In Situ X-ray Absorption Near-Edge Structure (XANES

(3) The presence of FeAl2O4 in these catalytic materials was most clearly ... To answer this question, an in situ iron XANES investigation was conduct...
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Energy & Fuels 2008, 22, 3620–3625

In Situ X-ray Absorption Near-Edge Structure (XANES) Spectroscopic Investigation of the Pre-reduction of Iron-Based Catalysts for Non-oxidative Alkane Dehydrogenation Frank E. Huggins,*,† Wenqin Shen,† Nicholas Cprek,† Naresh Shah,† Nebojsa S. Marinkovic,‡ and Gerald P. Huffman† Consortium for Fossil Fuel Science (CFFS)/Chemical and Materials Engineering (CME), UniVersity of Kentucky, Lexington, Kentucky 40506, and Center for Catalytic Science and Technology, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed July 17, 2008. ReVised Manuscript ReceiVed September 11, 2008

The reduction in a methane atmosphere of two as-prepared ferric oxide catalysts for the non-oxidative dehydrogenation of alkanes has been investigated by in situ X-ray absorption near-edge structure (XANES) spectroscopy using a novel X-ray transmission reaction cell. The two catalysts were prepared by different synthesis methods (incipient wetness and nanoparticle impregnation) and were supported on Al-substituted magnesium oxide obtained by decomposition of a synthetic hydrotalcite. The reduction of the ferric oxides by methane was followed by iron XANES spectroscopy at temperatures up to 650 °C complemented by a residual gas analyzer (RGA) used to track changes in the product gas. Results showed that the ferric oxides in the two catalysts underwent a stepwise reduction to first ferrous oxide, releasing mainly H2O in the case of the nanoparticle catalyst but H2 and CO in the case of the incipient wetness formulation at temperatures between 200 and 550 °C, and then more slowly to metallic iron at higher temperatures. Reaction of the ferrous oxide with the support to form magnesiowu¨stite also occurred in conjunction with the reduction. This in situ investigation confirms that metallic iron is the active catalytic phase for alkane dehydrogenation and that observations of ferric iron in samples investigated at room temperature after reduction and reaction are most likely due to re-oxidation of the iron in the catalyst upon exposure to air rather than incomplete reduction of the original ferric iron in the catalyst.

1. Introduction Non-oxidative dehydrogenation is an alternative method of obtaining hydrogen from methane and other hydrocarbons without the generation of significant quantities of CO2 or CO.1,2 Basically, the overall reaction is a decomposition and can be written in the general case as CnH2n+2 f (n + 1)H2 + nC

(1)

For methane, the most abundant hydrocarbon resource, this reaction occurs thermally at about 1000 °C, but use of catalysts can lower the reaction temperatures to less than 500 °C. Typically, such catalysts have been transition metals, either singly or in binary form, supported on γ-alumina or other oxide supports.1-3 Most recently, we have developed and examined various iron-based catalysts for possible application to this * To whom correspondence should be addressed. Telephone: +1-859257-4045. Fax: +1-859-257-7215. E-mail: [email protected]. † University of Kentucky. ‡ University of Delaware. (1) Shah, N.; Panjala, D.; Huffman, G. P. Hydrogen production by catalytic decomposition of methane. Energy Fuels 2001, 15, 1528–1534. (2) Shah, N.; Pattanaik, S.; Huggins, F. E.; Panjala, D.; Huffman, G. P. XAFS and Mo¨ssbauer spectroscopy characterization of supported binary catalysts for nonoxidative dehydrogenation of methane. Fuel Proc. Technol. 2003, 83, 163–173. (3) Shen, W.; Wang, Y.; Shi, X.; Shah, N.; Huggins, F. E.; Bollineni, S.; Seehra, M.; Huffman, G. P. Catalytic non-oxidative dehydrogenation of ethane over Fe-Ni and Ni catalysts supported on Mg(Al)O to produce pure hydrogen and easily purified carbon nanotubes. Energy Fuels 2007, 21, 3520–3529.

