Showing papers in "Nature Chemistry in 2011"

Single-atom catalysis of CO oxidation using Pt1/FeOx

Abstract: Platinum-based heterogeneous catalysts are critical to many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the surface active-site atoms are used. Catalysts with single-atom dispersions are thus highly desirable to maximize atom efficiency, but making them is challenging. Here we report the synthesis of a single-atom catalyst that consists of only isolated single Pt atoms anchored to the surfaces of iron oxide nanocrystallites. This single-atom catalyst has extremely high atom efficiency and shows excellent stability and high activity for both CO oxidation and preferential oxidation of CO in H-2. Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.

Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal–air batteries

Abstract: The prohibitive cost and scarcity of the noble-metal catalysts needed for catalysing the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries limit the commercialization of these clean-energy technologies. Identifying a catalyst design principle that links material properties to the catalytic activity can accelerate the search for highly active and abundant transition-metal-oxide catalysts to replace platinum. Here, we demonstrate that the ORR activity for oxide catalysts primarily correlates to σ-orbital (e(g)) occupation and the extent of B-site transition-metal-oxygen covalency, which serves as a secondary activity descriptor. Our findings reflect the critical influences of the σ orbital and metal-oxygen covalency on the competition between O(2)(2-)/OH(-) displacement and OH(-) regeneration on surface transition-metal ions as the rate-limiting steps of the ORR, and thus highlight the importance of electronic structure in controlling oxide catalytic activity.

Visible-light-enhanced catalytic oxidation reactions on plasmonic silver nanostructures

Abstract: Catalysis plays a critical role in chemical conversion, energy production and pollution mitigation. High activation barriers associated with rate-limiting elementary steps require most commercial heterogeneous catalytic reactions to be run at relatively high temperatures, which compromises energy efficiency and the long-term stability of the catalyst. Here we show that plasmonic nanostructures of silver can concurrently use low-intensity visible light (on the order of solar intensity) and thermal energy to drive catalytic oxidation reactions--such as ethylene epoxidation, CO oxidation, and NH₃ oxidation--at lower temperatures than their conventional counterparts that use only thermal stimulus. Based on kinetic isotope experiments and density functional calculations, we postulate that excited plasmons on the silver surface act to populate O₂ antibonding orbitals and so form a transient negative-ion state, which thereby facilitates the rate-limiting O₂-dissociation reaction. The results could assist the design of catalytic processes that are more energy efficient and robust than current processes.

Lessons from nature about solar light harvesting

Abstract: Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules; this electronic excitation is subsequently transferred to a suitable acceptor. For example, in photosynthesis, antenna complexes capture sunlight and direct the energy to reaction centres that then carry out the associated chemistry. In this Review, we describe the principles learned from studies of various natural antenna complexes and suggest how to elucidate strategies for designing light-harvesting systems. We envisage that such systems will be used for solar fuel production, to direct and regulate excitation energy flow using molecular organizations that facilitate feedback and control, or to transfer excitons over long distances. Also described are the notable properties of light-harvesting chromophores, spatial-energetic landscapes, the roles of excitonic states and quantum coherence, as well as how antennas are regulated and photoprotected.

Dynamic DNA nanotechnology using strand-displacement reactions

Abstract: The programmable and reliable hybridization of DNA strands has enabled the preparation of a wide variety of structures. This Review discusses how, in addition to these static assemblies, the process of displacing — and ultimately replacing — strands also makes possible the construction of dynamic systems such as logic gates or autonomous walkers.

