Cocatalyst Strategy Research Articles

Co-optimization of g-C3N4 with prolonging exciton lifetime strategy and co-catalyst strategy for enhanced photocatalytic H2 evolution activity

In this work, the photocatalytic activity of g-C3N4 is co-optimized for the first time using a strategy of prolonging exciton lifetime and a co-catalyst strategy. Amorphous CoB nanoparticles are loaded on OCN-K-CN, which is an O, K doped g-C3N4 material with van der Waals heterojunctions inside. The optimized OCN-K-CN-CoB sample, containing 12 wt% CoB and 1.25% K2SO4 in precursors, show photocatalytic H2 evolution rate approximately 27 times higher than that of GCN. The mechanism of co-optimization is detailedly investigated through comparative experiments on light absorption, charge separation performance, and HER activity, with optimization strategy and CoB loading ratio as controlled variables. New roles of the two strategies have been revealed: "flux-limited reduction activity sites" for CoB and ''buffer-enhancer'' for OCN-K-CN. These new roles illustrate a hidden relationship between the optimizing effects of the two strategies and can explain a surprising advantage of this co-optimization: achieving near-optimal hydrogen evolution activity with significantly reduced co-catalyst loading ratio.

Efficient hydrogen evolution from g-C3N4 under visible light by in situ loading Ag2Se nanoparticles as co-catalysts

The low charge separation and transfer of g-C3N4 hinders its industrial application in photocatalytic hydrogen evolution. Here, we design a novel co-catalyst strategy to integrate Ag2Se nanoparticles in situ on the surface of g-C3N4. The optimized photocatalyst, 15% Ag2Se/g-C3N4, demonstrates remarkable photocatalytic efficiency in the hydrogen evolution rate, reaching to 1102.8 μmol·g−1·h−1, 7 times higher than g-C3N4. To further elucidate the photocatalytic activity of 15% Ag2Se/g-C3N4, we present a possible mechanism based on various characterizations and density functional theory calculations. This research offers potential insights for the future development of silver chalcogenide composites in photocatalysis.

Titanium carbide (Ti3C2Tx) decorated molybdenum diselenide (MoSe2) nanoflower composite enhanced photo-electrocatalytic activity in hydrogen evolution

Two-dimensional titanium carbides (Ti3C2Tx) are part of the novel family of transition metal carbides/nitrides known as MXenes, which exhibit a broad range of photo-electrocatalytic applications. In this study, we present a facile hydrothermal synthesis method for Ti3C2Tx decorated on molybdenum diselenide (MoSe2) nanoflower composites, serving as an enhanced catalyst for hydrogen (H2) evolution. Ti3C2Tx, acting as a 2D conductive substrate, facilitates electron transfer, preventing MoSe2 agglomeration, while MoSe2 offers abundant active sites. The characterizations of surface analysis, optical properties, morphological nanostructure, and elemental composition performance of Ti3C2Tx nanosheets, MoSe2 nanoflowers, and Ti3C2Tx/MoSe2 nanoflower composites was conducted using FTIR, XRD, UV–Visible spectroscopy, SEM, EDS Mapping, TEM, and XPS techniques. The catalyst with optimal performance, the Ti3C2Tx/MoSe2 nanoflower composite demonstrates excellent functional properties in the H2 evolution, showing a lower overpotential of 135 mV and smaller Tafel slope of 72 mV/dec at 10 mA/cm2 compared to Ti3C2Tx nanosheets and MoSe2 nanoflowers. The Ti3C2Tx/MoSe2 nanoflower composite forms heterojunctions to inhibit the rapid recombination rate of photo-induced electrons and holes. In comparison to MoSe2 nanoflowers and Ti3C2Tx catalysts (where Tx = F, and O surface terminations), the optimized Ti3C2Tx/MoSe2 nanoflower composite exhibits significantly enhanced photocatalytic activity in H2 production, with a H2 production rate of 2146 μmolg−1h−1 superior than the Ti3C2Tx nanosheets (1367 μmolg−1h−1) and MoSe2 nanoflowers (986 μmolg−1h−1), respectively. Through this design, effective separation of photogenerated charge carriers is achieved, improving the activity of photocatalytic H2 production and demonstrating the promise of the cocatalyst strategy for photocatalysis.

