We demonstrate time-resolved rotational and vibrational temperature measurements to probe plasma non-equilibrium through a simple three-beam hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) system. A single pump/Stokes pair is employed to generate both the pure-rotational and ro-vibrational Raman coherence. A novel phase-matching scheme, to our knowledge, is employed to spatially overlap the two CARS signals, simplifying the optical design and allowing signal selection by tuning a single spectrometer grating. Measurements were performed near the electrodes in a N2 DC glow discharge. Both the rotational temperature and the vibrational populations up to v=8 were calculated from the separately measured single-shot pure-rotational and ro-vibrational CARS spectra. Strong rotation-vibration non-equilibrium was observed at both electrodes, with the cathode showing higher vibrational and rotational temperatures. In addition, non-Boltzmann behaviors were observed at both electrodes. This simplified approach enables measurements of rotational and vibrational temperatures with high spatial resolution in non-equilibrium flows.

Ammonia/methane ( NH_3 / CH_4 ) co-combustion offers a promising route toward high-efficiency, low-carbon, and low-NOx combustion, yet experimental insights into local displacement speeds in ammonia-containing flames remain scarce. Here, 2D particle image velocimetry (2D-PIV) was applied to turbulent premixed Bunsen flames at atmospheric pressure, covering ammonia fractions up to 0.6 and equivalence ratios between 0.7 and 1.0. Streamline-based decomposition of the progress-variable transport equation showed that the measured flame propagation speed sT is broadly consistent with the closure relation sR + sF at low turbulence intensities, supporting the applicability of the Bray-Moss-Libby (BML) thin-flame concept to NH_3 / CH_4 flames. The reaction and convection terms dominated the balance, while turbulent fluxes, though weaker, remained non-negligible. Closure errors increased with higher ammonia content or leaner mixtures but were greatly reduced by introducing Markstein corrections and approximating 2D flame surface density as 3D. Analysis of reaction rates further revealed that the near-axis region contributed most, while ammonia addition lowered local reactivity, increased flame height, and promoted local extinction at the root. The findings provide experimental guidance for source-term estimation and turbulence–flame interaction modeling.

We report the in situ formation of Ni nitride for plasma-assisted ammonia synthesis. Both the surface nitrogen concentration and the ammonia formation rate exhibit dependence on the N2:H2 feed ratio. The maximum surface nitrogen concentration occurs at a N2:H2 ratio of 4:1, and the maximum catalytic activity occurs at 2:1. In contrast, the formation of gas phase radicals is less sensitive to feed composition, indicating that Ni nitride is more kinetically relevant to ammonia production than gas-phase radicals. The plasma-induced formation of Ni nitride is therefore proposed to be a critical contributor to the synergistic effects in plasma-assisted catalytic ammonia synthesis. Additionally, Ni nitride alters the surface reaction mechanism of plasma-assisted ammonia synthesis, with the rate-determining-step (RDS) shifting to surface-bound NH3 formation rather than N2 activation at temperatures below 373 K. These findings provide mechanistic insight that opens opportunities for optimizing the performance of plasma-assisted catalytic ammonia synthesis.

Non-equilibrium plasma-assisted ammonia synthesis is investigated through enhanced active species production with ferroelectric discharge. Time-resolved in-situ diagnostics of femtosecond two-photon absorption laser-induced fluorescence, coherent anti-Stokes Raman scattering, and laser absorption spectroscopy, as well as optical emission spectroscopy, were conducted to probe the key intermediate species, such as H and N radicals as well as N2(ν), ions, and NH3 to achieve better understanding of non-equilibrium energy transfer and ammonia formation. The results reveal that ferroelectric discharge improved ammonia yield by four times. Results also show that ferroelectrics not only enhanced ions (N2+) production, radicals (N, H) number density, but also increased the N2 vibrational temperature. Further plasma modeling identified the couplings between elevated radical and ion production and enhanced vibrational excitation reactions, e.g., N + H2(ν)→NH + H, N2(ν)+H → NNH, N2+ + H2 → H + N2H+, and N2H++e→NH + N, facilitated by ferroelectric discharge. These findings provide critical insight into the mechanism of ferroelectric plasma catalysis and highlight their potential in advancing energy-efficient chemical synthesis. This work reports enhancements of radicals, vibrational states, and ammonia output in N2/H2 plasmas by ferroelectric barrier discharge, with evidence from time-resolved laser diagnostics, expanding plasma routes to cleaner chemical production.

