Photocatalytic reduction of CO2 for fuel production: Possibilities and challenges

One-step transformation of CO2 to methane by Escherichia coli with a synthetic biomethanation module

The biomethanation process is widely recognized as a significant approach to mitigating carbon dioxide emissions while simultaneously generating methane. However, only a few microorganisms that required intricate culturing conditions were identified for biomethanation. Here, Escherichia coli that featured easy cultivation and versatile chassis was genetically modified for biomethanation for the first time. The nif-related gene cluster with methanogenic capability from Rhodopseudomonas palustris was systematically analyzed, cloned, and integrated into a synthetic biomethanation module. As a result, E. coli BL21 (DE3) and Rosetta (DE3) carrying this synthetic biomethanation module exhibited significant methane production activity, with methane yields reaching 50 nmol/mL and 159 nmol/mL, respectively. This finding provided a simple route to construct synthetic strain for biomethanation, which would advance the fundamental research and be beneficial to further harness the power of biomethanation for practical application.

ZIF-8 metal-organic frameworks and their hybrid materials: emerging photocatalysts for energy and environmental applications

In the face of escalating environmental challenges such as fossil fuel dependence and water pollution, innovative solutions are essential for sustainable development. In this regard, zeolitic imidazolate frameworks (ZIFs), specifically ZIF-8, act as promising photocatalysts for environmental remediation and renewable energy applications. ZIF-8, a subclass of metal-organic frameworks (MOFs), is renowned for its large specific surface area, high porosity, rapid electron transfer ability, abundant functionalities, ease of designing, controllable properties, and remarkable chemical and thermal stability. However, its application as a standalone photocatalyst is limited by issues such as particle aggregation, poor water stability, and insufficient visible light absorption. By integrating ZIF-8 with various photoactive materials to form composite catalysts, these drawbacks can be mitigated, leading to enhanced photocatalytic efficiency. The review discusses the synthesis, properties, and applications of ZIF-8-based photocatalysts in light-driven H2 evolution, H2O2 evolution, CO2 reduction, and dye and drug degradation. It also highlights the challenges and future research directions in developing cost-effective, scalable, and environmentally friendly ZIF-8 composites for industrial applications. The potential of ZIF-8 composites to contribute to sustainable global energy solutions and environmental cleanup is significant, yet further exploration is required to harness their capabilities thoroughly.

Size and Morphology Dependent Activity of Cu Clusters for CO2 Activation and Reduction: A First Principles Investigation

Various Cu-based materials in diverse forms have been investigated as efficient catalysts for electrochemical reduction of CO2; however, they suffer from issues such as higher over potential and poor selectivity. The activity and selectivity of CO2 electro reduction have been shown to change significantly when the surface morphology (steps, kinks, and edges) of these catalysts is altered. In light of this, size and morphology dependent activity of selected copper clusters, Cun (n=2-20) have been evaluated for the activation and reduction of CO2 molecule. The phase-space of these copper clusters is rich in conformations of distinct morphologies starting from planar, 2D geometries to prolate-shaped geometries and also high-symmetry structures. The binding efficiency and the activation of CO2 are highest for medium sized clusters (n=9-17) with prolate-morphologies as compared to small or larger sized CunCO2 clusters that are existing mainly as planar (triangular, tetragonal etc.) or highly-symmetric geometries (icosahedron, capped-icosahedron etc.), respectively. The best performing (prolate-shaped) CunCO2 conformations are quite fluxional and also they are thermally stable, as demonstrated by the molecular dynamics simulations. Furthermore, on these CunCO2 conformations, the step-by-step hydrogenation pathways of CO2 to produce value-added products like methanol, formic acid, and methane are exceptionally favorable and energy-efficient.

