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The kinetics of NO oxidation was performed over well-defined catalysts, Pt 0 /SiO 2 and Pt x + /CeZrO 2 being chosen as model zero-valent and oxidized platinum active sites, respectively. On both catalysts, the global rate equation was determined, leading to the partial orders in NO and O 2 , and global rate constants. For both catalysts the orders in NO and O 2 are fractional and positive. Based on these data, two sequences of elementary steps are proposed. From the corresponding catalytic cycles, detailed kinetic rate laws were established leading to relevant kinetic constants and activation energies. Transient simulations were performed using the global power rate law and the detailed rate equations on both catalysts. A good fitting was observed up to 15% conversion on Pt 0 /SiO 2 , between 420 K and 500 K, with the global rate equation. The detailed kinetic data allowed a better fitting. For higher conversion the Arrhenius plots allowed us to conclude that diffusion limitations must be considered. A good fitting was observed over Pt x + /CeZrO 2 , between 420 K and 700 K, up to 25% conversion as expected with both global and detailed equations. Thermodynamic limitations are observed, in this case, above 30% conversion. |
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