Catalytic ozonation of NOx into HNO3 with low concentration ozone over MnOx-CeO2/TiO2: Two-phase synergistic effect of TiO2

Abstract

Anatase TiO2 (TiO2-A), TiO2-B, and TiO2-B/anatase mixed-phase TiO2 (TiO2-AB) are synthesized, and then utilized as supports for the preparation of Mn-Ce/TiO2-A, Mn-Ce/TiO2-B and Mn-Ce/TiO2-AB, respectively. These catalysts are investigated by various characterizations to understand different phase effect of TiO2 on physicochemical properties and catalytic activities for ozonation of NOx into HNO3, aiming to improve the total NOx removal efficiency during the subsequent wet absorption process. Characterizations reveal that the catalytic performances follow the order of Mn-Ce/TiO2-AB > Mn-Ce/TiO2-B > Mn-Ce/TiO2-A, which is relevant to the surface -OH content, oxygen vacancy amount, and reducibility property. The surface -OH groups participate in the transformation of O3 into reactive oxygen species (ROS), the oxygen vacancies increase the activity of surface -OH for ROS generation, and the superior reducibility property accelerates the cycling of Ce3+ → Ce4+ → Ce3+ reaction. Moreover, OH and HO2radicals are identified as the main reactive species involved in the ozonation of NOx into HNO3. Especially, Mn-Ce/TiO2-AB presents the best catalytic performance mainly because of the optimal reducibility property.