Ind. Eng. Chem. Res. 1991,30, 2105-2113
2105
Temperature-Programmed Reduction and X-ray Photoelectron Spectroscopy of Copper Oxide on Alumina following Exposure to Sulfur Dioxide and Oxygen Benoit Kartheuser and Benjamin K. Hodnett* Department of Materials and Industrial Chemistry, University of Limerick, Plassey Technological Park, Limerick, Ireland
Alfredo Riva and Gabriele Centi Department of Industrial Chemistry and Materials, V.le Risorgimento, 4, University of Bologna, 40136 Bologna, Italy
Haris Matralis, Marie Ruwet, and Paul Grange Groupe de Physico-Chimie Minerale et de Catalyse, Unite de Catalyse et Chimie des Materiaux Divises, Universite Catholique de Louvain, Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium
Nello Passarini EniMont Anic, 20097 Sun Donato Milanese, Milano, Italy
Sorbenta/catalysta based on alumina-supported CuO were prepared by impregnation using a copper sulfate solution, activated under Hz, and exposed to SOz and 0% The subsequent reduction of those samples (before and after sulfation) was studied by temperature-programmed reduction (TPR)using Hz or C H I as the reducing gas. The reaction products were analyzed by mass spectrometry. In addition, chemical analysis for copper and sulfate, X-ray photoelectron spectroscopy, and Fourier transform infrared analyses were carried out. Prior to sulfation, sulfate impregnated as copper sulfate is transferred to the alumina to form aluminum sulfate. The TPR profile then corresponds to the reduction of CuO or CuAlz04,followed by the reduction of A12(S04)3.Any residual CuS04is reduced to copper sulfide. After sulfation the sulfur to copper ratio achieved was usually greater than 2:l. In these circumstances the copper was present as CuS04. This compound was reduced to form metallic copper and SOz, but some copper sulfide also formed. At higher temperatures A12(S04)3 was reduced as well as the copper sulfide formed a t the lower temperatures. Methane is a less powerful reducing agent than hydrogen, but its utilization avoided the formation of copper sulfide.
Introduction The problem of cheap and efficient systems for the removal of SO2and NO, from flue gases continues as a major challenge @Eat and Schweiger, 1984). Many systems have been devised for the separate removal of these pollutanta, and some have been taken to commercial exploitation (Kohl and Riesenfeld, 1979). In general systems for SOz removal may be classified as wet or dry, but all essentially operate on the principle of SOzremoval by the formation of a stable sulfate or sulfite (Moser, 1981). This method is readily seen in the use of supported metal oxides as sorbents for SO2 in many so-called dry processes. This approach, while purifying the flue gas, results in a greater bulk of meterial to be disposed. Such is the scale of the problem that material disposal by simple dumping is not feasible. Hence attempts have been made to regenerate the sorbent and find a suitable end use for the sulfur compounds generated in the process. Interest has focused on the CuO/A1203system as a potential sorbent/catalyst for the removal of SOz (CuO acting as a sorbent, with the formation of CuS04) and for the selective catalytic reduction of NO,(CuSO, acting as the catalyst) (Lowell et al., 1971). The copper system is attractive because CuO readily reacts with SO2 and Ozto form the sulfate, and ita regeneration by reduction is less demanding than that required for many other metal sulfates (Lowell et al., 1971; Uysal et al., 1988). Uptake of SOz by copper oxide on alumina follows the same reactions as for all metal oxides, i.e., eq 1 followed by eq 2 of Table I. The formation of aluminum sulfate, by eq 3, on CuO/AlZO3working in a sulfur dioxide atmo-
Table I. Eauations l/zOz + SOz = SO3 (1) CuO + SO3 = CuSO, (2) A1203 + 3S03 = Alz(S04)3 (3) CUO + Hz = CUO + HzO (4) CuAlz04 + Hz = Cuo + Alz03+ HzO (4a) CuS04 + 2Hz = Cuo + SOz + 2Hz0 (5) 2CuS04 + 6Hz = CUZS + SO2 + 6HzO (6) cUso4 + 4Hz = CUS + 4H2O (7) CUS + HZ = CUO + HZS (8) CUZS+ HZ = ~ C U+" HZS (9) (10) Alz(SO& + 12Hz = A1203 + 3HzS + 9Hz0 A12(SO4)3+ 12Hz = Al& + 12Hz0 (11) CuSO, + 14/6Hz= '/&uO + l/sCu& + 6/aSOp+ 14/6Hz0 (12) CUO+ '/zHzO + '/&Oz (13) CUO + '/&HI + '/&HI = CUO+ SO2 + '/zCOz + H20 (14) CUSO~ Alz(S04)3 + CHI = Alz03+ 3soz + 2H20 + CO (15) A12(S04)3+ CHI = AlzS3 + 6Hz0 + 3 c o z (16) CH4 + HzO = CO + 3Hz (17) SO2 + 3Hz = HzS + 2Hz0 (18) SO2 t HZS = 25 + 2Hz0 (19)
sphere, has been observed by many workers (McCrea et al., 1970; Bhattacharyya et al., 1988; Nam et al., 1986). A limited amount of data is available in the chemical literature concerning the reduction of CuSO4/AlZOs. Copper sulfate is reduced in methane according to eq 14 (McCrea et al., 1970; Yeh et al., 1987; Pollak et al., 1988). Equation 5 is generally proposed for the reduction of CuSO, by hydrogen, but some authors have also proposed the formation of copper(1) sulfide (Dautzenberg et al., 1971) or copper(I1) sulfide (McCrea et al., 1970) by eqs 6 and 7. The reduction of copper sulfide (eqs 8 and 9) or aluminum sulfate (eqs 10 and 11)requires a higher tem-
