Ind. Eng. Chem. Res. 1992,31, 1947-1955
1947
SEPARATIONS Combined DeSO,/DeNO, Reactions on a Copper on Alumina Sorbent-Catalyst. 1. Mechanism of SO2 Oxidation-Adsorption Gabriele Centi,* Nello Passarini,' Siglinda Perathoner, and Alfred0 Riva Department of Industrial Chemistry and Materials, V.le Risorgimento 4, 40136 Bologna, Italy
The mechanim of SO2 oxidation-adsorption to form regenerable sulfates on a copper on alumina sorbentatalyst was analyzed. Studies were carried out both in a flow reactor with mass-quadrupole detection of the reaction products and by using Fourier-transform infrared (FT-IR) characterization of the nature of the sulfate species. Both in-situ in a flow infrared cell and ex-situ FT-IR studies of the evolution of the sulfate species as a function of time on stream and after regeneration of the sorbent-catalyst were carried out. The results indicate that copper catalyzes the oxidation of SO2 to SO3 in the presence of gaseous oxygen and that SO3, present probably as S-bonded chemisorbed sulfur oxide, further (i) reacts with near-lying sites of the support without desorption in the gas phase through a mechanism of surface transfer to form a sulfate linked to A1 or (ii) reacts with copper species to form a sulfate linked to copper. The relative rates of these two reactions of formation of sulfate species are strongly affected by the reaction temperature, which enhances in particular the former reaction. The latter sulfate species, however, is more easily regenerated by reduction.
Introduction The worldwide trend toward increasingly stringent levels of control of emissions from power-generating plants has spurred significant research on new less costly technologies capable of reducing emissions of SO2 (DeSO,) and NO, (DeNO,) (Ellison and Sedman, 1987;EPA, 1985;Holliden, 1988; Lu et al., 1988;Rentz, 1989). The combined S02/N0, removal in a single unit operation using a supported copper oxide sorbent-catalyst (Centi et al., 1990;Drummond et al., 1985;Kiel et al., 1989; Yeh et al., 1984, 1985, 1987) offers the potential for reducing the cost of environmental control. This technology can be considered as an evolution of the Shell Flue Gas Treating DeSO, process also based on CuO sorbents (Dautzenberg et al., 1971). SO2 reacts with CuO in the presence of O2 forming CuS04 (Drummond et al., 1985; Yeh et al., 1984,1985,1987),which catalyzes the reduction of NO to N2 in the presence of ammonia and 02. The copper sulfate can be then regenerated with hydrogen or methane to produce a concentrated stream of SO2 and metallic copper, which in contact with O2 quickly transform to initial CuO. The economical evaluation of the technology in comparison both with a process layout of separate flue gas desulfurization (FGD) and selective catalytic reduction (SCR) gas cleanup and with an integrated approach of combined removal of S02/N0, according to some of the technologies under development (NOXSO, electron beam, activated carbon, WSA-SNOX processes) has shown the cost advantage of the integrated technologies, and in particular of that based on CuO sorbents (Riva, 1987;Rubin et al., 1989). Various research projects are currently active in thisfield to demonstrate the reliability and operability of the proceas using various reactor codigurations, such as fluidized bed, radial mobile bed and gas*lid trickle flow reactors (Centi 'Div. Servizi Ausiliari, ENICHEM ANIC, S. Donato Milanese, Italy.
et al., 1990,Drummond et al., 1985;Kiel et al., 1989;Yeh et al., 1985,19871,but a more detailed investigation of the nature, process, and mechanism of interaction of the sorbent-catalyst with SOz and ",/NO, is necessary, because little information is available on these aspects (Centi et al., 1990;Kartheuser et al., 1991;McCrea et al., 1970;Pollack et al., 1988,Waqif et al., 1991). The process chemistry reported in the literature for this sorbent-catalyst (Drummond et al., 1985;McCrea et al., 1970;Patrick et al., 1989;Pollack et al., 1988;Stelman et al., 1986;Yeh et al., 1984,1985,1987)indicates only the copper oxide or copper ions in a low coordination environment as the active component for the DeSO, reaction, although the atomic S/Cu ratio in the sulfated catalyst may be higher than 1.0 (the stoichiometric value for complete transformation of CuO to CuS04) (Centi et al., 1990). This reaction is in agreement with spectroscopic evidence (Waqif et aL, 1991). The S/Cu ratio affecta the DeNO, behavior of the catalyst (Centi et al., 1990;Stelman et al., 1986). The aim of the research reported here was to analyze more in detail the mechanism of SO2 oxidation-adsorption of form sulfate species on the copper on alumina sorbent-catalyst, in order to provide the basis for a more rational tuning of catalytic performance and design of reactor processing technology. Preliminary evaluations about the feasibility of the process to the outlet of the economizers of fiie boilers have indicated a reaction temperature of around 570-600 K in order to maximize the economicity of the technology. For this reason, in the present study a reaction temperature of 570 K has been used for most of the tests, but in some cases, especially for infrared studies, lower temperatures were used for a better analysis of the reaction mechanism.
