The reaction kinetics of photocatalytic CO2 reduction is highly dependent on the transfer rate of electrons and protons to the CO2 molecules adsorbed on catalytic centers. Studies on uncovering the proton effect in catalysts on photocatalytic activity of CO2 reduction are significant but rarely reported. In this paper, we, from the molecular level, revealed that the photocatalytic activity of CO2 reduction is closely related to the proton availability in catalysts. Specifically, four dinuclear Co(II) complexes based on Robson-type ligands with different number of carboxylic groups (-nCOOH; n = 0, 2, 4, 6) were designed and synthesized. All these complexes show photocatalytic activity for CO2 reduction to CO in a water-containing system upon visible-light illumination. Interestingly, the CO yields increase positively with the increase of the carboxylic-group number in dinuclear Co(II) complexes. The one containing -6COOH shows the best photocatalytic activity for CO2 reduction to CO, with the TON value reaching as high as 10,294. The value is 1.8, 3.4, and 7.8 times higher than those containing -4COOH, -2COOH, and -0COOH, respectively. The high TON value also makes the dinuclear Co(II) complex with -6COOH outstanding among reported homogeneous molecular catalysts for photocatalytic CO2 reduction. Control experiments and density functional theory calculation indicated that more carboxylic groups in the catalyst endow the catalyst with more proton relays, thus accelerating the proton transfer and boosting the photocatalytic CO2 reduction. This study, at a molecular level, elucidates that more carboxylic groups in catalysts are beneficial for boosting the reaction kinetics of photocatalytic CO2 reduction.
Homonuclear dual-atomic catalysts showcase unique electronic modulation due to their dual metal centres, providing new direction in development of efficient catalysts for CO2 electroreduction. This article highlights a few cutting-edge homonuclear dual-atomic catalysts, focusing on their inherent advantages in efficient and selective CO2 electroreduction, to spotlight the potential application of dual-atomic catalysts in CO2 electroreduction.
Covalent organic frameworks (COFs) have been widely studied in photocatalytic CO2 reduction reaction (CO2 RR). However, pristine COFs usually exhibit low catalytic efficiency owing to the fast recombination of photogenerated electrons and holes. In this study, we fabricated a stable COF-based composite (GO-COF-366-Co) by covalently anchoring COF-366-Co on the surface of graphene oxide (GO) for the photocatalytic CO2 reduction. Interestingly, in absolute acetonitrile (CH3 CN), GO-COF-366-Co shows a high selectivity of 94.4 % for the photoreduction of CO2 to formate, with a formate yield of 15.8â mmol/g, which is approximately four times higher than that using the pristine COF-366-Co. By contrast, in CH3 CN/H2 O (v : v=4 : 1), the main product for the photocatalytic CO2 reduction over GO-COF-366-Co is CO (96.1 %), with a CO yield as high as 52.2â mmol/g, which is also approximately four times higher than that using the pristine COF-366-Co. Photoelectrochemical experiments demonstrate the covalent bonding of COF-366-Co and GO to form the GO-COF-366-Co composite facilitates charge separation and transfer significantly, thereby accounting for the enhanced catalytic activity. In addition, theoretical calculations and in situ Fourier transform infrared spectroscopy reveal H2 O can stabilize the *COOH intermediate to further form a *CO intermediate via O-H(aq)â â â O(*COOH) hydrogen bonding, thus explaining the regulated photocatalytic performance.
The development of hierarchically porous metal-organic frameworks (MOFs) with high stability is desirable to expand their applications but remains challenging. Herein, an anionic sodalite-type microporous MOF (Yb-TTCA; TTCA3- = triphenylene-2,6,10-tricarboxylate) was synthesized, which shows outstanding catalytic activities for the cycloaddition of CO2 into cyclic carbonates. Moreover, the microporous Yb-TTCA can be transformed into a hierarchical micro- and mesoporous Yb-TTCA by water treatment with the mesopore sizes of 2 to 12 nm. The hierarchically porous Yb-TTCA (HP-Yb-TTCA) not only exhibits a high thermal stability up to 500 °C but also shows a high chemical stability in aqueous solutions with pH values ranging from 2 to 12. In addition, the HP-Yb-TTCA displays enhanced performance for the removal of organic dyes in comparison with microporous Yb-TTCA. This work provides a facile way to construct hierarchically porous MOF materials.
