Oxidation Mechanisms of CF₂Br₂and CH₂Br₂Induced by Air Nonthermal Plasma

Abstract

Oxidation mechanisms in air nonthermal plasma (NTP) at room temperature and atmospheric pressure were investigated in a corona reactor energized by +dc, -dc, or +pulsed high voltage.. The two bromomethanes CF(2)Br(2) and CH(2)Br(2) were chosen as model organic pollutants because of their very different reactivities with OH radicals. Thus, they served as useful mechanistic probes: they respond differently to the presence of humidity in the air and give different products. By FT-IR analysis of the postdischarge gas the following products were detected and quantified: CO(2) and CO in the case of CH(2)Br(2), CO(2) and F(2)C ═ O in the case of CF(2)Br(2). F(2)C ═ O is a long-lived oxidation intermediate due to its low reactivity with atmospheric radicals. It is however removed from the NTP processed gas by passage through a water scrubber resulting in hydrolysis to CO(2) and HF. Other noncarbon containing products of the discharge were also monitored by FT-IR analysis, including HNO(3) and N(2)O. Ozone, an important product of air NTP, was never detected in experiments with CF(2)Br(2) and CH(2)Br(2) because of the highly efficient ozone depleting cycles catalyzed by BrOx species formed from the bromomethanes. It is concluded that, regardless of the type of corona applied, CF(2)Br(2) reacts in air NTP via a common intermediate, the CF(2)Br radical. The possible reactions leading to this radical are discussed, including, for -dc activation, charge exchange with O(2)(-), a species detected by APCI mass spectrometry.

Pollutant Degradation in Gas Streams by means of Non-Thermal Plasmas

“…Also evident is the better performance of dc-with respect to dc+ corona. Analogous results were obtained in similar experiments with other VOCs, including n-hexane [18,20] and dibromomethane [24]. The better efficiency of pulsed+ corona with respect to dc corona of either polarity is consistent with the results of an emission spectroscopy study which showed that at any specific input energy significantly greater average electron energy is obtained with pulsed corona than with dc coronas [25].…”

“…An example is shown in Figure 5. Other similar cases are reported in the literature [14,24,[26][27][28][29] and have been interpreted based on a simple scheme of inhibition by the intermediates formed in the VOC reaction [30]. Finally, the VOC chemical composition and structure also matters and different VOCs are oxidized with different efficiencies under the same experimental conditions.…”

“…The production of HNO 3 is high and ozone is almost completely absent in corona discharge treatment of air containing bromo derivatives CH 2 Br 2 and CF 2 Br 2 , ( Figure 10 d and e). These observations have been explained considering reactions and catalytic cycles involving BrOx (x = 0, 1) and NOx (x = 1, 2) species [24], which have been extensively investigated as major contributors to the depletion of stratospheric ozone [42].…”

Atmospheric Pressure Non-thermal Plasma for Air Purification: Ions and Ionic Reactions Induced by dc+ Corona Discharges in Air Contaminated with Acetone and Methanol

“…Corona discharges are indeed a very convenient means to produce ions at atmospheric pressure and are widely used for this purpose in applications ranging from electrostatic precipitators to commercial instrumentation for mass spectrometric analyses. While the role of ions and ion-molecule reactions in non-thermal plasma based air purification treatments has been recognized and highlighted in several studies [10][11][12][13][14][15][16][17][18], the majority of mechanistic investigations on NTP induced degradation of VOCs neglects their reactions with ions. Ions have been indeed classified as "less important plasma particles", in contrast with neutral reactive species (atoms, radicals and excited molecules), in reason of their very fast reactions, orders of magnitude faster than radical-molecule reactions, and of the typically dilute concentrations of VOCs [8].…”

Petroleomics by Direct Analysis in Real Time-Mass Spectrometry

“…In all cases, a similar Gaussian profile of m/z 250 to 600 was observed. The oxidized HCs species were probably generated at high temperature and atmospheric pressure such as OH and OOH radicals in contact with HC compound and they produced oxidized HCs (Ox classes) [4].…”