Energetics of Adsorbed CH3 and CH on Pt(111) by Calorimetry: Dissociative Adsorption of CH3I

The heat of adsorption and sticking probability of methyl iodide were measured on Pt(111) at 95, 215, 270, 300, and 320 K using single crystal adsorption calorimetry (SCAC). On clean Pt(111) at 95 K, the heat of adsorption for molecularly adsorbed methyl iodide was found to be 98.2 +/- 2.0 kJ/mol in the limit of low coverage, resulting in a standard enthalpy of formation (Delta H-f(0)) of CH3I3d of -83.6 +/- 2.2 kJ/mol. The rate of dissociative adsorption of methyl iodide was fast enough at 320 K for its heat to be accurately measured. The heat of adsorption measured in the low coverage regime (0-0.04 ML) yielded the energetics of adsorbed methyl and iodine adatoms, giving Delta H-f(0)(CH3,ad) = -53 kJ/mol (using reported energetics for I-ad, which has error bars of +/- 20 kJ/mol) and a Pt-CH3 bond strength of 200. kJ/mol. The measured integral heat of adsorption at 0.18 ML and 320 K yielded the energetics of adsorbed methylidyne (CHad), giving Delta H-f(0)(CHad) between +23 and +42 kJ/mol and a Pt-CH bond strength between 552 and 571 kJ/mol. Using these enthalpies of formation, the enthalpy for the dissociation of adsorbed methane to adsorbed methyl coadsorbed with a hydrogen adatom was found to be +1 kJ/mol almost thermoneutral. The further reduction of CH3,ad to CHad + 2 H-ad was found to be uphill by between +4 and +23 kJ/mol. Measured methane yields (which require the product H-ad from this step) imply that the equilibrium constant for this step lies far to the left, consistent with this reaction's enthalpy. The bond strengths measured here for CH3,ad and CHad are compared to previous DFT calculations.