The effect of gas composition on hydrogen assisted NH3-SCR over Ag/Al2O3

Journal article, 2013

In addition to high activity in hydrocarbon-SCR, Ag/Al2O3 catalysts show excellent activity for NOx reduction for H2-assisted NH3-SCR already at 200 °C. Here, we study the influence of different gas compositions on the activity of a pre-sulfated 6 wt% Ag/Al2O3 catalyst for NOx reduction, and oxidation of NO and NH3. The catalyst displays high initial activity for NOx reduction with a maximum of about 85% at 250 °C. Increasing the concentration of H2 results in further increased NOx reduction. Moreover, a global stoichiometry between NO: NH3: H2 equal to 1: 1: 2 is established during selective NOx reduction conditions. When increasing the concentration of one of the reducing agents only an increase of the H2 concentration leads to an increase in NOx reduction, while an increase of the NH3 concentration only is beneficial to a limit of an equimolar ratio between of NO and NH3. Under transient conditions at constant temperature, the concentration of NO reaches steady state fast, whereas it takes longer time for NH3 due to accumulated surface species, probably on the alumina. The oxidation of NO to NO2 is sensitive to the H2 concentration in similarity to the SCR reaction, while higher amounts of H2 suppress the oxidation of NH3. Moreover, the dependency on the O2 concentration is much higher for the NO and NH3 oxidation than for the SCR reaction. To explain all these features a reaction mechanism is proposed in which the role of H2 is to free silver from single oxygen atoms. Ammonia and nitric oxygen can adsorb on these sites and react probably on the border between the silver and alumina or on the alumina surface to N2.