Polymer Coated Functional Catalysts for Industrial Applications

Surface engineering of conventional catalysts using polymeric coating has been extensively explored for producing hybrid catalytic material with enhanced activity, high mechanical and thermal stability, enhanced productivity, and selectivity of the desired product. The present review discusses in detail the state-of-the-art knowledge on surface modification of catalysts, namely photocatalysts, electrocatalysts, catalysts for photoelectrochemical reactions, and catalysts for other types of reactions, such as hydrodesulfurization, carbon dioxide cycloaddition, and noble metal-catalyzed oxidation/reduction reactions. The various techniques employed for the polymer coating of catalysts are discussed and the role of polymers in enhancing the catalytic activity is critically analyzed. The review further discusses the applications of biodegradable and biocompatible natural polysaccharide-based polymers, namely, chitosan and polydopamine as prospective coating material.

Electrochemical Polymerization Provides a Function-Integrated System for Water Oxidation

Water oxidation is a key reaction in natural and artificial photosynthesis. In nature, the reaction is efficiently catalyzed by a metal-complex-based catalyst surrounded by hole-transporting amino acid residues. However, in artificial systems, there is no example of a water oxidation system that has a catalytic center surrounded by hole transporters. Herein, we present a facile strategy to integrate catalytic centers and hole transporters in one system. Electrochemical polymerization of a metal-complex-based precursor afforded a polymer-based material (Poly-1). Poly-1 exhibited excellent hole-transporting ability and catalyzed water oxidation with high performance. It was also revealed that the catalytic activity was almost completely suppressed in the absence of the hole-transporting moieties. The present study provides a novel strategy for constructing efficient molecule-based systems for water oxidation.

Control over Electrochemical Water Oxidation Catalysis by Preorganization of Molecular Ruthenium Catalysts in Self-Assembled Nanospheres

Oxygen formation through water oxidation catalysis is a key reaction in the context of fuel generation from renewable energies. The number of homogeneous catalysts that catalyze water oxidation at high rate with low overpotential is limited. Ruthenium complexes can be particularly active, especially if they facilitate a dinuclear pathway for oxygen bond formation step. A supramolecular encapsulation strategy is reported that involves preorganization of dilute solutions (10-5  m) of ruthenium complexes to yield high local catalyst concentrations (up to 0.54 m). The preorganization strategy enhances the water oxidation rate by two-orders of magnitude to 125 s-1 , as it facilitates the diffusion-controlled rate-limiting dinuclear coupling step. Moreover, it modulates reaction rates, enabling comprehensive elucidation of electrocatalytic reaction mechanisms.

Production of solar chemicals: gaining selectivity with hybrid molecule/semiconductor assemblies

Research on the production of solar fuels and chemicals has rocketed over the past decade, with a wide variety of systems proposed to harvest solar energy and drive chemical reactions. In this Feature Article we have focused on hybrid molecule/semiconductor assemblies in both powder and supported materials, summarising recent systems and highlighting the enormous possibilities offered by such assemblies to carry out highly demanding chemical reactions with industrial impact. Of relevance is the higher selectivity obtained in visible light-driven organic transformations when using molecular catalysts compared to photocatalytic materials.

Optimization of Photoanodes for Photocatalytic Water Oxidation by Combining a Heterogenized Iridium Water-Oxidation Catalyst with a High-Potential Porphyrin Photosensitizer

The development of water-splitting dye-sensitized photoelectrochemical cells has gained interest owing to their ability to generate renewable fuels from solar energy. In this study, photoanodes were assembled from a SnO₂ film sensitized with a combination of a high-potential CF₃ -substituted porphyrin dye with a tetrahydropyranyl-protected hydroxamic acid surface-anchoring group and a Cp*Ir (Cp*=pentamethylcyclopentadienyl) water-oxidation catalyst containing a silatrane anchoring group. The dye/catalyst ratios were varied from 2:1 to 32:1 to optimize the photocatalytic water oxidation. Photoelectrochemical measurements showed not only more stable and reproducible photocurrents for lower dye/catalyst ratios but also improved photostability. O₂ production was confirmed in real time over a 20 h period with a Clark electrode. Photoanodes prepared from 2:1 and 8:1 dye/catalyst sensitization solutions provided the most active electrodes for photocatalytic water oxidation and performed approximately 30-35 turnovers in 20 h.

