Znd. Eng. C h e m . R e s . 1988, 27, 2276-2282
2276
MATERIALS AND INTERFACES Properties of Copper/Alumina Sorbents Used for the Removal of Sulfur Dioxide Sidney S. Pollack,*it William P. Chisholm,$Richard T. Obermyer,s Sheila W. Hedges,t Mohan Ramanathan," and Pedro A. Montana"," US Department of Energy, Pittsburgh Energy Technology Center, Box 10940, Pittsburgh, Pennsylvania 15236, US Department of Energy, Morgantown Energy Technology Center, Box 880, Morgantown, W e s t Virginia 26505, Physics Department, T h e Pennsylvania S t a t e University, McKeesport, Pennsylvania 15132, Physics Department, W e s t Virginia University, Morgantown, W e s t Virginia 26506, and Physics Department, Brooklyn College of C U N Y , Brooklyn, N e w York 11210
An attempt has been made to determine the structure of Culalumina sorbents used for the removal of SOz from flue gas. Fresh, used, and regenerated Grace 7-alumina 1/16-in,spheres containing 7.2% CU were studied by various techniques. X-ray and electron diffraction showed that the copper compounds are not crystalline. The magnetic susceptibilities were much higher than those of any of the copper compounds that might be expected in these sorbents. Extended X-ray absorption fine-structure analysis indicated that the copper is surrounded by average of 2.4-2.7 oxygens and that the CuO units are very small or amorphous. The low coordination number may be related to the material being a good sorbent for SO2. Considerable work has been done on the use of CuO supported on alumina as a sorbent for the removal of sulfur dioxide from flue gas (McCrea et al., 1970; Bienstock et al., 1961; Van Heldon and Nabor, 1969). Extensive studies of SOz and NO, removal by Cu/AlZO3sorbent at the Pittsburgh Energy Technology Center have been reported by Yeh et al. (1985). In the Shell patent, at least some of the CuO is considered to be in bulk form (Van Heldon and Nabor, 1969),while other studies have indicated that the copper is in the alumina structure in a way that is not easily described (Friedman et al., 1978; Selwood and Dallas, 1948; Strohmier et al., 1985). The following reactions are usually written for the removal of sulfur dioxide by the sorbent and for regeneration of the sorbent with hydrogen or methane (McCrea et al., 1970):
+ so2 + '/OZ = CUSO, CUSO, + 2Hz = CU + SO2 + 2H20 CUSO+ ~ '/CH4 = CU + SO2 + 1/CO2 + H20 CUO
In a very detailed study of Cu/alumina catalysts, many properties were measured of materials very similar to the copper oxide sorbents (Friedman et al., 1978). That work found that, if the alumina support had a surface area of 200 m2/g and less than 8% Cu, the copper was present not as crystalline CuO or CuA1204but as a less well-defined entity. The extended X-ray absorption fine-structure (EXAFS) studies indicated that most of the copper was present as cupric ions that were predominantly in a tetragonally distorted octahedral environment, although some US Department of Energy, Pittsburgh.
* US Department of Energy, Morgantown. 8 The
Pennsylvania State University. West Virginia University. 'I Brooklyn College of CUNY.
0888-5885/88/ 2627-2276$01.50/0
tetrahedral ions were also present. A recent surface spectroscopic study of Cu/alumina catalysts has confirmed that the Cu is present in a species different from CuO or CuA1204(Strohmier et al., 1985). The purpose of the present work was to determine if the fresh sorbent used for SOz removal had properties similar to the catalysts whose exact structure remains unknown and to ascertain the compounds present in the spent and regenerated sorbents used in the flue gas cleanup process at the Pittsburgh Energy Technology Center (PETC). X-ray diffraction patterns of the fresh and used sorbents did not show crystalline CuO in the former or crystalline CuSO, in the latter, while magnetic susceptibilities were much higher than those of bulk CuO, indicating that probably the same difficult-to-describe material reported by Selwood and Dallas (1948) was present in both the fresh and used sorbents. Several additional techniques were then used in an attempt to gain more information about the structure of used, regenerated, and fresh sorbent.
