294 research outputs found

    Mechanistic Insight into the Facet-Dependent Adsorption of Methanol on a Pt<sub>3</sub>Ni Nanocatalyst

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    There is great interest in the design of catalysts for use in direct methanol fuel cells (DMFCs). However, our atomistic understanding of the adsorption and oxidation mechanism of CH<sub>3</sub>OH at the different crystal planes of metal catalysts is far from satisfactory. This study endeavors to gain mechanistic insight into the intrinsic facet-dependent adsorption properties of CH<sub>3</sub>OH on the surface of a Pt<sub>3</sub>Ni catalyst. We analyzed the adsorption characteristics of CH<sub>3</sub>OH by optimizing the structures and calculating the adsorption energies at each site on the (111), (100), and (110) surfaces of a Pt<sub>3</sub>Ni catalyst using density functional theory (DFT) calculations with van der Waals (vdW) corrections to the total energy. Our results indicate that the adsorption strength of CH<sub>3</sub>OH at the surface of the Pt<sub>3</sub>Ni catalyst is facet-dependent, following the order (110) > (111) > (100). The mechanism of the facet-dependent adsorption of CH<sub>3</sub>OH on the catalyst is rationalized in terms of shifts of the <i>d</i>-band center of the Ni component relative to the Fermi level, density of states, changes to the work function of each surface of the Pt<sub>3</sub>Ni catalyst, and polarization effects of the adsorbed CH<sub>3</sub>OH. We believe that the present results will provide useful information to help guide the rational design and construction of nanoarchitectured catalyst surfaces for optimal heterogeneous catalysis

    Extraction of Tryptophan with Ionic Liquids Studied with Molecular Dynamics Simulations

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    Extraction of amino acids from aqueous solutions with ionic liquids (ILs) in biphasic systems is analyzed with molecular dynamics (MD) simulations. Extraction of tryptophan (TRP) with the imidazolium-based ILs [C<sub>4</sub>mim][PF<sub>6</sub>], [C<sub>8</sub>mim][PF<sub>6</sub>], and [C<sub>8</sub>mim][BF<sub>4</sub>] are considered as model cases. Solvation free energies of TRP are calculated with MD simulations and thermodynamic integration in combination with an empirical force field, whose parametrization is based on the liquid-phase charge distribution of the ILs. Calculated solvation free energies reproduce successfully all observed experimental trends according to the previously reported partition of TRP between water and IL phases. Water is present in ILs as a cosolvent, due to direct contact with the aqueous phase during extraction, and is found to play a major role in the extraction of TRP. Water improves solvation of cationic TRP by 7.8 and 5.1 kcal/mol in [C<sub>4</sub>mim][PF<sub>6</sub>] and [C<sub>8</sub>mim][PF<sub>6</sub>], respectively, which is in the case of [C<sub>4</sub>mim][PF<sub>6</sub>] sufficient to extract TRP. Extraction in [C<sub>8</sub>mim][PF<sub>6</sub>] is not feasible, since the hydrophobic octyl groups of the cations limit the water concentration in the IL. The solvation of cationic TRP is 2.4 kcal/mol less favorable in [C<sub>8</sub>mim][PF<sub>6</sub>] than in [C<sub>4</sub>mim][PF<sub>6</sub>]. Water improves the solvation of TRP in ILs mostly through dipole–dipole interactions with the polar backbone of TRP. Extraction is most efficient with [C<sub>8</sub>mim][BF<sub>4</sub>], where hydrophilic BF<sub>4</sub><sup>–</sup> anions substantially increase the water concentration in the IL. Additionally, stronger direct electrostatic interactions of TRP with BF<sub>4</sub><sup>–</sup> anions improve its solvation in the IL further. The solvation of cationic TRP in [C<sub>8</sub>mim][BF<sub>4</sub>] is 3.4 kcal/mol more favorable than in [C<sub>8</sub>mim][PF<sub>6</sub>]. Overall, the extractive power of the ILs correlates with the water saturation concentration of the IL phase, which in turn is determined by the hydrophilicity of the constituting ions. The results of this work identify relations between the extraction performance of ILs and the basic chemical properties of the ions, which provide guidelines that could contribute to the design of improved novel ILs for amino acid extraction

