THE ELECTROCHEMICAL OXIDATION OF 4-NITROANILINE AND 4-NITROPHENOL ON MODIFIED PbO2-ELECTRODES

The electrochemical oxidation of p-nitroanilline and p-nitrophenol on lead dioxide anodes, modified by different ionic dopants has been investigated. The general mechanism of the oxidation of organic compounds of aromatic nature includes oxidizing of compounds to the intermediates with quinoid structure, reactions of aromatic ring opening and formation of aliphatic products (mainly acids) and in ideal case – the complete mineralization to CO2 and H2O. According to obtained results one can conclude that both reactions occur via formation of p-benzoquinon. Calculations, based on kinetic studies of the reaction, have shown that the rate constant of the degradation of the organics involved depends on the composition of the electrode material and varies due to the nature and the content of ionic additives in lead dioxide. The maximum interest for the electrochemical destruction of organic substances represents lead dioxide electrodes modified by bismuth to which a rate constant of p-nitroaniline oxidation increases in 1.6 times compared with nonmodified electrodes. Maximum electrocatalytic activity is achieved by increasing the proportion of α-phase, on the one hand, and increase the crystalline zone of oxide on the other, which leads to increased amounts of oxygen containing particles strongly bounded to the electrode surface that participate in the electrochemical oxidation of aromatic compounds.


Introduction
The line of research involved in the manuscript belongs to the world's key development priorities of modern chemistry and attracted considerable attention of researchers as indicated by the large number of publications. The results on the use of advanced catalytic oxidation method (advanced oxidation process  AOP) achieved over the last 5 years for the destruction of chemical and biological toxins and pollutants listed in review [1]. Based on the information provided in the publications [24], clearing of the aquatic environment from pollution by anthropogenic organic chemicals are known to be a very difficult problem even in industrial enterprises, which are widely used chemical reagent methods. Attempts to transfer this experience in agriculture in general has not been successful because the traditional methods and technologies are not adapted to the conditions of agriculture require huge capital costs for associated infrastructure and industrial engineering of the necessary reagents of shipping them over long distances to the place of use.
An alternative to traditional methods of water purification from toxic aromatics are electrochemical technologies that should be attributed to relatively reagent less, as latter are formed at the time of use. Other advantages include their efficiency, ease of use, ease of automation, modularity structures and the flexibility to scale based on the needs in use [5]. These techniques are promising for the treatment of water from a wide range of organic compounds of different types [6], for example, phenolic compounds [7; 8] and pesticides [9]. The electrochemical degradation of toxic pollutants is achieved both through direct transfer of electric charge between the electrode and the organic compounds and secondary chemical reactions, the oxidant in which are oxygen-containing radicals formed during electrolysis of water molecules. In both cases the optimal choice of anode material is critical, not only ratio, and direction of oxidation depends on it. For example, some electrodes in the oxidation of organic compounds may form polymers due to the nucleophilic attack of neutral molecules by radicals [10]. In recent years, a large number of materials have been used for the selective and non-selective anodic oxidation of resistant organic compounds, but the problem of choosing the optimal and chemically stable material still remains an open question requiring further study.

Experimental and Methods
All chemicals were reagent grade. Platinized titanium was used as substrate. Titanium sheet was treated as described in [11] before platinum layer depositing. Lead dioxide coatings were electrodeposited at anodic current density 10 mА · сm -2 from methanesulfonate electrolytes that contained Having in mind that the challenge in PbO2 research is to obtain an electrochemically active and durable material, in this work we electrodeposited PbO2 from CH3SO3  -containing medium.
Since methanesulfonate is becoming the most popular electrolyte for PbO2 electrosynthesis due to probability of the deposition of coatings up to 2 mm thick with low internal stresses [12], we chose these electrolytes because they are easy to prepare and work with; and PbO2 obtained in this medium has satisfactory mechanical properties and significantly different electrocatalytical activity in respect to coatings, obtained in traditional nitrare baths.
Qualitative and quantitative phase analysis was performed using the PowderCell program [14]. For selected samples with relatively high degree of crystallinity the Rietveld refinement was carried out using FullProf.2k (version 5.40) program [15].
XPS studies were carried out on a PHI 5000 spectrometer using monochromatic AlKα radiation for excitation. The BE value of C(1s), due to adventtious carbon and residual solvent, is 284. 8 The electrooxidation of organic compounds was carried out in undivided cell at jа = 50mА сm -2 . The volume of anolyte was 50 сm 3 . Solution, containing phosphate buffer (0.25 М Na2HPO4 + 0.1 М KH2PO4) + 2•10 -4 М organic compound, (рН = 6.55) was used as electrolyte. Stainless steel was used as cathode. Modified lead dioxide electrodes were used as anodes. Electrode surface area was 1 сm 2 .
Analyses of the reaction products were conducted by HPLC using a Shimadzu RF-10A xL instrument equipped with a Ultraviolet SPD-20AV detector and a 30 cm Discovery ® C18 column. Ozone analysis was carried out mostly by iodometric titration [16]. In some cases the results were checked by the spectrophotometric method. The formation of colored compounds during electrolysis was followed by UV-visible spectroscopy. The changing of the concentration of the organic substance during the electrolysis was measured by sampling (volume of 5 cm 3 ) at regular intervals and measuring the optical density of the solution in the ultraviolet and visible region (wavelength range 200-570 nm) using a Kontron Uvikon 940 spectrometer. Solution, containing phosphate buffer, was used as reference solution.