Rapid room-temperature synthesis of nanocrystalline spinels as oxygen reduction and evolution electrocatalysts

Abstract: Spinels can serve as alternative low-cost bifunctional electrocatalysts for oxygen reduction/evolution reactions (ORR/OER), which are the key barriers in various electrochemical devices such as metal-air batteries, fuel cells and electrolysers. However, conventional ceramic synthesis of crystalline spinels requires an elevated temperature, complicated procedures and prolonged heating time, and the resulting product exhibits limited electrocatalytic performance. It has been challenging to develop energy-saving, facile and rapid synthetic methodologies for highly active spinels. In this Article, we report the synthesis of nanocrystalline M(x)Mn(3-x)O(4) (M = divalent metals) spinels under ambient conditions and their electrocatalytic application. We show rapid and selective formation of tetragonal or cubic M(x)Mn(3-x)O(4) from the reduction of amorphous MnO(2) in aqueous M(2+) solution. The prepared Co(x)Mn(3-x)O(4) nanoparticles manifest considerable catalytic activity towards the ORR/OER as a result of their high surface areas and abundant defects. The newly discovered phase-dependent electrocatalytic ORR/OER characteristics of Co-Mn-O spinels are also interpreted by experiment and first-principle theoretical studies.

Activating efficient phosphorescence from purely organic materials by crystal design

Abstract: Phosphorescence is among the many functional features that, in practice, divide pure organic compounds from organometallics and inorganics. Considered to be practically non-phosphorescent, purely organic compounds (metal-free) are very rarely explored as emitters in phosphor applications, despite the emerging demand in this field. To defy this paradigm, we describe novel design principles to create purely organic materials demonstrating phosphorescence that can be turned on by incorporating halogen bonding into their crystals. By designing chromophores to contain triplet-producing aromatic aldehydes and triplet-promoting bromine, crystal-state halogen bonding can be made to direct the heavy atom effect to produce surprisingly efficient solid-state phosphorescence. When this chromophore is diluted into the crystal of a bi-halogenated, non-carbonyl analogue, ambient phosphorescent quantum yields reach 55%. Here, using this design, a series of pure organic phosphors are colour-tuned to emit blue, green, yellow and orange. From this initial discovery, a directed heavy atom design principle is demonstrated that will allow for the development of bright and practical purely organic phosphors.

The effect of gold loading and particle size on photocatalytic hydrogen production from ethanol over Au/TiO2 nanoparticles

Abstract: Catalytic hydrogen production from renewables is a promising method for providing energy carriers in the near future. Photocatalysts capable of promoting this reaction are often composed of noble metal nanoparticles deposited on a semiconductor. The most promising semiconductor at present is TiO₂. The successful design of these catalysts relies on a thorough understanding of the role of the noble metal particle size and the TiO₂ polymorph. Here we demonstrate that Au particles in the size range 3-30 nm on TiO₂ are very active in hydrogen production from ethanol. It was found that Au particles of similar size on anatase nanoparticles delivered a rate two orders of magnitude higher than that recorded for Au on rutile nanoparticles. Surprisingly, it was also found that Au particle size does not affect the photoreaction rate over the 3-12 nm range. The high hydrogen yield observed makes these catalysts promising materials for solar conversion.

Strong exchange and magnetic blocking in N₂³⁻-radical-bridged lanthanide complexes.

Abstract: Single-molecule magnets approach the ultimate size limit for spin-based devices. These complexes can retain spin information over long periods of time at low temperature, suggesting possible applications in high-density information storage, quantum computing and spintronics. Notably, the success of most such applications hinges upon raising the inherent molecular spin-inversion barrier. Although recent advances have shown the viability of lanthanide-containing complexes in generating large barriers, weak or non-existent magnetic exchange coupling allows fast relaxation pathways that mitigate the full potential of these species. Here, we show that the diffuse spin of an N(2)(3-) radical bridge can lead to exceptionally strong magnetic exchange in dinuclear Ln(III) (Ln = Gd, Dy) complexes. The Gd(III) congener exhibits the strongest magnetic coupling yet observed for that ion, while incorporation of the high-anisotropy Dy(III) ion gives rise to a molecule with a record magnetic blocking temperature of 8.3 K at a sweep rate of 0.08 T s(-1).

