The activities of Pt catalysts on carbon nanofibers with different nitrogen contents were compared for hydrogen production by formic acid decomposition. The catalysts contained a fraction of Pt clusters with a mean size of 1.0-2.3 nm and possibly a considerable fraction of Pt clusters with a diameter of less than 0.75 nm that were invisible by transmission electron microscopy. The activities of N-doped catalysts with low Pt contents (<= 1 wt.%) were 10 times higher than the activities of undoped catalysts. The N-doped catalysts demonstrated an improved selectivity to hydrogen and an increased resistance to-CO inhibition. However, they were inactive for ethylene hydrogenation. These results are explained by the presence of electron-deficient, two-dimensional sub-nm sized Pt clusters stabilized by pyridinic nitrogen on vacancy sites. In accordance, the Pt-4f(7/2) binding energies measured by X-ray photoelectron spectroscopy were 0.6 eV higher for the N-doped samples than for the undoped ones. (C) 2013 Elsevier Inc. All rights reserved.