The tpr spectra of the other catalysts did not

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Unformatted text preview: it such a shoulder. In contrast to the pure Ni/Al catalyst all magnesium containing samples exhibited only one reduction peak, which was shifted from 480°C for Ni72C900 to 730°C for Ni50C900 and to even higher temperatures with increasing Mg content. The decrease in the H2 consumption with decreasing Ni content corresponded to the decreasing total amount of Ni in the catalysts. The lower temperature peak of the catalyst Ni72C900, the pure Ni/Al catalyst, is attributed to the reduction of Ni2+ ions in the divalent metal oxide phase (NiO) while the higher temperature peak results from the reduction of Ni 2+ ions in the spinel phase (NiAl2O4) [6, 8, 9, 60]. This model for the interpretation of the reduction has been proposed in the literature on the basis of the similarity of the positions of the TPR peaks of HT derived Ni/Al catalysts to those of pure NiO and NiAl2O4 [6, 60]. Further evidence was presented by Alzamora et al., who observed by XRD that a Ni/Al HT 87 3. Results and discussion - catalyst preparation and characterisation ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp derived catalysts consisting of NiO and NiAl2O4 phases after its calcination at 1000°C, still contained NiAl2O4 after subsequent reduction at 600°C in a flow of hydrogen [59]. 1m mol H2 / (s·g) reaction rate [m mol H2 / (s·g)] Ni72C900 Ni50C900 Ni30C900 Ni15C900 Ni10C900 Ni5C900 0 200 400 600 temperature [°C] Fig. 17 TPR profiles of the Ni containing catalysts calcined at 900°C 800 1000 3. Results and discussion - catalyst preparation and characterisation 88 The reduction peak in the spectra of each Ni/Mg/Al catalyst is attributed to the reduction of the Ni2+ ions in the divalent mixed metal oxide phase with periclase structure [9]. The total amount of reducible Ni in the divalent metal oxide phase reflected the total Ni/Mg ratio in the samples, suggesting no strong preference of either Ni2+ or Mg2+ ions in the formation of the divalent metal oxide phase or the spinel phase. The increase in the reduction temperature with decreasing...
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