Monobenzyl toluene (H 0 -MBT) is an interesting material as a liquid organic hydrogen carrier, which is commercially available as heat transfer oil. In H 2 storage of H 0 -MBT via the hydrogenation reaction, supported Ru catalysts were reported to show superior performance. Herein, ZrO 2 -supported Ru nanoparticles were synthesized by using triruthenium dodecarbonyl Ru 3 (CO) 12 as Ru precursor and precipitated zirconium hydroxide subjected to different ageing periods. Although the extended ageing resulted in a larger BET surface area of Ru/ZrO 2 , the hydrogenation activity and Ru dispersion turned out to be in a volcano-shaped dependence on the ageing period. The characterization work on zirconium hydroxide revealed that the relative number of surface to bridging oxygen species decreased upon prolonged ageing and the hydroxylation degree largely affected the interaction with CO ligand of adsorbed Ru species. Consequently, the hydrogenation activity trend of the present Ru/ZrO 2 was a combined consequence of the surface OH content and surface area of zirconium hydroxide.Heat transfer fluids of homocyclic aromatic structures in an oligomeric form are a synthetic, organic medium widely used in a great variety of heat management systems for many decades. They are not only technically available at low prices under various trade names including Marlotherm (Sasol), Farolin (Aral), Diphyl (Lanxess), etc., but show very wide liquid ranges and low flammability. Since these attractive properties appeared to meet the requirements for liquid organic hydrogen carrier (LOHC) systems, Brü ckner et al. evaluated the potential of Marlotherm LH and SH (monobenzyl toluene and dibenzyl toluene regioisomers, respectively) providing a H 2 storage capacity of 6.2 wt % and proposed these oils as a new class of LOHC materials. [1] Based on their work, Hydrogenious Technologies GmbH, Erlangen manufactured a commercial demonstrator for LOHC-based hydrogen logistics using dibenzyl toluene. [2] Hydrogen storage and release for LOHC materials are achieved by the reversible hydrogenation and dehydrogenation reactions, respectively. However, the aforementioned LOHC materials suffer from the relatively high heat of hydrogenation and dehydrogenation; for instance, the hydrogenation enthalpy values of monobenzyl toluene (H 0 -MBT) and dibenzyl toluene (H 0 -DBT) were determined to be approximately À65 kJ per mol H 2 . [3] Due to this feature, the dehydrogenation of endothermic nature should take place at temperatures above 250 8C, and the hydrogenation catalyst has to sustain fairly the abundant heat released upon H 2 storage. However, less attention has been paid to the latter reaction because the dehydrogenation catalyst suffers more from thermal stress than the hydrogenation catalyst.In this communication we focused on supported Ru catalysts for the hydrogenation of H 0 -MBT (purity: > 99 %, an isomeric mixture as represented in Figure S1). Ru, the cheapest among noble metals, was examined the most active for several LOHC materials including N...