Study On The Doping Effect On SnO₂-based Solid Solution And Its Supported PdO Catalysts

SnO₂ catalysts possess Lewis acidic sites and active oxygen species on the surface,and also have good thermal stability.A wide range of cations can be dissolved into the SnO₂ lattice to form stable solid solutions.Therefore,catalytic chemistry on SnO₂-based materials has attracted researchers'attention in recent years.On the one hand,to investigate ion doping SnO₂,Cu²⁺-doped SnO₂ solid solution catalysts have been prepared and used for NO_x-SCR by NH₃.The lattice capacity effect on structure and reactivity of the catalysts has been elucidated.On the other hand,to explore ion doping SnO₂ as supports on an active component,Zr⁴⁺,Ce⁴⁺and Ti⁴⁺-doped SnO₂ supported PdO catalysts have been prepared and used for CO oxidation.The effects of active oxygen species of the surface support on active components and CO catalytic performance have been revealed.The main results are as follows:?1?To unravel Cu²⁺promotion effects on the structure and reaction performance of SnO₂,a series of Sn-Cu binary oxide catalysts were prepared by a coprecipitation method and used for NOx-SCR by NH₃.It was found that Cu²⁺cations can be dissolved into the lattice of tetragonal rutile SnO₂ to form a solid solution phase with a lattice capacity.Using an XRD extrapolation method,the SnO₂ lattice capacity for Cu²⁺has been quantified to be 0.1 g CuO per gram SnO₂,equaling a Sn/Cu molar ratio of 84/16.Interestingly,when Cu content is below the lattice capacity,Sn-Cu oxide catalysts present a pure solid solution phase,for which the amount of active oxygen and surface acidic sites increase with the increase of lattice Cu²⁺cation content.In contrast,after Cu content is above the lattice capacity,the excess CuO will be formed on the solid solution surface,which decreases the amount of the two sites.As a consequence,SnCu8.5-1.5,with the Cu content closest to the lattice capacity,owns the largest amount of pure solid solution phase,finally achieving the best activity for NO_x-SCR by NH₃,due to the catalyst possessing the largest quantity of active oxygen and surface acidic sites.More importantly,the results suggest that there exists a distinct threshold effect on the structure and reactivity of Sn-Cu oxide catalysts at the lattice capacity.Therefore,we proposed that the optimal reaction performance of a solid solution catalyst could be rationally designed by the formation of the largest amount of a pure solid solution phase by reaching the lattice capacity of a solvent oxide for a doping cation.?2?To explore solid solution doping effect of SnO₂ as supports on an active component,PdO catalysts supporting on pure SnO₂ and Zr⁴⁺,Ce⁴⁺and Ti⁴⁺-doped SnO₂-based solid solution have been prepared and used for CO oxidation.XRD,HR-TEM,STEM-mapping and XPS results demonstrated that those cations can be dissolved into the lattice of tetragonal rutile SnO₂ to form a solid solution phase,thus SnM9-1 supports possessing more surface active oxygen species,higher specific surface area and smaller pore size structure in comparison with un-doped SnO₂.As a result,the PdO catalysts supporting on SnM9-1 show higher CO reaction performance than the catalyst supporting on un-doped SnO₂,with the best activity obtained on the PdO/SnTi9-1 catalyst.CO-TPD?O₂-TPD and XPS results demonstrated that solid solution doping of the SnO₂ support can effectivly improve the PdO dispersion and increase CO adsorption capacity and the amount of active oxygen species,which are main factors accounting for better activity of PdO/SnTi9-1catalysts for CO oxidation in comparison with PdO/SnO₂.