Carbon/Iron Carbide Transformations in Highly Active Fe and FePt Fischer-Tropsch Catalysts during...
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Transcript of Carbon/Iron Carbide Transformations in Highly Active Fe and FePt Fischer-Tropsch Catalysts during...
Carbon/Iron CarbideTransformations in Highly Active Fe and FePt Fischer-Tropsch Catalysts during Pretreatment and Reaction
Calvin H. Bartholomew
Chemical Engineering Department
Brigham Young University
Chemical Engineering DepartmentBrigham Young University
Provo, UT
Presentation Outline
● Background
● PtFe/C: the beginning in 1972
● Controversy/consensus on active phase(s) of Fe FTS catalysts
● Activity/structure relationships● Statistically designed FBR activity tests
● Mössbauer investigations on spent catalysts
● High pressure in-situ Mössbauer studies
● Carbon species identification with TPSR-MS
● Conclusions
Surface Composition and Chemistry of Supported PtFe Alloys
● Dissertation of Calvin H. Bartholomew, Stanford University,1972; C.H. Bartholomew and M. Boudart, J. Catal. 29, 278-291 (1973).
● Objectives ● To use 57Fe as a probe to observe surface chemistry of Pt/C
● Determine surface composition of a supported alloy
● Approach● Used Mossbauer and H2-O2 titration to determine dispersion and surface
composition
● Use Mossbauer and magnetic measurements to confirm alloy
Particle Size d, Magnetic Transition Temperature TC, and Hyperfine Field H at 77 K for 50 Atomic% Iron in Platinum
Sample ReductionTemperature
d (A) TC (K) H at 77 K (kOe)
Pt-Fe Foil - Bulk 733 293
12.1% Pt-Fe/C 900°C 127 600 298
1.0% Pt-Fe/C 900°C 46 500 298
12.1% Pt-Fe/C 500°C 30 200 316
1.0% Pt-Fe/C 500°C 16 20 -
Dispersion and Surface Composition of Pt-Fe/C Alloy Catalysts
Total Metal wt% %DT %DFe %DPt χ χs
After exposure to O2 at 300°C, 10 min
1.0 62 79 45 0.51 0.65
1.8 61 68 57 0.34 0.38
1.0 64 85 57 0.25 0.33
9.4 40 72 36 0.101 0.182
After exposure of reduced catalyst to air at 25°C
1.0 62 57 68 0.51 0.47
1.8 61 56 63 0.34 0.31
1.0 64 65 64 0.25 0.25
9.4 40 53 38 0.101 0.135
Weight % Metal
Atomic % Fe DT % d Åa
1.0 50 62 16
3.9 50 35 28
12.1 48 31 31
1.8 34 61 17
1.0 25 64 18
9.4 10 40 26
a Average particle diameters were calculated assuming spherical particles and average site densities for Pt and Fe of 8.4 and 9.4 Å2/atom, respectively.
Platinum Iron Catalysts Supported on Carbon: Composition Total Dispersion DT and Average Particle Diameter d
The Binary Phase Diagram for Pt-Fe Alloys
Mössbauer Spectra of 50% Fe in Pt (a) foil at 298 K, (b) 1.0% Pt-Fe/C (reduced at 900°C, d=46 Å at 77K
Typical Computer-fitted Room Temperature Mössbauer spectrum for Pt-Fe/C
Peaks (1) and (4) form the outer surface doublet
Peaks (2) and (3) form the inner bulk doublet
Room Temperature Mössbauer Spectra for 1.0% Pt-Fe (50/50)/C
(a) After reduction in flowing hydrogen during 4 hours at 400°C and cooling to 25°C in hydrogen (1 atm)
(b) After evacuation and exposure to air at 25°C
