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In Situ XAFS Spectroscopic Studies of Direct Coal Liquefaction Catalysts Naresh Shah,* Jianmin Zhao, Frank E. Huggins, and Gerald P. Huffman Consortium for Fossil Fuel Liquefaction Science, University of Kentucky, 341 Bowman Hall, Lexington, Kentucky 40506-0059 Received August 7, 1995X
An in situ cell capable of withstanding typical coal liquefaction reaction conditions was designed and built for XAFS investigations. Three different Fe-based catalysts for direct coal liquefaction were studied at the reaction conditions of 1000 psig (cold) hydrogen pressure and up to 500 °C temperature. Both near-edge (XANES) and radial structure function (EXAFS) data showed gradual conversion from iron oxyhydroxide to pyrrhotite. The starting temperature and the temperature range over which such transformations occur vary significantly with the catalyst system being investigated.
Introduction To understand the role of catalysts in direct coal liquefaction (DCL), samples are normally examined before and after liquefaction and by quenching the reaction at different time intervals. By inferring the probable state of existence, valuable information has been obtained about the roles of catalysts from investigation of such samples at ambient conditions. The catalyst characterization capabilities of X-ray absorption fine structure (XAFS) spectroscopy have been demonstrated in our previous investigations1 of the nature of Fe-based DCL catalysts. We have observed that, irrespective of the starting form of Fe-based catalysts, after liquefaction conditions in the presence of sufficient sulfur, pyrrhotite (Fe1-xS) is the dominant phase formed. This suggests that Fe1-xS may be the active catalytic species. However, since most of the samples were investigated at ambient temperature after liquefaction and all of the samples at ambient pressure conditions, we can only infer the form of the active catalytic phase at liquefaction conditions. To understand liquefaction catalysis better, observation of the actual active catalytic species at liquefaction conditions is essential. However, few attempts have been made in this direction due to the experimental difficulties. In the current paper, the design and initial results for a high-temperature, highpressure in situ XAFS cell capable of reproducing direct coal liquefaction conditions are presented. In Situ XAFS Cell Constructing an in situ cell capable of achieving the temperatures of liquefaction reactions is relatively easy and we have previously reported results obtained using a high-temperature, ambient pressure in situ cell.1 To contain the high pressures of liquefaction conditions, it is essential to construct such cells with large wall thicknesses. Moreover, due to the high reactivity of hydrogen gas, additional safety measures have to be taken to X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Huffman, G. P.; Ganguly, B.; Zhao, J.; Rao, K. R. P. M.; Shah, N.; Zhen, F.; Huggins, F. E.; Taghiei, M. M.; Lu, F.; Wender, I.; Pradhan, V. R.; Tierney, J. W.; Seehra, M. S.; Ibrahim, M. M.; Shabtai, J. S.; Eyring, E. M. Energy Fuels 1994, 7, 285-296.
0887-0624/96/2510-0417$12.00/0
avoid any leaks and possible accumulation of gas outside the cell. These requirements have posed great experimental difficulties. Koningsberger and Cook2 have proposed a two-compartment cell. The sample is treated at reaction conditions in the upper compartment and is lowered into the adjoining compartment with X-ray transparent thin windows maintained at ambient pressure by opening a valve attached to the bottom of the sample holder and between two compartments. Boudart et al.3 constructed one of the first in situ XAFS cells capable of achieving 250 °C at 1050 psi by using 0.5 mm thick water-cooled Be windows to study the Mo K-edge. Because of the relatively long penetration distance of high energy (20 keV) Mo radiation, they were able to get good results. Montano et al.4 constructed an in situ Mo¨ssbauer spectroscopy cell using two 0.25 in. thick graphite-Al-Cu windows. These windows could withstand 2000 psi pressure at 450 °C but transmitted only 10% of the 14.4 keV gamma rays used for 57Fe Mo¨ssbauer spectroscopy and would transmit less than 10-6% of the radiation at 7.1 keV, the Fe K-edge energy. Neils and Burlitch5 designed their in situ cell by using specially designed graphite cloth/Al sandwich windows capable of withstanding 2000 psi, and the entire cell was externally heated up to 473 °C. They studied the XAFS spectra of Cu and Zn catalysts at X-ray energies of 8.9 and 9.6 keV, respectively. As shown in Figure 1, we have modified Neils and Burlitch design to construct our in situ cell for XAFS investigation of DCL catalysts. Two layers of graphite cloth6 were glued7 to each other and were then sandwiched between two copper gaskets. The graphite cloth (2) Koningsberger, D. C.; Cook, J. W. EXAFS and Near Edge Structure; Chem. Phys. Vol. 27; Bianconi et al., Eds.; Springer: New York, 1983; p 412. (3) Boudart, M.; Betta, R. D.; Foger, K.; Loffler, D. G. EXAFS Near Edge Struct. III 1984, 187. (4) Montano, P. A. et al. Fuel 1981, 60, 624-628, 703-711, 712716, 1022-1026. (5) Neils, T. L.; Burlitch, J. M. J. Catal. 1989, 118, 79-84. (6) G105/42.25′′/8HS cloth made with BASF Celion 3K graphite by Textile Technologies, Inc. 2800 Turnpike Dr. Hatboro, PA 19040. (7) Aremco-Bond 526 high-temperature organic adhesive from Aremco Products, Inc. P.O. Box 429, Ossining, NY 10562.
