The enhanced catalytic activities of octahedral layer birnessite-type manganese oxide synthesized via precipitation method for the degradation of methylene blue

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The catalytic activities of the octahedral layer birnessite-type manganese oxides for the degradation of methylene blue (MB) dye have been studied. The oxide catalysts were prepared via a facile precipitation method using the reaction between KMnO4 (oxidant) and citric acid or oxalic acid (reductant), where the acid is added dropwise into KMnO4 until certain volume achieved to generate a brownish black precipitate. The solid product was treated with either calcination or without calcination process, respectively. The samples without calcination display poor crystallinity with low intensity and broad peak and are identified as birnessite-type manganese oxides. The calcination, however, led to the formation of the highly crystalline birnessite materials. The birnessite catalysts synthesized by the different reductants displayed different crystallinity, morphology, average oxidation state (AOS) of manganese, zero-point charge pH (pHzpc) and particle size that affect their catalytic activities. The results indicated that the activities of birnessite-type manganese oxide catalysts for MB degradation correlated with the nature of catalysts. The citric acid-prepared manganese oxide catalyst (CAC) showed slight higher crystalline phase compared with the oxalic acid-prepared manganese oxide catalyst (OAC). The data from AOS of manganese suggested that the CAC catalysts have a lower value of AOS of manganese than OAC catalysts, indicating more defects due to the presence of more oxygen vacancy or more Mn3+ content. The smaller particle size and AOS of manganese for CAC catalysts are also considered to be responsible for their higher catalytic activities for MB degradation. The birnessite-type manganese oxide catalysts prepared without calcinations also showed much enhanced catalytic activities for MB degradation.