Page 8 - The Effects of Confinement inside Carbon Nanotubes on Catalysis

Effects of Confinement inside CNTs on Catalysis Pan and Bao

FIGURE 10. NMR spectra for the 129 Xe signal at ∼66 ppm inside
SWCNTs during adsorption and desorption of methanol at 0 °C. The 0
ppm signal results from gas-phase 129 Xe.
FIGURE 9. The concentration of CO and H 2 molecules inside and
outside of SWCNT(10, 10) channels as a function of pressure. The inset adsorption and desorption, the methanol diffusion rate was
shows the CO/H 2 ratio inside SWCNTs with the dashed line indicating estimated to be ten times as high as that inside MCM-41,
the bulk ratio. 43
which has a comparable pore size (∼3 nm). The fast diffusion
rate within CNTs was probably due to low friction and
Enrichment of Reactants inside CNTs. Molecules such as weakened hydrogen bonds among methanol molecules.
H 2 , alkanes, alkenes, and carbon tetrachloride have been MC and molecular dynamic simulation confirmed faster
reported to bind more strongly on the interior surface of diffusion coefficient of methanol in a SWCNT (10, 10) than
9
CNTs. Combining first principles calculations with Monte in the pores of VFI zeolite (1.5 nm in size). 44 More rapid
diffusion of other molecules in CNTs than other nanoporous
Carlo (MC) simulation, we showed that both CO and H 2
molecules are enriched in a pressure range 19 MPa inside materials has also been reported earlier. 45
SWCNT channels as a consequence (Figure 9). 43 Further-
more, CO was more enriched than H 2 due to stronger Conclusions and Prospects
interaction of CO with the CNT interior surface resulting in The well-defined nanosized channels of CNTs formed by
a CO/H 2 ratio higher than that in the bulk syngas feed. This graphene layers provide an intriguing confinement environ-
enrichment generally became greater inside smaller nano- ment for catalysis. They not only exert a spatial restriction on
tubes. The increased concentration of CO and H 2 could help metal particles hampering their sintering but also make it
possible to tune the particle size simply by changing the
accelerate the reaction rate, and the altered ratio of CO/H 2
could also lead to modified product selectivities. channel diameters. For example, subnanometer-sized clus-
Diffusion inside CNT channels. When the mean free ters can be obtained within SWCNTs and DWCNTs. The
path of molecules is larger than the tube diameters and electronic interaction of the confined catalysts with the
the density of the gas is low, the transport follows the CNT walls modifies their properties, which can influence
Knudsen diffusion mechanism, which is characterized by a the adsorption activation of reactants and hence the cata-
diffusion rate 23 orders of magnitude lower than that in lytic activity. In addition, the interaction between the reac-
the gas phase. This may cause severe transport resistance in tants and the CNT surface can modify the diffusion behavior
catalytic reactions. The diffusion inside CNTs has been and lead to enriched reactants inside the CNT channels,
studied employing solid-state nuclear magnetic resonance which create further opportunities to modulate the catalytic
(NMR) using hyperpolarized 129 Xe as the probe molecule. 44 performance. These effects may influence reaction rates to
When SWCNTs (i.d. 22.5 nm) preadsorbed with 129 Xe different extents depending on the selected metals, the
were exposed to a mixture of 129 Xe and methanol diameter of CNTs and specific reactions. Insights into the
(the switch-on sign in Figure 10), methanol gradually drove nature of such confinement effects could provide a novel
129
Xe out of and occupied the channels. Consequently approach to tune the catalytic activity or selectivity.
the intensity of the ∼66 ppm resonance corresponding to In addition to the CNT diameters, the confinement effect
the inside 129 Xe declined. When methanol was switched off, can be further modulated by modifying the electronic struc-
the intensity of 129 Xe increased due to desorption of metha- ture of the curved graphene walls. This can be achieved by
nol (Figure 10). From this intensity change during methanol doping CNTs with heteroatoms. For instance, doping with

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