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 (ΔH_f⁰) of CH₃I_ad 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 ΔH _f⁰(CH_3,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 (CH_ad), giving ΔH_f⁰(CH_ad) 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 CH _3,ad to CH_ad + 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 CH_3,ad and CH_ad are compared to previous DFT calculations.