Local electronic structure rearrangements and strong anharmonicity in YH3 under pressures up to 180 GPa.

The discovery of superconductivity above 250 K at high pressure in LaH10 and the prediction of overcoming the room temperature threshold for superconductivity in YH10 urge for a better understanding of hydrogen interaction mechanisms with the heavy atom sublattice in metal hydrides under high pressure at the atomic scale. Here we use locally sensitive X-ray absorption fine structure spectroscopy (XAFS) to get insight into the nature of phase transitions and the rearrangements of local electronic and crystal structure in archetypal metal hydride YH3 under pressure up to 180 GPa. The combination of the experimental methods allowed us to implement a multiscale length study of YH3: XAFS (short-range), Raman scattering (medium-range) and XRD (long-range). XANES data evidence a strong effect of hydrogen on the density of 4d yttrium states that increases with pressure and EXAFS data evidence a strong anharmonicity, manifested as yttrium atom vibrations in a double-well potential.

The reduction of copper in porous matrices--the role of electrostatic stabilisation.

The redox properties of Cu(II) species in FAU matrices have been studied by temperature programmed reduction (TPR) in hydrogen and by XAFS analysis of the products obtained after (stationary) reduction treatments at various temperatures. The influence of the matrix polarity was investigated by comparing aluminosilicate FAU (Y zeolite) with siliceous FAU. In addition, the influence of Zn ions on the reduction process was studied. It was found that both the matrix composition and the presence of zinc ions exert a significant influence on the course of the reduction. In Y zeolite, heat treatment which is known to transfer Cu(II) ions to remote sites (SI, SI', SII') affects the reduction process dramatically. Cu(II) is most easily reduced in siliceous FAU, but the reduction proceeds in two clearly separated steps. Between these steps, small Cu(0) nuclei coexist with Cu(I) species, apparently unable to activate hydrogen for the autocatalytic reduction of the remaining Cu ions. The polarity of the matrix causes an upshift of the Cu(II) reduction temperature (in TPR by ca. 80 K for sites in the large cavity, by ca. 105 K for the remote sites), but the reduction of Cu(I) depends strongly on the simultaneous presence of Cu(0) and on its ability to activate hydrogen and induce an autocatalytic reduction mechanism. While Cu(I) species in the large cavities are easily reduced to the metal, tending to segregate from the zeolite lattice, Cu(I) ions in remote sites are strongly stabilized towards further reduction and even traces of Cu metal form only at very high temperatures. In the presence of zinc ions, the Cu metal particles formed were found to be smaller than in zinc-free samples.

Reduction of copper in porous matrixes. Stepwise and autocatalytic reduction routes.

The reduction of Cu(II) oxide species in siliceous matrixes of different porosity (MFI, FAU, MCM-48) and in alumosilicate MFI was studied by temperature-programmed reduction in hydrogen (TPR), by X-ray absorption fine structure (after stationary hydrogen treatments), and by transmission electron microscopy. It was found that the reduction may proceed in one or in two reduction steps. The two-step scheme known for zeolites was observed also for Cu(II) in siliceous microporous matrixes, with similar temperature of Cu(II) reduction onset as for the alumosilicate MFI. Therefore, the two-step scheme cannot be explained by the stabilization of Cu ions by intra-zeolite electrical fields. CuOx clusters in MCM-48 were reduced in a one-step scheme (similar to bulk CuO) at high Cu content (6 wt %) but in a two-step scheme at low Cu content (1 wt %). The two reduction steps observed with most samples cannot be identified with the transitions of all Cu(II) to Cu(I) and of Cu(I) to Cu(0). Instead, Cu(0) nuclei were observed already at low reduction temperatures and were found to coexist with Cu ions over temperature ranges of different extension. This coexistence range was narrow in materials that favor aggregation of the Cu nuclei into particles: Cu-MCM-48 of low Cu content and Cu-ZSM-5. In the latter, metal segregation from the pore system was found to be accompanied by an autocatalytic initiation of the second reduction step. In the siliceous microporous matrixes, the Cu(0) nuclei were observed to coexist with Cu ions over wide temperature ranges (100 K for MFI) at temperatures far above that of Cu reduction in the bulk oxide. These observations suggest that oligomeric Cu metal nuclei which may have been formed, e.g., at the intersections of the MFI channel system, may be unable to activate hydrogen, which would be required for rapid reduction of the coexisting Cu ions.