Inelastic neutron scattering evidence for anomalous H-H distances in metal hydrides.

Hydrogen-containing materials are of fundamental as well as technological interest. An outstanding question for both is the amount of hydrogen that can be incorporated in such materials, because that determines dramatically their physical properties such as electronic and crystalline structure. The number of hydrogen atoms in a metal is controlled by the interaction of hydrogens with the metal and by the hydrogen-hydrogen interactions. It is well established that the minimal possible hydrogen-hydrogen distances in conventional metal hydrides are around 2.1 Å under ambient conditions, although closer H-H distances are possible for materials under high pressure. We present inelastic neutron scattering measurements on hydrogen in [Formula: see text] showing nonexpected scattering at low-energy transfer. The analysis of the spectra reveals that these spectral features in part originate from hydrogen vibrations confined by neighboring hydrogen at distances as short as 1.6 Å. These distances are much smaller than those found in related hydrides, thereby violating the so-called Switendick criterion. The results have implications for the design and creation of hydrides with additional properties and applications.

Hydrogen in methanol catalysts by neutron imaging.

Although of pivotal importance in heterogeneous hydrogenation reactions, the amount of hydrogen on catalysts during reactions is seldom known. We demonstrate the use of neutron imaging to follow and quantify hydrogen containing species in Cu/ZnO catalysts operando during methanol synthesis. The steady-state measurements reveal that the amount of hydrogen containing intermediates is related to the reaction yields of CO and methanol, as expected from simple considerations of the likely reaction mechanism. The time-resolved measurements indicate that these intermediates, despite indispensable within the course of the reaction, slow down the overall reaction steps. Hydrogen-deuterium exchange experiments indicate that hydrogen reduction of Cu/ZnO nano-composites modifies the catalyst in such a way that at operating temperatures, hydrogen is dynamically absorbed in the ZnO-nanoparticles. This explains the extraordinary good catalysis of copper if supported on ZnO by its ability to act as a hydrogen reservoir supplying hydrogen to the surface covered by CO2, intermediates, and products during catalysis.

Hydride Formation Diminishes CO2 Reduction Rate on Palladium.

The catalytic hydrogenation of CO2 includes the dissociation of hydrogen and further reaction with CO2 and intermediates. We investigate how the amount of hydrogen in the bulk of the catalyst affects the hydrogenation reaction taking place at the surface. For this, we developed an experimental setup described herein, based on a magnetic suspension balance and an infrared spectrometer, and measured pressure-composition isotherms of the Pd-H system under conditions relevant for CO2 reduction. The addition of CO2 has no influence on the measured hydrogen absorption isotherms. The pressure dependence of the CO formation rate changes suddenly upon formation of the ß-PdH phase. This effect is attributed to a smaller surface coverage of hydrogen due to repulsive electronic interactions affecting both bulk and surface hydrogen.

Rationally Designed Blue Triplet Emitting Gold(III) Complexes Based on a Phenylpyridine-Derived Framework.

A series of blue-emitting phosphorescent mono-cyclometalated AuIII complexes have been successfully synthesized. Tailoring the substitutions on the phenylpyridine (ppy) ligand scaffold with electron-withdrawing fluorine groups on the phenyl ring to achieve stabilization of the HOMO and an electron-donating dimethylamino group on the pyridine ring to destabilize the LUMO resulted in a large energy gap and bestowed on the gold(III) complexes high-energy emission and high quantum efficiencies. The results of cyclic voltammetry studies suggested a predominantly redox event localized on the cyclometalated ligand. Thermogravimetric analysis of selected complexes revealed a high stability up to 280 °C, thus the complexes are suitable for device fabrication through vacuum-deposition. Photophysical investigations performed on all the derivatives revealed phosphorescence emission in neat solid, solution, doped in poly(methyl methacrylate) (PMMA) films at room temperature as well as in rigidified glass media (2-MeTHF) at 77 K. A high photoluminescent quantum efficiency of 28 % was obtained for a complex in PMMA, the highest quantum yield reported for a blue-emitting gold(III) complex.

Neutron Insights into Sorption Enhanced Methanol Catalysis.

Sorption enhanced methanol production makes use of the equilibrium shift of the CO 2 hydrogenation reaction towards the desired products. However, the increased complexity of the catalyst system leads to additional reactions and thus side products such as dimethyl ether, and complicates the analysis of the reaction mechanism. On the other hand, the unusually high concentration of intermediates and products in the sorbent facilitates the use of inelastic neutron scattering (INS) spectroscopy. Despite being a post-mortem method, the INS data revealed the change of the reaction path during sorption catalysis. Concretely, the experiments indicate that the varying water partial pressure due to the adsorption saturation of the zeolite sorbent influences the progress of the reaction steps in which water is involved. Experiments with model catalysts support the INS findings.