A. Van Neste

Catalytic effect of transition metals on hydrogen sorption in nanocrystalline ball milled MgH2-Tm (Tm=Ti, V, Mn, Fe and Ni) systems

Abstract Intensive mechanical milling was used to make MgH2–Tm (Tm=3d-transition elements Ti, V, Mn, Fe, Ni) nanocomposite powders. The hydrogen storage properties of these composite powders were evaluated. The five 3d-elements Ti, V, Mn, Fe and Ni showed different catalytic effects on the reaction kinetics of Mg–H system. Desorption was most rapid for MgH2–V, followed by MgH2–Ti, MgH2–Fe, MgH2–Ni and MgH2–Mn at low temperatures. The composites containing Ti exhibited the most rapid absorption kinetics, followed in order by Mg–V, Mg–Fe, Mg–Mn and Mg–Ni. Formation enthalpy and entropy of magnesium hydride were not altered by milling with transition metals, while the activation energy of desorption for magnesium hydride was reduced drastically.

Structural study and hydrogen sorption kinetics of ball-milled magnesium hydride

It has recently been discovered that energetic ball milling of hydrides can improve their hydrogen sorption properties significantly. In this work, we present a systematic study of structural modifications and hydrogen absorption–desorption kinetics of ball-milled magnesium hydride. Structural investigations showed that after only 2 h of milling, a metastable orthorhombic (γ) magnesium hydride phase is formed. A Rietveld analysis of the X-ray diffraction spectrum of the 20 h milled sample gave a proportion of 74 wt.% MgH2, 18 wt.% γ MgH2 and 8 wt.% MgO. The hydrogen capacity and sorption kinetics were measured before and after milling. We found that the sorption kinetics are much faster for the milled sample compared to the unmilled one. This explains the fact that the hydrogen desorption temperature of the ball-milled sample as measured by pressured differential scanning calorimetry (PDSC), is reduced by 64 K compared to the unmilled sample. There is no significant change of the storage capacity upon milling and the absorption plateau pressure does not change. From the desorption curves, the activation energy was deduced. The milling also increased the specific surface area. This was confirmed by SEM micrographs and BET measurements. Possible mechanisms explaining the improved kinetics are presented.

Hydrogen absorption properties of a mechanically milled Mg–50 wt.% LaNi5 composite

Abstract Mechanical milling was used to make composite Mg–50 wt.% LaNi 5 powders. The structural changes during the milling process and the hydrogen storage properties of the mechanically milled composite were characterized. Mechanical milling leads to a nano-composite, which is not stable upon high temperature (573 K) hydriding and dehydriding cycling. The nano-composite transforms to a new Mg+LaH x +Mg 2 Ni composite, which is stable upon further cycling. The new composite has excellent hydrogen absorption kinetics at low temperatures. The storage capacity reaches 2.5 wt.% in 500 s under 1.5 MPa hydrogen at 302 K. The optimum capacity is 4.1 wt.% at intermediate temperatures (523–573 K). The high absorption rate is explained by the high quantity of phase boundaries and the porous surface structure.

Hydrogen storage in mechanically milled Mg–LaNi5 and MgH2–LaNi5 composites

Magnesium and magnesium hydride were mechanically milled with LaNi5 to make a Mg–Ni–La ternary alloy for hydrogen storage. Mechanical milling of MgH2+LaNi5 or milling of Mg+LaNi5 followed by a full hydrogenation leads to a composite of MgH2+LaH3+Mg2Ni. Upon hydrogen absorption/desorption cycling, a mixture of Mg+LaH3+Mg2Ni phases is obtained in both cases, but with different powder sizes. The powder size is greatly reduced by using MgH2 instead of Mg in the milling process. The reduction in powder size gives faster absorption kinetics, and slower desorption kinetics. Adding both Ni and La to Mg-based alloys produces a synergetic effect on the hydrogen absorption/desorption. The ternary Mg–Ni–La alloy showed much better absorption and desorption kinetics than the binary alloys Mg–La and Mg–Ni. Lanthanum hydride has strong catalytic effects on absorption of Mg, but weak effects on desorption. Mg2Ni has better catalytic effect than lanthanum hydride at temperatures above 373 K.

Structural transformations of alumina by high energy ball milling

Room temperature, high energy ball milling was applied to various transition aluminas ( γ, K , χ) , producing thermodynamically stable α-alumina–a phenomenon that could otherwise be achieved only by high temperature (1100–1200 °C) heat treatment. The transformation proceeds in two steps. The first one consists of rapid microstructural rearrangements with continuously increasing α-transformation rate. In the second step (1–2 h from the start), only relatively small changes in morphology are observed with a constant α-transformation rate. The rate is influenced only by the milling intensity. The presence or the absence of oxygen in the milling atmosphere has a large influence on the final surface area of α-alumina.

