Ligand-stabilized and atomically precise gold nanocluster catalysis: A case study for correlating fundamental electronic properties with catalysis

摘要

We present results from our investigations into correlating the styrene-oxidation catalysis of atomically precise mixed-ligand biicosahedral-structure [Au_(25)(PPh_3)_(10)(SC _(12)H_(25))_5Cl_2]~(2+)0 (Au _(25)-bi) and thiol-stabilized icosahedral core-shell-structure [Au _(25)(SCH_2CH_2Ph)_(18)]~- (Au_(25)-i) clusters with their electronic and atomic structure by using a combination of synchrotron radiation-based X-ray absorption fine-structure spectroscopy (XAFS) and ultraviolet photoemission spectroscopy (UPS). Compared to bulk Au, XAFS revealed low Au-Au coordination, Au-Au bond contraction and higher d-band vacancies in both the ligand-stabilized Au clusters. The ligands were found not only to act as colloidal stabilizers, but also as d-band electron acceptor for Au atoms. Au_(25)-bi clusters have a higher first-shell Au coordination number than Au_(25)-i, whereas Au_(25)-bi and Au_(25)-i clusters have the same number of Au atoms. The UPS revealed a trend of narrower d-band width, with apparent d-band spin-orbit splitting and higher binding energy of d-band center position for Au_(25)-bi and Au_(25)-i. We propose that the differences in their d-band unoccupied state population are likely to be responsible for differences in their catalytic activity and selectivity. The findings reported herein help to understand the catalysis of atomically precise ligand-stabilized metal clusters by correlating their atomic or electronic properties with catalytic activity.