Surface structure of crystalline and amorphous chromia catalysts for the selective catalytic reduction of nitric oxide III. Diffuse reflectance ftir study of ammonia desorption from Brønsted and Lewis acid sites

Abstract

Ammonia has been adsorbed onto oxidized and reduced surfaces of crystalline α-Cr 2O 3 and of amorphous chromia, respectively. Temperature-dependent changes in the diffuse reflectance FTIR spectrum are monitored and are correlated with temperature-programmed reaction and desorption (TPRD) spectra reported in part I of this study (1). Two types of NH3 molecules bound to Brønsted acidic sites on the surface of the chromia catalysts, as well as two species bound to Lewis sites, are identified from the IR spectra. The oxidized surface of α-Cr 2O 3 is characterized by a high surface coverage of strongly Lewis-bound ammonia molecules, which desorb at temperatures above 400 K. In contrast, weakly bound NH 3 prevails on the oxidized surface of amorphous chromia. The desorption of the latter species starts at 380 K and is completed around 480 K. A reductive pretreatment of the chromia surfaces by hydrogen (10% in Ar) decreases the number of surface hydroxyl groups and devoids the surface of adsorbed oxygen. As a consequence, the number of Brønsted acidic sites is reduced both on the crystalline and the amorphous catalyst. On the reduced surface of α-Cr 2O 3, strongly bound ammonia coordinated to Lewis acidic sites is observed to persist to temperatures beyond 560 K. In contrast, weakly Lewis-bound NH 3 prevails on reduced amorphous chromia, which is quantitatively desorbed at temperatures below 410 K. The formation of NH 3 oxidation products on α-Cr 2O 3 is manifested by the observation of two characteristic surface species, an N 2O 2 dimer and an NO 2 chelate surface complex. Corresponding signals are not observed on the surface of amorphous chromia. Apparently the removal of labile oxygen from the surface of α-Cr 2O 3 at moderate temperatures generates strongly Lewis acidic chromium sites, and the firm bonding of NH 3 to these sites is related to the undesired direct oxidation of ammonia.