A1 A2 A3 A4 A5 A6 A7 A8 A9 B1 B2 B4 B5 B7 B8

A7: Plasma catalysis for conversion of volatile organic compounds (VOC)

Principal Investigators: M. Muhler, P. Awakowicz

Team


Awakowicz, Peter
(Principal Investigator)

Muhler, Martin
(Principal Investigator)

Peters, Niklas
(PhD)

Schücke, Lars
(PhD)

Description

ImageDirect and indirect exposure of catalytically active surfaces to atmospheric pressure air plasmas allows for manifold physical and chemical interactions resulting in enhanced catalytic performance. Generally, cross-sections for gas phase reactions leading to degradation of pollutants - e.g. volatile organic compounds (VOCs) - are rather poor because of high energetic barriers. Shifting the site for chemical reactions to take place from the gas phase to a catalytically active surface yields significantly higher degrees of conversion. Plasma-assisted catalysis belongs to heterogenous catalysis and can be divided into two sub-categories: Post-plasma catalysis based on two consecutive reactors (PPC) and the more demanding in-plasma catalysis (IPC). In environmental applications, the efficient removal of VOCs by total oxidation to carbon dioxide (CO2) and water (H2O) for air purification is one of the major challenges. By means of high-energetic electrons, chemically reactive oxygen species (ROS) such as ozone (O3), hydroxyl radicals (OH), superoxide radicals (O2-) and atomic oxygen (O) are generated in the presence of oxygen molecules(O2) as well as reactive nitrogen species (RNS) in oxygen-nitrogen mixtures, which then react with pollutants. The main goal of this project is the basic understanding and optimisation the plasma-assisted total oxidation of VOCs to CO2 and H2O over transition metal oxides. The presented project consists of two sub-projects. One of them (Muhler) is devoted to the synthesis and application of catalyst materials which can help to solve the stated aim of the project. The other one (Awakowicz) deals with with quantification and optimisation of the plasma conditions and processes. Manganese oxide-based catalysts will be synthesised using a micromixer coupled to a spray dryer. In a T-shaped mixer rapid mixing of the precursor solutions with a defined mixing time is ensured followed by rapid quenching using the spray dryer to prevent further particle growth. The catalysts will be characterised by N2 physisorption, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and various temperature-programmed reactions. A surface dielectric barrier discharge (SDBD) in gas flow consisting of an a-Al2O3 plate with tinned, grid-structured copper traces on each side (see project A5) in conjunction with catalysts like manganese dioxide (MnO2) or titanium oxide (TiO2) will be applied and characterised using optical emission spectroscopy (OES) and voltage-current measurements. The SDBD discharge will be sustained using ms rise time damped sine wave pulses since this is the most energy efficient method known so far. In order to establish close contact and therefore high fluxes of radicals between the surface discharge and the catalyst whilst minimising the total dimensions of the air purification system, the catalytic materials will be deposited on the dielectric used as a barrier in the SDBD. A spray coating procedure will be developed to produce homogeneous thin films of the catalysts. The influence of the catalyst layer on the plasma conditions will be studied using OES with temporal and spatial resolution. The flux of radicals produced in the active volume of the SDBD will be determined using optical emission and Absorption spectroscopy (OES/OAS) and zero dimensional simulation of the plasma chemical kinetics (A9). By partial dissociation of VOCs in the SDBD a lot of different hydrocarbon radicals will be produced, which posses low energetic barriers for oxidation by ROS. In addition, ROS like O, O3 or OH adsorb on the catalytic surfaces and initiate further oxidation of the VOC fragments. Therefore, the interplay of plasma physics and surface chemistry is crucial, and a large variety of time and length scales has to be considered. In humid air, the Diameter of the SDBD’s single filaments is in the range of mm. While a single discharge takes place on the timescale of 10 to 30 ns, the total duration of discharge pulses in a single half-wave of current-voltage-characteristics can easily sum up to ms. This is due to the fact that discharge does not necessarily take place exactly at the same time all over the dielectric surface. While the discharge physics has to be looked at on a timescale of ns-ms, the desired chemical conversions take place on a timescale of ms-s. Therefore, residence times have a huge impact on the whole system. The catalytic properties of the catalysts will be investigated using fixed beds and thin coatings for the thermal oxidation of different classes of VOCs. The degrees of conversion and the selectivities of the formed products will be identified by fast online mass spectrometry, flame ionisation detectors (FIDs), gas chromatography coupled with mass spectrometry (GC-MS) and Fourier transform infrared spectroscopy (FTIR). This combination of spectroscopic and analytical methods yields detailed insight into the composition of the product gas. The occuring reaction pathways will be deduced from the densities of reactive species, intermediates and products.

Publications

Thesis

  • Lars Schücke
    Plasma catalysis for conversion of volatile organic compounds (VOC) (PhD thesis) - ongoing
  • Niklas Peters
    Plasma catalysis for conversion of volatile organic compounds (VOC) (PhD thesis) - ongoing
  • Christian Oberste-Beulmann
    Entwicklung einer Methode zur Beschichtung von Plattenelektroden mit heterogenen Katalysatoren für die oxidative Plasmakatalyse (BA) - 07/2018
  • Kevin Ollegott
    Plasma-assisted removal of oxygen traces from coke oven gas using a twin-surface dielectric barrier discharge (PhD thesis) - 01/2020
  • Philipp Wirth
    Plasma Catalysis in DBDs (PhD thesis) - ongoing
  • Jens Kallähn
    Entwurf einer Messstrecke zur optischen Absorptionsspektroskopie von Ozon an einer dielektrischen Oberflächenentladung (BA) - 01/2020
  • Jan-Luca Gembus
    Untersuchung der Konversion von flüchtigen Kohlenwasserstoffen in einer dielektrischen Oberflächenentladung (BA) - 09/2019
  • Christian Oberste-Beulmann
    Investigations on Plasma-catalytic Removal of Oxygen Traces from Hydrogen-rich Gas Mixtures (MA) - 12/2020
  • Niklas Friedrichs
    Untersuchung der Sauerstoffradikaldichten einer dielektrischen Oberflächenentladung (BA) - 04/2021

Further reading

  • Oxygen-Plasma-Functionalized Carbon Nanotubes as Supports for Platinum-Ruthenium Catalysts Applied in Electrochemical Methanol Oxidation,
    R. Chetty, K. K. Maniam, W. Schuhmann, and M. Muhler,
    ChemPlusChem 80, 130-135 (2015)

Project presentation video

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