Molybdenum carbide is polycrystalline powder with BET surface areas from 1 to 200 m2/g. Mo2C may substitute for precious metal catalysts in the hydrodenitrogenation, hydrogenation, water-gas shift, Fischer-Tropsch, hydro-isomerization, and methane reforming reactions, as well as use as fuel cells electrodes. The H2 producing water-gas shift (WGS) reaction is useful for fuel cells, ammonia synthesis, Fisher Tropsch, and hydrocarbon reforming. This work studies the relationships between Mo2C surface composition and its WGS activity. Metal carbide catalysts have not been widely accepted in industry due to deactivation issues. This study varies synthesis conditions, air exposure, and H2-TPR activation temperatures to generate a variety of surface compositions which are characterized using XPS prior to testing as WGS catalysts.

Molybdenum Carbide and Oxycarbide Hydrogen Production CatalystsPreparation, Characterization, and Evaluation

University of California at Santa Barbara, Departments of Chemistry and Materials

(Below) Electron micrographs of Mo2C prepared under different conditions. (a) APM precursor heated at 2˚C/min. (b) APM precursor heated at 0.2˚C/min. (c) MoO3 precursor heated at 0.2˚C/min. In A, B, and C the scale bar represents 200 nm. d) TEM lattice image of Mo2C illustrating lattice fringes from (321) planes. e) TEM image reveals nanopores in Mo2C.

Catalysts prepared under slow ramp conditions (0.2 °C/min) appear to have rougher surfaces (b) in SEM micrographs than samples brought to the final temperature rapidly (a).

Robert Savinel l i , Jun Li, Ram Seshadri, Susannah L. Scott

The reactants are a flowing mixture of 3.0% CO and 4.5% H2O balanced by Ar. The reaction occurs at a packed Mo2C catalyst bed (ca. 0.08 cm3)supported between two quartz wool plugs. Gas flow rates of 20, 40, or 60 mL/min correspond to space velocities between 15,000 and 45,000 hr-1.

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80 160 240 320 400 480 560 640

1 2 3 4 5

Time (hr)

MS

Sig

nal

Wei

ght l

oss

(%)

Temperature (oC)

CH4 (m/e=16)

NH3 (m/e=17)

H2O (m/e=18)

CO (m/e=28)

CO2 (m/e=44)

H2 (m/e=2)

Component B.E. (eV) FWHM Mo(II) 228.20 0.59Mo(IV) 229.25 1.22Mo(V) 230.80 1.18Mo(VI) 232.60 1.28

Water-gas shift (WGS) performance varies with surface area, crystallite size, and surface contamination (carbon and oxide deposits). Low activity Mo2C catalysts can form during temperature programmed carburization (TPC) if the maximum temperature is above 700°C and the temperature ramp is slow, (≥0.5°C/min). These conditions are conducive to the formation of larger Mo2C crystallites and thick graphitic carbon layers.

Understanding the role of carbon deposition on the deactivation of Mo2C catalysts is critical for improving the overall viability of transition metal carbide catalysts. Many industrial catalysts are also deactivated by carbon deposition. Advances in surface carbon analysis will benefit these heterogeneous systems as well.

In the water-gas shift reaction, oxygen atoms “shift” from water to carbon monoxide. The energy barrier for this reaction is decreased in the presence of Mo2C. Catalysts 1-6 were evaluated for catalytic activity before and after activation with H2 at 600 ˚C for 1 h and 825 ˚C for 0.5 h.

Introduction

H2O + CO H2 + CO2

Temperature Programmed CarburizationXRD and BET

Surface Structure by XPS

d) e)

SEM and TEM Images

(Left) XPS survey and C 1s spectra of catalysts 1, 2, and 3 as prepared, after H2-TPR-600 °C, and after H2-TPR-825 °C. The O atomic percent (at%) is initially low in catalysts 1and 3. The O at% decreases in catalyst 2 after H2-TPR at 600 °C. The O at% increased in all of the catalysts after H2-TPR at 825 °C. This must be due to the exposure of sub-surface oxides and oxycarbides upon removal the graphite layer. Depth profiles of many Mo2C powders prepared by TPC (below) indicate that the carbon at% is higher at shallow probe depths. The C at% is initially high in catalysts 3. It decreases in all spectra after H2-TPR at 825 °C. The near surface carbidic carbon (C-Mo) is highest in catalyst 1. Oxycarbide (C-Mo-O) is highest in catalyst 2. The C-Mo-O content decreased in catalyst 2 and increased in catalyst 3 after H2-TPR. Amorphous carbon was formed from graphite during the H2-TPR at 825 °C of catalyst 3.

