Current Research

Project A7 - Catalysis

Catalytic carbon monoxide oxidation over \newline potassium-doped manganese dioxide nanoparticles synthesized by spray drying

One of the major objectives of the CRC 1316 is the investigation and understanding of the complex interactions between a non-thermal plasma and heterogeneous catalysts. The first results of project A7 describing the thermocatalytic oxidation of CO over MnO2 catalysts were recently published in Emission Control Science and Technology. Particularly, the effect of the incorporated alkali ions on K+ and Na+ on the structural properties and the catalytic performance was emphasized to derive structure-activity correlations.

The MnO2 catalysts were synthesized by a semi-continuous spray drying procedure based on the comproportionation reaction of Mn(NO3)2 and KMnO4. Solutions of both compounds were continuously mixed in a micromixer and the emerging suspension was rapidly quenched by spray drying to inhibit further particle growth. In order to exchange the K+ ions by Na+ ions, NaMnO4 was used instead of KMnO4 during the synthesis. After washing and drying of the catalysts a fine brown powder was obtained, which was used as prepared or calcined at 450°C or 500°C for 4 h in synthetic air.

As shown by the XPS results and TPO profiles Mn (IV) is the predominant oxidation state of all samples prior to calcination proving that all catalysts consist of MnO2. However, the XRD patterns of the uncalcined catalysts reveal an X-ray amorphous structure preventing a more in-depth phase identification. After calcination the phase structure strongly depends on the type and amount of the incorporated alkali ion. The presence of K+ promotes the formation of crystalline alpha-MnO2 and stabilizes its tunnel structure up to temperatures of 500°C. Lower amounts of K+ or the exchange with Na+ lead to less crystalline phases after calcination at 450°C and to the formation of crystalline alpha-Mn2O3 after calcination at 500°C.

The catalytic CO oxidation was performed in a microreactor set up equipped with a non-dispersive IR detector. All uncalcined catalysts revealed a similar catalytic performance regardless of the type or amount of the incorporated alkali ion. Even though the specific surface area of the catalyst decreased from 77 m2/g to 37 m2/g during calcination the pure alpha-MnO2 phase exhibited a superior catalytic activity. Over alpha-MnO2 the temperature at which full conversion was achieved was shifted towards lower temperatures by more than 100°C. In contrast the catalyst containing alpha-Mn2O3 show a catalytic activity similar to the uncalcined catalysts indicating that not only the higher degree of crystallinity but also the structural properties of alpha-MnO2 cause its high catalytic activity. The incorporation of K+ ions is required to stabilize the tunnel structure of alpha-MnO2

PLASMA RESEARCH, project B7

Lightning bolt underwater 

© RUB, Kramer

A plasma tears through the water within a few nanoseconds. It may possibly regenerate catalytic surfaces at the push of a button.

Electrochemical cells help recycle CO2. However, the catalytic surfaces get worn down in the process. Researchers at the Collaborative Research Centre 1316 “Transient atmospheric plasmas: from plasmas to liquids to solids” at Ruhr-Universität Bochum (RUB) are exploring how they might be regenerated at the push of a button using extreme plasmas in water. In a first, they deployed optical spectroscopy and modelling to analyse such underwater plasmas in detail, which exist only for a few nanoseconds, and to theoretically describe the conditions during plasma ignition. They published their report in the journal Plasma Sources Science and Technology on 4 June 2019.

A plasma tears through the water within a few nanoseconds. Following plasma ignition, there is a high negative pressure difference at the tip of the electrode, which results in ruptures forming in the liquid. Plasma then spreads across those ruptures.

Video: Experimentalphysik II

Plasmas are ionised gases: they are formed when a gas is energised that then contains free electrons. In nature, plasmas occur inside stars or take the shape of polar lights on Earth. In engineering, plasmas are utilised for example to generate light in fluorescent lamps, or to manufacture new materials in the field of microelectronics. “Typically, plasmas are generated in the gas phase, for example in the air or in noble gases,” explains Katharina Grosse from the Institute for Experimental Physics II at RUB.

Ruptures in the water

In the current study, the researchers have generated plasmas directly in a liquid. To this end, they applied a high voltage to a submerged hairline electrode for the range of several billionth seconds. Following plasma ignition, there is a high negative pressure difference at the tip of the electrode, which results in ruptures forming in the liquid. Plasma then spreads across those ruptures. “Plasma can be compared with a lightning bolt – only in this case it happens underwater,” says Katharina Grosse.

