J. Phys. Chem. 1991,95,4051-4058
4051
Nature and Mechanism of Formation of Sulfate Specles on Copper/Alumlna Sorbent-Catalysts for SO2 Removal M. Waqif, 0. Saur, J. C. Lavalley,* URA CNRS 414 "Catalyse et spectrochimie", ISMRA, 6 Bd Marichal Juin, 14050-Caen Cedex, France
S. Perathoner, and C.Centi Dipartimento di Chimica Industriale e dei materiali, Universita degli studi di Bologna, viale del Risorgimento, 4, 40136 Bologna, Italy (Received: July 12, 1990)
The nature, thermal stability, and reducibility in H2 of sulfate species on copper-on-alumina and their mechanism of formation during interaction of the sorbent-catalyst with SO2antaining flows were studied in a flow microreactor and by Fourier-transform infrared spectroscopy. Spectra of the sulfate species formed on A1203,CuO, CuAI20,, and CuO/AI20, samples either by impregnation with various amounts of sulfate salts or by direct sulfation with SO2+ O2are compared. The results indicate that, on pure alumina, two types of surface sulfate species form, one more stable at low surface coverage attributed to a type with only one double S - 0 bond and the second less stable and more easily decomposed by water vapor, attributed to a SO3 group linked to an AI-0 pair site or to an oligomer species as S207. Sulfation of CuO leads to bulklike CuS04, whereas sulfation of copper aluminate leads to three types of sulfate species, one linked to AI3+ ions, another to Cu2+ions, and the third to sulfate species in interaction both with AI3+and Cu2+ions. The latter species does not appear in the spectra of Cu supported on AI2O3. The analysis of the formation of sulfate species on copper-on-alumina sorbent-catalysts suggests the following mechanism: in the presence of gaseous oxygen, copper performs catalytically the first step of oxidation of SO2 to SO, which then forms a stable surface sulfate at either the copper site or the aluminum site. During the first cycle of interaction of the sorbent catalyst with the S02-containing flow, a sulfate linked mainly to aluminum sites forms in an amount (about 300-400 pmol/g) equivalent to the limiting value of sulfate species on pure AI2O3. This species is more stable against reduction than the other sulfate species and is not reduced by H2 at 420 O C . During the first cycle of reduction, copper aluminate sites are reduced to metallic copper which, in the consecutive step of interaction with the SO2 + O2containing flow, give rise to the formation of surface species mainly on copper that are completely regenerated in the consecutive treatment with H2 at 420 OC.
Introduction Emission of sulfur oxides from combustion of fuels or cracking units causes a real air pollution problem. The removal of such pollutants has been the subject of much attention during the past years. The dry recovery processes seem the most advantageous as they permit flue gas treatment at relatively high temperatures but the regeneration of the sorbents is sometimes complex. Copper oxide supported on alumina is largely used for the removal of H#J or SOF3 The chemisorbed species are oxidized and sulfate species are formed. These species are then reduced by H2 or CH4 during the regeneration cycles. Much work has been done on these solids but it specially focuses on the characterization of CuO and its interaction with the s ~ p p o r t .The ~ nature of the SO,Z-species and of the adsorption sites on CuO or A1203support and their relative stability and transformation during regeneration do not seem well known yet. Knowledge of the nature of the sulfate species is particularly important in order to improve the performances of the sorbent as well as to perform efficiently the simultaneous removal of SO, and NO,, copper sulfate catalyzing the reduction of NO, by NH3.' Infrared spectroscopy and thermogravimetry are suitable methods for such studies and they were used to study the sulfation of pure oxides by SO, 02.*-i0 In particular, IR spectroscopy allowed us to differentiate bulklike sulfate from surface species, as shown on MgO,'O and to determine the structure of surface species. Only tridentate species have been shown from SO2 O2reaction on A1203and Ti02whereas bidentate sulfates occur on MgOIO and Si02.11 Moreover, in the simpler cases like on A1203, IR spectroscopy can be used to determine the amount of sulfates formed.I2 In the experimental conditions used, we observed a limit: 2.2 pmol m-2 ( I 3 x 1 0 ~ 3ions ~ m - ~ ) .Introducing '~ sulfates by (NH4)2S04impregnation, Okamoto et aI.I4 observed by XPS the formation of two S O : - species on alumina: (i) a well dispersed surface species for concentrations up to 12 X I O l 3 S042-
+
+
* T o whom correspondence should be addressed.
