Pd Metal Particles Research Articles

Improvement of a Real Gas-Sensor for the Origin of Methane Selectivity Degradation by µ-XAFS Investigation.

We have directly investigated the chemical state of the Pd species in a real μ-gas sensor device by examining the μ-fluorescence X-ray absorption fine structure. The μ-gas sensor device was heavily damaged by a heating process in which the temperature was ill-controlled, resulting in decrease of methane selectivity. We found that the PdO in the fresh μ-gas sensor was reduced to Pd metal particles as the methane selectivity decreased. Based on the investigation results, we modified the device structure so as to heat up homogeneously. The lifetime of the sensor was then successfully increased by more than 5 years.

Performance and durability of carbon black-supported Pd catalyst covered with silica layers in membrane-electrode assemblies of proton exchange membrane fuel cells

Pd metal particles supported on a high surface area carbon black (Pd/CB) were covered with silica layers to improve the durability under severe cathode condition of proton exchange membrane fuel cells (PEMFCs). The performance and the durability of the silica-coated Pd/CB (SiO2/Pd/CB) were investigated by rotating disk electrode (RDE) in aqueous HClO4 and single cell test of the membrane-electrode assemblies (MEAs). SiO2/Pd/CB showed excellent durability exceeding Pt/CB during potential cycle in single cell test as well as in RDE measurement while Pd/CB significantly degraded. Furthermore, the MEA using SiO2/Pd/CB as the cathode catalyst showed higher performance than that using Pd/CB even in the initial state. The catalytic activity of SiO2/Pd/CB was higher than that of Pd/CB, and the drop of the cell performances due to the inhibition of electron conduction, proton conduction, and oxygen diffusion by the silica layer was not significant. It has been shown that the silica-coating is a very practical technique that can stabilize metal species originally unstable in the cathode condition of PEMFCs without a decrease in the cell performance.

Toluene hydrogenation over Pd and Pt catalysts as a model hydrogen storage process using low grade hydrogen containing catalyst inhibitors

The effects of CO and H2S as catalyst inhibitors on the rate of toluene hydrogenation were studied as a means of hydrogen storage using low-grade hydrogen. Pd/SiO2 suffered serious negative effects from catalyst inhibitors; however, Pd/TiO2–SiO2 exhibited high CO and H2S tolerance because the acidic support decreased the electron density of the Pd metal particles, which, in turn, decreased the interaction between the Pd surface and CO (or H2S). The TiO2–SiO2-supported Pd catalyst exhibited activity greater than that of the TiO2–SiO2-supported Pt catalyst in the presence of CO; however, it exhibited lower activity in presence of H2S. Catalyst characterization after sulfidation with H2S revealed that Pd particles were fully sulfided, whereas Pt particles were sulfided only on their surface. We concluded that Pd catalysts supported on acidic oxides exhibit excellent activity toward toluene hydrogenation in the presence of CO and that Pt catalysts exhibit excellent activity in the presence of H2S.

Selective hydrodeoxygenation of cyclic vicinal diols to cyclic alcohols over tungsten oxide-palladium catalysts.

Hydrodeoxygenation of cyclic vicinal diols such as 1,4-anhydroerythritol was conducted over catalysts containing both a noble metal and a group 5-7 transition-metal oxide. The combination of Pd and WOx allowed the removal of one of the two OH groups selectively. 3-Hydroxytetrahydrofuran was obtained from 1,4-anhydroerythritol in 72 and 74% yield over WOx -Pd/C and WOx -Pd/ZrO2 , respectively. The WOx -Pd/ZrO2 catalyst was reusable without significant loss of activity if the catalyst was calcined as a method of regeneration. Characterization of WOx -Pd/C with temperature-programmed reduction, X-ray diffraction, and transmission electron microscopy/energy-dispersive X-ray spectroscopy suggested that Pd metal particles approximately 9 nm in size were formed on amorphous tungsten oxide particles. A reaction mechanism was proposed on the basis of kinetics, reaction results with tungsten oxides under an atmosphere of Ar, and density functional theory calculations. A tetravalent tungsten center (W(IV) ) was formed by reduction of WO3 with the Pd catalyst and H2 , and this center served as the reductant for partial hydrodeoxygenation.