reaction because a further benefit of this reaction is that, at the temperatures of the catalytic reaction using iron, the carbon product is present predominantly as multiwalled carbon nanotubes (CNTs).2-4 The iron-based catalysts are usually prepared as ferric oxides using incipient wetness or other nanoparticle synthesis methods and incorporated on the support in smallparticle forms. Prior to reaction, however, the ferric iron is normally reduced to the metallic state in a hydrogen prereduction stage at a temperature between 500 and 700 °C to avoid the reaction of methane and iron oxides forming CO2 or CO during hydrogen generation CH4 + Fe2O3 f Fe + (CO2, CO, C, H2O, H2)

(2)

One major problem that occurs during pre-reduction of the ironbased oxides is a competing reaction with the support, resulting in the formation of significant FeAl2O4 (hercynite) in the case of a γ-Al2O3 support.2 Not only is FeAl2O4 more difficult to reduce than simple ferric iron oxide, but it also reduces and/or retards the amount of metallic iron available for catalyzing the methane decomposition and presents significant difficulties for purification of the generated CNTs.3 The presence of FeAl2O4 in these catalytic materials was most clearly demonstrated when the supported catalyst materials were analyzed after prereduction and reaction by iron Mo¨ssbauer spectroscopy or X-ray absorption fine structure spectroscopy at room temperature.2 (4) Shen, W.; Huggins, F. E.; Shah, N.; Jacobs, G.; Wang, Y.; Shi, X.; Huffman, G. P. Novel Fe-Ni nanoparticle catalyst for the production of CO- and CO2-free H2 and carbon nanotubes by dehydrogenation of methane. Appl. Catal., A (DOI: 10.1016/j.apcata.2008.09.004).

10.1021/ef800569w CCC: $40.75  2008 American Chemical Society Published on Web 10/28/2008

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These techniques, especially Mo¨ssbauer spectroscopy, have revealed other potential complications that may occur in this catalyzed reaction by indicating the presence of significant ferric iron in the catalyst after reduction and reaction using a MgObased support. Does this ferric oxide result from oxidation of the metal in the catalyst during cooling in the reactor and exposure to air or is it a remnant of the original iron oxide present initially in the sample? To answer this question, an in situ iron XANES investigation was conducted. In this work, we describe an experimental reaction cell that enables in situ X-ray absorption fine structure (XAFS) spectra to be obtained under reaction conditions in absorption geometry. The cell is capable of reaching 1000 °C and has been used in this investigation for examination of the reduction by methane of two iron-based catalysts, one prepared by incipient wetness and the other prepared by a nanoparticle impregnation method. 2. Experimental Section 2.1. Preparation of Catalysts. The preparation and characterization of the iron-oxide-based catalysts and the Mg(Al)O support are described in detail in other papers.3-5 The support was prepared by calcination of a synthetic hydrotalcite prepared with a Mg/Al ratio of 5:1, resulting in an aluminum-substituted magnesium oxide, Mg(Al)O.3,4 The presence of aluminum in MgO ensures that the support has a small particle size and high surface area. The incipient wetness iron oxide catalyst was prepared by calcining at 550 °C for 5 h a mixture consisting of the Mg(Al)O powder, to which iron had been added by incipient wetness techniques from a solution of ferric nitrate. This catalyst, denoted as Fe IW/Mg(Al)O, had an Fe loading of 5 wt %.5 The second catalyst was prepared by a nanoparticle impregnation method similar to those developed previously.6-8 As described elsewhere,5 the particle size distribution of this catalyst was very uniform and tight, approximately 9 ( 2 nm, and consisted of particles of γ-Fe2O3 separated by surfactant molecules. It was incorporated onto the Mg(Al)O support by an ultrasonication method without calcination. This catalyst, denoted as Fe np/Mg(Al)O, had an Fe loading of 3.2 wt %.5 Both catalysts could be pressed into thin self-supporting pellets for use with the in situ XAFS reaction cell. 2.2. Description of Reaction Cell and Ancillary Equipment for in Situ XAFS Studies. A sketch of the reaction cell is provided in Figure 1. The reaction cell consists of a 1 in. diameter quartz tube about 12 in. in length, which protrudes by 3 in. at each end of a 6 in. long single-zone, electrically wound furnace. A flange assembly at both ends of the quartz tube is used for gas inlet and outlet purposes and for making gastight Mylar windows, through which the monochromatic X-rays from the synchrotron can be transmitted. Two smaller quartz tubes, 3/4 in. in diameter, are placed inside the 1 in. quartz tube to support the sample pellet and maintain its vertical orientation normal to the X-ray beam. Temperatures are measured and controlled by means of a type K chromel-alumel thermocouple connected to a temperature controller (not shown). The reaction cell has been successfully operated at temperatures as high as 650 °C for a number of hours and is capable of reaching 1000 °C. The gas inlet of the reaction cell is connected to a manifold that controls gas mixing and gas flow rates using four mass-flow controllers, whereas the gas outlet is connected to a residual gas (5) Shen, W.; Huggins, F. E.; Shah, N.; Huffman, G. P. Non-oxidative methane dehydrogenation over novel supported Fe/Mg(Al)O nanoparticle catalysts: Effect of particle size. Paper in preparation. (6) Hyeon, T. Chemical synthesis of magnetic nanoparticles. Chem. Commun. 2003, 22, 927–934. (7) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. Monodisperse MFe2O4 (M ) Fe, Co, Mn) nanoparticles. J. Am. Chem. Soc. 2004, 126, 273–279. (8) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N.-M.; Hyeon, T. Ultra-large-scale syntheses of monodisperse nanocrystals. Nat. Mater. 2004, 3, 891–895.

