• Green technology breakthrough: Hematite

    From ScienceDaily@1:317/3 to All on Wed Apr 27 22:30:50 2022
    Green technology breakthrough: Hematite photocatalyst using sunlight
    energy simultaneously produces hydrogen and hydrogen peroxide

    Date:
    April 27, 2022
    Source:
    Kobe University
    Summary:
    Hydrogen production using sunlight energy (solar-water
    splitting) has gained much attention in the quest to move
    towards carbon-neutral technologies. If chemical products with
    applications in the health and food industries could be produced
    at the same time as hydrogen, this would help reduce the cost
    of solar-water splitting, as well as increasing the technology's
    range of applications.



    FULL STORY ========================================================================== Hydrogen production using sunlight energy (solar-water splitting)
    has gained much attention in the quest to move towards carbon-neutral technologies. If chemical products with applications in the health and
    food industries could be produced at the same time as hydrogen, this
    would help reduce the cost of solar-water splitting, as well as increasing
    the technology's range of applications. In this study, Kobe University's Associate Professor Tachikawa et al. found that by modifying the surface
    of their previously-developed hematite photocatalyst, they could safely, cheaply and stably produce hydrogen peroxide as well as hydrogen. Hydrogen peroxide is used for many purposes including disinfecting, bleaching
    and soil improvement.


    ========================================================================== Using a hematite (*1) photocatalyst (*2), a joint research team has
    succeeded in producing both hydrogen gas and hydrogen peroxide (*3) at
    the same time from sunlight and water. The team included the following
    members from Kobe University: Associate Professor TACHIKAWA Takashi (of
    the Molecular Photoscience Research Center) Professor TENNO Seiichiro
    (Graduate School of System Informatics/ Graduate School of Science,
    Technology, and Innovation), Associate Professor TSUCHIMOCHI Takashi
    (Graduate School of System Informatics) et al.

    In the quest to make a carbon neutral society a reality, CO2-free hydrogen production using sunlight energy has gained attention. If chemical
    products with applications in the health and food industries could be
    produced at the same time as hydrogen through photocatalyst-mediated solar water-splitting, it would be possible to develop a solar water-splitting utilization system with even greater added value.

    Hematite mesocrystals (*4) can absorb a wide range of visible light. In
    this study, Associate Professor Tachikawa et al. found that by preparing electrodes with mesocrystals doped (*5) with two different metal ions,
    it was possible to safely, cheaply and stably produce hydrogen peroxide
    as well as hydrogen.

    Hydrogen peroxide is used for many purposes including disinfecting,
    bleaching and soil improvement.

    The research group's next aim is to implement this technology. While
    continuing to improve the high efficiency of the developed photocatalyst electrode, they will try to assemble the cells into a compact module as
    a step towards societal implementation. They also plan to develop this mesocrystal technology with various materials and reaction systems.

    This was a joint research project with Nagoya University's Institute
    of Materials and Systems for Sustainability (Professor MUTO Shunsuke)
    and the Japan Synchrotron Radiation Research Institute (JASRI) (Chief Researcher OHARA Koji and Researcher INA Toshiaki).



    ==========================================================================
    The results were given advanced online publication in Nature
    Communications (Nature Publishing Group) on March 23, 2022.

    Main Points
    * Hematite on its own is not suitable for producing hydrogen
    peroxide. By
    doping the hematite with different metal ions (tin and titanium)
    and sintering it, the researchers developed a highly active
    composite oxide co-catalyst (*6).

    * The ability to produce hydrogen peroxide on-site in addition
    to hydrogen
    will contribute towards reducing the cost of solar
    water-splitting, as well as increasing the technology's range of
    applications. Hydrogen peroxide is used for many purposes including
    disinfecting, bleaching and soil improvement.

