Redox Catalytic Processes Research Articles

Identifying Metal-Oxo/Peroxo Intermediates in Catalytic Water Oxidation by In Situ Electrochemical Mass Spectrometry.

Molecular catalysis of water oxidation has been intensively investigated, but its mechanism is still not yet fully understood. This study aims at capturing and identifying key short-lived intermediates directly during the water oxidation catalyzed by a cobalt-tetraamido macrocyclic ligand complex using a newly developed an in situ electrochemical mass spectrometry (EC-MS) method. Two key ligand-centered-oxidation intermediates, [(L2-)CoIIIOH] and [(L2-)CoIIIOOH], were directly observed for the first time, and further confirmed by 18O-labeling and collision-induced dissociation studies. These experimental results further confirmed the rationality of the water nucleophilic attack mechanism for the single-site water oxidation catalysis. This work also demonstrated that such an in situ EC-MS method is a promising analytical tool for redox catalytic processes, not only limited to water oxidation.

Precursor engineering of hydrotalcite-derived redox sorbents for reversible and stable thermochemical oxygen storage

Chemical looping processes based on multiple-step reduction and oxidation of metal oxides hold great promise for a variety of energy applications, such as CO2 capture and conversion, gas separation, energy storage, and redox catalytic processes. Copper-based mixed oxides are one of the most promising candidate materials with a high oxygen storage capacity. However, the structural deterioration and sintering at high temperatures is one key scientific challenge. Herein, we report a precursor engineering approach to prepare durable copper-based redox sorbents for use in thermochemical looping processes for combustion and gas purification. Calcination of the CuMgAl hydrotalcite precursors formed mixed metal oxides consisting of CuO nanoparticles dispersed in the Mg-Al oxide support which inhibited the formation of copper aluminates during redox cycling. The copper-based redox sorbents demonstrated enhanced reaction rates, stable O2 storage capacity over 500 redox cycles at 900 °C, and efficient gas purification over a broad temperature range. We expect that our materials design strategy has broad implications on synthesis and engineering of mixed metal oxides for a range of thermochemical processes and redox catalytic applications.

Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis.

Homogeneous electrochemical catalysis of N2O reduction to N2 is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes. An activation-driving force correlation is revealed with the organic species characteristic of a redox catalysis involving an outer-sphere electron transfer from the radical anions or dianions of the reduced catalyst to N2O. Taking into account the previously estimated reorganization energy required to form the N2O radical anions leads to an estimation of the N2O/N2O˙− standard potential in acetonitrile electrolyte. The direct reduction of N2O at a glassy carbon electrode follows the same quadratic activation driving force relationship. Our analysis reveals that the catalytic effect of the mediators is due to a smaller reorganization energy of the homogeneous electron transfer than that of the heterogeneous one. The physical effect of “spreading” electrons in the electrolyte is shown to be unfavorable for the homogeneous reduction. Importantly, we show that the reduction of N2O by low valent rhenium and manganese bipyridyl carbonyl complexes is of a chemical nature, with an initial one-electron reduction process associated with a chemical reaction more efficient than the simple outer-sphere electron transfer process. This points to an inner-sphere mechanism possibly involving partial charge transfer from the low valent metal to the binding N2O and emphasizes the differences between chemical and redox catalytic processes.

Recent developments and prospectives of acid-base and redox catalytic processes by metal oxides

This review paper deals mainly with acid-base, oxidation catalytic reactions, but also more general catalytic properties of metal oxides and their recent developments in the field of heterogeneous catalysis, a field largely covered by Prof. Guido Busca, during his scientific career. It includes a description of the main metal oxide catalysts used for acid-base (various reactions), partial and total oxidations, and other reactions, and of the main industrial processes using or expected to use them. Some case studies have been chosen as examples of recent progresses in metal oxides syntheses leading to new 2D, 3 D materials and these reactions. Particular attention is borne on recent and future researches and perspectives, mainly monitored by actual society regulations related to environmental issues, uses of biomass derivatives, carbon neutral processes and sustainability.