0888-5885/91/2630-2105$02.50/00 1991 American Chemical Society
2106 Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991 Table 11. CuOCuSOJAl~Os Samples Used in This Study impregnation of alumina by CU(CH&OO)~,calcined Ac 45OC at 450 OC Ac 450CS Ac 45043 sulfated at 300 "C IC impregnation of alumina spheres by immersion in a copper sulfate solution at room temperature and then ground IG 45OC IG calcined at 450 OC in air for 4 h IG 45OCS IC 45OC after sulfation incipient wetness impregnation of ground alumina GW at room temperature GW 45OC GW calcined at 450 "C in air for 4 h GW 450CS GW 450C after sulfation IG 400R IC reduced at 400 "C and dried at 150 OC IG 400RS IG 400R after sulfation IG 450R IG reduced at 450 "C IG 450RS ' IG 450R after sulfation IG reduced at 500 "C IG 500R IG 500RS IG 500R after sulfation
perature than the reduction of CuS04 (McCrea et al., 1970). Although many equations have been proposed in the literature for the reduction of CuSOI/AlZO3sorbents/ catalysts, the factors that favor individual reaction routes are not understood. This study was undertaken in an attempt to elucidate the factors that determine the contributions of the equations listed above and others in the reductive regeneration of alumina-supported copper sulfate.
Experimental Section Origin of the Sorbents/Catalysts. The catalyst samples used in this work were prepared by impregnation of alumina (325 m2 g-') by copper sulfate. The alumina was impregnated in two forms, i.e., as spheres (diameter = 2 mm) and subsequently ground prior to use or as spheres and ground prior to impregnation. After drying the samples were reduced in flowing hydrogen between 400 and 500 OC, again dried in air at 150 O C , and subjected to a sulfation treatment which involved their exposure to a flow of sulfur dioxide and oxygen at the temperatures to be specified below. Table I1 gives some preparation characteristics of the samples, and associates each sample with a code, so that IG 4OORS refers to a sample prepared by impregnation (I) of alumina pellets, ground (G), reduced in hydrogen (R) at 400 OC and then sulfated (SI. Sorption of SOz. Uptake of SOz was followed by passing a mixture of SOz and Ozover the sorbents in the temperature range 250-350 OC. Weight changes were followed by thermogravimetry. Temperature-ProgrammedReduction (TPR). TPR analysis was carried out by passing a gas mixture comprising 5 % H2in Nzor 5% CHI in Ar over a sample of sorbent/catalyst held between two quartz wool plugs in a straight tube quartz reactor. This assemble was located in a Stanton Redcroft furnace controlled by a Cambridge Process Controller 702. Hz or CHI consumption was monitored by use of a thermal conductivity detector (TCD). All samples were treated at 400 OC in a flow of nitrogen for 1 h prior to TPR analysis and cooled to 100 OC under nitrogen. In typical operation 50 mg of sample was used,the gas flow rate was 20 mL min-', and the linear heating rate was 10 "C min-'. In normal operation TPR profiles were run to 800 OC, but in some cases the program was stopped at a lower temperature: The resulting samples were cooled in Nzand passivated in a flow of 2% Oz in Ar at 12 "C and then analyzed by X-ray photoelectron Spectroscopy (XPS). The TPR peak of CuO (reduced according to eq 4) was used as a standard for the quan-
titative analysis of the TPR profiles. The TPR profiles of CuO, CuSO,, and A12(S0& and 1:l mechanical mixtures thereof were also recorded and will be referred to below as model compounds or mixtures. Mass Spectrometry (MS). A V.G. Micromass 12B magnetic focusing spectrometer was used for a detailed analysis of the reduction produds of catalyst regeneration. For this analysis, the gas exit line from the TCD was connected to the inlet of the maas spectrometer via a needle valve. The following mass numbers were used to monitor the individual reaction products: SOz (64 m u ) ; Ha (34 amu); 02 (32 m u ) ; CHI (16 m u ) ; HzO (28 m u ) ; Nz (28 m u ) ; Ar (40m u ) ; COZ (44m u ) ; CO (28 m u ) . CO was very difficult to monitor due to a large interfering background at 28 amu. Fourier Transform Infrared Spectroscopy (FTIR). The Fourier transform (FI'IR) spectra were recorded using a Perkin Elmer 1750 FTIR spectrometer and the KEh disk technique. Calibrated amounts of samples (around 0.1% w/w with respect to KBr) were used, and electronically calibrated subtraction of the Alz03 contribution to the spectrum was carried out. X-ray