Light-driven CO2 Reduction Research Articles

Controllable synthesis of CuPc/N-rich doped (0 0 1) TiO2 S-scheme nanosheet heterojunctions for efficiently wide-visible light-driven CO2 reduction

Inspired by natural photosynthesis, the rational design of efficient S-scheme heterojunctions holds promises for CO2 conversion by utilizing solar light. Herein, nitrogen-rich doped 001-exposed anatase nanosheets (NPT) have been designed and fabricated by post-treatment with NH3 at a high temperature of 600 °C to phosphate modified TiO2 firstly, and then controllably coupled with copper phthalocyanine (CuPc) via a hydroxyl-induced assembly process to construct a wide-visible-light responsive heterojunction towards CO2 conversion. The optimized CuPc/NPT heterojunction achieves a high production rate of CO (∼5 μmol/g/h) under visible light irradiation, with ∼9-fold enhancement compared with generally N-doped TiO2. Based on the experimental results mainly from photocurrent action spectra, electron paramagnetic resonance measurements, electrochemical reduction curves and in-situ diffuse reflectance infrared Fourier transform spectra, it is confirmed that the exceptional photoactivity is attributed to the N-rich doping for increasing visible-light absorption of TiO2, the effective S-scheme charge transfer from the closely-contacted CuPc/NPT heterojunction with wide visible-light absorption, and subsequently to the electron transfer from the Pc ligand to central metal Cu2+ with good catalytic function for CO2 reduction. This work provides feasible routes for the design and preparation of TiO2-based photocatalysts for solar-driven CO2 conversion.

Photocatalytic CO2 reduction with aminoanthraquinone organic dyes

The direct utilization of solar energy to convert CO2 into renewable chemicals remains a challenge. One essential difficulty is the development of efficient and inexpensive light-absorbers. Here we show a series of aminoanthraquinone organic dyes to promote the efficiency for visible light-driven CO2 reduction to CO when coupled with an Fe porphyrin catalyst. Importantly, high turnover numbers can be obtained for both the photosensitizer and the catalyst, which has not been achieved in current light-driven systems. Structure-function study performed with substituents having distinct electronic effects reveals that the built-in donor-acceptor property of the photosensitizer significantly promotes the photocatalytic activity. We anticipate this study gives insight into the continued development of advanced photocatalysts for solar energy conversion.

Variable valence Mo5+/Mo6+ ionic bridge in hollow spherical g-C3N4/Bi2MoO6 catalysts for promoting selective visible light-driven CO2 photoreduction into CO

Herein, the catalysts of ultrathin g-C3N4 surface-modified hollow spherical Bi2MoO6 (g-C3N4/Bi2MoO6, abbreviated as CN/BMO) were fabricated by the co-solvothermal method. The variable valence Mo5+/Mo6+ ionic bridge in CN/BMO catalysts can boost the rapid transfer of photogenerated electrons from Bi2MoO6 to g-C3N4. And the synergy effect of g-C3N4 and Bi2MoO6 components remarkably enhance CO2 adsorption capability. CN/BMO-2 catalyst has the best performances for visible light-driven CO2 reduction compared with single Bi2MoO6 and g-C3N4, i.e., its amount and selectivity of CO product are 139.50 μmol g−1 and 96.88% for 9 h, respectively. Based on the results of characterizations and density functional theory calculation, the photocatalytic mechanism for CO2 reduction is proposed. The high-efficient separation efficiency of photogenerated electron-hole pairs, induced by variable valence Mo5+/Mo6+ ionic bridge, can boost the rate-limiting steps (COOH*-to-CO* and CO* desorption) of selective visible light-driven CO2 conversion into CO. It inspires the establishment of efficient photocatalysts for CO2 conversion.

CdO decorated CdS nanorod for enhanced photocatalytic reduction of CO2 to CO.

Solar-driven CO2 reduction into fuels and sustainable energy has attracted increasing attention around the world. However, the photoreduction efficiency remains low due to the low efficiency of separation of electron-hole pairs and high thermal stability of CO2. In this work, we prepared a CdO decorated CdS nanorod for visible light driven CO2 reduction. The introduction of CdO facilitates the photoinduced charge carrier separation and transfer and acts as an active site for adsorption and activation of CO2 molecules. Compared with pristine CdS, CdO/CdS exhibits a nearly 5-fold higher CO generation rate (1.26 mmol g-1 h-1). In situ FT-IR experiments indicated that CO2 reduction on CdO/CdS may follow a COOH* pathway. This study reports the pivotal effect of CdO on photogenerated carrier transfer in photocatalysis and on CO2 adsorption, which provides a facile way to enhance photocatalytic efficiency.

