Single-Site vs. Cluster Catalysis in High Temperature Oxidations.

The behavior of single Pt atoms and small Pt clusters was investigated for high-temperature oxidations. The high stability of these molecular sites in CHA is a key to intrinsic structure-performance descriptions of elemental steps such as O2 dissociation, and subsequent oxidation catalysis. Subtle changes in the atomic structure of Pt are responsible for drastic changes in performance driven by specific gas/metal/support interactions. Whereas single Pt atoms and Pt clusters (> ca. 1 nm) are unable to activate, scramble, and desorb two O2 molecules at moderate T (200 °C), clusters <1 nm do so catalytically, but undergo oxidative fragmentation. Oxidation of alkanes at high T is attributed to stable single Pt atoms, and the C-H cleavage is inferred to be rate-determining and less sensitive to changes in metal nuclearity compared to its effect on O2 scrambling. In contrast, when combustion involves CO, catalysis is dominated by metal clusters, not single Pt atoms.

Continuous Partial Oxidation of Methane to Methanol Catalyzed by Diffusion-Paired Copper Dimers in Copper-Exchanged Zeolites.

Copper-exchanged zeolites can continuously and selectively catalyze the partial oxidation of methane to methanol using only oxygen and water at low temperatures, but the genesis and nature of the active sites are currently unknown. Herein, we demonstrate that this reaction is catalyzed by a [Cu-O-Cu]2+ motif that forms via a hypothesized proton-aided diffusion of hydrated Cu ions within the cages of SSZ-13 zeolites. While various Cu configurations may be present and active for methane oxidation, a dimeric Cu motif is the primary active site for selective partial methane oxidation. Mechanistically, CH4 activation proceeds via rate-determining C-H scission to form a surface-bound C1 intermediate that can either be desorbed as methanol in the presence of H2O/H+ or completely oxidized to CO2 by gas-phase O2. High partial oxidation selectivity can be obtained with (i) high methane and water partial pressures and (ii) maximizing Cu dimer formation by using zeolites with high Al content and low Cu loadings.

Reversible Transformation of Pt Nanoparticles into Single Atoms inside High-Silica Chabazite Zeolite.

We report the encapsulation of platinum species in highly siliceous chabazite (CHA) crystallized in the presence of N,N,N-trimethyl-1-adamantammonium and a thiol-stabilized Pt complex. When compared to Pt/SiO2 or Pt-containing Al-rich zeolites, the materials in this work show enhanced stability toward metal sintering in a variety of industrial conditions, including H2, O2, and H2O. Remarkably, temperatures in the range 650-750 °C can be reached without significant sintering of the noble metal. Detailed structural determinations by X-ray absorption spectroscopy and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy demonstrate subtle control of the supported metal structures from ∼1 nm nanoparticles to site-isolated single Pt atoms via reversible interconversion of one species into another in reducing and oxidizing atmospheres. The combined used of microscopy and spectroscopy is critical to understand these surface-mediated transformations. When tested in hydrogenation reactions, Pt/CHA converts ethylene (∼80%) but not propylene under identical conditions, in contrast to Pt/SiO2, which converts both at similar rates. These differences are attributed to the negligible diffusivity of propylene through the small-pore zeolite and provide final evidence of the metal encapsulation.

Molecular metal catalysts on supports: organometallic chemistry meets surface science.

