Study On The Preparation Of Controllable Structure Nickel Catalysts And The Corresponding Catalytic Performance In Plasma-assisted CH₄/CO₂ Reforming

Dry reforming of methane to generate synthetic gas(syngas, CO and H2) is one of important routes to reduce the emission and to realize the utilization of greenhouse gases(CH4 and CO2). Due to the limitation of high energy consumption, harsh reaction conditions, special requirements on equipment materials, and catalyst deactivated by coke formation, the thermal reforming of CH4 and CO2 has not yet been applied to industrial process. As a potential technology of the dry reforming of methane, plasma-catalysis hybrid reaction has attracted increasingly attention around the world in recent years. However, dry reforming of CH4 and CO2 assisted by the combination between catalysts and cold plasma suffers from several shortcomings, such as low reactants conversion, complex distribution of products, low selectivity of desired products, and carbon deposition. The scientific challenges in the industrial application of plasma catalytic dry reforming of methane are to improve the catalytic performance and stability of catalysts in plasma-catalysis hybrid reaction. Based on above discussed object, the following researches have thus been carried out in this thesis, which focuses on the preparation of controllable structure nickel catalysts and the corresponding catalytic performance in plasma-assisted CH4/CO2 reforming.①Core-shell La Ni O3@Si O2 nanoparticles were prepared and characterized. The Ni–La2O3@Si O2 catalyst reduced from La N i O3@Si O2 precursor was applied to the dry reforming of CH4 and CO2 in dielectric barrier discharge plasma. Compared with supported catalysts N i/Si O2, N i–La2O3/Si O2, and N i/La2O3 generated from perovskite La N i O3, the core-shell N i–La2O3@Si O2 catalyst showed high catalytic performance and stability for the plasma catalytic dry reforming of methane. This arises from the strong interaction between metallic N i0 particles and the protective effect of Si O2 shell. The metallic N i0 particles and La2O3 particles generated from the reduction of perovskite La N i O3 particles are located at the inner wall of porous silica shell, of which the strong core-shell interaction and porous silica shell can inhibit metallic nickel sintering and coke formation. In addition, the cavities generated between Ni–La2O3 core and Si O2 shell can serve as a micro-reactor, leading to the enhancement of surface adsorption and catalytic reactivity.②Core-shell Ni Co2O4@Si O2 nanoparticles were prepared and characterized. The catalytic performance of bimetallic N i–Co@Si O2 catalyst reduced from N i Co2O4@Si O2 precursor was investigated in the plasma-assisted dry reforming of CH4 and CO2 in dielectric barrier discharge plasma. Results suggested that core-shell N i–Co@Si O2 catalyst presented better catalytic activity and durability than those of supported Ni-based catalysts, such as N i/Si O2 and N i–Co/Si O2. The strong interaction of silica shell and N i–Co alloy core can suppress the sintering of N i–Co alloy particles and carbon deposition, and porous Si O2 shell also shows positive effect on the resistance to carbon formation. These account for the high catalytic performance and excellent stability of core-shell Ni–Co@Si O2 nano-catalyst.③Core-shell Ni Fe2O4@Si O2 nanoparticles were prepared and characterized. The bimetallic N i–Fe@Si O2 catalyst reduced from N i Fe2O4@Si O2 precursor was employed to the dry reforming of CH4 and CO2 in dielectric barrier discharge reactor. Silica-coated Ni–Fe bimetallic catalysts showed high activity and stability for the plasma-assisted catalytic reforming reaction. The main reasons are that N i–Fe particles can enhance the dispersion of metallic nickel particles to form much more N i0 active sites, and the cavities of N i–Fe particles and silica shell can serve as microcapsule- like reactor, inducing to facilitate the plasma-catalysis reforming of CH4 and CO2. What’s more, the strong interaction of N i–Fe particles and Si O2 shell and the coated effect of Si O2 shell can effectively hinder the sintering and carbon formation of active metal particles.④Embedded structure Ni Fe2O4#Si O2 nanoparticles were prepared by the modified St?ber method, in which ultrafine spinel Ni Fe2O4 nanoparticles(< 10 nm) were embedded in Si O2 shell. The Embedded structure N i–Fe#Si O2 generated from the reduction of N i Fe2O4#Si O2 presented much more active metallic sites with high dispersion compared to the supported N i-based catalysts. The catalytic reactivity of bimetallic N i–Fe#Si O2 catalyst can be enhanced by the metallic N i–Fe sites with small particle size and the cavities generated from the N i–Fe particles and Si O2 shell. In addition, the porous Si O2 shell and the strong interaction of N i–Fe particles and Si O2 shell can inhibit the sintering and carbon formation of ultrafine N i–Fe particles, and also the Fe-rich on the surface of N i–Fe particles can disperse metallic nickel particles and provide active oxygen actives to remove carbon, leading to the high activity and stability for the plasma-catalysis reaction.⑤The synergistic mechanism of controllable structure Ni-based catalyst and dielectric barrier discharge plasma for the dry reforming of CH4 and CO2 was discussed in this part. The catalyst particles packed at discharge zone affect the discharge behavior of cold plasma, and the non-thermal plasma can in turn change the physicochemical properties of catalyst particles located at the plasma field. The lattice defect and vacancies of active metal particles of catalyst can easily generated when catalyst suffers from the dual forces of plasma filed force and lattice force of metal particle, which favor the catalytic activity of catalyst for the plasma catalytic reaction. The excited particles CHx(x=1~4) and COx(x=1, 2) are preferential adsorbed on the active sites of catalyst and then react with each other on the catalyst surface. The plasma-catalysis hybrid reaction is a combination process between the free radical reactions based on inelastic collision of energetica electrons and gas molecules and the surface-catalyzed reactions of catalyst.The above results suggested that the controllable structure nickel catalysts placed at discharge zone presented better catalytic activity and stability for the reforming of CH4/CO2 than those of supported nickel-based catalysts, due to the “encapsulation effect” of Si O2 shells in core-shell structure or embedded structure. The synergy mechanism of catalysts and plasma is the free radiacal reactinons of excited species and the catalytic reaction. The activied species, CHx(x=1~4) and COx(x=1 、 2), are preferential adsorbed and surface-catalyzed to generate desired products of CO and H2.