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Yong-Wang Li

Publications and source records attributed to Yong-Wang Li.

16 recordsLinked to original sources
PubMed2006

Structure and energy of Mo27SxCy clusters: a density functional theory study.

For understanding the carburization processes of MoSx catalysts, the structures and energies of Mo27SxCy cluster models have been computed at the level of density functional theory. The surface sulfur atoms on the Mo edge and S edge as well as bulky sulfur atoms have been replaced by atomic carbon, and the corresponding structures have S/C ratios in the range of 0.8-55. The formation of all Mo27SxCy structures is favored thermodynamically. It is also found that the formation of CS and C2 bridging units is more favored than the individual or separated replacements and that the formation of C2 bridging units is more favored than that of CS units. In contrast, the replacement of sulfur on the Mo edge is least favored. Furthermore, the replacement of the bulky sulfur on the Mo edge is equally favored as those of sulfur on the S edge. For aiding further experimental studies, the C=S and C=C stretching frequencies have been computed.

Journal Article↗
PubMed2006

Thiophene adsorption and activation on MoP(001), gamma-Mo2N(100), and Ni2P(001): density functional theory studies.

The adsorption and dissociation of thiophene on the MoP(001), gamma-Mo(2)N(100), and Ni(2)P(001) surfaces have been computed by using the density functional theory method. It is found that thiophene adsorbs dissociatively on MoP(001), while nondissociatively on gamma-Mo(2)N(100) and Ni(2)P(001). On MoP(001), the dissociation of the C-S bonds is favored both thermodynamically and kinetically, while the break of the first C-S bond on gamma-Mo(2)N(100) has an energy barrier of 1.58 eV and is endothermic by 0.73 eV. On Ni(2)P(001) there are Ni(3)P(2)- and Ni(3)P-terminated surfaces. On the Ni(3)P(2)-terminated surface, the dissociation of the C-S bonds of adsorbed thiophene is endothermic, while it is exothermic on the Ni(3)P-terminated surface.

Journal Article↗
PubMed2006

NO adsorption on MoS(x) clusters: a density functional theory study.

The density functional theory (DFT) method has been used to investigate NO probe molecule adsorption on the stoichiometric (Mo(16)S(32)) and nonstoichiometric (Mo(16)S(34) and Mo(16)S(29)) clusters. The calculated adsorption energies indicate that the stoichiometric cluster has stronger NO affinity than the nonstoichiometric surfaces. It is also found that mononitrosyl adsorption is favored at low NO coverage, while dinitrosyl (germinal) and (NO)(2) dimer adsorption at high NO coverage are possible. Strong repulsive interaction has been found for the adsorbed dinitrosyl and (NO)(2) dimer species. In addition, the computed NO stretching frequencies for the mononitrosyl and dinitrosyl species agree well with the experimental data, while those of the dimer species are much lower than the suggested experimental data.

Journal Article↗
PubMed2006

Density function theory study of CO adsorption on Fe3O4(111) surface.

Density functional theory calculations have been carried out for CO adsorption on the Fe(oct2)- and Fe(tet1)-terminated Fe(3)O(4)(111) surfaces, which are considered as active catalysts in water-gas shift reaction. It is found that the on-top configurations are most stable on these two surfaces. Some bridge configurations are also stable in which the new C-O bond formed between the surface O atom and the C atom of CO. The adsorption on the Fe(oct2)-terminated surface is more stable than on the Fe(tet1)-terminated surface. The density of state reveals the binding mechanism of CO adsorption on the two surfaces. Our calculations have also shown that the absorbed CO can migrate from the on-top site to the bridge site or 3-fold site. The oxidation of CO via surface oxygen atoms is feasible, which is in good agreement with experimental results.

Adsorption↗
PubMed2006

CO2 reforming of CH4 on Ni(111): a density functional theory calculation.

CO(2) reforming of CH(4) on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH(4) sequential dissociation into surface CH (CH(4) --> CH(3) --> CH(2) --> CH) and hydrogen, and CO(2) dissociation into surface CO and O (CO(2) --> CO + O). The second step is CH oxygenation into CHO (CH + O --> CHO), which is more favored than dissociation into C and hydrogen (CH --> C + H). The third step is the dissociation of CHO into surface CO and H (CHO --> CO + H). This can explain the enhanced selectivity toward the formation of CO and H(2) on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO = C((ads)) + CO(2)) is more favored than by CH(4) sequential dehydrogenation. The major problem of CO(2) reforming of CH(4) is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH(4) activation into CH(3) and H as the rate-determining step of CO(2) reforming of CH(4) on Ni(111), in agreement with the isotopic experimental results.

Journal Article↗
PubMed2006

Structure and stability of neutral polyoxometalate cages: (Mo2O6)m (m=1-13).

