Electrocatalytic Oxidation of Methanol, Ethanol, and Glycerol on Ni(OH

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Energy, Environmental, and Catalysis Applications

Electrocatalytic oxidation of methanol, ethanol, and glycerol on Ni(OH) nanoparticles encapsulated with poly[Ni(salen)] film 2

José Luiz Bott-Neto, Thiago Serafim Martins, Sergio A. S. Machado, and Edson A. Ticianelli ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08441 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Applied Materials & Interfaces

Electrocatalytic Oxidation of Methanol, Ethanol, and Glycerol on Ni(OH)2 Nanoparticles Encapsulated with Poly[Ni(salen)] Film José L. Bott-Neto*†, Thiago S. Martins, Sérgio A. S. Machado, Edson A. Ticianelli.

São Carlos Institute of Chemistry, University of São Paulo, P.O. box 780, São Carlos, 13560-970, São Carlos SP, Brasil.

KEYWORDS. fuel cell, electrocatalyst, nickel, nanoparticles, alcohol, in situ FTIR.

ABSTRACT. This study describes a systematic investigation of the electrocatalytic activity of poly[Ni(salen)] films, as catalysts for the electro-oxidation of Cn alcohols (Cn = methanol, ethanol and glycerol) in alkaline medium. The [Ni(salen)] complex was electropolymerized on a glassy carbon surface and electrochemically activated in NaOH solution by cyclic voltammetry. X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) results indicate that during the

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activation step the polymeric film hydrolyzes, leading to the formation of β-Ni(OH)2 spherical nanoparticles (NPs), with an average size of 2.4 ± 0.5 nm, encapsulated with the poly[Ni(salen)] film. Electrochemical results obtained together with the in situ Fourier transform infrared spectroscopy (FTIR) confirm that the electro-oxidation of methanol, ethanol, and glycerol occurs involving a cycling oxidation of β-Ni(OH)2 with the formation of β-NiOOH species, followed by the charge transfer to the alcohols, which regenerates β-Ni(OH)2. Analyses of the oxidation products at low potentials indicate that the major product obtained during the oxidation of methanol and glycerol is the formate, while the oxidation of ethanol leads to the formation of acetate. On the other hand, at high potentials (E = 0.6 V), there are evidence that the oxidation of Cn alcohols leads to carbonate ions as an important product.

1. INTRODUCTION The development of technologies for the production and storage of energy in a sustainable way is essential for future generations. Even today, world energy demand is supplied mainly by petroleum derivatives. Thus, in order to face the predictable depletion of such resources and to reduce the impact caused by the burning of these fuels, the use of renewable sources is essential.1–3 As a consequence, a great deal of work has been directed to investigate alcohols electro-oxidation


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reactions for various applications such as (i) energy conversion devices (e.g. fuel cells);4–7 (ii) processes involving electrosynthesis of organic compounds8,9; (iii) electrolyzers, aiming at the production of hydrogen with co-generation of high-value chemicals (mainly in the case of glycerol).10–12 In several recent studies, the electro-oxidation of Cn alcohols on noble-metal-based such as Pt, Au and Pd and their alloys have been carried out because they are more active and stable electrocatalysts.13–16 However, these metals are easily poisoned by COads formed during alcohol oxidation,17 which slows down the kinetics of surface reactions. In this way, many efforts have been devoted to increase the activity of electrocatalysts derived from non-noble metals or to design more efficiently and lower cost new materials.18 In this context, Schiff base transition metal complexes have been explored as prospective materials for energy storage devices19–23, catalytic systems18,21,24, and electrochemical sensors.25,26 It has been observed that these materials exhibit high control and homogeneity of the layer when electropolymerized on the electrode surface, high conductivity, reversible electrochemical oxidation, high thermal stability and, more recently, a way of obtaining homogeneous and lowsize nanoparticles.19,27 In particular, the N,N'-bis(salicylidene)ethylenediaminonickel(II) complex, [Ni(salen)], has been found to be active for the electro-oxidation of alcohols, such as methanol, ethanol, and glycerol in alkaline medium24,28. The vast application of nickel-based electrocatalysts is because they present electrocatalytic activity, anti-poisoning capabilities, long-term stability in alkaline solutions, as well they are competitively priced materials, making them attractive candidates for numerous applications.1 Previous work has demonstrated that an uncycled pentadentated nickel(II)-shift base catalyst graphed on polystyrene presents quite reasonable activity for the oxidation of alcohols, such as methanol and ethanol, but the effect of previous