Emerging ZnO Semiconductors for Photocatalytic CO2 Reduction to Methanol

Carbon recycling is poised to emerge as a prominent trend for mitigating severe climate change and meeting the rising demand for energy. Converting carbon dioxide (CO2) into green energy and valuable feedstocks through photocatalytic CO2 reduction (PCCR) offers a promising solution to global warming and energy needs. Among all semiconductors, zinc oxide (ZnO) has garnered considerable interest due to its ecofriendly nature, biocompatibility, abundance, exceptional semiconducting and optical properties, cost-effectiveness, easy synthesis, and durability. This review thoroughly discusses recent advances in mechanistic insights, fundamental principles, experimental parameters, and modulation of ZnO catalysts for direct PCCR to C1 products (methanol). Various ZnO modification techniques are explored, including atomic size regulation, synthesis strategies, morphology manipulation, doping with cocatalysts, defect engineering, incorporation of plasmonic metals, and single atom modulation to boost its photocatalytic performance. Additionally, the review highlights the importance of photoreactor design, reactor types, geometries, operating modes, and phases. Future research endeavors should prioritize the development of cost-effective catalyst immobilization methods for solid-liquid separation and catalyst recycling, while emphasizing the use of abundant and non-toxic materials to ensure environmental sustainability and economic viability. Finally, the review outlines key challenges and proposes novel directions for further enhancing ZnO-based photocatalytic CO2 conversion processes.

Exploring Pt-Impregnated CdS/TiO2 Heterostructures for CO2 Photoreduction

This work focuses on the production of methane through the photocatalytic reduction of carbon dioxide using Pt-doped CdS/TiO2 heterostructures. The photocatalysts were prepared using P25 commercial titania and CdS synthesized through a solvothermal methodology, followed by the impregnation of Pt onto the surface to enhance the physicochemical properties of the resulting photocatalysts. The pure and heterostructure-based materials were characterized using X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible spectroscopy (UV-Vis), ultraviolet photoelectron spectroscopy (UPS), and photoluminescence spectroscopy (PL). The obtained results show the successful synthesis of the heterostructure impregnated with Pt. Moreover, the observed key role of CdS and Pt nanoparticles in the final semiconductor is to reduce the electron-hole pair recombination rate by acting as an electron sink, which slows down the recombination process and increases the photocatalyst efficiency. Thus, Pt-doped CdS/TiO2 heterostructures with the best observed composition presents better catalytic activity than P25 titania with methane production values being 460 and 397 µmol CH4/g·h, respectively.

Catalytic conversion of carbon dioxide (CO2) using coal-based nano-carbon materials

Carbon dioxide (CO2) is a prominent greenhouse gas and a widely available carbon resource. The chemical conversion of CO2 into high-value chemicals and fuels is a significant approach for mitigating carbon emissions and attaining carbon neutrality. However, enhancing CO2 adsorption and conversion rates remains a primary challenge in CO2 recycling. The development of high-performance catalysts is pivotal for the catalytic conversion of CO2. In this context, coal-based carbon materials, characterized by their extensive specific surface area and adaptable chemical composition, can offer more reactive active sites and have robust CO2 adsorption capabilities. They can function as either standalone catalysts or as components of composite catalysts, making them promising materials for CO2 reduction. The use of affordable and abundant coal as a precursor for carbon materials represents a crucial avenue for achieving clean and efficient coal utilization. This paper reviews the progress of research on coal-based carbon materials and examines their advantages and challenges as catalysts for CO2 reduction.

Utilization of waste tire derived activated carbon as CO2 capture and photocatalyst for CO2 conversion

The aims of this research were to prepare activated carbon (AC) impregnated with tetraethylenepentamine (TEPA) for use in carbon dioxide (CO2) capture and to then develop the AC-TEPA sorbent with titanium dioxide (TiO2) as a catalyst for photocatalytic reduction. The AC was impregnated with TEPA at three loading levels (2.5, 5, and 10% [w/w]) and then examined for its CO2 adsorption capacity under an ambient temperature and atmospheric pressure. The use of 5% (w/w) TEPA-impregnated AC (AC_5T) provided the highest CO2 adsorption capacity and long-term operation with a regeneration ability for up to 10 cycles. Then, AC_5T-doped TiO2 (AC_5T-TiO2) was prepared as a photocatalytic reduction catalyst, since the presence of carbon and nitrogen in AC_5T could reduce the band gap energy and so enhance the photocatalytic reduction. In addition, the CO2-saturated AC_5T was used as a CO2 source that could be directly converted to valuable chemicals using the AC_5T-TiO2 catalyst under photocatalytic reduction. Products were obtained in both the liquid (methanol) and gaseous (methane, carbon monoxide, and hydrogen) phases. Accordingly, the challenge of this research was to make valuable products from CO2 and to manage waste tires, following the circular economy concept.

