THE EFFECT OF REFORMING GASES ON THE STABILITY OF … · 2002. 7. 22. · Anna Rini Sekar Darujati,...

i

THE EFFECT OF REFORMING GASES ON THE STABILITY

OF MOLYBDENUM CARBIDE CATALYSTS

By

ANNA RINI SEKAR DARUJATI

A thesis submitted in partial fulfillment of

the requirements for the degree of

MASTER OF SCIENCE IN CHEMICAL ENGINEERING

WASHINGTON STATE UNIVERSITY Department of Chemical Engineering

August 2002

ii

To the Faculty of Washington State University:

The members of the Committee appointed to examine the thesis of ANNA RINI

SEKAR DARUJATI find it satisfactory and recommend that it be accepted.

______________________________ Chair

______________________________

______________________________

iii

ACKNOWLEDGEMENT

I would like to thank to my great advisor Dr. William J. Thomson for his encouragement

and advice through this course of study and for passing his knowledge to me in one of the

most exciting research areas. I would also like to thank to Dr. Anand Chellappa for his

valuable guidance during initial period of this research. Dave LaMont, my great friend

and colleague for all dynamic discussion on research and life. And finally, to my father

and my mother who deserve my greatest appreciation for their endless support and

encouragement. This thesis is dedicated to their patience and hard work for many years.

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THE EFFECT OF REFORMING GASES ON THE STABILITY

OF MOLYBDENUM CARBIDE CATALYSTS

Abstract

Anna Rini Sekar Darujati, M.S.

Washington State University

August 2002

Chair: William J. Thomson

The oxidation stability of a low surface area Mo2C catalyst has been studied in the

presence of gases associated with the steam and dry (CO2) reforming of methane, at

temperatures up to 850°C and pressures to 8 bar. The oxidation onset temperatures were

found to be about 600°C when the carbide was exposed to either steam or CO2. There

appears to be two distinct mechanisms for Mo2C oxidation: direct oxidation at low

temperatures and thermal decomposition followed by oxidation of the Mo metal at

temperatures above 750°C. Although onset temperatures were similar, CO2 was a

stronger oxidant than steam at the higher temperatures. Both H2 and CO were found to

inhibit oxidation and the effect can be explained by their influence on the prevailing

kinetics. Trace concentrations of methane were found to completely stabilize the carbide

from steam oxidation but significantly higher concentrations were required to stabilize it

from CO2 oxidation and this is attributed to the higher rates of CO2 oxidation relative to

v

carburization rates. The effect of pressure on the onset temperature of CO2 oxidation of

the carbide was found to be negligible, even when inhibited by CO.

vi

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENT………………………………………………………………..iii

ABSTRACT……………………………………………………………………………...iv

LIST OF TABLES……………………………………………………………………...viii

LIST OF FIGURES………………………………………………………………………ix

CHAPTER

1. INTRODUCTION……………………………………………………………1

2. EXPERIMENTAL……………………………………………………………4

2.1. Catalyst Characterization………………………………………………...4

2.2. Experimental Procedures………………………………………………...4

3. RESULTS.…………………….……………………………………………...8

3.1. The Thermal Decomposition of Mo2C ………………………………….8

3.2. Oxidation of Mo2C in Steam…………………………………………….10

3.2.1. Oxidation of Mo2C in Steam/H2…………………………………13

3.2.2. Oxidation of Mo2C in Steam/CH4……………………………….16

3.3. Oxidation of Mo2C in CO2……………………………………………...17

3.3.1. Oxidation of Mo2C in CO2/CO………………………………….19

3.3.2. Oxidation of Mo2C in CO2/CH4…………………………………20

3.4. Oxidation of Mo2C in CO2/CO at Elevated Pressure……………………23

4. DISCUSSION AND CONCLUSIONS...……………………………………25

vii

BIBLIOGRAPHY………………………………………………………………………..29

APPENDICES…………………………………………………………………………...30

A. EXPERIMENTAL EQUIPMENT…………………………………………...31

B. RUN CATALOGS…………………………………………………………...35

C. CATALYST CHARACTERIZATION……………………………………...45

viii

LIST OF TABLES

Page

1. Feed gas compositions of oxidation of Mo2C experiments in various gases………..….6 2. The last stable temperature of Mo2C in oxidation of Mo2C in steam/CH4 and

CO2/CH4………………………………………………………………………………23

3. Comparison of the last stable temperature and normalized MoO2 area at 825°C for

oxidation of Mo2C in CO/CO2 at 1 and 8 bars………..………………………………24

ix

LIST OF FIGURES

Page

1. In-situ DXRD scans (Co Kα radiation) of the Aesar Mo2C and the Mo metal……….10 2. Equilibrium constants for reactions (3) – (6) and (9)…………………………………12 3. Normalized area of Mo2C and MoO2 during oxidation of Mo2C in steam…………....13

4. Normalized area of MoO2 during oxidation of Mo2C in steam/H2 at PH2O = 0.25 atm.15

5. Normalized area of Mo during oxidation of Mo2C in steam/H2 at PH2O = 0.25 atm….16

6. Normalized area of MoO2 during oxidation of Mo2C in CO2………………………...18

7. Comparison of normalized MoO2 area during oxidation of Mo2C in CO2 and H2O….19

8. Normalized area of MoO2 during oxidation of Mo2C in CO2/CO at PCO2 = 0.25 atm..20

9. Normalized MoO2 area during oxidation of Mo2C in CO2/CH4 at PCO2 = 0.25 atm …21

10. Stability of Mo2C in H2/H2O and CO/CO2 at Poxid = 0.25 atm………………………22

A1. Schematic diagram of DXRD system……………………………………………….33

A2. Vaporizer unit……………………………………………………………………….34

B1. Average peak area of MoO2 during oxidation of Mo2C in CO2……………………..44

C1. XRD scan at 40°C of Aesar Mo2C catalyst used in this study………………………45

1

1. Introduction

The vast potential of fuel cells and their appealing benefits as an alternative

energy source, such as high efficiency and near zero emissions, have increased interest in

finding new catalysts for producing hydrogen from a variety of fuels. Two catalysts that

have been used primarily for fuel reforming are Ni and noble metals. However, the low

resistance of Ni catalysts to carbon formation and the high cost of the noble metal

catalysts, provide incentives for alternative catalyst investigations.

High surface area metal carbides, particularly of the group VI transition metals,

have been reported to possess catalytic properties comparable to those of noble metal

catalysts for a variety of reactions [1-9]. York et. al. [6], using temperature programmed

reaction (TPR), synthesized high surface area carbide catalysts and reported that the

carbides of molybdenum and tungsten were active catalysts for dry methane reforming

(DMR), steam-methane reforming (SMR) and the partial oxidation of methane under

stoichiometric feed conditions at 8 bar without any signs of catalyst deactivation.

However, they reported that the molybdenum carbide catalyst deactivated at atmospheric

pressure due to oxidation, forming MoO2 after about 3 hours on stream. They suggested

that the longer residence time at the higher pressure, which leads to the increased

exposure of the catalyst to carburizing gases, contributed to the stability of the carbide

catalysts at elevated pressure.

