Pd Al2O3 Research Articles

Atomic-Scale 3D Structure of a Supported Pd Nanoparticle Revealed by Electron Tomography with Convolution Neural Network-Based Image Inpainting.

Electron tomography based on scanning transmission electron microscopy (STEM) is used to analyze 3D structures of metal nanoparticles on the atomic scale. However, in the case of supported metal nanoparticle catalysts, the supporting material may interfere with the 3D reconstruction of metal nanoparticles. In this study, a deep learning-based image inpainting method is applied to high-angle annular dark field (HAADF)-STEM images of a supported metal nanoparticle to predict and remove the background image of the support. The inpainting method can separate an 11nm Pd nanoparticle from the θ-Al2O3 support in HAADF-STEM images of the θ-Al2O3-supported Pd catalyst. 3D reconstruction of the extracted images of the Pd nanoparticle reveals that the Pd nanoparticle adopts a deformed structure of the cuboctahedron model particle, resulting in high index surfaces, which account for the high catalytic activity for methane combustion. Using the xyz coordinate of each Pd atom, the local Pd-Pd bond distance and its variance in a real supported Pd nanoparticle are visualized, showing large strain and disorder at the Pd-Al2O3 interface. The results demonstrate that 3D atomic-scale analysis enables atomic structure-based understanding and design of supported metal catalysts.

Thermodynamic and kinetic isotope effects on hydrogen absorption by Pd–Al2O3 pellets

Palladium has been widely served as the separation material in the hydrogen isotope separation process on account of its unique hydrogenation characteristics. To reduce Pd pulverization effect and improve the gas flow rate, Pd–Al2O3 (Pd deposited on Al2O3) is commonly used as the separation material. It is crucial to grasp the hydrogen absorption and desorption thermodynamics and kinetic isotope effect of Pd–Al2O3 material, which favours raising the separation efficiency. In this work, the effect of PdO existing in the fresh Pd–Al2O3 on the thermodynamic character of Pd–Al2O3–H(D) system and the porous substrate effect on the hydrogen absorption kinetics of Pd–Al2O3 were investigated. It shows that PdO could totally reduce to Pd after primary hydrogen adsorption-desorption, leading to decreased hydrogen absorption capacity. However, it has no effect on the enthalpy and entropy values of Pd–Al2O3–H (D) system. The kinetic analysis of hydrogen adsorption of Pd–Al2O3 shows that the hydrogen absorption of Pd–Al2O3 could be fitted by Mample model, a special case of the JMAK model, and the interface-controlled model can be deduced. The porous substrate influences the rate of hydrogen adsorption of Pd–Al2O3. The SEM and XRD results show that after 60 hydrogen absorption-desorption cycles, the surface of Pd–Al2O3 pellet has a slight change with a pulverization rate of 3.3 wt%, although the high-strength alumina support can withstand the stress caused by Pd expansion during hydrogen absorption.

Effect of alumina on thermodynamic performance of palladium-H2 (D2) system

Palladium (Pd) has a strong hydrogen isotope effect, widely used in the Thermal Cycling Absorption Process (TCAP) separation column. However, due to pulverization and high gas resistance, Pd was usually dispersed in backing materials, such as alumina (Al2O3), to improve the service performance. In this paper, the microstructure of sponge palladium and Pd–Al2O3 pellet was observed by Scanning Electron Microscopy (SEM), and the crystalline structure before and after hydrogeneration was characterized by X-Ray Diffractometer (XRD). To study the support effect of backing material Al2O3 on the hydrogen/deuterium thermal absorption performance of palladium, the pressure-composition-temperature (P–C-T) curves were measured by series experiments. The result shows that Al2O3 acts as a support material between palladium particles to suppress the effect of the pellet expansion-shrinking during H2 (D2) absorption/desorption cycling. Alumina could provide free position for hydrogen atoms. And the inflection points of α phase to α+β phase shift from 0.02 to 0.09 for Pd–Al2O3 pellet/hydrogen system. The entropy and enthalpy variation of Pd/H (Pd/D) system in α+β phase are respectively −18.34 (−16.83) kJ/mol and −45.65 (−42.44) J/mol/K, which show no correlation with alumina. The H-D separation factor positively correlates with the quantity of hydrogen in the solid phase, and alumina could restrain the separation effect.

