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Dual-Site Pd/Perovskite Monolithic Catalysts for Methane Catalytic Combustion Stefano Cimino,*,† Luciana Lisi,† Raffaele Pirone,† and Gennaro Russo‡ Istituto Ricerche sulla Combustione, CNR, Piazzale V. Tecchio 80, 80125 Napoli, Italy, and Dipartimento Ingegneria Chimica, Universita` di Napoli Federico II, Napoli, Italy
Novel monolithic catalysts for the high-temperature combustion of methane have been developed with a dual-site nature, to combine low and high temperature activity within a single system and widen its operating range. Such catalysts are based on strongly interacting Pd and LaMnO3 active phases supported onto La/γ-alumina and applied as a thick washcoat on ceramic honeycombs. Optimization of the preparation procedure and catalyst characterization were performed with respect to the effects of Pd deposition method, catalyst activation strategy, and thermal history in comparison with previously developed catalysts based only on supported perovskite. Extensive methane combustion tests (steady state and transient) were run under both isothermal and autothermal lean premixed conditions in order to study intrinsic oxidation activity, light-off behavior, and real life performance in the middle to high temperature range. Experimental evidence is presented for improved activity, stability, and durability of the catalytic system, and for a self-regenerative behavior of Pd active sites through reversible interaction with perovskite lattice. Implications of novel combined catalyst functionalities are addressed with respect to operation of a fully catalytic burner. 1. Introduction Catalytic combustion has been hailed for many years as one of the most promising technologies for clean combustion.1,2 It prevents pollutants formation through a lower and uniform combustion temperature, thus avoiding thermal NOx formation while high efficiency is kept with CO and unburnt hydrocarbons at singledigit levels.3,4 However catalytic combustion has failed to reach a large commercial diffusion in power and heat production applications (i.e., gas turbines, radiant burners and furnaces, cooking stoves) essentially because of poor catalyst stability and durability at high temperatures or, vice versa, insufficient activity at low temperatures.1,3,5 Typically, catalyst compositions have included precious metals and in particular Pd,1-7 since it is commonly recognized as the most active component for the oxidation of methane,1,6-9 the preferred fuel in many industrial and domestic uses. Despite the large research effort on the Pd/alumina system, there are still debated issues concerning, for example, the Pd/PdO redox dynamic mechanism during reaction, or even about the most active state for high-temperature methane oxidation.1,8,9 Catalyst activity is indeed strongly influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatments,8,9 giving rise to some unique features, such as temperature selfcontrol,1,10 or potentially dangerous strong oscillating behavior.9,11 In fact, a single Pd-based catalytic system, despite the high metal loadings usually employed,3 is not able to fulfill all process requirements, especially * To whom correspondence should be addressed. Tel.: +39 081 7682235. Fax: +39 081 5936936. E-mail: stcimino@ unina.it. † Istituto Ricerche sulla Combustione. ‡ Universita ` di Napoli Federico II.
at high temperatures;11 therefore it must be coupled with either homogeneous combustion sections10,12 or additional catalytic stages12 that in turn increase the overall complexity and/or cost of the combustor up to unacceptable levels for most applications. The use of catalytically active support materials, such as Mn-substituted hexaaluminates13-15 or transition metal and rare earth doped aluminas,14,16,17 has been proposed as a possible way to overcome the unsatisfactory catalytic behavior of supported Pd systems at high temperature. In general, it is possible to avoid the activity drop connected to PdO dissociation above 700 °C,13-15 but no further beneficial interaction with the active support has been found either in terms of increased low-temperature activity17 or better stability of Pd species in the whole temperature range.18 On the other hand, Nishihata et al.19 have recently reported that automotive three-way catalysts containing small amounts of Pd supported on perovskite-type oxides show a remarkable enhancement of durability with respect to conventional Pd/alumina catalysts due to suppression of metallic particle growth. In particular, they claim favorable interactions between Pd and the perovskite conferring an outstanding self-regeneration capability to the catalyst, even after high-temperature long-term aging, due to reversible movement of Pd in and out of the perovskite lattice as a response to typical redox and/or temperature fluctuations in operating conditions.19-21 Perovskites themselves (general formula A1-yA′yB1-x B′xO3+δ) have attracted a large interest as partial and total oxidation catalysts owing to their good activity and structural stability,5,22,23 but they usually suffer from high-temperature sintering and deactivation.23 Nevertheless, it has been recently demonstrated that the specific activity and thermal stability of LaMnO3 perovskite-based monolithic catalysts used for the total and partial oxidation of light alkanes can be successfully
10.1021/ie049656h CCC: $27.50 © 2004 American Chemical Society Published on Web 09/09/2004
Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004 6671 Table 1. Denomination of Monolithic Catalysts and Corresponding Active Washcoat Loading, Composition, Preparation Strategy, and BET Surface Area after Repeated Calcination in Air at 800 °C
catalyst
washcoat loading (wt %)
LM PLM-1
39.1 40.6
PLM-2
18.0
PLM-3
38.6
a
active phase nominal compositiona,b (wt %)
Pd loadinga by ICP-MS (wt %)
LaMnO3 (30.0) LaMnO3 (26.4) Pd (0.95) LaMnO3 (32.8) Pd (1.29) LaMnO3 (30.7) Pd (1.02)
0.49 2.22 1.03
deposition route La, Mn: co-imp. La, Mn: co-imp. Pd: imp. pH ) 2 La, Mn: co-imp. Pd: imp. pH ) 11 La, Mn, Pd: co-imp.
no. of impregnations
BETa (m2/g)
10 10 5 10 6 10
104 55 61 65
With respect to the weight of active washcoat layer, monolithic substrate excluded. b Balance: La-stabilized γ-Al2O3.
