CO2 Photoconversion Research Articles

Defect Engineering of Photocatalysts towards Elevated CO2 Reduction Performance.

Photocatalytic CO2 reduction provides a promising solution to address the crises of massive CO2 emissions and fossil energy shortages. As one of the most effective strategies to promote CO2 photoconversion, defect engineering shows great potential in modulating the electronic structure and light absorption properties of photocatalysts while increasing surface active sites for CO2 activation and conversion. This Review summarizes the recent progress in defect engineering of photocatalysts to promote CO2 reduction performances from the following four aspects: 1) Approaches to defect (mainly vacancy and dopant) generation in photocatalysts; 2) defect structure characterization techniques; 3) physical and chemical properties of defect-engineered photocatalysts; 4) CO2 reduction performance enhancements in activity, selectivity, and stability of photocatalysts by defect engineering. This Review is expected to present readers with a comprehensive view of progress in the field of photocatalytic CO2 reduction through defect engineering for elevated CO2 -to-fuels conversion efficiency.

In-situ fabrication of Cu3(MoO4)2(OH)2 films decorated with MO (M=Zn, Cu, and Ni) for CO2 photoconversion into value-added compounds

CO2 photoreduction is an emerging technology to produce value-added compounds using abundant resources such as CO2, H2O, sunlight, and photocatalyst materials. Here, Cu3(MoO4)2(OH)2 (CMO) films were proposed as photocatalysts for CO2 photoconversion into value-added compounds, such as methanol, formic acid, and hydrogen. The CMO films were decorated with simple oxides MO (M=Zn, Cu, and Ni) by an in-situ microwave-hydrothermal method. The CMO films exhibited a bar morphology, high crystallinity, band gap suitable for solar-light activation (2.8 eV), and valence and conduction band potential of 1.3 eV and 1.5 eV vs NHE, respectively. After decoration with the simple oxides, the films retained the bar morphology and resulted in a more efficient charge transfer by forming type-I heterostructure between Cu3(MoO4)2(OH)2 and CuO; while, type-II heterostructures were produced between CMO and NiO and ZnO. The CO2 was photoconverted to HCOOH and CH3OH in the liquid phase using CMO reference and the decorated films as photocatalysts. Cu3(MoO4)2(OH)2 decorated with CuO (CMO/Cu) favored the production of 155 µmol g−1 HCOOH, 109 µmol g−1 for CH3OH, and 127 µmol g−1 H2 after 1 h under visible-light irradiation. The yields obtained were up to 8 times higher than other conventional photocatalysts. The stability and recyclability of the films were demonstrated after three consecutive cycles. The photocatalytic activity for value-added compounds production increased as follows: CMO/Cu > CMO/Ni > CMO/Zn > CMO, associated to a higher CO2 adsorption, 2D morphology, adequate conduction band potential, and a more efficient charge transfer.

The CO2 photoconversion over reduced graphene oxide based on Ag/TiO2 photocatalyst in an advanced meso-scale continuous-flow photochemical reactor.

This study aims at examining the use of an advanced meso-scale continuous-flow photochemical reactor for the photocatalytic conversion of CO2 with water into fuel over TiO2 (P25), Ag/TiO2, and Ag/TiO2/RGO catalysts. The silver loaded photocatalysts were prepared by one-step process via hydrothermal method. The prepared photocatalysts were characterized by various characterization techniques in order to identify the morphological, chemical, physical, and optical properties. The photocatalytic activity of the as-prepared catalysts was firstly examined by the photoelectrochemical (PEC) measurements and secondly by the photocatalytic reduction of CO2 in the proposed setup. Liquid products were analyzed using gas chromatography-mass spectrometry (GC-MS) and total organic carbon (TOC) techniques. It was found that the ternary composite revealed an outstanding performance towards CO2 photocatalytic reduction, where its selectivity was directed towards methanol production. The incorporation of graphene nanosheets enhanced the photocatalytic reduction of CO2 by 3.3 and 9.4 times compared with Ag/TiO2 and bare TiO2, respectively, using the proposed photochemical reactor in a continuous mode. This study sheds the light on a novel type of a photocatalytic reactor where CO2 conversion over Ag/TiO2/RGO ternary composite was evaluated. A meso-scale continuous-flow photochemical reactor.

