CO Oxidation by Neutral Gold-Vanadium Oxide Clusters

Oxidation of CO by gas-phase atomic clusters is being actively studied to understand the molecular-level mechanisms of heterogeneous CO oxidation over related catalytic surfaces. However, it is experimentally challenging to study CO oxidation by neutral heteronuclear metal oxide clusters because of the difficulty of cluster ionization and detection without fragmentation. Herein, the neutral AuVO_2-4 clusters were experimentally generated and their reactions with CO and O₂ were studied. The experimental results showed that CO adsorption is the dominant channel on the interactions of AuVO₄ and AuVO₃ with CO, and AuVO₂ can pick up an O₂ molecule to generate AuVO₄. Theoretical studies indicated that the oxidation of the trapped CO in AuVO_3,4CO into CO₂ is exothermic while the reaction barriers have to be overcome at the elevated temperatures. A catalytic cycle for CO oxidation by AuVO_2-4 is proposed.     

Rate determination of the catalytic oxidation of methane and propane in an electrochemical solid-electrolyte reactor

The possibility of using a solid-electrolyte reactor in kinetic studies of the catalytic oxidations of hydrocarbon with molecular oxygen was investigated. A theoretical analysis of processes in a catalytically asymmetric gas-diffusion cell in N₂ + O₂ + CH₄ and N₂ + O₂ + C₃H₈ gas mixtures was performed. Analytical expressions are presented for calculating the oxygen, methane, and propane concentrations and the methane and propane oxidation rates in the inner space of the cell from the emf of the latter. The potentiometric response was studied experimentally after the addition of methane and propane in the gas mixture in a reactor with silver electrodes and samples with applied catalytic materials. The concentrations of the components in the inner space of the reactor and the oxidation rates of hydrocarbons were calculated from the experimental data.     

Noble‐Metal‐Free Single‐Atom Catalysts CuAl4O7–9− for CO Oxidation by O2

The single copper atom doped clusters CuAl₄O_7–9^? can catalyze CO oxidation by O₂. The CuAl₄O_7–9^? clusters are the first group of experimentally identified noble‐metal free single atom catalysts for such a prototypical reaction. The reactions were characterized by mass spectrometry and density functional theory calculations. The CuAl₄O₉CO^? is much more reactive than CuAl₄O₉^? in the reaction with CO to generate CO₂. One adsorbed CO is crucial to stabilize Cu of CuAl₄O₉^? around +I oxidation state and promote the oxidation of another CO. The widely emphasized correlation between the catalytic reactivity of CO oxidation and Cu oxidation state can be understood at the strictly molecular level. The remarkable difference between Cu catalysis and noble‐metal catalysis was discussed.     

Photocatalytic Oxidation of Carbon Monoxide Using Synergy of Redox-Separated Photocatalyst and Ozone

Separating the redox centers of photocatalysts is the most promising strategy to enhance photocatalytic oxidation efficiency. Herein, I investigate a site-selective loading of Pt on facet-engineered TiO₂ to achieve carbon monoxide (CO) oxidation at room temperature. Spatially loaded Pt on {101} facets of TiO₂ attracts photoinduced electrons efficiently. Thereby, oxygen dissociation is facilitated on the Pt surface, which is confirmed by enhanced oxidation of CO by 2.4 times compared to the benchmark of Pt/TiO₂. The remaining holes on TiO₂ can be utilized for the oxidation of various gaseous pollutants. Specifically, gaseous ozone, which is present in indoor and ambient air, is converted to a hydroxyl radical by reacting with the hole; thus, the poisoned Pt surface is continuously cleaned during the CO oxidation, as confirmed by in situ diffuse reflectance infrared transform spectroscopy. While randomly loaded Pt can act as recombination center, reducing photocatalytic activity, redox-separated photocatalyst enhances charge separation, boosting CO oxidation and catalyst regeneration via simultaneous ozone decomposition.     

Oxidation of hydrogen on palladium: Chemicurrents in the Schottky nanodiode

The oxidation of hydrogen on palladium was studied by the chemicurrents method using the nanosized catalytic Pd/n-Si Schottky diode. The chemicurrent was found to be generated when the reactions H₂+O₂ and H + O + H₂ + O₂ occurred on the palladium surface, occasionally in the auto-oscillation mode. A model was created that describes the complex kinetic behavior of the reaction. Mathematical modeling was performed and showed the possibility of complex auto-oscillations of chemicurrent similar to those obtained in experiments. The catalytic Schottky nanodiode method was shown to be effective for reaction visualization and can be used as a new physical method for investigating the chemical processes on the catalyst surface.     

