Effects of metal doping on the methanol deep oxidation activity of the Pd/CeO2 monolithic catalyst†

Abstract

Methanol fuel can be used as a clean alternative to conventional gasoline. However, vehicles using methanol fuel typically exhibit low exhaust temperatures during the cold start and idle phases, which may result in the emission of unburned methanol vapor. Herein, a series of CeO₂-based solid solutions doped with different metal ions (CeM, M = Mg, La, Bi, Zr) are synthesized by a hydrothermal synthesis method, and supported Pd/CeM catalysts with regular interfacial structures are prepared by a special assembly method for the low-temperature deep oxidation of methanol. The results of XRD, N₂ adsorption/desorption, Raman spectroscopy, XPS and H₂-TPR show that the crystal structure, specific surface area, defect concentration, surface oxygen vacancy content, high-valence Pd^δ+ (δ > 2) species content and redox performance of the Pd/CeM catalyst are closely related to the type of doped metal ions. The catalytic performance results show that the Pd/CeLa catalyst exhibits the best low-temperature methanol oxidation activity, with a light-off temperature (T₅₀) of 118 °C and full conversion temperature (T₉₀) of 155 °C, which is at a high level under the same conditions reported in the literature. This is mainly attributed to its high defect concentration, high oxygen vacancy and more hypervalent Pd^δ+ (δ > 2) species content as well as excellent low-temperature reduction performance. The results of this study demonstrate the promise of the Pd/CeLa catalyst for methanol oxidation and may offer guidelines for designing efficient catalysts for purification of methanol fuel from vehicle exhaust.