Effect of precipitating agent on the properties of cobalt catalysts promoted with cerium and barium for NH3 synthesis obtained by co-precipitation

Abstract

Cobalt catalysts singly promoted with cerium and doubly promoted with cerium and barium were prepared by co-precipitation using different precipitating agents, i.e. potassium hydroxide or potassium carbonate, subsequent calcination and impregnation. The obtained materials were characterized at each step of preparation using selected techniques: X-ray fluorescence (XRF), N2 physisorption, X-ray powder diffraction (XRPD), temperature-programmed reduction coupled with mass spectrometry (TPR-MS) and temperature-programmed hydrogen desorption (H2-TPD). The activity in ammonia synthesis was measured also under conditions close to the industrial ones (T=400–470°C, p=6.3MPa, H2/N2=3:1). The materials obtained with the use of K2CO3 as a precipitant have better textural parameters (smaller Co3O4 crystallites) than the analogous materials obtained with the use of KOH. The differences in textural properties of the materials derived from KOH or K2CO3 as a precipitant decrease with a progress of catalysts preparation and finally the active phase surface area of the catalysts promoted with cerium and barium is similar. Regardless of the precipitating agent used the surface activities (TOF) and NH3 synthesis reaction rates for these catalysts are nearly the same. However, the TOF values for the doubly promoted catalysts are about one order of magnitude higher than that for the catalysts promoted only with cerium. Therefore, it seems that not a type of precipitating agent used during preparation, but a composition of the studied promoted cobalt catalysts is more important factor influencing the activity of these catalytic systems.