Abstract:

Palladium, a key component of three-way catalysts used in automobile exhaust treatment, plays a pivotal role in complete oxidation of alkanes and low-temperature oxidation of CO. Under oxygen-rich conditions, a thin oxide layer spontaneously forms on the palladium surface. To understand the influence of the oxidized palladium surface on the active phase of low-temperature hydrocarbon oxidation, we have conducted a comprehensive study of the oxidation process on Pd(100) and its impact on propane oxidation. Our experimental results reveal that under varying oxidation conditions, the Pd(100) surface sequentially forms three different monolayered oxide phases, namely, (2 × 2)-O, (5 × 5)-PdO and ($ \sqrt{\text{5}} $ × $ \sqrt{\text{5}} $) R27°-PdO. The oxygen coverage correspondingly increases in the order of 0.25 monolayer, 0.56 monolayer and 0.8 monolayer. Notably, (5 × 5)-PdO is identified for the first time by high-resolution scanning tunneling microscopy. Experiments show this structure exhibits typical chiral features with its two enantiomers observed in the experiments. The chiral features of the (5 × 5)-PdO structure may bear significant implications in practical applications like chiral catalysis. Based on high-resolution atomic imaging, we have proposed a new (5 × 5)-PdO structure model. In addition, it's experimentally revealed that upon thermal treatments the (5 × 5)-PdO structure decomposes into the ($ \sqrt{\text{5}} $ × $ \sqrt{\text{5}} $) R27°-PdO and the (2 × 2)-O structures, and the ($ \sqrt{\text{5}} $ × $ \sqrt{\text{5}} $) R27°-PdO decomposes into the (2 × 2)-O structure as well. These results suggest that the thermal stability of these oxide phases is in the order of (2 × 2)-O > ($ \sqrt{\text{5}} $ × $ \sqrt{\text{5}} $) R27°-PdO > (5 × 5)-PdO. We have also compared the catalytic activities of these three oxidation phases in low-temperature propane oxidation. The results show that only the ($ \sqrt{\text{5}} $ × $ \sqrt{\text{5}} $) R27°-PdO could catalyze propane oxidation near room temperature. We have observed a significant number of oxygen defects in the ($ \sqrt{\text{5}} $ × $ \sqrt{\text{5}} $) R27°-PdO, which also forms the reduced (2 × 2)-O phase. Both complete oxidation products H₂O and CO₂ are detectable by temperature-programmed desorption within two main temperature slots around 285 and 315 K. Neither the (2 × 2)-O nor the (5 × 5)-PdO phase shows an obvious catalytic activity. The superior oxidation activity of the ($ \sqrt{\text{5}} $ × $ \sqrt{\text{5}} $) R27°-PdO phase might be associated with its higher O density and hence more active O species within the structure. Our study indicates that the ($ \sqrt{\text{5}} $ × $ \sqrt{\text{5}} $) R27°-PdO serves as the active phase for the low-temperature propane oxidation. These insights would help understand the working mechanisms of three-way catalysts and should be of great importance for the development of efficient and low-temperature three-way catalysts.

Key words: Palladium oxide, Chiral oxide, Low-temperature propane oxidation, Scanning tunneling microscopy