THE ADSORPTION OF C2H4 ON THE MO(110) SURFACE AND THE EVOLUTION OF THE SURFACE WITH TEMPERATURE

The adsorption of C2H4 on the Mo(110) surface near room temperature and at 1100 K was studied with Auger electron spectroscopy, low energy electron diffraction, thermal desorption spectroscopy and electron energy loss spectroscopy. At both temperatures, adsorption saturated after exposures of about 6 langmuirs, (1 L = 10(-6) Torr.s), indicating the formation of a single layer which passivates the surface to further C2H4 adsorption. During thermal desorption, following adsorption near room temperature, C2H4 dissociates completely by 750 K to surface carbon and desorbed H2; no hydrocarbon desorption was observed. For adsorption above 750 K, the hydrogen desorbs at once leaving only carbon on the surface. No ordered overlayer was found at 240 K or during subsequent heating; however, following a vacuum anneal after saturation at either 800 or 1100 K a Mo(110)-(4 x 4)-C structure was formed. The ratio of the number of carbon to molybdenum atoms on this surface was C:Mo = 0.37 +/- 0.06, assuming unity sticking probability for C2H4 on the clean surface. Thermal indiffusion of carbon from the (4 x 4) surface begins at about 1400 K and becomes very rapid by 1600 K. When the C:Mo ratio fell to about 0.2, a second previously unreported structure. Mo(110)-(-44(33))-C formed. These overlayers probably result from chemisorbed carbon rather than from the formation of a true surface carbide.