Highly Efficient Photoreduction of Low‐Concentration CO 2 to Syngas by Using a Polyoxometalates/Ru II Composite

Post-synthetic Metalation on the Ionic TiO2 Surface to Enhance Metal-CO2 Interaction During Photochemical CO2 Reduction

During the photochemical CO2 reduction reaction, CO2 adsorption on the catalyst's surface is a crucial step where the binding mode of the [metal-CO2] adduct directs the product selectivity and efficiency. Herein, an ionic TiO2 nanostructure stabilized by polyoxometalates (POM), ([POM]x@TiO2), is prepared and the sodium counter ions present on the surface to balance the POMs' charge are replaced with copper(II) ions, (Cux[POM]@TiO2). The microscopic and spectroscopic studies affirm the copper exchange without altering the TiO2 core and weak coordination of copper (II) ions to the POMs' surface. Band structure analysis suggests the photo-harvesting efficiency of the TiO2 core with the conduction band edge higher than the reduction potential of CuII/I and multi-electron CO2 reduction potentials. Photochemical CO2 reduction with Cux[POM]@TiO2 results in 30 μmol gcat. -1 CO (79 %) and 8 μmol gcat -1 of CH4 (21 %). Quasi-in-situ Raman study provides evidence in support of CO2 adsorption on the Cux[POM]@TiO2 surface. 13C and D2O labeling studies affirm the {Cu-[CO2]-} adduct formation. Despite the photo-harvesting ability of Nax[POM]@TiO2 itself, the poor CO2 adsorption ability of sodium ions highlights the crucial role of copper ion CO2 photo-reduction. Characterization of the {M-[η2-CO2]-} species via surface tuning validates the CO2 activation and photochemical reduction pathway proposed earlier.

Tunable CO2-to-syngas conversion via strong electronic coupling in S-scheme ZnGa2O4/g-C3N4 photocatalysts

The conversion of CO2 into syngas, a mixture of CO and H2, via photocatalytic reduction, is a promising approach towards achieving a sustainable carbon economy. However, the evolution of highly adjustable syngas, particularly without the use of sacrifice reagents or additional cocatalysts, remains a significant challenge. In this study, a step-scheme (S-scheme) 0D ZnGa2O4 nanodots (∼7 nm) rooted g-C3N4 nanosheets (denoted as ZnGa2O4/C3N4) heterojunction photocatalyst was synthesized vis a facial in-situ growth strategy for efficient CO2-to-syngas conversion. Both experimental and theoretical studies have demonstrated that the polymeric nature of g-C3N4 and highly distributed ZnGa2O4 nanodots synergistically contribute to a strong interaction between metal oxide and C3N4 support. Furthermore, the desirable S-scheme heterojunction in ZnGa2O4/C3N4 efficiently promotes charge separation, enabling strong photoredox ability. As a result, the S-scheme ZnGa2O4/C3N4 exhibited remarkable activity and selectivity in photochemical conversion of CO2 into syngas, with a syngas production rate of up to 103.3 μ mol g-1 h-1, even in the absence of sacrificial agents and cocatalyst. Impressively, the CO/H2 ratio of syngas can be tunable within a wide range from 1:4 to 2:1. This work exemplifies the effectiveness of a meticulously designed S-scheme heterojunction photocatalyst for CO2-to-syngas conversion with adjustable composition, thus paving the way for new possibilities in sustainable energy conversion and utilization.

N-N-Bridged Polynuclear POM-Based Coordination Polymers Based on a V-Type Ligand: Proton Conduction and Magnetism

The construction of polyoxometalate (POM)-based coordination polymers, in the presence of a nitrogen heterocyclic ligand, is intriguing due to the potential for obtaining diverse structures. These structures exhibit extensive application possibilities in the fields of proton conductivity and magnetism. Herein, four new POM-based polynuclear coordination polymers with the formulas of {[Fe2(btb)3(H2O)2(SiW12O40)]·3H2O}n (1), {[Cd2(btb)2(H2O)6(HPMoVI10MoV2O40)]·2H2O}n (2), {[Co3(OH)2(btb)2(H2O)5(HPMoVI10MoV2O40)]·7H2O}n (3), and {[Cu3(OH)(btb)2(H2O)(HP2Mo5O23)]·6H2O}n (4) have been prepared using the V-type 1,3-bis(4H-1,2,4-triazole-4-yl)benzene (btb) ligand. Compounds 1 and 2 feature similar two-dimensional (2D) structures, derived from the binuclear Fe2N6 and Cd2N4 subunits connected by tridentate btb ligands. Meanwhile, in compound 3, hexanuclear Co6(OH)4 units are bound by quadridentate btb ligands forming a 2D layer with the same 4-c sql topology simplification as compounds 1 and 2. In compound 1, Keggin-type polyoxoanions are monodentate-coordinated to metal ions and suspended on the 2D structure, while, in compounds 2 and 3, they act as discrete counterions residing in the interstitial spaces between two adjacent layers, thereby extending the 2D structures into 3D structures through hydrogen bonding interactions. In compound 4, trinuclear Cu3(OH) subunits are further constructed into a 3D framework through cooperation with four tridentate and quadridentate btb ligands as well as Strandberg-type anions. Furthermore, the proton conduction of the four compounds has been investigated. They display high proton conductivities at 358 K and 98% RH with powdered samples, which are 1.26 × 10-3, 1.24 × 10-3, 3.24 × 10-4, and 2.57 × 10-4 S cm-1, respectively. Interestingly, by mixing with Nafion, the composite membranes of compounds 2 and 4 exhibit enhanced proton conductivities, measuring at 4.87 × 10-2 and 1.28 × 10-2 S cm-1, respectively, at 358 K and 98% RH, which suggests excellent potential for applications. In addition, compounds 1, 3, and 4 display antiferromagnetic behaviors due to similar magnetic interactions. This work can provide research insights into the assembly of 2D POM-based coordination polymers with nitrogen heterocyclic ligands and Keggin-type POMs and further promote their research progress in proton conduction.

