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 (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, EOvf) 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 EOvf values were found to decrease with increasing tensile strain. The Ce–La–Cu–O(111) surface exhibited the lowest EOvf 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, Rmax(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 (Eads), 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 Eg drop in the presence of double Ov 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.


INTRODUCTION
In heterogeneous catalysis, strained surfaces can be found in both metal-supported catalysts and multi-elemental metal oxides due to a lattice constant mismatch between metal− support and host−guest interactions, respectively. 1 Such lattice strain enhances the chemisorption properties of the catalytic surface significantly, 2 either for the adsorbate or for the intermediate species formed under reaction conditions. As a strain normally manipulates the surface ability to form bonds, there is a great possibility in using a strain as a catalyst reactivity modifier/descriptor. 2 Ball milling is a mature mechanical activation technique that can lead to the fabrication of nanocatalysts with anomalous properties compared to their bulk counterparts. Through mechanical activation, solid-state chemical reactions can be initiated/accelerated while causing various transformations and reactions, such as grain boundary disordering, amorphization, defect generation/migration, polymorphic transformations, ion coordination sphere change, and reduction in particle size. 3 The main function that takes place during ball milling is that the material's potential or its stored mechanical energy is enhanced in the presence of ball milling forces. 4 This is translated into defects (point, line, and volume ones), surface and interface formation, strain and structural disorder, and changes in electronic states. Some of the parameters in the ball milling process that can alter the impact of mechanical activation in terms of the final catalyst properties (e.g., surface area, active metal dispersion, and binding strength of the intermediates) are the ball size, number of balls used, ball-to-powder ratio, rotational speed, milling time, and milling atmosphere. 5,6 It is well known that the ceria morphology and thus its properties can be tuned through synthesis. A variety of synthetic methods have been used spanning from mild hydrothermal to form ceria nanotubes 7 and nanorods with different types and distributions of oxygen vacancies 8 to template-free microwave-assisted hydrothermal synthesis and urea homogeneous precipitation; the latter ceria materials were used for the preparation of Pt/ CeO 2 catalysts for the WGS reaction, 9 demonstrating the synthesis impact on the electronic/catalytic properties.
In the context of sustainable development, there is an increasing interest in the utilization of the captured CO 2 from a flue gas to form valuable chemicals and products. In this sense, two of the catalytic reactions of most interest are dry reforming of methane (DRM) and CO 2 hydrogenation. The DRM is very relevant in this context as it simultaneously tackles the abatement of two greenhouse gases (CH 4 and CO 2 ) while leading to the production of hydrogen (energy carrier). The CO 2 hydrogenation leads to the valorization of CO 2 into different value-added products. However, it is well known that the inert property and stability of the CO 2 molecule pose a challenge in both aspects: thermodynamics and kinetics. 10 Upon CO 2 adsorption on metal and metal oxide surfaces, the molecule's stability is reduced. The adsorption configuration is highly dependent on the surface Lewis acidity/basicity. In an ideal scenario, a bent configuration is favored by charge transfer (CT) from the surface (Lewis base, electron reservoir) to the CO 2 molecule. There are reports showing that both CO 2 linear and bent configurations on the ceria surface are favored. 11,12 It has also been reported that reduced ceria surfaces are favorable for carbon dioxide reduction reactions due to the involvement of polarons (Ce 3+ −O v pairs) in the CO 2 molecular adsorption at oxide surfaces, thus enhancing CT that is the initiator of such chemical reactions. It is therefore important to obtain insights into the surface chemical reactivity and its CO 2 -philicity, as well as how this can be steered. Acidic surfaces favor the formation of linear-type CO 2 adsorption configuration, whereas basic surfaces favor the CO 2 − formation (bent species and reactive ones). 13 From the aspect of catalyst design for CO 2 activation, some criteria need to be considered, such as (a) the use of metal oxide supports with high basicity (e.g., MgO and La 2 O 3 ) can enhance the dissociative adsorption of CO 2 , which inhibits carbon formation by creating a higher number of oxygen atoms around the catalyst-active metal surface. 14 The improvement of CO 2 dissociation on catalysts by increasing the surface basicity can similarly deactivate the catalyst. 14 It has been verified lately that excessive surface basicity/acidity can cause deactivation due to carbon formation in reactions such as DRM, 15 where the importance of moderate acidity and basicity along with the homogeneous dispersion that ideally defines catalytic conversion in the DRM reaction and the long-term stability of supported metal catalysts was demonstrated. (b) Oxygen vacancies (O v ) can be classified as intrinsic, naturally existing in the material (e.g., due to the presence of Ce 4+ /Ce 3+ ), or extrinsic, induced by the doping of CeO 2 lattice with aliovalent metal cations for better ionic conductivity. 16, 17 Among the doped ceria catalysts, the Ce−Cu−O system exhibits the (i) high redox properties and the ability to switch between Ce 3+ / Ce 4+ and Cu 2+ /Cu 1+ , (ii) increasing population of oxygen vacant sites (O v ), and (iii) increase of labile surface and bulk oxygen species compared to the single-phase oxide. 18 Therefore, it can be stated that O v has a critical role in the CO 2 dissociation on ceria surfaces and thus is expected to have a leading role in catalytic reactions where CO 2 is a reactant or a co-reactant. 19 It is also well established that strain can modify the defect (e.g., O v ) structure and electronic properties. 20 While the tensile strain leads to surface vacancies having polarons as the next neighbor (NN), the compressive strain is associated with subsurface vacancies having polarons as NN, whereas dimer vacancies are also favored over compressive strain.
Mechanical forces are used either to prepare or to actuate ceria-based catalysts and devices. For instance, in the preparation of ceria-based catalysts, the latter used for environmental and energy applications has been reported in many review articles using mechanochemical methods. 21 Demonstration of the superiority of Pd/CeO 2 ball-milled catalysts for the methane oxidation reaction at low temperature, above 95% conversion, over the traditional wet impregnation catalysts has been discussed by Trovarelli's research group even at conditions which are challenging for the particular reaction. 22,23 Furthermore, there is extensive literature on the impact of different types of external stimuli on thin films of ceria; for example, the electrochemomechanical effect has been reported by Lubomirsky and his colleagues 24,25 to produce stress that can potentially deteriorate them. 22,23 Furthermore, strain engineering has been reported to increase the CT and electronic conductivity. 26 However, the effect of applying compressive or tensile stress on doped ternary ceria (111) surfaces has not being comprehensively investigated in the literature yet in the context of steering of its catalytic chemistry and adsorption behavior. The oxide surfaces (Ce−La−Cu−O) studied in the present work exhibit a versatile catalytic functionality that spans from reforming 27 to oxidation chemistry, 18 given their noble-metal-free nature is worthy of more attention. Finding a way to engineer the vacancy population and structure, and, hence, the CO 2 −surface interaction, can be an additional tool toward catalyst design for CO 2 activation, thus contributing to a more rational design of catalysts for reactions such as DRM and CO 2 hydrogenation.
In the present work, microwave-prepared Ce−La−Cu−O ternary metal oxides were subjected into mechanochemical activation under dry and wet ball milling targeting their intrinsic property modification, namely, structure, crystallite size, specific surface area, redox properties, CO 2 activation energy, and pathway. The main emphasis was given on exploring how chemical strain, ε C (doping effect), and mechanical strain, ε M (compressive or tensile, originated by ball milling), can impact the oxygen vacancy formation and population [O v ] and their chemical reactivity toward CO 2 activation. Experimental and density functional theory (DFT) studies were performed in an effort to deconvolute the ε C and ε M impact. A versatile toolbox including X-ray powder diffraction, Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), 18 O 2 -transient isothermal isotopic exchange (TIIE), in situ CO 2 diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) spectroscopy, H 2 -temperature-programmed reduction (H 2 -TPR), and CO 2 -temperatureprogrammed desorption (CO 2 -TPD), along with intensive and systematic ab initio calculations was employed to fully characterize and analyze the performance of the solids. The effect of supports during ball milling on the performance of Ce− La−Cu−O supported Ni catalysts toward the DRM reaction was evaluated to provide evidence for the value that mechanochemistry can bring into catalysis.

