Preparation Of Ni/SiO₂ Catalyst And Its Application In Catalytic Decomposition Of Methane

Catalytic methane decomposition（CMD）is a promising route for hydrogen production owing to simple operation,easy separation of the products and no CO_x emission.For highly efficient catalytic reactions,the innovative design and prepartion of catalysts is the key.Nickel-based catalysts have many advantages,such as low price,environmental friendliness,good catalytic performance and so on,which are widely used in the dehydrogenation or hydrogenation reactions of hydrocarbons.However,its poor thermal stability and easy sintering,are not conducive to industrial applications.In this thesis,in order to solve the fast deactivation and low catalytic stability of nickel-based catalysts,it is expected to synthesize catalyst with low-cost and high-performance for decomposition of methane to produce hydrogen by adjusting the preparation process,reaction process and adding metal promoters.The Ni/Si O₂ catalyst was prepared using impregnation method,and the effects of calcination temperature,metal loading,reaction temperature and regeneration cycles on catalytic methane decomposition were systematically investigated.The results showed that Ni/Si O₂ catalysts prepared at different calcination temperatures exhibited mesoporous structure.Catalytic activity first increased with increasing the metal loading up to 30%and then declined.Reaction temperature remarkably affected the activity,stability and the state of the produced carbon.High temperature resulted in the decrease of the stability and the formation of encapsulated carbon.About 9.8%conversion of methane was obtained at 500 ^oC after 1000 min over 30Ni/Si O₂,and the yield of carbon nanofiber is about 7.2 times as that at 650 ^oC.Through the decomposition-regeneration cycle with CO₂ as the regeneration medium,it is found that catalytic activity of Ni/Si O₂ decreases with the number increase of successive regeneration cycles.The Ni（NO₃）₂ was impregnated on vanadium carbide（VC）to prepare composite Ni-VC/Si O₂ catalyst by one-step method.All the Ni-VC/Si O₂ samples exhibited H2-type hysteresis loop with cylindrical pore structure,corresponding to a typical mesoporous material.Compared with the single metal Ni/Si O₂ catalyst,the addition of VC reduced the specific surface area of the catalyst,but the pore volume and pore diameter increased significantly.Ni-VC/Si O₂ catalyst exhibited good performance in decreasing the deactivation rate compared with Ni/Si O₂ catalyst.The catalytic activity in the methane decomposition strongly depends on the size of metallic nickel particles.The addition of vanadium affects the agglomeration mechanism of Ni particles by changing the interaction between the metal and the support,and then inhibits the growth of Ni crystallites during the CMD process.With the increase of VC loading,the catalyst activity first increased and then decreased.The 10Ni-5VC/Si O₂ catalyst showed the best catalytic methane decomposition performance.At 500 ^oC,the methane conversion remained around 13.7%after 2300 min of reaction,and the hydrogen yield was up to 2231 mmol/g_cat during CMD.Mesoporous silica nanoparticles（MSN）with excellent surface properties,controllable pore size and relatively uniform pore structure were prepared through a facile,scalable and low-cost method,using tetraethyl orthosilicate as the silicon source,cetyltrimethylammonium bromide and urea as the template and mineralizing agent,respectively.The resultant MSN has high specific surface area（429.3 m²/g）and pore volume（1.00 cm³/g）,which is beneficial to improve the dispersibility of metal particles.All the Ni/MSN samples exhibited a typical type IV isotherm,with an obvious mesoporous structure.The activity of the catalyst first increased with increasing the metal loading up to 20%and then declined.About 32%conversion of methane was obtained at 550 ^oC after 600 min over 20Ni/MSN.The increase of the reaction temperature leads to faster catalyst deactivation rate and the decrease in stability.