Kazuhide Koike

Artificial Z-Scheme Constructed with a Supramolecular Metal Complex and Semiconductor for the Photocatalytic Reduction of CO2

A hybrid for the visible-light-driven photocatalytic reduction of CO2 using methanol as a reducing agent was developed by combining two different types of photocatalysts: a Ru(II) dinuclear complex (RuBLRu′) used for CO2 reduction is adsorbed onto Ag-loaded TaON (Ag/TaON) for methanol oxidation. Isotope experiments clearly showed that this hybrid photocatalyst mainly produced HCOOH (TN = 41 for 9 h irradiation) from CO2 and HCHO from methanol. Therefore, it converted light energy into chemical energy (ΔG° = +83.0 kJ/mol). Photocatalytic reaction proceeds by the stepwise excitation of Ag/TaON and the Ru dinuclear complex on Ag/TaON, similar to the photosynthesis Z-scheme.

Photochemical Reduction of CO2 Using TiO2: Effects of Organic Adsorbates on TiO2 and Deposition of Pd onto TiO2

Reduction of CO(2) using semiconductors as photocatalysts has recently attracted a great deal of attention again. The effects of organic adsorbates on semiconductors on the photocatalytic products are noteworthy. On untreated TiO(2) (P-25) particles a considerable number of organic molecules such as acetic acid were adsorbed. Although irradiation of an aqueous suspension of this TiO(2) resulted in the formation of a significant amount of CH(4) as a major product, it was strongly suggested that its formation mainly proceeded via the photo-Kolbe reaction of acetic acid. Using TiO(2) treated by calcination and washing procedures for removal of the organic adsorbates, CO was photocatalytically generated as a major product, along with a very small amount of CH(4), from an aqueous suspension under a CO(2) atmosphere. In contrast, by using Pd (>0.5 wt %) deposited on TiO(2) (Pd-TiO(2)) on which organic adsorbates were not detected, CH(4) was the main product, but CO formation was drastically reduced compared with that on the pretreated TiO(2). Experimental data, including isotope labeling, indicated that CO(2) and CO(3)(2-) are the main carbon sources of the CH(4) formation, which proceeds on the Pd site of Pd-TiO(2). Prolonged irradiation caused deactivation of the photocatalysis of Pd-TiO(2) because of the partial oxidation of the deposited Pd to PdO.

Photocatalytic CO2 reduction with high turnover frequency and selectivity of formic acid formation using Ru(II) multinuclear complexes

Previously undescribed supramolecules constructed with various ratios of two kinds of Ru(II) complexes—a photosensitizer and a catalyst—were synthesized. These complexes can photocatalyze the reduction of CO2 to formic acid with high selectivity and durability using a wide range of wavelengths of visible light and NADH model compounds as electron donors in a mixed solution of dimethylformamide–triethanolamine. Using a higher ratio of the photosensitizer unit to the catalyst unit led to a higher yield of formic acid. In particular, of the reported photocatalysts, a trinuclear complex with two photosensitizer units and one catalyst unit photocatalyzed CO2 reduction (ΦHCOOH = 0.061, TONHCOOH = 671) with the fastest reaction rate (TOFHCOOH = 11.6 min-1). On the other hand, photocatalyses of a mixed system containing two kinds of model mononuclear Ru(II) complexes, and supramolecules with a higher ratio of the catalyst unit were much less efficient, and black oligomers and polymers were produced from the Ru complexes during photocatalytic reactions, which reduced the yield of formic acid. The photocatalytic formation of formic acid using the supramolecules described herein proceeds via two sequential processes: the photochemical reduction of the photosensitizer unit by NADH model compounds and intramolecular electron transfer to the catalyst unit.

Activation of graphitic carbon nitride (g-C3N4) by alkaline hydrothermal treatment for photocatalytic NO oxidation in gas phase

Photocatalytic activity of graphitic carbon nitride (g-C3N4) was significantly improved by an alkaline hydrothermal treatment. The specific surface area of g-C3N4 obtained by heating melamine at 550 °C was only 7.7 m2 g−1, which was too small for it to be utilized as a catalyst for air purification. By the hydrothermal treatment with NaOH solution at 90–150 °C, the surface area was increased up to 65 m2 g−1, and the oxidation rate of nitrogen oxide (NO) under visible light (380 < λ < 480 nm) was increased by 8.6 times. XRD, ESR, elemental analysis and electron microscopy showed that unstable domains of not-well-ordered carbon nitride were removed by hydrolysis to form a mesoporous structure with a higher surface area. Deactivation of g-C3N4 was not observed during the experimental period, although a small part of carbon nitride was decomposed by self-oxidation.

Efficient photocatalytic CO2 reduction using [Re(bpy) (CO)3{P(OEt)3}]+

Abstract [Re(bpy) (CO)3{P(OEt)3}]+ (1+) (bpy, 2,2′-bipyridine) is the most efficient homogeneous photocatalyst for the selective reduction of CO2 to CO reported to date. Both the quantum yield and turnover number of the photocatalytic reaction are strongly dependent on the irradiation light intensity and wavelength because of the unusual stability of the one-electron-reduced species [Re(bpy.−) (CO)3{P(OEt)3}] (1).

Preparation of a visible light-responsive photocatalyst from a complex of Ti4+ with a nitrogen-containing ligand

An anatase type of TiO2 photocatalyst containing N atoms was synthesized by a new technique using a complex of Ti4+ with a nitrogen-containing ligand as a precursor. The TiO2-like photocatalyst prepared by calcination of a Ti4+–bipyridine complex exhibited high photocatalytic activity for NOx removal under both ultraviolet and visible-light (λ < 645 nm) illumination. The doping of the N atom into the anatase lattice, which is expected from UV-VIS spectroscopy, XRD, and XPS, is inferred as an important factor for the visible light absorption and NOx removal activity under a wide range of visible light illumination. The bipyridine ligand acted as the source of N and C atoms and additionally inhibited sintering of the photocatalyst during heat treatment.

