Kinetics of deep oxidation of n-hexane and toluene over Pt/Al2O3 catalysts -: Platinum crystallite size effect

The kinetics of n-hexane and toluene complete oxidation has been studied over 0.12% Pt/Al2O3 catalysts. Measurements were performed in an external recycle reactor. Reaction rates were measured as a function of temperature and reactants concentrations for two platinum crystallite sizes, 1.0 and 15.5 nm. The turnover frequency (TOF) for the oxidation of both hydrocarbons is about 10 times higher for the 15.5 nm than for the 1.0 nm Pt crystallite size. The reaction constants are calculated according to Mars van Krevelen (MVK) mechanism. The oxygen chemisorption rate constant (k(O)) is the same during the n-hexane and the toluene oxidation for each sizes of platinum crystallite. For both hydrocarbons the oxygen chemisorption rate constant and the surface reaction rate constant (k(i)) are higher for 15.5 nm than for 1.0 nm crystallite size. The activation energy for k(O) decreases with the increase of the Pt crystallite size. The activation energy for k(i) is independent on the Pt crystallite size for both hydrocarbons. The values of kinetic constants reveal that the increase of the reaction rate for large Pt crystallites could be related to the lower activation energy for k(O). This implies that on large Pt crystallites oxygen is held on Pt atoms with a lower bond strength that on small Pt crystallites, which explains the structure sensitivity found for n-hexane and toluene oxidation. (C) 2004 Elsevier B.V. All rights reserved.