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electrode cyclings and the possibility of forming Ni-based nanocatalyst centers were not considered.29 In this work, the electrocatalytic activity of electrodes modified with Ni(II)-salen polymer film in alkaline medium was investigated for the electro-oxidation of Cn alcohols (Cn = methanol, ethanol, and glycerol). The prepared catalyst was characterized by XRD, TEM, XPS, ultravioletvisible spectroscopy (UV-Vis) and cyclic voltammetry (CV). Here in situ FTIR experiments coupled to electrochemical experiments were used to monitor the electro-oxidation of these alcohols on poly[Ni(salen)]ATV, so to improve the understanding of the reaction mechanism and of the electrode electrokinetic dynamics. 2. EXPERIMENTAL 2.1. Synthesis of [Ni(salen)] complex The [Ni(salen)] complex was obtained from the condensation of salicylaldehyde (Sigma-Aldrich, 99.9 %) with an amine, followed by the step of coordinating nickel metal ions, as described previously.25,30–33 In brief, an ethanolic solution of ethylenediamine (2:5 volume ratio) and salicylaldehyde (3:5 volume ratio) were mixed and refluxed during 2 hours at 40 °C under a nitrogen atmosphere. After this, a yellow precipitate was collected by vacuum filtration, recrystallized on methanol (Sigma-Aldrich, 99.9%) and cooled to room temperature.34 Next, 100 mL of ethanol (Synth, 99%) was placed in a round-bottom flask and left at 40 °C for 30 min under an inert atmosphere to remove air. Afterward, 6.4 mmol of prepared H2salen ligand and 6.4 mmol of nickel (II) acetate (Vetec Quimica Fina Ltda, 99%) were dissolved in 100 mL of treated ethanol. The solution was refluxed for 2 hours at 40 °C under nitrogen atmosphere.30–33


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2.2. Electrode modification and electrochemical measurements All electrochemical experiments were carried out in a three-electrode cell at room temperature (23 ± 1 °C). A glassy carbon disk (0.196 cm²) modified with poly[Ni(salen)] film was used as a working electrode, a Pt foil as a counter electrode and Ag/AgCl (3.0 M KCl) as a reference electrode. Initially, glassy carbon electrode was polished with 0.05 µm alumina (Buehler) and sonicated with ethanol and ultrapure water (Purelab Ultra, Elga-Vivendi) during 5 minutes each. The oxidative electrochemical deposition was performed by cyclic voltammetry. Six cycles were taken from a mixture containing 5 mM [Ni(salen)] complex in a 0.1 M tetrabutylammonium perchlorate (TBAP, Sigma-Aldrich, 99.9%) / acetonitrile (CH3CN, Sigma-Aldrich, 99.9%) solution from 0 V to 1.1 V at 100 mV s-1 and immediately afterwards, the electrode was rinsed with ethanol. After the electropolymerization step, the electrode was electrochemically activated in N2-saturated 1 M NaOH solution between 0 and 0.6 V at 50 mV s-1. One hundred cycles were required until stable

voltammograms

were

obtained.

This

electrode

was

arbitrarily

called

here

poly[Ni(salen)]ATV. Finally, the electrocatalytic activity of the poly[Ni(salen)]ATV was investigated for the electro-oxidation of the Cn alcohols in the concentration of 0.1 M of alcohol in N2-saturated 1.0 M NaOH solution. It is important to note that all parameters used in the electropolymerization and activation steps have been optimized, as discussed in more detail in a previous publication.25 2.3. Characterization The crystalline structures of the [Ni(salen)] complex and of the poly[Ni(salen)]ATV film were determined by using the XRD technique. XRD patterns were collected using a diffractometer (Brucker, D8 advance) with Cu-kα radiation (λ = 1.5418 Å) operating at 40 kV/40 mA. The


