Nowadays, we are facing increasingly serious energy and environmental problems, which urgently need more efficient chemical industry technologies to meet the requirements of low cost, high yield and sustainability. Developing efficient catalysts is of great significance for improving production efficiency, expanding economic benefits, optimizing energy structure, and ameliorating industrial structure. Single-atom catalysts (SACs), featuring unique properties arising from their single-atom dispersion on support surface, have demonstrated exceptional activity, selectivity and stability in energy catalysis, environmental catalysis and organic catalysis. Therefore, preparation methods and catalytic mechanisms of SACs have become a hot research topic on the international catalytic community. This review describes three strategies for preparing SACs: bottom-up synthesis, top-down synthesis and quantum dots cross-linking/self-assembly. Specifically, methods such as co-precipitation, immersion, atomic layer deposition, high-temperature atom thermal transfer, and high-temperature pyrolysis are presented in detail. These approaches precisely control the location and distribution of metal atoms, maximizing their utilization and catalytic efficiency. In addition, the challenges and development prospects faced by SACs related to stability, integrated control and industrial scalability are also summarized.

Direct polymerization of CO₂ with diols for synthesis of carbonates represents a sustainable and efficient approach for CO₂ utilization, in which CeO₂ exhibits favorable catalytic properties in the reaction system. In this study, nanometer-sized CeO₂ catalysts were synthesized via a hydrothermal method utilizing NaOH as precipitating agent. The effects of sintering temperatures (500, 600, and 700 ℃) and surfactants (cationic, anionic, and nonionic) on structural and physicochemical properties of CeO₂ were thoroughly investigated. When the sintering temperature was 600 ℃, CeO₂ displayed an optimal crystallinity and a higher concentration of defect sites compared to the other temperatures. The surfactants significantly increased oxygen vacancy concentration on the surface of CeO₂, leading to a maximum CO₂ uptake of 0.532 mmol/g at 25 ℃. Building upon these findings, a series of synthesized CeO₂ catalysts were applied in the one-step synthesis of polycarbonate from CO₂ and diol, resulting in significant improvements in both conversion and selectivity within the reaction system. The results demonstrated that catalytic activities of CeO₂ prepared at various sintering temperatures with different surfactants displayed notable differences. Notably, the CeO₂ catalyst sintered with cetyltrimethylammonium bromide (CTAB) as the surfactant at 600 ℃ exhibited the highest catalytic activity and selectivity, achieving a conversion of 91.0% for 1,6-hexanediol and a selectivity of 76.6% for poly(6-hydroxyhexyl) carbonate. The outstanding catalytic performance of CeO₂ with the high yield can be primarily attributed to its favorable structural characteristics, abundant defect sites, and high CO₂ uptake capacity.

Among all options of carbon neutrality, conversion of CO₂ into valuable chemicals by electrocatalytic reduction exhibit outstanding performance. However, due to the numerous products and complex pathways of CO₂ electrocatalytic reduction, the exact factors affecting the activity of CO₂ electrocatalytic reduction have not yet been identified. In addition, the CO₂ electrocatalytic reduction process is often accompanied by hydrogen evolution reaction (HER). Therefore, it is still challenging to design a catalyst with high selectivity and high activity for specific product. Herein, this study systematically investigated the potential of 3d transition metal-based single-atom catalysts (SACs) positioned at graphene single vacancies (TM@C_SV), as well as double vacancies (TM@C_DV), for the CO₂ reduction reaction (CO₂RR) using first-principles. The exploration encompassed substrate stability, CO₂ adsorption, and the HER as the main competing reaction. Through the careful screening of 20 catalysts formed by Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn doped graphene defects, several promising catalysts were identified: Sc@C_SV situated on graphene single vacancies, Sc@C_DV and Ti@C_DV situated on graphene double vacancies. They could not only effectively adsorb CO₂ molecules, but also inhibit HER, the main competing reaction. In assessing their performance in CO₂RR, all exhibited selectivity toward HCOOH. Notably, Sc@C_DV demonstrated the best selectivity, requiring the lowest ΔG (0.96 eV) for efficient CO₂ conversion to HCOOH. Electronic structure analysis revealed that Sc@C_DV outperforms due to its optimal balance between ΔG of hydrogenation and the product desorption achieved through a moderate number of active electrons.

Electrochemical reduction of CO₂ to high value-added hydrocarbon fuels and chemicals has emerged as an effective strategy to achieve carbon neutrality. In conventional electrocatalytic powder-coated electrodes fabricated by spraying method, poor contact between electrocatalyst and substrate can severely impact the electrocatalytic activity and stability. Herein, a self-supporting nanotree electrode (Bi@Cu NTs) for efficient electroreduction from CO₂ to formate was structured by combing facile electrodeposition method and galvanic replacement reaction. The advantages of self-supporting nanotree structure including: 1) minimization of the interfacial resistance and improvement of the spatial structure stability; 2) rich active sites and plentiful pore structures. The charge transfer resistant could be effectively reduced while ensuring the stability of the electrode operation. Results demonstrated that the prepared Bi@Cu NTs electrode exhibited outstanding performance for CO₂ conversion in both electrochemical activity and long-term operation stability. In a wide operating potential window from -1.4 to -0.8 V (vs. RHE), the proposed Bi@Cu NTs electrode presented excellent formate selectivity, where the Faradaic efficiency of CO₂-to-formate (FE_Formate) at each operating potential was above 90%. Typically, at -1.2 V, the proposed electrode achieved a high FE_Formate of 97.9% and a current density of 170.6 mA·cm^-2, simultaneously. Meanwhile, the self-supporting Bi@Cu NTs electrode also revealed excellent stability in a long-term operation, as evidenced by maintaining an average FE_Formate of more than 90% and an average current density higher than 110 mA·cm^-2 over 50 h of continuous electrolysis at a controlled potential of -1.0 V without any degradation in performance.

