Reductive regeneration of sulfated CuO/Al 2 O 3 catalyst-sorbent in ammonia

Transcription

1 Applied Catalysis B: Environmental 45 (2003) Reductive regeneration of sulfated CuO/Al 2 O 3 catalyst-sorbent in ammonia Guoyong Xie, Zhenyu Liu, Zhenping Zhu, Qingya Liu, Jianrong Ma State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi , PR China Received 1 November 2002; accepted 24 March 2003 Abstract Reductive regeneration of a sulfated CuO/Al 2 O 3 catalyst-sorbent suitable for simultaneous SO 2 and NO x removal from flue gases was carried out in 5 vol.% NH 3 /Ar. Effect of regeneration temperature on SO 2 removal activity of the regenerated catalyst-sorbent was investigated. Chemical morphology and physical structure of the catalyst-sorbent before and after the regeneration were characterized using elemental analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and physical absorption. The results show that copper sulfate, the main copper species in the sulfated catalyst-sorbent, can be effectively regenerated at C. Aluminum sulfate, resulted from the reaction between Al 2 O 3 and SO 2 in flue gas, is unreductable under the conditions, which leads to reduced SO 2 removal activity of the regenerated catalyst-sorbent compared to the fresh catalyst-sorbent. The main copper species after the regeneration at 400 CisCu 3 N. The nonexistance of copper sulfide suggests that over-reduction occurring in H 2 is avoided in NH 3. BET surface area and pore size distribution of the regenerated catalyst-sorbent are almost the same as that of the fresh one when the reduction temperature is 400 C or higher. Besides as a reductant, ammonia also reacts with SO 2 formed in the regeneration to form hydroxyamine sulfate at the outlet of the reactor Elsevier Science B.V. All rights reserved. Keywords: Flue gas desulfurization; CuO/Al 2 O 3 catalyst-sorbent; Regeneration in NH 3 1. Introduction Sulfur oxides (SO x ) and nitrogen oxides (NO x )in flue gas are the major air pollutants to the atmosphere. Various processes are under investigation to remove them from flue gas, among which dry and catalytic methods for combined SO 2 and NO x removal are advantageous compared to individual approaches due to economical benefit [1 3]. Interest has focused on Corresponding author. Tel.: ; fax: address: zyl@public.ty.sx.cn (Z. Liu). copper on alumina (CuO/Al 2 O 3 ) catalyst-sorbent, which offers a good capacity for SO 2 adsorption and a high catalytic activity for selective catalytic reduction (SCR) of NO with ammonia in the presence of oxygen in the temperature range of C [4 8]. After adsorbing SO 2, the CuO in the CuO/Al 2 O 3 catalyst-sorbent transfers into CuSO 4 and must be regenerated for reusing. The regeneration efficiency is closely related to subsequent sulfur removal activity and stability of the catalyst-sorbent. To date, the regeneration methods for sulfated CuO/Al 2 O 3 catalyst-sorbent are mainly thermal decomposition and reductive regeneration [4]. Because the thermal /$ see front matter 2003 Elsevier Science B.V. All rights reserved. doi: /s (03)

2 214 G. Xie et al. / Applied Catalysis B: Environmental 45 (2003) decomposition has to be carried out at temperatures much higher than the desulfurization does, the catalyst-sorbent suffers from loss of sulfur removal activity and shortening of life-span [9,10]. The reductive regeneration is regarded to be better than the thermal decomposition due to relatively low regeneration temperatures. In reductive regeneration, hydrogen and alkane are used conventionally [4,5,11,12] which convert the sulfur in CuSO 4 into gaseous SO 2, and, therefore, requires an additional unit to convert SO 2 into elemental sulfur or sulfuric acid. This makes the whole process complicated. Furthermore, hydrogen is a strong reductant which usually results in over-reduction of copper to form Cu 2 S and elemental Cu at temperatures of 400 C and higher [6,13,14]. The sulfide species is oxidized to sulfate species in the desulfurization step and has no ability to further adsorb SO 2, thus lowering the sulfur removal capacity of the CuO/Al 2 O 3 catalyst-sorbent [14]. The elemental Cu is oxidized to form CuO in desulfurization, which releases a large amount of heat. This is unwanted especially when a large amount of such catalyst is being used. Comparatively, alkane is a weak reducing