Perovskite-type oxides synthesized by reactive grinding: Part IV. Catalytic properties of LaCo1−xFexO3 in methane oxidation

Abstract In this part of the present work, the catalytic properties in methane oxidation of perovskite-type mixed-oxides, LaCo 1− x Fe x O 3 prepared by reactive grinding of the component oxides were studied. A fine control of the nature and concentration of the various sites active in oxido-reduction catalysis was performed by applying the comprehensive picture of the effects of thermal pretreatment of the mixed-oxides generated by this technique, developed in part I of this contribution. These properties which were previously illustrated by the kinetics of complete oxidation of n -hexane over five nanocrystalline LaCo 1− x Fe x O 3 catalysts treated in parts II and III are now demonstrated by a kinetic study of methane oxidation over the same catalysts.

Hydrogen storage properties of nanocrystalline Mg1.9Ti0.1Ni made by mechanical alloying

The mechanical alloying technique was used to make nanocrystalline Mg2Ni and Mg1.9Ti0.1Ni powders. Their hydrogen storage properties were characterized and compared with that of polycrystalline Mg2Ni made by casting. It was found that the ball milled Mg2Ni and Mg1.9Ti0.1Ni can absorb hydrogen at 473 K in the first cycle rapidly without prior activation. The nanocrystalline Mg1.9Ti0.1Ni showed the best kinetics, followed in order by mechanically alloyed Mg2Ni and cast Mg2Ni. Mg1.9Ti0.1Ni absorbs more than 3 wt% H2 in 2000 s at 423 K while the mechanically alloyed and the cast Mg2Ni do not form hydride at this temperature. Partial substitution of Mg by Ti reduced the activation energy of desorption from 69 kJ mol−1 for nanocrystalline Mg2Ni to 59 kJ mol−1 for Mg1.9Ti0.1Ni.

NANOCRYSTALLINE NI-MO ALLOYS AND THEIR APPLICATION IN ELECTROCATALYSIS

The structural and electrocatalytic properties of metastable Ni-Mo alloys have been investigated for the hydrogen evolution reaction in alkaline solutions. Amorphous and nanocrystalline phases have been prepared by mechanically alloying the elemental components under various milling conditions. Fcc nanocrystals are formed when the Mo concentration is smaller than 30 at. %. The nanocrystalline state becomes unstable with respect to the amorphous phase when the Mo content in the solid solution exceeds 30 at. %. The electroactive phase for the hydrogen evolution reaction in alkaline solutions is the nanocrystalline supersaturated solid solution. The presence of oxygen during the milling process improves the properties of the alloys.

Effect of high-energy ball milling on the structural stability, surface and catalytic properties of small-, medium- and large-pore zeolites

Abstract Samples of NaA, CaA, KL, Silicalite-1, H-ZSM-5 and HY zeolites were submitted to high-energy ball milling and their properties were examined as functions of milling time. BET surface area, micropore volumes and external surface area as well as x-ray diffraction crystallinity and catalytic activity in the micropulse conversion of hydrocarbons with various kinetic diameters ( n -hexane, isooctane and toluene) were monitored. The results indicate that the mechanical resistance of the zeolite lattice is clearly correlated with its Si Al ratio. For small-pore zeolites an initial increase in catalytic activity is associated with an increase of the accessible surface area.

Perovskite-type oxides synthesised by reactive grinding: Part III. Kinetics of n-hexane oxidation over LaCo(1−x)FexO3

The catalytic properties in the n-hexane oxidation reaction of five nanocrystalline1 LaCo(1−x)FexO3 perovskites prepared by mechanosynthesis were studied. The characterisation of such samples was reported in Part I of this contribution. One of the advantages of this mechanosynthesis technique stems from the low preparation temperature, which allows to stabilise high surface area materials. In Part I, the unusual properties of these new materials were illustrated for LaCoO3 and LaCo(1−x)FexO3 perovskites [1]. It was shown that controlling the calcination conditions allows to tailor the surface properties of these materials. In Part II, a preliminary account of the catalytic behaviour of such catalysts in n-hexane oxidation was reported [2]. In the present contribution, the fine control of the nature and the concentration of the various sites active in redox catalysis is demonstrated through a kinetic study of the total oxidation of n-hexane by oxygen. The roles of two oxygen species O2− and O−, respectively, associated with α and β oxygens are discussed.