Efficient electrocatalytic reduction of nitrite species on zeolite modified electrode with Cu-ZSM-5

Electrochimica Acta 108 (2013) 583–590

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Efficient electrocatalytic reduction of nitrite species on zeolite modified electrode with Cu-ZSM-5 Ariel Guzmán-Vargas a,∗ , Miguel A. Oliver-Tolentino a,b , Enrique Lima c , Jorge Flores-Moreno d a ESIQIE-IPN, Departamento de Ingeniería Química – Laboratorio de Investigación en Materiales Porosos, Catálisis Ambiental y Química Fina, UPALM Edif.7 P.B. Zacatenco, México, DF 07738, Mexico b UPIBI-IPN, Departamento de Ciencias Básicas, Av. Acueducto s/n, Barrio La Laguna, Col. Ticomán, México, DF 07340, Mexico c IIM-Universidad Nacional Autónoma de México, Circuito exterior s/n, Cd. Universitaria, Del., CP 04510, México, DF, Mexico d UAM-Azcapotzalco, Área de Química de Materiales, Av. San Pablo 180, Col. Reynosa Tamaulipas, 02200, México, DF, Mexico

a r t i c l e

i n f o

Article history: Received 12 April 2013 Received in revised form 11 June 2013 Accepted 8 July 2013 Available online xxx Keywords: Electrocatalytic Nitrite Reduction Zeolite modified electrode Cu-ZSM-5

a b s t r a c t The electrocatalytic reduction of nitrite has been studied on the surface of zeolite modified electrode using exchanged ZSM-5 with copper (ME/Cu-ZSM-5). The process of reduction and kinetics studies were investigated through cyclic voltammetry and chronoamperometry techniques. The i-E characteristics of ME/Cu-ZSM-5 presented the faradic process associated to redox couple Cu2+ /Cu+ with a E = 0.064 V/SCE. On the other hand, voltammetric studies showed that in presence of nitrite, the cathodic peak current of ME/Cu-ZSM-5 increases followed by a decrease in the corresponding anodic current. This indicated that nitrite species were reduced by a cooperative effect of the acidic properties of zeolite and copper that acts as a redox mediator being immobilized on the electrode surface via an electrocatalytic mechanism. Further experiments showed that electrocatalytic activity increases as Si/Al ratio decreases, the electrocatalytic activity was improved when the material Cu-ZSM-5 was calcined prior to electrode preparation. The values of the rate constant of the catalytic reduction of nitrite and the detection limit obtained were 5.9 × 102 cm3 mol−1 s−1 and 1.4 × 10−5 mol L−1 , respectively. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction The nitrite ion, NO2 − , is an intermediate species in the nitrogen cycle, resulting from the oxidation of ammonia or from reduction of nitrate [1]. It is used as an additive in some types of food and its occurrence in soils, waters, foods and physiological systems is prevalent [2]. Nitrite species are widely involved in environmental chemistry and public health; its role was recognized long time ago [3]. Although, naturally occurring concentrations of nitrite are usually of no health significance, wastes from fertilizers or intentional additions of nitrite for corrosion control are potential sources of contamination [4,5]. On the other hand, the nitrite can be combined with blood pigments to produce meta-hemoglobin, which leads to oxygen depletion to the tissues [1,6]. In aqueous solutions, nitrite displays pH-dependent homogeneous-phase equilibria, such as the acid–base equilibrium HNO2 /NO2 − (pKa = 3.16 [7], 3.37 [8]). Therefore NO2 − predominates in neutral/alkaline pH media, while HNO2 is the dominant species at pH < 2; accordingly, the electrochemical response of an electrode material will be greatly affected by the

∗ Corresponding author. Tel.: +52 5557296000. E-mail address: [email protected] (A. Guzmán-Vargas). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.07.030

shift from HNO2 to NO2 − [9–11]. Besides, HNO2 decomposes to NO in acidic media [10,12,13], which is a reactive molecule on most metal and non-metal electrodes [9–12]. The interest in the electrochemical reduction of nitrite started 10 years ago [14]. The direction electroreduction of the nitrite ion on the most bare electrode surfaces is thermodynamically favorable, however, the charge-transfer kinetics associated with the reaction is slow, consequently, the reduction potential of nitrite is extremely negative [15]. In acidic media, the main focus was on gaseous products: all noble metals react with solution-phase NO (from HNO2 decomposition) to give N2 O to E < −0.1 V vs. SCE [16–18]. Therefore, this electrochemical reduction of nitrite should be catalyzed by a convenient catalyst, which acts like a charged mediator, reducing the potential for nitrite electroreduction [19,20]. In this area, electroreduction of nitrite on modified electrodes, using catalysts such as various metal complexes and polyoxometalates (POM) of both Keggin-type and Dawson-type [15,21–23] are usually immobilized on the electrode surface. On the other hand, in the so-called zeolite modified electrode (ZME) the charge transfer mediator guest can be immobilized inside of zeolite framework, the ZME’s have been employed in oxidation of ascorbic and uric acid, cysteine, methanol and ethanol [24–27], and in the reduction of H2 O2 [28].

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Particularly, the copper ion in solution and inside of framework of different materials as Cu(II)–hexacyanoferrate(III) and in Dawson heteropolyanion have been employed as charge mediator in the electrochemical reduction of NO2 − [29–31], however, in these reports the acid media in the solution is necessary for electroreduction of nitrite species, adding HCl or H2 SO4 . In this context, the ZME modified with Cu-ZSM-5 can be used for this purpose because the redox process attributed to Cu2+ /Cu+ couple occurs in a potential window between −0.2 and 0.5 V/SCE and the zeolite acid properties [32]. Furthermore, the ZSM-5 zeolite exchanged with Fe3+ and Cu2+ has been studied in the NOx removal from gas emissions, pointing out that the system Cu-ZSM-5 showed high catalytic activity and stability in the NOx reduction to nitrogen [33–36]. So that, the aim of this work is to study the electrocatalytic activity of zeolite modified electrode with zeolite ZSM-5 exchanged with copper toward nitrite electroreduction. 2. Experimental 2.1. Cu-ZSM-5 preparation The parent zeolite was NH4 -ZSM-5 with Si/Al ratio = 15 (Zeolyst CBV3024). H-ZSM-5 form was obtained by calcination of parent zeolite on air at 773 K/5 h. The Cu-ZSM-5 was prepared by ionexchange with a 15% of theoretical exchange; the materials were labeled Cu-ZSM-5, Briefly, 2 g of NH4 -ZSM-5 were added to a 500 mL of an aqueous solution of Cu(NO3 )2 ·2.5H2 O (0.07 M) and stirred for 24 h at room temperature. Then, the solid was filtered, washed three times with de-ionized water and dried at 353 K in air atmosphere. Other NH4 -ZSM-5 samples from Zeolyst with different Si/Al ratios were also used (25, 40 and 70). 2.2. Working electrode preparation A polymer suspension was obtained by dissolving 3 mg of polymethacrylic acid methyl ester, PMMA, (Aldrich, USA) in 1 mL of methyl acrylate, MA, (Aldrich, USA) and 0.2 g of Cu-ZSM-5 or NH4 ZSM-5 were added. The resulting mixture was homogeneously dispersed in an ultrasound bath for 30 min. Then, 0.5 ␮L of the resulting suspension were deposited on the surface of glassy carbon electrode and dried with argon at room temperature. The obtained zeolite modified electrodes (ZME) were labeled as ME/Cu-ZSM-5, ME/NH4 -ZSM-5 and ME/Cu-ZSM-5C when the zeolite was calcined prior to electrode preparation. 2.3. Solid-state Nuclear Magnetic Resonance 27 Al and 29 Si MAS Solid-state 27 Al and 29 Si Nuclear Magnetic Resonance (NMR) single excitation experiments were performed on a Bruker Avance 400 spectrometer at frequencies of 104.2 and 79.4 MHz, respectively. 29 Si NMR spectra were acquired using the combined techniques of Magic Angle Spinning (MAS) and Proton Dipolar Decoupling (HPDEC). Direct pulsed NMR excitation was used throughout, employing 90◦ observing pulses (3 ␮s) with a pulse repetition time of 40 s. Powdered samples were packed in zirconia rotors. The spinning rate was 5 kHz. Chemical shifts were referenced to TMS. 27Al MAS NMR spectra were acquired using short single pulses (␲/12) and a delay of 0.5 s. The samples were spun at 10 kHz, and the chemical shifts were referenced to an aqueous 1 M AlCl3 solution.

