Kinetics of deuteration of the Pd 0.772 Ag 0.228 alloy with α/β phase transition by in-situ neutron diffraction

The deuteration reaction of a Pd 0.772 Ag 0.228 foil was studied by neutron powder diffraction measurements vs. time in different T and p(D 2 ) conditions (D20 and D2B high-resolution diffractometers at ILL, France). The system has great importance as permeable membrane for hydrogen separation. At p = 1 bar and T < 200 °C Bragg peak splitting indicates the presence of two cubic phases, to be identified with the α and β modifications of palladium hydride. Rietveld refinements provided the corresponding a α and a β lattice constants, the ν α and ν β occupancies of D, and the β phase volume fraction vs. time during deuteration progress. A continuous α → β transformation takes place till reaction end. At T = 200 °C or p > 1 bar a single phase was always observed. The phase diagram of Pd 0.772 Ag 0.228 D ν is thus shown to present a α+β two-phase domain less extended but similar to that of PdH ν , at variance with most thermodynamic measurements suggesting full single-phase behavior. The ν(t) kinetic data of deuteration at 80 °C/1 bar can be interpreted by a mechanism based on a first-order nucleation step followed by a zero-order step of surface adsorption and dissociation of the D 2 molecule.