(Above) Greater concentrations of carbon atoms were detected closer to the catalyst surface using XPS depth profiling based on Mo 3s (506.3 eV), Mo 3d (228.2 eV), and C 1s (284.8 eV) photoelectron intensities. The average C : Mo ratio is 2.46 ± 0.18 times larger at Mo 3s analysis depths.

Hydrogen Production

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640

42

44

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2 3 4 5 6 7 8 9 Mo2C*

C 1s : Mo 3s

C 1s : Mo3d

C:M

o ra

tio

Sample

(Above) Representative high-resolution C 1s spectra of castalysts as prepared. Feature A is a combination of amorphous and graphitic carbon. Feature B is carbidic carbon. Catalyst 3 is similar to the C 1s spectrum for pure graphite. The C 1s spectra of catalyst 2 is similar to catalyst8 and to the commercial Mo2C. * 7 and 8 are not described in this poster.

(Above) Deconvoluted high-resolution C 1s spectra of catalysts 4, 5, and 6 (as prepared). C=O, and C-O are carbonate and carbonyl components, a-C-C is sp3

amorphous carbon, g-C-C is sp2 graphitic carbon, C-Mo-O is oxycarbidic carbon, and C-Mo is carbidic carbon. Catalyst 4 has the most a-C-C (64%).

(Left) Representative high-resolution O 1s spectra of Mo2C catalysts. (Below) O 1s binding energy assignments.

(Left) Representative high-resolution Mo 3dspectra of Mo2C catalysts. Mo(II) is the most prevalent molybdenum oxidation state. Mo(VI) is detected in sample 1 and is the dominant oxidation state in the commercial Mo2C.

(Right) The lid from the air free sample stage is removed under vacuum inside the XPS transfer chamber. Powders are mounted in a nitrogen glove box.

Mo2C was prepared from ammonium paramolybdate heated to temperatures between 630 °C and 700 °C in a flowing (1:1) mixture of hydrogen and methane. This carbon-rich mixture was selected to increase surface carbon deposition. The carb-urization process was monitored by thermogravimetric analysis (TGA) and by mass spectrometry (MS) of the gas phase products. Typical TPC-MS and TGA results are shown to the right. TGA weight loss is shown as a red line with brown arrows. Intermediate phases are to the right of the TPC-MS plot.

Water-gas shift equation

(Above) Quartz microreactor(Hiden) inside a N2 glovebox. It is connected to online mass spectrometer and was used during TPC, H2-TPR, and WGS.

Catalysts were evaluated for water-gas shift (WGS) activity in a quartz tube micro-reactor, monitoring CO, CO2, H2 and H2O concentration by MS before and after activation in pure H2 at 600 °C and 825 °C for 1 h.

Catalysts 1 and 2 were prepared using the same TPC recipe. Catalyst 1 was passivated in 1% O2/N2 prior to air exposure. Catalyst 2 was not. As evidenced by the XRD patterns, catalysts 1-4 have similar bulk. Catalyst 5 contains bulk phases of both Mo2C and MoO2. Catalyst 6 was treated with H2 at 675 °C for 2 h and was passivated in 1% O2/N2 for 4 h. Catalyst 6 is more crystalline than the other catalysts though its domains are smaller than theMo2C standard obtained from Aldrich.

650°C0.2°C/min

650°C

700°C

630°C

640°C

675°C

0.2°C/min

0.5°C/min

0.5°C/min

2.0°C/min

1.0°C/min

5 h

5 h

1 h

1 h

0.15 h

5 h

Temperature Programmed Reduction

(NH4)6Mo7O24·4H2O

MoO3

Mo2C

MoO2

(NH4)6Mo7O2

4(NH4)2Mo4O1

3

100 200 300 400 500 600 700 800

CH4 (m/e=16) x 25

H2O (m/e=18) x 50

CO (m/e=28) x 200

CO2 (m/e=44) x 400

Temperature (°C)

MS

Sig

nal (

a.u.