(Invited) Increased Oxide Ion Diffusivity and Surface Exchange on Pr2NiO4 Base Oxide by Au Dispersion

Influence of the metal dispersion in the dense Pr2NiO4-base oxides on the charge and mass transport was investigated in this study. Ag, Au, Pd, and Pt metal particles were dispersed into the Cu- and Ga-doped Pr2NiO4, and it was found that Au dispersion improves the electric conductivity. In addition, Au dispersion also enhances the oxide ion diffusivity and oxygen surface exchange property.

One-pot selective conversion of furfural into 1,5-pentanediol over a Pd-added Ir–ReOx/SiO2bifunctional catalyst

One-pot selective conversion of furfural into 1,5-pentanediol (1,5-PeD) was carried out over Pd-added Ir–ReOx/SiO2catalysts through two-step reaction temperatures. The Pd(0.66 wt%)–Ir–ReOx/SiO2catalyst showed the best performance in the production of 1,5-PeD from furfural. The maximum yield of 1,5-PeD was 71.4%. The furfural conversion and yield of 1,5-PeD was almost maintained during four repeated tests when the catalyst was calcined again. The characterization results from TPR, XRD, XANES, EXAFS and FT-IR of adsorbed CO indicated that Pd–Ir–ReOx/SiO2catalysts consisted of ReOx-modified Pd metal particles and ReOx-modified Ir metal particles. The lower-temperature reaction step was very crucial for the total hydrogenation of furfural into a tetrahydrofurfuryl alcohol intermediate, which was converted into 1,5-PeD by hydrogenolysis during the high temperature step over the ReOx-modified Ir metal particles.

Unique effect of surface area of support on propene combustion over Pd/ceria

The effects of calcination temperature and surface area of ceria support on the propene oxidation over Pd/ceria were investigated. Very unique dependence of the oxidation activity of Pd/ceria on surface area was observed, i.e. lower surface area of ceria is more preferable for the higher catalytic activity depending on the pretreatment conditions. The structure of supported Pd species was characterized by Pd K-edge EXAFS. Isolated Pd ions were stable on ceria with high surface area after the propene oxidation, which was caused by the easy oxidation of Pd species and formation of Pd–O–Ce bond. On the other hand, moderately dispersed Pd metal particles were abundant on ceria with low surface area. It was suggested that the weaker interaction with sintered ceria is suggested to be preferable for the formation of active Pd metal particles by the reduction at low temperatures.

Selective Oxidation of Glycerol over Titania Supported AuPd Bimetallic Catalysts

Oxidation of gycerol to form various types of important short chain oxygenated derivatives became an important reaction to support biodiesel industries. In this study, a series of AuPd nanoparticles at different metal mole ratios which were 9:1 and 8:2 supported on titania (TiO2) were successfully prepared, characterized and tested for its activity and selectivity in liquid phase oxidation reaction of glycerol. All the catalysts were prepared by deposition-precipitation method with decomposition of urea. AuxPdy/TiO2 catalysts were characterized using XRD, TEM and XPS. The formation of alloy Au-Pd phase was ascertained by XRD and XPS analysis. TEM analyses have shown that the Au and Pd metal particles in the range of 10-30 nm in size were uniformly dispersed on the TiO2 support with narrower size distribution. Higher catalytic activity observed for the catalysts was attributed to the presence of metallic Au0 and PdO phases together with the ‘synergistic’ effect of Au-Pd alloy. The highest selectivity to tartronic acid (55%) was obtained by using Au9Pd1/TiO2 catalyst after 8 h of reaction time at 50oC reaction temperature.

