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The catalytic reduction of SO2 by CO over lanthanum oxysulphide
~
APPLIED CATALYSS I AG : ENERAL
Applied Catalysis A: General 150 (1997) 253-268
ELSEVIER
The catalytic reduction of SO 2 by CO over lanthanum oxysulphide Jianxin Ma l, Ming Fang *, Ngai Ting Lau Research Centre, The Hong Kong University of Science and Technology, Clearwater Bay, Hong Kong
Received 15 January 1996; revised 30 July 1996; accepted 13 August 1996
Abstract Lanthanum oxysulphide was found to be an effective catalyst for the reduction of SO 2 by CO to elemental sulphur. Over 98% in SO 2 conversion and selectivity to elemental sulphur can be achieved under the following conditions: temperature above 500°C, stoichiometric CO/SO 2 feed ratio, and a space velocity 21,600 cc g-1 h - l . COS in the low ppm level was detected as a by-product similar to our earlier work with perovskite. The catalyst is resistant to oxygen and water vapour. In addition, it was recognised that the lanthanum oxysulphide is bifunctional, i.e., not only is it active in the reduction of SO 2 by COS [J.A. Baglio, Ind. Eng. Chem., Prod. Res. Dev., 21 (1982) 38], but also its lattice sulphur atoms are mobile enough to form COS with CO. This suggests that lanthanum oxysulphide functions as a catalyst for the reduction of SO 2 by CO via the intermediate COS. Keywords: Catalytic desulphurisation;Rare earth catalyst
1. I n t r o d u c t i o n
L a n t h a n u m - c o n t a i n i n g p e r o v s k i t e o x i d e s h a v e b e e n s u g g e s t e d as catalysts for the r e d u c t i o n o f S O 2 b y C O to e l e m e n t a l s u l p h u r [ 1 - 7 ] . S e v e r a l a u t h o r s r e p o r t e d that the p e r o v s k i t e s d e c o m p o s e i n t o a series o f c o m p l i c a t e d m i x t u r e o f s u l p h i d e s a n d s u l p h a t e s after r e a c t i o n a n d l a n t h a n u m o x y s u l p h i d e has b e e n i d e n t i f i e d as o n e o f the m a j o r p h a s e s [ 5 - 7 ] .
* Correspondingauthor. Tel: (852) 2358 6916. Fax: (852) 2358 1334. E-mail: [email protected]. l Present address: Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China. 0926-860X/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0926-860X(96)00312-2
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Baglio [5] reported that a commercial lanthanum oxysulphide (GTE Sylvania) was catalytically inactive for this reaction; however, it was active when COS was used instead of CO. He suggested that when a transition metal sulphide such as CoS 2 was incorporated into the lanthanum oxysulphide, the reduction of SO 2 by CO could be more easily catalysed. This is due to the ability of the transition metal sulphides to interact with CO to generate COS. In our previous study of composite sulphides in the reduction of SO 2 by CO, we were able to confirm a synergistic effect between CoS 2 and La202S, and in contrast to Baglio's conclusions, we found that lanthanum oxysulphide alone is active in the reaction between SO 2 and CO [8]. The active lanthanum oxysulphide can be prepared by activating lanthanum oxide in the reaction gas stream by means of a rehydration pretreatment step [9]. In this paper, we will focus our attention on the catalytic behaviour of the lanthanum oxysulphide, and some mechanistic aspects of the reaction system will be discussed.
2. Experimental 2.1. Materials and characterisation Two La202S samples denoted as La202S-RC and La202S-JM were used to catalyse the reduction of SO 2 by CO to elemental sulphur. The La2OES-RC sample was prepared from La203 powder (Yaolong Non-Ferrous Metals, 99.99% pure) by exposing the sample to air saturated in water vapour for one week to transform the oxide into La(OH) 3. The hydrated lanthanum oxide was sulphidised in a gas mixture composed of 0.5% mol SO 2 and 1.0% mol CO in nitrogen at 600°C for 2 h. A detailed description of the preparation method can be found in our earlier paper [9]. The LazO2S-JM sample was purchased from Johnson Matthey, USA and was 99.9% pure, but its preparation method was unknown. The bulk structure of the samples was checked using X-ray powder diffraction (XRD) (Philips MPD-1880I X-ray diffractometer, Cu K a radiation, A = 1.542,~). In both samples, the only detectable phase was LazOzS and there was almost no difference in the crystal parameters. The surface area of the samples was measured using BET (Micromeritics, ASAP 2010). The specific surface area of the La202S-RC sample was 4.0 m 2 g-1 and the La202S-JM sample was 16.0 m 2 g-1.
2.2. Apparatus The reaction was carried out in a fixed-bed flow reactor (a 2 cm diameter by 50 cm length quartz tube) heated externally in an electrical furnace. The
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
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temperature of the catalyst bed was controlled to within I°C. The feed gas contained 0.5% mol SO 2 and 1.0% mol CO in ultra-high purity grade nitrogen. In some experiments, oxygen or water vapour was added to the feed gas. Unless otherwise specified, the gas flow rate was held constant at 180 ml minmeasured at standard temperature and pressure. Water vapour was introduced by continuously pumping deionised water through a 0.3 mm id quartz capillary into the preheating zone of the reactor using a Harward Apparatus syringe pump. The gas mixture leaving the reactor was directed to three on-line nondispersive infrared gas analysers in series: SO 2 (CFA-321A, Horiba), CO and CO 2 (both VIA-510, Horiba), and in parallel to a HP 5890 Series II gas chromatograph with two columns: one molecular sieve and one Porapak Q, and two TCD detectors to measure 0 2, CO, CO 2 and the sulphur-containing compounds: SO 2, COS, H2S and CS 2. Elemental sulphur was condensed from the product gas stream by passing the effluent through an ice-bath trap and a filter with a pore size of 2 Ixm. For each experiment 0.5 g of catalyst was used except otherwise specified.
3. Results
3.1. Temperature effect One gram of the La202S-RC sample was used in this experiment. The two by-products, COS and CS 2, were detected; however, the concentration of CS 2 was too low to be ratable. Fig. 1 shows the effect of temperature on the conversions of SO 2 and CO, and on COS formation. It can be seen that the conversions of SO 2 and CO were insignificant up to 350°C, which was also the temperature when COS began to form. Above 350°C, a small increase in temperature, even just a few degrees, caused a sharp increase in the conversions and COS formation. However, the COS concentration dropped rapidly after reaching a maximum of 450 ppmv at about 375°C while the conversions increased to over 90%. At about 450°C, the SO 2 conversion was about 98% and was almost constant with further increases in temperature. This temperature dependency of the conversion of SO 2 and CO, and COS formation suggests that COS could be involved in the reaction between SO 2 and CO. The SO 2 conversion data are shown in Fig. 2 as an Arrhenius plot. It is not surprising to find two distinct temperature-dependent reaction regimes. Usually, the reaction in the low temperature zone is kinetic-limited and the one in the higher temperature zone is diffusion-limited [10]. It is clear that the transition temperature between the two regimes corresponds to the maximum COS formation temperature.