Experimental Section Sample Preparation. The sorbents studied in this work were prepared by the Davison Division of the Grace Corporation. 11-Alumina pellets, in. in diameter, were immersed in a 22.5 wt % solution of cupric nitrate trihydrate in water. The pellets were allowed to air dry and then were calcined at 500 "C. The final product contained 7.2 w t % Cu. The fresh sorbent had a BET surface area of 230 m2/g. After exposure to flue gas, this decreased to 190 m2/g. Regeneration did not change the latter value. Exposure and regeneration of the sorbents were carried out in a test facility that was described by Yeh et al. (1985). Only a brief description of the treatments of the sorbents is presented here. The sample studied after exposure to flue gas had been exposed to flue gas 3 times but regenerated only twice. In the last test 95% SO2and 90% NO, were removed. The regenerated sample studied had been through two cycles of SOz absorption and then regenera0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 2277 tion with CH4. All the absorption and regeneration tests were made in a 500 lb/h process development unit coal combustor. Absorption took place a t 399 "C, and the target temperature for regeneration was 426 'C. This temperature was never reached, and the actual temperature was between 371 and 399 "C. Regeneration was not complete, and the regenerated sample contained 1.2% S; the sample after the third flue gas treatment contained 2.3% S. The flue gas used for these tests contained 1900 ppm SOz, 570 ppm NO,, 15% COP, 7% HzO vapor, and 4% Oz, and the balance was nitrogen. X-ray Diffraction. X-ray diffraction patterns were obtained with a Rigaku computer-controlled diffractometer equipped with a long fie-focus Cu X-ray tube, a diffracted beam graphite monochromator to provide monochromatic Cu Ka radiation, and a scintillation detector. When data were collected from the samples, step scans were made at 0.1-deg intervals, and counting times varied from 5 to 60 s/step according to the intensity of the diffraction peak. TG. The thermal decomposition of used sorbents was investigated by using thermogravimetry analysis (TG). For this purpose, a Perkin-Elmer TGS-2 thermogravimetric analyzer equipped with a TADS Series thermal analysis data station was used. All of the thermal curves were recorded at a heating rate of 10 "C/min in either highpurity nitrogen or dry "air" (20% Oz + 80% NJ. Ground samples of 8-9 mg were placed on a quartz sample pan. Gas flow monitored by a calibrated rotometer was as follows: 100 cm3/min Nz to purge the balance housing and 50 cm3/min of either Nz or air in the sample chamber. Raman. The Raman spectra were recorded on a Spex Ramalog spectrometer equipped with holographic gratings. The 5145-A line from a Spectra-Physics Model 165 argon-ion laser was used as the exciting source. The spectral slit width was typically 4 cm-l, and laser source powers of approximately 45 mW, measured at the sample, were used. The samples were pressed into pellets with Teflon as a support. Rotation of the sample to provide noncontinuous irradiation of any given spot on the sample was the method used to avoid sample decomposition. IR. Infrared spectra were measured by using a Digilab FTS 20 spectrometer. Samples were mixed with KBr and pressed into thin wafers. ESR. A Varian Associates Model E112 spectrometer equipped with a field frequency lock and a Model E232 dual-sample cavity was used in this study. The magnetic field strength and the field scan were calibrated with peroxylamine disulfonate (PADS) in aqueous sodium carbonate solution. A g value of 2.005 50 was assumed for the field calibration, and the hyperfine splitting was taken to be 1.3 mT for the scan calibration. The microwave frequency was measured with an EIP Model 35010 highfrequency counter. Intensities were estimated by comparison of peak-to-peak amplitudes of first-derivative signals from samples and CuS04.5H20 standards. Magnetic Susceptibility. The magnetic measurements were performed by using a PAR vibrating sample magnetometer (VSM). Samples were investigated at liquid nitrogen and room temperatures in applied fields up to 20 kOe. Calibration of the most sensitive VSM range was made using various weights of a standard Ni sample. Susceptibility corrections were made using values obtained from blank runs of the sample container, both with and without the alumina support. In addition, diamagnetic corrections were made for oxygen and Cu ion effects. The copper loading in these samples was 7.2 wt % or less. EXAFS. EXAFS and X-ray absorption near-edge structure (XANES) measurements were made at beam line
I
32
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34
36 28 CuKa,
38
1
40
Figure 1. Smoothed X-ray diffraction patterns from 32' to 40' 28 of sorbents containing 7.2% Cu and synthetic mixtures of CuO and alumina used as support for sorbent. A, fresh sorbent; B, regenerated sorbent; C,used sorbent; D, 0.5% CuO mixed with alumina, for peak at 35.6', peak minus background intensity = 18 cps; E, 1% CuO mixed with alumina, for peak at 35.6', peak minus background intensity = 39 cps; F, 5% CuO mixed with alumina, for peak at 35.6', peak minus background intensity = 210 cps.
