Catalytic selective reduction of NO(x) to N(2) using a hydrocarbon in an oxidizing atmosphere is challenging because of the competitive reaction of hydrocarbon combustion. Alumina is a rather effective catalyst for this reaction, but it requires high operating temperatures. The activity can be enhanced by adding other catalytic components, such as Pt, Ag, CoO(x), or SnO(2). Many of these catalyst systems are bifunctional. A substantial portion of the reaction pathway for propene reduction of NO(x) has been elucidated for SnO(2)/Al(2)O(3). The function of SnO(2) is to oxidize the propene to acrolein, which is then transferred to alumina where it reacts to form acetaldehyde. Reduction of NO(x) by acetaldehyde on alumina leads to N(2). The bifuncational aspect of the catalyst systems offers the potential to improve the catalytic performance by optimizing the individual components.