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 $\ensuremath{\sim}0.1$ MPa of hydrogen pressure $({P}_{{\mathrm{H}}_{2}})$, we investigated the electronic states in Pd-$M$ $(M=\text{Ru},\text{Rh},\text{Ag},\text{and Au})$ alloys and their hydrides element-selectively by using x-ray absorption spectroscopy at the ${L}_{2,3}$ edges. Spectral changes near the absorption edge demonstrate that both Pd and $M$ atoms form bonds with H atoms in the Pd-$M$ $(M=\text{Ru} \text{and Rh})$ alloys even at ${P}_{{\mathrm{H}}_{2}}\ensuremath{\sim}0.1$ 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-$M$ $(M=\text{Ag} \text{and Au})$ alloys. Therefore, the hydrogen-induced changes in the electronic states differ between $M\mathrm{s}$ with fully occupied $d$ shells and $M\mathrm{s}$ 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 $M$.