Larger Pd Nanoparticles Research Articles

The Origin of the Size Effect in the Oxidation of CO on Supported Palladium Nanoparticles

Two Pd/TiO2 catalysts with mean particle sizes of 1 and 3 nm were prepared and tested in the low-temperature oxidation of CO. It was found that the first catalyst with higher dispersion is more active. Turnover frequencies varied for these catalysts by almost six times. In contrast, the apparent activation energy of the oxidation of CO on the catalyst with smaller Pd nanoparticles was estimated at 76 kJ/mol, and for the catalyst with larger Pd nanoparticles at 58 kJ/mol. According to in situ XANES studies, the particle size effect originates from the oxidation of small palladium nanoparticles under reaction conditions, whereas larger nanoparticles are stable and consist of palladium atoms mainly in the metallic state. Palladium oxide is more active in the low-temperature oxidation of CO than metallic palladium. This means that the origin of size-dependent activity of Pd nanoparticles in the low-temperature oxidation of CO is associated with the change in the chemical composition of nanoparticles that leads to a change in the reaction mechanism and, as a result, in their activity.

Structure sensitivity reaction of chloroform hydrodechlorination to light olefins using Pd catalysts supported on carbon nanotubes and carbon nanofibers

The upgrading of wasted chloroform in hydrodechlorination for the production of olefins such as ethylene and propylene is studied by employing four catalysts (PdCl/CNT, PdCl/CNF, PdN/CNT, and PdN/CNF) prepared by different precursors (PdCl2 and Pd(NO3)2) supported on carbon nanotubes (CNT) or carbon nanofibers (CNF). TEM and EXAFS-XANES results confirm that Pd nanoparticle size increases in the order: PdCl/CNT < PdCl/CNF ∼ PdN/CNT < PdN/CNF, descending the electron density of Pd nanoparticles in the same order. It illustrates that PdCl-based catalysts show donation of electrons from support to Pd nanoparticles, which is not observed in PdN-based catalysts. Moreover, this effect is more evident in CNT.The smallest and well-dispersed Pd nanoparticles (NPs) on PdCl/CNT with high electron density favor an excellent and stable activity and a remarkable selectivity to olefins. In contrast, the other three catalysts show lower selectivity to olefins and lower activities which suffer strong deactivation due to the formation of Pd carbides on their larger Pd nanoparticles with lower electron density, compared to PdCl/CNT.

Highly dispersed Pd clusters/nanoparticles encapsulated in MOFs via in situ auto-reduction method for aqueous phenol hydrogenation

In this work, a novel in situ auto-reduction strategy was developed to encapsulate uniformly dispersed Pd clusters/nanoparticles in MIL-125-NH2. It is demonstrated that the amino groups in MIL-125-NH2 can react with formaldehyde to form novel reducing groups (-NHCH2OH), which can in situ auto-reduce the encapsulated Pd2+ ions to metallic Pd clusters/nanoparticles. As no additional reductants are required, the strategy limits the aggregation and migration of Pd clusters and the formation of large Pd nanoparticles via controlling the amount of Pd2+ precursor. When applied as catalysts in the hydrogenation of phenol in the aqueous phase, the obtained Pd(1.5)/MIL-125-NH-CH2OH catalyst with highly dispersed Pd clusters/nanoparticles with the size of around 2 nm exhibited 100% of phenol conversion and 100% of cyclohexanone selectivity at 70 °C after 5 h, as well as remarkable reusability for at least five cycles due to the large MOF surface area, the highly dispersed Pd clusters/nanoparticles and their excellent stability within the MIL-125-NH-CH2OH framework.