Regulating CO and H2 Ratios in Syngas Produced from Photocatalytic CO2/H2O Reduction by Cu and Co Dual Active Centers on Carbon Nitride Hollow Nanospheres

For photocatalytic CO2 reduction to produce syngas, there are challenges in achieving a high catalytic efficiency and precise control over the product ratio. In this study, two non-noble metal complexes Cobpy and Cubpy (bpy = 2,2'-bipyridine) as cocatalysts for CO2 reduction and hydrogen evolution, respectively, were in situ supported on carbon nitride hollow nanospheres to construct a hybrid system for photocatalytic syngas production. The resulting CO/H2 ratio can be precisely regulated within a wide range of 0:1-9:1 by accurately controlling the content of the two complexes. The presence of the two complexes promotes the migration of photogenerated electrons of the carbon nitride. CO2 can be reduced to CO on the photoreduced species Co(bpy)2+ of Cobpy on CNHS, and H+ can be reduced to H2 on the photoreduced species Cu(bpy)2+ of Cubpy. Furthermore, this method is also applicable to other photocatalysts, such as CdS and TiO2 for generating syngas and regulating product ratios.

Surface Ru-H Bipyridine Complexes-Grafted TiO2 Nanohybrids for Efficient Photocatalytic CO2 Methanation

A series of novel surface Ru-H bipyridine complexes-grafted TiO₂ nanohybrids were for the first time prepared by a combined procedure of surface organometallic chemistry with post-synthetic ligand exchange for photocatalytic conversion of CO₂ to CH₄ with H₂ as electron and proton donors under visible light irradiation. The selectivity toward CH₄ increased to 93.4% by the ligand exchange of 4,4'-dimethyl-2,2'-bipyridine (4,4'-bpy) with the surface cyclopentadienyl (Cp)-RuH complex and the CO₂ methanation activity was enhanced by 4.4-fold. An impressive rate of 241.2 μL·g^-1·h^-1 for CH₄ production was achieved over the optimal photocatalyst. The femtosecond transient IR absorption results demonstrated that the hot electrons were fast injected in 0.9 ps from the photoexcited surface 4,4'-bpy-RuH complex into the conduction band of TiO₂ nanoparticles to form a charge-separated state with an average lifetime of ca. 50.0 ns responsible for the CO₂ methanation. The spectral characterizations indicated clearly that the formation of CO₂^•- radicals by single electron reduction of CO₂ molecules adsorbed on surface oxygen vacancies of TiO₂ nanoparticles was the most critical step for the methanation. Such radical intermediates were inserted into the explored Ru-H bond to generate Ru-OOCH species and finally CH₄ and H₂O in the presence of H₂.

Tunable green syngas generation from CO2 and H2O with sunlight as the only energy input

The carbon-neutral synthesis of syngas from CO₂ and H₂O powered by solar energy holds grand promise for solving critical issues such as global warming and the energy crisis. Here we report photochemical reduction of CO₂ with H₂O into syngas using core/shell Au@Cr₂O₃ dual cocatalyst-decorated multistacked InGaN/GaN nanowires (NWs) with sunlight as the only energy input. First-principle density functional theory calculations revealed that Au and Cr₂O₃ are synergetic in deforming the linear CO₂ molecule to a bent state with an O-C-O angle of 116.5°, thus significantly reducing the energy barrier of CO₂RR compared with that over a single component of Au or Cr₂O₃. Hydrogen evolution reaction was promoted by the same cocatalyst simultaneously. By combining the cooperative catalytic properties of Au@Cr₂O₃ with the distinguished optoelectronic virtues of the multistacked InGaN NW semiconductor, the developed photocatalyst demonstrated high syngas activity of 1.08 mol/g_cat/h with widely tunable H₂/CO ratios between 1.6 and 9.2 under concentrated solar light illumination. Nearly stoichiometric oxygen was evolved from water splitting at a rate of 0.57 mol/g_cat/h, and isotopic testing confirmed that syngas originated from CO₂RR. The solar-to-syngas energy efficiency approached 0.89% during overall CO₂ reduction coupled with water splitting. The work paves a way for carbon-neutral synthesis of syngas with the sole inputs of CO₂, H₂O, and solar light.

