Recent Advances in Porphyrin-Based Systems for Electrochemical Oxygen Evolution Reaction

Unveiling the potential of tailored β-substituted iron-porphyrins for highly efficient oxygen reduction reactions (ORR)

A series of β-substituted iron porphyrins, including Fe(III)ClTPPMe4, Fe(III)ClTPPPh4, and Fe(III)ClTPPBr4, featuring electron-withdrawing and electron-donating groups, were synthesized, characterized, and evaluated for their catalytic activity in the oxygen reduction reaction (ORR). These catalysts exhibited onset potentials of 0.77 V, 0.98 V, and 0.79 V (RHE), comparable to platinum-based electrodes, along with high stability over 11 000 s and a 4e-/4H+ transfer pathway. Electron-withdrawing β-substituents combined with steric effects enhance ORR performance, with the phenyl-substituted porphyrin exhibiting unique catalytic behavior due to steric distortions in the porphyrin ring. DFT calculations revealed that a reduced HOMO-LUMO gap facilitates electron transfer which in turn improves catalytic efficiency.

Metalloporphyrins in bio-inspired photocatalytic conversions

Numerous natural systems contain porphyrin derivatives that facilitate important catalytic processes; thus, developing biomimetic photocatalytic systems based on synthetic metalloporphyrins constitutes a rapidly advancing and fascinating research field. Additionally, porphyrins are widely investigated in a plethora of applications due to their highly versatile structure, presenting advantageous photoredox, photophysical and photochemical properties. Consequently, such metallated tetrapyrrolic macrocycles play a prominent role as photosensitizers and catalysts in developing artificial photosynthetic systems that can store and distribute energy through fuel forming reactions. This review highlights the advances in the field of metalloporphyrin-based biomimetic photocatalysis, particularly targeting water splitting, including both hydrogen and oxygen evolution reactions, carbon dioxide reduction and alcohol oxidation. For each photocatalytic system different approaches are discussed, concerning either structural modifications of the porphyrin derivatives or the phase in which the process takes place, i.e. homogenous or heterogenous. The most important findings for each porphyrin-based photocatalytic reaction are presented and accompanied by the analysis of mechanistic aspects when possible. Finally, the perspectives and limitations are discussed, providing future guidelines for the development of highly efficient metalloporphyrin-based biomimetic systems towards energy and environmental applications.

Recent Advances in Bonding Regulation of Metalloporphyrin-Modified Carbon-Based Catalysts for Accelerating Energy Electrocatalytic Applications

Metalloporphyrins modified carbon-based materials, owing to the excellent acid-base resistance, optimal electron transfer rates, and superior catalytic performance, have shown great potential in energy electrocatalysis. Recently, numerous efforts have concentrated on employing carbon-based substrates as platforms to anchor metalloporphyrins, thereby fabricating a diverse array of composite catalysts tailored for assorted electrocatalytic processes. However, the interplay through bonding regulation of metalloporphyrins with carbon materials and the resultant enhancement in catalyst performance remains inadequately elucidated. Gaining an in-depth comprehension of the synergistic interactions between metalloporphyrins and carbon-based materials within the realm of electrocatalysis is imperative for advancing the development of innovative composite catalysts. Herein, the review systematically classifies the binding modes (i.e., covalent grafting and non-covalent interactions) between carbon-based materials and metalloporphyrins, followed by a discussion on the structural characteristics and applications of metalloporphyrins supported on various carbon-based substrates, categorized according to their binding modes. Additionally, this review underscores the principal challenges and emerging opportunities for carbon-supported metalloporphyrin composite catalysts, offering both inspiration and methodological insights for researchers involved in the design and application of these advanced catalytic systems.

Design of a New Catalyst, Manganese(III) Complex, for the Oxidative Degradation of Azo Dye Molecules in Water Using Hydrogen Peroxide

