Computational catalysis in nanotubes - Catalysis - page 116

diameter below 1 nm. By an atomistic simulation method, Kang et al. 32
found that when the radius of CNTs increases, Na metal confined in
CNTs exhibits several phases from an atomic strand to multi-shell
packing structures composed of coaxial cylindrical shells and in both
helical and layered structures. The simulations obtained by Cheng et al. 33
using a canonical Monte Carlo method demonstrated that the melting
temperature of the confined icosahedral Pt 55 clusters increases with the
diameters of CNTs. Especially, the four-layer stacked structures are ob-
served for the encapsulated Pt clusters at 600 K. Arcidiacono et al. 34
carried out a pioneering work on Au nanoparticle confined in CNTs using
MD simulation technique, and found that the solidification temperatures
of Au nanoparticles confined in (n, 0)-CNTs are higher than those of the
corresponding free clusters and lower than that of its bulk counterpart.
What's more, the solidification temperature depends mainly on the
length of the particle with a minor dependence on the tube radii (R CNT )in
the R CNT ranges of 0.5-1.6 nm.
We 35 carried out MD simulations to investigate the structure, phase
transition, and nucleation on Au nanoparticles (Au N , N = 467, 818, 1522,
and 2230) confined in (n, n)-CNTs (n = 15, 19, 25, 30), applying the sec-
ond-moment approximation of the tight-binding (TB-SMA) potentials for
metal-metal interactions and Lennard-Jones potential for the metal-
carbon interactions. From Fig. 2 sharp increases in the total energy curves
are observed, corresponding to the melting transformations. Au 467 ,Au 818 ,
Au 1522 ,andAu 2230 within CNTs melt around 780, 813, 830, and 836 K,
respectively, demonstrating that the melting temperatures of confined Au
nanoparticles tend to increase with nanoparticle size. Moreover, the
melting temperatures are lower than that (1336 K) 36 of free bulk Au. The
depression of the melting point can be attributed to a large surface-to-
volume ratio and low-dimensional structures for confined Au nano-
particles. Due to the confinement environment provided by CNTs, the
solid Au nanoparticles in CNTs exhibit cylindrical multi-shelled structures
(Fig. 3), different from those of free Au clusters or bulk Au. Although there
Fig. 2 Total energy as a function of temperature for Au N confined in CNTs. The curves of
467(Au)-15, 818(Au)-19, and 1522(Au)-25 are shifted upward by 15, 10, and 5 kJ/mol, re-
spectively. The different systems N(Au)-n are distinguished by displaying the number
of gold atoms N(Au) and the index n of the (n,n)-CNTs in this and the following. 35