Colloidal Pd Research Articles

Synthesis of colloidal Pd nanoparticles immobilised on poly(N-vinylacetamide): characterisation and application in catalysis

This work demonstrates immobilisation of palladium nanoparticles (Pd NPs) on a vinyl polymer bearing amide side chains to give a Pd NPs-immobilised poly(N-vinylacetamide).

Porous Thin Films with Large Specific Surface Area on the Basis of Spray-Coated Metal Aerogels

The preparation of aerogels in the form of thin films deposited on a substrate (e.g., as an electrode material) is an attractive but challenging task. Such thin films are expected to combine the highly porous gel nanostructure, good mechanical stability, and a remarkably high specific surface area with efficient charge-carrier transport via the interconnected nanoparticle chains. All together, they make them attractive for the fabrication of (electro)catalysts or sensors for microfluidics, flexible electrodes for microelectronics, soft electrodes compatible with a stress-sensitive bioenvironment, etc. In this work, thin aerogel films of colloidal Pd and PtNi building blocks were prepared from the corresponding wet gels using a spray technique. The films were fabricated on substrates of around 1.5 cm2. During the modification of the substrates, the film thickness can be adjusted by controlling the ink content (flow rate, gas pressure, and temperature) and the number of spray cycles. In this way, both the electrical (down to Rs = ∼0.2 kOhm/sq) and optical parameters of the thin porous films can be tuned. Comparative studies of the pore and surface structures of the films have confirmed their integrity and the highly porous structure typical for aerogels. In perspective, spray-coated aerogels can be easily adapted to industrial applications because upscaling is possible and expensive equipment such as supercritical or freeze-dryers is not required. It is important to note that the spray-coating technique is applicable through a mask or can be converted to inkjet printing, which allows for the desired micropatterning of aerogel thin films.

Phase-Controlled Pd-Sn Nanostructures via Co-Digestive Ripening: Catalytic Performance for Base-Free Oxidation of Benzyl Alcohol.

Diluting the precious metal to be used for catalysis with an abundant and non-precious metal to form alloy/intermetallic nanostructures is of great interest owing to the cost-effectiveness of the catalyst. The physicochemical properties of such bimetallic nanostructures are influenced by the atomic ordering of different atoms in the system, generally enhancing the catalytic activity, selectivity, and durability compared to their monometallic counterparts. To understand the relationship between the structure and the activity of the catalyst, phase-controlled synthesis of alloy/intermetallic nanostructures is crucial. Easy and scalable synthesis of such nanostructures with phase control presents a true challenge. We exploited a colloidal-based synthetic route termed 'co-digestive ripening' to prepare Pd-Sn alloy/intermetallic nanostructures. Oleylamine capped Pd and Sn colloids were utilized to obtain network-like Pd3 Sn and grape-like Pd2 Sn nanostructures. Temperature and the stoichiometric ratio between Pd and Sn played significant roles in achieving phase control. The mixture of ligands (oleylamine and trioctylphosphine) in the synthetic procedure resulted in the formation of well-separated nanoparticles (2.9±0.5 nm) in the case of Pd3 Sn and few nm-sized particles along with aggregates in the case of Pd2 Sn. Pd-Sn nanostructures showed enhanced activity and selectivity as compared to their monometallic counterparts for the catalytic performance towards oxidation of benzyl alcohol.

Advanced Ag nanoparticles for the catalyzation of glyoxylic acid oxidation in through-holes electroless copper metallization

Advanced Ag nanoparticles (Ag NPs) were prepared by wet chemical oxidation–reduction method, using mainly the tannic acid as reducing agent and carboxymethylcellulose sodium as stabilizer. The prepared Ag NPs uniformly disperse and are stable for more than one month without agglomeration. The studies of transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) absorption spectroscopy indicate that the Ag NPs are in homogeneous sphere with only 4.4 nm average size and narrow particle size distribution. Electrochemical measurements reveal that the Ag NPs behave excellent catalytic activity for electroless copper plating using glyoxylic acid as reducing agent. In situ fourier transform infrared (in situ FTIR) spectroscopic analysis combined with density functional theory (DFT) calculation illustrate that the molecular oxidation of glyoxylic acid catalyzed by Ag NPs is as the following routes: glyoxylic acid molecule first is adsorbed on Ag atoms with carboxyl oxygen terminal, then hydrolyzed to diol anionic intermediate, and last oxidized to oxalic acid. Time-resolved in situ FTIR spectroscopy further reveals the real-time reactions of electroless copper plating as follows: glyoxylic acid is continuously oxidized to oxalic acid and releases electrons at the active catalyzing spots of Ag NPs, and Cu(II) coordination ions are in situ reduced by the electrons. Based on the excellent catalytic activity, the advanced Ag NPs can replace the expensive Pd colloids catalyst and successfully apply in through-holes metallization of printed circuit board (PCB) by electroless copper plating.

