The quantum instanton approximation is extended to investigate dynamical processes of hydrogen on surface, from surface to subsurface, and between interior sites in nickel lattice. The path integral Monte Carlo and adaptive umbrella sampling techniques are employed to manipulate the quantum instanton formula. The free energy profiles along reaction paths, temperature dependence of free energies, and rates as well as diffusion coefficients are calculated for each process. The results manifest that the motions of nickel atoms beneath the surface have little effect on the hydrogen diffusion on Ni(111), and the hydrogen at the fcc binding site is much easier to get into bulk nickel than the one at the hcp site. The temperature dependence of free energy profiles also reveals that the hydrogen in the subsurface octahedral vacancy and interior tetrahedral vacancy becomes unstable at low temperatures, which proposes a temperature dependence of reaction mechanism. In addition, the relaxations of the lattices dramatically lower the free energy barriers except for the process of the hydrogen diffusion on Ni(111). The quantum motions of the lattice atoms affect the free energies little at 300 K, but they hinder the rates by 20%-40% compared with the classical motions of lattice atoms.
[Show abstract][Hide abstract] ABSTRACT: Thermal rate constants and kinetic isotope effects for the title reaction are calculated by using the quantum instanton approximation within the full dimensional Cartesian coordinates. The obtained results are in good agreement with experimental measurements at high temperatures. The detailed investigation reveals that the anharmonicity of the hindered internal rotation motion does not influence the rate too much compared to its harmonic oscillator approximation. However, the motion of the nonreactive methyl group in C(2)H(6) significantly enhances the rates compared to its rigid case, which makes conventional reduced-dimensionality calculations a challenge. In addition, the temperature dependence of kinetic isotope effects is also revealed.
[Show abstract][Hide abstract] ABSTRACT: Quantum instanton (QI) approximation is recently proposed for the evaluations of the chemical reaction rate constants with use of full dimensional potential energy surfaces. Its strategy is to use the instanton mechanism and to approximate time-dependent quantum dynamics to the imaginary time propagation of the quantities of partition function. It thus incorporates the properties of the instanton idea and the quantum effect of partition function and can be applied to chemical reactions of complex systems. In this paper, we present the QI approach and its applications to several complex systems mainly done by us. The concrete systems include, (1) the reaction of H+CH4→H2+CH3, (2) the reaction of H+SiH4→H2+SiH3, (3) H diffusion on Ni(100) surface; and (4) surface-subsurface transport and interior migration for H/Ni. Available experimental and other theoretical data are also presented for the purpose of comparison.
Advances in Physical Chemistry 01/2012; 2012(1687-7985). DOI:10.1155/2012/483504
[Show abstract][Hide abstract] ABSTRACT: We present a detailed ring polymer molecular dynamics (RPMD) study of the diffusion of hydrogen on Ni(100) using the well-established embedded atom method (EAM) interaction potential. We pay particular attention to the effects of lattice motion, transition state recrossing, and multiple hops. We show that all these effects can be assessed within a unified theoretical framework using RPMD. First, we study the low-temperature regime where the diffusion coefficient can be calculated by the random walk model. The crossover from thermally activated diffusion to almost temperature-independent quantum diffusion is found at around 70 K, in agreement with earlier quantum instanton calculations. We show that the recrossings of the transition state dividing surface become significant only below the crossover temperature in our RPMD calculations. The lattice motion slightly increases the diffusion coefficient above and slightly decreases it below the crossover temperature. We also show that quantising the motion of the metal atoms has a negligible effect, even at very low temperatures. These last two observations are at variance with previous theoretical results obtained using the same interaction potential. We argue that this is primarily due to the different lattice models employed in the various calculations. Second, we studied the high-temperature regime. The diffusion coefficients are computed using the Einstein and Green-Kubo relations. Comparison of these results with those generated by the random walk model allows us to examine the role of correlated dynamical events in the diffusion. We find a noticeable contribution of correlated rebound events in our Einstein and Green-Kubo calculations leading to a decrease in the diffusion coefficient compared to the random walk estimate at temperatures above 300 K.
The Journal of Physical Chemistry A 05/2012; 116(20). DOI:10.1021/jp302453z · 2.69 Impact Factor
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