Copper Selenophosphate, Cu3PSe4, Nanoparticle Synthesis: Octadecane Is the Key to a Simplified, Atom-Economical Reaction.

Nanoparticle syntheses are designed to produce the desired product in high yield but traditionally neglect atom-economy. Here we report that the simple, but significant, change of the solvent from 1-octadecene (1-ODE) to the operationally inert octadecane (ODA) permits an atom-economical synthesis of copper selenophosphate (Cu3PSe4) nanoparticles. This change eliminates the competing selenium (Se) delivery pathways from our first report that required an excess of Se. Instead Se0powder is dispersed in ODA, which promotes a formal eight-electron transfer between Cu3-xP and Se0. Powder X-ray diffraction and transmission electron microscopy confirm the purity of the Cu3PSe4, while 1H and 13C NMR indicate the absence of oxidized ODA or Se species. We utilize the direct pathway to gain insights into stoichiometry and ligand identity using thermogravimetric analysis and X-ray photoelectron spectroscopy. Given the prevalence of 1-ODE in nanoparticle synthesis, this approach could be applied to other chalcogenide reaction pathways to improve stoichiometry and atom-economy.

Computational Insights into the Mechanism of Nitric Oxide Generation from S-Nitrosoglutathione Catalyzed by a Copper Metal-Organic Framework.

The controlled generation of nitric oxide (NO) from endogenous sources, such as S-nitrosoglutathione (GSNO), has significant implications for biomedical implants due to the vasodilatory and other beneficial properties of NO. The water-stable metal-organic framework (MOF) Cu-1,3,5-tris[1H-1,2,3-triazol-5-yl]benzene has been shown to catalyze the production of NO and glutathione disulfide (GSSG) from GSNO in aqueous solution as well as in blood. Previous experimental work provided kinetic data for the catalysis of the 2GSNO → 2NO + GSSG reaction, leading to various proposed mechanisms. Herein, this catalytic process is examined using density functional theory. Minimal functional models of the Cu-MOF cluster and glutathione moieties are established, and three distinct catalytic mechanisms are explored. The most thermodynamically favorable mechanism studied is consistent with prior experimental findings. This mechanism involves coordination of GSNO to copper via sulfur rather than nitrogen and requires a reductive elimination that produces a Cu(I) intermediate, implicating a redox-active copper site. The experimentally observed inhibition of reactivity at high pH values is explained in terms of deprotonation of a triazole linker, which decreases the structural stability of the Cu(I) intermediate. These fundamental mechanistic insights may be generally applicable to other MOF catalysts for NO generation.

Understanding the "Anti-Catalyst" Effect with Added CoOx Water Oxidation Catalyst in Dye-Sensitized Photoelectrolysis Cells: Carbon Impurities in Nanostructured SnO2 Are the Culprit.

In 2017, we reported a dye-sensitized, photoelectrolysis cell consisting of fluorine-doped tin oxide (FTO)-coated glass covered by SnO2 nanoparticles coated with N,N'-bis(phosphonomethyl)-3,4,9,10-perylenediimide (PMPDI) dye and then a photoelectrochemically deposited CoOx water oxidation catalyst (WOCatalyst), FTO/nano-SnO2/PMPDI/CoOx. This system employed nanostructured SnO2 stabilized by a polyethyleneglycol bisphenol A epichlorohydrin (PEG-BAE) copolymer and other C-containing additives based on a literature synthesis to achieve a higher surface area and thus greater PMPDI dye absorption and resultant light collection. Surprisingly, the addition of the well-established WOCatalyst CoOx resulted in a decrease in the photocurrent, an unexpected "anti-catalyst" effect. Two primary questions addressed in the present study are (1) what is the source of this "anti-catalyst" effect? and (2) are the findings of broader interest? Reflection on the synthesis of nano-SnO2 stabilized by PEG-BAE, and the large, ca. 10:1 ratio of C to Sn in synthesis, led to the hypothesis that even the annealing step at 450 °C in of the FTO/SnO2 anode precursors was unlikely to remove all the carbon initially present. Indeed, residual carbon impurities are shown to be the culprit in the presently observed "anti-catalyst" effect. The implication and anticipated broader impact of the results of answering the two abovementioned questions are also presented and discussed along with a section entitled "Perspective and Suggestions for the Field Going Forward."

