Dual Promotional Effects of TiO₂-Decorated Acid-Treated MnO_x Octahedral Molecular Sieve Catalysts for Alkali-Resistant Reduction of NO_x

Alkali metals generated during waste incineration in power stations are not conducive to the control of nitrogen oxide (NOx) emission. Hence, improved selective catalytic reduction of NOx with ammonia (NH3-SCR) in the presence of alkali metals is a major issue for practical NOx removal. In this work, we developed a novel TiO2-decorated acid-treated MnOx octahedral molecular sieve (OMS-5(H)@TiO2) catalyst with improved alkali-resistant NOx reduction at low temperature, and the dual promotional effects of OMS-5(H)@TiO2 catalysts were clarified. It was found that the special structure of the acid-treated MnOx octahedral molecular sieve (OMS-5(H)) was responsible for the trapping of alkali metals and high deNOx activity at low temperature. Subsequently, the decoration by TiO2 further improved the redox properties by accelerating the high ratio of Mn4+ and Oα on the surface of the highly active (OMS-5(H)@TiO2) catalyst. Moreover, a thorough mechanism study via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs) demonstrated that the acid treatment led to remarkable increment of acid sites, which enabled the catalyst to resist alkali metals in the form of ion exchange. Meanwhile, the decoration of TiO2 further increased the strength of the Lewis acid sites, assisting more active intermediate species to effectively take part in the deNOx reaction. Besides, a “fast SCR” process was observed to certify that the decoration of TiO2 promoted the improvement of low-temperature activity in the presence of alkali metals. The dual effects combining OMS-5(H) with TiO2 decoration in terms of alkali metal resistance and high catalytic activity at low temperature proved that the high-performance deNOx catalyst was successfully developed in this work. The work paves a way for the development of superior low-temperature SCR catalysts with improved NOx reduction efficiency in the presence of alkali metals.