Non-Faradaic Electrochemical Modification of Catalytic Activity: VIII. Rh-Catalyzed C 2H 4 Oxidation

Abstract

The catalytic activity of polycrystalline Rh films deposited on 8 mol% Y 2O 3-stabilized ZrO 2, (YSZ), an O 2-− conductor, can be altered dramatically and reversibly by varying the potential of the Rh catalyst film. The complete oxidation of ethylene was investigated as a model reaction in the temperature range 300 to 400°C and at atmospheric total pressure. The rate of C 2H 4 oxidation can be reversibly enhanced by up to 100 times by supplying O 2− to the catalyst via positive potential application (up to 1.5 V). This is the highest rate enhancement observed so far with in situ electrochemical promotion studies. The steady-state rate increase is typically 10 4 times larger than the steady-state rate of O 2− supply to the catalyst. It was also found that varying the catalyst potential causes the appearance of the well-known compensation effect with an isokinetic point at 372°C. As in previous studies of the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) the observed behaviour is due to the promotional action of back-spillover oxide ions which migrate from the YSZ solid electrolyte onto the catalyst surface under the influence of the applied potential. The back-spillover oxide ions are less reactive with C 2H 4 than normally chemisorbed oxygen and act as promoters by affecting the binding strength of chemisorbed oxygen and ethylene.