enhanced by dispersing the active phase onto high surface area refractory oxides (La/γ-Al2O3, MgO, ZrO2)24,25 and coating suitable substrates to obtain structured catalytic reactors with an enlarged operating temperature range, especially on the upper side.26,27 Combining features of Pd and supported perovskite catalysts might be particularly useful in combustion applications for power generation, where a continuous regeneration of the low-temperature activity is required to ensure stable and repeatable ignition of the fuel/air mixture, while complete conversion must be assured by catalytic sites that are stable and active at high temperature. The aim of this study was to investigate the preparation and catalytic oxidation activity of novel dual-site Pd/LaMnO3 perovskite catalysts dispersed onto Lastabilized γ-Al2O3 and applied as a thick washcoat over ceramic honeycomb reactors. The effects of Pd deposition methods, catalyst activation strategy, and thermal history were analyzed through both isothermal and autothermal (steady state and transient) methane combustion tests run under lean premixed conditions. Experimental evidence was searched and found for the bifunctional nature of the novel catalyst and for the regenerative behavior of its Pd active sites through comparison with early studies on pure perovskite-based monolithic catalysts27,30 looking at implications with respect to real life operation of a fully catalytic burner. 2. Experimental Section Catalyst Preparation. Commercial cordierite monoliths (Corning) with a cell density of 400 cpsi were cut to obtain samples of variable lengths (20-75 mm) with a 25-channel cross section. γ-Al2O3 was applied as a thick washcoat layer onto the structured substrate using a modified dip-coating procedure and successively stabilized with La2O3 [∼7% (w/w)] by impregnation followed by calcination in air at 800 °C.24 LaMnO3 precursors were deposed on the stabilized alumina washcoat through impregnation with an aqueous equimolar solution (0.23 M) of La(NO3)3‚6H2O (Aldrich, >99.99%) and Mn(CH3COO)2‚4H2O (Aldrich, >99%) followed by drying at 120 °C and calcination at 800 °C for 3 h under flowing air25 (in the following denoted as LM monolith). The process was repeated 10 times in order to achieve a perovskite loading of ∼30% (w/w) with respect to the active washcoat layer, monolithic substrate excluded. Pd was finally dispersed onto coated monoliths by repeated impregnation cycles, performed in order to improve metal dispersion28 [target loading 1% (w/w) of active washcoat, monolithic substrate excluded]. Samples were dipped in a diluted (0.027 M) water solution of Pd(NO3)2‚2H2O (Fluka,
purum) as the metal precursor, and then excess solution was removed by gently blowing with compressed air. The initial pH of the solution was either set at 2 or raised to 11 (by progressive ammonia addition) following literature indications28,29 in order to change the metalsupport interaction through modification of surface charge, aiming to achieve a better control of metal penetration deep inside the porous washcoat (respectively referred to as PLM-1 and PLM-2 catalysts). In a third preparation procedure, palladium nitrate was added in the desired amount (0.017 M) to the solution containing both manganese acetate and lanthanum nitrate (initial pH ) 5), and all the three elements deposed at the same time by repeated impregnations of alumina-washcoated monoliths (named PLM-3). In each case, monoliths were dried in a stove at 120 °C with periodic rotation on the axis, prior to calcination at 800 °C for 3 h under flowing air. Table 1 reports catalyst denomination, loading of the active washcoat, its nominal weight composition, and the impregnation strategy employed. To study catalyst thermal activation, some monoliths were heated under flowing air, nitrogen, or reaction mixture at 1000 °C for 1 h, before cooling down to 350 °C; at this temperature methane oxidation rate was measured under standard pseudo-isothermal conditions (see Combustion Test Procedure). Catalyst Characterization. Physicochemical characterization was performed directly on monolith catalysts, opportunely sectioned when needed. The specific surface area of the samples was assigned only to the active washcoat layer (SSA of cordierite substrate e 1 m2/g): it was evaluated by N2 adsorption at 77 K according to the BET method using a Carlo Erba 1900 Sorptomatic apparatus after degassing under vacuum at 200 °C. SEM analysis was performed using a Philips XL30 microscope equipped with an EDAX detector for EDS microanalysis. The concentration of the metal precursors in the solutions was checked before and after impregnation by flame atomic absorption spectrometry on a Varian SpectrAA 220 instrument. Actual Pd content in each monolith catalyst was quantitatively determined by inductively coupled plasma spectrometry on a Agilent 7500 ICP-MS instrument, after MWassisted dissolution of samples in nitric/hydrochloric acid solution. Experimental Setup and Combustion Test Procedure. Combustion experiments were carried out on monolith catalysts cut to a standard length of 22 mm and wrapped by high-temperature ceramic wool in order to hold them in position inside a lab scale quartz reactor (inner d ) 10 mm), which was positioned inside a threezone electrical tubular furnace.27 The central monolith channel was blocked for catalyst wall temperature
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measurement with one or two K-type thermocouples (d ) 0.5 mm) entering from the top of the reactor, while an additional thermocouple was located upstream of the catalyst in order to evaluate inlet gas temperature.27 The axial position (Tgas, -10 mm; Tin, 5 mm; Tout, 15 mm from reactor entrance) was determined by comparing gradations on the thermocouples against a length scale,30,31 with an accuracy estimated within (1 mm. The gaseous flow rates were regulated by Brooks 5850 mass flow controllers and premixed at atmospheric pressure to obtain an inlet CH4 concentration variable in the range 0.1-5% by volume, while oxygen was set to 10% by volume (i.e., below the minimum oxygen content for safety reasons; the balance was N2), unless otherwise stated. Total flow rate was varied in the range 16-80 NL/h corresponding to a GHSV 32 000-160 000 h-1 under normal conditions, based on monolith empty volume. Depending on the operating conditions, the reactor could be run in either of two ways: (i) pseudoisothermally (low CH4 flow rate and conversion, low to moderate temperatures), in order to obtain reliable data for kinetic analysis taking advantage of the favorable enhanced heat and mass transfer characteristics of monoliths with controlled fluid dynamics and low washcoat depths; or (ii) autothermally, with the aim to reproduce conditions of interest for high temperature premixed, fully catalytic burners. In fact, it must be noted that during autothermal experiments, maximum monolith temperatures are influenced by total heat released (flow rate) and are limited by radiative and conductive heat exchange from outer hot catalytic surfaces toward colder surroundings: such phenomenon is significant because of the relatively high external surface-to-volume ratio characteristic of a lab-scale monolithic reactor.30,31 The feed and product streams were continuously analyzed (after passing through a condenser and CaCl2 trap in series) using an Hartmann & Braun Advance Optima apparatus equipped with five independent nondispersive infrared detectors for CH4 (high and low concentrations), CO2, CO, and NO. Spatiotemporal temperature patterns in the reactor and concentrations of exit products were simultaneously acquired and recorded on a personal computer. Transient experiments were carried out by adding stepwise the fuel on the preheated flowed catalyst while oven temperature was kept constant. 