Hydrogen Production by Steam Reforming of Ethanol over Mesoporous Nickel-Alumina-Zirconia Catalysts

Abstract

As a renewable energy, hydrogen has received much attention on the basis of their potential contribution to the sustainable energy development. High energy density and environmental compatibility is main advantages of hydrogen utilization. Hydrogen production technologies can be classified into thermal processes, electrolyte processes, and photolytic processes. Although splitting of water through photo-catalysis and electrolysis is known as an ultimate method for hydrogen production, most hydrogen production is based on LNG (liquefied natural gas) reforming process which is still based on non-renewable fossil fuel system. Therefore, development of hydrogen production technology using renewable source is required to attain sustainability and fuel flexibility. In this situation, hydrogen production by bio-derived liquids reforming has been studied, which is the most viable renewable hydrogen pathway in the near future. Among the bio-derived liquids, ethanol is the most feasible source for hydrogen production in the near term due to its non-toxicity, storage facility, and biodegradable nature. Moreover, bio-ethanol can be used as a source for SOFC vehicle which has potential to become an alternative transportation in the future. As a consequence, ethanol steam reforming is a promising technology for hydrogen production which is expected to mitigate the environmental problems. For steam reforming of ethanol, Ni-based catalyst has been extensively used as a non-noble metal catalyst due to its low cost and high activity in C-C cleavage reaction. In general, Ni/Al2O3 catalyst has been widely employed for steam reforming of ethanol, because Al2O3 has large surface area and strong metal-support interaction with Ni particles. However, Al2O3 support with acidic nature induces ethanol dehydration, leading to coke formation. Therefore, many attempts have been made to increase both catalytic activity and durability of Ni/Al2O3 catalysts through modification of supporting materials. Alkaline earth metal oxides such as MgO and CaO have been used as promoters because they can suppress coke deposition by neutralizing acid sites of Al2O3. Addition of lanthanide oxides can also increase the catalytic activity by promoting gasification reaction of dissolved carbon species on the surface of nickel catalysts. Among various metal oxides, ZrO2 is known to be the effective promoter for Ni/Al2O3 catalysts in the steam reforming of ethanol, because ZrO2 can not only enhance the stability of the catalysts but also promote adsorption and dissociation of water on the surface of nickel catalysts. In this work, in order to achieve efficient catalyst for steam reforming of ethanol, mesoporous nickel-alumina-zirconia catalysts were designed by various catalyst compositions and preparation methods, including epoxide-driven sol-gel method, P123-assissted sol-gel method, supercritical CO2 drying method, and copper addition. First of all, a series of mesoporous nickel-alumina-zirconia xerogel catalysts (denoted as Ni-AZ-X) with different Zr/Al molar ratio (X) were prepared by a single-step epoxide-driven sol-gel method, and they were applied to the hydrogen production by steam reforming of ethanol. Surface area of Ni-AZ-X catalysts decreased with increasing Zr/Al molar ratio due to the lattice contraction of ZrO2 caused by the incorporation of Al3+ into ZrO2. Interaction between nickel oxide species and support (Al2O3-ZrO2) decreased with increasing Zr/Al molar ratio through the formation of NiO-Al2O3-ZrO2 composite structure. Acidity of reduced Ni-AZ-X catalysts decreased with increasing Zr/Al molar ratio due to the loss of acid sites of Al2O3 by the addition of ZrO2. Among the catalysts tested, Ni-AZ-0.2 (Zr/Al = 0.2) catalyst with an intermediate acidity exhibited the best catalytic performance in the steam reforming of ethanol. A series of mesoporous nickel-alumina-zirconia xerogel catalysts (denoted as XNiAZ) with different nickel content (X, wt%) were prepared by a single-step epoxide-driven sol-gel method. All the XNiAZ catalysts exhibited a well-developed mesoporous structure and they dominantly showed an amorphous NiO-Al2O3-ZrO2 composite phase, leading to high dispersion of NiO. Nickel surface area and reducibility of XNiAZ catalysts showed volcano-shaped trends with respect to nickel content. Among the catalysts tested, 15NiAZ catalyst with the highest nickel surface area exhibited the best catalytic performance in the steam reforming of ethanol. A mesoporous nickel-alumina-zirconia aerogel (Ni-AZ) catalyst was prepared by a single-step epoxide-driven sol-gel method and a subsequent supercritical CO2 drying method. For comparison, a mesoporous alumina-zirconia aerogel (AZ) support was prepared by a single-step epoxide-driven sol-gel method, and subsequently, a mesoporous nickel/alumina-zirconia aerogel (Ni/AZ) catalyst was prepared by an incipient wetness impregnation method. Although both catalysts retained a mesoporous structure, Ni/AZ catalyst showed lower surface area than Ni-AZ catalyst. From TPR, XRD, and H2-TPD results, it was revealed that Ni-AZ catalyst retained higher reducibility and higher nickel dispersion than Ni/AZ catalyst. In the hydrogen production by steam reforming of ethanol, Ni-AZ catalyst with superior textural properties, high reducibility, and high nickel surface area showed a better catalytic performance than Ni/AZ catalyst. A series of mesoporous nickel-alumina-zirconia xerogel (denoted as X-NAZ) catalysts were prepared by a P123-assisted epoxide-driven sol-gel method under different P123 concentration (X, mM), and they were applied to the hydrogen production by steam reforming of ethanol. All the catalysts retained a mesoporous structure. Pore volume of the catalysts increased with increasing P123 concentration. Nickel surface area and ethanol adsorption capacity of X-NAZ catalysts exhibited volcano-shaped trends with respect to P123 concentration. Among the catalysts tested, 12-NAZ catalyst with the highest Ni surface area and the largest ethanol adsorption capacity showed the best catalytic performance in the steam reforming of ethanol. A series of mesoporous copper-nickel-alumina-zirconia (XCNAZ) xerogel catalysts with different copper content (X, wt%) were prepared by a single-step epoxide-driven sol-gel method, and they were applied to the hydrogen production by steam reforming of ethanol. All the calcined XCNAZ catalysts retained a mesoporous structure, and their surface area increased with increasing copper content. Metal-support interaction of XCNAZ catalysts decreased with increasing copper content due to the electronic effect. Nickel surface area and ethanol adsorption capacity of the catalysts exhibited volcano-shaped trends with respect to copper content. Among the catalysts, 0.2CNAZ catalyst exhibited the highest nickel surface area and the largest ethanol adsorption capacity. Catalytic performance in the steam reforming of ethanol over XCNAZ catalysts showed a volcano-shaped trend with respect to copper content. This result was well matched with the trend of nickel surface area. Among the catalysts tested, 0.2CNAZ catalyst with the highest nickel surface area showed the highest hydrogen yield. In summary, various physicochemically-modified nickel-alumina-zirconia catalysts were designed and they were applied to the hydrogen production by steam reforming of ethanol in this study. In order to elaborate the effect of physicochemical properties of catalyst on catalytic performance in the steam reforming of ethanol, several characterizations such as N2 adsorption-desorption, XRD, TPR, TEM, XPS, H2-TPD, EtOH-TPD, and in-situ FT-IR analyses were conducted. It was concluded that nickel surface area served as a crucial factor determining the catalytic performance in the hydrogen production by steam reforming of ethanol.