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INTRODUCTION

Bimetallic Ru-Pt/C catalysts prepared by strong electrostatic adsorption and electroless deposition for direct methanol fuel cell application.

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INTRODUCTION

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  1. Bimetallic Ru-Pt/C catalysts prepared by strong electrostatic adsorption and electroless deposition for direct methanol fuel cell application John Meynard M. Tengco, BaharehAlsadatTavakoliMehrabadi, WeijianDiao, Taylor R. Garrick, John W. Weidner, John R. Monnier, and John R. Regalbuto University of South Carolina NAM 25 Denver, CO 5 June 2017

  2. INTRODUCTION SUPPORTED METAL CATALYSTS- more efficient metal utilization - greater amount of active surface Small Particles more metal atoms exposed (higher dispersion) Large Particles atoms inside are not utilized

  3. INTRODUCTION: STRONG ELECTROSTATIC ADSORPTION (SEA) - Inducing surface charge on support by adjusting pH of impregnating solution - SEA at incipient wetness is also called Charge Enhanced Dry Impregnation (CEDI) anion uptake support pH > PZC O- [Pt(NH3)4]2+ cationic complex cation uptake pH @ PZC OH metal uptake (per support area) [PtCl6]2- anionic complex pH < PZC OH2+ @ PZC pH > PZC pH < PZC support reduction treatment Pt0 [PtCl6]2- support H2O - resulting close packed monolayer of ionic complex (retaining hydration sheaths) with strong interaction with support - decreased mobility of metal atoms result in smaller catalyst particles (compared to simple impregnation)

  4. INTRODUCTION: BIMETALLIC CATALYSTS • Addition of another metal can enhance catalytic activity • Bimetallic Effects • Bifunctional • Electronic • Ensemble • Usual method of co-impregnation does not ensure interaction between component metals vs Using a method that synthesizes a bimetallic catalyst with the required high degree of metal 1 – metal 2 interaction, such as Electroless Deposition can result in better catalysts

  5. INTRODUCTION: ELECTROLESS DEPOSITION (ED) FOR BIMETALLIC CATALYSTS • Targeted deposition of secondary metal on the surface of primary/seed catalyst Immersion of seed catalyst in ED bath Activation of reducing agent (RA) on the surface of seed catalyst Reduction and deposition of secondary metal Catalytic deposition Auto-catalytic deposition - Necessary to have proper combination of reducing agent, metal precursor, and ED conditions

  6. Pt based catalysts for Fuel Cells DMFC: Anode Reaction: CH3OH+ H2O → CO2 + 6H+ + 6e- • Cathode Reaction: 3/2O2 + 6H+ + 6e-  → 3H2O • Cell Reaction: CH3OH+ 3/2O2 → CO2 + 2H2O • Significantly large quantities of Pt used in fuel cells • Poor dispersion gives low S.A., thus the need to increase metal loading

  7. COMMERCIALLY AVAILABLE CARBON SUPPORTED PLATINUM CATALYST Commercial 20% Pt/Carbon (Vulcan XC72) 20nm dN = 3.1nm dXRD = 2.5nm - large particles, wide distribution

  8. Development of Electroless Developer Bath for Ru on Pt/C Reduction of secondary metal on the surface Activation of reducing agent (RA) on base catalyst Further deposition of secondary metal Reducing Agent: Formic Acid, HCOOH Ru Precursor: Hexaammineruthenium(III) chloride, Ru(NH3)6Cl3 pH condition: Acidic, below PZC of Carbon Support (8.5), to prevent adsorption of [Ru(NH3)6]3+ on carbon surface T. R. Garrick, W. Diao, J. M. Tengco, J. R. Monnier and J. W. Weidner, Elec. Acta., 2016, 195, 106 R. P. Galhenage, K. Xie, W. Diao, J. M. M. Tengco, G. S. Seuser, J. R. Monnier and D. A. Chen, Phys. Chem. Chem. Phys., 2015, 17, 28354 W. Diao, J. M. M. Tengco, J. R. Regalbuto, and J. R. Monnier, ACS Catal., 2015, 5, 5123