Hotter than the sun

Using fast optical spectroscopy in combination with a fluid dynamics model, the research team identified the variations of power, pressure, and temperature in these plasmas. “In the process, we observed that the consumption inside these plasmas briefly amounts to up to 100 kilowatt. This corresponds with the connected load of several single-family homes,” points out Professor Achim von Keudell from the Institute for Experimental Physics II. In addition, pressures exceeding several thousand bars are generated – corresponding with or even exceeding the pressure at the deepest part of the Pacific Ocean. Finally, there are short bursts of temperatures of several thousand degrees, which roughly equal and even surpass the surface temperature of the sun.

Water is broken down into its components

Such extreme conditions last only for a very short time. “Studies to date had primarily focused on underwater plasmas in the microsecond range,” explains Katharina Grosse. “In that space of time, water molecules have the chance to compensate for the pressure of the plasma.” The extreme plasmas that have been the subject of the current study feature much faster processes. The water can’t compensate for the pressure and the molecules are broken down into their components. “The oxygen that is thus released plays a vital role for catalytic surfaces in electrochemical cells,” explains Katharina Grosse.  “By re-oxidating such surfaces, it helps them regenerate and take up their full catalytic activity again. Moreover, reagents dissolved in water can also be activated, thus facilitating catalysis processes.”

By Meike Drießen, Translated by Donata Zuber
RECENT RESEARCH ACHIEVEMENT, project B8

How bacteria protect themselves from plasma treatment 

© Daniel Sadrowski

Plasmas are applied in the treatment of wounds to combat pathogens that are resistant against antibiotics. But bacteria know how to defend themselves.

Considering the ever-growing percentage of bacteria that are resistant to antibiotics, interest in medical use of plasma is increasing. In collaboration with colleagues from Kiel, researchers at Ruhr-Universität Bochum (RUB) investigated if bacteria may become impervious to plasmas, too. They identified 87 genes of the bacterium Escherichia coli, which potentially protect against effective components of plasma. “These genes provide insights into the antibacterial mechanisms of plasmas,” says Marco Krewing. He is the lead author of two articles that were published in the Journal of the Royal Society Interface this year.

A cocktail of harmful components stresses pathogens

Plasmas are created from gas that is pumped with energy. Today, plasmas are already used against multi-resistant pathogens in clinical applications, for example to treat chronic wounds. “Plasmas provide a complex cocktail of components, many of which act as disinfectants in their own right,” explains Professor Julia Bandow, Head of the RUB research group Applied Microbiology. UV radiation, electric fields, atomic oxygen, superoxide, nitric oxides, ozone, and excited oxygen or nitrogen affect the pathogens simultaneously, generating considerable stress. Typically, the pathogens survive merely several seconds or minutes.

In order to find out if bacteria, may develop resistance against the effects of plasmas, like they do against antibiotics, the researchers analysed the entire genome of the model bacterium Escherichia coli, short E. coli, to identify existing protective mechanisms. “Resistance means that a genetic change causes organisms to be better adapted to certain environmental conditions. Such a trait can be passed on from one generation to the next,” explains Julia Bandow.

Mutants missing single genes

For their study, the researchers made use of so-called knockout strains of E. coli. These are bacteria that are missing one specific gene in their genome, which contains approximately 4,000 genes. The researchers exposed each mutant to the plasma and monitored if the cells kept proliferating following the exposure.

“We demonstrated that 87 of the knockout strains were more sensitive to plasma treatment than the wild type that has a complete genome,” says Marco Krewing. Subsequently, the researchers analysed the genes missing in these 87 strains and determined that most of those genes protected bacteria against the effects of hydrogen peroxide, superoxide, and/or nitric oxide. “This means that these plasma components are particularly effective against bacteria,” elaborates Julia Bandow. However, it also means that genetic changes that result in an increase in the number or activity of the respective gene products are more capable of protecting bacteria from the effects of plasma treatment.

Heat shock protein boosts plasma resistance

The research team, in collaboration with a group headed by Professor Ursula Jakob from the University of Michigan in Ann Arbor (USA), demonstrated that this is indeed the case: the heat shock protein Hsp33, encoded by the hslO gene, protects E. coli proteins from aggregation when exposed to oxidative stress. “During plasma treatment, this protein is activated and protects the other E. coliproteins – and consequently the bacterial cell,” Bandow points out. An increased volume of this protein alone results in a slightly increased plasma resistance. Considerably stronger plasma resistance can be expected when the levels of several protective proteins are increased simultaneously.

