Water Reduction Catalyst Research Articles

Oxygen octahedral void-confined iodine single-site RuO2 catalysts for water reduction

Iodine (I), with abundant extra-nuclear electrons and high electronegativity, can enhance the performance of catalysts by electronic tuning effects of I single-site. However, I atoms are always located at shallow surfaces of catalysts, leading to fast volatilization and subsequent poor catalytic stability. Herein, we conceive a single-site I doped RuO2 catalyst via heating RuI3 in air, wherein the I concentration gradually increases from shallow to deep surfaces. This ensures that partial I atoms are located in oxygen octahedral voids of RuO2 as single-site at deep surfaces of catalyst, avoiding I loss during catalysis. Impressively, I-introduction decreases the valence of Ru and strengthens the adsorption of *H on Ru actives, and then superior hydrogen evolution activity (14 mV @ 10 mA cm−2) is achieved in alkaline media. Additionally, both the confined effect of oxygen octahedral voids on I single atoms and stable I-O bonds make the catalyst durable for 50 h.

Light-driven hydrogen evolutionviaa novel pincer/no pincer mechanism including a possible concerted proton electron transfer

New water reduction catalysts containing secondary amines in the backbones show turnover numbers for photochemical hydrogen evolution up to 2237.

Electrochemistry in the Teaching Laboratory to Synthesize and Evaluate Catalysts for Water Oxidation and Reduction

Electrochemical analysis is an important skill to teach in chemistry curricula because it is a critical tool in current high-impact chemical research. Electrochemistry enables researchers to analyze a variety of systems extending from molecules to materials that encompass research themes ranging from clean energy to substrate activation in biological systems. Specifically, it can reveal information about catalytic efficiency, the role of catalysts, and the nature of chemical reduction and oxidation (redox) processes. Researchers working in the area of catalysis rely heavily on electrochemistry, using it to identify effective catalysts and to optimize reaction conditions. This talk with describe a laboratory experiment designed for undergraduate chemistry majors. The laboratory experiment begins with an introductory tutorial to electrochemistry by guiding students through the use of several electrochemical techniques using the ferricyanide/ferrocyanide redox couple as a model system. The techniques covered in the tutorial include cyclic voltammetry, differential pulse voltammetry, Tafel analysis, and bulk electrolysis. The protocol then applies these techniques to electrocatalysis by identifying and characterizing catalysts for hydrogen and oxygen generation in the water electrolysis reaction. Electrochemical methods are connected to current chemical research by focusing on catalysis in the context of renewable energy, which is a current societal and curricular imperative. The entire laboratory protocol is suitable for upper-level physical, analytical or inorganic chemistry. Figure 1

WO3 dehydration and phase transition as the catalytic driver of hydrogen production by non-calcinated WO3

In this study, WO3 (trioxide tungsten) was investigated as a catalyst for water reduction by NaBH4 in basic conditions, which are not typical for NaBH4 reduction processes. The results indicate that the structure of the WO3 phase strongly affects the catalytic process. The uncalcinated WO3, ncWO3, which contains primarily tungstite with traces of hydro-tungstite is the only WO3 form that acts as an effective catalyst. A cubic WO3 is formed or exposed during the reduction process. Hydrogen production increases during reaction cycles; an interesting and important result. Plausible mechanisms of the catalytic process observed are outlined.The results have far-reaching effects that should be further investigated in other systems, such as the phase structures acting as a catalyst for treating toxic organic compounds.

Effects of different dithiolate ligands on electrocatalytic hydrogen production of nickel complexes in acetic acid or water

