Yusuke Yamada

Highly active water oxidation catalysts based on earth-abundant elements are strongly demanded for practical application of artificial photosynthesis. An iron complex with a chlorin ligand derived from chlorophyll-a demonstrated the highest turnover frequency (= 21.3 ± 0.3 s–1) and turnover number (= 6920 ± 110) among homogeneous catalysts reported for photocatalytic water oxidation utilizing [RuII(bpy)3]2+ (bpy: 2,2′-bipyridine) and S2O82– as a photosensitizer and a sacrificial electron acceptor, respectively. The molecular nature of the Fe chlorin complex during the photocatalytic water oxidation was confirmed by dynamic light scattering analysis and electrospray ionization time-of-flight mass spectrometry. Also, kinetic studies suggested that the rate-determining step is O–O bond formation via coupling of two high-valent iron–oxo species formed by three-electron oxidation of the mononuclear Fe chlorin unit with water. The high planarity of the chlorin ligand enables the high-valent iron–oxo species to approach another iron–oxo unit closely, thereby facilitating O–O bond formation.

Cs ₂ O-modified CeO ₂ exhibited high catalytic performance for the synthesis of β-alkoxyacrylates from alkynes and dialkyl carbonates as the first heterogeneous catalyst, affording the desired acrylates in high yields (up to 99%).

Photocatalytic water oxidation systems employing [Ru ^II (bpy) ₃ ] ²⁺ as a photosensitizer and S ₂ O ₈ ²⁻ as a sacrificial electron acceptor are widely used to evaluate the activity of various catalysts. However, the stoichiometric amount of O ₂ hardly evolves due to the decomposition of [Ru ^II (bpy) ₃ ] ²⁺ during the reaction. We report herein that the addition of 5–20 vol% of acetonitrile (CH ₃ CN) to the reaction solution increases the O ₂ yield irrespective of water oxidation catalysts (WOCs). For example, the addition of 20 vol% of CH ₃ CN increased the O ₂ yield from 38% to 93% in a borate buffer containing [Ru ^II (bpy) ₃ ] ²⁺ , S ₂ O ₈ ²⁻ and a cobalt porphyrin complex working as a homogeneous WOC. An investigation into the effect of organic‐solvent addition on each reaction step indicated that the oxidative decomposition of [Ru ^II (bpy) ₃ ] ²⁺ observed in a buffer without organic solvents was suppressed by the sacrificial oxidation of the organic solvents. In the presence of a WOC, the oxidation of MeOH and acetone more readily proceeds than water oxidation, resulting in low O ₂ yields. In contrast, the lower oxidation susceptibility of CH ₃ CN scarcely interfered with water oxidation, thereby affording high O ₂ yields. CH ₃ CN addition to the reaction solution is promising for the evaluation of intrinsic catalysis of each WOC.

Transition-metal complexes featuring multiple redox-active centers have the ability to cooperatively accumulate positive charges, making them promising candidates for water oxidation catalysis (WOCs). Herein, we report the high water oxidation activity and detailed reaction mechanism of a dinuclear iron complex bearing a doubly N-confused hexaphyrin ligand ([Fe2(DNCH)]) in a photocatalytic system employing tris(2,2′-bipyridine)ruthenium(II) ([RuII(bpy)3]2+) as a photosensitizer and peroxydisulfate ion (S2O82−) as a sacrificial oxidant. Furthermore, we synthesized and investigated the catalytic performance of the heterobimetallic analogue [FeZn(DNCH)], in which one of the FeIII ions of [Fe2(DNCH)] is substituted with the redox-inactive ZnII. Among the molecular WOCs previously studied under similar photocatalytic conditions, [Fe2(DNCH)] showed a high turnover frequency (TOF = 8.7 s−1) and turnover number (TON = 2730). Notably, [Fe2(DNCH)] displayed a sixfold enhancement in TOF and a fifteenfold increase in TON relative to [FeZn(DNCH)] (TOF = 1.4 s−1, TON = 180). Kinetic analyses of water oxidation reactions catalyzed by [Fe2(DNCH)] and [FeZn(DNCH)] support the hypothesis that O−O bond formation proceeds via an atom-proton-transfer (APT) mechanism. Electrochemical measurements revealed that both [Fe2(DNCH)] and [FeZn(DNCH)] promote water oxidation at potentials up to approximately 1.2 V, comparable to the RuII/RuIII redox couple of [RuII(bpy)3]2+. Water oxidation by [Fe2(DNCH)] and [FeZn(DNCH)] involves the removal of four and three electrons, respectively, suggesting that the [Fe2(DNCH)] system generates a more electrophilic oxidized species. Density functional theory (DFT) calculations further reveal that the DNCH ligand acts as an effective electron-accepting scaffold, stabilizing key transition states during the O−O bond-forming steps. Moreover, the FeIV=O moiety plays a pivotal role in reducing the overall energy barrier, thereby facilitating a markedly faster water-oxidation process relative to its FeZn counterpart. Our study revealed that the high TOF and TON values observed for [Fe2(DNCH)] originate from the synergistic interplay between the dinuclear iron centers and the π-conjugated ligand.

