Richard G. Finke

CSU Mentor, Long Beach, California, United States

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Publications (212)691.38 Total impact

  • William W Laxson, Richard G Finke
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    ABSTRACT: 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", An, 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 k1obs), then (ii) fast autocatalytic surface growth (A + B → 2B, rate constant k2obs). Relatively precise homogeneous nucleation rate constants, k1obs, examined as a function of the amount of precatalyst, A, reveal that k1obs has an added dependence on the concentration of the precursor, k1obs = k1obs(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, k1obs(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.
    Journal of the American Chemical Society 12/2014; · 10.68 Impact Factor
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    ABSTRACT: A novel perylene diimide dye functionalized with phosphonate groups, N,N'-bis(phosphonomethyl)-3,4,9,10-perylenediimide (PMPDI), is synthesized and characterized. Thin films of PMPDI spin-coated onto indium tin oxide (ITO) substrates are further characterized, augmented by photoelectrochemically depositing a CoOx catalyst, and then investigated as photoanodes for water oxidation. These ITO/PMPDI/CoOx electrodes show visible-light-assisted water oxidation with photocurrents in excess of 150 μA/cm(2) at 1.0 V applied bias vs. Ag/AgCl. Water oxidation is confirmed by the direct detection of O2, with a faradaic efficiency of 80 ± 15% measured under 900 mV applied bias vs. Ag/AgCl. Analogous photoanodes prepared with another PDI derivative with alkyl groups in place of PMPDI's phosphonate groups do not function, providing evidence that PMPDI's phosphonate groups may be important for efficient coupling between the inorganic CoOx catalyst and the organic dye. Our ITO/PMPDI/CoOx anodes achieve internal quantum efficiencies for water oxidation ∼1%, and for hydroquinone oxidation of up to ∼6%. The novelty of our system is that, to the best of our knowledge, it is the first device to achieve photoelectrochemically driven water oxidation by a single-layer molecular organic semiconductor thin film coupled to a water-oxidation catalyst.
    ACS Applied Materials & Interfaces 03/2014; · 5.90 Impact Factor
  • Jordan J. Stracke, Richard G. Finke
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    ABSTRACT: Polyoxometalates (POMs) have been proposed to be excellent homogeneous water oxidation catalysts (WOCs) due to their oxidative stability and activity. However, recent literature indicates that even these relatively robust compounds can be transformed into heterogeneous, metal-oxide WOCs under the oxidizing reaction conditions needed to drive O2 evolution. This review covers the experimental methodology for distinguishing homogeneous and heterogeneous WOCs; it then addresses the “what is the true catalyst?” problem for POMs used as precatalysts in the oxidation of water to O2. These results are also compared to the broader WOC literature. The primary findings in this review are the following: (1) Multiple, complementary experiments are needed to determine the true catalyst, including determination of catalyst stability, speciation, and kinetics under operating conditions. (2) Controls with hypothetical heterogeneous metal-oxide catalysts are required to determine their kinetic competence in the reaction and support the conclusion of either a homogeneous or heterogeneous catalyst. (3) Although many studies observe qualitative stability of the starting POM under the reaction conditions, there is a lack of quantitative stability studies; if one does not know where the (pre)catalyst mass lies, then it is very difficult to rule out the possibility of an alternative species as the true catalyst. (4) The stability of POMs is dependent on the polyoxometalate, the metal center, and the reaction conditions. And, (5) as a result of the variable stability of POMs under different reaction conditions, those different conditions can influence the dominant catalyst identity. Overall, knowledge of which POMs (or other starting materials) tend to transform into heterogeneous WOCs, and how they do so, is therefore critical to developing the next generation of higher stability, higher activity, and truly long-lived POM and other water oxidation catalysts.