Atomic/molecular layer deposition strategies for enhanced CO2 capture, utilisation and storage materials

Elevated levels of carbon dioxide (CO2) in the atmosphere and the diminishing reserves of fossil fuels have raised profound concerns regarding the resulting consequences of global climate change and the future supply of energy. Hence, the reduction and transformation of CO2 not only mitigates environmental pollution but also generates value-added chemicals, providing a dual remedy to address both energy and environmental challenges. Despite notable advancements, the low conversion efficiency of CO2 remains a major obstacle, largely attributed to its inert chemical nature. It is imperative to engineer catalysts/materials that exhibit high conversion efficiency, selectivity, and stability for CO2 transformation. With unparalleled precision at the atomic level, atomic layer deposition (ALD) and molecular layer deposition (MLD) methods utilize various strategies, including ultrathin modification, overcoating, interlayer coating, area-selective deposition, template-assisted deposition, and sacrificial-layer-assisted deposition, to synthesize numerous novel metal-based materials with diverse structures. These materials, functioning as active materials, passive materials or modifiers, have contributed to the enhancement of catalytic activity, selectivity, and stability, effectively addressing the challenges linked to CO2 transformation. Herein, this review focuses on ALD and MLD's role in fabricating materials for electro-, photo-, photoelectro-, and thermal catalytic CO2 reduction, CO2 capture and separation, and electrochemical CO2 sensing. Significant emphasis is dedicated to the ALD and MLD designed materials, their crucial role in enhancing performance, and exploring the relationship between their structures and catalytic activities for CO2 transformation. Finally, this comprehensive review presents the summary, challenges and prospects for ALD and MLD-designed materials for CO2 transformation.

Nitrogen Doping-Roused Synergistic Active Sites in Perovskite Enabling Highly Selective CO2 Photoreduction into CH4

The intricate protonation process in carbon dioxide reduction usually makes the product unpredictable. Thus, it is significant to control the reactive intermediates to manipulate the reaction steps. Here, we propose that the synergistic La-Ti active sites in the N-La2Ti2O7 nanosheets enable the highly selective carbon dioxide photoreduction into methane. In the photoreduction of CO2 over N-La2Ti2O7 nanosheets, in situ Fourier transform infrared spectra are utilized to monitor the *CH3O intermediate, pivotal for methane production, whereas such monitoring is not conducted for La2Ti2O7 nanosheets. Also, theoretical calculations testify to the increased charge densities on the Ti and La atoms and the regulated formation energy barrier of *CO and *CH3O intermediates by the constructed synergistic active sites. Accordingly, the methane formation rate of 7.97 μL h-1 exhibited by the N-La2Ti2O7 nanosheets, along with an electron selectivity of 96.6%, exceeds that of most previously reported catalysts under similar conditions.

Recent Progress on Ligand-Protected Metal Nanoclusters in Photocatalysis

The reckless use of non-replenishable fuels by the growing population for energy and the resultant incessant emissions of hazardous gases and waste products into the atmosphere have insisted that scientists fabricate materials capable of managing these global threats at once. In recent studies, photocatalysis has been employed to focus on utilizing renewable solar energy to initiate chemical processes with the aid of semiconductors and highly selective catalysts. A wide range of nanoparticles has showcased promising photocatalytic properties. Metal nanoclusters (MNCs) with sizes below 2 nm, stabilized by ligands, show discrete energy levels and exhibit unique optoelectronic properties, which are vital to photocatalysis. In this review, we intend to compile information on the synthesis, true nature, and stability of the MNCs decorated with ligands and the varying photocatalytic efficiency of metal NCs concerning changes in the aforementioned domains. The review discusses the photocatalytic activity of atomically precise ligand-protected MNCs and their hybrids in the domain of energy conversion processes such as the photodegradation of dyes, the oxygen evolution reaction (ORR), the hydrogen evolution reaction (HER), and the CO₂ reduction reaction (CO₂RR).

