Tailoring the oxidizing intermediate in the Fenton reaction, OH• or FeIV=Oaq, by modifying the pKa of the (H2O)5FeII(H2O2) complex, and the case of PDS - a DFT study.

A DFT analysis of the Fenton and Fenton-like reactions points out that the pH effect on the nature of the oxidizing intermediate formed is due to a pKa of the peroxide when hydroperoxides are used. When S2O82- is used, the pH effect is due to the pKa of one of the water ligands of the central iron cation. The results suggest that the choice of the hydroperoxide and the ligands present affects the pH at which the transition from the formation of hydroxyl radicals to the formation of FeIV=Oaq occurs.

Hydrogen adsorption on various transition metal (111) surfaces in water: a DFT forecast.

The hydrogen adsorption and hydrogen evolution at the M(111), (M = Ag, Au Cu, Pt, Pd, Ni & Co) surfaces of various transition metals in aqueous suspensions were studied computationally using the DFT methods. The hydrogens are adsorbed dissociatively on all surfaces except on Ag(111) and Au(111) surfaces. The results are validated by reported experimental and computational studies. Hydrogen atoms have large mobility on M(111) surfaces due to the small energy barriers for diffusion on the surface. The hydrogen evolution via the Tafel mechanism is considered at different surface coverage ratios of hydrogen atoms and is used as a descriptor for the hydrogen adsorption capacity on M(111) surfaces. All calculations are performed without considering how the hydrogen atoms are formed on the surface. The hydrogen adsorption energies decrease with the increase in the surface coverage of hydrogen atoms. The surface coverage for the H2 evolution depends on each M(111) surface. Among the considered M(111) surfaces, Au(111) has the least hydrogen adsorption capacity and Ni, Co and Pd have the highest. Furthermore, experiments proving that after the H2 evolution reaction (HER) on Au0-NPs, and Ag0-NPs surfaces some reducing capacity remains on the M0-NPs is presented.

On The Nature of FeIV =Oaq in Aqueous Media: A DFT analysis.

FeIV =Oaq is a key intermediate in many advanced oxidation processes and probably in biological systems. It is usually referred to as FeIV =O2+ . The pKa's of FeIV =Oaq as derived by DFT are: pKa1=2.37 M06 L/6-311++G(d,p) (SDD for Fe) and pKa2=7.79 M06 L/6-311++G(d,p) (SDD for Fe). This means that in neutral solutions, FeIV =Oaq is a mixture of (H2 O)4 (OH)FeIV =O+ and (H2 O)2 (OH)2 FeIV =O. The oxidation potential of FeIV =Oaq in an acidic solution, E0 {(H2 O)5 FeIV =O2+ /FeIII (H2 O)6 3+ , pH 0.0} is calculated with and without a second solvation sphere and the recommended value is between 2.86 V (B3LYP/Def2-TZVP, with a second solvation sphere) and 2.23 V (M06 L/Def2-TZVP without a second solvation sphere). This means that FeIV =Oaq is the strongest oxidizing agent formed in systems involving FeVI O4 2- even in neutral media.

Heterogeneous Electrocatalytic Oxygen Evolution Reaction by a Sol-Gel Electrode with Entrapped Na3 [Ru2 (µ-CO3 )4 ]: The Effect of NaHCO3.

The Na3 [Ru2 (µ-CO3 )4 ] complex is acting as a water oxidation catalyst in a homogeneous system. Due to the significance of heterogeneous systems and the effect of bicarbonate on the kinetic, we studied the bicarbonate effect on the heterogeneous electrocatalyst by entrapping the Na3 [Ru2 (µ-CO3 )4 ] complex in a sol-gel matrix. We have developed two types of sol-gel electrodes, which differ by the precursor, and are demonstrating their stability over a minimum of 200 electrochemical cycles. The pH increases affected the currents and kcat for both types of electrodes, and their hydrophobicity, which was obtained from the precursor type, influenced the electrocatalytic process rate. The results indicate that NaHCO3 has an important role in the catalytic activity of the presented heterogeneous systems; without NaHCO3 , the diffusing species is probably OH- , which undergoes diffusion via the Grotthuss mechanism. To the best of our knowledge, this is the first study to present a simple and fast one-step entrapment process for the Na3 [Ru2 (µ-CO3 )4 ] complex by the sol-gel method under standard laboratory conditions. The results contribute to optimizing the WSP, ultimately helping expand the usage of hydrogen as a green and more readily available energy source.

