Amorphous Alloy Precursors Research Articles

Microstructure, surface properties, and CO oxidation properties of oxidized Zr2Pd alloy glass

Pd-dispersed composite catalysts were derived from amorphous Zr2Pd-based alloys; Zr65Pd35, Zr65Pd30Pt5, and Zr60Pd35Ce5 by heat treatment at 800 °C in air, and their CO oxidation reactivities were studied. The results of the X-ray diffraction (XRD) analysis indicated that the PdO phase mainly formed in the oxidized Zr65Pd35, and the metallic Pd phase formed in the oxidized Zr65Pd30Pt5 and Zr60Pd35Ce5. The scanning electron microscopic (SEM) images showed that the size of the Pd (PdO) particles increases by the addition of Pt or Ce into Zr–Pd metallic glass. The light-off CO oxidation reactivity and the frequency factor of oxidized Zr65Pd35 were higher than the others. This result was consistent with the amount of adsorbed CO, corresponding to the specific surface area of Pd phase. The CO oxidation reactivity was influenced not only by the morphology and the microstructure but also by the chemical state. Raman spectra and X-ray photoelectron spectra (XPS) indicated the existence of active oxygen species on the surface of Pd (PdO) particles in these catalysts. Thus, a novel type oxidation catalyst was achieved on the Pd-based materials by developing from amorphous alloy precursors and characterized by various methods.

Binary Magnesium-Based Amorphous Alloy Precursors in the Synthesis of Methyl Isobutyl Ketone

Results of our recent studies on the use of M-Mg(O) (M = Cu, Pd or Ni) amorphous alloy powders as catalyst precursors in the one-step conversion of acetone to methyl isobutyl ketone (MIBK) in the presence of hydrogen are summarized. Samples prepared by mechanical alloying and mechanically induced self-sustaining reaction (MSR) produce catalyst materials with varying catalytic properties. Precursor materials, in most cases, could be transformed by controlled oxidation into catalysts of enhanced activities and selectivities.

Microstructural and Auger Microanalytical Characterization of Cu-Hf and Cu-Ti Catalysts

Degradation processes occurring at the surface and in the bulk of Cu-based amorphous alloys during cathodic hydrogen charging were used for promoting the catalytic activity of such alloys. These processes modifying the structure, composition, and morphology of the substrate proved to be useful methods for transforming Cu-Hf and inactive Cu-Ti amorphous alloy precursors into active and durable catalysts. Indeed, their catalytic activity for dehydrogenation of 2-propanol increased up to a conversion level of approximately 60% at selectivities to acetone of about 99% for Cu-Ti and to conversion of approximately 90% at selectivities of approximately 95% for Cu-Hf. Previous attempts carried out by aging in air or hydrogen charging from the gas phase resulted in a maximum conversion level up to 15% for Cu-Hf and up to 3% for Cu-Ti. High resolution Auger spectroscopy allowed changes occurring during the activation process to be identified, namely, the formation of small Cu particles on the HfO2 surface and the formation of highly porous particles containing mostly Cu and some Ti and O (Cu-Ti-O) on a Cu-Ti substrate. Differences in the chemistry and structure of both catalysts are discussed, and the implications for catalytic function are considered. A probable configuration of active sites on the Cu-Ti-O/Ti-O-Cu catalyst for dehydrogenation of 2-propanol is proposed.

Effect of tetragonal ZrO 2 on the catalytic activity of Ni/ZrO 2 catalyst prepared from amorphous Ni–Zr alloys

In zirconia-supported nickel catalysts, the influence of the ZrO 2 phase on the catalytic activity for CO 2 methanation has been investigated by TPD and TPR measurements. Ni/ZrO 2 catalysts with various amounts of the tetragonal ZrO 2 polymorph were prepared from amorphous Ni–Zr alloys by an oxidation–reduction treatment. The fraction of tetragonal ZrO 2 in the Ni/ZrO 2 catalysts increased with increasing nickel content of the amorphous alloy precursors. The methanation activity of Ni/ZrO 2 was strongly influenced by the ZrO 2 phase change. The tetragonal zirconia-supported nickel catalysts showed a higher turnover frequency for the methanation reaction and a greater CO 2 adsorption than the monoclinic zirconia-supported nickel catalysts.