scanning at angular intervals of 0.08◦ . For this purpose the X-ray diffraction patterns samples were obtained on a Siemens D500 diffractometer with a molybdenum X-ray anode tube. The K␣ radia˚ was selected with a diffracted beam tion (wavelength of 0.70930 A) monochromator. 2.5. Raman spectroscopy The Raman spectroscopic measurements were performed at room temperature in a Labram HR800 spectrometer equipped with a laser operating at 784.29 nm, the experiment was obtained in a range between 200 and 1800 cm−1 . The microscope used was OLYMPUS BX41 with 100×. 2.6. Electrochemical measurements The electrochemical analyses were carried out at room temperature in a potentiostate-galvanostate VERSASTAT3-400 (Princeton Applied Research). A three-electrode standard electrochemical cell was used for the cyclic voltammetry (CV) measurements at 20 mV s−1 . A carbon rod and a calomel (SCE) electrode were used as counter and reference electrode, respectively. Prior to use, the solution was purged with argon for at least 15 min. The i-E characteristics were recorded in the interval from −0.2 V to 0.5 V/SCE. The initial potential was fixed at open circuit potential toward cathodic direction. Solution 0.1 M of NaCl was used as supporting electrolyte. 3. Results and discussion 3.1. Solid-state Nuclear Magnetic Resonance 27 Al and 29 Si MAS In a previous work, XRD results of Cu-ZSM-5, did not show major changes in the zeolite framework after copper incorporation [37], however, subtle structural changes cannot be characterized by XRD. In this sense, NMR is most useful as the 27 Al and 29 Si resonances are very sensitive to chemical environments of aluminum and silicon atoms, respectively. The 29 Si MAS NMR spectra displayed in Fig. 1A reveal that the zeolite framework does not change as a consequence of copper incorporation. A careful deconvolution (figure not shown) obtained from spectra reveals the four characteristic peaks associate with the zeolite framework at −113.8 and −107.8 ppm which are normally assigned to resonances from the Si in the SiO4 tetrahedral around with 4 and 3 Si atoms, i.e., 4Si, 0Al and 3Si, 1Al units, respectively. The shoulder peak centered at about −117.2 ppm is attributed to the existence of crystallographically inequivalent sites in the zeolite. The peak at −103 ppm in the 29 Si MAS NMR spectrum could be assigned to the silanols, where the terminal hydroxyl

A

B

-113.8

-117.2

-103 -107.8

-95 -100 -105 -110 -115 -120 -125

ppm

2.4. Radial distribution function (RDF) -80

The radial distribution functions were calculated from the full diffraction patterns in order to obtain high values of the angular parameter h = 4␲ sin /, the diffractogram was measured by step

-90

-100

-110

/ppm

-120

-130

200

100

0

-100

/ppm

Fig. 1. (A) 29 Si MAS RMN and (B) 27 Al MAS RMN of Cu-ZSM-5.

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585

0

1

2

3

4

5

6

7

8

9

470

805

a 1407 1456

b

291

Intensity /a.u

RDF

a

440

380

878

1635

b c

10

R / Amstrong

200

400

600

800

3.2. Radial distribution function (RDF) Short and long range order of zeolitic materials, and interatomic distances can be determined from the radial distribution functions. In any zeolite system, the distances between the atoms in the zeolite (Si, Al, O and compensating cations) change due to different cation exchange degree, structure of zeolite and chemical composition. Fig. 2 exhibits the RDF profiles for H-ZSM-5 (curve a) and Cu-ZSM-5 (curve b), both presented the same radial distances showing that the interatomic distances corresponding to the first neighbors are similar, they correspond indeed to the distances in the primary silicon–oxygen tetrahedral. The peaks were assigned following the reported theoretical results [39]. The positions c.a. 1 A´˚ is attributed to the O H bond, which is localized in each terminal O atom in the direction of the next T (tetrahedral) site, the peak c.a. 1.6 and 4.1 A´˚ suggest the (Al, Si) O interaction, the O O bond can ´˚ the peaks around 3.25 and 4.6 A´˚ can be associbe observed in 2.4 A, ated to the contribution of the O O and (Al, Si) Si bond, the Al Al ´˚ The shift of radius between H-ZSM-5 bond can be found to 5.5 A. and Cu-ZSM-5 is associated to the presence of copper in the zeolite framework and is due to the contributions of coordination bonds ´˚ this can be verified ˚ Cu Al (2.87 A) ˚ and Cu Si (3.13A), Cu O (2.1 A), in the different profiles observed in the region between 6 and 8 A´˚ [40].