Reactivity Patterns of Olefins in the Matsuda‐Heck‐Reaction and their Impact on the Formation of Pd‐Nanoparticles

AbstractA detailled study of the impact of stereoelectronic effects of olefins in the Matsuda‐Heck‐reaction was addressed by kinetic studies. In addition to the stereoelectronic effects of the olefins used, their influence on the formation of the catalytic Pd colloids was also investigated. Depending on the olefin used, different particle size distributions were obtained on the one hand and, on the other hand, inhibition effects due to adsorption of the olefins on the Pd colloids were observed. Based on these findings, it was possible to optimize reaction conditions for the respective olefins. The presented kinetic data were supplemented by STEM‐measurements.

Micelle-Derived Palladium Nanoparticles for Suzuki–Miyaura Coupling Reactions in Water at Room Temperature

Despite the wide utility of colloidal palladium (Pd) nanoparticle (NP)-catalyzed Suzuki–Miyaura cross-coupling reactions in laboratory and industrial synthesis, the easy construction of colloidal Pd NPs and the detailed role of surfactants remain the focus of attention. Here, we present a simple and sustainable strategy to construct a nanoreactor by connecting micelles with Pd NPs. The strategy enables us to employ a pH-induced morphological transition of sugar-based surfactant aggregates, such as worm-like, spherical, and tubular micelles, to investigate the solubilization of organic substrates. Pd NPs are mainly synthesized by the reduction of phenylboronic acid, further stabilized by the sugar-based surfactant micelles. A kinetic model is established, and the apparent activation energy of this reaction in aqueous micellar solutions is determined to be 46.7 kJ/mol. Notably, the recycled aqueous reaction mixture containing the micelles and Pd NPs can be reused.

Effect of Photocharging on Catalysis of Metallic Nanoparticles

The photocharging effect is widely known across different research fields, has rarely been quantified as a background contribution in photocatalysis, and has often been overlooked in mechanistic interpretation of nanoparticle photocatalysts. To address these issues, this work presents a two-step experiment: charging colloidal Pd nanoparticles with light and hole scavengers and using the charged particles to catalyze the reduction of 4-nitrophenol by NaBH4 under non-irradiation conditions. Experimental kinetics demonstrated a proportional correlation between accumulated electrons and catalytic improvement of Pd nanoparticles. This work reminds us that photocharged nanoparticles may still catalyze chemical reactions as a background phenomenon even when they are not undergoing photoexcitation.

Atmosphere-Induced Transient Structural Transformations of Pd-Cu and Pt-Cu Alloy Nanocrystals.

We have investigated the transformations of colloidal Pd–Cu and Pt–Cu bimetallic alloy nanocrystals (NCs) supported on γ-Al2O3 when exposed to a sequence of oxidizing and then reducing atmospheres, in both cases at high temperature (350 °C). A combination of in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopy was employed to probe the NC surface chemistry and structural/compositional variations in response to the different test conditions. Depending on the type of noble metal in the bimetallic NCs (whether Pd or Pt), different outcomes were observed. The oxidizing treatment on Pd–Cu NCs led to the formation of a PdCuO mixed oxide and PdO along with a minor fraction of CuOx species on the support. The same treatment on Pt–Cu NCs caused a complete dealloying between Pt and Cu, forming separate Pt NCs with a minor fraction of PtO NCs and CuOx species, the latter finely dispersed on the support. The reducing treatment that followed the oxidizing treatment largely restored the Pd–Cu alloy NCs, although with a residual fraction of CuOx species remaining. Similarly, Pt–Cu NCs were partially restored but with a large fraction of CuOx species still located on the support. Our results indicate that the noble metal present in the bimetallic Cu-based alloy NCs has a strong influence on the dealloying/migrations/realloying processes occurring under typical heterogeneous catalytic reactions, elucidating the structural/compositional variations of these NCs depending on the atmospheres to which they are exposed.

