Following the Thermal Activation of Au₂₅(SR)₁₈ Clusters for Catalysis by X-ray Absorption Spectroscopy

Abstract

We show the thermal activation of phenylethanethiolate (L = SC 8 H 9 ) and hexanethiolate (L = SC 6 H 13 ) Au 25 L 18 monolayer protected clusters (MPCs) on carbon black supports, followed by characterization with extended X-ray absorption fine structure (EXAFS) spectroscopy and transmission electron microscopy (TEM). EXAFS analysis shows that the thiolate stabilizers can be partially removed from the surface under mild heating conditions without significant changes in the cluster size. The resulting clusters are shown to be catalytically active for the reduction catalysis of 4-nitrophenol. EXAFS and TEM data show that thermal treatment under air at 200 °C leads to nearly complete removal of all of the thiolate stabilizers with little to no growth in cluster size, while cluster sintering was seen at higher calcination temperatures. The maximum catalytic activity for Au 25 (SC 8 H 9 ) 18 MPCs was seen at 250 °C activation conditions. These results are consistent with results reported earlier for Au 25 (SR) 18 clusters on ceria by Jin et al., who suggested that cluster integrity was maintained during mild activation conditions. Here, EXAFS data unambiguously show that while the cluster size does not grow significantly, such mild heating conditions do lead to nearly complete removal of the thiolate stabilizers.