Novel route of synthesis of ultra-small Au nanoparticles on SiO2 supports

A novel route to prepare monodispersed 1–2 nm gold nanoparticles (NPs), based on the use of extremely small SiO2 NPs (2–4 nm) as a support and increasing their metal-support interaction with surface modifier oxides is presented. The influence of modifier (La, Ce and Fe oxides) and modification method (impregnation (i) or direct synthesis (s)) on the formation of ultra-small Au NPs and their structural and electronic properties was studied. The samples were characterized by N2 adsorption (BET), FTIR of adsorbed CO, XRD and HR-TEM methods, and tested for the catalytic selective oxidation of 1-octanol. Preparation of monodispersed Au NPs with 1 nm diameter was successfully achieved for all the modified samples studied, with exception of Au/Ce/SiO2-i, where CeO2 was not homogeneously distributed. The Au NPs have high degree of monodispersity and are stable when treated in H2 up to 300 °C. Formation of these Au NPs depended on the strong interactions between the cationic gold complex precursor and the surface of modified SiO2 NPs. Modifiers changed electronic properties of supported gold; favoring the formation and stabilization of Auδ+ states, which are probable gold active sites of selective liquid-phase oxidation of alcohols in redox catalytic processes. 1-octanol oxidation was used as a model reaction for oxidation of fatty alcohols obtained during biomass transformation. The best performance for 1-octanol oxidation was found for gold nanoparticles supported on the ultra-small SiO2 modified cerium oxide by impregnation method. The relative order of activity was: AuCeSiO2-i > AuFeSiO2-i ≫ AuLaSiO2-i ≈ AuLaSiO2-s > AuSiO2 > AuFeSiO2-s≫ AuCeSiO2-s. The obtained results open the possibility of further development of high-performance catalysts for conversion of secondary products of biomass processing into valuable chemicals.

Recovery of high purity elemental sulfur from coal syngas by liquid redox catalytic process using tannin extracts

Liquid redox sulfur recovery (LRSR) processes using tannin extracts and vanadium pentoxide as dual redox catalysts are widely employed for removal of hydrogen sulfide and recovery of elemental sulfur from coal syngas in China. Difficulties in melting sulfur pastes, poor quality of sulfur and the running stability are the major concern of the process. In this study, tannin extracts obtained from valonia and black wattle and an industrial benchmark tannin extract designated as the LRSR's catalyst were investigated for their composition and the kinetics of sulfide oxidation. Industry samples of sulfur pastes from the LRSR processes were analyzed. The results demonstrate that the accumulation of impurities of tannin extracts along with byproducts of sulfide oxidation in scrubbing liquid prevents sulfur froth separation and affects the purity of sulfur. The pretreatment of sulfur pastes with nitric acid reduces the accumulation of the impurities and improves the purity of sulfur from 60% to 99%, leading to cleaner production of sulfur by the LRSR process.

Merging Visible Light Photoredox Catalysis with Metal Catalyzed C-H Activations: On the Role of Oxygen and Superoxide Ions as Oxidants.