(c) After evacuation and exposure to hydrogen at 25°C
(d) After exposure to oxygen (160 Torr) at 300°C, 10 minutes
Fischer-Tropsch Synthesis (FTS)● Discovered by Fischer and Tropsch in 1925
● Reaction of synthesis gas (H2 and CO) over a catalyst to produce a wide range of hydrocarbons:
● CO+2H2 = H2O + -CH2-
● Syngas can be produced through steam methane reforming and gasification/partial oxidation from nearly any carbon-bearing feedstock:
● natural gas
● coal
● Biomass
www.bp.com
Fischer-Tropsch Synthesis Chemistry
● Cheaper
● Remarkable WGS activity for handling syngas with low H2/CO from coal
● Highly olefinic C2-C6 fraction
Advantages of Iron FTS Catalysts
Li, Iglesia, et al. (2001) rapid rapidFe2O3Fe3O4FeCx
Fe2O3
Fe3O4
FeCx
In situ Fe K-Edge XANESSample: 1 mg Fe2O3
CO flow: 107 mol/g-atom Fe-hTPR and Carburization Studiesin CO25
Similar result obtained in:H2/CO = 2
Jackson, Datye, et al. (1997)
Zhang, O’Brien., et al. (1999)
“Active surface carbon on small iron carbide clusters
of appropriate size”
Iron Carbide -Fe2.5C is probably the active phase
Techniques Used:
● In situ Fe K-edge X-ray Absorption
● Isothermal Transient Measurements of FTS Rate with on-line Mass spectrometry
● CO chemisorptions and Surface Area Measurements
Recent Development (Iglesia et al. 2001):
Reaction is taking place on small clusters of iron carbides. Active phase could be assigned to either Fe3O4 or FeCx (or metallic Fe)
“Any ex situ techniques without concurrent measurement of the products evolved during activation and FTS can lead to misleading structure-function relations”
Pretreatment Effects on Catalyst Activity
0
10
20
30
40
50
60
70
80
90
100
0.00 50.00 100.00 150.00 200.00Time of Reaction, h
CO
con
vers
ion,
vol
%
H2
CO
H2/CO=1.0
11% Fe/1.0%Pt/0.9% K/SiO2
Reaction:265°C, 150 hH2/CO = 1.01.92 NL/g-cat/h
Pretreated:280°C, 16 h
Our Study:
• Design, preparation and characterization
• Non-aqueous Evaporation Deposition Technique
• Promotion with Pt
• Statistically designed experiments for activity tests
Catalyst Codes and Compositions
Catalyst Code Support Fe wt% K wt% Pt wt%
Fe-S-201 Davisil 644
10.7 - -
FePtK-S-218 Davisil 644
9.25 0.21 1.01
FePt-S-220 Davisil 644
11.54 - 1.01
H2 chemisorption, dispersion, and BET
surface area measurements
Catalyst Code Extent of Reductionat 300°C
(%)
H2 Uptake
(mole/g catalyst)
Dispersion (%)
BET SA
(m2/g)
Fe/SiO2 (calcined) 60 44.5 3.2 8.3 0.6 242±2
FePt/SiO2 (calcined) 80(Fe), 100(Pt)
51.1 14.9 6.5 1.9 266
FePtK/SiO2 (calcined) 70(Fe), 100(Pt)
56.5 15.7 8.2 2.0 296
Statistically Designed Fixed Bed Run Conditions (L18 Orthogonal Array)
Pretreatment Gas Composition
H2/CO
Pretreatment Temperatures
°C
Fixed Bed Temp
°C
Catalyst
0.1 0.5 1.0 250 280 320 250 265
Run Order
1 2 3
6
8
12
10 % Fe/SiO2
17
4 9 13 15 20 21
10 % Fe//1.0 % Pt/SiO2
22 5 7
10 11
14 16 18
19
10 % Fe//1.0 % Pt/0.2 % K/SiO2
23
Statistically Designed Fixed Bed Runs2 NL/g-cat/h
1501401301201101009080706050403020100
01020304050
60
70
80
90
100
1234567891011121314151617181920