Experimental Section Sample Preparation. The copper on alumina samples were prepared by incipient wet impregnation of pure yAlz03pellets, 1.5 mm in diameter, using a solution con-
088~-5885/92/2631-194~~03.oo/o @ 1992 American Chemical Society
1948 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
taining copper acetate in such an amount as to have the required amount of copper oxide in the final catalyst. Usually an amount correspondingto 4.8 wt % CuO (3.9% as Cu) was used. After impregnation and drying, the particles were calcined at 773 K to decompose the acetate anions. An alternative preparation involved fvst grinding the pellets and then using a preparation procedure as before. No differences in the sorption and reactivity characteristics of the samples prepared using the two procedures were observed. After calcination the surface area of the sorbent-catalyst was 117 m2/g for the sample containing 4.9 wt % CuO, and no significant difference in the surface area was determined in samples containing more copper oxide up to a loading of about 15 w t 9%. After calcination the samples were usually characterized by scanning electron miscroscopy and electron probe microanalysis to verify the homogeneity of the preparation of uniform dispersion of copper along the particle diameter. In all cases X-ray diffraction showed no crystalline materials present other than the alumina support. The preparations using other oxide supports were made as for the case of copper on alumina samples. The support oxides were commercial pure samples, sometimes calcined in order to decrease the surface area. Reactivity Tests. The tests of adsorption-regeneration were carried out using a packed-bed quartz downflow reactor and amounts of the sorbent-catalyst in the 0.05-1-g range (grain size of the particles in the 0.25442-mm range). Tests carried out using particles with a diameter of 1.5 mm or using a different linear velocity of the gas indicate that mass-transfer limitations were negligible in our experimental conditions. Calculations indicate that heat-transfer limitations can also be considered negligible. A thermocouple sliding inside the catalytic bed was utilized to verify that the axial temperature profile in the reactor was uniform within 2-3 "C. An on-line mass-quadrupole detector (VG Micromass SX200)was used for the analysis of the inlet and outlet stream. A calibrated gas flow (normally about 2 L/h at STP conditions) containing usually 0.8% SO2,3% 02,and 96.2% He (sulfation step) or 2% H2 in helium (reduction step) was used as the reacting mixture in the flow reactor experiments. The same reagent composition was also used in thermobalance and infrared experiments. Tests parallel to those carried out in the packed-bed reactor were also made using a thermobalance apparatus (Perkin Elmer TG2) which had the advantage of a uniform gas and solid composition. The results using both procedures were quite comparable. The same thermobalance apparatus was also used to study the thermal stability of the sulfate species as well as the possible presence of adsorbed species on the sorbent-catalyst. Pulse reactor tests were carried out using a microreactor containing 0.05 g of sample maintained under vacuum (around 0.1 Pa) and connected on-line with a mass-quadrupole detector. Pulses (0.1 mL) containing 1% SO2 in helium or 1% SO2and 3% O2in helium were sent to the sample monitoring system where the formation of the reaction products as a function of time from the inlet of the pulse was studied by mass-quadrupole analysis. Infrared Studies. Infrared spectra were measured using a Fourier-transformPerkin Elmer 1750 spectrometer, with a 2-cm-' resolution. The spectrum of the sample before adsorption of SO2 was subtracted as well as any contributions of the gas phase. Two types of experiments were carried out to analyze the evolution of the species on the copper on alumina sorbent-catalyst after contact with a S02/02flow. &-situ characterizationof the samples was
IS02vIS02b
0.75
0.5
i
."
P
1
0.25 -
0
50
100
150 Time, min
200
250
300
Figure 1. Breakthrough curve of SOz removal using a copper on alumina sorbent-catlyst. Experimental conditions: 1.0 g of sample containing 4.8 wt % CuO;reaction temperature 573 K reaction mixture, 1400 ppm SOz, 3% Oz and the remaining helium; total flow rate at STP conditions, 6 L/h.
carried out using the KBr disk technique and calibrated amounts of the samples, usually 0.1 w t %, diluted with KBr. In-situ experiments were carried out in a flow infrared cell as a function of time on stream in contact with the flow containing S02/02. In the latter case, samples were pressed into wafers weighing about 20 mg,which were preconditioned in the cell in an helium flow at the same reaction temperature of the experiment. The background spectrum of the gas phase, in addition to that of the sorbent-catalyst at zero time on stream, was subtracted. The spectra of the reference samples were recorded using the KBr technique after suitable pretreatment of the samples as specified in the legends of the figures. The pretreatment was usually carried out in the thermogravimetric apparatus using a flowing reaction mixture as for the reactivity tests.
Results and Discussion Reactivity of the Sorbent-Catalyst in SO2 Oxidation-Adsorption. Reported in Figure 1 is a typical breakthrough curve obtained in a packed-bed flow microreactor using 1.0 g of copper on alumina and a flow containing 1.4% SO2and 3%02. After around 60 min SO2 starts to be detected in the outlet stream by a massquadrupole detector, and after about 150 min the absorption capacity of the sorbent bed falls below 5%. During this entire experiment, no SO3 slip was detected, suggesting that oxidation of SO2 occurs at the same time as the formation of sulfate from SOs. Thermogravimetric analysis of the exhausted adsorbent-catalyst indicated that the temperature of decomposition of the sulfate formed on the surface of the copper on alumina after exposure to the S02/02stream is around 1120 K, in good agreement with the data reported by Pollack et al., (1988). The decomposition temperature is intermediate with respect to those of pure CuS04 (around 1020 K) and A2(so4)3 (around 1190K)(Brittain et al., 1989; Pollack et al., 1988, Sacks et al., 1984). The total moles of SO2adsorbed per mole of copper ions present in the sorbent may be estimated from the integral of the breakthrough curve. Figure 2 summarizes the results showing the dependence of the S02/Cu mole ratio (moles of SO2adsorbed up to a [SO2]/[SO2l0= 0.95 per mole of Cu present in the sorbent) as a function of the reaction temperature. The results reported were obtained in both the presence and absence of gaseous oxygen in the inlet feed. The main conclusions which derive from these
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1949 2
05 025
SO2Cu Moles Ratio 7
'b
-
A
HWCUor HBS02 Moles Ratio
SO2/Cu Moles Ratio 2,
A
A
n 0.5
I*
L * "2IS02
0
0:
0
0 6 0 Nr of Cycles of Adsorption-Regeneration
2
10
4
12
Figure 3. Chauge in amount of SO2adsorbed (expressed as S03/Cu mole ratio in exhausted sorbent-catalyata) and in consumption of H2 (exprewed as moles of H2consumed per mole of Cu in the catalytic bed) as a function of number of cycles of adsorption at 673 K (experimental Conditions as in Figure 1) and regeneration at 743 K using a flow containing 2% H2 in helium. SOZCu Moles Ratio 4 . 3 .
