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.

With the continuous progress of global industrialization, fossil fuels have been over-consumed, which results in a large amount of CO₂ discharged into the atmosphere and therefore causes negative effects such as global warming and ecological imbalance. Reducing CO₂ emissions and converting recycled CO₂ to value added chemicals have become important tasks. Around 2010, led by United States of America, followed by Europe and Japan, tens of countries started their national scientific research projects entitled “artificial photosynthesis”, with an investment as much as 100 million USDs. Since 2011, China has also funded similar projects by the National Natural Science Foundation of China and the Ministry of Science and Technology. In September of 2020, the Chinese government even put forward the goal of “carbon emission peak” by 2030 and “carbon neutrality” by 2060.
Green plants or microorganisms make organics from CO₂ and H₂O through “photosynthesis”. The photocatalysts can convert CO₂ and H₂O/H₂ into fuels or chemicals under light irradiation, which simulates the natural “photosynthesis” and is entitled as “artificial photosynthesis”. Nevertheless, the conversion of CO₂ is not limited to be driven by solar energy; alternatively, the electricity generated by non-fossil fuels to drive efficient electrocatalytic or thermocatalytic CO₂ reduction is also a feasible way. At present, the hot spots in basic research are photocatalysis, photothermocatalysis and electrocatalysis for CO₂ reduction. “Green conversion of CO₂” defines the technological approaches with at least one of the features of low energy consumption, low environmental load and high efficiency, which enables them competitive in future industrial applications.
In recent years, the study on CO₂ conversion has made rapid progresses, but some key problems are still to be solved. Photocatalysis provides the mildest way to convert CO₂ into fuels or chemicals, which means the lowest cost in the future application, but currently faces great challenge in efficiency and stability. Not until photothermocatalytic CO₂ reduction was reported in 2014, it has attracted extensive attention. The catalysts realize light-to-thermal conversion and subsequently drive thermocatalysis. Compared with photocatalysis, the efficiency and stability of photothermocatalytic CO₂ reduction are significantly increased, but the consumption of H₂ as feedstock will result in additional cost. Electrocatalytic CO₂ reduction is also a mild process, and its energy utilization efficiency is the highest among three methods. However, due to the reaction environment of aqueous solution, there inevitably occurs the competition of CO₂ reduction and proton reduction; therefore, the improvement of product selectivity is still faced with a bottleneck. Moreover, the separation of liquid or gas phase products somewhat increases production cost. Importantly, the economy of CO₂ conversion should take account of the yield of value-added product. The products of photocatalysis and electrocatalysis mainly concentrated in C1 chemicals (mainly including carbon monoxide, methane, methanol, formic acid salt,etc.), and a few studies reported the production of ethane. However, the photothermocatalysis exhibits superior advantage in this aspect; for instance, the Fischer-Tropsch synthesis with CO₂ and H₂ as feedstocks to produce C2 - C7 products has been reported. Each technological approach has its own advantages but faces different scientific or technical difficulties, which may complement each other and show its strengths in different application fields in the future.
Shortly after China put forward the goal of “carbon emission peak” and “carbon neutrality”, the editorial board ofJournal of Inorganic Materialsorganizes the special issue on “Green Conversion of CO₂”. Although the investigation on CO₂ conversion has made great progress, but still faces various challenges. Looking forward to that more researchers devote to this study to promote it from the basic research to the industrial application, our continuous effort will make our country reverse the disadvantage of terrible carbon emission to the advantage of converting recycled CO₂ to resources effectively.

The conversion of CO₂ into high value-added chemicals is an effective way to realize the carbon cycle, alleviate energy crisis and environmental problems. The preparation of metal-semiconductor electrodes provides a new idea for CO₂ conversion by photoelectric coupling technology. In this study, Si was etched in alkali solution, on which a bimetallic Bi, Zn co-modified Si photoelectrocathode (BiZn_x/Si) was prepared via the electrodeposition process, for the photoelectrochemical reduction CO₂. The results show that the introduction of Bi and Zn can improve the light absorption performance, reduce the electrochemical impedance, and increase the electrochemical active surface area (ECSA). Especially, the ECSA of the best photoelectrocathode BiZn₂/Si is 0.15 mF·cm^-2. Besides, the cooperative effect of Bi and Zn can improve the adsorption performance toward the intermediates *OCHO. During the photoelectrochemical reduction of CO₂, the Faradaic efficiency for HCOOH reaches up to 96.1% on the best photoelectrocathode BiZn₂/Si at the potential of -0.8 V (vs. RHE). Moreover, the photocurrent intensity on the photoelectrocathode BiZn₂/Si remained -13 mA·cm^-2 during the 10 h photoelectric stability test, suggesting its high stability.

Since the beginning of the 21st century, energy shortage and environmental pollution have been the major challenges faced by human beings. Photocatalytic carbon dioxide (CO₂) reduction is one of the promising strategies to solve the energy crisis and promote the carbon cycle, in which semiconductor captures solar energy to obtain hydrocarbon fuel. However, the low activity and poor selectivity of the products greatly limit the practical application of this technology. Thus, it is of great significance to regulate product selectivity, improve photocatalytic efficiency, and deeply understand the mechanism of CO₂ reduction reaction. In recent years, ultrathin materials have attracted extensive attention from researchers due to their high specific surface area, abundant unsaturated coordination surface atoms, shortened charge migration path from inside to surface, and tailorable energy band structure, and have achieved promising results in the field of photocatalytic CO₂ reduction. In this paper, the reaction mechanism of photocatalytic CO₂ reduction is firstly summarized. Next, the research results of promoting electron hole separation and regulating charge transport path of ultrathin nanostructures by constructing heterostructures, designing Z-scheme systems, introducing co-catalysts, and defect engineering are introduced. Finally, the prospect and challenge of improving the efficiency of photocatalytic CO₂ reduction and optimizing the product selectivity are pointed out.

