Kinetics of deep oxidation of n-hexane and toluene over Pt/Al2O3 catalysts: Oxidation of mixture

The oxidation of n-hexane and toluene mixtures in air over Pt/Al2O3 catalysts with mean Pt crystallite sizes of 1.0 and 15.5 nm has been studied. Measurements were performed in an external recycle reactor at temperatures in the range of 120–181 °C, in a concentration range of one hydrocarbon from 500 to 10,000 ppm. The mutual inhibition of the oxidation activity of n-hexane and toluene in the mixtures was observed for both Pt crystallite sizes. The structure sensitivity previously established for the oxidation of single components remains valid for their mixture. The mixture model oxidation based on a Mars van Krevelen mechanism fits very well experimental data at lower temperatures (for n-hexane: 148–160 °C; for toluene: 120–140 °C), whereas at higher temperatures the model predicts higher values for catalysts with both Pt crystallite sizes. The mixture model based on equilibrium adsorption of hydrocarbons and strong adsorption of oxygen fits exceptionally well experimental data in the whole investigated temperature range. For catalysts with a mean Pt crystallite size of 1.0 nm the evaluated ratio of adsorption coefficients of n-hexane and toluene is 9, and for the catalysts with a mean Pt crystallite size of 15.5 nm the ratio is 0.13. The evaluated ratios of adsorption coefficients point out that n-hexane is more strongly bound than toluene for catalysts with smaller Pt crystallites, while at larger Pt crystallites toluene is more strongly adsorbed than n-hexane.