    ACS Catalysis 02/2014; 4(3):909–933. · 5.27 Impact Factor
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    William W Laxson, Saim Ozkar, Richard G Finke
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    ABSTRACT: Reproducible syntheses of high-purity [(n-C4H9)4N]9P2W15Nb3O62 and, therefore, also the supported [(1,5-COD)Ir(I)](+) organometallic precatalyst, [(n-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 [(n-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 (31)P NMR now more routinely possible. Herein we (1) verify problems in reproducing ≥90% purity [(n-C4H9)4N]9P2W15Nb3O62, (2) determine three critical variables for the successful production of [(n-C4H9)4N]9P2W15Nb3O62, (3) optimize the synthesis to achieve 91-94% pure [(n-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 [(n-C4H9)4N]5H4P2W15Nb3O62 intermediate. These three insights have allowed improvement of the synthesis to a 91-94% final purity [(n-C4H9)4N]9P2W15Nb3O62 product by high S/N (31)P NMR. Moreover, the synthesis provided both is very detailed and has been independently checked (by Dr. Özkar) using only the written procedures. 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 level of written reporting necessary to make such challenging polyoxometalate inorganic syntheses reproducible in the hands of others.
    Inorganic Chemistry 02/2014; · 4.59 Impact Factor
  • Patrick D Kent, Joseph E Mondloch, Richard G Finke
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    ABSTRACT: 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.
    Journal of the American Chemical Society 01/2014; · 10.68 Impact Factor
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    Jordan J. Stracke, Richard G. Finke
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    ABSTRACT: Stoichiometry and kinetics are reported for catalytic water oxidation to O2 beginning with the cobalt polyoxometalate Co4(H2O)2(PW9O34)210– (Co4POM) and the chemical oxidant ruthenium(III)tris(2,2′-bipyridine) (Ru(III)(bpy)33+). This specific water oxidation system was first reported in a 2010 Science paper (Yin et al. Science 2010, 328, 342). Under standard conditions employed herein of 1.0 μM Co4POM, 500 μM Ru(III)(bpy)33+, 100 μM Ru(II)(bpy)32+, pH 7.2, and 0.03 M sodium phosphate buffer, the highest O2 yields of 22% observed herein are seen when Ru(II)(bpy)32+ is added prior to the Ru(III)(bpy)33+ oxidant; hence, those conditions are employed in the present study. Measurement of the initial O2 evolution and Ru(III)(bpy)33+ reduction rates while varying the initial pH, [Ru(III)(bpy)33+], [Ru(II)(bpy)32+], and [Co4POM] indicate that the reaction follows the empirical rate law: −d[Ru(III)(bpy)33+]/dt = (k1 + k2)[Co4POM]soluble[Ru(III)(bpy)33+]/[H+], where the rate constants k1 0.0014 s–1 and k2 0.0044 s–1 correspond to the water oxidation and ligand oxidation reactions, and for O2 evolution, d[O2]/dt = (k1/4)[Co4POM]soluble[Ru(III)(bpy)33+]/[H+]. Overall, at least seven important insights result from the present studies: (i) Parallel WOC and Ru(III)(bpy)33+ self-oxidation reactions well documented in the prior literature limit the desired WOC and selectivity to O2 in the present system to ≤28%. (ii) The formation of a precipitate from 2 Ru(II)(bpy)32+/3 Co4(H2O)2(PW9O34)210– with a Ksp = (8 ± 7) × 10–25 (M5) greatly complicates the reaction and interpretation of the observed kinetics, but (iii) the best O2 yields are still when Ru(II)(bpy)32+ is preadded. (iv) CoOx is 2–11 times more active than Co4POM under the reaction conditions, but (v) Co4POM is still the dominant WOC under the Co4POM/Ru(III)(bpy)33+ and other reaction conditions employed. The present studies also (vi) confirm that the specific conditions matter greatly in determining the true WOC and (vii) allow one to begin to construct a plausible WOC mechanism for the Co4POM/Ru(III)(bpy)33+ system.
