Development Of Highly Active And Stable Ni-based Catalyst For CO₂ Methanation

Global warming is mainly caused by CO2 emission,one interesting approach to utilize CO2 and produce high-quality products is CO2 methanation.The CO2 methanation or Sabatier reaction is an exothermic reaction to utilize CO2 and produce methane as the product.In particular,as the CO2 has strong carbon-oxygen bonds(O=C=O;782.4 kJ mol-1),it is hard to transform the C4+ of CO2 to C4-of CH4 with the eight-electron carrier's reduction process.Thus,this reaction required active catalysts to expedite the reaction.In addition,the CO2 methanation was less reactive at low temperatures and the deactivation problem occurred at high temperature.So,the challenge in the CO2 methanation is to improve the CO2 methanation performance at lower reaction temperature or to prevent the deactivation of catalysts.In this study,several approaches to enhance the performance of Ni-based catalysts for the CO2 methanation were systematically investigated.The catalyst properties and the reaction pathways were also studied.The first approach is to develop the Ni-based catalyst by incorporate the Ni into the CeO2-ZrO2 lattice and studied the effect of different Ni incorporation ratios.The small amount of Ni(2.4 wt.%)improves the basic properties especially the medium basic site,reducibility,and also the oxygen vacancies.On the other hand,an increase in the Ni incorporation of more than 2.4 wt.%resulted in a decrease in basicity as well as the interaction between Ni and CeO2-ZrO2 lattice,oxygen vacancies,and surface area.The decrease in surface area was caused by the pore blockage of Ni species.Ni agglomerated after increased Ni incorporation,resulting in a larger crystallite size.Even if the amount of Ni2+increased,the H2 activation to the active H species might not be improved.The Ni005CZO was found to be the most active catalyst.However,all of the Ni incorporated CeO2-ZrO2 catalysts exhibit the medium basic sites which are necessary for the CO2 methanation.In order to further improve the CO2 methanation performance,the influence of various amounts of Ni loading on Ni-modified CeO2-ZrO2 support was studied.The Ni-based catalysts were prepared by the impregnation method and successfully improve the characteristic properties of catalysts such as basicity,the amount of surface Ni species,and oxygen vacancies.A gradual increase in Ni loading from 15 to 45 wt%was found to increase medium-strong basic sites,and surface Ce3+ and Ni0 species along with oxygen vacancies favor high activity.The CO2 methanation activity was related to the amount of Ni loadings where the 45Ni/Ni005CZO catalyst was reported to be the most active catalyst.This is due to the high amount of Ni surface suitable for H2 activation and high medium basic sites to accommodate CO2 activation,leading to high catalytic activity.This approach also lowered carbon deposition due to the proper pore size and high amount of oxygen vacancies.In order to improve the CO2 methanation performance at low temperatures,a series of Ni-Zr catalysts were prepared by one-pot hydrothermal method using the high concentration of NaOH as a reducing agent.The t-ZrO2 and high surface area was obtained even at high Ni content.With the increase of Ni content,the amount of weak-medium basic sites and the oxygen vacancies increased,promoting the CO2 adsorption at low temperature.The increase in Ni content observed the increase in Ni0 which activates H species.The strong interaction between Ni-Zr along with the oxygen vacancies,which promotes CO2 and H2 dissociation,resulted in enhanced the CO2 methanation activity.The Ni-(0.2)Zr catalyst was displayed as the most active catalyst with a superior low temperature CO2 conversion of 89.16%and almost 100%CH4 selectivity at 250?.Thus,the increase in the Ni content with the one-pot hydrothermal method enhances the CO2 methanation performance at low temperature.Besides,the in-situ DRIFTS analysis confirms that the increase in the Ni contents promotes the hydrogenation of carbonate species to formate species,and improves the CO2 methanation performance.To enhanced the low temperature CO2 methanation performance,the tailored Mg-,Ca-,Zn-doped Ni-Al/hydrotalcite-derived catalysts were developed.The effects of various metals addition on the physicochemical properties and catalytic activities were evaluated.The addition of Mg2+and Ca2+,especially Mg2+,enhances the basicity,reducibility,metal dispersion,and metal-support interaction,and significantly improves the CO2 methanation activity.While the addition of Zn2+decreases the basicity and the metal-support interaction,which decreases the CO2 methanation performance.The relationship between the basic properties and the CO2 conversion showed that the CO2 conversion increased with the amount of alkali.The NiMgAl catalyst had the highest conversion of 91.8%at 250?.The in-situ DRIFTS found that the addition of Mg2+promotes the CO2 adsorption and the generation of monodentate formate,which actively contributed to the improvement of low-temperature performance.As a result,the ternary mixed oxides catalyst derived from LDHs precursors is a promising catalyst for CO2 methanation at low temperature.Overall,this study developed series of Ni-based catalysts,mainly studied the impact of Ni content,basicity,and other catalyst characteristic properties on the CO2 methanation reaction.The developed catalyst enhanced the CO2 methanation performance,the study provided new insights into their catalyst properties and behaviors.