Pd Zn Research Articles

Simultaneous Measurement of X-ray Absorption Spectra and Kinetics: A Fixed-bed, Plug-flow Operando Reactor

An inexpensive fixed-bed, plug-flow operando reactor is described in which X-ray absorbance and kinetic data can be measured simultaneously. Pt L3 (11.56 keV) XANES and EXAFS data were obtained on a 1.5% Pt/silica catalyst in borosilicate glass reactors of different diameters, 3–6 mm, and thicknesses, 0.3–1.2 mm, some of which are capable of operation at pressures up to about 40 atm. Additionally, polyimide tubular reactors with low absorbance can be used for lower energy edges of the 3d transition metals, or fluorescence detection for low concentration or highly absorbing supports. With the polyimide reactor, however, the pressure is limited to ~3.5 atm and the reaction temperature to about 300 °C. To validate the reactor, the rate and activation energies for the water-gas shift reaction on 2% Pd, 13.7% Zn on Al2O3 catalyst were within 15% of those obtained in a standard laboratory reactor, which is within laboratory reproducibility. In addition, the Pd K edge (24.35 keV) XANES and EXAFS data on pre-reduced catalyst were identical to that previously determined on a regular cell. The EXAFS data show that the degree of Pd–Zn alloy formation changes with reaction temperature demonstrating the importance of characterizing the catalyst under reaction conditions.

Studies on metal/n-GaAs Schottky barrier diodes: The effects of temperature and carrier concentrations

Metal/Semiconductor Schottky diodes were fabricated to study the effect of temperature and carrier concentrations on diode parameters, such as ideality factor and barrier heights. The diodes were formed on the epitaxial layers of metal organic chemical vapor deposition (MOCVD) grown n-GaAs with metals such as Al, Pd, and Zn–Pd deposited onto n-GaAs by thermal evaporation technique. Trimethyl gallium and AsH3 were used as Ga and As precursors, respectively, to grow GaAs on semi-insulating GaAs substrates at 600 °C and H2S was used for n-type doping in a horizontal reactor atmospheric pressure MOCVD system. The Schottky diodes were characterized by forward bias current-voltage measurements in the temperature range 130–300 K and capacitance-voltage measurement at room temperature and diode parameters such as ideality factor and barrier height have been evaluated. It is found that the Schottky barrier height decreases with decrease in temperature while the ideality factor increases. It is also observed that the barrier height increases linearly with the applied forward bias voltage and the rate of change of barrier height with voltage increases for higher carrier concentration of the semiconductor. The carrier concentration of n-GaAs was chosen in the regime 1×1016–8.2×1016 cm−3 so that the depletion region extends inside the semiconductor and the diode can be used as a III–V photovoltaic device.

A comparative study on the performance of mesoporous SBA-15 supported Pd–Zn catalysts in partial oxidation and steam reforming of methanol for hydrogen production

Abstract Using mesoporous SBA-15 (Santa Barbara Amorphous No. 15, a mesoporous material) as support, Pd–Zn nanocatalysts with varying Pd and Zn content were tested for hydrogen production from methanol by partial oxidation and steam reforming reactions. The physico-chemical characteristics of the synthesized SBA-15 support were confirmed by XRD, N 2 adsorption, SEM and TEM analyses. The PdZn alloy formation during the reduction of Pd–Zn/SBA-15 was revealed by XRD and DRIFT study of adsorbed CO. Also, the correlation between Pd and Zn loadings and PdZn alloy formation was studied by XRD and TPR analyses. The metallic Pd surface area and total uptakes of CO and H 2 were measured by chemisorption at 35 °C. The metallic Pd surface area values are in linear proportion with the Pd loading. The formation of PdZn alloy during high temperature reduction was confirmed by a shift in absorption frequency of CO on Pd sites to lower frequency due to higher electron density at metal particles resulted from back-donation. The reduced Pd–Zn/SBA-15 catalysts were tested for partial oxidation of methanol at different temperatures and found that catalyst with 4.5 wt% Pd and 6.75 wt% Zn on SBA-15 showed better H 2 selectivity with suppressed CO formation due to the enhanced Pd dispersion as well as larger Pd metallic surface area. The O 2 /CH 3 OH ratio is found to play a significant role in CH 3 OH conversion and H 2 selectivity. The performance of 4.5 wt% Pd–6.75 wt% Zn/SBA-15 catalyst in steam reforming of methanol was also tested. Comparatively, the H 2 selectivity is significantly higher than that in partial oxidation, even though the CH 3 OH conversion is less. Finally, the long term stability of the catalyst was tested and the nature of PdZn alloy after the reactions was found to be stable as revealed from the XRD pattern of the spent catalysts.