3. Results and Discussion It has already been reported27 that the washcoating procedure here employed allows the deposition of a uniform and firmly attached γ-alumina layer onto cordierite monoliths, characterized by a thickness of about 30-40 µm on the walls, which raises up to 80100 µm in the corners, giving the classical rounded shape to the originally square channels. EDS microanalysis of LM catalyst prepared by coimpregnation of La and Mn precursors indicates a uniform dispersion of all metal species along the channels and inside the washcoat layer; similar results were previously obtained with the more sophisticated deposition-precipitation route.27 On the other hand, addition of Pd metal to the preformed LM monoliths was not a trivial matter. At laboratory scale the most common strategy to depose the precious metal on alumina or zirconia supports is to disperse the precursor, commonly an acid nitrate solution, on a fine (micronic) powder of the porous support13-18 and eventually to apply it as a washcoat
Figure 1. EDS analysis of the Pd distribution along a cross section of the washcoat layer (35 µm) in PLM-1 monolith. The outer surface of the washcoat is at the top of the SEM image, while the cordierite wall is at the bottom.
on structured carriers.28 In this way the problem of precursor distribution inside the pores of the support is greatly circumvented due to the very small penetration depth required. On the other hand, it was already reported that better properties, or even unique features, may arise when the active phase is deposed directly on preformed and washcoated carriers;24,28 furthermore, such strategy appears highly desirable for catalyst preparation on an industrial scale.32 Nevertheless, various catalyst maldistributions32,33 are likely to arise starting from thicker preformed washcoat layers (100200 µm) or pellets (several millimeters): this factor is too often neglected or left uncontrolled when dealing with structured catalysts.32 Modification of the surface charge of the catalyst support can be used to achieve a better control of the penetration depth inside the porous washcoat.28,29 In general, water solutions of the Pd nitrate salt are acidic, since partly covalent bonds are formed with coordinated water molecules, with the consequence of charge transfer from the water ligands, which increases their acid properties and can cause hydrolysis.29 Pd2+ ions favor the formation of square planar PdL4 complexes and in water exist as Pd(H2O)42+ at pH < 1, while insoluble polynuclear palladium species are progressively formed at pH > 1.29 For this reason, acidic Pd(NO3)2/water solutions have been reported to be most effective for preparation of highly dispersed Pd/ γ-alumina catalysts,28 though some support dissolution has to be expected, and more important, a repulsive interaction with the positively charged surface (pH well below its zero charge point) occurs, possibly inhibiting deposition of Pd deep inside the pores of the support. Indeed, this circumstance was verified to occur in the case of the PLM-1 catalyst. Upon SEM inspection of the longitudinal section of the monolith, the channels appeared homogeneously covered by the active washcoat layer, and EDS microanalysis evidenced the presence of uniform superficial Pd coverage all along the channels. On the other hand, the analysis performed along the cross section of the washcoat layer (Figure 1) revealed that Pd penetration was low and limited to roughly 5 µm from the surface, as opposed to what observed for Mn and La, whose concentrations were
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almost constant along the whole depth. The incipient formation of polynuclear Pd species in the impregnating solution, which cannot be excluded at pH ) 2, may have contributed to limit the total amount of Pd deposed to roughly half of the target loading (ICP-MS analysis in Table 1). To increase metal-support interaction without moving to metal complexes that usually contain chlorine and in view of the basic nature expected for the surface of LaMnO3/La/γ-Al2O3, ammonia was added to the palladium nitrate impregnating solution and the pH was raised to 11 (PLM-2 catalyst); under such conditions the complex ion Pd(NH3)42+ is reported to be stable29 and should in principle have a stronger interaction with the porous support surface characterized by an overall slightly negative charge.28 Unfortunately, SEM observation of the inner surface of PLM-2 monolith showed that the original LaMnO3/La/γ-Al2O3 washcoat partly came off from the cordierite channels, which in some areas were left uncovered but from Pd. Such circumstance was confirmed by a partial reduction in sample weight; at the same time some Mn was detected by AAS in the impregnating solution, and both Si and Mg from the original cordierite substrate were observed by SEMEDS in the washcoat overlayer. The total amount of Pd deposed on the monolith was found by ICP-MS to be higher than expected if compared to the weight of the washcoat layer, probably because Pd was impregnated also on the partially uncovered cordierite substrate. Although EDS microanalysis showed some distinct improvements in the penetration depth of Pd deep inside the porous layer, this preparation method does not appear feasible due to the relative mechanical and chemical instability of the LaMnO3/La-γ-Al2O3-coated monoliths in this basic impregnating media. The best results in terms of adhesion and uniformity were obtained through the simultaneous impregnation of all three La, Mn, and Pd precursors onto monoliths coated only with La-stabilized γ-alumina layer. After 10 preparation cycles the active washcoat appeared still firmly anchored to the underlayer cordierite, without any significant detachment from channel walls. Preferential adsorption and/or depletion of any of the three precursors was excluded by AAS performed on the residual impregnating solution, which was characterized by constant and unchanged values of metal molar ratios; loading of precious metal checked by ICP-MS analysis (Table 1) perfectly corresponds to its nominal value. Moreover, EDS maps (Figure 2) recorded at several different cross sections of PLM-3 catalyst show a very homogeneous distribution of all three active elements in the whole active layer. In particular, the penetration depth of the noble metal is quite satisfactory: Figure 3 shows that atomic Pd content is in line with its nominal value and presents a rather constant profile as a function of the distance from the surface. As reported in Table 1, alumina-supported LaMnO3 prepared by the coimpregnation method exhibits a BET surface area of 104 m2/g, indicating a partly lower dispersion efficiency with respect to the depositionprecipitation route, which was previously found27 to give values as high as 127 m2/g for the same catalyst composition. All PLM catalysts containing also Pd are characterized by a further reduction of their specific surface area by 40-50%, which suggests some kind of negative effect of the noble metal also after moderate heat treatments of the samples (800 °C in air). More
Figure 2. SEM image of a cross section of PLM-3 catalyst together with the corresponding EDS maps showing the uniform distribution of Mn, La, and Pd inside the active washcoat layer above the cordierite substrate.