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 (S2O82-, PDS) with Fe(H2O)62+ (FeaqII). This paper presents results on the kinetics and mechanisms of reactions between FeaqII and PMS or PDS in the absence and presence of bicarbonate (HCO3-) at different pH. In the absence of HCO3-, FeaqIV, 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 FeaqIV by the FeaqII-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 FeaqIV and CO3•- in the FeaqII-PMS/PDS systems. The results shed significant light on advancing the persulfate-based AOPs to oxidize pollutants in natural water.

The Competition between 4-Nitrophenol Reduction and BH4- Hydrolysis on Metal Nanoparticle Catalysts.

Assessing competitive environmental catalytic reduction processes via NaBH4 is essential, as BH4- is both an energy carrier (as H2) and a reducing agent. A comprehensive catalytic study of the competition between the borohydride hydrolysis reaction (BHR, releasing H2) and 4-nitrophenol reduction via BH4- on M0- and M/M' (alloy)-nanoparticle catalysts is reported. The results reveal an inverse correlation between the catalytic efficiency for BH4- hydrolysis and 4-nitrophenol reduction, indicating that catalysts performing well in one process exhibit lower activity in the other. Plausible catalytic mechanisms are discussed, focusing on the impact of reaction products such as 4-aminophenol and borate on the rate and yield of BH4- hydrolysis. The investigated catalysts were Ag0, Au0, Pt0, and Ag/Pt-alloy nanoparticles synthesized without any added stabilizer. Notably, the observed rate constants for the 4-nitrophenol reduction on Ag0, Ag-Pt (9:1), and Au0 are significantly higher than the corresponding rate constants for BH4- hydrolysis, suggesting that most reductions do not proceed through surface-adsorbed hydrogen atoms, as observed for Pt0 nanoparticles. This research emphasizes the conflicting nature of BH4- hydrolysis and reduction processes, provides insights for designing improved catalysts for competitive reactions, and sheds light on the catalyst properties required for each specific process.

Overlooked Formation of Carbonate Radical Anions in the Oxidation of Iron(II) by Oxygen in the Presence of Bicarbonate.

Iron(II), (Fe(H2 O)6 2+ , (FeII ) participates in many reactions of natural and biological importance. It is critically important to understand the rates and the mechanism of FeII oxidation by dissolved molecular oxygen, O2 , under environmental conditions containing bicarbonate (HCO3 - ), which exists up to millimolar concentrations. In the absence and presence of HCO3 - , the formation of reactive oxygen species (O2 ⋅- , H2 O2 , and HO⋅) in FeII oxidation by O2 has been suggested. In contrast, our study demonstrates for the first time the rapid generation of carbonate radical anions (CO3 ⋅- ) in the oxidation of FeII by O2 in the presence of bicarbonate, HCO3 - . The rate of the formation of CO3 ⋅- may be expressed as d[CO3 ⋅- ]/dt=[FeII [[O2 ][HCO3 - ]2 . The formation of reactive species was investigated using 1 H nuclear magnetic resonance (1 H NMR) and gas chromatographic techniques. The study presented herein provides new insights into the reaction mechanism of FeII oxidation by O2 in the presence of bicarbonate and highlights the importance of considering the formation of CO3 ⋅- in the geochemical cycling of iron and carbon.

DFT Study of the BH4 - Hydrolysis on Au(111) Surface.

