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Ethylene is an important feedstock in the chemical industry, but currently requires production from fossil resources. The electrocatalytic oxidative decarboxylation of succinic acid offers in principle an environmentally friendly route to generate ethylene. Here, a detailed investigation of the role of different carbon electrode materials and characteristics revealed that a flat electrode surface and high ordering of the carbon material are conducive for the reaction. A range of electrochemical and spectroscopic approaches such as Koutecky-Levich analysis, rotating ring-disk electrode (RRDE) studies, and Tafel analysis as well as quantum chemical calculations, electron paramagnetic resonance (EPR), and in situ infrared (IR) spectroscopy generated further insights into the mechanism of the overall process. A distinct reaction intermediate was detected, and the decarboxylation onset potential was determined to be 2.2-2.3 V versus the reversible hydrogen electrode (RHE). Following the mechanistic studies and electrode optimization, a two-step bio-electrochemical process was established for ethylene production using succinic acid sourced from food waste. The initial step of this integrated process involves microbial hydrolysis/fermentation of food waste into aqueous solutions containing succinic acid (0.3 M; 3.75 mmol per g bakery waste). The second step is the electro-oxidation of the obtained intermediate succinic acid to ethylene using a flow setup at room temperature, with a productivity of 0.4-1 μmol ethylene cmelectrode -2 h-1. This approach provides an alternative strategy to produce ethylene from food waste under ambient conditions using renewable energy.

Two series of novel carbon nitride photocatalysts, Rho‐CN (“rhodizonate‐doped carbon nitride”) and Rho‐CN‐TC (Rho‐CN treated in potassium thiocyanate melt), are synthesized in a multistep fashion via copolymerization of cyanamide with potassium rhodizonate. The formed ionic carbon nitrides are composed of poly(triazine imide) (PTI/Li+Cl−) and potassium poly(heptazine imide) (K‐PHI) phases and provide a broad absorption range up to 800 nm. The photocatalysts are characterized by several techniques (including diffuse reflectance ultraviolet–visible, powder X‐ray diffraction, Fourier transform infrared, scanning electron microscopy, and electrochemical methods) and studied in a series of photocatalytic reactions, including red light‐promoted benzylamine oxidation, dual photoredox/nickel C–N cross‐coupling, and hydrogen peroxide evolution. The optimal ratio of rhodizonate dopant in its mixture with cyanamide is found to be 0.5 mol%. The performance of the newly synthesized materials is comparable to the activities of the benchmark catalysts K‐PHI and CN‐OA‐m (defective poly(heptazine imide) doped with oxamide), while not requiring more expensive nitrogen sources for preparation, like 5‐aminotetrazole, or multiple oven cycles.

The production of clean fuels and chemicals from waste feedstocks is an appealing approach towards creating a circular economy. However, waste photoreforming commonly employs particulate photocatalysts, which display low product yields, selectivity, and reusability. Here, a perovskite‐based photoelectrochemical (PEC) device is reported, which produces H2 fuel and simultaneously reforms waste substrates. A novel Cu30Pd70 oxidation catalyst is integrated in the PEC device to generate value‐added products using simulated solar light, achieving 60–90% product selectivity and ≈70–130 µmol cm−2 h−1 product formation rates, which corresponds to 102–104 times higher activity than conventional photoreforming systems. The single‐light absorber device offers versatility in terms of substrate scope, sustaining unassisted photocurrents of 4–9 mA cm−2 for plastic, biomass, and glycerol conversion, in either a two‐compartment or integrated “artificial leaf” configuration. These configurations enable an effective reforming of non‐transparent waste streams and facile device retrieval from the reaction mixture. Accordingly, the presented PEC platform provides a proof‐of‐concept alternative towards photoreforming, approaching more closely the performance and versatility required for commercially viable waste utilization.

