The role of MnO2 crystal morphological scale and crystal structure in selective catalytic degradation of azo dye

MnO2, as a representative manganese-based catalyst with many kinds of crystal forms, has been widely used to activate PMS. However, the role of morphological scale and crystal structures on the catalytic capability of MnO2 still lacks further study. In this study, four different crystal forms of MnO2 (alpha-MnO2, beta-MnO2, gamma-MnO2, and delta-MnO2) are succeeded in being fabricated via hydrothermal processes and evaluated by activating PMS for the removal of Reactive Yellow X-RG, typical azo dye. Experiment results indicate that alpha-MnO2 with a one-dimensional structure exhibits the best catalytic performance among the four as-prepared MnO2, which can be attributed to its broadest crystal interplanar distance (0.692), the highest portion of Mn (III)/Mn (IV) (4.194), and lowest value of average oxidation state AOS (2.696). Correlation analysis confirms that interplanar distance is the most relative factor with the catalytic activity of MnO2 among the three studied factors (R-2 =0.99715). Meanwhile, the morphological scale structure of alpha-MnO2 can also account for its highest catalytic ability among the four as-prepared MnO2, including its large specific area and advantageous one-dimensional nanostructure. Furthermore, according to the response surface methodology, when the dosage of PMS is 2.369 g/L, the dosage of alpha-MnO2 is 0.991 g/L, and the initial dye concentration is 1025 mg/L, the maximum removal rate of Reactive Yellow X-RG is up to 97.38%.