Electro-oxidation of chlorophenols at glassy carbon electrodes modified with polyNi(II)complexes

Abstract

The effect of the ligand macrocycle (phenylporphyrin (PP) or phthalocyanine (Pc)) and of the ligand substituent (NH₂ or SO₃⁻) on the catalytic activity for the electro-oxidation in a pH 11 buffer electrolyte of 2- and 4-chlorophenol (2-CP and 4-CP), 2,4- and 2,6-dichlorophenol (2,4-DCP and 2,6-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and pentachlorophenol (PCP) at glassy carbon electrodes modified with electropolymerized Ni(II) macrocycles was studied. The polyphenolic residue deposited at the electrode surface was characterized by cyclic voltammetry, impedance measurements, ex situ Fourier transform infrared spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). A band of aliphatic CO stretching in the IR spectrum of the fouling film produced by potential cycling in 2,4,6-TCP indicated that the aromatic ring had been broken, yielding ketones, aldehydes and/or carboxylic acids. The sulphonated Ni(II) polymers, which showed the Ni(III)/Ni(II) process in the CV, had XP spectra typical of paramagnetic Ni(II), indicating that they contained Ni(OH)₂ clusters. On the contrary, the CVs of the amino Ni(II) did not show the Ni(III)/Ni(II) process at all, this process appearing only after previous activation by potential cycling, and only to a small extent. As was to be expected, the XP spectra of activated amino films corresponded to diamagnetic Ni(II), showing that the concentration of Ni(OH)₂ clusters was very small. The amino films were less active than the sulpho films for the oxidation of chlorophenols, in agreement with the lower concentration of Ni(OH)₂ clusters in the former films. For all electrodes the highest activity was observed for 2,4,6-TCP, since its oxidation yields a phenolic residue which is much more porous than those produced by the other CPs.

Introduction

Chlorophenols (CPs), common pollutants in many industrial wastewaters, are used as pesticides, paint preservatives and cellulose bleaching agents. They are highly toxic and persistent, and thus their recovery or mineralization is a must. The phenolic OH group confers the CPs solubility in water, the ionized form being more hydrophilic, while the neutral form is more lipophilic and has higher membrane permeability [1], [2]. The pK_a value depends on the number of chlorine atoms and on their position with respect to the OH group [3], [4], [5], increasing from 4.7 for pentachlorophenol up to 9.4 for 2-chlorophenol. We reported a linear correlation between the anodic peak potential of several CPs on glassy carbon (GC) at pH 11 and the pK_a, the oxidation being the more facile the less acidic was the CP [4]. The formation of the polyphenolic film began at a lower potential for 2-CP and 2,4-DCP than for 2,4,6-TCP, and the film on the latter was so porous that it allowed the oxidation of 2,4,6-TCP to continue [4]. The first charge-transfer step is rate-determining and yields the corresponding phenoxy radical.

Abgoola et al. [6] studied the electro-oxidation of CPs at gold electrodes modified with electropolymerized Ni(II)tetrakisbenzylmercapto- and Ni(II)dodecylmercapto-phthalocyanines, and found that activation of the “ONiO” bridge yielded polyNi(OH)NiPcs, which showed a better catalytic activity than the unactivated polyNiPcs, although eventually all the electrodes became fouled by deposition of oligomers [7], [8], [9], [10], [11]. Coteiro et al. [12] propose that the electro-oxidation of 4-CP in acidic media on Ti/Ru_0.3M_0.7O₂ with MTi or Sn could yield the easily mineralized oxalic acid via 4-chlorocatechol or via hydroquinone–benzoquinone–malic acid–maleic acid.

Nickel tetraazamacrocyclic complexes are exceptionally efficient and selective electrocatalysts [6], [13], [14], probably because nickel easily changes from a square planar to an octahedral conformation [15], [16]. When planar Ni(II)-phthalocyanine and -porphyrin tetrasulphonated complexes are electrodeposited in aqueous media they form a stacked structure on the electrode surface [6], film growth being proposed to occur via NiONi bridges, with the formation of some nickel hydroxide nanostructures [17], [18]. In these modified electrodes the oxidation of Ni(II) occurs with water expulsion, probably due to the hydrophobic character of the macrocyclic ligands [19]. Revenga-Parra et al. [20] found that GC electrodes modified with films of Ni(II)-(N,N′-bis(2,5-dihydroxybenzylidene)-1,2-diaminobenzene) (Ni(II)-DHS]/GC) show electrocatalytic activity for methanol oxidation.

Previous studies in our laboratory indicate that polyNiTSPc-modified GC electrodes are less fouled by the electro-oxidation of 2-CP than bare GC [21]. Other authors have reported similar results for the oxidation of organic molecules at electrodes modified with different Ni(II) complexes [17], [22], [23]. We have reported [16] that the half-sum potential of the Ni(III)/Ni(II) process at GC modified electrodes in a pH 11 buffer electrolyte is almost independent of the ligand (phthalocyanines (Pc) or phenylporphyrins (PP)) and of the substituent (amino or sulphonated group). Francisco et al. [24] attributed to the Ni(III)/Ni(II) redox couple the activity for the oxidation of hydroquinone and 4-aminophenol of a carbon-paste electrode modified with SiO₂/Nb₂O₅-phosphate on which Ni(II)porphyrin had been previously adsorbed. However, Yi et al. [25] and Agboola et al. [6] recently reported the electro-oxidation of dopamine and CPs on electrodes modified with different electropolymerized films of nickel(II) complexes, although the Ni(III)/Ni(II) couple did not appear in the CVs. Probably the activity was due to a π–π interaction similar to that reported for polymeric adsorbents chemically modified with porphyrins, on which the retention of phenol and CPs is increased by this interaction [26].

In this work we have studied the effect of the ligand macrocycle, and of the ligand substituent, on the catalytic activity for oxidation at pH 11 of CPs at polyNi(II)phthalocyanine/GC (polyNiPc/GC) and polyNi(II)phenylporphyrin/GC (polyNiPP/GC) electrodes, both tetrasulpho- (TS) and tetraamino-(TA) substituted.