Hydrogen Spillover. Facts and Fiction

Publisher Summary This chapter describes the interaction between metal and support, which may complicate the interpretation of data obtained with supported metals, the critical problem of determining the surface area of the metal, and illustrates the use of supported metals for the elucidation of the problem of catalyst specificity. Work on the series of platinum catalysts suggests that many reactions of hydrogenation, dehydrogenation, and hydrogenolysis are structure-insensitive. If, these same catalysts are now used in reactions involving oxygen—decomposition of hydrogen peroxide, oxidation of methanol and ethanol, oxidation of acetaldehyde—the picture is substantially altered: while specific rates are again almost the same for highly dispersed samples with percentages of dispersion between 80 and 100%, these specific rates change by at least one order of magnitude on samples with a smaller dispersion (50%) and on platinum black.

Catalysis by Supported Metals

Polymer electrolyte fuel cells (PEFCs) are promising power sources in portable and transportation applications because of their high energy densities, low operating temperatures and ease of transportation and storage. However, high cost, low activity and short durability of electrocatalysts are restricting the commercialization of PEFCs. Metal oxides have abundant sources, low cost, high chemical and electrochemical stability, abundant hydroxyl groups on their surfaces, and strong interactions with metal nanoparticles, and thus are promising to reduce the current problems of PEFCs. In this review, binary metal oxides such as titanium oxide, tungsten oxide, molybdenum oxide, ruthenium oxide, tin oxide, cerium oxide and manganese oxide, and multi-component perovskite oxides are chosen based on the selection criteria of metal oxides and introduced as independent electrocatalysts, co-catalysts and supports for various anode oxidation and cathode reduction reactions in PEFCs. The effects of composition, morphology, doping and mixing of metal oxides on their electrocatalytic performance in PEFCs have also been summarized. Finally, several of the most promising metal oxides and possible research trends of metal oxides are recommended for future PEFCs.

An overview of metal oxide materials as electrocatalysts and supports for polymer electrolyte fuel cells

Hydrogen Spillover from a Metal Oxide Catalyst onto Carbon Nanotubes—Implications for Hydrogen Storage

Various experimental results on hydrogen spillover which have been obtained since the first evidence for this phenomenon are discussed concerning the nature of the activated hydrogen species. It will be demonstrated that the physical nature of the spilt-over species, especially their charge, can only be described by considering their interaction with the solid. A new model is proposed which is based on the description of spilt-over hydrogen as electron donor located at the surface. Consequently, H+ ions and H atoms coexist on the surface of the catalyst as also stated experimentally. Their ratio is determined by the electronic properties of the adsorbate/solid system. Considering atomic and recombinative desorption of spilt-over hydrogen, the adsorption isotherms are calculated exemplary for titania. The model is applied to interpret some experimental results related to hydrogen spillover, especially the partial electron transfer from the hydrogen species to the solid.

H.W. Kohn

Papers

On the reduction of tungsten trioxide accelerated by platinum and water