Ethanol steam reforming with zirconia based catalysts

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2014
Arslan, Arzu
Production of hydrogen, which has been considered as an environmentally clean ideal energy carrier, from abundant energy resources cleanly and renewably is essential to support sustainable energy development. Hydrogen production from bio-ethanol by steam reforming process is a promising approach, since bio-ethanol is the most available bio-fuel in the world and steam reforming of ethanol yields formation of 6 moles of hydrogen per mole of ethanol. Support material used for nickel based catalysts plays a crucial role for determining the activity and the stability of the catalyst, in ethanol steam reforming reaction. The main objective of this study was to achieve high purity hydrogen production with minimum coke deposition by using zirconia-silicate and ceria-zirconia supported nickel catalysts. Mesoporous zirconia and zirconia-silicate structured materials, such as Zr-SBA-15, Ce-Zr-SBA-15, Mg-Zr-SBA-15 and Zr-MCM-41, were synthesized following surfactant assisted synthesis routes and nickel was impregnated (6 wt.%) on these catalyst supports. Despite its highly acidic nature, Ni@ZrO2 catalyst resulted in reasonably high hydrogen production and medium-level graphitic carbon deposition. Zirconia-silicate supported catalysts showed much higher ethanol steam reforming activity and carbon deposition due to their higher surface area (>300 m2/g). Ni@Zr-SBA-15 catalyst resulted in very high catalytic activity, with hydrogen yield values approaching to 90% of the maximum possible yield of 6 and stability at 600 ℃ and 650 ℃. Ce4+ or Mg2+ incorporated Zr-SBA-15 supported nickel catalyst suffered from high carbon deposition rates, especially at 600 ℃, compared to Ni@Zr-SBA-15. Reduction of water adsorption-dissociation capability caused by magnesia and ceria limited the carbon gasification capability of zirconia in Zr-SBA-15 type material. In order to obtain maximum hydrogen yield with minimum coking, both acidity and ability of catalyst for dissociation of water to gasify the deposited carbon should be adjusted in ethanol steam reforming catalysts. In order to achieve high purity hydrogen production with long term stability, reaction temperature of ethanol steam reforming process and catalyst properties requires optimization. Properties of mesoporous CeO2-ZrO2 catalysts such as calcination and reduction temperatures and CeO2/ZrO2 ratio of the catalyst were adjusted according to activity test results. Calcination, reduction and reaction temperatures were investigated in the range of 450-650 ℃ and ZrO2/CeO2 ratio was changed from 0 to 1/6. With CeO2-ZrO2 supported nickel catalyst having CeO2/ZrO2 ratio of 6, 450 ℃ was found to be the optimum for calcination, reduction and operating temperatures. Calcination at higher temperatures resulted in collapse of the mesoporous structure of the materials due to sintering. Activity tests at 450 ℃ revealed the importance of crystal size in ethanol steam reforming reaction. CeO2-ZrO2 supported nickel catalyst having CeO2/ZrO2 ratio of 6 calcined and reduced at 650 ℃ caused 34% graphitic carbon deposition with very low hydrogen production and only 2% coke deposition was occurred with the material calcined and reduced at 450 ℃ with a 5.15 hydrogen formation per mole of ethanol which corresponds to 86% of the maximum possible value of 6. Investigation of optimum CeO2/ZrO2 ratio lead to the selection of 4 due to its best catalytic activity with over 90% hydrogen production per mole of ethanol, despite the formation of 30% graphitic carbon.