Decoupling the Chemical and Mechanical Strain Effect on Steering the CO<sub>2</sub> Activation over CeO<sub>2</sub>-Based Oxides: An Experimental and DFT Approach (2022)

Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO<sub>2</sub> is involved. Thus, it is important to understand how to tailor their CO<sub>2</sub> adsorption behavior. In this work, steering the CO<sub>2</sub> 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 (La<sup>3+</sup> or La<sup>3+</sup>/Cu<sup>2+</sup>) and post-synthetic ball milling were considered as the origin of the chemical and mechanical strain of CeO<sub>2</sub>, 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 CO<sub>2</sub> surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chemical strain (ε<sub>C</sub>) and mechanical strain (ε<sub>M</sub>) on the modification of the Ce-La-Cu-O surface reactivity toward CO<sub>2</sub> activation. During the ab initio calculations, the stability (energy of formation, <i>E</i><sub>O<sub>v</sub></sub><sup>f</sup>) of different configurations of oxygen vacant sites (O<sub>v</sub>) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO<sub>2</sub>-philicity of the surface was assessed at different levels of the imposed mechanical strain. The <i>E</i><sub>O<sub>v</sub></sub><sup>f</sup> values were found to decrease with increasing tensile strain. The Ce-La-Cu-O(111) surface exhibited the lowest <i>E</i><sub>O<sub>v</sub></sub><sup>f</sup> values for the single subsurface sites, implying that O<sub>v</sub> may occur spontaneously upon Cu addition. The mobility of the surface and bulk oxygen anions in the lattice contributing to the O<sub>v</sub> population was measured using <sup>16</sup>O/<sup>18</sup>O transient isothermal isotopic exchange experiments; the maximum in the dynamic rate of <sup>16</sup>O<sup>18</sup>O formation, <i>R</i><sub>max</sub>(<sup>16</sup>O<sup>18</sup>O), was 13.1 and 8.5 μmol g<sup>-1</sup> s<sup>-1</sup> for pristine (chemically strained) and dry ball-milled (chemically and mechanically strained) oxides, respectively. The CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> adsorption energy (<i>E</i><sub>ads</sub>), 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 CO<sub>2</sub> 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 <i>E</i><sub>g</sub> drop in the presence of double O<sub>v</sub> 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 CH<sub>4</sub> and CO<sub>2</sub> conversions. This study opens new possibilities to manipulate the CO<sub>2</sub> activation for a portfolio of heterogeneous reactions.