Results and Discussion
Electrochemical degradation of organics in the wastewater is known to be a very important task. The development of electrode materials used for wastewater treatment is recognized as the subject of many investigations. Synthetic diamond electrodes modified with boron (BDD), for example, commonly used as electrocatalysts [17], as well as anodes based on PbO2 [18]. It should be noted that in the first case the basic problem is the high power consumption due to the low conductivity of modified synthetic diamond, which makes them unsuitable for use as anodes in industry. Thus, more promising are materials based on lead dioxide, the more so because of their electrocatalytic activity, which can be significantly increased by the modification.
The process of oxidation of organic substances is not necessarily a direct electrochemical process. Quite likely, it occurs via oxidants generated in the primary electrochemical reaction, for example, the formation of hydroxyl radicals and ozone. So this process is a secondary chemical process. It should also be noted, that it is not always taken into account the fact that the oxygen evolution reaction on the electrode can occur in conjunction with other reactions with the transfer of oxygen, such as oxidation of organic [17; 19] or inorganic compounds [18], which are not necessarily implemented independently of other.
The effectiveness of such processes depends on the ratio of the rates of reactions both of the formation and the disappearance of OH-radicals. The accumulation of a sufficient amount of radicals on the electrode surface and the near electrode layer facilitates their interaction with inorganic and organic compounds, causing partial or complete destruction of these compounds [12].
The synthesis of strong oxidants such as ozone can be assigned to another group of anodic reactions occurring at high anodic potentials with oxygen-containing particles participation. Since ozone formation occurs simultaneously with the oxidation processes of organic compounds [20; 21], its synthesis in the electrolysis process can contribute to the destruction of toxic organic substances. As one can conclude from the obtained results, the current efficiencies of ozone evolution on electrodes deposited from methanesulfonate electrolytes are approximately three times lower than on the materials obtained from nitrate bath [22]. Modification by ionic additives increases the current efficiency of ozone, but the latter is characterized by the values in the range of a few percent [12].
An observed phenomenon is caused by differences in the chemical and phase composition of deposits obtained from nitrate and methanesulfonate electrolytes, namely, in the degree of surface hydroxylation. As the oxygen species strongly bounded with the electrode surface are involved in the process of ozone evolution [23], the decrease in their number would lead to a decrease in current efficiencies of ozone evolution, which is observed in the case of coatings obtained from the methanesulfonate electrolytes [22; 24; 25].
In order to determine the influence of deposition conditions and the composition of the anode materials, based on lead dioxide, on their electrocatalytic activity in respect to the oxidation of organic toxicants 4-nitroaniline and 4-nitrophenol were selected as model aromatics. This choice was due to the fact that the electrochemical incineration of phenolic compounds on the different electrodes is well studied process, so the attention can be focused only on the clarifying of the role of the anode material.
Thus, in particular, a considerable number of publications are devoted to aromatic compounds electrooxidation, in which PbO2 and other oxides of noble metals are used as anode materials [2; 12; 26]. The anodic oxidation of phenols may occur in two pathways, depending on the acidbase properties of the system [27][28][29][30][31].

