Correlation between band structure and hydride formation in dilute palladium alloys

Abstract

Data on the free energy change Δ G, following solution of hydrogen in dilute Pd-alloys Pd 1− x M x have been reviewed for different concentrations of M (M = Au, Ag, Pt, Ir, Rh, V, Cu, Ni, Pb, Sn and Ti) in both the α and β phases. The dependence of Δ G values upon the nature of the substituents (transition metals) is consistently explained within the framework of a metal-hydrogen bonding mechanism in the hydrides. For the β-hydride the Δ G values can be calculated on the basis of the equation ΔG = ΔG pd + a( T)(〈 ϵ M LB〉 − 〈 ϵ Pd LB〉) x, where ΔG Pd = − 0.0489 eV H atom and is the free energy change of solution of hydrogen in pure Pd, a( T) = 0.194 at T = 298 K, 〈ϵ m LB〉 and 〈ϵ pd LB〉 are the average energies of the lowest band of the pure constituents ( 〈ϵ Pd LB 〉 = −9.15 eV atom ). The stability of the palladium-hydrogen bond in dilute Pd-alloys depends on the value of 〈ϵ M LB〉; for substituents having lower 〈ϵ M LB〉 values than Pd the bond will strengthen, while for those having higher 〈ϵ M LB〉 values it will weaken. This behaviour agrees well with the general trend of the stability of the stoichiometric hydrides predicted by Gelatt, Ehrenreich and Weiss using band structure results.