Z Ma et al./Applied Catalysis A: General 150 (1997) 253-268
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T e m p e r a t u r e , °C Fig. 1. Reactant conversions and COS concentration as a function of temperature - Catalyst: La202S-RC (Feed: 0.5% tool SO 2 and 1.0% rnol CO; space velocity: 10,800 cc g - l h - l ; zx SO 2 conversion; O CO conversion; [] COS concentration).
3.2. Effect of C O / S O 2 ratio in the feed The formation of COS has been shown to have an important role in the desulphurisation reaction of SO 2 and CO via the COS intermediate mechanism; however, it is desirable to limit its concentration in the effluent to as low a level as possible because it is highly toxic. It has been reported that the formation of COS is dependent not only on the catalyst composition [ 1,6] but also on the CO partial pressure [1]. In the present work, the influence of the C O / S O 2 feed ratio on SO 2 conversion and COS concentration in the effluent was investigated and the results are presented in Table 1. Fig. 3 is a graphical representation of Table 1 and shows the production of COS as a function of C O / S O 2 ratio in the feed. The production of COS is defined as the amount of COS produced divided by the amount of SO 2 reduced in percent. Thus, it is inversely proportional to the selectivity for producing elemental sulphur. It can be seen that the production of COS was very small (not more than 4%) when the C O / S O 2 ratio was at or below the stoichiometric ratio. However, once above the stoichiometric ratio, it increased rapidly and became very high (32 to 49%) at a ratio of 2.6. This same effect was observed by
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
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Happel et al. [1] when lanthanum titanate was used. Furthermore, the production of COS decreased with an increase in temperature and this is consistent with the findings of Khalafalla and Haas [10] but contradicts Bazes et al. [2]. We noticed that a reaction temperature range of 400 to about 900°C was used in Ref. [11] while in Ref. [2] it was below 420°C. Our results, reported in Ref. [8] and in Fig. 1 of this paper, show that the formation of COS increased with temperature below 400°C and decreased at higher temperatures.
Table 1 Summary of conversion and concentration data at different C O / S O 2 ratios (0.5% mol of SO z in feed; Space velocity: 21,600 cc g i h - l ) C O / S O 2 ratio SO2 conversion, %
COS concentration, ppmv
450°C 500°C 550°C 600°C 450°C 500°C 550°C 600°C
1.5 49.6 57.0 63.0 66.7 100 73 41 23
1.8 54.0 63.0 70.0 73.4 108 73 61 67
2.0 85.0 95.7 98.0 98.2 87 75 87 89
2.2 32.5 44.4 76.2 89.7 170 135 125 123
2.6 69.6 80.8 99.2 99.2 1698 1468 1890 1659
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CO/SO 2Ratio Fig. 3. Temperature dependency of COS production at different CO/SO 2 ratio (SO s in feed: 0.5% tool; space velocity: 21,600 cc g - l h - l ; O 450°C; [] 500°C; A 550°C; '7 600°C).
The data in Table 1 can be used to calculate the conversion efficiency. This is defined as the total amount of sulphur-containing substances in the outlet gas stream including unconverted SO z and by-products, if any, and is therefore proportional to the yield of elemental sulphur. It can be seen in Fig. 4 that the most effective removal of SO 2 occurred when the C O / S O 2 ratio was near the stoichiometric ratio. Below this ratio insufficient CO impaired the efficiency due to low SO 2 conversion, while above which the efficiency was depressed by low selectivity due to the high formation of COS. This same result was reported for Cu/A1203 [12] and Lal_xSrxCoO 3 [6,13].
3.3. Effect of space velocity The effect of space velocity on the catalytic activity was investigated by varying the amount of catalyst used. The experiments were conducted using a feed at stoichiometric ratio and at a reaction temperature of 600°C; results are shown in Fig. 5. The range of space velocity studied was from 10,800 to 43,200 cc g-~ h-1. It can be seen that the SO 2 conversion decreased linearly with the increase in space velocity while the selectivity was not affected. The decrease in SO 2 conversion was 18% (from 96 to 78%) when there was a fourfold increase
Z Ma etal./Applied CatalysisA: General150(1997) 253-268
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CO/SO2 Ratio Fig. 4. Influence of C O / S O 2 ratio on the conversion efficiency at 600°C (SO 2 in feed: 0.5% mol; space velocity: 21,600 cc g-1 h l).
in space velocity showing that the effect of space velocity on SO 2 conversion was not severe.
3.4. Water uapour effect Any hydrogen-containing fuel will produce water vapour in the flue gas. Previous experience indicates that the effect of water vapour on the activity of desulphurisation catalyst can be significant [14]. The influence of water vapour on our catalyst was studied by pumping water directly into the preheating zone of the reactor. Fig. 6 shows that the SO 2 conversion decreased gradually from 96 to 66% and held when the feed contained 7% of water vapour. It was also found that the bone-dry conversion could be recovered in a very short time when the water vapour in the feed was removed. Furthermore, XRD analysis showed that there was no structural change in the catalyst after over 20 h of "wet reaction ". This suggests that water vapour does not poison the catalyst. A process mass spectrometer (MS250 Gas Analyzer, Extrel) was used to detect and measure the concentration of the by-products so that the influence of water vapour on the reaction could be further studied. It was found that there
260
J. Ma et al. / A p p l i e d Catalysis A: General 150 (1997) 253-268
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Fig. 5. I n f l u e n c e o f s p a c e v e l o c i t y on c o n v e r s i o n a n d selectivity (Feed: 0 . 5 % m o l S O / and 1.0% m o l CO; t e m p e r a t u r e 600°C; O S O 2 c o n v e r s i o n ; [] selectivity).
was a significant amount of H2S and a small amount of H 2 while the formation of COS decreased. These phenomena suggest the presence of three possible reactions when water is added. (1) The reverse Claus reaction, 32 8 2 -t-
2H20
~
2H2S + SO 2
(1)
which will cause an increase in the steady-state SO z concentration while decrease the selectivity in forming elemental sulphur. (2) The hydrolysis of COS, COS + H 2 0
~
HzS -t- CO 2
(2)
which will decrease the COS concentration with an equivalent increase in HzS. (3) The water-gas shift reaction CO + H 2 0
----) H 2 +
CO 2
(3a)
which consumes the reducing agent CO, thus causing a decrease in SO 2 conversion. The hydrogen produced can react readily with elemental sulphur to form hydrogen sulfide according to the following reaction, 1
H 2 + ~- S 2 ~
HzS
(3b)
which will also decrease the sulphur formation selectivity. Reactions (2) and (3)
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
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have been confirmed in experiments conducted using feeds containing COS or CO and water vapour. Since the catalyst was not deactivated by water vapour, the selectivity also recovered once the feed was dried. This reversible water vapour effect was also reported by Okay and Short [15] for the C u / A l z O 3 catalyst system, although they reported no hydrogen or H2S in the "wet reaction". Their thermodynamic calculations indicated that the water-gas shift reaction could take place at the reaction conditions used.
