Hot Electrons at Solid–Liquid Interfaces: A Large Chemoelectric Effect during the Catalytic Decomposition of Hydrogen Peroxide

The study of energy and charge transfer during chemical reactions on metals is of great importance for understanding the phenomena involved in heterogeneous catalysis. Despite extensive studies, very little is known about the nature of hot electrons generated at solid–liquid interfaces. Herein, we report remarkable results showing the detection of hot electrons as a chemicurrent generated at the solid–liquid interface during decomposition of hydrogen peroxide (H₂O₂) catalyzed on Schottky nanodiodes. The chemicurrent reflects the activity of the catalytic reaction and the state of the catalyst in real time. We show that the chemicurrent yield can reach values up to 10^?1 electrons/O₂ molecule, which is notably higher than that for solid–gas reactions on similar nanodiodes.     

Effects of precipitants and surfactants on catalytic activity of Pd/CeO<Subscript>2</Subscript> for CO oxidation

A series of precipitants and commercial surfactants (soft templates) were employed to synthesize mesoporous/nano CeO₂ by a hydrothermal method. As-prepared CeO₂ was impregnated with palladium and employed for low-temperature catalytic oxidation of CO. It was found that both soft templates and precipitants had significant effects on the morphology, particle size, crystallinity, and porous structure of the CeO₂, having a significant effect on the surface palladium abundance, molar ratios of surface species, and catalytic activity of the final impregnated Pd/CeO₂. Using ammonia as precipitant could facilitate increased surface palladium abundance and surface molar ratios of PdO/Pd _SMSI , Ce³⁺/(Ce³⁺ + Ce⁴⁺), and O_surface/O_lattice. The catalytic activity of the final Pd/CeO₂ catalysts could be enhanced as well. The optimal P123-assisted ammonia-precipitated Pd/CeO₂ catalyst exhibited over 99% catalytic conversion of CO at 50 °C.     

Three-way catalytic converter reactions aspects at near-ambient temperatures on modified Pd-surfaces

The dissolution of oxygen in palladium plays an important role in palladium catalysis. The present study shows that the surface modification (SM) due to the dissolution of atomic oxygen into the subsurfaces of palladium can be used as a control to tune its catalytic activity. CO oxidation and NO + H₂ + O₂ reaction was separately carried out on metallic Pd and on surface modified Pd using a molecular beam instrument and the results were compared. The metallic Pd does not show activity below 400 K for both reactions, whereas the SM-Pd shows activity at near-ambient temperatures. The electronic change due to SM was investigated using ambient pressure photoelectron spectroscopy, and the investigation clearly shows the effect of subsurface oxygen in the ambient temperature activity of palladium.     

The mechanism of selective NO<Subscript>x</Subscript> reduction by hydrocarbons in excess oxygen on oxide catalysts: VI. Spectroscopic and kinetic characteristics of surface complexes on a Ni-Cr oxide catalyst

The reaction mechanism of the selective catalytic reduction of NO_x by propane in the presence of O₂ on a commercial Ni-Cr oxide catalyst was studied using in situ IR spectroscopy. It was found that nitrite, nitrate, and acetate surface complexes occurred under reaction conditions. Considerable amounts of hydrogen were formed in the interaction of NO + C₃H₈ + O₂ or C₃H₈ + O₂ reaction mixtures with the catalyst surface. The rates of conversion of the surface complexes detected under reaction conditions were measured. The resulting values were compared to the rate of the process. It was found that, at temperatures lower than 200°C, nitrate complexes reacted with the hydrocarbon to form acetate complexes; in this case, the formation of reaction products was not observed. In the temperature region above 250°C, two reaction paths took place. One of them consisted in the interaction of acetate and nitrate complexes with the formation of reaction products. The decomposition of NO on the reduced surface occurred in the second reaction path. Nitrogen atoms underwent recombination, and oxygen atoms reoxidized the catalyst surface and reacted with the activated hydrocarbon to form CO₂ and H₂O in a gas phase.     