Geochemical controls on the distribution of PGE mineralisation in skarns formed during emplacement of the Platreef in the Northern limb of the Bushveld Complex, South Africa

The skarns in drill core SS330, extracted at the contact zone between the Platreef magmas from the Northern limb of the Bushveld Igneous Complex and the country rock dolomite, are investigated to understand processes that were involved in their genesis and have influenced the distribution of the platinum group element (PGE) mineralisation.The skarn lithologies are diopside-rich rocks (i.e. clinopyroxenite and feldspathic clinopyroxenite), and forsterite-rich rocks (olivine clinopyroxenite and serpentinite). The geochemical data show that the skarns were formed by metasomatic transformations of dolomite; such that Ca was pivotal in the genesis of diopside-rich rocks whereas Mg played a major role in the genesis of forsterite-rich rocks when compared to magmatic rocks from the Platreef. The metasomatic fluids involved in the skarn- and reef-forming processes were Si–Al–Fe-rich with variable amounts of the base metal sulphide (BMS) elements (e.g. Cu, Ni, Zn, Co) and originated from three mafic magmas. It is suggested that the fluids evolved along three crystallisation trends (i.e. upwards, downwards and inwards) leading to SiO2, Na2O, Fe2O3, Cu, Ni, Pt and Pd enrichments and Al2O3 depletion which resulted in the formation of the PGE and PGE-BMS reefs. On these bases, the proposed indices (e.g. SiO2/Al2O3, Na2O/Al2O3, Fe2O3/Al2O3, Si + Na + Fe/Al) can be used as lithogeochemical vectors to locate the position of the PGE reefs downhole in drill core SS330.

Biorefining of platinum group metals from model waste solutions into catalytically active bimetallic nanoparticles.

SummaryBacteria can fabricate platinum group metal (PGM) catalysts cheaply, a key consideration of industrial processes and waste decontaminations. Biorecovery of PGMs from wastes is promising but PGM leachates made from metallic scraps are acidic. A two‐step biosynthesis ‘pre‐seeds’ metallic deposits onto bacterial cells benignly; chemical reduction of subsequent metal from acidic solution via the seeds makes bioscaffolded nanoparticles (NPs). Cells of Escherichia coli were seeded using Pd(II) or Pt(IV) and exposed to a mixed Pd(II)/Pt(IV) model solution under H2 to make bimetallic catalyst. Its catalytic activity was assessed in the reduction of Cr(VI), with 2 wt% or 5 wt% preloading of Pd giving the best catalytic activity, while 1 wt% seeds gave a poorer catalyst. Use of Pt seeds gave less effective catalyst in the final bimetallic catalyst, attributed to fewer and larger initial seeds as shown by electron microscopy, which also showed a different pattern of Pd and Pt deposition. Bimetallic catalyst (using cells preloaded with 2 wt% Pd) was used in the hydrogenation of soybean oil which was enhanced by ~fourfold using the bimetallic catalyst made from a model waste solution as compared to 2 wt% Pd preloaded cells alone, with a similar selectivity to cis C18:1 product as found using a Pd‐Al2O3 commercial catalyst.

Synthesis of Mg-Doped Ordered Mesoporous Pd–Al2O3 with Different Basicity for CO, NO, and HC Elimination

Employing NH4HCO3 as pore-enlarging agent and P123 as a template agent, ordered mesoporous Mg-doped γ-Al2O3 with different basicity and large surface area have been successfully synthesized by a facile sol–gel approach by adjusting the content of magnesium and pH value. Specifically, the result alumina calcined at 1000 °C with large specific surface area (234.4 m2 g–1) and high pore volume (0.72 cm3 g–1) was obtained when the Mg content was 9 wt %. These materials with ordered mesostructure and advantageous structural properties were utilized as carriers of Pd–Al2O3 catalysts for the catalytic reaction of simulated exhaust automobile gases including CO, NO, and hydrocarbon (HC). The results revealed that the high catalytic performance of Pd/9Mg-OMA originated from its more appropriate basicity, greater PdO dispersion, and higher surface area.

Activity of varying compositions of Co–Ni–P catalysts for the methanolysis of ammonia borane

