Coadsorptlon of ethylene and potassium on Pt(111). 2. Influence of potassium on the decomposition of ethylene

The effect of preadsorbed potassium on the decomposition of ethylene on the Pt(111) single-crystal surface has been studied by temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Potassium decreases the amount of ethylene decomposition and increases the activation energy for C-H bond cleavage. Low coverages of potassium (θ_K ≅ 0.05 monolayer) cause a 2-3 kcal mol^-1 increase in the activation energy for C-H bond breaking, and hydrogen evolution at 310 K characteristic of the dehydrogenation of ethylene to ethylidyne (CCH₃) on Pt(111) is eliminated entirely. At 300 K and θ_K = 0.12 monolayer, we observe a metastable intermediate in the decomposition of ethylene to form ethylidyne, which we propose to be ethylidene (CHCH₃). High potassium coverages (θ_K ≥ 0.50 monolayer) cause the activation energy for C-H bond breaking to increase by 13 kcal mol^-1 (from 18 to 31 kcal mol^-1), resulting in ethylidene stabilization on the surface over the range 300-420 K. Complete dehydrogenation of ethylene occurs with an activation energy similar to that for breaking the first C-H bond, without forming a stable ethylidyne species. We suggest that the strong influence of potassium on the activation energies of these surface reactions, i.e. the inhibition of dehydrogenation of surface hydrocarbons, is due to the destabilization of carbon-metal σ bonds in the dehydrogenated hydrocarbon species due to charge transfer from potassium to platinum.