Solid electrolytes as active catalyst supports: Electrochemical modification of catalytic activity and selectivity.

摘要

With a conventional promoter it is possible to tune the degree of promotion by adjusting its concentration or changing it chemically. For scientific investigations of promoter/poison effects, as well as for industrial operations that depend on them, it would be highly effective and efficient if one could have a continually variable promoter which could be dosed or removed just by "turning a knob". The long sought goal of controlled promotion of catalyst surfaces appears now to be possible using non-Faradaic electrochemical modification of catalytic activity (NEMCA).; Two different systems were studied during the course of this research: methanol oxidation on Pt/YSZ cells and benzene hydrogenation on {dollar}rm Pt/betasp{lcub}primeprime{rcub}(Na)Alsb2Osb3.{dollar} The systems, in terms of NEMCA, exemplified the two extremes of behavior in electrochemical promotion: while one system could be understood in terms of promotional effects and rate enhancements due to {dollar}rm Osp{lcub}2-{rcub}{dollar} pumping to the catalyst; the other, benzene hydrogenation, exemplified a case of strong electrocatalytically-induced poisoning due to sodium doping on the catalyst surface. Both behaviors, although opposite, represented a clear example of the fact that NEMCA is not restricted to a specific reaction or to a specific electrolyte and provided some more general understanding for this phenomena. Additionally, in each system, a spectroscopic probe was used to identify and, in some cases, quantify the NEMCA-induced spillover species.; It was found that the catalytic activity and selectivity of Pt for the oxidation of methanol to formaldehyde and {dollar}rm COsb2{dollar} can be altered significantly and reversibly by depositing a Pt catalyst film on an yttria-stabilized-zirconia (YSZ) disc and by applying current or potential between the catalyst film and a Ag film deposited on the other side of the {dollar}rm Osp{lcub}2-{rcub}{dollar}-conducting YSZ disc. Both the catalyst film and the Ag counter and reference electrodes are exposed to the reacting {dollar}rm CHsb3OH{dollar}-{dollar}rm Osb2{dollar} mixture. The observed increase in the rate of {dollar}rm Hsb2CO{dollar} production was typically a factor of 100 higher than the rate of {dollar}rm Osp{lcub}2-{rcub}{dollar} supply to the catalyst with a concomitant two-fold increase in selectivity.; The catalytic hydrogenation of benzene to cyclohexane was studied in a Pt-single-pellet electrocatalytic reactor configuration. By interfacing the catalyst with {dollar}betasp{lcub}primeprime{rcub}{dollar}-{dollar}rm (Na)Alsb2Osb3,{dollar} a {dollar}rm Nasp+{dollar} conductor, and by supplying or removing sodium ions to or from the catalyst surface by applying potential to the cell, the catalytic activity of Pt can be strongly and reversibly modified.; It was found that by setting the catalyst potential more positive than its open-circuit emf (i.e., potentiostatic stripping of sodium from the Pt surface) caused a beneficial effect in activity, increasing the rate to two times the open-circuit value. Application of more negative potentials (i.e., potentiostatically-controlled sodium dosage onto the Pt surface) caused a negative effect on the rate, reducing it to almost zero for coverages of sodium of around 0.02, as calculated by integration of the potentiostatically induced ionic-current. The results suggested an electrophobic behavior for this reaction, with sodium acting as a poison for this system, and with the chemisorptive properties of Pt altered ("knob-tuned") by application of an external bias. The average poison-index experimentally calculated for this system was {dollar}-{dollar}130, thus suggesting poison effects of an electronic nature. This new result is the first one, at least with {dollar}betasp{lcub}primeprime{rcub}{dollar}-{dollar}rm (Na)Alsb2Osb3,{dollar} showing the feasibility of NEMCA in hydrogenation reactions. (Abstract shortened by UMI.)