Various compositions of Co–Ni–P catalysts supported on a palladium-activated Al2O3 (Pd–Al2O3) substrate were synthesized, characterized and investigated for catalytic methanolysis of ammonia-borane (AB, H3NBH3). The Co–Ni–P/Pd–Al2O3 catalysts were synthesized by polymer-stabilized Pd nanoparticle-catalyzation and activation of the Al2O3 substrate support and the electroless deposition of cobalt-nickel (Co–Ni) metal particles on the surface of the Al2O3 support for a plating time of 30 min. The Co–Ni–P/Pd–Al2O3 catalysts are stable enough to be isolated as solid materials and characterized by X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDS) and Scanning Electron Microscopy (SEM). At 40 ± 0.5 °C, the isolable, re-dispersible and reusable catalysts were found active in the methanolytic dehydrogenation of ammonia-borane retaining up to 65% of its initial activity after five cycles. Rates of hydrogen evolution were used to determine the kinetics of methanolysis reaction. The ranges of examined catalyst particle amounts, AB concentrations and temperatures were 15.75–63 mg, 50–200 mM, and 30–55 °C, respectively. Hydrogen desorption was identified as the rate controlling step in the methanolysis reaction and using the data, the kinetic rate constant (k¨H2), the hydrogen desorption equilibrium constant (KH2), and the overall equilibrium constant (K1) parameters in a Langmuir–Hinshelwood rate expression were determined to be 1.4 mol/g-cat. s, 1.5918 L/mol and 1.5986 L/mol, respectively. Activation parameters such as enthalpy of activation (ΔH), entropy of activation (ΔS), and activation energy (Ea) that were obtained by Eyring and Arrhenius equations are reported for the various catalyst ratios.

Electrochemical Reduction of Carbon Dioxide to Ethane Using Nanostructured Cu2O-Derived Copper Catalyst and Palladium(II) Chloride

A method to facilitate the electrochemical reduction of carbon dioxide (CO2) to ethane (C2H6) was developed. The electrolyte used was aqueous 0.1 M KHCO3. Chronoamperometry, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, online gas chromatography, and nuclear magnetic resonance spectroscopy were used to characterize the electrochemical system and products formed. Carbon dioxide reduction using a Cu2O-derived copper working electrode gave ethylene (C2H4) and ethanol as main C2 products, with optimized faradic efficiencies (FE) of 32.1 and 16.4% at −1.0 V vs RHE. The active catalysts were ∼500 nm-sized crystalline Cu0 particles, which were formed via the reduction of the Cu2O precursor during the initial phase of the CO2 reduction reaction. When palladium(II) chloride was added to the electrolyte, C2H6 formation could be achieved with a significant FE of 30.1% at the said potential. The production of C2H4 was, on the other hand, suppressed to a FE of 3.4%. The alternate use of Pd0, PdO, or Pd–Al2O3 dopants did not afford the same conversion efficiency. Extensive mechanistic studies demonstrate that C2H4 was first produced from CO2 reduction at the Cu0 sites, followed by hydrogenation to C2H6 with the assistance of adsorbed PdClx. Interestingly, we discover that both Cu and PdClx sites are necessary for the efficient reduction of C2H4 to C2H6. The PdCl2 was “consumed” during the reaction, and a hypothesis for how it contributes to the reduction of CO2 to ethane is proposed.

Improved Stability of Pd/Al2O3 Prepared from Palladium Nanoparticles Protected with Carbosilane Dendrons in the Dimethyl Ether Steam Reforming Reaction

AbstractThe dimethyl ether steam reforming reaction to generate hydrogen was tested over 1 wt % Pd/Al2O3 catalysts prepared from different precursors. The conventional catalyst prepared by incipient wetness impregnation from palladium nitrate underwent strong deactivation under reforming conditions at 823 K because of metal sintering and carbon deposition. The same occurred over the catalyst prepared from preformed Pd nanoparticles protected with dodecanethiol. In contrast, catalysts prepared from Pd nanoparticles protected with carbosilane dendrons showed enhanced stability and good performance for the production of hydrogen. This was because of the formation of SiO2 at the Pd–Al2O3 interface that acted as a pinning center and prevented Pd migration under the reaction conditions as well as the accumulation of carbon.

Synthesis of polyethylene/polyester copolymers through main chain exchange reactions via olefin metathesis

The synthesis and characterization of linear polyethylene/polyester copolymers produced by olefin cross metathesis and subsequent catalytic hydrogenation are described. The success of the scrambling reaction between 1,4-polybutadiene and unsaturated polyester via olefin cross metathesis was confirmed by 1H NMR. Differential scanning calorimetric (DSC) measurements indicated that the size of the copolymer blocks varied gradually with the exchange ratio. Catalytic hydrogenation of the unsaturated copolymers with different exchange ratios was carried out using Pd–Al2O3. The crystallinity of the resulting hydrogenated polymers depended on the exchange ratios achieved during olefin cross metathesis. This suggests that the unique sequential structure imparted to the unsaturated polymers during metathesis was retained during the hydrogenation process.