In order to solve the problems of fossil energy depletion and environmental pollution, it is necessary to find new energy sources that are recyclable and environmentally friendly. Among them, hydrogen is a high calorific value, clean and sustainable energy. At present, hydrogen is mainly obtained by electrolysis of water. The use of electrocatalysts in this process can effectively reduce the consumption of electrical energy. In this study, three molecular electrocatalysts based on nickel complexes, [BzTPP]2[Ni(mnt)2](1), [BzTPP]2[Ni(i-mnt)2](2), and [BzTPP]2[Ni(tdas)2](3) ([BzTPP]+ = 1-benzyltriphenylphosphinium, mnt = 1,2-dicyanoethylene-1,2-dithiolate, i-mnt = 1,1-dicyanoethylene-2,2-dithiolate and, and tdas = 1,2,5-thiadiazole-3,4-dithiolate), were systemically investigated. Electrochemical study showed three complexes can electrocatalyze hydrogen generation from acetic acid or aqueous buffer solution, and afford a turnover frequency (TOF) of 504.76, 591.52 and 1022.93 mol of hydrogen per mole of catalyst per hour at an overpotential (OP) of 0.838 V from a neutral buffer. It is shown that [NiII(tdas)2]2−exhibits higher electro-catalytic hydrogen production activity than [NiII(mnt)2]2− and [NiII(i-mnt)2]2−. This provides a new chemical paradigm for the design and fabrication of efficient molecular catalysts for water reduction.

Supramolecular Co(II) complexes based on dithiolate and dicarboxylate ligands: Crystal structures, theoretical studies, magnetic properties, and catalytic activity studies in photocatalytic hydrogen evolution

Two supramolecular Co(II)-based coordination compounds have been harvested from dithiolate and dicarboxylate ligands. They have been structurally characterized by FT-IR, elemental analysis, and single crystal X-ray diffraction. Salt 1 consists of two ([tris (2-aminoethyl) amine Co] 1,1-dicyano-2,2-ethylenedithiolate)+ per thiosulphate ion linked through intermolecular hydrogen bonds. Compound 2, (pyridine-2,6-dicarboxylate)2Co-5H2OCo).2H2O, is stabilized by intramolecular O−H···O hydrogen bonds, building ribbons that propagate along the [100] direction in the crystals. From HS analysis, it is observed that the major non-covalent interactions present in 1 are C−H···O, N−H···N, N−H···S, and N−H···O hydrogen bonds, which play an important role in stabilizing the crystal structure. In 2, out of all the non-covalent interactions, O···H/H···O interactions have major contributions to stabilize the crystal structure. Theoretical investigation on the molecular structures of the crystals also revealed that the major stabilizing factor for 1 is H-bonding along with π-π stacking while that for 2 is co-ordinate bond between water and cobalt(II) ion. Direct current variable-temperature magnetic susceptibility measurements carried out on polycrystalline samples of 1 and 2 in the temperature range of 1.8–300 K shows the presence of magnetic anisotropy of the cobalt(II) ion in 1 and weak intermolecular exchange in 2. Further, both the compounds 1 and 2 are found to be highly efficient water reduction catalysts in terms of per Co turn-over-numbers at lower concentrations.

A {Co9}-Added Polyoxometalate for Efficient Visible-Light-Driven Hydrogen Evolution

A polyanion cluster H6Na8Cs3[Co9(μ3-OH)3(H2O)6(HPO4)2(B-α-PW9O34)3]Cl·40H2O (1) was made with the guidance of the lacunary directing strategy under hydrothermal conditions. Compound 1 was characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and thermogravimetric analysis, respectively. Single-crystal X-ray diffraction analyses showed that 1 consists of three anions [B-α-PW9O34]9- and a cyclic cationic [Co9(μ3-OH)3(H2O)6]15+ and two anions HPO42-. Variable-magnetic properties indicate antiferromagnetic interactions in 1. Visible-light-driven hydrogen evolution tests demonstrated that 1 was an efficient water reduction catalyst with an H2 evolution rate of 1217.6 μmol h-1 g-1.

The influence of trinuclear complexes on light-induced hydrogen production

New trinuclear water reduction catalysts show turnover numbers for photochemical water splitting of up to 8899, a turnover frequency of 2737 h−1 and an incident photon conversion efficiency of 2.1%, thus outperforming mononuclear analogues.

Identifying and Interpreting Geometric Configuration-Dependent Activity of Spinel Catalysts for Water Reduction.

