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Catalytic conversion of CO, NO and SO2 on supported sulfide catalysts
Applied Catalysis B: Environmental 31 (2001) 133–143
Catalytic conversion of CO, NO and SO2 on supported sulfide catalysts II. Catalytic reduction of NO and SO2 by CO S.-X. Zhuang1 , M. Yamazaki, K. Omata, Y. Takahashi, M. Yamada∗ Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Received 29 February 2000; received in revised form 7 November 2000; accepted 7 November 2000
Abstract To investigate the possibility of simultaneous catalytic reduction of NO and SO2 by CO, reactions of NO, NO–CO, and NO–SO2 –CO were performed on ␥-alumina-supported sulfides of transition metals including Co, Mo, CoMo and FeMo. NO was decomposed into N2 O and N2 accompanied with the formation of SO2 ; this serious oxidation of lattice sulfur resulted in the deactivation of the catalysts. The addition of CO to the NO stream suppressed SO2 formation and yielded COS instead. A stoichiometric conversion of NO and CO to N2 and CO2 was observed above 350◦ C on the CoMo and the FeMo catalysts. Although the CO addition lengthened catalyst life, it was not enough to maintain activity. After the NO–CO reaction, an XPS analysis showed the growth of Mo6+ and SO4 2− peaks, especially for the sulfided FeMo/Al2 O3 ; the FeMo catalyst underwent strong oxidation in the NO–CO reaction. The NO and the NO–CO reactions proceeded non-catalytically, consuming catalyst lattice sulfur to yield SO2 or COS. The addition of SO2 in the NO–CO system enabled in situ regeneration of the catalysts; the catalysts oxidized through abstraction of lattice sulfur experienced anew reduction and sulfurization through the SO2 –CO reaction at higher temperature. NO and SO2 were completely and catalytically converted at 400◦ C on the sulfided CoMo/Al2 O3 . By contrast, the sulfided FeMo/Al2 O3 was easily oxidized by NO and hardly re-sulfided under the test conditions. Oxidation states of the metals before and after the reactions were determined. Silica and titania-supported CoMo catalysts were also evaluated to study support effects. © 2001 Elsevier Science B.V. All rights reserved. Keywords: NO; SO2 ; CO; Catalytic reduction; Sulfide catalysts; Cobalt molybdenum catalyst
1. Introduction In a previous paper [1], we have reported a complete catalytic reduction of SO2 by CO on a sulfided CoMo/␥-Al2 O3 . In the SO2 –CO reaction, a carbonyl ∗ Corresponding author. Tel.: +81-22-217-7214; fax: +81-22-217-7293. E-mail address: [email protected] (M. Yamada). 1 Present address: Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR China.
sulfide (COS) intermediate produced by abstraction of catalyst lattice sulfur works as stronger reductant than CO. Although the catalyst seems to be temporarily oxidized and lose its activity through the COS-forming reaction, it is assumed that the oxidized catalyst is in situ regenerated by elemental sulfur produced by the COS–SO2 reaction. The nature of catalyst metal–sulfur bond therefore strongly influences catalytic performances of sulfide catalysts in the SO2 –CO reaction. Sulfided CoMo/Al2 O3 catalysts, a well-known HDS catalyst, have the CoMoS structure
0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 0 ) 0 0 2 7 5 - 7
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with a moderate CoMo–S bond strength. This moderate bonding facilitates both the formation of the COS intermediate and in situ regeneration of oxidized catalysts, thus promoting the catalytic reduction of SO2 by CO. The present work is an extension of the previous work to the simultaneous catalytic reduction including NO. In industry the emission of SO2 is usually accompanied by the NOx emission, and hence the simultaneous reduction of NO and SO2 by CO is of keen interest. Its practical importance is increasing with growing concern about environmental issues. Owing to complex nature of this reaction, however, systematic studies have still been lacked in the literature. Detailed understanding of the reaction mechanism and its application have been desired. Reactions between NO and CO have been a subject of numerous publications. Zhdanov and Kasemo [2] have comprehensively discussed the mechanism and kinetics of the NO–CO reaction on Rh in a recent review. Taylor [3], Shelef and Graham [4] have investigated the catalytic removal of NO and CO from automotive tailpipe emissions, particularly over so-called three-way catalysts containing Rh, Pt and Pd. Viswanathan [5] has reviewed CO oxidation, NO reduction and NO–CO reaction on perovskite oxides. Compared with NO–CO reaction, few literatures have been published on the reduction of SO2 . Hibbert [6] has studied a poisoning effect of SO2 in reactions of CO, NO and hydrocarbons on perovskite oxides: owing to strong aggression caused by SO2 , the perovskite oxides are converted to mixtures of sulfates, sulfides and oxysulfides of transition and lanthanide elements depending on whether the atmosphere is oxidizing or reducing. Sulfides are predominant species in the reducing atmosphere, for example, in the presence of CO as described in the previous paper [1]. Further fewer papers have been published on the simultaneous reduction of NO and SO2 by CO. A group of Chevron [7] has studied NO–SO2 –CO reaction on alumina-supported Cu, Ag, and Pd, and silica-supported Mn, Ni, Ag, and Cu at 540◦ C at space rates approaching 10,000/h. They used reactant gas mixtures containing approximately 0.5% SO2 , 1–2% CO, 5–10% CO2 , and 150 ppm NO, assuming a flue gas. They observed substantial reduction of NO and SO2 , and formation of COS by the reaction be-
tween CO and elemental sulfur. The amount of COS produced, however, is not clear. Kittrell et al. [8,9] have also studied NO–SO2 –CO reaction on alumina-supported Cu. After activating the catalyst by introducing a gas containing CO and SO2 (no NO present), they observed conversions of 43% of SO2 and 97% of NO, associated with 13% yields of COS for a virtual stack gas containing NO/SO2 /CO = 908/2200/6624 ppm at 400◦ C. An increase in the NO level of the feed resulted in a significant decrease in the rate of SO2 reduction. They pointed out that NO is preferentially adsorbed on SO2 reduction sites, most probably on copper sites where CO is to be adsorbed. These observations motivated us to investigate the direct reduction of NO and SO2 by CO on the sulfided CoMo/Al2 O3 catalyst. Over the sulfided CoMo/Al2 O3 the SO2 –CO reaction is easy to operate in a single step when stoichiometrically sufficient amount of CO is present in the gas stream [1]. In the present work, we have studied the simultaneous NO–SO2 –CO reaction on various alumina-supported sulfides of transition metals including CoMo, FeMo, Co, and Mo. NO decomposition and NO–CO reaction were also studied. 2. Experimental 2.1. Preparation of catalysts All the catalysts were prepared by the conventional incipient wetness method. The preparation procedures were described elsewhere [1]. The prepared catalysts were CoMo/Al2 O3 (CoO 4.4 mass%, MoO3 15.9 mass%), FeMo/Al2 O3 (FeO 4.2 mass%, MoO3 15.0 mass%), Mo/Al2 O3 (MoO3 15.8 mass%), and Co/Al2 O3 (CoO 14.3 mass%). Titania (Ishihara Sangyo, surface area 72.6 m2 /g) and silica (Fuji Davison, ID gel, surface area 257 m2 /g) were also used as supports for CoMo catalysts. An oxide catalyst charged in a reactor was in situ sulfided in a flow of 5 vol.% H2 S/H2 at 400◦ C for 1 h, and then cooled down to room temperature with helium purge to avoid the adsorption of H2 S while cooling the catalyst. 2.2. Activity measurement and analysis A fixed-bed flow reactor made of Pyrex tube was used under atmospheric pressures. 0.15 g of oxide
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catalyst powder 0.15–0.25 mm (100–60 mesh) in diameter was packed in the reactor and in situ pre-sulfided. Feed gases used were 10.1% NO, 5.0% NO/5.0% CO, 2.0% NO/2.0% SO2 /6.0% CO and 2.5% NO/1.2% SO2 /5.0% CO each in balanced helium. The feed gas was passed through the catalyst bed and then entered an on-line trap cooled in an ice bath to condense sulfur before a sampling valve. Gaseous products were analyzed with a QP-mass spectrometer and a GC-TCD using Porapak Q column to separate CO2 , N2 O, COS, and SO2 , and an active carbon column to CO, NO and N2 . The feed gas was heated up around 500◦ C in 50◦ C steps, being held for 10 min at each step. Products were analyzed at the end of each step. An XPS analysis of catalysts was performed with a Shimadzu ESCA 750 system. Binding energies were referenced to Al (2p) at 74.5 eV.
