Photocatalytic Systems For CO2 Reduction Research Articles

Construction of Bi2WO6/g-C3N4/Cu foam as 3D Z-scheme photocatalyst for photocatalytic CO2 reduction

Photocatalytic CO2 reduction to hydrocarbon fuels represents a promising solution to CO2 emission challenges. However, photocatalytic CO2 reduction systems face hurdles in practical utilization for the low efficiency of photocatalysts and challenge in catalyst immobilization. Therefore, a novel monolithic three-dimensional Z-scheme photocatalyst was synthesized by fabricating Bi2WO6/g-C3N4 heterojunction on Cu foam substrate (Bi2WO6/g-C3N4/Cu foam) via thermal polymerization and electrophoresis deposition. A series of characterization results, including X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and N2 adsorption–desorption, illustrate the crystalline properties, pore structure, and morphology of the composite photocatalyst. Consequently, Bi2WO6/g-C3N4/Cu foam achieved CO production yield of 33.84 μmol/g cat under visible light illumination for 6 h and retained 96.2 % of initial photocatalytic activity after 30 h of reaction. UV–Vis diffuse reflection spectra, photoluminescence spectra and photoelectrochemical analysis was involved to reveal the mechanism of the photocatalytic activity of composite photocatalyst.The synergistic effect of the Z-scheme mechanism and the hierarchical macroporous structure improves the light absorption, light-induced electron-hole pair separation, and charge transmission. This catalyst construction strategy provides a viable approach for facilitating practical photocatalytic CO2 reduction.

Directed Electron Delivery from a Pb-Free Halide Perovskite to a Co(II) Molecular Catalyst Boosts CO2 Photoreduction Coupled with Water Oxidation.

The development of high-performance photocatalytic systems for CO2 reduction is appealing to address energy and environmental issues, while it is challenging to avoid using toxic metals and organic sacrificial reagents. We here immobilize a family of cobalt phthalocyanine catalysts on Pb-free halide perovskite Cs2AgBiBr6 nanosheets with delicate control on the anchors of the cobalt catalysts. Among them, the molecular hybrid photocatalyst assembled by carboxyl anchors achieves the optimal performance with an electron consumption rate of 300±13 μmol g-1 h-1 for visible-light-driven CO2-to-CO conversion coupled with water oxidation to O2, over 8 times of the unmodified Cs2AgBiBr6 (36±8 μmol g-1 h-1), also far surpassing the documented systems (<150 μmol g-1 h-1). Besides the improved intrinsic activity, electrochemical, computational, ex-/in situ X-ray photoelectron and X-ray absorption spectroscopic results indicate that the electrons photogenerated at the Bi atoms of Cs2AgBiBr6 can be directionally transferred to the cobalt catalyst via the carboxyl anchors which strongly bind to the Bi atoms, substantially facilitating the interfacial electron transfer kinetics and thereby the photocatalysis.

Directed Electron Delivery from a Pb‐Free Halide Perovskite to a Co(II) Molecular Catalyst Boosts CO2 Photoreduction Coupled with Water Oxidation

AbstractThe development of high‐performance photocatalytic systems for CO2 reduction is appealing to address energy and environmental issues, while it is challenging to avoid using toxic metals and organic sacrificial reagents. We here immobilize a family of cobalt phthalocyanine catalysts on Pb‐free halide perovskite Cs2AgBiBr6 nanosheets with delicate control on the anchors of the cobalt catalysts. Among them, the molecular hybrid photocatalyst assembled by carboxyl anchors achieves the optimal performance with an electron consumption rate of 300±13 μmol g−1 h−1 for visible‐light‐driven CO2‐to‐CO conversion coupled with water oxidation to O2, over 8 times of the unmodified Cs2AgBiBr6 (36±8 μmol g−1 h−1), also far surpassing the documented systems (&lt;150 μmol g−1 h−1). Besides the improved intrinsic activity, electrochemical, computational, ex‐/in situ X‐ray photoelectron and X‐ray absorption spectroscopic results indicate that the electrons photogenerated at the Bi atoms of Cs2AgBiBr6 can be directionally transferred to the cobalt catalyst via the carboxyl anchors which strongly bind to the Bi atoms, substantially facilitating the interfacial electron transfer kinetics and thereby the photocatalysis.

