Design And Performance Of Nickel/Boron Nitride-Based Structure-Confined Catalysts For Dry Reforming Of Methane

Dry reforming of methane（DRM）can convert CH₄ and CO₂ into syngas（H₂:CO=1:1）which can be directly used in Fischer-Tropsch synthesis reaction and hence has raise great interests of researches.Nickel-based catalysts are considered to be the most promising catalysts for DRM due to their low cost,abundant reserves,and high catalytic activity.However,the serious Ni sintering and coke deposition during high temperature DRM reaction decrease the activity sites and lead to serious deactivation.Therefore,it is urgent to design and prepare highly sintering-and coking-resistance nickel-based catalysts for efficient DRM reaction.In this thesis,combined with boron nitride which possessed the excellent coking-resistance and high thermal stability,various Nickle/boron nitride-based catalysts were designed based on confinement strategy which have been proved as an effective protocol to improve the coking-and sintering-resistance and learning the anti-coke mechanism.The main results can be summarized as follows:（1）Using the coking-resistance of BN and redox property of ZrO₂,an interfacial confinement catalyst was designed and prepared by mounting the nickel species in the interface between ZrO₂ and BN,with the sintering-and coking-resistance was enhanced significantly.Ni/ZrO₂-BN with confined structure maintained about 70%CH₄ conversion during 200 h DRM reaction.TEM,XRD,XPS and H₂-TPR results showed that the sintering of nickel particles for Ni/ZrO₂-BN in high temperature DRM reaction was effectively prevented and the interaction between metal and support was enhanced through constructing the interface confinement structure.The amount and structure of carbon deposited on the surface of the catalyst after reaction were characterized by TG and Raman,the results of which indicated that the catalyst with BN-ZrO₂ interface had better coking-resistance during the reaction.（2）To limit the size of Ni nanoparticles,we rationally designed and originally developed an efficient and stable boron nitride interface-confined and layered double hydroxides（LDHs）-derived Ni catalysts（NiMA-BN-M-R）for DRM.It was demonstrated that the confinement derived from the interface between BN and LDHs-derived（Ni,Mg）Al₂O₄-sheets were responsible for well-dispersed Ni nanoparticles.The target catalyst showed very high conversion for CH₄ and robust stability with only 1%activity drop after 100 h testing.CO₂-TPD and in situ DRIFTs results indicated the strong CO₂adsorption with enhancement of both CH₄ and CO₂ activation over NiMA-BN-M-R,resulting in the fast formation of carbonate and hydroxyl species which is beneficial for methane dry reforming reaction.The hydroxyl species can spontaneously react with the CH_x*to form desired products and prevent coke formation,thus improving the stability of Ni-based catalysts.Eventually,the NiMA-BN-M-R catalyst showed excellent stability that it was employed in the reaction for 20 h and regenerated for another 100 h.The confinement effect of the BN/（Ni,Mg）Al₂O₄-sheets interface and the strong metal support interaction are primary reasons for the excellent stability of the NiMA-BN-M-R catalyst.This work unlocks the development of a highly thermal stability composites catalysts for methane dry reforming.（3）To further understand the role of BN in DRM,BN was peeled off by ultrasonic peeling with monohydric alcohols.Specific defect sites were introduced into BN.The BN with abundant defect sites were used as supports for Ni nanoparticles through impregnation.Among them,the Ni nanoparticles loaded on the isopropanol（i-PrOH）treated BN（Ni/d-BN-p）showed the highest stability,catalytic activity and excellent coking-resistance.EELS and EPR results showed that ultrasound-assisted alcoholysis of BN can effectively break the B-N bond in BN and promote the formation of N vacancies in BN,helping to expose more B terminal sites.XRD,TEM,XPS and H₂-TPR results showed that the defect sites of BN can anchor nickel and improve its dispersion of nickel,significantly enhancing the interaction between Ni and BN,and effectively preventing the sintering of Ni nanoparticles in high temperature reaction.Ni/d-BN-p catalyst with abundant defect sites and defect confinement structure exhibited excellent coking-resistance in DRM.TG results shows that Ni/d-BN-p with defect-confinement catalyst exhibited excellent coking-resistance in DRM.In addition,the defect sites of BN were found to play an important role in the adsorption and activation of reaction gases through in situ DRIFTs spectroscopy combined with CO₂-TPD characterization.The existence of B-terminal sites can be directly used as active sites for DRM reaction and it also can effectively promote the formation of B-OH*,thus increasing the rate of conversion of carbonate species and CH_x*species into formate species,greatly promoting the formation of CO and effectively preventing coke deposition.