Recent advances in the synthesis and characterization of small, essentially molecular metal complexes and metal clusters on support surfaces have brought new insights to catalysis and point the way to systematic catalyst design. We summarize recent work unraveling effects of key design variables of site-isolated catalysts: the metal, metal nuclearity, support, and other ligands on the metals, also considering catalysts with separate, complementary functions on supports. The catalysts were synthesized with the goal of structural simplicity and uniformity to facilitate incisive characterization. Thus, they are essentially molecular species bonded to porous supports chosen for their high degree of uniformity; the supports are crystalline aluminosilicates (zeolites) and MgO. The catalytic species are synthesized in reactions of organometallic precursors with the support surfaces; the precursors include M(L)2(acetylacetonate)1-2, with M = Ru, Rh, Ir, or Au and the ligands L = C2H4, CO, or CH3. Os3(CO)12 and Ir4(CO)12 are used as precursors of supported metal clusters, and some such catalysts are made by ship-in-a-bottle syntheses to trap the clusters in zeolite cages. The simplicity and uniformity of the supported catalysts facilitate precise structure determinations, even in reactive atmospheres and during catalysis. The methods of characterizing catalysts in reactive atmospheres include infrared (IR), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and nuclear magnetic resonance (NMR) spectroscopies, and complementary methods include density functional theory and atomic-resolution aberration-corrected scanning transmission electron microscopy for imaging of individual metal atoms. IR, NMR, XANES, and microscopy data demonstrate the high degrees of uniformity of well-prepared supported species. The characterizations determine the compositions of surface metal complexes and clusters, including the ligands and the metal-support bonding and structure, which identify the supports as ligands with electron-donor properties that influence reactivity and catalysis. Each of the catalyst design variables has been varied independently, illustrated by mononuclear and tetranuclear iridium on zeolite HY and on MgO and by isostructural rhodium and iridium (diethylene or dicarbonyl) complexes on these supports. The data provide examples resolving the roles of the catalyst design variables and place the catalysis science on a firm foundation of organometallic chemistry linked with surface science. Supported molecular catalysts offer the advantages of characterization in the absence of solvents and with surface-science methods that do not require ultrahigh vacuum. Families of supported metal complexes have been made by replacement of ligands with others from the gas phase. Spectroscopically identified catalytic reaction intermediates help to elucidate catalyst performance and guide design. The methods are illustrated for supported complexes and clusters of rhodium, iridium, osmium, and gold used to catalyze reactions of small molecules that facilitate identification of the ligands present during catalysis: alkene dimerization and hydrogenation, H-D exchange in the reaction of H2 with D2, and CO oxidation. The approach is illustrated with the discovery of a highly active and selective MgO-supported rhodium carbonyl dimer catalyst for hydrogenation of 1,3-butadiene to give butenes.

Formation of supported rhodium clusters from mononuclear rhodium complexes controlled by the support and ligands on rhodium.

Extremely small supported rhodium clusters were prepared from rhodium complexes on the surfaces of solids with contrasting electron-donor properties. The samples were characterized by infrared and extended X-ray absorption fine structure spectroscopies to determine the changes occurring in the rhodium species resulting from treatments in hydrogen. Rhodium cluster formation occurred in the presence of H2, and the first steps are controlled by the electron-donor properties of the support--which acts as a ligand--and the other ligands bonded to the rhodium. The cluster formation begins at a lower temperature when the support is zeolite HY than when it is the better electron-donor MgO, provided that the other ligands on rhodium are ethene. In contrast, when these other ligands are CO, the pattern is reversed. The choice of ligands including the support also allows regulation of the stoichiometry of the surface transformations in H2 and the stability of the structures formed in the presence of other reactants. The combination of MgO as the support and ethene as a ligand allows restriction of the rhodium cluster size to the smallest possible-and these were formed in high yields. The data presented here are among the first characterizing the first steps of metal cluster formation.

Selective reductive coupling of nitro compounds with aldehydes to nitrones in H2 using carbon-supported and -decorated platinum nanoparticles.

Nitrones were synthesized in high yields directly from nitro compounds, aldehydes, and H2 using carbon-decorated platinum nanoparticles. The high selectivity for nitrone synthesis contrasts that of common supported metal catalysts and corresponds to an increase from roughly 6 to 97%. The catalytic performance is tuned by precise control of the structure of the active sites and the characteristics of the support.

Investigation of AFeO3 (A=Ba, Sr) Perovskites for the Oxidative Dehydrogenation of Light Alkanes under Chemical Looping Conditions.

