Highly Durable and Fully Dispersed Cobalt Diatomic Site Catalysts for CO2 Photoreduction to CH4

Precisely Tuning Band Gaps of Hexabenzocoronene-Based MOFs Toward Enhanced Photocatalysis

Precise adjusting the band gaps in metal-organic frameworks (MOFs) is crucial for improving their visible-light absorption capacity during photocatalysis, presenting both a formidable challenge and a charming opportunity. This present study employed a symmetry-reduction strategy to pre-design six novel 4-connected ligands with systematic substituents (-NO2, -H, -tBu, -OCH3, -OH and -NH2) and synthesized the corresponding pillared-layer Zr-MOFs (NKM-668) retaining the hexaphenylbenzene fragment. Subsequently, the NKM-668 MOFs were transformed into large-π-conjugated hexabenzocoronene-based MOFs (pNKM-668) via the Scholl reaction. These twelve MOFs exhibited broad and tunable band gaps over 1.41 eV (ranging from 3.25 eV to 1.84 eV), and the photocatalytic CO2 conversion rate raised by 33.2-fold. This study not only enriches the type of hexaphenylbenzene-based MOFs, but also paves the way for nanographene-containing MOFs in the further application of photocatalysis.

Enhanced Photocatalytic Degradation of Chlorsulfuron by MoS2/MIL101(Fe) S-Scheme Heterojunction: A Conversion Mechanism Dominated by Electrons and Long-Lifetime Reactive Species

Altering the generation route of reactive species is a potent means to augment the photocatalytic activity. In this study, MoS2/MIL-101(Fe) S-scheme heterojunction (MF2) is prepared using a water/solvent thermal method for photocatalytic degradation of chlorsulfuron. Driven by the internal electric field, the local electron density of MF2 is redistributed, thus enhancing the adsorption of O2. This promoted charge transfer to generate e-, •O 2 - ${\mathrm{O}}_{\mathrm{2}}^ - $ , and H2O2 for efficient oxidation of chlorsulfuron. It is confirmed that photogenerated electrons and long-lifetime reactive species (•O 2 - ${\mathrm{O}}_{\mathrm{2}}^ - $ and H2O2) played a major role. The degradation activity of MF2 for chlorsulfuron is much higher than MoS2 (42.21 times) and MIL-101(Fe) (4.06 times). The charge transfer mechanism of the MF2 S-scheme heterojunction is verified by experimental studies and Density Functional Theory simulation calculations. In addition, MF2 exhibited great potential for practical applications. This work provided new insights into the construction of S-scheme heterojunctions and long-lifetime reactive species-dominated conversion mechanisms.

Synergy of Copper Doping and Carbon Defect Engineering in Promoting C-C Coupling for Enhanced CO2 Photoreduction to Ethanol Activity

Photocatalytic conversion of carbon dioxide (CO2) to fuel provides an ideal pathway to achieving carbon neutrality. One significant hindrance in achieving the reduction of CO2 to higher energy density multicarbon products (C2+) was the difficulty in coupling C-C bonds efficiently. Copper (Cu) is considered the most suitable metal catalyst for C-C coupling to form C2+ products in the CO2 reduction reaction (CO2RR), but it encounters challenges such as low product selectivity and slow catalytic efficiency. Herein, we constructed a carbon defect on Cu-doped carbon nitride (Cu-CvN), as an efficient catalyst for photocatalytic CO2RR. The optimized catalyst (Cu-CvN-550) with a carbon defect shows high photocatalytic activity for CO2 reduction to ethanol, with an ethanol production rate of 122.6 μmol g-1 h-1 and a selectivity of 93.7%. The yield was 4.5 times higher than that of the Cu-CN-550 without carbon defect. The ratio of Cu+/Cu0 in Cu species changes regularly with calcination temperature, which was linearly correlated with the selectivity of the liquid product of CO2RR. DFT calculations combined with experimental results revealed that Cu doping promoted CO2 activation, followed by enhanced *CO adsorption and weakened hydrogenation and desorption. Carbon defects lower the free energy and greatly accelerate the *CO transfer process by promoting the formation of a six-membered ring intermediate state, serving as an intramolecular catalyst for *CO dimerization. Synergistic thermodynamic and kinetic interactions were realized through Cu doping and the introduction of carbon defects, thereby enhancing the catalytic performance of photocatalytic reduction of CO2 for ethanol production.

Regulating the electronic structure of Pt SAs-Ni2P for enhanced hydrogen evolution reaction

The single atom catalysts (SACs) show immense promise as catalytic materials. By doping the single atoms (SAs) of precious metals onto substrates, the atomic utilization of these metals can be maximized, thereby reducing catalyst costs. The electronic structure of precious metal SAs is significantly influenced by compositions of doped substrates. Therefore, optimizing the electronic structure through appropriate doping of substrates can further enhance catalytic activity. Here, Pt single atoms (Pt SAs) are doped onto transition metal sulfide substrate NiS2 (Pt SAs-NiS2) and phosphide substrate Ni2P (Pt SAs-Ni2P) to design and prepare catalysts. Compared to the Pt SAs-NiS2 catalyst, the Pt SAs-Ni2P catalyst exhibits better hydrogen evolution catalytic performance and stability. Under 1 M KOH conditions, the hydrogen evolution mass activity current density of the Pt SAs-Ni2P catalyst reaches 0.225 A mgPt-1 at 50 mV, which is 33 times higher than that of commercial Pt/C catalysts. It requires only 44.9 mV to achieve a current density of 10 mA cm-2. In contrast, for the Pt SAs-NiS2 catalyst, the hydrogen evolution mass activity current density is 0.178 A mgPt-1, requiring 77.8 mV to achieve a current density of 10 mA cm-2. Theoretical calculations indicate that in Pt SAs-Ni2P, the interaction between Pt SAs and the Ni2P substrate causes the Pt d-band center to shift downward, enhancing the H2O desorption and providing optimal H binding sites. Additionally, the hollow octahedral morphology of Ni2P provides a larger surface area, exposing more reactive sites and improving reaction kinetics. This study presents an effective pathway for preparing high-performance hydrogen evolution electrocatalysts by selecting appropriate doped substrates to control the electronic structure of Pt SAs.

Unique Oxygen-Bridged Nickel Atomic Pairs Efficiently Boost Electrochemical Reduction of Carbon Dioxide

Benefiting from the synergism between adjacent bimetallic atoms, in comparison with single atom catalysts, the dual atom catalysts have displayed great potential in electrocatalytic CO2 reduction reaction (CO2RR). However, the further modulation of the electronic structure of dual atom sites to enhance CO2RR performance still remains a challenge. Herein, an atomically dispersed oxygen-bridged Ni2N6O/NC catalyst with unique Ni-O-Ni sites is successfully synthesized through the microwave pyrolysis of the supported mixture containing the dinuclear nickel phthalocyanine and glucose on N-doped carbon nanosheets. Experiments and density functional theory calculation reveal that the Ni-O-Ni sites can adsorb H+ from the KHCO3 electrolyte to in situ-form the unique Ni-OH-Ni sites without Ni─Ni bonding interaction, which effectively lowers the energy barrier towards the formation of *COOH from CO2. As a result, the Ni2N6OH/NC catalyst exhibits a 99.4% of CO Faradaic efficiency with a 32.4 mA·cm-2 of CO partial current density at -0.7 V versus RHE in H-cell, much superior to the Ni2N6/NC with a Ni-Ni bonding interaction prepared by a similar procedure to that for Ni2N6O/NC but replacing microwave pyrolysis by a traditional heating process.

