Reducible tungsten(VI) oxide-supported ruthenium(0) nanoparticles: highly active catalyst for hydrolytic dehydrogenation of ammonia borane.

Reducible WO3 powder with a mean diameter of 100 nm is used as support to stabilize ruthenium(0) nanoparticles. Ruthenium(0) nanoparticles are obtained by NaBH4 reduction of ruthenium(III) precursor on the surface of WO3 support at room temperature. Ruthenium(0) nanoparticles are uniformly dispersed on the surface of tungsten(VI) oxide. The obtained Ru0/WO3 nanoparticles are found to be active catalysts in hydrolytic dehydrogenation of ammonia borane. The turnover frequency (TOF) values of the Ru0/WO3 nanocatalysts with the metal loading of 1.0%, 2.0%, and 3.0% wt. Ru are 122, 106, and 83 min-1, respectively, in releasing hydrogen gas from the hydrolysis of ammonia borane at 25.0 °C. As the Ru0/WO3 (1.0% wt. Ru) nanocatalyst with an average particle size of 2.6 nm provides the highest activity among them, it is extensively investigated. Although the Ru0/WO3 (1.0% wt. Ru) nanocatalyst is not magnetically separable, it has extremely high reusability in the hydrolysis reaction as it preserves 100% of initial catalytic activity even after the 5th run of hydrolysis. The high activity and reusability of Ru0/WO3 (1.0% wt. Ru) nanocatalyst are attributed to the favorable metal-support interaction between the ruthenium(0) nanoparticles and the reducible tungsten(VI) oxide. The high catalytic activity and high stability of Ru0/WO3 nanoparticles increase the catalytic efficiency of precious ruthenium in hydrolytic dehydrogenation of ammonia borane.

A review of the catalytic conversion of glycerol to lactic acid in the presence of aqueous base.

Lactic acid is a high-value-added chemical with large production, which is used in many industries including the production of pyruvic and acrylic acids. Lactic acid is largely obtained from the oxidation of glycerol, which is a prevalent by-product of biodiesel production. However, the oxidation of glycerol to lactic acid requires harsh reaction conditions such as high temperature and pressure as well as the use of a hefty strong base. In the presence of suitable catalysts, the production of lactic acid from glycerol can be achieved under mild conditions with 1 equivalent base per mole of glycerol. Herein, we review the reports of the catalytic conversion of glycerol to lactic acid in an aqueous alkaline medium considering the reaction conditions, catalytic activity for glycerol conversion and selectivity for lactic acid. We start first with the reports on the use of homogeneous catalysts that have high catalytic activity but miserable recovery. Next, we discuss the employment of colloidal metal(0) nanoparticles as catalysts in glycerol oxidation. The papers on the use of supported metal(0) nanoparticles are reviewed according to the type of support. We then review the polymetallic and metal/metal oxide nanocatalysts used for the conversion of glycerol to lactic acid in an alkaline medium. The catalysts tested for glycerol conversion to lactic acid without any additional bases are also discussed to emphasize the importance of a strong base for catalytic performance. The proposed mechanisms of glycerol oxidation to lactic acid in the presence or absence of catalysts as well as for the formation of side products are discussed. The available experimental kinetics data are shown to fit the mechanism with the formation of glyceraldehyde from glycerol alkoxide as the rate-determining step.

Palladium nanoparticles supported on cobalt(II,III) oxide nanocatalyst: High reusability and outstanding catalytic activity in hydrolytic dehydrogenation of ammonia borane.