Estimating reaction parameters in mechanism-enabled population balance models of nanoparticle size distributions: A Bayesian inverse problem approach.

In order to quantitatively predict nano- as well as other particle-size distributions, one needs to have both a mathematical model and estimates of the parameters that appear in these models. Here, we show how one can use Bayesian inversion to obtain statistical estimates for the parameters that appear in recently derived mechanism-enabled population balance models (ME-PBM) of nanoparticle growth. The Bayesian approach addresses the question of "how well do we know our parameters, along with their uncertainties?." The results reveal that Bayesian inversion statistical analysis on an example, prototype Ir0n nanoparticle formation system allows one to estimate not just the most likely rate constants and other parameter values, but also their SDs, confidence intervals, and other statistical information. Moreover, knowing the reliability of the mechanistic model's parameters in turn helps inform one about the reliability of the proposed mechanism, as well as the reliability of its predictions. The paper can also be seen as a tutorial with the additional goal of achieving a "Gold Standard" Bayesian inversion ME-PBM benchmark that others can use as a control to check their own use of this methodology for other systems of interest throughout nature. Overall, the results provide strong support for the hypothesis that there is substantial value in using a Bayesian inversion methodology for parameter estimation in particle formation systems.

Pseudoelementary Steps: A Key Concept and Tool for Studying the Kinetics and Mechanisms of Complex Chemical Systems.

The concept of a pseudoelementary step (PEStep) is reviewed, a key concept for approaching the analysis of kinetics data and associated, underlying mechanisms of complex chemical systems. Following a brief Introduction, a definition of a PEStep is given: a PEStep is an initial building block for more complex reactions, that is a starting point for the initial analysis of the observed kinetics and then constructing initial, deliberately minimalistic mechanistic models for complex reactions. PESteps are, therefore and typically, composites of underlying elementary step reactions and can be very useful if not required for the inverse problem of discovering mechanisms from experimental observables for complex reactions. It is the use of PESteps in the inverse problem of mechanism determination that is a primary focus of this review. After a section detailing the results of a literature search of "pseudoelementary step" and related terms such as "pseudoelementary process", pedagogically illustrative examples are given of the use of the PEStep concept in approaching and elucidating the mechanisms of complex reactions. This review shows how the underlying elementary steps of a catalytic cycle were successfully uncovered via a PEStep approach, addresses the classic case of the use of PESteps in determining the mechanisms of oscillating reactions, and examines a well-studied case of an Ir(0)n nanoparticle formation reaction. This latter example is illustrative in that the Ir(0)n nanoparticle formation reaction consisting of thousands of underlying elementary steps that, however, can be treated initially kinetically as just two PESteps, a reduction in complexity of 3 orders of magnitude. Known weaknesses and caveats of the PEStep approach are also summarized and discussed. A short summary of Horituti's "Stoichiometric Number" concept is provided, a concept that would appear to merit further investigation and use in the study of complex reactions. Finally, a section is provided that lists a few, selected areas where the PEStep concept and methodology are expected to prove especially important in the future, and a Conclusions section is provided that lists 11 bullet points. The latter serves as a summary of this first review of the PEStep concept and its importance in dealing with the kinetics and in elucidating the mechanisms of more complex, multistep reactions.

Copper Metal-Organic Framework Surface Catalysis: Catalyst Poisoning, IR Spectroscopic, and Kinetic Evidence Addressing the Nature and Number of the Catalytically Active Sites En Route to Improved Applications.

