Comparative study on sulfur poisoning of V₂O₅-Sb₂O₃/TiO₂ and V₂O₅-WO₃/TiO₂ monolithic catalysts for low-temperature NH₃-SCR

Highlights

• The V_2.7Sb₂Ti presents remarkable sulfur resistance at 250 °C.

• The NH₄HSO₄ deposited on V_2.7W₄T is twice larger than that on V_2.7Sb₂Ti.

• The lower SO₂ oxidation inhibits the NH₄HSO₄ deposition on V_2.7Sb₂Ti catalyst.

• The higher reactivity of NH₄HSO₄ with NO decreases its accumulation on V_2.7Sb₂Ti.

Abstract

The V₂O₅-Sb₂O₃/TiO₂ monolithic catalyst prepared by an impregnation method showed superior deNO_x performance compared with the commercial V₂O₅-WO₃/TiO₂ catalyst in the presence of sulfur dioxide. In addition, it exhibited remarkable sulfur resistance at 250 °C for 30 h test due to the low deposition of NH₄HSO₄, suggested by the TG and MS results. The lower SO₂ oxidation and higher reactivity of NH₄HSO₄ with NO are the key factors for the low deposition of NH₄HSO₄ over this Sb-modified monolithic catalyst.

Introduction

The nitrogen oxides (NO_x) emitted from stationary and mobile source are important pollutants, which resulting in photochemical smog, acid rain, ozone depletion and greenhouse effects [1]. Among the developed deNO_x technologies, the selective catalytic reduction of NO with NH₃ (NH₃-SCR) is an efficient method to remove NO_x, and V₂O₅-WO₃/TiO₂ (VWTi) is a widely used catalyst for decades owing to its stability and high catalytic activity at 300–400 °C [2], [3]. The SCR unit is usually located upstream of the desulfurized and particulate control devices because the commercial catalyst is usually used in 350–400 °C. However, high concentrations of ash and SO₂ could damage and deactivate the monolithic catalyst. When the SCR unit is located after cleaning and desulfurization with a lower reaction temperature, there is a strong need to develop low-temperature SCR catalysts, especially for flue gases from industry boilers and ship diesel engines [1]. Several V-free catalysts, including Mn- [4], [5], Cu- [6] and Ce-based catalysts [7], [8], have been widely researched for their catalytic performances at low temperatures (< 300 °C). However, the poor resistance to SO₂ of those catalysts limits their application in industries [2], [9]. Nowadays, V-based catalysts with relative high loading of V₂O₅ are still a preferable choice for SO₂-containing exhaust purification at low temperatures [10].

The predominant factor for sulfur poisoning varies at different temperatures. For NH₃-SCR at high temperatures, sulfation of active metals is generally responsible for deactivation of catalysts. However, NH₄HSO₄ deposition on the catalysts surface is reported as the primary cause for the NH₃-SCR deactivation at low temperatures in practical applications. The SO₂ in sulfurous fuels can be oxidized into SO₃, which further reacts with NH₃ in stream to form NH₄HSO₄ [11]. Magnusson et al. [12] found that in the presence of both H₂O and SO₂, the V₂O₅-WO₃/TiO₂ monolithic catalyst did not show any signs of deactivation at temperatures above 300 °C, but at a lower temperature (250 °C) the catalytic performance for NO_x reaction decreased with time. They ascribed such a decline to the formation of ammonium sulfate salts at low temperatures, which could cover parts of the active sites on the catalyst surface. Xi et al. [13] reported that the main deactivation reason at low temperature was physical poisoning by the formation of NH₄HSO₄, cutting off the NH₃-SCR reaction, rather than the poisoning of active species. Therefore, one of the primary goals for industrialization is to develop low-temperature NH₃-SCR catalysts with low NH₄HSO₄ deposition.

Recently, antimony (Sb) has attracted increasing attention for its good performance in the modification of V₂O₅/TiO₂ catalyst. Phil et al. [14], [15] reported that Sb₂O₃ (2 wt.%) contained V₂O₅/TiO₂ catalyst showed better NH₃-SCR performance than other promoter (B, P and Cu) contained catalysts. Besides, the addition of Sb on Mn/TiO₂ catalyst increased the low-temperature NH₃-SCR activity by improving the reducibility and generating strong Lewis acid sites for NH₃ desorption [16]. Meanwhile, the existence of Sb on Mn/TiO₂ catalyst also exhibited high Na-resistance. In this study, V₂O₅ (2.7 wt.%)-Sb₂O₃ (2.0 wt.%)/TiO₂ (V_2.7Sb₂Ti) and V₂O₅ (2.7 wt.%)-WO₃ (4.0 wt.%)/TiO₂ (V_2.7W₄Ti) monolithic catalysts were prepared. The V_2.7Sb₂Ti catalyst presented superior NH₃-SCR activity and strong SO₂ resistance, which could be a competitive catalyst for practical application, especially in high SO₂ containing exhausts at low temperatures.