Electroreductions on Silver-Based Electrocatalysts: The Use of Ag Nanoparticles for CHCl 3 to CH 4...

Electroreductions on Silver-BasedElectrocatalysts: The Use of AgNanoparticles for CHCl3 to CH4Conversion**S. Rondinini1*, G. Aricci1, Z. Krpetic2, C. Locatelli1, A. Minguzzi1, F. Porta2, A. Vertova1

1 Department of Physical Chemistry and Electrochemistry, University of Milan, Via Golgi 19, 20133 Milano, Italy2 Department of Inorganic, Metallorganic and Analytical Chemistry “Lamberto Malatesta”, University of Milan, Via Venezian,21–20113 Milano, Italy

Received August 28, 2008; accepted November 02, 2008

1 Introduction

The electrocatalytic properties of silver as a cathode mate-rial for the dehalogenation of several classes of organichalides have been extensively documented in the past 10–15years [1, 2].

Together with the accumulation of additional evidence ofAg electrocatalytic activity towards selected substrates of syn-thetic or environmental relevance [3], the research interestsare now focusing on the preparation methodologies of theelectrode material, both to improve the catalyst performanceand reduce the silver content.

Although most of the reported applications are performedon massive Ag electrodes, there is evidence [4, 5] that micro-and nanostructured particles may exhibit similar or even bet-ter behaviour than massive silver, while allowing a substan-tial reduction of Ag loading. Moreover, as evidenced by [6]the Ag nanoparticle size deeply affects the dissociation of gas-eous CHCl3 on silver. In fact, this surface reaction shifts fromweak on bulk Ag(111) to strong on 20–50 nm Ag particles,and medium Ag coverage, suggesting that the process is con-trolled by the number of surface defect sites.

The present contribution reports the preparation and char-acterisation of Ag nanoparticles (Ag_NP), and their use as

–[*] Corresponding author, [email protected]

AbstractA preliminary investigation on a new class on electrocataly-tic materials for the electroreduction of organic halides ispresented and discussed. The electrocatalysts are based onsilver nanoparticles (Ag_NP), ad hoc synthesised by chemicalreduction of an aqueous silver salt in the presence of six dif-ferent stabilising agents. The colloids are then supported oncarbon powder (10% loading) for further characterisationand use. The electrocatalytic properties of the Ag_NP/car-bon composites towards the dehalogenation of halocom-pounds are tested by cyclic voltammetry and by preparativeelectrolysis. The hydrodehalogenation of trichloromethane,extensively studied by this group, is selected as a modelreaction, because of its relevance for the detoxification of

wastes. The voltammetric characterisation is performed inan aqueous solution, supporting the composites on cavitymicroelectrodes. Gas-diffusion electrodes (GDE) based onthe most promising Ag_NP composite – and, for reference,on a commercial Ag/C oxygen reduction electrocatalyst –are then tested in an electrolytic process for the progressiveconversion of gaseous trichloromethane to less chlorinatedcompounds, and ultimately to methane.

Keywords: Composite Materials, Dehalogenation, Nanos-tructured Electrocatalyst, Silver Nanoparticles, Trichloro-methane, Waste Detoxification

–[**] Paper presented at the 3rd German-Italian-Japanese Meeting of

Electrochemists, May 25–28 2008, Taormina-Messina, Italy

FUEL CELLS 09, 2009, No. 3, 253–263 © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 253

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DOI: 10.1002/fuce.200800083

Rondinini et al.: Electroreductions on Silver-Based Electrocatalysts: The Use of Ag Nanoparticles

carbon-supported composite electrodes for the electroreduc-tion of organic halides.

Although, Ag_NP are used in a large variety of applica-tions that range from biosensors [7–9] to oxygen reductionelectrocatalysts [10–12], to antimicrobial agents [13], to thebest of our knowledge, this is the first time that these Ag_NPcomposites have been synthesised and applied to the electro-chemical reduction of organic halides.

Numerous different techniques of Ag_NP preparation areknown. They include chemical reduction methods [14–20],thermal methods [21], irradiation [22–24] or laser ablation [25,26]. In spite of the above-mentioned variety in Ag_NPs prep-aration the chemical reduction of silver salt by various reduc-ing agents (e.g. borohydride, citrate, ascorbic acid, formalde-hyde, hydrazine, reducing sugars) nowadays prevails. Theparticle size, size distribution (polydispersity) as well as thestability of the system can also be modified by additionalcomponents such as surfactants (surface-active molecules) orpolymers [27–29].

