Richard G. Finke

Colorado State University, Fort Collins, Colorado, United States

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Publications (244)1412.22 Total impact

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    W.W. Laxson · S. Özkar · S. Folkman · R.G. Finke
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    ABSTRACT: Reproducibility is the hallmark of reliable science. Reproducible synthetic procedures are of central importance in the chemical sciences, yet ⩾12% of syntheses submitted to publications that explicitly check procedures before their publication, such as Inorganic Syntheses and Organic Syntheses, are reported as having to be rejected since the submitted synthesis could not be repeated. In the present contribution paper we re-examine our own 2012 synthesis of [Ir(1,5-COD)(μ-H)]4 which, frustratingly, we were unable to reproducible once the first author of the original publication completed his stent in our labs. We detail the approach we took to uncover the problems in the synthesis, a key step in which was constructing a “paper mechanism” that led to the key hypothesis of what was going wrong in our attempts to repeat the published synthesis. The results have led to an improved synthesis, one shown to be reproducible by a second researcher working only from the detailed written procedure. Also detailed are the 4 conceptual Steps that were used to find the main problems in the synthesis, as well as the 7 specific alternative hypotheses that were involved and 20 experimental trials which discovered the missing detail in the original synthesis—the previously unknown need to employ a closed reaction system—and which led to the present, further improved synthesis which we demonstrate can be reproduced by an independent researcher with no prior Schlenk-technique experience working from only the written procedure. The hard-won insight of the need for a closed system is likely of broader significance and applicability to analogous syntheses involving metal-reductions by LiBEt3H and probably other anionic hydrides. A summary and conclusion section is included, one striving toward assisting the design and reporting of more reproducible syntheses in the chemical sciences.
    Inorganica Chimica Acta 06/2015; 432. DOI:10.1016/j.ica.2015.04.015 · 2.04 Impact Factor
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    ABSTRACT: Determining the kinetically dominant catalyst in a given catalytic system is a forefront topic in catalysis. The [RhCp*Cl2]2 (Cp* = [η5-C5(CH3)5]) system pioneered by Maitlis and co-workers is a classic precatalyst system from which homogeneous mononuclear Rh1, subnanometer Rh4 cluster, and heterogeneous polymetallic Rh(0)n nanoparticle have all arisen as viable candidates for the true hydrogenation catalyst, depending on the precise substrate, H2 pressure, temperature, and catalyst concentration conditions. Addressed herein is the question of whether the prior assignment of homogeneous, mononuclear Rh1Cp*-based catalysis is correct, or are trace Rh4 subnanometer clusters or possibly Rh(0)n nanoparticles the dominant, actual cyclohexene hydrogenation catalyst at 22 °C and 2.7 atm initial H2 pressure? The observation herein of Rh4 species by in operando-X-ray absorption fine structure (XAFS) spectroscopy, at the only slightly more vigorous conditions of 26 °C and 8.3 atm H2 pressure, and the confirmation of Rh4 clusters by ex situ mass spectroscopy raises the question of the dominant, room temperature, and mild pressure cyclohexene hydrogenation catalyst derived from the classic [RhCp*Cl2]2 precatalyst pioneered by Maitlis and co-workers. Ten lines of evidence are provided herein to address the nature of the true room temperature and mild pressure cyclohexene hydrogenation catalyst derived from [RhCp*Cl2]2. Especially significant among those experiments are quantitative catalyst poisoning experiments, in the present case using 1,10-phenanthroline. Those poisoning studies allow one to distinguish mononuclear Rh1, subnanometer Rh4 cluster, and Rh(0)n nanoparticle catalysis hypotheses. The evidence obtained provides a compelling case for a mononuclear, Rh1Cp*-based cyclohexene hydrogenation catalyst at 22 °C and 2.7 atm H2 pressure. The resultant methodology, especially the quantitative catalyst poisoning experiments in combination with in operando spectroscopy, is expected to be more broadly applicable to the study of other systems and the “what is the true catalyst?” question.Keywords: catalysis; determination of the dominant catalyst; catalyst poisoning studies; rhodium; organometallic complex catalysis; subnanometer cluster catalysis; nanoparticle catalysis; XAFS; in operando spectroscopic studies