Thermochemistry of Pd–In, Pd–Sn and Pd–Zn alloy systems

The standard enthalpy of formation of several Pd–M alloys (M = In, Sn and Zn) has been measured using a high temperature direct drop calorimeter. The reliability of the calorimetric results has been determined and supported by using different analytical techniques: light optical microscopy, scanning electron microscopy equipped with electron probe microanalysis (EPMA with EDS detector) and X-ray powder diffraction analysis. The values of Δ f H (kJ/mol atoms) for the following phases were obtained for the formation in the solid state at 300 K: PdIn (49 at.%In): −69.0 ± 1.0; Pd 2In 3 −57.0 ± 1.0; Pd 3In 7: −43.0 ± 1.0; PdSn 2: −50.0 ± 1.0; Pd 2Zn 9 (77 at.%Zn): −33.7 ± 1.0; Pd 2Zn 9 (78 at.%Zn): −34.0 ± 1.0; Pd 2Zn 9 (80 at.%Zn): −35.0 ± 1.0. The results show exothermic values which increase from the Pd–Zn to the Pd–Sn and Pd–In systems; the data obtained have been discussed in comparison with those available in literature.

Stability of bimetallic Pd–Zn catalysts for the steam reforming of methanol

ZnO-supported palladium-based catalysts have been shown in recent years to be both active and selective towards the steam reforming of methanol, although they are still considered to be less active than traditional copper-based catalysts. The activity of PdZn catalysts can be significantly improved by supporting them on alumina. Here we show that the Pd/ZnO/Al 2O 3 catalysts have better long-term stability when compared with commercial Cu/ZnO/Al 2O 3 catalysts, and that they are also stable under redox cycling. The Pd/ZnO/Al 2O 3 catalysts can be easily regenerated by oxidation in air at 420 °C followed by re-exposure to reaction conditions at 250 °C, while the Cu/ZnO based catalysts do not recover their activity after oxidation. Reduction at high temperatures (>420 °C) leads to Zn loss from the alloy nanoparticle surface resulting in a reduced catalyst activity. However, even after such extreme treatment, the catalyst activity is regained with time on stream under reaction conditions alone, leading to highly stable catalysts. These findings illustrate that the nanoparticle surface is dynamic and changes drastically depending on the environment, and that elevated reduction temperatures are not necessary to achieve high CO 2 selectivity.

Effect of Zn addition on the water–gas shift reaction over supported palladium catalysts

