Nitric acid-treated birnessite-type MnO2: An efficient and hydrophobic material for humid ozone decomposition

MnO2 catalysts are important for low-temperature removal of air pollutants. Sometimes the unavoidable moisture is not conductive to the desired reactions, therefore, water-resistant materials are desirable. However, the reported methods are not easily fulfilled for practical applications and the underlying mechanism for the improved hydrophobicity has not been clarified. In this study, highly water-resistant birnessite-type MnO2 was designed for humid ozone (O-3) decomposition, which is important for alleviating global O-3 pollution. The sample treated by nitric acid (H-MnO2) exhibited stable O-3 conversion of similar to 50% within 24 h under 50% of relative humidity (115 ppm of O-3, 600 L.g(-1).h(-1) of space velocity, 25 degrees C). However, the pristine MnO2 was quickly deactivated with O-3 conversion dropping to similar to 10% within 30 min. Detailed characterizations show that higher amount of acid sites and oxygen vacancies together with their improved water-resistant properties facilitates adsorption and subsequent decomposition of humid O-3 over the H-MnO2. DFT calculations demonstrate that compared with the pristine MnO2, oxygen vacancies were more easily formed over the H-MnO2, and the surface of the H-MnO2 was more hydrophobic and more attractive to O-3 adsorption. Finally, a mechanism involving acid sites and oxygen vacancies for gaseous O-3 decomposition was proposed. (C) 2018 Elsevier B.V. All rights reserved.