(337x) Fundamental Understanding of Atmospheric Pressure Plasma-Catalyst Interaction for Sustainable Value-Added Chemicals Production

I am interested in developing catalytic materials and designing plasma-catalytic systems for the sustainable production of value-added chemicals at the site of interest.

Abstract

Atmospheric pressure non-thermal plasmas (NTPs) produce reactive chemical environments, including electrons, ions, radicals, and vibrationally-excited molecules at low bulk temperatures where such environments are thermally inaccessible. There has been a growing interest in the integration of highly energetic NTPs with conventional heterogeneous catalysis for potentially carry out novel chemical transformations (e.g., C-N coupling from CH₄ and N₂) that are difficult using either plasma or conventional catalysis alone. Despite the advantages, progress in plasma catalysis is hindered by the lack of a detailed understanding of the underlying processes. The interactions between the plasma and surface material play major roles in plasma catalysis for selective chemical transformations toward a desired product. Thus, to improve a plasma-catalytic system, it is essential to understand the fundamentals of the underlying plasma-catalytic surface coupling. Specifically, how the reactive chemical environment produced in the plasma-phase interacts with catalytic and/or non-catalytic surface materials in the elementary steps of surface chemical reactions, including (1) adsorption of plasma-induced reactive species, (2) different types of surface reactions (i.e., Langmuir-Hinshelwood and Eley-Rideal), and (3) desorption of the products. For a comprehensive fundamental understanding of the plasma-surface interactions using model surface (Ni group and Cu group metals) in C-N coupling from CH₄ and N₂, I have designed a multi-modal spectroscopy tool. The tool combines polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS), optical emission spectroscopy (OES), and mass spectrometry (MS) to identify surface-adsorbed intermediates, plasma-activated species, and gas-phase products, respectively.

• Direct observation of plasma-stimulated activation of surface CH_x species: Using a multi-modal spectroscopic tool, I investigated the mechanisms of plasma-assisted non-oxidative coupling of CH₄ to C₂ and C₃ hydrocarbons over Ni and SiO₂ surfaces and subsequent activation of carbonaceous deposits under inert plasma exposure to produce H₂ and C₂ The comprehensive results indicate that the formation of C=C surface species and the production of H₂ and C₂ hydrocarbons on Ni is proposed to be most likely a surface phenomenon that does not occur on surfaces on SiO₂.

• Accessing metastable surface-adsorbed nitrogen species by coupling N₂ plasma and metal surface: Adsorbed nitrogen species formed during exposure of polycrystalline Ni, Pd, Cu, Ag, and Au surfaces to NTP-activated N₂ were characterized with multi-modal spectroscopy and density functional theory (DFT) in collaboration. The results indicate the formation of metastable surface nitrogen-containing species on all metals at room temperature, highlighting the potential of NTP to produce such metastable species at a mild condition.

• Investigation of NTP-activated C-N coupling on a catalytic surface from CH₄ and N₂: I investigated the interaction between NTP-activated CH₄/N₂ species and model surfaces (Ni, Pd, Cu, Ag, and Au) for C-N coupling using multi-modal spectroscopy. CH₄/N₂ feed gas was introduced sequentially to minimize the plasma-phase C-N coupling: (1) CH₄-N₂ in sequence and (2) N₂-CH₄ in sequence. The results indicate that different sequences lead to CN signatures in OES spectra corresponding to C-N coupled products forming on a catalytic surface. Also, the C-N coupled products were formed from simultaneous CH₄/N₂ Further analysis of the C-N coupled species on the surfaces using both sequential and simultaneous exposures indicate that depending on the procedure to form C-N coupled species, product selectivity can be tuned.