The catalytic activity of transition metal-based catalysts is overwhelmingly dependent on the geometric configuration. Identification and interpretation of different geometric configurations' contributions to catalytic activity plays a pivotal role in catalytic performance elevation. Spinel structured AB2X4, consisting of tetrahedral (A2+-X)Td and octahedral (B3+-X)Oh geometric configurations, is a prototypical category of multi-geometric-configuration featured catalysts. However, it is still under debate about the predominant geometric configuration responsible for spinel catalyst activity, and the mechanistic origin of specific activity discrepancy among varied geometric configurations also remains ambiguous. Herein, CoTd2+ and CoOh3+ in Co3O4 are replaced by catalytically inert Zn2+ and Al3+ to yield ZnCo2O4 and CoAl2O4, respectively, thus ensuring the manipulable exposure of monotypic active configurations. By means of pulse voltammetry and in situ extended X-ray absorption fine structure, (Co3+-O)Oh is identified to be dominant for alkaline HER. In-depth theoretical investigation in combination with X-ray absorption spectroscopy further interprets the synergistic effect between Co and O sites in (Co3+-O)Oh configuration on water reduction kinetics upon both water dissociation and hydrogen desorption steps. Furthermore, specific facet dependence of catalytic activity is also deciphered based on precise facet exposure identification and serial theoretical analysis. This work unambiguously figures out the subtle geometric configuration dependence of spinel catalyst activity for water reduction and highlights the synergistic relationship among different components confined in geometric configuration, thereby shedding new light on the rational design of advanced catalysts from the atomic level of geometric configuration optimization.

Ligand Substituent Effects on Light‐Driven Hydrogen Evolution by Cobalt(II) Tripodal Iminopyridine Catalysts under Precious‐Metal‐Free Conditions

AbstractCobalt tripodal iminopyridine complexes [Co(L)]2+ (1–4), where L=cis,cis,–1,3,5‐tris(pyridine‐2‐carboxaldimino)‐cyclohexane (1) and its derivatives, bearing substituents of varying electronic properties at the pyridine moieties, act as water reduction catalysts (WRCs) under visible light (λ>420 nm) in aqueous acetonitrile (2.5 %, v/v), using tri(n‐butyl)triazatriagulenium (TATA+) as the organic photosensitizer and triethylamine (TEA) as the sacrificial donor. In the presence of added acetic acid, enhanced turnover numbers (TONs) of 1430–2810 were recorded with the substituted catalysts (2–4), which are attributed to the varied catalytic potential (Ecat) upon the introduction of the substituents.

Two Novel Dinuclear Cobalt Polypyridyl Complexes in Electro- and Photocatalysis for Hydrogen Production: Cooperativity Increases Performance.

Syntheses and mechanisms of two dinuclear Co‐polypyridyl catalysts for the H2 evolution reaction (HER) were reported and compared to their mononuclear analogue (R1). In both catalysts, two di‐(2,2’‐bipyridin‐6‐yl)‐methanone units were linked by either 2,2’‐bipyridin‐6,6’‐yl or pyrazin‐2,5‐yl. Complexation with CoII gave dinuclear compounds bridged by pyrazine (C2) or bipyridine (C1). Photocatalytic HER gave turnover numbers (TONs) of up to 20000 (C2) and 7000 (C1) in water. Electrochemically, C1 was similar to the R1, whereas C2 showed electronic coupling between the two Co centers. The E(CoII/I) split by 360 mV into two separate waves. Proton reduction in DMF was investigated for R1 with [HNEt3](BF4) by simulation, foot of the wave analysis, and linear sweep voltammetry (LSV) with in‐line detection of H2. All methods agreed well with an (E)ECEC mechanism and the first protonation being rate limiting (≈104 m −1 s−1). The second reduction was more anodic than the first one. pK a values of around 10 and 7.5 were found for the two protonations. LSV analysis with H2 detection for all catalysts and acids with different pK a values [HBF4, pK a(DMF)≈3.4], intermediate {[HNEt3](BF4), pK a(DMF)≈9.2} to weak [AcOH, pK a(DMF)≈13.5] confirmed electrochemical H2 production, distinctly dependent on the pK a values. Only HBF4 protonated CoI intermediates. The two metals in the dualcore C2 cooperated with an increase in rate to a competitive 105 m −1 s−1 with [HNEt3](BF4). The overpotential decreased compared to R1 by 100 mV. Chronoamperometry established high stabilities for all catalysts with TONlim of 100 for R1 and 320 for C1 and C2.

