Promotion of Binary Nitride Catalysts: Isothermal N2 Adsorption, Microkinetic Model, and Catalytic Ammonia Synthesis Activity

The kinetics of ammonia synthesis and N2 dissociation over cesium-promoted Co3Mo3N are studied. The results support recent DFT calculations and explain why Co3Mo3N shows a higher ammonia synthesis activity than either constituting metal. The ammonia synthesis activity of cesium-promoted Co3Mo3N is reported for a series of conditions, with H2 : N2 ratios of both 3 : 1 and 1 : 1, total pressures of 50, 25, and 10 bar, and temperatures from 593 to 713 K. Isothermal N2 adsorption experiments were carried out and the results were well described by a simple model. The total number of surface sites on the catalyst capable of binding nitrogen was found to be approximately 30 μmol/g of catalyst. When only 1% of these surface sites is active for N2 dissociation, the initial sticking coefficient, s0, of dinitrogen on the active sites was found to be s0=0.1727·exp((−42.8±5 kJ/mol)/RT). This result was used as input to a microkinetic model for ammonia synthesis where it was assumed that N* and H* are the only surface intermediates. In this way it was possible to obtain a good fit to the ammonia synthesis activity data with the nitrogen-binding energy as the only adjustable parameter.