Converting CO2 into Value-Added Products by Cu2O-Based Catalysts: From Photocatalysis,Electrocatalysis to Photoelectrocatalysis

Converting CO₂ into value-added products by photocatalysis, electrocatalysis, and photoelectrocatalysis is a promising method to alleviate the global environmental problems and energy crisis. Among the semiconductor materials applied in CO₂ catalytic reduction, Cu₂O has the advantages of abundant reserves, low price and environmental friendliness. Moreover, Cu₂O has unique adsorption and activation properties for CO₂, which is conducive to the generation of C₂₊ products through C C coupling. This review introduces the basic principles of CO₂ reduction and summarizes the pathways for the generation of C₁, C₂, and C₂₊ products. The factors affecting CO₂ reduction performance are further discussed from the perspective of the reaction environment, medium, and novel reactor design. Then, the properties of Cu₂O-based catalysts in CO₂ reduction are summarized and several optimization strategies to enhance their stability and redox capacity are discussed. Subsequently, the application of Cu₂O-based catalysts in photocatalytic, electrocatalytic, and photoelectrocatalytic CO₂ reduction is described. Finally, the opportunities, challenges and several research directions of Cu₂O-based catalysts in the field of CO₂ catalytic reduction are presented, which is guidance for its wide application in the energy and environmental fields is provided.     

Alumina‐Supported CoFe Alloy Catalysts Derived from Layered‐Double‐Hydroxide Nanosheets for Efficient Photothermal CO2 Hydrogenation to Hydrocarbons

A series of novel CoFe‐based catalysts are successfully fabricated by hydrogen reduction of CoFeAl layered‐double‐hydroxide (LDH) nanosheets at 300–700 °C. The chemical composition and morphology of the reaction products (denoted herein as CoFe‐x) are highly dependent on the reduction temperature (x). CO₂ hydrogenation experiments are conducted on the CoFe‐x catalysts under UV–vis excitation. With increasing LDH‐nanosheet reduction temperature, the CoFe‐x catalysts show a progressive selectivity shift from CO to CH₄, and eventually to high‐value hydrocarbons (C₂₊). CoFe‐650 shows remarkable selectivity toward hydrocarbons (60% CH₄, 35% C₂₊). X‐ray absorption fine structure, high‐resolution transmission electron microscopy, Mössbauer spectroscopy, and density functional theory calculations demonstrate that alumina‐supported CoFe‐alloy nanoparticles are responsible for the high selectivity of CoFe‐650 for C₂₊ hydrocarbons, also allowing exploitation of photothermal effects. This study demonstrates a vibrant new catalyst platform for harnessing clean, abundant solar‐energy to produce valuable chemicals and fuels from CO₂.     

Emerging Carbon‐Based Heterogeneous Catalysts for Electrochemical Reduction of Carbon Dioxide into Value‐Added Chemicals

The electrocatalytic reduction of CO₂ provides a sustainable way to mitigate CO₂ emissions, as well as store intermittent electrical energy into chemicals. However, its slow kinetics and the lack of ability to control the products of the reaction inhibit its industrial applications. In addition, the immature mechanistic understanding of the reduction process makes it difficult to develop a selective, scalable, and stable electrocatalyst. Carbon‐based materials are widely considered as a stable and abundant alternative to metals for catalyzing some of the key electrochemical reactions, including the CO₂ reduction reaction. In this context, recent research advances in the development of heterogeneous nanostructured carbon‐based catalysts for electrochemical reduction of CO₂ are summarized. The leading factors for consideration in carbon‐based catalyst research are discussed by analyzing the main challenges faced by electrochemical reduction of CO₂. Then the emerging metal‐free doped carbon and aromatic N‐heterocycle catalysts for electrochemical reduction of CO₂ with an emphasis on the formation of multicarbon hydrocarbons and oxygenates are discussed. Following that, the recent progress in metal–nitrogen–carbon structures as an extension of carbon‐based catalysts is scrutinized. Finally, an outlook for the future development of catalysts as well as the whole electrochemical system for CO₂ reduction is provided.     

