Advanced oxidation of organic pollutants in air non-thermal plasmas

Transcription

1 UNIVERSITÀ DEGLI STUDI DI PADVA Department of Chemical Sciences Advanced oxidation of organic pollutants in air non-thermal plasmas Ester Marotta and Cristina Paradisi PlasTEP, Berlin, December 5-6, 212

2 Air non-thermal plasma and environmental remediation Air non-thermal plasmas are strongly oxidizing environments they are useful means of activation of advanced oxidation processes for air and water remediation N 2, 2 ambient air (humid) HV discharge e high en photons N 2, 2 N 2 * 2 * N excitation/ionization/ electron attachment/ dissociation e 2 - N + - ion/molecule and atom/molecule reactions H H H our goal reactions with organic pollutants to gain better control of process features efficiency selectivity to C 2 no side products how? basic research study of mechanisms C 2

3 The core process reaction of an organic carbon radical with 2, as in natural mechanisms of tropospheric oxidation of VCs R C. + 2 R C. R C. R=H 2. H + C R = H, alkyl group, Cl, Br,... organic carbon peroxo organic radical radical oxo organic radical R. + C The key organic carbon radical intermediate can be formed from the VC via any of possible initiation steps including reactions with: - radicals and atoms - ions - electrons - molecules in excited states - photons Schiorlin M., Marotta E., Rea M., Paradisi C., Environ. Sci. Technol. 29, 43, 9386

4 Advanced oxidation of volatile organic compounds (VCs) in air non-thermal plasmas The mechanistic approach study of Efficiency of VC decomposition Products and intermediates of VC decomposition (GC/FID, GC/MS, GC/TCD, LC/MS, FT/IR, IC) Short-lived reactive species, neutral and charged (MS, ES, chemical reactivity probes) under different experimental conditions with regard to Power supply and corona regime VC type (hydrocarbons, halogen-containing VCs, -containing VCs, ) VC concentration level of humidity in the air processing of more VCs together

5 Corona reactor for Volatile rganic Compounds (VCs) treatment Wire-cylinder geometry (stainless steel, 4 cm i.d. x 6 cm) Power supply +dc ( +25 kv) -dc ( -25 kv) +pulsed (pulsed high voltage power with dc bias, frequency of pulses: -3 Hz) Monitoring of corona (visual observation, V and i measurements) Flow-through mode of operation (15-8 ml/min) n-line and off-line monitoring of reactant and products (FT/IR, GC/MS, GC/FID, GC/TCD, LC/MS, IC) ptical emission spectroscopy

6 aria aria Ions and of ion chemistry in atmospheric plasma APCI/MS (Atmospheric Pressure Chemical Ionization/MS) R1 Corona discharge (electrons) Ionization 1 s Primary ions (air) 2 +, N 2 +, 2,.. 5 s Reagent ions (water) H 3 + (H 2 ) n N + (H 2 ) n ion/molecule reactions with M 1 % 1 % 1 % Scan AP+ 1.33e4 Scan AP+ 3.14e4 Scan AP+ 631 m/z A. Donò, C. Paradisi, G. Scorrano Rapid Commun. Mass Spectrom. 1997, 11, E. Marotta and C. Paradisi J. Am. Soc. Mass Spectrom. 29, 2, and refs. therein.

7 Emission Spectroscopy in collaboration Emission with P. Sonato Spectroscopy & B. Zaniol CNR in -atmospheric Padova plasma reactor ptic, F = 5 mm Focus = 3 mm Quartz fibre optic 7 m Entrance Slit 5-1 μm PC CCD camera C-T spectrometer Spectral windows investigated: From 35 nm (H emission) To 78 nm (triplet from I) The region: 33 4 nm for the evaluation of T e, T v, T r from the N 2 and N 2 + molecular emission spectra B. Zaniol, M. Schiorlin, E. Gazza, E. Marotta, X. Ren, M. E. Puiatti, M. Rea, P. Sonato, C. Paradisi, Int. J. Pla. Env. Sci. Tech. 28, 2, Marotta E., Schiorlin M., Rea M., Paradisi C., J. Phys. D: Appl. Phys. 21, 12411

