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Achille De Battisti Carlos Alberto Martinez-Huitle Sergio Ferro Laboratory of Electrochemistry University of Ferrara, Italy Implementation of Advanced.

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Presentation on theme: "Achille De Battisti Carlos Alberto Martinez-Huitle Sergio Ferro Laboratory of Electrochemistry University of Ferrara, Italy Implementation of Advanced."— Presentation transcript:

1 Achille De Battisti Carlos Alberto Martinez-Huitle Sergio Ferro Laboratory of Electrochemistry University of Ferrara, Italy Implementation of Advanced Electrodes to the Wastewaters Treatment Implementation of Advanced Electrodes to the Wastewaters Treatment Palić, Serbia, 17 – 22 Sept., 2006 ESSEE 4

2 Urban legends against the electrochemical way to wastewater remediation its not useful in case of poorly-conducting electrolytes it requires a huge consumption of electricity electrodes are expensive chemicals are easily available reactors are complex and/or difficult to manage …

3 1.1.Cl - + S S-Cl + e Volmer 2.S-Cl + S-Cl 2 S + Cl 2 Tafel 1.Cl - + S S-Cl + e 2'.S-Cl + Cl - S + Cl 2 + eHeyrovsky 1.Cl - + S S-Cl + e 2".S-Cl S-Cl + + e 3.S-Cl + + Cl - S + Cl 2 in all cases, S-Cl Cl ads Krishtalik the Chlorine evolution reaction

4 1. S + H 2 O S-OH + H + + e 2. S-OH + S-OH S-O + S + H 2 O 3. S-O + S-O 2 S + O 2 1. S + H 2 O S-OH + H + + e 2'. S-OH S-O + H + + e 3. S-O + S-O 2 S + O 2 the Oxygen evolution reaction DIAGNOSTIC PARAMETERS Tafel slope (b ) Tafel slope (b ) Reaction orders Reaction orders Kinetic study of a reaction chemical formation of the oxide electrochemical formation of the oxide

5 O 2 and Cl 2 evolution reactions in electrochemical incineration major links with the fundamentals of electrocatalysis major links with the fundamentals of electrocatalysis a solid background for developments from bench to commercial scale a solid background for developments from bench to commercial scale Electrochemical incineration: how to follow it? Traditional analytical approaches: Traditional analytical approaches: NMR, IR, UV-Vis., mass spectrometry, different chromatographies… NMR, IR, UV-Vis., mass spectrometry, different chromatographies… The alternative: The alternative: global analytical parameters, like COD and TOC global analytical parameters, like COD and TOC efficiency parameters (ICE, EOD) efficiency parameters (ICE, EOD)

6 the biological (aerobic/anaerobic) treatment biodegradable effluent toxic or non-biodegradableeffluent

7 3.1 DIRECT ELECTROCHEMICAL OXIDATION Strongly oxidant hydroxyl radicals are formed at high oxygen overvoltage anodes (PbO 2, Sb(V) or F - doped-SnO 2, diamond electrodes…) In case of metal-oxide electrodes, we can distinguish two kind of electrode material:mineralizing and converting anodes, depending on the available oxidation state of the metal. When the latter can increase its valence, radicals are stabilized by interaction with the electrode surface and their oxidation power is slower (effect: partial oxidation). On the contrary, when the oxide lattice cannot be expanded, the hydroxyl radicals exhibit larger reactivity (effect: complete mineralization, i.e. transformation into CO 2 ). Ch. Comninellis, Electrochim. Acta, 39 (1994) 1857

8 3.2 INDIRECT ELECTROCHEMICAL OXIDATION Due to a lower oxygen overvoltage (higher catalytic activity towards the OER), other anodic materials generally exhibit low faradaic yields. It is the case of galvanic Pt and IrO 2 -based DSAs ®. The performance of these stable anodes can be improved by using inorganic mediators of the oxidation. Active chlorine is of particular interest: oxidation of chlorides requires lower anode potentials, compared with those necessary for OH formation. The contemporaneous formation of the two reactive species (Cl and OH radicals) may produce hypochlorous acid, which is a strong oxidant. A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592 A possible consequence of chloride mediation: more electrode materials for electrochemical incineration

