Kinetic and mechanistic studies of supported-nanoparticle heterogeneous catalyst formation in contact with solution

2011 Summer.Includes bibliographical references.This dissertation begins with a comprehensive and critical review of the literature addressing the kinetics and mechanism(s) of supported-nanoparticle heterogeneous catalyst formation. The review chapter that follows makes apparent that routine kinetic monitoring methods, as well as well-defined supported-nanoparticle formation systems, are needed in order to gain fundamental insights into the mechanisms of supported-nanoparticle heterogeneous catalyst formation--a somewhat surprising finding given the long history as well as commercial importance of heterogeneous catalysis. Hence, the research presented within this dissertation is focused on (i) developing a kinetic monitoring method (i.e., in what follows, the cyclohexene reporter reaction method) capable of measuring supported-nanoparticle formation in contact with solution, and (ii) developing a well-defined supported-nanoparticle formation system, also in contact with solution, that is amenable to rigorous mechanistic studies. Development of the cyclohexene reporter reaction has allowed for the rapid and quantitative monitoring of the kinetics of Pt(0)n/Al2O3 and Pt(0)n/TiO2 supported-nanoparticle heterogeneous catalyst formation in contact with solution from H2PtCl6/Al2O3 and H2PtCl6/TiO2 respectively. Importantly, those kinetic studies revealed conditions where the most desirable, chemical-reaction-based, supported-nanoparticle formation conditions are present rather than diffusional-limited kinetic regimes. The largest drawback when utilizing the H2PtCl6 as a supported-precatalyst is its speciation--that is, other solvated Pt-based species form when in contact with solution. Such non-uniform speciation leads to a large variation in the supported-nanoparticle formation kinetics, observations that were obtained through the use of the cyclohexene reporter reaction kinetic monitoring method. Due to the large variability in the formation kinetics associated with the H2PtCl6 precatalyst speciation, synthesized next as a part of this dissertation work was the well-defined, fully characterized, speciation-controlled supported-organometallic precatalyst, Ir(1,5-COD)Cl/γ;-Al2O3. When in contact with acetone, cyclohexene and H2 this supported-precatalyst was found to evolve into a highly active and long-lived Ir(0)~900/γ;-Al2O3 supported-nanoparticle catalyst. The kinetics of Ir(0)~900/γ-Al2O3 formation were successfully followed by the cyclohexene reporter reaction method and found to be well-fit by a two-step mechanism consisting of nucleation (A → B, rate constant k1) followed by autocatalytic surface growth (A + B → 2B, rate constant k2) previously elucidated by Finke and Watzky. More specifically, nucleation was found to occur in solution from Ir(1,5-COD)Cl(solvent), while nanoparticle growth occurs on the γ-Al2O3 support, but in a reaction that involves the Ir(1,5-COD)Cl(solvent) species in solution. Most importantly, the fits to the two-step mechanism suggest that the nine synthetic and mechanistic insights, of nanoparticle formation in solution, should now be applicable to the formation of supported-nanoparticle heterogeneous catalysts in contact with solution. That is, it seems reasonable to expect that these studies will allow a more direct avenue for transferring both the mechanistic and synthetic insights that have resulted from the modern revolution in nanoparticle science to the synthesis of size, shape and compositionally controlled supported-nanoparticle catalysts under the nontraditional, mild and flexible conditions where supported organometallics and other precursors are in contact with solution

Water-stable zirconium-based metal-organic framework material with high-surface area and gas-storage capacities.

We designed, synthesized, and characterized a new Zr-based metal-organic framework material, NU-1100, with a pore volume of 1.53 ccg(-1) and Brunauer-Emmett-Teller (BET) surface area of 4020 m(2) g(-1) ; to our knowledge, currently the highest published for Zr-based MOFs. CH4 /CO2 /H2 adsorption isotherms were obtained over a broad range of pressures and temperatures and are in excellent agreement with the computational predictions. The total hydrogen adsorption at 65 bar and 77 K is 0.092 g g(-1) , which corresponds to 43 g L(-1) . The volumetric and gravimetric methane-storage capacities at 65 bar and 298 K are approximately 180 vSTP /v and 0.27 g g(-1) , respectively.OKF, JTH and RQS thank DOE ARPA-E and the Stanford Global Climate and Energy Project for support of work relevant to methane and CO2, respectively. TY acknowledges support by the U. S. Department of Energy through BES Grant No. DE-FG02-08ER46522. WB acknowledges support from the Foundation for Polish Science through the “Kolumb” Program. DFJ acknowledges the Royal Society (UK) for a University Research Fellowship. This material is based upon work supported by the National Science Foundation (grant CHE-1048773).This is the accepted manuscript. The final version is available as 'Water-Stable Zirconium-Based Metal–Organic Framework Material with High-Surface Area and Gas-Storage Capacities' from Wiley at http://onlinelibrary.wiley.com/doi/10.1002/chem.201402895/abstract