A new palladium(0) nanocatalyst is developed to enhance the catalytic efficiency of precious metal catalysts in hydrogen generation from the hydrolytic dehydrogenation of ammonia borane. Magnetically separable Pd0/Co3O4 nanocatalyst can readily be obtained by the reduction of palladium(II) cations impregnated on cobalt(II, III) oxide at room temperature. The obtained Pd0/Co3O4 nanocatalyst with 0.25% wt. palladium loading has outstanding catalytic activity with a record turnover frequency of 3048 min-1 in the releasing H2 from the hydrolysis of ammonia borane at 25.0 °C. They also provide outstanding reusability even after the tenth run of the hydrolysis of ammonia borane at 25.0 °C. The high activity and superb stability of magnetically isolable Pd0/Co3O4 nanoparticles are attributed to the favorable interaction of palladium with the surface of reducible cobalt oxide.

A review on platinum(0) nanocatalysts for hydrogen generation from the hydrolysis of ammonia borane.

This review reports a survey on the progress in developing highly efficient platinum nanocatalysts for the hydrolytic dehydrogenation of ammonia borane (AB). After a short prelude emphasizing the importance of increasing the atom efficiency of high cost, precious platinum nanoparticles (NPs) which are known to be one of the highest activity catalysts for hydrogen generation from the hydrolysis of AB, this article reviews all the available reports on the use of platinum-based catalysts for this hydrolysis reaction covering (i) early tested platinum catalysts, (ii) platinum(0) NPs supported on oxides, (iii) platinum(0) NPs supported on carbonaceous materials, (iv) supported platinum single-atom catalysts, (v) bimetallic- and (vi) multimetallic-platinum NP nanocatalysts, and (vii) magnetically separable platinum-based catalysts. All the reported results are tabulated along with the important parameters used in the platinum-catalyzed hydrolysis of AB. In the section "Concluding remarks and a look towards the future" a discussion is devoted to the approaches for making high cost, precious platinum catalysts as efficient as possible, ultimately lowering the cost, including the suggestions for the future research in this field.

Magnetically Isolable Pt0/Co3O4 Nanocatalysts: Outstanding Catalytic Activity and High Reusability in Hydrolytic Dehydrogenation of Ammonia Borane.

The development of a new platinum nanocatalyst to maximize the catalytic efficiency of the precious noble metal catalyst in releasing hydrogen from ammonia borane (AB) is reported. Platinum(0) nanoparticles are impregnated on a reducible cobalt(II,III) oxide surface, forming magnetically isolable Pt0/Co3O4 nanocatalysts, which have (i) superb catalytic activity providing a record turnover frequency (TOF) of 4366 min-1 for hydrogen evolution from the hydrolysis of AB at room temperature and (ii) excellent reusability, retaining the complete catalytic activity even after the 10th run of hydrolysis reaction. The outstanding activity and stability of the catalyst can be ascribed to the strong interaction between the platinum(0) nanoparticles and reducible cobalt oxide, which is supported by the results of XPS analysis. Pt0/Co3O4 exhibits the highest TOF among the reported platinum-nanocatalysts developed for hydrogen generation from the hydrolysis of AB.

Cobalt ferrite supported platinum nanoparticles: Superb catalytic activity and outstanding reusability in hydrogen generation from the hydrolysis of ammonia borane.

In this work, platinum(0) nanoparticles are deposited on the surface of magnetic cobalt ferrite forming magnetically separable Pt0/CoFe2O4 nanoparticles, which are efficient catalysts in H2 generation from the hydrolysis of ammonia borane. Catalytic activity of Pt0/CoFe2O4 nanoparticles decreases with the increasing platinum loading, parallel to the average particle size. Pt0/CoFe2O4 (0.23% wt. Pt) nanoparticles have an average diameter of 2.30 ± 0.47 nm and show an extraordinary turnover frequency of 3628 min-1 in releasing 3.0 equivalent H2 per mole of ammonia borane from the hydrolysis at 25.0 °C. Moreover, the magnetically separable Pt0/CoFe2O4 nanoparticles possess high reusability retaining 100% of their initial catalytic activity even after ten runs of hydrolysis. The superb catalytic activity and outstanding reusability make the Pt0/CoFe2O4 nanoparticles very attractive catalysts for the hydrogen generation systems in portable and stationary fuel cell applications.

