Size-controlled Pd Nanoparticles Research Articles

Discovery of a Neutral 40-PdII-Oxo Molecular Disk, [Pd40O24(OH)16{(CH3)2AsO2}16]: Synthesis, Structural Characterization, and Catalytic Studies.

We report on the synthesis and structural characterization of a giant, discrete, and neutral molecular disk, [Pd40O24(OH)16{(CH3)2AsO2}16] (Pd40), comprising a 40-palladium-oxo core that is capped by 16 dimethylarsinate moieties, resulting in a palladium-oxo cluster (POC) with a diameter of ∼2 nm. Pd40, which is the largest known neutral Pd-based oxo cluster, can be isolated either as a discrete species or constituting a 3D H-bonded organic-inorganic framework (HOIF) with a 12-tungstate Keggin ion, [SiW12O40]4- or [GeW12O40]4-. 1H and 13C NMR as well as 1H-DOSY NMR studies indicate that Pd40 is stable in aqueous solution, which is also confirmed by ESI-MS studies. Pd40 was also immobilized on a mesoporous support (SBA15) followed by the generation of size-controlled Pd nanoparticles (diameter ∼2-6 nm, as based on HR-TEM), leading to an effective heterogeneous hydrogenation catalyst for the transformation of various arenes to saturated carbocycles.

Mesocellular silica foams supported size-controlled Pd nanoparticles for racemic and asymmetric iodomethyl dihydrobenzofuran synthesis

Supported metal nanoparticles (NPs) catalysts in constructing benzene-fused cyclic framework with chirality is very challenging. Dihydrobenzofuran and its derivatives are significant heterocyclic units in biologically molecules. Herein, size-controlled Pd NPs fixed on mesoporous silica (Pd/MS-xT) SBA-15, FDU-12 and mesocellular silica foams (MCFs) were utilized to synthesize racemic and asymmetric iodomethyl dihydrobenzofuran for the first time. The size of Pd NPs was regulated by increasing NaBH4 reduction treatments times (-xT) and enlarging supports pore. Based on large mesopore of MCF-3, supported catalyst 3%Pd/MCF-3-4T possessed larger Pd size gave the excellent performance at 92% racemic yield. Under chiral phosphine modifier N–Me-Xu3, we implemented more deserving asymmetric reaction over 3%Pd/MCF-3-1T with smaller Pd NPs. TGA for organic surface coverage demonstrated that the substrate preferred larger Pd NPs, N–Me-Xu3 capped more easily on smaller ones. Besides, effects of reaction medium and N–Me-Xu3 additive quantity further revealed the coordination of chiral modifier on Pd/MCF-3 was the determinant. For broader visions, the performance dependence based on surface properties of heterogeneous catalyst would play crucial contributions towards construction of complexed organic molecules.

Regulating the Catalytic Activity of Pd Nanoparticles by Confinement in Ordered Mesoporous Supports

AbstractStudying structure‐sensitive reactions requires the synthesis of catalytic nanoparticles with precisely controlled sizes. In this work we demonstrate the facile production of size controlled Pd nanoparticles by confinement in the pores of mesoporous silica nanoparticles (MSN). We show that Pd particles 2.1 nm in size have a higher catalytic activity than larger nanoparticles for the Suzuki‐Miyaura cross‐coupling between 4’‐bromoacetophenone and phenyl boronic acid and for the hydrogenation of phenol in aqueous phase. The enhanced activity can be explained in terms of increased number of coordinatively unsaturated sites and higher back donation capacity of the small nanoparticles.

Surface Structure Characterization of Shape and Size Controlled Pd Nanoparticles by Cu UPD: A Quantitative Approach.

The search for new surface sensitive probes that characterize the surface structure of shape and size-controlled nanoparticles is an interesting topic to properly understand the correlations between electrocatalytic properties and surface structure at the nanoscale. Herein, we report the use of Cu UPD to characterize, not only qualitatively but also quantitatively, the surface structure of different Pd nanoparticles with controlled particle shape and size. Thus, Pd nanoparticles with cubic, octahedral and rhombic dodecahedral shapes, that is, with preferential {100}, {111}, and {110} surface structures, respectively, were prepared. In addition, cubic Pd nanoparticles with different particles sizes and spherical (2–3 nm) Pd nanoparticles were also synthesized. Based on the Cu UPD results on Pd single crystals, a new approach is proposed to qualitatively and quantitatively determine the percentages of {100}, {111}, and {110} surface domains present at the surface of the different shape and size controlled Pd nanoparticles. The results reported clearly show the benefits of this Cu UPD to get detailed information of the surface structure of the nanoparticles according to their particle shape and size.