)

100 200 300 400 500 600 700 800Temperature (°C)

MS

Sig

nal (

a.u.

)

CH4 (m/e=16) x 10

H2O (m/e=18) x 5

CO (m/e=28) x 10

CO2 (m/e=44) x 60

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MS

Sig

nal (

a.u.

)

CH4 (m/e=16) x 10

H2O (m/e=18) x 15

CO (m/e=28) x 20

CO2 (m/e=44) x 250

H2-TPR Catalyst 1, pure H2 (25mL/min), 825 °C, 0.5 h, 10 °C/min ramp

Water-gas shift Catalysis

TPC pathway

Mo2C surfaces are covered with carbon and oxygen atoms that were present during carburization. These contaminates inhibit catalytic performance. The procedure for removing surface contaminates, prior to catalysis, is temperature programmed reduction in flowing H2 (H2-TPR) at temperatures ≥600°C. H2-TPR results depend on the composition of surface contaminates. MoOxand amorphous carbon contamination are reduced to CH4 and H2O between 500 °C and 700 °C. The Mo2C catalyzed reduction of graphitic carbon is observed between 750 °C and 825 °C. Broad methane desorption peaks in catalysts 1 and 2 (above) are attributed to the presence of amorphous and graphitic carbon phases. The surface carbon on catalyst 3 is almost entirely graphitic carbon. H2-TPR at 600 °C has little effect on graphitic carbon.

CO2 and CO levels were also measured in an FTIR gas flow cell (above).

0

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1.5

200020502100215022002250230023502400

298 C250 C200 C153 C101 C25 C

Abso

rban

ce

wavenumbers (cm -1)

°°°°°

°

CO2

CO

35 45 55 65 75

2θ/degrees Cu-Kα1

Mo2C Aldrich

MoO 2

Strem

1

2

3

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nsity

(a.u

.)

5

4

6

Catalyst

Surface aream2/g

XRD Crystallitediameter

41 13 nm

57 14 nm

* 17 nm

* 18 nm

29 13 nm

* 26 nm

0.7 46 nm

16 nm

12 nm

23 nm

950 nm

BET particlediameter

600 ˚C

The activity of Mo2C catalysts are influenced by changes in synthesis, handling, activation, and reaction conditions due to changes in the surface structure and composition. Evidence of these changes should be detected by XPS, BET, SEM, and TEM. XRD

Powders were mounted on conductive carbon tape. High binding energy resolution was achieved using a monochromatic Al anode, an 8-channel hemispherical detector set to low pass energies (above). Samples were referenced to the Fermi level.

H2-TPR Catalyst 2, pure H2 (25mL/min), 825 °C, 0.5 h, 10 °C/min ramp

H2-TPR Catalyst 3, pure H2 (25mL/min), 825 °C, 0.5 h, 10 °C/min ramp

Heating Profiles

Synthesis Procedure (TPC)Temperature (630 °C - 700 °C)Ramp rate (0.2 – 2 °C/min)Soak time (0.15 – 5 h)HandlingAir exposure (rapid, none, 1% O2)Activation (H2-TPR)Temperature (600 °C or 825 °C)Reaction flow rate (20, 40, or 60 mL/min)

Carbon XPS standards

Hypothesis

is a supplement to these techniques as it sensitive to changes in the bulk structure. The conditions to the right are varied in order to elucidate the catalyst activation and deactivation pathways.

(Above) Cartoon of phases that may be present near the Mo2C surface. The XPS technique provides an average from the X-ray beam footprint (15-110 µm) and the information depth (5-10 nm).

XPS spectra of catalysts ramped slowly have a higher ratio of graphite (g-C-C) to amorphous (a-C-C) carbon near the surface than those heated rapidly. BET surface areas were between 1 and 57 m2/g. The surface area of samples with more a-C-C and g-C-C were lower indicating that elevated surface areas are a result of the Mo2C structure and not the surface carbon. XPS spectra indicate that there is an increase in g-C-C after air exposure.

