Selective Catalytic Reduction of NOx over Alumina-Supported Silver and Indium During Lean Operation
Catalytic emission control for vehicles was first applied in the 1970’s. The first such catalysts were designed to oxidize unburned hydrocarbons (HC) and carbon monoxide (CO). A few years later, the oxidation catalyst was further developed into the three-way catalyst (TWC), which efficiently removes CO, HC and nitrogen oxides (NOx under stoichiometric air-to-fuel ratios. However, the awareness of climate changes, caused by anthropogenic emissions of carbon dioxide (CO2), is a major motivator for the development of fuel-efficient engines, operating in excess oxygen (lean) combustion. At these air-to-fuel ratios, the TWC is ineffective for NOx reduction, promoting the development of lean NOx reduction techniques. Among the most promising today is selective catalytic reduction (SCR), for which the silver-alumina (Ag/Al2O3) catalyst shows promising results, both with HC and ammonia (NH3), as the reducing agent.
The current work focuses on the nature of the active sites of the Ag/Al2O3 catalyst and the impact of the structure of the reductant on the selective catalytic reduction of NOx. For this purpose, five different hydrocarbons and oxygenates, all containing two carbon atoms in the structure, as well as NH3, are investigated as reducing agents, with and without the presence of hydrogen. Furthermore, the influence of the active phase is elucidated by exchanging silver for the equivalent molar amount of indium. The catalysts are prepared by incipient wetness impregnation, characterized with regard to specific surface area, crystalline structure, concentration and strength of acidic sites, SCR activity and surface species. The latter are studied by diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy, where spectra from both the fresh samples and from samples subjected to various gas-phase pretreatments, mimicking conditions of the SCR reaction environment, are recorded. The connection between the silver and indium species, and the gas-phase environment is discussed, and it is proposed that species important for the activation of the reducing agent are essential for high SCR activity, both during HC- and NH3-SCR.
Furthermore, quantification of the NO reduction and NH3 slip over Ag/Al2O3 is performed for different locations of the reductant injection spray. The probability of a stoichiometric ammonia dose is higher when the spray is positioned in the center of the exhaust pipe, compared to at the pipe wall. Moreover, the NO conversion increases rapidly with increasing ammonia dose, however, NH3 doses of several times the stoichiometric amount do not improve NO reduction significantly but increase the NH3 slip almost linearly.