Figure 1. Sketch of the in situ reaction cell for absorption XAFS experiments. The flange assembly is shown as separated components on the inlet side and assembled on the outlet side.

analyzer (RGA) that measures the partial pressures of component gases after reaction by means of a quadrupole mass spectrometer. A number of gaseous components were monitored simultaneously with the RGA, including hydrogen (2 amu), helium (4 amu), various CH4-n+ ions derived from methane (13, 14, 15, and 16 amu), water (18 amu), CO/N2 (28 amu), and oxygen (32 amu). A detailed description of the gas mixing components and the RGA can be found at the synchrotron beam-line website.9 2.3. Iron XAFS Spectroscopy. XAFS spectroscopy was conducted at the iron K absorption edge at beam-line X-18B at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, Upton, NY. Spectra were collected in transmission geometry using the reaction cell described above. XAFS spectra were typically collected on a 15 min cycle from the catalyst pellet held in the center of the hot zone of the reaction chamber. XAFS spectra were collected over the range from 7012 to 7900 eV, with stepping over the edge region set to 0.3 eV/step. Analysis of the iron XAFS spectra followed the usual steps10,11 and was carried out using both ATHENA12 and SIXPack13 software packages for the personal computer. First, the energy axis of each XAFS spectrum was calibrated against the zero point of energy defined by the first major inflection point in the spectrum of metallic R-Fe, which is taken to occur at 7112 eV. After calibration, the spectra were divided into separate X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. However, the low loading of iron (3-5 wt %) on the catalysts coupled with the thickness of the pellets and the 12 in. passage of the X-rays through the methane atmosphere resulted in a relatively poor signal/noise ratio for the Fe XAFS spectra, such that the EXAFS regions could not be profitably analyzed. Hence, only analysis of the XANES regions was carried out. This was performed by least-squares (LSQ) fitting of the iron (9) Marinkovic, N. http://www.nsls.bnl.gov/beamlines/x18b/flow_ controllers_and_rga_setup.htm. (10) Lee, P. A.; Citrin, P. H.; Eisenberger, P.; Kincaid, B. M. Extended X-ray absorption fine structuresIts strengths and limitations as a structural tool. ReV. Mod. Phys. 1981, 53, 769–808. (11) X-ray Absorption. Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES; Koningsberger, D. C., Prins, R., Eds.; Wiley: New York, 1988. (12) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537–541. (13) Webb, S. M. SIXPack: A graphical user interface for XAS analysis using IFEFFIT. Phys. Scr., T 2005, 115, 1011–1014.

