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2007 ACS NORM Kinetic Study of Formic Acid Oxidation using PtRu-CNT and PtBi-CNT

2007 ACS NORM Kinetic Study of Formic Acid Oxidation using PtRu-CNT and PtBi-CNT. Kenichi Shimizu; I. Frank Cheng; Clive Yen; Byounghoon Yoon; Chien M. Wai Dept. Chemistry University of Idaho, Moscow, ID 83844-2343 shim8976@vandals.uidaho.edu. Direct Formic Acid Fuel Cell.

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2007 ACS NORM Kinetic Study of Formic Acid Oxidation using PtRu-CNT and PtBi-CNT

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  1. 2007 ACS NORMKinetic Study of Formic Acid Oxidation using PtRu-CNT and PtBi-CNT Kenichi Shimizu; I. Frank Cheng; Clive Yen; Byounghoon Yoon; Chien M. Wai Dept. Chemistry University of Idaho, Moscow, ID 83844-2343 shim8976@vandals.uidaho.edu

  2. Direct Formic Acid Fuel Cell • Largely available in nature (Renewable). • Higher fuel concentration than DMFC. • Up to ~20 M HCOOH vs. up to ~2 M CH3OH • Less fuel crossover than methanol. • Higher theoretical cell potential. • 1.45 V for DFAFC, 1.2 V for DMFC http://www.tekion.com/business/index.htm Kang, S.; et al.; J. Phys. Chem. B, 2006, 110, 7270. Rice, C.; et al.; J. Power Sources,2003, 115, 229.

  3. Formic acid oxidation as a part of methanol oxidation process Cao, D.; et al. J. Phys. Chem.2005, 109, 11622.

  4. Formic acid oxidation HCOOH  CO2 + 2H+ + 2e- E0 = -0.25 VNHE Rxn 2 Rxn 1 COads + H2O H2O  OHabs+ H+ + e- COads + OHads CO2 +H+ +e-

  5. Binary catalysts and their effects • Bi-functional effect (Active secondary catalyst). • Pt, PtPd, PtRu • Third body effect (Inert secondary catalyst). • PtBi, PtPb, PtAu Catalyst support is either Pt itself or carbon black. Inert to oxidation of small organic solvent. Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369 Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, 3607-3614 Conway, B.E.; et al. Zeitschrift für physikalische Chemie Neue Folge1978, 112, 195-214.

  6. Active secondary metal catalyst such as Ru. Dissociative absorption of CO onto Pt. Pt-CH3OHads Pt-COads + 4H+ + 4e- Or (Pt + HCOOH  Pt-COads + H2O) Absorption of OH onto Ru through dissociation of H2O. H2O + Ru  Ru-OHads + H+ +e- Pt-COads + Ru-OHads  Pt + Ru + CO2 +H+ + e- Secondary catalyst must have lower dissociation potential than Pt. (Ru has 0.2 -0.3 V lower reaction potential than Pt) Rate determining step is 3. Bi-functional Effect Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, 3607-3614. Christensen, P.A. et al. J. Electroanal. Chem.1993, 362, 207-218.

  7. Dissociative absorption on Pt takes more than one active site Pt + CH3OH  Pt-CH2OH + H +e- Pt-CH2OH  Pt2-CHOH + H+ + e- Pt2-CHOH  Pt3-COH + H+ + e- Pt3-COH  Pt-CO + 2Pt +H+ + e- Catalytically inert catalyst, such as Bi, sterically hinders absorption of poisonous carbon species. Third Body Effect Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, 3607-3614

  8. Research question • How can overall reaction be improved. Chen, X.-Y., et al., J. Angew. Chem., Int. Ed.2006, 45, 981.

  9. PtRu and PtBi CNT Pt42Ru58CNT Pt38Bi62CNT Atomic ratio of Pt:Ru is 1:1.4. Atomic ratio of Pt:Bi is 1:1.6.

  10. Catalytic effect of PtRuCNT • 1 M H2SO4 • 0.1 M HCOOH • Forward peaks are not resolved as well as using PtCNT. • Peak current was enhanced with PtRu CNT.

