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Sulphur poisoning of LaMn1−xMgxO3·yMgO catalysts for methane combustion
Applied Catalysis B: Environmental 34 (2001) 29–41
Sulphur poisoning of LaMn1−x Mgx O3·yMgO catalysts for methane 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 28 January 2001; received in revised form 12 May 2001; accepted 12 May 2001
Abstract LaMn1−x Mgx O3 ·yMgO catalysts (x = 0, 0.2, 0.5, y = 2, 6, 17) were prepared, characterised (X-ray diffraction (XRD), BET, scanning electron microscopy (SEM)–energy dispersion spectroscopy (EDS), transmission electron microscopy (TEM), FTIR, chemical analysis and atomic absorption), tested for high-temperature methane combustion and aged in presence of SO2 to investigate the effect of sulphur on their catalytic activity. Two important roles of MgO have been assessed: (i) MgO acts as promoter for the catalytic activity of fresh LaMn1−x Mgx O3 ·yMgO catalysts up to a certain MgO excess (y = 6); (ii) MgO slows down sulphur poisoning of all LaMn1−x Mgx O3 ·yMgO catalysts. LaMn1−x Mgx O3 ·17MgO catalysts show the highest resistance to sulphur poisoning and their deactivation is appreciable only after 32 days of exposure to 200 ppmv SO2 at 800◦ C. They, then, can be regenerated completely by aqueous NH3 washing. Sulphate formation on both perovskite and the MgO phase is proposed to be responsible of catalyst deactivation. An explanation of the nature of these sulphates, also based on FTIR investigation, is given. © 2001 Elsevier Science B.V. All rights reserved. Keywords: LaMn1−x Mgx O3 ·yMgO; Perovskite; Methane catalytic combustion; Sulphur poisoning
1. Introduction Catalytic combustion of methane outperforms conventional flame combustion because of lower emission of pollutants (HC, CO and NOx ) and easier controllability (wider range of air-to-fuel ratios). Despite these well known advantages [1,2], industrial applications have been limited, so far, to diffusive-type catalytic heaters [3], because the other promising applications (gas turbines [4] and premixed catalytic burners for heat generation purposes [5]) require very strict properties of catalytic materials: low ∗
Corresponding author. Tel.: +39-11-5644710; fax: +39-11-5644699. E-mail address: [email protected] (I. Rosso).
ignition temperature, high-temperature resistance (up to 1300◦ C) and long-term stability. Since rather high operating temperatures cannot be tolerated by conventional oxidation catalysts like noble metals or simple metal oxides, perovskite-type oxides with suitable thermal stability are being developed. These oxides (A1−x Ax B1−y By O3±δ , where A, A = La, Sr, Ba, etc. and B, B = Co, Mn, Cr, etc.) are promising also for their good catalytic activity and low cost. Several compositions with perovskites and related-type structures have been prepared by various techniques and their activity evaluated [6–10]. Perovskite catalysts, however, are known to be sensitive to sulphur compounds [11,12]. Since commercial natural gas is odorised by small amounts of sulphur compounds (about 8 ppmv of tetrahydrothio-
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 1 ) 0 0 1 9 6 - 5
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phene in Italy), it is important to know the effect of these latter on the catalytic activity. Previous studies on the effect of sulphur on the catalytic activity of LaMn1−x Mgx O3 perovskites [13] showed a rather drastic deactivation of each catalyst, because of metal sulphate formation, after exposure to 200 ppmv of SO2 at 800◦ C for 24 h. The extent of poisoning as well as the catalyst regenerability depended on the catalysts composition and, in particular, a content in oxygen and magnesium higher at the surface than in the bulk of the LaMn0.5 Mg0.5 O3 catalyst seemed to favour a mechanism of SO2 adsorption preserving the nature of the active sites. The present paper describes the effect of sulphur on the catalytic activity of LaMn1−x Mgx O3 ·yMgO catalysts, where an excess of magnesium oxide is introduced on purpose during catalyst preparation. A previous paper [14] showed that the MgO excess in LaMn1−x Mgx O3 ·17MgO catalysts acted as a textural promoter by avoiding perovskite crystals sintering at high-temperature. In this report, the possible protecting role of MgO against sulphur poisoning of LaMn1−x Mgx O3 ·yMgO catalysts is investigated and a simple and rapid regeneration method, already proposed for LaMn1−x Mgx O3 catalysts [13], is extended to the MgO-enriched ones.
2. Experimental 2.1. Catalyst preparation A series of catalysts of the perovskite-type LaMn1−x Mgx O3 (x = 0, 0.2, 0.5) with different excess of MgO compared to the stoichiometric value (perovskite–MgO excess ratios: 25–75, 50–50 and 75–25 wt.%) was prepared via a modified version of the so-called “citrates method” described in [13]. Solid mixtures of La(NO3 )3 ·6H2 O, Mn(CH3 COO)2 ·4H2 O and Mg(NO3 )3 ·6H2 O (from Fluka), dosed in MgO/LaMn1−x Mgx O3 molar ratios of about 17/1, 6/1 and 2/1, respectively, were mixed with a 40 wt.% amount of glycerine and 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 are produced in huge
amounts thereby causing the formation of a solid scum, quite friable and porous. Each catalyst was then finely ground in an agate mortar and calcined in an electric oven at 900◦ C for 8 h in calm air. The same procedure was followed to prepare samples of MgO and of the mixed oxide Mg6 MnO8 . 2.2. Catalyst ageing tests Different samples of LaMn1−x Mgx O3 ·yMgO powdered catalysts and of MgO were treated at 800◦ C for 1, 2, 4, 8, 16 and 32 days in a controlled-atmosphere oven. A flow of air with 200 ppmv of SO2 was forced to pass at a rate of 20 N cm3 min−1 over 1 g of powdered material located in a porcelain combustion boat. A high SO2 level (several times higher than that corresponding to the 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 and regeneration studies. 2.3. Fresh/aged catalyst characterisation Chemical analysis (dissolution+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, Mg) was consistent with the nominal content. X-ray diffraction (XRD) analyses (Philips X’Pert apparatus equipped with a monochromator for Cu K␣ radiation) were performed on all fresh catalysts (to check the crystallisation of perovskite and MgO phases) and on the aged ones (to check the possible formation of new phases). The specific surface area, as determined by the BET method using N2 , was measured on all the fresh LaMn1−x Mgx O3 ·yMgO catalysts, as well as MgO and Mg6 MnO8 oxides (Micromeritics ASAP 2010M apparatus). BET measurements were also performed on some selected aged catalysts to assess the influence of SO2 poisoning on the specific surface area. Fresh and aged catalysts were examined by transmission electron microscopy (TEM, Philips EM 400 apparatus), which provided evidence on the microstructure of the different powdered perovskites. Scanning electron microscopy (SEM) and energy
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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 of the sample, simultaneous EDS and SEM measurements were not possible because the X-ray emission of gold overlaps those of other elements of interest (e.g. sulphur). Infrared spectroscopy spectra (Bruker Equinox 55 FTIR, equipped with MCT detector) were recorded in the trasmittance mode on pelletised samples of fresh, differently aged and washed LaMn1−x Mgx O3 ·yMgO and MgO powders, after degassing in vacuo at room temperature for 30 min.
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(T50 ) was regarded to as an index of the catalytic activity. 2.5. Regeneration procedure LaMn1−x Mgx O3 ·17MgO catalysts, aged in SO2 atmosphere for 32 days, were regenerated by NH3 leaching, according to the procedure described in [13], 1 g of each catalyst was washed by 60 cm3 of a 2.5% NH3 aqueous solution at room temperature in a stirred becker, the powder was then filtered on a paper filter, washed by water and dried in an oven at 120◦ C. The leachate was then neutralised by a diluted HCl solution. Cations and soluble sulphur were detected in the so acidified leaching medium by atomic absorption.
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 [13]. After 30 min stay at 800◦ C in air flow as a common pre-treatment, a gas flow rate of 50 N cm3 min−1 (composition: CH4 = 2%, O2 = 18%, He = balance) was fed to a fixed-bed of 0.5 g of catalyst particles (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 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. The reactor temperature was then lowered at a 3◦ C min−1 rate 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 the methane conversion. Methane conversion data plotted versus temperature values give typical sigma-shaped curves. 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 300 to 800◦ C). The half-conversion temperature
Fig. 1. X-ray diffraction patterns of the following catalysts (calcination temperature = 900◦ C): (a) LaMnO3 ·2MgO; (b) LaMnO3 ·6MgO; (c) LaMnO3 ·17MgO. Legend: (䉬) rhombohedral perovskite; (䊉) MgO; (䉱) Mg6 MnO8 .
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3. Results 3.1. Catalysts characterisation The diffraction patterns of LaMnO3 ·yMgO catalysts after 900◦ C calcination are shown, as an example, in Fig. 1. The perovskite phase is well defined by rhombohedral diffraction lines (JCPDS card 32-0484) and the diffractions peaks of MgO (JCPDS card 45-0946) are clearly seen in each diffraction pattern. The intensity of these latter peaks increases from curve a to c, because of the increasing MgO excess in the catalysts (from 25 to 75 wt.%, respectively). Weak peaks due to a third phase, Mg6 MnO8 (JCPDS card 19-0766), are present in all catalysts. Suitable tests showed that it also forms after 900◦ C calcination of a mechanical mixture of LaMnO3 and different amounts of MgO, separately prepared according to the experimental procedure described above. Similar diffraction patterns were obtained for LaMn0.8 Mg0.2 O3 ·yMgO and LaMn0.5 Mg0.5 O3 ·yMgO catalysts. Fig. 2 reports TEM micrographs of powdered LaMnO3 ·yMgO catalysts. The dark perovskite crystals are surrounded by light-grey MgO ones; the MgO excess is 50 wt.% in sample a and 75 wt.% in sam-
ple b. The average crystal size of perovskite phase is about 50–70 nm for both catalysts. Fig. 3 shows TEM micrographs of powdered LaMn0.5 Mg0.5 O3 ·yMgO catalysts where the MgO excess is 25 wt.% (Fig. 3a), 50 wt.% (Fig. 3b) and 75 wt.% (Fig. 3c), respectively. The perovskite crystals are randomly dispersed among those of MgO: the average perovskite crystal size is, in this case, smaller than that of the LaMnO3 ·yMgO catalysts. The average perovskite crystal size varies somewhat according to the different MgO excess: about 30 nm for MgO 25 wt.% (Fig. 3a), about 35 nm for MgO 50 wt.% (Fig. 3b) and about 20 nm for MgO 75 wt.% (Fig. 3c). These results are in good agreement with the BET results (Table 1). LaMnO3 ·yMgO and LaMn0.8 Mg0.2 O3 ·yMgO catalysts (y = 2, 6, 17) have a specific surface area of about 5–6 m2 g−1 , not changing substantially by increasing the MgO excess, whereas LaMn0.5 Mg0.5 O3 ·yMgO catalysts (y = 2, 6, 17) have a much larger specific surface area, which diminishes with increasing MgO excess up to 50 wt.% and then rises again when the MgO excess is 75 wt.%. Comparing these results with the BET data of LaMn1−x Mgx O3 catalysts [13], it results that MgO excess (specific surface area of 34.5 m2 g−1 ) only causes a limited increase of the specific surface area
Fig. 2. TEM micrographs of fresh powdered LaMnO3 ·yMgO catalysts: (a) LaMnO3 ·6MgO (220,000×); (b) LaMnO3 ·17MgO (135,000×).