In this study, one-dimensional single-crystal TiO₂ nanobelt arrays with surface oxygen vacancies were constructed by Pd-catalyzed oxygen reduction method in anoxic environment to address the problems of insufficient surface active sites and slow reaction kinetics of TiO₂, low yield and poor selectivity of hydrocarbons in CO₂ reduction products. The effects of surface oxygen vacancies and hydrogen spillover of Pd on the separation and transport of photogenerated carrier and the selectivity of reduction product were investigated from morphological structure, carrier behavior and photocatalytic performance. With high CO₂ reduction activity of Pd-Ov-TNB, yields of CH₄, C₂H₆ and C₂H₄ are 40.8, 32.09 and 3.09 µmol·g^-1·h^-1, respectively, and selectivity of hydrocarbons is as high as 84.52%, showing great potential in C-C coupling. Its excellent photocatalytic activity is attributed to the one-dimensional single-crystal nanobelt structure that increases the active specific surface area and crystallinity of the material, provides more active sites for the CO₂ reduction and accelerates the segregated transport of photogenerated charges. Meanwhile, the oxygen vacancies enhance the surface accumulation of photogenerated charges, providing an electron-rich environment for CO₂ reduction. In addition, Pd nanoparticles increase concentration of H* in the reaction system, and then transfer H* to active sites of CO₂ adsorption on the catalyst surface through the hydrogen spillover effect, promoting the hydrogenation of reaction intermediates. Comprehensive advantages of Pd-nanoparticals contribute to the efficient conversion of CO₂ to hydrocarbons.

Cu/Mg-MOF-74 has several advantages, such as high specific surface area, adjustable microporous structure, alkali metal active site, excellent CO₂ adsorption, and good photocatalytic activity. However, how the molar ratio of Cu/Mg (Cu/Mg ratio) affects its CO₂ adsorption selectivity in a simulated flue gas is still unclear. Here, a synthesized Cu/Mg-MOF-74, with series of Cu/Mg ratios, using the solvothermal method was analyzed about its CO₂ photocatalytic performance, CO₂ and N₂ uptake, and pore structure. The CO₂ adsorption selectivity was calculated to reveal the effect of Cu/Mg ratio on CO₂ and N₂ uptake and selectivity. The results indicate that the photocatalytic activity of Cu/Mg-MOF-74 for CO₂ reduction to CO and H₂ initially increases and then decreases with Cu/Mg ratio decreasing. At the Cu/Mg ratio of 0.6/0.4, the yield of CO and H₂ by photocatalytic reduction is the highest, showing up to 10.65 and 5.41 μmol·h⁻¹·g_cat⁻¹ (1 MPa, 150 ℃), respectively. Furthermore, CO₂ and N₂ uptakes of Cu/Mg-MOF-74 increase as the Cu/Mg ratio decreases, and the increase in CO₂ uptake is more pronounced. At the Cu/Mg ratio of 0.1/0.9, the CO₂ and N₂ uptakes are the largest, reaching 9.21 and 1.49 mmol·g⁻¹ (273.15 K, 100 kPa), respectively. Their area and volume of micropore (d₁ ≥ 0.7 nm) and ultramicropore (d₂ < 0.7 nm) increase as the Cu/Mg ratio decreases. At the Cu/Mg ratio of 0.22/0.78, the area and volume of micropores and ultramicropores are larger than those of Mg-MOF-74. The selectivity of Cu/Mg-MOF-74 increases correspondingly with Cu/Mg ratio decreasing and CO₂ concentration increasing. CO₂ adsorption on Cu/Mg-MOF-74 is a combination process of pore-filling and Mg²⁺ chemical adsorption in which the micropore volume is the key factor affecting its adsorption performance. All above data demonstrate that modulating the Cu/Mg ratio can promisingly regulate the pore structure of Cu/Mg-MOF-74, CO₂ uptake, and selectivity.

One of the basic challenges of CO₂ photoreduction is to develop efficient photocatalysts. As an effective strategy, constructing heterostructure photocatalysts with intimate interfaces can enhance interfacial charge transfer for realizing high photocatalytic activity. Herein, a novel photocatalytic material, Bi₄O₅Br₂/CeO₂ composite fiber (B@C-x, x refers to the amount of reactant), was constructed by embeding CeO₂ nanofibers on Bi₄O₅Br₂ nanosheets via an electrospinning combined with hydrothermal method. Its composition, morphology and photoelectric properties were characterized. The results show that Bi₄O₅Br₂/CeO₂ heterojunction with appropriate Bi₄O₅Br₂ content can significantly improve the photocatalytic performance of CeO₂ nanofibers. Compared with pure Bi₄O₅Br₂ and CeO₂, B@C-2 exhibited the best photocatalytic activity under simulated sunlight. The Bi₄O₅Br₂/CeO₂ exhibited improved photocatalytic CO₂ reduction performance with a CO generation rate of 8.26 μmol·h⁻¹·g⁻¹ without using any sacrificial agents or noble co-catalysts. This can be attributed to the tight interfacial bonding between Bi₄O₅Br₂ and CeO₂ and the formation of S-scheme heterojunction, which enables the efficient spatial separation and transfer of photogenerated carriers. This work provides a simple and efficient method for directional synthesis of Bi-based photocatalytic composites with S-scheme heterojunction and illustrates an applicable tactic to develop potent photocatalysts for clean energy conversion.