Resistive Oxidation Kinetics of Iron(II) Thiochelate used as a Nitric Oxide Absorbent in Flue Gas

Abstract

Iron(II) thiochelates, especially iron(II) 2,3-bis(sulfanyl)propane-1-sulfonate (Fe(II)(DMPS) 2 ), have received particular attention as nitric oxide (NO) absorbents because they maintain high NO removal efficiency even in the presence of O 2 in flue gases. Herein, we explored the O 2 -inducing oxidation kinetics of Fe(II) and thiol groups (R−SH) in Fe(II)(DMPS) 2 and analyzed the resultant products to understand the resistive behavior toward O 2 . Quantitative analysis revealed that 4 mM of Fe(II) in 10 mM Fe(DMPS) 2 solution was maintained without further oxidation even after 12 h under 5% O 2 , but instead thiol group concentration continued to decrease. In comparison to the commonly used Fe(II)(EDTA) absorbent, in which all the 10 mM Fe(II) was fully oxidized within 40 min reaction by 5% O 2 , this distinctive oxidative behavior of iron(II) thiochelate was attributed to the self-sacrificing thiol-containing ligands. This phenomenon was verified by the rapid and almost complete redox reaction between the oxidized Fe(III) and thiol groups in DMPS. Fourier transform infrared spectra indicated that the oxidation products of thiols were disulfide (S−S) and some sulfonates (−SO 3 − ). Finally, based on all the experimental data, possible oxidation pathways of Fe(II)(DMPS) 2 by O 2 were proposed. This study provides both fundamental and applied insights of harnessing iron(II) thiochelate as a competitive NO absorbent for mitigating air pollution.

Effective Electroregeneration of the Oxidized Iron(II) Thiochelate Absorbent in the Wet NOX Absorption Process

“…The voltammogram of Fe­(II)­(DMPS) 2 showed a pair of distinct redox peaks with the half-wave potential at E 1/2 = −0.115 V vs Ag/AgCl, while Fe­(III)­(DMPS) 2 displayed a much lower peak current density at a nearby potential. It is known that Fe­(III)­(DMPS) 2 undergoes autoreduction, during which Fe­(III) and DMPS in the complex react to generate a Fe­(II) and DMPS dimer linked by a disulfide (RS–SR) bond. ,− This chemical reaction is related to the breakage of the complex and could deteriorate the electrochemical activity of Fe­(III)­(DMPS) 2 . The discrepancy in the shape between the reduction and oxidation peaks was also noticeable, suggesting the different reduction and oxidation environments of Fe­(DMPS) 2 at the electrode.…”

“… In the group of iron­(II) chelate, thiochelate was proposed as a stable alternative to the most well-studied Fe­(II)­(EDTA), as it is less susceptible to the undesired oxidation of Fe­(II) to Fe­(III) by O 2 contained in flue gases (2–6% by volume) and has a higher NO absorption capacity (2.1 × 10 7 L mol –1 at 55 °C). , The representative thiol-containing ligand is 2,3-dimercapto-1-propanesulfonate (DMPS), which possesses two thiol groups in a molecule. We previously identified that its oxidation resistivity stems from the sacrificing oxidation of the thiol groups, thereby shielding Fe­(II) from O 2 -induced oxidation within the complex . The resultant oxidized iron thiochelate, however, eventually loses its binding affinity to NO; thus, the regeneration process must be accompanied to maintain the original NO binding capacity .…”

“…We previously identified that its oxidation resistivity stems from the sacrificing oxidation of the thiol groups, thereby shielding Fe(II) from O 2 -induced oxidation within the complex. 5 The resultant oxidized iron thiochelate, however, eventually loses its binding affinity to NO; thus, the regeneration process must be accompanied to maintain the original NO binding capacity. 6 Even so, the reductive regeneration of iron(II) thiochelate inactivated by O 2 has not been systematically studied until recently, although there have been several reports on the reduction of the bound NO in the iron−nitrosyl complex.…”

Perspectives in Adsorptive and Catalytic Mitigations of NOx Using Metal–Organic Frameworks

“…One could refer to the recent review by Yang et al for discussions on the DeNO x methods such as SCR, SNCR, adsorption, storage reduction, and especially on direct catalytic decomposition of NO using catalysts involving noble metals, metal oxides, perovskite-type composite oxide, hydrotalcite materials, heteropoly acid, and Cu-ZSM-5. Various NO reduction chemical absorption systems have also been explored by different researchers using absorbents including Fe­(II)­EDTA, , Fe­(II) thiochelates, and 1,3-dimethylthiourea-derived eutectic solvents. , More specifically, abatement of NO x using SCR with ammonia is the most developed process because of its cost saving and efficiency in the presence of other competitive gases such as O 2 , H 2 O, and SO 2 . − …”

“…Various NO reduction chemical absorption systems have also been explored by different researchers using absorbents including Fe(II)EDTA, 21,22 Fe(II) thiochelates, and 1,3-dimethylthiourea-derived eutectic solvents. 23,24 More specifically, abatement of NO x using SCR with ammonia is the most developed process because of its cost saving and efficiency in the presence of other competitive gases such as O 2 , H 2 O, and SO 2 . 25−27 Three types of catalyst have been developed and are commercially used: noble metal-based catalyst, mixed oxides (V 2 O 5 /TiO 2 ), and zeolites.…”