- Email: [email protected]
Methane combustion on Mg-doped LaMnO3 perovskite catalysts
Applied Catalysis B: Environmental 20 (1999) 277±288
Methane combustion on Mg-doped LaMnO3 perovskite catalysts Guido Saracco*, Francesco Geobaldo, Giancarlo Baldi Dipartimento di Scienza dei Materiali e Ingegneria Chimica ± Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129, Turin, Italy Received 13 September 1998; received in revised form 8 December 1998; accepted 8 December 1998
Abstract LaMn1ÿxMgxO3 perovskite catalysts (x0±0.5) were synthesised by the so-called ``citrates method'', characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO3 and LaMn0.8Mg0.2O3). The major results obtained are: (i) Mg substitution in the basic LaMnO3 perovskite has a positive effect on the catalytic activity only at low x values (x0.2); (ii) as opposed to the results of previous studies on the LaCr1ÿxMgxO3 system, the role of MgO as a textural promoter is not always signi®cant and depends strongly on the calcination temperature of the samples (800±12008C) and on the value of x; (iii) an Eley±Rideal mechanism could satisfactorily ®t the experimental kinetics results for both catalysts, even though, as opposed to LaMnO3, the catalytic combustion over LaMn0.8Mg0.2O3 seems to involve two different types of adsorbed oxygen species, depending on the operating temperature. # 1999 Elsevier Science B.V. All rights reserved. Keywords: LaMnO3 (lanthanum manganate); Perovskite; Methane catalytic combustion; Citrates method
1. Introduction In the last years, under the spur of more and more severe HC, CO and NOx regulations, a considerable interest in catalytic combustion for energy production purposes has grown. As detailed in a number of review papers [1±4], catalytic combustion can burn more ef®ciently (low HC and CO emissions) and at lower temperatures (lower NOx emissions) than gas-phase combustion. Futher, it allows easy controllability since wider air-to-fuel ratios are permitted. The application ®elds of methane catalytic combustion can be classi*Corresponding author. Tel.: +39-11-5644654; fax: +39-115644699; e-mail: [email protected]
®ed as follows, depending on the type of operating conditions: (i) lean-burn premixed adiabatic combustion (e.g. catalytic burner for gas-turbine cycles [5,6]); (ii) premixed non-adiabatic combustion (e.g. radiant burners for industrial or domestic applications [7,8] or ¯uidised-bed burners [9]); (iii) non-adiabatic diffusive-type combustion (e.g. catalytic heaters [10,11]). Catalytic heaters reached a commercial stage a long time ago (70s); however their application is limited to low thermal output and the catalyst requirements are not severe. Conversely, catalysts for type (i) and (ii) applications, despite some promising results [12], are still at a pre-commercial stage of development. Current hurdles to the commercial success of these materials are [4]: (i) high catalytic activity: low ignition
0926-3373/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-3373(98)00118-0
278
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
temperature and high speci®c surface area are required despite the high calcination temperatures; (ii) longterm thermal stability at temperatures ranging from 8008C to 13008C depending on the application context: the catalyst should not sinter, volatilise, be poisoned by sulphur-based odorising compounds or react with the support; (iii) low cost: the catalyst should not be intrinsically too expensive or involve uneconomical preparation routes. Perovskite-type catalysts (A1ÿxAx0 B1ÿyBy0 O3, where A, A0 La, Sr, Ba, . . . and B, B0 Co, Mn, Cr, . . .) show particular promise in this context [13], owing to their good stability (at least below 10008C) and catalytic activity. B cations, generally transition metals, are co-ordinated octahedrally with six oxide ions, whereas A cations, generally rather large rare- or alkaline-earths, are located in the cavity formed by the octahedra in a 12-coordinated site. In this structure, A and/or B replacement with lowervalency cations causes formation of defects (either oxygen vacancies or variations in the oxidation state of transition metals) that modify the oxygen adsorption± desorption properties and therefore the catalytic activity [14]. Following previous studies on the LaCr1ÿxMgxO3 system [15], this paper is focused on assessing the role of Mn/Mg substitution on the catalytic properties of LaMnO3, a much more active oxidation catalyst according to the literature [13]. Further, since low surface area is a limiting factor of catalytic activity of perovskites, the use of MgO as a textural promoter [16], limiting sintering of the catalyst crystals by interposition among them, has also been assessed, on the grounds of the promising results obtained for the twin system based on lanthanum chromite. The last part of the paper is then focused on the de®nition of the reaction mechanism and of methane oxidation kinetics as a function of the main affecting parameters (methane and oxygen concentrations, temperature) for the LaMnO3 and LaMn0.8Mg0.2O3 compounds. 2. Experimental 2.1. Catalyst preparation A series of catalysts of the perovskite type LaMn1ÿxMgxO3 (0
and without an excess of Mg compared to the stoichiometric value (perovskite content: 25 wt%; excess MgO: balance), via the so called ``citrates method'' described in [15]. Brie¯y, solid mixtures of La(NO3)36H2O, Mn(NO3)26H2O and Mg(NO3)26H2O (dosed in stoichiometric ratio for the unsupported catalysts, and in 17/1 MgO/ LaMn1ÿxMgxO3 molar ratio for the series of MgOrich catalysts) are mixed with a 30 wt% amount of citric acid and to 40 wt% of water. The obtained solution is slowly heated up to 1208C until a slight gas emission (NOx, CO2 and water vapour) starts. The very viscous liquid is then rapidly poured in a stainless steel vessel, kept in an oven at 1808C, hereby causing immediate and massive gas formation, which leads to the formation of a solid scum, quite friable and porous. The scum is then ®nely ground in an agate mortar and kept in an oven for 4 h in calm air at a given calcination temperature, varied on purpose in the range 800± 12008C to check its in¯uence on catalyst performance. 2.2. Catalyst characterisation Chemical analysis (dissolutionatomic absorption, oxygen titration), performed on either supported or unsupported samples, con®rmed that the amount of the various elements of interest (La, Mn, Mg, O) was consistent with that used in the precursors and was compatible with the phases detected by X-ray diffraction with a 4% deviation. XRD analyses (Philips PW1710 apparatus equipped with a monochromator for the Cu K radiation), performed on all the LaMn1ÿxMgxO3 samples prepared at 8008C or higher calcination temperatures, showed that the typical ABO3 structure was formed in all cases, even at the highest Mn/Mg substitution level (x0.5). At 7008C crystallisation was very poor, hence this temperature was not considered. Fig. 1 (curve a), referring to LaMnO3 and to a calcination temperature of 8008C, shows how rather well-de®ned rhombohedral diffraction lines (JCPDS card 32-0484) are present with or without the excess of MgO. The diffraction peaks of MgO (JCPDS card 45-0946) are clearly noticeable in Fig. 1(b), concerning an MgOsupported LaMnO3 sample. No other side products were detectable. At 11008C or higher calcination temperatures an orthorhombic structure replaced the low-temperature rhombohedral one.
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 1. X-ray diffraction patterns of the following catalysts (calcination temperature8008C): (a) LaMnO 3 ; (b) LaMnO317MgO; (c) LaMn0.5Mg0.5O3. Legend: (~) rhombohedral perovskite; (*) MgO.
