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Testing of the CuO/Al2O3 Catalyst-Sorbent in Extended Operation for the Simultaneous Removal of NOx and SO2 from Flue Gases C. Macken and B. K. Hodnett* Department of Chemical and Environmental Sciences and the Materials and Surface Science Institute, University of Limerick, Limerick, Ireland
G. Paparatto EniChem, Bollate, via S. Pietro 50, 20021 Bollate (MI), Italy
A series of CuO/Al2O3 catalyst-sorbents have been subjected to a large number of reactionregeneration cycles and their physical and chemical properties compared before and after operation. The reaction phase of the cycle involved exposing the catalyst-sorbents to a mixture of SO2, NO, NH3, O2, and steam at 350 °C, during which time NO conversion was always greater than 90% and the SO2 was converted into CuSO4. The regeneration phase involved reduction of CuSO4 to copper metal by CH4 at 500 °C, followed by oxidation in air at 450 °C. In some tests the reaction or regeneration conditions were made more severe in an attempt to accelerate deactivation. The SO2 sorption capacity of one sample, which had been subjected to 1525 cycles, was reduced by about 25%, but generally there was no significant change in reactivity. Small changes in physicochemical properties are interpreted in terms of redispersion of the supported copper phase. CuSO4 + 2H2 f Cu + SO2 + 2H2O
Introduction CuO/Al2O3 has been examined as a potential catalystsorbent for the simultaneous removal of SO2 and NOx from flue gases, in a number of different reactor configurations. The Pittsburgh Energy Technology Centre (PETC), developed a CuO/Al2O3 catalyst-sorbent for the removal of SO2 in a fluidized-bed reactor. NH3 is injected into the flue gas stream for the simultaneous reduction of NOx gases to N2.1 A similar technology was developed by the Netherlands Energy Research Foundation (ECN) to the laboratory stage, utilizing CuO supported on SiO2 as the catalyst-sorbent. NH3 was added here also as a reductant for the simultaneous removal of NOx.2 A gas-solid trickle flow reactor was proposed where the solid falls from the top of the reactor through a series of trays, counter current to the flue gas flow. A third system envisages the use of the CuO/ Al2O3 system operating in dual mobile-bed reactors, in one of which SO2 and NOx elimination would occur followed by reductive regeneration in a second reactor.3 An essential feature of each of the proposed processes is a cyclic operation whereby the catalyst-sorbent would operate for a time in SO2 capture (eqs 1 and 2) and deNOx mode (eqs 3 and 4) at one set of reaction conditions, followed by reductive regeneration (eq 5 or 6) and oxidation (eq 7) in a different set of conditions: CuO
SO2 + 1/2O298 SO3
(1)
SO3 + CuO f CuSO4
(2)
CuO-CuSO4
4NO + 4NH3 + O298 4N2 + 6H2O CuO-CuSO4
2NO2 + 4NH3 + O298 3N2 + 6H2O
(3) (4)
CuSO4 + 1/2CH4 f Cu + SO2 + 1/2CO2 + H2O Cu + 1/2O2 f CuO
(5) (6) (7)
There are also many chemical transformations occurring during reaction-regeneration-oxidation cyclic operation (CuO f CuSO4 f Cu). Therefore, the CuO/Al2O3 catalyst-sorbent requires exceptional physicochemical and mechanical properties that should remain stable over a large number of reaction-regeneration cycles. A small number of previous studies have investigated the stability of the CuO/Al2O3 catalyst-sorbent system over extended reaction-regeneration cycles. Johannes et al. tested a 9.0CuO/Al2O3 sorbent (s.a. > 200 m2 g-1) over 800 sorption-regeneration cycles in a fixed-bed reactor (i.d. 20 mm).4 After completion of the test, 30% of the CuO was present as crystallites greater than 900 Å in size, which required longer regeneration times for the sulfated sorbent. At the Pittsburgh Energy Technology Center (PETC), McCrea et al. tested a 6.3CuO/Al2O3 sorbent (considered the optimum CuO loading) for over 200 sorption-regeneration cycles in a fixed-bed benchscale reactor (i.d. 100 mm).5 After the test, there was no evidence of chemical or physical deactivation. Continued research at the PETC by Yeh et al. led to the testing of the same catalyst-sorbent for 75 reactionregeneration cycles in a fluidized bed (i.d. 150 mm).6 After these tests, there was no evidence of any decrease in the chemical activity of the sorbent but some physical attrition was observed. This paper outlines the effect of operating under various reaction and regeneration regimes on the physicochemical and mechanical properties of the CuO/Al2O3 catalyst-sorbent. The effects of extended cyclic operation on the SO2 sorption capacity and regeneration behavior was investigated. In addition, a
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Figure 1. Typical events program of the laboratory-scale testing facility for the standard reaction-regeneration cyclic testing of CuO/ Al2O3 catalyst-sorbents.