Recently, Rostrup-Nielsen et. al. [10] demonstrated that high surface area

molybdenum carbides were stable for 22 hours with 85% conversion under DMR

conditions at 8 bar with excess CO2 (CO2/CH4=1.7) in a single pass, packed bed reactor.

2

However, XRD scans of the post reacted catalysts indicated that the carbide was partly

oxidized, with oxidation more prevalent at the reactor inlet; presumably due to exposure

to net oxidizing conditions in that region. The oxidation of Mo2C by CO2 to form MoO2

was hypothesized to be faster than the carburization of MoO2 with CH4 to form Mo2C.

Therefore, they concluded that the carbide stability was due to the reaction of CO with

MoO2 to form Mo2C, and these kinetics were faster at higher pressures. They evaluated

this hypothesis by operating at low pressures, but with a recycle stream that produced

“back-mix” behavior. The carbide catalyst was found to be stable in this back-mix mode,

for about 60 hours at high conversion with a 1.17 feed ratio of CO2/CH4. However, it

deactivated at longer times on stream, and since the spent catalyst was found to be pure

Mo2C, the deactivation was attributed to the loss of surface area.

Since low surface area molybdenum carbides have been considered to be the main

obstacles for catalytic applications, most of the previous research in this area has focused

on the synthesis of high surface area catalysts [11-15]. However, LaMont et. al. [16]

reported that the activity of Mo2C in SMR was independent of the surface area. They

showed that with a slight excess methane, a low surface area molybdenum carbide

catalyst was stable for 96 hours at SMR equilibrium conversion conditions. By

comparison, high surface area molybdenum carbides catalysts, synthesized by two

different methods (TPR and solution derived), deactivated after 47 hours under identical

conditions. The lower stability of the high surface area molybdenum carbides was

attributed to the excess carbon deposited on the catalysts during synthesis. It was

proposed that this mesoporous carbon was not only responsible for the high surface areas,

but also provided nucleation sites which enhanced methane cracking, leading to the

3

deposition of low surface area carbon which blocked active sites, thereby causing

deactivation. They also suggested that the presence of this excess carbon could also lead

to carbide oxidation when the carbon competed with active sites for CO2 and H2O.

This prior work suggests that the stability of carbide catalysts under fuel

reforming conditions may be related to redox chemistry and this, in turn, has prompted

the present study, which is directed at the determination of conditions under which

carbide oxidation can be prevented. Specifically, the stability of low surface area Mo2C

has been investigated as a function of temperature in gaseous environments containing

the constituent gases typically encountered in fuel reforming; viz., CH4, H2, CO, CO2 and

steam. In addition, separate experiments have also been conducted to evaluate the thermal

stability (decomposition) of low surface area Mo2C, when exposed to high temperatures

in the presence of inert gases, with and without H2. These results are also compared to

selected experiments at 8 bars, in order to evaluate the hypothesis that higher pressures

increase resistance to oxidation.

4

2. Experimental

2.1. Catalyst Characterization

The molybdenum carbides used in this study were commercial Alfa Aesar Mo2C

lot# K17J11 (99.5% metals purity, <325 mesh). The bulk “Aesar” Mo2C catalyst was

characterized by X-Ray powder diffraction and shown to be pure β-Mo2C (hcp) with an

average crystallite size of 40 nm as estimated by the Debye-Scherrer method. The BET

surface area of the fresh Aesar catalyst (Coulter BET Analyzer) was found to be less than

5 m2/g and the carbon content of the Aesar Mo2C was calculated to be 6.200 ± 0.001

wt.%, slightly higher than the stoichiometric value of 5.9%.

2.2. Experimental Procedures

All of the experiments were conducted in-situ using a Phillips X’Pert XRD

System (Co Kα source), operated dynamically (DXRD) and equipped with a RAYTECH

Position Sensitive Detector. The hot stage was an Anton Parr XRK 900, capable of

operating at temperatures up to 900°C, pressures to 10 bar and with flow-through gases

so that it could be used to dynamically monitor the crystalline changes of the catalyst

under actual reforming conditions. In all cases, two high-resolution XRD scans at 45 – 49

and 27.5 – 32.5 °2Θ, were used to monitor changes in the Mo2C and MoO2 phases,

respectively. The short scan between 45 – 49 °2Θ was also used to identify Mo metal

phase formation which is located at 47.4 °2Θ. Typically, 0.4 grams of Mo2C were loaded

5

on the sample holder and was pretreated in 42 SCCM of UHP H2 (99.999%) at 700°C for

one hour to remove any oxygen impurities prior to performing experiments. The outlet

gas compositions were monitored by a Shimadzu GC 14A equipped with a Hayesep D

column operating at 60 to 175°C. A bubble flow meter was used to measure the flow

rates of exit gases. The flow of the feed gases was controlled by Brooks model 5850E

mass flow controllers.

The carbide catalysts were exposed to 5% and 25% of either steam or CO2,

diluted in pre-purified N2 (99.998%) to test the carbide stability under oxidizing

environments. Water was delivered to a steam generator, using a syringe pump and was

premixed with the other gases in a stainless-steel heated line before introducing the

mixture to the chamber. A total gas flow rate of 38 SCCM was used for all experiments.

After H2 pretreatment, the carbide catalyst was cooled to 40°C and then exposed to pre-

purified N2 while heating to 500°C at 10°C/minute, at which point, either steam or CO2

was introduced into the chamber and the temperature was raised to 600°C. The

temperature was then increased to 850°C, in 25°C increments, with XRD scans taken

every 25°C. The effect of adding H2 and carburizing gases (CH4 and CO) on the

oxidation of Mo2C in steam and in CO2 was also studied, using identical experimental

protocols. The feed gas compositions for all oxidation experiments are shown in Table 1.

6

Table 1. Feed Gas Compositions of Oxidation of Mo2C Experiments in Various Gases (P = 1 atm, Total flow rate = 38 SCCM)

Experiments C[H2O] (%)

C[CO2] (%)

C[CO] (%)

C[H2] (%) C[CH4] (%)

Oxidation in steam 25 5 Oxidation in steam/H2 25 5 25 8.5 25 10 25 12 25 16 Oxidation in steam/CH4 25 5 25 3 25 1 Oxidation in CO2 25 5 Oxidation in CO2/CO 25 5 25 10 25 15 25 20 Oxidation in CO2/CH4 25 3 25 5 25 8

Thermal stability experiments were conducted using 100% pre-purified N2 with a

total flow rate of 38 SCCM. DXRD scans were initiated at 600°C and the temperature

was then raised in 50°C increments, to 850°C. Decomposition was monitored by

following the formation of the XRD peak corresponding to the Mo metal phase (47.5

o2Θ). The effect of H2 partial pressure on the stability of Mo2C was determined in a

similar manner, but by also holding at 850°C for 4 hours, using gas compositions of 5%

H2 in N2 and 50% H2 in N2.

7

A limited set of CO2 oxidation experiments were also conducted with the same

protocol at a total pressure of 8 bar. These experiments were designed to evaluate the

hypothesis of York et. al. [6], that higher pressures result in increased resistance to

carbide oxidation.

