Solution of hydrogen and deuterium in palladium phosphide, Pd6P

Abstract

Pd₆P dissolves hydrogen to form an interstitial solution which has the composition Pd₆PH_0.15 at 1 atm and 298 K. The hydrogen solubilities follow Sieverts' law of ideal solubility at low hydrogen contents showing that molecular hydrogen dissociates upon entering the phosphide lattice. The equilibration of hydrogen with the phosphide is rapid at 273 K indicating that the H₂→ 2H(surface) reaction occurs readily. Pre-exposure of Pd₆P to CO(g) does not significantly inhibit the rate of hydrogen uptake. This suggests that phosphorus, which is a known poison for hydrogenation catalysts, does not act as such when chemically combined with palladium.

Pressure–composition–temperature data have been obtained for the Pd₆P–H system from which thermodynamic parameters for the solution of hydrogen have been derived. It is known from neutron diffraction studies that deuterium enters both the octahedral and square pyramidal interstices in Pd₆P. A statistical thermodynamic model is proposed to describe the pressure–composition data for this system and it is compared with experimental data.