Similar diffraction patterns were obtained for all twin samples LaMn1ÿxMgxO3/LaMn1ÿxMgxO3 17MgO. Fig. 1(c) shows how LaMn0.5Mg0.5O3 keeps the same rhombhohedral structure of the basic LaMnO3 perovskite. The d-values of all the perovskite samples were practically coincident with those of the basic perovskite LaMnO3, a likely sign that the crystal lattice dimensions are not affected by the Mn/Mg substitution, despite the fact that Mg2 is a larger Ê ) and has a lower valency ion (ionic radius0.72 A 3 Ê ). A probable explathan Mn (ionic radius0.645 A nation of this lies in the formation, in direct proportion to Mg2 content, of the rather small Mn4 (ionic Ê ), which compensates for either the radius0.540 A sterical or the electronic effects of Mg-doping. However, as considered later on in the discussion, the elimination of a certain amount of cation vacancies, also playing a role in the catalyst activity, cannot be excluded. The low-temperature rhombohedral structure of LaMnO3 is indeed rich with cation vacancies and correspondingly characterised by a high oxygen excess (0.1±0.2) with respect to stoichiometry, accompanied by the spontaneous transition of 15± 30% of Mn3 to Mn4 so as to accomplish electroneutrality. Conversely, the orthorhombic phase is characterised by <0.05 and much lower catalytic activity towards oxidation processes [17]. BET analysis showed that perovskite samples prepared at 8008C without MgO excess have a speci®c
279
surface area ranging from 7 to 8 m2 gÿ1 (with the only exception of the LaMn0.5Mg0.5O3 sample for which a value of 9.5 m2 gÿ1 was measured). BET analysis cannot be regarded as highly reliable at these rather low speci®c surface area values, however this last parameter should not affect very much the catalytic activity of the various unsupported compounds tested. Shifting to MgO-supported samples, BET surface areas of 18±20 m2 gÿ1 were measured. Furthermore, TEM observations, described in the following, showed that, in these last samples, MgO crystals are much larger than perovskite ones. As a consequence, the speci®c surface area exposed per unit perovskite weight in the LaMn1ÿxMgxO317MgO samples should be signi®cantly higher than 20 m2 gÿ1. Unfortunately, calcination at 12008C reduces markedly the speci®c surface area of textural promoter free catalysts (about 1 m2 gÿ1), whereas all biphasic samples maintained at 12008C a speci®c surface area of 6±7 m2 gÿ1, a likely effect of the anti-sintering properties of MgO. TEM observations have also been performed so as to clarify the micro-structure of the prepared catalysts. Fig. 2 concerns the LaMnO3 catalyst in the absence and in the presence of MgO (textural promoter), after calcination at 8008C. By comparing the two mentioned micrographs some preliminary comments can be made: the perovskite crystals of the pure LaMnO3 are much larger compared to those of the same perovskite (dark crystals) in the presence of MgO (light crystals); particularly, on the basis of exten-
Fig. 2. TEM micrographs of the following catalysts (calcination temperature8008C). Left: LaMnO3 (100 000); right: LaMnO3 17 MgO (150 000).
280
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 3. TEM micrographs of the catalyst LaMn0.5Mg0.5O317MgO (calcination temperature8008C): left: 300 000; right: 600 000.
sive image analysis, the average particle size of the first samples was about 80 nm, whereas that of the second ones was just 25 nm; in MgO-rich catalysts, the perovskite crystals are randomly dispersed among the MgO crystals, which should effectively guarantee a hindering effect on high-temperature sintering. However, when the LaMn0.5Mg0.5O317MgO sample is considered (Fig. 3), it can be immediately observed that, compared with the LaMnO317MgO catalyst: the perovskite crystals are even smaller; the crystals of either MgO or LaMn0.5Mg0.5O3 appear once again rather homogeneously dispersed, even if the perovskite crystals are tightly surrounded by MgO ones, which, as discussed later on, might prevent or at least hamper the reacting gases from reaching the active catalyst surface. TEM observations on the samples heated at 12008C allowed to verify the effect of sintering on MgO-free samples, which lead to an increase of the average particle size up to 800 nm, against just 400 nm typical of those belonging to the LaMnO317MgO catalyst. Finally, the surrounding effect of MgO crystals with respect to perovskite ones was still present at this rather high calcination temperature. High-resolution TEM observation (up to 800 000) showed that unsupported samples exclusively consti-
tuted of the LaMn1ÿxMgxO3 perovskite compounds, and that no other nanosized crystals or oxide clusters were detectable over the perovskite crystal surfaces. The same holds for the unsupported samples made of LaMnO3 and MgO, for which these two species only could be seen even at the above high magni®cation. The occasional presence of some Mn or La oxides cannot though be completely excluded and might play a role (likely marginal) on the catalytic activity. Conversely, owing to the complex texture of the LaMn1ÿxMgxO3MgO samples (®ne crystals tightly bound to one another to form agglomerates), nothing could be concluded on the basis of HRTEM observation and the presence of simple oxides different from MgO cannot be excluded, even if they are not detectable by X-ray diffraction. As mentioned above, lanthanum manganate and related perovskite compounds are known to undergo reversible oxygen chemisorption, which represents a key feature of their catalytic activity towards oxidation reactions. Some temperature-programmed oxygen desorption runs were performed so as to get evidence of this, possibly enlightening different types of oxygen chemisorption on the perovskites having different x values. One g samples of MgO-free catalysts calcined at 8008C and characterised by three Mn/Mg substitution levels (x0, 0.2, 0.5) were pre-treated in a quartz tube at 8008C for 0.5 h under chromatographic air ¯ow, so as to allow desorption of any gas present on
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 4. Results of the temperature-programmed desorption tests on three selected perovskite catalysts (calcination temperature 8008C).
the catalyst surface. Temperature was then progressively lowered down to room temperature at a 38C minÿ1 rate, thereby allowing complete oxygen adsorption on the catalyst surface. Helium was then fed to the reactor at a 50 cm3 minÿ1 ¯ow rate through a mass ¯ow controller in order to wash out any oxygen molecule not chemisorbed. After 10 min at 258C, the temperature was raised to 9008C once again at a 38C minÿ1 rate, meanwhile monitoring the oxygen outlet concentration through gas-chromatographic analysis (Hewlett-Packard, mod. 5890 series II, equipped with a Porapak QS column and a TCD detector). Fig. 4 shows the variation of the outlet oxygen concentration as a function of temperature for the three catalysts tested. In line with the work of Seyama [14], two types of chemisorbed oxygen species can be noticed accompanied by related desorption peaks: a low-temperature species, named hereafter a, desorbed in the 300±5008C range, and a high-temperature one, named b in the following, desorbed at 600±8008C. The higher the x value, the lower the area of the b-oxygen desorption peaks, whereas a-oxygen type is only present in partially substituted samples. The role of these two types of oxygen on the catalytic activity and kinetics will be discussed in part 3. 2.3. Catalytic activity screening tests Some catalytic activity tests were then performed on all the prepared catalysts in the experimental apparatus, and according to the procedures, carefully described in [15]. After 30 min stay in air ¯ow as a common pre-treatment, a gas ¯ow rate (1.2 cm3 sÿ1 of
281
the following composition: CH41.5%, O218%, Hebalance) was fed to a ®xed-bed of 1 g of catalyst particles (obtained by pressing the perovskite powders into tablets, then crushed into 0.2±0.5 mm granules). The ®xed bed was enclosed in a quartz tube (internal diameter: 4 mm) and sandwiched between two quartzwool layers. The reactor temperature was then lowered at a 38C minÿ1 rate down to 3008C, meanwhile monitoring methane conversion by analysing the outlet concentration of CO2 (the only carbon oxidation product) by use of an FTIR analyser (Hartmann and Braun URAS 10E). The typical sigma-shaped curves obtained are shown in Fig. 5 for the catalysts calcined at 8008C. Each data point is the average of twin runs performed on two different samples of the same material. The deviation between the conversion measured at the same temperature in such twin runs was always less than 10%. No signi®cant hysteresis in these curves was observed performing the catalytic tests upward (from 3008C to 8008C); at least any difference in the results of downward runs was within the above deviation value. In the legend of this ®gure indication is given about the half-conversion temperatures (T50), a parameter which could be regarded as the simplest index of catalytic activity. Fig. 6 shows how, for the different calcination temperatures tested, T50 is affected by the extent of Mn/Mg substitution in either MgO-free (Fig. 6(a)) or MgO-rich (Fig. 6(b)) catalysts. In order to fully appreciate the catalytic effect of the perovskite, it has to be considered that the T50 value of a non-catalytic run performed with silica granules instead of the catalytic ones is about 7708C in the experimental apparatus employed, whereas that of pure MgO (calcined at 8008C) is about 6758C. This last ®gure, in particular, shows how magnesium oxide does not contribute signi®cantly to the catalytic activity of the LaMn1ÿxMgxO317MgO catalysts. 2.4. Reaction kinetics studies on selected catalysts Reaction kinetics were assessed on two catalysts, LaMnO3 and LaMn0.8Mg0.2O3, the former chosen as a reference, the second for its prevalent catalytic activity (Fig. 6(a)). The experimental apparatus used for this purpose is a recycle micro-reactor described in detail in [15]. The reactor, hosting 0.3 g of pelletised catalyst, was operated in the temperature range between
282
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 5. Results of the catalytic activity test towards methane combustion of all catalysts prepared at the calcination temperature of 8008C.