Formation of endogenous MgO and MgAl2O4 particles and their possibility of acting as substrate for heterogeneous nucleation of aluminum grains

Aluminum containing 4 wt.% magnesium was oxidized at a temperature for different oxidation times and analyzed by high‐resolution electron microscopy. A thin oxidized layer of about 5 µm, which is composed of MgO, forms at short oxidation time and gradually increases. High‐resolution microstructures reveal that the oxidized layers are porous regardless of oxidation time. After extended oxidation time, discrete MgAl₂O₄ particles formed as a result of the reaction of initially formed MgO, liquid aluminum, and oxygen introduced from air through the porous MgO. Furthermore, it is clear by high‐resolution lattice images that MgAl₂O₄ particles are covered with thin Al₂O₃, whereas MgO is bonded intimately to aluminum. Therefore, MgAl₂O₄ particles that form naturally during oxidation are difficult to act as a direct substrate for nucleation of aluminum grains because of the coverage of Al₂O₃. In contrast, MgO shows the possibility of acting as a substrate for the aluminum nucleation. The formation mechanism of MgO and MgAl₂O₄ and their possibility of acting as substrates for nucleation of aluminum grains suggest that atomic level bonding and mismatches of nucleant/nucleus metal should be considered for correct evaluation of the possibility of heterogeneous nucleation of metallic matrix on a potent nucleant. © 2015 The Authors. Surface and Interface Analysis Published by John Wiley & Sons, Ltd.     

Enabling visible-light water photooxidation by coordinative incorporation of Co(II/III) cocatalytic sites into organic-inorganic hybrids: quantum chemical modeling and photoelectrochemical performance

Coordinative incorporation of Co(II/III) cocatalytic sites into organic–inorganic hybrids of TiO₂ and “polyheptazine” (PH, poly(aminoimino)heptazine, melon, or “graphitic carbon nitride”) has been investigated both by quantum chemical calculations and experimental techniques. Specifically, density-functional theory (DFT) calculations (PBE/def2-TZVPP) suggest that Co(II/III) and Zn(II) ions adsorb in nanocavities at the surface of the hybrid PH–TiO₂ cluster, a prediction which can be further confirmed experimentally by ¹⁵N nuclear magnetic resonance in the case of the Zn complex. The absorption spectra of the complexes were characterized by time-dependent DFT calculations, suggesting a change of color upon Co ion binding which can in fact be observed with the naked eye. Hybrid TiO₂–PH photoelectrodes were impregnated with Co(II) ions from aqueous cobalt nitrate solutions. Optical absorption data suggest that Co(II) ions are predominantly present as single ions coordinated within the nitrogen cavities of TiO₂–PH, and any undesired blocking of light absorption is negligible. The cobalt-induced cocatalytic sites can efficiently couple to the holes photogenerated by visible light in TiO₂–PH, leading to complete oxidation of water to dioxygen. Our results indicate that coordinative incorporation of metal ions into well-designed surface sites in the light absorber is sufficient to drive complex multielectron transformations in artificial photosynthetic systems.     

Kinetics and Mechanism of low-Temperature Oxidation of H2S with Oxygen in the gas Phase

The possibility of the formation of polysulfides during oxidation of H₂ S with oxygen on oxide catalysts has been checked, and the sequence of the reaction stages at temperatures below the sulfur dew point determined. The amount of polysulfides formed during H₂ S oxidation has been found to exceed significantly that obtained in the reaction of sulfur with H₂ S. Polysulfides are concluded to be intermediates in H₂ S oxidation to sulfur. The rate of formation of SO₂ from sulfur vapor is shown to be negligibly low at 100-200°C. A reaction scheme involving the formation of sulfur from polysulfides and the formation of sulfur dioxide by direct oxidation of H₂ S is suggested.     

Mechanism of the reaction of 1,1′-diethylferrocene and decamethylferrocene with peroxides in organic solvents

Kinetic relationships of oxidation of 1,1′-diethylferrocene and decamethylferrocene with peroxides ROOR (R = H, t-C₄H₉) in organic solvents were studied and the composition of oxidation products was established. It is shown that reactivity of (C₅H₄C₂H₅)₂Fe is significantly lower than that of Cp*₂Fe [Cp* = η⁵-C₅(CH₃)₅] which is seen from the ability of the first metal complex to undergo oxidation with a notable rate only in the presence of Brønsted acids, whereas the second substance is oxidized both in the presence and in the absence of these acids. Two possible mechanisms of oxidation of metal complexes in the presence of strong acids are discussed. One of them is based on the ability of ferrocene to fast and equilibrium protonation with the formation of Cp₂Fe⁺—H structure. Another one considers the ability of metal complexes to coordinate peroxides with the formation of weakly bound charge transfer complexes of the composition Cp₂Fe^δ+·ROOR^δ-. The possibility of their formation is confirmed by the oxidation of Cp₂Fe with hydroperoxides in the absence of acids.     

