Photocatalytic Proton Reduction Research Articles

Light-Driven Hydrogen Evolution Reaction Catalyzed by a Molybdenum-Copper Artificial Hydrogenase.

Orange protein (Orp) is a small bacterial metalloprotein of unknown function that harbors a unique molybdenum/copper (Mo/Cu) heterometallic cluster, [S2MoS2CuS2MoS2]3-. In this paper, the performance of Orp as a catalyst for the photocatalytic reduction of protons into H2 has been investigated under visible light irradiation. We report the complete biochemical and spectroscopic characterization of holo-Orp containing the [S2MoS2CuS2MoS2]3- cluster, with docking and molecular dynamics simulations suggesting a positively charged Arg, Lys-containing pocket as the binding site. Holo-Orp exhibits excellent photocatalytic activity, in the presence of ascorbate as the sacrificial electron donor and [Ru(bpy)3]Cl2 as the photosensitizer, for hydrogen evolution with a maximum turnover number of 890 after 4 h irradiation. Density functional theory (DFT) calculations were used to propose a consistent reaction mechanism in which the terminal sulfur atoms are playing a key role in promoting H2 formation. A series of dinuclear [S2MS2M'S2MS2](4n)- clusters, with M = MoVI, WVI and M'(n+) = CuI, FeI, NiI, CoI, ZnII, CdII were assembled in Orp, leading to different M/M'-Orp versions which are shown to display catalytic activity, with the Mo/Fe-Orp catalyst giving a remarkable turnover number (TON) of 1150 after 2.5 h reaction and an initial turnover frequency (TOF°) of 800 h-1 establishing a record among previously reported artificial hydrogenases.

A Stable Alkylated Cobalt Catalyst for Photocatalytic H2 Generation in Liposomes

AbstractPhotocatalytic proton reduction is a promising way to produce dihydrogen (H2) in a clean and sustainable manner, and mimicking nature by immobilising proton reduction catalysts and photosensitisers on liposomes is an attractive approach for biomimetic solar fuel production in aqueous solvents. Current photocatalytic proton reduction systems on liposomes are, however, limited by the stability of the catalyst. To overcome this problem, a new alkylated cobalt(II) polypyridyl complex (CoC12) was synthesised and immobilised on the lipid bilayer of liposomes, and its performance was studied in a photocatalytic system containing an alkylated ruthenium photosensitiser (RuC12) and a 1 : 1 mixture of sodium ascorbate and tris‐2‐carboxyethylphosphine hydrochloride as sacrificial electron donors. Several parameters (concentration of CoC12 and RuC12, pH, membrane composition) were changed to optimise the turnover number for H2 production. Overall, CoC12 was found to be photostable and the optimised turnover number (161) was limited only by the decomposition of the ruthenium‐based photosensitiser.

A cucurbit[8]uril-stabilized 3D charge transfer supramolecular polymer with a remarkable confinement effect for enhanced photocatalytic proton reduction and thioether oxidation

A water-soluble porous supramolecular polymer is assembled through a CB[8]-based 2 + 2 host–guest binding motif, which can greatly increase the efficiency of photocatalysis.

Dynamic Control of Photocatalytic Proton Reduction through the Mechanical Actuation of a Hydrogel Host Matrix.

This paper describes a photocatalytic hydrogen evolution system that is dynamically and reversibly responsive to the pH of the surrounding solution through the actuation of a microhydrogel (microgel) matrix that hosts the photocatalysts (CdSe/CdS nanorods). The reversible actuation occurs within 0.58 (swelling) and 1.7 s (contraction). ΔpH = 0.01 relative to the pKa of the tertiary amine on the microgel polymer (7.27) results in a reversible change in the average diameter of the microgel hosts by a factor of 2.4 and a change in the photocatalytic turnover frequency (TOF) by a factor of 5. Kinetic isotope effect and photoluminescence quenching experiments reveal that the scavenging of the photoexcited hole by sulfite ions is the rate-limiting step and leads to the observed response of the TOF to pH through the actuation of the microgel. Molecular dynamics simulations quantify a greater local concentration of sulfite hole scavengers for pH < pKa.

Towards synthetic unimolecular [Fe2S2]-photocatalysts sensitized by perylene dyes

A new set of perylene-based [Fe2S2(CO)6] photocatalyst dyads was synthesized and characterized. Four different strategies were employed to facilitate photoinduced electron transfer from the chromophore to the catalytically active site, increasing the chance for successful photocatalytic proton reduction. X-ray photoelectron spectroscopy measurements demonstrated the covalent attachment of the catalyst and provided insights on oxidation states of the iron. Visible light absorption maxima of the presented complexes reached up to 550 nm with reduction potentials between −1.14 V and −1.20 V because of increased electron density of the photoactive moiety. Quantum chemical calculations were performed to evaluate the employed design strategies based on the energy levels of the frontier orbitals and allowed to elucidate the electronic states involved in the initial photoactivation as well as to assess the charge transfer phenomena within the Franck-Condon point.

