Sulphur poisoning of LaMn1−xMgxO3 catalysts for natural gas combustion

Applied Catalysis B: Environmental 30 (2001) 61–73

Sulphur poisoning of LaMn1−x Mgx O3 catalysts for natural gas combustion Ilaria Rosso∗ , Edoardo Garrone, Francesco Geobaldo, Barbara Onida, Guido Saracco, Vito Specchia Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129 Turin, Italy Received 1 April 2000; received in revised form 7 August 2000; accepted 7 August 2000

Abstract Ageing treatments were carried out on LaMn1−x Mgx O3+δ (x = 0, 0.2, 0.5) catalysts for methane combustion to throw light on their deactivation mechanism in the presence of tetrahydrothiophene — used in Italy as odorant of natural gas — or directly in the presence of SO2 . Various characterisation techniques were used in concert: XRD, BET, TEM, SEM-EDS, FTIR, TPD, chemical analysis and atomic absorption. The effects of treatment temperature and exposure on catalytic activity were evaluated: exposure to 200 ppmv SO2 at 800◦ C for 24 h causes a rather drastic deactivation of each LaMn1−x Mgx O3 catalyst. Metal sulphate formation is the major responsible for deactivation. However, the extent of poisoning as well as the catalyst regenerability depends on the catalyst composition. The LaMn0.5 Mg0.5 O3 catalyst can in fact be regenerated completely by water leaching, while LaMn0.8 Mg0.2 O3 and LaMnO3 perovskites can be regenerated significantly only by leaching with aqueous NH3 solutions. An explanation of these results, also based on the values of activation energy for each fresh, poisoned and regenerated catalysts, is suggested and a mechanism for catalyst regeneration by water and NH4 OH solution is proposed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: LaMn1−x Mgx O3+δ ; Perovskite; Methane catalytic combustion; Sulphur poisoning

1. Introduction Catalytic combustion for energy production purposes has received considerable attention during the past decades. Research efforts have been promoted by the need to meet more stringent governmental demands concerning pollution prevention and the wish to use energy sources more efficiently. The two main advantages offered by catalytic combustion, as opposed to conventional thermal combustion, consist of a wider range of air-to-fuel ratio and lower flame ∗ Corresponding author. Tel.: +39-11-5644710; fax: +39-11-5644699. E-mail address: [email protected] (I. Rosso).

temperatures, which enable easier controllability and less NOx emissions, respectively. Applications of natural gas catalytic combustion can be classified according to operating conditions: (i) lean-burn premixed adiabatic combustion (catalytic burner for gas turbines [1,2]); (ii) premixed nonadiabatic combustion (radiant burners for industrial or domestic applications [3]); (iii) non-adiabatic diffusive-type combustion (catalytic heaters [4]). Catalytic heaters have been on sale since the early 1970s: catalyst requirements are not severe and their application is limited because of their low thermal output. In contrast the development of catalytic burners for applications (i) and (ii) is still in progress as their commercial success depends on the following strict

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 2 2 - 8

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requirements on the catalyst materials: (i) high catalytic activity: low ignition temperature; (ii) long-term thermal stability: tolerance to high operating temperatures, ranging from 800 to 1300◦ C, without sintering or volatilisation. Other important properties for the catalysts are good resistance to poisoning agents and low cost. Perovskite-type oxides represent a promising alternative to noble metal catalysts because of their low cost, thermo-chemical stability at comparatively high temperatures (900–1100◦ C) and catalytic activity [5–8]. These oxides are characterised by an ABO3 stoichiometry, where A is generally a rare-earth-metal cation in the +3 oxidation state, whereas B is a transition-metal in the same valence state. The number of different perovskites available is very large owing to the different A and B metals that are able to enter the perovskite structure; moreover A and/or B cations may be the partially substituted by other cations. The A and/or B replacement with non-equivalent ions may cause the stabilisation of an unusual valence state or mixed valence state of transition metal ions in the B site or, lastly, the variation of lattice defects density. These changes in the electronic properties will in turn modify the catalytic performance of the resulting perovskite [9–11]. Previous studies on LaCr1−x Mgx O3 [12] and on LaMn1−x Mgx O3 [13] perovskites (x = 0–0.5) showed a promoting effect of Cr/Mg and Mn/Mg substitution on the catalytic activity of the resulting material. Because of Cr/Mg substitution catalytic activity of LaCr1−x Mgx O3 perovskites increased linearly up to x = 0.5, whereas that of LaMn1−x Mgx O3 perovskites increased up to x = 0.2, then diminished and rose again up to x = 0.5. On the other hand, perovskites suffer from some restrictions that limit their applicability in industrial processes: a rather low specific surface area (hardly exceeding 20 m2 g−1 above 900◦ C) and a remarkable susceptibility to poisoning by sulphur dioxide. Originally proposed as potential automotive catalysts [14], the early perovskites compositions, mostly manganates and cobaltates, have failed to become practical because of their low resistance to sulphur, often present in relatively high concentrations in combustion products of automotive fuels. During the last decade the interest in perovskites catalysts has been to some extent revived for application in catalytic