complete characterization of the textural properties (surface area and pore size distributions) of the fresh and aged catalyst-sorbents was carried out to identify possible deactivation phenomena. Experimental Section Preparation of the CuO/Al2O3 Catalyst-Sorbents. Three catalyst-sorbents with CuO loadings of 4.3%, 4.9%, and 7.0%, respectively, were prepared, by deposition of the appropriate concentration of CuSO4 precursor onto calcined RP535 Al2O3 support pellets by the incipient wetness technique, followed by drying (90 °C for 4 h) and calcining (250 °C for 8 h followed by 500 °C for 3 h). The lowest CuO loading was close to that required for monolayer coverage of the alumina and higher loadings were tested to explore the sorption capacity of the system. A fully automatic laboratory-scale testing facility was designed and constructed to assess the performance of CuO/Al2O3 catalyst-sorbents, suitable for the simultaneous removal of SO2 and NOx from a simulated flue gas (F ) 75 L h-1) over a large number of reactionregeneration cycles. The SO2 and NOx removal reactions were usually carried out at 350 °C, while the sulfated catalyst-sorbent was regenerated at 500 °C with 10% CH4. The temperature cycles, gas flow rates, and compositions were fully programmable for continuous unattended cyclic operation and are shown in Figure 1. A quartz fixed-bed reactor of internal diameter 1.5 cm and length 30 cm was used with a catalyst change of ca. 15 g. For the simultaneous SO2 and NOx removal reaction, water vapor was added to the simulated flue gas composition using a saturator, while NH3 was added at the entrance to the reactor in a separate line, to avoid the deposition of ammonium sulfate. To obtain a uniform aging of the catalyst-sorbent bed, a periodic inversion of the flow from upward to downward was possible (with a consequent inversion of the NH3 flow). The continuous analysis of the inlet and outlet reactor gaseous streams was made using a microprocessorcontrolled combustion analyzer based on precalibrated electrochemical cells. The gases analyzed were O2, CO2, SO2, NO, and NO2 with a resolution of 1 ppm. Ammonia breakthrough was never observed during testing. A summary of the number of reaction-regeneration cycles completed and the conditions employed during
Table 1. Overview of Conditions Employed during the Lifetime Testing of the CuO/Al2O3 Catalyst-Sorbents sample
cycles
4.3CuO 4.3CuO-370 4.3CuO-750 4.9CuO 4.9CuO-117 4.9CuO-1525 7.0CuO 7.0CuO-100 4.3CuO-36
fresh 370 750b fresh 117 1525 fresh 100 36
W/F (g/(mL min-1))
test conditionsa
3/250 3/250
a b
15/1250 15/1250
c a
3/250 600/1.6 m3 h-1
a d
a See Table 2. b Conditions were the same as those used for 4.3CuO-370 for the first 370 cycles.
