XPS STUDIES WITH AMMONIA-SYNTHESIS CATALYSTS

The chemical nature and compositions of the surfaces of industrial Fe ammonia catalysts were investigated by means of X-ray photoelectron spectroscopy (XPS). Unreduced catalysts exhibit a strong enrichment of K and Al and only rather small Fe concentrations in the surface region. Treatment in N2 + H2 mixtures (1 atm) at 350-400°C causes complete reduction of iron and a substantial increase of the surface concentration of this element whereas the other cations remain in their oxidized state. The establishment of the synthesis equilibrium was followed by means of a mass spectrometer. Under reaction conditions only relatively small stationary concentrations of atomic nitrogen are present on the surface which supports the conclusion that dissociative chemisorption of nitrogen is the rate-limiting step. After cooling the catalyst to room temperature in the reaction gas mixture in addition NH2 adsorbed to the oxidic components as well as NH and/or NH2 attached to iron were discovered. © 1979.