Pd Surface Coverage Research Articles

Unexpected Negative Performance of PdRhNi Electrocatalysts toward Ethanol Oxidation Reaction.

Direct ethanol fuel cells (DEFCs) need newly designed novel affordable catalysts for commercialization. Additionally, unlike bimetallic systems, trimetallic catalytic systems are not extensively investigated in terms of their catalytic potential toward redox reactions in fuel cells. Furthermore, the Rh potential to break the ethanol rigid C-C bond at low applied potentials, and therefore enhance the DEFC efficiency and CO2 yield, is controversial amongst researchers. In this work, two PdRhNi/C, Pd/C, Rh/C and Ni/C electrocatalysts are synthesized via a one-step impregnation process at ambient pressure and temperature. The catalysts are then applied for ethanol electrooxidation reaction (EOR). Electrochemical evaluation is performed using cyclic voltammetry (CV) and chronoamperometry (CA). Physiochemical characterization is pursued using X-ray diffraction (XRD), transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Unlike Pd/C, the prepared Rh/C and Ni/C do not show any activity for (EOR). The followed protocol produces alloyed dispersed PdRhNi nanoparticles of 3 nm in size. However, the PdRhNi/C samples underperform the monometallic Pd/C, even though the Ni or Rh individual addition to it enhances its activity, as reported in the literature herein. The exact reasons for the low PdRhNi performance are not fully understood. However, a reasonable reference can be given about the lower Pd surface coverage on both PdRhNi samples according to the XPS and EDX results. Furthermore, adding both Rh and Ni to Pd exercises compressive strain on the Pd lattice, noted by the PdRhNi XRD peak shift to higher angles.

Thermal annealing effects on palladium-decorated gold nanoparticle catalysts

Palladium metal supported on gold in the form of surface ensembles have enhanced catalytic properties compared to monometallic Pd, as exemplified by Pd-decorated Au nanoparticles (Pd-on-Au NPs) for various room-temperature reactions. Whereas the catalytic properties and nanostructure of Pd-on-Au NPs are not known at higher temperatures, this work focuses on thermal annealing effects on the Pd-on-Au NP nanostructure, bimetal distribution, and room-temperature water-phase trichloroethene hydrodechlorination (TCE HDC) as the model reaction. Analysis of the average coordination environment of Pd and Au atoms through x-ray absorption spectroscopy showed that as-synthesized Pd-on-Au NPs transitioned from a Au core/Pd shell structure to Au-rich core/PdAu surface alloy or PdAu mixed alloy structures depending on the Pd surface coverage (30–150 sc%) and annealing temperature (100–400 °C). The HDC activity strongly correlated with Pd ensemble size, where the as-formed Pd islands exhibited one order of magnitude enhanced activity compared to monometallic Pd. Higher annealing temperatures led to a surface/mixed alloy structure with smaller Pd ensemble size, resulting in lower activity but still ∼3 times more active than monometallic Pd. These results illustrate the importance of catalyst structure on activity and the usefulness of metal-decorated metal catalysts for higher-temperature reactions.

Modification of boron-doped diamond with gold-palladium nanoparticles for CO2 electroreduction

Modification of boron-doped diamond (BDD) with gold-palladium nanoparticles (AuPdNPs) was performed for an application in electrochemical reduction of CO2. Preparation of the colloidal AuPdNPs involved two steps, including the synthesis of gold nanoparticles (AuNPs) and the growth of palladium on the surface of AuNPs. BDD, which had been terminated with allylamine under UV light irradiation for6h, then, was immersed in the colloidal AuPdNPs. Characterization of the nanoparticles by using Tunneling Electron Microscopy (TEM) showed flowers-like nanoparticles with an average size of 40 nm, while characterization of the modified BDD showed the Au and Pd surface coverage of 4.4 % and 6.0 %, respectively, on BDD surface. The electrochemical reduction of CO2at the modified BDD was performed in a two-compartments cell with 0.1 M NaCl solution in the cathode chamber and 0.1 M Na2SO4solution in the anode chamber. By applying the potential -0.9 V (vs. Ag/AgCl) in the system for 60 min, it showed the faradaic efficiency of the system at approximately 84.65 % with acetic acid as one of the products.

