REDUCTION, CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF CO3-XALXO4-DELTA LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-2.0, -THAN-OR-EQUAL-TO-DELTA-LESS-THAN-OR-EQUAL-TO-1.0) - COMPARISON WITH THE CO GAMMA-AL2O3 FISCHER-TROPSCH CATALYST/

Spinel type samples Co3-xAlxO4 (0.0 less than or equal to x less than or equal to 2.0) were synthesized from citrate solutions and used as r eference compounds for characterization of a 20% Co/gamma-Al2O3 Fische r-Tropsch catalyst. With increasing Al content for the observed solid solution phase between Co3O4 and CoAl2O4, the unit cell expands smooth ly, the antiferromagnetic ordering temperature decreases (T-N less tha n or equal to 40 K), and the temperature required for reduction increa ses. Low-temperature reduction yields a partially reduced intermediate (Co-II) phase of the NaCl type. The crystal structure of such a parti ally reduced phase (Co5/6Al1/6)(0.92)O has been determined on the basi s of powder X-ray and neutron diffraction data: space group <Fm(3)over bar m>; a = 422.98(4)pm; R-wp=4.0%, R-p=3.1%. (Co5/6Al1/6)(0.92)O ord ers antiferromagnetically below T-N=320 +/- 5 K with a magnetic struct ure similar to that of CoO, mu(AF) = 3.57(3) mu(B); R-M=7.2%. TPR data for the oxidized catalyst show, consistent with PXD data, the presenc e of Co3O4 supported on gamma-Al2O3. An additional mixed Co/Al oxide p hase probably exists as an interface layer of Co3-xAlxO4 with x approx imate to 0.5. The latter composition is consistent with the observed a ntiferromagnetic ordering temperature T-N=27+/-1 K for the oxidized ca talyst. The activated catalyst obtained on reduction at 573 K was foun d to consist of fine particles (28 nm) of ferromagnetic cobalt (fcc) t ogether with a partially reduced, NaCl-type phase (Co0.88Al0.08)O supp orted on gamma-Al2O3.