- Email: [email protected]
Preparation, characterisation and catalytic activity of pure and substituted La-hexaaluminate systems for high temperature catalytic combustion
Applied Catalysis B: Environmental 35 (2001) 137–148
Preparation, characterisation and catalytic activity of pure and substituted La-hexaaluminate systems for high temperature catalytic combustion Gianpiero Groppi∗ , Cinzia Cristiani, Pio Forzatti Dipartimento di Chimica Industriale ed Ingegneria Chimica “G. Natta”, Politecnico di Milano, CIIC, Piazza Leonardo da Vinci 32, I 20133 Milano, Italy Received 12 March 2001; received in revised form 10 July 2001; accepted 29 July 2001
Abstract La-Al-O, La-Mg-Al-O, La-Mn-Al-O and La-Mg-Mn-Al-O hexaaluminates have been prepared using the carbonates route previously developed for M-substituted and unsubstituted Ba--Al2 O3 . Starting from amorphous precursors, the formation of a final magnetoplumbite (MP) phase is observed upon calcination at T ≥ 1100◦ C. In the case of LaMn1 Al11 O19 , the MP phase already forms at 900◦ C evidencing a promotion effect of Mn ions. Upon calcination at 1300◦ C, monophasic samples can be obtained only for Mg-substituted samples (LaMg1 Al11 O19 and LaMg0.5 Mn0.5 Al11 O19 ), whereas in the other samples the presence of LaAlO3 is always detected. This behaviour is associated with the stabilisation, via a charge compensation mechanism, of the MP phase due to the introduction of Mg2+ ions in the structure. The co-presence of Mg and Mn in the final catalyst has resulted in a higher specific catalytic activity per Mn mol. Such a behaviour is likely associated with the stabilisation of Mn ions at high oxidation state due to the co-presence of Mg2+ . © 2001 Elsevier Science B.V. All rights reserved. Keywords: Catalytic combustion; Hexaaluminates; Mn-Mg substituted; CH4 combustion
1. Introduction Hexaaluminate materials are of interest as combustion catalysts for gas turbine applications, due to the high thermal stability associated with their peculiar layered structure that consists of ␥-Al2 O3 spinel block intercalated by planes (mirror planes) in which the largest cations (Ba, Ca, La and Sr) are located. Introduction of transition metal ions (preferably Mn) in the spinel blocks provides good combustion activity [1–3] without decrease of sintering resistance.
∗ Corresponding author. Tel.: +39-2-2399-3258; fax: +39-2-7063-8173. E-mail address: [email protected] (G. Groppi).
Several compositions have been investigated in [3]. Among these La-hexaaluminates have received much interest either as supports [4,5] or as bulk catalysts [6–8], in view of their higher resistance towards poisoning by sulphur compounds and carbonates with respect to Ba- and Sr-hexaaluminates [9]. Furthermore, La-containing Mn-substituted hexaaluminates have been reported to be slightly more active in CH4 combustion reactions than the Ba-based ones [6–8]. Two routes have been proposed to prepare La-hexaaluminates with structural and morphological properties appropriate for catalytic purposes, namely the alkoxides [6,10] and the carbonates route [4,7]. However, according to characterisation data reported in [1–4,8,10], single phase hexaaluminate materials can hardly be obtained. Segregation of other phases,
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 2 4 8 - X
138
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
i.e. LaAlO3 , perovskite or ␣-Al2 O3 is typically reported, that might result in additional sintering with losses of surface area of the final material. According to the crystallographic literature in non substituted La-hexaaluminates, that are generally described by the formula LaAl11 O18 , only a very distorted magnetoplumbite (MP) structure forms, due to the high defectivity of the material [11]. Such a structure consists of [Al11 O16 ]+ spinel blocks intercalated by mirror planes of composition [LaAlO3 ]0 . This composition results in an electrically charged cell [LaAl12 O19 ]+ , so that defectivity is required to maintain the electroneutrality. According to Iyi et al. [11] in the mirror planes of the defective cells the La3+ site is vacant and one Al3+ position is replaced by two oxygen ions bonded to interstitial Al pairs forming bridges between the spinel blocks above and below the mirror plane. The final structure consists of non defective [LaAl12 O19 ]+ and defective [Al11 O19 ]5− cells in ratio 1 to 5 that corresponds to a neutral composition La0.833 Al11.833 O19 (i.e. Al/La = 14.2), that is characterised by a wide Al excess with respect to the nominal LaAl11 O18 composition. Considering the structural model above, it is evident that the introduction of bivalent ions in the spinel blocks, replacing Al3+ , can be an effective alternative to Al excess in order to obtain electrically neutral structure through a charge compensation mechanism. Indeed, monophasic Ln-hexaaluminates (Ln = La, Pr, Nd, Sm, Eu, Gd) have been obtained by partial substitution of Al3+ with Mg2+ ions [12,13]. Further crystallographic studies have shown that also transition metal ions such as Mn, Fe, Co and Ni can enter the structure with divalent oxidation state providing a similar stabilisation effect [14–19]. These pieces of crystallographic evidences have been collected on materials synthesised through crystallisation from melted precursor to obtain monocrystals or via high temperature solid state reaction to obtain strongly sintered materials which, however, are not adequate for catalytic uses. On the other hand, no systematic studies have been published on the possibility to obtain polycrystalline monophasic La-hexaaluminate, to be used as bulk catalysts or support, depending on the La/Al and on the amount and the nature of the Al substituent. This paper is devoted to the characterisation of La-Al-O, La-Mg-Al-O, La-Mn-Al-O and La-Mg-Mn-
Al-O hexaaluminates, prepared via an aqueous coprecipitation route suitable for catalytic purposes [3]. Thermal evolution of the precursor, and the structure and the morphology of the final materials have been studied to provide insight in the influence of the preparation chemistry on the formation and the properties of the final hexaaluminates. The effect of the catalysts composition on activity in CH4 combustion has also been addressed.
2. Experimental LaAl11 O18 , LaAl14.2 O22.81 and LaMgx Mn1−x Al11 O19 with x = 0, 0.5, and 1 have been prepared using the coprecipitation method known as carbonates route described elsewhere [20,21]. Briefly, an acid solution of the nitrates of the ions is poured, under vigorous stirring, into a solution of excess (NH4 )2 CO3 hold at 60◦ C. Upon contact, a precipitate immediately forms and the pH of the slurry stabilises at 7–8 depending on the composition. The slurry is aged at 60◦ C for 3 h and then filtered to separate the solid. The obtained cake is washed to remove nitrates and excess carbonates, re-suspending the solid in cold water and filtering up to negative results of the nitrates qualitative assay [22]. The resulting solid is then dried at 110◦ C overnight under re-circulation of air, grinded in a mortar and then calcined at 500, 700, 900, 1000, 1100, and 1300◦ C for 10 h (heating ramp 60◦ C/h, cooling ramp 100◦ C/h). In the following, the samples will be identified by a notation that account for chemical composition and calcination temperature: e.g. notation LaMg0.5 Mn0.5 Al11–700 corresponds to sample with composition La/Mg/Mn/Al = 1/0.5/0.5/11 calcined at 700◦ C. The dried and the calcined samples have been characterised by X-ray diffraction (XRD) using a Nifiltered Cu K␣ radiation (Philips vertical goniometer PW 1050-70) and surface area determination by N2 adsorption (Fison Sorptomatic 1900 instruments). XRD patterns for determination of phase composition have been collected in the angular range 10–70◦ (2θ ), with a step size of 0.05◦ (2θ ), a counting time of 12 s per step and a divergent slit of 1◦ (2θ ). The relative amounts of LaAlO3 and La-hexaluminates in the samples calcined at 1300◦ C have been
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
calculated as the ratio of the XRD intensity of the (1 1 0) reflection of LaAlO3 to the (1 1 0) reflection of LaAl11 O18 . For this purpose spectra have been collected in step scan mode, a selected angular range of 30–40◦ (2θ ), step size of 0.01◦ (2θ ), counting time of 9 s per step and a divergence slit of 0.5◦ . The intensities of the reflections have been calculated using a profile fitting routine provided by Philips. Temperature-programmed reduction (TPR) measurements have been performed on a TPDRO 1100 ThermoQuest apparatus. About 300 mg of fine powder samples (d p = 0.1 mm). have been loaded for each measurements. The reactor has been fed with 60 cm3 /min (at STP) of a 5% v/v H2 in Ar mixture. Heating rate has been set to 15◦ C/min from room temperature to 1100◦ C. Catalytic activity in CH4 combustion has been investigated over the Mn-containing samples LaMn1 Al11 and LaMg0.5 Mn0.5 Al11 calcined at 1300◦ C. A commercial catalyst with nominal composition Sr0.8 La0.2 Mn1 Al11 O19 by Süd-Chemie has been also tested as a reference sample. Catalytic activity tests have been performed in a laboratory rig, equipped with a quartz microreactor that is described in details elsewhere [23]. The reactor (i.d. = 0.8 cm.) has been loaded with 0.45 g of fine catalyst powders (100–150 mesh) diluted with quartz with the same particle size (V cat /V quartz = 2/1). The reactor has been fed with 1% of CH4 in air at GHSV = 54000 cm3 /gcat h (at STP). Analyses of products and reactants have been performed by an on-line GC using two in parallel packed columns filled with 5 Å molecular sieves and Porapak QS® , both equipped with TCD detectors. 3. Results 3.1. Dried samples The XRD spectra of all the samples dried at 110◦ C (Fig. 1) exhibit a set of reflections whose positions correspond to (NH4 )2 Al6 (CO3 )3 ·H2 O (ammonium–aluminium–carbonate–hydroxi–hydrate (AACHH)) [20] with calculated mean crystallite dimensions of about 100 Å. This phase composition is similar to that already detected in Ba-hexaaluminate
139
Fig. 1. XRD spectra of samples dried at 110◦ C: (a) LaAl11 ; (b) LaAl14 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) LaMg1 Al11 . (×) (NH4 )2 Al6 (CO3 )3 ·H2 O and (#) MnCO3 .
