Photochemical hydrogen generation with porphyrin-based systems

Dye-sensitized photocatalyst for effective water splitting catalyst

Renewable hydrogen production is a sustainable method for the development of next-generation energy technologies. Utilising solar energy and photocatalysts to split water is an ideal method to produce hydrogen. In this review, the fundamental principles and recent progress of hydrogen production by artificial photosynthesis are reviewed, focusing on hydrogen production from photocatalytic water splitting using organic-inorganic composite-based photocatalysts.

ZnTPPS demetalation: Role of polyelectrolytes on aggregation after protonation in acid

Acid-base properties of tetra-anionic zinc meso-5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin (ZnTPPS) in presence of cationic and anionic polyelectrolytes are studied by UV-visible spectroscopy. In fact, spectral modifications related to the out-of-plane deformation of porphyrins after protonation allow for an easy probing of the protonation event that, in presence of a metal derivative, requires a deprotonation step. Interactions with poly-D-glutamate and poly-L-lysine, modulated by system electrostatics and by the presence of an axially-coordinated central metal ion, trigger both protonation occurrences as well as porphyrin self-aggregation. The crucial role of electrostatic interactions experienced by the metalated inner core in strong acid solution, probed by UV-visible spectroscopy, are discussed.

Photosensitized H2 Production Using a Zinc Porphyrin-Substituted Protein, Platinum Nanoparticles, and Ascorbate with No Electron Relay: Participation of Good's Buffers

Development of efficient light-driven splitting of water, 2H₂O → 2H₂ + O₂, often attempts to optimize photosensitization of the reductive and oxidative half-reactions individually. Numerous homogeneous and heterogeneous systems have been developed for photochemical stimulation of the reductive half reaction, 2H⁺ + 2e^- → H₂. These systems generally consist of various combinations of a H⁺ reduction catalyst, a photosensitizer (PS), and a "sacrificial" electron donor. Zinc(II)-porphyrins (ZnPs) have frequently been used as PSs for H₂ generation, but they are subject to various self-quenching processes in aqueous solutions. Colloidal platinum in nanoparticle form (Pt NP) is a classical H⁺ reduction catalyst using ZnP photosensitizers, but efficient photosensitized H₂ generation requires an electron relay molecule between ZnP and Pt NP. The present report describes an aqueous system for visible (white) light-sensitized generation of H₂ using a protein-embedded Zn(II)-protoporphyrin IX as PS and Pt NP as H⁺ reduction catalyst without an added electron relay. This system operated efficiently in piperazino- and morpholino-alkylsulfonic acid (Good's buffers), which served as sacrificial electron donors. The system also required ascorbate at relatively modest concentrations, which stabilized the Zn(II)-protoporphyrin IX against photodegradation. In the absence of an electron relay molecule, the photosensitized H₂ generation must involve formation of at least a transient complex between a protein-embedded Zn(II)-protoporphyrin IX species and Pt NP.

A molecular approach to an electrocatalytic hydrogen evolution reaction on single-layer graphene

A major challenge in the development of electrocatalysts is to determine a detailed catalysis mechanism on a molecular level for enhancing catalytic activity. Here, we present bottom-up studies for an electrocatalytic hydrogen evolution reaction (HER) process through molecular activation to systematically control surface catalytic activity corresponding to an interfacial charge transfer in a porphyrin monolayer on inactive graphene. The two-dimensional (2D) assembly of porphyrins that create homogeneous active sites (e.g., electronegative tetrapyrroles (N₄)) on graphene showed structural stability against electrocatalytic reactions and enhanced charge transfer at the graphene-liquid interface. Performance operations of the graphene field effect transistor (FET) were an effective method to analyse the interfacial charge transfer process associated with information about the chemical nature of the catalytic components. Electronegative pristine porphyrin or Pt-porphyrin networks, where intermolecular hydrogen bonding functioned, showed larger interfacial charge transfers and higher HER performance than Ni-, or Zn-porphyrin. A process to create surface electronegativity by either central N₄ or metal (M)-N₄ played an important role in the electrocatalytic reaction. These findings will contribute to an in-depth understanding at the molecular level for the synergetic effects of molecular structures on the active sites of electrocatalysts toward HER.

Cobalt, nickel, and iron complexes of 8-hydroxyquinoline-di(2-picolyl)amine for light-driven hydrogen evolution

Novel first-row transition metal complexes based on the 8-hydroxyquinoline-di(2-picolyl)amine ligand were prepared and tested as potential HECs in light-driven experiments.

Photocatalytic hydrogen production based on a water-soluble porphyrin derivative as sensitizer and a series of Wilkinson type complexes as catalysts

Herein, we report photochemical hydrogen evolution systems consisting of various rhodium based catalysts with Wilkinson type structures, Zn metalated porphyrins and fluorescein as photosensitizers and triethanolamine as a sacrificial electron donor in acetonitrile/H2O (1:1) solution. Since rhodium complexes 1 and 2 are used for the first time as catalysts in this type of systems, a systematic study was performed in order to elucidate the best conditions for H2 production. Upon visible irradiation hydrogen production was detected and the best results were obtained at pH 7 when dye P3 and catalyst 1 were used with a TON of 61, after 48 h and in the presence of dye P1 and catalyst 2 with a TON of 69, after the 48 h.

