Sulphur poisoning of LaCr0.5−xMnxMg0.5O3·yMgO catalysts for methane combustion

Applied Catalysis B: Environmental 40 (2003) 195–205

Sulphur poisoning of LaCr0.5−x Mnx Mg0.5O3·yMgO catalysts for methane combustion Ilaria Rosso∗ , Guido Saracco, Vito Specchia, Edoardo Garrone Dipartimento di Scienza dei Materiali e Ingegneria Chimica—Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129 Turin, Italy Received 8 February 2002; received in revised form 20 May 2002; accepted 28 May 2002

Abstract LaCr0.5−x Mnx Mg0.5 O3 ·yMgO catalysts (x = 0, 0.25, y = 0, 17) were prepared, characterised (X-ray diffraction, BET analysis, scanning electron microscopy–energy dispersion spectroscopy, transmission electron microscopy, FTIR spectroscopy, chemical and atomic absorption analyses), tested for high-temperature methane combustion and aged in presence of SO2 to investigate the effect of sulphur on their catalytic activity. The role of chromium presence in the perovskite structure have been assessed by comparison with earlier studies on Cr-free perovskites: (i) it worsens the activity of catalysts in the fresh state; (ii) owing to its acidic nature, it makes perovskites less prone to adsorb SO2 and so more resistant to sulphur poisoning; (iii) it renders catalyst regeneration by NH3 leaching less effective because of the partial solubility of chromium oxide in basic media. Conversely, MgO promotes catalytic activity of fresh LaCr0.5 Mg0.5 O3 ·yMgO catalysts and slows down sulphur poisoning of the LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst only. © 2002 Elsevier Science B.V. All rights reserved. Keywords: LaCr0.5−x Mnx Mg0.5 O3 ·yMgO; Perovskite; Methane catalytic combustion; Sulphur poisoning

1. Introduction Catalytic combustion of methane represents the most effective strategy to significantly reduce pollutants (HC, CO and NOx ), produced during homogeneous combustion [1–3]. Innovative and reliable technologies have thought to be developed to make catalytic combustion widely acceptable [4]. The development of high-performance low-cost catalysts is essential in this context. Our current research efforts are addressed to developing cost effective catalyst for application in premixed natural gas catalytic burners for domestic boilers [5] and in the ∗ Corresponding author. Tel.: +39-11-5644710; fax: +39-11-5644699. E-mail address: [email protected] (I. Rosso).

treatment of the exhaust gases of compressed natural gas (CNG) cars, characterised by the presence of significant unconverted methane concentrations (having a 35-fold higher greenhouse potential than CO2 ) [6]. High activity, long-term thermal stability and good resistance to poisoning agents such as sulphur compounds (either present in natural gas ground reservoirs or introduced as odorants) are the required properties of combustion catalysts suitable for both applications. Perovskite-type mixed oxides can meet these requirements, owing to their low cost, thermo-chemical stability at comparatively high temperatures (900– 1100 ◦ C), and good catalytic activity [7–9]. Perovskites have the ABO3 stoichiometry (where A is generally a rare-earth-metal cation in the +3 oxidation state and B is a transition-metal cation in the same

