METAL-SUPPORT INTERACTIONS IN HYDROUS TITANIUM OXIDE-SUPPORTED NICKEL-CATALYSTS

A combination of techniques, including AES, SIMS, FTIR, and hydrogen chemisorption, has been used to investigate the activation of nickel ions supported on hydrous titanium oxide (HTO) ion-exchange materials. HTO supports allow metal ions to be loaded via ion exchange such that atomic dispersion is attained in the as-prepared material, even for high metal loadings. The results presented here support earlier work indicating that nickel forms large, 10-20-nm particles during hydrogen reduction of Ni/HTO at temperatures of 300 degrees C or greater. During reduction, these particles become covered by an amorphous film which inhibits catalytic activity. Evidence is presented which supports the theory that this film is composed of carbonaceous residue which originates from the organometallic precursors and organic solvents used to synthesize the HTO support. Reduction/oxidation cycles result in oscillations in the nickel surface concentration which are attributed to decoration of the particles by partially reduced TiOx species, in a manner similar to a strong metal-support interaction (SMSI). This SMSI occurs at temperatures as low as 300 degrees C, well below the temperatures typically required to induce SMSI on crystalline titania supports. This low onset temperature appears to be related to the amorphous nature of the HTO support, which may be more easily reduced than crystalline titania. Mild oxidation (300 degrees C) of Ni/HTO prior to reduction increases the amount of metal surface area which is formed during reduction by altering the interaction of the nickel ions with the support such that smaller nickel particles are formed. More severe oxidation treatments (>400 degrees C) lead to a decrease in active surface area by increasing the interaction of nickel ions with the support such that a smaller fraction of the nickel ions can be reduced and participate in particle formation.