The size-brightness correspondence:evidence for crosstalk among aligned conceptual feature dimensions

The same core set of cross-sensory correspondences connecting stimulus features across different sensory channels are observed regardless of the modality of the stimulus with which the correspondences are probed. This observation suggests that correspondences involve modality-independent representations of aligned conceptual feature dimensions, and predicts a size-brightness correspondence, in which smaller is aligned with brighter. This suggestion accommodates cross-sensory congruity effects where contrasting feature values are specified verbally rather than perceptually (e.g., where the words WHITE and BLACK interact with the classification of high and low pitch sounds). Experiment 1 brings these two issues together in assessing a conceptual basis for correspondences. The names of bright/white and dark/black substances were presented in a speeded brightness classification task in which the two alternative response keys differed in size. A size-brightness congruity effect was confirmed, with substance names classified more quickly when the relative size of the response key needing to be pressed was congruent with the brightness of the named substance (e.g., when yoghurt was classified as a bright substance by pressing the smaller of two keys). Experiment 2 assesses the proposed conceptual basis for this congruity effect by requiring the same named substances to be classified according to their edibility (with all of the bright/dark substances having been selected for their edibility/inedibility, respectively). The predicted absence of a size-brightness congruity effect, along with other aspects of the results, supports the proposed conceptual basis for correspondences and speaks against accounts in which modality-specific perceptuomotor representations are entirely responsible for correspondence-induced congruity effects

“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 H-2, in acetone. The results suggest that H+Cl--stabilized Ir(0)(n) nanoparticles, herein Ir(0)(n)center dot(H+Cl-)(a), serve as a WLLL nanoparticle for the preparation of, as illustrative examples, five specific nanoparticle products: Ir(0)(n)center dot(Cl-Bu3NH+)(a), Ir(0)(n)center dot(Cl(-)Dodec(3)NH(+))(a), Ir(0)(n)center dot(POct(3))(0.2n)(Cl-H+)(b), Ir(0)(n)center dot(POct(3))(0.2n), and the gamma-Al2O3-supported heterogeneous catalyst, Ir(0)(n)center dot(gamma-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 POct(3) as a key, prototype example, an important feature is that a minimum, desired, experimentally determinable amount of ligand (e.g., just 0.2 equiv POct(3) per mole of Ir) can be added, which is shown to provide sufficient stabilization that the resultant Ir(0)(n)center dot(POct(3))(0.2n)(Cl-H+)(b) is isolable. Additionally, the initial labile ligand stabilizer HCl can be removed to yield Ir(0)(n)center dot(POct(3))(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)center dot(H+Cl-)(a) as a WLLL nanoparticle

Kinetic Evidence for Bimolecular Nucleation in Supported-Transition-Metal-Nanoparticle Catalyst Formation in Contact with Solution: The Prototype Ir(1,5-COD)Cl/γ-Al₂O₃ to Ir(0)_∼900/γ-Al₂O₃ System