0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 1 5 3 - 4

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valence state) and the number of different perovskites available from various preparation techniques is quite large owing to both the variety of A and B metals that can enter the perovskite structure and the possibility to substitute partially either A or B [10–13]. Unfortunately, perovskite catalysts are sensitive to sulphur compounds. For this reason, they failed to become practical as automotive catalysts, as sulphur is often present in relatively high concentrations in the combustion products of automotive fuels [14], but they are promising in the field of natural gas combustion because much more resistant to the low levels of sulphur compounds (about 10 ppm) present in ground reservoirs or added on purpose to commercial natural gas [15]. Previous studies on the effect of sulphur on the catalytic activity of LaMn1−x Mgx O3 perovskites [16] showed drastic catalyst deactivation after a 200 ppmv SO2 exposure at 800 ◦ C for 24 h because of sulphates formation. The introduction of magnesium oxide in LaMn1−x Mgx O3 ·yMgO catalysts [17] improved sulphur resistance, because MgO represented the preferential adsorption site for the SO2 molecules: deactivation of such catalysts was appreciable only after 32 days of exposure to 200 ppmv SO2 at 800 ◦ C (accelerated ageing conditions). It can be expected that the use of a more acidic element than Mn in the above perovskite could improve the catalyst stability towards sulphur poisoning. On the ground of previous studies on the LaCr1−x Mgx O3 system (x = 0–0.5) [18], this paper is addressed to investigating the sulphur resistance of these materials, with or without the use of the above mentioned MgO promoter, and to the study of LaCr0.5−x Mnx Mg0.5 O3 perovskites, so bridging the two families of Mg-substituted lanthanum manganites and chromites.

2. Experimental 2.1. Catalyst preparation A series of catalysts of the perovskite-type LaCr0.5−x Mnx Mg0.5 O3 (x = 0, 0.25) with different excess of MgO compared to the stoichiometric value (perovskite-MgO excess ratios: 25/75, 50/50 and 75/25 wt.%, corresponding to 1/17, 1/6 and 1/2 molecular ratios, respectively) was prepared via a

modified version of the so-called “citrates method” described in [16]. Calcination was performed in calm air at 900 ◦ C for 8 h for MgO-free LaCr0.5−x Mnx Mg0.5 O3 catalysts and at 1000 ◦ C for 3 h for MgO-rich ones, in order to avoid the formation of less active phases, containing Cr6+ compounds such as La2 CrO6 [18], which are formed at lower calcination temperatures and times. Pure MgO was prepared with the same experimental procedure and calcinated at 900 ◦ C for 8 h. 2.2. Catalyst ageing tests Different samples of LaCr0.5−x Mnx Mg0.5 O3 ·yMgO powdered catalysts were treated at 800 ◦ C for 1, 2, 4, 8, 16 and 32 days in a controlled-atmosphere oven. As previously done [16,17], a flow of air with 200 ppmv of SO2 was forced to pass at a rate of 20 N cm3 /min over 1 g of powdered material located in a porcelain combustion boat. A high SO2 level (about 20 times higher than that corresponding to odorants usually added to commercial natural gas) was chosen in order to accelerate the ageing effect. Fractions of the SO2 -treated catalysts were used for characterisation, catalytic activity tests and catalyst regeneration studies. 2.3. Fresh/aged catalyst characterisation Chemical analysis (dissolution in acid + atomic absorption) was performed by a Perkin-Elmer 1100B atomic absorption spectrometer on each catalyst to confirm that the amount of the various elements of interest (La, Mn, Cr, Mg) was consistent with the nominal content. XRD analyses (Philips X’Pert apparatus equipped with a monochromator for Cu K␣ radiation) were performed on all fresh and aged catalysts. The typical ABO3 structure was formed in all cases, showing well-defined orthorhombic diffraction lines (JCPDS card 33-0701). The diffractions peaks of MgO (JCPDS card 45-0946) were clearly seen in all diffraction patterns of MgO-rich catalysts. Weak diffraction peaks attributable to La2 CrO6 phase (JCPDS card 26-0817) were present only in the LaCr0.5 Mg0.5 O3 spectrum. The diffraction patterns of fresh LaCr0.25 Mn0.25 Mg0.5 O3 and LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO are shown, as an example, in Fig. 1a and b, respectively.

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Fig. 1. X-ray diffraction patterns of: (a) fresh LaCr0.25 Mn0.25 Mg0.5 O3 perovskite calcinated for 8 h at 900 ◦ C; (b) fresh LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst calcinated for 3 h at 1000 ◦ C. (䉬) perovskite; (䊉) MgO.

