Atmospheric Plasma-Assisted Ammonia Synthesis Enhanced via

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Atmospheric plasma-assisted ammonia synthesis enhanced via synergistic catalytic absorption Peng Peng, Paul Chen, Min Addy, Yanling Cheng, Erik Anderson, Nan Zhou, Charles Schiappacasse, Yaning Zhang, Dongjie Chen, Raymond Hatzenbeller, Yuhuan Liu, and Roger Ruan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03887 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 17, 2018

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ACS Sustainable Chemistry & Engineering

Atmospheric plasma-assisted ammonia synthesis enhanced via synergistic catalytic absorption Peng Peng†, Paul Chen†, Min Addy†, Yanling Cheng†, Erik Anderson†, Nan Zhou†, Charles Schiappacasse†, Yaning Zhang†, ‡, Dongjie Chen†, Raymond Hatzenbeller†, Yuhuan LiuΔ, and Roger Ruan†, *

† Center

for Biorefining, and Department of Bioproducts and Biosystems Engineering,

University of Minnesota Twin Cities, 1390 Eckles Ave., St. Paul, Minnesota. USA, 55108 ‡

Harbin Institute of Technology, 92 Xidazhi St, Nangang Qu, Haerbin Shi, Heilongjiang Sheng,

China, 150001 Δ MOE

Biomass Engineering Research Center, Nanchang University, Qingshanhu, Nanchang,

China, 330000 *

Corresponding author, Distinguished Guest Professor, Nanchang University, and Professor,

University of Minnesota.. [email protected]

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Plasma-assisted ammonia synthesis has been one of the promising alternatives for building a sustainable ammonia production infrastructure. This study improved this process by introducing an in-situ catalytic absorption mechanism using magnesium chloride. Other than its catalytic functions, the absorption mechanism involved two pathways of forming Mg3N2 and Mg(NH3)6Cl2. Meanwhile, the pulse density modulation (PDM) was introduced to improve the energy performance of system. The series of efforts improved the energy efficiency of the system by approximately one-fold and achieved the highest value of 20.5 g/kwh.

Keywords: Sustainable ammonia synthesis; Atmospheric conditions; Plasma nitridation; Ammonia absorption; Pulse density modulation;

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Introduction The use of ammonia as a clean fuel and energy storage material has been intensively studied in the recent years. However, the industrial ammonia production method, namely known as the Haber-Bosch process, is highly centralized due to its high temperature and pressure conditions, while the current ammonia usage is distributed 1. Along with its centralization problem, the process emits a large amount of greenhouse gases and consumes 1%-2% of the world’s total energy usage 2. On the other hand, the distributed synthesis of ammonia is highly beneficial for its sustainable power generation and smarter grid applications 3. Therefore, researchers have been developing a process that can produce ammonia in a cleaner fashion and is accessible to small industries and local farms. Non-thermal plasma (NTP) has recently been proved to be a sustainable solution for the distributed ammonia production, as it circumvents the high temperature and pressure requirements of the Haber-Bosch process 4-6. In previous studies, various catalysts, including ruthenium-based 7-8, transitional metal 9, and alkaline oxide 10, have been investigated. However, one of the most critical challenges raised in previous research of using NTP to synthesize ammonia was the dissociation of the product within the plasma region 11. This study makes the first attempt on trying to enhance this process by adding separation inside the reactor. To achieve this, a novel and environmental-friendly synergistic catalytic absorption process to synthesize ammonia under non-thermal plasma using magnesium chloride (MgCl2) was proposed and studied. As its name explains, the synergistic mechanism consists of two parts, catalysis and absorption. For the catalytic part, magnesium oxide (MgO) was previously incorporated in to the plasma-based ammonia synthesis as a catalyst that decrease the

