Showing papers on "Catalysis published in 2004"
TL;DR: A review of gold nanoparticles can be found in this article, where the most stable metal nanoparticles, called gold colloids (AuNPs), have been used for catalysis and biology applications.
Abstract: Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346
11,752 citations
TL;DR: In this article, the photocatalytic degradation of azo dyes containing different functionalities has been reviewed using TiO2 as photocatalyst in aqueous solution under solar and UV irradiation.
Abstract: The photocatalytic degradation of azo dyes containing different functionalities has been reviewed using TiO2 as photocatalyst in aqueous solution under solar and UV irradiation. The mechanism of the photodegradation depends on the radiation used. Charge injection mechanism takes place under visible radiation whereas charge separation occurred under UV light radiation. The process is monitored by following either the decolorization rate and the formation of its end-products. Kinetic analyses indicate that the photodegradation rates of azo dyes can usually be approximated as pseudo-first-order kinetics for both degradation mechanisms, according to the Langmuir–Hinshelwood model. The degradation of dyes depend on several parameters such as pH, catalyst concentration, substrate concentration and the presence of electron acceptors such as hydrogen peroxide and ammonium persulphate besides molecular oxygen. The presence of other substances such as inorganic ions, humic acids and solvents commonly found in textile effluents is also discussed. The photocatalyzed degradation of pesticides does not occur instantaneously to form carbon dioxide, but through the formation of long-lived intermediate species. Thus, the study focuses also on the determination of the nature of the principal organic intermediates and the evolution of the mineralization as well as on the degradation pathways followed during the process. Major identified intermediates are hydroxylated derivatives, aromatic amines, naphthoquinone, phenolic compounds and several organic acids. By-products evaluation and toxicity measurements are the key-actions in order to assess the overall process.
3,692 citations
TL;DR: Nine transition metals were tested for the activation of three oxidants and the generation of inorganic radical species such as sulfate, peroxymonosulfate, and hydroxyl radicals to postulate the rate-determining step of the redox reactions taking place when a metal is coupled with an oxidant in aqueous solution.
Abstract: Nine transition metals were tested for the activation of three oxidants and the generation of inorganic radical species such as sulfate, peroxymonosulfate, and hydroxyl radicals. From the 27 combinations, 14 M/Ox couples demonstrated significant reactivity toward transforming a model organic substrate such as 2,4-dichlorophenol and are further discussed here. It was found that Co(II) and Ru(III) are the best metal catalysts for the activation of peroxymonosulfate. As expected on the basis of the Fenton reagent, Fe(III) and Fe(II) were the most efficient transition metals for the activation of hydrogen peroxide. Finally, Ag(I) showed the best results toward activating persulfate. Quenching studies with specific alcohols (tert-butyl alcohol and ethanol) were also performed to identify the primary radical species formed from the reactive M/Ox interactions. The determination of these transient species allowed us to postulate the rate-determining step of the redox reactions taking place when a metal is coupled with an oxidant in aqueous solution. It was found that when Co(II), Ru(III), and Fe(II) interact with peroxymonosulfate, freely diffusible sulfate radicals are the primary species formed. The same was proven for the interaction of Ag(I) with persulfate, but in this case caged or bound to the metal sulfate radicals might be formed as well. The conjunction of Ce(III), Mn(II), and Ni(II) with peroxymonosulfate showed also to generate caged or bound to the metal sulfate radicals. A combination of sulfate and hydroxyl radicals was formed from the conjunction of V(III) with peroxymonosulfate and from Fe(II) with persulfate. Finally, the conjunction of Fe(III), Fe(II), and Ru(III) with hydrogen peroxide led primarily to the generation of hydroxyl radicals. It is also suggested here that the redox behavior of a particular metal in solution cannot be predicted based exclusively on its size and charge. Additional phenomena such as metal hydrolysis as well as complexation with other counterions present in solution might affect the thermodynamics of the overall process and are further discussed here.