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Fig. 1. NO decomposition on sulfided 4.4% Co 15.9% Mo/Al2 O3 . Feed gas: 10.1% NO/He; SV = 3000 ml/g-cat/h; (䊉): NO; (): N2 O; (×): SO2 ; dotted line: N2 (calculation).
2.3. TPD measurements Sample catalysts were in situ sulfided as mentioned above. First, a gas was introduced into the reactor by the pulse method at room temperature. The amount of saturation adsorption was determined with a GC-TCD. After the saturation adsorption, the catalyst was heated up at a rate of 10◦ C/min in 30 ml/min of He stream. Desorbed gases were analyzed with a QP-mass spectrometer.
Fig. 2. NO decomposition on sulfided 4.2% Fe 15.0% Mo/Al2 O3 . Feed gas: 10.1% NO/He; SV = 3000 ml/g-cat/h; (䊉): NO; (): N2 O; (×): SO2 ; dotted line: N2 (calculation).
3. Results and discussion 3.1. NO decomposition 3.1.1. Product distribution A fundamental reactivity of NO was examined by feeding a gas only containing 10.1% NO in helium prior to the study on multi-component reactions. Figs. 1–4 show the temperature dependence of product distributions for the CoMo, FeMo, Mo, and Co catalysts. Output of NO (XNO ) and yields of products (Yproduct ) were calculated from the following equations: XNO
[NO]out = [NO]in
(1)
Fig. 3. NO decomposition on sulfided 15.8% Mo/Al2 O3 . Feed gas: 10.1% NO/He; SV = 3000 ml/g-cat/h; (䊉): NO; (): N2 O; (×): SO2 ; dotted line: N2 (calculation).
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Fig. 4. NO decomposition on sulfided 14.3% Co/Al2 O3 . Feed gas: 10.1% NO/He; SV = 3000 ml/g-cat/h; (䊉): NO; (): N2 O; (×): SO2 ; dotted line: N2 (calculation).
YN2 O =
2[N2 O]out [NO]in
(2)
YSO2 =
2[SO2 ]out [NO]in
(3)
YN2 = 1 − XNO − YN2 O
(4)
The products observed were N2 O, N2 , and SO2 ; NO2 and O2 were not detected. Fig. 1 shows the temperature dependence of NO output, N2 O and SO2 production on the CoMo/Al2 O3 . Main products are N2 O at lower temperature, and N2 and SO2 at higher temperature. The reaction, however, is suddenly inhibited above 450◦ C, and the CoMo/Al2 O3 seems to lose its activity. The CoMo (Fig. 1) and the FeMo (Fig. 2) are similar in activity, but they show different activities from the Mo (Fig. 3) and the Co (Fig. 4) catalysts. While the Co/Al2 O3 shows the lowest activity, the reaction is similar in scheme to the others. It is noticed that all the catalysts lose their activities at higher temperature. It was observed that the colors of the catalysts after the NO decomposition were those of corresponding oxides. An XPS analysis also proved the oxidation of the catalysts (not shown). These results clearly indicate that the sulfide catalysts show high activity for the NO decomposition and lose their activity through the oxidation. Hence, it is interesting to elucidate the role of sulfur in the reactions including NO.
Fig. 5. TPD spectra of NO adsorbed on sulfided CoMo/Al2 O3 . A: NO; B: N2 O; C: N2 ; D: SO2 ; E: H2 S; F: H2 O.
3.1.2. TPD measurements Concerning to the role of sulfide, it is noticed that, on every catalyst, SO2 is formed without O2 production. Combining the characteristic formation of SO2 with the oxidation of the catalyst, it is indicated that the following oxidation of lattice sulfur occurs: Slattice + 2Oad = SO2
(5)
Oad will be produced by NO dissociation (6). We have investigated the reaction mechanism by measuring TPD spectra for the CoMo/Al2 O3 . Fig. 5 shows a TPD spectrum of NO adsorbed on the sulfided CoMo/Al2 O3 . NO is desorbed at temperatures between room temperature and 300◦ C. Above 350◦ C H2 O and H2 S are desorbed. Judging from simultaneous production of H2 O and H2 S, H2 S should be produced through the reaction between lattice sulfur and H2 O that comes from alumina. Vacant sites will induce the dissociation of NO, Eq. (6). Adsorbed NO and nitrogen adatoms will react to form N2 O, Eq. (7). Fig. 5 shows that some NO molecules remain adsorbed and the others are desorbed below 300◦ C; the coexistence of adsorbed NO and vacant sites, which induce the reaction (6) and yield nitrogen adatoms, will allow the production of N2 O at this low temperature range (Fig. 1). Thus, the N2 O production at lower temperature is in good agreement with the TPD results. The dissociation of NO and the resulting formation of oxygen adatoms can lead to the oxidation of sulfide catalysts, Eq. (5): MoS2 is oxidized at temperatures even below 30◦ C [10]. Thus, the reaction scheme can be written as follows
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At lower temperature NOad + { } = Nad + Oad
(6)
NOad + Nad = N2 O
(7)
At higher temperature 2NOad = N2 + 2Oad
(8)
where subscript ‘ad’ stands for the adsorbed species on the catalysts and { } for vacant sites. Iwamoto et al. [11] have pointed out that accumulation of oxygen on a catalyst lowers catalytic activity in NO decomposition on Cu–NaY, because oxygen is not evolved below 500◦ C. In the present NO reaction, it is assumed that oxygen is consumed by the reaction with lattice sulfur; therefore, the sulfided catalysts may show high activity as long as lattice sulfur exists. A drastic deactivation of the Co catalyst in Fig. 4 might be caused by a low content of sulfur in the catalyst. In order to prove such a role of sulfur, it is necessary to show that the sulfide catalysts have stable activity under the reaction conditions as long as lattice sulfur exists. At the same time, to clarify the characteristic activity of sulfide catalysts, the present work tries to (i) retard the oxidation of the catalysts by introducing CO and (ii) compensate sulfur loss by introducing SO2 in the feed gas. 3.2. NO–CO reaction As shown in Fig. 6, when an equivalent molar amount of CO was added in the inlet NO gas, the
Fig. 6. NO–CO reaction on sulfided CoMo/Al2 O3 . Feed gas: 5% NO/5% CO/He; SV = 4500 ml/g-cat/h; (䊉): NO; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
Fig. 7. NO–CO reaction on sulfided FeMo/Al2 O3 . Feed gas: 5% NO/5% CO/He; SV = 4500 ml/g-cat/h; (䊉): NO; (䊏): CO; (): N2 O; (+): COS; (×): SO2 ; dotted line: N2 (calculation).
situation changed greatly. That is, NO is consumed at lower temperature in the NO–CO reaction than in the NO decomposition reaction. It is also noticed that SO2 formation was not observed. CO output X and COS yield Y are defined by XCO =
[CO]out [NO]in
YCOS =
[COS]out [NO]in
(9) (10)
For all the catalysts in Figs. 6–9, N2 O is the main product at lower temperature. SO2 formation is completely suppressed and COS is formed instead, except
Fig. 8. NO–CO reaction on sulfided Mo/Al2 O3 . Feed gas: 5% NO/5% CO/He; SV = 4500 ml/g-cat/h; (䊉): NO; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
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probably correlated with the stronger metal–sulfur bonds in these catalysts [1]. Slattice + CO = COS
Fig. 9. NO–CO reaction on sulfided Co/Al2 O3 . Feed gas: 5% NO/5% CO/He; SV = 4500 ml/g-cat/h; (䊉): NO; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
for the FeMo/Al2 O3 , where a small amount of SO2 is still observed. The lack of SO2 indicates that oxygen adatoms predominantly react with CO to produce CO2 , Eq. (11). CO might instantaneously remove oxygen adatoms and reproduce vacant sites, thus promoting NO dissociation, Eq. (6). An overall reaction at lower temperature is written as reaction (12). The preceding NO consumption ahead of CO consumption as shown in Figs. 6–9 will be explained with Eq. (12); that is, 2 mol of NO is consumed by 1 mol of CO.
