CO Oxidation Catalyzed by Oxide-Supported Au₂₅(SR)₁₈ Nanoclusters and Identification of Perimeter Sites as Active Centers

In this work, we explore the catalytic application of atomically monodisperse, thiolate-protected Au(25)(SR)(18) (where R = CH(2)CH(2)Ph) nanoclusters supported on oxides for CO oxidation. The solution phase nanoclusters were directly deposited onto various oxide supports (including TiO(2), CeO(2), and Fe(2)O(3)), and the as-prepared catalysts were evaluated for the CO oxidation reaction in a fixed bed reactor. The supports exhibited a strong effect, and the Au(25)(SR)(18)/CeO(2) catalyst was found to be much more active than the others. Interestingly, O(2) pretreatment of the catalyst at 150 °C for 1.5 h significantly enhanced the catalytic activity. Since this pretreatment temperature is well below the thiolate desorption temperature (~200 °C), the thiolate ligands should remain on the Au(25) cluster surface, indicating that the CO oxidation reaction is catalyzed by intact Au(25)(SR)(18)/CeO(2). We further found that increasing the O(2) pretreatment temperature to 250 °C (above the thiolate desorption temperature) did not lead to any further increase in activity at all reaction temperatures from room temperature to 100 °C. These results are in striking contrast with the common thought that surface thiolates must be removed-as is often done in the literature work-before the catalyst can exert high catalytic activity. The 150 °C O(2)-pretreated Au(25)(SR)(18)/CeO(2) catalyst offers ~94% CO conversion at 80 °C and ~100% conversion at 100 °C. The effect of water vapor on the catalytic performance is also investigated. Our results imply that the perimeter sites of the interface of Au(25)(SR)(18)/CeO(2) should be the active centers. The intact structure of the Au(25)(SR)(18) catalyst in the CO oxidation process allows one to gain mechanistic insight into the catalytic reaction.