precursors prepared using the same procedure [20,21]. In the case of LaMn1 Al11–110 , small amounts of MnCO3 [JCPDS 5-378] are also detected. Besides all the dried samples show high surface areas, 200–300 m2 /g, which indicate the presence of amorphous compounds along with the above crystalline phases. 3.2. Calcination at intermediate temperatures (500–1000◦ C) Calcination at 500◦ C results in the thermal decomposition of the crystalline phases, (NH4 )2 Al6 (CO3 )3 · H2 O and MnCO3 , whose reflections are no longer detected in the XRD spectra. Thus, amorphous samples are obtained at this temperature with high surface area values of 200–300 m2 /g. Further calcination at 700◦ C does not result in marked modification of phase composition. However, broad modulations of the base-line are detected in the XRD spectra that can be tentatively attributed to ␥-Al2 O3 [JCPDS 10-425]. In the case of LaMn1 Al11–700 , the most intense reflection (2θ = 33◦ ) of ␣-Mn2 O3 [JCPDS 10-69] is also evident. Although the microcrystalline ␥-Al2 O3 phase is still the main component in all the samples calcined at 900◦ C, incipient phase transformation with different features depending on sample composition are evidenced by the XRD spectra (Fig. 2) in the
140
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
900◦ C:
Fig. 2. XRD spectra of samples calcined at (a) LaAl11 ; (b) LaAl14 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) LaMg1 Al11 . (@) ␥-Al2 O3 ; (+) MgAl2 O4 ; and ( ) La-hexaaluminate.
substituted La-hexaaluminates. In LaMn1 Al11–900 the narrow reflection of ␣-Mn2 O3 at 2θ = 33◦ has disappeared, whereas several peaks are evident that can be associated with traces of a La-hexaaluminate phase [JCPDS 36-1317]; in LaMg1 Al11–900 and LaMg0.5 Mn0.5 Al11–900 reflections at 2θ = 32.0, 37.2 and 45.2◦ are observed that correspond to small amounts MgAl2 O4 [JCPDS 21-1151]. Despite of such slight differences in phase composition all the samples exhibit similar surface areas around 100 m2 /g. This suggests that morphological properties are still associated with those of the ␥-Al2 O3 matrix. Differences in phase composition become more marked upon calcination at 1000◦ C (Fig. 3). The unsubstituted LaAl11–1000 and LaAl14–1000 samples consist of microcrystalline ␥-Al2 O3 and well crystallised LaAlO3 [JCPS 31-22]. In LaMn1 Al11–1000 , only narrow reflections of the MP phase are observed. MP is the major phase in LaMg0.5 Mn0.5 Al11–1000 as well, but in this sample small reflections are also observed associated with residual amount of MgAl2 O4 . The broad reflections at 2θ = 32.0, 37.2 and 45.2◦ in the XRD spectrum of LaMg1 Al11–1000 are associated with poorly crystalline MgAl2 O4 (τ = 60 Å). Besides modulations associated with residual ␥-Al2 O3 are still observed. Higher surface areas of 70 and 100 m2 /g are measured for LaAl11 and LaAl14 , respectively, that are in
Fig. 3. XRD spectra of samples calcined at 1000◦ C: (a) LaAl11 ; (b) LaAl14 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) LaMg1 Al11 . (@) ␥-Al2 O3 ; (+) MgAl2 O4 ; (䊊) LaAlO3 ; and ( ) La-hexaaluminate.
line with the residual presence of a microcrystalline ␥-Al2 O3 matrix detected by XRD. In Mn-containing samples, surface area values of about 30–35 m2 /g have been found indicating that a marked drop of surface area has occurred in these samples, likely due to the formation of the final MP phase. Finally, LaMg1 Al11–1000 shows a quite high surface area of 70 m2 /g in line with the poor cristallinity of the segregated MgAl2 O4 . 3.3. Calcination at high temperatures (1100–1300◦ C) The XRD spectra of the samples calcined at 1100◦ C are reported in Fig. 4. At this temperature, the formation of the final MP LaAl11 O18 [JCPDS 33-0699] phase is observed also for the unsubstituted LaAl11–1100 and LaAl14–1100 samples. However, considerable amounts of well crystallised LaAlO3 are still present in these samples. In LaMn1 Al11 , no changes of phase composition are detected and calcination at 1100◦ C only results in a higher crystallisation of the MP phase. Also in LaMg0.5 Mn0.5 Al11–1100 and in LaMg1 , Al11–1100 the reflections associated with a well crystallised MP phase are the only features in the XRD spectra, while MgAl2 O4 is no longer detected. It is worth noting that in the XRD spectra of LaAl11 and LaAl14 not all the reflections associated with the MP phase are well evident. It is reported in the literature that unsubstituted La-hexaaluminates show a
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
Fig. 4. XRD spectra of samples calcined at 1100◦ C: (a) LaAl11 ; (b) LaAl14 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) LaMg1 Al11 . (䊊) LaAlO3 ; and ( ) La-hexaaluminate.
high disorder of the crystal lattice due to the presence of the defective cells discussed above, that originates scattering diffusion in (0 0 l) crystallographic planes [24]. This could result in the broadening of the corresponding diffraction peaks that consequently become poorly evident in the XRD spectra. According to the same authors, this effect is not observed in the samples containing Mn and/or Mg, where all the reflections are well evident due the presence of divalent cations in the structure, which lowers the structural disorder. In LaAl11 , LaAl14 , monophasic samples are not obtained even upon calcination at 1300◦ C. This is clearly evident in Fig. 5. The XRD spectra reported in the figure have been recorded more accurately in the 30–40◦ (2θ ) range to allow a better separation of the reflections of LaAlO3 and LaAl11 O18 . The amount of LaAlO3 phase decreases on increasing Al content from LaAl11–1300 to LaAl14–1300 : the ratio of the calculated intensity of the (1 1 0) reflection of LaAlO3 to the calculated intensity of the (1 1 0) reflection of the MP phase are equal to 1/1 in LaAl11–1300 and 1/6 in LaAl14–1300 . Accurate inspection of Fig. 5 evidences that traces of LaAlO3 are also present in LaMn1 Al11–1300 , whereas no other phase than MP are detected in the Mg-containing samples. Upon calcination at 1300◦ C the following surface have been measured: 18 m2 /g for LaAl14–1300 ; 19 m2 /g
Fig. 5. XRD (b) LaAl14 ; LaMg1 Al11 . LaAlO3 ; and
141
spectra of samples calcined at 1300◦ C: (a) LaAl11 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) Dotted line corresponds to (1 1 0) reflection of ( ) La-hexaaluminate.
for LaAl11–1300 ; 9 m2 /g LaMn1 Al11–1300 ; 15 m2 /g for LaMg0.5 Mn0.5–1300 . These values compare well with 19 m2 /g measured on a Mn-substituted Ba-hexaaluminates (BaMn1 Al11–1300 sample) prepared via the same procedure [23]. 3.4. Catalytic activity The catalytic activity in CH4 combustion of the Mn-containing samples LaMn1 Al11–1300 and LaMg0.5 Mn0.5–1300 has been investigated in comparison with that of a commercial Sr0.8 La0.2 Mn1 Al11–1300 sample. In Fig. 6, the conversion versus temperature curves obtained over the different catalysts are reported along with that collected over a BaMn1 Al11–1300 sample prepared via the same carbonates route [23]. Over all the catalysts, CH4 combustion at the investigated conditions starts in the 480–550◦ C temperature range and proceeds with similar apparent activation energy in the 87–94 kJ/mol range. Comparison of catalytic performances among samples with equal Mn-content shows that the combustion activity of the LaMn1 Al11 sample is markedly higher than that of BaMn1 Al11 prepared with the same carbonate route and it is slightly higher than that of the commercial catalyst with composition Sr0.8 La0.2 Mn1 Al11 O19 that is reported in the litera-
142
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
Fig. 6. Catalytic activity in CH4 conversion of LaMg0.5 Mn0.5 Al11 , LaMn1 Al11 , BaMn1 Al11 calcined at 1300◦ C and of Sr0.8 La0.2 Mn1 Al11 commercial catalysts calcined at 1300◦ C.
ture to provide the maximum catalytic activity among Mn-substituted hexaaluminates [6]. Despite of the large difference of Mn content, the La-hexaaluminate samples, LaMn1 Al11–1300 and LaMg0.5 Mn0.5 Al11–1300 , show very similar conversion curves over all the temperature range, i.e. the partially Mg-substituted sample exhibits roughly twice the specific activity for Mn atom than LaMn1 Al11 .
3.5. TPR measurements In order to collect evidences on the oxidation state of Mn ions within the layered alumina structure of the different samples H2 -TPR measurements have been performed on LaMn1 Al11–1300 , LaMg0.5 Mn0.5 Al11–1300 and BaMn1 Al11–1300 . The TPR profiles are compared in Fig. 7. For the La-substituted samples
Fig. 7. Temperature programmed reduction (TPR) experiments on LaMn1 Al11 (solid); LaMg0.5 Mn0.5 Al11 (dash dot dot); and BaMn1 Al11 (dash) calcined at 1300◦ C.
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
143
Table 1 Total H2 consumptions and average oxidation state of Mn for the different samples Sample
Sample weight (mg)
Experimental H2 consumption (mmol)
Average Mn oxidation state
BaMn1 LaMn1 LaMg0.5 Mn0.5
299 301 299
0.025 0.033 0.028
2.13 2.17 2.28
two reduction peaks are present. In both the samples, the first peak of comparable area shows a maximum at 420◦ C. On the other hand, the second peak is much more pronounced in LaMn1 Al11–1300 than in LaMg0.5 Mn0.5 Al11–1300 and slightly anticipated (maximum at 700◦ C versus 760◦ C). BaMn1 Al11–1300 shows a quite different TPR profile exhibiting a very broad reduction peak with a maximum at 510◦ C and a small shoulder centred at 770◦ C. Total H2 consumptions for the different samples have been derived from the peak area by calibration with the reduction of copper(II) oxide and are reported in Table 1 along with the average oxidation state of Mn calculated under the assumption that all Mn in the structure has been reduced to Mn(II) upon heating in H2 up to 1100◦ C [2]. The average oxidation state decreases with the following order LaMg0.5 Mn0.5 Al11–1300 > LaMn1 Al11–1300 > BaMn1 Al11–1300 . For the latter sample, the calculated average oxidation state agrees well with the results obtained in a previous crystallographic study on BaMnx Al12−x O19 samples (x = 0.5, 1, 2, and 3) based on the refinement of the XRD spectra collected in the proximity of the Mn absorption K-edge [26]. In such a study, it was pointed out that Mn preferentially enters the structure as Mn(II) in tetrahedral holes and as Mn(III) in octahedral sites. In the case of the BaMn1 Al11–1300 , sample herein considered the relative fractions of 0.84 of Mn(II) and 0.16 of Mn(III) were calculated which corresponded to an average oxidation state of 2.16 (versus 2.14 from the TPR results in this work). Structural analogy between Mn-substituted -Al2 O3 and MP along with comparison with literature data on pure Mn oxides [27] suggest that the first reduction peak in LaMn1 Al11 and LaMg0.5 Mn0.5 samples can be associated with Mn3+ → Mn2+ in octahedral sites. Comparison of TPR profiles in Fig. 7 shows that the reducibility of such Mn species is increased
in La-substituted samples with MP structure with respect to Ba-substituted hexaaluminates with -Al2 O3 structure. On the other hand, there is no clear attribution to the second reduction peak which, however, likely plays a minor role on the combustion activity in the 500–700◦ C temperature range herein investigated. Noticeably the higher average oxidation state of Mn in LaMg0.5 Mn0.5 Al11–1300 is mainly associated with the most reactive Mn species, which is qualitatively and quantitatively comparable to the same species in LaMn1 Al11–1300 . According to literature indications [15,24] Mg2+ enters the structure in tetrahedral sites. This suggests that displacement of Mn ion from tetrahedral to octahedral position could occur by Mg substitution which is in line with the increase of the first reduction peak associated with octahedral Mn3+ .
