At lower temperatures poorly crystallised divalent

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Unformatted text preview: l of the gases formed during the calcination process and slow heating rate had a beneficial influence on the catalysts’ preparation. The studies of the activation process by the reduction in hydrogen of the catalysts calcined at 900°C showed that the reduction takes place in two distinct steps. First the Ni2+ ions in the divalent metal oxide phase with periclase structure are reduced and then at higher temperatures also the Ni2+ ions in the spinel phase. At temperatures up to 950°C the Ni2+ ions in the divalent metal oxide phase were reduced in all the catalysts, the Ni2+ ions in the spinel phase only in the magnesium free one. The catalysts with a lower nickel content were more difficult to reduce but exhibited higher nickel dispersion. These findings are attributed to the substitution of Mg2+ by Ni2+ leading to the formation of a NiO-MgO solid solution upon calcination, which stabilises 5. Conclusions 142 ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp the Ni2+ ions against reduction and sintering. The formed metallic nickel particles are not stabilised by the MgO matrix against consecutive oxidation. Investigations of the influence of the calcination temperature on the properties of the catalyst Ni10 revealed that the reduction temperature increases slightly with increasing crystallinity of the catalysts. In the catalysts with a well crystallised structure only the Ni2+ ions in the divalent metal oxide phase were reducible below 950°C, whereas in the catalysts showing only a poorly crystallised structure almost all Ni2+ ions could be reduced. For the catalysts with a well crystallised spinel phase the dispersion decreased with increasing calcination temperature of the solid. In the case of the catalyst without a well crystallised spinel phase the total amount of surface Ni is higher, but the dispersion lower, which is attributed to the higher total amount of the reduced Ni atoms. Investigations of the basicity by temperature programmed desorption of CO2 after adsorption at 100°C for 1 hour revealed no d...
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This document was uploaded on 10/07/2013.

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