Strategies for Achieving Carbon Neutrality: Dual-Atom Catalysts in Focus

Carbon neutrality is a fundamental strategy for achieving the sustainable development of human society. Catalyzing CO2 reduction into various high-value-added fuels serves as an effective pathway to achieve this strategic objective. Atom-dispersed catalysts have received extensive attention due to their maximum atomic utilization, high catalytic selectivity, and exceptional catalytic performance. Dual-atom catalysts (DACs), as an extension of single-atom catalysts (SACs), not only retain the advantages of SACs, but also produce many new properties. This review initiates its exploration by elucidating the mechanism of CO2 reduction reaction (CO2RR) from CO2 adsorption and CO2 activation. Then, a comprehensive summary of recently developed preparation methods of DACs is presented. Importantly, the mechanisms underlying the promoted catalytic performance of DACs in comparison to SACs are subjected to a comprehensive analysis from adjustable adsorption capacity, tunable electronic structure, strong synergistic effect, and enhanced spacing effect, elucidating their respective superiorities in CO2RR. Subsequently, the application of DACs in CO2RR is discussed in detail. Conclusively, the prospective trajectories and inherent challenges of CO2RR are expounded upon concerning the continued advancement of DACs. This thorough review not only enhances the comprehension of DACs within CO2RR but also accentuates the prospective developments in the design of sophisticated catalytic materials.

Polarization-Induced Internal Electric Field-Dominated S-Scheme KNbO3-CuO Heterojunction for Photoreduction of CO2 with High CH4 Selectivity

The polarization-induced internal electric field (IEF) in ferroelectric materials could promote photogenerated charge transfer across the heterojunction interface, but the effect of polarization-induced IEF on the mechanism of photogenerated charge transfer is ambiguous. In this study, a KNbO3-CuO heterojunction was synthesized by depositing copper oxide (CuO) onto KNbO3. Incorporating CuO broadens the light absorption of KNbO3, thereby enhancing the dissociation of the photogenerated charges. The results show that the polarization-induced IEF in KNbO3 determines that the charge transport mechanism in the KNbO3-CuO heterojunction follows the S-scheme. Owing to the S-scheme heterojunctions and efficient CO2 capture and activation by CuO, the CH4 production rate of KNbO3-CuO increased by nearly 26 times compared to KNbO3. Additionally, the CH4 selectivity of KNbO3-CuO could reach up to 97.80%. This research offers valuable insights into enhancing the photogenerated charge separation and constructing heterojunctions.

Geminal Synergy in Pt-Co Dual-Atom Catalysts: From Synthesis to Photocatalytic Hydrogen Production

Dual-atom catalysts (DACs) have garnered significant interest due to their high atom utilization and synergistic catalysis. However, developing a precise synthetic method for DACs and comprehending the underlying catalytic mechanisms remain challenging. In this study, we employ a photoinduced anchoring strategy to precisely synthesize PtCo DAC on graphitic carbon nitride (CN). A Co atom was anchored on CN through the lone-pair electrons of nitrogen. Upon light irradiation, photoelectrons gathering at the Co site can anchor Pt metal ions nearby, accurately facilitating the formation of heteronuclear DACs. The PtCo DAC demonstrates a remarkably high H2 generation rate from ammonia borane (AB) hydrolysis, with a TOF of 3130 molH2 molPt-1 min-1 at 298 K. This TOF value is approximately 3.2 times higher than that of the Pt single-atom photocatalyst. Importantly, the PtCo DAC shows good stability, achieving a turnover number as high as 307,982 molH2 molPt-1 at room temperature. The experimental and theoretical calculation results demonstrate that the synergy between Pt and Co optimizes the adsorption energy of AB and H2 molecules while reducing the energy barrier of the rate-determining step, thus accelerating H2 evolution from AB hydrolysis. Additionally, the introduced Co species stabilize the Pt active sites by enhancing the stability of the Pt-N bond, preventing leaching, aggregation, and deactivation. The excellent catalytic performance, good stability, and low cost of the catalysts in this work open new prospects for their practical application in hydrogen production.

Efficient photoreduction of CO2 to CO by Co-ZIL-L derived NiCo-OH with ultrathin nanosheet assembled 2D leaf superstructure

The photocatalytic reduction of CO2 into valuable chemicals and fuels is considered a promising solution to address the energy crisis and environmental challenges. In this work, we introduce a Co-ZIL-L mediated in situ etching and integration process to prepare NiCo-OH with an ultrathin nanosheet-assembled 2D leaf-like superstructure (NiCo-OH UNLS). The resulting catalyst demonstrates excellent photocatalytic performance for CO2 reduction, achieving a CO evolution rate as high as 309.5 mmol g-1 h-1 with a selectivity of 91.0%. Systematic studies reveal that the ultrathin nanosheet structure and 2D leaf-like architecture not only enhance the transfer efficiency of photoexcited electrons but also improve the accessibility of active reaction sites. Additionally, the Ni-Co dual sites in NiCo-OH UNLS accelerate CO2 conversion kinetics by stabilizing the *COOH intermediate, significantly contributing to its high activity. This work offers valuable insights for designing advanced photocatalysts for CO2 conversion.

Confinement Synthesis of Atomic Copper-Anchored Polymeric Carbon Nitride in Crystalline UiO-66-NH2 for High-Performance CO2-to-CH3OH Photocatalysis

Photocatalytic CO2 reduction to value-added fuels displays an attractive scenario to enhance energy supply and reduce global warming. We report herein the confinement synthesis of polymeric carbon nitride (PCN) incorporating with Cu single atoms (CuSAs) inside the crystalline UiO-66-NH2, which combines the merits of heterojunction photocatalysis and single-atom catalysis (SAC) to achieve high-performance CO2-to-CH3OH conversion. A series of spectral studies displays the formation of CuSAs@PCN inside the crystalline UiO-66-NH2. Remarkably, the ternary composite shows an excellent photocatalytic turnover frequency of 4.15 mmol ⋅ h-1 ⋅ g-1 for CO2-to-CH3OH conversion. Theoretical and experimental studies demonstrate the doping of CuSAs, as well as the formation of type-II heterojunction, are causal factors to achieve CH3OH generation. The study provides new insights designing high-performance photocatalyst for CO2 conversion to fuels at atomic scale.

Engineering of Graphitic Carbon Nitride (g-C3N4) Based Photocatalysts for Atmospheric Protection: Modification Strategies, Recent Progress, and Application Challenges

Graphitic carbon nitride (g-C3N4) is a prominent photocatalyst that has attracted substantial interest in the field of photocatalytic environmental remediation due to the low cost of fabrication, robust chemical structure, adaptable and tunable energy bandgaps, superior photoelectrochemical properties, cost-effective feedstocks, and distinctive framework. Nonetheless, the practical application of bulk g-C3N4 in the photocatalysis field is limited by the fast recombination of photogenerated e--h+ pairs, insufficient surface-active sites, and restricted redox capacity. Consequently, a great deal of research has been devoted to solving these scientific challenges for large-scale applications. This review concisely presents the latest advancements in g-C3N4-based photocatalyst modification strategies, and offers a comprehensive analysis of the benefits and preparation techniques for each strategy. It aims to articulate the complex relationship between theory, microstructure, and activities of g-C3N4-based photocatalysts for atmospheric protection. Finally, both the challenges and opportunities for the development of g-C3N4-based photocatalysts are highlighted. It is highly believed that this special review will provide new insight into the synthesis, modification, and broadening of g-C3N4-based photocatalysts for atmospheric protection.

