Effect of palladium precursor and preparation method on the catalytic performance of Pd/SiO2 catalysts for benzene hydrogenation

Supported Pd catalysts on silica were prepared by different synthesis methods using Pd(Ac)₂ and PdCl₂ as salts precursors. The obtained materials were characterized by X-Ray Diffraction (XRD), H₂ chemisorption, and temperature programmed desorption of hydrogen (H₂-TPD). The catalytic performances of these catalysts have been evaluated in the hydrogenation of benzene. The obtained results show that metal dispersion and catalytic activity are strongly dependent on the salts precursor and the method of preparation of the catalyst. The catalysts prepared by hydrazine reduction exhibit higher activity in benzene hydrogenation than that by the polyol reduction method. Moreover, the catalyst prepared with palladium acetate showed higher catalytic activity than those prepared with palladium chloride. The superior catalytic performance of this catalyst in the hydrogenation of benzene was ascribed to a significantly better dispersion of Pd particles on the silica support.     

钯纳米颗粒的形貌控制合成

为优化钯纳米颗粒的化学还原法制备工艺,本文以氯钯酸(H₂PdCl₄)为前驱体,抗坏血酸(C₆H₈O₆)为还原剂,聚丙烯酸钠(PAAS)为表面活性剂制备钯纳米颗粒。采用正交实验探究不同工艺参数对钯纳米颗粒粒径和形貌的影响。通过 X射线粉末衍射仪(XRD)、场发射扫描电子显微镜(FE-SEM)、透射电子显微镜(TEM)及电化学工作站对制备产物的结构、物相、形貌、电催化性能进行了表征。结果表明:在相同的工艺体系下,通过温度的改变,40 ℃条件下可以得到粒径大小为64.5 nm,球形度较好,分散性高的钯纳米颗粒;90 ℃条件下可以得到粒径大小为45.9 nm的立方体钯纳米颗粒。所制备的球形和立方体钯纳米颗粒对甲酸的电氧化催化活性分别为商业钯黑的1.57倍和1.49倍,在催化剂制备领域有广泛的应用前景。     

Effect of palladium on hydrogen charging of titanium cathodically polarized in acid media

A study of cathodic hydrogen charging of titanium in a 3N H₂SO₄ solution at 25° C showed that the intensity of hydrogen charging of cathodically polarized titanium is sharply reduced if a layer of palladium is electrodeposited on the titanium surface. A similar effect is observed if trace quantities (10^–6 mole/l) of PdCl₂ are introduced into the acid solution or when titanium is in an electrical contact with a palladium specimen.     

Physicochemical and catalytic properties of palladium supported on poly(o-methoxyaniline)

Palladium was introduced into a conjugated polymer poly(o-methoxyaniline) (POM) by reacting the powdered polymer with aqueous solution of PdCl₂ of low acidity (PdCl₂: 2.3 × 10⁻³ mol/dm³, HCl: 0.66 × 10⁻³ mol/dm³). Various Pd²⁺ complexes with Cl⁻, H₂O, OH⁻ ligands coexisted in this solution but predominated [PdCl₂(H₂O)₂] ones. Several techniques like X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron and Raman spectroscopy, extended X-ray absorption fine structure have been used to characterize the poly(o-methoxyaniline)-Pd systems. In particular, the state of Pd species in the Pd/POM of various content of palladium (2-8 wt.% Pd) and chemical changes in the polymer matrix induced by insertion of palladium were studied. The protonation and redox reactions involved on palladium incorporation resulted in palladium ions and Pd metal in the final samples. Metallic Pd produced due to spontaneous reduction of palladium ions by the polymer formed large crystalline particles 200-1000 nm in size. The Pd²⁺ species in the form of anionic complexes like [PdCl₄]²⁻ acted as the counterions at low content of palladium (2-4 wt.% Pd). At high palladium content (8 wt.% Pd), several atoms like Cl, N and/or O were identified by extended X-ray absorption fine structure (EXAFS) technique in the nearest environment of Pd atoms. The structural groups of POM (like N groups and/or OCH₃) as well as H₂O, OH⁻ molecules are, therefore, considered as probable species in the coordination sphere of palladium. The catalytic properties were studied for the as-prepared Pd/POM and the samples additionally reduced with aqueous solution of NaH₂PO₂. They were used in the hydrogenation of CC bonds in maleic acid (MAC) and CO groups in 2-ethylanthraquinone (eAQ) at 60 °C and atmospheric pressure of hydrogen using xylene-octanol-2 or water medium. The correlation between Pd/POM activities and the content of Pd metal was found. Activation of the as-prepared Pd/POM with NaH₂PO₂ improved their catalytic properties. Much higher and much stable activities were then obtained in both MAC and eAQ hydrogenation reactions.     