8

3. Results

3.1. The Thermal Decomposition of Mo2C

At the high temperatures required for methane reforming, the thermal

decomposition of Mo2C:

Mo2C 2Mo + C (1)

is thermodynamically favored and has been observed in previous studies [5,16-18]. The

DXRD results for a thermal decomposition experiment carried out in inert N2 are shown

in Figure 1. As can be seen, the decomposition of Mo2C in an inert atmosphere, occurs at

temperatures above 750°C and is similar to earlier experiments [16] carried out with a

high surface area Mo2C which had a smaller crystallite size (20 nm vs. 40 nm).

Under reforming conditions, hydrogen concentrations would be high and

consequently decomposition experiments were also carried out under H2 pressures of

0.05 and 0.5 atm. In both cases the carbide was stable at temperatures up to 850°C and

for as long as 4 hours at that temperature. This is in disagreement with the results of

Ledoux et. al. [5] and Nagai et. al. [17], who observed decomposition of high surface

area Mo2C catalysts in 1.0 atm of H2, at 650 and 800°C, respectively. However, the

difference in the analytical technique could contribute to this discrepancy. Both Ledoux

and Nagai used XPS to determine the occurring of decomposition of Mo2C in H2. While

the reduction of Mo2C in H2 to form Mo took place on the surface, the bulk of Mo2C

9

could be remained stable. Given that the following reaction can also occur in the

presence of H2:

Mo2C + 2H2 2Mo + CH4 (2)

and that its equilibrium constant is 1.35 x 10-4 atm-1 (825°C), ppm levels of CH4 could

easily prevent decomposition. In fact, a separate study of the sintering of low surface area

Mo2C [19] found that all samples were stable up to 1500°C in H2 and Mo metal was only

detected in a vacuum, lending credence to the suggestion that trace quantities of methane

are able to prevent decomposition. Interestingly, Leclercq et. al. [18] carried out

experiments with a low surface area Mo2C catalyst (9 m2/g) Mo2C and found it to be

stable in 1.0 atm of hydrogen for 15 hours at 700°C.

10

0

10

20

30

40

50

45.0 45.5 46.0 46.5 47.0 47.5 48.0 48.5 49.0

600650

700750

800850

Inte

nsity

[Cou

nts/

s]

Angle [o2Θ]

Tem

pera

ture

[o C]

Mo

Mo2C

Figure 1. In-situ DXRD scans (Co Kα radiation) of the Aesar Mo2C phase at 2Θ = 46.2° and the Mo metal phase at 2Θ = 47.4° during the decomposition of Mo2C in N2.

3.2. Oxidation of Mo2C in Steam

With Mo2C in a steam-methane reforming environment, any number of redox

reactions can occur. For example,

Oxidation of Mo2C in steam:

Mo2C + 5H2O 2MoO2 + CO + 5H2 (3)

11

Carburization of MoO2 by CO:

2MoO2 + 6CO Mo2C + 5CO2 (4)

Carburization of MoO2 by methane:

2MoO2 + 5CH4 Mo2C + 4CO + 10H2 (5)

Reduction of MoO2 by H2:

MoO2 + 2H2 Mo + 2H2O (6)

Steam-methane reforming (SMR):

CH4 + H2O CO + 3H2 (7)

Water-gas Shift Reaction (WGSR):

CO + H2O CO2 + H2 (8)

The equilibrium constants corresponding to the most significant of these reactions have

been calculated between temperatures of 427 – 927°C, using standard thermodynamic

data [20], and are presented below in Figure 2.

The effect of steam partial pressure on the oxidation of Mo2C was studied by

exposing Mo2C to 5% and 25% steam diluted in N2, while the temperature was raised in

25°C increments. The oxidation of Mo2C was monitored by following the formation of

the MoO2 peak and the results in terms of the peak areas of Mo2C and MoO2, normalized

to the Mo2C peak at 600°C, are shown in Figure 3. As can be seen, in both 5% and 25%

steam, the last temperature at which Mo2C was stable was 600°C, and thus it appears that

the initial onset of oxidation of Mo2C is not affected by steam partial pressure.

12

1000/T [1/K]0.80.91.01.11.21.31.41.5

K eq

1e-101e-91e-81e-71e-61e-51e-41e-31e-21e-11e+01e+11e+21e+31e+41e+51e+61e+71e+81e+91e+10

Temperature [oC]400 500 600 700 800 900

(5)

(9)

(6)

(3)

(4)

Figure 2. Equilibrium constants for reactions (3) – (6) and (9).

Neither the Mo metal phase or the MoO3 phase was observed in these experiments,

indicating that Mo2C was directly oxidized to MoO2 in steam, via equation (3). Although

it is possible for the MoO3 phase to melt/vaporize at high temperatures, its pure-phase

melting point is 795°C, well below the initial oxidation temperatures observed here.

13

0.84

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

600 625 650 675 700 725 750 775 800 825 850

Temperature [oC]

Nor

mal

ized

Mo 2

C A

rea

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Nor

mal

ized

MoO

2 Are

a

Figure 3. Normalized area of Mo2C and MoO2 during oxidation of Mo2C in steam; ( ) Mo2C in 5% steam; ( ) Mo2C in 25% steam; ( ) MoO2 in 5% steam; ( ) MoO2 in 25% steam. Connecting lines added for clarity.

3.2.1. Oxidation of Mo2C in Steam/H2

Since H2 is a product of reaction (3), the effect of H2 on the oxidation of Mo2C in

steam was studied by exposing Mo2C to H2/H2O ratios of 0 - 0.64, while maintaining a

constant steam feed pressure of 0.25 atm. The normalized MoO2 and Mo peak areas that

were formed during oxidation of Mo2C in steam/H2 are plotted in Figures 4 and 5. It

should be noted that the Mo and MoO2 peak areas have been corrected for their relative

diffraction intensities, so that a direct quantitative comparison between the two can be

14

made. As can be seen, the addition of H2 increases the Mo2C oxidation resistance to

steam, with the oxidation onset temperature of Mo2C increasing as the H2/H2O ratio

increased. In fact, there were no signs of MoO2 at 850°C in H2/H2O =0.64. However, as

can be seen in Figure 5, the Mo metal begins to appear once the H2/H2O ratio reaches

0.48, and is the only product at a ratio of 0.64.

Although the metal could appear as a result of reduction of the oxide by H2

(reaction (6)), the gas phase composition, even at the highest H2/H2O ratio, is inconsistent

with thermodynamics. Consequently, the following mechanism is hypothesized.

Whereas, based on the decomposition results discussed earlier, decomposition of the

carbide to the metal would not be expected in a hydrogen atmosphere, in this case any

methane that forms via reaction (2) can be reformed by the large quantity of excess steam

(reaction (7)). In fact, as will be discussed below, small concentrations of methane readily

reform at temperatures between 600 and 850°C. Thus, reaction (2) proceeds to the right,

producing Mo, and the Mo metal subsequently oxidizes via the reverse of reaction (6).

The overall thermodynamics of this mechanism is favored under these conditions and

further evidence of its occurrence is the fact that the metal appears prior to the oxide at

H2/H2O = 0.48. It appears that the relative occurrence of metal versus oxide is determined

by the prevailing kinetics of these reactions. At low H2 concentrations, the kinetics of

reaction (3) are greater than that of reactions (2) and (6), and the Mo metal phase does not

appear. However, at high H2 concentrations the kinetics of reaction (3) are inhibited,

possibly by adsorption phenomena that is typical of many heterogeneous reactions.

