Precise localization of metal nanoparticles in dendrimer nanosnakes or inner periphery and consequences in catalysis

Abstract

Understanding the relationship between the location of nanoparticles (NPs) in an organic matrix and their catalytic performances is essential for catalyst design. Here we show that catalytic activities of Au, Ag and CuNPs stabilized by dendrimers using coordination to intradendritic triazoles, galvanic replacement or stabilization outside dendrimers strongly depends on their location. AgNPs are found at the inner click dendrimer periphery, whereas CuNPs and AuNPs are encapsulated in click dendrimer nanosnakes. AuNPs and AgNPs formed by galvanic replacement are larger than precursors and only partly encapsulated. AuNPs are all the better 4-nitrophenol reduction catalysts as they are less sterically inhibited by the dendrimer interior, whereas on the contrary CuNPs are all the better alkyne azide cycloaddition catalysts as they are better protected from aerobic oxidation inside dendrimers. This work highlights the role of the location in macromolecules on the catalytic efficiency of metal nanoparticles and rationalizes optimization in catalyst engineering.

Figure 1. Formation of the group-11 metal NPs encapsulated and stabilized by the dendrimer 1.

(a) Formula of dendrimer 1; (b) TEM picture of the nona-Au(III) complex of 1; (c) Syntheses of the metal CDENs and galvanic displacement reactions.

Figure 2. Microscopy images of the group-11 metal NPs intradendritically encapsulated by dendrimer 1.

(a) TEM image of AgNP-1; (b) TEM image of CuNP-1; (c) TEM image of AuNP-1; (d) STEM image of CuNP-1.

Figure 3. Microscopy images and spectra for AgNP-1.

(a) STEM image of AgNP-1; (b) Ag EDX compositional map of AgNP-1; (c) combined (Ag, Si and C) EDX compositional map; (d) XPS of AgNP-1; (e) UV–visible spectrum (SPB at 425 nm) and photograph of AgNP-1.

Figure 4. Synthesis of AuNP-2 and CuNP-2.

The lack of intradendritic ligands in dendrimer 2 induces the formation of large TMNPs AuNP-2 and CuNP-2 stabilized by clusters of dendrimers.

Figure 5. Compared photographs and UV–visible spectra of CuNP-1 and CuNP-2 upon standing in air.

(a) Photograph and UV–visible spectrum of CuNP-1 after 1 min; (b) photograph and UV–visible spectrum of CuNP-1 after 2 months; (c) photograph of CuNP-2 after 1 min; (d) photograph and UV–visible spectrum of CuNP-2 after 5 min.

Figure 6. XPS and X-ray diffraction spectra of CuNP-1 and CuNP-2.

(a) XPS spectrum of CuNP-1 showing its long-term aerobic stability as Cu (0); (b) XPS spectrum of CuNP-2 showing its oxidation as Cu (I) and Cu (II); (c) X-ray diffraction spectrum of CuNP-1 showing that only Cu (0) is present; (d) X-ray diffraction spectrum of CuNP-2 showing the presence of both Cu (0) and Cu₂O.

Figure 7. Catalytic reactions conducted with the AuPs and CuNPs as catalysts.

In equation (1), the 4-NP reduction to 4-AP by NaBH₄ is catalysed by AuNPs, and in equation (2) the copper-catalysed azide alkyne cycloaddition (CuAAC, click reaction) is catalysed by CuNPs.