Roles of ZrO₂ in SO₂-poisoned Pd/(Ce-Zr)O₂ catalysts for CO oxidation

This work aimed at understanding the roles of ZrO₂ in Pd/(Ce-Zr)O₂ catalysts for CO oxidation during sulfur poisoning and regeneration treatments. Pd (2 wt%) were loaded on four support materials with various Ce/Zr ratios (CeO₂, Ce_0.70Zr_0.30O₂, Ce_0.58Zr_0.42O₂ and Ce_0.32Zr_0.68O₂) by using conventional incipient wetness impregnation method. Several treatments were subsequently applied to the samples (hydrothermal aging, sulfur aging and regeneration). To get information of the physicochemical properties of catalysts, many analyses were carried out by the means of X-ray diffraction (XRD), N₂-adsorption-desorption with BET and BJH method, Fourier transform infrared spectroscopy (FT-IR), elemental analysis, CO-chemisorption, temperature programmed reduction with hydrogen (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption for the sulfur aged sample (SO₂-TPD). In addition, CO oxidation was performed for these differently treated catalysts. It was found that the amount of sulfur adsorption depends strongly on Ce content of the sample. Most of sulfur on sulfur aged catalysts exists in the form of bulk Ce(SO₄)₂, as confirmed by FTIR, while small peak arising from surface sulfate species was also observed. The regeneration process at 750 °C can remove most of bulk Ce(SO₄)₂, however, surface type sulfate species remains as residual sulfur which inhibits the recovery of CO oxidation ability. The addition of ZrO₂ in CeO₂ support reduces sulfur poisoning and improves structural stability during regeneration. Among the catalysts, it was found that Pd/Ce_0.58Zr_0.42O₂ demonstrates superior CO oxidation performance after sulfur aging and regeneration treatment.