AC

Determining Chemical Reaction Systems in Plasma-Assisted Conversion of Methane Using Genetic Algorithms

Even for processes with only a few gas species involved the detailed description of plasma-assisted conversion processes in gas mixtures requires a large amount of processes to be taken into account and a large number of neutral and charged particles must be considered. In addition, setting up the corresponding reaction kinetics model needs the knowledge of the rate coefficients and their temperature dependence for all possible reactions between those species. Reduced reaction networks offer a simplified and pragmatic way to obtain an overall reaction kinetics model, already useful for the analysis of experimental data even if not all details of chemistry can be covered. In this paper we present a derivation of a data driven reduced model for plasma-assisted conversion of methane in an helium environment. By consideration of a small number of elementary reactions, a simple model is set up. Experimental data are analyzed by a genetic algorithm that provides best-fit approximations for the open parameters of the model. In a further step non-relevant parameters of the model are identified and a further model reduction is achieved. The data driven analysis of methane conversion serves as an illustrative example of the proposed method. The parameters and reaction channels found are compared with known results from the literature. The method is described in detail. The main goal of this work is to present the potential of this data driven method for a simplified and pragmatic modeling in the increasingly important field of plasma-assisted catalytic processes.

Publisher: 
EP2
IEK-4
Project: 
SFB 1316
Authors: 
D. Reiser
A. von Keudell
T. Urbanietz

Determination of NO densities in a surface dielectric barrier discharge using optical emission spectroscopy

A new computationally assisted diagnostic to measure NO densities in atmospheric-pressure microplasmas by Optical Emission Spectroscopy (OES) is developed and validated against absorption spectroscopy in a volume Dielectric Barrier Discharge (DBD). The OES method is then applied to a twin surface DBD operated in N2 to measure the NO density as a function of the O2 admixture (0:1%–1%). The underlying rate equation model reveals that NO(A2Σ+) is primarily excited by reactions of the ground state NO(X2Π) with metastables N2(A3Σ+u).

Publisher: 
AEPT
Project: 
SFB 1316
Authors: 
B. Offerhaus
F. Kogelheide
D. Jalat
N. Bibinov
J. Schulze
K. Stapelmann
P. Awakowicz

Intra-cavity dynamics in a microplasma channel by side-on imaging

Here, a microplasma channel was investigated. The setup consists of three stacked layers: a magnet, a dielectric foil and two nickel foils that are separated by a 120 μm wide gap. The magnet is grounded while the two nickel foils are powered. The setup was operated with a triangular voltage with a frequency of 10 kHz and an amplitude of up to 700 V in Helium at atmospheric pressure. Phase resolved emission images were used to investigate the microplasma channel dynamics with line of sight from the top and from the side to the inside of the cavity. The top view images revealed that the discharge in the microplasma channel and the microplasma arrays behave similar. The already known asymmetric discharge behavior, the self-pulsing and the wavelike ignition was also observed in the microplasma channel. For the wavelike ignition in the channel a simple one dimensional model was proposed. With the additional side view images the asymmetric discharge behavior was examined more thoroughly. Unlike in the microplasma arrays, the discharge expands here in both half periods of the applied voltage above the upper edge of the powered electrodes. The discharge extends over a larger width in the half period, in which the potential of the upper electrodes is increasing, while it extends over a larger height in the other half period. Phase resolved images were also used to investigate the ignition phase of the discharge. The discharge ignites in the two half periods on a different height. This was explained by modeling the drift and diffusion of the charged particles between two discharge pulses.

Publisher: 
EP2
Project: 
SFB 1316
Authors: 
S. Kreuznacht
M. Böke
V. Schulz-von der Gathen

Characterisation of volume and surface dielectric barrier discharges in N2–O2 mixtures using optical emission spectroscopy

A volume and a twin surface dielectric barrier discharge (VDBD and SDBD) are generated in different nitrogen–oxygen mixtures at atmospheric pressure by applying damped sinusoidal voltage waveforms with oscillation periods in the microsecond time scale. Both electrode configurations are located inside vacuum vessels and operated in a controlled atmosphere to exclude the influence of surrounding air. The discharges are characterised with different spatial and temporal resolution by applying absolutely calibrated optical emission spectroscopy in conjunction with numerical simulations and current–voltage measurements. Plasma parameters, namely the electron density and the reduced electric field, and the dissipated power are found to depend strongly on the oxygen content in the working gas mixture. Different spatial and temporal distributions of plasma parameters and dissipated power are explained by surface and residual volume charges for different O2 admixtures due to their effects on the electron recombination rate. Thus, the oxygen admixture is found to strongly influence the breakdown process and plasma conditions of a VDBD and a SDBD.