0022-3654/91/2095-405 I.$02.50/0
cm-2, (ii) bulklike or subsurface compounds with further addition of S042-ions. It therefore appears that the amount and the nature of the sulfate depend on the preparation method used. Reported in this paper are the results of a systematic study of the nature of sulfate species formed on A1203,CuO, CuAI2O4, and CuO/AI2O3 samples prepared either by impregnation with various amounts of sulfate salts or by direct sulfation with SO2 02,carried out in order to analyze the type of sulfate species formed by the interaction of SO2 with the copper-on-alumina sorbent+atalyst. These results also allow a better understanding of the nature and the evolution of sulfate species present on the
+
(1) Patrick, V.; Gavalas, G. R.; Flytzani-Stephanopoulos,M.; Jothimurugesan, K. Ind. Eng. Chem. Res. 1989, 28, 931. (2) Tamhankar, S.S.; Bagajewicz, M.; Gavalas, G. R.; Sharma, P. K.; Flytzani-Stephanopoulos,M. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 429. (3) Zuhtuysal, B.; Aksahin, I.; Yucel, H. Ind. Eng. Chem. Res. 1988, 27, 434. (4) Friedman, R. M.; Freeman, J. J.; Lytle, F. W. J . Coral. 1978,55. 10. (5) Strohmeier, B. R.; Leyden, D. E.; Scott Field, R.; Hercules, D. M. J . Cotol. 1985, 94, 5 14. (6) Pollack, S. S.;Chisholm, W. P.; Obermyer, R. T.; Hedges, S. W.; Ramanathan. M.; Montano, P. A. Ind. Ena. Chem. Res. 1988, 27, 2276. (7) Drummond, C . J.; Yeh, J. T.; JouGrt, J. 1.; Ratafia-Brown, J. A.
Presented at the 78th Annual Meeting of the Air Pollution Control Association, June 1985. (8) Saur, 0.:Bcnsitel, M.; Mohammed Saad, A. B.; Lavalley, J. C.; Tripp, C. P.; Morrow, B. A. J . Cotol. 1986, 99, 104. (9) Bensitel, M.; Saur, 0.;Lavalley, J. C.; Morrow, B. A. Mater. Chem. Phys. 1988. 19, 147. (IO) Bensitel, M.; Waqif, M.; Saur, 0.; Lavalley, J . C. J . Phys. Chem. 1989, 93, 658 I . ( I I ) Morrow, B. A.; McFarlane, R. A.; Lion, M.; Lavalley, J. C. J . Cotol. 1987, 107, 232. (12) Preud'homme. J.: Lamotte. J.; Janin. A.; Lavalley, J. C. Bull. Soc. Chim..Fr. 11981,433. (13) Saussey, J.; Vallet, A,; Lavalley, J. C. Muter. Chem. Phys. 1983, 9, 457. (14) Okamoto, Y.; Imanaka, T. J . Phys. Chem. 1988, 92, 7102.
0 199 1 American Chemical Society
Waqif et al.
4052 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991 SKUA t a i c Ratio
micnnnoleslg -.
1.25
1-
1.5 0.75 1
05
-
0.25
-
500 0.5
0'4
I
I
I
I
!