Surface reconstruction of AgPd nanoalloy particles during the electrocatalytic formate oxidation reaction.

Formate is a kind of carbon-neutral fuel that can be synthesized by electrochemical conversion of CO2, however, the generated aqueous formate electrolyte is still short of potential application. Here, formate solution is proposed to be utilized as anode fuels of direct formate fuel cells through the formate oxidation reaction (FOR), and graphene supported AgPd nanoalloys (AgPd/rGO) are prepared to catalyze the FOR. Specifically, the mass activity of the as-prepared Ag49Pd51/rGO catalyst is 4.21 A mg-1Pd and the retention activity of Ag49Pd51/rGO is 49.1% of initial activity after successive 500 cycles, which is 2.48 and 3.03 times higher than that of unsupported Ag51Pd49 nanoalloys. When increasing the positive scan limit from 0.0 to 0.8 V, the mass activity of the Ag49Pd51/rGO catalyst increases from 2.32 to 6.03 A mg-1Pd and Pd surface coverage increases from 51.87% to 62.42%, indicating the occurrence of surface reconstruction where Pd atoms migrate to the surface of AgPd nanoalloys, and less intensive reconstruction is observed in unsupported Ag51Pd49 nanoalloys, whose mass activity increases from 1.35 to 2.49 A mg-1Pd. The driving force and kinetic path are calculated for the surface reconstruction induced by the adsorption of H, O and C atoms, in the case of C atoms on graphene, the segregation energy of surface Pd atoms in the AgPd nanoalloy is -1.16 eV, and the activation energy for the migration of subsurface Pd atoms to the surface is 0.54 eV, which are lower than the segregation (0.03 eV) and activation (2.06 eV) energy on a clean alloy surface.

Toward glucuronic acid through oxidation of methyl-glucoside using PdAu catalysts

The production of glucuronic acid via enzyme catalysis from biomass is slow. Here we studied the oxidation of methoxy-protected glucose (MG) using Pd-on-Au nanoparticle model catalysts to generate methoxy-protected glucuronic acid (MGA), a precursor to glucuronic acid. Pd-on-Au showed volcano-shape activity dependence on calculated Pd surface coverage (sc). The 80 sc% Pd-on-Au catalyst composition showed maximum initial turnover frequency (413 mol-MG mol-surface-atom−1 h−1) that was 5× higher than that of Au/C, while Pd/C was inactive. This Pd-on-Au composition gave the highest MGA yield (46%), supporting a bimetallic approach to glucuronic acid production.

Hydrogen-generating behavior of Pd-decorated gold nanoparticles via formic acid decomposition

Formic acid is a promising hydrogen storage material where hydrogen is generated via metal-catalyzed decomposition. Bimetallic catalysts are active for this reaction, but the mechanism has not been fully proven. Palladium metal supported on gold nanoparticles (Pd-on-Au NPs) has structural properties that are advantageous for studying aqueous-phase catalytic reactions. In this work, a series of Pd-on-Au NPs of varying Pd loadings (calculated in terms of Pd surface coverage, sc%) were synthesized, immobilized onto carbon, and studied for formic acid decomposition at room temperature. Pd-on-Au NPs were catalytically active, with a reaction rate constant as high as 137 m L-H2/gPd/min (corresponding to an initial turnover frequency TOF of 123 h−1) at a Pd loading of 300 sc%. In contrast, Au NPs were inactive, and Pd NPs were slightly active (5 mL-H2/gPd/min and TOF of 38 h−1). The Pd metal of Pd-on-Au catalysts are partially oxidized, and is readily reduced without changing the metal-on-metal structure during reaction, according to in situ x-ray adsorption spectroscopy measurements. CO formation was inhibited at a Pd loading of 300 sc%, suggesting that three-dimensional Pd ensembles favored the desired dehydrogenation pathway while single-atom and small two-dimensional Pd ensembles are active for the undesired dehydration pathway.

Systematic Control of Redox Properties and Oxygen Reduction Reactivity through Colloidal Ligand-Exchange Deposition of Pd on Au.

