Monitoring Supported-Nanocluster Heterogeneous Catalyst Formation: Product and Kinetic Evidence for a 2-Step, Nucleation and Autocatalytic Growth Mechanism of Pt(0)n Formation from H2PtCl6 on Al2O3 or TiO2

The Role of Hydrogen Bonds in Thermally Responsive Crystallization-Driven Template Autocatalysis

Autocatalysis has been recognized to be involved in the emergence of life and intrinsic to biomolecular replication. Recently, an efficient template autocatalysis driven by solvent-free crystallization has been reported. Herein, we unveil the role of intermolecular hydrogen bonds formed by amides in crystallization-driven template autocatalysis (CDTA), which involves the autocatalytic activity, template selectivity, and thermal responsiveness. We found that the thermal-induced cis-trans isomerization of amides possibly affects the H-bonding-mediated template ability of products for autocatalytic transformation. As a result, CDTA can be reversibly inhibited and activated by tuning the reaction temperatures. Our work sheds light on the significance of noncovalent H-bonding interactions in artificial self-replicators.

An Autocatalytic DNA Circuit Based on Hybridization Chain Assembly for Intracellular Imaging of Polynucleotide Kinase

Polynucleotide kinase (PNK) plays an essential role in various cellular events by regulating phosphorylation processes, and abnormal homeostasis of PNK could cause many human diseases. Herein, we proposed an autocatalytic hybridization system (AHS) through the elaborate integration of hybridization chain assembly (HCA) and catalytic DNA assembly (CDA) that enables a highly efficient positive feedback amplification. The PNK-targeting AHS biosensor is composed of three modules: a recognition module, an HCA amplification module, and a CDA autocatalytic module. In the presence of PNK, the recognition module could transform the PNK input into an exposed nucleic acid initiator (I). Then the initiator strand I could trigger the autonomous HCA process in the amplification module, and the resulted HCA products could reassemble the split CDA trigger strand T, subsequently inducing the CDA process in the autocatalytic module to form abundant DNA duplex products. Consequently, the embedded initiator strand I was liberated from the CDA duplex product to autonomously trigger the new rounds of HCA circuit. The rational integration and cooperative cross-activation between the HCA and CDA module could prominently accelerate the reaction and realize the exponential amplification efficiency by initiator regeneration. As a result, the self-sustainable AHS amplifier could implement the sensitive detection of PNK in vitro and in biological samples and further fulfill accurate monitoring of the intracellular PNK activity and the effective screening of PNK inhibitors. This work paves a way for exploiting highly efficient artificial DNA circuits to analyze low-abundance biomarkers, holding great potential in biochemical research and clinical diagnosis.

"Weakly Ligated, Labile Ligand" Nanoparticles: The Case of Ir(0) n ·(H+Cl-) m

It is of considerable interest to prepare weakly ligated, labile ligand (WLLL) nanoparticles for applications in areas such as chemical catalysis. WLLL nanoparticles can be defined as nanoparticles with sufficient, albeit minimal, surface ligands of moderate binding strength to meta-stabilize nanoparticles, initial stabilizer ligands that can be readily replaced by other, desired, more strongly coordinating ligands and removed completely when desired. Herein, we describe WLLL nanoparticles prepared from [Ir(1,5-COD)Cl]₂ reduction under H₂, in acetone. The results suggest that H⁺Cl^--stabilized Ir(0) _n nanoparticles, herein Ir(0) _n ·(H⁺Cl^-) _a , serve as a WLLL nanoparticle for the preparation of, as illustrative examples, five specific nanoparticle products: Ir(0) _n ·(Cl^-Bu₃NH⁺) _a , Ir(0) _n ·(Cl^-Dodec₃NH⁺) _a , Ir(0) _n ·(POct₃)_0.2n (Cl^-H⁺) _b , Ir(0) _n ·(POct₃)_0.2n , and the γ-Al₂O₃-supported heterogeneous catalyst, Ir(0) _n ·(γ-Al₂O₃) _a (Cl^-H⁺) _b . (where a and b vary for the differently ligated nanoparticles; in addition, solvent can be present as a nanoparticle surface ligand). With added POct₃ as a key, prototype example, an important feature is that a minimum, desired, experimentally determinable amount of ligand (e.g., just 0.2 equiv POct₃ per mole of Ir) can be added, which is shown to provide sufficient stabilization that the resultant Ir(0) _n ·(POct₃)_0.2n (Cl^-H⁺) _b is isolable. Additionally, the initial labile ligand stabilizer HCl can be removed to yield Ir(0) _n ·(POct₃)_0.2n that is >99% free of Cl^- by a AgCl precipitation test. The results provide strong support for the weakly ligated, labile ligand nanoparticle concept and specific support for Ir(0) _n ·(H⁺Cl^-) _a as a WLLL nanoparticle.

