mixture need not be chemically pure to reduce no x suitable selective catalyst reducing scr materials suitable for use with ethanol reductant include alumina supported tin or tin oxides as disclosed in u s pat no 6 030 590 and silver based catalysts as described in u s pat nos 6 045 765 6 057 259 and 6 284 211 and in sae papers 2000 01 2813 and 2001 01 1935 all incorporated herein by reference however because catalyst materials need to be developed in association with the reductant type different fuel borne reductants i e propane butane etc would likely require other catalyst formulations for optimal no x conversion the chemical process whereby no and no 2 are reduced on catalysts have been studied but a series of actual steps has been postulated only multiple steps are known to be involved and they have been shown to vary by type of catalyst type of support chemical nature of the reductant and temperature of the catalyst an excellent review is r burch et al applied catalysis b environmental 39 283 2002 each control system of scr and reductant must be optimized to maximize n 2 yield while minimizing n 2 o a potential greenhouse gas and nh 3 optimization includes matching the amount of reductant to the concentration of no x in the exhaust at a given time and maintaining the temperature of the scr catalyst to maximize conversion to n 2 the inherent difficulty in reducing the no x lies in the fact that modern compression ignition internal combustion engines operate using the lean burn principle to maximize fuel economy and the exhaust gas contains.