Core-shell nanoparticles of Au@Pd with precise submonolayer, monolayer, or multilayer structure were synthesized using ligand-exchange reactions of palladate ions onto colloidal Au nanocrystals. Decoupling the palladate adsorption step from the subsequent reduction enables excellent precision, uniformity, and tunability in the Pd shell thickness. The redox properties of the surface Pd are directly correlated to the thickness of the Pd shell with a >+200 mV shift in the PdO reduction potential for submonolayer Au@Pd nanoparticles compared to pure Pd. Using these precisely controlled core-shell materials, the oxygen reduction catalytic activity can be directly correlated to PdO reduction potential and Pd surface coverage on Au. When the Pd oxide reduction peak is shifted by +240 mV compared to pure Pd, a 50 mV reduction in overpotential and a 4-fold increase in kinetic current density for oxygen reduction are observed. Colloidal ligand-exchange synthesis may be particularly useful for noble metal core-shell catalysts as a strategy to subtly tune the electronic properties of surface atoms in order to lower overpotential and increase catalytic turnover.

A high stability AuPd-ZrO2-multiwall carbon nanotubes supported-catalyst in a formic acid electro-oxidation reaction

Catalytic activity and stability in a formic acid electro-oxidation, chemical and structural properties of AuPd nanoparticles deposited (a polyol method) on ZrO2 decorated functionalised multiwall carbon nanotubes (f-MWCNTs) (a hydrothermal method) were investigated using a fuel cell test, scanning transmission electron microscopy (STEM), high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).Non-stoichiometric ZrOx nanoparticles of a cubic phase (C-phase) and average size of 5–10 nm (STEM, HR-TEM), 4.5–5.0 nm (XRD) are anchored by f-MWCNTs carboxylic group through Zr-O-C bonds. Decoration by AuPd nanoparticles leads to ternary Pd/AuPd/Au phase of 6.9 nm crystallite size (XRD) with metallic Pd, oxidised (PdO, PdO2) and Pd-O-Zr phases (XPS). Oxidation/reduction at 300 °C/200 °C leads to nanoparticles sintering, increasing/decreasing Pd surface coverage, oxides/oxygen groups content and ZrOx stoichiometry. Catalysts after oxidation (the largest nanoparticle size, Pd oxides content, ZrOx stoichiometry) showed the highest activity and stability. Activity of AuPd-ZrO2/f-MWCNTs catalysts is smaller than that of these catalysts without ZrO2, however, the stability is remarkably larger, i.e. AuPd-ZrO2/f-MWCNTs > Pd-ZrO2/f-MWCNTs > AuPd/f-MWCNTs > Pd/f-MWCNTs, what is attributed to electronic properties of AuPd and role of ZrOx and oxygen functional groups in COad desorption and oxidation to CO2 releasing catalyst active sites.

Room temperature response and enhanced hydrogen sensing in size selected Pd-C core-shell nanoparticles: Role of carbon shell and Pd-C interface

In the present work, the effect of carbon shell around size selected palladium (Pd) nanoparticles on hydrogen (H2) sensing has been studied by investigating the sensing response of Pd-C core-shell nanoparticles having a fixed core size and different shell thickness. The H2 sensing response of sensors based on Pd and Pd-C nanoparticles deposited on SiO2 and graphene substrate has been measured over a temperature range of 25 °C–150 °C. It is observed that Pd-C nanoparticle sensor shows higher sensitivity with increase in shell thickness and faster response/recovery in comparison to that of Pd nanoparticle samples. Pd-C nanoparticles show room temperature H2 sensitivity in contrast to Pd nanoparticles which respond only at higher temperatures. Role of carbon shell is also understood by investigating H2 sensing properties of Pd and Pd-C nanoparticles on graphene substrates. These results show that higher catalytic activity and electronic interaction at Pd-C interface, a complete coverage and protection of Pd surface by carbon and presence of structural defects in nanoparticle core are important for room temperature and higher sensing response.