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Fig. 3. TEM micrographs of fresh powdered LaMn0.5 Mg0.5 O3 ·yMgO catalysts: (a) LaMn0.5 Mg0.5 O3 ·2MgO (280,000×); (b) LaMn0.5 Mg0.5 O3 ·6MgO (280,000×); (c) LaMn0.5 Mg0.5 O3 ·17MgO (135,000×).
of LaMn1−x Mgx O3 ·yMgO catalysts, with the exception of LaMn0.5 Mg0.5 O3 ·yMgO catalysts which reach relatively high values of the specific surface area. As a comparison, the specific surface area of Mg6 MnO8 powder was 5.8 m2 g−1 . Table 1 also lists the half-conversion temperatures of all LaMn1−x Mgx O3 ·yMgO catalysts. The activity of all the catalysts is not too different; though, all of them are more active than the single MgO or Mg6 MnO8 phases, for which T50 = 582 and 536◦ C, respectively. SEM micrographs of powdered LaMn0.5 Mg0.5 O3 · yMgO catalysts are shown in Fig. 4: LaMn0.5 Mg0.5 O3 · 2MgO (Fig. 4a), LaMn0.5 Mg0.5 O3 ·6MgO (Fig. 4b)
and LaMn0.5 Mg0.5 O3 ·17MgO (Fig. 4c). Each catalyst shows a spongy appearance, basically not changing with the catalyst composition. EDS analysis reveals that the composition of each LaMn1−x Mgx O3 ·yMgO catalyst is not very homogeneous: in different analysed zones of the same sample, in fact, the amount of each element varies in a range of ±20 wt.% of the stoichiometric value. 3.2. Effect of SO2 exposure and of NH3 leaching on catalyst properties The XRD patterns of each LaMn1−x Mgx O3 ·yMgO catalyst do not change after any time of exposure to
Table 1 Specific surface area (BET, m2 g−1 ) and methane half-conversion temperatures (T50 , ◦ C) of fresh powdered LaMn1−x Mgx O3 ·yMgO perovskitesa yMgO
LaMnO3 ·yMgO BET
0b 2 6 17
4.6b 5.9 6.1 6.0 a b
(m2 g−1 )
LaMn0.8 Mg0.2 O3 ·yMgO T50
(◦ C)
537b 500 470 484
BET 4.5b 5.3 6.6 5.7
(m2 g−1 )
LaMn0.5 Mg0.5 O3 ·yMgO T50
(◦ C)
520b 462 466 497
BET (m2 g−1 )–T50 (◦ C) values of MgO and Mg6 MnO8 are: 34.5–582 and 5.8–536, respectively. Results of previous paper [13].
BET (m2 g−1 )
T50 (◦ C)
7.0b 32.8 20.8 40.3
455b 489 468 498
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Fig. 5. Methane half-conversion temperatures (T50 , ◦ C) of LaMn1−x Mgx O3 ·yMgO catalysts after progressively longer exposure periods to 200 ppmv SO2 at 800◦ C.
Fig. 4. SEM micrographs of fresh powdered LaMn0.5 Mg0.5 O3 ·yMgO catalysts: (a) LaMn0.5 Mg0.5 O3 ·2MgO (20,000×); (b) LaMn0.5 Mg0.5 O3 ·6MgO (20,000×); (c) LaMn0.5 Mg0.5 O3 ·17MgO (20,000×).
SO2 at 800◦ C. No new peaks, e.g. those attributable to sulphate phases, were observed. The LaMnO3 ·17MgO and LaMn0.8 Mg0.2 O3 ·17MgO catalysts do not change their specific surface area after 32 days of SO2 exposure at 800◦ C (6.1 and 5.9 m2 g−1 ,
respectively), whereas the LaMn0.5 Mg0.5 O3 ·17MgO catalyst looses after the same exposure some specific surface area: from 40.3 to 28.5 m2 g−1 . 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−1 after 16 days at 800◦ C. The effect of SO2 exposure for progressively longer contact times at 800◦ C on the catalyst activity is shown in Fig. 5: the half-conversion temperatures of LaMnO3 ·yMgO (Fig. 5a), LaMn0.8 Mg0.2 O3 ·yMgO (Fig. 5b) and LaMn0.5 Mg0.5 O3 ·yMgO (Fig. 5c) catalysts are reported. The T50 data of fresh and 1-day SO2 -aged LaMn1−x Mgx O3 catalysts [13] are also drawn for a comparison.
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All the LaMn1−x Mgx O3 catalysts face a drastic deactivation after 1 day of SO2 exposure, whereas a comparable deactivation is reached after about 4–8 days by the catalysts with 25 wt.% of MgO excess, only after about 16 days by those with 50 wt.% of MgO excess and it is not yet reached completely after 32 days by the catalysts with 75 wt.% of MgO excess. The variation of T50 values for progressively longer SO2 contact times is quite similar for the LaMn1−x Mgx O3 ·2MgO, LaMn1−x Mgx O3 ·6MgO and LaMn1−x Mgx O3 ·17MgO catalysts. The catalytic activity of MgO after 16 days of SO2 exposure at 800◦ C has been also measured, and a T50 value equal to 682◦ C has been obtained. The thermal effect on catalytic activity of LaMn0.5 Mg0.5 O3 ·17MgO catalyst after 32 days at 800◦ C without exposure to SO2 results almost negligible: the T50 value of aged catalyst is 524◦ C, whereas the T50 value of the fresh catalyst is 498◦ C. EDS analysis on aged catalysts reveals a significant sulphur presence only on LaMn0.5 Mg0.5 O3 ·yMgO catalysts. Particularly, the LaMn0.5 Mg0.5 O3 ·17MgO catalyst shows increasing amounts of sulphur with time: from 0.2 wt.% after 4 days of SO2 exposure up to about 1.3 wt.% after 32 days of SO2 exposure. This means that about 8–9 atoms of sulphur per 1 nm2 can
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be estimated on the LaMn0.5 Mg0.5 O3 ·17MgO catalyst after 32 days of SO2 exposure. FTIR spectra recorded on pelletised LaMn1−x Mgx O3 ·17MgO catalysts show the growth of absorbance peaks on all aged catalysts, particularly evident on the aged LaMn0.5 Mg0.5 O3 ·17MgO ones because of their high specific surface area. Fig. 6 reports the spectra after degassing in vacuo at room temperature for 30 min concerning this catalyst, both before and after SO2 ageing. Curve a is the spectrum of fresh LaMn0.5 Mg0.5 O3 ·17MgO catalyst. Comparison with curve b (spectrum of fresh MgO) shows that the two spectra are quite similar because of the high MgO excess in the catalyst. No bands due to the perovskite phase alone are evident. Both spectra show two broad bands at about 1500 and 1400 cm−1 , two bands at about 1225 and 1090 cm−1 and one band at about 870 cm−1 assignable to carbonates species [15] (all these bands are pointed out in Fig. 6 with the symbol (䊏)). The weak shoulder at about 1620 cm−1 (symbol (䊐) in Fig. 6) is probably due to molecular water. Both carbonates species and water arise from atmospheric CO2 and water vapour. The spectra of the 16- and 32-day-aged catalysts (curves c and d, respectively) are very similar to that observed for surface sulphates species on ZrO2 [16].
Fig. 6. Absorbance FTIR spectra in the 2000–800 cm−1 region (corresponding to the modes of sulphates and carbonates) of: (a) LaMn0.5 Mg0.5 O3 ·17MgO catalyst in the fresh mode; (b) MgO in the fresh mode; (c) the aged ones at 800◦ C in 200 ppmv SO2 atmosphere for 16 days; (d) for 32 days.
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Four sharp bands at 1192, 1108, 1069 and 993 cm−1 (pointed out in Fig. 6 with the symbol (䊉)) can be assigned to bidentate SO4 2− ions in C2v symmetry (species A). Another quadruplet of broader bands at about 1218, 1131, 1080 and 982 cm−1 (symbol (䊊) in Fig. 6) can be assigned to another sulphate species (species B). The two broad bands at about 1400 and 1500 cm−1 and that at about 860 cm−1 (symbol (䊏) in Fig. 6) are again due to carbonate species. The other two carbonate bands at 1225 and 1090 cm−1 are overlapped to those due to sulphate species in the same region. The comparison among the spectra of fresh (curve a) and aged LaMn0.5 Mg0.5 O3 ·17MgO catalysts (curves c and d) shows that the carbonate bands at about 1400 and 1500 cm−1 have weakened whereas the sulphate bands have increased. Note that also the bands at 1225 and 1090 cm−1 due to carbonates will decrease when the amount of sulphates increase, although this effect is not visible for the overlapping of sulphate bands. A competition between CO2 and SO2 adsorption on the catalyst can be suggested. As LaMn1−x Mgx O3 ·17MgO catalysts keep the best catalytic activity after progressively longer SO2 times of contact, the regeneration method by aqueous NH3 leaching, proposed in [13], was applied only to these catalysts after 32 days of SO2 ageing. Fig. 7 shows the results of the catalytic activity tests towards methane combustion performed with LaMnO3 ·17MgO (Fig. 7a), LaMn0.8 Mg0.2 O3 ·17MgO (Fig. 7b) and LaMn0.5 Mg0.5 O3 ·17MgO (Fig. 7c) catalysts in fresh state, after exposure to 200 ppmv SO2 at 800◦ C for 32 days and after aqueous NH3 leaching. Aqueous NH3 leaching is quite effective in restoring the catalytic activity of all LaMn1−x Mgx O3 ·17MgO catalysts. LaMn0.5 Mg0.5 O3 ·17MgO catalyst was washed also with a more concentrated NH3 aqueous solution (25% m/v) which provides a comparable recovery of catalytic activity (T50 values of 514 and 507◦ C for catalysts washed with diluted and concentrated NH3 solutions, respectively). Accordingly, atomic absorption analysis detected S and Mg elements in all neutralised leachates. EDS analysis on the LaMn0.5 Mg0.5 O3 ·17MgO catalyst washed with diluted NH3 solution still detected a residual amount of sulphur of 0.65 wt.%, i.e. about one-half of the amount present before leaching. Fig. 8 compares the FTIR spectra of the LaMn0.5 Mg0.5 O3 ·17MgO catalyst (aged and washed
Fig. 7. Catalytic activity test results for methane combustion on fresh, poisoned and regenerated LaMn1−x Mgx O3 ·17MgO catalysts.