8 Efficiency of non-thermal plasma processing 1. [Toluene]/[Toluene] dc.43 L kj -1 In the literature there are different ways to express the process efficiency (k E, b, G, etc.) k E, energy constant, is preferred in mechanistic studies for analogy with chemical kinetics: large k E means high efficiency SIE (kj L -1 ) VC VC e ke SIE SIE = Specific Input Energy SIE can be changed by varying the applied voltage while maintaining constant the flow rate or by varying the flow rate while maintaining a constant applied voltage

9 [Toluene]/[Toluene] I 777nm triplet intensity (a.u.) electron temperature (ev) Process efficiency, k E, depends on power supply L kj -1 +dc dc +pulsed SIE (kj L -1 ).14 L kj L kj -1 Better efficiency of pulsed corona is consistent with higher electron temperature and density in plasma as revealed by ES measurements Schiorlin M., Marotta E., Rea M., Paradisi C., Environ. Sci. Technol. 29, 43, 9386 B. Zaniol, M. Schiorlin, E. Gazza, E. Marotta, X. Ren, M. E. Puiatti, M. Rea, P. Sonato, C. Paradisi, Int. J. Pla. Env. Sci. Tech. 28, 2, Marotta E., Schiorlin M., Rea M., Paradisi C., J. Phys. D: Appl. Phys. 21, ,5 3 2,5 2 1,5 1, , , PC DC+ +PC region DC- 1 pulsed dry air humid air,5 DC+ dry air + VC humid air + VC DC Energy density (kj/l) +DC &-DC region pulsed disch. DC+ disch. DC- disch Energy density (kj/l)

10 Process efficiency, k E, depends on VC For example, with dc: 1. n-hexane 5 ppm v [VC]/[VC] toluene 5 ppm v CH 2 Br 2 5 ppm v CF 2 Br 2 5 ppm v VC k E (LkJ -1 ) +DC DC +Pulsed n-hexane Toluene CH 2 Br CF 2 Br SIE (kj L -1 ) Air non-thermal PLASMA has high REACTIVITY but also some SELECTIVITY Marotta E., Schiorlin M., Rea M., Paradisi C., J. Phys. D: Appl. Phys. 21, Schiorlin M., Marotta E., Dal Molin M., Paradisi C. Environ. Sci. Technol. 212,

11 k E (L kj -1 ) Process efficiency, k E, depends on [VC] [CH 2 Br 2 ]/[CH 2 Br 2 ] Decomposition of CH 2 Br 2 in +DC with Dry Air CH 2 Br 2 1 ppm v CH 2 Br 2 1 ppm v SIE (kj L -1 ),2,175,15,125,1,75,5,25,,,25,5,75,1 1/[CH 2 Br 2 ] Efficiencies are to be compared at equal initial concentrations To interpret the dependence of efficiency on initial concentration it is helpful to use a kinetic model If energy input is constant in time one can shift from efficiency to kinetics SIE kj = P kws L F L SIE VC ke VC e VC e k t

12 The kinetic model of Slater and Douglas-Hamilton air X + VC I + X X d dt S k S k 1 k 2 d VC dt X (reactive species) I + other products Z + other products X VC k 2 X I k X VC 1 1 ke (L kj -1 ) /[CH 2 Br 2 ] S t k t VC VC VC e VC e VC e k E SIE predicts that k E increases with decreasing [VC] this prediction is generally confirmed by the experiments Slater and Douglas-Hamilton J. Appl. Phys. 1981, 52, 582

13 C ppm Absorbance Absorbance Abs Abs Product studies - n-line FT-IR analysis C ppm C ppm.4.3 T ue Jul 19 11:56:25 25 (GM T +2:) esano std n-hexane C-H n-hexane stretching T ue Jul 19 16:32:19 25 (GM T +2:) esano -19kV n-hexane -19 kv C C Wavenumbers (cm-1) 2 15 Wavenumbers (cm -1 ) C 2 -DC +DC +pulsed C -DC +DC +pulsed DC +DC 3 +pulsed [n-hexane]/[n-hexane] [n-hexane]/[n-hexane] [n-hexane]/[n-hexane] Marotta et al., Environ. Sci. Technol. (27)

