http://www.paper.edu.cn J. Phys. Chem. B 2005, 109, 17281-17289 17281 Thermodynamic Analysis of Decomposition of Thiourea and Thiourea Oxides Shun Wang,†,‡ Qingyu Gao,*,† and Jichang Wang*,§ College of Chemical Engineering, China UniVersity of Mining and Technology, Xuzhou, Jiangsu, China, 221008, Department of Chemistry and Material Science, Wenzhou Normal College, Wenzhou, China, 325027, and Department of Chemistry and Biochemistry, UniVersity of Windsor, Ontario, Canada N9B 3P4 ReceiVed: March 30, 2005; In Final Form: June 29, 2005 Thiourea has exhibited extremely rich dynamical behavior when being oxidized either through a chemical approach or via an electrochemical method. In this study, thermodynamic properties of thiourea and its oxides are investigated by measuring their thermogravimetry (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC) simultaneously. Online FT-IR measurements show that products of the thermal decomposition vary significantly with the reaction temperature. In addition to the determination of their apparent activation energy (E), preexponential factor (A), and entropy (∆Sq), enthalpy (∆Hq), and Gibbs energy (∆Gq) of thermal decomposition, our investigation further illustrates that the decomposition kinetics of thiourea and thiourea oxides follows the Johnson-Mehl-Avrami Equation, f (R) ) n(1 -R)- [-ln(1 -R)]1-1/n and G(R) ) [-ln(1 -R)]1/n with n equal to 2, 3.43, and 3, respectively. 1. Introduction sulfenyl, then sulfinic and sulfonic acids, and finally sulfate ions. Yet, whether these rich nonlinear phenomena observed in Thiourea (TU) and its oxides are important reagents in various thiourea oxidations arise from sulfur chemistry or from oxidants industrial productions.1-7 For example, thiourea has been used remains to be understood.29,30 In particular, both simple and widely as an additive to improve coating quality and to inhibit complex oscillations have been observed when thiourea was corrosion.1,2 It has also been used as an efficient reactant to oxidized through an electrochemical approach.31 Apart from synthesize heterocycles and to extract precious metals.3,4 scientific interests, knowing the oxidation kinetics of thiourea Thiourea dioxide, aminoiminomethanesulfinic acid (AIMSA), and thiourea oxides may also be critical in the understanding is commonly employed as a powerful bleaching reagent in the of the physiological effects of sulfur-containing species. In recent textile industry5 and as an effective initiator for various studies, Simoyi and his group conducted a series of experiments polymerizations.6,7 Thiourea trioxide, aminoiminomethane- on the decomposition of thiourea dioxide in alkaline solution sulfonic acid (AIMSOA), however, is found to be a convenient and their study suggests that the reactive oxygen species, guanidylating agent, which is known to present as a subunit in superoxide, peroxide, and hydroxyl radicals that result from the the amino acid arginine, the pyrimidine base of DNA, and many cleavage of the C-S bond to give the sulfoxylate ion are other biologically significant molecules.8-11 responsible for the inherent toxicities of thiourea.32 In addition to the practical applications mentioned above, To shed light on the oxidation kinetics of thiourea and its thiourea and thiourea oxides are also valuable reactants in impacts on physiological and environmental aspects, in this fundamental scientific researches, especially in the study of study we characterized the thermal stabilities of thiourea and nonlinear reaction dynamics.12-15 The oxidation of thiourea has its oxides. It is motivated by the fact that although it is well displayed various fascinating nonlinear dynamical behaviors known that violent oxidations or combustion of thiourea and such as oligooscillations, autocatalysis in a closed system, thiourea oxides may result in the production of poisonous gases mixed-mode oscillations, bistability, birhythmicity, quasiperi- such as SO and SO ,33 surprisingly little is known about their odicity and chaos in a continuously flow stirred tank reactor 2 3 thermal decomposition kinetics. In the following, we investigate (CSTR), and travelling waves in a reaction-diffusion me- the pyrolysis of thiourea, AIMSA, and AIMSOA under noniso- dium.16-21 Only a few chemical systems are known to be thermal conditions by a simultaneous TG/DTG/DSC-FT-IR capable of exhibiting such rich dynamics. Consequently, studies method. Our study leads to the establishment of three kinetic of the kinetics and mechanisms of the oxidation of thiourea and equations to account for their decompositions. thiourea derivatives have attracted a great deal of attention in the past two decades, in which various oxidants such as iodate, 2. Experimental Procedure bromate, chorite, chlorine dioxide, ferrate (VI), hydroxyl 22-29 radicals, and peracetic acid have been characterized. These Reagent grade SC(NH2)2 was recrystallized twice with hot earlier investigations led to a general conclusion that the water that had been distilled and then purified through a Milli-Q oxidation of thiourea goes through S-oxygenation to form system (18.2 MΩ). AIMSA (Aldrich) was used without further purification. AIMSOA was prepared according to literature and * Corresponding authors. E-mail: [email protected]; fax: 86-516- was identified by IR spectra measured with a Nicolet Avatar 3995758. E-mail: [email protected]; fax: 1-519-973-7098. 