Energetics of Adsorbed Methyl and Methyl Iodide on Ni(111) by Calorimetry: Comparison to Pt(111) and Implications for Catalysis

The heat of adsorption and sticking probability of methyl iodide were measured on Ni(111) at 100 and 160 K using single-crystal adsorption calorimetry (SCAC). At 100 K, methyl iodide adsorbs molecularly with a heat of 102 kJ/mol on terrace sites in the low-coverage limit, giving a standard enthalpy of formation (ΔHf°) of CH3I(ad) of −87 kJ/mol. A heat of 122 kJ/mol is also measured on defect sites, probably step edges. Calorimetry of the dissociative adsorption of methyl iodide on Ni(111) at 160 K yielded an integral heat of adsorption of −270 kJ/mol at 0.04 ML, providing the energetics of adsorbed methyl, with ΔHf°[CH3(ad)] = −71 kJ/mol and a CH3–Ni(111) bond enthalpy of 218 kJ/mol. This is 22 kJ/mol stronger than the reported value for H3C–Pt(111) bonds, explaining the greater activity of Ni catalysts for hydrogenolysis in comparison to Pt. The measured energetics for methyl were compared to density functional theory (DFT) calculations from previous literature, showing that these methods systematically underestimate the bond energy of methyl to Ni(111).