Combined electrochemical and spectroscopic investigations of carbonate-mediated water oxidation to peroxide

The development of electrosynthetic technologies for H2O2 production is appealing from a sustainability perspective. The use of carbonate species as mediators in water oxidation to peroxide has emerged as a viable route to do so but still many questions remain about the mechanism that must be addressed. To this end, this work combines electrochemical and spectroscopic methods to investigate reaction pathways and factors influencing the efficiency of this reaction. Our results indicate that CO32- is the key species that undergoes electrochemical oxidation, prior to reacting with water away from the catalyst. Through spectroelectrochemical experiments, we noted that CO32- depletion is a factor that limits the selectivity of the process. In turn, we showed how the application of pulsed electrolysis can augment this, with an initial set of optimized parameters increasing the selectivity from 20% to 27%. In all, this work helps pave the way for future development of practical H2O2 electrosynthetic systems.

Reaction of FeaqII with Peroxymonosulfate and Peroxydisulfate in the Presence of Bicarbonate: Formation of FeaqIV and Carbonate Radical Anions

Many advanced oxidation processes (AOPs) use Fenton-like reactions to degrade organic pollutants by activating peroxymonosulfate (HSO₅^-, PMS) or peroxydisulfate (S₂O₈^2-, PDS) with Fe(H₂O)₆²⁺ (Fe_aq^II). This paper presents results on the kinetics and mechanisms of reactions between Fe_aq^II and PMS or PDS in the absence and presence of bicarbonate (HCO₃^-) at different pH. In the absence of HCO₃^-, Fe_aq^IV, rather than the commonly assumed SO₄^•-, is the dominant oxidizing species. Multianalytical methods verified the selective conversion of dimethyl sulfoxide (DMSO) and phenyl methyl sulfoxide (PMSO) to dimethyl sulfone (DMSO₂) and phenyl methyl sulfone (PMSO₂), respectively, confirming the generation of Fe_aq^IV by the Fe_aq^II-PMS/PDS systems without HCO₃^-. Significantly, in the presence of environmentally relevant concentrations of HCO₃^-, a carbonate radical anion (CO₃^•-) becomes the dominant reactive species as confirmed by the electron paramagnetic resonance (EPR) analysis. The new findings suggest that the mechanisms of the persulfate-based Fenton-like reactions in natural environments might differ remarkably from those obtained in ideal conditions. Using sulfonamide antibiotics (sulfamethoxazole (SMX) and sulfadimethoxine (SDM)) as model contaminants, our study further demonstrated the different reactivities of Fe_aq^IV and CO₃^•- in the Fe_aq^II-PMS/PDS systems. The results shed significant light on advancing the persulfate-based AOPs to oxidize pollutants in natural water.

Hydrothermal Synthesis of Siderite and Application as Catalyst in the Electro-Fenton Oxidation of p-Benzoquinone

A weak aspect of the electro-Fenton (EF) oxidation of contaminants is the dependence of the Fenton reaction on acidic pH values. Therefore, the rationale of this work was to develop a novel catalyst capable of promoting the EF oxidation process at near-neutral and basic pH values. In this framework, rhombohedral FeCO₃ was synthesized hydrothermally and used as a catalyst in the EF oxidation of p-benzoquinone (BQ). The catalyst was characterized using various surface and spectroscopic methods. Moreover, the effects of applied current (100-500 mA), time (1-9 h), catalyst dosage (0.25-1.00 g L^-1), and initial concentration of BQ (0.50-1.00 mM) on the total organic carbon removal efficiency were determined. The results indicated that a 400 mA current was sufficient for a 95% total organic carbon removal and that the increase in catalyst dosage had a positive effect on the mineralization of BQ. It was determined that at pH 3, FeCO₃ behaved like a homogeneous catalyst by releasing Fe³⁺ ions; whereas, at the pH range of 5-7, it shifted to a homogeneous/heterogeneous catalyst. At pH 9, it worked solely as a heterogeneous catalyst due to the decrease of Fe ions passing into the solution. Finally, the spent catalyst did not undergo structural deformations after the EF treatment at higher pH values and could be regenerated and used several times.

