Decoupling the Chemical and Mechanical Strain Effect on Steering the CO Activation over CeO-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 is involved. Thus, it is important to understand how to tailor their CO adsorption behavior. In this work, steering the CO 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 or La/Cu) and post-synthetic ball milling were considered as the origin of the chemical and mechanical strain of CeO, 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 surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chemical strain (ε) and mechanical strain (ε) on the modification of the Ce-La-Cu-O surface reactivity toward CO activation. During the ab initio calculations, the stability (energy of formation, ) of different configurations of oxygen vacant sites (O) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO-philicity of the surface was assessed at different levels of the imposed mechanical strain. The values were found to decrease with increasing tensile strain. The Ce-La-Cu-O(111) surface exhibited the lowest values for the single subsurface sites, implying that O may occur spontaneously upon Cu addition. The mobility of the surface and bulk oxygen anions in the lattice contributing to the O population was measured using O/O transient isothermal isotopic exchange experiments; the maximum in the dynamic rate of OO formation, (OO), was 13.1 and 8.5 μmol g s for pristine (chemically strained) and dry ball-milled (chemically and mechanically strained) oxides, respectively. The CO 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 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 adsorption energy (), 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 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 drop in the presence of double O 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 and CO conversions. This study opens new possibilities to manipulate the CO activation for a portfolio of heterogeneous reactions.