The structure and stability of neutral polyoxometalate cages (Mo2O6)m (m=1-13) have been computed systematically. These neutral cages can be viewed topologically as polyhedra containing triangles (f3) and squares (f4). The relative stability of these polyhedra is associated with the location and separation of the f3. The initial stable isomers were preselected by the number of shared triangle edges (N33), and the predicted stability was validated further at the GGA-PW91/DND level of density function theory with the fine quality of mesh size. For large clusters, the square neighbor signature (P4444), which is similar to the hexagon neighbor rule for fullerene, becomes more applicable. The calculated disproportionation energies indicate that Mo6O18 (O(h), Lindqvist), Mo12O36 (O(h), alpha Keggin), Mo18O54 (D(3h), Wells-Dawson) and Mo24O72 (O(h)) cages have enhanced stability. [structure in text]. Mo6O18 (O(h)), Mo12O36 (O(h)), Mo18O54 (D(3h)) and Mo(24)O72 (O(h)) are the most stable neutral polyoxometalate cages on the basis of the structural and energetic criteria. They can therefore be considered as the inorganic fullerenes.

Electrons↗
PubMed2005

Chemisorption of CO2 on nickel surfaces.

CO2 chemisorption on the Ni(111), Ni(100), and Ni(110) surfaces was investigated at the level of density functional theory. It was found that the ability of CO2 chemisorption is in the order of Ni(110) > Ni(100) > Ni(111). CO2 has exothermic chemisorption on Ni(110) and endothermic chemisorption on Ni(111), while it is thermally neutral on Ni(100). It is also found that there is no significant lateral interaction between the adsorbed CO2 at 1/4 monolayer (ML) coverage, while there is stronger repulsive interaction at 1/2 ML. On all surfaces, the chemisorbed CO2 is partially negatively charged, indicating the enhanced electron transfer, and the stronger the electron transfer, the stronger the C=O bond elongation. The bonding nature of the adsorbed CO2 on nickel surfaces has been analyzed. The thermodynamics of CO2 dissociative chemisorption, compared with CO and O adsorption, has been discussed, and the thermodynamic preference is in the sequence Ni(100) > Ni(111) > Ni(110).

Journal Article↗
PubMed2005

Surface structure and stability of MoSx model clusters.

Density functional theory (DFT) computations have been carried out to study the structure and stability of MoSx clusters with the change of sulfur coverage at both Mo and S edges. DFT shows that adding sulfur to the Mo edge is always exothermic. However, deleting corner sulfur from the S edge is exothermic for 67 and 50% sulfur coverages, while deleting edge sulfur from the S edge is endothermic for 33 and 0% sulfur coverages. On the basis of the computed free energies along a wide range of H2S/H2 ratios, it is found that there are two stable structures with 33 and 50% sulfur coverages on the Mo edge by having 100% sulfur coverage on the S edge and one stable structure with 67% sulfur coverage on the S edge by having 0% sulfur coverage on the Mo edge. Under fully sulfiding atmosphere or at a very high H2S/H2 ratio, triangle MoSx structures with 100% sulfur coverage on the Mo edge are computed to be more stable than those with 100% sulfur coverage on the S edge, in agreement with the observation of scanning tunneling microscopy. In addition, the effects of cluster sizes on the surface structures are discussed.

Journal Article↗
PubMed2005

Surface structure and energetics of hydrogen adsorption on the Fe(111) surface.

Spin-polarized density functional theory calculations have been performed to characterize the hydrogen adsorption and diffusion on the Fe(111) surface at 2/3-, 1-, and 2-monolayer (ML) coverages. It is found that the most favored adsorption site for atomic hydrogen (H) is the top-shallow bridge site (tsb), followed by the quasi 4-fold site (qff) with the energy difference of about 0.1 eV, while the top site (t) is not competitive. Furthermore, the adsorbed atomic hydrogen (H) has a high mobility, as indicated by the small diffusion barriers. The local density of state (LDOS) analysis reveals that the Fe-H (tsb or qff) bond involves mainly the Fe 4s and 4p and H 1s orbitals with less contribution of the Fe 3d orbital, while the Fe 4s, 4p, and 3d orbitals all participate in the Fe-H (top) bond. In addition, the coverage effects on the adsorption configurations and adsorption energies are addressed.

Journal Article↗
PubMed2005

Density functional theory study of triangular molybdenum sulfide nanocluster and CO adsorption on it.

A systematic density functional theory study has been carried out on the structure and stability of triangular molybdenum sulfide (MoS(x)()) models. On the basis of the structural and energetic comparison, the triangle Mo(28)S(84) (VII) cluster has been identified as a reasonable structure for triangular MoS(x)() model. Under reductive atmosphere, the most stable structure has bridging sulfur on edge sites and two H(2)S at each corner site. It is found that CO adsorption at the corner site represents the most stable conformation. Along with other stretching modes, the computed frequency at 2102 cm(-1) for CO at the corner agrees perfectly with the experimental observation.

Journal Article↗
PubMed2005

Structures and energies of coadsorbed CO and H2 on Fe5C2(001), Fe5C2(110), and Fe5C2(100).

