Supported Palladium Catalysts Research Articles

Controllable synthesis of mesoporous alumina as support for palladium catalysts and reconstruction of active sites during methane combustion

Uniform mesoporous alumina (MA) nanospheres with excellent thermal stability and controllable pore size distribution were facilely obtained through ultrasonic assisted sol-gel method then taken as carriers for synthesizing a series of supported palladium catalysts. Results evidenced that the pore size distribution of MA significantly affected the catalytic performance of as-prepared catalysts. More importantly, the methane pretreatment tuned valence state of palladium species and distribution of surface oxygen species, along with promoting the adsorption of CH4 on catalysts and the generation of CH3∗ group to form methane-derived adsorbates. Moreover, the synergistic effect of PdO and surface adsorbed oxygen effectively activated CH4 and invoked the oxidation of methane-derived adsorbates, hence boosting the catalytic activity with the 90% CH4 conversion temperature (T90) decreased by 40 °C relative to the unpretreated catalyst. Noticeably, the T90 of Pd/MA(CH4) catalyst could be attained at 380 °C after subsequent long term and cyclic stability test.

Understanding Pore Formation in ALD Alumina Overcoats.

AlOX thin films deposited by atomic layer deposition (ALD) have previously been used to increase both stability and selectivity of supported palladium catalysts and are known to develop nanoscale porosity upon heating. Understanding the factors that affect ALD thin-film porosityenables future design of layered catalytic structures with tunable nanoscale features on industrially-relevant high-surface-area materials. In this study, porous and nonporous aluminum oxide supports with and without palladium nanoparticles were overcoated with thin films of 2-7 nm AlOX by ALD deposited at temperatures of 100, 200, and 300 °C. Hydroxyl loss and changes in surface chemistry were observed upon heating the films, and changes in surface area and pore volume of the annealed films were correlated to AlOX deposition temperature and the presence of Pd. Crystallization of the overcoat to γ-Al2O3 is shown to occur separately from hydroxyl loss and pore formation. A mechanistic understanding of pore formation in AlOX ALD films is obtained by reference to studies of the structural transformations accompanying the formation of transition aluminas from hydroxide precursors. Additionally, a direct and tunable correlation is established between pore development and the overall hydroxyl content of AlOX ALD coatings.

Improved catalytic activity over P-doped ceria-zirconia-alumina supported palladium catalysts for methane oxidation

A series of Pd/xP-CZA catalysts were prepared to investigate the impacts of phosphorus on the structures and catalytic activity. It was found that increasing the amount of phosphorus incorporation leads to the transformation of CexZr1-xO2 phase into ZrO2 and CePO4 at high temperature by the reaction of P species with CexZr1-xO2 composites, which induces the declined reducibility. The addition of phosphorus effectively adjusts the surface Pd2+/(Pd2++Pd0) ratios and generates larger Pd particle sizes, synergistically promoting catalytic oxidation of methane.

Rectifying the chemisorption – XRD discrepancy of carbon supported Pd: Residual chloride and/or carbon decoration

A conspicuous discrepancy is prevalent in the characterization of carbon supported palladium catalysts: particle size estimated by XRD and electron microscopy agree, while the chemisorption of hydrogen is substantially suppressed with respect to the small size measured by XRD/STEM. In this work, a systematic study of carbon materials, pretreatments, and characterization has isolated the contributions of carbon decoration and residual chloride, both of which can cause a chemisorption shortfall. The degree of decoration decreases with graphitization of the carbon supports due to stronger CC interaction, whereas increased density of oxygen functional groups on the surface increases decoration, due to enhanced Pd-C interactions. The discrepancy can be rectified by employing a low temperature burnoff of the decorating carbon layers prior to chemisorption, and by using metal precursors without chloride ligands or counterions.

Zeolite-Enhanced Sustainable Pd-Catalyzed C-C Cross-Coupling Reaction: Controlled Release and Capture of Palladium.

