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

We investigate the catalytic properties of water-soluble Au-n(SG)(m) nanocluster catalysts (H-SG = glutathione) of different sizes, including Au-15(SG)(13), Au-18(SG)(14), Au-25(SG)(18), Au-38(SG)(24), and captopril-capped Au-25(Capt)(18) nanoclusters. These Au-n(SR)(m) 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 similar to 100% selectivity in water using H-2 gas (20 bar) as the hydrogen source. These nanocluster catalysts, except Au-18(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-n(SR)(m) nanoclusters are moderately strong and similar in strength. The DFT results suggest that the catalytic activity of the Au-n(SR)(m) 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.