CO2 Reduction Research Articles

CO Reduction Process Technology and Development of Iron Ore Sintering Process

CO Reduction Process Technology and Development of Iron Ore Sintering Process

Scalable Electro‐Biosynthesis of Ectoine from Greenhouse Gases

Converting greenhouse gases into valuable products has become a promising approach for achieving a carbon‐neutral economy and sustainable development. However, the conversion efficiency depends on the energy yield of the substrate. In this study, we developed an electro‐biocatalytic system by integrating electrochemical and microbial processes to upcycle CO2 into a valuable product (ectoine) using renewable energy. This system initiates the electrocatalytic reduction of CO2 to methane, an energy‐dense molecule, which then serves as an electrofuel to energize the growth of an engineered methanotrophic cell factory for ectoine biosynthesis. The scalability of this system was demonstrated using an array of ten 25 cm2 electrochemical cells equipped with a high‐performance carbon‐supported isolated copper catalyst. The system consistently generated methane at the cathode under a total partial current of approximately −37 A (~175 mmolCH4 h–1) and O2 at the anode under a total partial current of approximately 62 A (~583 mmolO2 h–1). This output met the requirements of a 3‐L bioreactor, even at maximum CH4 and O2 consumption, resulting in the high‐yield conversion of CO2 to ectoine (1146.9 mg L–1). This work underscores the potential of electrifying the biosynthesis of valuable products from CO2, providing a sustainable avenue for biomanufacturing and energy storage.

In Situ Oxidative Ring-Opening of Calix[8]arene to Construct Stable Bismuth-Oxo Clusters with Exposed Catalytic Sites for Specific Electroreduction of CO2 to HCOOH.

Nanocluster catalysts typically face challenges in balancing stability with catalytic efficiency. This study introduces a unique bismuth-oxo cluster, solely protected by two ring-opened calixarenes, which demonstrates not only enhanced structural stability but also superior catalytic performance in the sustained conversion of CO2 to HCOOH via electrocatalysis. For the first time, we reveal that under specific solvothermal conditions, tert-butylcalix[8]arene (TBC[8]) can undergo in situ oxidative cleavage of its C-C bond, leading to ring-opened polyphenolic molecules. These molecules serve as protective ligands for the bismuth-oxo cluster, bestowing exceptional structural stability and offering a more flexible and diverse configuration compared to intact TBC[8]. This adaptability promotes the exposure of active bismuth sites on the cluster surface, enhancing catalytic efficiency. Notably, the Bi10 cluster, featuring a monobismuth active site, achieves an exceptional formate production efficiency of 98.79% at -1.25 V vs RHE while maintaining superb durability over 8 h. The stability and catalytic processes of Bi10 surpass those of the Bi13 cluster, which is structurally reinforced by two intact TBC[8] molecules and stabilized by four benzoic ligands. Through in situ infrared spectroscopy and density functional theory calculations, we demonstrate that the monobismuth active site in Bi10 more effectively stabilizes the *OCHO intermediate, thereby promoting the electrocatalytic reduction of CO2 to HCOOH compared to Bi13. This comparative performance underscores the potential of ring-opened calixarene ligands in enhancing the functionality of nanocluster catalysts.

Scalable Electro-Biosynthesis of Ectoine from Greenhouse Gases.

Converting greenhouse gases into valuable products has become a promising approach for achieving a carbon-neutral economy and sustainable development. However, the conversion efficiency depends on the energy yield of the substrate. In this study, we developed an electro-biocatalyticsystem by integrating electrochemical and microbial processes to upcycle CO2 into a valuable product (ectoine) using renewable energy. This system initiates the electrocatalytic reduction of CO2 to methane, an energy-dense molecule, which then serves as an electrofuel to energize the growth of an engineered methanotrophic cell factory for ectoine biosynthesis. The scalability of this system was demonstrated using an array of ten 25 cm2 electrochemical cells equipped with a high-performance carbon-supported isolated copper catalyst. The system consistently generated methane at the cathode under a total partial current of approximately -37 A (~175 mmolCH4 h-1) and O2 at the anode under a total partial current of approximately 62 A (~583 mmolO2 h-1). This output met the requirements of a 3-L bioreactor, even at maximum CH4 and O2 consumption, resulting in the high-yield conversion of CO2 to ectoine (1146.9 mg L-1). This work underscores the potential of electrifying the biosynthesis of valuable products from CO2, providing a sustainable avenue for biomanufacturing and energy storage.