    Research Background With the world facing increasing environmental and
    energy issues, hydrogen has gained attention as one of the possible next generation energy sources. Ideally, photocatalysts could use sunlight and
    water to produce hydrogen, however it is necessary to achieve a conversion
    rate of 10% to enable such a system to be adopted industrially. It has
    been pointed out that even if this efficiency is achieved, the cost of
    hydrogen will not reach the desired value. To overcome these issues, there
    is strong demand for the development of a competitive next generation
    solar water-splitting system with high added value that can produce
    other useful chemicals at the same time as hydrogen.

    In their previous research, Tachikawa et al. developed 'mesocrystal technology', which involves precisely aligning nanoparticles in
    photocatalysts to control the flow of electrons and their holes. Recently,
    they have succeeded in dramatically increasing the light energy conversion efficiency by applying this technology to hematite.

    Up until now, hematite has not been applied to the production of hydrogen peroxide. In this study, the researchers discovered that by modifying
    the surface of the hematite with a composite oxide of tin and titanium
    ions it was possible to produce both hydrogen and hydrogen peroxide in
    a highly efficient and selective manner.



    ========================================================================== Research Methodology Mesocrystal technology: The main problem that
    causes a conversion rate decline in photocatalytic reactions is that the electrons and holes produced by light recombine before they can react
    with the molecules (in this case, water).

    Tachikawa et al. created 3D structures of hematite mesocrystals
    with highly oriented nanoparticles via solvothermal synthesis
    (*8). Furthermore, they were able to develop mesocrystal photoelectrodes
    for water splitting by coating and sintering the mesocrystals on the
    conductive glass substrate.

    Formation of a co-catalyst for producing hydrogen oxide via dopant
    segregation: Normally, photocatalytic water-splitting using hematite
    results in oxygen being produced from the oxidation of the water. Doping
    this hematite with tin ions (Sn2+) and titanium ions (Ti4+) and then
    sintering it at 700DEGC causes segregation of the tin and titanium
    dopants, leading to the formation of a composite oxide (SnTiOx)
    co-catalyst with high selectivity for hydrogen peroxide production). This structural change was revealed by performing synchrotron-based X-ray
    total scattering measurements using beamlines BL01B1 and BLO4B2 at the
    SPring-8 (*9) facility, and by using a high-resolution electron microscope incorporating electron energy loss spectroscopy (*10).

    Photocatalyst formation and performance: The water-splitting reaction
    was promoted when voltage was applied to the photocatalyst electrode illuminated by artificial sunlight. The researchers investigated the photoelectric current density and the Faradiac efficiency (*11) which
    indicate the hydrogen production efficiency and the hydrogen peroxide selectivity, respectively. It was revealed that there were positive
    and negative effects on hydrogen and hydrogen peroxide production
    if the photocatalyst was doped with only one of the metal ions. On
    the other hand, hematite doped with both Sn2+ and Ti4+ could produce
    hydrogen and hydrogen peroxide at the same time in a highly efficient
    and highly selective manner. In addition, first principle calculations
    (*12) suggested that the SnTiOxco-catalyst on the hematite consisted of SnO2/SnTiO3 layers of a few nanometers in thickness.

    Further Developments By modifying the surface of the hematite used for
    the photocatalyst, the research group succeeded in producing hydrogen
    peroxide, which hasn't been produced in this manner before, in a highly efficient and selective way. Next, the researchers plan to further
    optimize the photocatalytic electrode and collaborate with industry to
    develop an onsite system for the production of hydrogen and hydrogen
    peroxide utilizing sunlight. They also plan to develop its applications
    to other metal oxides and reaction systems.

    Glossary 1. Hematite (a-Fe2O3): A type of iron oxide ore. In addition
    to being safe, inexpensive and stable (pH > 3), Hematite can absorb a
    wide range of visible light (approx. under 600nm).