Outstanding CO2 Photoreduction in Single-Atom Thulium Modified Carbon Nitride

CO2 reduction photocatalysts are favorable for obtaining renewable energy. Enriched active sites and effective photogenerated-carriers separation are keys for improving CO2 photo-reduction. A thulium (Tm) single atom tailoring strategy introducing carbon vacancies in porous tubular graphitic carbon nitride (g-C3N4) surpassing the ever-reported g-C3N4 based photocatalysts, with 199.47 µmol g-1 h-1 CO yield, 96.8% CO selectivity, 0.84% apparent quantum efficiency and excellent photocatalytic stability, is implemented in this work. Results revealed that in-plane Tm sites and interlayer-bridged Tm-N charge transfer channels significantly enhanced the aggregation/transfer of photogenerated electrons thus promoting CO2 adsorption/activation and contributing to *COOH intermediates formation. Meanwhile, Tm atoms and carbon vacancies both benefit for rich active sites and enhanced photogenerated-charge separation, thus optimizing reaction pathway and leading to excellent CO2 photo-reduction. This work not only provides guidelines for CO2 photo-reduction catalysts design but also offers mechanistic insights into single-atom based photocatalysts for solar fuel production.

Supported Au single atoms and nanoparticles on MoS2 for highly selective CO2-to-CH3COOH photoreduction

Effectively controlling the selective conversion of CO2 photoreduction to C2 products presents a significant challenge. Here, we develop a heterojunction photocatalyst by controllably implanting Au nanoparticles and single atoms into unsaturated Mo atoms of edge-rich MoS2, denoted as Aun/Au1-CMS. Photoreduction of CO2 results in the production of CH3COOH with a selectivity of 86.4%, which represents a 6.4-fold increase compared to samples lacking single atoms, and the overall selectivity for C2 products is 95.1%. Furthermore, the yield of CH3COOH is 22.4 times higher compared to samples containing single atoms and without nanoparticles. Optical experiments demonstrate that the single atoms domains can effectively capture photoexcited electrons by the Au nanoparticles, or the local electric field generated by the nanoparticles promotes the transfer of photogenerated electrons in MoS2 to Au single atoms, prolonging the relaxation time of photogenerated electrons. Mechanistic investigations reveal that the orbital coupling of Au5d and Mo4d strengthens the oxygen affinity of Mo and carbon affinity of Au. The hybridized orbitals reduce energy splitting levels of CO molecular orbitals, aiding C-C coupling. Moreover, the Mo-Au dual-site stabilize the crucial oxygen-associated intermediate *CH2CO, thereby enhancing the selectivity towards CH3COOH. The cross-scale heterojunctions provide an effective strategy to simultaneously address the kinetical and thermodynamical limitations of CO2-to-CH3COOH conversion.

Asymmetric Atomic Dual-Sites for Photocatalytic CO2 Reduction

Atomically dispersed active sites in a photocatalyst offer unique advantages such as locally tuned electronic structures, quantum size effects, and maximum utilization of atomic species. Among these, asymmetric atomic dual-sites are of particular interest because their asymmetric charge distribution generates a local built-in electric potential to enhance charge separation and transfer. Moreover, the dual sites provide flexibility for tuning complex multielectron and multireaction pathways, such as CO2 reduction reactions. The coordination of dual sites opens new possibilities for engineering the structure-activity-selectivity relationship. This comprehensive overview discusses efficient and sustainable photocatalysis processes in photocatalytic CO2 reduction, focusing on strategic active-site design and future challenges. It serves as a timely reference for the design and development of photocatalytic conversion processes, specifically exploring the utilization of asymmetric atomic dual-sites for complex photocatalytic conversion pathways, here exemplified by the conversion of CO2 into valuable chemicals.

Toward Functionality and Deactivation of Metal-Single-Atom in Heterogeneous Photocatalysts

Single-atom heterogeneous catalysts (SAHCs) provide an enticing platform for understanding catalyst structure-property-performance relationships. The 100% atom utilization and adjustable local coordination configurations make it easy to probe reaction mechanisms at the atomic level. However, the progressive deactivation of metal-single-atom (MSA) with high surface energy leads to frequent limitations on their commercial viability. This review focuses on the atomistic-sensitive reactivity and atomistic-progressive deactivation of MSA to provide a unifying framework for specific functionality and potential deactivation drivers of MSA, thereby bridging function, purpose-modification structure-performance insights with the atomistic-progressive deactivation for sustainable structure-property-performance accessibility. The dominant functionalization of atomically precise MSA acting on properties and reactivity encompassing precise photocatalytic reactions is first systematically explored. Afterward, a detailed analysis of various deactivation modes of MSA and strategies to enhance their durability is presented, providing valuable insights into the design of SAHCs with deactivation-resistant stability. Finally, the remaining challenges and future perspectives of SAHCs toward industrialization, anticipating shedding some light on the next stage of atom-economic chemical/energy transformations are presented.

Manipulating the Coordination Structure of Molecular Cobalt Sites in Periodic Mesoporous Organosilica for CO2 Photoreduction

Photocatalytic CO2 reduction, including reaction rate, product selectivity, and longevity, is highly sensitive to the coordination structure of the catalytic active sites, and the precise design of the active site remains a challenge in heterogeneous catalysts. Herein, we report on the modulation of the coordination structure of MN x -type active sites (M = Co or Ni; x = 4 or 5) anchored on a periodic mesoporous organosilica (PMO) support to improve photocatalytic CO2 reduction. The PMO was functionalized with pendant 3,6-di(2'-pyridyl)pyridazine (dppz) groups to allow immobilization of molecular Co and Ni complexes with polypyridine ligands. A comparative analysis of CO2 photoreduction in the presence of an organic photosensitizer (4CzIPN, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene) and a conventional [Ru(bpy)3]Cl2 sensitizer revealed strong influence of the coordination environment on the catalytic performance. CoN5-PMO demonstrated a superior CO2 photoreduction activity than the other materials and displayed a cobalt-based turnover number (TONCO) of 92 for CO evolution at ∼75% selectivity after 3 h irradiation in the presence of 4CzIPN. The hybrid CoN5-PMO catalyst exhibited better activity than its homogeneous [CoN5] counterpart, indicating that the heterogenization promotes the formation of isolated active sites with improved longevity and faster catalytic rate.