2,453 citations
TL;DR: In this article, the authors demonstrate the efficient chemical vapor deposition synthesis of single-walled carbon nanotubes where the activity and lifetime of the catalysts are enhanced by water.
Abstract: We demonstrate the efficient chemical vapor deposition synthesis of single-walled carbon nanotubes where the activity and lifetime of the catalysts are enhanced by water. Water-stimulated enhanced catalytic activity results in massive growth of superdense and vertically aligned nanotube forests with heights up to 2.5 millimeters that can be easily separated from the catalysts, providing nanotube material with carbon purity above 99.98%. Moreover, patterned, highly organized intrinsic nanotube structures were successfully fabricated. The water-assisted synthesis method addresses many critical problems that currently plague carbon nanotube synthesis.
2,405 citations
2,110 citations
TL;DR: Practical guidelines for the preparation and use of different Schiff base metal complexes in the field of catalytic transformations are discussed in this tutorial review.
Abstract: Schiff base ligands are considered “privileged ligands” because they are easily prepared by the condensation between aldehydes and imines. Stereogenic centres or other elements of chirality (planes, axes) can be introduced in the synthetic design. Schiff base ligands are able to coordinate many different metals, and to stabilize them in various oxidation states, enabling the use of Schiff base metal complexes for a large variety of useful catalytic transformations. Practical guidelines for the preparation and use of different Schiff base metal complexes in the field of catalytic transformations are discussed in this tutorial review.
1,642 citations
TL;DR: The current state of the art of the application of ionic liquids in catalysis is reviewed in this article, where examples of the different ways in which ionic liquid have been applied in Catalysis, i.e. as the catalyst itself, as a co-catalyst or catalyst activator, as the source of a new ligand for a catalytic metal centre, or just as the solvent for the reaction.
1,423 citations
1,398 citations
TL;DR: A number of methods using various copper complexes and salts to carry out cross-coupling reactions leading to the formation of C heteroatom (C N, C O, C S, C P, C Se), C C, and C metal bonds have been proposed as discussed by the authors.
1,361 citations
TL;DR: The C3-symmetric derivative of polytriazolylamines, TBTA, was shown to be a powerful stabilizing ligand for copper(I), protecting it from oxidation and disproportionation, while enhancing its catalytic activity.
1,344 citations
TL;DR: Palladium oxidase catalysis combines the versatility of Pd(II)-mediated oxidation of organic substrates with dioxygen-coupled oxidation of the reduced palladium catalyst to enable a broad range of selective aerobic oxidation reactions.
Abstract: Selective aerobic oxidation of organic molecules is a fundamental and practical challenge in modern chemistry. Effective solutions to this problem must overcome the intrinsic reactivity and selectivity challenges posed by the chemistry of molecular oxygen, and they must find application in diverse classes of oxidation reactions. Palladium oxidase catalysis combines the versatility of Pd(II)-mediated oxidation of organic substrates with dioxygen-coupled oxidation of the reduced palladium catalyst to enable a broad range of selective aerobic oxidation reactions. Recent developments revealed that cocatalysts (e.g. Cu(II), polyoxometalates, and benzoquinone) are not essential for efficient oxidation of Pd(0) by molecular oxygen. Oxidatively stable ligands play an important role in these reactions by minimizing catalyst decomposition, promoting the direct reaction between palladium and dioxygen, modulating organic substrate reactivity and permitting asymmetric catalysis.
TL;DR: In this paper, it was shown that the reaction rate under given reaction conditions shows a maximum as a function of dissociative adsorption energy of the key reactant, and that for most conditions this maximum is in the same range of reaction energies.
TL;DR: In this article, a detailed investigation of photocatalytic degradation of acid red 14 (AR14) was presented, and it was shown that zinc oxide appears to be a suitable alternative to TiO2 for water treatment.