Publisher: 
AEPT
Project: 
SFB 1316
Authors: 
F. Kogelheide
B. Offerhaus
N. Bibinov
P. Krajinski
L. Schücke
J. Schulze
K. Stapelmann
P. Awakowicz

In-situ control of microdischarge characteristics in unipolar pulsed plasma electrolytic oxidation of aluminum

Microdischarges occurring during plasma electrolytic oxidation are the main mechanism promoting oxide growth compared to classical anodization. When the dissipated energy by microdischarges during the coating process gets too large, high-intensity discharges might occur, which are detrimental to the oxide layer. In bipolar pulsed plasma electrolytic oxidation a so called 'soft-sparking' mode limits microdischarge growth. This method is not available for unipolar pulsing and for all material combinations. In this work, the authors provide a method to control the size- and intensity distributions of microdischarges by utilizing a multivariable closed-loop control. In-situ detection of microdischarge properties by CCD-camera measurements and fast image processing algorithms are deployed. The visible size of microdischarges is controlled by adjusting the duty cycle in a closed-loop feedback scheme, utilizing a PI-controller. Uncontrolled measurements are compared to controlled cases. The microdischarge sizes are controlled to a mean value of A = 510^-3 mm^2 and A = 710^-3 mm^2, respectively. Results for controlled cases show, that size and intensity distributions remain constant over the processing time of 35 minutes. Larger, high-intensity discharges can be effectively prevented. Optical emission spectra reveal, that certain spectral lines can be influenced or controlled with this method. Calculated black body radiation fits with very good agreement to measured continuum emission spectra (T = 3200 K). Variance of microdischarge size, emission intensity and continuum radiation between consecutive measurements is reduced to a large extent, promoting uniform microdischarge and oxide layer properties. A reduced variance in surface defects can be seen in SEM measurements, after coating for 35 minutes, for controlled cases. Surface defect study shows increased number density of microdischarge impact regions, while at the same time reducing pancake diameters, implying reduced microdischarge energies compared to uncontrolled cases.

Publisher: 
AEPT
Project: 
SFB 1316
Authors: 
P. Hermanns
S. Böddeker
V. Bracht
N. Bibinov
P. Awakowicz

Atomic oxygen generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and O<sub>2</sub>

Absolute atomic oxygen densities measured space resolved in the active plasma volume of a COST microplasma reference jet operated in He/O2 and driven by tailored voltage waveforms are presented. The measurements are performed for different shapes of the driving voltage waveform, oxygen admixture concentrations, and peak-to-peak voltages. Peaks- and valleys-waveforms constructed based on different numbers of consecutive harmonics, N, of the fundamental frequency f0 = 13.56 MHz their relative phases and amplitudes are used. The results show that the density of atomic oxygen can be controlled and optimized by voltage waveform tailoring (VWT). It is significantly enhanced by increasing the number of consecutive driving harmonics at fixed peak-to-peak voltage. The shape of the measured density profiles in the direction perpendicular to the electrodes can be controlled by VWT as well. For N > 1 and peaks-/valleys-waveforms, it exhibits a strong spatial asymmetry with a maximum at one of the electrodes due to the spatially asymmetric electron power absorption dynamics. Thus, the atomic oxygen flux can be directed primarily towards one of the electrodes. The generation of atomic oxygen can be further optimized by changing the reactive gas admixture and by tuning the peak-to-peak voltage amplitude. The obtained results are understood based on a detailed analysis of the spatio-temporal dynamics of energetic electrons revealed by phase resolved optical emission spectroscopy (PROES).

Publisher: 
AEPT
Project: 
SFB 1316
Authors: 
I. Korolov
D. Steuer
L. Bischoff
G. Hübner
Y. Liu
M. Böke
T. Mussenbrock
J. Schulze

Dissipated electrical power and electron density in an RF atmsopheric pressure helium plasma jet

Here we present a method for the operando determination of absolute absorbed power in an RF atmospheric pressure helium plasma discharge using miniaturized probes. A detailed error analysis demonstrates the reliability of the measured power values. With the help of a global model, the sheath width and electron density (4 × 1016^(–11) × 1016 m^(−3)) are derived from these power measurements and compared to literature. The results and thus the validity of the electrical model are confirmed by a second, independent characterization method using optical emission spectroscopy and time-averaged imaging.