Preparation of Ternary Metal Oxides.
A microwaveaccelerated reaction system (MARS-6) was used to synthesize the catalysts through microwave-assisted reflux synthesis. The microwave system had a power output of 0−1800 W ±5% (IEC 705 Method-1988). 18 Materials were prepared using precursor salts Ce(NO 3 ) 3 · 6H 2 O (Aldrich 99.95%), La(NO 3 ) 3 ·6H 2 O (Aldrich 99.95%), Sm-(NO 3 ) 3 ·6H 2 O (Aldrich 99.95%), and Cu(NO 3 ) 2 ·3H 2 O (Aldrich 99.95%) and dissolved in distilled water. The prepared mixed metal oxides had 10 at. % Cu content, while the Ce/M (M: La and Sm) ratio was maintained at unity. The total molar ratio was maintained at 0.03 mol in all cases. The complexing agent ethylene glycol (EG) was added to the solution with a ratio of EG to water retained at 2; that is, for each 100 mL of EG, 50 mL of distilled water was added.
The final solution was added to an open 1000 mL round-bottom flask in an open vessel mode, equipped with an oval magnetic stirring bar (length 20 mm and diameter 6 mm) made with polytetrafluoroethylene coating. A reflux system was attached to the open round flask of the MW system, allowing water to pass through the reflux for condensation. Parameters, such as stirring, reaction temperature, MW power, and heating/cooling ramp, were all adopted from a previous study. 1 The temperature of the reaction was monitored by built-in fiber optic probes. The solution was heated in the microwave reactor at two stages: (i) 130°C for 2 h and (ii) 170°C for 1 h at 800 W of magnetron power. Following microwave heating, all synthesized materials were calcined at 500°C for 6 h under ambient conditions to form the mixed metal oxide catalyst. In what follows, the wet ball-milled and dry ballmilled samples were coded as wet ball milling (WBM) and dry ball milling (DBM), respectively.

Ternary Oxides Post-Synthesis Mechanochemical
Treatment (Ball Milling). The synthesized materials were ball-milled using a planetary ball mill (Planetary Mill PULVERISETTE 5, Fritsch) under ambient conditions. A relevant amount of catalysts was mixed with distilled water, maintaining a weight ratio of 12:1. The same catalysts were ball-milled in a dry medium to track any structural changes. The catalysts were milled at 250 rpm at different milling times in a zirconia jar with a mass ratio of balls to solid powder equal to 100. The balls were made of zirconia and had a mass of around ∼3 g each. The ball-milled catalysts under wet condition were dried at 150°C for 3 h. Wet milling took place for 4 and 10 h, whereas DBM took place from 0 to 10 h at an interval of 2 h.

Supported Ni Catalyst Preparation.
Pristine Ce−La−Cu− O oxides along with the ones following DBM (Ce−La−Cu−O) and WBM (Ce−La−Cu−O) were used as supports for the deposition of Ni metal phase (supported Ni catalysts) following the wet impregnation method (5 wt % loading) as described in our previous work. 27 2.4. Characterization of Ternary Metal Oxides. Powder X-ray diffraction (XRD), Raman spectroscopy, electron paramagnetic resonance (EPR), scanning electron microscopy, TEM, H 2 -TPR, CO 2 -TPD, and XPS were employed to study the structural, textural, redox, and CO 2 adsorption properties in the pristine (following calcination) and ball-milled (wet and dry) ternary metal oxides. The instrumentation and the experimental protocol are provided in the Supporting Information (see Section S1.3).

18 O/ 16 O TIIE.
The surface and bulk oxygen mobility/diffusion in the mixed metal oxides, particularly Ce−La−10Cu−O, was investigated using 18 O 2 -TIIE experiments. The step-gas isotopic switch, 2 mol % 16 O 2 /2 mol % Kr/Ar/He (T, 30 min) → 2 mol % 18 O 2 /Ar/He (T, t), was conducted over a 20 mg sample with a total volume flow rate of 50 N mL/min. The sample was first pre-calcined under 20 vol % 16 O 2 /He gas flow at 800°C for 2 h, then Ar was passed over the sample for 10 min, and the temperature was then decreased to 350°C in Ar gas flow, followed by the TIIE step-gas switch. During the TIIE experiment, the dynamic evolution of the rates of oxygen exchange between the gasphase oxygen, lattice oxygen, and oxygen vacant sites was recorded.
The transient response curves of the three oxygen isotopic gases and of Kr tracer (inert gas) were continuously recorded by an on-line mass spectrometer (Balzers, Omnistar, 1−300 amu) for the mass numbers (m/z) 32 (1) In eqs 1 and 2, F T is the total molar flow rate (mol/s) of the feed gas stream, y 16  DRIFTS spectra were recorded in 5 vol % CO 2 /Ar gas flow at 350°C and after 30 min in Ar gas flow at 350°C for various times in Ar flow. The catalyst sample (∼80 mg) in a very fine powder form was placed firmly into the ceramic cup of the DRIFTS cell and the temperature was increased to 500°C in Ar gas flow and kept for 30 min. Then, the sample was cooled to 350°C in Ar gas flow, and the spectrum of the solid was recorded at 350°C. The latter spectrum was subtracted from the spectrum of the solid recorded in the CO 2 /Ar gas mixture or Ar gas flow at 350°C. Deconvolution and curve fitting procedures of DRIFTS spectra were performed considering the Gaussian peaks. DRIFTS spectra when necessary were smoothed to remove the high-frequency noise and further analyzed using the software Spectrum10 for Windows. ). The first-principles spin-polarized calculations, based on the DFT, 29,30 were performed after employing the Vienna Ab Initio Simulation Package (VASP). 31−33 The electronic exchange was used under the generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof. 34 To include the strong correlation effects of 4f electrons, GGA with the Hubbard U correction method 35,36 was used with a value of U = 5.0 eV. 37−39 The DFT + U method accounts for the O 2p states and has been previously followed in DFT studies as a common practice to accurately describe the oxygen electronic behavior rising from the doping effect. 40,41 The plane-wave cut-off energy was set to 400 eV 42 in all the calculations, and the projector-augmented wave pseudopotentials 43 were used to describe the core electrons. The (111) slab was built from the optimized CeO 2 unit cell with a lattice parameter equal to 5.46 Å, in agreement with the literature. 44 The slab consisted of nine (9) layers terminating with oxygen with a vacuum of 15 Å in the vertical direction to avoid interaction between neighboring images due to periodic boundary conditions. A γ-centered k-point of 3 × 3 × 1 in the full Brillouin zone, with an energy convergence criterion of 10 −6 eV, was

Two-Dimensional Planar Lattice Strain.
