Herein, the overall structure of a nanocluster coprotected by phosphine and mercaptan ligands [Au₇Ag₈(SPh)₆((p-OMePh)₃P)₈]NO₃ (Au₇Ag₈) was reported. For comparison, a previously reported nanocluster with the same structure, but a different metal composition, [Au₁₃Cu₂(TBBT)₆((p-ClPh)₃P)₈]SbF₆ (Au₁₃Cu₂), was synthesized. In addition, their optical and electrocatalytic CO₂ reduction properties were comprehensively compared. The results reveal that the photoluminescence quantum yield (PLQY) of the Ag-doped Au₇Ag₈ nanocluster is 1.62%, which is seven times greater than that of the Cu-doped Au₁₃Cu₂ nanocluster (PLQY = 0.23%). Furthermore, the Au₁₃Cu₂ nanocluster demonstrates significantly enhanced catalytic selectivity for CO, with a CO Faradaic efficiency ranging from 79.7% to 90.4%, compared with that of the Au₇Ag₈ nanocluster (CO Faradaic efficiency: 67.2%–77.7%) within a potential range of 0.5 to −1.1 V. From structural analyses, the superior CO selectivity of Au₁₃Cu₂ is attributed to the copper dopant.