Hetero-Motif Molecular Junction Photocatalysts: A New Frontier in Artificial Photosynthesis.

ConspectusTo cope with the increasingly global greenhouse effect and energy shortage, it is urgent to develop a feasible means to convert anthropogenic excess carbon dioxide (CO2) into energy resources. The photocatalytic CO2 reduction reaction (CO2RR) coupled with the water oxidation reaction (WOR), known as artificial photosynthesis, is a green, clean, and promoting strategy to deal with the above issues. Among the reported photocatalytic systems for CO2 reduction, the main challenge is to achieve WOR simultaneously due to the limited charge separation efficiency and complicated dynamic process. To address the problem, scientists have assembled two nanosemiconductor motifs for CO2RR and WOR into a heterojunction photocatalyst to realize artificial photosynthesis. However, it is difficult to clearly explore the corresponding catalytic mechanism and establish an accurate structure-activity relationship at the molecular level for their aperiodic distribution and complicated structural information. Standing on the shoulders of the heterojunction photocatalysts, a new-generation material, hetero-motif molecular junction (HMMJ) photocatalysts, has been developed and studied by our laboratory. A hetero-motif molecular junction is a class of crystalline materials with a well-defined and periodic structure, adjustable assembly mode, and semiconductor-like properties, which is composed of two predesigned motifs with oxidation and reduction, respectively, by coordination or covalent bonds. The intrinsic properties make these catalysts susceptible to functional modifications to improve light absorption and electrical conductivity. The small size and short distance of the motifs can greatly promote the efficiency of photogenerated electron-hole separation and migration. Based on these advantages, they can be used as potential excellent photocatalysts for artificial photosynthesis. Notably, the explicit structural information determined by single-crystal or powder X-ray diffraction can provide a visual platform to explore the reaction mechanism. More importantly, the connection number, spatial distance, interaction, and arrangement mode of the structural motifs can be well-designed to explore the detailed structure-activity relationship that can be hardly studied in nanoheterojunction photocatalyst systems. In this regard, HMMJ photocatalysts can be a new frontier in artificial photosynthesis and serve as an important bridge between molecular photocatalysts and solid photocatalysts. Thus, it is very important to summarize the state-of-the-art of the HMMJ photocatalysts used for artificial photosynthesis and to give in-depth insight to promote future development.In this Account, we have summarized the recent advances in artificial photosynthesis using HMMJ photocatalysts, mainly focusing on the results in our lab. We present an overview of current knowledge about developed photocatalytic systems for artificial photosynthesis, introduce the design schemes of the HMMJ photocatalysts and their unique advantages as compared to other photocatalysts, summarize the construction strategies of HMMJ photocatalysts and their application in artificial photosynthesis, and explain why hetero-motif molecular junctions can be promising photocatalysts and show that they provide a powerful platform for studying photocatalysis. The structure-activity relationship and charge separation dynamics are illustrated. Finally, we bring our outlook on present challenges and future development of HMMJ photocatalysts and their potential application prospects on other photocatalytic reaction systems. We believe that this Account will afford important insights for the construction of high-efficiency photocatalysts and guidance for the development of more photocatalytic systems in an atom-economic, environmentally friendly, and sustainable way.

Selective CO2 Photoreduction Enabled by Water-stable Cu-based Metal-organic Framework Nanoribbons.

