Effects of Co and Ni co-doping on the physicochemical properties of cryptomelane and its enhanced performance on photocatalytic degradation of phenol

Cryptomelane exhibits excellent photocatalytic activity for the degradation of organic pollutants. Incorporation of transition metals (TMs) into the structure of Mn oxides will cause changes to their substructure and physicochemical properties. However, the synergetic effects of incorporation of two kinds of metals have yet to be investigated. Here, cobalt and nickel co-doped cryptomelanes were synthesized with different molar ratios of Co/Ni, and characterized by powder X-ray diffraction, elemental analysis, N-2 physical adsorption and UV-Visible diffuse reflectance spectroscopy, and their photocatalytic performance for the degradation of phenol was investigated. These studies demonstrate that, doping of Co and Ni did not change the crystal structure of cryptomelane, but resulted in decreased crystallinity, and generally increased the specific surface area. Co(III) and Ni(II) were incorporated into cryptomelane and substituted for Mn(IV) and Mn(III), respectively, leading to a change in Mn average oxidation state. Compared with non-doped and singly doped samples, Co and Ni co-doped cryptomelanes have large increase in optical absorption properties and increase the rate of phenol degradation, i.e. the TOC removal rate was increased by 28-38%. Co and Ni co-doped cryptomelane has potential applications in the remediation of natural waters contaminated by organic pollutants. (C) 2014 Elsevier B.V. All rights reserved.