Promoters effect in Ru/C ammonia synthesis catalyst

A series of carbon-supported, ruthenium-based catalysts, variously promoted with alkali and alkali-earth compounds, were prepared, aiming at investigating the effect of such promoters on catalyst activity and stability. It was found that a simultaneous action of three promoters (K+Ba+Cs) maximised both activity and thermal resistance of the catalyst. In particular, Ba is very effective in providing catalyst activity and resistance to methanation, while Cs strongly improves resistance to metal sintering. A further activity improvement is given by K as third promoter. Through a XPS study, the promoter effect was confirmed to be essentially of electronic nature. The optimal catalyst composition, in terms of activity, thermal resistance in the reaction environment and cost, corresponds to Ru ca. 5 wt.%, Ba/Ru=0.6, Cs/Ru=1 and K/Ru=3.5 atomic ratios.