Hydrogen transport by group 5 metals: Achieving the maximal flux density through a vanadium membrane

Hydrogen transport by 100-μm-thick vanadium and palladium membranes was studied in the pressure range from 1 × 10^?8 to 4.5 × 10^?1 MPa at a temperature of 400°C. Both sides of the vanadium membrane were covered by 2 μm of palladium (Pd-V-Pd) for facilitating the dissociative absorption and associative desorption of H₂ molecules. At low pressures, hydrogen flux densities through vanadium and palladium membranes are nearly the same; at high pressures, the flux through the vanadium membrane becomes 16 times larger than the flux through the palladium membrane and attains a value of 2.4 scc cm^?2 s^?1. This flux of permeating hydrogen is larger than all values ever observed earlier for membranes made of group 5 metals or any other unsupported metal membranes.     

A novel route for the synthesis of nanosize particles of metallic palladium

A novel route for the synthesis of metallic palladium consisting of nanosize particles has been reported. The synthesis is based on (a) the addition of tetramethylammonium hydroxide (TMAH) to the aqueous solution of PdCl₂, and (b) autoclaving of this precipitation system at 160 °C. The distribution of the nanosize particles of metallic palladium has its mean value at about 18 nm. The mechanism of the metallic palladium formation is briefly discussed.     

High-temperature mechanical properties of Inconel-625: role of carbides and delta phase

Based on the temperature sensitivity characteristics, high-temperature tensile tests were performed at different temperatures and after different solid solution treatments to investigate the effect of Cr-rich M₂₃C₆, Nb-rich MC and the delta phase on the mechanical properties of Inconel 625. The experimental results indicated that the Cr-rich M₂₃C₆ carbides and the Nb-rich MC carbides decomposed at 700°C, which could be the reason for resulting tensile strength anomaly that was observed in a narrow temperature range from 650 to 700°C during the tensile tests at different temperatures. For the samples subjected to a prior solid solution treatment, the size of the δ phase was found to increase with the solution treatment temperature, whereas the elongation at fracture decreased.     

Microwave-assisted synthesis of palladium nanocubes and nanobars

Microwave was employed in the shape-controlled synthesis of palladium nanoparticles. Palladium nanocubes and nanobars with a mean size of about 23.8 nm were readily synthesized with H₂PdCl₄ as a precursor, tetraethylene glycol (TEG) as both a solvent and a reducing agent in the presence of PVP and CTAB in 80 s under microwave irradiation. The structures of the as-prepared palladium nanoparticles were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and ultraviolet-visible (UV-vis) absorption spectroscopy. The formation of PdBr₄²⁻due to the coordination replacement of the ligand Cl⁻ ions in PdCl₄²⁻ ions by Br⁻ ions in the presence of bromide was responsible for the synthesis of Pd nanocubes and nanobars. In addition, a milder reducing power, a higher viscosity and a stronger affinity of TEG were beneficial to the larger sizes of Pd nanocubes and nanobars.     

Preparation of metallic silver from Ag2S slurry by direct hydrogen reduction under hydrothermal conditions

Metallic silver has been prepared by direct hydrogen reduction of Ag₂S in basic slurry under hydrothermal conditions. The XRD pattern and SEM micrographs of the reaction product are given. Metallic silver was predominant in the reduction product obtained by using ZnO as additive and PdCl₂ as catalyst at 220 °C, pH 12.0 and 2–3 MPa of partial hydrogen pressure for 4 h.     

XPS study of palladium sensitized nano porous silicon thin film

Nano porous silicon (PS) was formed on p-type monocrystalline silicon of 2–5 Ω cm resistivity and (100) orientation by electrochemical anodization method using HF and ethanol as the electrolytes. High density of surface states, arising due to its nano structure, is responsible for the uncontrolled oxidation in air and for the deterioration of the PS surface with time. To stabilize the material PS surface was modified by a simple and low cost chemical method using PdCl₂ solution at room temperature. X-ray photoelectron spectroscopy (XPS) was performed to reveal the chemical composition and the relative concentration of palladium on the nanoporous silicon thin films. An increase of SiO₂ formation was observed after PdCl₂ treatment and presence of palladium was also detected on the modified surface. I–V characteristics of Al/PS junction were studied using two lateral Al contacts and a linear relationship was obtained for Pd modified PS surface. Stability of the contact was studied for a time period of around 30 days and no significant ageing effect could be observed.     

