Methylcyclohexane to Benzene Conversion over K-Promoted Pt(111)

The adsorption, desorption, and reaction of methylcyclohexane (MCH) were studied on clean and potassium- modified Pt(111) surfaces. The desorption of reversibly chemisorbed MCH on clean Pt(111) occurs at 240 K, and physisorbed MCH desorbs at 140–150 K. At 164 K, the MCH sticking coefficient is near unity and stays constant until about half-coverage of the chemisorbed layer before gradually dropping to 0, indicating precursor-mediated adsorption kinetics. Potassium at a coverage up to θ_k = 0.15 has almost no effect on the sticking coefficient or saturation coverage of MCH on Pt(111) at 164 K, but stabilizes chemisorbed MCH on the surface by about 20 K in TPD. At higher θ_k, MCH is more weakly bound and no chemisorbed MCH is formed for θ_k ⁼ 0.26. Potassium adatoms increase reversible MCH adsorption and promote the desorption of benzene as a product for 0.1 < θ_k < 0.25. The cleavage of the C-C bond between the ring and methyl group in MCH, which is responsible for the benzene formation, was observed for θ_k = 0.19 at a remarkably low temperature of less than 230 K. Preadsorbed carbon increases the amount of reversible MCH adsorption but does not change the bonding energy nor cause benzene desorption. These results indicate that MCH → benzene conversion and particularly benzene desorption are enhanced by the combined electronic effect of coadsorbed alkali adatoms and the local postdosing effect of CH_x from the methyl group.