Developing Superstructures to Study Heterogeneous Catalysis with Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Abstract

In this project, superstructures consisting of colloidal catalyst nanoparticles deposited on shell-isolated nanoparticles (SHINs) were developed. These superstructures were applied to study catalytically active platinum-group metal nanoparticles with shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), using CO oxidation as a model reaction. Raman spectroscopy is a valuable tool to study catalysts under working conditions.

Plasmonic gold nanoparticles were synthesised by a seeded growth method. These Au NPs were applied to enhance the Raman signal. In order to improve the stability and reduce plasmon-driven side reactions, the Au NPs were coated with an ultrathin silica shell, producing Au@SiO2 shell-isolated nanoparticles (SHINs). The silica shell can function as a support for catalyst materials, enabling the study of surface reactions over the catalyst nanoparticles with SHINERS. Platinum-group metal nanoparticles (Pd, Pt, Rh and Ru) with controlled morphology were then prepared by colloidal synthesis. These catalyst nanoparticles must be assembled on the silica shell of SHINs to obtain the desired superstructures. This has proven to be a challenging task. Several ligand exchange methods were evaluated in order to replace the isolating capping agents from the colloidal catalyst nanoparticles with smaller ligands. After ligand exchange, self-assembly of catalyst nanoparticles on Au@SiO2 SHINs should take place, driven by electrostatic interaction. Of the methods evaluated, ligand exchange with NOBF4 has proven to be the best for this purpose. This method resulted in successful assembly of colloidal nanoparticles on SHINs.

Platinum and ruthenium superstructures were treated with UV/Ozone and reduced, after which CO adsorption was observed in SHINERS. This indicates that even though assembly on the SHINs was not optimal or reproducible, these superstructures can be applied to study surface reactions. For Rh superstructures, the Raman signal was too weak to observe CO adsorption signals in SHINERS, but successful detection of CO adsorption on Au@Rh core-shell nanoparticles indicated that this can be achieved. In conclusion, the superstructures that were developed in this work can be applied to study surface reactions with SHINERS.

It is thought that the Raman signal intensity provided by the superstructures will become even stronger after optimisation of the assembly process, leading to a better ability to detect species in situ. Additionally, SHINERS can be applied to study structure-sensitive reactions by preparing superstructures for catalyst nanoparticles with varying morphology. Therefore, the developed superstructures show great promise for application to study heterogeneous catalysis.