Table 2. Standard Reaction-Regeneration Conditions (a) and Deviations from These Conditions Designed To Accelerate the Aging of the CuO/Al2O3 Catalyst-Sorbents (i.e., b, c, and d) test conditions a b c d
NOx and SO2 removal reactions
regeneration
T ) 500 °C, T ) 350 °C, 10% H2O 10% CH4 0.2% SO2, 3% O2, 0.1% NO, 0.1% NH3, and 10% CO2. T ) 450 °C T ) 550 °C T )450 °C, 20% H2O 100% CH4 0.13% SO2, 600 ppm NO, and 0.11% NH3
oxidation T ) 400 °C in air T ) 480 °C T ) 480 °C
the lifetime testing of the 4.3CuO/Al2O3, 4.9CuO/Al2O3, and 7.0CuO/Al2O3 catalyst-sorbent pellets is presented in Tables 1 and 2. The code for each catalyst-sorbent consists of the wt % CuO followed by the number of reaction-regeneration cycles completed. In addition to the standard cyclic conditions, some attempts were made to accelerate catalyst-sorbent aging. In the first of these, lifetime testing of a standard 4.3CuO/Al2O3 catalyst-sorbent was performed under (i) standard reaction-regeneration conditions (370 cycles) and (ii) an additional 380 reaction-regeneration cycles in accelerated aging conditions. The accelerated conditions consisted of utilizing a higher reaction (SO2 and NOx removal) temperature (450 °C) and a higher oxidation temperature (480 °C). The higher sorption temperature was employed with the intention of promoting bulk aluminum sulfate formation,7 deemed to adversely affect the stability of the textural properties of the catalyst-sorbent during extended reaction-regeneration cycles.8
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In a second series of accelerated aging tests, 4.9CuO/ Al2O3 was tested in conditions (117 cycles) where SO2 and NOx removal was carried out with higher water vapor content present in the simulated flue gas (i.e., 20% H2O) and at a higher reaction temperature (450 °C). The regeneration of the sulfated catalyst-sorbent was carried out at 550 °C with 10% CH4, thereby promoting possible sintering of the copper metal phase and the Al2O3 support. A 4.9CuO/Al2O3 catalyst-sorbent was also tested for a total number of 1525 reaction-regeneration cycles under standard experimental conditions and a slightly higher loading in CuO, namely, 7.0CuO/Al2O3 catalystsorbent, was tested for 100 reaction-regeneration cycles under standard conditions. A bench-scale testing facility was also used to test a standard 4.3CuO/Al2O3 catalyst-sorbent for a number of reaction-regeneration cycles using real flue gases at flow rates of 1.6 m3 h-1. The internal diameter of the reactor was 8 cm, giving a 30-cm bed height for a 1-kg change of catalyst-sorbent. A commercial gas-oil burner was utilized to provide the required flue gas flow rates and composition. The oil mixture contained 70% paraffin oil, 25% n-hexane, 5% thiophene, and small quantities of pyridine. Thiophene was added to the oil mixture to increase the SO2 concentration in the flue gas to 1300 ppm, while the addition of pyridine gave a NOx concentration of 600 ppm. Following the treatment described in Tables 1 and 2, the catalyst/sorbents were removed from the reactor in the sulfated state, ground, sieved to a particle size of 212 < x < 500 µm, and subjected to a number of evaluations of sorption and regeneration characteristics on a microscale in which the following testing conditions applied: W/F reaction regeneration oxidation
200 mg/40 mL min-1 T ) 350 °C, 4800 ppm SO2, and 5200 ppm O2 T ) 500 °C, 5% CH4 T ) 500 °C with 0.52% O2
A replicate sorption-regeneration cycle was completed for each catalyst-sorbent. The chemical analysis of the Cu content of the fresh and aged catalyst-sorbent particles was determined by atomic absorption spectroscopy. The surface areas of some of the catalyst sorbents particles (212 < x < 500 µm) were measured using a Micromeritics Gemini 2370 surface area analyzer. Procedures for the recording of temperature-programmed reduction (TPR) profiles in 5% CH4 or 5% H2 have been fully described elsewhere.9 Results Physicochemical Characterization of the Fresh and Aged Catalyst-Sorbents. The CuO concentration measured by atomic absorption spectroscopy given in Table 3 indicated no difference between intended and measured loadings of the fresh CuO/Al2O3 catalystsorbents, demonstrating that the impregnation technique was successful in depositing the intended CuO concentration onto the Al2O3 support. There was also no significant difference in the CuO loadings for the fresh and aged catalyst-sorbents, indicating no loss of the CuO active phase during cyclic reaction-regeneration testing. In general, the textural analysis of the aged catalystsorbents highlights a decrease in the BET surface area
Table 3. Physicochemical Characterization of the Fresh and Aged Catalyst-Sorbent Pellets, Including the CuO Loadings, Surface Areas, Total Pore Volume, and SO2:CuO Molar Ratio Achieved for the Stated Number of Cycles
sample 4.3CuO 4.3CuO-370 4.3CuO-750 4.9CuO 4.9CuO-117 4.9CuO-1525 7.0CuO 7.0CuO-100 4.3CuO-36 a
measured surface area wt % CuO (m2 g-1) 4.3 4.3 4.3 4.9 4.9 4.9 6.9 7.0 4.2
117 99 100 117 104 86 113 109 112
total pore volumea SO2:CuO (cm3 g-1) molar ratio 0.92 1.06 1.07 0.92 n.m 1.10 n.m 0.97 1.01
1.04 1.08 1.09 1.02 1.02 0.76 0.87 1.06 0.94
n.m ) not measured.