Strained endotaxial nanostructures with high thermoelectric figure of merit

Abstract: Thermoelectric materials can directly generate electrical power from waste heat but the challenge is in designing efficient, stable and inexpensive systems. Nanostructuring in bulk materials dramatically reduces the thermal conductivity but simultaneously increases the charge carrier scattering, which has a detrimental effect on the carrier mobility. We have experimentally achieved concurrent phonon blocking and charge transmitting via the endotaxial placement of nanocrystals in a thermoelectric material host. Endotaxially arranged SrTe nanocrystals at concentrations as low as 2% were incorporated in a PbTe matrix doped with Na(2)Te. This effectively inhibits the heat flow in the system but does not affect the hole mobility, allowing a large power factor to be achieved. The crystallographic alignment of SrTe and PbTe lattices decouples phonon and electron transport and this allows the system to reach a thermoelectric figure of merit of 1.7 at ~800 K.

Contemporary strategies for peptide macrocyclization

Abstract: Peptide macrocycles have found applications that range from drug discovery to nanomaterials. These ring-shaped molecules have shown remarkable capacity for functional fine-tuning. Such capacity is enabled by the possibility of adjusting the peptide conformation using the techniques of chemical synthesis. Cyclic peptides have been difficult, and often impossible, to prepare using traditional synthetic methods. For macrocyclization to occur, the activated peptide must adopt an entropically disfavoured pre-cyclization conformation before forming the desired product. Here, we review recent solutions to some of the major challenges in this important area of contemporary synthesis.

Carbon oxidation state as a metric for describing the chemistry of atmospheric organic aerosol

Abstract: A detailed understanding of the sources, transformations, and fates of organic species in the environment is crucial because of the central roles that organics play in human health, biogeochemical cycles, and Earth's climate. However, such an understanding is hindered by the immense chemical complexity of environmental mixtures of organics; for example, atmospheric organic aerosol consists of at least thousands of individual compounds, all of which likely evolve chemically over their atmospheric lifetimes. Here we demonstrate the utility of describing organic aerosol (and other complex organic mixtures) in terms of average carbon oxidation state (OSC), a quantity that always increases with oxidation, and is readily measured using state-of-the-art analytical techniques. Field and laboratory measurements of OSC , using several such techniques, constrain the chemical properties of the organics and demonstrate that the formation and evolution of organic aerosol involves simultaneous changes to both carbon oxidation state and carbon number (nC).

Efficient water oxidation catalysts based on readily available iron coordination complexes

Abstract: One of the bottlenecks for the development of sustainable artificial photosynthesis is the water oxidation reaction, which too often relies on expensive and toxic metals. Now, coordination complexes of readily available, environmentally benign iron are found to catalyse homogeneous water oxidation to O2 with high efficiency.

Macroscopic self-assembly through molecular recognition

Abstract: Molecular recognition plays an important role in nature, with perhaps the best known example being the complementarity exhibited by pairs of nucleobases in DNA. Studies of self-assembling and self-organizing systems based on molecular recognition are often performed at the molecular level, however, and any macroscopic implications of these processes are usually far removed from the specific molecular interactions. Here, we demonstrate that well-defined molecular-recognition events can be used to direct the assembly of macroscopic objects into larger aggregated structures. Acrylamide-based gels functionalized with either host (cyclodextrin) rings or small hydrocarbon-group guest moieties were synthesized. Pieces of host and guest gels are shown to adhere to one another through the mutual molecular recognition of the cyclodextrins and hydrocarbon groups on their surfaces. By changing the size and shape of the host and guest units, different gels can be selectively assembled and sorted into distinct macroscopic structures that are on the order of millimetres to centimetres in size.

Cytocompatible click-based hydrogels with dynamically tunable properties through orthogonal photoconjugation and photocleavage reactions

Abstract: To provide insight into how cells receive information from their external surroundings, synthetic hydrogels have emerged as systems for assaying cell function in well-defined microenvironments where single cues can be introduced and subsequent effects individually elucidated. However, as answers to more complex biological questions continue to be sought, advanced material systems are needed that allow dynamic alteration of the three-dimensional cellular environment with orthogonal reactions that enable multiple levels of control of biochemical and biomechanical signals. Here, we seek to synthesize one such three-dimensional culture system using cytocompatible and wavelength-specific photochemical reactions to create hydrogels that allow orthogonal and dynamic control of material properties through independent spatiotemporally regulated photocleavage of crosslinks and photoconjugation of pendant functionalities. The results demonstrate the versatile nature of the chemistry to create programmable niches to study and direct cell function by modifying the local hydrogel environment.