Catalysts featuring dinuclear metal sites are regarded as superior systems compared with their counterparts with mononuclear metal sites. The dinuclear metal sites in catalysts with appropriate spatial separations and geometric configurations can confer the dinuclear metal synergistic catalysis (DMSC) effect, and thus boost the catalytic performance, in particular for reactions involving multiple reactants, intermediates and products. In this review, we summarize the related reports on the design and synthesis of both homogeneous and heterogeneous dinuclear metal catalysts, and their applications in energy conversion reactions, including photo-/electro-catalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), CO2 reduction reaction (CO2RR), and N2 reduction reaction (N2RR). Particularly, we focus on the analysis of the relationship between the catalyst structure and catalytic performances, where the design principles are presented. Finally, we discuss the challenges in the design and preparation of dinuclear metal catalysts with the DMSC effect and present a perspective on the future development of dinuclear metal catalysts in energy conversion. This review aims to comprehensively summarize the up-to-date research progress on the synthesis and energy-related application of dinuclear metal catalysts and provide guidance for designing energy-conversion catalysts with superior performances.
The mismatched fast-electron-slow-proton process in the electrocatalytic oxygen evolution reaction (OER) severely restricts the catalytic efficiency. To overcome these issues, accelerating the proton transfer and elucidating the kinetic mechanism are highly sought after. Herein, inspired by photosystem II, we develop a family of OER electrocatalysts with FeO6 /NiO6 units and carboxylate anions (TA2- ) in the first and second coordination sphere, respectively. Benefiting from the synergistic effect of the metal units and TA2- , the optimized catalyst delivers superior activity with a low overpotential of 270â mV at 200â mA cm-2 and excellent cycling stability over 300â h. A proton-transfer-promotion mechanism is proposed by in situ Raman, catalytic tests, and theoretical calculations. The TA2- (proton acceptor) can mediate proton transfer pathways by preferentially accepting protons, which optimizes the O-H adsorption/activation process and reduces the kinetic barrier for O-O bond formation.
Dual-atom catalysts (DACs) have emerged as efficient electrocatalysts for CO2 reduction owing to the synergistic effect between the binary metal sites. However, rationally modulating the electronic structure of DACs to optimize the catalytic performance remains a great challenge. Herein, we report the electronic structure modulation of three Ni2 DACs (namely, Ni2 -N7 , Ni2 -N5 C2 and Ni2 -N3 C4 ) by the regulation of the coordination environments around the dual-atom Ni2 centres. As a result, Ni2 -N3 C4 exhibits significantly improved electrocatalytic activity for CO2 reduction, not only better than the corresponding single-atom Ni catalyst (Ni-N2 C2 ), but also higher than Ni2 -N7 and Ni2 -N5 C2 DACs. Density functional theory (DFT) calculations revealed that the high electrocatalytic activity of Ni2 -N3 C4 for CO2 reduction could be attributed to the electronic structure modulation to the Ni centre and the resulted proper binding energies to COOH* and CO* intermediates.
A carbazolide-bis(NHC) NiII catalyst (1; NHC, N-heterocyclic carbene) for selective CO2 photoreduction was designed herein by a one-stone-two-birds strategy. The extended π-conjugation and the strong σ/π electron-donation characteristics (two birds) of the carbazolide fragment (one stone) lead to significantly enhanced activity for photoreduction of CO2 to CO. The turnover number (TON) and turnover frequency (TOF) of 1 were ninefold and eightfold higher than those of the reported pyridinol-bis(NHC) NiII complex at the same catalyst concentration using an identical Ir photosensitizer, respectively, with a selectivity of â¼100%. More importantly, an organic dye was applied to displace the Ir photosensitizer to develop a noble-metal-free photocatalytic system, which maintained excellent performance and obtained an outstanding quantum yield of 11.2%. Detailed investigations combining experimental and computational studies revealed the catalytic mechanism, which highlights the potential of the one-stone-two-birds effect.
2D lamellar materials can offer high surface area and abundant reactive sites, thus showing an appealing prospect in photocatalytic hydrogen evolution. However, it is still difficult to build cost-efficient photocatalytic hydrogen evolution systems based on 2D materials. Herein, an in situ growth method is employed to build 2D/2D heterojunctions, with which 2D Ni-based metal-organic layers (Ni-MOLs) are closely grown on 2D porous CdS (P-CdS) nanosheets, affording traditional P-CdS/Ni-MOL heterojunction materials. Impressively, the optimized P-CdS/Ni-MOL catalyst exhibits superior photocatalytic hydrogen evolution performance, with an H2 yield of 29.81 mmol g-1 h-1 . This value is 7 and 2981 times higher than that of P-CdS and Ni-MOLs, respectively, and comparable to those of reported state of the art catalysts. Photocatalytic mechanism studies reveal that the enhanced photocatalytic performance can be attributed to the 2D/2D intimate interface between P-CdS and Ni-MOLs, which facilitates the fast charge carriers' separation and transfer. This work provides a strategy to develop 2D MOL-based photocatalysts for sustainable energy conversion.