4. Discussion Phase composition data on thermal evolution of the different samples are summarised in Table 2. All the samples dried at 110◦ C consist of an amorphous matrix and of crystalline (NH4 )2 Al6 (CO3 )3 · H2 O. In LaMn1 Al11 also traces of MnCO3 are detected. The crystalline phases easily decompose to give amorphous samples already at 500◦ C in all the investigated samples except in LaMn1 Al11 where small amounts of crystalline ␣-Mn2 O3 are present, likely originated from the MnCO3 precursor. The absence of crystalline phases along with the high value of surface area clearly indicates that the coprecipitation method herein adopted provides small dimension of the aggregates. Small dimensions guarantee an high interspersion of the constituents that promote the solid state reactions occurring at higher temperature to give the final phase. Such high interspersion is main-
144
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
Table 2 Phase composition of the different samples at the different calcination temperatures Calcination temperature (◦ C) 110 500 700 900 1000 1100 1300
LaAl11
LaAl14
LaMn1 Al11
LaMg0.5 Mn0.5 Al11
LaMg1 Al11
(NH4 )2 Al6 (CO3 )3 · H2 O Amorphous ␥-Al2 O3s ␥-Al2 O3 , LaAlO3 ␥-Al2 O3 , LaAlO3 MP, LaAlO3 MP, LaAlO3
(NH4 )2 Al6 (CO3 )3 · H2 O Amorphous ␥-Al2 O3s ␥-Al2 O3 ␥-Al2 O3 , LaAlO3 MP, LaAlO3 MP, LaAlO3
(NH4 )2 Al6 (CO3 )3 · H2 O, MnCO3 Amorphous ␥-Al2 O3 , ␣-Mn2 O3 ␥-Al2 O3 , MP (traces) MP MP MP, LaAlO3 (traces)
(NH4 )2 Al6 (CO3 )3 · H2 O Amorphous ␥-Al2 O3 ␥-Al2 O3 , MgAl2 O4 (tr.) MP, MgAl2 O4 MP MP
(NH4 )2 Al6 (CO3 )3 · H2 O Amorphous ␥-Al2 O3 ␥-Al2 O3 , MgAl2 O4 (tr.) ␥-Al2 O3 , MgAl2 O4 MP MP
tained upon calcination at 700◦ C that results in the formation microcrystalline ␥-Al2 O3 as indicated by the broad modulations observed in the XRD spectra. At 900◦ C the incipient formation of La-hexaaluminate is observed in the Mn-substituted LaMn1 Al11 sample while that of MgAl2 O4 is observed in Mgcontaining sample LaMg1 Al11 and LaMg0.5 Mn0.05 Al11 . At 1000◦ C differences in thermal evolution depending on sample composition become markedly evident. In both Mn-containing samples, the formation of the MP phase proceeds to a large extent so that the modulations associated with ␥-Al2 O3 are no longer detected. However, LaMg0.5 Mn0.5 Al11 still contains residual amount of MgAl2 O4 that is the dominant phase along with ␥-Al2 O3 in LaMgAl11 . Segregation of LaAlO3 is well evident in both the unsubstituted LaAl11 and LaAl14 samples where the MP phase has not appeared yet. A marked drop of surface area is associated with the formation of the MP phase in the Mn-containing samples whereas relatively high value are retained in the unsubstituted sample and in LaMg1 Al11 . Calcination at 1100◦ C brings about the complete formation of the MP phase in the LaMg1 Al11 sample, where the diffraction peaks associated with MgAl2 O4 completely disappear in favour of the reflections of the La-hexaaluminate phase. Reflections of MgAl2 O4 also disappear in the LaMg0.5 Mn0.5 Al11 indicating that the formation of the MP phase has been completed. MP also appears in the unsubstituted samples where important amounts of LaAlO3 are still present. No significant modifications of phase composition occur in the Mn- and Mg-substituted samples upon
calcination at 1300◦ C, whereas the formation of the MP phase extensively proceeds in the unsubstituted LaAl11 and LaAl14 . In these latter samples, however, LaAlO3 is still segregated in increasing amount on decreasing the Al/La ratio. Noteworthy traces of LaAlO3 are also present in LaMn1 Al11–1300 . All the samples are well sintered at 1300◦ C showing values of surface areas about 10–20 m2 /g that are in line with the typical values of hexaaluminate materials prepared according to the carbonates routes. For Ba-hexaaluminates prepared via the carbonates route it is reported in [20], that the final Ba--Al2 O3 forms via solid state reactions between ␥-Al2 O3 and BaAl2 O4 and between ␥-Al2 O3 and Ba dispersed species starting from 1100◦ C, i.e. the threshold temperature at which the mobility of Ba ions inside the ␥-Al2 O3 matrix is activated. The presence of Ba dispersed species has been hypothesised as a key factor to obtain final materials with the required structural and morphological properties. Dispersed Ba, in fact, preserves the microcrystalline ␥-Al2 O3 matrix up to the temperature of formation of the Ba--Al2 O3 . It is also reported that Mn ions promote the formation of the final phase that occurs 100◦ C below the threshold temperature of 1100◦ C observed for unsubstituted Ba-Al-O samples [21]. Besides lower amount of intermediate BaAl2 O4 are observed in the presence of Mn. Such effects have been associated with an increased ion mobility in the ␥-Al2 O3 spinel blocks due to Mn doping that promotes the formation of the Ba--Al2 O3 via diffusion of Ba ions in the ␥-Al2 O3 matrix. Similar mechanistic features can be invoked for the formation of the final MP phase in the La-hexalumi-
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
nates samples herein investigated. High threshold temperatures are required to start the formation of the MP phase suggesting that also in this case the kinetics is controlled by the slow diffusion of large La ions in the ␥-Al2 O3 matrix. Actually the threshold temperatures in La-hexaaluminates are slightly lower (900–1050◦ C versus 1000–1100◦ C) than in Ba-hexaaluminates, that is consistent with the smaller dimensions of the La3+ cations with respect to the Ba2+ ones (1.05 Å versus 1.35 Å). Dispersed La ions on the surface of the ␥-Al2 O3 aggregates, allowed by the coprecipitation method herein adopted, can play the same role reported for Ba dispersed species in Ba-hexaaluminates. Indeed, it is well known that La ions, similarly to Ba, effectively hinder the ␥-Al2 O3 sintering, thus preventing ␥- → - → ␣-Al2 O3 phase transitions [25] up to the threshold temperature of the MP phase formation. In the unsubstituted samples segregation of intermediate LaAlO3 is observed that parallels the formation of BaAl2 O4 in Ba-hexaluminates. Finally, Mn markedly promotes the formation of the MP phase, likely according to a similar mechanism to that reported for Ba--Al2 O3 , i.e. increasing of ion mobility in the ␥-Al2 O3 spinel blocks. Noteworthy in La-hexaaluminate partial substitution with Mn has a more marked effect on LaAlO3 segregation than that observed on segregation of BaAl2 O4 in Ba-hexaaluminates. In fact segregation of LaAlO3 , that the main phase at 1000–1100◦ C and it is still present at 1300◦ C in the unsubstituted samples, is completely avoided in LaMnAl11 ; whereas segregation of BaAl2 O4 is only decreased in Mn-substituted Ba-hexaluminates: comparison of BaAl12 with BaMn1 Al11 shows that at 1000◦ C the segregated BaAl2 O4 decreases from 40 to 20% of the total Ba-content and at 1300◦ C it completely reacts to form Ba--Al2 O3 in both the samples [20,21]. This difference could be related to the following reasons: (i) the easier formation of the La-containing MP phase, that actually compete with LaAlO3 segregation, associated with the lower size of La3+ ions with respect to the Ba2+ ones; (ii) the high defectivity of the MP structure in unsubstituted La-hexaaluminates that hinders LaAlO3 re-incorporation. Thermal evolution of Mg-substituted samples deserves for specific discussion. Both in LaMgAl11 and LaMg0.5 Mn0.5 Al11 the formation of a MgAl2 O4 spinel phase is observed starting from 900◦ C. In the
145
di-substituted sample, the promoting effect of Mn on the formation MP phase prevails so that at 1000◦ C the formation of this latter phase is almost completed. Only residual amount of MgAl2 O4 are still present in the LaMg0.5 Mn0.5 Al11–1000 sample that completely disappear upon calcination at 1100◦ C. On the other hand in LaMg1 Al11 the segregation of MgAl2 O4 also starts at 900◦ C but, differently from what observed in LaMg0.5 Mn0.5 Al11 , further proceeds at 1000◦ C. However also in this sample upon calcination at 1100◦ C MgAl2 O4 completely disappears in favour of LaMgAl11 O19 hexaaluminate. Direct reaction between MgAl2 O4 , ␥-Al2 O3 and dispersed La species can be reasonably invoked due to the strong similarity of the MgAl2 O4 spinel structure with the spinel blocks in the Mg-substituted MP phase. In fact, according to the literature also in this latter phase Mg2+ ions substitute Al3+ in tetrahedral site which exactly corresponds to the position occupied by Mg in the MgAl2 O4 spinel [16,24]. Noteworthy segregation of massive amounts of MgAl2 O4 does not result in irreversible drop of surface area due to the good sintering resistance of this phase up to 1000–1100◦ C. Data on phase composition of the samples calcined at 1300◦ C confirm that, to stabilise the final MP structure either a marked defect of La content with respect to Al/La = 11 stoichiometry or partial substitution of Al3+ with divalent ions is required, this latter being the most effective mechanism. According to the literature, these stabilisation effects both occur via charge compensation mechanisms. In the unsubstituted La-hexaaluminates, the increasing of the Al/La ratio allows for a higher concentration of [Al11 O19 ]5− defective cells that compensate the excess positive charge of [Al11 O16 ]+ spinel blocks with lower segregation of La as LaAlO3 . Accordingly, smaller amounts of LaAlO3 are present in LaAl14–1300 than in LaAl11–1300 . However, such a mechanism of charge balance is not effective enough to obtain monophasic materials possibly due to the strong defectivity of the resulting structure. The partial substitution of Al3+ with bivalent ions in the structure can activate a more efficient charge compensation mechanism that does not require the presence of [Al11 O19 ]5− defective cells. Accordingly, monophasic materials are obtained in Mg-substituted samples thanks to the stable divalent oxidation state of the alkaline earth cation. A similar
146
