The influence of Rh nanoparticle size and type of support on the catalytic performance in steam methane reforming has been investigated to clarify the nature of the rate-controlling step. A set of Rh catalysts was prepared using ZrO(2), CeO(2), CeZrO(2) and SiO(2) supports. The nature and dispersion of the active Rh metal phase was studied by H(2)-chemisorption, TEM and X-ray absorption spectroscopy. The particle size was varied between 1 and 9 nm. The degree of Rh reduction depends on the particle size and the support. Very small particles cannot be fully reduced, especially when ceria is the support. The intrinsic rate per surface metal atom increases linearly with the Rh metal dispersion and does not depend on the type of support. With the support of kinetic data, it is concluded that dissociative CH(4) adsorption is the single rate-controlling step at least at reaction temperatures above 325 degrees C. This implies that the overall rate is controlled by the density of low-coordinated edge and corner metal atoms in the nanoparticles. These particles contain sufficient step edge sites to provide an easy reaction pathway for C-0 recombination reactions. Catalysts with Rh nanoparticles smaller than 2.5 nm deactivate more strongly than catalysts with larger nanoparticles. Characterization of spent catalysts by X-ray absorption spectroscopy shows that deactivation is due to the oxidation of very small particles under the steam methane reforming reaction conditions. (C) 2011 Elsevier Inc. All rights reserved.