The metal-organic framework (MOF) H3[(Cu4Cl)3-(BTTri)8, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene] (CuBTTri) is a precatalyst for biomedically relevant nitric oxide (NO) release from S-nitrosoglutathione (GSNO). The questions of the number and nature of the catalytically most active, kinetically dominant sites are addressed. Also addressed is whether or not the well-defined structural geometry of MOFs (as solid-state analogues of molecular compounds) can be used to generate specific, testable hypotheses about, for example, if intrapore vs exterior surface metal sites are more catalytically active. Studies of the initial catalytic rate vs CuBTTri particle external surface area to interior volume ratio show that intrapore copper sites are inactive within the experimental error (≤1.7 × 10-5% of the observed catalytic activity)-restated, the traditional MOF intrapore metal site catalysis hypothesis is disproven for the current system. All observed catalysis occurs at exterior surface Cu sites, within the experimental error. Fourier transform infrared (FT-IR) analysis of CN--poisoned CuBTTri reveals just two detectable Cu sites at a ca. ≥0.5% detection limit, those that bind three or one CN- ("Cu(CN)3" and "CuCN"), corresponding to the CN- binding expected for exterior surface, 3-coordinate (Cusurface) and intrapore, 5-coordinate (Cupore) sites predicted by the idealized, metal-terminated crystal structure. Two-coordinate Cu defect sites are ruled out at the ≥0.5% FT-IR detection limit as such defect sites would have been detectable by the FT-IR studies of the CN--poisoned catalyst. Size-selective poisoning studies of CuBTTri exterior surface sites reveal that 1.3 (±0.4)% of total copper in 0.6 ± 0.4 µm particles is active. That counting of active sites yields a normalized turnover frequency (TOF), TOFnorm = (4.9 ± 1.2) × 10-2 mol NO (mol Cusurface)-1 s-1 (in water, at 20 min, 25 °C, 1 mM GSNO, 30% loss of GSNO, and 1.3 ± 0.4 mol % Cusurface)-a value ∼100× higher than the TOF calculated without active site counting. Overall, Ockham's razor interpretation of the data is that exterior surface, Cusurface sites are the catalytically most active sites present at a 1.3 (±0.4)% level of total Cu.

Dust Effects on Ir(0)n Nanoparticle Formation Nucleation and Growth Kinetics and Particle Size-Distributions: Analysis by and Insights from Mechanism-Enabled Population Balance Modeling.

The effects of microfiltration removal of filterable dust on nanoparticle formation kinetics and particle-size distribution, in a polyoxometalate polyanion (P2W15Nb3O629-)-stabilized Ir(0)n nanoparticle formation system, are analyzed by the newly developed method of Mechanism-Enabled Population Balance Modeling (ME-PBM). The [(Bu4N)5Na3(1,5-COD)Ir·P2W15Nb3O62] precatalyst system produces on average Ir(0)∼200 nanoparticles of 1.74 ± 0.33 nm and hence a particle-size distribution (PSD) of ±19% dispersion when the precatalyst is reduced under H2 in unfiltered propylene carbonate solvent. But if the precatalyst is reduced in microfiltered solvent and microfiltered reagent solutions (where the filtered solvent is then also used to rinse dust from the glassware), then larger Ir(0)∼300 1.96 ± 0.16 nm nanoparticles are produced with a remarkable, 2.4-fold lowered ±8% dispersion. The results and effects of the microfiltration reduction of dust are analyzed by the newly developed method of ME-PBM. More specifically, the studies reported herein address eight outstanding questions that are listed in the Introduction. Those questions include: how easy or difficult it is to fit PSD data? What is the ability of the recently discovered alternative termolecular nucleation and two size-dependent growth steps mechanism to account for the effects of dust on the PSD? What types and amount of PSD kinetics data are needed to deconvolute the PSD into the parameters of the ME-PBM? What is the reliability of the resulting rate constants? Additional questions addressed include: if the ME-PBM results offer insights into the remarkable 2.4-fold narrowing of the PSD post simple microfiltration lowering of the dust, and if the results are likely to be more general? The Summary and Conclusions section lists nine specific insights that include comments on needed future studies.

Mechanism-Enabled Population Balance Modeling of Particle Formation en Route to Particle Average Size and Size Distribution Understanding and Control.