Using personal glucose meters and functional DNA sensors to quantify a variety of analytical targets

Abstract: Portable sensors for the rapid quantitation of a variety of analytical targets could revolutionize both medical diagnostics and environmental monitoring. Here, functional DNA sensors that release the enzyme invertase in response to an analyte of choice are described. The enzyme converts sucrose to glucose which can then be easily detected using a widely available personal glucose meter.

Covalent bulk functionalization of graphene

Abstract: Efforts to make graphene more useful for applications include altering its bandgap and increasing its processability. Both of these can be solved by chemically modifying the material, and now a wet chemical method has been developed that functionalizes graphene in bulk starting from pristine graphite.

A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of dinitrogen into ammonia

Abstract: Nitrogen fixing is an extremely energy-consuming industrial process so there is much effort underway to develop better catalytic methods. Now, a dimolybdenum–dinitrogen complex bearing a PNP pincer ligand has been found to work as an effective catalyst for the formation of ammonia from dinitrogen.

High-efficiency dye-sensitized solar cells with ferrocene-based electrolytes.

Abstract: Dye-sensitized solar cells based on iodide/triiodide (I(-)/I(3)(-)) electrolytes are viable low-cost alternatives to conventional silicon solar cells. However, as well as providing record efficiencies of up to 12.0%, the use of I(-)/I(3)(-) in such solar cells also brings about certain limitations that stem from its corrosive nature and complex two-electron redox chemistry. Alternative redox mediators have been investigated, but these generally fall well short of matching the performance of conventional I(-)/I(3)(-) electrolytes. Here, we report energy conversion efficiencies of 7.5% (simulated sunlight, AM1.5, 1,000 W m(-2)) for dye-sensitized solar cells combining the archetypal ferrocene/ferrocenium (Fc/Fc(+)) single-electron redox couple with a novel metal-free organic donor-acceptor sensitizer (Carbz-PAHTDTT). These Fc/Fc(+)-based devices exceed the efficiency achieved for devices prepared using I(-)/I(3)(-) electrolytes under comparable conditions, revealing the great potential of ferrocene-based electrolytes in future dye-sensitized solar cells applications. This improvement results from a more favourable matching of the redox potential of the ferrocene couple with that of the new donor-acceptor sensitizer.

Multi-hierarchical self-assembly of a collagen mimetic peptide from triple helix to nanofibre and hydrogel.

Abstract: Replicating the multi-hierarchical self-assembly of collagen has long-attracted scientists, from both the perspective of the fundamental science of supramolecular chemistry and that of potential biomedical applications in tissue engineering. Many approaches to drive the self-assembly of synthetic systems through the same steps as those of natural collagen (peptide chain to triple helix to nanofibres and, finally, to a hydrogel) are partially successful, but none simultaneously demonstrate all the levels of structural assembly. Here we describe a peptide that replicates the self-assembly of collagen through each of these steps. The peptide features collagen's characteristic proline-hydroxyproline-glycine repeating unit, complemented by designed salt-bridged hydrogen bonds between lysine and aspartate to stabilize the triple helix in a sticky-ended assembly. This assembly is propagated into nanofibres with characteristic triple helical packing and lengths with a lower bound of several hundred nanometres. These nanofibres form a hydrogel that is degraded by collagenase at a similar rate to that of natural collagen.