    ACS Catalysis 05/2015; 5(6):3876-3886. DOI:10.1021/acscatal.5b00315 · 7.57 Impact Factor
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    ABSTRACT: Agglomerative sintering of an atomically dispersed, zeolite Y-supported catalyst, Ir1/zeolite Y, formed initially from the well-characterized precatalyst [Ir(C2H4)2]/zeolite Y, and in the presence of liquid-phase reactants, was monitored over three cycles of 3800 turnovers (TTOs) of cyclohexene hydrogenation at 72 °C. The catalyst evolved and sintered during each cycle even at the relatively mild temperature of 72 °C in the presence of the cyclohexene plus H2 reactants and cyclohexane solvent. Post each of the three cycles of catalysis, the resultant sintered catalyst was characterized by extended X-ray absorption fine structure spectroscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The results—the first quantitative investigation of sintering of an atomically dispersed catalyst—show that higher-nuclearity iridium species, Irn, are formed during each successive cycle. The progression from the starting mononuclear precursor, Ir1, is first to Ir~4-6, then, on average, Ir~40, and, finally, on average, Ir~70, the latter more accurately described as a bimodal dispersion of on-average Ir~40-50 and on-average Ir~1600 nanoparticles. The size distribution and other data disprove Ostwald Ripening during the initial and final stages of the observed catalyst sintering. Instead, the diameter-dispersion data plus quantitative fits to the cluster or nanoparticle diameter vs. time data provide compelling evidence for the underlying, pseudo-elementary steps of bimolecular agglomeration, B + B  C, and autocatalytic agglomeration, B + C  1.5C, where B represents the smaller, formally Ir(0) nanoparticles, and C is the larger (more highly agglomerated) nanoparticles (and where the 1.5 coefficient in the autocatalytic agglomeration of B + C necessarily follows from the definition, in the bimolecular agglomeration step), that 1 C contains the Ir from 2 B). These two specific, balanced chemical reactions are of considerable significance in going beyond the present state-of-the-art, but word-only, “mechanism”—that is, actually and instead, just a collection of phenomena—for catalyst sintering of “Particle Migration and Coalescence”. The steps of bimolecular plus autocatalytic agglomeration provide two specific, balanced chemical equations useful for fitting sintering kinetics data—as is done herein—thereby quantitatively testing proposed sintering mechanisms. These two pseudo-elementary reactions also define the specific words and concepts for sintering of bimolecular agglomeration and autocatalytic agglomeration. The results are also significant as the first quantitative investigation of the agglomeration and sintering of an initially atomically dispersed metal on a structurally well-defined (zeolite) support and in the presence of liquid reactants (cyclohexene substrate and cyclohexane solvent) plus H2. A list of additional specific conclusions is also provided in a summary section.
    ACS Catalysis 05/2015; 5(6):3514. DOI:10.1021/acscatal.5b00321 · 7.57 Impact Factor
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    ABSTRACT: The Ziegler-type hydrogenation precatalyst dimer, [(1,5-COD)Ir(μ-O2C8H15)]2 (1,5-COD = 1,5-cyclooctadiene; O2C8H15 = 2-ethylhexanoate) plus added AlEt3 stabilizer has recently been shown to form AlEt3-stabilized, Ziegler-type Ir(0)∼4-15 nanoparticles initially, which then grow to larger Ziegler-type Ir(0)∼40-50 nanoparticles during the catalytic hydrogenation of cyclohexene (Alley, W. M.; Hamdemir, I. K.; Wang, Q.; Frenkel, A. I.; Li, L.; Yang, J. C.; Menard, L. D.; Nuzzo, R. G.; Özkar, S.; Johnson, K. A.; Finke, R. G. Inorg. Chem. 2010, 49, 8131-8147). An interesting observation for this Ziegler-type nanoparticle catalyst system is that the apparent TOF (TOFapp = kobs/[Ir]) for cyclohexene hydrogenation increases with decreasing concentration of the precatalyst, [Ir] (defined as 2[{(1,5-COD)Ir(μ-O2C8H15)}2], that is, twice the starting precatalyst concentration since that dimer contains 2 Ir). A perusal of the literature reveals that