A. Knop-Gericke Fritz-Haber-Institut der Max-Planck-Gesellschaft, Depertment of Inorganic Chemistry, D-14195 Berlin, Germany Subsurface Species in Heterogeneous Catalytic Reactions: Subsurface Species in Heterogeneous Catalytic Reactions: Insights by in situ Photoelectron Spectroscopy Insights by in situ Photoelectron Spectroscopy Outline • Technical aspects • Methanol oxidation over Cu • Ethylene Epoxidation over Ag Collaborators LBNL & ALS: D.F. Ogletree, G. Lebedev, H. Bluhm Z. Hussain, C.S. Fadley, M. Salmeron FHI: M. Hävecker, K. Ihmann, E. Kleimenov, D. Teschner, S. Zafeiratos, E. Vass, P. Schnörch R. Schlögl Boreskov Inst. of Catalysis: V.I. Bukhtiyarov
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A. Knop-GerickeFritz-Haber-Institut der Max-Planck-Gesellschaft, Depertment of Inorganic Chemistry,
D-14195 Berlin, Germany
Subsurface Species in Heterogeneous Catalytic Reactions: Subsurface Species in Heterogeneous Catalytic Reactions: Insights by in situ Photoelectron SpectroscopyInsights by in situ Photoelectron Spectroscopy
Outline
• Technical aspects
• Methanol oxidation over Cu
• Ethylene Epoxidation over Ag
Collaborators
LBNL & ALS: D.F. Ogletree, G. Lebedev, H. BluhmZ. Hussain, C.S. Fadley, M. Salmeron
FHI: M. Hävecker, K. Ihmann, E. Kleimenov,D. Teschner, S. Zafeiratos, E. Vass, P. Schnörch R. Schlögl
Boreskov Inst. of Catalysis: V.I. Bukhtiyarov
• Very few methods can investigatethe solid-gas interfaceat high pressures- non-linerar optics (SFG, SHG)- scanning probe microscopies- X-ray diffraction
Why in situ XPS ?Why in situ XPS ?
15
10
5
-40 -20 0 20
temperature (ºC)
vapo
rpr e
s sur
e(t o
r r)w
ater Tm
• Photoelectron spectroscopy is very powerful⇒ Goal: XPS at pressures of at least 5 torr
• Many processes cannot beinvestigated in UHV:“Pressure Gap”- environmental chemistry- catalysis- corrosion- electrochemistry- biological samples
Fundamental limit:elastic and inelasticscattering of electronsin the gas phase
Technical issues: - Differential pumping to keep analyzer in high vacuum- Sample preparation and control in a flow reactor
In situ XPS: obstaclesIn situ XPS: obstacles
30x10-21
25
20
15
10
5
0Io
niza
tion
cros
s se
ctio
n (m
2 )
101
102
103
104
105
106
Electron kinetic energy (eV)
in situSEM in situ
TEM
in situEXAFSin situ
XPS
O2 exp. data from Schram et al. (1965) extrapolation of Schram's data
• Photons enterthrough a window
• Electrons and a gasjet escape throughan aperture tovacuum
In situ XPS: basic conceptIn situ XPS: basic concept
H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom. , 319 (1973)2
• H. Siegbahn et al. (1973- )• M.W. Roberts et al. (1979)• M. Faubel et al. (1987) • M. Grunze et al. (1988)• P. Oelhafen (1995)
H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom. , 205 (1981)24
In situ XPS using differentially pumped electrostatic lensesIn situ XPS using differentially pumped electrostatic lenses
D.F. Ogletree, H. Bluhm, G. Lebedev, C.S. Fadley, Z. Hussain, M. Salmeron, Rev. Sci. Instrum. 73 (2002) 3872.
