Photocatalytic Hydrogen Generation from a Visible-Light-Responsive Metal–Organic Framework System: Stability versus Activity of Molybdenum Sulfide Cocatalysts

We report the use of two earth abundant molybdenum sulfide-based cocatalysts, Mo(3)S(13)(2-)clusters and 1T-MoS2 nanoparticles (NPs), in combination with the visible-light active metal-organic framework (MOF) MIL-125-NH2 for the photocatalytic generation of hydrogen (H-2) from water splitting. Upon irradiation (lambda >= 420 nm), the best-performing mixtures of Mo3S132-/MIL-125-NH2 and 1T-MoS2/MIL-125-NH2 exhibit high catalytic activity, producing H-2 with evolution rates of 2094 and 1454 mu mol h(-1) g(MOF)(-1) and apparent quantum yields of 11.0 and 5.8% at 450 nm, respectively, which are among the highest values reported to date for visible-light-driven photocatalysis with MOFs. The high performance of Mo3S132- can be attributed to the good contact between these clusters and the MOF and the large number of catalytically active sites, while the high activity of 1T-MoS2 NPs is due to their high electrical conductivity leading to fast electron transfer processes. Recycling experiments revealed that although the Mo3S132-/MIL-125-NH2 slowly loses its activity, the 1T-MoS2/MIL-125-NH2 retains its activity for at least 72 h. This work indicates that earth-abundant compounds can be stable and highly catalytically active for photocatalytic water splitting, and should be considered as promising cocatalysts with new MOFs besides the traditional noble metal NPs

Parametric sensitivity in the Sabatier reaction over Ru/Al2O3 - theoretical determination of the minimal requirements for reactor activation

The methanation of carbon dioxide is an option for chemical storage of renewable energy together with greenhouse gas reutilization because it offers a product with a high energy density. The reaction CO2 + 4H(2) CH4 + 2H(2)O is performed on a Ru/Al2O3 catalyst and is strongly exothermal. For this reason, the reactor design must take into account an efficient thermal management system to limit the maximal temperature and guarantee high CO2 conversion. Additionally, the methanation reactor is subject to parameter sensitivity. This phenomenon can generate instability in the operation of a power to gas plant, due to the variability in the hydrogen production rate. Here we present a parametric study of the thermal properties of the reaction and determine the minimal feed temperature for the normal operation of a reactor. The minimal temperature required is determined by several parameters, such as pressure, space velocity and properties of the cooling system. For adiabatic reactors, the required feed temperature is 210 degrees C for a space velocity of 3000 h(-1) and a pressure of 10 bar. The space velocity strongly affects the positioning of the ignition point, causing a large variability of the feed temperature required. At the same time, the optimal working point of the reactor is at the minimal activation temperature. The properties of cooled reactors are elucidated, showing how the interrelationship between cooling and feed temperature makes the management of this class of reactors more challenging. On the base of the modelling results, we propose a reactor configuration that adjusts the thermodynamic limitations and respects the minimal requirements for reaction ignition, allowing a more stable operation and avoiding the functioning at excessive temperature

Hydrogen storage and electrochemical properties of LaNi5-xCux hydride-forming alloys

LaNi5-type alloys are commercial materials for the negative electrode in Ni-MH rechargeable batteries. Partial substitution of La by mischmetal (Mm) and of Ni by elements like Co, Al, and Mn significantly improve the cycle stability and high-rate discharge capacity of the electrodes. The partial substitution of Ni by Cu was studied previously for several selected ternary alloys with a special focus on crystal structure change upon substitution and gas phase hydrogen absorption. We present in this paper the results of the study of the electrochemical activation, discharge kinetics, equilibrium charge/discharge, and cycle life of electrodes made from four different LaNi5-xCux (x = 0.1, 0.5, 0.9, 1) alloys in order to provide full insights into utilization of these alloys. (C) 2018 Elsevier B.V. All rights reserved

Fast real time and quantitative gas analysis method for the investigation of the CO2 reduction reaction mechanism

We present a new fast real time and quantitative gas analysis method by means of mass spectrometry (MS), which has approximately an order of magnitude faster sampling rate in comparison with a traditional gas chromatography. The method is presented and discussed on the example of the CO2 reduction reaction. The advantages of the method are the possibility to analyze the reaction kinetics, where the kinetically determined reaction range is often only tens of degrees wide. Furthermore, due to the fast sampling rate, the experiments are much shorter and effects due to possible aging of the catalyst are significantly reduced. The quantification of the gas partial pressures is achieved by calibrating the Faraday detector in the quadrupole MS for the expected reactants and products. One major challenge to achieve a quantitative measurement with the MS is to correct for the pressure fluctuations over the probing capillary over the course of the experiment. This fluctuation is compensated in the analysis by normalizing the sum of all calculated partial pressures to the measured reaction pressure for every measured spectrum. With that, a precise, fast, and quantitative gas analysis is achieved. This is the fundament for, e.g., the kinetic reaction analysis where a high data point density is required. The method is discussed on the example of the CO2 hydrogenation reaction to CH4 on a commercial Ru/Al2O3 catalyst. Additionally, the key features of the gas controlling and analysis setup built for the CO2 hydrogenation reaction are described. Published by AIP Publishing

