Effects of La, Ce, and Y Oxides on SnO2 Catalysts for CO and CH4 Oxidation

SnO2-based catalysts modified by La, Ce, and Y with a Sn/Ln (Ln=La, Ce, Y) atomic ratio of 2:1 were prepared by using a co-precipitation method and used for CO and CH4 oxidation. The catalysts were characterized by N-2 adsorption-desorption, XRD, energy dispersive X-ray spectroscopy (EDS)-SEM, H-2 temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis differential scanning calorimetry (TGA-DSC) techniques. All three rare earth metal oxides were found to improve the thermal stability of SnO2, which resulted in catalysts with much higher surface areas and smaller crystallite and particle sizes. However, only the addition of Ce resulted in a catalyst with improved activity for both CO and CH4 oxidation. In contrast, La and Y modification resulted in samples with decreased activity for both reactions. For the Ce-modified sample, Ce cations were found to dope into the lattice of rutile SnO2 to form a solid-solution structure. As a lattice impurity, ceria, the well-known oxygen storage component (OSC), led to the formation of more defects in the matrix of SnO2 and impeded the crystallization process, which resulted in a catalyst with a higher surface area and more active oxygen species. In contrast, XRD proved that the addition of La and Y mainly led to the formation of more stable and inert pyrochlore compounds, Sn2La2O7 and Sn2Y2O7, which disrupted a major part of the active sites based on SnO2. Consequently, the oxidation activity was impaired, although these two samples also have higher surface areas than pure SnO2. The Ce-modified sample showed not only high activity but also good reaction durability and thermal stability. Furthermore, Sn-Ce binary oxide is a better support than SnO2, CeO2, and traditional Al2O3 supports for Pd, which gives it the potential to be applied in some real after-treatment applications.