(13)
A small amount of SO2 was formed on the sulfided FeMo/Al2 O3 . Oxidation of the FeMo/Al2 O3 was confirmed by the XPS analysis for Mo (3d), Fe (2p3/2 ), and S (2p) of the catalyst after the reaction (Fig. 10(c)). The intensity of the Mo4+ (3d5/2 ) peak at 229 eV is drastically attenuated and Mo6+ at 233 eV becomes the main component among surface Mo species. A strong emission peak of SO4 2− appears at 169 eV in the S (2p) region [12]; SO2 , which is adsorbed initially on alumina basic sites [1] to form adsorbed sulfite species, should be converted to sulfate species in the oxidation [13]. By contrast, while the intensity of the Mo4+ peak of the CoMo/Al2 O3 is reduced after the reaction, it is still dominant (Fig. 10(a)). Thus, the sulfided FeMo/Al2 O3 is found to be more susceptible to oxidation than the CoMo/Al2 O3 . The reactivity of lattice sulfur to oxygen (yields SO2 ) and CO (yields COS) in the sulfided FeMo/Al2 O3 seems rather complicated, because COS further reacts with SO2 [1]. We presently have no idea about the competitive production of SO2 and COS. A stoichiometrically complete conversion of NO and CO towards N2 and CO2 seems to be achieved
At lower temperature NOad + { } = Nad + Oad
(6)
NOad + Nad = N2 O
(7)
Oad + COad = CO2
(11)
Overall reaction 2NOad + COad = N2 O + CO2
(12)
The production of COS indicates the reaction between CO and catalyst lattice sulfur Eq. (13), [1]. COS yield, however, is little in the NO–CO reaction compared with the SO2 –CO reaction: at most 10% on the CoMo (Fig. 6) and 16% on the FeMo (Fig. 7), and almost negligible for the Mo (Fig. 8) and the Co (Fig. 9), in contrast to 30–70% in the SO2 –CO reaction [1]. CO must be more reactive with oxygen adatoms Eq. (11) than lattice sulfur Eq. (13). The negligible COS formation over the Co and the Mo catalysts is
Fig. 10. XPS spectra after NO–CO reaction over (a) sulfided CoMo/Al2 O3 and (c) sulfided FeMo/Al2 O3 , and NO–SO2 –CO reaction over (b) sulfided CoMo/Al2 O3 and (d) sulfided FeMo/Al2 O3 .
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Reactant outputs and product yields are defined on the basis of the inlet concentration of SO2 . For the feed gases containing NO/SO2 /CO = 1/1/3 (Fig. 12(a)) and NO/SO2 /CO = 2/1/4 (Fig. 12(b)) both on the sulfided CoMo/Al2 O3 , and NO/SO2 /CO = 1/1/3 on the sulfided FeMo/Al2 O3 (Fig. 13), a lot of N2 O and COS is formed below 400◦ C. Extra SO2 , i.e. SO2 out –SO2 in , is also formed on the sulfided FeMo/Al2 O3 . The sole reaction below 250◦ C is the decomposition of NO into N2 O. SO2 begins to react at 250◦ C accompanying CO consumption and COS formation; this simultaneous behavior indicates that SO2 is consumed by the reaction with COS (Eq. (16)), [1]. Fig. 11. Deactivation of sulfided CoMo/Al2 O3 in NO–CO reaction. Feed gas: 5% NO/5% CO/He. Solid line: SV = 22000 ml/g-cat/h; broken line: SV = 9200 ml/g-cat/h.
SO2 + 2COS = 2CO2 + 1.5S2
(16)
at 350◦ C or higher in Figs. 6–9, but the catalysts lost their activities after a long operation with a high space velocity (SV) at 500◦ C (Fig. 11). It can therefore be said that the suppression of the catalyst oxidation by the addition of CO is temporarily effective, but is not enough to maintain activity of the sulfide catalysts for NO conversion. 3.3. NO–SO2 –CO reaction As mentioned above, it is noticed that the NO–CO reaction produces COS. Since COS reacts with SO2 into CO2 and elemental sulfur on the sulfide catalysts [1], we can expect the simultaneous catalytic reduction of NO and SO2 by adding SO2 into the NO–CO mixture. Furthermore, we can expect that the lattice sulfur abstracted as COS will be supplied with elemental sulfur produced in the COS–SO2 reaction. 3.3.1. Product distribution A feed gas with the NO/SO2 molar ratio of 1/1 (2.0% NO/2.0% SO2 /6.0% CO in He) or 2/1 (2.5% NO/1.2% SO2 /5.0% CO in He) was fed over the CoMo and the FeMo catalysts. The following stoichiometric conversions (Eqs. (14) and (15)) are expected: NO + SO2 + 3CO = 0.5N2 + 0.5S2 + 3CO2
(14)
2NO + SO2 + 4CO = N2 + 0.5S2 + 4CO2
(15)
Fig. 12. (a) NO–SO2 –CO reaction on sulfided CoMo/Al2 O3 for NO/SO2 /CO = 1/1/3. Feed gas: 2% NO/2% SO2 /6% CO/He. SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation). (b) NO–SO2 –CO reaction on sulfided CoMo/Al2 O3 for NO/SO2 /CO = 2/1/4. Feed gas: 2.5% NO/1.2% SO2 /5% CO/He. SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
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S.-X. Zhuang et al. / Applied Catalysis B: Environmental 31 (2001) 133–143 Table 1 Amount of gases adsorbed on sulfided catalysts and supports Catalyst
Feed gas
Uptake (ml/g-cat) NO
Sulfided CoMo/␥-Al2 O3
Sulfided FeMo/␥-Al2 O3
Fig. 13. NO–SO2 –CO reaction on sulfided FeMo/Al2 O3 for NO/SO2 /CO = 1/1/3. Feed gas: 2% NO/2% SO2 /6% CO/He. SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
Thus the NO–SO2 –CO reaction can be explained by the combination of the characteristics of two fundamental reactions NO–CO and SO2 –CO: the dissociative adsorption of NO and the formation of the COS intermediate. This is because, NO, SO2 and CO little affect the adsorption one another as shown in a TPD study; a TPD spectra in Fig. 14 and the amount of adsorption in Table 1 show that each component gas of a NO/SO2 /CO mixture (a molar ratio of 1/1/1) is adsorbed on the sulfided CoMo/Al2 O3 at almost the same amount as the adsorption amount in each single-component gas stream. A stoichiometric catalytic reduction to N2 and elemental sulfur is achieved on the sulfided CoMo/Al2 O3 at a temperature of 400◦ C, about 100◦ C higher than
Fig. 14. TPD spectra of NO/SO2 /CO = 1/1/1 over sulfided CoMo/Al2 O3 . A: NO; B: CO2 + N2 O; C: CO + N2 ; D: SO2 ; E: H2 S; F: H2 O.
␥-Al2 O3 TiO2 SiO2
NO 6.5 SO2 CO NO/SO2 /CO ∼7.6a = 1/1/1 NO 9.7 SO2 CO NO/SO2 /CO ∼6.7a = 1/1/1 SO2 SO2 SO2
SO2
CO
6.4 5.9
0.3 ∼0a
5.0 3.6
0.2 ∼0a
13.4 1.8 0.03
a Although NO and CO have almost the same retention time and were not separable, CO adsorption was negligible in the mass spectroscopic study as shown in Fig. 14.
that in the SO2 –CO reaction [1]; the temperature range of COS formation shifts about 100◦ C higher. In Fig. 12(a) and (b), almost the same profiles of NO and SO2 outputs were obtained when the NO/SO2 ratio was increased from 1/1 to 2/1, while the N2 O and the N2 yield curves were expanded to higher temperature. This should be responsible for the increase in the NO/CO ratio from 1/3 to 1/2. Some adjustments of gas composition etc. are imperatively needed to get optimum reaction conditions. Fig. 13 shows that an extra amount of SO2 is produced on the sulfided FeMo/Al2 O3 in the NO–SO2 –CO reaction as in the NO–CO reaction. The increase in the NO/CO ratio from 1/3 to 1/2 results in a larger amount of extra SO2 (not shown). Since the extra production of SO2 was also observed for the NO–SO2 –CO reaction on a sulfided 4% Fe/Al2 O3 , iron-based sulfide catalysts must be oxidized by NO even in the presence of SO2 . 3.3.2. In situ regeneration The gas containing NO, SO2 , and CO was continuously fed to prove that the reaction is stable and catalytic at 500◦ C at SVs of 9000 and 36,000 ml gas/g-cat/h. In Fig. 15, reaction temperature and time are changed while feeding the gas on the sulfided CoMo/Al2 O3 . The catalyst temporarily loses its activity at 350◦ C, where COS is predominantly produced
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Fig. 15. In situ regeneration of sulfided CoMo/Al2 O3 for NO/SO2 /CO = 1/1/3. Feed gas: 2% NO/2% SO2 /6% CO/He SV = 36, 000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (+): COS.