agent, temperatures higher than 500 C is required for effective regeneration of sulfated CuO/Al 2 O 3. Based on these facts, it can be concluded that there is no appropriate method to regenerate the sulfated CuO/Al 2 O 3 at around 400 C, the optimal temperature for simultaneous SO x and NO x removal. Our earlier work showed that NH 3 is an appropriate reductant for regeneration of CuO/AC and Fe 2 O 3 /AC desulfuziers and can directly convert the SO 2 into solid ammonium sulfur salts [15,16]. It is likely that NH 3 can be used to effectively regenerate the sulfated CuO/Al 2 O 3 catalyst-sorbent at temperatures around 400 C, if so, significant economical advantages can be realized due to regenerating the sulfated CuO/Al 2 O 3 at temperatures of desulfurization and elimination of SO 2 conversion units. In this study, NH 3 is used to regenerate sulfated CuO/Al 2 O 3 catalyst-sorbent. Effect of regeneration temperature on SO 2 removal activity of the regenerated catalyst-sorbent is investigated. Chemical morphology and physical structure of the catalyst-sorbent before and after the regeneration are characterized using elemental analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and physical absorption. 2. Experimental 2.1. Catalysts preparation The CuO/Al 2 O 3 catalyst-sorbent was prepared by pore volume impregnation of pure -Al 2 O 3 pellets (30 40 mesh, BET surface area of 203 m 2 g 1 ) with an aqueous solution of copper(ii) nitrate. The prepared catalyst-sorbent contains 8.0 wt.% of Cu and is termed Cu8. After drying at 50 C for 8 h and 120 C for 5 h, the pellets was calcined at 400 C for 8 h in a muffle furnace. The calcined CuO/Al 2 O 3 catalyst-sorbent has a BET surface area of 166 m 2 g 1 and shows no crystalline materials in X-ray diffraction except the alumina support Activity test Desulfurization tests were carried out in a fixed-bed quartz reactor (16 mm i.d.). The experimental apparatus (shown in Fig. 1) consists of three sections: a reactor, a gas feeding system, and a gas analyzer. The CuO/Al 2 O 3 catalyst-sorbent was placed between two quartz wool plugs held in the reactor which was heated by an vertical electrical furnace. A thermocouple was inserted inside the reactor for actual temperature measurement. The desulfurization temperature was controlled at 400 C for all the runs. The sulfated sample was termed as Cu8(400S). A simulated flue gas (1550 ppm SO 2, 5.6 vol.% O 2, 3.0 vol.% H 2 O, balance Ar) regulated by a mass flow controller and rotameters, flowed through the reactor bed. The concentrations of SO 2 and O 2 at the inlet and outlet of the reactor were simultaneous monitored by an online flue gas analyzer (KM9006 Quintox, Kane international Limited) Regeneration Regeneration of the sulfated CuO/Al 2 O 3 catalystsorbent was carried out in the same apparatus as the desulfurization in 5 vol.% NH 3 /Ar at a flow rate of 200 ml min 1 for about 60 min. The temperatures were 300, 350, 400, 450 and 500 C. The CuO/Al 2 O 3 catalyst-sorbent regenerated at a temperature T is labeled as Cu8(400ST), such as Cu8(400S300) for regeneration at 300 C. After the regeneration, the apparatus was purged with Ar and

3 G. Xie et al. / Applied Catalysis B: Environmental 45 (2003) Fig. 1. Schematic diagram of experimental apparatus for desulfurization and regeneration. 1: rotameter; 2: mass flow controller; 3: water bath; 4: mixing chamber; 5: preheater; 6: furnace; 7: reactor; 8: quartz wool; 9: catalyst-sorbent; 10: program temperature controller; 11: flue gas analyzer. the temperature of the catalyst-sorbent was adjusted to 400 C. The feed was then changed to the simulated flue gas for the consecutive desulfurization cycle Characterization Temperature-programmed reduction Temperature-programmed reduction (TPR) experiments were carried out in 5 vol.