The goal of photocatalytic CO2 reduction system is to achieve near 100 % selectivity for the desirable product with reasonably high yield and stability. Here, two-dimensional metal-organic frameworks are constructed with abundant and uniform monometallic active sites, aiming to be an emerged platform for efficient and selective CO2 reduction. As an example, water-stable Cu-based metal-organic framework nanoribbons with coordinatively unsaturated single CuII sites are first fabricated, evidenced by X-ray diffraction patterns and X-ray absorption spectroscopy. In situ Fourier-transform infrared spectra and Gibbs free energy calculations unravel the formation of the key intermediate COOH* and CO* is an exothermic and spontaneous process, whereas the competitive hydrogen evolution reaction is endothermic and non-spontaneous, which accounts for the selective CO2 reduction. As a result, in an aqueous solution containing 1 mol L-1 KHCO3 and without any sacrifice reagent, the water-stable Cu-based metal-organic framework nanoribbons exhibited an average CO yield of 82 μmol g-1 h-1 with the selectivity up to 97 % during 72 h cycling test, which is comparable to other reported photocatalysts under similar conditions.

The photo-decomposition and self-restructuring dynamic equilibrium mechanism of Cu2(OH)2CO3 for stable photocatalytic CO2 reduction

Developing suitable photocatalysts and understanding their intrinsic catalytic mechanism remain key challenges in the pursuit of highly active, good selective, and long-term stable photocatalytic CO2 reduction (PCO2R) systems. Herein, monoclinic Cu2(OH)2CO3 is firstly proven to be a new class of photocatalyst, which has excellent catalytic stability and selectivity for PCO2R in the absence of any sacrificial agent and cocatalysts. Based on a Cu2(OH)213CO3 photocatalyst and 13CO2 two-sided 13C isotopic tracer strategy, and combined with in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis and density functional theory (DFT) calculations, two main CO2 transformation routes, and the photo-decomposition and self-restructuring dynamic equilibrium mechanism of Cu2(OH)2CO3 are definitely revealed. The PCO2R activity of Cu2(OH)2CO3 is comparable to some of state-of-the-art novel photocatalysts. Significantly, the PCO2R properties can be further greatly enhanced by simply combining Cu2(OH)2CO3 with typical TiO2 to construct composites photocatalyst. The highest CO and CH4 production rates by 7.5 wt% Cu2(OH)2CO3-TiO2 reach 16.4 μmol g−1h−1 and 116.0 μmol g−1h−1, respectively, which are even higher than that of some of PCO2R systems containing sacrificial agents or precious metals modified photocatalysts. This work provides a better understanding for the PCO2R mechanism at the atomic levels, and also indicates that basic carbonate photocatalysts have broad application potential in the future.

(Invited) Dye-Sensitized Molecular Photoelectrodes for CO2 Reduction Using Water as a Reductant

Photocatalytic CO2 reduction using visible light and water as a reductant has been one of the important targets for solving global warming problem and shortage of energy and carbon resources. In this presentation, I talk about two projects, i.e., (1) supramolecular photocatalysts with both functions as photosensitizer and catalysts and (2) photocatalytic CO2 reduction coupled to water oxidation.(1) Highly efficient supramolecular photocatalysts for CO2 reduction1-3 Since CO2 is the most oxidized state of carbon, one-electron reduction of CO2 is an extraordinarily high endergonic reaction. From the viewpoints of low-energy light application, the multi-electron reduction of CO2 via chemical reactions promises to be a more valuable process. This is a main reason why both a photosensitizer and a catalyst are required to promote efficient photocatalytic CO2 reduction. We have developed so-called supramolecular photocatalyts that have both the photosensitizer and the catalyst units. This bonding in the supramolecular photocatalysts can accelerate electron transfer between the two units, which improves the performance of the photocatalytic system, especially on the surface of photofunctional solid materials as described below.(2) Molecular photoelectrodes for photocatalytic CO2 reduction with water as a reductant 1,4 The molecular photocatalytic systems have a weakness, i.e., weak oxidation power in the excited state. For practical application, water should be used as an electron donor. For overcoming this problem, we have developed a hybrid system consisting of the supramolecular photocatalyst and semiconductor electrodes. The hybrid photoelectrochemical cell consisting of this hybrid photocathode and n-type semiconductor photoanode induced visible-light-driven CO2 reduction with water as a reductant to generate CO and O2. References “Photocatalytic Systems for CO2 Reduction: Metal-Complex Photocatalysts and Their Hybrids with Photofunctional Solid Materials” Kumagai, H.; Tamaki, Y.; Ishitani, O., Acc. Chem. Res. 2022, 55, 978-990.“Mechanistic study of photocatalytic CO2 reduction using a Ru(II)-Re(I) supramolecular photocatalyst” Kamogawa, K.; Shimoda, Y.; Miyata, K.; Onda, K.; Yamazaki, Y.; Tamaki, Y.; Ishitani, O., Chem. Sci. 2021, 12, 9682-9693.“Supramolecular Photocatalysts for the Reduction of CO2” Tamaki, Y.; Ishitani, O., ACS Cat. 2017, 7, 3394-3409.“Supramolecular photocatalysts fixed on the inside of the polypyrrole layer in dye sensitized molecular photocathodes: application to photocatalytic CO2 reduction coupled with water oxidation. Kuttassery, F.; Kumagai, H.; Kamata, R.; Ebato, Y.; Higashi, M.; Suzuki, H.; Abe, R.; Ishitani, O., Chem. Sci. 2021, 12, 13216-13232. Figure 1

