ChemInform Abstract: Infrared Studies of CO and/or NO Adsorption on Reduced W/Al2O3 Catalysts.

Abstract

Infrared spectroscopy (IR) has been used to investigate CO and/or NO adsorption on reduced W/Al2O3 catalysts. The effect of reduction temperature and tungsten-loading was studied. Three different kinds of surface W-sites were identified according to the IR spectra (possibly W5+, W4+ (I), and W4+ (II)). Up to three IR bands at 2198, 2176, and 2154 cm−1 for adsorbed CO, which are best assigned to W5+ (CO), W4+ (I) (CO), and W4+ (II) (CO), respectively, were observed. Only two kinds of surface complexes, W5+ (NO) with a single band at 1843 cm−1 and W4+ (I and II) (NO)2 with coupled bands at 1780 and 1691 cm−1, were formed upon NO contact. In the case of CO + NO and NO + CO (added in sequence) coadsorption, a surface complex in the form of (CO) W4+ (I) (NO)2, which gave three IR bands at about 2117, 1801, and 1725 cm−1, was evidenced in the spectra in addition to W5+ (CO) and W4+ (II) (NO)2 on the surface. These observations suggest that W4+ (I)-sites differ from W4+ (II)-sites in the coordination state and the coadsorption of CO + NO or NO + CO allows more information about the surface W-sites to be achieved than the corresponding CO or NO single adsorption in IR studies. The adsorption and desorption mechanisms are discussed for the surface complex of (CO) W4+ (I) (NO)2. Laser Raman spectroscopy (LRS) and temperature-programmed reduction (TPR) were also employed as complementary techniques to characterize the W/Al2O3 catalysts. The results from these two measurements are in good agreement with our IR results.