Maria Flytzani-Stephanopoulos

Reduction characteristics of copper oxide in cerium and zirconium oxide systems

Abstract The reduction of CuO dispersed on fluorite-type oxide catalysts, namely La-doped CeO2 and Y-doped ZrO2 was studied in this work. On both supports distinct copper species were identified as a function of copper content by temperature-programmed reduction (TPR) by H2 and CH4, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and scanning transmission electron microscopy/energy dispersive X-ray (STEM/EDX) analyses. At low copper loading ( 15 at%), in addition to clusters, larger CuO particles are present which are reduced at higher temperature close to the reduction temperature of bulk CuO. At copper loading lower than ca. 5 at%, copper is present as highly dispersed clusters or isolated Cu ions, which interact strongly with the fluorite-type oxide, thus requiring higher reduction temperature. However, the latter is still below the bulk CuO reduction temperature. Copper is more stabilized when dispersed in Ce(La)O2 than in Zr(Y)O2 matrix, so that reduction of copper oxide species requires lower temperatures on the Zr(Y)O2-based catalysts. The reducibility of the doped ceria is enhanced by the presence of copper in both H2- and CH4-TPR. On the other hand no such interaction is present in CuZr(Y)O2 system. The activity of various copper species for methane oxidation is discussed.

Complete oxidation of carbon monoxide and methane over metal-promoted fluorite oxide catalysts

Abstract Cu-Ce-O binary oxide catalysts were found to have high activity for the oxidation of CO and methane by oxygen to carbon dioxide and water as well as oxidation of CO by sulfur dioxide to carbon dioxide and elemental sulfur. The catalysts were characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy. It was found that copper in the Cu-Ce-O system existed in the form of clusters dispersed in the cerium oxide matrix and in the form of segregated agglomerates (> 10nm). The former are present in all catalyst compositions, while the latter increased with the copper content. The dispersed copper clusters may be stabilized by diffusion of copper ions into interstitial metal ion sites of the cerium oxide lattice forming Cu +1 species in the process. Cu +1 species were identified by XPS. A synergistic reaction model is proposed to explain the enhanced catalytic activity and stability. In the model, the Cu +1 species on the copper cluster provide surface sites for CO adsorption and the cerium oxide provides the oxygen source. The adsorbed CO reacts with oxygen at the interface of the two materials.

Reduction of sulfur dioxide by carbon monoxide to elemental sulfur over composite oxide catalysts

Abstract The catalyst activity of fluorite-type oxide, such as ceria and zirconia, for the reduction of sulfur dioxide by carbon monoxide to elemental sulfur can be significantly promoted by active transition metals, such as copper. More than 95% elemental sulfur yield, corresponding to almost complete sulfur dioxide conversion, was obtained over a Cu-Ce-O oxide catalyst with a feed gas of stoichiometric composition ([CO]/[SO 2 ]=2) at temperatures above 450°C. This type of mixed metal oxide catalyst has stable activity and is resistant to water and carbon dioxide poisoning. XPS analysis found copper in the Cu-Ce-O oxide in a reduced oxidation state (Cu 1+ , Cu 0 ). The stable fluorite-type structure, regarded as the backbone structure of the catalyst, existed in both the fresh and the spent catalyst. The high activity resulted from the strong interaction of transition metal and fluorite oxide.

THE REDUCTION OF ZINC TITANATE AND ZINC OXIDE SOLIDS

Abstract The reduction of bulk mixed oxides of zinc and titanium of various compositions and ZnTiO crystalline phases was studied in a thermogravimetric apparatus in H 2 H 2 ON 2 gas mixtures at 550–1050°C. Comparative reduction experiments with ZnO were also performed. In the absence of water vapor, activation energies of 24 and 37 kcal mol −1 were obtained for ZnO and ZnTiO, respectively. The addition of water vapor inhibited reduction and resulted in a change in activation energy to 44 kcal mol −1 for both ZnO and ZnTiO solids. Similar to H 2 O, H 2 S inhibited the initial reduction rate of ZnO and ZnTiO materials. Based on kinetic experiments, a two-site model is proposed for ZnO reduction. One type of sites is characterized by a rapid reduction rate but is poisoned by water vapor as well as by the presence of titanium atoms in the solid. The other type of sites has a lower reduction rate, is not poisoned by H 2 O, and is slowly eliminated by the presence of titanium.

