Molecular Catalysts Immobilized on Semiconductor Photosensitizers for Proton Reduction toward Visible-Light-Driven Overall Water Splitting

Novel nanohybrid iron (II/III) phthalocyanine-based carbon nanotubes as catalysts for organic pollutant removal: process optimization by chemometric approach

In the present study, two iron phthalocyanine (FePc)-based nanocatalysts were synthesized and fully characterized. The carbon nanotubes (CNT) functionalized in an easy way with either Fe(II)Pc or Fe(III)Pc exhibit a very good catalytical activity. The activity in real wastewater effluent was comparable with the activity in distilled water. The procedure of modeling and optimizing with the assistance of chemometrics, utilizing design of experiments (DOE) and response surface methodology (RSM), revealed the conditions of optimum for decaying Reactive Yellow 84 on the nanocatalysts FePc_CNT. These optimal conditions included a catalyst dose of 1.70 g/L and an initial concentration (C0) of 20.0 mg/L. Under the indicated optimal conditions, the experimental findings verified that the removal efficiency was equal to Y = 98.92%, representing the highest observed value in this study. Under UVA light, after only 15 min of reaction, over 94% of dye was removed using both catalysts. The reuse experiments show that the activity of both nanohybrid material based on FePc-CNT slightly decreases over four consecutive runs. The quenching experiments show that RY84 was removed through radical pathways (O2•- and •OH) as well as non-radical pathways (1O2 and direct electron transfer).

Enhanced Photo/Electrocatalytic Hydrogen Evolution by Hydrothermally Derived Cu-Doped TiO2 Solid Solution Nanostructures

Highly efficient nanocatalysts with a high specific surface area were successfully synthesized by a cost-effective and environmentally friendly hydrothermal method. Structural and elemental purity, size, morphology, specific surface area, and band gap of pristine and 1 to 5% Cu-doped TiO2 nanoparticles were characterized by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), energy dispersive X-ray analysis (EDAX), inductively coupled plasma mass spectrometry (ICP-MS), liquid chromatography-high resolution mass spectrometry (LC-HRMS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET surface area, Raman spectroscopy, photoluminescence spectroscopy (PL) and UV-visible diffused reflectance spectroscopy (UV-DRS) studies. The XPS and EPR findings indicated the successful integration of Cu ions into the TiO2 lattice. UV-DRS and BET surface area investigations revealed that with an increase in dopant concentration, Cu-doped TiO2 NPs show a decrease in band gap (3.19-3.08 eV) and an increase in specific surface area (169.9-188.2 m2/g). Among all compositions, 2.5% Cu-doped TiO2 has shown significant H2 evolution with an apparent quantum yield of 17.67%. Furthermore, the electrochemical water-splitting study shows that 5% Cu-doped TiO2 NPs have superiority over pristine TiO2 for H2 evolution reaction. It was thus revealed that the band gap tuning with the desired dopant concentration led to enhanced photo/electrocatalytic sustainable energy applications.

Solar-Driven CO2 Reduction Using a Semiconductor/Molecule Hybrid Photosystem: From Photocatalysts to a Monolithic Artificial Leaf

The synthesis of organic chemicals from H₂O and CO₂ using solar energy is important for recycling CO₂ through cyclical use of chemical ingredients produced from CO₂ or molecular energy carriers based on CO₂. Similar to photosynthesis in plants, the CO₂ molecules are reduced by electrons and protons, which are extracted from H₂O molecules, to produce O₂. This reaction is uphill; therefore, the solar energy is stored as the chemical bonding energy in the organic molecules. This artificial photosynthetic technology mimicking green vegetation should be implemented as a self-standing system for on-site direct solar energy storage that supports CO₂ recycling in a circular economy. Herein, we explain our interdisciplinary fusion methodology to develop hybrid photocatalysts and photoelectrodes for an artificial photosynthetic system for the CO₂ reduction reaction (CO₂RR) in aqueous solutions. The key factor for the system is the integration of uniquely different functions of molecular transition-metal complexes and solid semiconductors. A metal complex catalyst and a semiconductor appropriate for a CO₂RR and visible-light absorption, respectively, are linked, and they function complementary way to catalyze CO₂RR under visible-light irradiation as a particulate photocatalyst dispersion in solution. It has also been proven that Ru complexes with bipyridine ligands can catalyze a CO₂RR as photocathodes when they are linked with various semiconductor surfaces, such as those of doped tantalum oxides, doped iron oxides, indium phosphides, copper-based sulfides, selenides, silicon, and others. These photocathodes can produce formate and carbon monoxide using electrons and protons extracted from water through potential-matched connections with photoanodes such as TiO₂ or SrTiO₃ for oxygen evolution reactions (OERs). Benefiting from the very low overpotential of an aqueous CO₂RR at metal complexes approaching the theoretical lower limit, the semiconductor/molecule hybrid system demonstrates a single tablet-formed monolithic electrode called "artificial leaf." This single electrode device can generate formate (HCOO^-) from H₂O and CO₂ in a water-filled single-compartment reactor without requiring a separation membrane under unassisted or bias-free conditions, either electrically or chemically. The reaction proceeds with a stoichiometric electron/hole ratio and stores solar energy with a solar-to-chemical energy conversion efficiency of 4.6%, which exceeds that of plants. In this Account, the key results that marked our milestones in technological progress of the semiconductor/molecule hybrid photosystem are concisely explained. These results include design, proof of the principle, and understanding of the phenomena by time-resolved spectroscopies, synchrotron radiation analyses, and DFT calculations. These results enable us to address challenges toward further scientific progress and the social implementation, including the use of earth-abundant elements and the scale-up of the solar-driven CO₂RR system.

