Decoupling the Chemical and Mechanical Strain Effect on Steering the CO₂ Activation over CeO₂-Based Oxides: An Experimental and DFT Approach

Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO₂ is involved. Thus, it is important to understand how to tailor their CO₂ adsorption behavior. In this work, steering the CO₂ activation behavior of Ce-La-Cu-O ternary oxide surfaces through the combined effect of chemical and mechanical strain was thoroughly examined using both experimental and ab initio modeling approaches. Doping with aliovalent metal cations (La³⁺ or La³⁺/Cu²⁺) and post-synthetic ball milling were considered as the origin of the chemical and mechanical strain of CeO₂, respectively. Experimentally, microwave-assisted reflux-prepared Ce-La-Cu-O ternary oxides were imposed into mechanical forces to tune the structure, redox ability, defects, and CO₂ surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chemical strain (ε_C) and mechanical strain (ε_M) on the modification of the Ce-La-Cu-O surface reactivity toward CO₂ activation. During the ab initio calculations, the stability (energy of formation, E_{O_v}^f) of different configurations of oxygen vacant sites (O_v) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO₂-philicity of the surface was assessed at different levels of the imposed mechanical strain. The E_{O_v}^f values were found to decrease with increasing tensile strain. The Ce-La-Cu-O(111) surface exhibited the lowest E_{O_v}^f values for the single subsurface sites, implying that O_v may occur spontaneously upon Cu addition. The mobility of the surface and bulk oxygen anions in the lattice contributing to the O_v population was measured using ¹⁶O/¹⁸O transient isothermal isotopic exchange experiments; the maximum in the dynamic rate of ¹⁶O¹⁸O formation, R_max(¹⁶O¹⁸O), was 13.1 and 8.5 μmol g^-1 s^-1 for pristine (chemically strained) and dry ball-milled (chemically and mechanically strained) oxides, respectively. The CO₂ activation pathway (redox vs associative) was experimentally probed using in situ diffuse reflectance infrared Fourier transform spectroscopy. It was demonstrated that the mechanical strain increased up to 6 times the CO₂ adsorption sites, though reducing their thermal stability. This result supports the mechanical actuation of the "carbonate"-bound species; the latter was in agreement with the density functional theory (DFT)-calculated C-O bond lengths and O-C-O angles. Ab initio studies shed light on the CO₂ adsorption energy (E_ads), suggesting a covalent bonding which is enhanced in the presence of doping and under tensile strain. Bader charge analysis probed the adsorbate/surface charge distribution and illustrated that CO₂ interacts with the dual sites (acidic and basic ones) on the surface, leading to the formation of bidentate carbonate species. Density of states (DOS) studies revealed a significant E_g drop in the presence of double O_v and compressive strain, a finding with design implications in covalent type of interactions. To bridge this study with industrially important catalytic applications, Ni-supported catalysts were prepared using pristine and ball-milled oxides and evaluated for the dry reforming of methane reaction. Ball milling was found to induce modification of the metal-support interface and Ni catalyst reducibility, thus leading to an increase in the CH₄ and CO₂ conversions. This study opens new possibilities to manipulate the CO₂ activation for a portfolio of heterogeneous reactions.