Impact of Interactions between Metal Oxides to Oxidative Reactivity of Manganese Dioxide
2012
Taujale, Saru | Zhang, Huichun
Manganese oxides typically exist as mixtures with other metal oxides in soil–water environments; however, information is only available on their redox activity as single oxides. To bridge this gap, we examined three binary oxide mixtures containing MnO₂ and a secondary metal oxide (Al₂O₃, SiO₂ or TiO₂). The goal was to understand how these secondary oxides affect the oxidative reactivity of MnO₂. SEM images suggest significant heteroaggregation between Al₂O₃ and MnO₂ and to a lesser extent between SiO₂/TiO₂ and MnO₂. Using triclosan and chlorophene as probe compounds, pseudofirst-order kinetic results showed that Al₂O₃ had the strongest inhibitory effect on MnO₂ reactivity, followed by SiO₂ and then TiO₂. Al³⁺ ion or soluble SiO₂ had comparable inhibitory effects as Al₂O₃ or SiO₂, indicating the dominant inhibitory mechanism was surface complexation/precipitation of Al/Si species on MnO₂ surfaces. TiO₂ inhibited MnO₂ reactivity only when a limited amount of triclosan was present. Due to strong adsorption and slow desorption of triclosan by TiO₂, precursor-complex formation between triclosan and MnO₂ was much slower and likely became the new rate-limiting step (as opposed to electron transfer in all other cases). These mechanisms can also explain the observed adsorption behavior of triclosan by the binary oxide mixtures and single oxides.
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