Although the spectra of ni30c900 were less intensive

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Unformatted text preview: . Before the reduction process of Ni30C900 only reflections due the divalent metal oxide phase with periclase structure and the spinel phase were observed. At 600°C the spectrum was unchanged. At 900°C the reflections that were entirely due to the divalent metal oxide phase or both phases decreased drastically, whereas the reflection due to the spinel phase alone seemed to be unchanged. Simultaneously two new reflections due to metallic Ni were observed. The results show that the existing model for the reduction of the Ni/Al and Ni/Mg/Al catalysts is correct, first reduction of the Ni 2+ ions in the divalent metal phase with periclase structure to metallic Ni and second the reduction of Ni 2+ ions in the spinel phase. They also confirm the enhanced resistance of the catalysts against reduction upon the substitution of the Ni 2+ by Mg2+ ions. The fact that the divalent metal oxide phase did not disappear after reduction at 900°C is attributed to the presence of the not reducible MgO phase. The fact that no reflections of an Al3+ containing phase were observed, may result from the formation of an amorphous or a poorly crystallised Al2O3 phase not detectable due to the X-ray adsorption by the high amounts of Ar present in the reduction mixture (95%). 3. Results and discussion - catalyst preparation and characterisation 93 3.2.5. Ni dispersion The active surface area of a catalyst (i.e. the surface area of the active phase) is often expressed relatively as the dispersion of the active phase, defined as the ratio of the number of the active sites (or the superficial atoms of the active phase) to the total amount of the atoms of the active phase. This meaning of dispersion should not be confused with that used for characterisation of particle size distribution in a particulate matter. The dispersion of the active phase is an important parameter of the catalyst, related to the total number of the active sites and the average particle size of the active phase that both determine the catalyst’s performance. The knowledge of the metal dispersion in the Ni catalysts is important, since it may influence the cataly...
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This document was uploaded on 10/07/2013.

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