1 1
0
1
100
200 300 Surface Area. mag
A1203
A
400
Ti02
500
Figure 4. Moles of SO2 adsorbed per mole of Cu as a function of surface area and type of oxide support. Data refer to the amount adsorbed after 30 min at 523 K in contact with a flow containing 0.8% SO2 and 3% O2 In all samples the amount of copper corresponded to 4.8 wt % as CuO.
Figure 3 reports also the moles of hydrogen consumed during regeneration per mole of copper in the catalyst. A clear change during the first four cycles is found; then the reactivity of the sorbent-catalyst remains quite constant over several cycles. In the first two cycles parallel to a slight decrease in the SOz/Cu ratio a considerable change in the Hz/Cu ratio is observed, indicating a significant change in the sample properties during the first cycles of adsorption-regeneration. It should be noted that the Hz/Cu ratio in the first cycles is lower than the stoichiometric one for complete regeneration of the adsorbed copper sulfates, according to the equation proposed by McCrea et al. (1970) and Drummond et al. (1985): CuSOd + 2Hz 4 CU + SO2 + 2Hz0 (1) During the first three regeneration cycles, therefore, not all the sulfate surface species are removed from the sorbent After approximately four cycles, the H2/Cu ratio and especially the Hz/S ratio (Figure 3) become slightly higher than the stoichiometric value of 2.0, suggesting (i) a possible removal of sulfate species not regenerated during the first cycles and/or (ii) the creation of oxygen vacancies in the alumina lattice. Reported in Figure 4 is the effect of the surface area of the support on the DeSO, activity of the copper-baaed sorbent. As an index of activity, the SOz/Cu ratio at 523 K after 30 min of interaction with a flow containing 0.8%
1950 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
SO2 and 3% O2is reported. The amount of CuO in the sorbent is the same in all cases and corresponds to 4.9 w t % CuO. This amount is equivalent to approximately 1 monolayer of copper oxide on a surface area of the support of around 110 m2/g. The surface area does not influence the activity of the CuO/Si02 samples, which are much less reactive in the formation of sulfate species as compared to alumina and titania supporta. The activity of the silica-based samples is nearly equivalent to that of pure microcrystalline copper oxide. On the contrary, for both N203and Ti02supports the surface area has a pronounced influence on the DeSO, activity, even for values of the surface area higher than those required for the formation of a copper oxide monolayer and thus for a complete hypothetical dispersion of copper ions. In fact, it is known (Friedman et al., 1978; Marion et al., 1990; Lo Jacono et al., 1982; Strohmeier et al., 1985, Wolberg and Roth, 1969) that for a loading of copper oxide on alumina of about 5% and a surface area of around 100 m2/g the copper ions are well dispersed, with predominantly tetragonally distorted octahedal environments and the absence of Cu-0-Cu bonds. For higher loadings, microcrystallinecopper oxide appears to be present. Similar conclusions are valid for copper on titania samples (Bond et al., 1991). This indicates that in the DeSO, reaction not only the distribution of copper on the support (i.e., the specific surface area) is important, but the presence of support sites near welldispersed copper ions is necessary. Furthermore, it should be noted that the interaction of S02/02with pure TiOz and A1203 gives rise to the formation of surface sulfate species (Chang, 1978; Saur et al., 1986), whereas silica is much more inert toward SO2 (Morrow et al., 1987). The role of the support is confirmed in Figure 5 where the effect of the amount of copper on a alumina support (surface area around 110 m2/g) is reported. Shown in Figure 5A is the dependence on time on stream of the total amount of sulfate species adsorbed (,asgrams of equilvalent SO3 in order to consider that in aerobic conditions the formation of 2 mol of sulfate requires 2 mol of SO2 and 1 mol of oxygen), whereas Figure 5B shows the specific activity per Cu ion (as S02/Cu ratio of moles). The DeSO, activity per atom of Cu increases even for amounta of CuO much below those required for the complete dispersion of copper ions on the surface (Friedman et al., 1978) (around 5 w t % CuO for a surface area of the support of around 110 m2/g). The data show that the global DeSO, activity of the sorbent-catalyst increases with increasing copper content in the sample, but the specific activity per atom of Cu decreases. This clearly shows that not only the dispersion of copper ions on the surface is important, but also the presence of available sites to form surface sulfate species. Spectroscopic Evidence on the Reaction Mechanism. Sulfates, sulfites, and adsorbed sulfur oxide species show several vibrational bands in the 1500-600-cm-' infrared region sensible to the coordination and symmetry of adsorbed species, and thus an infrared characrerization of the species formed during the interaction of S02/02with the sorbentatalyst can provide useful information about the reaction mechanism. In order to have better indications on the evolution of the species with time on stream, samples were characterized both ex situ after various treatmenta in the same flow reactor as for previous experiments, but using much smaller amounts of sample in order to have an uniform axial SOz profie, and in situ in a special flow reador cell. Furthermore, in order to obtain a better analysis of the nature of the species formed during interaction of S02/02with the copper on alumina, refer-
g S03('E2)/g sorbent
____
7,
AT
cuo
%wt 162
4r 360
+ 491 8 820 h
0
0 0
10
30 40 Time, min
20
50
70
60
SOP/Cu Moles Ratio
o
B ' 1 6 -114 I
-12
0
n -1
C
h
3
V
v
cuo
SIC
w1
0
162
2
491
77
820
360
V
-08 -06
404 -02
0
10
20
30 40 Time, min
50
60
70
Figure 5. Amount (grams per lo2) of SOz adsorbed per gram of sorbent-xtalyst (A) and mole of SOzadsorbed per mole of Cu in the sorbent-catalyst (B)aa a function of time on stream for samples having different amounts of CuO. Experimental conditions aa in Figure 1.