Ppm level methane detection using micro-thermoelectric gas sensors with Pd/Al2O3 combustion catalyst films

The CH4 combustion performance of a micro-thermoelectric gas sensor (TGS) with Pd–Al2O3 catalysts prepared using two different methods, impregnation and colloid mixing, has been investigated. Three crystal phases of Al2O3, α, θ, and γ, were used for the catalyst support; the Pd/θ-Al2O3 catalyst showed the best combustion performance. The Pd/θ-Al2O3 catalyst exhibited the best dispersion of Pd nanoparticles as analyzed by both a pulse adsorption technique and transmission electron microscopy. The effect of the catalyst thickness on the H2 and CH4 detection performance of the TGS using this Pd/θ-Al2O3 catalyst prepared using a colloid mixing method was investigated. It was found that the response voltage, ΔV, of the TGS to CH4 increased with increasing film thickness, but the ΔV to H2 decreased. The TGS with the Pd/θ-Al2O3 catalyst whose thickness was 35μm could detect as low as 1ppm of CH4 in air by the thermoelectric conversion of the combustor heat to an electrical voltage.

Hydrogen isotopes separation using frontal displacement chromatography with Pd–Al2O3 packed column

Separation or purification of tritium isotopes is one of the key technologies in ITER. A set of frontal displacement chromatography (FDC) device was designed and constructed for hydrogen isotopes separation using palladium loading on/in alumina (Pd–Al2O3) as the separation material. The hydrogen isotopes separation experiments were carried out. It was found that deuterium abundance of the product was up to 98.5% and the average separation factor was as high as 64, under the condition of 273 K column temperature and 15 mL(NTP)/min flow rate, for a feed gas of 5%H2-5%D2-90%Ar. The deuterium recovery ratio was 42% in this separation test. The results showed that the separation performance of our FDC device was good by using Pd–Al2O3 as separation materials, and it suggested considerable potential for the applicability of FDC in hydrogen isotopes separation.

Highly dispersed heterogeneous palladium catalysts by the introduction of plant tannin into porous Al2O3 supports

Abstract A new strategy was provided by the introduction of plant tannins in porous Al2O3 to solve the problems of thermal migration of Pd during catalyst preparation, which ensured the preparation of heterogeneous Pd catalysts with well dispersion and superior activity. Compared with the conventional Pd–Al2O3 catalyst, the as-prepared heterogeneous Pd catalyst exhibited considerably improved Pd dispersion, which was highly active for the catalytic hydrogenation of olefins.

Deactivation behaviors of hybrid Fischer–Tropsch catalysts in the production of middle distillate from synthesis gas in a dual-bed reactor

Production of middle distillate (C10–C20) from synthesis gas (CO + H2) through hydrocracking of wax (>C21+) was carried out in a dual-bed reactor. Fischer–Tropsch catalyst (Co/TiO2) was used in the first-bed reactor to produce wax from synthesis gas, and a mesoporous Pd–alumina composite catalyst (Pd–Al2O3) was used in the second-bed reactor to produce middle distillate through hydrocracking of wax. Both Fischer–Tropsch synthesis function of Co/TiO2 catalyst and hydrocracking function of Pd–Al2O3 catalyst were deactivated during 100 h-hybrid Fischer–Tropsch synthesis reaction. It was revealed that deactivation behaviors of Co/TiO2 and Pd–Al2O3 catalysts were governed by different factors. Wax accumulation and Co sintering were responsible for deactivation of Co/TiO2 catalyst in the Fischer–Tropsch synthesis reaction. Loss of Pd dispersion and Pd surface area of Pd–Al2O3 catalyst was responsible for its decreased catalytic performance in the production of middle distillate through hydrocracking of wax.

Comprehensive Characterization and Permeation Analysis of Thin Pd/Al2O3 Composite Membranes Prepared by Suction‐Assisted Electroless Deposition

In this work, suction‐assisted electroless deposition technique is proposed to prepare high hydrogen permselective Pd/Al2O3 composite membranes. Effects of suction pressure and plating time on the surface morphology and denseness of the resulting composite membranes have been investigated. Scanning electron microscopy (SEM), electron probe microscope analysis (EPMA), x‐ray diffractometry (XRD), and nitrogen permeability technique were used to characterize these membranes. Furthermore, the hydrogen permeability and selectivity of the composite membranes were also studied. The experimental result shows that the nitrogen permeability of the palladium (Pd) top layer is decreased with lowering the suction pressure. Under suction pressure of 3 kPa and plating time of 1 hr, a membrane with a perfectly dense 4.5 µm thick Pd layer can be obtained, whose H2/N2 selectivity is approaching infinite. As compared to the conventional electroless deposition, via suction assistance, the adhesion between the Pd top layer and Al2O3 support of the resulting membranes can be remarkably improved, and the thickness attained for complete denseness can also be reduced effectively. Based on the hydrogen permeation results, it is found that the rate‐limiting step for the hydrogen permeation through the Pd top layer is controlled by the surface reaction. Furthermore, the activation energy is estimated as 18 kJ/mol. This value is in a good agreement with those reported in literature for Pd layers thinner than 10 µm.