ConspectusThe development of efficient catalytic systems for direct aromatic C–H bond functionalization is a long-desired goal of chemists, because these protocols provide environmental friendly and waste-reducing alternatives to classical methodologies for C–C and C–heteroatom bond formation. A key challenge for these transformations is the reoxidation of the in situ generated metal hydride or low-valent metal complexes of the primary catalytic bond forming cycle. To complete the catalytic cycle and to regenerate the C–H activation catalyst, (super)stoichiometric amounts of Cu(II) or Ag(I) salts have often been applied. Recently, “greener” approaches have been developed by applying molecular oxygen in combination with Cu(II) salts, internal oxidants that are cleaved during the reaction, or solvents or additives enabling the metal hydride reoxidation. All these approaches improved the environmental friendliness but have not overcome the obstacles associated with the overall limited functional group and substrate tolerance. Hence, catalytic processes that do not feature the unfavorable aspects described above and provide products in a streamlined as well as economically and ecologically advantageous manner would be desirable.In this context, we decided to examine visible light photoredox catalysis as a new alternative to conventionally applied regeneration/oxidation procedures. This Account summarizes our recent advances in this expanding area and will highlight the new concept of merging distinct redox catalytic processes for C–H functionalizations through the application of visible light photoredox catalysis. Photoredox catalysis can be considered as catalytic electron-donating or -accepting processes, making use of visible-light absorbing homogeneous and heterogeneous metal-based catalysts, as well as organic dye sensitizers or polymers. As a consequence, photoredox catalysis is, in principle, an ideal tool for the recycling of any given metal catalyst via a coupled electron transfer (ET) process.Here we describe our first successful endeavors to address the above challenges by combining visible light photoredox catalysis with different ruthenium, rhodium, or palladium catalyzed C–H activations. Since only small amounts of the oxidant are generated and are immediately consumed in these transformations, side reactions of substrates or products can be avoided. Thus, usually oxidant-sensible substrates can be used, which makes these methods highly suitable for complex molecular structure syntheses. Moreover, mechanistic studies shed light on new reaction pathways, intermediates, and in situ generated species. The successful development of our dual catalysis concept, consisting of combined visible light photoredox catalysis and metal catalyzed C–H functionalization, provides many new opportunities for further explorations in the field of C–H functionalization.

Redox Flow Lithium-Oxygen Battery with Unprecedently Low Charging Overpotentials

Lithium oxygen battery (LOB), as a promising next generation energy storage system, has attracted tremendous attention. However, for aprotic systems due to the deposition of insoluble and insulating discharging product Li2O2, surface passivation and pore clogging of the cathode occur. As a result, the charging overpotential of LOB is generally intolerably high. While people have tried to use homogeneous catalysts — redox mediators dissolved in the electrolyte to address the above issues, the passivation and clogging problems remain especially at deep discharge. Inspired by the redox targeting concept in redox flow lithium battery, we have recently proposed a new battery concept — redox flow lithium oxygen battery (RFLOB), which elegantly resolves the surface passivation and pore clogging issues.1-3 As shown in Figure 1, with a pair of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) redox catalysts, the formation of Li2O2 during discharge and oxidation during charge could be performed in a separate gas diffusion tank rather than on the cathode in the cell. Therefore, the passivation and clogging problems of the electrode are feasibly obviated.4 Here, we will report a series of robust redox mediators as efficient ORR and OER catalysts in the RFLOB system. By optimizing the electrolyte compositions, extremely low charging overpotential (~0.20 V) has been achieved, which results of unprecedently high voltage efficiency. The underlying mechanism of the redox catalytic processes toward ORR and OER will be discussed in detail. We anticipate RFLOB provide a new vista to the development of Li-air battery with high round-trip efficiency for practical applications. Figure 1. The structure of a redox flow lithium oxygen battery. Reference: (1) Wang, Q.; Zakeeruddin, S. M.; Wang, D.; Exnar, I.; Grätzel, M. Angewandte Chemie International Edition 2006, 45, 8197. (2) Huang, Q.; Li, H.; Gratzel, M.; Wang, Q. Physical Chemistry Chemical Physics 2013, 15, 1793. (3) Pan, F.; Yang, J.; Huang, Q.; Wang, X.; Huang, H.; Wang, Q. Advanced Energy Materials 2014, 4, n/a. (4) Zhu, Y. G.; Jia, C.; Yang, J.; Pan, F.; Huang, Q.; Wang, Q. Chemical Communications 2015. Figure 1

ChemInform Abstract: Progress on Layered Hydrotalcite (HT) Materials as Potential Support and Catalytic Materials

AbstractReview: 193 refs.