Figure 3. EDS analysis of the Pd distribution along a cross section of the washcoat layer in PLM-3 monolith.
detailed studies are needed to shed light on the mechanism (sintering, formation of solid-state solution, pore blocking) behind this phenomenon. Among PLM catalysts the surface area increases with the degree of uniformity and penetration of Pd, reaching a maximum for the PLM-3 sample prepared by coimpregnation at mild acidic pH conditions. Total Oxidation Activity. Methane combustion activity of monolith catalysts calcined in air at 800 °C was evaluated first in pseudo-isothermal, steady-state tests under very lean conditions: Figure 4a reports conversion plots obtained over LM and PLM monoliths at the same contact time (66 000 Ncm3/g‚h referred to the total weight of active washcoat deposed). All conversion curves increase monotonically in the whole temperature range explored and are associated with the total selectivity toward CO2 and water. Surprisingly, the initial activity of all PLM catalysts containing Pd is lower than measured for the basic LM catalyst containing only LaMnO3 perovskite as active phase. Among the Pd/perovskite catalysts, PLM-3 monolith prepared by coimpregnation of all three elements shows the best performances even if it still requires roughly 15 °C more than LM monolith in order to reach the same conversion
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Figure 5. Comparison of initial methane combustion rates at 350 °C over LM and PLM-3 catalysts treated in air at 800 °C (a) or at 1000 °C under different atmospheres (b, air; c, N2; d, reaction mix). Feed: CH4 ) 0.4% by volume; O2 ) 10%. Rates referred to the total weight of washcoat deposed, the monolithic substrate being excluded.
Figure 4. (a) Methane conversion as a function of temperature for LM and PLM catalysts. Inlet conditions: CH4 ) 0.4%; O2 ) 10%; F/W ) 66 000 Ncm3/g‚h. (b) Corresponding Arrhenius plots obtained under the assumption of first-order kinetics for CH4 and zero-order for O2. Activation for PLM-3 sample was performed at 1000 °C for 1 h under reaction mixture.
levels. This circumstance suggests some form of interaction between the perovskite phase and the noble metal, which appears to be blocked in an unusual and rather inactive form, since normally methane oxidation rates over supported PdO catalysts are ∼2 orders of magnitude faster than over LaMnO3 systems on a mass basis. In fact, it has been reported that Pd can strongly interact with perovskite-type oxides, easily forming solid-state solutions.19-21,34 Figure 4b reports the Arrhenius plots obtained for all the catalysts from conversion data under the assumption of ideal plug flow conditions and first-order kinetics with respect to methane (zero-order for O2). Plots are linear over the whole temperature range covered and are representative of a pure kinetic regime, having ruled out the presence of both external and internal mass transfer limitations. The resulting values of apparent activation energy are almost identical for LM and PLM-3 samples (respectively 19.8 and 20.1 kcal/mol) and increase following the opposite trend of activity for PLM-1 and PLM-2, respectively being 22.5 and 26.1 kcal/mol. Since values of apparent activation energy for methane oxidation are reported to be in the range 1720 kcal/mol over crystalline PdO (for initially dry conditions),8 40-45 kcal/mol over metallic Pd,8 and 1924 over supported LaMnO3 oxides,24,27 no direct conclusion on the nature of the active sites in PLM systems can be drawn, apart from excluding any significant contribution of metallic Pd to the overall methane conversion. Methane combustion rates over PLM-1 and PLM-3 catalysts differ by no more than 10-20%, regardless the
large difference in Pd loading, dispersion, and uniformity inside the washcoat. Such circumstances suggest that the main contribution to the overall activity for both catalysts comes from the perovskite phase rather than the noble metal; in fact, the difference in activity agrees with the larger surface area of the catalytic layer in PLM-3 monolith vs PLM-1 (65 vs 55 m2/g). On the other hand, the activity gap is more evident for the PLM-2 system. Indeed, the perovskite active phase was probably modified due to the partial chemical leaching of its components observed during Pd impregnation cycles. The larger apparent activation energy measured on this sample (26.1 kcal/mol) supports the hypothesis of major phase reconstruction and is in line with the formation of Al-enriched Mn/Al mixed perovskites.35 Once again, the noble metal appears to play a minor role, if any. Due to its lower activity and the unsatisfactory properties of the washcoat adhesion, PLM-2 catalyst was not tested any further. To sum up, the limited activity recorded even on the best performing PLM catalyst treated at 800 °C in air may be well assigned simply to its perovskite phase, characterized by a reduced specific surface area with respect to the base LM catalyst without Pd (65 vs 104 m2/g). Nevertheless, we have found that specific thermal pretreatments can greatly enhance methane combustion rates over PLM catalysts. For example, Figure 4a shows that the conversion plot for PLM-3 monolith is shifted toward lower temperatures by as much as 150 °C with respect to the initial sample after 1 h treatment at 1000 °C under reaction atmosphere. Indeed, the activated form of PLM-3 catalyst is able to convert methane already at 325 °C (T10), roughly 80 °C below the LM sample. Results of oxidation activity measurements carried out over PLM-3 catalyst after three different high-temperature (1000 °C) treatments respectively in air, nitrogen, or reaction mixture are presented in Figure 5 in terms of the reaction rate at 350 °C per unit weight of washcoat (r350 w ), through the comparison with those observed over LM and PLM-3 monoliths after their initial calcination at 800 °C in air. While methane combustion on LM catalyst proceeds faster than on PLM-3 calcined at 800 °C, the situation reverses after any of the activating treatments performed, which all enhance the reaction rate by at least 1 order of magnitude. The intrinsic activity more than doubles if the
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Figure 6. Apparent CH4 reaction order calculated for LM and PLM-3 catalysts (treated at 800 °C or activated at 1000 °C in reaction) under the assumption of ideal PFR behavior and power low rate expression. Feed: CH4 ) 0.1-1%; O2 ) 10%; F/W ) 66 000 Ncm3/g‚h.