Publisher: 
PIP
Project: 
SFB 1316
Authors: 
J. Golda
F. Kogelheide
P. Awakowicz
V. Schulz-von der Gathen

Oxygen removal from a hydrocarbon containing gas stream by plasma catalysis

Hydrocarbon exhaust gases containing residual amounts of oxygen may pose challenges for their conversion into value added chemicals downstream, because oxygen may affect the process. This could be avoided by plasma treating the exhaust to convert O2 in presence of hydrocarbons into CO or CO2 on demand. The underlying reaction mechanisms of plasma conversion of O2 in the presence of hydrocarbons are
analysed in a model experiment using a radio frequency atmospheric pressure helium plasma in a plug flow design with admixtures of O2 and of CH4. The plasma process is analysed with infrared absorption spectroscopy to monitor CH4 as well as the reaction products CO, CO2 and H2O. It is shown that the plasma reaction for oxygen (or methane removal) is triggered by the formation of oxygen atoms from O2 by electron.
Oxygen atoms are eciently converted into CO, CO2 and H2O with CO being an intermediate in that reaction sequence. However, at very high oxygen admixtures to the gas stream, the conversion efficiency saturates because electron induced O2 dissociation in the plasma seems to be counterbalanced by a reduction of the efficiency of electron heating at high admixtures of O2. The impact of a typical industrial manganese oxide catalyst is evaluated for methane conversion. It is shown that the conversion effciency is enhanced by 15% to 20% already at temperatures of 430 K.

Publisher: 
EP2
Project: 
SFB 1316
Authors: 
T. Urbanietz
C. Stewig
M. Böke
A. von Keudell

Dedicated setup to isolate plasma catalysis mechanisms

Plasma catalysis is the combination of plasma and catalysis to reach an efficient conversion of molecules for flexible operating parameters and flexible feed gases. By combining plasmas with conventional thermal catalysis, the temperature windows may be different and the process may be insensitive to catalyst poisoning. The understanding of plasma catalysis mechanisms, however, is an extremely difficult task due to the strong coupling between plasma, gas phase chemistry and surface. A multitude of reaction pathways may be enhanced or reduced by the presence of a plasma that provides excited species as reaction partners. We developed a robust setup to analyse those processes based on a parallel plate atmospheric pressure plasma jet that allows a plug flow design. The plasma chemistry is analysed by Fourier transform infrared absorption spectroscopy and mass spectrometry. The electrodes in contact with the plasma are temperature controlled and can easily be replaced to apply a catalyst on top of them. The basic characteristics of the setup are discussed as well as three examples for its application are given (i) the analysis of methane oxidation using the plug flow scheme, (ii) the plasma catalytic conversion of CO2, and (iii) the plasma catalytic conversion of methane in methane oxygen mixtures.

Publisher: 
EP2
Project: 
SFB 1316
Authors: 
C. Stewig
T. Urbanietz
L. Chauvet
M. Böke
A. von Keudell

Plasma‐driven in situ production of hydrogen peroxide for biocatalysis

Peroxidases and peroxygenases are promising classes of enzymes for biocatalysis because of their ability to carry out one‐electron oxidation reactions and stereoselective oxyfunctionalizations. However, industrial application is limited, as the major drawback is the sensitivity toward the required peroxide substrates. Herein, we report a novel biocatalysis approach to circumvent this shortcoming: in situ production of H2O2 by dielectric barrier discharge plasma. The discharge plasma can be controlled to produce hydrogen peroxide at desired rates, yielding desired concentrations. Using horseradish peroxidase, it is demonstrated that hydrogen peroxide produced by plasma treatment can drive the enzymatic oxidation of model substrates. Fungal peroxygenase is then employed to convert ethylbenzene to (R)‐1‐phenylethanol with an ee of >96 % using plasma‐generated hydrogen peroxide. As direct treatment of the reaction solution with plasma results in reduced enzyme activity, the use of plasma‐treated liquid and protection strategies are investigated to increase total turnover. Technical plasmas present a noninvasive means to drive peroxide‐based biotransformations.

Publisher: 
Applied Microbiology
Project: 
SFB 1316
Authors: 
A. Yayci
A. Gomez Baraibar
M. Krewig
E. Fernandez Fueyo
F. Hollmann
M. Alcaide
R. Kourist
J. Bandow