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.