Quantum chemical analysis of the Eley-Rideal and Langmuir-Hinshelwood mechanisms of the catalytic oxidation of carbon monoxide on the nickel surface

The studies concerned with the oxidation of carbon monoxide on the nickel surface are reviewed. The Eley-Rideal (ER) collision and Langmuir-Hinshelwood (LH) adsorption mechanisms of the oxidation are analyzed. Calculations of the activation barriers of the oxidation of carbon monoxide on the Ni (111), (100), and (110) faces were performed for the first time and involved optimization of the reaction paths by the collision and adsorption mechanisms. It is shown that on the Ni (111) and (110) faces the ER collision mechanism of the reaction is preferable with the activation barriers ΔE _dis ^O ₂=62 kJ/mole and ΔE _trans ^O A21F5001₂x=25 kJ/mole for Ni (111) and ΔE _dis ^O ₂=72 kJ/mole and ΔE _trans ^O ₂=20 kJ/mole for Ni (110); on the Ni (100) face, the LH adsorption mechanism with the activation barriers ΔE _dis ^O ₂=75 kJ/mole and ΔE _trans ^O ₂=42 kJ/mole is favored. Analysis of the potential barriers for the catalytic oxidation of carbon monoxide on the Ni surfaces suggests the LH mechanism to be preferential, although insignificant differences in the activation barries can lead to the oscillatory reaction mechanism, which is confirmed experimentally. The calculations were performed by the LCAO MO SCF method in the MINDO/3 approximation. Kiev Polytechnical Institute. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 4, pp. 628–645, July–August, 1996. Translated by I. Izvekova     

On the Upper Limits of Oxidation States in Chemistry

The concept of oxidation state ( OS ) is based on the concept of Lewis electron pairs, in which the bonding electrons are assigned to the more electronegative element. This approach is useful for keeping track of the electrons, predicting chemical trends, and guiding syntheses. Experimental and quantum‐chemical results reveal a limit near +8 for the highest OS in stable neutral chemical substances under ambient conditions. OS =+9 was observed for the isolated [IrO₄]⁺ cation in vacuum. The prediction of OS =+10 for isolated [PtO₄]²⁺ cations is confirmed computationally for low temperatures only, but hasn't yet been experimentally verified. For high OS species, oxidation of the ligands, for example, of O^?2 with formation of ^.O^?1 and O?O bonds, and partial reduction of the metal center may be favorable, possibly leading to non‐Lewis type structures.     

Activation of H2O2 by methyltrioxorhenium(VII) inside β-cyclodextrin

The effect of β-cyclodextrin on the catalytic stability and reactivity of methylrhenium trioxide (MTO), CH₃ReO₃, which has been used for activation of hydrogen peroxide toward oxidation and epoxidation reactions, was studied using UV–Vis spectrophotometery. The stability and reactivity of the new catalytic system (MTO/β-CD) to activate H₂O₂ toward oxidation of indigo blue dye were investigated in basic media. Furthermore, effects of inclusion stoichiometry, temperature and concentrations of hydrogen peroxide on the stability and reactivity of the MTO/β-CD system were investigated. The formation of the inclusion complex between MTO and β-CD was confirmed experimentally using the changes in the UV–Vis absorption spectra. The results of this study demonstrate that the complexation process was better guaranteed when the amount of β-CD is higher than that of MTO, using a 1:2 molar ratio of MTO:β-CD enhances both the activity and stability of the catalyst. The results showed that the stability of the catalytic system was enhanced in presence of β-CD with maintaining good reactivity of the MTO even in the presence of high concentration of NaOH. The changes of thermodynamic activation parameters (ΔH^≠ and ΔS^≠) for the oxidation reaction of indigo with H₂O₂ catalyzed by MTO/β-CD system were determined on the basis of the experimental data.     