Electroless Deposition of Iron–Phosphorus on Copper: Experimental and Modeling Studies of the Critical Role of Substrate Activation By Palladium

Electroless deposition of amorphous iron-phosphorus (Fe–P) is investigated. Fe–P is a potential benign alternative for replacing nickel-phosphorus films commonly used in electronics and surface finishing applications. In the present talk, the role of substrate (Cu) activation by palladium (Pd) in enabling electroless Fe–P deposition will be discussed. Specifically, it will be demonstrated that a critical Pd surface coverage is essential for spontaneous electroless Fe–P deposition to commence. Below this critical Pd coverage, the surface is catalytically inactive for Fe–P deposition. A mechanistic model that incorporates surface heterogeneity due to partial Pd coverage of Cu and the effect of this heterogeneity on electrocatalysis of the reductant oxidation reaction during electroless deposition will be presented. Model predictions will be compared to experimental observations of the electroless surface mixed potential to gain insights into the mechanism by which Pd coverage affects the efficacy of electroless Fe–P deposition. Implications of the findings to the optimization of pretreatment and activation processes commonly used in electroless deposition systems will also be discussed.

Improving gold catalysis of nitroarene reduction with surface Pd

Nitroarene reduction reactions are commercialized catalytic processes that play a key role in the synthesis of many products including medicines, rubbers, dyes, and herbicides. Whereas bimetallic compositions have been studied, a better understanding of the bimetallic structure effects may lead to improved industrial catalysts. In this work, the influence of surface palladium atoms supported on 3-nm Au nanoparticles (Pd-on-Au NPs) on catalytic activity for 4-nitrophenol reduction is explored. Batch reactor studies indicate Pd-on-Au NPs exhibit maximum catalytic activity at a Pd surface coverage of 150 sc%, with an initial turnover frequency of ∼3.7 mol-nitrophenol/mol-metalsurface/s, which was ∼5.5× and ∼13× more active than pure Au NPs and Pd NPs, respectively. Pd NPs, Au NPs, and Pd-on-Au NPs below 175 sc% show compensation behavior. Three-dimensional Pd surface ensembles (with ∼4–5 atoms) previously identified through X-ray adsorption spectroscopy provide the active sites responsible for the catalytic maximum. These results demonstrate the ability to adjust systematically a structural feature (i.e., Pd surface coverage) to yield a more active material.

Unsupported PVA- and PVP-stabilized Pd nanoparticles as catalyst for nitrite hydrogenation in aqueous phase

Pd colloids stabilized with polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP) have been prepared, characterized with TEM, zeta potential measurements, CO chemisorption in aqueous phase, and ATR-IR spectroscopy using CO as a probe molecule, and finally tested for performance in nitrite hydrogenation as unsupported nanoparticles. Polymer coverage on Pd surface was significant for both Pd-PVA and Pd-PVP colloid, blocking active sites. The turnover frequency (TOF) of the remaining sites as well as selectivity to ammonium was not influenced by coverage of the Pd surface with PVA. However, the ammonium selectivity increased with increasing Pd particle size. In contrast, increasing coverage of Pd surface with PVP increased the TOF, while the ammonium formation was suppressed. This has been attributed to an effect of PVP on the surface reaction intermediates, based on the observation that chemisorbed CO was also influenced by PVP.

Volcano-shape glycerol oxidation activity of palladium-decorated gold nanoparticles

Activity and selectivity of glycerol oxidation were highly dependent upon Pd surface coverage of Pd-on-Au bimetallic catalysts.

Reductive dechlorination of tetrachlorobisphenol A by Pd/Fe bimetallic catalysts

The Pd/Fe bimetallic catalysts of micron sizes were synthesized and the rates of tetrachlorobisphenol A (TCBPA) degradation were measured under various conditions using a batch reactor system. The results showed that TCBPA was rapidly dechlorinated to tri-, di- and mono-chlorobisphenol A and to bisphenol A (BPA). The observed rate constants (kobs) were found to increase as functions of the Pd coverage on the Fe particles and the dosages of the catalysts within the reactors. The kobs value decreased as the initial TCBPA concentration increased, suggesting that the TCBPA dechlorination may follow a surface-site limiting Langmuir–Hinshelwood rate model. The weakly acidic solution, especially at or near pH 6.0, also favored the dechlorination of TCBPA. At pH 6.0, Pd coverage of 0.044wt% and catalyst dosage of 5gL−1, TCBPA with an initial concentration of 20μM was completely transformed within 60min, and BPA was detected as the major product through the reaction time. Meanwhile, the kobs values measured at constant solution pH correlated linearly with the mass of particle-bound Pd introduced to the reactors, regardless of Pd/Fe catalyst dosage or Pd surface coverage. This study suggested that Pd/Fe catalysts could be potentially employed to rapidly degrade TCBPA in the contaminated environment.

