Ruthenium(0) Nanoparticles Supported on Multiwalled Carbon Nanotube As Highly Active Catalyst for Hydrogen Generation from Ammonia–Borane

Ruthenium­(0) nanoparticles supported on multiwalled carbon nanotubes (Ru(0)@MWCNT) were in situ formed during the hydrolysis of ammonia–borane (AB) and could be isolated from the reaction solution by filtration and characterized by ICP-OES, XRD, TEM, SEM, EDX, and XPS techniques. The results reveal that ruthenium(0) nanoparticles of size in the range 1.4–3.0 nm are well-dispersed on multiwalled carbon nanotubes. They were found to be highly active catalyst in hydrogen generation from the hydrolysis of AB with a turnover frequency value of 329 min^–1. The reusability experiments show that Ru(0)@MWCNTs are isolable and redispersible in aqueous solution; when redispersed they are still active catalyst in the hydrolysis of AB exhibiting a release of 3.0 equivalents of H₂ per mole of NH₃BH₃ and preserving 41% of the initial catalytic activity even after the fourth run of hydrolysis. The lifetime of Ru(0)@MWCNTs was measured as 26400 turnovers over 29 h in the hydrolysis of AB at 25.0 ± 0.1 °C before deactivation. The work reported here also includes the kinetic studies depending on the temperature to determine the activation energy of the reaction (<i>E</i>_a = 33 ± 2 kJ/mol) and the effect of catalyst concentration on the rate of the catalytic hydrolysis of AB, respectively

Palladium(0) Nanoparticle Formation, Stabilization, and Mechanistic Studies: Pd(acac)₂ as a Preferred Precursor, [Bu₄N]₂HPO₄ Stabilizer, plus the Stoichiometry, Kinetics, and Minimal, Four-Step Mechanism of the Palladium Nanoparticle Formation and Subsequent Agglomeration Reactions

Palladium­(0) nanoparticles continue to be important in the field of catalysis. However, and despite the many prior reports of Pd(0)_n nanoparticles, missing is a study that reports the kinetically controlled formation of Pd(0)_n nanoparticles with the simple stabilizer [Bu₄N]₂HPO₄ in an established, balanced formation reaction where the kinetics and mechanism of the nanoparticle-formation reaction are also provided. It is just such studies that are the focus of the present work. Specifically, the present studies reveal that Pd­(acac)₂, in the presence of 1 equiv of [Bu₄N]₂HPO₄ as stabilizer in propylene carbonate, serves as a preferred precatalyst for the kinetically controlled nucleation following reduction under 40 ± 1 psig initial H₂ pressure at 22.0 ± 0.1 °C to yield 7 ± 2 nm palladium(0) nanoparticles. Studies of the balanced stoichiometry of the Pd(0)_n nanoparticle-formation reaction shows that 1.0 Pd­(acac)₂ consumes 1.0 equiv of H₂ and produces 1.0 equiv of Pd(0)_n while also releasing 2.0 ± 0.2 equiv of acetylacetone. The inexpensive, readily available HPO₄^2– also proved to be as effective a Pd(0)_n nanoparticle stabilizer as the more anionic, sterically larger, “Gold Standard” stabilizer P₂W₁₅Nb₃O₆₂^9–. The kinetics and associated minimal mechanism of formation of the [Bu₄N]₂HPO₄-stabilized Pd(0)_n nanoparticles are also provided, arguably the most novel part of the present studies, specifically the four-step mechanism of nucleation (A → B, rate constant <i>k</i>₁), autocatalytic surface growth (A + B → 2B, rate constant <i>k</i>₂), bimolecular agglomeration (B + B → C, rate constant <i>k</i>₃), and secondary autocatalytic surface growth (A + C → 1.5C, rate constant <i>k</i>₄), where A is Pd­(acac)₂, B represents the growing, smaller Pd(0)_n nanoparticles, and C represents the larger, most catalytically active Pd(0)_n nanoparticles. Additional details on the mechanism and catalytic properties of the resultant Pd(0)_n·HPO₄^2– nanoparticles are provided in this work

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 <i>microfiltration of the removable dust</i> on the two steps and their rate constants are uncovered and quantitated for the first time, including the finding that the <i>presence of dust accelerates <u>both</u> steps by ca. 10-fold or more</i>. 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

Triniobium, Wells–Dawson-Type Polyoxoanion, [(<i>n</i>‑C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂: Improvements in the Synthesis, Its Reliability, the Purity of the Product, and the Detailed Synthetic Procedure

Reproducible syntheses of high-purity [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂ and, therefore, also the supported [(1,5-COD)­Ir^I]⁺ organometallic precatalyst, [(<i>n</i>-C₄H₉)₄N]₅Na₃(1,5-COD)­Ir­(P₂W₁₅Nb₃O₆₂), have historically proven quite challenging. In 2002, Hornstein et al. published an improved synthesis reporting 90% pure [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂ in their hands. Unfortunately, 36 subsequent attempts to replicate that 2002 synthesis by four researchers in our laboratories produced material with an average purity of 82 ± 7%, albeit as judged by the improved S/N ³¹P NMR now more routinely possible. Herein we (1) verify problems in reproducing ≥90% purity [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂, (2) determine three critical variables for the successful production of [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂, (3) optimize the synthesis to achieve 91–94% pure [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂, and (4) successfully reproduce and verify the synthesis via another researcher (Dr. Saim Özkar) working only from the written procedure. The key variables underlying previously irreproducible syntheses are (i) a too-short and incomplete, insufficient volume washing step for Na₁₂[α-P₂W₁₅O₅₆]·18H₂O that (previously) failed to remove the WO₄^2– byproduct present, (ii) inadequate reaction time and the need for a slight excess of niobium­(V) during the incorporation of three niobium­(V) ions into α-P₂W₁₅O₅₆^12–, and (iii) incomplete removal of protons from the resultant [(<i>n</i>-C₄H₉)₄N]₅H₄P₂W₁₅Nb₃O₆₂ intermediate. These three insights have allowed improvement of the synthesis to a 91–94% final purity [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂ product by high S/N ³¹P NMR. Moreover, the synthesis provided both is very detailed and has been independently checked (by Dr. Özkar) <i>using only the written procedures</i>. The finding that prior syntheses of Na₁₂[α-P₂W₁₅O₅₆] are contaminated with WO₄^2– is one of the seemingly simple, but previously confounding, findings of the present work. An explicit check of the procedure is the second most important, more general feature of the present paper, namely, recognizing, discussing, and hopefully achieving a <i>level of written reporting</i> necessary to make such challenging polyoxometalate inorganic syntheses reproducible in the hands of others

Copper(0) Nanoparticles Supported on Silica-Coated Cobalt Ferrite Magnetic Particles: Cost Effective Catalyst in the Hydrolysis of Ammonia-Borane with an Exceptional Reusability Performance

Herein we report the development of a new and cost-effective nanocomposite catalyst for the hydrolysis of ammonia-borane (NH₃BH₃), which is considered to be one of the most promising solid hydrogen carriers because of its high gravimetric hydrogen storage capacity (19.6% wt) and low molecular weight. The new catalyst system consisting of copper nanoparticles supported on magnetic SiO₂/CoFe₂O₄ particles was reproducibly prepared by wet-impregnation of Cu­(II) ions on SiO₂/CoFe₂O₄ followed by in situ reduction of the Cu­(II) ions on the surface of magnetic support during the hydrolysis of NH₃BH₃ and characterized by ICP-MS, XRD, XPS, TEM, HR-TEM and N₂ adsorption–desorption technique. Copper nanoparticles supported on silica coated cobalt­(II) ferrite SiO₂/CoFe₂O₄ (CuNPs@SCF) act as highly active catalyst in the hydrolysis of ammonia-borane, providing an initial turnover frequency of TOF = 2400 h^–1 at room temperature, which is not only higher than all the non-noble metal catalysts but also higher than the majority of the noble metal based homogeneous and heterogeneous catalysts employed in the same reaction. More importantly, they were easily recovered by using a permanent magnet in the reactor wall and reused for up to 10 recycles without losing their inherent catalytic activity significantly, which demonstrates the exceptional reusability of the CuNPs@SCF catalyst

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 <i>j</i> = 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

Hydrocarbon-Soluble, Isolable Ziegler-Type Ir(0)_<i>n</i> Nanoparticle Catalysts Made from [(1,5-COD)Ir(μ-O₂C₈H₁₅)]₂ and 2–5 Equivalents of AlEt₃: Their High Catalytic Activity, Long Lifetime, and AlEt₃-Dependent, Exceptional, 200 °C Thermal Stability

Hydrocarbon-solvent-soluble, isolable, Ziegler-type Ir(0)_<i>n</i> nanoparticle hydrogenation catalysts made from the crystallographically characterized [(1,5-COD)­Ir­(μ-O₂C₈H₁₅)]₂ precatalyst and 2–5 equiv of AlEt₃ (≥2 equiv of AlEt₃ being required for the best catalysis and stability, vide infra) are scrutinized for their catalytic properties of (1) their isolability and then redispersibility without visible formation of bulk metal; (2) their initial catalytic activity of the isolated nanoparticle catalyst redispersed in cyclohexane; (3) their catalytic lifetime in terms of total turnovers (TTOs) of cyclohexene hydrogenation; and then also and unusually (4) their relative thermal stability in hydrocarbon solution at 200 °C for 30 min. These studies are of interest since Ir(0)_<i>n</i> nanoparticles are the currently best-characterized example, and a model/analogue, of industrial Ziegler-type hydrogenation catalysts made, for example, from Co­(O₂CR)₂ and ≥2 equiv of AlEt₃. Eight important insights result from the present studies, the highlights of which are that Ir(0)_<i>n</i> Ziegler-type nanoparticles, made from [(1,5-COD)­Ir­(μ-O₂C₈H₁₅)]₂ and AlEt₃, are (i) quite catalytically active and long-lived; (ii) thermally unusually stable nanoparticle catalysts at 200 °C, vide infra, a stability which requires the addition of at least 3 equiv of AlEt₃ (Al/Ir = 3), but where (iii) the Al/Ir = 5 Ir(0)_<i>n</i> nanoparticles are even more stable, for ≥30 min at 200 °C, and exhibit 100 000 TTOs of cyclohexene hydrogenation. The results also reveal that (iv) the observed nanoparticle catalyst stability at 200 °C appears to surpass that of any other demonstrated nanoparticle catalyst in the literature, those reports being limited to ≤130–160 °C temperatures; and reveal that (v) AlEt₃, or possibly surface derivatives of AlEt₃, along with [RCO₂·AlEt₃]⁻ formed from the first equiv of AlEt₃ per 1/2 equiv of [(1,5-COD)­Ir­(μ-O₂C₈H₁₅)]₂ are main components of the nanoparticle stabilizer system, consistent with previous suggestions from Shmidt, Goulon, Bönnemann, and others. The results therefore also (vi) imply that either (a) a still poorly understood mode of nanoparticle stabilization by alkyl Lewis acids such as AlEt₃ is present or, (b) that reactions between the Ir(0)_<i>n</i> and AlEt₃ occur to give initially surface species such as (Ir_surface)_<i>x</i>–Et plus (Ir_surface)_<i>x</i>–Al­(Et)₂Ir, where the number of surface Ir atoms involved, <i>x</i> = 1–4; and (vii) confirm the literature’s suggestion that the activity of Ziegler-type hydrogenation can be tuned by the Al/Ir ratio. Finally and perhaps most importantly, the results herein along with recent literature make apparent (viii) that isolable, hydrocarbon soluble, Lewis-acid containing, Ziegler-type nanoparticles are an underexploited, still not well understood type of high catalytic activity, long lifetime, and unusually if not unprecedentedly high thermal stability nanoparticles for exploitation in catalysis or other applications where their unusual hydrocarbon solubility and thermal stability might be advantageous

Unintuitive Inverse Dependence of the Apparent Turnover Frequency on Precatalyst Concentration: A Quantitative Explanation in the Case of Ziegler-Type Nanoparticle Catalysts Made from [(1,5-COD)Ir(μ‑O₂C₈H₁₅)]₂ and AlEt₃

The Ziegler-type hydrogenation precatalyst dimer, [(1,5-COD)­Ir­(μ-O₂C₈H₁₅)]₂ (1,5-COD = 1,5-cyclooctadiene; O₂C₈H₁₅ = 2-ethylhexanoate) plus added AlEt₃ stabilizer has recently been shown to form AlEt₃-stabilized, Ziegler-type Ir(0)_∼4–15 nanoparticles initially, which then grow to larger Ziegler-type Ir(0)_∼40–50 nanoparticles during the catalytic hydrogenation of cyclohexene (Alley, W. M.; Hamdemir, I. K.; Wang, Q.; Frenkel, A. I.; Li, L.; Yang, J. C.; Menard, L. D.; Nuzzo, R. G.; Özkar, S.; Johnson, K. A.; Finke, R. G. <i>Inorg. Chem.</i> <b>2010</b>, 49, 8131–8147). An interesting observation for this Ziegler-type nanoparticle catalyst system is that the apparent TOF (TOF_app = <i>k</i>_obs/[Ir]) for cyclohexene hydrogenation <i>increases</i> with <i>decreasing</i> concentration of the precatalyst, [Ir] (defined as 2­[{(1,5-COD)­Ir­(μ-O₂C₈H₁₅)}₂], that is, twice the starting precatalyst concentration since that dimer contains 2 Ir). A perusal of the literature reveals that such an intuitively backward, inverse relationship between the apparent turnover frequency, TOF_app, and the concentration of precatalyst or catalyst has been seen at least eight times before in other, disparate systems in the literature. However, this effect has previously never been satisfactorily explained, nor have the mixed, sometimes opposite, explanations offered in the literature been previously tested by the disproof of all reasonable alternative explanations/mechanistic hypotheses. Herein, five alternative mechanistic explanations have been tested via kinetic studies, Z-contrast STEM microscopy of the nanoparticle product sizes, and other evidence. Four of the five possible explanations have been ruled out en route to the finding that the only mechanism of the five able to explain all the evidence, as well as to quantitatively curve-fit the inverse TOF_app vs [Ir] data, is a prior, dissociative equilibrium, in which <i>x</i> ≈ 3 equiv of the surface-bound, AlR₃-based nanocluster stabilizer is dissociated, Ir(0)_<i>n</i>·[AlEt₃]_<i>m</i> ⇄ <i>x</i>AlEt₃ + Ir(0)_<i>n</i>·[AlEt₃]_{<i>m</i>−<i>x</i>}, with the resulting, more coordinatively unsaturated Ir(0)_<i>n</i>·[AlEt₃]_{<i>m</i>−<i>x</i>} being the faster, kinetically dominant catalyst. The implication is that such unusual, inverse TOF_app vs [precatalyst or catalyst] concentration observations in the literature are, more generally, likely just unintentional, unwitting measurements of a component of the rate law for such systems. The results herein are significant (i) in providing the first quantitative, disproof-tested explanation for the inverse TOF_app vs [precatalyst or catalyst] observation; (ii) in providing precedent and, therefore, a plausible explanation for the eight prior examples of this phenomenon in the literature; and (iii) in demonstrating for one of those additional eight literature cases, a commercial cobalt-based polymer hydrogenation catalyst, that the prior dissociative equilibrium uncovered herein can also quantitatively fit the inverse TOF_app vs [precatalyst] data for that case, as well. The results herein are additionally significant (iv) in making apparent that the rigorous interpretation of any TOF requires that the rate law for the processes under study be known, a point that bears heavily on the confusion and current controversy in the literature over the proper use of the “TOF” concept; (v) in making apparent the usefulness and value of the TOF_app concept employed herein; and (vi) in uncovering the insight that the true, most active catalyst present in AlEt₃-stabilized, Ziegler-type Ir(0)_<i>n</i> nanoparticle catalysts is the more coordinatively unsaturated Ziegler-type Ir(0)_<i>n</i>·[AlEt₃]_{<i>m</i>−<i>x</i>} nanoparticle formed from the dissociative loss of ∼3 AlEt₃

