Coupled Heterogeneous and Homogeneous Hydrocarbon Oxidation Reactions in Model Diesel Oxidation Catalysts

In testing model diesel oxidation catalysts to treat exhaust from low temperature combustion (LTC) vehicle engines, the simulated exhaust conditions were such that both homogeneous and heterogeneous oxidation of some hydrocarbons took place. When homogeneous oxidation occurred, NO was readily oxidized to NO₂ and the larger hydrocarbon species were partially oxidized to aldehyde, alcohol, and alkene intermediates. The homogeneous oxidation potentials of several hydrocarbons were compared in the absence and presence of a model oxidation catalyst. Of the hydrocarbons evaluated, diethyl ether led to the best NO to NO₂ oxidation in terms of temperature and extent of conversion, and it did not inhibit the oxidation of other hydrocarbon species. The clear evidence of homogeneous reactions occurring within the monolith catalyst channels, and the formation of associated reaction intermediates, may impact reactions that would normally be considered isolated to the catalyst surface. These findings demonstrate that, in modeling such reactions, the homogeneous reactions need to be considered and may influence future catalyst design, especially considering the importance of NO₂ for downstream selective catalytic reduction and particulate filter catalyst systems.