% NH 3 /Ar (20 ml min 1 ) on a thermogravimetric analyzer (TGA92 thermoanalyzer, SETARAM, France). The sample loading was 50 mg. The heating rate was 5 C min 1. The weight change of the sulfated CuO/Al 2 O 3 catalyst-sorbent with time was recorded on a personal computer. The TPR conditions selected above meet the criteria defined by Malet and Caballero [17] ((βs 0 )/(FC 0 ) 20 K) to avoid distortions in the TPR profiles, where β is heating rate (K min 1 ), S 0 the initial amount of reducible species ( mol), F the flow rate (ml min 1 ), and C 0 the initial NH 3 concentration ( mol ml 1 ) X-ray diffraction X-ray diffraction patterns were obtained on a Rigaku computer-controlled D/max 2500X using Cu K as the radiation source. The applied current and voltage were 30 ma and 40 kv, respectively. During the analysis, the sample was scanned from 15 to 85 at a speed of 0.4 min X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy measurements were carried out at room temperatures on an VG- Scientific Escalab 220 I-XL interfaced to a Hewlett- Packard 9000/310 computer. The residual pressure in the spectrometer was in the range (1 6) kpa. A mono-chromated A1 anode (energy of the A1 K- line ev), powered at 10 kv and 20 ma, was used for X-ray production. The binding energies were calculated with respect to the C 1s peak set at ev Chemical analysis Copper was analyzed by atomic absorption spectroscopy (AAS). For the AAS analysis, two solutions were used to dissolve copper components of the samples, H 2 SO 4 for all the copper present and water for copper sulfate only. The sulfur and nitrogen contents of the catalyst samples were measured on a Vario EL from Elementar Analysensysteme GmbH Surface area and pore structure analysis BET surface area and average pore diameter of the catalyst-sorbent were measured through nitrogen adsorption at liquid-nitrogen temperature (77 K) by a surface area analyzer (ASAP2000) Fourier transform infrared analysis Fourier transform infrared (FT-IR) spectra were recorded on a Bio-Rad FTS 165 FT-IR spectrometer.

4 216 G. Xie et al. / Applied Catalysis B: Environmental 45 (2003) The samples were mixed with potassium bromide, ground and palletized. The weight ratio of the sample to potassium bromide is 1:300. Thirty-two scans were made and averaged to yield a spectrum with resolution of 4 cm 1 over the spectral range of cm Results and discussion 3.1. TPR study To determine the regeneration temperature range of the sulfated CuO/Al 2 O 3 catalyst-sorbent in NH 3, TPR experiment was performed. As shown in Fig. 2, the weight loss begins at about 300 C, and there are two main DTG peaks, one located at around 390 C and another 600 C. According to the literature [7,18], two types of sulfate species are formed during desulfurization process in the presence of O 2, one links to Cu 2+ ion, another to Al 3+ ion. Since the latter is more stable against reduction than the former [18,19], the first peak at around 390 C is from the reduction of copper sulfate, and the second peak around 600 C is from the reduction of aluminum sulfate. Macken and Hodnett [12] studied the regeneration of the sulfated CuO/Al 2 O 3 catalyst-sorbent in 5 vol.% H 2 /He or 5 vol.% CH 4 /He and found that the first weight loss peaks, corresponding to regeneration of CuSO 4, occurred at about 320 and 520 C, respectively. The peak temperature in the TPR profile can be used to assess the reducing strength of the reductants. Clearly, NH 3 is a reducing agent of intermediate strength compared to H 2 and CH 4. Since the transformation of aluminum sulfate to Al 2 O 3 requires higher temperatures, it may lead to surface reconstruction of alumina and sintering of copper species, and reduces catalyst stability in continued cycles of sulfation regeneration [18]. It should be pointed out that an appropriate quantity of aluminum sulfate on the surface may have a positive role on the stability of copper species against sintering [18]. Hence, the present work will mainly investigate the reductive regeneration of sulfated CuO/Al 2 O 3 catalyst-sorbents in the temperature range of C in 5 vol.% NH 3 /Ar Effect of regeneration temperature on sulfur removal activity Fig. 3 shows conversion curves of CuO/Al 2 O 3 catalyst-sorbent after been regenerated at different temperatures, as well as that of a fresh sample. The sulfur removal capacity (amount of SO 2 adsorbed per g of CuO/Al 2 O 3 catalyst-sorbent) may be estimated from the integration of the conversion curve. The figure shows four important features: (1) the SO 2 removal efficiency is 100% at the initial time for all the samples; (2) the sulfur removal capacity of the regenerated CuO/Al 2 O 3 catalyst-sorbent increases with increasing regeneration temperature; (3) the dependence of sulfur removal capacity on temperature Fig. 2. TG and DTG curves of sulfated CuO/Al 2 O 3 at a heating of 5 C min 1 in 5 vol.% NH 3 /Ar.