Synthesis of magnetically recyclable Fe3O4@[(EtO)3Si–L1H]/Pd(II) nanocatalyst and application in Suzuki and Heck coupling reactions

A Pd(II) Schiff base complex as an efficient and highly heterogeneous catalyst was developed by immobilization of a palladium complex on the surface of modified Fe₃O₄ magnetite nanoparticles. These surface‐modified nanoparticles were characterized using various techniques such as transmission electron microscopy, X‐ray diffraction, thermogravimetric analysis, vibrating sample magnetometry, elemental analysis and Fourier transform infrared spectroscopy. The palladium catalyst exhibited efficient catalytic activity in Suzuki and Heck coupling reactions. This method has notable advantages such as excellent chemoselectivity, mild reaction conditions, short reaction times and excellent yields. The yields of the products were in the range 85–100%. Also, the nanocatalyst can be easily recovered with a permanent magnet and reused at least five times without noticeable leaching or loss of its catalytic activity. Copyright © 2016 John Wiley & Sons, Ltd.     

Catalytic Active Site for NO Decomposition Elucidated by Surface Science and Real Catalyst

Comprehensive studies combining surface science and real catalyst were performed to get further insight into catalytic active site and reaction mechanism for NO decomposition over supported palladium and cobalt oxide-based catalysts. On palladium single-crystal model catalysts, adsorption, dissociation and desorption behavior of NO was found to be closely related to the surface structures, the stepped surface palladium being active for dissociation of NO. In accordance with this result, the activity of powder Pd/Al₂O₃ catalysts for NO decomposition was directly related to the number of step sites exposed on the surface, suggesting that the step sites act as the catalytic active site for NO decomposition on Pd/Al₂O₃. NO decomposition over cobalt oxide was found to be significantly promoted by addition of alkali metals. Surface science study and catalyst characterization led to the same conclusion that the interface between the alkali metal and Co₃O₄ serves as the catalytic active site. From the results of in situ Fourier transform infrared (FT-IR) spectroscopy and isotopic transient kinetic analysis, a reaction mechanism was proposed in which the reaction is initiated by NO adsorption onto alkali metals to form NO₂⁻ species and then NO₂⁻ species react with the adsorbed NO species to form N₂ over the interface between the alkali metal and Co₃O₄.     

Immobilization of transition metal ion complexes and their catalytic activity towards the activation of hydrogen peroxide

Transition metal ion-imidazole complexes have been immobilized on silica, silica–alumina (25%Al₂O₃), and alumina supports by adsorption and functionalization methods. The catalytic activity of these supported complexes in the decomposition of H₂O₂ has been studied. The reaction exhibits first-order kinetics with respect to [H₂O₂] and the quantity of catalyst. The rate of reaction decreases as [H₂O₂]₀ increases. The order of catalytic reactivity is strongly dependent on the type of metal ion, support, and the immobilization method. The complex anchored via adsorption exhibited a higher activity compared to the corresponding complex anchored via functionalization of the surface. The reaction proceed via formation of the peroxo-intermediate, which has an inhibiting effect on the reaction rate. The reaction is enthalpy-controlled as is concluded from the isokinetic relationship. A mechanism is proposed involving the generation of HO₂ radicals from the peroxo-intermediate in the rate-determining step.     

Domino Rhodium/Palladium‐Catalyzed Dehydrogenation Reactions of Alcohols to Acids by Hydrogen Transfer to Inactivated Alkenes

The combination of the d⁸ Rh^I diolefin amide [Rh(trop₂N)(PPh₃)] (trop₂N=bis(5‐H‐dibenzo[a,d]cyclohepten‐5‐yl)amide) and a palladium heterogeneous catalyst results in the formation of a superior catalyst system for the dehydrogenative coupling of alcohols. The overall process represents a mild and direct method for the synthesis of aromatic and heteroaromatic carboxylic acids for which inactivated olefins can be used as hydrogen acceptors. Allyl alcohols are also applicable to this coupling reaction and provide the corresponding saturated aliphatic carboxylic acids. This transformation has been found to be very efficient in the presence of silica‐supported palladium nanoparticles. The dehydrogenation of benzyl alcohol by the rhodium amide, [Rh]N, follows the well established mechanism of metal–ligand bifunctional catalysis. The resulting amino hydride complex, [RhH]NH, transfers a H₂ molecule to the Pd nanoparticles, which, in turn, deliver hydrogen to the inactivated alkene. Thus a domino catalytic reaction is developed which promotes the reaction R‐CH₂‐OH+NaOH+2 alkene→R‐COONa+2 alkane.     

