Oxidation of an organic dye catalyzed by MnOx nanoparticles
Graphical abstract
The catalysis of the oxidation of morin by H2O2 in the presence of colloidal manganese oxide particles is studied. It is demonstrated that this reaction proceeds on surface of the MnOx particles.
Research highlights
► We model the catalytic oxidation of morin by hydrogenperoxide in the presence of colloidal MnOx in a quantitative fashion. ► We demonstrate that the ultrathin MnOx particles present highly active catalysts for this oxidation. ► We demonstrate that the rate-determining step takes place on the surface of the colloidal MnOx. ► We analyze the dependence of this reaction on temperature and determine the respective kinetic and thermodynamic parameters.
Introduction
In the last decade, manganese oxide nanostructures have become systems of particular interest as catalysts for oxidation reactions [1], [2], [3], [4], [5]. There is a wide variety of polymorphs of Mn(IV) oxides such as α-, β-, γ- and δ-type MnO2 that differ in their respective linkage of the basic structure, the [MnO6] octahedron [6]. In general, manganese oxide nanoparticles (MnOxNP) are good catalysts for the catalytic decomposition of hydrogen peroxide (H2O2) [7], [8], [9]. For layered manganese oxides, the total surface can be increased by exfoliation that leads to an enhanced catalytic activity [10], [11]. However, only a few synthetic routes are known by now which create delaminated or highly exfoliated birnessite nanoparticles, most of them being multi-step approaches [12], [13], [14]. Since nanoparticles are mostly generated by solution-based methods, aggregation may occur under the conditions of catalytic reactions. This process may lead to a marked decrease in the catalytic activity with time.
Recently, we presented a new and facile room temperature method to generate and stabilize nanometer scale layered MnOxNP onto cationic spherical polyelectrolyte brushes (SPB) [15]. Fig. 1 displays the schematic representation of the composite particles MnOxNP@SPB with a corresponding cryogenic transmission electron microscopy (cryoTEM) micrograph. The SPB consist of a solid polystyrene (PS) core onto which cationic polyelectrolyte chains are densely grafted [16]. By adding potassium permanganate to a cationic SPB at room temperature, MnOxNP are formed directly on the carrier particles. The reduction of KMnO4 leads to platelets of birnessite that are affixed to the core particles by interaction with the cationic chains. This fixation prevents the coagulation or coarsening of the nanoparticles in an effective way [16]. The composite particles that consist of the SPB together with the immobilized MnOxNP exhibit an excellent colloidal stability [15].
Here, we present a study of the catalytic activity of these MnOxNP@SPB [15]. The oxidation of morin with H2O2 was chosen as a model reaction to analyze the mechanism of the catalysis in the presence of the MnOxNP@SPB. Morin belongs to a group of flavonoid plant dyes (see Fig. 2) [17]. These polyphenolic dyes are present in tea, fruits and vegetables and can be used as model compounds for studying bleaching processes in laundry detergents [18]. Previous work has demonstrated that the oxidation of dyes by hydrogen peroxide is catalyzed by colloidal manganese oxide. Moreover, clear evidence was found that this oxidation is related to the surface of the particles. For example, Segal et al. demonstrated that cyanine dyes can be decomposed using well-defined manganese oxide catalysts and concluded that the reaction takes place on the surface of the particles [19]. Gemeay et al. studied the oxidative decolorization of organic dyes on polyaniline/MnO2 composites [20]. During this study, evidence for the competing adsorption of both the dye and H2O2 on the surface of the catalyst was given. This was argued from the fact that the rate constant goes through a distinct maximum as the function of the concentration of H2O2, while it decreases with increasing concentration of the dye. Zhang et al. analyzed the oxidation of methylene blue with H2O2 on β-MnO2 nanorods [21] and concluded that methylene blue and H2O2 adsorb onto the manganese oxide surface where the reaction takes place. It is also interesting to note that the heterogeneous epoxidation of various alkenes with H2O2 in the presence of manganese oxides studied by Qi et al. is surface-controlled as well [22]. Here, a reaction mechanism that involves a surface-bound Mn2+ was proposed. Up to now, however, a full kinetic study of the oxidation of a dye in the presence of manganese oxide is still missing.
The present investigation gives a full kinetic study of the decomposition of a dye in the presence of MnOx nanoparticles. Since the composite particles MnOxNP@SPB exhibit an excellent colloidal stability, a precise analysis of their catalytic activity in solution can be done. No problems as, for example, coagulation or coarsening which would lead to a much smaller active surface are hampering the analysis. Hence, the influence of the surface on the kinetics can be studied quantitatively. Moreover, all results will be compared to recent studies on the catalytic activity of manganese ions in aqueous solution [17], [23], [24], [25]. The kinetic study presented here will also provide a firm basis for future technical application of these systems in detergent formulations.
Section snippets
Materials
All chemicals were of analytical grade and used without further purification. 2-Trimethylammonium ethyl methacrylate chloride (TMAEMC) was received from Polysciences. KMnO4, Na2CO3 and NaHCO3 were purchased by Fluka, and H2O2 and morin hydrate were received from Sigma–Aldrich. The boric acid buffer solution was purchased from Carl Roth. The water used here was 18 MΩ Millipore water.
Synthesis of MnOxNP immobilized on SPB
The cationic SPB TMAEMC-40 was synthesized and characterized as described recently [26]. The synthesis of the
Results and discussion
The synthesis and characterization of the colloidal carrier particles has been described in detail in earlier work [15]. TEM and dynamic light scattering analysis gave a PS core radius of 42.7 ± 0.3 nm and an average thickness of the polyelectrolyte shell of 42.0 ± 0.8 nm. After the addition of KMnO4 solution to the aqueous dispersion of the cationic SPB, the onset of nanoparticle formation could directly be followed by a change in the color from purple to brown due to OH−-catalyzed reduction of the
Conclusion
We presented a kinetic study of the catalytic oxidation of morin by H2O2 in aqueous solution using c*-disordered birnessite nanoparticles immobilized on cationic SPB as catalyst. The analysis of the kinetic data suggested that the rate-determining step takes place on the surface of the nanoparticles. Both reactants need to be adsorbed onto the surface of the catalyst in order to react. The adsorption process and the surface reaction are described by the thermodynamic adsorption constants K of
Acknowledgments
We thank the Deutsche Forschungsgemeinschaft, and the Henkel AG & Co. KGaA for the financial support. The authors are indebted to W. von Rybinski and A. Hätzelt for helpful discussion.
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