Physicochemical Improvement of Nickel-Alumina Catalysts for Hydrogen Production by Steam Reforming of Liquefied Natural Gas (LNG)

Abstract

Hydrogen has attracted much attention as the most promising energy carrier because it is clean and does not emit any pollutants such as NOx and SOx during the combustion. High energy density of hydrogen is another advantage of hydrogen utilization. These characteristics of hydrogen lead to a development of several hydrogen-related products such as hydrogen vehicle, combustion engine, and fuel cell. Although splitting of water through photo-catalysis and electrolysis is known as an ultimate method for hydrogen production, low productivity and high cost make it unfavorable for commercial hydrogen production. Instead, several catalytic reforming processes for commercial hydrogen production from hydrocarbons have been extensively investigated, including steam reforming, partial oxidation, auto-thermal reforming, and dry reforming. Among these reforming processes, steam reforming has been widely employed for hydrogen production due to its high economical advantage. Moreover, liquefied natural gas (LNG), which is abundant and mainly composed of methane, can be used as a primary source for hydrogen production by steam reforming reaction. As LNG pipelines are more widespread in the modern cities, therefore, LNG will become an important hydrogen source for fuel cell system equipped with fuel processing unit. Nickel-based catalyst has been considered as the most feasible catalyst for steam reforming reactions due to its high intrinsic activity and low price. Conventional steam reforming reaction has been carried out at high reaction temperature (> 800 oC), high reaction pressure (> 20 bar), and high steam to carbon ratio (steam/carbon > 2) in order to achieve a maximum catalytic performance of nickel-based catalyst. However, such severe reaction conditions are not favorable for on-site hydrogen production due to safety problems. For this reason, developing an efficient steam reforming catalyst with high catalytic activity and durability at moderate reaction conditions is of great importance. It has been reported that well-developed mesoporous structure of nickel-alumina catalyst not only enhanced the dispersion of active nickel site on the alumina support, but also facilitated the heat/mass transfer over the catalyst, resulting in a high catalytic performance. Thus, mesoporous nickel-alumina catalysts have received much attention for improving both catalytic activity and stability in the reforming reactions. In this work, in order to derive high hydrogen production efficiency in the catalytic steam reforming of LNG, mesoporous nickel-alumina catalysts were physicochemically modified by various preparation methods, including epoxide-driven sol-gel method, supercritical CO2 drying method, carbon-templating method, phosphorus addition, evaporation-induced self-assembly method, and chemical immobilization of nickel. First of all, a mesoporous Ni-Al2O3 aerogel catalyst was prepared by a single-step epoxide-driven sol-gel method and a subsequent supercritical CO2 drying method (NA-ES catalyst). For comparison, a mesoporous Ni-Al2O3 aerogel catalyst was also prepared by a single-step alkoxide-based sol-gel method and a subsequent supercritical CO2 drying method (NA-AS catalyst). Nickel species were highly dispersed in alumina through the formation of nickel aluminate phase in both NA-ES and NA-AS catalysts. However, nickel species in the NA-ES catalyst exhibited high reducibility and high dispersion compared to those in the NA-AS catalyst. In the steam reforming of LNG, NA-ES catalyst exhibited better catalytic performance than NA-AS catalyst in terms of LNG conversion and hydrogen yield. A series of mesoporous nickel-alumina xerogel catalysts (denoted as CNAX) were prepared by a single-step carbon-templating sol-gel method using different amount of carbon template (X). Textural properties of CNAX catalysts were improved with increasing the amount of carbon template. It was revealed that the interaction between nickel species and alumina in the CNAX catalysts became weakened with increasing the amount of carbon template. Crystallite size of metallic nickel in the reduced CNAX catalysts showed a volcano-shaped trend with respect to the amount of carbon template. In the steam reforming of LNG, CNAX (X = 0, 