As prepared

600 °C

825 °C

Cou

nts

As prepared

600 °C

825 °C

Cou

nts

As prepared

600 °C

825 °C

Cou

nts

As prepared

600 °C

825 °C

59%

56%

35%

7%

7%

21%

at% O C

600 °C

825 °C

48%

54%

33%

26%

12%

23%

at% O C

As prepared

600 °C

82%

80%

41%

6%

7%

31%

at% O C

As prepared

825 °C

Catalyst 1

Catalyst 2

Catalyst 3

A B

282283284285286287288289290B.E. (eV)

Cou

nts

Aldrich Mo2C

3

8

graphite

5

1

7

1.31288.76C=O

1.36286.21C-O

1.00284.77a-C-C

0.59284.26g-C-C

0.63283.81C-Mo-O

0.65283.31C-Mo

FWHMB.E. (eV)Component

C 1s

O-H O

-C-M

o

O=C

O-M

o

O-C

55 2 85 3 05 3 25 3 45 3 6

B .E . (e V )

7

5

1

3

1.59536.18O-H1.57533.70O=C1.34532.37O-C1.44531.49O-C-Mo1.29530.14O-Mo

FWHMB.E. (eV)Component

O 1s

Cou

nts

Mo 3dB.E. Assignment

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graphitic carbon

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amorphous carbon

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carbide carbon

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Oxygen

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Molybdenum

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Carbon

(Right) C 1s XPS. A decrease in gra-phitic carbon (g-C-C) is observed after H2-TPRat 825 °C.

(Right) C 1s XPS. The amorphous carbon (a-C-C) de-creases after H2-TPR at 600 °C. The change in a-C-C is mixed after H2-TPR at 825 °C

(Right) C 1s XPS. An increase in ca-rbidic carbon (C-Mo) is observed after H2-TPR at 600 °C and 825 °C.

(Above) C 1s XPS spectra of graphite, carbon black , and activated carbon. The B.E. and FWHM of pure graphite is used for the g-C-C component.

(Left) Particles sizes are estimated from surface areas using the spherical approximation and from XRD broadening using the Scherrer equation. * indicates that BET results were unavailable. Post H2-TPR XRD patterns were also unavailable.

Survey 160 eV 2 0.35 eV 100 msC 1s 10 eV 15-20 0.1 eV 400 msMo 3d, O 1s 20 eV 3-4 0.1 eV 400 ms

Spectra pass energy sweeps steps size dwell time

CO Consumption

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150 200 250 300 350 400 450Temperature (oC)

mol

%

H2O (m/e=18)

CO (m/e=28)

CO2 (m/e=44)

H2 (m/e=2)

Catalyst 5

(Above) Typical water–gas shift reactant and product gases vs. temperature plot. CO conversions at 250 °C are between 30 and 80%. MS plots are corrected for fluct-uations in total pressure and m/e 28 signal contributions from CO2 fragments. The CO signals are reduced by 11% of m/e 44 signal.

Cou

nts

Cou

nts

Cou

nts

Water-gas shift reaction monitored by MS using (catalyst 1, 98 mg, left) Mo2C and (catalyst 2, 107 mg, right) in flowing 3% CO/Ar 3-4% H2O (20, 40, 60 mL/min) ramped to 450 ˚C at 5 ˚C/min. Catalyst was treated for one hour at 600 ˚C or 825 ˚C under flowing H2 20mL/min. Upward triangles (∆) indicate an increasing temperature.

Water-gas shift reaction monitored by MS using (catalyst 3, 92 mg, left) Mo2C and (catalyst 4, 106 mg, right) in flowing 3% CO/Ar 3-4% H2O (20 or 40 mL/min) ramped to 450 ˚C at 5 ˚C/min. Catalyst was treated for one hour at 600 ˚C or 825 ˚C under flowing H2 20mL/min. Upward triangles (∆) indicate an increasing temperature.

Water-gas shift reaction monitored by MS using (catalyst 5, 75 mg, left) Mo2C and (catalyst 6, 99 mg, right) in flowing 3% CO/Ar 3-4% H2O (20 or 40 mL/min) ramped to 450 ˚C at 5 ˚C/min. Catalyst was treated for one hour at 600 ˚C under flowing H2 20mL/min. Upward triangles (∆) indicate an increasing temperature.