A study of furfural decarbonylation on K-doped Pd/Al 2O 3 catalysts

Vapor-phase decarbonylation of furfural to furan was performed on K-doped Pd/Al 2O 3 catalysts in a fixed-bed reactor. A series of K-doped catalysts were prepared by impregnation method with various potassium salt precursors and different potassium content. The catalyst evaluation results revealed that the doping of K not only promoted the furfural decarbonylation, but also suppressed the hydrogenation side reaction. Among the investigated catalysts, Pd–K 2CO 3/Al 2O 3 (K% = 8 wt.%) presented the best performance and achieved the furan yields up to 99.5% at 260 °C. The selectivity of 2-methylfuran would approach zero at high K content (K% > 4 wt.%). Furthermore, the catalysts were characterized by BET, SEM, ICP, H 2-TPR, H 2-TPD, H 2-chemisorption, CO-FTIR, furfural-TPSR, and furfural-FTIR experiments. It was shown that the dopant phase had a remarkable effect on the precursor Pd 2+ and the reduced Pd metal particles. Furfural-TPSR and furfural-FTIR exhibited that K-doped catalysts enhanced the furfural adsorption and promoted the decarbonylation reaction, which was due to the different adsorption mode and intensity of furfural on K-doped and undoped samples. Besides, the decrease of C–O adsorption of furfural ( η 2(C,O)-furfural) by K doping is the main reason for suppressing the furfural hydrogenation.

Non-thermal RF plasma effects on surface properties of Pd/TiO2 catalysts for selective hydrogenation of acetylene

Non-thermal RF plasma modification has been applied to Pd/TiO2 catalysts for selective hydrogenation of acetylene in the presence of ethylene. High ethylene selectivity and high acetylene conversion were obtained over the plasma-treated catalysts. To understand the plasma effect, the catalysts were characterized by differential scanning calorimetry in hydrogen (H2-DSC), pulse H2 chemisorption, X-ray photoelectron spectroscopy (XPS), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) with CO and temperature-programmed desorption with ethylene (C2H4-TPD). XPS and H2-DSC results confirmed that supported Pd precursors could be effectively reduced to the metallic state during the room temperature plasma treatment. Plasma treatments also enhanced the surface active sites of Pd/TiO2 catalysts and improved the dispersion of Pd metal particles. In addition, DRIFTS and C2H4-TPD results indicated that plasma treatments could induce strong metal-support interaction with lower reduction temperature (200°C), which lead to an enhanced catalytic performance on selective hydrogenation of acetylene. It demonstrates that the RF non-thermal plasma treatment is an effective way to manipulate surface properties and the interaction between metals and supports of supported Pd catalysts for selective hydrogenation.

Palladium-based bifunctional membrane reactor for one-step conversion of benzene to phenol and cyclohexanone

On the one-step hydroxylation of benzene to phenol over a Pd membrane reactor, the bifunctional effects between Pd membrane and metal particles were investigated. In this reaction system, the active oxygen species are formed by the reaction with the permeated hydrogen and adsorbed oxygen over Pd membrane. The active oxygen species are reacted with benzene and directly converts into phenol. For improving this system, various active metals were loaded on the α-Al 2O 3 porous tube which was the substrate of the thin Pd membrane. The loading of noble metals resulted in the enhancement of the activity of side reactions, i.e., complete oxidation and hydrogenation, and a decrease in the hydroxylation activity. The negative effect on hydroxylation was due to the imbalance between the catalytic activities of highly dispersed noble metal particles and the Pd membrane, which has a relatively small surface area. In contrast, the loading of Cu suppressed complete oxidation and enhanced the hydroxylation activity. This effect was mainly due to the increase in the hydrogenation activity without significant acceleration of oxidation. In addition, the potential for the selective conversion of benzene to cyclohexanone by this reactor was discussed.

