Relationship between element-selective electronic states and hydrogen absorption properties of Pd- M ( M = Ru , Rh , Ag , and Au ) alloys

To understand how the constituent atoms participate in the hydrogenation of Pd-based alloys at ∼0.1 MPa of hydrogen pressure (PH2), we investigated the electronic states in Pd-M (M=Ru,Rh,Ag,and Au) alloys and their hydrides element-selectively by using x-ray absorption spectroscopy at the L2,3 edges. Spectral changes near the absorption edge demonstrate that both Pd and M atoms form bonds with H atoms in the Pd-M (M=Ru and Rh) alloys even at PH2∼0.1 MPa. This is a striking result because high pressures of more than 1 GPa are required for the hydrogenation of Rh and Ru pure metals. In contrast, only Pd atoms bond with H atoms and the M-H bond is absent in the case of Pd-M (M=Ag and Au) alloys. Therefore, the hydrogen-induced changes in the electronic states differ between Ms with fully occupied d shells and Ms with partially occupied d shells. This study reveals that the thermodynamic hydrogenation properties of Pd-M alloys can be determined by a combination of the formation of the M-H bond and lattice expansion or compression by alloying Pd metal with M.