Preparation effects on the activity of Cu-ZSM-5 catalysts for NO decomposition

Effects of Cu-ZSM-5 catalyst preparation on the activity of “over-exchanged” copper for NO decomposition are reported. The Cu-ZSM-5 catalysts were prepared by incorporating Cu2+ cations into ZSM-5 zeolites from an aqueous cupric acetate solution adjusted to different pH values by adding either acetic anhydride or aqueous ammonia in the solution. The Cu2+ exchange levels increased with increasing pH level. STEM/EDX analysis identified CuO particles (5–6 nm) on the zeolite surface for the materials exchanged at pH>6. Conversion and kinetics measurements of NO decomposition to N2 over these catalysts showed that the “over-exchanged” copper was not active. Short-time wash with aqueous ammonia removed this copper. The catalyst activity correlated very well with the amount of copper remaining in the ZSM-5 channels.

Vapor etching of GaAs and AlGaAs by CH3I

With the objective of developing an improved process for in situ etching of GaAs‐based materials in organometallic vapor phase epitaxy reactors, GaAs wafers and AlxGa1−xAs epilayers have been etched with CH3I vapor in a horizontal reactor operated at atmospheric pressure with H2 or He carrier gas. For a H2 flow rate of 2.1 s lpm, etching temperatures from 400 to 625 °C, and CH3I mol fractions (yCH3I)from 0.0012 to 0.015, the measured GaAs etch rate r (in A min−1) is given by r = k0 y0.83CH3I exp[ − 45(kcal mol−1)/RT] with k0=3.2×1016 A min−1. The value of k0 depends on the type of carrier gas, flow rate, total pressure, and reactor geometry. The etch rate appears to be controlled mainly by the decomposition of CH3I to CH3 and I, for which the activation energy has been reported to be 43.5 kcal mol−1. The etch rate of AlxGa1−xAs epilayers with x up to 0.7, which was measured at 480 °C with yCH3I= 0.015, does not depend on Al content. The surface morphology of etched GaAs wafers improves with decreasing tem...

OMVPE regrowth of CH 3 I-vapor-etched GaAs

In experiments on the interrupted growth of GaAs by organometallic vapor phase epitaxy (OMVPE), we have compared the properties of two types of epilayers: those grown on CH3I-vapor-etched first epilayers and those grown on first epilayers that were either untreated or etched with H2SO4. The OMVPE growth and CH3I etching were performed in two different reactors, and each sample was briefly exposed to air immediately before being placed in the OMVPE reactor for growth of the second epilayer. For CH3I etch temperatures below 500° C, the two types of second epilayers are comparable in surface morphology, as characterized by Nomarski interference microscopy, and in optical quality, as characterized by low-temperature photoluminescence measurements. Electrical characterization by C-V depth profiling shows that electron accumulation at the regrowth interface is increased very little by CH3I etching. Such etching prior to regrowth eliminates most of the electron accumulation resulting from H2SO4 etching of the first epilayer.

CH3I vapor etching of masked and patterned GaAs

CH3I vapor etching of masked and patterned GaAs substrates has been experimentally investigated. For GaAs samples masked with silicon nitride stripes that are wider than 30 tzm, the etch depth increased compared to unmasked samples, the magnitude of which increased with increasing mask width. Etching of bulk substrates of (lll)Ga and (lll)As GaAs revealed a dependence of etch rate on crystal orientation, with (lll)Ga > (100)GaAs > (lll)As. Increasing etch temperature reduced the orientation dependence of etch rates. Orientation dependence of etch rates was also observed on non-planar GaAs substrates patterned to expose different orientations on wet-etched groove structures. In this case, etch rate differences between the different orientations were amplified when compared to the bulk substrate results. Finally, it was found that the extent of mask undercutting depended on the direction of mask stripes in a fashion consistent with the orientation reactivity results. Mask stripes on (100)GaAs oriented in the [011] direction were severely undercut whereas stripes oriented in the [011] direction were undercut less.

CH3I vapor etching of GaAs in a vertical rotating-disk reactor

A detailed computer model which predicts etch rates and spatial uniformity for chemical vapor etching of GaAs with CH3I in a vertical rotating-disk organometallic vapor phase epitaxy (OMVPE) reactor has been developed. Etch rate predictions compare favorably with experiments performed in the vertical reactor at several temperatures ranging from 525 to 590°C and CH3I mole fractions from 0.005 to 0.050 in an H2 carrier gas. Agreement between the data and the model verifies our earlier assertion that gas phase decomposition of CH3I is the rate-limiting step, and that the etchant species are I and to a lesser extent CH3 radicals. Predictions further show that radial etch rate uniformity improves from a 5% variation over 50% of the substrate at low rotation (20–100 rpm) to a 5% variation over 80% of the substrate at 500 rpm. Increasing rotation rate also reduces the etch rate, a result that is in contrast to earlier results for OMVPE growth from TMGa and AsH3. These contrasting results arise because growth is rate limited by gas phase diffusion of TMGa, whereas CH3I vapor etching is rate limited by gas phase decomposition of CH3I.