The role of Rh, Ir and Ru promoters on the structural properties and catalytic behavior of Nilet-Al2O3 catalyst during steam methane reforming (SMR) was investigated. The catalysts were subjected to aging treatments at 800 degrees C and P-H2O = 22 barg in order to induce sintering of the metal nanoparticles and alloying between Ni and the promoters. The crystallite sizes and the extent of alloying were determined in the reduced and aged catalysts by combination of in situ extended X-ray absorption fine structure (EXAFS), X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (STEM-XEDS) mapping. The results reveal that the degree of alloying between Ni and Rh or Ir promoters increases during the aging process, as a result of the high mobility of the nickel species and diffusion into the FCC structures of Rh and Ir. Moreover, the formation of Ni-Rh and Ni-Ir alloy particles has a positive effect in mitigating the sintering of the nickel particles. In contrast, sintering inhibition effect of Ru was poorer due to its lower miscibility in nickel as well as its lower sintering resistance under the aging conditions employed. Based on density functional theory (DFT) calculations the diffusion of the Ni atoms into the lattice structure of Ru is energetically not favorable, whereas Ni atoms are equally stable in the first and second surface layers of Ir and Rh. Hence, a synergistic effect between Ni and Rh or Ir is achieved due to the alloy formation leading to conservation of larger active metal surface area and consequently to superior SMR activity with respect to the monometallic systems. (C) 2015 Elsevier B.V. All rights reserved.