An S-scheme heterointerface-engineered high-performance ternary NiAl-LDH@TiO2/Ti3C2 MXene photocatalytic system for solar-powered CO2 reduction to produce energy-rich fuels

While there is much potential for photocatalytic CO2 reduction, poor light absorption and high recombination rates of photogenerated charges limit its effectiveness. To address these challenges, we systematically developed a heterointerface-engineered ternary hybrid photocatalyst comprising NiAl-layered double hydroxide (LDH), titanium dioxide (TiO2), and titanium carbide (Ti3C2) MXene via an in situ growth approach. As a result of the unique combination of these three components, the synthesized ternary NiAl-LDH@TiO2/Ti3C2 photocatalyst demonstrated broad light absorption spanning across the ultraviolet, visible, and near-infrared regions, as well as elevated CO2 adsorption capacity. In situ-irradiated X-ray photoelectron spectroscopy and electron paramagnetic resonance analyses provided compelling evidence for an unconventional S-scheme charge transfer mechanism in the ternary system that effectively separates the charges and suppresses recombination, allowing NiAl-LDH to maintain its strong reducing capacity and TiO2 to maintain its robust oxidizing capacity. Utilizing the complementary and synergistic properties of these three components (NiAl-LDH, TiO2, and Ti3C2), an optimized ternary NiAl-LDH@TiO2/Ti3C2 photocatalyst with 30 wt% Ti3C2 exhibited extraordinary solar-driven CO2 reduction performance with a remarkable 99 % CO selectivity against competitive H2 production and a high apparent quantum yield of 0.81 at 365 nm. Additionally, the ternary photocatalyst exhibited excellent stability, maintaining its performance capacity over multiple CO2 reduction cycles. This work provides a fresh perspective on designing and creating efficient ternary S-scheme photocatalytic systems for solar-driven CO2 reduction and highlights the potential for energy-rich fuel production.

Multicomponent Self-assembly of a Multinuclear Heterometal–Organic Polymer for CO2 Reduction

To address the greenhouse effect, scientists have never stopped exploring efficient photocatalytic CO2 reduction systems, and metal-organic compounds have recently emerged as a solution for overcoming global warming and climate issues. Under the irradiation of visible light, the catalytic activity, stability, and selectivity of most of the non-noble metal complexes reported so far are unsatisfactory. To efficiently fix CO2, Ru multinuclear heterometal–organic compound was obtained for carbon dioxide reduction. The as-synthesized M-Zn complex (M =Ru) catalysts exhibit special catalytic activity for the reduction of CO2 because of the immobilized metal ions serving as active sites during the reaction. This study proposes a general strategy for CO2 reduction through high levels in metal-organic polymers to achieve effective environmental protection and governance.

Self-Photosensitizing Dinuclear Ruthenium Catalyst for CO2 Reduction to CO.