A comparative study of the catalytic activity of [Cu(NH3)4]2+ sorbed on zirconium phosphate and zirconium phenylphosphonate

Summary The complex cation [Cu(NH₃)₄]²⁺ has been sorbed on to two different supports: zirconium phosphate and zirconium phenylphosphonate, and the products characterized by elemental analysis, FTIR, e.s.r., reflectance spectra, t.g.a. and surface area measurements (BET method). The catalytic activity of these materials has been studied through the disproportionation of H₂O₂. The kinetic data and calculated energy of activation show that the complex cation, supported on zirconium phenylphosphonate, exhibits enhanced catalytic activity.Author to whom all correspondence should be directed.     

Ion Exchangers as catalysts I. Catalytic decomposition of hydrogen peroxide in presence of Dowex 50 W resin in the form of ethylamine-copper(II) complex ion in aqueous medium

Summary Dowex 50 W resin in the form of an ethylamine-Cu¹¹ complex ion was used as potentially active catalyst for the decomposition of H₂O₂ in aqueous medium. The stoichiometry of the amine-Cu¹¹ complex on the resin, determined experimentally, was found to have the total [Cu²⁺]: [ethylamine]=14 concentration ratio. The kinetics of the decomposition was studied and the calculated rate constant (per g of dry resin) was found to decrease with increase the degree of resin crosslinking. The active species, formed as an intermediate at the beginning of the reaction, had an inhibiting effect on the reaction rate. The brown peroxo-copper complex formed as a result of H₂O₂ decomposition, was found to contain the catalytic active species. The order of the reaction increased with decreasing initial H₂O₂ concentration, a sign of a step-wise mechanism. A quantitative treatment of the decomposition of H₂O₂ was provided in terms of activation parameters.     

Fast Oxygen Reduction Catalyzed by a Copper(II) Tris(2‐pyridylmethyl)amine Complex through a Stepwise Mechanism

Catalytic pathways for the reduction of dioxygen can either lead to the formation of water or peroxide as the reaction product. We demonstrate that the electrocatalytic reduction of O₂ by the pyridylalkylamine copper complex [Cu(tmpa)(L)]²⁺ in a neutral aqueous solution follows a stepwise 4 e^?/4 H⁺ pathway, in which H₂O₂ is formed as a detectable intermediate and subsequently reduced to H₂O in two separate catalytic reactions. These homogeneous catalytic reactions are shown to be first order in catalyst. Coordination of O₂ to Cu^I was found to be the rate‐determining step in the formation of the peroxide intermediate. Furthermore, electrochemical studies of the reaction kinetics revealed a high turnover frequency of 1.5×10⁵ s^?1, the highest reported for any molecular copper catalyst.     

Pd(111), Pd(100)及Pd(110)表面H2和O2直接合成H2O2的密度泛函理论研究

采用周期性密度泛函理论研究了H₂和O₂在Pd(111),Pd(100)及Pd(110)表面上直接合成H₂O₂的反应机理,对反应的主要基元步骤进行了计算和分析.结果表明,Pd(111)表面对H₂O₂直接合成的催化选择性最好,表面原子密度较低的Pd(100)表面和Pd(110)表面上含有O-O键的表面物种解离严重,不利于H₂O₂的生成.H₂O₂的选择性与含有O-O键表面物种的O-O键能和表面物种的结合能有关.含有O-O键的表面物种在表面的结合能越大,越容易发生解离,不利于形成H₂O₂.     

Synthesis of recoverable palladium composite as an efficient catalyst for the reduction of nitroarene compounds and Suzuki cross-coupling reactions using sepiolite clay and magnetic nanoparticles (Fe3O4@sepiolite-Pd2+)

Clays are nontoxic, inexpensive, abundant, and have great potential as catalytic carriers because of their special structure, surface, and suitability for supporting transition metals. In this study, sepiolite was used as a ligand for the heterogenization of palladium chloride on Fe₃O₄ nanoparticle surface as a novel, high temperature stable, and recoverable green catalyst (Fe₃O₄@sepiolite-Pd²⁺). The catalytic activity of this system was tested in the reduction of nitroarene compounds and the Suzuki cross-coupling reaction. The catalyst structure was characterized using spectroscopic data and magnetic and thermal techniques such as Fourier transform infrared, scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction, vibrating sample magnetometer (VSM), and thermogravimetric analysis.     