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analyzes of the poly[Ni(salen)]ATV film were obtained in the grazing incidence X-ray diffraction mode (GIXRD). In this case, a platinum foil (1 cm² x 0.1 cm) was used as a substrate for the preparation of the polymeric film. The morphology and particle size of the freshly activated materials were analyzed by TEM and high-resolution TEM (HR-TEM) in a JEOL JEM-2100 (LaB6) microscope. The complex was also characterized by ex situ infrared spectroscopy (FTIR) in a Shimadzu IRAffinity 1 and by ultraviolet-visible spectroscopy in a JASCO V-630 spectrophotometer with 1 cm path quartz cuvette optical. The oxidations states of the elements composing the complex and poly[Ni(salen)]ATV film were investigated by X-ray photoemission spectroscopy. XPS spectra were obtained with a Scientia Omicron ECSA+ spectrometer equipped with an EA125 hemispherical analyzer and a XM1000 monochromated X-ray source using Al Kα radiation (hv = 1486.6 eV). X-ray sources with a power of 280 W and a constant pass energy of 50 eV were used. A Cn 10 Omicron Charge neutralizer with a beam energy of 1.6 eV was used. Survey and high-resolution XPS spectra were obtained with a pass energy of 0.5 and 0.05 eV, respectively. These results were fitted using CASA XPS® program and the spectral energy was calibrated at 284.8 eV for C 1s. 2.4. In situ FTIR The electro-oxidation reaction of the Cn alcohols was investigated by in situ FTIR. These measurements were performed on a Bruker VERTEX 70v spectrometer equipped with an MCT detector coupled to a Metrohm Autolab PGSTAT302N potentiostat. A home-made three-electrode spectroelectrochemical cell with a platinum gauze counter electrode and Ag/AgCl (3.0 M KCl) reference electrode was used. A modified glassy carbon electrode as described in section 2.2 was


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employed as working electrode. To perform the measurements, this electrode was carefully pressed against the infrared windows of ZnSe, to minimize the absorption of the infrared radiation by the electrolyte, resulting in the formation of a thin electrolytic film of ca. 1-10 µm.35 In situ FTIR

spectra

were

recorded

during

chronoamperometry

measurements

in

1.0 M NaOH + 0.1 M alcohol. Initially, the working electrode was polarized at 0 V for 600 s and then potentiostatic jumps were performed every 120 s at the potentials values of 0.3 V, 0.35 V, 0.45 V, 0.50 V, 0.55 V, and 0.60 V. A mean of 64 interferograms with a resolution of 4 cm-1 was used. Finally, the obtained spectra were plotted as the reflectance ratio R/R0, where R and R0 are the reflectance corresponding to the sample at a given potential, respect to a reference potential (0 V vs. Ag/AgCl (3.0 M KCl)). 3. RESULTS 3.1. Characterization and electrochemical behavior The [Ni(salen)] complex was synthetized through azo coupling reaction and characterized by UVVis absorption and ex situ FTIR, as shown in Figure S1 (supplementary material). From these results, the following observations can be made: (i) the UV-Vis absorption spectrum (Figure S1A) show a peak at 244, 255, 330 and 406 nm which are characteristic of the [Ni(salen)] complex36 (ii) the FTIR spectra, in Figure S1B, display bands in the region of 2800 to 3100 cm-1 (which corresponds to aliphatic and aromatic stretching of -C-H and =C-H, respectively),37 bands between 1450 and 1600 cm-1 ( typical of C-C stretching in aromatic ring),33 and bands at 1624 and 1127 cm1

(related to stretching vibrations of the azomethine and phenolics groups, both coordinated with

nickel), which are all characteristic of the [Ni(salen)] complex.38 Other chemical and physical properties of this complex have been discussed in more detail in a previous publication.25


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Figure 1A shows consecutive cyclic voltammograms taken during the electropolymerization of the [Ni(salen)] monomer on the glassy carbon electrode in N2-saturated 0.1 M TBAP/CH3CN solution at 100 mV s-1. In the first cycle, an anodic process (a1) at ca. 1.1 V is observed, which is attributed to the electro-oxidation of [Ni(salen)] monomer, resulting in the formation of an oligomers/polymer film on electrode surface.39 In the reverse scan, the cathodic process observed at ca. 0.8 V (c1) corresponds to the reduction of the polymeric film electrodeposited in the previous anodic scan. During subsequent potential cyclings, new anodic (a2) and cathodic (c2) peaks arise due to the oxi-reduction of the previously deposited polymer film. The magnitude of this current peak increases with potential cycling, reflecting accumulation of the electroactive polymer film on electrode surface.39,40 As already proposed in the literature, these features are consistent with a film growth mechanism the formation of the polymer film begins with the adsorption of the [Ni(salen)] monomer on the surface of the electrode in the positive scan; next the monomers present in the solution react with the adsorbed monomers, resulting in the electropolymerization via stacking of monomers.41 Figure S2 shows the typical electrochemical CV profile of poly[Ni(salen)] film in monomer-free 0.1 M TBAP/CH3CN electrolyte, showing the features related to the Ni oxi-reduction process in the polymeric film21.