such an intuitively backward, inverse relationship between the apparent turnover frequency, TOFapp, and the concentration of precatalyst or catalyst has been seen at least eight times before in other, disparate systems in the literature. However, this effect has previously never been satisfactorily explained, nor have the mixed, sometimes opposite, explanations offered in the literature been previously tested by the disproof of all reasonable alternative explanations/mechanistic hypotheses. Herein, five alternative mechanistic explanations have been tested via kinetic studies, Z-contrast STEM microscopy of the nanoparticle product sizes, and other evidence. Four of the five possible explanations have been ruled out en route to the finding that the only mechanism of the five able to explain all the evidence, as well as to quantitatively curve-fit the inverse TOFapp vs [Ir] data, is a prior, dissociative equilibrium, in which x ≈ 3 equiv of the surface-bound, AlR3-based nanocluster stabilizer is dissociated, Ir(0)n·[AlEt3]m ⇄ xAlEt3 + Ir(0)n·[AlEt3]m−x, with the resulting, more coordinatively unsaturated Ir(0)n·[AlEt3]m−x being the faster, kinetically dominant catalyst. The implication is that such unusual, inverse TOFapp vs [precatalyst or catalyst] concentration observations in the literature are, more generally, likely just unintentional, unwitting measurements of a component of the rate law for such systems. The results herein are significant (i) in providing the first quantitative, disproof-tested explanation for the inverse TOFapp vs [precatalyst or catalyst] observation; (ii) in providing precedent and, therefore, a plausible explanation for the eight prior examples of this phenomenon in the literature; and (iii) in demonstrating for one of those additional eight literature cases, a commercial cobalt-based polymer hydrogenation catalyst, that the prior dissociative equilibrium uncovered herein can also quantitatively fit the inverse TOFapp vs [precatalyst] data for that case, as well. The results herein are additionally significant (iv) in making apparent that the rigorous interpretation of any TOF requires that the rate law for the processes under study be known, a point that bears heavily on the confusion and current controversy in the literature over the proper use of the “TOF” concept; (v) in making apparent the usefulness and value of the TOFapp concept employed herein; and (vi) in uncovering the insight that the true, most active catalyst present in AlEt3-stabilized, Ziegler-type Ir(0)n nanoparticle catalysts is the more coordinatively unsaturated Ziegler-type Ir(0)n·[AlEt3]m−x nanoparticle formed from the dissociative loss of ∼3 AlEt3.Keywords: turnover frequency; Ziegler-type nanoclusters; hydrogenation catalysis; kinetics and mechanism; inverse dependence of turnover frequency on catalyst concentration
    ACS Catalysis 05/2015; 5(6):3342. DOI:10.1021/acscatal.5b00347 · 7.57 Impact Factor
  • 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; 136(50). DOI:10.1021/ja510263s · 11.44 Impact Factor
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    William W. Laxson · Saim Oezkar · Richard G. Finke
    ChemInform 05/2014; 45(19):no-no. DOI:10.1002/chin.201419016
  • Joel T Kirner · Jordan J Stracke · Brian A Gregg · Richard G Finke
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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; 6(16). DOI:10.1021/am405598w · 6.72 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. DOI:10.1021/cs4011716 · 7.57 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; 53(5). DOI:10.1021/ic403057k · 4.79 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; 136(5). DOI:10.1021/ja410194r · 11.44 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. DOI:10.1021/cs4006925 · 7.57 Impact Factor
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    Isil K Hamdemir · Saim Özkar · Richard G Finke
