Hsin-Tsung Chen

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 Au29 nanoparticle carry slightly negative charges, which enhance the O2 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 O2(ads) + CO(gas) → O2(ads) + CO(ads) → OOCO(ads) → O(ads) + CO2(gas) is calculated to be exothermic by 3.72 eV; the potential energies of the two transition states (TSLH1 and TSLH2) are smaller than the reactants, indicating that no net activation energy is required for this process. The CO oxidation via ER reaction Au29 + O2(gas) + CO(gas) → Au29–O2(ads) + CO(gas) → Au29–CO3(ads) → Au29–O(ads) + CO2(gas) requires an overall activation barrier of 0.19 eV, and the formation of Au29–CO3(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 Au29 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.

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 (O22−) 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.

Computational investigation of O2 reduction and diffusion on 25% Sr-doped LaMnO3 cathodes in solid oxide fuel cells.

The oxygen reduction reaction (ORR) and diffusion mechanisms on 25% Sr-doped LaMnO(3) (LSM) cathode materials as well as their kinetic behavior have been studied by using spin-polarized density functional theory (DFT) calculations. Bader charge and frequency analyses were carried out to identify the oxidation state of adsorbed oxygen species. DFT and molecular dynamics (MD) results show that the fast O(2) adsorption/reduction process via superoxide and peroxide intermediates is energetically favorable on the Mn site rather than on the Sr site. Furthermore, the higher adsorption energies on the Mn site of the (110) surface compared to those on the (100) surface imply that the former is more efficient for O(2) reduction. Significantly, we predict that oxygen vacancies enhance O(2) reduction kinetics and that the O-ion migration through the bulk is dominant over that on the surface of the LSM cathode.

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).

Role of hydroxyl groups in the NHx (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.

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.

Adsorption and Dissociation of NH3 on Clean and Hydroxylated TiO2 Rutile (110) Surfaces: A Computational Study

The adsorption and dissociation of NH3 on the clean and hydroxylated TiO2 rutile (110) surfaces have been investigated by the first‐principles calculations. The monodentate adsorbates such as H3NTi(a), H2NTi(a), NTi(a), H2N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 H2N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 NHx (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 NHx (x = 1 − 2) on the two‐fold‐coordinated O atoms (2c‐O) decreasing its basicity. According to potential energy profile, the NH3 dissociation on the TiO2 surface is endothermic and the hydroxyl group is found to lower the energetics of H2N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 NH3 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 NH3 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.

Investigation of the mechanical properties and local structural evolution of Ti60Zr10Ta15Si15 bulk metallic glass during tensile deformation: a molecular dynamics study

Ti60Zr10Ta15Si15 bulk metallic glass (BMG) has been proven to have potential for use in orthopedic bone fixation devices, and further studies on its structural properties and deformation mechanism under uniaxial tension have been conducted using molecular dynamics (MD) simulations. The Honeycutt–Andersen (HA) index analysis, Voronoi tessellation method and Warren–Cowley short-range order parameter are employed to investigate its structural properties. The results show a high content of icosahedral-like structures, which suggests an amorphous state and a trend for silicon to pair with a metal atom. In its tensile test, the Ti60Zr10Ta15Si15 bulk metallic glass showed good ductility and an estimated Youngs modulus of about 93 GPa, which is close to the experimental value. Local strain distribution was used to analyze the deformation mechanism, and the results show that shear bands develop homogeneously, which enhances the plasticity. The Voronoi tessellation analysis and HA index were used to further investigate the plastic/elastic deformation mechanism. The results of the HA analysis show that icosahedral local structures (1551, 1541, 1431) transfer to less dense structures (1422 and 1311), which shows an increase of open volume which can be attributed to the formation of the shear bands. In addition, the Voronoi tessellation analysis also shows a notable change from perfect icosahedra to distorted icosahedra. Further investigation shows the variations of the Voronoi index are mostly the Ti and Si-centered clusters. This suggests that the structures around Ti and Si atoms undergo a severe evolution during the tension process.

Observation of the amorphous zinc oxide recrystalline process by molecular dynamics simulation

The detailed structural variations of amorphous zinc oxide (ZnO) as well as wurtzite (B4) and zinc blende (B3) crystal structures during the temperature elevation process were observed by molecular dynamics simulation. The amorphous ZnO structure was first predicted through the simulated-annealing basin-hopping algorithm with the criterion to search for the least stable structure. The density and X-ray diffraction profiles of amorphous ZnO of the structure were in agreement with previous reports. The local structural transformation among different local structures and the recrystalline process of amorphous ZnO at higher temperatures are observed and can explain the structural transformation and recrystalline mechanism in a corresponding experiment [Bruncko et al., Thin Solid Films 520, 866-870 (2011)].

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.