Biaxial strain was isotropically applied by stretching or shrinking the simulation cell in the x, y directions while relaxing the cell along the z direction. The structure optimizations were performed until the energy-convergence criterion was satisfied, ca. 1 × 10 −6 eV. The atoms in the top six layers were allowed to relax in all the three directions (x, y, and z). A biaxial strain of −5 to 5% was applied to investigate the effect of biaxial strain on the oxygen vacancy formation energy (E Od v f ) in the case of doped/co-doped ceria surface layer of the slab. The E slab CO 2 is the total energy of the slab with the adsorbed CO 2 molecule, E slab pure is the total energy of the slab without the adsorbate (CO 2 ), and E CO 2(g) is the total energy of the isolated CO 2 molecule. As linear CO 2 is not activated, the hypothesis was to study how the chemical and mechanical strain enhances the CT, which will be evident by the bending and activation of CO 2 after relaxing the full configuration. A negative value of the adsorption energy means that the molecule is exothermically adsorbed (E ads < 0), while a positive value indicates that the molecule is endothermically adsorbed (E ads > 0); the more stable adsorption state is implied by the more negative value. The coordinates of the unit cell were obtained from materials project, 45 the slabs were viewed through VESTA, 46 and the CeO 2 (111) surface was cleaved using VESTA. All the above calculations were run multiple times to estimate the possible errors introduced from the VASP calculations; the error was <1%. Figure 1A compares the powder XRD patterns of the Ce−La−10Cu−O mixed metal oxide composition before (0 h) and after DBM for different milling times (ca. 2−10 h) with an interval of 2 h. A detailed presentation of the (111) diffraction peak position is presented in Figure 1B. For the reference oxides (CeO 2 and Ce−La−O), the corresponding XRD patterns are provided in Figure S1. For the case of Ce−La−O due to the La content (50%) and anticipated heavy ceria doping, the La 2 Ce 2 O 7 pyrochlore structure of fluorite type can be formed. Even though the present XRD pattern does not allow for its determination, more discussion is provided in the Raman section.  Figure 1A,B). It is also important that for the pristine Ce−La−10Cu−O (absence of ball milling), a small peak corresponding to the presence of CuO can be observed at 38°2θ (noted with *), which is vanishing with ball milling ( Figure 1A). Figure 1C,D presents the Ce−O and Cu−O bond lengths, respectively, as those have been calculated through modeling approach; their trends are discussed shortly. Figure 1E  From the detailed analysis of the (111) reflection shown in Figure 1B, a peak shift to higher 2θ angles can be observed for milling times up to 4 h due to a compressive strain developed along the Ce−O bond axis in the lattice and possible move of atoms to interstitial sites under the mechanochemical forces. 47 The compression of the Ce−O bonds was also supported through the ab initio calculations (see Figure 1C). It has to be mentioned that an opposite trend was noticed along the Cu−O bond (elongation, see Figure 1D). As the ball milling duration was increased beyond 4 h, either emerging of new peaks corresponding to impurity phases (La(OH) 3 and La 2 O 3 ) is noticed (WBM) or vanishing of the CuO heterophase peak (DBM) ( Figure 1B,E). An increase of milling time is expected to lead to an increased misorientation between neighboring grains causing the formation of high-angle boundaries. As the highenergy milling continues, the crystallite size reaches a critical value. Further energy input to the critical size crystals leads to additional crystal deformation, energy accumulation at the surface, followed by some extent of amorphization. 2 Figure 1F presents the EPR spectra obtained over the DBM and WBM samples. The sharp signal at ∼3400 mT is usually associated with [Ce 3+ −O − −Ce 4+ ] (S = 1/2) 48,49 species on the surface. Since the presence of Ce 3+ is usually linked to the presence of Ov, it can be stated that Ce 3+ can also be linked with high surface activity. The signal at 2600−3300 mT is linked to Cu 2+ (S = 1/2 and I = 3/2) ions. The shape of the EPR line shows the Cu−Cu dipolar interactions, that is, in a distance of 10 ± 3 Å from each other. 50 Based on the powder XRD studies, two types of crystallite size were calculated, and the obtained results are listed in Table 1. Using the Scherrer formula (broadening of the XRD peak originates from the crystallite size only), the crystallite size, D S (nm), was calculated after neglecting the strain. In addition, accepting strain-induced broadening of the XRD peaks, due to crystal imperfections and distortion, the Williamson−Hall (W− H) method was used. 51 In the W−H plot method, D W−H (nm) does not depend on 1/cos(θ), but it does change with tan(θ). This difference is important when small crystalline size and strain co-exist. The equation used for the W−H calculations represents the uniform deformation model (UDM), where a uniform strain across crystallographic orientations is assumed. It is observed that the UDM crystallite size systematically decreases as the milling time increases up to 8 h, and then saturation occurs that leads to an increase of the size after 10 h of milling. This trend can also be related with the equilibrium state of ball milling, where the particle size reduction is escorted by particle size enlargement. In such a case, smaller particles are agglomerated. This is quite consistent with the high-resolution TEM (HRTEM) analysis and the observed refinement of the solid solution nanodomain observed following DBM, as it will be discussed later.

Microstructural Studies.
It is also noticed that the Ce−La−10Cu−O lattice parameter is generally decreased in the case of WBM (5.488−5.440 Å) and DBM (5.488−5.454 Å), more likely indicating a greater extent of substitution of Ce 4+ by La 3+ and Cu 2+ guest cations under the severe milling conditions. The observed change in the lattice parameter can be justified with a preferred uptake in the crystal lattice of Cu (decrease) or La (increase) with a concomitant change in the lattice parameter, depending on the applied mechanical forces. The energy input introduced through ball milling enhances the solubility of the two dopant elements (La and Cu) in the host ceria matrix, 3 but we need to keep in mind that the diffusion is highly dependent on the atomic size of the diffusing species, as well as of host matrix atoms. 2 The W−H method was also used to study the lattice strain (ε) of the catalysts. The W−H plots are provided in Figure S2. It is observed that after doping ceria with La and La/Cu induces a different lattice strain, namely, from 0.0012 (pure ceria) to 0.0038 (tensile strain in Ce−La−O) and −0.0030 (compressive strain in Ce−La−Cu−O). Mechanical ball milling changed the overall lattice strain in the Ce−La−Cu−O oxide from compressive to tensile. The calculated W−H strain trends are in agreement with the sizes of the dopant elements (Cu 2+ , La 3+ vs Ce 4+ , Ce 3+ ). In addition, after increasing the time of DBM (0−10 h), this leads to a profile that includes an initial increase of the lattice strain (DBM 2 h), followed by decrease (4−8 h, DBM) and a final significant increase after 10 h of DBM. This trend/profile can be due to the high pressure and high temperature spots ("hot spots") developed under ball milling, thus causing some relaxation in the lattice. The hot spots can also drive the process of phase transformations. 52 It is also known that during the milling process, a drop of milling effectiveness can occur. According to the ball milling fundamentals, during the milling process, three stages can take place, namely, the Rittinger stage (a), where particle interaction is minimal; the aggregation stage (b), where particle interaction leads to aggregation, and thus, the surface area produced does not correspond to the energy input; and the agglomeration stage (c), where the dispersion is reduced and then vanished. 53 Raman spectroscopy studies were conducted to investigate the strain, phenomena such as phonon confinement and stoichiometry defects, based on the shifting and broadening of the spectral lines. In addition, they were conducted to probe the O sublattice (CeO 8 ) distortions due to doping and mechanochemical treatment. The Raman spectra for Ce−La−10Cu− O, before (t = 0 h) and after DBM for different milling durations (t = 0−10 h), are shown in Figure 2A. The Raman spectra for the reference oxides (CeO 2 and Ce−La−O) are provided in Figure  S3. The F 2g peak centered at 469 cm −1 (Figure 2A) is the characteristic band for the cubic fluorite lattice of ceria and reflects the vibrational mode of oxygen surrounding Ce 4+ ions in the CeO 8 coordination environment. 54 It is well documented that doping ceria, in the present study with La 3+ and Cu 2+ cations, results in an intensity attenuation and red shift of the F 2g peak to lower values, namely, from 469 to 452 and 447 cm −1 for the case of Ce−La−O and Ce−La−10Cu−O, respectively ( Figure S3A). This shift can be justified by the increased lattice strain due to the incorporation of the dopants, for example, due to La 3+ addition (tensile), O , that generated defects, such as oxygen vacancies and lattice distortion/expansion. 55 It has also been reported that in La-doped systems, the F 2g shift is about 6 cm −1 , whereas in heavily doped systems, the shift can reach up to 15 cm −1 ; such a case is found in the La 2 Ce 2 O 7 pyrochlore type of structure. 56,57 Doped ceria with metal cations can improve the oxygen mobility of the catalyst by lowering the barrier for oxygen migration and minimizing the activation energy for ceria reduction (Ce 4+ → Ce 3+ ), 58 as it will be discussed later (Section 3.7). Furthermore, recent density of states (DoS) studies from our group 59 showed that doping ceria leads to the formation of energy states, which host the electrons left behind in the oxygen vacant site, thus facilitating the formation of O v . For each Cu 2+ ion introduced into the lattice, one oxygen vacancy is expected to be formed, 55 which is anticipated to cause tensile strain that leads to the elongation of M−O bond and therefore an increase in the number/density of mobile oxygen vacancies. 60,61 This has been demonstrated to apply in the case of Ce−La−Cu−O solid by elongated Cu−O and compressed Ce−O bonds in the tensile strain region ( Figure 1C,D) according to the present DFT calculations (Table S1).
