Abstract
Catalytic performances of Cr/Al2O3 catalysts were investigated in Selective Catalytic Reduction (SCR) of NO by NH3 in the presence of oxygen. Solids were prepared either by sol–gel method or by impregnation and were characterized by different techniques such as chemical analysis, N2 physisorption, XRD, 27Al MAS NMR, DRIFT, UV–Vis Diffuse Reflectance (DRS), Raman Spectroscopy and Temperature Programmed Reduction by H2 (H2-TPR). The physisorption of N2 at 77 K reveals that solids are mesoporous. On the other hand, XRD shows that xerogels are amorphous but supercritical drying leads to a nanosized crystallite state. No crystalline α-Cr2O3 was found which indicate that metal species reside essentially on the surface of Al2O3 and their size measured less than 4 nm. Furthermore, 27Al MAS NMR reveals that part of chromium ions occupies sites on/in Al2O3 in close vicinity of tetrahedral 27Al. This, apparently, is not the case for aerogels. DRIFT results show that there is a consumption of hydroxyl groups of alumina after calcination. The esterification reaction between hydroxyl groups and chromium oxide during calcination leads to the formation of anchored (poly-)chromates according to DRS, Raman, and H2-TPR results. The catalysts are active in the studied reaction by NH3 and the activity is principally governed by preparation method and operating conditions. When compared to xerogels, aerogels are more active in NO reduction and less selective toward N2O. Preparation method and drying mode seem to involve the predominance of active species which are essentially mono and polychromates but Cr3+ ions incorporated inside alumina seem to be inactive.