Adsorption, desorption, and reaction of methyl radicals on surface terminations of α-Fe₂O₃

The adsorption, desorption, and reaction of gas phase methyl radicals were studied on the (0001)-oriented α-Fe₂O₃ surface in ultrahigh vacuum. Two different surface terminations were compared: An Fe ₃O₄ (111) layer and the so-called "biphase" surface thought to be a mixture of FeO and Fe₂O₃ terminations. Gas phase methyl radicals were prepared by pyrolysis of azomethane. On Fe₃O₄ (111) methyl radical adsorption forms surface methoxide species as determined by the C(1s) XPS binding energy. Temperature programmed reaction spectroscopy produced direct desorption of methyl radicals at all coverages and the formation of ethane at high coverages in two desorption peaks at 331 and 439 K. The activation energies for desorption were 84 and 133 kJ/mol in the two regimes. The two surface terminations exhibit saturation coverages that differ by ca., 30× : 1.5 × 10 ¹⁴ and 5.2 × 10¹² per cm² for the Fe ₃O₄(111) and "biphase" terminations, respectively. These results are interpreted in terms of bonding models and differences in atomic structure for the two terminations.