Photoelectroreduction CO2 to Ethanol Over BiFeO3 with Synergistic Effect of Self‐Polarization and External Electric Field

The exploitation of semiconductor self‐polarization is an effective mean of improving the efficiency of electron utilization. In this work, the synthesis of BiFeO3 (BFO) samples with varying photoelectric properties is achieved by modulating the concentration of KOH. The experimental results reveal that BFO‐4 (4 m KOH) has excellent carrier efficiency. Interestingly enough, the ethanol yield of up to 7.05 μmol cm−2 h−1 in the photoelectrocatalytic process, which is 1.4 times than that of single electrocatalysis. Based on the characterization data, the external electric field forms a multi‐electric field coupling in the BFO (external electric field can enhance the self‐polarization electric field), which enhances charge separation efficiency and facilitates surface reactions, and the CO2 reduction performance is improved. Finally, the free energy in the key step of CC coupling on the surface of BiFeO3 is calculated by DFT. This study offers a valuable reference for the application of ferroelectric materials in the photoelectrocatalytic reduction of CO2 and the design of catalysts for C2+ products.

Boosting selectivity for multi-carbon products in electrochemical CO2 reduction via enhanced hydroxide adsorption in ultrafine CuOx/CeOx nanoparticles

The electricity-driven reduction of CO2 presents a unique prospect of mitigating atmospheric CO2 levels and converting renewable energy into chemical fuels. However, the lack of an efficient catalyst that can transform CO2 into desired products with appreciable selectivity at commercially relevant current densities (ID) impedes the practical implications of this technology. This work describes a scalable approach for producing ultrafine nanoparticles of CuOx-CeOx (CuCe) composites via rapid coprecipitation under oxygen-deprived conditions. In this process, Ce(OH)3 acts as a reducing agent converting Cu(OH)2 into ultrafine Cu2O nanoparticles. Thus, the Ce: Cu ratio in composite effectively controls the particle size and bulk structural fixture of the CuCe composites. Despite having lower ECSA, The CuCe composites exhibit superior catalytic performance compared to CuO, with significantly higher selectivity for ethanol production and lower selectivity for CO and H2. The Cu1.5Ce prepared using a Cu: Ce ratio of 1.5:1, demonstrated remarkable catalytic activity with Faradaic efficiencies (F. E.) of 34.2 % for ethanol, 44.1 % for ethylene, and 81.08 % for multi-carbon (C2+) products. Moreover, while operating at the ID of 500 mA cm−2, Cu1.5Ce achieves a single pass CO2 conversion (SPCC) of 19.9 %, and a half-cell energy efficiency (E. E. half-cell) of 31.5 % for the C2+ products. It produces the C2+ products at the partial current density (pID) of 405.4 mA cm−2. The superior performance of Cu1.5Ce can be attributed to the improved hydroxide adsorption and partial retention of oxidized copper species under cathodic bias.

Sustainable operation of large scale heat pumps in cogeneration dominated district heating networks

Large scale heat pumps (LSHP) have seen a rapid proliferation in recent years, with projects and initiatives aimed at integrating electrically driven LSHP into existing district heating networks (DHN). The financial advantages have driven most LSHP installations to be powered by electricity generated by on-site cogeneration (CHP), given CHP’s dominant role in DHN. This LSHP + CHP combination poses the question, to what extent ecological improvements, measured as a reduction in specific CO2 emissions through LSHP operation can be achieved, contingent on the source of electricity and heat source employed. In this paper, the LSHP + CHP combination was generally analysed under usage of exergy analysis and a 2nd law derived method is presented. This method allows to evaluate the magnitude of CO2 reduction through LSHP operation across all common heat sources in 1st to 4th generation DHNs in conjunction with CHP. Results show, that in LSHP + CHP combination, heat from LSHP using either of the 2 heat sources ambient air or surfaced water, inevitably comes with higher specific CO2 emissions over the CHP heat. Only when using a sufficiently hot and emissions-free heat source, the LSHP heat from a LSHP + CHP combination can yield to an annual best case of half the CO2 emissions of the CHP. Importantly, the choice of power source has a far more significant impact, as LSHP powered by electricity from renewable energy sources achieve more than an eight-fold reduction in emissions compared to the LSHP + CHP combination. Therefore, power consumption with sufficiently low specific CO2 emissions out of external source is imperative to fully realize the ecological benefit of LSHP in DHN, underlining that the successful heat transition through LSHP relies heavily on the transformation of the electricity supply.Graphical

Radiation-Synthesized Metal-Organic Frameworks with Ligand-Induced Lewis Pairs for Selective CO2 Electroreduction.

