A.E. Hughes

The electrochemical performance of LSM/zirconia–yttria interface as a function of a-site non-stoichiometry and cathodic current treatment

Abstract The adhesion and the electrochemical performance of Sr doped LaMnO 3 (LSM) electrode with zirconia–yttria electrolyte has been investigated as a function of the A-site non-stoichiometry. In addition the effect of cathodic current treatment on the electrode resistance and the mechanism of oxygen reduction have been carefully studied. The LSM/electrolyte interface region was examined by SEM/EDS and XRD. The adhesion of porous LSM electrode coating is critically dependent on free La activity near the interface region and the formation of a pyrochlore phase, La 2 Zr 2 O 7 . A slight A-site deficiency (∼0.1) is effective in inhibiting the formation of such a pyrochlore phase and it greatly improves the adhesion to the electrolyte. The cathodic current treatment significantly enhances the electrochemical activity of LSM and also modifies the reaction mechanism for oxygen reduction. This effect appears to be associated with the surface composition of LSM, near the electrode/electrolyte interface, which changes with prevailing oxygen partial pressure near the three-phase boundary region under the condition of current flow.

Chromate Conversion Coatings on 2024 Al Alloy

X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and potentiodynamic measurements have been made on chromate conversion-coated Al 2024-T3 alloy. X-ray photoelectron spectroscopy measurements indicated that the conversion coating had a surface of CrOOH and Cr(VI), enriched in ferricyanide. The bulk of the coating was an equal mixture of CrOOH and Cr2O3 with significant levels of F− and Fe, the latter implying the presence of ferricyanide throughout the coating. Copper(II) ion was present at the interface between the conversion coating and the alloy, as well as Al3+. During ageing experiments, potentiodynamic measurements indicated that the corrosion current (icorr) decreased from ∽0.4 to ∽0.04 μA cm-2 during the first 40 h after preparation but thereafter slowly increased. No significant changes were observed in the chemistry of the coating by XPS for ageing times longer than 40 h, although morphological changes were observed with SEM. As the coating aged, a network of microcracks developed across the surface. It is believed that Cr6+ is consumed in the process in which plugs of hydrated chromium oxide form at the base of these cracks.

Impurity and yttrium segregation in yttria-tetragonal zirconia

Abstract X-ray photoelectron spectroscopy, scanning electron microscopy and impedance dispersion spectroscopy have been used to study the segregation behaviour of yttrium and impurities in yttria-tetragonal zirconia polycrystals (Y-TZP). Sintered and polished specimens of two Y-TZPs with different levels of impurities were annealed at temperatures between 800 and 1500°C. On annealing, significant amounts of impurity silicates and yttrium incorporated into the silicate, segregated to the external surface of specimens, even at low annealing temperatures. The segregation layer increased with increasing annealing temperature. Impedance spectroscopy indicated that grain boundary cleaning accompanied redistribution of the silicate phase to the external surface. The segregation of silicate phases at the external surface led to yttrium redistribution in the surface grains and enhanced grain growth.

The structure of Cu/SiO2 catalysts prepared by the ion-exchange technique

A series of CuSiO2 catalysts prepared by the ion-exchange method and containing 2.1 to 9.5 wt% copper are characterised in detail by TEM, X-ray diffraction analysis, XPS, and TPRTPO cycles. Two distinctly different copper species are identified, i.e., isolated copper atoms attached to two neighbouring silanol groups (ion-exchanged) representing 10–25% of the copper loading, and the remainder concentrated in numerous small copper particles (<6 nm, typically 1–3 nm diameter). In the calcined but unreduced samples, copper is present as both Cu(II) ions and copper oxide agglomerates of flat disc or hemispherical shape. Upon reduction in H2 and at temperatures up to 673 K, the ion-exchanged species reduce to Cu(I) only and retain their isolated nature. The CuO particles reduce to metallic copper. During reduction very little particle migration and agglomeration is observed, which we attribute to an interaction between the particles and the Cu(I)-modified support. Whereas the TPR results do not clearly show sequential stages in the reduction of CuO particles, the TPO experiments reveal three clearly defined reoxidation peaks which give information on the relative surface-to-volume ratios of the copper particles on the different catalysts. We suggest that the low deactivation rate for this type of catalyst is related to a strong interaction between the small copper particles and the ion-exchanged Cu(I) present on the SiO2 surface.