1200

1800

Fig. 3. Raman spectroscopy of (a) Cu-ZSM-5, (b) Cu-ZSM-5/MA and (c) Cu-ZSM5/MA/PMMA.

observed c.a. 1460 cm−1 and the CH2 scissors deformation can be found c.a. 1400 cm−1 , and the band at 878 cm−1 is attributed to CH3 vibration [42]. 3.4. Electrochemical reduction of nitrite on the glassy carbon surface The electrochemical reduction of nitrite on the surface of a glassy carbon electrode (GC) was studied using a solution of NaCl (0.1 M), as supporting electrolyte; the results are depicted in the inset of Fig. 4. The i-E characteristics of GC in NaCl with and without NO2 − , curve b and a respectively are showed, any faradic processes associate to nitrite reduction were not observed in the windows potential employed. With the purpose to evaluate the redox Cu2+ /Cu+ couple as charge mediator for the reduction of NO2 − , a solution of CuCl2 (0.01 M) was employed the pH obtained in the solution was 4.55. The electrochemical behavior of copper on the surface of GC without NO2 − is shown in Fig. 4 (curve a), the cathodic peak (Rd) c.a. to −0.06 V/SCE is attributed to the reduction of Cu2+ to Cu+ , whereas the anodic process (Ox) c.a. to 0.11 V/SCE indicates the oxidation of Cu+ to Cu2+ , on the other hand, the electrochemical profiles of copper in presence of 0.01 M of NO2 − (curve b), exhibited a little increase of the cathodic density current (RdN ), suggesting a chemical redox reaction between nitrite and Cu+ , which occurs at the surface of GC, where the NO2 − is reduced and the Cu+ is oxidized; this behavior can be verified during anodic scan where the current 1500 1000

Ox

OxN

a

b

-2

0 60

-500 -2 j / A cm

j / A cm

Fig. 3 compares the Raman spectra of Cu-ZSM-5 (curve a), the mixture Cu-ZSM-5/MA (curve b) and Cu-ZSM-5/MA/PMMA (curve c), each spectrum exhibited the band characteristics of the ZSM5 structure, the band c.a. 291 cm−1 is assigned to bending mode of 6-membered ring, the band at 380 cm−1 is associated to a five membered building unit of ZSM-structure zeolites, the vibration of 4-membered ring are observed to 440 and 470 cm−1 , and the 805 cm−1 band is related to the framework symmetric stretching vibration in ZSM-5 [41], on the other hand, the mixture Cu-ZSM5/MA (curve b) and Cu-ZSM-5/MA/PMMA (curve c) exhibited the bands associated to the presence of organic compounds in the CuZeolite, the Raman band around 1635 cm−1 is characteristic of C C stretching in methyl methacrylate, the C CH bending modes is

1600

Raman Shift /cm

500

3.3. Raman spectroscopy

1400

-1

Fig. 2. Radial distribution function of (a) H-ZSM-5 and (b) Cu-ZSM-5.

groups are connected directly to the Si atom in SiO4 tetrahedron (Si(–OH)1 (–OSi)3 units) [38]. The 27 Al MAS NMR spectra (Fig. 1B) showed an intense signal c.a. 53 ppm assigned to tetrahedral coordinated framework aluminum, AlIV . In addition, a low-intensity peak c.a. 0.5 ppm is associated to non-framework octahedral aluminum, AlVI , was also present. The AlVI signal is not consequence of copper incorporation, because this aluminum species are present in the calcined zeolites [36].

1000

-1000 -1500 -2000

Rd

b

40 20 0 -20 -40

RdN

a -0.2

0.0 0.2 0.4 E / V vs SCE

0.6

-2500 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

E / V vs SCE Fig. 4. Cyclic voltammetry of GC in NaCl (0.1 M) + CuCl2 (0.001 M), (a) without nitrite and (b) with nitrite (0.01 M) at pH = 4.55; inset Cyclic voltammetry of GC in NaCl (0.1 M) (a) without nitrite and (b) with nitrite (0.01 M).

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1400

b

a

OxN

OxN

700

0

0

-700

j / A cm-2

-2

1.4 1.2

-1400

1.0

Rd

-2100

EF

j / A cm

Ox

1400

Ox

700

-2800

0.8 0.6 0.4

RdN

-3500

-700

a

-1400 -2100

Rd

b

RdN

-2800 0

1

2

3

pH

4

5

-3500

RdNI

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.3 -0.2 -0.1 0.0

E / V vs SCE

0.1

0.2

0.3

0.4

0.5

E / V vs SCE Fig. 5. Cyclic voltammetry of GC in NaCl (0.1 M) + CuCl2 (0.001 M), (a) without nitrite and (b) with nitrite (0.01 M) at pH = 1; inset enhancement factor vs. pH of solution.

