Selective Photoreduction Of CO2 Research Articles

Microwave-assisted hydrothermal synthesis of mesoporous three-dimensional hexagonal graphitic carbon nitride for selective CO2 photoreduction

The selective photocatalytic reduction of CO2 to CO by using three-dimensional hexagonal and mesoporous graphitic carbon nitride (denoted as 3D H-g-C3N4-T) which are prepared via a microwave-assisted route is reported. The physicochemical and optical properties of prepared photocatalysts are characterized by various analytic tools and spectroscopies. Notably, the structure and morphology of 3D H-g-C3N4-T can be regulated by microwave-assisted hydrothermal temperatures (130–190 °C) in an ammonia solution. The obtained g-C3N4 and 3D H-g-C3N4-T are used as photocatalysts in the CO2 photoreduction under the visible-light irradiation. Among these photocatalysts, the 3D H-g-C3N4-130 shows the superior CO yield which is ca. 2.6-fold higher than the pristine g-C3N4. The formation of 3D hexagonal prisms with mesoporous structure observed for 3D H-g-C3N4-130 is responsible for the promoted visible-light absorption, greater surface area and abundant amino functional groups boosting the selective CO2-to-CO conversions. The present work demonstrates a time and energy-efficient route to prepare unique 3D g-C3N4 which may offer the possible applications in the solar photoreduction of CO2.

Assembling CdSe Quantum Dots into Polymeric Micelles Formed by a Polyethylenimine-Based Amphiphilic Polymer to Enhance Efficiency and Selectivity of CO2-to-CO Photoreduction in Water.

Colloidal quantum dots (QDs) as photocatalysts enable catalysis of CO2-to-CO conversion in the presence of electron donors. The surface and/or interfacial chemical environment of the QDs is essential for the activity and selectivity of the CO2 photoreduction. Various strategies, including exposing active metal sites or anchoring functional organic ligands, have been applied to tune the QDs' surface chemical environment and thus to improve both activity and selectivity of CO2 photoreduction, which occurs at surface of the QDs. However, the efficient and selective photocatalytic CO2 reduction with QD photocatalysts in water is still a challenging task due to low CO2 solubility and robust competing reaction of proton reduction in water. Different from state-of-the-art QDs' surface manipulation, we proposed to ameliorate the interfacial chemical environment of CdSe QDs via assembling the QDs into functional polymeric micelles in water. Herein, CdSe@PEI-LA assemblies were constructed by loading CdSe QDs into polymeric micelles formed by PEI-LA, a polyethylenimine (PEI)-based functional amphiphilic polymer. Due to self-assembly and high CO2 adsorption capacity of PEI-LA in water, the photocatalytic CO2-to-CO conversion efficiency and selectivity of the CdSe@PEI-LA assemblies in water were dramatically improved to 28.0 mmol g-1 and 87.5%, respectively. These two values increased 57 times and 1.5 times, respectively, compared with those of the pristine CdSe QDs. Mechanism studies revealed that CdSe QDs locate in polymeric micelles of high CO2 local concentration and the photoinduced electron transfer from the conduction band of CdSe QDs to Cd-CO2* species is thermodynamically and kinetically improved in the presence of PEI-LA. The CdSe@PEI-LA system represents a successful example of using a functionalized amphiphilic polymer to ameliorate interfacial microenvironments of nanocrystal photocatalysts and realizing efficient and selective CO2 photoreduction in water.

Cu–Ni core–shell bimetallic cocatalyst decorated polymeric carbon nitride for highly efficient and selective methane production from photocatalytic CO2 reduction