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Figure 2. Output of the residual gas analyzer as a function of time for the reduction of catalyst sample Fe np/Mg(Al)O by methane. Partial pressures of methane (CH4/10), hydrogen (H2), water (H2O), and mass 28 (N2/CO) are shown over the 4 h duration of the in situ experiment. Also indicated are the time intervals for temperature ramping from 25 to 550 °C and for a constant temperature of 550 °C.

XANES spectra to quantify the iron oxidation states using either SIXPack13 or WinXAS14 programs for the personal computer.

Huggins et al.

the relative proportions of Fe3+ and Fe2+ iron determined by LSQ fitting of the intermediate spectra. Figure 5 plots trends in the data for the percentages of Fe3+ and Fe2+ in the sample as a function of time and temperature. The spectra confirm that reduction of the ferric iron in methane does indeed commence at a relatively low temperature during the ramp-up, consistent with the trends in Figure 2. Furthermore, the spectra indicate that the reduction is largely complete after about 2.5 h and that only small changes occur after that time, which again is consistent with the RGA data. Interpretation of these data is as follows. Reduction of the ferric oxide nanoparticle catalyst in methane commences at a temperature between 150 and 200 °C, forming a Fe2+ oxide species, and continues for about 2 h, while the sample is being ramped up in temperature to 550 °C. The main reaction, on the basis of the RGA output (Figure 2), which shows H2O to be the dominant product, is CH4 + 2Fe2O3 f C + 2H2O + 4FeO

(3)

CH4 + 3Fe2O3 f CO + 2H2O + 6FeO

(4)

or

The minor observed H2 evolution could then be generated by the water-gas shift reaction

3. Results and Discussion

CO + H2O f CO2 + H2

3.1. In Situ Reduction of Nanoparticle Catalyst Fe np/ Mg(Al)O. The temperature-time profile shown in Figure 2 was followed to investigate reduction of the ferric oxides in the Fe np/Mg(al)O catalyst in a methane atmosphere. In this experiment, the sample temperature was ramped from 25 to 550 °C over about 1 h and then held at 550 °C for a further 2+ h. The trends in the gaseous species, notably H2O but also minor CO and H2, indicated that a significant reaction occurred between 0.5 and 2.5 h, with an appreciable tail for another hour before approaching steady state. Furthermore, the data suggested that the reaction started at a relatively low temperature (150-200 °C). Changes in the iron speciation of the Fe np/Mg(Al)O catalyst during the time/temperature sequence were monitored by iron XANES spectra. A total of 16 spectra were collected over the 4 h period of the experiment; these are shown in Figure 3. Three spectra were recorded prior to the start of the ramp-up in temperature; four spectra were collected during the temperature ramp; and a further nine spectra were collected at a temperature of 550 °C. The iron XANES spectra show significant variation over the 4 h period, although the differences between successive spectra are relatively subtle. However, it is clear that the preedge peak at about 7113 eV was significantly reduced in intensity over time, while the prominent shoulder at about 7120 eV increased in intensity. There was also a shift of the peak maximum from 7132 to 7130 eV as the temperature and time increased and the peak changed shape, becoming noticeably sharper. All of these features indicate that the ferric iron in the catalyst undergoes reduction to ferrous iron during the reaction. There was no indication of any further reduction to the metallic state. To quantify this trend, the XANES spectra were LSQ fit using the first and last spectra as the representative of the ferric and ferrous states, respectively. An example of such LSQ fitting is shown in Figure 4 for the sample that exhibited the spectrum closest to 50:50 of the two extreme spectra. Table 1 summarizes

