Controlled Preparation Of Molybdenum Carbide Catalytic Materials Through Single-Source Route

Molybdenum carbide belongs to the class of interstitial alloy compound, which forms upon C atoms inserting into the lattice of the metallic Mo. 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 very basic hydrogenation/dehydrogenation reactions to the catalytic decomposition reaction and electrocatalysis, molybdenum carbide has already exhibited extraordinary catalytic performance resembling to that of noble metals. Developing the preparation methodology for molybdenum carbide catalytic materials is under any circumstances of vital importance to the extension of the catalytic applications and the promotion of the feasibility of molybdenum carbide in catalysis.The dissertation presented the work in developing the single-source preparation methodology for molybdenum carbide catalytic materials with novel structure and extraordinary catalytic properties on the basis of the reaction between molybdate and melamine. In the beginning, microwave irradiation was used to thermally treat a mechanical mixture of Mo and carbon sources which could be seen as a resemblance to the single-source precursor for molybdenum carbide. The mixture was transformed into supported molybdenum carbide of high dispersion within a few minutes under microwave irradiation, representing an effective way to synthesize highly dispersive molybdenum carbide rapidly. Then, the chemical connection was set between Mo and carbon sources, resulting in MoⅥ-melamine hybrid, a real single-source precursor for molybdenum carbide. A single-source route to single-phase molybdenum carbide was presented through the direct pyrolysis of the MoⅥ-melamine hybrid. The interaction between the hybrid and the P123micelles was employed to accomplish the incorporation of molybdenum carbide into ordered mesoporous silica framework (OMSF), promoting the dispersion and practicality in catalytic use of the molybdenum carbide. By taking advantage of the reaction between the molybdate and melamine:1) eggshell molybdenum carbide catalyst was synthesized to promote the selectivity to the intermediate products of consecutive hydrogenations, enabling the control of the distribution of molybdenum carbide on support;2) molybdenum carbide nanowires were synthesized by using ethylene glycol as structure director, providing a template-free route to1D nanostructured molybdenum carbide and selective exposure of the specific lattice plane of molybdenum carbide;3) molybdenum carbide was organically combined with high-surface-area N-doped carbon (NDC) by the use of dehydration reactions, entitling molybdenum carbide with novel catalytic properties through the introduction of functional catalytic materials. The detailed research content is as below: (1) By taking advantage of microwave irradiation’s ability to elevate temperature in a fast manner, the mechanical mixture of Mo and carbon sources which could be seen as a resemblance to single-source precursor for molybdenum carbide was transformed into highly dispersed molybdenum carbide supported on CNTs and activated carbon (AC) within30min. The thermal sintering of the carbide particles was significantly reduced. The obtained Mo2C particles were3-5nm in diameter. The Mo2C/CNTs catalyst was proved to be an effective support for Pt. The interplay between Mo2C and Pt facilitated the reduction of oxygen over Pt in oxygen reduction reaction involved in fuel cells. The Mo2C/AC catalyst showed ultra-high selectivity of>99%toward tetralin and high stability in the hydrogenation of naphthalene. The conversion of naphthalene over Mo2C/AC could be kept above80%in60hours.(2) The electrostatic interaction between molybdate and melamine was used to establish the chemical connection between Mo and C sources in aqueous phase, which gave rise to the ammonia molybdate-melamine hybrid (AHM-MA), a single-source precursor for molybdenum carbide. The AHM-MA was transformed into single-phase β-Mo2C through a single step of heat treatment without the use of external carbon sources thus eliminated the carburization process and inverted the carbon diffusion. The amount of the surface carbon deposit of molybdenum carbide was significantly reduced. The single-phase β-Mo2C possessed a Mo/C atomic ratio of2.8, indicating no unstoichiometric carbon existed. The less carbon deposit molybdenum carbide had on its surface, the higher activity it achieved in the hydrogenation of naphthalene.(3) MoCx@OMSF cmposite with hierarchical structure was synthesized through the complexation of molybdate within the AHM-MA by the ethylene oxides on the surface of the P123micelles. The pore diameter, volume and the surface area of the composites were found to increase with the Mo loading owning to the expanding effect of AHM-MA on the volume of the P123micelles. The secondary structure of the MoCx@OMSF composite was found to be a tube-like carbon framework within the silica tubes, which generated through the thermal decomposition of the P123micelles with the catalysis of the in situ formed Mo species during the pyrolysis of the MoCx@OMSF preform. The MoCx clusters were incorporated to the walls of the "carbon tube". The MoCx@OMSF composites showed high selectivity of～98%toward tetralin and significantly enhanced stability with respect to bulk Mo2C in the hydrogenation naphthalene.(4) A nano-sized shell of molybdenum carbide was formed on the outer layer of the γ-Al2O3by taking advantage of the rapid reaction between ammonium molybdate and melamine. The catalyst was applied in high-temperature gas phase hydrogenation of phenylacetylene and ahieved high selectivity of～96%toward styrene due to rapid desorption of reactants from the carbide shell. The relation between the distribution of Mo2C on the support and the catalytic performance of Mo2C was investigated. The elimination of phenylacetylene within excessive styrene was realized through three cycles of hydrogenation over the eggshell Mo2C/γ-Al2O3catalyst, while the ultimate loss of styrene was as low as4%. The eggshell Mo2C/γ-Al2O3catalyst possessed a big operation window in the high-temperature gas phase hydrogenation of phenylacetylene, showing great potential of application in the purification process of styrene monomers which was coupled with ethylbenzene dehydrogenation.(5) The synthesis of1D nanostrutured β-Mo2C nanowires based on the reaction between molybdate and melamine was realized through a template-free route which used ethylene glycol as the structure director for molybdate. The formation mechanism of the nanowires was proposed, and the influence of H2O content, reaction duration, and AHM concentration on the morphology of the nanowires was investigated. The β-Mo2C nanowires with large aspect ratio of～20and selective exposure of the (010) facet were obtained with the optimized fabricating parameters. The tune of the mophorlogy of β-Mo2C was realized by changing the H2O content.β-Mo2C plate and tubes were obtained at high and low H2O content respectively.(6) The respective combination of the pore formers and the single-source precursors for molybdenum carbide with the preform of N-doped carbon (NDC) was finished by taking advantage of dehydration reaction and the reaction between molybdate and melamine. The oganic combination of sub-nanometer MoCx clusters with the functional electrocatalyst NDC with ultra-high surface area of～800m2·g-1was obtained through one-step pyrolyzing the doped NDC preform. The hal-wave potential for oxygen reduction over MoCx/NDC in oxygen reduction reaction involved in fuel cells shifted positively with the increase of the MoCx loading. The strong interaction between Pt and MoCx induced much earlier reduction of oxygen over Pt on MoCx/NDC with respect to NDC. However, the interplay also induced change in properties of Pt, leading to irreversible oxidation of Pt during the oxygen reduction.