In the simplest definition, a catalyst is a substance which speeds up or causes a chemical reaction, but remains unchanged. There are two general types of catalytic systems; homogeneous and heterogeneous. Homogeneous catalysis refers to systems in which the catalyst and the reactants are in the same phase, such as a liquid catalyst acting upon liquid reactants. Heterogeneous systems are more common to air pollution control. These systems operate with the catalyst and the reactants in different phases. A good example of this is the catalytic converter in automobiles. In this case the catalyst is a solid material, while the reactants are contained as gases in the automobile exhaust. Some basic characteristics of catalysts are as follows.

1. In the catalytic reaction, the catalyst is unchanged at the final step of the reaction, although at different phases of the reaction, the catalyst may be present as a different chemical compound.
   
2. When more than one possible reaction can occur between reactants, the catalyst may help to promote a specific desired reaction. This phenomenon is called selectivity, and lends its name to "selective catalytic reduction" referring to the ability of SCR catalyst to catalyze primarily the nitrogen oxide reduction reaction.
   
3. The rate of reduction is generally proportional to the amount of catalyst present. For solid catalyst, parameters such as surface area, number of active sites, and porosity are all important to reaction rate.

BACK

Home	SCR Support	Additional Services	General SCR Info
Contact Us	Ammonia on Fly Ash	Catalyst Development	Tracking Ammonia Slip and Catalyst Deactivation