Photoelectron Spectroscopy (XPS).XPS was carried out at room temperature with an SSX-100 Model 206 photoelectron spectrometer from Surface Science Instruments (SSI),interfaced to a Hewlett Packard 9000/310 computer. The samples were pressed (3.5 metric tons cm-2) in a stainless steel trough of 6-mm diameter. The residual pressure in the spectrometer was in the range (1-5) X 1P Torr. A monochromated A1 anode (energy of the A1 Kline 1486.6 eV), powered at 10 kV and 20 mA, was used for X-ray production. The binding energy scale of the spectrometer was calibrated by using the Au 4f7fz line (binding energy 83.98 eV). The analyzer energy and spot size were, respectively, 50 eV and 1.4 mm2. These conditions gave a fwhm on Au 4f7f2 of 1 eV. The positive charge, developed on the samples due to the photoelectron process, was compensated by a charge neutralizer, a flood gun, whose energy was adjusted to 6 eV (50 A). The binding energies were calculated with respect to the C 1s peak set at 284.6 eV. The intensities were estimated by calculating the integral of each peak following "S-shape" background subtraction (Wagner, 1983). Atomic concentration ratios were calculated by correcting the intensity ratios with the theoretical sensitivity factors based on the Scoffield cross sections. The transmission function of the spectrometer was aasumed to be independent of the kinetic energy and the electron mean free paths were taken as a function of (E,) 0.7. DeconvoIution of the peaks was carried out with the best fitting routine of the SSI instrument. Chemical Analysis. Copper was analyzed by atomic absorption spectroscopy ( U S ) and sulfate by a colorimetric method. Copper Analysis. For the AAS analysis, two techniques were used to dissolve the copper component of the samples, i.e., using NHIOH to dissolve all the copper present and using water to dissolve copper sulfate only. Dissolution by NHIOH. About 0.03 g of each sample was dissolved by stirring in 20 mL of NH,OH (33%) for 2 or 3 h at room temperature. Then the solution was acidified with concentration HCl(20 mL), filtered into a 250-mL volumetric flask,and diluted with distilled water. A blank waa made in the same way. Dissolution by Water. About 0.03 g of the sample was dissolved in 50 mL of distilled water for 5 h on a vapor bed, filtered into a 250-mL volumetric flask, and diluted with distilled water. Copper contents were determined by using
Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991 2107 S:Cu Atoa~icRatio
Table 111. Chemical Analysis of Sorbents/Catalysts Mmol of pmol of CuS04: rmol of SO, tow: sample Cut4t4IIg cuso4/&! ~ut4t4I sod&! cut4t4I 750 1.13 IG 661 90 0.14 816 1.30 87 0.14 IG 45OC 630 1875 2.91 IG 450CS 577 0.89 645 740 1.02 724 91 0.13 GW 701 1.11 85 0.13 630 GW 450C 1698 2.51 577 0.85 677 GW 450CS 719 1.27 567 167 0.29 IG 400R 1313 2.38 319 0.58 551 IC 400RS 42 0.07 542 0.91 598 IG 450R 1542 2.72 567 491 0.87 IG 450RS 396 0.66 26 0.04 598 IG 500R 1063 1.87 567 319 0.56 IG 500RS
25OC
TCD Response
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Time / min Figure 1. Sorption of SO?by Ac 4MC between 250 and 350 "C.
a Varian SpectrAA atomic absorption spectrophotometer with a graphite furnace. Analysis was carried out at 324.7 nm. Standards were made up by using standard cupric nitrate solutions to which was added distilled water or blank solution. Sulfate Analysis. The sulfate analysis was made by using a colorimetric method proposed by Krachmer (1970). This method is based on the use of barium chloranilate as a colorimetric reagent for sulfates to form barium sulfate and a soluble chloranilate which is determined photometrically. Ethanol was added to suppress the solubility of barium chloranilate, which otherwise contributes a high background. An ion-exchange column was used to remove interfering cations, like copper. After dissolution of copper oxide and all sulfate by NH40H (33%), the solution was filtered into a 100-mL volumetric flask. The solid was washed many times and diluted to 100 mL with distilled water. This solution was passed through the ion-exchange resin three times. Between each pass through the ionexchange resin, the column was regenerated with HCl(1 M). The pH of the effluent was adjusted to a value of 4 with dilute HC1 or NH40H solution; 20 mL was transferred to a 100-mL volumetric flask and 10 mL of potassium acid phthalate buffer (pH 4) and 50 mL of 95% ethanol were added. The solution was shaken vigorously for 10 min and filtered to remove insoluble barium chloranilate and sulfate, and the absorbance was measured in the UV region (312 nm) with the use of a Varian DMS 100 S UV/visible spectrophotometer. The sulfate present was determined by reference to a standard curve prepared from pure potassium sulfate.
