Deactivation Mechanism of Spent TiO2-Based Selective Catalytic Reduction (SCR) Catalysts and State-of-the-Art Recycling Technologies

Nitrogen oxides (NO x ) make up a group of gases that are mainly formed during the combustion of fossil fuels at high temperatures. NO x contributes to environmental degradation by forming acid rain and enhancing global warming. Exposure to air with a high concentration of NO x can aggravate respiratory diseases, particularly asthma. The elimination of NO x in practical applications mainly proceeds through selective catalytic reduction (SCR) of NO x with NH3 to harmless N2 and H2O. One practical issue of the SCR process is the deactivation of TiO2-based SCR catalysts. In addition, growing numbers of spent SCR catalysts present a serious waste-management challenge for recyclers at end of life. It is therefore crucial to disclose the deactivation mechanism of the spent TiO2-based NH3-SCR catalysts and propose effective recycling technologies for waste valorization. Here we outline the catalyst deactivation pathways for TiO2-based SCR catalysts and critically review methods of improving the direct sustainability of spent catalysts, in areas such as direct regeneration and recovery of spent catalysts. Through this review, the challenges, solutions, and future strategies for handling spent SCR catalysts are clarified for future studies of application on an industrial scale.