Conversion of CO₂ into fuels by photocatalysis is promising in solving the energy crisis and the greenhouse effect. Among various photocatalytic materials, Zn_1-2x(CuGa)_xGa₂S₄ materials possess visible light response and high conduction band potential, which are ideal CO₂ reduction materials from thermodynamics aspect. However, their photocatalytic CO₂ reduction activity is still low which is urgent to improve its activity in terms of kinetics. In this study, Zn_0.4(CuGa)_0.3Ga₂S₄ was synthesized and composited with CdS nanoparticles with different proportions to form Zn_0.4(CuGa)_0.3Ga₂S₄/CdS heterojunction photocatalysts. A series of characterizations suggest that CdS is uniformly grown on surface of Zn_0.4(CuGa)_0.3Ga₂S₄ microcrystals to form a Z-scheme type all-solid heterojunction composite materials. Such a structure effectively suppresses the recombination of electron-hole pairs and improve the photocatalytic performance. In the solution CO₂ reduction system, the as-prepared Zn_0.4(CuGa)_0.3Ga₂S₄/CdS can effectively reduce CO₂ into CO under visible light irradiation. The optimal molar ratio of Zn_0.4(CuGa)_0.3Ga₂S₄ and CdS in composite materials is 2 : 1, whose photocatalytic performance is 1.7 times of that of Zn_0.4(CuGa)_0.3Ga₂S₄/ CdS and 1.6 times of that of CdS. This work constructs all solid Z-scheme type Zn_0.4(CuGa)_0.3Ga₂S₄/CdS heterojunction materials with enhanced photocatalytic activity for CO₂ reduction, which is promising for designing novel photocatalysts in field of artificial photosynthesis.

Highly light absorptive photocatalysts are of great significance to boost the photothermal conversion efficiency. Light trapping effect of nanoarray structured photothermal catalysts can enhance the light absorption and improve the photothermal conversion efficiency. However, the practical applications of array-based catalysts are hindered by very low loadings of active metal catalysts per unit illumination area. Herein, we develop a SiO₂-protected MOFs pyrolysis method for the preparation of powder-form cobalt plasmonic superstructures that enable a 90% absorption efficiency of sunlight and tunable metal loading per unit area. Its high light absorption capacity was confirmed by time-domain finite-difference simulation calculations due to the plasmonic hybridization effect of nanoparticles. Compared with nanoarray-structured plasmonic superstructures, the powder-form catalyst exhibit enhanced catalytic activity and stability, resulting in the increase of CO₂ conversion efficiency from 0.9% to 26.2%. This study lays the foundation for the practical application of non-precious metal photothermal catalysts.

The electrocatalytic carbon dioxide reduction reaction can convert the greenhouse gas carbon dioxide into chemical raw materials or organic fuels, providing a feasible way to overcome global warming and the conversion of electrical energy to chemical energy. The main challenge of this technology is the wide product distribution, resulting in low selectivity of a single product, however, modulating the surface properties of the catalyst is an efficient strategy to solve this problem. In this study, the precursors of Cu₂O and Cu₂S were oxidized to the CuO catalysts with different surface properties. The CuO-FS catalyst derived from Cu₂S delivered the improved activity of electro-reduction of carbon dioxide and selectivity for formic acid product. This catalyst exhibited a higher total current density and the Faraday efficiency of formic acid > 70% in a wide test voltage range of -0.8 - -1.1 V; the Faraday efficiency for formic acid could reach a maximum of 78.4% at -0.9 V. The mechanism study indicated that the excellent performance of CuO-FS for electro-reduction of carbon dioxide could be attributed to the large electrochemically active surface area, which provided a large number of surface active sites, resulting in a higher total current density; moreover, the less zero-valent Cu was produced over the surface of CuO-FS during the electrocatalytic process, which reduced the production of ethylene and thus promoted the production of formic acid.

Carbon-neutral fuel production by solar-driven two-step thermochemical carbon dioxide splitting provides an alternative to fossil fuels as well as mitigates global warming. The success of this technology relies on the advancements of redox materials. Despite the recognition of the entropic effect, usually energy descriptors (enthalpy of formation or energy of oxygen-vacancy formation) were used for computational assessment of material candidates. Here, in the first step, the criteria was derived based on the combination of solid-state change of entropy and formation enthalpy, and was used to thermodynamically assess the viability of material candidates. In the thermodynamic map, a triangular region, featuring large positive solid-state changes of entropy and small enough solid-state changes of formation enthalpy, was found for qualified candidates. Next, a first-principles DFT+U method was presented to fast and reasonably predict the solid-state changes of entropy and formation enthalpy of candidate redox materials, exemplified for pure and Samaria-doped ceria, so that new redox materials can be added to the thermodynamic map. All above results highlight the entropic contributions from polaron-defect vibrational entropy as well as ionic (oxygen vacancies) and electronic (polarons) configurational entropy.