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
effect is obtained with Mn that, according to the literature, can enter the MP structure as Mn2+ in tetrahedral sites [16,24]. However in the Mn-substituted sample traces of LaAlO3 are still evident at 1300◦ C. In line with literature indications for Mn-substituted Ba--Al2 O3 [26], this is possibly due to the presence of a fraction of Mn3+ that is not effective in charge compensation. On the other hand, the presence of Mn in different oxidation states is required to obtain good catalytic activity through the Mn redox process. Indeed, similar apparent activation energies have been calculated for all the Mn-substituted catalysts, suggesting that the combustion activity is governed by the same process, i.e. the Mn2+ /Mn3+ redox process [2]. Despite of such analogy, significant activity variations are observed among the different samples. As a general trend, at fixed Mn-content, catalysts with MP structure (LaMn1 Al11 , Sr0.8 La0.2 Mn1 Al11 ) are more active than those with Ba--Al2 O3 (BaMnAl11 ). Machida et al. [6] proposed that the higher surface area obtained through heterogeneous composition of the mirror planes of their optimum composition Sr0.8 La0.2 Mn1 Al11 plays a key role in determining the high activity of such a catalyst. By investigation of the series Sr1−x Lax Mn1 Al11 through CH4 combustion activity, surface area, TPD and TG (in H2 ) measurements [6] the same Authors found that structural modifications can also increase the oxidation number of Mn ions, which promotes active oxygen desorption and eventually could result in higher catalytic activity. Along similar lines, Jang et al. [7] suggest that the nature of the large cation in the mirror plane (Ba2+ versus La3+ ) can affect the Mn2+ /Mn3+ redox process. Noteworthy, these latter authors, who prepared the material via the same carbonate route herein adopted also found maximum catalytic activity for LaMn1 Al11 which for some reasons provided poor performances in the study of Machida et al. [6] who prepared the catalyst via hydrolysis of alkoxides. The data herein collected provide further evidences in favour of the key role of the effect of the mirror plane composition on Mn2+ /Mn3+ redox process while diminishing the role of surface area. Indeed BaMn1 Al11–1300 possesses the highest surface area among the Mn-substituted samples despite of its worse catalytic performances, whereas H2 -TPR measurements indicate that reduction of the most reactive
Mn species occurs at significantly lower temperatures in LaMn1 Al11–1300 than in BaMn1 Al11–1300 . Noteworthy partial substitution with Mg, which stabilises the final phase, also promotes the catalytic activity in CH4 combustion. Indeed, LaMn1 Al11 and LaMg0.5 Mn0.5 Al11 exhibit the same combustion performances suggesting that the latter sample posses approximately twice the specific activity per Mn atom of the former one. A synergic effect of double Mn-Mg substitution in hexaaluminate based catalyst has been recently claimed in a patent [8] as a method to obtain catalysts with improved activity performances. This has been related to the increased capability to reversibly de-adsorb and re-adsorb oxygen of Mg-Mn substituted hexaaluminate samples than in purely Mn-substituted ones. Along the lines of the MP phase stabilisation mechanism sketched above, such promotion effect can be explained as follows: in the presence of Mg2+ divalent Mn ions are no more required to stabilise the MP final structure so that a higher fraction of Mn3+ is available for the redox process likely responsible for the observed catalytic activity. This explanation is supported by H2 -TPR experiments. Such measurements has pointed out the higher average oxidation state of Mn ions in LaMg0.5 Mn0.5 Al11–1300 than in LaMn1 Al11–1300 (2.28 versus 2.17) which is associated with a larger relative amount (with respect to the overall Mn content) of the most reactive Mn3+ species. Alternatively the high activity of LaMg0.5 Mn0.5 Al11–1300 could be partly associated with the improved sintering resistance obtained upon Mg introduction (15 m2 /g of LaMg0.5 Mn0.5 Al11–1300 versus 9 m2 /g of LaMn1 Al11–1300 ) which is possibly related to the stabilisation of the MP structure. It was stated in the Introduction that higher resistance to sulphur poisoning is one of the major advantages of La-hexaaluminates with respect to Ba-hexaaluminates [9]. It can be argued that this property could be lost upon introduction of alkaline metal earth Mg2+ ions in the structure. Although this problem must be addresses through direct investigation on sulphur poisoning, it can be conceived that also Mg-substituted hexaaluminate could exhibit good sulphur resistance properties on the basis of considerations on the relative stability of the sulphates of the different cations. Indeed MgSO4 exhibits stability similar to La2 (SO4 )3 [28], both of them being much less stable than BaSO4 .
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
In conclusion, the good combustion activity, phase composition and morphological properties of the LaMgx Mn1−x Al11 O19 catalyst prepared in this work via the coprecipitation carbonates routes make these systems an interesting alternative to commercial formulation Sr0.8 La0.2 Mn1 Al11 O19 as highly stable hexaaluminate materials for high temperature catalytic combustion.
147
can be associated to Mn ions that are stabilised at high oxidation state by Mg2+ . Accordingly LaMgx Mn1−x Al11 O19 represents a promising alternative to commercial formulation Sr0.8 La0.2 Mn1 Al11 O19 as highly stable hexaaluminate material for high temperature catalytic combustion.
Acknowledgements 5. Conclusions From what has been discussed in so far it can be concluded that: 1. The preparation method, carbonates route, is effective to prepare La-hexaaluminates unsubstituted and substituted with transition metal ions. This route provides a high interspersion of the constituents in the amorphous precursors that guarantees the achievement of a final phase with the MP structure and surface areas of about 10–15 m2 /g upon calcination at 1300◦ C. 2. Formation of the final MP phase likely occurs via diffusion of La3+ ions in the ␥-Al2 O3 spinel blocks. Within this mechanism dispersed La species on the surface of the small ␥-Al2 O3 aggregates play a key role, hindering ␥-Al2 O3 sintering and phase transitions up the threshold temperature of La3+ ions diffusion. 3. Mn promotes the formation of the MP phase by enhancing ion mobility in the ␥-Al2 O3 structure. Introduction of Mg results in the segregation of MgAl2 O4 , which however easily reacts to form the MP phase. This is likely due to strong similarity of the MgAl2 O4 spinel structure with that of the spinel blocks in the MP phase. 4. Monophasic samples can be obtained only when a stable divalent cation is present in the structure. The introduction of a divalent cation stabilises the MP structure via a charge compensation mechanism that lowers the defectivity of the materials and favours the formation of the MP phase. In this respect, the introduction of Mg, that can only be 2+, is more effective than Mn that can enter the structure both in the 2+ and 3+ oxidation state. 5. The co-presence of Mg and Mn results in a higher specific catalytic activity. Such a behaviour
Financial support for this work has been provided by MURST, Rome, Italy. The Authors also acknowledge Dr. Elisabetta Finocchio (Università di Genova) for the experimental support in TPR measurements. References [1] M. Machida, K. Eguchi, H. Arai, J. Catal. 103 (1987) 385. [2] M. Machida, K. Eguchi, H. Arai, J. Catal. 120 (1989) 377. [3] G. Groppi, C. Cristiani, P. Forzatti, Catalysis 13 (1997) 85. [4] S. Matzuda, A. Kato, M. Mizumoto, H. Yamashita, in: Proceedings of the 8th ICC, Vol. IV, Verlag, Basel, Berlin, 1984, pp. 4–879. [5] J.G. McCarty, M. Gusman, D.M. Lowe, D.L. Hildenbrand, K.N. Lau, Catal. Today 47 (1999) 5. [6] M. Machida, K. Eguchi, H. Arai, J. Catal. 123 (1990) 477. [7] B.W.L. Jang, R.M. Nelson, J.J. Spivey, M. Ocal, R. Oukaci, G. Marcelin, Catal. Today 47 (1999) 103. [8] P. Euzen, European Patent EP 0,689,870 A1 (1996); US Patent 5,830,822 (1998). [9] M. Johansson, Ph.D. Thesis, KTH Stockholm, Sweden, 1998. [10] R.C. Ropp, B. Carol, J. Am. Ceram. Soc. 63 (1980) 416. [11] N. Iyi, Z. Inue, S. Takekawa, S. Kimura, J. Solid State Chem. 54 (1984) 70. [12] M. Gasperin, M.C. Saine, A. Kahn, F. Laville, M. Lejus, J. Solid State Chem. 54 (1984) 61. [13] D. Saber, A.M. Lejus, Mater. Res. Bull. 16 (1981) 1325. [14] N. Iyi, Z. Inue, S. Kimura, J. Solid State Chem. 61 (1986) 236. [15] L. Laville, D. Gourier, A.M. Lejus, D. Vivien, J. Solid State Chem. 49 (1983) 180. [16] A.M. Lejus, F. Laville, J. Thery, Mater. Res. Bull. 23 (1988) 913. [17] F. Laville, A.M. Lejus, J. Cryst. Growth 63 (1983) 426. [18] E. Tronc, F. Laville, M. Gasperin, A.M. Lejus, D. Vivien, J. Solid State Chem. 81 (1989) 192. [19] F. Laville, M. Perrin, A.M. Lejus, M. Gasperin, R. Moncorge, D. Vivien, J. Solid State Chem. 65 (1986) 301. [20] G.Groppi.M. Bellotto, C. Cristiani, P. Forzatti, J. Mater. Sci. 29 (1994) 3441. [21] G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, P.L. Villa, J. Mater. Sci. 34 (1999) 2609.
148
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
[22] F.P. Treadwell, Chimica Analitica, VII Edition, Vol. I, F. Vallardi, Milano, 1966 (in Italian). [23] G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, P.L. Villa, Appl. Catal. A: Gen. 104 (1993) 101. [24] J.-G. Park, A.N. Cormack, J. Solid State Chem. 130 (1997) 199. [25] P. Burtin, J.P. Brunnelle, M. Pijolat, M. Sustelle, Appl. Catal. 34 (1987) 225.
[26] M. Bellotto, G. Artioli, C. Cristiani, P. Forzatti, G. Groppi, J. Catal. 179 (1998) 597. [27] E.R. Stobbe, B.A. de Boer, J.W. Geus, Catal. Today 47 (1999) 161. [28] J.W. Ward, Molecular sieve catalyst, in: B.E. Leach (Ed.), Applied Industrial Catalysis, Vol. III, Academic Press, New York, 1984.
