A better fundamental understanding of the plasma-catalyst interaction and the reaction mechanism is vital for optimizing the design of catalysts for ammonia synthesis by plasma-catalysis. In this work, we report on a hybrid plasma-enhanced catalytic process for the synthesis of ammonia directly from N2 and H2 over transition metal catalysts (M/Al2O3, M = Fe, Ni, Cu) at near room temperature (∼35 °C) and atmospheric pressure. Reactions were conducted in a specially designed coaxial dielectric barrier discharge (DBD) plasma reactor using water as a ground electrode, which could cool and maintain the reaction at near-room temperature. The transparency of the water electrode enabled operando optical diagnostics (intensified charge-coupled device (ICCD) imaging and optical emission spectroscopy) of the full plasma discharge area to be conducted without altering the operation of the reactor, as is often needed when using coaxial reactors with opaque ground electrodes. Compared to plasma synthesis of NH3 without a catalyst, plasma-catalysis significantly enhanced the NH3 synthesis rate and energy efficiency. The effect of different transition metal catalysts on the physical properties of the discharge is negligible, which suggests that the catalytic effects provided by the chemistry of the catalyst surface are dominant over the physical effects of the catalysts in the plasma-catalytic synthesis of ammonia. The highest NH3 synthesis rate of 471 μmol g–1 h–1 was achieved using Ni/Al2O3 as a catalyst with plasma, which is 100% higher than that obtained using plasma only. The presence of a transition metal (e.g., Ni) on the surface of Al2O3 provided a more uniform plasma discharge than Al2O3 or plasma only, and enhanced the mean electron energy. The mechanism of plasma-catalytic ammonia synthesis has been investigated through operando plasma diagnostics combined with comprehensive characterization of the catalysts using N2 physisorption measurements, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), NH3-temperature-programmed desorption (TPD), and N2-TPD. Four forms of adsorbed NH x (x = 0, 1, 2, and 3) species were detected on the surfaces of the spent catalysts using XPS. It was found that metal sites and weak acid sites could enhance the production of NH3 via formation of NH2 intermediates on the surface.

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In this study, the samples of ordinary Portland cement mortar were fully immersed in sodium sulfate solutions of different concentrations (0%, 5% and 15%) for up to 270 d. The evolution of the properties of the samples (compressive strength, static modulus of elasticity, stress–strain behaviour, hardened density, water permeability coefficient and length change) was monitored. The surface deterioration of the samples was also observed and the morphological features of the products were investigated using scanning electron microscopy. Water immersion was found to improve for the properties of the samples and reduce the water permeability coefficient. Moreover, at the initial stage of sodium sulfate attack, the presence of sulfate ions had a positive effect on the evolution of the properties due to the formation of ettringite and gypsum. Afterwards, the samples immersed in sodium sulfate solution showed a degradation in the properties, induced by the expansion of expansive products and the generation of micro-cracks. Furthermore, the mortar immersed in sodium sulfate solution showed higher expansion and a larger hardened density than the mortar immersed in water. An increase in the concentration of the sodium sulfate solution was found to reduce the time needed to reach peak properties and to accelerate deterioration of the properties at the late stage.