SnO₂–based Solid Solution Catalysts For Co And CH₄ Oxidation:Using XRD Extrapolation Method To Quantify The Lattice Capacity Of Solid Solutions

Tin oxide?SnO₂? is an n–type semiconductor with a wide–band gap?Eg = 3.6 eV?, which has been widely used as gas sensors, lithium–ion batteries, electrodes for photovoltaic devices and catalytic materials. Because of its facile surface and lattice oxygen and high thermal stability?melting point, 1630 °C?, the catalytic properties of SnO₂ have gotten more and more attentions. Former studies also revealed that the catalytic properties of SnO₂ can be enhanced by substituting part of the Sn4+ in the crystal lattice with other cations.Part I: A series of SnO₂–based catalysts modified by Mn, Zr, Ti and Pb oxides with different Sn/M molar ratios were prepared by co–precipitation method and used for CH₄ and CO oxidation. It is revealed that Mn3+, Zr4+, Ti4+ and Pb4+ cations are incorporated into the lattice of tetragonal rutile SnO₂ to form solid solution structure. As a consequence, the surface areas and thermal stability of the catalysts are improved. Moreover, the oxygen species of the modified catalysts become easier to be reduced. Therefore, the oxidation activity of the catalysts is improved except for the one modified by Pb oxide, possibly due to the nature of Pb as a catalyst poisoner. Mn oxide demonstrates the best promotional effects for SnO₂. To gain deeper understanding and to optimize the catalyst formulation, a series of Sn–Mn mixed oxide catalysts with different Sn/Mn molar ratios were also prepared and subjected to CH₄ deep oxidation. Using XRD extrapolation method, it is revealed that the lattice capacity of SnO₂ for Mn2O3 is 0.135 g Mn2O3/g SnO₂, which equals to a Sn/Mn molar ratio of 79/21,indicates that to form stable solid solution, only 21% Sn4+ cations in the lattice can be maximally replaced by Mn3+. If the amount of Mn3+ cations is over the capacity, Mn2O3 will be formed, which is not favorable for the activity of the catalysts. Those Sn rich samples with only Sn–Mn solid solution phase show generally better activity than the ones with excess Mn2O3 species.Part II: A series of Sn–Ce mixed oxide catalysts with different Sn/Ce molar ratios were prepared by co–precipitation method and subjected to CH₄ deep oxidation. Using XRD extrapolation method, it is revealed that the lattice capacity of SnO₂ for CeO₂ is 0.338 g CeO₂/g SnO₂, which equals to a Sn/ Ce molar ratio of 77/23; and the lattice capacity of CeO₂ for SnO₂ is 0.364 g SnO₂/g CeO₂, which equals to a Sn/Ce molar ratio of 29/71. It is revealed that if the solvent cations have a bigger radius than that of the solute cations, a larger lattice capacity will be achieved.Part III: The SnMn solid solution nanorods were successfully fabricated by hydrothermal method. The samples have uniform sizes in the range of 10¹30 nm and lengthes in the range of 0.2¹ ?m. After dopping Mn into the crystallite lattice of SnO₂ nanorods, the diameters become smaller; the surface areas and thermal stability of the catalysts are improved. The best Sn/Mn nanorod catalyst with a molar ratio of 8/2?SnMn8–2 NR? is not only active but also resistant to water vapor deactivation.