High-temperature Reduction Treatment Research Articles

Nanoscale compositional and structural evolution in ceria zirconia during cyclic redox treatments

We report changes in nanoscale composition and structure in individual ceria zirconia nanoparticles (nominal composition Ce0.7Zr0.3O2), before and after redox treatment, determined by energy dispersive X-ray (EDX) analysis using scanning transmission electron microscopy (STEM) and high resolution transmission electron microscopy (HRTEM) imaging. According to the chemical analysis of individual nanoparticles, the redox treatments promote the formation of more compositionally homogenous ceria zirconia solid solution nanoparticles, resulting in a uniform low-temperature reduction. Local pyrochlore-type cation ordering in individual nanoparticles was also observed after the high temperature reduction step. For nanoparticles with 70% ceria content, redox cycling treatment was critical to obtain material which exhibits a strong low temperature reducibility. The origin of significant X-ray diffraction line broadening and particle sintering mechanism in the solid solution nanopowders are discussed based on compositional and structural variation within and between individual nanoparticles and oxygen vacancies formed during high temperature reduction treatment.

Citral hydrogenation over Ir/TiO 2 and Ir/TiO 2/SiO 2 catalysts

Catalytic hydrogenation of citral over iridium supported on TiO 2, SiO 2 and mixed oxides TiO 2/SiO 2 has been studied. The effect of the reduction temperatures (473 or 773 K) and the successive enrichment of TiO 2 were analyzed. The solids were characterized by H 2 chemisorption at room temperature, N 2 adsorption at 77 K, XRD, TEM, FTIR and XPS. The obtained results show that the high temperature reduction treatment (HT) produces an important enhancement in both the catalytic activity and selectivity towards the desired product. The behavior is explained on the basis of a surface enrichment in titania due to the migration of TiO x species on the Ir crystallites, which does not occur in those catalysts reduced at low temperature (LT) or in the catalyst supported on an inert oxide such as SiO 2.

Sunflower oil hydrogenation on Pt catalysts: Comparison between conventional process and homogeneous phase operation using supercritical propane

The hydrogenation of sunflower oil on TiO 2 supported by Pt catalysts was studied. The Pt/TiO 2 catalyst was modified: (a) with K and (b) by high temperature reduction treatment, in order to observe its influence on the activity and the cis-isomers selectivity. The catalysts were characterized by atomic absorption, H 2 chemisorption, TPR and XRD. The hydrogenation of edible oils at conventional conditions is diffusion controlled. The addition of propane as supercritical (SC) solvent allows to perform the reaction in a homogeneous phase in contact with the solid catalyst. A significant increase in the reaction rate of the homogeneous system was observed. Regarding selectivity, the formation of trans-isomers was higher in the reaction under conventional conditions, whereas the production of saturated compound was favored in the homogeneous operation. Pt-K/TiO 2 and Pt/TiO 2 catalyst presented a similar TON, although the cis-isomer selectivity increased with the alkali addition. This behavior is explained in terms of a chemical modification of the noble metal.

The activation of supported Au catalysts prepared by impregnation

In this study, Au/TiO2, Au/γ-Al2O3, and Au/C catalysts were prepared by an incipient-wetness impregnation method. The CO oxidation activity after different pretreatment was analyzed. Two pretreatments are found possible to activate these catalysts for CO oxidation: (i) high-temperature hydrogen reduction and (ii) aqueous base treatment using NH3(aq). The high-temperature hydrogen reduction is effective for Au/TiO2 and Au/C, but not Au/Al2O3. The base treatment is effective for Au/Al2O3 and Au/TiO2, but not Au/C. Small Au particles of ca. 2 nm size were observed in activated Au catalysts from both pretreatments; however, the high-temperature H2 reduction procedure also resulted in large Au particles of ca. 25 nm size which makes it a less efficient pretreatment than the base treatment. The activated Au/TiO2 catalysts show comparable turnover frequency as the Au/TiO2 catalyst prepared by a deposition precipitation method. The effective pretreatments were found to be accompanied by a reduced acidity, which is measured by the pH of aqueous catalyst suspension. Reasons for such activation are discussed.

