Nano-Testtube Does Some Tricks

"Tuning of Redox Properties of Iron and Iron Oxides via Encapsulation within Carbon Nanotubes"

Wei Chen, Xiulian Pan, and Xinhe Bao*

J. Am. Chem. Soc. 2007, ASAP published on 05/18/2007.

10.1021/ja0713072

Comment:

It would be more perfect if the i.d. dependence of oxidation could be shown. But a beautifully done piece of work already from Xinhe Bao at Dalian Inst. of Chemical Physics (China)!

Synthesis:

MWNTs of various i.d. (2-5, 4-8, 8-12 nm, from Chengdu Organic Chemicals. Tag 1: It sounds and looks nice. But how accurate was the i.d. control?) were refluxed with conc. HNO3 for 14 h, heat at 60C for 12 h to "open the cap". The solid was thrown into a Fe(NO3)3 pot, sonicated and stirred for 2 h. Solvent was evaporated at ambient conditions, and the mixture was heated to 140C in air for 8h, then heated to 350C in He at 2C/min and held for 3 h. This gives Fe2O3-encap.-MWNT. (80% encap. yield from TEM)

Characterization:

TEM:

• Non-encapsulated Fe/MWNT sample: 5-8 nm
• Encapsulated Fe/MWNT samples: sizes of encap. Fe NPs decrease with tube inner diameter, while those of non-encap. Fe NPs do not change.

Raman:

• Fe-O Eg vibrations (~280 cm-1, ~385 cm-1) shifts to higher frequency when the encap. Fe NPs become smaller; the positions were in the lower end when Fe NPs were not encap.
• Normally, the dependence trend for vibration frequency-Fe NP diameter is toward the other direction, which means the observed shifts could be attributed to the interactions from CNT host.

For the reaction that Fe2O3 reduced by nanotube carbon ("autoreduction"):

• TPR shows the CO evoluted (Fe2O3 reduced) at lower temp. if the nanotube inner diameter was smaller (~600C for 4-nm i.d. MWNTs).
• in situ XRD shows beautifully the conversion from Fe2O3 (2theta = 34.5/42.6) to Fe (2theta = 44.1) (also tracked by Raman peaks).

Proposed Mechanism:

Inner nanotube surface is more electron deficient.

How about oxidation?

• Occurs at a much lower temp (1% O2) than reduction
• Encap. Fe are more stable (fully oxidized at ~400C vs ~290C for extern. Fe), as expected. But is it due to difficulty in diffusion?
• Using an "artificial" channel with similar tubular characteristics ("SBA", a mesoporous silica sample), it was shown that geometrical constriction (which may slow down oxygen diffusion) did not slow down the Fe oxidation (Tag 2: Interesting control. But perhaps need to be more cautious with the generalized conclusion). Thus the interactions of Fe NPs with CNT must have slowed things down. Again, the electron deficiency of CNT inner surface seems to explain it.
• It is interesting that evolution of CO2 (CNT oxidation) is accompanied along with the Fe oxidation, suggesting the Fe2O3 catalyzing the CNT oxidation.