Clamshell Palladium(II) Complexes: Suitable Precursors for Photocatalytic Hydrogen Production from Water

AbstractA series of carboxylato‐bridged dinuclear palladium(II) complexes with cyclopalladated 2‐phenylbenzothiazole ligands were prepared and characterized by elemental analyses, NMR, IR, and UV‐vis spectroscopies, cyclic voltammetry, and X‐ray crystallography. All the complexes have clamshell structures in which two cyclopalladated 2‐phenylbenzothiazole ligands bound to palladium atoms are in the anti‐configuration arranging the ligands in opposite orientations from each other. The Pd−Pd distances of these complexes are shorter than the sum of the van der Waals radii, indicative of the presence of metal‐metal bonding interactions. These dinuclear complexes were used as water reduction catalysts in the photocatalytic system for hydrogen evolution and regarded as precatalysts that are converted into highly catalytically active species.

Pd2Ga nanorods as highly active bifunctional catalysts for electrosynthesis of acetic acid coupled with hydrogen production

The production of hydrogen from water splitting is hampered by the sluggish oxygen evolution reaction (OER). To overcome the OER limitation, herein we propose the electrosynthesis of value-added acetic acid from ethanol as the anodic reaction using high activity catalyts. For this strategy to be cost-effective, we develop a bifunctional catalyst based on Pd2Ga nanorods. Such Pd2Ga/C-based catalyst presents outstanding activity, selectivity and also stability for the electrocatalytic ethanol-to-acetic acid conversion with a current density above 164 mA cm−2 and a mass activity of 1.97 A mg−1Pd. Besides, its activity for hydrogen production is comparable to that of commercial Pt/C catalysts. Using Pd2Ga/C as a bifunctional catalyst for both water reduction at the cathode and ethanol oxidation at the anode, these two coupled reactions are demonstrated to be an energy-efficient approach for the simultaneous production of high purity acetic acid and hydrogen. The assembled electrolyzer requires a small voltage input of 0.62 V to reach a current density of 10 mA cm−2, much lower than that of cells based on commercial Pt/C or Pd/C catalyst. Density functional theory calculations reveal that the high performance of the coupled system relies on a combination of an electronic and bifunctional effect of Ga, reducing the hydrogen-binding energy on the Pd site and at the same time actively participating in the reaction by providing OH− binding sites and reducing the energy barrier of the ethanol oxidation rate-determining step.

Distinct Bimetallic Cooperativity Among Water Reduction Catalysts Containing [CoIII CoIII ], [NiII NiII ], and [ZnII ZnII ] Cores.

Three binuclear species [LCoIII 2 (μ-Pz)2 ](ClO4 )3 (1), [LNiII 2 (CH3 OH)2 Cl2 ]ClO4 (2), and [LZnII 2 Cl2 ]PF6 (3) supported by the deprotonated form of the ligand 2,6-bis[bis(2-pyridylmethyl) amino-methyl]-4-methylphenol were synthesized, structurally characterized as solids and in solution, and had their electrochemical and spectroscopic behavior established. Species 1-3 had their water reduction ability studied aiming to interrogate the possible cooperative catalytic activity between two neighboring metal centers. Species 1 and 2 reduced H2 O to H2 effectively at an applied potential of -1.6 VAg/AgCl , yielding turnover numbers of 2,820 and 2,290, respectively, after 30 minutes. Species 3 lacked activity and was used as a negative control to eliminate the possibility of ligand-based catalysis. Pre- and post-catalytic data gave evidence of the molecular nature of the process within the timeframe of the experiments. Species 1 showed structural, rather than electronic cooperativity, while species 2 displayed no obvious cooperativity. DFT methods complemented the experimental results determining plausible mechanisms.

