Atomic Structure of Carbon and Nitrogen on the Pt(111) Surface
The structure and reactivity of elemental carbon and nitrogen on transition metal surfaces are important to a variety of problems in heterogeneous catalysis. Many of the surface chemical properties of both carbon and nitrogen can be deduced through studies that employ techniques that average over monolayers, while scanning tunneling microscopy (STM) can provide direct information on the structure of surface layers, often with atomic resolution. The techniques of reflection adsorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and low energy electron diffraction (LEED) have been used to study the formation of carbon and nitrogen on Pt(111) through dehydrogenation reactions.
In the case of carbon, the dehydrogenation of acetylene and ethylene was found to first produce ethylidyne (CCH3), which then decomposes to form CxHy clusters of various sizes as observed with room temperature STM. At higher temperatures, these clusters would undergo further dehydrogenation to form graphene islands. Under conditions that resulted in complete coverage of the Pt(111) surface with graphene, various rotational domains of graphene were observed. The boundaries between graphene domains provide nucleation sites for the growth of Pt nanoclusters when Pt is deposited onto the graphene covered Pt(111) surface.
In the case of nitrogen, it was found that reaction between ammonia and molecularly adsorbed O2 would result in the formation of H2O, which desorbs below 200 K to leave behind a well-ordered p(2×2)-N layer on Pt(111). This N layer readily reacts with H2 to form NH molecules on the surface, as observed with RAIRS. Through collaborative research with a group in Japan, a low temperature STM operated at 5 K was used to obtain atomically resolved images of the p-(2×2)-N layer and of its hydrogenation to NH.