Surface Of Pd Nanoparticles Research Articles

Enhancement of hydrogen response by forming an Au submonolayer on nanogap Pd nanoparticles

When Pd nanoparticles dispersed on a glass substrate with nanometer order gaps are exposed to H2 gas, H atoms are adsorbed on the nanoparticle surface, and the electrical resistance between the nanoparticles increases because of the tunneling current suppression. In contrast, when Au nanoparticles are exposed to H2 gas, the resistance remains unchanged because H atoms are not adsorbed on the Au surface. Considering these behaviors, the change ratio of the electrical resistance is expected to be smaller when the surface of Pd nanoparticles is partially covered with Au. However, the experimental results show the opposite resistance change. Density functional theory simulation indicates that H atoms are adsorbed and absorbed on the pure Pd surface, but H atoms are adsorbed and tend to remain on the partially covered Pd surface. These results indicate that the decrease in the resistance due to the gap narrowing by hydrogen absorption occurs in Pd nanoparticles, but it does not occur in Au/Pd nanoparticles, resulting in a larger resistivity increase compared with the Pd nanoparticles. This result implies that in certain cases, the low reactivity of Au to H2 contributes to the enhancement of the electrical resistance response.

Bond Selective Photochemistry at Metal Nanoparticle Surfaces: CO Desorption from Pt and Pd.

The use of visible photon fluxes to influence catalytic reactions on metal nanoparticle surfaces has attracted attention based on observations of reaction mechanisms and selectivity not observed under equilibrium heating. These observations suggest that photon fluxes can selectively impact the rates of certain elementary steps, creating nonequilibrium energy distributions among various reaction pathways. However, quantitative studies validating these hypotheses on metal nanoparticle surfaces are lacking. We examine the influence of continuous wave visible photon fluxes on the CO desorption rates from 1 to 2 nm diameter Pt and Pd nanoparticle surfaces supported on γ-Al2O3. Temperature-programmed desorption measurements quantified via diffuse reflectance infrared Fourier transform spectroscopy demonstrate that visible photon fluxes significantly enhanced the rate of CO desorption from Pt nanoparticles in a wavelength-dependent manner. 440 nm photons most efficiently promoted CO desorption from Pt nanoparticle surfaces, aligning with the excitation energy for the interfacial electronic transition within the Pt-CO bond. Conversely, visible photon fluxes had no measurable influence on CO desorption rates from Pd nanoparticle surfaces after accounting for photon-induced heating. Density functional theory calculations demonstrate that the Pt-CO bond exhibits a narrower LUMO resonance, stronger coupling between the photoexcitation and forces induced on the metal-C bond, and vibrational energy dissipation that more effectively couples to desorption as compared to Pd-CO. These results demonstrate the specificity photons provide in facilitating chemical reactions on metal nanoparticle surfaces and substantiate the idea that photon fluxes can steer processes and outcomes of catalytic reactions in ways not achievable by equilibrium heating.

Mechanistic analysis of transformative Pd/NHC catalyst evolution in the 1,2-diphenylacetylene semihydrogenation using molecular hydrogen

This study explored the dynamic transformations of the Pd/NHC catalytic system during the semihydrogenation of alkynes using molecular hydrogen under mild reaction conditions. Focusing on the appearance of a “cocktail”-type system, we harnessed advanced characterization methods, such as NMR, ESI-HRMS, and SEM/TEM. In the hydrogenation process of 1,2–diphenylacetylene, the Pd/NHC complex evolves to produce novel palladium-based compounds and a heterogeneous phase, which partially precipitates into catalytically active nanoparticles. A pivotal finding revealed that the modification of the Pd nanoparticle surface by organic ligands under reaction conditions significantly influenced the catalytic activity of the Pd particles. The split test results suggest an enhanced selectivity toward (Z)-stilbene when the precipitate is maintained in the reaction mixture, highlighting that the palladium nanoparticles act as active catalysts (or reservoirs of active species) synergistically with dissolved molecular complexes. This research reports important findings for understanding the dynamics of the Pd/NHC system, contributing to the development of more efficient catalytic systems.

Circumventing the activity–selectivity trade-off via the confinement effect from induced potential barriers on the Pd nanoparticle surface

Inducing a potential barrier alters the potential energy surface of the flat Pd catalyst, resulting in a “confinement” effect that achieves a simultaneous optimization of the selectivity and activity of the catalyst in acetylene semi-hydrogenation.