What Are the Oxidizing Intermediates in the Fenton and Fenton-like Reactions? A Perspective

The Fenton and Fenton-like reactions are of major importance due to their role as a source of oxidative stress in all living systems and due to their use in advanced oxidation technologies. For many years, there has been a debate whether the reaction of Fe^II(H₂O)₆²⁺ with H₂O₂ yields OH^• radicals or Fe^IV=O_aq. It is now known that this reaction proceeds via the formation of the intermediate complex (H₂O)₅Fe^II(O₂H)⁺/(H₂O)₅Fe^II(O₂H₂)²⁺ that decomposes to form either OH^• radicals or Fe^IV=O_aq, depending on the pH of the medium. The intermediate complex might also directly oxidize a substrate present in the medium. In the presence of Fe^III_aq, the complex Fe^III(OOH)_aq is formed. This complex reacts via Fe^II(H₂O)₆²⁺ + Fe^III(OOH)_aq → Fe^IV=O_aq + Fe^III_aq. In the presence of ligands, the process often observed is L_n(H₂O)_5−nFe^II(O₂H) → L^•+ + L_n−1Fe^III_aq. Thus, in the presence of small concentrations of HCO₃⁻ i.e., in biological systems and in advanced oxidation processes—the oxidizing radical formed is CO₃^•−. It is evident that, in the presence of other transition metal complexes and/or other ligands, other radicals might be formed. In complexes of the type L_n(H₂O)_5−nM^III/II(O₂H⁻), the peroxide might oxidize the ligand L without oxidizing the central cation M. OH^• radicals are evidently not often formed in Fenton or Fenton-like reactions.

The Role of Carbonate in Catalytic Oxidations

CO₂, HCO₃^-, and CO₃^2- are present in all aqueous media at pH > 4 if no major effort is made to remove them. Usually the presence of CO₂/HCO₃^-/CO₃^2- is either forgotten or considered only as a buffer or proton transfer catalyst. Results obtained in the last decades point out that carbonates are key participants in a variety of oxidation processes. This was first attributed to the formation of carbonate anion radicals via the reaction OH^• + CO₃^2- → CO₃^•- + OH^-. However, recent studies point out that the involvement of carbonates in oxidation processes is more fundamental. Thus, the presence of HCO₃^-/CO₃^2- changes the mechanisms of Fenton and Fenton-like reactions to yield CO₃^•- directly even at very low HCO₃^-/CO₃^2- concentrations. CO₃^•- is a considerably weaker oxidizing agent than the hydroxyl radical and therefore a considerably more selective oxidizing agent. This requires reconsideration of the sources of oxidative stress in biological systems and might explain the selective damage induced during oxidative stress. The lower oxidation potential of CO₃^•- probably also explains why not all pollutants are eliminated in many advanced oxidation technologies and requires rethinking of the optimal choice of the technologies applied. The role of percarbonate in Fenton-like processes and in advanced oxidation processes is discussed and has to be re-evaluated. Carbonate as a ligand stabilizes transition metal complexes in uncommon high oxidation states. These high-valent complexes are intermediates in electrochemical water oxidation processes that are of importance in the development of new water splitting technologies. HCO₃^- and CO₃^2- are also very good hole scavengers in photochemical processes of semiconductors and may thus become key participants in the development of new processes for solar energy conversion. In this Account, an attempt to correlate these observations with the properties of carbonates is made. Clearly, further studies are essential to fully uncover the potential of HCO₃^-/CO₃^2- in desired oxidation processes.