Reactions 3 and 4 go to completion while the catalyst is being held at 550 °C; however, over the last hour or so, the Fe2+ oxide undergoes a slow change that at most involves rearrangement or recrystallization rather than significant reduction. Previous studies3,5 have shown that the Fe nanoparticle catalyst is principally deposited as small-particle γ-Fe2O3 (maghemite) on the Mg(Al)O support. It is likely then that the reduction initially forms the predominantly ferrous oxide, wu¨stite (Fe1-xO), and, as the temperature is raised further, the wu¨stite reacts with the Mg(Al)O support to form magnesiowu¨stite, MgxFe1-xO. The transition from nonstoichiometric wu¨stite to more stoichiometric magnesiowu¨stite does entail a further small degree of reduction of ferric iron. Figure 6 shows the variation of the goodness of fit parameter, reduced χ2, as a function of time. As can be seen, the LSQ fits for the catalyst sample as the temperature was increasing have much larger χ2 values, indicating that the fits are more complex than the simple two-component fitting. It is possible that the higher χ2 values reflect the fact that the spectrum of the ferrous oxide differs significantly over the temperature range from the final spectrum at 550 °C. It is well-documented15,16 that the iron XANES spectrum of nonstoichiometric wu¨stite is very different from that of magnesiowu¨stite (Mg1-xFexO, x ) 0.0-0.5), which itself varies significantly with composition.15 Furthermore, the Fe XANES spectra will reflect any changes in the Fe3+/Fe2+ ratio in magnesiowu¨stite induced by changing the temperature or oxygen fugacity.17,18 However, other explanations must also be considered. The fact that each scan takes 15 min to complete means

(14) Ressler, T. WinXAS: A program for X-ray absorption spectroscopy data analysis under MS-Windows. J. Synchrotron Radiat. 1998, 5, 118– 122.

(5)

(15) Waychunas, G. A.; Apted, M. J.; Brown, G. E., Jr. X-ray K-edge absorption spectra of Fe minerals and model compounds: near-edge structure. Phys. Chem. Miner. 1983, 10, 1–9. (16) Wilke, M.; Farges, F.; Petit, P.-E.; Brown, G. E., Jr.; Martin, F. Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study. Am. Mineral. 2001, 86, 714–730. (17) Waychunas, G. A. Mo¨ssbauer, EXAFS, and X-ray diffraction study of Fe3+ clusters in MgO:Fe and magnesiowu¨stite (Mg, Fe) 1-xOsEvidence for specific cluster geometries. J. Mater. Sci. 1983, 18, 195–207. (18) Asakura, K.; Iwasawa, Y. A structure model as the origin of catalytic properties of metal-doped MgO systems. Mater. Chem. Phys. 1988, 18, 499–512.

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Figure 3. Fe XANES spectra collected every 15 min from catalyst Fe np/Mg(Al)O during the time and temperature sequence shown in Figure 1. (a) Fe XANES spectra. (b) Expanded detail of the iron absorption edge region. Successive spectra are displaced vertically each time.

Figure 5. Results of LSQ fitting showing variation in % Fe3+ and % Fe2+ with time and temperature. Figure 4. Example of LSQ fitting of iron XANES spectra of catalysts. Spectrum is fitted to 59% Fe3+ and 41% Fe2+. Reduced χ2 for LSQ fit is 8.7 × 10-5. Table 1. Results Obtained from LSQ Fitting of Iron XANES Spectra for the Fe np/Mg(Al)O Catalysta temperature (°C)b time (h)b % Fe3+ % Fe2+ ∆(% Fe) 25 25 25 start ramp 42 150 300 440 end ramp 550 550 550 550 550 550 550 550 550

-0:30 -0:15 0:00 0:16 0:24 0:39 0:54 1:09 1:23 1:25 1:40 2:03 2:19 2:34 2:50 3:05 3:21 3:36

∆E

reduced χ2 (×10-5)

100 100 100

0 0

0 0

-0.02 0.01

0.4 0.4

100 77 73 59

0 24 28 41

0 24 4 13

0.03 -0.18 -0.33 -0.28

2.5 10.8 12.4 8.7

23 15 11 6 4 3 2 1

77 85 89 94 96 97 98 99 100

36 8 4 5 2 1 1 1

-0.1 -0.21 -0.1 -0.07 -0.03 -0.02 -0.02 0

2.1 2.6 1.1 1.2 1.1 0.8 0.8 0.8

a Zero of time coincides with the start of the RGA data collection shown in Figure 2. b Times and temperatures are given for the start of each XAFS scan, which lasted for 15 min.