of oxidation peak (OxN ) decreases. As the electrochemical reduction of nitrite depends on the pH value of the aqueous solution, several experiments to different pH were carried out and the results are showed in Fig. 5. At pH = 1, the curve a shows the electrochemical behavior of redox couple Cu2+ /Cu+ without nitrite, it is evident that the pH has not any effect over faradic process of copper, however, the electrochemical behavior in the presence of 0.01 M of NO2 − , as observed in Fig. 5, the cathodic peak density current was greatly increased over ordinarily observed just for redox couple Cu2+ /Cu+ , while the corresponding anodic peak was substantially depressed on the reverse cycle, demonstrating that the electroreduction of nitrite can be catalyzed by copper as a mediator. This behavior is typical of that expected for electrocatalytic reduction (EC mechanism) as the following scheme proposed by Ojani et al. [20]. Cu2+ + e− → Cu+

E

Cu+ + HNO2 → NO + Cu2+ + H+

C

In order to evaluate the electrocatalytic activity toward nitrite electroreduction, the enhancement factor (EF) was calculated using Eq. (1) [28]. EF =

IprdN − Iprd Iprd

(1)

where the Iprd is the cathodic current intensity in absence of nitrites and IprdN is the cathodic current in the presence of different nitrite concentrations. Experiments at different pH were carried out using HCl, the results are observed in the inset of Fig. 5, from this figure it is evident a direct pH effect, higher EF value is obtained for pH near to zero, this behavior can be attributed to the electroreduction of nitrite is controlled by the simultaneous transfer of the electron and proton from a proton donor, localized in the reaction zone [43,44], in this case, the proton was donated by the acid in solution. 3.5. Electroreduction of nitrite on the ME/NH4 -ZSM-5 In Fig. 6, curve a displays the electrochemical behavior of ME/NH4 -ZSM-5, in a solution of NaCl (0.1 M) and CuCl2 (0.01 M) at pH = 4.55, at a potential scan rate of 0.02 V s−1 . Cathodic and anodic peak potentials (Epc , Epa ) and peak separation potential (Ep ) associated to redox process of copper were: 0.103, 0.164 and 0.065 V, respectively. In Fig. 6b, c.a. 0.064 V/SCE is observed that the cathodic peak (RdN ) current was considerably increased with respect to that

Fig. 6. Cyclic voltammetry of ME/NH4 -ZSM-5 in NaCl (0.1 M) + CuCl2 (0.001 M), (a) without nitrite and (b) with nitrite (0.01 M).

observed just for the redox couple Cu2+ /Cu+ (Ox/Rd), while the corresponding anodic peak (OxN ) was substantially depressed during anodic scan, the EF obtained was 1.25 this value is similar to 1.29 which was calculated from Fig. 5 at pH = 1, this behavior demonstrated that the acid conditions required for the electroreduction are provided by the zeolite surface. Besides, near to −0.09 V/SCE (RdNI ) an increment in the faradic current is evident; this phenomenon can be attributed to the second reduction process, which will be discussed below. 3.6. Electroreduction of nitrite on the ME/Cu-ZSM-5 The electrochemical profiles of ME/Cu-ZSM-5 in the absence of NO2 − (figure not shown) present the characteristic electrochemical behavior of redox couple Cu2+ /Cu+ as reported before [37], whereas in presence of nitrite the cathodic current density associated to the nitrite reduction increases, the EF value was 1.56, suggesting that the copper inside of zeolite present a higher catalytic activity toward nitrite reduction, than when copper is in solution. This behavior can be explained in base to work published by our group [37], in the case of ME/NH4 -ZSM-5 (i) the copper and the other cation in solution begin to exchange with NH4 + inside of zeolite, at the same time (ii) the copper in solution together NO2 − and electrolyte support permeates through the PMMA in the aperture between zeolite particles until reach the uncovered area in the glassy carbon were the copper faradic process occurs, whereas in the ME/Cu-ZSM-5 the copper inside of zeolite is exchanged by the cation in solution but more slowly than the NH4 + , in this modified electrode the reduction of copper takes place in the contact area and the nitrite exhibited more interaction with the zeolite particle and its acid property. 3.7. Effect of the acid properties of zeolite over the nitrite reduction The type of acidity presented by the zeolite Cu-ZSM-5 without thermal treatment is the Brønsted acidity, however, when the zeolite is calcined can occur some dealumination of the zeolite framework and the deposition of extraframework alumina species in the structure [45,46]. Taking into account this fact, the electrochemical reduction of nitrite was carried out on a calcined material, ex situ treated samples prior to electrode preparation, the thermal treatment of Cu-ZSM-5 (Si/Al = 15) was performed at 823 K in air for 8 h (50 cm3 min−1 , ramp: 10 K min−1 ), then, the electrode was labeled as ME/Cu-ZSM-5C.

