New Magic Au24 Cluster Stabilized by PVP: Selective Formation, Atomic Structure, and Oxidation Catalysis

Abstract

An unprecedented magic number cluster, Au24Clx (x = 0–3), was selectively synthesized by the kinetically controlled reduction of the Au precursor ions in a microfluidic mixer in the presence of a large excess of poly(N-vinyl-2-pyrrolidone) (PVP). The atomic structure of the PVP-stabilized Au24Clx was investigated by means of aberration-corrected transmission electron microscopy (ACTEM) and density functional theory (DFT) calculations. ACTEM video imaging revealed that the Au24Clx clusters were stable against dissociation but fluctuated during the observation period. Some of the high-resolution ACTEM snapshots were explained by DFT-optimized isomeric structures in which all the constituent atoms were located on the surface. This observation suggests that the featureless optical spectrum of Au24Clx is associated with the coexistence of distinctive isomers. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy of CO adsorbates revealed the electron-rich nature of Au24Clx clusters due to the interaction with PVP. The Au24Clx:PVP clusters catalyzed the aerobic oxidation of benzyl alcohol derivatives without degradation. Hammett analysis and the kinetic isotope effect indicated that the hydride elimination by Au24Clx was the rate-limiting step with an apparent activation energy of 56 ± 3 kJ/mol, whereas the oxygen pressure dependence of the reaction kinetics suggested the involvement of hydrogen abstraction by coadsorbed O2 as a faster process.