Abstract: The degradation of acid red 14 (AR14), commonly used as a textile dye, can be photocatalysed by ZnO. Using advanced oxidation processes (AOPs), zinc oxide appears to be a suitable alternative to TiO2 for water treatment. In this study, a detailed investigation of photocatalytic degradation of acid red 14 is presented. Photodegradation efficiency was small when the photolysis was carried out in the absence of ZnO and it was also negligible in the absence of UV light. The semi-log plot of dye concentration versus time was linear, suggesting first order reaction (K=0.0548 min−1). The effects of some parameters such as pH, amount of photocatalyst, hydrogen peroxide and ethanol concentration were also examined. The addition of proper amount of hydrogen peroxide improved the decolorization, while the excess hydrogen peroxide could quenched the formation of hydroxyl radicals ( OH). As our results indicated that ethanol inhibited the photodegradation of dye, we concluded from the inhibitive effect of ethanol that hydroxyl radicals played a significant role in the photodegradation of dye. This should not undermine direct oxidation caused by positive holes.
TL;DR: It was found that the phosphoric acid derivatives of general structure 1 serve as highly effective catalysts for the direct addition of acetyl acetone to N-Boc-protected arylimines and functions as an excellent catalyst.
Abstract: It was found that the phosphoric acid derivatives of general structure 1 serve as highly effective catalysts for the direct addition of acetyl acetone to N-Boc-protected arylimines. The beneficial effects of the 3,3‘-bisaryl substituents of the catalysts on the enantioselectivity are greatly appreciated, and thus 1d functions as an excellent catalyst. The Bronsted acid-catalyzed direct Mannich reactions presented herein provide an attractive way to construct β-aminoketones under extremely mild conditions. The stereochemical course of this reaction was established through the synthesis of Boc-(S)-phenylglycine methylester. The transformation thus demonstrated is applicable to a useful method for the synthesis of various phenylglycine derivatives.
TL;DR: In this article, the predictions of the properties of mixtures of ionic liquids using COSMOtherm, an approach based on unimolecular quantum chemical calculations of the individual molecules, are presented.
TL;DR: The surfaces investigated in this study had no lateral strain in them, demonstrating that strain is not a necessary factor in the modification of bimetallic surface properties.
Abstract: The modification of the electronic and chemical properties of Pt(111) surfaces by subsurface 3d transition metals was studied using density-functional theory. In each case investigated, the Pt surface d-band was broadened and lowered in energy by interactions with the subsurface 3d metals, resulting in weaker dissociative adsorption energies of hydrogen and oxygen on these surfaces. The magnitude of the decrease in adsorption energy was largest for the early 3d transition metals and smallest for the late 3d transition metals. In some cases, dissociative adsorption was calculated to be endothermic. The surfaces investigated in this study had no lateral strain in them, demonstrating that strain is not a necessary factor in the modification of bimetallic surface properties. The implications of these findings are discussed in the context of catalyst design, particularly for fuel cell electrocatalysts.
TL;DR: In this paper, the use of N-heterocyclic carbene ligands (NHCs) based on imidazolium ions and related heterocycles has emerged as an alternative to phosphines in the design of new organometallic catalysts.
TL;DR: Well-defined catalysts for epoxide-CO(2) copolymerization and related reactions are reviewed here.
Abstract: Most synthetic polymers are made from petroleum feedstocks. Given the non-renewable nature of these materials, there is increasing interest in developing routes to polymeric materials from renewable resources. In addition, there is a growing demand for biodegradable polymeric materials. Polycarbonates made from CO(2) and epoxides have the potential to meet these goals. Since the discovery of catalysts for the copolymerization of CO(2) and epoxides in the late 1960's by Inoue, a significant amount of research has been directed toward the development of catalysts of improved activity and selectivity. Reviewed here are well-defined catalysts for epoxide-CO(2) copolymerization and related reactions.
TL;DR: In this paper, the active nanowire sensor element in such devices can be configured either as resistors whose conductance is altered by charge transfer processes occurring at their surfaces or as field effect transistors whose properties can be controlled by applying an appropriate potential onto its gate.
