Reaction Kinetics of C3H6 Oxidation for Various Reaction Pathways Over Diesel Oxidation Catalysts

摘要

Reaction kinetics studies of C3H6 oxidation over Pt/Al2O3 and Pt/SiO2 catalysts were characterized using temperature-programmed oxidation with different oxidants: O2, NO2 and surface nitrates/Activation energies and conversion performance were compared in order to determine which hydrocarbon oxidation reaction pathway(s) is relevant in diesel exhaust gas aftertreatment applications. NO_x adsorption did not occur on the SiO2 surface so the reaction between C3H6 and NO2 could be isolated, i.e. no nitrate effect would complicate the analysis and their significance could be decoupled. These results were then compared with Pt/Al2O3 where surface nitrates did form upon exposure to NO_x. The onset of C3H6 oxidation was observed at a lower temperature with O2 than with NO2, but the activation energy was lower with NO2. This apparent discrepancy is related to the different oxidant concentrations used and the different adsorption pathways. The results indicate that hydrocarbons must be activated first for oxidation to begin, for either NO2 or O2. In analyzing the reaction between C3H6 and nitrates, the reaction did not occur until NO_x started to desorb from the catalyst at higher temperatures, i.e. when nitrates became unstable and decomposed, thus providing a readily available oxidant source. However, when O2 was added to the nitrate/C3H6 system, the reaction began at even lower temperature than with just C3H6 and O2. Nitrate consumption was also observed once oxidation began. The presence of the combination of nitrates and O2 resulted in a lower C3H6 oxidation activation energy.