Showing papers in "ACS Catalysis in 2012"

Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and Its Potential Applications

Abstract: Nitrogen doping has been an effective way to tailor the properties of graphene and render its potential use for various applications. Three common bonding configurations are normally obtained when doping nitrogen into the graphene: pyridinic N, pyrrolic N, and graphitic N. This paper reviews nitrogen-doped graphene, including various synthesis methods to introduce N doping and various characterization techniques for the examination of various N bonding configurations. Potential applications of N-graphene are also reviewed on the basis of experimental and theoretical studies.

Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials

Abstract: A comparative investigation was performed to examine the intrinsic catalytic activity and durability of carbon supported Ru, Ir, and Pt nanoparticles and corresponding bulk materials for the electrocatalytic oxygen evolution reaction (OER). The electrochemical surface characteristics of nanoparticles and bulk materials were studied by surface-sensitive cyclic voltammetry. Although basically similar voltammetric features were observed for nanoparticles and bulk materials of each metal, some differences were uncovered highlighting the changes in oxidation chemistry. On the basis of the electrochemical results, we demonstrated that Ru nanoparticles show lower passivation potentials compared to bulk Ru material. Ir nanoparticles completely lost their voltammetric metallic features during the voltage cycling, in contrast to the corresponding bulk material. Finally, Pt nanoparticles show an increased oxophilic nature compared to bulk Pt. With regard to the OER performance, the most pronounced effects of nanosca...

Polymeric Graphitic Carbon Nitride for Heterogeneous Photocatalysis

Abstract: Polymeric graphitic carbon nitride (for simplicity, g-C3N4) is a layered material similar to graphene, being composed of only C, N, and some impurity H. Contrary to graphenes, g-C3N4 is a medium band gap semiconductor and an effective photocatalyst for a broad variety of reactions, and it possesses a high thermal and chemical stability In this Perspective, we describe the polycondensation of this structure, how to modify band positions and band gap by doping and copolymerization, and how to texture the organic solid to make it an effective photocatalyst. We then describe the photochemical splitting of water and some mild and selective photooxidation reactions catalyzed by g-C3N4.

Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production: Insights into the Origin of their Catalytic Activity

Abstract: We present a scalable wet chemical synthesis for a catalytically active nanostructured amorphous molybdenum sulfide material. The catalyst film is one of the most active nonprecious metal materials for electrochemical hydrogen evolution, drawing 10 mA/cm2 at ∼200 mV overpotential. To identify the active phase of the material, we perform X-ray photoelectron spectroscopy after testing under a variety of conditions. As deposited, the catalyst resembles amorphous MoS3, but domains resembling MoS2 in composition and chemical state are created under reaction conditions and may contribute to this material’s high electrochemical activity. The activity scales with electrochemically active surface area, suggesting that the rough, nanostructured catalyst morphology also contributes substantially to the film’s high activity. Electrochemical stability tests indicate that the catalyst remains highly active throughout prolonged operation. The overpotential required to attain a current density of 10 mA/cm2 increases by o...

Hydrothermal synthesis of graphene-TiO 2 nanotube composites with enhanced photocatalytic activity

Abstract: In this study, TiO2 nanotube (TNT)/reduced graphene oxide (hGO) composites were prepared by an alkaline hydrothermal process. This was achieved by decorating graphene oxide (GO) layers with commercially available TiO2 nanoparticles (P90) followed by hydrothermal synthesis, which converts the TiO2 nanoparticles to small diameter (∼9 nm) TNTs on the hGO surface. The alkaline medium used to synthesize the TNTs simultaneously converts GO to deoxygenated graphene oxide (hGO). Compared to GO, the hGO has a ∼70% reduction of oxygenated species after alkaline hydrothermal treatment. The graphene nature of hGO in the composites was confirmed by X-ray diffraction (XRD), Raman, FTIR, and X-ray photoelectron spectroscopy (XPS) analysis. The photocatalytic performance of the hGO-TNT composites was evaluated for the photodegradation of malachite green. It was found that the ratio of hGO to TNT in the composites significantly affects the photocatalytic activity. Higher amounts of hGO in hGO-TNT composites showed lower p...

