Surface and catalytic investigations of Co3O4-MoO3/Al2O3 system

A series of Co3O4-MoO3/Al2O3 solids were prepared by the incipient wetness impregnation using AI(OH)(3) solid and ammonium molybdate and cobalt nitrate. The amounts of cobalt and molybdenum oxides were varied between 9.9 and 32.1 wt-\% CO3O4 and 0.48 and 5.9 wt.\% MoO3. The prepared solids were calcined at 600-1000 degreesC. The solid-solid interactions between the different constituents of the prepared samples were studied using XRD and XPS analyses. The surface properties of various samples were investigated using nitrogen adsorption at -196 degreesC and their catalytic activities were determined using CO-oxidation by O-2 at 100-200 degreesC. The results showed that the investigated adsorbents are mesoporous solids and measure specific surface areas ranged between 52 and 367 m(2) g(-1) depending on extent of cobalt oxide loading, dopant concentration (MoO3) and calcination temperatures. Treatment of Co3O4/Al2O3 system with small amount of MoO3 conducted at 600 degreesC increased the specific surface area and total pore volume of most of the investigated adsorbents. This treatment resulted in a progressive increase in the activation energy of sintering of various samples subjected to heat treatment at 600 to 1000 degreesC. The concentration of cobalt and molybdenum species on top surface layers of the samples precalcined at 600 degreesC are greater than those in bulk of the solids. MoO3-doping of CO3O4/Al2O3 solids conducted at 600 degreesC brought about a significant increase in the concentration of cobalt species on the outermost surface layers of the treated samples. Cobalt present in the samples containing about 10 wt.\% and calcined at 600 degreesC existed as CoAl2O4 and CoMoO4 compounds as evidenced from XPS measurements. The samples more rich in cobalt and calcined at 600 or 800 degreesC consisted of a mixture of Co3O4, CoAl2O4, CoMoO4 and gamma-Al2O3 compounds. MoO3-doping of the investigated system much enhanced the crystallization of alumina into alpha-Al2O3 (corrundum) by heating the treated solids at 1000 `C. The treatment of various solids with MoO3 (about 1 wt.\%) followed by calcination at 600 degrees1 effected a significant increase in their catalytic activity towards CO-oxidation with O-2 which decreased upon increasing the amount of dopant above this limit. The doping process following by heat treatment at 800 and 1000 degreesC brought about a measurable decrease in the catalytic activity of the treated catalyst samples. Doping of Co3O4/Al2O3 with MoO3 did not modify the mechanism of the catalytic reaction but changed the concentration of catalytically active sites without changing their energetic nature. (C) 2003 Elsevier Science B.V. All rights reserved.