natural gas combustion, where the levels of sulphur compounds, used as odorants, are comparatively low (<10 ppm). However, only few new data on the deactivation of the catalysts by sulphur-bearing compounds have recently appeared in the literature [15], if not once again related to automotive catalysts [16]. A good knowledge of the resistance of the promising perovskite catalysts (as LaMn1−x Mgx O3 ) to deactivation by sulphur-bearing compounds and of the possible regeneration techniques is necessary to finally assess their potential in the natural gas combustion field. The present report describes the effect on catalytic activity of LaMnO3 , LaMn0.8 Mg0.2 O3 and LaMn0.5 Mg0.5 O3 catalysts (chosen for their high catalytic activity [13]) by ageing in the presence of tetrahydrothiophene (THT) — used in Italy as odorant of natural gas — or directly under SO2 -rich atmospheres. The sulphur poisoning mechanism is studied using several analytical techniques and a simple and rapid regeneration method is proposed. 2. Experimental 2.1. Catalyst preparation A series of perovskite catalysts of the system LaMn1−x Mgx O3 (x = 0, 0.2, 0.5) was prepared via a modified version of the so called “citrates method” described in [17]. Solid mixtures of La(NO3 )3 ·6H2 O, Mn(NO3 )3 ·6H2 O and Mg(NO3 )3 ·6H2 O (from Fluka), dosed in stochiometric ratio, were mixed to a 40 wt.% amount of glycerine and to a 40 wt.% amount of water. The mixture was slowly heated up to 120◦ C until a slight NOx emission started, then rapidly poured into a stainless steel vessel and kept in an oven at 180◦ C for 30 min. Under such conditions NOx , CO2 and water vapour form in huge amounts thus causing the formation of a solid scum, quite friable and porous. This operating procedure was the result of several preliminary tests, since the specific surface area and the catalytic activity of LaMnO3 strongly depend on the preparation method (e.g. glycerine and water amount). The quantity of catalyst obtained per preparation cycle was also found to represent a critical factor for LaMnO3 catalytic performance. An optimum catalyst amount of 3 g was prepared each time. Each catalyst was then finely ground in an agate mortar

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and calcined in an electric oven at 900◦ C for 8 h in calm air. 2.2. Catalyst ageing tests Samples of LaMn1−x Mgx O3 powdered catalyst were treated at 400, 600 and 800◦ C for 24 h in a controlled-atmosphere oven. A flow of air with 200 ppmv of THT or SO2 was forced to pass over 1 g of catalyst located in a porcelain combustion boat at a rate of 20 Ncm3 min−1 . Further times of contact were 8, 16, 32, 48 h at 400◦ C and 2, 8, 16, 32, 48 h at 800◦ C, so as to study their poisoning evolution. Additional ageing experiments in air flowing at the same rate in the absence of sulphur compounds at 800◦ C for 24 h were performed on all the LaMn1−x Mgx O3 perovskites to evaluate their thermal ageing effects. The high THT/SO2 level (several times higher than that usually added to commercial natural gas: 8 ppmv) was chosen in order to accelerate the poisoning effect. Fractions of the THT/SO2 treated catalysts were used for characterisation, catalytic activity and regeneration studies. 2.3. Fresh/aged catalyst characterisation XRD analyses (Philips PW1710 apparatus equipped with a monochromator for the Cu K␣ radiation) were performed on all fresh catalysts (to check complete crystallisation of perovskites) and on the aged ones (to check the possible appearance of new phases). The specific surface area, as determined by the BET method using N2 , was measured on all the fresh perovskites (Micromeritics ASAP 2010 M apparatus). BET measurements were also performed on some selected aged perovskites in order to verify the influence of the THT/SO2 poisoning or thermal treatment on the specific surface area. Fresh and aged catalysts were examined by transmission electron microscopy (TEM, Philips EM 400 apparatus) to draw a better definition of the micro-structure of the different powdered perovskites. Scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) (Philips, 515 SEM equipped with EDAX 9900 EDS) were used to investigate the morphology as well as the elemental composition and distribution of all the catalysts. Since the morphological analysis needs gold metallisation

