Kinetics and mechanisms for the adsorption, dissociation, and diffusion of hydrogen in Ni and Ni/YSZ slabs: a DFT study.

The adsorption, dissociation, and diffusion of hydrogen in Ni(100) and Ni(100)/YSZ(100) slabs with two different interfaces (Ni/cation and Ni/O interface) have been studied by the density functional theory (DFT) with the Perdew-Wang functional. The H(2) molecule is found to preferentially absorb on a Top (T) site with side-on configuration on the Ni(100) surface, while the H-atom is strongly bound at a fcc Hollow (H) site. The barrier for the H(2) dissociation on both surfaces is calculated to be only ~0.1 eV. The potential energy pathways of H diffusion on pure Ni and Ni/YSZ with the two different interfaces are studied. Our calculated results show that the H-atom diffusion occurs via surface path rather than the bulk path. For the bulk path in Ni/YSZ, H-atom migration can occur more readily at the Ni/cation interface compared to the Ni/O interface. The existence of vacancy in the interface region is found to improve the mobility of H-atoms at the interface of Ni/YSZ slab. The rate constants for hydrogen dissociation and diffusion in pure Ni and Ni/YSZ are predicted.

Structure-dependent mechanical properties of ultrathin zinc oxide nanowires.

Mechanical properties of ultrathin zinc oxide (ZnO) nanowires of about 0.7-1.1 nm width and in the unbuckled wurtzite (WZ) phase have been carried out by molecular dynamics simulation. As the width of the nanowire decreases, Young's modulus, stress-strain behavior, and yielding stress all increase. In addition, the yielding strength and Young's modulus of Type III are much lower than the other two types, because Type I and II have prominent edges on the cross-section of the nanowire. Due to the flexibility of the Zn-O bond, the phase transformation from an unbuckled WZ phase to a buckled WZ is observed under the tensile process, and this behavior is reversible. Moreover, one- and two-atom-wide chains can be observed before the ZnO nanowires rupture. These results indicate that the ultrathin nanowire possesses very high malleability.

Adsorption and dissociation of NH3 on clean and hydroxylated TiO2 rutile (110) surfaces: a computational study.

The adsorption and dissociation of NH(3) on the clean and hydroxylated TiO(2) rutile (110) surfaces have been investigated by the first-principles calculations. The monodentate adsorbates such as H(3)N-Ti(a), H(2)N-Ti(a), N-Ti(a), H(2)N-O(a), HN-O(a), N-O(a) and H-O(a), as well as the bidentate adsorbate, Ti-N-Ti(a) can be formed on the clean surface. It is found that the hydroxyl group enhances the adsorption of certain adsorbates on the five-fold-coordinated Ti atoms (5c-Ti), namely H(2)N-Ti(a), HN-Ti(a), N-Ti(a) and Ti-N-Ti(a). In addition, the adsorption energy increases as the number of hydroxyl groups increases. On the contrary, the opposite effect is found for those on the two-fold-coordinated O atoms (2c-O). The enhanced adsorption of NH(x) (x = 1-2) on the 5c-Ti is due to the large electronegativity of the OH group, increasing the acidity of the Ti center. This also contributes to diminish the adsorption of NH(x) (x = 1-2) on the two-fold-coordinated O atoms (2c-O) decreasing its basicity. According to potential energy profile, the NH(3) dissociation on the TiO(2) surface is endothermic and the hydroxyl group is found to lower the energetics of H(2)N-Ti(a)+H-O(a) and HN-Ti(a)+2{H-O(a)}, but slightly raise the energetic of Ti-N-Ti(a)+3{H-O(a)} compare to those on the clean surface. However, the dissociation of NH(3) is found to occur on the hydroxylated surface with an overall endothermic by 31.8 kcal/mol and requires a barrier of 37.5 kcal/mol. A comparison of NH(3) on anatase surface has been discussed. The detailed electronic analysis is also carried out to gain insights into the interaction nature between adsorbate and surface.

The scratch behaviors of copper bi-layers by a diamond tip: a molecular statics study.

The scratch deformation behaviors of two bicrystal coppers (Cu(100)/Cu(110) and Cu(110)/Cu(100)) during the nanoscratching process were explored and compared with their single crystal ingredients by the molecular statics simulations. The effects of lattice configuration and scratch depth were investigated in this study. The results showed that the motion of dislocations was blocked in the bicrystal interface until the dislocations accumulated enough energy to move. From the study, it was found that the bicrystal interfaces can provide resistance to the motion of dislocations, and can strengthen the mechanical properties of copper materials.

