The knowledge of the metal dispersion in the ni

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Unformatted text preview: tic activity and selectivity as well as the resistance against coking, as found for the steam reforming process. In order to estimate the degree of dispersion of the Ni particles after reduction the catalysts were passivated at low temperatures (> 40°C) using 5% O2 diluted with He. Such a passivation results in the formation of only two or three monolayers of nickel oxide [110]. After passivation the catalysts were reduced again, the results are shown in Figure 21. On the basis of these results it was possible to estimate the dispersion of Ni particles expressed as the ratio of the amount of Ni reduced after passivation to the total amount of reducible Ni after calcination (Fig. 22). For a better comparison the scale of the y-axis is the same as in the TPR profiles of the calcined catalysts. All passivated catalysts exhibited reduction peaks in the temperature region 200-225°C. The amount of consumed H2 was about 5 - 23% of the amount consumed in the reduction of the calcined catalysts. The dispersion was highest for the catalyst ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp 3. Results and discussion - catalyst preparation and characterisation 94 ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp with a low to medium Ni/Mg ratio with more than 10% and decreased at higher Ni/Mg ratio to 5% at its lowest for Ni72C900 the pure Ni/Al catalyst. The low reduction temperature and the considerably lower H2 consumption compared to the TPR of the calcined catalysts confirm that by the passivation process no bulk Ni was oxidised [59]. Borowiecki, who investigated the reducibility and Ni dispersion of alumina supported Ni/Mg steam reforming catalysts also observed a rapid decline in the Ni dispersion at Ni/Mg ratio •  WKH VDPH UDWLR DV LQ WKH FDWDO\VW 1L& >@ Although such an effect may be expected as the Ni dispersion generally decreases with increasing Ni content due to enhanced sintering of the Ni particles, Choudhary et al. found higher Ni dispersion for a supported catalyst precoated with MgO than precoated...
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