Nickel versus cobalt catalysts for hydrogen production by ethanol steam reforming: Ni–Co–Zn–Al catalysts from hydrotalcite-like precursors

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• 作者：Guido Busca ; Umberto Costantino ; Tania Montanari ; Gianguido Ramis ; Carlo Resini ; Michele Sisani
• 关键词：Ethanol steam reforming ; Low temperature ethanol steam reforming ; Hydrtalcite catalyst precursors ; ESR ; Nickel catalysts for ESR ; Cobalt catalysts for ESR ; Selectivity in ESR ; Mechanism of ESR
• 刊名：International Journal of Hydrogen Energy
• 出版年：2010
• 出版时间：June 2010
• 年：2010
• 卷：35
• 期：11
• 页码：5356-5366
• 全文大小：939 K

文摘

The urea hydrolysis method allowed to prepare well-crystallized Ni–Co–Zn–Al Layered Double Hydroxides to be used as precursors of mixed oxide catalysts for the Ethanol Steam Reforming (ESR) reaction. The calcination of the layered precursors gives rise to high surface area mixed oxides, being actually a mixture of a rock salt phase (NiO), a wurtzite phase (ZnO) and a spinel phase.

The steam reforming of ethanol has been investigated over these catalysts after calcination at 973 K in flow reactor experiments. All these catalysts are active for ESR. At 820 K the selectivity to hydrogen increases with cobalt content. The most selective catalyst is the Ni-free Co–Zn–Al mixed oxide essentially constituted by a single spinel type phase Zn_0.55Co_0.45[Al_0.45Co_0.55]₂O₄. At 770 K the most active catalyst is constituted by a predominant spinel phase with composition near Zn_0.58Ni_0.42[Al_0.44Co_0.56]₂O₄ and small amounts of NiO and ZnO. Yield to hydrogen exceeding 90 % is obtained with this catalyst at 823 K with a water to ethanol feed ratio of 6. The most relevant selectivity determining step is supposed to be the evolution of acetate ions, assumed to be the key intermediate. The high selectivity to hydrogen and CO₂ obtained in the 770–870 K range is attributed to the less reduced state of cobalt in reaction conditions, allowing the evolution of acetate species to CO₂ and hydrogen. On more or completely reduced surfaces (with Ni and/or less water in the feed) acetates decompose preferentially producing CH₄ and COx.