Aerobic Conditions Enhance the Photocatalytic Stability of CdS/CdOx Quantum Dots.

Photocatalytic H2 production through water splitting represents an attractive route to generate a renewable fuel. These systems are typically limited to anaerobic conditions due to the inhibiting effects of O2 . Here, we report that sacrificial H2 evolution with CdS quantum dots does not necessarily suffer from O2 inhibition and can even be stabilised under aerobic conditions. The introduction of O2 prevents a key inactivation pathway of CdS (over-accumulation of metallic Cd and particle agglomeration) and thereby affords particles with higher stability. These findings represent a possibility to exploit the O2 reduction reaction to inhibit deactivation, rather than catalysis, offering a strategy to stabilise photocatalysts that suffer from similar degradation reactions.

A Poly(cobaloxime)/Carbon Nanotube Electrode: Freestanding Buckypaper with Polymer-Enhanced H2-Evolution Performance.

A freestanding H2-evolution electrode consisting of a copolymer-embedded cobaloxime integrated into a multiwall carbon nanotube matrix by π-π interactions is reported. This electrode is straightforward to assemble and displays high activity towards hydrogen evolution in near-neutral pH solution under inert and aerobic conditions, with a cobalt-based turnover number (TON(Co)) of up to 420. An analogous electrode with a monomeric cobaloxime showed less activity with a TON(Co) of only 80. These results suggest that, in addition to the high surface area of the porous network of the buckypaper, the polymeric scaffold provides a stabilizing environment to the catalyst, leading to further enhancement in catalytic performance. We have therefore established that the use of a multifunctional copolymeric architecture is a viable strategy to enhance the performance of molecular electrocatalysts.

Photocatalytic Formic Acid Conversion on CdS Nanocrystals with Controllable Selectivity for H2 or CO.

Formic acid is considered a promising energy carrier and hydrogen storage material for a carbon-neutral economy. We present an inexpensive system for the selective room-temperature photocatalytic conversion of formic acid into either hydrogen or carbon monoxide. Under visible-light irradiation (λ>420 nm, 1 sun), suspensions of ligand-capped cadmium sulfide nanocrystals in formic acid/sodium formate release up to 116±14 mmol H2 g(cat)(-1) h(-1) with >99% selectivity when combined with a cobalt co-catalyst; the quantum yield at λ=460 nm was 21.2±2.7%. In the absence of capping ligands, suspensions of the same photocatalyst in aqueous sodium formate generate up to 102±13 mmol CO g(cat)(-1) h(-1) with >95% selectivity and 19.7±2.7% quantum yield. H2 and CO production was sustained for more than one week with turnover numbers greater than 6×10(5) and 3×10(6), respectively.

Proton reduction by molecular catalysts in water under demanding atmospheres.

The electrocatalytic proton reduction activity of a Ni bis(diphosphine) (NiP) and a cobaloxime (CoP) catalyst has been studied in water in the presence of the gaseous inhibitors O2 and CO. CoP shows an appreciable tolerance towards O2, but its activity suffers severely in the presence of CO. In contrast, NiP is strongly inhibited by O2, but produces H2 under high CO concentrations.

Reversible interconversion of CO2 and formate by a molybdenum-containing formate dehydrogenase.

CO2 and formate are rapidly, selectively, and efficiently interconverted by tungsten-containing formate dehydrogenases that surpass current synthetic catalysts. However, their mechanism of catalysis is unknown, and no tractable system is available for study. Here, we describe the catalytic properties of the molybdenum-containing formate dehydrogenase H from the model organism Escherichia coli (EcFDH-H). We use protein film voltammetry to demonstrate that EcFDH-H is a highly active, reversible electrocatalyst. In each voltammogram a single point of zero net current denotes the CO2 reduction potential that varies with pH according to the Nernst equation. By quantifying formate production we show that electrocatalytic CO2 reduction is specific. Our results reveal the capabilities of a Mo-containing catalyst for reversible CO2 reduction and establish EcFDH-H as an attractive model system for mechanistic investigations and a template for the development of synthetic catalysts.

Development and understanding of cobaloxime activity through electrochemical molecular catalyst screening.

Electrochemical molecular catalyst screening (EMoCS) has been developed. This technique allows fast analysis and identification of homogeneous catalytic species through tandem catalyst assembly and electrochemistry. EMoCS has been used to study molecular proton reduction catalysts made from earth abundant materials to improve their viability for water splitting systems. The efficacy of EMoCS is proven through investigation of cobaloxime proton reduction activity with respect to the axial ligand in aqueous solution. Over 20 axial ligands were analysed, allowing rapid identification of the most active catalysts. Structure-activity relationships showed that more electron donating pyridine ligands result in enhanced catalytic currents due to the formation of a more basic Co-H species. The EMoCS results were validated by isolating and assaying the most electroactive cobaloximes identified during screening. The most active catalyst, [Co(III)Cl(dimethyl glyoximato)2(4-methoxypyridine)], showed high electro- and photoactivity in both anaerobic and aerobic conditions in pH neutral aqueous solution.