Relationship between element-selective electronic states and hydrogen absorption properties of Pd-$M$ $(M=\mathrm{Ru},\text{Rh},\text{Ag},\text{and Au})$ alloys

To understand how the constituent atoms participate in the hydrogenation of Pd-based alloys at $\sim 0.1$ MPa of hydrogen pressure $\left(P_{{\mathrm{H}}_2}\right)$, 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 $P_{{\mathrm{H}}_2}\sim 0.1$ 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 $M$.

Figure 1

XRD patterns of the polycrystalline ${\mathrm{Pd}}_{0.99}{\mathrm{Ru}}_{0.01}$ disk (black) and its hydride (red). A Cu $K\alpha$ x ray was used for the measurements.

Figure 2

Lattice constant of Pd-$M$ alloys as a function of $M$ content $y$. Open symbols represent the lattice constants before hydrogenation, and closed symbols denote the lattice constants after hydrogenation. Solid and broken lines are guides for the eye. The inset shows hydrogen content $x$ as a function of $y$. The hydrogen content, $x$, was evaluated from the hydrogen-induced expansion of lattice constants.

Figure 3

Total DOS of (a) AgH, (b) PdH, (c) RhH, and (d) RuH (red, thick lines) together with the DOS of pure metals (black, thin lines). The vertical lines indicate the Fermi energy, $E_{\text{F}}$.

Figure 4

Calculated XAS profiles of (a) RuH, (b) RhH, (c) PdH, and (d) AgH at the $M$ $L_3$ edge (red, thick lines) together with XAS of pure metals (black, thin lines). The vertical lines indicate the absorption edge $E_0$ determined from the energy gap between the $2p_{3/2}$ core level and Fermi energy of the $M$. The inset in panel (d) displays calculated profiles of Ag and AgH at the Ag $L_2$ edge.

Figure 5

XAS spectra of Pd, Pd-$M$ alloys and their hydrides at the Pd $L_{2,3}$ edges. Thin (black) lines and thick (red) lines represent the spectra of alloy and hydrides, respectively. The inset in the right panel summarizes the energy shift of the absorption edge $E_0$ at $M$ $L_{2,3}$ edges after hydrogenation.

Figure 6

XAS spectra of the Pd-Rh alloys at the Rh $L_{2,3}$ edges. Thin (black) lines and thick (black) lines represent the spectra of alloys and hydrides, respectively.

Figure 7

XAS spectra of the Pd-Ru alloys at the Ru $L_{2,3}$ edges. Thin (black) lines and thick (red) lines represent the spectra of alloy and hydrides, respectively.

Figure 8

XAS spectra of the Pd-$M$ $(M$ = Ag, Au) alloys at the (a) Ag $L_2$, (b) Au $L_3$, and (c) Au $L_2$ edges. Thin (black) lines and thick (red) lines represent the spectra of alloy and hydrides, respectively.

Figure 9

(a) Calculated enthalpies, $\mathrm{\Delta }{H}_{M\mathrm{H}}$, of $M$s for the reaction $M+\frac{1}{2}{\mathrm{H}}_2+\mathrm{\Delta }{H}_{M\mathrm{H}}=M\mathrm{H}$. (b) The $M$ composition dependence of the relative enthalpies $\mathrm{\Delta }{H}_{\mathrm{plat}}$ for Pd-$M$ alloys experimentally determined elsewhere. These enthalpies are evaluated from the plateau pressures $P_{H2}$ of each alloy. The $\mathrm{\Delta }{H}_{\mathrm{plat}}$ of pure Pd is indicated by the horizontal dashed line. Each value of $\mathrm{\Delta }{H}_{\mathrm{plat}}$ is taken from Ref. [23] for Cu and Au, Ref. [24] for Pt, Ref. [6] for Ag, and Ref. [25] for Rh.