, l o
0 1 0 2 0 3 0 4 0 5 0 6 0
0
0
250
300
350
250
300
350
Reaction temperature.'C
Time. min
Figure 1. Amount of SO2adsorbed at 300 OC (micromoles of SO2per gram of sample and corresponding S/Cu atomic ratio) as a function of time on stream in contact with a gas flow containing 0.6%SO2,2.9%02, and 96.5%He for a 4.88 wt 96 CuO on y-alumina sorbent-catalyst in a series of consecutive cycles of sulfation-regeneration (regeneration with 29%H2 in helium at 420 OC). For cycles other than the first one, the reported amounts do not take into account the amount of sulfate species remained on the surface after regeneration.
copper-on-alumina sorbent-catalyst during consecutive cycles of sulfat ion-regenera t ion. Experimental Section Pure A1203was Degussa-C (100 m2 g-l). CuO (5 m2 g-l) was obtained from an exhydroxycarbonate sample by heating it at 400 OC. Copper aluminate (220 m2 g-I) was kindly given by the French Institute of Petroleum (IFP); it contained 685 ppm of Na. The CuO/AI2O2 sample (1 12 m2 g-l; 4.88%w/w CuO on A1203)was prepared by wet adsorption of copper acetate on A1203 followed by a thermal decomposition in air at 450 OC. Sulfate ions were added by using different methods: (i) the samples were evacuated at 450 OC and then heated with known amounts of SO2and a large excess of O2at 450 OC for more than 12 h; (ii) they were impregnated by (NH4)2S04,A12(S04)3,or CuS04 solutions of variable molarities, dried, and calcined at 150 or 450 OC. The molarities of the solutions are expressed in moles per gram of impregnated catalyst. For IR studies, samples were pressed into wafers weighing about 20 mg (diameter 16 mm). They were evacuated at increasing temperature, generally up to 450 OC for 2 h. The IR spectra were scanned at room temperature, using a Nicolet MX-1 FT-IR spectrometer. The spectra that are reported are those of adsorbed species, the absorbance of the support (AI2O3,CuO, CuA1204, CuO/A1203) being subtracted. For the sulfated samples which were prepared by wet impregnation, the spectra of the support were obtained after total H2 reduction of sulfate species. Reduction of hydrogen was carried out in situ, under static conditions with a very large excess of hydrogen (P 3 X 104 Pa in the cell; Le., -500 mol H2 for 1 mol S042-). Thermogravimetric tests were carried out isothermally in a Perkin Elmer TG2 apparatus using about 20 mg of sample and a calibrated gas flow (about 1.8 L/h) containing usually 0.8% v/v SO2,3% 02,and 96.2%He (sulfation step) or 2%H2 in helium (reduction step). Flow reactor studies were carried out in a quartz microreactor (0.5 g of sample with grain size in the 30-60 mesh range) coupled with an on-line mass quadruple detector (VG Micromass SX200) for the analysis of the inlet and outlet stream. The reagent composition was the same as that used for the thermogravimetric tests.
-
Results Flow Reactor Studies. A determination of the amount of sulfate species adsorbed on the samples as a function of time on stream and of reaction temperature was carried out both in a flow thermobalance apparatus and in a flow microreactor with on-line mass quadruple analysis. Both techniques give comparable results.
Figure 2. Amount of SO2adsorbed at 300 OC (micromoles of SO2per gram of sample and corresponding S/Cu atomic ratio) by a copper-onalumina sample during the first cycle as a function of the temperature of reaction after 2 h in contact with gas flow containing the same concentration of SO2 (0.8%),but different amounts of oxygen: 2.9%,420 ppm, and less than 10 ppm.
