Article pubs.acs.org/est
Oxidation Mechanisms of CF2Br2 and CH2Br2 Induced by Air Nonthermal Plasma Milko Schiorlin, Ester Marotta, Marta Dal Molin, and Cristina Paradisi* Department of Chemical Sciences, Università di Padova, via Marzolo 1, 35131 Padova, Italy 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 CF2Br2 and CH2Br2 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: CO2 and CO in the case of CH2Br2, CO2 and F2CO in the case of CF2Br2. F2CO 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 CO2 and HF. Other noncarbon containing products of the discharge were also monitored by FT-IR analysis, including HNO3 and N2O. Ozone, an important product of air NTP, was never detected in experiments with CF2Br2 and CH2Br2 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, CF2Br2 reacts in air NTP via a common intermediate, the CF2Br radical. The possible reactions leading to this radical are discussed, including, for −dc activation, charge exchange with O2−, a species detected by APCI mass spectrometry.
1. INTRODUCTION Air quality control is a major issue of our time, which stimulates the search of ever more efficient and “greener” remediation procedures. According to green chemistry concepts and principles, air remediation should achieve mineralization of any organic carbon contaminant with as limited as possible wastes and energy costs.1 It is well-known indeed that often oxidation intermediates are worse pollutants than the original precursor, as is the case, for example of various acyl derivatives formed from saturated hydrocarbons.2 Advanced oxidation processes (AOPs), which take advantage of the high oxidizing power of reactive species obtained from molecular oxygen or water, are the obvious choice. One promising approach exploits nonthermalizing electrical discharges, like corona and dielectric barrier discharges, in ambient air at room temperature and atmospheric pressure to produce high energy electrons in a gas which remains virtually at room temperature.3−5 The ensuing electron-molecule interactions generate highly reactive nonthermal plasmas (NTP, also called nonequilibrium plasmas), which are strongly oxidizing environments due to the presence of ozone and, most importantly, of short-lived oxygen-based species, such as O atoms, OH and OOH radicals, and ions (O2+, H3O+, O2−, O3−).4 A schematic view of the relevant processes and species involved is offered in Scheme 1. Many of these species can be detected and determined by means of direct spectroscopic analysis and of indirect chemical probes. In our own research, we are using optical emission spectroscopy to monitor short-lived excited species, like N2+ (which provides information about the plasma electronic, vibrational and rotational temperatures) and O atoms;6,7 mass © 2012 American Chemical Society
spectrometry to study negative and positive ions including, most interestingly, species derived from organic pollutants present in the system;7−10 FT-IR to determine ozone.11,12 The direct observation of OH radicals requires more sophisticated methods like laser induced fluorescence.13,14 However OH radicals can also be determined by an indirect method on the basis of their reaction with CO.15,16 The efficiency and selectivity of a discharge induced AOP depend on many factors:4 the reactor used, especially insofar as the electrode configuration, material constitution and reciprocal distance (interelectrode gap) are concerned; the type of energization used (dc, ac, pulsed high voltage supply; positive or negative polarity); the amount of humidity present in the processed air; the chemical nature of the volatile organic compound (VOC) being treated and its initial concentration. Optimization of efficiency and selectivity requires therefore an integrated approach and a multidisciplinary effort spanning from the engineering of the device to the understanding of the physics and chemistry involved. To gain insight into the chemical mechanisms of discharge induced AOPs we have developed a reactor capable of sustaining different corona regimes and are using it to compare their performance in the oxidative processing of selected model VOCs.11,12,16,17 In designing the reactor and auxiliary apparatus, our major goal was not the optimization of energy efficiency but rather the Received: Revised: Accepted: Published: 542
September 1, 2012 November 26, 2012 November 28, 2012 November 28, 2012 dx.doi.org/10.1021/es303561n | Environ. Sci. Technol. 2013, 47, 542−548
Environmental Science & Technology
Article
Scheme 1
2. EXPERIMENTAL SECTION 2.1. Chemicals. “Pure air” used in the experiments was a synthetic mixture (80% nitrogen/20% oxygen) from Air Liquide with specified impurities of H2O (