with the two different NH3 solutions) and of pure MgO after 16 days of ageing in SO2 atmosphere. Curve b in Fig. 8 (catalyst washed with diluted NH3 aqueous solution) still shows the four well-separated bands (symbol (䊉)) assigned to sulphates (species A). The four additional shoulders at 1218, 1131, 1080 and 982 cm−1 , assigned to the other sulphates (species B, symbol (䊊)), have instead nearly disappeared. These bands do disappear after washing with a more concentrated NH3 solution (25% m/v) (curve c). The bands assigned to A sulphate species can be recognised on the spectrum of 16 days aged MgO (curve d); in this last case the bands have a weaker intensity because of the lower specific surface area of aged MgO.
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Fig. 8. Absorbance FTIR spectra in the 2000–800 cm−1 region (corresponding to the modes of sulphates and carbonates) of either LaMn0.5 Mg0.5 O3 ·17MgO catalysts: (a) aged at 800◦ C for 16 days in 200 ppmv SO2 atmosphere; (b) regenerated by NH3 (2.5% m/v) leaching; (c) regenerated by NH3 (25% m/v) leaching; (d) MgO aged at 800◦ C for 16 days in 200 ppmv SO2 atmosphere.
Finally, the specific surface area of the 32-day-aged LaMn0.5 Mg0.5 O3 ·17MgO catalyst (28.5 m2 g−1 ) increases after leaching with diluted NH3 solution up to 33.6 m2 g−1 . 4. Discussion The appearance of the Mg6 MnO8 species after calcination at 900◦ C indicates that the catalysts are not just a mechanical mixture of perovskite and magnesium oxide but, to some extent, also the result of solid phase reactions. For this reason, LaMn1−x Mgx O3 ·yMgO seemed a more proper notation than LaMn1−x Mgx O3 + yMgO, and therefore it has been adopted throughout the paper. The Mg6 MnO8 phase has a catalytic activity not negligible but nonetheless lower than that of each LaMn1−x Mgx O3 ·yMgO catalyst. Moreover, the amount present is so small that its effect on the overall catalytic activity is limited, so that it will be neglected in the following. In contrast, the catalytic activity of MgO is rather low. Therefore, the catalytic activity has been entirely ascribed to the perovskite phase.
The formation of the Mg6 MnO8 phase implies that some amount of Mn leaves the perovskite phase, compensated by some amount of Mg entering the perovskite structure of each MgO promoted catalyst. Due to the Mn/Mg substitution, which always takes place (0 < x ≤ 0.5) to some extent, the chemical formula of each MgO promoted compound LaMn1−x Mgx O3 ·yMgO is consistent with the overall composition of the material but not with the actual composition of the perovskite. This consideration is vital in explaining the methane half-conversion temperatures of fresh catalysts (Table 1). Summarising, three mechanisms are likely to affect the catalytic activity of fresh catalysts: • mechanism 1: the Mn/Mg substitution into the perovskite structure — it causes an improvement in the catalytic activity [13]; • mechanism 2: the progressive dilution of the perovskite active phase in LaMn1−x Mgx O3 ·yMgO (y = 2, 6, 17) catalysts, due to increasing MgO content — it implies a decrease of catalytic activity; • mechanism 3: the reduction of the average perovskite crystal size, due to the coprecipitation of MgO itself — it implies a higher specific surface
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area, generally favourable for the catalytic activity. Figs. 2 and 3 confirm that MgO effectively promotes the formation of small perovskite crystals (20–50 nm), as compared to the 100–120 nm crystal size of MgO-free LaMn1−x Mgx O3 perovskite [13]. LaMnO3 ·yMgO catalysts have T50 values lower than those of the corresponding catalysts without the MgO promoter, probably because of the Mn/Mg substitution and the crystal size reduction (mechanisms 1 and 3, respectively), but this improvement tends to decrease for the catalyst with the highest MgO excess because of the increasing importance of the active phase dilution (mechanism 2). When compared to LaMnO3 ·yMgO, the activity of LaMn0.8 Mg0.2 O3 ·yMgO catalysts improves even more due to the larger influence of the Mn/Mg substitution (mechanism 1). The T50 values are very low at small MgO excesses, as well, likely due to the crystal size reduction (mechanism 3). Once again, the improvement of catalytic activity tends to decrease for the catalyst with the highest MgO excess because of the increasing importance of the active phase dilution (mechanism 2). LaMn0.5 Mg0.5 O3 ·yMgO catalysts have, instead, T50 values higher than that of the corresponding catalyst without a MgO excess, probably because in the latter the Mg amount is at the optimal value. The active phase dilution (mechanism 2) seems to be the prevailing effect, whereas the effect due to the Mn/Mg substitution (mechanism 1) becomes negligible. The large gain in the specific surface area obtained by introducing the MgO promoter (LaMn0.5 Mg0.5 O3 ·yMgO catalysts) seems not to have a direct positive effect on the catalytic activity. TEM micrographs of Fig. 3 show that the perovskite crystals are tightly surrounded by MgO ones and this mixture is so strict that MgO crystals could cover the perovskite ones and, as already proposed [14], hamper the reacting gases from reaching the active catalyst surface. Another interesting information comes from the pore size distribution (Fig. 9), calculated by the BJH method from the adsorption (Fig. 9a) and the desorption (Fig. 9b) isotherms. As an example, Fig. 9 reports the ratio of pore volume per unit mass/pore diameter, which may be considered proportional to the specific area versus the pore diameter of both the powder (solid lines) and the pellets (dashed lines) of
Fig. 9. Pore size distribution of powder (solid lines) and pellets (dashed lines) of the fresh LaMn0.5 Mg0.5 O3 ·17MgO catalyst calculated by the BJH method: (a) from the adsorption isotherm; (b) from the desorption isotherm.
the LaMn0.5 Mg0.5 O3 ·17MgO catalyst, selected on the basis of its high specific surface area. The pore area internally exposed of the catalyst powder (Fig. 9a, solid line) is provided for about the same extent by pores of about 2.8 nm and by those of 30 nm diameter; in contrast, the area of pores that face the external surface (Fig. 9b, solid line) is mainly given by large pores (diameter of 32 nm) with a little contribution of the small ones (pores of about 3.5 nm diameter). This means that for the powder of the LaMn0.5 Mg0.5 O3 ·17MgO catalyst the main part of the large pores (about 30 nm diameter) face the external surface. These pore size values are of the same order of magnitude of the size of the perovskite crystals (20 nm). It can thus be argued that the inter-crystalline spaces are directly accounted for by the BJH data. If the pore size distribution of catalyst pellets (0.2–0.5 m) is considered (dashed lines in Fig. 9a
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and b), a very similar trend can be observed, except a larger contribution of small pores to internal pore area. It is important to underline that the experimental procedure performed to obtain catalyst pellets for the catalytic activity tests is not suitable for obtaining a good microstructural pore distribution, as concerns the reduction of intra-pellet mass transfer resistance. If catalyst pellets were produced with a clusterised pore distribution (macropores entering the pellet from its surface, with side micropores exploiting the high specific surface area), the effectiveness factor of the pellets would have been improved. Instead, by pressure compaction into tablets of the catalyst powders, as obtained from the preparation procedure, it results a pore structure inside the pellet of pore size analogous to the inter-crystalline spacing mentioned earlier. The small size pores of the prepared pellets hinder the reactants from reaching the pellet core and should in part reduce the catalyst effectiveness factor. This negative feature compensates for the high specific surface area leading to less than expected T50 values. Therefore, an intermediate MgO excess can represent a good compromise among all the mentioned phenomena: the best catalytic activity is, in fact, reached by all of LaMn1−x Mgx O3 ·6MgO catalysts. Sulphur poisoning of the LaMn1−x Mgx O3 ·yMgO catalysts brings about no consistent structural modifications on the catalyst, as indicated by the XRD results and already shown for the non-promoted LaMn1−x Mgx O3 catalysts [13]. As far as the LaMn1−x Mgx O3 ·17MgO catalysts are concerned, variations specific surface area are evident only for the LaMn0.5 Mg0.5 O3 ·17MgO catalyst, for which a surface area decrease after 32 days of SO2 exposure at 800◦ C is obtained (from 40.3 to 28.5 m2 g−1 ). On the one hand, this result confirms the effectiveness of MgO as a textural promoter, which hinders catalyst sintering [14]; on the other hand, the partial decrease of the specific surface area of the 32-day-aged LaMn0.5 Mg0.5 O3 ·17MgO catalyst is at least in part due to a thermal effect besides the sulphur presence. The recovery of specific surface area is in fact incomplete also after sulphur removal by the regeneration procedure with the diluted NH3 solution (33.6 m2 g−1 ). As far as the effects of SO2 exposure on the catalytic activity are concerned, Fig. 5 clearly shows that the MgO-free and MgO-rich catalysts have a
39