Supported palladium catalysts have attracted significant attention for use in cross-coupling reactions due to their recyclability. However, the inevitable progressive loss of Pd that occurs in the catalytic process deactivates the catalysts, which hinders their sustainable application. Herein, we report a zeolite-enhanced sustainable Pd catalyst for C-C cross-coupling reactions. Zeolite does a good job of acting as a sink for Pd2+ ions. This catalyst exhibits an excellent homogeneous catalytic performance by releasing Pd species from zeolite. In addition, the Pd2+ ions were successfully recaptured in a controlled catalytic system by combining the uniform microporous structure and good adsorption features of zeolite. The release/capture mechanism of the Pd species guaranteed the high loading and high dispersion of Pd on the recycled catalyst. The 0.84%Pd@USY catalysts were reused at least 10 times in water without an appreciable reduction in activity. This study presents a new perspective toward the design of a highly efficient and sustainable supported metal catalyst.

C–C Cross‐Coupling Reactions of Organosilanes with Terminal Alkenes and Allylic Acetates Using PdII Catalyst Supported on Starch Coated Magnetic Nanoparticles

Starch coated magnetic nanoparticles supported palladium catalyst has been explored to perform C–C cross coupling reactions, such as oxidative Heck coupling and Tsuji–Trost allylic coupling using organosilicon compounds as one of the coupling partners. The biopolymer coated magnetic catalyst was very easy to recover magnetically and was efficiently recycled in the subsequent batches. All the reactions were performed in air and thus the necessity of air and moisture free reaction condition is avoided. The present protocols show wide substrate scope and good yields of the products.

Catalytic activity of maghemite supported palladium catalyst in nitrobenzene hydrogenation

A maghemite supported palladium catalyst was prepared and tested in nitrobenzene hydrogenation. The catalyst support was made by a newly developed combined technique, where sonochemical treatment and combustion have been used. As a first step, maghemite nanoparticles were synthesized. Iron(II) citrate was treated in polyethylene glycol by high-intensity ultrasound cavitation to get a homogeneous dispersion, then the product was combusted. The produced powder contained maghemite nanoparticles with 21.8 nm average particle size. In the second step of catalyst preparation, the magnetic nanoparticles were dispersed in the ethanolic solution of palladium(II) nitrate. The necessary energy for the reduction of Pd2+ ions was achieved in the “hot spots” by acoustic cavitation, thus catalytically active palladium was formed. The prepared maghemite supported Pd catalyst have been tested in nitrobenzene hydrogenation at three different temperatures (283 K, 293 K and 303 K) and constant pressure (20 bar). At 293 K and 303 K, the conversion and selectivity of nitrobenzene was above 99% and 96%, respectively. However, the selectivity was only 73% at 273 K because the intermediate species (azoxybenzene and nitrosobenzene) have not been transformed to aniline. All in all, the prepared catalyst is successfully applied in nitrobenzene hydrogenation and easily separable from the reaction media.

Effect of an Alumina Supported Palladium Catalyst on the Magnesium Sulfate Decomposition Kinetics

The sulfur related thermochemical water-splitting cycles are an important class of chemical processes considered for hydrogen production. Recently, the magnesium sulfate thermal decomposition has been reported as a potential unit operation in one of these cycles. Therefore, some interest has been observed in the use of catalysts to lower the activation energy of such reactions. In this context, the present manuscript reports the thermodynamics and kinetics modeling results associated with this reaction system in the presence of a Pd catalyst supported over γ-Al2O3. The presence of such species is responsible for shifting the decomposition temperature to lower values in at least 100 °C. It was observed that the magnesium content is still oriented towards MgO formation. The obtained results indicate that the Pd/Al2O3 catalyst could be a good alternative in reducing the thermal decomposition temperature as its presence was responsible for diminishing the process activation energy from 368.2 to 258.8 kJ.mol−1.

Amidoxime modified PAN supported palladium complex: A greener and efficient heterogeneous catalyst for heck reaction

Development of a facile and greener approach for Heck reaction using a modified PAN supported palladium catalyst is described. The preparation of the catalyst involves a simple process through amidoxime modification of nitrile group in Polyacrylonitrile (PAN), followed by the complexation with Palladium chloride (PdCl2). The resulting supported catalyst was characterized by spectroscopic analyses and further by computational studies. This polymer supported Palladium complex displayed excellent catalytic activity in Heck coupling reaction of a series of activated alkenes and aryl halides and afforded the corresponding coupling products upto 96% isolated yield. After the reaction, the catalyst was easily recovered by simple filtration and recycled with excellent stability and activity even after 5 runs. This is the first report on the use of amidoxime functionalized PAN- Palladium (II) complex as a heterogeneous catalyst in Heck coupling reaction.