The electrochemical activation of inert CO2 molecules through C─C coupling reactions under ambient conditions remains a significant challenge but holds great promise for sustainable development and the reduction of CO2 emission. Lewis pairs can capture and react with CO2, offering a novel strategy for the electrosynthesis of high-value-added C2 products. Herein, an electron-beam irradiation strategy is presented for rapidly synthesizing a metal-organic framework (MOF) with well-defined Lewis pairs (i.e., Cu- Npyridinic). The synthesized MOFs exhibit a total C2 product faradic efficiency of 70.0% at -0.88V versus RHE. In situ attenuated total reflection Fourier transform infrared and Raman spectra reveal that the electron-deficient Lewis acidic Cu sites and electron-rich Lewis basic pyridinic N sites in the ligand facilitate the targeted chemisorption, activation, and conversion of CO2 molecules. DFT calculations further elucidate the electronic interactions of key intermediates in the CO2 reduction reaction. The work not only advances Lewis pair-site MOFs as a new platform for CO2 electrochemical conversion, but also provides pioneering insights into the underlying mechanisms of electron-beam irradiated synthesis of advanced nanomaterials.

Theory-driven design of local spin-state modulation at atomically dispersed iron sites for enhanced CO2 electroreduction

Herein, density functional theory (DFT) was used to guide the systematic design of highly efficient single-atom electrocatalysts to improve the kinetics of the CO2 reduction reaction (CO2RR). We present a novel class of catalysts that exhibit magnetic-proximity-induced exchange splitting (spin polarization). The design of these catalysts involved the precise modulation of d-shell energy levels at metallic sites and the subsequent alteration of the atomic/molecular orbitals of the adsorbed intermediates. To validate the effectiveness of our design strategy, we introduced neighboring nitrogen (N) doping near the Fe-N4 coordination environment, demonstrating the successful manipulation of the local spin state of the active sites. The results of the theoretical calculations align with the experimental results, with remarkable selectivity exceeding 90 % at low potentials (<-0.5 V vs. RHE) and sustained electrocatalytic activity for over 24 h. In situ characterization further revealed that a modulated local spin state led to a decrease in the occupation of the Fe 3d spin-down electronic states below the Fermi level. Concurrently, electrons migrated to the frontier antibonding orbitals of *COOH. This dual effect resulted in a lower reaction energy (ΔG), representing a thermodynamic benefit, and accelerated proton transfer to *COOH, representing a kinetic benefit, collectively enhancing the electrocatalytic process.

Exploring Electrocatalytic CO2 Reduction Over Materials Derived from Cu‐Based Metal‐Organic Frameworks

AbstractThe direct valorization of carbon dioxide (CO2) into value‐added chemicals offers an efficient and attractive approach to promoting carbon neutrality. Among the available methods, the electrocatalytic CO2 reduction reaction (eCO2RR) for producing multicarbon products (C2+) is gaining attention owing to its simplicity. However, achieving selective control over product formation remains a challenge. One key issue is the lack of a reliable correlation between the physicochemical properties of electrocatalytic materials and their activity and selectivity. To address this gap, we conducted a model study in which carbonized CuxZny@C materials, derived from metal‐organic frameworks (MOFs), were synthesized with varying Cu/Zn ratios. The pyrolyzed bimetallic MOFs retained key properties of the original MOFs while also developing new characteristics. These subtle changes in physicochemical properties influenced product selectivity. The findings of our study revealed that higher Zn doping favors the formation of single‐carbon (C1) products, whereas it is less favorable for multicarbon (C2+) products. Optimizing the Cu/Zn ratio was emphasized through characterization techniques, which will help guide the design of improved electrocatalytic systems for the eCO2RR process.

Converting Energy with Glycerol and CO2 in a Microfluidic Fuel Cell Equipped with CuBiO4/CuO Photocathode: Bypassing Bubbles Challenge of Concurrent Water Splitting.

