Sustainable Catalytic Activity of Ag-Coated Chitosan-Capped gamma-Fe2O3 Superparamagnetic Binary Nanohybrids (Ag-gamma-Fe2O3@CS) for the Reduction of Environmentally Hazardous Dyes-A Kinetic Study of the Operating Mechanism Analyzing Methyl Orange Reduction

The formation of binary nanohybrids consisting of environmentally benign components, gamma-Fe2O3, chitosan (CS), and Ag (Ag-gamma-Fe2O3@CS) (CSIOAg), containing very low concentration of Ag NPs (<= 1.2 mu M), has been reported. In the as-synthesized nanohybrids, the presence of gamma-Fe2O3 (8.5 +/- 0.8 nm) and Ag (5.9 +/- 0.5 nm) are revealed by optical, XRD, TEM, and XPS analyses, and their presence in cubic phase is determined by XRD and SAED measurements. The catalytic activity of CSIOAg has been analyzed by performing the reduction of certain toxic dyes. Under all kinetic conditions, the reaction is attended by an induction period, which is reduced upon increasing [Ag] and [Dye] in a specific concentration range, as well as temperature, suggesting restructuring of the surface prior to reduction. In case of methyl orange (MO), the reduction results in its cleavage to produce N, N-dimethyl-1,4-phenylenediamine and sodium sulfanilate in a significantly higher (> 97%) yield in a bimolecular reaction between MO and BH4-. The duration of induction period is regularly decreased and the rate of reduction (k(app)) increases linearly with increasing Ag in the wide concentration range (0.03-2.4 mu M). The reduction takes place with a second-order rate constant of 2.7 x 10(4) dm(3) mol(-1) s(-1), which is > 3.5-fold higher than that in the absence of chitosan (IOAg) under identical experimental conditions. The kinetics of reduction of MO is controlled by the nature and extent of its adsorption on the catalyst surface. The weaker binding between MO and Ag catalyst only allows its effective reduction. The XPS analysis of CSIOAg and IOAg containing the same amount of Ag (1.2 mu M) showed its higher amount on the surface of CSIOAg (0.12%) as compared to that of IOAg (0.09%). Detailed kinetic analysis of MO reduction, performed under pseudo-kinetic conditions for both the nanohybrids revealed them to follow Langmuir-Hinshelwood kinetic model and exhibited the recyclability up to 10 cycles with fairly high reaction efficiency and TOF, suggesting it to be a sustainable green nanosystem.