Switching Between Enantiomers by Combining Chromoselective Photocatalysis and Biocatalysis

<a></a><a></a><a></a><a></a><a></a><a>Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (<i>S</i>)- or the (<i>R</i>)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from <i>Agrocybe aegerita,</i> green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (<i>R</i>)-1-phenylethanol (99% <i>ee</i>). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from <i>Rhodococcus ruber </i>to form<i> </i>(<i>S</i>)-1-phenylethanol (93% <i>ee</i>).</a><a></a>

Switching Between Enantiomers by Combining Chromoselective Photocatalysis and Biocatalysis

<a></a><a></a><a></a><a></a><a></a><a>Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (<i>S</i>)- or the (<i>R</i>)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from <i>Agrocybe aegerita,</i> green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (<i>R</i>)-1-phenylethanol (99% <i>ee</i>). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from <i>Rhodococcus ruber </i>to form<i> </i>(<i>S</i>)-1-phenylethanol (93% <i>ee</i>).</a><a></a>

Boosting thermo-photocatalytic CO2 conversion activity by using photosynthesis-inspired electron-proton-transfer mediators

Natural photosynthesis proceeded by sequential water splitting and CO2 reduction reactions is an efficient strategy for CO2 conversion. Here, mimicking photosynthesis to boost CO2-to-CO conversion is achieved by using plasmonic Bi as an electron-proton-transfer mediator. Electroreduction of H2O with a Bi electrode simultaneously produces O2 and hydrogen-stored Bi (Bi-Hx). The obtained Bi-Hx is subsequently used to generate electron-proton pairs under light irradiation to reduce CO2 to CO; meanwhile, Bi-Hx recovers to Bi, completing the catalytic cycle. This two-step strategy avoids O2 separation and enables a CO production efficiency of 283.8 μmol g−1 h−1 without sacrificial reagents and cocatalysts, which is 9 times that on pristine Bi in H2 gas. Theoretical/experimental studies confirm that such excellent activity is attributed to the formed Bi-Hx intermediate that improves charge separation and reduces reaction barriers in CO2 reduction.

Electron Beam Modified Organic Materials and their Applications

The interaction of electron-beam with organic materials (e.g. Polymers, organic solvents, organic acids etc.) is known to modify their physico-chemical properties and, in many cases, these electron-beam modified materials are used for variety of societal applications. In this review article, we first describe the various types of accelerators to generate electron-beams of different energies, i.e. low (0.3 – 0.75 MeV), medium (0.75– 5 MeV) and high (5 – 10 MeV) energies, and emphasis is laid on various accelerators developed by Bhabha Atomic Research Center (BARC), Trombay, India. The energetic electrons on interaction with organic materials create free radicals that lead to modifications in material through various mechanisms such as, cross-linking, scissioning, curing and grafting. An overview of these mechanisms is presented by citing appropriate examples. Applications of electron beam-modified organic materials in different areas including bio-medical, textile, environment protection, electrical, radiation dosimetry, etc. are reviewed. The prospects and challenges involved in the electron-beam processing of organic materials are presented.

Metal Doped TiO2 Photocatalysts for CO2 Photoreduction

Greenhouse gases such as CO2, CH4 and CFCs are the primary causes of global warming. Worldwide, people are exploring techniques to reduce, capture, store CO2 gas and even convert this gas in to some useful chemicals. CO2 can be transformed into hydrocarbons in a photocatalytic reaction. The advantage of photo reduction of CO2 is to use inexhaustible solar energy. Knowledge of elementary steps in photocatalytic CO2 reduction under UV irradiation is required in order to improve the photo efficiency of the photocatalyst. A semiconductor photocatalyst mediating CO2 reduction and water oxidation needs to absorb light energy, generate electron hole pairs, spatially separate them, transfer them to redox active species across the interface and minimize electron hole recombination. This requires the semiconductor to have its conduction band electrons at higher energy compared to the CO2 reduction potential while the holes in the valence band need to be able to oxidize water to O2. A single semiconductor does not usually satisfy these requirements. Some recent developments in this field have been moves towards rational photocatalyst design, the use of highly active isolated Ti-species in mesoporous and microporous materials, metal-doping of TiO2, development of catalysts active at longer wavelengths than can be achieved with commercially available titania etc. The use of transition-metal loaded titanium dioxide (TiO2) has been extensively studied as a photocatalyst in photoreactions. Unlike traditional catalysts drive chemical reactions by thermal energy, semiconducting photocatalysts can induce chemical reactions by inexhaustible sunlight and convert CO2 in to the useful hydrocarbons. In this review article we will cover different aspects of metal doped nano structured TiO2 photocatalysts, used to convert/reduce CO2 in to useful hydrocarbons.

Dyes Degradation with Fe-Doped Titanium Nanotube Photocatalysts Prepared from Spend Steel Slag

TiO2has been studied most commonly because it has high stability, nontoxicity, high catalytic activity, and high conductivity. Many studies have shown that TiO2would generate electron-hole pairs illuminated with UV and surround more energy than that before being illuminated. In this study, the titanium nanotube (TNT) photocatalysts were prepared to increase the surface area and adsorption capacity. The Fe TNT was also prepared from a slag iron since many slag irons cause waste treatment problems. In this study, a different Fe loading was also assessed since TNT doped with metals can be used to improve the degradation efficiency. Furthermore, five kinds of dye concentration, including 10, 20, 100, 200, and 400 ppm, and five kinds of Fe-doped content, including 0, 0.77, 1.13, 2.24, and 4.50%, were tested. Different kinds of reaction time and dye species were also assessed. In this result, Direct Black 22 was the most difficult to be degraded, although the concentration was decreased or the dose amount was increased. The degradation efficiency of 10 ppm Direct Black 22 was below 40% with 0.04 gL−1TNT under 365 nm UV irradiation.

Progress in biological cryo-SEM

Field-emission SEM's of the in-lens type generate electron beam diameters of less than 1 nm at high accelerating voltages and of ca. 3-5 nm at 1 kV. These instruments, combined with a proper understanding of the involved signal generation and detection processes, represent one of the means to investigate biological surfaces with a structural information at the macromolecular level. In order to obtain this information, the biological structures have to be preserved as close as possible to the native state. This can be achieved with cryotechniques, e.g. cryoimmobilization, eventually followed by freeze-drying of the sample, metal coating at low specimen temperatures and lowtemperature observation in the SEM.Thin beam transparent samples can be visualized in this way with a structural resolution of ca. 3 nm. Fig. 1 shows a micrograph of differently polymerized human calcitonin: (a) a single fibril of ca. 8 nm diameter, exhibiting helical windings, and (b) a ribbon-like higher order structure, which is composed of individual fibrils.

X-ray and Gamma-ray Radiation from the Switched-off Radiopulsars

It is shown that pulsars that have ceased to generate electron-positron pairs (switched-off radiopulsars) may be the sources of X-ray and γ-ray radiation. The magnetic dipole radiation from these rotating neutron stars is transformed near the “light radius” into hard radiation by the plasma that is created due to ionization of interstellar neutral hydrogen.