than vanadium contacts. In addition to NH3-SCR and hydrolysis catalysts, the system comprises an ammonia oxidation stage, because the Euro-6 standard provides for the restriction of ammonia at 10 ppm. A number of computational studies has been performed to investigate the effectiveness of the selective catalyst in a gas engine running on lean fuel mixtures. The computational studies used a mathematical model describing the basic chemical processes occurring in the catalytic conversion system. The reactions of nitrogen oxides reduction (Winkler et al., 2003; Wurzenberger and Wanker, 2005) and conversion of ammonia are as follows: reduction of nitrogen oxides: 4NН3 + 4NO + О2 = 4N2 + 6Н2О; (1) combined reduction of nitrogen oxide and dioxide: 4NН3 + 2NO + 2NО2 = 4N2 + 6Н2О; (2) reduction of nitrogen dioxide: 8NН3 + 6NО2 = 7N2 + 12Н2О; (3) hydrolysis of isocyanic acid: HNCO + H2O = NH3 + CO2; (4) oxidation of ammonia: 4NН3 + 3О2 = 2N2 + 6Н2О. (5) The formulas for calculating reaction rates are as follows: