One-step synthesis of nanostructured Pd-doped mixed oxides MOx-CeO2 (M = Mn, Fe, Co, Ni, Cu) for efficient CO and C3H8 total oxidation

A series of nanostructured Pd-doped mixed oxides MOx-CeO2 (M = Mn, Fe, Co, Ni, Cu), with uniform mesoporous structure and large surface area exceeding 115 m2 g−1, were synthesized in one step by a surfactant-assisted co-precipitation. Their catalytic performance was investigated using total oxidation of CO and C3H8 as the model reactions. The results show that, a synergism exists between even trace amounts of exposed Pd and 3d-transition metal oxides for CO oxidation, whereas such an effect is absent for C3H8 oxidation. In situ diffuse reflectance infrared spectroscopy (DRIFTS) study reveals that the synergistic essential for CO oxidation should be the interaction-assisted generation of active oxygen species between Pd and MOx, which react readily with CO, forming bidentate carbonate (1587 and 1285 cm−1) as intermediates. Moreover, structural characterization results indicate that a solid solution is formed between CeO2 and Mn2O3 or Fe2O3, resulting in the very strong interaction between Pd and MOx, as well as the greatly improved CO oxidation. The light-off temperatures for Pd-doped Mn and Fe-containing catalysts, as compared with the Pd-free catalysts, are decreased by more than 70 and 100 °C, respectively. In particular, a CO conversion as high as 80% can be achieved even at room temperature on Pd-doped Mn-containing catalyst. While for C3H8 oxidation, the C–H bond activation, but not the oxygen activation, plays a crucial role. The C–H bond activation ability of the catalysts is largely determined by the d-electron configurations of the M cations. A ‘double-peak’ phenomenon can be derived with the increase of d-electron number.