Engineering Spatially Adjacent Redox Sites with Synergistic Spin Polarization Effect to Boost Photocatalytic CO2 Methanation

The integration of oxidation and reduction half-reactions to amplify their synergy presents a considerable challenge in CO2 photoconversion. Addressing this challenge requires the construction of spatially adjacent redox sites while suppressing charge recombination at these sites. This study introduces an innovative approach that utilizes spatial synergy to enable synergistic redox reactions within atomic proximity and employs spin polarization to inhibit charge recombination. We incorporate Mn into Co3O4 as a catalyst, in which Mn sites tend to enrich holes as water activation sites, while adjacent Co sites preferentially capture electrons to activate CO2, forming a spatial synergy. The direct H transfer from H2O at Mn sites facilitates the formation of *COOH on adjacent Co sites with remarkably favorable thermodynamic energy. Notably, the incorporation of Mn induces spin polarization in the system, significantly suppressing the recombination of photogenerated charges at redox sites. This effect is further enhanced by applying an external magnetic field. By synergizing spatial synergy and spin polarization, Mn/Co3O4 exhibits a CH4 production rate of 23.4 μmol g-1 h-1 from CO2 photoreduction, showcasing a 28.8 times enhancement over Co3O4. This study first introduces spin polarization to address charge recombination issues at spatially adjacent redox sites, offering novel insights for synergistic redox photocatalytic systems.

Noble-Metal-Free Single- and Dual-Atom Catalysts for Artificial Photosynthesis

Artificial photosynthesis enables direct solar-to-chemical energy conversion aimed at mitigating environmental pollution and producing solar fuels and chemicals in a green and sustainable approach, and efficient, robust, and low-cost photocatalysts are the heart of artificial photosynthesis systems. As an emerging new class of cocatalytic materials, single-atom catalysts (SACs) and dual-atom catalysts (DACs) have received a great deal of current attention due to their maximal atom utilization and unique photocatalytic properties, whereas noble-metal-free ones impart abundance, availability, and cost-effectiveness allowing for scalable implementation. This review outlines the fundamental principles and synthetic methods of SACs and DACs and summarizes the most recent advances in SACs (Co, Fe, Cu, Ni, Bi, Al, Sn, Er, La, Ba, etc.) and DACs (CuNi, FeCo, InCu, KNa, CoCo, CuCu, etc.) based on non-noble metals, confined on an arsenal of organic or inorganic substrates (polymeric carbon nitride, metal oxides, metal sulfides, metal-organic frameworks, carbon, etc.) acting as versatile scaffolds in solar-light-driven photocatalytic reactions, including hydrogen evolution, carbon dioxide reduction, methane conversion, organic synthesis, nitrogen fixation, hydrogen peroxide production, and environmental remediation. The review concludes with the challenges, opportunities, and future prospects of noble-metal-free SACs and DACs for artificial photosynthesis.

Research progress of dual-atom site catalysts for photocatalysis

Dual-atom site catalysts (DASCs) have sparked considerable interest in heterogeneous photocatalysis as they possess the advantages of excellent photoelectronic activity, photostability, and high carrier separation efficiency and mobility. The DASCs involved in these important photocatalytic processes, especially in the photocatalytic hydrogen evolution reaction (HER), CO2 reduction reaction (CO2RR), N2/nitrate reduction, etc., have been extensively investigated in the past few years. In this review, we highlight the recent progress in DASCs that provides fundamental insights into the photocatalytic conversion of small molecules. The controllable preparation and characterization methods of various DASCs are discussed. Subsequently, the reaction mechanisms of the formation of several important molecules (hydrogen, hydrocarbons and ammonia) on DASCs are introduced in detail, in order to probe the relationship between DASCs's structure and photocatalytic activity. Finally, some challenges and outlooks of DASCs in the photocatalytic conversion of small molecules are summarized and prospected. We hope that this review can provide guidance for in-depth understanding and aid in the design of efficient DASCs for photocatalysis.

Preparation and Application of Single-Atom Cobalt Catalysts in Organic Synthesis and Environmental Remediation

The development of high-performance catalysts plays a crucial role in facilitating chemical production and reducing environmental contamination. Single-atom catalysts (SACs), a class of catalysts that bridge the gap between homogeneous and heterogeneous catalysis, have garnered increasing attention because of their unique activity, selectivity, and stability in many pivotal reactions. Meanwhile, the scarcity of precious metal SACs calls for the arrival of cost-effective SACs. Cobalt, as a common non-noble metal, possesses tremendous potential in the field of single-atom catalysis. Despite their potential, reviews about single-atom Co catalysts (Co-SACs) are lacking. Accordingly, this review thoroughly summarized various preparation methodologies of Co-SACs, particularly pyrolysis; its application in the specific domain of organic synthesis and environmental remediation is discussed as well. The structure-activity relationship and potential catalytic mechanism of Co-SACs are elucidated through some representative reactions. The imminent challenges and development prospects of Co-SACs are discussed in detail. The findings and insights provided herein can guide further exploration and development in this charming area of catalyst design, leading to the realization of efficient and sustainable catalytic processes.

Current Status and Perspectives of Dual-Atom Catalysts Towards Sustainable Energy Utilization

The exploration of sustainable energy utilization requires the implementation of advanced electrochemical devices for efficient energy conversion and storage, which are enabled by the usage of cost-effective, high-performance electrocatalysts. Currently, heterogeneous atomically dispersed catalysts are considered as potential candidates for a wide range of applications. Compared to conventional catalysts, atomically dispersed metal atoms in carbon-based catalysts have more unsaturated coordination sites, quantum size effect, and strong metal-support interactions, resulting in exceptional catalytic activity. Of these, dual-atomic catalysts (DACs) have attracted extensive attention due to the additional synergistic effect between two adjacent metal atoms. DACs have the advantages of full active site exposure, high selectivity, theoretical 100% atom utilization, and the ability to break the scaling relationship of adsorption free energy on active sites. In this review, we summarize recent research advancement of DACs, which includes (1) the comprehensive understanding of the synergy between atomic pairs; (2) the synthesis of DACs; (3) characterization methods, especially aberration-corrected scanning transmission electron microscopy and synchrotron spectroscopy; and (4) electrochemical energy-related applications. The last part focuses on great potential for the electrochemical catalysis of energy-related small molecules, such as oxygen reduction reaction, CO2 reduction reaction, hydrogen evolution reaction, and N2 reduction reaction. The future research challenges and opportunities are also raised in prospective section.

Dual-Atom Catalyst with N-Colligated Zn1Co1 Species as Dominant Active Sites for Propane Dehydrogenation

Dual-atom catalysts (DACs) with paired active sites can provide unique intrinsic properties for heterogeneous catalysis, but the synergy of the active centers remains to be elucidated. Here, we develop a high-performance DAC with Zn1Co1 species anchored on nitrogen-doped carbon (Zn1Co1/NC) as the dominant active site for the propane dehydrogenation (PDH) reaction. It exhibits several times higher turnover frequency (TOF) of C3H8 conversion and enhanced C3H6 selectivity compared to Zn1/NC or Co1/NC with only a single-atom site. Various experimental and theoretical studies suggest that the enhanced PDH performance stems from the promoted activation of the C-H bond of C3H8 triggered by the electronic interaction between Zn1 and Co1 colligated by N species. Moreover, the dynamic sinking of the Zn1 site and rising of the Co1 site, together with the steric effect of the dissociated H species at the bridged N during the PDH reaction, provides a feasible channel for C3H6 desorption through the more exposed Co1 site, thereby boosting the selectivity. This work provides a promising strategy for designing robust hetero DACs to simultaneously increase activity and selectivity in the PDH reaction.

