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 <i>k</i>₁), autocatalytic surface growth (A + B → 2B, rate constant <i>k</i>₂), bimolecular agglomeration (B + B → C, rate constant <i>k</i>₃), and secondary autocatalytic surface growth (A + C → 1.5C, rate constant <i>k</i>₄), 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

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 <i>microfiltration of the removable dust</i> on the two steps and their rate constants are uncovered and quantitated for the first time, including the finding that the <i>presence of dust accelerates <u>both</u> steps by ca. 10-fold or more</i>. 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

Kinetic Evidence for Bimolecular Nucleation in Supported-Transition-Metal-Nanoparticle Catalyst Formation in Contact with Solution: The Prototype Ir(1,5-COD)Cl/γ-Al₂O₃ to Ir(0)_∼900/γ-Al₂O₃ System

Kinetic and mechanistic studies of the formation of supported-nanoparticle catalysts in contact with solution hold promise of driving the next generation syntheses of size, shape, and compositionally controlled catalysts. Recently, we studied the kinetics and mechanism of formation of a prototype Ir(0)_∼900/γ-Al₂O₃ supported-nanoparticle catalyst from Ir­(1,5-COD)­Cl/γ-Al₂O₃ in contact with solution (Mondloch, J.E.; Finke, R.G. <i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 7744). Key kinetic evidence was extracted from γ-Al₂O₃- and acetone-dependent kinetic curves in the form of rate constants for nucleation (A → B, rate constant <i>k</i>_1obs) and autocatalyic surface growth (A + B → 2B, rate constant <i>k</i>_2obs), where A is nominally the Ir­(1,5-COD)­Cl/γ-Al₂O₃ and B the growing, supported Ir(0)_<i>n</i>/γ-Al₂O₃ nanoparticle. The resultant data provided evidence for a mechanism consisting of four main steps: Ir­(1,5-COD)­Cl­(solvent) dissociation from the γ-Al₂O₃ support, then Ir­(1,5-COD)­Cl­(solvent) solution-based nucleation, fast nanoparticle capture by the γ-Al₂O₃ and then subsequent nanoparticle growth between Ir(0)_<i>n</i>/γ-Al₂O₃ and Ir­(1,5-COD)­Cl­(solvent) in solution. While the <i>k</i>_2obs vs [γ-Al₂O₃]_sus and [acetone] autocatalytic surface growth rate constants were nicely accounted for by the proposed mechanism, the <i>k</i>_1obs nucleation rate constants were only “roughly” accounted for by the previously proposed <i>unimolecular</i> solution-based nucleation mechanism. Hence, in the present work we have reexamined that γ-Al₂O₃- and acetone-dependent nucleation data in light of the hypothesis that nucleation is actually <i>bimolecular</i>. Extracting bimolecular, <i>k</i>_1obs(bimol), rate constants by curve-fitting yields qualitative (i.e., visual inspection) as well as quantitative (i.e., increased <i>R</i>² values) <i>evidence consistent with and strongly supportive of solution-based bimolecular nucleation</i> (A + A → 2B, rate constant <i>k</i>_1obs(bimol)) for the Ir­(1,5-COD)­Cl/γ-Al₂O₃ to Ir(0)_∼900/γ-Al₂O₃ system in contact with acetone. The extracted <i>k</i>_1obs(bimol) vs [γ-Al₂O₃]_sus and [acetone] data in turn rule out the solution-based unimolecular mechanism (as well as a hypothetical termolecular nucleation mechanism). This study is significant in that (i) it is the first evidence for bimolecular nucleation in transition-metal nanoparticle formation in any system, be it ligand- or support-stabilized nanoparticle formation in solution or on solid-supports in gas–solid systems, and since (ii) it shows that mechanism-based nanoparticle size control, previously demonstrated to depend on <i>k</i>_1obs, is hereby shown to actually depend on 2<i>k</i>_1obs(bimol)[A]¹. Furthermore, the results presented are of broad significance since (iii) they are part of a growing literature suggesting that simple, bimolecular nucleation may well be closer to the rule, rather than the exception, in a range of systems across nature, and since the results herein (iv) disprove, for at least the present system, the higher nuclearity nucleation kinetics suggested by nucleation theory and its often discussed critical nucleus concept. The results also (v) argue for the new concept of a “kinetically effective nucleus”, in this case binuclear M₂ (M = metal)

Sensitization of Nanocrystalline Metal Oxides with a Phosphonate-Functionalized Perylene Diimide for Photoelectrochemical Water Oxidation with a CoO_<i>x</i> 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 (CoO_<i>x</i>) 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-MO_<i>x</i>) 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 λ_max of the dye, and absorbed photon-to-current efficiency of 13% with H₂Q, a 2-fold improvement over the previous thin-film architecture. A CoO_<i>x</i> 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 CoO_<i>x</i> 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

Electrochemical Water Oxidation Catalysis Beginning with Co(II) Polyoxometalates: The Case of the Precatalyst Co₄V₂W₁₈O₆₈^10–

