Page 6 - The Effects of Confinement inside Carbon Nanotubes on Catalysis

Effects of Confinement inside CNTs on Catalysis Pan and Bao

FIGURE 7. (a) Ti 2p XPS spectra and (b) XANES features of TiO x -in-D as function of temperature.

NH 3 conversion over CoFe 5 -out was roughly half of that over transition. The modified electronic structure and the stabili-
CoFe 5 -in during ∼16 h time on stream. The superior thermal zation of small clusters might contribute to the higher
stability of the inside particles was proposed to be respon- epoxidation activity for TiO x -in-D.
sible for their higher conversion. 35 The electron transfer within
thegraphenewallsisexpectedtobemuchsmallerinlargeCNTs Liquid-Phase Hydrogenation Reactions
because of their weaker curvature. 32,33 Thus it likely has little Dependence of catalytic activity on the relative location of
influence on the electronic structure of metal particles inside catalysts on CNTs was first noticed in liquid-phase hydro-
wide CNTs. The above results show that the activity difference formylation of propylene. 36 A catalyst consisting of a
between the inside and outside catalysts differs in ammonia [HRh(CO)(PPh 3 ) 3 ] complex deposited on open CNTs yielded
synthesis and decomposition. This could be due to confine- a TOF = 0.10 s 1 and a molar ratio of the normal/branched
ment effects on different metals in different diameter CNTs. products n/i = 9 in comparison to 0.06 s 1 (TOF) and n/i = 6
Epoxidation of Propylene in DWCNTs. Subnanometer over the complex on closed CNTs. Although no direct
titania clusters confined inside DWCNT channels (TiO x -in-D) evidence was provided for the location of Rh, the higher
exhibited a significantly higher activity for catalyzing propy- activity and regioselectivity of the open catalyst were sug-
lene epoxidation compared with titania outside of DWCNTs gested to be attributed to the Rh species inside CNTs and its
(TiO x -out-D) and titania inside MWCNTs (i.d. 48 nm) (TiO x - surface consisting of six-membered C-rings. 36
in) 25 The formation rate of propylene oxide (PO) over TiO x - Pd particles were introduced inside MWCNTs (i.d. 5
in-D was 54.0 g PO/(kg cat h). It was 8 times higher than that 10 nm) for benzene hydrogenation, which exhibited a TOF
3
over TiO x -out-D, twice as high as that over TiO x -in, and more twice as high as that over zeolite Y and AC-supported Pd
than 20 times higher than titania supported on commercial catalysts although zeolite Y and AC have much higher sur-
P25 under the same reaction conditions. Note that no face areas than CNTs. 37 Confinement of Pd particles inside
conversion was detected over blank DWCNTs. MWCNTs (40 nm average i.d.) was also found to benefit
X-ray photoelectron spectroscopy (XPS), X-ray absorption selective hydrogenation of cinnamaldehyde with a faster
near-edge spectroscopy (XANES), and Raman spectroscopy hydrogenation rate and a much higher selectivity (90%) to
suggested an electronic interaction between the confined hydrocinnamaldehyde compared with an AC-supported Pd
titania clusters and the DWCNT interior wall. 25 This electron catalyst. 38 Likewise Serp and co-workers observed a much
transfer was temperature dependent and was strong en- better catalytic performance for PtRu particles inside MWCNTs
ough to be observed above 300 °C. For example, the Ti 2p (40 nm average i.d.) for the same reaction but with opposite
XPS peak shifted 0.5 eV to a higher binding energy for TiO x - selectivity. 15 The TOF was almost 3 times and the selectivity
in-D (Figure 7a), whereas that of TiO x -out-D did not exhibit toward cinnamyl alcohol was more than twice of that over
any changes. Correspondingly, the 4970.5 eV peak in the an unsupported PtRu catalyst with a similar particle size.
pre-edge region also downshifted to 4970.0 eV for TiO x -in-D Moreover, the selectivity was linearly correlated with the
at 300 °C (Figure 7b) suggesting a lower Ti 3d electron percentage of nanoparticles located inside CNTs. 15 A higher
density since the XANES result from the 1s f 3d dipole selectivity toward cinnamyl alcohol was also observed over

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