Adsorption and catalytic properties of CoxFe3–xO4 spinels. Part 2.—Hydrogen chemisorption on precursors to ammonia synthesis catalysts

Abstract

At temperatures in the range 323–423 K H₂ adsorbs largely irreversibly onto the surfaces of Co_xFe_3–xO₄(0 ⩽x⩽ 3) spinels to an extent (0.91–4.76 molecules nm^–2) which increases with x and temperature. Although the enthalpy of adsorption (60–210 kJ mol H₂) is greater than bulk spinel reduction at these temperatures the latter process is not thought to be significant. Adsorption isotherms obey the Langmuir model well. Crystal-field considerations suggest sites responsible for adsorbing H₂ which are contrary to recent reports (J-P. Beaufils, Y. Barbaux and B. Saubat, J. Chem. Soc., Chem. Commun., 1982, 1212). Cobalt additions improve the extent and enthalpy of the fast adsorption of hydrogen on these spinel precursors of non-promoted ammonia-synthesis catalysts.