    ACS Catalysis 12/2013; 4(1):79–89. · 5.27 Impact Factor
  • Jordan J. Stracke, Richard G. Finke
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    ABSTRACT: Evidence for the true water oxidation catalyst (WOC) when beginning with the cobalt polyoxometalate [Co4(H2O)2(PW9O34)2]10– (Co4–POM) is investigated at deliberately chosen low polyoxometalate concentrations (2.5 μM) and high electrochemical potentials (≥1.3 V vs Ag/AgCl) in pH 5.8 and 8.0 sodium phosphate electrolyte at a glassy carbon working electrode—conditions which ostensibly favor Co4–POM catalysis if present. Multiple experiments argue against the dominant catalyst being CoOx formed exclusively from Co2+ dissociated from the parent POM. Measurement of [Co2+] in the Co4–POM solution and catalytic controls with the corresponding amount of Co(NO3)2 cannot account for the O2 generated from 2.5 μM [Co4(H2O)2(PW9O34)2]10– solutions. This result contrasts with our prior investigation of Co4–POM under higher concentration and lower potential conditions (i.e., 500 μM [Co4(H2O)2(PW9O34)2]10–, 1.1 V vs Ag/AgCl, as described in Stracke, J. J.; Finke, R. G. J. Am. Chem. Soc.2011, 133, 14872) and highlights the importance ofreactionconditions in governing the identity of the true, active WOC. Although electrochemical studies are consistent with Co4–POM being oxidized at the glassy carbon electrode, it is not yet possible to distinguish a Co4–POM catalyst from a CoOx catalyst formed via decomposition of Co4–POM. Controls with authentic CoOx indicate conversion of only 3.4% or 8.3% (at pH 8.0 and 5.8) of Co4–POM into a CoOx catalyst could account for the O2-generating activity, and HPLC quantification of the Co4–POM stability shows the postreaction Co4–POM concentration decreases by 2.7 ± 7.6% and 9.4 ± 5.1% at pH 8.0 and 5.8. Additionally, the [Co2+] in a 2.5 μM Co4–POM solution increases by 0.55 μM during 3 min of electrolysis—further evidence of the Co4-POM instability under oxidizing conditions. Overall, this study demonstrates the challenges of identifying the true WOC when examining micromolar amounts of a partially stable material and when nanomolar heterogeneous metal-oxide will account for the observed O2-generating activity.
    ACS Catalysis 05/2013; 3(6):1209–1219. · 5.27 Impact Factor
  • Ercan Bayram, Richard G. Finke
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    ABSTRACT: Quantitative catalyst poisoning studies are of fundamental interest and importance because (a) knowledge of the number of true active sites is required for calculation of the true turnover frequency = (moles of product)/(moles of actual active sites)(time), and because (b) quantitative catalyst poisoning is proving to be a key, required piece of data en route to distinguishing single metal (M1), small metal cluster (e.g., M4), or metal nanoparticle (Mn) catalysis. In evidence of the latter point, quantitative catalyst poisoning experiments using 1,10-phenanthroline as the poison proved to be crucial in the recent identification of Rh4 subnanometer clusters as the true benzene hydrogenation catalyst in a system beginning with [RhCp*Cl2]2 (Cp*: (η5-C5(CH3)5)) at 100 °C and 50 atm initial H2 pressure (Bayram et al. J. Am. Chem. Soc.2011, 133, 18889). However and despite the success of those quantitative poisoning studies, five questions about such poisoning studies remained unanswered, questions posed and then addressed herein. In addition, the analysis herein of the 1,10-phenanthroline poisoning of both Rh(0) nanoparticle and Rh4 subnanometer benzene hydrogenation catalysts results in kinetic models for, respectively, strong-binding and weak-binding poisons. Also provided are quantitiative estimates of the poison binding constants, of the number of equivalents required to completely poison each catalyst, and of the number of active sites on each catalyst. The weak-binding poison kinetic model is then shown to have immediate applicability toward analyzing extant literature data via its application to literature CS2 quantitative poisoning data for ammonia-borane dehydrocoupling beginning with a [Ru(cod)(cot)] (cod: cyclooctadiene and cot: cyclooctatriene) precatalyst. The significance of the results is then summarized in a Conclusions section.
    ACS-Catalysis. 08/2012; 2(9):1967.
