Kinetic Studies of NO Oxidation and Reduction over Silver-Alumina Catalyst
Licentiate thesis, 2013
In line with growing concerns to manufacture more environmentally friendly vehicles, the use of internal combustion engines operating with oxygen excess or so called lean-burn engines will continue to be increasingly used. For lean-burn operation, reduction of NOx (NO+NO2) emissions is a major challenge and it is therefore urgently required to develop efficient and reliable NOx reduction aftertreatment systems for a wide variety of lean-burn or diesel engines. The main goal of this thesis is to increase the understanding of the reaction mechanism of selective catalytic reduction (SCR) of NOx with a hydrocarbon (HC) reductant over Ag-Al2O3 catalysts.
As an important subsystem in the HC-SCR mechanism, H2 assisted NO oxidation over a monolith-supported Ag-Al2O3 catalyst was investigated by constructing a microkinetic model that accounted for heat and mass transport in the catalyst washcoat. The effect of H2 examined in the kinetic model, was to reduce self-inhibiting surface nitrate species on active sites. A reduced factorial design of the inlet experimental conditions was used to generate transient experimental data. In general, the modelling results could reproduce the transient experimental data well with correct levels of outlet concentrations and time scales for transient responses. When H2 was present in the feed, the kinetic model showed that H2 was consumed rapidly in the front part of the monolith. This indicated that the H2 promotion of the NO oxidation reaction may have been isolated to only a portion of the catalyst.
A series of temperature-programmed desorption (TPD) studies of NOx were conducted over Ag-Al2O3 catalysts to quantify and characterize the stability of surface NOx species. Formation of two general groups of surface NOx species were found to be present: a less thermally stable group of so called “low temperature (LT) nitrates” and a more thermally stable group of “high temperature (HT) nitrates”. The LT NOx desorption peak could be attributed to the decomposition of nitrate species formed on the active sites. Elimination or decrease in quantities of these LT nitrates either thermally or by reaction with H2 resulted in higher NO oxidation and NOx reduction conversion. The HT NOx desorption peak primarily corresponded to the decomposition of nitrates on the Al2O3 support and could be considered spectator surface species. It was also found that H2 facilitates formation of nitrate on the Al2O3 support and it was indicative that the mechanism of NOx storage on the Al2O3 support was mainly via NO2 readsorption. From TPD studies of C3H6-SCR in the presence and absence of H2, it was shown that the presence of H2 not only eliminated LT nitrates but also promoted the formation of adsorbed hydrocarbons. Therefore, the dual role of H2 to both eliminate nitrates from active sites and promote NOx storage was elucidated.