Oxidative dehydrogenation (ODH) of light alkanes to produce C2-C3 olefins is a promising alternative to conventional steam cracking. Perovskite oxides are emerging as efficient catalysts for this process due to their unique properties such as high oxygen storage capacity (OSC), reversible redox behavior, and tunability. Here, we explore AFeO3 (A=Ba, Sr) bulk perovskites for the ODH of ethane and propane under chemical looping conditions (CL-ODH). The higher OSC and oxygen mobility of SrFeO3 perovskite contributed to its higher activity but lower olefin selectivity than its Ba counterpart. However, SrFeO3 perovskite is superior in terms of cyclic stability over multiple redox cycles. Transformations of the perovskite to reduced phases including brownmillerite A2Fe2O5 were identified by X-ray diffraction (XRD) as a cause of performance degradation, which was fully reversible upon air regeneration. A pre-desorption step was utilized to selectively tune the amount of lattice oxygen as a function of temperature and dwell time to enhance olefin selectivity while suppressing CO2 formation from the deep oxidation of propane. Overall, SrFeO3 exhibits promising potential for the CL-ODH of light alkanes, and optimization through surface and structural modifications may further engineer well-regulated lattice oxygen for maximizing olefin yield.

Prediction of Aggregate Packing with Tubular Macrocapsules in the Inert Structure of Self-Healing Concrete Based on Dewar's Particle Packing Model.

This paper brings a new insight into understanding the influence of macrocapsules in packing systems, which can be useful in designing the inert structure of self-healing concrete. A variety of tubular macrocapsules, in terms of types and sizes, was used to assess the capsules' effect in the packing, together with various aggregate types and fractions. The voids ratios (U) of aggregate mixtures were evaluated experimentally and compared with the prediction via the particle packing model of Dewar. The packing of coarse particles was found to be considerably affected by the presence of macrocapsules, while no capsules' effect on the packing of fine particles was attained. A higher capsule dosage and capsule aspect ratio led to a higher voids ratio. In the formulation of the inert structure, the packing disturbance due to capsules can be minimised by increasing the content of fine aggregates over coarse aggregates. Dewar's model showed a good compatibility with experimental results in the absence of capsules. However, the model needed to be upgraded for the introduction of tubular macrocapsules. Accordingly, the effect of macrocapsules was extensively analysed and a 'U model' for capsules (with some limitations) was finally proposed, offering a high predicting accuracy.

Surface-mediated synthesis of dimeric rhodium catalysts on MgO: tracking changes in the nuclearity and ligand environment of the catalytically active sites by X-ray absorption and infrared spectroscopies.

The preparation of dinuclear rhodium clusters and their use as catalysts is challenging because these clusters are unstable, evolving readily into species with higher nuclearities. We now present a novel synthetic route to generate rhodium dimers on the surface of MgO by a stoichiometrically simple surface-mediated reaction involving [Rh(C(2)H(4))(2)] species and H(2). X-ray absorption and IR spectra were used to characterize the changes in the nuclearity of the essentially molecular surface species as they formed, including the ligands on the rhodium and the metal-support interactions. The support plays a key role in stabilizing the dinuclear rhodium species, allowing the incorporation of small ligands (ethyl, hydride, and/or CO) and enabling a characterization of the catalytic performance of the supported species for the hydrogenation of ethylene as a function of the metal nuclearity and ligand environment. A change in the nuclearity from one to two Rh atoms leads to a 58-fold increase in the catalytic activity for ethylene hydrogenation, a reaction involving unsaturated, but stable, dimeric rhodium species.

Preliminary Study of the Fresh and Hard Properties of UHPC That Is Used to Produce 3D Printed Mortar.

Three-dimensional printed concrete (3DPC) is a relatively recent technology that may be very important in changing the traditional construction industry. The principal advantages of its use are more rapid construction, lower production costs, and less residues, among others. The choice of raw materials to obtain adequate behavior is more critical than for traditional concrete. In the present paper a mixture of cement, silica fume, superplasticizer, setting accelerator, filler materials, and aggregates was studied to obtain a 3DPC with high resistance at short curing times. When the optimal mixture was found, metallic fibers were introduced to enhance the mechanical properties. The fresh and hard properties of the concrete were analyzed, measuring the setting time, workability, and flexural and compressive strength. The results obtained demonstrated that the incorporation of fibers (2% in volume) enhanced the flexural and compressive strength by around 163 and 142%, respectively, compared with the mixture without fibers, at 9 h of curing. At 28 days of curing, the improvement was 79.2 and 34.7% for flexural and compressive strength, respectively.

Zeolite-supported rhodium complexes and clusters: switching catalytic selectivity by controlling structures of essentially molecular species.