The effect of Zn addition on the Pd turnover rate (TOR) for the water–gas shift (WGS) reaction was studied on 2 wt% Pd on alumina with Zn content varying from 0 to 19% Zn by weight. Over 85% of the Zn reacted with alumina to form zinc aluminate with a fraction reacting with Pd. The average Pd Zn coordination number as measured by extended X-ray absorption fine structure (EXAFS) increased monotonically from 0 to 4.5 as the Zn loading on alumina increased. At a loading of 2% Pd and 19% Zn on alumina the molar fraction of Zn in the Pd particles was 0.34, and the turnover rate increased by a factor of 13, as compared with the pure metallic Pd supported on alumina, to 3.6 × 10 −2 mol H 2 s −1 (mol exposed Pd) −1 at 280 °C, 6.8% CO, 8.5% CO 2, 21.9% H 2O, and 37.4% H 2 at 1 atm. The formation of the ordered 1:1 PdZn intermetallic compound by deposition of Pd on zinc oxide increased the average Pd Zn coordination number to 5.9 and the WGS turnover rate by a factor of 20 to 5.3 × 10 −2 mol H 2 s −1 (mol exposed Pd) −1. The bonding of adsorbed CO changed from 80% bridging on pure Pd to 90% linear on the 1:1 PdZn intermetallic compound as determined by diffuse reflectance infrared spectroscopy performed under WGS conditions. The fraction of CO binding linearly to Pd correlated linearly with the WGS rate per Pd on the surface. Because the dispersion of the samples was about the same, this result implies that the number of active sites can be counted by the fraction of CO binding linearly. A simple random substitution model for PdZn distribution was used to estimate the concentration of Pd and Zn on the surface, based on the coordination numbers measured by EXAFS. The correlation of the reaction rate with the types of sites on catalysts with six different Pd Zn compositions shows that the data is consistent with an active site composed of surface Pd atoms with surface coordination to 2 Pd and 4 Zn atoms, which is the coordination expected for the (111) plane of the 1:1 PdZn intermetallic compound. An alternate explanation for the promotional effect of Zn on Pd/alumina is that the formation of zinc aluminate breaks a deleterious Pd–alumina interaction.

Hydrogen production by oxidative methanol reforming on Pd/ZnO: Catalyst preparation and supporting materials

Pd and Pd–Zn alloy were supported on various supporting materials using impregnation, co-precipitation and microemulsion methods, and their catalytic performances in oxidative methanol reforming (OMR) were investigated. Pd/ZnO exhibited much higher selectivity than either Pd/Al 2O 3 or Pd/ZrO 2 in the OMR for hydrogen production. This was attributed to the presence of Pd–Zn alloy on the ZnO support. Elemental Pd on Al 2O 3 or ZrO 2 promotes methanol decomposition reaction and increases CO formation. Using a microemulsion method, a highly selective Pd/ZnO can be obtained with much lower Pd loading than that in samples prepared by co-precipitation. Modification of Al 2O 3 with ZnO produced a ZnAl 2O 4 phase, which was found to be a good support for the Pd/ZnO catalyst. Highly active and selective Pd/ZnO/ZnAl 2O 4 catalysts for the OMR reaction, containing much lower Pd loadings have been developed by impregnation of the supports with an aqueous solution of Pd(NO 3) 2 + Zn(NO 3) 2.

Direct synthesis of LPG from synthesis gas over Pd–Zn–Cr/Pd-β hybrid catalysts

The reaction of synthesis gas to liquefied petroleum gas (LPG) over Zn–Cr-based hybrid catalysts was systematically investigated in this paper. The results show that the synergistic effect, existed in the Zn–Cr-based methanol synthesis component and β zeolite, results in the high performance of hybrid catalysts for synthesis gas to LPG. Active metal Pd in the β zeolite mainly play the role of olefin hydrogenation, which promotes the reaction of the title-mentioned overall reaction. The ratio of SiO 2/Al 2O 3 in Pd-β has a large influence on the stability of hybrid catalysts for syngas to LPG. As the SiO 2/Al 2O 3 ratio increases, CO conversion slightly decreases, however, the catalytic stability of Zn–Cr-based hybrid catalysts is largely improved. The characterization results indicated that during the reaction, the decreasing of acidic sites is the main reason of catalyst deactivation.

