Synthesis Of Ni-Al₂O₃/CaO-CaZrO₃ Composite Catalyst And Modulation Of Its Sorption Enhanced Hydrogen Production Performance

CO₂ sorption enhanced CH₄/H₂O reforming is an efficient and low-energy emerging technology that couples reaction and separation by in situ CO₂ removal to produce high-purity H₂.Bifunctional composite catalysts have received wide attention because of their potential advantages such as low heat loss and low mass transfer resistance.The adsorptive component in the composite catalyst needs to be regenerated after the sorption saturation so as to constitute a reaction-regeneration cycle.However,the hydrogen production performance of the composite catalyst decreased significantly over several reaction-regeneration cycles.There are two main reasons for the degradation of the hydrogen production performance of the composite catalyst:1.The volume of the composite catalyst expands in the reaction and contracts in the regeneration process due to the twofold molar volume difference between Ca O and Ca CO₃.The cyclic volume change leads to structural collapse and encapsulation of active sites,which in turn leads to the degradation of the hydrogen production performance.2.The regeneration of composite catalysts is often performed at high temperatures（800-950℃）,and the harsh regeneration environment leads to sintering and deactivation of composite catalysts.Therefore,the development of efficient and stable composite catalysts is the core of CO₂ sorption enhanced CH₄/H₂O reforming hydrogen production process.In order to enhance the cycling stability of the composite catalyst,this work provided structural support for the composite catalyst by constructing a stable inert framework to prevent the active sites from being encapsulated,thus have achieved stable operation over multiple reaction-regeneration cycles.The reaction and regeneration temperatures were also optimized to slow down the sintering of each component and further improve the cycling stability of the composite catalyst.Finally,the methods to effectively reduce the regeneration temperature of the composite catalyst were investigated.The main contents and summary of this work are as follows:（1）Ni-Al₂O₃/Ca O-Ca Zr O₃ composite catalyst with high activity and cycling stability was prepared by a modified distributed sol-gel method.The high temperature resistant inert component Ca Zr O₃ was highly dispersed in the composite catalyst in a cross-linked structure,providing stable framework support for the volume change during cycling,preventing structural collapse and active site encapsulation and maintaining structural stability;at the same time,it can physically separate the active sites to slow down sintering.The Ni-Al₂O₃/Ca O-Ca Zr O₃composite catalyst was able to operate stably over 10 SESMR cycles.（2）Investigated the causes and patterns of reaction and regeneration temperatures affecting the performance of composite catalysts for hydrogen production and further improved the cycling stability of the composite catalyst.The reaction temperature was found to affect the matching of the performance of the catalytic and adsorptive components,the low activity of Ni at low temperatures led to the low conversion of CH₄;at high temperatures,CO₂ cannot be removed in time,resulting in low H₂ concentration.The regeneration temperature was found to affect the performance of the catalytic and adsorptive components in cyclic hydrogen production.At low temperatures,the degree of regeneration was low,and the CO₂ capture capacity in the subsequent reaction decreased,resulting in limited sorption enhanced progress;at high temperatures,sintering occurred,and the catalytic and adsorptive components were deactivated sequentially,resulting in a decrease in hydrogen production efficiency.For Ni-Al₂O₃/Ca O-Ca Zr O₃,when reaction at 600℃ and regeneration at 750℃,both catalytic and adsorptive components were highly active（initial CH₄ conversion 97.0%,H₂ concentration98.8%）and could remain stable in the cycles,and the CH₄ conversion and H₂ concentration could still reach 97.3%and 98.9%,respectively,over 10 SESMR cycles.（3）The regeneration process of the composite catalyst was analyzed,and it was found that doping the composite catalyst with inert components,reducing the partial pressure of CO₂ in the regeneration environment,and slowing down the heating rate of the regeneration process could effectively reduce the regeneration temperature of the composite catalyst.Among them,doping Zr can reduce the activation energy of Ca CO₃ decomposition and lower the regeneration temperature from 769.1℃ to 743.7℃;reducing the volume fraction of CO₂ in the regeneration environment from 30%to 0%can lower the regeneration temperature from 870.1℃ to 743.7℃;reducing the temperature heating rate from 10℃/min to 2℃/min of the regeneration process can lower the regeneration temperature from 743.7.1℃ to 681.2℃.Among the three ways to reduce the regeneration temperature,the CO₂ partial pressure had the greatest effect on the regeneration temperature.