quite different behaviour. While MgO-free catalysts deactivate drastically after 1 day of SO2 exposure, MgO-rich catalysts deactivate more slowly and the higher is the MgO content, the slower is the deactivation rate. A protecting role of MgO against sulphur poisoning of LaMn1−x Mgx O3 ·yMgO catalysts is to be inferred: after 32 days of SO2 exposure the catalysts with the higher MgO excess are less deactivated than the corresponding MgO-free catalyst after only 1 day of SO2 exposure. MgO possibly represents a preferential adsorption medium for SO2 molecules [12], so that the MgO-richer catalysts allow the active sites of the perovskite phase to avoid sulphur poisoning for a longer time. After prolonged exposure to SO2 , also the active sites of the perovskite phase are involved in sulphur poisoning and their catalytic activity is somehow depressed. However, since a sample of MgO, aged for 16 days under 200 ppmv of SO2 , showed a drastic increase of the T50 value (from 582 to 682◦ C), the catalytic activity of aged LaMn1−x Mgx O3 ·yMgO catalysts seems to be still due to the perovskite phase not yet poisoned by sulphur. Therefore, if the LaMn1−x Mgx O3 ·6MgO catalysts show the best performance in the fresh state, the LaMn1−x Mgx O3 ·17MgO catalysts keep the highest catalytic activity after prolonged exposure to sulphur. As a consequence, the utilisation of these latter catalysts for application purposes would be preferred in order to assure a sufficient performance durability of catalytic reactors (e.g. catalytic burners [17]). Sulphate formation probably takes place on all the LaMn1−x Mgx O3 ·yMgO catalysts, although the sulphur presence has been clearly detected by EDS analysis only on LaMn0.5 Mg0.5 O3 ·17MgO catalysts, owing to their high specific surface area. FTIR spectra give evidence of SO4 2− species on all the aged catalyst, but very strong bands, assignable to sulphates and carbonates species, are particularly evident only on LaMn0.5 Mg0.5 O3 ·17MgO catalysts (Fig. 6, curves c and d) thanks, once again, to their high specific surface area. As the spectra of the fresh LaMn0.5 Mg0.5 O3 ·17MgO catalyst and of fresh MgO (Fig. 6, curves a and b) are very similar, the FTIR spectra of all the LaMn0.5 Mg0.5 O3 ·17MgO catalysts are probably attributable only to the MgO phase. In such samples, perovskite phase is present in a low amount (25 wt.%); moreover, it is well possible that perovskite crystals are embedded into the MgO matrix,
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so that a fraction of them are not accessible. As a consequence, carbonate or/and sulphate species formed on the perovskite phase are, probably, not visible. The decrease of the intensity of carbonate bands when the intensity of sulphate bands increases, suggests a competition between adsorption of CO2 and SO2 (both being acidic anhydrides) on the same basic MgO sites. Strong bands assignable to carbonates are, in fact, present on the fresh LaMn0.5 Mg0.5 O3 ·17MgO catalyst, because they form easily at room temperature on basic MgO by reaction with atmospheric CO2 . Carbonates thermally decompose during exposure to SO2 at 800◦ C and sulphates form; the higher is the amount of sulphates formed, the lower is the amount of carbonates that will form at room temperature at the end of the ageing test. From the results of EDS analysis, it can be calculated that after 32 days of SO2 poisoning 8–9 SO2 molecules per 1 nm2 are at the surface of LaMn0.5 Mg0.5 O3 ·17MgO catalyst and that are halved by washing with diluted NH3 solution. Assuming crudely that: (i) all the sulphur sits on the MgO phase; (ii) the MgO microcrystals expose externally the (1 0 0) face of their cubic structure; (iii) due to the area of this face, about six MgO pairs are present per 1 nm2 , it results that there are about 1.33–1.5 S atoms per MgO surface unit. This figure would indicate that sulphation is not only confined to the external surface. Indeed, two different kinds of sulphate species are seen on the spectra of aged LaMn1−x Mgx O3 ·17MgO catalysts (Fig. 6): species A (sharp bands at 1192, 1108, 1069 and 993 cm−1 ; symbol (䊉)) and species B (broader bands at 1218, 1131, 1080 and 982 cm−1 ; symbol (䊊)). Species B, easily removed by washing with diluted NH3 solution and in a more complete way with concentrated NH3 solution, probably sits on the external surface, while species A, remaining on the washed catalyst, probably has a bulk or sub-superficial nature. The larger width of species B bands can be reasonably explained by the heterogeneity of these surface sulphates due to the nature of the MgO surface, carrying, for instance, Mg(OH)2 species. This effect should be less for sulphates formed inside the MgO phase as bulk or, at least, sub-superficial species, which therefore show sharper bands. Bands of species B are seen also on the spectrum of 16-day-aged MgO (Fig. 8, curve d): the overall weakness of SO2 - and CO2 -related bands is due to the low
specific surface area of the aged oxide. Species B are sulphate species formed on the surface of the MgO. Ageing of MgO in a SO2 atmosphere does not go beyond the formation of surface sulphates: species A are not found and even less probable is the formation of bulk MgSO4 which has a rather different FTIR spectrum. Previous work on SO2 deactivation of pure perovskite LaMnO3 [13] has shown that also in that case SO2 poisoning should take place through sulphate formation, probably because La2 O3 is known to be strongly basic. In the present case, as well, deactivation of the catalyst is probably attributable to sulphate formation on the perovskite phase. Unfortunately, no direct evidence of such sulphates is available, unless their IR features coincide with those of the species found on the MgO phase. Indeed, the variability in the peak position of sulphates is not large, and the same frequencies have been found for the mentioned species B in this study and on sulphated zirconia [16]. A further speculative possibility is that the outer layers of the perovskite phase might be rich in the basic compounds (La2 O3 and MgO) so that the chemical environment of sulphates would not be greatly different from the perovskite and the MgO phase. Anyway, although FTIR investigation does not give evidence of sulphates formed and removed from perovskite phase, which is the active phase of all the LaMn1−x Mgx O3 ·yMgO catalysts, it can help to explain the catalytic performance of the aged and differently washed LaMn0.5 Mg0.5 O3 ·17MgO catalysts. Bulk or sub-superficial sulphate species on MgO phase, species A, are not involved in the catalytic activity and, in fact, they are present either on poisoned or on completely regenerated LaMn0.5 Mg0.5 O3 ·17MgO catalyst spectra. On the contrary, surface sulphate species on the MgO phase, species B, can be important for catalytic activity and, in fact, they are present on the spectrum of poisoned LaMn0.5 Mg0.5 O3 ·17MgO catalyst, but disappear from the spectra of the same regenerated catalyst. It can be argued that these surface sulphate species on the MgO phase might have a shielding effect on the perovskite surface thereby preventing the reacting gases from reaching the active sites of perovskite phase, that, after sulphate removal, becomes once again available for catalytic reaction.
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5. Conclusions LaMn1−x Mgx O3 ·yMgO catalysts have been prepared, characterised and tested as catalysts for high-temperature methane combustion; up to a limited amount of MgO excess (y = 6), a promoting effect of MgO on the catalytic activity has been observed. More precisely, MgO alone gives a low contribution to catalytic activity, but the simultaneous synthesis of perovskite and MgO crystals has been proved to cause a Mn/Mg substitution into the perovskite structure of each catalyst and a reduction of the average perovskite crystal size (20–50 nm). Both these occurrences could effectively improve the catalytic activity. For high MgO excess (75 wt.%) the significant dilution of perovskite active phase balances, at least in part, the above advantage. Exposure to 200 ppmv SO2 at 800◦ C up to 32 days causes a deactivation of each LaMn1−x Mgx O3 ·yMgO catalyst, but the higher is the MgO content the slower is the catalyst deactivation, so that a protecting role of MgO against sulphur poisoning of LaMn1−x Mgx O3 ·yMgO catalysts can be argued. MgO represents possible preferential adsorption site for the SO2 molecules and formation of sulphate species on the MgO phase was proved by FTIR spectra of 32 days poisoned LaMn1−x Mgx O3 ·17MgO catalysts. The activity of each of these catalysts can be regenerated completely by aqueous NH3 washing. FTIR investigation allows to distinguish two kinds of sulphate species formed on the MgO phase: a surface species, important for catalytic activity, and a bulk or sub-superficial species, not affecting the catalytic activity. Catalyst deactivation should be caused by formation of sulphate species on both perovskite (direct deactivation) and MgO phases (shielding effect). Therefore, if LaMn1−x Mgx O3 ·6MgO catalysts show the best performance in fresh state, LaMn1−x Mgx O3 ·17MgO ones should be the most promising
41
for practical application purposes. Thanks to their high resistance to sulphur poisoning, these perovskite systems are being applied in the development of premixed catalytic burner prototypes for domestic boiler applications [17]. References [1] D.L. Trimm, Appl. Catal. 7 (1984) 249. [2] L.D. Pfefferle, W.C. Pfefferle, Catal. Rev. Sci. Eng. 32 (1987) 219. [3] S.W. Radcliffe, R.G. Hickman, J. Inst. Fuel 48 (1975) 208. [4] G. Ertl, R.L. Garten, R.A. Dalla Betta, J.C. Schlatter, in: G. Ertl, H. Kn¨ozinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Vol. 1, VCH, Weinheim, 1997, p. 1668. [5] I. Cerri, G. Saracco, F. Geobaldo, V. Specchia, Ind. Eng. Chem. Res. 39 (2000) 24. [6] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26 (1986) 265. [7] J. Kirchnerova, D. Klvana, J. Vaillancourt, J. Chaouki, Catal. Lett. 21 (1993) 77. [8] M. Daturi, G. Busca, G. Groppi, P. Forzatti, Appl. Catal. B 12 (1997) 325. [9] L. Lisi, G. Bagnasco, P. Ciambelli, S. De Rossi, P. Porta, G. Russo, M. Turco, J. Solid State Chem. 146 (1999) 176. [10] P. Ciambelli, S. Cimino, S. De Rossi, M. Faticanti, L. Lisi, G. Minelli, I. Pettiti, P. Porta, G. Russo, M. Turco, Appl. Catal. B 24 (2000) 243. [11] L.J. Tejuca, J.L.G. Fierro, J.M.D. Tascon, in: D.D. Eley, H. Pines, P.B. Weisz (Eds.), Advances in Catalysis, Vol. 36, Academic Press, New York, 1989, p. 237. [12] L. Wan, in: L.G. Tejuca, J.L.G. Fierro (Eds.), Properties and Applications of Perovskite-Type Oxides, Marcel Dekker, New York, 1993, p. 145. [13] I. Rosso, E. Garrone, F. Geobaldo, B. Onida, G. Saracco, V. Specchia, Appl. Catal. B 30 (2001) 61. [14] G. Saracco, F. Geobaldo, G. Baldi, Appl. Catal. B 20 (1999) 277. [15] J.C. Lavalley, Catal. Today 27 (1996) 377. [16] F. Babou, G. Coudurier, J.C. Vedrine, J. Catal. 152 (1995) 341. [17] I. Cerri, G. Saracco, V. Specchia, D. Trimis, Chem. Eng. J. 82 (2001) 73–85.