Preparation of supported palladium catalyst from hydrotalcite-like compound for dicyclopentadiene resin hydrogenation

Hydrogenated dicyclopentadiene (DCPD) resin has been aroused widely concerned with developing of polymer material science. Due to its cross-linked molecular structure, large steric hindrance, and high activation energy of unsaturated bonds, the design of hydrogenation catalyst with fine structure, high activity, and excellent stability poses great challenges. Herein, basing on the concept of strong metal support interaction, supported palladium catalyst (Pd-MgAlO-HT) derived from thermal decomposition of hydrotalcite-like compound has been synthesized and the related chemical processing conditions have been optimized in a stainless autoclave at 150−270 °C, 1−9 MPa H2 pressure and 0.5−6 h reaction time. The optimal product shows colorless transparent with a degree of hydrogenation over 96.5 %. As comparing with Pd/MgAlO-IM catalyst prepared by conventional impregnation method, the Pd-MgAlO-HT catalyst presents higher activity and stability for DCPD resin hydrogenation. Combined with the XRD, TEM, H2-TPR analysis results, the enhanced catalytic performance of Pd-MgAlO-HT catalyst could ascribe to the small particle size, high dispersion of Pd particles and strong metal support interaction between Pd species and the support.

Poly (3, 4-ethylene dioxythiophene) Supported Palladium Catalyst prepared by Galvanic Replacement Reaction for Methanol Tolerant Oxygen Reduction

Herein, we propose a facile electrochemical approach for the synthesis of Pd loaded poly 3, 4-ethylenedioxythiophene (PEDOT) electrodeposited on glassy carbon electrode (GCE) resulting in high surface area. The catalyst preparation is initiated with EDOT polymerization on GCE surface by electrochemical potential cycling method, followed by the electrodeposition of Cu from a 2 mM solution of CuSO4 in 0.1 M NaClO4 at a constant potential of +0.34 V vs. SHE in the form of Cu nanocubes on the PEDOT surface. Pd-PEDOT catalyst was then prepared by the partial substitution of copper by galvanic displacement with various concentrations of PdCl2. The prepared Pd/PEDOT electrocatalyst is found to be methanol resistant indicating its usefulness as fuel cell cathode. The prepared catalyst supports two electron transfer of oxygen reduction reaction in 0.5 M H2SO4. The effects of Pd and Cu contents and the quantity of PEDOT, mass and specific activities were studied. At a relatively low Pd loading of 0.57 ng/cm2, the Pd/PEDOT should be a cost-effective alternative cathode catalyst for direct methanol fuel cells, DMFCs. This work explains the usefulness of PEDOT as good catalyst supporting material which is prepared by an eco-friendly electrochemical route.

Structure-sensitivity of alumina supported palladium catalysts for N2O decomposition

The catalytic activity of Pd/γAl2O3 for N2O decomposition was found to be highly dependent on the preparation methodology and the number of reaction cycles. Chloride species on the surface of a 2.6 wt. % Pd-Al2O3 catalyst prepared by wet impregnation were identified as inhibiting the activity. Multiple reaction cycles were shown to remove these species, and a subsequent increase in activity was observed; 10.3 molN2O h−1 kgcat−1 in the first use, increased to 28.7 molN2O h−1 kgcat−1 on the fourth use both at 550 °C. Additionally, removal of physisorbed water from the support prior to metal deposition increased the thermal stability of Pd nanoparticles and increased the catalyst activity. Catalysts were subsequently prepared using a deposition technique with an increased concentration of Cl- ions resulted in increased Pd-Cl species in the final catalyst and the catalytic activity was consequently increased due to the improved control of Pd particle size.

Aqueous Carbonylation of Furfural-Derived 5-Bromofuroic Acid to 2,5-Furandicarboxylic Acid with Supported Palladium Catalyst

Developing new catalytic technologies to utilize biomass as the carbon source of chemical and material industries attracts more attention than ever because of the rapid depletion of fossil feedstoc...

