Effects of Sulfate Ion on Crystal Structure and Activity for Methane Oxidation of Iron Oxide Prepared from Goethite

Abstract

Iron oxides were prepared from goethite (α-FeO(OH)) as the precursor and tested for methane oxidation. Goethite was prepared by air oxidation of suspension obtained by mixing solutions of sodium hydroxide and iron (II) sulfate and then thermally decomposed to form iron oxide. Activity for methane oxidation over iron oxide was suppressed by the sulfate ion (SO₄²⁻). In order to clarify the effects of the residual sulfate ion on the catalytic activity and crystal structure of iron oxide prepared from goethite, we prepared sulfated goethite and compared with non-sulfated one. In the case of sulfated goethite, the temperature of phase transition to hematite (α-Fe₂O₃) was higher than that of non-sulfated sample by about 30 degrees. Electrons were donated from the sulfate ion to iron ions in hematite to reduce oxidizing power of irons. This led to deactivation of methane oxidation in a temperature range between 300 °C and 500 °C and evolution of partial methane oxidation. The line broadenings of XRD peaks due to stacking faults were observed on the lattice planes except for (113l) (l : 0, 1, 2) planes, which depicts directly the skeletal arrangement of irons in hematite, suggesting that the number of the disorder of skeletal arrangement of oxygen formed during the heat treatment. Above 550 °C, the activity of hematite prepared from sulfated goethite was higher than that from non-sulfated one. At a temperature above 550 °C, the sulfate groups were eliminated from the hematite, whereas the stacking faults were remained. We conclude that the high activity of hematite prepared from sulfated goethite at high temperature due to the presence of these stacking faults.