D091 Characterization of a Ce/Zr Mixed Oxide Material - A Comparison of Offline and In-Situ High Temperature Reduction Treatments — Invited

An abstract is not available for this content so a preview has been provided. Please use the Get access link above for information on how to access this content.

Study of preparation parameters of powder and pelletized Pt/KL catalysts for n-hexane aromatization

Pt/KL powder and pelletized catalysts were synthesized by different methods including ion exchange (IE), incipient wetness impregnation (IWI), and vapor phase impregnation (VPI) methods. A detailed characterization was conducted on the various samples in order to determine the distributions of Pt particle size and location resulting from each preparation, as well as the relationships between these distributions and the catalytic performance. The catalysts were pretreated at two different reduction temperatures — 400 and 500°C — to investigate the sensitivity of each catalyst to thermal treatment. All catalysts showed high dispersion, with H/Pt ratios greater than unity. However, FTIR of adsorbed CO showed that more important than dispersion, it is the distribution of Pt cluster size and location, which influences the resulting catalytic stability. Standard IE catalysts were found to have a high fraction of Pt particles external to the L-zeolite and were the most sensitive to thermal treatment, displaying Pt migration out of the L-zeolite. These catalysts deactivated rapidly by coke formation. The addition of excess K + ions in the exchange solution and a longer aging period, as suggested in the patent literature, improves the Pt dispersion. IWI and VPI catalysts showed a majority of Pt clusters inside the L-zeolite channels. However, after high temperature reduction treatment, the IWI catalysts displayed particle growth inside the channels, in contrast with the VPI. The IWI catalysts were found to deactivate to about half their initial activity, while the VPI maintained the highest activity and stability. Different VPI methods including a moderate vacuum and a helium flow technique were examined and they showed similar Pt cluster distributions and catalytic performance to the catalysts synthesized using high vacuum. Preparation directly on the pellet for the methods investigated in this work (standard IE, IWI, and VPI) resulted in large fractions of Pt clusters external to the pores and likely on the binder. Thus, rapid deactivation by coke formation was evident. These results indicate that pelletized catalysts prepared by any of these techniques would require preparation on the powder prior to pellet extrusion.

Gold and iron supported on Y-type zeolite for carbon monoxide oxidation

Gold and iron supported on Y-type zeolite for carbon monoxide oxidation have been studied. They were prepared by ion exchange of gold(III) compounds with Y-type zeolite, iron-impregnated Y-type zeolite, or iron-exchanged Y-type zeolite in a chloroauric acid solution. The as-prepared gold/Y sample possessed high activity for carbon monoxide oxidation, but it gradually and irreversibly deactivated during the reaction. The deactivation was attributed to the transformation of some gold ions to gold metal. The activities of as-prepared gold/iron/Y samples were poor when compared with that of gold/Y, but their activities were markedly promoted by a high-temperature reduction treatment at 570°C. This is ascribed to the fact that the reduced gold/iron/Y catalyst could catch and release oxygen at low temperature.

Effect of the oxidation state of LiNbO_3:Fe on the diffraction efficiency of multiple holograms

We show that the oxidation state of Fe in LiNbO(3) has two competing effects on the diffraction efficiency of multiple holograms in 90 degrees -geometry holographic storage. For crystals with moderate absorption, the saturation space-charge field is larger after high-temperature reduction treatment. However, reduction also increases absorption, which reduces the overall diffraction efficiency. We develop a theoretical model that predicts achievable diffraction efficiency as a function of oxidation state, doping level, photovoltaic field, crystal length, and region of beam overlap. We compare this model with experimental results for achievable diffraction efficiency and erasure-time constant.

