Photoelectrochemical Hydrogen Production in Alkaline Solutions Using Cu2O Coated with Earth‐Abundant Hydrogen Evolution Catalysts

The splitting of water into hydrogen and oxygen molecules using sunlight is an attractive method for solar energy storage. Until now, photoelectrochemical hydrogen evolution is mostly studied in acidic solutions, in which the hydrogen evolution is more facile than in alkaline solutions. Herein, we report photoelectrochemical hydrogen production in alkaline solutions, which are more favorable than acidic solutions for the complementary oxygen evolution half‐reaction. We show for the first time that amorphous molybdenum sulfide is a highly active hydrogen evolution catalyst in basic medium. The amorphous molybdenum sulfide catalyst and a Ni–Mo catalyst are then deposited on surface‐protected cuprous oxide photocathodes to catalyze sunlight‐driven hydrogen production in 1 M KOH. The photocathodes give photocurrents of ?6.3 mA cm^?2 at the reversible hydrogen evolution potential, the highest yet reported for a metal oxide photocathode using an earth‐abundant hydrogen evolution reaction catalyst.     

A Flexible Photoactive Titanium Metal–Organic Framework Based on a [TiIV3(μ3‐O)(O)2(COO)6] Cluster

The synthesis of titanium–carboxylate metal–organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to titanium‐based MOFs by the use of titanocene dichloride to synthesize COK‐69, the first breathing Ti MOF, which is built up from trans‐1,4‐cyclohexanedicarboxylate linkers and an unprecedented [Ti^IV₃(μ₃‐O)(O)₂(COO)₆] cluster. The photoactive properties of COK‐69 were investigated in depth by proton‐coupled electron‐transfer experiments, which revealed that up to one Ti^IV center per cluster can be photoreduced to Ti^III while preserving the structural integrity of the framework. The electronic structure of COK‐69 was determined by molecular modeling, and a band gap of 3.77 eV was found.     

AFM investigations on the influence of CO2 exposure on Ba0.5Sr0.5Co0.8Fe0.2O3–δ

In this work, fresh and CO₂-exposed specimens of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ (BSCF) are examined by atomic force microscopy (AFM) using amplitude-modulated Kelvin probe force microscopy (KPFM) and also electrostatic force microscopy (EFM) to characterize the early stages of the formation of reaction products due to reaction with gaseous CO₂. A comparison is made with results from electron microscopy on the same samples. BSCF specimens exposed for 24 and 240 h to an atmosphere of 99.9 % CO₂ at 900 °C, respectively, were analyzed and compared with non-exposed specimens. The observation of interconnected carbonate islands on BSCF forming a continuous carbonate layer after some exposure to CO₂ indicates a Stranski–Krastanov or Volmer–Weber growth mechanism of the carbonate layer. Our results demonstrate that the measurement of surface potential variations by means of KPFM and EFM constitutes a very sensitive technique to detect the formation of reaction layers on gas permeation membranes such as BSCF. In contrast to electron microscopy techniques, scanning probe techniques permit the investigation of the topography and of electrochemical characteristics of the sample surface as received and without further preparation.     

Elastic waves in suspensions

A model of heterogeneous medium taking into account the friction between the particles and liquid, as well as the relaxation of the small-size particles to the equilibrium on the stress, has been proposed to describe the propagation of the elastic waves in a suspension. A system of wave equations describing the propagation of a plane longitudinal wave has been formulated for the components of the medium. Analytical expressions for the sound velocity in a suspension has been obtained in the approximation in which the particles are completely carried away by liquid in the limiting cases in which the particles are in equilibrium under stress with the liquid or equilibrium is absent. The dependence of the sound velocity in the medium on the volumetric portion and the size of the inclusions has been studied. The obtained results agree with the experimental data and obtained analytical expressions for the sound velocity. The dynamics of the components of the medium at the propagation of the plane longitudinal monochromatic wave has been studied.     

Stereoselective Synthesis and Retentive Trapping of α‐Chiral Secondary Alkyllithiums Leading to Stereodefined α,β‐Dimethyl Carboxylic Esters

The treatment of α‐chiral secondary alkyl iodides with tBuLi at ?100 °C leads to the corresponding secondary alkyllithiums with high retention of configuration. Subsequent quenching with various electrophiles such as Bu₂S₂, DMF, MeOB(OR)₂, or Et₂CO provides the desired products with retention of configuration. Furthermore, a transmetalation with CuBr?P(OEt)₃ also allows retentive trapping with acid chlorides and ethylene oxide. The quenching of the resulting alkyllithiums with ClCO₂Et furnishes stereoselectively syn‐ and anti‐ethyl‐2,3‐dimethyl ester carboxylates (d.r.>94 %). Related esters bearing three adjacent stereo‐controlled centers (stereotriads) have also been prepared. This method has been applied to the synthesis of the ant pheromone (±)‐lasiol in 26 % overall yield (four steps) with d.r.=97:3 starting from commercially available cis‐2,3‐epoxybutane.     

Efficient unbiased variance reduction techniques for Monte Carlo simulations of radiative transfer in cloudy atmospheres: The solution

We present five new variance reduction techniques applicable to Monte Carlo simulations of radiative transfer in the atmosphere: detector directional importance sampling, n-tuple local estimate, prediction-based splitting and Russian roulette, and circum-solar virtual importance sampling. With this set of methods it is possible to simulate remote sensing instruments accurately and quickly. In contrast to all other known techniques used to accelerate Monte Carlo simulations in cloudy atmospheres - except for two methods limited to narrow angle lidars - the presented methods do not make any approximations, and hence do not bias the result. Nevertheless, these methods converge as quickly as any of the biasing acceleration techniques, and the probability distribution of the simulation results is almost perfectly normal. The presented variance reduction techniques have been implemented into the Monte Carlo code MYSTIC (“Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres”) in order to validate the techniques.