3.5. Oxygen effect Oxygen is always present in the flue gas of combustion processes and it is known to deactivate or destroy reduction catalysts. The oxygen effect was studied by introducing 0 2 at different concentrations into the feed gas while keeping C O / O 2 at stoichiometric ratio. Up to 2.5% mol of net oxygen was added to the reaction. The results are plotted in Fig. 7. The presence of oxygen caused a decrease in SO 2 conversion and the selectivity to form elemental sulphur for all temperatures studied, however, the effect was significantly lower at higher temperatures. For example, for a feed containing 2% mol of oxygen and 600°C, there was a decrease of 40% in SO~
262
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Oxygen Content, % Fig. 7. Effect of oxygen on conversion and selectivity (Feed: 0.5% tool SO 2 and 1.0% rnol CO; space velocity: 21,600 cc g-~ h - l ; • conversion at 600°C; [] selectivity at 600°C; • conversion at 650°C; © selectivity at 650°C).
conversion and a 9% reduction in selectivity, while the decrease was 10 and 6%, respectively, at 650°C. XRD analysis revealed that after 20 h of reaction in the presence of oxygen, the bulk structure of the lanthanum oxysulphide did not change showing that it is an oxygen-resistant catalyst. The decrease of SO 2 conversion in the presence of oxygen may be due to the following competitions: CO+Sac O 2 q- Sac 1
~ ~
CO+~-O 2 ~
COS SO 2
COg
(4) (5)
(6)
where Sac represents active sulphur atoms on the catalyst surface. The concentration of the intermediate COS is lower when Reaction (5) occurs, and this will lead to a lower SO 2 conversion via reduction by COS. However, Reaction (5) can be inhibited by Reaction (6), which is favoured at higher temperatures, thus leaving the active sulphur atoms to react with CO to form COS. 3.6. Comparison with a commercial sample Baglio [5] used two samples of La202S in his work: one was obtained from the Chemical and Metallurgical Division, GTE Sylvania, Towanda, Pa. and the
J. Ma et a l . / Applied Catalysis A: General 150 (1997) 253-268
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600
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T e m p e r a t u r e , °C Fig. 8. Reactant conversions and COS concentration as a function of temperature - Catalyst: La202S-JM (Feed: 0.5% mol SO 2 and 1.0% mol CO; space velocity: 10,800 cc g-1 h - l ; A SO 2 conversion; O CO conversion; [] COS concentration).
other he prepared himself. He observed that using the GTE sample at 650°C, there was only a minimal reduction of SO 2 by CO and no COS was generated. There was, however, no report of any results on his sample. To verify Baglio's results, we conducted experiments using a 99.9% pure La2OzS commercial sample (LazO2S-JM). The bulk structure of the commercial sample was determined using XRD and it was confirmed that the structure was the same as La202S-RC. The catalytic activity of La2OzS-JM was measured in the same manner as La202S-RC and the results are presented in Fig. 8. Compared to the results shown in Fig. 1, it is evident that this commercial sample possessed almost the same catalytic behaviour as the sample prepared in the present work. This contradicted Baglio's findings.
4. Discussion
We have previously found that sulphidised lanthanum cobaltate is an active catalyst for the reduction of SO 2 by CO, and the active catalyst consists of two major phases: La202S and CoS 2. Based on these discoveries, a reaction mechanism based on the COS intermediate and the "remote control" concepts
264
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was proposed [7]. According to this mechanism, CO first reacts with CoS 2 to form an intermediate, COS. The intermediate reduces SO 2 catalysed by La202S to CO 2 and elemental sulphur, and a portion of the elemental sulphur formed spills over from the surface of La202S to the CO reduced sites on the surface of CoS 2. The role of the active La202S phase is twofold: to catalyse the reduction of SO 2 by COS and to maintain (or control) the reactivity of the CoS 2 phase with CO. We also studied the synergistic effect between the two phases by conducting experiments on separately prepared La202S and COS2; the results provided additional evidence in support of this mechanism [8]. In the previous and current studies, however, we discovered that the lanthanum oxysulphide by itself is active in the reaction between SO 2 and CO. If the second active phase is absent, then the proposed mechanism for the SO 2 and CO reactions needs to be revised. As presented earlier in this paper, there was a rapid increase in the conversion of both SO 2 and CO at a temperature around which the formation of COS was maximum. Moreover, when the COS concentration went over the maximum and fell off, the reaction became diffusion-limited. Therefore, it is most likely that COS played an important role in the reduction of SO 2 by CO over La202S. Assuming for the moment that the reaction follows the intermediate mechanism, the COS concentration in the product gas stream tends to depend on the competition between COS formation and the consumption of COS. At lower temperatures, the interaction of CO with active sulphur species on the catalyst is weak, implying a low formation of COS. Therefore, the overall reaction rate will be determined by the formation of COS, and we observed the increase in SO 2 conversion with higher COS concentrations. At higher temperatures, both the formation of COS and the reaction between the COS and SO 2 are accelerated, thus the overall reaction will be controlled by the diffusion of the reactants from the gas phase to the surface of the catalyst. It was reported in Ref. [11] that COS is a stronger Lewis base than CO and will readily react with S O 2. In one series of experiments, we replaced CO with COS as the reducing agent, and the results are shown in Fig. 9. The reaction started to accelerate at about 250°C, and complete reduction was achieved at temperatures above 350°C. The result clearly shows that La2OzS is very active in the reduction of SO 2 by COS. In other words, COS is a more reactive reagent than CO. This implies that the formation of COS is a favoured step in the reduction of SO 2 by CO on La2OzS, if there is sufficient interaction between CO and the lattice sulphur atoms on La2OzS. This relationship between the complete reduction of SO 2 at temperatures above 350°C and maximum COS formation temperature is consistent with what is shown in Fig. 1. In another experiment, the reactivity of La202S with CO was measured according to the following procedure: after the reduction of SO 2 by CO at 600°C, the catalyst was purged using UHP nitrogen for 2 h at 600°C. A CO-containing (1.0% tool) nitrogen gas stream was then introduced and the
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
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O
O
450
500
265
80
60
~
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20
0 200
~ 250
300
350
400
550
600
650
T e m p e r a t u r e , °C Fig. 9. Conversion as a function of temperature for the reduction of SO 2 by COS (Feed: 0.5% mol SO 2 and 1.0% mol COS; space velocity: 21,600 cc g - l h - l ; [] SO 2 conversion; C) COS conversion).