The development of non-noble bimetallic cocatalysts for the efficient and selective CO2 photoreduction is extremely desirable yet challenging. Herein, we rationally synthesized a series of non-noble Cu–Ni bimetallic cocatalysts by varying the Cu and Ni molar ratios, and then, we decorated these cocatalysts on polymeric carbon nitride (CN) for photocatalytic CO2 reduction. Elemental distribution maps demonstrated that the Cu–Ni-11 bimetallic cocatalyst with a Cu:Ni molar ratio of 1:1 has a perfect core–shell configuration. Notably, this Cu–Ni-11 core–shell cocatalyst-decorated CN (Cu–Ni-11@CN) exhibited remarkable CO2 photoreduction activity for selective CH4 production, outperforming bare CN, Cu@CN, Ni@CN, and Cu–Ni-13@CN and Cu–Ni-31@CN catalysts with different metal proportions. Additionally, the Cu–Ni-11@CN catalyst was remarkably stable and durable over multiple test runs. The remarkable CO2 conversion activity and stability of the developed catalyst are predominantly due to the synergistic effect of the Cu–Ni-11 core–shell cocatalyst and CN, which affords significantly improved optical absorption, CO2 adsorption, and photoexcited charge separation. Therefore, the study results elucidate the rational design and development of non-noble bimetallic cocatalysts for photocatalytic CO2 conversion.

Selective CO2 Photoreduction to CH4 via Pdδ+‐Assisted Hydrodeoxygenation over CeO2 Nanosheets

AbstractHere, noble‐metal‐doped two‐dimensional metal oxide nanosheets are designed to realize selective CO2 photoreduction to CH4. As a prototype, Pd‐doped CeO2 nanosheets are fabricated, where the active sites of Pdδ+ (2<δ<4) and Ce3+−Ov are revealed by quasi in situ X‐ray photoelectron spectra and in situ electron paramagnetic resonance spectra. Moreover, in situ Fourier‐transform infrared spectra of D2O photodissociation and desorption verify the existence of the Pd−OD bond, implying that Pdδ+ sites can participate in water oxidation to deliver H* species for facilitating the protonation of the intermediates. Furthermore, theoretical calculations suggest the Pd doping could regulate the formation energy barrier of the key intermediates CO* and CH3O*, thus making CO2 reduction to CH4 become the favorable process. Accordingly, Pd‐doped CeO2 nanosheets achieve nearly 100 % CH4 selectivity of CO2 photoreduction, with the raising CH4 evolution rate of 41.6 μmol g−1 h−1.

Selective CO2 Photoreduction to CH4 via Pdδ+ -Assisted Hydrodeoxygenation over CeO2 Nanosheets.

Here, noble-metal-doped two-dimensional metal oxide nanosheets are designed to realize selective CO2 photoreduction to CH4 . As a prototype, Pd-doped CeO2 nanosheets are fabricated, where the active sites of Pdδ+ (2<δ<4) and Ce3+ -Ov are revealed by quasi in situ X-ray photoelectron spectra and in situ electron paramagnetic resonance spectra. Moreover, in situ Fourier-transform infrared spectra of D2 O photodissociation and desorption verify the existence of the Pd-OD bond, implying that Pdδ+ sites can participate in water oxidation to deliver H* species for facilitating the protonation of the intermediates. Furthermore, theoretical calculations suggest the Pd doping could regulate the formation energy barrier of the key intermediates CO* and CH3 O*, thus making CO2 reduction to CH4 become the favorable process. Accordingly, Pd-doped CeO2 nanosheets achieve nearly 100 % CH4 selectivity of CO2 photoreduction, with the raising CH4 evolution rate of 41.6 μmol g-1 h-1 .

PtAg alloys as an efficient co-catalyst for CO2 deep photoreduction with H2O: Synergistic effects of Pt and Ag

Solar driven conversion of CO2 into carbonaceous chemicals is a promising strategy to reduce greenhouse effect and obtain valuable chemical fuel. However, due to the lack of efficient active site, the selective CO2 photoreduction towards multi-electron products remains challenging. Here, we reported PtAg alloy acting as a novel cooperative co-catalyst to boost the CH4 formation from photocatalytic CO2 reduction over HNb3O8 nanosheets (HNb3O8 NS). The PtAg alloy can trap the electrons and improve the charge separation efficiency of the photocatalyst, mitigating the kinetical challenge of eight-electron transfer process for CH4. Further, PtAg alloy acts the synergistic sites for the CO2 reduction, namely, CO2 is reduced on Ag sites into CO intermediates, which then spillover or sequential adsorption on adjacent Pt sites and undergo further hydrogenation into CH4. Due to the synergic effect of Pt and Ag, the optimal PtAg-2/HNb3O8 NS catalyst achieves a selectivity of 74.3% for CH4 with an evolution rate of 93.6 μmol·g−1·h−1, which is about 34.7-fold of pure HNb3O8 NS (2.7 μmol·g−1·h−1). This work provides new insights into the role of synergistic effect of alloy co-catalyst for selective CO2 photoreduction and offers a new idea into the catalytic site design for CH4 formation.