In the current work, chloro(meso-tetrakis(phenyl)porphyrin) manganese(III) [Mn(TPP)Cl] was synthesized following two steps: the preparation of meso-tetraphenylporphyrin (H⁠2TPP) and the insertion of manganese into the free porphyrin H2TPP. The compounds were characterized using SEM, FT-IR, UV, TGA/DTA, and XRD analyses. Manganese(III) meso-porphyrins exhibited hyper-type electronic spectra with a half-vacant metal orbital with symmetry, such as [dπ:dxz and dyz]. The thermal behavior of [Mn(TPP)(Cl)] changed (three-step degradation process) compared to the initial H2TPP (one-step degradation process), confirming the insertion of manganese into the core of the free porphyrin H2TPP. Furthermore, [Mn(TPP)Cl] was used to degrade calmagite (an azo dye) using H2O2 as an oxidant. The effects of dye concentration, reaction time, H2O2 dose, and temperature were investigated. The azo dye solution was completely degraded in the presence of [Mn(TPP)(Cl)]/H2O2 at pH = 6, temperature = 20 °C, C0 = 30 mg/L, and H2O2 = 40 mL/L. The computed low activation energy (Ea = 10.55 Kj/mol) demonstrated the efficiency of the proposed catalytic system for the azo dye degradation. Overall, based on the synthesis process and the excellent catalytic results, the prepared [Mn(TPP)Cl] could be used as an effective catalyst for the treatment of calmagite-contaminated effluents.

Self-Assembled Nanostructure of Ionic Sn(IV)porphyrin Complex Based on Multivalent Interactions for Photocatalytic Degradation of Water Contaminants

[Sn(H2PO4)2(TPyHP)](H2PO4)4∙6H2O (2), an ionic tin porphyrin complex, was synthesized from the reaction of [Sn(OH)2TPyP] (1) with a dilute aqueous solution of a polyprotic acid (H3PO4). Complex 2 was fully characterized using various spectroscopic methods, such as X-ray single-crystal crystallography, 1H NMR spectroscopy, elemental analysis, FTIR spectroscopy, UV-vis spectroscopy, emission spectroscopy, EIS mass spectrometry, PXRD, and TGA analysis. The crystal structure of 2 reveals that the intermolecular hydrogen bonds between the peripheral pyridinium groups and the axially coordinated dihydrogen phosphate ligands are the main driving force for the supramolecular assembly. Simultaneously, the overall association of these chains in 2 leads to an open framework with porous channels. The photocatalytic degradation efficiency of methyl orange dye and tetracycline antibiotic by 2 was 83% within 75 min (rate constant = 0.023 min-1) and 75% within 60 min (rate constant = 0.018 min-1), respectively. The self-assembly of 2 resulted in a nanostructure with a huge surface area, elevated thermodynamic stability, interesting surface morphology, and excellent catalytic photodegradation performance for water pollutants, making these porphyrin-based photocatalytic systems promising for wastewater treatment.

Polyimide covalent organic frameworks bearing star-shaped electron-deficient polycyclic aromatic hydrocarbon building blocks: molecular innovations for energy conversion and storage

Polyimide covalent organic frameworks (PI-COFs) are outstanding functional materials for electrochemical energy conversion and storage owing to their integrated advantages of the high electroactive feature of polyimides and the periodic porous structure of COFs. Nevertheless, only anhydride monomers with C2 symmetry are generally used, and limited selectivity of electron-deficient monomers has become a major obstacle in the development of materials. The introduction of polycyclic aromatic hydrocarbons (PAHs) is a very effective method to regulate the structure-activity relationship of PI-COFs due to their excellent stability and electrical properties. Over the past two years, various star-shaped electron-deficient PAH building blocks possessing different compositions and topologies have been successfully fabricated, greatly improving the monomer selectivity and electrochemical performances of PI-COFs. This paper systematically summarizes the recent highlights in PI-COFs based on these building blocks. Firstly, the preparation of anhydride (or phthalic acid) monomers and PI-COFs related to different star-shaped PAHs is presented. Secondly, the applications of these PI-COFs in energy conversion and storage and the corresponding factors influencing their performance are discussed in detail. Finally, the future development of this meaningful field is briefly proposed.

A Polymer-Derived Co(Fe)Ox Oxygen Evolution Catalyst Benefiting from the Oxidative Dehydrogenative Coupling of Cobalt Porphyrins

Thin films of cobalt porphyrin conjugated polymers bearing different substituents are prepared by oxidative chemical vapor deposition (oCVD) and investigated as heterogeneous electrocatalysts for the oxygen evolution reaction (OER). Interestingly, the electrocatalytic activity originates from polymer-derived, highly transparent Co(Fe)Ox species formed under operational alkaline conditions. Structural, compositional, electrical, and electrochemical characterizations reveal that the newly formed active catalyst greatly benefited from both the polymeric conformation of the porphyrin-based thin film and the inclusion of the iron-based species originating from the oCVD reaction. High-resolution mass spectrometry analyses combined with density functional theory (DFT) calculations showed that a close relationship exists between the porphyrin substituent, the extension of the π-conjugated system cobalt porphyrin conjugated polymer, and the dynamics of the polymer conversion leading to catalytically active Co(Fe)Ox species. This work evidences the precatalytic role of cobalt porphyrin conjugated polymers and uncovers the benefit of extended π-conjugation of the molecular matrix and iron inclusion on the formation and performance of the true active catalyst.

Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Triazine-Based Porous Organic Polymers: Synthesis and Application in Dye Adsorption and Catalysis

The scientific community has been developing promising materials to increase the sustainability and efficiency of production processes and pollutant environmental remediation strategies. Porous organic polymers (POPs) are of special interest, as they are insoluble custom-built materials at the molecular level, endowed with low densities and high stability, surface areas, and porosity. This paper describes the synthesis, characterization, and performance of three triazine-based POPs (T-POPs) in dye adsorption and Henry reaction catalysis. T-POPs were prepared by a polycondensation reaction between melamine and a dialdehyde (terephthalaldehyde (T-POP1) or isophthalaldehyde derivatives with a hydroxyl group (T-POP2) or both a hydroxyl and a carboxyl group (T-POP3)). The crosslinked and mesoporous polyaminal structures, with surface areas between 139.2 and 287.4 m² g^-1, positive charge, and high thermal stability, proved to be excellent methyl orange adsorbents, removing the anionic dye with an efficiency >99% in just 15-20 min. The POPs were also effective for methylene blue cationic dye removal from water, reaching efficiencies up to ca. 99.4%, possibly due to favorable interactions via deprotonation of T-POP3 carboxyl groups. The modification of the most basic polymers, T-POP1 and T-POP2, with copper(II) allowed the best efficiencies in Henry reactions catalysis, leading to excellent conversions (97%) and selectivities (99.9%).

Conjugated porphyrin polymer films with nickel single sites for the electrocatalytic oxygen evolution reaction

Directly fused nickel(ii) porphyrins are successfully investigated as heterogeneous single-site catalysts for the oxygen evolution reaction (OER). Conjugated polymer thin films from Ni(ii) 5,15-(di-4-methoxycarbonylphenyl)porphyrin (pNiDCOOMePP) and Ni(ii) 5,15-diphenylporphyrin (pNiDPP) showed an OER onset overpotential of 270 mV, and current densities of 1.6 mA cm-2 and 1.2 mA cm-2 at 1.6 V vs. RHE, respectively, representing almost a hundred times higher activity than those of monomeric thin films. The fused porphyrin thin films are more kinetically and thermodynamically active than their non-polymerized counterparts mainly due to the formation of conjugated structures enabling a dinuclear radical oxo-coupling (ROC) mechanism at low overpotential. More importantly, we have deciphered the role of the porphyrin substituent in the conformation and performance of porphyrin conjugated polymers as (1) to control the extension of the conjugated system during the oCVD reaction, allowing the retention of the valence band deep enough to provide a high thermodynamic water oxidation potential, (2) to provide a flexible molecular geometry to facilitate O2 formation from the interaction between the Ni-O sites and to weaken the π-bond of the *Ni-O sites for enhanced radical character, and (3) to optimize the water interaction with the central metal cation of the porphyrin for superior electrocatalytic properties. These findings open the scope for molecular engineering and further integration of directly fused porphyrin-based conjugated polymers as efficient heterogeneous catalysts.

2,6-Diaminopyridine-Based Polyurea as an ORR Electrocatalyst of an Anion Exchange Membrane Fuel Cell

In order to yield more Co(II), 2,6-diaminopyridine (DAP) was polymerized with 4,4-methylene diphenyl diisocyanates (MDI) in the presence of Co(II) to obtain a Co-complexed polyurea (Co-PUr). The obtained Co-PUr was calcined to become Co, N-doped carbon (Co-N-C) as the cathode catalyst of an anion exchange membrane fuel cell (AEMFC). High-resolution transmission electron microscopy (HR-TEM) of Co-N-C indicated many Co-Nx (Co covalent bonding with several nitrogen) units in the Co-N-C matrix. X-ray diffraction patterns showed that carbon and cobalt crystallized in the Co-N-C catalysts. The Raman spectra showed that the carbon matrix of Co-N-C became ordered with increased calcination temperature. The surface area (dominated by micropores) of Co-N-Cs also increased with the calcination temperature. The non-precious Co-N-C demonstrated comparable electrochemical properties (oxygen reduction reaction: ORR) to commercial precious Pt/C, such as high on-set and half-wave voltages, high limited reduction current density, and lower Tafel slope. The number of electrons transferred in the cathode was close to four, indicating complete ORR. The max. power density (P_max) of the single cell with the Co-N-C cathode catalyst demonstrated a high value of 227.7 mWcm^-2.

