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1. 2-D Finite-Element Modeling of Surface Dielectric Barrier Plasma Discharge Devices to Understand the Influence of Design Parameters on Sterilization Applications

   https://doi.org/10.1109/TPS.2022.3156031  

   Bose, Arnesh K.; Maddipatla, Dinesh; Atashbar, Massood Z. (January 2022, IEEE Transactions on Plasma Science)

   Lopez, J. L. (Ed.)

   In this study, voltage distribution and surface dielectric barrier discharge (DBD) of a microplasma discharge device (MDD) were modeled in 2-D domain using finite-element analysis (FEA). Initially, the voltage distribution across comb-, H-tree-, and honeycomb-structured MDD was analyzed. Then, the cross section of an MDD consisting of a polyimide-based dielectric sandwiched between two copper electrodes was used for modeling the microplasma discharge characteristics in an argon environment. A sinusoidal voltage was applied to one of the copper electrodes while the other electrode was grounded. The spatial distributions of electron temperature (ET) across the electrodes for varying input voltages were simulated to demonstrate the importance of breakdown voltage. A detailed analysis on the effect of varying electrode and dielectric barrier thicknesses on electron density and ET was also performed to understand the importance of optimizing device configurations for microplasma discharge. Moreover, MDD was also simulated in varying ambient temperature and pressure conditions to evaluate their effect on ET and density across the electrodes. The results from these simulations provide a better understanding of parameters such as varying input voltage, electrode, and dielectric thickness on ET and electron density. This enables us to optimize design parameters for fabricating MDDs and the operating conditions for effective sterilization applications. 

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2. Contrasting the characteristics of atmospheric pressure plasma jets operated with single and double dielectric material: physicochemical characteristics and application to bacterial killing

   https://doi.org/10.1088/1361-6463/acb602  

   Ghimire, Bhagirath; Briggs, Elanie F; Sysoeva, Tatyana A; Mayo, John A; Xu, Kunning G (February 2023, Journal of Physics D: Applied Physics)

   Abstract This study reports an experimental comparison of two types of atmospheric pressure plasma jets in terms of their fundamental plasma characteristics and efficacy in bacterial sterilization. The plasma jets are fabricated by inserting a high voltage electrode inside a one-end closed (double DBD plasma jet) or both ends open (single DBD plasma jet) quartz tubes which are further enclosed inside a second quartz tube containing a ground electrode. Both plasma jets are operated in contact with water surface by using a unipolar pulsed DC power supply with helium as the working gas. Results from electrical and time-resolved imaging show that the single DBD configuration induces 3–4 times higher accumulation of charges onto the water surface with significantly faster propagation of plasma bullets. These results are accompanied by the higher discharge intensity as well as stronger emissions from short-lived reactive species which were analyzed through optical emission spectroscopy at the plasma-water interface. The rotational temperature for the single DBD configuration was observed to be higher making it unsafe for direct treatments of sensitive biological targets. These characteristics of the single DBD configuration result in the production of more than two times higher concentration of H 2 O 2 in plasma activated water. Shielding of the HV electrode reduces the plasma potential which in turn reduces the electric field & electron energy at the plasma-water interface. The reduced electric field for the double DBD configuration was lower by ≈463 Td than the single DBD configuration. The bactericidal efficacy of the two configurations of the plasma jets were tested against Escherichia coli , a well studied Gram-negative bacterium that can be commensal and pathogenic in human body. Our results demonstrate that although single DBD plasma jet result in stronger antibacterial effects, the double DBD configuration could be safer. 

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3. Non-equilibrium plasma-assisted dry reforming of methane over shape-controlled CeO ₂ supported ruthenium catalysts

   https://doi.org/10.1039/D3TA01196H  

   Ahasan, Md Robayet; Hossain, Md Monir; Ding, Xiang; Wang, Ruigang (May 2023, Journal of Materials Chemistry A)

   In this report, CeO 2 and SiO 2 supported 1 wt% Ru catalysts were synthesized and studied for dry reforming of methane (DRM) by introducing non-thermal plasma (NTP) in a dielectric barrier discharge (DBD) fixed bed reactor. From quadrupole mass spectrometer (QMS) data, it is found that introducing non-thermal plasma in thermo-catalytic DRM promotes higher CH 4 and CO 2 conversion and syngas (CO + H 2 ) yield than those under thermal catalysis only conditions. According to the H 2 -TPR, CO 2 -TPD, and CO-TPD profiles, reducible CeO 2 supported Ru catalysts presented better activity compared to their irreducible SiO 2 supported Ru counterparts. For instance, the molar concentrations of CO and H 2 were 16% and 9%, respectively, for plasma-assisted thermo-catalytic DRM at 350 °C, while no apparent conversion was observed at the same temperature for thermo-catalytic DRM. Highly energetic electrons, ions, and radicals under non-equilibrium and non-thermal plasma conditions are considered to contribute to the activation of strong C–H bonds in CH 4 and C–O bonds in CO 2 , which significantly improves the CH 4 /CO 2 conversion during DRM reaction at low temperatures. At 450 °C, the 1 wt% Ru/CeO 2 nanorods sample showed the highest catalytic activity with 51% CH 4 and 37% CO 2 conversion compared to 1 wt% Ru/CeO 2 nanocubes (40% CH 4 and 30% CO 2 ). These results clearly indicate that the support shape and reducibility affect the plasma-assisted DRM reaction. This enhanced DRM activity is ascribed to the surface chemistry and defect structures of the CeO 2 nanorods support that can provide active surface facets, higher amounts of mobile oxygen and oxygen vacancy, and other surface defects. 

