Noble Metals as Catalysts Oxidation of Methanol at the anode of a DMFC Zach Cater-Cyker 4/20/2006 MS&E 410.

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Presentation transcript:

Noble Metals as Catalysts Oxidation of Methanol at the anode of a DMFC Zach Cater-Cyker 4/20/2006 MS&E 410

Outline Overview of DMFC Reaction mechanism of methanol decomposition at anode Problems CO Poisoning Efficiency Lifetime Cost Advancements Structure effects Alloys

Overall Reaction At the anode CH 3 OH+H 2 O→CO 2 +6H + +6e - At the cathode 6H + +6e O 2 →3H 2 O Overall CH 3 OH+ 1.5O 2 →2H 2 O+ CO 2

The workings of DMFCs Anode Cathode 1 CH 3 OH 1.5 O 2 6e-6e- 1 H 2 O 6 H + CO 2 6e-6e- 6 H + Nafion Membrane 3 H 2 O 2H2O2H2O

Anodic Catalysis Mechanism Step 1 Methanol Decomposition

UHV vs. Electrochemical UHV or gaseous environment 1 st step is OH bond dissociation Formation of CH 3 O Electrochemical environment 1 st step is CH bond scission Formation of CH 2 OH Hartnig et. al.

Anodic Catalysis Mechanism Step 2 Water Activation CO Removal

Problems CO Poisoning Water activation is limited on Pt Removal of CO requires adsorbed OH Buildup of CO on surface Catalytic sites occupied by CO can’t be used to further decompose methanol Leads to loss of catalytic activity (efficiency) Causes higher loadings of Pt to be used (cost)

Aims to improve Anode Reduce CO poisoning Improve catalytic activity and efficiency Lower the amount of Pt used Improved efficiency (see above) Support structure Finding alternative material Alloys!!

Advancements Structure Crystallographic surface effects Environment Anionic deactivation Alloying Ru increases CO to CO 2 conversion rate

Surface Planes Pt(111) least CO poisoning Pt(110) high initial activity but deactivates quickly due to CO formation Herrero (111) (110)

(111) Peak Assignment Low CO poisoning A2 peak Increase amount of CO build up, see increase in a2 Assign to CO oxidation A1 and A3 peak More surface defect on electrode shows increase in a3 and decrease in a1 A3 is methanol oxidation on defect site A1 is methanol oxidation on (111) lattice

Why is (111) inactive? 2 possible first steps O-H Bond scission CH 3 O is endothermic C-H Bond scission CH 2 OH has large activation energy CH 3 OH would rather desorb Only catalytic sites on (111) exist at surface defects

(110) In comparison Thermodynamics on (110) much more favorable Intermediates can form before methanol desorbs Interesting behavioral comparison between (2X1) and (1x1) Pt(110) layers (2X1) shows no C-O bond scission Waszczuk

Anionic Deactivation Acidic medium, similar to actual fuel cell environment Different acids used Anions of acid have different adsorption strengths Competition between anion and methanol “Poisoning” or deactivation of catalyst surface Comparable in magnitude to surface geometry effects Oxidation rates HClO 4 >H 2 SO 4 >H 3 PO 4 Adsorption Strength H 3 PO 4 >H 2 SO 4 >HClO 4

Alloys Idea here is that adding second metal will combine benefits of both metals Stability of alloy Things to look at: Identity of second metal Composition Ru More easily activates water to OH than Pt

Composition of Pt-Ru Pt-rich or Ru-rich alloys have broad peak in voltammogram Onset of CO stripping dependent on Ru composition Lowest onset potential seems to be around a 1:1 Pt-Ru ratio Specifically 46% Ru Making a true alloy (mixture at the atomic level) much more efficient than surface decoration with Ru particles

Pt-Ru Mechanism Bifunctional Mechanism Pt breaks down methanol to form adsorbed CO Ru responsible for activated water complexes on surface, OH Pt-CO and Ru-OH react to form CO 2 and a proton Ru can activate water at potentials 300mV less than Pt Ligand Mechanism When Ru is alloyed with Pt, electronic effect arises Energy levels needed for Pt to activate water drop Pt-Ru bond creates decrease in Fermi level and weakens Pt-CO bond

Evidence of Ligand effect CO stretching frequency measured against Ru coverage CO stretching frequency related to strength of adsorbed CO Decrease in slope for Pt-Ru compared to Pt Some electrochemical effect takes place Pt Ru-Pt

Photocatalytic Enhancement Electrode made of Pt- Ru and TiO 2 Under UV radiation the TiO2 increases the current produced by methanol oxidation Can greatly reduce the loading of noble metals Reduce cost of fuel cell

References Lamy, Leger, Srinivasan, in: Bockris, Conway, Whits (Eds.), Modern Aspects of Electrochemistry, vol. 34, 2001, pp Beden, Leger, Lamy, in: Bockris, Conway, Whits (Eds.), Modern Aspects of Electrochemistry, vol. 22, 1992, pp Frelink et. al. Langmuir. 12 (1996) Chen and Tsao. Int. J. of Hydrogen Energy. 31 (2006) 391. Liu et. Al. J. of Power Sciences. 155 (2006) 95. Herrero et. Al. J. Phys. Chem. 98 (1994) Waszczuk et. Al. Electrochemica Acta. 47 (2002) 3637 Hartnig and Spohr. Chemical Physics. 319 (2005) 185. Krewer et. Al. J. Electroanalytical Chemistry. 589 (2006) 148. Malliard et. Al. J. Phys. Chem. B. 109 (2005) Stiegerwalt et. Al. J. Phys. Chem. B. 106 (2002) 760. Drew et. al. J. Phys. Chem. B. 109 (2005)