Increasing Pd Loading Research Articles

Efficient liquid-phase hydrogenation of bromate over nanosized Pd catalysts supported on TpBD-Me2 covalent organic framework

The formation of toxic and carcinogenic bromate, BrO3–, during the disinfection of fresh or seawater by ozonation poses a threat to the human health, and therefore the development of methodologies for its removal from water is of high importance. Heterocatalytic bromate hydrogenation has been widely employed to decrease the BrO3– ion concentration in water below the strict legal limit imposed. In this work, we evaluate for the first time the performance of a covalent organic framework (COF) for this reaction. TpBD-Me2 COF was synthesized on gram scale and used as both catalyst and support for Pd nanocatalysts with different Pd loadings for bromate hydrogenation reaction.TpBD-Me2 was found to present considerable catalytic activity for BrO3– reduction with 60% conversion after 180 min. The conversion increased with increasing Pd loading of the COF, with the sample with 5% Pd on COF reaching 93% conversion after 180 min. The catalytic activity was further improved by performing an in situ pre-reduction step before the reaction, resulting in complete conversion of BrO3– already after 60 min of reaction. These results show that COF materials are efficient catalysts for this application.

Non-oxidative coupling reaction of methane to hydrogen and ethene via plasma-catalysis process

The plasma pyrolysis and plasma catalysis of methane coupling were performed under room temperature and atmosphere. The coupling of methane through gliding arc plasma with different H2/CH4 ratios and N2 flow rates exhibited conversion from 25% to 40% and high selectivity of acetylene above 90%. Plasma emission spectra displayed lowest IHβ/IHα ratio at H2/CH4 ratio of 3, while IHβ/IHα ratio decreased with increasing N2 feed rate from 0 to 2 L/min, which had similar variation with selectivity of acetylene. X-ray photoelectron spectra for reduced catalysts demonstrated that metallic Pd species (335.4 eV) were transferred to electron-rich Pd species (334.8eV and 334.1 eV) with increasing Pd loading and Ag/Pd ratio, corresponding to red shift of linear adsorbed CO stretching frequency from 2060 to 2068 cm−1 to 2055-2015 cm−1. Superior plasma catalytic behavior over 0.1Pd0.5Ag/Al2O3 catalyst has been reached with 86.5% selectivity of ethene and 36.2% conversion of methane.

Simple and efficient: performance of palladium-loaded sepiolite for electrocatalytic ethanol oxidation

AbstractHigh-performance electrodes with outstanding catalysts play a vital role in the commercial application of direct ethanol fuel cells. In the present study, a supported catalyst with controllable Pd loading, prepared using a facile impregnation method with sepiolite as a carrier, was synthesized and tested for electrocatalytic oxidation of ethanol. Physical characterization revealed the pore structure and large specific surface area of the sepiolite, which provided excellent conditions for the loading of nanometal clusters. The Pd-sepiolite had greater electrocatalytic ethanol activity and anti-intermediate product poisoning performance than a metallic Pd disc electrode under alkaline conditions. Under these experimental conditions, the electrochemical activity in terms of ethanol oxidation increased significantly with increasing Pd loading. Considering both the activity and stability of the electrodes, 23 wt.% Pd loading on sepiolite was selected with a coating amount of 140 μg cm–2 on glassy carbon. Factors such as ethanol/potassium hydroxide concentration, scanning rate and temperature had direct impacts on peak current densities as well as on reaction kinetics as depicted by Tafel plots. The electrochemical impedance test showed that Pd intercalation could improve significantly the conductivity of sepiolite and reduce the electron-transfer resistance in the electrocatalytic process. Thus, Pd-loaded sepiolite is a simple and effective catalyst for direct ethanol fuel cells.