XPS and TPD study of CO interaction with Pd–Al and Pd–Al2O3 systems

The metal–substrate and metal–metal interactions represent important effects determining the properties of supported catalysts. We investigate the CO adsorption mechanism on Pd particles supported on Al2O3 and Al by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). CO–metal interaction is investigated by TP desorption of CO and by XPS of C 1s intensities that exhibit three CO-related components. The dissociation activity is monitored as a rise of C 1s signal at 285 eV while the molecularly adsorbed CO exhibits the intensity at 287 eV. Pd–substrate interaction is determined from the Pd core- and valence-level binding energy variations. The influence of the Pd–Al alloying process on the Pd MNN Auger transition is shown. The studies show that the metal–substrate and metal–metal interactions play an important role in CO–Pd adsorption process.

Novel sputtering method for Pd–Al2O3 UV transparent conductive coatings

A thin film structure of Pd–Al2O3 was developed as a signal plate for ultraviolet (UV) vidicon television picture tubes. The Pd–Al2O3 films were deposited using a novel sputtering method. The 20Å thick Pd layer served as a conducting plate while the 30–50Å thick Al2O3 layer functioned as a transparent protective passivation film. The obtained signal plates had a sheet resistance of 2.5kΩ and a transparency coefficient of 71% at a wavelength of 250nm, which is suitable for near-UV applications. The main advantage of vidicons equipped with such signal plates is its less than 0.05nA dark current.

Temperature Oscillations Characterized by An Overall Activation Energy

Temperature oscillations obtained during the heterogeneous catalytic oxidation of ethanol on Pd-Al2 O3 in a dynamic calorimeter were characterized by an overall activation energy. This parameter was determined by a non-isothermal kinetic method using the minimum and maximum values of the oscillations temperature. Using the bifurcation diagram with the oxygen as a bifurcation parameter an E value between 27.6 and 28.2 kJ mol-1 was obtained. With ethanol as bifurcation parameter the E values lies between 28.1 and 31.1 kJ mol-1 for 3.5 to 4.0 vol% ethanol and between 25.8 and 27.6 kJ mol-1 for 4.0 to 4.7 vol% ethanol. These results have been discussed.

Pd/Ni–Al 2O 3–Al Tunnel Diode as High-Concentration-Hydrogen Gas Sensor

Pd/Ni–Al2O3–Al tunnel diodes were fabricated on a glass substrate for detection of high-concentration hydrogen gas. For Pd–Al2O3–Al diodes, the output signals were saturated at hydrogen partial pressures higher than approximately 0.1 Torr, whereas a Pd/Ni(22%)–Al2O3–Al diode showed output signals even at 50 Torr hydrogen partial pressure at room temperature. The concentration of hydrogen detectable with the Pd/Ni alloy metal–insulator–metal (MIM) diodes increased with an increase in the Ni content. The change in the barrier height at the Pd/Ni–Al2O3 interface upon introduction of hydrogen gas was also measured for various Ni contents in Pd/Ni alloys by means of the logarithmic derivative of the tunnel current and was found to be associated with the hydrogen sensitivity of the Pd/Ni alloy MIM junctions.

Kinetics and stereochemistry in the catalytic hydrogenation of acridine

Hydrogenation of acridine (1) using a commercial Pd–Al2O3 catalyst was kinetically and stereochemically studied under variable conditions. A consecutive pathway, (1)→ 9,10-dihydroacridine (2)→ 1,2,3,4,4a,9,9a,10-octahydroacridine (5), which competes with hydrogenative isomerization of (2) to 1,2,3,4,5,6,7,8-octahydroacridine (4)via 1,2,3,4-tetrahydroacridine (3) or (5) was suggested from the observed product distributions over the time course of the reactions and their kinetic simulation. Thus, selectivity for the hydrogenated products is found to be controlled by either kinetics or thermodynamics according to the reaction conditions. Thermodynamic stabilities of the products are discussed based on quantum chemical (MNDO) and molecular mechanics (MM2) calculations to rationalize the reaction scheme. The stereoisomers of (5) and perhydroacridine (6) were identified and quantified by detail analyses using g.c.–i.r. and 13C n.m.r. The stereoselectivity is governed by the preference in adsorption of the intermediates on the catalyst surface at a lower reaction temperature (150 °C); however, a higher temperature (250 °C) produces the thermodynamically stable products through equilibrium control.