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

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.