First-principles Thermodynamic Calculations For Phase Characterization And Reaction Synthesis Mechanism Of Ag/Al₂O₃ Powder Composite Materials

The powder composite Ag/Al2O3 is widely used in industrial catalysis.It is an essential catalysis to catalyze C2H4,C3H6 to be C2H4O,C3H6O.Many preparation method of Ag/Al2O3 has been developed recently,including impregnation,sol-gel method and others,however,the main defect of these methods is that the nano-Ag is not stable during used in high temperature.So,in this paper,based on the early work(in-situ synthesis method),the phase transformation and microstructue of Ag/Al2O3 were analyzed and characterized by the First principle calculation and following equipment.All that would lay the foundation for the future application.The SEM(Tungsten filament)and FESEM(field emission scanning electron microscopy)equipped with EDS(energy dispersion spectrometry),X-ray,catalyst characterization system were used to characterized the powder composite Ag/Al2O3 by in-situ systhesis preparation.After acid cleaning,the oxygen absorption and desorption were analyzed by XPS(X-ray photoelectron spectroscopy).The experiments result show that the best heat treatment temperature in oxygen atmosphere was 600?,After heat treatment,the powder particle structure changed into shell and core,in that the Ag precipitated from matrix largely,and Ag cluster(300-400nm)distributed evenly on the powder's shell.After acid cleaning,the Ag exposed from the Al2O3 film and the size was 50-100nm.The absorption oxygen detached from the surface in 395? and 510? which greater than the temperature in practical application of Ag/a-Al2O3 catalyst.The size of powder and Ag,the distribution of Ag,and the oxygen degree can further change the material's performance.The possible occurred intermediate phases Ag3Al,AgAlO2,Ag2O,Ag2Al and final phases Al2O3,Ag,Ag2Al were selected to calculate the phase transformation process by the CASTEP module in MS software.From crystal lattice,cohesive energy,electrical population,electrical density of state,the calculation results show that,the thermostability for oxides is:Al2O3>AgAlO2>Ag2O,and that for intermediate phases is Ag2Al-Neumann>Ag2Al-Howe>Ag3Al-1>Ag3Al-4>Ag3Al-8,and that for solid solution is 63AllAg>47Al1Ag>31AllAg>15Al1Ag>7Al1Ag>3Al1Ag.From the crystal structure and energy of solid solution,the result show that,although the O concentration in elemental Al is higher than that in Ag2Al,0 atoms are more likely to be dissolved in elemental Al,in other word,elemental A1 is easier to oxidate than Ag2Al.The function of the thermodynamics parameters(?H,T*S,?G)related to the temperature were built based on CASTEP and common thermodynamic data calculation.After tested the reliability of the function,the oxygen mechanism,the reaction synthesis of Ag/Al2O3,were given.The result is that the Al solid solution bearing Ag can not precipitated Ag directly after heat treatment under O2 atmosphere,but the intermediate phases Ag2Al occurred.Then Ag2Al were oxidized to be Ag and Al2O3,not intermediate high temperature phases Ag3Al.The Al solid solution bearing Ag oxidized before Ag2Al,and the more content of it,the easier of the reaction.The ternary AgAlO2 did not emerge in the reaction.