ence experiments using pure alumina or copper oxide were also carried out. Due to the strong absorption of skeletal vibrations of the alumina support for frequencies below IO00 cm-', only the 1600-1OOO cm-' region was analyzed. The free sulfate species is highly symmetrical, and in the spectral region analyzed only the u3 fundamental is active. A lowering of the symmetry, such as that due to the coordination of sulfur oxide with an oxide surface, can form different surface complexes with a splitting and shifting of the v3 band. For example, sulfate-bridged bidentate complexes show three bands in the region 1500-1OOO cm-'( N h o t o , 1978; Horn et al., 1970). Figure 6 shows spectrum a obtained by sulfation of pure alumina at 723 K (14 h) in a flow reactor with a S02/02 stream. Five bands are observed in the 1100-1200-cm-' region, namely, at 1095,1125, 1140,1160 (shoulder), and 1200 cm-', and two bands near 1400 cm-'. The spectrum of AlzSO4*18H20shows a single band centered at 1095 cm-', but after calcination at 573 K two further bands appear at 1180 and 1200 cm-l that become the principal ones after calcination at 723 K (spectra c-e of Fig. 6). The spectra differ from those reported by Saw et al. (1986) and obtained under vacuum, which show a main sharp band at 1390 cm-' and a second weaker and broader band centered at 1040 cm-' with a shoulder near 1100 cm-'. Waqif et al. (1991), heating at 723 K an evacuated A1203 samples in the presence of SO2 and 02,found also two bands centered at 1410 and 1050 cm-'. However, 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-'. The species characterized by the band at 1380 cm-'
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1951
I
I
I
L
n
1300
N
.i.cm-'
Figure 6. Infrared spectra (KBr technique) of (a) pure alumina after sulfation with a S02/02flow at 723 K for 14 h, (b) pure alumina impregnated with ammonium sulfate and subsequent calcination at 723 K (the amount of ammonium sulfate deposited corresponds to that necessary to have 5 w t % sulfate species after calcination) and H room 2 0 temperature (c) or after calcination at 573 of ~ , ~ o 4 ~ 1 8 at K (d) or 723 K (e).
was assigned to a single oxo surface sulfate species,whereas the species characterized from the band near 1410 cm-I was assigned to a second sulfate species less stable and easily decomposed by water vapor. Tentatively, this second sulfate species was assigned to an oligomer species such as S20T2-or an SO3 group linked to an Al-0 pair site (Waqif et al., 1991). The differences between the spectra of Figure 6 and those reported in the literature (Saur et al., 1986, Waqif et al., 1991) can be interpreted taking into consideration that the interaction of water vapor with sulfate ions on alumina completely modifies the spectrum (Proud'homme et al., 1981): the band near 1400 cm-' disappears and a series of new bands appear in the 11001300-cm-' region. This change in the spectrum can be interpreted as a change in the coordination from a sulfate species having a single sulfoxo group and three bridging S U M bonds to a bidentate sulfate species coordinating water (Saur et al., 1986). We may therefore conclude that in our case the interaction at 723 K of S02/02with alumina in a flow reactor leads to a complex surface situation. The bands at 1095 and 1200 cm-I can be attributed to bulklike aluminum sulfate, whereas the three bands in the 1120-1175cm~'region as well as the bands near 1400 cm-' can be attributed to surface sulfate species with a low symmetry. The latter species are similar to those formed by impregnation of alumina with ammonium sulfate and consecutive calcination at 723 K (spectrum b of Figure 6). The attribution to a low-symmetry surface sulfate is in agreement with the data reported for covalent organic sulfatea (Nakamoto, 1978) that show a band near 1400 cm-' and a second band between 1210 and 1180 cm-'. The presence also of a relatively sharp band at 1370 cm-' may suggest that two surface sulfate species are present. Chang
(1978), in fact, has observed that SO2 adsorbed on pure alumina gives rise to a broad band at 1060 cm-', attributed to the formation of surface sulfite species, and for higher coverage to a second much weaker band at 1326 cm-' attributed to asymmetric u3 of S-bonded chemisorbed SO2. Upon heating the sample in the presence of gaseous oxygen, these species evolve to the formation of sulfate species. In particular, upon heating at about 670 K main bands are found at 1065 and 1365 cm-l and upon heating at about 770 K two stronger bands form at 1090 and 1400 cm-'. The latter bands are also found by adsorption of a large amount of SO3 on alumina and can thus be reasonably attributed to v1 and v3 vibrations of surface sulfates linked to alumina sites, but in a slightly different environment as compared to the sites found for lower surface coverage. Similarly the spectra observed by sulfation of pure alumina (Figure 6) can be interpreted as due to the presence of the same two surface sulfate species observed by Chang (1978), in addition to bulklike aluminum sulfate. It should however be noted that a sharp band at 1370 cm-' may also be attributed to a carbonate species, even though in our experiments we operated in such a way as to avoid any carbon dioxide contamination. The ex-situ spectrum of not supported CuO (spectrum a of Figure 7) after treatment at 523 K for 14 h with a SOZ/O2flow shows two well-resolved sharp bands with equal intensity, centered at 1100 and 1145 cm-I. It should be noted, however, that a long time was necessary to have a reasonable intensity in the spectrum (14 h), an indication that the rate of formation of significant amounts of sulfate species on pure CuO is very low, in agreement with flow reactor results (Centi et al., 1990). In CuSO4-2Hz0two strong adsorptions with equal intensity at 1200 and 1160 cm-I are present, but after calcination at 573 K a new band at 1100 cm-' appears and the band at 1160 cm-' shifts to slightly lower frequencies (spectra b and c of Figure 7). Alumina impregnated with copper sulfate and calcined at 573 K shows a main band at 1140 cm-l with broader shoulders at 1120 and 1160 cm-I (spectrum d of Figure 7) and two bands around 1400 cm-l. When the amount of copper sulfate deposited on the alumina support is increased, (i) the bands at 1400 cm-l increase in intensity, (ii) the band at 1120 cm-' increases in relative intensity as compared to the bands at 1140 cm-', (iii) a shoulder near 1090 cm-1 appears, and (iv) the shoulder at 1160 cm-' becomes stronger. The evolution of the bands near 1400 cm-l is in agreement with the previously discussed evolution of the spectra of surface sulfates on alumina as a function of surface coverage. It can be tentatively suggested that the interaction of copper sulfate with alumina below the amount necessary for monolayer coverage (amount corresponding to that deposited on the sample of Figure 7d) leads to the formation of a sulfate species similar to that attributed to surface sulfate species in a low coordination symmetry found by direct sulfation of pure alumina. For amounts of copper sulfate higher than those required for monolayer coverage (spectra e and f of Figure 7), together with the first species, bulklike CuS04appears to be present (bands at 1110 and 1050 cm-'). This also indicates that the sulfation of copper on alumina samples containing an amount of copper oxide higher than that required to form a complete monolayer coverage after sulfation (the unit cell dimensions of copper sulfate are higher than those of copper oxide and thus an expansion of the cell volume occurs during the transformation) probably leads to segregation of bulk copper sulfate particles in the process of sulfation. The segregation of these particles may cause a loss of DeSO, activity over multiple
1952 Ind. Eng. Chem. Res., Vol. 31,No. 8,1992
v)O *O
c-c
I I
1
1500
1
1300
I
1
1100
'
l! 1 0
I
V
I
1300
1100
Figure 7. Infrared spectra (KBr technique) of CuO after reaction with a flow of SOz/02 at 523 K for 14 h (a), of CuS04.2H20at room temperature (b) or after calcination at 573 K (c), of alumina impregnated with 5 wt % (d), 10 wt % (e), or 15 wt % (f) CuSO, and subsequent calcination at 473 K.
