# Oxygen vacancy engineering by La-incorporation into Co3O4 matrix for boosting catalytic oxidation of vinyl chloride: experimental and DFT evidences

H. Zhang, H. Cao, G. Li, V. Meille, S. Gil, C. Zhang

► To cite this version:

H. Zhang, H. Cao, G. Li, V. Meille, S. Gil, et al. Oxygen vacancy engineering by La-incorporation into Co3O4 matrix for boosting catalytic oxidation of vinyl chloride: experimental and DFT evidences. Journal of Catalysis, 2026, 453, 10.1016/j.jcat.2025.116524. hal-05379104

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Submitted on 18 Feb 2026

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Research article

# Oxygen vacancy engineering by La-incorporation into Co₃O₄ matrix for boosting catalytic oxidation of vinyl chloride: experimental and DFT evidences

Hongliang Zhang<sup>b</sup>, Haijie Cao<sup>c</sup>, Guanghao Li<sup>b</sup>, Valérie Meille<sup>b</sup>, Sonia Gil<sup>b,*</sup>, Chuanhui Zhang<sup>a,c,**</sup> <sup>©</sup>

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

Keywords:

Catalytic oxidation

La incorporation

ZIF-67 template

Oxygen vacancy engineering

Electronic structure modulation

Catalytic activity enhancement

## ABSTRACT

The development of high-performance transition metal oxides catalysts is one of the promising topics in the application field of chlorinated volatile organic pollutants abatement. Herein, La-incorporated Co₃O₄ catalysts with variable lanthanum contents were prepared by using ZIF-67 as the sacrificial template, giving rise to successful oxygen vacancies engineering and the construction of a hollow-cage structure. Experimental results revealed that La-incorporation into Co₃O₄ structure remarkably improved the catalytic activity for vinyl chloride (VC) oxidation with a close dependence on the lanthanum content. Among all catalysts, 5La-Co (the molar ratio of La in the total metal was 5 %) exhibited an optimal catalytic activity achieving 90 % of VC conversion at 276 °C in comparison with the pristine Co₃O₄ (0La-Co in this work) at 290 °C. Moreover, this catalyst presented an adequately high catalytic stability under complex humid conditions. Characterization results and DFT calculations revealed the modulation of the intrinsic electronic structure and spinel lattice parameters on the Co₃O₄ matrix by La incorporation, leading to the valence states redistribution of Co³⁺/Co²⁺, more oxygen vacancy generation originating from the lower formation energy (1.84 eV), and more abundant surface oxygen species originating from the lower gas-phase oxygen adsorption energy (−1.59 eV). All these properties enabled the observable catalytic activity enhancement of La-incorporated Co₃O₄ catalysts. In addition, the formation of enol groups and carboxylic acid as key organic intermediates during the VC oxidation process was highlighted by the *in-situ* DRIFT spectroscopy. The more plentiful surface oxygen species available on La-incorporated Co₃O₄ catalyst surface were favorable to the formation and further oxidation of these organic intermediates.

## 1. Introduction

Volatile organic compounds (VOCs) are considered as one typical category of atmospheric pollutants, and the industrial sources of VOCs emissions are usually characterized by high throughput, low concentration and complex multicomponent, which have been demonstrated to be the main contributors to air pollution [1,2]. The majority of VOCs are biologically toxic and can cause severe hazards to human health, especially triggering the potential risks for carcinogenicity, teratogenicity and mutagenicity. Furthermore, VOCs can photo-chemically react with other substances in the atmosphere to generate secondary pollutants,

which would induce more serious environmental issues [3,4]. For instance, the atmospheric chemical reactions of VOCs and nitrogen oxides (NOₓ) result in the formation of photochemical smog, which is even more harmful than the individual components. In addition, it has been reported that VOCs play a pivotal role in the formation of ozone and organic aerosols. As one category of VOCs that requires special attention, chlorinated volatile organic compounds (CVOCs) have the unique C-Cl structural units that enhance their polarity and water solubility [5,6]. Consequently, CVOCs molecules are more readily soluble in an ambient atmosphere and aquatic environmental conditions than the majority of other VOCs, and these characteristics make them easier to

<footnote><sup>a</sup> Faculty of Chemical Engineering and Energy Technology, Shanghai Institute of Technology, Shanghai 201418, China</footnote>

<footnote><sup>b</sup> Universite Claude Bernard Lyon 1, CNRS, IRCELYON, UMR 5256, Villeurbanne F-6910, France</footnote>

<footnote><sup>c</sup> Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao 266071, China</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author at: Faculty of Chemical Engineering and Energy Technology, Shanghai Institute of Technology, Shanghai 201418, China.</footnote>

<footnote>E-mail addresses: sonia.gil@ircelyon.univ-lyon1.fr (S. Gil), zhangch@qdu.edu.cn (C. Zhang).</footnote>
continuously accumulate in the environment [7,8]. Moreover, CVOCs are more biologically toxic than other VOCs, thus leading to more hazardous effect on living organisms.

In order to efficiently remove VOCs emissions, several physical, chemical and biological removal technologies have been developed during the past decades, such as adsorption, membrane separation, photocatalytic oxidation and biofiltering [1,9,10]. Amongst, thermal catalytic oxidation is capable to completely convert VOCs into harmless carbon dioxide (CO₂), water (H₂O) and other inorganics at low temperature, which shows widespread application prospects beneficial from its several advantages including high purification efficiency, low energy consumption and green environmental features. As the critical element in catalytic oxidation technology, the development of novel catalytic materials with high performances but inexpensive costs has always been a hot topic in the research community of VOCs abatement [11,12]. Currently, supported noble metals and transition metal oxides are commonly employed as VOCs oxidation catalysts [13]. However, considering the precious price and easy poisoning of noble metals, exploring suitable transition metal oxides and upgrading their catalytic activities remain a challenging object in current and future research activities [14–16].

As a typical transition metal oxide, spinel Co₃O₄ exhibited high catalytic oxidation activity and stability towards a wide range of VOCs, making it a promising and efficient catalyst [3,4]. Co₃O₄ possesses the spinel structure (AB₂O₄) that are occupied by Co²⁺ and Co³⁺ at the A and B sites, respectively. These cations are commonly located in tetrahedral and octahedral sites, which are tightly surrounded by lattice oxygen [17–19]. The significant differences in the outer electronic structures and oxygen coordination numbers of Co²⁺ and Co³⁺ in Co₃O₄ would result in two redox centers, exhibiting variable catalytic activities. Currently, it is widely accepted that Co³⁺ are the main adsorption sites for olefin-type VOCs since the C=C bond in olefins has electron donor properties [20]. Furthermore, the interconversion between Co²⁺ and Co³⁺ during catalytic reactions enhances the low-temperature redox performance, promoting the oxygen cycle in catalytic oxidation process [21,22]. It has been previously proved that the incorporation of heteroatom, such as alkali metals, alkaline-earth metals, rare-earth metals, non-metals and other transition metals, on Co₃O₄ is an effective approach to modify its physicochemical and catalytic properties [23–26]. In recent years, there are numerous reports on CVOCs oxidation by using Co₃O₄-based catalysts. González-Prior et al. synthesized nanocube-shaped Co₃O₄ and employed it for the oxidation of 1,2-dichloroethane. Characterization results revealed that the nanocube-shaped Co₃O₄ surface with abundant defects and oxygen vacancies can stably decompose 1,2-dichloroethane into inorganic small molecules at 400 °C [27]. While Deng et al. synthesized cobalt oxide-based catalysts via layered double hydroxides (LDH) for the oxidation of chlorobenzene. High surface area CoCr mixed oxides derived from LDHs exhibited significant activity toward chlorobenzene oxidation. Meanwhile, the addition of Cr effectively increased acidic and basic sites on the catalyst surface, thereby efficiently suppressing the formation of polychlorinated byproducts such as dichlorobenzene [28]. In this work, lanthanum (La) was selected as the dopant element. In contrast to cerium (Ce), which is often used as a catalytic support and dopant, La cations in metal oxides have a single stable chemical valence state (+3), which prevents them from participating in the redox process of the catalyst through valence state changes [6]. Therefore, La incorporation provides an opportunity to investigate the effect of heteroatom doping by non-variable-valence rare earth metals on the structure and properties of Co₃O₄ catalyst.