Redox Non-Innocent Ligands: Versatile New Tools to Control Catalytic Reactions

Abstract: In this (tutorial overview) perspective we highlight the use of “redox non-innocent” ligands in catalysis. Two main types of reactivity in which the redox non-innocent ligand is involved can be specified: (A) The redox active ligand participates in the catalytic cycle only by accepting/donating electrons, and (B) the ligand actively participates in the formation/breaking of substrate covalent bonds. On the basis of these two types of behavior, four main application strategies of redox-active ligands in catalysis can be distinguished: The first strategy (I) involves oxidation/reduction of the ligand to tune the electronic properties (i.e., Lewis acidity/basicity) of the metal. In the second approach (II) the ligand is used as an electron reservoir. This allows multiple-electron transformations for metal complexes that are reluctant to such transformations otherwise (e.g., because the metal would need to accommodate an uncommon, high-energy oxidation state). This includes examples of (first row) transition ...

Two Different Roles of Metallic Ag on Ag/AgX/BiOX (X = Cl, Br) Visible Light Photocatalysts: Surface Plasmon Resonance and Z-Scheme Bridge

Abstract: Ag/AgX/BiOX (X = Cl, Br) three-component visible-light-driven (VLD) photocatalysts were synthesized by a low-temperature chemical bath method and characterized by X-ray diffraction patterns, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and UV–vis diffuse reflectance spectra. The Ag/AgX/BiOX composites showed enhanced VLD photocatalytic activity for the degradation of rhodamine B, which was much higher than Ag/AgX and BiOX. The photocatalytic mechanisms were analyzed by active species trapping and superoxide radical quantification experiments. It revealed that metallic Ag played a different role for Ag/AgX/BiOX VLD photocatalysts, surface plasmon resonance for Ag/AgCl/BiOCl, and the Z-scheme bridge for Ag/AgBr/BiOBr.

Electrocatalysis for Polymer Electrolyte Fuel Cells: Recent Achievements and Future Challenges

Abstract: Fuel cell technology is currently shifting very fast from fundamental research to real development. In addition to other aspects, this transition is possible because of the important improvements achieved in the field of electrocatalysis in the past decade. This perspective will give a focused overview summarizing the most outstanding contributions in the last 10 years in terms of activity and durability of the catalyst materials for ethanol oxidation and oxygen reduction reaction, respectively. In addition, it provides an outlook about new catalyst support materials with improved performance/stability, advanced characterization techniques, and fundamental studies of reaction mechanisms and degradation processes. All the studies referred to in this perspective significantly contribute to reaching the technical targets for PEFC commercialization.

Photocatalytic CO2 Reduction with H2O on TiO2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and Exploration of Surface Chemistry

Abstract: CO2 photoreduction with water vapor has been studied on three TiO2 nanocrystal polymorphs (anatase, rutile, and brookite) that were engineered with defect-free and oxygen-deficient surfaces, respectively. It was demonstrated that helium pretreatment of the as-prepared TiO2 at a moderate temperature resulted in the creation of surface oxygen vacancies (VO) and Ti3+ sites on anatase and brookite but not on rutile. The production of CO and CH4 from CO2 photoreduction was remarkably enhanced on defective anatase and brookite TiO2 (up to 10-fold enhancement) as compared to the defect-free surfaces. Defective brookite was photocatalytically more active than anatase and rutile, probably because of a lower formation energy of VO on brookite. The results from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggested that (1) defect-free TiO2 was not active for CO2 photoreduction since no CO2– is generated, and (2) CO2 photoreduction to CO possibly underwent different reaction ...

Metal–Organic Frameworks for Light Harvesting and Photocatalysis

Abstract: Metal–organic frameworks (MOFs), a new class of crystalline molecular solids built from linking organic ligands with metal or metal-cluster connecting points, have recently emerged as a versatile platform for developing single-site solid catalysts. MOFs have been used to drive a range of reactions, including Lewis acid/base catalyzed reactions, redox reactions, asymmetric reactions, and photocatalysis. MOF catalysts are easily separated from the reaction mixtures for reuse, and yet their molecular nature introduces unprecedented chemical diversity and tunability to drive a large scope of catalytic reactions. This Perspective aims to summarize recent progress on light harvesting and photocatalysis with MOFs. The charge-separated excited states of the chromophoric building blocks created upon photon excitation can migrate over long distances to be harvested as redox equivalents at the MOF/liquid interfaces via electron transfer reactions or can directly activate the substrates that have diffused into the MO...

