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Metal Nanoparticle/Carbon Nanotube Catalysts
Brian Morrow
School of Chemical, Biological and Materials EngineeringUniversity of Oklahoma
Introduction
A. Kongkanand, K. Vinodgopal, S. Kuwabata, P. V. Kamat, J, Phys. Chem. B 110 (2006) 16185-16188
Carbon nanotubes have many properties which make them ideal supports for catalytic metal nanoparticles.
However, the surfaces of nanotubes are relatively inert, and they tend to form bundles which reduces their surface areas.
Metal nanoparticle/carbon nanotube materials are being investigated for use in catalytic and electrocatalytic applications such as fuel cells.
Armchair Zigzag Chiral
Baughman et al., Science 297 (2002) 787
Example
Anode (methanol oxidation): CH3OH + H2O → CO2 + 6H+ + 6e-
Cathode (oxygen reduction):(3/2)O2 + 6H+ + 6e- → 3H2O
Overall:CH3OH + (3/2)O2 → CO2 + 2H2O
K. Kleiner, Nature 441 (2006) 1046-1047
Possibility for powering devices such as cell phones and computers:- Potentially 3-10 times as much power as a battery- Methanol cheaper and easier to store than hydrogen
Problems:- Methanol crossover- Requires catalysts, usually platinum – expensive!
Example
Methanol oxidation - anode of direct methanol fuel cells
A. Kongkanand et al., J. Phys. Chem. B 110 (2006) 16185-16188
Langmuir 22 (2006) 2392-2396
Oxygen reduction - cathode of direct methanol fuel cells
Wildgoose et al., Small 2 (2006) 182-193
Other Examples
Selective hydrogenation
Oxidation of formic acid and formaldehyde
Hydrogen peroxide oxidation
Environmental catalysis
Synthesis of 1,2-diphenylethane
Synthesis
- Precursor metal salts (H2PtCl6, H2PdCl6, etc.) heated and reduced
- Particle size can be controlled by temperature and reducing conditions
- Particles can be anchored by oxidizing nanotubes (via acid treatment or microwave irradiation), but this can also damage the nanotubes
Georgakilas et al., J. Mater. Chem. 17 (2007) 2679-2694
Other techniques include chemical vapor deposition, electrodeposition, laser ablation, thermal decomposition, substrate enhanced electroless deposition
Metal particles can be grown directly on the carbon nanotubes
SynthesisAlready-grown metal particles can be connect to the carbon nanotubes
Covalent Linkage
Coleman et al., J. Am. Chem. Soc. 125 (2003) 8722
Hydrophobic interactions and hydrogen bonds
π-stacking
Han et al. Langmuir 20 (2004) 6019
Ou and Huang, J. Phys. Chem. B 110 (2006) 2031
CharacterizationTEM/SEM
Bittencourt et al., Surf. Sci. 601 (2007) 2800-2804
AFM
Hrapovic et al., Analytical Chemistry 78 (2006) 1177-1183
D.-J. Guo and H.-L. Li, Journal of Power Sources 160 (2006) 44-49
XRD
CharacterizationXPS
Lee et al., Langmuir 22 (2006) 1817-1821
Raman spectroscopy
Lee et al., Chem. Phys. Lett. 440 (2007) 249-252
Future Directions
- Minimizing use of expensive metals
- Synthesis techniques that yield nearly monodisperse nanoparticle size distributions
- Synthesis techniques that can control final structure of nanoparticles
- Better understanding of metal-carbon nanotube interactions
Characterization
A. Kongkanand et al., J. Phys. Chem. B 110 (2006) 16185-16188
“X-ray photoelectron spectroscopywas employed to investigate the binding energy of d-bandelectrons of Pt. As shown in Figure 6, a shift of 0.4 eV to ahigher binding energy was found in both 4d and 4f electrons of Pt deposited on PW-SWCNT, proving the role of SWCNTs inmodifying the electronic properties of Pt.”