Surface modifications with heteropoly and isopoly oxometalates: Part III. electrode modification procedures. the necessity of proton interference during the electrodeposition of the h.e.r. catalysts

Abstract

Heteropoly and isopoly oxometalates and tungstate anion can be used to modify persistently the surface of electrode materials. The test reaction chosen for demonstrating a satisfactory and useful derivatization is the hydrogen evolution reaction (h.e.r). From a practical point of view, the standard modification method in aqueous acid, which succeeds whatever the oxometalate, is obtained by poising the working electrode potential at -1.2 V vs. SCE. However, this method is too crude; to get a first insight into the various steps of such a modification, the gradual variation of the electrolysis potential has proved very helpful. The results show that no useful derivatization of the electrode surface for the h.e.r. is obtained, unless the potential of the working electrode is at least negative of a proton-dependent wave of the oxometalate. At more positive potentials, only adsorption products which are non-catalytic towards the h.e.r. are observed on the surface. Confirmation has been obtained by the use of a non-aqueous solvent such as dimethylformamide (DMF). These results, in water as well as in DMF, show that numerous nuances of the modification procedures can be devised, which all end up in a satisfactory derivatization, the choice depending on whether it is desired to optimize the duration or to try and understand some of the phenomena. In particular, direct adsorption of the oxometalate on the electrode surface prior to electrolysis in pure acidic solution has also proved successful. The possibility that the chemical integrity of the starting material be preserved during modification is discussed.