Efficient visible light initiated one-pot syntheses of secondary amines from nitro aromatics and benzyl alcohols over Pd@NH2-UiO-66(Zr)

Pd@NH2-UiO-66(Zr), with small-sized Pd nanoparticles encapsulated inside the cavities of NH2-UiO-66(Zr), was obtained via a double-solvent impregnation followed by a photoreduction process, which shows superior activity for the visible light initiated syntheses of secondary amines from nitro compounds and alcohols, via a sequential photocatalytic hydrogenation of nitro compounds/dehydrogenation of alcohols, condensation of amines and aldehydes to imines, and the hydrogenation of imines. Simultaneous consumption of photogenerated electrons/holes in the photocatalytic hydrogenation of nitro compounds to amines and dehydrogenation of alcohols to aldehydes promotes the whole reaction. Due to the confinement effect of the cavity and the small-sized Pd nanoparticles, Pd@NH2-UiO-66(Zr) shows significantly superior performance as compared with Pd/NH2-UiO-66(Zr), in which larger Pd nanoparticles are deposited on the surface. This study provides an efficient and green strategy for the production of secondary amines and highlights the great potential of using M/MOFs nanocomposites as multifunctional catalysts for light induced one-pot tandem reactions.

Iodide-Coordinated Single-Site Pd Catalysts for Alkyne Dialkoxycarbonylation

Heterogeneous single-metal-site catalysts (HSMSCs) have attracted considerable interest, but most studies have focused on the metal atoms in the active site while ignoring the key role of ligands. The unique coordination environment of a single-site catalyst is crucial for realizing its potential. Constructing this kind of catalyst via a feasible and practical fabrication method is challenging. Herein, a single-site Pd catalyst with iodide ligands supported on activated carbon (Pd1/AC) was successfully fabricated by atomic dispersion of large Pd nanoparticles (NPs). Intermediate I• radicals were detected during the atomic dispersion process of Pd NPs by in situ imaging photoelectron photoion coincidence spectroscopy (in situ iPEPICO) with vacuum ultraviolet synchrotron radiation. The molecular structure of single-site Pd was established as [Pd(CO)I4(O═AC)]2– through combined characterization. Alkyne dialkoxycarbonylation with high selectivity toward 1,4-dicarboxylic acid esters (>94%) and high acetylene conversion (>99%) was achieved. A sulfonic promoter on the Pd1/AC catalyst for alkyne dialkoxycarbonylation was avoided because of the iodide ligand. Good durability and a broad substrate scope were successfully achieved.

Size-Controlled Synthesis of Pd Nanocatalysts on Defect-Engineered CeO2 for CO2 Hydrogenation.

The size effects of metal catalysts have been widely investigated to optimize their catalytic activity and selectivity. However, the size-controllable synthesis of uniform supported metal nanoparticles without surfactants and/or additives remains a great challenge. Herein, we developed a green, surfactant-free, and universal strategy to tailor the sizes of uniform Pd nanoparticles on metal oxides by an electroless chemical deposition method via defect engineering of supports. The nucleation and growth mechanism suggest a strong electrostatic interaction between the Pd precursor and low-defective CeO2 and a weak reducing capacity for low-defective CeO2, resulting in small Pd nanoparticles. Conversely, large Pd nanoparticles were formed on a highly defective CeO2 surface. Combined with various ex situ and in situ characterizations, a higher intrinsic activity of Pd for the CO2-to-CO hydrogenation was found on large Pd nanoparticles with higher electron density owing to their stronger H2 dissociation ability and H-spillover effects, as well as the larger number of oxygen vacancies generated in situ for CO2 activation under hydrogenation conditions.

Improving the activity in hydrodechlorination of Pd/C catalysts by nitrogen doping of activated carbon supports

Aqueous phase 4-chlorophenol hydrodechlorination reaction was used to study the effect of N-doping of activated carbon support on the catalytic activity of Pd catalysts. Activated carbon was doped using pyridine and 1,10-phenantroline, reaching nitrogen contents of 0.42–1.22 and 1.35–4.19 % (w), respectively. All catalysts (0.75 % Pd w, carbon basis) showed relatively large Pd nanoparticles (35−55 nm), but they exhibited fast and complete 4-chlorophenol disappearance in batch experiments. In runs at 30 °C 4-chlorophenol disappearance was mainly ascribed to hydrodechlorination, although N-doping of the support also increased adsorption. Catalysts with supports doped with pyridine yielded higher 4-chlorophenol disappearance rate in spite of lower bulk nitrogen content, however they showed higher concentration of nitrogen species at the external surface and lower loss of surface area during the doping. 4-chlorophenol disappearance rate was boosted at 60 °C, with minor differences between catalysts with undoped and N-doped supports, but generation of cyclohexanone was only observed for the ones with doped support. Phenol generation simultaneous to 4-chlorophenol disappearance was observed with all the catalysts. However, subsequent hydrogenation to cyclohexanone ocurred only with the catalysts supported on N-doped activated carbon.