5 G. Xie et al. / Applied Catalysis B: Environmental 45 (2003) Fig. 3. Effect of the regeneration temperature on sulfur removal capacity of CuO/Al 2 O 3 catalyst-sorbent. Experimental conditions: 2.0 g catalyst-sorbent; reaction temperature 400 C. Reaction mixture: 1550 ppm SO 2, 5.6 vol.% O 2, 3 vol.% H 2 O. Total flow rate: 460 ml min 1. is more profound in the low temperature region than in the high temperature region; and (4) the sulfur removal capacities of the regenerated CuO/Al 2 O 3 catalyst-sorbent are lower than that of the fresh one. Chemical analysis of the CuO/Al 2 O 3 before and after regeneration, in Table 1, shows that the regeneration conversion, defined as the sulfur removed from the catalyst-sorbent in percentage, monotonously increases with increasing temperature. At regeneration temperature of 300 C, less than 40% of copper sulfate, ( )/1.13, on the sulfated CuO/Al 2 O 3 is regenerated in 1 h which results in a low sulfur removal capacity in the subsequent desulfurization (Fig. 3). The sharp increase in regeneration conversion from 23.5 to 57.0% with increasing regeneration temperature from 300 to 350 C accounts for the sudden increase in sulfur removal capacity. At temperatures greater than 350 C, almost no copper sulfate is found in the regenerated samples, indicating that copper sulfate can be completely regenerated. However, Table 1 also shows that the samples regenerated at these temperatures still contain about 0.6 mmol g 1 sulfur. This stable sulfur may exist in the forms of aluminum sulfate [18] and/or copper sulfide (confirmed later in XPS analysis), which leads to low sulfur removal capacity of the regenerated CuO/Al 2 O 3 in comparison with the fresh one. To understand the stability of the CuO/Al 2 O 3 catalyst-sorbent, sulfation regeneration experiments were carried out for four cycles at sulfation and regeneration temperature of 400 C. The results, in Fig. 4A, show similar SO 2 conversion behavior after each regenerations. Fig. 4B shows that the sulfur removal capacity decreases from 1.79 to 1.10 mmol g 1 through the first regeneration, and remains roughly at 1.10 mmol g 1 in the consecutive cycles. These results indicate that the CuO/Al 2 O 3 catalyst-sorbent is stable at the sulfation and regeneration temperatures of 400 C XPS characterization XPS was used to characterize chemical morphology of S in the fresh and regenerated CuO/Al 2 O 3 catalysts-sorbents. As shown in Fig. 5, all the samples exhibit a S 2p peak at about 169 ev corresponding to sulfur in the sulfate form. The sulfated sample (Cu8(400S)) contains the largest amount of SO 4 2 and the 300 C regenerated sample (Cu8(400S300)) contains the next largest amount of SO 4 2. The samples regenerated at 400 and 500 C result in the least and similar SO 4 2 content. These results are Table 1 Chemical analysis of CuO/Al 2 O 3 catalyst-sorbents Sample Cu total (mmol g 1 ) S (mmol g 1 ) N (mmol g 1 ) CuSO 4 (mmol g 1 ) Regeneration conversion a (%) Cu Cu8(400S) Cu8(400S300) Cu8(400S350) Cu8(400S400) Cu8(400S450) Cu8(400S500) a Calculated by the formula of (S sulfated sample S regenerated sample )/S sulfated sample.

