oxide combustion. The microkinetic model reproduces experimental heats of adsorption, sticking, TPD, and TPR measurements. The kinetics of epoxidation, ethylene combustion, and ethylene oxide combustion has been simulated by the model. The model has been validated successfully by comparing model output to measured initial rates for both oxygen- and ethylene-rich mixtures at different temperatures. Selectivity, apparent activation energies, isotope effects, and reaction orders are reproduced by the model. The rate and selectivity controlling steps in the reaction mechanism and the critical parameters of the model have been identified.