Bimetallic palladium-tin nanoclusters, PdSn(2 0 0) and PdSn(1 0 1), templated with cationic surfactant for electrochemical denitrification toward N2 and NH4+ selectivity

Effects of Pd nanoclusters in PdxSn100-x/Ni electrodes with preferred diffraction planes of Sn(101) and Sn(200) plated on a nickel foam were studied on electrochemical reduction reaction of nitrate (NO3–). NO3– converted to NO2–, N2 and NH4+ by surface Hads was assessed at different Pd to Sn ratio through the electroanalysis and batch electrolysis at constant current mode. Pd loading obviously enhanced the current density of proton diffusion-controlled Sn0/Sn(II) transition. NO3– reduction was much more efficient on PdSn(200) than on PdSn(101); 53% of N2 yield on Sn(200)/Ni could increase up to 79% on Pd5Sn(200)95/Ni, while further increasing Pd loading would gradually increase NH4+ yield. The steady-state kinetics proposed a scheme of adsorption and deoxygenation of NO3– on Sn(200) and hydrogenation of NO2– to N2 on Pd NPs beside the Sn crystals.

Highly efficient debromination of 4,4'-dibrominated diphenyl ether by organic palygorskite-supported Pd/Fe nanoparticles.

Organic palygorskite (OP)-supported Pd/Fe nanoparticles composite (OP-Pd/Fe) was prepared by stepwise reduction method. The removal capacity of 4,4'-dibrominated diphenyl ether (BDE15) by OP-Pd/Fe was compared with other various materials. For better understanding the possible mechanism, the synthesized and reacted OP-Pd/Fe materials were characterized by TEM, SEM, XRD, and XPS, respectively. The effects of major influencing parameters on the degradation of BDE15 were also studied. Benefit from the synergistic effect of the carrier and bimetallic nanoparticles, BDE15 could be completely debrominated into diphenyl ether (DE) under suitable conditions. A two-stage adsorption/debromination removal mechanism was proposed. The degradation of BDE15 with OP-Pd/Fe was mainly stepwise debromination reaction, and hydrogen transfer mode was assumed as the dominated debromination mechanism. The removal process fitted well to the pseudo first-order kinetic equation. The observed rate constants increased with increasing Pd loading and OP-Pd/Fe dosage while decreased with increasing initial BDE15 concentration, the tetrahydrofuran/water ratio, and the initial pH of the solution. The work provides a new approach for the treatment of PBDEs pollution.

Dynamic Estimation Method of Effective Active Site on Palladium Methane Oxidation Catalyst in Exhaust Gas of Marine Lean Burn Gas Engine

Abstract Lean-burn gas engines have recently attracted attention in the maritime industry, because they can reduce NOx, SOx, and CO2 emissions. However, since methane (CH4) is the main component of natural gas, the slipped methane, which is the unburned methane, likely contributes to global warming. It is thus important to make progress on exhaust after-treatment technologies for lean-burn gas engines. A Palladium (Pd) catalyst for CH4 oxidation is expected to provide a countermeasure for the slipped methane, because it can activate at lower exhaust temperature comparing with platinum. However, a de-activation in higher water (H2O) concentration should be overcome because H2O inhibits CH4 oxidation. This study was performed to investigate the effects of exhaust temperature or gas composition on active Pd catalyst sites to clarify CH4 oxidation performance in the exhaust gas of lean-burn gas engines. The authors developed the method of estimating effective active sites for the Pd catalyst at various exhaust temperatures. The estimation method is based on the assumption that active sites used for CH4 oxidation process can be shared with the active sites used for carbon mono-oxide (CO) oxidation. The molecular of chemisorbed CO on the active sites of the Pd catalyst can provide effective active sites for CH4 oxidation process. This paper introduces experimental results and verifications of the new method, showing that chemisorbed CO volume on a Pd/Al2O3 catalyst is increased with increasing Pd loading in 250–450 °C, simulated as a typical exhaust temperature range of lean-burn gas engines.

Plasma Catalytic Conversion of CH4 to Alkanes, Olefins and H2 in a Packed Bed DBD Reactor

Methane is activated at ambient conditions in a dielectric barrier discharge (DBD) plasma reactor packed with Pd/γ-alumina catalyst containing different loadings of Pd (0.5, 1, 5 wt%). Results indicate that the presence of Pd on γ-alumina substantially abates the formation of deposits, leads to a notable increase in the production of alkanes and olefins and additionally improves the energy efficiency compared to those obtained for the non-packed reactor and the bare γ-alumina packed reactor. A low amount of Pd (0.5 and 1 wt%) favors achieving a higher production of olefins (mainly C2H4 and C3H6) and a higher yield of H2. Increasing Pd loading to 5 wt% promotes the interaction of H2 and olefins, which consequently intensifies the successive hydrogenation of unsaturated compounds, thus incurring a higher production of alkanes (mainly C2H6 and C3H8). The substantial abatement of the deposits is ascribed to the role of Palladium in moderating the strength of the electric and shifting the reaction pathways, in the way that hydrogenation reactions of deposits’ precursors become faster than their deposition on the catalyst.