UiO-66 MOF-Derived Ru@ZrO2 Catalysts for Photo-Thermal CO2 Hydrogenation

The use of metal–organic frameworks (MOFs) as templates or precursors in the manufacture of heterogeneous catalysts is highly attractive due to the transfer of MOFs’ inherent porosity and homogeneous metallic distribution to the derived structure. Herein, we report on the preparation of MOF-derived Ru@ZrO2 catalysts by controlled thermal treatment of zirconium-based MOF UiO-66 with ruthenium moieties. Ru3+ (3 or 10 mol%) precursor was added to UiO-66 synthesis and, subsequently, the as-synthesized hybrid structure was calcined in flowing air at different temperatures (400–600 °C) to obtain ZrO2-derived oxides doped with highly dispersed Ru metallic clusters. The materials were tested for the catalytic photo-thermal conversion of CO2 to CH4. Methanation experiments were conducted in a continuous flow (feed flow rate of 5 sccm and 1:4 CO2 to H2 molar ratio) reactor at temperatures from 80 to 300 °C. Ru0.10@ZrO2 catalyst calcined at 600 °C was able to hydrogenate CO2 to CH4 with production rates up to 65 mmolCH4·gcat.–1·h–1, CH4 yield of 80% and nearly 100% selectivity at 300 °C. The effect of the illumination was investigated with this catalyst using a high-power visible LED. A CO2 conversion enhancement from 18% to 38% was measured when 24 sun of visible LED radiation was applied, mainly due to the increase in the temperature as a result of the efficient absorption of the radiation received. MOF-derived Ru@ZrO2 catalysts have resulted to be noticeably active materials for the photo-thermal hydrogenation of CO2 for the purpose of the production of carbon-neutral methane. A remarkable effect of the ZrO2 crystalline phase on the CH4 selectivity has been found, with monoclinic zirconia being much more selective to CH4 than its cubic allotrope.

Mechanism of CO2 photoreduction by selenium-doped carbon nitride with cobalt clusters as cocatalysts

Doping is an efficient strategy for improving the photocatalytic activity and tuning the electronic structure of carbon nitride. Selenium-doped melon carbon nitride (Se-doped melon CN) as a promising photocatalyst for CO₂ reduction is investigated using density functional theory calculations. In addition, considering the special role of a cocatalyst in CO₂ reduction, we have explored the electronic and optical properties of Co₄ clusters loaded on the Se-doped melon CN surface. After loading cobalt clusters, CO₂ activation is significantly improved, with preference for the 8-electron product CH₄, as the 2-electron products have higher desorption energies. Overall, this work provides a microscopic understanding of the CO₂ reduction mechanism on Se-doped melon CN with cobalt as the co-catalyst.

Research status, challenges and future prospects of renewable synthetic fuel catalysts for CO2 photocatalytic reduction conversion

In recent years, with global climate change, the utilization of carbon dioxide as a resource has become an important goal of human society to achieve carbon peaking and carbon neutrality. Among them, the catalytic conversion of carbon dioxide to generate renewable fuels has received great attention. As one of these methods, photocatalysis has its unique properties and mechanism, which can only rely on sunlight without inputting other energy. It is an emerging discipline with great development prospects. The core of photocatalysis lies in the development of photocatalysts with high activity, high selectivity, low cost, and high durability. This review first introduces the background and mechanism of photocatalysis, then introduces various types of photocatalysts based on different substrates, and analyzes the methods and mechanisms to improve the activity and selectivity of photocatalysts. Finally, combining the plasmon effect with photocatalysis, the review analyzes the promoting effect of the plasmon effect on the photocatalytic carbon dioxide synthesis of renewable fuels, which provides a new idea for it.

Improved water splitting efficiency of Au-NP-loaded Ga2O3 thin films in the visible region under strong coupling conditions

We fabricate a novel photoanode consisting of TiO₂/Au nanoparticles (Au-NPs)/Ga₂O₃/TiN/Au-film (TAGA), efficiently increasing light absorption and electron transfer from Au-NPs to Ga₂O₃ under modal strong coupling. A TiN thin layer deposited on an Au film enables stable high-temperature deposition of Ga₂O₃ onto the reflective Au film mirror. Modal strong coupling is observed when the resonance wavelength of the Ga₂O₃/TiN/Au-film Fabry-Pérot cavity overlaps with the plasmon resonance wavelength of Au-NPs partially inlaid in a thin TiO₂ layer. Under strong coupling conditions, the light absorption and photoelectrochemical conversion efficiency in the visible region increased more than in the samples without coupling. In this structure, the TiO₂ layer partially inlaying Au-NPs plays a vital role in effectively enhancing the coupling strength. We accomplish water splitting at zero bias potential by taking advantage of the intrinsically negative conduction band potential of Ga₂O₃.