Evaluation of Pd-based catalysts and the influence of operating conditions for autothermal reforming of dimethyl ether

A series of different Pd-based catalysts supported on ceramic monoliths were synthesized and tested in a screening study for autothermal reforming of dimethyl ether (DME). Alumina-supported Pd was shown to be very active for this reaction at temperatures between 350 and 400°C. Adding Zn to Pd/γ-Al2O3 decreased the activity of decomposition reactions leading to better reforming activity, and resulting in high selectivity to carbon dioxide. Pd-Zn/γ-Al2O3 was further evaluated in a parameter study varying oxygen-to-DME ratio, steam-to-DME ratio and temperature. The effect on the reformer performance of changing the operating conditions is discussed. The Pd-Zn/γ-Al2O3 catalyst generated carbon monoxide concentrations below 5%, and hydrogen concentrations close to 50%. The catalyst performance was significantly improved by preconditioning in hydrogen. Results from a first round of catalyst characterization studies suggest that Pd–Zn species are formed on the alumina support following reduction.

An easy way to Pd–Zn nanoalloy with defined composition from a heterobimetallic Pd(μ–OOCMe)4Zn(OH2) complex as evidenced by XAFS and XRD

The heterobimetallic lantern complex Pd(μ-OOCMe)4Zn(OH2) was found to be readily reduced with H2 under fairly mild conditions (150–250 °C, 5–10% H2/He) both in the carbon-supported and crystalline states to afford a Pd–Zn nanoalloy as evidenced by the ICP elemental analysis, EXAFS, XANES and XRD data.

One-step incorporation of Pd–Zn catalytic sites into organized mesoporous alumina for use in the oxidative steam reforming of methanol

In this paper we report the preparation of an organized mesoporous Pd–Zn/alumina system, by a new surfactant-assisted single-step sol–gel synthesis, performed in alcohol, using stearic acid as a chemical template, aluminum sec-butoxide as Al source and metal stearates both as Pd–Zn sources and as structural directing agents. The prepared materials have been characterized using a variety of techniques such as thermogravimetric analysis, atomic absorption spectroscopy, X-ray powder diffraction and N 2 adsorption–desorption, and preliminarily tested in the oxidative steam reforming of methanol (OSRM) in the temperature range 100–400 °C.

Controlled Synthesis of 3d–4d Heterobimetallic Complexes of a Symmetrical Tetraiminodiphenolate Macrocycle – Structural, Spectroscopic, and Redox Properties

AbstractThe mononuclear complexes [Rh(LH2)Cl2](ClO4)·3H2O (1) and [Pd(LH2)](ClO4)2 (2) of the tetraiminodiphenolate macrocycle L2– have been synthesized by the transmetallationreaction between [Pb(LH2)](ClO4)2, and RhCl3·xH2O and Na2[PdCl4], respectively. In these compounds, the uncoordinated imino nitrogen atoms are protonated and are hydrogen bonded to the phenolate oxygen atoms to stabilize them against hydrolytic cleavage. Using these mononuclear complexes, the following heterodinuclear complexes: [RhIIICl2(L)PdII](X) [X = ClO4 (3), PF6 (4)], [PdII(L)MII(H2O)2](ClO4)2 [M = Mn (5), Fe (6), Co (7), Ni (8)], [PdII(L)MII](ClO4)2 [M = Cu (9), Zn (10)]; and the dipalladium(II) complex [Pd2L](ClO4)2 (11) have been synthesized. The crystal structure determined for [RhCl2(L)Pd](PF6) (4) shows an axially elongated octahedral geometry for RhIII and a square‐planar geometry for PdII. In [PdII(L)CuII](ClO4)2 (9), both the metal ions lie in the flat plane of the macrocyclic ligand and they are positionally disordered with equal occupancies. The CuII center has an apically elongated square‐pyramidal geometry due to the weak perturbation by an oxygen atom of a perchlorate anion. Complexes 2, 10, and 11 on excitation at 400 nm exhibit luminescence at room temperature at 505, 437, and 443 nm, respectively. Spectrofluorimetric titrations of [PdII(LH2)]2+ with the acetate salts of zinc(II) and palladium(II) have shown that the formation of [Pd(L)Zn]2+ and [Pd2L]2+ complex species are accompanied by blue shift of luminescence with increased and reduced emission intensities, respectively. The hyperfine‐shifted 1H NMR spectral features of the paramagnetic compounds 7–9 have been analyzed by determining their longitudinal (T1) and transverse (T2) relaxation times. Cyclic and square‐wave voltammetric measurements have been carried out for complexes 7–9 in acetonitrile. For complexes 8 and 9, reversible one‐electron reduction occurs with E1/2 = –0.10 V for PdIICuII/PdIICuI and –0.80 V for PdIINiII/PdIINiI versus Ag/AgCl. Complex 7 undergoes irreversible reduction for cobalt(II) at –0.78 V. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Pd-Zn/Cu-Zn-Al catalysts prepared for methanol oxidation reforming in microchannel reactors