A closer look into Figure 2A (DBM) and Figure 2B (WBM) shows intense changes in the F 2g band shape and size following the mechanochemical treatment. For both DBM and WBM, there is an optimum in the duration of the mechanochemical treatment, ca. 4 h, where either the F 2g peak intensity is maximum (DBM) or there is no formation of phase impurity (WBM), in agreement with the XRD studies. Further increase of the mechanochemical treatment time either induces changes to the structure (WBM) or decreases the F 2g band intensity (DBM). It can be noticed that 10 h treatment leads to almost vanishing of the F 2g peak under DBM conditions, whereas under WBM, the peaks that correspond to the appearance of the hexagonal La(OH) 3 phase are noticed; the latter is most likely due to the dehydroxylation of the oxide surface and a subsequent reaction of the OH entities with La 3+ (La 3+ + 3 OH − → La(OH) 3 ) under milling forces. 53 The bands in the region below <400 cm −1 (e.g., 250 cm −1 ) are usually assigned to oxygen defects in ceria due to Brillouin zone scattering. 61 The increase of the intensity of those bands up to a milling time of 4 h agrees with the above observations in agreement with the XRD interpretations. These bands are more evident under DBM, suggesting a larger amount of oxygen defects (O v ) present in DBM oxides compared to the WBM ones. A detailed analysis of the F 2g band is presented in Figure  2C, showing the effect of lattice strain and phonon confinement on the band shape and oxygen stoichiometry. 62 It is worth noting that Raman band broadening implies the reduction in the size of the nanocrystallites, whereas a shift in Raman frequency (Δ cm −1 ) is associated with tensile or compressive strain when a red/blue shift, respectively, occurs. The combination of the phonon confinement and strain on the Ce−La−Cu−O nanocrystallites can make the interpretation hard. In this study, Ce−La−Cu−O oxides in the 4−12 nm size range were employed, making the phonon confinement effect significant in all the cases presented herein. However, the broadening and shift of Raman peaks are predominantly due to the strain and O v defects originated from the milling treatment. Figure 2C presents a drop in the lattice parameter in the presence of mechanical strain (ball milling, DBM/WBM), and this is in agreement with the Ce−O bond shrinkage estimated under tensile strain conditions (DBM was found to induce tensile strain, Table 1). Only the cases of 4 h milling are presented as this was found to preserve to the highest degree of the fluorite structure. In addition, in Figure 2D, a general increase of FWHM Fd 2g was observed with the milling time ranging from 0 to 10 h. This broadening of Raman peak full width at halfmaximum (FWHM) coincides with the size reduction and strain development in the crystallites. At the same time, a shift in the peak position is observed with the milling duration.
Important from the catalysis perspective is the defect region (550−600 cm −1 ), which is associated with the LO mode (F 1u symmetry). Even though this symmetry is Raman inactive, a relaxation of selection rules 63,64 makes the LO mode visible. The presence of defects causes a significant lowering in symmetry and thus relaxation in selection rules. It is anticipated that ball milling tunes the oxygen defects as it will be demonstrated through the ab initio calculations (Section 3.7). Deconvolution of the Raman spectral defect region led to quantitative estimation of the O v /F 2g ratio for the pristine, DBM, and WBM Ce−La−10Cu−O oxides ( Figure S4). This ratio is considered as a good descriptor of the O v population in the oxide. In particular, the O v /F 2g intensity ratio was found to be 0.24 (0 h), 0.15 (2 h), 0.11 (4 h), 0.04 (6 h), 0.03 (8 h), and 0.38 (10 h) for different increasing DBM durations. In addition, Raman spectra deconvolution allowed us to spot the phase impurity (LaO 8 ) as shown in Figures S3 and S4. Figures S5 and 3 show the HRTEM (A), scanning TEM highangle annular dark-field (STEM-HAADF) (B), red-green-blue (RGB) analysis (C), selected area (electron) diffraction (SAED) (D), and electron energy loss spectroscopy (EELS) (E) micrographs of the Ce−La−10Cu−O oxide before ( Figure  S5) and after DBM (t = 4 h, Figure 3). The DBM oxide (t = 4 h) was chosen for this analysis due to its structural features, which make it most promising from the catalysis perspective (preservation of the fluorite ceria lattice and hence higher oxygen storage capacity). A spongy morphology with some extent of agglomeration was noticed. In the pristine ternary oxide ( Figure S5), La seems to prefer to be segregated (RGB analysis, Figure S5C). Additionally, as shown in Figure S5C, La can be found as a dopant (green inside the bulk of the shown particle, area 3), as a layer surrounding the surface of the particle (decorator, area indicated by red arrows), and as a segregate (pure green upper left, area 1). The much lower Cu content (10 at. %) secures that the role of copper is mostly as a dopant into the ceria lattice. This is reflected through the purple color in the RGB image (purple as a result of the R (red) + B (blue) combination, area 2 in Figure S5C). Dopant distribution is most likely nonuniform. The above inhomogeneity in the nanoscale is in agreement with the results obtained from powder XRD and Raman; the latter showed the formation of the LaO 8 phase impurity peak. Additionally, line-scan EDX data (two lines, (i) and (ii)) are presented in Figure S5C(i,ii) aligned with the above La and Cu role.
Following ball milling, La and Cu dopants are fairly dispersed over the ceria catalyst. As demonstrated, all materials are polycrystalline with high crystallinity, based on the SAED studies ( Figure 3D and Tables S2 and S3) and in agreement with the XRD studies. Crystallinity is also maintained after ball milling. Compared to the pristine one, after DBM, a more homogeneous mixing of Ce, La, and Cu is achieved based on the RGB analysis (compare Figures 3C and S5C). Ce M4,5 EELS edges ( Figure 3E) also show a chemical shift to a lower energy and change of the fine structure corroborating the presence of Ce 3+ , while Ce 4+ is predominant in the pristine oxide ( Figure  S5E). This is an important finding as it confirms the contribution of milling process to the M−O bond breaking and formation of oxygen vacancies (O v ), which always accompany the presence of Ce 3+ oxidation state. This result is in agreement with the EPR results presented earlier, and with the DFT estimated Ce−O bond lengths under 0, −5, and +5% strain levels (Table S1). Analysis of the SAED ring structure (Tables S2 and S3) is in good agreement with the fluorite lattice.
An insightful look and analysis of the pristine and DBM oxides is presented in Figure 4, where the dislocations ( Figure 4F areas enclosed in boxes), atomic mobility ( Figure 4D, areas enclosed in circles), and local refaceting (compare Figure 4A vs Figure  4C) are demonstrated. Such features are more intense in the case of DBM oxide ( Figure 4C,D). The surface steps shown in the image demonstrate the surface/atomic mobility. In the ballmilled sample (DBM), faceting appears as the particle size is small (<10 nm, i.e., 5−8 nm) with sharp edges (shown with yellow arrows), whereas the pristine sample is smoother ( Figure  4A,B). Additionally, based on the SAED data, in the case of pristine oxide ( Figure S5D), the complete rings showcase the presence of more crystalline material exhibiting larger in size crystals.

Textural Studies.
