Pd Zn Research Articles

Hydrogen production by oxidative methanol reforming on Pd/ZnO catalyst: effects of the addition of a third metal component

Pd/ZnO was modified using a co-precipitation method by introducing a third metal component: Mg, Al, Zr, Ce, La, Ru or a first row transition metal element. The catalytic performance of each combination in oxidative methanol reforming was evaluated at atmospheric pressure. The addition of Cr, Fe or Cu into Pd/ZnO increases the selectivity to hydrogen production by decreasing CO formation. Conversely, CO formation was markedly increased by the addition of Co, Ni and Mg. Modification with Al, Zr, La, Ce, Ru or Mn also increased CO formation to different extent. Pre-reduction of the samples at a higher temperature resulted in much less formation of CO in most cases, other than Co and Mg. This was confirmed, from XRD characterization of the spent catalysts, to be related to Pd–Zn alloy formation.

Bi- and trinuclear oxalamidinate complexes of palladium as catalysts in the copper-free Sonogashira reaction and in the Negishi reaction

The binuclear complex [(acac)Pd( oxam)Pd(acac)] 1 ( oxam: tetraphenyl oxalic amidinate) has been prepared from H 2 oxam and Pd(acac) 2 in excellent yield. The complex was characterized by elemental analyses, mass spectroscopy, 1H NMR, 13C NMR spectroscopy and in the solid state by X-ray single crystal diffraction analyses. 1 consists of a bimetallic centrosymmetric unit in which the planar oxam ligand acts in a bis-chelating fashion. Each palladium center is in a planar environment. The complex 1 acts as highly selective pre-catalyst in the copper-free Sonogashira reaction between 4-bromoacetophenone and phenylacetylene. Its long-time catalytic activity is higher than that of the related binuclear complex 2 ( oxam: tetra- p-tolyl oxalic amidinate) or that of the trinuclear compound [(acac)Pd( oxam)Zn( oxam)Pd(acac)] ( 3), the solid-state structure of which was also determined by an X-ray structural analysis of single crystals. In addition, 2 is an active and extremely selective pre-catalyst for the Negishi reaction between 3,5,6,8-tetrabromophenanthroline and R–C C–ZnCl (R: Ph, ( i prop) 3Si) to form tetra-alkyne-substituted derivatives.

Comparative study between Zn?Pd/C and Pd/ZnO catalysts for steam reforming of methanol

Catalytic activity and CO2 selectivity over carbon black-supported Pd with the addition of Zn (Zn–Pd/C) was comparable to those of Pd/ZnO in methanol steam reforming. From the characterization of fresh Zn–Pd/C after H2 reduction pretreatment by means of BET surface area, XRD, TEM and the measurement of CO adsorption, Pd–Zn alloy particles were detected on Zn–Pd/C. This is also supported by high CO2 selectivity in methanol steam reforming over Zn–Pd/C as well as Pd/ZnO. In addition, Zn–Pd/C exhibited higher stability than Pd/ZnO in methanol steam reforming. After the reaction for the life test, the formation of zinc carbonate hydroxide such as Zn4CO3(OH)6·H2O was clearly observed on Pd/ZnO. It is expected that the deactivation is caused by covering active Pd–Zn alloy particles with zinc carbonate hydroxide. Although this phenomenon can also occur on Zn–Pd/C, the deactivation rate was much lower than Pd/ZnO because the amount of free ZnO is much smaller on Zn–Pd/C.

Comparative study between Zn–Pd/C and Pd/ZnO catalysts for steam reforming of methanol

Catalytic activity and CO2 selectivity over carbon black-supported Pd with the addition of Zn (Zn–Pd/C) was comparable to those of Pd/ZnO in methanol steam reforming. From the characterization of fresh Zn–Pd/C after H2 reduction pretreatment by means of BET surface area, XRD, TEM and the measurement of CO adsorption, Pd–Zn alloy particles were detected on Zn–Pd/C. This is also supported by high CO2 selectivity in methanol steam reforming over Zn–Pd/C as well as Pd/ZnO. In addition, Zn–Pd/C exhibited higher stability than Pd/ZnO in methanol steam reforming. After the reaction for the life test, the formation of zinc carbonate hydroxide such as Zn4CO3(OH)6·H2O was clearly observed on Pd/ZnO. It is expected that the deactivation is caused by covering active Pd–Zn alloy particles with zinc carbonate hydroxide. Although this phenomenon can also occur on Zn–Pd/C, the deactivation rate was much lower than Pd/ZnO because the amount of free ZnO is much smaller on Zn–Pd/C.

