Kinetics of palladium nano-particles catalyzed reduction of Methylene Green by hydrazine: Role of induction period in determining mechanistic pathway

Methylene Green (MG), a thiazine dye, is catalytically reduced by N2H4 in presence of palladium nanoparticles (Pdn) in buffer media. The observed rate, k(o) increases with increase in [Pdn], [N2H4] and pH but decrease with increase in ionic strength (maintained with NaNO3). Unlike other thiazine dyes, the catalyzed reduction of MG shows an induction period which is caused by the restructuring of Pdn surface. The rate of adsorbate-induced surface restructuring which increase with increase in [N2H4] but independent of [MG], suggest that only N2H4 is adsorbed on Pdn surface during the catalyzed reaction. Interestingly, NO3 ions are also adsorbed on the Pdn surface and turn the surface negatively charged. The reductant, N2H4 is adsorbed on this negatively charged Pdn surface before it reacts with MG. The surface restructuring energy of Pdn, though is constant (78.8 +/- 1.6 kJ M-1) over the temperature range of 288-313 K but the activation energy for the catalyzed reduction varies with temperatures. The convex Arrhenius plot illustrates that activation energy is different in lower (288-304 K, 123.8 +/- 12.6 kJ M-1) and higher temperature range (304-313 K, 69.2 +/- 2.1 kJ M-1) and around the inversion temperature, Tinv (304 K) a change in the nature of transition state takes place. (C) 2015 Elsevier B.V. All rights reserved.