Preparation, characterisation and catalytic activity of pure and substituted La-hexaaluminate systems for high temperature catalytic combustion Gianpiero Groppi∗ , Cinzia Cristiani, Pio Forzatti Dipartimento di Chimica Industriale ed Ingegneria Chimica “G. Natta”, Politecnico di Milano, CIIC, Piazza Leonardo da Vinci 32, I 20133 Milano, Italy Received 12 March 2001; received in revised form 10 July 2001; accepted 29 July 2001
Abstract La-Al-O, La-Mg-Al-O, La-Mn-Al-O and La-Mg-Mn-Al-O hexaaluminates have been prepared using the carbonates route previously developed for M-substituted and unsubstituted Ba--Al2 O3 . Starting from amorphous precursors, the formation of a final magnetoplumbite (MP) phase is observed upon calcination at T ≥ 1100◦ C. In the case of LaMn1 Al11 O19 , the MP phase already forms at 900◦ C evidencing a promotion effect of Mn ions. Upon calcination at 1300◦ C, monophasic samples can be obtained only for Mg-substituted samples (LaMg1 Al11 O19 and LaMg0.5 Mn0.5 Al11 O19 ), whereas in the other samples the presence of LaAlO3 is always detected. This behaviour is associated with the stabilisation, via a charge compensation mechanism, of the MP phase due to the introduction of Mg2+ ions in the structure. The co-presence of Mg and Mn in the final catalyst has resulted in a higher specific catalytic activity per Mn mol. Such a behaviour is likely associated with the stabilisation of Mn ions at high oxidation state due to the co-presence of Mg2+ . © 2001 Elsevier Science B.V. All rights reserved. Keywords: Catalytic combustion; Hexaaluminates; Mn-Mg substituted; CH4 combustion
1. Introduction Hexaaluminate materials are of interest as combustion catalysts for gas turbine applications, due to the high thermal stability associated with their peculiar layered structure that consists of ␥-Al2 O3 spinel block intercalated by planes (mirror planes) in which the largest cations (Ba, Ca, La and Sr) are located. Introduction of transition metal ions (preferably Mn) in the spinel blocks provides good combustion activity [1–3] without decrease of sintering resistance.
∗ Corresponding author. Tel.: +39-2-2399-3258; fax: +39-2-7063-8173. E-mail address: [email protected] (G. Groppi).
Several compositions have been investigated in [3]. Among these La-hexaaluminates have received much interest either as supports [4,5] or as bulk catalysts [6–8], in view of their higher resistance towards poisoning by sulphur compounds and carbonates with respect to Ba- and Sr-hexaaluminates [9]. Furthermore, La-containing Mn-substituted hexaaluminates have been reported to be slightly more active in CH4 combustion reactions than the Ba-based ones [6–8]. Two routes have been proposed to prepare La-hexaaluminates with structural and morphological properties appropriate for catalytic purposes, namely the alkoxides [6,10] and the carbonates route [4,7]. However, according to characterisation data reported in [1–4,8,10], single phase hexaaluminate materials can hardly be obtained. Segregation of other phases,
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 2 4 8 - X
138
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
i.e. LaAlO3 , perovskite or ␣-Al2 O3 is typically reported, that might result in additional sintering with losses of surface area of the final material. According to the crystallographic literature in non substituted La-hexaaluminates, that are generally described by the formula LaAl11 O18 , only a very distorted magnetoplumbite (MP) structure forms, due to the high defectivity of the material [11]. Such a structure consists of [Al11 O16 ]+ spinel blocks intercalated by mirror planes of composition [LaAlO3 ]0 . This composition results in an electrically charged cell [LaAl12 O19 ]+ , so that defectivity is required to maintain the electroneutrality. According to Iyi et al. [11] in the mirror planes of the defective cells the La3+ site is vacant and one Al3+ position is replaced by two oxygen ions bonded to interstitial Al pairs forming bridges between the spinel blocks above and below the mirror plane. The final structure consists of non defective [LaAl12 O19 ]+ and defective [Al11 O19 ]5− cells in ratio 1 to 5 that corresponds to a neutral composition La0.833 Al11.833 O19 (i.e. Al/La = 14.2), that is characterised by a wide Al excess with respect to the nominal LaAl11 O18 composition. Considering the structural model above, it is evident that the introduction of bivalent ions in the spinel blocks, replacing Al3+ , can be an effective alternative to Al excess in order to obtain electrically neutral structure through a charge compensation mechanism. Indeed, monophasic Ln-hexaaluminates (Ln = La, Pr, Nd, Sm, Eu, Gd) have been obtained by partial substitution of Al3+ with Mg2+ ions [12,13]. Further crystallographic studies have shown that also transition metal ions such as Mn, Fe, Co and Ni can enter the structure with divalent oxidation state providing a similar stabilisation effect [14–19]. These pieces of crystallographic evidences have been collected on materials synthesised through crystallisation from melted precursor to obtain monocrystals or via high temperature solid state reaction to obtain strongly sintered materials which, however, are not adequate for catalytic uses. On the other hand, no systematic studies have been published on the possibility to obtain polycrystalline monophasic La-hexaaluminate, to be used as bulk catalysts or support, depending on the La/Al and on the amount and the nature of the Al substituent. This paper is devoted to the characterisation of La-Al-O, La-Mg-Al-O, La-Mn-Al-O and La-Mg-Mn-
Al-O hexaaluminates, prepared via an aqueous coprecipitation route suitable for catalytic purposes [3]. Thermal evolution of the precursor, and the structure and the morphology of the final materials have been studied to provide insight in the influence of the preparation chemistry on the formation and the properties of the final hexaaluminates. The effect of the catalysts composition on activity in CH4 combustion has also been addressed.
2. Experimental LaAl11 O18 , LaAl14.2 O22.81 and LaMgx Mn1−x Al11 O19 with x = 0, 0.5, and 1 have been prepared using the coprecipitation method known as carbonates route described elsewhere [20,21]. Briefly, an acid solution of the nitrates of the ions is poured, under vigorous stirring, into a solution of excess (NH4 )2 CO3 hold at 60◦ C. Upon contact, a precipitate immediately forms and the pH of the slurry stabilises at 7–8 depending on the composition. The slurry is aged at 60◦ C for 3 h and then filtered to separate the solid. The obtained cake is washed to remove nitrates and excess carbonates, re-suspending the solid in cold water and filtering up to negative results of the nitrates qualitative assay [22]. The resulting solid is then dried at 110◦ C overnight under re-circulation of air, grinded in a mortar and then calcined at 500, 700, 900, 1000, 1100, and 1300◦ C for 10 h (heating ramp 60◦ C/h, cooling ramp 100◦ C/h). In the following, the samples will be identified by a notation that account for chemical composition and calcination temperature: e.g. notation LaMg0.5 Mn0.5 Al11–700 corresponds to sample with composition La/Mg/Mn/Al = 1/0.5/0.5/11 calcined at 700◦ C. The dried and the calcined samples have been characterised by X-ray diffraction (XRD) using a Nifiltered Cu K␣ radiation (Philips vertical goniometer PW 1050-70) and surface area determination by N2 adsorption (Fison Sorptomatic 1900 instruments). XRD patterns for determination of phase composition have been collected in the angular range 10–70◦ (2θ ), with a step size of 0.05◦ (2θ ), a counting time of 12 s per step and a divergent slit of 1◦ (2θ ). The relative amounts of LaAlO3 and La-hexaluminates in the samples calcined at 1300◦ C have been
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
calculated as the ratio of the XRD intensity of the (1 1 0) reflection of LaAlO3 to the (1 1 0) reflection of LaAl11 O18 . For this purpose spectra have been collected in step scan mode, a selected angular range of 30–40◦ (2θ ), step size of 0.01◦ (2θ ), counting time of 9 s per step and a divergence slit of 0.5◦ . The intensities of the reflections have been calculated using a profile fitting routine provided by Philips. Temperature-programmed reduction (TPR) measurements have been performed on a TPDRO 1100 ThermoQuest apparatus. About 300 mg of fine powder samples (d p = 0.1 mm). have been loaded for each measurements. The reactor has been fed with 60 cm3 /min (at STP) of a 5% v/v H2 in Ar mixture. Heating rate has been set to 15◦ C/min from room temperature to 1100◦ C. Catalytic activity in CH4 combustion has been investigated over the Mn-containing samples LaMn1 Al11 and LaMg0.5 Mn0.5 Al11 calcined at 1300◦ C. A commercial catalyst with nominal composition Sr0.8 La0.2 Mn1 Al11 O19 by Süd-Chemie has been also tested as a reference sample. Catalytic activity tests have been performed in a laboratory rig, equipped with a quartz microreactor that is described in details elsewhere [23]. The reactor (i.d. = 0.8 cm.) has been loaded with 0.45 g of fine catalyst powders (100–150 mesh) diluted with quartz with the same particle size (V cat /V quartz = 2/1). The reactor has been fed with 1% of CH4 in air at GHSV = 54000 cm3 /gcat h (at STP). Analyses of products and reactants have been performed by an on-line GC using two in parallel packed columns filled with 5 Å molecular sieves and Porapak QS® , both equipped with TCD detectors. 3. Results 3.1. Dried samples The XRD spectra of all the samples dried at 110◦ C (Fig. 1) exhibit a set of reflections whose positions correspond to (NH4 )2 Al6 (CO3 )3 ·H2 O (ammonium–aluminium–carbonate–hydroxi–hydrate (AACHH)) [20] with calculated mean crystallite dimensions of about 100 Å. This phase composition is similar to that already detected in Ba-hexaaluminate
139
Fig. 1. XRD spectra of samples dried at 110◦ C: (a) LaAl11 ; (b) LaAl14 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) LaMg1 Al11 . (×) (NH4 )2 Al6 (CO3 )3 ·H2 O and (#) MnCO3 .
precursors prepared using the same procedure [20,21]. In the case of LaMn1 Al11–110 , small amounts of MnCO3 [JCPDS 5-378] are also detected. Besides all the dried samples show high surface areas, 200–300 m2 /g, which indicate the presence of amorphous compounds along with the above crystalline phases. 3.2. Calcination at intermediate temperatures (500–1000◦ C) Calcination at 500◦ C results in the thermal decomposition of the crystalline phases, (NH4 )2 Al6 (CO3 )3 · H2 O and MnCO3 , whose reflections are no longer detected in the XRD spectra. Thus, amorphous samples are obtained at this temperature with high surface area values of 200–300 m2 /g. Further calcination at 700◦ C does not result in marked modification of phase composition. However, broad modulations of the base-line are detected in the XRD spectra that can be tentatively attributed to ␥-Al2 O3 [JCPDS 10-425]. In the case of LaMn1 Al11–700 , the most intense reflection (2θ = 33◦ ) of ␣-Mn2 O3 [JCPDS 10-69] is also evident. Although the microcrystalline ␥-Al2 O3 phase is still the main component in all the samples calcined at 900◦ C, incipient phase transformation with different features depending on sample composition are evidenced by the XRD spectra (Fig. 2) in the
140
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
900◦ C:
Fig. 2. XRD spectra of samples calcined at (a) LaAl11 ; (b) LaAl14 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) LaMg1 Al11 . (@) ␥-Al2 O3 ; (+) MgAl2 O4 ; and ( ) La-hexaaluminate.