Single-Site Metal–Organic Framework and Copper Foil Tandem Catalyst for Highly Selective CO2 Electroreduction to C2H4

Tandem catalysis is a promising way to break the limitation of linear scaling relationship for enhancing efficiency, and the desired tandem catalysts for electrochemical CO₂ reduction reaction (CO₂RR) are urgent to be developed. Here, a tandem electrocatalyst created by combining Cu foil (CF) with a single-site Cu(II) metal–organic framework (MOF), named as Cu–MOF–CF, to realize improved electrochemical CO₂RR performance, is reported. The Cu–MOF–CF shows suppression of CH₄, great increase in C₂H₄ selectivity (48.6%), and partial current density of C₂H₄ at −1.11 V versus reversible hydrogen electrode. The outstanding performance of Cu–MOF–CF for CO₂RR results from the improved microenvironment of the Cu active sites that inhibits CH₄ production, more CO intermediate produced by single-site Cu–MOF in situ for CF, and the enlarged active surface area by porous Cu–MOF. This work provides a strategy to combine MOFs with copper-based electrocatalysts to establish high-efficiency electrocatalytic CO₂RR.     

Exploring Bi2Te3 Nanoplates as Versatile Catalysts for Electrochemical Reduction of Small Molecules

The electroreduction of small molecules to high value-added chemicals is considered as a promising way toward the capture and utilization of atmospheric small molecules. Discovering cheap and efficient electrocatalysts with simultaneously high activity, selectivity, durability, and even universality is desirable yet challenging. Herein, it is demonstrated that Bi₂Te₃ nanoplates (NPs), cheap and noble-metal-free electrocatalysts, can be adopted as highly universal and robust electrocatalysts, which can efficiently reduce small molecules (O₂, CO₂, and N₂) into targeted products simultaneously. They can achieve excellent activity, selectivity and durability for the oxygen reduction reaction with almost 100% H₂O₂ selectivity, the CO₂ reduction reaction with up to 90% Faradaic efficiency (FE) of HCOOH, and the nitrogen reduction reaction with 7.9% FE of NH₃. After electrochemical activation, an obvious Te dissolution happens on the Bi₂Te₃ NPs, creating lots of Te vacancies in the activated Bi₂Te₃ NPs. Theoretical calculations reveal that the Te vacancies can modulate the electronic structures of Bi and Te. Such a highly electroactive surface with a strong preference in supplying electrons for the universal reduction reactions improves the electrocatalytic performance of Bi₂Te₃. The work demonstrates a new class of cheap and versatile catalysts for the electrochemical reduction of small molecules with potential practical applications.     

Cocatalysts in Semiconductor‐based Photocatalytic CO2 Reduction: Achievements,Challenges, and Opportunities

Ever‐increasing fossil‐fuel combustion along with massive CO₂ emissions has aroused a global energy crisis and climate change. Photocatalytic CO₂ reduction represents a promising strategy for clean, cost‐effective, and environmentally friendly conversion of CO₂ into hydrocarbon fuels by utilizing solar energy. This strategy combines the reductive half‐reaction of CO₂ conversion with an oxidative half reaction, e.g., H₂O oxidation, to create a carbon‐neutral cycle, presenting a viable solution to global energy and environmental problems. There are three pivotal processes in photocatalytic CO₂ conversion: (i) solar‐light absorption, (ii) charge separation/migration, and (iii) catalytic CO₂ reduction and H₂O oxidation. While significant progress is made in optimizing the first two processes, much less research is conducted toward enhancing the efficiency of the third step, which requires the presence of cocatalysts. In general, cocatalysts play four important roles: (i) boosting charge separation/transfer, (ii) improving the activity and selectivity of CO₂ reduction, (iii) enhancing the stability of photocatalysts, and (iv) suppressing side or back reactions. Herein, for the first time, all the developed CO₂‐reduction cocatalysts for semiconductor‐based photocatalytic CO₂ conversion are summarized, and their functions and mechanisms are discussed. Finally, perspectives in this emerging area are provided.     

