Theoretical study of acetate decomposition on Au(111) surface: Oxygen-assisted γ-CH activation mechanism

Inspired by the recent experimental results that the oxygen atoms adsorbed on Au(111) surface have a great influence on the mechanism and path of the decomposition of acetate(ACS Catal, 2014, 4(9): 3281?3288), the density functional theory was performed to simulate the decomposition of acetate on Au(111) surface with and without oxygen atom. The present calculation resents show that the pre-adsorbed oxygen atoms on Au(111) surface can activate the γ-CH bond of acetate and reduce the activation energy of the reaction, then finally get the product of carbon dioxide and formaldehyde. While without adsorbed oxygen atoms, acetate on Au(111) surface breaks down into carbon dioxide and methyl through C-C bond cleavage. In addition, the decomposition of acetate on Ag(111) surface with pre-adsorbed oxygen atoms has also been simulated and that of Au(111), and it was found that the oxygen atoms on Ag(111) assist γ-CH bond activation more efficiently due to its more negatively charged. The present study highlights the importance of oxygen-assisted γ-CH bond activation for oxygen-containing molecular on Au(111) and provide a new route for the synthesis of ester.     

Ultra-low voltage adenine based gas sensor to detect H2 and NH3 at room temperature: First-principles paradigm

The molecular system-level detection of H₂ and NH₃ gas using an electrically doped Adenine bio-molecular gas sensor has been proposed and investigated using Density Functional Theory (DFT) combined with Non-Equilibrium Green's Function (NEGF) formalisms. First-principles calculations were applied and the structures and electronic properties of the Adenine gas sensor have calculated. This sensor reveals that the current-voltage response and conductivity of the bio-molecules increased evidently after the adsorption of these gas molecules. The Adenine sensor offers approximately 1800 times and 3300 times better current response during H₂ and NH₃ adsorption respectively. The significant gap between Highest Occupied Molecular Orbital (HOMO) and Lowest Un-occupied Molecular Orbital (LUMO) indicates the system's thermodynamic stability. Therefore, we hope that the Adenine monolayer could be a room temperature H₂ and NH₃ sensor with high selectivity and sensitivity and fast response and recovery time. Therefore, we hope that the Adenine monolayer will be a good candidate forH₂ and NH₃ work-function-type gas sensors.     

Improving hydrogen storage properties of covalent organic frameworks by substitutional doping

We proposed a possible way of promoting the binding of H₂ molecules on covalent organic frameworks crystals via substituting the bridge C₂O₂B rings with different metal-participated rings, which can naturally avoid the clustering of metal atoms. First-principles calculations on both crystalline phase and molecular fragments show that the H₂ binding energy can be enhanced by a factor of four with regard to the undoped crystal, i.e. reaching about 10 kJ/mol. Grand canonical Monte Carlo simulations further confirm that such substitutional doping would improve the room temperature hydrogen storage capacity by a factor of two to three.     

First-principles study on the dehydrogenation properties and mechanism of Al-doped Mg2NiH4

Mg₂NiH₄, with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg₂NiH₄, and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg₂Ni and a destabilization of the hydride Mg₂NiH₄, which significantly alters the hydrogen desorption enthalpy ΔH_des for the reaction Mg₂NiH₄ → Mg₂Ni + 2H₂. A desirable enthalpy value of ∼0.4 eV/H₂ for application can be obtained for a doping level of x ≥ 0.35 in Mg_2−xAl_xNi alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg₂Ni and Mg₂NiH₄ exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg₂NiH₄ is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg₂Ni and Mg₃AlNi₂ as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg₂NiH₄ can be attributed to the weakened Mg-Ni and Ni-H interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.     

Fundamental influence of hydrogen on various properties of α-titanium

First-principles calculation reveals that the Ti–H interactions are energetically favorable with negative heats of formation, and H atoms could occupy octahedral and tetrahedral interstitial sites of α-Ti simultaneously due to their small energy difference. Calculation also shows that hydrogen concentration plays an important role in determining brittle/ductile behavior of Ti–H phases, and densities of states suggest that a bonding transition from mainly covalent to mainly metallic appear for Ti–H phases when the H/Ti ratio reaches about 1/8. The calculated results agree well with experimental observations and could clarify the controversies of the Ti–H system in the literature.     

The catalytic effect of the Au(111) and Pt(111) surfaces to the sodium borohydride hydrolysis reaction mechanism: A DFT study

In this research, hydrolysis mechanism of sodium borohydride (NaBH₄) have been studied theoretically on Au (111) and Pt (111) noble metal surfaces by periodic density functional theory calculations. Elementary reaction steps have been generated based on study of borohydride oxidation. Reaction intermediates which have plethora of hydroxyl (OH) radical(s) have been produced by decomposition of water molecule(s). In order to investigate surface effect, we have followed two different routes. The first route is that the atomic and molecular structures in the reaction steps have been optimized in 3-d box without a catalyst. At second one, they were interacted with the Au (111) and Pt (111) surfaces to compare relative behavior with reference to the non-catalytic medium. The relative energy diagrams were produced by relative energy differences which is useful to generate energy landscape using required/released energies in order to pursue the reaction. Three main peaks that means considerable energy changes have been observed to proceed the reaction in the non-catalytic medium. Then, changes in the energy differences depending on surfaces have been discussed. Although acquired relative energies are not within chemical accuracy, they are very successful to show the affect of the OH radical concentration to the potential energy diagram. Pt (111) surface have been found more reactive than Au (111) surface for Sodium Borohydride Hydrolysis reaction, as it is obviously coherent with the literature.     