Apparently, H2 inhibition is not significant until H2/H2O = 0.48. The fact that the Mo

metal formation rate is lower at H2/H2O = 0.64, may be due to the high concentrations of

15

H2 which would tend to slow down the reversible reforming reaction, reaction (7), thus

allowing for some Mo metal formation.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

600 625 650 675 700 725 750 775 800 825 850Temperature [oC]

Nor

mal

ized

MoO

2 Are

a

Figure 4. Normalized area of MoO2 during oxidation of Mo2C in steam/H2 at PH2O = 0.25 atm. ( ) H2/steam = 0; ( ) H2/steam = 0.2; ( ) H2/steam = 0.28; ( ) H2/steam = 0.4; ( ) H2/steam = 0.48; ( ) H2/steam = 0.64; Connecting lines added for clarity.

16

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

600 625 650 675 700 725 750 775 800 825 850

Temperature [oC]

Nor

mal

ized

Mo

Are

a

Figure 5. Normalized area of Mo during oxidation of Mo2C in steam/H2 at PH2O = 0.25 atm. ( ) H2/steam = 0; ( ) H2/steam = 0.2; ( ) H2/steam = 0.28; ( ) H2/steam = 0.4; ( ) H2/steam = 0.48; ( ) H2/steam = 0.64. Connecting lines added for clarity.

3.2.2. Oxidation of Mo2C in Steam/CH4

The role of CH4 in the oxidation of Mo2C in steam/CH4 was first tested by

exposing Mo2C to a CH4/steam ratio of 0.2 at temperatures between 600 and 850°C. The

carbide was stable at 850°C with no signs of oxidation or metal formation. The CH4/H2O

ratio was further decreased to as low as 0.04, and again, the Mo2C remained stable at

850°C. In both experiments, SMR took place, with CH4 conversions ranging from about

20% at 600°C to greater than 99% at 850°C. At a CH4/H2O feed ratio of 0.04,

equilibrium methane conversions were reached at 775°C, and still the carbide remained

17

stable even in the presence of such high levels of excess steam. Carbide stabilization by

CH4 is also supported by thermodynamic predictions. From Figure 2, the equilibrium

constant of reaction (5) (carburization of MoO2 by CH4) is several orders of magnitude

greater than that of reaction (3), favoring a stable carbide at temperatures greater than

700°C. Thus, while H2 is capable of inhibiting oxidation once the H2/H2O ratio reaches

0.64, it only takes trace quantities of CH4 to completely stabilize the carbide at reforming

temperatures. This suggests that a viable strategy for preventing the type of oxidation

reported by York et. al. [6] during SMR, is to operate at less than equilibrium

conversions.

3.3. Oxidation of Mo2C in CO2

Carbon dioxide is also a potential oxidant of Mo2C, whether supplied in dry

methane reforming or produced by the water gas shift reaction during SMR. The

oxidation of Mo2C in CO2 can be described by the following reaction:

Mo2C + 5CO2 2MoO2 + 6CO (9)

and its equilibrium constant is also given in Figure 2. Figure 6 shows the normalized peak

areas of MoO2 as the temperature was raised from 600 to 850°C in 5% and 25% CO2. It

can be seen that the last stable temperature of Mo2C in both 5% and 25% CO2 is 600°C,

essentially the same results obtained with steam. In both experiments, the oxidation of

Mo2C in CO2 was confirmed by the appearance of CO in the exit gas at about 625°C, and

18

there was no indication of the formation of either Mo or MoO3. Figure 7 shows a

comparison of the progress of oxidation in CO2 and H2O. As can be seen, initially there is

little difference between the oxidation rates in steam versus CO2, but CO2 is a stronger

oxidizing agent at the higher temperatures. This difference is significant, as can be seen

from the error bars associated with the experiment in 5% CO2. This conclusion is

consistent with a comparison of the equilibrium constants of reactions (3) and (9), which

show that CO2 oxidation becomes more highly favored at the higher temperatures.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

600 625 650 675 700 725 750 775 800 825 850

Temperature [oC]

Nor

mal

ized

MoO

2 Are

a

Figure 6. Normalized area of MoO2 during oxidation of Mo2C in CO2; ( ) MoO2 in 5% CO2 with 95% confidence error bars; ( ) MoO2 in 25% CO2. Connecting lines added for clarity.

19

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

600 625 650 675 700 725 750 775 800 825 850

Temperature [oC]

Nor

mal

ized

MoO

2 Are

a

Figure 7. Comparison of normalized MoO2 area during oxidation of Mo2C in CO2 and H2O; ( ) MoO2 in 5% H2O; ( ) MoO2 in 25% H2O; ( ) MoO2 in 5% CO2 with 95% confidence error bars; ( ) MoO2 in 25% CO2.

3.3.1. Oxidation of Mo2C in CO2/CO

Given that CO is a product of CO2 oxidation in reaction (9), experiments were

conducted in CO/CO2 mixtures in order to study the effect of CO addition on the

oxidation of Mo2C. Specifically, the carbide was exposed to CO/CO2 ratios between 0.4

and 0.8 at temperatures of 600 – 850°C, using a constant feed partial pressure of CO2

equal to 0.25 atm, and the experimental results are shown in Figure 8. From these results

it can be seen that the addition of CO retards oxidation of Mo2C in a manner similar to

the effect of H2 on the oxidation by steam.

20

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

600 625 650 675 700 725 750 775 800 825 850

Temperature [oC]

Nor

mal

ized

MoO

2 Are

a

Figure 8. Normalized area of MoO2 during oxidation of Mo2C in CO2/CO at PCO2 = 0.25 atm. ( ) CO/CO2 = 0; ( ) CO/CO2 = 0.2; ( ) CO/CO2 = 0.4; ( ) CO/CO2 = 0.6; ( ) CO/CO2 = 0.8. Connecting lines added for clarity.

3.3.2. Oxidation of Mo2C in CO2/CH4

Figure 9 shows the effect of CH4 on the oxidation of Mo2C by CO2, using

CH4/CO2 ratios between 0.12 and 0.32 at a constant feed CO2 pressure of 0.25 atm. The

only product formed was MoO2, but unlike the effect of CH4 on steam oxidation, CH4

was not able to completely stabilize the carbide from CO2 oxidation. Rather, there was a

direct retardation of the oxidation onset temperature as the methane concentration was

raised, with stabilization at 850°C achieved only at the highest CH4/CO2 ratio.

21

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

600 625 650 675 700 725 750 775 800 825 850

Temperature [oC]

Nor

mal

ized

MoO

2 Are

a

Figure 9. Normalized MoO2 area during oxidation of Mo2C in CO2/CH4 at PCO2 = 0.25 atm. ( ) CH4/CO2 = 0; ( ) CH4/CO2 = 0.12; ( ) CH4/CO2 = 0.2; ( ) CH4/CO2 = 0.32. Connecting lines added for clarity. Dry methane reforming was observed at all temperatures, with the CH4 conversions

ranging from 40% to 90%. The explanation for the fact that CH4 is not as effective in

retarding CO2 oxidation as it is in steam oxidation, lies in their respective reaction rates.

