Abstract: The costly separation of 1,2-propanediol (1,2-PDO), an unavoidable byproduct in the hydrogenation of dimethyl oxalate (DMO), significantly hampers the economic viability of coal-to-ethylene glycol (EG) technology. To address this challenge, the formation mechanism of the side product 1,2-PDO on the Cu(111) and Cu₂O(111) surfaces during DMO hydrogenation was investigated, which focused on the active sites of copper catalyst and the dominant pathway through density functional theory calculation. The thermodynamics of each elementary step and the adsorption behavior of various species involved in the reaction network along with the local density of states and charge density difference were systematically analyzed. The results indicate that 1,2-PDO is generated more favorably on the Cu₂O(111) surface than that on the Cu(111) surface, owing to the Lewis acid-base pairs, i.e. \rmCu_\rmus^+ and \rmO_\rmsuf^- sites, present on the Cu₂O(111) surface, which strengthens the binding of reactants, products, and reaction intermediates to the substrate. EG reacts primarily with methanol (MeOH) to form 1,2-PDO through Guerbet alcohol condensation reaction through three consecutive steps: alcohol dehydrogenation, aldol condensation, and unsaturated aldehyde hydrogenation. The \rmO_\rmsuf^- sites promote the dehydrogenation of alcohols into aldehydes, the generation of enolates during aldol condensation and the hydrogenation of unsaturated aldehydes, while the \rmCu_\rmus^+ sites are responsible for the C–C coupling reaction. These findings may shed light on the mechanism of 1,2-PDO formation over Cu catalyst and provide fundamental knowledge for the development of more efficient catalysts and process optimization.