Elsevier

Catalysis Today

Volume 59, Issues 1–2, 10 June 2000, Pages 163-177
Catalysis Today

Catalytic combustion of methane on substituted barium hexaaluminates

https://doi.org/10.1016/S0920-5861(00)00281-9Get rights and content

Abstract

A sol–gel method using metallic barium, aluminum alkoxides and metal nitrates has been used to synthesize barium hexaaluminate partially substituted by either manganese, iron or both metal ions. The β-alumina structure was obtained by calcination under oxygen at 1200°C. X-ray analysis revealed that formation of a pure single phase BaMxAl12−xO19 occurred up to x=4 for Fe, x=3 for Mn and for Fe1Mn1 in the case of mixed substituted hexaaluminates. Incorporation of Mn in excess leads to another phase formation (manganese oxide or spinel). As far as the valence state of transition metal ions is concerned, the introduced Fe ions were always trivalent, whereas the Mn ones were either divalent or trivalent. In the latter case, the first Mn ions were introduced in the matrix essentially as Mn2+ and only for BaMn3Al9O19 does manganese exist exclusively as Mn3+, the higher the Mn concentration, the higher the proportion of Mn3+. All solids were aged at 1200°C under water and oxygen and showed a good thermal resistance. Activity for methane combustion has been measured for fresh and aged solids, light-off temperatures were observed in the 560–640°C range. However, the highest activity was obtained for catalysts containing either 3 Mn, 2 Fe or 1 Fe+1 Mn ions per unit cell. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species.

Introduction

The new regulations concerning the emission of pollutants in gaseous emissions are becoming more and more stringent. The catalytic combustion of gaseous fuels appears to be a powerful process for environmental protection [1]. A lot of work has been performed on the catalytic combustion of methane which is one of the cleanest way to obtain thermal energy without any harmful CO, NOx or unburned hydrocarbon emissions [2]. For such a catalysis, special devices are required, and due to high temperature needed for very high efficiency output like for gas turbine [3], thermally stable catalysts are required [4]. Among various ceramics stable at high temperature in the presence of water, hexaaluminates, also named β-alumina for historical reason [5], have been selected.

The formula of all hexaaluminates is M2O(M′O)6Al2O3, where M or M′ stand for alkaline or alkaline-earth metal. The structure is lamellar and consists of layers of spinel blocks separated by a monolayer of oxides issued from either bulky alkaline cations, or bulkier alkaline-earth ones [6]. Such a lamellar structure prevents any contact between spinel blocks. Spinels are organized as an f.c.c. structure of oxygen anions, where both octahedral and tetrahedral sites are more or less filled [7]. On the other hand, α-alumina has an h.c.p. structure of oxygen anions with 23 octahedral sites occupied by aluminum cations [7], [8]. Formation of α-alumina at the expense of other forms (γ, δ, etc.) needs a restructuring of all the anion layers. This is not possible for β-alumina due to the blocking introduced by the alkaline(-earth) cation [9]. So, transformation of alumina is not possible and sintering is strongly hindered: these mixed oxides are exceptionally stable [10], but they are also almost inactive except at very high temperature. The introduction of active transition metal ions in the structure may be very worthwhile for activity [11]. But substitution is not easy because of charge and radius stress locally introduced. The aim of the present study is to investigate the catalytic activity as well as the thermal stability of barium hexaaluminate substituted at various levels by Mn and Fe ions.

Section snippets

Preparation

A sol–gel method, derived from that already published by Arai and co-workers [12] was used. Different solids have been synthesized: the parent barium hexaaluminate matrix and the solids resulting from the substitution of 1–4 Al3+ ions per unit cell by Fe3+, Mn3+ or both ions.

The required amounts of metallic barium and aluminum isopropoxide (Aldrich) were suspended into 2-propanol leading to a mixture of the two alkoxides which are only partly soluble. The suspension was refluxed for 3 h at 80°C,

Physicochemical characterizations

(a) Chemical analysis. Results concerning chemical analysis are reported in Table 1, where chemical formulae are also indicated as deduced from analysis. Elements (Al, Ba and Mn) were analyzed by atomic absorption spectroscopy after having dissolved solid in a mixture of concentrated acids (HF+HCl+HNO3).

(b) X-ray diffraction. X-ray diffraction (XRD) has been used to identify the crystallographic phases and to calculate the unit cell parameters. In order to avoid any shift of the diffraction

Catalytic activity measurements

The catalytic activity of the different solids was measured in the combustion of methane. Moreover, in order to check their thermal resistance, the catalysts were submitted to an accelerated aging. Differences between catalytic properties of as-prepared solids and aged ones associated with further physicochemical characterizations allowed to classify the catalysts according to their thermal stability. Experimental conditions were as follows:

  • Catalytic activity. Mass of catalyst: 500 mg;

Results

(a) Chemical analysis. After calcination at 1200°C, the acidic attack is difficult and may lead to false values in the case of incomplete dissolution. The results reported in Table 1 showed that, in every case, the M/Ba and M/Al experimental ratios are very close to the theoretical ones. Since oxygen has never been analyzed, formulae reported here are right as far as oxygen stoichiometric ratios are not considered.

(b) Structure of reference matrix. Structure of BaAl12O19 was only obtained after

Catalytic activity measurements

The fresh and aged solids were tested in the catalytic combustion of methane (Fig. 5, Fig. 6, Fig. 7) and a general increase in the catalytic activity when increasing the amount of active cations is observed. However, some differences appear due to variations of surface areas.

The matrix itself is not inactive since conversions of 10 and 20% are observed at 700 and 750°C, respectively, a full conversion is obtained near 800°C. The T10, T50 and T90 temperatures corresponding to 10, 50 and 90%

Structure

It is clear that the sol–gel method previously described by Arai and co-workers [12] is very worthwhile for preparing substituted hexaaluminates. However, the structure experimentally observed is always referred in the ICDD files as BaO·6.6Al2O3. According to the stoichiometric Ba/Al=6 ratio of the initial alkoxides, this would correspond to a structure where BaO should be in excess outside, and according to the temperature of calcination should not escape the X-ray analysis. Previous studies

Conclusion

Synthesis of barium hexaaluminate via metal alkoxides hydrolysis is an efficient method for obtaining pure single phased solids with a rather well-developed SSA. Introduction of Mn+ ions during the hydrolysis step leads to a gel very homogeneous in Ba, M and Al. Further calcination at 1200°C leads to the exactly desired structure. Substitution of aluminum by iron or manganese ions, having the same charge (+3), the same local symmetry (Oh) and comparable radius, is possible at least for 3 ions

Acknowledgements

The authors are grateful to GAZ DE FRANCE, Direction de la Recherche, Département des Etudes Générales, Pôle Combustion Catalytique et Mécanique des Fluides, for the stimulating discussion and financial support as well as to the European Community for a grant from the Brite Euram project No. 5846.

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    Present address: Battelle, Geneva Research Center, 7 route de Drize, CH 1227, Carouge Geneva, Switzerland.

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