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    ABSTRACT: This work addresses the question of what is the true catalyst when beginning with a site-isolated, atomically dispersed precatalyst for the prototype catalytic reaction of cyclohexene hydrogenation in the presence of cyclohexane solvent: is the atomically dispersed nature of the zeolite-supported, [Ir(C2H4)2]/zeolite Y precatalyst retained, or are possible alternatives including Ir4 subnanometer clusters or larger, Ir(0)n, nanoparticles the actual catalyst? Herein we report the (a) kinetics of the reaction; (b) physical characterizations of the used catalyst, including extended X-ray absorption fine structure spectra plus images obtained by high-angle annular dark-field scanning transmission electron microscopy, demonstrating the mononuclearity and site-isolation of the catalyst; and the (c) results of poisoning experiments, including those with the size-selective poisons P(C6H11)3 and P(OCH3)3 determining the location of the catalyst in the zeolite pores. Also reported are quantitative poisoning experiments showing that each added P(OCH3)3 molecule poisons one catalytic site, confirming the single-metal-atom nature of the catalyst and the lack of leaching of catalyst into the reactant solution. The results (i) provide strong evidence that the use of a site-isolated [Ir(C2H4)2]/zeolite Y precatalyst allows a site-isolated [Ir1]/zeolite Y hydrogenation catalyst to be retained even when in contact with solution, at least at 22 °C; (ii) allow a comparison of the solid–solution catalyst system with the equivalent one used in the solid–gas ethylene hydrogenation reaction at room temperature; and (iii) illustrate a methodology by which multiple, complementary physical methods, combined with kinetic, size-selective poisoning, and quantitative kinetic poisoning experiments, help to identify the catalyst. The results, to our knowledge, are the first identifying an atomically dispersed, supported transition-metal species as the catalyst of a reaction taking place in contact with solution.
    ACS-Catalysis. 08/2012; 2(9):1947.
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    ABSTRACT: The most extensive and highest quality Au0n nanocluster agglomeration size vs time TEM data set yet obtained are analyzed by a nanoparticle size vs time equation that is derived herein for parallel bimolecular (B + B → C, rate constant k3) and autocatalytic (B + C → 1.5C, rate constant k4) agglomeration steps of preformed nanoclusters, B. The results show that the size vs time data are well fit by the new size vs time equation. The fits and resultant k3 and k4 rate constants yield several interesting insights that are presented and discussed, including the finding that to date k4 > k3, that is, that the autocatalytic agglomeration rate constant is faster than the bimolecular rate constant, at least for the cases examined to date. The results of the effects of added TOABr (tetraoctylammonium bromide) on the 180 °C agglomeration k3 and k4 rate constants in unstirred diphenylmethane solvent are also presented and discussed, the TOABr being added originally to compact the nanoclusters double layer thereby helping induce agglomeration. The observed different [TOABr] effects on k3 vs k4 also provide prima facie evidence that the two agglomeration steps are fundamentally different and unique. Literature size vs time data, from El-Sayed et al. for Pd nanocluster agglomeration, are also fit as a further test of the new, mechanism-based size vs time equation. The combined results, showing good fits by the k3 and k4 steps to the Au0n as well as literature Pd, Pt, and Ir nanocluster data, provide good support for the underlying B + B → C and B + C → 1.5C agglomeration steps themselves as well as for the assumptions and math behind the new size vs time equation. The significance of the results in general, as well as for future measurements of k3 and k4 rate constants as a preferred way to quantitate nanocluster stability in solution, are also presented and discussed. Most significant, however, is that as a result of the present work one can now use chemical equations and associated, mechanistically rigorously defined concepts of bimolecular (B + B → C; rate constant k3) and autocatalytic (B+ C → 1.5C; rate constant k4) agglomeration to analyze and describe nanoparticle agglomeration rather than the harder to interpret, more obscure n and k parameters from an Avrami-type, semiempirical curve fit.
    Chemistry of Materials 05/2012; 24(10):1718–1725. · 8.24 Impact Factor
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    ABSTRACT: Hydrocarbon-solvent-soluble, isolable, Ziegler-type Ir(0)n 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)n 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)n 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)n 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)n and AlEt3 occur to give initially surface species such as (Irsurface)x–Et plus (Irsurface)x–Al(Et)2Ir, where the number of surface Ir atoms involved, x = 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.
