Novel Applications of X-Ray Photoelectron Spectroscopy - Controlled Differential Charging in Composite Materials and Near Ambient Pressure in-situ Studies of Catalysts Used in Exhaust Aftertreatment

Abstract

XPS holds a pivotal place in surface science due to its ability to resolve chemical states at the surface level. A prominent difficulty in its application is the buildup of positive surface charge as photoelectrons are emitted from the surface during spectral acquisition. This phenomenon causes unpredictable shifts in peak shape and position in non-conducting samples that cannot be easily resolved from changes in chemical state. In this work, we explore a way to control the surface charge during XPS measurements by application of an external bias to the sample holder. Using this technique, we can resolve components of a sample not simply based on chemical identity, but also their electrical properties.

The technique is applied to a composite catalyst support used in nanoplasmonic sensing, and we show how differential charging can be used to separate the nanoplasmonic disks from their silica support. We then identify a charge transfer barrier between the components and model the surface to show that the support acts as an RC element inducing a similar but delayed behavior in the disks. We also extend the application of the technique to polycrystalline and thin film materials, identifying a two-layer structure present in powders during spectral acquisition and a charge transfer barrier with dynamics on the order of 1-10 s between the layers that can be correlated with grain size.

Finally, we identify differential charging in industrial grade catalysts used in exhaust aftertreatment and show how it can be corrected. We further investigate these catalysts in-situ by NAP-XPS and show that the Pt/Al2O3 surface restructures under reaction conditions using a NOx atmosphere, causing the Pt to oxidize while the support remains inert.