cycles of adsorption-regeneration. Reported in Figure 8 are the ex-situ spectra of the time-on-stream evolution of sulfated species formed by interaction at 573 K of S 0 2 / 0 2(0.7% SO2 and 5% OJ with a 4.9 w t % copper on alumina sorbent-catalyst (spectra a-c). The following observations can be made: (a) The bands due to bulldike aluminum or copper sulfatea are not present. The spectrum is very similar to those previously attributed to surface sulfates on alumina. (b) A time-onstream evolution of the spectra can be observed. The sharp band at 1130 cm-' is the predominant one after 10 min of interaction, whereas the broader band centered at 1115 cm-I becomes the principal one at higher time on stream. At the same time the intensity of the shoulder at 1150cm-I increases. This suggests an evolution of the surface sulfate species during the DeSO, reaction, and in particular the rapid formation of a first sulfate surface species and then the slower formation of a second sulfate species. The attribution of these species cannot be unambiguous, but on the basis of that previously discussed, they can be tentatively attributed to low-symmetry surface sulfate species linked to Al or Cu, respectively. After several cycles of adsorption-regeneration the spectra of sulfate species become more intense and the relative intensities of the bands also change (spectrum d of Figure 8). In particular, two bands at 1125 and 1135 cm-I increase in intensity. These bands are similar to those found by sulfation of pure alumina and thus suggest that after several cycles the relative presence of surface sulfate species linked to Al increases as compared to other sulfate species. Furthermore, the bands at 1090 and 1200 cm-I become relatively intense, indicating the formation after several cycles of reaction-regeneration also of bulklike aluminum sulfate. After consecutive regeneration (spectrum e of Figure 8),bands due to sulfate species are still present, indicating that not all sulfate species can be regenerated. It should be noted, furthermore, that the two bands at 1125 and 1135 cm-I become more intense, suggesting that the sulfate species linked to A1 are more difficult to regenerate as compared to the other sulfate species present on the surface, and reasonably linked to
I600
1300
1100
1000
F , cm-'
Figure 8. Infrared spectra (KBr technique) of the time-on-stream evolution of a copper on alumina sorbent-catalyst in contact at 573 K with a flow containing 0.7% SOz and 5% 02:(a) after 10 min, (b) after 50 min, and (c) after 180 min. The spectra of the samples after 10 cycles of reaction-regeneration after the stage of SO2 adeorption-oxidation (d) and coneecutiveregeneration (e) me also reported.
copper, in agreement with the results using pure CuO (Figure 7a). This observation agrees with the results of Waqif et al. (1991). In conclusion, these IR data evidence a change in the nature of the sulfate species from the first to the other cycles of adsorption-regeneration. It can be reasonably
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1963
v ,cm"
1800
0,Cm"
1400
1000
Figure 9. In-situ infrared spectra of the time-on-atream evolution of a copper on alumina sorbent-catalyst in contact at 573 K with a flow containing 0.8%SO2and 3% 02.Time-on-stream,in minutes, is reported in the figure for the corresponding spectra The spectrum of the sample at zero time on stream (sample treated only in a helium flow) and the spectrum of the gas phase have been subtracted from the spectra of the sample at various times on stream.
suggested, in agreement also with previous results, that in
the first cycle the SO3 formed by oxidation of SO2 on copper immediately shifts to near-lying Al species, and then sulfate species linked to copper also form. This agrees with the stronger bond of sulfate species with Al as compared to Cu, which also makes the reduction of the former sites more difficult. In fact, sulfate species linked to Al are less regenerable than the species linked to copper (Waqif et al., 1991). The formation of bulklike copper or alumina sulfates can be considered negligible on the basis of IR evidence. Furthermore, as shown in the sulfation of pure CuO (Figure 7), the reaction proceeds slowly due to the higher cell volume of CuS04with respect to CuO; after formation of the first sulfated layer the further reaction of subsurface layers is inhibited. In order to obtain further verification of these indications, tests were also carried out in-situ in a flow infrared cell in order to follow directly the evolution of sulfate species as a function of time on stream in contact with a flow containing about 1% SO2and 3% OF Furthermore, using this cell, it is possible also to obtain evidence about the possible presence of S-bonded chemisorbed SO3, characterized by a band near 1375 cm-* as discussed previously. Reported in Figure 9 are the results obtained at 523 K as a function of time on stream. It should be noted that the spectra are quite different from those obtained ex situ using the KBr technique (Figure 8), but as stated previously, the spectra of sulfate species in this region are influenced considerably by the presence of water. As a general feature, spectra under vacuum of sulfate on copper on alumina are characterized by a most intense band near 1400 cm-l (Waqif et al., 1991). Upon admission of water, this band decreases in intensity and bands form in the 100&1350-cm-' region. The spectra recorded under ambient conditions using the KBr technique can thus be considered representative of a hydrated surface situation for sulfate species (very weak bands near 1400 cm-l), whereas those recorded in situ, due to (i) the presence of adsorbed water on the sorbent-catalyst, (ii) the possible presence also of traces of water in the feed, and (iii) the reaction temperature which induces a partial desorption of adsorbed water, can be assumed as an intermediate
Figure 10. (A) In-situ infrared spectra of the time-on-stream evolution of a copper on alumina sorbent-catalyst in contact at 623 K with a flow containing 0.8% SO2 and 3% 02.(B)Spectrum of the sample aftar 200 min of time on stream and subsequent evacuation (0.1 Pa) at the same reaction temperature. Time on stream, in minutes, is reported in the f i e for the correspondingspectra The spectrum of the sample at zero time on stream (sample treated only in a helium flow) and the spectrum of the gas phase have been subtracted from the spectra of the sample at various timea on stream.