Rational Regulation of Electronic Structure in Layered Double Hydroxide Via Vanadium Incorporation to Trigger Highly Selective CO2 Photoreduction to CH4

To realize excellent selectivity of CH₄ in CO₂ photoreduction (CO₂ PR) is highly desirable, yet which is challenging due to the limited active sites for CH₄ generation and severe electron-hole recombination on photocatalysts. Herein, based on the theoretically calculated effects of vanadium incorporation into the laminate of layered double hydroxides (LDHs), V into NiAl-LDH to synthesize a series of LDHs with various V contents is introduced. NiV-LDH is revealed to afford a high CH₄ selectivity (78.9%), and extremely low H₂ selectivity (only 0.4%) under λ > 400 nm irradiation. By further tuning the molar ratio of Ni to V, a CH₄ selectivity of as high as 90.1% is achieved on Ni₄ V-LDH, and H₂ is completely prohibited on Ni₂ V-LDH. Fine structural characterizations and comprehensive optical and electrochemical studies uncover V incorporation creates the lower-valence Ni species as active sites for generating CH₄ , and enhances the generation, separation, and transfer of photogenerated carriers.

Nickel metal-organic frameworks for visible-light CO2 reduction under mild reaction conditions

Photochemical CO₂ conversion into carbon fuel is a promising route to explore renewable energy and relieve climate change. However, it is still a key challenge to achieve high selectivity to CO and simultaneously achieve high conversion efficiency in photochemical CO₂ reduction. Herein, we demonstrate the effect of Ni metal centers as catalytic active sites for the photocatalytic conversion of CO₂ to CO by designing and constructing Ni metal-organic framework (Ni-MOF) materials. In pure CO₂, Ni-MOF catalyst exhibits outstanding performance for visible-light-driven reductive CO₂ deoxygenation with a high CO evolution rate of 19.13 μmol h^-1 (per 1 mg of catalyst) and CO selectivity of 91.4%, which exceeds those of most reported systems. Upon using isostructural Co-MOF as the catalyst to replace Ni-MOF, a moderate performance towards CO₂ photoreduction and low CO selectivity (40.1%) were observed, implying that the performance of CO₂ photoreduction and CO selectivity are dependent on unsaturated metal centers.

Selective photocatalytic CO2 reduction in aerobic environment by microporous Pd-porphyrin-based polymers coated hollow TiO2

Direct photocatalytic CO2 reduction from primary sources, such as flue gas and air, into fuels, is highly desired, but the thermodynamically favored O2 reduction almost completely impedes this process. Herein, we report on the efficacy of a composite photocatalyst prepared by hyper-crosslinking porphyrin-based polymers on hollow TiO2 surface and subsequent coordinating with Pd(II). Such composite exhibits high resistance against O2 inhibition, leading to 12% conversion yield of CO2 from air after 2-h UV-visible light irradiation. In contrast, the CO2 reduction over Pd/TiO2 without the polymer is severely inhibited by the presence of O2 ( ≥ 0.2 %). This study presents a feasible strategy, building Pd(II) sites into CO2-adsorptive polymers on hollow TiO2 surface, for realizing CO2 reduction with H2O in an aerobic environment by the high CO2/O2 adsorption selectivity of polymers and efficient charge separation for CO2 reduction and H2O oxidation on Pd(II) sites and hollow TiO2, respectively.

Challenges and prospects in the selective photoreduction of CO2 to C1 and C2 products with nanostructured materials: a review

Solar fuel generation through CO₂ hydrogenation is the ultimate strategy to produce sustainable energy sources and alleviate global warming. The photocatalytic CO₂ conversion process resembles natural photosynthesis, which regulates the ecological systems of the earth. Currently, most of the work in this field has been focused on boosting efficiency rather than controlling the distribution of products. The structural architecture of the semiconductor photocatalyst, CO₂ photoreduction process, product analysis, and elucidating the CO₂ photoreduction mechanism are the key features of the photoreduction of CO₂ to generate C1 and C2 based hydrocarbon fuels. The selectivity of C1 and C2 products during the photocatalytic CO₂ reduction have been ameliorated by suitable photocatalyst design, co-catalyst, defect states, and the impacts of the surface polarisation state, etc. Monitoring product selectivity allows the establishment of an appropriate strategy to generate a more reduced state of a hydrocarbon, such as CH₄ or higher carbon (C2) products. This article concentrates on studies that demonstrate the production of C1 and C2 products during CO₂ photoreduction using H₂O or H₂ as an electron and proton source. Finally, it highlights unresolved difficulties in achieving high selectivity and photoconversion efficiency of CO₂ in C1 and C2 products over various nanostructured materials.

