Correlating Oxidation State and Surface Ligand Motifs with the Selectivity of CO2 Photoreduction to C2 Products

Unlocking Copper-Free Interfacial Asymmetric C-C Coupling for Ethylene Photosynthesis from CO2 and H2O

Solar-driven carbon dioxide (CO2) reduction into C2+ products such as ethylene represents an enticing route toward achieving carbon neutrality. However, due to sluggish electron transfer and intricate C-C coupling, it remains challenging to achieve highly efficient and selective ethylene production from CO2 and H2O beyond capitalizing on Cu-based catalysts. Herein, we report a judicious design to attain asymmetric C-C coupling through interfacial defect-rendered tandem catalytic centers within a sulfur-vacancy-rich MoSx/Fe2O3 photocatalyst sheet, enabling a robust CO2 photoreduction to ethylene without the need for copper, noble metals, and sacrificial agents. Specifically, interfacial S vacancies induce adjacent under-coordinated S atoms to form Fe-S bonds as a rapid electron-transfer pathway for yielding a Z-scheme band alignment. Moreover, these S vacancies further modulate the strong coupling interaction to generate a nitrogenase-analogous Mo-Fe heteronuclear unit and induce the upward shift of the d-band center. This bioinspired interface structure effectively suppresses electrostatic repulsion between neighboring *CO and *COH intermediates via d-p hybridization, ultimately facilitating an asymmetric C-C coupling to achieve a remarkable solar-to-chemical efficiency of 0.565% with a superior selectivity of 84.9% for ethylene production. Further strengthened by MoSx/WO3, our design unveils a promising platform for optimizing interfacial electron transfer and offers a new option for C2+ synthesis from CO2 and H2O using copper-free and noble metal-free catalysts.

Pivotal Impact Factors in Photocatalytic Reduction of CO2 to Value-Added C1 and C2 Products

Over the past decades, CO2 greenhouse emission has been considerably increased, causing global warming and climate change. Indeed, converting CO2 into valuable chemicals and fuels is a desired option to resolve issues caused by its continuous emission into the atmosphere. Nevertheless, CO2 conversion has been hampered by the ultrahigh dissociation energy of C=O bonds, which makes it thermodynamically and kinetically challenging. From this prospect, photocatalytic approaches appear promising for CO2 reduction in terms of their efficiency compared to other traditional technologies. Thus, many efforts have been made in the designing of photocatalysts with asymmetric sites and oxygen vacancies, which can break the charge distribution balance of CO2 molecule, reduce hydrogenation energy barrier and accelerate CO2 conversion into chemicals and fuels. Here, we review the recent advances in CO2 hydrogenation to C1 and C2 products utilizing photocatalysis processes. We also pin down the key factors or parameters influencing the generation of C2 products during CO2 hydrogenation. In addition, the current status of CO2 reduction is summarized, projecting the future direction for CO2 conversion by photocatalysis processes.

Recent progress in solar-driven CO2 reduction to multicarbon products

Currently, most catalysts used for photoconverting carbon dioxide (CO2) typically produce C1 products. Achieving multicarbon (C2+) products, which are highly desirable due to their greater energy density and economic potential, still remains a significant challenge. This difficulty is primarily due to the kinetic hurdles associated with the C-C coupling step in the process. Given this, devising diverse strategies to accelerate C-C coupling for generating multicarbon products is requisite. Herein, we first give a classification of catalysts involved in the photoconversion of CO2 to C2+ fuels. We summarize metallic oxides, metallic sulfides, MXenes, and metal-organic frameworks as catalysts for CO2 photoreduction to C2+ products, attributing their efficacy to the inherent dual active sites facilitating C-C coupling. In addition, we survey covalent organic frameworks, carbon nitrides, metal phosphides, and graphene as cocatalysts for CO2 photoreduction to C2+ products, owing to the incorporated dual active sites that induce C-C coupling. In the end, we provide a brief conclusion and an outlook on designing new photocatalysts, understanding the catalytic mechanisms, and considering the practical application requirements for photoconverting CO2 into multicarbon products.

