In catalytic reactions involving molecular oxygen, activation of oxygen by dissociative adsorption can be an important step. When the rate of this step is slow compared to other steps in the reaction, the activation of other oxygen sources and the formation of products that require smaller number of oxygen atoms become more competitive. This can be accomplished by highly dispersing the redox centers in the catalyst, which are commonly transition metal ions. This hypothesis can be used to explain the results of the catalytic lean NOx reduction by hydrocarbon over mixed metal oxides, as well as catalytic selective oxidation of butane and selective epoxidation of alkenes. However, when the activation of molecular oxygen is a crucial step in determining the overall activity of the catalyst, possibly as in the NH3-SCR reaction over V/TiO2, a balance between the ability of the catalyst to activate molecular oxygen and the selectivity to N-2 when NO is reduced will determine the form of a desirable catalyst.