Autocatalytic Surface Growth Research Articles

Study of the Kinetics of Nucleation and Growth of Silver Nanoparticles: UV-Visible Spectroscopy vs. Atomic Force Microscopy and Transmission Electron Microscopy

Silver nanoparticles have been the subjects of research interest because of their unique properties and extensive use in modern-day applications. A variety of preparation techniques have been reported for their synthesis but among them mostly recognized is a modified version of the Turkevich citrate method. In this study, we also chose this method of silver nanoparticle synthesis. The synthesized nanoparticles were characterized first, traditionally with UV-Visible spectroscopy and then their sizes and shapes were identified employing Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). From the UV-Vis data were extracted the rate constants for the nucleation ($$k_1$$) and growth ($$k_2$$) of the silver nanoparticles, and then these constants were further used to fit those obtained from AFM and TEM. Using the values of $$k_1$$ and $$k_2$$ independently in the two experimental methods validated and supported the well-acknowledged two-step mechanism of slow nucleation and fast autocatalytic surface growth.

Mechanisms of controlled stabilizer-free synthesis of gold nanoparticles in liquid aerosol containing plasma.

The interaction between low-temperature plasma and liquid enables highly reactive solution phase chemistry and fast reaction kinetics. In this work, we demonstrate the rapid synthesis of stabilizer-free, spherical and crystalline gold nanoparticles (AuNP). More than 70% of gold ion complex (AuCl- 4) conversion is achieved within a droplet residence time in the plasma of ∼10 ms. The average size of the AuNPs increases with an increase in the droplet residence time and the particle synthesis showed a power threshold effect suggesting the applicability of the classical nucleation theory. Leveraging UV-vis absorption and emission spectroscopy, and nanoparticle size distributions obtained from TEM measurements, we showed that the AuCl- 4 conversion exceeded by 250 times the maximum faradaic efficiency. We identified important roles of both short-lived reducing species including solvated electrons and possibly vacuum ultraviolet (VUV) photons, and long-lived species, H2O2, in the reduction of AuCl- 4. A quantitative investigation was performed by a 1-D reaction-diffusion model which includes transport, plasma-enabled interfacial reduction of AuCl- 4, classical nucleation, monomer absorption and autocatalytic surface growth enabled by H2O2. The model shows good agreement with the experimental results. The timescale analysis of the simulation revealed that nucleation is enabled by fast reduction of gold ions, and autocatalytic growth mainly determines the particle size and is responsible for the majority of the ion precursor conversion while also explaining the excessively large faradaic efficiency found experimentally.

The in-situ XAS study on the formation of Pd nanoparticles via thermal decomposition of palladium (II) acetate in hydroxyl functionalized ionic liquids

The formation of palladium nanoparticles (NPs) via thermal decomposition of palladium (II) acetate in hydroxyl functionalized ionic liquid (IL) [C2OHmim]+[NTf2]− was studied using in-situ energy dispersive x-ray absorption fine structure spectroscopy (DXAFS) and complementary methods such as transmission electron microscopy (TEM) and inductively coupled plasma mass spectrometry. Detailed XAFS spectra analysis unraveled that the palladium acetate trimer precursor underwent dissociation after being dispersed in ILs, which accelerated the thermal decomposition compared with other organic solvents. Based on the reaction kinetics, extened x-ray absorption fine structure spectroscopy fitting and TEM characterization results, the thermal decomposition process can be divided three successive stages namely the explosive nucleation, autocatalytic surface growth and NP attachment growth.

Ligand-Mediated Nanocluster Formation with Classical and Autocatalytic Growth

We present a systematic study of ligand-mediated nanocluster (NC) formation using a kinetic model, which provides atomic insight into sub-nanometer cluster (S-NC) and NC formation. Our model describes the role of ligand-mediated nucleation and growth in obtaining monodisperse NCs. Nucleation includes metal ion reduction, reversible ligand association to the metal ion/atom, and formation of dimer nuclei. Growth can occur through autocatalytic surface growth and ligand-associated monomer addition to the cluster, depending on the rate of metal ion to neutral metal atom conversion. Furthermore, we studied the effect of the initial concentration of metal ions on NC formation using fast and slow reducing agents in the presence of slowly and rapidly binding ligands. The model shows that fast nucleation, slow growth, and a high molar ratio of rapidly binding ligand to metal ions promote the formation of S-NCs and NCs. Our results can guide experiments in the synthesis of ultrasmall clusters.