The imperative to address CO2 emissions has prompted the search for alternative approaches to capture this gas with minimal energy consumption. In this context, leveraging the CO2 reduction reaction (CO2RR) as an oxidant in fuel cells has emerged as a sophisticated strategy to convert this gas into usable energy. This study introduces a hybrid microfluidic photo fuel cell (μPFC) designed for the efficient conversion of CO2 and glycerol into electrical energy. The prototype integrates 3D-printed components with glass sealing, enabling precise control over the reactant flow and the use of light-sensitive catalysts. The anodic glycerol electrooxidation was investigated on Pt/C dispersed on carbon paper (CP), while the CO2RR was carried out on CuBiO4/CP and CuBiO4/CuO/CP in the presence of solar light. Half-cell measurements demonstrate the photoactivity of CuBiO4/CuO/CP and CuBiO4/CP electrodes for the CO2RR under light exposure at low onset potential in a neutral pH solution, generating a positive theoretical open-circuit voltage of 0.89-0.91 V when coupled to glycerol electrooxidation in an alkaline medium. The use of the mixed medium in the membraneless system equipped with the photosensitive catalysts allowed the building of this galvanic cell. However, the feasibility of using CuBiO4/CP is hindered by the disruption of the colaminar channel caused by hydrogen bubbles produced during concurrent water splitting. In contrast, the μPFC equipped with a CuBiO4/CuO/CP photocathode demonstrates a stable and reproducible performance, delivering a maximum power density of 0.9 mW cm-2. The formation of the CuBiO4/CuO heterojunction effectively suppresses photocatalytic water splitting, allowing for efficient CO2 conversion without disruption of the laminar flow channel. This innovative approach highlights the potential of μPFCs as sustainable energy converters for the utilization of CO2 in aqueous solutions, offering a pathway toward carbon-neutral energy production.

Efficient electrocatalytic CO2 reduction to ethylene using cuprous oxide derivatives.

Copper-based materials play a vital role in the electrochemical transformation of CO2 into C2/C2+ compounds. In this study, cross-sectional octahedral Cu2O microcrystals were prepared in situ on carbon paper electrodes via electrochemical deposition. The morphology and integrity of the exposed crystal surface (111) were meticulously controlled by adjusting the deposition potential, time, and temperature. These cross-sectional octahedral Cu2O microcrystals exhibited high electrocatalytic activity for ethylene (C2H4) production through CO2 reduction. In a 0.1M KHCO3 electrolyte, the Faradaic efficiency for C2H4 reached 42.0% at a potential of -1.376V vs. RHE. During continuous electrolysis over 10h, the FE (C2H4) remained stable around 40%. During electrolysis, the fully exposed (111) crystal faces of Cu2O microcrystals are reduced to Cu0, which enhances C-C coupling and could serve as the main active sites for catalyzing the conversion of CO2 to C2H4.

Electrodeposition of Cu 3D structures suitable for CO2 reduction

Porous Cu foam electrodes, suitable for cathodic CO2 reduction, were deposited in an acidic sulphate solution with different additives to obtain structures with a high real surface area and an adequate mechanical stability. The influence of the electrodeposition time and solution composition on the porosity parameters, microstructure and stiffness of Cu 3D structures was evaluated. Neither ammonium acetate nor polyethylene glycol were found to be effective additives to the Cu sulphate electrolyte to achieve the main objectives. Only the presence of Cl– ions in the deposition solution resulted in a threefold increase in the real surface area and the achievement of a sufficient mechanical stability of the Cu 3D structure. The latter effect is related to the specific influence of Cl– ions during the electrodeposition process on the microstructural characteristics, such as the size of micropores in the walls of holes and crystallite aggregates that form dendritic branches. These structural changes, in contrast to the Cu samples deposited in a solution without additives, resulted in larger real surface areas, while the denser structures deposited in the presence of Cl– ions ensured the mechanical stability of the 3D structure.

Enhanced photoelectrochemical CO2 reduction activity towards selective generation of alcohols over CuxO/SrTiO3 heterojunction photocathodes