    ACS Catalysis 03/2012; 2(4):632–641. · 5.27 Impact Factor
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    ABSTRACT: Reported herein is the synthesis of the previously unknown [Ir(1,5-COD)(μ-H)](4) (where 1,5-COD = 1,5-cyclooctadiene), from commercially available [Ir(1,5-COD)Cl](2) and LiBEt(3)H in the presence of excess 1,5-COD in 78% initial, and 55% recrystallized, yield plus its unequivocal characterization via single-crystal X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy, electrospray/atmospheric pressure chemical ionization mass spectrometry (ESI-MS), and UV-vis, IR, and nuclear magnetic resonance (NMR) spectroscopies. The resultant product parallels--but the successful synthesis is different from, vide infra--that of the known and valuable Rh congener precatalyst and synthon, [Rh(1,5-COD)(μ-H)](4). Extensive characterization reveals that a black crystal of [Ir(1,5-COD)(μ-H)](4) is composed of a distorted tetrahedral, D(2d) symmetry Ir(4) core with two long [2.90728(17) and 2.91138(17) Å] and four short Ir-Ir [2.78680 (12)-2.78798(12) Å] bond distances. One 1,5-COD and two edge-bridging hydrides are bound to each Ir atom; the Ir-H-Ir span the shorter Ir-Ir bond distances. XAFS provides excellent agreement with the XRD-obtained Ir(4)-core structure, results which provide both considerable confidence in the XAFS methodology and set the stage for future XAFS in applications employing this Ir(4)H(4) and related tetranuclear clusters. The [Ir(1,5-COD)(μ-H)](4) complex is of interest for at least five reasons, as detailed in the Conclusions section.
    Inorganic Chemistry 03/2012; 51(5):3186-93. · 4.59 Impact Factor
  • Joseph E. Mondloch, Ercan Bayram, Richard G. Finke
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    ABSTRACT: Nanoparticles supported on high surface area materials are commonly used in many industrially relevant catalytic reactions. This review examines the existing literature of the mechanisms of formation of practical, non-ultra high vacuum, supported-nanoparticle heterogeneous catalysts. Specifically, this review includes: (i) a brief overview of the synthesis of supported-nanoparticles, (ii) an overview of the physical methods for following the kinetics of formation of supported-nanoparticles, and then (iii) a summary of the kinetic and mechanistic studies of the formation of supported nanoparticle catalysts, performed under the traditional synthetic conditions of the gas–solid interface. This review then also discusses (iv) the synthesis, (v) physical methods, and (vi) the extant kinetic and mechanistic studies under the less traditional, less examined conditions of a liquid–solid system. A summary of the main insights from each section of the review is also given. Overall, surprisingly little is known about the mechanism(s) of formation of the desired size, shape and compositionally controlled supported-nanoparticle catalysts.
    Journal of Molecular Catalysis A Chemical 03/2012; 355:1. · 3.19 Impact Factor
  • Joseph E. Mondloch, Richard G. Finke
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    ABSTRACT: 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. J. Am. Chem. Soc.2011, 133, 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 k1obs) and autocatalyic surface growth (A + B → 2B, rate constant k2obs), where A is nominally the Ir(1,5-COD)Cl/γ-Al2O3 and B the growing, supported Ir(0)n/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)n/γ-Al2O3 and Ir(1,5-COD)Cl(solvent) in solution. While the k2obs vs [γ-Al2O3]sus and [acetone] autocatalytic surface growth rate constants were nicely accounted for by the proposed mechanism, the k1obs nucleation rate constants were only “roughly” accounted for by the previously proposed unimolecular 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 bimolecular. Extracting bimolecular, k1obs(bimol), rate constants by curve-fitting yields qualitative (i.e., visual inspection) as well as quantitative (i.e., increased R2 values) evidence consistent with and strongly supportive of solution-based bimolecular nucleation (A + A → 2B, rate constant k1obs(bimol)) for the Ir(1,5-COD)Cl/γ-Al2O3 to Ir(0)900/γ-Al2O3 system in contact with acetone. The extracted k1obs(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 k1obs, is hereby shown to actually depend on 2k1obs(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).
