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

MnO 2 , 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 MnO 2 still lacks further study. In this study, four different crystal forms of MnO 2 (α-MnO 2 , β-MnO 2 , γ-MnO 2 , and δ-MnO 2 ) 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 α-MnO 2 with a one-dimensional structure exhibits the best catalytic performance among the four as-prepared MnO 2 , 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 MnO 2 among the three studied factors ( R 2  = 0.99715). Meanwhile, the morphological scale structure of α-MnO 2 can also account for its highest catalytic ability among the four as-prepared MnO 2 , 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 α-MnO 2 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%. Graphical abstract