Surface enrichment phenomenon in the Ba-doped cobalt catalyst for ammonia synthesis

Abstract

Barium promoted cobalt catalysts (Co/Ba) for NH3 synthesis were synthesized by co-precipitation of a mixture of cobalt and barium carbonates. Various pretreatment method of the obtained precursors were applied: subsequent calcination and in situ reduction or in situ reduction only (without calcination). The catalytic materials were subjected to detailed characterization studies by nitrogen physisorption, X-ray powder diffraction (XRPD), temperature-programmed techniques (TPR-MS, TPD-H2, TPD-N2, TPSR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM-EDX). The Co/Ba catalysts were tested in catalytic ammonia synthesis reaction under conditions close to the industrial ones (400 °C, 6.3 MPa, H2/N2 = 3). Studies revealed that the method of catalysts precursors pretreatment has a significant influence on the properties of the final Co/Ba systems. The more effective method is the one without subsequent calcination – this material exhibited the most favorable catalytic properties. Detailed surface studies showed that calcination process of the co-precipitated carbonates mixture adversely affects the final properties of the catalyst, because it favors a migration of the barium promoter to the catalyst's surface during its reduction (surface enrichment phenomenon) limiting the amount of active phase (metallic cobalt) available to reactants during the catalytic reaction and thus substantially decreasing the catalyst's activity.