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- Cao, Xinrui, et al.
(författare)
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Spin Polarization-Induced Facile Dioxygen Activation in Boron-Doped Graphitic Carbon Nitride
- 2020
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 12:47, s. 52741-52748
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Tidskriftsartikel (refereegranskat)abstract
- Dioxygen (O-2) activation is a vital step in many oxidation reactions, and a graphitic carbon nitride (g-C3N4) sheet is known as a famous semiconductor catalytic material. Here, we report that the atomic boron (B)-doped g-C3N4 (B/g-C3N4) can be used as a highly efficient catalyst for O-2 activation. Our first-principles results show that O-2 can be easily chemisorbed at the B site and thus can be highly activated, featured by an elongated O-O bond (similar to 1.52 angstrom). Interestingly, the O-O cleavage is almost barrier free at room temperatures, independent of the doping concentration. It is revealed that the B atom can induce considerable spin polarization on B/g-C3N4, which accounts for O-2 activation. The doping concentration determines the coupling configuration of net-spin and thus the magnitude of the magnetism. However, the distribution of net-spin at the active site is independent of the doping concentration, giving rise to the doping concentration-independent catalytic capacity. The unique monolayer geometry and the existing multiple active sites may facilitate the adsorption and activation of O-2 from two sides, and the newly generated surface oxygen-containing groups can catalyze the oxidation coupling of methane to ethane. The present findings pave a new way to design g-C3N4-based metal-free catalysts for oxidation reactions.
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- Chen, Tao, et al.
(författare)
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Formation of polyynes and ring-polyyne molecules following fragmentation of polycyclic aromatic hydrocarbons
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 486:2, s. 1875-1881
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we perform molecular dynamic (MD) simulations to investigate the stability and fragmentation processes of vibrationally excited linear polycyclic aromatic hydrocarbons (PAHs). The program of CP2K in combination with the semi-empirical method PM3 is utilized for the MD simulations. The simulations show that the formation of molecular hydrogens (H-2) is different than previous studies, in particular, different than compact PAHs. At high temperatures, linear PAHs tend to open aromatic rings and convert the sp(3) C-C or sp(2) C=C bonds to sp C C bonds by removing H-2; i.e. polyynes are formed in such process. Besides polyynes, PAHs attached with sp-bonded polyyne chains are commonly observed at high temperatures. We notice that due to the addition of flexible tails (polyynes), the ring-polyyne molecules do not dissociate for a long period of time at high temperatures. Such structures facilitate the molecules to survive in the harsh environment of the interstellar medium. In addition, the ring-polyyne structures induce dipole moments that could, in principle, be detected by radio astronomy.
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- Chen, Tao, et al.
(författare)
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Fragmentation and isomerization of polycyclic aromatic hydrocarbons in the interstellar medium : Coronene as a case study
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 633
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Tidskriftsartikel (refereegranskat)abstract
- Aims. Due to the limitations of current computational technology, the fragmentation and isomerization products of vibrationally-excited polycyclic aromatic hydrocarbon (PAH) molecules and their derivatives have been poorly studied. In this work, we investigate the intermediate products of PAHs and their derivatives as well as the gas-phase reactions relevant to the interstellar medium, with coronene as a case study. Methods. Based on the semi-empirical method of PM3 as implemented in the CP2K program, molecular dynamics simulations were performed to model the major processes (e.g., vibrations, fragmentations, and isomerizations) of coronene and its derivatives (e.g., methylated coronene, hydrogenated coronene, dehydrogenated coronene, nitrogen-substituted coronene, and oxygen-substituted coronene) at temperatures of 3000 K and 4000 K. Results. We find that the anharmonic effects are crucial for the simulation of vibrational excitation. For the molecules studied here, H-2, CO, HCN, and CH2 are the major fragments. Following the dissociation of these small units, most of the molecules could maintain their ring structures, but a few molecules would completely break into carbon chains. The transformation from a hexagon to a pentagon or a heptagon may occur and the heteroatomic substitutions (e.g., N- or O-substitutions) would facilitate the transformation.
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- Chen, Tao, et al.