Catalytic activity and long-term stability of palladium oxide catalysts for natural gas combustion: Pd supported on LaMnO3-ZrO2

The catalytic activity and long-term stability of 2% Pd/LaMnO3-ZrO2 catalysts for natural gas combustion were deeply investigated. The catalyst, prepared via solution combustion synthesis, was completely characterized (XRD, BET, FESEM/EDS, TPC/TPD/TPR and FT-IR analysis) in the fresh status, and in the aged one, after prolonged treatment under hydro-thermal ageing and S-compounds poisoning (up to 3 weeks of hydro-thermal treatment at 800°C under a flow of domestic boiler exhaust gases typical composition of 9% CO2, 18% H2O, 2% O2 in N2, including 200ppmv of SO2). An increased catalytic activity towards NG combustion with ageing was detected: the T50, in fact, got lowered from 570 (fresh sample) to 465°C (after 3 weeks ageing). Highly dispersed Pd centers were predominant on fresh catalyst. Upon ageing, oxygen covered Pd metal particles formed, at the expense of dispersed cationic and zerovalent Pd atoms. The increase in the catalytic activity was associated to the phase modification occurring in the bulk support, where Mn oxides, active towards CH4 combustion, segregated. Moreover, bands due to sulfate species were detected in aged samples: IR analysis showed that Pd atoms did not interact significantly with these species. The bands of sulfate species decreased in intensity after 3 weeks ageing, likely mostly due to sintering of the catalyst, with the corresponding decrease in the surface area.

New Route for the Preparation of Pd and PdAu Nanoparticles Using Photoexcited Ti-Containing Zeolite as an Efficient Support Material and Investigation of Their Catalytic Properties

A simple and unique route to synthesize nanosize Pd particles using a zeolite support including a single-site Ti oxide moiety (TS-1) under UV-light irradiation has been developed. By the photoassisted deposition (PAD) method, a Pd precursor can be deposited directly on the photoexcited tetrahedrally coordinated Ti oxide moiety within the zeolite frameworks from an aqueous solution of Pd. The subsequent reduction with H2 generates the nanosized Pd metal particles with a narrow size distribution (PAD-Pd/TS-1). Characterization by XAFS and TEM analysis revealed that the size of the metal particles depends on the preparation methods and that the smaller sizes of Pd nanoparticles were formed on the photodeposited catalysts compared with the conventionally prepared impregnated catalysts. PAD also provides PdAu bimetallic nanoparticles from an aqueous solution of mixture of PdCl2 and HAuCl4. The catalytic activities in the direct synthesis of hydrogen peroxide (H2O2) using H2 and O2 gases under atmospheric pressure were strongly dependent on the preparation method and the presence of Au atoms. Here, both deposition of PdAu onto the TS-1 moiety under UV-light irradiation was the most efficient for the above reaction. The applicability of the present catalytic system is highlighted by the one-pot reaction of phenol using PAD-Pd/TS-1 in the presence of H2 and O2, in which both Pd nanoparticles and isolated Ti oxide moieties within the frameworks participate in the formation of H2O2 and oxidation of phenol, respectively.

Synthesis and Property of Carbon Nanotube-Supported Pd and Pt Nanoparticles

Carbon nanotubes (CNT) were used as a catalyst support where catalytically active Pd and Pt metal particles decorated the outside of the external CNT walls. In this study, Pd and Pt nanoparticles supported on <TEX>$HNO_3$</TEX>-treated CNT were prepared by microwave-assisted heating of the polyol process using <TEX>$PdCl_2$</TEX> and <TEX>$H_2PtCl_6{\codt}6H_2O$</TEX> precursors, respectively, and were then characterized by SEM, TEM, and Raman. Raman spectroscopy showed that the acid treated CNT had a higher intensity ratio of <TEX>$I_D/I_G$</TEX> compared to that of non-treated CNT, indicating the formation of defects or functional groups on CNT after chemical oxidation. Microwave irradiation for total two minutes resulted in the formation of Pd and Pt nanoparticles on the acid treated CNT. The sizes of Pd and Pt nanoparticles were found to be less than 10 nm and 3 nm, respectively. Furthermore, the <TEX>$SnO_2$</TEX> films doped with CNT decorated by Pd and Pt nanoparticles were prepared, and then the <TEX>$NO_2$</TEX> gas response of these sensor films was evaluated under <TEX>$1{\sim}5\;ppm$</TEX> <TEX>$NO_2$</TEX> concentration at <TEX>$200^{\circ}C$</TEX>. It was found that the sensing property of the <TEX>$SnO_2$</TEX> film sensor on <TEX>$NO_2$</TEX> gas was greatly improved by the addition of CNT-supported Pd and Pt nanoparticles.