© 1996 American Chemical Society
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Figure 2. Fe K-edge XANES and RSF of several standard compounds.
Figure 1. Schematic of high-pressure, high-temperature XAFS cell for in situ characterization of DCL catalysts.
provides the necessary mechanical strength to the window without substantial attenuation of the X-ray beam. The total hydrogen volume is kept low for safety considerations. A thin pellet of the sample (coal with the added catalyst) is placed in the sample cell and the window assemblies are attached to the sample cell with the outer two water-cooled flanges. Because of water cooling, we can use Viton O-rings for a leak-tight seal. The sample cell is charged with about 1000 psig of hydrogen (cold) and is then disconnected from the hydrogen supply. The sample within the cell is heated to reaction temperatures with four cartridge heaters8 embedded in the center flange. This design can reproduce the elevated temperature and hydrogen pressure (up to 1800 psig hydrostatic pressure) employed in an autoclave or microautoclave (tubing bomb) reactor while allowing a 7 keV synchrotron generated X-ray beam to pass through the cell windows and the sample. The only difference is the absence of a solvent and a shaking mechanism to improve mass transfer. Since the intention of the experiment is to study the transformations of the catalyst phase and not the reaction yields, the mass transfer limitations of the present setup are not considered critical. Catalyst Systems Investigated Three coal/catalyst systems were investigated with the in situ XAFS cell. To maintain realistic catalyst loadings, the total Fe content of all samples was kept e five weight percent. Since our previous studies1 indicated that only the sulfided forms of the Fe-based catalysts are good hydrogenation catalysts, enough sulfur (in the form of elemental sulfur) was added to all samples to achieve complete sulfidization of all iron present. 1. Carboxyl bound iron cations in Black Thunder mine (Wyoming subbituminous) coal were introduced by ion exchange from an iron acetate solution to a 3.78 (8) Firerod cartridge heaters (Code G1A38) from Watlow Electric Mfg. Co., 12001 Lackland Rd., St. Louis, MO 63146.