Effects of various acid catalysts in dimethyl isosorbide/water co-solvent system for lignocellulosic biomass pretreatment

Novel organosolv pretreatments of biomass using biobased green solvents have attracted increasing attention to realize sustainable biorefinery. We have demonstrated that the feasibility of dimethyl isosorbide/water co-solvent in fractionating lignocellulosic biomass (Eucalyptus) for multiple products under a mild condition (120 °C, 60 min). To take a further step towards revealing the effects of catalysts, we explored various Brønsted (e.g., H2SO4, HCl, and p-TsOH), Lewis acid catalysts (e.g., AlCl3 and FeCl3) and their combinations in this co-solvent system. An interesting finding was that the delignification efficiency of AlCl3 was lower than that of H2SO4 (69.9% vs 91.2%), while the cellulose conversion efficiency of the pretreated pulp was similar (∼80%). It is hypothesized that the presence of Al3+ could inhibit the binding of lignin to cellulase, which was supported by the quantum chemistry calculations. Results revealed the presence of cation− π interactions between Al3+ and lignin with interaction energy (ΔE) was 1022.04 kJ/mol and 1003.04 kJ/mol for S and G type of lignin, respectively. In addition, the pretreatment with H2SO4 -AlCl3 co-catalyst could achieve a high lignin removal efficiency of 93.0% and almost complete enzymatic hydrolysis of obtained cellulose pulp (∼100%). This research demonstrated that the co-catalyst strategy effectively improved the overall performance of the solvent-based biorefinery.

Spatially Separated Redox Cocatalysts on Ferroelectric Nanoplates for Improved Piezophotocatalytic CO2 Reduction and H2O Oxidation.

Utilizing solar and mechanical vibration energy for catalytic CO2 reduction and H2O oxidation is emerging as a promising way to simultaneously generate renewable energy and mitigate climate change, making it possible to integrate two energy resources into a reaction system for artificial piezophotosynthesis. However, the practical applications are hindered by undesirable charge recombination and sluggish surface reaction in the photocatalytic and piezocatalytic processes. This study proposes a dual cocatalyst strategy to overcome these obstacles and improve the piezophotocatalytic performance of ferroelectrics in overall redox reactions. With the photodeposition of AuCu reduction and MnOx oxidation cocatalysts on oppositely poled facets of PbTiO3 nanoplates, band bending occurs along with the formation of built-in electric fields on the semiconductor-cocatalyst interfaces, which, together with an intrinsic ferroelectric field, piezoelectric polarization field, and band tilting in the bulk of PbTiO3, provide strong driving forces for the directional drift of piezo- and photogenerated electrons and holes toward AuCu and MnOx, respectively. Besides, AuCu and MnOx enrich the active sites for surface reactions, significantly reducing the rate-determining barrier for CO2-to-CO and H2O-to-O2 transformation, respectively. Benefiting from these features, AuCu/PbTiO3/MnOx delivers remarkably improved charge separation efficiencies and significantly enhanced piezophotocatalytic activities in CO and O2 generation. This strategy opens a door for the better coupling of photocatalysis and piezocatalysis to promote the conversion of CO2 with H2O.

A Cocatalyst Strategy to Enhance Ruthenium‐Mediated Metathesis Reactivity towards Electron‐Deficient Substrates

Ruthenium-mediated olefin metathesis has been widely applied for the synthesis of various organic molecules and polymers. Inspired by the cocatalyst strategy for olefin polymerization, here we demonstrate that the abstraction of a chloride ion from various commercially available ruthenium catalysts significantly enhances their reactivity towards electron-deficient internal olefins. This cocatalyst strategy can be implemented in ethenolysis and cross-metathesis reactions of FG-CH=CH-FG type substrates bearing electron-withdrawing groups and the synthesis of telechelic polymers that can be converted to polyethylene-like materials with closed-loop recycling properties. The copolymerization of cyclic substrate with cycloolefins followed by hydrogenation afforded polyolefin materials with in-chain break points. Interestingly, switchable catalysis was achieved in the absence and presence of a cocatalyst, which allowed the polymer microstructure and material properties to be fine-tuned.