Sulfur-Containing Polymer As a Platform for Synthesis of Size-Controlled Pd Nanoparticles for Selective Semihydrogenation of Alkynes

Sulfur-containing polymer supported palladium nanoparticles (Pd/SPMB) were prepared by using the cross-linked poly(N,N′-methylene bis(acrylamide)) (SPMB) as the platform. The SPMB was synthesized via the reversible addition–fragmentation chain transfer (RAFT) polymerization, from which the sulfur inherently exists in the RAFT polymer due to the use of thiocarbonate as the chain transfer agent, and could be further utilized as a modifier of Pd nanoparticles. The Pd/SPMB showed size-dependent activity and selectivity in the semihydrogenation of alkynes to alkenes. The catalyst with an average Pd size of 5.3 nm was active in the semihydrogenation of both internal and terminal alkynes. Most of the surface of Pd NPs was covered (or poisoned) by sulfide and thiolate due to the moderate complexation strength between Pd NPs and SPMB, thus leading to the selectivity enhancement toward alkene. Furthermore, the catalyst was highly stable and can be reused 10 times without an appreciable loss in either activity or se...

Synthesis of Supported Pd0 Nanoparticles from a Single-Site Pd2+ Surface Complex by Alkene Reduction

A surface metal–organic complex, (-AlOx)Pd(acac) (acac = acetylacetonate), is prepared by chemically grafting the precursor Pd(acac)2 onto γ-Al2O3 in toluene at 25 °C. The resulting surface complex is characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and dynamic nuclear polarization surface-enhanced solid-state nuclear magnetic resonance spectroscopy (DNP SENS). This surface complex is a precursor in the direct synthesis of size-controlled Pd nanoparticles under mild reductive conditions and in the absence of additional stabilizers or pretreatments. Indeed, upon exposure to gaseous ethylene or liquid 1-octene at 25 °C, the Pd2+ species is reduced to form Pd0 nanoparticles with a mean diameter of 4.3 ± 0.6 nm, as determined by scanning transmission electron microscopy (STEM). These nanoparticles are catalytically relevant using the aerobic 1-phenylethanol oxidation as a probe reaction, with rates ...

Human Hair as Adsorbent of Palladium(II) in Solution: A Precursor of Well-Dispersed Size-Controlled Pd Nanoparticles

This work describes, for the first time, the preparation of palladium nanoparticles supported in thermally-treated human hair. Human hair showed to be an efficient adsorbent of Pd2+ in aqueous media, reaching nearly 100% of adsorption from a 100 ppm solution. The thermal treatment of hair containing Pd2+ at 200 oC under nitrogen atmosphere led to the formation of an N, S-containing material presenting 0.5 wt.% of palladium. The material was extensively characterized by elemental analysis (CHN and inductively coupled plasma optical emission spectroscopy (ICP OES)), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) and Raman spectroscopies, and by thermogravimetry (TG), thermogravimetry coupled to mass spectrometry (TG-MS), scanning electron miscroscopy (SEM), transmission electron microscopy (TEM) and dispersive X-ray spectroscopy (EDS) analyses. It was possible to observe that the surface structure of hair was preserved during thermal treatment, presenting palladium nanoparticles with particle sizes of approximately 4 nm. This material was used as heterogeneous catalyst in a preliminary application in nitrobenzene reduction to aniline in aqueous medium using sodium borohydride.

Tandem Site- and Size-Controlled Pd Nanoparticles for the Directed Hydrogenation of Furfural

The conversion of biomass to useful chemical products requires precise catalytic properties to achieve the required activity, selectivity, and durability. Here we show, through optimized colloidal synthesis, the tandem control of Pd size and site availability for the directed hydrogenation of the bioderived intermediate furfural. Adjusting the temperature of colloidal reduction dictates the size of Pd nanoparticles; in some instances ultrasmall clusters of <20 atoms are achieved. However, changing the solvent system affects the PVA–Pd interaction and relative proportion of available surface sites (corners, edges, planes), allowing us to control the selectivity to the valuable hydrogenation products furfuryl alcohol and tetrahydrofurfuryl alcohol. We demonstrate, through combined experimental and computational studies, that Pd nanoparticle planes are more prone to deactivation through the formation of Pd carbide, resulting in the reduced efficacy of furfural binding. This approach to nanoparticle optimizat...

