Investigation of the Adsorption and Reactions of Methyl Radicals on Transition Metal (M = Co, Ni, Pd, Pt) (111) Surfaces in Aqueous Suspensions

The DFT method was used to evaluate the adsorption of methyl radicals and the evolution of ethane on the M(111) (M = Co, Ni, Pd, Pt) surfaces, eight metal atoms, in aqueous medium. A maximum of five and four radicals can be adsorbed on Co(111) and Ni(111), respectively, and six on Pd(111) and Pt(111) (top site). The ethane evolution occurs via the Langmuir–Hinshelwood (LH) or Eley–Rideal (ER) mechanisms. The production of ethane through the interaction of two adsorbed radicals is thermodynamically feasible for high coverage ratios on the four surfaces; however, kinetically, it is feasible at room temperature only on Co(111) at a coverage of (5/5) and on Pd(111) at a coverage ratio of 4/6, 5/6, and 6/6. Ethane production occurs via the ER mechanism: a collision with solvated methyl radical produces either C₂H₆ or (Formula presented.). On Pd(111) the product is only C₂H₆, on Pt(111), both products (C₂H₆ or (Formula presented.)) are plausible, and on Co(111) and Ni(111), only (Formula presented.) is produced. Further reactions of (Formula presented.) with (Formula presented.) or (Formula presented.) to give (Formula presented.) or (Formula presented.) are thermodynamically plausible only on Pt(111); however, they are very slow due to high energy barriers, 1.48 and 1.36 eV, respectively.