Surface structure of crystalline and amorphous chromia catalysts for the selective catalytic reduction of nitric oxide I. Characterization by temperature-programmed reaction and desorption

Abstract

Amorphous and crystalline chromia catalysts used for the selective catalytic reduction (SCR) of NO by NH3 have been characterized using temperature-programmed reaction and desorption of preadsorbed NO and NH3. The acidity of the Lewis acid sites and the associated labile oxygen species are investigated using NH3 as a probe molecule. The degree of coordinative unsaturation of these sites is inferred from the reactions of NO. The effects of reduction, oxidation, and SCR treatment on the state of the Lewis acid sites and the labile oxygen are discussed. Lewis acid sites on crystalline chromia are more acidic than those on amorphous chromia. Different labile oxygen species are discernable based on their oxidizing strengths and the products formed. All labile oxygen is readily removed by reduction; SCR treatment leads to the partial removal of labile oxygen from both morphologies of chromia. The degree of lability of the different oxygen species is related to the morphology of the chromia. The proportions of one- and two-fold coordinatively unsaturated sites (1 and 2 c.u.s.) depends strongly on the morphology of the chromia and its pretreatment. Both types of sites show distinct differences in the acidity dependent on the morphology of the chromia. The most significant difference between the two morphologies is seen after SCR treatment. Crystalline chromia shows a very high density of 1 c.u.s. compared to amorphous chromia. These sites are generated from 2 c.u.s. by abstraction of oxygen from NO. Implications of these results on the SCR reaction are discussed.