An infinite chain, {[Ni(tn)2]3[Fe(CN)4(μ-CN)2]2}n, a new catalyst for electrochemical-driven water splitting and photochemical-driven hydrogen evolution from water under blue light

A new catalyst for both water reduction and oxidation, based on an infinite chain, {[Ni(tn)2]3 [Fe(CN)4 (μ-CN)2]2}n, is formed by the reaction of NiCl2, 1,3-propanediamine (tn) and K3 [Fe(CN)6]. {[Ni(tn)2]3 [Fe(CN)4 (μ-CN)2]2}n can electro-catalyze hydrogen evolution from a neutral aqueous buffer (pH 7.0) with a turnover frequency (TOF) of 1561 mol of hydrogen per mole of catalyst per hour (H2/mol catalyst/h) at an overpotential (OP) of 837 mV {[Ni(tn)2]3 [Fe(CN)4 (μ-CN)2]2}n also can electro-catalyze O2 production from water with a TOF of ∼45 mol O2 (mol cat)−1s−1 at an OP of 591 mV. Under blue light (λ = 469 nm), together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H2A) as a sacrificial electron donor, {[Ni(tn)2]3 [Fe(CN)4 (μ-CN)2]2}n can photo-catalyze hydrogen generation from an aqueous buffer (pH 4.0) with a turnover number (TON) of 11,450 mol H2 per mole of catalyst (mol of H2 (mol of cat)−1) during 10 h irradiation. The average of apparent quantum yield (AQY) is as high as 40.96% during 10 h irradiation. Studies indicate that {[Ni(tn)2]3 [Fe(CN)4 (μ-CN)2]2}n exists in two forms: a cyano-bridged chain ({[Ni(tn)2]3 [Fe(CN)4 (μ-CN)2]2}n) in solid, and a salt ([Ni(tn)2]3 [Fe(CN)6]2) in aqueous media; Catalytic reaction occurs on the nickel center of [Ni(tn)2]2+, and the introduction of [Fe(CN)6]3- can improve the catalytic efficiency of [Ni(tn)2]2+ for H2 or O2 generation. We hope these findings can afford a new method for the design of catalysts for both water reduction and oxidation.

Insights into Proton Coupled Electron Transfer in the Field of Artificial Photosynthesis

AbstractArtificial photosynthesis with respect to water splitting is usually divided into water oxidation catalysis (WOC) and the hydrogen evolution reaction (HER). Though in the combined redox dissociation of water into oxygen and hydrogen no protons and electrons occur, both half reactions show photoinduced proton coupled electron transfer (PCET). Regarding a classical approach, photosensitizers (PS) deliver electrons and protons that are accepted by water reduction catalysts (WRC) containing suitable basic atoms like nitrogen working as proton relays. However, the mechanisms of PCET reactions differ, where concerted proton electron transfer (CPET) is an elementary step. In CPET simultaneous electron and proton transfer occurs in the femtoseconds range, being rapid when compared to the periods for coupled vibrations and solvent modes. This has to be distinguished from stepwise electron and proton transfer, leading to underlying thermodynamics of the intermediates. DFT calculations based on X‐ray diffraction (XRD) data help to specify the different reaction pathways. A plethora of experimental procedures are used in order to verify the theoretical predictions. Among them femtosecond pump‐probe spectroscopic measurements play an important role. Furthermore, cyclic voltammetry (CV) has proven to be also a powerful tool. In the case of electrochemical PCET rotating‐disk electrode voltammetry, electrochemical impedance spectroscopy and spectroelectrochemistry complete the experimental tools. In this minireview a selection of examples, where PCET occurs is discussed with respect to possible mechanisms and used methods.

Pyrogallol[4]arene Coordination Nanocapsule Micelle as Bioinspired Water Reduction Catalyst

Pyrogallol[4]arene Coordination Nanocapsule Micelle as Bioinspired Water Reduction Catalyst

Engineering NiF3/Ni2P heterojunction as efficient electrocatalysts for urea oxidation and splitting

Exploring highly efficient and ideal stability electrocatalysts for urea oxidation reaction (UOR) is highly desired to achieve hydrogen production by an economic way. Herein, a novel flexible, self-supported NiF3-Ni2P heterostructure in-situ grown on carbon cloth (NiF3/Ni2P@CC) is controllable synthesized by hydrothermal and subsequent successive fluorination-phosphorization method. The as-obtained optimized NiF3/Ni2P@CC-2 is featured by large amounts of high valent Ni3+ active sites, reconfigured local charge density, superior hydrophilic surface and mesoporous property. Thanks to these merits, the heterojunction electrode demonstrates advanced UOR performance compared to pure NiF3@CC and Ni2P@CC, merely needs a potential of 1.36 V vs. RHE to deliver a current density of 10 mA cm−2 and keeps the catalytic activity after 5,000 cyclic voltammetry tests. Significantly, NiF3/Ni2P@CC-2 hybrid can be utilized as bi-functional catalysts for UOR and water reduction (HER). A symmetrical urea electrolyzer constructed by NiF3/Ni2P@CC-2 as anode and cathode offers 50 mA cm−2 at a potential of 1.83 V, and can work continuously for more than 10 h. This work provides an advanced UOR electrocatalyst and gives great promising for designing heterojunction catalysts for H2 generation in a more economic way.

