• • Au-Bi4Ti3O12 nanosheets achieve a C2H6 evolution rate of 2.58 μmol g−1 h−1 from 0.03% CO2, a first for single-catalyst photoreduction, offering a route to direct air-to-fuel conversion.
• • The Gibbs free energy for *CO–COH formation on Au-Bi4Ti3O12 is 2.23 eV, 1.36 eV lower than on Bi4Ti3O12 (3.59 eV), demonstrating that Au-Ti metal pair sites significantly lower the C–C coupling barrier.
• • In situ FTIR detected *OCCOH intermediate on Au-Bi4Ti3O12 but not on Bi4Ti3O12, providing direct spectroscopic evidence of the altered reaction pathway toward C2 products.
• • The catalyst operates under atmospheric CO2 concentration (0.03%) without sacrificial reagents, eliminating energy-intensive CO2 concentration steps and reducing overall process cost.