Cathodic hydrogen charging as a tool to activate Cu–Ti amorphous alloy catalysts

Degradation processes occurring at the surface and in the bulk of Cu-based amorphous alloys during cathodic hydrogen charging were used for enhancing their catalytic activity. Thus, the degradation processes caused by hydrogen charging (modifying the structure, composition and morphology of the substrate) proved to be useful methods of transforming Cu–Ti amorphous alloy precursors into active and stable catalysts. Hydrogen charging was applied to Cu60–Ti40 amorphous ribbons in an acid solution (0.1 M H 2SO 4) in order to bring about pronounced morphological and structural changes. The samples were then catalytically tested for dehydrogenation of 2-propanol. Catalytic activity increased up to a conversion level of ∼60% at selectivities to acetone of about 99%. The chemical and morphological changes were followed by SEM (Hitachi 3500N) and Auger electron microanalysis (Microlab 350). Porosity and specific surface area were measured (ASAP 2010 Sorptometer), and hydrogen content was determined by elemental analysis. The rather low average specific surface area and porosity, as well as a lack of Cu 0 both before and after the catalytic test suggest that the generally accepted mechanism for the dehydrogenation of alcohols over copper catalysts with the involvement of Cu 0 is not operative here.

Compositional dependence of the CO 2 methanation activity of Ni/ZrO 2 catalysts prepared from amorphous Ni [sbnd]Zr alloy precursors

Finely grained Ni/ZrO 2 catalysts were prepared from amorphous Ni Zr alloy precursors by oxidation and subsequent reduction pretreatment, and the catalytic activity for CO 2 methanation was examined as a function of precursor alloy composition and temperature. The catalysts thus prepared produce exclusively methane, apart from water as a by-product. The conversion of CO 2 increases with temperature in the range of 373–573 K. Among the catalysts examined, the maximum methanation rate is obtained on the catalysts prepared from the amorphous alloy precursors containing 40 and 50 at% zirconium. Further, the methanation rates of all the catalysts prepared from the amorphous alloy precursors are higher than that of a 3 at% Ni/ZrO 2 catalyst prepared by wet impregnation. The number of surface nickel atoms, determined by hydrogen chemisorption, increases with zirconium content in the catalysts, while, interestingly, the turnover number decreases with increasing zirconium content. In the catalysts prepared from the amorphous alloys, two types of zirconia are present: metastable tetragonal and stable monoclinic zirconia. The former zirconia phase is present predominantly in the catalyst prepared from the Ni-30 at% Zr alloy, but the relative amount of this oxide phase, with respect to the total amounts of zirconia, gradually decreases with an increase in zirconium content of alloys. Thus, the higher turnover number of the catalysts with higher nickel content can be attributed to nickel supported on metastable tetragonal zirconia. Increasing nickel content of the precursor alloys leads to an increase in tetragonal zirconia and to a decrease in the number of surface nickel atoms on the catalysts. This is responsible for the fact that the maximum conversion appears at medium contents of zirconium in the precursor alloys.

ChemInform Abstract: Decomposition of Nitrogen Monoxide over NiTa2O6‐Supported Palladium Catalysts Prepared from Amorphous Alloy Precursors.

