The spectra exhibited broad peaks starting at about

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Unformatted text preview: . 23). In contrast to the reduction temperature the oxidation temperature was almost independent from the Ni/Mg ratio of the catalyst. This again is attributed to the formation of separate metallic Ni particles by the reduction. These are not stabilised against oxidation by the MgO matrix. 97 3. Results and discussion - catalyst preparation and characterisation 1m mol O2 / (s·g) reaction rate [mmol O2 / (s·g)] Ni72C900 Ni50C900 Ni30C900 Ni10C900 0 200 400 temperature [°C] 600 800 Fig. 23 TPOx profiles of the catalysts after TPR. All catalysts were calcined at 900°C, reduced in TPR, passivated and reduced again 3. Results and discussion - catalyst preparation and characterisation 98 3.2.7. Influence of the calcination temperature on reducibility and Ni dispersion The catalysts Ni10 calcined at 800, 900 and 1000°C (Ni10C800, Ni10C900 and Ni10C1000) were used to investigate the influence of the calcination temperature on the reducibility and the Ni dispersion (Fig. 24, 25 and 26). The results show a shift in the reduction maxima to higher temperatures with increasing calcination temperature, from about 850°C for Ni10C800 to 900°C for Ni10C1000. The amount of reducible Ni2+ ions was higher for Ni10C800 than for Ni10C900 and Ni10C1000. The broad not intensive peak in the reduction spectrum of Ni10C800 at about 470°C is attributed to the destruction of the partially reconstructed HT phase in the catalyst. The total amount of passivated Ni was higher for Ni10C800 than for the other catalysts, but the dispersion decreased in the order Ni10C900 > Ni10C1000 > Ni10C800. The slight increase in the reduction temperature with increasing calcination temperature of the catalysts is attributed to the increasing crystallinity of the catalysts with increasing calcination temperature. The differences in the amount of reducible Ni2+ are in good agreement of the phase model for hydrotalcite-type derived catalysts. The catalysts calcined at temperatures ≥ 900°C consist of well crystallised divalent met...
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This document was uploaded on 10/07/2013.

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