The concept of Mechanism-Enabled Population Balance Modeling (ME-PBM) is reported, illustrated by its application to a prototype Ir(0)n nanoparticle formation reaction. ME-PBM is defined herein as the use of now available, experimentally established, disproof-based, deliberately minimalistic mechanisms of particle formation as the required input for more rigorous Population Balance models, critically including an experimentally established nucleation mechanism. ME-PBM achieves the long-sought goal of connecting such now available experimental minimum mechanisms to the understanding and rational control of particles size and size distributions. Twelve pseudoelementary step, particle-formation mechanisms are considered so that the approach to the ME-PBM is also extensively disproof-based. Resurrection of Smoluchowski's 1918 full Ordinary Differential Equation (ODE) approach to the PBM is another, critical aspect of our approach which, in turn, allows unbiased fitting of the information-laden particle-size distribution (PSD) including its shape. The results provide one solution to the "inverse problem" in which the PSD informs one as to the correct particle formation mechanism: A new, deliberately minimalistic 3-step particle-formation mechanism has been uncovered that is a single-additional-step expansion of the now broadly used Finke-Watzky (FW) 2-step mechanism, the new 3-step mechanism being: A → B (rate constant k1), A + B → C (rate constant k2), and A + C → 1.5C (rate constant k3), where A represents the monomeric nanoparticle precursor, B represents "small" nanoparticles, and C represents "larger" nanoparticles. The results strongly support three paradigm shifts for nucleation and growth of particles, the most critical paradigm shift being that the "burst" nucleation assumption in LaMer's 1950s model of particle formation is not required to produce narrow, near-monodisperse PSDs. Instead, narrow PSDs can be and are achieved despite continuous nucleation because smaller particles grow faster than larger ones, k2 > k3, thereby allowing the smaller particles to catch up in size to the more slowly growing larger particles.

Copper ion vs copper metal-organic framework catalyzed NO release from bioavailable S-Nitrosoglutathione en route to biomedical applications: Direct 1H NMR monitoring in water allowing identification of the distinct, true reaction stoichiometries and thiol dependencies.

Copper containing compounds catalyze decomposition of S-Nitrosoglutathione (GSNO) in the presence of glutathione (GSH) yielding glutathione disulfide (GSSG) and nitric oxide (NO). Extended NO generation from an endogenous source is medically desirable to achieve vasodilation, reduction in biofilms on medical devices, and antibacterial activity. Homogeneous and heterogeneous copper species catalyze release of NO from endogenous GSNO. One heterogeneous catalyst used for GSNO decomposition in blood plasma is the metal-organic framework (MOF), H3[(Cu4Cl)3-(BTTri)8, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl) benzene] (CuBTTri). Fundamental questions about these systems remain unanswered, despite their use in biomedical applications, in part because no method previously existed for simultaneous tracking of [GSNO], [GSH], and [GSSG] in water. Tracking these reactions in water is a necessary step towards study in biological media (blood is approximately 80% water) where NO release systems must operate. Even the balanced stoichiometry remains unknown for copper-ion and CuBTTri catalyzed GSNO decomposition. Herein, we report a direct 1H NMR method which: simultaneously monitors [GSNO], [GSH], and [GSSG] in water; provides the experimentally determined stoichiometry for copper-ion vs CuBTTri catalyzed GSNO decomposition; reveals that the CuBTTri-catalyzed reaction reaches 10% GSNO decomposition (16 h) without added GSH, yet the copper-ion catalyzed reaction reaches 100% GSNO decomposition (16 h) without added GSH; and shows 100% GSNO decomposition upon addition of stoichiometric GSH to the CuBTTri catalyzed reaction. These observations provide evidence that copper-ion and CuBTTri catalyzed GSNO decomposition in water operate through different reaction mechanisms, the details of which can now be probed by 1H NMR kinetics and other needed studies.

Electrochemically Driven Water-Oxidation Catalysis Beginning with Six Exemplary Cobalt Polyoxometalates: Is It Molecular, Homogeneous Catalysis or Electrode-Bound, Heterogeneous CoO x Catalysis?

