Selective Catalytic Reduction of Nitric Oxide in Diesel Exhaust
Jianming Liu, Ates Akyurtlu, and Jale F. Akyurtlu. Chemical Engineering, Hampton University, 168 Marshall Avenue, OLIN Engineering Building, Hampton, VA 23668
The aim of this work is to investigate the performance of catalysts consisting of two metal oxides supported on alumina (SOR10-III and SO10-IV) in the selective catalytic reduction (SCR) of NOx with two representative components of diesel exhaust, namely propylene and dodecane, as reductants. For the experiments with propylene, 2 grams of catalyst were used in a quartz microreactor (space velocity of 13700/hour). The reactant gas was composed of 1000ppm C3H6, 1000ppm NO, 1.5% O2, 7% H2O, and balance helium. Gas samples were analyzed using a Varian 3800 Gas Chromatograph for CO, CO2, N2 with TCD detector, and propylene and dodecane with FID detector. NO and NO2 were analyzed by a Thermo Environmental Instruments Model 42H Chemiluminescence NO- NO2- NOx analyzer. SOR10-III and SOR10-IV catalysts reached a maximum in activity at 400-425oC, and 450oC, respectively, when propylene is used as the reductant. Generally SOR10-III catalysts showed higher activities than those of SOR10-IV catalysts. Oxidation of propylene competes with the SCR reaction. The rate of oxidation of propylene is faster on SOR10-III catalyst than on SOR10-IV catalyst. Complete oxidation was realized at 370oC for the former catalyst while at around 425oC for the latter one. SOR10-III catalysts showed higher SCR selectivity than SOR10-IV catalysts under the same conditions, especially at temperatures lower than 500oC. The volcano-shaped activity curve for SCR is the result of the complete consumption of C3H6 by both NO and O2. Therefore, the conversion for NO reduction to N2 decreases for temperatures higher than that corresponding to the total conversion of the hydrocarbon reductant. The experimental results indicated a detrimental effect of water on the catalysts. This effect is due to the occupation of active sites by water, since at higher temperatures of operation, the deleterious effect of water disappears. In the absence of water vapor, SOR10-III catalyst reached a higher maximum for the conversion of NO (around 35%) at a lower temperature of about 300-325oC. One promoter improved the activity of both catalysts. However, promoted SOR10-III still showed a higher SCR activity than that of the promoted SOR10-IV. The promoter reduced the temperature at which maximum activity occurred for the SOR10-III catalyst, but not for the SOR10-IV; the temperature at maximum activity for the promoted SOR10-III catalyst was 400oC. This behavior is important for the use of this catalyst for diesel engine applications. These results indicate that the catalyst composition must be optimized to maximize SCR activity. The active promoter reduced the temperature for complete oxidation of propylene for both catalysts compared to their unpromoted counterparts, to 350oC for the promoted SOR10-III and to 400oC for SOR10-IV. The activity for oxidation of propylene seems to parallel the SCR activity in these catalysts. This may be due to a reaction mechanism which has the formation of adsorbed partially oxidized hydrocarbon as the rate determining step. The SOR10-III catalyst was sulfated to test the effect of sulfur on its SCR activity. The experiment with the sulfated catalysts is important since there is sulfur in the diesel fuel, and therefore, in the exhaust gases. It was observed that the SCR activity was not adversely affected by sulfur. This is of utmost importance in the use of these catalysts for diesel engine exhaust. Experiments on the kinetics of the SCR reaction with propylene indicated that NO conversion was independent of O2 concentration at 1000ppm NO, 1000ppm C3H6 in the range investigated. This is due to the fact that the reaction proceeds in the presence of excess oxygen. The reaction order is possibly negative with respect to NO, and positive with respect to propylene.