Atomic Distance Engineering in Metal Catalysts to Regulate Catalytic Performance

It is very important to understand the structure-performance relationship of metal catalysts by adjusting the microstructure of catalysts at the atomic scale. The atomic distance has an essential influence on the composition of the environment of active metal atom, which is a key factor for the design of targeted catalysts with desired function. In this review, we discuss and summarize strategies for changing the atomic distance from three aspects and relate their effects on the reactivity of catalysts. First, the effects of regulating bond length between metal and coordination atom at one single-atom site on the catalytic performance are introduced. The bond lengths are affected by the strain effect of the support and high-shell doping and can evolve during the reaction. Next, the influence of the distance between single-atom sites on the catalytic performance is discussed. Due to the space matching of adsorption and electron transport, the catalytic performance can be adjusted with the shortening of site distance. In addition, the effect of the arrangement spacing of the surface metal active atoms on the catalytic performance of metal nanocatalysts is studied. Finally, a comprehensive summary and outlook of the relationship between atomic distance and catalytic performance is given.

Asymmetric Cu-N-La Species Enabling Atomic-Level Donor-Acceptor Structure and Favored Reaction Thermodynamics for Selective CO2 Photoreduction to CH4

Photocatalytic CO2 reduction into ideal hydrocarbon fuels, such as CH4 , is a sluggish kinetic process involving adsorption of multiple intermediates and multi-electron steps. Achieving high CH4 activity and selectivity therefore remains a great challenge, which largely depends on the efficiency of photogenerated charge separation and transfer as well as the intermediate energy levels in CO2 reduction. Herein, we construct La and Cu dual-atom anchored carbon nitride (LaCu/CN), with La-N4 and Cu-N3 coordination bonds connected by Cu-N-La bridges. The asymmetric Cu-N-La species enables the establishment of an atomic-level donor-acceptor structure, which allows the migration of electrons from La atoms to the reactive Cu atom sites. Simultaneously, intermediates during CO2 reduction on LaCu/CN demonstrate thermodynamically more favorable process for CH4 formation based on theoretical calculations. Eventually, LaCu/CN exhibits a high selectivity (91.6 %) for CH4 formation with a yield of 125.8 μmol g-1 , over ten times of that for pristine CN. This work presents a strategy for designing multi-functional dual-atom based photocatalysts.

Non-Interacting Ni and Fe Dual-Atom Pair Sites in N-Doped Carbon Catalysts for Efficient Concentrating Solar-Driven Photothermal CO2 Reduction

Solar-to-chemical energy conversion under weak solar irradiation is generally difficult to meet the heat demand of CO2 reduction. Herein, a new concentrated solar-driven photothermal system coupling a dual-metal single-atom catalyst (DSAC) with adjacent Ni-N4 and Fe-N4 pair sites is designed for boosting gas-solid CO2 reduction with H2 O under simulated solar irradiation, even under ambient sunlight. As expected, the (Ni, Fe)-N-C DSAC exhibits a superior photothermal catalytic performance for CO2 reduction to CO (86.16 μmol g-1  h-1 ), CH4 (135.35 μmol g-1  h-1 ) and CH3 OH (59.81 μmol g-1  h-1 ), which are equivalent to 1.70-fold, 1.27-fold and 1.23-fold higher than those of the Fe-N-C catalyst, respectively. Based on theoretical simulations, the Fermi level and d-band center of Fe atom is efficiently regulated in non-interacting Ni and Fe dual-atom pair sites with electronic interaction through electron orbital hybridization on (Ni, Fe)-N-C DSAC. Crucially, the distance between adjacent Ni and Fe atoms of the Ni-N-N-Fe configuration means that the additional Ni atom as a new active site contributes to the main *COOH and *HCO3 dissociation to optimize the corresponding energy barriers in the reaction process, leading to specific dual reaction pathways (COOH and HCO3 pathways) for solar-driven photothermal CO2 reduction to initial CO production.

WS2 supported PtOx clusters for efficient photocatalytic CO2 reduction: a DFT study

Platinum (Pt) nanoparticles/nanoclusters are some of the most efficient cocatalysts for photocatalytic CO2 reduction. Nevertheless, the produced CO can lead to a poisoning effect due to the strong adsorption strength of the Pt cocatalysts. Using density functional theory, PtOx clusters with variable sizes (Pt4O6, Pt5O8, Pt7O10, and Pt8O13) are selected to load on WS2 (PtOx-WS2) for photocatalytic CO2 conversion. The calculated results demonstrate that PtOx-WS2 are highly stable, and the electron-rich PtOx clusters are beneficial for the photocatalytic CO2 reduction. All the PtOx-WS2 catalysts exhibit efficient photocatalytic performance for CO2 reduction. Especially, Pt4O6-, Pt5O8-, and Pt8O13-WS2 have acceptable or ultra-low ΔGmax (ΔG for the rate-determining step) of 0.57, 0.23, and 0.48 eV to produce CH3OH, HCOOH, and CH4, respectively. The photocatalytic activities of PtOx-WS2 are correlated with the adsorption strength of the key intermediates, and the strong interactions between PtOx-WS2 and *COOH or *HCOO can lower the free energy changes for the first hydrogenation step. More importantly, PtOx-WS2 can also weaken the adsorption strength of *CO and *HCOOH, which are conducive to forming *CHO. This work gives an in-depth insight to design novel catalysts and promote their catalytic activity for photocatalytic CO2 reduction.

Enhanced and synergistic catalytic activation by photoexcitation driven S-scheme heterojunction hydrogel interface electric field

The regulation of heterogeneous material properties to enhance the peroxymonosulfate (PMS) activation to degrade emerging organic pollutants remains a challenge. To solve this problem, we synthesize S-scheme heterojunction PBA/MoS2@chitosan hydrogel to achieve photoexcitation synergistic PMS activation. The constructed heterojunction photoexcited carriers undergo redox conversion with PMS through S-scheme transfer pathway driven by the directional interface electric field. Multiple synergistic pathways greatly enhance the reactive oxygen species generation, leading to a significant increase in doxycycline degradation rate. Meanwhile, the 3D polymer chain spatial structure of chitosan hydrogel is conducive to rapid PMS capture and electron transport in advanced oxidation process, reducing the use of transition metal activator and limiting the leaching of metal ions. There is reason to believe that the synergistic activation of PMS by S-scheme heterojunction regulated by photoexcitation will provide a new perspective for future material design and research on enhancing heterologous catalysis oxidation process.

Inter-Metal Interaction of Dual-Atom Catalysts in Heterogeneous Catalysis

Dual-atom catalysts (DACs) have been a new frontier in heterogeneous catalysis due to their unique intrinsic properties. The synergy between dual atoms provides flexible active sites, promising to enhance performance and even catalyze more complex reactions. However, precisely regulating active site structure and uncovering dual-atom metal interaction remain grand challenges. In this review, we clarify the significance of the inter-metal interaction of DACs based on the understanding of active center structures. Three diatomic configurations are elaborated, including isolated dual single-atom, N/O-bridged dual-atom, and direct dual-metal bonding interaction. Subsequently, the up-to-date progress in heterogeneous oxidation reactions, hydrogenation/dehydrogenation reactions, electrocatalytic reactions, and photocatalytic reactions are summarized. The structure-activity relationship between DACs and catalytic performance is then discussed at an atomic level. Finally, the challenges and future directions to engineer the structure of DACs are discussed. This review will offer new prospects for the rational design of efficient DACs toward heterogeneous catalysis.