Theoretical study of segregation of Zn and Pd in Pd–Zn alloys

Surface segregation of Zn and Pd in PdZn alloys was investigated based on density functional calculations using periodic slab models. The segregation propensity of Pd–Zn films was found to be support-dependent: Zn showed a stronger propensity to segregate on Zn support than on Pd support; likewise, for Pd the propensity was stronger on Pd support than on Zn support. In self-supported 1:1 alloy films, segregation of neither Zn nor Pd was favorable. Segregation was suppressed due to the fact that Pd–Zn heteronuclear bonds are stronger than the corresponding homonuclear bonds Pd–Pd and Zn–Zn. However, in films rich in Zn or Pd, segregation of the dominant component was possible. For instance, when a monolayer of Pd is deposited on a Zn(0 0 0 1) film, segregation of Zn is calculated energetically favorable, in agreement with experimental observations. With increasing Zn concentration in the top surface layer, the corrugation of alloy films was reduced.

Hydrogen production by oxidative methanol reforming on Pd/ZnO catalyst: effects of Pd loading

Catalytic performances of Pd/ZnO in oxidative methanol reforming reaction were studied as a function of Pd loading. It was confirmed that the formation of Pd–Zn alloy is essential to the selective production of hydrogen. High active Pd/ZnO, comparable to commercial Cu-Zn catalyst, was obtained with higher Pd loading. Selectivity of the reaction was greatly increased by increasing Pd loading on ZnO. At higher Pd loadings (>5%), co-precipitation was superior to impregnation for the catalyst preparation. The catalytic performances were also discussed based on results from X-ray diffraction (XRD) characterization.

The preparation and characterisation of Pd-ZnO catalysts for methanol synthesis

The preparation and characterisation of Pd–ZnO catalysts for the synthesis of methanol from carbon dioxide and hydrogen has been examined. Calcination and reduction temperatures have been shown to have a major effect on conversion and selectivity in the system. Extensive characterisation shows the catalyst performance is associated with ZnO islands on a Pd interface, best prepared by depositionprecipitation. Reduction or calcination at high temperatures leads to the formation of a Pd–Zn alloy and to loss of performance.

Steam reforming of methanol over highly active Pd/ZnO catalyst

Pd/ZnO catalysts were investigated for steam reforming of methanol. Unlike precious metal-based catalysts, Pd/ZnO catalysts not only exhibited high activity, but more importantly very low selectivity to CO for methanol steam reforming. Under the conditions examined, the decomposition activity is minimal. The novel function is attributed to the formation of highly structured Pd–Zn alloy at moderate temperatures under mild reducing environments. The current catalytic system was characterized by TPR, transmission electron microscopy (TEM), H 2 chemisorption, and X-ray diffraction (XRD).

Bimetallic Pd–Zn silica-supported catalyst for CO hydrogenation. In situ DRIFT study

We report the behaviour of a silica-supported Pd–Zn catalyst in the hydrogenation of CO (total pressure=20 bar, T=548–598 K). Unusual high selectivity to higher hydrocarbons and higher oxygenated products (ethanol and acetaldehyde) was observed. An in-situ DRIFT study under reaction conditions, with analysis by mass spectrometry of products evolved, was carried out. The presence of surface acetate species is related to the production of acetaldehyde, which is proposed to be the primary C 2-oxygenated product.

Hydrophilic-Hydrophobic Balance of Bis(octylethylenediamine) Zn(II), Cd(II), and Pd(II) Chlorides in Methanol/Water and Chloroform/Water Systems

Abstract Double-chained bis(N-octylethylenediamine ( = OE)) complexes, such as Zn(OE)2Cl2, Cd(OE)2Cl2, and Pd(OE)2Cl2, were prepared and their selective solubilities in water/chloroform and water/methanol solutions were studied. Although all the complexes are poorly soluble in water, they showed clearly different solubilities in the water/organic mixed solvents depending on each amphiphilic character.