Reductive debromination of tetrabromobisphenol A by Pd/Fe bimetallic catalysts

The bimetallic catalysts of micron sizes were synthesized and the rates of tetrabromobisphenol A (TBBPA) transformation were measured under various conditions using a batch reactor system. The results showed that TBBPA was rapidly debrominated to tri-, di- and mono-bromobisphenol A and to bisphenol A (BPA). The pseudo-first-order rate constants normalized to the specific surface area of the catalysts were found to increase as functions of the Pd coverage on the Fe particles and the dosages of the catalysts and the mass of Pd within the reactors. The rate constants were also decreasing as the solution pH increased. At pH 4.2, Pd coverage of 0.022wt% and catalyst dosage of 4gL−1, 99% of TBBPA was transformed within 2min, and tri-, di-, and mono-bromobisphenol A were detected as the major intermediate products. After reaction for 45min, BPA was the only compound found in this specific system. Meanwhile, the rate constants measured at constant solution pH correlated linearly with the Pd mass introduced to the reactors, regardless of Pd/Fe catalyst dosage or Pd surface coverage. The study suggested that Pd/Fe catalysts could be used for efficiently treating brominated flame retardants extracted from e-wastes.

Degrading perchloroethene at ambient conditions using Pd and Pd-on-Au reduction catalysts

Perchloroethene (PCE) is a common groundwater contaminant, due to its common use as a dry-cleaning solvent. Current treatment methods are limited in their ability to remove PCE from contaminated sites in an efficient and cost effective manner. Palladium-on-gold nanoparticles (Pd-on-Au NPs) have been shown to be highly catalytically active in the hydrodechlorination (HDC) of trichloroethene (TCE) and other chlorinated compounds. However, the catalytic chemistry of such nanoparticles for PCE HDC in water has not been systematically addressed in the literature. In this paper, we assess the catalytic properties of ∼4nm Pd-on-Au NPs, ∼4nm Pd NPs, and Pd/Al2O3 for water-phase PCE HDC under ambient conditions. The Pd-on-Au NPs exhibited volcano-shape activity as a function of Pd surface coverage (sc). Maximum activity was at 80±0.8 sc% (pseudo-first order rate constant of ∼5000L/gPd/min), which was ∼20x and ∼80x higher than that for Pd NPs and Pd/Al2O3 at room temperature and pH 7. A complete mechanistic model of PCE HDC that coupled gas–liquid mass transfer with the surface reactions was developed and found to be consistent with the observed concentration-time profiles for the 3 catalyst types. The formation and subsequent reaction of daughter products (TCE, dichloroethene isomers, vinyl chloride, and ethene) followed the stepwise dechlorination of the PCE chlorine groups. The final reaction products were ethane and minor amounts of n-butane/butenes. This study establishes the enhanced degradation chemistry of PCE using model Pd-on-Au catalysts and suggests the volcano-shape structure-activity dependence can be generalized from PCE and TCE to other organohalides.

Monolayer palladium supported on molybdenum and tungsten carbide substrates as low-cost hydrogen evolution reaction (HER) electrocatalysts

Active and low-cost hydrogen evolution reaction (HER) electrocatalysts are needed to minimize capital costs associated with large-scale hydrogen production from water electrolysis. Catalysts based on monolayer (ML) amounts of precious metals supported on carbides are a promising concept for this purpose. In the current study Pd supported on tungsten carbide (WC) and molybdenum carbide (Mo2C) were evaluated for HER activity. Carbide foils were synthesized using temperature programmed reaction of W or Mo in a CH4/H2 atmosphere. Physical vapor deposition was used to deposit Pd on WC or Mo2C while X-ray Photoelectron Spectroscopy (XPS) was used to determine the Pd surface coverage. Linear sweep voltammetry and chronopotentiometry were used to evaluate the HER activity and electrochemical stability of the catalysts, demonstrating the possibility of using ML Pd on either WC or Mo2C as active, stable and lower-cost HER catalysts.