    ACS Catalysis 01/2012; 2(2):298–305. · 5.27 Impact Factor
  • Chemistry of Materials 01/2012; 24:1718. · 8.24 Impact Factor
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    ABSTRACT: Determining the true, kinetically dominant catalytically active species, in the classic benzene hydrogenation system pioneered by Maitlis and co-workers 34 years ago starting with [RhCp*Cl(2)](2) (Cp* = [η(5)-C(5)(CH(3))(5)]), has proven to be one of the most challenging case studies in the quest to distinguish single-metal-based "homogeneous" from polymetallic, "heterogeneous" catalysis. The reason, this study will show, is the previous failure to use the proper combination of: (i) in operando spectroscopy to determine the dominant form(s) of the precatalyst's mass under catalysis (i.e., operating) conditions, and then crucially also (ii) the previous lack of the necessary kinetic studies, catalysis being a "wholly kinetic phenomenon" as J. Halpern long ago noted. An important contribution from this study will be to reveal the power of quantitiative kinetic poisoning experiments for distinguishing single-metal, or in the present case subnanometer Rh(4) cluster-based catalysis, from larger, polymetallic Rh(0)(n) nanoparticle catalysis, at least under favorable conditions. The combined in operando X-ray absorption fine structure (XAFS) spectroscopy and kinetic evidence provide a compelling case for Rh(4)-based, with average stoichiometry "Rh(4)Cp*(2.4)Cl(4)H(c)", benzene hydrogenation catalysis in 2-propanol with added Et(3)N and at 100 °C and 50 atm initial H(2) pressure. The results also reveal, however, that if even ca. 1.4% of the total soluble Rh(0)(n) had formed nanoparticles, then those Rh(0)(n) nanoparticles would have been able to account for all the observed benzene hydrogenation catalytic rate (using commercial, ca. 2 nm, polyethyleneglycol-dodecylether hydrosol stabilized Rh(0)(n) nanoparticles as a model system). The results--especially the poisoning methodology developed and employed--are of significant, broader interest since determining the nature of the true catalyst continues to be a central, often vexing issue in any and all catalytic reactions. The results are also of fundamental interest in that they add to a growing body of evidence indicating that certain, appropriately ligated, coordinatively unsaturated, subnanometer M(4) transition-metal clusters can be relatively robust catalysts. Also demonstrated herein is that Rh(4) clusters are poisoned by Hg(0), demonstrating for the first time that the classic Hg(0) poisoning test of "homogeneous" vs "heterogeneous" catalysts cannot distinguish Rh(4)-based subnanometer catalysts from Rh(0)(n) nanoparticle catalysts, at least for the present examples of these two specific, Rh-based catalysts.
    Journal of the American Chemical Society 11/2011; 133(46):18889-902. · 10.68 Impact Factor
  • Jordan J Stracke, Richard G Finke
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    ABSTRACT: The question of "what is the true catalyst?" when beginning with the cobalt polyoxometalate (POM) [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) in electrochemical water oxidation catalysis is examined in pH 8.0 sodium phosphate buffer at a glassy carbon electrode. Is [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) a true water oxidation catalyst (WOC), or just a precatalyst? Electrochemical, kinetic, UV-vis, SEM, EDX, and other data provide four main lines of compelling evidence that, under the conditions used herein, the dominant WOC is actually heterogeneous CoO(x) and not homogeneous [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-).
    Journal of the American Chemical Society 09/2011; 133(38):14872-5. · 10.68 Impact Factor
  • Kuang-Hway Yih, William M. Alley, Richard G. Finke
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    ABSTRACT: An improved synthesis procedure is reported for the compounds [(1,5-COD)M(μ-O2C8H15)]2 (COD = cyclooctadiene, M = Ir (1), Rh (2), O2C8H15 = 2-ethylhexanoate); that procedure provides a more effective method of product isolation resulting in easier crystallization, and 76% and 85% yields of pure, crystalline 1 and 2, respectively. The improved synthesis and simplified crystallization of high-purity 1 and 2 are significant, not only because 1 is useful as a model for industrial Ziegler-type hydrogenation precatalysts, but also because 1 and 2 are potentially useful in a variety of other areas.