(see Fig. 12(a)) and lattice sulfur is depleted. The resulting oxidized CoMo/Al2 O3 regains its activity at 500◦ C by in situ sulfiding with elemental sulfur produced by the COS–SO2 reaction. By contrast, in the same experiment on the sulfided FeMo/Al2 O3 , it catalytically works only at the lower SV (not shown) and was deactivated at the higher SV (Fig. 16). The results show that, the in situ regeneration of the catalyst much depends on the property of lattice sulfur. It is well known that iron has a less promoting effect than cobalt on Mo catalysts in the HDS reaction. As compared with CoMoS and NiMoS, MoS2 and FeMoS have higher sulfur binding energies corresponding to removal of the first S atom from S
Fig. 16. In situ regeneration of sulfided FeMo/Al2 O3 for NO/SO2 /CO = 1/1/3. Feed gas: 2% NO/2% SO2 /6% CO/He; SV = 36, 000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (+): COS.
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terminated edges (47 kJ/mol for MoS2 , 33 kJ/mol for FeMoS, and −34 kJ/mol for CoMoS) [14]. Sulfur atoms in the sulfided CoMo/Al2 O3 should be accordingly easy to dissociate and to recombine owing to the moderate CoMo–S bond strength; this is not the case with the sulfided FeMo/Al2 O3 whose strong FeMo–S bond prevents the regeneration. The XPS measurements were again performed for the sulfided CoMo and FeMo catalysts after the reaction (SV = 9000, Fig. 10(b) and (d)). Compared with the spectra obtained after the NO–CO reaction shown in Fig. 10(a) and (c), the intensity of Mo4+ (3d5/2 ) peak located at 229 eV increases, accompanied by attenuation of the SO4 2− peak; the oxidation of the sulfided FeMo/Al2 O3 is suppressed in the presence of SO2 . The sulfided CoMo/Al2 O3 catalyst has a spectrum quite similar to that obtained after the SO2 –CO reaction (see [1]). The spectrum shows that the sulfided CoMo/Al2 O3 is in situ regenerated after the reaction and still has high activity. It was found that the rejuvenation of the catalysts in the NO–SO2 –CO reaction presents a great contrast to the deactivation of the catalysts in the NO–CO reaction; lattice sulfur is supplied by the reaction of SO2 and COS in the NO–SO2 –CO system, while it is only consumed in the NO–CO system. 3.3.3. Support effects Finally, we have investigated support effects, in particular the role of alumina, using several types of supports. Khalafalla et al. [15] have proposed a dual reaction site mechanism for the SO2 –CO reaction on Fe/Al2 O3 : iron and alumina augment their specific sites activity at their interparticle contacts. Dalla Lana and coworkers [16] have indicated that COS on aluminum ion sites and SO2 on alumina hydroxyl sites react to form CO2 and sulfur. In the present reaction, COS needs to migrate from sulfided CoMo to aluminum ion sites. To confirm the mechanism, alumina, silica, and titania-supported sulfided CoMo catalysts were subjected to the NO–SO2 –CO reaction (NO/SO2 /CO = 1/1/3) (Fig. 17(a), (b)). The loadings of CoMo on silica and titania were adjusted to the same on alumina on the basis of unit surface area: that is, CoMo/Al2 O3 (CoO 4.4 mass%, MoO3 15.9 mass%), CoMo/SiO2 (CoO 4.4 mass%, MoO3 15.0 mass%), and CoMo/TiO2 (CoO 1.1 mass%, MoO3 3.6 mass%).
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sible routes of consuming SO2 on the CoMo/SiO2 and the CoMo/TiO2 . Although further investigations are required to clarify the reaction mechanism, the bifunctionality of the sulfided CoMo/Al2 O3 seems to be reasonable in the present study.
4. Conclusions
Fig. 17. (a) Support effects in NO–SO2 –CO reaction on sulfided CoMo: alumina (solid lines) vs. silica (broken lines). Feed gas: 1.7% NO/1.7% SO2 /5% CO/He; SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (+): COS. (b) Support effects in NO–SO2 –CO reaction on sulfided CoMo: alumina (solid lines) vs. titania (broken lines). Feed gas: 1.7% NO/1.7% SO2 /5% CO/He; SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (+): COS.
In Fig. 17(a), the reaction is retarded on the sulfided CoMo/SiO2 . In Fig. 17(b), the sulfided CoMo/TiO2 shows a comparable activity to the sulfided CoMo/Al2 O3 , but yields a considerable amount of COS at higher temperature. Titania adsorbs less amount of SO2 than alumina (Table 1), so that the inferior adsorptivity should suppress the SO2 –COS reaction, resulting in the large amount of COS production. In spite of the low adsorptivity of SO2 on silica and titania, however, SO2 is almost entirely removed from the inlet gas. Since the SO2 is supposed to be consumed by the stoichiometric reaction of SO2 –COS Eq. (16), this complete reduction of SO2 and the residual COS formation indicate a deviation from the above stoichiometric reaction mechanism Eqs. (14) and (15), that is, the presence of other pos-
NO was decomposed into N2 O and N2 with the formation of SO2 on ␥-alumina-supported sulfide catalysts (CoMo, FeMo, Mo and Co). The catalytic activity for the NO decomposition was lost abruptly at higher temperature with serious oxidation of the catalysts. The addition of CO to the NO stream clearly lowered the reaction temperature and suppressed SO2 formation, yielding COS instead, except for the FeMo catalyst, where a small amount of SO2 was still formed. The presence of NO drastically inhibited the COS formation; the amount of COS formed in the NO–CO reaction was considerably less than that in the SO2 –CO reaction in our previous report. A stoichiometric conversion of NO and CO to N2 and CO2 occurred above 350◦ C on the sulfided CoMo/Al2 O3 and the sulfided FeMo/Al2 O3 . The addition of CO to the NO stream lengthened catalyst life, but is not enough to maintain activity. After the NO–CO reaction, an XPS analysis showed the growth of Mo6+ and SO4 2− peaks, especially for the FeMo/Al2 O3 ; the FeMo catalyst underwent strong oxidation in the NO–CO reaction. The addition of SO2 in the NO–CO system enabled in situ regeneration of the sulfided catalysts that were oxidized through the formation of SO2 or COS. Stoichiometric catalytic reduction toward N2 and elemental sulfur was achieved at 400◦ C on the sulfided CoMo/Al2 O3 and the sulfided FeMo/Al2 O3 . The XPS analysis for the CoMo and the FeMo catalysts after the NO–SO2 –CO reaction revealed that all the spectra of Mo (3d) and S (2p) were similar to those observed after SO2 –CO reaction: Mo4+ was dominant, while the SO4 2− peak was small. The results indicated that the partially oxidized catalysts by NO at lower temperature experienced anew reduction and sulfurization through the SO2 –CO reaction at higher temperature in the NO–SO2 –CO reaction. The reaction proceeded catalytically at 500◦ C on the sulfided CoMo/Al2 O3 at the space velocity between 9000 and 36,000 ml/g-cat/h. The in situ rejuvenation
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of the CoMo catalyst was explained by the moderate strength of the CoMo–S bond; that is, sulfur atoms are easily dissociated from the moderate CoMo–S bond, and also easily bound to vacant sites to regenerate the oxidized CoMo catalyst. This was not the case with the sulfided FeMo/Al2 O3 : the catalyst remained inactive at higher temperature when the space velocity was increased to 36000 ml/g-cat/h. The NO–SO2 –CO reaction was retarded on the sulfided CoMo/SiO2 . The sulfided CoMo/TiO2 showed a similar reactivity to the sulfided CoMo/Al2 O3 except yielding a considerable amount of COS at high temperature. ␥-Al2 O3 seemed to be an optimum support under the test conditions. Acknowledgements The present work was supported partly by the Center for Interdisciplinary Research of Tohoku University and partly by ‘Research for the Future Program’ of Japan Society for the Promotion of Science (JSPS). One of the authors (S.-X. Zhuang) is grateful to Tohoku University for the financial support during his research stay of 6 months in Sendai.
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References [1] S.-X. Zhuang, H. Magara, M. Yamazaki, Y. Takahashi, M. Yamada, Appl. Catal. B 24 (2000) 89. [2] V.P. Zhdanov, B. Kasemo, Surf. Sci. Reports 29 (1997) 31. [3] K.C. Taylor, Catal. Rev.-Sci. Eng. 35 (1993) 457. [4] M. Shelef, G.W. Graham, Catal. Rev.-Sci. Eng. 36 (1994) 433. [5] B. Viswanathan, Catal. Rev.-Sci. Eng. 34 (1992) 337. [6] D.B. Hibbert, Catal. Rev.-Sci. Eng. 34 (1992) 391. [7] P.R. Ryason, J. Harkins, J. Air Poll. Control Assoc. 17 (1967) 796. [8] C.W. Quinlan, V.C. Okay, J.R. Kittrell, Ind. Eng. Chem. Process Des. Develop. 12 (1973) 359. [9] V.N. Goetz, A. Sood, J.R. Kittrell, Ind. Eng. Chem. Prod. Res. Develop. 13 (1974) 110. [10] S.-X. Zhuang, W.K. Hall, G. Ertl, H. Knozinger, J. Catal. 100 (1986) 167. [11] M. Iwamoto, S. Yokoo, K. Sakai, S. Kagawa, J. Chem. Soc., Faraday Trans. 1 77 (1981) 1629. [12] D.B. Hibbert, R.H. Campbell, Appl. Catal. 41 (1988) 289. [13] M. Waqif, O. Saur, J.C. Lavelley, S. Perathoner, G. Centi, J. Phys. Chem. 95 (1991) 4051. [14] L.S. Byskov, J.K. Nørskov, B.S. Clausen, H. Topsøe, Div. Petrol. Chem. ACS 43 (1998) 12. [15] S.E. Khalafalla, E.F. Foerster, L.A. Haas, Ind. Eng. Chem. Prod. Res. Develop. 10 (1971) 133. [16] T.T. Chuang, I.G. Dalla Lana, C.L. Liu, J. Chem. Soc., Faraday Trans. 1 69 (1973) 643.
