Paper SS-TuP29
CO Oxidation over Au/TiO₂ Model Catalyst

Tuesday, October 30, 2012, 6:00 pm, Room Central Hall

In this work we have investigated the reaction mechanism and active sites for CO oxidation over the Au/TiO₂ model surface and Au single crystal surfaces, along with the role of moisture CO.

We examined the effect of moisture on the CO₂ formation rate at the reaction temperature of 300 and 400 K. The CO₂ formation rate at 300 K was increased significantly with increasing H₂O partial pressure up to 0.1 Torr, and then gradually decreased with H₂O pressure. In contrast, no promotional effect of H₂O was observed at the reaction temperature of 400 K. The moisture has an essential role to promote the CO oxidation reaction over Au/TiO₂ catalyst at low temperature, whereas the CO oxidation reaction proceeded without moisture with high reaction temperature. This important observation indicates that the CO oxidation mechanism over Au/TiO₂ is different between 300 and 400 K, considering that the activation process of oxygen molecules strongly depended on a reaction temperature. That is, molecular oxygen has been activated directly over the Au surface at the high temperature while the moisture takes part in the activation of the oxygen molecule at low reaction temperature.

Next, we examined the turnover frequencies (TOFs) for CO₂ formation at the two reaction temperatures as a function of mean gold particle diameter. To determine whether the active sites for CO oxidation were exposed gold atoms on the gold particles or perimeter sites at the interface between the gold particles and the TiO₂ support, we calculated the TOFs in two ways: (i) by normalizing the total number of exposed Au atoms at the gold particles (TOF–S) and (ii) by normalizing the the total number of gold atoms at the perimeter interfaces (TOF–P). The results clearly showed that the relationship between TOF and mean gold particle diameter depended strongly on reaction temperature. At 300 K, TOF–S decreased with increasing mean gold particle diameter, whereas TOF–P remained nearly constant regardless of particle diameter, suggesting that the active sites for CO oxidation were the gold atoms located at the periphery of the gold particles attached to TiO₂. In contrast, TOF–S at 400 K remained nearly constant regardless of the mean gold particle diameter, indicating that the active sites for CO oxidation were newly created on the gold metal surface at the high temperature. Thus, we can conclude that both the reaction mechanisms and the active sites differed between the low temperature region and the high temperature region.