Energetics of Adsorbed CH2 and CH on Pt(111) by Calorimetry: The Dissociative Adsorption of Diiodomethane

The enthalpies of molecular and dissociative adsorption of CH2I2 on Pt(111) at 100–210 K were studied using single-crystal adsorption calorimetry and density functional theory (DFT). Gaseous CH2I2 was found to adsorb on the Pt(111) surface at 100 K to form CH2,ad + 2Iad, with a calorimetric heat of adsorption that decreases with coverage as 222–480θ kJ/mol for θ < 1/8, where θ is the coverage in monolayers (ML), defined as the number of dissociatively adsorbed CH2I2 molecules per Pt(111) surface atom. These coadsorbed iodine atoms greatly destabilize the methylene species even at the lowest coverage, which we attribute to their inability to diffuse away from the near-neighbor sites where they are initially produced on the short time scale of the heat measurement. A mixture of dissociative adsorption fragments of methylene and methylidyne were detected at elevated temperatures between 125 and 190 K. At 210 K, CH2I2 adsorption produced CHad, Had, and 2Iad, with the Iad now able to diffuse away to minimize repulsions on the time scale of heat measurements. The calorimetric heats for this reaction provide an estimate of the heat of formation of CHad on Pt(111) of −71 kJ/mol with a HC-Pt(111) bond enthalpy of 665 kJ/mol at 0.04 ML coverage. These results are used together with prior measurements to construct an energy landscape for the stepwise dehydrogenation of methane on Pt(111) and are compared to previous experimental results and theoretical predictions. DFT calculations reproduce the experimental stability of CHad and CH3,ad on Pt(111) to within ∼20 kJ/mol and the experimentally measured Iad–Iad repulsion energy between coadsorbed iodine adatoms to within 25%. DFT calculations also show that there is very strong repulsion between CH2,ad and Iad when in nearby sites, which seems to be the case when formed at 100 K from CH2I2, even at very low coverage.