0378-3

doi:10.

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Fuel Processing Technology 83 (2003) 163–173

XAFS and Mossbauer spectroscopy

characterization of supported binary catalysts for

nonoxidative dehydrogenation of methane

Naresh Shah, Siddhartha Pattanaik, Frank E. Huggins,Devadas Panjala1, Gerald P. Huffman*

University of Kentucky, Suite 107, Whalen Building, 533 S. Limestone St., Lexington, KY 40508-4005, USA

Accepted 16 January 2003

Abstract

The structure and speciation of nanoscale, binary catalysts (0.5% M–4.5% Fe/Al2O3, with

M=Pd, Mo, or Ni) that are very effective for the nonoxidative dehydrogenation of methane have

been investigated by X-ray absorption fine structure (XAFS) and Mossbauer spectroscopy. Results

are reported for the catalysts after precipitation from solution onto the alumina support, after pre-

reduction at 700 jC in hydrogen, and after several hours of reaction in flowing methane. It is found

that all three secondary metal additives enhance the reducibility of the catalysts. The phases

identified that are believed to have the greatest effect on activity are hercynite (FeAl2O4) and

nonmagnetic metallic alloys (Fe2Mo, Fe–Ni–C austenite, Fe–Pd–C austenite, and possibly Fe–

Mo–C austenite). Hercynite is believed to enhance the activity of the catalysts by binding the

catalyst particles to the alumina surface preventing demetallization. The nonmagnetic alloys are

believed to be the active phases for dehydrogenation of methane with simultaneous formation of

carbon nanotubes, which grow away from the surfaces of the bound catalyst nanoparticles,

preventing deactivation by coking.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Mossbauer spectroscopy; XAFS; Methane

820/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

1016/S0378-3820(03)00064-X

orresponding author.

ail address: Huffman@engr.uky.edu (G.P. Huffman).

resent address: Conoco Gas Solutions, Ponca City, OK 74601, USA.

N. Shah et al. / Fuel Processing Technology 83 (2003) 163–173164

1. Introduction

Production of pure hydrogen from hydrocarbons, particularly methane, the major

component of natural gas, has great practical importance. Traditionally, hydrogen has

been produced by reforming and partial oxidation of methane to produce synthesis gas,

followed by water–gas shift conversion of the CO to CO2 with the production of more

hydrogen, and subsequent removal of CO2. However, this hydrogen stream still contains

enough CO to poison the catalysts used in polymeric electrolyte proton exchange

membrane (PEM) electrochemical fuel cells. A reverse methanation or catalytic

oxidation reaction then has to be carried out to reduce the CO concentration to < 10

ppm.

Catalytic decomposition of methane to pure hydrogen and carbon is a promising

alternative approach for the conversion of methane to hydrogen. In recent studies [1], we

have investigated methane decomposition in the absence of any oxidizing medium over a

variety of alumina supported binary metal catalysts. Nanoscale, binary, 0.5% M–4.5% Fe

(M=Pd, Mo or Ni) catalysts supported on alumina were found to lower the decomposition

temperature of undiluted methane by 400–500 jC relative to noncatalytic thermal

decomposition. The product stream consisted of over 75 vol.% of hydrogen and

unconverted methane at reaction temperatures of 700 jC. Additionally, it was observed

that this high activity was maintained with little or no decrease for periods of up to 7 h.

The efficient removal of carbon from the catalyst surface in the form of nanotubes is

believed to be the key factor influencing catalyst performance [1].

Fig. 1 is a plot of hydrogen production by catalytic decomposition of methane at

various reactor temperatures using pure Fe, pure Mo, and binary Mo–Fe catalysts. The

synergistic role that binary metals play in methane decomposition is clearly evident.

Fig. 1. Comparison of alumina supported pure molybdenum and pure iron catalysts with binary Fe–Mo catalysts

pre-reduced at 700 jC for hydrogen production.

N. Shah et al. / Fuel Processing Technology 83 (2003) 163–173 165

Similar plots of hydrogen production by methane decomposition using other binary metal

catalysts (Ni–Fe and Pd–Fe) also show significantly higher activity than either Fe or the

secondary metal (Ni, Pd) alone [1].

