Preparation And Structure Regulation Of CaO-Ca₃Al₂O₆@Ni-SiO₂ Composite Catalyst For Adsorption-enhanced Hydrogen Production

Sorption-enhanced CH₄/H₂O reforming technology realizes one-step preparation of high-purity H₂ by in-situ removal of CO₂ generated by the conventional CH₄/H₂O reforming reaction by adding solid adsorbent,which greatly reduces energy consumption and requirements for process equipment,as well as greenhouse gas emissions.It is a new technology to realize pollution-free and efficient utilization of energy.However,the catalyst and adsorbent used in this technology will be sintered in the process of circulating hydrogen production,resulting in the reduction of catalytic and adsorption properties,and seriously affecting the purity of H₂production.Therefore,how to prepare dual-function composite catalysts with high cyclic stability is one of the key problems to be solved in this technology.The volume change of adsorption component Ca O in commonly used composite catalysts during CO₂ adsorption and desorption will cause the collapse of the structure of the composite catalyst,making the specific surface area and pore volume of Ca O smaller,resulting in the decline of CO₂ adsorption performance of the composite catalyst.At the same time,the active component Ni was also embedded in Ca CO_3,resulting in the degradation of catalytic performance and seriously affecting the purity of H₂ prepared by this technology.In response to this problem,this research uses the idea of"Si O₂ coats the adsorption component so that the repeated expansion and contraction of the volume during the reaction-regeneration cycle was carried out in the Si O₂ shell"to design a bifunctional composite catalyst.The advantage of this structure was that the volume changes of the Ca O-based adsorption components during the reaction and regeneration process are all carried out in the Si O₂ shell,which will not cause the collapse of the composite catalyst structure,and also prevent the entrapment of the catalytic components.On this basis,the structure of the composite catalyst was regulated by changing the Ca/Ni molar ratio and the p H value of the solution system,and the functional matching of the adsorption component and the catalytic component in the composite catalyst was realized.The main research contents and conclusions of this paper are as follows:（1）A bifunctional composite catalyst named Ca O-Ca₃Al₂O₆@Ni-Si O₂ was prepared by the self-template method and selective etching method.Under the conditions of 600℃ reaction,750℃ regeneration,1 atm,H₂O/CH₄ mole ratio of 4.8,space velocity of 968 h^-1,The concentration of hydrogen production only decreased by 2.3%（99.6%to 97.3%）in the10 cycles.Through characterization analysis,it was found that the reason for the high hydrogen production performance of the prepared composite catalyst Ca O-Ca₃Al₂O₆@Ni-Si O₂ was that Si O₂ was coated on the surface of the adsorption component,which played a role of structural stability.The composite catalyst maintained a high CO₂ adsorption performance in the reaction process.However,the CH₄ conversion rate decreased by 7.7%（99.5%to 91.8%）after 10 cycles,which may be due to the mismatch between the content of catalytic components and adsorption components.Meanwhile,it was found that only part of the catalytic components of the composite catalyst had a core-shell structure with Ni as the core and Si O₂ as the shell,and some Ni was directly loaded on the shell Si O₂.The above factors together lead to the reduction of CH₄ conversion rate.（2）In order to further improve the reusable stability of the composite catalyst.By adjusting the Ca/Ni molar ratio of the composite catalyst,the functional matching of adsorptive components and catalytic components of the composite catalyst was achieved.The results show that when the Ca/Ni mole ratio was 1:0.10,the CH₄ conversion rate and hydrogen production concentration of the prepared composite catalyst were 98.0%and 99.7%,respectively.After ten cycles of reaction,99.5%of the initial activity can still be retained,which can better achieve the functional matching of adsorption components and catalytic components in the composite catalyst.（3）In order to make the composite catalyst have higher reusable stability,the self-assembly process of CTAB-silane system was controlled by adjusting the p H value of the preparation system,and the composite catalyst with uniform structure was prepared.The study found that when the p H was 11.5,the prepared composite catalyst had more uniform catalytic components,and at this time the composite catalyst had the largest total pore volume and pore size distribution,and the composite catalyst showed the best hydrogen production.The performance and cycle stability can reach 99.3%CH₄ conversion and 99.5%H₂ concentration in the initial reaction,and 98.6%CH₄ conversion and 99.3%H₂ concentration after 10 cycles of reaction.