In our experimental conditions, novel pale-yellow silversols, Ag_NP(0–5), composed by Ag(0) nanoparticles havebeen obtained by reduction of an AgNO3 aqueous solution,under N2 atmosphere, with NaBH4 in the presence of six dif-ferent ligands, acting as stabilisers: namely, 0 = L(+)-glutamicacid, 1 = poly(vinyl alcohol), 2 = a-(nonylphenyl)-x-hydro-xypolyoxyethylene, Igepal, 3 = a-dodecyl-x-hydroxypolyox-yethylene, Brij 35, 4 = polyethyleneglycol bisphenol A epy-chlorohydrin copolymer, PEG-BDE, 5 = polyethylene glycol(PEG) (see Table 1). Stable colloidal systems were obtainedby adopting the silver/reducing agent molar ratio of 1:10 andthe silver/ligand (1–5) w/w ratio of 1:1, with the only excep-tion of Ag_NP0, for which the optimal molar ratios were sil-ver/reducing agent/ligand = 1:1:15.

The physicochemical characterisation was performed byusing UV–Visible spectroscopy and transmission electron mi-croscopy (TEM). UV–Visible measurements allowed thedetection of the plasmon resonance peak close to 390 nm, asenvisaged by the yellow colouration and in agreement withthe literature reports [30, 31]. TEM microscopy was used forthe dimensional and morphological analyses of the silverNPs.

The composite materials have been prepared by dippingthe support (carbon, Vulcan®XC72R) in the colloidal solution.The support amount was calculated in order to have 10 wt.-%final silver loading. This ratio is a balance between the typicalloading for catalysis (1–6% [32–34]) and our reference valueof 20% relevant to the commercial oxygen reduction cathodes.The Ag_NP/Carbon materials were characterised by X-raypowder diffraction analysis (XRPD).

The composites were electrochemically characterised bymeans of the cavity micro-electrode (C-ME), a device espe-cially developed for electrochemical studies of disperse sys-tems [5, 35–38]. This allows a preliminary evaluation of theirelectrocatalytic activity and, accordingly, the ranking of thesurfactants. The electrocatalytic properties were checkedagainst the model electroreduction of trichloromethane in an

aqueous medium, a well studied hydrodehalogenation reac-tion [2, 39] which leads to methane as almost unique product[40, 41].

On the basis of these results, the Ag_NP3_C compositewas used to prepare gas diffusion electrodes (GDE) to beapplied to the electrolytic reduction of CHCl3 from the gasphase, a reaction of environmental and energetic relevancefor the production of methane from wastes. A comparisonwith GDEs prepared with a commercial Ag/C compositeconfirms the very promising performances of the Ag_NPelectrocatalyst. The CHCl3 to CH4 conversion is taken as amodel reaction for the treatment of volatile organic halides(C1–C2), a class of compounds which could provide value-added fuels for energetic applications.

2 Experimental

2.1 Chemicals

Unless otherwise stated, all chemicals were purchasedfrom Aldrich and used as received. The aqueous solutionswere prepared using highly deionised water (MilliQ® Milli-pore System).

2.2 Spectroscopy and Microscopy

UV–Vis spectra of silver nanoparticles were recorded on aJASCO V-530 UV–Vis spectrophotometer. For the UV–Vis

Table 1 Stabiliser compounds selected for the synthesis of silver nanopar-ticles. The numbering is used in the text to denote the correspondingAg(0–5)/Vulcan®XC72R composite: C-No.

No. Ligand Chemical formula

0L(+)-glutamic acidM = 147 kg kmol—1 OH OH

O O

NH2

1PVAPoly(vinyl alcohol)M = 13,000–23,000 kg kmol–1

2

Igepal® CO-720 (branched)a-(nonylphenyl)-x-hydroxypolyoxyethyleneM ≈ 749 kg kmol—1 n ≈ 23

3

Brij 35®

a-Dodecyl-x-hydroxypolyoxyethyleneM ≈ 1,198 kg kmol–1 n ≥ 23

CH3(CH2)10CH2(OCH2CH2)nOH

4

PEG-BDEPolyethyleneglycol bisphenolA epichlorohydrin copolymerM = 7,000–9,000 kg kmol–1

[C15H16O2�C3H5ClO.(C2H4O)n.H2O]x

5PEGPoliethylene glycolM ≈ 200 kg kmol—1 n ≈ 4

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analysis, the Ag sol was diluted four times and the absorptionspectra were recorded in the range of 250–700 nm.

TEM micrographs of the colloidal dispersions wereobtained using a EF TEM LEO 912AB instrument operated atan accelerating voltage of 120 kV. Specimens for imaging byTEM were prepared by evaporating a droplet of silver col-loids onto carbon-formvar copper grids. A histogram of theparticle size distribution and the average particle diameterwere obtained by measuring about 100 particles.

XRPD spectra of the carbon support and Ag compositepowders were obtained using Philips PW 3020 powder dif-fractometer with horizontal scan having radiation Cu-Kaequipment with graphite monochromator operating onto adiffracted ray. The peak assignment has been performedusing DiffracPlus Evaluation EVA software, while the size ofthe crystal particles was calculated using the Scherrer for-mula after the interpolation of the peak profile using PhilipsAPD 3.6 software.