Precise synthesis and characterization of site-isolated rhodium complexes and extremely small rhodium clusters supported on zeolite HY allow control of the catalyst selectivity in the conversion of ethene to n-butene or ethane, respectively, as a result of tuning the structure of the active sites at a molecular level.

Supported molecular iridium catalysts: resolving effects of metal nuclearity and supports as ligands.

The performance of a supported catalyst is influenced by the size and structure of the metal species, the ligands bonded to the metal, and the support. Resolution of these effects has been lacking because of the lack of investigations of catalysts with uniform and systematically varied catalytic sites. We now demonstrate that the performance for ethene hydrogenation of isostructural iridium species on supports with contrasting properties as ligands (electron-donating MgO and electron-withdrawing HY zeolite) can be elucidated on the basis of molecular concepts. Spectra of the working catalysts show that the catalytic reaction rate is determined by the dissociation of H(2) when the iridium, either as mono- or tetra-nuclear species, is supported on MgO and is not when the support is the zeolite. The neighboring iridium sites in clusters are crucial for activation of both H(2) and C(2)H(4) when the support is MgO but not when it is the zeolite, because the electron-withdrawing properties of the zeolite support enable even single site-isolated Ir atoms to bond to both C(2)H(4) and H(2) and facilitate the catalysis.

A Study of the Flexural Behavior of Fiber-Reinforced Concretes Exposed to Moderate Temperatures.

The use of synthetic fibers in fiber-reinforced concretes (FRCs) is often avoided due to the mistrust of lower performance at changing temperatures. This work examines the effect of moderate temperatures on the flexural strengths of FRCs. Two types of polypropylene fibers were tested, and one steel fiber was employed as a reference. Three-point bending tests were carried out following an adapted methodology based on the standard EN 14651. This adapted procedure included an insulation system that allowed the assessment of FRC flexural behavior after being exposed for two months at temperatures of 5, 20, 35 and 50 °C. In addition, the interaction of temperature with a pre-cracked state was also analyzed. To do this, several specimens were pre-cracked to 0.5 mm after 28 days and conditioned in their respective temperature until testing. The findings suggest that this range of moderate temperatures did not degrade the behavior of FRCs to a great extent since the analysis of variances showed that temperature is not always a significant factor; however, it did have an influence on the pre-cracked specimens at 35 and 50 °C.

Characterization of Glass Powder from Glass Recycling Process Waste and Preliminary Testing.

This work studies the possibility of incorporating different proportions of glass powder from the waste glass (rejected material called fine cullet) produced during the glass recycling process into the manufacturing of mortar and concrete. For this purpose, the material is characterized by its chemical composition and pozzolanic activity, and the shape and size of its particles are studied. It is then incorporated as a substitute for cement into the manufacturing of mortar and concrete at 25% and 40% of cement weight, and its effect on setting times, consistency, and mechanical strength is analyzed. Its behavior as a slow pozzolan is verified, and the possibility of incorporating it into concrete is ratified by reducing its cement content and making it a more sustainable material.

Chemoselective synthesis of substituted imines, secondary amines, and beta-amino carbonyl compounds from nitroaromatics through cascade reactions on gold catalysts.

Substituted imines, alpha,beta-unsaturated imines, substituted secondary amines, and beta-amino carbonyl compounds have been synthesized by means of new cascade reactions with mono- or bifunctional gold-based solid catalysts under mild reaction conditions. The related synthetic route involves the hydrogenation of a nitroaromatic compound in the presence of a second reactant such as an aldehyde, alpha,beta-unsaturated carbonyl compound, or alkyne, which circumvents an ex situ reduction process for producing the aromatic amine. The process is shown to be highly selective towards other competing groups, such as double bonds, carbonyls, halogens, nitriles, or cinnamates, and thereby allows the synthesis of different substituted nitrogenated compounds. For the preparation of imines, substituted anilines are formed and condensed in situ with aldehydes to provide the final product through two tandem reactions. High chemoselectivity is observed, for instance, when double bonds or halides are present within the reactants. In addition, we show that the Au/TiO2 system is also able to catalyze the chemoselective hydrogenation of imines, so that secondary amines can be prepared directly through a three-step cascade reaction by starting from nitroaromatic compounds and aldehydes. On the other hand, Au/TiO2 can also be used as a bifunctional catalyst to obtain substituted beta-amino carbonyl compounds from nitroaromatics and alpha,beta-unsaturated carbonyl compounds. Whereas gold sites promote the in situ formation of anilines, the intrinsic acidity of Ti species on the support surface accelerates the subsequent Michael addition. Finally, two gold-catalyzed reactions, that is, the hydrogenation of nitro groups and a hydroamination, have been coupled to synthesize additional substituted imines from nitroaromatic compounds and alkynes.