A series of six exemplary cobalt-polyoxometalate (Co-POM) precatalysts have been examined to determine if they are molecular water-oxidation catalysts (WOCatalysts) or if, instead, they actually form heterogeneous, electrode-bound CoO x as the true WOCatalyst under electrochemically driven water-oxidation catalysis (WOCatalysis) conditions. Specifically, WOCatalysis derived from the following six Co-POMs has been examined at pH 5.8, 8.0, and 9.0: [Co4(H2O)2(PW9O34)2]10- (Co4P2W18), [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16- (Co9P5W27), [ ßß-Co4(H2O)2(P2W15O56)2]16- (Co4P4W30), [Co(H2O)PW11O39]5- (CoPW11), [α1-Co(H2O)P2W17O61]8- (α1-CoP2W17), and [α2-Co(H2O)P2W17O61]8- (α2-CoP2W17). The amount of Co(II)aq in 500 µM solutions of each Co-POM was measured after 3 h of aging as well as from t = 0 for pH = 5.8 and 8.0 by µM sensitive Co(II)aq-induced 31P NMR line broadening and at pH = 9.0 by cathodic stripping. The amount of detectable Co(II)aq after 3 h for the six Co-POMs ranges from ∼0.25 to ∼90% of the total cobalt initially present in the Co-POM. For 12 out of 18 total Co-POM and different pH cases, the amount Co(II)aq detected after 3 h forms heterogeneous CoO x able to account for ≥100% of the observed WOCatalysis activity. However, under 0.1 M NaPi, pH 5.8 conditions for CoPW11 and α1-CoP2W17 where ∼1.5% and 0.25% Co(II)aq is detectable, the measured Co(II)aq cannot account for the observed WOCatalysis. The implication is that these two Co-POMs are primarily molecular, Co-POM-based, WOCatalysts under electrochemically driven, pH 5.8, phosphate-buffer conditions. Even for the single most stable Co-POM, α1-CoP2W17, CoO x is still an estimated ∼76× faster WOCatalyst at pH = 5.8 and an estimated ∼740× faster WOCatalyst at pH = 8.

Alcohol Solvent Effects in the Synthesis of Co3O4 Metal-Oxide Nanoparticles: Disproof of a Surface-Ligand Thermodynamic Effect en Route to Alternative Kinetic and Thermodynamic Explanations.

The synthesis of Co3O4 core nanoparticles from cobalt acetate is explored in alcohol solvents plus limited water using O2 as oxidant and NH4OH as the base, all in comparison to controls in water alone employing the otherwise identical synthetic procedure. Syntheses in EtOH or t-BuOH cosolvents with limited water yield phase-pure and size-controlled (3 ± 1 nm) Co3O4-core nanoparticles. In marked contrast, the synthesis in water alone yields mixed phases of Co3O4 and ß-Co(OH)2 with a very large particle-size range (14-400 nm). Importantly, acidic reductive digestion of the Co3O4 particles followed by 1H NMR on the resultant solution yields no detectable EtOH in nanoparticles prepared in EtOH, nor any detectable t-BuOH in nanoparticles prepared in t-BuOH (∼5% detection limits for each alcohol), despite the dramatic effect of each alcohol cosolvent on the resultant cobalt-oxide product. Instead, in both cases HOAc is detected and quantified, indicative of OAc- as a surface ligand-and not EtO- or t-BuO- as the surface ligand. The resultant ROH cosolvent-derived particles were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, plus elemental analysis to arrive at an approximate, average molecular formula in the case of the particles prepared in EtOH, {[Co3O4(C2H3O2)]-[(NH4+)0.3(H+0.7)]+·(H2O)}∼216. The key finding is that, because EtOH and t-BuOH have a substantial effect on the phase- and size-dispersion of the cobalt-oxide nanoparticle product, yet the intact alcohol does not show up in the final Co3O4 nanoparticle product, the effect of these alcohols cannot be a surface-ligand thermodynamic effect on the net nanoparticle formation reaction. A careful search of the literature provided scattered, but consistent, literature in which anions or other additives have large effects on metal-oxide nanoparticle formation reactions, yet also do not show up in the nanoparticle products-that is, where the observed effects are again not due to binding by that anion or other additive in a surface-ligand thermodynamic effect on the overall reaction. Alternative hypotheses are provided as to the origin of ROH solvent effects on metal-oxide nanoparticles.