Sulphur poisoning of LaMn1−x Mgx O3·yMgO catalysts for methane 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 28 January 2001; received in revised form 12 May 2001; accepted 12 May 2001
Abstract LaMn1−x Mgx O3 ·yMgO catalysts (x = 0, 0.2, 0.5, y = 2, 6, 17) were prepared, characterised (X-ray diffraction (XRD), BET, scanning electron microscopy (SEM)–energy dispersion spectroscopy (EDS), transmission electron microscopy (TEM), FTIR, chemical analysis and atomic absorption), tested for high-temperature methane combustion and aged in presence of SO2 to investigate the effect of sulphur on their catalytic activity. Two important roles of MgO have been assessed: (i) MgO acts as promoter for the catalytic activity of fresh LaMn1−x Mgx O3 ·yMgO catalysts up to a certain MgO excess (y = 6); (ii) MgO slows down sulphur poisoning of all LaMn1−x Mgx O3 ·yMgO catalysts. LaMn1−x Mgx O3 ·17MgO catalysts show the highest resistance to sulphur poisoning and their deactivation is appreciable only after 32 days of exposure to 200 ppmv SO2 at 800◦ C. They, then, can be regenerated completely by aqueous NH3 washing. Sulphate formation on both perovskite and the MgO phase is proposed to be responsible of catalyst deactivation. An explanation of the nature of these sulphates, also based on FTIR investigation, is given. © 2001 Elsevier Science B.V. All rights reserved. Keywords: LaMn1−x Mgx O3 ·yMgO; Perovskite; Methane catalytic combustion; Sulphur poisoning
1. Introduction Catalytic combustion of methane outperforms conventional flame combustion because of lower emission of pollutants (HC, CO and NOx ) and easier controllability (wider range of air-to-fuel ratios). Despite these well known advantages [1,2], industrial applications have been limited, so far, to diffusive-type catalytic heaters [3], because the other promising applications (gas turbines [4] and premixed catalytic burners for heat generation purposes [5]) require very strict properties of catalytic materials: low ∗
Corresponding author. Tel.: +39-11-5644710; fax: +39-11-5644699. E-mail address: [email protected] (I. Rosso).
ignition temperature, high-temperature resistance (up to 1300◦ C) and long-term stability. Since rather high operating temperatures cannot be tolerated by conventional oxidation catalysts like noble metals or simple metal oxides, perovskite-type oxides with suitable thermal stability are being developed. These oxides (A1−x Ax B1−y By O3±δ , where A, A = La, Sr, Ba, etc. and B, B = Co, Mn, Cr, etc.) are promising also for their good catalytic activity and low cost. Several compositions with perovskites and related-type structures have been prepared by various techniques and their activity evaluated [6–10]. Perovskite catalysts, however, are known to be sensitive to sulphur compounds [11,12]. Since commercial natural gas is odorised by small amounts of sulphur compounds (about 8 ppmv of tetrahydrothio-
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 1 ) 0 0 1 9 6 - 5
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phene in Italy), it is important to know the effect of these latter on the catalytic activity. Previous studies on the effect of sulphur on the catalytic activity of LaMn1−x Mgx O3 perovskites [13] showed a rather drastic deactivation of each catalyst, because of metal sulphate formation, after exposure to 200 ppmv of SO2 at 800◦ C for 24 h. The extent of poisoning as well as the catalyst regenerability depended on the catalysts composition and, in particular, a content in oxygen and magnesium higher at the surface than in the bulk of the LaMn0.5 Mg0.5 O3 catalyst seemed to favour a mechanism of SO2 adsorption preserving the nature of the active sites. The present paper describes the effect of sulphur on the catalytic activity of LaMn1−x Mgx O3 ·yMgO catalysts, where an excess of magnesium oxide is introduced on purpose during catalyst preparation. A previous paper [14] showed that the MgO excess in LaMn1−x Mgx O3 ·17MgO catalysts acted as a textural promoter by avoiding perovskite crystals sintering at high-temperature. In this report, the possible protecting role of MgO against sulphur poisoning of LaMn1−x Mgx O3 ·yMgO catalysts is investigated and a simple and rapid regeneration method, already proposed for LaMn1−x Mgx O3 catalysts [13], is extended to the MgO-enriched ones.
2. Experimental 2.1. Catalyst preparation A series of catalysts of the perovskite-type LaMn1−x Mgx O3 (x = 0, 0.2, 0.5) with different excess of MgO compared to the stoichiometric value (perovskite–MgO excess ratios: 25–75, 50–50 and 75–25 wt.%) was prepared via a modified version of the so-called “citrates method” described in [13]. Solid mixtures of La(NO3 )3 ·6H2 O, Mn(CH3 COO)2 ·4H2 O and Mg(NO3 )3 ·6H2 O (from Fluka), dosed in MgO/LaMn1−x Mgx O3 molar ratios of about 17/1, 6/1 and 2/1, respectively, were mixed with a 40 wt.% amount of glycerine and 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 are produced in huge
amounts thereby causing the formation of a solid scum, quite friable and porous. Each catalyst was then finely ground in an agate mortar and calcined in an electric oven at 900◦ C for 8 h in calm air. The same procedure was followed to prepare samples of MgO and of the mixed oxide Mg6 MnO8 . 2.2. Catalyst ageing tests Different samples of LaMn1−x Mgx O3 ·yMgO powdered catalysts and of MgO were treated at 800◦ C for 1, 2, 4, 8, 16 and 32 days in a controlled-atmosphere oven. A flow of air with 200 ppmv of SO2 was forced to pass at a rate of 20 N cm3 min−1 over 1 g of powdered material located in a porcelain combustion boat. A high SO2 level (several times higher than that corresponding to the 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 and regeneration studies. 2.3. Fresh/aged catalyst characterisation Chemical analysis (dissolution+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, Mg) was consistent with the nominal content. X-ray diffraction (XRD) analyses (Philips X’Pert apparatus equipped with a monochromator for Cu K␣ radiation) were performed on all fresh catalysts (to check the crystallisation of perovskite and MgO phases) and on the aged ones (to check the possible formation of new phases). The specific surface area, as determined by the BET method using N2 , was measured on all the fresh LaMn1−x Mgx O3 ·yMgO catalysts, as well as MgO and Mg6 MnO8 oxides (Micromeritics ASAP 2010M apparatus). BET measurements were also performed on some selected aged catalysts to assess the influence of SO2 poisoning on the specific surface area. Fresh and aged catalysts were examined by transmission electron microscopy (TEM, Philips EM 400 apparatus), which provided evidence on the microstructure of the different powdered perovskites. Scanning electron microscopy (SEM) and energy
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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 of the sample, simultaneous EDS and SEM measurements were not possible because the X-ray emission of gold overlaps those of other elements of interest (e.g. sulphur). Infrared spectroscopy spectra (Bruker Equinox 55 FTIR, equipped with MCT detector) were recorded in the trasmittance mode on pelletised samples of fresh, differently aged and washed LaMn1−x Mgx O3 ·yMgO and MgO powders, after degassing in vacuo at room temperature for 30 min.
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(T50 ) was regarded to as an index of the catalytic activity. 2.5. Regeneration procedure LaMn1−x Mgx O3 ·17MgO catalysts, aged in SO2 atmosphere for 32 days, were regenerated by NH3 leaching, according to the procedure described in [13], 1 g of each catalyst was washed by 60 cm3 of a 2.5% NH3 aqueous solution at room temperature in a stirred becker, the powder was then filtered on a paper filter, washed by water and dried in an oven at 120◦ C. The leachate was then neutralised by a diluted HCl solution. Cations and soluble sulphur were detected in the so acidified leaching medium by atomic absorption.
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 [13]. After 30 min stay at 800◦ C in air flow as a common pre-treatment, a gas flow rate of 50 N cm3 min−1 (composition: CH4 = 2%, O2 = 18%, He = balance) was fed to a fixed-bed of 0.5 g of catalyst particles (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 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. The reactor temperature was then lowered at a 3◦ C min−1 rate 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 the methane conversion. Methane conversion data plotted versus temperature values give typical sigma-shaped curves. 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 300 to 800◦ C). The half-conversion temperature
Fig. 1. X-ray diffraction patterns of the following catalysts (calcination temperature = 900◦ C): (a) LaMnO3 ·2MgO; (b) LaMnO3 ·6MgO; (c) LaMnO3 ·17MgO. Legend: (䉬) rhombohedral perovskite; (䊉) MgO; (䉱) Mg6 MnO8 .
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3. Results 3.1. Catalysts characterisation The diffraction patterns of LaMnO3 ·yMgO catalysts after 900◦ C calcination are shown, as an example, in Fig. 1. The perovskite phase is well defined by rhombohedral diffraction lines (JCPDS card 32-0484) and the diffractions peaks of MgO (JCPDS card 45-0946) are clearly seen in each diffraction pattern. The intensity of these latter peaks increases from curve a to c, because of the increasing MgO excess in the catalysts (from 25 to 75 wt.%, respectively). Weak peaks due to a third phase, Mg6 MnO8 (JCPDS card 19-0766), are present in all catalysts. Suitable tests showed that it also forms after 900◦ C calcination of a mechanical mixture of LaMnO3 and different amounts of MgO, separately prepared according to the experimental procedure described above. Similar diffraction patterns were obtained for LaMn0.8 Mg0.2 O3 ·yMgO and LaMn0.5 Mg0.5 O3 ·yMgO catalysts. Fig. 2 reports TEM micrographs of powdered LaMnO3 ·yMgO catalysts. The dark perovskite crystals are surrounded by light-grey MgO ones; the MgO excess is 50 wt.% in sample a and 75 wt.% in sam-
ple b. The average crystal size of perovskite phase is about 50–70 nm for both catalysts. Fig. 3 shows TEM micrographs of powdered LaMn0.5 Mg0.5 O3 ·yMgO catalysts where the MgO excess is 25 wt.% (Fig. 3a), 50 wt.% (Fig. 3b) and 75 wt.% (Fig. 3c), respectively. The perovskite crystals are randomly dispersed among those of MgO: the average perovskite crystal size is, in this case, smaller than that of the LaMnO3 ·yMgO catalysts. The average perovskite crystal size varies somewhat according to the different MgO excess: about 30 nm for MgO 25 wt.% (Fig. 3a), about 35 nm for MgO 50 wt.% (Fig. 3b) and about 20 nm for MgO 75 wt.% (Fig. 3c). These results are in good agreement with the BET results (Table 1). LaMnO3 ·yMgO and LaMn0.8 Mg0.2 O3 ·yMgO catalysts (y = 2, 6, 17) have a specific surface area of about 5–6 m2 g−1 , not changing substantially by increasing the MgO excess, whereas LaMn0.5 Mg0.5 O3 ·yMgO catalysts (y = 2, 6, 17) have a much larger specific surface area, which diminishes with increasing MgO excess up to 50 wt.% and then rises again when the MgO excess is 75 wt.%. Comparing these results with the BET data of LaMn1−x Mgx O3 catalysts [13], it results that MgO excess (specific surface area of 34.5 m2 g−1 ) only causes a limited increase of the specific surface area
Fig. 2. TEM micrographs of fresh powdered LaMnO3 ·yMgO catalysts: (a) LaMnO3 ·6MgO (220,000×); (b) LaMnO3 ·17MgO (135,000×).