  9. Deposition Profiles for Ru ED on Commercial Pt/C - High temperature needed to completely deposit 1 ML (theoretical) coverage of Ru on Pt

  10. BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on commercial 20% Pt/C catalyst

  11. BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on commercial 20% Pt/C catalyst Representative electron micrographs and elemental maps of selected spots of 0.96 ML Ru on Pt/C HAADF-STEM XEDS Maps (C) (D) (A) (B) Pt@Ru/C Ruthenium Platinum 4nm 5nm Ru deposited is in good association with Pt on the surface (Ru and overlaid maps show a “shell”) XRD does not show alloy formation or large Ru phase TPR and XPS also confirm excellent association between component metals

  12. BASE CATALYST PREPARED BY SEA SEA 6.3% Pt/Carbon (Vulcan XC72R)

  13. COMPARISON OF BASE CATALYST PREPARED BY SEA WITH COMMERCIAL CATALYST 20nm Commercial 20% Pt/Carbon (Vulcan XC72) SEA 6.3% Pt/Carbon (Vulcan XC72R) vs. 20nm dN = 1.9nm dXRD ~ 1.5nm dN = 3.1nm dXRD ~ 2.5nm

  14. BIMETALLIC Ru-Pt/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on SEA prepared 6.3% Pt/C catalyst Ru-Pt/C

  15. BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on SEA prepared 6.3% Pt/C catalyst Representative electron micrographs and elemental maps of selected spots of 0.50 ML Ru on Pt/C Pt Ru Pt Ru

  16. ELECTROCHEMICAL EVALUATION OF BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) • Cyclic voltammetry • Methanol electrooxidation (0.5 M H2SO4 with 1 M methanol electrolyte, Hg/HgSO4 reference and Pt counter electrodes) • Repeated 10 times for peak current reproducibility

  17. ELECTROCHEMICAL EVALUATION OF BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on commercial 20% Pt/C catalyst Mass activities for DMFC Peak activity at 50% theoretical surface coverage (Ru/Pt = 1:1)

  18. ELECTROCHEMICAL EVALUATION OF BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – comparison of Commercial vs SEA prepared base catalyst Mass activities for DMFC Peak activity, same, at 50% theoretical surface coverage (Ru/Pt = 1:1)

  19. Additional Catalyst Synthesis Higher loading of platinum on carbon by cycling SEA/CEDI - Successive charge enhanced dry impregnation cycles done to produce 20% Pt/C (Darco G-60) - Small particles based on XRD (~1.5 nm) Pt (fcc)

  20. Additional Catalyst Synthesis Higher loading of platinum on carbon by cycling SEA/CEDI

  21. SUMMARY and CONCLUSIONS • Smaller, well dispersed, bimetallic nanoparticles of Ru and Pt were made by Electroless Deposition of Ru on Pt/C base catalyst — commercial and prepared by Strong Electrostatic Adsorption. • ED prepared Ru-Pt catalysts show enhanced activity for methanol electrooxidation • Electroless Deposition is a simple and viable method for the preparation of true bimetallic catalysts. Coupled with Strong Electrostatic Adsorption, well dispersed bimetallic catalysts can be made.

  22. ACKNOWLEDGEMENTS ELECTRON MICROSCOPY CENTER ASPIRE Thank you

  23. SUPPLEMENTARY INFORMATION

  24. TPR of O-precovered Ru-Pt Bimetallic Particles • 2.1% Ru corresponds to 1.0 monodisperse layer of Ru on Pt surface. • B. Absence of Ru-O reduction peak confirms Ru-Pt bimetallic particles.

  25. XRD: 6.3% Pt/XC72R SEA dxrd ≤ 1.5 nm

  26. XRD: 20% Pt/XC72 Premetek dxrd = 2.3 nm

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