New insights into the defective structure and catalytic activity of Pd/ceria

A surfactant-assisted synthesis method was applied to prepare nanosized ceria particles used as the three-way catalyst supports. The Rietveld refinements of crystalline structures of ceria calcined at various temperatures, together with XRD, FTIR, and TGA techniques, confirm that the surfactant cations are incorporated into the structural network of hydrous cerium oxide during the preparation stage, thus reducing the surface tension and diminishing the shrinkage of the mesoporous channels in the dried materials. The surface area is therefore enhanced but it provokes a significant distortion effect on the structure. It is evidenced by XRD analyses that a well-ordered cubic structure of ceria was formed at a temperature as low as 80 °C. The average crystallite size of ceria is determined by XRD within nanoscale range (i.e., 9-31 nm) when the calcination temperature increases from 80 to 800 °C, which is in good agreement with the data determined by TEM and electron diffraction methods. In addition, cationic defects are present in the crystalline structure of the solids, and the defect concentration in the unit lattice cell decreases as calcination temperature increases. CO chemisorption analysis by FTIR shows that CO may adsorb on Pd/ceria catalyst in different modes: linear Pd-CO and Ce⁴⁺-CO bonds, bridged Pd(CO)₂ bonds, and multibonded Pd₂(CO)₂ bonds. CO adsorption might lead to CO₂ production from CO reacting with lattice oxygen and/or CO disproportionation catalyzed by palladium crystals. The CO oxidzed over the Pd supported catalysts makes it that T₅₀ and T₉₀ for the ceria prepared in the presence of surfactant diminish at 80 °C and 110 °C respectively, which are much lower than the ones prepared without surfactant, mainly due to its defective structure, enhanced surface area, and different CO adsorption behavior.