Abstract: ▪ Abstract Metal-oxide nanowires can function as sensitive and selective chemical or biological sensors, which could potentially be massively multiplexed in devices of small size. The active nanowire sensor element in such devices can be configured either as resistors whose conductance is altered by charge-transfer processes occurring at their surfaces or as field-effect transistors whose properties can be controlled by applying an appropriate potential onto its gate. Functionalizing the surface of these entities offers yet another avenue for expanding their sensing capability. In turn, because charge exchange between an adsorbate and the nanowire can change the electron density in the nanowire, modifying the nanowire's carrier density by external means, such as applying a potential to the gate, could modify its surface chemical properties and perhaps change the rate and selectivity of catalytic processes occurring at its surface. Although research on the use of metal-oxide nanowires as sensors is still i...
TL;DR: The catalytic properties of the metal-organic framework compound Cu 3 (BTC) 2 (H 2 O) 3 ǫ x H 2 O (BTC=benzene 1,3,5-tricarboxylate) were explored in this article.
TL;DR: In this paper, a series of manganese-cerium oxide catalysts were prepared by co-precipitation method and used for low temperature selective catalytic reduction of NO x with ammonia in the presence of excess O 2.
Abstract: A series of manganese-cerium oxide catalysts were prepared by co-precipitation method and used for low temperature selective catalytic reduction (SCR) of NO x with ammonia in the presence of excess O 2 . These catalysts were characterized by X-ray diffraction (XRD), surface area measurement and FTIR. The experimental results showed that the best Mn-Ce mixed-oxide catalyst yielded 95% NO conversion at 150 °C at a space velocity of 42,000 h −1 . As the manganese content was increased from 0 to 40% (i.e. the molar ratio of Mn/(Mn+Ce)), NO conversion increased significantly, but decreased at higher manganese contents. The most active catalyst was obtained with a molar Mn/(Mn+Ce) ratio of 0.4. Only N 2 rather than N 2 O was found in the product when the temperature was below 150 °C. At higher temperatures, trace amounts of N 2 O were detected. A mechanistic pathway for this reaction was proposed based on earlier findings and FTIR results obtained in this work. The initial step was the adsorption of NH 3 on Lewis acid sites of catalyst, followed by reaction with nitrite species to produce N 2 and H 2 O. Possible intermediates are proposed and all the intermediates could transform into NH 2 NO, which could further react to produce N 2 and H 2 O.
TL;DR: In this article, the role of the shape of coordinating ligands and of different metal ions in directing the synthesis totally or preferentially towards mono-, di- or poly-nuclear entities is discussed.
TL;DR: The use of palladium-derived catalysts in the synthesis of fine chemicals, pharmaceutical inter mediates and active pharmaceutical ingredients (APIs) has become quite common in the last few decades as mentioned in this paper.
Abstract: The use of palladium-derived catalysts in the synthesis of fine chemicals, pharmaceutical inter- mediates and active pharmaceutical ingredients (APIs) has become quite common in the last few dec- ades The number of palladium-catalyzed synthetic re- actions (both achiral and chiral) available to chemists has provided access to more complex structures in fewer steps and with less waste, due to the catalytic nature of many of the methods An unfortunate side effect of using palladium is the potential for palladi- um-containing impurities to remain in the desired compound after isolation This is an especially signifi- cant problem for the pharmaceutical industry since there is a low limit for heavy metal impurities allowed in the drug substance Therefore, various methods of removing palladium impurities from organic com- pounds of pharmaceutical interest have been devel- oped This review will provide a survey of the publish- ed methods but is not meant to be inclusive of all pub- lished material in this area of research
TL;DR: The present Pd catalyst does not require additives to complete the catalytic cycle, and the diameters of the generated Pd nanoclusters can be controlled upon acting on the alcohol substrates used.