Nevertheless, in order to prevent the generation of LaCoO₃ perovskite and to ensure the uniform distribution of La in spinel Co₃O₄, ZIF-67 was employed as a sacrificial template for the preparation of Co₃O₄ catalysts in this work [6,14,29]. ZIF-67 is a typical metal-organic frameworks (MOFs) material with a unique dodecahedral hollow cage structure. After calcination under optimal temperature and atmospheric conditions, Co₃O₄ catalysts with high specific surface area and abundant

porous structure can be obtained, while retaining the unique structure of the ZIF-67 template. Furthermore, heteroatoms can be effectively dispersed on the template by adjusting the corresponding metal content in the synthesis process of ZIF-67 template.

In this work, La-incorporated Co₃O₄ catalysts with unique cage-like structures were prepared by using ZIF-67 template. Their physicochemical and catalytic properties for catalytic VC oxidation were extensively investigated by several characterization techniques, light-off and in-situ measurements and DFT calculations in order to highlight the role of oxygen vacancies in the reaction. Thus, this study provides more fundamental insights into the catalytic performances and modification mechanism of microstructure and electronic environment over Co₃O₄ based catalysts.

## 2. Experimental

### 2.1. Catalysts preparation

The La-incorporated Co₃O₄ catalysts were synthesized by a sacrificial template method in two steps [30,31]. Firstly, ZIF-67 templates with different amounts of La deposition were synthesized by a one-pot procedure. A total of 8 mmol of metal precursors containing Co(NO₃)₂·6H₂O and La(NO₃)₃·6H₂O was dissolved in 100 mL of methanol and the relative proportion of La(NO₃)₃·6H₂O in the total metal nitrates was adjusted at 0 mol.%, 2.5 mol.%, 5 mol.% and 7.5 mol.%. Meanwhile, 32 mmol of 2-methylimidazole was dissolved in 400 mL of methanol. The former metal nitrates-methanol solution was slowly poured into the latter 2-methylimidazole-methanol solution. After vigorous stirring for 5 min and further aging at room temperature for 24 h, the resultant purple precipitate was collected after centrifugation, methanol washing and drying at 60 °C for 10 h. According to the molar percentage of La(NO₃)₃·6H₂O in the total metal nitrate precursors, the obtained solid material was named as xLa-ZIF (x = 0, 2.5, 5, 7.5, with x represented the molar ratio of La).

Subsequently, La-Co composite oxides were prepared by the direct calcination of xLa-ZIF materials in air [31]. Specifically, the xLa-ZIF material was pyrolyzed in a muffle furnace at 350 °C for 4 h at a ramp of 2 °C min⁻¹. On the basis of the xLa-ZIF for pyrolysis, the obtained catalyst was successively named as xLa-Co (x = 0, 2.5, 5, 7.5).

For comparison, the single lanthanum oxide was prepared by a typical precipitation method [32]. 0.1 mol L⁻¹ NaOH aqueous solution was dropwise added into 100 mL of 0.1 mol·L⁻¹ La(NO₃)₃ aqueous solution under continuous stirring until the mixture pH reached 9. After aging for 24 h at room temperature, the precipitate was collected by centrifugation, washing with deionized water and drying at 60 °C for 10 h. The powder was also calcined in a muffle furnace at 350 °C for 4 h at a ramp of 2 °C min⁻¹, thus yielding the lanthanum oxide namely as La₂O₃.

### 2.2. Catalysts characterization

Various characterization techniques were conducted to investigate the physicochemical properties of all prepared catalysts, including X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), Raman spectroscopy, nitrogen physical sorption, temperature-programmed hydrogen reduction (H₂-TPR), temperature-programmed oxygen desorption (O₂-TPD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The adsorption and conversion behaviour of VC on the catalyst surface was analyzed by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs). More detailed information was provided in the Supplementary material.

### 2.3. Catalytic behavior evaluation

The catalytic performance for VC oxidation over the prepared catalyst was evaluated in a fixed-bed reactor (I.D. = 6 mm) under
atmospheric pressure. In the typical activity testing, 200 mg catalyst particles (40–60 mesh) were loaded on a quartz-wool plug inside the tubular quartz reactor. The gaseous feed containing 0.1 vol% of VC, 8 vol% of O₂ and Ar as the balance was set a total flow rate of 50 mL·min⁻¹, resulting in a space velocity (SV) of 15000 mL·g⁻¹·h⁻¹. Prior to each catalytic test, the reactor was thermally stabilized at 100 °C for an adsorption–desorption equilibrium of VC, and then heated up to 500 °C by temperature-programmed operation. The concentrations of VC, CO₂ and other possible organics were analyzed online using Fuli 9790II GC equipped with a hydrogen ionization detector (FID) and a thermal conductivity detector (TCD). The calculation equation for VC conversion was described below, where [C₂H₃Cl]in and [C₂H₃Cl]out represented the VC concentration at the inlet and outlet, respectively. And the concentration of VC completely mineralized into CO₂ at high temperatures was designated as the final concentration (c[CO₂]final). By comparing the CO₂ concentrations at different temperatures with the final concentration, the CO₂ yield (YCO₂) at a specific temperature was calculated. This yield reflects the ability of different catalysts to mineralize VC into CO₂. The calculation equations are as follows.

$$X_{C_2H_3Cl} = \frac{[C_2H_3Cl]_{in} - [C_2H_3Cl]_{out}}{[C_2H_3Cl]_{in}} \times 100\%$$

$$Y_{CO_2} = \frac{c[CO_2]}{c[CO_2]_{final}} \times 100\%$$

## 2.4. Computational details

All the spin-polarized calculations were performed using density functional theory (DFT) on the Vienna Ab initio Simulation Package

(VASP) [33,34]. Generalized gradient approximation functional with the Perdew-Burke-Ernzerh correction was used to describe the electron exchange–correlation energy [35]. Other computational details are in line with our previous work and can be found in the Supplementary material [36].