A Cocatalyst Strategy to Enhance Ruthenium‐Mediated Metathesis Reactivity towards Electron‐Deficient Substrates

Ruthenium-mediated olefin metathesis has been widely applied for the synthesis of various organic molecules and polymers. Inspired by the cocatalyst strategy for olefin polymerization, here we demonstrate that the abstraction of a chloride ion from various commercially available ruthenium catalysts significantly enhances their reactivity towards electron-deficient internal olefins. This cocatalyst strategy can be implemented in ethenolysis and cross-metathesis reactions of FG-CH=CH-FG type substrates bearing electron-withdrawing groups and the synthesis of telechelic polymers that can be converted to polyethylene-like materials with closed-loop recycling properties. The copolymerization of cyclic substrate with cycloolefins followed by hydrogenation afforded polyolefin materials with in-chain break points. Interestingly, switchable catalysis was achieved in the absence and presence of a cocatalyst, which allowed the polymer microstructure and material properties to be fine-tuned.

A general cocatalyst strategy for performance enhancement in nickel catalyzed ethylene (co)polymerization

The tuning of olefin-polymerization catalyst properties through ligand modifications is efficient but requires complicated and costly syntheses. In this contribution, a simple Bu2Mg-based cocatalyst strategy is designed that can simultaneously enhance the catalytic properties (activity, thermal stability, polymer molecular weight, branching density, melting point, etc.) of various nickel catalysts (α-diimine nickel, pyridine imine nickel and iminopyridine-N-oxide nickel) in ethylene polymerization, and enable great product morphology control. For example, a simple α-diimine nickel catalyst can demonstrate polymerization activity of up to 1.29×107 g mol−1 h−1 and molecular weight of up to 1.90×106 g/mol in the presence of Bu2Mg cocatalyst. The resulting polyethylenes exhibit excellent mechanical properties, with tensile stress of up to 47.4 MPa and strain of up to 1020%. This cocatalyst strategy is generally applicable to different nickel catalysts, and can lead to property enhancement in ethylene copolymerization with a series of polar comonomers such as methyl 10-undecylenate, 10-undecylenic acid and 10-undecenol.

Rules for Selecting Metal Cocatalyst Based on Charge Transfer and Separation Efficiency between ZnO Nanoparticles and Noble Metal Cocatalyst Ag/ Au/ Pt

AbstractLoading metal cocatalyst on semiconductor nanoparticles is a general strategy to enhance photocatalytic efficiency, while the consensus of selecting metal cocatalyst matching with semiconductor is still unclear. Herein, we investigated the charge transfer and separation efficiency between ZnO and cocatalysts Ag, Au, and Pt nanoparticles respectively using ultrafast mid‐IR transient absorption spectroscopy. We found that Ohmic contact of Ag with ZnO favoring electron transfer and charge separation, while Schottky junction of Pt or Au with ZnO preventing electron transfer from ZnO to metal. Thus Ohmic contact would be better than Schottky contact. Since photocatalytic efficiency is also determined by chemical catalytic efficiency, we proposed a dual metal cocatalyst strategy for improving the overall photocatalytic efficiency, with the inner metal forming Ohmic contact for efficient charge separation and shuttling electrons and the outer‐layer metal cocatalyst for optimizing the chemical reactivity.