Dust Effects on Ir(0)n Nanoparticle Formation Nucleation and Growth Kinetics and Particle Size-Distributions: Analysis by and Insights from Mechanism-Enabled Population Balance Modeling.

The effects of microfiltration removal of filterable dust on nanoparticle formation kinetics and particle-size distribution, in a polyoxometalate polyanion (P2W15Nb3O629-)-stabilized Ir(0)n nanoparticle formation system, are analyzed by the newly developed method of Mechanism-Enabled Population Balance Modeling (ME-PBM). The [(Bu4N)5Na3(1,5-COD)Ir·P2W15Nb3O62] precatalyst system produces on average Ir(0)∼200 nanoparticles of 1.74 ± 0.33 nm and hence a particle-size distribution (PSD) of ±19% dispersion when the precatalyst is reduced under H2 in unfiltered propylene carbonate solvent. But if the precatalyst is reduced in microfiltered solvent and microfiltered reagent solutions (where the filtered solvent is then also used to rinse dust from the glassware), then larger Ir(0)∼300 1.96 ± 0.16 nm nanoparticles are produced with a remarkable, 2.4-fold lowered ±8% dispersion. The results and effects of the microfiltration reduction of dust are analyzed by the newly developed method of ME-PBM. More specifically, the studies reported herein address eight outstanding questions that are listed in the Introduction. Those questions include: how easy or difficult it is to fit PSD data? What is the ability of the recently discovered alternative termolecular nucleation and two size-dependent growth steps mechanism to account for the effects of dust on the PSD? What types and amount of PSD kinetics data are needed to deconvolute the PSD into the parameters of the ME-PBM? What is the reliability of the resulting rate constants? Additional questions addressed include: if the ME-PBM results offer insights into the remarkable 2.4-fold narrowing of the PSD post simple microfiltration lowering of the dust, and if the results are likely to be more general? The Summary and Conclusions section lists nine specific insights that include comments on needed future studies.

Mechanism-Enabled Population Balance Modeling of Particle Formation en Route to Particle Average Size and Size Distribution Understanding and Control.

The concept of Mechanism-Enabled Population Balance Modeling (ME-PBM) is reported, illustrated by its application to a prototype Ir(0)n nanoparticle formation reaction. ME-PBM is defined herein as the use of now available, experimentally established, disproof-based, deliberately minimalistic mechanisms of particle formation as the required input for more rigorous Population Balance models, critically including an experimentally established nucleation mechanism. ME-PBM achieves the long-sought goal of connecting such now available experimental minimum mechanisms to the understanding and rational control of particles size and size distributions. Twelve pseudoelementary step, particle-formation mechanisms are considered so that the approach to the ME-PBM is also extensively disproof-based. Resurrection of Smoluchowski's 1918 full Ordinary Differential Equation (ODE) approach to the PBM is another, critical aspect of our approach which, in turn, allows unbiased fitting of the information-laden particle-size distribution (PSD) including its shape. The results provide one solution to the "inverse problem" in which the PSD informs one as to the correct particle formation mechanism: A new, deliberately minimalistic 3-step particle-formation mechanism has been uncovered that is a single-additional-step expansion of the now broadly used Finke-Watzky (FW) 2-step mechanism, the new 3-step mechanism being: A → B (rate constant k1), A + B → C (rate constant k2), and A + C → 1.5C (rate constant k3), where A represents the monomeric nanoparticle precursor, B represents "small" nanoparticles, and C represents "larger" nanoparticles. The results strongly support three paradigm shifts for nucleation and growth of particles, the most critical paradigm shift being that the "burst" nucleation assumption in LaMer's 1950s model of particle formation is not required to produce narrow, near-monodisperse PSDs. Instead, narrow PSDs can be and are achieved despite continuous nucleation because smaller particles grow faster than larger ones, k2 > k3, thereby allowing the smaller particles to catch up in size to the more slowly growing larger particles.

