(707c) Multi-Modal Operando Spectroscopic Characterization of Nitrogen Plasma Interactions with Metal Surfaces

The integration of atmospheric pressure non-thermal plasmas with heterogeneous catalysts has garnered significant interest due to its ability to facilitate transformations that are challenging or unattainable via conventional thermal catalysis. Nitrogen (N2) activation has emerged as a focal point in the plasma catalysis domain, leveraging the plasma's capacity to activate the robust dinitrogen triple bond, surmount thermodynamic barriers associated with thermal reactions, and sidestep scaling relations inherent to surface-catalyzed reactions. Despite the extensive attention dedicated to plasma-catalytic conversion of N2, the fundamental nature of nitrogen species involved in plasma-catalytic N2 chemistry remains a longstanding inquiry.

To probe surface species and plasma-phase dynamics during exposure of polycrystalline metal catalyst surfaces to plasma jets, we have developed a novel multi-modal operando spectroscopy instrument that integrates multiple diagnostic techniques: polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) to identify surface adsorbed species, optical emission spectroscopy (OES) to identify plasma phase radicals, and mass spectrometry to identify stable gas phase products. In this presentation, I will demonstrate that polycrystalline Ni, Cu, Au, Ag, and Pd surfaces exposed to N2 plasma jets exhibit a mysterious PM-IRAS vibrational band centered at ~2200 cm^-1. To unravel the identity of this enigmatic surface species, we employ a comprehensive suite of techniques including temperature-dependent experiments, sequential dosing, X-ray photoelectron spectroscopy, isotopic labeling, and density functional theory calculations. Our findings suggest that the predominant species is likely isocyanate (NCO), with carbon and oxygen potentially originating from trace impurities present in the ultra-high purity (UHP) Ar and/or N2 gas cylinders. This work not only underscores the potential of non-thermal plasmas to access adsorbates (NCO) unattainable via thermal means but also underscores the potential influence of trace impurities on the interpretation of plasma catalytic chemistry, thereby shedding light on critical considerations for future research and industrial applications.