No abstract

Metal/n-pyrite (metal=Pt, Au, Nb) Schottky barrier type diodes were fabricated on electrochemically reduced either synthetic or natural (100) and (111) surfaces of single crystalline n-FeS2. The temperature dependence of I-V curves in darkness were analyzed in the range of 200–350 K on the basis of thermionic emission and recombination models. The calculated effective barrier height was ∼0.60 eV and the activation energy for recombination ∼0.50 eV for all investigated n-FeS2/Pt samples. The doping density and the extrapolated potential (pseudo flatband situation) from the Mott–Schottky plot, obtained from capacities deduced from potentiostatic complex impedance measurements, were 2.0×1016 cm−3 and 0.25 eV vs Pt for the synthetic n-pyrite crystal, respectively. From the donor density and barrier height a band bending of 0.5 eV was deduced. Photovoltaic parameters like open-circuit photovoltage and short-circuit photocurrent were studied down to temperatures of 200 K. The main phenomenon preventing the generation of a photopotential approaching the band bending (0.50 eV) appears to be the pinning of the Fermi-level by recombination centers located in the middle of the band gap (Eg=0.95 eV) of pyrite.

Reduzierung der Treibhausgasemissionen und Begrenzung der Erderwärmung auf deutlich unter 2 °C sind die großen Ziele des Pariser Klimaschutzabkommens. Die technische Realisierung dieser Ziele stellt viele emissionsreiche Industriesparten weiterhin vor große Herausforderungen. Ein vielversprechender Ansatz ist der synergetische Verbund von Großindustrien in cross‐industriellen Netzwerken. Mit Carbon2Chem® ist erstmals der Zusammenschluss der Sparten Stahl, Chemie und Energie gelungen. Ziel der Initiative ist es, Hüttengase aus der Stahlproduktion als Ausgangsstoff für chemische Produkte zu nutzen.

The economic and ecological production of green hydrogen by water electrolysis is one of the major challenges within Carbon2Chem® and other power‐to‐X projects. This paper presents an evaluation of the different water electrolysis technologies with respect to their specific energy demand, carbon footprint and the forecast production costs in 2030. From a current perspective alkaline water electrolysis is evaluated as the most favorable technology for the cost‐effective production of low‐carbon hydrogen with fluctuating renewables.

The interaction of ammonia with (VO), P, O, prepared by calcination of the precursor compound VOHPO, 0.5H20 under nitrogen has been studied using temperature-programmed desorption of ammonia (TPDA), temperature-programmed reaction spectroscopy (TPRS), and IR and EPR spectroscopy. Massspectrometric detection was applied to observe possible ammonia decomposition or oxidation products. The investigation revealed that ammonia is not only adsorbed on but also reacts with (VO), P, O, in a redox process generating nitrogen, water and an amorphous V"'-containing compound, the concentration of which could be directly determined by potentiometric titration. The high amount of V"' found pointed towards reduction of V'" not only on the surface but also in deeper layers of the bulk. This was also confirmed by EPR spectroscopy.Furthermore, this reaction results in a change of the Brsnsted and Lewis acidity observed by IR spectroscopy.The concentration of the Brsnsted-acid OH groups was strongly enhanced by hydrolysis of P-0-P and/or V-0-P links by water formed during the redox reaction. The increased concentration of Lewis sites was caused by the removal of oxygen from surface vanadyl groups, probably creating additional coordinatively unsaturated sites. The influence of the observed redox reaction on the characterization of the acidity and the formation of VPO catalysts in the ammoxidation reaction are discussed.

The current/voltage curves of crystalline (100)‐ and (111)‐oriented surfaces of n‐pyrite/electrolyte and n‐pyrite/Pt junctions in darkness and under illumination are strongly influenced by cathodic hydrogen evolution before junction formation. Higher dark current and lower photocurrent on n‐pyrite (111)‐oriented surfaces in comparison to (100)‐oriented surfaces were obtained. The junction properties of the untreated surfaces change from a metal‐like behaviour to a rectifying one. During electrochemical treatment different etching patterns develop according to the exposed surface orientation. In the case of (111)‐oriented crystals these patterns are identified as new (100)‐surfaces. Quantum yields of ca. 0.7 at photon energies of 1.4 eV and almost similar photocurrent action spectra are obtained on n‐pyrite/electrolyte and n‐pyrite/metal junctions. The energetical position of the subbandgap response at 0.68 eV is not changed during etching. The increase of the quantum yield for photon energies higher than 1.4 eV on both types of junction points towards a reduction of surface recombination centers on etched samples. EBIC analysis on previously etched n‐pyrite in contact to vacuum deposited platinum films of 120 to 1000 Å thickness (Schottky barriers) indicate that the charge carrier collection efficiency is higher on etched areas. The role of surface recombination processes for photocurrent generation is demonstrated and discussed.

A significant amount of global carbon dioxide emissions is due to steel mill gases. One promising approach for the reduction of these emissions is their chemical conversion. In this regard, the conversion to urea is very advantageous, since common urea processes are already using carbon dioxide as a feedstock. In the present paper, urea capacities are determined that can be realized using steel mill gases. The capacities are estimated based on new process concepts that are presented in this paper. Exemplarily, the steel mill of thyssenkrupp Steel in Duisburg, Germany, is considered. It could be shown that reasonable urea capacities can be reached. By this means, a great impact can be made on the reduction of steel mill carbon dioxide emissions.

Quantum yields of illuminated single crystalline n-pyrite (FeS,)/electrolyte junctions were measured at A = 632.8 nm and found to be significantly higher in the presence of negatively charged redox species, such as Iand [Fe(CN)d4-in contrast to species like Fez+. Higher photocurrents and photovoltages at (100) n-pyrite rotating disc electrodes in comparison with (1 11) electrodes fabricated from the same single crystal are obtained. Further, (100) n-pyrite electrode surfaces were treated with pyrazine and 4,4'-bipyridine. A significant decrease of dark current and a shift of the equilibrium potential in 0.5 M FeSO, is ascribed to chemical complex formation between the electron donating ligands and the Lewis acidic Fe(I1) centres of the (100) n-pyrite surface. In the case of pyrazine treatment an enhancement of photocurrent and photovoltage could be observed. The observed phenomena are discussed in terms of coordinated chemical surface complex formation and charge transfer properties.