Bond selective dissociation of methane (CH₃D) on the steps and terraces of Pt(211)

The dissociative chemisorption of singly deuterated methane (CH₃D) has been studied on the steps and terraces of a Pt(211) surface by quantum state resolved molecular beam methods. At incident translational energy (E_t) below 50 kJ/mol, CH₃D dissociates only on the more reactive steps of Pt(211), where both C–H and C–D cleavage products CH₂D(ads) and CH₃(ads) can be detected by reflection absorption infrared spectroscopy. Vibrational excitation of a slow beam of CH₃D (E_t = 10 kJ/mol), prepared with one quantum of antisymmetric C–H stretch excitation by infrared laser pumping, allows for fully bond- and site-selective dissociation forming exclusively CH₂D(ads) on the step sites. At higher kinetic energies (E_t > 30 kJ/mol), bond selective dissociation by C–H bond cleavage is observed on the terrace sites for stretch excited CH₃D (ν₄) while on the steps, the C–H/C–D cleavage branching ratio approaches the statistical 3/1 limit. Finally, at E_t > 60 kJ/mol, both C–H and C–D cleavages are observed on both step and terrace sites of Pt(211). Our experiments show how careful control of incident translational and vibrational energy can be used for site and bond selective dissociation of methane on a catalytically active Pt surface.