The effect of H_2 and CO on the selective catalytic reduction of NO_x under real diesel engine exhaust conditions over Pt/Al_2O_3

摘要

In the presented work the selective catalytic reduction (SCR) of NO_x in a real diesel engine exhaust gas (O_2 present) from the engine operating at different conditions with and without H_2 and CO additions were studied. The tests were carried out using real diesel engine exhaust gas over lPt supported on alumina (Al_2O_3). The catalyst exhibits good NO_x reduction activity at a narrow temperature range of 200 to 300 ℃ when there is only a HC present. The maximum NO_x reduction of around 60 was achieved at temperature of 260 ℃. Although, the engine operating with EGR improves the percentage of NO_x converted in the SCR system due to increased HC:NO_x ratio and reduced NO_x concentration in the engine exhaust gas, the number of NO_x-ppm reduced over the catalyst was reduced. The cause of this effect is not yet clear, but there are evidences that this attributes to (a) lower NO_x coverage on the catalyst surface, which in turn makes its reduction by HC less probable and (b) the increased soot emissions which are blocking part of the catalyst active sites that are active in reducing NO_x. Hydrogen addition expands the SCR activity window towards lower temperatures (100-300 ℃) without affecting the maximum NO_x conversion. In contradiction to H_2 the CO addition is favourable to the H_2 oxidation reaction and the poisoning of the catalysts active sites and the good low temperature NO_x reduction activity cannot be seen. The incorporation of a mini-exhaust gas-reformer on-board a vehicle to provide the H_2 in the SCR reactor will require catalyst design and reactor engineering to maximise H_2 production and eliminate CO with the minimum penalty in the fuel economy.