14 area GC C ppm C ppm Sig. 2 in C:\HPCHEM\...\18LUG3.D Product studies - GC analysis with various detectors (MS, FID, TCD) hexane 4 acetaldehyde lower hydrocarbons propanaldehyde acetone butirraldehyde methanol ethanol hexanone 9 2-hexanone Time (min.) Propane and butane DC- DC+ +pulsed [n-hexane]/[n-hexane] Acetone -DC +DC +pulsed [n-hexane]/[n-hexane] Hexanone -DC +DC +pulsed [n-hexane]/[n-hexane] Marotta et al., Environ. Sci. Technol. (27)

15 Concentration (ppm) % n/n undetected carbon Product studies Carbon balance DC -DC +pulsed Carbon recovery: + DC > - DC > + pulsed n-hexane decomposed fraction Data suggest that nonvolatile C- containing products are formed These are in turn oxidized at higher energy input C 2 continues to be released even though all VC has been consumed 12 [C2], dry air 1 [C2], humid air ,4,6, [VC]/[VC]

16 Effect of humidity in the air Humidity is a major component of ambient air Water is a source of H radicals in air plasma H 2 + e - H + H + e - ( 1 D) + H 2 2 H N H 2 N 2 H + + H H N 2 H H 2 H H H 2 + M + 2 (H 2 ) + M + 2 (H 2 ) + H 2 H H + 2

17 Residual C (ppm) Measurement of relative H concentration via a chemical probe k C + H C 2 + H k(296 K) = cm 3 molecule -1 s -1 Method used in atmospheric chemistry research (M.J.Campbell et al.(1979)) and in nonthermal plasma research (Mizuno et al. J. Phys. D: Appl. Phys. 22 ) DC- DC-, RH=4% DC+ DC+, RH=4% pulsed E (kj/l) In humid air more H radicals are produced by -DC than by +DC Marotta et al., Plasma Process. Polymers.(28)C

18 Residual n-hexane fraction Residual n-hexane fraction Effect of humidity on efficiency of n-hexane (5 ppm in air) removal with -DC and +DC DC dry air -DC humid air Energy (kj/l) - DC: efficiency is greater in humid air consistent with known reactivity data k H = cm 3 molecule -1 s -1 k = cm 3 molecule -1 s DC dry air +DC humid air + DC: efficiency is not much affected by humidity Energy (kj/l) Marotta et al., Plasma Process. Polymers.(28)

19 k E (L/kJ) Similar results are found with other hydrocarbons, and in general with C-H containing VCs DC -DC Toluene k H = cm 3 molecole -1 s -1 k = cm 3 molecole -1 s -1 Schiorlin M., Marotta E., Rea M., Paradisi C., Environ. Sci. Technol. 29, 43, R H (%) Humidity induces a decrease in efficiency with +DC but an increase with DC Different mechanisms apply in +DC and DC processing Probing for ionic mechanisms by MS analysis [CH 2 Br 2 ]/[CH 2 Br 2 ] DC, dry air +DC, RH=4% -DC, dry air -DC, RH=4% SIE (kj/l) Marotta E., Schiorlin M., Rea M., Paradisi C., J. Phys. D: Appl. Phys. 21, Schiorlin M., Marotta E., Dal Molin M., Paradisi C. Environ. Sci. Technol. 212,

20 Current ( A) Current - voltage characteristics and ions in air plasma Hydrocarbons with +DC corona air air + hexane air + octane air air + hexane Voltage (kv) VC (5 ppm) greatly affects i vs V profile Mass spectrometry provides evidence for VC-derived ions air + octane Marotta et al. Plasma Processes Polym. 28 Marotta et al. J. Am. Soc. Mass Spectrom. 29