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5

3

a1

Electropolymerization

4

Electrochemical activation

2

3

cycles

2

j / mA cm-2

j / mA cm-2

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a2

1 i

0 -1

c2

-2 0.0

0.2

0.4

cycles c1

0.6

0.8

A 1.0

1.2

10th cycle 20th cycle 30th cycle 50th cycle

1 0

75th cycle 99th cycle 100th cycle

-1 0.0

0.1

E / VAg/AgCl (3 M KCl)

0.2

B 0.3

0.4

0.5

0.6


Figure 1. (A) electropolymerization of the [Ni(salen)] complex on glassy carbon in N2-saturated 0.1 M TBAP/CH3CN electrolyte at 100 mV s-1. (B) electrochemical activation of the polymeric film at 50 mV s-1 in N2-saturated 1.0 M NaOH solution.

After the electropolymerization step, the poly[Ni(salen)] electrode was electrochemically activated in 1.0 M NaOH solution (in absence of monomer) at 50 mV s-1 (Figure 1B), leading to the formation of a new electroactive material (poly[Ni(salen)]ATV). The voltammograms show a redox pair at ~ 0.43 and 0.35 V, which is characteristic of the Ni(OH)2 ⇋ NiOOH couple1,42,43. It is proposed that the observed increase in current during cycling is due to the enrichment of Ni(II)/Ni(III) electroactive species in the complex on the electrode film in NaOH solution, suggesting the successive generation of [Ni(II)(salen)(OH)2] and [Ni(III)(salen)OOH].18,24,28,44,45 In addition, the voltammetric profile is very similar to that reported in the literature for polycrystalline Ni electrodes and nickel hydroxide-based NPs.1,42,43 It is notable that the voltammetric profiles of the poly[Ni(salen)] film before and after activation are very different. Recently, Kuznetsov et al27 suggested that during the activation process, the polymeric film undergoes oxidation leading to the formation of NiOOH in the anodic scan, which in the reverse scan is cumulatively transformed into the Ni(OH)2 nanoparticles, whose presence in


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the present studies was searched by scanning electron microscopy measurement. In this sense, as the active material for the reactions of interest is poly[Ni(salen)]ATV, XRD, XPS and TEM analyses of the material at different stages of the poly[Ni(salen)]ATV film growth were performed, for samples collected in the reduced state. Figure 2A display TEM images obtained for the poly[Ni(salen)]ATV film, electropolymerized and activated directly on a Cu microscopic grid. The image shows the presence of spherical NPs with an average size of 2.4 ± 0.5 nm (Figure 2B). EDX analysis reveals that the chemical composition of these NPs includes nickel and oxygen species, Figure 2C. EDX spectra also indicate the presence of sodium and copper, which comes from sodium hydroxide used as electrolyte in the activation step and from the microscopic grid, respectively. The XRD patterns of electropolymerized and activated poly[Ni(salen)] on a platinum foil are shown in Figure 2D. Peaks observed in the region of 2θ 10° and 15° can be attributed to (001) and (003) planes of the Ni(OH)2,46,47 indicating a change in the structure of the polymer film. In addition, the absence of narrow and intense peaks suggests that the structure of Ni(OH)2 is poorly crystalline and/or their particles are very small. All other peaks are characteristic of Pt X-ray diffraction features.


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poly[Ni(salen)]

C

C

Cu

Ni(OH)2

O Ni Na Ni Cu

Ni

2

d = 2.4  0.5 nm

0.3 0.2 0.1 0.0

6 KeV

8

10

D

Intensity (a.u.)