Single-atom Pd dispersed on nanoscale anatase TiO2 for the selective hydrogenation of phenylacetylene

Combining the advantages of both heterogeneous and homogeneous catalysts, single-atom catalysts (SACs) with unique electronic properties have shown excellent catalytic properties. Herein, we report single-atom Pd dispersed on nanoscale TiO2 prepared by self-assembly method as efficient and selective catalysts for the hydrogenation of phenylacetylene to styrene. The catalysts were characterized by N2 adsorption/desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS). 0.2Pd-TiO2(150°C) possessing dominant single-atom Pd species, exhibited a turnover frequency (TOF) of over 8000 h−1 with 91% selectivity towards styrene at room temperature. Further increasing Pd loading from 0.2% to 0.5% and 1.5% resulted in the decrease of activity probably due to the formation of Pd nanoparticles. Besides, the 0.2Pd-TiO2 prepared by self-assembly strategy showed better catalytic performance than commercial 10%Pd/C and 0.2Pd-TiO2 synthesized by using impregnation method.

Glycerol oxidation on Pd nanocatalysts obtained on PEDOT-coated graphite supports

Glycerol oxidation is studied at Pd nanoparticles (NPs) obtained by electroless metal deposition on PEDOT-coated graphite electrodes. Pre-reduced PEDOT coatings with different polymerization charges and different doping ions (polystyrenesulfonate, PSS or dodecyl sulfate, SDS) are used to drive the electroless deposition of Pd NPs. Pd nanocatalysts with metal loadings in the order of 15–30 μgcm−2 and high electroactive surface area (EASA) are obtained. Oxidation of glycerol is studied in dependence of the type of doping (PSS or SDS) and polymerization charge. It is found that at constant Pd loading PEDOT/PSS-supported catalysts have a three times higher glycerol peak oxidation currents. The mass activity (MA) diminishes with increasing Pd loading on both types of PEDOT but for different reasons. Impeded access of glycerol to NPs deposited deeply in the polymer structure is suggested for the porous and hydrophilic structure of PEDOT/PSS. Aggregation of NPs and therefore smaller EASA due to the upper surface confined metal deposition process are operative at PEDOT/SDS coatings. These are also the reasons for the low MA values observed on this type of PEDOT support. MA values at PEDOT/PSS-coated graphite are found to range between 1100 and 1200 mAmg−1 and are between the best values obtained on Pd-supported catalysts.

Pd doped CaCoxZr1-xO3-δ perovskites for automotive emissions control

CaCoxZr1-xO3-δ (x=0, 0.3, 0.5, 0.7 and 0.9) perovskites with and without Pd doping were synthesized for the first time and reported here to show outstanding redox property and oxygen storage capacity (OSC) compared to the ceria-based oxygen storage materials (OSMs). The studied perovskites retain their main phase of orthorhombic Lakargiite CaZrO3 structure, and were characterized by x-ray fluorescence (XRF), x-ray diffraction (XRD), temperature programmed reduction (TPR) and dynamic redox cycle measurements. Partial substitution of Co by Zr at B sites enhanced the perovskite structural crystallinity and stability, material reducibility and OSC properties, giving optimized OSM composition when x was around 0.5. Further kinetic studies showed a first order reaction mechanism with an activation energy (Ea) of 0.159eV for CaCo0.5Zr0.5O3-δ. For Pd-containing samples, Pd was present in both forms of bulk Pd2+ and surface Pd0, and the amount of surface Pd0 increased with increasing Pd loading. Pd dopant also facilitated the Co reduction and improved the catalyst reducibility. Under simulated exhaust conditions at fuel lean-rich conditions (stoichiometric numbers of 1.16, 1.07 and 0.95), Pd-doped CaCo0.6-yZr0.4PdyO3 (y=0.05 and 0.1) samples showed profound catalytic activity towards C3H6 and CO oxidation (T50s<280°C at fuel lean-rich), suggesting their potential application for automotive emissions control.