21 Corrente (A) Current - voltage characteristics and ions in air plasma Hydrocarbons with +DC corona Dry air Benzene 5 ppm Toluene 5 ppm 1 % H 3 + (H 2 ) 2 73 H 3 + (H 2 ) 3 N + (H 2 ) N + (H 2 ) 2 m/z Tensione (kv) 1 % T + (T) 184 T + 92 (T+H) + (T+H) + T T + (N) 185 m/z VC (5 ppm) greatly affects i vs V profile Mass spectrometry provides evidence for VC-derived ions Schiorlin M., Marotta E., Rea M., Paradisi C., Environ. Sci. Technol. 29, 43, 9386

22 e5 Ionization of hydrocarbons in +DC air plasma % (d) Hydride abstraction % C 4 H + (e) (a) m/z % 69 M (b) [M - H] + + H % 2 bck N + (M) 139 (a) + M + 2 [M + 84 (b) ]* R + m/z + R' Scan AP e5 % (b) C 2 H [M-H] + [M-3H] 41 + C 3 H C 3 H + 7 M 41 + N [M - H] + + HN R + + neutral products Scan AP+ Scan AP+ Scan AP+ 6.86e4 3.6e4 N + (M) C 4 H + 7 (M) C 3 H + 5 (M) Scan AP+ 3.19e5 (c) 1 % 1 1 bck 66 [M-H] [M-H] + (H 2 ) 3 [M-H] N + (H + (M) C 4 H + 2 ) 7 (M) (b) (a) % % bck 48 bck m/z [M-H] (H 2 ) 3 [M-H] + (H + 2 ) (M) (a) C H Scan AP+ Scan 7.e4 AP+ 1.42e4 Scan AP+ 7.e4 C 2 H + 3 (c) M 43 + H % (d) M + R [M - H] + + H 2 + H 2 R + + R'H + H 2 [M - H] + + RH Scan AP+ 1.26e5 (d) Scan AP+ 6.86e4 E. Marotta, C. Paradisi. J. Am. Soc. Mass Spectrom. 29, 2,

23 Current (A) Current - voltage characteristics and ions in air plasma Hydrocarbons with -DC corona air air + hexane air + octane 1 % 1 air i-octane (H 2 ) ( 2 ) 3 (H 2 ) (H 2 ) Scan AP- 1.36e4 Scan AP- 2.16e Voltage (kv) % m/z VC (5 ppm) has no effect on i vs V profile Mass spectrometry provides evidence that no VC-derived ions are present in plasma Marotta et al. Plasma Processes Polym. 28

24 Corrente (A) Current - voltage characteristics and ions in air plasma Hydrocarbons with -DC corona Dry air Benzene 5 ppm Toluene 5 ppm Tensione (kv) VC (5 ppm) has no effect on i vs V profile Mass spectrometry provides evidence that no VC-derived ions are present in plasma Schiorlin M., Marotta E., Rea M., Paradisi C., Environ. Sci. Technol. 29, 43, 9386 air 1 % 1 % 1 % - 2 (H 2 ) 5-3 (H 2 ) (H 2 ) ( 2 ) m/z air + toluene (H 2 ) 2-2 (H 2 ) - 3 (H 2 ) 3 - Scan AP- 2.61e3 Scan AP- 3.61e ( 2 ) 84 m/z air + benzene (H 2 ) 2 - (H 2 ) 2 Scan AP- 5.91e (H 2 ) ( 2 ) m/z

25 Mechanisms of hydrocarbon oxidation in air plasma +DC process is initiated by ion-molecule reactions, such as charge transfer VC [VC + ]* R + + R but also hydride abstraction and other ion-molecule reactions -DC process is initiated by radical reactions RH + H RH + H 2 + R H + R both produce carbon centered radicals R which are oxidized according to the sequence R + 2 R R carbonyl derivatives, alcohols C 2 Marotta et al. Plasma Processes Polym. 28