2.3 Preparation of Ag_NP

2.3.1 Ag_NP (1–5)

In a typical preparation, to a 116 ml of water, an aqueoussolution of AgNO3 (58 mmol; 1.9 × 10–2 M) was added underN2 atmosphere in an amber glass round bottom flask. Undervigorous stirring, an aqueous solution of the stabiliser (1–5)(626 ll, 1% w/w stock solution) was added, in order to set upthe ratio between Ag and the ligand equal to 1:1 by weight.After 5 min, an aqueous solution of NaBH4 was added(1.16 ml, 0.5 M) and a pale yellow sol was immediatelyformed. The sol was left under stirring for further 30 min.The particles were then immobilised onto carbon powder(Vulcan®XC72R, Cabot Corporation) as described below.

2.3.2 Ag_NP0

To a 100 ml of water, an aqueous solution of AgNO3

(58 mmol; 1.9 × 10—2 M) was added under N2 atmosphere inan amber glass round bottom flask. Under vigorous stirring,an aqueous solution of L(+)-glutamic acid (0) (15.5 ml,56 mM) was added, in order to set up the molar ratiobetween Ag and the ligand 1:15. After 5 min, an aqueous so-lution of NaBH4 was added (1.16 mL, 0.5 M) and a pale yel-low sol was immediately formed. The sol was left under stir-ring for further 30 min. The particles were then immobilisedonto carbon powder (Vulcan®XC72R) as described below.

2.4 Preparation of Composites Ag_NP(0–5)_C

The sols were immobilised by adding a weighted amountof carbon powder (Vulcan®XC72R, Cabot Corporation) to thecolloidal solution. The powder amount was calculated toobtain a silver loading of 10% w/w. The slurry was magneti-cally stirred for 2 h and filtered off, and the absorptionchecked by ICP analyses of the filtrate. The pH of the slurry

was modified during immobilisation to 2 (by adding concen-trated H2SO4). The supported catalysts were rinsed with largeamounts of distilled water and dried under vacuum.

2.5 Cyclic Voltammetry

Voltammetric investigations were carried out using a PAR273A potentiostat/galvanostat driven by a PC controlled byPOWER SUITE® software, in a conventional three-electrodecell.

The electrolyte solution consisted of KClO4 (Fluka) 0.1 Min water. A double-bridge, aqueous saturated calomel elec-trode (SCE) and a platinum foil were used as the referenceand counter electrodes respectively. CVs were always firstperformed on background electrolyte. The desired trichloro-methane concentration of 0.01 M was obtained by adding theappropriate amount of CHCl3 (Merck ≥99.8%) to water. Dueto the high volatility of CHCl3, a new aqueous solutionwas prepared just before each run. Before any recording thesolutions were deaerated by N2 bubbling. To avoid anychange in CHCl3 concentration, nitrogen was presaturatedwith the same solution under test. All potentials are referredto SCE.

2.5.1 Preparation of Cavity Microelectrode (C-ME)

The C-ME were prepared as described by [36, 42] from aPt wire of 50 lm diameter. The typical recess depths were20–30 lm. The exact value of the depth, L/cm, was obtainedby measuring the steady-state reduction current inRu(NH3)6Cl3 1 mM solution (using aqueous KCl 0.1 M as asupporting electrolyte) at 1 mV s—1 and solving for L Eq. (1):

Il � 4pnFcbDr2

4L� pr(1)

relevant to planar diffusion in a recess [43]. Here Il is thesteady-state current intensity, L and r are cavity depth andnominal radius, respectively, cb and D are the Ru(III) complexconcentration and diffusion coefficient, respectively, F theFaraday constant and n is the number of moles of electronsper moles of reactant (n = 1).

The cavity was filled with material particles using the elec-trode as a pestle. The filling of the cavity was controlled withthe optical microscope, and at the same time, it was verifiedthat no particle remained on the head outside the cavity.

In addition to the Ag_NP(0-5) composites synthesised inthis work, a commercial microcrystalline Ag powder wasused (Aldrich 99.9%, nominal particle mean diameter of2–3.5 lm). C-ME containing commercial Ag will be denotedas Ag_powder. At the end of each experiment, the cavity wasemptied by sonication (Ultrasonic Falck UTD18) dipping theelectrode in a concentrated HNO3 (Fluka ≥65%) aqueous so-lution (10′) and then in water (2′). The cavity was then driedat 80 °C. Periodically the empty cavity was checked in theRu(III) complex solution for cleanliness.

