Page 1 - The Effects of Confinement inside Carbon Nanotubes on Catalysis

The Effects of Confinement inside Carbon
Nanotubes on Catalysis

XIULIAN PAN* AND XINHE BAO*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Zhongshan Road 457, Dalian 116023, China
RECEIVED ON DECEMBER 13, 2010

CONSPECTUS

he unique tubular morphology of carbon nanotubes (CNTs) has trig-
T gered wide research interest. These structures can be used as nano-
reactors and to create novel composites through the encapsulation of guest
materials in their well-defined channels. The rigid nanotubes restrict the
size of the encapsulated materials down to the nanometer and even the sub-
nanometer scale. In addition, interactions may develop between the
encapsulated molecules and nanomaterials and the CNT surfaces. The
curvature of CNT walls causes the π electron density of the graphene
layers to shift from the concave inner to the convex outer surface, which
results in an electric potential difference. As a result, the molecules and
nanomaterials on the exterior walls of CNTs likely display different
properties and chemical reactivities from those confined within CNTs.
Catalysis that utilizes the interior surface of CNTs was only explored
recently. An increasing number of studies have demonstrated that confining metal or metal oxide nanoparticles inside CNTs
often leads to a different catalytic activity with respect to the same metals deposited on the CNT exterior surface.
Furthermore, this inside and outside activity difference varies based on the metals used and the reactions catalyzed.
In this Account, we describe the efforts toward understanding the fundamental effects of confining metal nanoparticles inside
the CNT channels. This research may provide a novel approach to modulate their catalytic performance and promote rational
design of catalysts. To achieve this, we have developed strategies for homogeneous dispersion of nanoparticles inside nanotubes.
Because researchers have previously demonstrated the insertion of nanoparticles within larger nanotubes, we focused specifically
on multiwalled carbon nanotubes (MWCNTs) with an inner diameter (i.d.) smaller than 10 nm and double-walled carbon
nanotubes (DWCNTs) with 1.01.5 nm i.d. The results show that CNTs with well-defined morphology and unique electronic
structure of CNTs provide an intriguing confinement environment for catalysis.

4
Introduction synthesis, respectively. Inspired by the successful applica-
CNTs can be envisioned as rolled-up graphene layers tions of activated carbon (AC) as a catalyst support in
forming a tubular structure. According to the number of industrial processes, CNTs were widely studied as an alter-
layers, single-walled CNTs (SWCNTs), double-walled CNTs native for dispersion of transition metals on their exterior
(DWCNTs), and multiwalled CNTs (MWCNTs) are distin- walls for hydrogen involving reactions and electrocatalytic
guished. Their high electron and thermal conductivity, high reactions. A general conclusion can be drawn from those
surface area, and functionalizable surfaces have evoked studies that the activity or product selectivities are improved
wide interest for catalytic applications. 1,2 For example, in comparison to the metals supported on AC and conven-
MWCNTs were reported to catalyze oxidative dehydro- tional oxides. 1,2
genation of ethylbenzene to styrene due to the presence However, catalysis utilizing the interior surface of CNTs
3
of surface oxygen functional groups. As a catalyst additive, has been less explored, 5,6 although theoretical studies
MWCNTs promoted substantially the catalytic activities of predicted that chemical reactions (without additional
CuZnAl and CoMoCu for methanol and mixed alcohol catalysts) may be influenced inside such a significantly
Vol. 44, No. 8 ’ 2011 ’ 553–562 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 553
Published on the Web 06/27/2011 www.pubs.acs.org/accounts
10.1021/ar100160t & 2011 American Chemical Society