The increase in the reduction temperature with

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Unformatted text preview: Ni/Mg ratio shows that the lower the Ni/Mg ratio of the samples the more difficult it is to reduce them. Other authors also reported this general trend for similar HT derived catalysts [9, 125]. This effect is attributed to the substitution of Ni2+ by Mg2+ ions leading to the formation of a NiO-MgO solid solution upon calcination, where Ni2+ ions are stabilised against reduction by an inert MgO matrix. The stabilisation of Ni2+ ions by an inert MgO matrix has also been reported for solid NiO-MgO solutions [75, 77, 86, 88, 106]. The higher the substitution of Ni2+ by Mg2+ ions the more stabilised the Ni2+ ions are against reduction [9, 86, 88, 106]. The shoulder in the lower temperature peak of Ni72C900 was even better visible when a higher heating rate was applied, 10°C/min instead of 5°C/min (Fig. 18). It may be due to several factors. It could possibly result from the reduction of fine surface particles of NiO not detectable by XRD. It may also be due to a decrease in the accessibility of the NiO surface of the sample [8]. 89 3. Results and discussion - catalyst preparation and characterisation reaction rate [m mol H2 / (s·g)] 1m mol H2 / (s·g) 10°C/min 5°C/min 0 200 400 600 temperature [°C] 800 1000 Fig. 18 TPR profiles of Ni72C900 at different heating rates (5 and 10°C/min). The apparent higher amount of reduced Ni at 10°C/min results from the higher heating rate. Integration of the peaks over the time revealed the same amount of reduced Ni for both heating rates Verifications of the existing reduction model In order to verify the model of the reduction of the catalysts reduction combined with high temperature XRD was employed. The Ni/Al catalyst was used, since it was the catalyst easiest to reduce and it does not contain not reducible MgO and MgAl2O4 phases that would complicate the interpretation of the XRD spectra after the reduction. A catalyst with a medium Ni/Mg ratio (Ni30C900) was additionally investigated. The spectra were recorded before and after the distinct...
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