Microcalorimetric Studies of Surface Acid/Base Properties of Magnesium Iron Catalysts Prepared from Hydrotalcite-Type Precursors

Abstract Magnesium–iron mixed oxides with Mg/Fe molar ratios 1,3, and 6 were prepared from hydrotalcite-type precursors. Microcalorimetric adsorption of NH 3 and CO 2 showed that the surface acidity and basicity of the mixed oxides after calcination at 673 K are similar despite the different Mg/Fe ratios. Increasing calcination temperature from 673 to 773 K significantly decreased the surface area of the 3 Mg/Fe oxide, but the densities of both the acid and base sites were not changed. Mossbauer spectroscopy revealed that the reduction of the 3 Mg/Fe oxide (Fe 2 O 3 /MgO) in H 2 at 673 K converted all Fe 3+ to Fe 2+ . The resulted FeO/MgO exhibited the same acidity as that of the Fe 2 O 3 /MgO, but the basicity of the FeO/MgO was greatly enhanced. Reduction at 773 K resulted in the formation of 76% Fe 2+ and 24% Fe 0 as detected by Mossbauer spectroscopy. The Fe/FeO/MgO sample formed exhibited very low heat for the adsorption of NH 3 (40 kJ/mol) indicating that all iron atoms on the surface are Fe 0 . However, a substantial basicity remained on the surface of this sample that may account for its high olefin selectivity compared with pure iron catalyst in the Fischer–Tropsch synthesis.