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.

Author Correction: Self-adaptive dual-metal-site pairs in metal-organic frameworks for selective CO2 photoreduction to CH4

Author Correction: Self-adaptive dual-metal-site pairs in metal-organic frameworks for selective CO2 photoreduction to CH4

Engineering unsaturated coordination of conductive TiOx clusters derived from Metal–Organic–Framework incorporated into hollow semiconductor for highly selective CO2 photoreduction

Solar-driven CO2 conversion into fuels is a promising solution toward green energy conversion. It is a challenge to regulate the precise structure of photocatalysts at the atomic scale, facilitating the charge transfer and improving CO2 photoreduction performance. Herein, conductive Ti3+-rich TiOx clusters with unsaturated Ti–O5 coordination incorporated into nanosheets-assembled hollow sulfide semiconductors were prepared through a versatile transformation strategy by using NH2-MIL-125(Ti) metal–organic–framework as precursor. This typical photocatalysts exhibit highly selective photoreduction of CO2, achieving a super high visible-light photocatalytic CO evolution activity of 1100 μmol g−1h−1. The active sites were identified on the surface of conductive TiOx clusters, inducing efficient trapping of photogenerated electrons at the interface and suppressing H2 generation. This enhanced performance is ascribed to Ti3+-rich TiOx clusters as an efficient cocatalyst on sulfide semiconductor providing more active sites due to electrical conductivity and unsaturated coordination environment. This work provides a promising approach to enhance photocatalytic performance of the catalysts coupled by unique metal oxide clusters for selective photoreduction of CO2.

Nanocavity-assisted single-crystalline Ti3+ self-doped blue TiO2(B) as efficient cocatalyst for high selective CO2 photoreduction of g-C3N4

Two-dimensional (2D) graphitic carbon nitride (g-C3N4) has invoked significant interest for photocatalytic applications for its excellent features such as high surface area, visible light absorption, and easy transportation of photogenerated charge carriers, but the most reported g-C3N4 show relatively low photoactivity due to inferior conductivity and rapid recombination of carriers. These can be overcome by inducing porosity in g-C3N4, followed by exfoliation and combining with other materials. Herein, we synthesize nanocavity-assisted oxygen-deficient Ti3+ self-doped blue TiO2(B) nanorods (BT) and integrate them on exfoliated porous g-C3N4 (PCN). The synthesized materials are tested for photocatalytic conversion of CO2 into solar fuels (H2, CO, and CH4). The fabricated BT/PCN heterostructures exhibit higher photocatalytic CO2 conversion activity and 92% CO-evolving selectivity than BT and PCN. The enhancement in activity of BT/PCN can be attributed to the efficient separation and transportation of charge carriers, facilitated by the unique properties of BT, PCN, and their synergistic interactions. We believe that these results can contribute to the improvement of cost-effectiveness, feasibility, and overall performance for real photocatalytic systems.

Pd Nanosheet-Decorated 2D/2D g-C3N4/WO3·H2O S-Scheme Photocatalyst for High Selective Photoreduction of CO2 to CO.

