Structural Design And Property Studies Of Molybdenum Carbide-based Composite Nanomaterials

Transition metal carbides(TMCs)are produced by incorporating C atoms into the interstitial sites of transition metals,which belongs to the class of interstitial alloy compound.The insertion of C atoms modifies the d-band of the metallic Mo,entitling Mo with properties resembling to those of noble metals.From the basic hydrogenation/dehydrogenation reactions to the electrocatalytic reactions,molybdenum carbide has already exhibited extraordinary catalytic performance resembling or superior to that of noble metals.By taking advantage of the unique surface and electronic properties,TMCs exhibit synergistic interaction with admetal particles and often promote the formation of small,flat metal particles with novel catalytic properties.In this thesis,we break through the traditional ideas of metal/oxides catalytic systems and design novel metal/MoCx interfacial catalytic materials taking advantage of the strong interaction between the admetal and MoCx substrate.We deeply study the dispersion and stabilization mechanism of the active admetal on different crystal structure MoCx substrates through a variety of in-situ characterization techniques and DFT calculation methods,and also discuss the catalytic performance of these materials in different reactions.The main contents of this thesis are as follows:In view of the strong interaction between the admetal and transition metal carbides,we synthesized layered gold(Au)clusters on a molybdenum carbide(?-MoC)substrate to create an interfacial catalyst system for the ultralow-temperature WGS reaction.The results showed that the Au/?-MoC catalyst significantly reduced WGS reaction temperature to 120 ? and reached 0.62 molco/molAu/s WGS activity,which was more than 20 times higher than that of literature reported catalysts.Funthermore,in a single-run reaction(more than 143 h)under the full reformate gas feed,the 2%Au/?-MoC catalyst shows an excellent total turnover number(TTN),reaching up to 385,400 molco/molAu.Further structural characterizations show that layered Au clusters epitaxially grown on the substrate driven by this strong interaction,and Au forms an exceptionally high density of electron-deficient surface sites.So no obvious gold sintering and substrate structural difference was observed when the catalyst underwent the high temperature activation(700 ?)process for 2 h.Mechanism research shows that water was activated over a-MoC at 303 kelvin,whereas carbon monoxide adsorbed on adjacent Au sites was apt to react with surface hydroxyl groups formed from water splitting,leading to a high WGS activity at low temperatures.The novel catalytic system provides a new idea for the promotion of hydrogen economy and the purification process of hydrogen.On the basis of the foregoing research,we funther develop non-precious metal supported MoCx catalysts and explore a new effective synthesis path of metal/?-MoC catalysts,such as Cu,Ni,Ag,Co.We selecte Cu modified molybedenum carbides catalysts as the research object.The study shows that the differences of Cu precursors(Cu NPs,CuMoO4 or CuO)and Mo precursors(MoOxHy,MoO2 or MoO3)are the key parameter in synthesizing differen crystal structures of Cu/MoCx catalysts.And different crystal structure molybdenum carbides effectively control the active metal sites,leading to distinct WGS catalytic performances over different catalysts.The results show that higher a-MoC phase ratio in the sustrate leads to higher WGS performance over the Cu/MoC,catalysts.And the Cu/a-MoC catalyst exhibit the highest performance,reaching 31.0 ?molco/gcat/s at 150 ?.which is much higher than traditional Cu-based catalysts.The structural characterization and related mechanism study found that the dispersive state of the active metal copper was distinct different over different cystal structure Molybdenum carbide substrates.The Interface maximization of the admetal Cu and a-MoC substrate was responsible for the efficient WGS activity,which promotes the dispersion of Cu with an ultra-small particle size and the activation to the reactant.Previous studies has demonstrated that TMCs are excellent substrates to disperse metal.Herein,a series of novel Cu/?-Mo2C catalysts is synthesized exhibit extraordinary RWGS reaction activity,reaching 73.0 ?molCO2/gcat/s at 300 ? and 477.0 ?molCO2/gcat/s at 600 ?,which are far more active than that of trational Pt-oxide or Cu-oxide catalysts.Funthermore,no obvious deactivation is observed in a long time test,maintaining maintains 85%of initial activity after 40 h on stream.After detailed characterizations,we discover that the strong interaction between copper and hexagonal P-Mo2C substrate greatly enhances the metal dispersion and prevents the agglomeration of supported Cu particles under the working reaction condition,which accounts for the extraordinary catalytic performances.The excellent structural features of Cu/?-Mo2C catalyst show excellent catalytic activity and high temperature thermal stability in the RWGS reaction,which effectively solve the problem of sintering over Cu-based catalysts.The purpose of this chapter is to reduce the amount of precious metal Pt,so we synthesize a-MoC(FCC)catalyst,which shows excellent activity and stability in acid and alkaline medium for HER.Furthermore,we design and synthesize effective Pt/a-MoC catalyst,which exhibite better HER activity compared to 20wt%Pt/C with lower Pt loadings.The studies prove that the strong metal-support interaction between Pt and a-MoC substrate modifies dispersion of the metal Pt and improves the unification of Pt.A new preparation strategy for the replacement of platinum-based catalysts is successfully explored,which is expected to provide reliable material selection for the development of hydrogen production process.