Plastic waste is a growing problem, accumulating in landfills and the environment. Pyrolysis is a promising and industrially relevant approach for transforming plastic waste into value-added chemicals. However, the selectivity and yield of traditional plastic pyrolysis are poor, with products featuring broad molar mass distributions. Here we report a highly selective, energy-efficient and catalyst-free pyrolysis method that can upcycle plastic into value-added chemicals via pore-modulated pyrolysis. Using a Joule-heated carbon column, we demonstrate the pivotal role of the reactor’s graded porous structure in decreasing the polydispersity of the reaction intermediates, enabling high product selectivity and yield. The decreasing pore size of the reactor modulates the mass transport in an apparent gating effect—preventing high-molar-mass species from exiting the reactor before sufficient pyrolysis has occurred. Using polyethylene as a model reactant, we demonstrate a high yield of 65.9 ± 5.2 A pore-modulated pyrolysis reactor that enables catalyst-free and energy-efficient upcycling of plastic waste is demonstrated. The graded-pore structure imposes molecular-weight-dependent transport barriers, establishing a gating effect that enhances product selectivity and yields aviation fuel precursor (C8–C18) with high efficiency.

This study investigates the active species production, such as N and H atoms, in a non-equilibrium N-2/H-2 plasma using a ferroelectric barrier discharge in N-2/H-2 mixtures. By utilizing time-resolved in-situ femtosecond two-photon absorption laser-induced fluorescence diagnostic, this research provides kinetic data of active species production enhancements by ferroelectric electrodes. Results show that the N number density increases by a factor of two and one order of magnitude in pure N-2 and N-2/H-2 mixture, respectively, by the ferroelectric electrode due to the high electric field required for nitrogen dissociation. H production is also enhanced by both the increased electric field in ferroelectric barrier discharge in pure H-2 plasma, and the additional H-production reaction pathway via H-2(v)+N -> NH+H. The enhancements of N and H production in ferroelectric barrier discharge greatly promote its applicability in energy-efficient green manufacturing such as ammonia synthesis.

Vapour-phase synthesis methods have shown promise for the scalable synthesis of nanomaterials and coatings. However, the vaporization of different precursors for the synthesis of a broad nanomaterial space, particularly at atmospheric pressure, while maintaining compositional and structural control of the final product is challenging. Here we report the generation of an ultrahigh-temperature atomic vapour at atmospheric pressure based on electrified heating, for the growth of multi-elemental nanomaterials and thin films. This process relies on a reactor design whereby solid-state precursors are vaporized within a semi-confined space beneath an electrified heater that can reach ~3,000 K. The proximity of the heater rapidly breaks down the bonds of metal salt precursors and decomposes them into an atomic vapour that expands into a high-temperature (>2,000 K), highly reactive and high-flux vapour (1021–1022 atoms per cm2 per second) that travels upwards in a directional flow. When mixed with entrained ambient gases, the highly reactive atomic species rapidly nucleate and grow into the desired final products, including alloys, oxides, sulfides and thin films, which can be deposited on a low-temperature substrate. This EVD approach can synthesize a broad range of functional nanomaterials at atmospheric pressure, including single-phase multi-elemental nanomaterials formed under thermodynamically non-equilibrium conditions. Vapour-phase methods are promising for nanomaterial synthesis but the vaporization of different precursors for the synthesis of a broad nanomaterial space is challenging. Here electrified vapour deposition generates ultrahigh-temperature, high-flux atomic vapour at atmospheric pressure to rapidly vaporize diverse precursors, enabling the synthesis of multi-elemental nanomaterials with uniform compositions and tunable structures.

This work developed femtosecond two-photon absorption laser induced fluorescence for atomic nitrogen and hydrogen measurements, applied it to plasma aided ammonia synthesis, and unraveled the kinetic role of vibrational energy transfer of hydrogen molecules.