Selective hydrogenation of vegetable oil over supported noble metal nanocatalyst

Selective hydrogenation of canola oil was tested using a palladium metal catalyst on the graphene layer (G/Pd) nanocatalyst. The reaction was carried out in a batch reactor under mild operating conditions. G/Pd nanocatalyst was studied for the promotion of selectivity instearic acid (C18:0). The Pd catalysts were found to be much more active without the generation isomer such as transfatty acids, due to also the coating on the surface of Pd nanoparticles, during the hydrogenation experiment. The maximum conversion was obtained after 3 h of reaction at 100 °C, with a percentage of stearic acid of > 90 %.

Atomic NbOx overlayers on palladium nanoparticles enhance selective hydrodehydroxylation

The implementation of strong metal–support interactions (SMSI) is an effective design strategy that can be used to control reaction selectivity. In this study, we present evidence for the existence of SMSI between Nb2O5 and Pd, which results in the formation of an atomic thick layer of NbOx that coats the surface of Pd nanoparticles. The NbOx layer was found to profoundly affect the catalytic hydrodehydroxylation of 5-(hydroxymethyl)furfural (HMF) to 5-methylfurfural (MF). Under optimal reaction conditions, it significantly increased MF selectivity (from 6.4% to 81.0%) compared to an analogous control sample at iso-conversion. Through probing these two materials, the origin of this enhancement was attributed to the blocking of active sites responsible for the (undesirable) hydrogenation of C = O moieties. Importantly, however, the NbOx overlayer did not appear to influence the ability of the material to activate H2 and preferentially dissociate C – OH on Nb2O5, which are key steps in the hydrodehydroxylation mechanism.

Plasma Cleaning of Cationic Surfactants from Pd Nanoparticle Surfaces: Implications for Hydrogen Sorption

Plasma Cleaning of Cationic Surfactants from Pd Nanoparticle Surfaces: Implications for Hydrogen Sorption

Reaction of CO Oxidation on the Surface of Pd Nanoparticles: Optimization by Reinforcement Learning

Reaction of CO Oxidation on the Surface of Pd Nanoparticles: Optimization by Reinforcement Learning

Photo-thermal catalytic reverse water gas shift reaction over Pd/MaZrOx (M=Sr, SrMn) catalysts driven by "Cycle-double sites"

Constructing defect sites on the surface of a catalyst is an important strategy used to achieve efficient reverse water gas shift (RWGS) reaction. In this work, the oxygen vacancies (OVs) and Mn2+ site were successfully created on the surface of Pd/SrMnaZrOx catalysts to enhance the adsorption and activation of CO2. Based on the results of in-situ DRIFTS, quasi-situ EPR, and XPS measurements, which revealed that CO2 adsorbed on the OVs and Mn2+ sites and was activated into CO species. Under UV–vis light irradiation, photo-generated electrons from SrMnaZrOx migrated to the surface of Pd nanoparticles, which was favorable for the activation of H2. Meanwhile, visible light causes the conversion of Mn3+ to Mn2+, and promotes the formation of OVs around the converted Mn2+ sites. This formed "cycle-double sites" (both OVs regeneration and Mn2+ reformation) enhances the activity and stability of RWGS reaction on the prepared catalyst with UV–vis light.

Nanometals Templated by Tobacco Mosaic Virus Coat Protein with Enhanced Catalytic Activity

Bio-nano hybrid materials feature eco-friendly synthesis, various self-assembly patterns and numerous possibilities of surface functionalization. Owing to these unique advantages, biological scaffolds have been be regarded as green alternatives to fabricate metallic nanomaterials for catalytic applications. In this study, a bulky tobacco mosaic virus coat protein (TMVCP) has been employed as a versatile template to synthesize nanometals (Pd, Pt and Au) in alkaline buffer at room temperature. Protein-functionalized nanoparticles (NPs) with ca. 3 nm in diameter were obtained with uniform size distributions. These materials are green catalytic systems with exceptional performance for small organic molecule transformations in water. Remarkably, while Au NPs showed rapid kinetics in 4-nitrophenol reduction (rate constant with Au NPs: 0.01862 s-1), Pd NPs exhibited superior catalytic activity towards hydrogenation of allyl alcohol (Turnover frequency of Pd NPs: 6382 ± 200 mol product (mol Pd * h)-1) and a set of derivatives. The enhanced catalytic performance is strongly related to the protein ligand. To ascertain the protein structure-function effect on the catalytic performance, an in-depth analysis was performed for the TMVCP-Pd NP system via molecular dynamic (MD) simulations. Our results suggest that due to the moderate interaction between Pd and protein and the steric hindrance of the bulky protein, a large portion of the Pd NP surface remains approachable for the reactant molecules, thus resulting in enhanced reaction rate.

