Size-Dependent Oxygen Activation Efficiency over Pdn/TiO2(110) for the CO Oxidation Reaction

Abstract

The dissociative binding efficiency of oxygen over Pd_n/TiO₂(110) (n = 4, 7, 10, 20) has been measured using temperature programmed reaction (TPR) mass spectrometry and X-ray photoemission spectroscopy (XPS) following exposure to O₂ with varying doses and dose temperatures. Experiments were carried out following two different O₂ exposures at 400 K (10 L and 50 L) and for 10 L of O₂ exposure at varying temperatures (T_surf = 200, 300, and 400 K). During TPR taken after sequential O₂ and CO (5 L at 180 K) exposures, unreacted CO is found to desorb in three features at T_desorb ≈ 150, 200, and 430 K, while CO₂ is observed to desorb between 170 and 450 K. We show that Pd₂₀ has exceptionally high efficiency for oxygen activation, compared to other cluster sizes. As a consequence, its activity becomes limited by competitive CO binding at low O₂ exposures, while other Pd_n sizes are still limited by inefficient O₂ activation. This difference in mechanism can ultimately be related back to differences in electronic properties, thus making this question one that is interesting from the theoretical perspective. We also demonstrate a correlation between one of the two CO binding sites and CO₂ production, suggesting that only CO in that site is reactive.