Metal Poly(heptazine imides) as Multifunctional Photocatalysts for Solar Fuel Production

Solar-driven photocatalysis employing particulate semiconductors represents a promising approach for sustainable production of valuable chemical feedstock. Metal poly(heptazine imide) (MPHI), a novel 2D ionic carbon nitride, has been recognized as an emerging photocatalyst with distinctive properties. In this minireview, we first delineate the forefront innovations of MPHI photocatalysts, spanning from synthetic strategies and solving structures to the exploration of novel properties. We place special emphasis on the structural design principles aimed at developing high-performance MPHI systems toward photocatalytic solar fuel production such as H2 evolution, H2O oxidation, H2O2 production and CO2 reduction. Finally, we discuss crucial insights and challenges in leveraging highly active MPHIs for efficient solar-to-chemical energy conversion.

Single-Atom Titanium on Mesoporous Nitrogen, Oxygen-Doped Carbon for Efficient Photo-thermal Catalytic CO2 Cycloaddition by a Radical Mechanism

Developing efficient and earth-abundant catalysts for CO2 fixation to high value-added chemicals is meaningful but challenging. Styrene carbonate has great market value, but the cycloaddition of CO2 to styrene oxide is difficult due to the high steric hindrance and weak electron-withdrawing ability of the phenyl group. To utilize clean energy (such as optical energy) directly and effectively for CO2 value-added process, we introduce earth-abundant Ti single-atom into the mesoporous nitrogen, oxygen-doped carbon nanosheets (Ti-CNO) by a two-step method. The Ti-CNO exhibits excellent photothermal catalytic activities and stability for cycloaddition of CO2 and styrene oxide to styrene carbonate. Under light irradiation and ambient pressure, an optimal Ti-CNO produces styrene carbonate with a yield of 98.3 %, much higher than CN (27.1 %). In addition, it shows remarkable stability during 10 consecutive cycles. Its enhanced catalytic performance stems from the enhanced photothermal effect and improved Lewis acidic/basic sites exposed by the abundant mesopores. The experiments and theoretical simulations demonstrate the styrene oxide⋅+ and CO2⋅- radicals generated at the Lewis acidic (Tiδ+) and basic sites of Ti-CNO under light irradiation, respectively. This work furnishes a strategy for synthesizing advanced single-atom catalysts for photo-thermal synergistic CO2 fixation to high value products via a cycloaddition pathway.

In Situ Construction of Interface with Photothermal and Mutual Catalytic Effect for Efficient Solar-Driven Reversible Hydrogen Storage of MgH2

Hydrogen storage in MgH2 is an ideal solution for realizing the safe storage of hydrogen. High operating temperature, however, is required for hydrogen storage of MgH2 induced by high thermodynamic stability and kinetic barrier. Herein, flower-like microspheres uniformly constructed by N-doped TiO2 nanosheets coated with TiN nanoparticles are fabricated to integrate the light absorber and thermo-chemical catalysts at a nanometer scale for driving hydrogen storage of MgH2 using solar energy. N-doped TiO2 is in situ transformed into TiNxOy and Ti/TiH2 uniformly distributed inside of TiN matrix during cycling, in which TiN and Ti/TiHx pairs serve as light absorbers that exhibit strong localized surface plasmon resonance effect with full-spectrum light absorbance capability. On the other hand, it is theoretically and experimentally demonstrated that the intimate interface between TiH2 and MgH2 can not only thermodynamically and kinetically promote H2 desorption from MgH2 but also simultaneously weaken Ti─H bonds and hence in turn improve H2 desorption from the combination of weakened Ti─H and Ti─H bonds. The uniform integration of photothermal and catalytic effect leads to the direct action of localized heat generated from TiN on initiating the catalytic effect in realizing hydrogen storage of MgH2 with a capacity of 6.1 wt.% under 27 sun.