Nanoparticle Formation Kinetics and Mechanistic Studies Important to Mechanism-Based Particle-Size Control: Evidence for Ligand-Based Slowing of the Autocatalytic Surface Growth Step Plus Postulated Mechanisms

Ligands are known to affect the formation, stabilization, size, and size-dispersion control of transition-metal and other nanoparticles, yet the kinetic and mechanistic basis for such ligand effects remains to be elucidated and then coupled to predictions for improved particle size and narrower particle size distribution syntheses. Toward this broad goal, the effect of the added excess ligand (L) and the stabilizer, L = POM9– (= the polyoxometalate, P2W15Nb3O629–) is studied for the formation of POM9–-stabilized Ir(0)n nanoparticles, {Ir(0)n·(POM9–)m}9m−, synthesized from an atomically characterized precatalyst (COD)Ir·POM8– under H2. First, the balanced reaction stoichiometry and characterization of the nanoparticle products are established. Next, the kinetics of nanoparticle formation is analyzed initially by the FW 2-step minimum mechanism consisting of slow, continuous nucleation, A → B (rate constant k1obs), and autocatalytic surface growth, A + B → 2B (rate constant k2obs), where A is nominally (COD...

Continuous and Scalable Synthesis of Pt Multipods with Enhanced Electrocatalytic Activity toward the Oxygen Reduction Reaction

AbstractPlatinum multipods are attractive for catalytic and electrocatalytic applications owing to their highly open, branched structure and thus high specific surface area. A number of methods have been reported for the synthesis of Pt multipods, but they are all limited in terms of throughput due to the use of batch reactors. Here we report the use of a fluidic device for the continuous and scalable synthesis of Pt multipods with sizes controlled in the ranges of 3–5 nm and 2–3 nm for the length and width, respectively, of the branched arms. The facile protocol involves the use of as a precursor to Pt and oleylamine as a solvent, surfactant, and temperature‐dependent reductant. When a solution of these two components is pumped into the polytetrafluoroethylene tube immersed in an oil bath and held at 180 °C, Pt multipods are formed through fast autocatalytic surface growth and small particles attachment. Compared with the batch‐based synthesis, the throughput of the production in the flow system can readily be increased to 17 mg of Pt per hour while retaining a tight control over the quality of the products. When supported on carbon, the Pt multipods exhibit enhanced activity toward oxygen reduction relative to the commercial Pt/C catalyst.

Kinetics of Ag300 nanoclusters formation: The catalytically effective nucleus via a steady‐state approach

AbstractThe kinetics of formation of silver nanoparticles consisting of nearly 300 metal atoms is investigated, which were prepared by reduction of silver nitrate with hydrazine in ethylene glycol at 25°C without any stabilizer other than the glycol solvent. The resulting sigmoidal kinetic curves are analyzed by using the 1997 Finke–Watzky two‐step mechanism of slow continuous nucleation with subsequent fast autocatalytic surface growth. The kinetics of homogeneous nucleation of metal nanoparticles was analyzed using the assumption about the stepwise adjunction of precursor and the quasi steady‐state approximation. The equations were proposed to calculate the concentration of the formed metal nanoparticles and their mean size from the experimentally determined values of the Finke–Watzky rate constants. It is shown that a stepwise nucleation process can be described in the terms of the catalytically effective nucleus concept and that the number of atoms in the catalytically effective nucleus can be estimated.