Samples containing 4.88 wt % CuO on alumina were used in all these experiments. Reported in Figure 1 is the change of the S/Cu atomic ratio and of the micromoles per gram of adsorbed SO2as a function of time on stream during a series of consecutive cycles of SO2 adsorption (300 "C) and regeneration with H2 (420 "C). Following a rapid initial adsorption of SO2, the amount adsorbed reaches a nearly constant value after about 1-2 h of reaction. The amount of SO2is about 600 pmol g-l, corresponding to a S/Cu surface atomic ratio near to 1 .O. In all cases, no SO3was detected in the outlet stream. In the consecutive two cycles, the amount of SO2adsorbed decreased slightly as compared to the first cycles and then remained nearly constant in the consecutive cycles. Reported in Figure 2 is the effect of the reaction temperature (first cycle) on the amount of SO2adsorbed after 2 h. The amount adsorbed increases with the reaction temperature and values higher than the stoichiometric ratio of S/Cu = 1.0 (formation of only CuSO,) can be obtained at reaction temperature higher than about 300 OC. In all cases, the amount of SO2 adsorbed is in the 600-1200 pmol g-l range and thus more than 2 times higher than the amount of sulfate species formed on pure A1203after treatment with SO2and O2 at higher temperatures (450 "C). For oxygen concentrations higher than about 0.5-1%, the partial pressure of O2 had no effect on the amount of SO2 adsorbed, whereas for O2concentration below this value, a decrease in the amount of SO2adsorbed was observed. Reported in Figure 2 is the amount of SO2adsorbed as a function of the reaction temperature for samples in contact with a S02/helium flow. The oxygen concentration in this mixture, as determined by mass quadrupole analysis, was about 300-500 ppm. Values about half of those observed in the case of excess oxygen were observed. Furthermore, the rate of SO2adsorption was lower. When the traces of oxygen were removed by using a DEOXO system, the amount of SO2adsorbed decreased further. These data suggest that, in the absence of gaseous oxygen, the CuO/AI2O3 system does not oxidize SO2to SO3 (or oxidizes very little), at least in the 250-350 OC temperature range considered. This suggests that the copper catalyzes the oxidation of SO2 to SO3 by means of gaseous oxygen; SO3 reacts further with copper or with aluminum sites to form a stable sulfate. The amount of SO2removed at 420 OC during the regeneration step (2%H2 in helium) as a function of time on stream for the same samples in a series of consecutive cycles of SO2adsorption (300 "C) and regeneration (420 "C) is shown in Figure 3. Reduction under these conditions leads primarily to the formation of SO2. Only about half of the amount of SO2adsorbed in the first cycle was removed during the first reduction step, whereas in further cycles, more than 95%of the SO2adsorbed was removed during reduction. The rate of reduction during the first step was 3 times lower than in consecutive cycles. In all cases, an induction
Formation of Sulfate Species on Copper/Alumina Catalysts
The Journal of Physical Chemistry, Vol, 95, No. IO, 1991 4053
1
m i a a o k c SuWg I
O K ,
i
mI o
WRVENUMBERS
0.25
0 Lu
U
1460
a
:: m
a
1
30
'
1'300
'
URVENUMBEAS
1'1@0
'
do0
Figure 4. IR spectra of the SO4" species obtained by oxidation of SO2 on Al2O3at 450 O C (introduction of 2000 pmol g-' of SO2with an excess of 02)and then evacuation at (a) 450 OC;(b) 550 O C ; (c) 650 OC;(d) 750 OC;(e) 800 O C .
time was observed during the reduction, especially during the first cycle. These data thus indicate that a stable sulfate species was formed during the initial interaction between CuO/AI2O3and SO2. This species is more difficult to reduce than sulfate species formed during the concecutive cycles between SO2 and the sorbentcatalyst. The amount of sulfate which could not be regenerated, about 300-400 pmol of SO2per gram of sample, corresponds to the limiting value of the sulfate species on pure AI2O3.l3 Infrared Characterizations. A. Pure Alumina. (a) Sulfation by Oxidation of SO,. The spectra of the species adsorbed on an A1203 Degussa sample that has been heated at 450 OC for 14 h with 2000 pmol g-' of SO2and a large excess of 0,are shown in Figure 4. The sample was evacuated at 450 O C for 2 h prior to recording the spectrum at room temperature. Two bands centered at 1410 and 1050 cm-I were observed. By increasing the temperature of evacuation, the intensity of the two bands decreased while the band centered at 1410 cm-' was observed to shift to 1380 cm-I. It appears that some sulfates are stable on alumina up to 800 OC. This confirms revious studies relative to a small amount of sulfate on AI2O3! The same experiment
'
is80
imo
4054 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991 TABLE I: Cbracteristic IR
Waqif et al,
Bands (v/cm-'), Thermal Stability (TJOC), and
H2Reducibility (T,/OC) of Sulfate Speciesaon Mffemt Supports
AI203
SO, +02or impregn by
sulfa tion mode
A12(S04)3
sulfatesonAI,O,
1380 1410 120&1180b 650 550 T, 600
Y
T, 800
mixed Cu-AI sulfates sulfates on CuO
impregn by CuSO, sol 200 pmol g-I 1380 600-700 500-600
CuO/A1203
so2 + 0 2
pmo] g'l
1400-1380 1380 600-750 600-700 400-500 400-500
1410-1 380 600-750 400-500
Y
TS T* Y
T,
'The S O : -
I000
600 pmo] g-1
1220-1050 1250-1050 1200 (ep), 50&550 450-550 450-550 200-300 200-300 200-300
CUO
1370 700-750 550-600 1290 700-750 400 1100, I050 1200-1100 1220-I080 550-600 450-550
Tr were introduced by different methods. *Only when the sulfates are introduced by impregnation.