Transforming nonselective into chemoselective metal catalysts for the hydrogenation of substituted nitroaromatics.

It is generally accepted that good hydrogenation noble and nonnoble metal catalysts such as Pt, Ru, or Ni are not chemoselective for hydrogenation of nitro groups in substituted aromatic molecules. We have found that it is possible to transform nonchemoselective into highly chemoselective metal catalysts by controlling the coordination of metal surface atoms while introducing a cooperative effect between the metal and a properly selected support. Thus, highly chemoselective and general hydrogenation Pt, Ru, and Ni catalysts can be prepared by generating nanosized crystals of the metals on the surface of a TiO 2 support and decorating the exposed (111) and (100) crystal faces by means of a simple catalyst activation procedure. By doing this, it has been possible to change the relative rate for hydrogenating competitive groups present in the molecule by almost 2 orders of magnitude, increasing the chemoselectivity from less than 1% to more than 95%.

Integrating chemists preferences for shape-similarity clustering of series.

This study shows how chemistry knowledge and reasoning are taken into account for building a new methodology that aims at automatically grouping data having a chronological structure. We consider combinatorial catalytic experiments where the evolution of a reaction (e.g., conversion) over time is expected to be analyzed. The mathematical tool has been developed to compare and group curves taking into account their shape. The strategy, which consists on combining a hierarchical clustering with the k-means algorithm, is described and compared with both algorithms used separately. The hybridization is shown to be of great interest. Then, a second application mode of the proposed methodology is presented. Once meaningful clusters according to chemist's preferences and goals are successfully achieved, the induced model may be used in order to automatically classify new experimental results. The grouping of the new catalysts tested for the Heck coupling reaction between styrene and iodobenzene verified the set of criteria "defined" during the initial clustering step, and facilitated a quick identification of the catalytic behaviors following user's preferences.

A Molecular mechanism for the chemoselective hydrogenation of substituted nitroaromatics with nanoparticles of gold on TiO2 catalysts: a cooperative effect between gold and the support.

Nanoparticles of gold on TiO2 are highly chemoselective for the reduction of substituted nitroaromatics, such as nitrostyrene. By combining kinetics and in situ IR spectroscopy, it has been found that there is a preferential adsorption of the reactant on the catalyst through the nitro group. IR studies of nitrobenzene, styrene, and nitrostyrene adsorption, together with quantum chemical calculations, show that the nitro and the olefinic groups adsorb weakly on the Au(111) and Au(001) surfaces, and that although a stronger adsorption occurs on low-coordinated atoms in gold nanoparticles, this adsorption is not selective. On the other hand, an energetically and geometrically favored adsorption through the nitro group occurs on the TiO2 support and in the interface between the gold nanoparticle and the TiO2 support. Such preferential adsorption is not observed with nanoparticles of gold on silica which, contrary to the Au/TiO2 catalyst, is not chemoselective for the reduction of substituted nitroaromatic compounds. Therefore, the high chemoselectiviy of the Au/TiO2 catalyst can be attributed to a cooperation between the gold nanoparticle and the support that preferentially activates the nitro group.

Zeolite synthesis modelling with support vector machines: a combinatorial approach.

This work shows the application of support vector machines (SVM) for modelling and prediction of zeolite synthesis, when using the gel molar ratios as model input (synthesis descriptors). Experimental data includes the synthesis results of a multi-level factorial experimental design of the system TEA:SiO2:Na2O:Al2O3:H2O. The few parameters of the SVM model were studied and the fitting performance is compared with the ones obtained with other machine learning models such as neural networks and classification trees. SVM models show very good prediction performances and generalization capacity in zeolite synthesis prediction. They may overcome overfitting problems observed sometimes for neural networks. It is also studied the influence of the type of material descriptors used as model output.