  11. Summary of Peak Potentials • CNT supported catalysts resulted in slightly lower oxidation potential. • Peak 2 and 3 might be due to the same reaction. (3) Pt CNT (1) (2) *Carbon black supported Pt and PtRu from ETEK

  12. Summary of Peak Currents • Higher reverse peak than forward peak may indicate sluggish kinetic activity for Pt CNT and PtCB. (3) Pt CNT (1) (2) A/mg Pt *Carbon black supported Pt and PtRu from ETEK

  13. Influence of Temperature

  14. Activation Energy for intermediate oxidation

  15. Summary of Activation Energy 90% confidence interval *Carbon black supported Pt and PtRu from ETEK **Lovic, J.D.; et al.; J. Electroanal. Chem.,2005, 581, 294. ***Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369.

  16. Summary of Activation Energy • Results are agreeable to the others. • Mean activation energy was the smallest with Pt CNT. 90% confidence interval *Carbon black supported Pt and PtRu from ETEK **Lovic, J.D.; et al.; J. Electroanal. Chem.,2005, 581, 294. ***Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369.

  17. Summary of PtRuCNT • It is effective towards formic acid oxidation. (Highest peak current with PtRuCNT). • There is no significant difference in activation energies. • PtRuCNT has higher turn over rate.

  18. Catalytic Effect of PtBiCNT • 1 M H2SO4 • 0.1 M HCOOH • Position of the forward peak of PtBiCNT was almost same as the backward peak. • PtBiCNT had lower current output than PtCNT.

  19. Summary of Peak Potentials • Peak (1) was not observed for PtBi catalyst; no formation of Pt-CO. (3) Pt CNT (1) (2) *Carbon black supported Pt and PtRu from ETEK

  20. Summary of Peak Currents • Low catalytic activity of PtBi may be attributed to the larger particle size. • Better Efficiency than PtCNT and PtCB catalysts. (3) Pt CNT (1) (2) *Carbon black supported Pt and PtRu from ETEK

  21. Influence of Temperature • Peak current leveled off above 23 °C. *Lovic, J.D.; et al.; J. Electroanal. Chem.,2005, 581, 294. **Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369.

  22. Summary of Activation Energy • Bi secondary catalyst is not taking part of electro-oxidation process itself. • Restriction of Pt reaction site by Bi resulted in high activation energy. *Carbon black supported Pt and PtRu from ETEK **Lovic, J.D.; et al.; J. Electroanal. Chem.,2005, 581, 294. ***Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369.

  23. Summary of PtBiCNT • Only peak associated with reaction 1 was observed. • Low catalytic activity (low peak current). • Activation energy was significantly larger than Pt and PtRu electrocatalysts. • Bi suppresses reaction 2.

  24. Tafel analysis • Higher tafel slope indicated that the first electron transfer likely be the rate determining step. *Carbon black supported Pt and PtRu from ETEK *Lovic, J.D.; et al.; J. Electroanal. Chem.,2005, 581, 294. Maciá M.D.; et al.; J. Electroanal. Chem.2003, 554-555, 25.

  25. Conclusion • PtRu was possible to enhance formic acid oxidation through reaction 2. • Binary catalyst with bi-functional effect, i.e. PtRuCNT, did not affect on the activation energy of formic acid oxidation. • With PtBiCNT, major reaction was reaction 1, which had the lower oxidation potential than reaction 2. • PtBiCNT catalyst did not improve the catalytic activity towards formic acid oxidation. • PtBiCNT caused large increase in activation energy indicating effective suppression of reaction 2.

  26. Conclusion • Improvement of formic acid oxidation can be achieved by using catalyst with third body effect. • We need to prepare more active PtBi CNT.

  27. Dr. I. Frank Cheng Chris Roske Dr. Chen M. Wai Dr. Byunghoon Yoon Dr. Clive H. Yen Dept of Chemistry at the University of Idaho Financial support Electric Power Research Institute (EPRI) Innovative Small Grants Program Dr. and Mrs. Renfrew Summer Scholarship Acknowledgement

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