4508C and 6108C with an outlet pressure of 1 bar, preventing reaction kinetics from being affected by the internal mass transfer resistance (the pellet size was reduced down to 0.2 mm so as to minimise this effect) as well as by the external mass transfer resistance (the recycle ¯ow rate was increased on purpose up to 10 cm3 sÿ1 with a recycle ratio equal to 5.3). Methane
per-pass conversion was always lower than 10%, even at the highest operating temperatures. CH4 and O2 feed concentrations were varied through a set of mass ¯ow controllers (Brooks) in the ranges 0.5±2.5% and 2±15%, respectively, He being the balance. Methane conversion rate per unit catalyst mass RCH4 (mol gÿ1 sÿ1) could thus be measured as a function
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
283
Fig. 6. Effect of x and of the calcination temperature ((^)8008C; (&)9008C; (~)10008C; (*)11008C; (*)12008C) on the T50 values measured for the various catalysts prepared: (a) LaMn1ÿxMgxO3; (b) LaMn1ÿxMgxO317MgO.
of temperature and of the average partial pressures of oxygen and methane, calculated on the grounds of simple mass balances. The best kinetics law for data ®tting turned out to be of the Eley±Rideal type [18]: p KO2 pO2 p ; (1) RCH4 kr pCH4 1 KO2 pO2 where
Er Arrhenius; kr exp ÿ RT HO2 van0 t Hoff: KO2 KOo 2 exp ÿ RT kro
(2) (3)
The related reaction mechanism is based on the dissociative chemisorption of oxygen molecules at some active sites over the catalyst surface (regulated by a van't Hoff type equilibrium expression whose thermodynamic constant is indeed KO2 ). This chemisorption is then followed by reaction of methane with one of the derived oxygen atoms, which initiates the combustion process with a reaction rate governed by the kinetic constant kr. Some representative diagrams concerning model accordance to experimental data obtained at a given operating temperature for different oxygen and methane feed concentrations are reported in Fig. 7 for the LaMn0.8Mg0.2O3 catalyst. All model predic-
tions were evaluated by setting the two constants kr and KO2 at an optimum value, determined at each temperature by the least squares ®tting method. Arrhenius-type plots of such constants are reported in Figs. 8 and 9 for the LaMnO3 and LaMn0.8Mg0.2O3 catalysts, respectively. From these last data, estimation of the activation energy Er and of the heat of adsorption HO2 can be derived for both catalysts, based on Eqs. (2) and (3), once again by the least squares procedure. For the LaMnO3 catalyst a single value of Er (35.6 kcal molÿ1) and of HO2 (ÿ23.5 kcal molÿ1) turned out to give good model predictions all over the temperature range explored. Conversely, for the LaMn0.8Mg0.2O3 compound, the slope of kr and, especially, of KO2 variation versus 1/T varied abruptly around 5508C. As a consequence, two sets of Er and HO2 values were calculated above (48.3 and ÿ58.9 kcal molÿ1, respectively) and below (29.1 and ÿ4.5 kcal molÿ1) this critical temperature value. 3. Discussion In line with the previous studies on the LaCr1ÿxMgxO3 system, the three prevalent features of the work are listed here. 1. the assessment of the role of Mg-doping on the catalytic activity of the LaMnO3 perovskite, with
284
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 7. Dependency of RCH4 versus pCH4 (a) and pO2 (b) at different yCH4 and yO2 feed values for the LaMn0.8Mg0.2O3 perovskite at 5508C: experimental data and model predictions (Eq. (1)).
Fig. 8. Arrhenius plot of kr and KO2 constants for the LaMnO3 perovskite.
special concern about the effect of the calcination temperature of the catalyst; 2. the study of the potential of the dispersion of the perovskite crystals among MgO crystals so as to preserve the catalytic activity and stability at hightemperatures; 3. the assessment of methane combustion kinetics and reaction mechanism upon the above catalysts. As concerns point (a), it must be immediately considered that, as opposed to the LaCrO3 perovskite [19], lanthanum manganate possesses a considerable
intrinsic activity due to the already mentioned tendency of Mn3 to oxidise to Mn4 upon heating in air at ambient pressure. This transition is accompanied by the generation of cation vacancies and by the presence of the well-known excess of oxygen compared to the stoichiometric requirements [17]. In this context, the Mn/Mg substitution can play the following roles: 1. owing to the bivalent nature of the Mg2 ion, a further amount of manganese might shift from a III to a IV oxidation state, so as to accomplish a
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
285
Fig. 9. Arrhenius plot of kr and KO2 constants for the LaMn0.8Mg0.2O3 perovskite.
simple charge balance. Such a condition may promote an increase in the catalytic activity, owing to the enabled possibility of the lattice oxygen close to Mn4 to be easily reacted away by methane with contemporary regression of manganese to the Mn3 state. Fresh oxygen can then chemisorb on the generated oxygen vacancies, thus restoring the original situation. Similar effects, due to doping elements, were noticed on other perovskite systems in [20]. This effect should in any case be limited to the amount of manganese not related to the excess of oxygen . For instance, LaMnO3 samples calcinated at 8008C are characterised by a value equal to about 0.18 [17], which means that nearly 30% of the Mn content should have a IV valence. The maximum x value, at which no prejudice of cation vacancies might be obtained (equal value) and all Mn is turned to Mn4, should be thus close to 0.35. 2. On the other hand, the charge balance, disturbed by the introduction of the divalent Mg2 instead of the trivalent Mn3, can also be fulfilled by elimination of some excess oxygen and of the related cation vacancies. This second circumstance might negatively affect the catalytic activity of the system, since it reduces the structural defects of the LaMnO3, which are so important for its significant catalytic activity.
If the data shown in Fig. 4 are considered, it can be stated that both (i) and (ii) effects should take place simultaneously. In line with the theory outlined in [21,22], it might be deduced that the low-temperature a-type oxygen is related to type (i) effect, whereas btype oxygen is indeed the lattice oxygen related to the stoichiometric excess . When the degree of substitution is increased the b-oxygen peak is progressively reduced (type-ii) mechanism, whereas, simultaneously, an a-oxygen peak appears (type-i) mechanism. These combined effects turned to give the maximum bene®t to catalytic activity of pure perovskite compounds calcinated at 8008C, for an x value of 0.2 (Fig. 5(a) and Fig. 6(a)). Further analyses, by either the SIMS [23,24] or the XPS techniques, are planned for the near future in order to possibly get a deeper insight into the role played by the Mn3±Mn4 transition on the catalytic activity, as well as to check for possible differences between surface and bulk compositions. When the calcination temperature increases, the activity of pure perovskite samples diminishes for all x values. This can be easily explained by the occurrence of sintering, as demonstrated by the results of the BET and TEM analyses. Coming to point (b), on the basis of TEM observations (Figs. 2 and 3) and of catalytic activity data (Fig. 5(b) and Fig. 6(b)), the following two issues can be deduced:
286
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
1. for the LaMnO3 catalyst, MgO effectively promotes the formation of small perovskite crystals and hampers, to some extent, their sintering; as a result, the catalytic activity of the MgO-rich catalyst is, for certain calcination temperatures, even higher than its pure counterpart despite the presence of only about 25 wt% of the perovskite compound. The best results were obtained at a calcination temperature of 9008C at which the T50 values of the MgO-rich and MgO-free LaMnO3 samples are 4948C and 4708C, respectively. 2. For x60, despite the already-mentioned positive effect on the perovskite crystal size, the presence of excess MgO only occasionally entails higher catalytic activities than those of MgO-free compounds (see data points for x0.3, 0.4 at calcination temperatures 11008C). A likely explanation of this behaviour lies in the shielding effect on the perovskite surface, exerted by the MgO crystals which cover it very tightly, thereby limiting the flow rate of gas molecules reaching the catalytically active sites. MgO amounts lower than those here tested may perhaps give better results, by limiting this covering effect and still preventing sinterisation to an acceptable extent. The fact that LaMn1ÿxMgxO317MgO samples calcined at 9008C outperform those calcined at 8008C is rather puzzling, contrary to what should be expected on the grounds of the natural tendency of sintering phenomena to speed up with temperature. Further studies are in progress in order to clarify this point. 9008C is in any case a suitable temperature for premixed catalytic burner applications [7,8]. Considering the last point (c), the analysis of the kinetic data and the related theoretical arguments are strongly in favour of a remarkable effect of chemisorbed oxygen species on the catalytic activity, as also underlined by several authors for different perovskite systems, e.g. [21]. In fact, the earlier described Eley± Rideal mechanism was successfully employed in ®tting the experimental results of both LaMnO3 and LaMn0.8Mg0.2O3 perovskites, as shown in Figs. 7±9. Considering the activation energies and the heats of adsorption calculated on the basis of the data shown in Figs. 8 and 9, the following considerations can be drawn: The Er values are well inside the range reported for a variety of perovskite systems [14].