temperature of the catalyst bed was decreased stepwise from 600 to 350°C before increasing it back to 600°C. The concentration of COS was measured during the cooling as well as the heating stages. The results are presented in Fig. 10. It can be seen that the formation of COS decreased gradually during the cooling stage and was cut-off at 350°C. However, when the catalyst was reheated, the formation of COS increased again with temperature and a significant amount was detected after about 90 rain of reduction. Therefore, it has been confirmed that the lattice sulphur atoms in the La202S indeed were mobile enough to react with CO to form COS. Furthermore, it can also be found from Fig. 10 that the formation of COS started at above 350°C, which is the same temperature for the initiation of the reduction SO 2 by CO as shown in Fig. 1. Since XRD analysis revealed that the phase structure of L a 2 0 2 S after reaction did not change, the lattice sulphur on La202 S must have been replenished by the elemental sulphur formed by the reaction between SO 2 and COS. Based on the above mentioned observations, we postulate that lanthanum oxysulphide is bifunctional in the reduction of SO 2 by CO to elemental sulphur. It is not only active in the reaction between COS and SO z but also possesses the same function as the CoS 2 in providing mobile sulphur. Therefore, the overall
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300
initial
250
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150 = o
o
100
50
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0
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200
250
300
350
90 min A
f 400
450
500
550
600
650
o
Temperature, C Fig. 10. COS concentration during the reaction of CO with La202S (zx heating stage; v cooling stage).
reaction can still follow the COS intermediate mechanism as represented by the following equations: CO+La202 S ~ 2COS+SO z ~ 1
COS+La202[] 2CO~+3
L a 2 0 : [] + ~S z ~
La202S
(7)
(8) (9)
where [] is the lattice sulphur vacancy. The formation of vacancies in LazO2S may also be important to Reaction Eq. (8), because it may be the rate-determining step in the reduction of SO 2 by COS [5]. At steady-state the overall reaction is as follows: 2 C O + S O 2 ---> 2CO 2 + ~ S 2
5. C o n c l u s i o n
Lanthanum oxysulphide has been found to be an effective catalyst for the reduction of SO 2 by CO. The effectiveness may be due to its bifunctional characteristics which catalyses not only the reaction between SO 2 and COS but also the formation of COS with CO. Consequently, the reduction of SO 2 by CO
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
267
to elemental sulphur over La202S can also be described by the COS intermediate mechanism. Experimental results show that both water vapour and oxygen decrease the conversion of SO z and the selectivity to elemental sulphur. In particular, the selectivity can be significantly impaired by water vapour because of the formation of the by-product H2S via the reverse Claus reaction, the water-gas shift reaction and the hydrolysis of COS. However, the catalyst structure does not change after over 20 h of exposure to either water vapour or oxygen, and the catalytic activity can be recovered when the water vapour and oxygen are removed. This suggests that the catalyst is not destroyed by either water vapour or oxygen. Over 98% in SO 2 conversion can be obtained at temperatures > 450°C. Above this temperature, the concentration of the by-product COS decreases with the increase in temperature, the conversion of SO 2 virtually remains constant. Thus, the selectivity also increases at higher temperatures and over 98% can be reached. The effect of the f e e d - C O / S O 2 ratio is significant not only on the conversion of SO 2 but also on the formation of COS. The most effective removal of SO 2 is obtained near the stoichiometric ratio. The effect of space velocity on the selectivity is minimal and is relatively small on the conversion of SO 2. A linear decrease of 18% in the latter was observed when the space velocity increased fourfold from 10,800 to 43,200 cc g-1 h-1
Acknowledgements The authors appreciate the invaluable technical support provided by Messrs. K.L. To and H.S. Tsui of the Research Centre, HKUST. The Materials Preparation and Characterisation Centre has been more than generous in performing XRD and surface analysis on our behalf, and we are indebted to its staff.
References [1] [2] [3] [4] [5] [6] [7]
J. Happel, M.A. Hnatow, L. Bajars and M. Kundrath, Ind. Eng. Chem., Prod. Res. Dev., 14 (1975) 154. J.G.I. Bazes, L.S. Caretto and K. Nobe, Ind. Eng. Chem., Prod. Res. Dev., 14 (1975) 264. F.C. Palilla, US Pat. No. 3,931,393 (1976). L. Bajars, US Pat. No. 3,978,200 (1976). J.A. Baglio, Ind. Eng. Chem., Prod. Res. Dev., 21 (1982) 38. D.B. Hibbert and R.H. Charnpbell, Appl. Catal., 41 (1988) 289. J. Ma, M. Fang and N.T. Lau, Proceedings of the 1st World Congress on Environmental Catalysis, Pisa, Italy, 1-5 May 1995, p. 555.
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[8] J. Ma, M. Fang and N.T. Lau, On the Synergism between La202S and CoS z in the Reduction of SO2 to Elemental Sulphur by CO, J. Catal., 158 (1996) 251. [9] J. Ma, M. Fang and N.T. Lau, Activation of La203 for the Catalytic Reduction of SO 2 by CO, J. Catal., in press. [10] S.E. Khalafalla and L.A. Haas, J. Catal., 24 (1972) 121. [11] C.N. Satterfield, Heterogeneous Catalysis in Industrial Practice, McGraw-Hill, New York, 1991, p. 483. [12] D.B. Hibbert and C.C. Tseung, J. Chem. Soc. (London): Faraday Trans. 1, 74 (1978) 1981. [13] D.B. Hibbert and C.C. Tseung, J. Chem. Technol. Biotechnol., 29 (1979) 713. [14] W. Liu, A.F. Sarofim and M. Flytzani-Stephanopoulos, Appl. Catal. B: Environmental, 4 (1994) 167. [15] V.C. Okay and W.L. Short, Ind. Eng. Chem., Process Des. Develop., 12 (1973) 291.
