Structure sensitivity of dimethylamine deep oxidation over Pt/Al2O3 catalysts

Abstract

The deep oxidation of dimethylamine (DMA) was studied over Pt/Al2O3 catalysts with small (1 nm) and large (7.8-15.5 nm) Pt crystallite sizes. The turnover frequency (TOF) was higher for the large than for the small Pt crystallites, indicating that the reaction is structure sensitive. Two kinetic models were used to interpret the obtained results, i.e., the Mars van Krevelen and a mechanism based on the adsorption of oxygen and adsorption of dimethylamine on different active sites were employed. Both models showed that the activation energy for the oxygen chemisorption rate constant (k(o)) decreased with increasing of Pt crystallite size and that the activation energy for the surface reaction rate constant (k(i)) was independent of the Pt crystallite size. The structure sensitivity may be explained by differences in the reactivity of the oxygen adsorbed on these Pt crystallites. The Mars van Krevelen model fits the TOF values very well at concentrations of DMA higher than 1500 ppm, while in the lower concentrations region, the model under predicts the experimental data. The model based on the adsorption of oxygen and DMA on different active sites fits the experimental data quite well over the whole temperature and concentration range. The fitted values of the Henry adsorption constant are independent of the Pt crystallite size.