Nanoceria supported rhodium(0) nanoparticles as catalyst for hydrogen generation from methanolysis of ammonia borane

This work reports the preparation and catalytic use of nanoceria supported rhodium(0) nanoparticles, Rh(0)/nanoCeO2, as catalyst for hydrogen generation from the methanolysis of ammonia borane. Rh(0)/nanoCeO2 was in situ formed from the reduction of rhodium(II) octanoate on the surface of nanoceria during the catalytic methanolysis of ammonia borane at room temperature. The results of analysis using PXRD, TEM, STEM-EDS, XPS, SEM, SEM-EDX, N2 adsorption-desorption and ICP-OES reveal that rhodium(0) nanoparticles were well dispersed on the surface of nanoceria with an average size of 3.9 ± 0.6 nm. Rh(0)/nanoCeO2 shows high catalytic activity in the methanolysis of ammonia borane with a turnover frequency of 144 min−1. Note that hydrogen generation from the methanolysis of ammonia borane catalyzed by Rh(0)/nanoCeO2 is slightly less than 3.0 equivalent, due to the reduction of Ce4+ ions to Ce3+ ions on the surface of nanoceria during the methanolysis of ammonia borane. The reduction of Ce4+ ions leading to the formation of Ce3+ defects on the surface of nanoceria under the catalytic reaction conditions could be investigated by high resolution Ce 3d XPS analysis. Additionally, the formation kinetic of rhodium(0) nanoparticles could be studied by using the hydrogen generation from the methanolysis of ammonia borane as reporter reaction; thus, the rate constants for the slow nucleation, k1 and autocatalytic surface growth of rhodium(0) nanoparticles, k2 were determined. Our report also includes the results of kinetic study of the catalytic methanolysis of ammonia borane depending on rhodium concentration and temperature.

Quantitative Analysis of Different Formation Modes of Platinum Nanocrystals Controlled by Ligand Chemistry.

Well-defined metal nanocrystals play important roles in various fields, such as catalysis, medicine, and nanotechnology. They are often synthesized through kinetically controlled process in colloidal systems that contain metal precursors and surfactant molecules. The chemical functionality of surfactants as coordinating ligands to metal ions however remains a largely unsolved problem in this process. Understanding the metal-ligand complexation and its effect on formation kinetics at the molecular level is challenging but essential to the synthesis design of colloidal nanocrystals. Herein we report that spontaneous ligand replacement and anion exchange control the form of coordinated Pt-ligand intermediates in the system of platinum acetylacetonate [Pt(acac)2], primary aliphatic amine, and carboxylic acid ligands. The formed intermediates govern the formation mode of Pt nanocrystals, leading to either a pseudo two-step or a one-step mechanism by switching on or off an autocatalytic surface growth. This finding shows the importance of metal-ligand complexation at the prenucleation stage and represents a critical step forward for the designed synthesis of nanocrystal-based materials.

Nanoalumina-supported rhodium(0) nanoparticles as catalyst in hydrogen generation from the methanolysis of ammonia borane

Rhodium(0) nanoparticles were in situ formed from the reduction of rhodium(II) octanoate and supported on the surface of nanoalumina yielding Rh(0)/nanoAl2O3 which is highly active catalyst in hydrogen generation from the methanolysis of ammonia borane at room temperature. The kinetics of nanoparticle formation can be followed just by monitoring the volume of hydrogen gas evolved from the methanolysis of ammonia borane. The evaluation of the kinetic data gives valuable insights to the slow, continuous nucleation and autocatalytic surface growth steps of the formation of rhodium(0) nanoparticles. Rh(0)/nanoAl2O3 could be isolated and characterized by a combination of advanced analytical techniques including ATR-IR, PXRD, TEM, XPS, SEM, SEM-EDX and ICP-OES. The results reveal that rhodium(0) nanoparticles are highly dispersed on the surface of nanoalumina. The particle size of Rh(0)/nanoAl2O3 increases with the initial rhodium loading of nanoalumina. Rh(0)/nanoAl2O3 is highly active catalyst in hydrogen generation from the methanolysis of AB providing an exceptional initial turnover frequency of TOF=218min−1 at 25.0±0.5°C, which is the highest value ever reported for rhodium catalysts in hydrogen generation from the methanolysis of ammonia borane.

