Carbon/Iron Carbide Transformations in Highly Active Fe and FePt Fischer-Tropsch Catalysts during Pretreatment and Reaction

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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

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Chemical Engineering Department Brigham Young
University Provo, UT
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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

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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

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Particle Size d, Magnetic Transition Temperature
TC, and Hyperfine Field H at 77 K for 50 Atomic
Iron in Platinum
Sample Reduction Temperature d (A) TC (K) H at 77 K (kOe)
Pt-Fe Foil - Bulk 733 293
12.1 Pt-Fe/C 900C 127 600 298
1.0 Pt-Fe/C 900C 46 500 298
12.1 Pt-Fe/C 500C 30 200 316
1.0 Pt-Fe/C 500C 16 20 -
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Dispersion and Surface Composition of Pt-Fe/C
Alloy Catalysts
Total Metal wt DT DFe DPt ? ?s
After exposure to O2 at 300C, 10 min After exposure to O2 at 300C, 10 min After exposure to O2 at 300C, 10 min After exposure to O2 at 300C, 10 min After exposure to O2 at 300C, 10 min After exposure to O2 at 300C, 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 25C After exposure of reduced catalyst to air at 25C After exposure of reduced catalyst to air at 25C After exposure of reduced catalyst to air at 25C After exposure of reduced catalyst to air at 25C After exposure of reduced catalyst to air at 25C
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
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Platinum Iron Catalysts Supported on Carbon
Composition Total Dispersion DT and Average
Particle Diameter d
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.
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The Binary Phase Diagram for Pt-Fe Alloys
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Mössbauer Spectra of 50 Fe in Pt (a) foil at 298
K, (b) 1.0 Pt-Fe/C (reduced at 900C, d46 Å at
77K
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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

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Room Temperature Mössbauer Spectra for 1.0 Pt-Fe
(50/50)/C

1. After reduction in flowing hydrogen during 4
   hours at 400C and cooling to 25C in hydrogen (1
   atm)
2. After evacuation and exposure to air at 25C
3. After evacuation and exposure to hydrogen at 25C
   
4. After exposure to oxygen (160 Torr) at 300C, 10
   minutes

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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
  
• CO2H2 H2O -CH2-
• Syngas can be produced through steam methane
  reforming and gasification/partial oxidation from
  nearly any carbon-bearing feedstock
• natural gas
• coal
• Biomass

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Fischer-Tropsch Synthesis Chemistry
www.bp.com
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Advantages of Iron FTS Catalysts

• Cheaper
• Remarkable WGS activity for handling syngas with
  low H2/CO from coal
• Highly olefinic C2-C6 fraction

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Fe2O3 Fe3O4 FeCx
In situ Fe K-Edge XANES Sample 1 mg Fe2O3 CO
flow 107 mol/g-atom Fe-h TPR and Carburization
Studies in CO25 Similar result obtained in H2/CO
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Li, Iglesia, et al. (2001)
rapid rapid Fe2O3?Fe3O4?FeCx
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Iron Carbide ?-Fe2.5C is probably the active
phase
Jackson, Datye, et al. (1997)
Active surface carbon on small iron carbide
clusters of appropriate size
Zhang, OBrien., et al. (1999)
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Recent Development (Iglesia et al. 2001)

• 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

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
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Pretreatment Effects on Catalyst Activity
Reaction 265C, 150 h H2/CO 1.0 1.92 NL/g-cat/h
Pretreated 280C, 16 h
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Our Study

• Design, preparation and characterization
• Non-aqueous Evaporation Deposition Technique
• Promotion with Pt
• Statistically designed experiments for activity
  tests

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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
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H2 chemisorption, dispersion, and BET surface
area measurements
Catalyst Code Extent of Reduction at 300C () H2 Uptake (?mole/g catalyst) Dispersion () BET SA (m2/g)
Fe/SiO2 (calcined) 60 44.5 ? 3.2 8.3 ? 0.6 2422
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
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Statistically Designed Fixed Bed Run Conditions
(L18 Orthogonal Array)
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Statistically Designed Fixed Bed Runs2 NL/g-cat/h
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Correlations between Iron Carbide Content and
Catalyst Activity (after 150 h FBR run)
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Mössbauer Spectra after Different Pretreatments
and FBR Reaction (11 Fe/1.0Pt/0.9 K/SiO2)
H2/CO
H2
CO
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In-situ Mössbauer Spectra
Reacted
Fe2.5 C 25.7 Fe3O4 (sp)
74.4 Fe2 0.06
Pretreated in situ
Fe2.5 C 14.0 Fe3O4 (sp)
68.2 Fe2 7.5 Fe3O4 (FiM) 10.3
Untreated
Fe3O4 (sp) 100
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High Pressure In Situ Studies
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High Pressure In Situ Mössbauer Spectroscopy 10
Fe-1 Pt-0.2 K/SiO2
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Temperature-programmed Surface Reaction with
on-line Mass Spectrometry (TPSR-MS)
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Effect of Pretreatment Gases
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Carbon Species Transformation with Pretreatment
Time
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Pretreatment in 20 CO/He
ß
?
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Individual Peak Contributions from Previous TPSR
Spectra
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Fingerprinting FTS Catalysts during Reaction
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Isothermal Transient Measurements of FTS Rates
CO Pretreated
H2 Pretreated
H2/CO Pretreated
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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

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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

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Acknowledgements

• Dr. Calvin H. Bartholomew
• DOE (DE-FG26-98FT40110)
• Dr. Abaya K. Datye., Dr. Dragomir Bukur
• George Huber
• Matthew W. Stoker

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THANKS!
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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., 400C H2 0.305 0.325 0.423 0.987 54
(50 atomic Fe) Exp. air, 25C air 0.360 0.357 0.689 1.149 57
Exp. H2, 25C H2 0.381 0.390 0.682 1.128 52
Exp. O2, 300C 10 min O2 0.377 0.365 0.763 1.239 79
1.8 Pt-Fe/C red. 5 hr., 410C H2 0.336 0.352 0.380 0.925 60
(34 atomic Fe) Exp. air, 25C air 0.357 0.365 0.607 1.096 56
Exp. H2, 25C H2 0.304 0.335 0.345 0.884 60
Exp. O2, 300C 10 min O2 0.386 0.318 0.679 1.031 68
1.0 Pt-Fe/C (25 atomic Fe) Prev. red., 11 hr., 500C Red, 3 hr., 410C H2 0.324 0.350 0.262 0.747 56
Exp. air, 25C air 0.311 0.344 0.271 0.858 65
Exp. H2, 25C H2 0.317 0.353 0.282 0.826 58
Exp. O2, 300C 10 min O2 0.287 0.365 0.182 0.844 85
9.4 Pt-Fe/C red. 7 hr., 470C H2 0.349 0.449 0.245 0.732 47
(10 atomic Fe) Evac. 2 hr., 560C Vacuum 0.349 - 0.247 - -
Exp. air, 25C air 0.336 0.372 0.226 0.682 53
Exp. O2, 300C O2 0.329 0.330 0.221 0.928 72
Exp. H2, 25C H2 0.316 0.358 0.189 0.785 52