The FeII(citrate) Fenton reaction under physiological conditions

The Fenton reaction of Fe^II(citrate) in the presence and absence of bicarbonate (HCO₃^-) is studied. It is found that the rate constant of the Fenton reaction (k_obs) increases with increasing [citrate]. k_obs also increase with increasing [HCO₃^-]; this effect is most significant at biological citrate concentrations. Methane and ethane gases are formed from (CH₃)₂SO when the Fenton reaction is carried out in the presence of large [citrate] due to the reaction of the citrate radical, (^-₂OC)CH₂C(OH)(CO₂^-)CH(CO₂^-)/(^-₂OC)CH₂C(O)(CO₂^-)CH₂(CO₂^-) with (CH₃)₂SO. In the absence of citrate (CH₃)₂SO₂ is the main product of the Fenton reaction. However, in the presence of 0.10 mM citrate, no (CH₃)₂SO₂ is formed, some (CH₃)SOOH is formed, along with a low yield of beta-ketoglutaric acid. Formation of (CH₃)SOOH and beta-ketoglutaric acid are due to the citrate radical and Fe^IV(citrate). In the presence of bicarbonate formation of abundant beta-ketoglutaric acid confirms the formation of carbonate radical anion (CO₃^-). Thus, bicarbonate affects the mechanism and kinetics of the reaction dramatically. Hydroxyl radicals (OH) are not formed in the presence of bicarbonate and probably also not in its absence. These results point out that hydroxyl radicals, formed by the Fenton reaction, do not initiate oxidative stress in biological systems.

The role of carbonate in electro-catalytic water oxidation by using Ni(1,4,8,11-tetraazacyclotetradecane)2

NiL_i²⁺ are good electro-catalysts for water oxidation in phosphate or carbonate buffers. The results point out that the active oxidizing agents are L(X)Ni^IVOH^{4-(3-n+1)/(2-n+1)}, where X = PO₄H_n^(3-n)- or CO₃H_n^(2-n)- formed from LNi^IVX₂via a mechanism involving an acid catalyzed O-P or O-C bond heterolysis. Carbonate behaves differently from phosphate as it is a non-innocent ligand and it can be oxidized.

Carbonate and carbonate anion radicals in aqueous solutions exist as CO3(H2O)62− and CO3(H2O)6˙− respectively: the crucial role of the inner hydration sphere of anions in explaining their properties

An effort to reproduce the physical properties of CO₃²⁻ and CO₃˙⁻ in water proves that one has to include an inner hydration sphere of six water molecules for both anions.

Oxidative functionalization of C–H compounds induced by the extremely efficient osmium catalysts (a review)

In recent years, osmium complexes have found applications not only in thecis-hydroxylation of olefins but also very efficient in the oxygenation of C–H compounds (saturated and aromatic hydrocarbons and alcohols) by hydrogen peroxide as well as organic peroxides.

Mechanistic Studies on the Role of [CuII (CO3 )n ]2-2n as a Water Oxidation Catalyst: Carbonate as a Non-Innocent Ligand

Recently it was reported that copper bicarbonate/carbonate complexes are good electro-catalysts for water oxidation. However, the results did not enable a decision whether the active oxidant is a Cu^III or a Cu^IV complex. Kinetic analysis of pulse radiolysis measurements coupled with DFT calculations point out that Cu^III (CO₃ )_n^3-2n complexes are the active intermediates in the electrolysis of Cu^II (CO₃ )_n^2-2n solution. The results enable the evaluation of E°[(Cu^III/II (CO₃ )_n )_aq ]≈1.42 V versus NHE at pH 8.4. This redox potential is in accord with the electrochemical report. As opposed to literature suggestions for water oxidation, the present results rule out single-electron transfer from Cu^III (CO₃ )_n^3-2n to yield hydroxyl radicals. Significant charge transfer from the coordinated carbonate to Cu^III results in the formation of C₂ O₆^2- by means of a second-order reaction of Cu^III (CO₃ )_n^3-2n . The results point out that carbonate stabilizes transition-metal cations at high oxidation states, not only as a good sigma donor, but also as a non-innocent ligand.

Remarkable enhancement of Fenton degradation at a wide pH range promoted by thioglycolic acid

Thioglycolic acid efficiently recycles Fe(ii) and significantly enhances the Fenton degradation of organic and microbial pollutants at a broad pH range.

The plausible role of carbonate in photo-catalytic water oxidation processes

DFT calculations point out that the photo-oxidation of water on GaN is energetically considerably facilitated by adsorbed carbonate.