JOURNAL OF CATALYSIS 124, 30--40 (1990) Pt-Ru Anodes for Methanol Electrooxidation: A Ruthenium-99 M6ssbauer Study A. HAMNETT*, B. J. KENNEDY, *'1 AND F. E. WAGNER? *Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, United Kingdom, and tPhysik-Department El5, Technische UniversitgitMiinchen, D-8046 Garching, Germany Received September 18, 1989; revised January 2, 1990 99Ru M6ssbauer spectra have been obtained for a series of Pt-Ru methanol oxidation anodes. For the most catalytically active sample, XPS and M6ssbauer data indicate the presence of Ru(IV) species with a small quadrupole splitting. For highly dispersed samples the M6ssbauer data indi- cate that the Ru is present as a mixture of rutile-phase RHO2 and a second Ru(IV) species. Compari- son with electrocatalytic results suggests that the second Ru(IV) species is the active catalytic copromoter. The possible identity of this species is discussed. © 1990 Academic Press, Inc. INTRODUCTION It is well established that the catalytic ac- tivity of platinum for the electrooxidation of methanol to CO2 at low temperatures can be increased by the addition of a promoter such as Ru, Sn, or Ti (1-5). On pure plati- num a rapid poisoning is observed due to the build-up of a carbonaceous residue, and three possible mechanisms by which the promoters may work have been postulated. For Ru it has been proposed that the en- hanced behavior is due to the direct reac- tion of an oxide of the second metal with the chemisorbed Pt3-COH residue, yielding CO2 (6, 7). The metal oxide is then regener- ated via a second redox process. For Pb it is believed that the enhanced performance of the bimetallic electrode is due to a block- ing effect (8), in which the second metal is assumed to inhibit the strong chemisorption of the intermediate Pt3-COH species. Fi- nally there is the promoter model which appears appropriate for Ti and Sn, where the second metal promotes the formation of active Pt-O groups capable of completing the oxidation reaction (2, 6). Platinum-ruthenium mixtures have been i Current address: Department of Inorganic Chemis- try, University of Sydney, Sydney 2006, Australia. 0021-9517/90 $3.00 Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved. identified a one of the most promising cata- lysts, yet especially for carbon-supported electrodes the nature of the Ru atoms re- mains uncertain (9, 10). XPS studies have proved extremely valuable in the character- ization of other bimetallic systems (2, 6, 11); however, the binding energy of the di- agnostic Ru 3d line is approximately the same as that of the much stronger C ls line. Early temperature-programmed reduction (TPR) studies suggest that for carbon-sup- ported Pt-Ru electrocatalysts the Ru is present as an oxide species (12). The stable oxide of Ru is the rutile-phase RuO2 (•3); however, in confirmation of previous stud- ies (14) we have observed that this is a poor promoter for the methanol oxidation reac- tion. This apparent anomaly prompted us to undertake a detailed study of the physical properties of carbon-supported Ru and Pt- Ru systems. Mrssbauer spectroscopy with the 89-keV y-rays of 99Ru has proved a use- ful technique for the analysis of the chemi- cal state of ruthenium in metal-oxide-sup- ported Ru catalysts (15) and mixed metal Ru oxides (16, 17). The Mrssbauer parame- ters of many ruthenium compounds are known (15, 18). By comparison with these, unknown compounds can be identified, al- beit not always unequivocally. Moreover, the isomer shift depends in a systematic 30