Tandem Photocatalysis of CO2 to C2H4 via a Synergistic Rhenium-(I) Bipyridine/Copper-Porphyrinic Triazine Framework

The photocatalytic conversion of CO₂ into C₂₊ products such as ethylene is a promising path toward the carbon neutral goal but remains a big challenge due to the high activation barrier for CO₂ and similar reduction potentials of many possible multi-electron-transfer products. Herein, an effective tandem photocatalysis strategy has been developed to support conversion of CO₂ to ethylene by construction of the synergistic dual sites in rhenium-(I) bipyridine fac-[Re^I(bpy)(CO)₃Cl] (Re-bpy) and copper-porphyrinic triazine framework [PTF(Cu)]. With these two catalysts, a large amount of ethylene can be produced at a rate of 73.2 μmol g^-1 h^-1 under visible light irradiation. However, ethylene cannot be obtained from CO₂ by use of either component of the Re-bpy or PTF(Cu) catalysts alone; with a single catalyst, only monocarbon product CO is produced under similar conditions. In the tandem photocatalytic system, the CO generated at the Re-bpy sites is adsorbed by the nearby Cu single sites in PTF(Cu), and this is followed by a synergistic C-C coupling process which ultimately produces ethylene. Density functional theory calculations demonstrate that the coupling process between PTF(Cu)-*CO and Re-bpy-*CO to form the key intermediate Re-bpy-*CO-*CO-PTF(Cu) is vital to the C₂H₄ production. This work provides a new pathway for the design of efficient photocatalysts for photoconversion of CO₂ to C₂ products via a tandem process driven by visible light under mild conditions.

Rational design of atomic site catalysts for electrochemical CO2 reduction

Renewable-energy-powered electrochemical CO₂ reduction (ECR) is a promising way of transforming CO₂ to value-added products and achieving sustainable carbon recycling. By virtue of the extremely high exposure rate of active sites and excellent catalytic performance, atomic site catalysts (ASCs), including single-atomic site catalysts and diatomic site catalysts, have attracted considerable attention. In this feature article, we focus on the rational design strategies of ASCs developed in recent years for the ECR reaction. The influence of these strategies on the activity and selectivity of ASCs for ECR is further discussed in terms of electronic regulation, synergistic activation, microenvironmental regulation and tandem catalytic system construction. Finally, the challenges and future directions are indicated. We hope that this feature article will be helpful in the development of novel ASCs for ECR.

Ultrasmall Copper Nanoclusters in Zirconium Metal-Organic Frameworks for the Photoreduction of CO2

Encapsulating ultrasmall Cu nanoparticles inside Zr-MOFs to form core-shell architecture is very challenging but of interest for CO₂ reduction. We report for the first time the incorporation of ultrasmall Cu NCs into a series of benchmark Zr-MOFs, without Cu NCs aggregation, via a scalable room temperature fabrication approach. The Cu NCs@MOFs core-shell composites show much enhanced reactivity in comparison to the Cu NCs confined in the pore of MOFs, regardless of their very similar intrinsic properties at the atomic level. Moreover, introducing polar groups on the MOF structure can further improve both the catalytic reactivity and selectivity. Mechanistic investigation reveals that the Cu^I sites located at the interface between Cu NCs and support serve as the active sites and efficiently catalyze CO₂ photoreduction. This synergetic effect may pave the way for the design of low-cost and efficient catalysts for CO₂ photoreduction into high-value chemical feedstock.