In steam oxidation, the rate of forward reaction (3) is lower than the rate of carburization

via reaction (5), whereas, in CO2 oxidation, the rate of forward reaction (9) is higher than

the carburization rate. This explanation is also consistent with the observation that CO2 is

a stronger oxidant than steam at higher temperatures (Fig. 7).

The inhibiting effect of H2 and CO in the oxidation of Mo2C in steam and CO2,

respectively, can be summarized in terms of the “stability” plot in Figure 10. Here, the

last stable temperature is plotted versus the ratio of reductant to oxidant, resulting in three

22

stability lines. The region to the left of the highest stability line is the region in which the

oxidation of Mo2C to form MoO2 is favored while the region between the highest

stability line and the lowest stability line on the right is the region in which Mo2C is

stable from Mo formation. The region below all three stability lines is the region in which

Mo2C is stable from both oxidation and Mo metal formation. The effect of CH4 on the

600

625

650

675

700

725

750

775

800

825

850

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Reductant/Oxidant

Last

Sta

ble

Tem

pera

ture

[o C

]

Mo2C

MoO2

Mo

Figure 10. Stability of Mo2C in H2/H2O and CO/CO2 at Poxid = 0.25 atm. (−−−− −−−− −−−− −−−−), stability line in H2/H2O; ( ) CO/CO2 stability line in; ( ) Mo metal formation stability line.

oxidation of Mo2C is summarized in Table 2. The stability of Mo2C in steam can be

maintained in the presence of trace concentrations of CH4, while in CO2, much higher

CH4 concentrations are required to achieve stability at reforming temperatures.

23

Table 2. The last stable temperature of Mo2C in oxidation of Mo2C in steam/CH4 and CO2/CH4.

Oxidant

(0.25 atm.) CH4/Oxidant The Last Stable Temperature [°C]

Steam 0 600 0.04 >850 0.20 >850

CO2 0.12 700 0.20 800 0.32 >850

3.4. Oxidation of Mo2C in CO2/CO at Elevated Pressure

The effect of higher pressures on Mo2C oxidation was evaluated by studying the

effect of pressure on CO2 oxidation, using a 25% CO2 concentration at pressures from 1

to 8 bar, with the highest pressure chosen on the basis of York et. al.’s [6] conclusions

that 8 bar pressure was necessary to avoid carbide oxidation. In addition, the effect of

CO on the carbide oxidation was also evaluated at these same pressures, using a CO/CO2

ratio of 0.2, which was the highest ratio possible without experiencing CO cracking at 8

bar. Table 3 shows a comparison of the last stable temperature at both 1 and 8 bars total

pressure.

24

Table 3. Comparison of the last stable temperature and normalized MoO2 peak area at 825°C for oxidation of Mo2C in CO/CO2 at 1 and 8 bars.

CO/CO2 Pressure [bar] Last Stable Temperature [°C]

Normalized MoO2 peak area at 825°C

0 1 600 0.13

0 8 600 0.18

0.2 1 650 0.05

0.2 8 625 -

As can be seen, there are no significant effects of pressure on either direct oxidation in

CO2 or with CO inhibition. In fact, higher pressures appear to slightly reduce the

effectiveness of CO inhibition. However, it should be pointed out that these results only

apply to oxidation onset temperatures. Problems with CO cracking in the DXRD chamber

restricted the maximum temperature to 700°C, so the effect of pressure at temperatures

more typical of methane reforming, has not been evaluated.

25

4. Discussion and Conclusions

Based on these results, there appears to be two primary routes for the oxidation of

Mo2C catalysts in reforming atmospheres, reactions (3) and (9):

Mo2C + 5H2O 2MoO2 + CO + 5H2 (3)

Mo2C + 5CO2 2MoO2 + 6CO (9)

and reactions (1) and (2), followed by reactions (7) and (6):

Mo2C 2Mo + C (1)

Mo2C + 2H2 2Mo + CH4 (2)

CH4 + H2O CO + 3H2 (7)

MoO2 + 2H2 Mo + 2H2O (6)

Only reactions (3) and (9) occur at temperatures below 750°C, but they can be

significantly retarded by the presence of H2 and CO, respectively. Steam oxidation of the

carbide is more complex, since high H2 concentrations have a strong inhibiting effect, to

the point where the rate of reaction (3) becomes relatively slow. At above 750°C, reaction

(1) can occur, but, in the presence of H2, ppm levels of CH4 produced by reaction (2) act

to prevent carbide decomposition. However, when steam is also present, the CH4

produced by reaction (2) can be reformed to H2 and CO via reaction (7), thereby

removing the CH4 reaction product and allowing reaction (2) to proceed. The resulting

Mo metal is then susceptible to steam oxidation via the reverse of reaction (6). Evidence

for these two oxidation schemes can be found in Figure 4, where two distinct oxidation

rates are apparent at H2/H2O ratios less than 0.48. The initial rates are associated with

reaction (3), and the high temperature rates are associated with the rates of reactions (2-7-

26

6). However, H2 appears to have a significant negative effect on the rate of reaction (3).

Again referring to Figures 4 and 5, as the H2 concentrations are increased, oxidation by

reaction (3) is almost completely retarded, and the oxide appears only at temperatures

above 750°C, apparently through the sequence of reactions (2-7-6). Hydrogen can inhibit

this sequence as well, but in two distinctively different manners. At a H2/H2O ratio of

0.48, both MoO2 and Mo are produced, and the metal forms first, consistent with the

proposed reaction sequence of decomposition of Mo2C to Mo followed by Mo oxidation.

However, when the ratio is raised to 0.64, only the metal is present, and its formation rate

is much slower than at the lower ratio. Apparently, H2 also acts as a kinetic inhibitor of

the reverse of reaction (6), so that the metal is not easily oxidized at high H2

concentrations. The lower rate of Mo formation at a ratio of 0.64 can be attributed to the

negative influence of H2 on the reforming rate, so that the relative rates of reactions (2)

and (7) result in a slow production of Mo metal, which does not oxidize due to the

inhibition of the reverse of reaction (6) by the high H2 concentrations.

The oxidation of Mo2C by CO2 follows a similar pattern, except that there is no

H2 present to prevent thermal decomposition at the higher temperatures. Thus, the fact

that CO2 appears to be a stronger oxidant at higher temperatures is probably due to the

fact that oxidation can occur by two means, reaction (9) and, at temperatures above

750°C, the reaction sequence (1 and 10):

Mo2C + 5CO2 2MoO2 + 6CO (9)

Mo2C 2Mo + C (1)

Mo + 2CO2 MoO2 + 2CO (10)

27

Additions of CO were found to retard the net oxidation of the carbide, probably by

inhibiting both reactions (9) and (10), the latter at the higher temperatures. Since Mo

metal is not observed in any of the CO2 oxidation experiments, it is likely that the rates of

oxidation of the metal are fast relative to reaction (1) and thus Mo is immediately

converted to the oxide.