Production of 5-Hydroxymethylfurfural from Glucose Using a Combination of Lewis and Brønsted Acid Catalysts in Water in a Biphasic Reactor with an Alkylphenol Solvent

Abstract: We report the catalytic conversion of glucose in high yields (62%) to 5-hydroxymethylfurfural (HMF), a versatile platform chemical. The reaction system consists of a Lewis acid metal chloride (e.g., AlCl3) and a Bronsted acid (HCl) in a biphasic reactor consisting of water and an alkylphenol compound (2-sec-butylphenol) as the organic phase. The conversion of glucose in the presence of Lewis and Bronsted acidity proceeds through a tandem pathway involving isomerization of glucose to fructose, followed by dehydration of fructose to HMF. The organic phase extracts 97% of the HMF produced, while both acid catalysts remain in the aqueous phase.

Electrocatalysts for Nonaqueous Lithium–Air Batteries: Status, Challenges, and Perspective

Abstract: The Li–air battery has recently emerged as a potentially transformational energy storage technology for both transportation and stationary energy storage applications because of its very high specific energy; however, its practical application is currently limited by the poor power capability (low current density), poor cyclability, and low energy efficiency. All of these are largely determined by interfacial reactions on oxygen electrocatalysts in the air electrode. In this article, we review the fundamental understanding of oxygen electrocatalysis in nonaqueous electrolytes and the status and challenges of oxygen electrocatalysts and provide a perspective on new electrocatalysts' design and development.

Degradation Mechanisms of Pt/C Fuel Cell Catalysts under Simulated Start–Stop Conditions

Abstract: This manuscript investigates the degradation of a Pt/Vulcan fuel cell catalyst under simulated start–stop conditions in an electrochemical half-cell. Identical location transmission electron microscopy (IL-TEM) is used to visualize the several different degradation pathways occurring on the same catalyst material under potential cycling conditions. The complexity of degradation on the nanoscale leading to macroscopic active surface area lossis demonstrated and discussed. Namely, four different degradation pathways at one single Pt/Vulcan aggregate are clearly observed. Furthermore, inhomogeneous degradation behavior for different catalyst locations is shown, and trends in degradation mechanisms related to the platinum particle size are discussed in brief. Attention is drawn to the vast field of parameters influencing catalyst stability. We also present the development of a new technique to study changes of the catalyst not only with 2D projections of standard TEM images but also in 3D. For this purpose, i...

Surfactant Removal for Colloidal Nanoparticles from Solution Synthesis: The Effect on Catalytic Performance

Abstract: Colloidal nanoparticles prepared by solution synthesis with robust control over particle size, shape, composition, and structure have shown great potential for catalytic applications. However, such colloidal nanoparticles are usually capped with organic ligands (as surfactants) and cannot be directly used as catalyst. We have studied the effect of surfactant removal on the electrocatalytic performance of Pt nanoparticles made by organic solution synthesis. Various methods were applied to remove the oleylamine surfactant, which included thermal annealing, acetic acid washing, and UV-Ozone irradiation, and the treated nanoparticles were applied as electrocatalysts for the oxygen reduction reaction. It was found that the electrocatalytic performance, including electrochemically active surface area and catalytic activity, was strongly dependent on the pretreatment. Among the methods studied here, low-temperature thermal annealing (∼185 °C) in air was found to be the most effective for surface cleaning without...

Universality in Oxygen Reduction Electrocatalysis on Metal Surfaces

Abstract: In this work, we extend the activity volcano for oxygen reduction from the face-centered cubic (fcc) metal (111) facet to the (100) facet. Using density functional theory calculations, we show that the recent findings of constant scaling between OOH* and OH* holds on the fcc metal (100) facet, as well. Using this fact, we show the existence of a universal activity volcano to describe oxygen reduction electrocatalysis with a minimum overpotential, ηmin = 0.37 ± 0.1 V. Specifically, we find that the (100) facet of Pt is found to bind oxygen intermediates too strongly and is not active for oxygen reduction reaction (ORR). In contrast, Au(100) is predicted to be more active than Au(111) and comparable in activity to Pt alloys. Using this activity volcano, we further predict that Au alloys that bind OH more strongly could display improved ORR activity on the (100) facet. We carry out a computational search over candidate alloys and suggest that alloying Au with early transition metals could lead to materials t...

Zeolitic Imidazole Framework-8 Catalysts in the Conversion of CO2 to Chloropropene Carbonate

Abstract: The catalytic activity of zeolitic imidazole framework-8 (ZIF-8) and amine-functionalized ZIF-8 catalysts in the synthesis of chloropropene carbonate from CO2 and epichlorohydrin is demonstrated. In contrast to hitherto known catalysts, ZIF-8 catalysts displayed high epoxide conversions and moderate to high selectivities to chloropropene carbonate at reaction temperatures as low as 70 °C. No cocatalysts or solvents were required during the reaction. The incorporation of ethylenediamine in ZIF-8 enhanced its catalytic performance as a result of the higher CO2 adsorption capacity of the amine-functionalized samples. The ZIF-8 catalysts, however, lost their distinctive crystalline structure and superior catalytic performance when attempts were made to recycle them after use.

