Dual-nodes bridged cobalt-modified Keggin-type polyoxometalate-based chains for highly efficient CO2 photoconversion

The design of efficient catalysts for photocatalytic CO2 conversion is of great importance for the sustainable development of society. Herein, three polyoxometalate (POM)-based crystalline materials were formulated prepared by substituting transition metals and adjusting solvent acidity with 2-(2-pyridyl) benzimidazole (pyim) as the light-trapping ligand, namely {[SiW12O40][Co(pyim)2]2}·2C2H5OH (SiW12Co2), {[SiW12O40][Ni(pyim)2]2}·2C2H5OH (SiW12Ni2), and {[SiW12O40][Mn(pyim)2]2}·2C2H5OH (SiW12Mn2). X-ray crystallography diffraction analysis indicates that the three complexes exhibit isostructural properties, and form a stable one-dimensional chain structure stabilized by two [M(pyim)2]22+ (M = Co, Ni, and Mn) fragments serving as dual-nodes to the adjacent SiW12 units. A comprehensive analysis of the structural characterization and photocatalytic CO2 reduction properties is presented. Under light irradiation, SiW12Co2 exhibited a remarkable CO generation rate of 10 733 μmol g-1 h-1 with a turnover number of 328, outperforming most of the reported heterogeneous POM-based photocatalysts. Besides, cycling tests revealed that SiW12Co2 is an efficient and stable photocatalyst with great recyclability for at least four successive runs. This study proves that the successful incorporation of diverse transition metals into the POM anion could facilitate the development of highly efficient photocatalysts for the CO2RR.

Nanoscale covalent organic frameworks for enhanced photocatalytic hydrogen production

Nanosizing confers unique functions in materials such as graphene and quantum dots. Here, we present two nanoscale-covalent organic frameworks (nano-COFs) that exhibit exceptionally high activity for photocatalytic hydrogen production that results from their size and morphology. Compared to bulk analogues, the downsizing of COFs crystals using surfactants provides greatly improved water dispersibility and light-harvesting properties. One of these nano-COFs shows a hydrogen evolution rate of 392.0 mmol g-1 h-1 (33.3 μmol h-1), which is one of the highest mass-normalized rates reported for a COF or any other organic photocatalysts. A reverse concentration-dependent photocatalytic phenomenon is observed, whereby a higher photocatalytic activity is found at a lower catalyst concentration. These materials also show a molecule-like excitonic nature, as studied by photoluminescence and transient absorption spectroscopy, which is again a function of their nanoscale dimensions. This charts a new path to highly efficient organic photocatalysts for solar fuel production.

Two bimetal-doped (Fe/Co, Mn) polyoxometalate-based hybrid compounds for visible-light-driven CO2 reduction

Two polyoxometalate (POM)-based hybrid compounds have been successfully designed and constructed by the hydrothermal method with molecular formulas [K(H₂O)₂FeII0.33Co_0.67(H₂O)₂(DAPSC)]₂{[FeII0.33Co_0.67(H₂O)(DAPSC)]₂[FeII0.33Co_0.67(H₂O)₄]₂[Na₂FeIII4P₄W₃₂O₁₂₀]}·21.5H₂O (1), and [Na(H₂O)₂FeII0.33Mn_0.67(H₂O)₂(DAPSC)]₂{[FeII0.33Mn_0.67(H₂O)(DAPSC)]₂[FeII0.33Mn_0.67(H₂O)₄]₂[Na₂FeIII4P₄W₃₂O₁₂₀(H₂O)₂]}·24H₂O (2) (DAPSC = 2,6-diacetylpyridine bis-(semicarbazone)), respectively. Structural analysis revealed that 1 and 2 consisted of metal-organic complexes containing DAPSC ligands with dumbbell-type inorganic clusters, iron-cobalt (iron-manganese) and some other ions. By utilizing a combination of strongly reducing {P₂W₁₂} units and bimetal-doped centres the CO₂ photoreduction catalytic capacity of 1 and 2 was improved. Notably, the photocatalytic performance of 1 was much better than that of 2. In CO₂ photoreduction, 1 exhibited CO selectivity as high as 90.8%. Furthermore, for 1, the CO generation rate reached 6885.1 μmol g^-1 h^-1 at 8 h with 3 mg, and its better photocatalytic performance was presumably due to the introduction of cobalt and iron elements to give 1 a more appropriate energy band structure. Further recycling experiments indicated that 1 was a highly efficient CO₂ photoreduction catalyst, which could still possess catalytic activity after several cycles.