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Fig. 3. TEM micrographs of fresh powdered LaMn0.5 Mg0.5 O3 ·yMgO catalysts: (a) LaMn0.5 Mg0.5 O3 ·2MgO (280,000×); (b) LaMn0.5 Mg0.5 O3 ·6MgO (280,000×); (c) LaMn0.5 Mg0.5 O3 ·17MgO (135,000×).
of LaMn1−x Mgx O3 ·yMgO catalysts, with the exception of LaMn0.5 Mg0.5 O3 ·yMgO catalysts which reach relatively high values of the specific surface area. As a comparison, the specific surface area of Mg6 MnO8 powder was 5.8 m2 g−1 . Table 1 also lists the half-conversion temperatures of all LaMn1−x Mgx O3 ·yMgO catalysts. The activity of all the catalysts is not too different; though, all of them are more active than the single MgO or Mg6 MnO8 phases, for which T50 = 582 and 536◦ C, respectively. SEM micrographs of powdered LaMn0.5 Mg0.5 O3 · yMgO catalysts are shown in Fig. 4: LaMn0.5 Mg0.5 O3 · 2MgO (Fig. 4a), LaMn0.5 Mg0.5 O3 ·6MgO (Fig. 4b)
and LaMn0.5 Mg0.5 O3 ·17MgO (Fig. 4c). Each catalyst shows a spongy appearance, basically not changing with the catalyst composition. EDS analysis reveals that the composition of each LaMn1−x Mgx O3 ·yMgO catalyst is not very homogeneous: in different analysed zones of the same sample, in fact, the amount of each element varies in a range of ±20 wt.% of the stoichiometric value. 3.2. Effect of SO2 exposure and of NH3 leaching on catalyst properties The XRD patterns of each LaMn1−x Mgx O3 ·yMgO catalyst do not change after any time of exposure to
Table 1 Specific surface area (BET, m2 g−1 ) and methane half-conversion temperatures (T50 , ◦ C) of fresh powdered LaMn1−x Mgx O3 ·yMgO perovskitesa yMgO
LaMnO3 ·yMgO BET
0b 2 6 17
4.6b 5.9 6.1 6.0 a b
(m2 g−1 )
LaMn0.8 Mg0.2 O3 ·yMgO T50
(◦ C)
537b 500 470 484
BET 4.5b 5.3 6.6 5.7
(m2 g−1 )
LaMn0.5 Mg0.5 O3 ·yMgO T50
(◦ C)
520b 462 466 497
BET (m2 g−1 )–T50 (◦ C) values of MgO and Mg6 MnO8 are: 34.5–582 and 5.8–536, respectively. Results of previous paper [13].
BET (m2 g−1 )
T50 (◦ C)
7.0b 32.8 20.8 40.3
455b 489 468 498
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Fig. 5. Methane half-conversion temperatures (T50 , ◦ C) of LaMn1−x Mgx O3 ·yMgO catalysts after progressively longer exposure periods to 200 ppmv SO2 at 800◦ C.
Fig. 4. SEM micrographs of fresh powdered LaMn0.5 Mg0.5 O3 ·yMgO catalysts: (a) LaMn0.5 Mg0.5 O3 ·2MgO (20,000×); (b) LaMn0.5 Mg0.5 O3 ·6MgO (20,000×); (c) LaMn0.5 Mg0.5 O3 ·17MgO (20,000×).
SO2 at 800◦ C. No new peaks, e.g. those attributable to sulphate phases, were observed. The LaMnO3 ·17MgO and LaMn0.8 Mg0.2 O3 ·17MgO catalysts do not change their specific surface area after 32 days of SO2 exposure at 800◦ C (6.1 and 5.9 m2 g−1 ,
respectively), whereas the LaMn0.5 Mg0.5 O3 ·17MgO catalyst looses after the same exposure some specific surface area: from 40.3 to 28.5 m2 g−1 . 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−1 after 16 days at 800◦ C. The effect of SO2 exposure for progressively longer contact times at 800◦ C on the catalyst activity is shown in Fig. 5: the half-conversion temperatures of LaMnO3 ·yMgO (Fig. 5a), LaMn0.8 Mg0.2 O3 ·yMgO (Fig. 5b) and LaMn0.5 Mg0.5 O3 ·yMgO (Fig. 5c) catalysts are reported. The T50 data of fresh and 1-day SO2 -aged LaMn1−x Mgx O3 catalysts [13] are also drawn for a comparison.
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All the LaMn1−x Mgx O3 catalysts face a drastic deactivation after 1 day of SO2 exposure, whereas a comparable deactivation is reached after about 4–8 days by the catalysts with 25 wt.% of MgO excess, only after about 16 days by those with 50 wt.% of MgO excess and it is not yet reached completely after 32 days by the catalysts with 75 wt.% of MgO excess. The variation of T50 values for progressively longer SO2 contact times is quite similar for the LaMn1−x Mgx O3 ·2MgO, LaMn1−x Mgx O3 ·6MgO and LaMn1−x Mgx O3 ·17MgO catalysts. The catalytic activity of MgO after 16 days of SO2 exposure at 800◦ C has been also measured, and a T50 value equal to 682◦ C has been obtained. The thermal effect on catalytic activity of LaMn0.5 Mg0.5 O3 ·17MgO catalyst after 32 days at 800◦ C without exposure to SO2 results almost negligible: the T50 value of aged catalyst is 524◦ C, whereas the T50 value of the fresh catalyst is 498◦ C. EDS analysis on aged catalysts reveals a significant sulphur presence only on LaMn0.5 Mg0.5 O3 ·yMgO catalysts. Particularly, the LaMn0.5 Mg0.5 O3 ·17MgO catalyst shows increasing amounts of sulphur with time: from 0.2 wt.% after 4 days of SO2 exposure up to about 1.3 wt.% after 32 days of SO2 exposure. This means that about 8–9 atoms of sulphur per 1 nm2 can
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be estimated on the LaMn0.5 Mg0.5 O3 ·17MgO catalyst after 32 days of SO2 exposure. FTIR spectra recorded on pelletised LaMn1−x Mgx O3 ·17MgO catalysts show the growth of absorbance peaks on all aged catalysts, particularly evident on the aged LaMn0.5 Mg0.5 O3 ·17MgO ones because of their high specific surface area. Fig. 6 reports the spectra after degassing in vacuo at room temperature for 30 min concerning this catalyst, both before and after SO2 ageing. Curve a is the spectrum of fresh LaMn0.5 Mg0.5 O3 ·17MgO catalyst. Comparison with curve b (spectrum of fresh MgO) shows that the two spectra are quite similar because of the high MgO excess in the catalyst. No bands due to the perovskite phase alone are evident. Both spectra show two broad bands at about 1500 and 1400 cm−1 , two bands at about 1225 and 1090 cm−1 and one band at about 870 cm−1 assignable to carbonates species [15] (all these bands are pointed out in Fig. 6 with the symbol (䊏)). The weak shoulder at about 1620 cm−1 (symbol (䊐) in Fig. 6) is probably due to molecular water. Both carbonates species and water arise from atmospheric CO2 and water vapour. The spectra of the 16- and 32-day-aged catalysts (curves c and d, respectively) are very similar to that observed for surface sulphates species on ZrO2 [16].
Fig. 6. Absorbance FTIR spectra in the 2000–800 cm−1 region (corresponding to the modes of sulphates and carbonates) of: (a) LaMn0.5 Mg0.5 O3 ·17MgO catalyst in the fresh mode; (b) MgO in the fresh mode; (c) the aged ones at 800◦ C in 200 ppmv SO2 atmosphere for 16 days; (d) for 32 days.
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Four sharp bands at 1192, 1108, 1069 and 993 cm−1 (pointed out in Fig. 6 with the symbol (䊉)) can be assigned to bidentate SO4 2− ions in C2v symmetry (species A). Another quadruplet of broader bands at about 1218, 1131, 1080 and 982 cm−1 (symbol (䊊) in Fig. 6) can be assigned to another sulphate species (species B). The two broad bands at about 1400 and 1500 cm−1 and that at about 860 cm−1 (symbol (䊏) in Fig. 6) are again due to carbonate species. The other two carbonate bands at 1225 and 1090 cm−1 are overlapped to those due to sulphate species in the same region. The comparison among the spectra of fresh (curve a) and aged LaMn0.5 Mg0.5 O3 ·17MgO catalysts (curves c and d) shows that the carbonate bands at about 1400 and 1500 cm−1 have weakened whereas the sulphate bands have increased. Note that also the bands at 1225 and 1090 cm−1 due to carbonates will decrease when the amount of sulphates increase, although this effect is not visible for the overlapping of sulphate bands. A competition between CO2 and SO2 adsorption on the catalyst can be suggested. As LaMn1−x Mgx O3 ·17MgO catalysts keep the best catalytic activity after progressively longer SO2 times of contact, the regeneration method by aqueous NH3 leaching, proposed in [13], was applied only to these catalysts after 32 days of SO2 ageing. Fig. 7 shows the results of the catalytic activity tests towards methane combustion performed with LaMnO3 ·17MgO (Fig. 7a), LaMn0.8 Mg0.2 O3 ·17MgO (Fig. 7b) and LaMn0.5 Mg0.5 O3 ·17MgO (Fig. 7c) catalysts in fresh state, after exposure to 200 ppmv SO2 at 800◦ C for 32 days and after aqueous NH3 leaching. Aqueous NH3 leaching is quite effective in restoring the catalytic activity of all LaMn1−x Mgx O3 ·17MgO catalysts. LaMn0.5 Mg0.5 O3 ·17MgO catalyst was washed also with a more concentrated NH3 aqueous solution (25% m/v) which provides a comparable recovery of catalytic activity (T50 values of 514 and 507◦ C for catalysts washed with diluted and concentrated NH3 solutions, respectively). Accordingly, atomic absorption analysis detected S and Mg elements in all neutralised leachates. EDS analysis on the LaMn0.5 Mg0.5 O3 ·17MgO catalyst washed with diluted NH3 solution still detected a residual amount of sulphur of 0.65 wt.%, i.e. about one-half of the amount present before leaching. Fig. 8 compares the FTIR spectra of the LaMn0.5 Mg0.5 O3 ·17MgO catalyst (aged and washed
Fig. 7. Catalytic activity test results for methane combustion on fresh, poisoned and regenerated LaMn1−x Mgx O3 ·17MgO catalysts.