The mechanism of the catalytic hydrolysis of BH4 - on Au(111) as studied by DFT is reported. The results are compared to the analogous process on Ag(111) that was recently reported. It is found that the borohydride species are adsorbed stronger on the Au0 -NP surface than on the Ag0 -NP surface. The electron affinity of the Au is larger than that of Ag. The results indicate that only two steps of hydrolysis are happening on the Au(111) surface and the reaction mechanism differs significantly from that on the Ag(111) surface. These remarkable results were experimentally verified. Upon hydrolysis, only three hydrogens of BH4 - are transferred to the Au surface, not all four, and H2 generation is enhanced in the presence of surface H atoms. Thus, it is proposed that the BH4 - hydrolysis and reduction mechanisms catalyzed by M0 -NPs depend considerably on the nature of the metal.

Visible Light-Induced Catalyst-Free Activation of Peroxydisulfate: Pollutant-Dependent Production of Reactive Species.

Activation of peroxydisulfate (PDS, S2O82-) via various catalysts to degrade pollutants in water has been extensively investigated. However, catalyst-free activation of PDS by visible light has been largely ignored. This paper reports effective visible light activation of PDS without any additional catalyst, leading to the degradation of a wide range of organic compounds of high environmental and human health concerns. Importantly, the formation of reactive species is distinctively different in the PDS visible light system with and without pollutants [e.g., atrazine (ATZ)]. In addition to SO4•- generated via S2O82- dissociation under visible light irradiation, O2•- and 1O2 are also produced in both systems. However, in the absence of ATZ, H2O2 and O2•- are key intermediates and precursors for 1O2, whereas in the presence of ATZ, a different pathway was followed to produce O2•- and 1O2. Both radical and nonradical processes contribute to the degradation of ATZ in the PDS visible light system. The active role of 1O2 in the degradation of ATZ besides SO4•- is manifested by the enhanced degradation of contaminants and electron paramagnetic resonance spectroscopy measurements in D2O.

The Role of Carbonate in Catalytic Oxidations.

CO2, HCO3-, and CO32- are present in all aqueous media at pH > 4 if no major effort is made to remove them. Usually the presence of CO2/HCO3-/CO32- 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• + CO32- → CO3•- + OH-. However, recent studies point out that the involvement of carbonates in oxidation processes is more fundamental. Thus, the presence of HCO3-/CO32- changes the mechanisms of Fenton and Fenton-like reactions to yield CO3•- directly even at very low HCO3-/CO32- 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 CO32- 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-/CO32- in desired oxidation processes.

The Role of Common Alcoholic Sacrificial Agents in Photocatalysis: Is It Always Trivial?

Photocatalytic hydrogen production is proposed as a sustainable energy source. Simultaneous reduction and oxidation of water is a complex multistep reaction with high overpotential. Photocatalytic processes involving semiconductors transfer electrons from the valence band to the conduction band. Sacrificial substrates that react with the photochemically formed holes in the valence band are often used to study the mechanism of H2 production, as they scavenge the holes and hinder charge carrier recombination (electron-hole pairs). Here, we show that the desired sacrificial agent is one forming a radical that is a fairly strong reducing agent, and whose oxidized form is not a good electron acceptor that might suppress the hydrogen evolution reaction (HER). In an acidic medium, methanol was found to fulfill both these requirements better than ethanol and propan-2-ol in the TiO2 -(M0 -NPs) (M=Au or Pt) system, whereas in an alkaline medium, the alcohols exhibit a reverse order of activity. Moreover, we report that CH2 (OH)2 is by far the most efficient sacrificial agent in a nontrivial mechanism in acidic media. Our study provides general guidelines for choosing an appropriate sacrificial substrate and helps to explain the variance in the performance of alcohol scavenger-based photocatalytic systems.

Calculating the adsorption energy of a charged adsorbent in a periodic metallic system - the case of BH4- hydrolysis on the Ag(111) surface.