situation between that under high-vacuum conditions and that using the KBr technique. It is reasonable, however, to consider that the spectra recorded in situ are significant of the real nature and evolution of adsorbed species on the copper on alumina sorbent-catalyst. The zero time-onstream sample used for the subtraction of the other spectra was obtained with the sample treated in a helium flow at the same reaction temperature (523 K). In these conditions, not all the adsorbed water on the sorbent-catalyst is removed and part still remains on the sample. After admission of the flow containing S02/02, the interaction of sulfur oxide with sites of the sorbent-catalyst induces a partial removal of the adsorbed water, as shown by the negative peak around 1600 cm-'. This effect together with the previously discussed role of adsorbed water on the spectra of sulfate species can explain the apparent negative peak at about 1400 cm-' observed for the lower time on streams, but further suggests that sulfur oxide also reacts with Br~rnstedalumina sites, since these are the sites responsible for the strong adsorption of water. The evolution with time on stream of the spectra in the 1100-1350-~m-~ region clearly indicates the presence of at least two sulfate species which form with different rates. The first species, characterized by the main band at around 1330 cm-l, forms immediately, and the species characterized by the band at 1270 cm-' forms at a slower rate but becomes the principal one for higher times on stream. For higher reaction temperatures (Figure 10A) a similar situation is found, but it should be notd that (i) the intensity of the spectra is about twice that found at lower reaction temperatures (Figure 91, in agreement with flow reactor studies, (ii) the two sulfate species evolved with a different rate as in the spectra recorded at 523 K (Figure 9), but the species characterized by the band near 1330cm-'continues to form to the contrary of that observed for lower reaction temperatures, and (iii) due to the higher reaction temperature which enhances the desorption of water, the initial apparent negative peak at around 1400 cm-I is not longer observed. It should be noted that the presence of a very weak band near 1375 cm-* can be tentatively suggested (Figure 9 and lOA), indicating that S-bonded chemisorbed SO3 species can be present, but in a very small amount. After evacuation at the same temperature, the spectrum changes (Figure 1OB) c o n f i i the presence of coordinated water
1954 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
samples using the KBr technique, and evidence the different evolution of sulfate species linked to Cu or to A1 during the sulfation step and during the regeneration step.
.. J"
I
1400
I
i:,
1200
I
1(
v , cm" Figure 11. In-situ infrared spectra of the time-on-stream evolution of a copper on alumina sorbent-catalyst in contact at 723 K with a flow containing 2% H2in helium. Time on stream, in minutes, is reported in the figure for the corresponding spectra. The spectrum of the sample at zero time on stream (sample treated only in a helium flow) and the spectrum of the gas phase have been subtracted from the spectra of the sample at various times on stream.
on the sulfate groups, in particular coordinated to the species characterized by the band at about 1330 cm-' (Figure 10A). In the spectra obtained by interaction in vacuum of S 0 2 / 0 2with a copper on alumina sample and subsequent admission of water (Waqif et al., 1991),the bands in the 1000-1350-cm-' region of sulfate species linked to alumina or in interaction with both A13+and Cu2+are generally found at higher frequencies than those due to sulfate species linked to only copper. This may suggest tentatively that the two sulfate species evidenced in Figures 9 and 10 can be attributed to surface sulfate species, probably bidentate and coordinating water, linked to Al (characterized by a main band at 1330 cm-') or copper (characterized by a main band at 1270cm-'). This suggestion is in agreement with the change in the relative intensities of these species with time on stream and temperature, but no unequivocal attribution can be derived from the present data, due to the great sensibility of the surface sulfate species to coordination water. The in-situ spectra of the evolution of sulfate species on copper on alumina sorbent-catalyst during the regeneration procedure with 2% H2 in helium at 723 K are shown in Figure 11. It is evident that the two sulfate species which form with different rates in the sulfation step are also reduced with different rates in the regeneration step. The species, characterized by the intense band centered at 1270 cm-' (attributed to a sulfate species interacting with copper sites) is immediately reduced in contact with a Hzflow, whereas the second species (the first which forms in the sulfation step and is attributed to a surface species interacting with A1 sites) is reduced much more slowly. These results are in agreement with previous evidence on the reactivity of sulfate species in vacuum upon contact with a reducing agent (Waqif et al., 1991). The behvaior of the various sulfate species during regeneration is thus further support for the previous attribution. In conclusion, in-situ results are in good agreement with the conclusions obtained in the characterization of the