It is noticed that the mechanochemical treatment of Ce−La−10Cu−O oxide yields to at least 100% increase in the specific surface area (m 2 g −1 ) following both DBM and WBM conditions (Table 1 and Figure S6). The improvement of such textural property is consistent with previous studies, suggesting that the ball milling process facilitates the formation of more micropores and particle size reduction, particularly under dry conditions (harsh ones) due to the mechanical forces exerted on the grains. 65,66 The porosity trends for the reference oxides are provided in the Supporting Information ( Figure S6). Figure 5A presents the H 2 -TPR profiles of Ce−La−10Cu−O before (t = 0 h) and after the mechanochemical treatment in dry (DBM) and wet (WBM) atmospheres (t = 4 h). Ball milling after 4 h was only investigated since these conditions showed the preferred structural characteristics (based on XRD and Raman studies) among the wet ball and dry ball conditions. A profile with multiple peaks in three reduction regimes is observed, namely, T < 250°C (region α), 250°C < T < 500°C (region β), and 500°C < T < 700°C (region γ). Interestingly, the pristine sample (ε c only) presents larger concentration of easily reduced oxygen by hydrogen (increased mobility) in the regions α and β, whereas DBM (ε C + ε M ) increases the solid's reducibility at higher temperatures (region γ), as reflected by the relative amounts of H 2 consumed (mmol/g) ( Figure 5C). This demonstrates that the chemical strain activates mostly the easily reducible oxygen species, whereas the presence of mechanical strain contributes to the activation of the hardly reducible ones. The latter is important as a criterion of catalyst design for reactions taking place in the T > 500°C region (e.g., DRM). The enhancement of the redox properties following mechanochemical treatment is in agreement with the study of Yang et al. 67 who compared the asreceived commercial MnO 2 with a modified one in a top-down ball milling approach. Nonetheless, an increase in the concentration of oxygen vacancies was observed, which ultimately boosted the reactivity and mobility of surface lattice oxygen. The above results can be understood on the theoretical basis of the ball milling process, where it was reported that high temperatures (>1000°C) can be developed for periods of time in the scale of 10 −3 to 10 −4 s due to the friction between the balls and the materials. This leads to the formation of local hot spots which can drive the displacement of O lattice species leading to the formation of O v 3 .

Redox Properties.
Deconvolution of the H 2 -TPR profile for the Ce−La−10Cu− O oxide following DBM for 4 h is presented in Figure 5B. The H 2 -TPR profiles for the reference samples are given in Figure S7.  Figure 6 presents the XPS Cu 2p ( Figure 6A) and O 1s ( Figure 6B) core-level spectra of the oxides before and after WBM and DBM treatment. The Ce 3d along with the O 1s deconvoluted spectra for pristine, DBM, and WBM oxides are presented in Figures S8 and S9. According to the literature, 68 Cu ( Figure 6A) exists in the 2+ oxidation state, which is evident by the broad satellite peak of Cu 2p 3/2 arising at the high binding energy side (940−945 eV), clearly noticed after DBM and WBM modification. In the case of Ce 3d, all the intense peaks, namely, v0, v′, and v″ of Ce 3d 5/2 and u0, u′, and u″ of Ce 3d 3/2 spectra ( Figures S8 and S9), are attributed to Ce 4+ , indicating that the dominant species is Ce 4+ in all solids. As ceria exhibits high redox properties, it is useful to understand the effect of doping and mechanochemical treatment on the formation of Ce 3+ species. According to Ardelean et al., 69 the shoulder peaks appearing at ∼ 885 and 898 eV, labeled as v and u, respectively, are associated with Ce 3+ species, and their intensities relatively decrease upon doping. Interestingly, the intensities of Ce 3+ peaks increase after the mechanochemical treatment, which is consistent with the fact that the mechanical forces induce defects, such as oxygen vacancies. For the O 1s spectra ( Figure 6B,C), two components can be traced; the one at ∼529 eV corresponds to lattice oxygen, 68 whereas the one at ∼531.5 eV can be associated with surface hydroxyl species as well as carbonate or polarized O 2− ions surrounding the O vacant sites. 59 There are several reports associating this peak with neighbor to vacancies, as vacancies themselves cannot be traced using XPS. 70,71 It is observed that after mechanochemical treatment, there is a slight shift in the 529 eV peak maximum due to the change of the environment of the lattice oxygen and the introduced lattice distortion (in agreement with XRD and Raman studies). The calculated area proportion at 531.5 eV for these materials follows the order: WBM > DBM > P ( Figure  6D). Given the structural studies performed (XRD and Raman), it can be stated that in the case of WBM oxide, the predominance of oxygen species at 531.5 eV is originated from OH species rather than neighbor to vacancies species.
3.5. Oxygen Mobility Studies. As the lattice oxygen mobility is a descriptor for the surface reactivity of an oxide and the potential of oxygen vacancy formation, it is important to get an insight into the lattice oxygen (O L ) mobility (diffusivity). The oxides of this study were tested for their 16 18 O on the surface is slow, and this diffusionlimited process is clearly different among the three oxides studied (pristine, DBM, and WBM). For the pristine oxide, the  Table 2) for the pristine, DBM, and WBM oxides). Figure 7D presents the dynamic evolution of the α g (18) (t) descriptor parameter (see eq 5) with time upon exposure of the solids in the 2 mol % 18 O 2 isotopic gas mixture. The higher the value of this parameter, the slower the exchange of lattice oxygen with 18 O 2 , thus the smaller oxygen mobility (effective diffusivity) in the oxygen sublattice of the solid. It is shown that ball milling deteriorates oxygen mobility compared to the pristine Ce−La− 10Cu−O metal oxide according to the α g (18) (t) transient response curves (see the first 300 s of the transient). These results are consistent with those of Figure 7B, where larger t max is recorded for WBM compared to the other solids. 28 Furthermore, the pristine sample (chemically strained) appears to possess more labile surface and bulk lattice oxygen species ( Figure 7D) in harmony with the H 2 -TPR results (Figure 5), where the same solid showed the lowest concentration of hightemperature reducible lattice oxygen species.
3.6. CO 2 Metal Oxide−Surface Interaction. 3.6.1. Probing CO 2 -Philicity Using Thermal Desorption. Figure 8A presents the CO 2 -TPD profiles obtained over Ce−La−Cu−O pristine, DBM, and WBM, while those for the reference oxides are given in Figure S10. Generally, it is well known that CO 2 adsorption and desorption probe the strength of the basic sites (M δ+ −O δ− entities) on the surface. The peaks observed at temperatures below 200°C correspond to weak basic sites, the peaks at 200−450°C represent moderate basic sites, while peaks above 450°C are linked to strong basic sites. 74 Figure 8   , respectively. The numbers in parentheses correspond to basic sites concentration per unit surface area. As can be seen, the DBM enriches the surface basicity overall, while the WBM induces milder modification at temperatures below 250°C and a redistribution of the CO 2 -philic sites. In particular, the DBM increases the concentration of low-and high-strength basic sites, while it reduces that of the medium-strength basic sites ( Figure 8B). It is anticipated that ball milling through the imposed mechanochemical forces causes reduction of the particle size, as well as structural disordering, thus creating more adsorption sites per gram basis, as proved in what follows.
It has been reported that Lewis acidity (e − acceptor characteristic) and basicity (e − donor characteristic) have been correlated with the differences in powder processing method, as they can affect the triboelectric phenomenon, 75 powder flow, 76 and other surface and powder properties. 77 The milling process has been correlated with the increase of electrondonor characteristic of the surface and the increase of basic sites due to introduced defects, leading to higher oxidation activity. 78 3.6.2. Vibrational Spectroscopy Tools. To monitor the CO 2 activation pathway over the two surfaces of the most interest (pristine and DBM), in situ CO 2 -DRIFT spectra were recorded at 350°C on the unreduced catalysts as shown in Figures 9 and S11. The assignment of the adsorbed CO 2 IR bands was based on the open literature. Figure 9A     After spectra deconvolution, both samples show IR bands at 1590 cm −1 (peak 1) and 1554 cm −1 (peak 2), which correspond to bidentate carbonate species (Figure 9B,C). A closer look at the CO 2 adsorption IR bands for the pristine oxide (chemically strained) ( Figure 9B) gives the IR band recorded at 1554 cm −1 as the predominant one (area Peak1 /area Peak2 = 0.32). Following the DBM process, where both chemical and mechanical strain co-exist, the surface chemistry apparently changes, and the predominant IR band becomes the one at 1590 cm −1 ( Figure  9C; area Peak1 /area Peak2 = 1.96). According to the literature, the IR band recorded at 1554 cm −1 over unreduced ceria shifts to higher wavenumbers (1590 cm −1 ) over a reduced CeO 2 , and  after reoxidation, the IR band shifts back to 1554 cm −1 . 78 The areas ratio of peak 1 to peak 2 for the pristine sample was found to be 0.32, and for DBM 1.96, indicating that the DBM process (addition of mechanical strain) leads to an increase in the surface concentration of defects/CO 2 adsorption sites by ∼6 times compared to pristine (chemical strain only). The IR band at 1468 cm −1 (peak 3) and 1425 cm −1 (peak 4) can be assigned to polydentate carbonates and bicarbonate species, respectively. 79 The IR bands at 1322 cm −1 (peak 5) and 1285 cm −1 (peak 6) correspond to carbonates and bidentate carbonates, respectively. 80 These CO 2 adsorption-DRIFTS experiments were performed at 350°C, where medium-strength basic sites were present for both the pristine and milled solids. The concentration of medium-strength basic sites is higher for pristine compared to DBM ( Figure 8B), in agreement with the DRIFTS results ( Figure 9A).
