Catalysis of Nickel Ferrite for Photocatalytic Water Oxidation Using [Ru(bpy)₃]²⁺ and S₂O₈^2–

Single or mixed oxides of iron and nickel have been examined as catalysts in photocatalytic water oxidation using [Ru­(bpy)₃]²⁺ as a photosensitizer and S₂O₈^2– as a sacrificial oxidant. The catalytic activity of nickel ferrite (NiFe₂O₄) is comparable to that of a catalyst containing Ir, Ru, or Co in terms of O₂ yield and O₂ evolution rate under ambient reaction conditions. NiFe₂O₄ also possesses robustness and ferromagnetic properties, which are beneficial for easy recovery from the solution after reaction. Water oxidation catalysis achieved by a composite of earth-abundant elements will contribute to a new approach to the design of catalysts for artificial photosynthesis

Mesoporous Nickel Ferrites with Spinel Structure Prepared by an Aerosol Spray Pyrolysis Method for Photocatalytic Hydrogen Evolution

Submicron-sized mesoporous nickel ferrite (NiFe₂O₄) spheres were prepared by an aerosol spray pyrolysis method using Pluronic F127 as a structure-directing agent, and their photocatalytic performance for hydrogen (H₂) evolution was examined in an aqueous MeOH solution by visible light irradiation (λ > 420 nm). The structure of the spherical mesoporous nickel ferrites was studied by transmission electron microscopy, powder X-ray diffraction, and N₂ adsorption–desorption isotherm measurements. Mesoporous NiFe₂O₄ spheres of high specific surface area (278 m² g^–1) with a highly crystalline framework were prepared by adjusting the amount of structure-directing agent and the calcining condition. High photocatalytic activity of mesoporous NiFe₂O₄ for H₂ evolution from water with methanol was achieved due to the combination of high surface area and high crystallinity of the nickel ferrites

Water Oxidation Catalysis with Nonheme Iron Complexes under Acidic and Basic Conditions: Homogeneous or Heterogeneous?

Thermal water oxidation by cerium­(IV) ammonium nitrate (CAN) was catalyzed by nonheme iron complexes, such as Fe­(BQEN)­(OTf)₂ (<b>1</b>) and Fe­(BQCN)­(OTf)₂ (<b>2</b>) (BQEN = <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-bis­(8-quinolyl)­ethane-1,2-diamine, BQCN = <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-bis­(8-quinolyl)­cyclohexanediamine, OTf = CF₃SO₃^–) in a nonbuffered aqueous solution; turnover numbers of 80 ± 10 and 20 ± 5 were obtained in the O₂ evolution reaction by <b>1</b> and <b>2</b>, respectively. The ligand dissociation of the iron complexes was observed under acidic conditions, and the dissociated ligands were oxidized by CAN to yield CO₂. We also observed that <b>1</b> was converted to an iron­(IV)-oxo complex during the water oxidation in competition with the ligand oxidation. In addition, oxygen exchange between the iron­(IV)-oxo complex and H₂¹⁸O was found to occur at a much faster rate than the oxygen evolution. These results indicate that the iron complexes act as the true homogeneous catalyst for water oxidation by CAN at low pHs. In contrast, light-driven water oxidation using [Ru­(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) as a photosensitizer and S₂O₈^2– as a sacrificial electron acceptor was catalyzed by iron hydroxide nanoparticles derived from the iron complexes under basic conditions as the result of the ligand dissociation. In a buffer solution (initial pH 9.0) formation of the iron hydroxide nanoparticles with a size of around 100 nm at the end of the reaction was monitored by dynamic light scattering (DLS) in situ and characterized by X-ray photoelectron spectra (XPS) and transmission electron microscope (TEM) measurements. We thus conclude that the water oxidation by CAN was catalyzed by short-lived homogeneous iron complexes under acidic conditions, whereas iron hydroxide nanoparticles derived from iron complexes act as a heterogeneous catalyst in the light-driven water oxidation reaction under basic conditions

Mechanistic Insights into Homogeneous Electrocatalytic and Photocatalytic Hydrogen Evolution Catalyzed by High-Spin Ni(II) Complexes with S₂N₂‑Type Tetradentate Ligands

We report homogeneous electrocatalytic and photocatalytic H₂ evolution using two Ni­(II) complexes with S₂N₂-type tetradentate ligands bearing two different sizes of chelate rings as catalysts. A Ni­(II) complex with a five-membered SC₂S–Ni chelate ring (<b>1</b>) exhibited higher activity than that with a six-membered SC₃S–Ni chelate ring (<b>2</b>) in both electrocatalytic and photocatalytic H₂ evolution despite both complexes showing the same reduction potentials. A stepwise reduction of the Ni center from Ni­(II) to Ni(0) was observed in the electrochemical measurements; the first reduction is a pure electron transfer reaction to form a Ni­(I) complex as confirmed by electron spin resonance measurements, and the second is a 1e^–/1H⁺ proton-coupled electron transfer reaction to afford a putative Ni­(II)-hydrido (Ni^II–H) species. We also clarified that Ni­(II) complexes can act as homogeneous catalysts in the electrocatalytic H₂ evolution, in which complex <b>1</b> exhibited higher reactivity than that of <b>2</b>. In the photocatalytic system using [Ru­(bpy)₃]²⁺ as a photosensitizer and sodium ascorbate as a reductant, complex <b>1</b> with the five-membered chelate ring also showed higher catalytic activity than that of <b>2</b> with the six-membered chelate ring, although the rates of photoinduced electron-transfer processes were comparable. The Ni–H bond cleavage in the putative Ni^II–H intermediate should be involved in the rate-limiting step as evidenced by kinetic isotope effects observed in both photocatalytic and electrocatalytic H₂ evolution. Kinetic analysis and density functional theory calculations indicated that the difference in H₂ evolution activity between the two complexes was derived from that of activation barriers of the reactions between the Ni^II–H intermediates and proton, which is consistent with the fact that increase of proton concentration accelerates the H₂ evolution

