γ-Al2O3 in One Pot From

Research Article pubs.acs.org/acscatalysis

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 Patrick Kent, Joseph E. Mondloch,† and Richard G. Finke* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States S Supporting Information *

ABSTRACT: In 2010 we reported a two-step synthesis of a Ir0∼900/γAl2O3 supported-nanoparticle catalyst. In that study, a well-defined Ir(1,5-COD)Cl/γ-Al2O3 precatalyst was isolated 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(1,5COD)Cl]2 and γ-Al2O3 yields an order of magnitude increase in the observed nucleation rate constant, k1,obs, 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 presence of residual ethyl acetate 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 two, competing effects: a nucleation enhancing 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 Ir0n (or possibly IrxHy) nuclei to inhibit 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. KEYWORDS: supported-nanoparticle catalyst synthesis, reproducibility, one-pot syntheses, kinetics and mechanism, two effects of EtOAc

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INTRODUCTION Small metal nanoparticles fixed on high-surface-area supports constitute a large and important subset of heterogeneous catalysts.1 Despite their importance, the syntheses of these industrially significant catalysts are still largely empirical.2 Control over supported-nanoparticle catalyst syntheses is crucial, given that key catalyst properties, such as catalytic activity, selectivity, and lifetime, depend on the supportednanoparticle size, shape, and composition.3 Hence, a current goal in heterogeneous catalyst preparation is to transfer the synthetic,4 as well as developing mechanistic,5−11 insights from the modern revolution in nanoparticle scienceincluding control over nanoparticle composition, 12 size, 13 and shape14to the synthesis of supported-nanoparticle heterogeneous catalysts.15−19 A current, attractive synthetic approach to making supported-nanoparticle catalysts15−18,20 starts from discrete, supported molecular precursors. Running such syntheses in contact with solution in a liquid−solid reaction, rather than the traditional gas−solid reaction, allows the solution to influence © XXXX American Chemical Society

the nucleation and autocatalytic growth steps of the nanoparticle catalyst formation reaction.5,10,21 The synthesis of supported-nanoparticle catalysts in contact with solution potentially also offers superior control over the final nanoparticle composition, size, and shape by the careful selection of ligands and solvents that can, in the ideal scenario, serve as “weakly ligating/labile ligands”22,23 for the resultant supportednanoparticle catalysts. The weakly ligated/labile ligand nanoparticle synthesis strategy5,10,21 offers considerable flexibility, as well as shortened, more atom-economical syntheses, in comparison to the more widely investigated method of first making and isolating ligandor polymer-stabilized transition-metal nanoparticles in solution, subsequently depositing them on a chosen, high-surface-area support and then trying to remove the stabilizing ligands or polymers.24,25 Unfortunately, the harsh thermal, oxidative, Received: January 26, 2016 Revised: June 27, 2016

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COD)Cl]2 and γ-Al2O3 are slurried in ethyl acetate for 24 h (EtOAc being chosen as a seemingly innocuous solvent picked to limit the speciation of the [Ir(1,5-COD)Cl]2 to its dimeric form15). The resultant Ir(1,5-COD)Cl/γ-Al2O3 is then collected and dried under vacuum at room temperature to yield a yellow powder (Scheme 1, I1). (ii) Subsequently, 50 mg of that isolated Ir(1,5-COD)Cl/γ-Al2O3 precatalyst (corresponding to 0.97 mM Ir) is placed in acetone and cyclohexene and reduced under 40 psig of H2 while the mixture is stirred at 600 rpm to yield the dark gray Ir0∼900/γ-Al2O3 supportednanoparticle catalyst. This catalyst was also thoroughly characterized by its reaction stoichiometry, transmission electron microscopy (TEM), and XAFS (Scheme 1, I2). The nanoparticle formation kinetics for the net A (=Ir(1,5COD)Cl/γ-Al2O3) → B (=Ir0n/γ-Al2O3) reaction were obtained and were well-fit16 by the Finke−Watzky two-step (FW 2-step) mechanism5 of slow, continuous nucleation (A→ B, rate constant k1,obs) followed by fast autocatalytic surface growth (A + B → 2B, rate constant k2,obs). Note that A and B are unequivocally defined by the reaction stoichiometry established above: A is the Ir(1,5-COD)Cl/γ-Al2O3 precatalyst starting material, and B is Ir0 in the growing and final Ir0n/γAl2O3 product.16 The resultant Ir0∼900/γ-Al2O3 supported-nanoparticle catalyst is a prime example of what we have termed a weakly ligated/ labile ligand22,23 nanoparticle catalyst system in that (a) it was formed with only the reagents of interest for the resultant catalytic reaction being present during its formation (cyclohexene, H2, and acetone solvent), (b) it is, as a result and by that design, highly active (8200 turnovers/h), and (c) it is relatively long lived in the simple test reaction of cyclohexene hydrogenation (≥220000 total turnovers). Additionally, as documented in a recent review,19 this prototypical,31 weakly ligated/labile ligand22,23 supported-nanoparticle formation system16,18 is one of the best kinetically and mechanistically studied systems that is formed in contact with solution,32 as opposed to the more traditional gas−solid phase syntheses of supported-nanoparticle catalysts. While the previous isolation of the precatalyst Ir(1,5COD)Cl/γ-Al2O3 allowed for its valuable, full characterization,16 we wonderedfrom the perspective of a preferred synthesisif the isolation step shown in Scheme 1, I1 could be eliminated from the synthetic procedure? That is, could one simply mix the [Ir(1,5-COD)Cl]2 with γ-Al2O3 in acetone and cyclohexene and then reduce that precatalyst under H2 in a onepot synthesis (Scheme 1, II1)? Hence, the two supported precatalysts employed herein are either isolated prior to use in the catalyst formation step16,17 (defined as the isolated system for what follows) or generated in a one-pot solution reaction system (defined as the one-pot system). Despite the seemingly simple and ostensibly innocent nature of this change, the literature28−30 demonstrates that heterogeneous catalyst syntheses can be quite sensitive to the precise synthetic conditions. As such, our starting hypothesis for the present work evolved to be “that a simple change to a one-pot synthesis actually might not successfully reproduce the published16,17 Ir0∼900/γ-Al2O3 system”. Indeed, we have previously observed that nucleation in particular can be highly sensitive to trace solvent impurities.33 For instance, the Ir/ Al2O3 system appears to be extremely susceptible to trace peroxides in the cyclohexene substrate unless the cyclohexene is highly purified.34 Additionally, the autocatalysis, an established key part of our nanoparticle formation kinetics,5−10,15−18 is