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of the sample, EDS and SEM measurements were not possible at the same time because the X-ray emission of gold overlaps those of other elements (e.g. sulphur). Temperature programmed oxygen desorption (TPD) tests up to 1050◦ C in helium flow (10N cm3 min−1 ) were performed on the fresh LaMn1−x Mgx O3 catalysts by a TPD/R/O apparatus (Thermoquest TPD/R/O 1100 analyser, equipped with a Baltzer Quadstar 422 quadrupole mass spectrometer). Determination of oxygen excess (δ) with respect to stoichiometry of fresh LaMn1−x Mgx O3+δ catalysts was performed by return titration of 0.1N FeSO4 solution with 0.1N KMnO4 solution. Infrared spectroscopy analyses (Bruker, Equinox 55 FTIR, equipped with MCT criodetector) were recorded in the diffuse reflectance mode on fresh, aged and regenerated LaMn1−x Mgx O3 catalyst powders so as to investigate possible changes at their surface. 2.4. Catalytic activity screening tests Catalytic activity tests were performed on fresh, aged and regenerated catalysts in the experimental apparatus and according to the procedures described in detail in [12]. A fixed-bed of 0.5 g of catalyst particles (obtained by pressing the perovskite powders into tablets, which were then crushed and sieved to obtain 0.2–0.5 mm granules) was enclosed in a quartz tube (i.d.: 4 mm) and sandwiched between two quartz-wool layers. The reactor was placed in a PID-regulated oven and a thermocouple was inserted in the packed bed. After 30 min stay at 800◦ C in air flow (the common pre-treatment), a gas flow rate of 50N cm3 min−1 (composition: CH4 = 2%, O2 = 18%, He = balance) was fed to the reactor. The reactor temperature was then lowered at a 3◦ C min−1 rate down to 300◦ C, meanwhile the methane conversion was monitored by analysing the outlet CO2 concentration (the only carbon oxidation product) through an IR analyser (Hartmann & Braun URAS 10E). Typical sigma-shaped curves were obtained for methane conversion versus temperature. Twin runs were performed on two different samples of the same catalytic material and the results were then averaged. The deviation between the conversion measured at the same temperature in the twin runs was always less than 10%. Not significant hysteresis was observed in these curves performing the catalytic tests

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upward (from 300 to 800◦ C). The half-conversion temperature (T50 ) was generally regarded as an index of the catalytic activity. In some cases the activation energy of the oxidation process was calculated. The estimation of the activation energy Er of each fresh, poisoned and regenerated LaMn1−x Mgx O3 perovskite can be derived by interpolation of the initial data of the methane conversion curves by assuming a first-order kinetic expression towards methane partial pressure and a plug-flow pattern for the fixed bed reactor. Only the initial conversion levels (≤30%) were used in this context so as to avoid significant mass/heat transfer resistance effects.

of water at room temperature in a stirred beaker; then, the powder was filtered on a paper filter and dried in an oven at 120◦ C. Repeated poisoning/regeneration cycles were performed on some selected samples. As an alternative leaching medium, 60 cm3 of a 2.5% NH3 aqueous solution were used; the leachate was then neutralised by a diluted HCl solution. Cations and soluble sulphur were detected in the both water and NH3 leaching media by chemical analysis and atomic absorption (Perkin-Elmer 1100B atomic absorption spectrometer).

3. Results 2.5. Regeneration procedures 3.1. Characterisation Several attempts to regenerate the poisoned catalysts by simple heating at very high temperature in inert or reducing atmosphere were made. Temperature programmed desorption tests up to 1100◦ C in helium flow (20N cm3 min−1 ) and temperature programmed reduction tests up to 1100◦ C in hydrogen flow (20N cm3 min−1 ) were performed on 0.5 g of every poisoned LaMn1−x Mgx O3 catalyst. Moreover, the removal of sulphur compounds formed on LaMn1−x Mgx O3 catalysts was performed by water leaching: 1 g of each material was washed by 60 cm3

The diffraction patterns of the prepared LaMn1−x Mgx O3 catalysts after 900◦ C calcination are shown in Fig. 1. The typical ABO3 structure was formed in all cases: well-defined rhombohedral diffraction lines (JCPDS card 45-0946) are present either in the spectrum of LaMnO3 (curve a) or LaMn0.8 Mg0.2 O3 (curve b) or LaMn0.5 Mg0.5 O3 (curve c); i.e. the Mn/Mg substitution does not compromise the rhombohedral structure [13]. No other lines are observed. Identical diffraction patterns were obtained for all

Fig. 1. X-ray diffraction patterns of the following catalysts (calcination temperature = 900◦ C): (a) LaMnO3 ; (b) LaMn0.8 Mg0.2 O3 ; (c) LaMn0.5 Mg0.5 O3 .

