Exceptionally Fast Hydrogen Absorption and Desorption through Platinum Overlayers

Palladium alloys are widely used in heterogeneous catalysis, in this group Pd–Pt systems are of particular catalytic interest. They are active components for anodes in low temperature fuel cells and hydrogenation of aromatic compounds in fuels. Platinum monolayers deposited on palladium have been recognized for their unique catalytic properties towards oxygen reduction reaction. However, the hydrogen absorption has never been studied on platinized palladium. In this work we have demonstrated for the first time that platinum overlayers deposited on palladium surface are not only permeable to hydrogen but also enhance the kinetics of the hydrogen absorption/desorption by several orders of magnitude. In this study palladium-hydrogen was chosen as a model system forming nonstoichiometric hydrides.

The hydrogen electrosorption in thin palladium films (50-1000nm) has been investigated at electrodes covered with platinum overlayers. The highest absorption kinetics have been observed for Pd electrodes fully covered with 1 – 3 platinum monolayers. By means of electrochemical impedance spectroscopy (EIS) we have shown that the process is diffusion limited at platinized samples. Diffusion coefficient, D_H, determined in EIS, is two orders of magnitude higher than that previously reported for thin palladium films and approaches D_Hfor bulk palladium. The approach provides insight into the mechanism of hydrogen absorption and the process of hydrogen transport in metals-hydrogen systems.

Surprisingly high stability of the platinized palladium enable its use in heterogeneous catalysis, hydrogenation reactions, or hydrogen purification. It should be stressed that palladium has already been proven to serve as an efficient catalyst to facilitate hydrogen insertion into other metal hydrides. Therefore, another set of potential applications is related to hydrogen storage or sensing systems, where fast and selective reaction in the presence of hydrogen is required.