Species, pathways, and timescales for NH3 production by plasma catalysis over transition-metal wools are determined by measuring plasma-derived species densities [N, H, and N2(v)], quantitatively correlating consumption of these species with NH3 formation, and measuring consumption of plasma-derived species at different residence times. These findings are enabled by a capillary flow through Ar/N2/H2 plasma jet reactor setup that allows for the measurement of gas-phase species densities by molecular beam mass spectrometry. Surface-mediated reactions involving N radicals are responsible for NH3 formation over Fe, Ni, and Ag surfaces. N reacts to form NH3 with ∼100% selectivity over Ni and Ag when H/N > 3 and % H2 ≥ 0.5. The selectivity to ammonia drops as H and H2 densities decrease for each catalyst. A comparison between amounts of NH3 formed and N consumed with and without catalysts present shows that surface reactions enable higher and more selective conversion of N to NH3 than gas-phase reactions alone. The conversion of N to NH3 is negligible in the absence of H, demonstrating that H is required to produce NH3 at these operating conditions. The consumption of N occurs on the same timescale as NH3 formation, further confirming that reactions involving N contribute to NH3 formation. Though vibrationally excited N2 [N2(v)] is produced in quantities exceeding N by 100-fold, consumption of N2(v) on the catalytic surface does not contribute to NH3 formation. These findings show that for low-temperature atmospheric pressure plasma catalysis, surface-mediated reactions among radical N and H species drive NH3 formation.

The kinetic gas theory, in particular the equations of Chapman and Enskog, proved to be good and widely applicable approximations for modeling transport properties like diffusion coefficients, viscosities and thermal conductivities. However, these equations rely on at least the Lennard-Jones parameters and for polar gases also the dipole moment. In the scientific literature, the Lennard-Jones parameters are fitted to only one experimentally determined transport coefficient. This approach leads to good agreement between the Chapman Enskog equations employing the so obtained parameters with the experimental data for this specific transport property. However, utilizing the same parameters for modeling different transport properties oftentimes leads to distinct deviations. In this work, it is shown that the subset of Lennard-Jones parameters with which the Chapman Enskog equations can predict the experimental results with deviations comparable to the experimental uncertainty are not identical for each transport property. Hence, fitting towards one property doesn't necessarily yield parameters that are suited to describe the other transport properties. In this publication, the Lennard-Jones parameters and a temperature dependent Eucken correction factor, leading to a significantly higher accuracy than the classical Eucken correction and also its modification by Hirschfelder, are therefore fitted towards all three transport properties simultaneously for seven exemplary gases. This approach leads to a significantly better agreement with experimental data for the three transport properties than the classical approach that relies on fitting to one single transport property and can be utilized to determine accurate sets of Lennard-Jones parameters and Eucken correction factors for any gas species. It provides a computationally inexpensive and practical method for the precise calculation of transport properties over a wide range of temperatures relevant for processes in the chemical industry.