Results Sorption of SO2 by Cu/A1209. Figure 1 shows the uptake of SO2by a C u / 4 0 3 sample (Ac 45OC) monitored by thermogravimetry in the temperature range 250-350 OC. This behavior was typical of that observed for a large number of samples tested (Centi et al., 1990). Two important features will be pointed out here: the first is the rapid initial uptake of sulfur by the sorbent followed by a leaaening of the rate and eventual saturation. The second feature of interest is the high S:Cu ratios achieved above 300 "C.
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TEMP /C Figure 2. TPRH2 profdes of samples (a) GW (b) GW 45OC; (c) GW 450CS.
Chemical Analysis. Table 111gives the results of the chemical analysis of the sorbents/catalysts. The second column gives the total copper content of each sample. Copper determined as CuS04 (i.e., that part soluble in water) is shown in column 3, and the ratio CuS01:Cu,d is presented in column 4. The sulfate content is indicated in column 5, and S 0 4 : C u ratios are presented in column 6. These results clearly demonstrate the following points Almost all of the copper present in the nonsulfated samples is not present as CuS04, in spite of the fact that impregnation was carried out using this salt. The S04:Cubd ratio was always close to unity for the nonsulfated samples, so the impregnated sulfate did not decompose prior to analysis. For the sulfated samples, the S01:Cuhd ratio was consistently greater than 2:l and the ratio CuS04:Cuba was close to unity in some cases but never less than 0.6:l. By contrast this ratio in nonsulfated samples was never more than 0.3:l and usually less than 0.15:l. Increased reduction temperature in H2 during sample preparations for the sequence IC 400R, IG 450R, and IG 500R resulted in a reduction of the total sulfate content. Temperature-Programmed Reduction under H2 (TPRH2). Typical TPR profiles for the samples GW (before calcination), GW 450C (after calcination), and GW 450CS (after sulfation) are presented in Figure 2.
2108 Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991 TCD Response
TCD Response
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TEMP /c Figurs 3. TPRH2 profiles of samples (a) IC 400R (b) IC 450R (c) IG 500R.
Comparison before and after calcination shows a small increase in hydrogen consumption following calcination for temperatures below 350 "C. Hydrogen consumption started at lower temperatures and increased faster for the calcined vis-&vis the uncalcined samples. The major envelope of peaks observed for these samples irrespective of calcination was centered around 550 "C. A sharp contrast was observed for sulfated samples, as shown in Figure 2 for GW 45OCS. Hydrogen consumption did not commence until 300 "C and increased sharply to give a peak a t about 320 "C. Thereafter, hydrogen consumption continued as a pronounced background. In this case also an envelope of peaks was observed between 500 and 650 "C above the large background. This background continued until about 800 "C. Comparison of samples IG 4WR, IG 450R, and IG 5OOR (Figure 3) shows an increase in intensity and a sharpening of the peak at 260-280 "C with increased reduction temperature. There is also a shift of the second peak to higher temperatures as the reduction temperature was increased. This peak was observed at 550 "C for IG 400R, at 580 "C for IG 450R, and at 630 "C for IG 500R. For each of the sulfated samples IG 400RS, IG 450RS, and IG 5OORS (Figure 4) consumption of hydrogen started at higher temperatures than the corresponding nonsdfated samplea and clearly reached greater proportions. A notable feature of each profile was the shift, vis-&vis the nonsulfated samples, of the envelope of peaks in the midtemperature range to about 500 "C or slightly less. A comparison of the nonsulfated and sulfated samples shows important variations: The reduction of sulfated samples starts later than the nonsulfated samples; this was most apparent for IG 450R and IG 450RS. The first peak on the TPR profiles of sulfated samples occurs at about 310-320 "C as against 260-280 "C for nonsulfated. The peak in the midtemperature range was shifted to lower temperatures for the sulfated samples. Elemental sulfur was never detected at the reactor exit. Analysis of TPRHP Profilea by Mae8 Spectrometry (MS).The results of MS analysis of the TPRH2 profiles
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TEMP fC Figure 4. TPRH2 profiles of samples (a) IG 400RS;(b)IG 450RS (c) IG 500RS. TCD or MS Response
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TEMP/C Figure 5. TPRHP profiles of sample GW with MS analysis.
