The catalytic hydrogenations of carbazole, 9-carbazole-9H-ethanol and acridine over Ru black were studied and compared with the results obtained from 9-ethylcarbazole with the aim to elucidate the influence of the molecules structures on their ability to store hydrogen. The reactions were modelled with the rate constants derived for each of the elementary reactions observed. The specific catalytic activity and selectivity were found to be strongly dependent on variation in the structure of substrate due to both thermodynamic and kinetic reasons. The hydrogenation activity was found to increase in order: acridine > 9-H-carbazole-9H-ethanol > carbazole > 9-ethylcarbazole. The hydrogenation pathway of the molecule consisting of six-membered ring (acridine) was found to be more complex than that of a corresponding five-membered ring structure. The type of intermediates also appeared to depend on the basicity and steric environment of the nitrogen heteroatom, which influenced the geometry of the adsorption on the ruthenium surface. It is believed that this work can provide some general guidelines for the design of superior liquid organic carriers (LOCs). (C) 2014 Elsevier B.V. All rights reserved.