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2.6 Electrolysis

2.6.1 Electrodes

The cathodes are GDE prepared using either commercialsilver (E-TEK® C8-20, on Vulcan®XC72), normally employedfor oxygen cathodes, denoted as Ag_GDE, or the Ag_NP3_Ccomposite, denoted as Ag_NP3-GDE. Both electrodes havethe same structure of a gas diffusion layer and a catalyticlayer deposited on a silver-plated Ni screen, but they differfor the Ag content in the catalyst layer, which is 20 and 10%for Ag-GDE and Ag_NP3-GDE, respectively.

The GDE preparation, widely discussed in the literature[44–48], is briefly reported here, for the sake of completeness.An 80 mesh silver-plated Ni net is covered on both sides withlayers of silver loaded carbon + T30 Teflon emulsion (car-bon/PTFE = 1:1 w/w). A final coating of SAB® (ShawiniganAcetylene Black) + T30 Teflon emulsion (SAB/PTFE = 1:1 byweight) is applied on the gas-side to prevent the flooding ofthe electrode.

Although, the cell can accommodate GDE of differentsizes, up to 5 × 20 cm2 of exposed area, in the present case, inorder to limit the amount of Ag_NP3 to be synthesised, bothGDEs had an exposed area of 2 × 8 cm2 only.

The anode was a ruthenium oxide activated Ti (DSA,expanded mesh, 18 × 4 cm2), kindly provided by IndustrieDe Nora SpA.

2.6.2 Electrolytic Cell

Preparative electrolyses were performed in an in-housebuilt divided filter-press flow cell, presented in Figure 1. Itconsists in an anodic compartment (140 cm3) separated fromthe cathodic one by a cation exchange membrane (Nafion®

117). The cathodic section is in turn divided into two cham-bers by the GDE, namely, a gas chamber (up to 30 cm3, in thepresent case 4.8 cm3), connected to the gas feed, and a liquidchamber (140 cm3). The cell frames and end plates were ofpolymethylmethacrylate.

The cell, 30 cm × 10 cm × 8.5 cm overall size, is mountedwith the cathodic gas side upwards, in order to reduce the

liquid pressure on the GDE cathode, and slightly inclined toallow for the easy evacuation of the anodic gas.

The cathodic gas feed is N2 saturated with CHCl3 at20 ± 1 °C; by using the perfect gas state equation and theCHCl3 vapour pressure at 20 °C, the trichloromethane flowrate was of 0.8 mmol h–1.

The exhausted cathodic gas was sent to a gas chromato-graph HP5890 (GC/FID), equipped with a FID detector andwith a 0.32 mm capillary column AT624 (Alltech), for on-lineanalysis. The product peaks were previously assigned on thebasis of CG-Mass analysis [41].

All electrolyses were carried out at constant current(250 A m–2) with an ELIND 60HL10 power generator anddata were collected with a PC-Board CIO-DAS 1402/16(Measurement Computing Corporation).

GDE potential was measured versus SCE, connected via acapillary Luggin positioned in the bottom side of the cell plas-tic frame.

Aqueous 0.1 M Na2SO4 were used as supporting electro-lyte for catholyte and anolyte, and were continuously recircu-lated. Both catholyte and anolyte had a total volume (cellcompartment + recirculating tank + connecting tubes) of1,500 cm3.

3 Results and Discussion

The UV–Vis spectra of Ag_NP(0–5) are collected in Fig-ure 2. Spectra present plasmon resonance peaks close to390 nm (from 387 to 394 nm, see Table 2) in agreement withthe yellow colouration of the sols.

The dimensional and morphological analyses of Ag_NP(0–5) colloids were performed by TEM. As an example, wereport in Figure 3A a TEM micrograph of Ag_NP1, showingparticles of 2.5 nm averaged mean diameter (Figure 3B). Wefound that different stabilisers influence the particles size andpolydispersity, varying particle mean diameters in a range of2–15 nm.

The Ag_NP_C materials were characterised by XRPD anal-ysis highlighting their crystallinity by showing the presenceof one Ag(0) peak close to 38° (2 H) for all samples, as shownin Figure 4 in the case of Ag_NP5_C. The diameters ofAg_NP(0–5) nanoparticles on carbon support, calculatedfrom the Scherrer equation, have been found in the range of15–20 nm. As these values are larger than those measured byTEM, moderate agglomeration of the particles during theimmobilisation step is suggested.

Fig. 1 Scheme of the electrolysis flow cell. A: anolyte compartment; C:catholyte compartment; G: cathodic gas compartment. DSA: anode; GDE:gas-diffusion cathode; CM: cationic membrane. Arrows denote gas inand gas out in the cathodic gas compartment. Recirculation circuits of ano-lyte and catholyte not shown.

Table 2 kmax values of the plasmon resonance peaks of silver colloidsAg_NP(0–5).