Controlling the Production of Acid Catalyzed Products of Furfural Hydrogenation by Pd/TiO2

AbstractWe demonstrate a modified sol‐immobilization procedure using (MeOH)x/(H2O)1‐x solvent mixtures to prepare Pd/TiO2 catalysts that are able to reduce the formation of acid catalyzed products, e. g. ethers, for the hydrogenation of furfural. Transmission electron microscopy found a significant increase in polyvinyl alcohol (PVA) deposition at the metal‐support interface and temperature programmed reduction found a reduced uptake of hydrogen, compared to an established Pd/TiO2 preparation. We propose that the additional PVA hinders hydrogen spillover onto the TiO2 support and limits the formation of Brønsted acid sites, required to produce ethers. Elsewhere, the new preparation route was able to successfully anchor colloidal Pd to the TiO2 surface, without the need for acidification. This work demonstrates the potential for minimizing process steps as well as optimizing catalyst selectivity – both important objectives for sustainable chemistry.

A novel strategy for stabilization of sub-nanometric Pd colloids on kryptofix functionalized MCM-41: nanoengineered material for Stille coupling transformation

The stabilization of sub-nanometric metal particles (< 1 nm) with suitable distribution remained challenging in the catalytic arena. Herein, an intelligent strategy was described to anchoring and stabilizing sub-nanometric Pd colloids with an average size of 0.88 nm onto Kryptofix 23 functionalized MCM-41. Then, the catalytic activity of Pd@Kryf/MCM-41 was developed in Stille coupling reaction with a turnover frequency (TOF) value of 247 h−1. The findings demonstrate that porous MCM-41 structure and high-affinity Kryptofix 23 ligand toward adsorption of Pd colloids has a vital role in stabilizing the sub-nanometric particles and subsequent catalytic activity. Overall, these results suggest that Pd@Kryf/MCM-41 is a greener, more suitable option for large-scale applications and provides new insights into the stabilization of sub-nanometric metal particles.

Tri-tert-butyl(n-alkyl)phosphonium Ionic Liquids: Structure, Properties and Application as Hybrid Catalyst Nanomaterials

A series of sterically hindered tri-tert-butyl(n-alkyl)phosphonium salts (n-CnH2n+1 with n = 1, 3, 5, 7, 9, 11, 13, 15, 17) was synthesized and systematically studied by 1H, 13C, 31P NMR spectroscopy, ESI-MS, single-crystal X-ray diffraction analysis and melting point measurement. Formation and stabilization palladium nanoparticles (PdNPs) were used to characterize the phosphonium ionic liquid (PIL) nanoscale interaction ability. The colloidal Pd in the PIL systems was described with TEM and DLS analyses and applied in the Suzuki cross-coupling reaction. The PILs were proven to be suitable stabilizers of PdNPs possessing high catalytic activity. The tri-tert-butyl(n-alkyl)phosphonium salts showed a complex nonlinear correlation of the structure–property relationship. The synthesized family of PILs has a broad variety of structural features, including hydrophobic and hydrophilic structures that are entirely expressed in the diversity of their properties

The first example of anodic corrosion of Pd in aqueous ethylenediamine with formation of colloidal palladium

For the first time, corrosion of a Pd anode was detected during electrolysis in a weakly alkaline aqueous solution of ethylenediamine. The kinetics and mechanism of the process were investigated by the methods of cyclic voltammetry and gravimetry. Scanning electron microscopy showed corrosion of the Pd anode and deposition of Pd on the cathode surface, while the formation of colloidal Pd in the working solution was identified by transmission electron microscopy.

Highly Permeable Fluorinated Polymer Nanocomposites for Plasmonic Hydrogen Sensing.

Hydrogen (H2) sensors that can be produced en masse with cost-effective manufacturing tools are critical for enabling safety in the emerging hydrogen economy. The use of melt-processed nanocomposites in this context would allow the combination of the advantages of plasmonic hydrogen detection with polymer technology; an approach which is held back by the slow diffusion of H2 through the polymer matrix. Here, we show that the use of an amorphous fluorinated polymer, compounded with colloidal Pd nanoparticles prepared by highly scalable continuous flow synthesis, results in nanocomposites that display a high H2 diffusion coefficient in the order of 10–5 cm2 s–1. As a result, plasmonic optical hydrogen detection with melt-pressed fluorinated polymer nanocomposites is no longer limited by the diffusion of the H2 analyte to the Pd nanoparticle transducer elements, despite a thickness of up to 100 μm, thereby enabling response times as short as 2.5 s at 100 mbar (≡10 vol. %) H2. Evidently, plasmonic sensors with a fast response time can be fabricated with thick, melt-processed nanocomposites, which paves the way for a new generation of robust H2 sensors.

Solvent manipulation of the pre-reduction metal-ligand complex and particle-ligand binding for controlled synthesis of Pd nanoparticles.