Conclusions The combined infrared and flow reactor study of the formation and regeneration of sulfate species on copper on alumina sorbent-catalyst provides some useful information on the reaction mechanism that can be summarized as follows. 1. Copper catalyzes the oxidation of SOzto SO3, but SO3 does not desorb in the gas phase and remains linked to the surface probably as a S-bonded chemisorbed species. 2. After the oxidation stage, the SO3formed immediately reacts to form surface sulfates, probably bidentate species characterized by a low symmetry and partially coordinating water molecules. 3. Chemisorbed SO3 reacts with a high rate with neighboring A1 species to form a surface sulfate species linked to AI and reacts with a much slower rate with copper species to form sulfate species linked to Cu. 4. The sulfate species linked to Cu are more easily regenerated in the reduction step, whereas the regeneration of sulfate species linked to A1 is slower and part may remain after the reduction step. 5. The amount of Al species involved in the formation of surface sulfate species increases with increasing reaction temprature, as suggested from in-situ infrared experiments. This suggests that the number of accessible free A1 sites depends on the mean free path of surface diffusion of chemisorbed SO3 which has a high activation energy. 6. Surface sulfate species are the predominant onea for amounts of copper in the sorbent-catalyst equal to or below the amount necessary to form a complete monolayer of surface sulfate species (around 5 w t % CuO for a surface area of around 100 m2/g support). For higher loadings with copper, bulklike copper sulfate microparticles also form which, in consecutive cycles of adsorption-regeneration, may induce segregation and low of part of the copper with a consequent decrease in the sorbent-catalyst DeSO, performance. After several cycles of reaction-regeneration bulklike aluminum sulfate also forms. It should be noted that the alumina support stabilizes well-dispersed copper ions up to nearly monolayer coverage, probaby in the form of a surface spinel according to literature data on copper on alumina systems (Lo Jacono et al., 1982; Strohmeier et al., 1985; Wolberg and Roth, 1969), whereas for higher loadings, amorphous CuO microparticles also form. The analysis of the change in the specific rate of SOz removal per copper atom (Figure 5 ) suggests that CuO microparticles are less active in the DeSO, reaction due to the formation of a first surface sulfated layer which inhibits the further reaction of subsurface layers. The specific activity in SO2 oxidation-adsorption per Cu site thus decreases with increasingcopper content in the sorbent-catalyst. In addition, probably Bransted sites of the alumina support may be involved in the reaction mechanism, as suggested from in-situ infrared studies. This observation is in contrast with the findings of Chang (1978) in infrared experiments under high-vacuum conditions, but the difference in the surface siutation between tests in vacuum or during reaction (in-situ testa) can justify the contrast. These aspects, however, deserve more investigation. In conclusion, these data provide clear evidence that the role of the support is very important in determining the DeSO, activity of supported copper oxide. The support oxide, and in particular alumina, stabilizes well-dispersed copper ions and avoids the segregationof copper particles
Ind. Eng. Chem. Res., Vol. 31, No. 8,1992 1955
in multiple reaction-regeneration cycles. This segregation can have a detrimental effect on the mechanical strength and reactivity characteristics of the sorbent-catalyst. However, the support also has a second important role, because it is directly involved in the formation of sulfate species and contributes to the overall DeSO, activity, even though the sulfate species are less easily regenerated by reduction. The combination of all them aspects determines the general DeSO, reactivity of supported copper oxide samples and explains the considerable influence of the nature of the support as shown in Figure 4. The nature of the support (typeof oxide, but also surface area) and amount and distribution of copper species are thus very important factors to be taken into consideration for optimization of the sorbent characteristics and imporvement of the process. The temperature of reaction has also a considerable effect. Temperatures of reaction higher than about 600 K favor a deep sulfation of the alumina support with detrimental effects on both the regenerability of the sample and probably ita stability in a high number of cycles of reaction-regeneration. Acknowledgment
This work was sponsored by the European Community BRITE program (Contract RI 18B-0078-1). Registry NO.SOZ, 7446-09-5; Cu,7440-50-8;Al203,1344-2&1; SO,, 12624-32-7; NO,, 11104-93-1;SO3, 7446-11-9. Literature Cited Bond, G. C.; Namijo, S. N.; Wakeman, J. S. Thermal Analysis of Catalysis Precursors. 2. Influence of Support and Metal Precursor on the Reducibility of Copper Catalysts. J. Mol. Catal. 1991,64,305. Brittain, R. D.; Lan, K. H.; Hildenbrand, D. L. Effusion Studies of the Decomposition of CuSOl and CuO.CuS0,. J. Phys. Chem. 1989,93,5316. Centi, G.;Riva, A.; Pasearhi, N.; Brambilla, G.; Hodnett, B. K.; Delmon, B.;Ruwet, M. Simultaneous Removal of S02/N0, from Flue Gases. Sorbent/Catalyst Design and Performances. Chem. Eng. Sci. 1990,45,2679. Chang, C. C. Infrared Studies of SO2on Alumina. J. Catal. 1978, 53,374. Dautzenberg, F. M.; Naber, J. E.; van Ginneken, A. J. I. Shell's Flue Gas Desulfurization Process. Chern. Eng. Prog. 1971,67,86. Drummond, C. J.; Yeh, J. T.; Joubert, J. T.; Ratafii-Brown, J. A. The Design of a Dry, Regenerative Fluidized-Bed Copper Oxide Process for the Removal of SulfurDioxide and Nitrogen Oxides from Coal-Fired Boilers. Presented at the 78th Annual Meeting of the Air Pollution Control Association, Pittsburgh, PA, 1985. Ellison, W.; Sedman, C. B. 'German FGD/DeNOx Experience"; EPA Report PB87.147161,1987. EPA Report, 'Dry SO2 and Simultaneous SO,NO, Control Technologies"; Report PB85.23235,1985,Vol. 1. Friedman, R. M.; Freeman, J. J.; Lyttle, F. W. Characterization of Cu/A1203Catalysts. J. Catal. 1978,55,10. George, Z.M.; Bensitel, M.; Lion, M.; Saur, 0.; Lavalley, J. C. Effecta of Na+ on Sulphation and Related Reactions over a Commercial Claus Alumina Catalyst. Appl. Catal. 1988,43,167. Holliden, G. A. A Worldwide Review of Advanced SO2 Control Technologies. Proceedings 5th International Coal Conference, Pittsburgh; 1988. Horn, R. W.; Weissberger, E.; Collman, J. P. An l80Study of the Reaction between Iridium- and Platinum Oxygen Complexes and SO2To Form Coordinated Sulfate. Znorg. Chem. 1970,9,2367. Karthewr, B.; Hodnett, B. K.; Riva, A.; Centi, G.;Matralis, H.; Ruwet, M.; Grange, P.; Passarini, N. Temperature Programmed Reduction and X-Ray Photoelectron Spectroscopy of Copper Oxide on Alumina Sorbent/Catalyst following Exposure to SO2 and OP Znd. Eng. Chem. Res. 1991,30,2105.
