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

2. 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/CeO 2 –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 APPROACHMOTIVATION

3. Non-Faradaic Electrochemical Modification of Catalytic Activity WE O 2- CE RE VWCVWC VWRVWR I G- P YSZ 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] FARADAIC EFFICIENCY,  (-3x10 4 to 3x10 5 ) RATE ENHANCEMENT RATIO,  (0 to 150) Reproduced from Vayenas, Ind. Eng. Chem. Res., 2001, 40, 4209-4215. O 2 Spillover? Sub-surface O 2 ? Three phase boundary role?

4. Emitter Tip Studies of Metal / Solid Oxide / Fuel Reactions

5. PREVIOUS WORK 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 + (CH 3 OH) m (H 2 O) n Ion Mass H3O+H3O+ 0.39 0.44 0.55 1.10 APPLIED FIELD / VÅ -1

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

7. Magnetic Field (B) Electric Field (E) Lens: G.F. Rempter, J. Appl. Phys. 57 (1985) 2385. E x B Mass Separator: M. Kato and K. Tsuno, Nucl. Instr. Methods A298 (1990) 296. Wien Filter Ion Characterization Ion m+  m m-  m m Continuous Mode Ion Mass to Charge Resolution Easily Separate Distinct Ion Signals without Disturbing Formation Conditions WIEN SEPARATION Masses 19 and 37

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

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

10. Source Apparatus Pictures

11. Cerium Source Apparatus TUNGSTEN HEATING WIRE (0.35 mm) TUNGSTEN (95%) / RHENIUM (5%) WIRE (0.075 mm) CERIUM FOIL 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) Cerium Preparation

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

13. (111) (100) (110) Field Ion Micrograph 10 -4 Torr Neon 3.75 V/Å -0.43 V/Å-0.15 V/Å-0.22 V/Å CLEAN PLATINUM TIP (r T ~ 550 Å) AFTER CERIUM DEPOSITION CERIUM AFTER 350 K ANNEAL FIELD EMISSION MICROGRAPHS 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

14. (111) (100) (110) Field Ion Micrograph 10 -4 Torr Neon 3.75 V/Å TEMPERATURE RAMP (250 - 350 K) 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 DESORPTION OF Ce FROM Pt

15. 1000/V / (V -1 ) ln ( I /V 2 ) / (VA -2 ) Slope Pt = 32.2 Slope Ce = 16.0 From Fowler-Nordheim for our emitter tip this gives the slope of the line then is taking the clean Pt work function to be 5.65 eV gives the two slopes are related by Compare with literature value of 2.9 eV for clean Cerium. 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  A 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.

16. Press Fit or Lock-in O 2 Supply O 2 Supply Liquid N 2 Teflon Seal Translate to XPS In UHV Chamber UHV Electrode / Heater Leads Fuel To Vacuum O 2 Supply Engaged O 2 Supply Disengaged SOFC CHAMBER DESIGN

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

18. Temperature145K Pressure2*10-7 Torr 109 K145 K Low Temperature Ion Cluster Formation Evidence for 2-Step Ionization / Emission Mechanism APPLIED FIELD V/Å ION SIGNAL Time5 Minutes Thickness~100Å Tip Radius ~330Å H 2 O Deposition : 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.

19. 800 600 500 400 300 V Puls e 700 900 Ion Mass to Charge Ratio Ion Counts MeOH Cluster Formation: PULSE HEIGHT PROCEDURE Tip Temperature = 165 K V Tip at 3000 V; V CE at 2600 V V CE pulsed negative by V Pulse P MeOH = 6*10 -6 Torr Resolved with ToF m = 2 [65] m = 3 [97] m = 4 [129] H + (CH 3 OH) m RESULTS Protonated Methanol Clusters Behavior Similar to H 2 O –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 + (CH 3 OH) m (H 2 O) for m = 2,3

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

21. MeOH / H 2 O Cluster Formation: RESOLVED m = 1 PROCEDURE Tip Temperature = 165 K V Tip at 3000 V; V CE at 2600 V V CE pulsed negative by 600 V Resolved with ToF RESULTS Diversity of Peaks Near m = 1 H + (H 2 O) 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 Ion Mass to Charge Ratio Ion Counts Pure MeOH 4 : 1 MeOH / H 2 O

22. 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 Ceria / catalyst Coated Emitter Tip UHV Solid Oxide Fuel Cell

23. SUMMARY 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 + (CH 3 OH) m (H 2 O) 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 Results DoD Payoff 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.