Temperature-dependent synthesis of Pd@ZIF-L catalysts via an assembly method

The Pd@ZIF-L catalysts with uniform crosshair-star shape and a size of about 20 μm were synthesized by an assembly method, in which a two-dimensional layered zeolitic imidazolate framework-L (ZIF-L) was used as the support to immobilize Pd nanoparticles. Their catalytic activities were evaluated by the catalytic reduction of p-nitrophenol to p-aminophenol. The results highlight that the physical-chemical and catalytic properties of the Pd@ZIF-L catalysts are greatly affected by the synthesis temperature. The temperature variation can make the catalysts transform from two-dimensional flakes to zero-dimensional spheres. High temperature is beneficial for increasing Pd loading and encapsulating Pd nanoparticles within the ZIF-L matrix. However, under the temperature larger than 30 °C, the dense dia(Zn) structures are formed. Increasing Pd loading in the Pd@ZIF-L catalysts leads to a significantly higher p-nitrophenol conversion. The relative dense structure of the as-synthesized catalysts makes p-nitrophenol access the active centers difficultly and a lower catalytic activity is observed. These findings would aid the development of high-performance Pd@ZIF-L catalysts.

Pd supported on boron-doped mesoporous carbon as highly active catalyst for liquid phase catalytic hydrodechlorination of 2,4-dichlorophenol

Palladium catalysts supported on both ordered mesoporous carbon (CMK-3) and boron-doped mesoporous carbon (B-CMK-3) were synthesized via the complexing–reduction method. These catalysts were characterized using X-ray diffraction, N2 adsorption–desorption, transmission electron microscopy, and X-ray photoelectron spectroscopy, and their catalytic performance was examined for the liquid phase catalytic hydrodechlorination (HDC) of 2,4-dichlorophenol. Characterization results showed that for B-CMK-3 boron was introduced into the framework of the mesoporous carbon. Pd supported on B-CMK-3 had a smaller average Pd particle size and higher Pd2+/Pd0 ratio than that on CMK-3, although B-CMK-3 had slightly lower surface area and pore volume than CMK-3. For Pd/B-CMK-3, increasing Pd loading led to an increase in Pd particle size and a decrease in Pd2+/Pd0 ratio. The liquid phase catalytic HDC of 2,4-dichlorophenol over Pd/B-CMK-3 followed the Langmuir–Hinshelwood model, and the catalytic reaction proceeded in both stepwise and concerted pathways. The initial reaction rates of Pd(2.7)/B-CMK-3 and Pd(2.6)/CMK-3 were 0.608 and 0.207MgCat−1h−1, respectively, reflecting a much higher catalytic activity of Pd/B-CMK-3 than that of Pd/CMK-3. For Pd/B-CMK-3, increasing Pd loading from 1.6 to 2.7wt.% led to an increase in the initial rate from 0.260 to 0.608MgCat−1h−1, but further increase of the loading to 3.9wt.% resulted in a slight decrease in the catalytic activity.

An integrated catalyst of Pd supported on magnetic Fe3O4 nanoparticles: Simultaneous production of H2O2 and Fe2+ for efficient electro-Fenton degradation of organic contaminants

A novel electro-Fenton process based on Pd-catalytic production of H2O2 from H2 and O2 has been proposed recently for transforming organic contaminants in wastewaters and groundwater. However, addition of Fe(II) complicates the operation, and it is difficult to recycle Pd catalyst after treatment. This study attempts to synthesize an integrated catalyst by loading Pd onto magnetic Fe3O4 nanoparticles (Pd/MNPs) so that H2O2 and Fe2+ can be produced simultaneously in the electrolytic system. In an undivided electrolytic cell, phenol, a probe organic contaminant, is degraded by 98% within 60 min under conditions of 50 mA, 1 g/L Pd/MNPs (5 wt% Pd), pH 3 and 20 mg/L initial concentration. The degradation rate peaks at pH 3, increases with increasing Pd loading and electric current and decreases with increasing initial concentration. A distinct mechanism, reductive dissolution of solid Fe(III) in Fe3O4 by atomic H chemisorbed on Pd surface, is responsible for Fe2+ production from Pd/MNPs. The efficiency of phenol degradation can be sustained at the same level for ten times of repeated treatment using the Pd/MNPs catalyst. The variations of main crystal structure and magnetic property of catalysts are minimal after treatment, but low concentrations of Pd leached, which needs further evaluation.