The detailed catalyst synthesis and pretreatment procedures, as well as SEM/TEM

characterization of the produced nanotubes, have been reported elsewhere [1]. The current

paper primarily deals with characterization of the highly active nanoscale binary alumina

supported catalysts using X-ray absorption fine structure (XAFS) spectroscopy and

Mossbauer spectroscopy. Some preliminary results of this work were presented at a recent

conference [2]. Additional XRD and ESR characterization results for the same catalysts

are also reported in this volume [3].

2. Experimental details

Supported catalysts were prepared by adding aqueous solutions of metal salts to slurry

of alumina. The metal hydroxides were precipitated on alumina support by adding

ammonia to the slurry. Catalyst pellets extruded after dewatering the slurry to paste

consistency were vacuum-dried before use.

One gram of catalyst was supported in a vertical quartz reactor (12.5 mm OD� 300

mm long) by a plug of quartz wool. The reactor was externally heated by a three-zone

furnace and a reactant gas stream was passed through the bed and gas sampling loop of an

on-line GC before venting to exhaust.

XAFS experiments were carried out at beamlines X-18B of the National Synchrotron

Light Source (NSLS) and 4-3 of the Stanford Synchrotron Research Laboratory (SSRL).

Iron, nickel, molybdenum, and palladium K-edge XAFS spectra were collected at room

temperature in both transmission and fluorescence modes simultaneously using a Lytle

sample holder and detector [4]. XAFS data analysis was carried out following well-

established XAFS procedures [5] by means of WinXAS software [6]. In the usual fashion,

phase identifications were made on the basis of the X-ray absorption near edge structure

(XANES) within approximately 50–80 eV of the absorption edge and the radial structure

functions (RSF), derived by Fourier transformation of the extended X-ray absorption fine

structure (EXAFS) region of the XAFS spectrum [5–7].

Mossbauer spectroscopy was carried out at room temperature using a 57Co source in a Pd

matrix. A Halder drive was operated in triangular function mode to scan up to 12 mm/s

velocity range. Analysis procedures used for the Mossbauer spectra have been described in

detail elsewhere.

In the current study, XAFS and Mossbauer data were obtained from catalyst in three

states: ‘‘as-prepared’’, ‘‘reduced’’, and after ‘‘time on stream (TOS)’’. The ‘‘as-prepared’’

catalysts are synthesized by coprecipitation of ultrafine, binary ferrihydrite particles onto

high surface area alumina, as described previously [9,10]. The binary ferrihydrite

catalysts are then activated by reduction under conditions of 50 ml/min flowing

hydrogen, 700 jC reactor temperatures, for 2 h, followed by cooling to 25 jC in

flowing hydrogen in order to produce the ‘‘reduced’’ catalysts. The ‘‘TOS’’ samples

were from the actual experiments carried out using these pre-reduced catalysts to

produce hydrogen by nonoxidative methane decomposition. Typically, these TOS

Fig. 2. Fe K-edge XANES and radial structure functions of pure 5% Fe/Al2O3 catalysts before and after various treatments.

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samples spent several hours on stream at 700 jC in flowing methane. The various

catalyst samples were pulverized to fine powders and stored at ambient conditions for

XAFS and Mossbauer data acquisition.

XAFS characterization was carried out for all four elements. The zero points of energy

of the XAFS spectra for the elements were defined by the first inflection points of the

derivative of the respective metal foil spectra, as follows: Fe K-edge = 7112 eV, Ni

K-edge = 8333 eV, Mo K-edge = 20,000 eV, Pd K-edge = 24,350 eV.

The XAFS data for iron were complemented by results from 57Fe Mossbauer

spectroscopy. For samples containing greater than f 1% Fe, Mossbauer spectroscopy

provides more accurate iron phase identification and quantification than XAFS spectro-

scopy, particularly for samples that contain several different Fe-containing phases [7,8]. It

should be emphasized that the Mossbauer information was critical in achieving a final

designation of the important catalyst phases, since the interpretation of the XAFS spectra

was fairly qualitative in nature because of the presence of multiple phases of all elements

investigated.