Catalytic conversion of CO, NO and SO2 on supported sulfide catalysts II. Catalytic reduction of NO and SO2 by CO S.-X. Zhuang1 , M. Yamazaki, K. Omata, Y. Takahashi, M. Yamada∗ Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Received 29 February 2000; received in revised form 7 November 2000; accepted 7 November 2000
Abstract To investigate the possibility of simultaneous catalytic reduction of NO and SO2 by CO, reactions of NO, NO–CO, and NO–SO2 –CO were performed on ␥-alumina-supported sulfides of transition metals including Co, Mo, CoMo and FeMo. NO was decomposed into N2 O and N2 accompanied with the formation of SO2 ; this serious oxidation of lattice sulfur resulted in the deactivation of the catalysts. The addition of CO to the NO stream suppressed SO2 formation and yielded COS instead. A stoichiometric conversion of NO and CO to N2 and CO2 was observed above 350◦ C on the CoMo and the FeMo catalysts. Although the CO addition lengthened catalyst life, it was not enough to maintain activity. After the NO–CO reaction, an XPS analysis showed the growth of Mo6+ and SO4 2− peaks, especially for the sulfided FeMo/Al2 O3 ; the FeMo catalyst underwent strong oxidation in the NO–CO reaction. The NO and the NO–CO reactions proceeded non-catalytically, consuming catalyst lattice sulfur to yield SO2 or COS. The addition of SO2 in the NO–CO system enabled in situ regeneration of the catalysts; the catalysts oxidized through abstraction of lattice sulfur experienced anew reduction and sulfurization through the SO2 –CO reaction at higher temperature. NO and SO2 were completely and catalytically converted at 400◦ C on the sulfided CoMo/Al2 O3 . By contrast, the sulfided FeMo/Al2 O3 was easily oxidized by NO and hardly re-sulfided under the test conditions. Oxidation states of the metals before and after the reactions were determined. Silica and titania-supported CoMo catalysts were also evaluated to study support effects. © 2001 Elsevier Science B.V. All rights reserved. Keywords: NO; SO2 ; CO; Catalytic reduction; Sulfide catalysts; Cobalt molybdenum catalyst
1. Introduction In a previous paper [1], we have reported a complete catalytic reduction of SO2 by CO on a sulfided CoMo/␥-Al2 O3 . In the SO2 –CO reaction, a carbonyl ∗ Corresponding author. Tel.: +81-22-217-7214; fax: +81-22-217-7293. E-mail address: [email protected] (M. Yamada). 1 Present address: Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR China.
sulfide (COS) intermediate produced by abstraction of catalyst lattice sulfur works as stronger reductant than CO. Although the catalyst seems to be temporarily oxidized and lose its activity through the COS-forming reaction, it is assumed that the oxidized catalyst is in situ regenerated by elemental sulfur produced by the COS–SO2 reaction. The nature of catalyst metal–sulfur bond therefore strongly influences catalytic performances of sulfide catalysts in the SO2 –CO reaction. Sulfided CoMo/Al2 O3 catalysts, a well-known HDS catalyst, have the CoMoS structure
0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 0 ) 0 0 2 7 5 - 7
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with a moderate CoMo–S bond strength. This moderate bonding facilitates both the formation of the COS intermediate and in situ regeneration of oxidized catalysts, thus promoting the catalytic reduction of SO2 by CO. The present work is an extension of the previous work to the simultaneous catalytic reduction including NO. In industry the emission of SO2 is usually accompanied by the NOx emission, and hence the simultaneous reduction of NO and SO2 by CO is of keen interest. Its practical importance is increasing with growing concern about environmental issues. Owing to complex nature of this reaction, however, systematic studies have still been lacked in the literature. Detailed understanding of the reaction mechanism and its application have been desired. Reactions between NO and CO have been a subject of numerous publications. Zhdanov and Kasemo [2] have comprehensively discussed the mechanism and kinetics of the NO–CO reaction on Rh in a recent review. Taylor [3], Shelef and Graham [4] have investigated the catalytic removal of NO and CO from automotive tailpipe emissions, particularly over so-called three-way catalysts containing Rh, Pt and Pd. Viswanathan [5] has reviewed CO oxidation, NO reduction and NO–CO reaction on perovskite oxides. Compared with NO–CO reaction, few literatures have been published on the reduction of SO2 . Hibbert [6] has studied a poisoning effect of SO2 in reactions of CO, NO and hydrocarbons on perovskite oxides: owing to strong aggression caused by SO2 , the perovskite oxides are converted to mixtures of sulfates, sulfides and oxysulfides of transition and lanthanide elements depending on whether the atmosphere is oxidizing or reducing. Sulfides are predominant species in the reducing atmosphere, for example, in the presence of CO as described in the previous paper [1]. Further fewer papers have been published on the simultaneous reduction of NO and SO2 by CO. A group of Chevron [7] has studied NO–SO2 –CO reaction on alumina-supported Cu, Ag, and Pd, and silica-supported Mn, Ni, Ag, and Cu at 540◦ C at space rates approaching 10,000/h. They used reactant gas mixtures containing approximately 0.5% SO2 , 1–2% CO, 5–10% CO2 , and 150 ppm NO, assuming a flue gas. They observed substantial reduction of NO and SO2 , and formation of COS by the reaction be-
tween CO and elemental sulfur. The amount of COS produced, however, is not clear. Kittrell et al. [8,9] have also studied NO–SO2 –CO reaction on alumina-supported Cu. After activating the catalyst by introducing a gas containing CO and SO2 (no NO present), they observed conversions of 43% of SO2 and 97% of NO, associated with 13% yields of COS for a virtual stack gas containing NO/SO2 /CO = 908/2200/6624 ppm at 400◦ C. An increase in the NO level of the feed resulted in a significant decrease in the rate of SO2 reduction. They pointed out that NO is preferentially adsorbed on SO2 reduction sites, most probably on copper sites where CO is to be adsorbed. These observations motivated us to investigate the direct reduction of NO and SO2 by CO on the sulfided CoMo/Al2 O3 catalyst. Over the sulfided CoMo/Al2 O3 the SO2 –CO reaction is easy to operate in a single step when stoichiometrically sufficient amount of CO is present in the gas stream [1]. In the present work, we have studied the simultaneous NO–SO2 –CO reaction on various alumina-supported sulfides of transition metals including CoMo, FeMo, Co, and Mo. NO decomposition and NO–CO reaction were also studied. 2. Experimental 2.1. Preparation of catalysts All the catalysts were prepared by the conventional incipient wetness method. The preparation procedures were described elsewhere [1]. The prepared catalysts were CoMo/Al2 O3 (CoO 4.4 mass%, MoO3 15.9 mass%), FeMo/Al2 O3 (FeO 4.2 mass%, MoO3 15.0 mass%), Mo/Al2 O3 (MoO3 15.8 mass%), and Co/Al2 O3 (CoO 14.3 mass%). Titania (Ishihara Sangyo, surface area 72.6 m2 /g) and silica (Fuji Davison, ID gel, surface area 257 m2 /g) were also used as supports for CoMo catalysts. An oxide catalyst charged in a reactor was in situ sulfided in a flow of 5 vol.% H2 S/H2 at 400◦ C for 1 h, and then cooled down to room temperature with helium purge to avoid the adsorption of H2 S while cooling the catalyst. 2.2. Activity measurement and analysis A fixed-bed flow reactor made of Pyrex tube was used under atmospheric pressures. 0.15 g of oxide
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catalyst powder 0.15–0.25 mm (100–60 mesh) in diameter was packed in the reactor and in situ pre-sulfided. Feed gases used were 10.1% NO, 5.0% NO/5.0% CO, 2.0% NO/2.0% SO2 /6.0% CO and 2.5% NO/1.2% SO2 /5.0% CO each in balanced helium. The feed gas was passed through the catalyst bed and then entered an on-line trap cooled in an ice bath to condense sulfur before a sampling valve. Gaseous products were analyzed with a QP-mass spectrometer and a GC-TCD using Porapak Q column to separate CO2 , N2 O, COS, and SO2 , and an active carbon column to CO, NO and N2 . The feed gas was heated up around 500◦ C in 50◦ C steps, being held for 10 min at each step. Products were analyzed at the end of each step. An XPS analysis of catalysts was performed with a Shimadzu ESCA 750 system. Binding energies were referenced to Al (2p) at 74.5 eV.
