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      Insights into mesoporous metal phosphonate hybrid materials for catalysis

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          Abstract

          Mesoporous metal phosphonates have received increasing attention as promising heterogeneous catalysts due to their abundant framework compositions and controllable porosity.

          Abstract

          Catalysis is crucial for sustainable process management. The discovery of mesoporous metal phosphonate hybrid materials has opened great opportunities for new applications in catalysis, thanks to their homogeneous composition, combined merits of inorganic units and organic groups, and considerable porosity. In this review, we present the fundamentals of designing and optimizing hybrid mesostructures and the recent progress in the fields of metal phosphonate-based catalytic reactions. The catalytic reactions are divided into three categories based on the following elements, i.e., metal sites, organophosphonate moieties, and potential precursors of active metal phosphates. Each section highlights recent advances and the corresponding reactions are discussed in detail. Direct functionalization of the mesopore surfaces of the hybrids, such as sulfonation and supporting, features an elegant way to bridge the gap between heterogeneous and homogeneous catalysis. Moreover, remarks on the challenges and perspectives are presented for further exploration of mesoporous metal phosphonate hybrid heterogeneous catalysts.

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          Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions.

          The activities of the oxygen evolution reaction (OER) on iridium-oxide- and ruthenium-oxide-based catalysts are among the highest known to date. However, the OER activities of thermodynamically stable rutile iridium oxide (r-IrO2) and rutile iridium oxide (r-RuO2), normalized to catalyst mass or true surface area are not well-defined. Here we report a synthesis of r-IrO2 and r-RuO2 nanoparticles (NPs) of ∼6 nm, and examine their OER activities in acid and alkaline solutions. Both r-IrO2 and r-RuO2 NPs were highly active for OER, with r-RuO2 exhibiting up to 10 A/goxide at 1.48 V versus reversible hydrogen electrode. When comparing the two, r-RuO2 NPs were found to have slightly higher intrinsic and mass OER activities than r-IrO2 in both acid and basic solutions. Interestingly, these oxide NPs showed higher stability under OER conditions than commercial Ru/C and Ir/C catalysts. Our study shows that these r-RuO2 and r-IrO2 NPs can serve as a benchmark in the development of active OER catalysts for electrolyzers, metal-air batteries, and photoelectrochemical water splitting applications.
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            Radical generation by the interaction of transition metals with common oxidants.

            Nine transition metals were tested for the activation of three oxidants and the generation of inorganic radical species such as sulfate, peroxymonosulfate, and hydroxyl radicals. From the 27 combinations, 14 M/Ox couples demonstrated significant reactivity toward transforming a model organic substrate such as 2,4-dichlorophenol and are further discussed here. It was found that Co(II) and Ru(III) are the best metal catalysts for the activation of peroxymonosulfate. As expected on the basis of the Fenton reagent, Fe(III) and Fe(II) were the most efficient transition metals for the activation of hydrogen peroxide. Finally, Ag(I) showed the best results toward activating persulfate. Quenching studies with specific alcohols (tert-butyl alcohol and ethanol) were also performed to identify the primary radical species formed from the reactive M/Ox interactions. The determination of these transient species allowed us to postulate the rate-determining step of the redox reactions taking place when a metal is coupled with an oxidant in aqueous solution. It was found that when Co(II), Ru(III), and Fe(II) interact with peroxymonosulfate, freely diffusible sulfate radicals are the primary species formed. The same was proven for the interaction of Ag(I) with persulfate, but in this case caged or bound to the metal sulfate radicals might be formed as well. The conjunction of Ce(III), Mn(II), and Ni(II) with peroxymonosulfate showed also to generate caged or bound to the metal sulfate radicals. A combination of sulfate and hydroxyl radicals was formed from the conjunction of V(III) with peroxymonosulfate and from Fe(II) with persulfate. Finally, the conjunction of Fe(III), Fe(II), and Ru(III) with hydrogen peroxide led primarily to the generation of hydroxyl radicals. It is also suggested here that the redox behavior of a particular metal in solution cannot be predicted based exclusively on its size and charge. Additional phenomena such as metal hydrolysis as well as complexation with other counterions present in solution might affect the thermodynamics of the overall process and are further discussed here.
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              In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+.

              The utilization of solar energy on a large scale requires its storage. In natural photosynthesis, energy from sunlight is used to rearrange the bonds of water to oxygen and hydrogen equivalents. The realization of artificial systems that perform "water splitting" requires catalysts that produce oxygen from water without the need for excessive driving potentials. Here we report such a catalyst that forms upon the oxidative polarization of an inert indium tin oxide electrode in phosphate-buffered water containing cobalt (II) ions. A variety of analytical techniques indicates the presence of phosphate in an approximate 1:2 ratio with cobalt in this material. The pH dependence of the catalytic activity also implicates the hydrogen phosphate ion as the proton acceptor in the oxygen-producing reaction. This catalyst not only forms in situ from earth-abundant materials but also operates in neutral water under ambient conditions.
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                Author and article information

                Contributors
                Journal
                CSTAGD
                Catalysis Science & Technology
                Catal. Sci. Technol.
                Royal Society of Chemistry (RSC)
                2044-4753
                2044-4761
                2015
                2015
                : 5
                : 9
                : 4258-4279
                Affiliations
                [1 ]Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education)
                [2 ]Collaborative Innovation Center of Chemical Science and Engineering (Tianjin)
                [3 ]College of Chemistry
                [4 ]Nankai University
                [5 ]Tianjin 300071
                [6 ]School of Chemical Engineering and Technology
                [7 ]Hebei University of Technology
                [8 ]Tianjin 300130
                [9 ]China
                Article
                10.1039/C5CY00107B
                9e331127-2ebb-4415-b1c8-6a7b6fe2b76f
                © 2015
                History

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