Metal Oxide Cluster and Polyoxometallate Supports for Noble Metal Nanoparticles in Efficient Electrocatalysis

Inorganic metal oxides and related polyoxometallates have several useful characteristics for creating catalytic nanostructures on electrodes. Many oxides exhibit specific reactivity toward electroreductions (e.g., involving hydrogen peroxide, bromate, or chlorate) and electrooxidations (e.g., involving ascorbic acid or during water splitting). Efficient electrocatalytic systems often utilize dispersed noble metal nanoparticles or their alloys. Applicable systems would benefit from robust large-surface-area conducting (electronically, ionically) metal oxide supports capable of not only separating physically metal particles but also interacting with them to affect their chemisorptive and catalytic properties. In this respect, representative examples of metal nanoparticles supported on robust but reactive high surface area metal oxides (e.g., WO3, MoO3, TiO2, ZrO2, V2O5, or CeO2) or modified with ultra-thin films of heteropolytungstates or heteropolymolybdates are described. Electrocatalytic enhancement effects appearing during both reduction of oxygen and oxidation of small organic molecules or fuels (such as methanol, ethanol, or formic acid) are addressed here. Special attention is paid to the mechanistic considerations and stability of the discussed systems. The activity of metal oxide cluster and polyoxometallate supports should be also related to their ability to switch between different oxidation states, absorb and dissociate hydrogen, generate interfacial hydroxyl groups, and transfer oxygen atoms.