Chemistry Reference
In-Depth Information
and the lowest unoccupied molecular orbital (LUMO) in the Au
1
/pristine
CNTs was mainly composed of either Au or CNTs, respectively, further
indicating that no bond interaction existed between Au and pristine
CNTs. On defective CNTs, the HOMO and HOMO-1 were the resultant of
coupling of Au
1
and point defect of CNTs.
In addition, we
22
investigated the adhesion behaviors of bimetallic
Au-Pd clusters on CNTs. For the most stable Pd
10
/CNTs with E
B
of 2.01 eV,
five Pd atoms in the bottom layer were bonding with CNTs in the top or
bridge site, which was much more stable than Pd
10
in which four atoms
in the bottom layer, or three layers Pd
10
in which five Pd atoms were
directly bonding with CNTs or cubocatohedron Pd
10
cluster. The struc-
tures of Au
x
Pd
10
x
(x
=
1-5) supported on CNTs were obtained by con-
sidering all of possible substitution sites of Pd in the most stable
Au
x
1
Pd
10
x
þ
1
(x
=
1-5)/CNTs. The E
B
of the most stable Au
1
Pd
9
on CNTs
was 2.27 eV. The substitution of Pd by Au in the top layer was much more
stable than that in the bottom layer. In the top layer, Au prefered to the
lower coordinated edge site. In the bottom layer, the substituted Au did
not bond with carbon, in which the shortest bond length between Au and
carbon was 2.41 Å. For Au
2
Pd
8
/CNTs (E
B
=
2.36 eV), the most favorable
substitution site was the edge of the top layer and Au tended to segregate
rather than agglomerate. Au
3
Pd
7
/CNTs (E
B
=
2.25 eV) had the same rules
as Au
2
Pd
8
/CNTs. However, when the fourth Pd in the top layer was sub-
stituted by Au, one reconstructed structure had similar stability with the
regular two layers Au
4
Pd
6
one (E
B
=
2.32 eV). One Pd atommoved from the
bottom to top layer. As for Au
5
Pd
5
/CNTs (E
B
=
2.14 eV), these structures,
where four Pd atoms in the bottom layer were much more stable than
those five Pd atoms, were all located in the bottom. Overall, Au located on
the top layer was always more stable than that in the bottom layer. In
addition, the most unstable Au
x
Pd
10
x
(x
=
1-5) with Au in the top layer
was even more stable than the most stable Au
x
Pd
10
x
with Au in the
bottom layer. Despite of the small Pd and AuPd clusters, the bottom layer
was nearly all Pd, while the top layer was the mixture of AuPd, where Au
and Pd atom prefered the low-coordinated and high-coordinated edge
sites, respectively. It is seen that the structures of AuPd can be tuned by
the carbon support. After the acid treatment of carbon support, it is
anticipated that Au in the top layer could probably move into the bottom
layer which leads to more AuPd ensembles in these clusters.
It is well known that B and N elements can be incorporated into an sp
2
carbon network without significantly affecting the atomic arrangement in
the hexagonal two-dimensional lattice.
23,24
And the electronic properties
of CNTs can be modified by electron (N) and hole (B). It is expected that
the interactions between noble metal clusters and CNTs can be modu-
lated by B, N dopant, thus influsing the catalytic properties. In this sense,
we
25
conducted a systematically theoretical study on the adhesion
properties of various noble metals (Au, Ag and Pt monomer) on (5, 5)
CNTs doping by B, N. On the pristine, different from the stronger ad-
hesion of Pt, Au weakly binded atop carbon atom rather than the bridge
site for Pt. Ag can not bind at all. The nearest distance between Au and
carbon of CNTs was 2.24 Å, bigger than that of Pt-C (2.07 Å). DOS analysis