    Organometallics 08/2011; 30(18):5068–5070. · 4.15 Impact Factor
  • Joseph E Mondloch, Richard G Finke
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    ABSTRACT: A current goal in heterogeneous catalysis is to transfer the synthetic, as well as developing mechanistic, insights from the modern revolution in nanoparticle science to the synthesis of supported-nanoparticle heterogeneous catalysts. In a recent study (Mondloch, J. E.; Wang, Q.; Frenkel, A. I.; Finke, R. G. J. Am. Chem. Soc. 2010, 132, 9701-9714), we initialized tests of the global hypothesis that quantitative kinetic and mechanistic studies, of supported-nanoparticle heterogeneous catalyst formation in contact with solution, can provide synthetic and mechanistic insights that can eventually drive improved syntheses of composition-, size-, and possibly shape-controlled catalysts. That study relied on the development of a well-characterized Ir(1,5-COD)Cl/γ-Al(2)O(3) precatalyst, which, when in contact with solution and H(2), turns into a nonaggregated Ir(0)(~900)/γ-Al(2)O(3) supported-nanoparticle heterogeneous catalyst. The kinetics of the Ir(1,5-COD)Cl/γ-Al(2)O(3) to Ir(0)(~900)/γ-Al(2)O(3) conversion were followed and fit by a two-step mechanism consisting of nucleation (A → B, rate constant k(1)) followed by autocatalytic surface growth (A + B → 2B, rate constant k(2)). However, a crucial, but previously unanswered question is whether the nucleation and growth steps occur primarily in solution, on the support, or possibly in both phases for one or more of the catalyst-formation steps. The present work investigates this central question for the prototype Ir(1,5-COD)Cl/γ-Al(2)O(3) to Ir(0)(~900)/γ-Al(2)O(3) system. Solvent variation-, γ-Al(2)O(3)-, and acetone-dependent kinetic data, along with UV-vis spectroscopic and gas-liquid-chromatography (GLC) data, are consistent with and strongly supportive of a supported-nanoparticle formation mechanism consisting of Ir(1,5-COD)Cl(solvent) dissociation from the γ-Al(2)O(3) support (i.e., from Ir(1,5-COD)Cl/γ-Al(2)O(3)), solution-based nucleation from that dissociated Ir(1,5-COD)Cl(solvent) species, fast Ir(0)(n) nanoparticle capture by γ-Al(2)O(3), and then subsequent solid-oxide-based nanoparticle growth from Ir(0)(n)/γ-Al(2)O(3) and with Ir(1,5-COD)Cl(solvent), the first kinetically documented mechanism of this type. Those data disprove a solid-oxide-based nucleation and growth pathway involving only Ir(1,5-COD)Cl/γ-Al(2)O(3) and also disprove a solution-based nanoparticle growth pathway involving Ir(1,5-COD)Cl(solvent) and Ir(0)(n) in solution. The present mechanistic studies allow comparisons of the Ir(1,5-COD)Cl/γ-Al(2)O(3) to Ir(0)(~900)/γ-Al(2)O(3) supported-nanoparticle formation system to the kinetically and mechanistically well-studied, Ir(1,5-COD)·P(2)W(15)Nb(3)O(62)(8-) to Ir(0)(~300)·(P(2)W(15)Nb(3)O(62)(8-))(n)(-8n) solution-based, polyoxoanion-stabilized nanoparticle formation and stabilization system. That comparison reveals closely analogous, solution Ir(1,5-COD)(+) or Ir(1,5-COD)Cl-mediated, mechanisms of nanoparticle formation. Overall, the hypothesis supported by this work is that these and analogous studies hold promise of providing a way to transfer the synthetic and mechanistic insights, from the modern revolution in nanoparticle synthesis and characterization in solution, to the rational, mechanism-directed syntheses of solid oxide-supported nanoparticle heterogeneous catalysts, also in contact with solution.