A t
CUAIZO,
so2 + 02 so2 + 0 2
200
0.6
'
I500
'
1'300 ' 1'100 WWENUHBERS
Figure 7. IR spectra of SO," species on Alz03samples impregnated with AIz(S04)3solutions whose concentrations were calculated to obtain (a) 150; (b) 250; (c) 600; (d) 2000 pmol g-' of sulfate.
Figure 6. Effect of HzOaddition on the spectra of the Sod2species of a highly sulfated A1203sample: (a) sample evacuated at 450 O C ; (b) HzOgas (P, 5 X IO2 Pa) added at room temperature and sample heated at 450 O C ; (c) evacuation for 2 h at 450 OC;(d) (a) - (c) difference spectra.
-
(Figure 6c). Difference spectra show that the H 2 0 treatment suppresses the component responsible for the 1418-cm-' band (Figure 6d). Heating the AI2O3sample impregnated with the 600 Fmol g-' (NH4)$304 solution at increasing temperature shows species responsible for the high-wavenumter absorption band was less stable than that corresponding to the low-wavenumber species. Its intensity preferentially decreased at T > 650 O C whereas that of the 1390-cm-' band was stable up to 800 O C as shown in Figure 4. ( c ) fmpregnofion with A12(S04)3. The spectra of alumina samples impregnated with different amounts of A12(S04)3and then evacuated at 450 OC are compared in Figure 7. As in the case of (NH4)2S04impregnation, it appears that the position of the band near 1390 cm-' increased with the amount of sulfate. Additional bands appeared near 1200-1 180 cm-' in the spectra of the more sulfated samples. Such bands were observed on the spectrum of aluminum sulfate in KBr pellets evacuated at 450 OC. They disappeared when the samples were evacuated at T > 550 O C (Figure 8) and were characteristic of another type of sulfate (Table I). B. Sulfarion of CuO. The infrared spectrum of the species obtained by heating CuO with SO2 + O2 showed infrared absorption bands at 1200, 1160, 1080, and 960 cm-' (Figure 9A). They disappeared by evacuation at T > 500 OC. These bands
-
0.6
a b
C
d
ison
'
' I'm0 WCIVENUPIBERS
1'900
'
Figure 8. IR spectra of AIZO3impregnated with lOOOpmol g-I A12(S04), and then evacuated at increasing temperature: (a) 500 OC;(b) 550 OC; (c) 650 OC;(d) 750 OC.
are close to those observed for CuS04.5H20 in a KBr pellet (1 200 m, 1155 s, 1 1 IO s, 1090 sh, 995 m, 962 m cm-') or to those reported by Ferraro et al.ls for anhydrous CuSO, (1215, 1153, 1085, 962 cm-I). (IS) Ferraro, J. R.; Walker, A. J. Phys. Chem. 1965, 42, 1278.
The Journal of Physical Chemistry, Vol. 95, NO. IO, 1991 4055
Formation of Sulfate Species on Copper/Alumina Catalysts
4
0.07
1
1500
UCIVENUHBERS
0.41
1
v
'
Figure 10. IR spectra of A1203 impregnated with (a) 200, (b) 600, (c) 1000 pmol g-l solutions of CuS04. (d) (b) - (a) difference spectra. (e) (c) - (a) difference spectra.
0.4
1
"lk
a!