Highly Selective Photoreduction of CO2 with Suppressing H2 Evolution by Plasmonic Au/CdSe–Cu2O Hierarchical Nanostructures under Visible Light

Here, the photocatalytic CO₂ reduction reaction (CO₂RR) with the selectivity of carbon products up to 100% is realized by completely suppressing the H₂ evolution reaction under visible light (λ > 420 nm) irradiation. To target this, plasmonic Au/CdSe dumbbell nanorods enhance light harvesting and produce a plasmon‐enhanced charge‐rich environment; peripheral Cu₂O provides rich active sites for CO₂ reduction and suppresses the hydrogen generation to improve the selectivity of carbon products. The middle CdSe serves as a bridge to transfer the photocharges. Based on synthesizing these Au/CdSe–Cu₂O hierarchical nanostructures (HNSs), efficient photoinduced electron/hole (e^?/h⁺) separation and 100% of CO selectivity can be realized. Also, the 2e^?/2H⁺ products of CO can be further enhanced and hydrogenated to effectively complete 8e^?/8H⁺ reduction of CO₂ to methane (CH₄), where a sufficient CO concentration and the proton provided by H₂O reduction are indispensable. Under the optimum condition, the Au/CdSe–Cu₂O HNSs display high photocatalytic activity and stability, where the stable gas generation rates are 254 and 123 µmol g^?1 h^?1 for CO and CH₄ over a 60 h period.     

Boron Phosphide Nanoparticles: A Nonmetal Catalyst for High‐Selectivity Electrochemical Reduction of CO2 to CH3OH

Electrocatalysis has emerged as an attractive way for artificial CO₂ fixation to CH₃OH, but the design and development of metal‐free electrocatalyst for highly selective CH₃OH formation still remains a key challenge. Here, it is demonstrated that boron phosphide nanoparticles perform highly efficiently as a nonmetal electrocatalyst toward electrochemical reduction of CO₂ to CH₃OH with high selectivity. In 0.1 m KHCO₃, this catalyst achieves a high Faradaic efficiency of 92.0% for CH₃OH at ?0.5 V versus reversible hydrogen electrode. Density functional theory calculations reveal that B and P synergistically promote the binding and activation of CO₂, and the rate‐determining step for the CO₂ reduction reaction is dominated by *CO + *OH to *CO + *H₂O process with free energy change of 1.36 eV. In addition, CO and CH₂O products are difficultly generated on BP (111) surface, which is responsible for the high activity and selectivity of the CO₂‐to‐CH₃OH conversion process.     

A Highly Active Star Decahedron Cu Nanocatalyst for Hydrocarbon Production at Low Overpotentials

The electrochemical carbon dioxide reduction reaction (CO₂RR) presents a viable approach to recycle CO₂ gas into low carbon fuels. Thus, the development of highly active catalysts at low overpotential is desired for this reaction. Herein, a high‐yield synthesis of unique star decahedron Cu nanoparticles (SD‐Cu NPs) electrocatalysts, displaying twin boundaries (TBs) and multiple stacking faults, which lead to low overpotentials for methane (CH₄) and high efficiency for ethylene (C₂H₄) production, is reported. Particularly, SD‐Cu NPs show an onset potential for CH₄ production lower by 0.149 V than commercial Cu NPs. More impressively, SD‐Cu NPs demonstrate a faradaic efficiency of 52.43% ± 2.72% for C₂H₄ production at ?0.993 ± 0.0129 V. The results demonstrate that the surface stacking faults and twin defects increase CO binding energy, leading to the enhanced CO₂RR performance on SD‐Cu NPs.     

Promoting Electrocatalytic CO2 Reduction to CH4 by Copper Porphyrin with Donor–Acceptor Structures

Molecular catalysts have been receiving increasingly attention in the electrochemical CO₂ reduction reaction (CO₂RR) with attractive features such as precise catalytic sites and tunable ligands. However, the insufficient activity and low selectivity of deep reduction products restrain the utilization of molecular catalysts in CO₂RR. Herein, a donor–acceptor modified Cu porphyrin (CuTAPP) is developed, in which amino groups are linked to donate electrons toward the central CuN₄ site to enhance the CO₂RR activity. The CuTAPP catalyst exhibited an excellent CO₂-to-CH₄ electroreduction performance, including a high CH₄ partial current density of 290.5 mA cm⁻² and a corresponding Faradaic efficiency of 54.8% at –1.63 V versus reversible hydrogen electrode in flow cells. Density functional theory calculations indicated that CuTAPP presented a much lower energy gap in the pathway of producing *CHO than Cu porphyrin without amino group modification. This work suggests a useful strategy of introducing designed donor–acceptor structures into molecular catalysts for enhancing electrochemical CO₂ conversion toward deep reduction products.     

