Page 5 - The Effects of Confinement inside Carbon Nanotubes on Catalysis

Effects of Confinement inside CNTs on Catalysis Pan and Bao

FIGURE 5. (a) Ammonia synthesis activity and (b) particle size distributions of Ru-in and Ru-out.

FIGURE 6. The differential electron density isosurfaces of (a) Ru 6 -out and (b) Ru 6 -in. The insets show the cross-section of a full nanotube model. The
gray balls represent the carbon atoms and red bars the RuRu bonds. The blue and yellow areas suggest enriched and depleted electron density,
respectively, with respect to free-standing clusters.

s
over Ru-in was in the range of (1.62.6) 10 4 1 at 400 °C that the Ru 6 -out cluster donated 1.66 electrons to the CNT
and 20 mL min 1 flow rate in the pressure range of 1 convex surface whereas the inside cluster transferred 2.41
3
4 MPa. Remarkably, TOFs over Ru-out were much higher electrons to the concave surface. This is consistent with the
(Figure 5a). 20 TEM and chemisorption experiments con- electron structure of CNTs. Deviation from planarity causes
firmed that the particle size of Ru-in and Ru-out was π-electron density to shift from the concave interior to the
similar, with ∼90% particles falling in the range of 25nm convex exterior surface. 32,33 Thus, the outside metal cluster
(Figure 5b). Even after reaction, the particle size distribution donates less electrons to the relatively electron-enriched
only changed slightly, indicating that both catalysts were exterior surface than the inside cluster to the electron-
rather stable under reaction conditions. deficient interior surface. These results could explain the
Adsorption microcalorimetry showed that the initial dif- higher ammonia synthesis activity over Ru-out than over Ru-
ferential heat for CO adsorption on Ru-out was 12 kJ/mol in since a higher electron density facilitates the electrophilic
higher than that over Ru-in. 20 This implied stronger adsorp- process of N 2 dissociative adsorption. 34
tion sites for CO on Ru-out. The different strength of adsorp- On the other hand, it was recently reported that ammonia
tion sites was most likely related to the electron density on decomposition benefits from confinement of the bimetal
metal surfaces since Ru-in and Ru-out had a similar particle CoFe 5 inside CNTs with 40 nm average i.d. (CoFe 5 -in). 35 TEM
size. This was corroborated by first principles calculations. 20 indicated that 96 wt % particles of CoFe 5 -in were located
Figure 6 shows the differential electron density isosurfaces inside the channels and CoFe 5 -out had 73 wt % located on
of a Ru 6 cluster inside (Ru 6 -in) and outside (Ru 6 -out)of a the outside. The two fresh catalysts exhibited a similar
SWCNT(10,10). 20 Mulliken population analysis indicated particle size distribution and the same CoFe 2 O 4 (311) planes.

Vol. 44, No. 8 ’ 2011 ’ 553–562 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 557