Polymer-Stabilized Au<sub>38</sub> Cluster: Atomically Precise Synthesis by Digestive Ripening and Characterization of the Atomic Structure and Oxidation Catalysis
Shingo Hasegawa, Shinya Masuda, Shinjiro Takano, Koji Harano, Tatsuya Tsukuda
Abstract
Microfluidic mixing of AuCl4– and NaBH4 in the presence of a controlled amount of polyvinylpyrrolidone (PVP) yielded a mixture of Au34Clm and Au43Cln clusters, as revealed by matrix-assisted laser desorption/ionization mass spectrometry. These mixtures underwent digestive ripening by the addition of NaOH, leading to the conversion to atomically precise Au38 clusters that were absent in the as-prepared products. The atomic structure of the Au38 core of Au38:PVP thus obtained was studied by means of density functional theory (DFT) calculations and aberration-corrected transmission electron microscopy (ACTEM). A DFT-based search for Au38 found more than 10 noncrystalline structures containing 0–4 inner Au atoms as structural candidates. ACTEM video imaging with the help of transmission electron microscopy simulation for DFT-optimized model structures revealed the polydisperse and dynamic nature of the Au38 core structure. Fourier-transform infrared spectroscopy of CO adsorbed on Au38:PVP and X-ray photoelectron spectroscopy indicated that the Au38 core was negatively charged due to the interaction with PVP. Au38:PVP dispersed in basic water catalyzed the aerobic oxidation of p-substituted benzyl alcohols without degradation. The kinetic isotope effect and Hammett analysis indicated that hydride elimination from the α-carbon of alkoxide was the rate-determining step as in the case of Au24:PVP previously reported. The higher catalytic activity of Au38:PVP compared to that of Au24:PVP in alcohol oxidation was ascribed to the reduction in the apparent activation energy: 48 ± 1 and 56 ± 3 kJ mol–1 for Au38:PVP and Au24:PVP, respectively. DFT calculations suggested that the hydride transfer from alkoxide to Au38 was energetically more favored as compared to Au24 due to the higher capacity for excess electronic charge over the larger cluster volume.