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Ox

OxNI

j / A cm

-2

EF

a

-500 Rd

-1000

0.4

C

-1500 0.3

Rd N

EF

-2000

b

0.2

Rd NI

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 E / V vs SCE

A

OxI

B

10000

2000

8000 Ox 6000

1000

4000 0

OxNII

2000 0 Rd

0.1

RdN

-2000 0

150

300

450

Fig. 7. (A) Cyclic voltammetry of ME/Cu-ZSM-5C in NaCl (0.1 M) (a) without nitrite and (b) with nitrite (0.01 M). (B) Relationship between enhancement factor and Si/Al ratio and (C) Relationship between enhancement factor and nitrites concentration.

Fig. 7A exhibits the electrochemical profiles of ME/Cu-ZSM-5C in the absence (curve a) and in the presence (curve b) of 0.01 M of NO2 − , the experiments were carried out in a NaCl solution as supporting electrolyte. In the curve a, the reduction process (Rd) of copper take place at c.a. 0.096 with a E = 0.064 V/SCE, whereas in the curve b the cathodic process (RdN ) associated to the reduction of nitrite occurs at c.a. 0.032 V/SCE, further around −0.06 V/SCE the faradic current (RdNI ) started to increase. In contrast, during the anodic scan, it is evident that the anodic peak (OxN ) decreases, in addition, a new oxidation process appears c.a. 0.45 V/SCE (OxNI ), the EF value is 2.15 which was higher than EF value exhibited by the ME/Cu-ZSM-5. This behavior can be attributed to acidity of octahedral aluminum present in the calcined sample [45] and to the progressive rehydration of the zeolite when is in contact with the solution [37]. It is well known, that the acidity of the zeolite is provided by the presence of trivalent aluminum, which introduces a negative charge within the crystal structure. Counterions, such as H+ , are adsorbed over oxygen atoms close to the aluminum centers, in order to balance the charge [46]. Thus, the number of exchange sites and the acidity of zeolites depends on the Al content, as a consequence, when the Si/Al ratio increases as the acidity decreases [47], In order to examine the effect of Si/Al ratio on the electrocatalytic reduction of nitrite, experiments with ZME prepared with zeolite ZSM-5 with different Si/Al ratio were carried out, the results of enhancement factor vs. Si/Al ratio are showed in the inset of Fig. 7B, the EF of nitrite electroreduction increases as Si/Al ratio decreases, these results suggest that the material ZSM-5 with Si/Al = 15 ratio promotes the higher electrocatalytic activity; Fig. 7C exhibits a linear correlation between EF and nitrite concentration in the range of 20–500 ␮mol L−1 (R2 = 0.998), in addition, the detection limit of nitrite concentration was 14 ␮mol L−1 . The electrochemical behavior of ME/Cu-ZSM-5C without and in presence of nitrite, using NaCl (0.1 M) as supporting electrolyte, are depicted in Fig. 8, plot A and B, respectively, the experiments were carried out in a window potential between −0.7 and 0.7 V/SCE. The results exhibited the characteristic redox process of ME/Cu-ZSM5C without nitrite (Fig. 8A). The peaks Rd (c.a. 0.096 V/SCE) and RdI (c.a. −0.35 V/SCE) are attributed to the reduction of Cu2+ until Cu0 (metallic copper) by two reaction steps involving one electron of charge transfer, whereas, for the anodic scan are evident two oxidation peaks OxI and Ox associated to oxidation of Cu0 to Cu+ (c.a. −0.07 V/SCE) and Cu+ to Cu2+ (c.a. 0.16 V/SCE). On the other hand, in presence of nitrite a very complex profile was observed (Fig. 8B). During the cathodic scan, the peak RdN c.a.

RdNI

RdI

-2000

-1

C / mol L

OxNI OxN

-1000

-2500 -3000

12000

3000

B

500 0

3.0 2.5 2.0 1.5 1.0 0.5 0.0 10 20 30 40 50 60 70 Si/Al ratio

-2

OxN

A

j / A cm

1000

587

-0.6 -0.3

0.0

0.3

0.6

-4000

RdNII

-0.6 -0.3

RdNIII

0.0

0.3

0.6

E /V vs SCE Fig. 8. Cyclic voltammetry of ME/Cu-ZSM-5C in NaCl (0.1 M) and in: (A) absence of nitrite and (B) 0.01 M NO2 − .