Electrochemical Studies on Effects of Psychostimulants,Opioids and Alcohol Intake Towards Level of Hydrogen Peroxide in the Brain Striatum

Hydrogen peroxide (H₂O₂) was the main types of many peroxides produced in living mammalian cells that consumed oxygen. In the brain, the main source of H₂O₂ was the superoxide dismutase (SOD)‐catalyzed reaction in mitochondria. However, the level of H₂O₂ would be elevated through administration of control drugs and alcohol by dopamine metabolism of monoamine oxidase. In this study, a H₂O₂ microsensor was used to investigate the level of H₂O₂ in the brain striatum after administration of methamphetamine (MAP), morphine (MrP) or ethanol (Eth). The placement of microsensor in the brain was done at coordinates A/P 1.1 from bregma, M/L+2.6 and D/V‐1.5. A working potential of +0.05 V vs. Ag/AgCl was applied. The H₂O₂ concentration was measured direct from the current generated by its catalytic reaction at the electro active surface of the electrode. A significant increase of H₂O₂ level was observed after 7 successive injections of the controlled drugs or alcohol. The initial measurement of H₂O₂ is essential as excess dosage of H₂O₂ during treatment will contribute to the formation of neurotoxin oxygenated radicals. The H₂O₂ was the precursor of O_2? and OH radicals. Thus, this study provided a mean to monitor H₂O₂ level in the brain.     

Synthesis of γ-keto carboxylic acids from γ-keto-α-chloro carboxylic acids via carbonylation—decarboxylation reactions catalysed by a palladium system

A palladium-based catalytic system is highly active in the synthesis of γ-keto acids of type ArCOCH₂CH₂COOH via carbonylation-decarboxylation of the corresponding α-chloride. Typical reaction conditions are: P(CO) = 20–30 atm; substrate/H₂O/Pd = 100–400/800–1000/1 (mol); temperature: 100–110 °C; [Pd]=0.25 × 10⁻²−1 × 1O⁻² M; solvent: acetone; reaction time: 1–2 h. A palladium(II) complex can be used as catalyst precursor. Under the reaction conditions above, reduction of the precursor to palladium metal occurs to a variable extent. High catalytic activity is observed when the precursor undergoes extensive decomposition to the metal. Pd/C is also highly active. Slightly higher yields are obtainable when the catalytic system is used in combination with a ligand such as PPh₃. A mechanism for the catalytic cycle is proposed: (i) The starting keto chloride undergoes oxidative addition to reduced palladium with formation of a catalytic intermediate having a Pd-[CH(COOH)CH₂COPh] moiety. The reduced palladium may be the metal coordinated by other atoms of palladium and/or by carbon monoxide and/or by a PPh₃ ligand when catalysis is carried out in the presence of this ligand. It is also proposed that the keto group in the β-position with respect to the carbon atom bonded to chlorine weakens the CCl bond, easing the oxidative addition step and enhancing the activity of the catalyst. (ii) Carbon monoxide ‘inserts’ into the PdC bond of the above intermediate to give an acyl catalytic intermediate having a Pd-[COCH(COOH)CH₂COPh] moiety. (iii) Nucleophilic attack of H₂O to the carbon atom of the carbonyl group bonded to the metal of the acyl intermediate yields a malonic acid derivative as product intermediate. This, upon decarboxylation, gives the final product. Alternatively, the desired product may form without the malonic acid derivative intermediate, through the following reaction pathway: the acyl intermediate undergoes decarboxylation with formation of a different acyl intermediate, having a Pd-[CO-CH₂CH₂COPh] moiety, which, upon nucleophilic attack of H₂O on the carbon atom of the carbonyl group bonded to the metal, yields the final product.     

