Reaction pathway design and optimization in heterogeneous catalysis .2. Oscillating partial oxidation of propene on Pt/SiO2 designed by widely varying dispersion

The oxidation of propene by nitrous oxide has been studied in detail on Pt/SiO2 of widely varying dispersion (Dh) at 160-350 degrees C, using a tubular flow reactor under atmospheric pressure. The partial and total oxidations proceeded in parallel on two different active sites and the catalytic oscillation appeared only in the partial oxidation to form propanal (PA). For the total oxidation route, it was elucidated that the reaction was exact structure insensitive with an activation energy of 85 +/- 5 kJ/mol through all Dh's and that there were stable surface intermediates consisting of four different species each of which could produce CO2 and H2O by the reaction with N2O or produce propane by the consecutive reaction with H-2. The turnover frequency, amplitude, period and selectivity of the oscillating PA were significantly varied depending on the percentage of platinum surface exposed, clearly indicating a strong structure-sensitive oscillation. As a possible trigger for the oscillations, the surface coverage fluctuation of weakly adsorbed propene was a reasonable choice because of its actual oscillatory behavior from theta = 0.07 to 0.28 which was synchronized with the responses of the oscillatory reaction, The reaction pathway selectivity to form PA was found to be 89% for the catalyst with Dh = 80% indicating a better catalyst which can be designed by changing Dh.