treatment is performed under N2 rather than air, while the most effective catalyst is obtained upon exposure to high temperature reacting conditions, since its low temperature activity is roughly 40-fold higher than the corresponding initial state. Such large variations are most probably connected to the role played by the noble metal and the particular form in which it is present in the catalytic layer. It must be noted that both PLM-1 and PLM-3 catalysts can be activated upon proper treatments at temperatures above those quoted for PdO decomposition, but the reaction rate is always lower on PLM-1 catalyst, probably for its lower Pd content and dispersion. A preliminary kinetic study was carried out over a PLM-3 monolith before and after activation by progressively increasing CH4 concentration in the feed at several fixed temperature levels. Resulting conversions were modeled by a simple power law rate equation, rw ) kwpCH4n, assuming isothermal plug flow conditions, which gives values of n reported in Figure 6 as a function of temperature. The apparent reaction order for methane is always lower than 1, but the dependence of conversion on inlet CH4 concentration is far stronger in the case of the activated PLM-3 catalyst rather than for its initial counterpart, although the temperature ranges examined are only partly overlapped due to the large difference in activity (almost 2 orders of magnitude) found on the same sample. In fact, initial PLM-3 catalyst displays an apparent reaction order of 0.8, being almost constant with temperature, perfectly in line with the results obtained on LM catalyst.24 On the other hand, activated PLM-3 catalyst shows an apparent reaction order of 0.5 at 350 °C, which rapidly increases with temperature, in agreement with the literature indication for supported PdO catalysts, which suffer by a strong inhibition effect of water (desorption limiting step) at T e 450 °C.8,36 As shown in Figure 7, the activity gain is partly lost during exposure to lean reaction conditions, due to a progressive deactivation, which tends to bring the reaction rate back to the base level on a time scale of some hours at 350 °C. Apparent activation energy measured on fully activated and partially deactivated samples is unchanged (21.3 kcal/mol) and suggests a modification in the total number of accessible sites, rather than in their chemical nature. Deactivation
Figure 7. Time on stream dependence of methane combustion rate at 350 °C over PLM-3 catalyst treated in air at 800 °C (a) or at 1000 °C under different atmospheres (b, air; c, N2; d, reaction mix). Feed: CH4 ) 0.4% by volume; O2 ) 10%.
might be related to the sintering of fine PdO particles previously emerged on the surface, as already reported for pure and doped aluminas18,37 and active hexaaluminate supports,15 and/or to the formation of solid-state solution with the perovskite lattice.20,21 It is worth noting that the deactivation process is completely reversible: exposure to high temperature (above 800 °C) can fully restore the activity level previously attained. Therefore, PLM-3 catalyst can keep on cycling between two boundary activity levels: the lower is probably related to the perovskite phase and the upper to the noble metal, and they differ by more than 1 order of magnitude. Both boundaries appear stable with respect to the number of cycles (20) experienced by the catalyst and to its progressive aging at high temperature. On the other hand, the specific catalytic performance showed within this range strongly depends on the temperature history experienced by the sample. Many authors reported that intermediate Pd/PdO states can significantly increase (or decrease) activity for methane oxidation,3,8,37 probably depending on where Pd and PdO phases are located and possibly related to the presence of nonstoichiometric PdOx phases. In fact, contradictory observations were made when partial reduction (or oxidation) was obtained via chemical reaction (by H2 or methane pulses, activity increase) or thermal treatment (activity decrease upon heating and increase on cooling), pointing to the complexity of Pdbased systems.8 For alumina supported catalysts it is still questionable weather the reported increase in activity upon reoxidation of Pd to PdO is solely due to a surface area increase or to a true interaction between phases.8 In present case, the situation is made even more complicated by the presence of a very versatile, active, oxygen donor support (the LaMnO3 perovskite), which is also able to reversibly incorporate Pd inside its lattice,21 fixing it in some unusual intermediate oxidation state. It was reported that Pd/perovskite solid solutions can be readily formed by treating at 800 °C in air: EXAFS studies demonstrated that Pd can be present as Pd3+ or Pd4+, occupying the B site of the perovskite lattice.19,20 In this form the noble metal is a less active three-way catalysts than when supported on the surface.34 Nevertheless, segregation of metal Pd outside the lattice has been found to easily occur upon reducing treatments at high temperature (800 °C in 10%
