Projects

Abstract: Nonthermal plasma-surface interactions enable transformative advancements in green chemistry, healthcare, materials processing, pollution abatement, and the ever-growing area of plasma catalysis. In the context of plasma catalysis, the fate of the active sites during plasma treatment has remained enigmatic, and observation of low-temperature plasma-cataly events has been challenging. The induction of strong metal–support interactions (SMSI) through high-temperature hydrogen treatment is a well-documented and established, yet limited, method to impact selectivity and stability of noble metal catalysts on reducible supports. Thermally driven SMSI occurs through reduction and subsequent migration of the support to the surface of exposed metal sites, thus affecting the catalyst both electronically and geometrically and serving as an ideal system to evaluate dynamic plasma-catalyst interactions. In this study, a dielectric barrier discharge of hydrogen was used to successfully induce a plasma-SMSI state (P-SMSI) in niobia-supported platinum particles at bulk-gas temperatures as low as −30 °C, which enhances the selectivity for propane dehydrogenation and offers conclusive evidence of plasma-catalyst interactions. Time-resolved spectroscopic evidence of this phenomenon was obtained in situ using a cryogenically cooled plasma IR transmission cell, which provided evidence of diffusion-controlled surface migration. Collectively, P-SMSI constitutes a promising, low-impact technology for synthesizing SMSI-enhanced catalysts with controllable active sites, and knowledge of the nonthermal plasma-catalyst dynamics is critical in designing materials for specific applications or selecting conditions of operation.

Abstract: Nonthermal plasmas can directly activate and cleave the strong chemical bonds in molecular nitrogen and methane to facilitate the transformation of these inherently stable molecules through the application of electrical energy. Here, we report a low temperature, atmospheric pressure, nonthermal plasma for the “one-pot” synthesis of olefins, alkynes, higher molecular weight hydrocarbons, ammonia, and nitrogen-containing liquids from a representative shale gas feed enriched with nitrogen. We reproducibly observe a wide range of valuable and synthetically challenging gas-phase and liquid-phase products containing C–N, C–C, and N–H bonding by controlling the N2 concentration in the inlet feed stream. In nitrogen-lean regimes, hydrocarbon products dominate (e.g., ethylene, acetylene, etc.), while nitrogen-rich regimes promote incorporation of nitrogen into the products, leading to the formation of ammonia and liquid products containing a variety of functionalities (e.g., nitriles, amines, heterocycles). High resolution electrospray ionization mass spectrometry was used to measure molecular weights and identify the chemical formulas of the liquid products. Van Krevelen diagrams were created and showed many products with compositions around H/C = 2 and N/C = 0.5, indicating the potential importance of intermediate species with these ratios for liquid formation (e.g., CH3CN + H). 

Abstract: Electrification of the methane dehydroaromatization reaction with the use of nonthermal plasmas could alleviate the high-temperature requirement for this process while promoting the formation of valuable aromatics. Here, we evaluate the use of nonthermal plasma to investigate methane activation and conversion to aromatics by systematically varying bulk gas temperature in a one-pot, plasma-stimulated catalytic reactor over Mo/H-ZSM-5 and metal-free H-ZSM-5 catalysts. We report that Mo is not required for methane activation under low-temperature plasma conditions (573–773 K), and methane conversions up to ∼15% with a 1:1 methane/N2 feed are obtained under a 10 W plasma. However, Mo contributes to the formation of aromatics in the presence of a plasma at 773 K, achieving close to a 2-fold increase in the production of aromatics when compared to unmodified H-ZSM-5. Further, the exposure of as-prepared Mo/H-ZSM-5 to the methane plasma feed induces the formation of Mo-carbide phases in the temperature range studied. These findings highlight the complex roles of nonthermal plasmas in the direct activation of methane and the importance of plasma-catalyst design to facilitate aromatization reactions under plasma-assisted reaction conditions.

Abstract: Nonthermal plasma activation of light alkanes is an encouraging decarbonization strategy to produce chemicals or fuels from abundant and/or flared carbon sources. However, prolific carbon growth on both the catalyst and electrode has limited its practicality, requiring additional knowledge of the carbon structure and growth mechanism before breakthroughs are realized. Here, visual evidence is provided for nonuniform diamond-like carbon (DLC) microstructures that materialize in a coaxial dielectric barrier discharge (DBD) reactor flowing ethane and He at 278 K. Through a connection to known behaviors of DBD microdischarge patterns, the microstructure spacing was controlled by altering the applied voltage (ΔV) of the plasma or the burning voltage (Ub). Additionally, carbon valorization through nitrogen incorporation from N2 was explored as an orthogonal solution to carbon mitigation, with N/C values >0.25 achieved and both sp2 and sp3 C–N bonding observed in the microstructures. 

Abstract: Ethylene oligomerization is an important step for upgrading light olefins to liquid fuels or value-added chemicals such as butene monomers. Ethylene oligomerization catalyzed by mono-Ni substituted polyoxometalate (POM) catalysts is promising due to the observed selectivity toward linear butene products and stability of these well-defined materials. In this work, two approaches to tailor the catalytic properties of isolated Ni sites in POMs were experimentally tested, both of which perturbed the surrounding molecular environment of the active sites. First, structural modifications were performed to alter the size of the polyoxometalates, evaluating between Wells-Dawson and Keggin structures while maintaining the same elemental components. When comparing these two structures, it was observed that the Ni active sites were kinetically independent from the effect of POM size, even when the POM consisted of the same elements. This was evidenced by the identical product distributions and kinetic parameters observed with both structures. However, a localized kinetic effect was observed when Keggin POM structures containing different internal heteroatoms (e.g, P, Si, Al) were included in close proximity to the Ni active site. Specifically, a notable periodic trend was observed between the electronegativity of internal heteroatoms and the measured apparent activation energy for the catalyzed ethylene coupling reaction to butene. 

Abstract: Nonthermal plasma activation of N2 can facilitate nitrogen adsorption on metal catalysts at low bulk temperatures and atmospheric pressure. We apply a plasma-assisted temperature-programmed reaction (plasma-TPRxn) for ammonia (NH3) synthesis using sequential exposure of a silica-supported metal catalyst to N2 plasma followed by thermal hydrogen treatment while ramping the temperature to decouple the plasma activation of N2 from surface catalyzed hydrogenation steps. This approach eliminates the effects from bulk plasma phase reactions, thereby allowing for direct interrogation of plasma activated nitrogen on the active metal surfaces. We confirm previously reported spectroscopic observations that show plasma-generated surface nitrogen can be converted to NH3 through surface catalyzed pathways. Further, we demonstrate that the ammonia desorption peak temperature is sensitive to metal, with Pt desorbing NH3 at the lowest temperature. Unsteady state microkinetic models of desorption kinetics as a function of initial N coverage and metal recover observed trends in NH3 desorption temperatures and confirm that observed results reflect hydrogenation of plasma-induced N accommodation at each surface. In total, we show that the hydrogenation ability of the catalyst after plasma activation of N2 is responsible for the reactivity trends observed in plasma-assisted NH3 synthesis.