Advanced Metal/Ceramic Catalysts for Hydrogen Generation by Steam Reforming of Hydrocarbons
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structure formation as related to activity. EXPERIMENTAL As starting compounds, powdered Al of the PA-4 grade, LaNiO 3 synthesized either by the high-frequency induction plasma thermolysis of the mixed nitrates solution [4] (> perovskite. According to XRD data, all three types of the most strong reflexes typical for the fcc lattice, were observed for powdered Al. A phase of the hexagonally distorted LaNiO 3 perovskite with small admixtures of the phase with a layered-type structure (La 2NiO 4 ) and NiO oxide were revealed in the ceramic sample. For «(plasmochemical> sample, the layered-type perovskite phases were the most abundant along with the oxides of lanthanum and nickel. Vol. % 20
10
20
20 -
10
0 2,7
4.3
I.
101.8 17,1
27.2
1:1,2 68.6
di,pm Fig. 1. Particle size distribution in different powders: a-Al, b-La-Ni-O ceram, c-La-Ni-O plasm.
122
Composite catalyst formation During hydrothermal treatment, a partial oxidation of Al and formation of boehmite particles on its surface occurs [3]. The resulting increase of the solid phase volume in a confined space causes a self-pressing of powders into a monolith. Fig. 2 demonstrates the thermal analysis data for catalysts samples with the LaNiO 3 -based active component. For comparison, the data for a pure cermet matrix are also shown here. For all samples, endoeffects at 120 C° and 520 'C accompanied by a weight loss were observed and assigned to desorption of water and boehmite decomposition, respectively [3]. In all cases, exoeffect at - 330 'C was also observed. Earlier, [3], it was assigned to a structural relaxation in the metastable aluminum oxohydroxide phase formed by the hydrothermal oxidation of aluminum. More pronounced exoeffects for samples containing lanthanum nickelates imply that nickel and lanthanum cations are in part incorporated into the aluminum oxohydroxide lattice thereby increasing is disordering. A second exoeffect situated at 640 'C is assigned to aluminum oxidation. It is superimposed on a large endoeffect at 660 'C caused by the melting of aluminum. At higher temperatures, a continuous weight increase due to the melt oxidation is observed. The last phenomenon was much less pronounced for the perovskite - containing composites, indicating a retardation of the aluminum oxidation due to the presence of Ni and La in the oxidic layer encapsulating the melt droplets. For perovskite containing systems, two new endoeffects accompanied by a weight loss were revealed at - 350 'C and -385 - 400 'C. These features can be assigned to decomposition of lanthanum hydroxide and/or mixed nickel-lanthanum oxohydroxides, which usually decompose at temperatures intermediate between those corresponding to individual hydroxides [7,8].
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120
330
,
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f
640 ' 665
dashed. curve
....... ./
DTA.120
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535
400
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Fig. 2. Differential thermal analysis data of powdered precursor after HTT treatment and drying at 120 'C: I - At, 2 - AI÷(La-Ni-O plasm), 3 - AI+(La-Ni-O ceram). Full curve
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