Mesoporous Sn(IV) doping MCM-41 supported Pd nanoparticles for enhanced selective catalytic oxidation of 1,2-propanediol to pyruvic acid

The conversion of biomass derivative 1,2-propanediol (PDO) to value-added chemicals by selective catalytic oxidation is of great research interest. Herein, we provide an alternative and effective strategy for selectively catalyzing the PDO oxidation to pyruvic acid using Sn(IV) doping MCM-41 (SnM) supported Pd nanoparticles (2%Pd/SnM) as catalyst, in which the SnM with the Si/Sn ratios ranging from 6 to 50 are prepared by direct hydrothermal synthesis (DHS) method and the Pd nanoparticles are in situ reduced on the SnM surfaces. When the Si/Sn ratios are above 12, decreasing the Si/Sn ratio favors the incorporation of more Sn(IV) species into the framework, the formation of smaller Pd nanoparticles with higher distribution and the increment of basicity, contributing to the significant enhancement in the selective oxidation of PDO to pyruvic acid, but if the ratio is further reduced to 6, larger Pd nanoparticles are formed in the 2%Pd/SnM6 with less ordered mesostructure due to excess Sn(IV) resulting in the formation of SnO2 crystallites, displaying much lower catalytic activity than the 2%Pd/SnM12. The Sn(IV) species incorporated into MCM-41 can be used as effective promoter for enhancing the selective oxidation of PDO to pyruvic acid over the Pd/SnM catalyst.

Size-Dependent Adsorption of Styrene on Pd Clusters: A Density Functional Theory Study

When the size of a reactant molecule is comparable to that of a nanometer catalyst (such as an aromatic compound and a nanometer or subnanometer metal catalyst), the adsorption configuration of the molecule on the catalyst becomes different from that on their larger counterparts. Consequently, distinct catalytic performances can be observed for catalysts of different sizes. Therefore, it is crucial to understand the adsorption behaviors of molecules on various metal nanocatalysts. Here, the size-dependent adsorption of a representative system (i.e., styrene on Pd clusters) was systematically explored using the density functional theory calculations. For large Pd nanoparticles (between 31 and 73 atoms), the adsorption becomes weaker as the particle size increases. For particles with fewer than 31 atoms, the adsorption weakens rapidly with a decrease in size, which was also observed for Cu-supported Pd clusters. The peaks of adsorption strength of middle-sized particles are caused by the combination of the ...

Insights into the Role of Poly(vinylpyrrolidone) in the Synthesis of Palladium Nanoparticles and Their Electrocatalytic Properties.

Four types of palladium (Pd) nanoparticles were prepared from the systems containing PdCl2 or Na2PdCl4 with or without the assistance of poly(vinylpyrrolidone) (PVP). Two types of Pd nanoparticles obtained in the absence of PVP were obviously larger than those synthesized with the assistance of PVP. The former large Pd particles showed typical features in cyclic voltammetry in H2SO4 solution, whereas two types of small Pd nanoparticles did not. However, small nanoparticles treated first in an electrochemical way in 0.5 M KOH solution displayed the adsorption and desorption peaks similar to those of typical Pd-modified electrodes in H2SO4 solution. Large Pd nanoparticles from the PdCl2 synthesis system showed a catalytic specific current of 629 mA/mg in the electrocatalysis of ethanol, whereas large particles from the Na2PdCl4 system showed a current of 262 mA/mg. The maximum catalytic currents of small Pd nanoparticles without surface cleaning treatment were 1382 and 1019 mA/mg for samples from the Na2PdCl4 and PdCl2 systems, respectively, higher than those being treated in KOH solution first, and the electrocatalytic stability of the two untreated samples was better. However, small nanoparticles after the electrochemical treatment can reach the maximum catalytic current faster. The synthesis and structure-property relation of four types of Pd nanoparticles have been discussed and analyzed on the basis of systematically experimental data.