reductive, plasma, or other severe treatments needed to attempt to remove the massive polymer or other ligands can alter the supported-nanoparticle composition, size, and shape.24,25 Additionally, it is typically impossible to completely remove residual C, N, or O overlayers often left behind, overlayers which alter the heterogeneous catalysts in unknown, and often undesired, ways.26,27 Despite the attractiveness of the weakly ligated/labile ligand nanoparticle synthesis strategy,22,23 little work has appeared investigating factors that might influence the reproducibility of supported-nanoparticle heterogeneous catalyst syntheses. Even simple questions, such as whether one needs to preisolate molecular, supported-organometallic precatalysts en route to supported nanoparticle catalysts, remain unanswered. Can one simply make the precatalyst in one pot en route to the identical resultant supported catalyst, one wonders, thereby saving a step in the synthesis? Or will such changes introduce unexpected consequences? These simple questions are the starting point of the present contribution.28−30 One additional goal of the present studies is to examine supported-nanoparticle catalyst synthesis via the relativity little studied19 probe of the nucleation and growth kinetics and mechanism of the supported-nanoparticle formation reaction.19 What variables contribute to reproducibility or irreproducibility there? The Preisolated, Well-Characterized Ir(1,5-COD)Cl/γAl2O3 to Ir0∼900/γ-Al2O3 Prototype31 Supported-Nanoparticle Catalyst Synthesis. In a series of recent papers,16−18 we have prepared a “prototype”31 Ir0∼900/γ-Al2O3 heterogeneous catalyst system (Scheme 1), resulting in 2.9 ± 0.4 nm Ir0 nanoparticles supported on γ-Al2O3. As Scheme 1 shows, the established synthesis involves preisolation and unequivocal characterization of a Ir(1,5-COD)Cl/γ-Al2O3 precatalyst. The resultant yellow precatalyst Ir(1,5-COD)Cl/γ-Al2O3 was characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES), CO/IR spectroscopy trapping experiments, and X-ray absorbance fine structure spectroscopy (XAFS). The Ir(1,5-COD)Cl/γ-Al2O3 precatalyst is then converted, under H2 and in contact with solution, to the Ir0∼900/γ-Al2O3 product in a second step. Specifically, in this isolated-precatalyst, two-step synthesis, we note the following details. (i) [Ir(1,5Scheme 1. Two-Step Synthesis of the SupportedNanoparticle Catalyst Ir∼900/γ-Al2O316 Involving (I) an Isolated Precatalyst System and Its Comparison to the (II) One-Pot Catalyst Synthesis System

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under the dry N2 atmosphere. Gas−liquid chromatography (GLC) was performed using a Hewlett-Packard 5890 Series II chromatograph, along with a flame-ionization detector, that was equipped with a Supelco SPB-1 (Aldrich, 30 m × 0.25 mm × 0.25 μm) fused-silica column. The GLC parameters were as follows: initial oven temperature, 50 °C; initial time, 3.0 min; rate, 10 °C/min; final temperature, 160 °C; injector temperature, 180 °C; detector temperature, 200 °C; injection volume, 2 μL. TEM analysis was conducted at Colorado State University with Dr. Roy Geiss on a JEOL JEM2100F instrument at 200 kV. UV−vis spectroscopy experiments were run on a Hewlett-Packard 8452A diode array spectrophotometer, and the data were analyzed via Hewlett-Packard’s UV−visible ChemStation software. Hydrogenation Apparatus and Data Handling. Hydrogenation experiments for monitoring the H2 reduction of [Ir(1,5-COD)Cl]2 in contact with acetone, γ-Al2O3, and cyclohexene were carried out in a previously described apparatus5,6 which continuously monitors the H2 pressure loss. Briefly, the apparatus consists of a Fischer−Porter (FP) bottle modified with Swagelock TFE-sealed Quick-Connects to both a H2 line and an Omega PX621 pressure transducer. The pressure transducer was interfaced to a PC through an Omega D1131 5 V A/D converter with a RS-232 connection. Reactions were run at a constant temperature by immersing the FP bottle in a 500 mL jacketed reaction flask containing dimethyl silicon fluid (Thomas Scientific); the temperature was regulated by a thermostated recirculating H2O bath (VWR). Pressure uptake data were collected using LabVIEW 7.1. The H2 uptake curves were converted to cyclohexene curves using the previously established 1/1 H2/cyclohexene stoichiometry.5,33 The data were also corrected for the acetone solvent vapor pressure using the previously established protocol37 by back-extrapolating the experimental vapor pressure rise (seen in the induction period) of a control run using just the acetone solvent.37 The cyclohexene kinetic curves were fit to the analytic eq 1 for the Finke−Watzky two-step mechanism of first-order nucleation (A → B, rate constant k1,obs)5 and autocatalytic surface growth (A + B → 2B, rate constant k2,obs) using nonlinear leastsquares fitting in Origin 7.0. The k2,obs values were then corrected by the mathematically required ∼1700 = 1.65 M cyclohexene/0.00097 M Ir stoichiometry factor, 1700k2,curvefit = k2,obs, to account for the cyclohexene reporter reaction and its pseudoelementary step methodology.5 To ensure that the hydrogenation kinetics were reporting on only the conversion of A, and not the accompanying olefin hydrogenation reporter reaction,5 just the first half of the data were fit so that the condition [A]t ≪ [cyclohexene]t was met.5 The resultant rate constants were then used to simulate a curve for the entire formation kinetics.5