Bimetallic Au/Ag Core-Shell Superstructures with Tunable Surface Plasmon Resonance in the Near-Infrared Region and High Performance Surface-Enhanced Raman Scattering.

Due to the larger surface area and the synergistic effects between two noble metals, the bimetallic superstructures exhibit enhanced distinctive optical, catalytic, and photothermal performances and surface-enhanced Raman scattering (SERS) "hot-spot" effect, and thus have attracted great interest in various applications. Compared with the common Pd, Pt hierarchical structures coated onto Au nanoparticles (NPs), easily synthesized via fast autocatalytic surface growth arising from intrinsic properties of Pd and Pt metals, precisely controlling the hierarchical Ag growth onto Au NPs is rarely reported. In our present study, the reducing agent dopamine dithiocarbamate (DDTC) was covalently capped onto the first metal core (Au) to delicately control the growth model of the second metal (Ag). This results in heterogeneous nucleation and growth of Ag precursor on the surface of Au nanorods (NRs), and further formation of cornlike bimetallic Au/Ag core-shell superstructures, which usually cannot be achieved from traditional epitaxial growth. The thickness of the hierarchical Ag shell was finely tuned in a size range from 8 to 22 nm by simply varying the amount of the ratio between Ag ions and DDTC capped on Au NR core. The tunable Ag shell leads to anisotropic bimetallic Au/Ag core-shell superstructures, displaying two distinctive plasmonic resonances in the near-infrared region (NIR). In particular, the longitudinal surface plasmon resonance exhibits a broadly tunable range from 840 to 1277 nm. Additionally, the rich hot spots from obtained Au/Ag superstructures significantly enhance the SERS performance.

Formation Mechanism of Gold Nanoparticles Synthesized by Photoreduction in Aqueous Ethanol Solutions of Polymers Using In Situ Quick Scanning X-ray Absorption Fine Structure and Small-Angle X-ray Scattering

The photoreduction process of [AuCl4]− precursors in poly(N-vinyl-2-pyrrolidone) solution was investigated by utilizing a combination of in situ quick scanning X-ray absorption fine structure (QXAFS) and small-angle X-ray scattering (SAXS) measurements to elucidate the nucleation and subsequent aggregative particle growth of colloidal gold nanoparticles (Au NPs). The sequence of stages for the reduction–nucleation and association process (autocatalytic surface growth and aggregative particle growth) of Au atoms to generate Au NPs was observed during the photoreduction. QXAFS analysis revealed that the reduction of [AuCl2]− species to Au0 atoms is a slower step than that of [AuCl4]− to [AuCl2]−, and the reduction of [AuCl2]− to Au0 atoms and the association of Au0 atoms to produce Au nucleates concurrently proceeds during short-duration photoirradiation. A sigmoidal curve expressed by the solid-state kinetic model, specifically the Avrami-Erofe’ev model, was applicable to demonstrate the aggregative partic...

Rhodium(0) nanoparticles supported on nanosilica: Highly active and long lived catalyst in hydrogen generation from the methanolysis of ammonia borane

Nanosilica stabilized rhodium(0) nanoparticles (Rh(0)/nanoSiO2), in situ formed from the reduction of rhodium(II) octanoate impregnated on the surface of nanosilica, are active catalyst in hydrogen generation from the methanolysis of ammonia borane at room temperature. Monitoring the hydrogen evolution enables us to follow the kinetics of nanoparticles formation. The resulting sigmoidal kinetic curves are analyzed by using the 2-step mechanism of the slow, continuous nucleation and autocatalytic surface growth. By using the temperature dependent kinetic data, we could calculate the activation energy for the nucleation and autocatalytic surface growth of rhodium(0) nanoparticles as well as for the catalytic methanolysis of ammonia borane. Rh(0)/nanoSiO2 could be isolated and characterized by a combination of advanced analytical techniques including XRD, TEM, EDX, XPS, and N2 adsorption–desorption. The results reveal that rhodium(0) nanoparticles are highly dispersed on nanosilica surface and have tunable particle size depending on the initial metal concentration. An increase in the mean particle size of rhodium(0) nanoparticles is observed when the initial metal concentration increases. Rh(0)/nanoSiO2 are highly active and long lived catalyst in hydrogen generation from the methanolysis providing an exceptional initial turnover frequency of TOF=168min−1 (504min−1 corrected for the surface atoms) at 25.0±0.5°C, which is the highest value ever reported for rhodium catalysts. An inverse dependence of TOF on the initial rhodium concentration is observed and ascribed to the increasing size of rhodium(0) nanoparticles. Carbon disulfide poisoning and filtration experiments unequivocally demonstrate that rhodium(0) nanoparticles are the true heterogeneous catalyst in hydrogen generation from the methanolysis of ammonia borane.