Single-Atom Cr-N4 Sites with High Oxophilicity Interfaced with Pt Atomic Clusters for Practical Alkaline Hydrogen Evolution Catalysis

Although dispersing Pt atomic clusters (ACs) on a conducting support is a promising way to minimize the Pt amount required in hydrogen evolution reaction (HER), the catalytic mass activity and durability of Pt ACs are often unsatisfactory for alkaline HER due to their unfavorable water dissociation and challenges in stabilizing them against agglomeration and detachment. Herein, we report a class of single-atom Cr-N4 sites with high oxophilicity interfaced with Pt ACs on mesoporous carbon for achieving a highly active and stable alkaline HER in an anion-exchange-membrane water electrolyzer (AEMWE). The as-made catalyst achieves the highest reported Pt mass activity (37.6 times higher than commercial Pt/C) and outstanding operational stability. Experimental and theoretical studies elucidate that the formation of a unique Pt-Cr quasi-covalent bonding interaction at the interface of Cr-N4 sites and Pt ACs effectively suppresses the migration and thermal vibration of Pt atoms to stabilize Pt ACs and contributes to the greatly enhanced catalytic stability. Moreover, oxophilic Cr-N4 sites adjacent to Pt ACs with favorable adsorption of hydroxyl species facilitate nearly barrierless water dissociation and thus enhance the HER activity. An AEMWE using this catalyst (with only 50 μgPt cm-2) can operate stably at an industrial-level current density of 500 mA cm-2 at 1.8 V for >100 h with a small degradation rate of 90 μV h-1.

Dual Atom Catalysts for Energy and Environmental Applications

The pursuit of high metal utilization in heterogeneous catalysis has triggered the burgeoning interest of various atomically dispersed catalysts. Our aim in this review is to assess key recent findings in the synthesis, characterization, structure-property relationship and computational studies of dual-atom catalysts (DACs), which cover the full spectrum of applications in thermocatalysis, electrocatalysis and photocatalysis. In particular, combination of qualitative and quantitative characterization with cooperation with DFT insights, synergies and superiorities of DACs compare to counterparts, high-throughput catalyst exploration and screening with machine-learning algorithms are highlighted. Undoubtably, it would be wise to expect more fascinating developments in the field of DACs as tunable catalysts.

Dual-Active-Sites Single-Atom Catalysts for Advanced Applications

Dual-Active-Sites Single-Atom catalysts (DASs SACs) are not only the improvement of SACs but also the expansion of dual-atom catalysts. The DASs SACs contains dual active sites, one of which is a single atomic active site, and the other active site can be a single atom or other type of active site, endowing DASs SACs with excellent catalytic performance and a wide range of applications. The DASs SACs are categorized into seven types, including the neighboring mono metallic DASs SACs, bonded DASs SACs, non-bonded DASs SACs, bridged DASs SACs, asymmetric DASs SACs, metal and nonmetal combined DASs SACs and space separated DASs SACs. Based on the above classification, the general methods for the preparation of DASs SACs are comprehensively described, especially their structural characteristics are discussed in detail. Meanwhile, the in-depth assessments of DASs SACs for variety applications including electrocatalysis, thermocatalysis and photocatalysis are provided, as well as their unique catalytic mechanism are addressed. Moreover, the prospects and challenges for DASs SACs and related applications are highlighted. The authors believe the great expectations for DASs SACs, and this review will provide novel conceptual and methodological perspectives and exciting opportunities for further development and application of DASs SACs.

Dual Lewis Acid-Base Sites Regulate Silver-Copper Bimetallic Oxide Nanowires for Highly Selective Photoreduction of Carbon Dioxide to Methane

Highly selective photoreduction of CO2 to valuable hydrocarbons is of great importance to achieving a carbon-neutral society. Precisely manipulating the formation of the Metal1 ⋅⋅⋅C=O⋅⋅⋅Metal2 (M1 ⋅⋅⋅C=O⋅⋅⋅M2 ) intermediate on the photocatalyst interface is the most critical step for regulating selectivity, while still a significant challenge. Herein, inspired by the polar electronic structure feature of CO2 molecule, we propose a strategy whereby the Lewis acid-base dual sites confined in a bimetallic catalyst surface are conducive to forming a M1 ⋅⋅⋅C=O⋅⋅⋅M2 intermediate precisely, which can promote selectivity to hydrocarbon formation. Employing the Ag2 Cu2 O3 nanowires with abundant Cu⋅⋅⋅Ag Lewis acid-base dual sites on the preferred exposed {110} surface as a model catalyst, 100 % selectivity toward photoreduction of CO2 into CH4 has been achieved. Subsequent surface-quenching experiments and density functional theory (DFT) calculations verify that the Cu⋅⋅⋅Ag Lewis acid-base dual sites do play a vital role in regulating the M1 ⋅⋅⋅C=O⋅⋅⋅M2 intermediate formation that is considered to be prone to convert CO2 into hydrocarbons. This study reports a highly selective CO2 photocatalyst, which was designed on the basis of a newly proposed theory for precise regulation of reaction intermediates. Our findings will stimulate further research on dual-site catalyst design for CO2 reduction to hydrocarbons.

Polarized Active Pairs at Grain Boundary Boost CO2 Chemical Fixation

The chemical fixation of CO2 as a C1 feedstock is considered one of the most promising ways to obtain long-chain chemicals, but its efficiency was limited by the ineffective activation of CO2. Herein, we propose a grain boundary engineering strategy to construct polarized active pairs with electron poor-rich character for effective CO2 activation. By taking CeO2 as a model system, we illustrate that the polarized "Ce4+-Ce3+-Ce4+" pairs at the grain boundary can simultaneously accept and donate electrons to coordinate with O and C, respectively, in CO2. By the combination of synchrotron radiation in situ technique and density functional theory calculations, the mechanism of the catalytic reaction has been systematically investigated. As a result, the CeO2 nanosheets with a rich grain boundary show a high DMC yield of 60.3 mmol/gcat with 100% atomic economy. This study provides a practical way for the chemical fixation of CO2 to high-value-added chemicals via grain boundary engineering.

Atomic Interface Engineering of Single-Atom Pt/TiO2 -Ti3 C2 for Boosting Photocatalytic CO2 Reduction

Solar-driven CO2 conversion into valuable fuels is a promising strategy to alleviate the energy and environmental issues. However, inefficient charge separation and transfer greatly limits the photocatalytic CO2 reduction efficiency. Herein, single-atom Pt anchored on 3D hierarchical TiO2 -Ti3 C2 with atomic-scale interface engineering is successfully synthesized through an in situ transformation and photoreduction method. The in situ growth of TiO2 on Ti3 C2 nanosheets can not only provide interfacial driving force for the charge transport, but also create an atomic-level charge transfer channel for directional electron migration. Moreover, the single-atom Pt anchored on TiO2 or Ti3 C2 can effectively capture the photogenerated electrons through the atomic interfacial PtO bond with shortened charge migration distance, and simultaneously serve as active sites for CO2 adsorption and activation. Benefiting from the synergistic effect of the atomic interface engineering of single-atom Pt and interfacial TiOTi, the optimized photocatalyst exhibits excellent CO2 -to-CO conversion activity of 20.5 µmol g-1  h-1 with a selectivity of 96%, which is five times that of commercial TiO2 (P25). This work sheds new light on designing ideal atomic-scale interface and single-atom catalysts for efficient solar fuel conversation.