Developing post-modified Ce-MOF as a photocatalyst: a detail mechanistic insight into CO2 reduction toward selective C2 product formation

Post-synthetically modified defect-regulated Ce-UiO-66 MOF produces selective C2-based product (acetic acid) by visible light-driven CO2 reduction reaction.

TaOx electron transport layers for CO2 reduction Si photocathodes

TaOx electron transport layers used in photocathodes for light-driven CO2 reduction have good electronic transport, are stable, and are catalytically inert for the competing hydrogen evolution reaction.

Cobalt-based tripodal complexes as molecular catalysts for photocatalytic CO2 reduction.

Construction of artificial photosynthetic systems including CO2 reduction is a promising pathway to produce carbon-neutral fuels and mitigate the greenhouse effect concurrently. However, the exploitation of earth-abundant catalysts for photocatalytic CO2 reduction remains a fundamental challenge, which can be assisted by a systematic summary focusing on a specific catalyst family. Cobalt-based complexes featuring tripodal ligands should merit more insightful discussion and summarization, as they are one of the most examined catalyst families for CO2 photoreduction. In this feature article, the key developments of cobalt-based tripodal complexes as molecular catalysts for light-driven CO2 reduction are discussed to offer an upcoming perspective, analyzing the present progress in electronic/steric tuning through ligand modification and dinuclear design to achieve a synergistic effect, as well as the bottlenecks for further development.

Reduced TiO2 quantum dots/graphene for solar light driven CO2 reduction into precisely controlled C1 vs C2 hydrocarbon products without noble Co-catalyst

Photocatalytic CO2 reduction is a logical approach to overcome the energy crisis as well as to curb the anthropogenic CO2 emission by converting atmospheric CO2 with water vapour under irradiation into hydrocarbon fuels. Herein, for the first time, a noble metal-free photocatalyst comprised of reduced TiO2 quantum dots (TQDs) dispersed over graphene sheets has been successfully synthesized and employed for this reaction. EELS investigation reveals the addition of graphene has imparted oxygen vacancies inside TQDs, which in turn helps to enhance broad solar light absorption. TRPL spectroscopy revealed a prolonged charge separation in the heterostructure. Depending on graphene content, the ratio of the products has been changed; CH4:C2H6 from 3.41:1 to 1:1.62. The optimized sample exhibits 2.8- and 3.7-fold increment of CH4 and C2H6 yields compared to TQDs. Under the continuous mode, the photocatalyst has shown excellent stability of 72 h.

P and Cu Dual Sites on Graphitic Carbon Nitride for Photocatalytic CO2 Reduction to Hydrocarbon Fuels with High C2 H6 Evolution.

The light-driven CO2 reduction to multi-carbon products is especially meaningful, while the low efficiency of multi-electron transfer and sluggish C-C coupling greatly hinder its development. Herein, we report a photocatalyst comprising of P and Cu dual sites anchored on graphitic carbon nitride (P/Cu SAs@CN), which achieves a high C2 H6 evolution rate of 616.6 μmol g-1 h-1 in reducing CO2 to hydrocarbons. The detailed spectroscopic characterizations identify the formation of charge-enriched Cu sites, where the isolated P atoms serve as hole capture sites during photocatalysis. Theoretical simulations combined with in situ FTIR measurement reveal a kinetically feasible process for the formation of C-C coupling intermediate (*OC-COH) and confirm the favorable production of C2 H6 on the P/Cu SAs@CN photocatalyst. This work offers new insights into the photocatalyst design with atomic precision toward highly efficient photocatalytic CO2 conversion to high value-added carbon products.

Copper(II)-Doped Two-Dimensional Titanium-Based Metal-Organic Frameworks toward Light-Driven CO2 Reduction to Value-Added Products.