Density functional theory calculations have been carried out on the CO/H2 coadsorption on the (001), (110), and (100) surfaces of Fe5C2 for the understanding of the Fischer-Tropsch synthesis (FTS) mechanism. The stable surface species changes with the variation of the H2 and CO coverage. Along with dissociated hydrogen and adsorbed CO in 2-, 3-, and 4-fold configurations, methylidyne (C(s)H) (C(s), surface carbon), ketenylidene (C(s)CO), ketenyl (C(s)HCO), ketene (C(s)H2CO), and carbon suboxide (C(s)C2O2) are computed as thermodynamically stable surface species on Fe5C2(001) and Fe5C2(110) containing both surface iron and carbon atoms. These surface carbon species can be considered as the preliminary stages for FTS. On Fe5C2(100) with only iron atoms on the surface layer, the stable surface species is dissociated hydrogen and CO with top and 2-fold configurations. The bonding nature of these adsorbed carbon species has been analyzed.

Journal Article↗
PubMed2005

Theoretical ONIOM2 study on pyridine adsorption in the channels and intersection of ZSM-5.

The structures of the acid sites in the channels and intersections of H-, Li-, and Na-ZSM-5 (ZSM = zeolite socony mobil) and their interactions with pyridine molecule have been computed by using three corresponding models containing 22 tetrahedral sites. The calculated adsorption energies of pyridine in the intersection regions of H-, Li-, and Na-ZSM-5 are 197.0, 172.5, and 122.3 kJ/mol, respectively, in good agreement with the respective experimental values of 200 +/- 5, 155-195, and 120 kJ/mol, while those in the straight and sinusoidal channels are much smaller (157.9 and 127.6, 152.2 and 149.4, and 150.4 and 109.9 kJ/mol, respectively). These indicate that the most probable adsorption site for pyridine in ZSM-5 is the acidic site located in the intersection region. The structural parameters of the adsorption complexes show that the acidic proton in the three models of H-ZSM-5 has been transferred to the nitrogen of pyridine, while in alkali cation-exchanged ZSM-5, the coordination of the alkali cation to the nitrogen atom of pyridine dominates the overall interaction. In addition, the adsorption complexes were further stabilized by the long-range electrostatic interaction between the positively charged pyridine hydrogen atoms and the negatively charged lattice oxygen atoms of the zeolite framework. In the intersection regions of H-, Li-, and Na-ZSM-5, the coordination energy of the charge-compensating cation to the pyridine nitrogen amounts to 58, 60, and 68% of the total adsorption energy, respectively, while another 42, 40, and 32%, respectively, is due to long-range electrostatic interactions. This indicates that the zeolite lattice framework surrounding the adsorption site has important contributions to the adsorption energy of the pyridine molecule.

Journal Article↗
PubMed2005

Density functional theory study of CO adsorption on molybdenum sulfide.

CO adsorption on four MoSx (stoichiometric and nonstoichiometric) clusters has been investigated by using density functional method. It is found that CO prefers adsorption on the coordinatively unsaturated (1010) surface. The adsorption energy of high coverage shows the additivity as compared with that of one CO adsorption, and there is no significant repulsive interaction between the end-on adsorbed CO probes. The computed CO stretching frequencies (2000-2080 cm(-1)) agree perfectly with the experimental data (a broad band centered at 2070 cm(-1) with a tail extent to 2000 cm(-1)). No bridged CO adsorption is favored energetically under high CO concentration, and this might explain the catalytic ability of MoSx for C1 products instead of higher hydrocarbons and alcohols.

Journal Article↗
PubMed2005

Hydroformylation and isomerization of allene and propyne: a density functional theory study.

The [HCo(CO)3]-catalyzed hydroformylation of allene and propyne has been investigated at the B3LYP level of density functional theory. It is found that hydroformylation of allene favors the linear anti-Markovnikov product in high regioselectivity both kinetically and thermodynamically. The origin of this regioselectivity comes from the enhanced stability of the eta3-allylic intermediate [(eta3-CH2CHCH2)Co(CO)3]. By contrast, propyne does not show any regioselectivity. The possible interconversion between allene and propyne mediated by [HCo(CO)3] has been explored.

Aldehydes↗
PubMed2005

Density functional theory study of hydrogen adsorption on Fe5C2(001), Fe5C2(110), and Fe5C2(100).

Density functional theory calculations have been carried out for hydrogen adsorption on the (001), (110), and (100) surfaces of Fe5C2. At 1/3 and 2/3 monolyer (ML) on (001), the most stable hydrocarbon species is CsH, while CsH and CsH3 can coexist at 1 ML. On (110), only dissociated hydrogen is found at 2/5 ML, while CsH is the most stable hydrogen carbon species at 4/5 ML, and CsH and CH3 coexist at 6/5 ML. On (001) and (110) surfaces, CsH2 is less stable and can dissociate into CsH or convert into CsH3, respectively. These results are in agreement with the experimental observations. On the metallic Fe5C2(100) surface which lacks surface carbon atoms on the surface monolayer, dissociated hydrogen is found at 1/2 ML, while both dissociated hydrogen and activated H2 are found at 1 ML.

Journal Article↗