Reconstruction of Highly Dense Cu−N 4 Active Sites in Electrocatalytic Oxygen Reduction Characterized by Operando Synchrotron Radiation

The emerging star of single atomic site (SAS) catalyst has been regarded as the most promising Pt-substituted electrocatalyst for oxygen reduction reaction (ORR) in anion-exchange membrane fuel cells (AEMFCs). However, the metal loading in SAS directly affects the whole device performance. Herein, we report a dual nitrogen source coordinated strategy to realize high dense Cu-N₄ SAS with a metal loading of 5.61 wt% supported on 3D N-doped carbon nanotubes/graphene structure wherein simultaneously performs superior ORR activity and stability in alkaline media. When applied in H₂ /O₂ AEMFC, it could reach an open-circuit voltage of 0.90 V and a peak power density of 324 mW cm^-2 . Operando synchrotron radiation analyses identify the reconstruction from initial Cu-N₄ to Cu-N₄ /Cu-nanoclusters (NC) and the subsequent Cu-N₃ /Cu-NC under working conditions, which gradually regulate the d-band center of central metal and balance the Gibbs free energy of *OOH and *O intermediates, benefiting to ORR activity.

Microenvironment Modulation in Carbon-Supported Single-Atom Catalysts for Efficient Electrocatalytic CO2 Reduction

The electrocatalytic CO 2 reduction reaction (ECRR) becomes an effective way to reduce excess CO 2 in the air and a promising strategy to maintain carbon balance. Carbon-supported single-atom catalysts (C-SACs) is a kind of cost savings and most promising catalysts for ECRR. For C-SACs, the key to achieving efficient ECRR performance is to adjusting the electronic structure of the central metal atoms by modulating their microenvironment of the catalysts. Not only the coordination numbers and hetero-atom coordination, but also the regulation of diatomic sites have a great influence on the performance of C-SACs. This review mainly focuses on recent studies for the microenvironment modulation in C-SACs for efficient ECRR. We hope that this review can contribute readers a comprehensive insight in the current research status of C-SACs for ECRR, as well as provide help for the rational design of C-SACs with better ECRR performance.

Atomic- and Molecular-Level Modulation of Dispersed Active Sites for Electrocatalytic CO2 Reduction

Global climate changes have been impacted by the excessive CO 2 emission, which exacerbates the environmental problems. Electrochemical CO 2 reduction (CO 2 RR) offers the solution for utilizing CO 2 as feedstocks for value-added products while potentially mitigating the negative effects. Owing to the extreme stability of CO 2 , selectivity and efficiency are crucial factors in the development of CO 2 RR electrocatalysts. Recently, single-atom catalysts have emerged as potential electrocatalysts for CO 2 reduction. They generally comprise of atomically- and molecularly dispersed active sites over conductive supports, which enable atomic-level and molecular-level modulations. In this minireview, catalyst preparations, principle of modulations, and reaction mechanisms are summarised together with related recent advances. The atomic-level modulations are first discussed, followed by the molecular-level modulations. Finally, the current challenges and future opportunities are provided as guidance for further developments regarding the discussed topics.

Molecular Stabilization of Sub-Nanometer Cu Clusters for Selective CO2 Electromethanation

Electrochemical CO₂ methanation powered by renewable electricity provides a promising approach to utilizing CO₂ in the form of a high-energy-density, clean fuel. Cu nanoclusters have been predicted by theoretical calculations to improve methane selectivity. Direct electrochemical reduction of Cu-based metal-organic frameworks (MOFs) results in large-size Cu nanoparticles which favor multi-carbon products. This study concerns an electrochemical oxidation-reduction method to prepare Cu clusters from MOFs. The derived Cu clusters exhibit a faradaic efficiency of 51.2 % for CH₄ with a partial current density of >150 mA cm^-2 . High-resolution microscopy, in situ X-ray absorption spectroscopy, in situ Raman spectroscopy, and a range of ex situ spectroscopies indicate that the distinctive CH₄ selectivity is due to the sub-nanometer size of the derived materials, as well as stabilization of the clusters by residual ligands of the pristine MOF. This work offers a new insight into steering product selectivity of Cu by an electrochemical processing method.

