Solid State Ionics 136–137 (2000) 677–685 www.elsevier.com / locate / ssi Electrochemical promotion of catalytic reactions using alkali ion conductors * Richard M. Lambert , Alejandra Palermo, Federico J. Williams, Mintcho S. Tikhov Chemistry Department, Cambridge University, Cambridge, CB21EW , UK Abstract Electro-pumping of Na or K to the surfaces of metal catalyst films can lead to very large and reversible changes in both catalytic activity and selectivity for a range of different reactions. This is illustrated with respect to the catalytic reduction of NO by CO and by propene over Pt, Rh and Cu, the combustion of propene over Pt, and the selective hydrogenation of acetylene over Pt. The results are interpreted in terms of alkali-induced changes to the adsorption state of reactants (NO reduction, propene combustion) or intermediate species (acetylene hydrogenation). At low alkali coverages, changes in catalyst potential scale linearly with the measured work function change. XP and Auger spectroscopy shows unambiguously that the changes in catalytic behaviour are due to changes in the surface concentration of the alkali. These electron spectroscopic data, supplemented by X-ray absorption spectra, show that the chemical state of the alkali promoter is dependent on the reactive gas environment. The fundamental insight provided by electrochemical promotion experiments can be used to guide the synthesis of practical dispersed catalysts.  2000 Elsevier Science B.V. All rights reserved. Keywords: Promotion; Electrochemical; NO reduction; Acetylene hydrogenation; Reaction mechanisms; XPS; Alkali effect 1. Introduction retical basis in terms of which the EP effect may be interpreted. The technique entails electrochemical The NEMCA phenomenon, also referred to as pumping of ions from a solid electrolyte to the electrochemical promotion (EP), discovered, de- surface of a porous, catalytically active metal film: veloped and discussed extensively by Vayenas and the resulting changes in work function ( DF ) of the his school has been applied to a wide variety of catalyst alter the adsorption enthalpies of adsorbed catalytic systems, including reactions taking place in species and the activation energies of reactions solution. These studies have illuminated many im- involving these species. Electrochemically induced portant aspects of the underlying phenomena and laid changes in catalyst potential measured with respect the basis for a fundamental understanding of the to a reference electrode ( D V ) are related to DF. WR NEMCA effect [1]. Ref. [1] provides a detailed The effects on activity and selectivity are reversible, account of the phenomenology and outlines a theo- and the phenomenon provides an effective and controllable method for the in situ tuning of working catalytic systems. Thus, in addition to its potential *Corresponding author. Tel.: 144-1223-336-467; fax: 144- practical utility, EP also provides a means for the 1223-336-362. E-mail address: rml1@cam.ac.uk (R.M. Lambert). systematic investigation of certain aspects of promot- 0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738(00)00509-9