The values of the heats of adsorption (ÿ23.5 kcal molÿ1) derived for the LaMnO3 perovskite is high enough to entail a chemisorption rather than a simple physisorption of oxygen. Similar values were directly measured by Fierro and co-workers [25] for this compound. On the basis of the outcomes of temperature-programmed desorption runs, it may be stated that the above oxygen chemisorption regards b-type lattice oxygen, which may thus be reacted away by CH4 molecules and recovered by dissociative adsorption of oxygen molecules from the gas phase. The value of the enthalpy of adsorption (ÿ4.5 kcal molÿ1) derived for the LaMn0.8Mg0.2O3 catalyst below 5508C suggests that, at low temperatures, a-type oxygen, characterised by a rather low heat of adsorption, governs the reaction mechanism. Since the above heat of adsorption is quite close to values typical of simple physisorption processes, the kinetic data determined at all temperatures below 5508C were also fitted with the kinetic expression RCH4 kr pCH4
KO2 pO2 ; 1 KO2 pO2
(4)
accounting for a non-dissociative chemisoprtion of oxygen. However, the results were not satisfactory since the least squares sum was always higher than that obtained with the Eley±Rideal expression (by 20% on an average) at any operating temperature, and unrealistic positive values of the heat of adsorption were drawn. For these reasons, the authors are prone to consider a-oxygen adsorption on the above samples as a weak chemisoprtion. This is indeed confirmed by temperature-programmed desorption runs (Fig. 4), which show how this kind of oxygen is released at temperatures higher than 3008C, frankly incompatible with a simple physisorption. Conversely, at high temperatures (above 5508C), the residual b-type oxygen, remaining after Mn/ Mg substitution, is playing a major role. Further, the HO2 value derived above 5508C (ÿ58.9 kcal molÿ1), quite high if compared with that calculated for the unsubstituted LaMnO3 perovskite, may suggest that the lower the boxygen amount in the perovskite, the more difficult its release for reaction purposes becomes.
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
4. Conclusions Mg-doped LaMnO3 perovskite catalysts were prepared by the citrates method, characterised and tested as catalysts for high-temperature methane combustion, showing, up to a limited degree of Mn/Mg substitution (x0.2), a promoting effect of x on the catalytic activity. The simultaneous synthesis of perovskite and MgO crystals was proved to be an effective mean to reduce the sintering tendency of the catalyst and thus allowing to keep small-size perovskite crystals and fair catalytic activity at high temperatures. In fact, the presence of MgO dispersing particles, in thermodynamical equilibrium with the perovskite ones, hampers, to some extent, the contact between different perovskite crystals and the subsequent sintering phenomena. Such effect was nonetheless strictly dependent on the calcination temperature, the best results being obtained at 9008C. Further, for high x values a clear tendency of magnesium crystals to cover perovskite ones thereby limiting their availability to gas molecules can be noticed, which, at least in part, eliminates the above advantages. A kinetics analysis concerning methane combustion on the selected perovskite catalysts showed that the Elay±Rideal mechanism provides satisfactory interpretation of the kinetic data as a function of oxygen and methane partial pressure for both unsubstituted and substituted LaMnO3 samples. However, as opposed to the basic LaMnO3 perovskite in which only the excess oxygen is taking part in the reaction, a different type of chemisorbed oxygen is involved below 5508C by the LaMn0.8Mg0.2O3 catalyst, likely linked to partial transition of Mn3 to Mn4 forced by the presence of Mg2 inside the perovskite structure. Further studies are though needed to elucidate this point. Under the spur of the above promising results the studied perovskite systems are being applied in the development of premixed catalytic burners for domestic boiler applications [8]. 5. Nomenclature Er
activation energy of the reaction constant (kcal molÿ1)
HO2 kr KO2 kro KOo 2 p RCH4 T x y
287
enthalpy of oxygen adsorption (kcal molÿ1) reaction constant (mol barÿ1 gÿ1 sÿ1) oxygen adsorption coefficient (barÿ1) pre-exponential value of the reaction constant (mol barÿ1 gÿ1 sÿ1) pre-exponential value of oxygen adsorption coefficient (barÿ1) partial pressure (bar) conversion rate of methane (mol gÿ1 sÿ1) temperature (K) degree of Mn/Mg substitution in LaMn1ÿxMgxO3 mole fraction
Greek letters
excess oxygen in the perovskite lattice methane conversion
References [1] D.L. Trimm, Appl. Catal. 7 (1983) 249. [2] L.D. Pfefferle, W.C. Pfefferle, Catal. Rev.-Sci. Eng. 29 (1987) 219. [3] Z.R. Ismagilov, M.A. Kerzhentsev, Catal. Rev.-Sci. Eng. 32 (1990) 51. [4] M.F.M. Zwinkels, S.G. JaÈraÊs, P. Govind Menon, Catal. Rev.Sci. Eng. 35 (1993) 319. [5] M.F.M. Zwinkels, High-temperature Catalytic Combustion, Ph.D. Thesis, KTH University, Stockholm, 1996. [6] R.A. Dalla Betta, Catal. Today 35 (1997) 129. [7] J.D. Sullivan, Basic Research on Radiant Burners, Alzeta Corporation Report no. 91-7027-167, Santa Clara, CA, 1991. [8] G. Saracco, I. Cerri, V. Specchia, R. Accornero, Chem. Eng. Sci. (1998), in press. [9] M. Foka, J. Chaouki, C. Guy, D. Klvana, Chem. Eng. Sci. 49 (1995) 4269. [10] S.W. Radcliffe, R.G. Hickman, J. Inst. Fuel 48 (1975) 208. [11] V. Specchia, S. Sicardi, A. Gianetto, Chem. Eng. Commun. 10 (1981) 189. [12] R.A. Dalla Betta, US Patent 5 405 260, 11 April 1995. [13] L.G. Tejuca, J.L.G. Fierro (Eds.), Properties and Applications of Perovskite-type Oxides, Marcel Dekker, New York, 1993. [14] T. Seiyama, Catal. Rev.-Sci. Eng. 34 (1992) 281. [15] G. Saracco, G. Scibilia, A. Iannibello, G. Baldi, Appl. Catal. B 8 (1996) 229. [16] C.N. Satterfield, Heterogeneous Catalysis in Industrial Practice, McGraw-Hill, New York, 1991, p. 127. [17] J. ToÈpfer, J.B. Goodenough, J. Solid State Chem. 130 (1997) 117. [18] J.N. Thomas, W.J. Thomas, Introduction to the Principles of Heterogeneous Catalysis, Academic Press, London, 1967.
288
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
[19] B.K. Flandermeyer, M.M. Nasrallah, A.K. Agarwal, H.U. Anderson, J. Am. Ceram. Soc. 67 (1984) 195. [20] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26 (1986) 265. [21] L.G. Tejuca, J.L.G. Fierro, J.M.D. TascoÂn, Adv. Catal. 36 (1989) 237. [22] T. Seiyama, N. Yamazoe, K. Eguchi, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 19.
[23] L.A. Isupova, V.A. Sadykov, V.P. Ivanov, A.A. Rar, S.V. Tsybulya, M.P. Andrianova, V.N. Kolomilchuk, A.N. Petrov, O.F. Kononchuck, React. Kinet. Catal. Lett. 53 (1994) 223. [24] L.A. Isupova, V.A. Sadykov, S.V. Tsybulya, G.N. Kryukova, V.P. Ivanov, A.N. Petrov, O.F. Kononchuck, React. Kinet. Catal. Lett. 62 (1997) 129. [25] J.L.G. Fierro, J.M.D. TascoÂn, L.G. Tejuca, J. Catal. 89 (1984) 209.
