Feng-Qi Zhao

DFT studies of the adsorption and dissociation of H2O on the Al13 cluster: origins of this reactivity and the mechanism for H2 release

AbstractA theoretical study of the chemisorption and dissociation pathways of water on the Al13 cluster was performed using the hybrid density functional B3LYP method with the 6-311+G(d, p) basis set. The activation energies, reaction enthalpies, and Gibbs free energy of activation for the reaction were determined. Calculations revealed that the H2O molecule is easily adsorbed onto the Al13 surface, forming adlayers. The dissociation of the first H2O molecule from the bimolecular H2O structure via the Grotthuss mechanism is the most kinetically favorable among the five potential pathways for O–H bond breaking. The elimination of H2 in the reaction of an H2O molecule with a hydrogen atom on the Al cluster via the Eley–Rideal mechanism has a lower activation barrier than the elimination of H2 in the reaction of two adsorbed H atoms or the reaction of OH and H. Following the adsorption and dissociation of H2O, the structure of Al13 is distorted to varying degrees. FigurePotential energy surface along the reaction coordinate for steps 5–9, calculated at the B3LYP/6-311+G(d,p) level

DFT Studies on Doping Effect of Al12X: Adsorption and Dissociation of H2O on Al12X Clusters

The adsorption and reaction of H2O molecule on neutral X-centered icosahedronal Al12X clusters (X = Al, Mg, Zn, Ga, Ni, Fe, B, C, Si, P) were investigated by PW91, PBE, and PWC methods. Reaction energies and reaction barriers were determined. The spin states and the doped atoms have important influences on the Al12X geometries, density, electronic properties, and energy density of reaction between Al12X with a single H2O molecule. The energies of the neutral X-centered Al12X are lower than that of surface X-replaced Al12X with the exception of Al12Mg. The H2O dissociation on the Al12X (X = Mg, Zn, Ga, Ni, Fe) clusters have relatively low activation barriers, but large activation barriers for Al12X (X = B, C, Si, P). The activation barrier of water dissociation on the singlet Al12Fe cluster is the lowest, whereas the highest barrier is with the Al12C. The reaction of H2O with Al12Fe is the most exothermic. The center-Fe atom can move out to the surface after the adsorption and dissociation of H2O with an energy barrier of 172 kJ/mol. The results showed that the water dissociation on the Al12X cluster can be tuned by controllable X doping.

Theoretical study of the geometries and decomposition energies of CO2 on Al12X: Doping effect of Al12X

The adsorption and decomposition of CO₂ molecule on X-centered icosahedronal Al₁₂X clusters (doping atom X=Al, Be, Zn, Fe, Ni, Cu, B, C, Si, P) were investigated by the DFT methods of PW91 and PWC. Adsorption energies, chemisorption energies and energy barriers of physic- and chemisorptions for CO₂ were determined. It was found that the doping atoms and spin states have important influences on the Al₁₂X geometries, electronic properties and energies of the adsorption processes. CO₂ chemisorption on the Al₁₂C cluster is energetically and kinetically unfavorable. CO₂ decomposition on the metallic doping Al₁₂X (X=Fe, Ni, Cu) clusters has relatively low energy barriers. On contrary, the barriers are large when X=B, C, Si and P. The energy barriers for CO₂ chemisorption and decomposition on the Al₁₂Fe cluster are 5.23 kJ/mol and 38.53 kJ/mol, respectively. These values are the lowest among all the clusters being discussed. The adsorption and decomposition of CO₂ on the Al₁₂X cluster can be tuned by X doping.

Adsorption of carbon dioxide on Al12X clusters studied by density functional theory: effect of charge and doping.