The development of the global economy in recent years, environmental problems, greenhouse effect, and so forth have been of concern for countries all over the world. The key for solving the greenhouse effect is the reduction of CO2. With the development of photocatalytic reduction of CO2, hybrid photocatalytic nanostructures composed of noble metals and plasmonic semiconductors are being widely studied. In this work, S-scheme photocatalysts with a g-C3N4/WO3·H2O/Pd heterostructure was constructed by introducing ultrathin Pd nanosheets into the optimized 2D/2D g-C3N4/WO3·H2O binary system. The S-scheme charge transfer generated by the matched band gap of g-C3N4 and WO3·H2O can effectually improve the electron transfer rate and the redox ability of photogenerated carriers. The introduction of Pd nanosheets can inject a large number of hot electrons into the semiconductor on the basis of the S-scheme heterojunction to participate in the reaction. The S-scheme electron transfer method is used to improve the utilization rate of thermionic electrons and achieve the effect of widening the near-infrared-light absorption area of the composite material. Moreover, the reaction was carried out in water without the addition of any sacrificial agent, which can better reflect the green environmental protection of the experiment. This investigation will promote the broad-spectrum application of new and environment-friendly thermoelectron-assisted S-scheme photocatalysts, and on this basis, the possible reaction mechanism is discussed.

Bandgap engineered g-C3N4 and its graphene composites for stable photoreduction of CO2 to methanol

Carbon nitride (g-C3N4) is a two-dimensional material with several advantages over other photocatalysts, such as metal-free, biocompatible, chemically and thermally stable, to name a few. However, it usually suffers from low charge carrier mobility, high recombination rate, low electrical conductivity, and, more importantly, low absorption in the visible range. To address the multiple shortcomings, a simple and cost-effective copolymerization strategy was developed to synthesize g-C3N4 by selecting the appropriate precursors and optimizing the synthesis parameters, which resulted in lowering the bandgap from 2.80 eV to as narrow as 2.40 eV. To further improve the charge separation and conductivity, g-C3N4 and reduced graphene oxide (rGO) based composites were synthesized. The obtained composite catalysts were studied for photocatalytic CO2 reduction. It is important to note that g-C3N4/rGO composites resulted in the selective photoreduction of CO2 to methanol as the only liquid product with evolution rates of ∼114 μmol g−1 h−1 along with H2 (68 μmol g−1 h−1) under scavenger free conditions and exhibited robust stability.

Hierarchical CuS@ZnIn2S4 Hollow Double-Shelled p-n Heterojunction Octahedra Decorated with Fullerene C60 for Remarkable Selectivity and Activity of CO2 Photoreduction into CH4.

In this work, a hollow double-shelled architecture, based on n-type ZnIn2S4 nanosheet-coated p-type CuS hollow octahedra (CuS@ZnIn2S4 HDSOs), is designed and fabricated as a p-n heterojunction photocatalyst for selective CO2 photoreduction into CH4. The resulting hybrids provide rich active sites and effective charge migration/separation to drive CO2 photoreduction, and meanwhile, CO detachment is delayed to increase the possibility of eight-electron reactions for CH4 production. As expected, the optimized CuS@ZnIn2S4 HDSOs manifest a CH4 yield of 28.0 μmol g-1 h-1 and a boosted CH4 selectivity up to 94.5%. The decorated C60 both possesses high electron affinity and improves catalyst stability and CO2 adsorption ability. Thus, the C60-decorated CuS@ZnIn2S4 HDSOs exhibit the highest CH4 evolution rate of 43.6 μmol g-1 h-1 and 96.5% selectivity. This work provides a rational strategy for designing and fabricating efficient heteroarchitectures for CO2 photoreduction.

Size-dependent selectivity and activity of highly dispersed sub-nanometer Pt clusters integrated with P25 for CO2 photoreduction into methane fuel

Integration of sub-nanometer–sized Pt cocatalysts on semiconductor materials is a promising approach to improve the activity and selectivity of CO2 photoreduction. However, the practical realization of such an integrated catalyst is a challenging task. We rationally integrated sub-nanometer Pt nanoclusters (Pt NCs) on the surface of commercial P25 catalyst by adjusting the amount of Pt loading. The lateral reduction in the size of Pt NCs resulted in high metal dispersion, increased active metal surface area, better CO2 adsorption, and hindered photoexcited charge-carrier recombination. The sub-nanometer Pt NCs integrated with P25 with a weight percentage of 0.5% (Pt-NC/P25-0.5) exhibited exceptional CO2 photoreduction activity for CH4 production and excellent stability during successive test runs. This catalyst outperformed its counterparts with larger Pt NCs and several previously reported state-of-the-art photocatalysts in terms of CH4 generation. Importantly, compared to its counterparts, Pt-NC/P25-0.5 had a higher proportion of edge and corner sites, which have a strong affinity for CO molecules, resulting in a high CH4 selectivity of 95% against H2. The size-dependent selectivity and activity of Pt NCs for CO2 reduction demonstrated in this study offer insights into the development of sub-nanometer metal-based catalysts for photocatalytic energy applications.