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Because of the significance of energy-related small molecule activation, this review covers recent progress in hydrogen evolution, oxygen evolution, and oxygen reduction reactions catalyzed by porphyrins and corroles.

Photosensitized H2 generation from "one-pot" and "two-pot" assemblies of a zinc-porphyrin/platinum nanoparticle/protein scaffold

We report photosensitized H2 generation using a protein scaffold that nucleates formation of platinum nanoparticles (Pt NPs) and contains "built-in" photosensitizers. The photosensitizers, zinc-protoporphyrin IX or zinc-mesoporphyrin IX (ZnP) were incorporated in place of the naturally occurring heme in the 24-subunit iron storage protein bacterioferritin (Bfr) when the ZnPs were added to the E. coli expression medium. We engineered a stable dimeric Bfr variant with two protein subunits sandwiching a ZnP. Ten glycines were also substituted in place of residues surrounding the vinyl side of the porphyrin in order increase access of solvent and/or redox agents. An optimized "one-pot" reaction of this glycine-substituted ZnMP-Bfr dimer with a Pt(iv) salt and borohydride resulted in a ∼50 : 50 mixture of protein in the form of Pt-free glycine-substituted ZnP-Bfr dimers and re-assembled 24-mers surrounding Pt NPs formed in situ. H2 production occurred upon visible light irradiation of this "one-pot" product when combined with triethanolamine as sacrificial electron donor and methyl viologen as electron relay. An analogous "two-pot" system containing mixtures of separately prepared Pt-free glycine-substituted ZnP-Bfr dimer and porphyrin-free Pt NP@Bfr 24-mer also showed robust photosensitized H2 generation. The glycine-substituted-ZnP-Bfr dimer thus served as photosensitizer for catalytic reduction of methyl viologen by triethanolamine, and the reduced methyl viologen was able to transfer electrons across the Bfr 24-mer protein shell to generate H2 at the enclosed Pt NP in a "dark" reaction. Our results demonstrate that Bfr is a readily manipulatable and versatile scaffold for photosensitized redox chemistry.

Photochemical hydrogen production and cobaloximes: the influence of the cobalt axial N-ligand on the system stability

We report on the first systematic study of cobaloxime-based hydrogen photoproduction in mixed pH 7 aqueous/acetonitrile solutions and demonstrate that H2 evolution can be tuned through electronic modifications of the axial cobalt ligand or through introduction of TiO2 nanoparticles. The photocatalytic systems consist of various cobaloxime catalysts [Co(dmgH)2(L)Cl] (L = nitrogen-based axial ligands) and a water soluble porphyrin photosensitizer. They were assayed in the presence of triethanolamine as a sacrificial electron donor. Optimal turnover numbers related to the photosensitizer are obtained with electron-rich axial ligands such as imidazole derivatives (1131 TONs with N-methyl imidazole). Lower stabilities are observed with various pyridine axial ligands (443 TONs for para-methylpyridine), especially for those containing electron-acceptor substituents. Interestingly, when L is para-carboxylatopyridine the activity of the system is increased from 40 to 223 TONs in the presence of TiO2 nanoparticles.

Photochemical hydrogen evolution using Sn-porphyrin as photosensitizer and a series of Cobaloximes as catalysts

Herein, we report a photochemical hydrogen evolution system consisting of various cobalt based catalysts, a metallated Sn porphyrin as photosensitizer and a triethanolamine as a sacrificial electron donor in acetonitrile/H2O (1:1) solution. Since a tin metallated porphyrin is used for the first time as photosensitizer in this type of systems, a systematic study was performed in order to elucidate the best conditions for H[Formula: see text]production. Upon visible irradiation hydrogen production was detected with the best result obtained at pH 7 with a TON of 150, after 100 h in the presence of catalyst 1. Moreover, when TiO2 nanoparticles were added at the catalytic system 2 the hydrogen production was increased from 53 TON to 131 TON, under the same experimental conditions.

Photoinduced hydrogen evolution with new tetradentate cobalt(ii) complexes based on the TPMA ligand

New cobalt(ii) complexes based on the TPMA ligand have been synthesized and characterized as molecular catalysts for photoinduced hydrogen evolution.

Molecular-structure control of electron transfer dynamics of push–pull porphyrins as sensitizers for NiO based dye sensitized solar cells

Zn(ii)-porphyrin dyes for NiO dye-sensitized solar cells showed surprisingly rapid charge recombination, in spite of their push–pull character. Appending a secondary acceptor prolonged charge separation and led to improved photovoltaic performance.

Photocatalytic hydrogen evolution with ruthenium polypyridine sensitizers: unveiling the key factors to improve efficiencies

An alternative benchmark sensitizer for photochemical hydrogen generation is identified within a series of ruthenium complexes, capable of outperforming the standard Ru(bpy)₃²⁺.

Antimony porphyrins as red-light powered photocatalysts for solar fuel production from halide solutions in the presence of air

Stable light-harvesting sensitizers for the two-electron oxidation of halide ions are reported. Photocatalysis is studied in solution, in aqueous micellar medium and with surface immobilized samples for convenient photocatalyst recycling.