Fine Adjustment Of The Shell Structure Of Yolk-shell Structure Ni@SiO₂ Catalyst And Its Effect On The Performance Of CH₄ Dry Reforming Reaction

Excessive human activities and large amounts of energy consumption have led to the continuously deteriorate of the global greenhouse effect.As we all know,CO₂ plays the significant role in the greenhouse effect.In 2010,the global CO₂ emissions exceeded 31 billion tons/year.The CO₂ utilization through CO₂ dry reforming of methane（DRM）reaction can not only effectively slow down the global greenhouse effect,but also create huge economic value.However,the high temperature characteristics of the DRM reaction led to the inevitable thermodynamic sintering of the catalyst metal particles and the formation of carbon deposits.Therefore,the above problems can be effectively addressed by designing the catalyst of a core/yolk-shell structure.However,the structural characteristics of core/yolk-shell must be thoroughly understood and improved before it can be applied the real harsh industrial conditions.In this paper,we synthesized a novel type of Ni@Si O₂ nanocapsule catalysts with different shell thicknesses by changing the amount of silica source added in the preparation process and used them for the DRM reaction under high space velocity conditions to explore the important influence of the shell structure parameters on the catalyst performance.The results show that a much thin shell layer cannot effectively exert the anti-sintering and anti-carbon deposition ability due to the limiting confinement effect.When the shell layer is thicker,the capsule structure of the catalyst became more stable,and the anti-sintering and anti-carbon deposition ability will be obviously enhanced.However,an excessively thick shell layer will unfortunately inhibit the catalytic performance.The Ni@Si O₂ nanocapsule catalyst with a medium shell thickness maintains effective anti-sintering and anti-carbon deposition capabilities while exhibiting excellent DRM catalytic performance under high space velocity conditions.For the core/yolk-shell structure,the reactant gas must pass through the shell and enter the cavity to contact the metal core and complete the interface conversion.The structural properties of the shell will inevitably affect the diffusion rate of reactant gas molecules in and out of the catalyst surface,and this effect will be more pronounced under high space velocity reaction conditions.Therefore,we carried out direct characterizations to explore the influence of the shell diffusion effect on the catalyst performance over the Ni@Si O₂ nanocapsule catalysts with different shell thicknesses.Results showed that when the thickness of the shell layer is appropriate,the resistance for gas molecules to enter the cavity of the capsule through the shell layer is weak,the diffusion effect is easily overcome,and the diffusion effect of the shell layer has little effect on the performance of the catalyst.When the shell layer is too thick,on the one hand,the path for gas molecules to pass through the shell layer is prolonged,and on the other hand,the available paths for gas molecules to pass through the shell layer are reduced.The increased diffusion effect of the shell layer will adversely affect the catalyst.In addition,in view of the excessive metal support interaction formed by some metal particles embedded in the inner interface of the shell in the core-shell confinement.Although it can greatly improve the sintering resistance and carbon deposition resistance of metal sites,this excessive restriction also causes the metal species to be difficult to activate in the actual reaction process,thus the accessibility of metal sites is insufficient.In this regard,we reported a citric acid etching method which could effectively weaken the strong metal support interaction while has no negative effect on the metal composition.Our results proved the dual modification effect of citric acid etching activation:weakening the interaction of strong metal interaction,and dispersing Ni metal particles to smaller sizes.The yolk-shell catalyst modified by citric acid etching can exhibit excellent catalytic performance under mild metal support interaction and small size effect.