Electrocatalysis on bimetallic and alloy surfaces

Abstract

Bimetallic surfaces and alloys are well known to have unique catalytic properties for many important chemical transformations [1]. In electrocatalysis, bimetallic and alloy catalysts have been a particularly active area of research in relation to low-temperature fuel cells [2]. On the anode side, bi- or even tri-metallic electrodes are used to improve the CO tolerance of the hydrogen oxidation or to enhance methanol oxidation activity [3], whereas on the cathode side it has recently been established that for oxygen reduction certain bimetallic surfaces have superior activity compared to pure platinum [4 and 5]. The enhanced catalytic activity of bimetallic surfaces in comparison to pure metal surfaces is usually ascribed to two effects, or a combination thereof: the bifunctional effect in which the unique catalytic properties of each of the elements in the alloy combine in a synergetic fashion to yield a surface which is more active than each of the elements alone, and the ligand or electronic effect, in which one of the elements alters the electronic properties of the other so as to yield a more active catalytic surface. In relation to the issue of CO tolerance and the electrochemical oxidation of CO, bimetallic surfaces have been studied since the early work of Bockris and Wroblowa [6], Janssen and Moolhuysen [7], and, most notably, Watanabe and Motoo [8]. These issues have become more important recently, with the increased emphasis on fuel cells for our energy future. It is now well established that for the electrochemical oxidation of CO on bimetallic surfaces such as PtRu, PtSn and PtMo, the bifunctional effect is the most dominant mechanism. The more oxophylic element Ru or Sn provides the oxygen donor, usually believed to be adsorbed hydroxyl, by activating water at reduced overpotential,