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 (ε_C) and mechanical strain (ε_M) 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 (O_v) 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 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 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 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 CH₄ and CO₂ conversions. This study opens new possibilities to manipulate the CO₂ activation for a portfolio of heterogeneous reactions.

Keywords: CO2 activation; DFT; DRIFTS; DRM; ball milling; ceria; mechanochemistry; oxygen vacancies; strain engineering; surface tuning; ternary oxides.

Figure 1

(A) XRD patterns of Ce–La–10Cu–O ternary oxides following DBM treatment for 0, 2, 4, 6, and 10 h; (*) denotes the CuO(111) phase impurity; (B) zoom in of the XRD patterns of (A) in the 26–30° 2θ region corresponding to the (111) diffraction plane; (C) Ce–O bond length (Å) in CeO₂, Ce–La–O, and Ce–La–10Cu–O oxides calculated through the ab initio studies; (D) Cu–O bond length (Å) in Ce–Cu–O and Ce–La–10Cu–O oxides calculated through the ab initio studies; (E) XRD patterns of the Ce–La–10Cu–O ternary oxides following WBM treatment for 0, 4, and 10 h; (*) denotes the CuO(111) phase impurity, and (#) denotes the La₂O₃ phase impurity; and (F) EPR spectra of the Ce–La–10Cu–O ternary oxide following DBM and WBM treatment for 4 h.

Figure 2

(A) Raman spectra obtained over the Ce–La–10Cu–O ternary oxides following DBM treatment for t = 0, 2, 4, 6, and 10 h; (B) Raman spectra of the Ce–La–10Cu–O ternary oxides following WBM treatment for t = 0, 4, and 10 h (*, ^ denote the CuO- and CeO₂-based phases); (C) effect of ball milling conditions/atmosphere on the F_2g position (Raman band), O_v/F_2g ratio (Raman bands ratio), and lattice parameter, Å (based on the XRD); and (D) effect of dry milling time on the F_2g position, O_v/F_2g ratio, and lattice parameter (Å).

Figure 3

(A) HRTEM, (B) HAADF-STEM, (C) RGB mapping, (D) SAED, and (E) EELS obtained over the DBM, 4 h Ce–La–10Cu–O metal oxide.

Figure 4

(A,B) HRTEM of the pristine CeLaCuO; (C,D) HRTEM of the DBM CeLaCuO; (E) fast Fourier transformed (FFT) analysis of (B); and (F) inverse FFT analysis of (B) across the planes (111).

Figure 5

(A) H₂-TPR profiles of pristine-, DBM-, and WBM-treated Ce–La–10Cu–O oxides; (B) deconvoluted TPR profile of DBM-treated Ce–La–10Cu–O oxide; and (C) redox site distribution over the pristine, DBM, and WBM oxides in the low-, medium-, and high-temperature regime.

Figure 6

(A) Cu 2p core-level spectra for pristine and ball-milled Ce–La–10Cu–O oxides; (B) O 1s core-level spectra for the pristine and ball-milled Ce–La–10Cu–O oxides; (C) deconvoluted O 1s spectrum of the Ce–La–10Cu–O following DBM; and (D) calculated O 2p areas as per the deconvoluted spectra in (C).

Figure 7

(A) Transient rates (μmol g^–1 s^–1) of ¹⁶O₂ formation as a function of time estimated from the TIIE experiment: 2 mol % ¹⁶O₂/2 mol % Kr/Ar/He (350 °C, 30 min) → 2 mol % ¹⁸O₂/Ar/He (350 °C, t) for the Ce–La–10Cu–O oxides. W = 0.02 g; F = 50 N mL/min; (B) transient rates (μmol g^–1 s^–1) of ¹⁶O¹⁸O formation as a function of time estimated from the TIIE experiment: 2 mol % ¹⁶O₂/2 mol % Kr/Ar/He (350 °C, 30 min) → 2 mol % ¹⁸O₂/Ar/He (350 °C, 15 min) for Ce–La–10Cu–O solids. W = 0.02 g; F = 50 N mL/min; (C) evolution of the total amount of ¹⁶O exchange (N(¹⁶O), mmol g^–1) as a function of time estimated from the TIIE experiment: 2 mol % ¹⁶O₂/2 mol % Kr/Ar/He (350 °C, 30 min) → 2 mol % ¹⁸O₂/Ar/He (350 °C, 15 min) for the Ce–La–10Cu–O oxides. W = 0.02 g; F = 50 N mL/min; (D) α_g⁽¹⁸⁾ descriptor parameter as a function of time (t) estimated from the TIIE experiment: 2 mol % ¹⁶O₂/2 mol % Kr/Ar/He (350 °C, 30 min) → 2 mol % ¹⁸O₂/Ar/He (350 °C, 15 min) for the Ce–La–Cu–O oxides. W = 0.02 g; F = 50 N mL/min.