Kiel, J. H. A.; Prins, W.; van Swaaij, W. P. M. Flue Gas Desulfphurization in a Gas-Solid Trickle Flow Reactor with a Regenerable Sorbent. Proceedings, First International Conference on Cas Separation Technology, Antwerp, Belgium; Elsevier: Amsterdam, 1989; in press. Lo Jacono, M.; Cimino, A.; Inversi, M. Oxidation States of Copper on Alumina Studied by Redox Cycles. J. Catal. 1982, 76,320. Lu, G. Q.;Do, D. D.; Beltramini, J. A New Process for SO2and NO, removal from Gases. Proceedings, Australia's Bicentennial Znternational Conference for the Process Industries, Sydney; The Institution of Engineers: Australia, 1988;Vol. 88/16,p 1010. Marion, M.C.; Garbowski, E.; Primet, M. Physicochemical Properties of Copper Oxide Loaded Alumina in Methane Combustion. J . Chem. SOC.,Faraday Trans. 1990,86,3027. McCrea, D. H.; Forney, A. J.; Myers, J. G. Recovery of Sulfur from Flue Gases using a Copper Oxide Adsorbent. J.Air Pollut. Control Assoc. 1970,20,819. Morrow, B. A.; McFarlane, R.A.; Lion, M.; Lavalley, J. C. An Infrared Study of Sulfated Silica. J. Catal. 1987,107,232. Nakamoto, K. Infrared and Ramun Spectra of Inorganic and Coordination Compounds, 3rd ed.; Wiley-Interscience: New York, 1978; p 239. Patrick, V.; Gavalas G. R.;Flytzam-Stephanopoulos, M.; Jothimurugesan, K. High Temperature Sflidation-Regeneration of Cu0-A1203 Sorbents. Ind. Eng. Chem. Res. 1989,28,931. Pollack, S . S.;Chisholm, W. P.; Obermyer, R. T.; Hedges, S. W.; Romanathan, M.; Montano, P. A. Properties of Copper/Alumina Sorbents used for the Removal of Sulfur Dioxide. Znd. Eng. Chem. Res. 1988,27,2276. Proud'homme, J.; Lamotte, J.; Jamin, A.; Lavalley, J. C. Dosage des Sulfates sur Alumine par Spectroscopic Infrarouge. Bull. SOC. Chim. Fr. 1981,11,433. Rentz, 0. European Application of Integrated Design for Combined Environmental Control: EPRI NO,/SOz Removal. In Integrated Pal; 46, CA, 1989. Riva, A. Simultaneous Removal of NO, and SO2through Mobile Bed Catalytic Processes. Presented at the first BRITE Technological Days; Brussel, Belgium, 1987. Rubin, E. S.; Salmento, J. S.; Frey, H. C. Evaluation of Combined S02/N0, Processes. In Integrated Environmental Control; EPRI: Palo Alto, CA, 1989. Sacks, M. D.; Tseng, T.-Y.; Lee, S. Y. Thermal Decomposition of Spherical Hydrated Basic Alumium Sulfate. Ceram. Bull. 1984, 63,301. Saur, D.; Bensitel, M.; Mohammed Saad, A. B.; Lavalley, J. C.; Tripp, C. P.; Morrow, B. A. The Structure and Stability of Sulfated Alumina and Titania. J. Catal. 1986,99,104. Saussey, H.; Vallet, A.; Lavalley, J.-C. Comparative Study of Alumina Sulfation from H a and SO2Oxidation. Mater. Chern.Phys. 1983,9,474. Stelman, D.; Ampaya, J. P.; Heredy, L. A.; Kohl, A. L.; Newcomb, J. C. Presented at the Second Meeting on Coal Utilization and Environmental Control, Pittsburgh, PA, 1986. Strohmeier, B. R.; Leyden, D. E.; Scott Field, R.;Hercules, D. M. Surface Spectroscopic Characterization of CuO/A1203Catalysts. J. Catal. 1985,94,514. Waqif, M.; Saur, 0.;Lavalley, J. C.; Perathoner, S.; Centi, G. Nature and Mechanism of Formation of Sulfate Species on Copper/Alumina Sorbent-Catalyst for SO2Removal. J. Phys. Chern. 1991,95, 4051. Wolberg, A.; Roth, J. F. Copper Oxide Supported on Alumina. I11 X-Rav K-Adsomtion Edge - Studies of the Cu2+SDecies. J. Catal. 1969,-15,250. Yeh, J. T.; Demaki, R.J.; Strakey, J. P.; Joubert, J. I. PETC Fluidized-bed comer oxide Drocess for combined SOJNO. removal from flue g&s. Presented at the AIChE 1984 6nte;National Meeting, Atlanta, GA, 1984. Yeh, J. T.; Demski, R. J.; Strakey, J. P.; Joubert, J. I. Combined S02/N0, Removal from Flue Gases. Environ. Prog. 1985,4,223. Yeh, J. T.; Drummond, C. J.; Joubert, J. I. Process Simulation of the Fluidized-Bed Copper-Oxide Process Sulfation Reaction. Enuiron. Prog. 1987,6,44. I
Received for review December 30, 1991 Revised manuscript received April 21, 1992 Accepted May 12,1992