Methanol oxidation reforming is an important reaction in the process of hydrogen generation for PEMFC. In this paper, we report the wall coated catalysts in a microchannel reactor for methanol oxidation reforming. The preparation method of the wall coating catalyst was studied in detail, i.e., the sol–gel and solution-coating techniques. To prepare the catalysts for methanol oxidation reforming, the washing-coating layer of CuZnAl was prepared by the sol–gel technique, and then the active layer was coated on it by solution-coating technique with emulsion colloid containing Pd–ZnO particles. Both the supporting layer and catalyst on the stainless steel foils were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The formed wash-coating layer with ordered arrays of petaloid microcrystallites in layer surface and Pd–Zn alloy crystal phase can be clearly observed. The reaction experimental results indicated that the catalyst prepared have high activity and relatively high stability.

Investigation of the thermal properties of [Cd(hydet-en)2Pd(CN)4] AND [Zn(hydet-en)2Pd(CN)4] single crystals

Thermal properties of the single crystals have been investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques. The thermodynamic parameters such as activation energy and enthalpy and thermal stability temperature of the samples were calculated from the differential thermal analysis (DTA) and TG data. The activation energies for first peak of DTA curves were found as 496.65 (for Cd–Pd) and 419.37 kJ mol–1 (for Zn–Pd). For second peak, activation energies were calculated 116.56 (for Cd–Pd) and 173.96 kJ mol–1 (for Zn–Pd). The thermal stability temperature values of the Cd–Pd and Zn–Pd compounds at 10°C min–1 heating rate are determined as approximately 220.7 and 203°C, respectively. The TG results suggest that thermal stability of the Cd–Pd complex is higher than that of the Zn–Pd complex.

Growth and structural stability of well-ordered PdZn alloy nanoparticles

Despite many recent attempts to unravel the structure of novel PdZn alloys, which are promising catalysts in methanol steam reforming, a detailed study on the formation of Pd–Zn alloy particles and of their structural and thermal stability is still missing. We take advantage of the unique properties of epitaxially grown Pd particles embedded in layers of amorphous ZnO and mechanically stabilized by SiO 2, and present an electron microscopy study of the preparation of well-ordered PdZn alloy nanoparticles at surprisingly low reduction temperatures. These are formed by topotactic growth on the surface of the Pd nanoparticles and are structurally and thermally stable in a broad temperature range (473–873 K). At and above 873 K, partial decomposition of PdZn and the start of interaction with the SiO 2 support are observed.

The Experimental and Theoretical Study of Phase Equilibria in the Pd–Zn (‐Sn) System

The binary Pd‐Zn system was assessed in the scope of this paper. The experimental data from the literature were used for the assessment, together with our own data obtained by SEM‐EDS analysis and by the DTA. The paper is focused particularly on the Pd‐Zn system, but the Pd‐Sn and Sn‐Zn systems are also analysed in this paper. Finally, first prediction of the isothermal sections of the ternary Pd‐Sn‐Zn system was made.

