Lean NOx reduction over Ag/Al2O3 catalysts using future fuels

The focus of this work is to increase the understanding of the lean NOx reduction over silver-alumina catalysts using hydrocarbons as the reductant (HC-SCR). In particular, the effect of silver loading, the role of platinum doping, the effect of the reducing agent as well as the effects of aging and hydrothermal treatment on the catalytic activity, in particular at low temperatures, and selectivity, were investigated. Silver-alumina samples were prepared using a sol-gel method, including freeze-drying, and evaluated for HC-SCR. The samples were synthesized with varying silver loading, with and without addition of trace amounts of platinum. Activity studies, hydrothermal treatment and oxidation experiments were performed in a synthetic gas-bench reactor using model hydrocarbons (n-octane, ethane, ethane, ethyne) and a commercial bio-diesel (NExBTL) as the reducing agent. The samples were characterized with respect to surface area (BET) and nature and relative amount of surface silver species (UV-vis, XPS). The results show that silver-alumina catalysts, prepared via sol-gel synthesis, display a high consistency regarding surface area and the relative amount of different silver species, as the silver loading is varied. It can be seen that activation for oxidation generally proceeds more easily with increasing bond order of the hydrocarbon. Furthermore, the use of hydrocarbons with high bond order, that is ethyne, as reductant for NOx results not only in the highest peak activity for lean NOx reduction but also in considerable high activity in a wide temperature range mainly thanks to high activity at low temperatures. With increasing silver loading, the oxidation reactions are favored such that both the hydrocarbon and the NO activation occur at lower temperatures. As the hydrocarbon chain-length is increased, the NOx reduction decreases, however, high activity can be seen even when using the commercial bio-diesel. For the pure silver-alumina samples tested with NExBTL, the 6 wt% Ag sample displays the highest activity reaching 60% at 350°C. Furthermore, it is concluded that as the samples are doped with trace-amounts of platinum, the activity for lean NOx reduction at low temperatures is enhanced. The catalyst composition showing the highest activity for NOx reduction, using n-octane as reducing agent, is found to be a 2 wt% loaded Ag/Al2O3 sample doped with 500 ppm platinum. This catalyst also displays the highest low-temperature activity (51% NOx reduction at 300°C), most likely owing to an increased ability to partially oxidize the hydrocarbon reductant as well as higher adsorption of the hydrocarbon on the surface, attributed to the Pt doping.