The thermal stability of the adsorbed species formed after CO 2 interaction at 350°C can be studied based on the data reported in Figures 9D and S11. The IR bands for both materials decrease with time in Ar gas flow following a 30 min CO 2 gas treatment ( Figure S11). The ratio in the integral band area of peak 1 and peak 2 recorded after 30 min in CO 2 /Ar to that after 10 min in Ar gas flow remains the same for both samples, indicating that the IR band at 1590 cm −1 was not shifted to lower wavenumbers (1554 cm −1 ) and that both materials were not reoxidized. Figure 9D shows the dynamic evolution at 350°C of the ratio of areas of peak 1 or peak 2 under Ar gas flow to that at time zero (under CO 2 /He gas mixture). As illustrated in Figure  9D, more stable carbonates are formed over the pristine (chemically strained) compared to the DBM (chemically and mechanically strained) solids. This thermal stability should be related to the CO 2 −surface interaction as described using the theoretical approach. Additionally, upon the CO 2 adsorption on the surface, the O−C−O angle for 0, 5, and −5% was found to be 120.04, 119.88, and 124.75°, respectively. The value for the tensile strain (119.88°) is more realistic as discussed earlier, that upon DBM, tensile strain was developed in the solids (Table 1). However, the difference between the two angles (120.04 vs 119.88°) is insignificant and hence not conclusive. Furthermore, the C COd 2 −O surface distance (Å) (Tables S19 and S21) was found to be shorter under 5% tensile strain compared to zero and compressive strain, corroborating for a higher interaction/CT in the tensile strain conditions. This difference is more pronounced  Figure 10 and Table 3) to have a qualitative comparison and deeper insights regarding lattice strain (doping effect) and biaxial strain (external stimulus) effects on the defective systems' stability. The pure CeO 2 (111) depicted in Figure 10A was doped with La in the first layer and labeled Ce−La−O, which is shown in Figure 10B. Figure 10C illustrates the addition of copper (Cu) and lanthanum (La) to the ceria slab to resemble the oxides discussed in the experimental part (section 2.1). In this work, the same annotations as in our previous publication 59 was followed; the single vacancies on the surface layer are denoted as black, and the vacancies generated in the first subsurface layer are represented by cyan. The following labels were used for various vacancies: single surface (1,2,3,4), single subsurface (5,6,7,8), double of surface and subsurface (combination of the corresponding locations of the vacancies, e.g., 1,7), double of surface vacancies (combination of the corresponding locations of the vacancies, e.g., 1,2), and double vacancies both occurring at the subsurface (combination of the corresponding locations of the vacancies, e.g., 6,8). These distinct scenarios of surface defects were considered (see Table 3), and their effect on the slabs' stability was systematically investigated. All the obtained values are provided in Tables S4−S18. Figure 11A shows that the oxygen vacancy formation energy ( Figure 11B demonstrates the effect of chemical strain (doping) and biaxial strain (mechanical forces) on the single vacancies created in the first subsurface (SSSVs) of the three studied oxides. The profile of the SSSV is similar to that of the SSV, but the slope of the energy−strain curves for the SSSV is smaller (less steep) than for the SSV. For CeO 2 , under zero strain, the subsurface oxygen vacancies (SSSVs) are more favorable than the single surface vacancies (SSVs  Table S1 and Figure 1C,D). The double vacancies (DSV) in the surface and subsurface (DSSV) slabs suggest lower stability of CeO 2 in all ranges of tensile and compression strains applied than the single surface vacancy. Moreover, by comparing between the double vacancy configurations in CeO 2 ( Figure  11C−E), the stability appears to be the highest when both vacancies are created in the subsurface (0%: 2.17−2.37 eV) and to be the lowest when a combination of surface and subsurface vacancies are suggested (zero strain: 2.72−3.03 eV). This  It was also noticed that the behavior of the O v formation energies at 5% tension was not consistent throughout the suggested configurations in the double surface (see Table S7). For instance, 1,7, 7,4, and 2,5 cases of vacancies exhibited low stability at +5% strain (0.17 eV), while 3,8, 1,5, and 2,6 showed the opposite behavior at −5% strain.  83 This part of the study aims to enlighten the chemical strain effect on the CO 2 adsorption, which is reflected on the doping. As clearly shown in Figure 12A,B and Table 4, La doping drops the E ads from −0.97 eV (CeO 2 defective structure) to −1.01 eV; La and Cu ceria co-doping further enhances the spontaneity of the CO 2 adsorption process (E ads = −1.40 eV) (see Figure 12C). It should be noted that the adsorption energy in the presence of the oxygen vacancy in the CeO 2 was approximately as that reported in the literature 83  ). 83 Further calculations have been conducted on different sites which showed that CO 2 adsorption on vacancy sites takes place with a low adsorption energy. Therefore, the rest of the calculations were established assuming the above chemical environment (adsorption site) since it was reported to be the most stable compared to the one where the CO 2 is directly adsorbed onto an oxygen vacancy. 83 It should be mentioned at this point that the present study did not focus on optimizing all the possible CO 2 adsorption sites on the surface, and thus, only the linear configuration of CO 2 was studied.

Single Surface Vacancies.
Based on the E Od v f results presented above (Section 3.7.1), it is expected that the CO 2 adsorption is enhanced with the presence of La, Cu dopants due to the facility of the O v formation ( Figure  12A), which means activation of CO 2 will be more efficient. Figure 12 shows the activation of the linear CO 2 molecule, which is accompanied by the decrease in the O−C−O angle and the elongation of the O�C bonds (Table 4). In particular, the intramolecular C−O bonds of the adsorbed species appeared to be elongated to 1.22−1.32 Å compared to 1.13 Å in the free CO 2 molecule; this corresponds to a strong deformation and CT to be discussed later. ≠ 0). CO 2 adsorption energy (E ads ) calculations were also performed on the three surfaces (CeO 2 , Ce−La−O, and Ce−La−Cu−O) in the absence of oxygen vacancies. The CO 2 adsorption was considered under one effect at a time (e.g., doping effect vs external strain effect) in order to decouple the combined effects of each of these two factors and their role on the CO 2 adsorption. In the herein calculations, the chosen adsorption site was also on top of a surface oxygen atom (O surf ) as in the previous case. The calculations were carried out regarding this specific site, starting from adsorbing the CO 2 on the clean CeO 2 for which a good agreement with the literature (−0.36 eV) was found (formation of carbonate species). Figure 12D shows that the adsorption of the CO 2 molecule on CeO 2 (111) and Ce− La−O (111) surfaces follows the same behavior, whereas as the biaxial tensile strain (external stimulus) increases, the E ads value drops, indicating stronger adsorption. Introduction of the dopant in the ceria lattice facilitated the adsorption significantly.   A fluctuating behavior of the adsorption energy was observed, where the E ads value varied between a decrease (more negative value) as the compressive strain decreased (−3 to −1%), to reach its strongest binding at 0% strain, and then an increase (less negative value) as the tensile strain increased. It is also important to point out that the E ads values at zero strain were more negative with O v present on the surface compared to the cases where O v were absent (as can be observed in Figure 13). This is due to the enhanced CT from the surface to the CO 2 molecule. 84 In other words, the high basicity of the surface (CO 2 -TPD earlier studies) is indicative of the CT from the surface to the CO 2 molecule in order for the latter to be activated; the CT occurs due to the electrons residing from the O 2p shell upon creating the O v . 85 The presence of dopants  weakened the M−O bond (see Table S1) and facilitated the generation of oxygen vacancies that strengthened the E ads of CO 2 . 86

Charge Transfer.