Polystyrene Waste Thermochemical Hydrogenation to Ethylbenzene by a N-Bridged Co, Ni Dual-Atom Catalyst

Recycling waste plastics requires the degradation of plastics into small molecules. However, various products are widely distributed using traditional methods of depolymerizing polystyrene (PS) such as catalytic pyrolysis and hydrogenolysis. Here, we creatively report a N-bridged Co, Ni dual-atom (Co-N-Ni) catalyst for the targeted conversion of waste PS plastics to ethylbenzene via a pressurized tandem fixed-bed reactor where hydropyrolysis is coupled with downstream vapor-phase hydrotreatment. The Co-N-Ni catalyst achieves 95 wt % PS conversion with 92 wt % ethylbenzene yield, significantly superior to the corresponding single-atom catalysts, and enables degradation of real PS plastics. Theoretical calculations and experimental results demonstrate that the d-band center of metal atoms is well regulated in the Co-N-Ni catalyst. The Co site activates the C═C bond more easily, while the Ni site spatially optimizes the adsorption configuration of the styrene molecule due to the electronic interaction. This Co-N-Ni catalyst in the tandem reactor also shows excellent durability and provides a new direction for real plastic degradation.

Asymmetric gradient orbital interaction of hetero-diatomic active sites for promoting C - C coupling

Diatomic-site catalysts (DACs) garner tremendous attention for selective CO₂ photoreduction, especially in the thermodynamical and kinetical mechanism of CO₂ to C₂₊ products. Herein, we first engineer a novel Zn-porphyrin/RuCu-pincer complex DAC (ZnPor-RuCuDAC). The heteronuclear ZnPor-RuCuDAC exhibits the best acetate selectivity (95.1%), while the homoatomic counterparts (ZnPor-Ru₂DAC and ZnPor-Cu₂DAC) present the best CO selectivity. In-situ spectroscopic measurements reveal that the heteronuclear Ru-Cu sites easily appear C₁ intermediate coupling. The in-depth analyses confirm that due to the strong gradient orbital coupling of Ru4d-Cu3d resonance, two formed ^*CO intermediates of Ru-Cu heteroatom show a significantly weaker electrostatic repulsion for an asymmetric charge distribution, which result from a side-to-side absorption and narrow dihedral angle distortion. Moreover, the strongly overlapped Ru/Cu-d and CO molecular orbitals split into bonding and antibonding orbitals easily, resulting in decreasing energy splitting levels of C₁ intermediates. These results collectively augment the collision probability of the two ^*CO intermediates on heteronuclear DACs. This work first provides a crucial perspective on the symmetry-forbidden coupling mechanism of C₁ intermediates on diatomic sites.

Retrospective insights into recent MXene-based catalysts for CO2 electro/photoreduction: how far have we gone?

The electro/photocatalytic CO₂ reduction reaction (CO₂RR) is a long-term avenue toward synthesizing renewable fuels and value-added chemicals, as well as addressing the global energy crisis and environmental challenges. As a result, current research studies have focused on investigating new materials and implementing numerous fabrication approaches to increase the catalytic performances of electro/photocatalysts toward the CO₂RR. MXenes, also known as 2D transition metal carbides, nitrides, and carbonitrides, are intriguing materials with outstanding traits. Since their discovery in 2011, there has been a flurry of interest in MXenes in electrocatalysis and photocatalysis, owing to their several benefits, including high mechanical strength, tunable structure, surface functionality, high specific surface area, and remarkable electrical conductivity. Herein, this review serves as a milestone for the most recent development of MXene-based catalysts for the electrocatalytic and photocatalytic CO₂RR. The overall structure of MXenes is described, followed by a summary of several synthesis pathways classified as top-down and bottom-up approaches, including HF-etching, in situ HF-formation, electrochemical etching, and halogen etching. Additionally, the state-of-the-art development in the field of both the electrocatalytic and photocatalytic CO₂RR is systematically reviewed. Surface termination modulation and heterostructure engineering of MXene-based electro/photocatalysts, and insights into the reaction mechanism for the comprehension of the structure-performance relationship from the CO₂RR via density functional theory (DFT) have been underlined toward activity enhancement. Finally, imperative issues together with future perspectives associated with MXene-based electro/photocatalysts are proposed.

Identification of the Active Sites on Metallic MoO2-x Nano-Sea-Urchin for Atmospheric CO2 Photoreduction Under UV, Visible, and Near-Infrared Light Illumination

We report an oxygen vacancy (V_o )-rich metallic MoO_2-x nano-sea-urchin with partially occupied band, which exhibits super CO₂ (even directly from the air) photoreduction performance under UV, visible and near-infrared (NIR) light illumination. The V_o -rich MoO_2-x nano-sea-urchin displays a CH₄ evolution rate of 12.2 and 5.8 μmol g_catalyst ^-1  h^-1 under full spectrum and NIR light illumination in concentrated CO₂ , which is ca. 7- and 10-fold higher than the V_o -poor MoO_2-x , respectively. More interestingly, the as-developed V_o -rich MoO_2-x nano-sea-urchin can even reduce CO₂ directly from the air with a CO evolution rate of 6.5 μmol g_catalyst ^-1  h^-1 under NIR light illumination. Experiments together with theoretical calculations demonstrate that the oxygen vacancy in MoO_2-x can facilitate CO₂ adsorption/activation to generate *COOH as well as the subsequent protonation of *CO towards the formation of CH₄ because of the formation of a highly stable Mo-C-O-Mo intermediate.

Plasmonic Metal Mediated Charge Transfer in Stacked Core-Shell Semiconductor Heterojunction for Significantly Enhanced CO2 Photoreduction

Construction of core-shell semiconductor heterojunctions and plasmonic metal/semiconductor heterostructures represents two promising routes to improved light harvesting and promoted charge separation, but their photocatalytic activities are respectively limited by sluggish consumption of charge carriers confined in the cores, and contradictory migration directions of plasmon-induced hot electrons and semiconductor-generated electrons. Herein, a semiconductor/metal/semiconductor stacked core-shell design is demonstrated to overcome these limitations and significantly boost the photoactivity in CO₂  reduction. In this smart design, sandwiched Au serves as a "stone", which "kills two birds" by inducing localized surface plasmon resonance for hot electron generation and mediating unidirectional transmission of conduction band electrons and hot electrons from TiO₂  core to MoS₂  shell. Meanwhile, upward band bending of TiO₂  drives core-to-shell migration of holes through TiO₂ -MoS₂  interface. The co-existence of TiO₂  → Au → MoS₂  electron flow and TiO₂  → MoS₂  hole flow contributes to spatial charge separation on different locations of MoS₂  outer layer for overall redox reactions. Additionally, reduction potential of photoelectrons participating in the CO₂  reduction is elaborately adjusted by tuning the thickness of MoS₂  shell, and thus the product selectivity is delicately regulated. This work provides fresh hints for rationally controlling the charge transfer pathways toward high-efficiency CO₂  photoreduction.

