METHYLCYCLOHEXANE TO BENZENE CONVERSION OVER K-PROMOTED PT(111)

The adsorption, desorption, and reaction of methylcyclohexane (MCH) we re studied on clean and potassium-modified Pt(111) surfaces. The desor ption 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 c oefficient is near unity and stays constant until about half-coverage of the chemisorbed layer before gradually dropping to 0, indicating pr ecursor-mediated adsorption kinetics. Potassium at a coverage up to th eta(K) = 0.15 has almost no effect on the sticking coefficient or satu ration coverage of MCH on Pt(111) at 164 K, but stabilizes chemisorbed MCH on the surface by about 20 K in TPD. At higher theta(K), MCH is m ore weakly bound and no chemisorbed MCH is formed for theta(K) = 0.26. Potassium adatoms increase reversible MCH adsorption and promote the desorption of benzene as a product for 0.1 < theta(K) < 0.25. The clea vage of the C-C bond between the ring and methyl group in MCH, which i s responsible for the benzene formation, was observed for theta(K) = 0 .19 at a remarkably low temperature of less than 230 K. Preadsorbed ca rbon increases the amount of reversible MCH adsorption but does not ch ange the bonding energy nor cause benzene desorption. These results in dicate that MCH --> benzene conversion and particularly benzene desorp tion are enhanced by the combined electronic effect of coadsorbed alka li adatoms and the local postdosing effect of CH(x) from the methyl gr oup.