Highly dispersed Pd/modified-Al₂O₃ catalyst on complete oxidation of toluene: Role of basic sites and mechanism insight

Noble metal catalysts face high cost and limited resources challenge. Developing a high efficiency catalyst with low noble metal loading is an important way to meet this challenge. In this study, Mg-, Ba- and Zr-doped Pd/Al₂O₃ catalysts with Pd loading of 0.5 wt% were prepared by stepwise wet impregnation method and used for complete oxidation of toluene. These catalysts were characterized by powder X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), CO chemisorption, hydrogen temperature-programmed reduction (H₂-TPR), temperature-programmed desorption of CO₂ (CO₂-TPD) and temperature-programmed desorption of oxygen (O₂-TPD). Catalytic reaction of toluene and isopropyl alcohol were carried out in a fixed bed micro-reactor under the condition of GHSV = 24,000 ml h⁻¹g_cat ⁻¹ with 500 ppm toluene or isopropyl alcohol. The results of XRD, Raman and XPS revealed that PdO is the major species in all of the catalysts. The size of PdO remarkably decreased on MgO-doped Al₂O₃ surface. The results of H₂ temperature-programmed reduction and O₂ temperature-programmed desorption indicated that the reducibility of PdO and the active oxygen species were enhanced by doping basic element. In situ diffusion reflectance-infrared Fourier transform spectroscopy proved that benzaldehyde and carboxylates were the intermediate products of toluene oxidation. The results of X-ray photoelectron spectroscopy on used catalysts demonstrated that many more Pd²⁺ species were detected in Pd/MgO-Al₂O₃ sample, suggesting that a Pd²⁺/Pd⁰ pairs was formed which is favored to the combustion of toluene. The results show that the catalytic performance of Pd/Al₂O₃ was promoted by modifying basic oxides in the order of Pd/MgO-Al₂O₃ > Pd/BaO-Al₂O₃ > Pd/ZrO₂-Al₂O₃ > Pd/Al₂O₃. The catalytic activity of 0.5 wt% Pd/MgO (2.0 wt%)-Al₂O₃ was similar with that of 1.0 wt% Pd/Al₂O₃.