Membrane electrode assembly fabricated with the combination of Pt/C and hollow shell structured-Pt-SiO2@ZrO2 sphere for self-humidifying proton exchange membrane fuel cell

The Pt-supported hollow structured Pt-HZrO2 with the shell thickness of 27 nm is successfully synthesized. The water retention ability of Pt-HZrO2 is significantly enhanced compared with that of SiO2@ZrO2 due to the hydrophilic hollow structured HZrO2 with high BET surface area. Pt-C and Pt-HZrO2 are combined with different weight fractions to prepare the double catalyst electrode (DCE). The membrane electrode assembly with the DCE is fabricated and applied to both anode and cathode or anode side only. The water flooding and thus rapid voltage drop is affected by the presence/or absence of the DCE at the cathode side. The cell test and visual experiment suggests that the Pt-HZrO2 layer adsorb the water molecules generated by the oxygen reduction reaction (ORR), preventing the water flooding. The power generation under RH 0% strongly suggests the back-diffusion of water molecules generated by the ORR. The flow rate to the cathode significantly affects the water flooding and cell performance. Higher flow rate to the cathode is advantageous to expel the water generated by the ORR, thus preventing water flooding and enhancing the cell performance. Therefore, the weight fraction of Pt-C to Pt-HZrO2 and the flow rate to the cathode should be well balanced. (C) 2017 Elsevier B.V. All rights reserved

The Origin of the Catalytic Activity of a Metal Hydride in CO2 Reduction

Atomic hydrogen on the surface of a metal with high hydrogen solubility is of particular interest for the hydrogenation of carbon dioxide. In a mixture of hydrogen and carbon dioxide, methane was markedly formed on the metal hydride ZrCoHx in the course of the hydrogen desorption and not on the pristine intermetallic. The surface analysis was performed by means of time-of-flight secondary ion mass spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy, for the in situ analysis. The aim was to elucidate the origin of the catalytic activity of the metal hydride. Since at the initial stage the dissociation of impinging hydrogen molecules is hindered by a high activation barrier of the oxidised surface, the atomic hydrogen flux from the metal hydride is crucial for the reduction of carbon dioxide and surface oxides at interfacial sites

Influence of Composition on Performance in Metallic Iron-Nickel-Cobalt Ternary Anodes for Alkaline Water Electrolysis

Metallic electrodes based on iron, nickel, and/or cobalt have re-emerged as promising cost-effective anodes for the alkaline oxygen evolution reaction (OER) due to their simplicity and their in situ formation of a highly active oxy-hydroxide surface catalyst layer, which exhibits state-of-the-art overpotentials for the OER. However, the effect of alloy composition has not been systematically studied. Herein, using metallic anodes with defined Fe-Ni-Co atomic ratios prepared via arc melting, we report the relationship between the initial alloy composition, the OER performance, and the emergent active catalyst composition. After 50 h operation at 0.5 A cm(-2) the most active initial alloys (having a moderate amount of cobalt <40 at. %, an iron proportion between 30 and 80 at. % and a nickel ratio below 60 at. %) gave average overpotentials for 10 mA cm(-2) ca. 300-320 mV and Tafel slopes of 35-50 mV dec(-1). Iron and nickel-rich alloys performed poorer. The oxyhydroxide OER catalyst formed on the anode surface generally showed an increased concentration of Co and Ni and a depletion of Fe compared to the initial metal composition, giving the most active OER catalyst at a composition of Ni and Co of ca. 40 at. % with Fe at ca. 20 at. %. However, the initial alloy composition of Fe12.5Co12.5Ni75, showed a nearly invariant surface metal composition, indicating this as the most stable composition. Further analysis of the surface identified no correlation of the mass of metals leached from the anode surface, the electrochemically active surface area, or the presence of active Ni2+/3+ redox surface sites to the OER performance suggesting these factors do not influence the results

Effect of Co-Substitution on Hydrogen Absorption and Desorption Reactions of YMgNi4-Based Alloys

YMgNi4-based alloys exhibit reversible hydrogen absorption and desorption reactions at near room temperature. Here, we report that Co-substituted YMgNi4-based alloys exhibited higher hydrogen contents and lower hydrogen absorption and desorption reaction pressures than unsubstituted alloys. The effects of Co-substitution viewed from atomic arrangements were particularly clarified by synchrotron radiation powder X-ray diffraction, neutron diffraction, and inelastic neutron scattering. Powder neutron diffraction of the Co-substituted alloy at 5 MPa of D-2 pressure suggested the formation of gamma-phase deuteride (higher deuterium content) from beta-phase deuteride (lower deuterium content). However, no gamma-phase deuteride was observed in the unsubstituted alloys at 5 MPa. Therefore, the gamma-phase deuteride formation of the Co-substituted alloy at lower pressure led to higher hydrogen contents than the unsubstituted alloys. The combined results of powder neutron diffraction and inelastic neutron scattering suggested that the gamma-phase hydride of the Co-substituted alloy was continuously generated due to additional H atoms at the H atom sites in the beta-phase hydride because of the disordered H atomic arrangement involving H-H interactions. As a result, hydrogen absorption and desorption reaction pressures for the gamma-phase deuteride formation with higher hydrogen storage capacity were lowered