Fabrication of ultrafine Pd nanoparticles on 3D ordered macroporous TiO2 for enhanced catalytic activity during diesel soot combustion

Nanocatalysts consisting of three-dimensionally ordered macroporous (3DOM) TiO2-supported ultrafine Pd nanoparticles (Pd/3DOM-TiO2-GBMR) were readily fabricated by gas bubbling-assisted membrane reduction (GBMR) method. These catalysts had a well-defined and highly ordered macroporous nanostructure with an average pore size of 280 nm. In addition, ultrafine hemispherical Pd nanoparticles (NPs) with a mean particle size of 1.1 nm were found to be well dispersed over the surface of the 3DOM-TiO2 support and deposited on the inner walls of the material. The nanostructure of the 3DOM-TiO2 support ensured efficient contact between soot particles and the catalyst. The large interface area between the ultrafine Pd NPs and the TiO2 also increased the density of sites for O2 activation as a result of the strong metal (Pd)-support (TiO2) interaction (SMSI). A Pd/3DOM-TiO2-GBMR catalyst with ultrafine Pd NPs (1.1 nm) exhibited higher catalytic activity during diesel soot combustion compared with that obtained from a specimen having relatively large Pd NPs (5.0 nm). The T10, T50 and T90 values obtained from the former were 295, 370 and 415 °C. Both the activity and nanostructure of the Pd/3DOM-TiO2-GBMR catalyst were stable over five replicate soot oxidation trials. These results suggest that nanocatalysts having a 3DOM structure together with ultrafine Pd NPs can decrease the amount of Pd required, and that this approach has potential practical applications in the catalytic combustion of diesel soot particles.

Coupling MOF-based photocatalysis with Pd catalysis over Pd@MIL-100(Fe) for efficient N-alkylation of amines with alcohols under visible light

Pd nanoparticles with an average size of 1.7nm confined inside a MIL-100(Fe) cavity (Pd@MIL-100(Fe)) were prepared via double-solvent impregnation combined with a photoreduction process. Due to efficient coupling of photocatalytic dehydrogenation and Pd-based hydrogenation, the resultant Pd@MIL-100(Fe) showed significantly superior performance in light-induced N-alkylation of amines with alcohols over Pd/MIL-100(Fe), in which larger Pd nanoparticles (6–12nm) deposited on the external surface of MIL-100(Fe) were prepared via a conventional single-solvent impregnation followed by a similar photoreduction process. Kinetic studies and controlled experiments revealed that the whole N-alkylation reaction is limited by the photocatalytic alcohol-to-aldehyde dehydrogenation reaction. This is the first demonstration of light-induced N-alkylation of amines by alcohols over M/MOFs nanocomposites and also highlights the great potential of using M/MOFs nanocomposites as multifunctional catalysts for light-induced organic syntheses.

Bromide (Br−) ion-mediated synthesis of anisotropic palladium nanocrystals by laser ablation

Anisotropic growth of Pd nanoparticles in bromine (Br) containing solution has been studied by pulsed laser ablation. For size and shape control different solutions like water, sodium dodecyl sulphate (SDS) (anionic surfactant), and (Br−) ion containing cetyltrimethylammonium bromide (CTAB) (cationic surfactant) and electrolyte (KBr) were used. In laser ablation surrounding liquid plays a dominant role in controlling size and directional growth. Absorption spectra of as-generated Pd nanoparticles undergo modification with time in different solutions due to Br− ion-mediated directional growth. In water and SDS quasi-spherical and spherical Pd nanoparticles with mean size of 14 and 8nm, respectively, and in CTAB decahedron and icosahedron shape Pd nanocrystals with mean size 65nm were observed. When strong Br− ion source KBr was used sharp edged cuboid shaped large Pd nanoparticles were observed. Surface energy modification due to preferential chemisorption of Br− ions onto {100} planes of Pd resulted in formation anisotropic Pd nanostructures enclosed with {100} planes. The nanocubes exhibit broad plasmon resonance around 250–280nm. Further, size of nanocuboids were controlled by using mixed solutions of KBr with SDS and CTAB for tunable plasmon resonance wavelength from 230 to 550nm.

