PROMOTED IRON FISCHER-TROPSCH CATALYSTS - CHARACTERIZATION BY THERMAL-ANALYSIS

The effects of metal and alkali promoters on precipitated iron oxide F ischer-Tropsch catalyst precursors are examined using thermal gravimet ry (TG) and differential thermal analysis (DTA). A distinct exotherm c orresponding to what appears to be the transformation from FeOOH to Fe 2O3 is observed for unpromoted and promoted iron oxides when the promo ter's ionic radius is less than approximately 0.7 Angstrom. The heat r eleased during the exotherm is similar for unpromoted and promoted iro n oxides corresponding to this size range. For promoters having ionic radii larger than 0.7 Angstrom, the exotherm occurs over a broad tempe rature range. Silica exhibits a unique propensity to stabilize against this transition, and appears to be present as two species. Increased loadings of Si cause a shift from the sharp, narrow temperature range exotherm to the broad transition observed for promoters with larger io nic radii.