High selective photocatalytic CO2 conversion into liquid solar fuel over a cobalt porphyrin-based metal-organic framework.

In the present study, porphyrin-based metal-organic framework (Co/PMOF) was synthesized and characterized by different spectra analyses. The photoluminescence properties of porphyrin and Co/PMOF revealed that the photoluminescence of Co/PMOF was quenched compared to the porphyrin, indicating that the lifetime of photo-generated charge carriers in Co/PMOF is longer than porphyrin. The prepared Co/PMOF was applied as an efficient photocatalyst for CO2 photoconversion to formate in the presence of triethanolamine (TEOA) as a sacrificial agent under visible-light irradiation. The photoreaction results showed that 23.21µmol HCOO- was produced over Co/PMOF during the 6h photocatalytic reaction under visible illumination, showing much better activity than the porphyrin, 4.56µmol HCOO-. No other products were detected, suggesting that this reaction over Co/PMOF has high selectivity. Co/PMOF reusability and stability were examined through recycling tests and there were no remarkable losses of photoactivity even after three cycles of photoreaction. Moreover, FTIR measurement and UV-Vis spectra demonstrated no notable changes in Co/PMOF structure. As a result, superior photocatalytic behavior of Co/PMOF was implied for CO2 photoreduction which highlights the great potential of assembly porphyrin and cobalt into MOFs for CO2 photoreduction.

Standalone photoconversion of CO2 using Ti and TiOx-sandwiched heterojunction photocatalyst of CuO and CuFeO2 films

Artificial photoconversion of carbon dioxide to value-added chemicals remains a challenge. In this work, we synthesize heterojunction copper oxide and copper iron oxide (CuO/CuFeO2; CFO) films on transparent conducting substrates with metallic Ti layers, and deposit disordered TiOx particles on the CFO films. The Ti/CFO/TiOx samples convert CO2 into formate with ∼100% selectivity and drive O2 evolution under simulated sunlight in the absence of any electrical and chemical biases, achieving the overall solar-to-chemical energy conversion efficiency of ∼5%. The primary roles of the Ti underlayer are to create a robust contact between CFO and substrates and facilitate hole transfer. The TiOx (with unsaturated Ti atoms) delocalizes the electron density of the CFO and enhances electron transfer to the adsorbed CO2. Density functional theory calculations reveal that Ti/CFO/TiOx is the most suitable among the bare and modified CFO samples for efficient CO2 adsorption, formiloxyl intermediate formation, and formate desorption.

Recent Progress in Plasmonic Hybrid Photocatalysis for CO2 Photoreduction and C–C Coupling Reactions

Plasmonic hybrid nanostructures have been investigated as attractive heterogeneous photocatalysts that can utilize sunlight to produce valuable chemicals. In particular, the efficient photoconversion of CO2 into a stable hydrocarbon with sunlight can be a promising strategy to achieve a sustainable human life on Earth. The next step for hydrocarbons once obtained from CO2 is the carbon–carbon coupling reactions to produce a valuable chemical for energy storage or fine chemicals. For these purposes, plasmonic nanomaterials have been widely investigated as a visible-light-induced photocatalyst to achieve increased efficiency of photochemical reactions with sunlight. In this review, we discuss recent achievements involving plasmonic hybrid photocatalysts that have been investigated for CO and CO2 photoreductions to form multi-carbon products and for C–C coupling reactions, such as the Suzuki–Miyaura coupling reactions.