The interfacial behavior of water/PMMA thin film under normal compression.

Molecular dynamics simulation (MD) has been used to investigate the structure property of water/PMMA interface under compression and compression release. A virtual repulsive wall was employed to generate a normal compression strain on the simulation model, leading a compressive system. In order to understand the difference of interfacial phenomenon between the system under strain and under release, the hydrogen bond and density distributions of water and PMMA along the normal direction are calculated. The results show that the hydrogen bond distribution profile of compressive system is different from that of the release system at the same strain. It demonstrates that the characteristic structure of water/PMMA interface under a compression-release cycle is not reversible.

Adsorption and dissociation of the O2 on W(111) surface: a density functional theory study.

The adsorption and dissociation of O2 molecules on W(111) surface have been studied at the density functional theory (DFT) level in conjunction with the projector augmented wave (PAW) method. All passable dissociation reaction paths of O2 molecule on W(111) surface are considered. The nudged elastic band (NEB) method is applied to locate transition states, and minimum energy pathways (MEP). We find that there is an existing of little barriers for the dissociations reaction of O2 molecule. Ab initio molecular dynamics simulation is also preformed to study the adsorption and dissociation mechanism of O2 molecules on the W(111) surface. Our results indicate that O2 molecule will be dissociated by inclined deposition at temperature of 10 K, but can stable adsorb on top site by normal deposition. The change of bond length and adsorption energy in process of dissociation of O2 molecules on the W(111) surface are also calculated. The O2 coverage effect is also discussed in this paper.

Hydrogen-bond structure at the interfaces between water/poly(methyl methacrylate), water/poly(methacrylic acid), and water/poly(2-aminoethylmethacrylamide).

The molecular dynamics approach was employed to study the structural characteristics at the interface of water/poly(methyl methacrylate) (PMMA), water/poly(methacrylic acid) (PMAA), and poly(2-aminoethylmethacrylamide) (PAEMA). It is found that the water on the PMAA surface shows a significant increase in the density at the interface, with a greater number of water molecules permeating into the bulk of the substrate region. The structure of hydrogen bonds of water and the radial distribution function for given polar atoms in the polymer substrate are calculated. We found that a network structure of hydrogen bonding between water and the polar atom of the polymer forms at the interface. PMAA exhibits a more hydrophilic property than PMMA and PAEMA because it generates a shell-like structure of water molecules around its functional group. Finally, the hydrogen bond numbers of PMMA, PMAA, and PAEMA are also analyzed. The results detail the hydrogen bond structure of each specific atom and find that, in all three cases, the carboxyl oxygen attracts the greatest number of water molecules compared with other atoms.

Oxygen vacancy formation and migration in Ce(1-x)Zr(x)O2 catalyst: a DFT+U calculation.

Spin-polarized density functional theory with the inclusion of on-site Coulomb correction (DFT+U) calculation is carried out to study the oxygen vacancy and migration of Ce(1-x)Zr(x)O(2) in a series of Ce/Zr ratios. Substitution of Zr(4+) ion in CeO(2) creates activated oxygen in Ce(1-x)Zr(x)O(2), leading to higher oxygen storage capacity (OCS) compared to CeO(2) due to its structural and electronic modifications. It is found that the oxygen vacancy formation energy (E(f)) is lowered even by small amounts of zirconia; the oxide with a content of 50% zirconia exhibits the lowest E(f) and the best OCS. This indicates that the O vacancy is most easily created near the Zr centers. In addition, the activation energy calculations for oxygen vacancy migration around Zr dopant show facile oxygen migration through the Ce(1-x)Zr(x)O(2) materials, especially for 50% Zr-doped ceria. The detailed electronic analysis is also carried out to gain insights into the higher OCS of the Ce(1-x)Zr(x)O(2) catalyst.

Computational study on reaction mechanisms and kinetics of diazocarbene radical reaction with NO.