APPLIED CATALYSS I AG : ENERAL
Applied Catalysis A: General 150 (1997) 253-268
ELSEVIER
The catalytic reduction of SO 2 by CO over lanthanum oxysulphide Jianxin Ma l, Ming Fang *, Ngai Ting Lau Research Centre, The Hong Kong University of Science and Technology, Clearwater Bay, Hong Kong
Received 15 January 1996; revised 30 July 1996; accepted 13 August 1996
Abstract Lanthanum oxysulphide was found to be an effective catalyst for the reduction of SO 2 by CO to elemental sulphur. Over 98% in SO 2 conversion and selectivity to elemental sulphur can be achieved under the following conditions: temperature above 500°C, stoichiometric CO/SO 2 feed ratio, and a space velocity 21,600 cc g-1 h - l . COS in the low ppm level was detected as a by-product similar to our earlier work with perovskite. The catalyst is resistant to oxygen and water vapour. In addition, it was recognised that the lanthanum oxysulphide is bifunctional, i.e., not only is it active in the reduction of SO 2 by COS [J.A. Baglio, Ind. Eng. Chem., Prod. Res. Dev., 21 (1982) 38], but also its lattice sulphur atoms are mobile enough to form COS with CO. This suggests that lanthanum oxysulphide functions as a catalyst for the reduction of SO 2 by CO via the intermediate COS. Keywords: Catalytic desulphurisation;Rare earth catalyst
1. I n t r o d u c t i o n
L a n t h a n u m - c o n t a i n i n g p e r o v s k i t e o x i d e s h a v e b e e n s u g g e s t e d as catalysts for the r e d u c t i o n o f S O 2 b y C O to e l e m e n t a l s u l p h u r [ 1 - 7 ] . S e v e r a l a u t h o r s r e p o r t e d that the p e r o v s k i t e s d e c o m p o s e i n t o a series o f c o m p l i c a t e d m i x t u r e o f s u l p h i d e s a n d s u l p h a t e s after r e a c t i o n a n d l a n t h a n u m o x y s u l p h i d e has b e e n i d e n t i f i e d as o n e o f the m a j o r p h a s e s [ 5 - 7 ] .
* Correspondingauthor. Tel: (852) 2358 6916. Fax: (852) 2358 1334. E-mail: [email protected]. l Present address: Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China. 0926-860X/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0926-860X(96)00312-2
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Baglio [5] reported that a commercial lanthanum oxysulphide (GTE Sylvania) was catalytically inactive for this reaction; however, it was active when COS was used instead of CO. He suggested that when a transition metal sulphide such as CoS 2 was incorporated into the lanthanum oxysulphide, the reduction of SO 2 by CO could be more easily catalysed. This is due to the ability of the transition metal sulphides to interact with CO to generate COS. In our previous study of composite sulphides in the reduction of SO 2 by CO, we were able to confirm a synergistic effect between CoS 2 and La202S, and in contrast to Baglio's conclusions, we found that lanthanum oxysulphide alone is active in the reaction between SO 2 and CO [8]. The active lanthanum oxysulphide can be prepared by activating lanthanum oxide in the reaction gas stream by means of a rehydration pretreatment step [9]. In this paper, we will focus our attention on the catalytic behaviour of the lanthanum oxysulphide, and some mechanistic aspects of the reaction system will be discussed.
2. Experimental 2.1. Materials and characterisation Two La202S samples denoted as La202S-RC and La202S-JM were used to catalyse the reduction of SO 2 by CO to elemental sulphur. The La2OES-RC sample was prepared from La203 powder (Yaolong Non-Ferrous Metals, 99.99% pure) by exposing the sample to air saturated in water vapour for one week to transform the oxide into La(OH) 3. The hydrated lanthanum oxide was sulphidised in a gas mixture composed of 0.5% mol SO 2 and 1.0% mol CO in nitrogen at 600°C for 2 h. A detailed description of the preparation method can be found in our earlier paper [9]. The LazO2S-JM sample was purchased from Johnson Matthey, USA and was 99.9% pure, but its preparation method was unknown. The bulk structure of the samples was checked using X-ray powder diffraction (XRD) (Philips MPD-1880I X-ray diffractometer, Cu K a radiation, A = 1.542,~). In both samples, the only detectable phase was LazOzS and there was almost no difference in the crystal parameters. The surface area of the samples was measured using BET (Micromeritics, ASAP 2010). The specific surface area of the La202S-RC sample was 4.0 m 2 g-1 and the La202S-JM sample was 16.0 m 2 g-1.
2.2. Apparatus The reaction was carried out in a fixed-bed flow reactor (a 2 cm diameter by 50 cm length quartz tube) heated externally in an electrical furnace. The
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
255
temperature of the catalyst bed was controlled to within I°C. The feed gas contained 0.5% mol SO 2 and 1.0% mol CO in ultra-high purity grade nitrogen. In some experiments, oxygen or water vapour was added to the feed gas. Unless otherwise specified, the gas flow rate was held constant at 180 ml minmeasured at standard temperature and pressure. Water vapour was introduced by continuously pumping deionised water through a 0.3 mm id quartz capillary into the preheating zone of the reactor using a Harward Apparatus syringe pump. The gas mixture leaving the reactor was directed to three on-line nondispersive infrared gas analysers in series: SO 2 (CFA-321A, Horiba), CO and CO 2 (both VIA-510, Horiba), and in parallel to a HP 5890 Series II gas chromatograph with two columns: one molecular sieve and one Porapak Q, and two TCD detectors to measure 0 2, CO, CO 2 and the sulphur-containing compounds: SO 2, COS, H2S and CS 2. Elemental sulphur was condensed from the product gas stream by passing the effluent through an ice-bath trap and a filter with a pore size of 2 Ixm. For each experiment 0.5 g of catalyst was used except otherwise specified.
3. Results
3.1. Temperature effect One gram of the La202S-RC sample was used in this experiment. The two by-products, COS and CS 2, were detected; however, the concentration of CS 2 was too low to be ratable. Fig. 1 shows the effect of temperature on the conversions of SO 2 and CO, and on COS formation. It can be seen that the conversions of SO 2 and CO were insignificant up to 350°C, which was also the temperature when COS began to form. Above 350°C, a small increase in temperature, even just a few degrees, caused a sharp increase in the conversions and COS formation. However, the COS concentration dropped rapidly after reaching a maximum of 450 ppmv at about 375°C while the conversions increased to over 90%. At about 450°C, the SO 2 conversion was about 98% and was almost constant with further increases in temperature. This temperature dependency of the conversion of SO 2 and CO, and COS formation suggests that COS could be involved in the reaction between SO 2 and CO. The SO 2 conversion data are shown in Fig. 2 as an Arrhenius plot. It is not surprising to find two distinct temperature-dependent reaction regimes. Usually, the reaction in the low temperature zone is kinetic-limited and the one in the higher temperature zone is diffusion-limited [10]. It is clear that the transition temperature between the two regimes corresponds to the maximum COS formation temperature.