Exploring the Gas-Phase Formation and Chemical Reactivity of Highly Reduced M8 L6 Coordination Cages

Coordination cages with well-defined cavities show great promise in the field of catalysis on account of their unique combination of molecular confinement effects and transition-metal redox chemistry. Here, three coordination cages are reduced from their native 16+ oxidation state to the 2+ state in the gas phase without observable structural degradation. Using this method, the reaction rate constants for each reduction step were determined, with no noticeable differences arising following either the incorporation of a C60 -fullerene guest or alteration of the cage chemical structure. The reactivity of highly reduced cage species toward molecular oxygen is "switched-on" after a threshold number of reduction steps, which is influenced by guest molecules and the structure of cage components. These new experimental approaches provide a unique window to explore the chemistry of highly-reduced cage species that can be modulated by altering their structures and encapsulated guest species.

The pH Influence on the Water-Splitting Electrocatalytic Activity of Graphite Electrodes Modified with Symmetrically Substituted Metalloporphyrins

Hydrogen, considered to be an alternative fuel to traditional fossil fuels, can be generated by splitting water molecules into hydrogen and oxygen via the use of electrical energy, in a process whose efficiency depends directly on the employed catalytic material. The current study takes part in the relentless search for suitable and low-cost catalysts relevant to the water-splitting field by investigating the electrocatalytic properties of the O₂ and H₂ evolution reactions (OER and HER) of two metalloporphyrins: Zn(II) 5,10,15,20-tetrakis(4-pyridyl)-porphyrin and Co(II) 5,10,15,20-tetrakis(3-hydroxyphenyl)-porphyrin. The TEM/STEM characterisation of the porphyrin samples obtained using different organic solvents revealed several types of self-assembled aggregates. The HER and OER experiments performed on porphyrin-modified graphite electrodes in media with different pH values revealed the most electrocatalytically active specimens. For the OER, this specimen was the electrode manufactured with one layer of Co-porphyrin applied from dimethylsulfoxide, exhibiting an overpotential of 0.51 V at i = 10 mA/cm² and a Tafel slope of 0.27 V/dec. For the HER, it was the sample obtained by drop casting one layer of Zn-porphyrin from N,N-dimethylformamide that displayed a HER overpotential of 0.52 V at i = -10 mA/cm² and a Tafel slope of 0.15 V/dec.

Recent Advances in the Photoreactions Triggered by Porphyrin-Based Triplet–Triplet Annihilation Upconversion Systems: Molecular Innovations and Nanoarchitectonics

Triplet–triplet annihilation upconversion (TTA-UC) is a very promising technology that could be used to convert low-energy photons to high-energy ones and has been proven to be of great value in various areas. Porphyrins have the characteristics of high molar absorbance, can form a complex with different metal ions and a high proportion of triplet states as well as tunable structures, and thus they are important sensitizers for TTA-UC. Porphyrin-based TTA-UC plays a pivotal role in the TTA-UC systems and has been widely used in many fields such as solar cells, sensing and circularly polarized luminescence. In recent years, applications of porphyrin-based TTA-UC systems for photoinduced reactions have emerged, but have been paid little attention. As a consequence, this review paid close attention to the recent advances in the photoreactions triggered by porphyrin-based TTA-UC systems. First of all, the photochemistry of porphyrin-based TTA-UC for chemical transformations, such as photoisomerization, photocatalytic synthesis, photopolymerization, photodegradation and photochemical/photoelectrochemical water splitting, was discussed in detail, which revealed the different mechanisms of TTA-UC and methods with which to carry out reasonable molecular innovations and nanoarchitectonics to solve the existing problems in practical application. Subsequently, photoreactions driven by porphyrin-based TTA-UC for biomedical applications were demonstrated. Finally, the future developments of porphyrin-based TTA-UC systems for photoreactions were briefly discussed.