Design of Pd-Decorated SrTiO3/BiOBr Heterojunction Materials for Enhanced Visible-Light-Based Photocatalytic Reactivity.

The development of photocatalytic materials that exploit visible light is imperative for their sustainable application in environmental remediation. While a variety of approaches have been attempted, facile routes to achieve such structures remain limited. In this contribution, a direct route for the production of a SrTiO3/BiOBr/Pd heterojunction is presented that employs a low temperature, sustainable production method. The materials were produced in a two-step process wherein BiOBr nanoplates are fabricated in the presence of the SrTiO3 nanospheres, generating a highly integrated composite material. Pd nanoparticle surface decoration was subsequently employed to facilitate and enhance charge separation lifetimes to optimize reactivity. The structures were fully characterized via a suite of approaches to confirm the final material composition and arrangement. Their reactivity was explored for the degradation of both colored and colorless model environmental pollutants, where the SrTiO3/BiOBr/Pd demonstrated significant reactivity using visible light, leading to substrate degradation in <10 min in some cases. The enhanced reactivity was attributed to the significant integration between materials, facilitating electron transfer. Such studies provide key information for the development of new materials with optimized visible-light-driven photocatalytic reactivity for sustainable environmental remediation.

Waste Eggshell with naturally-functionalized sulfonic groups as excellent support for loading Pd and Ag nanoparticles towards enhanced 1,3-butadiene hydrogenation

• Eggshell was used as novel green support in selective 1,3-butadiene hydrogenation. • Pd-Ag/eggshell catalysts demonstrated superior stability. • The sulfonic groups on eggshell altered the Pd nanoparticles electronic properties. • The sulfonic groups result in the weaker adsorption of butenes and 1,3-butadiene. • The eggshell can be used as a natural modified support for hydrogenation. In recent years, high value utilization of waste biomass has been a hot topic in scientific research. Herein, waste eggshell (ES) rich in sulfonic groups was utilized as a natural modified support to load Pd and Ag nanoparticles (NPs), leading to efficient Pd-Ag/ES catalysts for the catalytic hydrogenation of 1,3-butadiene. Through series of characterizations such as SEM, TEM, BET, XPS, and FTIR, it was found that the porous ES not only has hierarchical structures to facilitate the metal dispersion, but also interacts strongly with Pd and Ag NPs via the inherent surface sulfonic groups, which are responsible for the enhanced catalytic performance. The optimized Pd-Ag/ES catalyst exhibited butenes selectivity of 95.8% and 1,3-butadiene conversion of 95% at 45 °C with excellent stability for at least 30 h on-stream. Furthermore, the first-principles density functional theory (DFT) calculation and in situ DRIFTS were performed to explore the plausible mechanism underlying the enhanced catalytic performance due to sulfonic groups. It is suggested that the sulfonic groups can effectively reduce the adsorption energy of butenes and 1,3-butadiene on the surface of Pd NPs which are beneficial to the selectivity of butenes. This work exemplifies that the waste ES with inherent sulfonic groups can be used as excellent support materials for loading active metal nanoparticles for selective hydrogenation reactions. Collectively, we verified that biological waste like eggshell has a great potential in serving as catalyst carrier due to its unique advantages.

Facile Redox Synthesis of Novel Bimetallic Crn+/Pd0 Nanoparticles Supported on SiO2 and TiO2 for Catalytic Selective Hydrogenation with Molecular Hydrogen

The bimetallic Crn+/Pd0 nanoparticles have been synthesized for the first time by a two-step redox method. The method includes the deposition of Pd0 nanoparticles on the surface of SiO2 and TiO2 carriers followed by the deposition of Crn+ on the surface of Pd0 nanoparticles using the redox procedures, which are based on the catalytic reduction of Crn+ with H2 in aqueous suspensions at ambient conditions. Transmission (TEM) and scanning (SEM) electron microscopy, X-ray photoelectron spectroscopy (XPS), Fourie-transformed infrared spectroscopy of adsorbed CO (FTIR-CO), and CO chemisorption studies were performed to characterize the morphology, nanoparticle size, element, and particle distribution, as well as the electronic state of deposited metals in the obtained catalysts. A decrease in nanoparticle size from 22 nm (Pd/SiO2) to 2–6 nm (Pd/TiO2) makes possible deposition of up to 1.1 wt.% Cr most likely as Cr3+. The deposition of CrOx species on the surface of Pd nanoparticles was confirmed using FTIR of adsorbed CO and the method of temperature-programmed reduction with hydrogen (TPR-H2). The intensive hydrogen consumption in the temperature ranges from −50 °C to 40 °C (Cr/Pd/SiO2) and from −90 °C to −40 °C (Cr/Pd/TiO2) was first observed for the supported Pd catalysts. The decrease in the temperature of β-PdHx decomposition indicates the strong interaction between the deposited Crn+ species and Pd0 nanoparticle after reduction with H2 at 500 °C. The novel Crn+/Pd/TiO2 catalysts demonstrated a considerably higher activity in selective hydrogenation of phenylacetylene than the Pd/TiO2 catalyst at ambient conditions.