Hydrodechlorination catalysis of Pd-on-Au nanoparticles varies with particle size

Trichloroethene (TCE), a common carcinogen and groundwater contaminant in industrialized nations, can be catalytically degraded by Au nanoparticles partially coated with Pd (“Pd-on-Au NPs”). In this work, we synthesized Pd-on-Au NPs using 3, 7, and 10nm Au NPs with Pd surface coverages between 0–150% and studied how particle size and composition influenced their TCE hydrodechlorination (HDC) activity. We observed volcano-shape dependence on both Au particle size and Pd surface coverage, with 7nm Au NPs with Pd coverages of 60–70% having maximum activity. Using extended X-ray absorption fine-structure spectroscopy, we found a strong correlation between catalytic activity and the presence of 2-D Pd ensembles (as small as 2–3 atoms). Aberration-corrected scanning transmission electron microscopy further confirmed the presence of Pd ensembles. The Pd dispersion and oxidation state generally changed from isolated, metallic Pd atoms to metallic 2-D Pd ensembles of varying sizes, and to partially oxidized 3-D Pd ensembles, as Pd surface coverage increased. These changes occurred at different surface coverages for different Au particle sizes. These findings highlight the importance of controlling particle size and surface coverage in bimetallic catalysts.

Characteristics and Catalytic Properties of Mesocellular Foam Silica Supported Pd Nanoparticles in the Liquid-Phase Selective Hydrogenation of Phenylacetylene

An ultra-large pore mesocellular foam silica (MCF) was employed as a support for preparation of supported Pd catalysts for the liquid-phase selective hydrogenation of phenylacetylene. The catalysts were prepared by three different routes: (i) incipient wetness impregnation using Pd(II)acetate solution (Pd/MCF-imp), (ii) impregnation of colloidal Pd nanoparticles obtained by the solvent reduction method (Pd/MCF-col), and (iii) in situ synthesis of MCF in the presence of the Pd colloid (Pd/MCF-ss). The conventional impregnation method resulted in more agglomeration of Pd particles and partial collapse of MCF structure, hence the Pd/MCF-imp exhibited the lowest selectivity towards styrene at total conversion of phenylacetylene. Only the Pd/MCF-ss, in which most of the Pd nanoparticles were encapsulated by the silica matrix, was found to retain high styrene selectivity (>80%) after complete conversion of phenylacetylene. Comparing to the other highly efficient Pd catalysts reported in the literature under similar reaction conditions, it can be emphasized that coverage of Pd surface by the support produces great beneficial effect for enhancing styrene selectivity, regardless of the type of supports used (i.e., TiO2, carbon nanotubes, or mesostructured silica). The direct incorporation of palladium nanoparticles before hydrolysis of the silica precursor during the synthesis of mesocellular foam silica (Pd/MCF-ss) resulted in superior catalytic performance in the selective hydrogenation of phenylacetylene, comparing to those obtained via impregnation of Pd acetate solution (Pd/MCF-imp) and deposition of Pd colloid (Pd/MCF-col). High styrene selectivity (> 80%) was retained after complete conversion of phenylacetylene over the Pd/MCF-ss.

Influence of flame conditions on the dispersion of Pd on the flame spray-derived Pd/TiO 2 nanoparticles

The Pd/TiO 2 nanoparticles containing 5 wt.% Pd were synthesized by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) results, the average particle sizes of Pd/TiO 2 were increased from 9.7 to 24.6 nm with increasing the precursor concentration and the feed flow rate as well as reduction of the O 2 dispersing gas during FSP synthesis. Although the BET surface area and %anatase phase content were decreased with increasing Pd/TiO 2 particle size, %Pd dispersion as determined from the amounts of CO chemisorption were higher on the larger size FSP-made Pd/TiO 2 nanoparticles. It is suggested that the shorter residence time in flame and/or the lower combustion energy (enthalpy density) resulted in more coverage of Pd surface by the formation of Ti–O groups, rendering lower CO chemisorption ability of the smaller size Pd/TiO 2.