N2O formation in the ammonia oxidation and in the SCR process with V2O5-WO3 catalysts
Introduction
For many years, nitrous oxide (N2O) was considered as a relatively innocuous chemical species due to the ignorance of this compound as a potential contributor to environmental problems and as such received little interest among the scientific and technological community [1]. However, during the last decade, a growing concern for the emissions of this gas has been registered since its identification as one of those responsible for the destruction of the stratospheric ozone layer, also as a promoter of the greenhouse effect.
The formation of nitrous oxide has been observed in the catalytic elimination of ammonia traces from gaseous effluents [2], such as those from the selective catalytic reduction (SCR) of nitrogen oxides with ammonia [3]. Catalysts based on V2O5-WO3 supported on TiO2 are generally used in the SCR process to eliminate nitrogen oxides present in waste gases from fossil fuel fired power stations [4]. Furthermore, the use of V2O5/TiO2 catalysts has been generalized in purification units that operate at low temperature (∼200 °C) for the treatment of effluent gases from nitric acid plants. In a series of pioneer works on the oxidation of ammonia, a great number of catalysts in which V2O5, WO3 and TiO2 were found to be active oxides in this process were compared [5].
Recently, the formation of nitrous oxide during ammonia oxidation with vanadia-tungsta catalysts has been described, where it was observed that the selectivity to this compound increased with the vanadia loading in the catalyst and the reaction temperature [6]. However, the mechanism by which this phenomenon took place was not discussed. The formation of nitrous oxide has been observed also in the SCR processes, with V2O5/TiO2 catalysts [7] and V2O5-WO3/TiO2 catalysts [8].
Bearing in mind these considerations, the purpose of this work was to study the formation of nitrous oxide in the total oxidation of ammonia traces and in the selective catalytic reduction process of nitrogen oxides using catalysts, based on titania as support with different contents of vanadia and tungsta as active phase. In this work, we attempt to illustrate the criterion by which the optimum composition of these catalysts can be chosen since until recently their design has only been based on the benefits that they present for the main reaction without any thought for the eventual formation of any undesirable products, such as nitrous oxide, whose emissions participate in the degradation of the atmosphere.
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Catalyst preparation
For this study, a series of vanadia and tungsta supported on titania catalysts were prepared. The raw materials were a hydroxylated titanium gel, with approximately 50% water content and an average particle size of 90% <40 μm after drying and calcining at 500 °C (manufactured by Tioxide, UK), ammonium metavanadate (Panreac) and ammonium metatungstate (Panreac). The titanium oxide content of all the catalysts was maintained at 90 wt.%, varying the vanadia and tungsta contents to produce the five
Ammonia oxidation
In order to determine the catalyst behaviour in the ammonia oxidation process, a series of experiments between 180 and 350 °C were carried out. As may be observed, from the results shown in Fig. 1, the ammonia conversions increased as the vanadia loading was raised up to 8 wt.%. The sample with only tungsta had a very low activity indicating that although ammonia could be adsorbed over the Brønsted acid sites, since this material has no redox capability the ammonia cannot be reduced. However, in
Conclusions
The catalytic oxidation of ammonia increases with the reaction temperature and the vanadia content in catalysts of V2O5-WO3/TiO2. However, the increased vanadium oxide content leads to nitrous oxide selectivity values greater than 40% under the studied operating conditions. The selection of a catalyst for the elimination of ammonia traces should be made, therefore, keeping in mind not only ammonia conversion values but also the selectivity to nitrogen. A reaction scheme that justifies the
Acknowledgements
The realisation of this work has been possible thanks to the financing granted by the CICYT (Refs. MAT 2001-1597 and MAT 2000-0080-P4-02) of the Spanish Ministry of Education and Science. The authors wish to dedicate this work to the memory of Professors Ricardo Linarte and Paul Grange.
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