An in situ oxidation route to fabricate graphene nanoplate-metal oxide composites

We report our studies on an improved soft chemical route to directly fabricate graphene nanoplate-metal oxide (Ag₂O, Co₃O₄, Cu₂O and ZnO) composites from the in situ oxidation of graphene nanoplates. By virtue of H⁺ from hydrolysis of the metal nitrate aqueous solution and NO₃⁻, only a small amount of functional groups were introduced, acting as anchor sites and consequently forming the graphene nanoplate-metal oxide composites. The main advantages of this approach are that it does not require cumbersome oxidation of graphite in advance and no need to reduce the composites due to the lower oxidation degree. The microstructures of as-obtained metal oxides on graphene nanoplates can be dramatically controlled by changing the reaction parameters, opening up the possibility for processing the optical and electrochemical properties of the graphene-based nanocomposites.     

Electrochemical investigation of flavonolignans and study of their interactions with DNA in the presence of Cu(II)

Flavonolignans, silybin and its derivatives (2,3-dehydrosilybin, 7-O-methylsilybin, 20-O-methylsilybin) and isosilybin were studied using ex situ (adsorptive transfer, AdT) cyclic and square wave voltammetry (SWV). The two oxidation steps were described for flavonolignans at potentials E_p1 + 0.5 V and E_p2 + 0.85 V depending on experimental conditions. An additional oxidation peak at E_p3 + 0.35 V was observed only for 2,3-dehydrosilybin. The anodic currents of flavonolignans are related to their electron transfer processes (oxidation of hydroxyl groups), which was supported by density functional theory (DFT) and B3P86 theory level. Our electrochemical results confirmed that 2,3-dehydrosilybin is a relatively strong antioxidant, which is strictly associated with oxidation at E_p3. The oxidation processes and antioxidant parameters of flavonolignans can be affected by transition metal complexation via hydroxyl groups. We found that silybin and 2,3-dehydrosilybin are able to chelate transition metals, especially Cu²⁺. The formation of silybin/Cu complexes was studied by AdT SWV and the observation was also confirmed using fluorescence spectroscopy. The electrochemical investigation of DNA interactions and damage caused in the presence of silybin/Cu complex and hydrogen peroxide is described. We present evidence that flavonolignans are involved not only in antioxidant abilities but also in the prooxidation effects under in vitro conditions.     

Catalytic Water Splitting with an Iridium Carbene Complex: A Theoretical Study

Catalytic water oxidation at Ir (OH)⁺ ( Ir =IrCp*(Me₂NHC), where Cp*=pentamethylcyclopentadienyl and Me₂NHC=N,N′‐dimethylimidazolin‐2‐ylidene) can occur through various competing channels. A potential‐energy surface showing these various multichannel reaction pathways provides a picture of how their importance can be influenced by changes in the oxidant potential. In the most favourable calculated mechanism, water oxidation occurs via a pathway that includes four sequential oxidation steps, prior to formation of the O?O bond. The first three oxidation steps are exothermic upon treatment with cerium ammonium nitrate and lead to formation of Ir ^V(?O)(O ^. )⁺, which is calculated to be the most stabile species under these conditions, whereas the fourth oxidation step is the potential‐energy‐determining step. O?O bond formation takes place by coupling of the two oxo ligands along a direct pathway in the rate‐limiting step. Dissociation of dioxygen occurs in two sequential steps, regenerating the starting material Ir (OH)⁺. The calculated mechanism fits well with the experimentally observed rate law: v=k_obs[ Ir ][oxidant]. The calculated effective barrier of 24.6 kcal mol^?1 fits well with the observed turnover frequency of 0.88 s^?1. Under strongly oxidative conditions, O?O bond formation after four sequential oxidation steps is the preferred pathway, whereas under milder conditions O?O bond formation after three sequential oxidation steps becomes competitive.     

Electrochemical oxidation of phenol on boron-doped diamond electrode

The process of phenol oxidation on a boron-doped diamond electrode (BDD) is studied in acidic electrolytes under different conditions of generation of active oxygen forms (AOFs). The scheme of phenol oxidation known from the literature for other electrode materials is confirmed. Phenol is oxidized through a number of intermediates (benzoquinone, carboxylic acids) to carbon dioxide and water. Comparative analysis of phenol oxidation rate constants is performed as dependent on the electrolysis conditions: direct anodic oxidation, with oxygen bubbling, and addition of H₂O₂. A scheme is confirmed according to which active radicals (OH^·, HO₂^·, HO₂⁻) are formed on a BDD anode that can oxidize the substrate which leads to formation of organic radicals interacting with each other and forming condensation products. Processes with participation of free radicals (chain-radical mechanism) play an important role in electrochemical oxidation on BDD. Intermediates and polymeric substances (polyphenols, quinone structures, and resins) are formed. An excess of the oxidant (H₂O₂) promotes a more effective oxidation of organic radicals and accordingly inhibition of the condensation process.     