Crystal Engineering of MOF-Derived Bimetallic Oxide Solid Solution Anchored with Au Nanoparticles for Photocatalytic CO2 Reduction to Syngas and C2 Hydrocarbons

Considering that CO2 reduction is mostly a multielectron reaction, it is necessary for the photocatalysts to integrate multiple catalytic sites and cooperate synergistically to achieve efficient photocatalytic CO2 reduction to various products, such as C2 hydrocarbons. Herein, through crystal engineering, we designed and constructed a metal-organic framework-derived Zr/Ti bimetallic oxide solid solution support, which was confirmed by X-ray diffraction, electron microscopy and X-ray absorption spectroscopy. After anchoring Au nanoparticles, the composite photocatalyst exhibited excellent performances toward photocatalytic CO2 reduction to syngas (H2 and CO production rates of 271.6 and 260.6 μmol g-1 h-1) and even C2 hydrocarbons (C2H4 and C2H6 production rates of 6.80 and 4.05 μmol g-1 h-1). According to the control experiments and theoretical calculations, the strong interaction between bimetallic oxide solid solution support and Au nanoparticles was found to be beneficial for binding intermediates and reducing CO2 reduction, highlighting the synergy effect of the catalytic system with multiple active sites.

Synchrotron Radiation Based X-ray Absorption Spectroscopy: Fundamentals and Applications in Photocatalysis

Photocatalysis is one of the most promising green technologies to utilize solar energy for clean energy achievement and environmental governance. There is a knotty problem to rational designing high-performance photocatalyst, which largely depends on an in-depth insight into their structure-activity relationships and complex photocatalytic reaction mechanisms. Synchrotron radiation based X-ray absorption spectroscopy (XAS) is an important characterization method for photocatlayst to offer the element-specific key geometric and electronic structural information at the atomic level, on this basis, time-resolved XAS technique has a huge impact on mechanistic understanding of photochemical reaction owing to their powerful ability to probe, in real-time, the electronic and geometric structures evolution within photocatalysis reactions. This review will focus on the fundamentals of XAS and their applications in photocatalysis. The detailed applications obtained from XAS is described through the following aspects: 1) identifying local structure of photocatalyst; 2) uncovering in situ structure and chemical state evolution during photocatalysis; 3) revealing the photoexcited process. We will provide an in depth understanding on how the XAS method can guide the rational design of highly efficient photocatalyst. Finally, a systematic summary of XAS and related significance is made and the research perspectives are suggested.

Advances in Defect Engineering of Metal Oxides for Photocatalytic CO2 Reduction

Photocatalytic CO2 reduction technology, capable of converting low-density solar energy into high-density chemical energy, stands as a promising approach to alleviate the energy crisis and achieve carbon neutrality. Semiconductor metal oxides, characterized by their abundant reserves, good stability, and easily tunable structures, have found extensive applications in the field of photocatalysis. However, the wide bandgap inherent in metal oxides contributes to their poor efficiency in photocatalytic CO2 reduction. Defect engineering presents an effective strategy to address these challenges. This paper reviews the research progress in defect engineering to enhance the photocatalytic CO2 reduction performance of metal oxides, summarizing defect classifications, preparation methods, and characterization techniques. The focus is on defect engineering, represented by vacancies and doping, for improving the performance of metal oxide photocatalysts. This includes advancements in expanding the photoresponse range, enhancing photogenerated charge separation, and promoting CO2 molecule activation. Finally, the paper provides a summary of the current issues and challenges faced by defect engineering, along with a prospective outlook on the future development of photocatalytic CO2 reduction technology.

Tandem Synergistic Effect of Cu-In Dual Sites Confined on the Edge of Monolayer CuInP2 S6 toward Selective Photoreduction of CO2 into Multi-Carbon Solar Fuels

One-unit-cell, single-crystal, hexagonal CuInP2 S6 atomically thin sheets of≈0.81 nm in thickness was successfully synthesized for photocatalytic reduction of CO2 . Exciting ethene (C2 H4 ) as the main product was dominantly generated with the yield-based selectivity reaching ≈56.4 %, and the electron-based selectivity as high as ≈74.6 %. The tandem synergistic effect of charge-enriched Cu-In dual sites confined on the lateral edge of the CuInP2 S6 monolayer (ML) is mainly responsible for efficient conversion and high selectivity of the C2 H4 product as the basal surface site of the ML, exposing S atoms, can not derive the CO2 photoreduction due to the high energy barrier for the proton-coupled electron transfer of CO2 into *COOH. The marginal In site of the ML preeminently targets CO2 conversion to *CO under light illumination, and the *CO then migrates to the neighbor Cu sites for the subsequent C-C coupling reaction into C2 H4 with thermodynamic and kinetic feasibility. Moreover, ultrathin structure of the ML also allows to shorten the transfer distance of charge carriers from the interior onto the surface, thus inhibiting electron-hole recombination and enabling more electrons to survive and accumulate on the exposed active sites for CO2 reduction.

