Electrocatalytic reduction of nitroaromatic compounds by activated graphite sheets in the presence of atmospheric oxygen molecules

Highlights

• The novel activation process was demonstrated by cerium nitrate activation.

• The effect of oxygen reduction was performed to the reduction of nitrobenzene.

• The interference properties of aGR/SPCE were investigated in the NACs.

• The overall reaction rate was discussed based on HOMO-LUMO interaction.

Abstract

The anthropogenic climate and environmental issues incite the remediation of nitroaromatic compounds (NACs). Therefore, most of the researches have been devoted to the electrocatalytic reduction of NACs. Mainly, the carbonaceous materials have been employed as an electrode material for the determination of NACs. However, the effect of oxygen reduction is necessary to address with the electrocatalytic reduction of NACs. Hence, this work mainly focuses the reduction of NACs with the saturated oxygen and atmospheric oxygen conditions. The graphite and nitrobenzene (NB) were chosen as a model electrode and analyte for assess the effect of oxygen reduction. Though, the graphite is no longer express high electrocatalytic activity thus the activated graphite sheets were employed in the determination of NACs. Interestingly, the activated graphite sheets exhibited an excellent performance to the reduction of nitrobenzene and revealed an acceptable linear range around 0.05–1100 µM, 0.05–147 µM and 0.05–145 µM, in the N₂, O₂ and aerated atmospheres, respectively. Moreover, the lowest detection limits were obtained around 0.026 µM, 0.031 µM and 0.032 µM, for the N₂, O₂ and aerated atmospheres respectively. In order to investigate the electrochemical reactivity of activated graphite, four different NACs were selected such as flutamide (anti-cancer drug), methyl parathion (pesticide), 4-nitro aniline and 4-nitro phenol.

Introduction

Nitroaromatic compounds (NACs) are lethal environmental pollutants that cause severe adverse effects to human health [1]. These NACs are employed in chemical industries to prepare various profitable products such as explosives, plastics, dyes, drugs and pesticides [2], [3], [4]. Nowadays, these products are very essential to the country’s development and security. However, the waste water from these industries are polluted the soil and water. Moreover, the usage of explosives and pesticides also affect the environment [5], [6], [7]. If these NACs are exposed in environment for a long time that contaminates the ground water. Consumption of this contaminated water leads to mutagenesis and carcinoma to humans [8], [9], [10]. Therefore, earliest researchers have been developed various techniques to determine these NACs, such as chromatographic, spectroscopic, spectrophotometric and electrochemical methods [11], [12], [13]. Among all, the electrochemical method is very interesting because of its rapid detection and low cost. In addition, it provides acceptable sensitivity and selectivity over the other methods [14]. In most of the cases, the carbonaceous materials or its composites were used as electrode material for the determination of pesticides and explosives [15], [16], [17]. These electrodes can possibly determine the pesticides and explosives in waste water. However, a critical selectivity problem arises in their determination with the interference of dissolved oxygen molecules, because the electrocatalytic reduction of oxygen at the carbon matrix is highly active [18]. Not only carbon, some of noble metals such as platinum, gold, silver and palladium also affected by this phenomenon [19], [20], [21], [22], [23]. Therefore, the present work concentrates the effect of electrocatalytic activity of dissolved oxygen molecules with NACs. Thereby, we have chosen the graphite as a model carbonaceous material for the detection of NACs. Generally, graphite is a three dimensional and naturally occurring carbon allotrope which has high thermal stability and conducting behavior [24]. This graphite consists of individual graphene sheets which orderly arranged with the weak Van der Waal’s bonds. Therefore, the graphite was easily isolated into graphene sheets. In contrast with graphite, the graphene exhibited good electrical conductivity and better electrochemical performances [25]. This graphene is mainly prepared by Hummer’s method, moreover, some other methods are also available such as mechanical, thermal and electrochemical exfoliation [26], [27], [28]. However, it is highly risk to direct use of graphene for the fabrication and modification. Hence, we have concentrated on the synthesis of activated graphite sheets (aGR, few layers of graphene) for the detection of NACs. For the exfoliation, we have avoided the corrosive chemicals and explosive methodologies. Thereby, we have simply used the cerium nitrate as unzipping agent. The electrostatic attraction of cerium with graphite weakens the Van der Waal’s force between the graphite layers then the nitrate ions are separated the layers (see Fig. 1). Moreover, this manuscript mainly focuses the electrocatalytic properties of aGR to the detection of NACs in the presence of dissolved oxygen.