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.

Cataluminescence in Er-Substituted Perovskites.

Thermophotovoltaic devices have promising applications for energy conversion. However, current conversion efficiency of chemical energy to light is very low, limited by the competing process of heat dissipation released as black body radiation. From a fundamental point of view, the direct conversion of chemical energy into light without this detour is possible. This so called cataluminescence from methanol combustion over Er-substituted SrTiO3 with high efficiency is demonstrated. The catalytically active quaternary perovskites Er0.15 La0.15 Sr0.55 Ti0.95 Cu0.05 O3 - δ exsolute and reabsorb metallic Cu particles onto the surface in reducing and oxidizing conditions, respectively. Thus, it is able to manipulate the surface structure and investigate its influence on the catalytic as well as luminescent properties. The fuel to air ratio around the stoichiometry point changes the conditions from reducing to oxidizing and thereby alters the surface properties. This is evidenced by post mortem X-ray diffraction and X-ray photoemission as well as operando optical spectroscopy. Cataluminescence takes place under oxidizing conditions (lean fuel to air mixture) on the Er-perovskite oxide with a strong selective near infrared emission, while reducing conditions stimulate formation of plasmonic Cu-nanoparticles, which emit black body radiation.

Discrimination of nano-objects via cluster analysis techniques applied to time-resolved thermo-acoustic microscopy.

Time-effective, unsupervised clustering techniques are exploited to discriminate nanometric metal disks patterned on a dielectric substrate. The discrimination relies on cluster analysis applied to time-resolved optical traces obtained from thermo-acoustic microscopy based on asynchronous optical sampling. The analysis aims to recognize similarities among nanopatterned disks and to cluster them accordingly. Each cluster is characterized by a fingerprint time-resolved trace, synthesizing the common features of the thermo-acoustics response of the composing elements. The protocol is robust and widely applicable, not relying on any specific knowledge of the physical mechanisms involved. The present route constitutes an alternative diagnostic tool for on-chip non-destructive testing of individual nano-objects.

Coherent narrowband light source for ultrafast photoelectron spectroscopy in the 17-31 eV photon energy range.

Here, we report on a novel narrowband High Harmonic Generation (HHG) light source designed for ultrafast photoelectron spectroscopy (PES) on solids. Notably, at 16.9 eV photon energy, the harmonics bandwidth equals 19 meV. This result has been obtained by seeding the HHG process with 230 fs pulses at 515 nm. The ultimate energy resolution achieved on a polycrystalline Au sample at 40 K is ∼22 meV at 16.9 eV. These parameters set a new benchmark for narrowband HHG sources and have been obtained by varying the repetition rate up to 200 kHz and, consequently, mitigating the space charge, operating with ≈ 3 × 10 7 electrons/s and ≈ 5 × 10 8 photons/s. By comparing the harmonics bandwidth and the ultimate energy resolution with a pulse duration of ∼105 fs (as retrieved from time-resolved experiments on bismuth selenide), we demonstrate a new route for ultrafast space-charge-free PES experiments on solids close to transform-limit conditions.

Dynamics of correlation-frozen antinodal quasiparticles in superconducting cuprates.

Many puzzling properties of high-critical temperature (Tc) superconducting (HTSC) copper oxides have deep roots in the nature of the antinodal quasiparticles, the elementary excitations with wave vector parallel to the Cu-O bonds. These electronic states are most affected by the onset of antiferromagnetic correlations and charge instabilities, and they host the maximum of the anisotropic superconducting gap and pseudogap. We use time-resolved extreme-ultraviolet photoemission with proper photon energy (18 eV) and time resolution (50 fs) to disclose the ultrafast dynamics of the antinodal states in a prototypical HTSC cuprate. After photoinducing a nonthermal charge redistribution within the Cu and O orbitals, we reveal a dramatic momentum-space differentiation of the transient electron dynamics. Whereas the nodal quasiparticle distribution is heated up as in a conventional metal, new quasiparticle states transiently emerge at the antinodes, similarly to what is expected for a photoexcited Mott insulator, where the frozen charges can be released by an impulsive excitation. This transient antinodal metallicity is mapped into the dynamics of the O-2p bands, thus directly demonstrating the intertwining between the low- and high-energy scales that is typical of correlated materials. Our results suggest that the correlation-driven freezing of the electrons moving along the Cu-O bonds, analogous to the Mott localization mechanism, constitutes the starting point for any model of high-Tc superconductivity and other exotic phases of HTSC cuprates.