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Fig. 2. TEM micrographs of LaMnO3 catalyst: (a) fresh (220 000×); (b) aged at 800◦ C for 24 h in air (220 000×); (c) aged at 800◦ C for 24 h in 200 ppmv SO2 atmosphere (170 000×).

LaMn1−x Mgx O3 samples after THT/SO2 ageing at any time and temperature of exposure and after thermal treatment. Fig. 2 concerns TEM micrographs of the LaMnO3 catalyst, as prepared (Fig. 2a), aged at 800◦ C for 24 h in air (Fig. 2b) and aged at 800◦ C for 24 h in SO2 atmosphere (Fig. 2c). Shape, distribution and size of crystals do not vary significantly after either thermal ageing or SO2 treatment: some sintering is evident on all the samples and crystals always have a variable size distribution centred at a range of 100–120 nm. These results are in agreement with BET data. The fresh LaMnO3 catalyst has a specific surface area of 4.6 m2 g−1 ; after thermal ageing (24 h at 800◦ C in air) its surface area remains practically the same (4.5 m2 g−1 ) and after SO2 exposure (24 h at 800◦ C) it is only somewhat lower: 3.8 m2 g−1 . The other fresh LaMn1−x Mgx O3 catalysts have a specific surface area ranging from 5 to 7 m2 g−1 . Fig. 3 shows SEM micrographs of aged LaMn1−x Mgx O3 catalysts: thermally aged LaMnO3 (Fig. 3a) and LaMn0.5 Mg0.5 O3 aged at 800◦ C for 24 h in SO2 atmosphere (Fig. 3b). As far as the morphology is concerned, poor homogeneity was noticed in every fresh or aged sample: globular, foam like or smooth surfaces appear without precise correlation to the catalyst com-

position or the ageing treatment. LaMn0.5 Mg0.5 O3 catalyst (Fig. 3b) seems to present a lighter superficial phase which could be confirmed by TEM micrographs shown in Fig. 4. LaMn0.5 Mg0.5 O3 powder in fact presents either large well-defined perovskite crystals (Fig. 4c) or small less-defined crystals (Fig. 4a); moreover the crystal aggregate shown in Fig. 4b reminds TEM micrographs of LaMn1−x Mgx O3 + 17MgO catalysts reported in a previous paper [13]. However, XRD spectra show only one phase (the rhombohedral perovskite structure) probably because the amount of the light superficial phase is too small to be detected by XRD analysis. EDS analysis on LaMn0.5 Mg0.5 O3 catalyst showed an amount of magnesium and oxygen in the light superficial phase higher than that in the dense bulk one. Amounts of desorbed oxygen, resulted by TPD tests on the fresh LaMn1−x Mgx O3 catalysts, are: 263 ␮mol g−1 for LaMnO3 catalyst, 166 ␮mol g−1 for LaMn0.8 Mg0.2 O3 catalyst and 23 ␮mol g−1 for LaMn0.5 Mg0.5 O3 catalyst. These data correspond to an oxygen excess (δ) in LaMn1−x Mgx O3+δ catalysts of 0.13, 0.076 and 0.011 (for LaMnO3 , LaMn0.8 Mg0.2 O3 and LaMn0.5 Mg0.5 O3 catalysts, respectively) that are in perfect agreement with the results of oxygen excess titration: δ = 0.14, 0.075

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Fig. 3. SEM micrographs of LaMn1−x Mgx O3 catalysts: (a) thermal aged LaMnO3 (20 000×); (b) aged at 800◦ C for 24 h in 200 ppmv SO2 atmosphere LaMn0.5 Mg0.5 O3 (2500×).

and 0.01 (for the same catalysts, respectively) and with literature data [17–19]. 3.2. Effect of THT/SO2 exposure on catalyst properties The effect of THT/SO2 exposure and of prolonged treatment in air at 800◦ C on sample catalytic activity is documented by the half-conversion temperatures of fresh and aged LaMn1−x Mgx O3 catalysts listed in

Table 1. The thermal ageing at 800◦ C for 24 h in air has no significant influence on the catalytic activity. This is not surprising since the catalysts underwent a calcination treatment of 8 h at 900◦ C. On the average, the catalytic activity of all LaMn1−x Mgx O3 perovskites is not reduced significantly after 24 h of 200 ppmv sulphur exposition at 400 and 600◦ C, whereas T50 rises drastically at 800◦ C. Table 2 lists the T50 values obtained on catalysts poisoned under SO2 flow for progressively longer

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Fig. 4. TEM micrographs of LaMn0.5 Mg0.5 O3 catalyst (280 000×).