Selective CO2 photoreduction to CH4 mediated by dimension-matched 2D/2D Bi3NbO7/g-C3N4 S-scheme heterojunction

Discovering highly selective catalysts is key to achieve effective CO2 photoreduction to hydrocarbon fuels. In this work, we construct an ultrathin dimension-matched S-scheme Bi3NbO7/g-C3N4 heterostructure, which permits the highly selective photocatalytic reduction of CO2 to CH4, as shown by 13C isotopic measurements. Density functional theory calculations combined with solid-state characterization confirm the electron transfer from g-C3N4 nanosheets to Bi3NbO7, establishing an internal electric field. The internal electric field drives photogenerated electrons from Bi3NbO7 to g-C3N4, as revealed by in-situ X-ray photoelectron spectroscopy, demonstrating the presence of an S-scheme charge transfer path in Bi3NbO7/g-C3N4 heterostructures allowing efficient and selective CO2 photoreduction. As a result, the optimized sample achieved a CH4 evolution rate of 37.59 μmol·g-1·h-1, a ca. 15-fold enhancement compared to ultrathin g-C3N4 nanosheets, and also retained stability after 10 reaction cycles and 40 h of simulated solar irradiation with no sacrificial reagents. The optimized Bi3NbO7/g-C3N4 composites achieve almost 90% selectivity for CH4 production over CO.

Spatially separated oxygen vacancies and nickel sites for ensemble promotion of selective CO2 photoreduction to CO

The CO2 reduction reaction (CRR), which is key to addressing global environmental and energy issues, must overcome the strong C = O bond and is hindered by low CRR selectivity and activity. Here, we unravel the ensemble effect of catalytically active sites for promotion of CRR while suppressing hydrogen evolution reactions (HERs). We design a 2D/2D hybrid consisting of an ordered assembly of nickel-rich Ni-Mn layered double hydroxide (LDH) nanosheets (NSs) and oxygen vacancies (OVs)-rich Mn3O4 (OVs-Mn3O4) NSs. On its own, Ni-Mn LDH demonstrates high CRR selectivity but mediocre CRR efficiency, whereas OVs-Mn3O4 gives a higher efficiency but poorer selectivity. The 2D/2D hybrid inherits advantages from both components, attaining high CRR selectivity and markedly higher efficiency than Ni-Mn LDH and OVs-Mn3O4. Calculations and in situ Fourier transform infrared spectroscopy (FT-IR) measurements reveal that nickel sites in the 2D/2D hybrid activate CO2 to generate CO2•– via a one-electron injection, whereas OVs in OVs-Mn3O4 promote light harvesting and charge transport without activating unwanted HERs.

Dynamic Active Sites in Bi5O7I Promoted by Surface Tensile Strain Enable Selective Visible Light CO2 Photoreduction.

Surface defects with abundant localized electrons on bismuth oxyhalide catalysts are proved to have the capability to capture and activate CO2. However, bismuth oxyhalide materials are susceptible to photocorrosion, making the surface defects easily deactivated and therefore losing their function as active sites. Construction of deactivation-resistant surface defects on catalyst is essential for stable CO2 photoreduction, but is a universal challenge. In this work, the Bi5O7I nanotubes with surface tensile strain are synthesized, which are favorable for the visible light-induced dynamic I defects generation. The CO2 molecules absorbed on I defects are constantly reduced by the incoming photogenerated electrons from I-deficient Bi5O7I nanotubes and the successive protonation of CO2 molecules is thus highly promoted, realizing the selective CO2 conversion process via the route of CO2-COOH--CO. The efficient and stable photoreduction of CO2 into CO with 100% selectivity can be achieved even under visible light (λ >420 nm) irradiation benefited from the dynamic I defects as active sites. The results presented herein demonstrate the unique action mechanism of light-induced dynamic defects during CO2 photoreduction process and provide a new strategy into rational design of deactivation-resistant catalysts for selective CO2 photoreduction.