(författare)
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The infrared bands of polycyclic aromatic hydrocarbons in the 1.6-1.7 mu m wavelength region
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 632
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Tidskriftsartikel (refereegranskat)abstract
- Context. The 3.3 mu m aromatic C-H stretching band of polycyclic aromatic hydrocarbon (PAH) molecules seen in a wide variety of astrophysical regions is often accompanied by a series of weak satellite bands at similar to 3.4-3.6 mu m. One of these sources, IRAS 21282 +5050, a planetary nebula, also exhibits a weak band at similar to 1.68 mu m. While the satellite features at similar to 3.4-3.6 mu m are often attributed to the anharmonicities of PAHs, it is not clear whether overtones or combination bands dominate the 1.68 mu m feature. Aims. In this work, we examine the anharmonic spectra of eight PAH molecules, including anthracene, tetracene, pentacene, phenanthrene, chrysene, benz[a]anthracene, pyrene, and perylene, to explore the origin of the infrared bands in the 1.6-1.7 mu m wavelength region. Methods. Density functional theory (DFT) in combination with the vibrational second-order perturbation theory (VPT2) was used to compute the anharmonic spectra of PAHs. To simulate the vibrational excitation process of PAHs, the Wang-Landau random walk technique was employed. Results. All the dominant bands in the 1.6-1.7 mu m wavelength range and in the 3.1-3.5 mu m C-H stretching region are calculated and tabulated. It is demonstrated that combination bands dominate the 1.6-1.7 mu m region, while overtones are rare and weak in this region. We also calculate the intensity ratios of the 3.1-3.5 mu m C-H stretching features to the bands in the 1.6-1.7 mu m region, I3.1-3:5/I1.6-1.7, for both ground and vibrationally excited states. On average, we obtain < I3.1-3.5=/I1.6-1.7 > approximate to 12.6 and < I3.1-3.5=/I1.6-1.7 > approximate to 17:6 for PAHs at ground states and at vibrationally excited states, respectively.
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- Cheng, Xiao
(författare)
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Multiplet computation methods for core level X-ray spectroscopy of transition metal and rare earth elements
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the development of new generation synchrotron facilities, the performances of various X-ray spectroscopies have become more advanced. In order to interpret the X-ray spectrum experiments of various novel materials related to transition metal and rare earth elements, new advanced theoretical methods are required. The present thesis incorporates four modus operandi based on the classic multiplet theory to study the core level X-ray spectroscopy of transition metal and rare earth element. The four approaches consist of new methods developed from classic multiplet approach to high level first-principles method assisted multiplet calculation. Some methods are selected from previous researches and some are invented by original researches. These methods are integrated together to form a complete set of multiplet computational methods. This set of multiplet computational methods can perform calculations on various X-ray spectroscopies such as XAS, XPS, XES and RIXS related to the core-level electron. These wide range of spectroscopic methods coupled to different multiplet theory approaches serve as efficient tools to understand the electronic structure of metal sites and their unique contribution to the physical/chemical properties of the materials.The thesis creatively improves the classic multiplet theory on several aspects: (1) the relation between crystal field parameters and local structure factors; (2) the difficulty of processing point group symmetry branching chain in low symmetric structure; (3) the first-principles calculation of semi-empirical parameters. Four modus operandi are presented in this thesis: the first is the classic multiplet theory consisting of the multiplet effect, crystal field effect and charge transfer effect via several semi-empirical parameters as description for these effects. The second level multiplet theoretical approach analyze the crystal field potential matrix in various symmetries according to the point group symmetry branching rules. Then the crystal field effect parameters used in classic multiplet theory are linked analytically to the specific structural factors such as bond length and angles. This approach is a good tool to study the structural distortion from higher to lower order symmetry with analysis of X-ray spectral feature changes in experiment. The third modus operandi adopts large cluster model consisting of point charges at equivalent atoms position to simulate the crystal field effect on the center metal site. This approach handles low order symmetric crystal field with long range effect in multiplet calculation in an easier way than the classic multiplet theory. The fourth modus operandi initially studies the system of interest in first-principles calculation for the electronic wavefunctions. Then the electronic wavefunctions are used to derive the maximally localized Wannier functions at metal/ligand sites. The analysis of these Wannier functions provide a lot of semi-empirical parameters required in the classic multiplet calculation approach in a first-principles way. This modus operandi has substantially resolved the problem of finding the best set of semi-empirical parameters to fit the calculated X-ray spectrum with experimental data.In order to study the core electrons of the light elements (such as C/N/O) around center metal ions, a theoretical calculation method used to study the core electrons' vibrationally-resolved X-ray spectroscopy is also introduced as a complementary research and applied to C1s core ionized XPS calculation as an example.
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