Ammonia synthesis over cobalt catalysts doped with cerium and barium. Effect of the ceria loading

Abstract

The effect of ceria loading on the activity and thermal stability of a cobalt catalyst for ammonia synthesis has been studied. The concentration of CeO2 in the Co3O4+CeO2 mixtures ranged from 0 to 29wt%, whereas the barium content was constant and equaled 13wt% in relation to Co3O4. Kinetic measurements in NH3 synthesis were carried out in a flow differential reactor operating under standard conditions (p=6.3MPa, T=400°C, H2:N2=3:1). The kinetic measurements of NH3 synthesis were supplemented with the characterization studies of the obtained materials – N2 physisorption, XRPD, H2 chemisorption, TPR-TG/DTG-MS and SEM-EDS. It has been found that ceria plays the role of a structural promoter; it hinders the sintering of Co3O4 during calcination and stabilizes the surface area of Co under reaction conditions. Moreover, it seems that ceria stabilizes the hcp phase of metallic cobalt, which results in a higher activity of the cobalt catalysts. The co-promoted cobalt catalyst containing 11.5wt% of CeO2 (symbol Co/Ce(11.5)/Ba) was shown to be more active in NH3 synthesis than the conventional fused iron catalyst (KM I, H. Topsoe). Moreover, the catalyst Co/Ce(11.5)/Ba is very resistant to overheating. Heat treatment at 600°C for 160h (3H2:N2=30.0l/h, p=0.1MPa) results in a decrease of its activity in NH3 synthesis by only a few percent.