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Quantum dynamics of hydrogen interacting with single-walled carbon nanotubes: Multiple H-atom adsorbates

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10.1063/1.3537793

### Abstract

In a previous paper [J. L. McAfee and B. Poirier, J. Chem. Phys.130, 064701 (2009)], using spin-polarized density functional theory(DFT), the authors reported a binding energy of 0.755 eV, for a single hydrogen atom adsorbed on a pristine (unrelaxed) (5,5) single-walled carbon nanotube(SWNT) substrate. A full three-dimensional (3D) potential energy surface (PES) for the SWNT–H system was also developed, and used in a quantum dynamics calculation to compute all rovibrational bound states, and associated equatorial and longitudinal adsorbate migration rates. A highly pronounced preference for the latter migration pathway at ambient temperatures was observed. In this work, we extend the aforementioned study to include multiple H-atom adsorbates. Extensive DFT calculations are performed, in order to ascertain the most relevant dynamical pathways. For two adsorbates, the SWNT–H–H system is found to exhibit highly site-specific binding, as well as long-range correlation and pronounced binding energy enhancement. The latter effect is even more pronounced in the full-hydrogenation limit, increasing the per-adsorbate binding energy to 2.6 eV. To study migration dynamics, a single-hole model is developed, for which the binding energy drops to 2.11 eV. A global 3D PES is developed for the hole migration model, using 40 radial × 18 cylindrical *ab initio* geometries, fit to a Fourier basis with radially dependent expansion coefficients (rms error 4.9 meV). As compared with the single-adsorbate case, the hole migration PES does not exhibit separate chemisorption and physisorption wells. The barrier to longitudinal migration is also found to be much lower. Quantum dynamics calculations for all rovibrational states are then performed (using a mixed spectral basis/phase-space optimized discrete variable representation), and used to compute longitudinal migration rates. Ramifications for the use of SWNTs as potential hydrogen storagematerials are discussed.

© 2011 American Institute of Physics

Received 05 July 2010
Accepted 21 December 2010
Published online 16 February 2011

Acknowledgments: This work was supported by an award from The Welch Foundation (D-1523), and also a New Directions Grant from the American Chemical Society Petroleum Research Fund (PRF # 49031-ND6). The authors also wish to acknowledge Texas Tech University's High Performance Computing Center for vast amounts of CPU time.

Article outline:

I. INTRODUCTION

II. THEORY, METHODOLOGY, AND NUMERICAL DETAILS

A. Density functional theory calculations

B. Mechanistic pathways for multiple adsorbates

1. Two H-atom adsorbates

2. Full coverage limit

3. Concerted migration model

4. Hole migration model

C. Potential energy surface fitting

D. Quantum dynamics calculation

III. RESULTS AND DISCUSSION

A. SWNT–H–H system: Secondary binding enhancement

B. SWNT–H_{20} system: Full coverage limit

C. SWNT–H_{20} system: Hole migration model

1. Potential energy surface

2. Rovibrational bound states

3. Migration

IV. SUMMARY AND CONCLUSIONS

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2011-02-16

2014-04-17

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