The effect of H22 and CO on the selective catalytic reduction of NOxx under real diesel engine exhaust conditions over Pt/Al22O33

In the presented work the selective catalytic reduction (SCR) of NOxx in a real diesel engine exhaust gas (O22 present) from the engine operating at different conditions with and without H22 and CO additions were studied. The tests were carried out using real diesel engine exhaust gas over 1%Pt supported on alumina (Al22O3)3). The catalyst exhibits good NOxx reduction activity at a narrow temperature range of 200 to 300 ∘∘C when there is only a HC present. The maximum NOxx reduction of around 60% was achieved at temperature of 260 ∘∘C. Although, the engine operating with EGR improves the percentage of NOxx converted in the SCR system due to increased HC:NOxx ratio and reduced NOxx concentration in the engine exhaust gas, the number of NOxx-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 NOxx 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 NOxx. Hydrogen addition expands the SCR activity window towards lower temperatures (100–300 ∘∘C) without affecting the maximum NOxx conversion. In contradiction to H22 the CO addition is favourable to the H22 oxidation reaction and the poisoning of the catalysts active sites and the good low temperature NOxx reduction activity cannot be seen. The incorporation of a mini-exhaust gas-reformer on-board a vehicle to provide the H22 in the SCR reactor will require catalyst design and reactor engineering to maximise H22 production and eliminate CO with the minimum penalty in the fuel economy.