Catalytic activity of bimetal-containing Co,Pd systems in the oxidation of carbon monoxide

The catalytic activity of low-percentage Co,Pd systems on ZSM-5, ERI, SiO₂, and Al₂O₃ supports in the oxidation of CO was studied. The activity of bimetal-containing catalysts was shown to depend on the nature of the catalyst and the amount and ratio of their active components. According to the results of thermoprogrammed reduction with H₂ (H₂ TPR) and X-ray photoelectron spectroscopy (XPS) data, the metals are distributed as isolated cations or Co^δ+-O-Pd^δ+ clusters with cobalt and palladium cations surrounded by off-lattice oxygen in Co,Pd systems. The 0.8% Co,0.5% Pd-ZSM-5 bimetal catalysts were found to be more active due to the presence of clusters.     

Mechanistic Study of Ozone-Water Reaction and Their Reaction Products with Gas Phase Elemental Mercury:A Computational Study

The interaction of oxygen of water and central oxygen of ozone produces stable H₂O‐O₃ complex with no barrier. With decomposition of this complex through H‐abstraction by O₃ and O‐abstraction by H₂O, four possible product channels have been found. The reaction of mercury and the products of water‐ozone reaction have been studied. All geometrical and AIM parameters of intermediate, transition states, and the products of reactions are calculated and thermodynamic parameters are obtained. The negative value of free energy show that channels Hg+H₂OO, Hg+H₂O₂ and Hg+H₂O₄ in hydrogen tetroxide form (HTO) may be the main reaction channels.     

Decomposition of a cation exchange resin with hydrogen peroxide

Ion exchange resins are widely used in the field of nuclear industry. The present work aimed at the development of a method for complete decomposition of cation exchange resins with H₂O₂ in the presence of Fe³⁺ ion. The decomposition reaction proceeded at ambient temperature and decomposition time was greatly shortened with increasing concentration of Fe³⁺ ion rather than that of H₂O₂. The catalytic action of Fe³⁺ ion was suppressed with increase of HNO₃ concentration. As much as 4 g of the air-dried resin could be decomposed with 8 ml of 30% H₂O₂, and the use of about 60 ml of 30% H₂O₂ resulted in the complete decomposition of organic carbon to CO₂. Absence of any orgnaic carbon in the residual solution will simplify the final disposal.     

Characteristics of the Oxidation of Carbon Monoxide on Platinum and Palladium Acetylacetonates Immobilized on a Silica Surface

Characteristics of the kinetics of the oxidation of carbon monoxide on acetylacetonates of palladium and platinum immobilized on a silica surface have been studied. The bound metal complexes show no hysteresis in the dependence of the rate of reaction on the concentration of CO and O₂ and have a higher catalytic activity than Pt/SiO₂ and Pd/SiO₂. A mechanism is proposed for the oxidation of carbon monoxide on platinum and palladium complexes bound to a SiO₂ surface.     

Hydrocarbon oxidation with an oxygen-hydrogen mixture: Catalytic systems based on the interaction of platinum or palladium with a heteropoly compound

A series of studies of hydrocarbon oxidation by the O₂ + H₂ mixture in the presence of catalytic systems based on Pt or Pd and a heteropoly compound (HPC) is reviewed. The catalytic systems were prepared from Pd(II) complexes with the heteropoly tungstate anions PW₁₁O ₂₉ ^7? and PW₉O ₃₄ ^9? , the complex salt [Pt(NH₃)₄][H₂Mo₁₂O₄₀]₂ · 7H₂O, mixtures of H₂PtCl₄ or H₂PtCl₆ with H_{3 + n} PMo_{12 ? n} V _n O₄₀ (n = 0–3) heteropoly acids, or supported platinum dispersed in HPC solutions. The interaction of metal ions and particles with HPCs in the initial state and after thermal and redox treatments was investigated by NMR, IR spectroscopy, XPS, EXAFS, HREM, and TPR. The catalytic systems were tested in the liquid-phase oxidation of alkanes, cyclohexane, cycloalkenes, benzene, toluene, and phenol with the O₂ + H₂ mixture at low temperatures. Effective supported catalysts based on platinum nanoparticles associated with the redox-active HPCs H₃PMo₁₂O₄₀ and H₄PMo₁₁VO₄₀ were prepared for gas-phase benzene oxidation into phenol. The oxidation mechanism includes the interaction between dioxygen and platinum (or palladium) and the participation of the HPC in the formation of active oxygen species of radical nature.