known to amplify sensitivity to the reaction conditions and lead to irreproducibility,35 as Epstein and co-workers have convincingly demonstrated.36 Furthermore, we previously found that simply changing just the order of addition of the iridium to the γ-Al2O3 in the isolated precatalyst synthesis slowed the catalyst formation kinetics.16 We also reasoned that the kinetic and mechanistic handles for the Ir(1,5-COD)Cl/γ-Al2O3 to Ir0n/γ-Al2O3 prototypical supported-nanoparticle formation system, as well as this system’s published kinetic and mechanistic information,16−18 provide an unparalleled opportunity to probe whetherand if so, howthe change to a one-pot, in situ precatalyst formation might alter the nanoparticle formation nucleation and growth steps. Focus of the Present Contribution. The first question addressed herein then is as follows: does the omission of a simple, isolation step in the synthesis of the Ir(1,5-COD)Cl/γAl2O3 precatalyst change either the kinetics or the Ir0n/γ-Al2O3 products? The answer will be that as the synthesis was originally performed, it does. The second question addressed is “why and how”? After considerable “mechanistic detective work” (vide infra) we were able to track down the cause for the change: the presence of ethyl acetate used in the synthesis of the isolated Ir(1,5-COD)Cl/γ-Al2O3 vs its absence in the onepot synthesis. We demonstrate the unprecedented effect of EtOAc on the nucleation kineticsa change in the apparent rate constant for nucleation of ∼101! The third question addressed is the following: if the EtOAc variable is controlled (i.e., either by having it present in both syntheses or omitting it from both syntheses), can then the nanoparticle formation kinetics and resultant products from these two synthetic routes be made reproducible? The answer to this third question will be “yes”, a pleasing end result. The results are of synthetic significance in showing that, if one controls all the known, relevant variables of the system, and if one has handles to monitor the nucleation and growth kinetics, then a simpler, one-pot synthesisof highly active, long-lived, weakly ligated/ labile ligand, supported-nanoparticle catalystsis both possible and synthetically readily achieved.

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EXPERIMENTAL SECTION Materials. All solvents and compounds were stored in a drybox prior to use. Used as received and packed under N2 were [Ir(1,5-COD)Cl]2 (STEM, 99%), acetone (Aldrich, Chromasolv Plus, water content 7 year period from data obtained with the P2W15Nb3O629− polyoxoanion-stabilized Ir0∼300 nanoparticle system.37 d σ as used here is the breadth of the particle size distribution for a single set of microscopy data.

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experiments studying the effect of the [Al2O3]sus in the prototype, isolated precatalyst system.17,18 The data were fit to eqs 3 and 4 derived previously38 for solution second-order nucleation (A + A → 2B), which yield k1(bimol) and k2 as defined by eqs 3 and 4 (and which thereby have the [Al2O3]sus effect factored out).17,18 The fits to eqs 3 and 4 account well for the data (as they did before for our prior work18), giving k1(bimol) and k2 as [15(5)] × 101 and [2.0(2)] × 104 h−1 M−1, respectively.17,18 The growth rate constant, k2, for the one-pot system is within experimental error (1σ) of the [4(1)] × 104 h−1 M−1 previously reported17 for the isolated precatalyst. More importantly, the bimolecular nucleation rate constant k1(bimol) = [15(5)] × 101 h−1 M−1 for the EtOAc-free, One-Pot system is different by 2σ and approximately 7.5 times larger than that of [2.0(1.6)] × 101 r−1 M−1 which we previously measured for the Isolated Precatalyst system.18 Differences in the Two Supported Catalysts Produced By the One-Pot Synthesis vs the Synthesis Employing Isolated Precatalyst. In the present work, Standard Conditions One-Pot precatalysts were prepared on a 50 mg scale with 24 h of stirring before commencing reduction. On the other hand, the Ir(1,5-COD)Cl/γ-Al2O3 isolated precatalyst was prepared on a 1020 mg scale by dissolving and dispersing in ethyl acetate followed by drying under vacuum before 50 mg was returned to acetone solution for reduction, all as described in Table 1.16 Table 2 summarizes the experimental results for the Standard Conditions One-Pot system in comparison to those obtained when starting with the Standard Conditions Isolated Precatalyst system. The one-pot precatalyst system’s reductive catalyst formation (Scheme 1) occurs modestly faster, with the entire reaction taking two-thirds the time as when using the isolated precatalyst system. The observed nucleation rate constant, k1,obs, is a bit more than 1 order of magnitude (precisely, 15 times) larger for the one-pot precatalyst in comparison to the isolated system, while the two systems’ growth rate constants, k2,obs, are identical within experimental error.39 This enhanced nucleation rate for the one-pot synthesis is retained to the extent of ∼7.5-fold in the k1(bimol) rate constants extracted using eq 3: [15(5)] × 101 h−1 M−1 for the one-pot synthesis vs [2.0(2)] × 101 h−1 M−1 for the isolated precatalyst system. As previously noted, microscopy of the Ir0n/Al2O3 products from the two syntheses show distinct particle size distributions (Table 2 and Figure S5 in the Supporting Information). The nanoparticles from the one-pot synthesis are on average Ir0∼600/ Al2O3, whereas the isolated system’s particles are an on average ∼300 Ir0 atoms larger, Ir0∼900/Al2O3. Consistent with this, comparing the two systems’ TOFs on a total Ir basis reveals that the one-pot system gives a larger TOF indicative of smaller particles with a higher percentage of Ir exposed for catalysis. However, and pleasingly, following correction of the two apparent TOF values for the number of surface atoms based on the average particle sizes of 600 and 900 atoms, the site-corrected TOFs are identical within experimental error between the two systems, as expected for a so-called “structure-insensitive” (i.e., particle-size-insensitive) reaction such as olefin hydrogenation.40 Uncovering the Reason behind the Changes in the Nucleation Kinetics and Resultant Products in the Two Systems. Several experiments were performed to tease out the explanation for the differences summarized in Table 2 for the two systems. The first two alternative hypotheses investigated

precatalyst, the nucleation rate constant for the one-pot synthesis is an order of magnitude largeran important, previously unavailable observation. Initial Analysis of the More Intimate Formation Mechanism of Ir0∼600/γ-Al2O3 in Contact with Solution. Has It Changed As Well? Our previous work provided evidence for a dissociative prior equilibrium, Ir(1,5-COD)Cl/ Al2O3 + 2 solvent ⇌ Ir(1,5-COD)Cl(solvent) + Al2O3, followed by H2 reduction of the Ir(1,5-COD)Cl(solvent) in solution, as the more intimate mechanism of the nucleation and growth process.17 To test whether the [Ir(1,5-COD)Cl]2 dimer starting complex undergoes a similar scission to two solvated monomers under the one-pot conditions, UV−visible spectra were recorded of the 1.46 mM [Ir(1,5-COD)Cl]2 stock solution (diluted to 0.49 mM) and then of the one-pot precatalyst solution after 24 h stirring. The results were then compared to the UV−visible for the isolated precatalyst solution (Figures S2−S4 in the Supporting Information). After the one-pot precatalyst solution was stirred for 24 h in contact with γ-Al2O3, the [Ir(1,5-COD)Cl]2 dimer stock solution’s λmax value at 460 nm changed to give the absorbance at 396 nm, characteristic of the isolated precatalyst solution. This confirms scission of the [Ir(1,5-COD)Cl]2 dimer to form Ir(1,5-COD)Cl(solvent) in the one-pot system and, hence, qualitatively similar Ir speciation in both systems. Next, evidence was obtained in the one-pot system for the role of a dissociative prior equilibrium in the more intimate pathway of nanoparticle nucleation and growth. The needed experiments followed closely previous studies performed17 with the isolated Ir0900/γ-Al2O3 system. Specifically, the “concentration” of suspended γ-Al2O3 support,17 [Al2O3]sus, was varied over a factor of 5 to determine its effect on the nucleation and growth (Figure 3), again all paralleling our prior, successful

