Thermal Stability of Potassium-Promoted Cobalt Molybdenum Nitride Catalysts for Ammonia Synthesis

The application of cobalt molybdenum nitrides as ammonia synthesis catalysts requires further development of the optimal promoter system, which enhances not only the activity but also the stability of the catalysts. To do so, elucidating the influence of the addition of alkali metals on the structural properties of the catalysts is essential. In this study, potassium-promoted cobalt molybdenum nitrides were synthesized by impregnation of the precursor CoMoO4·3/4H2O with aqueous KNO3 solution followed by ammonolysis. The catalysts were characterized with the use of XRD and BET methods, under two conditions: as obtained and after the thermal stability test. The catalytic activity in the synthesis of ammonia was examined at 450 °C, under 10 MPa. The thermal stability test was carried out by heating at 650 °C in the same apparatus. As a result of ammonolysis, mixtures of two phases: Co3Mo3N and Co2Mo3N were obtained. The phase concentrations were affected by potassium admixture. The catalytical activity increased for the most active catalyst by approximately 50% compared to non-promoted cobalt molybdenum nitrides. The thermal stability test resulted in a loss of activity, on average, of 30%. Deactivation was caused by the collapse of the porous structure, which is attributed to the conversion of the Co2Mo3N phase to the Co3Mo3N phase.

Influence of Final Nitriding Temperature on the Preparation and the Catalytic Performance of CoMoNx/CNTs for Ammonia Decomposition

A series of cobalt-molybdenum nitride catalysts were prepared using Co-Mo oxide precursors via temperature-programmed reaction in N2-H2 mixed gases. The catalysts were characterized by N2 physical adsorption, X-ray diffraction, temperature-programmed desorption of H2. Their catalytic performance was evaluated in the model reaction of ammonia decomposition. The influence of the final nitriding temperatures on the surface properties and the catalytic perfomance of CoMoNx/CNTs were described. The catalyst nitrided at 650°C shows the best catalytic performance. The results indicated that a suitable final nitriding temperature contributes directly to the formation of nitrides and favor the catalyst activity.