Bromide-Mediated Photoelectrochemical Epoxidation of Alkenes Using Water as an Oxygen Source with Conversion Efficiency and Selectivity up to 100.

In a photoelectrochemical (PEC) cell, the production of solar fuels such as hydrogen is often accompanied either by the oxidation of water or by the oxidation of organic substrates. In this study, we report bromide-mediated PEC oxidation of alkenes at a mesoporous BiVO4 photoanode and simultaneous hydrogen evolution at the cathode using water as an oxygen source. NaBr as a redox mediator was demonstrated to play a dual role in the PEC organic synthesis, which facilitates the selective oxidation of alkenes into epoxides and suppresses the photocorrosion of BiVO4 in water. This method enables a near-quantitative yield and 100% selectivity for the conversion of water-soluble alkenes into their epoxides in H2O/CH3CN solution (v/v, 4/1) under simulated sunlight without the use of noble metal-containing catalysts or toxic oxidants. The maximum solar-to-electricity efficiency of 0.58% was obtained at 0.39 V vs Ag/AgCl. The obtained epoxide products such as glycidol are important building blocks of the chemical industry. Our results provide an energy-saving and environment-benign approach for producing value-added chemicals coupled with solar fuel generation.

Aqueous CO2 Reduction on Si Photocathodes Functionalized by Cobalt Molecular Catalysts/Carbon Nanotubes.

Photoelectrochemical reduction of CO2 is a promising approach for renewable fuel production. We herein report a novel strategy for preparation of hybrid photocathodes by immobilizing molecular cobalt catalysts on TiO2 -protected n+ -p Si electrodes (Si|TiO2 ) coated with multiwalled carbon nanotubes (CNTs) by π-π stacking. Upon loading a composite of CoII (BrqPy) (BrqPy=4',4''-bis(4-bromophenyl)-2,2' : 6',2'' : 6'',2'''-quaterpyridine) catalyst and CNT on Si|TiO2 , a stable 1-Sun photocurrent density of -1.5 mA cm-2 was sustained over 2 h in a neutral aqueous solution with unity Faradaic efficiency and selectivity for CO production at a bias of zero overpotential (-0.11 V vs. RHE), associated with a turnover frequency (TOFCO ) of 2.7 s-1 . Extending the photoelectrocatalysis to 10 h, a remarkable turnover number (TONCO ) of 57000 was obtained. The high performance shown here is substantially improved from the previously reported photocathodes relying on covalently anchored catalysts.

A Ribulose-5-phosphate Shunt from the Calvin-Benson Cycle to Methylerythritol Phosphate Pathway for Enhancing Photosynthetic Terpenoid Production.

Cyanobacteria are attractive hosts for photosynthetic terpenoid production, using CO2 as the sole carbon source. Although the methylerythritol phosphate (MEP) pathway is superior to the mevalonate pathway for cyanobacterial terpenoid synthesis, the first reaction of the MEP pathway, which is catalyzed by 1-deoxy-d-xylulose-5-phosphate (DXP) synthase, involves complex regulation and carbon loss. Here, we constructed a direct route linking ribulose-5-phosphate (Ru5P) in the Calvin-Benson (CB) cycle with DXP in the MEP pathway in a cyanobacterium to increase the terpenoid yield from CO2 and bypass the DXS-targeted regulations. By employing the adaptive laboratory evolution, we identified new RibB variants including RibB 90-92del with a high activity of synthesizing DXP from Ru5P. These RibB variants were introduced into Synechococcus elongatus, resulting in the significantly increased photosynthetic production of isopentenol. The 13C tracer experiments demonstrated a direct carbon flow from Ru5P in the CB cycle to the MEP pathway; thus, this direct route was denoted as the Ru5P shunt. The strain harboring the Ru5P shunt produced 105.2 mg L-1 of isopentenol with an average rate of 17.5 mg L-1 d-1 under continuous light conditions, which is higher than those ever reported for five-carbon alcohol production by photoautotrophic microorganisms. Utilization of the Ru5P shunt in cyanobacterial cells also improved the pinene production, which demonstrates that this shunt can be used to enhance the photosynthetic production of diverse terpenoids.

Electrocatalytic nitrate reduction to ammonia via amorphous cobalt boride.

Electrocatalytic nitrate reduction reaction (NitRR) is an energy-saving and environmentally benign approach to synthesizing ammonia under ambient conditions. However, the development of noble metal-free catalysts with high activity and selectivity is still a significant challenge. In this study, uniformly dispersed amorphous CoBx nanoparticles supported on carbon paper were synthesized VIA a simple wet chemical reduction method. As an efficient nitrate reduction electrocatalyst, CoBx exhibited a maximum faradaic efficiency of 94.00 ± 1.67% and a yield rate of up to 0.787 ± 0.028 mmol h-1 cm-2 for ammonia production. The enhanced NitRR performance could be attributed to a partial electron transfer from B to Co, which is necessary for optimizing the adsorption energies of the reaction intermediates and facilitating electron transport. Thus, selective and cost-effective electroreduction of nitrates to ammonia can be achieved using CoBx nanoparticles.

Iron carbonate hydroxide templated binary metal-organic frameworks for highly efficient electrochemical water oxidation.

Metal-organic frameworks (MOFs) are promising catalysts for electrochemical reactions. Herein, self-supported NiFe-MOF nanoplates grown on Ni foam (NF) were prepared with iron carbonate hydroxide nanosheets (FeCH NSs) as a semisacrificial template and evaluated for the electrocatalytic oxygen evolution reaction (OER). In this approach, the porous FeCH NSs not only serve as the iron source of NiFe-MOF, but also slow down the leaching of Ni ions from the substrate, thus playing a unique role in regulating the morphology of NiFe-MOF with reduced thickness and sizes, enabling rapid electron transfer and mass transport. The resultant NiFe-MOF/FeCH-NF electrode showed higher activity than FeCH template-free electrodes and superior OER performance over other MOF based binder-free OER electrodes. A current density of 10 mA cm-2 was obtained at a low overpotential of 200 mV with excellent durability in alkaline solution. Raman and TEM measurements reveal the partial transformation of NiFe-MOF to hydroxide during water oxidation.