Surface properties and catalytic performance in methane combustion of Sr-substituted lanthanum manganites

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Abstract

Perovskite type La1  xSrxMnO3 (x = 0–0.5) oxides were prepared by the amorphous citrate process, characterised by X-ray diffraction, oxygen desorption, temperature-programmed reduction, infrared and X-ray photoelectron spectroscopic techniques, and tested for methane combustion within the 473–1073 K temperature range. Since catalyst activity was found to depend strongly on BET areas and to a lesser extent, on the degree of substitution (x), intrinsic activities were computed for La1  xSrxMnO3 catalyst series. Among the compositions investigated, the degree of substitution x = 0.2 showed the highest intrinsic activity within the temperatures explored. Characterisation techniques made possible to correlate catalytic performance with the structural characteristics of the oxides. The stability of Mn4+ is probably the most important parameter, but excess of oxygen and atomic surface composition should also be taken into account.

Introduction

Interest in catalytic combustion is growing fast due to strong environmental legislation on gaseous emissions from either fossil fuel burners or automobile exhausts. For example, in gas turbine systems air and natural gas are compressed and preheated up to ca. 600 K and then ignited in a combustion chamber where adiabatic combustion temperatures higher than 2000 K are generated. Unfortunately, at 2000 K, nitrogen is oxidised by the excess of oxygen into NOx, producing about 165 ppm in the turbine exhaust [1]. A solution to this problem is catalytic combustion at temperatures below 1600 K, where NOx formation is reduced to almost zero [1], [2], [3], [4], [5]. To make catalytic combustion widely acceptable, reliable technologies need to be developed [2], [6]. Among the catalysts suitable for this purpose, supported noble metals initially seemed to be good candidates [2], [4], [7]. However, they are inadequate because of the evaporation or sintering of the catalytic component during the high temperature reached in the hydrocarbon combustion. In this regard, single or mixed metal oxides are desired for the catalytic combustion of hydrocarbons. Among the single oxides, NiO, Co3O4 and MnO2 have been found to be very good catalysts for the oxidation of hydrocarbons. Despite this, mixed oxide of appropriate composition often afford higher catalytic activity and greater stability properties for hydrocarbon combustion than single oxides [8], [9], with a conversion level in the order of that of Pt/Al2O3 catalysts [8], [10], [11].

In the perovskite-type oxide, represented by the general formula ABO3, the B cations have octahedral coordination with oxygen and the A cations are located in the dodecahedral sites of this structure. The cations in the perovskite lattice can be partially replaced by foreign cations with no large change in crystalline structure, producing substituted perovskites, i.e. A1  xAx′BO3 or AB1  xBx′O3. This property has frequently been exploited in catalysis because the substitution of foreign cations in A and/or B sites usually promotes catalytic activity [8], [9], [10], [11], [12], [13]. Nitadori et al. [12] found that the activity of unsubstituted ABO3 perovskite oxides is mainly determined by component B, and that the most active catalysts are those containing Mn and Co. Similarly, McCarty and Wise [9] found that, in the LaMO3 series, LaNiO3 is the most active catalyst in the combustion of methane, followed by LaCoO3, LaFeO3 and LaMnO3. Substitution of the cation B in the ABO3 proved to be far more effective for optimising catalytic activity than choosing different lanthanides at the A site of the structure. This was confirmed by the results of Nitadori et al. [12], who found that the reaction rate for propane oxidation is two orders of magnitude higher for LaCoO3 than for LaFeO3, while the change of La for other lanthanides modifies the rate less than one order. Nevertheless, partial substitution of the A3+ ion with Sr2+ (La1  xSrxBO3) generally results in a substantial increase in activity for hydrocarbon combustion [8], [9], [10], [11], [12], [13]. Nitadori et al. [12] observed an increase of the reaction rate of propane combustion on LnCoO3 catalysts (Ln: lanthanide) of ca. one order magnitude due to substitution of Ln by Sr. For these authors, the most active catalyst was La0.8Sr0.2CoO3, whereas McCarty and Wise [9] found the highest catalytic activity for x = 0.5 substitution in methane combustion. Furthermore, Zhang et al. [14], [15] found that the citrate process is superior to the nitrate process for the preparation of La1  xSrxMnO3 oxides supported on lanthanum hexaluminate.