Table 1 Methane half-conversion temperatures (T50 (◦ C)) of fresh and aged perovskitesa

Fresh 800◦ C in air Ageing temperature 400◦ C 600◦ C 800◦ C a

LaMnO3

LaMn0.8 Mg0.2 O3

LaMn0.5 Mg0.5 O3

537 567

520 546

455 460

SO2

THT

SO2

THT

SO2

THT

564 583 635

574 532 636

615 541 630

537 547 642

472 485 645

511 462 574

Treatment time: 24 h; SO2 /THT concentration: 200 ppmv.

time exposure periods at 400 and 800◦ C. The catalytic activity during ageing at 400◦ C remains the same for about 24 h, then it decreases significantly. At 800◦ C, the deactivation becomes fully appreciable already after 8 h and reaches a constant level after about 24 h. The FTIR spectra obtained for the fresh and aged LaMn0.5 Mg0.5 O3 catalyst after exposition to 200 ppmv SO2 at 800◦ C for 24 h are shown in Fig. 5. This catalyst was the only one to show by FTIR analysis clear evidence of surface modification after SO2 exposure. Curve b is characterised by a broad band centred around 1110 cm−1 with shoulders at 1066 and 1155 cm−1 assigned to S–O stretching and two weaker bands at 1380 and 1480 cm−1 assigned

to S=O stretching [20], confirming literature data reported on magnesium oxide and its interaction with SO2 [21]; the weak band at 1620 cm−1 is due to water vibrations, as FTIR spectra were registered in air. Table 2 Methane half conversion temperatures (T50 (◦ C)) of selected perovskites aged in 200 ppmv SO2 atmosphere for various exposure time and two different temperature Ageing time (h)

Fresh catalyst

LaMn0.5 Mg0.5 O3 455 at 400◦ C LaMn0.8 Mg0.2 O3 520 at 800◦ C

2

8

16

24

32

48

–

464

462

472

658

670

584

625

630

630

640

653

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Fig. 5. Reflectance IR spectra of LaMn0.5 Mg0.5 O3 catalyst in S–O and S=O stretching region: (a) fresh; (b) aged at 800◦ C for 24 h in 200 ppmv SO2 atmosphere.

3.3. Catalyst regeneration Temperature programmed desorption or reduction up to 1100◦ C on every aged LaMn1−x Mgx O3 compound did not cause catalyst regeneration. Moreover, no ion fragments in the gas phase attributable to sulphur compounds were detected by mass spectrometry. If poisoning by SO2 brings about sulphates formation, corresponding metal sulphates could be soluble in water. Therefore, each aged catalyst was water washed [22]. Fig. 6 shows the results of the catalytic activity tests towards methane combustion performed with LaMnO3 (Fig. 6a), LaMn0.8 Mg0.2 O3 (Fig. 6b) and LaMn0.5 Mg0.5 O3 (Fig. 6c) catalysts in fresh state, after exposition to 200 ppmv SO2 at 800◦ C for 24 h and after water leaching. Water leaching is practically uneffective on LaMnO3 and on LaMn0.8 Mg0.2 O3 catalysts, while the original activity of the LaMn0.5 Mg0.5 O3 catalyst was restored. Successive poisoning/regeneration cycles with 200 ppmv SO2 at 800◦ C for 24 h and with water at room temperature, respectively, were performed on the same LaMn0.5 Mg0.5 O3 catalyst; the original activity was completely restored after the first and the

second poisoning cycle, only partially restored after the third and practically not restored after the fourth cycle (Fig. 7). The estimated Er values of each fresh, poisoned and regenerated LaMn1−x Mgx O3 perovskite are presented in Table 5. Whereas the activation energy of fresh LaMnO3 and LaMn0.8 Mg0.2 O3 catalysts varies significantly during the poisoning and regeneration cycle, the activation energy of LaMn0.5 Mg0.5 O3 catalyst fresh, poisoned and regenerated remains almost unaffected. Moreover, the estimated Er values of LaMn0.5 Mg0.5 O3 catalyst do not vary significantly also during the successive poisoning/regeneration cycles, as shown in Fig. 9. Infrared spectrum of the regenerated LaMn0.5 Mg0.5 O3 catalyst shows the removal of S–O and S=O stretching bands and it coincides with the spectrum of fresh LaMn0.5 Mg0.5 O3 catalyst (Fig. 5, curve a); no variations compared with fresh and poisoned catalyst spectra are detected by FTIR analysis on other washed LaMn1−x Mgx O3 perovskites. Finally, chemical analysis with atomic absorption detected S and Mg elements in all water leachates of successive poisoning cycles (Table 3).

