Non-thermal, atmospheric pressure plasmas contain a rich mixture of reactive species, including ions, electrons, radicals, and vibrationally- and electronically-excited molecules. These plasma-activated species may experience lowered activation barriers for reaction on catalyst surfaces, and can bypass scaling relations and thermodynamic limitations that are inherent to thermal catalysis. However, a key bottleneck in the progress of plasma-enhanced catalysis is the limited understanding of the fundamental mechanisms by which plasmas interact with catalytic surfaces to enhance catalysis. To address this fundamental knowledge gap, we have developed a new multi-modal operando spectroscopy tool that is capable of simultaneously detecting: (1) plasma-activated species in the gas-phase using optical emission spectroscopy (OES), (2) intermediate species adsorbed on the catalyst surface using polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), and (3) gas-phase final products using mass spectrometry. Thus, this tool is capable of correlating formation of plasma-activated species to surface-adsorbed species and, ultimately, to the final products. In this presentation, the unique design and operation of this new tool is described, and its utility is demonstrated by exploring plasma-activated non-oxidative coupling of methane to C2 (ethane, ethylene, acetylene) and C3 products (propane, propylene) over Ni model surfaces. The relationships between gas-plasma species, surface-adsorbed species, and gas-phase final products will be discussed to elucidate how the plasma interacts with the Ni surface to give the final products.

Plasma-assisted non-oxidative coupling of methane: Mechanistic insights from multi-modal operando PM-IRAS+OES spectroscopy

ACS Spring Meeting 2021

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