Abstract
Nanostructured manganese oxides, e.g. MnO2, have shown laccase-like catalytic activities, and are thus promising for pollutant oxidation in wastewater treatment. We have systematically compared the laccase-like reactivity of manganese oxide nanomaterials of different crystallinity, including α-, β-, γ-, δ-, and ɛ-MnO2, and Mn3O4, with 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) and 17β-estradiol (E2) as the probing substrates. The reaction rate behaviors were examined with regard to substrate oxidation and oxygen reduction to evaluate the laccase-like catalysis of the materials, among which γ-MnO2 exhibits the best performance. Cyclic voltammetry (CV) was employed to assess the six MnOx nanomaterials, and the results correlate well with their laccase-like catalytic activities. The findings help understand the mechanisms of and the factors controlling the laccase-like reactivity of different manganese oxides nanomaterials, and provide a basis for future design and application of MnOx-based catalysts.
Highlights
Enzyme-catalyzed oxidative reactions are increasingly examined as an alternative approach to water/wastewater treatment and soil remediation for decomposing organic pollutants, but are limited by enzyme denaturation and cost
Certain manganese oxides (MnOx) can oxidize substrates via single electron transfer[10, 11, 17, 18], while the resultant reduced manganese oxides MnOxred can be re-oxidized to MnOx by dissolved oxygen that is reduced to water under certain conditions[19,20,21] (Fig. 1 part b), leading to a net result of electron shuttling from substrates to oxygen, like laccase
It should be noted that a manganese oxide, or its nano-sized form, could not be regarded as a catalyst, unless it can undergo both the substrate oxidation and the oxygen reduction reactions illustrated in Fig. 1(b), so that MnOx can complete a full redox cycle to return to its oxidation state
Summary
Enzyme-catalyzed oxidative reactions are increasingly examined as an alternative approach to water/wastewater treatment and soil remediation for decomposing organic pollutants, but are limited by enzyme denaturation and cost. Manganese oxides nanomaterials have recently been found to exhibit reactivity similar to laccase, a phenol oxidase that has promising application in pollution control, and have been named “nanozymes” in recent studies[5,6,7] Such naming may be premature, given that no comprehensive information is available regarding the laccase-like activity of manganese oxides nanomaterials. It is well known that the copper-cluster enzyme laccase catalyzes one-electron oxidative reaction of substrates, while molecular oxygen undergoes four-electron reduction to water as shown in Fig. 1 (part a), in which the copper redox reactivity plays a critical role in shuttling the electrons from substrates to oxygen[15, 16]. It should be noted that the catalytic performance of a manganese oxide is dependent on both sides of the redox cycle shown in Fig. 1(b), i.e. the substrate oxidation and the oxygen reduction, and the overall rate is limited by the slower side
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