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Table 4 Atomic absorption data on the leachate obtained on the poisoned LaMn1−x Mgx O3 catalysts by aqueous NH3 leaching Sample

mg S g−1 catalyst

mg Mg g−1 catalyst

LaMnO3 LaMn0.8 Mg0.2 O3 LaMn0.5 Mg0.5 O3

1.9 1.1 6.4

– 0.04 0.125

effective on all LaMn1−x Mgx O3 catalysts. Lastly, atomic absorption data on the neutralised leachates are listed in Table 4.

4. Discussion

Fig. 6. Results of the catalytic activity test towards methane combustion of fresh, poisoned and water washed LaMn1−x Mgx O3 catalysts.

T50 values obtained on LaMn1−x Mgx O3 catalysts regenerated by aqueous NH3 leaching after poisoning by 200 ppmv SO2 at 800◦ C for 24 h are drawn in Fig. 8, showing that aqueous NH3 leaching is Table 3 Atomic absorption data on the water leachate obtained on successive poisoning/regeneration cycles on the LaMn0.5 Mg0.5 O3 catalyst Poisoning/regeneration cycle

mg S g−1 catalyst

mg Mg g−1 catalyst

First Second Third Fourth

3.7 3.6 2.9 1.4

0.50 0.50 0.33 0.53

Sulphur poisoning of the LaMn1−x Mgx O3 catalysts brings about neither any consistent structural modifications on the samples, as indicated by XRD results, nor marked variation of micro-structure and specific surface area of the catalysts, as shown by TEM and BET results. Table 1 shows clearly that the effects of THT or SO2 exposure on the catalytic activity are comparable, as the sulphur in THT, just like as other natural gas odorant [15], is likely to turn into SO2 by oxidation in air at high temperature. 200 ppmv SO2 exposure at 800◦ C for 24 h causes a rather drastic deactivation of each LaMn1−x Mgx O3 catalyst (and especially of the LaMn0.5 Mg0.5 O3 one that, among the three tested catalysts, has the higher catalytic activity in the fresh state), while SO2 ageing at 400 and 600◦ C does not cause a significant catalytic activity decrease. The information about the effect of SO2 ageing on catalytic activity at different temperatures is very useful for application purposes of LaMn1−x Mgx O3 perovskites, as premixed catalytic burners work in a large range of temperatures (from 400 to 900◦ C). The data reported in Table 2 demonstrate that LaMn1−x Mgx O3 catalyst deactivation depends on SO2 exposure time as also demonstrated by other Authors [15]. If structural variations cannot be invoked to explain sulphur poisoning, surface modifications should therefore be involved. While EDS analysis (1000 ppm sensitivity) does not detect sulphur on each poisoned catalyst, reflectance FTIR spectra (Fig. 5) give evidence of S–O and S=O species only on

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Fig. 7. Results of the catalytic activity test towards methane combustion of LaMn0.5 Mg0.5 O3 catalyst fresh and after successive poisoning/water regeneration cycles.

LaMn0.5 Mg0.5 O3 catalyst, where, on the basis of the results of EDS, SEM and TEM investigation (Figs. 3b and 4), magnesium cations do not seem to enter completely into the perovskite structure but to remain partially on the surface as, probably, magnesium

oxide phase. The little amount of oxygen desorbed by LaMn0.5 Mg0.5 O3 catalyst in the TPD test and the little oxygen excess resulted by titration could confirm the incomplete introduction of magnesium cations into the perovskite structure. Magnesium/oxygen

Fig. 8. T50 values measured for LaMn1−x Mgx O3 catalysts fresh, poisoned (800◦ C, 24 h, 200 ppmv SO2 ) and regenerated by aqueous NH3 leaching.

