Plasma and catalysts. Part-financed by the European Union (European Regional Development Fund

Transcription

1 Plasma and catalysts David Cameron Professor of Material Technology Advanced Surface technology Research Laboratory (ASTRaL) University of Lappeenranta Finland Part-financed by the European Union (European Regional Development Fund

2 Outline Basic parameters of plasma catalytic operation Reactor configurations Combination of plasma and catalysis VOC removal NOx removal 1

3 Pollutant removal Inorganic compounds SO 2, NO x Volatile organic compounds (VOCs) many different compounds test compounds toluene, benzene, ethyl acetate, 2

4 Efficiency of removal of VOCs Carbon balance Carbon balance (%) n 0 CO2 CO VOC VOC 100 n = no. of C atoms in VOC molecule [VOC] 0 is input concentration of VOC molecules CO 2 selectivity s CO 2 Specific input energy (SIE) (%) 2 CO2 CO CO 100 SIE P Q dis f discharge gas power (W) flow rate (l/s) J / l 3

5 Removal of NOx, VOCs, etc. by non-thermal plasmas Difficulties due to energy efficiency poor mineralisation of VOCs formation of by-products CO 2 selectivity can be low higher hydrocarbons can also be formed e.g. from toluene can form benzaldehyde, benzyl alcohol, methyl-nitrophenols,. Exploit synergy of plasma with heterogeneous catalyst 4

6 Non-thermal plasma decomposition of VOCs Parameter Kinetics Overall efficiency Residence time of gas Aerosol formation Carbon balance Decomposition removal efficiency v. rate constants of OH and O radicals (ionisation potential) Oxygen content Non thermal plasma d ln First order C / de ke C E C / C0 ke E = SIE (J.l -1 ) 1 exp E 100 (%) No influence reaction time << residence time (energy dependent) Kim et al., IEEE Trans. Plasma Sci. 34 (2006) 984 k E in l. J Yes 1 k E Poor VOCs in aerosol particles Highly related (gas phase chemistry) Minor influence 5

7 Time scale of reactions Kim et al, Plasma Proc. Polymers 1 (2004) 91 6

8 Non-thermal plasma decomposition of VOCs Parameter Kinetics Overall efficiency Residence time of gas Aerosol formation Carbon balance Decomposition removal efficiency v. rate constants of OH and O radicals (ionisation potential) Oxygen content Non thermal plasma d ln First order C / de ke C E C / C0 ke E = SIE (J.l -1 ) 1 exp E 100 (%) No influence reaction time << residence time (energy dependent) Kim et al., IEEE Trans. Plasma Sci. 34 (2006) 984 k E in l. J Yes 1 k E Poor VOCs in aerosol particles Highly related (gas phase chemistry) Minor influence 7

9 Aerosol SD surface discharge PDC Single-stage plasma catalysis Aerosol formation from the decomposition of benzene (200 ppm benzene, 0.5 vol.% water vapour) Kim, Plasma Process. Polym. 2004, 1,

10 Non-thermal plasma decomposition of VOCs Parameter Kinetics Overall efficiency Residence time of gas Aerosol formation Carbon balance Decomposition removal efficiency v. rate constants of OH and O radicals (ionisation potential) Oxygen content Non thermal plasma d ln First order C / de ke C E C / C0 ke E = SIE (J.l -1 ) 1 exp E 100 (%) No influence reaction time << residence time (energy dependent) Kim et al., IEEE Trans. Plasma Sci. 34 (2006) 984 k E in l. J Yes 1 k E Poor VOCs in aerosol particles Highly related (gas phase chemistry) Minor influence 9

11 SPC decomposition of VOCs Parameter Kinetics Overall efficiency Residence time of gas Carbon balance Aerosol formation Decomposition removal efficiency v. rate constants of OH and O radicals (ionisation potential) Oxygen content Kim et al., IEEE Trans. Plasma Sci. 34 (2006) 984 Single-stage plasma catalysis d Zeroth order C / de C k E E = SIE k E No influence (energy dependent) Good No k k Unrelated (surface catalytic chemistry) E E E in ppm. l. J E (%) 100 C 0 1 Large influence 10

12 Energy dependent decomposition Kim et al., App. Catal. B 56 (2005)

13 SPC decomposition of VOCs Parameter Kinetics Overall efficiency Residence time of gas Carbon balance Aerosol formation Decomposition removal efficiency v. rate constants of OH and O radicals (ionisation potential) Oxygen content Kim et al., IEEE Trans. Plasma Sci. 34 (2006) 984 Single-stage plasma catalysis d Zeroth order C / de C k E E = SIE k E No influence (energy dependent) Good No k k Unrelated (surface catalytic chemistry) E E E in ppm. l. J E (%) 100 C 0 1 Large influence 12

