Surface science based microkinetic analysis of ammonia synthesis over ruthenium catalysts

A microkinetic model for catalytic ammonia synthesis over nonpromoted ruthenium is developed, based mostly on results from surface science investigations. Nitrogen dissociation is assumed to be the rate-determining step.In the preceding paper this process was investigated over an Ru(0001) surface, and it was found that only steps present in an amount of about 1% at the surface showed any significant activity. It is therefore assumed that only a small fraction of the surface sites on ruthenium catalysts are active, and for the nonpromoted catalysts a nitrogen dissociation rate equal to the rate at the steps of the Ru(0001) surface is used. The catalytic activities of an Ru/MgAl2O4 catalyst were measured for a wide range of conditions in order to thoroughly test the model. The model describes very well the activities of the Ru/MgAl2O4 catalyst as well as other experimental observations on ruthenium catalysts. As a result of fitting the observed negative reaction order for ammonia, the dominating nitrogen containing surface species is NH*. This agrees well with the observed kinetic effects of promoting ruthenium with alkali metals which will interact repulsively with NH*. The dominating surface species at low ammonia concentration is H*, giving rise to the negative reaction order observed for hydrogen.