To gain understanding on the adsorbate/surface charge distribution upon CO 2 adsorption on the best performing surface of doped CeO 2 (111) with lanthanum and copper, Bader charge analysis along with charge density difference has been performed. Results of the Ce−La− Cu−O surface considering first the effect of oxygen vacancies at zero strain followed by the Ce−La−Cu−O surface at different levels of strain in the absence of oxygen vacancies are shown in Figure 14.
Ef fect of oxygen vacancy: The effect of oxygen vacancy (O v ) on the CO 2 -derived carbon atom (C δ+ ) and the CO 2 -derived oxygen atoms (O δ− ) and charge changes on the surface are discussed next. Upon CO 2 adsorption on the Ce−La−Cu−O surface and in the presence of O v , the charge associated with the CO 2 -derived carbon atom was found to be +2.10 |e − | ( Figure  14A), whereas in the absence of O v was calculated to be +2.20 |e| ( Figure 14D), both being compared to the 0% strain condition. Thus, the presence of O v enhances the electron accumulation on the carbon atom (C δ+ ) as its value is found to be less positive. In Figure 14B,C, the charge accumulation region on the C atom is designated with the blue region.
In the case of CO 2 -derived oxygen atoms, one of the oxygen atoms (O 1 ) shows similar values (−1.04 and −1.03 |e|) in both cases, whereas the second oxygen atom (O 2 ) had distinct charges of −1.07 and −0.82 |e| in the case of the Ce−La−Cu−O surface with and without oxygen vacancy, respectively. Therefore, the O 2 atom in the adsorbed CO 2 had higher charge accumulation in the presence of O v ; hence, in the presence of O v , a stronger electric field is formed due to the excess electrons surrounding the oxygen atoms belonging to CO 2 . Additionally, the charge density difference plot reveals the region of electron accumulation around the O 2 in the CO 2 molecule ( Figure  14B,C), anticipating the participation of this oxygen atom in charge-exchange events with the surface (surface acting as the electron donor).
Ef fect of applied strain: Comparing the tensile (+5%) and compressive (−5%) strain to the zero strain condition in the case of Ce−La−Cu−O, it can be seen that +5% strain has a negligible effect on the CO 2 electron transfer toward/from the surface based on similar values of charges on the adsorbed carbon and oxygen atoms (2.22 for carbon, −1.04 and −0.84 |e| for both oxygens) compared to the 0% strain case (2.22 for carbon and −1.03 and −0.87 |e| for both oxygens). In the above reported values, the O atoms with the low charge value are the ones pulled upward along the z direction. However, upon applying compressive strain (−5%), both oxygens are pulled toward the surface having more negative charge than in the 0 and +5% strain cases.
The CO 2 -derived C atom (C δ+ ) is also more enriched with electrons having a charge of 2.12 |e| ( Figure 14F) compared to that of 2.22 |e| (Figure 14D,E). Thus, these charges under −5% applied strain suggest the formation of covalent bonds within the CO 2 /La−Ce−Cu−O system, thus indicating strong CO 2 adsorption.

Electronic DOS: Influence of Chemical Strain (ε C ) and Mechanical Strain (ε M ) on the Electronic Structure of Doped
CeO 2 (111) Surfaces. In this section, the role of O v as well as that of tensile and compressive strain on the DOS of the pristine and doped (La)/co-doped (La, Cu) ceria is discussed. The relative position of the CB and VB is important when adsorbent− adsorbate interactions have a covalent characteristic. In such a case, strain can alter the energetic positions of the bands and thereby the adsorption properties. Furthermore, covalent interaction as that dictated by the VB−CB splitting means higher dependence of basicity from the strain. 87  configurations is expected to originate from the O 2p and Ce 5d orbitals allocated below and above the Fermi level, respectively. The Fermi level is represented by a dashed vertical line in all DOS plots. It is important to note that in most cases, the value of the band gap (E g ) calculated using DFT is underestimated, mainly due to the exchange−correlation derivative discontinuity. 88 In the case of clean CeO 2 (111) (absence of oxygen vacancies), no gap states were observed in the wide band gap region between the VB and CB (O 2p and Ce 5d). 89,90 Such a DOS profile is characteristic of an insulator with empty forbitals, the case of no surface defect formation.
However, in the reduced CeO 2 case, it is noted that the presence of both single O v ( Figure S13B) and double O v ( Figure  S13C) results in an observable reduction of the O 2p −Ce 5d band gap. This may be due to the fact that oxygen vacancies increase the number of readily available excess electrons in both surface and subsurface layers. This in turns induces charge compensation by Ce atoms to minimize the bonds between neighboring oxygen atoms that ultimately result in the smaller O 2p −Ce 5d gap. Prominently, in the case of reduced ceria surfaces with single and double oxygen vacancies, the −5% applied strain reveals the greatest reduction in band gap energy. Besides, a high peak of spin-up defected state at around 0.4 eV is witnessed with −5% surface −C COd 2 entities. 13 CO 2 can react with a metal oxide acid/base surface toward the formation of carbonate or bicarbonate species through the participation of O surface or OH surface , respectively, as well as linear adsorbed species, the latter being perpendicular or parallel to the surface. It has been reported that the surface with acidic characteristic promotes the linear-type adsorbed CO 2 species, whereas basic characteristic promotes the bent configurations (e.g., CO 2 − or HCO 3 − species). During the CO 2 − species (bent configuration) formation, electron transfer to the π antibonding orbitals of CO 2 takes place. 13 3  Figures 12 and 13. Adsorption of CO 2 onto the surface leads to the formation of a CO 2 complex, identified as carbonates; in the latter complex entity, two of the C−O bonds are elongated and the third one corresponds to the C (coming from CO 2 ) and the O from the surface (O surf ) (see Tables 4, 5, and S19−S21).
The  Having a closer look on the effect of strain on the CO 2 −surface interaction, it can be seen that tensile strain leads to the same charge distribution for the C COd 2 and the O COd 2 atoms. However, it seems that compressive strain enhances the basic characteristic of the surface, as C COd 2 is more electron enriched, allowing us to speculate for more participation of the surface basic sites in the interaction with the CO 2 and thus for a possible surface acidity− basicity tuning upon strain imposed. The above are also supported from the data listed in Table 5, where both under compressive (−5%) and tensile (5%) strain level, the O−C−O angle and the C�O bond are distorted. The lower value of the O−C−O angle under tensile (119.88°) compared to the same angle value under compressive (124.75°) allows us to discuss the strength of the interaction (strength of acid/basic sites). This is supported by the DOS (Figure 15B) of Ce−La−Cu−O(111) reduced surface, where in both tensile and compressive fields, the formation of new gap states can be noticed, leading to an apparent narrowing of the band gap (VB−CB splitting). The latter has been reported to have an instrumental role in the strain tuning of surface basicity, when covalent bonding is involved. 87 The new gap states have different shapes and width implying different electron distributions on the surface upon different types of strain imposed on the surface. More focused studies using near-ambient pressure XPS and extended X-ray absorption fine structure can enlighten more the role of strain on the acid/ base characteristic of the surface. 94,95 Based on the present experimental findings of in situ DRIFTS studies, the bidentate carbonates were found as predominant species on the pristine (chemically strained) and DBM (chemically and mechanically strained) surfaces, justifying the dual-site interaction of CO 2 with the surface. 80 Addition of mechanical strain led to an increase in the population of bidentate carbonate species. It has been reported that the free carbonate ion (D 3h symmetry) has an IR band at 1415 cm −1 . Upon adsorption, lowering of the symmetry gives rise to two ν(CO) bands on both sides of the band at 1415 cm −1 . The separation between the two bands is known as Δν 3 splitting, 96 which is used as a descriptor of the surface basic strength. Usually, a smaller splitting accounts for stronger basic sites; for the different carbonate species, different values are reported; unidentate (Δν 3 = 100 cm −1 ), bidentate (Δν 3 = 300 cm −1 ), and bridged species (Δν 3 = 400 cm −1 ). The Δν 3 splitting in Figure 9 is 268 cm −1 , characteristic of bidentate species formed in moderate-strength basic sites. Bidentate carbonates are favored over the medium-strength basic sites (see Figure 8); the latter are present in the Ce−La−Cu−O surface and tuned upon mechanochemical processing (ball milling) (see Figure 8B). The pristine and the DBM solids is another example of isostructural oxides with a different behavior upon strain. The latter is demonstrated not only by the different population of the carbonates formed but also from their different thermal stabilities. The different thermal stability reflects the strength of interaction and is justified by the different O surf −C−O 1 and O surf −C−O 2 angles (deformation) in zero and 5% strain levels.
