Xueling Lei

Structures, Stability, Vibration Entropy and IR Spectra of Hydrated Calcium Ion Clusters [Ca(H2O)n]2+ (n = 1-20, 27): A Systematic Investigation by Density Functional Theory

The low-lying candidates of hydrated calcium ion clusters, [Ca(H(2)O)(n)](2+) with n = 1-20 and 27, have been extensively sought by using density functional theory (DFT) at BLYP/6-311+G(d,p) level. The results showed that the first hydration shell around the calcium ion was fully occupied by six water molecules, whereas the second hydration shell might be fully occupied with different numbers of water molecules. This just corresponds to different growth patterns of the hydrated calcium ion clusters. Furthermore, we revealed that the vibration entropy contributed to the free energy of an isomer significantly. As a result, the stability of some low-lying candidates at zero-temperature was not maintained at finite temperatures. Therefore, we suggested that, at finite temperatures, the realistic products of [Ca(H(2)O)(n)](2+) should be a mixture of the best candidate and some of metastable isomers for a given cluster size. For a cluster having second and/or third shell of water molecules, we found structural transitions between a low-lying structure and the lowest-energy structure undergoing much lower energy barriers. In addition, the IR spectra of the best candidates were predicted, in which the evolution of hydrogen-bond configurations with the cluster size was revealed.

Is silicene stable in O2???First-principles study of O2 dissociation and O2-dissociation?induced oxygen atoms adsorption on free-standing silicene

The stability of free-standing silicene in O2 is an open question. In this letter, the O2 dissociation and O2-dissociation–induced O atoms adsorption on free-standing silicene are studied by using first-principles calculations. Our results show that the O2 molecule dissociates on the free-standing silicene surface easily from both the thermodynamic and kinetic points of view, which is different from the case of graphene. The dissociation reaction is an exothermic process, and the dissociated O atoms form strong bonds with Si atoms, which lowers the energy of the system substantially. On the other hand, the dissociation reaction occurs spontaneously on the free-standing silicene without overcoming any energy barrier. Furthermore, the migration and desorption of O atoms are relatively difficult under room temperature due to the strong Si-O bonds in the O-adsorbed silicene, which is in favor of forming silicon oxides. Our results provide convictive evidence to show that free-standing silicene is unstable in O2.

Amorphous Na2Si2O5 as a fast Na+ conductor: an ab initio molecular dynamics simulation

The present work uses the ab initio molecular dynamics (AIMD) methodology to simulate ionic transport in amorphous and crystalline Na2Si2O5 from 573 to 973 K. The results suggest that amorphous Na2Si2O5 is primarily a Na+ conductor with negligible O2− and Si4+ contributions to ionic conduction, whereas crystalline Na2Si2O5 is virtually an electrical insulator. The favorable pathway for Na+ transport in amorphous Na2Si2O is along the two-dimensional channels formed by the SiO4 tetrahedral layers. The disrupted Na–O coulombic attraction by the long-range disorder in amorphous Na2Si2O5 contributes to the enhanced Na+ conduction.

The role of Cu in degrading adsorption of CO on the PtnCu clusters.

The platinum copper alloy nanocrystals (NCs) have generated much interest because of their wide applications in fuel cells due primarily to their good catalytic performance and to decreasing sensitivity toward CO poisoning. The exact atomic-level morphology of platinum copper alloy NCs is still not clear in the literature, and research to understanding the poisoning mechanism is still insufficient to date. In this article, we report on density functional calculations of small PtnCu clusters and their adsorption of a CO molecule that provide evidence for degrading adsorption of the CO molecule compared to pure platinum clusters. The lowest-energy geometries of PtnCu and PtnCuCO clusters have been identified. The CO molecule prefers to be adsorbed on the nearest platinum atom by the C-end-on mode, forming linear or quasi-linear O-C-Pt structures. The adsorption energies indicate that the introduction of a copper atom decreases the adsorption ability of the CO molecule. The local density of states of the representative clusters is used to characterize the adsorption properties of the CO molecule on the PtnCu clusters. Results from our theoretical calculations can be helpful for understanding the poisoning mechanism of the CO molecule on the platinum copper alloy NCs.

Remarkable O2 permeation through a mixed conducting carbon capture membrane functionalized by atomic layer deposition

The development of energy-efficient and cost-competitive carbon capture technology is of vital importance to effective reduction of carbon emissions and mitigation of global climate change. The present work reports that a highly energy-efficient electrochemical carbon capture membrane consisting of a carbonate and silver phase exhibits a remarkably high rate of O2 permeation after the silver phase is functionalized with a nanoscale layer of Al2O3 by atomic layer deposition. The resulting permeation flux ratio of O2 to CO2 was 1.5:1, a complete reversal from 1:2 of the conventional membrane. The mechanisms of this exceptionally high rate of O2 permeation were investigated by in situ Raman spectroscopy and theoretical DFT calculations. The results revealed that LiCO4− as the active surface species was responsible for the enhanced O2 permeation. A new CO2/O2 transport model based on a “cogwheel” migration mechanism of CO42− was presented to explain the experimental results with excellent agreement.

