9 research outputs found
Boosting the performance of single-atom catalysts via external electric field polarization
Single-atom catalysts represent a unique catalytic system with high atomic utilization and tunable reaction pathway. Despite current successes in their optimization and tailoring through structural and synthetic innovations, there is a lack of dynamic modulation approach for the single-atom catalysis. Inspired by the electrostatic interaction within specific natural enzymes, here we show the performance of model single-atom catalysts anchored on two-dimensional atomic crystals can be systematically and efficiently tuned by oriented external electric fields. Superior electrocatalytic performance have been achieved in single-atom catalysts under electrostatic modulations. Theoretical investigations suggest a universal “onsite electrostatic polarization” mechanism, in which electrostatic fields significantly polarize charge distributions at the single-atom sites and alter the kinetics of the rate determining steps, leading to boosted reaction performances. Such field-induced on-site polarization offers a unique strategy for simulating the catalytic processes in natural enzyme systems with quantitative, precise and dynamic external electric fields
Spectra of Subdivision Vertex-Edge Join of Three Graphs
In this paper, we introduce a new graph operation called subdivision vertex-edge join (denoted by G 1 S â–ą ( G 2 V ∪ G 3 E ) for short), and then the adjacency spectrum, the Laplacian spectrum and the signless Laplacian spectrum of G 1 S â–ą ( G 2 V ∪ G 3 E ) are respectively determined in terms of the corresponding spectra for a regular graph G 1 and two arbitrary graphs G 2 and G 3 . All the above can be viewed as the generalizations of the main results in [X. Liu, Z. Zhang, Bull. Malays. Math. Sci. Soc., 2017:1⁻17]. Furthermore, we also determine the normalized Laplacian spectrum of G 1 S â–ą ( G 2 V ∪ G 3 E ) whenever G i are regular graphs for each index i = 1 , 2 , 3 . As applications, we construct infinitely many pairs of A-cospectral mates, L-cospectral mates, Q-cospectral mates and L -cospectral mates. Finally, we give the number of spanning trees, the (degree-)Kirchhoff index and the Kemeny’s constant of G 1 S â–ą ( G 2 V ∪ G 3 E ) , respectively
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Boosting the performance of single-atom catalysts via external electric field polarization.
Single-atom catalysts represent a unique catalytic system with high atomic utilization and tunable reaction pathway. Despite current successes in their optimization and tailoring through structural and synthetic innovations, there is a lack of dynamic modulation approach for the single-atom catalysis. Inspired by the electrostatic interaction within specific natural enzymes, here we show the performance of model single-atom catalysts anchored on two-dimensional atomic crystals can be systematically and efficiently tuned by oriented external electric fields. Superior electrocatalytic performance have been achieved in single-atom catalysts under electrostatic modulations. Theoretical investigations suggest a universal "onsite electrostatic polarization" mechanism, in which electrostatic fields significantly polarize charge distributions at the single-atom sites and alter the kinetics of the rate determining steps, leading to boosted reaction performances. Such field-induced on-site polarization offers a unique strategy for simulating the catalytic processes in natural enzyme systems with quantitative, precise and dynamic external electric fields
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Selective Formation of Acetic Acid and Methanol by Direct Methane Oxidation Using Rhodium Single-Atom Catalysts
Atomically dispersed catalysts such as single-atom catalysts have been shown to be effective in selectively oxidizing methane, promising a direct synthetic route to value-added oxygenates such as acetic acid or methanol. However, an important challenge of this approach has been that the loading of active sites by single-atom catalysts is low, leading to a low overall yield of the products. Here, we report an approach that can address this issue. It utilizes a metal-organic framework built with porphyrin as the linker, which provides high concentrations of binding sites to support atomically dispersed rhodium. It is shown that up to 5 wt% rhodium loading can be achieved with excellent dispersity. When used for acetic acid synthesis by methane oxidation, a new benchmark performance of 23.62 mmol·gcat-1·h-1 was measured. Furthermore, the catalyst exhibits a unique sensitivity to light, producing acetic acid (under illumination, up to 66.4% selectivity) or methanol (in the dark, up to 65.0% selectivity) under otherwise identical reaction conditions
Selective Methane Oxidation by Heterogenized Iridium Catalysts
Oxidative
methane (CH4) carbonylation promises a direct
route to the synthesis of value-added oxygenates such as acetic acid
(CH3COOH). Here, we report a strategy to realize oxidative
CH4 carbonylation through immobilized Ir complexes on an
oxide support. Our immobilization approach not only enables direct
CH4 activation but also allows for easy separation and
reutilization of the catalyst. Furthermore, we show that a key step,
methyl migration, that forms a C–C bond, is sensitive to the
electrophilicity of carbonyl, which can be tuned by a gentle reduction
to the Ir centers. While the as-prepared catalyst that mainly featured
Ir(IV) preferred CH3COOH production, a reduced catalyst
featuring predominantly Ir(III) led to a significant increase of CH3OH production at the expense of the reduced yield of CH3COOH
Highly Selective Photocatalytic Methane Coupling by Au-Modified Bi<sub>2</sub>WO<sub>6</sub>
Photocatalytic
oxidative coupling of methane (OCM) to ethane promises
a route to value-added C2 products from an abundant and
low-cost feedstock. However, selective activation of the C–H
bond of CH4 without overoxidation to CO2 has
been a major challenge. In this work, we present the use of Au-modified
Bi2WO6 as a prototypical photocatalyst, demonstrating
a high performance of OCM through photocatalysis. A C2H6 production rate at 1.69 × 103 μmol·g–1·h–1 with approximately 85%
selectivity was achieved, which ranks among the top-performing photocatalytic
OCM systems. Efforts were also made in establishing a correlation
between improved OCM performance and the photocatalyst system by examining
the nature of the oxide photocatalyst. Our findings indicated that
oxygen within the oxide surface, likely from adsorbed and subsequently
dissociated oxygen at the vacancy sites, afforded a desired reactivity
to selectively activate the C–H bond without significant overoxidation.
Surprisingly, it was revealed that the Au cocatalyst plays dual roles
of activating the oxide photocatalyst for enhanced CH4 activation
and promoting C–C coupling to yield C2H6 as the main product