Figure 3. Change in k1,obs(bimol) and k2,obs rate constants against [γAl2O3]sus. The red line fits are to eqs 3 and 4 derived first as part of our previous works.17,18 The resultant k1(bimol) and k2 values for the present study of the one-pot system are [1.5(5)] × 102 and [2.0(2)] × 104 h−1 M−1, respectively, as defined in eqs 3 and 4.17,18 5455

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ACS Catalysis were that the observed changes are somehow due to one or both of the following differences in the two syntheses: (a) the lack of 24 h of precatalyst solution aging in the isolated precatalyst system immediately prior to its hydrogenation, or (b) the lack of mechanical grinding of the γ-Al2O3 support due to continual mechanical stirring in the isolated precatalyst system (which was dried under vacuum). The latter could have resulted in a changed surface area for the alumina support in the one-pot synthesis, thereby conceivably influencing the synthesis. To test these two alternative hypotheses, two control experiments were done: first an isolated precatalyst solution was stirred for 24 h prior to olefin addition. Separately, γ-Al2O3 was mechanically ground and fragmented by stirring dry for 1 day prior to its subsequent use in a one-pot catalyst formation reaction. Both experiments produce kinetics unchanged from the Standard Conditions Isolated Precatalyst and Standard Conditions One-Pot systems, respectively (Figures S6 and S7 in the Supporting Information). Hence, the above two conceivable alternative hypotheses are ruled out. Next, the third and in the end winning hypothesis was investigated: namely, the hypothesis (c) that the lack of any ethyl acetate in the one-pot precatalyst preparation is affecting the nucleation kinetics somehowat the time, pretty much a “last ditch, Hail Mary” hypothesis without any precedent in the literature that we could find (nor any precedent via subsequent, exhaustive literature searches; vide infra). Analysis of past gas chromatograms from the cyclooctane evolution experiments for both systems, against a control chromatogram of 2 μL ethyl acetate in 2.5 mL of acetone and 0.5 mL of cyclohexene, found a peak corresponding to ethyl acetate only in chromatograms f rom the isolated precatalyst (i.e., but not in one-pot reactions examined as a control, as expected since EtOAc is not used in the one-pot synthesis). The influence of ethyl acetate was then tested in both systems by reversing when EtOAc is and is not present. Specifically, the “reverse syntheses” of (i) synthesizing an isolated precatalyst using acetone in place of the ethyl acetate solvent (denoted “EtOAc-Free Isolated Precatalyst” in Table 3)

Figure 4. Nanoparticle formation kinetics of the EtOAc-free isolated system (circles; FW 2-step fit and simulation, red line) against the EtOAc-Al2O3 one-pot synthesis system (diamonds; FW 2-step fit and simulation). The key finding is that the new, now “EtOAc-Free Isolated Precatalyst” system now has the faster kinetics, while the modified, EtOAc-containing, “EtOAc-Al2O3 One-Pot” synthesis system shows the slower kinetics.

Supporting Information). In short, controlling the absence, or presence, of EtOAc is indeed the key to seeing the larger vs smaller k1,obs nucleation rate constant. In summary, two conceivable alterative hypotheses for the observed differences in the Standard Conditions systems were disproved. Instead, the slowed nucleation and overall slower kinetics correlate only and precisely with the presence of EtOAc (Table 3). The results provide seemingly incontrovertible evidence that an EtOAc effect on the nucleation step is the source of the difference in the particle-formation kinetics and the resultant supported-nanoparticle size difference. Deeper Look at the Effect of EtOAc. Is it Due to Perturbation of the Dissociative Equilibrium, KDiss, That Yields the Readily Reduced and Hence Nucleated Ir(1,5COD)Cl(solvent), or Is It Due to Some Other Effect or Combination of Effects? Next, a return to the literature was made for an exhaustive literature search seeking any report of the influence of EtOAc on metal−nanoparticle formation kinetics. No such report of any kind was found. Indeed, the only literature we could find describes ethyl acetate as having a small promoting effect on heterogeneous catalytic hydrogenations.41,42 Hence, a more thorough investigation of the effect(s) of EtOAc was undertaken as described nextwhich in turn uncovered two, opposing, previously unknown effects of EtOAc on the supported-nanoparticle catalyst formation reaction and its nucleation kinetics. Our initial hypothesis for the more detailed cause of the observed EtOAc effect was that EtOAc is somehow decreasing the amount of Ir(1,5-COD)Cl(solvent) dissociated off of the Al2O3 support and, therefore, slowing nucleation. The basis of this hypothesis is the mechanistic knowledge that nucleation for the Ir(1,5-COD)Cl/γ-Al2O3 system has been shown to take place in solution:17,18 that is, off of the support via the kinetically more readily reduced Ir(1,5-COD)Cl(solvent) (i.e., and not via the supported complex, Ir(1,5-COD)Cl/γ-Al2O3). More specifically, the first hypothesis was that the slower nucleation in the EtOAc-employing, Standard Conditions Isolated Precatalyst system must somehow be due to a decreased KDiss,