FTIR and density functional study of NO interaction with reduced ceria: Identification of N3− and NO2− as new intermediates in NO conversion

To design new deNOx materials and processes we need to know in details the mechanism of NO interaction with solid surfaces. Many NOx conversion catalysts contain ceria as a component and cerium changes its oxidation state in the course of redox catalytic processes. Here we show that the interaction between NO and reduced ceria leads to formation of azides (N3−, 2044–2042cm−1) and nitric oxide dianion (NO2−, 1010–980cm−1) with the simultaneous oxidation of Ce3+. These species are established for the first time after NO adsorption on solid surfaces and their relative concentrations strongly depend on the sample morphology. At ambient temperature N3− does not interact with NO or O2 alone but easily disappears in the co-presence of the two gases, thus demonstrating reactivity similar to that of isocyanates. In contrast, NO2− accepts one NO molecule and is converted into hyponitrite (bands in the 1030–970cm−1 region). At further stages of interaction, N2O, N2, nitrites and nitrates are formed. The present findings enrich the current opinions of catalytic NO conversion and impose some revisions.

Direct comparison of electrochemical and spectrochemical kinetics for catalytic oxygen reduction.

We describe here a direct comparison of electrochemical and spectrochemical experiments to determine rates and selectivity of oxygen reduction catalyzed by iron 5,10,15,20-meso-tetraphenylporphyrin chloride. Good agreement was found between the two methods, suggesting the same mechanism is occurring under both conditions, with the same third-order rate law, similar selectivity, and the derived rate constants agreeing within a factor of at most 4, with k(cat) ≅ 2 × 10(6) M(-2) s(-1). This Communication provides a rare example of a redox catalytic process characterized by two common but very different methods.

Composite materials {glassy carbon/graphene/zero-valent metal (Pd0)} for building three-dimensional redox electrodes

When glassy carbon (GC) is mechanically covered with micrometric layers of graphene, the reduction of primary alkyl halides (RX) can shift to the catalytic mode due to the fact that cathodically charged graphene may act as a redox reducing agent triggering a classical redox catalytic process with a moderate gain (<0.2V) in potential. Alternatively, the injection of very small amounts of transition metals (not only Pd, but also Au, Ag or Pt) at zero-valent state into the graphene layer may induce catalytic reduction of RIs and RBrs that allows the formation of free radicals R and their concomitant grafting, not only at the GC/graphene interface but also in bulk graphene that plays the role of a three-dimensional radical scavenger. Therefore, by use of this simple process, GC/graphene layers could be converted into a 3D active interface. In order to quantify the global grafting/insertion of R, ferrocene and aryl probes (Fc-n-alkyl-I and pyrene-CH2-Br) were used, attesting to significant modification of the interface. Thus, for the first time, a bulky graphene–ferrocene layer (a redox system fully usable as a 3D modified electrode) was built. This model could quite hopefully be envisaged for electrochemically incorporating many other redox centers in graphene within the potential range of −2V to +1V vs. Ag/AgCl.

Surface modification imparts selectivity, facilitating redox catalytic studies: quinone mediated oxygen reduction

Modifying a gold electrode surface with hydroxy-alkyl-thiols significantly reduces the observed rate of electron transfer. This designed and controlled decrease in electron transfer rate readily allows reversible and irreversible electrode processes to be more clearly delineated. Separation of such voltammetric responses can facilitate the direct study of redox catalytic processes, which would, under other experimental conditions be obscured. The reduction of oxygen by anthraquinone hydroxy derivatives are used as a paradigmatic example, demonstrating the importance of the hydroxyl groups in the reduction process. Specifically, the reduced form of 1,8-dihydroxy-anthraquinone is shown to be significantly more reactive towards oxygen reduction than the 1,4-dihydroxyl analogue.

