Fuel effects on the activity of silver hydrocarbon-SCR catalysts

The hydrocarbon-SCR activity of Ag/Al2O3 has been correlated with the chain length of three model fuels (n-octane, n-decane, n-dodecane) and with the alkane/aromatic composition of two fully formulated diesel fuels. The diesel fuels were US06 (ultra-low sulphur but 24 wt.% aromatics) and Fisher-Tropsch Gas-to-Liquid (FT-GTL) (negligible sulphur and only 0.3 wt.% aromatics). As the hydrocarbon chain length increases, the low-temperature NOx-conversion activity improves, while the reactant selectivity (hydrocarbon SCR versus hydrocarbon oxidation) decreases at higher temperatures. With diesel fuel, the catalyst initially achieves high NOx conversion at low temperature, but the conversion degrades with time due to coking. The rate of degradation is dependent on the nature of the hydrocarbons. Both the long-chain alkanes and the aromatics present in US06 diesel fuel promote the deposition of coke on the catalyst surface. However, high NOx conversion can be maintained using a fuel injection strategy that tunes the hydrocarbon/NOx ratio to the inlet temperature, so avoiding coking and improving both selectivity and fuel economy. At low temperatures the hydrocarbon/NOx ratio is kept low, and then increased as a function of temperature to compensate for the loss of hydrocarbon when the direct oxidation by 02 begins to dominate. Optimisation of the fuel injection strategy is different for each type of fuel, but in general the NOx-conversion activity is higher for FT-GTL fuel at comparable hydrocarbon/NOx ratios. (c) 2007 Elsevier B.V. All rights reserved.