Z Ma et al./Applied Catalysis A: General 150 (1997) 253-268
256
I00
1000
2
/
/
~
fa
80
800
60
600
ga,
, ~0
~
400
40
\
1
"[3 --_..C~ ~
8
1
20
200
\
J J ~O
200
250
300
350
400
450
500
550
600
650
T e m p e r a t u r e , °C Fig. 1. Reactant conversions and COS concentration as a function of temperature - Catalyst: La202S-RC (Feed: 0.5% tool SO 2 and 1.0% rnol CO; space velocity: 10,800 cc g - l h - l ; zx SO 2 conversion; O CO conversion; [] COS concentration).
3.2. Effect of C O / S O 2 ratio in the feed The formation of COS has been shown to have an important role in the desulphurisation reaction of SO 2 and CO via the COS intermediate mechanism; however, it is desirable to limit its concentration in the effluent to as low a level as possible because it is highly toxic. It has been reported that the formation of COS is dependent not only on the catalyst composition [ 1,6] but also on the CO partial pressure [1]. In the present work, the influence of the C O / S O 2 feed ratio on SO 2 conversion and COS concentration in the effluent was investigated and the results are presented in Table 1. Fig. 3 is a graphical representation of Table 1 and shows the production of COS as a function of C O / S O 2 ratio in the feed. The production of COS is defined as the amount of COS produced divided by the amount of SO 2 reduced in percent. Thus, it is inversely proportional to the selectivity for producing elemental sulphur. It can be seen that the production of COS was very small (not more than 4%) when the C O / S O 2 ratio was at or below the stoichiometric ratio. However, once above the stoichiometric ratio, it increased rapidly and became very high (32 to 49%) at a ratio of 2.6. This same effect was observed by
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
257
-6
-7
e~ Q
Q
O
-9
,
1.0
I
,
1.2
I 1.4
r
I
,
1.6
1.8
1Fr * l 0 s Fig. 2. Arrhenius-type plot of SO 2 conversion rate.
Happel et al. [1] when lanthanum titanate was used. Furthermore, the production of COS decreased with an increase in temperature and this is consistent with the findings of Khalafalla and Haas [10] but contradicts Bazes et al. [2]. We noticed that a reaction temperature range of 400 to about 900°C was used in Ref. [11] while in Ref. [2] it was below 420°C. Our results, reported in Ref. [8] and in Fig. 1 of this paper, show that the formation of COS increased with temperature below 400°C and decreased at higher temperatures.
Table 1 Summary of conversion and concentration data at different C O / S O 2 ratios (0.5% mol of SO z in feed; Space velocity: 21,600 cc g i h - l ) C O / S O 2 ratio SO2 conversion, %
COS concentration, ppmv
450°C 500°C 550°C 600°C 450°C 500°C 550°C 600°C
1.5 49.6 57.0 63.0 66.7 100 73 41 23
1.8 54.0 63.0 70.0 73.4 108 73 61 67
2.0 85.0 95.7 98.0 98.2 87 75 87 89
2.2 32.5 44.4 76.2 89.7 170 135 125 123
2.6 69.6 80.8 99.2 99.2 1698 1468 1890 1659
258
J. Ma et al. / A p p l i e d Catalysis A: General 150 (1997) 253-268 50
O
40
,fill
30
o 20 0 10 1
O . . . -:.2" . - O • .... 0"
I
I
I
1.0
- ' /" " J~ I
I
1.5
2.0
I
2.5
I
I
I
3.0
CO/SO 2Ratio Fig. 3. Temperature dependency of COS production at different CO/SO 2 ratio (SO s in feed: 0.5% tool; space velocity: 21,600 cc g - l h - l ; O 450°C; [] 500°C; A 550°C; '7 600°C).
The data in Table 1 can be used to calculate the conversion efficiency. This is defined as the total amount of sulphur-containing substances in the outlet gas stream including unconverted SO z and by-products, if any, and is therefore proportional to the yield of elemental sulphur. It can be seen in Fig. 4 that the most effective removal of SO 2 occurred when the C O / S O 2 ratio was near the stoichiometric ratio. Below this ratio insufficient CO impaired the efficiency due to low SO 2 conversion, while above which the efficiency was depressed by low selectivity due to the high formation of COS. This same result was reported for Cu/A1203 [12] and Lal_xSrxCoO 3 [6,13].
3.3. Effect of space velocity The effect of space velocity on the catalytic activity was investigated by varying the amount of catalyst used. The experiments were conducted using a feed at stoichiometric ratio and at a reaction temperature of 600°C; results are shown in Fig. 5. The range of space velocity studied was from 10,800 to 43,200 cc g-~ h-1. It can be seen that the SO 2 conversion decreased linearly with the increase in space velocity while the selectivity was not affected. The decrease in SO 2 conversion was 18% (from 96 to 78%) when there was a fourfold increase
Z Ma etal./Applied CatalysisA: General150(1997) 253-268
259
2000
t-L 1500
Q
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500
0 1.0
,
,
,
,
I
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,
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,
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,
,
~
,
3.0
CO/SO2 Ratio Fig. 4. Influence of C O / S O 2 ratio on the conversion efficiency at 600°C (SO 2 in feed: 0.5% mol; space velocity: 21,600 cc g-1 h l).
in space velocity showing that the effect of space velocity on SO 2 conversion was not severe.
3.4. Water uapour effect Any hydrogen-containing fuel will produce water vapour in the flue gas. Previous experience indicates that the effect of water vapour on the activity of desulphurisation catalyst can be significant [14]. The influence of water vapour on our catalyst was studied by pumping water directly into the preheating zone of the reactor. Fig. 6 shows that the SO 2 conversion decreased gradually from 96 to 66% and held when the feed contained 7% of water vapour. It was also found that the bone-dry conversion could be recovered in a very short time when the water vapour in the feed was removed. Furthermore, XRD analysis showed that there was no structural change in the catalyst after over 20 h of "wet reaction ". This suggests that water vapour does not poison the catalyst. A process mass spectrometer (MS250 Gas Analyzer, Extrel) was used to detect and measure the concentration of the by-products so that the influence of water vapour on the reaction could be further studied. It was found that there
260
J. Ma et al. / A p p l i e d Catalysis A: General 150 (1997) 253-268
100
90
80
L
70
g. 60
50
,
,
,
,
r ,
,
,
1
,
I ,
,
,
,
2
Space Velocity, h a
I . . . .