14 Carbon balance open symbols dry closed symbols humid Kim et al., IEEE Trans. Ind. Appl. 41 (2005)

15 SPC decomposition of VOCs Parameter Kinetics Overall efficiency Residence time of gas Carbon balance Aerosol formation Decomposition removal efficiency v. rate constants of OH and O radicals (ionisation potential) Oxygen content Kim et al., IEEE Trans. Plasma Sci. 34 (2006) 984 Single-stage plasma catalysis d Zeroth order C / de C k E E = SIE k E No influence (energy dependent) Good No k k Unrelated (surface catalytic chemistry) E E E in ppm. l. J E (%) 100 C 0 1 Large influence 14

16 Aerosol SD surface discharge PDC Single-stage plasma catalysis Aerosol formation from the decomposition of benzene (200 ppm benzene, 0.5 vol.% water vapour) Kim, Plasma Process. Polym. 2004, 1,

17 SPC decomposition of VOCs Parameter Kinetics Overall efficiency Residence time of gas Carbon balance Aerosol formation Decomposition removal efficiency v. rate constants of OH and O radicals (ionisation potential) Oxygen content Kim et al., IEEE Trans. Plasma Sci. 34 (2006) 984 Single-stage plasma catalysis d Zeroth order C / de C k E E = SIE k E No influence (energy dependent) Good No k k Unrelated (surface catalytic chemistry) E E E in ppm. l. J E (%) 100 C 0 1 Large influence 16

18 Effect of plasma + catalyst Plasma will produce many active species electrons (1-25 ev) (effective temperature up to 250,000 K) ions ( ev) ( C) radicals (e.g. HO 2 ) excited atoms (e.g. O ) excited molecules (e.g. N 2 *) molecular fragments (e.g. OH) photons (up to 10 ev or more) All these may affect the catalyst Presence of catalyst may also affect plasma discharge Effect will depend on the reactor and catalyst types 17

19 Reactor configurations Single-stage Plasma Catalysis (SPC) IPC (In-Plasma Catalysis) IPCR (In-Plasma Catalysis Reactor) PDC (plasma-driven Catalysis) CPC (Combined Plasma Catalysis) Kim, Plasma Process. Polym. 2004, 1,

20 Reactor configurations Two-stage Plasma Catalysis (TPC) PEC (Plasma-Enhanced Catalysis) PPCR (Post-Plasma Catalysis Reactor) PPC (Post-Plasma Catalysis) Kim, Plasma Process. Polym. 2004, 1, (may also be pre-plasma) 19

21 Two-Stage Plasma Catalysis Catalyst usually placed downstream of plasma plasma reactions are in gas phase catalyst reactions are on the surface Can optimise conditions for plasma and catalyst independently e.g. optimum temperatures may be different Role of plasma partial conversion of the reactant formation of ozone partial oxidation of reactant is important for NO x reduction ozone enhances VOC oxidation to CO 2 over the catalyst 20

22 Single-Stage Plasma Catalysis Combination of gas phase and surface reactions Reactions take place simultaneously Complicated reactions and difficult to optimise Maximise surface area of catalyst Recent work mostly in SPC using dielectric barrier discharges (DBD) 21

23 Types of catalyst Typically metal oxides. Many have no catalytic action except at high temperatures but when exposed to the plasma they promote surface reactions Al 2 O 3 -Al 2 O 3 (hexagonal), -Al 2 O 3 (cubic) MnO 2 silica gel zeolites TiO 2 (catalytic at low temperatures) CuO May have metal particle loading. Ag, Ni, Pt, Pd 22

24 Methods of incorporation of catalyst in reactor Coating on reactor walls or electrodes Deposition by CVD, ALD, sol-gel, slurry coating, etc Van Durme et al., Appl. Catal. B 78 (2008)

25 Methods of incorporation of catalyst in reactor Packed bed Material is in the form of powder, pellets, coated fibres or porous solid foam Van Durme et al., Appl. Catal. B 78 (2008)

26 Methods of incorporation of catalyst in reactor Layer of catalyst material Powder, pellets, granules, coated fibres 25

27 Van Durme et al., Appl. Catal. B 78 (2008)

28 Effect on performance in SPC Chen et al., Environ. Sc. Technol. 43 (2009)

29 Effects of plasma and catalyst Change of work function can change likelihood of a reaction on the surface and can change the electric field in a catalyst Surface sputtering of catalyst cleaning of surface by removal of poisoning compounds and exposure of new active material Photogeneration of electrons and their chemical reactions Plasma-induced adsorption and desorption Increased gas residence times by adsorption Modification of plasma properties by the catalyst Activation of lattice oxygen Local heating of catalyst by the plasma Modification of the mass transfer properties 28

