Design and fabrication of highly reducible PtCo particles supported on graphene-coated ZnO

International audienc

Electron Deficient Monomers that Optimize Nucleation and Enhance the Photocatalytic Redox Activity of Carbon Nitrides

International audienc

Surface states, surface potentials, and segregation at surfaces of tin-doped In2O3

Surfaces of In2O3 and tin-doped In2O3 (ITO) were investigated using photoelectron spectroscopy. Parts of the measurements were carried out directly after thin film preparation by magnetron sputtering without breaking vacuum. In addition samples were measured during exposure to oxidizing and reducing gases at pressures of up to 100 Pa using synchrotron radiation from the BESSY II storage ring. Reproducible changes of binding energies with temperature and atmosphere are observed, which are attributed to changes of the surface Fermi level position. We present evidence that the Fermi edge emission observed at ITO surfaces is due to metallic surface states rather than to filled conduction band states. The observed variation of the Fermi level position at the ITO surface with experimental conditions is accompanied by a large apparent variation of the core level to valence band maximum binding energy difference as a result of core-hole screening by the free carriers in the surface states. In addition segregation of Sn to the surface is driven by the surface potential gradient. At elevated temperatures the surface Sn concentration reproducibly changes with exposure to different environments and shows a correlation with the Fermi level position

Ionothermal Synthesis of Triazine-Heptazine Based Co-frameworks with Apparent Quantum Yields of 60 % at 420 nm for Solar Hydrogen Production from "Sea Water"

Polymeric carbon nitride (PCN), either in triazine or heptazine forms, has been regarded as promising metal?free, environmental benign and sustainable photocatalysts for solar hydrogen production. However, PCN in most cases only exhibits moderate activities due to the inherent properties such as rapid charge carrier recombination. Here we present a triazine?heptazine copolymer synthesized from simple post?calcination of PCN in eutectic salts, i.e. NaCl/KCl, to modulate the polymerization process and optimize the structure. The construction of internal triazine?heptazine donor?acceptor (D?A) heterostructures is affirmed to significantly accelerate the charge transfer (CT) and thus corporately boost the photocatalytic activity (AQY= 60 % at 420 nm). This study highlights the construction of intermolecular D?A copolymers in NaCl/KCl molten salts with higher melting points but absence of lithium to modulate the polymerization process and chemical structure of PCN

CPC Solar collectors with flat bifacial absorbers

Summarization: The design, construction and test results of non-evacuated stationary CPC solar collectors with flat absorbers are presented and discussed. The proposed collector design is based on a truncated asymmetric CPC reflector, consisting of a parabolic and a circular part. A flat bifacial absorber is installed at the upper part of the collector, parallel to the glazing to form a thermal trap space between the reverse absorber surface and the circular part of the mirror. Two prototypes based on the same collector geometry were constructed and tested. The first model consists of one mirror–absorber unit and the second of three smaller units integrated in one collector device. The truncated CPC mirror and the installation of the absorber parallel to the glazing keep the optical efficiency at a satisfactory level. The reduction of radiative thermal losses by using selective absorbers and the suppression of convection thermal losses from the reverse absorber surface to the collector cover result to a significant decrease of the total collector thermal losses. The experimental results showed that the proposed CPC collector could achieve a maximum efficiency of 0.71 and a stagnation temperature of about 180°C, with the multiunit collector device being more efficient and practical.Παρουσιάστηκε στο: Solar Energ

Combined in situ XPS and in situ soft XAS study of Cu/ZnO catalysts for methanol steam reforming

Copper-Zinc oxide (alumina) catalysts are industrially relevant for a variety of processes (low temperature methanol synthesis, low temperature water-gas shift reaction, methanol steam reforming). We used high pressure X-ray photoemission spectroscopy (XPS) and high pressure soft X-ray absorption spectroscopy (XAS) for an extensive in situ study of Cu/Zn catalysts during the activation process in H2 and during the methanol steam reforming reaction. Analysis of XP core level spectra and the Cu L2,3 – and Zn L2,3 near edge X-ray absorption fine structure revealed detailed information about the catalyst surface under the presence of the reactive gas phase. We found that the reduction process (activation) plays a decisive role for the state of the catalysts under methanol steam reforming conditions. The Zn3p/Cu3p intensity ratio altered during the reduction process. A Zn species different to ZnO was found after calcination. Furthermore, a surface oxygen species (additional to ZnO) was observed after reduction and under reaction conditions. This investigation highlights the potential of surface sensitive in situ methods like high pressure XPS and soft XAS in general

In situ X-ray photoelectron spectroscopy of methanol steam reforming on Cu/ZnO catalysts

Cu/ZnO (alumina) catalysts have found industrial use for the low-temperature methanol synthesis, for the low-temperature water-gas shift reaction, and for the steam reforming of methanol. An important application could be the onboard production of hydrogen for fuel cell application. It is the proposed synergistic effect in the binary copper/zinc oxide that makes this system interesting for investigation. The knowledge of the relationship between the catalytic activity, surface structure, and bulk structure is necessary in order to elucidate synthesis pathways to new and improved catalysts. Cu/ZnO catalysts have been studied during the activation process and during the methanol steam reforming reaction by in situ XPS and in situ NEXAFS at U49/2-PGM1