In situ scanning tunneling microscopy (STM) is used to explore the spatial structures of a ordered platinum adlayer deposited on an Au(111) electrode under potential control in double layer region in 0.1 M sulfuric acid. The Pt deposit is prepared by chemical reduction of PtCl₆^2- complexes by carbon monoxide (CO) molecules, which generates a one-atom-thick Pt deposit capped by a CO adlayer which impeded the further deposition. The superstructure of the CO on Pt film is (√7×√7)R19.1° in CO free electrolyte, the same as observed on a Pt(111) electrode, suggesting that Pt adatoms are arranged spatially similar to the Pt(111), not Au(111), plane. The CO adlayer is stripped off completely by pulsing the potential to 0.96 V (vs. hydrogen reversible electrode) within 3 s in H₂-satruated 0.1 M H₂SO₄, resulting in a Pt adlayer still organized in a hexagonal lattice with an atomic distance of 0.287 nm which is 3.7% smaller than Au(111)’s and 3.2% larger than Pt(111)’s. The mismatch in lattice dimension between the Pt adlayer and Au(111) substrate yields intricate commensurate structures, exhibiting a corrugated surface morphology, as observed with the reconstructed Pt(111). This phase, dubbed the R phase, is stable only if the potential is more negative than 0.8 V; otherwise, it is converted into a Moiré pattern (M phase). This M phase appears elastic as the interatomic spacing decreases from 0.281 to 0.277 nm from 0 to 0.3 V, and stays at 0.277 nm till 0.95 V, where oxidation and restructuring occur. These R and M phases allotropes show different electrocatalytic activities toward hydrogen evolution and oxidation reactions (HER and HOR), indicating that the atomic structure of the Pt deposit is important in determining its electrochemical properties.