Preparation And Properties Of Molybdenum-based Nanomaterials By Solution Combustion Synthesis

Owing to unique physical and chemical properties,molybdenum-based materials are widely used in many fields such as metallurgy,chemical engineering,and aerospace.Besides,they also show good application prospects in emerging fields such as catalysis,energy storage,semiconductor,and medical.It has become an important research topic to develop advanced preparation methods for nanocrystallization,structural regulation,and uniform doping of molybdenum-based materials.Solution combustion synthesis,a new type of simple,efficient,low energy consumption,and low-cost wet chemical method,can be applied synthesize single or multicomponent nanomaterials rapidly.In this study,the solution combustion synthesis method was applied to the preparation of molybdenum-based materials.The main research contents are as follows:(1)Porous foamed molybdenum dioxide nanomaterials were prepared by solution combustion synthesis method with ammonium molybdate as the molybdenum source,ammonium nitrate as the oxidant,and glycine as the fuel.The effects of glycine fuel on the phase composition and microstructure of product were systematically evaluated,and the synthesis mechanism was elucidated.The results show that when the molar ratio of glycine to ammonium nitrate is 0=0.50,the reaction has the highest combustion temperature(683 ℃),and a foamed molybdenum dioxide nanomaterial composed of 20-30nm nanoparticles is formed by a reduction reaction of large amount of NH3 produced by the reaction with the MoO3produced by the pyrolysis of AMH.When Φ<0.50,it is a lean fuel reaction with the lowest combustion temperature,and it is impossible to synthesize MoO2.When Φ>0.50,it is a fuel-rich reaction.The combustion temperature drops to 457-527℃ because of excess glycine consuming a lot of heat.The product of reaction is a mixture of 10 nm MoO2 nanoparticles and amorphous carbon(Φ=0.75)or a molybdenum oxide/carbon composite with all-amorphous structure(Φ=1.0,1.25).(2)The porous foamed molybdenum dioxide nanomaterials were used as photocatalysts to systematically evaluate the catalytic activity of catalytic degradation of methyl orange(MO),methylene blue(MB),Rhodamine B(RhB),and phenol,revealing the catalytic mechanism.The results show that foamed molybdenum dioxide nanomaterials can successfully degrade four organic pollutants in 30 min,and the rate constants are 0.10567 min-1(MO),0.08269 min-1(MB),0.06763 min-1(RhB),and 0.10658 min-1(phenol)with excellent photocatalytic performance.The conduction and valence bands of molybdenum dioxide nanomaterials are located at 0.56 eV and 2.56 eV,respectively,and the forbidden band width is 2.0 eV.The main active species are photogenerated cavities and hydroxyl radicals.The excellent photocatalytic performance of molybdenum dioxide nanomaterials can be attributed to a high specific surface area,wide photoresponse range,and high electron-hole pair separation efficiency.(3)The effect of glucose addition on the phase and microstructure of product was studied,and the catalytic performance of hydrogen evolution was evaluated by introducing a glucose carbon source into the combustion reaction system and synthesizing the MoOx/C composite of amorphous structure.The results show that with the increase in glucose content,the phase and morphology of MoOx were evolved from coarse particles to fine particles and partially amorphous to completely amorphous.When the molar ratio of glucose to molybdenum source is 2.5,MoOx/2.5C composite material with an amorphous structure can be synthesized.As a hydrogenation catalyst,the composite material has a starting potential of 132 mV under 0.5 M H2SO4,an overpotential(η10)of 212 mV,and a Tafel slope of 84.9 mV/dec.Its excellent hydrogen evolution catalytic performance and stability can be attributed to rich catalytic active sites provided by the amorphous structure and the improved electrical conductivity and structural stability achieved by introducing carbon.(4)Porous molybdenum dioxide nanomaterials were used as the precursors to prepare nanomolybdenum powders using hydrogen reduction method.The effects of reduction temperature on the phase composition and morphology of products were systematically investigated,and the synthesis mechanism was elucidated.The results show that the irregular shape of nanomolybdenum powder can be successfully prepared at a reduction temperature of 600 ℃ and holding time of 2h,and the powder maintains the morphology of precursor as a whole.When the reduction temperature is increased to 650 ℃ and 700 ℃,the molybdenum particles are sintered and grow up,the size of molybdenum particles increases to 100-180 nm,and the carbothermal-hydrogen reduction reaction of residual carbon and MoO2leads to the formation of Mo2C impurities.Because the precursor of nanostructure has a high reducing activity,the reduction temperature obtained in this study is lower than the temperatures reported in literature.(5)The nanomolybdenum-based composite powder uniformly doped with lanthanum oxide(lanthanum nitrate as the doping source in SCS)was prepared by hydrogen reduction,and the formation mechanism was elucidated.The results show that the precursors with different cerium oxide doping amounts have porous foamy structures composed of 20-30 nm nanoparticles,and the main phase is MoO2.The nanomolybdenum-based composite powder without Mo2C impurity and uniformly doped with lanthanum oxide with a size of 40-70 nm can be prepared by argon calcination at 500℃/1 h and hydrogen reduction at 600℃/2h of precursor.Calcination at argon atmosphere promotes the consumption of residual carbon by carbothermal reduction reaction with MoO3 and Mo4O11,effectively preventing the formation of Mo2C impurities.