The effect of CH4 on the retardation of Mo2C oxidation was overwhelming in the

case of steam oxidation. Total stabilization could be achieved at very low CH4

concentrations, and this is attributed to the high equilibrium constant of reaction (5),

which acts to carburize any oxide that may be formed. The effect of CH4 on CO2

oxidation was significantly less, and a CH4/CO2 ratio of 0.32 was required to achieve

stability at 850°C. Since the metal will not form, even with trace concentrations of

methane, the only prevailing oxidation reaction is reaction (9). Therefore, it is suggested

that methane is less effective at preventing carbide oxidation because CO2 is a stronger

oxidant, and the rate of recarburization by reaction (5):

2MoO2 + 5CH4 Mo2C + 4CO + 10H2 (5)

requires higher CH4 concentrations before it is able to overcome the oxidation rate of

reaction (9).

Finally, the results of a selected set of CO2 oxidation experiments at pressures up

to 8 bar showed no significant differences with respect to oxidation onset temperatures.

However, the ultimate stability of Mo2C under the reforming conditions employed here,

has been shown to depend on the relative reaction rates of a number of reactions.

28

Consequently, the negligible effect of pressure on the onset oxidation temperatures

cannot necessarily be extended to what may be the effect at reforming temperatures, and

in the complex gas mixtures associated with reforming.

29

References

[1] M. Nagai, K. Oshikawa, T. Kurakami, T. Miyao, S.Omi, J. Catal. 180 (1998) 14. [2] J. Patt, D.J. Moon, C. Philips, L. Thompson, Catal. Ltr. 65 (2000) 193. [3] S. Ramanathan, S.T. Oyama, J. Phys. Chem. 99 (1995) 16365. [4] J. Choi, J.R. Brenner, L.T. Thompson, J. Catal. 154 (1995) 33. [5] M.J. Ledoux, C.P. Huu, J. Guille, H. Dunlop, J. Catal. 134 (1992) 383. [6] J. B. Claridge, A.P.E. York, A.J. Brungs, C. Marques-Alvarez, J. Sloan, S.C. Tsang,

M.L.H. Green, J. Catal. 180 (1998) 85. [7] C.C. Yu, S. Ramanathan, B. Dhandhapani, J.G. Chen, S.T. Oyama, J. Phys. Chem. B

101 (1997) 512. [8] K.R. McCrea, J.W. Logan, T.L. Tarbuck, J.L. Heiser, M.E. Bussell, J. Catal. 171

(1997) 255. [9] B. Dhandhapani, S. Ramanathan, C.C. Yu, B. Fruberger, J.G. Chen, S.T. Oyama, J.

Catal. 176 (1998) 61. [10] J. Sehested, C.J.H. Jacobsen, S. Rokini, J.R. Rostrup-Nielsen, J. Catal 201 (2001)

206. [11] Volpe, L., Boudart, M., J. Solid State Chem. 59 (1985) 332. [12] Lee, J.S., Oyama S.T., Boudart, M.J., J. Catal. 106 (1987) 125. [13] Ledoux, M.J., Pham-Huu, C., Catal. Today 15 (1992) 263. [14] Preiss, H., Meyer, B., Olchewski, C., J. Mater Sci. 33 (1998) 713. [15] Choi, J.G., Curl, R.L., Thompson, L.T., J. Catal. 146 (1994) 218. [16] David C. LaMont, Andrew J. Gilligan, Anna R.S. Darujati, Anand S. Chellappa,

William J. Thomson, J. Catal. (submitted to J. Catal). [17] Nagai, M., Kurakami, T., Omi, S., Catal. Today 45 (1998) 235. [18] Leclercq, G., Kamal, M., Lamonier, J. F., Feigenbaum, L., Malfoy, P., Leclercq, L.,

Applied Catal. 121 (1995) 169 [19] Hojo, J., Tajika, M., Kato,A., J. of the Less-Common Metals, 75 (1980) 11 [20] Barin, Knacke, Kubaschewski, Thermochemical Properties of Inorganic Substances,

vol. 1, Springer-Verlag, Berlin, 1977.

APPENDICES

31

Appendix A: Experimental Equipment

The DXRD system used in this study is shown in Figure A1. The flow rates of the

feed gases were controlled by two Brooks Mass Flow controllers model 5850E, and two

Brooks Mass Flow Sensors model 5860E, equipped with two needle valves, and were

monitored by Brooks Read Out and Control Electronic 0154. Steam was generated by

pumping water via a Cole-Parmer 74900 series syringe pump to a vaporizer. The detailed

vaporizer schematic is shown in Figure A2. Once the steam generation was steady, the

three-way valve was used to introduce the steam to the pre-mixer where it was mixed

with the feed gases. The mixture was then delivered to the Anton Parr XRK 900 hot

stage in a heated line. The hot stage was uniquely designed so that solid state-gas

reactions can be studied under various gaseous atmospheres and under pressures as high

as 10 bar. The maximum allowable temperature for this hot stage was 900°C. In the case

of experiments at elevated pressures, a back pressure regulator was installed at the outlet

of the hot stage to control the pressure in the hot stage. A blank run in a mixture of He

and Ar was used to determine the gas composition and that would result in thermal

conductivity necessary to allow the temperature to be heated up to 850°C in the hot stage.

The thermal conductivity required to heat the XRK 900 hot stage to 850C was found to

be 0.29 W/m-K. The temperature in the hot stage was controlled by a Paar Physica TCU

750 Temperature Control Unit. An ice bath was used to condense water out of the exit

gas prior to introducing it to the GC. The exit gas composition was analyzed by a

Shimadzu GC 14-A equipped with a thermal conductivity detector and a Hayesep D 20’ x

1/8 ‘’ nickel column and coupled to a Shimadzu Chromatopac GC Integrator. A mixture

32

of 10% H2 in He was used as a carrier gas. This carrier gas was chosen because with this

mixture, the H2 peaks were completely inverted, so as to allow for accurate integration.

33

Brooks Mass Flow Controller

Needle valve

Brooks Mass Flow Sensor

Syringe Pump

Vaporizer Heated line

Phillips X’pert DXRD Ice Bath Shimadzu GC 14-A

Sample

Pattern of gas flow through

Figure A1. Schematic diagram of DXRD system

34

Figure A2. Vaporizer unit

Heating tape

Gas to hot stage

Feed gas in

Water in

Aluminum rod

Beaker glass

35

Appendix B: Run Catalogs

Tables B1, B2, B3 and B4 show the run catalog for all runs were considered valid

for analysis and discussion. In most cases (except in oxidation in CO2/N2 and steam/N2

runs), 25% oxidant (steam or CO2) was used. This oxidant partial pressure was

determined based on two factors: first, the maximum steam loading that can be handled

by the vaporizer to avoid pulsing and second, the optimum steam partial pressure that can

be used safely to prevent any condensation in the XRD hot stage. Nitrogen was used as a

dilution gas for all experiments because of its relatively low thermal conductivity and

especially because of its low x-ray mass absorption coefficient (11.09 cm2/g). The low x-

ray absorption of N2 allows the intensity of a diffracted beam to be proportionally

measured by the detector.

