SCR CATALYST

Activity map – By varying the molar ratio of NH3 to NOx a map of SCR activity as function of temperature and alpha ratio can acquired。 Figure 4 shows this map for the SCR catalyst at the standard conditions defined in the previous section。 At temperatures above 300 C the NOx conversion increases linearly with the alpha ratio as is expected for the stoichiometric reaction between NO and NH3。 At lower temperatures, however, the conversion increases only linearly with the alpha ratio up to a certain critical alpha ratio above which an increase in alpha ratio does not result in a significant increase in NOx conversion。 Moreover, increasing the alpha ratio above the critical alpha ratio results in a stationary NH3 slip。 Measured N2O concentrations after the catalyst were always below 10 v。-ppm and resulted partially from impurities in the gases used and also from oxidation of NH3 in the equipment itself。

Effect of hydrocarbons – Figure 5 shows the effect of hydrocarbons on the NOx conversion over the catalyst at standard conditions。 In this case n-decane (C10H22) was taken as a model component。 With other hydrocarbons similar effects were observed。 It is clear that the curve for NOx conversion versus temperature is shifted to higher temperatures with higher hydrocarbon concentrations。 The n-decane is oxidized to CO and CO2 over the catalyst between 250 and 400 C。 However when no NH3 is added no significant conversion of NOx is observed, which leads to the conclusion that the hydrocarbons are not involved in the reduction of NOx on

the SCR catalyst。

Effect of NO2 fraction in NOx – The effect of increasing the NO2 fraction of NOx at a constant NOx concentration of 500 v。-ppm was studied in the model gas setup at an increased space velocity of 60000 hr-1。 The results of these tests are visualized in figure 6。 Upon increasing the NO2 content up to 50 vol。-% a clear increase in the low temperature activity is observed。 At higher contents, however, the high temperature activity and the maximum NOx conversion are reduced significantly and stationary NH3 slip is observed。 It is clear that at NO2 contents of up to 50%, NOx can be converted more efficiently at lower temperatures and higher space velocities than without NO2。 At higher contents however the higher stoichiometry needed to convert NO2, i。e。 1。33 compared to 1 for NO, and the slower reaction of NO2 with NH3

which results in NH3 slip, limits the maximum conversion without secondary emissions。

Stability – To determine the thermal stability of the SCR formulation, cores were aged for 100 hours in a furnace in air containing 10 vol。-% water and 20 v。-ppm SO2 at temperatures of 650 , 700 and 750 C。 Figure 7 shows clearly that this aging procedure up to aging temperatures of 700 C has no significant effect on the activity。 At higher temperatures the activity of the catalyst is dramatically reduced。 At these temperatures the V2O5 catalyzes the phase transition of the high surface area anatase TiO2 to the low surface area rutile TiO2。

PREOXIDATION CATALYST – Using a preoxidation catalyst it is possible to increase the NO2 fraction in the NOx up to the maximum given by the thermodynamic equilibrium between NO and NO2 in O2。 Figure 8 gives the thermodynamic equilibrium for different O2 concentrations together with the composition after an oxidation catalyst (containing 50 g/ft3 Pt) as function of

temperature。 The gas at the inlet contained 6 vol。-% O2, 270 v。-ppm NO, 10 vol。-% H2O and N2 as balance。 The space velocity during this test was 50000 hr-1。 At temperatures above 280 C this catalyst completely equilibrates the gas phase and hence the NO2 content is thermodynamically controlled。 At lower temperatures the activity of the catalyst is not sufficient to convert enough NO to reach the thermodynamic equilibrium and the NO2 content is kinetically controlled。 At temperatures below 150 C Pt based oxidation catalysts are not active enough to give any significant oxidation of NO to NO2 at space velocities typical for automotive exhaust aftertreatment。