wt % Fe loading, as discussed by Taghiei et al.9,10 Typical Black Thunder coal contains less than 0.2 wt % Fe as pyrite. For the current sample, this pyrite was removed during a demineralization step prior to the ionexchange process. Mo¨ssbauer spectroscopy showed that the iron added during ion exchange ends up primarily in a ferric oxyhydroxide phase. Cryogenic Mo¨ssbauer spectroscopy further confirmed that this iron oxyhydroxide phase was present in a finely dispersed superparamagnetic form. 2. Blind Canyon (DECS-17) coal was physically mixed with two ferrihydrite catalysts. DECS-17 is a specially selected coal that contains essentially no pyrite or other iron-bearing phases. The two ferryhydride catalysts were prepared by the following procedure. (1) Ferric nitrate solution was precipitated with ammonia and was treated with coprecipitated silica gel to obtain 5 wt % SiO2 loading on the ferrihydrite.11 (2) Ferric nitrate solution was precipitated with ammonia and subsequently treated with citric acid to produce surface acidity.12 XAFS Results XAFS experiments were carried out at beam line X-19A of the National Synchrotron Light Source (NSLS), at the Brookhaven National Laboratory. Iron K-edge XAFS spectra were collected with a focused (1 mm × 1 mm) beam in the transmission mode using a Si[111] double crystal monochromator. The XAFS data analysis procedure for finely dispersed iron-based catalysts has been previously described by Zhao et al.13 Figure 2 shows typical X-ray absorption near edge structure (XANES) and radial structure functions (RSF) of pure ferrihydrite and monoclinic pyrrhotite standards taken at cryogenic temperatures. Data are also shown for a number of additional iron standard compounds. The zero energy point for all spectra is taken as the first inflection point of the spectrum of iron metal (7112 eV). The ferrihydrite XANES shows a well-defined pre-edge (9) Taghiei, M. M.; Huggins, F. E.; Ganguly, B.; Huffman, G. P. Energy Fuels 1993, 7, 399-405. (10) Taghiei, M. M.; Huggins, F. E.; Mahajan, V.; Huffman, G. P. Energy Fuels 1994, 8, 31-37. (11) Zhao, J.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 38-43. (12) Zhao, J.; Zeng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 1152-1153. (13) Zhao, J.; Huggins, F. E.; Feng, Z.; Lu, F.; Shah, N.; Huffman, G. P. J. Catal. 1993, 143, 499-509.
XAFS Spectra of Direct Coal Liquefaction Catalysts
Figure 3. Fe K-edge in situ XANES and RSF of Fe ion exchanged Black Thunder coal; 650 psig (cold) of hydrogen pressure.
Figure 4. Fe K-edge in situ XANES and RSF of citric acid treated ferrihydrite catalyst in Blind Canyon coal; 950 psig (cold) of hydrogen pressure.
peak at 1 eV and a relatively sharp white line peak at 20 eV with a shoulder at 35 eV. The pyrrhotite XANES has a white line peak at 9 eV, with the pre-edge peak appearing as a minor shoulder at 1 eV. The main peak at 20 eV is relatively broad. The RSF of ferrihydrite exhibits a peak at 1.5 Å derived from the nearestneighbor oxygen shell and a peak at 2.8 Å from the second neighbor iron shell. The pyrrhotite RSF exhibits a peak from the nearest neighbor sulfur shell at 2.0 Å and a weak peak, presumably from an iron shell, at 2.5 Å. Figures 3, 4, and 5 show a series of Fe K-edge XANES and RSF for the three different catalyst systems investigated. Each spectrum required about 30 min to obtain. All spectra were recorded with the samples inside the in situ cell under a pressurized hydrogen atmosphere (∼1000-2000 psig). Before the cell is heated (i.e., at room temperature) both the XANES and the RSF are characteristic of very finely dispersed iron oxyhydroxide.12 As the temperature of the sample is raised, the finely dispersed iron starts reacting with sulfur, presumably as hydrogen sulfide (H2S). The XAFS spectra do not indicate any significant sintering of catalyst particles or reduction to the metallic state. As seen in Figures 3-5, the main peak of the XANES spectra broadens and the inflection point moves to lower energies suggesting reduction of the formal oxidation state. The pre-edge feature also becomes weaker and appears more like a
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Figure 5. Fe K-edge in situ XANES and RSF of ferrihydrite catalyst containing 5 wt % SiO2 in Blind Canyon (DECS-17) coal; 1000 psig (cold) of hydrogen pressure.