No new peaks were observed in the spectra of all the aged catalysts. The specific surface area was measured by the BET method using N2 on all fresh LaCr0.5−x Mnx Mg0.5 O3 · yMgO catalysts (Micromeritics ASAP 2010 M apparatus). Results are listed in Table 1. MgO-free catalysts have a specific surface area of 6–14 m2 /g, which rises by increasing the MgO excess up to 37–38 m2 /g for LaCr0.5−x Mnx Mg0.5 O3 ·17MgO catalysts. BET

data obtained on 32-day-aged catalysts show that MgO-free perovskites with low BET values in the fresh state do not change their specific surface area: from 6.1 to 5.9 m2 /g for LaCr0.5 Mg0.5 O3 perovskite. MgO-richest perovskites with highest BET data in the fresh state, instead, loose after the same exposure some specific surface area: from 36.9 to 30 m2 /g for LaCr0.5 Mg0.5 O3 ·17MgO catalyst and from 38 to 27 m2 /g for LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst.

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Table 1 Specific surface area (BET, m2 /g) and methane half-conversion temperatures (T50 , ◦ C) of fresh LaCr0.5−x Mnx Mg0.5 O3 ·yMgO perovskites LaCr0.5 Mg0.5 O3 ·yMgO yMgO (m2 /g)

BET T50 (◦ C)

LaCr0.25 Mn0.25 Mg0.5 O3 ·yMgO

0

2

6

17

0

17

6.08 577

13.6 552

24.2 545

36.9 529

14.4 506

38.0 532

A pure thermal effect in the absence of SO2 was evaluated on the specific surface area of MgO: it diminishes from 34.5 to 19.6 m2 /g after 16 days at 800 ◦ C [17]. Transmission electron microscopy (TEM, Philips EM 400 apparatus) was performed on the different powdered perovskites. Fig. 2 shows TEM micrographs of LaCr0.5 Mg0.5 O3 (Fig. 2a) and LaCr0.5 Mg0.5 O3 · 17MgO (Fig. 2b) catalysts. The perovskite crystals size is about 90–120 nm in MgO-free catalyst (Fig. 2a) and about 20–45 nm in MgO-rich one (Fig. 2b), where the dark perovskite crystals are surrounded by light-grey MgO ones. The perovskite crystal size was estimated also by the measurement of the full-width half-maximum (FWHM) of the perovskite lines in the diffraction patterns (Fig. 1) according to

Debye–Scherrer equation [19]. The resulting average perovskite particle size is about 60 nm in MgO-free catalyst and about 50 nm in MgO-rich one; the average MgO particle size results to be about 44 nm. Whereas a rather good consistency between the sizes obtained by both procedures for MgO-rich catalysts is observed, a larger discrepancy occurs for MgO-free catalysts, presumably because the perovskite particles of MgO-free catalyst examined by TEM consist of aggregates of many particles. Scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) (Philips 515 SEM equipped with EDAX 9900 EDS) were performed to investigate the morphology as well as the elemental composition and distribution of all the catalysts. Since the morphological analysis (SEM) needs gold

Fig. 2. TEM micrographs (280,000×) of fresh catalysts: (a) LaCr0.5 Mg0.5 O3 ; (b) LaCr0.5 Mg0.5 O3 ·17MgO.

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Fig. 3. SEM micrographs of fresh catalysts: (a) LaCr0.5 Mg0.5 O3 (10,000×); (b) LaCr0.5 Mg0.5 O3 ·6MgO (5000×).

metallisation of the sample and the X-ray emission (EDS) of gold overlaps those of other elements of interest (e.g. sulphur), simultaneous EDS and SEM measurements on the same sample were not possible. Fig. 3 shows SEM micrographs of fresh LaCr0.5 Mg0.5 O3 (Fig. 3a) and LaCr0.5 Mg0.5 O3 ·6MgO (Fig. 3b) catalysts. Each catalyst shows a spongy microstructure, particularly evident in the MgO-rich one (Fig. 3b). EDS analysis reveals a significant presence of sulphur on each 32-day-aged catalyst: from 0.9 to 1 wt.% for LaCr0.5−x Mnx Mg0.5 O3 catalysts to 1.7–1.8 wt.% for LaCr0.5−x Mnx Mg0.5 O3 ·17MgO ones.

