Dual Ag/Co cocatalyst synergism for the highly effective photocatalytic conversion of CO2 by H2O over Al-SrTiO3

金属ナノ粒子で光触媒のモチベーションを上げることに成功 --人工光合成で二酸化炭素(CO2)の再資源化の新展開--. 京都大学プレスリリース. 2021-03-11.Loading Ag and Co dual cocatalysts on Al-doped SrTiO3 (AgCo/Al-SrTiO3) led to a significantly improved CO-formation rate and extremely high selectivity toward CO evolution (99.8%) using H2O as an electron donor when irradiated with light at wavelengths above 300 nm. Furthermore, the CO-formation rate over AgCo/Al-SrTiO3 (52.7 μmol h−1) was a dozen times higher than that over Ag/Al-SrTiO3 (4.7 μmol h−1). The apparent quantum efficiency for CO evolution over AgCo/Al-SrTiO3 was about 0.03% when photoirradiated at a wavelength at 365 nm, with a CO-evolution selectivity of 98.6% (7.4 μmol h−1). The Ag and Co cocatalysts were found to function as reduction and oxidation sites for promoting the generation of CO and O2, respectively, on the Al-SrTiO3 surface

La5Ti2Cu1-xAgxS5O7 photocathodes operating at positive potentials during photoelectrochemical hydrogen evolution under irradiation of up to 710 nm

A photoelectrochemical (PEC) cell based on a series-connected photocathode and photoanode made of particulate semiconductors is a potentially scalable and inexpensive device for renewable solar hydrogen production via PEC water splitting without any external power supply. The realisation of such PEC devices hinges on the development of photoelectrodes that operate at a small applied voltage. In this study, solid solutions of La5Ti2CuS5O7 (LTC) and La5Ti2AgS5O7 (LTA) were synthesised, and their physical, optical, and PEC properties in the water splitting reaction were discussed. LTC and LTA formed a La5Ti2Cu1-xAgxS5O7 solid solution (LTC(1-x)A(x)) over the whole compositional range. The indirect bandgap energy of LTC(1-x)A(x) changed nonlinearly with respect to composition, attaining its minimum value (ca. 1.8 eV) at a composition of x approximate to 0.16. Photoelectrodes of Al-doped LTC(1-x)A(x) solid solution powder fabricated using the particle transfer method exhibited a photocathodic response regardless of the Ag content. 1% Al-LTC(0.9)A(0.1) photocathodes exhibited the best PEC properties in the hydrogen evolution reaction and yielded a hypothetical half-cell solar-to-hydrogen energy conversion efficiency of 0.25% at +0.6 V vs. RHE, three times higher than the previously reported 1% Sc-LTC. In addition, 1% Al-LTC(0.9)A(0.1) photocathodes were fairly stable at + 0.7 V vs. RHE without any protective modifications. Owing to the positive operational electrode potential of 1% Al-LTC(0.9)A(0.1), unassisted PEC water splitting was accomplished using series-connected photoelectrodes made of 1% Al-LTC(0.9)A(0.1) and BaTaO2N, particulate semiconductors with absorption edge wavelengths of 710 and 660 nm, respectively, at a Faradaic efficiency of unity and a solar-to-hydrogen energy conversion efficiency of approximately 0.1%.ArticleEnergy & Environmental Science.8(11):3354-3362(2015)journal articl

Minimizing energy demand and environmental impact for sustainable NH3 and H2O2 production—A perspective on contributions from thermal, electro-, and photo-catalysis

There is an urgent need to provide adequate and sustainable supplies of water and food to satisfy the demand of an increasing population. Catalysis plays important roles in meeting these needs by facilitating the synthesis of hydrogen peroxide that is used in water decontamination and chemicals production, and ammonia that is used as fertilizer. However, these chemicals are currently produced with processes that are either very energy-intensive or environmentally unfriendly. This article offers the perspectives of the challenges and opportunities in the production of these chemicals, focusing on the roles of catalysis in more sustainable, alternative production methods that minimize energy consumption and environmental impact. While not intended to be a comprehensive review, the article provides a critical review of selected literature relevant to its objectives, discusses areas needed for further research, and potential new directions inspired by new developments in related fields. For each chemical, production by thermal, electro-, and photo-excited processes are discussed. Problems that are common to these approaches and their differences are identified and possible solutions suggested

Unveiling charge dynamics of visible light absorbing oxysulfide for efficient overall water splitting

Oxysulfide semiconductor, Y2Ti2O5S2, has recently discovered its exciting potential for visible-light-induced overall water splitting, and therefore, imperatively requires the probing of unknown fundamental charge loss pathways to engineer the photoactivity enhancement. Herein, transient diffuse reflectance spectroscopy measurements are coupled with theoretical calculations to unveil the nanosecond to microsecond time range dynamics of the photogenerated charge carriers. In early nanosecond range, the pump-fluence-dependent decay dynamics of the absorption signal is originated from the bimolecular recombination of mobile charge carriers, in contrast, the power-law decay kinetics in late microsecond range is dominated by hole detrapping from exponential tail trap states of valence band. A well-calibrated theoretical model estimates various efficiency limiting material parameters like recombination rate constant, n-type doping density and tail-states parameters. Compared to metal oxides, longer effective carrier lifetime ~6 ns is demonstrated. Different design routes are proposed to realize efficiency beyond 10% for commercial solar-to-hydrogen production from oxysulfide photocatalysts

Origin of the overall water splitting activity of Ta3N5 revealed by ultrafast transient absorption spectroscopy

