Nanoalloying Effect in the Preferential Oxidation of CO over Ir–Pd Catalysts

H₂-free CO oxidation and preferential CO oxidation in the presence of H₂ (PROX) have been studied on Ir_{100–<i>x</i>}Pd_<i>x</i> nanoalloys (<i>x</i> = 0, 11, 45, 89, and 100) supported on alumina or amorphous silica–alumina. The catalysts were prepared by impregnation of acetylacetonates and characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. It is shown that, depending on the temperature conditions, Ir-rich catalysts undergo oxidation or reduction, leading to complex reaction kinetics upon heating or cooling. The surface acidity of the support does not significantly affect the CO oxidation performances, but decreases the CO PROX activity due to the probable promoting role of basic OH groups in this reaction. In addition, although the H₂-free CO oxidation and the PROX turnover frequencies increase with the Pd content, the CO₂ selectivity of Ir_{100–<i>x</i>}Pd_<i>x</i> catalysts (<i>x</i> = 11 and 45) is found superior to those of Ir catalysts, Pd catalysts, and an Ir + Pd mechanical mixture. Moreover, the selectivity of these catalysts is retained at higher temperatures and the CO conversion is higher. This synergistic alloying effect is discussed in terms of ligand effect within the hypothesized Ir@Pd core-shell structure and Ir-induced inhibition of hydride formation