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.

The Role of Composition and Phase upon the Lattice Nitrogen Reactivity of Ternary Molybdenum Nitrides

The reactivity of the lattice nitrogen in the filled β-Mn structured Co2Mo3N and the η-carbide structured Co3Mo3N and Fe3Mo3N has been investigated under 3:1 H2/Ar at temperatures up to 900 °C. The lattice nitrogen in Co3Mo3N was found to be reactive, as reported previously, whereas Co2Mo3N was shown to be stable up to 800 °C. Upon H2/Ar treatment at 900 °C, the Co2Mo3N, Co3Mo3N and Fe3Mo3N phases decomposed. These results suggest that both metal composition and phase have an influence on the bulk lattice nitrogen reactivity of the ternary nitrides.