# 3. Results and discussion

## 3.1. Crystalline structure and textural property

The XRD patterns of the prepared xLa-ZIF precipitates and xLa-Co catalysts are illustrated in Fig. 1. As shown in Fig. 1a, all xLa-ZIF precipitates exhibited multiple diffraction peaks corresponding to the typical zeolitic imidazolate framework of ZIF-67, which indicated the successful generation of ZIF-67 crystals even after the addition of small amounts lanthanum cations [37]. Furthermore, it was also observed that the peaks intensity of the xLa-ZIF precipitates gradually decreased with the increasing addition of La, which indicated that the addition of La cations would reduce the crystallinity of ZIF-67 crystals [20,38,39].

The XRD patterns of the prepared xLa-Co catalysts are illustrated in Figs. 1b and c. It can be noted that all xLa-Co catalysts exhibited diffraction peaks typical of Co₃O₄ spinel phases (JCPDS No. 43–1003), with the peaks assignable to specific crystal planes labelled in Fig. 1b. There were no peaks attributable to LaOₓ/La(OH)ₓ/LaCoO₃, which indicated that La cations were well inserted into the spinel Co₃O₄ matrix after calcination. According to previous reports, La₂O₃ was usually formed when the concentration of La exceeded 5 mol.% in the solid materials [14,22,32,40]. It has also been shown that lanthanum oxides can react with cobalt oxides under calcination conditions to reform the LaCoO₃ perovskite structure [20,41,42]. However, no diffraction signal

<figure><img src="image_3.png" /><figcaption>Fig. 1. XRD patterns of (a) xLa-ZIF precursor, (b) xLa-Co catalysts in the 2θ range of 10-90°, (c) xLa-Co catalysts in the 2θ range of 35-40° and (d) Raman spectra of xLa-Co catalysts.</figcaption></figure>
assignable to La₂O₃, La(OH)ₐ or LaCoO₃ perovskite was detected in this work, even when the molar ratio of La in the xLa-ZIF precipitate reached 7.5 mol.%. This indicated that either no independent La₂O₃ structure formed, or only a small amount of fine La₂O₃ crystals formed below the XRD detection limit. To validate the actual La content, ICP-OES measurements were performed over all La-incorporated Co₃O₄ catalysts, and the results are presented in Table S1. Due to the 2-methylimidazole greater tendency to coordinate with Co during ZIF-67 formation, the actual La content obtained was slightly lower than the feed ratio. Nevertheless, at a feed ratio of 7.5 mol%, the actual La content reached 5.97 mol%. This phenomenon showed that the use of ZIF-67 as the sacrificial template and its further pyrolysis can effectively facilitate the good incorporation of La into the Co₃O₄ matrix [29,43]. Additionally, it can be observed that the peaks intensity of xLa-Co catalysts gradually decreased with the increasing molar ratio of La, suggesting that a greater incorporation of La into Co₃O₄ lattice can result in the significant structural changes of spinel Co₃O₄ and presumably formation of lattice defects [19,44].

In order to facilitate the observation about mentioned changes in peak positions and widths, the diffraction patterns corresponding to the (311) crystal planes of xLa-Co catalysts are magnified in Fig. 1c. It can be observed that the peak central position of the (311) crystal plane slightly shifted towards a higher angle orientation with the increasing lanthanum content from 0La-Co to 7.5La-Co. Since the ionic radius of La³⁺ (1.03 Å) is much larger than those of Co²⁺ (0.65 Å) and Co³⁺ (0.55 Å), the incorporation of La³⁺ into Co₃O₄ lattice should theoretically result in the XRD peak shift towards the lower angle [16,45]. However, the observed experimental results in Fig. 1c were exactly the opposite. This peculiar phenomenon could be explained by the fact that the incorporation of La³⁺ with a larger ionic radius would induce a significant lattice mismatch in the spinel structure of Co₃O₄, thereby causing strong lattice strain. Thus, in order to relax the lattice strain, the cobalt cations at the A- and B-sites in the spinel matrix (AB₂O₄) have been reasonably rearranged, resulting in the reduction of the lattice parameters and the high angle shift of the XRD peak [20].

The crystalline size of the xLa-Co catalysts was calculated by the Debye-Scherrer equation and are listed in Table 1. The crystalline size of xLa-Co catalysts gradually decreased with the increasing content of La, which indicated that the incorporation of La was beneficial for the aggregation of small-sized Co₃O₄ particles [46,47]. It can be speculated that the nanoscale aggregation may be conducive to the generation of more lattice defects, thereby contributing to the potential increase of catalytic activity.

We also utilized XRD results to calculated lattice parameters and micro strains through XRD patterns. As shown in Table S2, the lattice parameter of Co₃O₄ decreased slightly from 8.047 Å in the 0La-Co sample to 8.0309 Å in the 5La-Co sample, indicating a minor lattice contraction. This can be attributed to the low-doped La³⁺ ions preferentially occupying interstitial sites, forming a small number of defects that cause localized contraction. As the La incorporation level increased, 7.5La-Co exhibited a lattice parameter of 8.0609 Å. This expansion was attributed to the larger La³⁺ ions (1.16 Å) substitution for Co³⁺ (0.545 Å) within the spinel lattice structure. Meanwhile, Table S2 also lists the

<table><caption>**Table 1**
The crystalline size and the relative intensity ratio of A₁g and F₂g peaks over xLa-Co catalysts.</caption><thead><tr><th>Catalysts</th><th>Crystalline size<sup>a</sup> (nm)</th><th>I<sub>671</sub>/I<sub>189</sub><sup>b</sup></th></tr></thead><tbody><tr><td>0La-Co</td><td>18.9</td><td>4.4</td></tr><tr><td>2.5La-Co</td><td>16.7</td><td>7.7</td></tr><tr><td>5La-Co</td><td>13.8</td><td>12.3</td></tr><tr><td>7.5La-Co</td><td>14.9</td><td>11.3</td></tr></tbody></table>

calculation results of micro strains. Compared with the pristine Co₃O₄, the La-incorporated Co₃O₄ showed a slightly higher micro strain, suggesting that La incorporation caused minor lattice distortion due to ionic radius mismatch, but no severe structural stress or amorphization occurred.