Photoelectrochemical reduction (PECR) of CO2 to value-added chemicals and fuels will reduce the dependency on fossil fuels, also it will solve environmental issues that arise due to greenhouse gases. A heterojunction of CuxO/SrTiO3 with varying post-annealing temperature ranges from 300 to 600 °C (CS300-600) was synthesized by overlying the spin-coated SrTiO3 on electrodeposited Cu2O over the FTO glass substrate. The synthesized photoelectrode's crystallinity and phase formation significantly varied by varying the post-annealing temperature, which was characterized via XRD and Raman spectroscopy. Optical, morphological, type of heterojunction formation and elemental surface oxidation states of photoelectrodes were studied through UV–visible DRS spectrum, FE-SEM, and XPS analysis respectively. Electrocatalytic analysis such as Linear Sweep Voltammetry (LSV), and Electrochemical Impedance Spectrometry (EIS) was employed, thus it conforms to the highest photocurrent density (−1.38 mA/cm2 at −0.6V vs. Ag/AgCl), and low charge transfer resistance (RCT=0.412 kΩ) at the electrode-electrolyte interfaces for CS500 photoelectrode as compared to others photoelectrodes. PECR of CO2 to liquid product formation was evaluated by applying different potential ranges (0 to −0.6 V vs. Ag/AgCl). Highest methanol formation of 48.69 μmol.cm−2hr−1, which is approximately 9 times enhanced as compared to the pure Cu2O (5.62 μmol.cm−2hr−1) photoelectrode at 0 V vs Ag/AgCl for CS500 heterojunction. Optimization of overlaying SrTiO3 synthesis temperature for crystallization formation on Cu2O and applied photoelectrode potential findings could pave the way for designing other new heterojunction types for selective liquid alcohol production.

Ag-Mediated Growth of Au/Ag-Cu Ternary Heterostructures for Selective Electrochemical CO2 Reduction.

Copper (Cu)-based nanocatalysts play crucial roles in the electrochemical CO2 reduction reaction (ECO2RR) for sustainable energy resources. Particularly, Cu-based nanostructures incorporating Au and Ag are promising, offering enhanced activity, selectivity, and stability. However, precise control over the structure and composition of heterostructures remains challenging, hindering the development of highly efficient catalysts. Herein, we present a silver (Ag) transition-layer-mediated approach to synthesize ternary heterostructures with two specific morphologies, namely, Au/Ag-Cu-side and Au/Ag-Cu-tip, which exhibit different Ag-Cu interface epitaxial patterns. The two heterostructures achieve high C2 product selectivity in ECO2RR. Especially, the Au/Ag-Cu-side structure achieves 50.3% C2 selectivity with 35.5% ethanol, while the tip structure shows higher ethylene selectivity. Our study reveals the impact of the Ag layer in directing deposition sites on heterostructure growth and further facilitating the design of multicomponent Cu-based catalysts with enhanced structural integrity and ECO2RR performance.

Integration of Plasmonic Ag(I) Clusters and Fe(II) Porphyrinates into Metal–Organic Frameworks for Efficient Photocatalytic CO2 Reduction Coupling with Photosynthesis of Pure H2O2

AbstractEfficient photocatalytic CO2 reduction coupled with the photosynthesis of pure H2O2 is a challenging and significant task. Herein, using classical CO2 photoreduction site iron porphyrinate as the linker, Ag(I) clusters were spatially separated and evenly distributed within a new metal–organic framework (MOF), namely Ag27TPyP‐Fe. With water as electron donors, Ag27TPyP‐Fe exhibited remarkable performances in artificial photosynthetic overall reaction with CO yield of 36.5 μmol g−1 h−1 and ca. 100 % selectivity, as well as H2O2 evolution rate of 35.9 μmol g−1 h−1. Since H2O2 in the liquid phase can be more readily separated from the gaseous products of CO2 photoreduction, high‐purity H2O2 with a concentration up to 0.1 mM was obtained. Confirmed by theoretical calculations and the established energy level diagram, the reductive iron(II) porphyrinates and oxidative Ag(I) clusters within an integrated framework functioned synergistically to achieve artificial photosynthesis. Furthermore, photoluminescence spectroscopy and photoelectrochemical measurements revealed that the robust connection of Ag(I) clusters and iron porphyrinate ligands facilitated efficient charge separation and rapid electron transfer, thereby enhancing the photocatalytic activity.

Soft X-ray spectromicroscopic proof of a reversible oxidation/reduction of microbial biofilm structures using a novel microfluidic in situ electrochemical device

In situ electrochemistry on micron and submicron-sized individual particles and thin layers is a valuable, emerging tool for process understanding and optimization in a variety of scientific and technological fields such as material science, process technology, analytical chemistry, and environmental sciences. Electrochemical characterization and manipulation coupled with soft X-ray spectromicroscopy helps identify, quantify, and optimize processes in complex systems such as those with high heterogeneity in the spatial and/or temporal domain. Here we present a novel platform optimized for in situ electrochemistry with variable liquid electrolyte flow in soft X-ray scanning transmission X-ray microscopes (STXM). With four channels for fluid control and a modular design, it is suited for a wealth of experimental conditions. We demonstrate its capabilities by proving the reversible oxidation and reduction of individual microbial biofilm structures formed by microaerophilic Fe(II)-oxidizing bacteria, also known as twisted stalks. We show spectromicroscopically the heterogeneity of the redox activity on the submicron scale. Examples are also provided of electrochemical modification of liquid electrolyte species (Fe(II) and Fe(III) cyanides), and in situ studies of electrodeposited copper nanoparticles as CO2 reduction electrocatalysts under reaction conditions.

