Synergistic effect of vanadium and zirconium oxides in the Pd-only three-way catalysts synthesized by sol-gel method

Various Pd-only three-way catalysts containing vanadium and zirconium oxides were prepared by the sol-gel method and investigated with XRD, XPS, ⁵¹V NMR, ²⁷Al MAS NMR, Raman spectrometry, BET analysis and three-way catalytic reaction. In the Pd-only three-way catalyst prepared by the sol-gel method, vanadium and zirconium were good promoters to improve the resistance against SO₂ poisoning and to enhance the thermal stability, respectively. Excellent properties such as high activity at low-temperature, improved durability against SO₂ gas and enhanced thermal stability at high temperature were found only on the catalyst containing the specific mole ratio of V/Zr = 0.36, Pd (1 wt.%)-V (2 wt.%)-Zr (10 wt.%)-Al₂O₃ catalyst, which was ascribed to some synergism between V and Zr like a formation of V-O-Zr bonds. Because of the synergism on the catalyst, the advantageous textural forms such as highly dispersed Pd clusters, Zr-O-X bond (X = V and Al), γ-Al₂O₃ and amorphous tetrahedrally coordinated (M-O)₃V double bond O were not deteriorated significantly to the disadvantageous textural forms such as large crystalline Pd clusters, monoclinic ZrO₂, κ-Al₂O₃ and θ-Al₂O₃, and microcrystalline octahedrally coordinated V₂O₅ species, in spite of thermal aging at 1000 °C.