Table 3. Comparison of the Four Synthetic Conditions: Isolated or One-Pot System vs Presence or Absence of Ethyl Acetate system

presence of ethyl acetate

standard conditions isolated standard conditions one-pot EtOAc-free isolated EtOAc-Al2O3 one-pot

yes: in precatalyst synthesis no: only acetone, cyclohexene no: acetone in precatalyst synth yes: Al2O3 preprocessed in EtOAc

and also (ii) processing the starting γ-Al2O3 in ethyl acetate prior to its use in a one-pot synthesis (denoted “EtOAc-Al2O3 One-Pot Synthesis” in Table 3). The synthetic changes in the isolated and one-pot precatalyst conditions caused the respective kinetics to shift toward those observed in the other, Standard Conditions One-Pot and Standard Conditions Isolated Precatalyst systems (Figure 4). Specifically, the new “EtOAc-Free Isolated Precatalyst” now exhibits the larger k1,obs value similar to the case for the original, Standard Conditions One-Pot synthesis. Equally significant and telling, the new, now “EtOAc-Al2O3 One-Pot Synthesis” system now has the smaller k1,obs value similar to the original, Standard Conditions Isolated Precatalyst system (Figures S8 and S9 and Table S1 in the 5456

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ACS Catalysis defined17,18 by the dissociative equilibrium Ir(1,5-COD)Cl/γAl2O3 ⇌ Ir(1,5-COD)Cl(solvent) + [Al2O3]sus. To test this first hypothesis, both Standard Conditions OnePot and Isolated Precatalyst solutions were prepared. Each solution was then passed through a 0.2 μm syringe filter to remove the solid support. The filtrate from each was then analyzed by UV−visible for the amount of solvated Ir(1,5COD)Cl(solvent) present. In addition, to yield a second determination of the KDiss value for each system, each filtered solution was also then hydrogenated to measure the amount of cyclooctane produced from the [Ir(1,5-COD)Cl(solvent)] present (as a proxy for the amount of Ir(1,5-COD)Cl(solvent) present) (Table 4). Further details of these experiments and resultant KDiss measurements are available in the Experimental Section for the interested reader.

experimentally this ef fect of an otherwise seemingly innocent solvent on the ability of a solid oxide to bind an organometallic precatalyst. As such, this (first, vide infra) “EtOAc effect” is itself an interesting and valuable result of the present studies and mechanistic detective work. Uncovering the Secondand DominantEffect of the Presence of Ethyl Acetate: EtOAc Inhibition of Nucleation. The above findings, namely that the effects of EtOAc on the KDiss values are opposite the direction needed to explain the ca. order-of-magnitude faster nucleation in the (EtOAc-free) original, Standard Conditions One-Pot system, leads one to the next hypothesis we investigated. Specifically, we hypothesized next that the presence of relatively low levels of residual EtOAc must be inhibiting nucleation. To test this next hypothesis, precatalyst solutions made from a Standard Conditions One-Pot synthesis and then from a Standard Conditions Isolated Precatalyst synthesis were separately prepared, and each was separately spiked with 1 equiv of ethyl acetate per equivalent of iridium present. Their supported-nanoparticle formation kinetics were then measured. The spiked isolated system’s kinetics showed a decreased nucleation as expected for an EtOAc inhibition effect (Figure S11 in the Supporting Information), but somewhat surprisingly the original (EtOAc-free) one-pot system showed no detectable change in the kinetics (Figure S12 in the Supporting Information). Reflection on these results led to the subhypothesis that greater than 1 equiv of ethyl acetate can react with or otherwise be adsorbed to the support, fully consistent with the interpretation above that EtOAc can lead to acylation of Al− OH sites on the support. Hence, next we tested the effect of added EtOAc in the absence of any oxide support. In this experiment, just the [Ir(1,5-COD)Cl]2 dimer (i.e., without any γ-Al2O3 present) was reduced under H2 in acetone and cyclohexene in comparison to an identical solution which had been spiked with 1 equiv of EtOAc per equivalent of iridium present. The solution containing 1 equiv of EtOAc exhibited a lower k1,obs value for nucleation (by about a factor of ∼2 in this rough model system where this experiment is possible). Hence, while not the perfect experiment of examining pure Ir(1,5-COD)Cl(solvent) monomer with EtOAc, this available and useful control experiment indicates (i) that EtOAc alone can and does indeed inhibit nucleation of a Ir(1,5-COD)-based system and (ii) that the γ-Al2O3 support is not needed for the nucleationinhibiting effect (Figure 5). In addition, these control experiments (iii) offer additional evidence consistent with and supportive of a reaction of EtOAc with the γ-Al2O3 support. In short, the above control experiments demonstrate that EtOAc slows the nucleation of Ir0 nanoparticles. A plausible explanation for this inhibition is oxidative addition of EtOAc to the low-valent, formally Ir0, thereby oxidizing it and stopping its participation in the nanoparticle formation reaction.47 Oxidative addition of ethyl acetate has previously been observed in Ir(II)48 and Ni(0) complexes,49 and other ester additions have been observed with Rh(0)50 and Pt(0)51 complexes, but not previously for multimetallic Ir0. An alternative explanation is that a plausible iridium hydride, IrxHy, intermediate21 is intercepted by reaction with EtOAc. The latter explanation is actually favored by the unexpected and interesting finding that the autocatalytic growth k2obs values are largely unchanged by the presence of EtOAc (see Table S1 and Figures S11 and S12 in the Supporting Information), given that

Table 4. Comparison of the Kdiss Values Measured by UV− Visible and GC for the Standard Conditions Isolated Precatalyst (EtOAc-Exposed) and Standard Conditions OnePot (EtOAc-Free) Syntheses, as Well as for the “Reverse Syntheses” of “EtOAc-Free Isolated Precatalyst” and the “EtOAc-Al2O3 One-Pot Synthesis” precatalyst system