Electrochemical production of Palladium and its use as indicator electrode

The electrodeposition of palladium from electrolytes containing palladium chloride along with potassium-dihydrogen phosphate, sodium fluoride or oxalic acid was studied under a variety of condition. The conditions used were: 1.0 M palladium chloride, 0.15 M potassium dihydrogen phosphate, pH 1.5 at 298 K and current density 0.85 mA cm^?2/ The current efficiency was calculated and the crystal structure confirmed by X-ray analysis. Spectrophotomteric and chemical analysis reveals that the purity of the metal is better than 99%. Palladium electrode can function satisfactorily as indicator electrode for hydrogen ions in acid-base titrations. Titration of acids with sodium hydroxide and of salts of weak acids with hydrochloric acid showed that palladium can indicate very accurately the end point. The end point coincide to great extent with the visual values with an error not more than ± 2%.     

Charge Ordered Insulating State in DODHT Salts

Radical salts based on DODHT [(1,4-dioxane-2,3-diyldithio)dihydrotetrathiafulvalene] are semiconductors at ambient pressure, but some salts (PF₆, AsF₆, and BF₄·H₂O salts) exhibit supuerconducting transition under hydrostatic pressure. X-ray diffraction and magnetic susceptibility measurements at ambient pressure for PF₆ salt revealed that the insulating phase at ambient pressure corresponds to a charge ordering (CO) state with stripe-type charge pattern. While the similar magnetic behavior was observed for BF₄ salt, temperature dependence of magnetic susceptibility of AsF₆ salt is quite different from those of the other salts.     

Preparation and catalytic activity of nanostructured Pd catalysts supported on hydrogenotitanate nanotubes

Nanostructured palladium particles (nanorods, icosahedra, cubes) were synthesized in aqueous solution using a seeding-mediated approach with a structure-directing agent. These nanostructured Pd particles were then impregnated onto hydrogenotitanate nanotubes using two different impregnation procedures. The as-prepared catalysts were then tested in the selective hydrogenation of cinnamaldehyde at 323 K under 10 bars of H₂. The selectivity is influenced by the morphology of the Pd nanostructured particles with a higher selectivity into saturated alcohols when the proportion of (111) Pd sites increases.     

Superconducting and electrical properties of rhenium nitride and amorphous rhenium prepared by ion implantation

Different rhenium nitrides were prepared by homogeneous implantation of nitrogen ions into rhenium thin films at various temperatures as a function of N concentration. At 9 at % N new x-ray lines appeared in addition to the hcp rhenium phase. The formation of this phase, which could not be identified, was accompanied by a large increase in stress, of the order of 10¹¹ dynes/cm². Rhenium nitride with fcc structure was formed between 13 and 50 at %. For ReN a lattice parameter of 4.021±0.002 Å, a maximumT _cvalue of 4.5–5.0 K, and a resistivity ρ_RT of 250 µΩ-cm were obtained. Both rhenium nitride phases decomposed above 645 K. An x-ray amorphous phase was obtained after implantation of 40 at % N at LNT. This amorphous phase had aT _cvalue of 6.0 K and a ρ_RT value of 300 µΩ-cm. Transformation into an amorphous Re phase was also observed after implantation of 20 at % P. The maximumT _cvalue of this phase was 7 K. This phase decomposed at about 925 K.     

Aqueous co-precipitation of Pd-doped cerium oxide nanoparticles: chemistry, structure, and particle growth

Nanoparticles of palladium-doped cerium oxide (Pd–CeO₂) have been prepared by aqueous co-precipitation resulting in a single phase cubic structure after calcination according to X-ray diffraction (XRD). Inhomogeneous strain, calculated using the Williamson–Hall method, was found to increase with palladium content, and the lattice contracts slightly, relative to nano-cerium oxide, as palladium content is increased. Moreover, high resolution transmission electron microscopy reveals some instances of defective microstructure. These factors combined imply that palladium is in solid solution with CeO₂ in these nanoparticles, but palladium (II) oxide (PdO) peaks in the Raman spectra indicate that solid solution formation is partial and that highly dispersed PdO is present as well as the solid solution. Nevertheless, the addition of palladium to the CeO₂ lattice inhibits the growth of the 6% Pd–CeO₂ particles compared to pure CeO₂ between 600 and 850 °C. Activation energies for grain growth of 54 ± 7 and 79 ± 8 kJ/mol were determined for 6% Pd–CeO₂ and pure CeO₂, respectively, along with pre-exponential Arrhenius factors of 10 for the doped sample and 600 for pure cerium oxide.     