1
ism f s o
izoo
UWENUHBERS
iosd
WCIVENUMBERS
Figure 9. (A) 1R spectra of sulfate species on a CuO sample obtained by heating 200 pmol g-' of SO, with an excess of 02.(B) IR spectrum of SO4* species formed by oxidation of SO2(- 100 pmol g-l introduced) on CuAI2O4. (C) Reduction by H2of a highly sulfated CuAI20, sample (1000 pmol g-l of SO2 + an excess of 0,)by increasing temperature (from top to bottom) (a) spectrum of the sample evacuated at 450 OC and after H2treatment at (b) 200 OC; (c) 300 OC; (d) 400 OC; (e) 500 OC. C. CuA1204.Various quantities of SO2were introduced on the CuA1,04 sample pretreated at high temperature (700 "C) in order to eliminate carbonate species. After heating with excess O2at 450 OC, the samples were evacuated and the spectra of the adsorbed species were recorded. All spectra show bands of medium intensity near 1375 and 1290 cm-l and a very intense band between 1200 and IO50 cm-I (Figure 9B,C). The intensity of the latter increased with the amount of SO2introduced (Figure 9B,C). It decreased by evacuation at 550 OC or by treatment with H2 at 200 OC (Figure 9C). The absorbance of the 1290-cm-I band decreased following reduction in H2 at a temperature of about 400 OC whereas the 1370 and 1060-cm-l bands disappeared at T 600 OC, suggesting that three types of adsorbed species were present on the surface (Table I). D. CuO/A120,. ( a ) Impregnation of A1203 with CuSO,. Aluminum was impregnated with solutions containing different amounts of CuSO, (200, 600, or 1000 pmol g-l). The spectra of the species adsorbed on these impregnated samples after evacuation at 450 OC are shown in Figure 10. The spectrum of the sample with the lower amount of sulfate shows bands near 1380 and 1040 cm-l. When the sulfate concentration was higher than 200 pmol g-I, bands were observed near 1200,1150, and 1075 cm-I (Figure IOb,d). After impregnation with the IO00 pmol g-l CuSO, solution a very broad and intense band appears at 1165 cm-' (Figure ]&,e). At the same time, the 1400-1380-cm-~range was modified. Impregnation by the 600 pmol g-' solution leads to the appearance of a new band at 1400 cm-' (Figure 10d) whereas more concentrated CuSO, solutions decrease the intensity of the 1380-cm-' band previously observed when introducing 200
-
e l
i400
'
1'200 UAVENUMBERS
iooo'
Figure 11. (a) IR spectra of SO4,- species on the CuO/A1203sample formed by oxidation of SO, (600 pmol g-l introduced) and then evacuated at 450 OC and then reduced by H2at (b) 200 OC, (c) 300 O C , (d) 400 O C , (e) 500 O C . pmol g-l of CuS04 (Figure 10e). The absorbance of the bands observed in the 1200-1 100 cm-I range decreased when the samples were evacuated at T > 550 OC. These bands were very sensitive to reduction by H2. Their intensity decreased at 200 OC and they completely disappeared at 300 OC under hydrogen whereas the 1380- and 1060-cm-' bands are stable up to 450 O C under the same conditions. (b) Sulfation of CuO/A1203Catalysrs. The CuO/AI2O3 catalyst (3.9% of Cu by weight) evacuated at 450 OC was sulfated by heating 50 or 600 pmol g-' of SO2with a large excess of O2 at 450 OC. The spectrum of the adsorbed species obtained by oxidation of 50 pmol g-' SO2 shows a band at 1375 cm-' with an integrated area corresponding to about 45 pmol g-' of on AI2O3l2and another band near 1070 cm-I. When larger amounts of SO2were used, the higher wavenumber band shifted to 1400 cm-l and a shoulder appeared near 1220 cm-I. The spectrum (Figure 1 la) was very similar to the one shown in Figure 10b for pure alumina sample impregnated with the 600 rmol g-l CuSO, solution and then evacuated at 450 OC. As noted above for CuS04impregnated samples, treatment of sulfate species by H2 at increasing temperature first leads to a decrease of the shoulders near 1220 and 1070 cm-' and shifts the 1400-cm-I band toward a lower wavenumber (Figure 11). When SO2was heated on CuO/A120, without 02,bands appear up to 220 OC at 1365 cm-l and in the range 1200-1000 cm-I.
