Adsorption chemistry of NO x on Ag/Al 2O 3 catalyst for selective catalytic reduction of NO x using hydrocarbons

Abstract

Ag/Al 2O 3 is an active catalyst for selective catalytic reduction (SCR) of NO x using long paraffins as reducing agents. In view of the important role of surface nitrates in this catalytic chemistry, their formation on the Ag/Al 2O 3 catalyst, i.e. on the alumina support and on the silver species, was quantified in this study under typical reaction conditions of 500–1000 ppm NO x , 6% O 2 and 12% H 2O. The NO x adsorption capacity and the stoichiometry of nitrate formation and elimination reactions were determined as function of the temperature and the composition of the NO x using volumetric determinations in a flow system. The silver chemistry was investigated using in situ DR UV–vis spectroscopy. In the presence of water, nitrate formation out of NO 2 proceeds according to two different mechanisms depending on the state of silver as Ag(0) or Ag(I) on the catalyst. Minute concentrations of hydrogen (3500 ppm) are responsible for Ag(I) reduction. The study reveals the origin of the promotion of the low temperature SCR-NO x activity of Ag/Al 2O 3 by hydrogen. Hydroxylation–dehydroxylation of the alumina surface of Ag/Al 2O 3 catalyst was found to be an oscillating reaction, whereas the alumina support itself did not display this phenomenon.