Efficient hydrogenation of organic carbonates, carbamates and formates indicates alternative routes to methanol based on CO2 and CO

Abstract: Catalytic hydrogenation of organic carbonates, carbamates and formates is of significant interest both conceptually and practically, because these compounds can be produced from CO2 and CO, and their mild hydrogenation can provide alternative, mild approaches to the indirect hydrogenation of CO2 and CO to methanol, an important fuel and synthetic building block. Here, we report for the first time catalytic hydrogenation of organic carbonates to alcohols, and carbamates to alcohols and amines. Unprecedented homogeneously catalysed hydrogenation of organic formates to methanol has also been accomplished. The reactions are efficiently catalysed by dearomatized PNN Ru(II) pincer complexes derived from pyridine- and bipyridine-based tridentate ligands. These atom-economical reactions proceed under neutral, homogeneous conditions, at mild temperatures and under mild hydrogen pressures, and can operate in the absence of solvent with no generation of waste, representing the ultimate 'green' reactions. A possible mechanism involves metal-ligand cooperation by aromatization-dearomatization of the heteroaromatic pincer core.

Facile removal of stabilizer-ligands from supported gold nanoparticles

Abstract: Metal nanoparticles that comprise a few hundred to several thousand atoms have many applications in areas such as photonics, sensing, medicine and catalysis. Colloidal methods have proven particularly suitable for producing small nanoparticles with controlled morphologies and excellent catalytic properties. Ligands are necessary to stabilize nanoparticles during synthesis, but once the particles have been deposited on a substrate the presence of the ligands is detrimental for catalytic activity. Previous methods for ligand removal have typically involved thermal and oxidative treatments, which can affect the size or morphology of the particles, in turn altering their catalytic activity. Here, we report a procedure to effectively remove the ligands without affecting particle morphology, which enhances the surface exposure of the nanoparticles and their catalytic activity over a range of reactions. This may lead to developments of nanoparticles prepared by colloidal methods for applications in fields such as environmental protection and energy production.

Biodegradable nanostructures with selective lysis of microbial membranes

Abstract: Macromolecular antimicrobial agents such as cationic polymers and peptides have recently been under an increased level of scrutiny because they can combat multi-drug-resistant microbes. Most of these polymers are non-biodegradable and are designed to mimic the facially amphiphilic structure of peptides so that they may form a secondary structure on interaction with negatively charged microbial membranes. The resulting secondary structure can insert into and disintegrate the cell membrane after recruiting additional polymer molecules. Here, we report the first biodegradable and in vivo applicable antimicrobial polymer nanoparticles synthesized by metal-free organocatalytic ring-opening polymerization of functional cyclic carbonate. We demonstrate that the nanoparticles disrupt microbial walls/membranes selectively and efficiently, thus inhibiting the growth of Gram-positive bacteria, methicillin-resistant Staphylococcus aureus (MRSA) and fungi, without inducing significant haemolysis over a wide range of concentrations. These biodegradable nanoparticles, which can be synthesized in large quantities and at low cost, are promising as antimicrobial drugs, and can be used to treat various infectious diseases such as MRSA-associated infections, which are often linked with high mortality.

Crystalline molecular flasks

Abstract: Crystalline networks containing empty cavities can host a variety of molecules but also promote reactions between guests. Through robust crystallinity and a pseudo-solution state (dynamic movements) within their pores, these crystalline molecular flasks enable the direct observation of species — including unstable intermediates — during a reaction by in situ X-ray diffraction.

Synthetic organic spin chemistry for structurally well-defined open-shell graphene fragments

Abstract: Graphene, a two-dimensional layer of sp(2)-hybridized carbon atoms, can be viewed as a sheet of benzene rings fused together. Three benzene rings can be combined in three different ways, to yield linear anthracene and angular phenanthrene, where the rings share two C-C bonds, and the phenalenyl structure where three C-C bonds are shared between the rings. This third structure contains an uneven number of carbon atoms and, hence, in its neutral state, an uneven number of electrons--that is, it is a radical. All three structures may be viewed as being sections of graphene. Extension of this concept leads to an entire family of phenalenyl derivatives--'open-shell graphene fragments'--that are of substantial interest from the standpoint of fundamental science as well as in view of their potential applications in materials chemistry, in particular quantum electronic devices. Here we discuss current trends and challenges in this field.

Self-reproduction of supramolecular giant vesicles combined with the amplification of encapsulated DNA.

Abstract: The self-replication process of a giant vesicle encapsulating double-stranded DNA has been observed, which represents a supramolecular approach to the construction of a protocell. Growth and division of the vesicle occurred rapidly on addition of a membrane precursor, and amplified DNA was distributed amongst the resulting daughter giant vesicles.