Study of PtOx/TiO₂ Photocatalysts in the Photocatalytic Reforming of Glycerol: The Role of Co-Catalyst Formation

In this study, relationships between preparation conditions, structure, and activity of Pt-containing TiO₂ photocatalysts in photoinduced reforming of glycerol for H₂ production were explored. Commercial Aerolyst^® TiO₂ (P25) and homemade TiO₂ prepared by precipitation-aging method were used as semiconductors. Pt co-catalysts were prepared by incipient wetness impregnation from aqueous solution of Pt(NH₃)₄(NO₃)₂ and activated by calcination, high temperature hydrogen, or nitrogen treatments. The chemico-physical and structural properties were evaluated by XRD, ¹H MAS NMR, ESR, XPS, TG-MS and TEM. The highest H₂ evolution rate was observed over P25 based samples and the H₂ treatment resulted in more active samples than the other co-catalyst formation methods. In all calcined samples, reduction of Pt occurred during the photocatalytic reaction. Platinum was more easily reducible in all of the P25 supported samples compared to those obtained from the more water-retentive homemade TiO₂. This result was related to the negative effect of the adsorbed water content of the homemade TiO₂ on Pt reduction and on particle growth during co-catalyst formation.

Gold Nanoparticle Formation Kinetics and Mechanism: A Critical Analysis of the "Redox Crystallization" Mechanism

A 2013 paper proposed a "redox crystallization" (R-C) mechanism for the formation of Au⁰ _n nanoparticles from the reduction of a AuCl₄ ^- precursor. That study used an unconventional analysis of the valuable, expertly obtained kinetics data reported, and came up with multiple claims and insights collected under the putatively new R-C mechanism. If confirmed, those claims and the R-C mechanism provide a valuable addition to the knowledge of gold nanoparticle formation kinetics and mechanisms. On the other hand, if the methodology used to support the R-C mechanism is flawed so that its resultant conclusions are incorrect, then the R-C mechanism needs to be discarded until compelling evidence for it can be gathered, evidence that would have to include the disproof of the other dominant mechanism(s) of nanoparticle formation. The present work provides a critical analysis of the evidence previously offered for the R-C mechanism, efforts that are of interest to the areas of Au⁰ _n nanoparticles, the kinetics and mechanisms of nanoparticle formation and, as it turns out, more generally to those interested in kinetic and mechanistic studies.

Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals

The growth of colloidal metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of colloidal metal nanocrystals.

Microwave Enhancement of Autocatalytic Growth of Nanometals

The desire for designing efficient synthetic methods that lead to industrially important nanomaterials has led a desire to more fully understand the mechanism of growth and how modern synthetic techniques can be employed. Microwave (MW) synthesis is one such technique that has attracted attention as a green, sustainable method. The reports of enhancement of formation rates and improved quality for MW driven reactions are intriguing, but the lack of understanding of the reaction mechanism and how coupling to the MW field leads to these observations is concerning. In this manuscript, the growth of a metal nanoparticles (NPs) in a microwave cavity is spectroscopically analyzed and compared with the classical autocatalytic method of NP growth to elucidate the underpinnings for the observed enhanced growth behavior for metal NPs prepared in a MW field. The study illustrates that microwave synthesis of nickel and gold NPs below saturation conditions follows the Finke-Watzky mechanism of nucleation and growth. The enhancement of the reaction arises from the size-dependent increase in MW absorption cross section for the metal NPs. For Ni, the presence of oxides is considered via theoretical computations and compared to dielectric measurements of isolated nickel NPs. The study definitively shows that MW growth can be modeled by an autocatalytic mechanism that directly leads to the observed enhanced rates and improved quality widely reported in the nanomaterial community when MW irradiation is employed.