Rechargeable Zn–CO2 Electrochemical Cells Mimicking Two‐Step Photosynthesis

Metal–CO₂ batteries represent a promising priority for sustainable energy and the environment. However, CO₂ utilization in nonaqueous electrolytes mostly involves difficult CO₂ electrochemistry, leading to poor selectivity and limited cycle performance. Herein, an aqueous rechargeable Zn–CO₂ electrochemical cell that tunably produced CO fuel gas (90% Faradaic efficiency) during cell discharge (cathodic reaction: CO₂ + 2e^? + 2H⁺ → CO + H₂O) and O₂ during cell charge at ≈2 V (cathodic reaction: H₂O → 1/2O₂ + 2e^? + 2H⁺), mimicking the separate steps of CO₂ fixation and water oxidation during photosynthesis while exhibiting the advantages of high efficiency, tunable products, and operation independent of sunlight is proposed and realized. The cell achieves a remarkable energy efficiency of 68% with fuel generation, providing an alternative for the green, efficient, and safe utilization of CO₂ by metal–CO₂ batteries.     

Continuously Flow Photothermal Catalysis Efficiently CO2 Reduction Over S-Scheme 2D/0D Bi5O7I-OVs/Cd0.5Zn0.5S Heterojunction with Strong Interfacial Electric Field

Using CO₂, water, and sunlight to produce solar fuel is a very attractive process, which can synchronously reduce carbon and convert solar energy into hydrocarbons. However, photocatalytic CO₂ reduction is often limited by the low selectivity of reduction products and poor photocatalytic activity. In this study, S-scheme Bi₅O₇I-OVs/Cd_0.5Zn_0.5S (Bi₅O₇I-OVs/CZS-0.5) heterojunction with strong interfacial electric field (IEF) is prepared by in situ growth method. The performance of reduction CO₂ to CO is studied by continuous flow photothermal catalytic (PTC) CO₂ reduction platform. 12.5% Bi₅O₇I-OVs/CZS-0.5 shows excellent CO yield of 58.6 µmol g⁻¹ h⁻¹ and selectivity of 98.4%, which are 35.1 times than that of CZS-0.5 under visible light. The charge transfer path of the S-scheme through theoretical calculation (DFT), in situ irradiation Kelvin probe force microscope (ISI-KPFM) and in situ irradiation X-ray photoelectron spectroscopy (ISI-XPS) analysis, is verified. The study can provide useful guidance and reference for improving activity by oxygen vacancy induced strong IEF and the development of a continuous flow PTC CO₂ reduction system.     

Hybrid Cu0 and Cux+ as Atomic Interfaces Promote High‐Selectivity Conversion of CO2 to C2H5OH at Low Potential

The mixing of charge states of metal copper catalysts may lead to a much improved reactivity and selectivity toward multicarbon products for CO₂ reduction. Here, an electrocatalyst model composed of copper clusters supported on graphitic carbon nitride (g‐C₃N₄) is proposed; the connecting Cu atoms with g‐C₃N₄ can be oxidized to Cu^{x +} due to substantial charge transfer from Cu to N atoms, while others stay as Cu⁰. It is revealed that CO₂ can be captured and reduced into *CO on the Cu_t⁰ site, owing to its zero oxidation state. More importantly, C–C coupling reaction of two *CHO species on the Cu_t⁰–Cu_b^{x +} atomic interface can occur with a rather low kinetic barrier of 0.57 eV, leading to the formation of the final C₂ product, namely, C₂H₅OH. During the whole process, the limiting potential is just 0.68 V. These findings may open a new avenue for CO₂ reduction into high‐value fuels and chemicals.     