4056 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991 I
Waqif et al. When the sample is hydrated the equilibrium
C
-
O\ -O-S=O -0
+
/
O\ k0 s-0 / Kb
He0
"+
-0-H
L
1500
,
,
,
,
,
,
,
,
,
,
1300
1400
,
.
,
1200
,
,
,
1 loo
WAVENUMBERS
Figure 12. IR spectra of the sulfate species on a CuO/AI2O3system arising from sulfation in a flow reactor (0.8% SO2 and 3% 0,)and subsequent evacuation at 450 OC in the IR cell: (a) first cycle of sulfation-regeneration, after 2-3 min; (b) first cycle, after 2 h; (c) after three cycles of sulfation-regeneration and a further sulfation step for 1 h.
Their absorbance increases slightly with temperature and the maximum in the first band shifts to 1375 cm-l. It can be noted that the intensity of these bands was weaker than that observed by heating SO2 with 02.It decreased beyond 500 O C . The spectra of the sulfate species formed following the interaction of SO2+ O2with the copper-on-alumina in a flow reactor and subsequent evacuation at 450 OC in the IR cell are shown in Figure 12. After interaction for a short time (about 2-3 min) with the SO2 + O2 containing flow during the first cycle of sulfation, a weak band centered at 1365 cm-I is present (Figure 12a) with a shoulder near 1380 cm-l. Following interactions for longer periods of time (about 2 h) still during the first cycle of sulfation, a more intense band develops centered at 1390 cm-I (Figure 12b). It was asymmetric to the low-frequency side. A weak shoulder was observed near 1205 cm-' on the high-frequency side of the stronger band in the 1100-IO00 cm-l range. The spectrum is quite similar to that of sulfate species on alumina at low surface coverage. The spectrum changes after three complete cycles of sulfation-regeneration and a consecutive step of further sulfation (Figure 12c). In particular, the intense asymmetric band shifts to 1405 cm-' and the shoulder on the main band in the 1 100-1 000-cm-l region increases in intensity and shifts to higher energies, in the 1280-1 200-cm-I range. This clearly indicates a change in the sulfate species present on the copper-on-alumina sorbent/catalyst (i) after the first cycle of sulfation and (ii) after sulfation of the same sample after some cycles of sulfation-regeneration. The consecutive reduction at 420 O C of this sample decreases the intensity of the bands but does not change their shape or position significantly. Discussion Nature of Surface Sulfate Species. Sulfate Species on Alumina. Previous studies8J2have shown that the spectrum of S042species formed by oxidation of less than 500 pmol g-l of H2S or SO2 on AI2O3depends on the degree of surface hydration. When the sample is highly dehydrated, the spectrum shows a very characteristic band at 1380 cm-I and another wider one near 1050 cm-I. This band is more difficult to analyze because the absorbance of the A1203support is important in this range. I6O I8O exchange experiments lead us to conclude the presence of surface species with only one S=O:
-
O\ -o-s"'=o ' 0 -
was proposed8 to explain the modification of the spectrum. The absorbance of the 1380-cm-l band has been shown to be proportional to the sulfate amount for quantities of S042-below about 2.2 pmol m-2.12From XPS measurements, Okamoto et aLf4 observed that the S(2p)/A1(2s) XPS intensity ratio increases cm-2 (2 linearly with the concentration up to ca 12 X IOl3 pmol m-2). Beyond this concentration, the slope decreased slightly and this was assigned to the formation of bulklike or subsurface compounds. However XRD shows an absence of a distinct phase assigned to sulfate ions.I4 IR spectra of samples highly sulfated by impregnation with a solution of A12(S04)3(Figure 7) show bands near 1200-1 180 cm-l which remain up to about 600 "C and are characteristic of bulklike A12(S04), in A1203. The spectra of highly dehydrated samples sulfated by oxidation of a large excess of SO2and O2or by the impregnation with a large amount of (NH4),S04 in solution do not show bands in the 1300-1 100cm-I range but a wide band whose maximum is higher than 1380 cm-'. Figures 5 and 6 show that it is formed by two components corresponding to two types of surface species formed by oxidation of S02(or H2S) or by the decomposition of (NH4)2S0,. (i) The type which first appears is characterized by the 1380-cm-l band; it is stable up to 800 O C and is reduced by H2 near 600 O C . (ii) The other type gives rise to a band near 1410 cm-I; it is less stable and is easily decomposed by water vapor. The structures of the two types are not very different. The first one presents only one v(S=O) as already observed8 but it is difficult to determine the number of AI3+ to which it is linked. Davydov et al., in a recent study suggested the structure
'0
Ti-0
's=o
' 0 '
for the sulfates on anatase.16 Okamoto et aI.l4 showed that the adsorption of one S042-group corresponds to the elimination of two OH groups. It is, therefore, possible that this first species is linked to one, two, or three AI3+. The hydrated structure is compatible with the observation of bands near 1200 and IO00 cm-I. The other species formed when the degree of sulfation is higher could be an oligomer species such as S2072-9or an SO3 group linked to an AI-0 pair site."