A homogeneous redox catalytic process for the paired synthesis of l-cysteine and l-cysteic acid from l-cystine

Redox catalytic process involved in the paired electrosynthesis of l-cysteine and l-cysteic acid from l-cystine is investigated by cyclic voltammetric technique and also confirmed by preparative electrolysis. The cyclic voltammetric behaviour shows that in the catholyte, in situ deposited tin (Sn) surface acts as a redox catalyst for the electro-reduction of l-cystine to l-cysteine whereas in the anolyte, the electro-generated bromine acts as a homogeneous redox mediator to enhance the electro-oxidation of l-cystine. l-Cysteine hydrochloride monohydrate (l-cysteine) and l-cysteic acid are prepared from l-cystine by preparative electrolysis with high purity and high yield using graphite cathode and DSA anode. At optimum concentration of l-cystine with 1:1 concentration ratio (catholyte:anolyte), the material yield obtained for l-cysteine is above 80% and that for l-cysteic acid is close to 60% in the paired electrosynthesis process in the batch operation. Scope for further experiments in conversion efficiency is also discussed.

Cleavage of aryl iodides at carbon cathodes in organic carbonates. Surfaces doped by transition metals at the nanometric scale: Electro or redox catalysis?

The cathodic reduction of aryl iodides ArIs at carbon electrodes was achieved in cyclic carbonates (e.g., propylene carbonate PC). The novelty of the present method simply lies in the ability of title compounds to create aromatic carbanions that may act as nucleophiles towards carbonates. Consequently, several aromatic esters ArC(O)OR are generated as side products that play the role of redox mediators for indirectly cleaving C–X bond at solid inert electrodes. This procedure surprisingly allows one to obtain, through this self-induced redox catalysis, Ar–Ar dimers sometimes in high yield. Meanwhile, the triggering of such processes could be boosted by doping inert electrodes by electro-deposition of metals such as palladium, nickel, or silver. Extremely thin layers of metals are well shown to be very efficient (even at average thickness < 0.1 nm) to induce catalytic steps presenting quite large potential shifts compared to bare carbon electrodes when employed in inert solvents. The probable concomitance of electrocatalytic and redox catalytic processes is discussed and argued.

Variety of available coordination sites for extra-framework iron in LTA and MFI zeolites

Variations in coordination states of extra-framework iron are studied in low iron content ferrisilicates (Si/57Fe ≈ 200) during various in situ treatments. In Fe-LTA complete Fe3+ ↔ Fe2 + reversibility is observed. In Fe-MFI extra-framework iron can be stabilized in Fe2 + state in spite of ambient oxidizing conditions (N2O, 620 K). Further, in Fe-MFI simultaneous stabilization of Fe2 + and Fe3 + may take place providing centres for redox catalytic processes.

Radiolytic Hydrogen Yields in Aqueous Suspensions of Gold Particles

The effect of high concentrations of large gold particles, in the hundreds of nanometer size regime, on the yields of molecular hydrogen, G(H(2)), produced in the radiolysis of several aqueous solutions was determined. In particular we look for direct effect of radiation absorbed by the solid particles on the yield of water products. These particles, however, are catalytically active in the conversion of reducing radicals to molecular hydrogen as well. A very small increase in G(H(2)) observed in bromide solutions upon addition of 50 wt % of gold particles indicates that the radiolysis of the solid particles does not affect the yields in the aqueous phase. Very little exchange of charge carriers or energy between the two phases occurs in these large particle suspensions. On the other hand, efficient catalytic conversion of (CH(3))(2)C(*)OH radicals to H(2) is shown to occur. The efficiency of the presently studied suspensions in the redox-catalytic process is similar to that of suspensions of small particles of similar total surface area. In the presence of radicals from hydrogen atom abstraction from tert-butyl alcohol the yield decreases significantly, again similar to the behavior in suspensions of small particles. We conclude that the redox catalysis does not depend on the size of the particles when their size exceeds a few nanometers.