30 Change of discharge type Corona discharge plasma streamers are extended along surface of the insulating catalyst corona electrode insulator VOC oxidation near insulators will be enhanced in this configuration Rodrigo et al.,!ee Proc. Sci. Mes. Technol. 152 (2005)

31 Change of discharge type Microdischarges generated inside pores microdischarge More discharge per unit volume Higher mean energy density Electrical properties of particles are important higher permittivity allows more field to be produced in the pores BaTiO3 (ferroelectric)has very high permitivity 30

32 Generation of new reactive species Catalyst can increase the production of active species Ozone decomposition leads to reactive atomic oxygen O atoms very efficient in oxidising VOCs O O2 Occurs with a range of catalysts Temperature hot spots formed in the catalyst bed facilitates decomposition of ozone 3 O Roland et al., Appl. Catal. B 58 (2005)

33 Generation of new reactive species Inverse relationship between VOC decomposition and ozone concentration discharge on Roland et al., Appl. Catal. B 58 (2005)

34 SPC v. TPC Plasma generates both shortand long-lived species DBD induced radical formation and conversion of gaseous compounds in synthetic diesel exhaust (13.7% O2, 4.5% CO2, 5.3% H2O, 76.5% N2, and 450 ppm NO at 212 C). Hammer, et al., Catalysis Today 89 (2004) 5 33

35 SPC v. TPC Short lived species cannot reach post-plasma catalyst N, N 2 *, O* only relevant in SPC Longer lived species can have an effect in post-plasma reactors O 3, O, OH lifetime is still short intermediate hydrocarbon species can be produced which are then degraded more easily by the catalyst 34

36 Effect on catalyst properties Catalyst properties affected in different ways enhance the dispersion of active catalyst components over the support the oxidation state of the catalyst may be changed e.g. Mn 2 O 3 was changed to Mn 3 O 4 which is more active* new types of active sites may be formed (e.g. Al-O-O* species on Al 2 O 31 ) before change structure or increase surface area* Manganese oxide/alumina/nickel foam After DBD process, finer particles. higher surface area, less perfect crystals with more vacancies. (However some reports of reduced area) *Guo et al., Journal of Molecular Catalysis A: Chemical 245 (2006) 93 1 Roland et al., Appl. Catal. B 58 (2005)

37 Thermal activation of catalyst Ambient gas temperatures increase due to electron-molecule collisions C too small to explain thermal activation effects Hot spots formed small local zones caused by strong microdischarges local surface temperature may be much higher than general temperature can enhance catalyst activity, e.g. ozone decomposition 36

38 Activation by photons Photocatalyst activation semiconductor catalysts titanium dioxide is the most active photostability, strong oxidising power,, non-toxicity, chemical and biological inertness, stability, low cost. Plasma generates various wavelengths from the excited species to activate TiO 2, wavelength must be shorter than 380 nm (3.2 ev) needs ultra-violet (UV) emission 37

39 UV emission from plasma Depends on gas mixture With nitrogen, strongest UV emissons are from second Positive System (SPS) at nm (3.7 ev) Dinescu et al.,, Journal of Optoelectronics and Advanced Materials Vol. 7, No. 5, October 2005, p

40 UV emission from plasma With oxygen, strong emission from atomic oxygen Zhao et al., Thin Solid Films 518 (2010)

41 UV emission from plasma With mixtures (e.g. air), other emissions can be present, e.g. NO Dinescu et al.,, Journal of Optoelectronics and Advanced Materials Vol. 7, No. 5, October 2005, p

42 Optical absorption in catalyst UV emission spectra outside reactor a) without catalyst b) with TiO2 c) absorbance of TiO2 Van Durme et al., Appl. Catal. B 78 (2008)

43 Catalyst performance Titanium dioxide is widely used May have a metal added for improved efficiency netal particles act as reaction sites on surface and increase the lifetime of electronhole pairs in the titanium dioxide. Example of Ag/TiO 2 (anatase) Ag incorporated by chemical impregnation 42

44 Effect of reactor type on benzene destruction Kim, Plasma Process. Polym. 2004, 1,

45 Reactor construction Kim, Plasma Process. Polym. 2004, 1,

46 Some results Kim et al., App. Catal. B 56 (2005)

47 Decomposition of benzene No plasma 1% Ag/TiO2 Needs high temperature for catalysis alone Kim et al., App. Catal. B 56 (2005)