Plasmon-enhanced light-driven water oxidation by a dye-sensitized photoanode

Dye-sensitized photoelectrosynthesis cells (DSPECs) provide a flexible approach for solar water splitting based on the integration of molecular light absorption and catalysis on oxide electrodes. Recent advances in this area, including the use of core/shell oxide interfacial structures and surface stabilization by atomic layer deposition, have led to improved charge-separation lifetimes and the ability to obtain substantially improved photocurrent densities. Here, we investigate the introduction of Ag nanoparticles into the core/shell structure and report that they greatly enhance light-driven water oxidation at a DSPEC photoanode. Under 1-sun illumination, Ag nanoparticle electrodes achieved high photocurrent densities, surpassing 2 mA cm^-2 with an incident photon-to-current efficiency of 31.8% under 450-nm illumination.

Two Electrode Collector-Generator Method for the Detection of Electrochemically or Photoelectrochemically Produced O2

A dual working electrode technique for the in situ production and quantification of electrochemically or photoelectrochemically produced O2 is described. This technique, termed a collector-generator cell, utilizes a transparent fluorine doped tin oxide electrode to sense O2. This setup is specifically designed for detecting O2 in dye sensitized photoelectrosynthesis cells.

Analysis of Homogeneous Water Oxidation Catalysis with Collector-Generator Cells

A collector-generator (C-G) technique has been applied to determine the Faradaic efficiencies for electrocatalytic O2 production by the homogeneous water oxidation catalysts Ru(bda)(isoq)2 (1; bda = 2,2'-bipyridine and isoq = isoquinoline) and [Ru(tpy)(bpz)(OH2)](2+) (2; tpy = 2,2':6',2″-terpyridine and bpz = 2,2'-bipyrazine). This technique uses a custom-fabricated cell consisting of two fluorine-doped tin oxide (FTO) working electrodes separated by 1 mm with the conductive sides facing each other. With a catalyst in solution, water oxidation occurs at one FTO electrode under a sufficient bias to drive O2 formation by the catalyst; the O2 formed then diffuses to the second FTO electrode poised at a potential sufficiently negative to drive O2 reduction. A comparison of the current versus time response at each electrode enables determination of the Faradaic efficiency for O2 production with high concentrations of supporting electrolyte important for avoiding capacitance effects between the electrodes. The C-G technique was applied to electrocatalytic water oxidation by 1 in the presence of the electron-transfer mediator Ru(bpy)3(2+) in both unbuffered aqueous solutions and with the added buffer bases HCO3(-), HPO4(2-), imidazole, 1-methylimidazole, and 4-methoxypyridine. HCO3(-) and HPO4(2-) facilitate water oxidation by atom-proton transfer (APT), which gave Faradaic yields of 100%. With imidazole as the buffer base, coordination to the catalyst inhibited water oxidation. 1-Methylimidazole and 4-methoxypyridine gave O2 yields of 55% and 76%, respectively, with the lower Faradaic efficiencies possibly due to competitive C-H oxidation of the bases. O2 evolution by catalyst 2 was evaluated at pH 12 with 0.1 M PO4(3-) and at pH 7 in a 0.1 M H2PO4(-)/HPO4(2-) buffer. At pH 12, at an applied potential of 0.8 V vs SCE, water oxidation by the Ru(IV)(O)(2+) form of the catalyst gave O2 in 73% yield. In a pH 7 solution, water oxidation at 1.4 V vs SCE, which is dominated by Ru(V)(O)(3+), gave O2 with an efficiency of 100%. The lower efficiency for Ru(IV)(O)(2+) at pH 12 may be due to competitive oxidation of a polypyridyl ligand.