1 . . . .
3
4
5
(/10,000)
Fig. 5. I n f l u e n c e o f s p a c e v e l o c i t y on c o n v e r s i o n a n d selectivity (Feed: 0 . 5 % m o l S O / and 1.0% m o l CO; t e m p e r a t u r e 600°C; O S O 2 c o n v e r s i o n ; [] selectivity).
was a significant amount of H2S and a small amount of H 2 while the formation of COS decreased. These phenomena suggest the presence of three possible reactions when water is added. (1) The reverse Claus reaction, 32 8 2 -t-
2H20
~
2H2S + SO 2
(1)
which will cause an increase in the steady-state SO z concentration while decrease the selectivity in forming elemental sulphur. (2) The hydrolysis of COS, COS + H 2 0
~
HzS -t- CO 2
(2)
which will decrease the COS concentration with an equivalent increase in HzS. (3) The water-gas shift reaction CO + H 2 0
----) H 2 +
CO 2
(3a)
which consumes the reducing agent CO, thus causing a decrease in SO 2 conversion. The hydrogen produced can react readily with elemental sulphur to form hydrogen sulfide according to the following reaction, 1
H 2 + ~- S 2 ~
HzS
(3b)
which will also decrease the sulphur formation selectivity. Reactions (2) and (3)
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
261
100
500
80
"~ 400
=- 60
-1 300
1
o
0
40
200
20 t
100
0
' 2
4
6
'
=
0 8
Content of Water Vapor, % Fig. 6. Effect of water vapour on the conversion and the concentration of by-products. (Feed: 0,5% mol SO 2 and 1.0% tool CO; temperature 600°C; space velocity: 21,600 cc g - i h-L; O SO 2 conversion; [] COS concentration; z~ H2S concentration* 10).
have been confirmed in experiments conducted using feeds containing COS or CO and water vapour. Since the catalyst was not deactivated by water vapour, the selectivity also recovered once the feed was dried. This reversible water vapour effect was also reported by Okay and Short [15] for the C u / A l z O 3 catalyst system, although they reported no hydrogen or H2S in the "wet reaction". Their thermodynamic calculations indicated that the water-gas shift reaction could take place at the reaction conditions used.
3.5. Oxygen effect Oxygen is always present in the flue gas of combustion processes and it is known to deactivate or destroy reduction catalysts. The oxygen effect was studied by introducing 0 2 at different concentrations into the feed gas while keeping C O / O 2 at stoichiometric ratio. Up to 2.5% mol of net oxygen was added to the reaction. The results are plotted in Fig. 7. The presence of oxygen caused a decrease in SO 2 conversion and the selectivity to form elemental sulphur for all temperatures studied, however, the effect was significantly lower at higher temperatures. For example, for a feed containing 2% mol of oxygen and 600°C, there was a decrease of 40% in SO~
262
J. Ma et al./Applied Catalysis A." General 150 (1997) 253-268
100
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0
'
0.0
I
.5
,
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,
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,
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Oxygen Content, % Fig. 7. Effect of oxygen on conversion and selectivity (Feed: 0.5% tool SO 2 and 1.0% rnol CO; space velocity: 21,600 cc g-~ h - l ; • conversion at 600°C; [] selectivity at 600°C; • conversion at 650°C; © selectivity at 650°C).
conversion and a 9% reduction in selectivity, while the decrease was 10 and 6%, respectively, at 650°C. XRD analysis revealed that after 20 h of reaction in the presence of oxygen, the bulk structure of the lanthanum oxysulphide did not change showing that it is an oxygen-resistant catalyst. The decrease of SO 2 conversion in the presence of oxygen may be due to the following competitions: CO+Sac O 2 q- Sac 1
~ ~
CO+~-O 2 ~
COS SO 2
COg
(4) (5)
(6)
where Sac represents active sulphur atoms on the catalyst surface. The concentration of the intermediate COS is lower when Reaction (5) occurs, and this will lead to a lower SO 2 conversion via reduction by COS. However, Reaction (5) can be inhibited by Reaction (6), which is favoured at higher temperatures, thus leaving the active sulphur atoms to react with CO to form COS. 3.6. Comparison with a commercial sample Baglio [5] used two samples of La202S in his work: one was obtained from the Chemical and Metallurgical Division, GTE Sylvania, Towanda, Pa. and the
J. Ma et a l . / Applied Catalysis A: General 150 (1997) 253-268
/
100
O'
0
263
1000
Q
80
800
60
600
ca.
d" Q .~
Q
400
40
-
20
- 2OO
0
200
,,,I..,
250
0
300
350
400
450
500
550
600
650
T e m p e r a t u r e , °C Fig. 8. Reactant conversions and COS concentration as a function of temperature - Catalyst: La202S-JM (Feed: 0.5% mol SO 2 and 1.0% mol CO; space velocity: 10,800 cc g-1 h - l ; A SO 2 conversion; O CO conversion; [] COS concentration).
other he prepared himself. He observed that using the GTE sample at 650°C, there was only a minimal reduction of SO 2 by CO and no COS was generated. There was, however, no report of any results on his sample. To verify Baglio's results, we conducted experiments using a 99.9% pure La2OzS commercial sample (LazO2S-JM). The bulk structure of the commercial sample was determined using XRD and it was confirmed that the structure was the same as La202S-RC. The catalytic activity of La2OzS-JM was measured in the same manner as La202S-RC and the results are presented in Fig. 8. Compared to the results shown in Fig. 1, it is evident that this commercial sample possessed almost the same catalytic behaviour as the sample prepared in the present work. This contradicted Baglio's findings.