The synthesis of fuels using sunlight offers a promising sustainable solution for chemical energy storage, but inefficient utilization of the solar spectrum limits its commercial viability. Apart from fundamental improvements to (photo)catalyst materials, solar fuel production systems can also be designed to improve solar energy utilization by integrating complementary technologies that more efficiently utilize the solar spectrum. Here we review recent progress on emerging complementary approaches to better modify, enhance or distribute solar energy for sunlight-to-fuel conversion, including advanced light management, integrated thermal approaches and solar concentrators. These strategies can improve the efficiency and production rates of existing photo(electro)chemical systems and, therefore, the overall economics of solar fuel production. More broadly, the approaches highlight the necessary collaboration between materials science and engineering to help drive the adoption of a sustainable energy economy using existing technologies.

The clean conversion of carbon dioxide and water to a single multicarbon product and O2 using sunlight via photocatalysis without the assistance of organic additives or electricity remains an unresolved challenge. Here we report a bio-abiotic hybrid system with the non-photosynthetic, CO2-fixing acetogenic bacterium Sporomusa ovata grown on a scalable and cost-effective photocatalyst sheet consisting of a pair of particulate semiconductors (La and Rh co-doped SrTiO3 (SrTiO3:La,Rh) and Mo-doped BiVO4 (BiVO4:Mo)). The biohybrid effectively produces acetate (CH3COO–) and oxygen (O2) using only sunlight, CO2 and H2O, achieving a solar-to-acetate conversion efficiency of 0.7% at ambient conditions (298 K, 1 atm). The photocatalyst sheet oxidizes water to O2 and provides electrons and hydrogen (H2) to S. ovata for the selective synthesis of CH3COO– from CO2. To demonstrate utility in a closed carbon cycle, the solar-generated acetate was used directly as feedstock in a bioelectrochemical system for electricity generation. These semi-biological approaches thus offer a promising strategy for sustainably and cleanly fixing CO2 and closing the carbon cycle. [Figure not available: see fulltext.]

The production of clean fuels and chemicals from waste feedstocks is an appealing approach towards creating a circular economy. However, waste photoreforming commonly employs particulate photocatalysts, which display low product yields, selectivity, and reusability. Here, a perovskite‐based photoelectrochemical (PEC) device is reported, which produces H2 fuel and simultaneously reforms waste substrates. A novel Cu30Pd70 oxidation catalyst is integrated in the PEC device to generate value‐added products using simulated solar light, achieving 60–90% product selectivity and ≈70–130 µmol cm−2 h−1 product formation rates, which corresponds to 102–104 times higher activity than conventional photoreforming systems. The single‐light absorber device offers versatility in terms of substrate scope, sustaining unassisted photocurrents of 4–9 mA cm−2 for plastic, biomass, and glycerol conversion, in either a two‐compartment or integrated “artificial leaf” configuration. These configurations enable an effective reforming of non‐transparent waste streams and facile device retrieval from the reaction mixture. Accordingly, the presented PEC platform provides a proof‐of‐concept alternative towards photoreforming, approaching more closely the performance and versatility required for commercially viable waste utilization.

The design and synthesis of faceted nanoparticles with a controlled composition is of enormous importance to modern catalyst engineering. Faceted FePt-Fe3O4 dumbbell nanoparticles have been prepared by a simple, one-pot technique that avoids the need for expensive additives or preformed seeds. The faceted product consists of an FePt octopod and a cubic Fe3O4 lobe, of mean diameter 13.6 and 14.9 nm, respectively. The mass normalized activity for electrocatalytic oxygen reduction shows that this new structure-type outperforms related catalysts in alkaline media. This work illustrates the power of morphology control and tailoring crystal facet abundance at the nanoparticle surface for enhancing catalytic performance.

The design and synthesis of faceted nanoparticles with a controlled composition is of enormous importance to modern catalyst engineering. Faceted FePt-Fe3O4 dumbbell nanoparticles have been prepared by a simple, one-pot technique that avoids the need for expensive additives or preformed seeds. The faceted product consists of an FePt octopod and a cubic Fe3O4 lobe, of mean diameter 13.6 and 14.9 nm, respectively. The mass normalized activity for electrocatalytic oxygen reduction shows that this new structure-type outperforms related catalysts in alkaline media. This work illustrates the power of morphology control and tailoring crystal facet abundance at the nanoparticle surface for enhancing catalytic performance.