More Accurate Method for Evaluating the Activity of Photocatalytic Hydrogen Evolution and Its Reaction Kinetics Equation

Photocatalytic water splitting to hydrogen is a sustainable energy conversion method. However, there is a lack of sufficiently accurate measurement methods for an apparent quantum yield (AQY) and a relative hydrogen production rate (r_H2) at the moment. Thus, a more scientific and reliable evaluation method is highly required to allow the quantitative comparison of photocatalytic activity. Herein, a simplified kinetic model of photocatalytic hydrogen evolution was established, the corresponding photocatalytic kinetic equation was deduced, and a more accurate calculation method is proposed for the AQY and the maximum hydrogen production rate v_H2,max. At the same time, new physical quantities, absorption coefficient k_L and specific activity SA, were proposed to sensitively characterize the catalytic activity. The scientificity and practicality of the proposed model and the physical quantities were systematically verified from the theoretical and experimental levels.

An isolated doughnut-like molybdenum(V) cobalto-phosphate cluster exhibiting excellent photocatalytic performance for carbon dioxide conversion

An isolated doughnut-like molybdenum(V) cobalto-phosphate cluster with the formula (C₁₁NH₁₀)₂{[Co(H₂O)₆]@[H₂₉Co₁₆Mo₁₆(H₂O)₁₆(PO₄)₂₄O₃₆]}(H₂PO₄)·25H₂O has been successfully synthesized by a hydrothermal method. Single crystal X ray diffraction analysis shows that four {Co₄O₆₀} tetramers and eight {Mo₂O₁₀} dimers are linked by oxygen atoms and phosphate groups to construct a doughnut-type structure for [Co@{Co₁₆Mo₁₆}], in which one [Co^II(H₂O)₆]²⁺ octahedron is enclosed. More importantly, [Co@{Co₁₆Mo₁₆}] exhibits promising photocatalytic performance for CO₂ reduction with the CO formation rate of 6764.3 μmol g^-1 h^-1 and the selectivity of 96.89%. In addition, the cycling test indicated that [Co@{Co₁₆Mo₁₆}] can be reused for at least four cycles without significant loss of catalytic activity. The result of this work may provide new insight for the synthesis of highly efficient POM-based photocatalysts for CO₂ reduction.

Figures of Merit for Photocatalysis: Comparison of NiO/La-NaTaO3 and Synechocystis sp. PCC 6803 as a Semiconductor and a Bio-Photocatalyst for Water Splitting

While photocatalysis is considered a promising sustainable technology in the field of heterogeneous catalysis as well as biocatalysis, figures of merit (FOM) for comparing catalytic performance, especially between disciplines, are not well established. Here, photocatalytic water splitting was conducted using a semiconductor (NiO/La-NaTaO3) and a bio-photocatalyst (Synechocystis sp. PCC 6803) in the same setup under similar reaction conditions, eliminating the often ill-defined influence of the setup on the FOMs obtained. Comparing the results enables the critical evaluation of existing FOMs and a quantitative comparison of both photocatalytic systems. A single FOM is insufficient to compare the photocatalysts, instead a combination of multiple FOMs (reaction rate, photocatalytic space time yield and a redefined apparent quantum yield) is superior for assessing a variety of photocatalytic systems.

A Purely Inorganic Quasi-Keggin Polyoxometalate for Photocatalytic Conversion of Carbon Dioxide to Carbon Monoxide

A pure inorganic cluster, H₄₇ Na₂ Co₄ Mo₂₄ (PO₄ )₁₁ O₇₂  ⋅ 15H₂ O (denoted as {Co₄ Mo₂₄ }), has been successfully synthesized by hydrothermal method. Notably, the assembly of a central {Co₂ PO₄ } tetrahedron and four peripheral {Co[P₄ Mo₆ ]} fragments gives rise to a rare "quasi-Keggin" structure of {Co₄ Mo₂₄ }, in which Co linkers continue to bridge adjacent substructures, resulting in the generation of 3D framework with large cavities. Benefitting from the combination of strong reductive {P₄ Mo₆ } units and Co active centers, the photocatalytic system with {Co₄ Mo₂₄ } as heterogeneous catalyst exhibits excellent activity for CO₂ conversion to CO, offering the CO formation rate of 1848.3 μmol g^-1 h^-1 with high selectivity of 97.0 %. Besides, thermogravimetric and X-ray diffraction analysis confirm that {Co₄ Mo₂₄ } can maintain stable during the photocatalytic reaction process.