Electrocatalytic Reduction of CO2 to Ethylene by Molecular Cu‐Complex Immobilized on Graphitized Mesoporous Carbon

The electrochemical reduction of carbon dioxide (CO₂) to hydrocarbons is a challenging task because of the issues in controlling the efficiency and selectivity of the products. Among the various transition metals, copper has attracted attention as it yields more reduced and C2 products even while using mononuclear copper center as catalysts. In addition, it is found that reversible formation of copper nanoparticle acts as the real catalytically active site for the conversion of CO₂ to reduced products. Here, it is demonstrated that the dinuclear molecular copper complex immobilized over graphitized mesoporous carbon can act as catalysts for the conversion of CO₂ to hydrocarbons (methane and ethylene) up to 60%. Interestingly, high selectivity toward C2 product (40% faradaic efficiency) is achieved by a molecular complex based hybrid material from CO₂ in 0.1 m KCl. In addition, the role of local pH, porous structure, and carbon support in limiting the mass transport to achieve the highly reduced products is demonstrated. Although the spectroscopic analysis of the catalysts exhibits molecular nature of the complex after 2 h bulk electrolysis, morphological study reveals that the newly generated copper cluster is the real active site during the catalytic reactions.     

Amorphizing of Cu Nanoparticles toward Highly Efficient and Robust Electrocatalyst for CO2 Reduction to Liquid Fuels with High Faradaic Efficiencies

Conversion of carbon dioxide (CO₂) into valuable chemicals, especially liquid fuels, through electrochemical reduction driven by sustainable energy sources, is a promising way to get rid of dependence on fossil fuels, wherein developing of highly efficient catalyst is still of paramount importance. In this study, as a proof‐of‐concept experiment, first a facile while very effective protocol is proposed to synthesize amorphous Cu NPs. Unexpectedly, superior electrochemical performances, including high catalytic activity and selectivity of CO₂ reduction to liquid fuels are achieved, that is, a total Faradaic efficiency of liquid fuels can sum up to the maximum value of 59% at ?1.4 V, with formic acid (HCOOH) and ethanol (C₂H₆O) account for 37% and 22%, respectively, as well as a desirable long‐term stability even up to 12 h. More importantly, this work opens a new avenue for improved electroreduction of CO₂ based on amorphous metal catalysts.     

Metal‐Organic Frameworks for CO2 Chemical Transformations

Carbon dioxide (CO₂), as the primary greenhouse gas in the atmosphere, triggers a series of environmental and energy related problems in the world. Therefore, there is an urgent need to develop multiple methods to capture and convert CO₂ into useful chemical products, which can significantly improve the environment and promote sustainable development. Over the past several decades, metal‐organic frameworks (MOFs) have shown outstanding heterogeneous catalytic activity due in part to their high internal surface area and chemical functionalities. These properties and the ability to synthesize MOF platforms allow experiments to test structure‐function relationships for transforming CO₂ into useful chemicals. Herein, recent developments are highlighted for MOFs participating as catalysts for the chemical fixation and photochemical reduction of CO₂. Finally, opportunities and challenges facing MOF catalysts are discussed in this ongoing research area.     

Ceo2/Cus Nanoplates Electroreduce Co2 to Ethanol with Stabilized Cu+ Species

Copper-based electrocatalysts effectively produce multicarbon (C₂₊) compounds during the electrochemical CO₂ reduction (CO₂RR). However, big challenges still remain because of the chemically unstable active sites. Here, cerium is used as a self-sacrificing agent to stabilize the Cu⁺ of CuS, due to the facile Ce³⁺/Ce⁴⁺ redox. CeO₂-modified CuS nanoplates achieve high ethanol selectivity, with FE up to 54% and FE_C2+ ≈ 75% in a flow cell. Moreover, in situ Raman spectroscopy and in situ Fourier-transform infrared spectroscopy indicate that the stable Cu⁺ species promote C C coupling step under CO₂RR. Density functional theory calculations further reveal that the stronger ^*CO adsorption and lower C C coupling energy, which is conducive to the selective generation of ethanol products. This work provides a facile strategy to convert CO₂ into ethanol by retaining Cu⁺ species.     

