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Noble Metals as Catalysts

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.

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Noble Metals as Catalysts

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

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

  3. Overall Reaction • At the anode • CH3OH+H2O→CO2+6H++6e- • At the cathode • 6H++6e-+1.5O2→3H2O • Overall • CH3OH+ 1.5O2→2H2O+ CO2

  4. CO2 6 H+ 3 H2O The workings of DMFCs 6 e- 6 e- 6 H+ 1.5 O2 1 CH3OH 1 H2O 2 H2O Nafion Membrane Cathode Anode

  5. Anodic Catalysis MechanismStep 1 • Methanol Decomposition

  6. UHV vs. Electrochemical • UHV or gaseous environment • 1st step is OH bond dissociation • Formation of CH3O • Electrochemical environment • 1st step is CH bond scission • Formation of CH2OH Hartnig et. al.

  7. Anodic Catalysis MechanismStep 2 • Water Activation • CO Removal

  8. 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)

  9. 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!!

  10. Advancements • Structure • Crystallographic surface effects • Environment • Anionic deactivation • Alloying • Ru increases CO to CO2 conversion rate

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

  12. (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

  13. Why is (111) inactive? • 2 possible first steps • O-H Bond scission • CH3O is endothermic • C-H Bond scission • CH2OH has large activation energy • CH3OH would rather desorb • Only catalytic sites on (111) exist at surface defects

  14. (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

  15. 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 • HClO4>H2SO4>H3PO4 • Adsorption Strength • H3PO4>H2SO4>HClO4

  16. 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

  17. 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

  18. 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 CO2 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 Pt-Ru Mechanism

  19. 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 Ru-Pt Pt

  20. Photocatalytic Enhancement • Electrode made of Pt-Ru and TiO2 • 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

  21. References • Lamy, Leger, Srinivasan, in: Bockris, Conway, Whits (Eds.), Modern Aspects of Electrochemistry, vol. 34, 2001, pp. 53-118. • Beden, Leger, Lamy, in: Bockris, Conway, Whits (Eds.), Modern Aspects of Electrochemistry, vol. 22, 1992, pp. 205-217. • Frelink et. al. Langmuir.12 (1996) 3702. • 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) 5074. • 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) 16230. • Stiegerwalt et. Al. J. Phys. Chem. B. 106 (2002) 760. • Drew et. al. J. Phys. Chem. B.109 (2005) 11851.

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