48 Benzene decomposition using Ag/TiO 2 Performance depends on SIE Does not depend on Ag concentration below 4% increased Ag may reduce exposed surface area of oxide reaction temperature = 100 C Kim et al., IEEE Trans. Plasma Sci. 34 (2006)

49 Effect on carbon balance For low or no Ag loading, carbon balance can be > or < 100% >100% means that some hydrocarbons are being deposited in the reactor which are then released later For higher Ag loading, 100% carbon balance even at low powers TiO 2 promotes initial benzene decomposition Ag promotes breakdown of intermediates reaction temperature = 100 C Kim et al., IEEE Trans. Plasma Sci. 34 (2006)

50 Effect of metal type Ni shows better benzene decomposition Ni also show poorer carbon balance Kim et al., IEEE Trans. Plasma Sci. 34 (2006)

51 Alumina catalyst Ag/Al 2 O 3 better than Pt/Al 2 O 3 or Pd/Al 2 O 3 both for decomposition and carbon balance Kim et al., IEEE Trans. Plasma Sci. 34 (2006)

52 Zeolite catalyst Similar to others Kim et al., IEEE Trans. Plasma Sci. 34 (2006)

53 Zeolite Zeolites are microporous, aluminosilicate minerals Porous structure Pore sizes up to ~1 nm Very large surface area 52

54 CO 2 selectivity Want a high selectivity CO is toxic Kim et al., IEEE Trans. Plasma Sci. 34 (2006)

55 Byproducts nitrogen oxides Mostly NO 2, less N 2 O, negligible NO Zeolites gave best results for NO 2 (may be because of absorption in the porous structure rather than purely by catalysis) Kim et al., IEEE Trans. Plasma Sci. 34 (2006)

56 Byproducts ozone No ozone with TiO 2 With Al 2 O 3, metal loading affects concentration With zeolite, little decomposition of ozone Doesn t affect benzene decomposition Kim et al., IEEE Trans. Plasma Sci. 34 (2006)

57 Effect of catalyst Type of catalyst is not very important Conversion of benzene determined by SIE Surface area of catalyst has apparently little influence Plasma causes initial step in benzene decomposition Catalyst causes further decomposition affecting carbon balance, carbon selectivity and ozone emission 56

58 Other VOCs Same effects found with toluene Kim et al., IEEE Trans. Plasma Sci. 34 (2006)

59 Ozone In plasma, ozone formed by two-step process 1. formation of atomic oxygen 2. recombination of atomic oxygen with oxygen molecule On catalyst, ozone decomposes O accelerates surface oxidation 3 2 electron O( D) O( O 2 N * O( D) O( P) 3 P) N O( P) O2 M O3 M (M O2, N2) O O 3 3 catalyst O( catalyst O 3 P) O O 2 2 catalyst 2 catalyst 58

60 NOx removal Ideally convert to N 2 rather than HNO 2, HNO 2. Difficult for plasma reactor, if oxygen content of gas is >3.4% Addition of a catalyst can improve efficiency with the addition of H 2 O 2 Kim, Plasma Process. Polym. 2004, 1,

61 NOx removal Effectiveness depends on the addition of other compounds to the gas stream to react with the NOx and reduce it. H2O2 C2H4 MeOH EtOH carbon (soot particles in diesel exhaust) NOx is converted to CO 2, N 2 and H 2 O 60

62 Probable reaction pathways to NOx reduction in plasma catalytic reactor Complex! Lin et al., Proc. Combustion Inst. 31 (2007)

63 Selectivity of NOx conversion Depends on added material H2O2 C2H4 MeOH EtOH Needs a reducing agent to form N 2 Kim, Plasma Process. Polym. 2004, 1,

64 Summary Plasma catalytic reactors increase the efficiency of VOC and NOx reduction Enable catalysis to take place at much lower temperatures than catalyst alone without plasma Reaction pathways are complex Different catalysts have varying effects Still need to improve selectivity, carbon balance and energy input. 63

65 Useful publications Combining non-thermal plasma with heterogeneous catalysis in waste gas treatment: A review. J Van Durme, J Dewulf, C Leys, H Van Langenhove, Appl. Catal. B 78 (2008) Removal of volatile organic compounds by single-stage and two-stage plasma catalysis systems: A review of the performance enhancement mechanisms, current status, and suitable applications. H L Chen, H M Lee, S H Chen, M B Chang, S J Yu and S N Li, Environ. Sc. Technol. 43 (2009) EFfect of different catalysts on the decomposition of vocs using flow-type plasma-driven catalysis. H-H Kim, A Ogata, and S Futamura, IEEE Trans. Plasma Sci. 34 (2006)