Enhancing d/p-2π* Orbitals Hybridization via Strain Engineering for Efficient CO2 Photoreduction.

The photoconversion of CO2 into valuable chemical products using solar energy is a promising strategy to address both energy and environmental challenges. However, the strongly adsorbed CO2 frequently impedes the seamless advancement of the subsequent reaction by significantly increasing the reaction activation energy. Here, we present a BiFeO3 material with lattice strain that collaboratively regulates the d/p-2π* orbitals hybridization between metal sites and *CO2 as well as *COOH intermediates to achieve rapid conversion of solidly adsorbed CO2 to critical *COOH intermediates, accelerating the overall CO2 reduction kinetics. Quasi in situ X-ray photoelectron spectroscopy and in situ Fourier Transform infrared spectroscopy combined with theoretical calculation reveals that the optimized Fe sites enhance the adsorption and activation effect of CO2, and continuous internal electrons are rapidly transferred to the reaction sites and injected into the surface *CO2 and *COOH under the condition of illumination, which promotes the rapid formation and stability of *COOH. Certainly, the performance of CO2 photoreduction to CO is improved by 12.81-fold compared with the base material. This work offers a new perspective for the rapid photoreduction process of strongly adsorbed CO2.

Enhancing d/p‐2π* Orbitals Hybridization via Strain Engineering for Efficient CO2 Photoreduction

AbstractThe photoconversion of CO2 into valuable chemical products using solar energy is a promising strategy to address both energy and environmental challenges. However, the strongly adsorbed CO2 frequently impedes the seamless advancement of the subsequent reaction by significantly increasing the reaction activation energy. Here, we present a BiFeO3 material with lattice strain that collaboratively regulates the d/p‐2π* orbitals hybridization between metal sites and *CO2 as well as *COOH intermediates to achieve rapid conversion of solidly adsorbed CO2 to critical *COOH intermediates, accelerating the overall CO2 reduction kinetics. Quasi in situ X‐ray photoelectron spectroscopy and in situ Fourier Transform infrared spectroscopy combined with theoretical calculation reveals that the optimized Fe sites enhance the adsorption and activation effect of CO2, and continuous internal electrons are rapidly transferred to the reaction sites and injected into the surface *CO2 and *COOH under the condition of illumination, which promotes the rapid formation and stability of *COOH. Certainly, the performance of CO2 photoreduction to CO is improved by 12.81‐fold compared with the base material. This work offers a new perspective for the rapid photoreduction process of strongly adsorbed CO2.

High-Selectivity Tandem Photocatalytic Methanation of CO2 by Lacunary Polyoxometalates-Stabilized *CO Intermediate.

Stabilizing specific intermediates to produce CH4 remains a main challenge in solar-driven CO2 reduction. Herein, g-C3N4 is modified with saturated and lacunary phosphotungstates (PWx, x = 12, 11, 9) to tailor the CO2 reduction pathway to yield CH4 in high selectivity. Increased lacuna of phosphotungstates leads to higher CH4 yield and selectivity, with a superior CH4 selectivity of 80% and 40.8 μmol·g-1·h-1 evolution rate for PW9/g-C3N4. Conversely, g-C3N4 and PWx alone show negligible CH4 production. The conversion of CO2 to CH4 follows a tandem catalytic process. CO2 is initially activated on g-C3N4 to form *CO intermediates, meanwhile photogenerated electrons derived from g-C3N4 transfer to PWx. Then the reduced PWx captures *CO, which is subsquently hydrogenated to CH4. With the injection of two photogenerated electrons, PW9 is capable of adsorbing and activating *CO. However, the reduced PW12 and PW11 are incapable of adsorbing *CO due to the small energy of occupied molecular orbitals, which is the reason for the poorer activity of PWx/g-C3N4 (x = 12, 11) compared with that of PW9/g-C3N4. This work provides new insights to regulate highly selective CO2 photoreduction to CH4 by utilizing lacuna of polyoxometalates to enhance the interaction of metals in polyoxometalates with key intermediates.