Recently, metal-organic framework (MOF)-based photocatalysts for an efficient CO2 reduction reaction have drawn wide attention in multidisciplinary fields and sustainable chemistry. In this work, a series of Cu2+-doped two-dimensional Ti-based MOFs were fabricated by a facile in situ solvothermal method. Cu2+ ions were doped in equal proportions and uniformly dispersed in the crystal structure of the MOF matrix. Interestingly, the doping content of Cu2+ ions and the photocatalytic performance displayed an obvious volcanic relationship, the medium-concentration Cu2+-doped sample (T1-2Cu) held the greatest activity with 100% carbonaceous product (CH4 and CO) formation, and the CH4 production rate was 3.7 μmol g-1 h-1 with 93% electron selectivity. The band structure, local electronic structure, carrier separation kinetics, and CO2 adsorption studies demonstrated that the excellent photocatalytic activity of T1-2Cu benefited from the appropriate amount of Cu2+ ion doping: (1) a doping amount of 2 atom % optimized the conduction band position of the MOF substrate and endowed T1-2Cu with strong reduction potential in thermodynamics, (2) doping Cu2+ ions tuned the local electronic environment around titanium oxide clusters and optimized the generation, separation, and migration processes of photoinduced carriers, and (3) the introduction of Cu2+ ions also provided more accessible active sites and more probabilities for the adsorption and activation of CO2 reactants.

Phosphorus Tailors the d-Band Center of Copper Atomic Sites for Efficient CO2 Photoreduction under Visible-Light Irradiation.

Photoreduction of CO2 into solar fuels has received great interest, but suffers from low catalytic efficiency and poor selectivity. Herein, two single-Cu-atom catalysts with unique Cu configurations in phosphorus-doped carbon nitride (PCN), namely, Cu1 N3 @PCN and Cu1 P3 @PCN were fabricated via selective phosphidation, and tested in visible light-driven CO2 reduction by H2 O without sacrificial agents. Cu1 N3 @PCN was exclusively active for CO production with a rate of 49.8 μmolCO gcat -1 h-1 , outperforming most polymeric carbon nitride (C3 N4 ) based catalysts, while Cu1 P3 @PCN preferably yielded H2 . Experimental and theoretical analysis suggested that doping P in C3 N4 by replacing a corner C atom upshifted the d-band center of Cu in Cu1 N3 @PCN close to the Fermi level, which boosted the adsorption and activation of CO2 on Cu1 N3 , making Cu1 N3 @PCN efficiently convert CO2 to CO. In contrast, Cu1 P3 @PCN with a much lower Cu 3d electron energy exhibited negligible CO2 adsorption, thereby preferring H2 formation via photocatalytic H2 O splitting.

First-principles studies of monolayers MoSi2N4 decorated with transition metal single-atom for visible light-driven high-efficient CO2 reduction by strain and electronic engineering

Solar-driven CO2 reduction into value-added hydrocarbons could promote CO2 utilization and solve related environmental issues, while the key lies in the modulation of the active site’s orbital electronic state. By performing first-principles calculations, we systematically explore MoSi2N4 decorated with 3d and 4d period transition metal single atoms (TMSAs) as efficient CO2 reduction photocatalysts and find that product selectivity is highly correlated with the d-band center of TMSAs. Compressive and tensile strains in mutually perpendicular directions are introduced in the horizontal plane to improve the catalytic performance. After filtering, V, Mn, and Ni were found to be the best TMSAs for CO, HCOOH, and CH4 production, respectively. Ni/MoSi2N4 under −6% tensile strain exhibited the lowest CO2 reduction limiting potential (UL) of 0.24 V. Notably, the time-dependent ab initio nonadiabatic molecular dynamics simulations were carried out to elucidate the photogenerated electron transport properties, where the photo-generated electrons can be efficiently transferred from MoSi2N4 to TMSAs active sites. We also identify that the applied strains affect the CO2 reduction performance by modulating the d-band center and valence electrons. These findings provide pioneering design schemes of precisely tailored photocatalytic systems for CO2 reduction under visible light.