Kinetic Analysis of Catalytic Reductive Dechlorination of Chlorobenzene in Aqueous Solutions

Deposition of Pd on the surface of zero-valent iron (Pd/Fe) further enhances the ability of the metal to reductively dechlorinate organic contaminants. This work determined the dechlorination of chlorobenzene in water by Pd/Fe and evaluated the effects of Pd loading in Fe, Pd/Fe dosage, solution pH and temperature on the reaction. Pseudo-first-order rate constants were obtained to analyze the reaction kinetics. Chlorobenzene was nearly completely dechlorinated within 60 min by Pd/Fe at room temperature. Benzene was the end product of the reaction, along with the release of chloride into water. The rate constant of chlorobenzene dechlorination increased with increasing Pd loading in Fe and Pd/Fe dosage within the tested ranges of 0.005 - 0.020% and 2.0 - 6.0 g/75 mL, respectively. The rate constant increased with decreasing solution pH over the tested pH range of 4.5 - 6.5, indicating the role of protons in dechlorination. The reaction was considered to occur primarily on the surface of Pd where protons were reduced to hydrogen species and chlorobenzene was subsequently dechlorinated by the hydrogen species. The rate of chlorobenzene dechlorination increased with increasing temperature. The estimated activation energy of the reaction was 47.94 kJ/mol within the temperature range of 15 - 40°C, indicating that the dechlorination of chlorobenzene by Pd/Fe readily occurs at room temperature. Pd/Fe may be a potential reductant for effective removal of chlorinated organic contaminants from water.

Effect of Pd Nanoparticle on the Thermal Degradation Kinetics of Nylon 6/Pd Nanocomposite Prepared by a Dry Process

Palladium (Pd) nanoparticles were incorporated into a nylon 6 film via a dry process which consisted of simultaneous vaporization, penetration and reduction processes of palladium (II) bis (acetylacetonate, Pd (acac)2) at 180°C for various exposure time. The even dispersion of the generated Pd nanoparticles were observed by transmission electron microscope (TEM) and the Pd loading weight of about 15~43 wt% was measured by thermogravimetric analysis (TGA). In order to study the catalytic effect of Pd nanoparticles on the thermal degradation kinetics of nylon 6, TGA data at various heating rates were introduced to Flynn &amp; Wall equation. The thermal degradation activation energy for neat nylon 6 was ca. 162~178 kJ/mol over the thermal degradation fraction of 0.05~0.40 while that of the nylon 6/Pd (26.5 wt%) nanocomposite was ca. 110~169 kJ/mol over the same fraction range. It meant the Pd nanoparticles were acted as a catalyst on the depolymerization of amide group in nylon 6. It was also found that the activation energy decreased slightly with the increasing Pd loading weight.

Tungsten carbide-supported Pd electrocatalysts for triglyceride hydrogenation in a solid polymer electrolyte reactor

Noble metals are often used for the electrochemical hydrogenation of organics, however, substitutes are being sought due to their high cost. Research described in this paper compared the performance of tungsten carbide-supported Pd (Pd/W 2C) catalysts to those of W 2C, Pd and other noble metals for the hydrogenation of triglycerides in a solid polymer electrolyte reactor. Rates for the Pd/W 2C catalysts increased with increasing Pd loading. The 6 wt.% Pd/W 2C catalyst exhibited the highest activity, and was more active than the Pd or W 2C catalysts, indicating a possible synergy. Hydrogenation rates for all of the catalysts were functions of the applied potential, suggesting that the rate-determining step was electrochemical.