Reductive Cycloisomerization of Diynes by Supported Palladium Catalysts and Subsequent [4+2] Cycloaddition for One‐Pot Synthesis of Cyclohexenes

AbstractAbstract: Supported Pd nanoparticles catalyzed the reductive cycloisomerization of diynes to give 1,3‐dienes, and subsequent intermolecular [4+2] cycloaddition with dienophiles led to cyclohexene derivatives. Various diynes and dienophiles participated in the one‐pot reaction to give the corresponding products in good to high yields. Al2O3‐supported Pd catalysts were highly tolerant toward air and reusable.

Water Inhibition in Methane Oxidation over Alumina Supported Palladium Catalysts

In situ diffuse reflectance infrared Fourier transform spectroscopy has been used to distinguish surface hydroxyl groups on Al2O3 and PdO/Al2O3 model catalysts calcined at 500-900 °C. Employing the operando approach, the formation of surface hydroxyl groups has been correlated to the methane oxidation activity for PdO/Al2O3 catalysts using a PdO powder sample as reference. The results show that the alumina support stabilizes active PdO particles leading to enhanced apparent methane turnover frequency (TOF), which decreases slowly in dry conditions due to alumina hydroxylation. Wet conditions cause severe hydroxylation that is detrimental for the methane TOF. The hydroxylation follows two different routes, i.e., spillover of hydrogen-containing species to the PdO-Al2O3 boundary and/or the close proximity of the supported PdO particles and under wet conditions also dissociation of gas phase water on the entire alumina surface. Both hydroxylation routes obey varying kinetics such that near saturation is reached quickly (minutes) followed by a continuous slow growth for prolonged exposure times (hours). At low temperatures, inhibition of palladium active sites on the rim of the PdO particles close to alumina seems to be of particular importance for the observed detrimental effect of water, whereas water induced morphological changes (no sintering observed) of the PdO particles play a minor role.

Investigation of supported palladium catalysts for combustion of methane: The activation effect caused by SO2

Catalytic combustion of methane was studied over Ce0.5Zr0.5O2 and CeO2 supported Pd (Pd/CZ and Pd/Ce) catalysts in the presence of SO2. The effect of sulfation (air including 100 ppm SO2 for 6 h at 500 °C) on the sulfur-tolerant performance of the catalysts was also investigated. Results indicated that there is a considerable promotion effect for CH4 complete oxidation on Pd/CZ catalyst, while the activity of Pd/Ce catalyst was inevitably inhibited. Further, the superior sulfur-tolerant behavior of Pd/CZ was verified through a 50 h time-on-stream stability test. Mechanistic insights into the synergistic effect between the support and Pd were unveiled by Raman, H2-TPR, XPS, NH3-TPD, DRIFTS, HAADF-STEM, etc. It was deduced that the formation of sulfates at the Pd-support interface could promote the dissociative adsorption of methane on Pd/CZ catalyst. Meanwhile, the higher relativeacid strength andthe acidamountwere also decisive factors for the high sulfur-resistant performance. The enhanced acid strength and acid amount prevented the further formation of bulk-like sulfate species for the SO2 treated Pd/CZ catalyst, thus enhancing the sulfur-tolerant ability of the catalyst.

Synthesis of hierarchically porous silica aerogel supported Palladium catalyst for low-temperature CO oxidation under ignition/extinction conditions

Abstract Synthesis of well-dispersed palladium nanoparticles within silica aerogel pores with controlled size was carried out using sol-gel synthesis under supercritical ethanol drying. The high concentration of silanol groups on silica (SiO2) surface facilitated a superior palladium (Pd) loading up to 10 wt %. The synthesized Pd/SiO2 nanocomposite aerogels were characterized by X-ray diffraction (XRD), N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopic methods. The silica aerogel supported catalysts were found to have a wide pore size distribution. TEM investigations confirmed that Pd nanocrystals were located within the SiO2 microspores and mesopores. The catalyst was evaluated for carbon monoxide (CO) oxidation reaction under ignition/extinction conditions. The synthesized catalyst demonstrated a high catalytic activity at low operating temperatures (

