Pd Supported on SnO₂-Al₂O₃ Composite Supports for CO Oxidation – Designing Thermally Stable and Active Supports for Pd

Abstract

A series of SnO₂-Al₂O₃ composite metal oxide supports with different Sn/Al atomic ratios were synthesized with co-precipitation method and used as supports for Pd to prepare catalysts for CO oxidation. The samples were characterized by N₂ adsorption-desorption, CO adsorption-desorption, X-ray Powder Diffraction (XRD), H₂ temperature programmed reduction (H₂ TPR), and X-ray photoelectron spectroscopy (XPS) techniques. It was found that Al³⁺ can be introduced into the crystalline lattice of tetragonal rutile SnO₂ to effectively form solid solution structure. As a consequence, SnO₂-Al₂O₃ binary supports with significantly improved thermal stability, enlarged surface areas, enhanced texture structure and containing more active surface oxygen species can be obtained. SnAl73, a support with a Sn/Al atomic ratio of 7/3, was found to be the best support in this study due to its certain large surface area and the presence of suitable amount of reducible oxygen species. With this support, the two major reasons accounting for the activity, that is, Pd dispersion and strong metal support interaction (SMSI) between Pd and the support, is believed to be tuned optimally. Water vapor has reversible effect on the CO oxidation activity of Pd/SnAl73 catalyst, but which is mitigated evidently by the presence of SnO₂. In addition, no any permanent deactivation occurs to Pd/SnAl73 catalyst during a 60 hours' test in the presence or absence of water vapor, indicating this optimized SnO₂-Al₂O₃ support has a strong potential to be applied in some practical processes for exhaust clean. In summary, SnO₂-Al₂O₃ composite metal oxides with suitable Sn/Al atomic ratios are better supports than individual Al₂O₃ and SnO₂, on which less amount of Pd could be used to achieve supported Pd catalysts with rival performance.