Dimethoxymethane (DMM) is a promising renewable fuel with low-carbon intensity and low tendencies for soot and NOx emissions, which is drawing increasing attention to meet the carbon-neutral requirements. In this work, DMM oxidation was studied by using a novel supercritical pressure jet-stirred reactor at 10 and 100 atm, with temperatures between 450 and 950 K, and equivalence ratios of 0.27 and 2.0. The experimental results show that the negative temperature coefficient (NTC) behavior becomes much weaker under 100 atm than the case of 10 atm. One reason is the significant shift of the intermediate-temperature HO2 chemistry to lower temperature at 100 atm and the other one is the increase of multi-oxygen addition reactions at 100 atm. Selected kinetic models in the literature show some discrepancies in comparison to the experimental results in this study. Thus, a new model updated from a previous study was developed to improve the prediction of the experimental data under high pressures. Reaction pathway and sensitivity analyses were performed to identify key reactions in DMM high-pressure oxidation. DMM H-atom abstraction at the primary C site by OH (DMM_1 radical) is found to be the most important reaction to promote oxidation, while the secondary site (DMM_2 radical) shows different sensitivity under different conditions. The reason is that under richer or lower pressure conditions, the decomposition of DMM_2 is favored over O2 addition, thus inhibits the oxidation process. DMM H-atom abstractions by CH3O and HO2 are found to be important under low- and intermediate-temperature, respectively. Therefore, more efforts in studying these reactions are suggested to further improve the model prediction. In addition, reaction 2HO2 = 2OH + O2, added in this work, is found to be important in promoting DMM oxidation at the early stage and improves model prediction on oxidation onset temperature.

Nonequilibrium generation of atomic nitrogen and hydrogen governing ammonia production both in the gas phase and on the catalyst surface is critical to plasma-aided ammonia synthesis. Here, this work studies the nonequilibrium generation of atomic nitrogen and hydrogen with a focus on the kinetic role of vibrational energy transfer of hydrogen molecules in plasma-aided ammonia synthesis. By combining two-photon absorption laser-induced fluorescence measurements and plasma kinetic modeling, we found that plasma not only generates ammonia but also produces critical H, N, and NH radicals via both electron impact and vibrational energy transfer excitations. The vibrational energy transfer from the excited hydrogen H-2(v = 1) to higher vibrational levels H-2(v = 2-3) via the V-V exchange (H-2(v)-H-2(v)) and V-V ' exchange (N-2(v)-H-2(v)) significantly enhances the H and NH production and then promotes the coupling between N and NH for ammonia synthesis both in the gas phase and on the catalyst surface.

We combine in situ laser spectroscopy, quantum chemistry, and kinetic calculations to study the reaction of a singlet oxygen atom with dimethyl ether. Infrared laser absorption spectroscopy and Faraday rotation spectroscopy are used for the detection and quantification of the reaction products OH, H2O, HO2, and CH2O on submillisecond time scales. Fitting temporal profiles of products with simulations using an in-house reaction mechanism allows product branching to be quantified at 30, 60, and 150 Torr. The experimentally determined product branching agrees well with master equation calculations based on electronic structure data and transition state theory. The calculations demonstrate that the dimethyl peroxide (CH3OOCH3) generated via O-insertion into the C-O bond undergoes subsequent dissociation to CH3O + CH3O through energetically favored reactions without an intrinsic barrier. This O-insertion mechanism can be important for understanding the fate of biofuels leaking into the atmosphere and for plasma-based biofuel processing technologies.

Manipulating surface charge, electric field, and plasma afterglow in a non-equilibrium plasma is critical to control plasma-surface interaction for plasma catalysis and manufacturing. Here, we show enhancements of surface charge, electric field during breakdown, and afterglow by ferroelectric barrier discharge. The results show that the ferroelectrics manifest spontaneous electric polarization to increase the surface charge by two orders of magnitude compared to discharge with an alumina barrier. Time-resolved in-situ electric field measurements reveal that the fast polarization of ferroelectrics enhances the electric field during the breakdown in streamer discharge and doubles the electric field compared to the dielectric barrier discharge. Moreover, due to the existence of surface charge, the ferroelectric electrode extends the afterglow time and makes discharge sustained longer when alternating the external electric field polarity. The present results show that ferroelectric barrier discharge offers a promising technique to tune plasma properties for efficient plasma catalysis and electrified manufacturing.