Organic-inorganic hybrids for CO2 sensing, separation and conversion

Motivated by the air pollution that skyrocketed in numerous regions around the world, great effort was placed on discovering new classes of materials that separate, sense or convert CO₂ in order to minimise impact on human health. However, separation, sensing and conversion are not only closely intertwined due to the ultimate goal of improving human well-being, but also because of similarities in material prerequisites -e.g. affinity to CO₂. Partly inspired by the unrivalled performance of complex natural materials, manifold inorganic-organic hybrids were developed. One of the most important characteristics of hybrids is their design flexibility, which results from the combination of individual constituents with specific functionality. In this review, we discuss commonly used organic, inorganic, and inherently hybrid building blocks for applications in separation, sensing and catalytic conversion and highlight benefits like durability, activity, low-cost and large scale fabrication. Moreover, we address obstacles and potential future developments of hybrid materials. This review should inspire young researchers in chemistry, physics and engineering to identify and overcome interdisciplinary research challenges by performing academic research but also - based on the ever-stricter emission regulations like carbon taxes - through exchanges between industry and science.

Judging the feasibility of TiO2 as photocatalyst for chemical energy conversion by quantitative reactivity determinants

In this study we assess the general applicability of the widely used P25-TiO₂ in gas-phase photocatalytic CO₂ reduction based on experimentally determined reactivity descriptors from classical heterogeneous catalysis (productivity) and photochemistry (apparent quantum yield/AQY). A comparison of the results with reports on the use of P25 for thermodynamically more feasible reactions and our own previous studies on P25-TiO₂ as photocatalyst imply that the catalytic functionality of this material, rather than its properties as photoabsorber, limits its applicability in the heterogeneous photocatalytic CO₂ reduction in the gas phase. The AQY of IrO_x/TiO₂ in overall water splitting in a similar high-purity gas-solid process was four times as high, but still far from commercial viability.

Single-site and nano-confined photocatalysts designed in porous materials for environmental uses and solar fuels

Silica-based micro-, meso-, macro-porous materials offer attractive routes for designing single-site photocatalysts, supporting semiconducting nanoparticles, anchoring light-responsive metal complexes, and encapsulating metal nanoparticles to drive photochemical reactions by taking advantage of their large surface area, controllable pore channels, remarkable transparency to UV/vis and tailorable physicochemical surface characteristics. This review mainly focuses on the fascinating photocatalytic properties of silica-supported Ti catalysts from single-site catalysts to nanoparticles, their surface-chemistry engineering, such as the hydrophobic modification and synthesis of thin films, and the fabrication of nanocatalysts including morphology controlled plasmonic nanostructures with localized surface plasmon resonance. The hybridization of visible-light responsive metal complexes with porous materials for the construction of functional inorganic-organic supramolecular photocatalysts is also included. In addition, the latest progress in the application of MOFs as excellent hosts for designing photocatalytic systems is described.

A spongy nickel-organic CO2 reduction photocatalyst for nearly 100% selective CO production

Solar-driven photocatalytic conversion of CO₂ into fuels has attracted a lot of interest; however, developing active catalysts that can selectively convert CO₂ to fuels with desirable reaction products remains a grand challenge. For instance, complete suppression of the competing H₂ evolution during photocatalytic CO₂-to-CO conversion has not been achieved before. We design and synthesize a spongy nickel-organic heterogeneous photocatalyst via a photochemical route. The catalyst has a crystalline network architecture with a high concentration of defects. It is highly active in converting CO₂ to CO, with a production rate of ~1.6 × 10⁴ μmol hour^-1 g^-1. No measurable H₂ is generated during the reaction, leading to nearly 100% selective CO production over H₂ evolution. When the spongy Ni-organic catalyst is enriched with Rh or Ag nanocrystals, the controlled photocatalytic CO₂ reduction reactions generate formic acid and acetic acid. Achieving such a spongy nickel-organic photocatalyst is a critical step toward practical production of high-value multicarbon fuels using solar energy.