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Fig. 1. NO decomposition on sulfided 4.4% Co 15.9% Mo/Al2 O3 . Feed gas: 10.1% NO/He; SV = 3000 ml/g-cat/h; (䊉): NO; (): N2 O; (×): SO2 ; dotted line: N2 (calculation).
2.3. TPD measurements Sample catalysts were in situ sulfided as mentioned above. First, a gas was introduced into the reactor by the pulse method at room temperature. The amount of saturation adsorption was determined with a GC-TCD. After the saturation adsorption, the catalyst was heated up at a rate of 10◦ C/min in 30 ml/min of He stream. Desorbed gases were analyzed with a QP-mass spectrometer.
Fig. 2. NO decomposition on sulfided 4.2% Fe 15.0% Mo/Al2 O3 . Feed gas: 10.1% NO/He; SV = 3000 ml/g-cat/h; (䊉): NO; (): N2 O; (×): SO2 ; dotted line: N2 (calculation).
3. Results and discussion 3.1. NO decomposition 3.1.1. Product distribution A fundamental reactivity of NO was examined by feeding a gas only containing 10.1% NO in helium prior to the study on multi-component reactions. Figs. 1–4 show the temperature dependence of product distributions for the CoMo, FeMo, Mo, and Co catalysts. Output of NO (XNO ) and yields of products (Yproduct ) were calculated from the following equations: XNO
[NO]out = [NO]in
(1)
Fig. 3. NO decomposition on sulfided 15.8% Mo/Al2 O3 . Feed gas: 10.1% NO/He; SV = 3000 ml/g-cat/h; (䊉): NO; (): N2 O; (×): SO2 ; dotted line: N2 (calculation).
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Fig. 4. NO decomposition on sulfided 14.3% Co/Al2 O3 . Feed gas: 10.1% NO/He; SV = 3000 ml/g-cat/h; (䊉): NO; (): N2 O; (×): SO2 ; dotted line: N2 (calculation).
YN2 O =
2[N2 O]out [NO]in
(2)
YSO2 =
2[SO2 ]out [NO]in
(3)
YN2 = 1 − XNO − YN2 O
(4)
The products observed were N2 O, N2 , and SO2 ; NO2 and O2 were not detected. Fig. 1 shows the temperature dependence of NO output, N2 O and SO2 production on the CoMo/Al2 O3 . Main products are N2 O at lower temperature, and N2 and SO2 at higher temperature. The reaction, however, is suddenly inhibited above 450◦ C, and the CoMo/Al2 O3 seems to lose its activity. The CoMo (Fig. 1) and the FeMo (Fig. 2) are similar in activity, but they show different activities from the Mo (Fig. 3) and the Co (Fig. 4) catalysts. While the Co/Al2 O3 shows the lowest activity, the reaction is similar in scheme to the others. It is noticed that all the catalysts lose their activities at higher temperature. It was observed that the colors of the catalysts after the NO decomposition were those of corresponding oxides. An XPS analysis also proved the oxidation of the catalysts (not shown). These results clearly indicate that the sulfide catalysts show high activity for the NO decomposition and lose their activity through the oxidation. Hence, it is interesting to elucidate the role of sulfur in the reactions including NO.
Fig. 5. TPD spectra of NO adsorbed on sulfided CoMo/Al2 O3 . A: NO; B: N2 O; C: N2 ; D: SO2 ; E: H2 S; F: H2 O.
3.1.2. TPD measurements Concerning to the role of sulfide, it is noticed that, on every catalyst, SO2 is formed without O2 production. Combining the characteristic formation of SO2 with the oxidation of the catalyst, it is indicated that the following oxidation of lattice sulfur occurs: Slattice + 2Oad = SO2
(5)
Oad will be produced by NO dissociation (6). We have investigated the reaction mechanism by measuring TPD spectra for the CoMo/Al2 O3 . Fig. 5 shows a TPD spectrum of NO adsorbed on the sulfided CoMo/Al2 O3 . NO is desorbed at temperatures between room temperature and 300◦ C. Above 350◦ C H2 O and H2 S are desorbed. Judging from simultaneous production of H2 O and H2 S, H2 S should be produced through the reaction between lattice sulfur and H2 O that comes from alumina. Vacant sites will induce the dissociation of NO, Eq. (6). Adsorbed NO and nitrogen adatoms will react to form N2 O, Eq. (7). Fig. 5 shows that some NO molecules remain adsorbed and the others are desorbed below 300◦ C; the coexistence of adsorbed NO and vacant sites, which induce the reaction (6) and yield nitrogen adatoms, will allow the production of N2 O at this low temperature range (Fig. 1). Thus, the N2 O production at lower temperature is in good agreement with the TPD results. The dissociation of NO and the resulting formation of oxygen adatoms can lead to the oxidation of sulfide catalysts, Eq. (5): MoS2 is oxidized at temperatures even below 30◦ C [10]. Thus, the reaction scheme can be written as follows
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At lower temperature NOad + { } = Nad + Oad
(6)
NOad + Nad = N2 O
(7)
At higher temperature 2NOad = N2 + 2Oad
(8)
where subscript ‘ad’ stands for the adsorbed species on the catalysts and { } for vacant sites. Iwamoto et al. [11] have pointed out that accumulation of oxygen on a catalyst lowers catalytic activity in NO decomposition on Cu–NaY, because oxygen is not evolved below 500◦ C. In the present NO reaction, it is assumed that oxygen is consumed by the reaction with lattice sulfur; therefore, the sulfided catalysts may show high activity as long as lattice sulfur exists. A drastic deactivation of the Co catalyst in Fig. 4 might be caused by a low content of sulfur in the catalyst. In order to prove such a role of sulfur, it is necessary to show that the sulfide catalysts have stable activity under the reaction conditions as long as lattice sulfur exists. At the same time, to clarify the characteristic activity of sulfide catalysts, the present work tries to (i) retard the oxidation of the catalysts by introducing CO and (ii) compensate sulfur loss by introducing SO2 in the feed gas. 3.2. NO–CO reaction As shown in Fig. 6, when an equivalent molar amount of CO was added in the inlet NO gas, the
Fig. 6. NO–CO reaction on sulfided CoMo/Al2 O3 . Feed gas: 5% NO/5% CO/He; SV = 4500 ml/g-cat/h; (䊉): NO; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
Fig. 7. NO–CO reaction on sulfided FeMo/Al2 O3 . Feed gas: 5% NO/5% CO/He; SV = 4500 ml/g-cat/h; (䊉): NO; (䊏): CO; (): N2 O; (+): COS; (×): SO2 ; dotted line: N2 (calculation).
situation changed greatly. That is, NO is consumed at lower temperature in the NO–CO reaction than in the NO decomposition reaction. It is also noticed that SO2 formation was not observed. CO output X and COS yield Y are defined by XCO =
[CO]out [NO]in
YCOS =
[COS]out [NO]in
(9) (10)
For all the catalysts in Figs. 6–9, N2 O is the main product at lower temperature. SO2 formation is completely suppressed and COS is formed instead, except
Fig. 8. NO–CO reaction on sulfided Mo/Al2 O3 . Feed gas: 5% NO/5% CO/He; SV = 4500 ml/g-cat/h; (䊉): NO; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
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probably correlated with the stronger metal–sulfur bonds in these catalysts [1]. Slattice + CO = COS
Fig. 9. NO–CO reaction on sulfided Co/Al2 O3 . Feed gas: 5% NO/5% CO/He; SV = 4500 ml/g-cat/h; (䊉): NO; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
for the FeMo/Al2 O3 , where a small amount of SO2 is still observed. The lack of SO2 indicates that oxygen adatoms predominantly react with CO to produce CO2 , Eq. (11). CO might instantaneously remove oxygen adatoms and reproduce vacant sites, thus promoting NO dissociation, Eq. (6). An overall reaction at lower temperature is written as reaction (12). The preceding NO consumption ahead of CO consumption as shown in Figs. 6–9 will be explained with Eq. (12); that is, 2 mol of NO is consumed by 1 mol of CO.
