Study On The Catalytic Structure-Activity Relationship Of SnO₂-Based Solid Solution Materials

Metal oxide solid solutions have better physical and chemical properties than the pure oxides.The formation of a solid solution structure by doping can effectively improve the stability of a catalyst,and can regulate its surface active sites.Since the solute content often affects the reaction performance of solid solution catalysts,we have previously developed an easy and simple XRD extrapolation method to quantify the lattice capacity of a solid solution catalyst,and correlate with its reaction performance.Based on this method,this thesis has measured the lattice capacities and crystalline structures of Sn-Ce-O solid solution,a mutual soluble system,and investigated the lattice capacity effect on the reaction performance.In order to improve the XRD extrapolation method for lattice capacity quantification,and elucidate the reasons influencing the formation of solid solutions systematically,the calcination temperature and the solute cation radius effects have also been invetigated by this method.The main results are summarized here:1.The Sn-Ce-O solid solution catalyst was synthesized by a redox coprecipitation method.By using XRD extrapolation method,its lattice capacities have been quantified,and the threshold effect has been discussed.It has been revealed that Sn-Ce-O is a mutual soluble solid solution system.When Sn is rich,Ce⁴⁺cations can be dissolved into SnO₂ lattice to form a tetragonal rutile solid solution with a lattice capacity of 0.339 g CeO₂/g SnO₂,corresponding to a molar ratio of Sn/Ce=77/23;When Ce is rich,Sn⁴⁺cations can be dissolved into CeO₂lattice to form a cubic flourite solid solution with a lattice capacity of 0.367 g SnO₂/g CeO₂,corresponding to a molar ratio of Sn/Ce=29/71.The formation of solid solution structures was confirmed by Raman and STEM-mapping.SnCe3-7 displays the best CO oxidation activity,and SnCe8-2 shows the best CH₄ oxidation activity,demonstrating evident lattice capacity threshold effect.XPS,O₂-TPD,CO-TPD and CH₄-TPD results testify that both SnCe3-7 and SnCe8-2 possess abundant surface active oxygen species.In addition,the former owns the largest amount of CO adsorption sites and the latter owns the largest amount of CH₄ adsorption sites,thus they display the highest activity for CO and CH₄ oxidation,respectively.2.On the basis of the first part of work,in order to probe the calcination temperature and the solute cation radius effects on the lattice capacity,Sn-Ce-O solid solution was calcined at different temperatures,and a series of SnM?M=Fe,Mn,Cr?solid solutions was synthesized by co-precipitation method,whose lattice capacities have also been measured by XRD extrapolation method.It is discovered that the lattice capacities of Sn-Ce-O solid solution decrease with the increasing of the calcination temperatures until 800 ^oC.Above this temperature,the lattice capacities keep constant,indicating the structure of Sn-Ce-O solid solution transforms from meta-stable to stable.For SnM?M=Fe,Mn,Cr?solid solution calcined at 800°C,in the crystalline lattice of SnO₂,a metal oxide solvent,the lattice capacities increase with the decreasing of the solute cation radii.To understand the strucrure-reactivity relationship,SnM9-1?M=Fe,Mn,Cr?catalysts with a pure solid solution phase were tested for CO oxidation.It is found that by decreasing the the solute cation radii in the order of r_Cr3+>r_Mn3+>r_Fe3+,the amount of surface active oxygen sites increases,thus improving both of the intrinsic and overall activity of the catalysts in the order of SnFe9-1>SnMn9-1>SnCr9-1>SnO₂.