Ionic covalent organic nanosheet anchoring discrete copper for efficient quasi-homogeneous photocatalytic proton reduction

Covalent organic nanosheets can be applied potentially to solar-to-hydrogen fuel conversion because of their long-range ordered structure and predictable regulation. Additionally, minor progress has been made in the precise synthesis of copper-loaded composite catalysts at the atomic level. Hence, we synthesized hybrid Cu@TpTG-iCON materials by anchoring copper(II) ions into the evenly dispersed tridentate chelation sites of the TpTG-iCON precursor, which maximized the exposure of active sites during the catalytic reaction. Taking advantage of the well-defined structural configuration and nanosheet morphology, the heterogeneous Cu@TpTG-iCON catalyst exhibited an excellent hydrogen evolution reaction (HER) rate and significant stability in a ternary photocatalytic system. The atomic-level coordination modulation strategy in the covalent organic nanosheet matrix successfully demonstrated here has made possible the synthesis of other non-precious metal-based hybrid materials, indicating a step forward in the generation of a recyclable and efficient HER catalyst.

Design of noble metal-free CoTiO3/Zn0.5Cd0.5S heterostructure photocatalyst for selective synthesis of furfuraldehyde combined with H2 production

The development of photocatalytic systems composed of earth-abundant metal-based catalysts for efficient production of clean fuel, H2 as well as value-added chemicals is of significant importance towards sustainable generation of energy resources. Consequently, herein we report rational construction of Z-scheme CoTiO3/xZn0.5Cd0.5S (x = 5 (S1), 10 (S2), 15 (S3) and 20 wt% (S4)) heterostructures featuring suitable band structure for efficient photocatalytic reduction of protons of water to H2 combined with selective oxidation of furfuryl alcohol (biomass derivative) to a value-added product, furfuraldehyde. Electron microscopy analysis of heterostructure S2 revealed that Zn0.5Cd0.5S nanoparticles are decorated over the surface of CoTiO3 microrods. The photocatalytic studies showed higher catalytic performance by S2, for selective oxidation of furfuryl alcohol to furfuraldehyde with 95% yield coupled with a H2 generation rate of 1929 μmol g−1h−1 which is about 4-fold higher than that of pristine Zn0.5Cd0.5S. The enhanced catalytic performance of heterostructure S2 has been ascribed to synergistic interaction aided by the Z-scheme heterojunction formation between CoTiO3 and Zn0.5Cd0.5S. Overall, this work demonstrates the application of noble metal-free photocatalyst for simultaneous production of H2 and value-added chemical under mild and environment-friendly conditions.

Fundamental Characterization, Photophysics and Photocatalysis of a Base Metal Iron(II)-Cobalt(III) Dyad.

A new base metal iron‐cobalt dyad has been obtained by connection between a heteroleptic tetra‐NHC iron(II) photosensitizer combining a 2,6‐bis[3‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]pyridine with 2,6‐bis(3‐methyl‐imidazol‐2‐ylidene)‐4,4′‐bipyridine ligand, and a cobaloxime catalyst. This novel iron(II)‐cobalt(III) assembly has been extensively characterized by ground‐ and excited‐state methods like X‐ray crystallography, X‐ray absorption spectroscopy, (spectro‐)electrochemistry, and steady‐state and time‐resolved optical absorption spectroscopy, with a particular focus on the stability of the molecular assembly in solution and determination of the excited‐state landscape. NMR and UV/Vis spectroscopy reveal dissociation of the dyad in acetonitrile at concentrations below 1 mM and high photostability. Transient absorption spectroscopy after excitation into the metal‐to‐ligand charge transfer absorption band suggests a relaxation cascade originating from hot singlet and triplet MLCT states, leading to the population of the 3MLCT state that exhibits the longest lifetime. Finally, decay into the ground state involves a 3MC state. Attachment of cobaloxime to the iron photosensitizer increases the 3MLCT lifetime at the iron centre. Together with the directing effect of the linker, this potentially makes the dyad more active in photocatalytic proton reduction experiments than the analogous two‐component system, consisting of the iron photosensitizer and Co(dmgH)2(py)Cl. This work thus sheds new light on the functionality of base metal dyads, which are important for more efficient and sustainable future proton reduction systems.