Spectroscopic Characterization of Mixed Fe–Ni Oxide Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Electrolytes

Abstract: Mixed Fe−Ni oxide electrocatalysts for the oxygen evolution reaction in alkaline electrolytes were synthesized using three different approaches: evaporation induced self-assembly, hard templating, and dip-coating. For each synthesis method, a peak in oxygen evolution activity was observed near 10 mol % Fe content, where the mixed metal oxide was substantially more active than the parent metal oxide electrocatalysts. X-ray diffraction (XRD) analysis showed the formation of a mixed NiO/NiFe2O4 phase at low Fe concentrations, and formation of Fe2O3 at compositions above 25 mol % Fe. Raman vibrational spectroscopy confirmed the formation of NiFe2O4, and did not detect Fe2O3 in the electrocatalysts containing up to 20 mol % Fe. X-ray absorption near edge structure (XANES) showed the Fe in the mixed oxides to be predominantly in the +3 oxidation state. Extended X-ray absorption fine structure (EXAFS) showed changes in the Fe coordination shells under electrochemical oxygen evolution conditions. Temperature programmed reaction spectroscopy showed the mixed oxide surfaces also have superior oxidation activity for methanol oxidation, and that the reactivity of the mixed oxide surface is substantially different than that of the parent metal oxide surfaces. Overall, the NiFe2O4 phase is implicated in having a significant role in improving the oxygen evolution activity of the mixed metal oxide systems.

Cycloaddition of Biomass-Derived Furans for Catalytic Production of Renewable p-Xylene

Abstract: A renewable route to p-xylene from biomass-derived dimethylfuran and ethylene is investigated with zeolite catalysts. Cycloaddition of ethylene and 2,5-dimethylfuran and subsequent dehydration to p-xylene has been achieved with 75% selectivity using H–Y zeolite and an aliphatic solvent at 300 °C. Competitive side reactions include hydrolysis of dimethylfuran to 2,5-hexanedione, alkylation of p-xylene, and polymerization of 2,5-hexanedione. The observed reaction rates and computed energy barriers are consistent with a two-step reaction that proceeds through a bicyclic adduct prior to dehydration to p-xylene. Cycloaddition of ethylene and dimethylfuran occurs without a catalytic active site, but the reaction is promoted by confinement within microporous materials. The presence of Bronsted acid sites catalyzes dehydration of the Diels–Alder cycloadduct (to produce p-xylene and water), and this ultimately causes the rate-determining step to be the initial cycloaddition.

Advanced Platinum Alloy Electrocatalysts for the Oxygen Reduction Reaction

Abstract: In the past decade, significant advancement has been made in the development of electrocatalysts for energy conversion and storage. Among various approaches, alloying Pt with 3d transition metals has shown great potential in tailoring the atomic and electronic structures of catalytically active materials toward improved catalytic performance. Here, we provide a brief overview of the recent advancements in the design and synthesis of electrocatalysts for the oxygen reduction reaction. Our focus is placed on the systematic studies of particle size, composition, and shape effect for the monodisperse and homogeneous platinum alloy electrocatalysts that have been synthesized by organic solution approaches.

Synthesis of Carbon Nitride Semiconductors in Sulfur Flux for Water Photoredox Catalysis

Abstract: Sulfur-mediated synthesis has been demonstrated as a simple but efficient pathway to control the texture and electronic structure of poly(tris-triazine) based graphitic carbon nitride semiconductors with improved photocatalytic reactivity over the pristine counterpart. Here, we advance this strategy by employing cheap and easily available elemental sulfur as the external sulfur species instead of sulfur-containing precursors for the sulfur-mediated synthesis of polymeric carbon nitride photocatalysts. Characterization results revealed that the multiple thermal condensations of carbon nitride precursors in the hot sulfur flux provided a facile means to promote the formation of graphitic-like carbon nitride conjugated systems, altering the traditional route of thermal-induced self-polymerization of melamine. The textural, electronic, and optical properties of the resultants organic semiconductors was therefore strongly modified to endow the materials with improved physical and chemical properties, as demons...

