preparation-polyoxometalate-based-photo-responsive-membranes-for

Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic ...

University of Yamanashi

Manganese oxides have been extensively investigated as model systems for the oxygen-evolving complex of photosystem II. However, most bioinspired catalysts are inefficient at neutral pH and functional similarity to the oxygen-evolving complex has been rarely achieved with manganese. Here we report the regulation of proton-coupled electron transfer involved in water oxidation by manganese oxides. Pyridine and its derivatives, which have pKa values intermediate to the water ligand bound to manganese(II) and manganese(III), are used as proton-coupled electron transfer induction reagents. The induction of concerted proton-coupled electron transfer is demonstrated by the detection of deuterium kinetic isotope effects and compliance of the reactions with the libido rule. Although proton-coupled electron transfer regulation is essential for the facial redox change of manganese in photosystem II, most manganese oxides impair these regulatory mechanisms. Thus, the present findings may provide a new design rationale for functional analogues of the oxygen-evolving complex for efficient water splitting at neutral pH. 

Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH.

We investigated the ability of β-iron silicide (β-FeSi2) to serve as a hydrogen (H2)-evolution photocatalyst due to the potential of its conduction band bottom, which may allow thermodynamically favorable H2 evolution in spite of its small band-gap of 0.80 eV. β-FeSi2 had an apparent quantum efficiency for H2 evolution of ∼24% up to 950 nm (near infrared light), in the presence of the dithionic acid ion (S2O6(2-)) as a sacrificial agent. It was also sensitive to infrared light (>1300 nm) for H2 evolution. 

Photocatalytic hydrogen evolution over β-iron silicide under infrared-light irradiation.

One challenge in artificial photosynthetic systems is the development of active oxygen evolution catalysts composed of abundant elements. The oxygen evolution activities of manganese pyrophosphate compounds were examined in electrochemical and photochemical experiments. Electrocatalysis using calcium-manganese pyrophosphate exhibited good catalytic ability under neutral pH and an oxygen evolution reaction was driven with a small overpotential (η<100 mV). UV-vis diffuse reflectance measurements revealed that manganese pyrophosphates exhibit weak absorption in the visible light region while commonly used oxygen evolution catalysts exhibit intense absorption. Therefore, the efficient light absorption of a photocatalyst was retained even after surface modification with a manganese pyrophosphate, and photochemical oxygen evolution was achieved by using magnesium ferrite modified with manganese pyrophosphate nanoparticles under the illumination of visible light at wavelength of over 420 nm.

Development of optically transparent water oxidation catalysts using manganese pyrophosphate compounds.

We recently reported the synthesis of a solid-state heterojunction photocatalyst consisting of zinc rhodium oxide (ZnRhO) and bismuth vanadium oxide (BiVO), which functioned as hydrogen (H) and oxygen (O) evolution photocatalysts, respectively, connected with silver (Ag). Polycrystalline BiVO (p-BiVO) powders were utilized to form ZnRhO/Ag/p-BiVO, which was able to photocatalyze overall pure-water splitting under red-light irradiation with a wavelength of 700 nm (R. Kobayashi et al., J. Mater. Chem. A, 2016, 4, 3061). In the present study, we replaced p-BiVO with a powder obtained by pulverizing single crystals of BiVO (s-BiVO) to form ZnRhO/Ag/s-BiVO, and demonstrated that this heterojunction photocatalyst had enhanced water-splitting activity. In addition, ZnRhO/Ag/s-BiVO was able to utilize nearly the entire range of visible light up to a wavelength of 740 nm. These properties were attributable to the higher O evolution activity of s-BiVO.

A heterojunction photocatalyst composed of zinc rhodium oxide, single crystal-derived bismuth vanadium oxide, and silver for overall pure-water splitting under visible light up to 740 nm.

The rate-determining step of the oxygen evolution reaction on hematite electrodes was switched from the sequential electron/proton transfer process to the concerted proton-coupled electron transfer (CPET) process by adding pyridine derivatives to the electrolyte. By inducing the CPET process, the overpotential for oxygen evolution at neutral pH decreased by approximately 250 mV.

Efficient oxygen evolution on hematite at neutral pH enabled by proton-coupled electron transfer.

We facilely prepared a solid-state heterojunction photocatalyst in which zinc rhodium oxide (ZnRh2O4) and bismuth vanadium oxide (Bi4V2O11) as hydrogen (H2) and oxygen (O2) evolution photocatalysts, respectively, were connected with gold (Au) to generate ZnRh2O4/Au/Bi4V2O11. ZnRh2O4/Au/Bi4V2O11 photocatalyzed the overall pure-water splitting under irradiation with red light at wavelengths of up to 740 nm.

Article	Year
Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH. Nature communications\| PubMed ID: 24977746	2014
Photocatalytic hydrogen evolution over β-iron silicide under infrared-light irradiation. Chemical communications (Cambridge, England)\| PubMed ID: 25585056	2015
Development of optically transparent water oxidation catalysts using manganese pyrophosphate compounds. Journal of photochemistry and photobiology. B, Biology\| PubMed ID: 25648929	2015
A heterojunction photocatalyst composed of zinc rhodium oxide, single crystal-derived bismuth vanadium oxide, and silver for overall pure-water splitting under visible light up to 740 nm. Physical chemistry chemical physics : PCCP\| PubMed ID: 27387092	2016
Efficient oxygen evolution on hematite at neutral pH enabled by proton-coupled electron transfer. Chemical communications (Cambridge, England)\| PubMed ID: 27853765	2016
Facile synthesis of a red light-inducible overall water-splitting photocatalyst using gold as a solid-state electron mediator. Chemical communications (Cambridge, England)\| PubMed ID: 29967911	2018

Toshihiro Takashima

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