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Eric M. Stuve Department of Chemical Engineering University of Washington FY2002

Surface Reaction Fundamentals in Direct Oxidation Hydrocarbon Fuel Cells. Eric M. Stuve Department of Chemical Engineering University of Washington FY2002 6.1 Electrochemistry Review March 4-6, 2002 Annapolis, MD. MOTIVATION. APPROACH.

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Eric M. Stuve Department of Chemical Engineering University of Washington FY2002

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  1. Surface Reaction Fundamentals in Direct Oxidation Hydrocarbon Fuel Cells Eric M. Stuve Department of Chemical Engineering University of Washington FY2002 6.1 Electrochemistry Review March 4-6, 2002 Annapolis, MD

  2. MOTIVATION APPROACH • Examine fundamental surface chemistry of electrolytic hydrocarbon oxidation reactions • NEMCA Effect • Intermediates • Reaction pathways • Kinetic parameters • Characterize fuel / catalyst combinations • Ceria / metal catalyzed direct oxidation of hydrocarbons • Gorte and Vohs: Direct oxidation on Cu/CeO2 • Catalysts for direct hydrocarbon oxidation < 700 °C • Bond breaking tendencies for C–C, C–H, and C–O • Role of surface / substrate oxygen in direct oxidation • Fuels for proton / oxygen ion conducting electrolytes • Ceria / catalyst coated field emitter tip • Work function studies by Field Emission Microscopy • Imaging with Field Ionization Microscopy / Field Desorption Microscopy • Ionization monitored by ToF and ExB filter • UHV Solid Oxide Fuel Cell (SOFC) • Surface analysis of catalyst / oxide (XPS, LEIS, etc.) • Reaction pathways and kinetics

  3. FARADAIC EFFICIENCY, L (-3x104 to 3x105) WE RE VWC G-P RATE ENHANCEMENT RATIO, r (0 to 150) O2- O2- O2- YSZ I CE VWR Reproduced from Vayenas, Ind. Eng. Chem. Res., 2001, 40, 4209-4215. Non-Faradaic Electrochemical Modification of Catalytic Activity Vayenas’ experimental setup for NEMCA. WE, RE, and CE are working (Pt), reference (Pt) and counter (Ag) electrodes, respectively; G-P is a galvanostat-potentiostat. [Adapted from Vayenas, 1993] O2 Spillover? Sub-surface O2? Three phase boundary role?

  4. Od- O O O Od- d+ MOx O O d+ d+ CATALYST d+ d+ d+ d+ d+ d+ O2– SOLID OXIDE O2– O2– O2– O2– O2– O2– TPB CxHy H2O Pt TIP CO2 SIDE VIEW FRONT VIEW Emitter Tip Studies of Metal / Solid Oxide / Fuel Reactions

  5. Water Ion Cluster Formation Low Temperature (<165 K) Field Desorption from Adsorbed Ice Layers (Amorphous and Crystalline) Field Ion Emission from Field Adsorbed Water Layers (>165 K) Developed 2-Step Ion Dissociation / Emission Mechanism Water / Methanol Ion Cluster Formation Field Ion Emission from Field Adsorbed Water/Methanol Mixtures (>165 K) Observed Mixed Cluster Formation H+(CH3OH)m (H2O)n PREVIOUS WORK 0.39 0.44 APPLIED FIELD / VÅ-1 0.55 1.10 Ion Mass H3O+

  6. UHV Chamber Configuration ANALYTICAL EQUIPMENT • Pulsed Potential ToF • Quadrupole Mass Spec • Wien Filter (ExB) • Rotatable Tip Assembly • FIM/FEM Imaging Tip Assembly LD 20 - 56 mm Variable Counter Electrode-Lens Distance Heating Loop (0.25 mm Pt) Thermocouple Leads Emitter Tip (0.13 mm Pt)

  7. Lens: G.F. Rempter, J. Appl. Phys.57 (1985) 2385. E x B Mass Separator: M. Kato and K. Tsuno, Nucl. Instr. MethodsA298 (1990) 296. m+dm Magnetic Field (B) m Wien Filter Ion Characterization m-dm • Continuous Mode Ion Mass to Charge Resolution • Easily Separate Distinct Ion Signals without Disturbing Formation Conditions Electric Field (E) Ion WIEN SEPARATION Masses 19 and 37

  8. METAL (Pt) LATTICE STEP IMAGE GAS (Ne) ION (Ne+) Potential Energy of Image Gas Electron In Applied Field Near Tip Surface V FERMI LEVEL I X f Neon on Pt 107 K 1x10-4 Torr ~3.75 V/Å Field Ion Microscopy • Spatial Resolution of Ion Emission • Field Clean Pt Surface to Prevent Possible Contamination MULTI-CHANNEL PLATES PHOSPHOR SCREEN TIP HV Adapted from Tsong,1990.