Colloidal nanoparticle size control: experimental and kinetic modeling investigation of the ligand-metal binding role in controlling the nucleation and growth kinetics

Despite the major advancements in colloidal metal nanoparticles synthesis, a quantitative mechanistic treatment of the ligand's role in controlling their size remains elusive. We report a methodology that combines in situ small angle X-ray scattering (SAXS) and kinetic modeling to quantitatively capture the role of ligand-metal binding (with the metal precursor and the nanoparticle surface) in controlling the synthesis kinetics. We demonstrate that accurate extraction of the kinetic rate constants requires using both, the size and number of particles obtained from in situ SAXS to decouple the contributions of particle nucleation and growth to the total metal reduction. Using Pd acetate and trioctylphosphine in different solvents, our results reveal that the binding of ligands with both the metal precursor and nanoparticle surface play a key role in controlling the rates of nucleation and growth and consequently the final size. We show that the solvent can affect the metal-ligand binding and consequently ligand coverage on the nanoparticles surface which has a strong effect on the growth rate and final size (1.4 nm in toluene and 4.3 nm in pyridine). The proposed kinetic model quantitatively predicts the effects of varying the metal concentration and ligand/metal ratio on nanoparticle size for our work and literature reports. More importantly, we demonstrate that the final size is exclusively determined by the nucleation and growth kinetics at early times and not how they change with time. Specifically, the nanoparticle size in this work and many literature reports can be predicted using a single, model independent kinetic descriptor, (growth-to-nucleation rate ratio)^1/3, despite the different metals and synthetic conditions. The proposed model and kinetic descriptor could serve as powerful tools for the design of colloidal nanoparticles with specific sizes.

Shape-controlled synthesis of 3D copper nicotinate hollow microstructures and their catalytic properties

View of the process of preparation and catalysis of Cu hollow microstructures.

Fabrication of Fe3O4@Au hollow spheres with recyclable and efficient catalytic properties

A view of the preparation process and the evaluation of the catalysis activity of Fe₃O₄@Au hollow spheres.

On the two-step mechanism for synthesis of transition-metal nanoparticles

The two-step particle synthesis mechanism, also known as the Finke-Watzky (1997) mechanism, has emerged as a significant development in the field of nanoparticle synthesis. It explains a characteristic feature of the synthesis of transition metal nanoparticles, an induction period in precursor concentration followed by its rapid sigmoidal decrease. The classical LaMer theory (1950) of particle formation fails to capture this behavior. The two-step mechanism considers slow continuous nucleation and autocatalytic growth of particles directly from precursor as its two kinetic steps. In the present work, we test the two-step mechanism rigorously using population balance models. We find that it explains precursor consumption very well, but fails to explain particle synthesis. The effect of continued nucleation on particle synthesis is not suppressed sufficiently by the rapid autocatalytic growth of particles. The nucleation continues to increase breadth of size distributions to unexpectedly large values as compared to those observed experimentally. A number of variations of the original mechanism with additional reaction steps are investigated next. The simulations show that continued nucleation from the beginning of the synthesis leads to formation of highly polydisperse particles in all of the tested cases. A short nucleation window, realized with delayed onset of nucleation and its suppression soon after in one of the variations, appears as one way to explain all of the known experimental observations. The present investigations clearly establish the need to revisit the two-step particle synthesis mechanism.

Evolution of atomic structure during nanoparticle formation

Understanding the mechanism of nanoparticle formation during synthesis is a key prerequisite for the rational design and engineering of desirable materials properties, yet remains elusive due to the difficulty of studying structures at the nanoscale under real conditions. Here, the first comprehensive structural description of the formation of a nanoparticle, yttria-stabilized zirconia (YSZ), all the way from its ionic constituents in solution to the final crystal, is presented. The transformation is a complicated multi-step sequence of atomic reorganizations as the material follows the reaction pathway towards the equilibrium product. Prior to nanoparticle nucleation, reagents reorganize into polymeric species whose structure is incompatible with the final product. Instead of direct nucleation of clusters into the final product lattice, a highly disordered intermediate precipitate forms with a local bonding environment similar to the product yet lacking the correct topology. During maturation, bond reforming occurs by nucleation and growth of distinct domains within the amorphous intermediary. The present study moves beyond kinetic modeling by providing detailed real-time structural insight, and it is demonstrated that YSZ nanoparticle formation and growth is a more complex chemical process than accounted for in conventional models. This level of mechanistic understanding of the nanoparticle formation is the first step towards more rational control over nanoparticle synthesis through control of both solution precursors and reaction intermediaries.

