Abstract
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.
Highlights
The Haber–Bosch process developed in the early years of the twentieth century had a great influence on the production of ammonia
In our previous studies [18,19] we reported the complexity of the phase composition of the cobalt molybdenum nitride catalysts, especially the occurrence of two active phases: Co2 Mo3 N and Co3 Mo3 N
Cobalt molybdenum nitride catalysts modified with an admixture of potassium, were obtained in a three-step process
Summary
The Haber–Bosch process developed in the early years of the twentieth century had a great influence on the production of ammonia. This process uses an iron catalyst that allows direct bonding of H2 and N2 and can be considered efficient; due to the huge worldwide production of ammonia, the continuous improvement of catalysts is required for both financial and environmental reasons [1]. An important approach to solving this problem was the application of ruthenium-based catalysts They exhibit high activity in ammonia synthesis but are burdened by several technological flaws, as well as high cost [2,3]. Transition metal nitrides proved to be very effective catalysts that can be obtained as binary [5], ternary [1] and very recently quaternary [6] systems that—with further improvements—can serve as excellent catalysts for ammonia synthesis
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