“Turning Over” Definitions in Catalytic Cycles

Abstract: “Indeed, the catalytic activity, for a valid comparison, must be referred to the number of exposed surface atoms of a specified kind. Thus a convenient way to express catalytic activity is by means of a turnover number equal to the number of reactant molecules converted per minute per catalytic site for given reaction conditions.” With these words of Boudart the first definition of what later was called the Turnover Frequency (TOF) entered into the realm of heterogeneous chemistry. It was a term borrowed from enzymatic kinetics, and slowly passed to homogeneous catalysis. Nowadays it is a ubiquitous term, focusing strictly on the catalytic center, as distinct from the classical term “rate of reaction”, which emphasizes the generation of products or the consumption of reactants. Despite its utility and common use, the TOF concept is still not well-defined and leads to confusion. IUPAC’s gold book, the most authoritative source of chemical terminology, has a very concise definition of the turnover frequency: “Commonly called the turnover number, N, and defined, as in enzyme catalysis, as molecules reacting per active site in unit time.” This description of the TOF has two main problems. The first is the difficulty of providing a one-to-one correspondence between name and function, since (as appeared in Boudart’s paragraph) the terms “turnover frequency” (TOF) and “turnover number” (TON) seem to have one and the same meaning. However, in typical catalytic jargon, both expressions have very different connotations. Sometimes also the terms “turnover rate” and “catalytic constant” (kcat) are used interchangeably in the literature with the same meaning. To make matters worse, the TOF is occasionally considered a rate-constant, since the rate of reaction (r = TOF × [Cat]) depends on the catalysts concentration. However, the TOF itself can depend on the concentration of reactants and products even at saturation, and in this sense it is closer to a rate than to a kinetic constant. In spite of this, from a strict terminological stance the TOF is a frequency, with units of time−1. All this debate evidently resembles the biblical story of the Tower of Babel and the confusion of languages. The second problem of IUPAC’s and Boudart’s definitions is a recurring expression of the TOF as a function of the number of reactants consumed, or even of the products generated. In most cases it is indeed an accurate way to derive the TOF, but for instance in bimolecular reactions this is not the case. Moreover, and from a philosophical perspective, when expressing the TOF as a function of produced or consumed molecules, the focus of the measure goes back to those molecules instead of emphasizing the role of the catalyst (see Scheme 1).

Recent Advances in Preferential Oxidation of CO Reaction over Platinum Group Metal Catalysts

Abstract: Preferential oxidation of CO (PROX) is an important reaction for removing small amounts of CO to a parts-per-million level from the hydrogen-rich stream, which will be ultimately supplied as a fuel to polymer–electrolyte membrane fuel cells. The key to the application of PROX is to develop a highly active and selective catalyst that operates well in a wide temperature window (e.g., 80–180 °C) and has good resistance to CO2 and steam. In the past decades, various catalyst formulations have been developed, among which platinum group metal catalysts, including Pt, Ru, and Ir—in particular, those modified with promoters such as alkali metals and reducible metal oxides—have received a great deal of attention for their significantly improved catalytic activities in the low-temperature range. In this minireview, the recent advances of the platinum group metal catalysts for the PROX reaction are summarized, including performances of unpromoted and promoted catalysts, reaction mechanisms, and kinetics. In addition...

Nitrogen-Doped Graphene Nanosheets as Metal-Free Catalysts for Aerobic Selective Oxidation of Benzylic Alcohols

Abstract: This work demonstrates the molecular engineering of active sites on a graphene scaffold. It was found that the N-doped graphene nanosheets prepared by a high-temperature nitridation procedure represent a novel chemical function of efficiently catalyzing aerobic alcohol oxidation. Among three types of nitrogen species doped into the graphene lattice—pyridinic N, pyrrolic N, and graphitic N—the graphitic sp2 N species were established to be catalytically active centers for the aerobic oxidation reaction based on good linear correlation with the activity results. Kinetic analysis showed that the N-doped graphene-catalyzed aerobic alcohol oxidation proceeds via a Langmuir–Hinshelwood pathway and has moderate activation energy (56.1 ± 3.5 kJ·mol–1 for the benzyl alcohol oxidation) close to that (51.4 kJ·mol–1) proceeding on the catalyst Ru/Al2O3 reported in literature. An adduct mechanism was proposed to be different remarkably from that occurring on the noble metal catalyst. The possible formation of a sp2 N–...