2.4. Catalytic activity screening tests Catalytic activity tests were performed on fresh, aged and regenerated (see Section 2.5) catalysts in the experimental apparatus and according to the procedures described in detail in [16]. After 30 min stay at 800 ◦ C in air flow as a common pre-treatment, a gas flow rate of 50 N cm3 /min (composition: CH4 = 2%, O2 = 18%, He = balance) was fed to a fixed-bed of 0.5 g of catalyst particles. Such pellets were obtained by pressing at 125 MPa the perovskite powders into tablets, which were then crushed and sieved to obtain 0.2–0.5 mm pellets. The fixed-bed was enclosed

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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 K-type thermocouple was inserted into the packed bed. After the pretreatment, the reactor temperature was lowered at a 3 ◦ C/min rate from 800 down to 300 ◦ C, meanwhile analysing the outlet CO2 concentration (the only detectable carbon oxidation product) through an NDIR analyser (Hartmann & Braun URAS

10E) in order to determine methane conversion. 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%. No significant hysteresis was observed for these curves performing the catalytic tests upward (from

Fig. 4. Methane half-conversion temperatures (T50 , ◦ C) vs. exposure time to 200 ppmv SO2 at 800 ◦ C of: (a) LaCr0.5 Mg0.5 O3 ·y MgO catalysts; (b) LaCr0.5 Mg0.5 O3 /LaCr0.5 Mg0.5 O3 ·17MgO, LaCr0.25 Mn0.25 Mg0.5 O3 /LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO and LaMn0.5 Mg0.5 O3 /LaMn0.5 Mg0.5 O3 ·17MgO catalysts.

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300 to 800 ◦ C) after the downward standard run. The half-conversion temperature (T50 ) was regarded as an index of the catalytic activity. Table 1 lists the half-conversion temperatures of all fresh LaCr0.5−x Mnx Mg0.5 O3 ·yMgO catalysts. By increasing the MgO excess, only the activity of the LaCr0.5 Mg0.5 O3 ·yMgO catalyst improves, whereas no improvement is observed for LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst. The effect of SO2 exposure for progressively longer times on-stream at 800 ◦ C on the. catalytic activity of LaCr0.5 Mg0.5 O3 ·yMgO catalysts is shown in Fig. 4a. LaCr0.5 Mg0.5 O3 catalyst shows a very slight deactivation; MgO-rich LaCr0.5 Mg0.5 O3 catalysts undergo a negligible deactivation up to 10–16 days of SO2 exposure and a more evident one after 32 days. Fig. 4b reports the T50 data of fresh and SO2 -aged LaCr0.5−x Mnx Mg0.5 O3 and LaCr0.5−x Mnx Mg0.5 O3 · 17MgO catalysts. Data concerning LaMn0.5 Mg0.5 O3 and LaMn0.5 Mg0.5 O3 ·17MgO catalysts, derived from [17], are also plotted for comparison. The MgO-free Mn containing-catalysts deactivate drastically after just 1 day of SO2 exposure, while deactivation is