can be found here: https://doi.org/10.1021/acscatal.8b01969

The selective reduction of CO 2 with inexpensive solar-driven photoelectrochemical devices is a contemporary challenge in the quest for renewable fuel production. Here, we report a molecular catalyst-based photocathode assembled from precious-metal-free components that is active towards aqueous CO 2 reduction. The reported photocathode is based on a phosphonated cobalt bis(terpyridine) catalyst that is interfaced via a mesoporous TiO 2 scaffold with a light-harvesting p-type silicon electrode. The hybrid photoelectrode reduces CO 2 to CO in both organic-water and purely aqueous conditions, achieving a turnover number of ~330 and maintaining stable activity for more than one day. Critically, in-depth electrochemical as well as in situ resonance Raman and infrared spectroelectrochemical investigations alluded to a catalytic mechanism that differs to that reported for the soluble metal bis(terpyridine) catalyst as the consequence of the immobilization. In addition, it further unlocks an earlier catalytic onset and better electrocatalytic performance while enabling aqueous CO 2 reduction with the reported photocathode.

The selective reduction of CO 2 with inexpensive solar-driven photoelectrochemical devices is a contemporary challenge in the quest for renewable fuel production. Here, we report a molecular catalyst-based photocathode assembled from precious-metal-free components that is active towards aqueous CO 2 reduction. The reported photocathode is based on a phosphonated cobalt bis(terpyridine) catalyst that is interfaced via a mesoporous TiO 2 scaffold with a light-harvesting p-type silicon electrode. The hybrid photoelectrode reduces CO 2 to CO in both organic-water and purely aqueous conditions, achieving a turnover number of ~330 and maintaining stable activity for more than one day. Critically, in-depth electrochemical as well as in situ resonance Raman and infrared spectroelectrochemical investigations alluded to a catalytic mechanism that differs to that reported for the soluble metal bis(terpyridine) catalyst as the consequence of the immobilization. In addition, it further unlocks an earlier catalytic onset and better electrocatalytic performance while enabling aqueous CO 2 reduction with the reported photocathode.

Thermal or photochemical metal-centered cycloaddition reactions of azidocobaltocenium hexafluoridophosphate or azidoferrocene with (cyclooctadiene)(cyclopentadienyl)cobalt- (I) afforded the first metallocenyl-substituted tetrazene cyclopentadienyl cobalt complexes together with azocobaltocenium or azoferrocene as side products. The trimetallic CpCo compounds are highly conjugated, colored, and redox-active metallo-aromatic compounds, as shown by their spectroscopic, structural, and electrochemical properties. The CpCo-tetrazenido complex with two terminally appended cobaltocene units catalyzes electrochemical proton reduction from acetic acid at a mild overpotential (0.35 V). Replacing cobaltocene with ferrocene moieties rendered the complex inactive toward catalysis., Austrian Science Fund (FWF), Christian Doppler Association (Austrian Federal Ministry for Digital and Economic Affairs, the National Foundation for Research, Technology and Development), OMV, Marie Skłodowska-Curie, Deutsche Forschungsgemeinschaft

Plastic waste is recycled to hydrogen using sunlight and a CdS photocatalyst.

Strong interest exists in the development of organic-inorganic lead halide perovskite photovoltaics and of photoelectrochemical (PEC) tandem absorber systems for solar fuel production. However, their scalability and durability have long been limiting factors. In this work, we reveal how both fields can be seamlessly merged together, to obtain scalable, bias-free solar water splitting tandem devices. For this purpose, state-of-the-art cesium formamidinium methylammonium (CsFAMA) triple cation mixed halide perovskite photovoltaic cells with a nickel oxide (NiOx) hole transport layer are employed to produce Field’s metal-epoxy encapsulated photocathodes. Their stability (up to 7h), photocurrent density (–12.1 0.3mAcm 2 at 0V vs.RHE) and reproducibility enables a matching combination with robust BiVO4 photoanodes, resulting in 0.25cm2 PEC tandems with an excellent stability of up to 20h and a bias-free solar-to-hydrogen efficiency of 0.35 0.14%. The high reliability of the fabrication procedures allows scaling of the devices up to 10cm2, with a slight decrease in bias-free photocurrent density from 0.39 0.15mAcm 2 to 0.23 0.10mAcm 2 due to an increasing series resistance. To characterise these devices, a versatile 3D-printed PEC cell was also developed. The modular PEC cell represents an affordable alternative to existing designs and can be easily adjusted for a broad range of samples. Overall, these findings shed further light on the factors required to bring both perovskite photovoltaics and photoelectrocatalysis into large-scale applications, revealing some key aspects for device fabrication, operation and implementation.

