Understanding the interactions between plasmonic gold (Au) nanoparticles and the adsorbate is essential for photocatalytic and plasmonic applications. However, it is often challenging to identify a specific reaction mechanism in the ground state and to explore the optical properties in the excited states because of the complicated pathways of carriers. In this study, photocatalytic reduction of carbon dioxide (CO2) to C1 products (for example, CO and CH4) on the Au(111) nanoparticle (NP) surface was studied based on reaction pathway analysis, adsorbate reactivity, and its ability to stabilize or deactivate the surface. The calculated reaction Gibbs free energies and activation barriers revealed that the first step in CO reduction via a direct hydrogen transfer mechanism on Au(111) is the formation of formyl (*CHO) instead of hydroxymethylidyne (*COH). Furthermore, the size enhanced and symmetry sensitive optical responses of cuboctahedral Au(111) NPs on localized surface plasmon resonance (LSPR) were investigated by using time-dependent DFT (TDDFT) calculations. Although near field enhancement around cuboctahedral Au(111) NPs is only weakly dependent on the morphology of NPs, it was observed that corner sites stabilize *C-species to drive the CO2 reduction to CO. The density of active surface states interacting with the adsorbate states near the Fermi level gradually decreases from the (111) on-top site toward the corner site of the Au(111) NP-CO system, which strongly affects the molecule's binding on catalytic sites and, in particular, electronic excitation. Finally, the spatial distribution of the charge oscillations was determined as a guide for the fabrication of Au NPs with an optimal LSPR response.