Promotion in ammonia synthesis: a pressure dependent phenomenon

The origin of the pressure dependence of potassium promotion of iron catalysts has been investigated by measuring the turnover numbers for ammonia synthesis on polycrystalline iron, potassium-doped polycrystalline iron and two triply promoted (K2O, Al2O3, CaO) industrial ammonia synthesis catalysts at 1, 2 and 31 bar in the temperature range 550-800 K. At 1 and 2 bar the turnover numbers of polycrystalline iron and potassium doped polycrystalline iron are greater than those of the industrial catalysts, suggesting that the promoters are merely decorating the surface of the iron, blocking off part of it for reaction. It is only at 31 bar and at temperatures above 670 K, that the promoting effect of the additions to the industrial catalyst becomes apparent, the turnover numbers of these materials being roughly 20 times greater than that of polycrystalline iron. Potassium doped polycrystalline iron, however, has the same turnover number as those of the industrial catalysts. It is concluded that at temperatures above 670 K in a hydrogen/nitrogen (3 : 1) mixture and at 31 bar the morphology of the surface of the industrial catalysts is changed with the iron, probably as iron nitride, coating the potassium aluminate, the identity of the turnover numbers of the catalysts and of potassium doped polycrystalline iron showing that potassium plays a key role in this promotion. The surface restructuring of the industrial catalysts does not produce dominantly the (111) surface, the observed turnover numbers being 10(3) times lower than that of the (111) surface. The activation energy for ammonia synthesis in the temperature range 670-770 K at 31 bar on these materials is much higher (32-37 kcal mol(-1)) than that of iron (111) (19.2 kcal mol(-1)) since it incorporates the activation energy for surface reconstruction of iron. The molecular nitrogen desorption spectra from the industrial catalysts exhibit three states: (i) N-2 bonded end on, (ii) N-2 pi-bonded and (iii) pi-bonded N-2 vicinal to some form of potassium.