Rational Design of Dot-on-Rod Nano-Heterostructure for Photocatalytic CO2 Reduction: Pivotal Role of Hole Transfer and Utilization.

Inspired by green plants, artificial photosynthesis has become one of the most attractive approaches toward carbon dioxide (CO2 ) valorization. Semiconductor quantum dots (QDs) or dot-in-rod (DIR) nano-heterostructures have gained substantial research interest in multielectron photoredox reactions. However, fast electron-hole recombination or sluggish hole transfer and utilization remains unsatisfactory for their potential applications. Here, the first application of a well-designed ZnSe/CdS dot-on-rods (DORs) nano-heterostructure for efficient and selective CO2 photoreduction with H2 O as an electron donor is presented. In-depth spectroscopic studies reveal that surface-anchored ZnSe QDs not only assist ultrafast (≈2ps) electron and hole separation, but also promote interfacial hole transfer participating in oxidative half-reactions. Surface photovoltage (SPV) spectroscopy provides a direct image of spatially separated electrons in CdS and holes in ZnSe. Therefore, ZnSe/CdS DORs photocatalyze CO2 to CO with a rate of ≈11.3µmol g-1 h-1 and ≥85% selectivity, much higher than that of ZnSe/CdS DIRs or pristine CdS nanorods under identical conditions. Obviously, favored energy-level alignment and unique morphology balance the utilization of electrons and holes in this nano-heterostructure, thus enhancing the performance of artificial photosynthetic solar-to-chemical conversion.

Fabricated hierarchical CdS/Ni-MOF heterostructure for promoting photocatalytic reduction of CO2

Fabricating the hierarchical heterostructure is efficient way for enhancing CO2 conversion of semiconductor photocatalysts. Herein, a simple strategy has been employed to prepare several 3D hierarchical CdS/Ni-MOF heterostructure photocatalysts, and the selective photoreduction of CO2 to CO under simulated sunlight in the gas–solid phase reactor. Among the prepared photocatalysts, 20%-CdS/Ni-MOF shows the best CO yield, which was 16 times and 7 times that of Ni-MOF and CdS, respectively. By controlling the morphology of the catalyst, we found that the significant improvement in photocatalytic activity can be ascribed to the synergistic effect of heterostructure in CdS/Ni-MOF and its unique hierarchical structure, which can improve the efficiency of charge transmission and provides abundant active sites, etc. Finally, the preliminary photocatalytic mechanism was discussed by in situ FTIR and liquid ultraviolet spectrophotometer.

Construction of Low-Cost Z-Scheme Heterostructure Cu2 O/PCN for Highly Selective CO2 Photoreduction to Methanol with Water Oxidation.

Solar-driven CO2 reaction with water oxidation into alcohols represents a promising approach to achieve real artificial photosynthesis. However, rapid recombination of photogenerated carriers seriously restricts the development of artificial photosynthesis. Herein, a facile method is explored to construct low-cost Z-Scheme heterostructure Cu2 O/polymeric carbon nitride (PCN) by in situ growth of Cu2 O hollow nanocrystal on PCN. The protective PCN layer and Z-schematic charge flow can make robust Cu2 O/PCN photocatalysts, and the spatial separation of electrons and holes with high redox potentials of ECB (-1.15eV) and EVB (1.65eV) versus NHE can efficiently drive CO2 photoreduction to methanol in pure water, which is further confirmed by DFT calculation. The Z-scheme heterostructure Cu2 O/PCN exhibits a high methanol yield of 276µmol g-1 in 8 h with ca. 100% selectivity, much superior to that of isolated Cu2 O and PCN, and all the reported Cu2 O-based heterostructures. This work provides a unique strategy to efficiently and selectively promote the conversion of CO2 and H2 O into high-value chemicals by constructing a low-cost Z-scheme heterostructure.