Magnetically separable rhodium nanoparticles as catalysts for releasing hydrogen from the hydrolysis of ammonia borane.

Magnetically separable catalysts attract considerable attention in catalysis due to their facile separation from the reaction medium. This propensity is crucial for efficient multiple use of precious noble metal nanoparticles in catalysis. In fact, the isolation of catalysts from the reaction medium by filtration and washing results usually in the loss of huge amount of activity in the subsequent run of catalysis. Although many transition metal nanoparticle catalysts have been reported for the H2 generation from the hydrolysis of ammonia borane, there is no study reporting the magnetically separable rhodium based catalysts for the hydrolytic dehydrogenation of ammonia borane. Here, we report the preparation of rhodium(0) nanoparticles supported on the surface of Fe3O4 and CoFe2O4 magnetic nanopowders as the first example of magnetically separable rhodium nanocatalysts. The resulting magnetically separable Rh0/Fe3O4 and Rh0/CoFe2O4 nanoparticles are highly active, long-lived and reusable catalysts in H2 generation from the hydrolysis of ammonia borane providing a turnover frequency value of 273 and 720  min-1, respectively, at 25.0 ±â€¯0.1 °C. These magnetically separable catalysts show high reusability and long-term stability in the hydrolysis reaction. They retain their complete initial activity even after the 5th use releasing exactly 3.0 equivalent H2 gas per mole of ammonia borane. The long-term stability tests show that Rh0/Fe3O4 and Rh0/CoFe2O4 nanoparticles provide a total turnover number of 125,000 and 245,000, respectively, in releasing H2 from the hydrolysis of ammonia borane at room temperature. The long term stability and reusability of magnetically separable Rh0/Fe3O4 and Rh0/CoFe2O4 nanoparticles make them attractive catalysts for hydrogen generation in fuel cell applications.

Ceria supported ruthenium(0) nanoparticles: Highly efficient catalysts in oxygen evolution reaction.

Ruthenium(0) nanoparticles were successfully prepared on the surface of ceria (Ru0/CeO2) and used as catalysts on glassy carbon electrode (GCE) in oxygen evolution reaction (OER) from water electrolysis at room temperature. Ru0/CeO2 on GCE exhibits high catalytic activity for OER in alkaline solution. It provides a low onset potential of 1.57 V vs. RHE and low overpotential of 420 mV vs. RHE to reach a current density of 10 mA cm-2. Ru0/CeO2 on GCE shows no change in the onset potential value even after 6.1 h electrochemical measurement in OER.

Titania, zirconia and hafnia supported ruthenium(0) nanoparticles: Highly active hydrogen evolution catalysts.

Designing a cost-effective catalyst with high activity and stability for hydrogen evolution reaction (2H+ + 2e- → H2) is a big challenge due to increasing demand for energy. Herein, we report the electrocatalytic activity of glassy carbon electrodes with group 4 metal oxides (TiO2, ZrO2, HfO2) supported ruthenium(0) nanoparticles in hydrogen evolution reaction. Electrochemical activity of modified electrodes is investigated by recording linear sweep voltammograms in 0.5 M H2SO4 solution. The results of electrochemical measurements reveal that among the three electrodes the glassy carbon electrode with Ru0/TiO2 (1.20% wt. Ru) exhibits the highest activity with a relatively small Tafel slope of 52 mV dec-1, the highest exchange current density of 0.728 mA cm-2, and the smallest overpotential of 41 mV at j = 10 mA cm-2. Furthermore, it demonstrates superior stability in acidic solution with an unaltered onset potential for long term electrochemical measurement.

Nanoceria-Supported Ruthenium(0) Nanoparticles: Highly Active and Stable Catalysts for Hydrogen Evolution from Water.