The Raman spectra of the xLa-Co catalysts are shown in Fig. 1d. Five vibration peaks in the range of 100–800 cm⁻¹ appeared over all xLa-Co catalysts, and the ones located at ca. 189, 466, 510, 606 and 671 cm⁻¹ were attributable to the F₂g, E₉, F₂g, F₂g, and A₁g vibration modes of Co₃O₄ spinel structure, respectively [39,48]. Among these vibration modes, the A₁g mode was related to the octahedral cobalt sites (CoO₆, Co³⁺–O²⁻), and the F₂g vibration mode was related to the tetrahedral cobalt sites (CoO₄, Co²⁺–O²⁻) in Co₃O₄ spinel structure. The relative abundance of Co³⁺ and Co²⁺ on the surface of each catalyst can be approximately determined by calculating the peak intensity ratio between the A₁g and F₂g modes [49,50], and the obtained intensity ratios (I₆₇₁/I₁₈₉) are listed in Table 1. It can be seen that the value of I₆₇₁/I₁₈₉ over the xLa-Co catalysts initially increased and then decreased with the continuous increasing content of La, reaching the highest value over 5La-Co. This trend suggests that an appropriate amount of La incorporation can initiate the formation of more Co³⁺ on the surface of Co₃O₄. Thus, 5La-Co catalyst showed the highest surface abundance of Co³⁺. At present, it is widely accepted that Co³⁺ exhibits higher efficiency in adsorbing VOCs with electron-donor properties such as aromatic hydrocarbons and alkenes in comparison with Co²⁺ [20]. Therefore, the higher abundance of Co³⁺ on the surface of this catalyst could be beneficial to the catalytic activity improvement. In addition, one can note that the Raman peak position of La-Co composite oxides (2.5La-Co, 5La-Co and 7.5La-Co) shifted to the higher wavenumber after La incorporation in comparison with the pure Co₃O₄ (0La-Co). Taking the A₁g vibration mode as an example, it was found that the peak positions were 670, 681, 684, and 686 cm⁻¹ in order from the catalyst of 0La-Co to 7.5La-Co. The blue shift of Raman spectra over the xLa-Co catalysts can be attributed to the generation of oxygen vacancies (O₅) on the Co₃O₄ surface because of the La incorporation [19].

Fig. 2 shows the nitrogen adsorption–desorption isotherms and pore size distributions of all xLa-Co catalysts, and the corresponding data including specific surface area (SSA), total pore volume and average pore diameter are summarized in Table 2. As shown in Fig. 2a, all catalysts exhibited type IV isotherms and H3 hysteresis loops in the relative pressure range (p/p₀) of 0.6–1.0, which indicated that all xLa-Co catalysts possessed the typical mesoporous structure. Fig. 2b showed that the pore size distribution of La-incorporated Co₃O₄ catalysts was largely concentrated in the range of 5–20 nm, which further evidenced their mesoporous characteristics [51,52].

## 3.2. Redox behavior

H₂-TPR and O₂-TPD measurements were carried out in order to investigate the redox behaviors of the xLa-Co catalysts, and the obtained results are illustrated in Fig. 3 and Table S3. As shown in Fig. 3a, the overall H₂-TPR profiles can be deconvoluted into three or four peaks with regards to the different reduction temperatures, which were namely grouped into peak I, II, III and IV. One can note that peak I appeared in the range of 100–230 °C over the catalysts of 2.5La-Co, 5La-Co and 7.5La-Co. This phenomenon can be observed more clearly in Fig. 3c. Considering that all these catalysts were pretreated at 300 °C under argon atmosphere before hydrogen reduction, the presence of physically absorbed oxygen on catalyst surface has been excluded. Thus, the reduction signal of peak I could be attributed to the hydrogen consumption with the surface chemically adsorbed oxygen species [45]. It was noteworthy that there was no obvious reduction signal of peak I observed on the pure Co₃O₄ catalyst (0La-Co). In contrast, the signal of peak I became more intense with the increasing lanthanum content in the La-Co composite oxides, which suggested that the incorporation of La into spinel Co₃O₄ significantly increased the amount of chemisorbed

<footnote>a The crystalline size was calculated by Debye-Scherrer equation from XRD patterns, which is based on the (220) peak.</footnote>

<footnote>b The relative ratio was calculated using the intensity in Raman spectra.</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions of xLa-Co catalysts.</figcaption></figure>

<table><caption>**Table 2**
Representative textural parameters from nitrogen sorption over xLa-Co catalysts.</caption><thead><tr><th>Catalysts</th><th>SSA<sup>a</sup><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th>Total pore volume<sup>b</sup><br>(cm<sup>3</sup> g<sup>-1</sup>)</th><th>Pore diameter<sup>b</sup><br>(nm)</th></tr></thead><tbody><tr><td>0La-Co</td><td>52.4</td><td>0.84</td><td>36.0</td></tr><tr><td>2.5La-Co</td><td>66.9</td><td>0.44</td><td>14.6</td></tr><tr><td>5La-Co</td><td>73.3</td><td>0.55</td><td>10.0</td></tr><tr><td>7.5La-Co</td><td>51.6</td><td>0.39</td><td>6.8</td></tr></tbody></table>

<sup>a</sup> Determined by the BET (Brunauer-Emmett-Teller) method.

<sup>b</sup> Determined by the BJH (Barrett-Joyner-Wallenda) method from the desorption branch.

oxygen species on the catalyst surface [44]. Given that the abundance of surface chemisorbed oxygen is closely related to the presence of oxygen vacancies ($O_v$), it can be inferred that La incorporation is beneficial to promoting the generation of $O_v$ on the catalyst surface [19,45]. The hydrogen uptakes of peak II and III could be attributed to the reduction of $Co^{3+}$ into $Co^{2+}$ and the further reduction of $Co^{2+}$ into metallic cobalt ($Co^0$). It can be seen that the maximum positions of peak II and III appeared at the similar temperature over all xLa-Co catalysts, suggesting that the introduction of La had almost no impact on the reduction behaviors of cobalt cations in these catalysts. It was worth noting that the width of peak III gradually increased with the more incorporation of La, which seem to indicate that the La-Co interaction could affect the reduction rate of cobalt cations [22]. Furthermore, the peak area of peak IV, which can be attributed to the reduction of lattice oxygen species chemically bonding with La, gradually increased with the addition of La. It was noteworthy that the temperature of peak IV was much lower than the reduction temperature of $La_2O_3$. This was due to that the energy of the La-O bond weakened in the La-O-Co structure after La incorporation into the $Co_3O_4$ lattice. In general, the $H_2-TPR$ results showed that La incorporation can effectively increase the surface capacity of chemisorbed oxygen species and the abundance of $O_v$ on the catalyst, but made little effect on the overall reducibility of $Co_3O_4$ [17].

As shown in Fig. 3b, the $O_2-TPD$ profiles can be divided into three distinct regions based on the desorption temperature of oxygen species, which were attributed to physically adsorbed oxygen ($O_{phy}$), chemically adsorbed oxygen ($O_{chem}$) and bulk lattice oxygen ($O_{latt}$). In particular, it has been demonstrated that the amount of $O_{chem}$ is closely related to the abundance of $O_v$ on the catalyst surface, which plays an important role in catalytic oxidation of VOCs [11,21,37]. Herein, one can find that the desorption peak intensity of $O_{chem}$ remarkably strengthened over the La-Co composite oxides in comparison with the pure $Co_3O_4$ (0La-Co). Moreover, this phenomenon was most pronounced over 5La-Co, reflecting that La incorporation evidently improved the storage

capacity of chemisorbed oxygen and emerged the most plentiful oxygen vacancies thanks to the lattice reconstruction of spinel $Co_3O_4$ [11,21,37]. According to the starting desorption temperatures of $O_{latt}$ marked in Fig. 3b, one can note that the starting desorption temperature appeared at the lower position after La incorporation in comparison with the single $Co_3O_4$ (0La-Co), while the lowest starting desorption temperature was observed over 5La-Co. Since the surface active oxygen can be readily replenished by lattice oxygen migration through oxygen vacancies, the lower starting desorption temperature indicated that the migration and transformation of lattice oxygen could be much easier during the catalytic reaction. This phenomenon beneficially resulted from the higher mobility of $O_{latt}$ and the more generation of $O_v$ after La incorporation [23,49].