and an increase in the total pore volume with the number of reaction-regeneration cycles. The 4.9CuO1525 sample, which had undergone 1525 reactionregeneration cycles, has the lowest surface area (86 m2 g-1) and the highest total pore volume (1.10 cm3 g-1), but the overall differences with the fresh samples were small. Scanning electron microscopy (S.E.M) analysis of 4.9CuO-1525 showed the presence of small voids, possibly accounting for the observed decrease in surface area and the increase in total pore volume.10 Sorption-Regeneration Tests on the Fresh and Aged Catalyst-Sorbents. During cyclic reactionregeneration testing, the CuO/Al2O3 catalyst-sorbents were capable of NO conversions in excess of 90% and SO2 capture capacities of close to 100% until the available CuO was exhausted. This behavior has been extensively reported previously. The SO2 sorption capacities presented in Figure 2 indicate that no deactivation occurred for the 4.3CuO/ Al2O3 catalyst-sorbent tested for a total number of 750 cycles (inc. 380 in accelerated aging conditions) or the 4.9CuO/Al2O3 catalyst-sorbent tested for 117 cycles in accelerated conditions. The 4.9CuO/Al2O3 catalystsorbent, which underwent the highest number of reaction-regeneration cycles (1525), exhibits the largest decrease in SO2 sorption activity (ca. 25%). The detailed results are presented in Figure 3 for this material in terms of SO2 capture capacity and SO2 release during reductive regeneration. Interestingly, the 7.0CuO/Al2O3 catalyst-sorbent showed an increase in SO2 sorption activity after 100 standard reaction-regeneration cycles (Figure 2). Finally, the 4.3CuO/Al2O3 catalyst-sorbent tested for 36 cycles on the bench-scale apparatus has lower SO2 sorption activity after aging, compared to the fresh sample. The data from Figure 2 are also presented as the SO2: CuO molar ratio in Table 3. Viewed from this perspective, the apparent increase in SO2 sorption for 7.0CuO/ Al2O3 after 100 cycles may be viewed as an increase from an initially low level for the fresh catalyst-sorbent. TPR Profiles of Fresh and Aged Sulfated Catalyst-Sorbents. The TPR profiles in 5% CH4 of the sulfated fresh and aged CuO/Al2O3 catalyst-sorbents presented in Figure 4, show a single broad reduction peak (SO2 evolution) beginning at 350 °C with a maximum at 515 °C. There is no significant difference in any of the TPR profiles of the fresh and aged catalyst-sorbents, except for the 4.3CuO-36 which has a maximum SO2 evolution peak at slightly lower temperatures (ca. 505 °C).
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Figure 2. SO2 sorption activities of the fresh and aged CuO/Al2O3 catalyst-sorbents.
Figure 4. TPR in 5% CH4 of fresh and aged sulfated CuO/Al2O3 catalyst-sorbents (temperature ramp rate ) 10 °C min-1, W ) 50 mg, F ) 20 mL min-1).
Figure 3. (A) Sorption of SO2 by fresh and aged 4.9CuO/Al2O3 catalyst-sorbents (T ) 350 °C, 4800 ppm SO2, and 5150 ppm O2), and (B) the regeneration-reaction (T ) 500 °C, 5% CH4).
