eric m. stuve department of chemical engineering university of washington fy2002 6.1...
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Eric M. StuveEric M. StuveDepartment of Chemical Department of Chemical
EngineeringEngineering
University of WashingtonUniversity of Washington
FY2002FY20026.1 Electrochemistry Review6.1 Electrochemistry Review
March 4-6, 2002March 4-6, 2002Annapolis, MDAnnapolis, MD
Surface Reaction Fundamentals in Direct Oxidation
Hydrocarbon Fuel Cells
•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
APPROACHMOTIVATION
Non-Faradaic Electrochemical Modification of Catalytic Activity
WE
O2- O2- O2-
CE
REVW
C
VW
R
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, (-3x104 to 3x105)
0r
r=ρ
RATE ENHANCEMENT RATIO, ρ(0 to 150)
( )FI
rr
2/0−=Λ
licElectrophi
bicElectropho
→<→>
11
Reproduced from Vayenas, Ind. Eng. Chem. Res., 2001, 40, 4209-4215.
O2 Spillover?
Sub-surface O2?
Three phase boundary role?
Pt TIP
CxHy
CO2
H2O
O2– O2–O2– O2–O2–SOLID OXIDE
O
OMOxO
O OO
O
O2–O2–
TPB
CATALYST
SIDE VIEW FRONT VIEW
Emitter Tip Studies of Metal / Solid Oxide / Fuel Reactions
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+(CH3OH)m (H2O)n
Ion Mass H3
O+
0.39
0.44
0.55
1.10
APPLI
ED
FIE
LD /
V
Å-1
Gas Handling
Turbomolecular Pump
Mass Spectrometer
WienFilter
Drift TubeLensFocus
Tip TranslationApparatus
Alternate Wien FilterConfiguration (no Drift Tube)
CoolantDown Tube
UHV ChamberConfiguration
20 - 56 mm Variable CounterElectrode-Lens Distance
LD
EntranceDiaphragm
FrontElectrode
CenterElectrode
BackElectrode
Lens Assembly
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
MagnetiMagneticc
Field (B)Field (B)
ElectricElectricField Field (E)(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 IonCharacterization
L1L
m
m0
Lens
Drift TubeE x BMass
Separator
Ion Detector
Tip
ΔxVt
VCE VL
IonIon
m+m+mmm-m-mm
mm
•Continuous Mode Ion Mass to Charge Resolution
•Easily Separate Distinct Ion Signals without Disturbing Formation Conditions
00 BeeE ν=0
000
2
m
eBE
φ= ⎟
⎟⎠
⎞⎜⎜⎝
⎛−⎥
⎦
⎤⎢⎣
⎡+=Δ
m
mLL
LEx 0
1
2
0
0 122φ
WIEN SEPARATION Masses 19 and 37
•Spatial Resolution of Ion Emission
•Field Clean Pt Surface to Prevent Possible Contamination
Field Ion Microscopy Neon on Pt107 K
1x10-4 Torr~3.75 V/Å
METAL (Pt)LATTICE STEPIMAGE GAS (Ne)
ION (Ne+)
Adapted from Tsong,1990.
TIP
HV
MULTI-CHANNELPLATES
PHOSPHORSCREEN
Potential Energy
of Image Gas
Electron In
Applied Field
Near Tip Surface
I X
V
FERMILEVEL
SourceApparatus
27 mm
CERAMICSUPPORT
TOCERIUM SOURCE
CURRENT SUPPLY
CERIUM SOURCE
TOLITHIUM SOURCECURRENT SUPPLY
LITHIUM SOURCE
TOGROUND
TANTALUM FOIL
22 mm
Source Apparatus Pictures
Cerium Source Apparatus
TUNGSTENHEATINGWIRE(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
14 mm
4.8 mm
TUNGSTENHEATING COIL
(0.25 mm)TANTALUM
FOILLITHIUMPELLET
1) CaO and Li2CO3 (1:4) powder pelleted2) Heated in vacuum to remove CO2
3) Degassed mixture and Al (2:1) powder pelleted4) Pellet placed in source5) Source heated in vacuum to de-gas
Pellet Preparation
Lithium Source Apparatus
(111)
(100)(110)
Field Ion Micrograph10-4 Torr Neon3.75 V/Å
-0.43 V/Å -0.15 V/Å -0.22 V/Å
CLEAN PLATINUM TIP (rT ~ 550 Å)
AFTER CERIUM DEPOSITION
CERIUM AFTER350 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
(111)
(100)(110)
Field Ion Micrograph10-4 Torr Neon3.75 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
QuickTime™ and aIntel Indeo® Video 5.0 decompressor
are needed to see this picture.