0.03 V/SCE indicates the electroreduction of NO2 − as mentioned in Section 3.1, near to −0.3 V/SCE the peak RdNI put in evidence the formation of Cu0 , while the reduction peak RdNII (c.a. −0.55 V/SCE) suggests a possible reduction of NO formed in the process RdN . Even as, during the anodic scan the peak OxNI is due to the oxidation of Cu0 to Cu+ , which is confirmed by the presence of RdNIII peak, this process is attributed to the electroreduction of NO2 − species due to formation of Cu+ in the process OxNI , in addition, the peak OxN appears as a result of the remaining Cu+ is oxidized to Cu2+ , and the peak OxNIII involves the oxidation of nitrated species formed in the anodic processes RdNIII and RdN . In previous work was proposed [37] that for ME the redox process takes place in the contact area, through the surface mediated electron transfer mechanism, in this case, the electroactive species situated at outer zeolite surface begin the charge transfer followed by electron hopping to the probes located on the bulk solid, while the charge is compensating by cation migration. Thus, the next scheme can describe, in general approach, the electroreduction of nitrite species. + + e− → Cu(I)esi + M+ Cu(II)esi + Mso1 Zeo1

Cu(I)esi + NO− 2 → Cu(II)esi + Reduced Nitrated Species esi is the electroactive species interface, zeol is the zeolite, and sol is the solution. 3.8. Kinetics parameters calculation for ME/Cu-ZSM-5C without nitrite Experiments to different scan rates in a 0.1 M NaCl solution for ME/Cu-ZSM-5C, were carried out. The current peaks obtained by the cyclic voltammetry profiles (figure not shown) present a linear relationship respect to scan rate (Fig. 9B), indicating a typical diffusional controlled process. On the other hand, the electrode reactions are controlled by an electron-transfer kinetics (ETK) as the peak-to-peak potential interval (Ep ) increases with respect to the sweep potential (Fig. 9, inset A) [48–50]. Therefore, ETK parameters such as the electron-transfer rate constants can be calculated according to Laviron model (Eq. (2)) [51]. This equation gives a general analysis for voltammetric-lineal sweep responses in the case of surface-confined electroactive species with small enough concentrations at modified electrodes. Epc = E 0 −

RT ln ˛nF

 ˛nF  RTks

(2)

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0.15 100

B

80 0.10 60 40 20

Ip / A

EP

0.05

0.00

0 -20 -40

-0.05 -60

A

-80

-0.10 5.2 5.4 5.6 5.8 6.0 6.2 6.4

6

9 12 15 18 21 24

1/2/(mVs-1)0.5

-1

Ln( ) / mVs

Fig. 9. (A) Ep vs. ln() and (B) ip vs. 1/2 of experiments to different scan sweep of ME/Cu-ZSM-5C in 0.1 M of NaCl.

In Eq. (2), ˛ is the electron-charge transfer coefficient,  is the scan rate (V s−1 ), n is the number of transferred electrons and ks the charge transfer rate constant (s−1 ) for electron transfer between the electrode and the surface deposited layer, E0 is the formal potential, T = 298 K, R = 8.314 J mol−1 K−1 and F = 96,480 C mol−1 . E0 is obtained from data given in an Epc vs.  plot at  = 0 (figure not shown). The calculated values for E0 in the cathodic and anodic 0 = 0.052 and E 0 = 0.146 V, respectively. From portion were Epc pc the linear portion of the plot [Ep = (Ep − E1/2 )] vs. ln(), inset A in Fig. 9, ks and ˛ can be calculated, taking into account that E1/2 = (Epa − Epc )/2, the values of ˛ and ks were 0.53 and 14.86 s−1 , respectively, for the ME/Cu-ZSM-5C in the presence of 0.1 M NaCl.

with the square root of scan rate (Fig. 10B) indicating that the transport of electroactive species is governed by diffusion. Besides, in Fig. 10A shows the variation of Ep vs. log() plot which exhibit an indicative shape typical of an EC catalytic process, also the Tafel slope, b, was obtained using the following equation valid for a totally irreversible diffusion-controlled process [25]: EP =

b 2

log  + constant

(3)

On the basis of Eq. (3), the slope of the Ep vs. log() plot is b/2, where b is the Tafel slope. The slope of the Ep vs. log() plot is ∂Ep /∂ log() was found to be 0.026 V in this work (Fig. 10A), so, b = 2(0.0403) = 0.0806 V. The value of Tafel slope indicates that a one-electron transfer process is the rate limiting step. The variation of the ratio ip × −1/2 vs.  (Fig. 10C) evidenced that the reduction of nitrite ions agrees with the case VII for catalytic reactions: Nicholson–Shain scheme, where occurs a chemical reaction preceding a quasi-reversible charge transfer [52].