The characterisation of Ce-Mo-based conversion coatings on Al-alloys: Part II

Abstract Scanning Electron Microscopy (SEM), X-Ray Photo-electron Spectroscopy (XPS) and Scanning Auger Electron Spectroscopy (AES) have been used to study the surface chemistry of a multistep process for the generation of cerium- and molybdenum-containing conversion coatings on aluminium alloys. After standard pretreatment, the conversion coating was produced on 2024-T351 and 6061-T6 Al alloys by (i) treatment for 24 h in air saturated with water vapour at 100 °C, (ii) 2 h in 10 mM Ce(NO 3 ) 3 at 90–100 °C, (iii) 2 h in 5 mM CeCl 3 at 90–100 °C and (iv) anodic polarisation for 2 h in Na 2 MoO 4 at 500 mV (SCE). XPS indicated that the coating generated on both alloys was predominantly a hydrated aluminium oxide. It appeared from SEM and Scanning AES that the cerium was concentrated locally on the oxide. XPS indicated that the cerium was present in a mixture of Ce(III) and Ce(IV) oxidation states. Many of these cerium rich areas were associated with the constituent particles (large intermetallics). Mo(VI) was detected with XPS and there was evidence from SEM and AES to suggest that there was local enrichment of Mo around the intermetallic particles.

Surface area control during the synthesis and reduction of high area ceria catalyst supports

Abstract Precipitation of ceria from (NH 4 ) 2 [Ce(NO 3 ) 6 ] with a ‘ammonium carbonate’ solution produces powders, with surface areas determined by the initial [Ce] concentrations. Samples, free of carbonate impurities and with reproducible surface areas of up to 200 m 2 g −1 , have been obtained after controlled dehydration and calcination to 600°C. Temperature-programmed reduction (in 3% H 2 N 2 ) of samples with a range of surface areas, has confirmed that the net hydrogen uptake by ceria between 120 and 700°C comprises two components. The first is a constant amount which represents sorption of interstitial hydrogen into the ceria lattice. The second is proportional to the surface area of the sample and originates from partial reduction of surface Ce IV and hydroxylation of the surface oxide.

Ruthenium promotion of fischer-tropsch synthesis over coprecipitated cobalt/ceria catalysts

Abstract A reduced co-precipitate of cobalt and cerium oxides has been found to be selective for production of lower alkenes and higher n-alkanes from syngas, with the advantage of low (ca. 1 wt.-%) carbon dioxide as a co-product. Activity is trebled by adsorption of as little as 0.3 wt.-% ruthenium, as Ru3(CO)12, without loss of selectivity. Activity is maintained almost unchanged over periods up to 100 h, operating at carbon monoxide conversions of over 40%. Both X-ray photoelectron spectroscopy and temperature-programmed reduction profiles show that for the unpromoted catalyst, the presence of Co3O4 lowers the reduction temperature of CeO2, while for the promoted catalyst, the addition of ruthenium lowers the reduction temperatures of both Co3O4 and CeO2 further, presumably via hydrogen spillover reduction. Consequently there is more Co0 on the ruthenium-promoted catalyst than the unpromoted catalyst leading to higher activity. Fuel parameters of the n-alkane condensate include high cetane index (80–100) and intermediate pour point (−4°C), showing suitability for fuel oils or blends.

An XPS study of coke distribution on ZSM-5

Two ZSM-5 catalysts were prepared with bulk SiAl ratios of 39 and 74. After conversion to the hydrogen form they were partially deactivated at 370 °C in a methanol/nitrogen stream. Coke contents were determined by combustion in a microbalance. Measurement of the surface and near-surface CSi ratio by X-ray photoelectron spectroscopy showed internal and external coke formation. Internal coke formation predominated during methanol conversion over ZSM-5 until the catalyst was essentially deactivated. External coke was then formed, probably by the thermal cracking of methanol.

An XPS study of Ru-promotion for Co/CeO2 Fischer-Tropsch catalyst

Abstract X-ray photoelectron spectroscopy has been used to examine the reduction behavior of co-precipitated Co3O4/CeO2 and of Ru-promoted Co3O4/CeO2. It was found, by evaluating the percentage of Co° and Ce3+, that the presence of Co3O4 lowered the reduction temperature of CeO2. After reduction at 550°C the stoichiometry of the cerium oxide in the Co3O4/CeO2 coprecipitate was CeO1.77 ± 0.01. Promotion with Ru lowered the reduction of both Co3O4 and CeO2. In the Ru-promoted case the reduction of ceria to CeO1.84 ± 0.01 was achieved at temperatures as low as 170°C.

X-Ray Photoelectron Spectroscopic Study of the Wool Fiber Surface:

The surface analysis technique of x-ray photoelectron spectroscopy (XPS) has been used to investigate the chemical composition and reactivity of the wool epicuticle. Approximate relative proportions of protein and lipid material in the epicuticle have been deduced from the surface elemental ratios; the sulfur content of the proteins in the epicuticle was estimated to be about 9% by weight. Treating wool with potassium t-butoxide in anhydrous t-butanol, a reagent that is reported to cause specific epicuticle modification, caused only minor changes in the elemental composition of the fiber surface. There were marked changes, however, in the sulfur (2p) photoelectron spec trum. Approximately one third of the cystine residues in the epicuticle were converted to cysteic acid residues. Evidence was also obtained for the partial conversion of cystine residues into lanthionine residues.