Methane combustion on Mg-doped LaMnO3 perovskite catalysts Guido Saracco*, Francesco Geobaldo, Giancarlo Baldi Dipartimento di Scienza dei Materiali e Ingegneria Chimica ± Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129, Turin, Italy Received 13 September 1998; received in revised form 8 December 1998; accepted 8 December 1998
Abstract LaMn1ÿxMgxO3 perovskite catalysts (x0±0.5) were synthesised by the so-called ``citrates method'', characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO3 and LaMn0.8Mg0.2O3). The major results obtained are: (i) Mg substitution in the basic LaMnO3 perovskite has a positive effect on the catalytic activity only at low x values (x0.2); (ii) as opposed to the results of previous studies on the LaCr1ÿxMgxO3 system, the role of MgO as a textural promoter is not always signi®cant and depends strongly on the calcination temperature of the samples (800±12008C) and on the value of x; (iii) an Eley±Rideal mechanism could satisfactorily ®t the experimental kinetics results for both catalysts, even though, as opposed to LaMnO3, the catalytic combustion over LaMn0.8Mg0.2O3 seems to involve two different types of adsorbed oxygen species, depending on the operating temperature. # 1999 Elsevier Science B.V. All rights reserved. Keywords: LaMnO3 (lanthanum manganate); Perovskite; Methane catalytic combustion; Citrates method
1. Introduction In the last years, under the spur of more and more severe HC, CO and NOx regulations, a considerable interest in catalytic combustion for energy production purposes has grown. As detailed in a number of review papers [1±4], catalytic combustion can burn more ef®ciently (low HC and CO emissions) and at lower temperatures (lower NOx emissions) than gas-phase combustion. Futher, it allows easy controllability since wider air-to-fuel ratios are permitted. The application ®elds of methane catalytic combustion can be classi*Corresponding author. Tel.: +39-11-5644654; fax: +39-115644699; e-mail: [email protected]
®ed as follows, depending on the type of operating conditions: (i) lean-burn premixed adiabatic combustion (e.g. catalytic burner for gas-turbine cycles [5,6]); (ii) premixed non-adiabatic combustion (e.g. radiant burners for industrial or domestic applications [7,8] or ¯uidised-bed burners [9]); (iii) non-adiabatic diffusive-type combustion (e.g. catalytic heaters [10,11]). Catalytic heaters reached a commercial stage a long time ago (70s); however their application is limited to low thermal output and the catalyst requirements are not severe. Conversely, catalysts for type (i) and (ii) applications, despite some promising results [12], are still at a pre-commercial stage of development. Current hurdles to the commercial success of these materials are [4]: (i) high catalytic activity: low ignition
0926-3373/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-3373(98)00118-0
278
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
temperature and high speci®c surface area are required despite the high calcination temperatures; (ii) longterm thermal stability at temperatures ranging from 8008C to 13008C depending on the application context: the catalyst should not sinter, volatilise, be poisoned by sulphur-based odorising compounds or react with the support; (iii) low cost: the catalyst should not be intrinsically too expensive or involve uneconomical preparation routes. Perovskite-type catalysts (A1ÿxAx0 B1ÿyBy0 O3, where A, A0 La, Sr, Ba, . . . and B, B0 Co, Mn, Cr, . . .) show particular promise in this context [13], owing to their good stability (at least below 10008C) and catalytic activity. B cations, generally transition metals, are co-ordinated octahedrally with six oxide ions, whereas A cations, generally rather large rare- or alkaline-earths, are located in the cavity formed by the octahedra in a 12-coordinated site. In this structure, A and/or B replacement with lowervalency cations causes formation of defects (either oxygen vacancies or variations in the oxidation state of transition metals) that modify the oxygen adsorption± desorption properties and therefore the catalytic activity [14]. Following previous studies on the LaCr1ÿxMgxO3 system [15], this paper is focused on assessing the role of Mn/Mg substitution on the catalytic properties of LaMnO3, a much more active oxidation catalyst according to the literature [13]. Further, since low surface area is a limiting factor of catalytic activity of perovskites, the use of MgO as a textural promoter [16], limiting sintering of the catalyst crystals by interposition among them, has also been assessed, on the grounds of the promising results obtained for the twin system based on lanthanum chromite. The last part of the paper is then focused on the de®nition of the reaction mechanism and of methane oxidation kinetics as a function of the main affecting parameters (methane and oxygen concentrations, temperature) for the LaMnO3 and LaMn0.8Mg0.2O3 compounds. 2. Experimental 2.1. Catalyst preparation A series of catalysts of the perovskite type LaMn1ÿxMgxO3 (0
and without an excess of Mg compared to the stoichiometric value (perovskite content: 25 wt%; excess MgO: balance), via the so called ``citrates method'' described in [15]. Brie¯y, solid mixtures of La(NO3)36H2O, Mn(NO3)26H2O and Mg(NO3)26H2O (dosed in stoichiometric ratio for the unsupported catalysts, and in 17/1 MgO/ LaMn1ÿxMgxO3 molar ratio for the series of MgOrich catalysts) are mixed with a 30 wt% amount of citric acid and to 40 wt% of water. The obtained solution is slowly heated up to 1208C until a slight gas emission (NOx, CO2 and water vapour) starts. The very viscous liquid is then rapidly poured in a stainless steel vessel, kept in an oven at 1808C, hereby causing immediate and massive gas formation, which leads to the formation of a solid scum, quite friable and porous. The scum is then ®nely ground in an agate mortar and kept in an oven for 4 h in calm air at a given calcination temperature, varied on purpose in the range 800± 12008C to check its in¯uence on catalyst performance. 2.2. Catalyst characterisation Chemical analysis (dissolutionatomic absorption, oxygen titration), performed on either supported or unsupported samples, con®rmed that the amount of the various elements of interest (La, Mn, Mg, O) was consistent with that used in the precursors and was compatible with the phases detected by X-ray diffraction with a 4% deviation. XRD analyses (Philips PW1710 apparatus equipped with a monochromator for the Cu K radiation), performed on all the LaMn1ÿxMgxO3 samples prepared at 8008C or higher calcination temperatures, showed that the typical ABO3 structure was formed in all cases, even at the highest Mn/Mg substitution level (x0.5). At 7008C crystallisation was very poor, hence this temperature was not considered. Fig. 1 (curve a), referring to LaMnO3 and to a calcination temperature of 8008C, shows how rather well-de®ned rhombohedral diffraction lines (JCPDS card 32-0484) are present with or without the excess of MgO. The diffraction peaks of MgO (JCPDS card 45-0946) are clearly noticeable in Fig. 1(b), concerning an MgOsupported LaMnO3 sample. No other side products were detectable. At 11008C or higher calcination temperatures an orthorhombic structure replaced the low-temperature rhombohedral one.
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 1. X-ray diffraction patterns of the following catalysts (calcination temperature8008C): (a) LaMnO 3 ; (b) LaMnO317MgO; (c) LaMn0.5Mg0.5O3. Legend: (~) rhombohedral perovskite; (*) MgO.
