STM Studies of Restructuring and Stability of Bimetallic Alloys

The optimum performance of a catalytic process can often be achieved with bimetallic catalysts. For example, PdAg catalysts are most effective for the partial hydrogenation of acetylene to ethylene by combining the high activity of Pd with the selectivity of Ag. The catalytic properties of a bimetallic catalyst will depend on the atomic structure of the surface, which is likely to change during the course of the reaction, particularly under reaction conditions at elevated temperatures. By using scanning tunneling microscopy (STM), we found that Ag encapsulation of Pd islands readily occurs even at room temperature on the time scale of minutes. We showed that growth of the Ag layer on top of the Pd islands is nucleated at sites near the centers of the islands. In the second project, the research delves into the initial stages of surface composition change and stability for Pd/Ag(111) and Pd/Au(111) bimetallic surfaces. Ag encapsulates Pd islands at room temperature within a brief period, forming vacancy pits on Ag(111). In contrast, there is no observed Au encapsulation of Pd islands. Subsequent annealing reveals that Pd islands exhibit greater stability on Au(111) compared to Ag(111). There has been a long-term need to modify the manipulator head of the Omicron STM to heat samples to high temperatures. Towards the end of the talk, a simple device is described for heating single-crystal samples to temperatures as high as 2000°C in ultrahigh vacuum. This device is compatible with the standard sample plates used in a common commercial scanning tunneling microscope (STM). Heating high-melting-point samples to temperatures higher than is possible with existing STM sample holders is necessary to obtain clean, well-ordered surfaces. Results are demonstrated for the (0001) surface of ZrB2, which has a melting point of 3,050°C.