to pump to pump
X-rays from synchrotron
In situ XPS systemIn situ XPS system
Experimental cellsupplied by gas lines (p0)
X-rays enter the cell at 55° incidence through an SiNx window(thickness ~ 1000 Å)
Analyzerinput lens
Focal point of analyzerinput lens
First differentialpumping stage (10-4p0)
Second differentialpumping stage (10-6p0)
Third differentialpumping stage (10-8p0)
Hemisphercalelectronanalyzer(10-9p0)
mass spectrometerand additional pumping
Gas phase composition canbe measured by XPS.gas phase signal: 1 torr·mm ~ a few monolayers
CloseClose--up of sampleup of sample--first aperture regionfirst aperture region
gas
z
d
hν
e-
p0
1.0
0.5
0.0-2 -1 0 1 2
z [d]
p/p 0
z=2 mm
d=1 mm
Gas Flow systemGas Flow system
gas1
gasn
LV /MFCn n
Vg
QMS
TPMS
PTRMS
air
BP TPPTP1
DS1
A0 A1
SCh
LVQMS
LVP
LVm1
LVm2
VP
VSCh
LV /MFC1 1
QMS
1 bar1 bar
1010--66 mbarmbar1010--66 mbarmbar
Gas Phase analysisGas Phase analysis
••Quadrupole Mass Spectrometry (QMS)Quadrupole Mass Spectrometry (QMS)•Simple in use•Real time investigation•Sensitive (10 ppb)•Relatively quantitative•High fragmentation rate at high masses
••Proton Transfer Reaction Mass Spectrometer (PTRProton Transfer Reaction Mass Spectrometer (PTR--MS)MS)•Very sensitive to volatile organic compounds ( < 1 ppb)•Low fragmentation rate•Selective to substance with proton affinities higher than water
••Micro Gas Chromatography (MicroMicro Gas Chromatography (Micro--GC)GC)•Detection sensitivity (1 ppm)•Quantitative analysis•Not real time detection (> 30 s)
Oxygen species on Ag (111) at 500 and 600 K in 0.13 mbar OOxygen species on Ag (111) at 500 and 600 K in 0.13 mbar O22
500 K500 K
600 K600 K
534 532 530 528 526 370 369 368 367
367.7368.1528.5
Binding Energy / eVBinding Energy / eV
530.4
Ag 3d5/2
hv=555 eV
O 1shv=720 eV
Inte
nsity
/ a.
u
×5
×5
*Sample was first cleaned (sputtered-annealed) then temp. raised and 0.13 mbar O2 introduced
~0.3 ML~0.3 ML
~0.2 ML~0.2 ML
Total O Total O coveragecoverage
Oxygen species on Ag (111) at 600 K in 0.13 mbar OOxygen species on Ag (111) at 600 K in 0.13 mbar O22Comparison between O 1s spectra recorded using different excitatComparison between O 1s spectra recorded using different excitation energiesion energies
Surface
Bulk
534 532 530 528 526
Inte
nsity
/ a.
u
Binding Energy / eV
Bulk Bulk (1220 eV)(1220 eV)
Surface Surface (720 eV)(720 eV)
O 1sO 1s
SurfaceSurface
BulkBulk
369 368 367
600 K
410 K
Ag 3d hv=555 eV
Inte
nsity
/ a.
u
Binding Energy / eV
Oxygen species on Ag (111) in 0.13 mbar OOxygen species on Ag (111) in 0.13 mbar O22Oxygen Species related with ionic silver formationOxygen Species related with ionic silver formation
534 532 530 528 526
Binding Energy / eV
Inte
nsity
/ a.