26 In contrast to hydrocarbons, halogen-containing VCs form negative ions in DC air plasma 1 CH 2 Cl 2 3 (M) Cl (M) 2 (M) (H 2 )(M) 2 (M) 2 % (M) Cl (H 2 )(M) APCI-MS Cl (M) 2 3 (M) 2 2 (H 2 )(M) 2 Cl (M) 3 Scan AP- 1.87e3 m/z 1) Ionization by reaction with 2 - MS/MS experiment with triple quadrupole 2 CH 2 Cl-CH 2 Cl + 2 [CH 2 Cl-CH 2 Cl ]* + 2 Cl + C 2 H 2 Cl Cl H H Cl C Cl C H H + 2 _ _ Cl + H H Cl C C Cl 2 Cl 2) Ionization by interaction with electrons CH 2 Cl-CH 2 Cl + e Cl + CH 2 -CH 2 Cl E. Marotta, G. Scorrano, C. Paradisi. Plasma Process. Polym. 25, 3,

27 Negative ions from CF 2 Br 2 in DC air plasma BKN (.653) Cm (7:39) % APCI-MS pure air Scan AP- 2.63e4 m/z N (.375) 5 Cm (7:24) Scan 6 AP e3 1 Br CF 2 Br 2 Br (M) Br 258 % (M) APCI-MS CF 2 Br 2 5 ppm m/z m/z CF 2 Br 2 [CF 2 Br 2 ]* 2 + CF 2 Br CF 2 Br + Br CF 2 Br Br + CF 2 = + Br * *R. Thomas et al. Int. J. Mass Spectrom. Ion Process. (1996)

28 Concentration (ppm) A b s o r b a n c e Absorbance CH 2 Br 2 processing with corona in air: products Concentration (ppm) CH 2 Br 2 DC (-21.5 kv) CH 2Br2, DC- 21,5k V, t =1' HN 3 HN HN 3 N 2 HN 3 C 2 C CH HN 2 Br Wavenumbers (cm-1) Wavenumbers (cm -1 ) C C DC, [C] DC, [C] +Pulsed, [C] DC, [C2] DC, [C2] +Pulsed, [C2] [CH 2 Br 2 ]/[CH 2 Br 2 ] [CH 2 Br 2 ]/[CH 2 Br 2 ] Marotta E., Paradisi C. et al. J. Phys. D: Appl. Phys. 21, Marotta E., Paradisi C. et al. Environ. Sci. Technol. 212,

29 Effect of CH 2 Br 2 on zone concentration 1 +DC 8 6 +DC, DC, +pulsed ppm ppm SIE (kj/l) +DC, Dry Air +DC, CH2Br ppm SIE (kj/l) +DC, Dry Air DC, Dry Air +Pulsed, Dry Air +DC, CH2Br ppm DC, CH2Br ppm +Pulsed, CH2Br ppm In the presence of CH 2 Br 2, ozone is completely absent In the case of DC and +pulsed and reduced in the case of +DC Marotta E., Paradisi C. et al. J. Phys. D: Appl. Phys. 21, Marotta E., Paradisi C. et al. Environ. Sci. Technol. 212,

30 A b s o r b a n c e Absorbance CF 2 Br 2 processing with corona in air: products.9.8 CF 2 Br 2 DC (-21.5 kv) CF2Br2, -DC 21.5kV t=1min HN 3 HN 3 CF 2 Br 2 CF 2 Br CF 2 = HN HN 3 HN 3 C 2 N 2 CF 2 =.1 -. Wavenumbers (cm-1) Wavenumbers (cm -1 ) CF 2 Br 2 decomposition products: The products of the decomposition of CF 2 Br 2 are C 2 and CF 2 = No C formation is detected by FT-IR ther products: 3 is not detected under any corona regime N 2 and HN 3 Marotta E., Paradisi C. et al. J. Phys. D: Appl. Phys. 21, Marotta E., Paradisi C. et al. Environ. Sci. Technol. 212,

31 Formation and fate of CF 2 = Formation via ion/molecule reaction CF 2 Br Br + CF 2 = + Br via radical reaction CF 2 Br M CF 2 Br + M CF 2 Br + N CF 2 Br + N 2 CF 2 Br CF 2 = + Br Reactivity of CF 2 = CF 2 = has a low reactivity toward radical species, while it reacts with water: CF 2 = + H 2 C HF No C can be formed from CF 2 = Marotta E., Paradisi C. et al. Environ. Sci. Technol. 212,