### 3.3. Morphological and structural characteristics

The SEM images and SEM-EDS mappings of the xLa-ZIF precipitates and xLa-Co catalysts are shown in Figs. 4 and 5, respectively. It can be seen that all xLa-ZIF precipitates exhibited the typical rhombic dodecahedral crystals with smooth surface, and the lanthanum addition didn't arise noticeable effect on the microstructure of ZIF-67 template. In addition, the crystal size of all synthesized xLa-ZIF precipitates were basically kept around 2 µm. In particular, numerous irregular particle-like projections appeared on the smooth surface of xLa-ZIF crystals after the lanthanum incorporation. As shown in Fig. 5e-h, the obtained xLa-Co catalysts still maintained the dodecahedral skeleton of ZIF-67 template after high temperature pyrolysis in an air atmosphere, but their skeleton dimensions approximately collapsed to ca. 700–800 nm comparable to the pristine ZIF-67 template [30,31].

In terms of the TEM images in Fig. 6, one can note that all xLa-Co catalysts retained the dodecahedral framework of ZIF-67 but with the appearance of hollow cage structures. It can be inferred that the unique hollow cage structure could facilitate the adsorption, internal diffusion and efficient conversion of VOC molecules on the catalyst [50,52]. Fig. 7 shows the HR-TEM images and SAED patterns of xLa-Co catalysts. Obviously, lattice fringes with the spacing of 0.24 and 0.46 nm were prevalent over all xLa-Co catalysts, which well corresponded to the (311) and (111) crystal planes of spinel $Co_3O_4$, respectively. These findings further confirmed the main spinel crystalline phases of $Co_3O_4$ without any other crystalline planes assignable to $La_2O_3$ or $LaCoO_3$ perovskite. Based on this, it can be believed that the La cations have been properly incorporated into the lattice of spinel $Co_3O_4$, which was consistent with the results of XRD and Raman analysis. Additionally, it was noteworthy that there appeared numerous heterogeneous interfaces in the HR-TEM images of all catalysts, which may be related to the nanoparticles accumulation of the catalysts.
<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) H₂-TPR and (b) O₂-TPD profiles as well as (c) zoomed H₂-TPR profile in low-temperature of xLa-Co catalysts.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. SEM images of (a) 0La-ZIF, (b) 2.5La-ZIF, (c) 5La-ZIF, (d) 7.5La-ZIF, (e) 0La-Co, (f) 2.5La-Co, (g) 5La-Co and (h) 7.5La-Co.</figcaption></figure>

### 3.4. Surface elemental analysis

The elemental compositions and valence states distribution on the surface of xLa-Co catalysts were investigated by XPS, and the full and deconvoluted XPS spectra are illustrated in Fig. 8. Moreover, the surface atomic ratios of metal elements (La/Co), cobalt cations (Co³⁺/Co²⁺) and oxygen species (Oads/Olatt) are summarized in Table 3. The O 1 s spectra of all catalysts in Fig. 8b were deconvoluted into three counterparts

consisting of surface adsorbed water and hydroxyl groups (Owat) at 533.1 eV, surface adsorbed oxygen (Oads) at 531.2 eV and surface lattice oxygen (Olatt) at 529.8 eV. The molar ratio of Oads/Olatt can reflect the relative abundance of Oads on the catalyst surface. As seen in Table 3, the molar ratio of Oads/Olatt over the catalyst gradually increased with the increasing addition of La, which indicated that La incorporation significantly boosted the surface abundance of Oads. These observations are well consistent with the O₂-TPD results in Fig. 3b. Since the Oads
<figure><img src="image_7.png" /><figcaption>Fig. 5. SEM-EDS mappings of (a) 2.5La-ZIF, (b) 5La-ZIF, (c) 7.5La-ZIF, (d) 2.5La-Co, (e) 5La-Co and (f) 7.5La-Co.</figcaption></figure>

abundance was closely linked with the amount of oxygen vacancies, it
can be deduced that the increasing abundance of Oads over La-Co cata-
lysts resulted from the more generation of surface oxygen vacancies after
La incorporation into Co3O4 lattice, which was also in good accordance
with the aforementioned conclusions from Raman and O2-TPD analysis.
Compared with Olatt in Co3O4 catalysts, Oads species available on the
catalyst surface have higher electrophilic ability, and can effectively
attack and activate VOCs molecules on the catalyst surface, which plays
a vital role in determining the catalytic activity for VOCs oxidation
[53,54]. These results suggest that the incorporation of La can improve
the catalytic activity in VOCs oxidation process since Oads species
available on the catalyst surface have higher electrophilic ability than
Olatt in Co3O4 catalysts [24,53].

As shown in Fig. 8c, the Co 2p spectra of all catalysts could be
deconvoluted into the subcomponents namely as Co²⁺ 2p₁/₂, Co³⁺ 2p₁/₂,
Co²⁺ 2p₃/₂, Co³⁺ 2p₃/₂ and several satellite peaks. The obtained atomic
ratios of Co³⁺/Co²⁺ in Table 3 showed that the relative proportion of
Co³⁺ on the catalyst surface gradually increased with the increasing
addition of La, reaching the maximum value over 5La-Co. Combined
with the XRD results in Fig. 1, it was speculated that a proper degree of
La incorporation was beneficial to inducing the lattice distortion of
Co₃O₄ and promoting the transformation of Co³⁺. It is generally believed
that the redox couple of Co³⁺/Co²⁺ is the main active center in the
catalytic oxidation of VOCs, and a higher surface concentration of Co³⁺
is known to facilitate the formation of oxygen vacancies [54,55].
Meanwhile, the higher relative ratio of Co³⁺/Co²⁺ on the catalyst re-
flected its stronger redox capability, which is also favorable for deliv-
ering higher catalytic activity. Therefore, the highest Co³⁺/Co²⁺ ration
observed on the 5La-Co catalyst surface can also suggest a highest cat-
alytic activity for VOCs removal. Finally, Fig. 8d illustrates the decon-
volution of La 3d spectra consisting of La³⁺ 3d₃/₂, La³⁺ 3d₅/₂ orbits and
two satellite peaks, which indicated that lanthanum species existed only

as La³⁺ cations in all xLa-Co catalysts.

3.5. *Catalytic performance for VC oxidation*

The light-off curves of VC conversion versus reaction temperature over all prepared catalysts are illustrated in Fig. 9a, and the representative catalytic activity data (T₁₀, T₅₀ and T₉₀ corresponding to the temperatures at 10, 50 and 90 % VC conversion) are listed in Table 4. One can find that the La-incorporated Co₃O₄ catalysts exhibited higher catalytic activity in comparison with the pure Co₃O₄ (0La-Co), showing a volcanic-type trend with the increasing content of La in the catalyst (Fig. S1). Based on the data in Table 4, the decreasing order of the catalytic activities was 5La-Co > 2.5La-Co > 7.5La-Co > 0La-Co ≫ La₂O₃, with the 5La-Co catalyst providing the optimum catalytic activity. It was noteworthy that the single La₂O₃ exhibited extremely poor catalytic activity with only ca. 5 % of VC conversion at 350 °C, indicating that cobalt was the primary metal active component in the La-Co composite oxides for catalytic VC oxidation. Table S6 summarizes the T₅₀ and T₉₀ data for catalysts used in VC catalytic oxidation in recent years. It can be seen that the 5La-Co catalyst in this work exhibited superior catalytic activity in transition metal oxide catalysts, but was still inferior to supported precious metal catalysts.