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activation energy of hydrogen and nitrogen 12-13. In more detail, the catalyst can promote electron excitation and vibrational excitation reactions in the gas phase, and facilitate interactions between the molecules at the solid catalyst surface 14. As for the absorption part, the mechanism utilizes two critical pathways. The first one utilizes the ability of MgCl2 to form a critical absorption intermediate compound, Magnesium nitride (Mg3N2). Although this process has recently demonstrated by magnesium oxide (MgO) 15, the lattice energy of MgCl2 is significantly lower than MgO. Therefore, it is reasonable to propose that the formation of Mg2N3 from MgCl2 smoother than MgO. Second, the ammonia absorption of MgCl2 under plasma conditions has not been studied prior to this study, although in several recent studies, it has been used as an efficient absorbent that could lower the operating pressure and enhance the performance of the HaberBosch process 16-19. This direct absorption of ammonia onto MgCl2 usually relies on the formation of Mg(NH3)6Cl2, which has a strong ammonia-holding capacity 20-23. Considering these two potential mechanisms, the hypothesis can be made that when applying MgCl2 to the plasma-assisted ammonia synthesis approach would lead to a better product formation and separation, and increase the synthesis efficiency. From the engineering standpoint, to generate the high frequency and high voltage electric field to create the plasma is a challenge to almost all the plasma-related processes. Besides the synergistic mechanism, this study introduced a pulse density modulation (PDM) to the traditional high frequency inverter. Also, the influences of the dielectric plasma parameters were studied under different experimental conditions. In addition to its absorption effects, the advantages of using the MgCl2 compared to other plasma catalysts, such as Ru deposited on dielectric substances, are its considerably low cost, and easy preparation and regeneration. Due to the series of efforts, the energy efficiency achieved in this study was 20.5 g NH3/kWh, which

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showed an improvement compared with other plasma-assisted NH3 synthesis processes from nitrogen and hydrogen 24-26.

Discussion Figure 1 a) to c) showed a schematic diagram of how the MgCl2 was placed at different positions relative to the plasma discharge region under the reaction time of 20 minutes. It was obvious that the ammonia synthesis rate in the gas phase (red), absorption rate in the solid phase (blue), and efficiency (dashed lines) achieved the highest in configuration a) among the different placements, which tentatively showed catalytic and absorption effects of MgCl2. Since for this configuration, MgCl2 was located at plasma discharge, it was reasonable to propose that MgCl2 provided both adsorption effect and dielectric effect. Here, around 6% of ammonia was absorbed by MgCl2 and was not dissociated in the plasma region. When MgCl2 was placed immediately below the plasma region or 5 cm below, the total ammonia synthesis rates also increased compared with the results without MgCl2. However, with a closer look into the rate composition, the enhancement of these two displacements was caused mainly by absorption, as the synthesis rates in the gas phase were approximately the same as the plasma-only synthesis. The dielectric effect was what contributed to the catalytic functions of the MgCl2. Within the plasma discharge region, the magnesium is capable of bonding at its surface with the free radicals (N*, H*) formed by the electron excitation and vibrational excitation in the gas phase, providing additional reaction sites with the NHx* free radicals to form ammonia 8, 14, 27. To further study how the change in discharge characteristics due to dielectric effect may contribute to improvement of efficiency, Figures 2 showed how the NH3 synthesis rate and efficiency were affected by dielectric parameters at the reaction time of 20 minutes. The higher discharge voltage

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(Figure 2 a) could promote the plasma catalytic reaction to form NH3, until the dissociation affect started to dominate 24. The frequency at 23 kHz was found to provide the resonance effect between the plasma reactor and high voltage and frequency generator, which led to a more homogeneous plasma discharge and offers the highest energy efficiency to the system 28. For the reactant concentration, more nitrogen was need (N2:H2 = 1:1) compared to the stoichiometric equation (N2:H2 = 1:3) to reach the optimum energy efficiency, as nitrogen dissociation is the rate-limiting step in the plasma-assisted ammonia synthesis approach 24, 29-30. Furthermore, Figure 2 d) showed that the ammonia absorption effect by the MgCl2 decreased at higher flow rates due to the decrease in the residence time. Due to the fast nature of plasma-related reactions, the solid-state reaction on MgCl2 should decrease with the increasing reaction time, leading to more products being formed in the region, which could further cause greater synthesis efficiency 24

. The optimum energy efficiency was reached at the flow rate of 4 L/min, which corresponded

to the reaction time of 24 ms. The PDM was another critical factor to the system improvement. Recent literature showed that adding the PDM control of high-frequency inverters could lead to high efficiency (by greatly reducing the switching loss), small size, and the ability of self-power factor correction 31. In other words, the PDM not only allowed a linear control of plasma according to the command signals, but also capable of reducing the power delivered to output of the inverter, which was the load, while maintaining the high frequency and voltage. As the consequence, the PDM increased the energy efficiency of the system as the discharge current from the invertor into the transformer was reduced, which further enhanced the efficiency of ammonia synthesis under plasma region, shown in Figure 3.