SignificanceThe photosensitizer is one of the important components in the photocatalytic system. Molecular photosensitizers have well-defined structures, which is beneficial in revealing the catalysis mechanism and helpful for further structural design and performance optimization. However, separation and recycling of the molecular photosensitizers is a great problem. Loading them into/on two/three-dimensional supports through covalent bonds, electrostatic interactions, and supramolecular interactions is a method that enhances their separation and recycling capability. Nonetheless, the structures of the resulting composites are unclear. Thus, the development of highly crystalline heterogeneity methods for molecular photosensitizers, albeit greatly challenging, is meaningful and desirable in photocatalysis, through which heterogeneous photosensitizers with well-defined structures, definite catalysis mechanisms, and good catalytic performance would be expected.
Photosensitizing Agents , Catalysis , Molecular Structure , Photosensitizing Agents/chemistry
To avoid the agglomeration of iron NPs and improve the dispersion of Fe SAs, we employed a mixed-ligand strategy to regulate the iron content in PCN-224(ZnxFey) and PCN-222(ZnxFey). Thanks to the sublimation of Zn and the Kirkendall effect, uniform dispersions of Fe SAs with 1.04-1.06 wt% were obtained in the pyrolysis products Zn0.5Fe0.5-N-C-224 and Zn0.5Fe0.5-N-C-222 with excellent CO2 â CO activity, super-stability, and recyclability.
The exploitation of highly stable and active catalysts for the conversion of CO2 into valuable fuels is desirable but is a great challenge. Herein, we report that the incorporation of chromophores into metal-organic frameworks (MOFs) could afford robust catalysts for efficient CO2 conversion. Specifically, a porous Nd(III) MOF (Nd-TTCA; TTCA3- = triphenylene-2,6,10-tricarboxylate) was constructed by incorporating one-dimensional Nd(CO2)n chains and TTCA3- ligands, which exhibits a very high stability, retaining its framework not only in the air at 300 °C for 2 h but also in boiling aqueous solutions at pH 1-12 for 7 days. More importantly, Nd-TTCA has achieved a 5-fold improvement in photocatalytic activity for reducing CO2 to HCOOH and a 10-fold improvement in catalytic activity for the cycloaddition of CO2 into cyclic carbonate in comparison to those of H3TTCA itself. This work gives a new strategy to design efficient artificial crystalline catalysts for CO2 conversion.
It is common that different crystal facets in metal and metal oxide nanocrystals display different catalytic performances, whereas such phenomena have been rarely documented in metal-organic frameworks (MOFs). Herein, we demonstrate for the first time that a nickel metal-organic layer (MOL) exposing rich (100) crystal facets (Ni-MOL-100) shows a much higher photocatalytic CO2 -to-CO activity than the one exposing rich (010) crystal facets (Ni-MOL-010) and its bulky counterpart (bulky Ni-MOF), with a catalytic activity up to 2.5 and 4.6 times more active than Ni-MOL-010 and bulky Ni-MOF, respectively. Theoretical studies reveal that the two coordinatively unsaturated NiII ions with a close distance of 3.50â Å on the surface of Ni-MOL-100 enables synergistic catalysis, leading to more favorable energetics in CO2 reduction than that of Ni-MOL-010.
Revealing the contribution of π-π stacking interactions in supramolecular assembly is important for understanding the intrinsic nature of molecular assembly fundamentally. However, because they are much weaker than covalent bonds, π-π stacking interactions are usually ignored in the construction of porous materials. Obtaining stable porous materials that are only dependent on π-π stacking interactions, despite being very challenging, could address this concern. Here, we present a porous supramolecular framework (π-1) stabilized only by intermolecular π-π stacking interactions. π-1 shows good thermal and chemical stability not only in various organic solvents but also in aqueous solution in a broad pH range. Furthermore, featuring one-dimensional channels with dangling thiolate groups, π-1 exhibits excellent Hg2+ removal performance, with adsorption capacity as high as 786.67 mg g-1 and an adsorption ratio as high as 99.998%. In addition, π-1 also shows high adsorption selectivity to Hg2+ in the presence of a series of interfering ions.
