Plasma Chemistry at Work: efficient plasma-assisted fuel conversion through control of vibrational excitation
See also our paper Faraday Discussions 178 (2015) 233-248
DIFFER's NFC-group researches in collaboration with R. Engeln from Eindhoven University of Technology and G. Berden from Radboud University the fundamental understanding of plasma chemical dissociation of CO2 to create a scientific basis for the development of a generic plasma chemical route as the first step in CO2-neutral fuel production. It comprises of a combined experimental and modeling programme to unravel the role and dynamics of vibrational excitation (i.e. of putting the energy preferably in the bond that is to be broken) in relation to electronic excitation (a possible shortcut for efficiently overcoming the energy barrier in CO2 dissociation).
The role of vibrational excitation is studied by perturbation of the vibrational distribution through resonant infrared laser pulses (FELIX facility, RUN). The main absorption band of CO2 is centred around 2350 cm-1 (4.26 µm), which is in the range of the infrared Free Electron Laser FELIX. The FEL can supply 20 mJ per pulse that results in a momentary power of 4 kW, which is enough to significantly perturb the distribution of the vibrational modes in the steady state plasma. A combination of advanced diagnostics is then used to measure the effect of the vibrational perturbation.
A comprehensive suite of advanced diagnostics with high spatial and temporal resolution is employed to probe all relevant parameters of the dissociation process. Active laser spectroscopy is used to measure temperature (Rayleigh-, Raman scattering) and CO-density (Laser Induced Fluorescence) in the plasma. Infrared absorption is used to measure CO-densities in the exhaust (FTIR, QCL) and to map the evolution of vibrational population in the plasma over time (StepScan FTIR, OPO). In a new collaboration with Frans Harren of the Trace Gas Research group of Radboud University, a state-of-the-art IR Dual Frequency Comb will be used to measure the vibrational distribution in the plasma with unprecedented temporal and spectral resolution.
Apart from measuring the vibrational properties of the plasma, enhanced control the plasma parameters is necessary to increase the energy efficiency of the process. A fine control over the Electron Energy Distribution Function will be achieved through the addition of alkali metal impurities. By controlling the EEDF the selectivity of reaction processed can be tailored, enabling optimization for the most efficient reaction pathways.
The goal of the research is to provide the fundamental insights required for scaling up the plasma approach to industrially relevant throughputs. This means migrating to atmospheric conditions with high conversion efficiencies, whilst maintaining high energy efficiency.
Non-oxidative coupling of methane via plasma catalysis
This projects is in collaboration with the group of L. Lefferts at the University of Twente and studies methyl radicals interaction with a catalyst surface in a plasma-assisted reactor to achieve non-oxidative coupling of methane. It aims at a novel approach to heterogeneous catalysis in which plasma is used to activate inert species and the catalyst action focusses on product formation (or moderation).
Methyl radical production is the rate determining step in coupling of methane to larger hydrocarbons. Non-thermal plasma is used to produce methyl radicals without the presence of oxygen, short-circuiting by-product formation. In the same reactor, catalyst surfaces are used to influence recombination and quenching of the radicals and to change the reaction pathways to selectively produce ethane, ethylene and propylene. Thus, we decouple the formation of (plasma) radicals from consecutive conversion of those radicals to address the selectivity issue. Resonantly enhanced multi-photon ionization, REMPI is used to quantify the methyl radical production and to investigate the relationship between the catalyst structure (surface area, porosity) and conversion/selectivity. Effluent gas measurements, infrared-spectroscopy (FTIR) and mass spectrometry (MS) are used to measure effluent gases and characterize the reactor performance.
Nitrogen fixation using non-equilibrium plasmas
The NFC (through FOM Institute DIFFER) is an active member of European wide consortium of industry and academia called MAPSYN which aims at commercializing plasma for industrial scale nitrogen fixation.