## Materials and Methods  
### Experimental Set-Up  
A scheme of the experimental arrangements with the AC-PPP reactor used in this study is shown in Figure 1.  
For the manufacture of this reactor, two copper pipes (of 10 mm diameter) were placed axially on both sides of the reactor structure and were connected to the two ring disks (of 50 mm diameter). The pipes performed the role of both gas inlet/outlets and electrodes which were connected to the AC power supply (0-22 kV). The two dielectric cylinders (of 50 mm diameter) were applied to fix these copper pipes. These dielectric cylinders were joined by a Pyrex tube. The axial distance between electrodes was kept at 10 mm (see Figure 2).  
Figure 1: Experimental arrangement.  
Figure 2: Photographic image and the 3D scheme of the AC-PPP reactor: (1) high voltage dielectric, (2) Pyrex tube, (3) copper disk, (4) copper pipe.  
A strong electric field was created between the copper electrodes when applying the AC high voltage. The applied voltage varied between 10-22 kV. In this way, a plasma was generated between the electrodes at atmospheric pressure.  
The gas entered via one pipe and exited by a separate one. The flow rate of CO\({}_{2}\) gas (purity > 99.5%) was low and constant (\(\sim\)0.15 SLM) to obtain improved control of the applied field. Therefore, the derived axial velocity could be neglected compared to other velocities, i.e., drift and diffusion velocities (\(V_{diff}>V_{drif}>>V_{axi}\)).  
The application of an electric potential between the two electrodes led to the formation of an axial electric field. Subsequent electron acceleration led to the ionization of background carbon dioxide. During the ionization process, free radicals, ions, and neutral atoms were created inside the AC-PPP reactor. The operating situation of the AC-PPP reactor for a flow rate of 0.15 SLM and an applied voltage fixed of 22 kV is shown in the photographic images in Figure 2.  
The consumed power in the discharge was measured with a current probe (Tektronix. Beaverton, OR, USA, TCP202) and a high voltage probe (Tektronix, P6015A) which were connected to an oscilloscope (HM1508).  
An HR4000 Ocean Optic spectrometer was used to detect the spectra and perform optical emission spectroscopy (OES). This spectrometer is a compact and flexible device with no moving parts and a linear CCD-array of 3648 pixels. It stores a full spectrum every millisecond with a wavelength range of 200-1100 nm. The optical features of this spectrometer include an input and output focal distance 101.6 mm, spatial groove density grating 2400 line/mm, slit width 5 um, and optical resolution of width \(\sim\)0.025 nm.  
A confocal quartz lens, with a diameter of 50 mm and focal length of 75 mm, was used to focus the emitted light of the plasma onto the optical fiber probe, which was placed on the focal point of the lens. This lens was focused on the highest intensity positions between electrodes (middle of radial positions, r = 15 mm). This configuration allowed the determination of species present in the plasma and enabled measurement of the plasma parameters, such as electron density, and the rotational, vibrational, and excitation temperatures in this zone.  
Gas chromatography was used to study the decomposition of CO\({}_{2}\) and the formation of CO and O\({}_{2}\) compounds. The feed and exhaust gases were analyzed using a compact-gas chromatograph (CGC) type GC, Agilent 6890 N, equipped with a flame ionization detector (FID) and the packed GC columns Molecular Sieve 139 (MS-139) and HayeSep type Q and N. The FID can evaluate hydrocarbons such as propane, acetylene, ethylene, ethane, and others. Moreover, a thermal detector connected by columns, was used to analyze the gas components such as CO\({}_{2}\), CO, O\({}_{2}\), etc.