Correlations between Iron Carbide Content and Catalyst Activity (after 150 h FBR run)
St #01 St #02 St #03 St #06 St #08 St #12 St #171416182022242628303234363840424446485052
Statistical Designed FBR Run Number
CO
Co
nvers
ion
(%
)
10
20
30
40
50
60
70
80
90
10% Fe/SiO2
Iron
Carb
ide C
on
ten
t (%)
St #19 St #05 St #11 St #07 St #10 St #14
50
60
70
80
Statistical Designed FBR Run Numbers
CO
Co
nvers
ion
(%
)
20
30
40
50
6010% Fe/1.0% Pt/0.2% K/SiO2
Iron
Carb
ide C
on
ten
t (%)
St #13 St #20 St #17 St #04 St #15 St #09
20
30
40
50
60
10% Fe/1.0% Pt/SiO2
Statistical Designed FBR Run Number
CO
Co
nvers
ion
(%
)
10
20
30
40
50
Iron
Carb
ide C
on
ten
t (%)
Mössbauer Spectra after Different Pretreatments and FBR Reaction (11% Fe/1.0%Pt/0.9% K/SiO2)
-10 -8 -6 -4 -2 0 2 4 6 8 1076543210
-1
Velocity (mm/sec)
CO, Tpretreat
=280°CFe
2.5C (41.6%)
Fe3O
4 (SP) (52.9%)
Fe2+ (5.5%)
-10 -8 -6 -4 -2 0 2 4 6 8 1076543210
-1
Per
cent
Abs
orpt
ion
H2, T
pretreat=280°C
Fe2.5
C (52.2%)Fe
3O
4 (SP) (46.3%)
Fe2+ (1.5%)
-10 -8 -6 -4 -2 0 2 4 6 8 1076543210
-1
H2/CO=1.0, T
pretreat=280°C
Fe2.5
C (19.3%)Fe
3O
4 (SP) (70.9%)
Fe3O
4 (FiM) (9.8%)
CO
H2
H2/CO
In-situ Mössbauer Spectra
-10 -5 0 5 10
121086420
Abs
orpt
ion
%
10 % Fe/SiO2
after 55 h FBR Run
Velocity Relative to Iron (mm s-1)
-10 -5 0 5 1076543210
-1
10 % Fe/SiO2
In-situ pretreatment(H
2/CO = 1.0, 280°C, 1 atm)
-10 -5 0 5 10
2.52.01.51.00.50.0
-0.5
10 % Fe/SiO2
Fe2.5 C : 25.7%Fe3O4 (sp): 74.4%Fe2+: 0.06%
Fe2.5 C : 14.0%Fe3O4 (sp): 68.2%Fe2+: 7.5%Fe3O4 (FiM): 10.3%
Fe3O4 (sp): 100% Untreated
Pretreatedin situ
Reacted
High Pressure In Situ Studies
O-rings
CO/H 2 gas inlet
gas outlet
Sliding Flanges
Catalyst Sample Wafer
Sliding Trough
Top Flange
Mylar Window
O-rings
TITLE
High Pressure Mössbauer In-Situ Mossbauer Cell
-10 -8 -6 -4 -2 0 2 4 6 8 10
Reaction for 10 h
Velocity (mm/sec)
Pretreated in syngas 32 h
Inte
nsi
ty (
No
rmal
ized
Co
un
ts)
Pretreated in syngas 8 h
Calcined
High Pressure In Situ Mössbauer Spectroscopy
10% Fe-1% Pt-0.2% K/SiO2
TPD/MS System
H 2
He
CO
SS-200-3
SS-200-6-1ZV
SS-200-6
MFC
MFC
MFC
SS-400-6-2
MASSSPECTROMETER
COMPUTER
1
2
3
4 5
6 5
7
5
7
8
9
10
11
12
13
14
15
16
17
1819
2021
1/8"
1/4"
5"
8"
8"
Temperature-programmed Surface Reaction with on-line Mass Spectrometry (TPSR-MS)
Effect of Pretreatment Gases
0 100 200 300 400 500 600 700 800 9000
5
10
15
20
25
Reaction 10 h after 16 h pretreatment in: H
2
H2/CO
CO
Temperature (°C)
Me
tha
ne
Ra
te (m
ole
/g-s
ec)
Carbon Species Transformation with Pretreatment Time
0 100 200 300 400 500 600 700 8000
2
4
6
8
10
12
14
16
18
FePtK-S-218 after Pretreatment(H
2/CO = 1.0) for:
1 h 2 h 6 h16 h
Temperature (°C)
Me
tha
ne
Ra
te (m
ol/g
-se
c)
Pretreatment in 20% CO/He
β
0 100 200 300 400 500 600 700 8000
5
10
15
20
Calcined Catalyst Pretreated in 20% CO/He for:
1 h 2 h 6 h 16 h
Temperature (°C)
Me
tha
ne
Ra
te (m
ol/g
-se
c)
0 100 200 300 400 500 600 700 800
Temperature (°C)
16 h
6 h
2 h
Met
hane
Rat
e (
mol
e/g-
sec)
1 h