3. Results and discussion

XAFS and Mossbauer results are summarized below for all four catalysts (5% Fe and

0.5% M–4.5% Fe supported on Al2O3) examined.

3.1. 5% Fe/Al2O3

Fig. 2 shows the Fe K-edge XANES and the RSF of the 5% Fe/Al2O3 catalyst before and

after various treatments. Both the Mossbauer and XAFS spectra confirmed that the iron in

the ‘‘as-prepared’’ catalysts exists in the ferrihydrite structure. The as-prepared catalysts

were not active for methane decomposition [1]. The ‘‘reduced’’ catalysts were prepared by

reduction at 700 jC for 2 h and cooling in flowing hydrogen. However, both Mossbauer and

Fe XAFS spectroscopic data show that the reduction of Fe to the metallic state is not

complete at 700 jC because iron reacts with the alumina support to form hercynite

(FeAl2O4). The Fe K-edge XANES spectra show a large shift in the absorption edge

position as well as changes in the overall shape of the spectrum, indicating significant

reduction of the Fe3 + to Fe2 +. Mossbauer data confirmed that pre-reduction under hydrogen

at 1000 jC for 2 h is required to complete the reduction of all the Fe to the metallic state.

Exposure of the pre-reduced catalysts to flowing methane at 700 jC did not result in any

significant changes in the Fe K-edge XANES spectra or the RSF of the catalysts.

Table 1

Mossbauer spectroscopy results for the percentages of the Fe contained in various phases in the 5% Fe/Al2O3

catalyst

Catalyst state MK File # Fe3 + Fe2 + Fe metal Austenite Carbide

As prepared 2574 100

Reduced 2717 22 74 4

TOS 2718 25 71 (minor) 4

Fig. 3. Mo K-edge and Ni K-edge radial structure functions of 0.5% Mo–4.5% Fe/Al2O3 and 0.5% Ni–4.5% Fe/Al2O3 catalysts before and after various treatments.

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Table 2

Mossbauer spectroscopy results for percentages of Fe in various phases in the (0.5% Mo–4.5% Fe)/Al2O3

catalysts

Fe3 + Fe2 + Fe

metal

MoFe2 and

austenite

Carbide

(Fe3C)

As prepared 100

Reduced 70 11 19

TOS 35 28 37

N. Shah et al. / Fuel Processing Technology 83 (2003) 163–173 169

The Mossbauer results summarized in Table 1 confirm the reduction of significant

amounts (but not all) of the Fe3 + in the ‘‘as-prepared’’ catalyst to Fe2 + in the ‘‘reduced’’

catalyst. The quadruple splitting and isomer shift identify the ferrous phase as hercynite

(FeAl2O4). Exposure to methane converts a small amount of metallic Fe into a carbide

phase (Fe3C) and may also produce some minor metallic austenite. Lack of reduction of

significant amounts of Fe to metallic state is believed to be the main cause for poor catalytic

behavior for methane decomposition to hydrogen and carbon nanotubes for the 5% Fe/

Al2O3 catalyst.

It is worth noting that pre-reduction of this catalyst at 1000 jC converted essentially all

Fe to metallic iron. During subsequent reaction of this catalyst in methane at 700 jC, most

of the metallic Fe was converted to (Fe3C) and Fe–C austenite.

3.2. (0.5% Mo–4.5% Fe)/Al2O3

Fig. 3 (left) shows the Mo K-edge RSF spectra of Fe–Mo catalysts before and after

various treatments. The RSF of the standard compounds MoO2 and Mo2C are shown for

comparison. Only the nearest neighbor oxygen shell is clearly detectable in the RSF of the

as-prepared Fe–Mo catalyst, indicating very small particles of an Fe–Mo ferrihydrite

phase with little long-range order. The oxygen shell in the RSF of the reduced catalyst is

shifted to a longer distance, close to the location of the oxygen shell in MO2, indicating

that Mo is predominantly contained in a more reduced oxide state. Additionally, a small

peak appears at approximately 2.15 A. The Mossbauer data (Table 2) for the reduced

catalyst show that the ferric ferrihydrite phase has been reduced primarily to a ferrous state