Formation of gold nanoparticles during the reduction of HAuBr4 in reverse micelles of oxyethylated surfactant: Influence of gold precursor on the growth kinetics and properties of the particles

Abstract

Nucleation is second order: an apparent kinetically effective nucleus of two for Ir(0)n nanoparticle formation from [(1,5-COD)Ir(I)·P2W15Nb3O62]8- plus hydrogen.

Nucleation initiates phase changes across nature. A fundamentally important, presently unanswered question is if nucleation begins as classical nucleation theory (CNT) postulates, with n equivalents of monomer A forming a "critical nucleus", A(n), in a thermodynamic (equilibrium) process. Alternatively, is a smaller nucleus formed at a kinetically limited rate? Herein, nucleation kinetics are studied starting with the nanoparticle catalyst precursor, [A] = [(Bu4N)5Na3(1,5-COD)Ir(I)·P2W15Nb3O62], forming soluble/dispersible, B = Ir(0)(∼300) nanoparticles stabilized by the P2W15Nb3O62(9-) polyoxoanion. The resulting sigmoidal kinetic curves are analyzed using the 1997 Finke-Watzky (hereafter FW) two-step mechanism of (i) slow continuous nucleation (A → B, rate constant k(1obs)), then (ii) fast autocatalytic surface growth (A + B → 2B, rate constant k(2obs)). Relatively precise homogeneous nucleation rate constants, k(1obs), examined as a function of the amount of precatalyst, A, reveal that k(1obs) has an added dependence on the concentration of the precursor, k(1obs) = k(1obs(bimolecular))[A]. This in turn implies that the nucleation step of the FW two-step mechanism actually consists of a second-order homogeneous nucleation step, A + A → 2B (rate constant, k(1obs(bimol))). The results are significant and of broad interest as an experimental disproof of the applicability of the "critical nucleus" of CNT to nanocluster formation systems such as the Ir(0)n one studied herein. The results suggest, instead, the experimentally-based concepts of (i) a kinetically effective nucleus and (ii) the concept of a first-observable cluster, that is, the first particle size detectable by whatever physical methods one is currently employing. The 17 most important findings, associated concepts, and conclusions from this work are provided as a summary.

Ruthenium(0) nanoparticles supported on magnetic silica coated cobalt ferrite: Reusable catalyst in hydrogen generation from the hydrolysis of ammonia-borane

Ruthenium(0) nanoparticles supported on magnetic silica-coated cobalt ferrite (Ru(0)/SiO2-CoFe2O4) were in situ generated from the reduction of Ru3+/SiO2-CoFe2O4 during the catalytic hydrolysis of ammonia-borane (AB). Ruthenium(III) ions were impregnated on SiO2-CoFe2O4 from the aqueous solution of ruthenium(III) chloride and then reduced by AB at room temperature yielding Ru(0)/SiO2-CoFe2O4 which were isolated from the reaction solution by using a permanent magnet and characterized by ICP-OES, XRD, TEM, TEM-EDX and XPS techniques. The resulting magnetically isolable Ru(0)/SiO2-CoFe2O4 were found to be highly reusable catalyst in hydrolysis of AB retaining 94% of their initial catalytic activity even after tenth run. Ru(0)/SiO2-CoFe2O4 provide the highest catalytic activity after the tenth use in hydrolysis of AB as compared to the other ruthenium catalysts. The work reported here also includes the formation kinetics of ruthenium(0) nanoparticles. The evaluation of rate constants for the nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles at various temperatures provides the estimation of activation energy for both reactions; Ea=116±7kJ/mol for the nucleation and Ea=51±2kJ/mol for the autocatalytic surface growth of ruthenium(0) nanoparticles. The report also includes the activation energy of the catalytic hydrogen generation from the hydrolysis of AB (Ea=45±2kJ/mol) determined from the evaluation of temperature dependent kinetic data and the effect of catalyst concentration on the rate of hydrolysis of AB.