Abstract: Treatment of a stoichiometric hydroxyapatite (HAP), Ca10(PO4)6(OH)2, with PdCl2(PhCN)2 gives a new type of palladium-grafted hydroxyapatite. Analysis by means of powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX), IR, and Pd K-edge X-ray absorption fine structure (XAFS) proves that a monomeric PdCl2 species is chemisorbed on the HAP surface, which is readily transformed into Pd nanoclusters with a narrow size distribution in the presence of alcohol. Nanoclustered Pd0 species can effectively promote the alcohol oxidation under an atmospheric O2 pressure, giving a remarkably high turnover number (TON) of up to 236,000 with an excellent turnover frequency (TOF) of approximately 9800 h(-1) for a 250-mmol-scale oxidation of 1-phenylethanol under solvent-free conditions. In addition to advantages such as a simple workup procedure and the ability to recycle the catalyst, the present Pd catalyst does not require additives to complete the catalytic cycle. The diameters of the generated Pd nanoclusters can be controlled upon acting on the alcohol substrates used. Oxidation of alcohols is proposed to occur primarily on low-coordination sites within a regular arrangement of the Pd nanocluster by performing calculations on the palladium crystallites.
TL;DR: Turnover rates for forward reactions of CH 4/CO 2 and CH 4 /H 2 O mixtures were proportional to CH 4 pressure (5-450 kPa) and independent of the partial pressure of the CO 2 or H 2 O coreactants (5 −450 kPA), indicating that these reactions are mechanistically equivalent as discussed by the authors.
TL;DR: This tutorial review deals with recent developments in the activation of C-C bonds in organic molecules that have been catalyzed by transition metal complexes and the strategies and tactics for C-B bond activation will be explained.
Abstract: This tutorial review deals with recent developments in the activation of C–C bonds in organic molecules that have been catalyzed by transition metal complexes. Many chemists have devised a variety of strategies for C–C bond activation and significant progress has been made in this field over the past few decades. However, there remain only a few examples of the catalytic activation of C–C bonds, in spite of the potential use in organic synthesis, and most of the previously published reviews have dwelt mainly on the stoichiometric reactions. Consequently, this review will focus mainly on the catalytic reaction of C–C bond cleavage by homogeneous transition metal catalysts. The contents include cleavage of C–C bonds in strained and unstrained molecules, and cleavage of multiple C–C bonds such as CC triple bonds in alkynes. Multiple bond metathesis and heterogeneous systems are beyond the scope of this review, though they are also fascinating areas of C–C bond activation. In this review, the strategies and tactics for C–C bond activation will be explained.
TL;DR: The synthesis of well-defined porous carbon films involves four steps: monomer-block copolymer film casting, structure refining through solvent annealing, polymerization of the carbon precursor, and carbonization.
Abstract: The synthesis of well-defined porous carbon films involves four steps: (1) monomer-block copolymer film casting, (2) structure refining through solvent annealing, (3) polymerization of the carbon precursor, and (4) carbonization. The resulting films, such as that depicted, have potential as separation membranes, chemical sensors, and catalysts.
TL;DR: Au catalysts have been used for deodorizers in rest rooms in Japan and will find growing applications in indoor air quality control, pollutant emission control, production of hydrogen energy carrier, and innovations in chemical processes as discussed by the authors.
Abstract: Gold can be deposited as nanoparticles on a variety of support materials by coprecipitation or deposition-precipitation of Au(OH)3, grafting of organo-gold complexes such as dimethyl-Au(III)-acetylacetonate, mixing of colloidal Au particles, and vacuum deposition. Owing to the moderate adsorption of at least one of reactants (for example, CO) on the edges and corners of Au nanoparticles and to the activation of the counter reactant (for example, O2) at the perimeter interface with the supports, supported Au nanoparticle catalysts exhibit unique and practically useful catalytic properties at relatively low temperature below 473K. They have already been commercially used for deodorizers in rest rooms in Japan and will find growing applications in indoor air quality control, pollutant emission control, production of hydrogen energy carrier, and innovations in chemical processes. Cluster science of Au may also open an exciting area of research showing some magic numbers for dramatic changes in reactivity.