(632d) Plasma-Assisted Ligand Removal for Improved Catalysis By Pd/SiO2

The morphology of supported nanoparticles plays an essential role in the reactivity of heterogeneous catalysis. Colloidal synthesis is an extensively used method in the synthesis of nanoparticles with well-defined size and morphology. Organic ligands, such as polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and polyamine (PAA), are essential in colloidal synthesis to control size and shape by preventing aggregation and selectively binding to different metal facets, respectively. However, organic ligands hinder (steric) reagent access to the active metal sites. Hence, several pretreatments have been developed to remove the organic ligands. Thermal annealing is frequently used, but the high temperatures may result in aggregation and shape modification. Ultraviolet ozone can successfully remove ligands without affecting nanoparticle shape and size. Still, the complete removal of ligands usually requires long treatments (more than 12 h). Finally, ligand exchange can replace the long-chain organic ligands with small ones, like acetic acid, diethylamine, and NaBH_4, but still leaves the surface crowded. Therefore, new, efficient methods of removing these organic ligands are frequently desired.

In this work, we implement an electrified approach, a non-thermal O₂/He plasma, to successfully remove PVP from Pd cubic nanoparticles supported on SiO₂. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) indicated almost complete removal of PVP after short plasma treatment. Transmission electron microscopy (TEM) revealed that the shape and size of Pd nanoparticles remained intact during plasma treatment. In contrast, their size increased, and their shape became polycrystalline upon mild calcination. Additionally, first-principles quantum mechanical calculations and air-free XPS experiments reveal the strong electronic modification of Pd by PVP. Finally, we compared traditionally made and plasma-treated catalysts in furfural hydrogenation. The results show that non-thermal plasma is a greener, more efficient alternative for catalyst pretreatment.