The activity of heterogeneous catalysts-which are involved in some 80 per cent of processes in the chemical and energy industries-is determined by the electronic structure of specific surface sites that offer optimal binding of reaction intermediates. Directly identifying and monitoring these sites during a reaction should therefore provide insight that might aid the targeted development of heterogeneous catalysts and electrocatalysts (those that participate in electrochemical reactions) for practical applications. The invention of the scanning tunnelling microscope (STM) and the electrochemical STM promised to deliver such imaging capabilities, and both have indeed contributed greatly to our atomistic understanding of heterogeneous catalysis. But although the STM has been used to probe and initiate surface reactions, and has even enabled local measurements of reactivity in some systems, it is not generally thought to be suited to the direct identification of catalytically active surface sites under reaction conditions. Here we demonstrate, however, that common STMs can readily map the catalytic activity of surfaces with high spatial resolution: we show that by monitoring relative changes in the tunnelling current noise, active sites can be distinguished in an almost quantitative fashion according to their ability to catalyse the hydrogen-evolution reaction or the oxygen-reduction reaction. These data allow us to evaluate directly the importance and relative contribution to overall catalyst activity of different defects and sites at the boundaries between two materials. With its ability to deliver such information and its ready applicability to different systems, we anticipate that our method will aid the rational design of heterogeneous catalysts.

The morphology of attack at and around the intermetallic compounds ͑IMC͒ present on bare AA 2024-T3 was studied in situ using confocal laser scanning microscopy. Exposures were conducted in 0.1 M Na 2 SO 4 ϩ 0.005 M NaCl at pH 3, 6, and 10 as well as near-neutral 0.5 M NaCl. The types of attack observed could be categorized as matrix and IMC pitting, trenching adjacent to IMC, and matrix etching. The electrochemical behavior of bulk synthesized Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic compounds as well as that of AA 2024-T3 was used to rationalize the observed attack metrology. The galvanic coupling between the AA2024-T3 matrix and the intermetallic particles controlled the attack rates. In Al-Cu-Mg, the strong polarization to the opencircuit potential of the alloy caused rapid dissolution ͑ca. 10 mA/cm 2 ͒, whereas for the Al-Cu-Fe-Mn the dissolution rates were on the order of 100 A/cm 2 . The limited dissolution rates of the Al-Cu-Fe-Mn phase were due to the cathodic polarization of these particles by the matrix under open-circuit conditions. Several pits were initiated at large Al-Cu-Mg particles. These pits were stable within the Al-Cu-Mg phase, but could not form stable pits in the alloy matrix during open-circuit corrosion. Calculation of growth rates and pit stability products for the individual IMC emphasized the role of metastable pitting in the observed corrosion metrology, which developed on AA2024-T3 during open-circuit corrosion.

The reversible intercalation of solvated Na-ions into graphite and the concomitant formation of ternary Na–graphite intercalation compounds (GICs) are studied using several in operando techniques, such as X-ray-diffraction (XRD), electrochemical scanning tunnelling microscopy (EC-STM) and the electrochemical quartz crystal microbalance technique (EQCM).