High-pressure XPS: a new tool for environmental science and catalysisD.F. Ogletree, F.G. Requejo, C.S. Fadley1, Z. Hussain2, M. Salmeron H. Bluhm, A. Knop-Gericke, M. Hävecker, R. Schlögl

Lawrence Berkeley National Laboratory, Materials Sciences Division, Berkeley, CA 94720.1also at Department of Physics, University of California, Davis, CA 95616.2 Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA 94720.

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abteilung Anorganische Chemie, Faradayweg 4-6,

D-14195 Berlin, Germany.

The Instrument Premelting of Ice

Observing Catalysts at Work

Inte

nsity

(a.

u.)

560555550545540535530

Incident photon energy (eV)

292 288 284

Fluorescence Yield,Ice, -25 ºC

con

tam

ina

tion

pe

ak

fre

e h

ydro

gen

pe

ak

-4 ºC

-25 ºC

XPSC1s

-25 ºC

0.40

0.35

0.30

0.25

0.20

0.15

0.10

Fre

e hy

drog

en p

eak

area

0.40.30.20.10.0

Contamination peak area

Water-----------------------------------

Ice-2 C -10 C-4 C -15 C-5 C -25 C-7 C -39 C

Why high-pressure XPS?

Many processes and samples cannot be investigated in UHV (“Pressure Gap”)

• catalysis, corrosion, some oxidation• environmental chemistry• biological samples under wet conditions (high vapor pressure of water!)

LEED-type calculation of elastic scattering cross sections for Oxygen atoms

1 Torr-mm of water vapor is 3.3 monolayers

Fundamental limit in high-pressure XPS:Elastic and inelastic scattering of electrons by gas

molecules

0 200 400 600 800 10000

2

4

6Elastic Mean Free Path in Oxygen Gas

Electron Kinetic Energy [eV]

Mea

n F

ree

Pat

h [T

orr-

mm

]

The solution: Differentially pumpedelectrostatic lens system

Experimental cellsupplied by gas lines

X-rays enter the cell at15° incidence through asilicon nitride window(100 nm thick, 4x4 mm wide, Δp ~50 torr)

„Nozzle“

„Control Electrode“

„Skimmer“

„Condenser“

„Quadrupole“Analyzer input lens;analyzer is pumpedindependently

Focal point of analyzerinput lens

First differentialpumping stage

Second differentialpumping stage

Iceliquid

TmeltTonset

Above a certain temperature the ice surface is covered bya thin liquid film

Our goal is the investigation of the influence of hydrocarbon contamination (using XPS) on the thickness (using NEXAFS) of the liquid-like layer.

Nor

mal

ized

Aug

er y

ield

(a.

u.)

555550545540535530

Incident photon energy (eV)

-25 ºC

-15 ºC

-10 ºC

-5 ºC

-4 ºC

ice, -2 ºC

water, -2 ºC

-39 ºC

“Clean” Ice SurfaceAuger yield NEXAFS spectra showing the premelting

transition

30

25

20

15

10

5

0

LLL

thic

knes

s (Å

)

-40 -30 -20 -10 0

Temperature (ºC)

Thickness of the liquid layer on “clean” ice samples as determined from the NEXAFS spectra

energy windowfor Auger Yield

NEXAFS

photoelectronsfor hv=530...560 eV

E of valencekin

2a1

1b3a

1

1

1b2

Oxygen KLL Auger spectra

550500450400

supercooled water-10 ºC, 1.95 torr

ice, -10 ºC, 1.95 torr

ice, -85 ºC, p < 0.01 torr(hv = 560 eV)

vapor, 0.5 torr 2.0 torr

Nor

mal

ized

Aug

er y

ield

(a.

u.)

Kinetic Energy (eV) (hv = 569 eV)

Comparison of NEXFAS and XPS: At both temperatures the more contaminated ice surface shows stronger signs of premelting than the less contaminated ice surface.

Area of the free hydrogen peak (535 eV) vs. contamination peak area (532 eV). Premelting of the ice surface thus depends strongly on both temperature and hydrocarbon contamination.

Acknowledgements: Gennadi Lebedev (GETOM, Petaluma, CA), initial lens design; John Bozek, Ed Wong, ALS; Eli Rotenberg, XPS software; Anders Nilsson, Satych Myneni for helpful discussions.

Example: oxidation of methanol over a copper catalyst.The two main reactions are:

(1) CH3OH + ½O2 CH2O+ H2O

(2) 2CH3OH + 3O2 2CO2+ 4H2O

Reaction (1) (partial oxidation) is an important process in the chemical industry. Our goal is to identify the composition and chemical nature of the surface of the copper catalyst for formaldehyde (CH2O) production under working conditions.

Contaminated Ice Surface

545 540 535 530

Binding Energy (eV)

O2 (g)

CH3OH (g)

CH3OH (surface)

Cu2O

CH3O-

XPS O1s spectrum of a Cu foil in a gas mixture of 0.07 torr O2 + 0.34 torr CH3OH @ 275 °C.

Due to the work function of the sample (4.5 eV) the surface peaks are shifted towards the gas phase peaks.

0,1

0,2

0,3

150 200 250 300 350 400 450

Temperature (°C)

0

0,1

0,2

0,3

0,4

0,5

0,6

CH

2 O yield (a.u.)

O1s

Cu 2

O in

tens

ity (

a.u.

)

Comparison of mass spec and XPS data. A more metallic (i.e., less oxidic) Cu surface and higher temperatures favor the production of formaldehyde (CH2O).