Thomas Godfroid

Optical characterization of a microwave pulsed discharge used for dissociation of CO2

Conversion of CO2 into CO and O is studied in a flowing gas surfaguide pulsed microwave discharge operating with CO2 and CO2 + N2 gas mixtures under different conditions. Optical emission spectroscopy, including actinometry (using N2), vibrational (N2 molecule) and rotational (CO and N2 molecules) analysis are utilized. Both time- and space-resolved measurements are performed. The results show the essential changes of the CO2 conversion rate, its energetic efficiency, and the gas and vibrational temperatures along the gas flow direction in the discharge. The spatial distribution of the power absorbed in the plasma is analyzed. It is also confirmed that the vibrational excitation is a key factor in the CO2 dissociation process in this type of plasma. It is suggested that the obtained dissociation rates can be further optimized by varying the gas composition, as well as the power applied to the discharge.

Functionalization of carbon nanotubes by atomic nitrogen formed in a microwave plasma Ar + N2 and subsequent poly(ε-caprolactone) grafting

Multi-walled carbon nanotubes (MWNTs) are placed under atomic nitrogen flow formed through an Ar + N2 microwave plasma in order to functionalize covalently their side walls with nitrogen-containing groups. The MWNT surface analyzed by X-ray photoelectron spectroscopy shows the presence of amides, oximes and mainly amine and nitrile functions grafted in this way. In order to highlight the actual location of the amine functions grafted on MWNTs, they were considered as initiation species in ring-opening polymerization of e-caprolactone using triethylaluminium as activator. The so-generated poly(e-caprolactone) chains remain grafted on the MWNTs via amide bonds and form polyester islets along the nanotubes surface. TEM images of these MWNT surfaces grafted with poly(e-caprolactone) show a good amino-sidewall distribution. This work demonstrates the side-wall amino-functionalization of carbon nanotubes readily achieved by microwave plasma with the possibility to reach within a short time period very high contents in nitrogen-based functions (∼10 at.%).

Functionalization of MWCNTs with atomic nitrogen.

In this study of the changes induced by exposing MWCNTs to a nitrogen plasma, it was found by HRTEM that the atomic nitrogen exposure does not significantly etch the surface of the carbon nanotube (CNT). Nevertheless, the atomic nitrogen generated by a microwave plasma effectively grafts amine, nitrile, amide, and oxime groups onto the CNT surface, as observed by XPS, altering the density of valence electronic states, as seen in UPS.

Time-resolved gas temperature evolution in pulsed Ar–N2 microwave discharge

Temporal evolution of the gas temperature (Tg) in a pulsed microwave surfaguide discharge is studied by measuring the N2 rotational temperature. We found that at high power applied per pulse, gas temperature grows linearly, and saturates after about 150 μs. This effect is absent at low power values, or at short pulse durations. Observed Tg behavior correlates with time-resolved measurements of the N2 vibrational temperature, as well as with N emission lines. Consequently, Tg time behavior was related to N atoms production in plasma. Using obtained Tg growth rates, the effective power used for plasma heating is determined.

Simple method for gas temperature determination in CO 2 -containing discharges

A simple gas temperature determination method based on the line ratio between two rotational peaks from the CO Angstrom rotational emission band is reported. A formula based on CO spectral synthesis provides a way for temperature control in plasmas containing CO molecules. This approach is validated in a CO2 flowing gas surfaguide microwave discharge operating at 2.45 GHz. The gas temperature results are compared with the ones obtained using a Boltzmann plot approach, as well as using direct comparison of measured and calculated rotational spectra of the same rotational band.

Study of Ar and Ar-CO2 microwave surfaguide discharges by optical spectroscopy

A surfaguide microwave discharge operating at 2.45 GHz in Ar and Ar-CO2 mixtures is studied using diagnostics methods based on optical emission spectroscopy. The population densities of Ar metastable and resonant states of the lowest group of excited levels ( 1sx) are investigated for several experimental conditions using the self-absorption technique. It is found that the densities of these levels, ranging from 1017 to 1016 m−3 for the pure Ar case, are dependent on the discharge pressure and applied power. The electron temperature and electron density are calculated via the balances of creation/loss mechanisms of radiative and metastable levels. In the range of the studied experimental conditions (50–300 W of applied power and 0.5–6 Torr of gas pressure), the results have shown that lower values of electron temperature correspond to higher values of power and pressure in the discharge. Adding CO2 to the argon plasma results in a considerable decrease (about 3 orders of magnitude) of the Ar metastable at...