Role of size and pretreatment of Pd particles on their behaviour in the direct synthesis of H2O2

Two families of catalysts, based on Pd nanoparticles supported on ceramic asymmetric tubular alumina membranes, are studies in the direct synthesis of H2O2. They are prepared by depositing Pd in two ways: (i) reduction with N2H4 in an ultrasonic bath and (ii) by impregnation-deposition. The first preparation leads to larger particles, with average size of around 11nm, while the second preparation leads to smaller particles, with average size around 4nm. The catalytic membranes were tested as prepared, after thermal treatment in air and after further pre-reduction with H2 in mild (100°C) conditions. Samples were characterized by TEM, CO-chemisorption monitored by DRIFTS method and TPR, while catalytic tests have been performed in a semi-batch recirculation membrane reactor. Experimental catalytic results were analysed using two kinetics models to derive the reaction constants for the parallel and consecutive reactions of the kinetic network. Smaller particles of Pd show lower selectivity due to the higher rate of parallel combustion, even if the better dispersion of Pd and thus higher metal surface area in the sample lead to a productivity in H2O2 similar or even higher than the sample with the larger Pd particles. Independently on the presence of smaller or larger Pd nanoparticles, an oxidation treatment leads to a significant enhancement in the productivity, although the catalyst progressively reduces during the catalytic process. The inhibition of the parallel combustion reaction (to water) induced from the calcination treatment remains after the in-situ reduction of the oxidized Pd species formed during the pre-treatment. This is likely due to the elimination of defect sites which dissociatively activate oxygen, and tentatively attributed to Pd sites able to give three- and four-fold coordination of CO.

Coarsening of Pd nanoparticles in an oxidizing atmosphere studied by in situ TEM

The coarsening of supported palladium nanoparticles in an oxidizing atmosphere was studied in situ by means of transmission electron microscopy (TEM). Specifically, the Pd nanoparticles were dispersed on a planar and amorphous Al2O3 support and were observed during the exposure to 10mbar technical air at 650°C. Time-resolved TEM image series reveal that the Pd nanoparticles were immobile and that a few percent of the nanoparticles grew or shrank, indicating a coarsening process mediated by the Ostwald ripening mechanism. The TEM image contrast suggests that the largest nanoparticles tended to wet the Al2O3 support to a higher degree than the smaller nanoparticles and that the distribution of projected particle sizes consequently broadens by the appearance of an asymmetric tail toward the larger particle sizes. A comparison with computer simulations based on a simple mean-field model for the Ostwald ripening process indicates that the observed change in the particle size distribution can be accounted for by wetting of the Al2O3 support by the larger Pd nanoparticles.

Investigating Unexpected Magnetism of Mesoporous Silica-Supported Pd and PdO Nanoparticles

The synthesis and magnetic behavior of matrix-supported Pd and PdO nanoparticles (NPs) are described. Mesoporous silica with hexagonal columnal packing is selected as a template, and the impregnation method with thermal annealing is used to obtain supported Pd and PdO NPs. The heating rate and the annealing conditions determine the particle size and the phase of the NPs, with a fast heating rate of 30 °C/min producing the largest supported Pd NPs. Unusual magnetic behaviors are observed. (1) Contrary to the general belief that smaller Pd NPs or cluster size particles have higher magnetization, matrix-supported Pd NPs in this study maintain the highest magnetization with room temperature ferromagnetism when the size is the largest. (2) Twin boundaries along with stacking faults are more pronounced in these large Pd NPs and are believed to be the reason for this high magnetization. Similarly, supported PdO NPs were prepared under air conditions with different heating rates. Their phase is tetragonal (P42/mmc) with cell parameters of a = 3.050 Å and c = 5.344 Å, which are slightly larger than in the bulk phase (a = 3.03 Å, c = 5.33 Å). Faster heating rate of 30 °C/min also produces larger particles and larger magnetic hysteresis loop, although magnetization is smaller and few twin boundaries are observed compared to the supported metallic Pd NPs.