are summarized in Figures 5 and 6 for samples GW and GW 450CS, respectively. These results may be summarized as follows: Water was detected in association with every peak. H20 was the only product detected from GW prior to 500 "C. H2S was detected above 570 "C. SOz was detected from about 280 "C from GW 450CS, and this stopped at about 500 "C. Thereafter production of H2S commenced and continued until 800 "C. TPR of the Model Compounds and Mixtures. Figure 7 presents the TPRH2 profiles of unsupported CuO, and of mixtures thereof. The main CuS04, and A12(S04)9 featurea of note for the individual compounds were the low temperature at which CuO could be reduced, the complex pattern observed for CuS04,and the very high temperature
Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991 2109
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TEMP /c Figure 6. TPRH2 profiles of sample GW 4 W S with MS analysis.
required to reduce A12(S04)3. The profile of CuO + CuS04 is clearly not the sum of the CuO and CuS04 profiles. The CuO component can be seen as before, but a difference can be seen in the rest of the profile from that expected for CuS04. Two peaks of high intensity at 410 and 445 O C were observed, but the peak present at 525 OC in the TPR profile of CuS04 had disappeared. The broad peak at very high temperatures, noted for pure CuS04, was still present. The TPR profile of the mixture CuS04 + A1(S04)3exhibited only two peaks: one at 420 OC and the second mth the maximum between 560 and 600 OC. Hydrogen consumption was almost finished at 650 OC. For the mixture of CuO + A1z(S04)3 a TPR profile more complex than the s u m of the profiles of CuO and A&?(so4)3 was recorded. Two peaks with maxima at 290, and 400 TCD ResDonse
OC were observed, followed by a third peak at 700 OC. The TPR profile of CuO + CuS04 + Alz(S04)3is composed of one peak at 265 OC (similar to that of CuO), followed by various peaks between 320 and 450 OC, two peaks at 560 and 610 OC, and ending with a small peak at 665 O C . Temperature-Programmed Reduction under CH4 (TPRCH4). The TPRCH4 profiles of samples GW, GW 450C, and GW 450CS are presented in Figure 8. The corresponding profiles for samples GW 450CS and IG 450CS with MS analysis are presented in Figures 9 arid 10. Several features of these profiles may be alluded to here. By comparison with the TPRH2 profiles of the same materials, the traces are much simpler. In the case of the nonsulfated samples a rather broad peak was observed between 500 and 700 O C ; SOz and COz production as well as CHI consumption were associated with the peak and at higher temperatures a strong negative peak began to evolve. The latter was a common feature at high temperatures of all TPRCH4 profiles and is probably due to the production of small amounts of H2 from CHI decornposition at these temperatures. For the sulfated samples a much more pronounced peak was observed in the TPRCH4 profiles with a maximum at 550 OC associated with the production of SOz and COz and with the consumption of CHI. Hydrogen sulfide was not detected in any conditions during reduction with CH,. This observation was also made by McCrea et al. (1970). XPS of the Sorbents/Catalysts. All catalysts exhibited two peaks for Cu 2~312at about 932.5 arid 935 eV with a large satellite peak between 941 and 944 eV. Comparison with the binding energies of reference materials shows that peaks at about 935 eV could be associated with CuA1204 and/or CuO and/or CuS04 (Strohmeier et al., 1985; Rosencwaig and Wertheim, 1972; Larson, 1974; Robert and Offergeld, 1972). The peak at 932.5 eV could be asaociated with CuzO or Cuoor CuzSand also with Cu+ produced in the vacuum system during XPS analysis. Therefore it is impossible to distinguish between these compounds on the basis of the Cu 2p3Iz peak alone. All samples also exhibited a S 2p peak at about 169 eV which corresponds to sulfur in the sulfate form. Those TCD Response
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Figure 7. TPRHS profiles of model compounds and mixtures: (a) CuO; (b) CuSO,; (c) A12(S0,),; (d) CuO + CuSO,; (e) CuSO, (0CUO + AI2(SO& (dCUO+ CUSO~+ A12(SOJp
+ Al,(SO,)B;
2110 Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991 TCD Response
TCD or MS Response
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GW
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Figure 10. TPRCH4 profiles of sample GW 45ocS with MS a d ysis. Table IV. Atomic Concentration Ratios from XPS Analysis sulfide: sample CU:MX 108 S:MX 108 sulfate IG 450C 32.9 74 0 IG 45OCS 25.2 156 0 GW 450C 36.8 68 0 GW 45OCS 25.8 165 0 IC 400R 34.8 65 0 IG 400RS 25.7 108 0 IG 450R 41.8 64 0.19 IC 450RS 25.1 117 0 IG 500R 41.6 46 0.17 IG 500RS 27.0 110 0
GW 450C
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TEMP/C Figure 8. TPRCH4 profiles of (a) G W (b) GW 45OC; (c) GW 450cs.