201

slower and evident only after 32 days of SO2 exposure for LaMn0.5−x Crx Mg0.5 O3 ·17MgO catalysts. Infrared spectroscopy spectra (Bruker Equinox 55 FTIR, equipped with MCT detector) were recorded in the trasmittance mode on pelletised samples of fresh and 32-day-aged LaCr0.5−x Mnx Mg0.5 O3 ·yMgO powders, after degassing in vacuum at room temperature for 30 min. Fig. 5 reports, as an example, the spectra of fresh and 32-day-aged LaCr0.25 Mn0.25 Mg0.5 O3 · 17MgO catalysts in the sulphate region. Both spectra are very similar to those observed for fresh and 32-day-aged LaMn0.5 Mg0.5 O3 ·17MgO catalysts [17], respectively. Four sharp bands at 1181, 1106, 1071 and 993 cm−1 can be assigned to ionic SO4 2− ions in C2v symmetry present in the bulk of the MgO matrix [17]. 2.5. Catalyst regeneration LaCr0.5−x Mnx Mg0.5 O3 ·yMgO catalysts, aged in SO2 atmosphere for 32 days, were regenerated by leaching with either distilled water or an aqueous NH3 solution (2.5% v/v), according to the procedure described in [16]: 1 g of each catalyst was washed

Fig. 5. FTIR absorbance spectra in the 1300–800 cm−1 region (corresponding to the modes of sulphates) of LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst in: (a) fresh state; (b) aged for 32 days at 800 ◦ C in 200 ppmv SO2 atmosphere.

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Table 2 Methane half-conversion temperatures (T50 , ◦ C) of fresh, 32-day-aged and washed LaCr0.5−x Mnx Mg0.5 O3 ·yMgO perovskites LaCr0.5 Mg0.5 O3 ·yMgO yMgO Fresh 32-day-aged Washed (NH3 ) Washed (H2 O)

LaCr0.25 Mn0.25 Mg0.5 O3 ·yMgO

0

2

6

17

0

17

577 607 605 585

552 644 582 565

545 618 583 563

529 623 588 566

506 724 554 505

532 687 532 532

Table 3 Atomic absorption data (mg element/g catalyst) on the leachate obtained on the 32-day-aged LaCr0.5−x Mnx Mg0.5 O3 ·yMgO perovskites by aqueous NH3 and water leaching LaCr0.5 Mg0.5 O3 ·yMgO yMgO

LaCr0.25 Mn0.25 Mg0.5 O3 ·yMgO

0

2

6

17

0

17

Cr

(NH3 ) (H2 O)

0.46 0.35

0.34 0.25

0.20 0.16

0.13 0.11

0.23 0.18

0.05 0.03

Mg

(NH3 ) (H2 O)

0.14 0.62

0.30 1.5

0.20 1.7

0.58 2.3

0.03 0.15

0.21 1.1

S

(NH3 ) (H2 O)

32 18

35 20

46 26

with 60 cm3 of the leachant at room temperature in a stirred becker; the powder was then filtered on a paper filter and dried in an oven at 120 ◦ C. Table 2 lists the half-conversion temperatures of all fresh, 32-day-aged and washed LaCr0.5−x Mnx Mg0.5 O3 ·yMgO catalysts. Aqueous NH3 leaching is effective in restoring the catalytic activity of LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst, while the other catalysts recover only partially their initial activity. Conversely, water leaching turned out to restore the catalytic activity completely for all LaCr0.25 Mn0.25 Mg0.5 O3 ·yMgO catalysts and partially (although better than aqueous NH3 ) for LaCr0.5 Mg0.5 O3 ·yMgO ones. The leachates obtained during catalyst regeneration were acidified by adding a diluted aqueous HCl solution. Cations and soluble sulphur were detected in the leaching medium by atomic absorption (Perkin-Elmer 1100B atomic absorption spectrometer) and the results are listed in Table 3. The sulphur amounts are much larger than the magnesium and the chromium ones and the NH3 leaching dissolves more sulphur and chromium and less magnesium than the water one.