Colloid kmax (nm)

Ag_NP0 390Ag_NP1 393Ag_NP2 390Ag_NP3 392Ag_NP4 394Ag_NP5 387

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3.1 Cyclic Voltammetry

The electrochemical behaviour of the composite powderssupported on C-ME, in aqueous KClO4 0.1 M, for the six dif-ferent electrode materials are reported in Figure 5A–F. Ineach case the CV recorded both in the absence and in thepresence of CHCl3 0.01 M are shown.

As can be seen, all the background curves show the typicalcapacitive responses of silver in the supporting electrolyte,until for sufficiently negative potentials (E≤–0.9 V) the onsetof the hydrogen evolution reaction (HER) is observed. No sig-nal can be attributed to the presence of the stabiliser mole-cules, which do not show any electroactivity in the relevantpotential window, but for their possible role as proton source,the modification of the relative permittivity of the surfacelayer and the partial shielding of the silver sites. Note that,with the only exception of Ag_NP3_C whose features will bediscussed below, the charging currents are quite similar forall composites, thus denoting a good homogeneity in silverloading, the small differences being easily attributed to thereproducibility of filling the cavity, which is ≥90%. In addi-tion, the d.c. capacitances have intermediate values betweenthe ones recorded on Ag_powder (compare with Figure 6A),which are about 10 times lower, and carbon powder (com-pare Figure 6B), which are about 10 times higher. This featureis connected with the high surface area provided by the car-bon support, which, nonetheless, is strongly modified by theaddition of the silver nanoparticles. The six composites showlarger differences in the hydrogen evolving region, due to thesuperposition of different phenomena, like the reduction ofcarbon side groups, small changes in the surface aciditybound to the different properties of the six stabilisers, differ-ences in the surface availability of Ag sites for HER. Interest-ingly, the largest differences are observed between the com-posites based on Ag_NP2_C, Ag_NP3_C and Ag_NP5_C,whose ligands, in turn, differ for the presence of a branchednonylphenyl and a linear dodecyl group in 2 and 3, respec-tively. Ag_NP3_C shows about one-half the d.c. capacitanceof the others. Since the good reproducibility of the recordingsrules out effects bound to the cavity filling, the high HER/charging current ratio rules out a reduced silver loading andthe well-shaped CV shape rules out an higher internal resis-tance of the material, the low capacitance could point to alarger shielding of the silver nanoparticles. This is very likelydue to the presence of the highly hydrophobic dodecylgroups in the Ag(0)-stabiliser micelles, which can deeplymodify the double-layer electric permittivity, while acting asgood anchoring tools to the carbon support. However, thesame does not apply to Ag_NP2_C, which does not show anyreduction in capacitance, while exhibiting, as Ag_NP3_C, ahigh HER/background current ratio. We believe that thepresence of the bulky branched nonylphenyl group togetherwith a reduced polyoxyethylene chain (n ≈ 12 vs n ≥ 23) has astrong effect on the micelle structure, which in turn governsthe carbon–nanoparticle interactions on one side, and the sil-ver surface accessibility on the other. The small size of 5 couldFig. 2 UV–Vis spectra of Ag_NP(0–5).

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be, as well, at the base of the corresponding composite behav-iour, which, at variance with the other two, shows a lowactivity for HER, thus evidencing a poor availability of silveractive sites. In comparison with Ag_NP2 and Ag_NP3, ahigher hydrophilic character of the micelle can be expected,which could be at the base of a more labile interaction withthe carbon support, and hence of a reduced silver loading. Amore complex hypothesis could involve the competitionbetween the carbon–ligand and the Ag–ligand interactions,and the loss of silver. This picture could also explain the dif-ferent behaviour of Ag_NP5_C and Ag_NP0_C, which pres-ent the lowest and the highest HER currents, respectively. Infact, thanks to the stronger interactions between silver and 0,due to the presence of the amidic nitrogen, neither micelledestabilisation nor the loss of nanoparticles is observed forthe corresponding composite, notwithstanding the small sizeof the ligand and its high hydrophilic character.

When considering the CV features in the presence ofCHCl3 0.01 M, we observe a general electroactivity of thecomposites towards the trichloromethane reduction, in com-parison with the bare carbon support. In fact, in all cases, amore or less well-shaped peak located at potentials more neg-ative than –0.8 V is observed, or at least, as in the case ofAg_NP0_C, a strong increase in the reduction current in com-parison with the background. Nonetheless, the peak positiontogether with the comparison between the first and thesteady-state cycles shows how the Ag_NP properties doheavily affect the electrode performances in the presence ofthe probe reaction, whose general reduction scheme, for thegeneric organic halide, can be summarised as

RX� e� �� RX��→R��X� (2)

RX� e�→R� � X� (3)

R� � e�→R� (4)

R��R�� HA→RH�A��A�� (5)

R� � RX→R� R� X� (6)

R��R�→R� R (7)

Here, reaction (2) represents the stepwise pathway, inwhich an intermediate radical anion is formed before thebreaking of the C–X bond, while reaction (3) represents thesimultaneous electron transfer (ET) + C–X cleavage of theconcerted pathway [49, 50]. Depending on the substrate andthe working conditions (solvent, supporting electrolyte, elec-trode material and potential), a second ET reaction (4) leadsto the organic anion. Both anion and/or radical can react withsolvent, hydrogen/proton donors and/or other RX molecules(or their organic moieties) to give propagation reaction prod-

Fig. 3 (a) TEM micrograph of Ag_NP1 and (b) the corresponding particle size distribution histogram.

