3.4. Catalysts with a higher temperature window of NO x conversion
In spite of having high NOx removal efficiency, the Pt/mordenite catalyst has a relatively low temperature window of NOx conversion efficiency and about a half of its NOx reaction byproducts is N20 at maximum NOx conversion temperature. NH 3 was
not detected in all our experiments. Since the ratio of products from NOx N2/N20 generally increases with increasing temperature, we investigated a catalyst with a higher temperature window of NOx conversion than the Pt/mordenite catalyst. Such a catalyst must have poor oxidization efficiency. As a result, we have found the Pt-Mo-Na/SiO 2 catalyst to be the one that meet the above requirements. The catalyst was prepared by impregnating an aqueous solution of Na and Mo with the SiO 2 support and Pt loading after calcination at 500°C in air.
The NOx conversion efficiency of Pt-Mo-Na/SiO 2 is shown in Fig. 5 as a function of temperature. The N2/N20 ratio of NOx reaction products on this catalyst at the maximum conversion temperature was about 4. Surprisingly, the Pt-Mo-Na/SiO 2 catalyst
removed NOx with CO as well as H2, while no other Pt catalyst did that in our previous experiments. To clear the origin of the difference of catalysis between Pt-Mo-Na/SiO 2 and other Pt catalysts, we characterized Pt-Mo-Na/SiO 2.