58 34

38 25

67 40

3. Discussion The calcination step in the catalyst preparation was performed at the least temperatures yielding the desired phases in a well-crystallised form, so to obtain specific surface areas as high as possible. MgO-free perovskites crystallise after 8 h at 900 ◦ C, while MgO-rich perovskites crystallisation needs 3 h at 1000 ◦ C. An excess of magnesium oxide slows down the perovskite crystallisation process, so that a higher calcination temperature is required; in spite of the higher calcination temperature, the specific surface area increases (Table 1), so confirming the textural promoting effect of MgO, already observed [12,16,17]. Fig. 2 shows, indeed, the reduction of the average perovskite crystal size, due to the co-crystallisation with MgO. As far as the catalytic activity of fresh catalysts is concerned (Table 1), it results that 1. the partial substitution of Cr with Mn in the perovskite structure improves the catalytic activity. That of LaCr0.5 Mg0.5 O3 perovskite is close

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to that of MgO (T50 = 577 and 582 ◦ C, respectively); the half-conversion temperature of LaCr0.25 Mn0.25 Mg0.5 O3 perovskite is definitively lower (T50 = 506 ◦ C). If the half-conversion temperature of LaMn0.5 Mg0.5 O3 perovskite is considered (T50 = 455 ◦ C [16]), it results that the presence of Mn brings about a progressive improvement of catalytic activity. This is not surprising as it is already known that in the absence of Mg, Mn-containing-catalysts are more active than Cr-containing ones [20]: in the present case, because the Cr/Mg and Mn/Mg substitutions force a fraction of either manganese or chromium to shift to the IV oxidation state [12,18], it can be stated that the Mn4+ species has an activity higher than that of the Cr4+ one; 2. MgO has a promoting effect on the catalytic activity of LaCr0.5 Mg0.5 O3 ·yMgO catalysts. The higher specific surface area of MgO-rich perovskites is favourable in terms of increase of the specific number of active sites and of an improvement of catalytic activity; 3. MgO does not have a promoting effect on the catalytic activity of LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst. The large gain in the specific surface area obtained by introducing MgO seems not to have a direct positive effect on the catalytic activity. The observed decrease of catalytic activity could be caused (i) by the dilution of the more active perovskite phase with the less active MgO phase, (ii) by the very strict mixture between MgO and perovskite crystals, observed in TEM micrographs (Fig. 2b), that, as already proposed [12] could hamper the reacting gases from reaching the active catalyst surface. These effects are not important for LaCr0.5 Mg0.5 O3 ·yMgO catalysts probably because the catalytic activity of LaCr0.5 Mg0.5 O3 perovskite is close to that of MgO, so that the increase of the specific surface area is the prevailing factor. If the LaCr0.5 Mg0.5 O3 ·17MgO catalyst, within the LaCr0.5 Mg0.5 O3 ·yMgO series, shows the best catalytic activity in the fresh state, the MgO-free LaCr0.5 Mg0.5 O3 catalyst preserves, among the catalysts tested, the best catalytic activity after 32 days of SO2 exposure at 800 ◦ C (Fig. 4a). The MgO-rich catalysts, in fact, show a negligible deactivation

