Pt Nanoparticle Facets Characterised by CO Adsorption with Infrared Spectroscopy

Abstract

Platinum nanoparticles (Pt NPs) are used extensively as catalysts in numerous catalytic reactions. The shape and size of these nanoparticles can affect the activity and selectivity of these reactions. It is, therefore, important to find a way to characterize these properties of the catalyst. Nowadays, Transmission Electron Microscopy (TEM) is often used to characterize the size and shape of nanoparticles. However, this only analyses a small sample size and is an expensive method. It is known that the Pt environment affects the Pt-CO band position in carbon monoxide infrared (CO-IR) spectroscopy. In this study, I researched if CO-IR spectroscopy can be used to characterize the shape and size of Pt NPs.

Depending on the nanoparticle’s shape, different facets are exposed. Platinum cubes and octahedrons both have only one surface facet, {100} and {111}, respectively. During this research, both Pt cubes and octahedrons were synthesized using the polyol method. The resulting nanoparticles were then characterized with TEM to obtain the average particle size and shape. Thereafter, CO-IR spectroscopy was done with the cubes and octahedrons. The Pt-CO band position was then studied to see if it corresponds to the TEM results.

Reaction temperature, stirring rate, and precursor and ligand addition methods seemed to be crucial factors in the resulting nanoparticle shape. Higher temperatures resulted in a blueshift in the Pt-CO band. However, peak fitting of the CO adsorption DRIFTS spectra gave inconclusive results, and no difference between cubes and octahedrons could be made. HR-TEM images of the cubes synthesized at 160 °C and 200 °C showed minimal differences but a possible difference in the straightness of the edges. The former showed more ridges formed on the surfaces, thus more under-coordinated Pt atoms on the surface. However, a small sample size was studied, and further research should be conducted. Octahedrons displayed low CO adsorption, attributed to residual silver bridges between the Pt nanoparticles blocking the CO adsorption sites and circular aggregate formation of the Pt nanoparticles reducing the total surface area. H2 pretreatments resulted in the band splitting into two distinct bands, attributed to the well-coordinated and under-coordinated sites. O2 pretreatments led to a blueshift of the Pt-CO band to 2100 cm-1.

In the future, more research should be done on the synthesis to get more uniform results. This will lead to a clearer picture of the CO-IR spectroscopy results. Therefore, the surface facets of more samples should be characterized with HR-TEM to determine the facets.