Temperature-programmed reduction study of NiO–MgO interactions in magnesia-supported Ni catalysts and NiO–MgO physical mixture

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Abstract
The reducibility of magnesia-supported nickel catalysts and NiO–MgO physical mixture has been studied by the temperature-programmed reduction (TPR) technique in the temperature range 373–1273 K. The influences of calcination temperature (673–1273 K), treatment time (1–48 h) and Ni loading (2.8–18 wt %) on NiO reduction have been evaluated. The TPR profiles of Ni/MgO catalysts reveal the presence of several forms of NiO, at the surface and in the near-surface regions (bulk-like) of the MgO, the reducibility of which is affected by the interaction strength between Ni2+ and underlying MgO support. A form of ‘unreacted’ NiO located on the MgO surface, which behaves as ‘bulk unsupported NiO’, was detected. Calcination at 673 K promotes a partial migration of Ni2+ ions into the MgO structure, which is enhanced by higher calcination temperatures, hindering the whole reducibility. At 1273 K an ‘ideal’ and non-reducible bulk NiO–MgO solid solution is formed. In contrast, the reducibility of the NiO–MgO physical mixture is slightly affected by calcination up to 1073 K and its TPR profile shows only a reduction peak shifted to higher T by calcination temperature. The different degree of interaction between NiO and MgO in supported and physically mixed systems has been explained in terms of different contact area arising from the preparation method. The importance of calcination time, in order to avoid erratic assessments due to unsteady states in the NiO–MgO reacting system, has been demonstrated. A linear correlation between the reducibility of Ni/MgO catalysts and the Ni loading was found, and was explained by inferring that higher Ni loading increases the fraction of ‘unreacted’ and ‘easy-reducible’ NiO. Experimental results indicate that TPR is a powerful technique for investigating the solid-state reaction between NiO and MgO.