Mesoporous Ga ₂ O ₃ -integrated γ-Al ₂ O ₃ nanocomposites with high dispersion of Ga ₂ O ₃ species produced by a sol–gel synthesis route are high-performing bifunctional oxide catalysts for the hydrogen production by dimethyl ether (DME) steam reforming.

Cerium dioxide nanoparticles (CeO ₂ NPs) have been reported to act as active species for various catalytic reactions including organophosphate hydrolysis. The catalytic activity of CeO ₂ NPs embedded in silica nanoparticles assemblies (SiO ₂ NPAs) for p ‐nitrophenylphosphate ( p ‐NPP) hydrolysis was enhanced to maintain high dispersity. CeO ₂ NPs capped with oleylamine with a size of 15–40 nm showed the low initial rate (−1 in a HEPES buffer containing p ‐NPP (8.0 mM) at 60 °C. CeO ₂ NPs calcined at 800 °C showed the increased initial rate of 1.1 mM ⁻¹ by removal of oleylamine, although CeO ₂ NPs partially sintered. Conventional SiO ₂ support was used for suppression of the sintering, however, the initial rate decreased to 0.75 mM h ⁻¹ by high contact between CeO ₂ and SiO ₂ . SiO ₂ NPAs were used to minimize contact between CeO ₂ and SiO ₂ . The initial rate with CeO ₂ /SiO ₂ NPAs prepared by the impregnation method was as high as 1.8 mM h ⁻¹ . Furthermore, the initial rate with CeO ₂ /SiO ₂ NPAs prepared by a premixed method increased to 2.6 mM h ⁻¹ by effective suppression of both CeO ₂ NPs aggregation and electronic interaction with SiO ₂ . Although the sintering of CeO ₂ NPs was effectively suppressed even at 1000 °C, the initial rate decreased because of the adsorption of p ‐NPP on SiO ₂ NPAs.

A cyano-bridged coordination polymer (CP), K{Co^II_1.5[Fe^II(CN)₆]} {(K)Co-Fe}, insoluble in water was successfully solubilized by countercation exchange with Me₄N⁺ ions. On the one hand, the resultant powder remained insoluble after countercation exchange with NH₄⁺ or Et₄N⁺ ions. Powder X-ray diffraction measurements indicated that solubility was closely related to the crystal structures of the obtained powder after countercation exchange. The penetration of a countercation with an appropriate size into the Co-Fe framework resulted in solubilization by depolymerization. Countercation exchange with RMe₃N⁺ ions (R = Et, n-Pr, or n-Bu), which have an orthogonal projection size similar to that of the Me₄N⁺ ion, solubilized the Co-Fe framework. On the other hand, n-HexMe₃N⁺ ions hardly penetrating the Co-Fe framework provided insoluble powder due to the longer alkyl chain. The solubilized cyano-bridged CPs exhibited higher catalytic activity for water oxidation.

Correction for ‘Enhanced catalytic activity of solubilised species obtained by counter-cation exchange of K{Co^II_1.5[Fe^II(CN)₆]} for water oxidation’ by Yusuke Seki et al., Chem. Sci., 2024, 15, 16760–16767, https://doi.org/10.1039/D4SC04390A.

Highly active catalysts for electrocatalytic and photocatalytic water oxidation are strongly demanded to realize artificial photosynthesis. A cobalt complex with a chlorin derivative ligand (Co^II(Ch)) exhibited high activity for electrocatalytic water oxidation with an overpotential of 0.45 V at pH 9.0. Spectroelectrochemistry (UV-vis) unveiled the formation of two intermediates by successive one-electron oxidations. Also, the Pourbaix diagram depicted by the pH dependence of redox potentials indicated that the water oxidation proceeded after the oxidation of both the central cobalt ion and chlorin ligand with proton-coupled electron transfer (PCET). Then, the photocatalytic activity of Co^II(Ch) was examined for water oxidation using [Ru^II(bpy)₃]²⁺ (bpy: 2,2'-bipyridine) and S₂O₈^2- as a photosensitizer and a sacrificial electron acceptor, respectively. The turnover number, turnover frequency, and oxygen yield reached as high as 980, 5.2 s^-1, and 98%, respectively, under optimized conditions. The O₂-evolution rates increased in proportion to the square of the catalyst concentration in the reaction solution, suggesting that the formation of the O-O bond regarded as the rate-determining step of water oxidation proceeded by the interaction of two metal centers (I2M) mechanism in which two molecules of high-valent metal oxo or oxyl radical species react with each other.