32 Intensity CF2= (a.u.) Processing of CF 2 Br 2 with +DC, DC and +pulsed corona in air Pure air CF 2 Br 2 5 ppm DC DC +Pulsed [CF 2 Br 2 ]/[CF 2 Br 2 ] 1. zone is not detected under any conditions (+DC, DC, +pulsed) 2. C is not formed under any conditions (+DC, DC, +pulsed) 3. CF 2 = is formed in very similar concentration under +DC, -DC and + pulsed, depending on CF 2 Br 2 conversion 4. Also C 2 is formed in very similar concentration under +DC, -DC and + pulsed, depending on CF 2 Br 2 conversion

33 Intensity N2 (a.u.) Intensity N2 (a.u.) Processing of CF 2 Br 2 with +DC, DC and +pulsed corona in air SIE (kj/l) DC DC +Pulsed [CF 2 Br 2 ]/[CF 2 Br 2 ] +DC, Dry Air DC, Dry Air +Pulsed, Dry Air +DC, CF2Br2 DC, CF2Br2 +Pulsed, CF2Br2 5. N 2 and HN 3 are formed in very similar concentration under +DC, -DC and + pulsed, depending on CF 2 Br 2 conversion Marotta E., Paradisi C. et al. J. Phys. D: Appl. Phys. 21, Marotta E., Paradisi C. et al. Environ. Sci. Technol. 212,

34 zone formation and destruction in air plasma containing CF 2 Br 2 or CH 2 Br 2 Formation : Destruction: 2 + ( 3 P) 3 Br + 3 Br + 2 Br + Br Cycle I Br + 3 Br + 2 Br + N Br + N N N Cycle II zone is destroyed by the reaction with or N through cycles catalyzed by Br atoms Marotta E., Paradisi C. et al. J. Phys. D: Appl. Phys. 21, Marotta E., Paradisi C. et al. Environ. Sci. Technol. 212,

35 N 2 and HN 3 in air plasma containing CF 2 Br 2 or CH 2 Br 2 Formation of N 2 and HN 3 in air plasma: N 2 + N N 2 N 2 + H HN 3 In the presence of CF 2 Br 2 or CH 2 Br 2, N 2 and HN 3 production is not influenced only by Cycle II Br + 3 Br + 2 Br + N Br + N N N but also by the reaction of N 2 with Br: N 2 + Br BrN 2 BrN 2 + H 2 HBr + HN 3

36 Mechanisms: comparison of CF 2 Br 2 vs CH 2 Br 2 Absence of H drastically changes the reactivity of CF 2 Br 2 with respect to CH 2 Br 2 +DC, -DC, +pulsed CF 2 Br 2, e, H CH 2 Br 2 -DC, +pulsed +DC e ions C and C 2 CF 2 Br CHBr 2 CH 2 Br CF 2 Br N 2 CF 2 Br N 2 2 CHBr 2 N CHBr 2 N 2 2 CH 2 Br N CH 2 Br N 2 CF 2 = + Br H 2 C HF CHBr= + Br, H CBr + Br CH 2 = + Br, H CH + Br C + Br C 2 Marotta E., Paradisi C. et al. J. Phys. D: Appl. Phys. 21, Marotta E., Paradisi C. et al. Environ. Sci. Technol. 212, 2 C and C 2

37 Conclusions Depending on the VC nature and on the specific corona regime different mechanisms of initiation can prevail in VC advanced oxidation induced by air non-thermal plasma ur results confirm the important role of H radicals in initiating such processes but also provide evidence for alternative mechanisms including ion-molecule reactions The strong oxidizing power of air non-thermal plasma is clearly demonstrated by its capability to decompose also the very persistent pollutant dibromodifluoromethane In the optimization of plasma induced VC advanced oxidation, product selectivity to C 2 is to be pursued alongside with efficiency of VC conversion