The TPR profiles of the fresh and aged sulfated CuO/ Al2O3 catalyst-sorbents in 5% H2 shown in Figure 5 offer more information regarding possible deactivation phenomena. In general, the reduction reaction for the sulfated materials in 5% H2 begins at approximately 290 °C with two SO2 peak maxima at approximately 315 and 325 °C and a peak above 340 °C. The multiple SO2 peaks may suggest the reduction of CuSO4 species with a bimodal particle size distribution. Interestingly, the second SO2 peak maximum in the TPR profile of 4.3CuO-750 occurs at approximately
20 °C lower than the corresponding peak in the reduction profile of the fresh sulfated 4.3CuO-3 catalystsorbent. This suggests that some of the CuSO4 species are more readily regenerated after the aging process. The 4.3CuO-36 and 4.9CuO-117 (to a lesser extent) show some SO2 evolution above 400 °C. TPR in 5% H2 of Fresh and Aged Oxidized Catalyst-Sorbents. In general, the TPR profiles shown in Figure 6 of the fresh and aged oxidized CuO/Al2O3 catalyst-sorbents in 5% H2 as the reducing agent possess one peak maximum for H2 consumption centered around 230 °C with a shoulder at 280 °C. The main reduction peak at 230 °C is present for the reduction of CuO on a number of supports (i.e., Al2O3, SiO2, TiO2) and relates to the reduction of small, highly dispersed CuO particles.11,12 The shoulder at 280 °C is assigned to the reduction of larger (possibly crystalline) CuO particles. In Figure 6, there is a definite shift of the first peak maximum to lower temperatures (240 f 220 °C) for the aged CuO/Al2O3 catalyst-sorbents, possibly due to an
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Figure 5. TPR in 5% H2 of fresh and aged sulfated CuO/Al2O3 catalyst-sorbents (temperature ramp rate ) 10 °C min-1, W ) 50 mg, F ) 20 mL min-1).
Figure 6. TPR in 5% H2 of fresh and aged oxidized CuO/Al2O3 catalyst-sorbents (T ) 10 °C min-1, W/F ) 50 mg/20 mL min-1).
improved dispersed and hence the formation of smaller more reactive CuO particles.12 There is also a definite decrease in the intensity of the feature at 280 °C with aging of the CuO/Al2O3 catalyst-sorbent, which suggests a reduction in the amount of larger (crystalline) CuO particles. Discussion TPR of Fresh and Aged Sulfated CatalystSorbents. The TPR profiles of the sulfated fresh and aged CuO/Al2O3 catalyst-sorbents with H2 were more sensitive to changes in the catalyst-sorbent composition than the corresponding profiles run in CH4. In these
conditions, three main SO2 evolution peaks are present. The highest temperature peak present above 350 °C for the fresh 4.3CuO-3 sample is shifted to lower temperatures for all the aged catalyst-sorbents (excluding the 4.3CuO-36), suggesting that some of the CuSO4 species are more readily reduced after the aging tests, possibly due to the presence of smaller more reactive CuSO4 particles. The aged 4.3CuO-36 and 4.9CuO-117 (to a lesser extent) show some SO2 evolution above 400 °C, which probably corresponds to the presence of some larger CuSO4 particles that are more difficult to reduce. Kartheuser et al.13 showed the reduction of bulk CuSO4 in TPR mode in 5% H2 under similar experimental conditions had its SO2 evolution peak at this temperature. This may indicate that some bulk CuSO4 is formed in the conditions employed during these reactionregeneration aging cycles, although no significant crystalline CuSO4 was detected by XRD for these samples. TPR in 5% H2 of Fresh and Aged Oxidized Catalyst-Sorbents. A peak maximum at 300 °C in the TPR profile in 5% H2 of a fresh oxidized 2.0CuO/Al2O3 catalyst-sorbent has been observed in several studies and is assigned to the reduction of Cu2+ ions that are strongly interacting with the Al2O3 support.11,12 At these low CuO loadings the predominant species present on the catalyst are isolated Cu2+ ions and two-dimensional CuO clusters.14,15 These Cu2+ ions are difficult to completely reduce to Cu0 because of their strong interaction with the support.16 The TPR profile in 5% H2 of a fresh oxidized 4.3CuO-2 catalyst-sorbent shown in Figure 6 has two reduction maxima with a major peak at ca. 230 °C and a minor peak (shoulder) at ca. 280 °C. The lower temperature peak can be assigned to the existence of small highly dispersed CuO particles or three-dimensional clusters, while the higher reduction peak is assigned to the reduction of larger CuO particles.11,12,14,17 The TPR profile of a fresh oxidized 7.0CuO-2 catalystsorbent also shows two reduction peak maxima (Figure 6). The peak maximum assigned to the small, highly dispersed CuO particles is shifted to higher temperatures (ca. 250 °C) compared to that of 4.3CuO-2, possibly due to a decrease in CuO dispersion and an increase in particle size.12 In addition, the shoulder at 280 °C becomes more significant, probably due to the existence of a greater number of larger CuO particles (i.e., crystalline CuO). The TPR profile of bulk CuO exhibits a single reduction peak maximum at ca. 280 °C, which indicates that the shoulder peak at 280 °C which increases in intensity with CuO loading is due to crystalline CuO.18,19 The first reduction peak maximum was progressively shifted to lower temperatures (250 f 220 °C) with the number of reaction-regeneration cycles performed on the catalyst-sorbents (Figure 6). Therefore, it is postulated here that the shift to lower temperatures for the aged catalyst-sorbents is related to an improved dispersion of the CuO active phase. There was also a significant reduction in the intensity with aging, of the second reduction peak maximum (at 280 °C), which was assigned to the presence of some larger CuO particles (Figure 6). This is particularly obvious for the fresh and aged 7.0CuO/Al2O3 catalystsorbent where more of these larger CuO particles are expected (Figure 6).