Reversible Redox Reaction Between Antiaromatic and Aromatic States of 32π‐Expanded Isophlorins

32π‐antiaromatic expanded isophlorins with a varying number of thiophene and furan rings adopt either planar, ring‐inverted, or twisted conformations depending on the number of furan rings in the macrocycle. However, they exhibit identical reactivity with respect to their oxidation to aromatic 30π‐dicationic species under acidic conditions. These 32π‐antiaromatic macrocycles can also be oxidized with [Et₃O⁺SbCl₆^?]and NOBF₄ to generate dications, thus confirming ring oxidation of macrocycles. Furthermore, they can be reduced back to their parent 32π‐antiaromatic state by triethylamine, Zn, or FeCl₂. Single‐crystal X‐ray diffraction analysis confirmed a figure‐eight conformation for a hexafuran system, which opens to a planar structure upon oxidation.     

In-situ active site formation in CO oxidation on alumina

For the development of automotive catalysts which may fit the condition of developing countries, catalytic activity of alumina for CO oxidation was studied. It was proposed that the carbon formed in-situ acted as an active site for CO oxidation. the carbon active site was also checked by methanol oxidation on alumina which showed temperature hysteresis during consecutive heating and cooling operations. Alkali-treated Alumina did not show any indication of the temperature hysteresis. The optimal temperature for maximum carbon depostion was confirmed by thermogravimetric analysis to be 450–500 C, which well explains the hysteresis. CeO₂−Al₂O₃ showed remarkably higher activities for complete oxidation. It seems that alumina has reasonably satisfactory activity in total clean-up of exhaust gas.     

Super-Oxidizing Covalent Triazine Framework Electrocatalyst for Two-Electron Water Oxidation to H2O2

Electrochemical two-electron water oxidation (2e WOR) is gaining surging research traction for sustainable hydrogen peroxide production. However, the strong oxidizing environment and thermodynamically competitive side-reaction (4e WOR) posit as thresholds for the 2e WOR. We herein report a custom-crafted covalent triazine network possessing strong oxidizing properties as a proof-of-concept metal-free functional organic network electrocatalyst for catalyzing 2e WOR. As the first-of-its-kind, the material shows a maximum of 89.9 % Faradaic Efficiency and 1428 μmol/h/cm² H₂O₂ production rate at 3.0 V bias potential (vs reversible hydrogen electrode, RHE), which are either better or comparable to the state-of-the-art electrocatalysts. We have experimentally confirmed a stepwise 2e WOR mechanism which was further computationally endorsed by density functional theory studies.     

Experimental study of intermediates and wave phenomena in co oxidation on platinum metal (Pt,Pd) surfaces

The mechanism of catalytic CO oxidation on Pt(100) and Pd(110) single-crystal surfaces and on Pt and Pd sharp tip (～10³ Å) surfaces has been studied experimentally by temperature-programmed reaction, temperature desorption spectroscopy, field electron microscopy, and molecular beam techniques. Using the density functional theory the equilibrium states and stretching vibrations of oxygen atoms adsorbed on the Pt(100) surface have been calculated. The character of the mixed adsorption layer was established by high resolution electron energy loss spectroscopy—molecular adsorption (O_2ads, CO_ads) on Pt(100)-hex and dissociative adsorption (O_ads, CO_ads) on Pt(100)-(1×1). The origin of kinetic self-oscillations for the isothermal oxidation of CO in situ was studied in detail on the Pt and Pd tips by field electron microscopy. The initiating role of the reversible phase transition (hex) ? (1 × 1) of the Pt(100) nanoplane in the generation of regular chemical waves was established. The origination of self-oscillations and waves on the Pt(100) nanoplane was shown to be caused by the spontaneous periodical transition of the metal from the low-active state (hex) to the highly active catalytic state (1 × 1). A relationship between the reactivity of oxygen atoms (O_ads) and the concentration of CO_ads molecules was revealed for the Pd(110) surface. Studies using the isotope label ¹⁸O_ads demonstrated that the low-temperature formation of CO₂ at 150 K is a result of the reaction of CO with the highly reactive state of atomic oxygen (O_ads). The possibility of the low-temperature oxidation of CO via interaction with the so-called “hot” oxygen atoms (O_hot) appearing on the surface at the instant of dissociation of O_2ads molecules was studied by the molecular beam techniques.