Fine-tuning inverse metal-support interaction boosts electrochemical transformation of methanol into formaldehyde based on density functional theory

Different from traditional metal-support heterogenous catalysts, inverse heterogeneous catalysts, in which the surface of metal is decorated by metal oxide, have recently attracted increasing interests owing to the unique interfacial effect and electronic structure. However, a deep insight into the effect of metal-oxide interaction on the catalytic performance still remains a great challenge. In our work, an inverse hematite/palladium (Fe2O3/Pd) hybrid nanostructure, i.e., the active Fe2O3 ultrathin oxide layers partially covering on the surface of Pd nanoparticles (NPs), exhibited superior electrocatalytic performance towards methanol oxidation reaction (MOR) as compared to the bare Pd NPs based on density functional theory calculation. The charge could transfer from Pd to Fe2O3 driven by the built-in potential at the interface of Pd and Fe2O3, which favors the downshift of d band center of Pd. With the assistance of interfacial hydroxyl OH*, the cleavage of OH and CH in CH3OH could take place much easily with lower barrier energy on Fe2O3/Pd than that on pure Pd via two electrons transferring reaction pathways. Our results highlight that the synergy of Pd and Fe2O3 at the interface could facilitate the electrochemical transformation of methanol into formaldehyde assisted with interfacial hydroxyl OH*.

Facile Synthesis of Ultrahigh Molecular Weight Poly(Methyl Methacrylate) by Organic Halides in the Presence of Palladium Nanoparticles.

A facile and versatile approach for the synthesis of ultrahigh molecular weight poly(methyl methacrylate) (PMMA) at mild conditions was developed. Certain organic halides combined with a catalytical amount of palladium nanoparticles (Pd NPs) were found to be very effective in initiating polymerizations of methyl methacrylate (MMA), methyl acrylate, vinyl acetate and other vinyl monomers. An ultrahigh molecular weight PMMA with a number-average molecular weight of 4.65 × 106 Da and a weight-average molecular weight of 8.08 × 106 Da was synthesized at 70 °C using 2-bromoisobutyric acid ethyl ester (EBiB) as an initiator in the presence of catalytical amount (10.1 ppm) of Pd NPs. A kinetic investigation found that the orders of polymerization with respect to EBiB, Pd NP and MMA were 0.23, 0.50, and 0.58, respectively. Proton nuclear magnetic resonance (1H NMR) combined with matrix-assisted laser desorption ionization time of flight mass spectroscopy (MALDI-TOF) and gel permeation chromatography (GPC) were used to prove that the macromolecular chain had an end-group of EBiB residue. The electron spin resonance (ESR), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS) results reveal that the reaction of EBiB with Pd NPs caused a bromo atom (Br) transfer from EBiB to Pd NPs and resulted in the generation of EBiB residue radical to initiate the polymerization of MMA and the formation of PdIIBr2 on the surface of Pd nanoparticles.

Selective Hydrogenation of Acetylene to Ethylene over the Surface of Sub-2 nm Pd Nanoparticles in Miscanthus sinensis-Derived Microporous Carbon Tubes

As one of the organic carbon-rich feedstocks, Miscanthus sinensis is an interesting candidate for producing many high-value materials and chemicals. In this study, M. sinensis-derived activated carbon (msAC) particles with ca. 3.4 nm average pore size have been prepared by a simple carbonation method for supporting Pd nanoparticles (NPs) to hydrogenate acetylene. Catalytic test results showed that the 0.25% Pd/msAC catalyst could convert ∼99.9% of acetylene with ∼99.9% of ethylene selectivity. The life test result strongly demonstrated that the catalyst had a good durability for long-term (>100 h) acetylene hydrogenation with high conversion (∼99.9%) and ethylene selectivity (∼93.1%) at 260 °C with a space velocity of 80,000 h–¹. The characterization results indicated that the msAC with the micro- and nanoscale pore structure and uniformly distributed oxygen-containing functional groups could efficiently disperse Pd nanoparticles and promote their Pd (111) crystal plane exposure. The above structure nature was co-contributing to the catalyst to choose the energetically favorable path for efficient acetylene hydrogenation and ethylene desorption. This work opens up valuable insights into the high-value utilization of M. sinensis biomass to design an efficient isolated Pd NP catalyst.