u
390 K
430 K
450 K
470 K
520 K
600 K
O 1s hv=720 eV
TT
Related with AgRelated with Ag++
AgAg++
Oxygen species on Ag (111) at 500 and 600 K in 0.13 mbar OOxygen species on Ag (111) at 500 and 600 K in 0.13 mbar O22
Ethylene epoxidation on SilverEthylene epoxidation on Silver Powder Samples Powder Samples SEM picturesSEM pictures**
*Commercially available samples (Sigma-Aldrich)
Activated Ag Activated Ag NanoNano--powder powder (~100 nm, high defect structure)(~100 nm, high defect structure)
SAMPLE 1 : SAMPLE 1 : AgAg--nanonano
Ag powder Ag powder (<600(<600 μμm)m)
3 3 μμmm
500 500 nmnm
SAMPLE 2 : SAMPLE 2 : AgAg--powderpowder
0 1 2 3 40.00
0.01
0.02
520 K
470 K420 K400 K
370 K
Ag nanopowder (~100 nm)(fresh)
C2H
4O P
artia
l Pre
ssur
e / m
bar
Time /hours
45
0.0 0.2 0.4 0.60.00
0.01
0.02 45
Time /hours
C2H
4O P
artia
l Pre
ssur
e / m
bar
520 K
Ag powder (~30 μm)
Powder Samples : Catalytic resultsPowder Samples : Catalytic resultsDetection of CDetection of C22HH44O (PTRMS) : O (PTRMS) : Reaction conditions : Reaction conditions : CC22HH44/O/O22 (1/2), P=0.5 mbar(1/2), P=0.5 mbar
Ag Ag nanonano
Ag powderAg powder
Silver particles agglomeration Silver particles agglomeration loss of surface arealoss of surface area
Highly active Highly active for ethylene for ethylene epoxidationepoxidation
Inactive for Inactive for ethylene ethylene
epoxidationepoxidationunder our under our conditionsconditions
Powder Samples : Post reaction characterizationPowder Samples : Post reaction characterization(C(C22HH44/O/O22 (1/2), P=0.5 mbar, @300 K)(1/2), P=0.5 mbar, @300 K)
Ag Ag nanonano
Ag powderAg powder
x 103
85
90
95
100
105
110
CPS
536 534 532 530 528Binding Energy (eV)
x 105
2
4
6
8
10
12
CPS
371 370 369 368 367 366 365Binding Energy (eV)
x 103
130
140
150
160
170
180
CPS
536 534 532 530 528Binding Energy (eV)
x 105
2
4
6
8
10
12
14
CPS
371 370 369 368 367 366 365Binding Energy (eV)
O 1sO 1s Ag 3dAg 3d••530.8 eV530.8 eV••532.7 eV532.7 eV
••529.1 eV529.1 eV••530.2 eV530.2 eV••531.6 eV531.6 eV
••368.15 eV368.15 eV
••368.15 eV368.15 eV
x 104
14
16
18
20
22
24
292 290 288 286 284 282Binding Energy (eV)
C 1sC 1s
EK=220 eVEK=220 eV
Intensity ratioIntensity ratio
O/Ag =0.138O/Ag =0.138
O/Ag =O/Ag = 0.0840.084
Powder Samples : Powder Samples : Comparison Post and inComparison Post and in--situ characterizationsitu characterization
O 1sO 1s
x 103
130
140
150
160
170
180
CPS
536 534 532 530 528Binding Energy (eV)
x 103
85
90
95
100
105
110
CPS
536 534 532 530 528Binding Energy (eV)
x 103
120
130
140
150
160
170
180
190
CPS
536 534 532 530 528Binding Energy (eV)
x 104
10
15
20
25
30
CPS
536 534 532 530 528Binding Energy (eV)
Ag powderAg powderAg Ag nanonano
In s
ituIn
situ
Pos
t rea
ctio
nP
ost r
eact
ion
x 105
2
4
6
8
10
12
14
CPS
371 370 369 368 367 366 365Binding Energy (eV)
x 105
2
4
6
8
10
12
CPS
371 370 369 368 367 366 365Binding Energy (eV)
x 104
10
20
30
40
50
60
70
80
CPS
371 370 369 368 367 366 365Binding Energy (eV)
x 104
10
15
20
25
30
35
CPS
371 370 369 368 367 366 365Binding Energy (eV)
Ag 3dAg 3d5/25/2 Ag powderAg powderAg Ag nanonano
In s
ituIn
situ
Pos
t rea
ctio
nP
ost r
eact
ion
Differences between Differences between in situin situand and post reactionpost reaction spectraspectra :•Oxygen species•Silver species•Oxygen amount•Adsorbed carbon
Differences between Differences between active active and and nonnon--active active catalystcatalyst :•Oxygen species•Silver species•Oxygen amount