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The improved dispersion of the CuO active phase occurs during the cyclic oxidation-sulfation-reduction cycles where the following chemical changes occur: CuO f CuSO4 f Cu0. XPS studies have shown that there is a 40% decrease in the surface Cu:Al ratio after reduction with H2 at 250 °C of CuO/Al2O3 catalysts, due to sintering,14,20,21 while electron diffraction studies also show the existence of Cu0 particles (80 Å) after reduction with H2.14,22 After oxidation there is a re-dispersion of Cu species; the XPS surface Cu:Al ratio increases, while electron diffraction studies (detection limit 10 Å) cannot detect any CuO particles.22 This constant restructuring of the CuO active phase during reaction-regeneration cyclic operation probably leads to an improved dispersion as observed in the TPR studies of the aged catalystsorbents (Figure 6). Waqif et al. have suggested that the presence of surface aluminum sulfate species, which are not readily regenerated, inhibit excessive sintering of the Cu particles in the regeneration step, resulting in the formation of highly dispersed amorphous CuO particles that are very reactive in the oxidative sulfation reaction.8 The incipient wetness impregnation technique gives dispersions of ca. 12% for a 8 wt % CuO/Al2O3 catalyst, measured by N2O titration of surface Cu atoms after reduction,12,19 while CuO dispersions up to 4 times higher have been achieved when the active phase was deposited onto the Al2O3 support by precipitation from a homogeneous solution using urea hydrolysis at 90 °C.21,23 This shows that it is reasonable to assume that some large CuO crystallites are formed (resulting in the presence of a peak at 280 °C and above in the TPR profile) because of the impregnation technique employed in the preparation step. With aging it is suggested that the shoulder (at 280 °C) in the TPR profile (Figure 6) disappears because of an improved dispersion of the CuO. Chemical Activity of the Fresh and Aged Catalyst-Sorbents. The higher SO2 sorption capacities (Figure 2) of the 4.3CuO-370 and 4.3CuO-750 catalystsorbents suggest an improved dispersion of the CuO active phase with aging. There was also a slight increase in the mean maximum rate of SO2 evolution during the regeneration reaction in 5% CH4 at 500 °C. The shift to lower temperatures of the TPR peak maximum of the oxidized 4.3CuO-370 and 4.3CuO-750 catalyst-sorbents is consistent with this proposal. The 4.9CuO-117 catalyst-sorbent was shown to have a stable SO2 sorption capacity (Figure 2) when tested in reaction-regeneration conditions designed to accelerate aging (Tables 1 and 2). A reaction temperature of 450 °C was utilized to promote the formation of bulk Al2(SO4)3 deemed to adversely affect the stability of the catalyst-sorbent during cyclic reaction-regeneration tests.8 Meanwhile, a higher regeneration temperature of 550 °C was also employed to promote sintering of the Cu0 crystallites. The 4.9CuO-1525 catalyst-sorbent exhibited a decrease (ca. 25%) in its SO2 sorption capacity (Figure 2). The lower mean rate of SO2 evolution during the regeneration of the aged catalyst-sorbent (Figure 3) compared with that of the fresh catalys--sorbent suggests that some unreactive CuO particles were formed during the aging tests. The X-ray Cu KR microprobe analysis of a 4.9CuO-1525 catalyst-sorbent pellet clearly shows the presence of some of these large CuO par-