Kinetic and mechanistic studies of the formation of supported-nanoparticle catalysts in contact with solution hold promise of driving the next generation syntheses of size, shape, and compositionally controlled catalysts. Recently, we studied the kinetics and mechanism of formation of a prototype Ir(0)_∼900/γ-Al₂O₃ supported-nanoparticle catalyst from Ir­(1,5-COD)­Cl/γ-Al₂O₃ in contact with solution (Mondloch, J.E.; Finke, R.G. <i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 7744). Key kinetic evidence was extracted from γ-Al₂O₃- and acetone-dependent kinetic curves in the form of rate constants for nucleation (A → B, rate constant <i>k</i>_1obs) and autocatalyic surface growth (A + B → 2B, rate constant <i>k</i>_2obs), where A is nominally the Ir­(1,5-COD)­Cl/γ-Al₂O₃ and B the growing, supported Ir(0)_<i>n</i>/γ-Al₂O₃ nanoparticle. The resultant data provided evidence for a mechanism consisting of four main steps: Ir­(1,5-COD)­Cl­(solvent) dissociation from the γ-Al₂O₃ support, then Ir­(1,5-COD)­Cl­(solvent) solution-based nucleation, fast nanoparticle capture by the γ-Al₂O₃ and then subsequent nanoparticle growth between Ir(0)_<i>n</i>/γ-Al₂O₃ and Ir­(1,5-COD)­Cl­(solvent) in solution. While the <i>k</i>_2obs vs [γ-Al₂O₃]_sus and [acetone] autocatalytic surface growth rate constants were nicely accounted for by the proposed mechanism, the <i>k</i>_1obs nucleation rate constants were only “roughly” accounted for by the previously proposed <i>unimolecular</i> solution-based nucleation mechanism. Hence, in the present work we have reexamined that γ-Al₂O₃- and acetone-dependent nucleation data in light of the hypothesis that nucleation is actually <i>bimolecular</i>. Extracting bimolecular, <i>k</i>_1obs(bimol), rate constants by curve-fitting yields qualitative (i.e., visual inspection) as well as quantitative (i.e., increased <i>R</i>² values) <i>evidence consistent with and strongly supportive of solution-based bimolecular nucleation</i> (A + A → 2B, rate constant <i>k</i>_1obs(bimol)) for the Ir­(1,5-COD)­Cl/γ-Al₂O₃ to Ir(0)_∼900/γ-Al₂O₃ system in contact with acetone. The extracted <i>k</i>_1obs(bimol) vs [γ-Al₂O₃]_sus and [acetone] data in turn rule out the solution-based unimolecular mechanism (as well as a hypothetical termolecular nucleation mechanism). This study is significant in that (i) it is the first evidence for bimolecular nucleation in transition-metal nanoparticle formation in any system, be it ligand- or support-stabilized nanoparticle formation in solution or on solid-supports in gas–solid systems, and since (ii) it shows that mechanism-based nanoparticle size control, previously demonstrated to depend on <i>k</i>_1obs, is hereby shown to actually depend on 2<i>k</i>_1obs(bimol)[A]¹. Furthermore, the results presented are of broad significance since (iii) they are part of a growing literature suggesting that simple, bimolecular nucleation may well be closer to the rule, rather than the exception, in a range of systems across nature, and since the results herein (iv) disprove, for at least the present system, the higher nuclearity nucleation kinetics suggested by nucleation theory and its often discussed critical nucleus concept. The results also (v) argue for the new concept of a “kinetically effective nucleus”, in this case binuclear M₂ (M = metal)

Synthesis of Heterogeneous Ir⁰_∼600–900/γ-Al₂O₃ in One Pot From the Precatalyst Ir(1,5-COD)Cl/γ-Al₂O₃: Discovery of Two Competing Trace “Ethyl Acetate Effects” on the Nucleation Step and Resultant Product

In 2010 we reported a two-step synthesis of a Ir⁰_∼900/γ-Al₂O₃ supported-nanoparticle catalyst. In that study, a well-defined Ir­(1,5-COD)­Cl/γ-Al₂O₃ precatalyst was <i>isolated</i> and characterized before being reduced in contact with acetone solvent and cyclohexene and under H₂ in a second step. Synthetically, one would like to remove the Ir­(1,5-COD)­Cl/γ-Al₂O₃ precatalyst isolation step, shortening the precatalyst synthesis and allowing the overall synthesis to be accomplished more efficiently in one pot. However, herein we report that the one-pot synthesis starting from commercially available [Ir­(1,5-COD)­Cl]₂ and γ-Al₂O₃ yields <i>an order of magnitude increase in the observed nucleation rate constant</i>, <i>k</i>_<i>1,obs</i>, as well as a decrease in the average particle size from Ir⁰_∼900 to Ir⁰_∼600. Mechanistic experiments reveal that the origin of this effect, amazingly, is the <i>presence of residual ethyl acetate</i> employed in the isolated precatalyst synthesis, which is not present in the one-pot synthesis. Additional mechanistic probing, along with multiple control experiments, reveals that the presence of even small levels of EtOAc has <i>two, competing effects</i>: a nucleation <i>enhancing</i> effect of increasing the amount of solvated Ir­(1,5-COD)­Cl­(solvent) dissociated off of the γ-Al₂O₃ support (a step known to be involved in nucleation in solution on the basis of a second paper published in 2011), but then also a more dramatic effect of EtOAc reacting with Ir⁰<i>_n</i> (or possibly Ir_<i>x</i>H_<i>y</i>) nuclei to <i>inhibit</i> nucleation. Armed with these mechanistic insights, we achieved the goal of one-pot syntheses by controlling the presence or absence of the EtOAc. Overall, seemingly innocent solvents such as EtOAc are hereby added to an increasing list of variables crucial to achieving reproducible nanoparticle nucleation- and growth-based syntheses. A conclusions section summarizes those variables along with five additional noteworthy findings and recommendations from the present study

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-Al₂O₃ to 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 <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 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