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

Abstract

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.

Aggregation of catalytically active Ru nanoparticles to inactive bulk, monitored in situ during an allylic isomerization reaction. Influence of solvent, surfactant and stirring

“…In the literature, it states that the C-NPs are often the true catalysts that are suitable for use in various systems. 16,21,22 In the fifth step, the agglomeration of C and B particles results in the formation of a C-particle, in agreement with the general model proposed by Finke. 23,24 The best fit theoretical profile shows good agreement between mechanism 1 and the data obtained experimentally, where C-NPs are the true isomerization reaction catalyst.…”

“…The proposed mechanism of this reaction differs from the conventional type reported in the literature. 6,[12][13][14][15][16][17][18] We demonstrated that in the case of a composite complex structure, the order of the reduction of metallic centers inuences the NPs self-assembly mechanism, and in its turn, affects the kinetic prole of isomerization reaction (1). Schematically speaking, the proposed kinetic model is as follows see Scheme 2 (mechanism 1).…”

Influence of Reduction Kinetics on the Preparation of Well-Defined Cubic Palladium Nanocrystals

“…Some decades ago, the generation process of crystalline materials has been studied and many relevant theories have been achieved. − On the basis of the classical nucleation theory (CNT), the generation process of metallic nanocrystals can be divided into three stages, namely, prenucleation, nucleation, and growth, where detailed discussion of different theories has been taking place in fields of chemistry, physics and mathematics. − To handle modern metallic nanoparticle formation phenomena, the Finke and Watzky (FW) 2-step nucleation mechanism, which has successfully dealt with many situations of metallic nanoparticle growth, is discovered. − In the FW theory, metallic nanoparticles mainly go through slow, continuous nucleation and autocatalytic surface growth, and the reaction kinetics of nucleation and growth can be facilely separated.…”

Controlled synthesis of silver nanopowders

“…Однак таке зростання полідисперсності частинок після диспергування нанопорошків срібла у воді можна вважати незначним, а самі порошки -придатними для використання у подальших дослідженнях, зокрема для перевірки їх каталітичної активності [24], як наповнювачів в електропровідних полімерних композиціях тощо. Імовірно, що великі розмір і полідисперсність частинок срібла, отримані за одноразового додавання 2,4 мл 0,4 М розчину AgNO 3 до золю зародків, спричинені неконтрольованими процесами гомогенної нуклеації, автокаталітичного росту, агрегації наночастинок срібла і вторинного росту утворюваних агрегатів [27] через надзвичайно високі концентрації реагентів -0,02 моль/л AgNO 3 і 0,08 моль/л N 2 H 4 . Однак для підтвердження такого припущення необхідним є проведення додаткових досліджень.…”