Photoreforming of lignocellulose is a promising approach toward sustainable H2 generation, but this kinetically challenging reaction currently requires UV-absorbing or toxic light absorbers under harsh conditions. Here, we report a cyanamide-functionalized carbon nitride, NCNCNx, which shows enhanced performance upon ultra-sonication. This activated NCNCNx allows for the visible-light driven conversion of purified and raw lignocellulose samples into H2 in the presence of various proton reduction co-catalysts. The reported room-temperature photoreforming process operates under benign aqueous conditions (pH ~2-15) without the need for toxic components.

Plastic waste is recycled to hydrogen using sunlight and a CdS photocatalyst.

Strong interest exists in the development of organic-inorganic lead halide perovskite photovoltaics and of photoelectrochemical (PEC) tandem absorber systems for solar fuel production. However, their scalability and durability have long been limiting factors. In this work, we reveal how both fields can be seamlessly merged together, to obtain scalable, bias-free solar water splitting tandem devices. For this purpose, state-of-the-art cesium formamidinium methylammonium (CsFAMA) triple cation mixed halide perovskite photovoltaic cells with a nickel oxide (NiOx) hole transport layer are employed to produce Field’s metal-epoxy encapsulated photocathodes. Their stability (up to 7h), photocurrent density (–12.1 0.3mAcm 2 at 0V vs.RHE) and reproducibility enables a matching combination with robust BiVO4 photoanodes, resulting in 0.25cm2 PEC tandems with an excellent stability of up to 20h and a bias-free solar-to-hydrogen efficiency of 0.35 0.14%. The high reliability of the fabrication procedures allows scaling of the devices up to 10cm2, with a slight decrease in bias-free photocurrent density from 0.39 0.15mAcm 2 to 0.23 0.10mAcm 2 due to an increasing series resistance. To characterise these devices, a versatile 3D-printed PEC cell was also developed. The modular PEC cell represents an affordable alternative to existing designs and can be easily adjusted for a broad range of samples. Overall, these findings shed further light on the factors required to bring both perovskite photovoltaics and photoelectrocatalysis into large-scale applications, revealing some key aspects for device fabrication, operation and implementation.

Visible-light driven H₂ evolution in water is achieved using catechol-photosensitised TiO₂ nanoparticles with a molecular nickel catalyst. Layer-by-layer immobilisation of catechol-TiO₂ onto tin-doped indium oxide electrodes generates photocathodic currents in the presence of an electron acceptor. This approach represents a new strategy for controlling photocurrent direction in dye-sensitised photoelectrochemical applications.

A series of diketopyrrolopyrrole (DPP) dyes with a terminal phosphonic acid group for attachment to metal oxide surfaces were synthesised and the effect of side chain modification on their properties investigated. The organic photosensitisers feature strong visible light absorption ($\lambda$ = 400 to 575 nm) and electrochemical and fluorescence studies revealed that the excited state of all dyes provides sufficient driving force for electron injection into the TiO$_2$ conduction band. The performance of the DPP chromophores attached to TiO$_2$ nanoparticles for photocatalytic H$_2$ evolution with co-immobilised molecular Co and Ni catalysts was subsequently studied, resulting in solar fuel generation with a dye-sensitised semiconductor nanoparticle system suspended in water without precious metal components. The performance of the DPP dyes in photocatalysis did not only depend on electronic parameters, but also on properties of the side chain such as polarity, steric hinderance and hydrophobicity as well as the specific experimental conditions and the nature of the sacrificial electron donor. In an aqueous pH 4.5 ascorbic acid solution with a phosphonated DuBois-type Ni catalyst, a DPP-based turnover number (TON$_{DPP}$) of up to 205 was obtained during UV-free simulated solar light irradiation (100 mW cm$^{-2}$ , AM 1.5G, $\lambda$ > 420 nm) after 1 day. DPP-sensitised TiO$_2$ nanoparticles were also successfully used in combination with a hydrogenase or platinum instead of the synthetic H$_2$ evolution catalysts and the platinum-based system achieved a TON$_{DPP}$ of up to 2660, which significantly outperforms an analogous system using a phosphonated Ru tris(bipyridine) dye (TON$_{Ru}$ = 431). Finally, transient absorption spectroscopy was performed to study interfacial recombination and dye regeneration kinetics revealing that the different performances of the DPP dyes are most likely dictated by the different regeneration efficiencies of the oxidised chromophores.