Development of highly efficient supramolecular CO2 reduction photocatalysts with high turnover frequency and durability.

New Ru(II)-Re(I) supramolecular photocatalysts with a rhenium(I) biscarbonyl complex as a catalyst unit were synthesized. They photocatalyzed CO2 reduction to CO using a wide-range of visible light, and their photocatalytic abilities were strongly affected by the phosphorus ligands on the Re site. Especially, Ru-Re(FPh), with two P(p-FPh)3 ligands, exhibited tremendous photocatalytic properties, i.e. TN(CO) = 207 and phi(CO) = 0.15, and, in addition, this is one of the fastest-operating photocatalysts for CO2 reduction to CO, with TF(CO) = 281 h(-1). We also clarified a balance of transferred electrons in this photocatalytic reaction and found that the two electrons necessary for CO formation were provided by two sequential reductive quenching processes of the excited Ru photosensitizer unit by the reductant BNAH.

Photocatalytic CO2 reduction to formic acid using a Ru(II)-Re(I) supramolecular complex in an aqueous solution.

In an aqueous solution, photophysical, photochemical, and photocatalytic abilities of a Ru(II)-Re(I) binuclear complex (RuReCl), of which Ru(II) photosensitizer and Re(I) catalyst units were connected with a bridging ligand, have been investigated in details. RuReCl could photocatalyze CO2 reduction using ascorbate as an electron donor, even in an aqueous solution. The main product of the photocatalytic reaction was formic acid in the aqueous solution; this is very different in product distribution from that in a dimethylformamide (DMF) and triethanolamine (TEOA) mixed solution in which the main product was CO. A (13)CO2 labeling experiment clearly showed that formic acid was produced from CO2. The turnover number and selectivity of the formic acid production were 25 and 83%, respectively. The quantum yield of the formic acid formation was 0.2%, which was much lower, compared to that in the DMF-TEOA mixed solution. Detail studies of the photochemical electron-transfer process showed back-electron transfer from the one-electron-reduced species (OERS) of the photosensitizer unit to an oxidized ascorbate efficiently proceeded, and this should be one of the main reasons why the photocatalytic efficiency was lower in the aqueous solution. In the aqueous solution, ligand substitution of the Ru(II) photosensitizer unit proceeded during the photocatalytic reaction, which was a main deactivation process of the photocatalytic reaction. The product of the ligand substitution was a Ru(II) bisdiimine complex or complexes with ascorbate as a ligand or ligands.

Effect of intramolecular π–π and CH–π interactions between ligands on structure, electrochemical and spectroscopic properties of fac-[Re(bpy)(CO)3(PR3)]+(bpy = 2,2′-bipyridine; PR3= trialkyl or triarylphosphines)

Intramolecular pi-pi and CH-pi interactions between the bpy and PR3 ligands of fac-[Re(bpy)(CO)3(PR3)]+ affect their structure, and electrochemical and spectroscopic properties. Intramolecular CH-pi interaction was observed between the alkyl groups on the phosphine ligand (R =nBu, Et) and the bpy ligand, and intramolecular pi-pi and CH-pi interactions were both observed between the aryl group(s) on the phosphorus ligand (R =p-MeOPh, p-MePh, Ph, p-FPh, OPh) and the bpy ligand, while no such interactions were found in the trialkylphosphite complexes (R = OiPr, OEt, OMe). The intramolecular interactions distort the pyridine rings of the bpy ligand as long as 3.7 x 10(-2)A in crystals. Molecular orbital calculations of the bpy ligand suggest that this distortion decreases the energy gap between its pi and pi* orbitals. An absorption band attributed to the pi-pi*(bpy) transition of the distorted rhenium complexes, measured in a KBr pellet, was red-shifted by 1-5 nm compared to the complexes without the distorted bpy ligand. Even in solution, similar red shifts of the pi-pi*(bpy) absorption were observed. The redox potential E1/2(bpy/bpy*-) of the complexes with the trialkylphosphine and triarylphosphine ligand are shifted positively by 110-120 mV and 60-80 mV respectively, compared with those derived from the electron-attracting property of the phosphorus ligand. In contrast with these properties, three nu(CO) IR bands, which are sensitive to the electron density on the central rhenium because of pi-back bonding, were shifted to higher energy, and a Re(I/II)-based oxidation wave was observed at a more positive potential according to the electron-attracting property of the phosphorus ligand.

Photochemistry and photocatalysis of rhenium(I) diimine complexes

Abstract Rhenium(I) diimine carbonyl complexes have been well investigated because of their functionalities, such as the intense emission properties, capabilities as a building block for multinuclear complexes, and photocatalytic activities. This chapter describes the following three topics of the rhenium complexes including recent reported works: photophysics, photochemical reactions, and photocatalyses. After Section I, the photophysical processes of the rhenium complexes are briefly summarized: the electronic structures, the photophysical relaxation processes, and the effects of intramolecular weak interaction between ligands on the photophysical properties. The next section about photochemical reactions includes ligand substitution, homolysis, and reactions of the ligand on the rhenium complexes. This section also includes synthesis of emissive multinuclear rhenium(I) complexes using the photochemical ligand substitution. The last section describes unique and high photocatalytic activities of the rhenium(I) diimine carbonyl complexes, especially for CO 2 reduction. The photocatalyses of mononuclear rhenium complexes, multicomponent systems, supramolecular systems with a Ru(II) complex as a photosensitizer, and a rhenium complex with periodic mesoporous organosilica as a light-harvesting system.