The promise of artificial photosynthesis to solve environmental and energy issues such as global warming and the depletion of fossil fuels has inspired intensive research into photocatalytic systems for CO2 reduction to produce value-added chemicals such as CO and CH3OH. Among the photocatalytic systems for CO2 reduction, self-photosensitizing catalysts, bearing the functions of both photosensitization and catalysis, have attracted considerable attention recently, as such catalysts do not depend on the efficiency of electron transfer from the photosensitizer to the catalyst. Here, we have synthesized and characterized a dinuclear RuII complex bearing two molecules of a tripodal hexadentate ligand as chelating and linking ligands by X-ray crystallography to establish the structure explicitly and have used various spectroscopic and electrochemical methods to elucidate the photoredox characteristics. The dinuclear complex has been revealed to act as a self-photosensitizing catalyst, which acts not only as a photosensitizer but also as a catalyst for CO2 reduction. The dinuclear RuII complex is highly durable and performs efficient and selective CO2 reduction to produce CO with a turnover number of 2400 for 26 h. The quantum yield of the CO formation is also very high─19.7%─and the catalysis is efficient, even at a low concentration (∼1.5%) of CO2.

Photocatalytic reduction of CO2 in aqueous phase using amino-functionalized MOFs loaded with hydroxy-functionalized graphene quantum dots

Hydroxy-functionalized graphene quantum dots (OH-GQDs) were loaded onto NH2-MIL-101(Fe) to prepare a novel binary OH-GQDs/NH2-MIL-101(Fe) (OGNM) composite with a simple solvothermal method. The obtained catalysts were characterized by SEM, TEM, XRD, FI-IR, XPS and BET to analyze the morphology, elemental composition, and specific surface area of the composite material. Notably, the morphology and micro-structures of OGMM-3 exhibited that OH-GQDs evenly dispersed on the surface of hexagonal micropillar NH2-MIL-101(Fe). The nanocomposite catalysts were tested in a photocatalytic CO2 reduction system using water and acetonitrile as solvents, without additional sacrificial agents. OGNM-3 composite possessed the optimum photoreduction CO2 activity, with a CO yield of 128.68 μmol·g−1 within 10 h, which was 5.3 times higher than that of pure NH2-MIL-101(Fe). After three cycles of reuse, the catalyst still maintained catalytic activity without any significant changes in its crystal structure, morphology or functional groups. In summary, this study proposed a design for synthesizing binary OGNM photocatalysts, offering useful insights for cost-effectively address CO2 emissions in greenhouse gas emissions.

Development of enzyme immobilization systems for CO2 bioconversion: advances and challenges

Enzyme-catalyzed CO2 reduction to value-added commodities is important for alleviating the global environmental issues and energy crises due to high selectivity and mild conditions. Owing to high energy density, formic acid or methanol produced from CO2 using formate dehydrogenase (FDH) or multi-enzyme cascades are promising target chemicals for CO2 utilization. However, the low activity, poor stability and low reusability of key enzymes involved in such process hampered its large-scale application. Enzyme immobilization provides an effective solution to these problems and significant progress have been made in immobilization carriers. Moreover, integration of enzyme immobilization with other catalysis techniques have been explored extensively. This review summarized the recent advances in the immobilization of enzymes using membranes, inorganic materials, metal-organic frameworks, covalent organic frameworks and other carriers, and illustrated the characteristics and advantages of different immobilization materials and immobilization methods. The synergistic effects and applications of immobilized enzymes and electrocatalytic or photocatalytic coupling reaction systems for CO2 reduction were further summarized. Finally, the current challenges of enzyme immobilization technology and coupling reaction systems were pointed out and their development prospects were presented.

