Facile homogeneous precipitation method to prepare MnO2 with high performance in catalytic oxidation of ethyl acetate

Abstract

Altering the urea decomposition rate in the homogeneous precipitation progress enabled the formation of various morphologies of α-MnO 2 . The decrease in crystalline size led to an increase in the surface area and dominance of the (2 1 1) crystal plane. The abundant surface oxygen species and high reducibility related to unsaturated manganese played critical roles in the decomposition of ethyl acetate. • A facile method driven by a synchronous reaction of urea hydrolysis and potassium permanganate reduction was proposed. • Pure α-MnO 2 with various morphologies was obtained by tuning reaction parameters. • Oxygen species on the (2 1 1) crystal plane were found to be most active in oxidation. • The optimal α-MnO 2 catalyst exhibited high and stable catalytic performance. A facile homogeneous precipitation method driven by synchronous reactions of urea hydrolysis and potassium permanganate reduction is proposed for preparation of MnO 2 catalysts. Pure α-MnO 2 nanostructures with different morphologies, including nanowires, nanorods and nanoparticles, were obtained by simply tuning the precipitation conditions. The evolution of materials derived from different preparation conditions was investigated via X-ray diffraction, transmission electron microscopy, N 2 adsorption, X-ray photoelectron spectroscopy, chemisorption and DFT calculations. The precipitation temperature and time significantly influenced the physiochemical properties of as-prepared catalysts. With increasing precipitation temperature and time, the degree of crystallinity, BET surface area and amount of surface-adsorbed oxygen of α-MnO 2 exhibited a substantial increase. The main exposed surface facets varied from (2 0 0), (3 1 0) to (2 1 1) as temperature and time increased. The best precipitation temperature and time were 90 °C and 24 h respectively. The optimal α-MnO 2 catalyst demonstrated 100% conversion of 1000 ppm ethyl acetate under the high space velocity of 78,000 h −1 during a 113-h test at 190 °C, outperforming other manganese oxide catalysts and many typical noble metal catalysts in terms of activity and stability. Experimental results and DFT calculations were consistent and indicated that surface oxygen species originating from the (2 1 1) surface of α-MnO 2 were most active. This study provides important insights for manipulating the morphology of MnO 2 by a facile method and remarkably promoting the performance for ethyl acetate VOC elimination.