Understanding how to control the nucleation and growth rates is crucial for designing nanoparticles with specific sizes and shapes. In this study, we show that the nucleation and growth rates are correlated with the thermodynamics of metal-ligand/solvent binding for the pre-reduction complex and the surface of the nanoparticle, respectively. To obtain these correlations, we measured the nucleation and growth rates by in situ small angle X-ray scattering during the synthesis of colloidal Pd nanoparticles in the presence of trioctylphosphine in solvents of varying coordinating ability. The results show that the nucleation rate decreased, while the growth rate increased in the following order, toluene, piperidine, 3,4-lutidine and pyridine, leading to a large increase in the final nanoparticle size (from 1.4 nm in toluene to 5.0 nm in pyridine). Using density functional theory (DFT), complemented by 31P nuclear magnetic resonance and X-ray absorption spectroscopy, we calculated the reduction Gibbs free energies of the solvent-dependent dominant pre-reduction complex and the solvent-nanoparticle binding energy. The results indicate that lower nucleation rates originate from solvent coordination which stabilizes the pre-reduction complex and increases its reduction free energy. At the same time, DFT calculations suggest that the solvent coordination affects the effective capping of the surface where stronger binding solvents slow the nanoparticle growth by lowering the number of active sites (not already bound by trioctylphosphine). The findings represent a promising advancement towards understanding the microscopic connection between the metal-ligand thermodynamic interactions and the kinetics of nucleation and growth to control the size of colloidal metal nanoparticles.

Anchoring and stabilization of colloidal PdNPs on exfoliated bis-thiourea modified graphene oxide layers with super catalytic activity in water and PEG

Anchoring and stabilization of colloidal noble metal particles (CNMPs) with good distribution have remained an enormous challenge in catalytic applications. However, designing a catalytic system with complementary properties including high surface area, high loading, extraordinary electronic properties, and easy separation offers a promising route for efficient utilization of layers nanostructures for various applications. Herein, an environmental benign approach was developed to fabricate novel catalysts consist of graphene oxide (GO) functionalized with bis-thiourea (BTU) linker. This methodology helps to stabilize and distribute colloidal palladium nanoparticles (Pd NPs) and increases the surface electron density of nanoparticles by electron-donating from BTU ligand. The catalytic activity of Pd NPs supported on the BTU surface was developed in carbon-carbon coupling reactions and nitroarenes reduction under very mild and sustainable reaction conditions. The outstanding catalytic performance can be ascribed to the stabilization of colloidal Pd with particle size 4.6 ± 0.8 nm, as well as the unique three-dimensional network structure of BTU-GO, being favorable for rapid mass transferring. Our finding indicates that the proposed catalyst is a greener, recyclable, and more suitable option for the large-scale application and could pave a way for construction of other transition metal-based catalysts for wide applications.

Engineering of Surface Environment of Pd Nanoparticle Catalysts on Carbon Support with Pyrene-Thiol Ligands for Semihydrogenation of Alkynes.

A new type of pyrene-thiol derivative-modified Pd nanoparticle (NP) catalyst on a carbon black support for the efficient semihydrogenation of alkynes to alkenes is reported herein. Colloidal Pd NPs surrounded by pyrene-thiol modifiers were prepared using the two-phase Brust method followed by impregnation of carbon black materials. Based on the structural characterization of the prepared catalyst (PyC12S-Pd/VC) by NMR, UV-vis, FT-IR, TEM, HAADF-STEM, Pd K-edge XAFS, XRD, N2 adsorption, and XPS, we show that highly dispersed Pd NPs are immobilized on the catalysts via π-π interaction between pyrene groups bound to the Pd NPs and carbon black supports. PyC12S-Pd/VC efficiently catalyzes the alkyne semihydrogenation reaction while maintaining high alkene selectivity; an alkene selectivity of 94% is attained at 98% conversion after 5 h of reaction, and the selectivity was retained around 80% in 10 h of reaction. This performance is superior to that of a catalyst without pyrene groups and that of a commercial Lindlar catalyst. The steric hindrance of pyrene groups restricts access of the substrates to Pd NP surfaces, suppressing the unfavorable overhydrogenation of alkenes to alkanes, which is revealed by the solvent and substrate dependency on the catalytic performance and a DFT calculation study. Furthermore, the high selectivity and stability of PyC12S-Pd/VC are caused by the strong interaction between pyrene groups and carbon supports, which prevents the separation of pyrene modifiers and the leaching or sintering of Pd NPs during the catalytic reaction. It is demonstrated that the combination of Pd NPs, pyrene-thiol modifiers, and carbon supports offers high activity, alkene selectivity, and stability in the semihydrogenation reaction.