X18B a t the National Synchroton Light Source, Brookhaven National Laboratory. The samples were pressed between two thin Mylar tapes for good thermal contact and mounted on the cryostat's copper sample holder. The cryostat has Kapton windows for passage of the X-ray beam. After evacuation, the cryostat was cooled to 77 K with liquid nitrogen and, with the sample cooled to 80 K, was aligned in the X-ray beam so that the beam passed through the sample without obstruction. Ion chambers 10 and 30 cm long, and filled with 0.4 and 1atm of neon, were used to measure the incident and transmitted beam. For EXAFS and XANES measurements, the monochromator was rotated to record the spectral range 8.7-10.1 keV that spans the copper K edge. The data were collected with a PDP-11/24 computer.
Results X-ray Diffraction. X-ray diffraction analyses of the fresh, used, and regenerated samples showed no crystalline materials present other than the ?-alumina support. Figure 1shows a portion of the diffraction patterns of fresh, regenerated, and used sorbents, as well as of physical mixtures of CuO and the alumina support. As little as 0.5% CuO gives a weak peak at 35.6' 28, but no CuO peaks are seen for any of the sorbents studied here, indicating that they are similar to those samples described by Friedman et al. (1978) in which the copper was not present as the oxide but was dispersed on the alumina support. Many of their samples did contain bulk CuO. To ensure that the samples did not contain very small crystallites of CuO, which would have been difficult to detect by X-ray diffraction, electron diffraction patterns were also obtained. They showed only the presence of the 7-alumina. Crystallites as small as 10 A can be detected by electron diffraction. The diffraction patterns of regenerated, used, and highly sulfated used sorbents are shown in Figure 2. All used sorbents showed no crystalline material except the highly
2278 Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 Table I. Susceotibilits
I 18
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22
20
1
24 28 c U
I
1
28
26
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30
K ~ ~
"From Foex et al., Vol. 7.
Figure 2. Smoothed X-ray diffraction patterns from 18' to 30' 2% showing strongest peak of CuS04-5H20for (A) highly sulfated sorbent, (B) used sorbent, and (C) regenerated sorbent.
D
7
918 28 C u K a ,
Gram of C o m e r (cm3/n) a t room samwle temD a t 77 K u. UR 17.7 X 10" 58.9 X lo* 1.59 unused unused, heated to 500 'C 15.6 X 10" 74.3 X lo* 1.63 14.2 X lo* 78.0 X 10" 1.62 used 62 X lo* 1.61 Selwood and Dallas (7.2% Cu 18 X lo* unused catalyst) CUO" 2.3 X 10" CUSO," 3.3 x 10* CuSO4*5H2O0 1.5 X 10" Der
1u 8 9 28 G u K a ,
Figure 3. Smoothed diffraction patterns from 18' to 19' 2%of sorbents with varied treatments and synthetic mixtures of CuS04. 5H20 and support used for sorbents. A, 1% CuSO4-5Hz0mixed with alumina, peak minus background = 12 cps; B, 5% CuS04.5Hz0 mixed with alumina, peak minus background = 108 cps; C, 10% CuSO4-5HZ0mixed with alumina, peak minus background = 270 cps; D, regenerated sorbent; E, used sorbent; F, highly sulfated sorbent; peak minus background = 54 cps.
