Hui-Lung 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.

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).

Ab Initio Chemical Kinetics for H + NCN: Prediction of NCN Heat of Formation and Reaction Product Branching via Doublet and Quartet Surfaces

The reaction of NCN with H atoms has been investigated by ab initio MO and RRKM theory calculations. The mechanisms for formation of major products on the doublet and quartet potential energy surfaces have been predicted at the CCSD(T) level of theory with the complete basis set limit. In addition, the heat of formation for NCN predicted at this rigorous level and those from five isogyric reactions are in close agreement with the best value based on the isodesmic process, (3)CCO + N2 = (3)NCN + CO, 109.4 kcal/mol, which lies within the two existing experimental values. The rate constants for the three possible reaction channels, H + NCN → CH + N2 (k(P1)), HCN + (4)N (k(QP1)), and HNC + (4)N (k(QP2)), have been calculated in the temperature range 298-3000 K. The results show that k(P1) is significantly higher than k(QP1) and k(QP2) and that the total rate constant agrees well with available experimental values in the whole temperature range studied. The kinetics of the reverse CH + N2 reaction has also been revisited at the CCSD(T)/CBS level; the predicted total rate constants at 760 Torr Ar pressure can be represented by kr = 4.01 × 10(-15) T(0.90) exp(-17.42 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) at T = 800-4000 K. The result agrees closely with the most recent experimental data and the best theoretical result of Harding et al. (J. Phys. Chem. A 2008, 112, 522) as well as that of Moskaleva and Lin (Proc. Combust. Inst. 2000, 28, 2393) evaluated with a steady-state approximation after a coding error correction made in this study.

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.

Mechanism and Kinetics for Ammonium Dinitramide (ADN) Sublimation: A First-Principles Study

The mechanism for sublimation of NH(4)N(NO(2))(2) (ADN) has been investigated quantum-mechanically with generalized gradient approximation plane-wave density functional theory calculations; the solid surface is represented by a slab model and the periodic boundary conditions are applied. The calculated lattice constants for the bulk ADN, which were found to consist of NH(4)(+)[ON(O)NNO(2)](-) units, instead of NH(4)(+)[N(NO(2))(2)](-), agree quite well with experimental values. Results show that three steps are involved in the sublimation/decomposition of ADN. The first step is the relaxation of the surface layer with 1.6 kcal/mol energy per NH(4)ON(O)NNO(2) unit; the second step is the sublimation of the surface layer to form a molecular [NH(3)]-[HON(O)NNO(2)] complex with a 29.4 kcal/mol sublimation energy, consistent with the experimental observation of Korobeinichev et al. (10) The last step is the dissociation of the [H(3)N]-[HON(O)NNO(2)] complex to give NH(3) and HON(O)NNO(2) with the dissociation energy of 13.9 kcal/mol. Direct formation of NO(2) (g) from solid ADN costs a much higher energy, 58.3 kcal/mol. Our calculated total sublimation enthalpy for ADN(s) → NH(3)(g) + HON(O)NNO(2)) (g), 44.9 kcal/mol via three steps, is in good agreement with the value, 42.1 kcal/mol predicted for the one-step sublimation process in this work and the value 44.0 kcal/mol computed by Politzer et al. (11) using experimental thermochemical data. The sublimation rate constant for the rate-controlling step 2 can be represented as k(sub) = 2.18 × 10(12) exp (-30.5 kcal/mol/RT) s(-1), which agrees well with available experimental data within the temperature range studied. The high pressure limit decomposition rate constant for the molecular complex H(3)N···HON(O)NNO(2) can be expressed by k(dec) = 3.18 × 10(13) exp (-15.09 kcal/mol/RT) s(-1). In addition, water molecules were found to increase the sublimation enthalpy of ADN, contrary to that found in the ammonium perchlorate system, in which water molecules were shown to reduce pronouncedly the enthalpy of sublimation.

