Catalytic oxidation characteristics of NO over Pt-based catalysts supported on TiO2 (anatase, rutile) have been investigated in a fixed bed reactor (8 mm-ID) of mainly 8 % water with various concentrations of oxygen, nitric oxide and nitrogen dioxide. In the given experimental conditions, the rutile supported catalyst seems to be less efficient compared to the anatase supported catalyst. The order of activity for oxidation reaction using all possible parameters is found to be: $Pt (1.6)/TiO_2 (anatase) > Pt (2.4)/TiO_2$ (rutile) due to smaller BET surface area and low porosity in the rutile. Anatase and rutile supports used in this study have also been investigated by the SEM images and found that rutile particles are large and not uniform compared with the anatase particles.
On treating these catalysts with different $O_2$ concentrations it is seen that the conversion of NO to $NO_2$ increases with increasing $O_2$ concentration from 3 to 12%, but it levels off at higher concentrations due to the saturation effect of $O_2$ concentration on the active site. The conversion to $NO_2$ decreases with increasing feed concentrations of NO and also decreases by the addition of $NO_2$ to the feed. These both observations suggest that the oxidation of NO on Pt based catalysts is auto-inhibited by the reaction product $NO_2$.
Further experiments were performed for the oxidation of CO and $SO_2$ and also their effect on NO oxidation over same catalysts. The activity order of these catalysts for the oxidation of CO and $SO_2$ has also been investigated and it was found that the activity order is the same order as that of NO oxidation. On treating these $Pt/TiO_2$ catalysts with $SO_2$, the catalytic activity of NO oxidation drops dramatically. Moreover it is also observed that anatase supported catalyst is not affected much by $SO_2$ and shows good conversion but rutile based structure is affected too much by $SO_2$. This may be caused by less stability of sulfates on the anatase surface due to the presence of $WO_3$ which displaces the $SO_3$ from the basic site of the catalyst. More over, with increasing $SO_2$ concentration, NO conversion decreases more rapidly. On the other hand, the presence of NO promotes $SO_2$ oxidation because $NO_2$ is a strong oxidizing agent and it reduces itself and oxidizes $SO_2$ to $SO_3$.
The effect of CO presence on NO oxidation has also been determined. It is reported that presence of CO exhibits remarkable positive effect on NO oxidation. This effect is more pronounced on the anatase supported catalyst than the rutile supported one because the presence of CO inhibits the auto-inhibition effect of NO conversion. Moreover high concentrations of CO also increase the NO conversion but on further increase it may decrease the conversion because most of the oxygen is caused by CO. On the other hand, the presence of NO decreases the CO oxidation. This decrease in the CO oxidation is due to the competition of CO and NO for the active sites.