(principally hercynite), with formation of some metallic Fe and a nonmagnetic alloy phase,

Fe2Mo. The TOS sample, obtained after reacting the pre-reduced catalyst for several hours

in flowing methane at 700 jC, shows conversion of half of the ferrous iron to metallic

Table 3

Mossbauer spectroscopy results for percentages of Fe in various phases in the (0.5% Ni–4.5% Fe)/Al2O3

catalysts

MK File # Fe3 + Fe2 + Fe metal Austenite Carbide

As prepared 2706 100

Pre-reduced 2678 68 9 23

TOS 2690 70 30

Fig. 4. 57Fe Mossbauer spectrum and Pd K-edge RSF of 0.5% Pd–4.5% Fe/Al2O3 catalysts before and after various treatments.

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N. Shah et al. / Fuel Processing Technology 83 (2003) 163–173 171

phases, conversion of iron metal to carbide (cementite, Fe3C), and formation of additional

nonmagnetic alloy phases.

The nonmagnetic alloy phases in the TOS sample are believed to include both Fe2Mo

and an Fe–Mo–C austenite, which is stabilized by the incorporation of carbon. The Mo

RSF for the TOS sample is in qualitative agreement with the Mossbauer results, exhibiting

peaks that can be identified as a reduced oxide state, a carbide (Mo2C), and a somewhat

larger peak at 2.15 A, which presumably corresponds to a contribution from the

nonmagnetic alloy phases.

3.3. 0.5% Ni–4.5% Fe/Al2O3

The RSF of bulk NiO shows a relatively small peak from the first shell, which arises

from six oxygen anions, and a much larger peak from the second shell because in this shell

Ni is coordinated by 12 Ni atoms, which have a much higher backscattering coefficient

than oxygen (Fig. 3, right). However, in the RSF of the as-prepared Ni–Fe catalyst, only

the peak due to the first nearest neighbor oxygen is evident, again implying that Ni is

distributed in very small oxyhydroxide particles. The RSF of the reduced sample (Fig. 3,

right) shows several peaks that can be tentatively identified as follows: O—1.6 A; Ni in a

metallic phase—2.1 A; and Ni in a reduced oxide phase (presumably NiO or possibly Ni

ferrite)—2.5 A. The Mossbauer data shown in Table 3 are consistent with these assign-

ments, showing 68% of the Fe in a ferrous state (hercynite), 9% in an Fe-rich martensitic

Fe(Ni) alloy, and 23% in an austenitic Fe–Ni alloy. After several hours TOS (700 jC in

flowing methane), the Mossbauer spectrum places 70% of the Fe in the ferrous phase

(mainly hercynite) and 30% in the austenitic Fe–Ni–C alloy, while the Ni RSF shows that

most of the Ni is present in the metallic Fe–Ni austenite. On the basis of the Mossbauer

phase percentages for the TOS sample, this would set the composition of the Fe–Ni–C

austenite at approximately 75% Fe–25% Ni, which is well into the gamma field of the

Fe–Ni phase diagram at 700 jC.Undoubtedly, the austenite also contains several percentages of interstitial carbon,

which would further stabilize the phase. Pertinent information on the Fe–Ni and Fe–C

phase diagrams can be found in the book by Hansen and Anderko [11].

3.4. 0.5% Pd–4.5% Fe/Al2O3

Similar to the other catalysts, the dominant feature of the Pd K-edge RSF of the as-

prepared Pd–Fe catalyst is one large peak due to the first oxygen shell (Fig. 4), again

indicating that the Pd is contained in ferrihydrite nanoparticles. Reduction at 700 jCconverts significantly more of the Pd and Fe to the metallic state than was observed for

the Fe–Ni catalyst. The Mossbauer results in Table 4 and the Mossbauer spectra in Fig.