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    ABSTRACT: In recent work we showed that Ziegler-type nanoparticles made from [Ir(1,5-COD)(␮-O 2 C 8 H 15)] 2 plus AlEt 3 are an unusually thermally stable (≥30 min at 200 • C), hydrocarbon-solvent soluble, high catalytic activity nanoparticle catalyst (I.K. Hamdemir, S. Özkar, K.-H. Yih, J.E. Mondloch, R.G. Finke, ACS Catal. 2 (2012) 632–641). As such, they are analogous to—and currently the cleanest and best characterized model system for—Ziegler-type nanoparticles made from Co or Ni precatalysts plus AlEt 3 which are used industrially to hydrogenate ∼1.7 × 10 5 metric tons of styrenic block copolymers per year (for a review of the area, see W.M. Alley, I.K. Hamdemir, K.A. Johnson, R.G. Finke, J. Mol. Catal.: A Chem. 315 (2010) 1–27). The key question addressed in the present paper is " What is the nature of the AlEt 3-derived stabilizer species? " for the unusually stable and active Ziegler-nanoparticles formed from [Ir(1,5-COD)(␮-O 2 C 8 H 15)] 2 plus AlEt 3. Specifically tested herein are four primary hypotheses for the AlEt 3-derived stabilizer(s) in the Ir(0) n Ziegler-nanoparticle system: (i) that the key stabilizer is neutral (i.e., uncharged) aluminum alkyl carboxylates following precedent from the work of Shmidt and Bönnemann; (ii) that the key stabilizer is anionic [AlEt 3 (O 2 C 8 H 15)] − ; (iii) that a key stabilizer is the AlEt 3 (or its derivatives) reacting with the Ir(0) n nanoparticle surface; or (iv) that an important AlEt 3-derived stabilizer is Al O Al containing alkylalumoxanes formed from any water present. The results obtained rule out (ii), but provide strong evidence for (iii), as well as evidence consistent with (i) and (iv), as stabilizers in Ziegler-nanoparticles. A pictorial scheme (Scheme 2) is provided as a working hypothesis for the stabilization mode(s) of Ziegler-nanoparticles and as a way to focus and expedite the needed additional composition and structural studies of Ziegler-nanoparticle stabilizers.
    Journal of Molecular Catalysis A Chemical 11/2013; 378:333-343. DOI:10.1016/j.molcata.2013.07.005 · 3.68 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. DOI:10.1021/cs400141t · 7.57 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. DOI:10.1021/cs300330c · 7.57 Impact Factor
  • Ercan Bayram · Jing Lu · Ceren Aydin · Alper Uzun · Bruce C. Gates · Richard G. Finke
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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. DOI:10.1021/cs300366w · 7.57 Impact Factor
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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. DOI:10.1021/cm203186y · 8.54 Impact Factor
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    ABSTRACT: Sustainability and green chem. are intimately connected to chem. catalysis. Moreover, there is no more important initial question in a given catalytic reaction than "what is the true catalyst", since optimizing catalyst activity, lifetime, selectivity, isolation, and regeneration all depend on knowledge of the identity and compn. of the true catalyst. The short talk will present recent evidence on the "who is the true catalyst" question for the title system based on in operando XAFS spectroscopy, kinetic studies, and esp. quant. kinetic poisoning data. The results provide a compelling case for Rh4, subnanometer cluster-based benzene hydrogenation catalysis at 100 °C and 50 atm H2 pressures.
    243rd ACS National Meeting & Exposition,, San Diego, USA; 03/2012
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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. DOI:10.1021/cs200688g · 7.57 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. DOI:10.1021/ic2026494 · 4.79 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(15):1. DOI:10.1016/j.molcata.2011.11.011 · 3.68 Impact Factor

Publication Stats

9k Citations
1,412.22 Total Impact Points

Institutions

  • 1994–2015
    • Colorado State University
      • Department of Chemistry
      Fort Collins, Colorado, United States
    • Kanagawa University
      • Faculty of Science
      Yokohama, Kanagawa, Japan
  • 2012
    • Washington University in St. Louis
      • Department of Chemistry
      San Luis, Missouri, United States
  • 1980–2007
    • University of Oregon
      • Department of Chemistry
      Eugene, OR, United States
  • 2003
    • University of Michigan
      • Department of Biological Chemistry
      Ann Arbor, MI, United States
  • 2002–2003
    • Middle East Technical University
      • Department of Chemistry
      Engüri, Ankara, Turkey
  • 1986
    • University of Illinois, Urbana-Champaign
      • Department of Chemistry
      Urbana, Illinois, United States
  • 1983
    • University of Colorado at Boulder
      • Department of Chemistry and Biochemistry
      Boulder, Colorado, United States
  • 1982
    • Stanford University
      • Department of Chemistry
      Palo Alto, California, United States