Another important factor to be considered is the true surface composition of these perovskite oxides. As the symmetry and coordination of A3+ and B3+ ions are lost at the surface, they show a strong tendency to become saturated through reaction with H2O and CO2. Thus, the O 1s core-level spectra of many ABO3 perovskite-type compounds exposed to the environment or reduced in a hydrogen flow indicate the appearance of more than one oxygen species, i.e. the lattice O2− ions are usually accompanied by other O-containing species such as OH and CO32− [16], [17], [18], [19], [20]. Depending on the preparation method, small amounts of the individual oxides A2O3 and B2O3 not completely incorporated into the perovskite structure during the synthesis may be still present at the surface. It is therefore expected that these differences in surface composition with respect to the bulk composition may significantly modify both the catalytic activity and adsorption properties.

In view of the large number of factors involved in the catalytic combustion of hydrocarbons on strontium-substituted manganese-based perovskites, this work was undertaken with the aim of analysing the effect of strontium-substitution in manganites for the combustion of methane for the La1  xSrxMnO3 series, prepared by decomposition of citrate precursors. Parallel characterisation of the precursors and catalyst structures was performed by infrared spectroscopy, X-ray diffraction, temperature-programmed reduction, photoelectron spectroscopy and temperature-programmed desorption in order to correlate surface (and bulk) properties with catalytic activity.

Section snippets

Catalysts preparation

Substituted La1  xSrxMnO3 (x = 0–0.5) perovskites were prepared by amorphous citrate decomposition as described earlier [21]. In the synthesis, La(NO3)3·6H2O, Mn(NO3)2·4H2O and citric acid monohydrate (p.a., from Merck), Sr(NO3)2 (reagent grade, from Aldrich-Chemie) were used. A concentrated solution of citric acid was prepared and then added to a solution of the metal nitrates of appropriate La, Sr and Mn concentrations, in such a way that the ratio of equivalent grams of metal to equivalent

Elemental chemical analysis and specific area

The elemental chemical analyses of La, Mn and Sr for the samples calcined at 973 K, expressed as atomic ratios relative to manganese, are summarised in Table 1. There is a close similarity between analytical and targeted values for all the samples. The specific areas of the catalyst series are also included in Table 1. The data clearly indicate a marked dependence of the area on the degree of substitution (x). For substitutions x = 0.1 and x = 0.2, specific area decreases by ca. 40 and 42%,

Discussion

The IR data presented in this work clearly show that, in the preparation of catalysts by the citrate process, the precursor is not merely a mixture of the different metal citrates. Nitrates and structural water are also present in the precursor. Different structural arrangement has been previously proposed to contain such species [38], [39], and the spectroscopic data in the present work confirm that this is indeed the case. As the temperature of calcination of the precursor increases, water is

Conclusions

Methane combustion on perovskite-type SrxLa1  xMnO3 (x = 0–0.5) oxides was found to depend markedly on their specific areas. Among the compositions investigated, the substitutions x = 0.1 and x = 0.2 showed the highest intrinsic activity within the explored temperature range (473–1073 K). The stability of Mn4+ in this catalyst seems to be one of the most important determining factors in the catalytic activity, but other structural parameters should also be considered. Among them, the excess of oxygen

Acknowledgements

Financial support from CICYT (Spain), projects QUI98–0877 and QUI98-1141-CE, is gratefully acknowledged. One of the authors (SP) is grateful to ICI (Spain) for a fellowship. Thanks are due to Dr. P. Terreros for fruitful help in the characterisation of the precursors.

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