ticles.10 It is clear from the TPR data in Figure 6 that there is a tendency within this system for the copper particles to change their physical and chemical form during cyclic operation, and the former must be facilitated by some transport of the copper phases over the support. For operation up to about 750 cycles this resuls in an improved dispersion and in some cases improved reactivity. However, for cyclic operations up to 1525 cycles, the same driving forces result in some sintering of the supported copper phases. Alternativiely, sintering of the support may have resulted in encapulation of copper particles, lowering the SO2 capture capacity. A small decrease in the SO2 sorption activity (Figure 3) of ca. 11% was observed for the 4.3CuO-36 catalystsorbent, which underwent 36 reaction-regeneration cycles on a bench-scale testing apparatus treating a real flue gas. The TPR of the sulfated catalyst-sorbent in 5% H2 reveals a SO2 evolution peak at 400 °C, possibly due to the presence of some crystalline CuSO4 species. The fresh 7.0CuO/Al2O3 catalyst-sorbent sorbed 35% extra SO2 compared to the fresh 4.3CuO/Al2O3 catalystsorbent (Figure 2), confirming the proposal by Yeh et al.1 that 30% lower recirculation rates are required for such an increase in the CuO loading. In addition, the TPR profile in 5% H2 in Figure 6 of the aged oxidized CuO/Al2O3 shows a significant reduction in the intensity of a shoulder peak at 290 °C, due to a decrease in the size of the larger CuO particles responsible for this shoulder. Conclusions (1) The 4.3CuO/Al2O3 catalyst-sorbent was shown to have stable SO2 sorption and NOx removal properties for over 750 reaction-regeneration cycles, 380 of which were performed in conditions designed to accelerate possible deactivation phenomena. The textural and mechanical properties of the catalyst-sorbent were not affected by the aging process. Preliminary estimates put the useful lifetime of this catalyst at greater than 1 year of continuous operation. (2) A 4.9CuO/Al2O3 catalyst-sorbent tested in reaction-regeneration conditions designed to accelerate aging for 117 cycles also exhibited no physicochemical deactivation. (3) The 4.3CuO/Al2O3 catalyst-sorbent, which was tested on a bench-scale apparatus treating a real flue gas, exhibited very small changes in its performance after 36 cycles. (4) The 4.9CuO-1525 catalyst-sorbent showed a decrease in the BET surface area (from 117 to 86 m2 g-1) during aging and in SO2 sorption capacity (ca. 25%), while the efficiency of the NOx removal reaction was not affected.10 This deactivation is associated with the generation of large CuO crystallites or encapsulation of the supported phase during support sintering. (5) Significantly, the SO2 sorption activity of the 7.0CuO/Al2O3 catalyst-sorbent increased by 27% after 100 standard reaction-regeneration cycles compared to the fresh catalyst-sorbent due to an improved dispersion of the CuO active phase. Acknowledgment This work was supported by the EU under the BRITE-EURAM Programme, Contract BREU CT91/ 0549.
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Literature Cited (1) Yeh, J. T.; Drummond, C. J.; Joubert, J. I. Process Simulation of the Fluidized-Bed Copper Oxide Process Sulfation Reaction. Environ. Prog. 1987, 6 (2), 44. (2) Kiel, J. H. A.; Prins, W.; Van Swaaij, W. P. M. Performance of Silica-Supported Copper Oxide Sorbents for SO2/NOX Removal from Flue Gas; Part I. Appl. Catal. B: Environ. 