Scanning Tunneling Microscopy Investigation of the Conversion of Ethylene to Carbon Clusters and Graphite on Pt(111)

The dehydrogenation of ethylene on Pt(111) was studied by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. Previous experiments have shown that thermal dehydrogenation following saturation exposure of ethylene on Pt(111) resulted in the formation of well-defined carbon clusters. The aggregation to form the carbon clusters leaves open Pt areas that could be available for additional adsorption. It had not been previously determined whether the adsorption and dehydrogenation of additional ethylene would lead to the growth of the initial clusters or the nucleation of additional clusters of the same size. The present study confirms previous reports that the initial carbon clusters are 15 +/- 2 angstrom in diameter, 2.5 +/- 0.3 angstrom in height, and contain an average of 34 +/- 9 carbon atoms per cluster. We show that exposing this surface to additional ethylene at room temperature and annealing leads to an increase in the number of particles of the same size, with no growth in size of the initial particles. Dosing and dehydrogenation/annealing cycles were repeated until the dehydrogenation activity of the Pt(111) surface was completely suppressed, which occurred after the fourth such cycle. Continued cycling leads to the beginning of the formation of a graphite adlayer on the platinum, presumably via agglomeration of the clusters at a high cluster density.