Noble metal free CdS@CuS-NixP hybrid with modulated charge transfer for enhanced photocatalytic performance

Constructing low-cost and highly active photocatalysts is extremely desirable for converting solar energy into chemical fuels. Herein, we report a one-dimensional (1D) CdS@CuS-NixP (CCN) core-shell heterostructure catalyst for effective photocatalytic hydrogen (H2) evolution with modulated charge transfer, which exhibits remarkably higher activity than bare CdS nanowires (NWs). The evenly distributed and closely contacted CdS@CuS (CC) heteroepitaxial nanomaterial constructs a type-I heterostructure for improved photoinduced charge carrier separation. The NixP nanoparticles (NPs), which act as a cocatalyst, not only contribute to constructing three-level charge transfer by capturing the photogenerated electrons, but also provide active sites for proton reduction. In addition, the cocatalyst strategy reported here can be extended to other transition metal phosphide (TMP). Mechanistic studies suggest that the cooperative synergy of three-level charge transfer and introduction of reaction sites results in a significant enhancement of photoactivity for H2 evolution.

Isotype heterojunction g-C3N4/g-C3N4 nanosheets as 2D support to highly dispersed 0D metal oxide nanoparticles: Generalized self-assembly and its high photocatalytic activity

Graphitic carbon nitride (g-C3N4) has attracted increasing interest in recent years as a metal-free polymeric photocatalyst for water splitting and degradation of organic pollutants. However, the relatively low photocatalytic activity and serious aggregation of g-C3N4 nanosheets substantially limit its applications. Herein, we propose a generalized and facile bottom-up approach to self-assembly synthesis of isotype heterojunction g-C3N4/g-C3N4 nanosheets/0D metal oxide nanoparticles hierarchical architectures, in which the highly dispersed 0D metal oxide (CeO2 and ZnO as representatives) nanoparticles are anchored on 2D flexible g-C3N4/g-C3N4 heterojunction nanosheets, and the former can be spatially separated by the latter as well. The pyrolysis-induced gases play a critical role in the construction of unique 0D metal oxide/2D g-C3N4/g-C3N4 (metal oxide/CN-UT) hierarchical architectures, which possess improved light harvesting and pollutant adsorption capacity, as well as enhanced separation of photoinduced charge carriers via the three-level electron–hole transfer process owing to the intimate contact between constituents and band alignment among them. As a result, the obtained metal oxide/CN-UT exhibits extraordinary photocatalytic performance under natural sunlight and indoor light illumination. This work demonstrates a generalized bottom-up synthetic method to design efficient composite photocatalysts by adopting integrative nanostructure engineering and a cocatalyst strategy and highlights the promising applications in the field of energy and environment.

Redox-Based Visible-Light-Driven Z-Scheme Overall Water Splitting with Apparent Quantum Efficiency Exceeding 10%

Summary For mimicry of natural photosynthesis, particulate-based Z-scheme overall water splitting (ZOWS) free of external bias is intriguing for hydrogen production, but it commonly confronts low efficiency with the huge challenge of charge transfer and separation. Here we report an assembly of unprecedentedly efficient ZOWS system with apparent quantum efficiency (AQE) of 10.3% at 420 nm using [Fe(CN)6]3−/[Fe(CN)6]4− as redox mediator. The breakthrough of AQE is mainly ascribed to an effective cocatalyst strategy that significantly promotes charge transfer and surface catalysis. Meanwhile, the metallic Au cocatalyst is first revealed to favor electron transfer from BiVO4 to [Fe(CN)6]3− shuttle ions, and site-selective deposition of Au and CoOx dual cocatalysts on the corresponding electron-rich {010} and hole-rich {110} facets of BiVO4 is crucial for promotion of water oxidation. This work highlights the importance of the cocatalyst strategy as well as site-selective deposition in accelerating charge separation and surface catalysis of artificial photocatalyst for enhanced solar-to-chemical energy conversion.