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TEMP IC Figure 9. TPRCH4 profiles of GW450C with MS analysis.
samples that had been reduced in H2above 400 O C , namely, IG 450R and IG 500R, exhibited in addition a S 2p peak at 162 eV which corresponds to a sulfur in the sulfide form. The su1fide:sulfate ratios are reported in Table IV. Table IV also shows that there was a reduction in the surface concentration of copper after sulfation; i.e., the ratio Cu:A1 decreased after sulfation. For all samples studied the ratio S:A1 increased as expected following sulfation. XPS of Samples after Partial TPR. Again, all samples exhibited two Cu 2p3,*peaka at 932.5 and 935 eV with satellite peaks. Before reduction Ac 45OCS exhibited a S 2p peak at 169.2 eV which corresponds to sulfate. After reduction in TPRHZ conditions to a maximum tempera-
Samples after Partial TPRH2 AC45OCSH,/470 "C 82.6 64 IG 45OCSH1/800 "C 54.9 40 GW 450CS:H2/800 "C 66.0 51
0.85 0.54 0.66
Samples after Partial TPRCH4 AC45OCSCH1/470 "C 62.1 148 GW 450CS:CH,/550 "C 52.7 89 GW 450CS:CHd/800 "C 56.2 37
0.0 0.02 0.43
ture of 470 "C and passivation, it exhibited an additional S 2p peak at 162 eV which corresponds to sulfide. This feature was also noted for the other samples subjected to partial TPR analysis, and all the calculated atomic ratios are presented in Table W . These data reveal that the ratio su1fide:sulfate increases with reduction temperature but the ratio S,w:Al decreases almost by a factor of 4 (Table IV)* The corresponding data for samples subjected to partial TPRCH4 analysis are presented in the last part of table IV. The main feature was the almost total absence of sulfide even at 550 OC. At 800 "C some sulfide was detected. This should be viewed against the background of a diminishing S:Al ratio. FTIR Investigation of the Catalysts. Figure 11 presents the results of the FTIR investigation of this system. Sulfate species show characteristic vibrational bands in the 1000-1500-~m~~ region. The free sulfate species is highly symmetrical, and in the spectral region examined only the V3 fundamental is active. At lower
Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991 2111
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Figure 11. FTIR spectra of copper-alumina samples (A) after regeneration with H2at 420 OC and (B)after sulfation; spectra of (C) CuSO, and (D)Alz(S0&,.
symmetry splitting and shifting of the V3 band occurs. Figure 11compares the spectra of bulk CuS04 (C) and of &(so4)3(D) with those of a copper-alumina samples after sulfation (B)and after regeneration with H2 at 420 OC (A). It is evident from the appearance in spectrum B of additional bands not corresponding to the presence of unsupported copper or aluminum sulfate and from the splitting that the bands could be attributed to low-symmetry surface sulfate species. After regeneration (A) a lowering of the intensity of the spectrum occurs, with the dominant presence of a doublet of bands at 1130 and 1150 cm-l, which are present also before reduction. This indicates that this species (tentatively at the Cu-SO4-A1 interface (Centi et al., 1990)) is more difficult to reduce than CuS04. Discussion This discussion will attempt to associate one of the basic chemical reactions listed in Table I with each peak of the TPRHP or TPRCH4 profiles in Figures 2-10. Initially nonsulfated samples will be considered followed by the sulfated samples. Identification will be on the basis of XPS, FTIR, chemical analysis, and the TPRH2 profiles of the model compounds. TPRH2 Peak Assignments for Nonsulfated Samples. For the nonsulfated samples, the TPRH2 profiles show two important features (see, for example, figure 2). The first is a peak in the range 200-350 O C . The second feature is an envelope of peaks located between 550 and 800 o c . Several factors indicate that the first peak corresponds to the reduction of an oxide, namely, CuO (eq 4) or CuA1204 (eq 4a). Neither SO2nor Hfi was evolved with this peak according to MS analysis (Figure 5), nor was sulfur (eq 19) observed at the exit to the reactor. The chemical
analysis (Table III) indicates that most of the copper added as CuS04 transforms into another form of copper, eventually CuO or CuA1204,on the support. In the TPRHB profile of model CuO (Figure 7) a single peak similar to the low temperature peak of the unsulfated sample was noted, and a value of 270 "C has been reported in the literature for the reduction of CuO (Jones and McNicol, 1986). The envelope of peaks between 550 and 800 "C is more difficult to assign, but the following factors indicate that they are caused by the reduction of A12(S04)3according to eq 10: 1. The chemical analysis indicates that most of the sulfate present on the samples is not associated with the copper component, i.e. it is not CuSOI. We propose then that sulfate is transferred from the copper component to the support to form A12(S04),. 2. At first sight the envelope of peaks in question is not consistent with the TPRH2 profile of model A12(S04)3 (Figure 7), which indicates a single peak ca. 800 OC. However, when A12(S04)3was mechanically mixed with CuO, the copper component of the mixture reduced in two steps, but more significantly, a definite shift (70 OC lower) in reduction temperature, to the range 600-800 OC, occurred for the aluminum component. It is proposed here that the presence of metallic copper, formed via eq 4 or 4a, may have aided the reduction of &(so.