Similar diffraction patterns were obtained for all twin samples LaMn1ÿxMgxO3/LaMn1ÿxMgxO3 17MgO. Fig. 1(c) shows how LaMn0.5Mg0.5O3 keeps the same rhombhohedral structure of the basic LaMnO3 perovskite. The d-values of all the perovskite samples were practically coincident with those of the basic perovskite LaMnO3, a likely sign that the crystal lattice dimensions are not affected by the Mn/Mg substitution, despite the fact that Mg2 is a larger Ê ) and has a lower valency ion (ionic radius0.72 A 3 Ê ). A probable explathan Mn (ionic radius0.645 A nation of this lies in the formation, in direct proportion to Mg2 content, of the rather small Mn4 (ionic Ê ), which compensates for either the radius0.540 A sterical or the electronic effects of Mg-doping. However, as considered later on in the discussion, the elimination of a certain amount of cation vacancies, also playing a role in the catalyst activity, cannot be excluded. The low-temperature rhombohedral structure of LaMnO3 is indeed rich with cation vacancies and correspondingly characterised by a high oxygen excess (0.1±0.2) with respect to stoichiometry, accompanied by the spontaneous transition of 15± 30% of Mn3 to Mn4 so as to accomplish electroneutrality. Conversely, the orthorhombic phase is characterised by <0.05 and much lower catalytic activity towards oxidation processes [17]. BET analysis showed that perovskite samples prepared at 8008C without MgO excess have a speci®c
279
surface area ranging from 7 to 8 m2 gÿ1 (with the only exception of the LaMn0.5Mg0.5O3 sample for which a value of 9.5 m2 gÿ1 was measured). BET analysis cannot be regarded as highly reliable at these rather low speci®c surface area values, however this last parameter should not affect very much the catalytic activity of the various unsupported compounds tested. Shifting to MgO-supported samples, BET surface areas of 18±20 m2 gÿ1 were measured. Furthermore, TEM observations, described in the following, showed that, in these last samples, MgO crystals are much larger than perovskite ones. As a consequence, the speci®c surface area exposed per unit perovskite weight in the LaMn1ÿxMgxO317MgO samples should be signi®cantly higher than 20 m2 gÿ1. Unfortunately, calcination at 12008C reduces markedly the speci®c surface area of textural promoter free catalysts (about 1 m2 gÿ1), whereas all biphasic samples maintained at 12008C a speci®c surface area of 6±7 m2 gÿ1, a likely effect of the anti-sintering properties of MgO. TEM observations have also been performed so as to clarify the micro-structure of the prepared catalysts. Fig. 2 concerns the LaMnO3 catalyst in the absence and in the presence of MgO (textural promoter), after calcination at 8008C. By comparing the two mentioned micrographs some preliminary comments can be made: the perovskite crystals of the pure LaMnO3 are much larger compared to those of the same perovskite (dark crystals) in the presence of MgO (light crystals); particularly, on the basis of exten-
Fig. 2. TEM micrographs of the following catalysts (calcination temperature8008C). Left: LaMnO3 (100 000); right: LaMnO3 17 MgO (150 000).
280
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 3. TEM micrographs of the catalyst LaMn0.5Mg0.5O317MgO (calcination temperature8008C): left: 300 000; right: 600 000.
sive image analysis, the average particle size of the first samples was about 80 nm, whereas that of the second ones was just 25 nm; in MgO-rich catalysts, the perovskite crystals are randomly dispersed among the MgO crystals, which should effectively guarantee a hindering effect on high-temperature sintering. However, when the LaMn0.5Mg0.5O317MgO sample is considered (Fig. 3), it can be immediately observed that, compared with the LaMnO317MgO catalyst: the perovskite crystals are even smaller; the crystals of either MgO or LaMn0.5Mg0.5O3 appear once again rather homogeneously dispersed, even if the perovskite crystals are tightly surrounded by MgO ones, which, as discussed later on, might prevent or at least hamper the reacting gases from reaching the active catalyst surface. TEM observations on the samples heated at 12008C allowed to verify the effect of sintering on MgO-free samples, which lead to an increase of the average particle size up to 800 nm, against just 400 nm typical of those belonging to the LaMnO317MgO catalyst. Finally, the surrounding effect of MgO crystals with respect to perovskite ones was still present at this rather high calcination temperature. High-resolution TEM observation (up to 800 000) showed that unsupported samples exclusively consti-
tuted of the LaMn1ÿxMgxO3 perovskite compounds, and that no other nanosized crystals or oxide clusters were detectable over the perovskite crystal surfaces. The same holds for the unsupported samples made of LaMnO3 and MgO, for which these two species only could be seen even at the above high magni®cation. The occasional presence of some Mn or La oxides cannot though be completely excluded and might play a role (likely marginal) on the catalytic activity. Conversely, owing to the complex texture of the LaMn1ÿxMgxO3MgO samples (®ne crystals tightly bound to one another to form agglomerates), nothing could be concluded on the basis of HRTEM observation and the presence of simple oxides different from MgO cannot be excluded, even if they are not detectable by X-ray diffraction. As mentioned above, lanthanum manganate and related perovskite compounds are known to undergo reversible oxygen chemisorption, which represents a key feature of their catalytic activity towards oxidation reactions. Some temperature-programmed oxygen desorption runs were performed so as to get evidence of this, possibly enlightening different types of oxygen chemisorption on the perovskites having different x values. One g samples of MgO-free catalysts calcined at 8008C and characterised by three Mn/Mg substitution levels (x0, 0.2, 0.5) were pre-treated in a quartz tube at 8008C for 0.5 h under chromatographic air ¯ow, so as to allow desorption of any gas present on
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 4. Results of the temperature-programmed desorption tests on three selected perovskite catalysts (calcination temperature 8008C).
the catalyst surface. Temperature was then progressively lowered down to room temperature at a 38C minÿ1 rate, thereby allowing complete oxygen adsorption on the catalyst surface. Helium was then fed to the reactor at a 50 cm3 minÿ1 ¯ow rate through a mass ¯ow controller in order to wash out any oxygen molecule not chemisorbed. After 10 min at 258C, the temperature was raised to 9008C once again at a 38C minÿ1 rate, meanwhile monitoring the oxygen outlet concentration through gas-chromatographic analysis (Hewlett-Packard, mod. 5890 series II, equipped with a Porapak QS column and a TCD detector). Fig. 4 shows the variation of the outlet oxygen concentration as a function of temperature for the three catalysts tested. In line with the work of Seyama [14], two types of chemisorbed oxygen species can be noticed accompanied by related desorption peaks: a low-temperature species, named hereafter a, desorbed in the 300±5008C range, and a high-temperature one, named b in the following, desorbed at 600±8008C. The higher the x value, the lower the area of the b-oxygen desorption peaks, whereas a-oxygen type is only present in partially substituted samples. The role of these two types of oxygen on the catalytic activity and kinetics will be discussed in part 3. 2.3. Catalytic activity screening tests Some catalytic activity tests were then performed on all the prepared catalysts in the experimental apparatus, and according to the procedures, carefully described in [15]. After 30 min stay in air ¯ow as a common pre-treatment, a gas ¯ow rate (1.2 cm3 sÿ1 of
281
the following composition: CH41.5%, O218%, Hebalance) was fed to a ®xed-bed of 1 g of catalyst particles (obtained by pressing the perovskite powders into tablets, then crushed into 0.2±0.5 mm granules). The ®xed bed was enclosed in a quartz tube (internal diameter: 4 mm) and sandwiched between two quartzwool layers. The reactor temperature was then lowered at a 38C minÿ1 rate down to 3008C, meanwhile monitoring methane conversion by analysing the outlet concentration of CO2 (the only carbon oxidation product) by use of an FTIR analyser (Hartmann and Braun URAS 10E). The typical sigma-shaped curves obtained are shown in Fig. 5 for the catalysts calcined at 8008C. Each data point is the average of twin runs performed on two different samples of the same material. The deviation between the conversion measured at the same temperature in such twin runs was always less than 10%. No signi®cant hysteresis in these curves was observed performing the catalytic tests upward (from 3008C to 8008C); at least any difference in the results of downward runs was within the above deviation value. In the legend of this ®gure indication is given about the half-conversion temperatures (T50), a parameter which could be regarded as the simplest index of catalytic activity. Fig. 6 shows how, for the different calcination temperatures tested, T50 is affected by the extent of Mn/Mg substitution in either MgO-free (Fig. 6(a)) or MgO-rich (Fig. 6(b)) catalysts. In order to fully appreciate the catalytic effect of the perovskite, it has to be considered that the T50 value of a non-catalytic run performed with silica granules instead of the catalytic ones is about 7708C in the experimental apparatus employed, whereas that of pure MgO (calcined at 8008C) is about 6758C. This last ®gure, in particular, shows how magnesium oxide does not contribute signi®cantly to the catalytic activity of the LaMn1ÿxMgxO317MgO catalysts. 2.4. Reaction kinetics studies on selected catalysts Reaction kinetics were assessed on two catalysts, LaMnO3 and LaMn0.8Mg0.2O3, the former chosen as a reference, the second for its prevalent catalytic activity (Fig. 6(a)). The experimental apparatus used for this purpose is a recycle micro-reactor described in detail in [15]. The reactor, hosting 0.3 g of pelletised catalyst, was operated in the temperature range between
282
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 5. Results of the catalytic activity test towards methane combustion of all catalysts prepared at the calcination temperature of 8008C.