All invalid runs, are presented in table B5. Typically, procedural errors or/and

equipment errors resulted in the invalid results. Procedural and equipment errors

consist of:

a. The failure to use H2 pre-treatment to remove the oxygen layer on the

catalysts surface prior to conducting experiment, which caused oxidation of

the catalyst at lower temperatures.

b. Using only low-resolution-long XRD scans to identify oxidation which caused

inaccurate oxidation detection.

c. Using a Ti frit which has an XRD pattern similar to Mo2C’s pattern. This

should be avoided because if the sample surface is disturbed so as to expose

the frit, such occurrence could not be identified immediately by looking at

subsequent XRD scans.

36

d. Discontinuing the runs immediately once oxidation occurred instead of

maintaining the runs up to the maximum temperature (850°C)

e. Using gas mixtures which had a thermal conductivity above the maximum

thermal conductivity of XRD hot stage so that the temperature was unable to

reach temperatures higher than 750°C.

f. Using polymer tubing which allows air to diffuse and mix with the feed gas.

Although the data from these experiments was invalid, these experiments provided

information as to how the experiments should be done to obtain useful results.

Blank runs were also conducted to determine the activity of the hot stage at

atmospheric pressure in the absence of catalyst. The steam/CH4 ratio used in these runs

was 2.6. The maximum blank conversion at 875°C was found to be 10%. Since steam-

methane reforming is thermodynamically favored at higher temperatures, the conversion

at lower temperatures would be less than 10%.

37

B.1. Oxidation of Mo2C in Steam, Steam/H2 and Steam/CH4 Experiments

Run Date Feed Gas

SD020425 25-Apr-02 25% Steam – N2

SD020426 26-Apr-02 5% Steam – N2

SD020413 13-Apr-02 25% Steam – 5% H2 – N2

SD020416 16-Apr-02 25% Steam – 7% H2 – N2

SD020418 18-Apr-02 25% Steam – 8.5% H2 – N2

SD020422 22-Apr-02 25% Steam – 10% H2 – N2

SD020424 24-Apr-02 25% Steam – 12% H2 – N2

SD020429 29-Apr-02 25% Steam – 16% H2 – N2

SD020502 2-May-02 25% Steam – 5% CH4 – N2

SD020504 4-May-02 25% Steam – 3% CH4 – N2

SD020506 6-May-02 25% Steam – 1% CH4 – N2

Notes:

1. The heating rate used in all experiments was 10°C/minute

2. The XRD scans were taken every 25°C from 600°C up to 850°C

3. The total flow rate was 38 SCCM

4. All catalysts were pretreated in H2 at 700°C for 1 hour

38

B.2. Oxidation of Mo2C in CO2, CO2/CO, CO2/CH4 experiments

Run Date Feed Gas

SD020518 18-May-02 5% CO2 – N2

SD020520 20-May-02 25% CO2 – N2

SD020522 22-May-02 25% CO2 – 10% CO – N2

SD020523 23-May-02 25% CO2 – 15% CO – N2

SD020524 24-May-02 25% CO2 – 20% CO – N2

SD020619 19-Jun-02 25% CO2 – 5% CO – N2

SD020530 30-May-02 25% CO2 – 5% CH4 – N2

SD020601 1-Jun-02 25% CO2 – 10% CH4 – N2

SD020603 3-Jun-02 25% CO2 – 15% CH4 – N2

Notes:

1. The heating rate used in all experiments was 10°C/minute

2. The XRD scans were taken every 25°C from 600°C up to 850°C



39

B.3. Oxidation of Mo2C in CO2 at Elevated Pressure

Run Date Pressure (atm) Feed Gas

SD020620 20-Jun-02 8 25% CO2 – 5% CO – N2

SD020626 26-Jun-02 8 25% CO2 – N2

Notes:

1. The heating rate used in all experiments was 10°C/min

2. The XRD scans were taken every 25°C from 600 – 850°C


4. All catalysts were pretreated in H2 at atmospheric pressure at 700°C for 1 hour.

40

B.4. Decomposition of Mo2C in H2 and Inert

Run Date Feed Gas Holding Time at 850°C

(hours)

SD020612 12-Jun-02 100%N2 -

SD020510 10-May-02 50% H2 – N2 4

SD020508 8-May-02 5% H2 – N2 4

Note:



3. XRD scans were taken every 50°C from 600 – 850°C

41

B.5. Oxidation and Decomposition Runs Run Date Feed Gas Total Flow

Rate (SCCM) Catalyst

SD001101 11-Nov-00 75% He – 25% H2O, 50% CH4 –50% H2O

42 Aesar Mo2C (L15H05)

SD010109 9-Jan-01 27% CH4 – 73% H2O 14 - SD010227 27-Jan-01 50% H2 – 50% He 42 Aesar Mo2C

(L15H05) SD010302 2-Mar-01 50% H2 – 50% Ar 42 Aesar Mo2C

(L15H05) SD010307 7-Mar-01 50% H2 – 50% Ar 20 Aesar Mo2C

(L15H05) SD010328 28-Mar-01 50% CO2 – 50% He 42 Aesar Mo2C

(L15H05) SD010527 27-May-01 100% CO2 42 Aesar Mo2C

(L15H05) SD010529 29-May-01 10% CO2 – 90% He 42 Aesar Mo2C

(L15H05) SD010531 31-May-01 100% air 42 Aesar Mo2C

(L15H05) SD010606 6-Jun-01 100% CO2 42 Aesar Mo2C

(K17J11) SD010608 8-Jun-01 20% CH4 – 80% Ar 42 Aesar Mo2C

(K17J11) SD010609 9-Jun-01 100% air 42 Aesar Mo2C

(K17J11) SD010611 11-Jun-01 10% CO2 – 90% He 42 Aesar Mo2C

(K17J11) SD010612 12-Jun-01 10% CH4 – 90% Ar 42 - SD010613 13-Jun-01 50% CO2 – 50% He 42 Aesar Mo2C

(K17J11) SD010615 15-Jun-01 100% Ar 42 Aesar Mo2C

(K17J11) SD010618 18-Jun-01 100% He 70 Aesar Mo2C

(L15H05) SD010620 20-Jun-01 100% Ar 70 MB059 SD010621 21-Jun-01 100% Ar 70 Aesar Mo2C

(L15H05) SD010623 23-Jun-01 100% Ar 70 Aesar Mo2C

(K17J11) SD10626 26-Jun-01 100% Ar 70 MB059 SD010630 30-Jun-01 10% CO2 – 10% CH4

– 80% Ar 42 Aesar Mo2C

(K17J11) SD010710 10-Jul-01 100% Ar 70 Aesar Mo2C

(K17J11)

42

Run Date Feed Gas Total Flow Rate (SCCM)

Catalyst

SD010716 16-Jul-01 50% air – 50% He 42 Aesar Mo2C (K17J11)

SD010717 17-Jul-01 5% air – 95% He 42 Aesar Mo2C (K17J11)

SD010718 18-Jul-01 5% CH4 – 95% CO2 42 Aesar Mo2C (K17J11)

SD010719 19-Jul-01 5% CH4 – 50% CO2 – 45% He

42 Aesar Mo2C (K17J11)

SD010721 21-Jul-01 5% CH4 – 10% - CO2 – 85% He


SD010724 24-Jul-01 50% air – 5% CH4 – 45% He


SD010725 25-Jul-01 100% He 42 Aesar Mo2C (K17J11)