Modifications Induced by Copper on the Nature of the Sulfate Species. Sulfation of CuO leads to bulklike CuS04 characterized by IR bands in the 1220--1000-~m-~ range (Figure 9A). Surface species were not observed, perhaps due to the low area of the sample. A sample with a higher area would be needed to observe them as in the case of MgO.Io Analysis of the spectra of the adsorbed species on copper aluminate after SO2+ O2treatment enables us to conclude the formation of three types of sulfate species. One could be linked to AI3+ ions (1 375, 1050 cm-' bands) and another one to Cu2+ ions (contribution to the broad 1 150-1000-cm-1 band), while the origin of the 1290-cm-l band is less obvious. A similar band has been observed for Na doped A1203samplesI8 and assigned to a
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(16) Khadjivanov, K.; Davydov, A.; Klissurski, D. Zh. Khim. 1988, 2f, 516.
(17) Khadjivanov, K.;Davydov, A. Kinef. Cofol. 1988, 29. 398. (18) Gcorge, 2. M.;Bensitel, M.; Lion, M.: Saw, 0.;Lavalley, J. C. Appl. Cafal. 1988, 43, 167.
Formation of Sulfate Species on Copper/Alumina Catalysts mixed AI-Na sulfate species. The amount of Na in the copper aluminate is too low (-680 ppm) to explain the occurrence of the 1290-cm-l band. We are inclined to suggest that it might be characteristic of sulfate species in interaction with both AI3+and Cu2+ ions. The band at 1290 cm-l does not appear on the spectrum of Cu supported on A1203sulfated catalysts whatever the introduced amount of SO2. These spectra essentially show bands due to sulfate species adsorbed either on alumina (1400-1380, 1050 cm-l) or on copper oxide (1220, 1200-1080 cm-I). For a low degree of sulfation, the species first formed, either by impregnation of A1203by CuS04 or by SO2oxidation on CuO/AI2O3 catalysts, followed by evacuation at 450 OC, are localized on AI2O3(Figure loa). This could be explained by their higher stability. Impregnation with a more concentrated solution of CuS04 (600 pmol g-I) increases their number (Figure lob) whereas a larger amount (1000 pmol g-l) reduces it (Figure IOc). This could be due to an almost total coverage of the alumina surface by copper oxide. Note that the spectrum of copper sulfate species depends on the amount of sulfate. The very broad band near 1 165 cm-l (Figure IOc) is characteristic of bulk copper sulfate. To explain the absence of the 1290-cm-I band in the case of CuO/AI2O3catalyst, we have to consider the arrangement of C d 2 ions different on the surface of copper aluminate and on Cu supported on A1203. If mixed Cu-AI sulfate species are formed on CuO/A1203, their structure might be different from that observed on CuA120.+ In their study, Strohmeier et al.5 concluded that for CuO/AI2O3 catalysts with low Cu contents (