Preparation of Pd-Modified Ni Foam Electrodes and Their Use as Anodes for the Oxidation of Alcohols in Basic Media

Pd-modified Ni foam electrodes were prepared by a spontaneous deposition method. Ni foam samples were immersed in acid PdCl2 solutions for different durations (tSD). The Pd loading and the surface area of the Pd deposits were determined as a function of [PdCl2] and tSD. SEM–EDX showed that Pd deposits were homogeneously formed on the walls of both the outer and the inner cells of the Ni foam. Pd-modified Ni foam electrodes were used as anodes for the oxidation of methanol, ethanol, ethylene glycol, and glycerol in basic media. Voltammetric curves for the oxidation of alcohols showed that the peak current increased with increasing Pd loading in a sub-linear way. For the lowest loading explored (ca. 1 mg of Pd in a 1 cm3 foam volume), the peak current per unit Pd mass was of the order of 650 A g−1 for 0.5 M methanol and ethanol, ca. 1,500 A g−1 for glycerol and higher than 2,000 A g−1 for ethylene glycol.

Pd/SBA-15 nanocomposite: Synthesis, structure and catalytic properties in Heck reactions

Pd nanoparticles supported in mesoporous silica SBA-15 (or Pd/SBA-15 nanocomposites) were prepared by ion-exchange with cationic Pd precursor in an alkaline solution on an uncalcined silica. The high Pd loading in these nanocomposites can be achieved up to 5.21 wt.% by adjusting the pH value of the solution. The surface area and the pore volume decrease with increasing Pd loading. The Pd nanoparticles equal to or smaller than 6 nm in size in the nanocomposites are distributed in the channels of the mesoporous SBA-15. The Pd/SBA-15 nanocomposites exhibit excellent catalytic activities and high reuse ability in air for the Heck carbon–carbon coupling reactions.

Aqueous bromate reduction by catalytic hydrogenation over Pd/Al2O3 catalysts

Bromate is recognized as an oxyhalide disinfection byproduct in drinking water. In this study, supported noble metal (Pd, Pt) catalysts with different supports of SiO2, Al2O3 and activated carbon (AC) were prepared and the catalytic hydrogenation of aqueous bromate was first investigated. Characterization results showed the isoelectric points (IEPs) of Pd/SiO2 and Pd/Al2O3 catalysts were around 2.0 and 8.0, respectively, whereas the IEP of Pd/AC was much lower than 2.0. In comparison with Pd/SiO2 and Pd/AC, Pd/Al2O3 exhibited a substantially higher catalytic activity at pH 5.6 for bromate reduction due to the electrostatic attractive interaction between the bromate ion and the catalyst. Moreover, bromate with an initial concentration of 0.39mM was removed by 80.2% over Pt/Al2O3 and nearly 100% over Pd/Al2O3 after reaction for 2h, indicative of a higher catalytic activity of Pd/Al2O3. For Pd/Al2O3, the bromate reduction followed the Langmuir–Hinshelwood model, reflecting an adsorption controlled reduction mechanism. Increasing Pd loading amount resulted in enhanced bromate reduction. In addition, the bromate reduction was found to be strongly pH-dependent and enhanced reduction rate could be achieved at low pH. In the presence of coexisting anions (Cl−, Br− and SO42−) the bromate reduction was suppressed, wherein SO42− exhibited the most marked inhibition effect, attributed to competitive adsorption for active surface sites. The present results indicate that catalytic hydrogenation can be used as a potential treatment technique for the removal of bromate in drinking water.

One-step synthesis of methyl isobutyl ketone from acetone over Pd/MCM-22 zeolites

MCM-22 zeolites with different Si/Al ratios were dynamically synthesized through hydrothermal method, and the Pd/MCM-22 catalysts were prepared by ion-exchange method as well. The physicochemical properties of the samples were well characterized by XRD, SEM, TEM, N 2 adsorption, NH 3-TPD, and FT-IR of d 3-acetonitrile adsorption. MCM-22 synthesized was markedly different from those in previous literatures, showing much smaller crystals. MCM-22 with Si/Al ratio of 25 showed the highest crystallinity and concentration of Brønsted acid sites. The loading of Pd particles had little influence on the structural and porous properties of MCM-22 zeolite. The specific surface area, pore volume, pore diameter, and the concentrations of the acid sites all decreased with increasing Pd loading as expected. One-step synthesis of methyl isobutyl ketone from acetone was investigated over Pd/MCM-22 catalysts. An acetone conversion of 34.0% and a selectivity of 86.9% for MIBK were realized. The catalytic results also suggested that a proper balance between metallic and acidic sites was necessary for obtaining high catalytic performance.