&possibly by some mechanism involving spillover hydrogen species. 3. The MS analysis associates the evolution of H2S with the envelope of peaks in the midtemperature range. Although the XPS results were inconclusive as to the nature of the copper components present, they do indicate the presence of the sulfide species after certain reductive treatments (Table IV) so that some of the Al2(SO4)3 may have transformed into A12S3. However, it is also possible that the observed sulfide is CuS or Cu2S. Possible reactions involving these species will be discussed below in connection with the sulfated samples. A final point in connection with the TPR profiles of the nonsulfated samples is the chemical analysis, which indicates that a low concentration of CuS04 was present (about 10% or less of the original amount used in the preparation). Its contribution to the TPR profile must have been very low and could be assigned to the broad intermediary part of the TPR profile. The MS results indicated the absence of SO2 in this region so reduction of the small amount of CuS04 may have proceeded via eq 7. Further reduction of CuS only becomes thermodynamically allowed at higher temperatures (Kartheuser, 1990). In conclusion, for the nonsulfated samples the evidence associates the first peak with eq 4 or 4a, with some reduction of residual CuS04according to eq 7. The envelope of peaks in the midtemperature range is assigned principally to eq 10. TPRH2 Peak Assignments of Sulfated Samples. A consistent finding in the TPRH2 profiles of the sulfated samples was the higher temperature required for the onset of reduction and the greater amount of H, consumed vis-&vis the nonsulfated analogues. In addition, SOz was detected by MS analysis in association with the first peak (Figure 6), i.e., between 300 and 450 "C. The chemical analysis (Table 111) indicates that most of the copper was present as CuS04 after sulfation. The TPR profile of model CuSO4 (Figure 7) was rather complex, but ita reduction largely coincided with the temperature range 300-450 "C. These considerations lead to the conclusion that eq 5 is principally responsible for the hydrogen con-
2112 Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991
sumption observed in the TPR profiles between 300 and 450 "C. The XPS analysis (Table IV) indicated the presence of sulfide on some samples, particularly those that had been reduced at 450 "C or more and passivated. This suggests that a part of the CuS04may have been transformed into CuS or CuzS according to eq 6 or 7. This hypothesis is consistent with quantitative analysis of the TPRH2 profiles which indicated that the molar ratio of Hz consumed to CuS04present in the model mixtures was always greater than 2:1, which is the theoretical ratio associated with eq 5 of Table I. In addition the ratio of H2 consumed prior to 550 OC to total copper contents for the sorbents/catalysts was in the range 3.1-5.6:l. These data imply that eq 7, theoretical ratio 4:1, is an important route for the reduction of CuS04,but this point cannot be pursued further in the quantitative sense because TPRH2 peak deconvolution was very difficult for the sorbents/catalysts. However, it is proposed that the low symmetry sulfate species, observed by FTIR spectroscopy (Figure 11B) and assigned to the Cu-SO4-A1 interface, and which is more difficult to reduce than CuS04,is the precursor species responsible for sulfide formation. It is proposed here that the reduction of CuO or of CuAlZO4contributes very little to the low-temperature peak in sulfated samples because the chemical analysis indicated that most of the copper was present as copper sulfate. The envelope of peaks in the midtemperature range (450-550 "C) for the sulfated samples is similar in appearance to the corresponding envelope of peaks observed for the nonsulfated samples, except that a shift to lower temperatures occurred upon sulfation. The chemical analysis implies the presence of Alz(S04)3following the same logic used for the nonsulfated samples. Just as mixing CuO with model A12(S04)3 provoked a shift in reduction of the latter to lower temperatures, so mixing CuS04 with A12(S04)3provoked an even greater shift (Figure 7). This finding is consistent with the shift to lower temperatures observed for the midtemperature envelope of peaks for the sulfated samples. The MS analysis associated HzS with these peaks (Figure 6) suggests that they arise from eq 10. Following the midtemperature-range peaks, hydrogen continued to be consumed although no distinct peak appeared in the TPRH2 profiles, but the XPS analysis (Table IV) indicated the presence of sulfide at this temperature. Reduction of CuS and CuzS by hydrogen only becomes allowed thermodynamically at high temperatures (Kartheuser, 1990). It appears reasonable to assign the continued consumption of Hz between 550 and 800 "C to eqs 8 and/or 9, especially since HzS production was observed by MS analysis up to 800 "C. In summary, the first peak in the TPR profiles of the sulfated samples is assigned to the reduction of CuS04via eq 5 to produce Cuoand SO2. There is also a contribution from eqs 6 and 7 in which the low symmetry sulfate species at the Cu-SO4-A1 interface is transformed into CuS or Cu2S, A1203,and HzO a t temperatures above 450 "C. Thereafter the envelope of peaks in the midtemperature range is assigned to eq 10 and the high-temperature H2 consumption is associated with the reduction of CuS and/or CuzS (eqs 8 and 9) produced during the reduction of the Cu-SO4-A1 species. Peak Shifts Observed in TPRH2. It is clear from the above that considerable shifts in peak positions occurred depending upon the precise composition of the catalysts