  9. Source Apparatus TO GROUND CERAMIC SUPPORT TO CERIUM SOURCE CURRENT SUPPLY TO LITHIUM SOURCE CURRENT SUPPLY TANTALUM FOIL 22 mm CERIUM SOURCE LITHIUM SOURCE 27 mm

  10. Source Apparatus Pictures

  11. Cerium Source Apparatus Cerium Preparation 1) Ce foil (0.62 mm x 1 mm x 3 mm) bound to W heating wire(0.35 mm) by WRe wire (0.075 mm) 2) Heated in vacuum to melt foil (>800K) TUNGSTEN HEATING WIRE (0.35 mm) TUNGSTEN (95%) / RHENIUM (5%) WIRE (0.075 mm) CERIUM FOIL

  12. Lithium Source Apparatus Pellet Preparation 1) CaO and Li2CO3 (1:4) powder pelleted 2) Heated in vacuum to remove CO2 3) Degassed mixture and Al (2:1) powder pelleted 4) Pellet placed in source 5) Source heated in vacuum to de-gas TUNGSTEN HEATING COIL (0.25 mm) LITHIUM PELLET TANTALUM FOIL 4.8 mm 14 mm

  13. (110) (100) (111) AFTER CERIUM DEPOSITION CERIUM AFTER 350 K ANNEAL CLEAN PLATINUM TIP (rT ~ 550 Å) Cerium Depostion on Pt Emitter Tip • Field cleaned and imaged in Neon • Field emission image of clean surface • Cerium deposited on Pt at 110K (~1 ML) • Field emission image of deposition • Anneal to 350 K during field emission Field Ion Micrograph 10-4 Torr Neon 3.75 V/Å FIELD EMISSION MICROGRAPHS -0.43 V/Å -0.15 V/Å -0.22 V/Å

  14. (110) (100) (111) Cerium Diffusion on Pt Emitter Tip • Field desorption of Cerium layer (~1.3 V/Å) • Imaged with Field Desorption Microscopy • Field emission picture after desorption • Temperature ramped from 110 K to 350 K to observe diffusion (0.4 to 0.2 V/Å) . Field Ion Micrograph 10-4 Torr Neon 3.75 V/Å FIELD DESORPTION OF Ce FROM Pt TEMPERATURE RAMP (250 - 350 K)

  15. Calculating the Change in Work Function φ after Deposition of Ce Total tip current was set to 0.1, 0.3, 0.5 and 1.0 mA for clean Pt and annealed Ce on Pt. Tip potential was recorded. Data is based on total current and therefore represents an average work function for the crystalline faces. From Fowler-Nordheim for our emitter tip this gives the slope of the line then is Slope Ce = 16.0 the two slopes are related by Slope Pt = 32.2 ln (I/V2) / (VA-2) taking the clean Pt work function to be 5.65 eV gives Compare with literature value of 2.9 eV for clean Cerium. 1000/V / (V-1)

  16. Press Fit or Lock-in O2 Supply SOFC CHAMBER DESIGN O2 Supply Disengaged Translate to XPS In UHV Chamber Electrode / Heater Leads O2 Supply Engaged Liquid N2 UHV Teflon Seal To Vacuum Fuel O2 Supply

  17. HIGH TEMPERATURE MACHINABLE CERAMIC (>1000 C) SOLID OXIDE PELLET (Ceria) HEATING ELEMENT SOFC TEST CELL DESIGN 0.8” 3” Reference Electrode Working Electrode Counter Electrode