The question is addressed of whether the cobalt polyoxometalate (Co-POM) precatalyst Co₄V₂W₁₈O₆₈^10– (hereafter <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b>) is a stable, homogeneous water oxidation catalyst under electrochemically driven conditions and in 0.1 M pH 5.8 and 8.0 NaPi buffer as well as pH 9.0 sodium borate (NaB) buffer. This question is of considerable interest since <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> has been reported to be highly stable and a 200-fold faster water oxidation catalyst than its P congener Co₄P₂W₁₈O₆₈^10– (hereafter <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b>), for reasons that were not specified. The nature of the true water oxidation catalyst with <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> as the starting material is of further fundamental interest because a recent report reveals that the ⁵¹V NMR peak at ca. −507 ppm assigned by others to <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> and used to argue for its solution stability is, instead, correctly assigned to the highly stable <i>cis</i>-V₂W₄O₁₉^4–, in turn raising the question of the true stability of <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> under water oxidation catalysis conditions. A battery of physical methods is used to address the questions of the stability and true water oxidation catalyst with <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> as the precatalyst: ³¹P line-broadening detection of Co­(II) present in solution from leaching or as a counterion impurity; a check of those Co­(II) concentration results by the second method of cathodic stripping; the O₂ yield (and, hence, Faradaic efficiency) of electrocatalytic water oxidation; electrochemical, SEM, EDX, and XPS characterization of CoO_<i>x</i> films produced on the electrode; and multiple controls and other experiments designed to test alternative hypotheses that might explain the observed results. The collective evidence provides a compelling case that Co­(II) derived from <b>Co</b>_<b>4</b><b>V</b>_<b>2</b><b>W</b>_<b>18</b> forms a CoO_<i>x</i> film on the electrode which, in turn, carries all the observed, electrochemically driven water-oxidation catalysis current within experimental error. A list of seven main findings is provided as a summary

Nucleation is Second Order: An Apparent Kinetically Effective Nucleus of Two for Ir(0)_<i>n</i> 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 <i>n</i> equivalents of monomer A forming a “critical nucleus”, A<i>_n</i>, 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 <i>k</i>_1obs), then (ii) fast autocatalytic surface growth (A + B → 2B, rate constant <i>k</i>_2obs). Relatively precise homogeneous nucleation rate constants, <i>k</i>_1obs, examined as a function of the amount of precatalyst, A, reveal that <i>k</i>_1obs has an added dependence on the concentration of the precursor, <i>k</i>_1obs = <i>k</i>_{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, <i>k</i>_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)_<i>n</i> 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

Water Oxidation Catalysis Beginning with 2.5 μM [Co₄(H₂O)₂(PW₉O₃₄)₂]^10–: Investigation of the True Electrochemically Driven Catalyst at ≥600 mV Overpotential at a Glassy Carbon Electrode

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 CoO_<i>x</i> 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. <i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 14872) and <i>highlights the importance of</i> <i>reaction</i> <i>conditions in governing the identity of the true, active WOC.</i> 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 CoO_<i>x</i> catalyst formed via decomposition of Co₄–POM. Controls with authentic CoO_<i>x</i> indicate conversion of only 3.4% or 8.3% (at pH 8.0 and 5.8) of Co₄–POM into a CoO_<i>x</i> 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 <i>Co</i>_<i>4</i><i>-POM instability under oxidizing conditions</i>. Overall, this study demonstrates the challenges of identifying the true WOC when examining micromolar amounts of a partially stable material and when <i>nanomolar</i> heterogeneous metal-oxide will account for the observed O₂-generating activity

Catalyst Sintering Kinetics Data: Is There a Minimal Chemical Mechanism Underlying Kinetics Previously Fit by Empirical Power-Law Expressionsand if So, What Are Its Implications?

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 <i>empirical</i> 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 <i>k</i>₃), followed by autocatalytic agglomeration, B + C → 1.5C (rate constant <i>k</i>₄), 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

Water Oxidation Catalysis Beginning with Co₄(H₂O)₂(PW₉O₃₄)₂^10– When Driven by the Chemical Oxidant Ruthenium(III)tris(2,2′-bipyridine): Stoichiometry, Kinetic, and Mechanistic Studies en Route to Identifying the True Catalyst

Stoichiometry and kinetics are reported for catalytic water oxidation to O₂ beginning with the cobalt polyoxometalate Co₄(H₂O)₂(PW₉O₃₄)₂^10– (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 <i>Science</i> paper (Yin et al. <i>Science</i> <b>2010</b>, <i>328</i>, 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⁺]/d<i>t</i> = (<i>k</i>₁ + <i>k</i>₂)­[Co₄POM]_soluble[Ru­(III)­(bpy)₃³⁺]/[H⁺], where the rate constants <i>k</i>₁ ∼ 0.0014 s^–1 and <i>k</i>₂ ∼ 0.0044 s^–1 correspond to the water oxidation and ligand oxidation reactions, and for O₂ evolution, d­[O₂]/d<i>t</i> = (<i>k</i>₁/4)­[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)₂^10– with a <i>K</i>_sp = (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) CoO_<i>x</i> 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