Fundamentals and Challenges of Engineering Charge Polarized Active Sites for CO2 Photoreduction toward C2 Products

ConspectusGlobal warming and climatic deterioration are partly caused by carbon dioxide (CO2) emission. Given this, CO2 reduction into valuable carbonaceous fuels is a win-win route to simultaneously alleviate the greenhouse effect and the energy crisis, where CO2 reduction into hydrocarbon fuels by solar energy may be a potential strategy. Up to now, most of the current photocatalysts photoconvert CO2 to C1 products. It is extremely difficult to achieve production of C2 products, which have higher economic value and energy density, due to the kinetic challenge of C-C coupling of the C1 intermediates. Therefore, to realize CO2 photoreduction to C2 fuels, design of high-activity photocatalysts to expedite the C-C coupling is significant. Besides, the current mechanism for CO2 photoreduction toward C2 fuels is usually uncertain, which is possibly attributed to the following two reasons: (1) It is arduous to determine the actual catalytic sites for the C-C coupling step. (2) It is hard to monitor the low-concentration active intermediates during the multielectron transfer step.Most traditional metal-based photocatalysts usually possess charge balanced active sites that have the same charge density. In this aspect, the neighboring C1 intermediates may also show the same charge distribution, which would lead to dipole-dipole repulsion, thus preventing C-C coupling for producing C2 fuels. By contrast, photocatalysts with charge polarized active sites possess obviously different charge distributions in the adjacent C1 intermediates, which can effectively suppress the electrostatic repulsion to steer the C-C coupling. Based on this analysis, higher asymmetric charge density on the active sites would be more beneficial to anchoring between the adjacent intermediates and active atoms in catalysts, which can boost C-C coupling.In this Account, we summarize various strategies, including vacancy engineering, doping engineering, loading engineering, and heterojunction engineering, for tailoring charge polarized active sites to boost the C-C coupling for the first time. Also, we overview diverse in situ characterization technologies, such as in situ X-ray photoelectron spectroscopy, in situ Raman spectroscopy, and in situ Fourier transform infrared spectroscopy, for determining charge polarized active sites and monitoring reaction intermediates, helping to reveal the internal catalytic mechanism of CO2 photoreduction toward C2 products. We hope this Account may help readers to understand the crucial function of charge polarized active sites during CO2 photoreduction toward C2 products and provide guidance for designing and preparing highly active catalysts for photocatalytic CO2 reduction.

Selective CO2 Photoreduction to Acetate at Asymmetric Ternary Bridging Sites

Photoreduction of CO₂ is a promising strategy to synthesize value-added fuels or chemicals and realize carbon neutralization. Noncopper catalysts are seldom reported to generate C₂ products, and the selectivity over these catalysts is low. Here, we design rich-interface, heterostructured In₂O₃/InP (r-In₂O₃/InP) for highly competitive photocatalytic CO₂-to-CH₃COOH conversion with a productivity of 96.7 μmol g^-1 and selectivity > 96% along with water oxidation to O₂ in pure water (no sacrificial agent) under visible light irradiation. The hard X-ray absorption near-edge structure (XANES) shows that the formation of r-In₂O₃/InP with the isogenesis cation adjusts the coordination environment via interface engineering and forms O-In-P polarized sites at the interface. In situ FT-IR and Raman spectra identify the key intermediates of OCCO* for acetate production with high selectivity. Density functional theory (DFT) calculations reveal that r-In₂O₃/InP with rich O-In-P polarized sites promotes C-C coupling to form C₂ products because of the imbalanced adsorption energies of two carbon atoms. This work reports an interesting indium-based photocatalyst for selective CO₂ photoreduction to acetate under strict solution and irradiation conditions and provides significant insights into fabricating interfacial polarization sites to promote the process.