Exploring Plasma Catalysis for Sustainable Production of Chemicals

Jakob Afzali Andersen*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Abstract

This thesis is dedicated to the exploration of plasma catalysis and to identifying possible processes for sus-tainable production of chemicals with this system. This has been done through experiments and kinetic mod-elling. In the experiments, investigations of process conditions and the influence of introducing different cat-alysts in the plasma zone was conducted.

Recent research has shown that an electrically generated non-thermal plasma is a promising alterna-tive to thermal activation of molecules, as the plasma can specifically energize the electrons of the molecules. Furthermore, it was reported that the introduction of packing material resulted in an intensified electric field around each contact point of the particles. In turn, this produced free electrons of increased energy.

The experiments in this thesis were conducted on a laboratory scale setup with a DBD reactor operated at near atmospheric pressure and room temperature. A sinusoidal wave form was used at all times for the applied voltage.

The use of catalytic material (Ag, Cu, and Pt supported on γ-Al2O3) for plasma-catalytic dry reforming was found to result in similar conversions of CH4 and CO2 (33% and 22%) as the plasma alone at a plasma power of 45 W and feed flow rate of 50 Nml/min with CH4:CO2 = 1:1. This indicates that the electric field enhancement caused by the packing materials could not compensate for the shorter residence time of the gas due to the introduction of the packing at these conditions. Furthermore, comparable selectivities of H2, CO, saturated and unsaturated hydrocarbons were observed for the different catalytic materials indicating that the catalytic activity was not significant compared to the plasma effect.

A similar conclusion was reached from plasma-assisted NH3 decomposition experiments, as a linear correlation between the conversion and the number of micro-discharges was observed for both plasma alone and in the presence of various solid packing materials (MgAl2O4, HZSM-5 (Si/Al=25), SiO2, γ-Al2O3, m-ZrO2, t-ZrO2/La2O3(8.2 wt%), TiO2, and BaTiO3). A plasma power of 21 W, frequency of 3 kHz, and 100% NH3 feed flow rate of 75 Nml/min was used. The primary function of the solids was thus to facilitate the gas phase reaction by assisting the formation of micro-discharges. Of the tested materials, MgAl2O4 yielded the highest NH3 conversion (15.1%).

From experiments with a diluted NH3 feed (2% NH3 in N2) it was observed that collisions with high energy electrons initiated the decomposition of NH3. This was found based on investigations of different process parameters (plasma power: 10-25W, frequency: 1.0-3.5 kHz, feed flow rate: 50-125 Nml/min, gas residence time: 1.25-20 s, and the presence of MgAl2O4 in the plasma). Altering the volume of the plasma was found to modify the plasma power density distribution between micro-discharges and the uniform plasma at a constant plasma power. Moreover, a smaller discharge volume was observed to result in a more non-uniform plasma and a higher number of micro-discharges per volume, with a more non-uniform plasma promoting the decomposition. The introduction of packing was found to result in a more uniform plasma with a lower electron density during micro-discharges and a lower average electron energy during the after-glow.

Using a 0D plasma kinetic model confirmed the experimental observations, as electron collisions in the micro-discharges and their afterglow were identified to dissociate the NH3. The rate of NH3 destruction was found to be 5 to 6 orders of magnitude higher during a micro-discharge compared to its afterglow for both the unpacked and packed setup. It was further identified by the model that N and H radicals are key reactants in the intermediate steps of the decomposition for converting NH2 and NH. Additionally, the packing material was found to introduce high concentrations of surface bound H, which led to a significant re-formation of NH3 through an Eley-Rideal reaction with NH2 causing a negative effect of the introduction of packing material on NH3 conversion.

Lastly, plasma-catalytic NH3 synthesis from N2 and H2 was investigated. Here, the effect of plasma power (10-30W), feed flow rate (40-100 Nml/min), N2:H2 feed ratio (3:1-1:3), temperature (100-200°C), and different packing materials (MgAl2O4, Ru/MgAl2O4, and Co/MgAl2O4) on the NH3 synthesis rate was examined via experiments. An improved NH3 synthesis rate was achieved when increasing plasma power, feed flow rate, and gas temperature (although not for plasma alone). At 200°C an optimum in the NH3 synthesis rate was observed at an equimolar feed ratio (N2:H2=1:1) for the plasma alone and MgAl2O4, while a N2-rich feed was favored for Ru/MgAl2O4 and Co/MgAl2O4. In contrast to the other reactions investigated, a high catalytic activity was observed for plasma-catalytic NH3 synthesis. When implementing the Co/MgAl2O4 catalyst, a 4 times higher NH3 synthesis rate compared to plasma alone was achieved at 25 W. The optimum in the syn-thesis rate with the N2-rich feed, where high energy electrons are more likely to collide with N2, suggests that the rate-limiting step is the dissociation of N2 in the gas phase. This was also supported by the 0D kinetic model when packing material was used for all tested feed ratios.
It was further indicated that the catalytic NH3 formation was not limited by the gas residence time at these conditions, but by the NH3 dissociation rate in the micro-discharges. Additionally, the model revealed that a higher N density in the gas phase with a similar H(s) coverage was obtained for a N2-rich feed compared to a H2-rich feed, when a packing material was present. In turn, this yielded a higher coverage of surface bound NH and NH2 (NH3 precursors), which was formed through direct adsorption from the gas phase and elementary Eley-Rideal and Langmuir-Hinshelwood reactions.
Based on the information presented in the thesis, it is evident that non-thermal plasma is capable of activating and converting a variety of gases due to the high energy of the electrons formed. While low energy efficiencies are generally reported for these plasma systems, the addition of a catalyst could, in some cases, increase this value. Still, further investigations into the kinetics and the synergy between plasma and catalysts are needed to increase the understanding of these systems and develop specific plasma-catalysts to improve the energy efficiency.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages151
Publication statusPublished - 2021

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