DISSOCIATIVE CHEMISORPTION OF METHANE ON CLEAN AND OXYGEN PRECOVERED PT(111)

Effects of adsorbed oxygen on dissociative chemisorption of methane mo lecules on a platinum (111) surface have been studied by using molecul ar beam surface scattering. It has been found that for an oxygen preco verage of 0.42 ML on Pt(lll), and for the clean surface, an increase i n translational energy (E(n)) of CH4 and in surface temperature (T-s) of the sample strongly enhance the methane activation in the range of parameters studied, 60 < E(n) < 125 kJ/mol and 300 K < T-s < 750 K. Ho wever, the absolute dissociation probability of CH4 is always much sma ller for the oxygen precovered surface than for the clean surface. Cle arly, oxygen poisons the C-H activation. This poisoning of the surface implies that oxygen sterically blocks the active surface sites for di ssociative chemisorption of CH4 or it inhibits the dissociation electr onically. Despite of surface poisoning, the CH4 activation mechanism i s probably the same for the clean and oxygen covered Pt(lll) surfaces.