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As proposed in section 1, ammonia could be theoretically absorbed in this process via two mechanisms, shown in Figure 4 b). The first mechanism involves the formation of the Mg3N2 intermediate via the nitridation mechanism 15, which was believed to be an ideal solid-state ammonia carrier under atmospheric conditions by forming additional NH3 molecules via the reactions with water after the plasma treatment under nitrogen environments. The SEM/EDS (supplementary section S3.1) and XRD pattern in Figure 4 a) supported the hypothesis by showing that not only the untreated sample matched closely with previous reports 32, but The diffraction peaks in the treated samples at 21,32, and 34 2-degree theta were associated to Mg3N2 33

. Due to MgCl2’s lower lattice energy, the ammonia synthesis rate and efficiency in this study

was higher than MgO15. The second mechanism was the rapid absorption of the produced ammonia by MgCl2 in the plasma region. The fast absorption was achieved via the formation of Mg(NH3)6Cl2, which was confirmed by the peaks at 16 and 22 2-degree theta in the XRD patterns of the treated samples 34-35. Intensity wise, the MgCl2-associateds peaks decreased after being treated in the plasma region, and further decreased when placed below the plasma discharge region. This result showed that MgCl2 absorbed more ammonia in the plasma region than below the region, which was corresponded with the findings from Figure 1 b). Therefore, it was reasonable that the concentration of pure MgCl2 would decrease compared with the untreated and in-discharge samples. Lastly, MgCl2 can easily absorb water and is more sensitive to moisture. Therefore, efficient pretreatment process would be needed to eliminate the moisture in the nitrogen and hydrogen feed gases, which could potentially add additional capital cost to this system, and should be taken into consideration for future optimization and potential scale-ups.

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Conclusions To conclude, this study demonstrated a novel, synergistic catalytic absorption strategy to improve the plasma-assisted ammonia synthesis. To reduce the effect of product deformation caused by plasma in this process. Beyond its catalytic effects, the absorption process using magnesium chloride involved two critical pathways that forms Mg3N2 and Mg(NH3)6Cl2. Moreover, the introduction of PDM caused improvements in energy efficiency to produce a more homogeneous plasma discharge. These enhancements led to a greatest energy efficiency of 20.5 g/kwh. The promising results could open a new pathway of improving the plasma-assisted ammonia synthesis process by better product separation methods and greater compatibility of the plasma generating systems.

Acknowledgement Funding for this project was provided in part by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR), Minnesota’s Discovery, Research, and Innovation Economy (MnDRIVE), and University of Minnesota Center for Biorefining.

Supporting information Experimental methods, energy efficiency calculation, SEM and EDS results, experimental parameters and results.

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In Below 5 cm below discharge discharge discharge

Figure 1 Schematic diagrams of the MgCl2 locations relative to the plasma discharge region a) in discharge; b) below discharge; c) 5cm below discharge d) Ammonia synthesis results with MgCl2 packed at various locations relative to the plasma discharge region

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Figure 2 The effects of different plasma discharge parameters and reactant properties on the NH3 synthesis results (synthesis rate, absorption rate, and efficiency) with MgCl2. a) plasma discharge voltage; b) plasma discharge frequency c) N2 composition d) reactant flow rate

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MgCl2 MgCl2 Ru with Ru Plasma Plasma With without PDM without only only PDM PDM PDM with without PDM PDM Figure 3 The effect of the pulse density modulation on the efficiency of the NTP ammonia synthesis system under various catalyst load

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a)

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Figure 4 a) XRD patterns of the MgCl2 samples under different conditions b) An illustrative diagram of the synergistic absorption mechanism of MgCl2 on plasma-assisted ammonia synthesis

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For Table of Contents Use Only

Synopsis The synergistic catalytic absorption and pulse density modulation improved the plasmaassisted technology for building a sustainable ammonia synthesis infrastructure

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Atmospheric Plasma-Assisted Ammonia Synthesis Enhanced via

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