One-pot synthesis of Ni/$Al_2O_3$ catalysts via spray pyrolysis process for methane reforming reaction

Abstract

basicity introduction into catalyst, oxygen carrier addition, strong interaction among components, nano-confinement and porosity of support, and bimetallic active sites. The basicity introduction into catalyst, which is mostly realized by introducing alkali or alkaline earth metal into the catalyst, is justified by the fact that the acidic sites are responsible for the coke formation. The oxygen carrier, representatively the cerium, changes the oxidation number under reaction condition and produces reactive oxygen that gasify the coke of the catalyst. The strong interaction, especially between the metal and the support prevents the active metal from being completely enclosed in the coke and also inhibits sintering of the active metal under high temperature reaction conditions. The porosity of the support provides a sufficiently large specific surface area to maximize the dispersion of the active metal, and to suppress sintering. The size of sintered metal particles does not exceed the pore size even if sintering occurs. The introduction of bimetallic active sites is to aim synergy of different kinds of active metals on the reaction mechanism. A combination of nickel and cobalt is mainly used. In this thesis, a spray pyrolysis is used as a method for synthesizing catalysts. In a spray pyrolysis reactor, droplets are generated from a homogeneous precursor solution by an ultrasonic nebulizer. A stream of droplets are carried by a carrier gas into a furnace reactor in which drying and pyrolysis occur. At the end of a spray pyrolysis reactor, a filtration system collects the catalyst particles. The synthesized particles have a uniform composition inherited from the stoichiometry of the precursor solution. These particles are thermodynamically metastable in spite of passing through a high temperature furnace because drying and pyrolysis take place in a very short time. Subsequent heat treatment converts these metastable particles into thermodynamically stable structures. In addition to a spray pyrolysis of a plain aqueous solution, an evaporation-induced self-assembly (EISA) is also used. In EISA, a pore structure of catalyst particles is modified by self-assembly of a mixture of salt solution and surfactant. Before the catalyst was synthesized by spray pyrolysis, a support was synthesized. Alumina was selected because of high physical stability, chemical inertness, and low cost. At high temperatures, the alumina changes its crystal structure from face-centered cubic to hexagonal close-packed and undergoes reconstruction of crystal structure, and eventually pores are collapsed. In this study, this problem was solved by doping heteroatom in alumina. In addition, in the spray pyrolysis method, the carbon source is gasified so that the pore size is larger than that of a liquid phase synthesis method although using the same structure directing agent. It was found that the expansion of the pores is achieved by introducing a proper amount of interactable material with structure directing agent without interfering with the pore forming ability. For one-pot synthesis of nickel supported alumina catalyst, nickel oleate solution was used in the evaporation induced self-assembly process. The nickel oleate molecule interacts with the hydrophobic group of the structure directing agent and generates “metal-in-pore” structure. Different from evaporation induced self-assembly carried out in an oven, nickel metal is located inside the catalyst without large nickel crystal outside the catalyst even though the structure directing agent is not sufficiently segregating phases. In addition, since the oleate group acts as a spacer during the formation of nickel aluminate in the calcination process, the dispersion of nickel metal was improved compared to the catalyst synthesized using nickel nitrate. Among the synthesized catalysts, nickel oleate-based catalysts exhibited the most stable and the highest reactivity and the least coke was formed due to the high dispersity and strong metal support interaction. Even higher dispersity of nickel metal was achieved by adding sucrose into the precursor solution. Added sucrose served as additional spacer before nickel aluminate formation. This study demonstrated the possibility of spray pyrolysis as a whole catalyst synthesis for dry reforming of methane. Due to the thermodynamic metastability of the particles synthesized by spray pyrolysis, the strong metal support interaction of the catalyst was improved over the conventional impregnation catalyst. The enhanced strong metal support interaction contributed to high dispersity of active metal and stable reactivity. Nickel oleate was used to effectively place the nickel metal in the pores, and oleate or sucrose served as a spacer and dispersed the active metal better than conventional methods.

A synthesis gas is a mixture of gases including hydrogen and carbon monoxide in various ratio. Chemical products such as ammonia, ethanol, and synthetic fuel, are produced from the synthesis gas. The production method of synthetic gas is mainly coal gasification and other method is reforming of natural gas. Among the various reactant combination for reforming reaction, methane and carbon dioxide are drawing attention from researchers because this process can contribute to the removal of greenhouse gases. The reaction between methane and carbon dioxide, which is generally called “dry reforming of methane” (DRM), is an endothermic reaction which means that it is thermodynamically favorable at high temperature and heating of a reactor is required to promote the forward reaction. The important side reaction of DRM is coke formation reaction through methane decomposition reaction that is favored at high temperatures or carbon monoxide disproportionation reaction that is favored at low temperatures. Nickel active metal catalysts are mainly used for the dry reforming reaction of methane, because of the cost issue. However, the catalyst is degraded due to sintering and coke formation under the reaction conditions. To slow down the degradation of the catalyst, many attempts have been made, which are categorized into 5 areas as following