Electroreduction of 2,4,6-Trinitrotoluene in Room Temperature Ionic Liquids: Evidence of an EC 2 Mechanism Colin Kang, Junqiao Lee, and Debbie S. Silvester* Nanochemistry Research Institute, Department of Chemistry, Curtin University, GPO Box U1987, Perth 6845, Australia * S Supporting Information ABSTRACT: The reduction of 2,4,6-trinitrotoluene (TNT) has been studied in eight room temperature ionic liquids (RTILs) on a gold (Au) microdisk electrode and a Au thin film electrode (TFE). Three reduction peaks were observed in all RTILs, corresponding to the reductions of each of the three nitro groups in the TNT structure. TNT was the easiest to reduce in imidazolium RTILs, followed by pyrrolidinium and then tetraalkylphosphonium. Diffusion coefficients (D) and electron counts (n) were calculated from potential-step chronoamperometry on the first reduction peak. D’s ranged from 0.7 × 10 -11 to 4.1 × 10 -11 m 2 s -1 , and a plot of D against the inverse of viscosity was linear, indicating that the Stokes- Einstein relation holds well for TNT in RTILs. The electron count was one in most RTILsin stark contrast to the widely accepted six-electron reduction in protic solvents. An electrogenerated red solid was formed after the first reduction peak, believed to be an azo (or azoxy) compound formed by dimerization of two TNT radicals, although characterization of the product(s) proved difficult. The behavior at different concentrations revealed different degrees of chemical reversibility of reduction peak. This evidence points toward the possibility of an EC 2 mechanism, which was supported by digital simulation of the experimental voltammograms. Understanding the reduction mechanism of TNT is essential if RTILs are to be used for TNT sensing applications, particularly at high concentrations. 1. INTRODUCTION 2,4,6-Trinitrotoluene (TNT) is an explosive compound commonly used for military, industrial, and mining applications. It has also been used in homemade explosives by terrorists due to its insensitivity to shock and friction (e.g., compared to other high explosives such as nitroglycerin), reducing the risk of accidental detonation. Additionally, if not completely removed after an explosion, its reduction products are known to be toxic and carcinogenic to humans and may contaminate drinking water. 1-3 Because of a range of security and environmental needs, the development of sensors for explosives such as TNT is of huge interest. Various techniques (both physical and chemical) have been developed to detect TNT, with electro- chemical methods offering the advantages of low-cost instrumentation, portability, durability, sensitivity, and rapid response times. 2,3 TNT is typically dissolved directly in the electrochemical solvent for sensing or can be volatilized to be detected in the gas phase. 2,3 From an electrochemical perspective, the presence of nitro groups on the aromatic ring results in TNT being redox active and able to accept electrons. The electrochemical reduction of TNT has therefore been studied extensively in aqueous media, 1,4-8 with the most accepted mechanism involving the transfer of 6 electrons and 6 protons in each reduction process (total of 18 electrons and 18 protons over the three processes). 1 Review articles on the electrochemical detection of TNT are also available. 2,3 Despite the fact that the electrochemical behavior of TNT has been well characterized in aqueous solvents, there has been very little work performed in aprotic solvents (e.g., acetonitrile) and also in room temperature ionic liquids (RTILs). Prabu et al. 9 reported the detection of TNT in acetonitrile using square wave stripping voltammetry (SWSV). Although their study was mostly analytical (i.e., no mechanistic investigations were performed), they mentioned that two peaks were observed on the reductive scan using cyclic voltammetry. In RTILs, Forzani et al. 10 reported cyclic voltammetry for the reduction of TNT in the RTIL 1-butyl-3-methylimidazolium hexafluoro- phosphate ([C 4 mim][PF 6 ]). Three reduction peaks were observed, and the reduction processes were found to produce distinctive red products, suggested to be azo and azoxy derivatives. 10 It was stated that the ionic liquid medium was essential to produce the colored reaction products (since the same products were not observed in aqueous solutions), but a study on the reduction mechanism was not performed. A later study from the same group 11 also reported cyclic voltammetry Received: March 23, 2016 Revised: April 25, 2016 Published: May 2, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 10997 DOI: 10.1021/acs.jpcc.6b03018 J. Phys. Chem. C 2016, 120, 10997-11005