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pairs represent preferential adsorption sites for the SO2 molecules [18], so that magnesium sulphate formation can be accounted for. Curve b of Fig. 5 was compared to experimental infrared spectrum of MgSO4 : the two spectra are characterised by the same bands, so confirming that magnesium sulphate formation on LaMn0.5 Mg0.5 O3 catalyst took place. The surface or bulk nature of formed magnesium sulphate is not clear yet, also because available literature data are contradictory: Morterra et al. [23] assign bands at about 1350 and 1150 cm−1 frequencies to surface sulphates, Waqif et al. [20] assign the bands between 1400 and 1340 cm−1 to surface sulphates and the band centred near 1160 cm−1 to bulk sulphates. Bulk and surface magnesium sulphates could coexist on poisoned LaMn0.5 Mg0.5 O3 catalyst: this issue will be investigated in further studies. The failure of heat treatment for catalyst regeneration can be accounted for the very high thermal stability of magnesium sulphate (>1100◦ C [24]). A further, even stronger validation of magnesium sulphate formation by SO2 poisoning comes from the water washing tests of poisoned LaMn0.5 Mg0.5 O3 perovskite. The atomic absorption results in Table 3, as well as chemical analyses of the leachate, detect sulphur and magnesium species. Conversely, lanthanum was not detected because of its high (25 ␮g ml−1 ) detection limit. Manganese species could not be detected as well, since they are ruled out by the low thermal stability of manganese(III–IV) sulphates. Table 3 shows that washing of LaMn0.5 Mg0.5 O3 provides sulphur amounts much larger than the corresponding magnesium ones. This is not surprising because, even if MgSO4 is water soluble at room temperature (270 g l−1 [25]), magnesium cations are linked to structural catalyst oxygen anions, while SO4 2− ions can be removed more easily by water. Moreover, the following mechanism of MgSO4 dissolution by water can be proposed: [MgSO4 ]s + 2H2 O ↔ [Mg(OH)2 ]s + H+ + HSO4 − (1) Indeed, the leachate is weakly acidic and Mg(OH)2 which is practically insoluble in water at room temperature (9 mg l−1 [25]). Magnesium hydroxide turns into MgO at high temperature, thus preserving the original composition of LaMn0.5 Mg0.5 O3 perovskite.

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The proposed mechanism is not completely efficient because Eq. (1) is not completely shifted to the right. The results of catalytic activity at several poisoning/regeneration cycles (Fig. 7) confirm that LaMn0.5 Mg0.5 O3 catalyst can be regenerated quite significantly up to three successive poisoning cycles. After the fourth cycle, the original LaMn0.5 Mg0.5 O3 catalytic activity could not be restored: the slope and the shape of the sigma-shaped curve (methane conversion versus temperature) is quite different, a likely consequence of a local variation of the perovskite composition. EDS analysis performed on this sample indicated, in fact, that the magnesium content was reduced in the surface light phase and in some areas of the bulk dense phase. EDS analysis on the LaMn0.8 Mg0.2 O3 catalyst does not show any superficial excess in the magnesium and oxygen content, so SO2 interaction, and consequent surface MgSO4 formation, are less favoured than with LaMn0.5 Mg0.5 O3 . The decrease in catalytic activity of LaMn0.8 Mg0.2 O3 and LaMnO3 after SO2 exposure is probably related to the formation of lanthanum sulphates. As mentioned earlier, manganese(III–IV) sulphates are not thermally stable, while lanthanum sulphate decomposes over 1150◦ C [24]. The less effectiveness of water washing regeneration of poisoned LaMn0.8 Mg0.2 O3 and LaMnO3 catalysts (Fig. 6) is accounted for the low solubility in water of La2 (SO4 )3 (30 g l−1 [25]). NH4 OH washing allows the complete regeneration of each poisoned LaMn1−x Mgx O3 perovskite (Fig. 8). Atomic absorption data listed in Table 4 show the sulphur presence in NH3 leachate for all the LaMn1−x Mgx O3 catalysts (again lanthanum was not detected because its high detection limit): the sulphur quantities are again much larger than the magnesium ones in LaMn0.8 Mg0.2 O3 and LaMn0.5 Mg0.5 O3 leachates. The comparison between LaMn0.5 Mg0.5 O3 atomic absorption data in Tables 3 and 4 points out that NH4 OH leaching dissolves more sulphur and less magnesium than the water one. These data are perfectly in line with the regeneration mechanism in Eq. (1) as the equilibrium is shifted to the right. The same regeneration mechanism can be proposed for the poisoned LaMn0.8 Mg0.2 O3 and LaMnO3 perovskites, because the higher OH− availability favours the La2 (SO4 )3 dissolution with consequent formation of insoluble La(OH)3 .

