Surface and catalytic properties of Co3O4/Al2O3 as influenced by ZnO

The effects of ZnO-treatment (2.1–7.7 wt.%) of Co3O4/Al2O3 system (32 wt.% Co3O4) on solid–solid interaction between the catalyst and its support, surface and catalytic properties were investigated. The techniques employed were XRD, nitrogen adsorption at −196 °C and catalysis of CO by O2 at 60–100 °C using a static method. The results obtained revealed that ZnO-treatment of the investigated system conducted at 400 °C increased the degree of dispersion of Co3O4 phase via decreasing its particle size. Opposite effect was observed upon treatment with 4 wt.% ZnO followed by heating at 600 °C. The doping process conducted at 400 and 600 °C resulted in no significant changes in the BET-surface areas of Co3O4/Al2O3 system, while the doping process conducted at 500 °C resulted in a significant progressive increase in the BET-surface areas of the treated adsorbents. The doping process followed by calcinations at 400 and 500 °C resulted in widening of the pores present. The catalytic activity was found to increase by increasing the amount of dopant present for the solids precalcined at 400 and 500 °C reaching to a maximum limit in presence of 4 wt.% ZnO. The increase was, however, more pronounced for the solids precalcined at 400 °C. The maximum increase in the catalytic activity, expressed as reaction rate constant (k), for the solids precalcined at 400 °C was 171, 288 and 350% for the reaction conducted at 60, 80 and 100 °C, respectively. The doping process conducted at 600 °C resulted in a progressive decrease in the catalytic activity of the treated solids. The observed increase in the catalytic activity due to ZnO-doping at 400 and 500 °C was discussed in terms of an effective increase in the degree of dispersion of Co3O4 crystallites. The decrease in the activity due to doping at 600 °C has been tentatively discussed in terms of a possible coverage of the surfaces of Co3O4 with ZnO film and also to a possible enhancement of diffusion of cobalt species in Al2O3 lattice sites. ZnO-doping of Co3O4/Al2O3 did not modify the mechanism of the catalysed reactions but changed the concentration of catalytically active constituent taking part in the catalysed reaction without changing their energetic nature.