Constructing 2D graphitic carbon nitride nanosheets/layered MoS2/graphene ternary nanojunction with enhanced photocatalytic activity

Two-dimensional (2D) nanosheets materials have attracted extensive attention because of their promising practical application and theoretical values. In order to enhance the photocatalytic activity of bulk C3N4, we designed a 2D fabrication in which g-C3N4 nanosheets was in the middle of MoS2/graphene layered structure. Here, we report an effective strategy to synthesize ternary g-C3N4/MoS2/graphene nanocomposite photocatalyst via an in-situ adsorption method, which exhibits superior photocatalytic activity owing to enhanced charge carrier separation via well-contacted interface and fast charge transfer pathway. This work indicates a new insight into the design of such 2D heterostructure and a promising cocatalyst strategy for designing a more efficient g-C3N4-based semiconductor photocatalyst toward degradation of organic pollutants.

Improving Photocatalytic Performance from Bi2WO6@MoS2/graphene Hybrids via Gradual Charge Transferred Pathway

The charge transfer from the main catalyst to the cocatalyst is a key factor to enhance catalytic activity for photocatalytic nanocomposite materials. In order to enhance the charge transfer between Bi2WO6 and graphene, we inlet MoS2 as a “stepping-stone” into Bi2WO6 and graphene. Here, we report an effective strategy to synthesize ternary Bi2WO6@MoS2/graphene nanocomposite photocatalyst by a facile two-step hydrothermal method, which is afforded by assembling two cocatalysts, graphene and MoS2, into the Bi2WO6 matrix with a nanoparticle morphology as a visible light harvester. Compared with Bi2WO6/graphene, Bi2WO6/MoS2 and pure Bi2WO6, the Bi2WO6@MoS2/graphene ternary composites exhibit superior photocatalytic activity owing to an enhanced charge carrier separation via gradual charge transferred pathway. This work indicates a promising cocatalyst strategy for designing a more efficient graphene based semiconductor photocatalyst toward degradation of organic pollutants.

Progressive Design of Plasmonic Metal-Semiconductor Ensemble toward Regulated Charge Flow and Improved Vis-NIR-Driven Solar-to-Chemical Conversion.

Surface plasmon resonance (SPR)-mediated photocatalysis without the bandgap limitations of traditional semiconductor has aroused significant attention in solar-to-chemical energy conversion. However, the photocatalytic efficiency barely initiated by the SPR effects is still challenged by the low concentration and ineffective extraction of energetic hot electrons, slow charge migration rates, random charge diffusion directions, and the lack of highly active sites for redox reactions. Here, the tunable, progressive harvesting of visible-to-near infrared light (vis-NIR, λ > 570 nm) by designing plasmonic Au nanorods and metal (Au, Ag, or Pt) nanoparticle codecorated 1D CdS nanowire (1D CdS NW) ensemble is reported. The intimate integration of these metal nanostructures with 1D CdS NWs promotes the extraction and manipulated directional separation and migration of hot charge carriers in a more effective manner. Such cooperative synergy with tunable control of interfacial interaction, morphology optimization, and cocatalyst strategy results in the distinctly boosted performance for vis-NIR-driven plasmonic photocatalysis. This work highlights the significance of rationally progressive design of plasmonic metal-semiconductor-based composite system for boosting the regulated directional flow of hot charge carrier and thus the more efficient use of broad-spectrum solar energy conversion.

Spatial separation of the hydrogen evolution center from semiconductors using a freestanding silica-sphere-supported Pt composite.

Spatial separation of the reduction center (photosystem I) and oxidation center (photosystem II) is an obvious characteristic of natural photosynthesis. Enlightened by this natural process, a simple material based on silica-sphere-supported Pt nanoparticles (SSP) was designed as a freestanding hydrogen evolution center for semiconductor photocatalysts. In situ photoluminescence characterization showed that the radiation recombination of electron-hole pairs in semiconductors (i.e. TiO2 and CdS) was well suppressed due to the presence of SSP. And the quenching efficiency increases with the hydrogen evolution rate of photocatalysts. These results indicated that SSP could effectively trap electrons from the photoexcited semiconductors during collision between SSP and semiconductor, and then complete the hydrogen evolution reaction over the Pt nanoparticles. Detailed investigation also showed that the performance of SSP was influenced by several factors, including the particle size of the silica sphere and the physical and chemical states of Pt nanoparticles. Besides, it was demonstrated that the loaded metal was not limited to Pt. Pd, Ni and Au can also be used as active sites in this freestanding cocatalyst strategy.