Fig. 9b shows the CO₂ yield as a function of reaction temperature in the catalytic reaction. In the catalytic oxidation of VC over these Co₃O₄ catalysts in this work, CO₂, HCl and H₂O were the predominant products in the overall reaction temperatures. In particular, a certain amount of Cl₂ was produced at temperatures above 300 °C due to the Deacon reaction. Additionally, trace amounts of chlorinated organic by-products, including dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃), tetrachloromethane (CCl₄) and 1,1,2-trichloroethane (CH₂Cl-CHCl₂), were formed in the temperature range of 200–300 °C and can be detected by gas chromatography with a flame ionisation detector (FID). The
<figure><img src="image_8.png" /><figcaption>Fig. 6. TEM images of (a) 0La-Co, (b) 2.5La-Co, (c) 5La-Co and (d) 7.5La-Co.</figcaption></figure>

concentrations of these chlorinated by-products were determined to be at the ppm level (maximum 20 ppm), as indicated by their extremely tiny peaks in the chromatograms. This indicates the strong catalytic ability of the Co₃O₄ catalyst to mineralize VC into CO₂.

According to the Arrhenius plots in Fig. S1 and the apparent activation energy (Eₐ) presented in Table S4, one can find that the catalysts of 2.5La-Co and 7.5La-Co possessed similar Eₐ values (91.8 and 91.9 kJ mol⁻¹) compared to that of obtained for pure Co₃O₄ (0La-Co, 89.1 kJ mol⁻¹), which was consistent with their approximate catalytic activities at low temperatures. By contrast, 5La-Co exhibited significantly lower apparent activation energy (69.8 kJ mol⁻¹) comparable to other catalysts, suggesting that the appropriate incorporation of La could significantly reduce the energy barrier for VC activation and conversion. The superior catalytic activity and the lowest Eₐ value of 5La-Co could be related to its abundant active oxygen species and excellent oxygen mobility on the surface [6].

Water vapor is a common component in industrial exhausts and is widely recognized for its potential to significantly inhibit catalytic activity, especially at low temperature (< 300 °C) [56,57]. In order to verify the water resistance of the catalyst, catalytic stability for VC oxidation over the optimal 5La-Co catalyst was evaluated at the representative temperatures of T₅₀ (ca. 250 °C) and T₉₀ (ca. 280 °C). Water vapor at variable concentrations (1 vol%, 3 vol%, 5 vol%) was alternately introduced and withdrawn during the reaction, and the corresponding catalytic results are illustrated in Fig. 10. One can note that the VC conversion obviously decreased after the addition of water vapor, while VC conversion can be rapidly recovered after the withdrawal of water vapor, which suggested the good regeneration capability of 5La-Co in this test [5,30]. Meanwhile, the higher concentration of water vapor addition caused more severely inhibitive effect on VC conversion,

leading to a greater reduction in catalytic activity. According to the literature, this phenomenon can be attributed to the competitive adsorption of VC and water molecules on the surface active sites [58]. Interestingly, the catalyst exhibited a transient increase in VC conversion compared to its initial value at both 250 and 280 °C after water vapor removal, although this transient increase was short-lived. This phenomenon reflected that the catalytic activity improvement could occur momentarily after the water vapor addition was stopped, which would be related to the cleaning effect of water molecules on surface active sites [59]. In fact, the adsorbed water molecules could provide additional protons for the protonation reaction with surface chlorine species, thus promoting the elimination of surface chlorine species and achieving the re-exposure of surface active sites [59]. Overall, the presence of water molecules in the reaction has a dual character. On one hand, water and VC molecules may compete for catalyst surface adsorption sites [60], inhibiting the activation and conversion of VC. On the other hand, water molecules appear to favor the removal of accumulated chlorine species on the catalyst surface, which is beneficial to the catalytic activity regeneration of the catalyst. Meanwhile, catalytic activity remained largely stable despite prolonged high-temperature reaction and multiple introduction and withdrawal of water vapor, indicating that 5La-Co exhibited excellent thermal stability and maintained structural integrity, without evidence of chlorine poisoning or structural deterioration at elevated temperature.

### 3.6. *In situ* DRIFTs measurements

In order to investigate VC adsorption behavior and elucidate the reaction intermediates, pathways and mechanism, *in situ* DRIFTs experiments were performed on 0La-Co and 5La-Co catalysts under both
<figure><img src="image_9.png" /><figcaption>Fig. 7. High-resolution (HR) TEM images and selected area electron diffraction (SAED) patterns of (a) 0La-Co, (b) 2.5La-Co, (c) 5La-Co and (d) 7.5La-Co.</figcaption></figure>

oxygen-free and oxygen-containing conditions. The system was gradually heated to 300 °C at first in an oxygen-free atmosphere to observe VC adsorption and intermediate formation, and then oxygen was introduced and IR spectra were recorded over time. The obtained in situ DRIFTs results are presented in Fig. 11. It can be noted that both catalysts show the IR peaks at 846 cm⁻¹ under oxygen-free conditions even with the temperature increase from 50 to 300 °C. This IR signal is attributed to the stretching vibration of the C-Cl bond, which may result from the VC adsorption on the catalyst surface and possible formation of other chlorinated organics during the reaction [61]. As previously mentioned, the IR band at 846 cm⁻¹ is persistently present even at high temperature, which indicates that the organic chlorine species on the catalyst surface could not be completely eliminated under oxygen-free conditions. The IR band at 1036 cm⁻¹ can be attributed to the carboxylic acid intermediate species (COO⁻) from VC oxidation [62]. It can be found that the intensity of this band is significantly higher over 5La-Co than that observed over 0La-Co. Furthermore, the intensity of the band associated to these COO⁻ intermediates gradually decreases with the increasing temperature, and completely disappears at 300 °C over 5La-Co. By contrast, the IR signal of COO⁻ shows no noticeable change over 0La-Co. This different phenomenon can be correlated to the presence of more active oxygen species available on the surface of 5La-Co catalyst compared to that presented on 0La-Co, which facilitates the further oxidation of those organic intermediates such as COO⁻ species at low temperature [63]. Meanwhile, the generation and elimination of COO⁻ intermediates were observable under an oxygen-free atmosphere, which indicated that the activation and partial oxidation of VC could occur with the participation of surface active oxygen species on both 0La-Co and 5La-Co catalysts by following the Mars-van Krevelen mechanism [64,65].

