Following the thermal activation of Au25(SR)18 clusters for catalysis by x-ray absorption spectroscopy

摘要

We show the thermal activation of phenylethanethiolate (L = SC\u2088H\u2089) and hexanethiolate (L = SC\u2086H\u2081\u2083) Au\u2082\u2085L\u2081\u2088 monolayer protected clusters (MPCs) on carbon black supports, followed by characterization with extended Xray absorption \ufb01ne 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 signi\ufb01cant 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 \ub0C 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\u2082\u2085(SC\u2088H\u2089)\u2081\u2088 MPCs was seen at 250 \ub0C activation conditions. These results are consistent with results reported earlier for Au\u2082\u2085(SR)\u2081\u2088 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 signi\ufb01cantly, such mild heating conditions do lead to nearly complete removal of the thiolate stabilizers.