Polyoxometalate-decorated nanoparticles

Polyoxometalate cluster anions (POMs) control formation and morphology, and serve as protecting ligands, for structurally and compositionally diverse nanostructures. While numerous examples of POM-protected metal(0) nanoparticle syntheses and reactions can now be found in the literature, the use of POMs to prepare nano-scale analogs of binary inorganic materials, such as metal-oxides, sulfides and halides, is a relatively recent development. The first part of this critical review summarizes the use of POMs as protecting ligands for metal(0) nanoparticles, as well as their use as templates for the preparation of new inorganic materials. Here, key findings that reveal general trends are given additional emphasis. In the second part of the review, new information concerning the structure of POM-protected metal(0) nanoparticles is systematically developed. This information, obtained by the combined use of cryogenic transmission microscopy (cryo-TEM) and UV-vis spectroscopy, provides a new perspective on the formation and structure of POM-decorated nanoparticles, and on the rational design of catalytic and other functional POM-based nano-assemblies.

Nucleation and aggregative growth process of platinum nanoparticles studied by in situ quick XAFS spectroscopy

The early stage in the nucleation and subsequent aggregative particle growth of the colloidal platinum (Pt) dispersions produced by photoreduction in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) was quantitatively investigated by means of in situ quick XAFS (QXAFS) measurements. The stages of the reduction-nucleation and the association process (aggregative particle growth and Ostwald ripening) of Pt atoms to produce Pt nanoparticles was successfully discriminated in course of the photoreduction time. The present QXAFS analysis indicated that Pt nuclei (i.e., (Pt(0))(m) nucleates approximately m = 4) were continuously produced in the reduction-nucleation process at the early time, followed by the aggregative particle growth with the autocatalytic reduction of Pt ionic species on the surface of Pt nuclei to produce Pt nanoparticles. Subsequently the particle growth proceeded via Ostwald ripening, resulting in the production of larger Pt nanoparticles at a later time. It was also found that the aggregative particle growth follows a sigmoidal profile well described either by the solid-state kinetic model or by the chemical-mechanism-based kinetic model, specifically the Avrami-Erofe'ev or Finke-Watzky models. The difference in terms of the formation mechanism was observed between the reduction of Pt(IV)Cl(6)(2-) and Pt(II)Cl(4)(2-) as a source material. Also presented is that the addition of the photoactivator such as benzoin, benzophenone, and acetophenone in the system is very effective to enhance the rate for the formation of Pt nanoparticles.

Rhodium(0) nanoparticles supported on nanocrystalline hydroxyapatite: highly effective catalytic system for the solvent-free hydrogenation of aromatics at room temperature

The hydrogenation of aromatics under mild conditions remains a challenge in the fields of synthetic and petroleum chemistry. Described herein is a new catalytic material that shows excellent catalytic performance in terms of activity, selectivity, and reusability in the hydrogenation of aromatics in solvent-free systems under mild conditions. The catalyst, consisting of rhodium nanoparticles supported on nanocrystalline hydroxyapatite, can quantitatively hydrogenate neat benzene to cyclohexane with exceptionally high rates (initial TOF > 10(3) h(-1)) at 298 K and 3 bars of initial H(2) pressure. This new material maintains its inherent catalytic activity after several reuses. Importantly, catalyst preparation does not require elaborate procedures because the active metal nanoparticles are readily formed from the in situ reduction of Rh(3+)-exchanged hydroxyapatite while submerged in the aromatic solvent at room temperature under 3 bars of H(2) pressure.

Metal nanoparticles in liquid phase catalysis; from recent advances to future goals

Metal nanoparticles have attracted much attention over the last decade owing to their unique properties, different to their bulk counterparts, which pave the way for their application in different fields from materials science and engineering to biomedical applications. Of particular interest, the use of metal nanoparticles in catalysis has brought superior efficiency in terms of activity, selectivity and lifetime to heterogeneous catalysis. This article reviews the recent developments in the synthesis routes and the catalytic performance of metal nanoparticles depending on the solvent used for various organic and inorganic transformations. Additionally, we also discuss the prevalent complications and their possible solutions plus future prospects in the field of nanocatalysis.