The mechanisms of the reaction of the diazocarbene radical (CNN) with the NO have been investigated by ab initio molecular orbital in conjunction with variational TST and RRKM calculations. The potential energy surface (PES) was calculated by the high-level CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) method. From the calculated potential energy surface, we have predicted that the formation of N(2)O + CN (P5) is most favorable, and the calculated energies of reaction intermediates and transition structures along this path are all below the starting reference point. The predicted total rate constants, k(total), at a 760 Torr Ar pressure can be represented by the equations: k(total) = 2.47 x 10(-17)T(1.20) exp(1.60 kcal mol(-1)/RT) at T = 300-650 K and 2.49 x 10(-19)T(1.82) exp(2.29 kcal mol(-1)/RT) at T = 660-3000 K cm(3) molecule(-1) s(-1). The calculated results also indicate that the branching ratio for R(P5) in the temperature range 300-3000 K has the largest value. In addition, the rate constants for key individual product channels are provided in different temperature and pressure conditions. To rationalize the scenario of the calculated results, we also employ the Fukui functions and HSAB theory to seek for the possible explanation.

Role of hydroxyl groups in the NH(x) (x = 1-3) adsorption on the TiO2 anatase (101) surface determined by a first-principles study.

A spin-polarized density functional theory calculation was carried out to study the adsorption of NH(x) species (x = 1-3) on a TiO2 anatase (101) surface with and without hydroxyl groups by using first-principles calculations. It was found that the present hydroxyl group has the effect of significantly enhancing the adsorption of monodentate adsorbates H2N-Ti(a) compared to that on a bare surface. The nature of the interaction between the adsorbate (NH(x)) and the hydroxylated or bare surface was analyzed by the Mulliken charge and density of states (DOS) calculations. This facilitation of NH2 is caused by the donation of coadsorbed H filling the nonbonding orbital of NH2, resulting in an electron gain in NH2 from the bonding. In addition, the upper valence band, which originally consisted of the mixing of O 2p and Ti 3d orbitals, has been broadened by the two adjacent H 1s and NH2 sigma(y)(b) orbitals joined to the bottom of the original TiO2 valence band. The results are important to understand the OH effect in heterogeneous catalysis.

Theoretical study on adsorption and dissociation of NO2 molecule on Fe(111) surface.

We applied periodic density-functional theory (DFT) to investigate the adsorption and dissociation of NO(2) on a Fe(111) surface. The most favorable adsorption configuration of NO(2)/Fe(111) is the FeNO(2)(S-mu(3)-N,O,O') configuration with NO(2) at the 3-fold-shallow site of the surface, which has an adsorption energy -64.59 kcal/mol. Of two geometries of NO(2)/Fe(111) for the stepwise NO(2) deoxygenation, one is the most stable structure, FeNO(2)(S-mu(3)-N,O,O'), with activation barriers 10.38 and 19.36 kcal/mol to break the first (ON-O bond activation) and second (N-O bond activation) nitrogen-oxygen bonds, respectively; another configuration FeNO(2)(B-mu(2)-N,O) has a smaller energy barrier (3.88 kcal/mol) to break the first ON-O bond. All these findings show that NO(2) can readily decompose on the Fe(111) surface. The rate constants for the two aforementioned processes were also predicted by VTST and RRKM theory, and the predicted total rate constants, k(total) (in units of cm(3) molecule(-1) s(-1)), can be represented by the equations k(total) = 5.61 x 10(-5)T(-2.060) exp(-0.639 kcal mol(-1)/RT) at T = 100-1000 K. To acquire insight into the great catalytic activity of the Fe(111) surface for the decomposition of NO(2), the nature of the interaction between the adsorbate and the substrate is subjected to a detailed electronic analysis.

First-principle calculations on CO oxidation catalyzed by a gold nanoparticle.

We have elucidated the mechanism of CO oxidation catalyzed by gold nanoparticles through first-principle density-functional theory (DFT) calculations. Calculations on selected model show that the low-coordinated Au atoms of the Au(29) nanoparticle carry slightly negative charges, which enhance the O(2) binding energy compared with the corresponding bulk surfaces. Two reaction pathways of the CO oxidation were considered: the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH). The overall LH reaction O(2(ads)) + CO((gas)) --> O(2(ads)) + CO((ads)) --> OOCO((ads)) --> O((ads)) + CO(2(gas)) is calculated to be exothermic by 3.72 eV; the potential energies of the two transition states (TS(LH1) and TS(LH2)) are smaller than the reactants, indicating that no net activation energy is required for this process. The CO oxidation via ER reaction Au(29) + O(2(gas)) + CO((gas)) --> Au(29)-O(2(ads)) + CO((gas)) --> Au(29)-CO(3(ads)) --> Au(29)-O((ads)) + CO(2(gas)) requires an overall activation barrier of 0.19 eV, and the formation of Au(29)-CO(3(ads)) intermediate possesses high exothermicity of 4.33 eV, indicating that this process may compete with the LH mechanism. Thereafter, a second CO molecule can react with the remaining O atom via the ER mechanism with a very small barrier (0.03 eV). Our calculations suggest that the CO oxidation catalyzed by the Au(29) nanoparticle is likely to occur at or even below room temperature. To gain insights into high-catalytic activity of the gold nanoparticles, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis.