4 indicate that all of the ferric phase has been reduced to either the ferrous or metallic

state, with the metallic Fe divided about equally between Fe metal and Fe in an austenite

phase. After several hours TOS in methane at 700 jC, the Pd RSF and 57Fe Mossbauer

spectra indicate that essentially all of the Pd is in the metallic state, ferrous Fe is further

reduced, most of the Fe metal is converted to carbide (Fe3C), and the amount of Fe

contained in metallic austenite has increased to 25%. Because the Fe metal spectrum is

Table 4

Mossbauer spectroscopy results for percentages of Fe in various phases in the (0.5% Pd–4.5% Fe)/Al2O3

catalysts

MK

File #

Fe3 + Fe2 + Fe

metal

Austenite Carbide

(Fe3C)

As prepared 2664 100

Pre-reduced 2673 58 23 19

TOS 2697 43 7 25 25

N. Shah et al. / Fuel Processing Technology 83 (2003) 163–173172

consistent with pure Fe, it is likely that most of the Pd is contained in an Fe–Pd

austenite. This would put the composition of the austenite phase at about 70% Fe–30%

Pd, which is consistent with gamma phase formation, according to the Fe–Pd phase

diagram [11].

4. Summary and conclusions

Binary nanoscale Fe–M/Al2O3 (4.5% Fe–0.5% M, M=Mo, Ni, or Pd) based catalysts

have been shown to be much more effective in producing hydrogen by nonoxidative

decomposition of methane than the individual metal catalysts [1]. To understand the

synergistic behavior of the two metals in these supported catalysts, they were analyzed

using XAFS and Mossbauer spectroscopy in their as-prepared, pre-reduced and after

reaction (TOS) states. The results of these catalyst characterization studies are briefly

summarized below.

1. Following pre-reduction in hydrogen at 700 jC, most of the Fe (60–70%) is present as

hercynite (FeAl2O4).

2. The secondary element significantly enhances the reducibility of the catalysts in the

approximate order: Fe < Fe–Mo < Fe–Ni < Fe–Pd.

3. For both the Fe–Ni and Fe–Pd catalysts, the dominant metallic phase is an

austenitic alloy containing approximately 70–75% of Fe and 25–30% of Ni or

Pd. It is highly likely that several percentages of C are incorporated into the

austenite during reaction in methane. For Fe–Mo, an intermetallic Fe2Mo phase

is formed during reduction that probably becomes mixed with an Fe–Mo–C

austenite during reaction. These nonmagnetic2 alloys are believed to be the active

phases.

These conclusions suggest that the key factors that account for the high activity and

comparatively long lifetimes of these catalysts are:

1 Binding of the catalyst particles to the alumina support by the formation of hercynite,

preventing deactivation by demetallization.

2 Nonmagnetic means ‘‘not magnetically ordered’’. These phases are probably paramagnetic.

N. Shah et al. / Fuel Processing Technology 83 (2003) 163–173 173

1 Dehydrogenation of methane with simultaneous formation of carbon nanotubes, which

grow away from the surfaces of the bound, nonmagnetic, Fe–M–C alloy catalyst

nanoparticles, preventing deactivation by coking.

Acknowledgements

This research was supported by the U.S. Department of Energy through both the

Division of Fossil Energy (FE) under the National Energy Technology Laboratory (NETL)

and the Division of Energy Efficiency and Renewable Energy (EE) under the Office of

Advanced Automotive Technologies, under contract No. DE-FC26-99FT40540. The

XAFS experiments were conducted at the Stanford Synchrotron Radiation Laboratory and

the National Synchrotron Light Source at Brookhaven National Laboratory, which are

supported by the U.S. Department of Energy.

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Preprints 47 (2002) 132–133.

[3] A. Punnoose, N. Shah, G.P. Huffman, M.S. Seehra, Elsewhere in this volume.

[4] F.W. Lytle, The EXAFS Company, http://www.exafsco.com/.

[5] D.C. Koningsberger, R. Prins (Eds.), X-Ray Absorption: Principles, Applications, Techniques of EXAFS,

SEXAFS, and XANES, Wiley-Interscience, New York, 1988.

[6] T. Ressler, http://www.winxas.de/.

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[8] G.P. Huffman, Chemtech 10 (1980) 504–511.

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[10] J. Zhao, F.E. Huggins, Z. Feng, G.P. Huffman, Clays and Clay Minerals 42 (1994) 737–746.

[11] M. Hansen, K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, 1958.