that the speciation of the catalysts can change significantly during the collection of the spectrum. Consequently, the LSQ fitting result is actually an “average” between the ferric oxide/ ferrous oxide ratio at the start of the spectrum and that at the

Figure 6. Variation of reduced χ2 statistic for LSQ fitting and ∆(% Fe) with time and temperature.

end of the spectrum, and such variation would be also expected to lead to larger χ2 values. However, it should be noted that the correlation depicted in Figure 6 between χ2 and the change in % Fe as Fe2+ between successive spectra, ∆(% Fe), is not that strong; the largest change in the latter parameter occurs once a constant temperature of 550 °C has been reached but does not correspond to a large value of χ2. Conversely, the two largest χ2 values occur over the temperature range of 150 and 400 °C, just as the reduction is beginning, and when it would be expected that the spectrum of the ferrous component would be more similar to that of wu¨stite. 3.2. In Situ Reduction of Incipient Wetness Catalyst Fe IW/Mg(Al)O. An in situ XANES investigation was also made

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CH4 + Fe2O3 f 2FeO + CO + 2H2

(6)

CH4 + 2Fe2O3 f 4FeO + CO2 + 2H2

(7)

or

Mass 44 for CO2 unfortunately was not recorded but was observed to be nonzero at this point in the experiment. In addition, H2O was seen to increase but does not show the same decay trend as H2 and CO. Water is likely being formed because of the reverse water-gas shift reaction (reaction 8) or to the reduction of CO to carbon (reaction 9) CO2 + H2 f CO + H2O

(8)

CO + H2 f C + H2O

(9)

Eventually, no ferric iron is left in the catalyst, and upon continuing exposure to methane, the next stage of the reduction of the iron commences, viz., the formation of metallic Fe from Fe2+. However, this reduction is much less facile, and only about 10-15% of the iron is reduced to the metallic state after 75 min at 600 °C. When the temperature is increased a further 50 °C, the reduction proceeds faster but still does not progress completely to metallic iron but stabilizes at about 50% metallic iron. However, the gas behavior is quite different, and hydrogen and CO appear to be the sole products CH4 + FeO f CO + 2H2 + Fe

(10)

It is also possible that CO is reduced because of the equilibrium with carbon and CO2 2CO a C + CO2 Figure 7. Fe XANES spectra collected every 15 min from catalyst Fe IW/Mg(Al)O during the time and temperature sequence shown in Figure 8. (a) Fe XANES spectra. (b) Expanded detail of the iron absorption edge region. Successive spectra are displaced vertically each time. The first spectrum shown at 600 °C was actually measured during the change in temperature from 300 to 600 °C.

of the Fe IW/Mg(Al)O catalyst. As before, iron XANES spectra (Figure 7) were collected every 15 min over the 4.5 h duration of the experiment. The temperature-time profile for this experiment is shown in Figure 8. In this experiment, the temperature of the catalyst was increased to 300 °C in a helium atmosphere before methane was introduced to the in situ reaction cell. As can be seen from the RGA output in the lower part of Figure 8, it took about 45 min to completely displace all of the helium from the reaction cell. However, no significant reaction was observed during this time. Once the methane level was constant, the temperature was quickly raised to 600 °C and held at that temperature for about 100 min; after which the temperature was raised to 650 °C for a further 80 min. A total of 14 iron XANES spectra were collected: 3 spectra were taken at a temperature of 300 °C, followed by 1 spectrum as the temperature was raised to 600 °C, 6 spectra while at 600 °C, and finally, 4 spectra while at 650 °C. The upper part of Figure 8 shows the variation of the iron oxidation state, as determined by LSQ fitting of the iron XANES spectra using the spectrum of metallic iron and the first spectra obtained at the constant temperatures of 300 and 600 °C to represent the Fe0, Fe3+, and Fe2+ oxidation states, respectively. From the large dip in the CH4 concentration as the temperature was raised to 600 °C, it is clear that a significant reaction occurred rapidly as the temperature was raised from 300 to 600 °C. On the basis of the change in the iron XANES spectra (Figure 7), Fe3+ was largely replaced by Fe2+; at the same time, the dip in CH4 was mirrored by spikes in H2 and CO, and a reaction for the reduction can be written as either

