Elucidating the Interaction Between Non-Thermal Plasma Activated Nitrogen and Catalytic Surfaces Using Multi-Modal Spectroscopy

Abstract

Non-thermal plasma (NTP) produce highly reactive chemical environments (i.e. excited species, ions, and radicals) at temperatures at which such species are thermally inaccessible. The integration of NTP with conventional heterogeneous catalysis has recently gained substantial interest for enhancing catalytic activities and/or selectivities, as well as for novel chemical transformations at mild conditions beyond either plasma or catalysis could deliver individually. Despite the promises of plasma catalysis, the fundamental mechanisms of the interactions between the plasma-activated species and the catalyst at the interface is still lacking. In this work, we utilize a newly-designed multi-modal spectroscopy tool combining polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS), mass spectrometry (MS), and optical emission spectroscopy (OES) to investigate the interaction between NTP-activated nitrogen species and model catalytic surfaces (Ni, Pd, and Au). We show that different surface temperatures lead to the adsorption of different nitrogen species onto a catalytic surface. These results are further supported by ex-situ X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and X-ray diffraction (XRD). The nature and the reactivity of different nitrogen species are investigated with temperature programmed oxidation (TPO) and temperature programmed reaction (TPR) with hydrogen to produce ammonia, respectively. Lastly, the nature and the reactivity of plasma-stimulated adsorbed nitrogen species are further interrogated with isotopically labelled N 2 .