Ammonia synthesis and related reactions over iron-cobalt and iron-nickel alloy catalysts. Part I. Catalysts reduced at 853 K

Studies have been undertaken of ammonia synthesis, nitrogen chemisorption and nitrogen isotope exchange over a series of iron-cobalt and iron-nickel alloy catalysts containing 3% of alumina as a structural stabiliser. Catalysts were reduced for 18 h at 853 K before use and metal surface areas were measured by adsorption of carbon monoxide. While nitrogen chemisorption could be observed below 673 K the rate decreased with coverage so markedly that temperatures of 773 K or more were required to attain adsorption equilibrium within a reasonable period. Ammonia synthesis was observed from 550 K but at 700 K or over the rate of synthesis was affected by the back reaction becoming significant. Higher temperatures in the range from 773 K to 853 K were required to give measurable rates of nitrogen isotope exchange. Thus the temperatures for ammonia synthesis correspond to the start of nitrogen chemisorption and those for exchange to the higher temperatures needed for reversible adsorption and the establishment of the adsorption equilibrium. The pattern of activity with composition for the cobalt alloys showed a small maximum at a composition of iron (95%) cobalt (5%) for all three processes (adsorption, synthesis and exchange). A similar but less well-defined maximum was found with catalysts containing from 5% to 15% of nickel. Possible reasons for the enhanced activity of these alloys are discussed.