The hydrolysis of borohydride on the Ag(111) surface is explored theoretically to obtain the in-depth reaction mechanism. Many heterogeneously catalyzed reactions like this involve the adsorption of charged species on metals. DFT calculations of charged systems, with periodic boundaries, face serious problems, concerning convergence and reliability of the results. To study the heterogeneously catalyzed reactions, a simple method to calculate the adsorption energy of charged systems in metallic periodic cells is proposed. In this method, a counter ion is placed at a non-interactive distance, in an aqueous medium, so that the calculated system is neutral. Bader analysis is used to validate that the calculated couple is charged correctly. Adsorption energies of F-, Cl-, Br-, OH-, BH4-, ClO4- and H- ions on the Ag(111) surface in an aqueous medium were determined using Na+ and K+ as counter ions, to evaluate the performance of this method. The adsorption of the divalent ions S2-, Se2- and SO42- on different surfaces was studied as well. Then this method was used to explore the hydrolysis of BH4- ions, which have a high theoretical hydrogen storage capacity, on the Ag(111) surface. The results point out that during the catalytic hydrolysis only one hydrogen atom from borohydride is transferred to the surface. In the first step one hydrogen atom from BH4- is transferred to the silver surface; this H atom reacts with a hydrogen atom that is released from an adsorbed water molecule; in addition, a hydrogen molecule is released in the second step (one atom from *BH4- and one from *H2O). Thus, the mechanisms of the catalyzed reductions by BH4- and the hydrogen evolution reactions must be reconsidered.

Redox Properties of CeIVDOTA in Carbonated Aqueous Solutions. A Radiolytic and an Electrochemical Study.

The redox chemistry of CeIIIDOTA in cage in carbonate solutions was studied using electrochemistry and radiolysis techniques (continuous radiolysis and pulse radiolysis). Spectroscopic measurements point out that the species present in the solutions at high bicarbonate concentrations are [CeIIIDOTA(CO3)]3- (or less plausible [CeIIIDOTA(HCO3)]2-) with the carbonate (bicarbonate) anion as the ninth ligand versus [CeIIIDOTA(H2O)]- present in the absence of bicarbonate. Electrochemical results show a relatively low increase in the thermodynamic stabilization of the redox couple CeIV/III in the presence of carbonate versus its aqueous analogue. [CeIVDOTA(CO3)]2- and [CeIVDOTA(H2O)], prepared electrochemically, decompose photolytically. However, kept in the dark, both are relatively long lived; [CeIVDOTA(H2O)], though, is orders of magnitude kinetically more stable (a considerably longer half-life). Thus, one concludes that the carbonate species have a different mechanism of decomposition depending also on the presence of dioxygen after its preparation (in deaerated/aerated solutions). The [CeIVDOTA(CO3)]2- species is produced radiolytically by oxidation of the trivalent species by CO3•- with a rate constant, measured using pulse radiolysis, of 3.3 × 105 M-1 s-1. This rate constant is at least 1 order of magnitude smaller than most of the rate constants so far reported for the reaction of CO3•- with transition metal/lanthanide (cerium)/actinide complexes. This result together with the bulkiness of the reactants might suggest an outer-sphere electron transfer rather than the inner-sphere one so far proposed. The lifetime of the tetravalent cerium species obtained radiolytically in the presence of carbonate is shorter than the electrochemical one, suggesting a different conformer involved.

Cobalt Carbonate as an Electrocatalyst for Water Oxidation.

CoII salts in the presence of HCO3 - /CO3 2- in aqueous solutions act as electrocatalysts for water oxidation. It comprises of several key steps: (i) A relatively small wave at Epa ≈0.71 V (vs. Ag/AgCl) owing to the CoIII/II redox couple. (ii) A second wave is observed at Epa ≈1.10 V with a considerably larger current. In which the CoIII undergoes oxidation to form a CoIV species. The large current is attributed to catalytic oxidation of HCO3 - /CO3 2- to HCO4 - . (iii) A process with very large currents at >1.2 V owing to the formation of CoV (CO3 )3 - , which oxidizes both water and HCO3 - /CO3 2- . These processes depend on [CoII ], [NaHCO3 ], and pH. Chronoamperometry at 1.3 V gives a green deposit. It acts as a heterogeneous catalyst for water oxidation. DFT calculations point out that Con (CO3 )3 n-6 , n=4, 5 are attainable at potentials similar to those experimentally observed.

