The high purity hydrogen, free from any traces of carbon monoxide is crucial for the efficient operation of hydrogen proton exchange membrane fuel cells (PEMFCs). In this study we report the low temperature (25-120 degrees C) carbon monoxide methanation in hydrogen-rich streams over 1 wt. % Pt nanoparticles supported on yttria-stabilized zirconia (YSZ) support. Pt nanoparticles with four average particle sizes 1.9, 3.0, 4.4 and 6.7 nm are investigated. The turnover frequency at 30 degrees C of CO methanation reaction rate doubles with decreasing Pt mean particle size from 6.7 to 1.9 nrn. The high activity of Pt/YSZ catalyst is due to the backspillover of ionic species O delta- from YSZ support to Pt active sites. Under the reaction conditions, O delta- forms an "effective" double layer at the catalyst surface that weakens CO (electron acceptor) adsorption and strengthens H-2 (electron donor) adsorption bonds and this effect becomes more pronounced as nanoparticle size decreases. Crown Copyright (C) 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. All rights reserved.