Bimetallic synergy in cobalt-ruthenium Fischer-Tropsch synthesis catalysts

The addition of small amounts of Ru to Co/TiO2 or Co/SiO2 catalysts (Ru/Co < 0.008 at.) increases their turnover rate and C5+ selectivity in the Fischer-Tropsch synthesis. Also, deactivated bimetallic Co-Ru catalysts can be regenerated by a hydrogen treatment at reaction temperature whereas monometallic Co catalysts cannot. On Co/TiO2, Ru addition increases turnover rates by a factor of three and C5+ selectivities from 84.5 to 91.1%, without an apparent change in cobalt dispersion. Activation energies and reaction kinetics are unaffected by Ru addition. These data suggest that the presence of Ru leads to higher Co site density during reaction without modifying the chemical reactivity of exposed Co surface atoms. Ru appears to inhibit the deactivation of surface Co ensembles. Ru atoms at the surface of Co crystallites increase the rate of removal of carbon and oxygen species during reaction and during regeneration of deactivated Co catalysts. The resulting higher Co site density leads to higher apparent turnover rates and to enhanced readsorption of α-olefins. As a result, Co-Ru catalysts yield a heavier and more paraffinic product than monometallic Co catalysts with similar initial dispersion. The observed bimetallic interactions require intimate contact between Co and Ru, a state that forms during oxidation of the bimetallic precursors at high temperature. This treatment induces migration of Ru oxide species and leads to mixed Co-Ru oxides. The reducibility and the carbon deposition rates on well-mixed bimetallic Co-Ru catalysts are very different from those of monometallic Co catalysts. Electron microscopy. X-ray absorption spectroscopy, and thermogravimetric studies confirm the intimate mixing required for catalytic rate and C5+ selectivity enhancements, for faster reduction of Co oxide precursor, and for inhibition of carbon deposits during reactions of H2/CO mixtures.