4. Discussion
We have previously found that sulphidised lanthanum cobaltate is an active catalyst for the reduction of SO 2 by CO, and the active catalyst consists of two major phases: La202S and CoS 2. Based on these discoveries, a reaction mechanism based on the COS intermediate and the "remote control" concepts
264
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
was proposed [7]. According to this mechanism, CO first reacts with CoS 2 to form an intermediate, COS. The intermediate reduces SO 2 catalysed by La202S to CO 2 and elemental sulphur, and a portion of the elemental sulphur formed spills over from the surface of La202S to the CO reduced sites on the surface of CoS 2. The role of the active La202S phase is twofold: to catalyse the reduction of SO 2 by COS and to maintain (or control) the reactivity of the CoS 2 phase with CO. We also studied the synergistic effect between the two phases by conducting experiments on separately prepared La202S and COS2; the results provided additional evidence in support of this mechanism [8]. In the previous and current studies, however, we discovered that the lanthanum oxysulphide by itself is active in the reaction between SO 2 and CO. If the second active phase is absent, then the proposed mechanism for the SO 2 and CO reactions needs to be revised. As presented earlier in this paper, there was a rapid increase in the conversion of both SO 2 and CO at a temperature around which the formation of COS was maximum. Moreover, when the COS concentration went over the maximum and fell off, the reaction became diffusion-limited. Therefore, it is most likely that COS played an important role in the reduction of SO 2 by CO over La202S. Assuming for the moment that the reaction follows the intermediate mechanism, the COS concentration in the product gas stream tends to depend on the competition between COS formation and the consumption of COS. At lower temperatures, the interaction of CO with active sulphur species on the catalyst is weak, implying a low formation of COS. Therefore, the overall reaction rate will be determined by the formation of COS, and we observed the increase in SO 2 conversion with higher COS concentrations. At higher temperatures, both the formation of COS and the reaction between the COS and SO 2 are accelerated, thus the overall reaction will be controlled by the diffusion of the reactants from the gas phase to the surface of the catalyst. It was reported in Ref. [11] that COS is a stronger Lewis base than CO and will readily react with S O 2. In one series of experiments, we replaced CO with COS as the reducing agent, and the results are shown in Fig. 9. The reaction started to accelerate at about 250°C, and complete reduction was achieved at temperatures above 350°C. The result clearly shows that La2OzS is very active in the reduction of SO 2 by COS. In other words, COS is a more reactive reagent than CO. This implies that the formation of COS is a favoured step in the reduction of SO 2 by CO on La2OzS, if there is sufficient interaction between CO and the lattice sulphur atoms on La2OzS. This relationship between the complete reduction of SO 2 at temperatures above 350°C and maximum COS formation temperature is consistent with what is shown in Fig. 1. In another experiment, the reactivity of La202S with CO was measured according to the following procedure: after the reduction of SO 2 by CO at 600°C, the catalyst was purged using UHP nitrogen for 2 h at 600°C. A CO-containing (1.0% tool) nitrogen gas stream was then introduced and the
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
100
O
O
450
500
265
80
60
~
4o
20
0 200
~ 250
300
350
400
550
600
650
T e m p e r a t u r e , °C Fig. 9. Conversion as a function of temperature for the reduction of SO 2 by COS (Feed: 0.5% mol SO 2 and 1.0% mol COS; space velocity: 21,600 cc g - l h - l ; [] SO 2 conversion; C) COS conversion).
temperature of the catalyst bed was decreased stepwise from 600 to 350°C before increasing it back to 600°C. The concentration of COS was measured during the cooling as well as the heating stages. The results are presented in Fig. 10. It can be seen that the formation of COS decreased gradually during the cooling stage and was cut-off at 350°C. However, when the catalyst was reheated, the formation of COS increased again with temperature and a significant amount was detected after about 90 rain of reduction. Therefore, it has been confirmed that the lattice sulphur atoms in the La202S indeed were mobile enough to react with CO to form COS. Furthermore, it can also be found from Fig. 10 that the formation of COS started at above 350°C, which is the same temperature for the initiation of the reduction SO 2 by CO as shown in Fig. 1. Since XRD analysis revealed that the phase structure of L a 2 0 2 S after reaction did not change, the lattice sulphur on La202 S must have been replenished by the elemental sulphur formed by the reaction between SO 2 and COS. Based on the above mentioned observations, we postulate that lanthanum oxysulphide is bifunctional in the reduction of SO 2 by CO to elemental sulphur. It is not only active in the reaction between COS and SO z but also possesses the same function as the CoS 2 in providing mobile sulphur. Therefore, the overall
266
J. Ma et al. / A p p l i e d Catalysis A: General 150 (1997) 253-268
300
initial
250
g14
200
6 min
o
150 = o
o
100
50
/
0
66 rain
200
250
300
350
90 min A
f 400
450
500
550
600
650
o
Temperature, C Fig. 10. COS concentration during the reaction of CO with La202S (zx heating stage; v cooling stage).
reaction can still follow the COS intermediate mechanism as represented by the following equations: CO+La202 S ~ 2COS+SO z ~ 1
COS+La202[] 2CO~+3
L a 2 0 : [] + ~S z ~
La202S
(7)
(8) (9)
where [] is the lattice sulphur vacancy. The formation of vacancies in LazO2S may also be important to Reaction Eq. (8), because it may be the rate-determining step in the reduction of SO 2 by COS [5]. At steady-state the overall reaction is as follows: 2 C O + S O 2 ---> 2CO 2 + ~ S 2
5. C o n c l u s i o n
Lanthanum oxysulphide has been found to be an effective catalyst for the reduction of SO 2 by CO. The effectiveness may be due to its bifunctional characteristics which catalyses not only the reaction between SO 2 and COS but also the formation of COS with CO. Consequently, the reduction of SO 2 by CO
J. Ma et al. /Applied Catalysis A: General 150 (1997) 253-268
267
to elemental sulphur over La202S can also be described by the COS intermediate mechanism. Experimental results show that both water vapour and oxygen decrease the conversion of SO z and the selectivity to elemental sulphur. In particular, the selectivity can be significantly impaired by water vapour because of the formation of the by-product H2S via the reverse Claus reaction, the water-gas shift reaction and the hydrolysis of COS. However, the catalyst structure does not change after over 20 h of exposure to either water vapour or oxygen, and the catalytic activity can be recovered when the water vapour and oxygen are removed. This suggests that the catalyst is not destroyed by either water vapour or oxygen. Over 98% in SO 2 conversion can be obtained at temperatures > 450°C. Above this temperature, the concentration of the by-product COS decreases with the increase in temperature, the conversion of SO 2 virtually remains constant. Thus, the selectivity also increases at higher temperatures and over 98% can be reached. The effect of the f e e d - C O / S O 2 ratio is significant not only on the conversion of SO 2 but also on the formation of COS. The most effective removal of SO 2 is obtained near the stoichiometric ratio. The effect of space velocity on the selectivity is minimal and is relatively small on the conversion of SO 2. A linear decrease of 18% in the latter was observed when the space velocity increased fourfold from 10,800 to 43,200 cc g-1 h-1
Acknowledgements The authors appreciate the invaluable technical support provided by Messrs. K.L. To and H.S. Tsui of the Research Centre, HKUST. The Materials Preparation and Characterisation Centre has been more than generous in performing XRD and surface analysis on our behalf, and we are indebted to its staff.
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