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

•Palladium nano-particles (Pdn) catalyze the reduction of Methylene Green (MG) by N2H4.•Catalyzed reaction shows an induction time which accounts for the restructuring of Pdn surface.•Rate of adsorbate induced surface restructuring depends on [N2H4] but is independent of [MG].•Only N2H4 is adsorbed on Pdn surface before it reacts with MG in rate step.•Convex Arrhenius plot indicates the change in nature of transition state with temperature.

Methylene Green (MG), a thiazine dye, is catalytically reduced by N2H4 in presence of palladium nano-particles (Pdn) in buffer media. The observed rate, ko 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.

Graphical abstractPalladium nano-particles (Pdn) catalyze the reduction of Methylene Green (MG) by N2H4 and the observed rates though increase with increase in [Pdn], [N2H4] and pH but decrease with increase in I (maintained with NaNO3). The catalyzed reaction shows an induction period (t0) and 1/t0, the surface restructuring rate depends on [N2H4] and [NO3−]. Adsorption of NO3− turns the surface negatively charged and N2H4 is adsorbed on this surface before reducing MG through a transient reactants-association.Figure optionsDownload full-size imageDownload as PowerPoint slide