Physicochemical surface and catalystic properties of Cr 2O 3/Al 2O 3 system

Abstract

Solid-solid interactions, surface and catalystic properties of Cr 2O 3/Al 2O 3 system were investigated using TG, DTA, XRD, nitrogen adsorption at −196°C and CO-oxidation by O 2 at 250–400°C. The amount of chromium oxide was varied between 4.3 and 42.7 wt.% Cr 2O 3. The results showed that heating of Al(OH) 3 pretreated with different amounts of CrO 3 at 400–900°C resulted in the formation of a mixture of γ-, k-,θ-aluminas, Cr 2O 5, CrO 2 and α-Cr 2O 3 phases. The rise in calcination temperature to 1000°C made the appearance of minute amounts of α-Al 2O 3 and Cr 3O 4 besides the other phases (α-Cr 2O 3 and θ-, k-Al 2O 3). The BET-surface area decreases by increasing the precalcination temperature in the range 400–1000°C. The mixed oxide adsorbents calcined at 400–600°C contain mainly narrow pores and turned to wide pores upon heating at 700–1000°C. The activation energy of sintering (Δ E s) decreases markedly from 21 to 7.6 kJ mol −1 by increasing the amount of Cr 2O 3 from 4.3 to 8.3 wt.% then increases progressively reaching to 33 kJ mol −1 in the presence of 42.7 wt.% Cr 2O 3. The catalystic activity increases as a function of Cr 2O 3 content. The curves relating the catalystic activity (reaction rate constant per unit surface area) and precalcination temperature exhibited a maximum at 700 or 800°C depending on the amount of Cr 2O 3 present. The rise in both Cr 2O 3 content and calcination temperature did not modify the mechanism of the catalystic reaction but changed the concentration of catalystically active constituents without altering their energetic nature.