Fig. 4 XRPD spectra of crystalline Ag_NP5_C, with the characteristicAg(0) peak at 38.06 2H.

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ucts like dimers and hydrodehalogenated species, as in reac-tions (5)–(7).

As already observed by several authors [1, 51], reactions(2) or (3) generally represent the slow step in the reductionprocess, reaction (4) being considered very fast and charac-terised by a less negative potential than the first step. Conse-quently, the CV curves generally exhibit highly irreversible 2-electron peaks for each halide-leaving group. In the case oftrichloromethane, up to three 2-electron peaks could then be

expected at increasingly negative potentials. Nonetheless, theuse of an aqueous electrolyte strongly reduces the accessiblepotential window, and the CHCl3 reduction peak(s) may beobserved only on highly electrocatalytic materials, as silver[39]. In fact the high affinity of silver towards the halides is atthe base of the formation of a “reaction cage” in which thehalide group acts as a bridge between the electrode surfaceand the organic moiety of the substrate, Ag� � �X� � �R [52, 53].This, in turn, could be at the base of the dramatic reduction of

(A) (B)

(C) (D)

(E) (F)

Fig. 5 Cyclic voltammetry of 0.01 M trichloromethane (full lines: heavy–first cycle; light–steady-state cycle), in aqueous 0.1 M KClO4 background elec-trolyte, on C-ME supported composites, at room temperature. Dotted line: background electrolyte only. Scan rate: 300 mV s–1. (A) Ag_NP0_C; (B)Ag_NP1_C; (C) Ag_NP2_C; (D) Ag_NP3_C; (E) Ag_NP4_C; (F) Ag_NP5_C.

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the activation energy of the first ET concerted step (Eq. 3)[54].

By comparing Figure 6A with B, the remarkable electroca-talytic behaviour of microcrystalline silver may be observed.In fact, between –0.6 and –1 V, a large reduction current isobtained in 0.01 M CHCl3, whose high Ip/Ibackground ratio isonly marginally reduced between the first and the subse-quent cycles. Rather than separate reduction peaks, a largeplateau is observed, which is attributed to the presence ofmultiple reaction sites on which the electron transfers rele-vant to the consecutive expulsion of the chloride groups takeplace. Vice versa, the carbon support shows almost no differ-ence in the absence and in the presence of CHCl3.

Although, to a lesser extent, the behaviour of the six com-posites, for which the silver loading is only the 10 wt.-% ofthe total electrode material, reproduces the Ag_powderbehaviour, the main differences being generally lowerIp/Ibackground ratios and more negative and more localisedreduction peaks. In particular, Ag_NP3 shows the most selec-tive behaviour between the reduction of trichloromethaneand water, its well-shaped peak being at a rather less negativepotential (–0.94 V at 300 mV s—1) than on the other materials.In addition, the steady-state current reduces to one-half withrespect to the first cycle, thus denoting a low deactivationeffect. In contrast, the remarkable reduction of the Ip,steady-state

in the case of Ag_NP0, Ag_NP2 and Ag_NP5 point to either atotal deactivation or to the loss of silver. A strong deactivationis also observed for Ag_NP1 and Ag_NP4, which, nonethe-less, may be considered for further investigation.

In summary, Ag_NP3 exhibits the most promising CV fea-tures, and consequently was chosen for a rapid test underelectrolysis condition.

3.2 Electrolysis

The selected electrolytic process consisted in the cathodichydrodehalogenation of gaseous trichloromethane, using the

cell set-up presented in Figure 1. The process was driven in acontinuous mode, by feeding the cathodic gas chamber withCHCl3 saturated N2 stream. The reacted gas mixture, continu-ously vented, is periodically analysed by gas chromatography(GC) to determine the reaction products and the trichloro-methane conversion.

