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Synthesis of Active Bimetallic Catalysts for Direct Methanol Fuel Cells

Synthesis of Active Bimetallic Catalysts for Direct Methanol Fuel Cells. Bahareh Alsadat Tavakoli, John. R. Regalbuto , John. W. Weidner and John. R. Monnier University of South Carolina, Columbia SC 29208. Direct Methanol Fuel Cell (DMFC). Anode:. Cathode. Catalyst. Ionomer.

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Synthesis of Active Bimetallic Catalysts for Direct Methanol Fuel Cells

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  1. Synthesis of Active Bimetallic Catalysts for Direct Methanol Fuel Cells Bahareh Alsadat Tavakoli, John. R. Regalbuto, John. W. Weidner and John. R. Monnier University of South Carolina, Columbia SC 29208

  2. Direct Methanol Fuel Cell (DMFC) Anode: Cathode Catalyst Ionomer Carbon support • Significantly large quantities of Pt used in fuel cells • It is known to be the best catalyst for breaking C-H and O-H bonds in alcohol. • The poisoning of Pt by CO , Slows kinetics of the anodic reaction Development of supported electrocatalysts that can increase performance. 1

  3. Bimetallic Catalysts • Addition of second metal can enhance catalytic activity Pt-Ru [1] • Bi-functional Mechanism Activation of water at the surface of second metal Second metal modifies the electronic structure of Pt by donating electron density • Electronic Mechanism Weakening the Pt-CO bond CH3OH Pt CO2 CH3OHad COad [1] M. P. Hogarth and T. R. Ralph, Platinum Metals Review, 46, 146-164 (2002). OHads Ru 2

  4. Catalyst Preparation Methods • An Effective control of particle size and composition is still a challenge. Conventional methods of catalyst synthesis often use simple impregnation • Poor dispersion gives low S.A., thus the need to increase metal loading Large Particles atoms inside are not utilized Strong Electrostatic Adsorption (SEA) • Small Particles • More metal atoms exposed (higher dispersion) Usual method of co-impregnation does not ensure interaction between component metals Electroless Deposition (ED) Using a method that synthesizes a bimetallic catalyst with the required high degree of metal 1 – metal 2 interaction 3

  5. Objectives • Preparation of different metal loadings of Pt on carbon-XC72 by using the SEA method. • Preparation of series of Ru-Pt bimetallic catalysts prepared by the ED method (controlled and variable amounts of Ru on the Pt surface prepared by SEA method). • Characterization and comparison of the prepared catalysts with a commercial Pt/XC72 catalysts. 4

  6. Strong Electrostatic Adsorption (SEA) - Inducing surface charge on support by adjusting pH of impregnating solution 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 Pt0 Reduction treatment [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) 5

  7. Electroless Deposition (ED) • 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 Bn+ RA B B B B B B B B B H H H H B B B B B B B B B B B B B B B B B A A A A A Support Support Support Support Support Autocatalytic Catalytic 6

  8. Chemisorption of Pt Dispersed on Carbon (H2 Titration) Pretreatment Titration Titration Surface Site Estimation by Chemisorption • H2 titration of O-pre-covered Pt 200°C 60min 120min 5°C/min 40°C 40°C 40°C 40°C RT RT 30min 30min 30min 30min 10% H2 in Ar (pulsed) 10% H2 in Ar (pulsed) 10% O2 in He 10% O2 in He He H2 Ar Ar Ar H2O H2 • H2-O2 titration: O2 O H O H O H Pt Pt Pt Pt O Pt Pt H Pt Pt Pt Pt Pt Pt Pt Pt Pt 3H2 + 2Pt-O → 2Pt-H + 2H2O Stoichiometry: 0.67 Pt : 1 H2 7

  9. X-ray Diffraction (XRD) 6.3% Pt/XC72 • The full patterns from the support and catalyst are shown along with the background subtractions and deconvoluted patterns 8

  10. 12.4 wt% Pt/XC72 6.3 wt% Pt/XC72 STEM Results • STEM micrographs and corresponding size distributions • Commercial Pt/XC72 catalyst showed larger particle sizes and wider size distribution. 20nm 20nm 20 nm 20 nm 20 wt% Pt/XC72 (Commercial 20.6 wt% Pt/XC72 9

  11. Cyclic Voltammetry The electrochemical surface area (ECSA) of the Pt catalyst is calculated from the charge density qPt(C/cm2electrode) obtained from the CV experiment Cyclic voltammetry (CV) at 5.0 mV/s in N2 saturated 0.5 M H2SO4. Metal loading The charge required to reduce a monolayer of protons on Pt = 210 μC/cm2Pt 10

  12. Methanol Oxidation Cyclic Voltammetry in 0.5 M H2SO4,1 M Methanol, Scan rate 5 mV/s • The peak current is indicative of the activity of the electrocatalysts for oxidation of methanol. • The SEA-derived Pt on carbon catalyst for methanol oxidation gives mass activity much higher than the commercial catalysts. 11

  13. Methanol Oxidation • The optimal coverage of Ru on Pt is 0.5 monolayers based on the performance. • Ru-Pt bimetallic catalysts prepared by SEA and ED show higher performance than commercial. 12

  14. Summary and Conclusions • Smaller, well dispersed, bimetallic nanoparticles of Ru and Pt were made by ED of Ru on Pt/XC72 base catalyst prepared by the SEA method. • ED prepared Ru-Pt catalysts show enhanced activity for methanol electrooxidation. • Different metal loadings of catalysts prepared by using the SEA and ED methods show the same value towards methanol electrooxidation. • By using ED coupled with the SEA method, well dispersed bimetallic catalysts can be made toward methanol oxidation. 13

  15. Thank you Support for this work by Qatar National Research Foundation and Qatar University (NPRP 9-219-2-105). Center for Rational Catalyst Synthesis (CERCAS) Dr. John M. Tengco Rembert D. White Umema Khan

  16. Future work • Test inside fuel cell • Durability tests • Pt-Ni bimetallic catalysts

  17. ECSA • If gaseous atoms or molecules adsorb on a solid surface, each atom or molecule of gas is attached by a real covalent bond to an adsorption site • The real surface area can be evaluated by measuring the amount of adsorbed gas and the area occupied by each gas atom Avogadro’s number Amount of gas adsorbed in moles The surface metal atom density

  18. Ru, Au, Pd, Sn, Os, Ni, Zn, Mo, Ir Activity plots for Pt alloy materials measured at 80 °C. Reproduced from [1]. [1] M. P. Hogarth and T. R. Ralph, Platinum Metals Review, 46, 146-164 (2002).

  19. J. Durst, A. Siebel, C. Simon, F. Hasché, J. Herranz, H. A. Gasteiger, Royal Society of Chemistry, 2014

  20. Dispersion, % = (No. Surface Atoms / Total No. Atoms) x 100 TCD signal for H₂ titration of surface Pt-O for 20 wt% Pt/XC72 Dispersion=S M N/100L where S is a stoichiometric factor. That means for example in the case of Pt and H2 chemisorption, one molecule of H2 dissociate to to give two atoms to be chemisorbed each atom over Pt Nis the amount of monolayer, experimentally obtained from Langmuir isotherm (in static experiments) or directly from dynamic (flow experiments) in mol/g M is the molecular weight of the metal (g/mol) L is the weight percentage of metal in your solid. This value should be measured (for example by ICP). You should not use the theoretical percentage, I mean the percentage that you suppose after preparation.

  21. Particle Size Comparison

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