Molecular-level understanding of interfacial carbonates in stabilizing CuO-ZnO(Al2O3) catalysts

A descriptor of active CuO-ZnO(Al2O3) methanol-synthesis and water–gas-shift catalysts is the presence of high-temperature carbonates (HT-CO3) in the oxidic catalyst precursor. Previous reports have shown that such HT-CO3 lead to an appropriate interaction between the oxides; as a result, a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) can be achieved. Yet, their nature is not well understood. In this study, the nature of these carbonates was investigated by experimental and theoretical methods in the oxidic precatalyst. A calcined Cu-Zn-Al hydrotalcite model compound revealed to have well-dispersed ZnO and CuO phases, together with highly stable HT-CO3. It was hypothesized that these HT-CO3 groups may be placed at critical locations at nano-scale as a glue, thus avoiding the growth of the oxide crystallites during calcination. This is an excellent pre-condition to achieve a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) upon reduction, and therefore a high activity. To prove that, first-principles calculations were carried out based on the density functional theory (DFT); alumina was not considered in the model as the experimental data showed it to be amorphous but it may still have an effect. Comprehensive calculations provided evidence that such carbonate groups favourably bind the CuO and ZnO together at the interface, rather than being isolated on the individual oxide surfaces. The results strongly suggest that the HT-CO3 groups are part of the structure, in the calcined precatalyst, where they would prevent thermal sintering through a bonding mechanism between CuO and ZnO particles, which is a novel interpretation of this important catalyst descriptor

Nurses' perceptions of aids and obstacles to the provision of optimal end of life care in ICU

Contains fulltext : 172380.pdf (publisher's version ) (Open Access

One-pot catalyst preparation: combined detemplating and Fe ion-exchange of BEA through Fenton’s chemistry

BEA zeolite has been simultaneously detemplated and Fe-exchanged by treating the parent zeolite with a Fenton’s-type reagent (Fe3+–H2O2) at low temperature. This one-pot process simplifies and speeds up considerably the preparation route. The catalyst shows excellent performance on N2O decomposition compared to conventionally prepared Fe-BEA.

Innovations in the synthesis of Fe-(exchanged)-zeolites

Several aspects on the preparation of Fe-zeolites are discussed. In contrast to the many studies highlighting the characterisation of the active sites, new approaches for incorporation of Fe are presented. Full utilization of exchange capacity of zeolites has been achieved by a controlled alkaline treatment of the parent sample. With this method, iron can be fully exchanged by liquid phase ion-exchange on ZSM5 without the formation of inactive Fe-oxides. The second topic is the use of a mild oxidant (H2O2, and peroxides in general) to break down strong complexating equilibria during ion-exchange by controlled redox titration of the ligands. Hydrogen peroxide oxidizes effectively chelating groups releasing Fe species at a controlled rate. The method is demonstrated for the preparation of Fe-FER through Ferric-citrate. The final concept discussed is the detemplating of the zeolite with the simultaneous incorporation of the iron (combined detemplation and ionexchange). This one-pot preparation minimizes the number of steps considerably. To realize this, a strong oxidant is necessary to remove the organic template, and Fe-cations for exchange. Both requirements are met with the Fenton’s-chemistry (Fe3+/2+/H2O2 mixtures) involving radical chemistry.

Catalyst studies on the hydrotreatment of fast pyrolysis oil

Catalytic hydrotreatment is considered an attractive technology for fast pyrolysis oil upgrading to liquid transportation fuels. We here report an experimental study to gain insights in catalyst stability when using Ru/C catalysts for the hydrotreatment of fast pyrolysis oil (350°C and 200bar) in a batch reactor set-up. A considerable reduction in the liquid yield (55-30wt.%), increased solids formation (3-20wt.%), a reduction in the H/C ratio (1.24-1.08) of the liquid product and a lowering of the extent of methane in the gas phase was observed after a number of catalyst recycles. Characterization of the catalyst before and after reaction using TEM, chemo- and physisorption showed significant coke deposition and a decrease in pore volume and metal dispersion. The application of in-house prepared Ru/C catalysts for both the hydrotreatment of fast pyrolysis oil as well as phenol using different Ru-precursors (RuCl3, Ru(NO)(NO3)3 and Ru(acac)3) gave different results for the various catalysts with respect to product yield (45-56wt.% for fast pyrolysis oil) and elemental composition of the liquid phase. A catalyst prepared from the precursor RuCl3 at a ruthenium loading of 5wt.% showed the highest activity for the hydrogenation reaction of fast pyrolysis oil (H/C of 1.32 vs. 1.24 for the commercial Ru/C catalyst) and the lowest reduction in BET area and metal dispersion after reaction.

Bulk and Surface Structures of Palladium-Modified Copper-Zinc Oxides ex Hydroxycarbonate Precursors

(Pd)-Cu-Zn ex hydroxycarbonate precursors were prepared and characterized by several bulk and surface techniques. A palladium-free Cu-Zn precursor (CZ) was prepared by coprecipitation. Two Pd-Cu-Zn samples were prepared by coprecipitation (PCZ-CP) and sequential precipitation (PCZ-SP). It is shown that the surface and bulk structures of Pd-modified precursors depend on the palladium incorporation methodology. This effect does not appear to be influenced by palladium incorporation itself but results from the conditions used in the preparation of the materials, more specifically the sodium concentration from the precipitating agent Na2CO3. A higher Na2CO3 concentration was needed in the preparation of PCZ-CP than for the other precursors. The higher Na concentration in the synthesis medium affects the growth of the sodium-zinc carbonate precursor and gives rise to the formation of independent Cu and Zn phases in the final precursor. The interaction between CuO and ZnO in the calcined state is then inhibited, resulting in an increase in the particle size of both oxidic phases. The lower Na concentration used during the synthesis of the materials CZ and PCZ-SP favors the development of an aurichalcite phase, a structure in which the Cu and Zn are atomically dispersed in a hydroxycarbonate matrix. This leads to finely dispersed copper and zinc mixed oxides with smaller particle sizes than those derived from independent Cu and Zn precursor phases, as in the case of PCZ-CP.