A Four-Step Mechanism for the Formation of Supported-Nanoparticle Heterogenous Catalysts in Contact with Solution: The Conversion of Ir(1,5-COD)Cl/γ-Al2O3to Ir(0)∼170/γ-Al2O3

Product stoichiometry, particle-size defocusing, and kinetic evidence are reported consistent with and supportive of a four-step mechanism of supported transition-metal nanoparticle formation in contact with solution: slow continuous nucleation, A → B (rate constant k1), autocatalytic surface growth, A + B → 2B (rate constant k2), bimolecular agglomeration, B + B → C (rate constant k3), and secondary autocatalytic surface growth, A + C → 1.5C (rate constant k4), where A is nominally the Ir(1,5-COD)Cl/γ-Al2O3 precursor, B the growing Ir(0) particles, and C the larger, catalytically active nanoparticles. The significance of this work is at least 4-fold: first, this is the first documentation of a four-step mechanism for supported-nanoparticle formation in contact with solution. Second, the proposed four-step mechanism, which was obtained following the disproof of 18 alternative mechanisms, is a new four-step mechanism in which the new fourth step is A + C → 1.5C in the presence of the solid, γ-Al2O3 support. Third, the four-step mechanism provides rare, precise chemical and kinetic precedent for metal particle nucleation, growth, and now agglomeration (B + B → C) and secondary surface autocatalytic growth (A + C → 1.5C) involved in supported-nanoparticle heterogeneous catalyst formation in contact with solution. Fourth, one now has firm, disproof-based chemical-mechanism precedent for two specific, balanced pseudoelementary kinetic steps and their precise chemical descriptors of bimolecular particle agglomeration, B + B → C, and autocatalytic agglomeration, B + C → 1.5C, involved in, for example, nanoparticle catalyst sintering.

Kinetic Factors in the Synthesis of Silver Nanoparticles by Reduction of Ag+ with Hydrazine in Reverse Micelles of Triton N-42

The growth kinetics of silver nanoparticles (AgNPs) during the reduction of AgNO3 by hydrazine in the droplets of dispersed aqueous phase encapsulated in the reverse micelles of oxyethylated surfactant Triton N-42 with decane as disperse medium was studied in situ by UV–vis spectroscopy. The mechanism of the process includes two steps that are slow, continuous nucleation and fast, autocatalytic surface growth. Both steps are under kinetic control and are limited by the rate of Ag+ reduction. The rate of nucleation is limited by reaction in the droplets of the aqueous phase forming the cores of reverse micelles, and the rate of the growth is limited by the reaction on the surface of AgNPs growing inside the micelles. The reduction of Ag+ is a second-order process with respect to N2H4. It includes the formation of the intermediate complex Ag(N2H4)+ and its reaction with another N2H4 molecule. The concentration effects of N2H4 (c′N2H4) and NH3 (c′NH3) as competing ligand, medium effects of ionic strength (I) and of the background salt in dispersed aqueous phase, the effect of solubilization capacity of the micellar solution (Vs/Vo), and the effect of temperature (T) on the observed rate constants for the steps were studied. An increase in c′N2H4, I, Vs/Vo and T can be used to accelerate the rates of both steps, whereas an increase in c′NH3 inhibits them. The background salts have a positive effect on the rate of nucleation, whereas their effect on the growth rate is small and has probably a negative trend. The size and composition of AgNPs were characterized by means of DLS, TEM, EDXA, XRD, UV–vis, and IR spectroscopy.