The adsorption of a CO2 molecule on neutral and charged X-centered icosahedron Al12X(±z) clusters (X = Al, Be, Zn, Ni, Cu, B, P; z = 0, 1) was investigated by the density functional PW91 and PWC methods. Optimized configurations corresponding to physisorption and chemisorption of CO2 were identified. The adsorption energies, activation barriers, and binding energies involving both the physisorption (Al12X(±z)·CO2-I) and chemisorption (Al12X(±z)·CO2-II) for CO2 were determined. The chemisorption of a CO2 molecule on the Al12X clusters (X is a metallic doping element) requires relatively low activation barriers. The lowest barrier was found to be with the Al12Be cluster. For the Al12X(-) clusters, the barriers are all higher than those of the neutral analogues. For the Al12X(+) clusters, two corresponding configurations are linked by a low-energy barrier, and CO2 molecule chemisorption on the Al12Be(+) cluster has the lowest barrier. The adsorption energies are larger than the energy barriers, which facilitates the chemisorption. The results show that carbon dioxide adsorbed on the Al12X(±z) clusters can be tuned by controllable X doping and the total number of valence electrons and suggest the potential application of Al12X(±z) nanostructures for carbon dioxide capture and activation.

Access to Novel Graphene-Like Sheet of Hydroboron: First-Principles Investigation

We designed a cyclic borane (B6 H12 ) molecule with a benzene-like structure, in which the six B atoms are located in the same plane. Three methods of B3LYP, MP2, and CCSD with the 6-311++G** basis were used to investigate its structure, electronic property, and stability. Next, we calculated the stability and electronic property of three hydroboron derivatives with fused rings of B10 H18 , B14 H24 , and B16 H26 . Finally, we investigated three types of novel two-dimensional infinite hydroboron sheets with diborane as a building block. The results of the phonon spectra ensure the dynamic stability of these predicted structures. Furthermore, the three types of hydroboron sheets are shown to have different band gap energies of less than 3.0 eV. Some investigations on the optical properties have also been performed. The predicted sheets are candidates for semiconductors, whose band gap energy can be tuned by the positions of the bridge hydrogen atoms in the sheets.

STRAIN ENERGY CALCULATIONS OF CAGED SILANES

Abstract Strain energies (SE) of typical caged silanes were evaluated at the B3LYP/6-31+G** and B3LYP/aug-cc-pVDZ levels, in combination with the isodesmic and homodesmotic reactions. The SE values of Si4H4 and Si8H8 are 512 kJ/mol and 336–338 kJ/mol, respectively, at the B3LYP/6-31+G** level, whereas the SE values of the caged silanes with five-number or larger rings are very small. In comparison, the SE value of Si8H8 is much smaller than 656–707 kJ/mol of cubane (C8H8), since the Si‒Si bond length (2.274 Å) of Si8H8 is much larger than the corresponding C‒C bond length of cubane. The large electron charge density at body-center of Si4H4 causes strong repulsion between the cage center and Si‒Si bond, which leads to the Si‒Si bond bending. The electron charge density at the face center of Si4H4 is more than two times of that of Si8H8, which also contributes significantly to the large SE difference between Si4H4 and Si8H8. GRAPHICAL ABSTRACT

Theoretical investigations on the phase transition of pure and Li-doped AlH3

In order to solve a contradiction between early theoretical prediction and experiments concerning the γ → α phase transition of aluminum hydride, models of Li-doped AlH3 were constructed and investigated theoretically. Thermodynamic calculations show that the γ → α transition of pure AlH3 absorbs energy, and the changes in Gibbs free energy are in range of 1.74–1.99 kJ mol−1 at 298–380 K. These are opposite to the experimental fact that the γ- to α-phase transition takes place at 380 K. However, the changes in enthalpy and Gibbs free energy in the γ → α phase transition of Li-doped AlH3 are negative. The doping of Li decreases the activation energy of the γ → α transition and introduces more metastable states between them. As the doping content increases, both the changes in enthalpy and Gibbs free energy (ΔHγ→α and ΔGγ→α) decrease. The experimental ΔHγ→α value (−2.83 kJ mol−1) is between those of doped AlH3 with 1/23 and 1/11 Li-content (−0.87 and −5.62 kJ mol−1 for Al23LiH70 and Al11LiH34, respectively). Heat capacity CP(T) increases as the Li-doping content increases. The CP(T) of Al23LiH70 is consistent with the experiments. Considering the thermodynamic evidence and the experimental conditions for AlH3 preparation, the aluminum hydride synthesized by the reaction of LiAlH4 + AlCl3 is probably Li-doped with a Li content of 1/23. The changes in enthalpy and Gibbs free energy, as well as the activation energy for the γ → α phase transition can be increased if the Li-doped AlH3 is purified.