Pd modified defective HNb3O8 with dual active sites for photocatalytic coproduction of hydrogen fuel and value-added chemicals

Developing photocatalysts for coproduction of hydrogen fuel and valuable chemicals is highly desired but still challenging. Herein, Pd-modified defective HNb3O8 photocatalyst (denote as Pd/HNb3O8-H2) consisting of dispersive Pd sites and abundant surface oxygen vacancies (VOs) was prepared via the thermal treatment of Pd2+-incorporated HNb3O8 in H2/Ar gas. The existence of Pd favors the formation of VOs on HNb3O8, enabling the synchronous construction of Pd and VO dual active sites. The spectroscopy and photoelectrochemical studies demonstrate the advantages of Pd/HNb3O8-H2 photocatalyst in promoting the separation and migration of photoinduced charge carries. The Pd/HNb3O8-H2 photocatalyst exhibits the outstanding performance toward photocatalytic proton reduction coupled with selective benzyl-alcohol oxidation, achieving the optimal production rates of H2 and benzaldehyde as high as 3.17 and 2.76 mmol·gcat−1 h−1, respectively. The experimental and theoretical investigations have expounded the synergistic mechanism of Pd and VO dual active sites in boosting photocatalytic coproduction of H2 and benzaldehyde.

Semiconductor nanocrystal photocatalysis for the production of solar fuels.

Colloidal semiconducting nanocrystals (NCs) are powerful elements of a photocatalytic system useful for enabling a variety of chemical transformations owing to their strong light-absorbing properties and high degree of size-, shape-, and composition-tunability. Key to their utility is our understanding of the photoinduced charge transfer processes required for these photochemical transformations. This Perspective will focus on the implementation of semiconductor NCs for photochemical fuel formation. Three general system designs for photocatalytic proton reduction using semiconductor NCs will be reviewed: metal-semiconductor heterostructures, NC photosensitizers with molecular catalysts, and hydrogenase-based systems. Other relevant reactions toward solar fuel targets, such as CO2 and N2 reductions with NCs, will also be highlighted. Illustrating the versatile roles that NCs can play in light-driven chemical reactions, advances made toward NC-catalyzed organic transformations will be discussed. Finally, we will share a few concluding thoughts and perspectives on the future of the field, with a focus on goals toward improving and implementing NC-based technologies for solar fuel development.

Promoting Photocatalytic Hydrogen Evolution Activity of Graphitic Carbon Nitride with Hole-Transfer Agents.

Visible light‐driven photocatalytic reduction of protons to H2 is considered a promising way of solar‐to‐chemical energy conversion. Effective transfer of the photogenerated electrons and holes to the surface of the photocatalyst by minimizing their recombination is essential for achieving a high photocatalytic activity. In general, a sacrificial electron donor is used as a hole scavenger to remove photogenerated holes from the valence band for the continuation of the photocatalytic hydrogen (H2) evolution process. Here, for the first time, the hole‐transfer dynamics from Pt‐loaded sol−gel‐prepared graphitic carbon nitride (Pt‐sg‐CN) photocatalyst were investigated using different adsorbed hole acceptors along with a sacrificial agent (ascorbic acid). A significant increment (4.84 times) in H2 production was achieved by employing phenothiazine (PTZ) as the hole acceptor with continuous H2 production for 3 days. A detailed charge‐transfer dynamic of the photocatalytic process in the presence of the hole acceptors was examined by time‐resolved photoluminescence and in situ electron paramagnetic resonance studies.

Metal-seed assistant photodeposition of platinum over Ta3N5 photocatalyst for promoted solar hydrogen production under visible light

Cocatalysts play a vital role in accelerating the reaction kinetics and improving the charge separation of photocatalysts for solar hydrogen production. The promotion of the photocatalytic activity largely relies on the loading approach of the cocatalysts. Herein, we introduce a metal-seed assistant photodeposition approach to load the hydrogen evolution cocatalyst of platinum onto the surface of Ta3N5 photocatalyst, which exhibits about 3.6 times of higher photocatalytic proton reduction activity with respect to the corresponding impregnation or photodeposition loading. Based on our characterizations, the increscent contact area of the cocatalyst/semiconductor interface with metal-seed assistant photodeposition method is proposed to be responsible for the promoted charge separation as well as enhanced photocatalytic H2 evolution activity. It is interesting to note that this innovative deposition strategy can be easily extended to loading of platinum cocatalyst with other noble or non-noble metal seeds for promoted activities, demonstrating its good generality. Our work may provide an alternative way of depositing cocatalyst for better photocatalytic performances.