    Study of Wetting on Chemically Soften Interfaces by Using Combined Solution Thermodynamics and DFT Calculations: Forecasting Effective Softening Elements

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    Despite recent progress in understanding the wetting principles on soft solids, the roles of chemical bonding in the formation of interfaces have been largely ignored, because most of these studies are conducted at room temperatures. Here we propose a universal wetting principle from solution thermodynamics to account for the softening of both the solid and liquid surfaces (stable or metastable). Density functional theory (DFT) calculations are applied to evaluate the stability and electron transportation across the interfaces. We find that wetting is dominated by the system entropy changes involving not only the stable liquid alloy phase but also the metastable liquid oxide phases. The state-of-art multicomponent solution thermodynamic models and databases are applied to describe the entropy changes and predict the wetting behaviors. Our results show that by chemically softening either the liquid or the solid phase, the wetting angle reduces. And an effective soften agent/additive (either in the form of chemical elements or molecules) will weaken the bonds within the liquid (or solid) phase and promote new bonds at the interfaces, thus increasing the interface entropy. Subsequently, as an example, Ti and Zr are proposed as effective softening elements to improve the wetting of aluminum liquid on B<sub>6</sub>Si(s). This approach provides a concept and tool to advance research in catalytic chemistry, nucleation (growth), elastowetting, and cell–substrate interactions

    Iron-Promoted Difunctionalization of Alkenes by Phenyl­selenylation/1,2-Aryl Migration

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    Iron-promoted difunctionalization of α,α-diaryl and α-aryl-α-alkyl allylic alcohols has been efficiently achieved by means of <i>N</i>-(phenyl­seleno)­phthalimide (N-PSP) under mild conditions. An <i>in situ</i> generated phenylselenium cation (PhSe<sup>+</sup>) was added to the olefinic CC bond to initiate the regioselective phenylselenylation with concomitant 1,2-aryl migration, following a migration preference contrary to the well-known radical pathway. Hydrazonation of the resultant alkene difunctionalization products, that is, α-aryl-β-phenylselenyl ketones, and subsequent copper-catalyzed dehydroselenylation efficiently afforded functionalized 2-pyrazoline derivatives

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    Iron-Promoted Difunctionalization of Alkenes by Phenyl­selenylation/1,2-Aryl Migration

    Full text link
    Iron-promoted difunctionalization of α,α-diaryl and α-aryl-α-alkyl allylic alcohols has been efficiently achieved by means of <i>N</i>-(phenyl­seleno)­phthalimide (N-PSP) under mild conditions. An <i>in situ</i> generated phenylselenium cation (PhSe<sup>+</sup>) was added to the olefinic CC bond to initiate the regioselective phenylselenylation with concomitant 1,2-aryl migration, following a migration preference contrary to the well-known radical pathway. Hydrazonation of the resultant alkene difunctionalization products, that is, α-aryl-β-phenylselenyl ketones, and subsequent copper-catalyzed dehydroselenylation efficiently afforded functionalized 2-pyrazoline derivatives

    Image_1_Genome-Wide Analysis Reveals Ancestral Lack of Seventeen Different tRNAs and Clade-Specific Loss of tRNA-CNNs in Archaea.PDF

