Ni-based Catalysts Derived From Hydrotalcites And Their Methanation Performance

Ni-based catalysts are widely used in methanation reactions due to their high activity and low cost.However,the disadvantages of the Ni-based catalysts are mainly the agglomeration of Ni nanoparticles at high temperature,easy formation of carbon deposits,and poor low-temperature activity(250 ?).Therefore,it is important and challenging to improve the low temperature catalytic performance of Ni-based catalysts in syngas methanation.Nickel carbonyl is easily formed under low temperature and high concentration of CO,thus the catalyst is deactivated at a low temperature.In view of this,the Ni-based catalysts embedded in AlOx,substrate were obtained by in-situ reduction of hydrotalcites and used in the syngas methanation reaction.The preparation of hydrotalcites and the influence of different additives on the methanation performance of the catalysts were investigated.The main research contents of this thesis are as follows:(1)By adding different ammonium salts in the synthesis process of NiAl-hydrotalcites,and then directly reducing the NiAl-hydrotalcites without calcination to obtain Ni-based catalysts and applied it to methanation reaction of syngas.The effect of different ammonium salts on the structure and catalytic performance of the hydrotalcites were investigated.The results of XRD,SEM,H2-TPR and TEM showed that the hydrotalcites synthesized by the addition of ammonium fluoride had poor crystallinity,the size of the nanosheets was large and the nanosheets were stacked on each other,and the interaction was strong,so the required reduction temperature was higher,and the size of Ni particles in the obtained catalyst was larger.The catalysts prepared by adding ammonium nitrate and ammonium chloride had a small Ni particle size,and high low-temperature catalytic performance in the syngas methanation reaction.(2)By changing the composition of the solvent during synthesis of the NiAl-hydrotalcites,and then directly reducing the NiAl-hydrotalcites without calcination to obtain the Ni-based catalysts and applied it to the methanation reaction of the syngas.The effect of different solvents on the structure and catalytic performance of the hydrotalcites were investigated.Combined with XRD and H2-TPR results,it was found that the NiAl-hydrotalcites layer prepared by used ethanol as a solvent had a large spacing,and the electrostatic interaction between the interlayer anions and the interlayer was weak,so it could be completely reduced at 400 ?.In the catalyst,the Ni particles with low dispersion and the H2 adsorption amount was small,and with poor low-temperature catalytic performance in the syngas methanation reaction.The NiAl-hydrotalcites prepared by using deionized water as a solvent contained more carbonate ions,so the required reduction temperature is higher,due to the stronger electrostatic interaction between the interlayer anions and the interlayer.The prepared catalyst possessed high dispersion of Ni particles,H2 adsorption amount,and high low-temperature methanation activity.(3)NiMnAl-LDHs,NiZrAl-LDHs and NiVAl-LDHs were successfully prepared by adding Mn,Zr and V in the synthesis process of NiAl-LDHs.The Ni-based catalysts modified with Mn,Zr and V promoters were obtained from in-situ LDHs.XRD,H2-TPD,XPS and TEM results showed that doped additives had entered the layered structure of hydrotalcites,and had no effect on the morphology and crystallinity of hydrotalcites.In addition,the addition of additives could effectively improve the dispersion of Ni particles,the number of active oxygen vacancies and the amount of H2 adsorption,so it is significant to improve the low-temperature catalytic performance,carbon deposition resistance and high-temperature stability of the catalysts.Among them,the doping Zr had the most obvious improvement on the dispersion of Ni particles and the adsorption amount of H2 in the catalyst,so it had the highest low-temperature catalytic performance.