(11)

No formation of water is observed during this stage of the reduction. A comparison of the two sections of Figure 8 shows an obvious parallel of the increasing trends for the H2 concentration and the amount of iron in the metallic state because of initiation of reaction 1 catalyzed by the just formed metallic iron. The drop-off in H2 production at the very end of the run is likely due to the increasing formation of iron carbide, which curtails catalytic activity. As reported elsewhere,1-3 secondary elements added to the iron prolong the catalytic decomposition of methane by destabilizing the formation of carbide relative to metallic iron, resulting in longer catalytic activity for methane dehydrogenation compared to pure iron catalysts. One further observation should be noted. A comparison of eqs 3 and 4 for the reduction of the Fe np/Mg(Al)O catalyst with eqs 6 and 7 for the reduction of the Fe IW/Mg(Al)O catalyst indicates that different reaction mechanisms may be operative for the two catalysts. In the case of the catalyst formed by nanoparticle impregnation, H2O is the major gaseous product, whereas for the incipient wetness catalyst, H2 and CO are the dominant products. This difference must relate to how the iron oxide interacts with the support in the two catalysts because this is the major difference between them. 4. Conclusions In this work, we have used in situ Fe XANES spectroscopy with a novel X-ray transmission reaction cell to examine the behavior and properties of two iron-based catalysts, both supported on Al-substituted MgO, as they underwent reduction in a methane atmosphere at temperatures up to 650 °C. The catalysts were prepared by two different methods: incipient wetness and a nanoparticle impregnation method. The two catalyst formulations exhibited, albeit by different reactions, an initial reduction step involving the relatively rapid and appar-

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catalyst, presumably because of the fact that it was exposed to higher temperatures than the Fe np/Mg(Al)O catalyst. Hence, we can conclude that the presence of significant ferric iron observed in room-temperature characterization3-5 of spent catalysts after hydrogen pre-reduction and methane dehydrogenation at temperatures above 600 °C for at least 5 h is due to re-oxidation of the iron in the catalysts upon exposure to air. Reaction of the iron with the Mg(Al)O support occurred simultaneously with the ferric to ferrous reduction, and the resulting formation of magnesiowu¨stite may be the major reason for the apparent lack of any observed further reduction of the ferrous iron to the metallic state for the Fe np/Mg(Al)O catalyst at temperatures up to 550 °C. These results suggest that the pre-reduction step of iron-based catalysts for the dehydrogenation of alkanes should be carried out not only at a high temperature (at least 650 °C) but also with as rapid as possible heat-up stratagem to promote the formation of the catalytic metallic state and minimize the reaction of the intermediate ferrous iron with the support. Alternatively, catalyst synthesis methods should be developed that result in the formation of nanoparticles of metallic iron directly on the support for use with this reaction to avoid the complications associated with the pre-reduction step and to minimize the formation of undesired H2O, CO, and CO2 species in the product gas. Figure 8. (Top) Variation of iron oxidation states determined by LSQ fitting of the XANES spectra shown in Figure 7. The abscissa applies to both plots. (Bottom) Output of residual gas analyzer as a function of time for the reduction of catalyst sample Fe IW/Mg(Al)O by methane. Partial pressures of helium (He), methane (CH4), hydrogen (H2), water (H2O), and mass 28 (N2/CO) are shown over the 4.5 h duration of the in situ experiment. Also indicated are the time intervals for constant temperature episodes at 300, 600, and 650 °C.

ently complete reduction of ferric iron to ferrous iron, while the second stage of the reduction to the metallic state was slower and incomplete and only observed with the Fe IW/Mg(Al)O

Acknowledgment. The authors are grateful to the Synchrotron Catalyst Consortium for use of equipment to conduct in situ experiments at beam-line X-18B of the National Synchrotron Light Source at Brookhaven National Laboratory. Catalysis research conducted by the CFFS at the University of Kentucky was supported by the U.S. Department of Energy under contract DE-FC2605NT42456. Use of the National Synchrotron Light Source at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886. EF800569W