A series of diketopyrrolopyrrole (DPP) dyes with a terminal phosphonic acid group for attachment to metal oxide surfaces were synthesised and the effect of side chain modification on their properties investigated. The organic photosensitisers feature strong visible light absorption (λ = 400 to 575 nm) and electrochemical and fluorescence studies revealed that the excited state of all dyes provides sufficient driving force for electron injection into the TiO2 conduction band. The performance of the DPP chromophores attached to TiO2 nanoparticles for photocatalytic H2 evolution with co-immobilised molecular Co and Ni catalysts was subsequently studied, resulting in solar fuel generation with a dye-sensitised semiconductor nanoparticle system suspended in water without precious metal components. The performance of the DPP dyes in photocatalysis did not only depend on electronic parameters, but also on properties of the side chain such as polarity, steric hinderance and hydrophobicity as well as the specific experimental conditions and the nature of the sacrificial electron donor. In an aqueous pH 4.5 ascorbic acid solution with a phosphonated DuBois-type Ni catalyst, a DPP-based turnover number (TONDPP) of up to 205 was obtained during UV-free simulated solar light irradiation (100 mW cm−2, AM 1.5G, λ > 420 nm) after 1 day. DPP-sensitised TiO2 nanoparticles were also successfully used in combination with a hydrogenase or platinum instead of the synthetic H2 evolution catalysts and the platinum-based system achieved a TONDPP of up to 2660, which significantly outperforms an analogous system using a phosphonated Ru tris(bipyridine) dye (TONRu = 431). Finally, transient absorption spectroscopy was performed to study interfacial recombination and dye regeneration kinetics revealing that the different performances of the DPP dyes are most likely dictated by the different regeneration efficiencies of the oxidised chromophores.

Attaching the phosphonated molecular catalyst [Re$^{I}$Br(bpy)(CO)₃]⁰ to the wide-band gap semiconductor TiO₂ strongly enhances the rate of visible-light driven reduction of CO₂ to CO in dimethyl formamide (DMF) with triethanolamine (TEOA) as sacrificial electron donor. Herein, we show by transient mid-IR spectroscopy that the mechanism of catalyst photoreduction is initiated by ultrafast electron injection into TiO₂, followed by rapid (ps-ns) and sequential two-electron oxidation of TEOA that is coordinated to the Re center. The injected electrons can be stored in the conduction band (CB) of TiO₂ on a ms-s time scale, and we propose they lead to further reduction of the Re-catalyst and completion of the catalytic cycle. Thus, the excited Re catalyst gives away one electron and would eventually get three electrons back. The function of an electron reservoir would represent a role for TiO₂ in photo-catalytic CO₂ reduction that has previously not been considered. We propose that the increase in photocatalytic activity upon heterogenisation of the catalyst to TiO₂ is due to the slow charge recombination and the high oxidative power of the ReII species after electron injection, as compared to the excited MLCT state of the unbound Re catalyst or when immobilized on ZrO₂, which results in a more efficient reaction with TEOA.