Ammonia (NH3) has been widely recognized as one of the carbon-neutral fuels. However, ammonia combustion suffers low reactivity and high N2O/NOx emissions. To overcome these issues, this work reports plasma assisted NH3/H2 oxidation and unveils the kinetics of fuel oxidation and N2O/NOx formation by combining time-resolved laser diagnostics with plasma modeling. Firstly, we found that the NH3 consumption is promoted with a H2 blending ratio of 0.3, due to enhancements of H and OH formation by plasma assisted H2 dissociation. Secondly, at a high reduced electric field, when the H2 blending ratio increases, the NH3 oxidation is promoted due to both the HO2 formation and strong NO kinetic enhancement via NO-HO2 and NO2-H pathways. In the meantime, it is shown that the NO mole fraction also increases with H2 blending ratio, because the NO formation is enhanced via N(2D)-O2 pathways, and the DeNOx chemistry is weakened with less NH2 production. By contrast, at a lower reduced electric field, when the H2 blending ratio increases, the decreased N(2D) formation does not produce enough NO to replenish the NO formation drop caused by lower NH3 concentration. Thirdly, the reduced electric field non-monotonically affects fuel consumption and N2O/NOx formation by manipulating electron energy deposition pathways. The NH3 consumption is maximized with an optimal reduced electric field where N2* excitation and O2 dissociation are most efficient. When the reduced electric field deviates from its optimum, the NH3 consumption decreases due to the discharge energy deposition to either vibrational excitation or dissociation of N2. The N2O/NOx emissions governed by the NH3 oxidation follow the above NH3 consumption trend.

In plasma discharges, the acceleration of electrons by a fast varying electric field and the subsequent collisional electron energy transfer determines the plasma dynamics, chemical reactivity, and breakdown. Current in situ electric field measurements require reconstruction of the temporal profile over many observations. However, such methods are unsuitable for non-repetitive and unstable plasmas. Here, we present a method for creating “movies” of dynamic electric fields in a single acquisition at sample rates of 500 × 106 fps. This ultrafast diagnostic was demonstrated in radio frequency electric fields between two parallel plates in air, as well as in Ar nanosecond-pulsed single-sided dielectric barrier discharges.

High pressure ammonia/methanol oxidation and NOx formations were investigated using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at 20 and 100 atm with temperatures between 550 and 950 K and equivalence ratios of 0.138 and 1.15. The experimental results show that NH3 oxidation at high pressure is significantly accelerated by the active OH radicals produced from CH3OH oxidation. Furthermore, the kinetic interactions between NH3 and CH3OH are governed mainly by the reactions CH3OH + NH2 = CH2OH + NH3, CH3OH + NH2 = CH3O + NH3, and CH2O + NH2 = HCO + NH3. A HP-Mech model for high-pressure NH3/ CH3OH oxidation was developed in this study. It consists of the most recent NH3 and CH3OH models including some new reactions and updated rate constants from the literature as well as NH3-CH3OH interactions where rate constants of CH3OH + NH2 = CH2OH + NH3, CH3OH + NH2 = CH3O + NH3, NH2 + CH2O = NH3 + HCO, and NH2 + CH2O = NH2CHO + H were theoretically calculated in this study. Our model with these updates improves the prediction for the measured N2O/NOx temperature dependence at 100 atm. In addition, the reaction pathway and sensitivity analysis show that N2O/NOx/HONO interactions with HO2 are very important, especially for a fuel-lean mixture at 100 atm. The HONO mole fraction for the fuel-lean mixture at 100 atm was then measured by off-axis integrated cavity output spectroscopy (ICOS) at wavenumber of 6638.26 cm-1. The experimental data show a significant HONO formation at intermediate temperature that is strongly underpredicted by numerical simulation at 100 atm. Therefore, the HONO related reactions with notable uncertainty at high pressure such as NO + OH (+M) = HONO (+M) and H2NO + NO2 = HONO + HNO need deeper exploration in the future.

View Video Presentation: https://doi.org/10.2514/6.2023-0351.vid Plasma represents one of the most attractive ways to synthesize ammonia (NH3), since it uses renewable electricity to produce NH3 at low temperatures. For plasma assisted NH3 synthesis, the underlying chemical kinetics involving gas phase chemistry is not well understood, especially for atomic species like nitrogen (N) and hydrogen (H) atoms. Therefore, this work reports quantitative measurements of atomic N and H in a N2/H2 AC plasma using femtosecond two-photon absorption laser induced fluorescence (code-named fs-TALIF). Effects of key AC plasma properties including frequency and applied voltage on the N and H number densities were studied. The results show two major observations. First, the N number density is approximately one order of magnitude lower than the H number density, indicating that producing N atom is critical in plasma assisted NH3 synthesis. Second, due to the competing factors including N2/H2 dissociations by excited species and electrons and N/H consumptions via radical quenching and propagation, the number density ratio of N to H (a key indicator to NH3 production) is nonlinearly dependent on frequency. The number density ratio of N to H first decreases as the frequency increases from 5 kHz to 15 kHz, and then rises back at 20 kHz.