Kinetics of deuteration of the Pd0.772Ag0.228 alloy with α/β phase transition by in-situ neutron diffraction

The deuteration reaction of a Pd0.772Ag0.228 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 degrees C Bragg peak splitting indicates the presence of two cubic phases, to be identified with the a and beta modifications of palladium hydride. Rietveld refinements provided the corresponding a(alpha), and a(beta) lattice constants, the nu(alpha), and nu(beta) occupancies of D, and the beta phase volume fraction vs. time during deuteration progress. A continuous alpha -> beta transformation takes place till reaction end. At T= 200 degrees C or p> 1 bar a single phase was always observed. The phase diagram of Pd0.772Ag0.228D nu is thus shown to present a alpha+beta two-phase domain less extended but similar to that of PdH nu, at variance with most thermodynamic measurements suggesting full single-phase behavior. The p(t) kinetic data of deuteration at 80 degrees 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. (C) 2019 Elsevier B.V. All rights reserved.