sulfated sample that had been exposed to sulfur dioxide for a much longer period of time, approximately 8 h. The highly sulfated used sorbent, exposed to flue gas at 399 'C, contained 3.7% sulfur (wet analysis), while the used sorbent contained only 2% sulfur. The diffraction peaks present in a highly sulfated sample are due to CuS04.5H20 except for the peak at 25.1' 28. This peak is too intense to be CuS04.5H20only and is probably due to both the pentahydrate and anhydrous copper sulfate. From the data in Figure 3 showing the strongest diffraction peak of CuSO4-5H20for the regenerated, used, highly sulfated used, and physical mixtures of the pentahydrate and the alumina support, the amount of pentahydrate can be estimated to be about 3%. However, since the pentahydrate contains only about 25% copper, it accounts for only about 10% of the copper in the sample. Estimating the amount of anhydrous copper sulfate is difficult because of interference from the pentahydrate. The strongest line of the anhydrate superimposes a strong line of the pentahydrate, and the second strongest line is close to a weak pentahydrate line. Probably only 1% at most is present, accounting for another 5% of the total Cu.
The Cu allotted to these two crystalline sulfates is only 15% of the copper in the highly sulfated sample, leaving about 85% of the copper in a noncrystalline form. The regenerated sample contains about 1% sulfur but, like the used sorbent with 2% sulfur, shows no diffraction lines from any copper compound. It is not known if the unsupported alumina would sorb sulfur dioxide from flue gas. Diffraction studies of the fresh sorbent after reduction with H2 for 3 h a t 450 "C showed the presence of approximately 200-A crystallites of elemental Cu. The ability of Cu atoms to migrate and form crystallites indicates that they probably are held at or close to the surface of the alumina crystallites. Magnetic Susceptibility. The magnetic susceptibility results at 295 and 77 K, as well as the average effective magnetic moment per atom of copper, are shown in Table I. Magnetic susceptibility measurements for fresh and used sorbents were very similar to values reported by Selwood and Dallas (1948) in their studies of a fresh Cu/alumina catalyst where no bulk CuO was detected by X-ray diffraction. These researchers reported anomalous susceptibilities for supported Cu/alumina when the copper content was 11 wt % or less. We also observed these anomalously large susceptibilities of copper in Cu/alumina as compared with bulk cupric oxide. Selwood and Dallas attributed this behavior to the formation of ionic assemblies rather than of a two-dimensional solid solution. Data for our samples were collected at two temperatures, and therefore Curie-Weiss behavior could not be confirmed definitively. Because the susceptibilities of the Selwood and Dallas (1948) samples followed the CurieWeiss law, and our results match theirs at the temperatures of interest, we assumed Curie-Weiss behavior with reasonable certainty and calculated the effective moment per copper atom. The values presented in Table I represent the average value for the two temperatures, and the experimental error is i0.2 FB. As can be seen, the effective magnetic moments per copper atom for the unused and used samples are in line, within experimental error, with the expected "spin-only'' value of 1.8 MB per copper atom. The fact that copper has a spin-only moment lends support to the idea that there is very little, if any, influence of the support upon the valence state of the copper. Although these moments agree with the copper spin-only value within experimental error, they appear to be on the low side. This could be due to the presence of some copper in the +1 or 0 valence state. An upper limit estimate of this copper would be between 1%and 10% of the total copper content. Note that this work and that of Selwood and Dallas are in contrast to the work of Lo Jacono and Schiavello (1976) on CuO/alumina systems. Using X-ray diffraction and magnetic techniques, they found cupric oxide present above 10 wt % copper and a copper/alumina spinel structure below 10 wt % copper. The type of phase that
Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 2279
D
0 2800 2200 1600 1000 400 WAVENUMBER, cm-1
Figure 5. Infrared spectra of (A) highly sulfated Cu/A1203sorbent, (B)CuS04*5Hz0,(C) CuS04.5Hz0 heated to 300 "C, and (D)CuO.
