Page 7 - The Effects of Confinement inside Carbon Nanotubes on Catalysis

Effects of Confinement inside CNTs on Catalysis Pan and Bao

FIGURE 8. (a) UVvis absorption spectra of SWCNT-confined [CoCp 2 ] indicating electron transfer from cobalt ions to the nanotube; 40 (b) ultraviolet
photoemission spectroscopy of the SWCNT-confined [FeCp 2 ] (spectrum B) in comparison with that of pristine SWCNT (spectrum A). 41

a CNT-confined Pt catalyst (60100 nm i.d.) compared with The above results demonstrate that an electronic inter-
the outside Pt catalyst. 39 action exists between metal species and the CNT surfaces
and the strength can vary with metals and with the size of
Toward Understanding the Confinement CNTs. The modified electronic structure of metal/metal
Effects in Catalysis oxide nanoparticles induced by confinement can influence

Electronic Interaction of the Confined Materials with their catalytic activity as redox reactions involve electron
CNTs. The effect of the electronic structure of CNTs on the transfer between reactants and catalysts.
transition states of a chemical reaction has been theoreti- Space Restriction. The nanosized channels of CNTs pro-
8
cally studied ealier. In addition, modified redox behavior of vide spatial restriction on metal particles, which can hamper
metals and metal oxides, as demonstrated in the previous their aggregation under reaction conditions. This is impor-
section, implied interactions between the confined mate- tant because aggregation of nanoparticles frequently results
rials and CNT surfaces. 20,23,26,27,31 Our CO adsorption in deactivation of catalysts. For example, the particles of
microcalorimetry and first principles calculation results RhMn-in were limited to the range of 58 nm in CNTs even
suggested that the inside Ru transferred more electrons after 112 h time on stream, which led to a rather steady
to the CNT interior surface than the outside Ru to the performance at syngas conversion conditions of 320 °C
exterior surface. 20 Such an electron transfer was observed and 5 MPa. 24 In contrast, the particles of RhMn-out aggre-
experimentally for CNT-confined metallocene molecules. gated noticeably as indicated by a broader distribution of 8
For example, Khlobystov and co-workers attributed a red- 10 nm size due to lack of space restriction. The superior
shift of the photoluminescence and UVvis absorp- stability of the confined nanoparticles was also observed for
tion spectra of CNT-encapsulated bis(cyclopentadienyl) FTS iron catalyst. 31 The inside particles remained in the
cobalt [CoCp 2 ] and bis(ethylcyclopentadienyl) cobalt mo- range of 611 nm while the outside particles grew and the
lecules [Co(EtCp) 2 ](Figure 8a)to a change of the cobalt largestparticles reached24nmafter125htimeonstream.
charge state due to electron transfer to the nanotubes. 40 Furthermore, the availability of nanotubes with varying
Similarly, Shiozawa et al. estimated that 0.14 electrons inner diameters enables tuning the particle size. For exam-
were transferred from confined [FeCp 2 ]to SWCNTs ple, inside small tubes such as DWCNTs, subnanometer
per [FeCp 2 ] molecule according to a shift to a higher titania particles had been well dispersed. 25 The capability
binding energy in the ultraviolet photoemission spectra of creating variable nanometer-sized metal particles and
(Figure 8b). 41 Lee et al. observed a modified electronic maintaining their high dispersity under reaction con-
structure of the nanotube in CNT-encapsulated Gd metal- ditions makes CNTs interesting support materials and could
lofullerenes, as evidenced by low-temperature scanning trigger further fundamental investigations on the nature of
tunneling microscopy. 42 nanocatalysis.

Vol. 44, No. 8 ’ 2011 ’ 553–562 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 559