Au2+ cannot catalyze conversion of methane to ethene at low temperature

Abstract

The previously reported conversion of methane to ethene catalyzed by Au₂⁺ at thermal energies, at odds with established thermodynamics, is investigated through a combination of experiments under both single-collision conditions using a guided ion beam tandem mass spectrometer (GIBMS) apparatus and at higher pressures using a selected-ion flow tube (SIFT) apparatus, as well as through density functional calculations. Production of Au₂(C₂H₄)⁺ (or m/z 422) or Au₂(C₂D₄)⁺ (or m/z 426) is significantly lower in the higher pressure SIFT experiments relative to previously reported results. The amount observed is consistent with a pathway initiating from a small fraction of a reactive species, potentially electronically excited Au₂⁺* or Au₂O⁺, isobaric with Au₂(CH₄)⁺. Extensive theoretical exploration of the potential energy surface for Au₂(CH₄)⁺ + CH₄ shows no low-energy pathway that is consistent with ethene formation, with prohibitive barriers of 1–2 eV calculated along all identified reaction coordinates, consistent with previous calculations. GIBMS data do show the production of m/z 422 in the reaction of Au₂(CH₄)⁺ + CH₄, consistent with the proposed key intermediate, but also provide evidence that this observed species is Au₂(CO)⁺ arising from Au₂O⁺, not Au₂(C₂H₄)⁺ arising from Au₂(CH₄)⁺. The present results are consistent with the established thermochemistry for methane-to-ethene conversion, which unambiguously demonstrates that such conversion cannot proceed at thermal energies regardless of the presence of Au₂⁺.