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.%)\u2013BaO (20 wt.%). The catalyst was synthesized by incipient-wetness impregnation of the support with a water solution of cobalt acetylacetonate and calcined at 550 \ub0C 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 \ub0C and successive TPO up to 550 \ub0C are active for the NO reduction at low temperature, N2O being the major product at 250 \ub0C, while N2 formation was favored at temperature higher than 400 \ub0C. The as prepared sample, calcined at 550 \ub0C for 5 h, exhibits significant activity only at temperature > 400 \ub0C, 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 \ub0C also N2 formation was observed. Then at temperature 65450 \ub0C only the reaction NO\u2192N2 takes place. Over CoO the reaction starts at 3c400 \ub0C 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 \ub0C