Particulate photocatalytic reactors with spectrum-splitting function for artificial photosynthesis

We have applied spectrum splitting, which is the most reliable way for highly efficient solar energy utilization, to particulate photocatalytic reactors. We have elucidated that the spectrum splitting is feasible using plural cells/compartments, in which photocatalyst particles of different bandgaps are suspended respectively, arranged optically in series. When the particles are sufficiently small (≤20 nm in diameter), high-energy photons are absorbed in the wide-gap cell/compartment on the solar illumination side while low-energy photons reach the backside narrow-gap cell/compartment with being scarcely diffuse-reflected. We have proposed two concrete configurations of the reactors: wide-gap cell/narrow-gap Z-scheme cell (WG/Z), and wide-gap cell/two-compartment cell of middle-gap and narrow-gap (WG/MG-NG), based on the previous configuration of a two-compartment cell of wide-gap and narrow-gap (WG-NG). We have constructed a new model of the carrier supply process from the semiconductor photocatalysts to the active sites, and calculated the practical upper limits of the carrier supply rates and solar-to-chemical conversion efficiencies. The spectrum-splitting reactors can yield higher efficiencies of artificial photosynthetic H2 and CO production by up to 1.5-1.6 times than the conventional Z-scheme reactors. The newly proposed WG/Z reactor widens the room of the material developments and improves the robustness against solar spectrum variation, and hence would be a promising practical solution, although the efficiency is slightly lower than that for the ideal WG-NG reactor. The WG/MG-NG reactor yields the highest efficiency among the three configurations, with high spectral robustness.

Metalloporphyrin immobilized CeO2: in situ generation of active sites and synergistic promotion of photocatalytic water oxidation

CoTCPP transfer photoexcited electrons to CeO₂ by d–f electron coupling. The in situ generation of catalytically active sites: reduction on CeO₂ accompanied with the creation of oxygen vacancies and oxidation on CoTCPP that transforms into CoOOH.

The effect of Fe(iii) ions on oxygen-vacancy-rich BiVO4 on the photocatalytic oxygen evolution reaction

The surface oxygen vacancies could promote the photocatalytic O2 evolution of BiVO4. Simultaneously, Fe3+ ions in solution could facilitate the holes' transfer to improve the water oxidation reaction.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Bioinspired metal complexes for energy-related photocatalytic small molecule transformation

Bioinspired transformation of small-molecules to energy-related feedstocks is an attractive research area to overcome both the environmental issues and the depletion of fossil fuels. The highly effective metalloenzymes in nature provide blueprints for the utilization of bioinspired metal complexes for artificial photosynthesis. Through simpler structural and functional mimics, the representative herein is the pivotal development of several critical small molecule conversions catalyzed by metal complexes, e.g., water oxidation, proton and CO₂ reduction and organic chemical transformation of small molecules. Of great achievement is the establishment of bioinspired metal complexes as catalysts with high stability, specific selectivity and satisfactory efficiency to drive the multiple-electron and multiple-proton processes related to small molecule transformation. Also, potential opportunities and challenges for future development in these appealing areas are highlighted.

Photoelectrochemical water-splitting over a surface modified p-type Cr2O3 photocathode

H₂ generation via solar photoelectrochemical water-splitting by Cr₂O₃ was successfully realized by surface modification with TiO₂ and the following Pt deposition.

Mechanistic insights into photocatalysis and over two days of stable H2 generation in electrocatalysis by a molecular cobalt catalyst immobilized on TiO2

Molecular and heterogeneous water reduction combined: Over 2 days of electrocatalysis of a cobalt polypyridyl catalyst immobilized on TiO₂.

Visible-Light-Driven CO2 Reduction by Mesoporous Carbon Nitride Modified with Polymeric Cobalt Phthalocyanine

The integration of molecular catalysts with low-cost, solid light absorbers presents a promising strategy to construct catalysts for the generation of solar fuels. Here, we report a photocatalyst for CO2 reduction that consists of a polymeric cobalt phthalocyanine catalyst (CoPPc) coupled with mesoporous carbon nitride (mpg-CNx ) as the photosensitizer. This precious-metal-free hybrid catalyst selectively converts CO2 to CO in organic solvents under UV/Vis light (AM 1.5G, 100 mW cm-2 , λ>300 nm) with a cobalt-based turnover number of 90 for CO after 60 h. Notably, the photocatalyst retains 60 % CO evolution activity under visible light irradiation (λ>400 nm) and displays moderate water tolerance. The in situ polymerization of the phthalocyanine allows control of catalyst loading and is key for achieving photocatalytic CO2 conversion.