Ruthenium(0) nanoparticles supported on nanoceria (Ru0/CeO2) were prepared by reduction of Ru3+ ions on the surface of ceria using aqueous solution of NaBH4. The Ru0/CeO2 samples were characterized by advanced analytical tools and employed as electrocatalysts on the glassy carbon electrode (GCE) in hydrogen evolution from water. The GCE, modified by Ru0/CeO2 (1.86 wt % Ru), provides an incredible electrocatalytic activity with a high exchange current density of 0.67 mA·cm-2, low overpotential of 47 mV at j = 10 mA·cm-2, and small Tafel slope of 41 mV·dec-1. Moreover, this modified GCE provides an unprecedented long-term stability without changing the onset potential (33 mV) even after 10 000 scans in acidic water splitting at room temperature. The hydrogen gas, evolved during the water splitting using the Ru0/CeO2 (1.86 wt % Ru) electrocatalyst, was also collected. The amount of the evolved H2 gas matches well with the calculated value, which indicates the achievement of nearly 100% Faradaic efficiency.

"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.

Nanozirconia supported ruthenium(0) nanoparticles: Highly active and reusable catalyst in hydrolytic dehydrogenation of ammonia borane.

Nanozirconia supported ruthenium(0) nanoparticles (Ru0/ZrO2) were prepared by impregnation of ruthenium(III) cations on the surface of zirconia followed by their reduction with sodium borohydride at room temperature. Ru0/ZrO2 was isolated from the reaction solution by centrifugation and characterized by ICP-OES, XRD, TEM, SEM-EDS and XPS techniques. All the results reveal that ruthenium(0) nanoparticles were successfully supported on zirconia and the resulting Ru0/ZrO2 is a highly active and reusable catalyst for hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value of 173 min-1 at 25 °C. The reusability and catalytic lifetime tests reveal that Ru0/ZrO2 is still active in the subsequent runs of hydrolysis of ammonia borane preserving 67% of the initial catalytic activity even after the fifth run and Ru0/ZrO2 provides 72,500 turnovers (mol H2/mol Ru) before deactivation at 25 °C. Our report also includes the results of kinetic studies depending on the catalyst concentration and temperature to determine the activation energy (Ea = 58 ±â€¯2 kJ/mol) for hydrolytic dehydrogenation of AB.

A Classic Azo-Dye Agglomeration System: Evidence for Slow, Continuous Nucleation, Autocatalytic Agglomerative Growth, Plus the Effects of Dust Removal by Microfiltration on the Kinetics.

An important but virtually ignored 1978 paper by Reeves and co-workers, which examined a dye-OAc hydrolysis and then agglomeration system, is reanalyzed in light of current state of knowledge of nucleation and growth/agglomeration phenomena. The Finke-Watzky two-step mechanism is used to account quantitatively for the kinetics data, in turn providing deconvolution of dye hydrolysis and nucleation of agglomerative growth, from the agglomerative growth step, including their separate rate constants. Significantly, the effects of microfiltration of the removable dust on the two steps and their rate constants are uncovered and quantitated for the first time, including the finding that the presence of dust accelerates both steps by ca. 10-fold or more. A postulated minimum mechanism able to account for all the observed results is provided. The results allow the excellently designed and executed, now nearly 40-years old, classic studies of Reeves and co-workers to be placed in its proper position in history, while at the same time providing six insights and conclusions detailed in the Discussion and Conclusions sections of the paper.

Nickel(0) nanoparticles supported on bare or coated cobalt ferrite as highly active, magnetically isolable and reusable catalyst for hydrolytic dehydrogenation of ammonia borane.

