Copper metal is in theory a viable oxidative electrocatalyst based on surface oxidation to Cu(III) and/or Cu(IV) , but its use in water oxidation has been impeded by anodic corrosion. The in situ formation of an efficient interfacial oxygen-evolving Cu catalyst from Cu(II) in concentrated carbonate solutions is presented. The catalyst necessitates use of dissolved Cu(II) and accesses the higher oxidation states prior to decompostion to form an active surface film, which is limited by solution conditions. This observation and restriction led to the exploration of ways to use surface-protected Cu metal as a robust electrocatalyst for water oxidation. Formation of a compact film of CuO on Cu surface prevents anodic corrosion and results in sustained catalytic water oxidation. The Cu/CuO surface stabilization was also applied to Cu nanowire films, which are transparent and flexible electrocatalysts for water oxidation and are an attractive alternative to ITO-supported catalysts for photoelectrochemical applications.
Chloride oxidation to chlorine is a potential alternative to water oxidation to oxygen as a solar fuels half-reaction. Ag(I) is potentially an oxidative catalyst but is inhibited by the high potentials for accessing the Ag(II/I) and Ag(III/II) couples. We report here that the complex ions AgCl2(-) and AgCl3(2-) form in concentrated Cl(-) solutions, avoiding AgCl precipitation and providing access to the higher oxidation states by delocalizing the oxidative charge over the Cl(-) ligands. Catalysis is homogeneous and occurs at high rates and low overpotentials (10 mV at the onset) with μM Ag(I). Catalysis is enhanced in D2O as solvent, with a significant H2O/D2O inverse kinetic isotope effect of 0.25. The results of computational studies suggest that Cl(-) oxidation occurs by 1e(-) oxidation of AgCl3(2-) to AgCl3(-) at a decreased potential, followed by Cl(-) coordination, presumably to form AgCl4(2-) as an intermediate. Adding a second Cl(-) results in "redox potential leveling", with further oxidation to {AgCl2(Cl2)}(-) followed by Cl2 release.
Simply mixing a Cu(II) salt and 1,2-ethylenediamine (en) affords precursors for both heterogeneous or homogeneous water oxidation catalysis, depending on pH. In phosphate buffer at pH 12, the Cu(II) en complex formed in solution is decomposed to give a phosphate-incorporated CuO/Cu(OH)2 film on oxide electrodes that catalyzes water oxidation. A current density of 1 mA/cm2 was obtained at an overpotential of 540 mV, a significant enhancement compared to other Cu-based surface catalysts. The results of electrolysis studies suggest that the solution en complex decomposes by en oxidation to glyoxal, following Cu(II) oxidation to Cu(III). At pH 8, the catalysis shifts from heterogeneous to homogeneous with a single-site mechanism for Cu(II)/en complexes in solution. A further decrease in pH to 7 leads to electrode passivation via the formation of a Cu(II) phosphate film during electrolyses. As the pH is decreased, en, with pK b ≈ 6.7, becomes less coordinating and the precipitation of the Cu(II) film inhibits water oxidation. The Cu(II)-based reactivity toward water oxidation is shared by Cu(II) complexation to the analogous 1,3-propylenediamine (pn) ligand over a wide pH range.
Copper metal is in theory a viable oxidative electrocatalyst based on surface oxidation to Cu III and/or Cu IV , but its use in water oxidation has been impeded by anodic corrosion. The in situ formation of an efficient interfacial oxygen-evolving Cu catalyst from Cu II in concentrated carbonate solutions is presented. The catalyst necessitates use of dissolved Cu II and accesses the higher oxidation states prior to decompostion to form an active surface film, which is limited by solution conditions. This observation and restriction led to the exploration of ways to use surface-protected Cu metal as a robust electrocatalyst for water oxidation. Formation of a compact film of CuO on Cu surface prevents anodic corrosion and results in sustained catalytic water oxidation. The Cu/CuO surface stabilization was also applied to Cu nanowire films, which are transparent and flexible electrocatalysts for water oxidation and are an attractive alternative to ITO-supported catalysts for photoelectrochemical applications.
