Decoupling the Interactions of Hydrocarbons and Oxides of Nitrogen Over Diesel Oxidation Catalysts

摘要

Oxidation of NO to NO_2 over a Diesel Oxidation Catalyst (DOC) plays an important role in different types of aftertreatment systems, by enhancing NOx storage on adsorber catalysts, improving the NOx reduction efficiency of SCR catalysts, and enabling the passive regeneration of Diesel Particulate Filters (DPF). The presence of hydrocarbon (HC) species in the exhaust is known to affect the NO oxidation performance over a DOC; however specific details of this effect, including its underlying mechanism, remain poorly understood. Two major pathways are commonly considered to be responsible for the overall effect: NO oxidation inhibition, due to the presence of HC, and the consumption of the NO_2 produced by reaction with hydrocarbons. In this work we have attempted to decouple these two pathways, by adjusting the catalyst inlet concentrations of NO and NO_2 to the thermodynamic equilibrium levels and measuring the composition changes over the catalyst in the presence of HC species. We also compared the effect of gaseous and stored HC on NO oxidation performance, at different concentrations and using different HC species. We found that the entire body of experimental evidence thus collected can be explained using a single combined inhibition mechanism, complicated by the integral (axial 1-d) nature of the DOC device, as follows. HC adsorbed on or in the close proximity of the active site of precious metal, appear to be effectively modifying its oxidation state by acting as a sink for oxygen atoms, thus both inhibiting NO oxidation and accelerating NO_2 reduction. However, in some cases, presence of stored HC can also minimize over-oxidation of Pt active sites, which is known to negatively affect the reaction kinetics. Axially, in the inlet segment of the catalyst with substantial presence of HC, the incoming NO_2 is effectively reduced, while production of NO_2 is also suppressed, thus erasing the potential benefit afforded by cases of high NO_2 /NOx ratio at the catalyst inlet. The downstream, virtually HC-free, section of the catalyst then acts as a generator of NO_2.