Surface chemistry and reactivity of ceria–zirconia-supported palladium oxide catalysts for natural gas combustion

The behavior of 2% Pd catalyst supported on ceria–zirconia, prepared by solution combustion synthesis, was investigated in fresh and aged status, after severe hydro-thermal ageing in presence of SO 2. A surface chemical characterization study was performed using methane temperature programmed combustion, oxygen temperature-programmed desorption, hydrogen temperature-programmed reduction and infrared spectroscopy of low temperature carbon monoxide adsorption. The fresh catalyst is mostly constituted by partly oxidized very small Pd metal particles and dispersed Pd oxide species. This situation gives rise to high catalytic activity for methane combustion at low temperature. The progressive oxidation of highly dispersed small Pd particles to PdO x , as well as the coalescence of dispersed Pd oxide species result in PdO x particles fully oxidized at least at the surface which are less active (in the low temperature range) with respect to the active species of the original catalyst.

Wacker-type oxidation in vapor phase using a palladium–copper chloride catalyst in a liquid polymer medium supported on silica gel

Pd(II) chloride and Cu(II) chloride in various liquid polymer media supported on silica gel were prepared and used in a catalyst system for vapor-phase synthesis of acetaldehyde by Wacker-type oxidation of ethylene. This catalyst system supported on silica gel prepared by impregnation was quickly deactivated, while use of polyethylene glycol (PEG) as a liquid polymer medium supported on silica gel showed stable catalytic activity. PEG inhibited the formation of Pd metal particles, which deactivate the catalyst system. Addition of alkali metal salts, especially LiCl, to the PdCl 2–CuCl 2 catalyst system with PEG enhanced catalytic activity for 22 h, even when the Pd content was high, leading to high activity but poor stability. LiCl also inhibited the formation of metal particles.

Interaction of NO2 with Model NSR Catalysts: Metal–Oxide Interaction Controls Initial NOx Storage Mechanism

Using scanning tunneling microscopy (STM), molecular-beam (MB) methods and time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we investigate the mechanism of initial NO(x) uptake on a model nitrogen storage and reduction (NSR) catalyst. The model system is prepared by co-deposition of Pd metal particles and Ba-containing oxide particles onto an ordered alumina film on NiAl(110). We show that the metal-oxide interaction between the active noble metal particles and the NO(x) storage compound in NSR model catalysts plays an important role in the reaction mechanism. We suggest that strong interaction facilitates reverse spillover of activated oxygen species from the NO(x) storage compound to the metal. This process leads to partial oxidation of the metal nanoparticles and simultaneous stabilization of the surface nitrite intermediate.

Flame-synthesized LaCoO 3-supported Pd : 2. Catalytic behavior in the reduction of NO by H 2 under lean conditions

A 0.5 wt% Pd/LaCoO 3, prepared by flame-spray pyrolysis (FP), was tested as catalyst for the low-temperature selective reduction of NO by H 2 in the presence of excess O 2. In particular, the effect of the precalcination and prereduction temperature on catalytic activity was compared with that of a similar Pd/LaCoO 3 sample prepared by impregnation with a Pd solution of FP-prepared LaCoO 3. The FP-made catalyst allowed full NO conversion at 150 °C, with 78% selectivity to N 2, thus outperforming the catalytic behavior of the corresponding sample prepared by impregnation. The higher activity of the FP-made catalyst has been attributed to the formation of segregated Co metal particles, not present in the impregnated sample, formed during the precalcination at 800 °C, followed by reduction at 300 °C. Two reaction mechanisms can be deduced from the temperature-programmed experiments. The first of these, occurring at lower temperatures, indicates cooperation between the Pd and Co metal particles, with formation of active nitrates on cobalt, successively reduced by hydrogen spillover from Pd. The second, occurring at higher temperature, allows 50% conversion of NO, with >90% selectivity to N 2, and involves N adatoms formed by dissociative NO adsorption over Pd. Prereduction at 600 °C led to a slight increase in catalytic activity, due to the formation of a Pd Co alloy, which is more stable on reoxidization compared with Pd alone. Moreover, the cooperative reaction mechanism seems to be favored by the proximity of Co and Pd in metal particles.

