Catalytic combustion of methane on nano-structured perovskite-type oxides fabricated by ultrasonic spray combustion

https://doi.org/10.1016/j.apcatb.2009.10.017Get rights and content

Abstract

The synthesis of perovskite-type oxides using ultrasonic spray combustion (USC) was investigated by adjusting the composition of the precursor solution and the temperature of synthesis and calcination. LaMnO3 was chosen as model perovskite to systematically analyze the effect of USC operating parameters. XRD, IR spectroscopy, TG-TPD, SEM, TEM and BET were used to characterize the samples with respect to phase composition, thermal stability, morphology and surface area. The catalytic properties were evaluated with respect to methane combustion as such materials are potential catalysts in the control of exhaust gases from mobile and stationary sources. Addition of citric acid to the precursor solution and calcination in air appeared to be the crucial parameters to produce spherical hollow particles composed of nano-sized LaMnO3 crystallites (down to 30 nm) with high catalytic activity and durability. Calcination was required in order to remove uncombusted precursors and to improve both the crystallinity of the materials and their catalytic activity. LaFeO3, LaCoO3 and La(M,Pd)O3 (M = Mn, Fe) with pure perovskite-phase were synthesized in a single step under the optimal conditions selected for LaMnO3. Though the order of catalytic activity within the two series of samples (with and without Pd) agreed with reported trends, the production of each single perovskite composition by USC may require further optimization around these synthesis conditions. We demonstrate that USC is a simple, versatile and reliable method with potential application in the one-step synthesis of heterogeneous catalysts.

Introduction

Perovskite-type oxides of general formula ABO3, in which A is usually an alkaline earth metal or a lanthanide ion and B a transition metal ion, are well-known materials with attractive physical and chemical properties, covering magnetic and (thermo)electric properties, catalytic properties and oxygen transport capability [1], [2]. Recently, perovskites have received renewed interest as catalysts for the abatement of exhaust gases from mobile and stationary sources [3], [4], [5], and seem to be able to maintain the original premise [6], [7] for the control of automotive emissions.

Activity and stability of catalysts are strongly related to material properties such as particle size distribution, morphology and crystallinity that are on their turn controlled by the method of synthesis [8], [9]. The classic solid state procedure for the preparation of perovskite-type oxides, involving mechanical grinding of binary oxides, presents the disadvantages of long processing time, low surface area, large particle size and limited degree of chemical homogeneity. In recent years, considerable interest has been paid to the development of suitable chemical solution methods [10], [11], [12], [13], [14], [15], [16], which can provide products of fine and homogeneous particles with relatively high specific surface area at low calcination temperature (below 800 °C). However, most of these routes involve multi-step reactions, can be elaborated and the final products may not be phase pure, which has consequences on catalytic activity [17], [18]. One-step processes are more desirable to avoid time-consuming preparation routes and to offer possibility for production scale-up. Solution combustion synthesis [19] is one such method, where an additive as urea is used to promote self-sustained combustion of the precursors solution. Solution-based aerosol processes in which the precursor solution is nebulized and decomposed (in a flame or in a furnace) [20], [21] are extremely promising because of their one-step and continuous character. Among few others, Co- and Ti-containing perovskites [22], [23], [24] have been prepared by flame synthesis and their catalytic properties have been evaluated prevalently in the catalytic methane combustion with respect to activity and stability.

Ultrasonic spray synthesis [20], [25] has demonstrated to be a unique route for the synthesis of various materials, including supported metals [20], [26], metal oxides [27], [28], [29], [30], composite powders [26], and films [31]. Ultrasonic spray synthesis is based on the ultrasound assisted generation of micro-droplets of an aqueous precursor solution containing easily available materials (i.e., nitrates). The micro-droplets are then carried through a furnace and decomposed into particles via evaporation, combustion and sintering. Typically, the resulting particles (>0.5 μm) are spherical and exhibit hollow structure, which is of interest for applications in drug delivery, catalysis and separation technology. Hollow particles are porous and have thin walls composed of nano-sized crystallites. Ultrasonic spray synthesis allows a precise control of stoichiometry and is highly reproducible, inexpensive, scalable and uncomplicated. Moreover, it can be realized in a single step and the products have narrow size distribution and display high phase purity despite the low residence time of the droplet at high temperature. Although several synonyms are currently used for the same technique [20], [32], we make use of the ultrasonic spray combustion (USC) designation because we have chosen to prepare the particles using citric acid (CA) as a fuel supporting decomposition of precursors droplets in the furnace.

