Decoupling the Chemical and Mechanical Strain Effect on Steering the CO 2 Activation over CeO 2 -Based Oxides: An Experimental and DFT Approach.

Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO 2 is involved. Thus, it is important to understand how to tailor their CO 2 adsorption behavior. In this work, steering the CO 2 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 3+ or La 3+ /Cu 2+ ) and post-synthetic ball milling were considered as the origin of the chemical and mechanical strain of CeO 2 , 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 2 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 CO 2 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 CO 2 -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 16 O/ 18 O transient isothermal isotopic exchange experiments; the maximum in the dynamic rate of 16 O 18 O formation, R max ( 16 O 18 O), 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 CO 2 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 2 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 2 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 CO 2 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 CH 4 and CO 2 conversions. This study opens new possibilities to manipulate the CO 2 activation for a portfolio of heterogeneous reactions.