Structure Controllable Preparation And Performance Of Molybdenum Carbides Catalytic Materials For Hydrodeoxygenation And Hydrogen Evolution

Molybdenum carbides are formed by the insert of carbon atoms into the crystal lattices of molybdenum.The introduction of carbon atoms leads to the change of electronic structure of parent molybdenum.In particular,the metallic d-band has been modified,which makes molybdenum carbides exhibit Pt-like properties.Thus,in many hydrogen-involving reactions,such as traditional hydrogenation reactions and emerging electrocatalytic reactions,molybdenum carbides present Pt-like performance.Optimizing the synthetic process of molybdenum carbides,improving their properties,expanding their application,and exploring the relationship between their structure and corresponding catalytic performance are pivotal for the development and application of molybdenum carbides catalytic materials.Basing on the interaction between ammonium heptamolybdate and organic ligands,series of molybdenum carbides catalytic materials with special structures and excellent performance have been constructed via the single-source method in this thesis.The effects of the crystal structure,Mo/C molar ratio and heteroatom N-doping,addition of the second metal Ni,and morphology of molybdenum carbides on their properties were discussed with various characterizations.Remarkably,the structure-performance relationship of the as-prepared molybdenum carbides-based catalysts has also been revealed in the hydrodeoxygenation of biomass-based compounds including diphenyl ether,palmitic acid,methyl palmitate,and hydrogen evolution reaction from water electrolysis.The study contents and results of this thesis are shown below:(1)With ammonium heptamolybdate(AHM)as molybdenum source,dopamine as carbon source,molybdenum carbides nanosheets were prepared via one-step pyrolysis of Mo-doplamine organic-inorganic hybrids from the single-source method.The crystal structure of molybdenum carbides was controlled by pyrolysis gases.?-MoC1-x and ?-M02C was obtained at Ar and Ar/H2 respectively,where the addition of H2 promoted the carburization process.The type of active sites of ?-Mo2C and ?-MoC1-x was similar,which was indicated by their accordant pathway for hydrodeoxygenation of diphenyl ether(DPE).The conversion of DPE over the as-prepared molybdenum carbides nanosheets was presented as the direct hydrogenolysis of C-O bonds first and hydrogenation of benzene in the end.However,according to the difference in crystal structure,?-Mo2C owned higher site density and exposed more actives sites than ?-MoC1-x,which made for the absorption and activation of hydrogen and reactants.Therefore,?-Mo2C performed higher activity(ca.100%)and benzene selectivity(ca.95%)in hydrodeoxygenation of diphenyl ether.In addition,the structure and catalytic performance of ?-Mo2C nanosheets could keep stable during the 55 h stability test.(2)With AHM as molybdenum source and amitrole as carbon source,N-doping molydebnum carbides nanowires(MoxCNy)were obtained via one-step pyrolysis of the 70-Mo-3atrz organic-inorganic hybrids from the single-source method.Mo/C molar ratio(x)and N-doping(y:N/C)in MoxCNy nanowires could be controlled by the carburization parameters(gas,temperature,and time),Single-phase molybdenum carbides(?-Mo2C)could be synthesized at 500?.The relationship between the structural formula and performance of MoxCNy nanowires catalysts was explored in the hydrodeoxygenation of palmitic acid.N-doping resulted in the electron-transfer within Mo,C and N,thus leading to the modification of the electron property of molybdenum carbides.Therefore,the structural formula ratio between Mo,C and N could be optimized by adjusting "x" and "y" in MoxCNy catalysts,which was beneficial to the modification of catalysts' properties,the exposure of more active sites,and finally improving the hydrodeoxygenation performance for palmitic acid.Therein,Mo2.56CN0.50 nanowires prepared at 500? for 8 h in Ar/H2 showed the optimized activity with ca.99%conversion and ca.99%alkanes selectivity.(3)With the amitrole as carbon source,AHM as molybdenum source and nickel nitrate as nickel source,nickel molybdenum bimetallic organic-inorganic hybrids were obtained at room temperature.Nickel molybdenum carbonitrides(NixMoCN)composed of ?-Mo2C and Ni2Mo3N were synthesized via one-step pyrolysis of the as-prepared precursors.Therein,the phase composition of carbonitrides was tunable by adjusting the addition of nickel source.The relationship between the addition of the second metal Ni and the catalytic performance was revealed during the hydrodeoxygenation of methyl palmitate(MPA).The introduction of Ni lowered the pyrolysis temperature of precursors,effected the phase composition of samples after pyrolysis,and changed the reaction pathway and catalytic activity of molybdenum carbides simultaneously.There existed a volcanic-dependent relationship within the catalytic activity of NixMoCN and Ni/Mo molar ratio(x),while the performance for cleaving C-C bonds abutting carboxyl or carbonyl groups was positively related to x.It was speculated that the addition of Ni increased the active sites over NixMoCN for cleaving C-C bonds,made for the DCO route,and there existed the synergistic effect between ?-Mo2C and Ni2Mo3N within the heterogeneous catalysts NixMoCN.Since the Ni/Mo molar ratio reached 0.30,the effect between the two phases(51 wt.%Ni2Mo3N and 49 wt.%?-Mo2C)reached a maximum,which resulted in the increase of the catalytic active sites and the concurrent enhancement of regioselectivity cleavage of C-C and C-O bonds over Ni0.30MoCN.Therein,Ni0.30MoCN exhibited ca.98%MPA conversion and 100%selectivity to alkanes including hexadecane and pentadecane.(4)With AHM as molybdenum source,melamine-formaldehyde resin as carbon source,and polyvinyl alcohol(PVA)as the soft template,Mo2C/Mo2N composites hollow nanospheres were prepared by combining soft-template method and single-source method,and performed in hydrogen evolution reaction for the exploration of structure-performance relationship.The morphology and phase composition of the as-prepared catalysts were affected by PVA and pyrolysis temperatures.The complete decomposition of PVA at high temperature was responsible for the formation of the hollow structure.Mo2C/Mo2N composites exhibited good performance during HER tests both in acid and alkaline electrolyte.Therein,there only needed overpotentials of 132 mV,176 mV and 192 mV to achieve 10 mA cm-2,100 mA cm-2 and 240 mA cm-2 current densities in 0.5 M H2SO4 over Mo2C/Mo2N composites respectively.The three-dimensional hollow structure and rough surface of Mo2C/Mo2N composites increased the electrochemical surface area and decreased the electron transport resistance,while the heterojunction accelerated the electron transport and improved the intrinsic activity for HER.