Metal Nanoparticle/Carbon Nanotube Catalysts Brian Morrow School of Chemical, Biological and Materials Engineering University of Oklahoma

Introduction A. Kongkanand, K. Vinodgopal, S. Kuwabata, P. V. Kamat, J, Phys. Chem. B 110 (2006) 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. ArmchairZigzagChiral Baughman et al., Science 297 (2002) 787

Example Anode (methanol oxidation): CH3OH + H2O → CO2 + 6H+ + 6e- Cathode (oxygen reduction): (3/2)O 2 + 6H+ + 6e- → 3H 2 O Overall: CH 3 OH + (3/2)O 2 → CO 2 + 2H 2 O K. Kleiner, Nature 441 (2006) 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) Langmuir 22 (2006) Oxygen reduction - cathode of direct methanol fuel cells

Wildgoose et al., Small 2 (2006) Other Examples Selective hydrogenation Oxidation of formic acid and formaldehyde Hydrogen peroxide oxidation Environmental catalysis Synthesis of 1,2-diphenylethane

Synthesis - Precursor metal salts (H 2 PtCl 6, H 2 PdCl 6, 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) 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

Synthesis Already-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

Characterization TEM/SEM Bittencourt et al., Surf. Sci. 601 (2007) AFM Hrapovic et al., Analytical Chemistry 78 (2006) D.-J. Guo and H.-L. Li, Journal of Power Sources 160 (2006) XRD

Characterization XPS Lee et al., Langmuir 22 (2006) Raman spectroscopy Lee et al., Chem. Phys. Lett. 440 (2007)

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

Questions?

Characterization A. Kongkanand et al., J. Phys. Chem. B 110 (2006) “X-ray photoelectron spectroscopy was employed to investigate the binding energy of d-band electrons of Pt. As shown in Figure 6, a shift of 0.4 eV to a higher binding energy was found in both 4d and 4f electrons of Pt deposited on PW-SWCNT, proving the role of SWCNTs in modifying the electronic properties of Pt.”