New insights into HER catalysis: the effect of nano-silica support on catalysis by silver nanoparticles.

Supported metal catalysts have recently attracted considerable attention in the field of catalysis. The effect of surface chemical groups (SiO-/SiOH2+) on SiO2-Ag0-NPs along with the average negative charge induced by (CH3)2COH˙ radicals on the catalytic reduction of H2O/H3O+ towards the hydrogen evolution reaction (HER) is reported. The results indicate that similar effects are observed both above and below the point of zero charge (PZC) of silica. More importantly, it is shown that a high concentration of this catalyst does not necessarily contribute to boosting the hydrogen formation, but instead, the density of charge on its surface is a decisive factor. A mechanistic explanation of the observed effect is given.

On the Differences in the Mechanisms of Reduction of AuCl2- and Ag(H2O)2+ with BH4.

The mechanism of reduction of AuCl4-/AuCl3OH- by BH4- was analyzed by density functional theory (DFT). The results point out that Auatoms0 are not intermediates in the process. The derived mechanism differs considerably from that reported for the analogous process involving the reduction of Ag(H2O)2+ by BH4-. Thus, though both processes follow the Creighton procedure, the detailed mechanism differs significantly. For Au, the agglomeration starts with AuH2-, whereas for Ag, it starts with (H2O)AgH. Stopped-flow measurements support the complicated mechanism.

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 CuIII or a CuIV complex. Kinetic analysis of pulse radiolysis measurements coupled with DFT calculations point out that CuIII (CO3 )n3-2n complexes are the active intermediates in the electrolysis of CuII (CO3 )n2-2n solution. The results enable the evaluation of E°[(CuIII/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 CuIII (CO3 )n3-2n to yield hydroxyl radicals. Significant charge transfer from the coordinated carbonate to CuIII results in the formation of C2 O62- by means of a second-order reaction of CuIII (CO3 )n3-2n . 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.

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 chemical and physical properties of carbonate and carbonate anion radicals in aqueous solutions by DFT proves that one has to include an inner hydration sphere of six water molecules for both anions. Application of the SMD model to CO3(H2O)62- and CO3(H2O)6˙- enables achievement of the experimental value of the redox potential of the CO3(H2O)6˙-/2- couple. This is a result of the direct inclusion of a considerable charge transfer (CT) from CO32- to its inner hydration sphere in the calculation of the hydration effect. The HOMO of clusters is an analogue of the non-bonding σ-type a2'-HOMO of the parent CO3 moiety with a σ*(OH) contribution. This is a MO manifestation of the CT to the first hydration shell. The localization of the CT on the first hydration shell also re-produces the very strong OHO stretch peak. Furthermore, the very large difference in the hydration energies of CO32- and CO3˙- which causes the very large differences in the length of the C-OH-O hydrogen bonds suggests that the oxidations by CO3˙- proceed via the inner sphere mechanism.

The Chemical Properties of Hydrogen Atoms Adsorbed on M0 -Nanoparticles Suspended in Aqueous Solutions: The Case of Ag0 -NPs and Au0 -NPs Reduced by BD4.

The nature of H-atoms adsorbed on M0 -nanoparticles is of major importance in many catalyzed reduction processes. Using isotope labeling, we determined that hydrogen evolution from transient {(M0 -NP)-Hn }n- proceeds mainly via the Heyrovsky mechanism when n is large (i.e., the hydrogens behave as hydrides) but mainly via the Tafel mechanism when n is small (i.e., the hydrogens behave as atoms). Additionally, the relative contributions of the two mechanisms differ considerably for M=Au and Ag. The results are analogous to those recently reported for the M0 -NP-catalyzed de-halogenation processes.