    Journal of the American Chemical Society 05/2011; 133(20):7744-56. · 10.68 Impact Factor
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    ABSTRACT: Ziegler-type hydrogenation catalysts are important for industrial processes, namely, the large-scale selective hydrogenation of styrenic block copolymers. Ziegler-type hydrogenation catalysts are composed of a group 8-10 transition metal precatalyst plus an alkylaluminum cocatalyst (and they are not the same as Ziegler-Natta polymerization catalysts). However, for ∼50 years two unsettled issues central to Ziegler-type hydrogenation catalysis are the nature of the metal species present after catalyst synthesis, and whether the species primarily responsible for catalytic hydrogenation activity are homogeneous (e.g., monometallic complexes) or heterogeneous (e.g., Ziegler nanoclusters defined as metal nanoclusters made from combination of Ziegler-type hydrogenation catalyst precursors). A critical review of the existing literature (Alley et al. J. Mol. Catal. A: Chem. 2010, 315, 1-27) and a recently published study using an Ir model system (Alley et al. Inorg. Chem. 2010, 49, 8131-8147) help to guide the present investigation of Ziegler-type hydrogenation catalysts made from the industrially favored precursors Co(neodecanoate)(2) or Ni(2-ethylhexanoate)(2), plus AlEt(3). The approach and methods used herein parallel those used in the study of the Ir model system. Specifically, a combination of Z-contrast scanning transmission electron microscopy (STEM), matrix assisted laser desorption ionization mass spectrometry (MALDI MS), and X-ray absorption fine structure (XAFS) spectroscopy are used to characterize the transition metal species both before and after hydrogenation. Kinetic studies including Hg(0) poisoning experiments are utilized to test which species are the most active catalysts. The main findings are that, both before and after catalytic cyclohexene hydrogenation, the species present comprise a broad distribution of metal cluster sizes from subnanometer to nanometer scale particles, with estimated mean cluster diameters of about 1 nm for both Co and Ni. The XAFS results also imply that the catalyst solutions are a mixture of the metal clusters described above, plus unreduced metal ions. The kinetics-based Hg(0) poisoning evidence suggests that Co and Ni Ziegler nanoclusters (i.e., M(≥4)) are the most active Ziegler-type hydrogenation catalysts in these industrial systems. Overall, the novelty and primary conclusions of this study are as follows: (i) this study examines Co- and Ni-based catalysts made from the actual industrial precursor materials, catalysts that are notoriously problematic regarding their characterization; (ii) the Z-contrast STEM results reported herein represent, to our knowledge, the best microscopic analysis of the industrial Co and Ni Ziegler-type hydrogenation catalysts; (iii) this study is the first explicit application of an established method, using multiple analytical methods and kinetics-based studies, for distinguishing homogeneous from heterogeneous catalysis in these Ziegler-type systems; and (iv) this study parallels the successful study of an Ir model Ziegler catalyst system, thereby benefiting from a comparison to those previously unavailable findings, although the greater M-M bond energy, and tendency to agglomerate, of Ir versus Ni or Co are important differences to be noted. Overall, the main result of this work is that it provides the leading hypothesis going forward to try to refute in future work, namely, that sub, M(≥4) to larger, M(n) Ziegler nanoclusters are the dominant, industrial, Co- and Ni- plus AlR(3) catalysts in Ziegler-type hydrogenation systems.
    Langmuir 05/2011; 27(10):6279-94. · 4.38 Impact Factor

Publication Stats

2k Citations
691.38 Total Impact Points

Institutions

  • 1999–2014
    • CSU Mentor
      • Department of Chemistry
      Long Beach, California, United States
  • 2012
    • Hungkuang University
      臺中市, Taiwan, Taiwan
  • 1994–2012
    • Colorado State University
      • Department of Chemistry
      Fort Collins, Colorado, United States
  • 2008
    • University of California, Berkeley
      • Department of Chemistry
      Berkeley, MO, United States
  • 1981–2007
    • University of Oregon
      • Department of Chemistry
      Eugene, OR, United States
  • 2005
    • Middle East Technical University
      • Department of Chemistry
      Ankara, Ankara, Turkey
  • 2003
    • University of Michigan
      • Department of Biological Chemistry
      Ann Arbor, MI, United States
  • 2002
    • Kanagawa University
      • Faculty of Science
      Yokohama, Kanagawa, Japan
  • 1996
    • University of Udine
      • Department of Medical and Biological Sciences
      Udine, Friuli Venezia Giulia, Italy
  • 1980
    • University of Vermont
      • Department of Chemistry
      Burlington, VT, United States