Fabrication of six-atom Pd clusters regulated with different short ligands and their surface structure-dependent...

As model catalysts, it is necessary to study the relationship between the structure and properties of ultra-small metal nanoclusters (MNCs) and to reduce their steric hindrance as much as possible, e.g. preparing ultrasmall MNCs protected by ultra-short ligands. However, it is challenging to attain various MNCs with the same cores but different surface stabilizing ligands. Additionally, shortening the chains of protecting ligands will lead to larger MNC cores. Here, four different Pd NCs (Pd₆(SC₄H₉)₁₂, Pd₆(SC₈H₁₇)₁₂, Pd₆(SC₆(C₂)H₁₇)₁₂ and Pd₆(SC₆H₁₃)₁₂) were successfully synthesized by a slow synthesis process. All these clusters consist of six Pd atoms and are stabilized by 12 thiols with different chain lengths and steric hindrance. The catalytic properties of the as-prepared Pd₆ NCs were evaluated using the catalytic reduction of p-nitroaniline to p-phenylenediamine as a model reaction. The outcomes indicated that shortening the chain length of the protecting thiols could enhance the catalytic activity of the Pd₆ NCs. Notably, stable and active ultra-small Pd₆ clusters stabilized by ultra-short ligands (HSC₄H₉) were successfully synthesized. Although the performance of Pd₆(SC₄H₉)₁₂ clusters protected by the ultra-short thiols is lower than that of commercial palladium on carbon (Pd/C), they display higher stability. Interestingly, the activity of Pd₆ NCs protected by ethyl-branched alkane thiols is also better than that of Pd₆ NCs protected by the alkane thiol ligands with the same chain length or the same number of carbon numbers. This work provides clear evidence that the catalytic activity of atomically precise MNCs can be controlled by regulating the surface stabilizing ligands.