Page 3 - The Effects of Confinement inside Carbon Nanotubes on Catalysis

Effects of Confinement inside CNTs on Catalysis Pan and Bao

FIGURE 2. (a) The effect of confinement in CNTs on the activity of FTS iron catalyst and (b) crystal-phase evolution of Fe-in in syngas. 23

are labeled as Metal-in, and metal particles located on the
outsideofCNTsasMetal-out in the following.
We found that filling of DWCNTs (i.d. 12 nm) was not
very successful with the just described wet chemistry meth-
od. Instead, using TiCl 4 vapor, we successfully dispersed
subnanometer-sized titania particles in DWCNTs. 25 Nano-
tubes were first evacuated before exposure to TiCl 4 . Anhy-
drous ethanol was used to affect slow hydrolysis of TiCl 4
under Ar atmosphere. A small amount of titanium species on
the outer walls was removed by hydrofluoric acid followed
by washing with deionized water. The bright dots in the
high-angle annulardark field (HAADF) electron microscopy
(providing sub-angstrom resolution) image corresponding to FIGURE 3. (a) Temperature-programmed desorption profiles in He for
heavy element titanium were neatly aligned in the resulting the CNT-confined Fe 2 O 3 particles with different inner diameters where
Fe 2 O 3 is reduced by carbon from CNTs and (b) their Raman spectra. 22
sample, and the size of the majority was around 0.2 nm,
even though some were probably bigger under the micro- CNT-confined catalyst favored CO conversion and forma-
scope due to overlapping of individual dots. Larger particles tion of long chain hydrocarbons. For example, CO conver-
were only observed inside a few bigger nanotubes. These sion was almost 1.5 times, and the yield of C 5þ hydro-
particles exhibited a rather good stability since no obvious carbons was twice as high as those over Fe-out at 6000 h 1 ,
aggregation was observed under the electron microscope and 5 MPa (Figure 2a). 23 Furthermore, the yield was 6 times
when the sample was in situ heated up to about 500 °C. 25 higher than that over the XC-72 carbon black supported
This method is expected to enable synthesis of other iron catalyst, which had a similar surface area as the
sub-nanometer-sized metal and metal oxide clusters. To MWCNTs.
distinguish from MWCNT samples, titania confined inside In situ HRTEM, XRD, Raman spectroscopy, and tempera-
DWCNTs are labeled as TiO x -in-D, and the outside catalysts ture-programmed desorption (TPD) experiments revealed
are denoted as TiO x -out-D. that the reduction of Fe 2 O 3 particles by CNTs was facilitated
within MWCNTs with respect to the outside oxide. 22,26 The
Gas-Phase Reactions narrower the tubes, the more facile was the reduction of the

Syngas Conversion. The confinement effect on Fischer confined Fe 2 O 3 (Figure 3a). Facilitated reduction was also
Tropsch synthesis (FTS) was studied by comparing iron observed for Fe 3 O 4 nanowires inside CNTs. 27 Even in H 2
confined (Fe-in) in MWCNTs (i.d. 48 nm) and on their and CO, 23 the reduction of the confined Fe 2 O 3 nanoparticles
outside (Fe-out). 23 TEM analysis indicated that over 70% of to FeO and metallic Fe occurred at a ca. 6090 °C lower
iron particles of Fe-in were distributed inside CNT channels temperature than that for the outside catalyst at each
while almost all particles of Fe-out were on the outside. This step. This facilitated reduction of metal oxide induced by

Vol. 44, No. 8 ’ 2011 ’ 553–562 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 555