substituted La-hexaaluminates. In LaMn1 Al11–900 the narrow reflection of ␣-Mn2 O3 at 2θ = 33◦ has disappeared, whereas several peaks are evident that can be associated with traces of a La-hexaaluminate phase [JCPDS 36-1317]; in LaMg1 Al11–900 and LaMg0.5 Mn0.5 Al11–900 reflections at 2θ = 32.0, 37.2 and 45.2◦ are observed that correspond to small amounts MgAl2 O4 [JCPDS 21-1151]. Despite of such slight differences in phase composition all the samples exhibit similar surface areas around 100 m2 /g. This suggests that morphological properties are still associated with those of the ␥-Al2 O3 matrix. Differences in phase composition become more marked upon calcination at 1000◦ C (Fig. 3). The unsubstituted LaAl11–1000 and LaAl14–1000 samples consist of microcrystalline ␥-Al2 O3 and well crystallised LaAlO3 [JCPS 31-22]. In LaMn1 Al11–1000 , only narrow reflections of the MP phase are observed. MP is the major phase in LaMg0.5 Mn0.5 Al11–1000 as well, but in this sample small reflections are also observed associated with residual amount of MgAl2 O4 . The broad reflections at 2θ = 32.0, 37.2 and 45.2◦ in the XRD spectrum of LaMg1 Al11–1000 are associated with poorly crystalline MgAl2 O4 (τ = 60 Å). Besides modulations associated with residual ␥-Al2 O3 are still observed. Higher surface areas of 70 and 100 m2 /g are measured for LaAl11 and LaAl14 , respectively, that are in
Fig. 3. XRD spectra of samples calcined at 1000◦ C: (a) LaAl11 ; (b) LaAl14 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) LaMg1 Al11 . (@) ␥-Al2 O3 ; (+) MgAl2 O4 ; (䊊) LaAlO3 ; and ( ) La-hexaaluminate.
line with the residual presence of a microcrystalline ␥-Al2 O3 matrix detected by XRD. In Mn-containing samples, surface area values of about 30–35 m2 /g have been found indicating that a marked drop of surface area has occurred in these samples, likely due to the formation of the final MP phase. Finally, LaMg1 Al11–1000 shows a quite high surface area of 70 m2 /g in line with the poor cristallinity of the segregated MgAl2 O4 . 3.3. Calcination at high temperatures (1100–1300◦ C) The XRD spectra of the samples calcined at 1100◦ C are reported in Fig. 4. At this temperature, the formation of the final MP LaAl11 O18 [JCPDS 33-0699] phase is observed also for the unsubstituted LaAl11–1100 and LaAl14–1100 samples. However, considerable amounts of well crystallised LaAlO3 are still present in these samples. In LaMn1 Al11 , no changes of phase composition are detected and calcination at 1100◦ C only results in a higher crystallisation of the MP phase. Also in LaMg0.5 Mn0.5 Al11–1100 and in LaMg1 , Al11–1100 the reflections associated with a well crystallised MP phase are the only features in the XRD spectra, while MgAl2 O4 is no longer detected. It is worth noting that in the XRD spectra of LaAl11 and LaAl14 not all the reflections associated with the MP phase are well evident. It is reported in the literature that unsubstituted La-hexaaluminates show a
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
Fig. 4. XRD spectra of samples calcined at 1100◦ C: (a) LaAl11 ; (b) LaAl14 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) LaMg1 Al11 . (䊊) LaAlO3 ; and ( ) La-hexaaluminate.
high disorder of the crystal lattice due to the presence of the defective cells discussed above, that originates scattering diffusion in (0 0 l) crystallographic planes [24]. This could result in the broadening of the corresponding diffraction peaks that consequently become poorly evident in the XRD spectra. According to the same authors, this effect is not observed in the samples containing Mn and/or Mg, where all the reflections are well evident due the presence of divalent cations in the structure, which lowers the structural disorder. In LaAl11 , LaAl14 , monophasic samples are not obtained even upon calcination at 1300◦ C. This is clearly evident in Fig. 5. The XRD spectra reported in the figure have been recorded more accurately in the 30–40◦ (2θ ) range to allow a better separation of the reflections of LaAlO3 and LaAl11 O18 . The amount of LaAlO3 phase decreases on increasing Al content from LaAl11–1300 to LaAl14–1300 : the ratio of the calculated intensity of the (1 1 0) reflection of LaAlO3 to the calculated intensity of the (1 1 0) reflection of the MP phase are equal to 1/1 in LaAl11–1300 and 1/6 in LaAl14–1300 . Accurate inspection of Fig. 5 evidences that traces of LaAlO3 are also present in LaMn1 Al11–1300 , whereas no other phase than MP are detected in the Mg-containing samples. Upon calcination at 1300◦ C the following surface have been measured: 18 m2 /g for LaAl14–1300 ; 19 m2 /g
Fig. 5. XRD (b) LaAl14 ; LaMg1 Al11 . LaAlO3 ; and
141
spectra of samples calcined at 1300◦ C: (a) LaAl11 ; (c) LaMn1 Al11 ; (d) LaMg0.5 Mn0.5 Al11 ; and (e) Dotted line corresponds to (1 1 0) reflection of ( ) La-hexaaluminate.
for LaAl11–1300 ; 9 m2 /g LaMn1 Al11–1300 ; 15 m2 /g for LaMg0.5 Mn0.5–1300 . These values compare well with 19 m2 /g measured on a Mn-substituted Ba-hexaaluminates (BaMn1 Al11–1300 sample) prepared via the same procedure [23]. 3.4. Catalytic activity The catalytic activity in CH4 combustion of the Mn-containing samples LaMn1 Al11–1300 and LaMg0.5 Mn0.5–1300 has been investigated in comparison with that of a commercial Sr0.8 La0.2 Mn1 Al11–1300 sample. In Fig. 6, the conversion versus temperature curves obtained over the different catalysts are reported along with that collected over a BaMn1 Al11–1300 sample prepared via the same carbonates route [23]. Over all the catalysts, CH4 combustion at the investigated conditions starts in the 480–550◦ C temperature range and proceeds with similar apparent activation energy in the 87–94 kJ/mol range. Comparison of catalytic performances among samples with equal Mn-content shows that the combustion activity of the LaMn1 Al11 sample is markedly higher than that of BaMn1 Al11 prepared with the same carbonate route and it is slightly higher than that of the commercial catalyst with composition Sr0.8 La0.2 Mn1 Al11 O19 that is reported in the litera-
142
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
Fig. 6. Catalytic activity in CH4 conversion of LaMg0.5 Mn0.5 Al11 , LaMn1 Al11 , BaMn1 Al11 calcined at 1300◦ C and of Sr0.8 La0.2 Mn1 Al11 commercial catalysts calcined at 1300◦ C.
ture to provide the maximum catalytic activity among Mn-substituted hexaaluminates [6]. Despite of the large difference of Mn content, the La-hexaaluminate samples, LaMn1 Al11–1300 and LaMg0.5 Mn0.5 Al11–1300 , show very similar conversion curves over all the temperature range, i.e. the partially Mg-substituted sample exhibits roughly twice the specific activity for Mn atom than LaMn1 Al11 .
3.5. TPR measurements In order to collect evidences on the oxidation state of Mn ions within the layered alumina structure of the different samples H2 -TPR measurements have been performed on LaMn1 Al11–1300 , LaMg0.5 Mn0.5 Al11–1300 and BaMn1 Al11–1300 . The TPR profiles are compared in Fig. 7. For the La-substituted samples
Fig. 7. Temperature programmed reduction (TPR) experiments on LaMn1 Al11 (solid); LaMg0.5 Mn0.5 Al11 (dash dot dot); and BaMn1 Al11 (dash) calcined at 1300◦ C.
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
143
Table 1 Total H2 consumptions and average oxidation state of Mn for the different samples Sample
Sample weight (mg)
Experimental H2 consumption (mmol)
Average Mn oxidation state
BaMn1 LaMn1 LaMg0.5 Mn0.5
299 301 299
0.025 0.033 0.028
2.13 2.17 2.28
two reduction peaks are present. In both the samples, the first peak of comparable area shows a maximum at 420◦ C. On the other hand, the second peak is much more pronounced in LaMn1 Al11–1300 than in LaMg0.5 Mn0.5 Al11–1300 and slightly anticipated (maximum at 700◦ C versus 760◦ C). BaMn1 Al11–1300 shows a quite different TPR profile exhibiting a very broad reduction peak with a maximum at 510◦ C and a small shoulder centred at 770◦ C. Total H2 consumptions for the different samples have been derived from the peak area by calibration with the reduction of copper(II) oxide and are reported in Table 1 along with the average oxidation state of Mn calculated under the assumption that all Mn in the structure has been reduced to Mn(II) upon heating in H2 up to 1100◦ C [2]. The average oxidation state decreases with the following order LaMg0.5 Mn0.5 Al11–1300 > LaMn1 Al11–1300 > BaMn1 Al11–1300 . For the latter sample, the calculated average oxidation state agrees well with the results obtained in a previous crystallographic study on BaMnx Al12−x O19 samples (x = 0.5, 1, 2, and 3) based on the refinement of the XRD spectra collected in the proximity of the Mn absorption K-edge [26]. In such a study, it was pointed out that Mn preferentially enters the structure as Mn(II) in tetrahedral holes and as Mn(III) in octahedral sites. In the case of the BaMn1 Al11–1300 , sample herein considered the relative fractions of 0.84 of Mn(II) and 0.16 of Mn(III) were calculated which corresponded to an average oxidation state of 2.16 (versus 2.14 from the TPR results in this work). Structural analogy between Mn-substituted -Al2 O3 and MP along with comparison with literature data on pure Mn oxides [27] suggest that the first reduction peak in LaMn1 Al11 and LaMg0.5 Mn0.5 samples can be associated with Mn3+ → Mn2+ in octahedral sites. Comparison of TPR profiles in Fig. 7 shows that the reducibility of such Mn species is increased
in La-substituted samples with MP structure with respect to Ba-substituted hexaaluminates with -Al2 O3 structure. On the other hand, there is no clear attribution to the second reduction peak which, however, likely plays a minor role on the combustion activity in the 500–700◦ C temperature range herein investigated. Noticeably the higher average oxidation state of Mn in LaMg0.5 Mn0.5 Al11–1300 is mainly associated with the most reactive Mn species, which is qualitatively and quantitatively comparable to the same species in LaMn1 Al11–1300 . According to literature indications [15,24] Mg2+ enters the structure in tetrahedral sites. This suggests that displacement of Mn ion from tetrahedral to octahedral position could occur by Mg substitution which is in line with the increase of the first reduction peak associated with octahedral Mn3+ .
