Non-equilibrium processes are the basis of a multitude of phenomena in nature such as transport, excitation of atoms and molecules and de-excitation and dissipation at surfaces. The non-equilibrium character of plasmas is especially pronounced due to the high energy density in these systems and the very selective excitation of, for example, only the electrons. If these plasmas are brought into contact with solids or liquids, the non-equilibrium character can be transferred to other states of matter. An excellent example are plasma chemistry processes that are directly coupled to catalytically active surfaces.
The use of non-equilibrium atmospheric pressure plasmas is most interesting since they can most easily be combined with standard chemical processes. The non-equilibrium character of these plasmas can be controlled by large gas flows or by short pulsed excitation assuring strong cooling mechanisms. Thereby, a huge variety of desired plasma chemistries or emission patterns can be adjusted following an empirical strategy. However, any further progress is hampered by the lack of a fundamental understanding of those discharges and their interaction with fluid and solid interfaces leading to many open questions:
- How to reach and maintain a stable atmospheric pressure non-equilibrium discharge in a range of different gas mixtures?
- How to efficiently transport the species from the plasma to the object to be treated or coated?
- What are the chemical non-equilibrium synthesis routes of new materials or species?
- How does the transfer of species and energy occur on the nanosecond timescale?
- What are the roles of gaseous, liquid, biological, and solid state catalysts in contact with those plasmas?
The Collaborative Research Centre (CRC) 1316 “Transient atmospheric plasmas – from plasmas to liquids to solids” addresses these research questions by combining expertise in plasma physics, surface physics, chemistry, biotechnology, and engineering. This CRC focuses on transient atmospheric plasmas at varying spatial and temporal scales for the nanostructuring and activation of catalytic surfaces, for the coupling to catalysis and biocatalysis, as well as for electrochemical processes. Due to the strong interaction between these plasmas and the confining interfaces, special in-situ, real-time, and in-operando methods will be employed. The research program follows three consecutive phases with reaching a basic understanding at the beginning to the optimum integration of plasma and active surface, until the up-scaling of these plasmas. The CRC 1316 seeks optimal solutions for systems for energy conversion (solar fuels, CO2 harvesting, photocatalysis), to health (removal of volatile organic compounds from air streams), for biotechnology (plasma-driven biocatalysis), and for technical chemistry (bottom-up synthesis from small molecules to valuable chemicals).
Project Area A Transient Plasmas
Project Area A addresses the fundamentals of non-equilibrium transient atmospheric plasmas on timescales between nanoseconds and seconds. This is illustrated in Figure 1.8 by describing the important timescales for the various physical systems such as plasma excitation on the nanosecond scale, the conversion of the electronic excitation in ro-vib states on the nanosecond to microsecond scale and the possible quenching by gas phase collisions. Finally, the species follow the flow patterns to the confining surfaces on the timescale of milliseconds to seconds. Those questions need to be addressed by combining expertise in the Physics of transient plasmas, atomic and molecular physics covering the excitation and de-excitation as well as the non-equilibrium chemistry and surface physics of the interaction of those species with for example catalyst surfaces.
The projects A1, A2, A3 cover the electronic, vibrational and rotational excitation of the species in ns-plasmas and RF plasmas. Project A4 deals with plasma excitation using variable waveform tailoring in RF driven plasma jets using microstuctured electrodes. Projects A5 and A6 address the transfer of ns-plasmas to dielectric barrier discharges either as 2D-filamentary discharges or as controllable plasma pixel arrays. A7 covers the Transfer of excitation including the interaction with catalytic surfaces. A8 and A9 address the modelling of the nonequilibrium chemistry and the transport of species.
Proect Area B - Plasma Interfaces
Project Area B ”Plasma-Liquid-Solid Interfaces” addresses the fundamentals of non-equilibrium transient atmospheric plasmas on spatial scales between nanometre and millimetre. This is illustrated in Figure 1.9 by describing the important length scales for the various physical systems such as nanometre sized reactive surface structures, triggering catalytic reactions on oxidised metals or functionalised carbon nanotubes to the formation of plasma streamers and plasmas in liquids on the micrometre scale. This is extended to the millimetre scale in plasma arrays or during plasma electrolytic oxidation in liquids. Those systems need to be addressed by combining expertise in surface physics, in physics of plasma liquid interaction, and in chemical Engineering.
Project Area B can be separated in two types of plasma interfaces, the plasma-solid interface and the plasma-liquid-solid interface. Plasma-solid interfaces are addressed in project B1 for the plasma induced Formation of catalytic nano structures, project B2 combines this process with external laser heating. In Project B3 alternative surfaces based on functionalised carbon nanotubes are explored by combining plasma induced grafting of reactive groups followed by atomic layer deposition of the catalyst itself. These systems are complemented by theoretical support for the exchange of excited plasma generated species with catalytic surfaces in project B4. Plasma-liquid-solid interfaces are addressed in project B5 for the case of plasma electrolytic oxidation (PEO), the modelling of those processes in project B6 and finally the fast analysis of the reaction chemistry at these interfaces in B7. The prospect of biocatalysis is studied project B8.