Surface acidity enhancement of CeO2 catalysts via modification with a heteropoly acid for the selective catalytic reduction of NO with ammonia

Abstract

Vanadium-free catalysts for ammonia selective catalytic reduction (NH₃-SCR), such as CeO₂-based catalysts, often have low activities over a wide reaction window, and low N₂ selectivity. These limitations can be overcome by increasing the surface acidity of the catalyst and adjusting its surface defects and vacancies. In this study, a series of silicotungstic acid (HSiW)-modified CeO₂ samples was prepared, and their NH₃-SCR performances were evaluated. The experimental results indicated that 10% HSiW/Ce achieved a NO conversion of over 90% in the temperature region of 226–400 °C with 99% N₂ selectivity. The HSiW was well-dispersed over the surface of CeO₂ and maintained its Keggin-type structure, leading to an increase in Brønsted acid sites in the catalyst. The adsorption of NH₃ during the SCR reaction was greatly improved, resulting in higher activity. In addition, the incorporation of HSiW changed the redox properties of CeO₂ by decreasing the concentration of Ce³⁺ and vacancies. During the reaction process, the peroxidation of NH₃ at high temperature was inhibited due to the decreased surface-adsorbed oxygen and Ce³⁺, leading to improved N₂ selectivity. The reaction mechanism was also studied using in situ diffuse reflectance infrared Fourier transform (DRIFT) experiments. Although both the L–H and E–R mechanisms could occur during the SCR reaction, the E–R mechanism may dominate under the experimental conditions. Overall, modification with HSiW is a satisfactory method to improve the SCR activity of CeO₂.