Oxygenic photosynthesis is the primary solar energy-conversion process that supports much of life on Earth. It is initiated by photosystem II (PSII), an enzyme that extracts electrons from H2O and feeds them into an electron-transport chain to result in chemical synthesis using the input of solar energy. PSII can be immobilized onto electrodes for photoelectrochemical studies, in which electrons photogenerated from PSII are harnessed for enzyme characterization, and to drive fuel-forming reactions by electrochemically coupling the PSII to a suitable (bio)catalyst. Research in PSII photoelectrochemistry has recently made substantial strides in electrode design and unravelling charge-transfer pathways at the bio–material interface. In turn, these efforts have opened up possibilities in the field of bio-photoelectrochemistry, expanding the range of biocatalysts that can be systematically interrogated, including biofilms of whole photosynthetic cells. Furthermore, these studies have accelerated the development of semi-artificial photosynthesis to afford autonomous, solar-driven, fuel-forming biohybrid devices. This Review summarizes the latest advancements in PSII photoelectrochemistry with respect to electrode design and understanding of the bio-material interface, on both the protein and cellular level. We also discuss the role of biological photosynthetic systems in present and future semi-artificial photosynthesis.

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 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 route to exploit the O2 reduction reaction to inhibit deactivation, rather than catalysis, offering a strategy to stabilize photocatalysts that suffer from similar degradation reactions.

Harvesting solar energy to convert CO2 into chemical fuels is a promising technology to curtail the growing atmospheric CO2 levels and alleviate the global dependence on fossil fuels; however, the assembly of efficient and robust systems for the selective photoconversion of CO2 without sacrificial reagents and external bias remains a challenge. Here we present a photocatalyst sheet that converts CO2 and H2O into formate and O2 as a potentially scalable technology for CO2 utilization. This technology integrates lanthanum- and rhodium-doped SrTiO3 (SrTiO3:La,Rh) and molybdenum-doped BiVO4 (BiVO4:Mo) light absorbers modified by phosphonated Co(ii) bis(terpyridine) and RuO2 catalysts onto a gold layer. The monolithic device provides a solar-to-formate conversion efficiency of 0.08 ± 0.01% with a selectivity for formate of 97 ± 3%. As the device operates wirelessly and uses water as an electron donor, it offers a versatile strategy toward scalable and sustainable CO2 reduction using molecular-based hybrid photocatalysts.

Natural photosynthesis stores sunlight in chemical energy carriers, but it has not evolved for the efficient synthesis of fuels, such as H2. Semi-artificial photosynthesis combines the strengths of natural photosynthesis with synthetic chemistry and materials science to develop model systems that overcome Nature’s limitations, such as low-yielding metabolic pathways and non-complementary light absorption by Photosystem (PS) I and II. Here, we report a bias-free semi-artificial tandem platform that wires PSII to hydrogenase for overall water splitting. This photoelectrochemical cell integrated the red and blue light-absorber PSII with a green light-absorbing diketopyrrolopyrrole dye-sensitised TiO2 photoanode enabling complementary panchromatic solar light absorption. Effective electronic communication at the enzyme-material interface was engineered using an Os complex-modified redox polymer on a hierarchically-structured TiO2. This system provides a design protocol for bias-free semi-artificial Z-schemes in vitro and provides an extended toolbox of biotic and abiotic components to re-engineer photosynthetic pathways.

Photocatalytic chemical synthesis by coupling abiotic photosensitizers to purified enzymes provides an effective way to overcome the low conversion efficiencies of natural photosynthesis while exploiting the high catalytic rates and selectivity of enzymes as renewable, earth-abundant electrocatalysts. However, the selective synthesis of multiple products requires more versatile approaches and should avoid the time-consuming and costly processes of enzyme purification. Here we demonstrate a cell-based strategy supporting light-driven H2 evolution or the hydrogenation of C═C and C═O bonds in a nonphotosynthetic microorganism. Methylviologen shuttles photoenergized electrons from water-soluble photosensitizers to enzymes that catalyze H2 evolution and the reduction of fumarate, pyruvate, and CO2 in Shewanella oneidensis. The predominant reaction is selected by the experimental conditions, and the results allow rational development of cell-based strategies to harness nature’s intrinsic catalytic diversity for selective light-driven synthesis of a wide range of products.