-
0 2800 2200 1600 1000 400 WAVENUMBER, cm-l
Figure 4. Infrared spectra of copper/alumina sorbents: A, alumina support (7-alumina); B, unused Cu/A1203sorbent; C, used sorbent; D, highly sulfated sorbent; E, regenerated sorbent.
is present appears to be related to the calcination procedures. Infrared. The infrared data presented in Figure 4 show the spectral changes produced by impregnating copper 4B on the p-alumina support 4A, absorbing flue gas 4C and 4D, and regenerating 4E. Spectra 4C and 4D, of used sorbents, both show the strong absorption band at about 1150 cm-l, which is characteristic of the sulfate ion. This band is very much weaker in the spectrum of the regenerated sorbent 4E. The spectra in 4C and 4D also contain a sharp spectral feature at about 1380 cm-', which is characteristic of the isolated NO3- ion. This is a strong indication of some reactive sorption of NOz from the flue gas. Spectra of the unused 4B and regenerated 4E samples show a pattern of three bands at 1620, 1530, and 1400 cm-'. The 1400-cm-' band suggests the presence of a carbonate, and this band with the 1530-cm-' band may indicate the presence of a hydrated basic copper carbonate. The band at -1500 cm-' in the spectrum of the unused sorbent (4B) is in a spectral region where a strong absorption for the mineral malachite occurs. This is consistent with the presence of a basic carbonate phase, but additional work is needed to verify this. The pure compound infrared spectra, 5B and 5C presented in Figure 5, illustrate the nature and magnitude of spectral changes when the degree of hydration or crystallinity is varied. Unfortunately, the sharp bands between 400 and 650 cm-' in crystalline sulfates are the most sensitive to changes in the environment, and they are obscured by the alumina support absorption.
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IO0 '
5 0 0 ' 700 TEMPERATURE, O C
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Figure 6. Thermogravimetry curves upon heating to 1000 OC. Dashed lines are the derivative thermogravimetry curves. A, CuSO46H20; B, used Cu/alumina sorbent (exposed to flue gas); C, A12(S04)3.18Hz0;D, unsupported ?-alumina.
TG. In Figure 6 are shown the weight loss curves for CuS04-5Hz0upon heating to 1000 "C, for a used Cu/ alumina sorbent, for A12(S04)3.18H20,and for the alumina support by itself. The derivatives of the curves are depicted by the dashed lines, and the minima in the derivative curves (DTG) between 700 and 1000 "C were chosen to indicate the temperatures for the decomposition of the sulfates. The CuS04 and Alz(S04)3lose SO3 at about 770 and 915 "C, respectively, while the used sorbent decomposes a t about 840 "C. To check that the concentration of sulfate did not affect the decomposition temperature, CuS04.5H20and A12(S04)3.18H20were each mixed with the alumina support so that the mixtures contained approximately 2% sulfur, the content of the used sorbent. The decomposition temperature of the CuS04 decreased to about 715 "C, and while it is difficult to measure the decomposition temperature of the Al2(SO4I3,it is approximately the same as the unmixed sample. These curves are shown in Figure 7, along with curves for the
2280 Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 L
