Metal Nanoparticle/Carbon Nanotube Catalysts

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

2. A. Kongkanand, K. Vinodgopal, S. Kuwabata, P. V. Kamat, J, Phys. Chem. B 110 (2006) 16185-16188 Introduction Armchair Zigzag Chiral 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. Baughman et al., Science 297 (2002) 787

3. 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!

4. A. Kongkanand et al.,J. Phys. Chem. B 110 (2006) 16185-16188 Example Methanol oxidation - anode of direct methanol fuel cells Oxygen reduction - cathode of direct methanol fuel cells Langmuir 22 (2006) 2392-2396

5. 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

6. Synthesis Metal particles can be grown directly on the carbon nanotubes - 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

7. Synthesis Already-grown metal particles can be connect to the carbon nanotubes Hydrophobic interactions and hydrogen bonds Covalent Linkage Han et al. Langmuir 20 (2004) 6019 π-stacking Coleman et al., J. Am. Chem. Soc. 125 (2003) 8722 Ou and Huang, J. Phys. Chem. B 110 (2006) 2031

8. XRD D.-J. Guo and H.-L. Li, Journal of Power Sources 160 (2006) 44-49 Characterization TEM/SEM Bittencourt et al., Surf. Sci. 601 (2007) 2800-2804 AFM Hrapovic et al., Analytical Chemistry 78 (2006) 1177-1183

9. XPS Lee et al., Langmuir 22 (2006) 1817-1821 Characterization Raman spectroscopy Lee et al., Chem. Phys. Lett. 440 (2007) 249-252

10. 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

11. Questions?

13. Characterization “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.” A. Kongkanand et al.,J. Phys. Chem. B 110 (2006) 16185-16188