Nickel(0) nanoparticles supported on cobalt ferrite (Ni0/CoFe2O4), polydopamine coated cobalt ferrite (Ni0/PDA-CoFe2O4) or silica coated cobalt ferrite (Ni0/SiO2-CoFe2O4) are prepared and used as catalysts in hydrogen generation from the hydrolysis of ammonia borane at room temperature. Ni0/CoFe2O4 (4.0% wt. Ni) shows the highest catalytic activity with a TOF value of 38.3min-1 in hydrogen generation from the hydrolysis of ammonia borane at 25.0±0.1°C. However, the initial catalytic activity of Ni0/CoFe2O4 catalyst is not preserved in subsequent runs of hydrolysis. Coating the surface of cobalt ferrite support with polydopamine or silica leads to a significant improvement in the stability of catalysts. The TOF values of Ni0/PDA-CoFe2O4 and Ni0/SiO2-CoFe2O4 are found to be 7.6 and 5.3min-1, respectively, at 25.0±0.1°C. Ni0/PDA-CoFe2O4 catalyst shows high reusability as compared to the Ni0/CoFe2O4 and Ni0/SiO2-CoFe2O4 catalysts in hydrolytic dehydrogenation of ammonia borane at room temperature. All the catalysts are characterized by using a combination of various advanced analytical techniques. The results reveal that nickel nanoparticles with an average size of 12.3±0.7nm are well dispersed on the surface of PDA-CoFe2O4. .

Dust Effects on Nucleation Kinetics and Nanoparticle Product Size Distributions: Illustrative Case Study of a Prototype Ir(0)n Transition-Metal Nanoparticle Formation System.

The question is addressed if dust is kinetically important in the nucleation and growth of Ir(0)n nanoparticles formed from [Bu4N]5Na3(1,5-COD)IrI·P2W15Nb3O62 (hereafter [(COD)Ir·POM]8-), reduced by H2 in propylene carbonate solvent. Following a concise review of the (often-neglected) literature addressing dust in nucleation phenomena dating back to the late 1800s, the nucleation and growth kinetics of the [(COD)Ir·POM]8- precatalyst system are examined for the effects of 0.2 µm microfiltration of the solvent and precatalyst solution, of rinsing the glassware with that microfiltered solvent, of silanizing the glass reaction vessel, for the addition of <0.2 µm γ-Al2O3 (inorganic) dust, for the addition of flame-made carbon-based (organic) dust, and as a function of the starting, microfiltered [(COD)Ir·POM8-] concentration. Efforts to detect dust and its removal by dynamic light scattering and by optical microscopy are also reported. The results yield a list of eight important conclusions, the four most noteworthy of which are (i) that the nucleation apparent rate "constant" k1obs(bimol) is shown to be slowed by a factor of ∼5 to ∼7.6, depending on the precise experiment and its conditions, just by the filtration of the precatalyst solution using a 0.20 µm filter and rinsing the glassware surface with 0.20 µm filtered propylene carbonate solvent; (ii) that simply employing a 0.20 µm filtration step narrows the size distribution of the resulting Ir(0)n nanoparticles by a factor of 2.4 from ±19 to ±8%, a remarkable result; (iii) that the narrower size distribution can be accounted for by the slowed nucleation rate constant, k1obs(bimol), and by the unchanged autocatalytic growth rate constant, k2obs(bimol), that is, by the increased ratio of k2obs(bimol)/k1obs(bimol) that further separates nucleation from growth in time for filtered vs unfiltered solutions; and (iv) that five lines of evidence indicate that the filterable component of the solution, which has nucleation rate-enhancing and size-dispersion broadening effects, is dust.

Nanoparticle Nucleation Is Termolecular in Metal and Involves Hydrogen: Evidence for a Kinetically Effective Nucleus of Three {Ir3H2x·P2W15Nb3O62}6- in Ir(0)n Nanoparticle Formation From [(1,5-COD)IrI·P2W15Nb3O62]8- Plus Dihydrogen.