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furnace exhaust and laboratory-oven-prepared samples is unclear, and it can only be concluded that the laboratory-prepared samples imperfectly model the real-world conditions. ESR. Bulk copper(I1) oxide does not show an ESR signal, although it does show a static paramagnetic susceptibility. When dispersed on alumina, roughly 19'0 of the copper ions is ESR detectable at room temperature. The spectra are very characteristic of cupric ions, with g 2 and g-factor anisotropy, and are very similar to the curves shown by Khulbe and Mann (1982). There is resolvable hyperfine structure at lower temperatures. Its ESR detectability implies that this fraction of the copper ions is not located in bulk copper oxide crystallites. Upon exposure to SOz in air or nitrogen a t ambient temperature, the number of ESR-detectable ions increases by one-third. When copper(I1) oxide is heated to 375 and 400 "C, the copper ESR line broadens and loses intensity. After a few minutes of exposure to SO2 in nitrogen, a strong narrow line appears near g = 2, attributed by other workers (Gonzdlez-Elipe and Soria, 1986) to SOz- radical anion. Upon exposure to SOz in air, the radical signal disappeared. After exposure to SOz in air, the sample was exposed to air saturated with water vapor, and the temperature was lowered to room temperature. At intervals of a few hours, an ESR spectrum of the sample, still under a flow of wet air, was obtained. The spectrum showed a nearly symmetric cupric ion signal, narrower than that of the signal in the unreacted sample. The intensity of this signal increased with time, reaching a maximum intensity in about 20 h; further hydration did not increase its intensity. At maximum, the intensity was approximately 10 times that of the unreacted sorbent. The position and symmetry of this signal identify it as CuS04.5Hz0. There was no background from any unreacted ESR-detectable copper oxide. On the basis of the earlier estimate that 1% of the copper present in the original unreacted sorbent was ESR-detectable, apparently only about 10% of the copper originally present has reacted to form CuS04. The rest of the copper reacts to form (unidentified) ESR-undetectable copper compounds, presumably sulfates. These other unknown sulfates are probably the progenitors of the three unassigned lines in the Raman spectrum at 1070, 1130, and 1160 cm-'. For purposes of comparison, a sample of pure CuS04. 5H20 powder was subjected to the same treatment of hydrogen reduction, sulfation, and hydration. Here, of course, all the copper ions initially present contributed to the characteristic ESR spectrum of this compound. Upon heating to -60 "C, the symmetric line broadened and became asymmetric as the sample lost some of its water. At higher temperatures, the copper ESR signal disappeared; the distorted tetrahedral structure of CuSO, generates such g-factor anisotropy that the broadened line is not detectable at either elevated or room temperature. After reduction with hydrogen in nitrogen a t 375 "C, the sample appears as bright metallic copper powder with no EPR signal. Oxidation and sulfation do not regenerate a signal until the sample is hydrated with humid air. Most, if not all, of the ESR signal intensity reappears. Starting the cycle with a sample of -&"-long, 1mm-diameter copper oxide wire produced similar results. However, not all of the sample reacted with oxygen and sulfur dioxide, and rehydration produced a cupric ion ESR line that, although symmetric, was broader than usually observed for CuSO4.5H20at room temperature. The intensity of this line was much less than would be expected if all the copper present was sulfated and hydrated. It can
-
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500 ' 700 ' TEMPERATURE, C '
300 '
900
'
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' 500 ' 700 ' TEMPERATURE, 'C
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Figure 7. Thermogravimetry curves upon heating to 1000 "C. Dashed lines are the derivative thermogravimetry curves. A, CuSO4.5H20mixed with alumina support to contain about 2% sulfur; B, used Cu/alumina sorbent (exposed to flue gas); C, alumina support which had been exposed to SOp and absorbed a small amount mixed with alumina support to conof the gas; D, A12(S04)3.18H20 tain about 2 % sulfur.
