Page 2 - The Effects of Confinement inside Carbon Nanotubes on Catalysis

Effects of Confinement inside CNTs on Catalysis Pan and Bao

FIGURE 1. Scheme for introducing nanoparticles inside MWCNTs (i.d. 48 nm) by a wet chemistry method. The TEM image displays Ru nanoparticles
confined in MWCNTs with the inset showing their particle size distribution.

reduced reaction volume, for example, the D exchange with below 100200 mN/m and thus could be filled through
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H 2. The Menshutkin reaction yielding the charged product open ends. 13 Discrete nanoparticles of various metals had
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([H 3 NCH 3 ] [Cl] ) also benefited from enclosure inside CNTs been successfully introduced inside the CNT channels with a
due to dipolar interaction of the product and CNT surface. In few tens of nanometer width. 14,15 However, difficulties were
addition, confinement has been demonstrated to modify the often encountered in small tubes with an inner diameter (i.d.)
structure and properties of confined materials. For instance, of a few nanometers. For example, Ugarte et al. observed
water molecules formed a layered cylindrical structure con- that nanotubes with a diameter smaller than 3 nm were
sisting of hydrogen-bonded heptagonal rings inside nano- barely filled due to the size constraint on the capillarity and
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tubes, which does not exist in the bulk phase. FeNi alloys the wetting behavior. 16 Yet, small nanotubes are antici-
crystallized inside CNTs exhibited several unusual morpho- pated to be more interesting for catalysis because one
logical, compositional, and structural features. 10 Therefore, may obtain metal particles in this size range and even
the CNT channels are anticipated to provide an intriguing down to a sub-nanometer scale due to the space restric-
confinement environment for metal catalysts and catalytic tion. Extraordinary catalytic activities have been fre-
reactions. quently demonstrated for metal particles at this scale,
Studying confinement effects in catalysis requires effec- for example, Au 17 and Ag. 18
tive techniques to disperse metal nanoparticles uniformly We have improved the wet chemistry method and
inside the CNT channels. This has remained a challenge at a achieved homogeneous dispersion inside small MWCNTs
practical macroscale, in particular inside small nanotubes. (i.d. 48 nm). 19 As depicted in Figure 1, the freshly produced
Therefore, the first section of this Account is dedicated to MWCNTs were opened at the ends and shortened via a
recent developments in introducing nanoparticles within controlled oxidation process catalyzed by Ag nanoparticles.
small nanotubes before experimental studies on catalytic Shortened tubes reduce the transport resistance for the
reactions over CNT-confined metal nanoparticles are dis- metal precursor solutions into the channels, as well as for
cussed, and finally the different effects on catalysis are reactants and products if reactions take place inside the
considered. channels. Ultrasonic treatment and stirring were employed
to facilitate the expulsion of the air and the entry of solution
Dispersion of Nanoparticles inside CNTs in the channels. Take filling of CNTs with Ru particles as an
The efforts to fill the CNT channels started shortly after their example, a RuCl 3 /acetone solution was employed. TEM
discovery and several techniques were developed including indicated that around 80% of Ru particles were distributed
in situ filling during arc discharge, vapor deposition of inside the channels following drying and reduction in H 2 , 20
volatile complexes, and wet chemistry methods. 1116 Wet which was confirmed by three-dimensional tomography. 21
chemistry methods are simple and most versatile for cata- The particles had a rather narrow size distribution with 90%
lysis since they are applicable to most metals. The filling falling in the range of 24 nm. Following the same method,
depends on the surface tension of the liquid and the contact iron 22,23 and rhodium 24 have been dispersed within MWCNTs
angle between the liquid and the pore walls. CNTs were too, demonstrating that this can be applied for many
predicted to be wettable by liquids with a surface tension transition metals. The MWCNT-confined metal particles

554 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 553–562 ’ 2011 ’ Vol. 44, No. 8