Nanostructure rod-like TiO2-reduced graphene oxide composite aerogels for highly-efficient visible-light photocatalytic CO2 reduction

In response to the worldwide over-standard carbon dioxide emission problem, this work synthesized a series of titanium dioxide/reduced graphene oxide composite aerogels (TiO2-rGO) for photoconversion of CO2 by a one-step hydrothermal and freeze-drying method. The prepared composite aerogel presents a high specific surface area of 287.3 m2/g and pore volume of 0.72 cm3/g, contributing to remarkable absorption capability of reactants and fast intraparticle molecular transfer. In the three-dimensional structure of rGO aerogel, TiO2 with nano-rod shape (10–20 nm × 100–150 nm) is uniformly interspersed. Through applying the composite catalytic aerogel for the photocatalysis reaction, CO2 was efficiently converted to MeOH, CH4, and EtOH, etc. The total yield of carbon generated by G-25Ti (TiO2-rGO with 25 mmol Ti4+) was found 15.7 times higher than that of the pure P25. The corresponding characteristic analysis demonstrated that the photocatalytic performance of TiO2-rGO composite aerogel has been highly improved, originated from two folds: (1) the introduction of 3-D rGO to nano-rod shape TiO2 promoted its light absorption efficiency, and more significantly (2) the specific chemical bonding sites and strong O˭C‒O‒Ti group between rGO and TiO2 effectively mitigate the recombination of photogenerated electron–hole pairs. In this work, rod-like TiO2-rGO composite aerogels prepared by using TiCl4 as precursor for the first time have been found a new application in CO2 reduction using visible sunlight.

Pt-decorated ZnMn2O4 nanorods for effective photocatalytic reduction of CO2 into methanol under visible light

The emergence of CO2 photoconversion into liquid fuels becomes a sustainable and ecological way to overcome future energy problems. Accordingly, a simple preparation of visible-light ZnMn2O4 (ZMO) nanorod harvesters is presented. These ZMO nanorods are decorated with small amount of Pt nanoparticles (0.5–2.0 wt%) to enhance the selective photoreduction of CO2 into methanol (MeOH). Depiction of the produced samples revealed a notable advance of structure and surface properties by introducing Pt. Moreover, the visible light absorption and photocharge separation were enhanced through the deposition of Pt with a reduction of the bandgap down to 1.98 eV for the 1.5% Pt/ZMO. This optimal Pt/ZMO showed a 1906 μmol/g selective photoreduction of CO2 to MeOH within 9 h at 2.4 g L-1. The prepared photocatalyst showed sustainable recyclability after five times reuse. The enhanced photocatalytic performance of Pt/ZMO is referring to the suppression of the charge recombination and improvement of the mobility as well. This work signifies the use of less noble metal supporting spinel-structured photocatalysts for renewable generation of fuels.

Direct synthesis of Ru3(BTC)2 metal-organic framework on a Ti/TiO2NT platform for improved performance in the photoelectroreduction of CO2

This work describes the use of a thin film of Ru3(BTC)2 metal-organic framework (MOF) deposited onto Ti/TiO2 nanotubes as a photocathode in CO2 reduction. This approach combines the advantages of the ruthenium MOF as a heterogeneous catalyst and as an agent for trapping CO2 on its active sites, resulting in an eco-friendly technology for photoelectrocatalysis and the photoconversion of CO2 to methanol in aqueous media. Morphological and crystallographic analyses showed that thin films of Ru3(BTC)2 MOF could be deposited homogeneously onto anatase TiO2 nanotubes, improving the photocurrent response of the electrode to on/off cycles of UV/Vis light illumination. Under optimized conditions, the Ti/TiO2NT-Ru3(BTC)2 electrode produced 314 μmol L−1 of methanol by photoelectrocatalysis after 3 h, which was 2.3-fold higher than obtained by a photocatalytic process. The results indicated that the construction of multifunctional materials, such as by the deposition of the porous crystalline Ru3(BTC)2 MOF onto Ti/TiO2 electrodes, can lead to a new generation of porous photoelectrocatalysts suitable for the capture of CO2 and its selective reduction to methanol.

