CO₂ activation begins when the linear carbon dioxide molecule adsorbs on the catalyst surface. Metal sites like copper provide electrons to the CO₂ pi-star antibonding orbital, causing the molecule to bend from 180 degrees to approximately 134 degrees, forming an activated CO₂ radical intermediate.
In the PCET process, protons from the electrolyte and electrons from the catalyst simultaneously transfer to the activated CO₂ radical. This creates two competing reaction pathways: the OCHO pathway leading to formate formation, and the COOH pathway leading to carbon monoxide. The selectivity depends on catalyst surface properties and local reaction conditions.
The formate formation pathway proceeds through three main steps. First, the OCHO intermediate undergoes further protonation and electron transfer to form HCOO. Then, C-O bond breaking occurs, followed by desorption of the formate product into solution. This pathway requires only two electrons and has favorable thermodynamics, making it the preferred route on sea cucumber-like Cu₂O/Cu@N-C catalysts.