Nanoscale Laser Production Of Transition Metal Carbides From Metal-Organic Frameworks

Transition metal carbides?TMCs?have unique physical and chemical properties,and possess extremely high hardness and brittleness,high melting points,tensile strength,electronic and magnetic properties.They also show good chemical stability and resistance against corrosion under reaction conditions,and making them good candidates for coatings,cutting tools,aerospace,energy storage and catalytic applications.Compared to Pt-group noble metals,TMCs have the advantages of comparatively low cost,much greater natural abundance and chemical stability.However,there are few studies focus on the synthesis of homogeneous nanosize-TMCs.And traditional preparing methods center on carbothermal reduction,temperature programmed reaction?TPR?,chemical vapor deposition?CVD?and sol-gel.TPR,CVD and carbothermal methods involve high temperature,the contact between metal sources and carbon sources are insufficiency,so the target product is easy to be aggregated,and large particles are formed,traditional preparation processes are frequently both time consuming and energy intensive.Sol-gel method can decrease the reaction temperature to some extent;however,complex processes and long periods of time are applied.These methods are limited for synthesis of multiple nano transition metal carbides,and until now,few investigations focus on it.Iron carbides and molybdenum carbides are reported by pyrolysis of metal-organic frameworks?MOFs?,but the metals are difficult to be completely reduced and the species of TMCs is restricted.Traditional heating pyrolysis method has low energy utilization due to the low thermal conductivity of MOFs,and thermal conductivity has an effect on the particle size and morphology of target product,so it is difficult to achieve homogeneous nanoparticles via traditional pyrolysis method.Aiming at these problems,we initiate a simple,rapid,versatile and low-cost method to synthesize a series of nano-transition metal carbides by MOFs precusors coupled with direct-write laser process.In this method,after MOF crystals adsorb laser photon,electrons jump to excited state,and energy is released in the form of heat,which has high utilization of energy,so it is easy to get homogeneous nano-TMCs rapidly.And the structures of carbon-based transition metal carbides are characterized by powder X-ray diffraction?PXRD?,scanning electron microscope?SEM?,transmission electron microscope?SEM?and X-ray photoelectron spectroscopy?XPS?and so on.We have developed the relationship between laser processing parameters and morphology or structures of TMCs.And iron carbides are used as catalyst in Fischer-Tropsch synthesis?FTS?,the relationship between the structures of iron carbides and catalytic performance are initially established.The main contents and innovations in this thesis including:1.Synthesis of carbon-based titanium carbide?TiC?.For the first time,the support materials are generated,via the direct-write laser of Ti metal-organic frameworks:MIL-125?Ti?-NH₂.Ultrasmall highly crystalline TiC nanoparticles are well dispersed on the surface of N-doped hollow carbon spheres.The isoreticular topology of MIL-125?Ti?without amino functional groups is also studied for the synthesis of TiC,and we find that it is easier to obtain TiC with amino functional groups.2.Synthesis of carbon-based zirconium carbide?ZrC?and hafnium carbide?HfC?.The carbon-based ZrC and HfC are obtained,for the first time,by using 2-dimension?2D?UMCM-309a and MOLs as solid precursors coupled with laser without destroying the pristine morphology of MOFs.The resulting materials are highly crystalline with average particle size of 10 nm,which is much smaller than traditional method.We also explore the influence of UiO-66?Zr?and UiO-67?Zr?with different topologies on the formation of ZrC.3.Synthesis of carbon-based vanadium carbide?V₈C₇?.The carbon-based V₈C₇ are obtained by laser treatment of MIL-47?V?.Highly crystalline and ultrasmall nanoparticles encapsulated in few layers of graphite carbon are homogeneous dispersed on the surface of carbon.And the effect of laser scan rates and power on the degree of conversion,morphology and particle size are systematic studied.4.Synthesis of carbon-based molybdenum carbide??-MoC?and chromium carbide?Cr₃C₂?.?-MoC and Cr₃C₂ carbon-base materials are converted from Mo-BTC and MIL-53?Cr?via nano-second pulsed laser,respectively.The particle size is about 7 nm and20 nm for?-MoC and Cr₃C₂,respectively.For comparison,another Cr-MOF with different topology,MIL-101?Cr?,composed of the same metal salt and organic linker with MIL-53?Cr?is explored.5.Synthesis of carbon-base iron carbides.Iron carbides with mixture phase of carbon supported Fe₂C and Fe₇C₃ are generated from isoreticular topology MOFs:MIL-101?Fe?,MIL-101?Fe?-NH₂ and MTV-MIL-101?Fe?.The absorption ability of functional groups for laser energy are studied,and we find that it is easier for MOF with amino functional groups to convert into iron carbide.6.Iron carbides exhibite high activity and stability in Fischer-Tropsch synthesis?FTS?.And the relationship between the structures of iron carbides and catalytic performance are initially established.N-doped carbon used as an electron donor of iron catalyst for CO hydrogenation can achieve high acticity,and the formed encapsulated carbon act as a shield to protect active species from deactivation.To sum up,the method for preparing varieties of transition metal carbides based on metal-organic framework and laser gives a new route to synthesize nano TMCs.This method is versatile and low cost,rapid and simple,the assembling of MOFs and laser process were carried out under ambient environment.The resulting carbon-based transition metal carbides are homogeneous with ultrasmall nanoparticles.And we use iron carbide as catalyst in FTS,it exhibites excellent reaction activity and stability,which shows great potential of this method for preparing nano TMCs.