With the depletion of fossil fuels and environmental contamination, photocatalytic H2 production has become an essential issue. Co‐catalysts play a critical role in improving photocatalytic H2 generation of photocatalysts. However, co‐catalysts frequently need additional synthesis steps for loading on the surface of photocatalysts, and the interface contact between the co‐catalyst and the photocatalyst is insufficient. Herein, a CdS/MoS2 nanooctahedron heterostructure is prepared through the in situ sulfidation of CdMoO4 nanooctahedrons. MoS2 as the co‐catalyst provides active sites for H2 generation and enhances the separation of photo‐generated carriers. Furthermore, the sulfidation of CdMoO4 precursors ensures a tight contact interface by S atoms between CdS and MoS2, which is beneficial to the electrons transfer from CdS to MoS2, thus markedly improving the photocatalytic H2 evolution activity. The obtained optimum CdS/MoS2 nanooctahedrons exhibit a better photocatalytic H2 generation activity than those of pure CdS, pure MoS2, and even CdS/MoS2 by hydrothermal synthesis under visible light irradiation. In addition, solar‐driven biomass upgrading of furfural alcohol, bacterial cellulose membrane, bioplastic wastes upgrading of polylactic acid (PLA), polyethylene terephthalate (PET), and their reforming to H2 are also performed and demonstrate an inexpensive route to drive aqueous proton reduction to H2 through waste biomass oxidation.
Tandem ammonia borane dehydrogenation and nitroarenes hydrogenation has been reported as a novel strategy for the preparation of aromatic amines. However, the practical application of this strategy is subjected to the high-cost and tedious preparation of supported noble metal nanocatalysts. The commercially available CuO powder is herein demonstrated to be a robust catalyst for hydrogenation of nitroarenes using ammonia borane as a hydrogen source under mild conditions. Numerous amines (even sterically hindered, halogenated, and diamines) could be obtained through this method. This monometallic catalyst is characteristic of support-free, excellent chemoselectivity, low-cost, and high recyclability, which will favor its future utilization in preparative reduction chemistry. Mechanistic studies are also carried out to clarify that diazene and azoxybenzene are key intermediates of this heterogeneous reduction.Aromatic amines serve as key intermediates for the production of a wide range of high value-added materials, including dyes, pharmaceuticals, polymers, agrochemicals, etc. The increasing demand for functionalized amines has continuously driven the development of green and economical synthetic methods for their scalable production. The sustainable reduction of aromatic nitro compounds represents one of the most straightforward, industrially applicable approaches to synthesize anilines. [1] Although remarkable progress has been made in the direct hydrogenation of nitroarenes using pressurized H 2 as a hydrogen source with non-noble metal catalysts, [2][3][4][5][6][7][8] the transfer hydrogenation could realize safe reductions under mild conditions without specialized reaction setups. Therefore, NaBH 4 , [9] HCOONH 4 , [10] HCOOH, [11] hydrazine hydrate, [12] silane, [13] and H 3 PO 3 [14] have emerged as alternative hydrogen storage materials for reduction of nitro compounds.Unlike the above hydrogen sources, ammonia borane (BH 3 · NH 3 , denoted as AB hereafter) has been generally considered as a practical reservoir for hydrogen storage and transport, due to its high hydrogen capacity (19.6 wt %), stability (solid at room temperature), safety, and nontoxicity. [15] Additionally, bulk quantities of AB are commercially available and affordable. Hydrogen evolution from AB can be expediently performed via hydrolysis in the presence of transition metals (BH 3 · NH 3 + 2H 2 O!NH 4 + + BO 2 À + 3H 2 "), and one-pot tandem AB dehydrogenation and unsaturated compound hydrogenation provides a viable strategy for the manufacture of valuable molecules. As a metal-free reductant, AB has been directly used to initiate the reduction of imines, [16] ketones, [17] aldehydes, [17] and even polarized olefins. [18] Although the reducing ability of AB is not strong enough on its own to initiate the direct hydrogenation of other unsaturated functional groups, the use of metal catalysts with this reagent resolves this challenge. There are numerous reports describing the use of AB as the hydrogen donor for the catalytic transfer hydrog...
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