with the two different NH3 solutions) and of pure MgO after 16 days of ageing in SO2 atmosphere. Curve b in Fig. 8 (catalyst washed with diluted NH3 aqueous solution) still shows the four well-separated bands (symbol (䊉)) assigned to sulphates (species A). The four additional shoulders at 1218, 1131, 1080 and 982 cm−1 , assigned to the other sulphates (species B, symbol (䊊)), have instead nearly disappeared. These bands do disappear after washing with a more concentrated NH3 solution (25% m/v) (curve c). The bands assigned to A sulphate species can be recognised on the spectrum of 16 days aged MgO (curve d); in this last case the bands have a weaker intensity because of the lower specific surface area of aged MgO.
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Fig. 8. Absorbance FTIR spectra in the 2000–800 cm−1 region (corresponding to the modes of sulphates and carbonates) of either LaMn0.5 Mg0.5 O3 ·17MgO catalysts: (a) aged at 800◦ C for 16 days in 200 ppmv SO2 atmosphere; (b) regenerated by NH3 (2.5% m/v) leaching; (c) regenerated by NH3 (25% m/v) leaching; (d) MgO aged at 800◦ C for 16 days in 200 ppmv SO2 atmosphere.
Finally, the specific surface area of the 32-day-aged LaMn0.5 Mg0.5 O3 ·17MgO catalyst (28.5 m2 g−1 ) increases after leaching with diluted NH3 solution up to 33.6 m2 g−1 . 4. Discussion The appearance of the Mg6 MnO8 species after calcination at 900◦ C indicates that the catalysts are not just a mechanical mixture of perovskite and magnesium oxide but, to some extent, also the result of solid phase reactions. For this reason, LaMn1−x Mgx O3 ·yMgO seemed a more proper notation than LaMn1−x Mgx O3 + yMgO, and therefore it has been adopted throughout the paper. The Mg6 MnO8 phase has a catalytic activity not negligible but nonetheless lower than that of each LaMn1−x Mgx O3 ·yMgO catalyst. Moreover, the amount present is so small that its effect on the overall catalytic activity is limited, so that it will be neglected in the following. In contrast, the catalytic activity of MgO is rather low. Therefore, the catalytic activity has been entirely ascribed to the perovskite phase.
The formation of the Mg6 MnO8 phase implies that some amount of Mn leaves the perovskite phase, compensated by some amount of Mg entering the perovskite structure of each MgO promoted catalyst. Due to the Mn/Mg substitution, which always takes place (0 < x ≤ 0.5) to some extent, the chemical formula of each MgO promoted compound LaMn1−x Mgx O3 ·yMgO is consistent with the overall composition of the material but not with the actual composition of the perovskite. This consideration is vital in explaining the methane half-conversion temperatures of fresh catalysts (Table 1). Summarising, three mechanisms are likely to affect the catalytic activity of fresh catalysts: • mechanism 1: the Mn/Mg substitution into the perovskite structure — it causes an improvement in the catalytic activity [13]; • mechanism 2: the progressive dilution of the perovskite active phase in LaMn1−x Mgx O3 ·yMgO (y = 2, 6, 17) catalysts, due to increasing MgO content — it implies a decrease of catalytic activity; • mechanism 3: the reduction of the average perovskite crystal size, due to the coprecipitation of MgO itself — it implies a higher specific surface
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area, generally favourable for the catalytic activity. Figs. 2 and 3 confirm that MgO effectively promotes the formation of small perovskite crystals (20–50 nm), as compared to the 100–120 nm crystal size of MgO-free LaMn1−x Mgx O3 perovskite [13]. LaMnO3 ·yMgO catalysts have T50 values lower than those of the corresponding catalysts without the MgO promoter, probably because of the Mn/Mg substitution and the crystal size reduction (mechanisms 1 and 3, respectively), but this improvement tends to decrease for the catalyst with the highest MgO excess because of the increasing importance of the active phase dilution (mechanism 2). When compared to LaMnO3 ·yMgO, the activity of LaMn0.8 Mg0.2 O3 ·yMgO catalysts improves even more due to the larger influence of the Mn/Mg substitution (mechanism 1). The T50 values are very low at small MgO excesses, as well, likely due to the crystal size reduction (mechanism 3). Once again, the improvement of catalytic activity tends to decrease for the catalyst with the highest MgO excess because of the increasing importance of the active phase dilution (mechanism 2). LaMn0.5 Mg0.5 O3 ·yMgO catalysts have, instead, T50 values higher than that of the corresponding catalyst without a MgO excess, probably because in the latter the Mg amount is at the optimal value. The active phase dilution (mechanism 2) seems to be the prevailing effect, whereas the effect due to the Mn/Mg substitution (mechanism 1) becomes negligible. The large gain in the specific surface area obtained by introducing the MgO promoter (LaMn0.5 Mg0.5 O3 ·yMgO catalysts) seems not to have a direct positive effect on the catalytic activity. TEM micrographs of Fig. 3 show that the perovskite crystals are tightly surrounded by MgO ones and this mixture is so strict that MgO crystals could cover the perovskite ones and, as already proposed [14], hamper the reacting gases from reaching the active catalyst surface. Another interesting information comes from the pore size distribution (Fig. 9), calculated by the BJH method from the adsorption (Fig. 9a) and the desorption (Fig. 9b) isotherms. As an example, Fig. 9 reports the ratio of pore volume per unit mass/pore diameter, which may be considered proportional to the specific area versus the pore diameter of both the powder (solid lines) and the pellets (dashed lines) of
Fig. 9. Pore size distribution of powder (solid lines) and pellets (dashed lines) of the fresh LaMn0.5 Mg0.5 O3 ·17MgO catalyst calculated by the BJH method: (a) from the adsorption isotherm; (b) from the desorption isotherm.
the LaMn0.5 Mg0.5 O3 ·17MgO catalyst, selected on the basis of its high specific surface area. The pore area internally exposed of the catalyst powder (Fig. 9a, solid line) is provided for about the same extent by pores of about 2.8 nm and by those of 30 nm diameter; in contrast, the area of pores that face the external surface (Fig. 9b, solid line) is mainly given by large pores (diameter of 32 nm) with a little contribution of the small ones (pores of about 3.5 nm diameter). This means that for the powder of the LaMn0.5 Mg0.5 O3 ·17MgO catalyst the main part of the large pores (about 30 nm diameter) face the external surface. These pore size values are of the same order of magnitude of the size of the perovskite crystals (20 nm). It can thus be argued that the inter-crystalline spaces are directly accounted for by the BJH data. If the pore size distribution of catalyst pellets (0.2–0.5 m) is considered (dashed lines in Fig. 9a
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and b), a very similar trend can be observed, except a larger contribution of small pores to internal pore area. It is important to underline that the experimental procedure performed to obtain catalyst pellets for the catalytic activity tests is not suitable for obtaining a good microstructural pore distribution, as concerns the reduction of intra-pellet mass transfer resistance. If catalyst pellets were produced with a clusterised pore distribution (macropores entering the pellet from its surface, with side micropores exploiting the high specific surface area), the effectiveness factor of the pellets would have been improved. Instead, by pressure compaction into tablets of the catalyst powders, as obtained from the preparation procedure, it results a pore structure inside the pellet of pore size analogous to the inter-crystalline spacing mentioned earlier. The small size pores of the prepared pellets hinder the reactants from reaching the pellet core and should in part reduce the catalyst effectiveness factor. This negative feature compensates for the high specific surface area leading to less than expected T50 values. Therefore, an intermediate MgO excess can represent a good compromise among all the mentioned phenomena: the best catalytic activity is, in fact, reached by all of LaMn1−x Mgx O3 ·6MgO catalysts. Sulphur poisoning of the LaMn1−x Mgx O3 ·yMgO catalysts brings about no consistent structural modifications on the catalyst, as indicated by the XRD results and already shown for the non-promoted LaMn1−x Mgx O3 catalysts [13]. As far as the LaMn1−x Mgx O3 ·17MgO catalysts are concerned, variations specific surface area are evident only for the LaMn0.5 Mg0.5 O3 ·17MgO catalyst, for which a surface area decrease after 32 days of SO2 exposure at 800◦ C is obtained (from 40.3 to 28.5 m2 g−1 ). On the one hand, this result confirms the effectiveness of MgO as a textural promoter, which hinders catalyst sintering [14]; on the other hand, the partial decrease of the specific surface area of the 32-day-aged LaMn0.5 Mg0.5 O3 ·17MgO catalyst is at least in part due to a thermal effect besides the sulphur presence. The recovery of specific surface area is in fact incomplete also after sulphur removal by the regeneration procedure with the diluted NH3 solution (33.6 m2 g−1 ). As far as the effects of SO2 exposure on the catalytic activity are concerned, Fig. 5 clearly shows that the MgO-free and MgO-rich catalysts have a