Water-soluble mono- and star-shaped hexanuclear functional organo-iron catalysts for nitrate and nitrite reduction in water: syntheses and electroanalytical study

Ten functional sandwich complexes [Fe II(η 5-C 5H 4CO 2H)(η 6-(arene)][PF 6] and [Fe +(η 5-C 5H 5)(η 6-(C 6(CH 2CH 2C 6H 4OH) 6)][PF 6] have been synthesized to provide water-soluble redox catalysts with a range of redox potentials for the Fe II–Fe I redox couple. The CpFe + induced CH benzylic activation and the carboxylation of hexamethylbenzene, the CpFe + induced one-pot polymethylation of mesitylene and hexamethylbenzene, and the one-pot CpFe + induced hexa- p-methoxybenzylation of hexamethylbenzene have led to the desired functionalized complexes. These compounds catalyze the cathodic reduction of nitrate and nitrite in a basic aqueous medium on a Hg cathode. The rates of the rate-limiting steps of the redox-catalytic processes involving the homogeneous reduction of nitrate and nitrite by the ‘19-electron’ states of the redox catalysts have been measured using enhancement of the intensity of the monoelectronic cathodic wave Fe II→Fe I in the presence of the substrate by cyclic voltammetry, polarography and chronoamperometry. For six iron sandwich complexes without excessive bulk around the iron center, a linear Marcus-type correlation between the kinetics and the thermodynamics was found by plotting the logarithm of this rate constant versus the redox potential, which indicates that the homogeneous electron-transfer from the 19-electron catalyst to the substrate is rate-limiting. With four complexes in which chains of the arene ligand introduced more bulk around the iron center, the rates were found to be an order of magnitude lower, showing the crucial effect of the distance on the outer-sphere electron transfer. A small inner-sphere contribution cannot be discarded, however. When the redox catalysts are attached to the termini of the branches of hexa-branch star-shaped cores, the rate constants are close to those of unencumbered catalysts, i.e. no significant loss of activity is observed.

CO Oxidation and NO Reduction on Perovskite Oxides

Abstract The search for substitutes of noble metal supported exhaust control catalysts has led to the study of a number of perovskite oxides (1). The mechanisms of redox processes on these surfaces are topotactic in nature, thus accounting forthe reversible loss and uptake of bulk oxygen or for the concomitant creation and annihilation of vacancies rendering these systems as attractive oxidation catalysts. The variable oxygen stoichiometry of perovskite structure is responsible for the exotic behavior of these materials as a new class of high Tc superconducting oxides (2). The extreme ease of removing oxygen from the framework but still retaining the basic ingredients of the perovskite structure and the possibility of deriving different structural frameworks from ideal perovskite structure (e.g. insertion of rock salt layer between perovskite layers results in a K2NiF4 structure, while insertion of double [Bi2O2] layers between perovskite layers results in an Aurivillius phase) and the possibility of substitution of either or both A and B site cations by foreign metal ions make them function as chemical chameleons (3) with a wide variety of solid state and catalytic properties. Among the various redox catalytic processes promoted by them the oxidation of CO and the reduction of NO seem to be interesting in view of their relevance for auto-exhaust emission control. The present chapter attempts to consider the generalities that have been evolved in these studies, which will be useful for the formulation of auto-exhaust control catalysts based on perovskites in the future.

Homogeneous redox catalysis of multielectron electrochemical reactions: Part II. Competition between homogeneous electron transfer and addition on the catalyst

Homogeneous redox catalytic processes in which catalysis competes with partial destruction of the catalyst are investigated. The kinetics are shown to depend upon three parameters: the excess factor (concentration of substrate over concentration of the catalyst), a dimensionless kinetic parameter representing the rate of the initial homogeneous electron transfer step and a dimensionless parameter representing the competition between the second electron transfer and the addition on the catalyst. Procedures are described that allow the rate constant of the initial electron transfer step and the ratio of the rate constants of the second electron transfer vs. the addition step to be extracted from the experimental data. The reduction of n-butyl chloride and bromide, mediated by aromatic anion radicals, is taken as an example illustrating the application of the proposed procedures to experimental systems.