Article history: Received 18 December 2015 Received in revised form 2 February 2016 Accepted 6 February 2016 Available online xxxx The nature of the nano-catalyzed hydrogenation of ethylene, yielding benchmark information pertaining to the concept of structure sensitivity/insensitivity and its applicability at the bottom of the catalyst particle size-range, is explored with experiments on size-selected Ptn (n= 7–40) clusters soft-landed onMgO, in conjunction with first-principles simulations. As in the case of larger particles both the direct ethylene hydrogenation channel and the parallel hydrogenation–dehydrogenation ethylidyne-producing route must be considered, with the fundamental uncovering that at the b1 nm size-scale the reaction exhibits characteristics consistent with structure sensitivity, in contrast to the structure insensitivity found for larger particles. In this size-regime, the chemical properties can be modulated and tuned by a single atom, reflected by the onset of low temperature hydrogenation at T N 150 K catalyzed by Ptn (n ≥ 10) clusters, withmaximum room temperature reactivity observed for Pt13 using a pulsed molecular beam technique. Structure insensitive behavior, inherent for specific cluster sizes at ambient temperatures, can be induced in the more active sizes, e.g. Pt13, by a temperature increase, up to 400 K, which opens dehydrogenation channels leading to ethylidyne formation. This reaction channel was, however found to be attenuated on Pt20, as catalyst activity remained elevated after the 400 K step. Pt30 displayed behavior which can be understood from extrapolating bulk properties to this size range; in particular the calculated d-band center. In the non-scalable sub-nanometer size regime, however, precise control of particle size may be used for atom-by-atom tuning and manipulation of catalyzed hydrogenation activity and selectivity. © 2016 Elsevier B.V. All rights reserved.