On the sulfur-aided metal-support interaction in [formula omitted] catalysts

The SMSI behavior displayed by sulfated Pt Al 2 O 3 Cl catalysts with relatively low sulfur content has been explained by the alternative reactions ▪ or ▪ Redox reaction (2) would be sulfur catalyzed, giving the so-called sulfur-aided SMSI. In this paper the SMSI behavior displayed by sulfided samples with only irreversibly held sulfur ( S Pt = 0.20–0.35 ) was studied by the changes in H 2 and O 2 chemisorption, infrared spectroscopy, temperature-programmed reduction and H 2S desorption at increasing temperatures. The changes in the 1380-cm −1 band (SO 2− 4 on the carrier) and in the 2060-cm −1 band (CO adsorption on Pt in linear form) were used to investigate the effects of the high-temperature reduction treatment. The TPR profiles of sulfated aluminas showed only one reduction peak at 650–680 °C; the presence of Pt gave rise to a low-temperature peak which appeared at 450–500 °C. Such a peak shifted to lower temperatures when the Pt content was increased. The data showed that the SMSI behavior of sulfated Pt Al 2 O 3 Cl samples is better explained in terms of redox reaction (1). During the HTR treatment Pt cata-lyzes the reduction of sulfate ions. The H 2S formed is adsorbed on the metal so that the H 2 chemisorption is suppressed by the S blockage of the surface. Subsequent oxidation treatment causes oxidized S species in the metal surface to migrate back to the support. The metallic fraction is regenerated and, as a consequence, the H 2 chemisorption capacity is restored.

Characterization of metal precipitates in magnesium oxide

Abstract A method, based on subtractive coloration or high-temperature reduction, has been developed to produce metal precipitates or colloids in oxides. The structure, symmetry, morphology and composition of precipitates were analysed by analytical electron microscopy and neutron scattering techniques. In the MgO: Ni system, the colloids were coherent nickel precipitates having a f.c.c. structure (ao = 3·52 A), and occasionally a b.c.c. structure (ao = 2·88A). The coherent precipitates were found to have either ⟨100⟩m‖⟨111⟩p or ⟨100⟩m‖⟨100⟩p matrix-precipitate relationship, depending on the high-temperature reduction treatment. From the neutron-magnetic-scattering measurements, the ratio of magnetic to nuclear scattering amplitudes for the (111) precipitate reflection was determined to be 17% below the value for pure nickel, indicating the presence of a magnetically ‘dead’ layer around the precipitates, a small amount of impurities incorporated into the precipitates, or a small fraction of another phase ...

Metal Colloids in Oxides*

ABSTRACTA method, based upon subtractive coloration or high-temperature reduction, was developed to produce metal precipitates or colloids in oxides. The structure, symmetry, morphology and composition of precipitates were analyzed by analytical electron microscopy and neutron scattering technioues. In the MgO:Ni system, the colloids were coherent nickel precipitates having a fcc structure (a0 = 3.52 Å), and occasionally a bcc structure (a0 = 2.88 Å). The coherent precipitates were analyzed to have either <100>m,∣∣ <111>p or <l00>m ∣∣<100>p matrix-precipitate orientation relationship depending upon the high-temperature reduction treatment. The optical absorption spectra of these materials were characterized by broad optical bands centered near 2.2 and 5.0 eV. By changing the size distribution of the precipitates, it was possible to obtain optical spectra of these materials suited for solar selective absorber applications. The dislocations and sub-boundaries provided nucleation sites for the formation of precipitates, and decreased the temperature of reduction. By providing sources of dislocations, the precipitates enhance the ductility and inhibit the propagation of cracks.

Microwave relaxation properties of Be 2+-doped yttrium iron garnet LPE films

A series of yttrium iron garnet epitaxial films containing an increasing concentration of beryllium has been grown and studied by the FMR technique in the microwave range. A nearly linear increase of the films line-width with the BeO content in the melt together with high temperature air annealings and reduction treatments in aqueous solutions evidences the presence of Fe 4+ Be 2+ pairs in the lattice to provide ion charge equilibrium.