shoulder than a peak. This transition of the XANES from that characteristic of iron oxyhydroxide to that characteristic of pyrrhotite is gradual and occurs over a range of time and temperature conditions. The EXAFS data corroborate this observation. The RSF (Figures 3-5) of the unreacted catalysts exhibit peaks at 1.5 and 2.8 Å for the oxygen and iron shells, respectively. With increasing temperature, the peak due to the iron shell (at 2.8 Å) diminishes rapidly, the peak due to oxygen (at 1.5 Å) decreases gradually, and a new peak due to the sulfur in pyrrhotite appears (at 2.0 Å). The magnitude of the sulfur peak continues to increase, while the peak for the oxygen shell decreases. It is observed that the transition conditions are quite different for different catalysts. If we identify the condition of approximately equal height of the oxygen and sulfur peaks in the RSF as a transition condition, the iron in the ion exchanged coal is highly reactive with a “transition temperature” of 325 °C, while the ferrihydrite catalysts have higher “transition temperatures” of 400 °C (citric acid treated catalyst) and 425 °C (SiO2 treated catalyst). Though they are somewhat harder to quantify, the XANES spectra support this catalyst reactivity ordering. Results from the back transform analysis of the RSF for the three catalysts are shown in Table 1. All spectra except the initial room temperature spectra were fitted with Fe-O (at 2 Å) and Fe-S (at 2.45 Å) shells. The number of atoms shown are normalized values, assuming a total of 6 coordination atoms in both shells. With increasing reaction severity, the average number of oxygen atoms in the Fe-O shell decreases from 6 to approximately 2, while the number of sulfur atoms in the Fe-S shell increases, reaching a value of 6 for the ion-exchanged coal and the citric acid treated ferrihydrite catalyst, but only a value of 3.7 for the SiO2 treated ferrihydrite catalyst. Using these numbers, we can calculate an approximate percentage of the iron that has transformed to pyrrhotite. However, because of the relatively large error in the coordination shell numbers ((1), these transformation percentages should only be viewed as qualitative. Conclusions A high-pressure, high-temperature XAFS cell has been developed for in situ investigations of the structure and reactions of DCL catalysts. This cell was used to
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Table 1. Least-Squares Fitting Analysis of the RSF Peaks from in Situ XAFS Spectra Fe-O shell (at 1.5 Å)
sample
temp (°C)
Fe ion exchanged Black Thunder coal
RT, start 325 375 RT, after RT, start 300 400 500 RT, after RT, start 300 350 400 450 500 RT, after
citric acid treated ferrihydrite in Blind Canyon coal
SiO2 treated ferrihydrite in Blind Canyon coal
a
no. of oxygen atoms in shella
radius of oxygen shell around Fe (Å)
6.0 2.4 2.2
1.95 2.02 2.12
6.0 6.0 3.4 2.1
1.94 1.96 2.01 2.01
6.0 4.9 4.4 4.2 3.1 2.3 2.3
1.97 1.97 2.02 2.05 2.02 2.02 2.02
Fe-S shell (at 2.0 Å) no. of sulfur atoms in shella
radius of sulfur shell around Fe (Å)
3.6 3.8 6.0
2.47 2.55 2.48
2.6 3.9 6.0
2.49 2.49 2.44
1.1 1.6 1.8 2.9 3.7 3.7
2.48 2.42 2.49 2.47 2.48 2.46
% transformn to pyrrhotite 0 60 63 100 0 0 43 65 100 0 18 27 30 48 62 62
Values are normalized to 6 (number of atoms in first shells of oxyhydroxide and pyrrhotite).
investigate the reactions of nanoscale iron oxyhydroxide catalysts at temperatures up to 500 °C and hydrogen pressure up to 1000 psig (cold). Three nanoscale iron catalysts were investigated: iron ion-exchanged into a subbituminous coal; a citric acid treated ferrihydrite; and a binary ferrihydrite containing 5 wt % SiO2. The ferrihydrite catalysts were mixed with a bituminous coal (Blind Canyon, DECS-17). Elemental sulfur was added to the coal-catalyst mixtures. All three catalysts exhibited a transformation of the iron oxyhydroxide phase to the pyrrhotite phase, which appears to be the active phase for hydrogenation. The transformation temperatures were significantly different for the three catalyst systems, with the ion-exchanged iron exhibiting the lowest transition temperature. For all of the catalysts, the transformation occurs over a range of temperature. The lack of a sharp transition is probably due to a range of particle sizes, with smaller catlayst
particles transforming at a lower temperature and larger particles at higher temperatures. Even though sufficient sulfur is present in the system, SiO2 treated ferrihydrite catalyst did not completely convert to pyrrhotite even after being exposed to severe reaction conditions. Thus, a treatment to improve specific surface area and dispersion of catalyst can hinder conversion of catalytic precursor into an active catalyst. Acknowledgment. This research was supported by the U.S. Department of Energy under DOE Contract No. DE-FC22-93PC93053 as part of the research program of the Consortium for Fossil Fuel Liquefaction Science. The National Synchrotron Light Source is also supported by the U.S. Department of Energy. EF950157Q