Improved methane oxidation activity of P-doped γ-Al2O3 supported palladium catalysts by tailoring the oxygen mobility and electronic properties

Micro-mesoporous P-doped γ-Al2O3 with cluster morphology was obtained via an efficient ultrasound-assisted sol-gel process and taken as carrier to construct palladium catalysts for methane oxidation. It was revealed that the structure and properties of catalysts were significantly influenced by the phosphorus precursors with diverse valence and acidity. Dissimilar reducibility of surface hydroxyl and oxygen species is observed in the catalysts derived from different phosphorus sources, indicating the difference in the oxygen mobility and the capacity of the catalysts to convert intermediate CO. The behavior of charge-transfer transition and d-d transition, the transfer ability of electrons from palladium particles into the antibonding 2π* orbitals of CO, together with the surface acidity and electronic density of palladium species was likewise tailored, which demonstrated the metal-support interaction could be tuned, making palladium species behave with diverse status and electronic structures. The optimized properties cooperatively provided an enhancement in catalytic performance of P-containing catalysts.

Complete dehalogenation of bromochloroacetic acid by liquid phase catalytic hydrogenation over Pd/CeO2 catalysts

Bromochloroacetic acid is classified as one of the typical disinfection byproducts (DBPs). In this work, supported palladium catalysts on different supports (CeO2, Al2O3, SiO2 and activated carbon (AC)) (labelled as Pd/support) were synthesized via the deposition-precipitation method (D-P method) and their activities for the complete dehalogenation of bromochloroacetic acid by liquid phase catalytic hydrogenation were evaluated. Comprehensive characterizations of the catalysts were conducted by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), point of zero charge (PZC), X-ray photoelectron spectroscopy (XPS) and CO chemisorption. Results indicated that the PZCs of the supports varied with each other. The stronger Pd-support interaction and higher Pd dispersion of Pd/CeO2 and Pd/Al2O3 than those of Pd/AC and Pd/SiO2 were confirmed by X-ray photoelectron spectroscopy and CO chemisorption. Pd/CeO2 had a higher ratio of positively charged Pd to metallic Pd (Pdn+/Pd0) than Pd/Al2O3 and Pd/AC due to a stronger metal-support interaction. Accordingly, a negligible bromochloroacetic acid conversion was observed on Pd/SiO2, whereas bromochloroacetic acid was found to be readily decomposed on Pd/CeO2, Pd/Al2O3 and Pd/AC. However, the dechlorination reaction could not further proceed on Pd/Al2O3 and Pd/AC catalysts after the bromine functionality was removed from bromochloroacetic acid. A complete dehalogenation of bromochloroacetic acid occurred only on Pd/CeO2. Furthermore, the dechlorination rate constants of monochloroacetic acid and bromochloroacetic acid over Pd(1.40)/CeO2 were 0.018 and 0.031 min−1 respectively, confirming an induced synergistic effect due to the existence of bromine atoms. It was worth noting that a stepwise-concerted pathway was verified during the liquid phase catalytic hydrodehalogenation of bromochloroacetic acid.

Reductive degradation of 2,4-dichlorophenoxyacetic acid using Pd/carbon with bifunctional mechanism

In this study, commercial carbon nanofibers with different graphitic structure and commercial multiwall carbon nanotubes (CNT) were used. Palladium catalysts were prepared using these supports. Subsequently, they were tested in the hydrodechloration reaction of 2,4-dichlorophenoxyacetic acid under ambient-like conditions. Thermogravimetric analyses (TGA), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and nitrogen adsorption at 77 K techniques were applied to characterize the different materials. The results have demonstrated the efficiency of a bifunctional material in an integrated process that synergically combines physical adsorption and catalytic degradation. During the process, the carbon surface provides active sites to get chlorophenoxyacetic adsorbed. After saturation of the nanocarbon, the compound was decomposed by the catalytic function of supported Palladium catalysts. The study is focused on analyzing the effects of the support surfaces and electronic state of supported palladium nanoparticles on the catalytic performances. High selectivity to dechlorinated product was obtained with the catalysts prepared over more graphitic supports, whereas no-selectivity to dechlorinated products took place over oxygen-containing support. The mechanistic aspects of this bifunctional process were postulated based on the characterisation of these catalytic materials.