Experimental and kinetic studies of H2 effect on lean exhaust aftertreatment processes: HC-SCR and DOC

Doktorsavhandling, 2015

With a growing concern to lower greenhouse gas emissions from road transportation, lean burn and diesel engines will keep playing an important role in the future. Development of a highly efficient and durable process to reduce NOx to N2 becomes a challenging issue especially in the presence of ample O2 concentration as in lean burn exhaust. One way to reduce NOx emissions in lean exhaust is by using hydrocarbon-selective catalytic reduction (HC-SCR). HC-SCR over Ag/Al2O3 catalysts appears to be a promising technology to abate NOx emission in lean burn exhaust. The function of the Diesel oxidation catalyst (DOC), as a part of a lean exhaust aftertreatment process, is to oxidize CO, HC and NO. Interestingly, addition of H2 has been shown to promote the HC-SCR activity over Ag/Al2O3 and NO oxidation activity over Pt/Al2O3 catalyst. The overall focus of this thesis was to increase understanding of the mechanisms of the H2 effect on the model catalysts of Ag-Al2O3 and Pt/Al2O3. A combination of experimental and kinetic modeling approaches was utilized as a way to examine mechanistic effects of H2. Temperature-programmed desorption (TPD) technique was used to characterize thermal stabilities of various surface NOx species formed during NO oxidation and C3H6-SCR conditions over Ag/Al2O3 catalyst. In addition, DRIFTS analysis was used to identify different types of nitrate species. These TPD results elucidated the dual roles of H2 to remove inhibiting nitrate on active sites and facilitate formation of inactive nitrate species mainly on the Al2O3 support. An initial development of a microkinetic model to describe H2-assisted NO oxidation over Ag/Al2O3 was conducted using a set of transient data. The single role of H2 to remove inhibiting nitrate species on active sites was examined. In the further model development, a global kinetic model of H2-assisted C3H6-SCR, including NO oxidation, C3H6 oxidation and C3H6-SCR in the presence and absence of H2, was proposed. This model was based on dual roles of H2 to remove inhibiting nitrates from active sites and simultaneously form more active Ag sites. The model could effectively capture a wide range of feed concentrations and temperatures, including temperature-programmed and transient experiments. The influence of H2 on NO oxidation over Pt/Al2O3 as a DOC catalyst was evaluated with various feed mixtures. Formation of Pt oxide has been known to lower the NO oxidation activity over Pt/Al2O3. The role of H2 to retard the Pt oxide formation was investigated. This resulted in a temporal enhancement in NO2 yield due to H2 addition during temperature ramp experiments. In addition, the effect of C3H6 and CO to influence the NO oxidation was also investigated. Addition of H2 mainly serves to weaken the inhibition effect of C3H6 and to a much lesser degree CO. This is mainly due to an enhancement of lower temperature C3H6 oxidation. The promotional effects of H2 to increase NO2 yield was proposed as a result of effects of H2 on surface chemistry and/or reactions. These effects could be clearly distinguished from exothermal heat effects from mainly H2 but also C3H6 and CO oxidation.