6 218 G. Xie et al. / Applied Catalysis B: Environmental 45 (2003) Fig. 4. Effect of the number of sulfation regeneration cycles on sulfur removal capacity of CuO/Al 2 O 3 catalyst-sorbent. Experimental conditions: 2.0 g catalyst-sorbent; reaction temperature 400 C. Reaction mixture: 1550 ppm SO 2, 5.6 vol.% O 2, 3 vol.% H 2 O. Total flow rate: 460 ml min 1. consistent with the data in Table 1 and suggest that there are at least two types of sulfur in the sulfated catalyst-sorbent, a regenerable type corresponding to copper sulfate and a non-regenerable type corresponding to aluminum sulfate. It is important to point out that the non-regenerable type of sulfur may also include metal sulfide as indicated by the small peak at 162 ev for the sample regenerated in 5 vol.% NH 3 /Ar at 500 C (Cu8(400S500)). This suggestion is supported by the appearance of the same peak (at 162 ev) for a sample regenerated in 5 vol.% H 2 /Ar at 400 C (line (e) in Fig. 5). These data also indicate that NH 3 is less active than H 2 and over-reduction will not occur at 400 C in 5 vol.% NH 3 /Ar XRD characterization The XRD patterns of the sulfated and regenerated CuO/Al 2 O 3 catalyst-sorbents at various regeneration temperatures are shown in Fig. 6. In the figure, the sulfated sample (a, Cu8(400S)) shows the characteristic peaks of copper sulfate (2θ = 21.2, 25.1 and 34.3 ). After regeneration at 300, 400 and 500 C in 5 vol.% NH 3 /Ar, the peaks of copper sulfate disappear, which indicates that most of the copper sulfate, if not all, is converted. It is interesting to note that lines (b) and (c) show peaks of copper nitride (Cu 3 N) (2θ = 23.4, 40.9 and 47.6 ), a solid product of a reaction between copper sulfate and NH 3 [20]. For the sample regenerated at 500 C (line (d)), the peaks of Cu 3 N disappear

7 G. Xie et al. / Applied Catalysis B: Environmental 45 (2003) Fig. 5. XPS spectra of S 2p for CuO/Al 2 O 3 sulfated and regenerated at different temperature. (a) Cu8(400S), (b) Cu8(400S300), (c) Cu8(400S400), (d) Cu8(400S500), (e) Cu8(400S400H). and peaks of Cu appear (2θ = 23.4, 40.9 and 47.6 ), which indicates that Cu 3 N has decomposed to form Cu at this temperature. This agrees with the result [20] that Cu 3 N is prone to decompose to copper and nitrogen at temperatures around 470 C. These results are also supported by the fact that the fresh and sulfated samples contain no nitrogen, and the samples regenerated at 300, 350 and 400 C contain nitrogen (Table 1). At regeneration temperature of 400 C, the mol ratio of nitrogen to copper is about 1:3, corresponding to the stoichiometric ratio of Cu 3 N, which indicates that all the Cu in the catalyst-sorbent is in the form of Cu 3 N after regeneration at 400 C. For the samples regenerated at 450 and 500 C, the nitrogen content is zero due to thermal decomposition of Cu 3 N. The line (e) in Fig. 6 is from a sample regenerated at 400 C (Cu8(400S400)) and then exposed to O 2 at the same temperature. No Cu 3 N peaks are found. This indicates that Cu 3 N in the sample Cu8(400S400) can easily be oxidized to CuO in the subsequent desulfurization. All the samples show no diffraction signal of aluminum sulfate although it is confirmed in chemical analysis that aluminum sulfate does exist. This suggests that the aluminum sulfate is in small size and highly dispersed on the surface of CuO/Al 2 O 3 catalystsorbent BET surface area and pore structure analysis The surface area and pore volume of the catalystsorbent before and after sulfation and after regeneration are given in Table 2. The sulfation decreases the surface area and pore volume of the fresh sample by 33 and 24%, respectively, due to size expansion of the Cu compound, from CuO to CuSO 4, and possibly pore plugging by the formed CuSO 4. At regeneration temperatures of 400 C or higher, both surface area and pore volume recover to the values of the fresh one. This result indicates that the CuO/Al 2 O 3 catalyst-sorbent shows stable physical structure when regenerated in 5 vol.