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H2) or as a consequence of the exposure to redoxfluctuating atmospheres typical for three-way applications.20 Such phenomenon is accompanied by a fine redispersion of the precious metal on the surface, thus representing a self-regeneration function for the catalyst, which therefore is not prone to sintering and agglomeration of Pd active phase.19,21 By analogy, it can be hypothesized that in PLM catalysts treated at 800 °C in air, Pd is almost completely present in solid solution within the perovskite lattice. In this form palladium either is not accessible to reacting molecules or is blocked in an oxidation state that is less active than the LaMnO3 perovskite. Since the thermodynamic decomposition temperature of PdO to Pd in air is ≈790 °C,37 each heating treatment up to 1000 °C is thought to completely reduce the noble metal, segregating it out from the perovskite lattice, on the surface of the catalyst; this phenomenon is clearly enhanced in the presence of inert (N2) or reacting atmospheres. Upon cooling (in air) or exposure to lean reacting conditions at temperatures below the decomposition threshold, partial reoxidation of the metal is most likely to occur, leading to the formation of highly dispersed and active PdO clusters and/or mixed PdO/Pd phases on the surface of the support.3,8 Such species appear to be only metastable because Pd can form once again solid solution with perovskite crystal. For this reason, the enhanced low-temperature catalytic activity lasts as long as some dispersed PdO is available on the surface. Moreover, if catalyst cooling after activation is too slow, the total number of active surface palladium sites may be strongly reduced due to formation of solid-state solution and/or sintering. Indeed, the best performance was obtained by heating in situ the catalyst at 1000 °C through autothermal methane combustion (see the next section) with low external preheating (e400 °C); the very rapid cooling (rate exceeding 60 °C/s in the early stage down to 600 °C), obtained by simply removing the fuel from the feed, preserves a larger number of active and accessible PdO sites on the surface. Autothermal Combustion. As expected, under autothermal operation with higher CH4 concentration (3%) at a GHSV ) 96 000 h-1, the conversion plot over PLM-3 monolith increases very steeply, jumping from 20% to over 99% as a consequence of only 10 °C increase in the preheating temperature, so that an ignition temperature of 505 °C can be assigned to the system. In comparison, a fresh LM catalyst tested under similar conditions27 was able to ignite the same mixture already at 485 °C, the difference being in line with the reported lower initial activity measured over PLM-3 under isothermal conditions. Figure 8 presents the transient profiles of surface temperature and outlet conversion recorded during this first ignition over PLM-3 monolith (curve 1). Exactly as reported,27 in the case of simple LaMnO3 perovskitebased monolith (LM), ignition proceeded through an initial, slow, warming-up phase of the reactor, followed by a much sharper increase in both temperature and conversion, also associated with the travelling of the reaction front backward from monolith outlet to inlet section. Under steady-state ignited conditions, the catalyst surface reaches a maximum temperature slightly above 900 °C, while CO and NO production is always extremely low, less than or equal to the detection limit of 2 ppm. The quite similar light-off (transient) behavior of LM and PLM-3 monoliths again points to the conclu-
Figure 8. Autothermal ignition transients over PLM-3 monolith in its initial state (1) or activated (2) after exposure to hightemperature reaction conditions. (a) Catalyst temperature 8 mm from the inlet section; (b) outlet CH4 conversion. Preheating 510 °C; CH4 ) 3%; O2 ) 10%; GHSV ) 96 000 h-1.
sion that only the perovskite phase plays a role in activating methane when PLM-3 catalyst is in its initial (not activated) state, while Pd contribution is minor. Nevertheless, exposure to high temperature reaction conditions during this first autothermal combustion test causes a marked activation of PLM-3 monolith, as already reported. Indeed, the activated catalyst exhibits a completely different light-off behavior, as clearly shown by the curves (2) in Figure 8, obtained by repeating the ignition test just after cooling the reactor to the same starting conditions. In this case, the warmup phase is completely absent, since surface temperature raises quite rapidly (within 50 s from the start) up to ∼780 °C, and then it abruptly slows down before eventually reaching the same final steady value as for the initial PLM-3 catalyst, within a comparable time. Methane conversion experiences an analogous time pattern, jumping up to 90% in the first minute, to then stop and even pass through a small minimum before slowly raising again up to its final value. The sharp transition recorded in light-off profiles appears related to the well-known endothermic, spontaneous decomposition of PdO to metallic Pd, which is reported to occur in the range 670-800 °C, depending on the partial pressure of oxygen and the kind of support.8 Indeed, metallic Pd formed by thermal decomposition of PdO is commonly considered much less active than its oxide or even completely inactive at this temperature.8,9 Nevertheless, perovskite active sites, characterized by an intermediate reactivity between the two, can sustain the reaction and ensure complete conversion at such temperatures, as demonstrated by the light-off profiles in Figure 8 (1) recorded over the initial form of PLM-3. The enhanced activity developed by PLM-3 monolith after exposure to high-temperature reacting conditions lowers the minimum preheating necessary for light-off down to 490 °C, a value in line with the temperature required over a fresh LM catalyst, but remarkably lower than over an equivalently thermally aged LM monolith, which needed from 530 to 560 °C to light-off.27 As shown
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Figure 9. Autothermal ignition transient at fixed preheating of 450 °C over the activated PLM-3 monolith (1) and repeated after cooling in flowing air (2). (a) Catalyst temperature 8 mm from the inlet section; (b) outlet CH4 conversion. CH4 ) 3%; O2 ) 10%; GHSV ) 96000 h-1.