1992, 1, 13. (3) Centi, G.; Hodnett, B. K.; Jaeger, P.; Macken, C.; Marella, M.; Tomaselli, M.; Paparatto, G.; Perathoner, S. Development of Copper-on-Alumina Materials for the Cleanup of Flue Gas and the Disposal of Diluted Ammonium Sulfate Solutions. J. Mater. Res. 1995, 10, 553. (4) Johannes, H.; Heldon, A. V.; Naber, J. E. Process for the Removal of SO2 Form an O2 Containing Gas Mixture. Inter. Patent Specification B 01 j 11/32; C 01 b 17/60, 1969. (5) McCrea, D. H.; Forney, A. J.; Myers, J. G. Recovery of Sulfur from Flue Gases Using a Copper Oxide Absorbent. J. Air Pollut. Control Assoc. 1970, 20, (12), 819. (6) Yeh, J. T.; Demski, R. J.; Strakey, J. P.; Joubert, J. I. Combined SO2/NOX Removal from Flue Gas. Environ. Prog. 1985, 4 (4), 223. (7) Yoo, K. S.; Kim, S. D.; Park, S. B. Sulfation of Al2O3 in Flue Gas Desulphurisation by CuO/Al2O3 Sorbent. Ind. Eng. Chem., Res. 1994, 33, 1786. (8) Waqif, M.; Saur, O.; Lavalley, J. C.; Perathoner, S.; Centi, G. Nature and Mechanism of Formation of Sulfate Species on Copper/Alumina Sorbent-Catalysts for SO2 Removal. J. Phys. Chem. 1991, 95, 4051. (9) Macken, C.; Hodnett, B. K. Reductive Regeneration of Sulfated CuO/Al2O3 Catalyst-Sorbents in Hydrogen, Methane, and Steam. Ind Eng Chem. Res. 1998, 37, 2611. (10) Paparatto, G. Development and Optimisation of Cu-based Catalytic Materials for the Simultaneous Removal of SO2 and NOX from Flue Gases. Technical Report BREU-CT91-0547, Project 4074, 1995. (11) Bond, G. C.; Namijo, S. N.; Wakeman, J. S. Thermal Analysis of Catalyst Precursors. J. Mol. Catal. 1991, 64, 305. (12) Robinson, W. R. A. M.; Mol, J. C. Characterisation and Catalytic Activity of Copper/Alumina Methanol Synthesis Catalysts. Appl. Catal. 1988, 44, 165. (13) Kartheuser, B. Study of the Reduction of Alumina Supported Copper Oxide and Copper Sulphate Catalysts Following
Exposure to Sulphur Dioxide. MS Thesis, University of Limerick, Limerick, Ireland, 1990. (14) Tikhov, S. F.; Sadykov, V. A.; Kryukova, G. N.; Paukshtis, E. A.; Popovskii, V. V.; Starostina, T. G.; Kharlamov, G. V.; Anufrienko, V. F.; Poluboyarov, V. F.; Razdobarov, V. A.; Bulgakov, N. N.; Kalinkin, A. V. Microstructural and Spectroscopic Investigations of the Supported Copper-Alumina Oxide System. J. Catal. 1992, 134, 506. (15) Centi, G.; Peranthoner, S.; Biglino, D.; Giamello, E. Adsorption and Reactivity of NO on Copper-on-Alumina CatalystsI. J. Catal. 1995, 151, 75. (16) Lo Jacono, M.; Cimino, A.; Inversi, M. Oxidation States of Copper on Alumina Studied by Redox Cycles. J. Catal. 1982, 76, 320. (17) Marion, M. C.; Garbowski, E.; Primet, M. Catalytic Properties of CuO Supported on Zinc Aluminate in Methane Combustion. J. Chem. Soc., Faraday Trans. 1991, 87 (11), 1795. (18) Hurst, N. W.; Gentry, S. J.; Jones, A. Temperature Programmed Reduction Catal. Rev.-Sci. Eng. 1982, 24 (2), 233. (19) Huang, T. J.; Yu, T. C.; Chang, S. H. Effect of Calcination Atmosphere on CuO/Al2O3 Catalyst for Carbon Monoxide Oxidation. Appl. Catal. 1989, 52, 157. (20) Strohmeier, B. R.; Leyden; D. E.; Field, R. S.; Hercules, D. M. Surface Spectroscopic Characterisation of Cu/Al2O3 Catalysts. J. Catal. 1985, 94, 514. (21) Kaushik, V. K.; Sivaraj, Ch.; Rao, P. K. ESCA Characterisation of Copper/alumina Catalysts Prepared by DepositionPrecipitation Using Urea Hydrolysis. Appl. Surf. Sci. 1991, 51, 27. (22) Pollack S. S.; Chisholm, W. P.; Obermyer, R. T.; Hedges, S. W.; Ramanathan, M.; Montano, P. A. Properties of Copper/ Alumina Sorbents Used for the Removal of SO2. Ind. Eng. Chem., Res. 1988, 27, 2276. (23) Shivaraj, C.; Kantarao, P. Characterisation of Copper/ Alumina Catalysts; Prepared by Deposition-Precipitation Using Urea Hydrolysis. Appl. Catal. 1988, 45, 103.
Received for review March 20, 2000 Revised manuscript received June 16, 2000 Accepted June 18, 2000 IE000342D