Hydroxyapatite supported ruthenium(0) nanoparticles catalyst in hydrolytic dehydrogenation of ammonia borane: Insight to the nanoparticles formation and hydrogen evolution kinetics

When a solution of ammonia borane is added to the suspension of ruthenium(III) ions supported on hydroxyapatite, both reduction of ruthenium(III) to ruthenium(0) nanoparticles and hydrogen release from the hydrolysis of ammonia borane occur concomitantly at room temperature. Using the hydrogen evolution from the hydrolysis of ammonia borane as reporter reaction provides valuable insights to the formation kinetics of ruthenium(0) nanoparticles. Thus, the rate constants for the slow nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles could be obtained. Furthermore, the evaluation of rate constants at various temperatures provides the estimation of activation energies for both reactions; Ea=166±7kJ/mol for the nucleation and Ea=59±2kJ/mol for the autocatalytic surface growth of ruthenium(0) nanoparticles. The ruthenium(0) nanoparticles, in situ formed during the hydrolysis of ammonia borane and supported on hydroxyapatite, could be isolated from the reaction solution and characterized by a combination of advanced analytical techniques. The results show that (i) highly dispersed ruthenium(0) nanoparticles of 4.7±0.7nm size were formed on the surface of hydroxyapatite, (ii) they are highly active catalyst in the hydrolytic dehydrogenation of ammonia borane with a turnover frequency value of 137min−1 at 25.0±0.1°C, and (iii) they are long lived and reusable catalyst providing 87,000 turnovers for hydrogen generation from the hydrolysis of ammonia borane and preserving 92% of their initial catalytic activity even after the fifth run of hydrolysis of ammonia borane at 25.0±0.1°C. The results of kinetic study on the hydrogen generation from the hydrolysis of ammonia borane were also reported including the activation energy of 58±2kJ/mol for the hydrolytic dehydrogenation of ammonia borane.

Hydrogen liberation from the hydrolytic dehydrogenation of dimethylamine–borane at room temperature by using a novel ruthenium nanocatalyst

Herein we report the discovery of an in situ generated, highly active nanocatalyst for the room temperature dehydrogenation of dimethylamine-borane in water. The new catalyst system consisting of ruthenium(0) nanoparticles stabilized by the hydrogenphosphate anion can readily and reproducibly be formed under in situ conditions from the dimethylamine-borane reduction of a ruthenium(III) precatalyst in tetrabutylammonium dihydrogenphosphate solution at 25 ± 0.1 °C. These new water dispersible ruthenium nanoparticles were characterized by using a combination of advanced analytical techniques. The results show the formation of well-dispersed ruthenium(0) nanoparticles of 2.9 ± 0.9 nm size stabilized by the hydrogenphosphate anion in aqueous solution. The resulting ruthenium(0) nanoparticles act as a highly active catalyst in the generation of 3.0 equiv. of H(2) from the hydrolytic dehydrogenation of dimethylamine-borane with an initial TOF value of 500 h(-1) at 25 ± 0.1 °C. Moreover, they provide exceptional catalytic lifetime (TTO = 11,600) in the same reaction at room temperature. The work reported here also includes the following results; (i) monitoring the formation kinetics of the in situ generated ruthenium nanoparticles, by using the hydrogen generation from the hydrolytic dehydrogenation of dimethylamine-borane as a catalytic reporter reaction, shows that sigmoidal kinetics of catalyst formation and concomitant dehydrogenation fits well to the two-step, slow nucleation and then autocatalytic surface growth mechanism, A → B (rate constant k(1)) and A + B → 2B (rate constant k(2)), in which A is RuCl(3)·3H(2)O and B is the growing, catalytically active Ru(0)(n) nanoclusters. (ii) Hg(0) poisoning coupled with activity measurements after solution infiltration demonstrates that the in situ generated ruthenium(0) nanoparticles act as a kinetically competent heterogeneous catalyst in hydrogen generation from the hydrolytic dehydrogenation of dimethylamine-borane. (iii) A compilation of kinetic data depending on the temperature and catalyst concentration is used to determine the dependency of reaction rate on catalyst concentration and the activation energy of the reaction, respectively.