203

up to 16 days of SO2 exposure but their catalytic activity decreases after 32 days. In this case the MgO introduction seems not to have a protecting role against sulphur poisoning although MgO actually furnishes preferential adsorption sites for SO2 molecules [14], as confirmed by the use of MgO derivatives as SOx adsorbents in catalytic cracking units [21]. The amount of sulphur detected by EDS analysis on the 32-day-aged LaCr0.5 Mg0.5 O3 ·17MgO catalyst is about twice the amount of sulphur detected on 32-day-aged MgO-free LaCr0.5 Mg0.5 O3 catalyst: this fact alone is not conclusive, as it could be due to the higher specific surface area of the former. Evidence of sulphates formation on MgO phase is given by FTIR spectrum of 32-day-aged LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst (Fig. 5, curve b). The four sharp bands of curve b (1181, 1106, 1071 and 993 cm−1 ), absent in curve a (fresh state), can be assigned to ionic SO4 − in a C2v symmetry, in according to very similar spectra observed for bulk sulphate species on MgO smoke (10 m2 /g) by other authors [22]. The high amount of sulphur detected by EDS analysis (1.8 wt.%) indicates that sulphation is not only confined to the external surface and is in good agreement with the assignation of FTIR bands to bulk sulphates. Sharp bands at the same frequencies (1192, 1108, 1069 and 993 cm−1 ) were found in FTIR spectrum of the 32-day-aged LaMn0.5 Mg0.5 O3 ·17MgO catalyst, assigned also to bulk sulphate species [17,23]. This similarity confirm that the FTIR spectra of all the LaMn0.5 Mg0.5 O3 ·17MgO and LaCr0.5−x Mnx Mg0.5 O3 ·17MgO catalysts are probably attributable only to MgO phase, as, in such samples, perovskite phase is present in a low amount (25 wt.%) and, considering perovskite crystals embedding into the MgO matrix, sulphate species formed on the perovskite phase can be not visible. In MgO-rich LaCr0.5 Mg0.5 O3 catalysts sulphate species formed on the MgO phase, after 32 days exposure to SO2 , have a negative extent on the catalytic activity of the overall system probably because they have a shielding effect on the perovskite surface thereby preventing the reacting gases from reaching the active sites of perovskite phase and causing a consequent decrease of the catalytic activity, as proposed for LaMn1−x Mgx O3 ·yMgO catalysts [17]. In contrast, the MgO-free LaCr0.5 Mg0.5 O3 catalyst, thanks

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to the acidic nature of chromium which is less prone than manganese to sulphur adsorption [15,16], shows a higher resistance to sulphur poisoning. The importance of Cr in the resistance to sulphur poisoning is confirmed by the behaviour of aged LaCr0.25 Mn0.25 Mg0.5 O3 catalyst showed in Fig. 4b. Thanks to the presence of Mn, this perovskite is more active than LaCr0.5 Mg0.5 O3 catalyst in the fresh state, but it deactivates drastically after only 1 day exposure to SO2 , because of the basic nature of elements such as La, Mg and Mn itself, which are very prone to adsorb acidic species like SO2 . This behaviour is similar to that of the LaMn0.5 Mg0.5 O3 catalyst [16], also reported as a comparison in Fig. 4b. By analogy, LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO and LaMn0.5 Mg0.5 O3 ·17MgO [17] catalysts, all reported in Fig. 4b, show comparable behaviours. Both these catalysts deactivate more slowly than the corresponding MgO-free ones, because of the presence of the basic MgO, which preferentially adsorbs sulphur and thus allows the perovskite active sites to avoid sulphur poisoning for a longer time. However, after prolonged exposure to SO2 , also the active sites of the perovskite phase are involved in sulphur poisoning, so that the catalytic activity is depressed by direct deactivation as well as by the already mentioned shielding effect of sulphate species formed on the MgO phase. On the grounds of the expertise derived from previous works on LaMn1−x Mgx O3 ·yMgO catalysts [16,17], where thermal treatment at very high temperature in inert or reducing atmosphere of poisoned catalysts were proved to be not effective for regeneration purposes, catalyst regeneration was performed by washing with both water [24] and an aqueous NH3 solution (Table 2). Surprisingly, the extent of regeneration is somehow better using the water leaching. A previous comparison on LaMn1−x Mgx O3 perovskites between the washing with water or with aqueous NH3 solution [16] demonstrated that ammonia leaching was more effective in restoring the catalytic activity of the catalysts. Assuming that metal sulphate (M2 (SO4 )n ) dissolution by water is based on the following mechanism [16]: [M2 (SO4 )n ]solid + nH2 O ↔ [M(OH)n ]solid + nH+ + nHSO4 −