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    <p>Transfer RNA (tRNA) is a category of RNAs that specifically decode messenger RNAs (mRNAs) into proteins by recognizing a set of 61 codons commonly adopted by different life domains. The composition and abundance of tRNAs play critical roles in shaping codon usage and pairing bias, which subsequently modulate mRNA translation efficiency and accuracy. Over the past few decades, effort has been concentrated on evaluating the specificity and redundancy of different tRNA families. However, the mechanism and processes underlying tRNA evolution have only rarely been investigated. In this study, by surveying tRNA genes in 167 completely sequenced genomes, we systematically investigated the composition and evolution of tRNAs in Archaea from a phylogenetic perspective. Our data revealed that archaeal genomes are compact in both tRNA types and copy number. Generally, no more than 44 different types of tRNA are present in archaeal genomes to decode the 61 canonical codons, and most of them have only one gene copy per genome. Among them, tRNA-Met was significantly overrepresented, with an average of three copies per genome. In contrast, the tRNA-UAU and 16 tRNAs with A-starting anticodons (tRNA-ANNs) were rarely detected in all archaeal genomes. The conspicuous absence of these tRNAs across the archaeal phylogeny suggests they might have not been evolved in the common ancestor of Archaea, rather than have lost independently from different clades. Furthermore, widespread absence of tRNA-CNNs in the Methanococcales and Methanobacteriales genomes indicates convergent loss of these tRNAs in the two clades. This clade-specific tRNA loss may be attributing to the reductive evolution of their genomes. Our data suggest that the current tRNA profiles in Archaea are contributed not only by the ancestral tRNA composition, but also by differential maintenance and loss of redundant tRNAs.</p

    Image_2_Genome-Wide Analysis Reveals Ancestral Lack of Seventeen Different tRNAs and Clade-Specific Loss of tRNA-CNNs in Archaea.PDF

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    <p>Transfer RNA (tRNA) is a category of RNAs that specifically decode messenger RNAs (mRNAs) into proteins by recognizing a set of 61 codons commonly adopted by different life domains. The composition and abundance of tRNAs play critical roles in shaping codon usage and pairing bias, which subsequently modulate mRNA translation efficiency and accuracy. Over the past few decades, effort has been concentrated on evaluating the specificity and redundancy of different tRNA families. However, the mechanism and processes underlying tRNA evolution have only rarely been investigated. In this study, by surveying tRNA genes in 167 completely sequenced genomes, we systematically investigated the composition and evolution of tRNAs in Archaea from a phylogenetic perspective. Our data revealed that archaeal genomes are compact in both tRNA types and copy number. Generally, no more than 44 different types of tRNA are present in archaeal genomes to decode the 61 canonical codons, and most of them have only one gene copy per genome. Among them, tRNA-Met was significantly overrepresented, with an average of three copies per genome. In contrast, the tRNA-UAU and 16 tRNAs with A-starting anticodons (tRNA-ANNs) were rarely detected in all archaeal genomes. The conspicuous absence of these tRNAs across the archaeal phylogeny suggests they might have not been evolved in the common ancestor of Archaea, rather than have lost independently from different clades. Furthermore, widespread absence of tRNA-CNNs in the Methanococcales and Methanobacteriales genomes indicates convergent loss of these tRNAs in the two clades. This clade-specific tRNA loss may be attributing to the reductive evolution of their genomes. Our data suggest that the current tRNA profiles in Archaea are contributed not only by the ancestral tRNA composition, but also by differential maintenance and loss of redundant tRNAs.</p

    Iron-Catalyzed Oxidative C–H Functionalization of Internal Olefins for the Synthesis of Tetrasubstituted Furans

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    Tetrasubstituted furans were efficiently synthesized from Fe­(OAc)<sub>2</sub>-catalyzed C–H/C–H cross-dehydrogenative-coupling (CDC) reactions of activated carbonyl methylenes with <i>S</i>,<i>S</i>-functionalized internal olefins, that is, α-oxo ketene dithioacetals and analogues, under oxidative conditions. β-Ketoesters, 1,3-dicarbonyls, β-keto nitrile, and amide derivatives were used as the coupling partners. The resultant alkylthio- and carbonyl-functionalized furans could be further transformed to diverse arylated furan derivatives and furan-fused <i>N</i>-heterocycles, respectively. The control experiments have revealed a radical reaction pathway
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