Continuous-Flow Sunlight-Powered CO2 Methanation Catalyzed by γ-Al2O3-Supported Plasmonic Ru Nanorods

Plasmonic CO2 methanation using γ-Al2O3-supported Ru nanorods was carried out under continuous-flow conditions without conventional heating, using mildly concentrated sunlight as the sole and sustainable energy source (AM 1.5, irradiance 5.5–14.4 kW·m−2 = 5.5–14.4 suns). Under 12.5 suns, a CO2 conversion exceeding 97% was achieved with complete selectivity towards CH4 and a stable production rate (261.9 mmol·gRu−1·h−1) for at least 12 h. The CH4 production rate showed an exponential increase with increasing light intensity, suggesting that the process was mainly promoted by photothermal heating. This was confirmed by the apparent activation energy of 64.3 kJ·mol−1, which is very similar to the activation energy obtained for reference experiments in dark (67.3 kJ·mol−1). The flow rate influence was studied under 14.4 suns, achieving a CH4 production plateau of 264 µmol min−1 (792 mmol·gRu−1·h−1) with a constant catalyst bed temperature of approximately 204 °C.

Semiconductor photocatalysts and mechanisms of carbon dioxide reduction and nitrogen fixation under UV and visible light

The review summarizes the current knowledge about heterogeneous semiconductor photocatalysts that are active towards photocatalytic reduction of carbon dioxide and molecular nitrogen under visible and near-UV light. The main classes of these photocatalysts and characteristic features of their application in the target processes are considered. Primary attention is given to photocatalysts based on titanium dioxide, which have high activity and stability in the carbon dioxide reduction. For the first time, the photofixation of nitrogen under irradiation in the presence of various semiconductor materials is considered in detail.

The bibliography includes 264 references.

Asymmetric Triple-Atom Sites Confined in Ternary Oxide Enabling Selective CO2 Photothermal Reduction to Acetate

Light-induced heat is largely neglected in traditional photocatalytic systems, especially for the thermodynamically and kinetically challenging CO₂ reduction to C₂ fuels. Herein, we first design asymmetric Metal₁-O-Metal₂ triple-atom sites confined in phenakite to facilitate C-C coupling and employ photoinduced heat to increase molecular thermal vibration and accelerate CO₂ reduction to C₂ fuels. Using O-vacancy-rich Zn₂GeO₄ nanobelts as prototypes, quasi in situ Raman spectra disclose the Zn-O-Ge triatomic sites are likely the reactive sites. Density functional theory calculations reveal that the asymmetric Zn-O-Ge sites could promote C-C coupling through inducing distinct charge distributions of neighboring C₁ intermediates, whereas the created O vacancies could lower the energy barrier of the rate-determining hydrogenation step from 1.46 to 0.67 eV. Catalytic performances under different testing conditions demonstrate that light initiates the CO₂ reduction reaction. In situ Fourier-transform infrared spectra and D₂O kinetic isotopic effect experiments disclose that light-induced heat kinetically triggers C-C coupling and accelerates OCCO* hydrogenation via providing abundant hydrogen species. Consequently, in a simulated air atmosphere under 0.1 W/cm² illumination at 348 K, the O-vacancy-rich Zn₂GeO₄ nanobelts demonstrate an acetate output of 12.7 μmol g^-1 h^-1, a high acetate selectivity of 66.9%, a considerable CO₂-to-CH₃COOH conversion ratio of 29.95%, and a stability of up to 220 h.

Enhanced light-driven CO2 reduction on metal-free rich terminal oxygen-defects carbon nitride nanosheets

Exploiting highly-efficient and metal-free photocatalyst for CO₂ conversion into useful chemicals is a promising pathway to solve the energy and environmental crises. In this work, through a facile exfoliation process, an ultra-thin and short-range order g-C₃N₄ nanosheet with rich terminal oxygen defects is successfully constructed, which presents total electron yield of 36.30 μmol g^-1h^-1, 3.05 times higher than that of bulk one. The results affirms that both the van der Waals forces between the C₃N₄ layers and the CN bonds on the periodic heptazine units could be disrupted during the sonication process, thus achieving the ultra-thin and ultra-small g-C₃N₄ nanosheet, which enables the improvement of optical absorption and carrier separation abilities. The π-conjugated triazine rings structure is still remained but the terminal active C radicals tend to transform into oxygen defects which become the sites to bind and activate CO₂. The in-situ DRIFTS provides the direct evidence that the size regulation and oxygen-defects design strategy can effectively promote the CO₂ adsorption and activation process upon the photocatalyst, thus turning out to boost the reactivity toward CO₂.