Catalysts based on large size-controlled Pd nanoparticles for aqueous-phase hydrodechlorination

This work is aimed at evaluating the catalytic performance of Pd/C catalysts based on size-controlled nanoparticles (mean size 10–50nm). Carbon black was used as support and the catalysts were tested in the aqueous phase hydrodechlorination of 4-chlorophenol. The Pd precursor was reduced with a methanol/water (10%vol.) mixture in the presence of PVP (PVP/Pd molar ratio 5–35) as capping agent. The performances of unsupported Pd–PVP colloidal nanoparticle and the catalysts prepared by impregnation of them on the carbon black (ex-situ synthesis), reduction of the precursor in the presence of that support (in-situ synthesis) and wetness impregnation were compared. The ex-situ preparation with less content of PVP (PVP/Pd molar ratio of 5) boosted the activity of the Pd–PVP nanoparticles from 16.8 to 167mmolgPd−1min−1 showing the role of the support in enhancing the catalytic performance. The in-situ synthesis with less content of PVP (PVP/Pd molar ratio of 5) resulted in catalysts with lower activity than that prepared by ex-situ synthesis, although they exhibited higher ability to hydrogenate phenol to cyclohexanone and cyclohexanol, which can be related to the morphology of the nanoparticles synthesized. Regardless of the type of synthesis, as PVP dose in synthesis increased, activity and selectivity to hydrogenation products decreased. High activity values were obtained (20–167mmolgPd−1min−1) taking into account the high mean size of the Pd nanoparticles in the catalysts.

Size-Controlled Pd Nanoparticles in 2-Butyne-1,4-diol Hydrogenation: Support Effect and Kinetics Study

Structured catalyst has been developed for C–C triple bond three-phase hydrogenation. The sintered metal fiber (SMF) coated by different oxides served as support for monodispersed Pd nanoparticles (6.4 ± 0.5 nm). The effect of acid–base properties and reducibility of metal oxide coating on catalytic performance in the liquid phase (T = 303–348 K; P = 1–20 bar) hydrogenation of 2-butyne-1,4-diol to 2-butene-1,4-diol (B2) has been studied. The oxides MgO, ZnO, Ga2O3, Al2O3, ZrO2, SnO2, and SiO2 and the mixtures of MgO + ZnO + Al2O3, MgO + Al2O3, and ZnO + Al2O3 were tested. The catalyst activity was higher up to 10-fold for Pd0 on acidic supports, like SiO2, but demonstrated lower selectivity to B2 as compared to the basic oxides. The highest yield (∼99%) of the target B2 and stability over four consecutive runs were attained over the 0.2% Pd0/ZnO/SMF catalyst. The high selectivity to B2 was attributed to the formation of an active phase containing intermetallic PdZn alloy as confirmed by XPS. The reaction ...

Size-controlled Pd Nanoparticles Supported on α-Al2O3 as Heterogeneous Catalyst for Selective Hydrogenation of Acetylene

Size-controlled Pd nanoparticles (PdNPs) were synthesized in aqueous solution, using sodium carboxymethyl cellulose as the stabilizer. Size-controlled PdNPs were supported on α-Al2O3 by the incipient wetness impregnation method. The PdNPs on α-Al2O3 support were in a narrow particle size distribution in the range of 1-6 nm. A series of PdNPs/α-Al2O3 catalysts were used for the selective hydrogenation of acetylene in ethylene-rich stream. The results show that PdNPs/α-Al2O3 catalyst with 0.03% (by mass) Pd loading is a very effective and stable catalyst. With promoter Ag added, ethylene selectivity is increased from 41.0% to 63.8% at 100 °C. Comparing with conventional Pd-Ag/α-Al2O3 catalyst, PdNPs-Ag/α-Al2O3 catalyst has better catalytic performance in acetylene hydrogenation and shows good prospects for industrial application.

Structure Sensitivity Study of Waterborne Contaminant Hydrogenation Using Shape- and Size-Controlled Pd Nanoparticles

