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1.
The reaction of N2 with trinuclear niobium and tungsten sulfide clusters Nb3Sn and W3Sn (n=0–3) was systematically studied by density functional theory calculations with TPSS functional and Def2-TZVP basis sets. Dissociations of N−N bonds on these clusters are all thermodynamically allowed but with different reactivity in kinetics. The reactivity of Nb3Sn is generally higher than that of W3Sn. In the favorite reaction pathways, the adsorbed N2 changes the adsorption sites from one metal atom to the bridge site of two metal atoms, then on the hollow site of three metal atoms, and at that place, the N−N bond dissociates. As the number of ligand S atoms increases, the reactivity of Nb3Sn decreases because of the hindering effect of S atoms, while W3S and W3S2 have the highest reactivity among four W3Sn clusters. The Mayer bond order, bond length, vibrational frequency, and electronic charges of the adsorbed N2 are analyzed along the reaction pathways to show the activation process of the N−N bond in reactions. The charge transfer from the clusters to the N2 antibonding orbitals plays an essential role in N−N bond activation, which is more significant in Nb3Sn than in W3Sn, leading to the higher reactivity of Nb3Sn. The reaction mechanisms found in this work may provide important theoretical guidance for the further rational design of related catalytic systems for nitrogen reduction reactions (NRR).  相似文献   
2.
A detailed study of the geometry, aromatic character, electronic and magnetic properties for a series of positively charged N-doped polycyclic aromatic hydrocarbons (PAHs) was performed. Magnetic properties of the examined molecules were analyzed by means of the magnetically induced current density calculated using the diamagnetic-zero version of the continuous transformation of origin of current density (CTOCD-DZ) method. The comparative study of the local aromaticity of the studied molecules was performed using several different indices: energy effect (ef), harmonic oscillator model of aromaticity (HOMA) index, six centre delocalization index (SCI) and nucleus independent chemical shifts (NICS). The presence of N-atoms in the inner rings was found to cause a planarity distortion in the studied N-doped systems. The geometric changes and charged nature of the studied N-doped systems do not significantly influence the current density and the local aromaticity distribution in comparison with the corresponding parent benzenoid hydrocarbons. The present study demonstrates how quantum chemical calculations can be used for rational design of novel PAHs and for fine tuning of their properties.  相似文献   
3.
Dehydrogenation of an organic compound is the first and the most fundamental elementary reaction in many organic reactions. In ethanol oxidation reaction (EOR) to form CO2, there are a total of 46 pathways in C2HxO (x=1–6) species leading to the removal of all six hydrogen atoms in five C−H bonds and one O−H bond. To investigate the degree of dehydrogenation in EOR under operando conditions, we performed density function theory (DFT) calculations to study 28 dehydrogenation steps of C2HxO on Ir(100). An activation energy surface was then constructed and compared with that of the C−C bond cleavages to understand the importance of the degree of dehydrogenation in EOR. The results show that there are likely 28 dehydrogenations in EOR under fuel cell temperatures and the last two hydrogens in C2H2O are less likely cleaved. On the other hand, deep dehydrogenation including 45 dehydrogenations can occur under ethanol steam reforming conditions.  相似文献   
4.
以过渡金属为催化衬底的化学气相沉积法(Chemical Vapor Deposition,CVD)已经可以制备与机械剥离样品相媲美的石墨烯,是实现石墨烯工业应用的关键技术之一。原子尺度理论研究能够帮助我们深刻理解石墨烯生长机理,为实验现象提供合理的解释,并有可能成为将来实验设计的理论指导。本文从理论计算的角度,总结了各种金属衬底在石墨烯CVD生长过程中的各种作用与相应的机理,包括在催化碳源裂解、降低石墨烯成核密度等,催化加快石墨烯快速生长,修复石墨烯生长过程中产生的缺陷,控制外延生长石墨烯的晶格取向,以及在降温过程中石墨烯褶皱与金属表面台阶束的形成过程等。在本文最后,我们对当前石墨烯生长领域中亟需解决的理论问题进行了深入探讨与展望。  相似文献   
5.
Yuanchao Huang 《中国物理 B》2022,31(4):46104-046104
The p-type doping efficiency of 4H silicon carbide (4H-SiC) is rather low due to the large ionization energies of p-type dopants. Such an issue impedes the exploration of the full advantage of 4H-SiC for semiconductor devices. In this study, we show that co-doping group-IVB elements effectively decreases the ionization energy of the most widely used p-type dopant, i.e., aluminum (Al), through the defect-level repulsion between the energy levels of group-IVB elements and that of Al in 4H-SiC. Among group-IVB elements Ti has the most prominent effectiveness. Ti decreases the ionization energy of Al by nearly 50%, leading to a value as low as ~0.13 eV. As a result, the ionization rate of Al with Ti co-doping is up to ~5 times larger than that without co-doping at room temperature when the doping concentration is up to 1018 cm-3. This work may encourage the experimental co-doping of group-IVB elements such as Ti and Al to significantly improve the p-type doping efficiency of 4H-SiC.  相似文献   
6.