Highly stable and durable ZnIn2S4 nanosheets wrapped oxygen deficient blue TiO2(B) catalyst for selective CO2 photoreduction into CO and CH4

Developing new and highly stable efficient photocatalysts is crucial for achieving high performance and selective photocatalytic CO2 conversion. In this paper, we designed a one-dimensional oxygen-deficient blue TiO2(B) (BT) catalyst for improved electron mobility and visible light accessibility. In addition, hexagonal ZnIn2S4 (ZIS) nanosheets with a low bandgap and great visible light accessibility are employed to produce effective heterostructures with BT. The synthesized materials are tested for photocatalytic conversion of CO2 into solar fuels (H2, CO and CH4). The optimized composite yields 71.6 and 10.3 μmol g−1h−1 of CO and CH4, three and ten times greater than ZIS, respectively. When ZIS nanosheets are combined with a one-dimensional oxygen-deficient BT catalyst, improved electron mobility and visible light accessibility are achieved, charge carriers are effectively segregated, and the transfer process is accelerated, resulting in efficient CO2 reduction. The photocatalytic CO2 conversion activity of the constructed BT/ZIS heterostructures is very stable over a 10-day (240-hour) period, and CO and CH4 production rates increase linearly with time; however, as time goes on, the rates of H2 production decrease. Further, a five-time recycling test confirmed this, revealing essentially equal activity and selectivity throughout the experiment. As a result, CO2 to CO and CH4 conversion has high selectivity and longer durability. The band structure of the BT/ZIS composite is determined using Mott-Schottky measurement, diffuse reflectance spectroscopy, and valence band X-ray photoelectron spectroscopy. This research demonstrates a novel approach to investigating effective, stable, and selective photocatalytic CO2 reduction systems for solar-to-chemical energy conversion.

Design and Development of Photocatalytic Systems for Reduction of CO2 into Valuable Chemicals and Fuels

This review presents the results of research in the field of photocatalytic reduction of carbon dioxide (CO2) to methane and methanol as valuable chemicals and fuels. CO2 reduction is a promising technology, but it is an endothermic process with unfavourable thermodynamics. Other limitations include the inertness of the CO2 molecule, the slow multielectron process, and the lack of understanding of the reaction mechanism, leading to low selectivity and insufficient efficiency. Tailoring reaction parameters such as CO2 adsorption, choice of reducing agent, development of photocatalysts in terms of composition, structural properties and morphology, energy band gap, and the presence of surface functional groups can affect the reaction mechanism and selectivity for the desired product. Therefore, the main challenges in this research area are the development of an active and selective catalyst for the photoreduction of CO2 to useful products with high added value and the optimization and development of a suitable photoreactor that allows successful contact between all key participants in the photocatalytic process. This review is intended to provide guidance for the future development of advanced photocatalysts and photocatalytic systems for CO2 reduction and to enable further breakthroughs in this field.

G‐C3N4 Nanosheet Nanoarchitectonics: H2 Generation and CO2 Reduction

AbstractFew‐layered graphitic carbon nitride (g‐C3N4) has been considered a popular base for constructing composite materials in photocatalytic CO2 reduction and H2 generation applications, particularly for constructing g‐C3N4 based composites based on the idea of nanoarchitectonics. The photocatalytic activity of these constructed g‐C3N4 composites relies on the composition, morphology, microstructure of the material. This review aims to focus on the summary of latest progress in the development of few‐layered g‐C3N4 nanosheets based noble metal clusters/nanoparticles and other semiconductor nanomaterials modified composite materials constructed using the technology of nanoarchitectonics for photocatalytic H2 generation and CO2 reduction. This review covers a brief introduction of the enhanced photogenerated carrier separation and transfer as well as extended light absorption of superior thin g‐C3N4 nanosheets based photocatalysts; discussions on the mechanism of g‐C3N4 based photocatalyst involved photocatalytic H2O splitting and CO2 reduction system; the latest progress in the study of nanoarchitectonics of a variety of g‐C3N4 based heterostructures including type II, Z‐scheme, S‐scheme, and Schottky junction. Major challenges on nanoarchitectonics of superior thin g‐C3N4 based heterostructures, as well as the recommendations for future studies of these materials to attain higher efficiency in photocatalytic H2 generation and CO2 reduction, are also concluded.