3D hierarchical architecture collaborating with 2D/2D interface interaction in NiAl-LDH/Ti3C2 nanocomposite for efficient and selective photoconversion of CO2

3D hierarchical NiAl-LDH/Ti 3 C 2 nanocomposite collaborating with 2D/2D heterogeneous interface is developed through precise tuning of the process for in situ loading of Ti 3 C 2 nanosheets on NiAl-LDH, contributing to efficient and selective photoconversion of CO 2 . Photocatalytic conversion of CO 2 into a special chemical fuel with high yield and selectivity is still a major challenge. Herein, a 3D hierarchical NiAl-LDH/Ti 3 C 2 MXene (LDH/TC) nanocomposite is constructed through in situ loading of Ti 3 C 2 nanosheets on the NiAl-LDH scaffold during the hydrothermal process. The formation of a uniform and well-defined 2D/2D heterogeneous interface can be realized by optimizing the ratio of Ti 3 C 2 and the precursors for NiAl-LDH. The 3D hierarchical scaffold with high specific surface area contributes to the favourable photon adsorption and utilization. The intimate contact between Ti 3 C 2 and NiAl-LDH with numerous interfaces effectively promotes the separation of the photoinduced electron-hole pairs in NiAl-LDH. Together with the highly exposed oxidation–reduction active sites and the enhanced CO 2 capture and activation. The maximum photocatalytic CO production rate on NiAl-LDH/Ti 3 C 2 reaches 11.82 μmol g −1 h −1 with 92% selectivity and superior stability. This work provides an effective approach for the development of an ideal photocatalyst by collaborative utilization of materials with different dimensionalities.

Engineering cyanobacteria with enhanced growth in simulated flue gases for high-yield bioethanol production

Abstract Flue gases emitted from industrial plants mainly contribute to global CO2 emissions. Sequestrations of CO2 from the flue gas could reduce the impact of CO2 on global warming. In this study, recombinant cyanobacterial strains with enhanced photosynthetic activity, cell growth, and ethanol production were generated by co-overexpressing ictB, ecaA, and groESL, along with a heterologous ethanol synthesis pathway (pdc-adhII genes from Zymomonas mobilis) in S. elongatus PCC7942. The engineered S. elongatus PCC7942 exhibited a significant improvement in cell growth and ethanol production under a simulated flue gas consisted of 25% CO2, 80−90 ppm SO2, 90−100 ppm NO. The present work represents the first attempt of direct photoconversion of CO2 from flue gases to ethanol via expression of ictB, ecaA, groESL, and pdc-adhII in S. elongatus. The transgenic cyanobacterium becomes useful for sequestrations of CO2 directly from flue gases with the simultaneous production of bioethanol.

Photoconversion of anthropogenic CO2 into tunable syngas over industrial wastes derived metal-organic frameworks

Industrial wastes such as electroplating sludges and anthropogenic CO2, are misplaced resources whose utilization is severely hampered by their complex compositions or low concentration. Herein, on the basis of the specific binding characteristic of metal ions with small molecules, we demonstrate gradient-upcycling of solid wastes into different metal-organic frameworks (MOFs) for anthropogenic CO2-to-syngas photoconversion. In low-concentration CO2 (10%), the as-prepared Ni MOFs exhibits record-high performance for CO2-to-CO photoconversion, while Fe MOFs favors for H2 production. By taking Fe/Ni MOFs as the modular catalysts and adjusting their ratio, syngas with a widely tunable CO/H2 ratio (1:15-14:1) was produced from CO2 photoreduction. The diverse catalytic performance of Fe/Ni MOFs originates from the different affinities towards CO2/H2O, which relates to the 3d electron numbers of metal ions. We further confirm the universality of the gradient recovery approach to upcycle metal resources from solid wastes and the modular catalysts design approach for CO2-to-syngas photoconversion.

2D PbS Nanosheets with Zigzag Edges for Efficient CO2 Photoconversion.

The rational design of transition-metal sulfide with two-dimensional (2D) structure and tunable edges on the nanoscale can effectively improve their activity for variously catalytic reactions. Herein, the 2D PbS nanosheets with abundant zigzag edges (e-PbS NS), which exhibited an excellent performance for CO2 photoconversion to CO, were constructed. The zigzag edges on the PbS NS are beneficial for exposing more active sites and promoting charge separation, thereby accelerating the kinetics process of CO2 photoreduction. This study provides a strategy to regulate structure with effective edge sites for the CO2 reduction.

Selective CO2 Photoconversion on Low-Coordinate TiO2

The selective conversion of CO2 into desired hydrocarbon products remains a challenge in photocatalysis. In this issue of Chem, Feng and colleagues report a strong-magnetic-field-assisted strategy for forming active low-coordinate (100) facets on TiO2 nanotubes, enabling selective photoconversion of CO2 into the C2 product C2H5OH under visible light.