The development of high-performance electrocatalytic systems for the controlled reduction of CO2 to value-added chemicals is a key goal in emerging renewable energy technologies. The lack of selective and scalable catalysts in aqueous solution currently hampers the implementation of such a process. Here, the assembly of a [MnBr(2,2′-bipyridine)(CO)3] complex anchored to a carbon nanotube electrode via a pyrene unit is reported. Immobilization of the molecular catalyst allows electrocatalytic reduction of CO2 under fully aqueous conditions with a catalytic onset overpotential of η = 360 mV, and controlled potential electrolysis generated more than 1000 turnovers at η = 550 mV. The product selectivity can be tuned by alteration of the catalyst loading on the nanotube surface. CO was observed as the main product at high catalyst loadings, whereas formate was the dominant CO2 reduction product at low catalyst loadings. Using UV–vis and surface-sensitive IR spectroelectrochemical techniques, two different intermediates were identified as responsible for the change in selectivity of the heterogenized Mn catalyst. The formation of a dimeric Mn0 species at higher surface loading was shown to preferentially lead to CO formation, whereas at lower surface loading the electrochemical generation of a monomeric Mn-hydride is suggested to greatly enhance the production of formate. These results emphasize the advantages of integrating molecular catalysts onto electrode surfaces for enhancing catalytic activity while allowing excellent control and a deeper understanding of the catalytic mechanisms.

See here: https://doi.org/10.1002/anie.201814419

Sunlight-driven CO2 reduction is increasingly considered as a promising approach to contribute toward a carbon-neutral fuel cycle, but most photocatalyst systems are currently studied individually under batch conditions with manual, labor-intensive analytical procedures. Here, we present the advantages of a continuous-flow setup to study photocatalytic CO2 to CO reduction systems, which also benefits from aspects of automation (using programmed in-line gas quantification of multiple samples in parallel). The capabilities of the methodology are demonstrated using a state-of-the-art light absorber platform based on ZnSe quantum dots (QDs) in combination with a series of molecular co-catalysts based on Ni and Co for visible-light-driven CO2 reduction in aqueous ascorbate solution. A newly synthesized Co-tetraphenylporphyrin featuring three sulfonate groups and one amine group (Co(tppS3N1)) is identified to exhibit a benchmark photocatalytic activity (18.6 μmol of CO, 79.7 mmol of CO gZnSe–1, TONCo (CO) of 619, external quantum efficiency (EQE) >5%). The utility of our methodology is further shown by applying the setup to study the photocatalyst systems under lower light intensities, low CO2 concentration, and aerobic conditions, which impact the photocatalytic activity and selectivity. Overall, this work reports an improved methodology for studying photocatalytic CO2 reduction alongside advancing the understanding of QD molecular co-catalyst hybrids using ZnSe QDs as a versatile light absorber based on earth-abundant components that operate under fully aqueous conditions.

Immobilized first-row transition metal complexes are potential low-cost electrocatalysts for selective CO2 conversion in the production of renewable fuels. Mechanistic understanding of their function is vital for the development of next-generation catalysts, although the poor surface sensitivity of many techniques makes this challenging. Here, a nickel bis(terpyridine) complex is introduced as a CO2 reduction electrocatalyst in a unique electrode geometry, sandwiched by thiol-anchoring moieties between two gold surfaces. Gap-plasmon-assisted surface-enhanced Raman scattering spectroscopy coupled with density functional theory calculations reveals that the nature of the anchoring group plays a pivotal role in the catalytic mechanism. Our in situ spectro-electrochemical measurement enables the detection of as few as eight molecules undergoing redox transformations in individual plasmonic hotspots, together with the calibration of electrical fields via vibrational Stark effects. This advance allows rapid exploration of non-resonant redox reactions at the few-molecule level and provides scope for future mechanistic studies of single molecules. Tracking immobilized molecular complexes under in situ conditions is vital for the development of next-generation catalysts, although the poor surface sensitivity of many techniques makes this challenging. Now, the role of the anchoring group in a nickel bis(terpyridine) complex has been elucidated by in situ gap-plasmon-assisted SERS coupled with DFT calculations.