Figure 8

(A) CO₂-TPD profiles over pristine, DBM, and WBM Ce–La–10Cu–O oxides and (B) distribution of basic sites (%) in the low, medium, and high strength basicity regions over the pristine (P), DBM, and WBM metal oxides.

Figure 9

(A) DRIFTS spectra recorded under 5 vol%CO₂/He (30 min) gas mixture in the 1750–1150 cm^–1 range over Ce–La–10Cu–O—DBM and Ce–La–10Cu–O—pristine solids. Deconvoluted IR spectra of Ce–La–10Cu–O—pristine (B) and Ce–La–10Cu–O—DBM (C) solids. (D) Ratio of area(t)/area(t = 0) related to peak 1 and peak 2 for both the pristine and DBM materials.

Figure 10

Schematic representation of oxygen vacancy (O_v) created in the uppermost surface (black) and subsurface (cyan): (A) pure CeO₂(111), (B) La-doped CeO₂ (Ce–La–O(111)), (C) La and Cu co-doped CeO₂ (Ce–La–Cu–O(111)), (D,E) first neighbor configurations of the possible oxygen vacancies in Ce–La–Cu–O(111), and (F) demonstration of the compressive and tensile biaxial strain.

Figure 11

(2 × 2) CeO₂(111) (red), Ce–La–O (blue), and Ce–La–Cu–O (green) oxygen vacancy energy of formation (E_Ov^f) under the compressive (−5 to 0%) and tensile (0–5%) biaxial strain for (A) single surface vacancy (SSV), (B) single subsurface (SSSV), (C) double surface (DSV), (D) double subsurface (DSSV), and (E) double surface and subsurface configurations (DSSSV).

Figure 12

(A) CO₂ adsorption on defective CeO₂(111); (B,C) CO₂ adsorption on the doped-CeO₂(111) surfaces at site 2 at zero strain (all the surfaces are considered with surface O_v); and (D) CO₂ adsorption on CeO₂(111) and doped-CeO₂(111) surfaces under the strain effect in the absence of oxygen vacancies (O_v).

Figure 13

CO₂ adsorption on CeO₂(111) and doped CeO₂(111) surfaces at the 0, 5, and −5% strain level in the absence of oxygen vacancies (O_v).

Figure 14

(A) Bader charge analysis, charge density difference (B) top view, and (C) side view of CO₂ adsorbed on Ce–La–Cu–O with oxygen vacancy at site 2 at zero strain. Bader charge analysis with the strain effect on CO₂ adsorbed on Ce–La–Cu–O at (D) 0, (E) +5, and (F) −5% strain in the absence of the oxygen vacancy (O_v). Yellow and blue regions in charge density difference plots denote charge accumulation and depletion, respectively. An isosurface value of 0.0025 eÅ^–3 is used.

Figure 15

DOS of Ce–La–Cu–O(111) under −5, 0, and +5% strain level with different configurations of (A) clean surface, (B) reduced surface, single oxygen vacancies, and (C) reduced surface, double oxygen vacancies. The dashed vertical line represents the Fermi level.

Figure 16

(A) Integral rate of CH₄ conversion (mmol g^–1_Ni s^–1) and H₂/CO gas product ratio obtained after 0.5 and 12 h of DRM (20 vol % CH₄/ 20% CO₂/He) at 750 °C over 5 wt % Ni/Ce–La–10Cu–O catalysts as a function of support pre-treatment (pristine, WBM, and DBM). (B) Transient response curves of CO₂ formation rate (μmol g^–1 s^–1) obtained during TPO of carbon formed after 12 h of DRM (20 vol % CH₄/20 vol % CO₂/He) at 750 °C over 5 wt % Ni/Ce–La–10Cu–O catalysts: (a) pristine, (b) WBM, and (c) DBM.