In situ controlled promotion of catalyst surfaces via solid electrolytes: Ethylene oxidation on Rh and propylene oxidation on Pt

Summarization: The kinetics of C2H4 oxidation on Rh and C3H6 oxidation on Pt were investigated on polycrystalline metal films interfaced with ZrO2(8mol%Y2O3) solid electrolyte in galvanic cells of the type: C2H4,O2,Rh/YSZ/Pt,O2andC3H6,O2,CO2,Pt/YSZ/Au,C3H6,O2,CO2 It was found that by applying positive potentials and thus, supplying O2- to the catalyst surface, up to 100-fold increases in catalytic rate can be obtained (electrophobic NEMCA effect) for the case of C2H4 oxidation on Rh. For the case of C3H6 oxidation on Pt, up to 6-fold increases in catalytic rate were observed by negative potentials, i.e. removing of O2- from the catalyst surface (electrophilic NEMCA effect). The induced changes in catalytic rates for both reactions were found to be 103 to 5×104 higher than the rates of ion transfer to or from the catalyst-electrode surface. For both reactions it was found that varying the catalyst potential, and thus work function, causes pronounced changes in activation energy and preexponential factor, leading to an interesting demonstration of the well-known “compensation” effect. The results can be rationalized on the basis of the theoretical considerations invoked to explain previous NEMCA studies, i.e. the effect of changing work function on chemisorptive bond strengths of electron acceptor and electron donor adsorbates.Presented on: Ionic

Electrochemical promotion of catalyst surfaces deposited on ionic and mixed conductors

Summarization: The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion (EP) was investigated on Pt films deposited on Y2O3-stabilized-ZrO2 (YSZ), an O2− conductor, TiO2, a mixed conductor, and Nafion 117 solid polymer electrolyte (SPE), a H+ conductor and also on Pd films deposited on YSZ and β″-Al2O3 a Na+ conductor. Four catalytic systems were investigated, i.e. C2H6 oxidation on Pt/YSZ, C2H4 oxidation on Pd/YSZ and Pd/β″-Al2O3, C2H4 oxidation on Pt/TiO2 and H2 oxidation on Pt/Nafion 117 in contact with 0.1 M aqueous KOH solution. In all cases pronounced and reversible non-Faradaic electrochemical modification of catalytic rates was observed with catalytic rate enhancement up to 2000% and Faradaic efficiency values up to 5000. All reactions investigated exhibit a pronounced electrophobic behaviour which is due to the weakening of chemisorptive oxygen bond at high catalyst potentials. Ethane oxidation, however, also exhibits electrophilic behaviour at low potentials due to weakened binding of carbonaceous species on the surface. The general features of the phenomenon are similar for all four cases presented here showing that the NEMCA effect is a general, electrochemically induced, promoting catalytic phenomenon not depending on the reaction and the type of supporting electrolyte.Presented on: Ionic