Water-oxidation catalysis by manganese in a geochemical-like cycle

Abstract: Water oxidation in all oxygenic photosynthetic organisms is catalysed by the Mn₄CaO₄ cluster of Photosystem II. This cluster has inspired the development of synthetic manganese catalysts for solar energy production. A photoelectrochemical device, made by impregnating a synthetic tetranuclear-manganese cluster into a Nafion matrix, has been shown to achieve efficient water oxidation catalysis. We report here in situ X-ray absorption spectroscopy and transmission electron microscopy studies that demonstrate that this cluster dissociates into Mn(II) compounds in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. Cycling between the photoreduced product and this mineral-like solid is responsible for the observed photochemical water-oxidation catalysis. The original manganese cluster serves only as a precursor to the catalytically active material. The behaviour of Mn in Nafion therefore parallels its broader biogeochemistry, which is also dominated by cycles of oxidation into solid Mn(III/IV) oxides followed by photoreduction to Mn²⁺.

Interfacial synthesis of hollow metal–organic framework capsules demonstrating selective permeability

Abstract: Metal–organic frameworks (MOFs) are a class of crystalline materials that consist of metal ions and organic ligands linked together by coordination bonds. Because of their porosity and the possibility of combining large surface areas with pore characteristics that can be tailored, these solids show great promise for a wide range of applications. Although most applications currently under investigation are based on powdered solids, developing synthetic methods to prepare defectfree MOF layers will also enable applications based on selective permeation. Here, we demonstrate how the intrinsically hybrid nature of MOFs enables the self-completing growth of thin MOF layers. Moreover, these layers can be shaped as hollow capsules that demonstrate selective permeability directly related to the micropore size of the MOF crystallites forming the capsule wall. Such capsules effectively entrap guest species, and, in the future, could be applied in the development of selective microreactors containing molecular catalysts.

Light-induced spin-crossover magnet

Abstract: The light-induced phase transition between the low-spin (LS) and high-spin (HS) states of some transition-metal ions has been extensively studied in the fields of chemistry and materials science. In a crystalline extended system, magnetically ordering the HS sites of such transition-metal ions by irradiation should lead to spontaneous magnetization. Previous examples of light-induced ordering have typically occurred by means of an intermetallic charge transfer mechanism, inducing a change of valence of the metal centres. Here, we describe the long-range magnetic ordering of the extended Fe(II)(HS) sites in a metal-organic framework caused instead by a light-induced excited spin-state trapping effect. The Fe-Nb-based material behaves as a spin-crossover magnet, in which a strong superexchange interaction (magnetic coupling through non-magnetic elements) between photo-produced Fe(II)(HS) and neighbouring Nb(IV) atoms operates through CN bridges. The magnetic phase transition is observed at 20 K with a coercive field of 240 Oe.

Nanocrystal bilayer for tandem catalysis

Abstract: Supported catalysts are widely used in industry and can be optimized by tuning the composition and interface of the metal nanoparticles and oxide supports. Rational design of metal–metal oxide interfaces in nanostructured catalysts is critical to achieve better reaction activities and selectivities. We introduce here a new class of nanocrystal tandem catalysts that have multiple metal–metal oxide interfaces for the catalysis of sequential reactions. We utilized a nanocrystal bilayer structure formed by assembling platinum and cerium oxide nanocube monolayers of less than 10 nm on a silica substrate. The two distinct metal–metal oxide interfaces, CeO2–Pt and Pt–SiO2, can be used to catalyse two distinct sequential reactions. The CeO2 –Pt interface catalysed methanol decomposition to produce CO and H 2 , which were subsequently used for ethylene hydroformylation catalysed by the nearby Pt–SiO2 interface. Consequently, propanal was produced selectively from methanol and ethylene on the nanocrystal bilayer tandem catalyst. This new concept of nanocrystal tandem catalysis represents a powerful approach towards designing high-performance, multifunctional nanostructured catalysts.