3.9. Kinetics parameters calculation for ME/Cu-ZSM-5C with nitrite The cyclic voltammogram of ME/Cu-ZSM-5C at various scan rates in the presence of 0.01 M NO2 − was performed (figure not shown). The reduction current for nitrites was increased linearly

0.04

0.00 0.0

A 0.03

B

C

-0.01

-0.3 -1/2

0.02

-0.9 -1.2

Ip*

0.00 -1.5 -0.01

-0.03 -0.04 -0.05

-1/2

0.01

/(mA mV

1/2

s

-0.6

Ip / mA

Ep / V vs SCE

)

-0.02

-0.06 -0.07

-1.8

-0.08 -0.02

-2.1 1.8 2.1 2.4 2.7 3.0 -1

Log( )/mV s

5

10 15 20 25 1/2

-1 1/2

/(mV s )

0

200 400 -1 / mV s

600

Fig. 10. (A) Ep vs. log(), (B) ip vs. 1/2 and (C) ip × −1/2 vs.  of experiments to different scan sweep of ME/Cu-ZSM-5C in 0.1 M of NaCl and in presence of 0.01 M of NO2 − .

A. Guzmán-Vargas et al. / Electrochimica Acta 108 (2013) 583–590

0 -50

Acknowledgments

b

a

ICYTDF PICS09-321, CONACYT 101319 and SIP-IPN 20131229 projects for financial support.

c

-100

d

IC / I L

i/ A

-150 -200 -250 -300

3.02 3.00 2.98 2.96 2.94 2.92 2.90 2.88 2.86 0.5 0.6 0.7 0.8 0.9 1.0 1/2 1/2 t /s

References

-350 0

1

589

2

3

4

5

6

7

8

9

10

t /s Fig. 11. Chronoamperograms obtained for the ME/Cu-ZSM-5C: (a) whit out nitrite, (b) 0.001 M, (c) 0.01 M, and (d) 0.1 M of NO2 − in NaCl (0.1 M), the applied potential was −0.05 V/SCE, inset: dependence of IC /IL vs. t1/2 . Inset information is derived from the profiles a and d of the main panel.

3.10. Chronoamperometric study Fig. 11 displays the chronoamperogram of NO2 − at ME/Cu-ZSM5 C, obtained by setting the working electrode potential at −0.05 V vs. SCE for several nitrite concentrations. Taking advantage of this technique, the rate constant for the chemical reaction between the nitrite and the redox sites of ME/Cu-ZSM-5C can be evaluated by chronoamperometry, according to the method described by Eq. (4) [26,53]: IC =  1/2 [ 1/2 erf( 1/2 ) + exp(−) 1/2 ] IL

(4)

where IC is the catalytic current of the ME/Cu-ZSM-5C in the presence of nitrite, IL is the limiting current in the absence of NO2 − and  = kC0 t (C0 is the bulk concentration of nitrite) is the argument of the error function. In the cases where  exceed 2, the error function is almost equal to 1 and Eq. (4) leads to [26,54]: Ic 1/2 =  1/2 1/2 = 1/2 (kC0 t) IL

(5)

where k, C0 and t are the catalytic rate constant (cm3 mol−1 s−1 ), nitrite concentration (mol cm−3 ) and time elapsed (s), respectively. We can simply calculate the value of k for a given concentration of substrate from the slop of the IC /IL vs. t1/2 plot. The inset of Fig. 11 shows one such plot, constructed from the chronoamperogram of the ME/Cu-ZSM-5C in the absence and presence of 0.1 M nitrite solution. The mean value for k was found 5.9 × 102 cm3 mol−1 s−1 . 4. Conclusions The experiment results reported in this work demonstrated that copper exchanged in ZSM-5 framework operates as a charge mediator for the electrocatalytic reduction of nitrite species on neutral conditions; the acid properties of zeolite improve the electrocatalytic activity, it means that any other acid substance in solution is necessary to add. Thus, the Cu-ZSM-5 zeolite modified electrode exhibited excellent electrocatalytic activity for reduction of nitrite. Furthermore, using cyclic voltammetry and chronoamperometry techniques, the kinetics parameters such as charge transfer coefficient (˛), standard rate constant of reaction (ks), the catalytic chemical reaction rate constant (k) and detection limit for reduction of nitrite were determined.

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