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 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 NHx (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.

Highlights

  • Ammonia is one of the most important chemicals used in modern society

  • As there were no notable differences in the discharge characteristics for the three different metal catalysts, these results suggest 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

  • An insight into the synergetic effects of plasma-catalytic synthesis of ammonia at near-room temperature and atmospheric pressure has been investigated via a series of characterization methods

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Summary

Introduction

Ammonia is one of the most important chemicals used in modern society. It is a vital precursor in the synthesis of many useful products, including fertilizers, plastics, resins, explosives, and synthetic fabrics. It has the potential to be used for energy storage[1−3] and as a hydrogen fuel.[4−6] It is produced on an industrial scale from N2 and H2 using the Haber-Bosch process, which is typically carried out at 450−600 °C and 150−300 bar in the presence of a highly active catalyst. This process emits over 300 million metric tons of CO2 each year and is highly energy-intensive, consuming 1−2% of the world’s primary energy supply: the largest in the chemical industry.[7,8]. Efforts have, Received: June 17, 2019 Revised: October 16, 2019 Published: October 18, 2019

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