Size Dependence of Atomically Precise Gold Nanoclusters in Chemoselective Hydrogenation and Active Site Structure

Abstract

We investigate the catalytic properties of water-soluble Au_<i>n</i>(SG)_<i>m</i> nanocluster catalysts (H-SG = glutathione) of different sizes, including Au₁₅(SG)₁₃, Au₁₈(SG)₁₄, Au₂₅(SG)₁₈, Au₃₈(SG)₂₄, and captopril-capped Au₂₅(Capt)₁₈ nanoclusters. These Au_<i>n</i>(SR)_<i>m</i> nanoclusters (SR represents thiolate generally) are used as homogeneous catalysts (i.e., without supports) in the chemoselective hydrogenation of 4-nitrobenzaldehyde (4-NO₂PhCHO) to 4-nitrobenzyl alcohol (4-NO₂PhCH₂OH) with ∼100% selectivity in water using H₂ gas (20 bar) as the hydrogen source. These nanocluster catalysts, except Au₁₈(SG)₁₄, remain intact after the catalytic reaction, evidenced by UV–vis spectra, which are characteristic of nanoclusters of each size and thus serve as spectroscopic “fingerprints”. We observe a drastic size dependence and steric effect of protecting ligands on the gold nanocluster catalysts in the hydrogenation reaction. Density functional theory (DFT) modeling of the 4-nitrobenzaldehyde adsorption shows that both the -CHO and -NO₂ groups closely interact with the S-Au-S staples on the gold nanocluster surface. The adsorptions of the 4-nitrobenzaldehyde molecule on the four different sized Au_<i>n</i>(SR)_<i>m</i> nanoclusters are moderately strong and similar in strength. The DFT results suggest that the catalytic activity of the Au_<i>n</i>(SR)_<i>m</i> nanoclusters is primarily determined by the surface area of the Au nanocluster, consistent with the observed trend of the conversion of 4-nitrobenzaldehyde versus the cluster size. Overall, this work offers molecular insight into the hydrogenation of 4-nitrobenzaldehyde and the catalytically active site structure on gold nanocluster catalysts