Site- and Spatial-Selective Integration of Non-noble Metal Ions into Quantum Dots for Robust Hydrogen Photogeneration

Summary Artificial photosystems consisting of semiconductor quantum dots (QDs) and non-noble metal ions (e.g., Fe2+, Co2+, Ni2+) show intrinsic advantages in efficient and cost-effective hydrogen (H2) evolution. However, well-controlled integration of these metal ions into ultra-small QDs is a major challenge. Here, we present a two-step strategy to realize site- and spatial-selective integration of earth-abundant Fe2+ ions in CdSe QDs; i.e., CdSe QDs are modified with a thin and anisotropic ZnS shell, and partial Zn2+ ions are selectively substituted by Fe2+ via cation exchange. The anisotropic ZnS layer not only passivates CdSe core but also works as an ideal medium to intimately anchor Fe2+. The multifunctional CdSe/Zn1−xFexS QD exhibits extraordinary activity and stability toward H2 photogeneration. Specifically, more than 880 mL (∼39,300 μmol) H2 gas is produced from 6 mL of aqueous solution during a consecutive 172 h (>1 week) experiment, thus achieving a turnover number of >600,000 per QD.

Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye.

Mesoporous NiO photocathodes containing the push–pull dye PB6 and alkyl-derivatized cobaloxime catalysts were prepared using surface amide couplings and analyzed for photocatalytic proton reduction catalysis. The length of the alkyl linker used to derivatize the cobalt catalysts was found to correlate to the photocurrent with the highest photocurrent observed using shorter alkyl linkers but the lowest one for samples without linker. The alkyl linkers were also helpful in slowing dye–NiO charge recombination. Photoelectrochemical measurements and femtosecond transient absorption spectroscopic measurements suggested electron transfer to the surface-immobilized catalysts occurred; however, H2 evolution was not observed. Based on UV–vis, X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS) measurements, the cobalt catalyst appeared to be limiting the photocathode performance mainly via cobalt demetallation from the oxime ligand. This study highlights the need for a deeper understanding of the effect of catalyst molecular design on photocathode performance.

Water structure near the surface of Weyl semimetals as catalysts in photocatalytic proton reduction.

In this work, second-generation Car–Parrinello-based mixed quantum-classical mechanics molecular dynamics simulations of small nanoparticles of NbP, NbAs, TaAs, and 1T-TaS2 in water are presented. The first three materials are topological Weyl semimetals, which were recently discovered to be active catalysts in photocatalytic water splitting. The aim of this research was to correlate potential differences in the water structure in the vicinity of the nanoparticle surface with the photocatalytic activity of these materials in light induced proton reduction. The results presented herein allow explaining the catalytic activity of these Weyl semimetals: the most active material, NbP, exhibits a particularly low water coordination near the surface of the nanoparticle, whereas for 1T-TaS2, with the lowest catalytic activity, the water structure at the surface is most ordered. In addition, the photocatalytic activity of several organic and metalorganic photosensitizers in the hydrogen evolution reaction was experimentally investigated with NbP as the proton reduction catalyst. Unexpectedly, the charge of the photosensitizer plays a decisive role for the photocatalytic performance.

Ruthenium Complex-Incorporated Two-Dimensional Metal-Organic Frameworks for Cocatalyst-Free Photocatalytic Proton Reduction from Water.

Ultrathin two-dimensional (2D) nanosheets with efficient light-driven proton reduction activity were obtained through the exfoliation of novel metal-organic frameworks (MOF), which were synthesized by using a bis(4'-carboxy-2,2':6',2″-terpyridine) ruthenium complex as a linker and 3d transition-metal (Mn, Co, Ni, and Zn) anions as nodes. The nanosheet of the Ni2+ node exhibits a photocatalytic hydrogen evolution rate of 923 ± 40 μmol g-1 h-1 at pH = 4.0, without the presence of any cocatalyst or cosensitizer. A combined experimental and theoretical study suggests a reductive quenched pathway for the photocatalytic hydrogen evolution by the nanosheet. The transition-metal nodes at the edge of the nanosheets are proposed as the active sites. Density functional theory (DFT) calculations attributed the different catalytic activities of the nanosheets to the discrepancy of H adsorption free energy at various transition-metal nodes.

Photocatalytic proton reduction by a computationally identified, molecular hydrogen-bonded framework

A hydrogen-bonded organic framework is an effective photocatalyst for producing hydrogen from water. Its crystal structure is key to its activity; a chemically identical, amorphous version is almost inactive, as rationalized by crystal structure prediction.