Reflective and electrically conductive palladium surface‐metallized polyimide nanocomposite membranes

AbstractA previous article (Southward and Thompson, Chem Mater 2004, 16, 2091) focused on the characterization of surface metallized palladium‐BTDA/4,4′‐ODA (3,3′,4,4′‐benzophenone tetracarboxylic dianhydride‐4,4′‐oxydianiline) hybrid membranes formed by in situ reduction of Pd2+ to Pd○ during thermal imidization of the poly(amic acid) via the protocol of St. Clair et al. (J Am Chem Soc 1980, 102, 876). The present work extended the Pd‐polyimide metallization synthesis to the BPDA/4,4′‐ODA (3,3′,4,4′‐biphenyltetracarboxylic dianhydride‐4,4′‐ODA) polymer. The effects of the Pd2+ ligand on the reductive metallization of BTDA/4,4′‐ODA were also examined for the PdCl2(SMe2)2, PdBr2(SMe2)2, and Pd(CF3COO)2 complexes. The hybrid films were characterized by specular and diffuse reflectivity and by conductivity measurement. Maximum specular reflectivity was in the range of 35%–55%, and conductivity was in the range of 1–10 Ω/square. Scanning electron microscopy and transmission electron microscopy revealed that the initial Pd metal particles formed within the films were in the 3–8 nm range in size. These particles increased in size with irregular shapes at the surface as the metallic exterior emerged. A fraction of the nanometer‐sized Pd particles remained uniformly distributed throughout the bulk of the film. The mechanism for the formation of a metallic surface required selective oxidative degradation of the polyimide at the polyimide–air interface. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2708–2716, 2006

Liquid phase catalytic hydrodechlorination of aryl chlorides over Pd–Al-MCM-41 catalyst

Supported Pd catalysts were prepared by direct hydrothermal (DHT) or template-ion exchange (TIE) method on Al-substituted MCM-41 as the support and tested in the hydrodechlorination of aryl chlorides such as 4-chloroanisole in liquid phase, together with impregnated Pd catalysts as references. When Pd was loaded on Al-MCM-41 with the Si/Al ratio of 150 by the TIE method, the catalyst showed the highest activity among the Pd catalysts prepared. PdO originally formed on the catalyst surface was in situ reduced to Pd metal during the reaction as the active form for the dechlorination. The activities of the supported Pd catalysts well correlated with the Pd dispersion on the TIE or impregnated Pd catalysts, whereas the DHT catalysts showed relatively high activities independently on the Pd dispersions. It was confirmed that the dechlorination proceeded by a heterogeneous mechanism catalyzed by Pd metal particles sized less than 10nm on the surface of the catalyst. Al substitution for Si on MCM-41 was effective for the loading of Pd metal with the high dispersion, none the less Pd was located on the surface of the TIE catalyst particles and no significant effect of mesoporous structure on the reaction was observed. Methanol was the most profitable as the solvent among the various solvents tested. Various types of arylchlorides bearing hydroxy, methoxy, methyl, nitro and phenylcarbonyl group at the p-position were efficiently dechlorinated over Pd–Al-MCM-41 catalyst at 40°C. Electrophilic attack of arylchloride was proposed as the rate determining step, where ionic mechanism positively worked and electron-releasing substituents increased the catalytic activity.