"Weakly Ligated, Labile Ligand" Nanoparticles: The Case of Ir(0) n ·(H+Cl-) m.

It is of considerable interest to prepare weakly ligated, labile ligand (WLLL) nanoparticles for applications in areas such as chemical catalysis. WLLL nanoparticles can be defined as nanoparticles with sufficient, albeit minimal, surface ligands of moderate binding strength to meta-stabilize nanoparticles, initial stabilizer ligands that can be readily replaced by other, desired, more strongly coordinating ligands and removed completely when desired. Herein, we describe WLLL nanoparticles prepared from [Ir(1,5-COD)Cl]2 reduction under H2, in acetone. The results suggest that H+Cl--stabilized Ir(0) n nanoparticles, herein Ir(0) n ·(H+Cl-) a , serve as a WLLL nanoparticle for the preparation of, as illustrative examples, five specific nanoparticle products: Ir(0) n ·(Cl-Bu3NH+) a , Ir(0) n ·(Cl-Dodec3NH+) a , Ir(0) n ·(POct3)0.2n (Cl-H+) b , Ir(0) n ·(POct3)0.2n , and the γ-Al2O3-supported heterogeneous catalyst, Ir(0) n ·(γ-Al2O3) a (Cl-H+) b . (where a and b vary for the differently ligated nanoparticles; in addition, solvent can be present as a nanoparticle surface ligand). With added POct3 as a key, prototype example, an important feature is that a minimum, desired, experimentally determinable amount of ligand (e.g., just 0.2 equiv POct3 per mole of Ir) can be added, which is shown to provide sufficient stabilization that the resultant Ir(0) n ·(POct3)0.2n (Cl-H+) b is isolable. Additionally, the initial labile ligand stabilizer HCl can be removed to yield Ir(0) n ·(POct3)0.2n that is >99% free of Cl- by a AgCl precipitation test. The results provide strong support for the weakly ligated, labile ligand nanoparticle concept and specific support for Ir(0) n ·(H+Cl-) a as a WLLL nanoparticle.

Gold Nanoparticle Formation Kinetics and Mechanism: A Critical Analysis of the "Redox Crystallization" Mechanism.

A 2013 paper proposed a "redox crystallization" (R-C) mechanism for the formation of Au0 n nanoparticles from the reduction of a AuCl4 - precursor. That study used an unconventional analysis of the valuable, expertly obtained kinetics data reported, and came up with multiple claims and insights collected under the putatively new R-C mechanism. If confirmed, those claims and the R-C mechanism provide a valuable addition to the knowledge of gold nanoparticle formation kinetics and mechanisms. On the other hand, if the methodology used to support the R-C mechanism is flawed so that its resultant conclusions are incorrect, then the R-C mechanism needs to be discarded until compelling evidence for it can be gathered, evidence that would have to include the disproof of the other dominant mechanism(s) of nanoparticle formation. The present work provides a critical analysis of the evidence previously offered for the R-C mechanism, efforts that are of interest to the areas of Au0 n nanoparticles, the kinetics and mechanisms of nanoparticle formation and, as it turns out, more generally to those interested in kinetic and mechanistic studies.

A Classic Azo-Dye Agglomeration System: Evidence for Slow, Continuous Nucleation, Autocatalytic Agglomerative Growth, Plus the Effects of Dust Removal by Microfiltration on the Kinetics.

An important but virtually ignored 1978 paper by Reeves and co-workers, which examined a dye-OAc hydrolysis and then agglomeration system, is reanalyzed in light of current state of knowledge of nucleation and growth/agglomeration phenomena. The Finke-Watzky two-step mechanism is used to account quantitatively for the kinetics data, in turn providing deconvolution of dye hydrolysis and nucleation of agglomerative growth, from the agglomerative growth step, including their separate rate constants. Significantly, the effects of microfiltration of the removable dust on the two steps and their rate constants are uncovered and quantitated for the first time, including the finding that the presence of dust accelerates both steps by ca. 10-fold or more. A postulated minimum mechanism able to account for all the observed results is provided. The results allow the excellently designed and executed, now nearly 40-years old, classic studies of Reeves and co-workers to be placed in its proper position in history, while at the same time providing six insights and conclusions detailed in the Discussion and Conclusions sections of the paper.

