Facet-Engineered MgO for Efficient Nonthermal Plasma Catalytic CO₂ Splitting: Dominant Role of the (111) Surface

The facet-dependent catalytic behavior of MgO in non-thermal plasma (NTP)-driven CO₂ decomposition is systematically investigated by combining experimental measurements and density functional theory (DFT) calculations. Three MgO catalysts with dominant exposure of the (100), (110), and (111) facets are synthesized. CO₂ temperature-programmed desorption (CO₂-TPD) shows that CO₂ adsorption capacity follows the order MgO(110) > MgO(111) > MgO(100), consistent with DFT-derived adsorption energies. DFT energy profiles reveal that although MgO(110) binds CO₂ most strongly, it suffers from excessively strong CO adsorption (5.84 eV), inhibiting product desorption. In contrast, MgO(111) offers a favorable CO₂ adsorption energy combined with a remarkably low CO desorption energy (0.71 eV), enabling rapid turnover. Electronic structure analyses demonstrate substantial charge transfer from MgO(111) to CO₂ (up to 1.76 |e|) and pronounced orbital hybridization near the Fermi level, which are further enhanced under plasma conditions. Plasma-catalytic tests at 0.8 W show that MgO(111) achieves the highest CO₂ conversion (60.7%) with excellent selectivity toward CO (95.3%) and O₂ (94.4%), outperforming MgO(110) and MgO(100). Increasing the input power from 0.8 to 2.5 W raises conversion to 78.1% but reduces energy efficiency due to increased gas heating or non-productive pathways. Overall, the (111)-enriched MgO is identified as an efficient and selective catalyst for NTP-based CO₂ splitting, owing to its optimal balance of adsorption strength, facile CO desorption, strong charge transfer, and plasma–catalyst synergy. This work highlights the importance of facet engineering and power optimization for designing oxide-based plasma catalysts toward energy-efficient CO₂ utilization.