Agglomerative Sintering of an Atomically Dispersed Ir₁/Zeolite Y Catalyst: Compelling Evidence Against Ostwald Ripening but for Bimolecular and Autocatalytic Agglomeration Catalyst Sintering Steps

Agglomerative sintering of an atomically dispersed, zeolite Y-supported catalyst, Ir₁/zeolite Y, formed initially from the well-characterized precatalyst [Ir­(C₂H₄)₂]/zeolite Y and in the presence of liquid-phase reactants, was monitored over three cycles of 3800 turnovers (TTOs) of cyclohexene hydrogenation at 72 °C. The catalyst evolved and sintered during each cycle, even at the relatively mild temperature of 72 °C in the presence of the cyclohexene plus H₂ reactants and cyclohexane solvent. Post each of the three cycles of catalysis, the resultant sintered catalyst was characterized by extended X-ray absorption fine structure spectroscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The results show that higher-nuclearity iridium species, Ir_<i>n</i>, are formed during each successive cycle. The progression from the starting mononuclear precursor, Ir₁, is first to Ir_∼4–6; then, on average, Ir_∼40; and finally, on average, Ir_∼70, the latter more accurately described as a bimodal dispersion of on-average Ir_∼40–50 and on-average Ir_∼1600 nanoparticles. The size distribution and other data disprove Ostwald ripening during the initial and final stages of the observed catalyst sintering. Instead, the diameter-dispersion data plus quantitative fits to the cluster or nanoparticle diameter vs time data provide compelling evidence for the underlying, pseudoelementary steps of bimolecular agglomeration, B + B → C, and autocatalytic agglomeration, B + C → 1.5C, where B represents the smaller, formally Ir(0) nanoparticles, and C is the larger (more highly agglomerated) nanoparticles (and where the 1.5 coefficient in the autocatalytic agglomeration of B + C necessarily follows from the definition, in the bimolecular agglomeration step, that 1C contains the Ir from 2B). These two specific, balanced chemical reactions are of considerable significance in going beyond the present state-of-the-art, but word-only, “mechanism”that is, actually and instead, just a collection of phenomenafor catalyst sintering of “Particle Migration and Coalescence”. The steps of bimolecular plus autocatalytic agglomeration provide two specific, balanced chemical equations useful for fitting sintering kinetics data, as is done herein, thereby quantitatively testing proposed sintering mechanisms. These two pseudoelementary reactions also define the specific words and concepts for sintering of bimolecular agglomeration and autocatalytic agglomeration. The results are also significant as the first quantitative investigation of the agglomeration and sintering of an initially atomically dispersed metal on a structurally well-defined (zeolite) support and in the presence of liquid reactants (cyclohexene substrate and cyclohexane solvent) plus H₂. A list of additional specific conclusions is provided in a summary section