Interstitial Hydrides, Focussing on Palladium Hydride

Several transition metals such as V, Nb, Ta, and Pd can form stable bulk hydrides, so-called interstitial hydrides the bonding in the hydride phase is not ionic but mostly metallic in character, and the hydrogen to metal ratio is not necessarily stoichiometric. Especially, nanoparticles of noble metals such as Pd are relatively easy to prepare by various methods, such as vapor phase deposition on substrates, reductions of salts in solution (electrochemically or electroless), and the inverse micelle templated growth. They are not easily oxidized, and, in recent years, several methods have been developed to precisely control the size of the particles or clusters. Furthermore, growth in solution in the presence of surfactants and stabilizers allows control over the shape of the final particles [35, 36, 42]. 

Palladium nanoparticles vfith a size of a few nanometers supported on carbon are widely used as catalysts, for instance in three-way automotive exhaust catalysts and fuel cells, and can easily be prepared by impregnation of a porous support body with a precursor solution, followed by drying, decomposition of the precursor and, if necessary, reduction. It is well-known that the activity and selectivity of these catalysts for hydrogenation reactions depend on the palladium dispersion for particles sizes in the range 1-10 nm. It is, hence, not surprising that the interaction of Pd with hydrogen, and the infiuence of nanosizing, have been widely studied. 

Many of the experimental observations can also be explained by considering that hydrogen sorption near to the surface is more favorable than absorption in the bulk. 

A very interesting question is what the influence is of particle size on the relative stability of the hydride. As indicated before, it is not really useful to talk about the heat of sorption, as there is a clear distribution in energies. Nevertheless, very generally speaking, one would expect the stability of the hydride to increase with decreasing particle size, due to the higher stability of the (sub) surface hydride. However, as can be seen there is a very large scatter in the data obtained experimentally. Pd nanoparticles as such are not stable at elevated temperatures as they tend 

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