Influence of organic additives on the properties of impregnation solutions and on nickel oxide particle size for Ni/Al2O3 catalysts

• Sorbitol, glucose and citric acid are introduced as additives in impregnation solutions.
• The main role played by these additives is to increase the solution viscosity.
• In solution, the additives do not behave as stable ligands toward Ni2+ ions.
• Crystallization of the precursor salt and formation of larger NiO nanoparticles are inhibited.
• The reducibility of smaller nanoparticles compares well with reference Ni/Al2O3 catalyst.

The introduction of organic additives, such as polyols, sugars or polyacids, in impregnation solutions has been suggested to better control the size of nanoparticles on supported catalysts. The present paper focuses on the roles played by three of these additives, sorbitol, glucose and citric acid, in the preparation of Ni/Al2O3 catalysts. ATR-IR, circular dichroism spectroscopy and viscosity measurements show that the additives act as modifiers of the physical properties of the solutions, by increasing their viscosity via hydrogen bonding, rather than as stable ligands toward Ni2+ ions. Recrystallization of the nickel precursor salt is prevented upon drying. As a consequence, no population of large particles is evidenced by XRD and TPR on the catalysts prepared with additives, unlike reference catalysts prepared by conventional impregnation of nickel nitrate. Adding these organic molecules in the impregnation solution is thus an easy way to narrow the size distribution of NiO nanoparticles without significantly affecting their reducibility.

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