A first-principles investigation of the origin of ferroelectricity in the Aurivillius compound Bi2VO5.5 is presented. Calculations with the density functional theory, in conjunction with the modern theory of polarization, allowed us to study the structural, electronic, and polar properties of two configurations built with oxygen vacancies, causing a charge imbalance and a subsequent displacement of the ions, generating two maximum polarizations, one of 14.75 μC/cm2 and one of 4.31 μC/cm2 along [011] direction. Electron localization function schemes were used to identify the asymmetry of charges in (001), (010) and (100) planes. The results obtained in this study establish that the origin of ferroelectricity is due to the displacement of the ions caused by oxygen vacancies and the asymmetric distribution of the isolated pair of Bi ions. On the other hand, the bandgap calculations and the results of Ps establish that Bi2VO5.5 is a candidate ferro-photovoltaic material.  相似文献   
7.
Compounds 1-2, the rare examples of polycyclic polyprenylated acylphloroglucinols (PPAPs) with unique 6/5/7 and 6/6 ring systems possessing unprecedented 2, 3, 12-trioxatricyclo-[5.3.1.15,11] dodecane and bicyclo-[4.4.0] decane, respectively, were isolated from the fruits of Garcinia bracteata and Garcinia multiflora, respectively. Their structures and absolute configurations were elucidated by the combination of gauge-independent atomic orbital (GIAO) NMR calculations and electronic circular dichroism (ECD) calculations. Furthermore, the hypoglycemic activity of 1 was assayed by the insulin resistance (IR)-HepG2 cell model. The results showed that compound 1 at the concentrations of 2, 4, and 6 μM could significantly promote glucose uptake dose-dependently and improve glucose metabolism disorder. Taken together, these findings suggested 1 could be a promising candidate for the treatment of diabetes mellitus.  相似文献   
8.
Photocatalytic reduction of CO2 is one important approach to alleviate greenhouse gas emission and energy crisis, which has gained huge attention in the past decades. However, the lack of understanding complex reaction mechanism impedes new catalysts design. It is also very difficult to understand the mechanism by using only experimental approaches. For this concern, theoretical calculations can effectively supplement the experimental deficiency and thus play an important role. Recently theoretical calculations have been performed on adsorption, migration and reduction of CO2 molecule on the photocatalyst surface, leading to useful information that have contributed greatly to this field. This review summarizes recent advances in first-principles calculations about CO2 photoreduction over various semiconductor photocatalysts like metal oxides, sulfides and g-C3N4. The methods, models, adsorption and reaction pathways have been discussed in detail. The perspective about future investigation on the photocatalytic reduction of CO2 using first principles calculations is also presented.  相似文献   
9.
《中国物理 B》2021,30(7):73101-073101
Ni-rich layered lithium transition metal oxides LiNi_xMn_yCo_zO_2(1-y-z ≥ 0.6) are promising candidates for cathode materials, but their practical applications are hindered by high-voltage instability and fast capacity fading. Using density functional theory calculations, we demonstrate that Na-, F-doping, and Na/F-co-doping can stabilize the structure and result into a higher open circuit voltage than pristine LiNi_(0.6)Mn_(0.2)Co_(0.2)O_2(NMC622) during the charging process, which may attain greater discharge capacity. F doping may inhibit the diffusion of Li ions at the beginning and end of charging; Na doping may improve Li ion diffusion due to the increase in Li layer spacing, consistent with prior experiments. Na/F-codoping into NMC622 promotes rate performance and reduces irreversible phase transitions for two reasons:(i) a synergistic effect between Na and F can effectively restrain the Ni/Li mixing and then enhances the mobility of Li ions and(ii) Ni/Li mixing hinders the Ni ions to migrate into Li layers and thus, stabilizes the structure. This study proposes that a layer cathode material with high electrochemical performance can be achieved via rational dopant modification, which is a promising strategy for designing efficient Li ion batteries.  相似文献   
10.
The first principle computational screening was performed to investigate the effect of selected dopants for Li3PS4 sulfide solid electrolyte on its ionic conductivity and stability toward moisture. The results suggest that substitution P5+ using isovalent cations whose electronegativity (EN) value is closer to the value of S has more significant effects on the ionic conductivity, whereby W5+ and Sb5+ can improve most. Similarly, aliovalent cation substitutions with compensating changes in the lithium-ion concentration, particularly those with a lower oxidation state and higher EN, such as Cu2+, effectively enhance the lithium-ion conductivity in this structure. For cation dopants, it is found that ionic conductivity improvement of Li3PS4 is the synergetic effect of EN and oxidation number of the dopant as well as the material's lattice parameter change. Oxides of the considered cation dopants can also improve the ionic conductivity of the material but have much lower lithium-ion conductivity than the cases of cation dopants. However, the metal oxide dopants, particularly those derived from soft Lewis' acid cations, show a marginal improvement in moisture stability of the Li3PS4 electrolyte. The effect of halides and metal halide dopants on the lithium-ion conductivity and moisture stability of Li3PS4 electrolyte are also studied. It is found that metal halides are more effective than any other dopants in improving the ionic conductivity of Li3PS4.  相似文献   
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