Type-II heterostructures of α-V2O5 nanowires interfaced with cadmium chalcogenide quantum dots: Programmable energetic offsets, ultrafast charge transfer, and photocatalytic hydrogen evolution.

We synthesized a new class of heterostructures by depositing CdS, CdSe, or CdTe quantum dots (QDs) onto α-V2O5 nanowires (NWs) via either successive ionic layer adsorption and reaction (SILAR) or linker-assisted assembly (LAA). SILAR yielded the highest loadings of QDs per NW, whereas LAA enabled better control over the size and properties of QDs. Soft and hard x-ray photoelectron spectroscopy in conjunction with density functional theory calculations revealed that all α-V2O5/QD heterostructures exhibited Type-II band offset energetics, with a staggered gap where the conduction- and valence-band edges of α-V2O5 NWs lie at lower energies (relative to the vacuum level) than their QD counterparts. Transient absorption spectroscopy measurements revealed that the Type-II energetic offsets promoted the ultrafast (10-12-10-11 s) separation of photogenerated electrons and holes across the NW/QD interface to yield long-lived (10-6 s) charge-separated states. Charge-transfer dynamics and charge-recombination time scales varied subtly with the composition of heterostructures and the nature of the NW/QD interface, with both charge separation and recombination occurring more rapidly within SILAR-derived heterostructures. LAA-derived α-V2O5/CdSe heterostructures promoted the photocatalytic reduction of aqueous protons to H2 with a 20-fold or greater enhancement relative to isolated colloidal CdSe QDs or dispersed α-V2O5 NWs. The separation of photoexcited electrons and holes across the NW/QD interface could thus be exploited in redox photocatalysis. In light of their programmable compositions and properties and their Type-II energetics that drive ultrafast charge separation, the α-V2O5/QD heterostructures are a promising new class of photocatalyst architectures ripe for continued exploration.

Size dependence of photocatalytic hydrogen generation for CdTe quantum dots.

CdTe quantum dots (QDs) are attractive photosensitizers for photocatalytic proton reduction due to their broad absorbance profile that can extend from the ultraviolet to near-infrared regions, providing access to a larger portion of the solar spectrum than possible with analogous CdSe and CdS QD photosensitizers. Here, the photocatalytic hydrogen (H2) generation from various sizes of dihydrolipoic acid (DHLA)-capped CdTe QDs, ranging from 2.5 to 7.5 nm in diameter, and a molecular Ni-DHLA catalyst in aqueous solutions was evaluated, and an unusual size-dependent photocatalytic activity with CdTe QDs was observed. Under optimized conditions, using 3.4 nm CdTe-DHLA and a 1:20 ratio of QD/Ni-DHLA catalyst, as many as 38 000 turnover numbers (mol H2 per mol QD) were achieved. However, below this critical size, the H2 production efficiency decreased; this behavior is attributed to the rapid oxidation of the QD surface, resulting in detrimental surface trap states. These results are consistent with ultrafast transient absorption spectroscopic measurements, which suggest the presence of extremely fast charge-trapping processes in the oxidized CdTe-DHLA QDs. While fast electron transfer from CdTe-DHLA QDs is observed in the presence of the Ni-DHLA catalyst, the charge trapping processes occur on a competitive time scale, thus lowering the efficiency of the CdTe/Ni-DHLA H2 production system. Understanding rapid charge trapping in CdTe QDs may help suggest potential improvements for the overall CdTe photocatalytic system.

Visible light-induced conversion of biomass-derived chemicals integrated with hydrogen evolution over 2D Ni2P–graphene–TiO2

Highly selective photocatalytic conversion of biomass-derived chemicals into value-added chemicals and clean hydrogen energy under mild conditions by simultaneously utilizing photogenerated holes and electrons is in line with the theme of sustainable development of green chemistry. Herein, a two-dimensional (2D) heterostructure of ternary Ni2P–graphene–lepidocrocite TiO2 (NPG–TiO2) with intimately interfacial contact has been successfully fabricated, which exhibits significantly elevated photocatalytic performance toward coevolution of benzaldehyde (2.17 mmol/h/g) and hydrogen (1.97 mmol/h/g) from biomass-derived benzyl alcohol in aqueous solution under the visible light irradiation. Mechanistic studies reveal that in this ternary heterostructure, graphene serves as an electron relay mediator to facilitate the flow of electrons from TiO2 to Ni2P due to its inherent electrical conductivity, and the Ni2P provides the active sites for photocatalytic proton reduction to hydrogen. It is anticipated that this work would make a contribution to rational design of 2D flat-structured multi-component photocatalysts for selective conversion of biomass-derived chemicals coupled with hydrogen evolution.