Chemistry Reference
In-Depth Information
averaged self-diffusion coecients of Au-Pt atoms in melted Au-Pt clus-
ters decrease as Pt composition increases, which are mainly caused by
relatively stronger Pt-CNTs interactions. The computed diffusion co-
ecients of the confined Pt
2230
are approximately larger than those of
free bulk Pt within 1600-1800 K, which should be related to a smaller
Pt
2230
nanoparticle. The derived nucleation rates of the confined Pt
2230
clusters are about (0.84-2.46)
10
36
m
1
s
1
, larger than those of con-
fined Au nanoparticles
35
and free Au nanoparticles.
82
The solid-liquid
interfacial free energies are 0.132-0.146 J/m
2
within the temperature
range of 1150-1300 K. The nucleation energy barriers are estimated as
4
10
19
J.
2.3 Reactions on the metal clusters/CNTs
H
2
O
2
, as an environmentally friendly oxidant, has a lot of applications in
chemical industry.
45,46
However, the current industrial process for the
H
2
O
2
production by the sequential hydrogenation and oxidation of an
alkyl anthraquinone is not environmentally benign.
45
The direct syn-
thesis of H
2
O
2
from molecular hydrogen and oxygen will certainly play an
increasing important role in green chemical synthesis in which the
formed H
2
O
2
can be directly applied in an oxidation reaction. Pd is the
most widely investigated and highly ecient catalyst in the direct syn-
thesis of H
2
O
2
.
47
However, it has a very low selectivity. So far, the addition
of a second metal such as Au, Pt, Ag into Pd has shown better catalytic
properties than monometallic Pd.
46-48
What's more, a significantly
higher productivity and selectivity of H
2
O
2
was observed when AuPd
alloys loaded on carbon materials rather than metal oxides like TiO
2
,
Al
2
O
3
, ZnO.
49
By means of DFT calculations, our group
22
investigated the
mechanism of H
2
O
2
synthesis on CNTs supported AuPd clusters.
2.3.1 The adsorption and dissociation of O
2
. Different adsorption
sites of O
2
are used to investigate the effects of coordination environment
and the carbon support on the adsorption energy, as shown in Fig. 8. The
most stable configuration of O
2
on Pd
10
/CNTs is the bridge site of Pd in
the top layer with the adsorption energy of 1.76 eV. The adsorption of O
2
on the bridge site of Pd in the bottom layer is much weaker, only 1.05 eV.
However, for noble metal clusters supported on reducible metal oxide, O
2
adsorption on the interfacial sites between metal and metal oxide is
always much stronger than that on only metal sites. Smaller adsorption
energy than 1.76 eV on other adsorption sites of Pd
10
/CNTs can be mainly
attributed to the effect of coordination environment. It was always found
that the adsorption energy of O
2
on low-coordinated Pd is much stronger
than that on high-coordinated one. With the increasing ratio of Au, the
adsorption energy of O
2
on most of the Pd sites slightly decreases due to
the electronic effect. Meanwhile, the adsorption energy of O
2
slightly
increases when these structures have obvious deformation. The ad-
sorption energy of O
2
dramatically reduces when O
2
is directly bonded
with Au due to the geometric effect. The adsorption site of O
2
shifts from
Pd-Pd site to Pd-Au or Au-Au site, leading to the great decline of ad-
sorption energy of O
2.