The reaction stoichiometry is given by the followingscheme:

At the cathode:

CHCl2Cl�g� � 2e� �H��aq�→CHCl2H�g� � Cl��aq� (8)

CH2ClCl�g� � 2e� �H��aq�→CH3Cl�g� � Cl��aq� (9)

CH3Cl�g� � 2e� �H��aq�→CH4�g� � Cl��aq� (10)

which sum up to:

CHCl3�g� � 6e� � 3H��aq�→CH4�g� � 3Cl��aq� (11)

At the anode:

3H2O�aq�→ 32

O2�g� � 6e� � 6H��aq� (12)

Membrane transport:

H��anolyte�→H��catholyte� (13)

In the hypothesis of total conversion of trichloromethaneto methane, the overall cell reaction (14) is obtained by thesum of reactions (11) + (12) + (13):

CHCl3�g� cathodicgaschamber� � 3H2O�aq� anolyte�→

32

O2�g� anolyte� � CH4�g� cathodicgaschamber�� 3HCl�aq� catholyte�

�14�

(A)(B)

Fig. 6 Cyclic voltammetry of 0.01 M trichloromethane (full lines: heavy–first cycle; light–steady-state cycle), in 0.1 M KClO4 background electrolyte, onC-ME supported materials, at room temperature. Dotted line: background electrolyte, only. Scan rate: 300 mV s–1. (A) Ag microcristalline powder; (B)VulcanXC72R powder.

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In the case of partial substrate reduction, di- and mono-chloromethanes are detected in the gaseous phase, while thechloride content of the catholyte is accordingly reduced. Byconsidering the substrate degree of conversion and the prod-uct distribution, the electroactivity of the GDE material andthe effectiveness of the electrolysis conditions are estimated[41].

To check the electroactivity of Ag_NP3_C (Ag_NP3 10%w/w), the most promising composite, an Ag_NP3_GDE wasprepared and tested under working electrolytic conditions.For comparison an Ag_GDE (Ag 20% w/w), based on com-mercial E-TEK silver powder, of the same exposed area wasalso prepared and used. The latter was selected as a referencematerial, because of the very good performances already ob-served [41] in extensive preceding tests.

The electrolyses were driven intensiostatically, at a currentdensity of 250 A m–2 for about 4 h. Before starting, thecathodic gas chamber was conditioned with the N2 + CHCl3gas feed until constant GC trichlorometane area was detectedat the gas outlet. In fact, since the substrate is partiallyabsorbed onto the carbon support, an initial reduction of thecorresponding GC area is observed. Once the current isswitched on, the excess of absorbed trichloromethane is rap-idly consumed and the process reaches the required opera-tional conditions.

Two runs for each electrode were performed; between thetwo runs, each electrode was removed from the cell, washedand dried overnight in the oven at 60 °C. The results are sum-marised in Figures 7 and 8. The CHCl3 conversion and thecorresponding CH4 production during the first run for thetwo electrodes are compared in Figure 7, while Figure 8 com-pares CHCl3 conversion of the two subsequent runs.

The first interesting outcome is that both electrodes exhibitsimilar performances, CHCl3 conversion ranging between 50and 60% and CH4 reaching almost 40% of the total reactionmixture. In both cases the trichloromethane conversion isbelow the almost complete conversion observed withAg_GDE in previous experiments. This is attributed to the

small size of the present electrodes (16 cm2) in comparisonwith the usual 100 cm2 of exposed area. In fact, the reducedarea would have required a proportional reduction of thesubstrate flow rate, too drastic to be obtained without gasrecycling. It has to be noted that, although we foresaw thepossible drawback, the reduced area was adopted to limit theamount of Ag_NP3 necessary for the GDE, an amount(≈120 mg) that, nonetheless, required a significant ‘scaling-up’ of the synthesis with respect to the ≈10 mg usually pre-pared for the CV experiments.

Given the moderate conversion of the substrate, in bothcases the product distribution points to methane as the mainproduct, with only a small fraction of CH2Cl2 and CH3Clpresent at the gas outlet and detected by GC. However, theircumulative contribution remained rather low and were there-fore not reported in the figures. An estimation of the by-prod-ucts content can also be derived from the parallel chloridedeterminations in the catholyte, whose Cl–/CHCl3 molar ratioat the end of the runs ranged between 2.14 and 2.58. Note thatthe chloride concentration, monitored at 20 min intervals,points to a current efficiency close to 100% in all cases but inthe second run of Ag_GDE, because of its evident deactiva-tion (Figure 8).

Now, comparing in more detail the two electrodes, it canbe noted that Ag_NP3_GDE requires at first an activation pe-riod longer than Ag_GDE (compare empty triangles and cir-cles in Figure 7), but subsequently shows no tendency todeactivate. In fact run 2 maintains the same CHCl3 conver-sion as run 1 (compare empty and full triangles in Figure 8).Moreover, the methane production remains systematicallyhigher than on Ag_GDE (compare full diamonds and squaresin Figure 7). Vice versa, comparing the empty and full circlesin Figure 8, Ag_GDE seems to be readily active, and tofurther improve its activity during the first 100 min of run 2,until the rapid decay of the CHCl3 conversion suggests a kindof surface deactivation. This behaviour is attributed to thegood wettability of Ag_GDE, which readily favours the gas–liquid contact and the exchange of chloride ions and protons

Fig. 7 CHCl3 conversion (empty symbols) and CH4 (full symbols) produc-tion during first electrolytic run using the two GDE electrodes:Ag_NP3_GDE triangles and diamonds; Ag_GDE circles and squares.Dotted lines are for easy reading.

