Lean NOx reduction over silver-alumina from lab-scale to real conditions - Effects of reducing agent and catalyst composition

Doktorsavhandling, 2014

Lean NOx reduction over silver-alumina from lab-scale to real conditions Effects of reducing agent and catalyst composition FREDRIK GUNNARSSON Applied surface chemistry Department of Chemical and Biological Engineering Chalmers University of Technology Abstract The focus of this work is to increase the understanding of lean NOx reduction over silver-alumina catalysts using hydrocarbons as the reducing agent (HC-SCR), in particular by investigating the effect of silver loading, the role of platinum doping, and the effect of the type of reducing agent. In addition, the effects of aging and hydrothermal treatment on the catalytic activity and selectivity, and the influence of real engine exhausts were investigated, with special attention to the low-temperature activity. 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. NOx reduction activity studies, hydrothermal treatment and oxidation experiments were performed in a synthetic gas bench reactor using model hydrocarbons (n-octane, ethane, ethene, ethyne) and commercially available biofuels (NExBTL, ethanol) as the reducing agent. The samples were characterized with respect to surface area (BET) as well as the type and relative amount of surface silver species (UV-vis, XPS). An optimized catalyst was also up-scaled and investigated in an engine bench reactor, with diesel exhausts, using a commercial biofuel as reducing agent, and with hydrogen supplied via a reformer catalyst. 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. Furthermore, it can be observed that activation for hydrocarbon oxidation generally proceeds more easily with increasing bond order of the hydrocarbon. For instance, the use of hydrocarbons with high bond order, e.g. 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 increased activity at low temperatures compared to a corresponding hydrocarbon with lower bond order. With increasing silver loading, the oxidation reactions are favored such that both the hydrocarbon and the NO activation occur at lower temperatures. This is also corroborated by experiments with oxidized hydrocarbons (ethanol) displaying a complete reduction of NOx over a wide temperature range. As the hydrocarbon chain-length is increased, the NOx reduction decreases, however, high activity can be seen even when using the commercial biodiesel, with a reduction of over 70 % in laboratory scale experiments. 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 when using n-octane. The catalyst composition showing the highest activity for NOx reduction, using n-octane as reducing agent, is found to be a 2 wt. % Ag/Al2O3 sample doped with 500 ppm platinum. This catalyst also displays the highest low-temperature activity, 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. The 4 wt. % Ag/Al2O3 sample doped with 100 ppm platinum shows high activity in close to real conditions with the addition of reformate. It can also be concluded that adding reformate instead of pure hydrogen, to the same H2 concentration, results in an equal or even higher activity for NOx reduction, likely owing to the presence of unreacted hydrocarbons in the reformate. In conclusion this work shows HC-SCR as a viable concept for NOx reduction, even though many challenges remain. Low-level platinum doping is shown to increase the activity for NOx reduction, especially at low temperatures and for higher hydrocarbons. In addition, the in-house synthesized catalysts are compatible with commercial biofuels for both heavy-duty (NExBTL) and light-duty (ethanol) vehicles. The real engine experiments also show a promising NOx reduction, both when using pure hydrogen and reformate.