In-situ growth of p-type Ag2O on n-type Bi2O2S with intimate interfacial contact for NIR light-driven photocatalytic CO2 reduction

Photocatalytic converting CO2 into organic carbon resources holds promise for achieving the utilization of CO2 resources and the development of clean energy. However, the low utilization efficiency of solar energy and poor separation efficiency of photo-generated carriers limit the application of photocatalytic CO2 reduction. Herein, Ag2O/Bi2O2S p-n heterojunction photocatalyst (xAO/BOS) was constructed by anchoring Ag2O on the surface of Bi2O2S through in-situ synthesis. The novel 27 %AO/BOS photocatalyst shows the enhanced photocatalytic CO2 reduction performance with 14.49 μmol g−1 of CO yield and 12.06 μmol g−1 of CH4 yield after 120 min of NIR light irradiation. The formed p-n heterojunction in 27 %AO/BOS photocatalyst by the in-situ synthesis contributes to facilitating the separation and transfer of photo-generated carriers and improving the adsorption and activation of CO2. The composite still exhibits excellent photocatalytic cycling stability, and Ag0 species are produced in the photocatalytic CO2 reduction reaction owing to the in-situ photoreduction of Ag2O. Based on the experimental analyses and theoretical calculations, the proposed mechanism for the improved photocatalytic CO2 reduction activity over in-situ synthesis of 27 %AO/BOS p-n heterojunction photocatalyst is finally given. This work paves a promising way to construct effective NIR light-responsive photocatalysts for CO2 photoreduction.

Electrochemical and light-driven CO2 reduction by amine-functionalized rhenium catalysts: A comparison between primary and tertiary amine substitutions

Properly positioned second-coordination sphere functional groups can play an integral role in enhancing catalytic activity, altering product selectivity, and/or reducing the energy cost for desired chemical transformations. In this context, we have investigated the effect of second-sphere tertiary amine groups in the fac-Re(bpy)(CO)3Cl (ReBpy) catalyst scaffold (bpy is 2,2′-bipyridine). A family of isomeric Re(bpy) catalysts bearing N,N-dimethylaniline (DMA) substituents at the 6 position of 2,2′-bipyridine has been synthesized and characterized, where the -NMe2 group of each complex is installed at the ortho, meta, or para position of the pendant phenyl ring to systematically vary the distance between the amine and the rhenium metal center. The para-substituted complex (Re-pNMe2) leads the series with the highest turnover frequency (168 s−1) and highest turnover number (80) measured under optimized electrochemical and photochemical conditions, respectively. Spectroelectrochemical experiments were performed to identify reduced intermediates that are involved in the catalytic cycles of these complexes. In addition, the catalytic parameters have been compared with the analogous and previously reported series of Re catalysts bearing –NH2 functionality in the second-coordination sphere. Our results demonstrate that, unlike the aniline-substituted primary amine systems, catalysis with the DMA-substituted tertiary amine complexes is predominantly governed by inductive electronic effects.

Integration of 3D macroscopic reduced graphene oxide aerogel with DUT-67 for selective CO2 photoreduction to CO in Gas-Solid reaction

Herein, DUT-67/RGO aerogel photocatalysts with three-dimensional (3D) macroscopic morphology were first prepared by a facile hydrothermal and freeze-drying process. It was observed that DUT-67 endowed aerogels with strong adsorption/activation capacity for CO2 molecules. Besides, RGO not only dispersed DUT-67 as a macroscopic carrier, but also improved the utilization rate of light energy. The high conductivity of RGO and the strong interaction with DUT-67 enhanced the electron coupling effect, which was conducive to the separation of carriers. 15D/R achieved near 99.6% selectivity at the gas–solid interface for UV–Vis light-driven CO2 reduction to CO, with a rate of 42.41 μmol·g−1. The adsorption behavior of products at the interface and formation process of intermediates were monitored based on CO-TPD and in situ Fourier transform infrared spectra. The extremely high selectivity for CO came from the weak adsorption of CO on the surface of 15D/R and formation of CH4 required more complex intermediates.

