Manh Hoang

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.

Ammonia removal by sweep gas membrane distillation.

Wastewater containing low levels of ammonia (100 mg/L) has been simulated in experiments with sweep gas membrane distillation at pH 11.5. The effects of feed temperature, gas flow rate and feed flow rate on ammonia removal, permeate flux and selectivity were investigated. The feed temperature is a crucial operating factor, with increasing feed temperature increasing the permeate flux significantly, but reducing the selectivity. The best-performing conditions of highest temperature and fastest gas flow rate resulted in 97% removal of the ammonia, to give a treated water containing only 3.3 mg/L of ammonia.

A review of water recovery by vapour permeation through membranes.

In vapour permeation the feed is a vapour, not a liquid as in pervaporation. The process employs a polymeric membrane as a semi-permeable barrier between the feed side under high pressure and the permeate side under low pressure. Separation is achieved by the different degrees to which components are dissolved in and diffuse through the membrane, the system working according to a solution-diffusion mechanism. The materials used in the membrane depend upon the types of compounds being separated, so water transport is favoured by hydrophilic material, whether organic or inorganic. The process is used for the dehydration of natural gas and various organic solvents, notably alcohol as biofuel, as well as the removal of water from air and its recovery from waste steam. Waste steam can be found in almost every plant/factory where steam is used. It is frequently contaminated and cannot be reused. Discharging the spent steam to the atmosphere is a serious energy loss and environmental issue. Recycling the steam can significantly improve the overall energy efficiency of an industry, which is responsible for massive CO(2) emissions. Steam separation at high fluxes and temperatures has been accomplished with a composite poly(vinyl alcohol) membrane containing silica nanoparticles, and also, less efficiently, with an inorganic zeolite membrane.

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 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.

Supported ruthenium nanoparticles on polyorganophosphazenes: preparation, structural and catalytic studies

Abstract Supported Ru nanoparticles on a number of polyorganophosphazenes were prepared and tested for the hydrogenation of unsaturated compounds. The complex Ru(η6-cycloocta-1,3,5-triene)(η4-cycloocta-1,5-diene) was found to be a suitable precursor to deposit metallic nanoparticles on polyorganophosphazenes; upon the removal of cycloolefin ligands under hydrogen atmosphere, highly dispersed metal particles on the polymeric support can be obtained. Ru on polydimethylphosphazene was found to be an active catalyst for the hydrogenation of a wide range of unsaturated substrates (olefins, carbonyl compounds and aromatic compounds) under mild conditions. Polyorganophosphazenes are interesting in their ability to act as either soluble or insoluble catalyst supports, depending upon the dispersing liquid; thus, it is possible to operate in heterogeneous phase as well as in homogeneous phase. The reaction in homogeneous phase can be performed in environmental-friendly solvents such as alcohols and water. The catalyst exhibits high stability towards agglomeration. No significant change in the ruthenium nanoparticles surface as well as catalyst activity was observed.

Formation and thermal decomposition of rare-earth carbonates

The preparation of carbonate-containing rare-earth compounds and their thermal decomposition in 0%–17% CO2/N2 gas streams has been studied. Three types of rare-earth carbonate or hydroxycarbonate were produced by precipitation; with ammonium bicarbonate as the precipitant, La2 (CO3)3,CeOCO3 and Ln (OH)x (CO3)y (Sm, Tb, and Yb) were obtained, whereas with Na2CO3, only the normal carbonate, Ln2(CO3)3 (La, Sm, Tb, Er and Yb) was found. Temperature programmed decomposition studies revealed that the normal carbonate decomposed stepwise via a dioxocarbonate, Ln2O2CO3, to the oxide. In contrast, the hydroxycarbonates decomposed directly to the oxide. The presence of CO2 during heating had minimal effect on the decomposition of Ln2(CO3)3 to La2O2CO3 but raised significantly the decomposition temperature of Ln2O2CO3 to the oxide. As CO2 is a major product of the rare-earth oxide catalysed oxidative coupling of methane, these observations indicate that the state of catalyst carbonation will be dependent on the reaction temperature, overall catalyst selectivity and preparative method.

Oxidative Dehydrogenation of Isobutane to Isobutylene Over Supported Transition Metal Oxide Catalysts

Various supported transition metal oxides have been tested for the oxidative dehydrogenation of isobutane to isobutylene. Chromium oxide supported on lanthanum carbonate has been found to be active and selective for this reaction.

Structure retention in cross-linked poly(ethylene glycol) diacrylate hydrogel templated from a hexagonal lyotropic liquid crystal by controlling the surface tension

Retaining hexagonal lyotropic liquid crystal (LLC) structures in polymers after surfactant removal and drying is particularly challenging, as the surface tension existing during the drying processes tends to change the morphology. In this study, cross-linked poly(ethylene glycol) diacrylate (PEGDA) hydrogels were prepared in LLC hexagonal phases formed from a dodecyltrimethylammonium bromide (DTAB)/water system. The retention of the hexagonal LLC structures was examined by controlling the surface tension. Polarized light microscopy, X-ray diffraction and small angle X-ray scattering results indicate that the hexagonal LLC structure was successfully formed before polymerization and well retained after polymerization and after surfactant removal when the surface tension forces remained neutral. Controlling the surface tension during the drying process can retain the nanostructures templated from lyotropic liquid crystals which will result in the formation of materials with desired nanostructures.

Ru-doped phosphazene membrane materials used for catalytic hydrogenation

Abstract Semicrystalline polydimethylphosphazene [−NP(CH3)2−]n (PDMP) and the catalyst Ru-doped PDMP, in both the virgin and hydrogenated forms, have been analysed using differential scanning calorimetry spectroscopy (DSC), 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction (XRD) and positron annihilation lifetime spectroscopy (PALS). These techniques in combination indicate that the catalyst Ru coordinates the polymer and alters the structural conformation of the polymeric chain. The PALS data show that this coordination induces structural ordering that results in a decrease in the size of the inter- and intra-chain spacing. This ordering is also evidenced by a decrease in the intensity of the 31P upfield resonance, an increase in Tg and the appearance of a pre-melting endotherm.