Study of CO2 decomposition in microwave discharges by optical diagnostic methods

The increasing of the carbon dioxide (CO2) release into the atmosphere is undeniably one of the biggest concerns for the twenty-first century. Among the different strategies proposed for reduction of CO2 emission (carbon capture and sequestration (CCS), renewable energies, etc.), low-temperature plasma technology offers an alternative and rather efficient way to convert CO2 into the valuables chemicals (e.g. syngas) which can be stored and used afterwards. Several CO2 decomposition plasma-related approaches have been proposed in the literature, all having a main task: increasing the energy efficiency associated to the decomposition process, while keeping the conversion rate at reasonably high level. This task is especially challenging since many kinetic mechanisms of CO2 decomposition in low-temperature discharges are not yet well-known, such as the vibrational excitation which plays a key role in achieving high decomposition rates. In this chapter our recent research efforts associated with the experimental study of the CO2 decomposition in microwave surfaguide low-temperature discharges are presented. The research was focused on the systematic investigation of the basic plasma parameters. The discharge area of the reactor was characterized by optical emission spectroscopy using the light emitted from spontaneous relaxation of excited species in plasma. The critical parameters such as gas temperature and dissociation rate were evaluated. In addition to this, the post-discharge area was characterized by two-photon absorption laser-induced fluorescence and gas chromatography techniques in order to investigate the exhaust gas composition. All together, the results overviewed in this chapter provide interesting insights into different kinetic mechanisms of CO2-containing discharges, which play an important role in the CO2 decomposition process.

Ion density evolution in a high-power sputtering discharge with bipolar pulsing

Time evolution of sputtered metal ions in high power impulse magnetron sputtering (HiPIMS) discharge with a positive voltage pulse applied after a negative one (regime called “bipolar pulse HiPIMS”—BPH) is studied using 2-D density mapping. It is demonstrated that the ion propagation dynamics is mainly affected by the amplitude and duration of the positive pulse. Such effects as ion repulsion from the cathode and the ionization zone shrinkage due to electron drift towards the cathode are clearly observed during the positive pulse. The BPH mode also alters the film crystallographic structure, as observed from X-ray diffraction analysis.Time evolution of sputtered metal ions in high power impulse magnetron sputtering (HiPIMS) discharge with a positive voltage pulse applied after a negative one (regime called “bipolar pulse HiPIMS”—BPH) is studied using 2-D density mapping. It is demonstrated that the ion propagation dynamics is mainly affected by the amplitude and duration of the positive pulse. Such effects as ion repulsion from the cathode and the ionization zone shrinkage due to electron drift towards the cathode are clearly observed during the positive pulse. The BPH mode also alters the film crystallographic structure, as observed from X-ray diffraction analysis.

Enhancing the Greenhouse Gas Conversion Efficiency in Microwave Discharges by Power Modulation

Scientific interest to the plasma-assisted greenhouse gas conversion continuously increases nowadays, as a part of the global Green Energy activities. Among the plasma sources suitable for conversion of CO2 and other greenhouse gases, the non-equilibrium (low-temperature) discharges where the electron temperature is considerably higher than the gas temperature, represent special interest. The flowing gas discharges sustained by microwave radiation are proven to be especially suitable for molecular gas conversion due to high degree of non-equilibrium they possess. In this Chapter the optimization of CO2 conversion efficiency in microwave discharges working in pulsed regime is considered. The pulsed energy delivery represents new approach for maximization of CO2 conversion solely based on the discharge “fine-tuning”, i. e. without the additional power expenses. In our work several discharge parameters along the gas flow direction in the discharge have been studied using various diagnostic techniques, such as optical actinometry, laser-induced fluorescence, and gas chromatography. The results show that CO2 conversion efficiency can be essentially increased solely based on the plasma pulse frequency tuning. The obtained results are explained by the relation between the plasma pulse parameters and the characteristic time of the relevant energy transfer processes in the discharge.

Role of plasma catalysis in the microwave plasma-assisted conversion of CO2

Climate change and global warming caused by the increasing emissions of greenhouse gases (such as CO 2 ) recently attract attention of the scientific community. The combination of plasma and catalysis is of great interest for turning plasma chemistry in applications related to pollution and energy issues. In this chapter, our recent research efforts related to optimization of the conversion of CO 2 and CO 2 /H 2 O mixtures in a pulsed surface‐wave sus‐ tained microwave discharge are presented. The effects of different plasma operating condi‐ tions and catalyst preparation methods on the CO 2 conversion and its energy efficiency are discussed. It is demonstrated that, compared to the plasma‐only case, the CO 2 conversion and energy efficiency can be enhanced by a factor of ∼2.1 by selecting the appropriate con‐ ditions. The catalyst characterization shows that Ar plasma treatment results in a higher density of oxygen vacancies and a comparatively uniform distribution of NiO on the TiO 2 surface, which strongly influence CO 2 conversion and energy efficiencies of this process. The dissociative electron attachment of CO 2 at the catalyst surface enhanced by the oxygen vacancies and plasma electrons may explain the increase of conversion and energy efficien‐ cies in this case. A mechanism of plasma‐catalytic conversion of CO 2 at the catalyst surface in CO 2 and CO 2 /H 2 O mixtures is proposed.