Electrochemical promotion of an IrO2 catalyst for the gas phase oxidation of ethylene
This work contributes to the generalization of the Non-faradaic Electrochemical Modification of Catalytic Activity effect, known under the acronym NEMCA. This effect is observed for the first time over an oxide catalyst, namely IrO2 supported on yttria-stabilized-zirconia. The activity of the IrO2 catalyst for the complete ethylene oxidation is remarkably and reversibly increased by polarizing the catalyst-solid electrolyte interface. The reaction rate in excess oxygen is enhanced by at least an order of magnitude, when the catalyst is polarized positively with respect to a reference electrode. The rate enhancement may be up to 1·104 times higher than the rate of O2- supply to the catalyst. The application of low negative potentials generally causes the rate to decrease. However, under a limited range of experimental conditions, namely temperatures above 370 °C and high overpotentials, the cathodic polarization induces a rate enhancement. The ethylene oxidation kinetics over an IrO2 catalyst exhibits a Langmuir-Hinshelwood behaviour with competitive adsorption of reactants. The rate dependence on the oxygen partial pressure indicates that the reaction may proceed via an intermediate. The electrochemical promotion of the catalyst is most effective in oxygen-rich gas phase compositions, where the rate is essentially limited by the ethylene adsorption. In-situ cyclic voltammetry is used to maintain the reaction rate at a higher value than under open-circuit conditions. This simple method of investigation has just entered in the world of catalysis and provides valuable information on the state of the catalyst surface. The redox processes involving very small amounts of electroactive species can only be studied in low oxygen pressures. The main finding of this work is the experimental evidence that the electrochemical promotion of the ethylene oxidation rate is linearly related to the electrical charge stored at the electrode-electrolyte interface, measured by cyclic voltammetry. The voltammetric analysis of the IrO2 catalyst in air shows that, upon positive polarization, the catalyst surface is oxidized to a higher oxide IrO2+δ which is responsible for the rate enhancement observed.