QuickTime™ and aIntel Indeo® Video 5.0 decompressor
are needed to see this picture.
1000/V / (V-1)
ln (
I/V
2)
/ (V
A-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 CeTotal 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.
Press Fit or Lock-in O2 Supply
O2 Supply
Liquid N2
Teflon Seal
Translate to XPSIn UHV Chamber
UH
V
Electrode / Heater Leads
FuelTo Vacuum
O2 SupplyEngaged
O2 SupplyDisengaged
SOFC CHAMBER DESIGN
CounterElectrode
ReferenceElectrode
WorkingElectrode
3”
0.8”
HIGH TEMPERATURE MACHINABLE CERAMIC (>1000 C)
SOLID OXIDE PELLET (Ceria)
HEATING ELEMENT
SOFC TEST CELL DESIGN
•Temperature 145K•Pressure 2*10-7 Torr
109 K 145 K
Low Temperature Ion Cluster Formation
Evidence for 2-Step Ionization / Emission Mechanism
APPLIED FIELD V/Å
ION
SIG
NA
L
•Time 5 Minutes•Thickness ~100Å•Tip Radius ~330Å
H2O 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.
0.00
0.25
0.50
0.75
1.00
100 150 200 250
Temperature, K
Applied Field (V/Å)
H2O+
H+(H2O)n1
23
4 -6
Dissociation
RFD 1
RFD 2
FIELD FREE CONDENSATION
0.00
0.25
0.50
0.75
1.00
100 150 200 250
Temperature, K
Applied Field (V/Å)
H2O+
H+(H2O)n1
23
4 -6
Dissociation
RFD 1
RFD 2
FIELD FREE CONDENSATION
0.0 0.2 0.4 0.6 0.8 1.0
Applied Field, V/Å
m=127
m=109
m=91
m=55
m=73
m=37m=19
H+(H2O)7
H+(H2O)6
H+(H2O)5
H+(H2O)4
H+(H2O)3
H+(H2O)2
H+(H2O)1
0.0 0.2 0.4 0.6 0.8 1.0
Applied Field, V/Å
m=127
m=109
m=91
m=55
m=73
m=37m=19
H+(H2O)7
H+(H2O)6
H+(H2O)5
H+(H2O)4
H+(H2O)3
H+(H2O)2
H+(H2O)1
800
600
500
400
300
VPul
se
700
900
Ion Mass to Charge Ratio
Ion
Cou
nts
0 20 40 60 80 100 120 140
MeOH Cluster Formation: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
m = 2[65]
m = 3[97]
m = 4[129]
H+(CH3OH)m
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
Ion Mass to Charge Ratio
Ion
Cou
nts
4 : 1
3 : 2
2 : 3
1 : 4
0 : 5
MeOH : H2O5 : 0
0 10 20 30 40 50 60 70 80 90 100 110 120
MeOH / H2O Cluster Formation:MIXTURE RATIOPROCEDURE• 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
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
a b cd
MeOH / H2O Cluster Formation:RESOLVED m = 1PROCEDURE• Tip Temperature = 165 K• VTip at 3000 V; VCE at 2600 V• VCE pulsed negative by 600
V• Resolved with ToF
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
Ion Mass to Charge Ratio
Ion
Cou
nts
Pure MeOH
4 : 1 MeOH / H2O
28 30 32 34 36 38 40
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 TipUHV Solid Oxide Fuel Cell
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+(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
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.