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

摘要

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 A1_2O_3(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 550degC for 5h.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 (Co_3O_4,Co~(3+),surface Co~(2+),CoAl_2O_4) 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 Co_3O_4 particles,formed after a TPR up to 1100degC and successive TPO up to 550degC are active for the NO reduction at low temperature,N_2O being the major product at 250degC,while N_2 formation was favored at temperature higher than 400degC.The as prepared sample,calcined at 550degC for 5 h,exhibits significant activity only at temperature >400degC,giving selective formation of N_2.The nature of the active species in the supported catalyst was confirmed by studying the CO+NO reaction and NO decomposition over Co_3O_4 and CoO oxides as reference materials.On Co_3O_4 the NO reduction occurs with different selectivity to N_2O/N_2,depending on the reaction temperature:at low temperature only N_2O was detected,whereas at temperature >350degC also N_2 formation was observed.Then at temperature >=450degC only the reaction NO->N_2 takes place.Over CoO the reaction starts at approx400degC and N_2 is the only nitrogen product detected in the whole range of temperature investigated.Hundred percent of NO conversion into N_2 is reached at 600degC.