Patricia Benito

Insights into the macroporosity of freeze-cast hierarchical geopolymers

Geopolymer monoliths with controlled lamellar macroporosity and total porosity ranging from 60% to 70% were prepared by ice-templating a partially geopolymerized slurry. Both the maturation treatment of the starting mixture and the water specifically added for freeze-casting were tailored to modify both the geopolymerization and viscosity of the slurry, and, consequently, its freezing behavior, in order to optimize the final lamellar architecture. Following a room temperature maturation treatment, a 50% water content added for freezing developed thick lamellae and wide pores. A lower water content (30%) and curing at 80 °C after maturation at room temperature (for both 50% and 30% H2O) was conducive to a narrow lamellar pore width distribution in the 30–130 μm range. However, the consumption of water due to geopolymerization in samples cured at 80 °C led to a decreased length and thickness of the lamellae. Lastly, the interparticle meso- and macropores (0.003 to 1 μm) within the geopolymer lamellae were only slightly modified by the maturation treatment.

Bimetallic Nanoparticles as Efficient Catalysts: Facile and Green Microwave Synthesis

This work deals with the development of a green and versatile synthesis of stable mono- and bi-metallic colloids by means of microwave heating and exploiting ecofriendly reagents: water as the solvent, glucose as a mild and non-toxic reducer and polyvinylpirrolidone (PVP) as the chelating agent. Particle size-control, total reaction yield and long-term stability of colloids were achieved with this method of preparation. All of the materials were tested as effective catalysts in the reduction of p-nitrophenol in the presence of NaBH4 as the probe reaction. A synergistic positive effect of the bimetallic phase was assessed for Au/Cu and Pd/Au alloy nanoparticles, the latter showing the highest catalytic performance. Moreover, monoand bi-metallic colloids were used to prepare TiO2- and CeO2-supported catalysts for the liquid phase oxidation of 5-hydroxymethylfufural (HMF) to 2,5-furandicarboxylic acid (FDCA). The use of Au/Cu and Au/Pd bimetallic catalysts led to an increase in FDCA selectivity. Finally, preformed Pd/Cu nanoparticles were incorporated into the structure of MCM-41-silica. The resulting Pd/Cu MCM-41 catalysts were tested in the hydrodechlorination of CF3OCFClCF2Cl to CF3OCF=CF2. The effect of Cu on the hydrogenating properties of Pd was demonstrated.

The reducibility of highly stable Ni-containing species in catalysts derived from hydrotalcite-type precursors

A study was conducted on the speciation and reducibility of Ni in catalysts derived from hydrotalcite-type (HT) precursors intercalated by silicates. Silicate and nickel contents in Ni/Mg/Al HT precursors varied and the products obtained by thermal decomposition in the 773–1373 K range were characterized. Sample properties were related to the amount of silicates and nickel. The former altered the formation of the spinel-type phase and decreased the ratio between MgO and MgAl2O4 phases in which the active species were stabilized. On the other hand, the Ni distribution depended on the Ni-loading. Ni-containing species in the spinel phase were more abundant for high Ni-loaded catalysts, and were readily reduced by H2 treatment at 1023 K, whereas those in the Ni1−xMgxO solid solution remained partially unreduced.

One-step electrodeposition of Pd–CeO2 on high pore density foams for environmental catalytic processes

Environmental catalytic processes are probably the best example for the application of structured catalysts based on activated open-cell metallic foams, and Pd/CeO2 based materials are one of the most active catalysts for different applications like three-way catalysts, methane combustion and water gas shift. Here, we report the one-step deposition of Pd and CeO2 on FeCrAlloy foams with very high pore densities (cell sizes equal to 580 and 1200 μm) by the electro-base generation method followed by calcination. The type of Pd and CeO2 species, the quality of the coating and the catalytic activity in the mass transfer limited CO oxidation as a model reaction were investigated. For comparison purposes, a CeO2 coating was also prepared. Reproducible and evenly distributed defective nano-CeO2 coatings containing PdxCe1−xO2−δ solid solution and Pd0 particles were electrodeposited in 500 s regardless of the foam pore size. The structure, morphology, and adhesion of the catalytic layer, as well as the Pd distribution, were almost constant after calcination at 550 °C, only some PdO segregated. The resulting structured catalysts showed high activity and stability after 48 h time-on-stream and different thermal cycles in CO oxidation even at high GHSV values (e.g. 4 × 106 h−1 referred to as the total foam disk volume). High volumetric mass transfer coefficients, widely outperforming those of conventional honeycomb monoliths, were obtained, especially for the small pore (580 μm) foam. The in situ formation of Pd0 nanoparticles and the homogeneous distribution and stability of the coating may contribute to the high catalytic performance, although these catalyst features depended on the position in the catalytic bed.