Electrochemical Reduction of CO2 to CO over Transition Metal/N-Doped Carbon Catalysts: The Active Sites and Reaction Mechanism

Electrochemical CO2 reduction to value-added chemicals/fuels provides a promising way to mitigate CO2 emission and alleviate energy shortage. CO2 -to-CO conversion involves only two-electron/proton transfer and thus is kinetically fast. Among the various developed CO2 -to-CO reduction electrocatalysts, transition metal/N-doped carbon (M-N-C) catalysts are attractive due to their low cost and high activity. In this work, recent progress on the development of M-N-C catalysts for electrochemical CO2 -to-CO conversion is reviewed in detail. The regulation of the active sites in M-N-C catalysts and their related adjustable electrocatalytic CO2 reduction performance is discussed. A visual performance comparison of M-N-C catalysts for CO2 reduction reaction (CO2 RR) reported over the recent years is given, which suggests that Ni and Fe-N-C catalysts are the most promising candidates for large-scale reduction of CO2 to produce CO. Finally, outlooks and challenges are proposed for future research of CO2 -to-CO conversion.

Insight into Structural Evolution, Active Site and Stability of Heterogeneous Electrocatalysts

The structure-activity correlation study of electrocatalysts is essential for improving conversion from electrical to chemical energy. Recently, increasing evidences obtained by operando characterization techniques reveal that the structural evolution of catalysts caused by the interplay with electric fields, electrolytes or reactants/intermediates brings about the formation of real active sites. Hence, it is time to summarize the structural evolution-related research advances and envisage their future developments. In this minireview, we first introduce the fundamental concepts associated with structural evolution ( e.g., catalyst, active site/center and stability/lifetime) and their relevance. Then, the multiple inducements of structural evolution and advanced operando characterizations are discussed. Lastly, a brief overview of structural evolution and its reversibility in heterogeneous electrocatalysis, especially for representative electrocatalytic oxygen evolution reaction (OER) and CO 2 reduction reaction (CO 2 RR), along with key challenges and opportunities, is highlighted.

Highly Efficient CO2 Electroreduction to Methanol through Atomically Dispersed Sn Coupled with Defective CuO Catalysts

Using renewable electricity to drive CO₂ electroreduction is an attractive way to achieve carbon-neutral energy cycle and produce value-added chemicals and fuels. As an important platform molecule and clean fuel, methanol requires 6-electron transfer in the process of CO₂ reduction. Currently, CO₂ electroreduction to methanol suffers from poor efficiency and low selectivity. Herein, we report the first work to design atomically dispersed Sn site anchored on defective CuO catalysts for CO₂ electroreduction to methanol. It exhibits high methanol Faradaic efficiency (FE) of 88.6 % with a current density of 67.0 mA cm^-2 and remarkable stability in a H-cell, which is the highest FE(methanol) with such high current density compared with the results reported to date. The atomic Sn site, adjacent oxygen vacancy and CuO support cooperate very well, leading to higher double-layer capacitance, larger CO₂ adsorption capacity and lower interfacial charge transfer resistance. Operando experiments and density functional theory calculations demonstrate that the catalyst is beneficial for CO₂ activation via decreasing the energy barrier of *COOH dissociation to form *CO. The obtained key intermediate *CO is then bound to the Cu species for further reduction, leading to high selectivity toward methanol.

The Controllable Reconstruction of Bi-MOFs for Electrochemical CO2 Reduction through Electrolyte and Potential Mediation

Monitoring and controlling the reconstruction of materials under working conditions is crucial for the precise identification of active sites, elucidation of reaction mechanisms, and rational design of advanced catalysts. Herein, a Bi-based metal-organic framework (Bi-MOF) for electrochemical CO₂ reduction is selected as a case study. In situ Raman spectra combined with ex situ electron microscopy reveal that the intricate reconstruction of the Bi-MOF can be controlled using two steps: 1) electrolyte-mediated dissociation and conversion of Bi-MOF to Bi₂ O₂ CO₃ , and 2) potential-mediated reduction of Bi₂ O₂ CO₃ to Bi. The intentionally reconstructed Bi catalyst exhibits excellent activity, selectivity, and durability for formate production, and the unsaturated surface Bi atoms formed during reconstruction become the active sites. This work emphasizes the significant impact of pre-catalyst reconstruction under working conditions and provides insight into the design of highly active and stable electrocatalysts through the regulation of these processes.

