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

Abstract

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 (εC) and mechanical strain (εM) 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 Ov f) of different configurations of oxygen vacant sites (Ov) 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 Ov f values were found to decrease with increasing tensile strain. The Ce–La–Cu–O(111) surface exhibited the lowest E Ov f values for the single subsurface sites, implying that Ov may occur spontaneously upon Cu addition. The mobility of the surface and bulk oxygen anions in the lattice contributing to the Ov 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 Ov and compressive strain, a finding with design implications in covalent type of interactions. To bridge this study ...

Advanced NH3 Detection by 1D Nanostructured La:ZnO Sensors with Novel Intrinsic p–n Shifting and Ultrahigh Baseline Stability

“…Due to the incongruity in ionic size and chemical properties between host (zinc) and foreign (lanthanum) ions, several kinds of internal strains, mainly lattice mismatch strain and chemical strain, may appear in the structure after the doping process. The former kind of strain happens due to the huge mismatch in ionic sizes, causing the crystal lattice to expand or contract locally around the dopant atoms and leading to the generation of local distortions and potential defects. , Yet, the latter kind of strain arises from the differences in charges, chemical interactions, and bonding preferences between La dopant and neighboring atoms in the ZnO host material, which distort bond lengths and angles and can affect oxygen vacancy formation and population and their chemical reactivity within the lattice . With Scherrer formulation D = K λ β .25em cos nobreak0em0.25em⁡ θ ε = β 4 .25em tan nobreak0em0.25em⁡ θ where β is the full-width at half-maximum (fwhm) and K = 0.941 is a shape factor, the films’ crystallite size and lattice strain based on the most intense peak were estimated as D = 39 nm and ε = 3.2 × 10 –3 for ZOL0 and D = 36 nm and ε = 3.3 × 10 –3 for ZOL3.…”

“…31,32 Yet, the latter kind of strain arises from the differences in charges, chemical interactions, and bonding preferences between La dopant and neighboring atoms in the ZnO host material, which distort bond lengths and angles and can affect oxygen vacancy formation and population and their chemical reactivity within the lattice. 33 With Scherrer formulation 34…”

Highly Efficient Hydrogenation of Nitrocyclohexane to Cyclohexanone Oxime over NiTi-Layered Double Hydroxide: The Key Role of Oxygen Vacancies

“…In Figure , the anatase TiO 2 (101) surface was used as the adsorption configuration. Moreover, the oxygen-defective TiO 2 (101) (TiO 2 –Ov) surface was built up by removing an O atom from the TiO 2 (101) surface. , Furthermore, as shown in Figure S10, the TiO 2 –Ov surface loaded with Ni (Ni-loaded TiO 2 –Ov) was constructed by adding a Ni atom to one of the Ov. Similarly, Ni-loaded TiO 2 was constructed.…”

“…Moreover, the oxygen-defective TiO 2 (101) (TiO 2 −Ov) surface was built up by removing an O atom from the TiO 2 (101) surface. 59,60 Furthermore, as shown in Figure S10 To further reveal the mechanism for the hydrogenation of NCH to CHO, the adsorption configuration of NCH on Niloaded TiO 2 and Ni-loaded TiO 2 −Ov surfaces was investigated. The adsorption models of NCH and CHO are depicted, and the calculated results are illustrated in Figure 14.…”

Construction of cerium-based oxide catalysts with abundant defects/vacancies and their application to catalytic elimination of air pollutants

“…Ball milling is a well-established mechanical activation technique in which the potential of the material or its stored mechanical energy is enhanced under the application of the ball milling forces, thereby causing various transformations and reactions, such as disordering and amorphization (including surface amorphization), grain boundary disordering, polymorphic transformations, and so forth. 64,65 Polychronopoulou et al 66 first prepared a Ce–La–Cu–O oxide catalyst by a microwave-assisted sol–gel method and then subjected the catalyst to ball milling. The ball milling process caused mechanical strain on the catalyst, which was found to facilitate oxygen vacancy formation and provide more adsorption sites.…”