Configuration of hetero-framework via integrating MOF and triazine-containing COF for charge-transfer promotion in photocatalytic CO2 reduction

Photocatalytic CO2 reduction into high value-added chemicals is of considerable prospect for tackling quandaries in energy scarcity. One potent strategy to raise up its conversion efficacy is engineering heterostructure by virtue of tunable substituent in organic structure to facilitate the charge delivery within the photocatalyst. Herein, three metal organic frameworks (MOFs) built by the same ligand but different metal ions, are in-situ grown onto a triazine-containing covalent framework (COF), TP-TA, resulting in three hetero-frameworks, namely, In-MOF@TP-TA, Zr-MOF@TP-TA and Fe-MOF@TP-TA (In-MOF, Zr-MOF and Fe-MOF are abbreviations for NH2-MIL-68(In), NH2-UiO-66(Zr) and NH2-MIL-101(Fe) MOFs, respectively). Owning to the configured type-II heterojunctions that apparently suppress the charge recombination, the light-driven CO2 reduction reaction (CO2RR) manipulated by the three hybrids behave higher catalytic performance by contrast to the pristine MOFs or COF. Amid the three heterostructure, In-MOF@TP-TA provides the optimal catalytic activity, giving the CO and CH4 production rate as 25 and 11.67 μmol·g−1·h−1, respectively. In addition, their photocatalytic activities follow the order as In-MOF@TP-TA > Zr-MOF@TP-TA > Fe-MOF@TP-TA, consisting with the CB potentials of the MOF components (from negative to positive) and in turn certifying the charge flowing orientation within the type-II heterostructure. Our work broadens the rational design for the covalently integrated heterostructures as well as their melioration in photocatalytic CO2RR.

Activating oxygen deficient TiO2 in the visible region by Bi2MoO6 for CO2 photoreduction to methanol.

Fast photogenerated charge recombination and inappropriate bandgap for visible light driven charge generation hinders the performance of TiO2. In this study, TiO2 was activated for visible light driven CO2 reduction in the presence of Bi2MoO6 as an electron donor. Furthermore, the introduction of oxygen vacancies resulted in enhanced CO2 adsorption and conversion. The best catalyst gives 27.1 μmol g-1 h-1 methanol formation. DRIFTS was used to explain the methanol formation mechanism on oxygen deficient TiO2.

2D Covalent Organic Frameworks with an Incorporated Manganese Complex for Light Driven Carbon Dioxide Reduction

AbstractCovalent organic frameworks (COFs) have emerged as a novel class of crystalline porous photocatalytic materials due to their unique properties such as large surface area, tunable porosity, and rigid structure. In this work, we report the direct incorporation of a manganese CO2 molecular catalyst (MC) into COFs (Mn−TTA‐COF) and the evaluation of its capability as photocatalyst for visible light driven CO2 reduction to form CO. We found that the photocatalytic activity of Mn−TTA‐COF is quite low, which mainly results from the elimination of the CO ligand in the Mn MC upon light illumination, rendering its short duration in the catalytic reaction. While this is a central concern for the further use Mn−TTA‐COF as a CO2 reduction catalyst, we found that the stability and efficiency of Mn MC is largely enhanced after being incorporated into COFs with respect to its homogeneous version, suggesting the capability of COFs as heterogeneous platform to incorporate MC and improve the catalytic performance of MC. Moreover, transient absorption spectroscopic studies show that the intramolecular charge transfer lifetime of the Mn‐incorporated COF is longer than that in the parent COF, which suggests that charge separation (CS) may occur from the parent COF to the Mn moiety. These results together suggest that COFs may show promise as a platform for creating next‐generation photocatalysts with a built‐in photosensitizer and MC, which can facilitate CS and enhance the stability and efficiency of the incorporated MC.

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.

Photodriven Catalytic Hydrogenation of CO2 to CH4 with Nearly 100% Selectivity over Ag25 Clusters.

The conversion of chemically inert carbon dioxide and its photoreduction to value-added products have attracted enormous attention as an intriguing prospect for utilizing the principal greenhouse gas CO2. Herein, we explore the use of Ag25 clusters with well-defined atomic structures for high-selectivity photocatalytic hydrogenation of CO2 to methane. Ag25 clusters, with molecular-like properties and surface plasmon resonance, exhibit competitive catalytic activity for light-driven CO2 reduction that yield an almost 100% product selectivity of methane at a relatively mild temperature (100 °C). DFT calculations reveal that the absorption of CO2 on Ag25 clusters is energetically favorable. The methanation of the Ag25 cluster catalyst has been investigated by operando infrared spectroscopy, verifying that methane was produced through a -H-assisted multielectron reaction pathway via the transformation of formyl and formaldehyde species to form surface CHx. This work presents a highly efficient strategy for high-performance CO2 methanation via well-defined metal cluster catalysts.