The production of two-dimensional (2D) ultrathin metal-organic framework (MOF) nanosheets with functionalized surfaces is significant for extending their applications. To date, no protocol has been developed yet to solve this problem. Herein, we report a facile, mild, and efficient method to produce 2D monolayer MOF nanosheets with hydrophobic surfaces from layer-pillared 3D MOFs. This approach is based on the replacement of weaker coordinating pillar ligands with stronger coordinating capping ligands with the assistance of a high concentration gradient of the latter. Utilizing this method, the replacement of the 4,4'-bipyridine (bpy) pillars in two cadmium-based layer-pillared MOFs with alkylpyridine derivatives has been achieved, producing 2D MOF nanosheets with monolayer thickness and double-sided hydrophobic surfaces. The resulting hydrophobic 2D MOF nanosheets exhibit good performance for the separation of oil and water.
There is a demand to develop molecular catalysts promoting the hydrogen evolution reaction (HER) with a high catalytic rate and a high tolerance to various inhibitors, such as CO and O2 . Herein we report a cobalt catalyst with a penta-dentate macrocyclic ligand (1-Co), which exhibits a fast catalytic rate (TOF=2210â s-1 ) in aqueous pHâ 7.0 phosphate buffer solution, in which proton transfer from a dihydrogen phosphate anion (H2 PO4 - ) plays a key role in catalytic enhancement. The electrocatalyst exhibits a high tolerance to inhibitors, displaying over 90 % retention of its activity under either CO or air atmosphere. Its high tolerance to CO is concluded to arise from the kinetically labile character of undesirable CO-bound species due to the geometrical frustration posed by the ligand, which prevents an ideal trigonal bipyramid being established.
Using solar energy to convert CO2 into valuable fuels or chemicals offers a powerful solution to urgent energy and environmental problems. However, the development of efficient and selective catalysts remains a considerable scientific challenge. To address this, catalytically active CoII centers can be anchored into the porous matrix of metal-organic frameworks (MOFs) by utilizing a robust Zr-based MOF (Zr-DMBD) functionalized with freestanding thiol groups to enable efficient post-synthetic metal insertion. The thus-prepared Zr-DMBD-Co MOF solids are modified by well-defined Co-thiolate units and have the capability of photocatalytically converting CO2 into CO with high efficiency and selectivity under visible-light irradiation in a water-containing system. The turnover number and CO selectivity reach as high as 97 941 and 98 %, respectively.
The solar-driven CO2 reduction is a challenge in the field of "artificial photosynthesis", as most catalysts display low activity and selectivity for CO2 reduction in water-containing reaction systems as a result of competitive proton reduction. Herein, we report a dinuclear heterometallic complex, [CoZn(OH)L1 ](ClO4 )3 (CoZn), which shows extremely high photocatalytic activity and selectivity for CO2 reduction in water/acetonitrile solution. It achieves a selectivity of 98 % for CO2 -to-CO conversion, with TON and TOF values of 65000 and 1.8â s-1 , respectively, 4, 19, and 45-fold higher than the values of corresponding dinuclear homometallic [CoCo(OH)L1 ](ClO4 )3 (CoCo), [ZnZn(OH)L1 ](ClO4 )3 (ZnZn), and mononuclear [CoL2 (CH3 CN)](ClO4 )2 (Co), respectively, under the same conditions. The increased photocatalytic performance of CoZn is due to the enhanced dinuclear metal synergistic catalysis (DMSC) effect between ZnII and CoII , which dramatically lowers the activation barriers of both transition states of CO2 reduction.
Herein, we report a mononuclear Co(ii) cryptate which exhibits highly efficient and selective photocatalytic CO2-to-CO conversion in H2O/CH3CN solution. The TON and selectivity reach as high as 51 392 and 98%, respectively.
Coordination polymers (CPs) are ideal precursors for synthesizing porous catalysts. However, the direct thermolysis of CPs is prone to generate agglomerates, greatly reducing the electrical conductivity and active sites of their derived catalysts. The construction of well-ordered CP nanostructures is a promising strategy for alleviating the above issue, but it remains challenging. Here, a facile chemical etching approach is developed for the fabrication of well-aligned three-dimensional (3D) bimetallic Prussian blue-analogue nanosheet arrays. Impressively, the derived porous metal oxide (Fe-NiO) acts as a remarkable oxygen evolution reaction (OER) catalyst, which merely requires overpotentials as low as 218 and 270â mV to achieve 10 and 100â mA cm-2 in 1.0 m KOH aqueous solution, respectively. The excellent electrocatalytic performance of Fe-NiO is ascribed to the 3D porous nanosheet array architecture, which endows the bimetallic catalyst with abundant electrocatalytic active sites, enhanced surface permeability, and high electronic conductivity. It is expected that the proposed strategy can pave a new way for fabricating highly efficient electrocatalysts for energy storage and conversion.