9 3.2 INDIRECT ELECTROCHEMICAL OXIDATION Redox Mediators: OH + H + + e H 2 O2.74 V O 3 + 2H + + 2e O 2 + H 2 O2.07 V S 2 O 8 = + 2e 2SO 4 = 2.05 V Ag 2+ + e Ag + in HClO 4 4N V Co 3+ + e Co 2+ in HNO 3 3M V H 2 O 2 + 2H + + 2e 2H 2 O1.776 V Ce 4+ + e Ce 3+ in HClO 4 1N 1.70 V MnO H + + 3e MnO 2 + 2H 2 O1.679 V HClO + H + + e 1/2Cl 2 + H 2 O1.63 V HBrO + H + + e 1/2Br 2 + H 2 O1.59 V Mn 3+ + e Mn V MnO H + + 5e Mn H 2 O1.491 V Cr 2 O 7 = +14H + + 6e 2Cr H 2 O1.33 V IO H + + 5e 1/2I 2 + 3H 2 O1.195 V

10 3.2 INDIRECT ELECTROCHEMICAL OXIDATION A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592

11 3.2 INDIRECT OXIDATION – Role of the different Parameters Glucose 10 g/l in 1M Na 2 SO M NaOH; Ti/Pt at 1200 A/m 2 and 25 °C S. Ferro et al., Electrochim. Acta, 46 (2000) 305

12 Glucose 10 g/l in 1M Na 2 SO 4 + NaCl 5g/l; Ti/Pt at 1200 A/m 2 and 25 °C 3.2 INDIRECT OXIDATION – Role of the different Parameters S. Ferro et al., Electrochim. Acta, 46 (2000) 305

13 Glucose 10 g/l in 1M Na 2 SO M NaOH + NaCl 3 g/l; Ti/Pt at 25 °C 3.2 INDIRECT OXIDATION – Role of the different Parameters S. Ferro et al., Electrochim. Acta, 46 (2000) 305

14 Glucose 10 g/l in 1M Na 2 SO M NaOH + NaCl 3 g/l; Ti/Pt at 1200 A/m INDIRECT OXIDATION – Role of the different Parameters S. Ferro et al., Electrochim. Acta, 46 (2000) 305

15 3.2 INDIRECT OXIDATION – WHAT HAVE WE LEARNT? relatively small amounts of chloride ions may inhibit the OER, causing relatively small amounts of chloride ions may inhibit the OER, causing an increase of the anode potential and therefore a higher reactivity of an increase of the anode potential and therefore a higher reactivity of adsorbed hydroxyl and chloride/oxychloride radicals adsorbed hydroxyl and chloride/oxychloride radicals increasing the chloride concentration above a certain critical value increasing the chloride concentration above a certain critical value would cause a potentiostatic buffering by the chlorine redox system, would cause a potentiostatic buffering by the chlorine redox system, and consequently a decrease of the anode potential and consequently a decrease of the anode potential …may inhibit… …would cause… Further investigation is needed!

16 The electrode material is becoming the main character …It means one more tough variable We have to simplify the experimental approach Let study a simpler substrate:Oxalic Acid

17 3.3 Electrochemical Oxidation – The electrode material Oxalic Acid – direct oxidation substrate concentration: 0.12M ; background electrolyte: 1N H 2 SO 4

18 Polarization curves for a Ti/IrO 2 -Ta 2 O 5 electrode, at different OA concentrations.Inset: elaboration of data in terms of Tafel plot. Anodic oxidation of Oxalic Acid (OA) at different electrode materials: IrO 2 -Ta 2 O 5 active coatings

19 Anodic oxidation of Oxalic Acid (OA) at different electrode materials: IrO 2 -2SnO 2 active coatings Polarization curves for a Ti/IrO 2 -2SnO 2 electrode, at different OA concentrations

20 . Polarization curves for a Ti/Ir 0.6 7Ru 0.33 O 2 -2SnO 2 electrode, at different OA concentrations. Inset: elaboration of data in terms of Tafel plot. Anodic oxidation of Oxalic Acid (OA) at different electrode materials: Ti/Ir 0.67 Ru 0.33 O 2 -2SnO 2 electrode

21 Anodic oxidation of Oxalic Acid (OA) at different electrode materials: IrO 2 -2SnO 2 active coatings Polarization curves for a Ti/IrO 2 -2SnO 2 electrode, at different OA concentrations