4508C and 6108C with an outlet pressure of 1 bar, preventing reaction kinetics from being affected by the internal mass transfer resistance (the pellet size was reduced down to 0.2 mm so as to minimise this effect) as well as by the external mass transfer resistance (the recycle ¯ow rate was increased on purpose up to 10 cm3 sÿ1 with a recycle ratio equal to 5.3). Methane
per-pass conversion was always lower than 10%, even at the highest operating temperatures. CH4 and O2 feed concentrations were varied through a set of mass ¯ow controllers (Brooks) in the ranges 0.5±2.5% and 2±15%, respectively, He being the balance. Methane conversion rate per unit catalyst mass RCH4 (mol gÿ1 sÿ1) could thus be measured as a function
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
283
Fig. 6. Effect of x and of the calcination temperature ((^)8008C; (&)9008C; (~)10008C; (*)11008C; (*)12008C) on the T50 values measured for the various catalysts prepared: (a) LaMn1ÿxMgxO3; (b) LaMn1ÿxMgxO317MgO.
of temperature and of the average partial pressures of oxygen and methane, calculated on the grounds of simple mass balances. The best kinetics law for data ®tting turned out to be of the Eley±Rideal type [18]: p KO2 pO2 p ; (1) RCH4 kr pCH4 1 KO2 pO2 where
Er Arrhenius; kr exp ÿ RT HO2 van0 t Hoff: KO2 KOo 2 exp ÿ RT kro
(2) (3)
The related reaction mechanism is based on the dissociative chemisorption of oxygen molecules at some active sites over the catalyst surface (regulated by a van't Hoff type equilibrium expression whose thermodynamic constant is indeed KO2 ). This chemisorption is then followed by reaction of methane with one of the derived oxygen atoms, which initiates the combustion process with a reaction rate governed by the kinetic constant kr. Some representative diagrams concerning model accordance to experimental data obtained at a given operating temperature for different oxygen and methane feed concentrations are reported in Fig. 7 for the LaMn0.8Mg0.2O3 catalyst. All model predic-
tions were evaluated by setting the two constants kr and KO2 at an optimum value, determined at each temperature by the least squares ®tting method. Arrhenius-type plots of such constants are reported in Figs. 8 and 9 for the LaMnO3 and LaMn0.8Mg0.2O3 catalysts, respectively. From these last data, estimation of the activation energy Er and of the heat of adsorption HO2 can be derived for both catalysts, based on Eqs. (2) and (3), once again by the least squares procedure. For the LaMnO3 catalyst a single value of Er (35.6 kcal molÿ1) and of HO2 (ÿ23.5 kcal molÿ1) turned out to give good model predictions all over the temperature range explored. Conversely, for the LaMn0.8Mg0.2O3 compound, the slope of kr and, especially, of KO2 variation versus 1/T varied abruptly around 5508C. As a consequence, two sets of Er and HO2 values were calculated above (48.3 and ÿ58.9 kcal molÿ1, respectively) and below (29.1 and ÿ4.5 kcal molÿ1) this critical temperature value. 3. Discussion In line with the previous studies on the LaCr1ÿxMgxO3 system, the three prevalent features of the work are listed here. 1. the assessment of the role of Mg-doping on the catalytic activity of the LaMnO3 perovskite, with
284
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
Fig. 7. Dependency of RCH4 versus pCH4 (a) and pO2 (b) at different yCH4 and yO2 feed values for the LaMn0.8Mg0.2O3 perovskite at 5508C: experimental data and model predictions (Eq. (1)).
Fig. 8. Arrhenius plot of kr and KO2 constants for the LaMnO3 perovskite.
special concern about the effect of the calcination temperature of the catalyst; 2. the study of the potential of the dispersion of the perovskite crystals among MgO crystals so as to preserve the catalytic activity and stability at hightemperatures; 3. the assessment of methane combustion kinetics and reaction mechanism upon the above catalysts. As concerns point (a), it must be immediately considered that, as opposed to the LaCrO3 perovskite [19], lanthanum manganate possesses a considerable
intrinsic activity due to the already mentioned tendency of Mn3 to oxidise to Mn4 upon heating in air at ambient pressure. This transition is accompanied by the generation of cation vacancies and by the presence of the well-known excess of oxygen compared to the stoichiometric requirements [17]. In this context, the Mn/Mg substitution can play the following roles: 1. owing to the bivalent nature of the Mg2 ion, a further amount of manganese might shift from a III to a IV oxidation state, so as to accomplish a
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
285
Fig. 9. Arrhenius plot of kr and KO2 constants for the LaMn0.8Mg0.2O3 perovskite.
simple charge balance. Such a condition may promote an increase in the catalytic activity, owing to the enabled possibility of the lattice oxygen close to Mn4 to be easily reacted away by methane with contemporary regression of manganese to the Mn3 state. Fresh oxygen can then chemisorb on the generated oxygen vacancies, thus restoring the original situation. Similar effects, due to doping elements, were noticed on other perovskite systems in [20]. This effect should in any case be limited to the amount of manganese not related to the excess of oxygen . For instance, LaMnO3 samples calcinated at 8008C are characterised by a value equal to about 0.18 [17], which means that nearly 30% of the Mn content should have a IV valence. The maximum x value, at which no prejudice of cation vacancies might be obtained (equal value) and all Mn is turned to Mn4, should be thus close to 0.35. 2. On the other hand, the charge balance, disturbed by the introduction of the divalent Mg2 instead of the trivalent Mn3, can also be fulfilled by elimination of some excess oxygen and of the related cation vacancies. This second circumstance might negatively affect the catalytic activity of the system, since it reduces the structural defects of the LaMnO3, which are so important for its significant catalytic activity.
If the data shown in Fig. 4 are considered, it can be stated that both (i) and (ii) effects should take place simultaneously. In line with the theory outlined in [21,22], it might be deduced that the low-temperature a-type oxygen is related to type (i) effect, whereas btype oxygen is indeed the lattice oxygen related to the stoichiometric excess . When the degree of substitution is increased the b-oxygen peak is progressively reduced (type-ii) mechanism, whereas, simultaneously, an a-oxygen peak appears (type-i) mechanism. These combined effects turned to give the maximum bene®t to catalytic activity of pure perovskite compounds calcinated at 8008C, for an x value of 0.2 (Fig. 5(a) and Fig. 6(a)). Further analyses, by either the SIMS [23,24] or the XPS techniques, are planned for the near future in order to possibly get a deeper insight into the role played by the Mn3±Mn4 transition on the catalytic activity, as well as to check for possible differences between surface and bulk compositions. When the calcination temperature increases, the activity of pure perovskite samples diminishes for all x values. This can be easily explained by the occurrence of sintering, as demonstrated by the results of the BET and TEM analyses. Coming to point (b), on the basis of TEM observations (Figs. 2 and 3) and of catalytic activity data (Fig. 5(b) and Fig. 6(b)), the following two issues can be deduced:
286
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
1. for the LaMnO3 catalyst, MgO effectively promotes the formation of small perovskite crystals and hampers, to some extent, their sintering; as a result, the catalytic activity of the MgO-rich catalyst is, for certain calcination temperatures, even higher than its pure counterpart despite the presence of only about 25 wt% of the perovskite compound. The best results were obtained at a calcination temperature of 9008C at which the T50 values of the MgO-rich and MgO-free LaMnO3 samples are 4948C and 4708C, respectively. 2. For x60, despite the already-mentioned positive effect on the perovskite crystal size, the presence of excess MgO only occasionally entails higher catalytic activities than those of MgO-free compounds (see data points for x0.3, 0.4 at calcination temperatures 11008C). A likely explanation of this behaviour lies in the shielding effect on the perovskite surface, exerted by the MgO crystals which cover it very tightly, thereby limiting the flow rate of gas molecules reaching the catalytically active sites. MgO amounts lower than those here tested may perhaps give better results, by limiting this covering effect and still preventing sinterisation to an acceptable extent. The fact that LaMn1ÿxMgxO317MgO samples calcined at 9008C outperform those calcined at 8008C is rather puzzling, contrary to what should be expected on the grounds of the natural tendency of sintering phenomena to speed up with temperature. Further studies are in progress in order to clarify this point. 9008C is in any case a suitable temperature for premixed catalytic burner applications [7,8]. Considering the last point (c), the analysis of the kinetic data and the related theoretical arguments are strongly in favour of a remarkable effect of chemisorbed oxygen species on the catalytic activity, as also underlined by several authors for different perovskite systems, e.g. [21]. In fact, the earlier described Eley± Rideal mechanism was successfully employed in ®tting the experimental results of both LaMnO3 and LaMn0.8Mg0.2O3 perovskites, as shown in Figs. 7±9. Considering the activation energies and the heats of adsorption calculated on the basis of the data shown in Figs. 8 and 9, the following considerations can be drawn: The Er values are well inside the range reported for a variety of perovskite systems [14].
