ABSTRACT
The removal of NOx from exhaust gases also containing oxygen is accomplished in two basic ways. One process, identified as selective catalytic reduction (SCR), involves the selective reduction of the NOx using ammonia as the reducing agent. The second type of method relies on the selective adsorption of the NOx and its subsequent release in a concentrated form in an O2-free gas flow, which then passes together with a reducing gas such as H2, CO, or methane over a reduction catalyst. The catalyst used by the SCR process is vanadium pentoxide and titanium dioxide on a support, usually aluminum oxide of the gamma species. This catalyst is actually a chemisorbent composed of an alkali carbonate impregnated into granular, spherical, or microspheroidal alumina. Catalysts that are useful are copper chromite or precious metals on a refractory oxide in the form of granules, cylinders, rings, or honeycomb.