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4. Energetics of Reaction Pathways Enabled by N and H Radicals during Catalytic, Plasma-Assisted NH3 Synthesis

   https://doi.org/https://doi.org/10.1021/acssuschemeng.1c05660  

   Tsung-Wei Liu; Fnu Gorky; Maria L. Carreon; and Diego A. Gómez-Gualdrón (February 2022, ACS sustainable chemistry engineering)

   Plasma-assisted catalysis is emerging as an alternative to several thermocatalytic processes. For ammonia synthesis, it could make the process milder, which would help production, decentralization, and compatibility with renewable energy. However, one major obstacle preventing optimization of the plasma-assisted process is the incipient mechanistic understanding of ammonia formation on plasma-exposed catalysts. Here, optical emission spectroscopy is consistent with only a weak effect of the metal on plasma composition and with the presence of small concentrations of plasma radicals in N2/H2 mixtures in dielectric barrier discharge (DBD) reactors, which are bound to enable new catalyst- involved pathways not considered in previous kinetic models for NH3 synthesis. Thus, we comprehensively examined, via density functional theory calculations, the energetics (favorability) of 51 reactions on Fe, Ni, Co, Pd, Ga, Sn, Cu, Au, and Ag. Enthalpic barriers for Eley−Rideal (ER) reactions involving N• and H• radicals were found to be negligible and hence supportive of the following: (i) plausible NNH formation and consequent prominent role of the associative pathway to form NH3 (consistent with some experimental reports detecting surface-bound NXHY species), (ii) likelihood of N• adsorptiontakingoverN*2 dissociationastheprimarysourceofsurfaceboundN*,and(iii)probabledominanceofER hydrogenation reactions over Langmuir−Hinshelwood ones. The energetics herein presented will allow thoroughly studying the pathway competition in future kinetic models, but numbers calculated here already suggest that the dominant pathway may change with the metal identity. For instance, N2HY dissociation favorability is more likely to become competitive with ER hydrogenation earlier in the hydrogenation sequence the more nitrophilic the metals. Yet, the calculated favorability of ER reactions is also already consistent with the weaker dependence of initial NH3 turnover frequencies (TOFs) on metal identity compared to the thermocatalytic scenario. With practical implications for computational catalyst screening, TOFs experimentally measured herein for an atmospheric DBD reactor linearly correlate with ΔErxn for the ER hydrogenation reaction H• + HNNH*2 → HNNH*3 . This descriptor may be robust to exact synthesis conditions, as its correlation with TOFs was maintained for earlier TOF data in a sub- atmospheric radio frequency reactor. KEYWORDS: OES, DBD, plasma composition, DFT, Eley−Rideal reactions, scaling relationships, catalyst descriptor 

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5. Managing photon flux in a miniaturized photoionization detector

   https://doi.org/10.1063/5.0193595  

   Meyer, Mackenzie; Huang, Xiaheng; Fan, Xudong; Kushner, Mark J (April 2024, Journal of Applied Physics)

   Miniaturized photoionization detectors (PIDs) are used in conjunction with gas chromatography systems to detect volatile compounds in gases by collecting the current from the photoionized gas analytes. PIDs should be inexpensive and compatible with a wide range of analyte species. One such PID is based on the formation of a He plasma in a dielectric barrier discharge (DBD), which generates vacuum UV (VUV) photons from excited states of He to photoionize gas analytes. There are several design parameters that can be leveraged to increase the ionizing photon flux to gas analytes to increase the sensitivity of the PID. To that end, the methods to maximize the photon flux from a pulsed He plasma in a DBD-PID were investigated using a two-dimensional plasma hydrodynamics model. The ionizing photon flux originated from the resonance states of helium, He(3P) and He(21P), and from the dimer excimer He2*. While the photon flux from the resonant states was modulated over the voltage pulse, the photon flux from He2* persisted long after the voltage pulse passed. Several geometrical optimizations were investigated, such as using an array of pointed electrodes. However, increasing the capacitance of the dielectric enclosing the plasma chamber had the largest effect on increasing the VUV photon fluence to gas analytes. 

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