In Situ Raman Spectroscopy of Plasma Electrochemical and Plasma Catalytic Reactors
Authors: Polprasarn Kasidapa , Pareek Pankaj , Sadi Dihya , Janda Mario , Guaitella Oliver , Pai David 
Published: August 2024

Plasma electrochemistry (PEC) has been under investigation for an expanding number of potential applications such as agriculture, water treatment, decarbonized chemical processing, and nanomaterials synthesis [1–4]. PEC in water leads to the formation of gas-phase species that undergo a cascade of processes in the liquid to produce aqueous species such as OH, H2O2, ONOOH, NO2
-, and NO3
- that are sought after in these applications. Plasma-assisted catalysis (PAC) is another growing research field for some of the same applications and others that occur in the gas phase, such as industrial catalyst decoking, VOC abatement, gas reforming, and CO2 conversion [5]. For such problems, popular catalysts include zeolites, TiO2, and CeO2 [6]. Similar to PEC, in PAC the operating conditions for catalysis become more relaxed (e.g. lower temperature to activate the catalyst). There can be plasma-catalyst synergistic effects that improve the process yield well beyond that using plasma or catalyst alone.
In both PEC and PAC, the plasma produces energetic photons, electrons, ions, and excited species, and enhances the electric field near plasma-liquid and plasma-solid interfaces. These strongly non-equilibrium conditions enable the plasma to perform the required processes for these applications in many cases in a single step and without assistance, compared to conventional methods that can require multiple steps, potentially toxic chemical agents (e.g. acids, reducing agents), and/or external heating.
A major challenge facing the development of PEC- and PAC-related applications is the direct and detailed experimental investigation of the plasma-liquid and plasma-catalyst interfacial regions, respectively. The majority of existing liquid- and solid-phase diagnostics must be performed ex situ, removed from the plasma reactor and after treatment. To extend the scope of diagnostics, we employ an in situ approach using multiple diagnostic techniques to study a wide range of physical and chemical properties at the plasma-water interface. The centrepiece of this platform is in situ spontaneous Raman microspectroscopy, which is advantageous because of its non-intrusiveness, selectivity, versatility, and straightforward calibration. Shaping the laser beam into a light sheet enables probing of the interfacial region with micron-scale spatial resolution. For PEC in pure water, in situ Raman spectra showed that the concentrations of aqueous H2O2 and NO3
- both exceed those of the bulk liquid at a depth of a few tens of microns from the plasma-liquid interface [7].
We will focus on transient changes to Raman spectra that appear in the presence of plasma, then disappear when the plasma is switched off. Such changes are observable in situ, not ex situ, and indicate the energy exchange from the plasma to water or the catalyst surface. The general approach will be to establish relationships between plasma properties and changes to the Raman spectra. Two primary cases will be studied. First, we will study PEC in air plasma-water systems at atmospheric pressure, using both batch reactor and electrospray configurations. We will focus on the spectral profile of the –OH stretch band of water and of probe molecules such as NO3
-. Analysis of the –OH stretch band reveals that the plasma weakens the hydrogen bonding network of water. To help pinpoint the cause, we will track the broadening of the N-O symmetric stretch mode (v
1) of NO3
- at less than 20 µm depth from the plasma-liquid interface. Second, we will investigate a PAC reactor consisting of a low- to medium-pressure CO2 plasma in contact with CeO2 as a catalyst. In this case, the catalyst temperature will be tracked using the Stokes-to-anti-Stokes ratio of the Raman intensities of the first-order optical phonon of CeO2, as well as the spectral shift of this spectral feature.

Acknowledgments

Financial support: Campus France Franco-Thai PhD fellowship (French Embassy in Thailand, E4C Engie sponsorship), IP Paris Energy4Climate “E4C” Interdisciplinary Center, Campus France PHC Stefanik project (49885SJ), CNRS-IEA “GRAFMET”, EUR PLASMAScience, Fédération Plas@Par, ANR grant ANR-15-CE06-0007-01

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