sample of the alumina support that had been exposed to SO2 and absorbed a small amount of the gas. This last sample gave curves very similar to those of A&(s04)3 mixed with the alumina support in the high-temperature range. The TG shows that the used sorbent decomposes at a temperature higher than does CuS04 and lower than does Alz(S04)3,indicating that neither is present in the sorbent. Raman. Sorbent pellets sulfated in the flue gas stream of one of PETC's coal-fired furnaces were removed from the gas stream and stored in air for several weeks. The pellets were then ground to a fine powder and pressed into a disk with a Teflon backing. A Raman spectrum of this sample obtained with 5145-A excitation showed five bands in the sulfate region, 900-1300 cm-l, two of which are characteristic of CuS04-5H20. The other three bands are not present in the spectrum of CuS04.5H20. Furthermore, these lines are not characteristic of the galumina substrate nor of CuO on ?-alumina. The spectra of numerous sulfates in the region 900-1300 cm-' were obtained in an attempt to identify these lines. The compounds investigated included CuS04,CuS04-H20,C U ~ S O ~ ( O H Cu3(S)~, Oq)(OH)4, C U ~ S O ~ ( O H ) ~ *CHU~~OS,O ~ ( O H ) B * ~CHU~~OS,040,A12(S04)3,CuA1204,C U K ~ ( S O ~H2S04, ) ~ , and polymorphous reagent-grade A1203. None of the compounds can account for the observed three lines. Therefore, all can be excluded as principal components of the Raman scatterers in the highly sulfated sorbent. Identical pellets of copper oxide dispersed on alumina were cycled in a laboratory oven using a flow of humidified air, 0.5% SO2 as the sulfation reagent, and nitrogen with 0.5% H, as the reducer gas. Two samples were repeatedly cycled, one at 400 "C and one at 350 "C. Exposure to the reactants was always in excess, and the reactions are therefore assumed to have gone to completion. Visual observation at 400 "C indicated that both the sulfation and reduction are diffusion controlled under the conditions listed above. Under a slow flow of the reactant gases, the color changes associated with reactions appear at the upstream end of the sample bed and propagate in minutes down the length of the bed. Raman spectra from the samples generated in the laboratory oven were difficult to obtain owing to an intense fluorescence background. Only the intense CuS04.5H20 band at 982 cm-' was present in the sample from the 400 "C experiment after SO2 exposure, and only the three unassigned bands at 1070, 1130, and 1160 cm-l were present in the 350 "C treatment. The origin of the difference in background fluorescence between
Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 2281
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Figure 8. X-ray absorption near-edge structure (XANES) spectra of (A) Cu20, (B)CuO, (C) Cu/alumina sorbent, exposed to flue gas, (D) Cu/alumina sorbent, unused, and (E) Cu/alumina sorbent, regenerated.
be concluded that bulk cupric oxide does not sulfate nor reduce in the same manner as the Cu dispersed on alumina or Cu formed by reducing powdered copper sulfate and then reoxidizing it. Several new conclusions can be drawn from this preliminary ESR work. First, major products other than CuS04 result from the reaction of dispersed Cu/alumina with SO2and air at 400 "C. Second, the generation of these products is dependent on the history of the sorbent pellets. Third, the SO2 radical anion does not play a role in the chemistry of this system in the presence of air at elevated temperatures. Fourth, the dispersed Cu/alumina behaves differently from bulk cupric oxide and, as other techniques have shown, should not be identified as CuO. EXAFS. Comparison of the near-edge absorption curves of the fresh, used (exposed to flue gas), and regenerated samples with those of CuO and CuzO clearly indicates the divalence of the copper (Figure 8). However, the inflections (marked by letters on the CuO curve) are much weaker or nonexistent for the sorbents, suggesting a less well-developed structure and a smaller number of neighbors. A complete analysis of the EXAFS data was accomplished by fitting the data to those of CuO to obtain the unknown parameters. Once the phase and amplitude parameters were extracted from the CuO data, those values were used to determine the Cu-0 distances and coordination numbers of the sorbent samples. These values are listed below: sample fresh used regenerated
Cu-0 distance, 1.93 1.95 1.92
A
f 0.02
coordination no. f 0.5 2.7 2.4 2.4
In crystalline CuO, each Cu(I1) ion is coordinated by four oxygens in a square-planar configuration at a distance of about 1.95 A according to Asbrink and Norby (1970). Earlier workers had described the oxygen coordination as
20
40 60 RADIUS
80
Figure 9. Fourier-transformed (Xk3)Cu K-absorption edge EXAFS data. The data are not corrected for the phase shift. A, CuO; B, Cu,O; C, Cu/alumina sorbent, unused; D, Cu/alumina sorbent, exposed to flue gas; E, Cu/alumina sorbent, regenerated.