Coupling Electrocatalytic CO2 Reduction with Thermocatalysis Enables the Formation of a Lactone Monomer

Carbonylation reactions that generate high-value chemical feedstocks are integral to the formation of many industrially significant compounds. However, these processes require the use of CO, which is invariably derived from fossil-fuel-reforming reactions. CO may also be generated through the electroreduction of CO₂ , but the coupling of these two processes is yet to be considered. Merging electrocatalytic reduction of CO₂ to CO with thermocatalytic use of CO would expand the range of the chemicals produced from CO₂ . This work describes the development of a system coupling a high-pressure CO₂ electrolytic cell containing a bimetallic ZnAg catalyst at the cathode for production of CO with a reactor with a Faradaic efficiency of >90 % where high pressure CO is used for carbonylating propylene oxide into β-butyrolactone by thermal catalysis, the latter step having a reaction yield above 80 %. Although the production of monomers and polymers from CO₂ is currently limited to organic carbonates, this strategy opens up the access to lactones from CO₂ , for the formation of polyesters.

Operando XAS/SAXS: Guiding Design of Single-Atom and Subnanocluster Catalysts

Single-atom and subnanocluster catalysts (SSCs) represent a highly promising class of low-cost materials with high catalytic activity and high atom-utilization efficiency. However, SSCs are susceptible to undergo restructuring during the reactions. Exploring the active sites of catalysts through in situ characterization techniques plays a critical role in studying reaction mechanism and guiding the design of optimum catalysts. In situ X-ray absorption spectroscopy/small-angle X-ray scattering (XAS/SAXS) is promising and widely used for monitoring electronic structure, atomic configuration, and size changes of SSCs during real-time working conditions. Unfortunately, there is no detailed summary of XAS/SAXS characterization results of SSCs. The recent advances in applying in situ XAS/SAXS to SSCs are thoroughly summarized in this review, including the atomic structure and oxidation state variations under open circuit and realistic reaction conditions. Furthermore, the reversible transformation of single-atom catalysts (SACs) to subnanoclusters/nanoparticles and the application of in situ XAS/SAXS in subnanoclusters are discussed. Finally, the outlooks in modulating the SSCs and developing operando XAS/SAXS for SSCs are highlighted.

Atomically Dispersed Fe-N4 Modified with Precisely Located S for Highly Efficient Oxygen Reduction

Immobilizing metal atoms by multiple nitrogen atoms has triggered exceptional catalytic activity toward many critical electrochemical reactions due to their merits of highly unsaturated coordination and strong metal-substrate interaction. Herein, atomically dispersed Fe-NC material with precise sulfur modification to Fe periphery (termed as Fe-NSC) was synthesized, X-ray absorption near edge structure analysis confirmed the central Fe atom being stabilized in a specific configuration of Fe(N3)(N-C-S). By enabling precisely localized S doping, the electronic structure of Fe-N4 moiety could be mediated, leading to the beneficial adjustment of absorption/desorption properties of reactant/intermediate on Fe center. Density functional theory simulation suggested that more negative charge density would be localized over Fe-N4 moiety after S doping, allowing weakened binding capability to *OH intermediates and faster charge transfer from Fe center to O species. Electrochemical measurements revealed that the Fe-NSC sample exhibited significantly enhanced oxygen reduction reaction performance compared to the S-free Fe-NC material (termed as Fe-NC), showing an excellent onset potential of 1.09 V and half-wave potential of 0.92 V in 0.1 M KOH. Our work may enlighten relevant studies regarding to accessing improvement on the catalytic performance of atomically dispersed M-NC materials by managing precisely tuned local environments of M-Nx moiety.