Experimental and Mechanistic Understanding of Aldehyde Hydrogenation Using Au25 Nanoclusters with Lewis Acids: Unique Sites for Catalytic Reactions.
The catalytic activity of Au25(SR)18 nanoclusters (R = C2H4Ph) for the aldehyde hydrogenation reaction in the presence of a base, e.g., ammonia or pyridine, and transition-metal ions M(z+), such as Cu(+), Cu(2+), Ni(2+) and Co(2+), as a Lewis acid is studied. The addition of a Lewis acid is found to significantly promote the catalytic activity of Au25(SR)18/CeO2 in the hydrogenation of benzaldehyde and a number of its derivatives. Matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry in conjunction with UV-vis spectroscopy confirm the generation of new species, Au25-n(SR)18-n (n = 1-4), in the presence of a Lewis acid. The pathways for the speciation of Au24(SR)17 from its parent Au25(SR)18 nanocluster as well as its structure are investigated via the density functional theory (DFT) method. The adsorption of M(z+) onto a thiolate ligand "-SR-" of Au25(SR)18, followed by a stepwise detachment of "-SR-" and a gold atom bonded to "-SR-" (thus an "Au-SR" unit) is found to be the most likely mechanism for the Au24(SR)17 generation. This in turn exposes the Au13-core of Au24(SR)17 to reactants, providing an active site for the catalytic hydrogenation. DFT calculations indicate that M(z+) is also capable of adsorbing onto the Au13-core surface, producing a possible active metal site of a different kind to catalyze the aldehyde hydrogenation reaction. This study suggests, for the first time, that species with an open metal site like adducts [nanoparticle-M]((z-1)+) or fragments Au25-n(SR)18-n function as the catalysts rather than the intact Au25(SR)18.
State Key Laboratory of Catalysis & Gold Catalysis Research Centre, Dalian Institute of Chemical Physics, Chinese Academy of Sciences , Dalian 116023, China.
Gao
G
Li
Department of Chemistry, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States.
Hadi
H
Abroshan
Department of Chemistry, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States.
Yuxiang
Y
Chen
Department of Chemistry, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States.
Rongchao
R
Jin
Department of Chemistry, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States.|School of Computational Sciences, Korea Institute for Advanced Study , Seoul 130-722, Korea.
Hyung J
HJ
Kim