a distorted octahedron, since there are also two oxygens 2.78 A from each Cu(II), but this distance is too long to be a bonding distance. The Fourier transforms of the samples are contained in Figure 9. They show the same Cu-0 distances as bulk CuO and no evidence of Cu-Cu neighbors even at liquid nitrogen temperature. Since there is a strong peak for only the nearest oxygen atoms, the data indicate that there is no long-range order and that the CuO units are amorphous or very small particles. The calculated coordination numbers are much lower than the four or six normally expected for Cu(I1) ions. This is probably due to two factors: first, the uncertainty in the value used for the mean free path of electrons in the calculations; and second, the presence of very small CuO particles. When the particles are very small, more of the Cu(I1) ions are close to the particle surface, and these ions may have fewer surrounding oxygens than Cu ions deep inside a large particle. The EXAFS measurements cannot distinguish between planar and nonplanar coordinations. Discussion. The various types of analyses that have been carried out on the fresh and used sorbents indicate that the fresh sorbent is something other than crystalline CuO/alumina and that the used sorbent is not CuSO,/ alumina. Infrared shows the presence of sulfate in the used sorbent, but the TGA measurements indicate the sulfate decomposes at a temperature between those expected for CuS04 and Al,(S04)3. The strongest positive evidence for the presence of something other than CuO on the alumina is the magnetic susceptibility work. These data show that the samples have a susceptibility much higher than that of CuO or CuS04. X-ray diffraction, Raman, and TG provide no evidence that CuO and CuS04 are present. The EXAFS data suggest that there are an average of 2.4 or 2.7 oxygens about the Cu(I1) ions but cannot reveal how they are arranged. In their studies of Cu/alumina catalysts, Friedman et al. (1978) described EXAFS studies only on samples that contained some crystalline CuO. Therefore, it is difficult
2282 Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988
to compare their coordination number with the coordination numbers of the samples in this work that were free of crystalline CuO. They did report coordination numbers of 4.4 f 0.5 for catalysts containing 8.8% Cu calcined a t 900 OC, and 5.2 f 0.5 for catalysts containing 6.1% Cu calcined at 500 “C. Assuming only the presence of tetrahedral and octahedral sites, they found 80% tetrahedral sites in the former and 40% in the latter. In most of the studies at PETC using Culalumina sorbent, the copper content has usually varied 5-7%. A t these levels, no crystalline CuO is usually present in the fresh sorbent and no CuS04 or CuSO4.5H20 in the used sorbent. Yeh et al. (1987) found that a sorbent containing 18% copper disintegrated after a few absorption-regeneration cycles even when there was no mechanical handling of the particles. They state “The phenomenon was not observed using sorbents containing 11% or less copper. Experience obtained during the microbalance kinetics study indicates that 8% to 9% sorbent copper may be an upper limit, although no quantitative data are available for an accurate assessment”. The disintegration cited above is probably due to the presence of crystalline CuO, which expands when exposed to flue gas to form CuSO, or CuS04.5H20 and then contracts when it is converted back to CuO with regeneration and exposure to air. The Cu loading used in this work is approximately a monolayer of Cu for a support with a surface area of 200 m2/g. Since crystalline CuO has a detrimental effect on the physical nature of the sorbent, a support with a much higher surface area would be needed to increase the Cu loading and thereby increase the effectiveness of the sorbent. Summary. The Cu in the Cu/alumina sorbent is a noncrystalline material, and no crystalline Cu compounds are formed during the sorption of SO2 or during regeneration. While the structure of the sorbents is not known, the EXAFS data indicate that the Cu(I1) ions are coordinated by 2.4 or 2.7 oxygens in an as-yet-unknown configuration. The EXAFS data indicate that there is no long-range order and that the CuO units are very small or amorphous. When the particles are very small, more of the Cu(I1) ions are close to the particle surface, and these ions may have fewer surrounding oxygens than Cu ions deep inside a large particle. The low coordination number of the Cu may be related to the material being a good sorbent for SO2.
Acknowledgment The late Dr. J. V. Sanders kindly made the TEM observations on the sorbents. M. Ramanathan and P. A.
Montan0 received support from the West Virginia Energy Research Center. D. Finseth prepared the infrared spectra. J. Yeh kindly provided the samples. Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favoring by the US Department of Energy. Registry No. AI20,, 1344-28-1;Cu, 7440-50-8; SOz,7446-09-5.
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Received f o r review May 6 , 1988 Accepted August 23, 1988
