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

A combination of techniques, including AES, SIMS, FTIR, and hydrogen c hemisorption, has been used to investigate the activation of nickel io ns supported on hydrous titanium oxide (HTO) ion-exchange materials. H TO supports allow metal ions to be loaded via ion exchange such that a tomic dispersion is attained in the as-prepared material, even for hig h metal loadings. The results presented here support earlier work indi cating that nickel forms large, 10-20-nm particles during hydrogen red uction of Ni/HTO at temperatures of 300 degrees C or greater. During r eduction, these particles become covered by an amorphous film which in hibits catalytic activity. Evidence is presented which supports the th eory that this film is composed of carbonaceous residue which originat es from the organometallic precursors and organic solvents used to syn thesize the HTO support. Reduction/oxidation cycles result in oscillat ions in the nickel surface concentration which are attributed to decor ation 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. Thi s 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 i ncreases the amount of metal surface area which is formed during reduc tion by altering the interaction of the nickel ions with the support s uch that smaller nickel particles are formed. More severe oxidation tr eatments (>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 i n particle formation.