Oxidation of p-nitroaniline.
As it is noted in [32], the general mechanism of the oxidation of organic compounds of aromatic nature will include oxidizing of compounds to the intermediates with quinoid structure, reactions of aromatic ring opening and formation of aliphatic products [mainly acids] and in ideal case -the complete mineralization to CO2 and H2O. According to [33], quite a number of intermediates are produced during anodic oxidation of p-nitroaniline. The primary intermediates include maleic acid and benzoquinone.
The mechanism of p-nitroaniline electrooxidation on modified lead dioxide electrodes was considered in detail in our previous publications [12]. The HPLC investigation has shown 1,4-benzoquinone as the major aromatic intermediate. Only aliphatic acids can be detected in a solution after long-term electrolysis.
Electronic absorption spectra of solutions at different electrolysis time were taken in order to determine the time of the disappearance of intermediate aromatic products and a change in the concentration of the initial compound. Fig. 1 shows the absorption spectra in the visible and UV regions obtained at different times of electrolysis in a phosphate buffer on nonmodified lead dioxide anode.
It should be noted that electrocatalytic activity of lead dioxide anodes in respect to the oxidation of p-nitroaniline depends on the concentration of methanesulfonate ions in the deposition electrolyte. The dependence has extreme character with a maximum at 0.1-0.3 M concentrations of CH3SO3Na (Fig. 2). As follows from the obtained results (Table 1), the chemical composition of lead dioxide is practically no changed. In this case, structural factors play a significant role. Maximum electrocatalytic activity is achieved by increasing the proportion of α-phase, on the one hand, and increase the crystalline zone of oxide on the other, which leads to increased amounts of oxygen containing particles strongly bounded to the electrode surface that participate in the electrochemical oxidation of aromatic compounds [12]. Table 1 The phase composition of lead dioxide coatings depending on deposition conditions Obtained results can be satisfactorily described in the framework of the mechanism [12], wherein the primary intermediate is benzoquinone. Since the electrochemical destruction of p-nitroaniline occurs via the formation of benzoquinone, electrochemical destruction of this compound was investigated further. Fig. 3 shows the absorption spectra of p-benzoquinone solution obtained at different time of electrolysis in a phosphate buffer on lead dioxide anode. Kinetic parameters of the electrochemical oxidation of p-benzoquinone were commented in [12].
The processes of p-nitroaniline electrochemical oxidation on unmodified and modified lead dioxide electrodes occur qualitatively the same and differ only in the rate. This suggests the invariability of the mechanism of p-nitroaniline oxidation on different materials that allows one for a correct comparison of their electrocatalytic activity.
According to calculations (Table 2), based on kinetic studies of the reaction rate constant of p-nitroaniline degradation depends on the composition of the electrode material and varies due to the nature and content of ionic additives in lead dioxide.
The maximum interest for the electrochemical destruction of organic substances represents lead dioxide electrodes modified by bismuth to which a rate constant of p-nitroaniline oxidation increases in 1.6 times compared with nonmodified electrodes (see Table 2). In other cases, the rate constants are comparable.
Oxidation of p-nitrophenol. Since mechanism of electrooxidation of p-nitrophenol was considered in detail previously, let's concentrate only of the effect of dopants on the reaction rate. Fig. 4 shows the absorption spectra of p-nitrophenol solution obtained at different time of electrolysis in a phosphate buffer on lead dioxide anode. As was suggested in [12], maleic acid and a stoichiometric amount of NO3 -were detected as electrolysis products by high performance liquid chromatography. The primary aromatic intermediate in this case was also a 1,4-benzoquinone, but its concentration was an order of magnitude higher than in the oxidation of p-nitroaniline [12]. The latter indicates a more effective destruction of the aromatic ring in the case of p-nitroaniline.
As one can conclude from obtained results ( Table 3) rate constants of p-nitrophenol oxidation on modified lead dioxide electrodes somewhat lower than those for p-nitroaniline [12].

Conclusions
The processes of electrochemical oxidation of investigated organic compound on unmodified and modified lead dioxide electrodes occur qualitatively the same and differ only in the rate. This suggests the invariability of the mechanism of their oxidation on different materials that allows one for a correct comparison of their electrocatalytic activity.
According to calculations, based on kinetic studies of the reaction rate constant of organic compounds degradation depends on the composition of the electrode material and varies due to the nature and content of ionic additives in lead dioxide.
The maximum interest for the electrochemical destruction of organic substances represents lead dioxide electrodes modified by bismuth to which a rate constant of p-nitroaniline oxidation increases in 1.6 times compared with nonmodified electrodes. In other cases, the rate constants are comparable.