Fundamental studies on the catalytic removal of nitrogen monoxide using reducing agents.

Abstract

The behavior of adsorbed nitrogen monoxide (NO) and the reactivity of NO with reducing agents were investigated. Two species with N-O stretching frequencies at 1, 790 and 1, 850 cm-1, respectively, were found from the infrared studies of adsorbed NO on a cobalt-silica catalyst. The former arose from the strongly chemisorbed species and it was the intermediate species in the reaction between NO and reduced surface. Bases such as NH3, NH2CH3, NH (CH3)2 and N(CH3)3 resulted in the lowering of the N-O stretching frequencies of preadsorbed NO, indicating weakening of the N-O bond. This was explained on the basis of a change in the electron density of metal-N-O molecular orbitals when a second gas was admitted.The roles of reducing agents such as H2, CO and NH3 in the catalytic reduction of NO over supported metal or metal oxide catalysts were studied from the viewpoint of reaction mechanism. The reaction of NO with H2 or CO was shown to proceed through a redox cycle of the catalyst, that is, the catalyst was oxidized by NO and then it was reduced by the reducing agents to complete the catalytic sequence. On the other hand, it was confirmed by means of isotope labelling techniques that NH3 or fragments of NH3 directly reacted with the adsorbed NO. Also, the reaction of NO with NH3 was greatly enhanced by preoxidation of a Cr2O3-Al2O3 catalyst and this effect was due to the oxygen with a high oxidation power which was produced on the catalyst surface.These differences in the roles of reducing agents in the reaction mechanism satisfactorily explain some aspects of catalytic removal of NO in the practical process where oxygen is present.