in Figure 9, if light-off is attempted on the activated PLM-3 monolith at a preheating temperature of 450 °C (i.e., 40 °C below the minimum required), the surface temperature raises again very steeply during the first seconds, reaching a maximum value of 710 °C, where it stops in correspondence with the beginning of the PdO to Pd phase transition. In this case, the reaction on the perovskite sites does not proceed fast enough to sustain the final part of the ignition process before the loss of activity (due to PdO sintering and/or formation of solidstate solutions) causes a drop of surface temperature down to 500 °C. The final steady state corresponds exactly to that obtained by repeating the experimental run after cooling the monolith in flowing air, indicating that PLM-3 catalyst has reached its initial, notactivated form. Since the low-temperature activity can be completely recovered after cooling the system from high temperature (above 800 °C), it appears that the novel catalyst possess a very interesting and easy autoregenerative functionality that might be successfully employed in real burners to pilot the ignition. It must be noted that the reduction in light-off temperature (20 °C) between the initial or activated states of PLM-3 catalyst does not correspond completely to the large difference in methane oxidation rates measured at low temperature (a factor ≈40; Figure 5). Such circumstance demonstrates how important is to preserve an elevated catalytic oxidation activity also at moderate to high temperatures in order to keep stable and fully ignited operation. Indeed, the combination of two types of catalytic sites, one (Pd based) acting at low to moderate temperature and the other (LaMnO3 based) from moderate to high temperatures, requires a good overlapping of the characteristic operating windows to be really effective at steady state. In fact, the temperature of 490 °C necessary to ignite a 3% CH4/10% O2 mix over the activated PLM-3 monolith is the same value previously measured as minimum preheating temperature required to keep full conversion (extinction
Figure 10. Transient temperature profiles and outlet methane conversion measured during light-off over a fully activated PLM-3 monolith at different preheating levels (1, 380 °C; 2, 390 °C; 3, 430 °C). CH4 ) 4%; O2 ) 10%; GHSV ) 96 000 h-1.
temperature) over a similarly aged perovskite LM catalyst operated under the same conditions.30 It must be also noticed that a higher degree of adiabaticity of the reactor would facilitate light-off at lower preheating.31 A larger decrease in the minimum ignition temperature could be achieved over the fully activated PLM-3 monolith by feeding a 4% CH4/10% O2 mix: under such conditions, the system is able to steadily ignite and completely convert the feed already at 375 °C preheating. Figure 10 reports the light-off transients recorded with such a mixture over activated PLM-3 at three different preheating levels (380, 390, and 430 °C). In analogy with experiments at lower (3%) CH4 concentration, all profiles show a sharp initial temperature rise caused by very fast heterogeneous methane combustion, which abruptly slows down as soon as surface temperatures reach values in the range 700-780 °C. From this point on, the overall CH4 conversion drops and passes through a well-pronounced minimum, whose depth and duration decrease at higher preheating. During this phase, surface temperatures stay almost constant, before starting to increase again following the trend of methane conversion. Calculations performed with the kinetic parameters estimated from isothermal runs on activated PLM-3 catalyst indicate that the reaction becomes increasingly mass transfer limited (first internally and then externally) at temperatures above 650-700 °C. Nevertheless,
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it was verified that at any temperature the residence time in the catalytic reactor is long enough to give complete conversion also under full mass transfer control. Therefore, the main reason for the sharp drop in activity must be related to catalyst reconstruction (PdO transformation into metal Pd and/or segregation in perovskite lattice), which progressively reduces the reaction rate from that of the activated catalyst to that of the initial state (1-2 orders of magnitude lower). The prolonged and deep valley recorded in methane conversion during this phase seems due to an induction period needed by perovskite sites to start contributing to the reaction. This might be tentatively related to the presence of residual highly active palladium sites that compete for reaction and/or adsorption of reactants, which therefore cannot penetrate deep inside the whole porous washcoat layer and reach perovskite centers. It has been reported that thermal reduction of PdO at such temperatures can last tens of minutes, even if the presence of reaction has been found to speed it up.8 In fact, with increasing preheating, catalyst temperatures during this phase reach progressively higher levels, so that catalyst reconstruction as well as light-off are completed in a progressively shorter time. Moreover, at steady state, the reaction front settles closer to the entrance of the monolith for higher preheating levels. Regarding catalyst durability, repeatable light-off performance was recorded over PLM-3 monolith during 30 ignition-extinction cycles corresponding to a total exposure of more than 50 h to temperatures in the range 800-1000 °C, suggesting a mutual stabilizing effect between Pd and perovskite phases. Indeed, it has been recently demonstrated that the reversible formation of solid solutions between Pd and perovskites in automotive catalysts not only suppresses or reverses the grain growth of Pd but even inhibits sintering of perovskite crystals after thermal aging at high temperatures.21 Steady-State Multiplicity. The existence of steady state multiplicity during autothermal (or adiabatic) catalytic combustion of methane in structured reactors has been already reported experimentally and theoretically for LM perovskite monoliths as well for PdO-based systems.30,31 Also PLM-3 monolith shows the occurrence of an hysteresis loop between ignition and extinction as a function of the preheating temperature (Figure 11) with at least two possible stable steady states, one at low and one at high (complete) conversion. In this case, the situation is complicated for the existence of two possible ignition branches, one corresponding to the activated form of the PLM-3 catalyst and the other to its initial state, which respectively require a minimum preheating temperature of 375 and 450 °C to light-off combustion of a 4% methane/air mix. On the other hand, the presence of only one extinction branch (blow-out estimated at 345 °C from transient cooling run) supports the hypothesis that during high-temperature combustion the catalyst reaches always the same final state regardless its starting form,3 with oxidation activity almost exclusively associated with LaMnO3 sites. As a consequence, the amplitude of the hysteresis loop is strongly reduced when comparing the initial to the activated form of PLM-3 catalyst, passing from roughly 100 °C to only 35 °C. Since the extinction branch must always occur at temperatures less than or equal to the ignition one, the zone of existence of multiple steady states progressively shrinks and finally disappears if the low-temperature catalytic functionality is strongly
Figure 11. Steady-state multiplicity during an autothermal ignition-extinction cycle over a PLM-3 monolith activated (1) or not (2). (a) Catalyst temperature 8 mm from the inlet section; (b) outlet CH4 conversion. CH4 ) 4%; O2 ) 10%; GHSV ) 96 000 h-1.