(13)
A small amount of SO2 was formed on the sulfided FeMo/Al2 O3 . Oxidation of the FeMo/Al2 O3 was confirmed by the XPS analysis for Mo (3d), Fe (2p3/2 ), and S (2p) of the catalyst after the reaction (Fig. 10(c)). The intensity of the Mo4+ (3d5/2 ) peak at 229 eV is drastically attenuated and Mo6+ at 233 eV becomes the main component among surface Mo species. A strong emission peak of SO4 2− appears at 169 eV in the S (2p) region [12]; SO2 , which is adsorbed initially on alumina basic sites [1] to form adsorbed sulfite species, should be converted to sulfate species in the oxidation [13]. By contrast, while the intensity of the Mo4+ peak of the CoMo/Al2 O3 is reduced after the reaction, it is still dominant (Fig. 10(a)). Thus, the sulfided FeMo/Al2 O3 is found to be more susceptible to oxidation than the CoMo/Al2 O3 . The reactivity of lattice sulfur to oxygen (yields SO2 ) and CO (yields COS) in the sulfided FeMo/Al2 O3 seems rather complicated, because COS further reacts with SO2 [1]. We presently have no idea about the competitive production of SO2 and COS. A stoichiometrically complete conversion of NO and CO towards N2 and CO2 seems to be achieved
At lower temperature NOad + { } = Nad + Oad
(6)
NOad + Nad = N2 O
(7)
Oad + COad = CO2
(11)
Overall reaction 2NOad + COad = N2 O + CO2
(12)
The production of COS indicates the reaction between CO and catalyst lattice sulfur Eq. (13), [1]. COS yield, however, is little in the NO–CO reaction compared with the SO2 –CO reaction: at most 10% on the CoMo (Fig. 6) and 16% on the FeMo (Fig. 7), and almost negligible for the Mo (Fig. 8) and the Co (Fig. 9), in contrast to 30–70% in the SO2 –CO reaction [1]. CO must be more reactive with oxygen adatoms Eq. (11) than lattice sulfur Eq. (13). The negligible COS formation over the Co and the Mo catalysts is
Fig. 10. XPS spectra after NO–CO reaction over (a) sulfided CoMo/Al2 O3 and (c) sulfided FeMo/Al2 O3 , and NO–SO2 –CO reaction over (b) sulfided CoMo/Al2 O3 and (d) sulfided FeMo/Al2 O3 .
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139
Reactant outputs and product yields are defined on the basis of the inlet concentration of SO2 . For the feed gases containing NO/SO2 /CO = 1/1/3 (Fig. 12(a)) and NO/SO2 /CO = 2/1/4 (Fig. 12(b)) both on the sulfided CoMo/Al2 O3 , and NO/SO2 /CO = 1/1/3 on the sulfided FeMo/Al2 O3 (Fig. 13), a lot of N2 O and COS is formed below 400◦ C. Extra SO2 , i.e. SO2 out –SO2 in , is also formed on the sulfided FeMo/Al2 O3 . The sole reaction below 250◦ C is the decomposition of NO into N2 O. SO2 begins to react at 250◦ C accompanying CO consumption and COS formation; this simultaneous behavior indicates that SO2 is consumed by the reaction with COS (Eq. (16)), [1]. Fig. 11. Deactivation of sulfided CoMo/Al2 O3 in NO–CO reaction. Feed gas: 5% NO/5% CO/He. Solid line: SV = 22000 ml/g-cat/h; broken line: SV = 9200 ml/g-cat/h.
SO2 + 2COS = 2CO2 + 1.5S2
(16)
at 350◦ C or higher in Figs. 6–9, but the catalysts lost their activities after a long operation with a high space velocity (SV) at 500◦ C (Fig. 11). It can therefore be said that the suppression of the catalyst oxidation by the addition of CO is temporarily effective, but is not enough to maintain activity of the sulfide catalysts for NO conversion. 3.3. NO–SO2 –CO reaction As mentioned above, it is noticed that the NO–CO reaction produces COS. Since COS reacts with SO2 into CO2 and elemental sulfur on the sulfide catalysts [1], we can expect the simultaneous catalytic reduction of NO and SO2 by adding SO2 into the NO–CO mixture. Furthermore, we can expect that the lattice sulfur abstracted as COS will be supplied with elemental sulfur produced in the COS–SO2 reaction. 3.3.1. Product distribution A feed gas with the NO/SO2 molar ratio of 1/1 (2.0% NO/2.0% SO2 /6.0% CO in He) or 2/1 (2.5% NO/1.2% SO2 /5.0% CO in He) was fed over the CoMo and the FeMo catalysts. The following stoichiometric conversions (Eqs. (14) and (15)) are expected: NO + SO2 + 3CO = 0.5N2 + 0.5S2 + 3CO2
(14)
2NO + SO2 + 4CO = N2 + 0.5S2 + 4CO2
(15)
Fig. 12. (a) NO–SO2 –CO reaction on sulfided CoMo/Al2 O3 for NO/SO2 /CO = 1/1/3. Feed gas: 2% NO/2% SO2 /6% CO/He. SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation). (b) NO–SO2 –CO reaction on sulfided CoMo/Al2 O3 for NO/SO2 /CO = 2/1/4. Feed gas: 2.5% NO/1.2% SO2 /5% CO/He. SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
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S.-X. Zhuang et al. / Applied Catalysis B: Environmental 31 (2001) 133–143 Table 1 Amount of gases adsorbed on sulfided catalysts and supports Catalyst
Feed gas
Uptake (ml/g-cat) NO
Sulfided CoMo/␥-Al2 O3
Sulfided FeMo/␥-Al2 O3
Fig. 13. NO–SO2 –CO reaction on sulfided FeMo/Al2 O3 for NO/SO2 /CO = 1/1/3. Feed gas: 2% NO/2% SO2 /6% CO/He. SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (䊏): CO; (): N2 O; (+): COS; dotted line: N2 (calculation).
Thus the NO–SO2 –CO reaction can be explained by the combination of the characteristics of two fundamental reactions NO–CO and SO2 –CO: the dissociative adsorption of NO and the formation of the COS intermediate. This is because, NO, SO2 and CO little affect the adsorption one another as shown in a TPD study; a TPD spectra in Fig. 14 and the amount of adsorption in Table 1 show that each component gas of a NO/SO2 /CO mixture (a molar ratio of 1/1/1) is adsorbed on the sulfided CoMo/Al2 O3 at almost the same amount as the adsorption amount in each single-component gas stream. A stoichiometric catalytic reduction to N2 and elemental sulfur is achieved on the sulfided CoMo/Al2 O3 at a temperature of 400◦ C, about 100◦ C higher than
Fig. 14. TPD spectra of NO/SO2 /CO = 1/1/1 over sulfided CoMo/Al2 O3 . A: NO; B: CO2 + N2 O; C: CO + N2 ; D: SO2 ; E: H2 S; F: H2 O.
␥-Al2 O3 TiO2 SiO2
NO 6.5 SO2 CO NO/SO2 /CO ∼7.6a = 1/1/1 NO 9.7 SO2 CO NO/SO2 /CO ∼6.7a = 1/1/1 SO2 SO2 SO2
SO2
CO
6.4 5.9
0.3 ∼0a
5.0 3.6
0.2 ∼0a
13.4 1.8 0.03
a Although NO and CO have almost the same retention time and were not separable, CO adsorption was negligible in the mass spectroscopic study as shown in Fig. 14.
that in the SO2 –CO reaction [1]; the temperature range of COS formation shifts about 100◦ C higher. In Fig. 12(a) and (b), almost the same profiles of NO and SO2 outputs were obtained when the NO/SO2 ratio was increased from 1/1 to 2/1, while the N2 O and the N2 yield curves were expanded to higher temperature. This should be responsible for the increase in the NO/CO ratio from 1/3 to 1/2. Some adjustments of gas composition etc. are imperatively needed to get optimum reaction conditions. Fig. 13 shows that an extra amount of SO2 is produced on the sulfided FeMo/Al2 O3 in the NO–SO2 –CO reaction as in the NO–CO reaction. The increase in the NO/CO ratio from 1/3 to 1/2 results in a larger amount of extra SO2 (not shown). Since the extra production of SO2 was also observed for the NO–SO2 –CO reaction on a sulfided 4% Fe/Al2 O3 , iron-based sulfide catalysts must be oxidized by NO even in the presence of SO2 . 3.3.2. In situ regeneration The gas containing NO, SO2 , and CO was continuously fed to prove that the reaction is stable and catalytic at 500◦ C at SVs of 9000 and 36,000 ml gas/g-cat/h. In Fig. 15, reaction temperature and time are changed while feeding the gas on the sulfided CoMo/Al2 O3 . The catalyst temporarily loses its activity at 350◦ C, where COS is predominantly produced
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Fig. 15. In situ regeneration of sulfided CoMo/Al2 O3 for NO/SO2 /CO = 1/1/3. Feed gas: 2% NO/2% SO2 /6% CO/He SV = 36, 000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (+): COS.