Temperature effect on the dynamic behavior of tricarboxylic acid derivatives in two-dimensional network structures.

The temperature effect on the adsorption behavior and the dynamic behavior of TCMB 2D network structure on the Au(111) substrate has been investigated. From the calculation of the adsorbed energy between the molecule and the Au(111) substrate, it can be found that there are significant changes in adsorbed energy as the temperature increase; moreover, different migration features are appeared at different specific temperature, owing to the deformation of the 2D network structure changed. The mean square displacement (MSD) and diffusion coefficient are calculated to study the migration property and dynamical behavior of 2D TCMB networks at specific temperatures.

Chain-length effects on conformations of methyl methacrylate-oligomer thin films on an Au(111) substrate.

Molecular dynamics simulations were employed to investigate chain-length effects on conformations of methyl methacrylate (MMA)-oligomer thin films on an Au(111) substrate. Some observations were obtained from the present research. For short chain films, there is a sharp peak in the density profile of the MMA monomers for the adsorption region and the thin films exhibit a flattened conformation in the adsorption and the surface regions. For long chain films, however, there is no sharp peak in the whole density profile and a snake-like conformation appears in the adsorption region, which shrinks and convolutes gradually in the bulk region and even more in the surface region of the thin film.

Identifying the O2 diffusion and reduction mechanisms on CeO2 electrolyte in solid oxide fuel cells: a DFT + U study.

The interactions and reduction mechanisms of O2 molecule on the fully oxidized and reduced CeO2 surface were studied using periodic density functional theory calculations implementing on-site Coulomb interactions (DFT + U) consideration. The adsorbed O2 species on the oxidized CeO2 surface were characterized by physisorption. Their adsorption energies and vibrational frequencies are within -0.05 to 0.02 eV and 1530-1552 cm(-1), respectively. For the reduced CeO2 surface, the adsorption of O2 on Ce4+, one-electron defects (Ce3+ on the CeO2 surface) and two-electron defects (neutral oxygen vacancy) can alter geometrical parameters and results in the formation of surface physisorbed O2, O2a- (0 < a < 1), superoxide (O2-), and peroxide (O(2)(2-)) species. Their corresponding adsorption energies are -0.01 to -0.09, -0.20 to -0.37, -1.34 and -1.86 eV, respectively. The predicted vibrational frequencies of the peroxide, superoxide, O2a- (0 < a < 1) and physisorbed species are 897, 1234, 1323-1389, and 1462-1545 cm(-1), respectively, which are in good agreement with experimental data. Potential energy profiles for the O2 reduction on the oxidized and reduced CeO2 (111) surface were constructed using the nudged elastic band method. Our calculations show that the reduced surface is energetically more favorable than the unreduced surface for oxygen reduction. In addition, we have studied the oxygen ion diffusion process on the surface and in bulk ceria. The small barrier for the oxygen ion diffusion through the subsurface and bulk implies that ceria-based oxides are high ionic conductivity at relatively low temperatures which can be suitable for IT-SOFC electrolyte materials.

A computational study on the decomposition of formic acid catalyzed by (H2O)x, x = 0-3: comparison of the gas-phase and aqueous-phase results.

The mechanisms for the water-catalyzed decomposition of formic acid in the gas phase and aqueous phase have been studied by the high-level G2M method. Water plays an important role in the reduction of activation energies on both dehydration and decarboxylation. It was found that the dehydration is the main channel in the gas phase without any water, while the decarboxylation becomes the dominant one with water catalyzed in the gas phase and aqueous phase. The kinetics has been studied by the microcanonical RRKM in the temperature range of 200-2000 K. The predicted rate constant for the (H 2O) 3-catalyzed decarboxylation in the aqueous phase is in good agreement with the experimental data. The calculated CO 2/CO ratio is 200-74 between 600-700 K and 178-303 atm, which is consistent with the average ratio of 121 measured experimentally by Yu and Savage (ref 3).

Computational study on kinetics and mechanisms of unimolecular decomposition of succinic acid and its anhydride.