enhanced but the high-temperature sites are not active enough to sustain the reaction. In this case, light-off and extinction branches would overlap with the result that steady ignition would be limited on the lower side by the extinction behavior of the system. Indeed, the rather small decrease in light-off temperature obtained when feeding 3% CH4 on the initial or the activated monolith (∆T ) 20 °C) is due to the occurrence of this circumstance. On the other hand, the two branches do not coincide at 4% CH4, since extinction over perovskitebased sites occurs at lower temperature than ignition over Pd-based ones, making it possible to take full advantage of this bifunctional catalyst. 4. Conclusions The novel Pd/LaMnO3 combustion catalysts developed possess unique features since they have a dual-site nature with matching operating ranges (low + mid/high temperature), combined with a self-regeneration capability of the low temperature oxidation activity, which is easily expressed under normal autothermal operation of a catalytic burner. It must be noted that if in principle the dual-site feature could be realized by a multimonolith-segmented configuration of the combustor, selfregeneration appears strongly connected to the simultaneous presence of both Pd and perovskite. Such interesting functionality is related to the exposure to high temperatures (above PdO reduction) and is possibly enhanced by the capability of reversibly forming solid solutions with perovskite lattice. With respect to this latter point, even small amounts of Pd are sufficient to obtain remarkable improvements of the low temperature (light-off) combustion performances of supported perovskite catalyst: continuous regeneration and spontaneous reformation of active Pd sites during normal operation ensures an efficient use of the expensive precious metal. Once ignited, combustion can proceed stably, relying on the high-temperature activity of perovskite-based active sites. Exposure to combustion
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atmospheres at temperatures as high as 1000 °C does not cause any deactivation issue but, on the contrary, is the key for subsequent catalyst regeneration. Moreover, preliminary results seem to confirm that the reversible formation of solid solutions with Pd stabilizes the perovskite phase as well, reducing its natural tendency to sintering and thus further improving the high-temperature durability of the system. Further studies are in progress to characterize the real nature of the most active sites on the catalyst surface in each phase, their deactivation-regeneration mechanisms, and long term durability. To sum up, the novel PLM catalysts appear particularly promising for the development of fully catalytic or hybrid premixed radiant burners for both domestic and industrial applications with improved thermal efficiency and even characterized by strongly discontinuous operation. Literature Cited (1) Choudhary, T. V.; Banerjee, S.; Choudhary, V. R. Catalysts for combustion of methane and lower alkanes. Appl. Catal. A 2002, 234, 1. (2) Zwinkels, M. F. M.; Jaras, S. G.; Menon, P. G.; Griffin, T. A. Catalytic materials for high-temperature combustion. Catal. Rev.-Sci. Eng. 1993, 35, 319. (3) Forzatti, P. Status and perspectives of catalytic combustion for gas turbines. Catal. Today 2003, 83, 3. (4) Dalla Betta, R. A. Catalytic combustion gas turbine systems: The preferred technology for low emissions electric power production and co-generation. Catal. Today 1997, 35, 129. (5) Klvana, D.; Kirchnerova, J.; Chaouki, J.; Delval, J.; Yaici, W. Fibre supported perovskites for catalytic combustion of natural gas. Catal. Today 1999, 47, 115. (6) Cadete Santos Aires, F. J.; Ramirez, S.; Garc_a Cervantes, G.; Rogemond, E.; Bertolini, J. C. Application of Pd/R-Si3N4 catalysts to radiant panels using methane catalytic combustion to obtain infrared emission. Appl. Catal. A 2003, 238, 289. (7) Cerri, I.; Pavese, M.; Saracco, G.; Specchia, V. Premixed metal fibre burners based on a Pd catalyst. Catal. Today 2003, 83, 19. (8) Ciuparu, D.; Lyubovsky, M. R.; Altman, E.; Pfefferle, L. D.; Datye, A. Catalytic combustion of methane over palladium-based catalysts. Catal. Rev. 2002, 44, 593. (9) Centi, G. Supported palladium catalysts in environmental catalytic technologies for gaseous emissions. J. Mol. Catal. A: Chem. 2001, 173, 287. (10) Dalla Betta, R.; Sohji, T.; Tsurumi, K.; Ezawa, N. Partial combustion process and a catalyst for use in the process. U.S. Patent 5,326,253, 1994. (11) Euzen, P.; LeGal, J.-H.; Rebours, B.; Martin, G. Deactivation of palladium catalyst in catalytic combustion of methane. Catal. Today 1999, 47, 19. (12) Zwinkels, M. F. M.; Jaras, S. G.; Menon, P. G. Catalytic fuel combustion in honeycomb monolith reactors. In Structured Catalysts and Reactors; Cybulski, A., Moulijn, J., Eds.; Marcel Dekker: New York, 1998; p 149. (13) Sekizawa, K.; Machida, M.; Eguchi, K.; Arai, H. Catalytic properties of Pd-supported hexaaluminate catalysts for hightemperature catalytic combustion. J. Catal. 1993, 142, 655. (14) Sekizawa, K.; Eguchi, K.; Widjaja, H.; Machida, M.; Arai, H. Properties of Pd-supported catalysts for catalytic combustion. Catal. Today 1996, 28, 245. (15) Jang, B. W. L.; Nelson, R. M.; Spivey, J. J.; Ocal, M.; Oukaci, R.; Marcelin, G. Catalytic oxidation of methane over hexaaluminates and hexaaluminate-supported Pd catalysts. Catal. Today 1999, 47, 103. (16) Groppi, G.; Cristiani, C.; Lietti, L.; Ramella, C.; Valentini, M.; Forzatti, P. Effect of ceria on palladium catalysts for hightemperature combustion of CH4 under lean conditions. Catal. Today 1999, 50, 399.
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Received for review April 28, 2004 Revised manuscript received July 20, 2004 Accepted July 23, 2004 IE049656H