Catalytic reduction with Pd has emerged as a promising technology to remove a suite of contaminants from drinking water, such as oxyanions, disinfection byproducts, and halogenated pollutants, but low activity is a major challenge for application. To address this challenge, we synthesized a set of shape- and size-controlled Pd nanoparticles and evaluated the activity of three probe contaminants (i.e., nitrite, N-nitrosodimethylamine (NDMA), and diatrizoate) as a function of facet type (e.g., (100), (110), (111)), ratios of low- to high-coordination sites, and ratios of surface sites to total Pd (i.e., dispersion). Reduction results for an initial contaminant concentration of 100 μM show that initial turnover frequency (TOF0) for nitrite increases 4.7-fold with increasing percent of (100) surface Pd sites (from 0% to 95.3%), whereas the TOF0 for NDMA and for diatrizoate increases 4.5- and 3.6-fold, respectively, with an increasing percent of terrace surface Pd sites (from 79.8% to 95.3%). Results for an initial nitrite concentration of 2 mM show that TOF0 is the same for all shape- and size-controlled Pd nanoparticles. Trends for TOF0 were supported by results showing that all catalysts but one were stable in shape and size up to 12 days; for the exception, iodide liberation in diatrizoate reduction appeared to be responsible for a shape change of 4 nm octahedral Pd nanoparticles. Density functional theory (DFT) simulations for the free energy change of hydrogen (H2), nitrite, and nitric oxide (NO) adsorption and a two-site model based on the Langmuir–Hinshelwood mechanism suggest that competition of adsorbates for different Pd sites can explain the TOF0 results. Our study shows for the first time that catalytic reduction activity for waterborne contaminant removal varies with the Pd shape and size, and it suggests that Pd catalysts can be tailored for optimal performance to treat a variety of contaminants for drinking water.

Microwave-Assisted Facile Synthesis of Palladium Nanoparticles in HEPES Solution and Their Size-Dependent Catalytic Activities to Suzuki Reaction

Palladium nanoparticles (NPs) were successfully synthesized via a rapid and facile microwave route in HEPES (2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid) buffer solution. The shape- and size-controlled Pd nanoparticles could be obtained by one-step method without dependence of seed-mediated growth. The capping agent plays a key role in the formation of Pd NPs with different shape and size, which could be tuned by varying capping agents such as polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), sodium citrate (Na3(cit)) and potassium bromide (KBr). The size-dependent catalytic activities of the obtained Pd NPs for Suzuki coupling reaction were also investigated. It demonstrated that the catalytic activity of Pd NPs was enhanced regularly with the decrease of particle size. Pd NPs less than 10 nm exhibited better catalytic activities for Suzuki reaction than the commercial Pd/C catalyst. Pd/MWCNTs and Pd/SBA-15 nanocomposites were also prepared by a facile method and afforded good catalytic activity and reusability. This "green" synthetic protocol could be used as a general method for the rapid synthesis of transition metal nanoparticles.

Size controlled synthesis of Pd nanoparticles inspired from the Wacker reaction and their catalytic performances

A novel and facile strategy for the synthesis of size-controlled Pd nanoparticles employing C2H4 as the reducing agent was inspired from the Wacker reaction. Uniform Pd nanoparticles with the size ranging from 3 nm to 50 nm were successfully synthesized by using different types of capping agents and optimizing the synthesis parameters. Pd nanoparticles with different sizes exhibit a size-dependent catalytic performance in the aerobic oxidation of benzyl alcohol that could be reasonably attributed to the surface blocking effect exerted by the capping agent chemisorbed on their surfaces and the likely electronic effect.

Preparation of Nanoparticles in Nanoporous Silica, FSM-16

Supercritical fluids (SCFs) are used for the preparation of size-controlled nanoparticles in micro- and meso-porous silicas with different pore sizes, CnFSM-16 (n = 8, 10, 12 and 16; pore diameter = 1.6–3.5 nm), where the value n denotes the carbon number in the alkyl chain of the surfactant template. Precursors, M(acac)n (M = Pd, Rh, Ru and Fe; acac = acetylacetonate), dissolved in supercritical CO2 with acetone were loaded on the FSM-16, followed by reduction in a flow of H2 at 673 K or oxidation in a flow of O2 at 673 K. Nanoparticles of Pd, Rh, Ru and Fe2O3 can be impregnated on nanoporous silica with a uniform pore size, i.e. FSM-16. The particles are highly dispersed and have a small size distribution. Metal/metal oxide nanoparticles were impregnated inside the pores. For the impregnation of Pd nanoparticles, size-controlled Pd nanoparticles can be prepared in the FSM-16 nanoporous silica with a uniform pore size. The particle size linearly increased with the increase in the pore size of the FSM-16.

Electrochemical Synthesis, Voltammetric Behavior, and Electrocatalytic Activity of Pd Nanoparticles

Size-controlled Pd nanoparticles were electrochemically synthesized in large quantities at room temperature through the direct electroreduction of bulk PdCl42- ions in the presence of poly(N-vinylpyrrolidone) (PVP), a water-soluble polymer. The technological key to electrochemical synthesis of Pd particles is that PVP greatly promoted the formation process of Pd particles and inhibited the electrodeposition process of Pd onto the Pt cathode at the same time besides effectively stabilizing the formed Pd nanoparticles. The as-synthesized Pd nanoparticles display the voltammetric behavior significantly different from the bulk Pd and also present a high catalytic activity toward the electrooxidation of methanol.