Sandia National Laboratories, Livermore, California 94550, USA.
J Am Chem Soc. 2013 Jul 10;135(27):10091-8. doi: 10.1021/ja402599t. Epub 2013 Jun 27.
We study how the (100) surface of magnetite undergoes oxidation by monitoring its morphology during exposure to oxygen at ~650 °C. Low-energy electron microscopy reveals that magnetite's surface steps advance continuously. This growth of Fe3O4 crystal occurs by the formation of bulk Fe vacancies. Using Raman spectroscopy, we identify the sinks for these vacancies, inclusions of α-Fe2O3 (hematite). Since the surface remains magnetite during oxidation, it continues to dissociate oxygen readily. At steady state, over one-quarter of impinging oxygen molecules undergo dissociative adsorption and eventual incorporation into magnetite. From the independence of growth rate on local step density, we deduce that the first step of oxidation, dissociative oxygen adsorption, occurs uniformly over magnetite's terraces, not preferentially at its surface steps. Since we directly observe new magnetite forming when it incorporates oxygen, we suggest that catalytic redox cycles on magnetite involve growing and etching crystal.
我们通过监测磁铁矿在 ~650°C 下暴露于氧气时的形貌来研究其（100）表面的氧化过程。低能电子显微镜揭示了磁铁矿表面台阶的连续推进。这种 Fe3O4 晶体的生长是通过形成体相 Fe 空位来实现的。通过拉曼光谱，我们确定了这些空位的汇，即α-Fe2O3（赤铁矿）的夹杂物。由于氧化过程中表面保持为磁铁矿，因此它仍然可以轻易地使氧气离解。在稳定状态下，超过四分之一的入射氧分子经历离解吸附并最终掺入磁铁矿中。从生长速率与局部台阶密度无关，我们推断出氧化的第一步，即离解氧吸附，均匀地发生在磁铁矿的平台上，而不是优先发生在其表面台阶上。由于我们直接观察到新的磁铁矿形成，当它吸收氧气时，我们认为磁铁矿上的催化氧化还原循环涉及晶体的生长和刻蚀。
