Luca Matteo Martini

Plasma catalysis combines the high‐energy chemistry of plasma with the speed and selectivity of chemical reactions in catalysis. However, unlike well‐established thermal catalysis, a better understanding of fundamental mechanisms is needed, as evidenced by the contrasting results reported in the literature. One main challenge is that not only the genuine catalytic effect may play a role, but both the support and the catalyst also impact the plasma, complicating the understanding. In this study, exploring the impact of support by comparing a single metal on various substrates made of the same material is focused on. Herein, silver on γ‐alumina is used to investigate ammonia synthesis in N₂/H₂ plasma discharges. Beyond confirming the beneficial role of silver in ammonia formation, it is also found that the influence of support is crucial and it is affected by the preparation method. These findings contribute to clarifying the discrepancies in the literature results despite using the same materials.

Context. CH₃CN (methyl cyanide) is one of the simplest and most abundant interstellar complex organic molecules (iCOMs), and has been detected in young solar analogues, shocked regions, protoplanetary discs, and comets. CH₃CN can therefore be considered a key species to explore the chemical connections between the planet-forming disk phase and comets. However, for such comparison to be meaningful, kinetics data for the reactions leading to CH₃CN formation and destruction must be updated. Aims. Here we focus on the destruction of methyl cyanide through collisions with He^+. . We employed a combined experimental and theoretical methodology to obtain cross sections (CSs) and branching ratios (BRs) as a function of collision energy, from which we calculated reaction rate coefficients k(T) in the temperature range from 10 to 300 K. Methods. We measured CSs and BRs using a guided ion beam setup, and developed a theoretical treatment based on an analytical formulation of the potential energy surfaces (PESs) for the charge exchange process. The method employs a Landau Zener model to obtain reaction probabilities at crossings between the entrance and exit PESs, and an adiabatic centrifugal sudden approximation to calculate CSs and k(T), from subthermal to hyper-thermal regimes. Results. k(T) and experimental BRs differ from those predicted from widely used capture models. In particular, the rate coefficient at 10 K is estimated to be almost one order of magnitude smaller than what is reported in the KIDA database. In addition, the charge exchange is completely dissociative and the most abundant fragments are HCCN⁺/CCNH⁺ , HCNH⁺ and CH₂⁺. Conclusions. Our results, combined with a revised chemical network for the formation of CH₃CN, support the hypothesis that methyl cyanide in protoplanetary discs could be mostly the product of gas-phase processes rather than grain chemistry, as currently proposed. These findings are expected to have implications in the comparison of the abundance ratios of N-bearing molecules observed in discs with cometary abundance ratios.

A combined experimental and theoretical study demonstrates the presence of stereodynamical effects in the charge exchange reactivity of He˙⁺ with CH₃OH, a key process in the astrochemistry of methanol.

Over the years, until the present days, a persistent mistake has been found in the literature: the use of the ‘vibrational temperature’ of an emitting electronic state as somewhat representative of the vibrational temperature of the gas in the discharge. Such a temperature is determined by fitting the spectra measured by optical emission spectroscopy. Besides the misuse of the word temperature , the results of such fittings are ambiguously named ‘vibrational temperature’ and sometimes used to argue about the vibrational non-equilibrium and its variation with discharge conditions. What has this temperature to do with the vibrational excitation of the molecules’ ground state, i.e. of the large majority of gas components? It is well established that the connection between the vibrational population of the excited and the ground state exists through the excitation process, the collisional quenching, and the vibrational relaxation in the manifold of the excited state. Nevertheless, this is very often ignored in the literature. In this note, we discuss this subject with the example of the ‘vibrational temperature’ of the N ₂ (C, v ) manifold, showing how much all the mentioned parameters can drive to incorrect deductions from an anyway conceptually wrong measurement.

Nanosecond pulsed discharge plasma shows a high degree of non-equilibrium, and exhibits relatively high conversions in the dry reforming of methane. To further improve the application, a good insight of the underlying mechanisms is desired. We developed a chemical kinetics model to explore the underlying plasma chemistry in nanosecond pulsed discharge. We compared the calculated conversions and product selectivities with experimental results, and found reasonable agreement in a wide range of specific energy input. Hence, the chemical kinetics model is able to provide insight in the underlying plasma chemistry. The modeling results predict that the most important dissociation reaction of CO ₂ and CH ₄ is electron impact dissociation. C ₂ H ₂ is the most abundant hydrocarbon product, and it is mainly formed upon reaction of two CH ₂ radicals. Furthermore, the vibrational excitation levels of CO ₂ contribute for 85% to the total dissociation of CO ₂ .

Time-resolved absolute OH density measurements in a millisecond-pulsed DC glow discharge from carbon dioxide with water admixture, by laser-induced fluorescence (LIF) spectroscopy, are presented. For this purpose, a novel technique is suggested to calibrate the OH LIF setup by means of two-photon absorption LIF spectroscopy measurements on a CO-filled gas cell. OH densities in the order of 1 × 10¹⁸ m⁻³ are reported at a pressure of 6.67 mbar with a water admixture of 20% and a discharge current of 50 mA. Furthermore, the time evolution of the CO₂ conversion is determined from collision energy transfer LIF and validated against the literature. The time-dependent rotational temperature of OH is obtained with LIF thermometry.