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      Plasma-Enhanced Catalytic Synthesis of Ammonia over a Ni/Al 2O 3 Catalyst at Near-Room Temperature: Insights into the Importance of the Catalyst Surface on the Reaction Mechanism

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          Abstract

          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 N 2 and H 2 over transition metal catalysts (M/Al 2O 3, 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 NH 3 without a catalyst, plasma-catalysis significantly enhanced the NH 3 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 NH 3 synthesis rate of 471 μmol g –1 h –1 was achieved using Ni/Al 2O 3 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 Al 2O 3 provided a more uniform plasma discharge than Al 2O 3 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 N 2 physisorption measurements, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), NH 3-temperature-programmed desorption (TPD), and N 2-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 NH 3 via formation of NH 2 intermediates on the surface.

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          Most cited references 57

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          Solving the Boltzmann equation to obtain electron transport coefficients and rate coefficients for fluid models

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            Ammonia synthesis at atmospheric pressure

            Ammonia was synthesized from its elements at atmospheric pressure in a solid state proton (H+)-conducting cell-reactor. Hydrogen was flowing over the anode and was converted into protons that were transported through the solid electrolyte and reached the cathode (palladium) over which nitrogen was passing. At 570 degreesC and atmospheric pressure, greater than 78 percent of the electrochemically supplied hydrogen was converted into ammonia. The thermodynamic requirement for a high-pressure process is eliminated.
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              Is Open Access

              Ammonia for power

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                Author and article information

                Journal
                ACS Catal
                ACS Catal
                cs
                accacs
                ACS Catalysis
                American Chemical Society
                2155-5435
                18 October 2019
                06 December 2019
                : 9
                : 12
                : 10780-10793
                Affiliations
                []Department of Electrical Engineering and Electronics, University of Liverpool , Liverpool L69 3GJ, United Kingdom
                []School of Chemical and Biomolecular Engineering, Sydney Nano Institute, The University of Sydney , Sydney, NSW 2037, Australia
                Author notes
                Article
                10.1021/acscatal.9b02538
                7011700
                3b155a8e-e359-45e8-b8b5-8fd1d5381cbf
                Copyright © 2019 American Chemical Society

                This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.

                Categories
                Research Article
                Custom metadata
                cs9b02538
                cs9b02538

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