Structural and morphological investigation of a cobalt catalyst supported on alumina-baria: effects of redox treatments on the activity in the NO reduction by CO

Temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO) have been used to study the various cobalt species formed in a cobalt catalyst (Co 1 wt.%) supported on alumina-baria, with composition Al2O3(80 wt.%)–BaO (20 wt.%). The catalyst was synthesized by incipient-wetness impregnation of the support with a water solution of cobalt acetylacetonate and calcined at 550 °C for 5 h. In order to investigate the structure and morphology of the sample, XRD, BET and SEM analyses were performed. Depending on the temperature and nature of pre-treatment (reductive or oxidative) different Co species (Co3O4, Co3+, surface Co2+, CoAl2O4) were formed. Activity tests in the reduction of NO by CO were carried out in order to investigate the effectiveness of the catalyst depending on the redox pre-treatment. Highly dispersed Co3O4 particles, formed after a TPR up to 1100 °C and successive TPO up to 550 °C are active for the NO reduction at low temperature, N2O being the major product at 250 °C, while N2 formation was favored at temperature higher than 400 °C. The as prepared sample, calcined at 550 °C for 5 h, exhibits significant activity only at temperature > 400 °C, giving selective formation of N2. The nature of the active species in the supported catalyst was confirmed by studying the CO + NO reaction and NO decomposition over Co3O4 and CoO oxides as reference materials. On Co3O4 the NO reduction occurs with different selectivity to N2O/N2, depending on the reaction temperature: at low temperature only N2O was detected, whereas at temperature >350 °C also N2 formation was observed. Then at temperature ≥450 °C only the reaction NO→N2 takes place. Over CoO the reaction starts at ∼400 °C and N2 is the only nitrogen product detected in the whole range of temperature investigated. Hundred percent of NO conversion into N2 is reached at 600 °C.