The IR bands at 1354, 1433 and 1543 cm⁻¹ are also observed on the 0La-Co catalyst, corresponding to the methylene groups (–CH₂), enol species (CH₂=CH-OH) and double bonds (C=C) in VC, respectively [66]. By contrast, there appears a noticeable broad band with multiple peaks in the range of 1250–1650 cm⁻¹ over the 5La-Co catalyst, suggesting a higher adsorption capacity of VC and the possible formation of abundant organic intermediates from VC activation on the surface [67]. Moreover, the IR bands at 2337 and 2364 cm⁻¹, which can be attributed to the vibration of CO₂ [68,69], are clearly observed over the 5La-Co catalyst under oxygen-free conditions. These bands could be hardly observed over the 0La-Co catalyst. These results indicate that the carboxylic acid intermediates on the surface of 5La-Co catalyst can be readily oxidized into CO₂ via the surface active oxygen species on this catalyst, but this transformation is significantly more difficult over 0La-Co catalyst because of its lower amount of surface active oxygen species.

After the oxygen introduction into the reaction atmosphere, the multiple IR peaks in the range of 1200–1700 cm⁻¹ rapidly disappear, suggesting a rapid removal of the resultant complex organic intermediate species over 5La-Co catalyst. This phenomenon reflects that the introduced gas-phase oxygen molecules undoubtedly participated in the activation, migration and oxidation processes on the surface of 5La-Co, thus providing more available active oxygen species to accelerate the further oxidation of these organic intermediates. By contrast, the IR signal corresponding to enol and carboxylate intermediates is still observable over the 0La-Co catalyst even after the oxygen introduction, which indicates that the organic intermediates could not be rapidly decomposed to CO₂ even under an oxygen-containing atmosphere due to the poor redox capacity of 0La-Co catalyst.

Based on DRIFTs measurements above-mentioned, Fig. 12 gives a clear illustration about the plausible conversion pathways of VC and
<figure><img src="image_10.png" /><figcaption>Fig. 8. (a) Full element, (b) O 1 s, (c) Co 2 p and (d) La 3 d XPS spectra of xLa-Co catalysts.</figcaption></figure>

<table><caption>**Table 3**

Quantitative results of XPS analysis over xLa-Co catalysts.</caption><thead><tr><th rowspan="2">Catalysts</th><th colspan="3">Surface atomic ratio</th></tr><tr><th>La/Co</th><th>Co<sup>3+</sup>/Co<sup>2+</sup></th><th>O<sub>ads</sub>/O<sub>latt</sub></th></tr></thead><tbody><tr><td>0La-Co</td><td>/</td><td>0.59</td><td>0.49</td></tr><tr><td>2.5La-Co</td><td>0.09</td><td>0.66</td><td>0.51</td></tr><tr><td>5La-Co</td><td>0.14</td><td>0.73</td><td>0.56</td></tr><tr><td>7.5La-Co</td><td>0.18</td><td>0.61</td><td>0.81</td></tr></tbody></table>

other organic intermediates and the role of oxygen vacancies in the catalytic reaction. With regards to the Mars-van Krevelen mechanism, VOCs oxidation usually occurs under the catalysis by active lattice

<table><caption>**Table 4**

Representative parameters for catalytic VC oxidation.</caption><thead><tr><th>Catalysts</th><th>T<sub>10</sub> (°C)</th><th>T<sub>50</sub> (°C)</th><th>T<sub>90</sub> (°C)</th></tr></thead><tbody><tr><td>0La-Co</td><td>215</td><td>259</td><td>290</td></tr><tr><td>2.5La-Co</td><td>205</td><td>251</td><td>281</td></tr><tr><td>5La-Co</td><td>202</td><td>246</td><td>276</td></tr><tr><td>7.5La-Co</td><td>207</td><td>255</td><td>287</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>Fig. 9. (a)Light-off curves and (b) CO₂ yield curve of pure Co₃O₄ (0La-Co), La₂O₃ and xLa-Co catalysts for catalytic VC oxidation.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 10. VC conversion as a function of reaction temperature and time-on-stream over the 5La-Co catalyst under dry and humid conditions.</figcaption></figure>

oxygen on the catalyst surface, and oxygen vacancies are recognized as the critical mediators for the activation and migration of both gas phase oxygen and bulk lattice oxygen [18]. In the absence of gas phase oxygen, the bulk lattice oxygen can be activated by oxygen vacancies in the form of internal migration from the bulk phase to the surface, and subsequently participates in the oxidation reactions of VC and other organic intermediates. In the presence of gas phase oxygen on the contrary, the presence of oxygen vacancies can facilitate the activation of surface adsorbed oxygen molecules from gas phase into the bulk lattice, which

becomes a necessary part of the redox cycle in the reaction. As illustrated in Fig. 12, one can note that there are more engineered oxygen vacancies available on the surface of 5La-Co catalyst after La incorporation into $Co_3O_4$ lattice structure, thus providing more surface active oxygen species involved in the reaction pathways. Thus, the abundant oxygen vacancies not only promote the formation of organic intermediates (carboxylic acid) from the oxidation of enol species, but also accelerate the further decomposition into $CO_2$, which thereby enhances the total catalytic activity. By contrast, the limited presence of oxygen vacancies on the surface of 0La-Co catalyst would deliver a restricted effect on the oxidation pathways of enol and carboxylic acid intermediates.

### 3.7. DFT calculations

DFT calculations were employed in order to clarify the contribution of La incorporation to the engineering of uniform abundant oxygen vacancies in La-Co composite oxides catalysts. Herein, the La-incorporated $Co_3O_4$ was constructed by La replacing a Co atom in the $(Co_3O_4)_8$ conventional cell. Generally, La cation is preferable to incorporating in the tetrahedron site [LaO$_4$] with a formation energy lower by 0.59 eV than incorporating in the octahedron site. The low incorporation concentration of La has slightly elongated the lattice parameter by 0.93 % and induced volume expansion by 2.82 % (Table S5). According to Fig. S3, the total and partial density of states (TDOS and PDOS) of the bulk structures exhibit the similar properties, implying that the La-incorporated $Co_3O_4$ catalyst maintains the inherent properties of the pristine $Co_3O_4$ at the current incorporation concentration (La:Co = 1:23). Then the properties of (001) surfaces were investigated in detail to decipher the sources for the catalytic behavior improvement. The TDOS and PDOS reveal that the active sites of La-incorporated $Co_3O_4$ (001) surfaces are Co and O species other than La since the Co 3d and O

<figure><img src="image_13.png" /><figcaption>Fig. 11. In situ DRIFTs of (a1, a2) 0La-Co and (b1, b2) 5La-Co for catalytic VC oxidation under variable reaction conditions.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Plausible conversion pathway of catalytic VC oxidation over 0La-Co and 5La-Co.</figcaption></figure>

2p orbitals mainly constitutes the states around Fermi energy (Fig. S3). However, La incorporation alters the electronic structures (Figs. 13a, e) and causes an upshift of the d-band center from –1.36 eV to –1.26 eV, indicating that the active species on La-incorporated Co₃O₄ (001) are more reactive. The Co–O bond lengths (Figs. 13b, f) elongate from 1.881 to 1.884 Å to 1.883–1.926 Å, which indicates that the interaction

between Co and O ions are weakened over La-incorporated Co₃O₄ (001). Hence, the surface lattice oxygen species away from La site are much easier to activate. As shown in Figs. 13c and g, the La-incorporated Co₃O₄ (001) surface is more propitious to generate oxygen vacancies, giving a lower formation energy of 1.84 eV.

