STM studies of cyclohexene hydrogenation/dehydrogenation and its poisoning by carbon monoxide on Pt(111)

A scanning tunneling microscope that can be operated in ultra high vacuum (< 10(-9) Torr) as well as at high pressures (1-10(3) Torr) has been utilized to study the structures formed by Cyclic C-6 hydrocarbons adsorbed on a platinum (111) crystal surface. Catalytic reactions of cyclohexene were also studied in the presence of hydrogen at pressures (up to 200 mTorr) and 300 K-350 K temperature range. Cyclohexane and cvclohexene produced the same adsorbed structure, which is attributed to the partially dehydrogenated pi-allyl (C6H9). 1,3-Cyclohexadiene produced structures similar to those produced by benzene. In contrast 1,4-cyclohexadiene forms a structure that we attribute to intact molecular 1.4-cyclohexadiene. During reaction the STM images appear disordered, indicative of rapid diffusion of surface species. Addition of 5 mTorr of CO stops the catalytic activity and forms an ordered structure on the surface. (c) 2006 Published by Elsevier B.V.