Individual Peak Contributions from Previous TPSR Spectra
Fingerprinting FTS Catalysts during Reaction
0 100 200 300 400 500 600 700 8000
10
20
30
40
50
Pretreatment for 16 hFollowed by reaction for
10 h 20 h 60 h150 h dewaxed
Temperature (°C)
Me
tha
ne
Ra
te (m
ole
/g-s
ec)
Isothermal Transient Measurements of FTS Rates
0 1000 2000 3000 4000
CH4
Time (s)
H2O
Form
atio
n of
Pro
duct
s (a
mps
)
CO2
Alkanes (M/Z=55)
0 200 400 600 800 1000 1200
CH4
Time (s)
H2O
CO2
Form
atio
n of
Pro
duct
s (a
mps
)
Alkanes (m/z=55)
CO Pretreated
0 200 400 600 800 1000 1200
CH4
Form
ation
of P
rodu
cts (a
mps
)
Time (s)
H2O
CO2
Alkanes (m/z=55)
H2 Pretreated H2/CO Pretreated
Conclusions● A statistical experimental design
● reduces the number of required activity tests● shows that CO conversion of Fe/silica is significantly
influenced by reaction temperature, addition of Pt and K promoters, and pretreatment temperature
● Pretreatment atmosphere ● greatly influences activity-time behavior
● Catalyst activity is not necessarily correlated with bulk (carbide) phase compositions
● Rapid rise in activity of H2-pretreated FePtK/SiO2 during first 2-3 hours of reaction may be due to rapid carbiding of small iron clusters generated during H2 reduction
Conclusions (continued)● Intimate association between Pt promoter and Fe on the catalyst is
supported by TGA data but no FePt alloy was detected by Mossbauer spectrosocpy on any catalysts, thus Pt is probably uniformly distributed along with highly dispersed iron oxides which improves facilitates reduction by H2 spillover to the neighboring iron atoms
● Not all Pt involves in the hydrogenolysis of carbon deposits on the surface of Fe catalytic sites for lack of a intimate contact in between
Acknowledgements
• Dr. Calvin H. Bartholomew • DOE (DE-FG26-98FT40110)• Dr. Abaya K. Datye., Dr. Dragomir Bukur• George Huber • Matthew W. Stoker
THANKS!
Room-Temperature Mossbauer Parameters (in mm/sec) for Pt-Fe/CSamples 1 atm of Air, H2, or O2
Sample Pretreatment Run Condition IS2-3 IS1-4 QS2-3 QS1-4 %DFe
1.0% Pt-Fe/C Red. 5 hr., 400°C H2 0.305 0.325 0.423 0.987 54
(50 atomic% Fe) Exp. air, 25°C air 0.360 0.357 0.689 1.149 57
Exp. H2, 25°C H2 0.381 0.390 0.682 1.128 52
Exp. O2, 300°C 10 min O2 0.377 0.365 0.763 1.239 79
1.8% Pt-Fe/C red. 5 hr., 410°C H2 0.336 0.352 0.380 0.925 60
(34 atomic% Fe) Exp. air, 25°C air 0.357 0.365 0.607 1.096 56
Exp. H2, 25°C H2 0.304 0.335 0.345 0.884 60
Exp. O2, 300°C 10 min O2 0.386 0.318 0.679 1.031 68
1.0% Pt-Fe/C(25 atomic% Fe)
Prev. red., 11 hr., 500°CRed, 3 hr., 410°C
H2 0.324 0.350 0.262 0.747 56
Exp. air, 25°C air 0.311 0.344 0.271 0.858 65
Exp. H2, 25°C H2 0.317 0.353 0.282 0.826 58
Exp. O2, 300°C 10 min O2 0.287 0.365 0.182 0.844 85
9.4% Pt-Fe/C red. 7 hr., 470°C H2 0.349 0.449 0.245 0.732 47
(10 atomic% Fe) Evac. 2 hr., 560°C Vacuum 0.349 - 0.247 - -
Exp. air, 25°C air 0.336 0.372 0.226 0.682 53
Exp. O2, 300°C O2 0.329 0.330 0.221 0.928 72
Exp. H2, 25°C H2 0.316 0.358 0.189 0.785 52