Confirmation of Suzuki-Miyaura Cross-Coupling Reaction Mechanism through Synthetic Architecture of Nanocatalysts.

Despite widespread use of heterogeneous Pd catalysts in Suzuki-Miyaura coupling reactions, detailed roles of Pd, especially the nature of its active species, are still a topic of controversial debate. While some studies showed an active surface of Pd nanoparticles or nanoclusters acting heterogeneously, others claimed soluble Pd species leached from the metallic Pd to be active species which are homogeneous in nature. Besides, within the homogeneous mechanism, how the Pd leaches and promotes the cross-coupling reaction is then another question that needs to be addressed. It could be envisioned that if the soluble Pd species and solid-phase Pd are physically separated, the mechanism of Suzuki-Miyaura coupling could then be confirmed through examining the catalytic activity in different reaction regions. Herein we use microporous Stöber silica as a membrane to separate the soluble Pd species from solid Pd and conduct size-selective reactions which allow the passage of leaching Pd species, but not of reactants or products larger than the membrane aperture. With this strategy, we have been able to differentiate the surface reaction from the solution cross-coupling. We find that the leached Pd species are the only genuine catalytic intermediate in the cross-coupling reactions. We also confirm that oxidative addition of aryl halides to the solid Pd leads to leaching of the soluble Pd species which is necessary to promote Suzuki-Miyaura reactions.

Kinetic Analysis of 4-Nitrophenol Reduction by "Water-Soluble" Palladium Nanoparticles.

The most important model catalytic reaction to test the catalytic activity of metal nanoparticles is the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride as it can be precisely monitored by UV–vis spectroscopy with high accuracy. This work presents the catalytic reduction of 4-nitrophenol (4-Nip) to 4-aminophenol (4-Amp) in the presence of Pd nanoparticles and sodium borohydride as reductants in water. We first evaluate the kinetics using classical pseudo first-order kinetics. We report the effects of different initial 4-Nip and NaBH4 concentrations, reaction temperatures, and mass of Pd nanoparticles used for catalytic reduction. The thermodynamic parameters (activation energy, enthalpy, and entropy) were also determined. Results show that the kinetics are highly dependent on the reactant ratio and that pseudo first-order simplification is not always fit to describe the kinetics of the reaction. Assuming that all steps of this reaction proceed only on the surface of Pd nanoparticles, we applied a Langmuir−Hinshelwood model to describe the kinetics of the reaction. Experimental data of the decay rate of 4-nitrophenol were successfully fitted to the theoretical values obtained from the Langmuir–Hinshelwood model and all thermodynamic parameters, the true rate constant k, as well as the adsorption constants of 4-Nip, and BH4− (K4-Nip and KBH4−) were determined for each temperature.

Selective Hydrogenation of Cinnamaldehyde over the Stepped and Plane Surface of Pd Nanoparticles with Controlled Morphologies by CO Chemisorption.

Carbon monoxide (CO) molecules are attracting attention as capping agents that control the structure of metal nanoparticles. In this study, we aimed to control the shape and surface structure of Pd particles by reducing the supported Pd precursor with CO. The reduction of Pd nanoparticles with CO promoted the exposure of step sites and generated spherical and concave-tetrahedral Pd particles on carbon and SiO2 supports. On the other hand, conventional H2-reduced Pd particles show a flattened shape. The preferential exposure of the step sites by the adsorbed CO molecules was supported by the density functional theory-calculated surface energy and the Wulff construction. Morphology- and surface-controlled Pd nanoparticles were used to study the surface structure and morphology effects of Pd nanoparticles on cinnamaldehyde (CAL) hydrogenation. With an increase in the fraction of step sites on Pd nanoparticles, the hydrogenation activity and selectivity of hydrocinnamaldehyde (HCAL) increased. On step sites, the adsorption of the C═C bond of CAL proceeded preferentially, and HCAL was efficiently and selectively generated.

A New Redox Technique for Depositing CrOx on the Surface of Pd(0) Nanoparticles: Catalysts for Selective Phenylacetylene Hydrogenation

A catalyst containing chromium oxide particles on the surface of palladium nanoparticles is synthesized for the first time using a redox technique for reduction with adsorbed hydrogen. The low-temperature reduction of chromium oxide and formation of palladium hydride is reported for the first time in such catalyst. A considerable difference between its catalytic properties in the selective hydrogenation of phenylacetylene and those of the initial palladium catalyst is shown.