DFT Study of Oxygen Dissociation in Molten Carbonate

Using density functional theory method, we have studied the oxygen dissociation in alkali molten carbonate at the B3LYP/6-31G(d) level. The calculated energies were then verified by MP4 and CCSD(T). A four-formula cluster (M2CO3)4, M = Li, Na, and K was used to describe the molten carbonate. It was found that the adsorption of oxygen to molten carbonate is of a chemical type and leads to the formation of CO5(2-) in MC, which was confirmed for the first time by DFT calculations. The energy barrier for its dissociation is calculated to be 197.9, 116.7, and 170.3 kJ/mol in the (M2CO3)4 cluster, M = Li, Na, and K, respectively. If the reaction of O2 + 2CO3(2-) → 2CO4(2-) is approximated as a one-step reaction, the activation energy is estimated to be 96.2, 15.1, and 68.6 kJ/mol, respectively. The reaction rate is first order to the pressure of oxygen. Surprisingly, the reaction of oxygen dissociation has the lowest energy barrier in sodium carbonate, which is consistent with the recent experimental findings. It is very clear that the molten carbonate salt has directly participated in the ORR process and plays an important role as a catalyst in the cathode of SOFCs. The oxygen reduction has been facilitated by MC and enhanced cell performance has been observed.

THE GEOMETRIES AND PROTON TRANSFER OF HYDRATED DIVALENT LEAD ION CLUSTERS [Pb(H2O)n]2+(n = 1–17)

The low-lying candidates of hydrated divalent lead ion clusters [Pb(H2O)n]2+ with up to n = 17 have been extensively studied by using density functional theory (DFT) at B3LYP level. The optimized structures show that for n = 5–13 the lowest-energy structures prefer tetracoordinate with hemi-directed geometries, while the best candidates with n = 14–17 are hexacoordinate with holo-directed geometries, which is just consistent with the experimental observation. Furthermore, the origin of hemi-directed and holo-directed geometries has been revealed. It is found that in the hemi-directed geometries, the lone pair orbital has p character and fewer electrons are transferred from the water molecules to the Pb2+ ion. Contrarily, in the holo-directed geometries, the lone pair orbital has little or no p character and more electrons are transferred to the Pb2+ ion. On the other hand, the proton transfer reactions of the [Pb(H2O)n]2+(n = 2, 4, 8) complexes have been examined, from which the predicted products of these complexes are in good agreement with the experimental observation.

Energetics of proton transfer in alkali carbonates: a first principles calculation

Recent development of dual-phase ceramic–carbonate composite electrolytes for intermediate-temperature solid oxide fuel cells (SOFCs) has prompted a pressing question as to whether H+ can transfer in molten carbonates and play a role in the enhanced ionic conductivity and improved SOFC performance. In the present study, we use a first principles approach to examine the energetics of H+-transfer in CO32−, Li2CO3 crystals and (Li2CO3)8 clusters. The results indicate that H+-transfer in solid carbonates is difficult, but very facile in a (Li2CO3)8 cluster, a surrogate of molten carbonates.

Bulk properties and transport mechanisms of a solid state antiperovskite Li-ion conductor Li3OCl: insights from first principles calculations

The excellent Li+ conductivity (0.85 × 10−3 S cm−1 at room temperature) and wide electrochemical stability window (>6 V) of the antiperovskite Li3OCl material make it a promising candidate electrolyte for rechargeable all-solid-state Li batteries. In this study, we systematically evaluate the electronic, mechanical, and thermodynamic properties of Li3OCl by first-principles density functional theory calculations. The defect chemistry and Li+ migration mechanisms are also discussed in the context since Li+ diffusion is strongly influenced by defects in the material. Our results show that Li3OCl is an indirect wide-band gap insulator in the equilibrium state with a band gap of ∼6.26 eV. Phonon spectral data confirm that Li3OCl is dynamically stable at its ground state. It is also revealed that Li3OCl is mechanically brittle. The bulk modulus of Li3OCl is greater than that of Li10GeP2S12, while it is comparable to that of Li0.5La0.5TiO3 and Li7La3Zr2O12. From quasi-harmonic approximation, the linear thermal expansion coefficient and thermal conductivity of the material are found to be 3.12 × 10−6 K−1 and 22.49 W m−1 K−1 at room temperature, respectively. With four types of point defect pairs in Li3OCl being considered, it is revealed that the LiCl defect pair has the lowest formation energy compared to Li2O, O substituted Cl, and Li/Li-vacancy Frenkel defect pairs. The LiCl and Frenkel defect pairs are the most important point defects responsible for the fast Li diffusion. Overall, our study provides fundamental and comprehensive insights into the bulk properties and transport mechanisms of Li3OCl for its practical application as a solid-state electrolyte in all-solid-state Li batteries.

Can molten carbonate be a non-metal catalyst for CO oxidation?

For the first time, we have examined molten carbonate as a non-metal catalyst for CO oxidation in the temperature range of 300–600 °C. The reaction mechanism was analyzed using a classic Langmuir–Hinshelwood model combined with DFT calculations. It was found that the conversion of CO is greatly enhanced by molten carbonate at about 400 °C and increased to 96% at 500 °C. The reaction process involves four steps, including (1) dissociative adsorption of oxygen, (2) adsorption of CO, (3) surface reaction, and (4) desorption of CO2. DFT modeling reveals the formation of (C2O4)2− and (CO4)2− as the intermediate species, and that the first two steps are exothermic and preferred by chemical equilibrium. The energy barrier of oxygen dissociation to form CO42− is calculated to be 23.0 kcal mol−1, which is in good agreement with the measured overall activation energy of 19.1 kcal mol−1. However, the surface reaction (step 3) has a low energy barrier of 10.8 kcal mol−1 only. This confirms that the oxygen dissociation is the rate determing step in the whole process. Further analysis of the reaction kinetics indicates that the reaction is affected by the CO concentration. With low CO concentration, the reaction is first order with respect to CO and half order to O2. From the current report, it has been proven that molten carbonate can serve as an efficient catalyst for CO oxidation and potentially for other oxidation reactions in the temperature range of 400–600 °C. More studies are demanded to further investigate the reaction mechanism and explore more potential industrial applications.