    2. Photocatalyst: A material that can be utilized as a catalyst for
    reactions involving light illumination. The photocatalyst is applied to a conductive glass substrate (FTO glass) which absorbs the light. Used as an electrode, it can also be called a photocatalyst anode or a photoanode. In
    this study, a photocatalyst was used for the reaction to produce hydrogen
    by splitting the water molecules.

    3. Hydrogen Peroxide: Hydrogen Peroxide (H2O2) is commonly used for a
    wide range of applications, such as disinfectants, detergents, cosmetics, bleach and in purifying water. The majority of hydrogen peroxide is
    produced using the antraquinone process which must be conducted in a large-scale chemical plant and generates organic waste and CO2. In
    addition, hydrogen peroxide is unstable, therefore it is expensive
    to transport it and there are concerns about its safety. However, this
    research group developed a method of synthesizing liquid H2O2 via a safe, low-cost and green process. H2O2 has a higher market value than O2 so
    producing hydrogen peroxide at the same time as hydrogen can also reduce hydrogen production costs.

    4. Mesocrystal: Porous crystal structures consisting of nanoparticles that
    are three dimensionally aligned. Hundreds of nanometers or micrometers
    small, they feature pores between the nanoparticles that are between 2
    to 50 nanometers.

    5. Doping: Adding a small quantity of another substance to the crystals
    to change their physical properties. Dopant diffusion occurs inside
    the crystal structure and the phenomenon whereby it is deposited on the
    surface is called dopant segregation.

    6. Co-catalyst: A substance which is combined with the photocatalyst
    to facilitate the reaction. In this study, a tin and titanium composite
    oxide was used to promote hydrogen peroxide production.

    7. Light energy conversion efficiency: The amount of light particles
    used in the reaction (output) divided by the amount of inputted light particles.

    8. Solvothermal method: A method of synthesizing solids using solvents
    at high temperatures and high pressures.

    9. SPring-8: Located in Harima Science Park in Hyogo Prefecture, Japan,
    SPring- 8 is a large synchrotron radiation facility which currently
    provides the most powerful synchrotron radiation in the world. Synchrotron radiation is produced when electron beams, accelerated to almost the
    speed of light, are forced to travel in a curved path by a magnetic
    field, producing highly-focused powerful electromagnetic radiation. A
    wide range of research using synchrotron radiation is conducted
    at Spring-8, including nanotechnology, biotechnology and industrial applications. SPring-8 is managed by RIKEN, with the Japan Synchrotron Radiation Research Institute (JASRI) in charge of promoting its use.

    10. Electron energy loss spectroscopy: A spectroscopy technique to
    analyze the composition of a sample and bonding state of its elements
    by measuring the energy lost when the incident electron beam excites
    the electrons in the sample. By combining this technique with scanning transmission electron microscopy, it is possible to analyze minute
    regions at high resolutions.

    11. Faradaic efficiency: The percentage of the total electric current
    that is transferred into a system facilitating an electrochemical reaction
    (in this case the production of hydrogen and hydrogen peroxide).

    12. First principle calculation: A method of calculating the movement
    of electrons inside a substance, based on Density Functional Theory. It
    enables the properties for surface energy absorption and the optimum
    structure of a solid or the particles to be calculated.

    13. Anode: In electro-chemistry, the electrode where the oxidation
    reaction occurs 14. Cathode: In electro-chemistry, the electrode where
    the reduction reaction occurs

    ========================================================================== Story Source: Materials provided by Kobe_University. Note: Content may
    be edited for style and length.


    ========================================================================== Journal Reference:
    1. Zhujun Zhang, Takashi Tsuchimochi, Toshiaki Ina, Yoshitaka Kumabe,
    Shunsuke Muto, Koji Ohara, Hiroki Yamada, Seiichiro L. Ten-no,
    Takashi Tachikawa. Binary dopant segregation enables hematite-based
    heterostructures for highly efficient solar H2O2 synthesis. Nature
    Communications, 2022; 13 (1) DOI: 10.1038/s41467-022-28944-y ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/04/220427100519.htm

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