Low-temperature C-C coupling of CH₄ and CO₂ over Rh₁/UiO-66-H single-atom catalysts: insight from DFT calculations

Direct C-C coupling of CH₄ and CO₂ to CH₃COOH is a promising 100% atom-economic route, but kinetically hindered by the high inertness of the two reactants. In this work, we systematically evaluated the structural stability of M₁/UiO-66-H (M = Fe, Co, Ni, Cu, Ru, Rh, Pd, and Ag) single-atom catalysts (SACs) as well as their adsorption capacities for CH₄ and CO₂ by DFT calculations. The reaction mechanism was identified to proceed through three consecutive steps, namely, CH₄ activation to form CH₃*, C-C coupling to generate CH₃COO*, and subsequent hydrogen transfer to yield CH₃COOH. The rate-determining step (RDS) was determined to be the C-C coupling step, and Rh₁/UiO-66-H exhibited a very low activation barrier of 0.56 eV. Microkinetic simulations further demonstrated that Rh₁/UiO-66-H achieves a turnover frequency (TOF) of 3.91 × 10^-3 s^-1 site^-1 per active site under industrially relevant conditions (394 K, 7 bar). The high catalytic activity of Rh₁/UiO-66-H originates from the regulation of the interaction between the two carbon atoms derived from CH₄ and CO₂ by the Rh₁ single atom, which effectively facilitates the formation of the C-C bond. This catalytic strategy demonstrates the great potential of Rh₁/UiO-66-H for the efficient conversion of CO₂ and CH₄.