Tantalum nitride (Ta3N5) is one of the few visible light absorbing photocatalysts capable of overall water splitting (OWS), by which the evolution of both H2 and O2 is possible. Despite favourable energetics, realizing the OWS or efficient H2 evolution in Ta3N5 prepared by the nitridation of tantalum oxide (Ta2O5) or Ta foil remains a challenge even after 15 years of intensive research. Recently our group demonstrated OWS in Ta3N5 when prepared by the short time nitridation of potassium tantalate (KTaO3). To obtain a mechanistic insight on the role of Ta precursor and nitridation time in realizing OWS, ultrafast dynamics of electrons (3435 nm probe) and holes (545 nm probe) is investigated using transient absorption spectroscopy. Electrons decay majorly by trapping in Ta3N5 prepared by the nitridation of Ta2O5, which do not show OWS. However, OWS activity in Ta3N5 prepared by 0.25 hour nitridation of KTaO3 is particularly favoured by the virtually absent electron and hole trapping. On further increasing the nitridation time of KTaO3 from 0.25 to 10 hour, trapping of both electron and hole is enhanced which concurrently results in a reduction of the OWS activity. Insights from correlating the synthesis conditions—structural defects—carrier dynamics—photocatalytic activity is of importance in designing novel photocatalysts to enhance solar fuel production

An Al-doped SrTiO3 photocatalyst maintaining sunlight-driven overall water splitting activity for over 1000 h of constant illumination

Photocatalytic water splitting is a viable approach to the large-scale production of renewable solar hydrogen. The apparent quantum yield for this reaction has been improved, but the lifespan of photocatalysts functioning under sunlight at ambient pressure have rarely been examined, despite the critical importance of this factor in practical applications. Herein, we show that Al-doped SrTiO3 (SrTiO3: Al) loaded with a RhCrOx (rhodium chromium oxide) cocatalyst splits water with an apparent quantum yield greater than 50% at 365 nm. Moreover, following the photodeposition of CoOOH and TiO2, this material maintains 80% of its initial activity and a solar-to-hydrogen energy conversion efficiency greater than or equal to 0.3% over a span of 1300 h under constant illumination by simulated sunlight at ambient pressure. This result is attributed to reduced dissolution of Cr in the cocatalyst following the oxidative photodeposition of CoOOH. The photodeposition of TiO2 further improves the durability of this photocatalyst. This work demonstrates a concept that could allow the design of longterm, large-scale photocatalyst systems for practical sunlight-driven water splitting.ArticleCHEMICAL SCIENCE.10(11):3196-3201(2019)journal articl

La5 Ti2 Cu0.9 Ag0.1 S5 O7 Modified with a Molecular Ni Catalyst for Photoelectrochemical H2 Generation.

The stable and efficient integration of molecular catalysts into p-type semiconductor materials is a contemporary challenge in photoelectrochemical fuel synthesis. Here, we report the combination of a phosphonated molecular Ni catalyst with a TiO2 -coated La5 Ti2 Cu0.9 Ag0.1 S5 O7 photocathode for visible light driven H2 production. This hybrid assembly provides a positive onset potential, large photocurrents, and high Faradaic yield for more than three hours. A decisive feature of the hybrid electrode is the TiO2 interlayer, which stabilizes the oxysulfide semiconductor and allows for robust attachment of the phosphonated molecular catalyst. This demonstration of an oxysulfide-molecular catalyst photocathode provides a novel platform for integrating molecular catalysts into photocathodes and the large photovoltage of the presented system makes it ideal for pairing with photoanodes.Japan Society for the Promotion of Science (JSPS); Christian Doppler Research Association (Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development); OMV group; EPSRC, Grants-in-Aids for Scientific Research and for Young Scientists from JSP

Linking in-situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen evolving photocatalysts

Recently, record solar to hydrogen efficiencies have been demonstrated using La,Rh co-doped SrTiO3 (La,Rh:SrTiO3) incorporated into a low cost and scalable Z-scheme device, known as a photocatalyst sheet. However, the unique properties that enable La,Rh:SrTiO3 to support this impressive performance are not fully understood. Combining in-situ spectroelectrochemical measurements with density functional theory and photoelectron spectroscopy produces a depletion model of Rh:SrTiO3 and La,Rh:SrTiO3 photocatalyst sheets. This reveals remarkable properties, such as deep flatband potentials (+2 VRHE) and a Rh oxidation state dependent reorganisation of the electronic structure, involving the loss of a vacant Rh 4d mid gap state. This reorganisation enables Rh:SrTiO3 to be reduced by co-doping without compromising p-type character. In-situ time resolved spectroscopies show the electronic structure reorganisation induced by Rh reduction controls electron lifetime in photocatalyst sheets. In Rh:SrTiO3 , enhanced lifetimes can only be obtained at negative applied potentials, where the complete Z-scheme operates inefficiently. La co-doping fixes Rh in the 3+ state, resulting in long-lived photogenerated electrons even at very positive potentials (+1 VRHE), where both components of the complete device operate effectively. This understanding of role of co-dopants provides new insight into the design principles for water splitting devices based on bandgap engineered metal oxides

Particle suspension reactors and materials for solar-driven water splitting

Reactors based on particle suspensions for the capture, conversion, storage, and use of solar energy as H_2 are projected to be cost-competitive with fossil fuels. In light of this, this review paper summarizes state-of-the-art particle light absorbers and cocatalysts as suspensions (photocatalysts) that demonstrate visible-light-driven water splitting on the laboratory scale. Also presented are reactor descriptions, theoretical considerations particular to particle suspension reactors, and efficiency and performance characterization metrics. Opportunities for targeted research, analysis, and development of reactor designs are highlighted