3.9. Case Study: DRM Reaction. In order to validate the effect of mechanochemistry concepts in catalysis, the DRM reaction was selected as a probe one. The supported Ni catalysts were prepared based on the procedure mentioned earlier (Section 2.3), where the pristine, dry ball-milled, and wet ballmilled Ce−La−Cu−O oxides were used as supports. Figure 16A and Table 6 present the experimental CH 4 and CO 2 integral rate values after 0.5 and 12 h of DRM reaction, as well as the amount of carbon accumulated on the catalyst surface after 12 h of DRM. In the case of the pristine-supported Ni catalyst, the conversions were lower than the equilibrium values when DRM is only considered but also for the DRM/reverse water gas shift (RWGS) reaction network (see Table 6). In our previous work, 27 it was found (use of transient isotopic experiments) that the decrease of carbon deposition in the pristine-supported Ni catalyst was due to carbon oxidation (or gasification) to CO by labile lattice oxygen, where CO 2 reoxidizes the reduced ceria-based support. In the case of all the catalysts listed in Table 6, for short time-on-stream (TOS) (ca. 0.5 h), the H 2 /CO ratio adopts a value slightly higher than unity, signifying the simultaneous presence of some side reactions, such as the RWGS, CH 4 decomposition, carbon oxidation to CO and CO 2 , and steam methane reforming. The RWGS explains the drop in the H 2 /CO gas ratio, the increase of X COd 2 , and the drop in the H 2 -yield after 12 h TOS, as shown in Table 6. Overall, DBM and WBM have increased the CH 4 and CO 2 conversions for TOS of 0.5 and 12 h to a different extent. This is linked to the modification of the Ni−support interface upon the milling process, as demonstrated through H 2 -TPD experiments performed over the three supported Ni catalysts (pristine, dry ball milled, wet ball milled; Figure S16). In the H 2 -TPD profiles of the milled catalysts (both DBM and WBM), low-temperature desorption peaks appeared, which is not the case for the pristine-supported Ni catalyst ( Figure S16 and Table  S22), linked with the presence of smaller size Ni crystallites grown on the milled supports. Ball milling was also found to alter the redox properties of the catalysts along with the distribution of labile oxygen species present on the surface and in the subsurface region; the latter can be inferred by the shape of the peaks ( Figure S17). Additionally, DBM seems to have an opposite effect on the catalyst reducibility compared to the DBM (Table S23).
The carbon accumulated on the catalyst surfaces was measured by temperature-programmed oxidation (TPO) (see Figure 16B). It is also noteworthy to mention the two prominent peaks that appear in the TPO-CO 2 trace of Ni/Ce−La−10Cu− O catalyst, ca. ∼550 and 600°C ( Figure 16B), indicating the formation of two types of carbon. This is in agreement with our previous findings on Ni/Ce−La−Cu−O-supported catalysts (pristine), 27 where the La-doped catalyst resulted in an asymmetric (i.e., two peaks) TPO-CO 2 trace at ∼550°C, indicating the formation of two different types of carbon. These peaks became more prominent and well defined after WBM (curve b).
Based on the above results, it can be suggested that ball milling could benefit several catalytic reactions involving ceria, such as chemical looping water splitting (CLWS) coupled with the decomposition of glycerol, 97 aqueous-phase reforming (APR) of glycerol, 98 and chemical looping steam reforming of glycerol, 99 where ball milling could induce the tuning of CO 2 (as product this time) interaction with the CeO 2 -related surface toward the preferred direction. In the study reported by Dou et al., 97 CeO 2 acts as a promoter, attributing to stronger metal−support interactions and enhancing the thermal stability of the Ni− CeO 2 /MCM-41 or SBA-15 catalysts. In the work of Wu et al., 98 the Ni−Cu bimetallic supported on mesoporous CeO 2 was used for APR of glycerol (biodiesel byproduct). The kinetic analysis conducted proved that after adding CaO onto the 1Ni2Cu/ CeO 2 catalyst, the apparent activation energy dropped, ca. 29.86 kJ mol −1 . Here, CaO is integrated as an absorbent to facilitate the WGS reaction and reduce methanation reactions via in situ CO 2 removal and capture. Last but not least, Lou et al. reported on Fe−Ce−Ni−O-based oxygen carriers (OCs) for CLWS coupled with glycerol decomposition in an attempt to simultaneously produce hydrogen and syngas. 99 A remarkable redox behavior was achieved by the prepared Fe−Ce−Ni−Obased OCs, leading to a significant catalytic function of partial oxidation and decomposition of glycerol at 750°C. The best oxygen-transfer capability and highest hydrogen and syngas production were attained by OC with a 100:10:3 molar ratio of Fe/Ce/Ni.

CONCLUSIONS
In this work, we presented the first example of how the chemical (ε C ) and mechanical (ε M ) strain can be used to tailor the CO 2 activation and adsorption onto a metal oxide surface by a combined experimental modeling approach. To demonstrate this, the Ce−La−Cu−O ternary oxide surface was prepared using a microwave coupled with sol−gel and then subjected into mechanochemical treatment (ball milling). Chemical strain is originated from the CeO 2 lattice doping (La 3+ or La 3+ /Cu 2+ ), whereas mechanical strain is originated from post-synthetic ball milling treatment. This study reveals the impact of the mechanical strain (ball milling) on the mobility of lattice oxygen (O L ) through state-of-the-art 16   The amount of carbon accumulated (mg C g −1 cat ) after 12 h of DRM is also presented. b Number in parentheses gives the equilibrium conversions of CH 4 and CO 2 and the H 2 /CO product gas ratio for the DRM reaction alone; feed gas composition (20% CO 2 /20% CH 4 /He); 750°C. c Number in parentheses gives the equilibrium conversions of CH 4 and CO 2 and the H 2 /CO product gas ratio when both the DRM and RWGS reactions participate in the reaction network; feed gas composition (20% CO 2 /20% CH 4 /He); T = 750°C. CO 2 adsorption. The interplay between the O v entities and the CO 2 activation and the role of mechanical strain were illustrated through in situ DRIFTS studies and Bader charge analysis. In particular, the DBM process (mechanical strain) was found to enhance by 6 times the population of the bidentate carbonates formed, though lowering their thermal stability. Geometric characteristics of the adsorbed CO 2  is proposed, through which the surface utilizes both its acidic and basic sites; the relative contribution of the sites in the interaction with CO 2 can be tuned by the mechanical strain applied. Having the possibility to tune the participation of the acidic or basic sites in the interaction with CO 2 , this leads to carbonates with different stabilities; the latter is of high importance in catalytic reactions, such as DRM and CO 2 hydrogenation, where the CO 2 activation and carbonate formation as active intermediates are important mechanistic steps. A proof of concept is herein demonstrated using the DRM reaction, where Ni-based catalysts supported on a ball-milled carrier exhibited different (smaller) Ni crystallite sizes and enhanced labile oxygen species features that led to higher CO 2 and CH 4 conversions and significantly lower carbon deposition.
Characterization details such as XRD and Raman of the reference materials, HRTEM over the fresh Ce−La− 10Cu−O oxide, textural studies, H 2 -TPR of the fresh and reference oxides, CO 2 -TPD of the reference oxides, XPS over the Ce−La−10Cu−O oxide before and after the ball milling, and IR spectra of Ce−La−10Cu−O�pristine oxide; some of the computational (DFT) data; DOS, analytical data of the energies of formation of oxygen vacancies at different positions, and different arrangements and different levels of strain; geometric characteristics of the adsorbed CO 2 species over the reference oxides; and dispersion of the Ni-supported catalysts as well as their H 2