Recently, the researchers from University of Science and Technology of China of the Chinese Academy of Sciences (CAS) revealed the conjugated dual size effect of core-shell bimetallic nanocatalysts for the first time, with the activity of the catalysts increasing with the core size in the benzyl alcohol oxidation reaction. The work was published in Nature Communications.  

Bimetallic catalysts are widely used in different chemical synthesis for their bimetallic synergy varying with compositions and structures. Compared with those of alloy catalysts, the peculiar lattice strains and ligand effects of core-shell catalysts can optimize the geometric and electronic properties. The shell thickness significantly affects bimetallic synergy because the ligand effects and the charge transfers between components usually happen at the core-shell interface.  

The lattice strains in core-shell catalyst occur due to lattice mismatch between the metal core and the shell, which have a prominent effect on the electronic structure of the metal shell and the overall activity of catalysts. When the core size is reduced, its lattice contracts considerably, which affects its mismatch with shell lattice, consequently modulates the lattice strains in the shell. This dual size effect of core-shell particles has not yet been explored due to the immense challenge in adjusting the core size and shell thickness at atomic level.  

In this study, the researchers chose solvent-free selective oxidation of benzyl alcohol (BzOH) as probe reaction, and revealed the dual size effect of Au@Pd catalyst using atomic layer deposition, multi-spectroscopy and density functional theory (DFT) calculations.  

The theoretical calculations showed that the lattices of Pd overlayer on Au particles expand and tend to enhance BzOH adsorption, while a reduced Au core size would reduce the strains on the Pd shell. Further calculations showed that the ligand effects of Au core would drastically weaken the BzOH adsorption but could became negligible when the Pd shell thickness reached 2 monolayers (ML) and above. Therefore, the highest adsorption energy can be obtained if the large-sized Au core is wrapped by 2 ML thick Pd shell.  

To experimentally examine the dual size effect, the researchers first synthesized Au/SiO₂ catalysts with different Au sizes and executed Pd ALD to yield Pd shells with varying thickness on Au particles at atomic level. Then, using transmission electron microscopy (TEM) and X-ray diffraction (XRD), they revealed the evolution of the atomic and electronic structure of Au and Pd in the core-shell structure with the Au core size and Pd shell thickness.  

Results showed that in the oxidation of BzOH, with a fixed Au core size, the activity of catalysts increased with Pd shell thickness quickly and peaked at 2.9 ML before it began to drop. With a fixed Pd shell thickness, the activity also improved with increasing core size. The researchers achieved a maximum activity higher than those of previous studies in Au6.8@2.9ML-Pd catalyst.  

Similar conjugated effect was found in hydrogenation of para-chloronitrobenzene (p-CNB), exhibiting the universality of the effect. This work provided significant guideline for the future design of highly efficient bimetallic catalysts.