(see Fig. 12(a)) and lattice sulfur is depleted. The resulting oxidized CoMo/Al2 O3 regains its activity at 500◦ C by in situ sulfiding with elemental sulfur produced by the COS–SO2 reaction. By contrast, in the same experiment on the sulfided FeMo/Al2 O3 , it catalytically works only at the lower SV (not shown) and was deactivated at the higher SV (Fig. 16). The results show that, the in situ regeneration of the catalyst much depends on the property of lattice sulfur. It is well known that iron has a less promoting effect than cobalt on Mo catalysts in the HDS reaction. As compared with CoMoS and NiMoS, MoS2 and FeMoS have higher sulfur binding energies corresponding to removal of the first S atom from S
Fig. 16. In situ regeneration of sulfided FeMo/Al2 O3 for NO/SO2 /CO = 1/1/3. Feed gas: 2% NO/2% SO2 /6% CO/He; SV = 36, 000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (+): COS.
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terminated edges (47 kJ/mol for MoS2 , 33 kJ/mol for FeMoS, and −34 kJ/mol for CoMoS) [14]. Sulfur atoms in the sulfided CoMo/Al2 O3 should be accordingly easy to dissociate and to recombine owing to the moderate CoMo–S bond strength; this is not the case with the sulfided FeMo/Al2 O3 whose strong FeMo–S bond prevents the regeneration. The XPS measurements were again performed for the sulfided CoMo and FeMo catalysts after the reaction (SV = 9000, Fig. 10(b) and (d)). Compared with the spectra obtained after the NO–CO reaction shown in Fig. 10(a) and (c), the intensity of Mo4+ (3d5/2 ) peak located at 229 eV increases, accompanied by attenuation of the SO4 2− peak; the oxidation of the sulfided FeMo/Al2 O3 is suppressed in the presence of SO2 . The sulfided CoMo/Al2 O3 catalyst has a spectrum quite similar to that obtained after the SO2 –CO reaction (see [1]). The spectrum shows that the sulfided CoMo/Al2 O3 is in situ regenerated after the reaction and still has high activity. It was found that the rejuvenation of the catalysts in the NO–SO2 –CO reaction presents a great contrast to the deactivation of the catalysts in the NO–CO reaction; lattice sulfur is supplied by the reaction of SO2 and COS in the NO–SO2 –CO system, while it is only consumed in the NO–CO system. 3.3.3. Support effects Finally, we have investigated support effects, in particular the role of alumina, using several types of supports. Khalafalla et al. [15] have proposed a dual reaction site mechanism for the SO2 –CO reaction on Fe/Al2 O3 : iron and alumina augment their specific sites activity at their interparticle contacts. Dalla Lana and coworkers [16] have indicated that COS on aluminum ion sites and SO2 on alumina hydroxyl sites react to form CO2 and sulfur. In the present reaction, COS needs to migrate from sulfided CoMo to aluminum ion sites. To confirm the mechanism, alumina, silica, and titania-supported sulfided CoMo catalysts were subjected to the NO–SO2 –CO reaction (NO/SO2 /CO = 1/1/3) (Fig. 17(a), (b)). The loadings of CoMo on silica and titania were adjusted to the same on alumina on the basis of unit surface area: that is, CoMo/Al2 O3 (CoO 4.4 mass%, MoO3 15.9 mass%), CoMo/SiO2 (CoO 4.4 mass%, MoO3 15.0 mass%), and CoMo/TiO2 (CoO 1.1 mass%, MoO3 3.6 mass%).
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sible routes of consuming SO2 on the CoMo/SiO2 and the CoMo/TiO2 . Although further investigations are required to clarify the reaction mechanism, the bifunctionality of the sulfided CoMo/Al2 O3 seems to be reasonable in the present study.
4. Conclusions
Fig. 17. (a) Support effects in NO–SO2 –CO reaction on sulfided CoMo: alumina (solid lines) vs. silica (broken lines). Feed gas: 1.7% NO/1.7% SO2 /5% CO/He; SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (+): COS. (b) Support effects in NO–SO2 –CO reaction on sulfided CoMo: alumina (solid lines) vs. titania (broken lines). Feed gas: 1.7% NO/1.7% SO2 /5% CO/He; SV = 9000 ml/g-cat/h; (䊉): NO; (䉱): SO2 ; (+): COS.
In Fig. 17(a), the reaction is retarded on the sulfided CoMo/SiO2 . In Fig. 17(b), the sulfided CoMo/TiO2 shows a comparable activity to the sulfided CoMo/Al2 O3 , but yields a considerable amount of COS at higher temperature. Titania adsorbs less amount of SO2 than alumina (Table 1), so that the inferior adsorptivity should suppress the SO2 –COS reaction, resulting in the large amount of COS production. In spite of the low adsorptivity of SO2 on silica and titania, however, SO2 is almost entirely removed from the inlet gas. Since the SO2 is supposed to be consumed by the stoichiometric reaction of SO2 –COS Eq. (16), this complete reduction of SO2 and the residual COS formation indicate a deviation from the above stoichiometric reaction mechanism Eqs. (14) and (15), that is, the presence of other pos-
NO was decomposed into N2 O and N2 with the formation of SO2 on ␥-alumina-supported sulfide catalysts (CoMo, FeMo, Mo and Co). The catalytic activity for the NO decomposition was lost abruptly at higher temperature with serious oxidation of the catalysts. The addition of CO to the NO stream clearly lowered the reaction temperature and suppressed SO2 formation, yielding COS instead, except for the FeMo catalyst, where a small amount of SO2 was still formed. The presence of NO drastically inhibited the COS formation; the amount of COS formed in the NO–CO reaction was considerably less than that in the SO2 –CO reaction in our previous report. A stoichiometric conversion of NO and CO to N2 and CO2 occurred above 350◦ C on the sulfided CoMo/Al2 O3 and the sulfided FeMo/Al2 O3 . The addition of CO to the NO stream lengthened catalyst life, but is not enough to maintain activity. After the NO–CO reaction, an XPS analysis showed the growth of Mo6+ and SO4 2− peaks, especially for the FeMo/Al2 O3 ; the FeMo catalyst underwent strong oxidation in the NO–CO reaction. The addition of SO2 in the NO–CO system enabled in situ regeneration of the sulfided catalysts that were oxidized through the formation of SO2 or COS. Stoichiometric catalytic reduction toward N2 and elemental sulfur was achieved at 400◦ C on the sulfided CoMo/Al2 O3 and the sulfided FeMo/Al2 O3 . The XPS analysis for the CoMo and the FeMo catalysts after the NO–SO2 –CO reaction revealed that all the spectra of Mo (3d) and S (2p) were similar to those observed after SO2 –CO reaction: Mo4+ was dominant, while the SO4 2− peak was small. The results indicated that the partially oxidized catalysts by NO at lower temperature experienced anew reduction and sulfurization through the SO2 –CO reaction at higher temperature in the NO–SO2 –CO reaction. The reaction proceeded catalytically at 500◦ C on the sulfided CoMo/Al2 O3 at the space velocity between 9000 and 36,000 ml/g-cat/h. The in situ rejuvenation
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of the CoMo catalyst was explained by the moderate strength of the CoMo–S bond; that is, sulfur atoms are easily dissociated from the moderate CoMo–S bond, and also easily bound to vacant sites to regenerate the oxidized CoMo catalyst. This was not the case with the sulfided FeMo/Al2 O3 : the catalyst remained inactive at higher temperature when the space velocity was increased to 36000 ml/g-cat/h. The NO–SO2 –CO reaction was retarded on the sulfided CoMo/SiO2 . The sulfided CoMo/TiO2 showed a similar reactivity to the sulfided CoMo/Al2 O3 except yielding a considerable amount of COS at high temperature. ␥-Al2 O3 seemed to be an optimum support under the test conditions. Acknowledgements The present work was supported partly by the Center for Interdisciplinary Research of Tohoku University and partly by ‘Research for the Future Program’ of Japan Society for the Promotion of Science (JSPS). One of the authors (S.-X. Zhuang) is grateful to Tohoku University for the financial support during his research stay of 6 months in Sendai.
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