The nucleation process yielding Ir(0)∼300 nanoparticles from (Bu4N)5Na3[(1,5-COD)Ir·P2W15Nb3O62] (abbreviated hereafter as (COD)Ir·POM8-, where POM9- = the polyoxometalate, P2W15Nb3O629-) under H2 is investigated to learn the true molecularity, and hence the associated kinetically effective nucleus (KEN), for nanoparticle formation for the first time. Recent work with this prototype transition-metal nanoparticle formation system ( J. Am. Chem. Soc. 2014 , 136 , 17601 - 17615 ) revealed that nucleation in this system is an apparent second-order in the precatalyst, A = (COD)Ir·POM8-, not the higher order implied by classic nucleation theory and its nA ⇌ An, "critical nucleus", An concept. Herein, the three most reasonable more intimate mechanisms of nucleation are tested: bimolecular nucleation, termolecular nucleation, and a mechanism termed "alternative termolecular nucleation" in which 2(COD)Ir+ and 1(COD)Ir·POM8- yield the transition state of the rate-determining step of nucleation. The results obtained definitively rule out a simple bimolecular nucleation mechanism and provide evidence for the alternative termolecular mechanism with a KEN of 3, Ir3. All higher molecularity nucleation mechanisms were also ruled out. Further insights into the KEN and its more detailed composition involving hydrogen, {Ir3H2xPOM}6-, are also obtained from the established role of H2 in the Ir(0)∼300 formation balanced reaction stoichiometry, from the p(H2) dependence of the kinetics, and from a D2/H2 kinetic isotope effect of 1.2(±0.3). Eight insights and conclusions are presented. A section covering caveats in the current work, and thus needed future studies, is also included.

Facile Synthesis of Three-Dimensional Pt-TiO2 Nano-networks: A Highly Active Catalyst for the Hydrolytic Dehydrogenation of Ammonia-Borane.

Three-dimensional (3D) porous metal and metal oxide nanostructures have received considerable interest because organization of inorganic materials into 3D nanomaterials holds extraordinary properties such as low density, high porosity, and high surface area. Supramolecular self-assembled peptide nanostructures were exploited as an organic template for catalytic 3D Pt-TiO2 nano-network fabrication. A 3D peptide nanofiber aerogel was conformally coated with TiO2 by atomic layer deposition (ALD) with angstrom-level thickness precision. The 3D peptide-TiO2 nano-network was further decorated with highly monodisperse Pt nanoparticles by using ozone-assisted ALD. The 3D TiO2 nano-network decorated with Pt nanoparticles shows superior catalytic activity in hydrolysis of ammonia-borane, generating three equivalents of H2 .

Ceria-supported ruthenium nanoparticles as highly active and long-lived catalysts in hydrogen generation from the hydrolysis of ammonia borane.

Ruthenium(0) nanoparticles supported on ceria (Ru(0)/CeO2) were in situ generated from the reduction of ruthenium(iii) ions impregnated on ceria during the hydrolysis of ammonia borane. Ru(0)/CeO2 was isolated from the reaction solution by centrifugation and characterized by ICP-OES, BET, XRD, TEM, SEM-EDS and XPS techniques. All the results reveal that ruthenium(0) nanoparticles were successfully supported on ceria and the resulting Ru(0)/CeO2 is a highly active, reusable and long-lived catalyst for hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value of 361 min(-1). The reusability tests reveal that Ru(0)/CeO2 is still active in the subsequent runs of hydrolysis of ammonia borane preserving 60% of the initial catalytic activity even after the fifth run. Ru(0)/CeO2 provides a superior catalytic lifetime (TTO = 135 100) in hydrogen generation from the hydrolysis of ammonia borane at 25.0 ± 0.1 °C before deactivation. The work reported here includes the formation kinetics of ruthenium(0) nanoparticles. The rate constants for the slow nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles were obtained using hydrogen evolution as a reporter reaction. An evaluation of rate constants at various temperatures enabled the estimation of activation energies for both the reactions, Ea = 60 ± 7 kJ mol(-1) for the nucleation and Ea = 47 ± 2 kJ mol(-1) for the autocatalytic surface growth of ruthenium(0) nanoparticles, as well as the activation energy of Ea = 51 ± 2 kJ mol(-1) for the catalytic hydrolysis of ammonia borane.