Breakthroughs in hard X-ray diffraction: towards a multiscale science approach in heterogeneous catalysis.

Breakthroughs in hard X-ray diffraction: towards a multiscale science approach in heterogeneous catalysis.

Kelvin Probe Force Microscopy in Surface Chemistry: Reactivity of Pd Nanoparticles on Highly Oriented Pirolytic Graphite

The reactivity of 10–50 nm large Pd nanoparticles (PdNPs) supported on the highly oriented pirolytic graphite (HOPG) surface has been studied under ultra-high-vacuum (UHV) conditions by monitoring the work function (WF) during Kelvin probe force microscopy. The WF of as-prepared PdNPs can vary by more than 600 meV under similar deposition conditions. Because of a chemical reaction between the PdNPs and the residual gas of the UHV at room temperature, the WF always continuously decreases until an equilibrium value is reached. We suspect carbon contamination resulting from the dissociation of CO and hydrocarbons for both phenomena. Smaller PdNPs exhibit a higher reactivity (contamination) than larger ones, and site effects can be observed. We show that annealing at high temperatures in an oxygen atmosphere is sufficient to obtain clean PdNPs, which exhibit the expected WF difference between HOPG and Pd.

Facile fabrication of hollow hydridosilica nanoparticles with mesoporous shell and their dual effect in Pd nanoparticles immobilization

Synthesis of hollow hydridosilica nanoparticles with mesoporous shell (HMHNs) employing a mixture of cationic fluorocarbon surfactant (FC-04) and nonionic triblock copolymer surfactant (P123) as template and triethoxysilane as silica precursor is reported. It is found that the content of the FC-04 in the surfactant mixture greatly influences the aggregation state of the FC-04 and P123 surfactant template and therefore influences the morphology of the synthesized hydridosilica nanoparticles. HMHNs can be synthesized with proper amount of FC-04 surfactants due to the formation of FC-04 and P123 complex micelle, where FC-04 surfactant locates at the outer shell and the P123 constructs the inner core. Ascribed to the inherent hydride sites within the HMHNs, facile immobilization of Pd nanoparticles is achieved by using HMHNs as both the reducing agent and scaffold. The lesser processes produce some smaller Pd nanoparticles (2–3nm) within the pore voids and relatively larger Pd nanoparticles (10–15nm) on the outer surfaces of HMHNs. The catalytic hydrogenation of 4-nitrophenol demonstrates that the heterogeneous Pd catalyst is efficient.

Single-step synthesis and characterization of single-walled carbon nanohorns hybridized with Pd nanoparticles using N 2 gas-injected arc-in-water method

Single-walled carbon nanohorns (SWCNHs) hybridized with palladium (Pd) nanoparticles were synthesized by a single-step gas-injected arc-in-water method (GI-AIW) with a Pd wire inserted inside the anode hole. In the arc zone, carbon and Pd were vaporized simultaneously, leading to the formation of hybrid material of SWCNHs and Pd nanoparticles due to effective quenching. Based on TEM and CO chemisorption analyses, Pd nanoparticles were found to be embedded inside SWCNH aggregates. The size of Pd nanoparticles, determined by X-ray diffraction, was in the range of 3–6 nm when Pd wires with diameters of 0.1 and 0.3 mm were used. Using a Pd wire with a diameter larger than 0.5 mm results in larger Pd nanoparticles which tend to be exposed to the outer surface of the hybrid material. According to thermogravimetric analyses, the weight fraction of Pd nanoparticles is increased by increasing the Pd wire diameter although the yield of Pd nanoparticles decreased. SWCNHs hybridized with dispersed Pd nanoparticles, synthesized with 0.1 mm Pd wire, exhibited strong anti-oxidation resistance with a highly graphitic structure.