A hierarchical heterostructure of CdS QDs confined on 3D ZnIn2S4 with boosted charge transfer for photocatalytic CO2 reduction

Metal sulfide based materials as photocatalysts for energy conversion are essential to produce value-added chemical fuels, but their intrinsically slow carrier dynamics and low activity are yet to be resolved. Herein, we developed a unique heterogeneously nanostructured ZnIn2S4-CdS heterostructure that involves zero-dimensional (0D) CdS quantum dots uniformly confined on three-dimensional (3D) ZnIn2S4 nanoflowers, which achieves an excellent catalytic performance of CO2 photoconversion under visible-light irradiation. The obtained hierarchical heterostructure can significantly enhance the light harvesting, shorten the migration distance of carriers, and obviously accelerate the transport of electrons. As evidenced by the ultrafast transient absorption spectroscopy, the formed interface can effectively facilitate charge separation and transport. This work opens up a new avenue to carefully design the elaborate heterostructures for achieving optimal charge separation efficiency by lowering interfacial kinetic barriers and energy losses at the interface.

Molecularly engineered photocatalyst sheet for scalable solar formate production from carbon dioxide and water

Harvesting solar energy to convert CO2 into chemical fuels is a promising technology to curtail the growing atmospheric CO2 levels and alleviate the global dependence on fossil fuels; however, the assembly of efficient and robust systems for the selective photoconversion of CO2 without sacrificial reagents and external bias remains a challenge. Here we present a photocatalyst sheet that converts CO2 and H2O into formate and O2 as a potentially scalable technology for CO2 utilization. This technology integrates lanthanum- and rhodium-doped SrTiO3 (SrTiO3:La,Rh) and molybdenum-doped BiVO4 (BiVO4:Mo) light absorbers modified by phosphonated Co(ii) bis(terpyridine) and RuO2 catalysts onto a gold layer. The monolithic device provides a solar-to-formate conversion efficiency of 0.08 ± 0.01% with a selectivity for formate of 97 ± 3%. As the device operates wirelessly and uses water as an electron donor, it offers a versatile strategy toward scalable and sustainable CO2 reduction using molecular-based hybrid photocatalysts.

Rational Design of FeNi Bimetal Modified Covalent Organic Frameworks for Photoconversion of Anthropogenic CO2 into Widely Tunable Syngas

Direct photoconversion of low-concentration CO2 into a widely tunable syngas (i.e., CO/H2 mixture) provides a feasible outlet for the high value-added utilization of anthropogenic CO2 . However, in the low-concentration CO2 photoreduction system, it remains a huge challenge to screen appropriate catalysts for efficient CO and H2 production, respectively, and provide a facile parameter to tune the CO/H2 ratio in a wide range. Herein, by engineering the metal sites on the covalent organic frameworks matrix, low-concentration CO2 can be efficiently photoconverted into tunable syngas, whose CO/H2 ratio (1:19-9:1) is obviously wider than reported systems. Experiments and density functional theory calculations indicate that Fe sites serve as the H2 evolution sites due to the much stronger binding affinity to H2 O, while Ni sites act as the CO production sites for the higher affinity to CO2 . Notably, the widely tunable syngas can also be produced over other Fe/Ni-based bimetal catalysts, regardless of their structures and supporting materials, confirming the significant role of the metal sites in regulating the selectivity of CO2 photoreduction and providing a modular design strategy for syngas production.

