Paper SS1-WeA9
Imaging of Transition Metal Oxidation and Catalysis

Wednesday, October 17, 2007, 4:20 pm, Room 608

Transition metals have attracted significant attention because of their high activity in oxidation catalysis. For several materials this activity is due to the formation of thin oxides under reaction conditions. The most prominent example of this type of activation is Ru(0001), which only turns into an excellent low-temperature oxidation catalyst at higher oxygen partial pressures.¹ Despite extensive efforts in characterizing this and other similar systems, a number of fundamental questions remain unanswered, e.g., regarding the initial oxidation mechanism, the nature of oxygen-rich near-surface structures, and their individual catalytic activities. Here we discuss major progress toward understanding both transition metal oxidation and the catalytic activity of the resulting oxygen-rich structures, made possible by a novel spectroscopic imaging technique: dynamic intensity-voltage low-energy electron microscopy (dIV-LEEM). In contrast to IV low-energy electron diffraction (IV-LEED), dIV-LEEM produces spectroscopic stacks of real-space images of a surface as a function of electron acceleration voltage. Hence, the technique combines the real-time nano-imaging capabilities of conventional LEEM with the structural sensitivity of IV-LEED, making it possible to obtain, for instance, local time-dependent IV-LEED characteristics at every image pixel. Combined with dynamic LEED theory, such data can be used to identify nanoscale surface phases, for real-time structural fingerprinting of complex structural transitions, and to explore cooperative effects in surface reactions. Specifically, we establish the pathway of initial oxidation of the 4d late transition metals. The initial oxidation of Ru(0001), for instance, was predicted to proceed via the formation of a thin surface oxide as a precursor to the RuO₂ bulk oxide.² Our dIV-LEEM movies show instead that bulk and surface oxides grow simultaneously without transforming into one another. This finding has far-reaching consequences for catalysis, which will be discussed on the basis of real-time dIV-LEEM movies during the catalytic cycle.³ Strikingly, the coexistence of several nanoscale structures induces cooperative effects that may be visualized and quantified by dIV-LEEM analysis.

¹ H. Over et al., Science 287, 1474 (2000).
² K. Reuter et al., Chem. Phys. Lett. 352, 311 (2002).
³ J. I. Flege and P. Sutter, submitted (2007).