Quantum Chemical Prediction of Pathways and Rate Constants for Reaction of Cyanomethylene Radical with NO

High-level ab initio calculations have been performed to study the mechanism and kinetics of the reaction of the cyanomethylene radical (HCCN) with the NO. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level, and their corresponding single-point energies are improved by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) approach. From the calculated potential energy surface, we have predicted the favorable pathways for the formation of several isomers of a HCCN-NO complex. Barrierless formation of HCN + NCO (P1) is also possible. Formation of HCNO + CN (P3) is endoergic but may become significant at high temperatures. To rationalize the scenario of our calculated results, we also employ the Fukui functions and hard-and-soft acid-and-base (HSAB) theory to seek possible clues. The predicted total rate coefficient, k(total), at He pressure 760 Torr can be represented with the equation k(total) = 1.40 × 10(-7) T(-2.01) exp(3.15 kcal mol(-1)/RT) at T = 298-3000 K in units of cm(3) molecule(-1) s(-1). The predicted total rate coefficients at some available conditions (He pressures of 6, 18, and 30 Torr in the temperature of 298 K) are in reasonable agreement with experimental observation. In addition, the rate constants for key individual product channels are provided in different temperature and pressure conditions.

Ab Initio Study on Mechanisms and Kinetics for Reaction of NCS with NO

The mechanisms and kinetics of the reaction of a thiocyanato radical (NCS) with NO were investigated by a high-level ab initio molecular orbital method in conjunction with variational RRKM calculations. The species involved were optimized at the B3LYP/6-311++G(3df,2p) level, and their single-point energies were refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311+G(3df,2p) method. Our calculated results indicate favorable pathways for the formation of several isomers of an NCSNO complex. Formation of OCS + N 2 also is possible, although this pathway involves a substantial energy barrier. The predicted total rate constants, k total, at a 2 torr He pressure can be represented by the following equations: k total = 9.74 x 10 (26) T (-13.88) exp(-6.53 (kcal mol (-1))/ RT) at T = 298-950 K and 1.17 x 10 (-22) T (2.52) exp(-6.86 (kcal mol (-1))/ RT) at T = 960-3000 K, in units of cm (3) molecule (-1) s (-1), and the predicted values are in good agreement with the experimental results in the temperature range of 298-468 K. The calculated results clearly indicate that the branching ratio for R M1 in the temperature range of 298-950 K has the largest value ( R M1 accounts for 0.53-0.39). However, in the higher temperature range (960-3000 K), the formation of OCS + N 2 ( P5) with branching ratio R P5 (0.40-0.79) becomes dominant. The rate constants for key individual product channels are provided for different temperature and pressure conditions.

Theoretical Study on Reaction Mechanisms and Kinetics of Cyanomidyl Radical with NO

The mechanisms and kinetics of the reaction of the cyanomidyl radical (HNCN) with the NO have been investigated by the high-level ab initio molecular orbital method in conjunction with VTST and RRKM theory. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level and their single-point energies are refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) method. Our calculated results indicate that the favorable pathways for the formation of several isomers of an HNCN-NO complex. Formations of HNC + N(2)O (P1) and HNCO + N(2) (P2) are also possible, although these two pathways involve little activation energy. Employing the Fukui functions and HSAB theory, we are able to rationalize the scenario of the calculated outcome. The predicted total rate constants, k(total), at a 760 Torr Ar pressure can be represented by the equations k(total) = 4.39 x 10(8) T(-7.30) exp(-1.76 kcal mol(-1)/RT) at T = 298-1000 K and 1.01 x 10(-32) T(5.32) exp(11.27 kcal mol(-1)/RT) at T = 1050-3000 K, respectively, in units of cm(3) molecule(-1) s(-1). In addition, the rate constants for key individual product channels are provided in a table for different temperature and pressure conditions. These results are recommended for combustion modeling applications.

Investigation of Zr and Si diffusion behaviors during reactive diffusion – a molecular dynamics study

Molecular dynamics simulation was used to investigate the diffusion behaviors of Zr and Si atoms during a reactive diffusion which produces Zr silicide. The simulation results were compared with those in Roys experimental results. The profiles of mean square displacements (MSDs) of Zr and Si atoms at different temperatures were first used to evaluate the melting point above which the significant inter-diffusions of Zr and Si atom occur. The diffusion coefficients near the melting point were derived by the Einstein equation from MSD profiles. On the basis of diffusion coefficients at different temperatures, the diffusion barriers of Zr and Si atoms can be calculated by the Arrhenius equation. Compared to the corresponding experimental values, the predicted diffusion barriers at the Zr–Si interface were 23 times lower than the measured values in Roys study. The main reason for this is that the Zr and Si atoms within the inter-diffusion region form different local ZrSi crystal alloys in the experiment, resulting in the lower diffusion coefficients and higher diffusion barriers found in the experimental observation.