Confined pyrolysis transformation of ZIF-8 to hierarchically ordered porous Zn-N-C nanoreactor for efficient CO2 photoconversion under mild conditions

Rational design and fabrication of metal–organic framework (MOF)-derived ordered hierarchical macro-/meso-/microporous N-doped carbon materials featured with the atomically dispersed metal atoms can effectively enhance their catalytic performance. Herein we develop a template-assisted strategy, namely the confined growth and pyrolysis of single-crystalline zeolite imidazolate framework-8 (ZIF-8) in three-dimensional ordered template voids, to create hierarchical porosity in MOFs-derived N-doped carbon materials. The as-synthesized single-atom Zn on N-doped hierarchically ordered porous carbon polyhedrons (Zn-N-HOPCPs) show integrated Lewis acid (single-atom Zn) and base sites (pyridinic N), high surface area (1128 m2·g−1), large pore volume (0.90 cm3·g−1), periodically interconnected open windows (~55 nm), highly accessible micro/mesopore channels (0.55, 0.8, 1.2, and 4 nm), and abundant Zn content (5.8 wt%). The hierarchical nanoarchitecture (macro-/meso-/micropores) significantly facilitates mass transfer for bulky-molecule-involved reactions and exposes inside active sites, resulting in satisfactory CO2 photoconversion under mild conditions. The strategy presented here sheds light on the design and fabrication of various hierarchically ordered porous carbon decorated with atomically dispersed metal atoms for other important applications.

Construction of Functionally Compartmental Inorganic Photocatalyst-Enzyme System via Imitating Chloroplast for Efficient Photoreduction of CO2 to Formic Acid.

Inorganic photocatalyst-enzyme systems are a prominent platform for the photoreduction of CO2 to value-added chemicals and fuels. However, poor electron transfer kinetics and enzyme deactivation by reactive oxygen species in the photoexcitation process severely limit catalytic efficiency. In chloroplast, enzymatic CO2 reduction and photoexcitation are compartmentalized by the thylakoid membrane, which protects enzymes from photodamage, while the tightly integrated photosystem facilitates electron transfer, promoting photocatalysis. By mimicking this strategy, we constructed a novel functionally compartmental inorganic photocatalyst-enzyme system for CO2 reduction to formate. To accomplish efficient electron transfer, we first synthesized an integrated artificial photosystem by conjugation of the cocatalyst (a Rh complex) onto thiophene-modified C3N4 (TPE-C3N4), demonstrating an NADH regeneration rate of 9.33 μM·min-1, 2.33 times higher than that of a homogeneous counterpart. The enhanced NADH regeneration activity was caused by the tightly conjugated structure of the artificial photosystem, enabling rapid electron transfer from TPE-C3N4 to the Rh complex. To protect formate dehydrogenase (FDH) from photoinduced deactivation, FDH was encapsulated into MAF-7, a metal-organic framework (MOF) material, to compartmentalize FDH from the toxic photoexcitation process, similar to the function of the thylakoid membrane. Moreover, the triazole linkers of MAF-7 possess both hydrophilicity and pH-buffering capacity providing a stable microenvironment for FDH, which could enhance enzyme stability in photosynthesis. The synergy between the enhanced electron transfer of TPE-C3N4 for NADH cofactor regeneration and MOF-protection of the redox enzyme enables the construction of a functionally compartmental inorganic photocatalyst-enzyme association system, promoting CO2 photoconversion to formic acid with a yield of 16.75 mM after 9 h of illumination, 3.24 times greater than that of the homogeneous reaction counterpart.

Synergistic effects of Pd–Ag bimetals and g-C3N4 photocatalysts for selective and efficient conversion of gaseous CO2

Photoconversion of CO2 has been a promising method to decrease the emission of CO2 and obtain useful fuel source. Yet, the conversion efficiency and the selectivity of CO2 conversion by the photocatalysts have not been fully understood. Here, graphitic carbon nitride (g–C3N4)–based photocatalysts is fabricated with the incorporation of Pd–Ag bimetallic nanoparticles for CO2 conversion. The production rate and selective conversion feature of the photocatalysts are highly dependent on the molar ratio of the deposited Pd–Ag bimetals. Higher Pd content of the bimetals is favorable in selectively producing CH4 while lower Pd content preferably contribute to selective production of CO. The sample with 1:2 M ratio of Pd–Ag remarkably produces 5.42 μmol/g·h of CO and 4.03 μmol/g·h of CH4. Especially, the CH4 production rate is 40 times higher than that of bare g-C3N4 photocatalysts. Further understanding of the selective conversion and higher production rate by the decorated Pd–Ag bimetallic nanoparticles onto g-C3N4 are provided, elucidating the importance on the activation of g-C3N4 through bimetal structure.