Various perovskite-type mixed oxides have been already produced in the form of nano-sized powders using ultrasonic spray synthesis, and included for example BaTiO3 [33], [34], [35], CaMn1−xNbxO3 [36], La1−xCaxTiO3 [32], [37], GdCo1−xCuxO3 [38], LaCrO3 [39] and LaMnO3 [40]. To the best of our knowledge none of these perovskite-type oxides was targeted to catalytic applications. To this end, the catalytic properties of powders prepared by ultrasonic spray synthesis have been evaluated only in the case of modified TiO2 photocatalysts [26]. The synthesis of Pt/Al2O3, Pt/ZrO2 [20] and Pt/SiO2 [41] has been also accomplished however without the mention of their possible catalytic application. Therefore, the potential of the ultrasonic spray technique for the production of heterogeneous catalysts still remains to be demonstrated. We endeavor this issue starting from the USC production of simple perovskite-type mixed oxides provided their relevance for environmental catalysis.

The most important parameters influencing the production of ceramic materials using ultrasonic spray synthesis are the flow rate of the carrier gas and therefore the residence time of the droplets in the furnace, the concentration of the precursor solution, the temperature of the furnace and the atmosphere in which decomposition occurs in the furnace. We found that the carrier gas flow, the citric acid-to-metal ions ratio and the overall solution concentration were crucial to optimize the textural properties of the thermoelectric material La0.95Ca0.05Fe0.95Ni0.05O3 [42].

The aim of this paper was the systematic investigation of selected relevant parameters for the USC method, i.e. the composition and concentration of the precursor solution, the temperature of synthesis and the temperature of calcination. LaMnO3 was chosen for this systematic study for the following reasons: (a) LaMnO3 has been prepared using numerous synthesis methods and is therefore a suitable model material; (b) it is the base for the synthesis of numerous catalysts formulations, by substitution at both A- and B-sites including addition of precious metals; (c) it is reported among the most active perovskite-type oxides for methane combustion. The present characterization focused on the relationship between the variation of the synthesis parameters and the morphological, textural and catalytic properties of the samples. Besides, LaFeO3 and LaCoO3 powders were synthesized via USC after selection of the optimal conditions based on the characterization of LaMnO3 samples. Finally, preliminary results have been obtained for the production of Pd-containing manganites and ferrites as these materials became increasingly attractive as potential catalysts for the removal of pollutant exhaust gases [3], [5], [43].

Section snippets

Ultrasound spray combustion

The homebuilt ultrasound spray combustion (USC) equipment is composed of an atomizer, a quartz reactor, a high temperature furnace and a vacuum pump [37]. Micro-droplets of the precursor solution were generated by an ultrasonic atomizer consisting of three piezoelectric oscillators at a fixed frequency of 1.67 MHz, and then transported by a flow of synthetic air (2 l/min) through a quartz tube (l = 1300 mm; dext = 26 mm) installed in a vertical high temperature furnace (Nabertherm), where they were

Synthesis and characterization

Fig. 1 shows the SEM images of LaMnO3 powders prepared by ultrasonic spray combustion (USC) from precursor solutions with total concentration of metal ions (CMN) set to 0.1 mol/l and with increasing citric acid (CA) concentration (CCA), i.e., the molar ratio CA/metal ions (MCA/MN) is 0, 1, 2, 4 and 8 from Fig. 1a, b, c, d and e, respectively.

In the absence of CA (Fig. 1a), micrometric spherical particles were produced, which had heterogeneous size in the 0.3–2.5 μm range. TEM data revealed that

Conclusion

USC is a reliable, simple and versatile method for the preparation of mixed oxides. The USC synthesis of LaMnO3 perovskite was systematically investigated by varying the content of the precursor solution and adjusting the temperature of synthesis and calcination. The results show that the addition of citric acid in sufficient excess to the metal precursors leads to micro-sized powders with hollow spherical morphology and thin porous walls composed of nano-sized perovskite crystallites (30–50 nm).

Acknowledgments

The authors gratefully acknowledge A. Eyssler for the preparation of LaMnO3 by the amorphous citrate method, Dr. D. Logvinovich for the TEM measurements, P. Hinz for some SEM measurements and L. Rotach for providing support for the electronics of the USC equipment. X.W. would like to thank BNF at University of Bern for financial support. The Swiss-Norwegian beamline at ESRF is kindly acknowledged for providing beam time and support during measurement.

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    Present address: Nanotechnology Group, ETH Zurich, Tannenstrasse 3, CH–8092 Zurich, Switzerland.

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