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

Questions?

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.”