KDiss by UV−visible

KDiss by GC

standard isolated standard one-pot EtOAc-free, isolated EtOAc-Al2O3, one-pot

[14(7)] × 10−3 [3.3(1.4)] × 10−3 [4.4(3)] × 10−3 [34(6)] × 10−3

[11(0.3)] × 10−3 [3.0(7)] × 10−3 [4.4(3)] × 10−3 [10(9)] × 10−3

These determinations of KDiss reveal that the EtOAc-exposed, Isolated Precatalyst system has a KDiss value 4.2 times greater not smaller!than the KDiss value for the original, Standard Conditions One-Pot system (Table 4). This translates to 2.6 times morenot lessIr(1,5-COD)Cl(solvent) being liberated under the reaction conditions from the γ-Al2O3 support in the original EtOAc-employing isolated precatalyst synthesis in comparison to the original EtOAc-free, one-pot system. As a check on the KDiss determinations and their correlation with the kinetics, KDiss values were determined for the previously described two “reverse syntheses”, the modified “EtOAc-Free Isolated Precatalyst” synthesis and the “EtOAcAl2O3 One-Pot” system. As Table 4 summarizes, the trend in KDiss is now reversed, again tracking with the exposure to EtOAc. The ethyl acetate containing, “EtOAc-Al2O3 One-Pot” synthesis now has the larger KDiss = [34(6)] × 10−3 while the smaller KDiss = [4.4(3)] × 10−3 value now corresponds to the modified, “EtOAc-Free Isolated Precatalyst” system. Hence, the trend in KDiss for each of the four possible syntheses is opposite that needed to account for the observed kinetics. Although unprecedented, the observed effect of EtOAc on the KDiss values is chemically reasonable. An exchange of acetyl groups from EtOAc to an Al−OH site on the support, EtO− C(O)CH3 + Al−OH ⇌ EtOH + Al−O−C(O)CH3, is chemically plausible. Acylation of the OH groups of the support could, in turn, block Al−OH sites in the γ-Al2O3 where Ir(1,5-COD)Cl would otherwise bind, in turn yielding an increased apparent KDiss, Ir(1,5-COD)Cl/γ-Al2O3 ⇌ Ir(1,5COD)Cl(solvent) + [Al2O3]sus. There is certainly literature precedent for acylation of solid oxides, specifically Al2O3 by acetic acid43−45 reacting with surface OH groups. Ethoxylation by ethanol also has precedent.46 Moreover (and regardless of the precisely correct explanation for the observed EtOAc effect on increasing KDiss), the present work unequivocally demonstrates 5457

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ACS Catalysis

Figure 5. Ir0 formation kinetics of the [Ir(1,5-COD)Cl]2 dimer by the cyclohexene reporter reaction method without γ-Al2O3 (black diamonds; FW 2-step fit and simulation, blue line), compared against the same reaction with 1 equiv of ethyl acetate added per equivalent Ir present (black circles; FW 2-step fit and simulation, red line). The k1,obs values are 4.8 × 10−2 and 2.6 × 10−2 h−1 for the dimer and the spiked dimer controls, respectively.

autocatalytic growth occurs atop Ir0 nanoparticle surface sites according to all the available evidence.5−9 Return to the Original Synthetic Goal: Successful Reproduction of Isolated System Supported Iridium Catalyst Product Size by Addition of EtOAc to the OnePot Route and the Associated Reverse Experiment of Reproduction of the Standard Conditions One-Pot Catalyst by Omitting EtOAc from the Isolated Precatalyst Synthesis. Finally, as a “synthetic test” of the mechanistic insights herein, we probed (i) whether the inclusion of EtOAc in a one-pot synthesis was able to reproduce the nanoparticle product seen in the original isolated precatalyst system by STEM as well as (ii) if the modified, EtOAc-free isolated precatalyst yields the same product as the original (EtOAc-free) one-pot system (by microscopy and within experimental error). The new microscopy results show that either the deliberate inclusion of EtOAc, or its removal, in the two respective original systems allows the successf ul reproduction of the particle size distribution of the counterpart system within experimental error (Figure 6 and Table S1 and Figure S10 in the Supporting Information), at least when these experiments are done side by side.52 Noteworthy here is that the above results demonstrate after considerable mechanistic detective work to uncover the key “offending” variable, EtOActhat one can in fact achieve the seemingly simple, but in fact experimentally quite challenging, original synthetic goal of removing the precatalyst isolation step16−18 so that the synthesis can be accomplished in one pot. The present work demonstrates that, yes, this “simple” improvement is achievable so long as one controls all the other relevant variables in the synthesis. Controlling the presence of EtOAc, or analogous, even mildly oxidizing solvents or other reagents, is a previously hidden key to attaining reproducible nanoparticle catalyst nucleation and growth-based syntheses. Finally, it merits mention that the sensitivity of nanoparticle nucleation to the reaction conditions and trace impurities is not unexpected. Indeed, for the present well-studied Ir0/Al2O3 nanoparticle system15−17,34 and its even better studied polyoxoanion-stabilized counterpart,5,7,21 factors that are now known to influence nucleation and growth include the

Figure 6. (top) Particle size distributions (PSDs) for the original, Standard Conditions One-Pot system (blue) against the modified, EtOAc-free isolated system (red). (bottom) PSDs for the modified, EtOAc-processed γ-Al2O3 one-pot (blue) system overlaid on the original, Standard Conditions Isolated system’s nanoparticle product (red). The PSDs are indistinguishable within experimental error in each case.52

following: water,33 impurities in acetone solvent,33 H+,33 H2gas-to-solution mass-transfer limitations (and, hence, stirring rate, pressure, and precursor concentrations),15,53 temperature,54 precursor concentration,34,54 residual peroxides in even carefully distilled cyclohexene used in excess as a substrate,34,52,55 and, now, noninnocent solvents such as EtOActhe last operating via two separate, opposing, effects.

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CONCLUSIONS The most important findings of the present study can be summarized as follows. (1) An attempt to remove the precatalyst isolation step from the synthesis of Ir0∼900/Al2O3 yielded, initially, different nucleation and growth kinetic curves plus somewhat smaller, Ir0∼600/Al2O3 average-size supported-nanoparticle catalysts. (2) Tracking down the underlying cause of the different nanoparticle-formation kinetics and resultant Ir0n products reveals that residual EtOAc from the original isolated precatalyst synthesis16 is the underlying cause of the differences in the two catalyst syntheses and their resultant supported nanoparticles. 5458