Sensitization of Nanocrystalline Metal Oxides with a Phosphonate-Functionalized Perylene Diimide for Photoelectrochemical Water Oxidation with a CoOx Catalyst.

A planar organic thin film composed of a perylene diimide dye (N,N'-bis(phosphonomethyl)-3,4,9,10-perylenediimide, PMPDI) with photoelectrochemically deposited cobalt oxide (CoOx) catalyst was previously shown to photoelectrochemically oxidize water (DOI: 10.1021/am405598w). Herein, the same PMPDI dye is studied for the sensitization of different nanostructured metal oxide (nano-MOx) films in a dye-sensitized photoelectrochemical cell architecture. Dye adsorption kinetics and saturation decreases in the order TiO2 > SnO2 ≫ WO3. Despite highest initial dye loading on TiO2 films, photocurrent with hydroquinone (H2Q) sacrificial reductant in pH 7 aqueous solution is much higher on SnO2 films, likely due to a higher driving force for charge injection into the more positive conduction band energy of SnO2. Dyeing conditions and SnO2 film thickness were subsequently optimized to achieve light-harvesting efficiency >99% at the λmax of the dye, and absorbed photon-to-current efficiency of 13% with H2Q, a 2-fold improvement over the previous thin-film architecture. A CoOx water-oxidation catalyst was photoelectrochemically deposited, allowing for photoelectrochemical water oxidation with a faradaic efficiency of 31 ± 7%, thus demonstrating the second example of a water-oxidizing, dye-sensitized photoelectrolysis cell composed entirely of earth-abundant materials. However, deposition of CoOx always results in lower photocurrent due to enhanced recombination between catalyst and photoinjected electrons in SnO2, as confirmed by open-circuit photovoltage measurements. Possible future studies to enhance photoanode performance are discussed, including alternative catalyst deposition strategies or structural derivatization of the perylene dye.

Dust Effects on Nucleation Kinetics and Nanoparticle Product Size Distributions: Illustrative Case Study of a Prototype Ir(0)n Transition-Metal Nanoparticle Formation System.

The question is addressed if dust is kinetically important in the nucleation and growth of Ir(0)n nanoparticles formed from [Bu4N]5Na3(1,5-COD)IrI·P2W15Nb3O62 (hereafter [(COD)Ir·POM]8-), reduced by H2 in propylene carbonate solvent. Following a concise review of the (often-neglected) literature addressing dust in nucleation phenomena dating back to the late 1800s, the nucleation and growth kinetics of the [(COD)Ir·POM]8- precatalyst system are examined for the effects of 0.2 µm microfiltration of the solvent and precatalyst solution, of rinsing the glassware with that microfiltered solvent, of silanizing the glass reaction vessel, for the addition of <0.2 µm γ-Al2O3 (inorganic) dust, for the addition of flame-made carbon-based (organic) dust, and as a function of the starting, microfiltered [(COD)Ir·POM8-] concentration. Efforts to detect dust and its removal by dynamic light scattering and by optical microscopy are also reported. The results yield a list of eight important conclusions, the four most noteworthy of which are (i) that the nucleation apparent rate "constant" k1obs(bimol) is shown to be slowed by a factor of ∼5 to ∼7.6, depending on the precise experiment and its conditions, just by the filtration of the precatalyst solution using a 0.20 µm filter and rinsing the glassware surface with 0.20 µm filtered propylene carbonate solvent; (ii) that simply employing a 0.20 µm filtration step narrows the size distribution of the resulting Ir(0)n nanoparticles by a factor of 2.4 from ±19 to ±8%, a remarkable result; (iii) that the narrower size distribution can be accounted for by the slowed nucleation rate constant, k1obs(bimol), and by the unchanged autocatalytic growth rate constant, k2obs(bimol), that is, by the increased ratio of k2obs(bimol)/k1obs(bimol) that further separates nucleation from growth in time for filtered vs unfiltered solutions; and (iv) that five lines of evidence indicate that the filterable component of the solution, which has nucleation rate-enhancing and size-dispersion broadening effects, is dust.