282284286288290

graphite(Alfa)

CP

S

B.E. (eV)

g-C

-C

C-O

a-C

-C

282284286288290

CP

S

B.E. (eV)

g-C

-C

C-O

a-C

-C

carbonblack

(Strem)

0.54eV

96%

0.68eV

82%

282284286288290

CPS

B .E. (eV )

g-C

-C

C-O

a-C

-C

ac tivated carbon(S trem )

0.98eV

54%

% sp2

graphitic carbon

Manifold FWHM

(Above) Survey XPS trends. Changes in near surface atomic composition due to H2-TPR at 600 °C and 825 °C. Mo 3d at% increases with H2-TPR. O 1sat% increases when the graphitic carbon layer is removed at temperaures above 750 °C. C 1s at% decreases slightly after 600 °C and significantly at 825 °C.

The overall performance of Mo2C WGS catalysts are enhanced by H2-TPR. CO consumption in catalysts 1 and 2 are highest when activated at 600 °C. In general, CO consumption increases with gas flow rate. The activity of catalyst 3 was unaffected by H2-TPR at 600 °C due the high g-C-C coverage (56%) remaining after H2-TPR. The activity of catalyst 3 increased after H2-TPR at 825 °C due to the decrease in g-C-C (5%). CO consumption measured with catalysts 4 and 5 increase after H2-TPR 600 °C. Catalysts 5 and 6 had appreciable activity without activation due to low a-C-C and g-C-C content (<32% combined).

In general, WGS activity was higher during reactor cooling. This indicates that another catalyst activation pathway is present during the reaction. A possible explanation for this observation is the formation of more active oxycarbide sites during water-gas shift.

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n (µ

mol

CO

sec

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-1)

as prepared20mL

600oC-20mL

Catalyst 5

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CO

sec

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-1)

as prepared-20mL

Catalyst 6

as prepared-40mL

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as prepared-20mL600oC-40mL

825oC-40mL

Catalyst 3

Temperature (oC)

CO

Con

sum

ptio

n (µ

mol

CO

sec

-1 g

-1)

0.0

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150 200 250 300 350 400 450 500Temperature (oC)

CO

Con

sum

ptio

n (µ

mol

CO

sec

-1 g

-1)

as prepared20mL

600oC-20mL

Catalyst 4

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CO

Con

sum

ptio

n (µ

mol

CO

sec

-1 g

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as prepared20mL

825oC-20mL

600oC-20mL

825oC-40mL

Catalyst 1

0.0

1.0

2.0

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6.0

100 150 200 250 300 350 400 450 500Temperature (oC)

CO

Con

sum

ptio

n (µ

mol

CO

sec

-1 g

-1)

as prepared-20mL

825oC-20mL

600oC-60mL

600oC-40mL825oC-60mL

600oC-20mL

825oC-40mL

Catalyst 2

XPS survey XPS C 1s

7 . 4 5 e V

22 823 023 223 423 6B .E . (e V )

Cou

nts

Aldrich Mo2C

5

7

1

(IV) (II)(V)(VI)Mo 3d 5/2

Results and ConclusionsCarbon deposition is the major cause of Mo2C catalyst deactivation. The type and quantity of carbon, blocking the active Mo2C surfaces, depend on synthesis temperature, ramp rate, soak time, and air exposure. Catalysts prepared at higher temperature have higher levels of carbon contamination. Slow heating rates result in rough surfaces with higher levels of graphite at the surface. Exposure to air increased

Mo2C Mo2C

Inactive active

H2-TPR

(Above) cartoon of catalyst activation by temperature programmed reduction in H2.

the amount of graphitic and oxycarbidic carbon. H2-TPR temperatures above 750 °C were required to remove graphite deposits. BET surface areas were higher for catalysts with more carbidic carbon atoms near the surface. WGS activity was highest in air free catalysts which were slowly heated to 650 °C followed by H2-TPR at 600 °C.

Mo 3d O 1s C 1s

a-C-C

g-C-C

C-Mo