Structure-Activity Relationship Of Ni-Based Catalyst Derived From Hydrotalcites For CO₂ Methanation

Due to the burning of a large amount of fossil fuels,the CO₂ content in the atmosphere has increased year by year and the greenhouse effect aggravates,further seriously destroying the ecological environment balance.Therefore,how to effectively reduce the concentration of CO₂ in the atmosphere and realize the recycling of carbon resources has gradually aroused interest of researchers.At present,the CO₂ methanation reaction has received extensive attention.This is mainly because the CO₂ methanation reaction is not only conducive to reduce the CO₂ concentration in the air,but also the product of CH₄ is a clean energy with high calorific value,which could realize the recycling and efficient utilization of carbon resources.Catalysts are the key problem to determine whether the CO₂ methanation crafts can be industrially applied.Although precious metal-based catalysts have ultra-high low temperature catalytic performance,their high price limits their industrial applications.However,non-precious metal Ni-based catalysts are widely used in CO₂ methanation reactions due to their low cost,high catalytic activities.However,studies have found that the Ni-based catalyst has poor low-temperature activity,high activation temperature,and the catalyst is prone to agglomeration,sintering and carbon deposition at high temperatures,which shortens the life of catalysts and increases industrial production cost.Therefore,the development of Ni-based catalyst with low temperature,high activity and high stability has very important practical significance,while it is also a very challenging research work.In summary,hydrotalcites are chosen the catalysts precursor in this work.And we prepared a series of Ni-based catalysts and used them in the CO₂ methanation reaction by adjusting the preparation process of hydrotalcites and adding additives,and explored the structure-activity relationships of catalysts.The core content of this work mainly includes:（1）Using NaY molecular sieve as an aluminum source,and the self-sacrificial template method was used to successfully prepare NaY-Nix-LDHs precursors with sea urchin-like and tremella-like shapes.The Ni-based catalysts obtained by in-situ reduction of the NaY-Nix-LDHs precursors still maintain the morphology of the LDHs precursor.At the same time,the catalysts also retain the mesoporous pore structure of the NaY molecular sieve.Combining the results of H₂-TPR,H₂-TPD and TEM,it is found that NaY-Ni5-R has the largest hydrogen adsorption capacity（622 umol/g）,the highest Ni nanoparticle dispersion（12.6%）and the most moderately basic sites.Therefore,the NaY-Ni5-R catalyst has the highest activity in the CO₂ methanation reaction.At 350℃,0.1 MPa,and a GSHV of 30,000 mL·g^-1·h^-1,the CO₂ conversion and CH₄ selectivity are 88%and 100%.The characterization results combined with the performance evaluation data clarified the dependence mechanism of catalytic performance,the number of active sites and CO₂ adsorption activation,and provided an effective strategy for controlling preparation of high activity and high stability methanation catalysts.（2）By adding a small amount of Co element in the synthesis process of LDHs precursor,a Ni-based catalyst containing a small amount of Co was successfully prepared and used for CO₂ methanation reaction.Combined with experimental data such as XRD,H₂-TPR,CO₂-TPD and TEM,it is found that doping with an appropriate amount of Co as the promoter can effectively control the size of the Ni nanoparticles in the catalyst,the interaction between the Ni species and the AlO_x substrate,and the alkalinity of catalysts surface,thereby significantly improving the low temperature catalytic activity of Ni-based catalysts.When the reaction temperature is 250℃,the CO₂ conversion rate on the NiCo0.5Al-R catalyst can reach 81%.In addition,the steric hindrance effect of the AlO_x substrate on the Ni nanoparticles effectively inhibits the agglomeration and sintering of the Ni particles during the CO₂ methanation reaction,thereby significantly improving the stability of the catalyst.In addition,the kinetic experiment results show that the catalyst NiCo_0.5Al-R has the smallest activation energy,so the CO₂ methanation reaction is easier to proceed on this catalyst.（3）Ni_xCo_yAl-LDHs precursors were successfully prepared by adjusting the molar ratio of Ni and Co.SEM results show that Ni_xCo_yAl-LDHs are nano-sheet structures with uniform size.Ni_xCo_yAl-LDHs precursor was reduced in situ to obtain NiCo alloy catalyst.Through the CO₂ methanation performance test,it is found that the CO₂ conversion rate of the Ni₇Co₃Al-R catalyst has reached 74%at 250℃,while the CO₂ conversion rate on the NiAl-R and CoAl-R catalysts is only 26%and 30%,under the.same conditions,respectively.According to the results of H₂-TPR,CO₂-TPD,XPS,and TEM-EDS,it is found the particle size of the active phase in the NiCo alloy catalyst is small,and the active sites are more exposed,the oxygen vacancies are more,and the interaction between the two metals in NiCo Synergistic promotion,so NiCo alloy has an excellent low temperature catalytic activity.In addition,the life test results show that the NiCo alloy catalyst has ultra-high stability in the CO₂ methanation reaction.This is mainly because the NiCo alloy catalyst prepared by the in-situ reduction of Ni_xCo_yAl-LDHs precursor has a unique mosaic structure,and-the NiCo alloy nanoparticles are highly uniformly dispersed in the AlO_x substrate.The steric hindrance effect of AOx substrate is beneficial to improve the stability of NiCo alloy nanoparticles.（4）Through the kinetic study of the CO₂ methanation reaction,it was found that the doping of Co and the formation of the alloy structure increased the reaction order of CO₂ and H₂,reduced the activation energy of CH₄,and promoted the formation of CH₄.Through the in situ FTTR characterization results,it is found that only the CoAl-R catalyst follows the reaction mechanism of HCOOH.The catalysts including of NaY-Nix-R,NiCo_xAl-R,and Ni_xCo_yAl-R all follow the reaction mechanism containing the CO intermediates.The possible reaction path is that CO₂→CO₂^*→HCO3^*/CO3^*→HCOO^*→CO^*→CH₄.Among them,HCO3^*,CO3^*,CO^*and HCOO^*are the important reaction intermediates,and these intermediates can further generate CO or CH₄ products through deoxygenation or hydrogenation.In addition,the experimental results show that the molar amount of Ni and the doping of Co do not significantly change the types of intermediates adsorbed on the catalyst surface,and therefore have almost no effect on the reaction mechanism of the Ni-based catalyst derived from LDHs.The study of the reaction mechanism provides theoretical support for the preparation of low-temperature and high-activity Ni-based catalysts.