Carbon Dioxide Promotes Dehydrogenation in the Equimolar C2H2‐CO2 Reaction to Synthesize Carbon Nanotubes

The equimolar C₂H₂‐CO₂ reaction has shown promise for carbon nanotube (CNT) production at low temperatures and on diverse functional substrate materials; however, the electron‐pushing mechanism of this reaction is not well demonstrated. Here, the role of CO₂ is explored experimentally and theoretically. In particular, ¹³C labeling of CO₂ demonstrates that CO₂ is not an important C source in CNT growth by thermal catalytic chemical vapor deposition. Consistent with this experimental finding, the adsorption behaviors of C₂H₂ and CO₂ on a graphene‐like lattice via density functional theory calculations reveal that the binding energies of C₂H₂ are markedly higher than that of CO₂, suggesting the former is more likely to incorporate into CNT structure. Further, H‐abstraction by CO₂ from the active CNT growth edge would be favored, ultimately forming CO and H₂O. These results support that the commonly observed, promoting role of CO₂ in CNT growth is due to a CO₂‐assisted dehydrogenation mechanism.     

Dual Engineering of Lattice Strain and Valence State of NiAl-LDHs for Photoreduction of CO2 to Highly Selective CH4

Converting CO₂ to clean-burning fuel such as natural gas (CH₄) with high activity and selectivity remains to be a grand challenge due to slow kinetics of multiple electron transfer processes and competitive hydrogen evolution reaction (HER). Herein, the fabrication of surfactants (C₁₁H₂₃COONa, C₁₂H₂₅SO₄Na, C₁₆H₃₃SO₄Na) intercalated NiAl-layered double hydroxides (NiAl-LDH) is reported, resulting in the formation of LDH-S1 (S1 = C₁₁H₂₃COO⁻), LDH-S2 (S2 = C₁₂H₂₅SO₄⁻) and LDH-S3 (S3 = C₁₆H₃₃SO₄⁻) with curved morphology. Compared with NiAl-LDH with a 1.53% selectivity of CH₄, LDH-S2 shows higher selectivity of CH₄ (83.07%) and lower activity of HER (3.84%) in CO₂ photoreduction reaction (CO₂PR). Detailed characterizations and DFT calculation indicates that the inherent lattice strain in LDH-S2 leads to the structural distortion with the presence of V_Ni/Al defects and compressed M O M bonds, and thereby reduces the overall energy barrier of CO₂ to CH₄. Moreover, the lower oxidation states of Ni in LDH-S2 enhances the adsorption of intermediates such as OCOH* and *CO, promoting the hydrogenation of CO to CH₄. Therefore, the coupling effect of both lattice strain and electronic structure of the LDH-S2 significantly improves the activity and selectivity for CO₂PR.     

Selective CO2 Electroreduction to Formate on Polypyrrole-Modified Oxygen Vacancy-Rich Bi2O3 Nanosheet Precatalysts by Local Microenvironment Modulation

Challenges remain in the development of highly efficient catalysts for selective electrochemical transformation of carbon dioxide (CO₂) to high-valued hydrocarbons. In this study, oxygen vacancy-rich Bi₂O₃ nanosheets coated with polypyrrole (Bi₂O₃@PPy NSs) are designed and synthesized, as precatalysts for selective electrocatalytic CO₂reduction to formate. Systematic material characterization demonstrated that Bi₂O₃@PPy precatalyst can evolve intoBi₂O₂CO₃@PPy nanosheets with rich oxygen vacancies (Bi₂O₂CO₃@PPy NSs) via electrolyte-mediated conversion and function as the real active catalyst for CO₂ reduction reaction electrocatalysis. Coating catalyst with a PPy shell can modulate the interfacial microenvironment of active sites, which work in coordination with rich oxygen vacancies in Bi₂O₂CO₃ and efficiently mediate directional selective CO₂ reduction toward formate formation. With the fine-tuning of interfacial microenvironment, the optimized Bi₂O₃@PPy-2 NSs derived Bi₂O₂CO₃@PPy-2 NSs exhibit a maximum Faradaic efficiency of 95.8% at −0.8 V (versus. reversible hydrogen electrode) for formate production. This work might shed some light on designing advanced catalysts toward selective electrocatalytic CO₂ reduction through local microenvironment engineering.