Two-dimensional heterostructures for photocatalytic CO2 reduction

The photocatalysis conversion of CO₂ into fuels has become an encouraging method to address climate and energy issues as a long-term solution. Single material suffers poor yield due to low light energy utilization and high recombination rate of photoinduced electron-hole pairs. It is an efficient approach to construct heterojunction through two or three materials to improve the photocatalytic performance. Recently, 2D-based heterojunction is getting popular for outstanding properties, such as special light collecting structure to enhance light harvest, intimate interface to facilitate charge transfer and separation, and large specific surface area to provide abundant reactive sites. Recently, some new 2D-based heterostructures materials (both structure and composition) have been developed with excellent performance. 2D materials exert structural and functional advantages in these fine composite photocatalysts. In this review, the literatures about the photocatalytic conversion of CO₂ are mainly summarized based on overall structure, interface type and material type of 2D-based heterojunction, with special attention given to the preparation, characterization, structural advantages and reaction mechanism of novel 2D-based heterojunction. This work is in hope of offering a basis for designing improved composite photocatalyst for CO₂ photoreduction.

P-Mediated Cu-N4 Sites in Carbon Nitride Realizing CO2 Photoreduction to C2 H4 with Selectivity Modulation

Photocatalytic CO₂ reduction to high value-added C₂ products (e.g., C₂ H₄ ) is of considerable interest but challenging. The C₂ H₄ product selectivity strongly hinges on the intermediate energy levels in the CO₂ reduction pathway. Herein, Cu-N₄ sites anchored phosphorus-modulated carbon nitride (CuACs/PCN) is designed as a photocatalyst to tailor the intermediate energy levels in the the C₂ H₄ formation reaction pathway for realizing its high production with tunable selectivity. Theoretical calculations combined with experimental data demonstrate that the formation of the C-C coupling intermediates can be realized on Cu-N₄ sites and the surrounding doped P facilitates the production of C₂ H₄ . Thus, CuACs/PCN exhibits a high C₂ H₄ selectivity of 53.2% with a yielding rate of 30.51 µmol g^-1 . The findings reveal the significant role of the coordination environment and surrounding microenvironment of Cu single atoms in C₂ H₄ formation and offer an effective approach for highly selective CO₂ photoreduction to produce C₂ H₄ .

Recent Progress of Diatomic Catalysts: General Design Fundamentals and Diversified Catalytic Applications

In recent years, some experiments and theoretical work have pointed out that diatomic catalysts not only retain the advantages of monoatomic catalysts, but also introduce a variety of interactions, which exceed the theoretical limit of catalytic performance and can be applied to many catalytic fields. Here, the interaction between adjacent metal atoms in diatomic catalysts is elaborated: synergistic effect, spacing enhancement effect (geometric effect), and electronic effect. With regard to the classification and characterization of various new diatomic catalysts, diatomic catalysts are classified into four categories: heteronuclear/homonuclear, with/without carbon carriers, and their characterization measures are introduced and explained in detail. In the aspect of preparation of diatomic catalysts, the widely used atomic layer deposition method, metal-organic framework derivative method, and simple ball milling method are introduced, with emphasis on the formation mechanism of diatomic catalysts. Finally, the effective control strategies of four diatomic catalysts and the key applications of diatomic catalysts in electrocatalysis, photocatalysis, thermal catalysis, and other catalytic fields are given.

Phosphorus Tailors the d-Band Center of Copper Atomic Sites for Efficient CO2 Photoreduction under Visible-Light Irradiation

Photoreduction of CO₂ into solar fuels has received great interest, but suffers from low catalytic efficiency and poor selectivity. Herein, two single-Cu-atom catalysts with unique Cu configurations in phosphorus-doped carbon nitride (PCN), namely, Cu₁ N₃ @PCN and Cu₁ P₃ @PCN were fabricated via selective phosphidation, and tested in visible light-driven CO₂ reduction by H₂ O without sacrificial agents. Cu₁ N₃ @PCN was exclusively active for CO production with a rate of 49.8 μmol_CO  g_cat ^-1  h^-1 , outperforming most polymeric carbon nitride (C₃ N₄ ) based catalysts, while Cu₁ P₃ @PCN preferably yielded H₂ . Experimental and theoretical analysis suggested that doping P in C₃ N₄ by replacing a corner C atom upshifted the d-band center of Cu in Cu₁ N₃ @PCN close to the Fermi level, which boosted the adsorption and activation of CO₂ on Cu₁ N₃ , making Cu₁ N₃ @PCN efficiently convert CO₂ to CO. In contrast, Cu₁ P₃ @PCN with a much lower Cu 3d electron energy exhibited negligible CO₂ adsorption, thereby preferring H₂ formation via photocatalytic H₂ O splitting.

Unpaired Electron Engineering Enables Efficient and Selective Photocatalytic CO2 Reduction to CH4

The photocatalytic CO₂ reduction to CH₄ reaction is a long process of proton-coupled charge transfer accompanied by various reaction intermediates. Achieving high CH₄ selectivity with satisfactory conversion efficiency therefore remains rather challenging. Herein, we propose a novel strategy of unpaired electron engineering to break through such a demanding bottleneck. By taking TiO₂ as a photocatalyst prototype, we prove that unpaired electrons stabilize the key intermediate of CH₄ production, i.e., CHO*, via chemical bonding, which converts the endothermic step of CHO* formation to an exothermic process, thereby altering the reaction pathway to selectively produce CH₄. Meanwhile, these unpaired electrons generate midgap states to restrict charge recombination by trapping free electrons. As an outcome, such an unpaired electron-engineered TiO₂ achieves an electron-consumption rate as high as 28.3 μmol·g^-1·h^-1 (15.7-fold with respect to normal TiO₂) with a 97% CH₄ selectivity. This work demonstrates that electron regulation holds great promise in attaining efficient and selective heterogeneous photocatalytic conversion.

Modulating the Electronic Coordination Configuration and d-Band Center in Homo-Diatomic Fe2N6 Catalysts for Enhanced Peroxymonosulfate Activation

The electronic coordination configuration of metal active sites and the reaction mechanism were investigated by constructing homo-diatomic Fe sites for visible-light-assisted heterogeneous peroxymonosulfate (PMS) activation. A novel Fe₂N₆ catalyst was synthesized by selecting uniform pyridinic-N of graphitic carbon nitride (g-C₃N₄) as anchoring sites. The results demonstrated that homo-diatomic Fe sites modulated the d-band center and electron delocalization and thus enhanced the PMS activation kinetics (3.58 times vs single-atom Fe catalyst) with k_obs of 0.111 min^-1 owing to the synergistic effect between adjacent Fe atoms. New Fe-Fe coordination significantly decreased the contribution of the antibonding state in the Fe-O bond due to the coupling of the Fe-3d orbitals, which facilitated the O-O bond cleavage of the Fe₂-HOO-SO₃ complex with a reduced thermodynamic energy barrier of only -0.29 eV. This work provided comprehensive mechanistic insights into developing homo-diatomic catalysts governed by the coordination configuration and radical pathway for efficient heterogeneous PMS catalysis.

Synergetic Dual-Atom Catalysts: The Next Boom of Atomic Catalysts

Dual-atom catalysts (DACs) are an important branch of single-atom catalysts (SACs), in which the former can effectively break the dilemma faced by the traditional SACs. The synergetic effects between bimetallic atoms provide many active sites, promising to improve catalytic performance and even catalyze more complex reactions. This paper reviews the recent research progresses of two kinds of DACs, including homonuclear and heteronuclear DACs, and their applications in oxygen reduction, carbon dioxide reduction, hydrogen evolution, oxygen evolution, Zn-air batteries, tandem catalytic reactions, and so on. In addition, in order to promote the further development of DACs, the challenges and perspectives of DACs are put forward.