Physicochemical surface and catalystic properties of Cr2O3/Al2O3 system

Solid-solid interactions, surface and catalystic properties of Cr2O3/Al2O3 system were investigated using TG, DTA, XRD, nitrogen adsorption at - 196 d egrees C and CO-oxidation by O-2 at 250-400 degrees C. The amount of chromi um oxide was varied between 4.3 and 42.7 wt.% Cr2O3. The results showed tha t heating of AI(OH), pretreated with different amounts of CrO3 at 400-900 d egrees C resulted in the formation of a mixture of gamma-, k-,theta-alumina s, Cr2O5, CrO2 and alpha-Cr2O3 phases. The rise in calcination temperature to 1000 degrees C made the appearance of minute amounts of alpha-Al2O3 and Cr3O4 besides the other phases (alpha-Cr2O3 and theta-, k-Al2O3). The BET-s urface area decreases by increasing the precalcination temperature in the r ange 400-1000 degrees C. The mixed oxide adsorbents calcined at 400-600 deg rees C contain mainly narrow pores and turned to wide pores upon heating at 700-1000 degrees C. The activation energy of sintering (Delta E-s) decreas es markedly from 21 to 7.6 kJ mol(-1) by increasing the amount of Cr2O3 fro m 4.3 to 8.3 wt.% then increases progressively reaching to 33 kJ mol(-1) in the presence of 42.7 wt.% Cr2O3. The catalystic activity increases as a fu nction of Cr2O3 content. The curves relating the catalystic activity (react ion rate constant per unit surface area) and precalcination temperature exh ibited a maximum at 700 or 800 degrees C depending on the amount of Cr2O3 p resent. The rise in both Cr2O3 content and calcination temperature did not modify the mechanism of the catalystic reaction but changed the concentrati on of catalystically active constituents without altering their energetic n ature. (C) 1999 Elsevier Science B.V. All rights reserved.