Tailoring of Lattice Parameter in Palladium By Alloying with Inert-Gases and Its Impact on Pd-H System

Metallic palladium is a commonly employed heterogeneous catalyst used in particular in reduction, hydrogenation and dehydrogenation reactions. The standard Gibbs energies of hydrogen chemisorption on palladium surface is strongly dependent on the lattice distortions <Δa/a> that have been demonstrated e.g. by the ccharacterization of pseudomorphic palladium layers deposited on foreign metals [1]; however, in such systems the strong chemical influence of the substrate by no means cannot be eliminate under experimental conditions. Similar conclusion can be drawn for bimetallic alloys, in which the charge transfer between both constituents affects their electronic structure [2]. Lattice contraction caused by the surface tension in palladium nanoparticles seems to be relatively easily achievable, however, also in this case the surface phenomena are hindered by the presence of the support and/or organic capping agents used for NPs stabilization so that the fundamental studies of adsorption processes are difficult.

Here we report a method for the lattice parameter expansion achieved by means of ion implantation of inert gases, such as N₂ and noble gases, in thin palladium layers (50-500 nm in thickness). By using this approach the lattice parameter can be tailored by changing the gas type, implantation ionic current or dose. By using noble gasses for implantation the chemical effects normally related to alloying was entirely eliminated. The nonmetallic element content in these alloys varied from 0.1 to 10 at. % depending on experimental conditions. Obtained layers were characterized by surprisingly good stability even after multiple cycles of hydrogen absorption and desorption [3].  Prepared Pd-X films were investigated by using Grazing Incidence X-ray Diffraction (GIXD). This method allowed us to get insight into depth-resolved material structure. After inert gases implementation we have observed diffraction lines from solid inert gases embedded into palladium structure most likely in form of nanoparticles. The studies of hydrogen absorption in those alloys were carried out using classical transient measurements and electrochemical impedance spectroscopy. The hydrogen insertion into the alloy was performed in an aqueous 0.1 M HClO₄ solution and electrosorption isotherms were then determined. The results indicate that although the total amount of the hydrogen dissolved in those alloys is only slightly lower with an increase of the nonmetallic element content, the hydrogen insertion in the alpa-phase was enhanced nearly by two orders of magnitude for PdXe alloy with respect to pure palladium (Figure 1). This result indicate that lattice distortion is an extremely important factor in metal/hydrogen systems and quite crucial in heterogeneous catalysis, e.g. in hydrogenation reactions and/or hydrogen storage.

Literature:

[1] L.A. Kibler, A.M El-Aziz, R. Hoyer, D.M. Kolb, 2005, Angewandte Chemie – Int. Ed 44 (14) , pp. 2080-2084

[2] J.A. Rodriguez, D.W. Goodman, Acc. Chem. Res. 28 (1995) 477

[3] P. Połczyński, G. Dercz, R. Jurczakowski, manuscript in preparation.