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

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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