By Meike Drießen, Translated by Donata Zuber
Project B7 - Nanosecond plasmas

Extreme Conditions in Plasma in Liquids

Nanosecond plasmas in liquids play an important role in the field of decontamination, electrolysis or plasma medicine. The understanding of these very dynamic plasmas requires information about the temporal variation of species densities and temperatures. This is analyzed by monitoring nanosecond plasmas that are generated by high voltages (HV) between 14 kV and 26 kV and pulse lengths of 10 ns applied to a tungsten tip with 50 µm diameter immersed in water. Ignition of the plasma causes the formation of a cavitation bubble that is monitored by shadowgraphy to measure the dynamic of the created bubble and the sound speed of the emitted acoustic waves surrounding this tungsten tip.

The temporal evolution of the bubble size is compared with cavitation theory yielding good agreement for an initial bubble radius of 25 µm with an initial pressure of GPa at a temperature of 1200 K for a high voltage of 18 kV. This yields an initial energy in the range of a few 10-5 J that varies with the applied high voltage. The dissipated energy by the plasma drives the adiabatic expansion of water vapor inside the bubble from its initial supercritical state to a low pressure, low temperature state at maximum bubble expansion reaching values of 10^3 Pa and 50 K, respectively. These predictions from cavitation theory are corroborated by optical emission spectroscopy (OES). After igniting the nanosecond plasma, the electrical power oscillates in the feed line between HV pulser and plasma chamber with a ring down time of the order of 60 ns. These reflected pulses re-ignite a plasma inside the expanding bubble periodically. Broadband emission due to recombination and Bremsstrahlung becomes visible within the first 100 ns. At later times, line emission dominates. Stark broadening of the spectral lines of H_alpha (656\,nm) and OI (777\,nm) is evaluated to determine both the electron density and the electron temperature in these re-ignited plasmas.

Project A3 - Excitation Transfer

Non-equilibrium excitation of CO2 in an atmospheric pressure helium plasma jet

First results on the non-equilibrium excitation and dissociation of CO2 in an atmospheric pressure helium RF plasma jet. The objective of the project A3 in the SFB 1316 and the BMBF project Carbon2Chem is the separation of plasma and surface chemistry studied for the example of CO2 plasma excitation admixed to the noble gas within the plasma jet. This method offers the possibility to control the gas temperature of the feed gas as well as the molecule excitation by low energy electrons or by Penning collisions with the excited noble gas atoms or dimers. The plasma jet is driven with varying absorbed plasma power and admixture levels of CO2 . The excitation of CO2 is monitored by in-situ set-up of Fourier-Transform Infrared Spectroscopy. Concetrations of CO2 and the produced CO are analysed. Furthermore, the vibrational and rotational temperatures of the possible degrees of freedom of the measured molecules are determined.

The gas feed of the atmospheric pressure plasma jet was helium, because of the large mass difference and, there- fore, poor momentum transfer to CO2. This results in the smallest collisional quenching of all noble gases. The plane parallel plasma jet is driven with RF. These kind of plasma source is already very well investigated in respect to their plasma physics and chemistry for different gas mixtures of noble gases and molecules.

Comsol Simluation of the He Flow Pattern and CO2 dissociation

The main result of this work is the clear non-equilibrium excitation of CO2 and CO. In detail, the rotational tem- perature of CO is below 400 K and, in contrast to this, the vibrational temperature reached values up to 1600 K, and the temperature of the excitation of the asymmetric vibration of CO2 is about 700 K. The impact of variable plasma power and admixture of CO2 to the He gas flow is rather weak. It is assumed that the vibrational and rotational excitation of CO mainly originates from the dis- sociation reaction either by direct electron impact of CO2 or by Penning dissociation between CO2 and excited helium metastables. From this electronic energy transfer to CO2, highly vibrational excited CO molecules are pro duced by dissociation.

The non-equilibrium is due to the nature of excitation of molecules by collisions with electrons with energies larger than 7 eV at the oscillating sheath edges and Penning collisions with excited heilum atoms. The low rotational gas temperature is explained by the helium plasma gas which acts as a buffer.

This non-equilibrium character offers more investigations within the field of plasma catalysis, through which the reaction rate of a desired catalytic reaction is supported. Moreover, it is important to ensure the en- hancement of the reaction rate through the impact of excited molecules and not due to an unintentional heating of the catalyst surface by the plasma itself.

In the future, the experiments will be extended to other gas mixtures and the impact of catalytically active surfaces will be explored.

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