A STUDY OF THE ADSORPTION OF METHYL RADICALS ON CLEAN AND OXYGEN-MODIFIED NI(100)

The adsorption of methyl radicals has been studied by X-ray photoelect ron spectroscopy and temperature-programmed desorption (TPD) on a clea n Ni(100) surface, Ni(100) with a chemisorbed oxygen overlayer, and Ni (100) with a NiO(111) oxide overlayer. Methyl radical dosing at surfac e temperatures of 120-170 K produces carbon coverages in excess of 5 m onolayers with a C(1s) peak position indicative of an adsorbed hydroca rbon species. The carbon coverage never saturated on any of surfaces s tudied, TPD indicating C-2-C-4 formation was observed following very l arge methyl radical exposures. These results are indicative of the for mation of surface hydrocarbon chains. The methyl radical gas temperatu re had no effect on the surface chemistry. However, hydrocarbon chains formed with higher selectivity on the oxygen-modified surfaces compar ed to the clean surface. These results suggest that a direct reaction between incoming methyl radicals and adsorbed hydrocarbon groups is no t involved. The chains are likely produced by polymerizaton of surface methylene species produced by dehydrogenation of chemisorbed methyl g roups via a mechanism similar to the Fischer-Tropsch synthesis reactio n. On NiO TPD results also indicate that surface alkoxy groups are for med based on the low-temperature desorption of CO.