Found higher ni dispersion for a supported catalyst

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Unformatted text preview: with CaO, BaO or Yb2O3 [106]. Therefore it can be concluded that the MgO matrix does not only stabilise the Ni2+ ions against reduction, but also against sintering, leading to higher reduction temperatures and resulting in a better dispersion of the Ni particles formed. Furthermore the reduction profiles of the passivated catalysts exhibited no significant temperature shift, which is in contrast to the reduction of the calcined catalysts, where the reduction temperature increased with decreasing Ni/Mg ratio of the catalyst. By the reduction separate metallic Ni particles are formed that are surfacially oxidised by the passivation process. The formed NiO is no longer stabilised against reduction by the MgO matrix. For the formation of a solid solution of NiO with the MgO matrix, the passivation temperature (< 40°C) was much too low. However, it must be pointed out that these dispersion measurements should not be used to draw detailed conclusions about the Ni crystallite size using geometrical models because it was not possible to exactly establish the depth of the surface oxide layer formed by the passivation. 95 3. Results and discussion - catalyst preparation and characterisation 1m mol H2 / (s·g) Ni72C900 reaction rate [m mol H2 / (s·g)] Ni50C900 Ni30C900 Ni15C900 Ni10C900 Ni5C900 0 100 200 temperature [°C] 300 400 Fig. 21 TPR profiles of the passivated catalysts. All catalysts were calcined at 900°C, reduced in TPR and passivated below 40°C 96 3. Results and discussion - catalyst preparation and characterisation 0.23 Ni dispersion 0.2 0.15 0.13 0.13 0.1 0.06 0.05 0 Ni5C900 Ni10C900 Ni15C900 Ni30C900 catalyst Ni50C900 Ni72C900 Fig. 22 Ni dispersion of the catalysts estimated from the TPR results 3.2.6. Temperature programmed oxidation The oxidation of the reduced catalysts Ni10C900, Ni30C900, Ni50C900 and Ni72C900 was investigated by temperature programmed oxidation (TPOx). The spectra exhibited broad peaks starting at about 150 – 200°C with maxima between 350-400°C (Fig...
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This document was uploaded on 10/07/2013.

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