Facet-dependent electrocatalysis in the HCOOH synthesis from CO₂ reduction on Cu catalyst: a density functional theory study

Copper (Cu) has unique hydrogenation properties in the CO₂ reduction reaction (CO₂RR) for the synthesis of valuable hydrocarbons and alcohols such as methane, ethylene, ethanol and formic acid. Among the C1 products, formic acid (HCOOH) is chemically stable in the liquid phase at room temperature and is the simplest energy source that can be synthesized from CO₂. The electrochemical CO₂RR on Cu has been widely studied due to its very high selectivity toward target products and mild reaction conditions. However, electrochemistry activity can be impacted by many catalytic surface properties, including the morphology, crystallinity and surface index. Cu has a face-centered cubic (FCC) structure, and the Cu(1 1 1) surface index tends to be naturally exposed. Since the number of dangling bonds on the surface of the catalyst is different for each exposed surface, each surface activity is different, and the reaction pathway may also be different. The surface indexes of Cu(1 0 0) and (2 1 1) can be observed as a supported catalyst, and Cu(1 1 0) can be observed in a rod shape nanoparticle. Herein, we performed CO₂RR over each surface and proposed an energy diagram of HCOOH formation via the COOH and HCOO intermediates using spin-polarized density functional theory. We elucidate the determining factor of catalytic activity and discuss the relationship between the coordination number/surface roughness (caused by the different surface facet) and the activity through electronic analysis of the d orbitals.