or model mixtures. In addition, there was a noticeable change in the degrees of reduction of the Alz(S04)3component that could be achieved depending also upon composition. TPRH2 Peak Shifts: Model Compounds and Mixtures. Adding a copper component to model A12(S04)3 brought about a shift in reduction temperature of the Alz(S04)3and an increase in degree of reduction. The effectiveness of the copper component followed the order CuS04 (200 "C (shift)) > CuS04 + CuO (160) > CuO (70) > none (0) We have implied previously that hydrogen spillover from metallic copper may be responsible for these effects, since atomic hydrogen formed by spillover would be a more effective reducing agent than Hz(Hodnett and Delmon, 1986). At the point in the TPR profiles where reduction of Alz(SO& started, it seems likely that the model mixtures comprised Cuo and A12(S04)3in the case of the CuO + Alz(so4)3mixture and Cuo, CuS, and/or CuzS and AlZ(S04)3 in the case of the CuS04 + A12(S04)3mixture. Our conclusion therefore is that a mixture of Cuo and CuS and/or C u a is a more effective donor of spillover hydrogen than pure Cuo. TPRH2 Peak Shifts: Sorbents/Catalysts. The peak shifts observed in the catalysts may now be interpreted in terms of the explanation offered above and used to interpret the influence of reduction treatment prior to TPR. Influence of Reduction Temperature Prior to TPRH2. The chemical analysis pointed to the presence of less total sulfate (as CuS04or Alz(SO&) and less CuS04 as the pretreatment temperature increased. The TPR profiles (Figures 2 and 3) featured a stronger peak due to the reduction of CuO or CuAlz04 as the pretreatment temperature was increased. (Note: a separate drying step in air at 150 "C followed this reductive treatment, thus oxidizing any Cuo formed to CuO or CuAlz04and CuS to CuS04.) This finding is consistent with more of the copper component of the samples being present as Cuorather than as CuS04as the reduction temperature was increased. The shift of the Alz(so4)3peak to higher temperatures as the reduction was increased is consistent with the production of less CuS/CuzS in TPRH2 analysis, so reducing the effectiveness of transfer of spillover hydrogen. The smaller size of the peak due to the reduction of A12(S04)3may have been simply due to the prior removal of more sulfates as the reductive pretreatment became more severe, but the effectiveness of transfer of spillover hydrogen may also have been a factor. TPRCHI of Nonsulfated Samples. For all the nonsulfated samples a rather broad peak was observed between 500 and 700 "C. Therefore a strong negative deflection was observed in the profile. This feature is probably due to the production of H2 above 700 "C according to eq 17. Our analytical system was not sensitive to H2 or CO and very small quantities of Hz would have produced a significant response by the thermal conductivity detector. Methane consumption, COz and SOz production, and HzO were associated with the broad peak between 500 and 700 "C. Hence this peak is associated with the reduction of CuO (or CUAIzO4)according to eq 13 and the reduction of the limited amount of CuS04 present according to eq 14. The roles of eqs 15 and 16 cannot be assessed on the basis of the data presently at hand. Calcination increased the intensity of the peak between 500 and 700 "C, so pointing further to the role of CuO or CuA1204reduction in making up most of this peak.
Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991 2113
TPRCH4 of Sulfated Samples. Following sulfation a more intense peak shifted to lower temperatures, 450-600 OC, was observed. The MS analysis associated CHI consumption, COP,SO2, and H20 production with the peak, suggesting that it arose principally from the reduction of CuSO, according to eq 14. H2Swas never detected during TPRCH4, nor was elemental sulfur. This observation was also made by McCrea et al. (1970). The XPS analysis of these samples suggested no sulfide formation prior to 470 OC (Table IV)and very little sulfide even at 550 OC, i.e., at the peak maximum. At 800 OC a substantial proportion of the sulfur was present as sulfide, but this must be viewed against a diminishing S:Al ratio (Table IV)as the reduction process proceeded. In fact the sulfide produced in this case is very likely due to A12S3 produced between 500 and 800 OC under methane according to eq 16. This situation contrasts sharply with TPRHZ conditions, where large amounts of sulfide were produced even at 470 "C. Conclusions When the Cu0/Al2O3system is exposed to a mixture of SO2and 02,a complex series of interactions occurs, with transfer of sulfate from the copper to the alumina component. Subsequent regeneration by H2 involves a series of complex reactions, with Cuoand CuS and/or C@ being formed. These species in turn aid in the reduction of A12(S04)3.The corresponding reduction in methane is much simpler without the formation of CuS. Acknowledgment This work was sponsored by the European Community BRITE program. Re&tm NO. CUO, 1317-38-0; S02,7446-09-5; HS,1333-74-0; CHI, 74-82-8; C U ~ S11115-78-9. ,
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Received for review November 2, 1990 Revised manuscript received April 21, 1991 Accepted May 8,1991