22 3.3 Electrochemical Oxidation – The electrode material Tafel plot for Oxalic acid electroxidation, in HClO 4, at different electrode materials [OA] = 750mM

23 3.3 Electrochemical Oxidation – The electrode material Considering the BDD anode material, Comninellis et al. [ref] have proposed a mechanism of OA oxidation that involves the participation of hydroxyl radicals generated at the electrode surface: H 2 O OH + H + + e - (r.d.s.) H 2 C 2 O 4 + OHHC 2 O 4 + H 2 O HC 2 O 4 CO 2 + COOH COOH + OHCO 2 + H 2 O COOH + OHCO 2 + H 2 O H2OH2O OH HOOC-COOH OH Ch. Comninellis et al., J. Appl. Electrochem., 30 (2000) 1345

24 3.3 Electrochemical Oxidation – The electrode material Mildly ox BDD GC 1 Ti/IrO 2 -2SnO 2 Pt Ti/Pt 2 Ti/Ir 0.67 Ru 0.33 Sn 2 O 6 3 Ti/IrO 2 -Ta 2 O mA/cm mA/cm 2 OH radicalsOH radicals mechanism mechanism H 2 C 2 O 4 (H 2 C 2 O 4 ) ads + H + + e - H 2 O (OH) ads + H + + e - (H 2 C 2 O 4 ) ads + 2(OH) ads 2CO 2 + 2H 2 O

25 3.3 Electrochemical Oxidation – The electrode material Oxalic Acid – direct and mediated oxidation at bulk Pt substrate concentration: 0.12M ; NaX concentration: 5 g/l background electrolyte: 0.25M NaOH + 0.5M Na 2 SO 4 A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35

26 3.3 Electrochemical Oxidation – The electrode material Tartaric Acid – direct and mediated oxidation at Ti/Pt substrate concentration: 0.10M ; NaX concentration: 5 g/l background electrolyte: 0.5M H 2 SO 4 or 0.25M NaOH + 0.5M Na 2 SO 4

27 3.3 Electrochemical Oxidation – The electrode material Effect of NaCl concentration on the current/potential characteristics, attained at the Pt electrode supporting electrolyte: 0.25M NaOH + 0.5M Na 2 SO 4 A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35

28 3.3 Electrochemical Oxidation – The electrode material Effect of NaBr concentration on the current/potential characteristics, attained at the Pt electrode supporting electrolyte: 0.25M NaOH + 0.5M Na 2 SO 4 A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35

29 3.3 Electrochemical Oxidation – The electrode material Effect of NaF concentration on the current/potential characteristics, attained at the Pt electrode supporting electrolyte: 0.25M NaOH + 0.5M Na 2 SO 4 A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35

30 3.3 Electrochemical Oxidation – The electrode material A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35

31 Halogenide-mediated (indirect) electrochemical incineration (alkaline media): Volume reaction of the substrate with electrogenerated strong oxidants (ClO 2, HClO, ClO -, BrO 3 - ); Surface reaction of the adsorbed substrate with electrosorbed species (e.g.: oxy-chloro radicals); Inhibition of the oxygen evolution reaction. Direct electrochemical incineration:Direct electrochemical incineration: Concomitant with oxygen evolution reaction; Good faradaic yields at high-oxygen overvoltage anodes; Weakly adsorbed hydroxyl radicals are the main factor leading to electrochemical incineration; As an extreme case, hydroxyl radicals may act within a reaction cage nearby the electrode surface. A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35

32 objectives sterilization of solutions for medical purposes low capacity municipal plants final treatment a real approach… for the potabilization of water

33 Generation of fresh water anolyte and catholyte

34 23 cm 29 cm Characteristics Characteristics produced water: 1 liter/minute redox potential: V SCE (potable water: 0.3 ÷ 0.4 V SCE ) (potable water: 0.3 ÷ 0.4 V SCE ) service life : liters (e.g. 20 l /day 274 years!!) (e.g. 20 l /day 274 years!!) a real approach… for the potabilization of water

35 W pH < 5pH > 9

36 applications Potable water Wastewater Swimming Pools, Spas, Hot tubs Cooling Towers Disinfection Agricultural Applications Food Processing