Fig. 8 CHCl3 conversion during first (empty symbols) and second (fullsymbols) electrolytic runs using the two GDE electrodes: Ag_NP3_GDE tri-angles; Ag_GDE circles. Dotted lines are for easy reading.

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between the two phases, as implied by reactions (8)–(10).Consequently, the reverse of the coin is a higher tendency toflooding, with reduction of active area and hence of perfor-mance. In summary, the lower wettability of Ag_NP3_GDE,very likely bound to the presence of 3 in the electrode matrix,would be responsible for both its longer activation periodand its lower tendency to flooding, thus pointing to a predic-tably longer service life.

Notwithstanding, the limited duration of the test, and thewide margins of improvement in the case of both materials,the satisfying behaviour of Ag_NP3_GDE, which containshalf the precious metal loading of Ag_GDE, strongly sup-ports the superior role that silver nanoparticles can ultimatelyplay for the preparation of electrocatalytic materials for theorganic halide electroreduction.

4 Conclusions

Silver nanoparticles were synthesised and successfullyapplied to the electroreductive dehalogenation of organichalides.

The potentialities of the new electrocatalytic compositematerials, obtained from colloidal Ag(0) nanoparticles stabi-lised by a selection of six different ligands and supported oncarbon powder, were tested by cyclic voltammetry in aque-ous medium, by means of a cavity-microelectrode.

In the supporting electrolyte, all the composites show a sil-ver-like behaviour, only moderately modulated by the differ-ent ligands.

A preliminary ranking of the electrocatalyst activity wasobtained using trichloromethane as a model probe, whoseelectroreduction is usually masked by the hydrogen evolu-tion background current, but for the most active materials, asconfirmed by parallel measurements performed on themicrocrystalline silver and plain carbon support, taken asblank electrodes. On the basis of the evaluation criteria, i.e.the shape and the potential value of the reduction peak,together with the extent of current decrease betweenthe first and the steady-state cycles, the most promisingcomposite seems to be Ag_NP3_C, followed byAg_NP1_C≥Ag_NP4_C≥Ag_NP2_C ≈ Ag_NP5_C ≈ Ag_N-P0_C. Note that by neglecting the deactivation of the materialevidenced by the dramatic decrease in the Ip,steady-state/Ip,first

ratio, the simple, frequently used, peak potential (first cycle)criterion would have led to: Ag_NP3_C ≈ Ag_NP1_-C>Ag_NP5_C ≥ Ag_NP2_C>Ag_NP4_C>Ag_NP0_C.

The electrocatalytic activity of Ag_NP3_C, with only10 wt.-% of silver loading, was then confirmed by the resultsof the electrolytic reduction of gaseous trichloromethane, per-formed in a two-compartment membrane cell, using gas-dif-fusion cathodes based on the new material. The successfulperformance was confirmed by the comparable behaviour ofa blank GDE based on a commercial Ag/C composite (silverloading 20 wt.-%), whose effectiveness has been extensivelytested in a previous work [41].

Work is still in progress to confirm these encouragingresults. Nonetheless, the preliminary tests point to a promis-ing stability of the electrocatalytic activity of carbon-support-ed Ag nanoparticles.

The present contribution opens the way to a new class ofelectrocatalysts for the electroreductive dehalogenation oforganic halide, in which the nanometric dimensions of the Agparticles allow the retention or even the improvement of thecatalytic activity, while dramatically lowering the preciousmetal content. Further, investigations will be devoted to elu-cidate the role of the nanoparticle morphology and its effecton the substrate–electrode interactions.

Acknowledgments

The contribution of MUR–The University of Milan (FIRSTfunds) is gratefully acknowledged. A. M. wish to thank Oron-zio e Niccolò De Nora Foundation (Milan, Italy) for the fel-lowship grant. Z. Krpetic and F. Porta thank Fondazione Cari-plo for the financial support (Materiali Nanostrutturati a BaseMetallica per la Catalisi)

List of Symbols

Latin Letters

cb Bulk concentration / mol cm3

D Diffusion coefficient / cm2 s–1

E Working electrode potential / VF Faraday’s constant / C mol–1

I Current intensity / AIbackground Supporting electrolyte current intensity / nAIl Steady-state current intensity / AIp Peak current intensity / nAIp,steady-state Steady-state peak current intensity / nAL Length / cmM Molar mass / kg kmol—1

n Number of moles of electrons per moles of reac-tant

n Number of repetitive unitsr C-ME radius / cmt Time / min

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Electroreductions on Silver-Based Electrocatalysts: The Use of Ag Nanoparticles for CHCl 3 to CH 4...

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