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quite different behaviour. While MgO-free catalysts deactivate drastically after 1 day of SO2 exposure, MgO-rich catalysts deactivate more slowly and the higher is the MgO content, the slower is the deactivation rate. A protecting role of MgO against sulphur poisoning of LaMn1−x Mgx O3 ·yMgO catalysts is to be inferred: after 32 days of SO2 exposure the catalysts with the higher MgO excess are less deactivated than the corresponding MgO-free catalyst after only 1 day of SO2 exposure. MgO possibly represents a preferential adsorption medium for SO2 molecules [12], so that the MgO-richer catalysts allow the active sites of the perovskite phase to avoid sulphur poisoning for a longer time. After prolonged exposure to SO2 , also the active sites of the perovskite phase are involved in sulphur poisoning and their catalytic activity is somehow depressed. However, since a sample of MgO, aged for 16 days under 200 ppmv of SO2 , showed a drastic increase of the T50 value (from 582 to 682◦ C), the catalytic activity of aged LaMn1−x Mgx O3 ·yMgO catalysts seems to be still due to the perovskite phase not yet poisoned by sulphur. Therefore, if the LaMn1−x Mgx O3 ·6MgO catalysts show the best performance in the fresh state, the LaMn1−x Mgx O3 ·17MgO catalysts keep the highest catalytic activity after prolonged exposure to sulphur. As a consequence, the utilisation of these latter catalysts for application purposes would be preferred in order to assure a sufficient performance durability of catalytic reactors (e.g. catalytic burners [17]). Sulphate formation probably takes place on all the LaMn1−x Mgx O3 ·yMgO catalysts, although the sulphur presence has been clearly detected by EDS analysis only on LaMn0.5 Mg0.5 O3 ·17MgO catalysts, owing to their high specific surface area. FTIR spectra give evidence of SO4 2− species on all the aged catalyst, but very strong bands, assignable to sulphates and carbonates species, are particularly evident only on LaMn0.5 Mg0.5 O3 ·17MgO catalysts (Fig. 6, curves c and d) thanks, once again, to their high specific surface area. As the spectra of the fresh LaMn0.5 Mg0.5 O3 ·17MgO catalyst and of fresh MgO (Fig. 6, curves a and b) are very similar, the FTIR spectra of all the LaMn0.5 Mg0.5 O3 ·17MgO catalysts are probably attributable only to the MgO phase. In such samples, perovskite phase is present in a low amount (25 wt.%); moreover, it is well possible that perovskite crystals are embedded into the MgO matrix,
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so that a fraction of them are not accessible. As a consequence, carbonate or/and sulphate species formed on the perovskite phase are, probably, not visible. The decrease of the intensity of carbonate bands when the intensity of sulphate bands increases, suggests a competition between adsorption of CO2 and SO2 (both being acidic anhydrides) on the same basic MgO sites. Strong bands assignable to carbonates are, in fact, present on the fresh LaMn0.5 Mg0.5 O3 ·17MgO catalyst, because they form easily at room temperature on basic MgO by reaction with atmospheric CO2 . Carbonates thermally decompose during exposure to SO2 at 800◦ C and sulphates form; the higher is the amount of sulphates formed, the lower is the amount of carbonates that will form at room temperature at the end of the ageing test. From the results of EDS analysis, it can be calculated that after 32 days of SO2 poisoning 8–9 SO2 molecules per 1 nm2 are at the surface of LaMn0.5 Mg0.5 O3 ·17MgO catalyst and that are halved by washing with diluted NH3 solution. Assuming crudely that: (i) all the sulphur sits on the MgO phase; (ii) the MgO microcrystals expose externally the (1 0 0) face of their cubic structure; (iii) due to the area of this face, about six MgO pairs are present per 1 nm2 , it results that there are about 1.33–1.5 S atoms per MgO surface unit. This figure would indicate that sulphation is not only confined to the external surface. Indeed, two different kinds of sulphate species are seen on the spectra of aged LaMn1−x Mgx O3 ·17MgO catalysts (Fig. 6): species A (sharp bands at 1192, 1108, 1069 and 993 cm−1 ; symbol (䊉)) and species B (broader bands at 1218, 1131, 1080 and 982 cm−1 ; symbol (䊊)). Species B, easily removed by washing with diluted NH3 solution and in a more complete way with concentrated NH3 solution, probably sits on the external surface, while species A, remaining on the washed catalyst, probably has a bulk or sub-superficial nature. The larger width of species B bands can be reasonably explained by the heterogeneity of these surface sulphates due to the nature of the MgO surface, carrying, for instance, Mg(OH)2 species. This effect should be less for sulphates formed inside the MgO phase as bulk or, at least, sub-superficial species, which therefore show sharper bands. Bands of species B are seen also on the spectrum of 16-day-aged MgO (Fig. 8, curve d): the overall weakness of SO2 - and CO2 -related bands is due to the low
specific surface area of the aged oxide. Species B are sulphate species formed on the surface of the MgO. Ageing of MgO in a SO2 atmosphere does not go beyond the formation of surface sulphates: species A are not found and even less probable is the formation of bulk MgSO4 which has a rather different FTIR spectrum. Previous work on SO2 deactivation of pure perovskite LaMnO3 [13] has shown that also in that case SO2 poisoning should take place through sulphate formation, probably because La2 O3 is known to be strongly basic. In the present case, as well, deactivation of the catalyst is probably attributable to sulphate formation on the perovskite phase. Unfortunately, no direct evidence of such sulphates is available, unless their IR features coincide with those of the species found on the MgO phase. Indeed, the variability in the peak position of sulphates is not large, and the same frequencies have been found for the mentioned species B in this study and on sulphated zirconia [16]. A further speculative possibility is that the outer layers of the perovskite phase might be rich in the basic compounds (La2 O3 and MgO) so that the chemical environment of sulphates would not be greatly different from the perovskite and the MgO phase. Anyway, although FTIR investigation does not give evidence of sulphates formed and removed from perovskite phase, which is the active phase of all the LaMn1−x Mgx O3 ·yMgO catalysts, it can help to explain the catalytic performance of the aged and differently washed LaMn0.5 Mg0.5 O3 ·17MgO catalysts. Bulk or sub-superficial sulphate species on MgO phase, species A, are not involved in the catalytic activity and, in fact, they are present either on poisoned or on completely regenerated LaMn0.5 Mg0.5 O3 ·17MgO catalyst spectra. On the contrary, surface sulphate species on the MgO phase, species B, can be important for catalytic activity and, in fact, they are present on the spectrum of poisoned LaMn0.5 Mg0.5 O3 ·17MgO catalyst, but disappear from the spectra of the same regenerated catalyst. It can be argued that these surface sulphate species on the MgO phase might have a shielding effect on the perovskite surface thereby preventing the reacting gases from reaching the active sites of perovskite phase, that, after sulphate removal, becomes once again available for catalytic reaction.
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5. Conclusions LaMn1−x Mgx O3 ·yMgO catalysts have been prepared, characterised and tested as catalysts for high-temperature methane combustion; up to a limited amount of MgO excess (y = 6), a promoting effect of MgO on the catalytic activity has been observed. More precisely, MgO alone gives a low contribution to catalytic activity, but the simultaneous synthesis of perovskite and MgO crystals has been proved to cause a Mn/Mg substitution into the perovskite structure of each catalyst and a reduction of the average perovskite crystal size (20–50 nm). Both these occurrences could effectively improve the catalytic activity. For high MgO excess (75 wt.%) the significant dilution of perovskite active phase balances, at least in part, the above advantage. Exposure to 200 ppmv SO2 at 800◦ C up to 32 days causes a deactivation of each LaMn1−x Mgx O3 ·yMgO catalyst, but the higher is the MgO content the slower is the catalyst deactivation, so that a protecting role of MgO against sulphur poisoning of LaMn1−x Mgx O3 ·yMgO catalysts can be argued. MgO represents possible preferential adsorption site for the SO2 molecules and formation of sulphate species on the MgO phase was proved by FTIR spectra of 32 days poisoned LaMn1−x Mgx O3 ·17MgO catalysts. The activity of each of these catalysts can be regenerated completely by aqueous NH3 washing. FTIR investigation allows to distinguish two kinds of sulphate species formed on the MgO phase: a surface species, important for catalytic activity, and a bulk or sub-superficial species, not affecting the catalytic activity. Catalyst deactivation should be caused by formation of sulphate species on both perovskite (direct deactivation) and MgO phases (shielding effect). Therefore, if LaMn1−x Mgx O3 ·6MgO catalysts show the best performance in fresh state, LaMn1−x Mgx O3 ·17MgO ones should be the most promising
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for practical application purposes. Thanks to their high resistance to sulphur poisoning, these perovskite systems are being applied in the development of premixed catalytic burner prototypes for domestic boiler applications [17]. References [1] D.L. Trimm, Appl. Catal. 7 (1984) 249. [2] L.D. Pfefferle, W.C. Pfefferle, Catal. Rev. Sci. Eng. 32 (1987) 219. [3] S.W. Radcliffe, R.G. Hickman, J. Inst. Fuel 48 (1975) 208. [4] G. Ertl, R.L. Garten, R.A. Dalla Betta, J.C. Schlatter, in: G. Ertl, H. Kn¨ozinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Vol. 1, VCH, Weinheim, 1997, p. 1668. [5] I. Cerri, G. Saracco, F. Geobaldo, V. Specchia, Ind. Eng. Chem. Res. 39 (2000) 24. [6] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26 (1986) 265. [7] J. Kirchnerova, D. Klvana, J. Vaillancourt, J. Chaouki, Catal. Lett. 21 (1993) 77. [8] M. Daturi, G. Busca, G. Groppi, P. Forzatti, Appl. Catal. B 12 (1997) 325. [9] L. Lisi, G. Bagnasco, P. Ciambelli, S. De Rossi, P. Porta, G. Russo, M. Turco, J. Solid State Chem. 146 (1999) 176. [10] P. Ciambelli, S. Cimino, S. De Rossi, M. Faticanti, L. Lisi, G. Minelli, I. Pettiti, P. Porta, G. Russo, M. Turco, Appl. Catal. B 24 (2000) 243. [11] L.J. Tejuca, J.L.G. Fierro, J.M.D. Tascon, in: D.D. Eley, H. Pines, P.B. Weisz (Eds.), Advances in Catalysis, Vol. 36, Academic Press, New York, 1989, p. 237. [12] L. Wan, in: L.G. Tejuca, J.L.G. Fierro (Eds.), Properties and Applications of Perovskite-Type Oxides, Marcel Dekker, New York, 1993, p. 145. [13] I. Rosso, E. Garrone, F. Geobaldo, B. Onida, G. Saracco, V. Specchia, Appl. Catal. B 30 (2001) 61. [14] G. Saracco, F. Geobaldo, G. Baldi, Appl. Catal. B 20 (1999) 277. [15] J.C. Lavalley, Catal. Today 27 (1996) 377. [16] F. Babou, G. Coudurier, J.C. Vedrine, J. Catal. 152 (1995) 341. [17] I. Cerri, G. Saracco, V. Specchia, D. Trimis, Chem. Eng. J. 82 (2001) 73–85.