The mechanisms and kinetics of unimolecular decomposition of succinic acid and its anhydride have been studied at the G2M(CC2) and microcanonical RRKM levels of theory. It was shown that the ZsgsZ conformer of succinic acid, with the Z-acid form and the gauche conformation around the central C-C bond, is its most stable conformer, whereas the lowest energy conformer with the E-acid form, ECGsZ, is only 3.1 kcal/mol higher in energy than the ZsgsZ. Three primary decomposition channels of succinic acid producing H2O + succinic anhydride with a barrier of 51.0 kcal/mol, H2O + OCC2H3COOH with a barrier of 75.7 kcal/mol and CO2 + C2H5COOH with a barrier of 71.9 kcal/mol were predicted. The dehydration process starting from the ECGCZ-conformer is found to be dominant, whereas the decarboxylation reaction starting from the ZsgsZ-conformer is only slightly less favorable. It was shown that the decomposition of succinic anhydride occurs via a concerted fragmentation mechanism (with a 69.6 kcal/mol barrier), leading to formation of CO + CO2 + C2H4 products. On the basis of the calculated potential energy surfaces of these reactions, the rate constants for unimolecular decomposition of succinic acid and its anhydride were predicted. In addition, the predicted rate constants for the unimolecular decomposition of C2H5COOH by decarboxylation (giving C2H6 + CO2) and dehydration (giving H3CCHCO + H2O) are in good agreement with available experimental data.

Adsorption of water molecules inside a Au nanotube: a molecular dynamics study.

A molecular dynamics simulation of water molecules through a Au nanotube with a diameter of 20 A at bulk densities 0.8, 1, and 1.2 gcm(3) has been carried out. The water molecules inside a nanoscale tube, unlike those inside a bulk tube, have a confined effect. The interaction energy of the Au nanotube wall has a direct influence on the distribution of water molecules inside the Au tube in that the adsorption of the water molecules creates shell-like formations of water. Moreover, the high number of adsorbed molecules has already achieved saturation at the wall of the Au nanotube at three bulk densities. This work compares the distribution percentage profiles of hydrogen bonds for different regions inside the tube. The structural characteristics of water molecules inside the tube have also been studied. The results reveal that the numbers of hydrogen bonds per water molecule influence the orientational order parameter q. In addition, the phenomenon of a group of molecules bonded inside the tube can be observed as the number of hydrogen bonds increase.

Adsorption mechanism of water molecules surrounding Au nanoparticles of different sizes.

Molecular dynamic simulation is used to investigate the adsorption mechanism of water molecules surrounding Au nanoparticles with different sizes. Our results show that the adsorption mechanism of the water molecules in the first water shell will be influenced by the size of the Au nanoparticle. For the larger Au nanoparticles, the hydrogen bonding of water molecules adsorbed on the surface of the Au nanoparticles are arranged in a two-dimensional structure, while those adsorbed on the edge of the surface of the Au nanoparticles are arranged in a three-dimensional structure. However, in the case of the smallest Au nanoparticle, the hydrogen bonding of the water molecules on the first adsorbed layer are arranged only in a three-dimensional structure. The arrangement of the water molecules in the first water shell can be determined by orientation order parameter. The water molecules that adsorb on the larger Au nanoparticles tend to arrange in an irregular arrangement, while those adsorbed on the smallest Au nanoparticle tend to arrange a regular arrangement. Interestingly, the water molecules adsorbed on the smallest nanoparticle are arranged in a bulklike structure in the first shell.

Modeling of polyethylene and poly (L-lactide) polymer blends and diblock copolymer: chain length and volume fraction effects on structural arrangement.

Dissipative particle dynamics (DPD), a mesoscopic simulation approach, has been used to investigate the chain length effect on the structural property of the immiscible polyethylene (PE)/poly(L-lactide) (PLLA) polymer in a polymer blend and in a system with their diblock copolymer. In this work, the interaction parameter in DPD simulation, related to the Flory-Huggins interaction parameter chi, is estimated by the calculation of mixing energy for each pair of components in molecular dynamics simulation. The immiscibility property of PE and PLLA polymers induces the phase separation and exhibits different architectures at different volume fractions. In order to observe the structural property, the radius of gyration is used to observe the detailed arrangement of the polymer chains. It shows that the structure arrangement of a polymer chain is dependent on the phase structure and has a significantly different structural arrangement character for the very short chains in the homopolymer and copolymers. The chain length effect on the degree of stretching or extension of polymers has also been observed. As the chain length increases, the chain exhibits more stretching behavior at lamellae, perforated lamellae, and cylindrical configurations, whereas the chain exhibits a similar degree of stretching or extension at the cluster configuration.