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Fig. 9. Estimated activation energy values (Er , kcal mol−1 ) for LaMn0.5 Mg0.5 O3 catalyst fresh and after successive poisoning/NH3 regeneration cycles.

The different nature of sulphur poisoning between the LaMn0.5 Mg0.5 O3 and the LaMn0.8 Mg0.2 O3 / LaMnO3 catalysts is validated by the estimated activation energy values shown in Table 5. As mentioned earlier, the variation of catalytic activity during successive poisoning/regeneration cycles of LaMn0.5 Mg0.5 O3 catalyst does not involve substantial changes of Er values (Table 5 and Fig. 9). As a consequence, it can be guessed that the variation of the number of catalytic sites active in the different poisoning/ regeneration steps is the major reason for reduced/ restored catalytic activity. In contrast, the variation of the catalytic activity of the LaMn0.8 Mg0.2 O3 and LaMnO3 perovskites involves an evident change in the activation energy (Table 5), suggesting that not only the number but also the intrinsic activity of the active sites changes in the fresh, poisoned and washed Table 5 Estimated activation energy values (kcal mol−1 ) for LaMn1−x Mgx O3 catalysts fresh, poisoned (800◦ C, 24 h, 200 ppmv SO2 ) and regenerated by aqueous NH3 leaching

Fresh Poisoned Regenerated

LaMnO3

LaMn0.8 Mg0.2 O3

LaMn0.5 Mg0.5 O3

23.9 27.3 22.5

27.1 31.8 28.0

23.4 25.2 27.2

catalysts. This could mean that surface magnesium oxide, only present significantly on LaMn0.5 Mg0.5 O3 perovskite, acts as a promoter [26] and preserves the nature of active sites. These active sites, probably for the steric hindrance of the formed MgSO4 , are hardly reachable by the reacting gases after the poisoning treatment (hence an apparent reduction of the number of active sites could take place), but become available once again after the regeneration process. After repeated regeneration (e.g. after four poisoning/regeneration cycles) the amount of magnesium oxide is decreased and its promoting effect is less important. The investigation on the surface magnesium oxide could be made deeper by TEM-nanodiffraction analysis; however these preliminary results suggest further studies on LaMn1−x Mgx O3 + yMgO catalysts, in order to verify a possible improved resistance to sulphur poisoning in the natural gas combustion, guaranteed by the presence of MgO introduced on purpose during catalyst preparation. Beyond this protecting role against sulphur poisoning, the mentioned MgO excess crystals should also allow to act as a textural structural promoter by avoiding perovskite crystal sintering at high temperatures, as demonstrated in a previous paper [13].

I. Rosso et al. / Applied Catalysis B: Environmental 30 (2001) 61–73

5. Conclusions The poisoning by SO2 or THT is temperature and time depending and a 200 ppmv SO2 exposure at 800◦ C for 24 h causes a rather drastic deactivation of each LaMn1−x Mgx O3 catalyst. The poor homogeneous distribution of magnesium and oxygen in the fresh LaMn1−x Mgx O3 compounds (x > 0.2) was found to be critical for their SO2 poisoning and for their regenerability via water leaching. The LaMn0.5 Mg0.5 O3 catalyst, which shows an oxygen and magnesium content higher in the surface than in the bulk, is mainly poisoned by SO2 via surface magnesium sulphate formation and it can be regenerated completely by water washing up to three successive poisoning cycles. After the fourth poisoning cycle, the original LaMn0.5 Mg0.5 O3 catalytic activity cannot be restored, probable because of local variations in the perovskite composition and a progressive depletion of the MgO excess. LaMn0.8 Mg0.2 O3 and LaMnO3 perovskites, which do not show a surface excess of magnesium and oxygen, are poisoned by SO2 prevalently via lanthanum sulphate formation and can be regenerated significantly only by aqueous NH3 washing. A mechanism for water and NH4 OH regeneration is proposed, based on SO4 2− substitution by OH− species. The higher OH− concentration in NH4 OH solution makes this washing more effective. Under the spur of the above promising results, further studies are in progress to clarify the role of the excess of magnesium oxide, already introduced as textural promoter [13], on the sulphur resistance of LaMn1−x Mgx O3 + yMgO perovskites. On the basis of the simple and rapid regeneration method proposed, the studied perovskite systems have been applied in the development of premixed catalytic burner prototypes for domestic boiler applications [3]. References [1] M.F.M. Zwinkels, S.G. Järås, P. Govin Menon, Catal. Rev.-Sci. Eng. 35 (1993) 319.

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