ETHYLENE OXIDATION OVER PLATINUM - IN-SITU ELECTROCHEMICALLY CONTROLLED PROMOTION USING NA-BETA ALUMINA AND STUDIES WITH A PT(111) NA MODELCATALYST/

Electrochemically modified ethylene oxidation over a PI film supported on the Na+ ion conductor beta '' alumina has been studied over a rang e of conditions encompassing both promotion and poisoning, The system exhibits reversible behavior, and the data are interpreted in terms of (i) Na-enhanced oxygen chemisorption and (ii) poisoning of the surfac e by accumulation of Na compounds. At low Na coverages the first effec t results in increased competitive adsorption of oxygen at the expense of ethylene, resulting in an increased rate, At very negative catalys t potentials (high Na coverage) both effects operate to poison the sys tem: the increased strength of the Pt-O bond and coverage of the catal ytic surface by compounds of Na strongly suppress the rate, Kinetic an d spectroscopic results for ethylene oxidation over a Pt(111)-Na model catalyst shed light on important aspects of the electrochemically con trolled system, Low levels of Na promote the reaction and high levels poison it, mirroring the behavior observed under electrochemical contr ol and strongly suggesting that sodium pumped from the solid electroly te is the key species, XP and Auger spectra show that under reaction c onditions, the sodium exists as a surface carbonate. Post-reaction TPD spectra and the use of (CO)-C-13 demonstrate that CO is formed as a s table reaction intermediate, The observed activation energy (56 +/- 3 kJ/mol) is similar to that measured for CO oxidation under comparable conditions, suggesting that the rate limiting step is CO oxidation. (C ) 1996 Academic Press, Inc.