0.4

4

Pt foil poly[Ni(salen)]atv on Pt foil

A

N / Ntotal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B 1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

10

20

30

40

50

60

70

80

90

2  ° (CuK)

Particle size (nm)

Figure 2. (A) TEM image of poly[Ni(salen)]ATV. (B) Particle size distribution histogram obtained from the count 400 nanoparticles. (C) EDX of poly[Ni(salen)]ATV (TEM image of EDX analysis region is shown in the insert); and (D) XRD patterns of the poly[Ni(salen)]ATV on a platinum foil; for the purpose of comparison, the XRD patterns of the pure Pt foil are also shown. Analyzes of XPS were also performed to investigate the changes in the oxidation state of Ni in the polymeric nanofilm after the activation step. Figure 3 shows the XPS survey spectra obtained for [Ni(salen)] and poly[Ni(salen)]ATV, as well as the spectra of the individual elemental regions of Ni 2p and N 1s. The XPS survey spectrum, Figure 3A, clearly show that there is a change in the structure and atomic composition of the polymer complex after the activation step, which is attributed to the formation of more oxygenated nickel species, as shown by the atomic content percentage data inserted in the graph. In the XPS survey spectra of the poly[Ni(salen)]ATV, the


11


presence of a peak in the energy region of Na 1s is observed, which certainly comes from the sodium hydroxide used as electrolyte. In Figure 3B, the spectrum in the N 1s binding energy region shows a peak at 398 eV, which almost disappears in the poly[Ni(salen)]ATV, probably due to the rupture of N-C and N-Ni bonds, originally present in the [Ni(salen)] complex, and the release of N-based species out of the film. Finally, results shown in Figure S3 (supplementary material), denote negligible incorporation of iron, coming from impurities in the electrolyte, takes placed in the present cyclings conditions.48

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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855.5 eV

N 1s

Composition atomic (%) C 1s O 1s N1s Ni 2p [Ni(salen)] 80.7 11.4 6.8 1.1 Poly[Ni(salen)]ATV 52.1 42.4 N.D 5.5

Ni 2p

398.9 eV 873.2 eV

861.3 eV

397.7 eV

O 1s

853.5 eV

Na 1s Ni 2p C 1s

N 1s

1200 1000 800 600 400 200 Binding energy (eV)

870.8 eV

A 0

C

B 415

410

405

400

395

390

Binding energy (eV)

880 875 870 865 860 855 850 845 Binding energy (eV)

Figure 3. (A) XPS survey spectra of the [Ni(salen)] complex (orange line) and poly[Ni(salen)]ATV (green line) and in the energy regions (B) N 1s, and (C) Ni 2p. High-resolution XPS spectrum in the Ni 2p energy region of the [Ni(salen)] monomer (Figure 3C) clearly shows the presence of a satellite-free 853.5 eV peak, which is characteristic of Ni-square planar complexes.49 In contrast, the XPS spectrum in the same energy region for the poly[Ni(salen)]ATV, shows a peak at 855.5 eV and the presence of a satellite peak at 861.3 eV, attributed to species of nickel with an oxidation state similar to nickel hydroxide, arising due to the breaking of the bond between the metal and the binder.50 In addition, a satellite peak located between 6 and 8 eV higher than the main binding energy peak (such as that at 861.4 eV) is attributed to multiple division in the energy levels of the transition metal and is typically used for


12

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confirming the presence of divalent nickel.51 Therefore, these results in conjunction with the XRD data, provide clear evidence that the NPs observed in TEM images for poly[Ni(salen)]ATV electrode are really related to Ni(OH)2 NPs. 3.2. Electrocatalytic oxidation of Cn alcohols The activity of the poly[Ni(salen)]ATV catalyst towards the electro-oxidation of Cn alcohols was investigated by cyclic voltammetry in N2-saturated 0.1 M NaOH solution, and the results are shown in Figure 4. As previously mentioned, the cyclic voltammograms in the absence of alcohols (black lines in Figure 4) present a typical profile of nickel-based electrodes in alkaline electrolyte: (i) a double layer region in the range of 0 to 0.2 V observed for β-Ni(OH)2; (ii) the Ni(II)/Ni(III) redox couple in the region between 0.2 and 0.5 V and; (iii) the oxygen evolution reaction in the region above 0.5 V. However, when the voltammograms are recorded in the presence of alcohols, the voltammetric profiles are quite different, presenting an increased current density, whose values at 0.59 V are 14.1, 16.3 and 31.4 mA cm-2 (positive-going scan) for methanol, ethanol, and glycerol, respectively. The onset potential for the electro-oxidation of methanol and ethanol cannot be defined from the cyclic voltammetric profiles, due to the overlap of potentials of β-NiOOH formation and of oxidation of these alcohols. In all these cases, it is seen that the electro-oxidation of alcohols clearly begins after the formation of β-NiOOH species, β-Ni(OH)2 + OH- ⇋ β-NiOOH + H2O + e-