Transition into Net-Zero Carbon Community from Fossil Fuels: Life Cycle Assessment of Light-Driven CO2 Conversion to Methanol Using Graphitic Carbon Nitride

Direct conversion of waste CO2 into CH3OH by solar irradiation has emerged as the most enthralling alternative to conventional steam methane reforming, which reduced the concentration of greenhouse gases and the consumption of fossil resources. A cradle-to-gate life cycle assessment was carried out to assess the environmental feasibility of implementing the current g-C3N4-based photocatalytic CO2 reduction system based on 1 kg CH3OH production. It was observed that the emerging photocatalytic CH3OH system induced 68 and 53% less carbon footprint and fossil usage, respectively, in producing the same amount of CH3OH as compared to the conventional approach. The CO2 reduction reaction step was identified as the main contributor in photocatalytic system, which accounted for more than 50% of the total environmental impact scores. In the conventional system, the score points of global warming and fossil depletion contributed the most by the product purification and desulfurization step, respectively. In addition, sensitivity analysis manifested that the lowest environmental burden of 46.50 kg of CO2 equiv carbon emission and 15.27 kg oil equiv fossil consumption was obtained in the photocatalytic CH3OH system upon replacing fossil-based energy with hydro-powered electricity. These results suggest a potent replacement of the incumbent industry system for the amelioration of climate benefits and resource conservation with the advanced development of the photocatalytic system.

Dynamics of Electron Transfers in Photosensitization Reactions of Zinc Porphyrin Derivatives

Photocatalytic systems for CO2 reduction operate via complicated multi-electron transfer (ET) processes. A complete understanding of these ET dynamics can be challenging but is key to improving the efficiency of CO2 conversion. Here, we report the ET dynamics of a series of zinc porphyrin derivatives (ZnPs) in the photosensitization reactions where sequential ET reactions of ZnPs occur with a sacrificial electron donor (SED) and then with TiO2. We employed picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption (TA) measurement to investigate the fast ET dynamics concealed in the steady-state or slow time-resolved measurements. As a result, Stern-Volmer analysis of fluorescence lifetimes evidenced that the reaction of photoexcited ZnPs with SED involves static and dynamic quenching. The global fits to the TA spectra identified much faster ET dynamics on a few nanosecond-time scales in the reactions of one-electron reduced species (ZnPs•-) with TiO2 compared to previously measured minute-scale quenching dynamics and even diffusion rates. We propose that these dynamics report the ET dynamics of ZnPs•- formed at adjacent TiO2 without involving diffusion. This study highlights the importance of ultrafast time-resolved spectroscopy for elucidating the detailed ET dynamics in photosensitization reactions.

A critical review on advances in TiO2-based photocatalytic systems for CO2 reduction

• CO 2 reactivity increases with an increase in the CO 2 :H 2 O ratio. • Acidic solutions rather than pure water are preferable for formic acid formation. • High temperatures promote the rate of thermally activated steps such as desorption. • Heating promotes high activity for the CO 2 hydrogenation reaction. • Monolithic photoreactor allows selective production of CO and H 2 . The growing world population and uncontrolled industrial growth have resulted in elevated energy consumption and harsh emission of toxic agents tearing down the ecosystem. The transformation of a greenhouse gas like CO 2 into energy sources, i.e., CO, CH 3 OH, CH 4 , etc. is a promising strategy to alleviate the interlinked issues of environmental pollution, global warming and climate change. This article presents an overview of the fundamentals of photocatalytic systems and the latest developments in CO 2 chemical reduction towards solar fuels over titanium dioxide (TiO 2 ). The basic principle of photocatalysis states new photocatalyst synthesis and how the current materials can be used to improve the existing catalysts, focusing on the surface area, electrical conductivity, and chemical stability of TiO 2 . In the study of the state of the art it was observed that at a lower CH 4 /CO 2 ratio, more H 2 was produced, while a higher feed ratio promoted CO production. The maximum production rate reported for CH 4 was 0.64 mmol g -1 h −1 , for CO was 158 mmol/h/g cat, and 102 mmol h -1 Kg cat _1 . Detailed insights for enhanced performance, especially factors affecting mass transfer, thermodynamics, selectivity, and reaction mechanism are provided and discussed. This review summarizes and analyzes research work on the photoreduction of CO 2 in the presence of TiO 2 over the past ten years. State of the art in photocatalytic CO 2 chemical reduction over TiO 2 emphasizes material design and reactor configurations. One section is dedicated to reactor design optimization and studying the synergic effect in photocatalysis. Also, brief perspectives and the main challenges in photocatalytic CO 2 reduction are outlined, which are vital for improving the conversion efficiency of CO 2 . The use of solar energy to drive CO 2 photocatalytic reactions and the recent progress in this area are also explored and discussed.

