Amorphous PdZr Research Articles

ChemInform Abstract: Oxidation of Carbon Monoxide Over Palladium on Zirconia Prepared from Amorphous Pd-Zr Alloy. Part 2. The Nature of the Active Surface.

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Effects of Pretreatments on the Structure of Palladium-Containing Amorphous Alloys Followed by DSC

Amorphous PdZr, PdCuZr and PdCuSi alloy ribbons and powders are characterized by DSC, XRD and XPS in the as-received state and after treatments with oxygen, hydrogen or dilute hydrogen fluoride solution. Zr-containing alloys are shown to undergo substantial structural changes resulting in palladium enrichment on their surface, whereas no apparent changes in the bulk structure are found for PdCuSi. Catalytic activity and selectivity of the pretreated samples were tested in the hydrogenation of phenylacetylene.

Oxidation of carbon monoxide over palladium on zirconia prepared from amorphous PdZr alloy: II. The nature of the active surface

The oxidation of dilute mixtures of CO with dilute oxygen to C0 2 procedes efficiently at moderate temperatures over a catalyst prepared from in-situ activation of amorphous PdZr 2 alloys. The process of activation and the nature of the activated surface are investigated by XPS, UPS, and low pressure reaction experiments. It turned out to be crucial to perform the activation “ in situ” in a special reaction chamber. The simultaneous presence of oxidizing and reducing components in the gas leads to the formation of particles of a solid solution of oxygen in Pd (Pd + O) in intimate contact to an ion conducting phase of zirconia. The interface between the two types of particles is large and influences the chemical stability of the Pd + O. It is preformed in the rapid solidification process which leads to a disc-shaped morphology of the micrograins in the alloy. The arrangement is capable of storing chemically active oxygen. The superior catalytic performance of this system relative to a conventional impregnated Pd/ZrO 2 catalyst is traced back to the intimate contact of the oxygen storage phase, zirconia, with the active phase, Pd + O, which is stabilized against reduction to the pure metal by a constant supply of oxygen ions through the bulk of the supporting matrix and the surface of the particle.

Oxidation of carbon monoxide over palladium on zirconia prepared from amorphous PdZr alloy I. Bulk structural, morphological, and catalytic properties of catalyst

Catalysts highly active for the oxidation of carbon monoxide have been prepared by oxidation of amorphous and crystalline Pd 1Zr 2 metal alloys. The oxidation was carried out in situ; i.e., the fresh metal alloys were exposed to CO-oxidation conditions at temperatures of 473–553 K. Under these conditions, the initially almost inactive metal alloys underwent a series of solid state reactions and were finally transformed into stable and highly active microporous catalysts. Bulk structural characterization using X-ray diffraction and electron microscopy revealed that the final catalysts were made up of small intergrown and poorly crystalline domains of different palladium and zirconia phases. Palladium was present in three forms in the active catalysts, as a solid solution of palladium and oxygen, as metallic palladium, and as PdO. Zirconia existed in monoclinic and tetragonal forms. Comparative kinetic measurements carried out in a differential fixed bed reactor in the temperature range 300–500 K indicated that the activity of the palladium species in the catalysts derived from the metal alloys was more than an order of magnitude higher than the activity of the palladium species in a palladium on zirconia catalyst prepared by impregnation. The comparison of the activities was based on turnover frequencies calculated on the basis of accessible palladium sites determined by CO chemisorption. The higher activity of the catalysts derived from the metal alloys is attributed to their unique structural and chemical properties leading to extremely large interfacial areas and possibly to enhanced oxygen transfer via the solid-solid interphase.

In-Situ X-Ray Diffraction Study of Metal Hydride Formation in Palladium and Palladium-Based Alloy Electrodes under Cathodic Hydrogen Charging

Abstract An in situ X-ray diffraction (XRD) technique reported earlier was improved and employed to study the hydride formation in Pd and Pd–Au alloy foils and amorphous Pd–Zr alloy ribbons during cathodic hydrogen charging or its reverse process. The phase transformation from α- to β-PdHx in the Pd electrode, accompanied by an increase in the cell volume, was demonstrated to take place by the formation of a β phase domain in the bulk during the cathodization. The lattice constant was discontinuously changed from a\simeq3.92 to 4.04 Å and such a discontinuous change was observed on the Pd90Au10 alloy as well. However, no phase transformation was observed on the Pd–Au alloys with the Pd content of 70 at.% or below, where the lattice constant increased continuously with an increase in the negative potential for the cathodic polarization. No crystallization in the amorphous Pd–Zr alloy ribbon electrodes was detected after hydrogen charging.

Amorphous PdZr alloys for water electrolysis cathode materials

Amorphous Pd 0.35Zr 0.65 alloys prepared by a melt—quench technique were investigated for their possible use as water electrolysis cathodes. The Tafel—Volmer reaction route of the hydrogen electrode reaction was concluded. Kinetic parameter values of the constituent steps were evaluated by a transient technique. The electrocatalytic activity for cathodic hydrogen evolution on the as-obtained electrode was 10 2 times lower than Pd foil electrodes, but it was improved after the electrodes were treated in acids, typically aqueous HF. The activity in the highest state exceeds that of Pd by one order of magnitude. SEM and XPS observations indicated that the improvement in activity is due to increased surface Pd concentration after removal of a Zr-enriched surface layer which was produced during fabrication of the amorphous alloy specimens. Crystalline alloy electrodes prepared by heat treatment of the amorphous alloy were very brittle but showed electrocatalytic activity close to that of the amorphous electrodes.

Short-range structure of hydrogenated amorphous PdZr alloys by pulsed neutron total scattering

The short-range structure of deuterided amorphous PdZr alloys was observed using pulsed neutron total scattering with a spallation neutron source installed at a 500 MeV proton booster synchrotron. The results show that deuterium atoms characteristically modify the local arrangements of metal atoms surrounding a deuterium atom in amorphous PdZrD alloys and are distributed in several different types of polyhedral hole. In amorphous Pd 0.35Zr 0.65D x , for example, a deuterium atom occupies a tetrahedral site at x < 0.2 and an octahedral site near x = 0.4. A further increase in the deuterium content leads to a local arrangement of five metal atoms surrounding a deuterium atom. Even in amorphous alloys metal atoms prefer to have hydrogen atom neighbours similar to the topological arrangement found in the corresponding crystalline metal hydrides.

Ultrasonic properties of superconducting amorphous PdZr

Propagation of longitudinal waves in superconducting amorphous Pd 30Zr 70 has been studied down to temperatures as low as 0.4 K. A strong decrease of the absorption is observed for temperatures lower than 1.5 K whereas no change of the absorption occurs when passing through the superconducting transition temperature. Below 1.5 K the absorption becomes strongly magnetic-field dependent. We compare its overall behaviour to a recently developed theory. At the lowest temperatures, the temperature variation of the sound velocity exhibits a contribution arising from resonant interaction of phonons with tunnelling centers, also present in this material.