Synthesis of Chiral, All-Carbon Quaternary Centers via Allylic Alkylation

Asymmetric allylic alkylations have been well explored; however, the enantioselective construction of all-carbon quaternary centers still remains a challenging goal. Using γ-acetoxy-α,β-unsaturated carbonyl compounds as π-allyl precursor and cyclic β-ketoesters as nucleophile in the presence of a Pd catalyst, Zn additive and chiral diaminophosphine oxide ligand (DIAPHOX), a variety of quaternary carbon compounds were synthesized with good to excellent enantioselectivity.

Zn1− x Pd x ( x =0.14–0.24): a missing link between intergrowth compounds and quasicrystal approximants

Our investigations in the Zn-rich portion of the Zn–Pd system are related to the idea that a prototypic 1-dimensional (1D) quasicrystal, as defined by the Fibonacci sequence, can be regarded both as a quasicrystal and as an incommensurately modulated crystal. Consequently, there are two different ways to achieve the atomic structure. Six different new structures, closely related to the γ-brass structure, in the Zn1−x Pd x system have been synthesized and contain Pd-centred Zn icosahedra. A misfit character of all structures with two distinct main subsets and satellite reflections is clear from the diffraction patterns. Among these six structures, two specific examples, Zn10.65Pd2.35 (Zn0.819Pd0.181) and Zn212Pd64 (Zn0.768Pd0.232), have been refined by single crystal X-ray diffraction using traditional refinement methods but also using the (3 + 1)D formalism (superspace group Xmmm(00γ)0s0). Analogies with the Fibonacci sequence are possible if we identify dimers of Pd-centred Zn icosahedra (DI) as short segments, S, and trimers of Pd-centred Zn icosahedra (TI) as long segments, L. By defining these S and L segments, Zn11Pd2 and Zn212Pd64 correspond to the two first simplest approximants of a hypothetical 1D quasicrystal.

Hydrogen production by oxidative methanol reforming on Pd/ZnO: Catalyst deactivation

The durability performance of Pd/ZnO for oxidative methanol reforming has been investigated. While Cu/ZnO lost its activity with a constant selectivity of CO formation, Pd/ZnO exhibited a more stable activity, but with increasing CO production throughout the reaction time on stream. Characterization of the catalyst using XRD and XPS revealed the formation of a Pd–Zn alloy, which was thought to be essential for the selective production of hydrogen. Based on the reaction and the catalyst characterization results, two routes were supposed for the deactivation of Pd/ZnO catalyst: surface fouling by deposited carbon and surface oxidation of the Pd–Zn alloy, breaking down the Pd–Zn alloy to produce more elemental Pd on a ZnO support. The catalyst can be regenerated in a H 2 reducing atmosphere at higher temperatures and in a diluted O 2 atmosphere at lower temperatures.

Hydrogen production by oxidative methanol reforming on Pd/ZnO

Pd/ZnO catalysts with Pd loadings of between 1 and 45% were prepared by impregnation and coprecipitation methods; their catalytic performances during oxidative methanol reforming for the production of hydrogen were investigated under various reaction conditions. The prepared catalysts showed high activity and selectivity towards hydrogen formation. At higher Pd loading (exceeding 5%), the coprecipitation method was superior to the impregnation method for the preparation of the catalyst. Highly active Pd/ZnO that is comparable to commercial Cu–Zn catalyst can be obtained with high Pd loadings. On the other hand, CO formation was greatly reduced by increasing the Pd loading. Excess amounts of Pd on the ZnO decreased the conversion of methanol and increased CO formation. XRD characterization of freshly prepared and spent catalysts confirmed the formation of the Pd–Zn alloy. The sizes of both the ZnO and the Pd–Zn crystallites affect the catalytic performance. A strong interaction between the Pd–Zn alloy and the ZnO support is believed to be essential for the selective formation of hydrogen. The reaction mechanism and the CO formation route are also discussed, based on the results that we obtained.