Recent Advances in TiO2-Based Heterojunctions for Photocatalytic CO2 Reduction With Water Oxidation: A Review

Photocatalytic conversion of CO₂ into solar fuels has gained increasing attention due to its great potential for alleviating the energy and environmental crisis at the same time. The low-cost TiO₂ with suitable band structure and high resistibility to light corrosion has proven to be very promising for photoreduction of CO₂ using water as the source of electrons and protons. However, the narrow spectral response range (ultraviolet region only) as well as the rapid recombination of photo-induced electron-hole pairs within pristine TiO₂ results in the low utilization of solar energy and limited photocatalytic efficiency. Besides, its low selectivity toward photoreduction products of CO₂ should also be improved. Combination of TiO₂ with other photoelectric active materials, such as metal oxide/sulfide semiconductors, metal nanoparticles and carbon-based nanostructures, for the construction of well-defined heterostructures can enhance the quantum efficiency significantly by promoting visible light adsorption, facilitating charge transfer and suppressing the recombination of charge carriers, resulting in the enhanced photocatalytic performance of the composite photocatalytic system. In addition, the adsorption and activation of CO₂ on these heterojunctions are also promoted, therefore enhancing the turnover frequency (TOF) of CO₂ molecules, so as to the improved selectivity of photoreduction products. This review focus on the recent advances of photocatalytic CO₂ reduction via TiO₂-based heterojunctions with water oxidation. The rational design, fabrication, photocatalytic performance and CO₂ photoreduction mechanisms of typical TiO₂-based heterojunctions, including semiconductor-semiconductor (S-S), semiconductor-metal (S-M), semiconductor-carbon group (S-C) and multicomponent heterojunction are reviewed and discussed. Moreover, the TiO₂-based phase heterojunction and facet heterojunction are also summarized and analyzed. In the end, the current challenges and future prospects of the TiO₂-based heterostructures for photoreduction of CO₂ with high efficiency, even for practical application are discussed.

Recent advances in visible-light-driven carbon dioxide reduction by metal-organic frameworks

Metal-organic frameworks (MOFs) have emerged as promising materials and have attracted researchers due to their unique chemical and physical properties-design flexibility, tuneable pore channels, a high surface-to-volume ratio that allow their distinct application in diverse research fields-gas storage, gas separation, catalysis, adsorption, drug delivery, ion exchange, sensing, etc. The rapidly growing CO₂ in the atmosphere is a global concern due to the excessive use of fossil fuels in the current era. CO₂ is the prime cause of global warming and should be ameliorated either through adsorption or conversion into value-added products to protect the environment and mankind. Nowadays, MOFs are exploited as a photocatalyst for applications of CO₂ reduction. Since the use of semiconductors limits the use of visible light for photocatalytic reduction of CO₂, MOFs are promising options. The current review describes recent development in the application of MOFs as host, composites, and their derivatives in photocatalytic reduction of CO₂ to CO and different organic chemicals (HCOOH, CH₃OH, CH₄). Efficient charge separation and visible light absorption by incorporation of active sites for efficient photocatalysis have been discussed. The selection of material for high CO₂ uptake and potential strategies for the rational design and development of high-performance catalysts are outlined. Major challenges and future perspectives have also been discussed at the last of the review.

Design, Fabrication, and Mechanism of Nitrogen-Doped Graphene-Based Photocatalyst

Solving energy and environmental problems through solar-driven photocatalysis is an attractive and challenging topic. Hence, various types of photocatalysts have been developed successively to address the demands of photocatalysis. Graphene-based materials have elicited considerable attention since the discovery of graphene. As a derivative of graphene, nitrogen-doped graphene (NG) particularly stands out. Nitrogen atoms can break the undifferentiated structure of graphene and open the bandgap while endowing graphene with an uneven electron density distribution. Therefore, NG retains nearly all the advantages of original graphene and is equipped with several novel properties, ensuring infinite possibilities for NG-based photocatalysis. This review introduces the atomic and band structures of NG, summarizes in situ and ex situ synthesis methods, highlights the mechanism and advantages of NG in photocatalysis, and outlines its applications in different photocatalysis directions (primarily hydrogen production, CO₂ reduction, pollutant degradation, and as photoactive ingredient). Lastly, the central challenges and possible improvements of NG-based photocatalysis in the future are presented. This study is expected to learn from the past and achieve progress toward the future for NG-based photocatalysis.