In situ formed "weakly ligated/labile ligand" iridium(0) nanoparticles and aggregates as catalysts for the complete hydrogenation of neat benzene at room temperature and mild pressures

"Weakly ligated/labile ligand" nanoparticles, that is nanoparticles where only weakly coordinated ligands plus the desired catalytic reactants are present, are of fundamental interest. Described herein is a catalyst system for benzene hydrogenation to cyclohexane consisting of "weakly ligated/labile ligand" Ir(0) nanoparticles and aggregates plus dry-HCl formed in situ from commercially available [(1,5-COD)IrCl](2) plus 40 +/- 1 psig (approximately 2.7 atm) H(2) at 22 +/- 0.1 degrees C. Multiple control and other experiments reveal the following points: (i) that this catalyst system is quite active with a TOF (turnover frequency) of 25 h(-1) and TTO (total turnovers) of 5250; (ii) that the BF(4)(-) and PF(6)(-) iridium salt precursors, [(1,5-COD)Ir(CH(3)CN)(2)]BF(4) and [(1,5-COD)Ir(CH(3)CN)(2)]PF(6), yield inferior catalysts; (iii) that iridium black with or without added, preformed HCl cannot achieve the TOF of 25 h(-1) of the in situ formed Ir(0)/dry-HCl catalyst. However and importantly, CS(2) poisoning experiments yield the same activity per active iridium atom for both the Ir(0)/dry-HCl and Ir black/no-HCl catalysts (12.5 h(-1) Ir(1-)), but reveal that the Ir(0)/dry-HCl system is 10-fold more dispersed compared to the Ir(0) black catalyst. The simple but important and key result is that "weakly ligated/labile ligand" Ir(0) nanoparticles and aggregates have been made in situ as demonstrated by the fact that they have identical, per exposed Ir(0) activity within experimental error to Ir(0) black and that they have no possible ligands other than those desired for the catalysis (benzene and H(2)) plus the at best poor ligand HCl. As expected, the in situ catalyst is poorly stabilized, exhibiting only 60% of its initial activity in a second run of benzene hydrogenation and resulting in bulk metal precipitation. However, stabilization of the Ir(0) nanoparticles with a ca. 2-fold higher catalytic activity and somewhat longer lifetime for the complete hydrogenation of benzene was accomplished by supporting the Ir(0) nanoparticles onto zeolite-Y (TOF of 47 h(-1) and 8600 TTO under otherwise identical conditions). Also reported is the interesting result that Cl(-) (present as Proton Sponge x H(+)Cl(-)) completely poisons benzene hydrogenation catalysis, but not the easier cyclohexene hydrogenation catalysis under otherwise the same conditions, results that suggest different active sites for these ostensibly related hydrogenation reaction. The results suggest that synthetic routes to "weakly ligated/labile ligand" nanoparticles exhibiting improved catalytic performance is an important goal worthy of additional effort.

Size-focusing synthesis, optical and electrochemical properties of monodisperse Au38(SC2H4Ph)24 nanoclusters

We report a facile, high yielding synthetic method for preparing truly monodisperse Au(38)(SC(2)H(4)Ph)(24) nanoclusters. The synthetic approach involves two main steps: first, glutathionate (-SG) protected polydisperse Au(n) clusters (n ranging from 38 to approximately 102) are synthesized by reducing Au(I)-SG in acetone; subsequently, the size-mixed Au(n) clusters react with excess phenylethylthiol (PhC(2)H(4)SH) for approximately 40 h at 80 degrees C, which leads to Au(38)(SC(2)H(4)Ph)(24) clusters of molecular purity. Detailed studies by mass spectrometry and UV-vis spectroscopy explicitly show a gradual size-focusing process occurred in the thermal etching-induced growth process. The formula and molecular purity of Au(38)(SC(2)H(4)Ph)(24) clusters are confirmed by electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry, and size-exclusion chromatography. The optical and electrochemical properties of Au(38)(SC(2)H(4)Ph)(24) clusters show molecule-like behavior and the HOMO-LUMO gap of the cluster was determined to be approximately 0.9 eV. The size focusing growth process is particularly interesting and may be exploited to synthesize other robust gold thiolate clusters.