Catalysts, Volume 2, Issue 3 (September 2012) – 7 articles , Pages 327-411

The effect of extended H₂ sorption cycles on the structure and on the hydrogen storage performances of MgH₂ powders with 5 wt% of Fe particle catalyst is reported. MgH₂ powders with and without Fe have been ball milled under Argon, the doped MgH₂ nanocomposite has been cycled under hydrogen pressure up to a maximum of 47 desorption and absorption cycles at 300 °C. After acceleration during the first 10 cycles, the kinetics behavior of doped MgH₂ is constant after extended cycling, in terms of maximum storage capacity and rate of sorption. The major effect of cycling on particle morphology is the progressive extraction of Mg from the MgO shell surrounding the powder particles. The Mg extraction from the MgO shell leaves the catalyst particles inside the hydride particles. Many empty MgO shells are observed in the pure ball milled MgH₂ upon cycling at higher temperature, suggesting that this enhancement of the extraction efficiency is related to the higher operating temperature which favors Mg diffusivity with respect to the H ion one. Full article

Cobalt talc doped with iron (Fe/Co~0.1) and dispersed in SiO₂ aerogel was prepared from silica alcogel impregnated with metal nitrates by supercritical drying. Catalytic honeycombs were prepared following the same procedure, with the alcogel synthesized directly over cordierite honeycomb pieces. The composite aerogel catalyst was characterized by X-ray diffraction, scanning electron microscopy, focus ion beam, specific surface area and X-ray photoelectron spectroscopy. The catalytic layer is about 8 µm thick and adheres well to the cordierite support. It is constituted of talc layers of about 1.5 µm × 300 nm × 50 nm which are well dispersed and anchored in a SiO₂ aerogel matrix with excellent mass-transfer properties. The catalyst was tested in the ethanol steam reforming reaction, aimed at producing hydrogen for on-board, on-demand applications at moderate temperature (573–673 K) and pressure (1–7 bar). Compared to non-promoted cobalt talc, the catalyst doped with iron produces less methane as byproduct, which can only be reformed at high temperature, thereby resulting in higher hydrogen yields. At 673 K and 2 bar, 1.04 NL_H2·mL_EtOH(l)⁻¹·min⁻¹ are obtained at S/C = 3 and W/F = 390 g·min·mol_EtOH⁻¹. Full article

Tetraethoxy orthosilicate (TEOS) was used not only as a precursor of silica, but also as an agent which reinforces the skeleton of silica-gel to prepare an aerogel and resultant silica and silica-alumina with large pore size and pore volume. In this gel skeletal reinforcement, the strength of silica aerogel skeleton was enhanced by aging with TEOS/2-propanol mixed solution to prevent the shrink of the pores. When silica aerogel was reinforced by TEOS solution, the pore diameter and pore volume of calcined silica could be controlled by the amount of TEOS solution and reached 30 nm and 3.1 cm³/g. The results from N₂ adsorption measurement indicated that most of pores for this silica consisted of mesopores. Silica-alumina was prepared by the impregnation of an aluminum tri-sec-butoxide/2-butanol solution with obtained silica. Mixed catalysts were prepared by the combination of β-zeolite (26 wt%) and prepared silica-aluminas with large mesopore (58 wt%) and subsequently the effects of their pore sizes on the catalytic activity and the product selectivity were investigated in catalytic cracking of n-dodecane at 500 °C. The mixed catalysts exhibited not only comparable activity to that for single zeolite, but also unique selectivity where larger amounts of branched products were formed. Full article

Alumina-supported manganese catalysts with cryptomelane and/or birnessite structure have been prepared using a simple method based on the thermal decomposition of potassium permanganate. The samples have been characterized by XRD, FTIR, TGA, DSC, N₂ adsorption at −196 °C, SEM, H₂-TPR and XPS, and their catalytic activity for soot combustion has been tested and compared to that of a reference Pt/alumina catalyst. The thermal decomposition of alumina-supported KMnO₄ yields a mixture of supported birnessite and potassium manganate which is the most effective, among those prepared, to lower the soot combustion temperature. However, this material is not useful for soot combustion because the accelerating effect is not based on a catalytic process but on the oxidation of soot by potassium manganate. A suitable soot combustion catalyst is obtained after potassium manganate is removed by water washing, yielding only the birnessite phase on the γ-Al₂O₃ support. This birnessite phase can be transformed into cryptomelane by calcination at 600 °C. These two samples, γ-Al₂O₃-supported birnessite and cryptomelane are suitable catalysts for soot combustion in NO_x/O₂ mixtures, as their catalytic activity is based on the NO₂-assited mechanism, that is, both catalysts accelerate the oxidation of NO to NO₂ and NO₂ promotes soot oxidation. The soot combustion temperatures obtained with these birnessite/cryptomelane alumina-supported catalysts are similar to that obtained with the reference Pt/alumina catalyst. Full article

We report a study on the desorption properties, crystallography and chemical state of MgH₂ and 1 mol% Nb₂O₅ ball-milled composites. Desorption temperatures of the composites decreased with increase of ball-milling time. Size of MgH₂ crystallites decreased during ball-milling. Reduction of Nb₂O₅ after ball-milling was confirmed by tracing the chemical state of Nb and was further supported by TEM observation. The reduced phases may act as more effective catalysts improving the desorption properties. Full article

The kinetics of hydrogen absorption/desorption in magnesium can be improved without any catalysis assistance and MgO was found to be more effective than the best catalyst reported so far, i.e., Nb₂O₅. Herein, a quantitative analysis of the hydrogen kinetics in magnesium modified with MgO was performed in order to identify possible rate controlling mechanisms. While hydrogen absorption was found to be diffusion controlled as commonly reported, hydrogen desorption evolved from nucleation and growth to an interface controlled process depending on the desorption temperature. Comparison with the effect of Nb₂O₅ indicates that similar rate limiting steps occur regardless of the oxide additive. These findings are reconciled by considering the tribological effect of solid oxide additives, as a correlation between oxides electronegativity and improvement in hydrogen kinetics was found. Such a correlation clearly highlights the mechanical effect of solid oxides in facilitating the grinding and stabilisation of small magnesium particles for efficient and fast hydrogen kinetics. Full article