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

• Published in: Journal of catalysis 2013-09, Vol.305, p.290-306
• Main Authors: Szijjártó, Gábor P., Pászti, Zoltán, Sajó, István, Erdőhelyi, András, Radnóczi, György, Tompos, András
• Format: Article
• Language: English
• Subjects: acetaldehyde; activation energy; active sites; Active sites in Ni-based catalysts; adsorption; Catalysis; Catalysts; catalytic activity; cations; Chemistry; cobalt; Ethanol; Exact sciences and technology; General and physical chemistry; High throughput experimentation; Hydrogen production; magnesium; Metals; Multi-component catalysts; nickel; oxygen; reflectance; Spinel; steam; Steam reforming of ethanol; synergism; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; transmission electron microscopy; X-ray diffraction; X-ray photoelectron spectroscopy
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2013.05.036

Metallic Ni is a key ingredient of oxide supported ethanol reforming catalysts. Metallic Ni content on MgAl2O4 support was optimized by addition of Co, which occupied exchangeable Mg sites and forced Ni to environments more accessible for reduction. Increased concentration of metallic sites and charge transfer from metallic particles to the support were responsible for the enhanced ethanol reforming performance of the bimetallic catalyst. [Display omitted] •Formation of oxygen vacancies (F-centers) in the MgAl2O4 support has been evidenced.•In the MgAl2O4 support, Mg2+ is partially exchanged by Co2+ and Ni2+.•Preferential exchange of Mg to Co forced Ni to sites more accessible for reduction.•A charge transfer from Ni to F-centers facilitates initial steps of reforming.•Higher amount of metallic Ni species accelerate direct steam reforming of CH3CHO. 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 adsorp