Catalyst coking during ethanol steam reforming is a key problem, especially during testing at relatively low temperature (773 K). Coking activity is related to both support acidity and the tendency of some active phases to form carbon filaments. The latter also depends on the metal support interaction, which determines metal dispersion and stability. In order to deepen this point, different active phases and precursors have been compared as in the following two examples. Hydrotalcite-like precursors were found to allow excellent dispersion of the active phase and promoter in steam reforming catalysts. The data here reported provide evidence of the good catalytic activity of Co-Zn-Al and Co-Ni-Zn-Al catalysts prepared from hydrotalcite-like LHD precursors for ethanol steam reforming. At 823 K the most active Co/Ni catalyst contains a predominant spinel phase with composition near Zn0.58Ni0.42[Al0.44Co0.56]2O4 and small amounts of NiO and ZnO. On the other side, at 873 K the selectivity to hydrogen increases with cobalt content, the most selective catalyst being the Ni-free Co-Zn-Al mixed oxide, essentially constituted by a single spinel type phase Zn0.55Co0.45[Al0.45Co0.55]2O4. Cobalt catalysts appear consequently to behave better than nickel based catalysts in this high temperature range. The key feature for high selectivity to hydrogen is proposed to be associated to the stability of a relatively high oxidation state of Co at the catalyst surface, the most relevant selectivity determining step being constituted by the evolution of surface acetate species. In fact, over oxidized catalyst surface the acetate species evolve producing CO2 and hydrogen while over a more reduced surface they evolve giving rise to methane and COx. Water is supposed to have the main role of allowing surface sites to stay in an unreduced state at least in the temperature range 720-870 K. Co based catalysts, in spite of the usually better carbon balance, proved in general less active than Ni-based ones when tested at lower temperature, i.e. 773 K. As a matter for comparison, TiO2 was also explored as possible support for Ni- and Co-based catalysts for the steam reforming of ethanol. Ni confirmed the most active metal for this reaction, within those here investigated, but many concerns rely on its coking activity. The comparison between the active phases confirm the role of Ni on carbon accumulation over the catalyst, in the form of multiwalled carbon nanotubes, with a more limited contribution of support acidity. Higher resistance to coking may be achieved when loading Ni in very dispersed form and strongly interacting with the support, which may lead to the stabilization of the active phase under the reaction conditions. One possibility is the formation of a mixed oxide, which allows the obtainment of very dispersed metal particles after reduction at high temperature. In other cases, the use of this support may be detrimental due to surface reconstruction and possible coverage of the active phase by the support itself, leading to practically inactive catalysts.
|Titolo:||Cobalt-based and Nichel-based catalysts for hydrogen production by steam reforming of ethanol|
|Data di pubblicazione:||2013|
|Settore Scientifico Disciplinare:||Settore CHIM/02 - Chimica Fisica|
Settore CHIM/04 - Chimica Industriale
|Citazione:||Cobalt-based and Nichel-based catalysts for hydrogen production by steam reforming of ethanol / G. Ramis, E. Finocchio, V. Nichele, M. Signoretto, J. Lasso, I. Rossetti, A. Di Michele. ((Intervento presentato al convegno National Congress of Catalysis GIC 2013 and National Congress of Zeolites Science and Technology tenutosi a Riccione nel 2013.|
|Appare nelle tipologie:||14 - Intervento a convegno non pubblicato|