Supported Intermetallic PdZn Nanoparticles as Bifunctional Catalysts for the Direct Synthesis of Dimethyl Ether from CO‐Rich Synthesis Gas

AbstractThe single‐step syngas‐to‐dimethyl ether (STD) process entails economic and technical advantages over the current industrial two‐step process. Pd/ZnO‐based catalysts have recently emerged as interesting alternatives to currently used Cu/ZnO/Al2O3 catalysts, but the nature of the active site(s), the reaction mechanism, and the role of Pd and ZnO in the solid catalyst are not well established. Now, Zn‐stabilized Pd colloids with a size of 2 nm served as the key building blocks for the methanol active component in bifunctional Pd/ZnO‐γ‐Al2O3 catalysts. The catalysts were characterized by combining high‐pressure operando X‐ray absorption spectroscopy and DFT calculations. The enhanced stability, longevity, and high dimethyl ether selectivity observed makes Pd/ZnO‐γ‐Al2O3 an effective alternative system for the STD process compared to Cu/ZnO/γ‐Al2O3.

Supported Intermetallic PdZn Nanoparticles as Bifunctional Catalysts for the Direct Synthesis of Dimethyl Ether from CO-Rich Synthesis Gas.

The single‐step syngas‐to‐dimethyl ether (STD) process entails economic and technical advantages over the current industrial two‐step process. Pd/ZnO‐based catalysts have recently emerged as interesting alternatives to currently used Cu/ZnO/Al2O3 catalysts, but the nature of the active site(s), the reaction mechanism, and the role of Pd and ZnO in the solid catalyst are not well established. Now, Zn‐stabilized Pd colloids with a size of 2 nm served as the key building blocks for the methanol active component in bifunctional Pd/ZnO‐γ‐Al2O3 catalysts. The catalysts were characterized by combining high‐pressure operando X‐ray absorption spectroscopy and DFT calculations. The enhanced stability, longevity, and high dimethyl ether selectivity observed makes Pd/ZnO‐γ‐Al2O3 an effective alternative system for the STD process compared to Cu/ZnO/γ‐Al2O3.

Homoleptic cis- and trans-palladium(II) bis(guanidinato) complexes derived from N-aryl-N′,N″-di(pyridin-2-yl)- and N-aryl-N′,N″-bis(6-methylpyridin-2-yl)guanidines: Catalysts for Heck-Mizoroki coupling reactions

N-Aryl-N′,N″-di(pyridin-2-yl)- and N-aryl-N′,N″-bis(6-methylpyridin-2-yl)guanidines (1–4 and 5–7) were isolated in 75%–81% yields. Reactions of Pd(OAc)2 with guanidines 2–7 carried out separately in toluene at 60 °C for 3 h afforded 8–13 respectively in 69%–80% yields. Compounds 1–13 were characterized by elemental analyses, HR-MS, IR and NMR (1H and 13C) spectroscopy. Molecular structures of guanidines 1, 4, 5 and 6 and those of 8–13 were determined by single crystal X-ray diffraction. The Pd(II) atom in 8–10 revealed trans geometry while that in 11–13 revealed cis geometry. DFT calculations were carried out on model compounds 9a (trans) and its hypothetical cis isomer, 9b and 12a (cis) and its hypothetical trans isomer 12b which indicated a very small energy difference between the 9a/9b pair (1.28 kcal/mol) whereas a large energy difference was observed between the 12a/12b pair (26.38 kcal/mol) in CH2Cl2. The catalytic utility of 9 in Heck-Mizoroki coupling reactions involving styrene and methyl acrylate and aryl bromides/aryl chlorides in the presence of NaOAc and excess of tetrabutylammonium bromide (TBAB) at 120 °C was explored. Both activated and de-activated aryl bromides and aryl chlorides were coupled with styrene and in addition, the aryl chlorides were coupled with methyl acrylate in the presence of 9 to afford the respective coupling products in 68% − >99% yields. Neat reaction carried out with 9 and TBAB under the optimized condition released the colloidal Pd black as verified by EDAX, PXRD and SEM techniques thereby implying the heterogeneous nature of catalysis.

In situ transformations of Pd/NHC complexes with N-heterocyclic carbene ligands of different nature into colloidal Pd nanoparticles

A series of Pd/NHC species undergoes R–NHC coupling reaction forming catalytically active NHC-free complexes and/or colloidal Pd nanoparticles.