Morphological Regulation Of Ni Species On Mesoporous SiO₂ Surface And The Effect On CH₄/CO₂ Reforming Performance

With the rapid development of modern society,the large consumption of fossil fuels has brought about increasingly serious environmental problems,especially global climate change caused by the massive emissions of greenhouse gases such as carbon dioxide.How to reduce the greenhouse effect is an urgent problem to be solved.The carbon dioxide reforming of methane can simultaneously use CO₂and CH₄greenhouse gases generating high-value-added industrial syngas,which effectively mitigates the greenhouse effect and has important research value in protecting the environment,resource utilization,and economic development.However,for the carbon dioxide reforming of methane reaction,the most industrially applicable Ni-based catalysts are prone to carbon deposition and sintering,leading to catalyst deactivation.In order to solve the problem,this paper uses mesoporous Si O₂as the carrier and metal Ni as the active component.The morphology of Ni species on the surface of the carrier is regulated by high-temperature heat treatment and doping of the second metal to obtain highly dispersed and small Ni particles,which achieved the stabilization of Ni active phase and obtained the excellent catalytic performance for carbon dioxide reforming of methane.The specific work is as follows:First,we explored the effect of high temperature heat treatment on the morphology of Ni species on the surface of the Si O₂carrier.We found that different heat treatment temperatures had a significant effect on the size and dispersion of Ni particles on the carrier.TEM results show that the catalyst particle sizes are 16.9 nm,14.5 nm,7.1 nm and 18.4 nm,and the metal dispersions are 4.9%,5.5%,23.4%and13.7%,after high temperature treatment at 700,800,900 and 1000°C,respectively.Due to the appropriate high temperature treatment to stimulate the reconstruction of Ni species on the surface of the carrier,the larger Ni particles are dispersed into evenly distributed small particles.According to the H₂-TPR results,the interaction between the small-sized Ni particles and the carrier is stronger,thereby forming a stable active center and significantly improving the catalytic performance.Therefore,the catalyst heat-treated at 900°C exhibited the highest TOF(1.1 s^-1),and the catalytic activity remained basically unchanged at 750°C for 34 h.It was worth noting that only a small amount of amorphous carbon deposits were generated after the reaction.Subsequently,we tried to use a simpler second metal doping method to control the Ni species on the surface of the Si O₂carrier,and explored the effects of alkali metal Ca O and different Ni/Ca on the methane dry reforming performance of the catalyst.We found that Ca O doping can not only enhance the metal-support interaction,but also facilitate the adsorption and activation of CO₂and accelerate the removal of intermediate C species.In addition,proper Ni/Ca can further optimize the catalyst surface structure.The catalyst with Ni/Ca of 3:2 exhibits the largest specific surface area and the smallest particle size?only 3.73 nm?,Which showed good stability during continuously reacting in the thermodynamic active zone of coke formation for 48 h.TG results show that the carbon deposits of the catalyst after the reaction are 31.7 wt%,8.6 wt%,4.2 wt%,and 12 wt%,respectively,with the Ni/Ca increasing,indicating that proper Ni/Ca can significantly improve the carbon resistance of the catalyst.Besides,excellent anti-sintering property is also obtained on the catalyst with Ni/Ca of 3:2 only with the increment of Ni particle size from 3.73 to3.99 nm.In summary,appropriate conditions can regulate the existence of Ni species on the surface of mesoporous Si O₂to increase the resistance to carbon deposition and sintering of catalysts.However,the catalysts still inevitably generated carbon deposition after a long period of stability testing.Therefore,the reaction mechanism and the carbon deposition mechanism of the catalysts still need to be thoroughly explored.