Boosting photocatalytic CO2 reduction by tuning photogenerated carrier kinetics in two-dimensional WOx/BiOCl S-scheme heterojunction with oxygen vacancies

Photocatalytic CO2 reduction driven by sustainable solar energy is a highly desirable route to achieve carbon recycling, to which charge separation in photocatalysts holds the key. In this work, two-dimensional (2D) WO3·H2O nanosheets with rich oxygen vacancies (Vo-WOx) were combined with 2D BiOCl nanosheets to form a 2D/2D Vo-WOx/BiOCl S-scheme heterojunction by a self-assembly process. The 2D/2D stacked structure combined with the introduction of oxygen vacancies in Vo-WOx could improve the light harvesting of the designed photocatalytic system. The dual transfer pathways in the S-scheme junction were induced by the fact that the photogenerated electrons on the conduction band and defect energy level of the Vo-WOx transfer across the interface to combine with the BiOCl, leading to an improved kinetic process of photogenerated carriers. Moreover, the S-scheme configuration can retain a strong redox potential for satisfying the thermodynamic requirement of CO2 reduction. As a result, the optimized 30Vo-WOx/BiOCl composite can achieve the highest CO yield rate of 8.82 μmol h−1 from CO2 reduction without any sacrifice reagents, which was 4.69 and 3.08 times higher than that of pure BiOCl and WO3/BiOCl without oxygen vacancy. This work provides a new view of defect engineering in S-scheme heterojunction for pursuing a highly efficient photocatalytic CO2 reduction system.

Advances and challenges in developing cocatalysts for photocatalytic conversion of carbon dioxide to fuels

The global adoption of efficient sustainable energy sources is a crucial step toward meeting energy demands while achieving carbon emission reduction targets. Solar energy has become a clean and cost-competitive alternative to traditional fossil fuels, but the intermittent nature of sunlight results in challenges associated with energy storage and transport. Photocatalytic carbon dioxide reduction intends to mimic natural photosynthesis for utilizing sunlight to chemically convert water and CO2 into fuels. In this process, the solar energy is captured and stored in fuels, so-called solar fuels, for widespread on-demand use. Heterogeneous solar fuel production systems are multi-component, comprising light-harvesting (photosensitizer) and catalytic (cocatalyst) units. Cocatalysts are indispensable for photocatalytic CO2 reduction systems, which promote charge carrier separation and transport, reduce the reaction activation energy, and alter the reaction route, thereby enhancing the activity and selectivity of the photocatalytic reactions. This review presents a comprehensive summary of the recent advancements in cocatalysts for photocatalytic CO2 reduction reaction (CO2RR), with the purpose of providing new insights and guidance to the field with regard to research directions and best practices. We summarize how various cocatalysts including inorganic nanoparticles, metal complexes, enzymes, and bacteria can be combined with semiconductor photosensitizer for light-driven photocatalytic CO2RR. Side-by-side comparisons reveal the strengths and limitations of each kind of cocatalysts and how lessons extracted from studying natural photosynthetic systems can be applied to investigations of artificial photosynthesis, presenting an outlook discussing possible future concepts for a more effective photocatalytic CO2 reduction process.