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Application of Sputter Deposition Technique to the Preparation of Amorphous Alloy-Derived Catalysts for NO Decomposition

Sputter deposition technique has been applied to the preparation of amorphous alloy precursors of catalysts on a fine oxide powder, in order to overcome the low surface area of the catalysts prepared from melt-spun amorphous alloy precursors. Amorphous Ni-Ta-Pd alloys have been sputter-deposited onto the γ-alumina powder with a high surface area, and then pre-oxidized at 1023 K in a 0.5% NO atmosphere. During the preoxidation, the amorphous alloys have been converted to palladium catalysts supported on the NiTa 2 O 6 double oxide. The BET areas of the catalysts thus prepared (sputter-deposited catalysts) are approximately 50 times higher than those prepared from a melt-spun Ni-40Ta-lPd alloy precursor (melt-spun catalyst). The catalytic activity of the sputter-deposited catalysts for NO decomposition becomes about twice as high as that of the melt-spun catalyst. Furthermore, the selectivity of nitrogen formation is also improved compared with the melt-spun catalyst. Accordingly, the application of sputtering technique is quite suited for preparing amorphous alloy catalyst precursors with a high surface area.

Decomposition of nitrogen monoxide over NiTa2O6-supported palladium catalysts prepared from amorphous alloy precursors

Active catalysts for the direct decomposition of nitrogen monoxide were prepared from amorphous Ni40TaPd alloys by HF-treatment and subsequent pre-oxidation. The pre-oxidized catalyst consists of NiO, Ta2O5 and PdO, and PdO is decomposed to fcc Pd by heating at temperatures higher than 600°C. At further higher temperatures NiO and Ta2O5 are transformed to a very fine grained double oxide NiTa2O6. The catalysts thus formed consist of three layers and show high catalytic activity for the decomposition of nitrogen monoxide in a wide temperature range from 550 to 850°C. The catalytic behavior is affected by the structural change in the catalyst. With the transformation from NiO and Ta2O5 to NiTa2O6, the catalytic activity and the nitrogen formation selectivity increase significantly. TEM observation of ultramicrotomed cross-sections revealed that finely dispersed palladium supported on very fine-grained NiTa2O6 is formed in the interface between the outer and intermediate layers. High catalytic activities and high nitrogen formation selectivities of the Ni40TaPd alloys are attributable to the formation of the NiTa2O6-supported palladium catalyst.

Decomposition of nitrogen monoxide over NiTa 2O 6-supported palladium catalysts prepared from amorphous alloy precursors

Active catalysts for the direct decomposition of nitrogen monoxide were prepared from amorphous Ni 40Ta Pd alloys by HF-treatment and subsequent pre-oxidation. The pre-oxidized catalyst consists of NiO, Ta 2O 5 and PdO, and PdO is decomposed to fcc Pd by heating at temperatures higher than 600°C. At further higher temperatures NiO and Ta 2O 5 are transformed to a very fine grained double oxide NiTa 2O 6. The catalysts thus formed consist of three layers and show high catalytic activity for the decomposition of nitrogen monoxide in a wide temperature range from 550 to 850°C. The catalytic behavior is affected by the structural change in the catalyst. With the transformation from NiO and Ta 2O 5 to NiTa 2O 6, the catalytic activity and the nitrogen formation selectivity increase significantly. TEM observation of ultramicrotomed cross-sections revealed that finely dispersed palladium supported on very fine-grained NiTa 2O 6 is formed in the interface between the outer and intermediate layers. High catalytic activities and high nitrogen formation selectivities of the Ni 40Ta Pd alloys are attributable to the formation of the NiTa 2O 6-supported palladium catalyst.

Mechanical Properties of Rapidly Solidified Nickel-Base Superalloys and Intermetallics

ABSTRACTThe improvements in the mechanical properties of nickel-base alloys that have been made possible by rapid solidification processing are reviewed. The results of processing by powder metallurgy, laser melting, low pressure plasma deposition and spray forming are examined. In general, the increased homogeneity obtained by rapid solidification allows for increased alloying and improved hot workability. The refined grain size improves the low and intermediate temperature strength, but leads to lower strengths at high temperature. For the high temperature applications, post solidification grain growth is required, as for example the directional recrystallization of powder metallurgy preforms. The development of a novel means of producing a fine dispersion from amorphous alloy precursors and the recent work attempting to improve the ductility of the intermetallic phases NiAl and Ni3A1 are also described.