Characterisation and reactivity of vanadia–titania supported SBA-15 in the SCR of NO with ammonia

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Abstract

TiOx, VOx and TiOx–VOx oxides were highly dispersed at different metal loadings on the mesoporous silica SBA-15 by the molecular designed dispersion (MDD) method. A combination of different techniques (N2-sorption, FT-Raman, TPR, UV–vis-DR) was used to verify the nature of the vanadia and/or titania species on the surface of the mesoporous support. Temperature-programmed desorption (NH3-TPD) was applied to characterise the surface acidity of these catalysts. Vanadia and titania were found to be present as isolated species, even at high metal oxide concentrations. The supported catalysts were tested in the selective catalytic reduction (SCR) of NO with ammonia. Among the studied samples, the highest activity was found for TiOx–VOx mixed oxides on SBA-15. The catalytic activity results were discussed in terms of loading and metal dispersion on the support material.

Introduction

Nitrogen oxides (NOx), are produced by both natural and anthropogenic sources. They contribute to a variety of environmental problems: the formation of acid rain and the resultant acidification of aquatic systems, ground-level ozone (smog), and general atmospheric degradation [1]. Selective catalytic reduction (SCR) of nitrogen oxides is an area of significant interest due to the increasingly severe emission limitations, which culminated in an agreement in Kyoto. The legal targets were set for reducing the emissions of six greenhouse gases in the period 2008–2012 [2].

Materials containing Ti and V are known as excellent catalysts for the SCR of NOx with ammonia. The commercial catalyst for the removal of NOx emitted from chemical industrial plants and stationary power stations are based on V2O5 supported on TiO2 (anatase) [3]. V2O5–TiO2 oxide system shows high catalytic performance resisting against poisoning with SO2 in real industrial conditions [4]. Vanadia is considered as the active phase for the SCR reaction. TiO2 is one of the more successful supports for vanadium oxide. It has been reported that the anatase form is very active probably because of the crystallographic matching between the structures of the two components [5]. However, the use of TiO2 as a support is limited by the fact that it posses low resistance to sintering, low surface area and high cost. Therefore, to improve their properties, many methods have been developed and applied [6], [7]. Almost all these studies consist on the deposition of vanadium on mixed TiO2–SiO2 oxides prepared either by coprecipitation or by sol–gel method [8], [9], [10], [11]. Silica has the advantage of a high specific surface area, which provides more surface active sites for reaction, and a higher resistance to sintering compared to TiO2. Moreover, it has been reported that the catalytic activity of V supported silica catalysts was improved when coating the silica support with a monolayer of TiO2 [12], [13].

In this work, the Ti- and/or V-containing catalysts were prepared using the mesoporous silica SBA-15 as support material and tested for the SCR of NO with ammonia. SBA-15 with an ordered hexagonal structure has very interesting properties to act as a catalytic support especially due to its high thermal and hydrothermal stability. Moreover, it is characterised by a narrow pore size distribution, high pore volume, thick pore walls, intrinsically combined micro- and mesoporous and high surface area (∼800 m2/g). Because the dispersion of active components plays a crucial role in the DeNOx process, TiOx and/or VOx were introduced on the SBA-15 surface by the molecular designed dispersion (MDD) using acetylacetonate (=acac) complexes [14], [15], [16]. This method was used to deposit the active elements on carefully prepared micelle templated structures [17]. The MDD is based on the reaction of the acac complexes of metals with the surface hydroxyls of the support material. The thermal decomposition of surface organometallic species results in a formation of highly dispersed metal oxides [18].

The influence of different metal loadings on the catalytic behaviour of the VOx/SBA-15, TiOx/SBA-15 and VOx–TiOx/SBA-15 samples in the SCR of NO with NH3 was investigated and discussed in this work.

Section snippets

Catalysts preparation

The SBA-15 support was prepared according to a procedure described in literature by Segura et al. [18]. Pluronic P123 triblock copolymer surfactant (EO20–PO70–EO20) was used as a template, which was dissolved in 2 M solution of HCl in water. Briefly, a suitable amount of TEOS (tetraethyl orthosilicate) was added. The resulting mixture was stirred for 8 h at 45 °C and then aged at 80 °C for 15 h. The white product was filtered, washed and dried. The sample was subsequently calcined at 550 °C with a

Catalysts characterisation

The VOx/SBA-15, TiOx/SBA-15, and TiOx–VOx/SBA-15 oxide catalysts present the typical XRD pattern of mesoporous materials. SBA-15 exhibits one strong reflection (1 0 0) at 2θ ∼1 and two weaker peaks (1 1 0), (2 0 0) at higher 2θ, associated with the hexagonal symmetry and likewise characteristic of the hexagonal ordered structure of SBA-15. The XRD pattern of the samples after metal deposition is very similar to that obtained for the parent blank SBA-15 support, although the intensity of the peaks is

Conclusions

The deposition of titanium and vanadium oxides on the SBA-15 by the MDD method using acetylacetonate complexes resulted in highly dispersed TiOx and VOx oxides on the silica support. The presence of the isolated V and/or Ti species on SBA-15, even for the samples with a high titanium and/or vanadium content, was evidenced by FT-Raman and UV–vis-DR spectroscopic measurements. For the samples with the higher V-loading, also the presence of polymeric chains of VOx was detected. Modification of the

Acknowledgements

The authors thank the Ministry of Flanders and the Polish Ministry of Scientific Research and Information Technology for financial support in the frame of bilateral Flemish–Polish project for 2004–2005. P. Cool also acknowledges the FWO-Flanders (Fund for Scientific Research-Flanders) for financial support.

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