Nitrogen oxides reduction with hydrocarbons under oxidizing conditions over alumina supported metal catalysts.

Abstract

A single step sol-gel procedure has been developed and applied to synthesize Pt/alumina, Ag/alumina and Au/alumina catalysts. Also, an activation procedure has been found to be crucial to get highly active Au/alumina and Ag/alumina catalysts. In contrast, Pt/alumina catalyst was active after the calcination step. Although all catalysts are calcined at 600&deg;C, their surface areas are &sim;300 m<super>2</super>/g with a uniform pore size distribution, having pores less than 10 nm. In addition, the metal particle size in all catalysts is found to be independent of the metal loading. The catalytic activity results show that the single step sol-gel prepared Pt/alumina, Ag/alumina and Au/alumina catalysts are very active and also selective toward N₂ in comparison to impregnation catalysts. In addition, the temperature window for activity of all sol-gel catalysts is larger than that for impregnation catalysts under the same reaction conditions. Their NO_x activity is observed to increase as a function of oxygen concentration. Especially, in the case of Pt/alumina catalyst, the N₂ selectivity at maximum NO_x conversion increases to &sim;80% when the oxygen concentration is 14%. The presence of water in the reactant gas mixture does not hinder the NO_x activity and N₂ selectivity of Pt and Au sol-gel catalysts but the activity of Ag sol-gel catalyst is affected by the presence of water. In addition, the presence of 25 ppm SO₂ slightly decreases the activity of Au and Ag sol-gel catalysts but it does not change the activity and N₂ selectivity of Pt sol-gel catalyst.