Production of Methane by Sunlight-Driven Photocatalytic Water Splitting and Carbon Dioxide Methanation as a Means of Artificial Photosynthesis

This article describes an experimental apparatus of artificial photosynthesis, which generates methane gas from water and carbon dioxide with the aid of sunlight energy. This apparatus was designed on the basis of our previous 100 m2-scale photocatalytic solar hydrogen production mini-plant, which continuously produced filtered hydrogen gas for more than several months. A catalytic CO2 methanator was attached, converting photogenerated H2 into CH4. The overall setup was successfully operated, and photosynthetic CH4 was accumulated. Several versions were examined by changing the sizes of the composing assemblies and choosing specific purposes for experiments. The performances of the water-splitting photocatalytic panels, the hydrogen filtration subsystem, and the methanator are illustrated. One of the versions was implemented in the competition of the European Innovation Council (EIC) Horizon Prize on Artificial Photosynthesis “Fuel from the Sun” in 2022. For future expansion as artificial photosynthetic plants, the technical issues related to scaling up the plant size are extracted and discussed from these results

Production of Methane by Sunlight-Driven Photocatalytic Water Splitting and Carbon Dioxide Methanation as a Means of Artificial Photosynthesis

This article describes an experimental apparatus of artificial photosynthesis, which generates methane gas from water and carbon dioxide with the aid of sunlight energy. This apparatus was designed on the basis of our previous 100 m2-scale photocatalytic solar hydrogen production mini-plant, which continuously produced filtered hydrogen gas for more than several months. A catalytic CO2 methanator was attached, converting photogenerated H2 into CH4. The overall setup was successfully operated, and photosynthetic CH4 was accumulated. Several versions were examined by changing the sizes of the composing assemblies and choosing specific purposes for experiments. The performances of the water-splitting photocatalytic panels, the hydrogen filtration subsystem, and the methanator are illustrated. One of the versions was implemented in the competition of the European Innovation Council (EIC) Horizon Prize on Artificial Photosynthesis “Fuel from the Sun” in 2022. For future expansion as artificial photosynthetic plants, the technical issues related to scaling up the plant size are extracted and discussed from these results

Photoelectrochemical Oxidation of Water Using BaTaO₂N Photoanodes Prepared by Particle Transfer Method

A photoanode of particulate BaTaO₂N fabricated by the particle transfer method and modified with a Co cocatalyst generated a photocurrent of 4.2 mA cm^–2 at 1.2 V_RHE in the photoelectrochemical water oxidation reaction under simulated sunlight (AM1.5G). The half-cell solar-to-hydrogen conversion efficiency (HC-STH) of the photoanode reached 0.7% at 1.0 V_RHE, which was an order of magnitude higher than the previously reported photoanode made from the same material. The faradaic efficiency for oxygen evolution from water was virtually 100% during the reaction for 6 h, attesting to the robustness of the oxynitride

Insights into the Proton Transfer Mechanism of a Bilin Reductase PcyA Following Neutron Crystallography

Phycocyanobilin, a light-harvesting and photoreceptor pigment in higher plants, algae, and cyanobacteria, is synthesized from biliverdin IXα (BV) by phycocyanobilin:ferredoxin oxidoreductase (PcyA) via two steps of two-proton-coupled two-electron reduction. We determined the neutron structure of PcyA from cyanobacteria complexed with BV, revealing the exact location of the hydrogen atoms involved in catalysis. Notably, approximately half of the BV bound to PcyA was BVH⁺, a state in which all four pyrrole nitrogen atoms were protonated. The protonation states of BV complemented the protonation of adjacent Asp105. The “axial” water molecule that interacts with the neutral pyrrole nitrogen of the A-ring was identified. His88 Nδ was protonated to form a hydrogen bond with the lactam O atom of the BV A-ring. His88 and His74 were linked by hydrogen bonds via H₃O⁺. These results imply that Asp105, His88, and the axial water molecule contribute to proton transfer during PcyA catalysis

Particulate Photocatalyst Sheets Based on Carbon Conductor Layer for Efficient Z‑Scheme Pure-Water Splitting at Ambient Pressure

Development of sunlight-driven water splitting systems with high efficiency, scalability, and cost-competitiveness is a central issue for mass production of solar hydrogen as a renewable and storable energy carrier. Photocatalyst sheets comprising a particulate hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) embedded in a conductive thin film can realize efficient and scalable solar hydrogen production using Z-scheme water splitting. However, the use of expensive precious metal thin films that also promote reverse reactions is a major obstacle to developing a cost-effective process at ambient pressure. In this study, we present a standalone particulate photocatalyst sheet based on an earth-abundant, relatively inert, and conductive carbon film for efficient Z-scheme water splitting at ambient pressure. A SrTiO₃:La,Rh/C/BiVO₄:Mo sheet is shown to achieve unassisted pure-water (pH 6.8) splitting with a solar-to-hydrogen energy conversion efficiency (STH) of 1.2% at 331 K and 10 kPa, while retaining 80% of this efficiency at 91 kPa. The STH value of 1.0% is the highest among Z-scheme pure water splitting operating at ambient pressure. The working mechanism of the photocatalyst sheet is discussed on the basis of band diagram simulation. In addition, the photocatalyst sheet split pure water more efficiently than conventional powder suspension systems and photoelectrochemical parallel cells because H⁺ and OH^– concentration overpotentials and an <i>IR</i> drop between the HEP and OEP were effectively suppressed. The proposed carbon-based photocatalyst sheet, which can be used at ambient pressure, is an important alternative to (photo)­electrochemical systems for practical solar hydrogen production