10 † 360 FT-IR spectrometer. The measured characteristic absorp- China University of Mining and Technology. -1 -1 ‡ Wenzhou Normal College. tion wavenumbers of AIMSOA are 1220 cm and 1050 cm , § University of Windsor. which are in good agreement with theoretical values.34 The 10.1021/jp051620v CCC: $30.25 © 2005 American Chemical Society 转载 Published on Web 08/20/2005 中国科技论文在线 http://www.paper.edu.cn 17282 J. Phys. Chem. B, Vol. 109, No. 36, 2005 Wang et al. Figure 1. TG/DTG/DSC curves of the thermal decomposition of thiourea. The flow rate of nitrogen is 30.0 mL min-1. The heating rate equals 8.0 °C min-1, and the initial amount of thiourea is 6.40 mg. Figure 2. Online FT-IR spectra of the off-gases of the pyrolysis of thiourea. All of the reaction conditions are the same as those in Figure 1. simultaneous TG/DTG/DSC measurements were carried out min-1. The DSC curve shows that there are two strongly with a Netzsch STA 409C thermoanalyzer under nitrogen overlapping endothermic processes in which the first endother- atmosphere. Throughout this study, the flow of nitrogen gas mic process spans the temperature range between 171.2 and - was fixed at 30.0 mL min 1. The heating rate has been studied 187.5 °C with the maximum at 180.2 °C. As is indicated by ° -1 at 2.0, 5.0, 8.0, 12.0, and 15.0 C min , respectively. Samples the TG curve, during the first endothermic process there is nearly - of 3.0 9.0 mg were placed in a Al2O3 crucible, and the gas no mass loss. Therefore, the first endothermic process is products of the thermal decomposition were analyzed online dominated by the melting of thiourea.35 Notably, in our with a Nicolet Avatar 470 FT-IR spectrometer. experiments the initial point, T0, at which the DSC curve deviates from the baseline is lower than the melting temperature 3. Experimental Results of thiourea, which is reported to be around 175.05 °C and 179.05 Figure 1 presents the typical TG, DTG, and DSC curves of °C.36,37 In addition, the calculated reaction enthalpies yield an thiourea decomposition conducted at a heating rate of 8.0 °C average value of 6.8 kJ‚mol-1, which is also less than 12.6 中国科技论文在线 http://www.paper.edu.cn Oxidation of Thiourea and Thiourea Oxides J. Phys. Chem. B, Vol. 109, No. 36, 2005 17283 temperature at 222.1 °C) governs more than 80% of the total weight loss. As is illustrated by the real time FT-IR spectra of the off-gases (see Figure 2), there are three major species in the thermal decomposition products, namely, carbon disulfide -1 (CS2) with absorption peaks at 1540 and 2171 cm , isothio- cyanic acid (HNCS) with absorption peaks at 2070 and 2041 -1 cm , and ammonia (NH3) with absorption peaks at 3331, 1635, 966, and 925 cm-1.38,39 Figure 3 presents the corresponding evolution curves of the three reagents as a function of temper- ature, which clearly shows that the thermal decomposition of thiourea begins at 187.5 °C. As is shown by the DTG curve in Figure 1, the decomposition rate of thiourea increases rapidly with temperature and reaches the maximum at 218.5 °C. The production of CS2, HNCS, and NH3 are presumably dominated by the following two steps: f + + 2SC(NH2)2(s) CS2(g) H2NCN(g) 2NH3(g) (R2) Figure 3. The absorbance vs temperature curves showing variations of concentrations of the thermal decomposition products as a function NH SCN(s) f NH (g) + HNCS(g) (R3) of temperature. The reaction conditions are listed in Figure 1. 4 3 kJ‚mol-1 reported by other groups.36 Because we had always used freshly prepared thiourea, the decomposition of thiourea According to reaction R2, cyanamide (H2NCN), which has - (i.e., aging) is unlikely responsible for the above abnormal a characteristic absorption peak at 2284 cm 1, shall also appear behavior. Isomerization of thiourea has been proposed earlier in the IR spectra; yet no such a peak was seen in our by Waddell in the study of the conversion of thiourea into experiments. An existing study suggested that H2NCN(g) may thiocyanate. The production of NH4SCN(s), which has a lower undergo a spontaneous polymerization reaction to produce 40 melting temperature around 149.0 °C, would consequently melamine. As a result, the escape of H2NCN is retarded. reduce T0 of the DSC curve. Thus, the isomeric reaction (R1) Figure 3 shows that despite the fact that the decomposition is suggested to take place within the temperature range between rate of thiourea reaches the maximum at T ) 218.5 °C, 171.2 and 187.5 °C.