Previous research on ammonia synthesis over ruthenium catalysts is reviewed with a particular emphasis on the action of alkali promoters. The concept of electron donation and the nature of the adsorbed species are examined, and their relationship with electronic and structural factors related to dinitrogen activation is considered. A kinetic analysis is performed and the reaction mechanism involved in ammonia synthesis over ruthenium catalysts is investigated. (C) 2016 Elsevier B.V. All rights reserved.