Bridging the structure -properties relationship of bimetallic catalysts is essential for the rational design of heterogeneous catalysts. Different from random alloys, intermetallic compounds (IMCs) present atomically -ordered structures, which is advantageous for catalytic mechanism studies. We used Pt based intermetallic nanoparticles (iNPs), individually encapsulated in mesoporous silica shells, as catalysts for the hydrogenation of nitroarenes to functionalized anilines. With the capping -free nature and ordered atomic structure, PtSn iNPs show >99% selectivity to hydrogenate the nitro group of 3-nitrostyrene albeit with a lower activity, in contrast to Pt3Sn iNPs and Pt NPs. The geometric structure of PtSn iNPs in eliminating Pt threefold sites hampers the adsorption/dissociation of molecular H-2 and leads to a non-Horiuti-Polanyi hydrogenation pathway, while Pt3Sn and Pt surfaces are saturated by atomic H. Calculations using density functional theory (DFT) suggest a preferential adsorption of the nitro group on the intermetallic PtSn surface contributing to its high selectivity. (C) 2017 Elsevier Inc. All rights reserved.