Turning Perspective in Photoelectrocatalytic Cells for Solar Fuels

The development of new devices for the use and storage of solar energy is a key step to enable a new sustainable energy scenario. The route for direct solar-to-chemical energy transformation, especially to produce liquid fuels, represents a necessary element to realize transition from the actual energy infrastructure. Photoelectrocatalytic (PECa) devices for the production of solar fuels are a key element to enable this sustainable scenario. The development of PECa devices and related materials is of increasing scientific and applied interest. This concept paper introduces the need to turn the viewpoint of research in terms of PECa cell design and related materials with respect to mainstream activities in the field of artificial photosynthesis and leaves. As an example of a new possible direction, the concept of electrolyte-less cell design for PECa cells to produce solar fuels by reduction of CO2 is presented. The fundamental and applied development of new materials and electrodes for these cells should proceed fully integrated with PECa cell design and systematic analysis. A new possible approach to develop semiconductors with improved performances by using visible light is also shortly presented.

Photo-induced reduction of biomass-derived 5-hydroxymethylfurfural using graphitic carbon nitride supported metal catalysts

Photo-induced reduction of biomass-derived 5-hydroxymethylfurfural was achieved by using graphitic carbon nitride-supported transition metal catalysts.

A Brown Mesoporous TiO2-x /MCF Composite with an Extremely High Quantum Yield of Solar Energy Photocatalysis for H2 Evolution

A brown mesoporous TiO2-x /MCF composite with a high fluorine dopant concentration (8.01 at%) is synthesized by a vacuum activation method. It exhibits an excellent solar absorption and a record-breaking quantum yield (Φ = 46%) and a high photon-hydrogen energy conversion efficiency (η = 34%,) for solar photocatalytic H2 production, which are all higher than that of the black hydrogen-doped TiO2 (Φ = 35%, η = 24%). The MCFs serve to improve the adsorption of F atoms onto the TiO2 /MCF composite surface, which after the formation of oxygen vacancies by vacuum activation, facilitate the abundant substitution of these vacancies with F atoms. The decrease of recombination sites induced by high-concentration F doping and the synergistic effect between lattice Ti(3+)-F and surface Ti(3+)-F are responsible for the enhanced lifetime of electrons, the observed excellent absorption of solar light, and the photocatalytic production of H2 for these catalysts. The as-prepared F-doped composite is an ideal solar light-driven photocatalyst with great potential for applications ranging from the remediation of environmental pollution to the harnessing of solar energy for H2 production.

Computational Screening of 2D Materials for Photocatalysis

Two-dimensional (2D) materials exhibit a range of extraordinary electronic, optical, and mechanical properties different from their bulk counterparts with potential applications for 2D materials emerging in energy storage and conversion technologies. In this Perspective, we summarize the recent developments in the field of solar water splitting using 2D materials and review a computational screening approach to rapidly and efficiently discover more 2D materials that possess properties suitable for solar water splitting. Computational tools based on density-functional theory can predict the intrinsic properties of potential photocatalyst such as their electronic properties, optical absorbance, and solubility in aqueous solutions. Computational tools enable the exploration of possible routes to enhance the photocatalytic activity of 2D materials by use of mechanical strain, bias potential, doping, and pH. We discuss future research directions and needed method developments for the computational design and optimization of 2D materials for photocatalysis.