37 DSA ® (Dimensionally Stable Anodes) support film interlayer Support the Oxide mixture Electrocatalytic Oxides (IrO 2, RuO 2, PtO x ) Valve-metal Oxides (SnO 2, Ta 2 O 5, TiO 2 ) Film A conductive metal, thermally stable (Ti, Ta) Interlayer A thin layer of a metal or oxide, having a high affinity toward the catalytic film High surface area High electrical conductivity High electrocatalytic activity/selectivity Chemical and mechanical stability Low cost Health safety Ideal features

38 The accelerated service-life test for oer DSA Need for quicker diagnostics, e.g.: 1-4 months Typical test example : Solution: 3 M H 2 SO 4 Temperature: 60°C Galvanostatic conditions, j = 10 – 50 kA m -2 Test end upon 1 V increase in cell potential (polarization curves and CVs recorded during the experiments)

39 The accelerated service-life test for oer DSA. A possible way to analize the results The passivation (deactivation) time can be misleading (the catalyst loading (film thickness) is not properly considered). Charge consumption per unit electrode-surface- area (e.g. kAh m -2 ) is more meaningful than time; Normalization to film thickness (catalyst loading) is mandatory. At fixed coating composition the amount of noble-metal (e.g.: g Ir m -2 ) can be used as normalizing factor

40 Limitations of DSAs used in the industry Service life index:

41 3800 h = 158 d = 5 m Service life index

42 Preparation (0 100% Mol IrO 2 ) Characterization Microstructure(XRD) (SEM) (EDX) (AFM) Electrochemical Activity Service life Building Blocks

43 Preparation (0 100% Ir, constant-mass deposits) Ti-support etching by conc. NaOH (acidic treatments lead to shorter s.l.) Interlayer deposition (thermal methods) precursor deposition (Ir(IV) and Sn(IV) chloro-aceto complexes, colloidal suspensions) Thermal decomposition: 450 °C)

44 35% Ir 1200X Scanning Electron Microscopy Images 50% Ir 1200X cracked mud 100% Ir Increase of the Ir % 30000X Complete absence of organisation

45 Element Weight % Atomic% O K Cl K Ti K Sn L Ir M Totals Nominal composition (%Ir) EDX results (%Ir) Correlation among EDX results and gravimetric data from precursor solutions Energy Dispersive X-rays Analysis The slope close to 1 indicates that no volatilization of Sn takes place during the pyrolysis step low thickness for the oxide film: 1m (presence of Ti from the support) 35 % Ir

46 Atomic Force Microscopy Images Ir 0% - Sn 100% No formation of nanoaggregates Formation of nanoaggregates Ir 35% - Sn 65%

47 Atomic Force Microscopy Images Formation of microaggregates Ir 20% - Sn 80%

48 X-ray Diffraction Analysis Gradual transition from the IrO 2 rutile structure to the rutile system of SnO 2 (i ) (ii ) Progressive shift of the 2 values, varying the percentage of Iridium Ti

49 XRD: test of the Vegard law Formation of a metastable solid solution

50 XRD - Particle size vs Composition Particle Size: 35% 2.5 nm Cell Volume

51 Cyclovoltammetric Characterization 92 C/g Ir Electrolyte: HClO 4 1N Potential range: V Scan rate: 100mV/sec

52 Cyclovoltammetric Characterization HClO 4 1N - Potential window: V - Scan rate: 100mV/sec Effect of the catalyst composition on the anodic charge

53 Cyclovoltammetric Characterization Supporting electrolyte: HClO 4 1N - Potential window: V Effect of the catalyst loading on the anodic charge

54 Role of the anodic material on the rate of the electrochemical process

55 Kinetic study: considerations % Irb exp (mV/dec) b exp b e ff S-OH « 1 Optimal catalytic activity (high TurnOver number)

56 Kinetic study of the Oxygen Evolution Reaction Hypothesis H 2 O (H 2 O) ads first step in equilibrium second step rate determining low overpotentials s-OH 0 K 1 (reql constant) Electrochemical Oxide formation

57 Dependence of current density for o.e. on anodic charge density: all electrode compositions

58 Dependence of current density for o.e. on anodic charge density: same composition, different thickness

59 The Group!: Martina Donatoni Sergio Ferro Fabio Galli Carlos Alberto Martinez-Huitle Davide Perelli Lourdes Vazquez-Gomez


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