TiC Powder Synthesis Through Chemical Vapor Deposition And Its Application To Electrocatalytic Hydrogen Evolution Process

Titanium carbide(TiC)has been widely used as functional and engineering ceramics under harsh conditions due to its high melting point,high hardness,good corrosion resistance and similar electronic structure to Pt,which gives excellent catalytic performance.The performance of ceramic device is determined by the quality of TiC powder,which needs to meet criteria of ultra-fine,high purity,controlled stoichiometric ratio and controlled morphology.However,the mass transfer barrier that existed in solid-phase carbonization process leads to the difficulty of synthesizing ultra-fine and high purity TiC powders with desired morphology and stoichiometric ratio.Although chemical vapor deposition(CVD)method can avoid the mass transfer barrier,the equilibrium constant of the traditional CVD reaction system(TiCl4-CH4-H2)is too low to provide sufficient supersaturation to synthesize TiC powder.Although TiC powder can be obtained by plasma or active metal(Na,Mg,etc.)vapors assisted CVD,the as-obtained TiC powder are of low quality,like low purity,uncontrollable morphology,etc..Therefore,it is still a great challenge to synthesize ultra-fine and high-purity TiC powder with controlled morphology and stoichiometric ratio.To tackle the above problems,new ideas of carbonizing TiClx(x=4,3,2)by fluidized bed chemical vapor deposition(FBCVD)were proposed to synthesize TiC powder.Firstly,the effects of particle size of TiC seed powder,synthesis temperature and ratios of reactants on the morphology,purity and stoichiometry of products from TiCl4-CH4-H2 system were systematically studied in a fluidized bed.Secondly,the thermodynamic and kinetic behaviors of reducing TiCl4 to TiCl3 or TiCl2 by H2,Ti etc.were investigated.The effects of temperatures and the ratios of TiClx(x=2,3)to CH4 on the nucleation and growth of TiC were also systematically studied.For the first time,high purity,ultra-fine TiC powders with cubic or spherical morphologies were successfully synthesized by CVD using the TiClx-CH4-H2 system(x=2,3).The main achievements are listed as follows.(1)TiC powder was synthesized by heterogeneous nucleation from TiCl4-CH4-H2 system on surface of the TiC seed particles with mean particle size of 37.6 ?m and 3.8?m using both a cylindrical and a conical fluidized bed,respectively.It was found that the 37.6 ?m coarse seed powder was fluidized in the form of bubbling state in the cylindrical bed,while the 3.8 ?m ultra-fine seed powder can only be fluidized in the conical bed in the form of agglomerate(34.8 ?m)fluidization.The deposition process does not trigger defluidization.The ultra-fine seed powder has larger specific surface area than the coarse seed powder(14.5 times)and thus provides more nucleation sites.Consequently,the deposition rate with ultra-fine seed powder is roughly 5.4 times that of the coarse seed powder.It was also found that the deposition rate at 1000? is about 3.5 times that at 900?.The content of Cl impurity decreases with increasing temperature.TiC0.87 powder with oxygen content(0.39 wt.%),which is 50%lower than that of the seed powder and is carbon free,has been synthesized at 1000? under CH4/TiCl4 of 0.6 using the ultra-fine TiC as seed powder.(2)Thermodynamic calculation indicates that the equilibrium constant of TiCl3-CH4-H2 system is?1000 times(1000?)as compared with the TiCl4-CH4-H2 system,which has greater homogeneous nucleation driving force.A new FBCVD method for synthesizing TiC by homogeneous nucleation from TiCl3-CH4-H2 system was thus developed.The effects of H2,Ti and TiH2 reductants on the yield of TiCl3 reduced from TiCl3 were studied.Ti was determined as the most efficient reductant.The effects of temperature and TiCl4 partial pressure on the productivity of TiCl3 in the Ti-TiCl4 system were investigated and the optimal synthesis conditions of TiCl3 were determined as 1000? and 9.8 kPa,respectively.Different from the TiCl4-CH4-H2 system,the TiCl3-CH4-H2 system can synthesize ultra-fine TiC powder through the homogeneous nucleation mechanism.Thus,TiC powder was synthesized for the first time by a thermal CVD process without plasma or active metal vapors assistance.The effects of reaction temperature and reactant ratios on the stoichiometric ratio,purity and particle size of the products were further studied.TiC0.95 powder,with the average particle size of 77.1 nm and the purity of 99.57 wt.%,was successfully synthesized under the optimal conditions of CH4/TiCl3=0.8,H2/TiCl3=5 and 1000?.A new method of synthesizing TiC powder by pre-reducing TiCl4 with H2,with the idea of the "instant production and consumption" of TiCl3,was further developed to simplify the process,resulting in the ultra-fine(103 nm)and high purity(99.65 wt.%)TiC0.87 powder.(3)The TiCl2-CH4-H2 system with even greater equilibrium constant,as compared with the above-mentioned processes,was further proposed for synthesizing TiC powder.TiCl2 can be obtained through reduction of TiCl4 by Ti particles.The effect of Ti particle size on TiC14 conversion was studied in a fluidized bed reactor,and it was found that TiCl2 yield could reach 95.36 wt.%when the Ti particle size decreases to 24.5 ?m.It was found that the CVD temperature has crucial effect on the morphology and the C/Ti ratio of the as-synthesized TiC powder in the TiCl2-CH4-H2 system,where the TiC shows near spherical with the C/Ti ration of 0.60 at 850?,while it exhibits cubic shape with the C/Ti ration of 0.95 at 1000?.Moreover,the TiC particle size was found to increase with increasing CH4 concentration,e.g.from 0.38 ?m at the CH4 concentration of 20 vol.%to 2.45 ?m at the CH4 concentration of 50 vol.%.It was also found that the carbon film in cubic TiC can be totally eliminated with the CH4 concentration below 20 vol.%at 1000?,by which the transformation of cubic TiC from polycrystal to single crystal was realized,and single crystal cubic TiC with exposed {100} crystal plane was synthesized.(4)The hydrogen evolution reaction(HER)performances of {100} and {111}faceted TiC powders were tested.It was found that the overpotential required for 10 mA/cm2 current density of the {100} faceted TiC powder is only 392 mV,which is 44%lower than that of the {111} faceted TiC powder,and 52%lower than that of the commercial irregular TiC powder.First principles calculation has been employed to calculate the Gibbs free energy of hydrogen atom(?GH*)and adsorption energy of H2 molecule(Eads)on different adsorption sites of Ti atoms,C atoms and vacancies on the{100} and {111} crystal planes.And it was found that the {100} facet has an ideal?GH*of-0.17 eV and weak adsorption strength for H2(Eads=0.018 eV),while the{111} facet has ?GH*of-1.02 eV and strong adsorption strength for H2(Eads=-1.057 eV).