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ACS Catalysis Notes

(3) More detailed mechanistic probing revealed two competing, each previously unprecedented ef fects of EtOAc: an increase of KDiss and hence the amount of [Ir(1,5-COD)Cl(solvent)] dissociated from the γ-Al2O3 but also a competing, dominating effect of EtOAc poisoning the nucleation event by reacting with Ir0n or conceivably its plausible precursors such as IrxHy.21 The net effect is the slowing of nucleation by 1 order of magnitude by just the residual EtOAc that is present. The results are significant, since a recent review of the literature reveals the somewhat surprisingly poor state of knowledge of the kinetics and mechanism, especially of the nucleation step, of supported-nanoparticle catalyst formation reactions and syntheses.19 (4) However, armed with the mechanistic insights in the effects of EtOAc, the desired result of performing the supported nanoparticle syntheses in one pot was achieved. (5) Our results (i) teach the importance of monitoring the nanoparticle formation, nucleation, and growth kinetics wherever possible, as well as (ii) the high sensitivity of particularly nanoparticle nucleation to trace reagents, especially trace solvents or other reagents that are even mild oxidants. (6) Our results suggest that it might well be a better general synthetic strategy to perform both isolated precatalyst and onepot syntheses simultaneously to start, as that would (at least in principle) allow one to uncover any hidden details (such as the two “EtOAc effects” discovered herein) upfront and as early as possible. The results presented herein have the important synthetic implication that one-pot syntheses can yield the same catalyst and underlying kinetics and mechanisms as preisolating and fully characterizing the precatalyst, so long as all the important synthetic variables are carefully and fully controlled. Overall, it is hoped that the present work will serve as a useful step in achieving reproducible, facile, and efficient onepot syntheses of high-activity, weakly ligated/labile ligand22,23 supported-nanoparticle catalysts such as the prototypical31 Ir0/ γ-Al2O3 supported-nanoparticle heterogeneous catalysts prepared herein.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant DE-FG02-03ER15688.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00265. Examples of STEM images of a Standard Conditions One-Pot product, overcoming challenges in measuring 1.0 equiv of cyclooctane evolution, UV−vis measurements of precatalyst solutions, comparing particle size distributions (PSDs) between the standard One-Pot and Isolated Precatalyst systems, controls testing two alternative hypotheses for the difference in the One-Pot and Isolated Systems’ nucleation kinetics, testing the effect of the presence or absence of ethyl acetate in OnePot and Isolated Precatalyst syntheses, calculation of the prior equilibrium Kdiss, and spiking the One-Pot and Isolated Ssystems with 1 equiv of ethyl acetate (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail for R.G.F.: [email protected]. Present Address

† Department of Chemistry, University of Wisconsin Stevens Point, Stevens Point, Wisconsin, 54481, United States

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Puntes, V. F.; Kiricsi, I.; Zhu, J.; Ager, J. W., III; Ko, M. K.; Frei, H.; Alivisatos, P.; Somorjai, G. A. Chem. Mater. 2003, 15, 1242−1248. (b) Zheng, N.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 14278− 14280. (c) Lee, I.; Morales, R.; Albiter, M. A.; Zaera, F. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 15241−15246. (d) Huang, X.; Guo, C.; Zuo, J.; Zheng, N.; Stucky, G. D. Small 2009, 5, 361−365. (e) Boualleg, M.; Basset, J.-M.; Candy, J.-P.; Delichere, P.; Pelzer, K.; Veyre, L.; Thieuleux, C. Chem. Mater. 2009, 21, 775−777. (f) Zhu, Y.; Qian, H.; Drake, B. A.; Jin, R. Angew. Chem., Int. Ed. 2010, 49, 1295−1298. (25) Examples from Somorjai and co-workers are available where they have made extensive, expert efforts to remove stabilizing ligand overlayers from both Pt0n and Rh0n nanoparticles with varyingalbeit never completedegrees of success in removing the stabilizers: (a) Rioux, R. M.; Song, H.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. J. Phys. Chem. B 2005, 109, 2192−2202. (b) Song, H.; Rioux, R. M.; Hoefelmeyer, J. D.; Komor, R.; Niesz, K.; Grass, M.; Yang, P.; Somorjai, G. A. J. Am. Chem. Soc. 2006, 128, 3027−3037. (c) Rioux, R. M.; Hsu, B. B.; Grass, M. E.; Song, H.; Somorjai, G. A. Catal. Lett. 2008, 126, 10−19. (d) Borodko, Y.; Jones, L.; Lee, H.; Frei, H.; Somorjai, G. A. Langmuir 2009, 25, 6665−6671. (e) Park, J. Y.; Aliaga, C.; Renzas, J. R.; Lee, H.; Somorjai, G. A. Catal. Lett. 2009, 129, 1−6. (f) Aliaga, C.; Park, J. Y.; Yamada, Y.; Lee, H. S.; Tsung, C.-H.; Yang, P.; Somorjai, G. A. J. Phys. Chem. C 2009, 113, 6150−6155. (g) Grass, M. E.; Joo, S. H.; Zhang, Y.; Somorjai, G. A. J. Phys. Chem. C 2009, 113, 8616−8623. (h) Borodko, Y. G.; Lee, H. Y.; Joo, S. H.; Zhang, Y.; Somorjai, G. A. J. Phys. Chem. C 2010, 114, 1117−1126. (i) Kuhn, J. N.; Tsung, C.−H.; Huang, W.; Somorjai, G. A. J. Catal. 2009, 265, 209−215. There are also extensive examples attempting to remove dendrimer stabilizers, from dendrimer-stabilized and supported nanoparticles; see for example: (j) Lang, H.; May, A.; Iversen, B. L.; Chandler, B. D. J. Am. Chem. Soc. 2003, 125, 14832−14836. (k) Liu, D.; Gao, J.; Murphy, C. J.; Williams, C. T. J. Phys. Chem. B 2004, 108, 12911−12916. (l) Singh, A.; Chandler, B. D. Langmuir 2005, 21, 10776. (m) Deutsch, D. S.; Siani, A.; Fanson, P. T.; Hirata, H.; Matsumoto, S.; Williams, C. T.; Amiridis, M. D. J. Phys. Chem. C 2007, 111, 4246−4255. (n) Albiter, M. A.; Zaera, F. Langmuir 2010, 26, 16204−16210. (26) Borodko, Y.; Lee, H. S.; Joo, S. H.; Zhang, Y.; Somorjai, G. J. Phys. Chem. C 2010, 114, 1117−1126. (27) Blavo, S. O.; Qayyum, E.; Bladyga, L. M.; Castillo, V. A.; Sanchez, M. D.; Warrington, K.; Barakat, M. A.; Kuhnm, J. N. Top. Catal. 2013, 56, 1835−1842. (28) Upon reflection, it was by no means obvious to start that the simple change to a one-pot synthesis should yield the same supportednanoparticle catalyst. Indeed, a perusal of the relevant literature documents the sensitivity of heterogeneous catalyst syntheses to the precise reaction conditions.29 For example, subtle differences in dayto-day preparations, such as ambient temperature and humidity, is known to alter “light-off temperatures” (e.g., the temperature at which 50% of flowing CO is converted to CO2) by up to 100 K through shifts in pH and drying conditions.29 Motivated by a lack of reproducibility in supported-nanoparticle catalyst syntheses, Cukic and co-workers systematically varied 10 parameters in the synthesis of Pdn/Al2O3 catalysts via an impregnation, drying, and calcination scheme in an attempt to identify the most sensitive catalyst synthesis variable(s).30 Those authors report that the ratio of the support pore volume to impregnating solution volume produced the largest variation in activity. A paper by Cukic and co-workers further suggests that stirring speed, pH, drying temperature, calcination temperature, and the calcination heat ramp are also important variables in producing the final Pdn/Al2O3 product. Overall, those authors note that: “Sometimes preparation even carried out in supposedly similar ways results in catalysts with different performance.”30 (29) Wolf, A.; Schueth, F. Appl. Catal., A 2002, 226, 1−13. (30) Cukic, T.; Holena, M.; Linke, D.; Herein, D.; Dingerdissen, U. Stud. Surf. Sci. Catal. 2006, 162, 195−202. (31) The eight prototype criteria previously developed16 are (i) a compositionally and structurally well-defined supported precatalyst (accomplished previously via inductively coupled plasma optical 5460

