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Cold atmospheric-pressure plasma (CAP) is a relatively new method being investigated for antimicrobial activity. However, the exact mode of action is still being explored. Here we report that CAP efficacy is directly correlated to bacterial cell wall thickness in several species. Biofilms of Gram positive Bacillus subtilis, possessing a 55.4 nm cell wall, showed the highest resistance to CAP, with less than one log10 reduction after 10 min treatment. In contrast, biofilms of Gram negative Pseudomonas aeruginosa, possessing only a 2.4 nm cell wall, were almost completely eradicated using the same treatment conditions. Planktonic cultures of Gram negative Pseudomonas libanensis also had a higher log10 reduction than Gram positive Staphylococcus epidermidis. Mixed species biofilms of P. aeruginosa and S. epidermidis showed a similar trend of Gram positive bacteria being more resistant to CAP treatment. However, when grown in co-culture, Gram negative P. aeruginosa was more resistant to CAP overall than as a mono-species biofilm. Emission spectra indicated OH and O, capable of structural cell wall bond breakage, were present in the plasma. This study indicates that cell wall thickness correlates with CAP inactivation times of bacteria, but cell membranes and biofilm matrix are also likely to play a role.

Plasma catalysis has drawn attention from the plasma and chemical engineering communities in the past few decades as a possible alternative to the long-established Haber–Bosch process for ammonia production. The highly reactive electrons, ions, atoms, and radicals in the plasma significantly enhance the chemical kinetics, allowing ammonia to be produced at room temperature and atmospheric pressure. However, despite the promise of plasma catalysis, its performance is still well short of that of the Haber–Bosch process. This is at least in part due to the lack of understanding of the complex mechanisms underlying the plasma–catalyst interactions. Gaining such an understanding is a prerequisite for exploiting the potential of plasma catalysis for ammonia production. In this perspective, we discuss possible benefits and synergies of the combination of plasma and catalyst. The different regimes of plasma discharges and plasma reactor configurations are introduced and their characteristics in ammonia synthesis are compared. Based on detailed kinetic modeling work, practical ideas and suggestions to improve the energy efficiency and yield of ammonia production are presented, setting future research directions in plasma catalysis for efficient ammonia production.

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Ammonia synthesis by plasma catalysis has emerged as an alternative process for decoupling nitrogen fixation from fossil fuels. Plasma activation can potentially circumvent the limitations of conventional thermocatalytic ammonia synthesis; however, the contribution of different reaction mechanisms to the production of ammonia at the catalyst surface remains unclear. Here, we identify the reaction intermediates adsorbed on γ-Al 2 O 3 -supported Ni and Fe catalysts during plasma-activated ammonia synthesis under various temperatures and reactor configurations using FTIR spectroscopy, steady-state flow reactor experiments, and computational kinetic modeling. Ammonia yield can be influenced by plasma-derived intermediates and their interactions with catalyst surfaces, which lead to different reaction pathways: Ni/γ-Al 2 O 3 enhances plasma-promoted NH 3 production and favors surface-adsorbed NH x species, while Fe/γ-Al 2 O 3 shows the presence of N 2 H y and a lower overall concentration of N-containing adsorbates. Plasma−catalyst interactions are probed to reveal that elevated temperature and plasma irradiation of the surfaces promote NH 3 desorption. The direct evidence of catalytic surface reactions occurring during a plasma-activated process provides mechanistic insight into plasma-activated ammonia synthesis.

Ammonia was synthesized from nitrogen and hydrogen in a dielectric-barrier discharge reactor packed with glass spheres and MgO pellets at atmospheric pressure. The addition of argon to nitrogen and hydrogen, and increasing the peak voltage, led to increases in discharge power and uniformity, gas temperature, and the fraction of hydrogen converted to ammonia. IndexTerms-Heterogeneous ammonia production, packed-bed dielectric barrier discharge, plasma catalysis.H ETEROGENEOUS catalysis, combining nonequilibrium atmospheric-pressure plasmas and a catalyst, is used in applications such as chemical synthesis and gas cleaning. The interaction between plasma species and the catalyst allows reactions to proceed at much lower temperatures than in conventional processes [1]. For example, the standard Haber process for ammonia production requires high pressures and temperatures, but plasma-catalyst systems can produce ammonia at atmospheric pressure and room temperature [2].We used a cylindrical dielectric barrier discharge reactor, shown in Fig. 1, with a stainless-steel central high-voltage (HV) electrode, a borosilicate glass tube as the dielectric barrier and a 200-mm long and 56-mm diameter stainlesssteel-mesh outer ground electrode. The 4-mm gap between the HV electrode and the dielectric barrier was filled with sodalime glass spheres (3-mm diameter) and MgO pellets (∼2-mm long and 1-mm diameter). A Trek 20/20C HV amplifier, fed by a 1-kHz sine wave, was used as the power source. The ammonia concentration in the exit gas was measured using a Fourier transform infrared (FT-IR) spectrometer (Perkin-Elmer Frontier), calibrated against measurements made using a Thermo Fisher Scientific Orion ammonia ion-

Sustainable ammonia synthesis at ambient conditions that relies on renewable sources of energy and feedstocks is globally sought to replace the Haber–Bosch process. Here, using nitrogen and water as raw materials, a nonthermal plasma catalysis approach is demonstrated as an effective power‐to‐chemicals conversion strategy for ammonia production. By sustaining a highly reactive environment, successful plasma‐catalytic production of NH3 was achieved from the dissociation of N2 and H2O under mild conditions. Plasma‐induced vibrational excitation is found to decrease the N2 and H2O dissociation barriers, with the presence of matched catalysts in the nonthermal plasma discharge reactor contributing significantly to molecular dissociation on the catalyst surface. Density functional theory calculations for the activation energy barrier for the dissociation suggest that ruthenium catalysts supported on magnesium oxide exhibit superior performance over other catalysts in NH3 production by lowering the activation energy for the dissociative adsorption of N2 down to 1.07 eV. The highest production rate, 2.67 mmol gcat.−1 h−1, was obtained using ruthenium catalyst supported on magnesium oxide. This work highlights the potential of nonthermal plasma catalysis for the activation of renewable sources to serve as a new platform for sustainable ammonia production.

A direct one-step nano-gold printing process from a HAuCl4 solution precursor is demonstrated using an atmospheric-pressure plasma jet.