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 photoelectron spectroscopy and temperature-programmed desorption (TPD) on a clean Ni(100) surface, Ni(100) with a chemisorbed oxygen overlayer, and Ni(100) with a NiO(111) oxide overlayer. Methyl radical dosing at surface temperatures of 120-170 K produces carbon coverages in excess of 5 monolayers with a C(1s) peak position indicative of an adsorbed hydrocarbon species. The carbon coverage never saturated on any of surfaces studied. TPD indicating C₂-C₄ formation was observed following very large methyl radical exposures. These results are indicative of the formation of surface hydrocarbon chains. The methyl radical gas temperature had no effect on the surface chemistry. However, hydrocarbon chains formed with higher selectivity on the oxygen-modified surfaces compared to the clean surface. These results suggest that a direct reaction between incoming methyl radicals and adsorbed hydrocarbon groups is not involved. The chains are likely produced by polymerizaton of surface methylene species produced by dehydrogenation of chemisorbed methyl groups via a mechanism similar to the Fischer-Tropsch synthesis reaction. On NiO TPD results also indicate that surface alkoxy groups are formed based on the low-temperature desorption of CO.