  18. Low Temperature Ion Cluster Formation Evidence for 2-Step Ionization / Emission Mechanism H2O Deposition : • Time 5 Minutes • Thickness ~100Å • Tip Radius ~330Å • Temperature 145K • Pressure 2*10-7 Torr 109 K 145 K Ramped Field Desorption 1 • Crystalline Ice Deposition • Field Ramp passes through Emission Fields for all clusters n  2 before Dissociation • When Ramp reaches Dissociation Field, clusters n  2 are emitted simultaneously. • Compare mass 55 peaks in 109 K and 145 K. Ramped Field Desorption 2 • Field Adsorbed Layer • Field Ramp activates Dissociation before Emission • Cluster n emission observed, each in turn. ION SIGNAL APPLIED FIELD V/Å

  19. MeOH Cluster Formation: H+(CH3OH)m m = 2 [65] m = 3 [97] m = 4 [129] VPulse 900 PULSE HEIGHT PROCEDURE • Tip Temperature = 165 K • VTip at 3000 V; VCE at 2600 V • VCE pulsed negative by VPulse • PMeOH= 6*10-6 Torr • Resolved with ToF 800 700 Ion Counts 600 RESULTS • Protonated Methanol Clusters • Behavior Similar to H2O • Large Clusters at Low Fields • Cluster Size  with Field  • Complicated Spectra Near m = 1 • Mass 33 to 32 Shift with Field  • Presence of masses 83 and 115 • H+(CH3OH)m(H2O) for m = 2,3 500 400 300 Ion Mass to Charge Ratio

  20. MeOH / H2O Cluster Formation: a b c d MeOH : H2O MIXTURE RATIO 5 : 0 PROCEDURE • Tip Temperature = 165 K • VTip at 3000 V; VCE at 2600 V • VCE pulsed negative by 600 V • PMeOH + PH2O = 5*10-6 Torr • Resolved with ToF 4 : 1 Ion Counts RESULTS • H+(CH3OH)m(H2O)n Observed • H3O+ Emission Enhancement • MeOH Lowers Emission Barrier? • 33 to 32 ratio  with H2O  • Mixed Cluster Formation (m,n) (a) 1 , 1 mass 51 (b) 1 , 2 mass 69 (c) 2 , 1 mass 83 (d) 1 , 3 mass 87 3 : 2 2 : 3 1 : 4 0 : 5 Ion Mass to Charge Ratio

  21. MeOH / H2O Cluster Formation: RESOLVED m = 1 Pure MeOH PROCEDURE • Tip Temperature = 165 K • VTip at 3000 V; VCE at 2600 V • VCE pulsed negative by 600 V • Resolved with ToF Ion Counts RESULTS • Diversity of Peaks Near m = 1 • H+(H2O)2 Peak at 37 • Primary Peaks at 32 and 33 • Other Peaks at 30, 31 and 35 • Secondary Peak Characteristics? • ~100 bins between 32 and 33 • 1600 Ion Count Maximum 4 : 1 MeOH / H2O Ion Mass to Charge Ratio

  22. UHV Solid Oxide Fuel Cell Ceria / catalyst Coated Emitter Tip FUTURE WORK • Characterize Layer Thickness of Cerium • Oxidize Cerium and Develop Ceria Preparation Technique • Deposit Pt on Ceria Coated Tip • Li Ion Imaging of Tip • Imaging with FIM / FDM • Investigate Surface Reactions and Fuel Oxidation • Work Function Studies by FEM • Ionization Monitored by ToF and ExB filter • Design and fabricate SOFC apparatus • Surface analysis of catalyst / oxide (XPS, LEIS, etc.) • NEMCA Studies • Reaction Pathways and Kinetics

  23. SUMMARY DoD Payoff Results Extended Understanding of Water Ion Cluster Formation on Pt Tip • 2 Step Mechanism Ionization / Emission Mechanism • Importance of Solvation for Dissociation and Emission Water / Methanol Ion Cluster Formation • Behavior Similar to Previous Water Results • Mixed H+(CH3OH)m(H2O)n Clusters Observed • Presence of MeOH Alters Emission and Solvation Successful Cerium Deposition on Pt Tip • Field Emission Spectroscopy Shows Deposition • Work Function of Tip Decreased Provide fundamental information about relative tendencies of bond breaking in electrocatalysis, surface reaction intermediates, carbon deposition, and the role of oxygen in direct hydrocarbon oxidation important for an overall understanding of direct oxidation hydrocarbon fuel cells.

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