Effect of SiO₂ addition on NH₄HSO₄ decomposition and SO₂ poisoning over V₂O₅–MoO₃/TiO₂–CeO₂ catalyst

Abstract

The deposition of NH₄HSO₄ and the poisoning effect of SO₂ on SCR catalyst are the main obstacles that restrict the industrial application of CeO₂-doped SCR catalysts. In this work, deposited NH₄HSO₄ decomposition behavior and SO₂ poisoning over V₂O₅–MoO₃/TiO₂ catalysts modified with CeO₂ and SiO₂ were investigated. By the means of characterization analysis, it was found that the addition of SiO₂ into VMo/Ti–Ce had an impact on the interaction existed between catalyst surface atoms and NH₄HSO₄. Temperature-programmed methods and in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments indicated that the doping of SiO₂ promoted the decomposition of deposited NH₄HSO₄ on VMo/Ti–Ce catalyst surface by reducing the thermal stability of NH₄HSO₄ and enhancing the NH₄HSO₄ reactivity with NO in low temperature. And this improvement may be the reason for the better catalytic activity than VMo/Ti–Ce in the case of NH₄HSO₄ deposition. Accompanied with cerium sulfate species generated over catalyst surface, the conversion of SO₂ to SO₃ was inhibited in SiCe mixed catalyst. The addition of SiO₂ could promote the decomposition of cerium sulfate, which may be a potential strategy to enhance the resistance of SO₂ poisoning over CeO₂-modifed catalysts.

Introduction

The selective catalytic reduction (SCR) technology is applied for NO_x emission control in both stationary and mobile sources like power plants and vehicles (Busca et al., 1998). V₂O₅/TiO₂ catalyst promoted by WO₃ or MoO₃ is widely commercialized due to high performance in de-NO_x activity and N₂ selectivity (Topsøe et al., 1995, Choo et al., 2003). However, the oxidation of SO₂ to SO₃ is inevitable over V₂O₅-based catalysts (Svachula et al., 1993, Du et al., 2018). It is generally believed that gaseous SO₃ will unavoidably react with reductant NH₃ and H₂O vapor to generate NH₄HSO₄, which will accumulate on the SCR catalyst surface in low temperature. The deposition of NH₄HSO₄ leads to the block of pore structures and coverage of active sites, consequently resulting in the drop of denitration efficiency (Guo et al., 2009). Therefore, the issues that SO₂ oxidation and NH₄HSO₄ deposition on catalysts surface are gaining increased concerned.

As we know, the oxidation of SO₂ to SO₃ and NH₄HSO₄ decomposition could be affected by the chemical composition of SCR catalyst (Zhu et al., 2000, Xiao et al., 2019). Kobayashi et al. (2005) pointed out that the mixed oxide of TiO₂ and SiO₂ as support for V₂O₅-based catalyst could noticeably decrease the SO₂ catalytic oxidation. The addition of GeO₂ could reduce SO₂ oxidation over V₂O₅/TiO₂ catalyst, as reported by Morikawa et al. (1981). Dunn et al., 1999, Dunn et al., 1999 believed that SO₂ catalytic oxidation was irrelevant to the VO bonds and the V–O–V bridge of polymeric polyvanadium species but mainly related to the V–O-M bonds. Ji et al. (2016) held the view that the VO bond played a critical role in the adsorption and oxidation of SO₂. Furthermore, for the purpose to promote the sulfur resistance of SCR catalysts, many researchers had focused on improving the SO₂ resistance and reducing the decomposition temperature of NH₄HSO₄ by catalyst component modification. The addition of Fe in V–Ti oxide catalyst showed excellent SO₂ tolerance, as proposed by Zhu et al. (2020). Ye et al. (2018) confirmed that the NH₄HSO₄ decomposition could be enhanced by Nb and Sb decoration. Huang et al. (2003) proved that carbon-based catalysts would make differences in the reactivity behavior of NH₄HSO₄ with NO in low temperature.

Recently, the promotion of CeO₂ on SCR activity over V-based catalysts has been investigated in depth (Cheng et al., 2014). Due to the better redox properties and acid-base properties, the addition of CeO₂ appeared to be promising for low temperature SCR catalysts (Xu et al., 2009, Peng et al., 2014, Peng et al., 2014). Nevertheless, the catalytic performance of catalysts containing CeO₂ would be inhibited easily in the present of SO₂ and H₂O at temperature below 300°C (Zhang et al., 2015, Gao et al., 2010). And CeO₂ could be directly sulfated by SO₂ under oxidizing conditions (Kullgren et al., 2014). It has been assumed that surface Ce(SO₄)₂, which is the most stable cerium sulfate species, will generate after SO₂ poisoning (Liu et al., 2012). Therefore, both NH₄HSO₄ and cerium sulfate species would form over catalysts surface after being treated in simulate flue gas conditions and the catalytic activity in low temperature was unrecoverable (Xu et al., 2018). Zhang et al., 2015, Gao et al., 2010 found that CeO₂ could facilitate the NH₄HSO₄ decomposition over V₂O₅–MoO₃/CeO₂–TiO₂ catalyst, while cerium sulfate species would generate in the process of decomposition and the accumulation of cerium sulfate would delay the release of SO₂ to higher temperature. Therefore, it is imperative to lower the sulfates stability over CeO₂-modified catalysts surface for industrial application.

It is widely agreed that the decoration of SiO₂ in Ti–Ce mixed catalysts can enlarge the specific surface area, which is helpful for the active component to be dispersed well on the support (Peng et al., 2013, Fang et al., 2014), as well as for deposited NH₄HSO₄. Besides, the enlargement of specific surface area means increased active sites could be accessible, which may affect the interaction between NH₄HSO₄ and catalyst surface. However, the details of NH₄HSO₄ decomposition behavior over Ti–Ce mixed catalysts doped with SiO₂, was seldom discussed. Moreover, as mentioned before, the addition of SiO₂ to SCR catalysts could lower the SO₂/SO₃ conversion, and the SO₂ tolerance properties of SiO₂-doped catalyst were enhanced (Ye et al., 2019). It was found that SO₂ preferentially connected with CeO₂ as sulfate species over CeO₂–TiO₂–SiO₂ catalyst after SO₂ poisoning (Peng et al., 2017), but the role SiO₂ played in the poison effect was unclear.

Based on the discussion above, in this work, SiO₂ was doped into V₂O₅–MoO₃/TiO₂–CeO₂ catalyst to investigate the decomposition behavior of deposited NH₄HSO₄ and SO₂ poisoning effect. V₂O₅–MoO₃/TiO₂ was also taken into consideration as a comparison. The samples were characterized by X-ray diffraction (XRD), N₂ adsorption-desorption, X-ray photoelectron spectroscopy (XPS), temperature-programmed methods. The effect of SO₂ poisoning over catalysts were revealed by SO₂ oxidation tests, thermogravimetry-differential thermogravimetry (TG-DTG) measurement, in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) and SO₂ temperature-programmed desorption (TPD).