Copper-Based Metal-Organic Framework with a Tetraphenylethylene-Tetrazole Linker for Visible-Light-Driven CO2 Photoconversion

The design of efficient and inexpensive photocatalysts for CO₂ photoreduction under visible light is of great significance for the sustainable development of the entire society. Herein, a copper-based metal-organic framework (MOF) (CUST-804) using a bulky tetraphenylethylene-tetrazole linker is synthesized and successfully used as a photocatalyst for CO₂ reduction. The structural characterizations, as well as the photophysical properties, are investigated systematically. In the heterogeneous catalytic system, CUST-804 exhibits a robust CO production activity up to 2.71 mmol g^-1 h^-1 with excellent recyclability along with a selectivity of 82.8%, which is comparable with those of the reported copper-based MOF system. Theoretical calculations demonstrated that, among three kinds of coordinated model, only the 5-coordinated Cu site is active for CO₂ reduction, in which the *COOH intermediate is stabilized and CO is readily desorbed. The results obtained herein can provide fresh insights into the realization of efficient copper-functionalized crystalline photocatalysts for CO₂ reduction.

Integration of zirconium-based metal-organic framework with CdS for enhanced photocatalytic conversion of CO2 to CO

It is a promising strategy to prepare composite photocatalysts based on MOFs and semiconductors for enhancing photocatalytic reduction of carbon dioxide (CO₂). A family of binary composite photocatalysts (CdS@UiO-66-NH₂) with different CdS contents have been designed and synthesized, which have been explored for photocatalytic reduction of CO₂. CdS@UiO-66-NH₂ can efficiently convert CO₂ into CO under visible light irradiation via the solid-gas mode in the absence of sacrificial agents and photosensitizers. The generation rate of CO can reach up to 280.5 μmol g^-1 h^-1, which is 2.13-fold and 2.9-fold improvements over the pristine CdS and UiO-66-NH₂, respectively, and the selectivity for CO is very high. Furthermore, this kind of photocatalysts can still maintain great photocatalytic activity in CO₂/N₂ mixed atmosphere with different CO₂ concentrations. The outstanding performances of CdS@UiO-66-NH₂ may be attributed to the existence of the direct Z-scheme heterojunction, which possesses the enhanced separation and migration of photo-generated charge carriers between UiO-66-NH₂ and CdS, available specific surface areas and improved visible light absorption ability as well as abundant reaction active sites. This case reveals that MOF-based composite photocatalysts exhibit promising potential applications in the field of CO₂ conversion.

Synthesis and CO2 photoreduction of two 3d–4f heterometal–organic frameworks

Two 3d–4f heterometal–organic frameworks with similar structures were synthesized by a steam-assisted conversion method and exhibited high activity and selectivity for the photoreduction of CO2.

Two three-dimensional polyanionic clusters [M(P4Mo6)2] (M = Co, Zn) exhibiting excellent photocatalytic CO2 reduction performance

Two captivating {P4Mo6}-based compounds, formulated as (H2bbi)2{[Co2(bbi)][Co2.33(H2O)4][H9.33CoP8Mo12O62]}·4H2O (1) and (H2bbi){[Zn(Hbbi)]2[Zn0.75(bbi)][K2Zn(H2O)4][H8.5ZnP8Mo12O62]} (2) [bbi = 1,1'-(1,4-butanediyl)bis(imidazole)], were successfully synthesized under hydrothermal conditions. Structural analysis demonstrates that compounds 1 and 2 are constructed from hourglass-shaped structures [M(P4Mo6O31)2]n- (M = Co, Zn), which are all made up of molybdophosphates and one transition metal ion as the central connecting node. Compounds 1 and 2 feature three-dimensional (3D) frameworks, which are all connected to form a 3D structure by metal ions and bbi ligands. More interestingly, compound 1 exhibits higher catalytic activity than 2 in CO2 photoreduction due to the suitable energy band structure of Co species in {P4Mo6} clusters. The CO yield was 3261 μmol g-1 with high selectivity in 8 h for compound 1 in photocatalytic CO2 reduction, which is highly promising in the photocatalytic field. Additionally, the photoluminescence properties of 2 were investigated.

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.