(Eq. 1)


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which is commonly reported to be the electrochemically active phase for the oxidation of small organic molecules43,52,53, involving electron transfer from the R-OH species to β-NiOOH, restoring the β-Ni(OH)2 species, which successively repets the cycle (reaction 1 and 2): β-NiOOH + R-OH → β-Ni(OH)2+ products

(Eq. 2)

where R is CH3- (for methanol), C2H5- (for ethanol) or C3H7O2- (for glycerol). This process explains the decrease of the cathodic current density in the reverse scan of the CV (backward of reaction 1), since the amount of β-NiOOH in the surface is reduced while it is participating in reaction 2. As pointed by Taraszeska et al.54 in the electrocatalytic process, the alcohol molecules penetrate the film of nickel hydroxide together with the OH- ions, which are trapped in the structure and are involved in the overall oxidation reaction of alcohols (reactions 1 and 2). As can be seen in Figure 4, the activity of the poly[Ni(salen)]ATV electrode for the glycerol oxidation at 0.59 V is 2.2 and 1.9 times higher compared to the methanol and ethanol oxidation, respectively. These higher values of current density for glycerol oxidation were also observed in gold-based electrodes. In this case, the higher activity has been explained in terms of the values of pKa and the number of vicinal hydroxyls.55,56 Kwon et al. found that for certain alcohols, the lower the pKa value, the greater the electrocatalytic activity of gold for alcohol oxidation reactions.55 However, they also report that only the pKa values are not enough to predict reactivity. On the other hand, Holze et al. proposed a mechanism that explains the higher activity of polyalcohols when compared to mono alcohols. The authors suggested that while electro-oxidation of mono alcohols proceeds only at adsorption sites adjacent to places occupied by adsorbed OH- species,


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the electro-oxidation of polyalcohols occurs by a cyclic transition stage which should be less dependent on the cooperation of adsorbed OH- species, and this enables faster kinetics.56 33 1.0 M NaOH 0.1 M MeOH

30

1.0 M NaOH 0.1 M GlyOH

1.0 M NaOH 0.1 M EtOH

27 24

j / mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21 18 15 12 9 6 3 0

B

A

-3 0.0

0.2 0.4 0.6 E / VAgAgCl (3 M KCl)

0.0

0.2 0.4 0.6 E / VAg/AgCl (3 M KCl)

C 0.0

0.2 0.4 0.6 E / VAg/AgCl (3 M KCl)

Figure 4. Cyclic voltammograms of the poly[Ni(salen)]ATV electrode in 1.0 M NaOH in the absence and present of 0.1 M alcohol at 50 mV s-1: (a) methanol; (B) ethanol; and (C) glycerol. The results discussed so far, indicate that the electrocatalysis of Cn alcohols oxidation occurs due to the presence of Ni(OH)2 nanoparticles, which are formed in situ during the activation step. On the other hand, it may be also possible that Ni(III) active species remaining in the polymeric matrix after the activation step contributes to the electrocatalytic activity observed. Figure 5 presents cyclic voltammograms of the non-activated poly[Ni(salen)] film in NaOH solution in the presence and absence of glycerol. In the absence of glycerol, the first cycle shows an anodic peak (E = 0.56 V) associated with the polymer irreversible oxidation process27, while in the presence of glycerol, higher current densities are obtained, indicating that the poly[Ni(salen)] film promotes the glycerol electrocatalysis. However, in the second cycle, an abrupt reduction of the currents for both


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situations was observed, indicating a large instability of the Ni(III) containing polymer in alkaline medium and in the studied potential range. Therefore, while the poly[Ni(salen)]ATV electrode remain activity along the potential multi-cycling, the non-activated electrode is unstable and presents current values around 18 times smaller than those for poly[Ni(salen)]ATV. Then it may be concluded that the high poly[Ni(salen)] electrocatalytic activity observed in Figure 4 is due to exclusively to the Ni(III) species from the Ni(OH)2 nanoparticles, as mentioned previously.

