Catalytic oxidation of nitrogen monoxide over Pt/SiO_2

摘要

The catalytic oxidation of NO was studied on a catalyst consisting of platinum supported on SiO_2.The kinetic behavior over Pt/SiO_2 with a platinum loading of 2.5 wt.% was investigated in a feed containing 5% water and various concentrations of oxygen,nitrogen monoxide and nitrogen dioxide.The conversion of NO to NO_2 increases when the oxygen concentration is increased from 0.1 to 10%,but levels off at higher concentrations.Increasing feed concentrations of NO lead to a decrease in the conversion to NO_2.The formation of NO_2 is also depressed by the addition of NO_2 to the feed.Both observations suggest that the oxidation of NO on Pt/SiO_2 is autoinhibited by the reaction product NO_2.Further experiments have shown that the inhibition caused by NO_2 is mostly persistent,i.e.a deactivation of the catalyst occurs.A pretreatment at 250 deg C in a feed containing 500 ppm NO_2 causes a very strong decrease in activity.However,the initial activity can be restored either by a thermal regeneration at 650 deg C in air or by a regeneration under reducing conditions at 250 deg C,e.g.in a feed containing NH_3.This suggests that the deactivation by NO_2 is due to the formation of a thin layer of platinum oxide covering the platinum surface at least partially.