04 Mar 2021-Physical Chemistry Chemical Physics (The Royal Society of Chemistry)-Vol. 23, Iss: 8, pp 4963-4974

Abstract: Density functional theory (DFT) calculations were performed to investigate the catalytic cycle of methane conversion to methanol over both [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites in the Cu-ZSM-5 catalyst. The [Cu2(O2)]2+ site is found to be active for the partial oxidation of methane to methanol, and although it has a higher energy barrier in the methane activation step, it involves a very low energy barrier in the methanol formation step (36.3 kJ mol-1) as well as a lower methanol desorption energy (52.5 kJ mol-1). As the [Cu2(O2)]2+ active site is also thermodynamically stable, it may play an important role during methane conversion to methanol. Furthermore, the methane activation step follows the homolytic route and the heterolytic route for the [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites, respectively, whereas the methanol formation step follows the direct radical rebound mechanism and the indirect rebound mechanism, respectively. Our calculations further indicate that the electronic properties of the reactive O atoms in the active site influence their reactivity toward methane oxidation. More specifically, the higher the spin density and the more negative the charge of the reactive O atom at the active site are, the lower the energy barrier for methane activation will be; and the more negative the charge of the hydroxyl group in the reaction intermediate during the partial oxidation of methane to methanol is, the higher energy barrier of the methanol formation step will be in the triplet state. Furthermore, we used a larger cluster model to predict the mechanism of the methane activation step and the effect of atomic charge of the O atom at the [Cu2(μ-O)]2+ and [Cu2(O2)]2+ active sites on the energy barriers of partial oxidation of methane to methanol, and the conclusions drawn employing the larger cluster model are consistent with those drawn using the smaller double-5T-ring cluster model. In addition, different from the traditional mechanism for methane activation at [Cu2(O2)]2+, which consists of two transition states, we find that the partial oxidation of methane at [Cu2(O2)]2+ can also occur via a single step by direct insertion of one of the O atoms at the active site into the C-H bond of methane.

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Topics: Anaerobic oxidation of methane (58%), Methane (55%), Partial oxidation (54%) ... read more

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5 results found

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01 Jan 2021-

Abstract: Computer-assisted rational design of heterogeneous catalysts has become a central theme of computational studies on industrial catalysis, which may contribute significantly to our impending transition from a fossil fuel-based energy and chemical industry into a renewable energy-based one. To this end, integrated research efforts in mechanistic elucidation of the relevant catalytic reactions remain essential, but emphasis must be further placed on the development of effective approaches in the rational design of industrial catalysts, where traditional methodologies must be combined with the new information-based technologies.

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Topics: Industrial catalysts (55%)

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Muhammad Haris Mahyuddin^{1}, Seiya Tanaka^{2}, Ryotaro Kitagawa^{2}, Arifin Luthfi Maulana^{1} +5 more•Institutions (2)

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Lakshitha Jasin Arachchige^{1}, Lakshitha Jasin Arachchige^{2}, Ani Dong^{1}, Tianyi Wang^{1} +6 more•Institutions (3)

26 Nov 2021-

Topics: Graphyne (58%), Atom (order theory) (51%)

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Abstract: In 2019, Diaz-Urrutia and Ott developed a high-yield method for direct conversion of methane to methanesulfonic acid and proposed a cationic chain reaction mechanism. However, Roytman and Singleton questioned this mechanism, and they favored a free-radical mechanism. In the present paper, we studied both the cationic chain and radical mechanisms and found the radical mechanism is more favorable, since it has a much lower energy barrier. However, the radical mechanism has not considered the effect of ions for the reaction taking place in oleum. Thus, we studied a simple model of a protonated radical mechanism, which further lowers the energy barrier. Although the true mechanism for the CH4 + SO3 reaction could be more complicated in electrolyte solutions, this model should be helpful for the further study of the mechanism of this reaction.

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Topics: Methanesulfonic acid (54%)

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58 results found

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Abstract: A correlation-energy formula due to Colle and Salvetti [Theor. Chim. Acta 37, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the local kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.

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Topics: Orbital-free density functional theory (56%), Density functional theory (56%), Electronic density (56%) ... read more

77,776 Citations

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Abstract: Current gradient-corrected density-functional approximations for the exchange energies of atomic and molecular systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy density. Here we report a gradient-corrected exchange-energy functional with the proper asymptotic limit. Our functional, containing only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of atomic systems with remarkable accuracy, surpassing the performance of previous functionals containing two parameters or more.

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Topics: Hybrid functional (62%), Orbital-free density functional theory (59%), Time-dependent density functional theory (52%) ... read more

42,343 Citations

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Abstract: In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlof, Taylor, and co‐workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects i f the exponents of the functions are optimized in atomic correlated calculations, although the primitive (s p) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations i f the appropriate primitive set is used. Test calculations on oxygen‐containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlof and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of c o r r e l a t i o n c o n s i s t e n t b a s i s s e t s, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron.

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Topics: STO-nG basis sets (65%), Basis set (57%), Quantum chemistry composite methods (53%) ... read more

24,491 Citations

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Abstract: Multiwfn is a multifunctional program for wavefunction analysis. Its main functions are: (1) Calculating and visualizing real space function, such as electrostatic potential and electron localization function at point, in a line, in a plane or in a spatial scope. (2) Population analysis. (3) Bond order analysis. (4) Orbital composition analysis. (5) Plot density-of-states and spectrum. (6) Topology analysis for electron density. Some other useful utilities involved in quantum chemistry studies are also provided. The built-in graph module enables the results of wavefunction analysis to be plotted directly or exported to high-quality graphic file. The program interface is very user-friendly and suitable for both research and teaching purpose. The code of Multiwfn is substantially optimized and parallelized. Its efficiency is demonstrated to be significantly higher than related programs with the same functions. Five practical examples involving a wide variety of systems and analysis methods are given to illustrate the usefulness of Multiwfn. The program is free of charge and open-source. Its precompiled file and source codes are available from http://multiwfn.codeplex.com.

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Topics: Population (52%), Plot (graphics) (51%)

9,777 Citations

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Chunyang Peng^{1}, Philippe Y. Ayala^{1}, H. Bernhard Schlegel^{1}, Michael J. Frisch•Institutions (1)

Abstract: A redundant internal coordinate system for optimizing molecular geometries is constructed from all bonds, all valence angles between bonded atoms, and all dihedral angles between bonded atoms. Redundancies are removed by using the generalized inverse of the G matrix; constraints can be added by using an appropriate projector. For minimizations, redundant internal coordinates provide substantial improvements in optimization efficiency over Cartesian and nonredundant internal coordinates, especially for flexible and polycyclic systems. Transition structure searches are also improved when redundant coordinates are used and when the initial steps are guided by the quadratic synchronous transit approach. © 1996 by John Wiley & Sons, Inc.

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Topics: Log-polar coordinates (62%), Generalized coordinates (62%), Coordinate system (56%) ... read more

2,461 Citations