DOI: 10.1021/acscatal.6b00265 ACS Catal. 2016, 6, 5449−5461

Research Article

ACS Catalysis

(52) “Interestingly”, the size difference in these STEMs is systematically shifted to smaller sizes, on average 2.5 nm for Ir∼600 and 2.2 nm for Ir∼400 (i.e., shifted from the prior on average 2.9 nm for Ir∼900 and 2.5 nm for Ir∼600). A different batch of cyclohexene (distilled as usual; see the Experimental Section) appears to be the causeagain showing the extreme sensitivity of nanoparticle nucleation and growth phenomenon to the precise reaction conditions. (53) Aiken, J. D., III; Finke, R. G. J. Am. Chem. Soc. 1998, 120, 9545− 9554. (54) Ott, L. S.; Finke, R. G. J. Nanosci. Nanotechnol. 2008, 8, 1551− 1556. (55) Weiner, H.; Trovarelli, A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 191, 217−252.

emission spectroscopy, CO/IR trapping experiments, and X-ray absorbance fine structure (XAFS) spectroscopy16), (ii) a system in contact with solution and formed under low-temperature conditions, and (iii) a system where a balanced stoichiometry of the supportednanoparticle formation reaction is established (e.g., bottom of Scheme 1 and as previously confirmed elsewhere16), leading to a well-defined Ir0∼900/γ-Al2O3 supported-nanoparticle heterogeneous catalyst (confirmed for the present system by transmission electron microscopy (TEM) and XAFS16). In addition, a prototype system should (iv) yield an active and long-lived catalyst, and hence (v) provide a system where the initial kinetic and mechanistic studies of the one-pot catalyst formation are worth the effort. Additionally, (vi) the prototype system should also yield reproducible and quantitative kinetic data so that quantitative conclusions and mechanistic insights can be drawn, and ideally (vii) comparison to a kinetically and mechanistically wellstudied nanoparticle formation system in solution should also be possible for any insights that comparison might allow.5 Finally, once that prototype system is in hand, one also wishes to (viii) systematically vary key synthetic variables such as the support, solvent, and metal precursor to reveal their effects on supported-nanoparticle formation in contact with solution. (32) Our first publication on the characterization of precatalyst and catalyst in the prototypical Ir(1,5-COD)Cl/γ-Al2O3 to Ir0900/γ-Al2O3 found that the reduction kinetics were accounted for by the Finke− Watzky two-step mechanism of slow, continuous nucleation and autocatalytic surface growth.16 Subsequent work on the prototypical system proposed a reaction pathway consisting of a solvation− dissociation prior equilibrium, bimolecular solution based nucleation, fast support capture, and autocatalytic surface growth by solvated Ir1 monomer.17,18 (33) Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 4891. (34) Kent, P. D.; Mondloch, J. E.; Finke, R. G. J. Am. Chem. Soc. 2014, 136, 1930−1941. (35) Nagy, I. P.; Bazsa, G. React. Kinet. Catal. Lett. 1991, 45, 15−25. (36) Nagypal, I.; Epstein, I. R. J. Phys. Chem. 1986, 90, 6285−6292. (37) Widegren, J. A.; Aiken, J. D., III; Ö zkar, S.; Finke, R. G. Chem. Mater. 2001, 13, 312−324. (38) Per a reviewer’s query, the interested reader will find the full derivation of eq 3 and the first-order form of eq 4 in pages S4−S7 in the Supporting Information of our 2011 JACS paper.17 The secondorder nature of eq 4 is the result of a ACS Catalysis full paper reanalyzing the data in the 2011 work.18 (39) Widegren, J. A.; Bennett, M. A.; Finke, R. G. J. Am. Chem. Soc. 2003, 125, 10301−10304. (40) Segal, E.; Madon, R. J.; Boudart, M. J. Catal. 1978, 52, 45−49. (41) Augustine, R. L.; Warner, R. W.; Melnick, M. J. J. Org. Chem. 1984, 49, 4853−4856. (42) Guo, Z.; Chen, Y.; Li, L.; Wang, X.; Haller, G. L.; Yang, Y. J. Catal. 2010, 276, 314−326. (43) Ma, Q.; Liu, Y.; Liu, C.; He, H. Phys. Chem. Chem. Phys. 2012, 14, 8403−8409. (44) Evans, H. E.; Weinberg, W. H. J. Chem. Phys. 1979, 71, 4789− 4798. (45) Carlos-Cuellar, S.; Li, P.; Christensen, A. P.; Kruger, G. J.; Burrichter, C.; Grassian, V. H. J. Phys. Chem. A 2003, 107, 4250−4261. (46) Fink, P. Z. Chem. 1967, 7, 284−285. (47) Collman, P. J.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Oxidative Additions and Reductive Eliminations. In Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1978. (48) Kundu, S.; Choi, J.; Wang, D. Y.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2013, 135, 5127−5143. (49) Yamamoto, T.; Ishizu, J.; Kohara, T.; Komiya, S.; Yamamoto, A. J. Am. Chem. Soc. 1980, 102, 3758−3764. (50) Komiya, S.; Suzuki, J.; Miki, K.; Kasai, N. Chem. Lett. 1987, 7, 1287−1290. (51) Manbeck, K. A.; Kundu, S.; Walsh, A. P.; Brennessel, W. W.; Jones, W. D. Organometallics 2012, 31, 5018−5024. 5461

DOI: 10.1021/acscatal.6b00265 ACS Catal. 2016, 6, 5449−5461