In situ attenuated total reflection-surface enhanced infrared adsorption spectroscopy (ATR-SEIRAS) was performed in Bruker Vertex 80 FT-IR spectrometer. Hemisphere ZnSe with a diameter of 2.5 mm was used as the attenuated total reflection (ATR) crystal. The Au deposition method was modified from the literature 53 : the ZnSe was firstly polished twice with 3 μm alumina and sonicated 3 times in a mixture of EtOH and acetone for 10 minutes. After drying the surface under an N2 flow, the crystal was heated to 40 °C. 1 mL 24 mM HAuCl4 solution (also pre-heated to 40 °C) was added onto the crystal surface for electroless Au deposition. After around 75 s, the solution was dumped and the Au-coated ZnSe was cleaned by ultrapure water and dried under an N2 flow. A successful Au deposition would give a surface resistance of 15-30 Ω. The catalyst sample (~0.4 mg) was dispersed in 0.2 mL EtOH and spread onto the Au-coated ZnSe. The crystal was then mounted onto a home-made cell (Supplementary Figure ). The electrolyte was 0.01 M KOH + 0.05 M K2SO4. Prior to the measurements, a -0.1 V vs RHE potential was applied to the electrode for 10 min under N2 purging to reduce any surface oxide and to make sure the catalyst was fully immersed. The background spectrum was collected after holding the electrode potential at 0.4 V vs RHE for 5 min. Each spectrum was co-added by adding 100 single spectra with a resolution of 4 cm -1 .