Peptide Cross-linkers: Immobilization of Platinum Nanoparticles Highly Dispersed on Graphene Oxide Nanosheets with Enhanced Photocatalytic Activities

For exerting potential catalytic and photocatalytic activities of metal nanoparticles (MNPs), immobilization of MNPs on a support medium in highly dispersed state is desired. In this Research Article, we demonstrated that surfactant-free platinum nanoparticles (PtNPs) were efficiently immobilized on graphene oxide (GO) nanosheets in a highly dispersed state by utilizing oligopeptide β-sheets as a cross-linker. The fluorenyl-substituted peptides were designed to form β-sheets, where metal-binding thiol groups and protonated and positively charged amino groups are integrated on the opposite sides of the surface of a β-sheet, which efficiently bridge PtNPs and GO nanosheet. In comparison to PtNP/GO composite without the peptide linker, the PtNP/peptide/GO ternary complex exhibited excellent photocatalytic dye degradation activity via electron transfer from GO to PtNP and simultaneous hole transfer from oxidized GO to the dye. Furthermore, the ternary complex showed photoinduced hydrogen evolution upon visible light irradiation using a hole scavenger. This research provides a new methodology for the development of photocatalytic materials by a bottom-up strategy on the basis of self-assembling features of biomolecules

A Molecular Surface Functionalization Approach to Tuning Nanoparticle Electrocatalysts for Carbon Dioxide Reduction

Conversion of the greenhouse gas carbon dioxide (CO₂) to value-added products is an important challenge for sustainable energy research, and nanomaterials offer a broad class of heterogeneous catalysts for such transformations. Here we report a molecular surface functionalization approach to tuning gold nanoparticle (Au NP) electrocatalysts for reduction of CO₂ to CO. The <i>N</i>-heterocyclic (NHC) carbene-functionalized Au NP catalyst exhibits improved faradaic efficiency (FE = 83%) for reduction of CO₂ to CO in water at neutral pH at an overpotential of 0.46 V with a 7.6-fold increase in current density compared to that of the parent Au NP (FE = 53%). Tafel plots of the NHC carbene-functionalized Au NP (72 mV/decade) vs parent Au NP (138 mV/decade) systems further show that the molecular ligand influences mechanistic pathways for CO₂ reduction. The results establish molecular surface functionalization as a complementary approach to size, shape, composition, and defect control for nanoparticle catalyst design

Homogeneous Photocatalytic Water Oxidation with a Dinuclear Co^III–Pyridylmethylamine Complex

A bis-hydroxo-bridged dinuclear Co^III-pyridylmethylamine complex (<b>1</b>) was synthesized and the crystal structure was determined by X-ray crystallography. Complex <b>1</b> acts as a homogeneous catalyst for visible-light-driven water oxidation by persulfate (S₂O₈^2–) as an oxidant with [Ru^II(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) as a photosensitizer affording a high quantum yield (44%) with a large turnover number (TON = 742) for O₂ formation without forming catalytically active Co-oxide (CoO_<i>x</i>) nanoparticles. In the water-oxidation process, complex <b>1</b> undergoes proton-coupled electron-transfer (PCET) oxidation as a rate-determining step to form a putative dinuclear bis-μ-oxyl Co^III complex (<b>2</b>), which has been suggested by DFT calculations. Catalytic water oxidation by <b>1</b> using [Ru^III(bpy)₃]³⁺ as an oxidant in a H₂¹⁶O and H₂¹⁸O mixture was examined to reveal an intramolecular O–O bond formation in the two-electron-oxidized bis-μ-oxyl intermediate, prior to the O₂ evolution

Homogeneous Photocatalytic Water Oxidation with a Dinuclear Co^III–Pyridylmethylamine Complex

A bis-hydroxo-bridged dinuclear Co^III-pyridylmethylamine complex (<b>1</b>) was synthesized and the crystal structure was determined by X-ray crystallography. Complex <b>1</b> acts as a homogeneous catalyst for visible-light-driven water oxidation by persulfate (S₂O₈^2–) as an oxidant with [Ru^II(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) as a photosensitizer affording a high quantum yield (44%) with a large turnover number (TON = 742) for O₂ formation without forming catalytically active Co-oxide (CoO_<i>x</i>) nanoparticles. In the water-oxidation process, complex <b>1</b> undergoes proton-coupled electron-transfer (PCET) oxidation as a rate-determining step to form a putative dinuclear bis-μ-oxyl Co^III complex (<b>2</b>), which has been suggested by DFT calculations. Catalytic water oxidation by <b>1</b> using [Ru^III(bpy)₃]³⁺ as an oxidant in a H₂¹⁶O and H₂¹⁸O mixture was examined to reveal an intramolecular O–O bond formation in the two-electron-oxidized bis-μ-oxyl intermediate, prior to the O₂ evolution