Amorphous Carbon with SO3H Groups as a Solid Brønsted Acid Catalyst

Abstract: Homogeneous Bronsted acid catalysts such as H2SO4 and HCl are used for the production of industrially important chemicals. However, their use requires significant energy costs for separation, reuse, and treatment of salt wastes. Alternatively, heterogeneous Bronsted acid catalysts are promising candidates that can decrease the environmental impact associated with chemical production. In this review, we highlight amorphous carbon bearing SO3H groups as an insoluble Bronsted acid available for various acid-catalyzed reactions.

Development of Preformed Pd Catalysts for Cross-Coupling Reactions, Beyond the 2010 Nobel Prize

Abstract: Strategies for the development of Pd catalysts based on utilization of L2Pd and LPd species, beyond the contributions of the 2010 Nobel Laureates Richard Heck, Ei-ichi Negishi, and Akira Suzuki, along with their contemporaries, are reviewed. These well-defined, preformed Pd catalysts improve the selectivity and activity of selected cross-coupling reactions by reducing the metal loading and the ligand-to-metal ratios. This review describes predominantly the development of Pd precatalysts over the last 10 years and highlights the benefits often observed when using well-defined preformed catalysts relative to those generated in situ.

Rhodium-Catalyzed Asymmetric Arylation

Abstract: Rhodium-catalyzed asymmetric arylation (RCAA) reactions provide one of the most straightforward and powerful ways to introduce aryl fragments in an enantioselective manner. The discovery of novel chiral ligands and catalytic systems is a major focus in generating optical chiralities for RCAA reactions. In the past decade, the chelating functionalities in ligands have been significantly expanded from traditional phosphorus to interesting diene, bissulfoxide, and their hybrids. Herein we highlight the research on these distinct families of chiral ligands and describe their applications in the RCAA of arylmetals to activated alkenes, aldehydes, ketones and imines, and RCAA-tandem reactions.

Enhancement in Aerobic Alcohol Oxidation Catalysis of Au25 Clusters by Single Pd Atom Doping

Abstract: Au25 and Pd1Au24 clusters on multiwalled carbon nanotubes were developed via adsorption of Au25(SC12H25)18 and Pd1Au24(SC12H25)18, respectively, on the nanotubes, followed by calcination. Comparison of their catalysis for the aerobic oxidation of benzyl alcohol showed that single Pd atom doping significantly improved the catalytic performance of Au25 for the first time.

Photocatalytic H2O2 Production from Ethanol/O2 System Using TiO2 Loaded with Au–Ag Bimetallic Alloy Nanoparticles

Abstract: TiO2 loaded with Au–Ag bimetallic alloy particles efficiently produces H2O2 from an O2-saturated ethanol/water mixture under UV irradiation. This is achieved via the double effects created by the alloy particles. One is the efficient photocatalytic reduction of O2 on the Au atoms promoting enhanced H2O2 formation, due to the efficient separation of photoformed electron–hole pairs at the alloy/TiO2 heterojunction. Second is the suppressed photocatalytic decomposition of formed H2O2 due to the decreased adsorption of H2O2 onto the Au atoms.

Transition Metal Nanoparticle Catalysis in Ionic Liquids

Abstract: The main “molecular” physicochemical aspects involved in the catalytic applications of “soluble” transition metal nanoparticles in ionic liquids are used to explain their properties. In particular, a special view on the concepts of coordination catalysis is used to describe the properties of these nanoscale catalysts in solution as well as their scope and limitations in multiphase catalysis.

Conversion of Xylose to Furfural Using Lewis and Brønsted Acid Catalysts in Aqueous Media

Abstract: The role of molecular structure on pentose dehydration to furfural has been examined using HCl as a Bronsted acid catalyst in a single phase aqueous media. It is shown that xylulose dehydration results in a much higher furfural yield than xylose dehydration under similar reaction conditions. Furthermore, a cascade of reactions for the efficient conversion of xylose to furfural in a single pot reactor is presented whereby a Lewis acid, CrCl3, is used to isomerize xylose to xylulose, and a Bronsted acid, HCl, is employed to dehydrate xylulose to furfural. Using the combination of Lewis and Bronsted acids, a furfural yield of ∼39% is achieved compared to ∼29% using HCl alone, at a moderate reaction temperature (∼418 K) in a single aqueous phase, with an associated decrease in the residence time by a factor of 5. With this combined catalyst functionalities, a much higher yield (76%) to furfural can be obtained in a biphasic system at low temperatures and short times. Aside from increasing performance, our res...