Cu-Ce-La-O_x as efficient CO oxidation catalysts: Effect of Cu content

Nanograins of Ce-La-xCu-O mixed metal oxides (where x = 3, 5, 7, 10, 20 at.%), of approximately 16 nm² area size, and having all the metal cations homogeneously distributed, are tested as ternary catalytic materials towards the CO oxidation in the 100–475 °C range. Preservation of the cubic ceria lattice structure in those catalysts following La³⁺ (heavy) and Cu²⁺ (light) metal cations doping is confirmed based on the powder X-ray diffraction and Raman shift studies. From X-ray photoelectron spectroscopy studies, the presence of mixed Ce³⁺/Ce⁴⁺ and Cu²⁺/Cu¹⁺ oxidation states was confirmed, which was more profound in the low Cu-content Ce-La-xCu-O catalysts. The copper doping is also found to increase the concentration of oxygen vacant sites in the doped-CeO₂ solid as demonstrated from the increase of the Raman intensity ratio I_Ov/I_F2g of 1.58 and 1.78 with the increase in copper content from 7 to 20 at.% as compared to the lower value of 0.44 obtained for the Cu-free catalyst (Ce-La). The mobility of the surface and bulk oxygen ions in the lattice of such doped-CeO₂ materials is further investigated using ¹⁶O/¹⁸O transient isothermal isotopic exchange (TIIE) experiments, and is found to be Cu at.% dependent. For the case of Ce-La-20Cu, the participation of the lattice oxygen (O_L) in the reaction mechanism has been demonstrated using transient experiments. Accordingly, the specific rate (μmol CO m⁻²s⁻¹) of the CO oxidation reaction is found to be higher for the Ce-La-20Cu and Ce-La-7Cu catalysts, corroborating thus the presence of more mobile/labile oxygen species in those ternary catalysts as opposed to the other lower copper compositions.