40

3.2 st

1 cycle in NaOH 2nd cycle in NaOH 3rd cycle in NaOH 1st cycle in NaOH + GlyOH 2nd cycle in NaOH+ GlyOH 3rd cycle in NaOH + GlyOH poly[Ni(salen)]ATV in NaOH + GlyOH

30 25 20 15 10

2.8

2nd cycle in NaOH 3rd cycle in NaOH 2nd cycle in NaOH + GlyOH 3rd cycle in NaOH + GlyOH

2.4

j / mA cm-2

35

j / mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fresh poly[Ni(salen)] in NaOH with/without glycerol

2.0 1.6 1.2

Fresh poly[Ni(salen)] in NaOH with/without glycerol

0.8 i

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0.5

0.6


Figure 5: Cyclic voltammograms of the non-activated poly[Ni(salen)] film in N2-saturared 1.0 M NaOH in the absence and present of 0.1 M glycerol at 50 mV s-1: (a) the first three cycles and (b) only the 2nd and 3rd cycle. The 3rd cycle of the voltammogram of poly[Ni(salen)]atv is also shown for comparation. 3.3. In situ FTIR The in situ FTIR measurements were conducted to determine the products and intermediates formed along the oxidation of the alcohols as a function of the applied potential. Figure 6 shows the spectra obtained during chronoamperometric measurements at different potentials, from 0 to 0.6 V. For all spectra when the electrode is polarized at 0.3 V, no well-defined bands appear. On


16

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the other hand, when the electrode is maintained at 0.4 V, different bands are seen in the spectra, indicating that the oxidation process of theses alcohols begins around this potential, so occurring together with the formation of the β-NiOOH species, in agreement with the results in Figure 4. Moreover, these results suggest that the magnitude of the FTIR signals of the detected products follows the same activity sequence as observed in the cyclic voltammograms (Figure 4), with methanol giving less intense bands, followed by ethanol and glycerol. The FTIR spectra characteristic of the different products or possible intermediates recorded in the alkaline medium for the oxidation of different alcohols have been already published and will be used here in the data analyses. The in situ FTIR spectra obtained during oxidation of methanol are shown in Figure 6A. It must be noted at first the appearance of a negative peak at 1017 cm-1 in the spectra in Figure 6A, which is characteristic of C-O stretching of methanol in NaOH solution57. This negative peak starts to appear above ca. 0.4 V and evidences a reduction of the methanol concentration in the solution near to the electrode surface, indicating the beginning of the occurrence of methanol oxidation reaction. At potentials above 0.4 V, positive bands located at 1350, 1380, 1580 and 1630 cm-1 become apparent. The band at 1350 cm-1 has been assigned to υa(OCO) stretching vibration of carboxylates/carbonates1,57, while that at 1380 may be related to δ(CH) or ρr(COO) stretching/bending vibrations of CH and carboxylate/carbonate groups57, respectively, and that at 1580 cm-1 must be assigned υs(OCO) stretching vibrations also of carboxylate/carbonates.1,57 This indicates that the methanol oxidation occurs via the production of formate (1350 and 1380 cm-1) and eventually of carbonate. It should be noted that the intensity of the band at 1380 suffers a prominent increase while those at 1350 and 1580 cm-1 are decreased when the electrode potential is moved from 0.55 to 0.6 V. This is consistent with a decrease of formate production and a large


17


increase of the CO2 production. In this way, it must be inferred that in alkaline media CO2 becomes a major product of the methanol oxidation in the β-Ni(OH)2 catalyst for potentials above 0.55 V. Finally, the band at 1630 cm-1, whose intensity grows with the increase of potential, may be assigned to bending deformations of the H-O-H structure of water, resulting from changes of the structure of the ultrathin reaction layer.57 Strong signals coming from water makes difficult recognition of any features related to reactant or oxidation products for wavenumbers below 2000 cm-1.

0.00 V 0.30 V 0.40 V 0.45 V 0.50 V

Transmitance (a.u.)

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Page 18 of 34

0.55 V

0.60 V

R/Ro = 0.5 %

A

R/Ro = 0.4 %

B

R/Ro = 1 %

C

2000 1500 1000

2000 1500 1000 2000 1500 1000 -1 Wavenumber (cm ) Figure 6. In situ FTIR spectra of the electro-oxidation of: (A) MeOH, (B) EtOH and (c) GlyOH.

In situ FTIR results obtained during the ethanol electro-oxidation on the poly[Ni(salen)]ATV electrode are presented in Figure 6B, for different electrode potentials (E =

Electrocatalytic Oxidation of Methanol, Ethanol, and Glycerol on Ni(OH

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