Surface and catalytic investigations of CuO–Cr 2O 3/Al 2O 3 system

Abstract

A series of CuO–Cr 2O 3/Al 2O 3 solids were prepared by the incipient wetness impregnation using Al(OH) 3 solid and chromium and copper nitrates. The amounts of copper and chromium oxides were varied between 12.2 and 13.5 wt.% CuO and 3.1 and 8.8 wt.% Cr 2O 3. The prepared solids were calcined at 400–1000 °C. The solid–solid interactions between the different constituents of the prepared solids were investigated using XRD and XPS analyses. The surface and catalytic properties of various solids were studied using nitrogen adsorption at −196 °C and CO-oxidation by O 2 at 50–250 °C. The results showed that the investigated adsorbents are mesoporous solids and measure specific surface areas within the range 73–356 m 2/g depending on the amount of dopant added and precalcination temperature. Cr 2O 3-treatment of the system investigated brought about a significant decrease in its BET-surface area which was found to decrease by increasing the precalcination temperature of the various solids. However, this treatment resulted in a progressive increase in the activation energy of sintering of different solids subjected to heat treatment at 400–800 °C. The concentration of copper and chromium species on top surface layers of the samples precalcined at 400 and 600 °C are greater than those present in the bulk of the solids. The doping process effected an increase in the surface concentration of copper species. The increase in precalcination temperature of pure and doped solids from 400 to 600 °C effected an inward diffusion of copper and chromium species towards their interior leading to a decrease in surface concentration of these species. Doping CuO/Al 2O 3 system with Cr 2O 3 decreased the crystallinity of CuO phase and enhanced its interaction with Al 2O 3 at 800 °C yielding CuAl 2O 4. XPS measurements showed also the possibility of formation of CuCrO 2 on the top surface layers of the heavily doped sample precalcined at 600 °C. The catalytic activity and durability of CuO/Al 2O 3 system increased by treatment with Cr 2O 3. Doping of this system did not modify the mechanism of the catalytic reaction but increased the concentrating of catalytically active sites without changing their energetic nature.