K2O is added to iron catalysts in order to accelerate nitrogen chemisorption (which is the rate-limiting step) in ammonia synthesis. This work reports on model studies in which the properties of K and K + O adsorbed on a polycrystalline Fe surface as well as the influence of these adlayers on dissociative nitrogen chemisorption were investigated by means of AES, work function, and thermal desorption measurements. Adsorbed K accelerates the rate of N adsorption markedly but would presumably desorb under stationary reaction conditions (T ≳ 670 K). Addition of oxygen to iron precovered with potassium causes at first a further lowering of the work function (to a minimum value) and thermally stabilizes the adlayer but simultaneously the sticking coefficient for nitrogen is lowered due to blocking of adsorption sites by oxygen. This latter effect is even more pronounced if the sequence of admission (O + K) is reversed. It is very likely that a composite Fe-O-K layer is the actual surface phase under working conditions of the catalyst instead of any known (bulk) potassium compound. The promoter action is ascribed to the enhanced adsorption energy of molecular nitrogen caused by a low local work function which has been explored in detail with single crystals as described elsewhere.
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