Highly sensitive room-temperature hydrogen sensor based on Pd-functionalized titania nanotubes prepared at appropriate electrolyte temperature

Hydrogen sensor based on palladium (Pd) nano-rings supported by titania nanotubes (TNTs) has been investigated at different electrolyte temperatures. The electrolyte temperature, which affected the morphology of the TNTs and the sensitivity of the hydrogen sensor, varied from 0 to 80 °C. When the TNTs were prepared at 30 °C, they had a relatively regular nanotube structure, and the follow-up prepared sensor presented a high room-temperature hydrogen sensitivity of 97.49% at 1.9% hydrogen concentration.     

Transformations of neat and heated struvite (MgNH4PO4⋅6H2O)

Struvite (MgNH₄PO₄?6H₂O) is the mineral phase of one of the urinary tract stones of humans and animals, besides being an important phase seen in waste water treatment and purification plants. Single-phase and highly crystalline struvite powders were synthesized in this study at room temperature in aqueous solutions containing dissolved salts of MgCl₂?6H₂O and (NH₄)₂HPO₄ at the Mg/P molar ratio of 1.00. As-synthesized, crystalline struvite powders (neat powders) completely transformed into cryptocrystalline apatitic calcium phosphate (Ap-CaP), in less than 72 h, when soaked in Ca-containing saline solutions at 37 °C. Moreover, crystalline struvite powders heated at temperatures from 90° to 200 °C became x-ray amorphous. Amorphous powders stirred for 4 h at room temperature in water containing dissolved diammonium hydrogen phosphate recrystallized back into struvite.     

Growth kinetics of Pd2Si from evaporated and sputter-deposited films

The growth kinetics of Pd₂Si formed by palladium films either vacuum deposited or sputter deposited onto vacuum-evaporated, sputter-deposited or single-crystal silicon substrates were investigated by backscattering spectrometry (BS). The impurity contents of the films and substrates were characterized by both secondary ion mass spectrometry and BS. In all cases, Pd₂Si is the only silicide phase detected after vacuum annealing up to 300°C and the growth mechanism is always diffusion limited. The rate of the diffusion was found to differ and the activation energies for Pd₂Si formation varied between 0.9 and 1.5 eV. These changes are correlated with the presence of impurities in the deposited palladium and silicon films. Also, sputtered palladium films do not form epitaxial Pd₂Si layers on (111) Si substrates as vacuum-deposited palladium films do. The conclusion is that rates of reaction and morphology of Pd₂Si layers grown by thermal annealing depend on the deposition system.     

Thermal reactions between aluminum and palladium layered films

Phase formation and reaction during annealing in thin film couples of aluminum and palladium were studied by transmission electron microscopy, X-ray diffraction and Auger depth profiling techniques in the temperature range 200– 500°C. The known bulk phases of Al₃Pd₂, AlPd, Al₃Pd₅ and AlPd₂ were confirmed and two additional aluminum-rich phases were tentatively identified as Al₃Pd and Al₄Pd. In all the samples studied the reactions were initiated by the formation of the β-AlPd phase, which was then followed by the formation of the Al₃Pd₂ phase. Subsequent reactions caused precipitation of AlPd₂ particles if unreacted palladium was present, otherwise Al₄Pd particles were precipitated. The reaction rate was usually reduced if single-crystal, instead of polycrystalline, palladium film was used.     

Synthesis and characterization of thorium phosphate phases by spray pyrolysis: chemistry of thorium phosphates

This paper describes the synthesis of some thorium phosphate compounds with different Th/P ratio (1/2, 2/3 and 3/4) by a spray pyrolysis technique. The so-prepared rough compounds were annealed at different temperatures for 2 h and then analyzed by mainly X-ray diffraction on powder and infrared spectroscopy. Every rough compound is composed by very badly crystallized ThO₂ phase polluted by carbon residue. An annealing treatment at 800 °C leads to the thorium diphosphate phase, α-ThP₂O₇ in every case. At 900 °C, such a phase is decomposed into a thorium phosphate diphosphate phase (Th₄(PO₄)₄P₂O₇, called TPD). However, a thorium excess in the initial mixture (Th/P = 3/4) leads also to observe the ThO₂ phase. The TPD phase is stable up to 1200 °C and does not react with the ThO₂ compound. Beyond 1200 °C, the TPD phase is slowly decomposed into a thorium phosphate compound which should be a thorium oxide phosphate; this compound does not contain any diphosphate species.