Heterogeneous catalysis and the challenges of powering the planet, securing chemicals for civilised life, and clean efficient utilization of renewable feedstocks

This article reviews, first, the prospects, practices and principles of generating solar fuels. It does so with an analysis of recent progress in the light-driven emission of H2 (and other fuels) as well as O2 from water. To place this challenge in perspective, some current practices entailing the use of well-proven solid catalysts developed for fossil-based feedstocks, are described. The massive differences between proven methods of generating fuel and chemicals from non-renewable and from solar radiation are emphasized with the aid of numerous quantitative examples. Whilst it is acknowledged that a key action in reducing the liberation of greenhouse gases (GHG) is to tackle the challenge of decreasing their evolution in power generation and in the production of steel, aluminium and other bulk commodities (metals, alloys, concrete and ceramics), nevertheless much can be done to diminish the emission of CO2 (and to use it as feedstock) through the agency of new, designed solid catalysts and microalgae. Solar-thermal converters are also attractive alternatives, even though they are more likely to be used centrally rather than in small modular units like 'artificial leaves,' some of which are promising for the purposes of generating energy (and perhaps fuel) in a delocalized, modular manner.

The coordination chemistry of organo-hydride donors: new prospects for efficient multi-electron reduction

In biological reduction processes the dihydronicotinamides NAD(P)H often transfer hydride to an unsaturated substrate bound within an enzyme active site. In many cases, metal ions in the active site bind, polarize and thereby activate the substrate to direct attack by hydride from NAD(P)H cofactor. This review looks more widely at the metal coordination chemistry of organic donors of hydride ion--organo-hydrides--such as dihydronicotinamides, other dihydropyridines including Hantzsch's ester and dihydroacridine derivatives, those derived from five-membered heterocycles including the benzimidazolines and benzoxazolines, and all-aliphatic hydride donors such as hexadiene and hexadienyl anion derivatives. The hydride donor properties--hydricities--of organo-hydrides and how these are affected by metal ions are discussed. The coordination chemistry of organo-hydrides is critically surveyed and the use of metal-organo-hydride systems in electrochemically-, photochemically- and chemically-driven reductions of unsaturated organic and inorganic (e.g. carbon dioxide) substrates is highlighted. The sustainable electrocatalytic, photochemical or chemical regeneration of organo-hydrides such as NAD(P)H, including for driving enzyme-catalysed reactions, is summarised and opportunities for development are indicated. Finally, new prospects are identified for metal-organo-hydride systems as catalysts for organic transformations involving 'hydride-borrowing' and for sustainable multi-electron reductions of unsaturated organic and inorganic substrates directly driven by electricity or light or by renewable reductants such as formate/formic acid.

Quantification of photocatalytic oxygenation of human blood

Photocatalytic oxygenation of human blood is an emerging concept based on the principle of photocatalytic splitting of water into oxygen and hydrogen. This communication reports: (i) a design of a photocatalytic cell (PC) that separates the blood from UV (incident) radiation source, (ii) a pH, temperature and flow controlled circuit designed for quantifying the oxygenation of human blood by photocatalysis and (iii) measuring the current efficacy of ITO/TiO2 nano thin films in oxygenating human blood in a dynamic circuit in real time. The average increase in oxygen saturation was around 5% above baseline compared to control (p<0.0005). We believe this is one of the first attempts to quantify photocatalytic oxygenation of human blood under controlled conditions.

Mesoporous TiN microspheres with hierarchical chambers and enhanced visible light-driven hydrogen evolution

Mesoporous titanium nitride (TiN) microspheres with tunable chamber structures were synthesized through NH3 nitridation of organotitania obtained via solvothermal alcoholysis. Owing to high content of organic species in organotitania, which stabilized the original structure and also promoted the nitridation at low temperature, TiN duplicated the solid, yolk-shell, and hollow chambers of organotitania in NH3 nitridation and also formed the mesoporous structure after removing organic species. During visible light-induced photocatalytic water splitting, the yolk-shell TiN demonstrated a higher H2 evolution efficiency than the solid, hollow, and crushed TiN; the yolk-shell TiN obtained through N2 nitridation; and other semiconductor photocatalysts reported so far without the aid of noble metals. This could be attributed mainly to the narrow energy band gap for absorbing visible light, high surface area, and excellent electrical conductivity, which facilitated light harvesting, reactant adsorption, and photoelectron-hole separation to suppress their recombination. Moreover, it could be easily recycled and thus used repetitively.

Photocatalytic reduction of CO2 on TiO2 and other semiconductors

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Semiconductor photocatalysis--mechanistic and synthetic aspects

Preceding work on photoelectrochemistry at semiconductor single-crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron-donor and -acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom-economic C-C and C-N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid-state chemistry, and heterogeneous catalysis.