(7d) Exploiting Metal-Support Interactions to Optimize Dispersion and Reducibility of a Highly Active & Selective Fischer-Tropsch Synthesis Eggshell Catalyst

In this study the optimum solution chemistry for synthesis of a highly active and selective Fischer-Tropsch (FT) eggshell cobalt-based catalyst is investigated. The precursor salt (Co(NO₃)₂*6H₂O) is dehydrated and dissolved in anhydrous ethanol under a low humidity nitrogen atmosphere and titrated with anhydrous ethanol-urea solution. It was found that the addition of small amounts of de-ionized water followed by titration ionizes the solution and greatly enhances active metal dispersion on the silica gel support. However, higher amounts of added water can either aid in the ionization of salt or lead to the recombination of ions during titration resulting in higher or lower metal-to-surface interaction respectively.

The silica gel surface can become negatively charged due to ion exchange with an ethanol-water mixture. If the water is present in small quantities, it will not form an immobilized aqua monolayer, allowing for vertical stacking of ethanol on the gel to leave vacant aligned spaces for salt attachment. After titration with urea, when a small amount of water is present, there is a net increment of salt ions which interact with the silica gel to form coordinated covalent bonds and eventually lead to higher dispersions. On the other hand, if the quantity of water is high, an immobilized aqua monolayer is formed on the silica gel before ethanol can interact. Also, this large amount of water leads to recombination of ions, thus reducing the free floating Co²⁺ ions near the surface. Under such circumstances, direct metal support interaction is not possible leading to poor non uniform metal distribution on the surface and eventual sintering.

In order to exploit this interaction, the optimum urea-water-ethanol proportions were found via conductivity tests of solutions throughout the titration process. Hydrogen chemisorption is used to test this hypothesis.