6, 12, and 18) catalysts exhibited a stable catalytic performance during the reaction, while CNA24 catalyst showed a significant catalyst deactivation. Initial LNG conversion and initial hydrogen yield increased with decreasing crystallite size of metallic nickel of the catalysts. A series of nickel catalysts supported on mesoporous phosphorus-modified alumina xerogel (Ni/XPA, X = P/Al molar ratio) were prepared by an epoxide-driven sol-gel method and a subsequent incipient impregnation method. Although the calcined Ni/XPA catalysts retained both nickel oxide phase and nickel aluminate phase, relative distribution of nickel species of the catalysts was different depending on P/Al molar ratio. Crystallite size of metallic nickel increased with increasing P/Al molar ratio in the reduced Ni/XPA catalysts. However, Ni/0.05PA catalyst showed the largest amount of strong hydrogen-binding sites and exhibited the largest amount of adsorbed methane in the H2-TPD and CH4-TPD measurements, respectively. Catalytic performance in the steam reforming of LNG over Ni/XPA catalysts showed a volcano-shaped trend with respect to P/Al molar ratio. This result was well correlated with the amount of adsorbed methane calculated from CH4-TPD measurements. A mesoporous nickel-phosphorus-alumina aerogel catalyst (NPAA) was prepared by a single-step epoxide-driven sol-gel method and a subsequent supercritical CO2 drying method. For comparison, a mesoporous nickel-phosphorus-alumina xerogel catalyst (NPAX) was also prepared by a single-step epoxide-driven sol-gel method and a subsequent evaporative drying method. It was found that supercritical CO2 drying method was effective for enhancing textural properties of NPAA catalyst. It was also observed that the reduced NPAA catalyst exhibited high nickel dispersion and large amount of methane adsorption compared to the reduced NPAX catalyst. In the steam reforming of LNG, NPAA catalyst with high affinity toward methane showed a better catalytic performance than NPAX catalyst. An ordered mesoporous nickel-alumina catalyst (denoted as OMNA) was prepared by a single-step evaporation-induced self-assembly method. For comparison, a nickel catalyst supported on ordered mesoporous alumina support (denoted as Ni/OMA) was also prepared by an impregnation method. Although both Ni/OMA and OMNA catalysts retained unidimensionally ordered mesoporous structure, textural properties of the catalysts were significantly affected by the preparation method. Nickel species in the OMNA catalyst exhibited not only high reducibility but also strong resistance toward sintering during the reduction process, compared to those in the Ni/OMA catalyst. In the catalytic tests, OMNA catalyst with small crystallite size of metallic nickel exhibited higher LNG conversion and hydrogen yield than Ni/OMA catalyst. Furthermore, OMNA catalyst was more active in the steam reforming of LNG than non-ordered mesoporous nickel-alumina catalysts prepared by common surfactant-templating methods using cationic, anionic, and non-ionic surfactants. Chemical immobilization of nickel was also attempted in order to increase nickel dispersion on alumina support. In conventional impregnation, nickel species are significantly aggregated on alumina support. However, nickel species can be finely dispersed on alumina support by chemical immobilization through Coulombic interaction between positively-charged alumina surface and Ni(EDTA)2- complex anion. The catalyst prepared by a chemical immobilization method (NiE/Al) showed larger nickel dispersion and larger methane adsorption capacity than the catalyst prepared by a conventional impregnation method (Ni/Al). Accordingly, NiE/Al catalyst exhibited better catalytic performance in the steam reforming of LNG than Ni/Al catalyst. In summary, various physicochemically-improved nickel-alumina catalysts were prepared and they were applied to the hydrogen production by steam reforming of LNG in this study. In order to explain catalytic performance of the prepared catalysts in the steam reforming of LNG, several characterizations such as N2 adsorption-desorption, XRD, TPR, TEM, H2-TPD, and CH4-TPD analyses were carried out. It was concluded that nickel dispersion and reactant affinity of the catalysts played as an important factors determining the catalytic performance in the hydrogen production by steam reforming of LNG.