The values of the heats of adsorption (ÿ23.5 kcal molÿ1) derived for the LaMnO3 perovskite is high enough to entail a chemisorption rather than a simple physisorption of oxygen. Similar values were directly measured by Fierro and co-workers [25] for this compound. On the basis of the outcomes of temperature-programmed desorption runs, it may be stated that the above oxygen chemisorption regards b-type lattice oxygen, which may thus be reacted away by CH4 molecules and recovered by dissociative adsorption of oxygen molecules from the gas phase. The value of the enthalpy of adsorption (ÿ4.5 kcal molÿ1) derived for the LaMn0.8Mg0.2O3 catalyst below 5508C suggests that, at low temperatures, a-type oxygen, characterised by a rather low heat of adsorption, governs the reaction mechanism. Since the above heat of adsorption is quite close to values typical of simple physisorption processes, the kinetic data determined at all temperatures below 5508C were also fitted with the kinetic expression RCH4 kr pCH4
KO2 pO2 ; 1 KO2 pO2
(4)
accounting for a non-dissociative chemisoprtion of oxygen. However, the results were not satisfactory since the least squares sum was always higher than that obtained with the Eley±Rideal expression (by 20% on an average) at any operating temperature, and unrealistic positive values of the heat of adsorption were drawn. For these reasons, the authors are prone to consider a-oxygen adsorption on the above samples as a weak chemisoprtion. This is indeed confirmed by temperature-programmed desorption runs (Fig. 4), which show how this kind of oxygen is released at temperatures higher than 3008C, frankly incompatible with a simple physisorption. Conversely, at high temperatures (above 5508C), the residual b-type oxygen, remaining after Mn/ Mg substitution, is playing a major role. Further, the HO2 value derived above 5508C (ÿ58.9 kcal molÿ1), quite high if compared with that calculated for the unsubstituted LaMnO3 perovskite, may suggest that the lower the boxygen amount in the perovskite, the more difficult its release for reaction purposes becomes.
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
4. Conclusions Mg-doped LaMnO3 perovskite catalysts were prepared by the citrates method, characterised and tested as catalysts for high-temperature methane combustion, showing, up to a limited degree of Mn/Mg substitution (x0.2), a promoting effect of x on the catalytic activity. The simultaneous synthesis of perovskite and MgO crystals was proved to be an effective mean to reduce the sintering tendency of the catalyst and thus allowing to keep small-size perovskite crystals and fair catalytic activity at high temperatures. In fact, the presence of MgO dispersing particles, in thermodynamical equilibrium with the perovskite ones, hampers, to some extent, the contact between different perovskite crystals and the subsequent sintering phenomena. Such effect was nonetheless strictly dependent on the calcination temperature, the best results being obtained at 9008C. Further, for high x values a clear tendency of magnesium crystals to cover perovskite ones thereby limiting their availability to gas molecules can be noticed, which, at least in part, eliminates the above advantages. A kinetics analysis concerning methane combustion on the selected perovskite catalysts showed that the Elay±Rideal mechanism provides satisfactory interpretation of the kinetic data as a function of oxygen and methane partial pressure for both unsubstituted and substituted LaMnO3 samples. However, as opposed to the basic LaMnO3 perovskite in which only the excess oxygen is taking part in the reaction, a different type of chemisorbed oxygen is involved below 5508C by the LaMn0.8Mg0.2O3 catalyst, likely linked to partial transition of Mn3 to Mn4 forced by the presence of Mg2 inside the perovskite structure. Further studies are though needed to elucidate this point. Under the spur of the above promising results the studied perovskite systems are being applied in the development of premixed catalytic burners for domestic boiler applications [8]. 5. Nomenclature Er
activation energy of the reaction constant (kcal molÿ1)
HO2 kr KO2 kro KOo 2 p RCH4 T x y
287
enthalpy of oxygen adsorption (kcal molÿ1) reaction constant (mol barÿ1 gÿ1 sÿ1) oxygen adsorption coefficient (barÿ1) pre-exponential value of the reaction constant (mol barÿ1 gÿ1 sÿ1) pre-exponential value of oxygen adsorption coefficient (barÿ1) partial pressure (bar) conversion rate of methane (mol gÿ1 sÿ1) temperature (K) degree of Mn/Mg substitution in LaMn1ÿxMgxO3 mole fraction
Greek letters
excess oxygen in the perovskite lattice methane conversion
References [1] D.L. Trimm, Appl. Catal. 7 (1983) 249. [2] L.D. Pfefferle, W.C. Pfefferle, Catal. Rev.-Sci. Eng. 29 (1987) 219. [3] Z.R. Ismagilov, M.A. Kerzhentsev, Catal. Rev.-Sci. Eng. 32 (1990) 51. [4] M.F.M. Zwinkels, S.G. JaÈraÊs, P. Govind Menon, Catal. Rev.Sci. Eng. 35 (1993) 319. [5] M.F.M. Zwinkels, High-temperature Catalytic Combustion, Ph.D. Thesis, KTH University, Stockholm, 1996. [6] R.A. Dalla Betta, Catal. Today 35 (1997) 129. [7] J.D. Sullivan, Basic Research on Radiant Burners, Alzeta Corporation Report no. 91-7027-167, Santa Clara, CA, 1991. [8] G. Saracco, I. Cerri, V. Specchia, R. Accornero, Chem. Eng. Sci. (1998), in press. [9] M. Foka, J. Chaouki, C. Guy, D. Klvana, Chem. Eng. Sci. 49 (1995) 4269. [10] S.W. Radcliffe, R.G. Hickman, J. Inst. Fuel 48 (1975) 208. [11] V. Specchia, S. Sicardi, A. Gianetto, Chem. Eng. Commun. 10 (1981) 189. [12] R.A. Dalla Betta, US Patent 5 405 260, 11 April 1995. [13] L.G. Tejuca, J.L.G. Fierro (Eds.), Properties and Applications of Perovskite-type Oxides, Marcel Dekker, New York, 1993. [14] T. Seiyama, Catal. Rev.-Sci. Eng. 34 (1992) 281. [15] G. Saracco, G. Scibilia, A. Iannibello, G. Baldi, Appl. Catal. B 8 (1996) 229. [16] C.N. Satterfield, Heterogeneous Catalysis in Industrial Practice, McGraw-Hill, New York, 1991, p. 127. [17] J. ToÈpfer, J.B. Goodenough, J. Solid State Chem. 130 (1997) 117. [18] J.N. Thomas, W.J. Thomas, Introduction to the Principles of Heterogeneous Catalysis, Academic Press, London, 1967.
288
G. Saracco et al. / Applied Catalysis B: Environmental 20 (1999) 277±288
[19] B.K. Flandermeyer, M.M. Nasrallah, A.K. Agarwal, H.U. Anderson, J. Am. Ceram. Soc. 67 (1984) 195. [20] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26 (1986) 265. [21] L.G. Tejuca, J.L.G. Fierro, J.M.D. TascoÂn, Adv. Catal. 36 (1989) 237. [22] T. Seiyama, N. Yamazoe, K. Eguchi, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 19.
[23] L.A. Isupova, V.A. Sadykov, V.P. Ivanov, A.A. Rar, S.V. Tsybulya, M.P. Andrianova, V.N. Kolomilchuk, A.N. Petrov, O.F. Kononchuck, React. Kinet. Catal. Lett. 53 (1994) 223. [24] L.A. Isupova, V.A. Sadykov, S.V. Tsybulya, G.N. Kryukova, V.P. Ivanov, A.N. Petrov, O.F. Kononchuck, React. Kinet. Catal. Lett. 62 (1997) 129. [25] J.L.G. Fierro, J.M.D. TascoÂn, L.G. Tejuca, J. Catal. 89 (1984) 209.