SD010727 27-Jul-01 100% He 42 Aesar Mo2C (K17J11)

SD010729 29-Jul-01 100% CO2 42 Aesar Mo2C (K17J11)

SD010802 2-Aug-01 100% Ar 70 Aesar Mo2C (K17J11)

SD010808 8-Aug-01 50% CO2 – 50% He 42 Aesar Mo2C (K17J11)

SD010811 11-Aug-01 100% CO2 42 Aesar Mo2C (K17J11)

SD010816 16-Aug-01 5% CH4 – 10% CO2 – 85% He


SD010821 21-Aug-01 5% CH4 – 50% CO2 – 45% He


SD010824 24-Aug-01 95% CO2 – 5% CH4 42 Aesar Mo2C (K17J11)

SD010921 21-Sep-01 10% CO2 – 5% CH4 – 85% He


SD010928 28-Sep-01 95% H2 – 5% CH4 42 Aesar Mo2C (K17J11)

SD11212 12-Dec-01 7% Ar – 95% He 70 Aesar Mo2C (K17J11)

SD011214 14-Dec-01 21% Ar – 79% He 70 Aesar Mo2C (K17J11)

SD011229 29-Dec-01 100% CO2 42 Aesar Mo2C (K17J11)

SD011231 31-Dec-01 10% CO2 – 90% He 42 Aesar Mo2C (K17J11)

SD020101 1-Jan-02 100% H2 42 Aesar MoO2 (F24I17)

43

Run Date Feed Gas Total Flow Rate (SCCM)

Catalyst

SD020123 23-Jan-02 25% H2O – 75% He 23 Aesar Mo2C (K17J11)



SD020130 30-Jan-02 3.46% H2O – 58.77% H2 – 37.78% Ar


SD020202 2-Feb-02 3.46% H2O – 2.59% H2 – 93.95% He + Ar


SD020606 6-Jun-02 25% CO2 – 15% H2 – N2


SD020326 26-Mar-02 25% H2O – 3.75% CO – N2


SD020328 28-Mar-02 25% H2O – 5% CO – N2


SD020403 3-Apr-02 25% H2O – 7% CO – N2


SD020407 7-Apr-02 25% H2O – 10% CO – N2


SD020525 25-May-02 25% CO2 – 15% H2 – N2


SD020526 26-May-02 25% CO2 – 20% H2 – N2


SD020527 27-May-02 25% CO2 – 5% H2 – N2


SD020605 5-Jun-02 25% CO2 – 10% H2 – N2


44

B.6. Repeatability Test Runs

Three runs (SD011006, SD011009, SD011011) were carried out to determine the

repeatability of the CO2 oxidation experiments. The XRK 900 hot stage was burned out

in 60 SCCM air at 850°C for 4 hours to remove excess coke. About 0.4 grams of Mo2C

(K17J11) were loaded on the sample holder and pre-treated in 42 SCCM H2 at 700°C for

1 hour. The temperature was then cooled-off to 40°C prior to switching-on 42 SCCM

CO2. After the hot stage was completely purged in CO2, the temperature was ramped to

500°C at a 10°C/min heating rate. The short XRD scans at 27.5 – 32.5 °2Θ were taken at

500 – 700°C in 10°C increments to monitor the formation of MoO2. The average peak

areas of MoO2 during this oxidation run are shown in figure B1. The error bars were

determined based on confidence level of 95% which calculated using the average value

and standard deviation obtained from three runs.

0

2000

4000

6000

8000

10000

12000

14000

16000

500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 650 660 670 680 690 700

Temperature, oC

MoO

2 Pea

k A

rea,

Cou

nts

Figure B1. Average peak area of MoO2 during oxidation of Mo2C in CO2 based on 3 runs: SD011006, SD011009, SD011011. Error bars: 5%.

45

Appendix C: Catalyst Characterization

The molybdenum carbide catalysts used in this study were purchased from Alfa

Aesar (lot# K17J11) and were used as is. Three methods used to characterize catalysts

were X-Ray diffraction (XRD), oxidative carbon analysis, and BET surface area.

C.1. X-Ray Diffraction

A long x-ray scan at room temperature of the fresh Mo2C catalyst at 10 – 110 °2Θ

was taken (figure C1). This XRD fingerprint compared well with Mo2C reference pattern

from ICDD (International Center for Diffraction Data) card number 35-787. The average

crystallite size of 40 nm was determined by calculating the width at an intensity equal to

half the maximum intensity, or as known as Debye-Scherer’s formula.

Figure C1. XRD scan at 40°C of Mo2C catalyst used in this study; (♦) Mo2C peak identified by ICDD card number 35-787

♦ ♦

♦

♦ ♦ ♦

♦ ♦ ♦

♦ ♦

♦ ♦

46

For all experiments, the Mo2C, MoO2 and Mo peak areas were determined by

integrating the most intense Mo2C peak at °2Θ = 46.2 and normalizing it to its

corresponding Mo2C peak at 600°C. Based on 0.95 confidence, the average peak area of

Mo2C at 600°C for all runs equals 473006.5 ± 4766.056 counts.

The sensitivity of XRD to MoO2 and Mo phases was determined by scanning pure

MoO2 (Alfa Aesar, lot# F24I17) and Mo (Alfa Aesar, lot# C07K23) at their most intense

peak at °2Θ = 30.5 and 47.4 respectively. The peak area of 286561 for MoO2 and 339142

counts for Mo were obtained by integrating these peaks. The correction value obtained

from these two peak areas was then applied to the experimental results.

C.2. Oxidative Carbon Analysis

Gravimetric measurements were used to determine the carbon content of fresh

catalyst. Measurement of the mass change during the complete oxidation of Mo2C to

MoO3 allowed for the calculation of excess carbon on the catalyst. This required the

catalyst phase to be pure Mo2C.

Three samples were used in this analysis. Ceramics crucibles were cleaned and

dried at about 100°C to remove any adsorbed water on the surface of the crucibles. The

crucibles were then cooled and weighed immediately prior to analysis. About 0.6 grams

of a sample was loaded into the crucibles, dried in an oven at 100°C for 2 hours and

weighed. The crucibles and the samples were then placed in the oven and heated to

600°C at 10°C/min heating rate for 2.5 hours. The samples were cooled to room

temperature, removed from the oven and then weighed for the last time. The calculation

of carbon content was made based on the weight gain during these different treatments.

47

The carbon content of Mo2C used in this study (Alfa Aesar, lot# K17J11) was 6.2% ±

0.00091. For comparison, the stoichiometric carbon content of Mo2C is 5.9 wt%.

C.3. Surface Area Measurement

A Coulter model SA3100+ BET surface area and pore analyzer was used to

determine the surface area of the catalyst. A 3 ml Coulter sample tube was dried in an

oven at 100°C to avoid water adsorption. The empty sample tube was then weighed after

removal from the oven. A Mo2C sample of about 0.3 g was loaded into a sample tube and

weighed. After outgassing for 1 hour at 300°C, the sample was capped with a rubber

stopper and was weighed again. The analysis was based on determination of the N2

adsorption isotherm.

THE EFFECT OF REFORMING GASES ON THE STABILITY OF … · 2002. 7. 22. · Anna Rini Sekar Darujati,...

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