Hydrides, surface properties

As many authors have indicated, it is well to be aware of the dangers involved in assuming that bulk and surface properties are the same differences in bond hydridization are to be expected. However, the bulk... 

Another consideration in the use of hydride materials in Ni/MH batteries is related to the electrochemical kinetics and transport processes. The power output of the battery depends critically on these processes. During discharge, hydrogen stored in the bulk metal must be brought to the electrode surface by diffusion. The hydrogen then must react with hydroxyl ions at the metal electrolyte interface. As a consequence, surface properties such as oxide thickness, electrical conductivity, surface area, porosity and the degree of catalytic activity... 

Engineering applications such as hydrogen storage in metal hydrides, the nickel-metal hydride rechargeable battery (Ni-MH), and the proton exchange membrane fuel cell (PEMFC) are basically dependent on the surface properties and characteristics. 

Mg oxidizes very easily so that it is a real challenge to study the surface properties of clean Mg and of H adsorption and hydride formation. 

A surface segregation model (Schlapbach et al, 1980) based on the analysis of surface properties by means of photoelectron spectroscopy and magnetic susceptibility measurements, very successfully explains the great reactivity of hydride-forming intermetallic compounds AB (e.g, LaNi ). Selective oxidation and lower surface energy of the electropositive component A (La) induces a surface segregation (Fig.12). 

Synthesis Tools to Produce Metal Nanoparticles 44 Organometallic complexes as the source of metal atoms 46 Ligands as stabilizers of metal nanoparticles 46 Tools for the Characterization of Metal Nanoparticles 47 Quantif cation of hydrides at metal nanoparticle surface 48 Coordination of CO to probe the surface properties of metal nanoparticles 48 Catalysis to probe the surface state of metal nanoparticles 50... 

High Surface Sodium. Liquid sodium readily wets many soHd surfaces. This property may be used to provide a highly reactive form of sodium without contamination by hydrocarbons. Powdered soHds having a high surface area per unit volume, eg, completely dehydrated activated alumina powder, provide a suitable base for high surface sodium. Other powders, eg, sodium chloride, hydride, monoxide, or carbonate, can also be used. 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

There are few systematic guidelines which can be used to predict the properties of AB2 metal hydride electrodes. Alloy formulation is primarily an empirical process where the composition is designed to provide a bulk hydride-forming phase (or phases) which form, in situ, a corrosion— resistance surface of semipassivating oxide (hydroxide) layers. Lattice expansion is usually reduced relative to the ABS hydrides because of a lower VH. Pressure-composition isotherms of complex AB2 electrode materials indicate nonideal behaviour. 

A short survey of information on formation, structure, and some properties of palladium and nickel hydrides (including the alloys with group IB metals) is necessary before proceeding to the discussion of the catalytic behavior of these hydrides in various reactions of hydrogen on their surface. Knowledge of these metal-hydrogen systems is certainly helpful in the appreciation, whether the effective catalyst studied is a hydride rather than a metal, and in consequence is to be treated in a different way in a discussion of its catalytic activity. 

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