Preparation And Modification Of Core-shell Ni@SiO₂ Catalysts And Its Performance In Dry Reforming Of Methane

The prosperity and development of human society are inseparable from fossil energy,but its excessive consumption has caused adverse problems such as energy shortage and global warming that seriously threaten human survival and development.The dry reforming with methane reaction?DRM?simultaneously converts greenhouse gases CH₄ and CO₂ to produce an important chemical intermediate synthesis gas?H₂/CO molar ratio is 1?,which is considered as an important chemical that can achieve greenhouse gas emission reduction and sustainable energy production.The response has become a research hotspot for scientists from various countries.Ni catalysts are the best catalysts for methane dry reforming due to their low cost and high activity.However,the sintering of nickel particles and the carbon deposition of the catalyst have greatly affected the stability of the DRM.Therefore,designing Ni catalysts with high activity and stability,and solving the problems of sintering of Ni particles and carbon deposition are the prerequisites for industrial application of DRM.In this thesis,in order to prepare a nickel-based catalyst with high activity and stability,and excellent sintering resistance and low carbon deposition capacity.We took Ni@SiO₂ catalysts as the representative,regulate the structure and properties of the catalyst from three aspects:size effect,metal support interaction and promoter modification to achieve the goal of stable catalytic methane dry reforming.The details are as follows:1.Core-shell Ni@SiO₂ catalysts with ultrafine Ni particle size?less than 5 nm?were synthesized by the microemulsion method and used in the methane dry reforming reaction.The results showed that the core-shell Ni@SiO₂ catalysts exhibited low sintering and low carbon deposition in the 1023 K DRM reaction,and the Ni@SiO₂-1h catalysts with the highest Ni loading,the smallest Ni nanoparticle size,and the highest specific surface area showed the most excellent activity and stability,the conversion of CH₄ and CO₂ reached 91%and 94%in the DRM reaction at 1073 K,and maintained stable for 50 h in the DRM reaction at 1023 K.The main reasons were that the size effect of Ni particles made Ni@SiO₂ catalysts had high DRM reactivity,and the limit of the core-shell structure on the range of movement of Ni particles significantly inhibited its sintering,so that it maintains good stability in DRM reactions.2.Core-shell structure Ni@SiO₂ catalysts with different nickel particle sizes and different metal support interactions were prepared by a simple calcination process and applied to high temperature methane dry reforming reactions.The results showed that the catalytic performance of Ni@SiO₂ depended on the size of Ni particles and the strength of Ni-SiO₂ interaction.Compared with Ni@SiO₂-T?T=823,923,1123,and1223?catalysts,the Ni@SiO₂-1023 catalysts,which had the best coupling of size effect and metal support interaction,had the highest CH₄ and CO₂ conversion(42.5mmol_i g^-1_Nisurfs^-1?42.9 mmol_i g^-1_Nisurfs^-1)and no deactivation during the 50 h DRM reaction.3.The novel Ni@SiO₂@CeO₂ catalysts were prepared by depositing an appropriate amount of CeO₂ nanocrystals on the surface of Ni@SiO₂ and applied to the low-temperature DRM reaction.The results of H₂-TPR and XPS showed that the deposition of CeO₂ on the surface of SiO₂ strengthened the interaction between the metal and the support,and generated a large number of active oxygen vacancies on the surface,which provided a guarantee for the improvement of the catalytic activity of Ni@SiO₂@CeO₂ in the low temperature DRM reaction and the inhibition of carbon deposition.The performance evaluation showed that Ni@SiO₂@CeO₂ exhibited more efficient catalytic performance than Ni@SiO₂ in DRM of 873 K and 673 K,and the carbon deposition amount were reduced from 8.0%to 4.0%.The relationship between the catalyst structure and the performance of DRM was preliminarily established.A dual function mechanism for CeO₂ activation of CO₂ and Ni particles activation of CH₄in DRM reaction was proposed.