(Invited) Surface Modification of Dye-Sensitized Hydrogen-Evolving Photocatalyst for Z-Scheme Solar Water Splitting

Z-scheme water splitting system composed of water reduction and oxidation photocatalysts coupled with redox-reversible electron mediator has attracted considerable attention because of the promising activity for water splitting under visible light irradiation. To improve the activity of Z-scheme water-splitting photocatalyst, how to achieve the one-way electron transfer from O2 evolution photocatalyst to H2 evolution photocatalyst through electron mediator is crucial. Dye-sensitization is one of the promising approaches not only to enhance the light absorption ability but also to improve the charge separation efficiency at the dye-semiconductor interface. We recently found that the multilayering of Ru(II) dyes on the surface of water reduction catalyst Pt-TiO2 nanoparticle (nRu@Pt-TiO2, n = number of layers of Ru(II) dyes) significantly improved the photocatalytic H2 evolution activity.[1] In this work, to enhance the electron donation from iodide, we have synthesized two different 2Ru@Pt-TiO2 nanoparticles with different surface structures: P-2Ru@Pt-TiO2 having phosphate groups on the surface and ZrP-2 Ru@Pt-TiO2 with Zr4+-phosphate groups on the surface as well as our previously reported 2Ru@Pt-TiO2 without any surface phosphonates.[2] Figure shows the results of photocatalytic H2 evolution reactions in 0.5 M aqueous KI solutions of three kinds of dye-double-layered nanoparticle photocatalysts with different surface structures. The amount of H2 produced after 6 h of light irradiation increased in the following order: 2Ru@Pt-TiO2 < P-2Ru@Pt-TiO2 < ZrP-2Ru@Pt-TiO2 . Notably, the AQY of ZrP-2Ru@Pt-TiO2 in the first hour of reaction was approximately 1%. The Ru(III)/Ru(II) redox potential of Ru(II)dye with phosphonic acid groups was reported to be shifted positively to less than 0.1 V by immobilization to the TiO2 surface or by coordination bond formation with Zr4+ cations[3], suggesting that the binding of Zr4+ to the phosphonic acid groups of Ru(II) dye has little effect on the driving force of the electron donation reaction from the iodide. The zeta potential of ZrP-2Ru@Pt-TiO2 was estimated to be +7 mV without KI, and it negatively shifted to −6 mV in the presence of 0.5 M KI. On the other hand, the shift of P-2Ru@Pt-TiO2 was estimated to be smaller (from −7 to −12 mV). The negative shifts in the 0.5 M KI aqueous solution could be ascribed to the attraction of iodide anions to the nanoparticle surface by electrostatic interactions with the surface Zr4+ cations in the case of ZrP-2Ru@Pt-TiO2, and by hydrogen-bonding interactions with the surface phosphonic acids for P-2Ru@Pt-TiO2. These results suggest that the surface structure of Ru(II) dye-sensitized Pt-TiO2 nanoparticle photocatalyst has a great influence on the reactivity with the redox mediator. Details will be discussed. Figure 1

Cobalt Complexes of Polypyridyl Ligands for the Photocatalytic Hydrogen Evolution Reaction.

The reductive part of artificial photosynthesis, the reduction of protons into H₂, is a two electron two proton process. It corresponds basically to the reactions occurring in natural photosystem I. We show in this review a selection of involved processes and components which are mandatory for making this light-driven reaction possible at all. The design and the performances of the water reduction catalysts is a main focus together with the question about electron relays or sacrificial electron donors. It is shown how an original catalyst is developed into better ones and what it needs to move from purely academic homogeneous processes to heterogeneous systems. The importance of detailed mechanistic knowledge obtained from kinetic data is emphasized.