Furthermore, both pristine Co₃O₄ (001) and La-incorporated Co₃O₄

<figure><img src="image_15.png" /><figcaption>**Fig. 13.** Surface properties of pristine Co₃O₄ (a–d) and La-incorporated Co₃O₄ (e–f): (a, e) bader charges; (b, f) bond lengths; (c, g) formation energies of oxygen vacancies; (d, h) charge density differences. The green, blue, red balls represent La, Co, O atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
(001) surfaces are thermodynamically feasible to chemically adsorb oxygen molecules. The adsorption energies decrease from -1.11 eV on Co<sub>3</sub>O<sub>4</sub> (001) to -1.59 eV on La-incorporated Co<sub>3</sub>O<sub>4</sub> (001), which evidences that La-incorporated Co<sub>3</sub>O<sub>4</sub> (001) surface shows a stronger adsorption ability to oxygen and accelerates the utilization of gas-phase oxygen molecules. The Bader charge analysis reveals that electrons slightly transfer from the catalyst to the chemisorbed oxygen (Figs. 13d and h), and the O=O bond lengths are elongated after the chemical adsorption. The charge density difference maps show that the electron density accelerates between Co and surface O species, while the electron density between O=O exhibits a notable depletion. Notably, the bond lengths of Co-O bonds are 1.876 and 1.877 Å over La-incorporated Co<sub>3</sub>O<sub>4</sub> (001), shorter than that over pristine Co<sub>3</sub>O<sub>4</sub> (001). The bond lengths of adsorbed O-O bonds are 1.336 and 1.347 Å over Co<sub>3</sub>O<sub>4</sub> (001) and La-incorporated Co<sub>3</sub>O<sub>4</sub> (001), which reveals that the surface adsorbed oxygen on Co<sub>3</sub>O<sub>4</sub> (001) are relatively easier to be activated with the formation of Co-O* species via the cleavage of O-O bond. These results confirm that La incorporation constitutes an efficient and feasible strategy to improve the catalytic reactivity of Co<sub>3</sub>O<sub>4</sub> by tailoring the electronic properties of catalyst surfaces.

### 3.8. Structure-performance relationship

It has been commonly accepted that the physico-chemical properties such as morphological characteristics, microscale and porous structure, redox behaviors and surface active species play critical roles in VOCs catalytic oxidation over Co<sub>3</sub>O<sub>4</sub> based catalysts [70]. In this work, La-incorporated spinel Co<sub>3</sub>O<sub>4</sub> was prepared by using ZIF-67 as a sacrificial template, and the SEM/TEM images showed that the unique hollow cage structure derived from ZIF-67 template provided xLa-Co catalysts with high special surface areas and rich porosity, thus facilitating the mass transfer efficiency of reactants and products molecules in the reaction. Furthermore, XRD and XPS results revealed that an appropriate amount of La incorporation increased the surface abundance of Co<sup>3+</sup>, which gave rise to sufficient active sites for VC adsorption and oxidation. However, excessive La incorporation reduced the surface abundance of Co<sup>3+</sup>, which was attributable to the maintenance charge balance on Co<sub>3</sub>O<sub>4</sub> surface. And the variable surface abundance of Co<sup>3+</sup> over the prepared catalysts was also in good agreement with their catalytic activities for VC oxidation. Notably, the results of Raman spectra, H<sub>2</sub>-TPR and O<sub>2</sub>-TPD measurements strongly reflected that the La incorporation effectively boosted the creation of oxygen vacancies on the catalyst. This experimental deduction was further confirmed by DFT calculations. DFT calculations also showed that the incorporation of La lengthens the Co-O bond and weakens the interaction between Co and O, accordingly facilitating the activation of surface oxygen species and promoting the formation of oxygen vacancies. Finally, *in situ* DRIFTs results revealed that VC oxidation over Co<sub>3</sub>O<sub>4</sub> catalysts followed the Mars-van Krevelen mechanism, in which VC molecules reacted with surface active oxygen species coming from catalyst lattice. In this process, oxygen vacancies play a significant role in the migration of lattice oxygen and the activation of gas phase oxygen. Overall, La incorporation remarkably altered the lattice structure of Co<sub>3</sub>O<sub>4</sub> and engineered appreciable density of oxygen vacancies on the catalyst surface, which effectively enhanced the catalytic activity for catalytic VC oxidation.

### 4. Conclusions

La-incorporated Co<sub>3</sub>O<sub>4</sub> catalysts with variable La contents were prepared by using ZIF-67 as the sacrificial template. La incorporation induces the appearance of lattice strain and prompts a rearrangement of Co<sup>3+</sup>/Co<sup>2+</sup> sites in the spinel Co<sub>3</sub>O<sub>4</sub> structure. The catalytic activity for VC oxidation over the prepared catalysts follows the ascending trend of 0La-Co < 7.5La-Co < 2.5La-Co < 5La-Co, where 5La-Co exhibits the optimum catalytic behavior with adequate catalytic stability and regeneration capacity under humid conditions. The catalytic activity

improvement over La-incorporated Co<sub>3</sub>O<sub>4</sub> catalysts can be attributed to the enhanced redox properties from valence state redistribution of cobalt cations, the more abundant surface oxygen species and higher lattice oxygen mobility from oxygen vacancies generation and probably by the faster reaction kinetics from the hollow cage-like structure of ZIF-67 template. Enol and carboxylic acid species are identified as the key organic intermediates in VC oxidation by *in situ* DRIFT spectroscopy. These measurements also prove that the engineered oxygen vacancies over La-incorporated Co<sub>3</sub>O<sub>4</sub> catalyst can promote the formation and further oxidation of these organic intermediates. DFT calculations give more evidences that La-incorporation can modulate the electronic structure of Co<sub>3</sub>O<sub>4</sub> with an upshift of the *d*-band center, thus enabling the lower formation energy of oxygen vacancies (1.84 eV) and the lower adsorption energy of gas-phase oxygen molecules (-1.59 eV) to facilitate the oxygen activation.

### CRediT authorship contribution statement

**Hongliang Zhang:** Writing – original draft, Methodology, Funding acquisition, Formal analysis. **Haijie Cao:** Methodology, Formal analysis, Data curation. **Guanghao Li:** Methodology, Formal analysis, Data curation. **Valérie Meille:** Validation, Formal analysis, Data curation. **Sonia Gil**. **Chuanhui Zhang:** Writing – original draft, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Chuanhui Zhang reports financial support was provided by National Natural Science Foundation of China. Hongliang Zhang reports financial support was provided by China Scholarship Council. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work was financially supported by the Natural Science Foundation of Shandong province (ZR2025MS164), the National Natural Science Foundation of China (No. 22076088) and the doctoral program of China Scholarship Council (No. 202308370181).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2025.116524.

### Data availability

The authors do not have permission to share data.

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