% NH 3 /Ar at 400 C. The result also suggests that the formation of Al 2 (SO 4 ) 3 does not alter the catalyst s structure. Fig. 6. XRD pattern of CuO/Al 2 O 3 catalyst-sorbent sulfated and regenerated at different temperature. (a) Cu8(400S), (b) Cu8(400S300), (c) Cu8(400S400), (d) Cu8(400S500), (e) Cu8(400S400) + O 2. Table 2 BET surface area and pore structure of CuO/Al 2 O 3 catalystsorbents after sulfation and regeneration Sample BET area (m 2 g 1 ) Cu Cu8(400S) Cu8(400S300) Cu8(400S350) Cu8(400S400) Cu8(400S450) Cu8(400S500) Total pore volume (ml g 1 )

8 220 G. Xie et al. / Applied Catalysis B: Environmental 45 (2003) to hydroxyamine sulfate ((NH 2 OH) 2 H 2 SO 4 ). The results of XRD and FT-IR characterization suggest the formation of hydroxyamine sulfate through reaction between SO 2 and NH 3 in the presence of H 2 O in the outlet of the reactor. Hydroxyamine sulfate is a useful precursor for pesticide and medicine synthesization. Formation of hydroxyamine sulfate not only simplifies the post-treatment of regeneration, but also improves economical feature of this process. 4. Conclusions Fig. 7. FT-IR spectra of the collected crystallites at the outlet of the reactor Characterization of the regeneration products No SO 2, NO and NO 2 were found at the outlet of the reactor during regeneration in NH 3 except white crystallites. The FT-IR spectra of the crystallites is shown in Fig. 7. The broad band in the cm 1 range is associated to OH stretching. Correspondingly, the band at 1259 cm 1 is attributed to OH deformation vibration. The bands near 1400, 1080 and 983 cm 1 are associated to sulfate. The bands at 1176 and 800 cm 1 are attributed to amine. As shown in Fig. 8, the XRD pattern of the white crystallites exhibits principal diffraction peaks at 2θ = 23.86, and 30.56, which are associated (1) Copper sulfates on the sulfated CuO/Al 2 O 3 catalyst-sorbents can be effectively regenerated at temperatures around 400 C in 5 vol.% NH 3 /Ar in 1 h. The aluminum sulfate is stable under the conditions. (2) The main copper species after regeneration at 400 CisCu 3 N. The nonexistence of copper sulfide at regeneration temperatures of 450 C and below suggests that over-reduction occurring in H 2 at 400 C is avoided in NH 3. BET surface area and pore size distribution of the regenerated catalyst-sorbent are almost the same as the fresh one when the reduction temperature is 400 Cor higher. (3) Ammonia is not only functioned as a reductant, but also reacts with SO 2 formed during the regeneration step to form hydroxyamine sulfate at the outlet of the reactor, which simplifies the overall process. Acknowledgements The authors gratefully acknowledge the financial support from the Natural Science Foundation of China ( and ), the National High-Tech Research and Development Program (the 863 Program, 2002AA529110), Chinese Academy of Sciences and the Shanxi Natural Science Foundation. References Fig. 8. XRD pattern of the collected crystallites at the outlet of the reactor. [1] J.N. Armor, Appl. Catal. B 1 (1992) [2] H. Bosch, F. Janssen, Catal. Today 2 (1988) 369.

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