Nature of the active sites in Ni/MgAl2O4-based catalysts designed for steam reforming of ethanol

Noble metal-free MgAl2O4-supported multicomponent catalysts containing Ni, Co, and Ce have been prepared by means of homogeneous precipitation. Origin of the synergistic effects in steam reforming of ethanol between different components in the catalyst has been investigated by means of kinetic tests, in situ Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy, X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM) measurements. Formation of oxygen vacancies in the MgAl2O4 support, so-called F-centers, has been evidenced, which have been proved to play a crucial role in the catalytic activity of our catalysts. In multicomponent systems, incorporation of Ni and Co to sites originally occupied by Mg cations was evidenced. In catalysts containing both Ni and Co, the relative amount of metallic Ni was considerably higher than in the Ni-only case, as preferential exchange of Mg to Co in the spinel support forced Ni to locations more accessible for reduction. From XPS data charge transfer from the metallic particles to the defect states of the support was inferred. A mechanism for co-operation of the defect sites of the electron-rich support and the metallic particles is proposed for facilitating initial steps of ethanol reforming leading to acetaldehyde while the higher number of metallic Ni species (prevailing to oxidized forms) accelerates direct steam reforming of acetaldehyde. Higher activation energy of water gas-shift reaction over the multicomponent catalysts is explained by the presence of oxidized Co species, which are less active in adsorption and activation of CO than the metallic forms. (C) 2013 Elsevier Inc. All rights reserved.