Difference in the reactivity of acetaldehyde intermediates in the dehydrogenation of ethanol over supported Pd catalysts

Catalytic performances of supported Pd catalysts for the dehydrogenation of ethanol were greatly modified upon the formation of Pd alloy phases. Over Pd–Zn, Pd–Ga and Pd–In alloys, acetaldehyde was selectively produced at lower conversion levels. With the increased conversion level, ethyl acetate was produced at the expense of acetaldehyde. The selectivities for the ethyl acetate formation exceeded that over a Cu/ZnO catalyst. Over metallic Pd, the decomposition of ethanol, C2H5OH → CO + CH4 + H2, occurred to a considerable extent. It was shown that the reactivity of acetaldehyde species over the Pd alloys was markedly different from that over metallic Pd. Over the Pd alloys, acetaldehyde species were stabilized and transformed into ethyl acetate by the nucleophilic addition of ethanol. By contrast, over metallic Pd, aldehyde species were rapidly decarbonylated to methane and carbon monoxide.

Selective PdZn Alloy Formation in the Reduction of Pd/ZnO Catalysts

Abstract Temperature programmed reduction (TPR) experiments were carried out over Pd/ZnO with different Pd loadings. Combined results with an X-ray diffraction method and X-ray photoelectron spectroscopy showed that a Pd–Zn alloy with a structure of PdZn was selectively produced in the course of TPR of Pd/ZnO. It was suggested that hydrogen retained in Pd spilled over to ZnO. The ZnO was reduced to metallic Zn, being converted to PdZn. The molar amounts of the ZnO reduced were practically the same as the those of Pd loaded. Hence, all the Pd loaded was completely transformed to the PdZn alloys upon reduction.

Palladium thin film contacts on p-type ZnSe: adjustment of electrical properties by reaction diffusion

The ternary system PdZnSe was investigated at 340°C. A complete isothermal phase diagram is given, containing a newly discovered ternary phase τ 2 of approximate composition Pd 62Zn 32Se 6. The reaction diffusion between Pd and ZnSe was investigated at 340°C and the diffusion path is described. Thin film palladium contacts were annealed at 250°C for 1–30 min. Lowest contact resistivities were reproducibly obtained after 2 min annealing. The results are related to the study of the ternary PdZnSe phase equilibria and Pd/ZnSe bulk reaction diffusion. A complex contact metallurgy is responsible for the attainment of good ohmic contact properties.

New catalytic functions of Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys in the conversions of methanol

Pd and Pt supported on ZnO, Ga2O3 and In2O3 exhibit high catalytic performance for the steam reforming of methanol, CH3OH+H2O→CO2+3HH2, and the dehydrogenation of methanol to HCOOCH3, 2CH3OH→HCOOCH3+2HH2. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities.

Ohmic contacts to p-ZnSe using Pd metallization

Thin film palladium contacts were prepared on etched p-ZnSe and annealed at 250°C for 1 to 30 min. Pd reacts vigorously with ZnSe as detected by optical and scanning electron microscopy as well as X-ray diffraction. Electrical contact properties were determined using a transmission line method which allowed for a statistical analysis of individual contact resistances. Annealing for only 1 min changed the initially nonlinear behaviour. Lowest contact resistivities were reproducibly obtained after 2 min of annealing. The results are related to an extensive study of the ternary Pd–Zn–Se phase equilibria and Pd/ZnSe bulk reaction diffusion. A complex contact metallurgy is considered to be responsible for the attainment of good ohmic contact properties.

Pd particles supported on ZnO (001) epitaxial thin films, evolution during HRTEM observations

Pd particles were condensed under UHV on ZnO (001) thin films epitaxially grown on mica single crystals. The epitaxial relationships between Pd and ZnO lattices were determined by electron diffraction, (111) Pd//(001) ZnO and [112] Pd//[100] ZnO. A tetrahedral shape of the particles larger than 20 nm was observed, using weak beam dark field electron microscopy. After strong irradiation conditions, an evolution of the structure was observed, which could be due to the formation of Pd Zn alloy.