Enhanced hydrogen storage properties and mechanisms of magnesium hydride modified by transition metal dissolved magnesium oxides

Magnesium hydride (MgH₂) is a promising on-board hydrogen storage material due to its high capacity, low cost and abundant Mg resources. Nevertheless, the practical application of MgH₂ is hindered by its poor dehydrogenation ability and cycling stability. Herein, the influences and mechanisms of thin pristine magnesium oxide (MgO) and transition metals (TM) dissolved Mg(TM)O layers (TM = Ti, V, Nb, Fe, Co, Ni) on hydrogen desorption and reversible cycling properties of MgH₂ were investigated using first-principles calculations method. The results demonstrate that either thin pristine MgO or Mg(TM)O layer weakens the MgH bond strength, leading to the decreased structural stability and hydrogen desorption energy of MgH₂. Among them, the Mg(Nb)O layer exhibits the most pronounced destabilization effect on MgH₂. Moreover, the Mg(Nb)O layer presents a long-acting confinement effect on MgH₂ due to the stronger interfacial bonding strength of Mg(Nb)O/MgH₂ and the lower brittleness of Mg(Nb)O itself. Further analyses of electronic structures indicate that these thin oxide layers coating on MgH₂ surface reduce the bonding electron number of MgH₂, which essentially accounts for the weakened MgH bond strength and enhanced hydrogen desorption properties of modified MgH₂ systems. These findings provide a new avenue for enhancing the hydrogen desorption and reversible cycling properties of MgH₂ by designing and adding suitable MgO based oxides with high catalytic activity and low brittleness.     

The effect of Ti atom on hydrogenation of Al(111) surface: First-principles studies

First-principles calculations were performed to investigate hydrogen dissociation and subsequent diffusion over both clean and Ti-doped Al(111) surfaces. The calculations show that it is energetically favorable to dope the surface or subsurface layer of Al(111) with Ti atom. Through calculations on the detailed process associated with hydrogen dissociation and diffusion, we found that Ti doping will decrease the hydrogen dissociation barrier by about 0.6 eV. Additionally, the mobility of hydrogen atoms on surface will be easier if Ti atom is placed in subsurface layer instead of top surface layer. The present results further contribute towards understanding the improved kinetics observed in recycling of hydrogen in Ti-doped NaAlH₄.     

Strain effect on structural and dehydrogenation properties of MgH2 hydride from first-principles calculations

The structure, stability, dehydrogenation thermodynamic and kinetic properties of MgH₂ hydride under different biaxial strain conditions were investigated by using first-principles calculations based on the density functional theory (DFT). The results show that either biaxial tensile or compressive strain is likely to cause the structural deformation of MgH₂ crystal, and its lattice distortion becomes severe with increasing magnitude of strain. Due to the contribution of strain energy, the biaxial strain not only weakens the structural stability of MgH₂, but also lowers its hydrogen desorption enthalpy and dehydrogenation temperature. Furthermore, the diffusion activation energy of hydrogen atom in MgH₂ host is also decreased, which results in a remarkable improvement of dehydrogenation properties. Noticeably, the effect of tensile strain in improving dehydrogenation thermodynamics is relatively superior to that of compressive one, while the role of the latter in enhancing dehydrogenation kinetics is relatively stronger than that of the former. Further analysis of electronic structures suggests the strain-induced changes in structural and dehydrogenation properties of MgH₂ are closely associated with the value of total densities of states at the Fermi level as well as the bonding electrons number below Fermi level. These results provide an insight for developing better MgH₂-based nanocomposite hydrogen storage materials by introducing suitable interface misfit strain.     

Alkali metal silanides α-MSiH3: A family of complex hydrides for solid-state hydrogen storage

The alkali metal silanides α-MSiH₃ appear to be a promising family of complex hydrides for solid-state hydrogen storage. Herein the structural, energetic and electronic properties of α-MSiH₃ silanides (M = Li, Na, K, Rb, Cs) and MSi Zintl phases are systematically investigated for the first time by using first-principles calculations method based on density functional theory. The structural parameters of α-MSiH₃ and MSi including lattice constants and atomic positions are determined through geometry optimization. The obtained results are close to the experimental data analysed from X-ray and neutron powder diffraction. The calculations of formation enthalpy show that α-KSiH₃, α-RbSiH₃ and α-CsSiH₃ silanides are easier to be synthetized relative to α-LiSiH₃ and α-NaSiH₃, which interprets well the lower thermostabilities of experimental α-LiSiH₃ and α-NaSiH₃. Nevertheless, LiSi, KSi and CsSi phases are easier to be formed relative to NaSi and RbSi. The calculations of hydrogen desorption enthalpy reveal that the dehydrogenation abilities of α-MSiH₃ silanides along the decomposition path of α-MSiH₃→MSi + H₂ are gradually enhanced in the order of α-CsSiH₃, α-RbSiH₃, α-KSiH₃, α-NaSiH₃ and α-LiSiH₃, which may be originated from their decreasing thermostabilities. From a comprehensive point of view including hydrogen storage capacity, thermostability and dehydrogenation ability, α-KSiH₃ (～4.29 wt%) is identified as the most promising alkali metal silanide for reversible hydrogen storage. Analysis of electronic structures indicates that a significant charge transfer leads to positively charged M ions and negatively charged SiH₃ complex, which constitutes the ionic bonding between them. The bonding within SiH₃ complex not only involves the covalent hybridization between Si (3s) (3p) and H (1s) orbitals, but also exhibits some ionic bond characteristics due to the partial charge transfer from Si to H. The covalent bonding interactions between H and Si atoms within SiH₃ mainly dominate the thermostabilities and dehydrogenation properties of α-MSiH₃ silanides.