4. Discussion Phase composition data on thermal evolution of the different samples are summarised in Table 2. All the samples dried at 110◦ C consist of an amorphous matrix and of crystalline (NH4 )2 Al6 (CO3 )3 · H2 O. In LaMn1 Al11 also traces of MnCO3 are detected. The crystalline phases easily decompose to give amorphous samples already at 500◦ C in all the investigated samples except in LaMn1 Al11 where small amounts of crystalline ␣-Mn2 O3 are present, likely originated from the MnCO3 precursor. The absence of crystalline phases along with the high value of surface area clearly indicates that the coprecipitation method herein adopted provides small dimension of the aggregates. Small dimensions guarantee an high interspersion of the constituents that promote the solid state reactions occurring at higher temperature to give the final phase. Such high interspersion is main-
144
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
Table 2 Phase composition of the different samples at the different calcination temperatures Calcination temperature (◦ C) 110 500 700 900 1000 1100 1300
LaAl11
LaAl14
LaMn1 Al11
LaMg0.5 Mn0.5 Al11
LaMg1 Al11
(NH4 )2 Al6 (CO3 )3 · H2 O Amorphous ␥-Al2 O3s ␥-Al2 O3 , LaAlO3 ␥-Al2 O3 , LaAlO3 MP, LaAlO3 MP, LaAlO3
(NH4 )2 Al6 (CO3 )3 · H2 O Amorphous ␥-Al2 O3s ␥-Al2 O3 ␥-Al2 O3 , LaAlO3 MP, LaAlO3 MP, LaAlO3
(NH4 )2 Al6 (CO3 )3 · H2 O, MnCO3 Amorphous ␥-Al2 O3 , ␣-Mn2 O3 ␥-Al2 O3 , MP (traces) MP MP MP, LaAlO3 (traces)
(NH4 )2 Al6 (CO3 )3 · H2 O Amorphous ␥-Al2 O3 ␥-Al2 O3 , MgAl2 O4 (tr.) MP, MgAl2 O4 MP MP
(NH4 )2 Al6 (CO3 )3 · H2 O Amorphous ␥-Al2 O3 ␥-Al2 O3 , MgAl2 O4 (tr.) ␥-Al2 O3 , MgAl2 O4 MP MP
tained upon calcination at 700◦ C that results in the formation microcrystalline ␥-Al2 O3 as indicated by the broad modulations observed in the XRD spectra. At 900◦ C the incipient formation of La-hexaaluminate is observed in the Mn-substituted LaMn1 Al11 sample while that of MgAl2 O4 is observed in Mgcontaining sample LaMg1 Al11 and LaMg0.5 Mn0.05 Al11 . At 1000◦ C differences in thermal evolution depending on sample composition become markedly evident. In both Mn-containing samples, the formation of the MP phase proceeds to a large extent so that the modulations associated with ␥-Al2 O3 are no longer detected. However, LaMg0.5 Mn0.5 Al11 still contains residual amount of MgAl2 O4 that is the dominant phase along with ␥-Al2 O3 in LaMgAl11 . Segregation of LaAlO3 is well evident in both the unsubstituted LaAl11 and LaAl14 samples where the MP phase has not appeared yet. A marked drop of surface area is associated with the formation of the MP phase in the Mn-containing samples whereas relatively high value are retained in the unsubstituted sample and in LaMg1 Al11 . Calcination at 1100◦ C brings about the complete formation of the MP phase in the LaMg1 Al11 sample, where the diffraction peaks associated with MgAl2 O4 completely disappear in favour of the reflections of the La-hexaaluminate phase. Reflections of MgAl2 O4 also disappear in the LaMg0.5 Mn0.5 Al11 indicating that the formation of the MP phase has been completed. MP also appears in the unsubstituted samples where important amounts of LaAlO3 are still present. No significant modifications of phase composition occur in the Mn- and Mg-substituted samples upon
calcination at 1300◦ C, whereas the formation of the MP phase extensively proceeds in the unsubstituted LaAl11 and LaAl14 . In these latter samples, however, LaAlO3 is still segregated in increasing amount on decreasing the Al/La ratio. Noteworthy traces of LaAlO3 are also present in LaMn1 Al11–1300 . All the samples are well sintered at 1300◦ C showing values of surface areas about 10–20 m2 /g that are in line with the typical values of hexaaluminate materials prepared according to the carbonates routes. For Ba-hexaaluminates prepared via the carbonates route it is reported in [20], that the final Ba--Al2 O3 forms via solid state reactions between ␥-Al2 O3 and BaAl2 O4 and between ␥-Al2 O3 and Ba dispersed species starting from 1100◦ C, i.e. the threshold temperature at which the mobility of Ba ions inside the ␥-Al2 O3 matrix is activated. The presence of Ba dispersed species has been hypothesised as a key factor to obtain final materials with the required structural and morphological properties. Dispersed Ba, in fact, preserves the microcrystalline ␥-Al2 O3 matrix up to the temperature of formation of the Ba--Al2 O3 . It is also reported that Mn ions promote the formation of the final phase that occurs 100◦ C below the threshold temperature of 1100◦ C observed for unsubstituted Ba-Al-O samples [21]. Besides lower amount of intermediate BaAl2 O4 are observed in the presence of Mn. Such effects have been associated with an increased ion mobility in the ␥-Al2 O3 spinel blocks due to Mn doping that promotes the formation of the Ba--Al2 O3 via diffusion of Ba ions in the ␥-Al2 O3 matrix. Similar mechanistic features can be invoked for the formation of the final MP phase in the La-hexalumi-
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
nates samples herein investigated. High threshold temperatures are required to start the formation of the MP phase suggesting that also in this case the kinetics is controlled by the slow diffusion of large La ions in the ␥-Al2 O3 matrix. Actually the threshold temperatures in La-hexaaluminates are slightly lower (900–1050◦ C versus 1000–1100◦ C) than in Ba-hexaaluminates, that is consistent with the smaller dimensions of the La3+ cations with respect to the Ba2+ ones (1.05 Å versus 1.35 Å). Dispersed La ions on the surface of the ␥-Al2 O3 aggregates, allowed by the coprecipitation method herein adopted, can play the same role reported for Ba dispersed species in Ba-hexaaluminates. Indeed, it is well known that La ions, similarly to Ba, effectively hinder the ␥-Al2 O3 sintering, thus preventing ␥- → - → ␣-Al2 O3 phase transitions [25] up to the threshold temperature of the MP phase formation. In the unsubstituted samples segregation of intermediate LaAlO3 is observed that parallels the formation of BaAl2 O4 in Ba-hexaluminates. Finally, Mn markedly promotes the formation of the MP phase, likely according to a similar mechanism to that reported for Ba--Al2 O3 , i.e. increasing of ion mobility in the ␥-Al2 O3 spinel blocks. Noteworthy in La-hexaaluminate partial substitution with Mn has a more marked effect on LaAlO3 segregation than that observed on segregation of BaAl2 O4 in Ba-hexaaluminates. In fact segregation of LaAlO3 , that the main phase at 1000–1100◦ C and it is still present at 1300◦ C in the unsubstituted samples, is completely avoided in LaMnAl11 ; whereas segregation of BaAl2 O4 is only decreased in Mn-substituted Ba-hexaluminates: comparison of BaAl12 with BaMn1 Al11 shows that at 1000◦ C the segregated BaAl2 O4 decreases from 40 to 20% of the total Ba-content and at 1300◦ C it completely reacts to form Ba--Al2 O3 in both the samples [20,21]. This difference could be related to the following reasons: (i) the easier formation of the La-containing MP phase, that actually compete with LaAlO3 segregation, associated with the lower size of La3+ ions with respect to the Ba2+ ones; (ii) the high defectivity of the MP structure in unsubstituted La-hexaaluminates that hinders LaAlO3 re-incorporation. Thermal evolution of Mg-substituted samples deserves for specific discussion. Both in LaMgAl11 and LaMg0.5 Mn0.5 Al11 the formation of a MgAl2 O4 spinel phase is observed starting from 900◦ C. In the
145
di-substituted sample, the promoting effect of Mn on the formation MP phase prevails so that at 1000◦ C the formation of this latter phase is almost completed. Only residual amount of MgAl2 O4 are still present in the LaMg0.5 Mn0.5 Al11–1000 sample that completely disappear upon calcination at 1100◦ C. On the other hand in LaMg1 Al11 the segregation of MgAl2 O4 also starts at 900◦ C but, differently from what observed in LaMg0.5 Mn0.5 Al11 , further proceeds at 1000◦ C. However also in this sample upon calcination at 1100◦ C MgAl2 O4 completely disappears in favour of LaMgAl11 O19 hexaaluminate. Direct reaction between MgAl2 O4 , ␥-Al2 O3 and dispersed La species can be reasonably invoked due to the strong similarity of the MgAl2 O4 spinel structure with the spinel blocks in the Mg-substituted MP phase. In fact, according to the literature also in this latter phase Mg2+ ions substitute Al3+ in tetrahedral site which exactly corresponds to the position occupied by Mg in the MgAl2 O4 spinel [16,24]. Noteworthy segregation of massive amounts of MgAl2 O4 does not result in irreversible drop of surface area due to the good sintering resistance of this phase up to 1000–1100◦ C. Data on phase composition of the samples calcined at 1300◦ C confirm that, to stabilise the final MP structure either a marked defect of La content with respect to Al/La = 11 stoichiometry or partial substitution of Al3+ with divalent ions is required, this latter being the most effective mechanism. According to the literature, these stabilisation effects both occur via charge compensation mechanisms. In the unsubstituted La-hexaaluminates, the increasing of the Al/La ratio allows for a higher concentration of [Al11 O19 ]5− defective cells that compensate the excess positive charge of [Al11 O16 ]+ spinel blocks with lower segregation of La as LaAlO3 . Accordingly, smaller amounts of LaAlO3 are present in LaAl14–1300 than in LaAl11–1300 . However, such a mechanism of charge balance is not effective enough to obtain monophasic materials possibly due to the strong defectivity of the resulting structure. The partial substitution of Al3+ with bivalent ions in the structure can activate a more efficient charge compensation mechanism that does not require the presence of [Al11 O19 ]5− defective cells. Accordingly, monophasic materials are obtained in Mg-substituted samples thanks to the stable divalent oxidation state of the alkaline earth cation. A similar
146
