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.

Catalyst sintering is an undesired, but general, and hence practically important catalyst deactivation process. Understanding sintering kinetics and, then, the associated mechanism(s) is an important goal, one crucial to being better able to limit and otherwise control catalyst sintering rationally. However, and despite the availability of atomic-based sintering models, the kinetics of sintering of practical catalysts are to this day most often accounted for by curve-fitting with empirical power laws. Such empirical kinetics treatments are, unfortunately, devoid of rigorous mechanistic insight because they lack the balanced chemical equations that are required to define the rate constants and to also define the proper concepts and associated words that, in turn, are crucial for being able to describe correctly the sintering process physically. Hence, addressed herein is the key, previously unanswered question: is there a disproof-based, Ockham’s razor-obeying, hence mechanistically rigorous, minimal chemical mechanism that can be used to curve-fit sintering kinetics data previously accounted for by empirical power law expressions? If so, then what are its implications? The results provided demonstrate that literature catalyst sintering data, previously fit using empirical power laws, can instead be quantitatively accounted for by a simple, deliberately minimalistic, two-step kinetic model consisting of bimolecular nucleation of agglomeration, B + B → C (rate constant k3), followed by autocatalytic agglomeration, B + C → 1.5C (rate constant k4), in which B is the average starting nanoparticle, and C is the average larger, agglomerated nanoparticle. The results and findings compellingly demonstrate that the two-step mechanism can account for a variety of sintering kinetics data previously fit only by empirical power laws. Evidence is presented that the kinetic model appears to correspond to what has been called Particle Migration and Coalescence (PMC) in the prior literature. Ten conclusions and hypotheses, as well as four caveats, are listed in the Conclusion section, along with suggestions for further research.

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.

A quantitative kinetics and mechanistic re-analysis is performed of an important 2016 paper that described the formation of Agn nanoparticles from the polyol reduction of silver nitrate in the presence of poly(N-vinylpyrrolidone) under microwave heating. Elegantly and expertly obtained, in operando synchrotron high-energy X-ray diffraction (HEXRD) data, integrated with the microwave heating for the first time, were used to follow the Agn nanoparticle formation reaction in real time and to obtain time-resolved, HEXRD peak areas for the formation of both Ag(111) and Ag(200) facets. Unfortunately, the subsequent kinetics and mechanistic analysis that resulted is far from the state-of-the-art and was done without citing nor using well-established literature of nanoparticle nucleation and growth kinetics and mechanisms that has been available for over 20 years. Herein, the data are re-analyzed and re-interpreted in light of the fitting of the kinetics data with the presently most widely cited and employed, deliberately minimalistic, disproof-based nanoparticle nucleation and growth mechanism, dating back to 1997, of the pseudoelementary steps of slow continuous nucleation, A → B (rate constant k1), and then fast, autocatalytic surface growth, A + B → 2B (rate constant k2), where A is the starting Ag+ and B is the Ag0 product. The two pseudoelementary step mechanism is shown to be able to account for the previously reported kinetics data even for these large, up to ∼100 nm (i.e., 0.1 μm) Agn nanoparticles, a remarkable result in its own right given that there are on the order of ∼107 Ag(0) atoms in a ∼100 nm particle formed from the reduction of ∼107 Ag+ atoms in what must be >107 actual elementary steps. However, the k2 rate constant in particular likely loses much of its value since it is an average over a large change in the percentage of surface atoms in the growing nanoparticle. The results lead to nine revisions of questionable to incorrect previous claims and conclusions, plus a series of eight insights and guidelines for future work in nanoparticle formation kinetics and mechanism. An extensive Supporting Information further discusses interesting questions regarding the issues in analyzing and understanding nanoparticle formation kinetics and the mechanism of such large, ∼0.1 μm-sized particles.

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.

Sigmoidal kinetic curves have been reported for a number of cooperative phenomena in nature. These curves may be fit by purely mathematical functions that, however, do not correspond to any physical model. The 1997 Finke–Watzky (F–W) two-step model of slow, continuous nucleation (A → B, rate constant k1) and fast, autocatalytic growth (A + B → 2B, rate constant k2) provides both a physical model and a mathematical solution. As a result, the F–W two-step kinetic model has been successfully applied to a large number of cooperative phenomena throughout nature that display sigmoidal kinetic curves. Herein, we derive formulas for the first, second, and third derivatives of the concentration of product versus time, [B]t, expressed in terms of the F–W parameters k1, k2, and the initial concentration of monomer, [A]0. Mathematical expressions are then derived for key empirical parameters in sigmoidal curves, including the induction period and (maximum) slope, which are then examined under the limit k1 ≪ k2[A]0 where nucleation and growth are well-separated in time. The resultant seven previously unavailable equations provide a better fundamental and intuitive understanding of sigmoidal curves across nature well-fit by the F–W two-step mechanism.

In 2010 we reported a two-step synthesis of a Ir0∼900/γ-Al2O3 supported-nanoparticle catalyst. In that study, a well-defined Ir(1,5-COD)Cl/γ-Al2O3 precatalyst was isolated and characterized before being reduced in contact with acetone solvent and cyclohexene and under H2 in a second step. Synthetically, one would like to remove the Ir(1,5-COD)Cl/γ-Al2O3 precatalyst isolation step, shortening the precatalyst synthesis and allowing the overall synthesis to be accomplished more efficiently in one pot. However, herein we report that the one-pot synthesis starting from commercially available [Ir(1,5-COD)Cl]2 and γ-Al2O3 yields an order of magnitude increase in the observed nucleation rate constant, k1,obs, as well as a decrease in the average particle size from Ir0∼900 to Ir0∼600. Mechanistic experiments reveal that the origin of this effect, amazingly, is the presence of residual ethyl acetate employed in the isolated precatalyst synthesis, which is not present in the one-pot synthesis. Additional mechanistic probing, along with multiple control experiments, reveals that the presence of even small levels of EtOAc has two, competing effects: a nucleation enhancing effect of increasing the amount of solvated Ir(1,5-COD)Cl(solvent) dissociated off of the γ-Al2O3 support (a step known to be involved in nucleation in solution on the basis of a second paper published in 2011), but then also a more dramatic effect of EtOAc reacting with Ir0n (or possibly IrxHy) nuclei to inhibit nucleation. Armed with these mechanistic insights, we achieved the goal of one-pot syntheses by controlling the presence or absence of the EtOAc. Overall, seemingly innocent solvents such as EtOAc are hereby added to an increasing list of variables crucial to achieving reproducible nanoparticle nucleation- and growth-based syntheses. A conclusions section summarizes those variables along with five additional noteworthy findings and recommendations from the present study.Keywords: kinetics and mechanism; one-pot syntheses; reproducibility; supported-nanoparticle catalyst synthesis; two effects of EtOAc

The vanadium-containing cobalt polyoxometalate (Co-POM) Co4V2W18O6810– (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, Co4P2W18O6810–. 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 51V) NMR peak widths in complexes that also contain paramagnetic metals such as Co(II), the incorrect assignment of a –506.8 ppm 51V 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.