Nanoparticle Nucleation Is Termolecular in Metal and Involves Hydrogen: Evidence for a Kinetically Effective Nucleus of Three {Ir3H2x·P2W15Nb3O62}6- in Ir(0)n Nanoparticle Formation From [(1,5-COD)IrI·P2W15Nb3O62]8- Plus Dihydrogen.

The nucleation process yielding Ir(0)∼300 nanoparticles from (Bu4N)5Na3[(1,5-COD)Ir·P2W15Nb3O62] (abbreviated hereafter as (COD)Ir·POM8-, where POM9- = the polyoxometalate, P2W15Nb3O629-) under H2 is investigated to learn the true molecularity, and hence the associated kinetically effective nucleus (KEN), for nanoparticle formation for the first time. Recent work with this prototype transition-metal nanoparticle formation system ( J. Am. Chem. Soc. 2014 , 136 , 17601 - 17615 ) revealed that nucleation in this system is an apparent second-order in the precatalyst, A = (COD)Ir·POM8-, not the higher order implied by classic nucleation theory and its nA ⇌ An, "critical nucleus", An concept. Herein, the three most reasonable more intimate mechanisms of nucleation are tested: bimolecular nucleation, termolecular nucleation, and a mechanism termed "alternative termolecular nucleation" in which 2(COD)Ir+ and 1(COD)Ir·POM8- yield the transition state of the rate-determining step of nucleation. The results obtained definitively rule out a simple bimolecular nucleation mechanism and provide evidence for the alternative termolecular mechanism with a KEN of 3, Ir3. All higher molecularity nucleation mechanisms were also ruled out. Further insights into the KEN and its more detailed composition involving hydrogen, {Ir3H2xPOM}6-, are also obtained from the established role of H2 in the Ir(0)∼300 formation balanced reaction stoichiometry, from the p(H2) dependence of the kinetics, and from a D2/H2 kinetic isotope effect of 1.2(±0.3). Eight insights and conclusions are presented. A section covering caveats in the current work, and thus needed future studies, is also included.

Cobalt Polyoxometalate Co4V2W18O68(10-): A Critical Investigation of Its Synthesis, Purity, and Observed (51)V Quadrupolar NMR.

The vanadium-containing cobalt polyoxometalate (Co-POM) Co4V2W18O68(10-) (hereafter Co4V2W18) has been reported to be a stable, homogeneous water-oxidation catalyst, one with a claimed record turnover frequency that is also reportedly 200-fold faster than its phosphorus congener, Co4P2W18O68(10-). The claimed superior water-oxidation catalysis activity of the vanadium congener, Co4V2W18, rests squarely on the reported synthesis of Co4V2W18, its purity, and its stability in both the solid-state and in solution. Attempts to repeat the preparation of Co4V2W18 by either of two literature syntheses, along with the other studies reported herein, led to the discovery of multiple, convoluted problems in the prior literature of Co4V2W18. The three most serious of those problems proved to be the prior misunderstanding of the quadrupolar (herein (51)V) NMR peak widths in complexes that also contain paramagnetic metals such as Co(II), the incorrect assignment of a -506.8 ppm (51)V NMR to Co4V2W18, and then the use of that -506.8 peak to argue for the stability of Co4V2W18 in solution. The results are reported in a somewhat historical, "story" fashion en route to elucidating and fully supporting the 11 insights and take-home messages listed in the Summary and Conclusions section.

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

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

Nucleation is second order: an apparent kinetically effective nucleus of two for Ir(0)n nanoparticle formation from [(1,5-COD)Ir(I)·P2W15Nb3O62]8- plus hydrogen.

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", A(n), 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 k(1obs)), then (ii) fast autocatalytic surface growth (A + B → 2B, rate constant k(2obs)). Relatively precise homogeneous nucleation rate constants, k(1obs), examined as a function of the amount of precatalyst, A, reveal that k(1obs) has an added dependence on the concentration of the precursor, k(1obs) = k(1obs(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, k(1obs(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.