Multifunctional photocatalytic performances of recyclable Pd-NiFe2O4/reduced graphene oxide nanocomposites via different co-catalyst strategy

In this report, magnetically recyclable, multifunctional Pd-NiFe2O4/reduced graphene oxide (Pd-NiFe2O4/rGO) photocatalysts have been successfully prepared by a one-step green hydrothermal method. The integration of NiFe2O4 with Pd nanoparticles supported on rGO enables efficient harvesting visible light for different catalytic reactions on the heterojunction structures. The Pd-NiFe2O4/rGO nanocomposites present significantly enhanced photocatalytic activity toward dyes degradation compared to the blank-NiFe2O4 and the optimum binary NiFe2O4/rGO, which is ascribed to the added Pd nanoparticles, acting as the mediator on the interfacial layer between NiFe2O4 and rGO, and transferring the charge carrier across the heterojunction interface. Another important role of Pd nanoparticles is as the electron reservoir, which can directly trap photogenerated electrons. It is interesting that the photogenerated electrons, which transfer to Pd nanoparticles, can increase the electron density of Pd. The energetic Pd nanoparticles thus raise their intrinsic catalytic activity, which can promote the Suzuki coupling under visible light irradiation. A possible catalytic mechanism for the multifunctional efficient photocatalytic performances is proposed.

Enhancing the visible light photocatalytic performance of ternary CdS–(graphene–Pd) nanocomposites via a facile interfacial mediator and co-catalyst strategy

By embedding noble metal palladium (Pd) into the interfacial layer matrix of graphene (GR) and semiconductor CdS, we have successfully constructed ternary CdS–(GR–Pd) nanocomposites with intimate interfacial contact. The CdS–(GR–Pd) nanocomposites show remarkably enhanced photocatalytic activity toward selective redox reactions under visible light irradiation as compared to blank-CdS and the optimum binary CdS–GR. It is revealed that the photocatalytic performance enhancement of CdS–(GR–Pd) is ascribed to the optimized spatial charge carrier transfer across the interface resulting from the introduction of Pd nanoparticles as mediators into the interfacial layer between GR and CdS. One role of Pd is to serve as electron reservoir to directly trap photogenerated electrons from CdS and the other role is as interfacial mediator to promote electron relay in the ternary CdS–(GR–Pd) photocatalysts along with conductive graphene as dual co-catalysts. Moreover, the negative light “shielding effect” of GR can be partially counterbalanced through such a facile strategy. This work substantiates the feasibility of adopting the “interfacial-mediator” strategy to optimize the interfacial charge carriers transfer pathway and efficiency for improved photoactivity of GR–semiconductor nanocomposites toward target photoredox reactions.

Visible-Light-Driven Oxidation of Primary C–H Bonds over CdS with Dual Co-catalysts Graphene and TiO2

Selective activation of primary C–H bonds for fine chemicals synthesis is of crucial importance for the sustainable exploitation of available feedstocks. Here, we report a viable strategy to synthesize ternary GR-CdS-TiO2 composites with an intimate spatial integration and sheet-like structure, which is afforded by assembling two co-catalysts, graphene and TiO2, into the semiconductor CdS matrix with specific morphology as a visible light harvester. The GR-CdS-TiO2 composites are able to serve as a highly selective visible-light-driven photocatalyst for oxidation of saturated primary C–H bonds using benign oxygen as oxidant under ambient conditions. This work demonstrates a wide, promising scope of adopting co-catalyst strategy to design more efficient semiconductor-based photocatalyst toward selective activation of C–H bonds using solar light and molecular oxygen.