(1)

it is evident that the NH4 OH leaching should be more effective because it shifts the equilibrium to the right side. Then, the formation of the metal hydroxide, which turns into metal oxide at high temperature, allows to preserve the original composition of the perovskite. Atomic absorption data listed in Table 3 show the presence of chromium, magnesium and sulphur in both water and NH3 leachate for all the LaCr0.5−x Mnx Mg0.5 O3 ·17MgO catalysts (manganese was absent, while lanthanum was not detected because of its high detection limit (25 ␮g/ml)). The sulphur amounts are much larger than the magnesium and the chromium ones, which allows the original composition of catalyst to be substantially preserved. The NH3 leaching dissolves more sulphur and less magnesium than the water leaching, confirming at least for MgSO4 the shift to the right side of Eq. (1), as observed for LaMn1−x Mgx O3 .catalysts [16]. However, it dissolves more chromium than the water leaching. Chromium oxide is probably partially dissolved by ammonia with formation of [Cr(NH3 )6 ]3+ complexes [25], that were actually noticed to provide the solution with a yellow colour. This fact could explain the worse effectiveness of ammonia leaching on the regeneration of LaCr0.5−x Mnx Mg0.5 O3 ·17MgO catalysts. Only the LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst, the catalysts with the lowest chromium content among those tested recovers, in fact, completely its starting catalytic activity also with ammonia leaching. The release of such amounts of chromium, however, could also be due to some unreacted phases escaping XRD analysis, as diffraction analysis on polycrystalline powders permits to detect phases only if their amount is higher than 5 wt.%. The slow deactivation by sulphur poisoning and the complete regenerability to the original high catalytic activity by water or NH3 leaching make the LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst performance almost comparable to that of LaMn1−x Mgx O3 ·17MgO catalysts [17], and therefore promising for application purposes. However, the LaCr0.5 Mg0.5 O3 catalyst, even if less active in the fresh state, seems to be perhaps preferable for its highest stability and resistance to sulphur poisoning in order to assure stable performance and durability to catalytic reactors (e.g. catalytic burners [5]).

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4. Conclusions

References

LaCr0.5−x Mnx Mg0.5 O3 ·yMgO catalysts have been prepared, characterised, tested for high-temperature methane catalytic combustion. Ageing in presence of SO2 was also accomplished to investigate the effect of sulphur on their catalytic activity. Experimental results demonstrate that different roles of chromium can be assessed: (i) the presence of chromium in the perovskite structure lowers the activity of catalysts in the fresh state; (ii) the acidic nature of chromium makes the perovskites less prone to adsorb SO2 and so more resistant to sulphur poisoning; (iii) the partial solubility of chromium oxide in ammoniacal media makes the leaching with aqueous NH3 solution not very effective to catalyst regeneration compared to previously studied perovskite catalysts [16,17]. MgO has a promoting effect on the catalytic activity of fresh LaCr0.5 Mg0.5 O3 ·yMgO catalysts, but, representing a preferential adsorption site for the SO2 molecules, its presence has a negative extent on the catalytic activity of the LaCr0.5 Mg0.5 O3 ·yMgO catalysts aged for 32 days in SO2 laden atmospheres, because of the shielding effect of sulphates formed on it. Conversely, for the LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO catalyst, the presence of MgO slows down sulphur poisoning, which becomes consistent only after 32 days of SO2 ageing. Therefore, the catalyst with the lowest Cr-content, LaCr0.25 Mn0.25 Mg0.5 O3 ·17MgO, results promising for application purposes thanks to the slow deactivation by sulphur poisoning and the complete regenerability to its original high catalytic activity by water or NH3 leaching. On the other hand, the catalyst with the highest Cr-content, LaCr0.5 Mg0.5 O3 , although less active in the fresh state, seems to assure a more stable performance because it shows the highest stability and resistance to sulphur poisoning and does not require regeneration leaching. This is a non-negligible feature for practical application in industrial catalytic reactors (e.g. catalytic burners, catalytic converters for gas turbines, . . . ) where long-term stability is an important prerequisite.

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