Optimization of Pd/Zn/Pd/Au Ohmic Contacts to p-Type inGaAs/ InP

ABSTRACTThe electrical properties of Pd/Zn/Pd/Au based ohmic contacts to p-type In0 47Ga0 53As/ InP with an interposed superlattice of 50Å In047Gao 53As/ 50 Å InP have been investigated. In this study, several configurations of the Pd/Zn/Pd/Au metallization were fabricated with varying thicknesses of the Zn and interfacial Pd layers in the range 0 to 400 Å. The lowest values of specific contact resistance, ρc, were 1.2 x 10-5 Ω cm2 as-deposited and 7.5 x 106 Ω cm2 for samples annealed at 500 °C. In the as-deposited structures, ρc was reduced by an increase in thickness of both the Zn and Pd layers to 300 Å. For annealed samples, a critical thickness of the Zn ≥ 50 Å and Pd ≥ 100 Å layers was required in order to significantly reduce the magnitude of ρc. These results are consistent with a model of Pd/Zn contacts based on Zn doping of the interface. Studies of thermal stability of the contacts at 400 °C and 500 °C have shown that the Zn/Pd/Au and Pd/Zn/Pd/Au configurations were significanty lower in ρc at extended ageing times than the Pd/Au contacts.

Hydrogen and oxygen at metal/oxide interfaces

Using mostly electrochemical methods the chemical potential, diffusion coefficient, partial molar volume and the resistivity increment of hydrogen were determined at room temperature in internally oxidized PdAl, PdMg, PdZn and PdZr alloys. In all samples hydrogen is trapped at the formed metal/oxide interfaces, where irreversible trapping (with respect to anodic polarization) can be distinguished from reversible one. In the cases of Pd Al 2 O 3 and Pd MgO phase boundaries the oxide precipitates were characterized by electron microscopy with regard to their size and their orientation and, therefore, the coverage of the phase boundaries with irreversibly bound hydrogen could be calculated as 1.5·10 15 H/cm 2 or 7·10 14 H/cm 2, respectively, which is about one monolayer of the densest packed oxygen planes of the corresponding oxides. This equivalence and the irreversible nature of the bonding indicates the formation of OH bonds at the phase boundaries. During irreversible trapping a larger volume change is observed as compared to dissolution of hydrogen in regular octahedral sites of palladium. The measured partial molar volumes correspond to an increase of the phase boundary “thickness” of 0.3 to 0.9 Å between Pd and oxide. The irreversible component of hydrogen trapping disapears after annealing in the vapor of the alloying addition (Al, Mg, …) or in vacuum and appears again after a second treatment in air. As an explanation of this effect we propose that annealing in metal vapor or in vacuum, respectively, changes the composition at the phase boundary. At high oxygen activities oxygen segregates to the phase boundaries. In comparison to the oxygen in the oxide the segregated one is less strongly bound and, therefore, can be desorbed at low O-activities. Furthermore the missing bonds to the stable oxide allow the formation of OH bonds.

Metallurgical aspects and corrosion behavior of yellow low-gold alloys

Three copper-free type III yellow low-gold alloys (AgAuInPdZn) were investigated. Microstructural analysis was performed with x-ray diffraction, light microscopy, and an electron x-ray micro-analysis. The electrochemical behavior of the alloys was tested with standard potentiodynamic techniques in 0.9% saline solution and an artificial saliva and compared with that of a traditional high-gold alloy. The three alloys turned out to be two-phase mixtures of a fcc (matrix) and a bcc (island) phase. The yellow color, present in spite of the low-gold content, was caused by the disordered PdIn, island-type structure. An additional third structure was seen after etching. However, no distinct phase could be correlated with it. The corrosion behavior for the experimental alloys, based on cyclic polarization, can be considered acceptable in both electrolytes used, but is inferior to that of the control alloy.

The PdZn system for ohmic contacts to p-type GaP

Pd-based metallizations on p-type GaP have been studied. It has been found that the multi-layered Pd/Zn/Pd structure and the co-evaporated Pd/PdZn structure show good ohmic characteristics after heat-treatment. Specific contact resistivity, ρ c, as low as 6.9 × 10 −5 Ω-cm 2 for the surface concentration in the range of 2 ∼ 3 × 10 17 cm −3 has been obtained. This is the lowest ρ c value ever obtained for the same dopant concentration. Moreover, this Pd-based contact system does not deform even after heat-treatment up to the temperature of 650°C.