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
effect is obtained with Mn that, according to the literature, can enter the MP structure as Mn2+ in tetrahedral sites [16,24]. However in the Mn-substituted sample traces of LaAlO3 are still evident at 1300◦ C. In line with literature indications for Mn-substituted Ba--Al2 O3 [26], this is possibly due to the presence of a fraction of Mn3+ that is not effective in charge compensation. On the other hand, the presence of Mn in different oxidation states is required to obtain good catalytic activity through the Mn redox process. Indeed, similar apparent activation energies have been calculated for all the Mn-substituted catalysts, suggesting that the combustion activity is governed by the same process, i.e. the Mn2+ /Mn3+ redox process [2]. Despite of such analogy, significant activity variations are observed among the different samples. As a general trend, at fixed Mn-content, catalysts with MP structure (LaMn1 Al11 , Sr0.8 La0.2 Mn1 Al11 ) are more active than those with Ba--Al2 O3 (BaMnAl11 ). Machida et al. [6] proposed that the higher surface area obtained through heterogeneous composition of the mirror planes of their optimum composition Sr0.8 La0.2 Mn1 Al11 plays a key role in determining the high activity of such a catalyst. By investigation of the series Sr1−x Lax Mn1 Al11 through CH4 combustion activity, surface area, TPD and TG (in H2 ) measurements [6] the same Authors found that structural modifications can also increase the oxidation number of Mn ions, which promotes active oxygen desorption and eventually could result in higher catalytic activity. Along similar lines, Jang et al. [7] suggest that the nature of the large cation in the mirror plane (Ba2+ versus La3+ ) can affect the Mn2+ /Mn3+ redox process. Noteworthy, these latter authors, who prepared the material via the same carbonate route herein adopted also found maximum catalytic activity for LaMn1 Al11 which for some reasons provided poor performances in the study of Machida et al. [6] who prepared the catalyst via hydrolysis of alkoxides. The data herein collected provide further evidences in favour of the key role of the effect of the mirror plane composition on Mn2+ /Mn3+ redox process while diminishing the role of surface area. Indeed BaMn1 Al11–1300 possesses the highest surface area among the Mn-substituted samples despite of its worse catalytic performances, whereas H2 -TPR measurements indicate that reduction of the most reactive
Mn species occurs at significantly lower temperatures in LaMn1 Al11–1300 than in BaMn1 Al11–1300 . Noteworthy partial substitution with Mg, which stabilises the final phase, also promotes the catalytic activity in CH4 combustion. Indeed, LaMn1 Al11 and LaMg0.5 Mn0.5 Al11 exhibit the same combustion performances suggesting that the latter sample posses approximately twice the specific activity per Mn atom of the former one. A synergic effect of double Mn-Mg substitution in hexaaluminate based catalyst has been recently claimed in a patent [8] as a method to obtain catalysts with improved activity performances. This has been related to the increased capability to reversibly de-adsorb and re-adsorb oxygen of Mg-Mn substituted hexaaluminate samples than in purely Mn-substituted ones. Along the lines of the MP phase stabilisation mechanism sketched above, such promotion effect can be explained as follows: in the presence of Mg2+ divalent Mn ions are no more required to stabilise the MP final structure so that a higher fraction of Mn3+ is available for the redox process likely responsible for the observed catalytic activity. This explanation is supported by H2 -TPR experiments. Such measurements has pointed out the higher average oxidation state of Mn ions in LaMg0.5 Mn0.5 Al11–1300 than in LaMn1 Al11–1300 (2.28 versus 2.17) which is associated with a larger relative amount (with respect to the overall Mn content) of the most reactive Mn3+ species. Alternatively the high activity of LaMg0.5 Mn0.5 Al11–1300 could be partly associated with the improved sintering resistance obtained upon Mg introduction (15 m2 /g of LaMg0.5 Mn0.5 Al11–1300 versus 9 m2 /g of LaMn1 Al11–1300 ) which is possibly related to the stabilisation of the MP structure. It was stated in the Introduction that higher resistance to sulphur poisoning is one of the major advantages of La-hexaaluminates with respect to Ba-hexaaluminates [9]. It can be argued that this property could be lost upon introduction of alkaline metal earth Mg2+ ions in the structure. Although this problem must be addresses through direct investigation on sulphur poisoning, it can be conceived that also Mg-substituted hexaaluminate could exhibit good sulphur resistance properties on the basis of considerations on the relative stability of the sulphates of the different cations. Indeed MgSO4 exhibits stability similar to La2 (SO4 )3 [28], both of them being much less stable than BaSO4 .
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
In conclusion, the good combustion activity, phase composition and morphological properties of the LaMgx Mn1−x Al11 O19 catalyst prepared in this work via the coprecipitation carbonates routes make these systems an interesting alternative to commercial formulation Sr0.8 La0.2 Mn1 Al11 O19 as highly stable hexaaluminate materials for high temperature catalytic combustion.
147
can be associated to Mn ions that are stabilised at high oxidation state by Mg2+ . Accordingly LaMgx Mn1−x Al11 O19 represents a promising alternative to commercial formulation Sr0.8 La0.2 Mn1 Al11 O19 as highly stable hexaaluminate material for high temperature catalytic combustion.
Acknowledgements 5. Conclusions From what has been discussed in so far it can be concluded that: 1. The preparation method, carbonates route, is effective to prepare La-hexaaluminates unsubstituted and substituted with transition metal ions. This route provides a high interspersion of the constituents in the amorphous precursors that guarantees the achievement of a final phase with the MP structure and surface areas of about 10–15 m2 /g upon calcination at 1300◦ C. 2. Formation of the final MP phase likely occurs via diffusion of La3+ ions in the ␥-Al2 O3 spinel blocks. Within this mechanism dispersed La species on the surface of the small ␥-Al2 O3 aggregates play a key role, hindering ␥-Al2 O3 sintering and phase transitions up the threshold temperature of La3+ ions diffusion. 3. Mn promotes the formation of the MP phase by enhancing ion mobility in the ␥-Al2 O3 structure. Introduction of Mg results in the segregation of MgAl2 O4 , which however easily reacts to form the MP phase. This is likely due to strong similarity of the MgAl2 O4 spinel structure with that of the spinel blocks in the MP phase. 4. Monophasic samples can be obtained only when a stable divalent cation is present in the structure. The introduction of a divalent cation stabilises the MP structure via a charge compensation mechanism that lowers the defectivity of the materials and favours the formation of the MP phase. In this respect, the introduction of Mg, that can only be 2+, is more effective than Mn that can enter the structure both in the 2+ and 3+ oxidation state. 5. The co-presence of Mg and Mn results in a higher specific catalytic activity. Such a behaviour
Financial support for this work has been provided by MURST, Rome, Italy. The Authors also acknowledge Dr. Elisabetta Finocchio (Università di Genova) for the experimental support in TPR measurements. References [1] M. Machida, K. Eguchi, H. Arai, J. Catal. 103 (1987) 385. [2] M. Machida, K. Eguchi, H. Arai, J. Catal. 120 (1989) 377. [3] G. Groppi, C. Cristiani, P. Forzatti, Catalysis 13 (1997) 85. [4] S. Matzuda, A. Kato, M. Mizumoto, H. Yamashita, in: Proceedings of the 8th ICC, Vol. IV, Verlag, Basel, Berlin, 1984, pp. 4–879. [5] J.G. McCarty, M. Gusman, D.M. Lowe, D.L. Hildenbrand, K.N. Lau, Catal. Today 47 (1999) 5. [6] M. Machida, K. Eguchi, H. Arai, J. Catal. 123 (1990) 477. [7] B.W.L. Jang, R.M. Nelson, J.J. Spivey, M. Ocal, R. Oukaci, G. Marcelin, Catal. Today 47 (1999) 103. [8] P. Euzen, European Patent EP 0,689,870 A1 (1996); US Patent 5,830,822 (1998). [9] M. Johansson, Ph.D. Thesis, KTH Stockholm, Sweden, 1998. [10] R.C. Ropp, B. Carol, J. Am. Ceram. Soc. 63 (1980) 416. [11] N. Iyi, Z. Inue, S. Takekawa, S. Kimura, J. Solid State Chem. 54 (1984) 70. [12] M. Gasperin, M.C. Saine, A. Kahn, F. Laville, M. Lejus, J. Solid State Chem. 54 (1984) 61. [13] D. Saber, A.M. Lejus, Mater. Res. Bull. 16 (1981) 1325. [14] N. Iyi, Z. Inue, S. Kimura, J. Solid State Chem. 61 (1986) 236. [15] L. Laville, D. Gourier, A.M. Lejus, D. Vivien, J. Solid State Chem. 49 (1983) 180. [16] A.M. Lejus, F. Laville, J. Thery, Mater. Res. Bull. 23 (1988) 913. [17] F. Laville, A.M. Lejus, J. Cryst. Growth 63 (1983) 426. [18] E. Tronc, F. Laville, M. Gasperin, A.M. Lejus, D. Vivien, J. Solid State Chem. 81 (1989) 192. [19] F. Laville, M. Perrin, A.M. Lejus, M. Gasperin, R. Moncorge, D. Vivien, J. Solid State Chem. 65 (1986) 301. [20] G.Groppi.M. Bellotto, C. Cristiani, P. Forzatti, J. Mater. Sci. 29 (1994) 3441. [21] G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, P.L. Villa, J. Mater. Sci. 34 (1999) 2609.
148
G. Groppi et al. / Applied Catalysis B: Environmental 35 (2001) 137–148
[22] F.P. Treadwell, Chimica Analitica, VII Edition, Vol. I, F. Vallardi, Milano, 1966 (in Italian). [23] G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, P.L. Villa, Appl. Catal. A: Gen. 104 (1993) 101. [24] J.-G. Park, A.N. Cormack, J. Solid State Chem. 130 (1997) 199. [25] P. Burtin, J.P. Brunnelle, M. Pijolat, M. Sustelle, Appl. Catal. 34 (1987) 225.
[26] M. Bellotto, G. Artioli, C. Cristiani, P. Forzatti, G. Groppi, J. Catal. 179 (1998) 597. [27] E.R. Stobbe, B.A. de Boer, J.W. Geus, Catal. Today 47 (1999) 161. [28] J.W. Ward, Molecular sieve catalyst, in: B.E. Leach (Ed.), Applied Industrial Catalysis, Vol. III, Academic Press, New York, 1984.