The Promoting Effect of Metal Vacancy on CoAl Hydrotalcite-Derived Oxides for the Catalytic Oxidation of Formaldehyde

Abstract

Formaldehyde (HCHO) is a major harmful volatile organic compound (VOC) that is particularly detrimental to human health indoors. Therefore, effectively eliminating formaldehyde is of paramount importance to ensure indoor air quality. In this study, CoAl hydrotalcite (LDH) was prepared using the co-precipitation method and transformed into composite metal oxides (LDO) through calcination. Additionally, a metal Al vacancy was constructed on the surface of the composite metal oxides (E_X-LDO and E_X-LDO/NF) using an alkaline etching technique. SEM demonstrated the successful loading of CoAl-LDO onto nickel foam surfaces (LDO/NF), and an extended etching time resulted in a greater number of porous structures in the samples. XRD confirmed the successful synthesis of the precursor materials, CoAl hydrotalcite (CoAl-LDH) and CoAl layered double oxides (CoAl-LDO). EDS analysis confirmed a reduction in aluminum content after alkaline etching. XPS analysis verified the presence of abundant Co²⁺ and surface oxygen as crucial factors contributing to the catalyst’s excellent catalytic activity. The experimental results indicated that catalysts containing metal cation vacancies exhibited superior catalytic performance in formaldehyde oxidation compared to conventional hydrotalcite-derived composite oxides. H₂-TPR confirmed a significant enhancement in the participation of lattice oxygen in the catalytic oxidation reaction; it was found that the proportion of surface lattice oxygen consumption by the E₅-LDO catalyst (30.2%) is higher than that of the LDO catalyst (23.4%), and the proportion of surface lattice oxygen consumption by the E₁-LDO/NF catalyst (27.5%) is higher than that of the LDO/NF catalyst (14.6%), suggesting that cation vacancies can activate the surface lattice oxygen of the material, thereby facilitating improved catalytic activity. This study not only reveals the critical role of surface lattice oxygen in catalytic oxidation activity, but also aids in the further development of novel catalysts for efficient room-temperature oxidation of HCHO. Moreover, it provides possibilities for developing high-performance catalysts through surface modification.

1. Introduction

Formaldehyde (HCHO) is a commonly found volatile organic compound (VOC) and a highly detrimental indoor air pollutant, primarily originating from wooden furniture and building materials. Moreover, the time spent indoors by individuals has increased (80–90%), especially after the outbreak of the coronavirus disease (COVID-19) [1]. Prolonged exposure to even low concentrations (ppm) of HCHO may lead to various health issues, such as inflammation of the eyes and throat, chronic respiratory diseases, neurological disorders, and even cancer [2,3,4]. To meet the requirements of increasingly stringent environmental regulations, the development of effective techniques for removing indoor formaldehyde is particularly important and urgent. Various strategies have been developed to reduce indoor HCHO levels. While traditional physical adsorption and ventilation are known for their removal efficiency of HCHO, unfortunately, adsorbents are limited in their removal capacity and can only be effective for a short period, while ventilation methods have low efficiency in removing indoor HCHO. In contrast, catalytic oxidation is considered the most effective method, as it can continuously and completely oxidize HCHO to water and carbon dioxide at room temperature [5,6,7,8,9].

Layered double hydroxides (LDHs) are a type of layered bimetallic hydroxide that possess characteristics such as interlayer ion exchange and alkalinity. Hydrotalcite, a representative LDH, is composed of layers and interlayer organic/inorganic anions. The divalent metal cations (Mg²⁺, Fe²⁺, Cu²⁺, Ni²⁺, etc.) are coordinated with hydroxyl groups to form octahedra, constituting the layers. The structure consists of positively charged host layers of metal hydroxides and interlayer regions with compensating anions and solvent molecules [10,11]. The general formula of LDH is [M_1−x²⁺M_x³⁺(OH)₂]^x+•A_x/nⁿ⁻•mH₂O, where M²⁺ and M³⁺ represent the divalent and trivalent metal cations; Aⁿ⁻ is the interlayer anion, X denotes the proportion of trivalent metal cations (usually 0.2 < X < 0.33), and m represents the number of solvent molecules (typically water) [10,11,12,13,14,15]. LDHs have received significant attention due to their high stability and structural diversity [12,13,14,15]. Li et al. [15] prepared Au-decorated cobalt (Co)-doped Mg/Al layered double hydroxide catalysts by depositing uniformly distributed Au nanoparticles onto preformed Co-LDH nanosheets using in situ deposition. The loading amount of Au was 2 wt% (2%Au/Co-LDH). The 2% Au/Co-LDH catalyst demonstrated higher activity for the complete oxidation of indoor formaldehyde (HCHO) to CO₂ compared to the undoped 2% Au/LDH binary sample and other control samples. The mixed oxides obtained by calcining LDHs are referred to as layered double oxides (LDOs). Compared to the pristine LDHs, LDOs exhibit unique structures characterized by high surface area, abundant active sites, and stable dispersed metals. These features make LDOs highly attractive as adsorbents, catalysts, or catalyst precursors. Particularly, in recent years, transition metal-containing LDOs have been widely employed as highly active catalysts for the degradation of air pollution [16,17,18,19]. Xie et al. [18] synthesized Co₂Ca₁Al₁-LDO as a type of layered double oxide (LDO) material. For the first time, Co₂Ca₁Al₁-LDO was employed for the activation of peracetic acid (PAA). The Co₂Ca₁Al₁-LDO/PAA system exhibited removal rates ranging from 90.4% to 100% for various micro-pollutants. Additionally, the catalyst demonstrated excellent reusability and stability. Chen et al. [19] incorporated Co into Mn₁Fe_0.25Al_0.75O_x-LDO and found that the N₂ selectivity for NH₃-SCR significantly improved within the low temperature range of 150–250 °C when the Co/Mn molar ratio was 0.5. Zeng et al. [17] prepared NiMnAl-LDO catalysts with different treatment times using a solution plasma-assisted method for CO₂ methanation reaction. The results demonstrated an enhancement in the low-temperature activity of the catalysts. Among them, the highly dispersed NiMnAl-LDO-P (20) catalyst exhibited the highest catalytic activity for CO₂ methanation (at 200 °C, CO₂ conversion rate of 81.3% and CH₄ selectivity of 96.7%); even after 70 h of operation, the catalyst maintained a high level of stability.

As a typical transition metal oxide, cobalt oxide (Co₃O₄) exhibits significant performance in the catalytic oxidation of formaldehyde due to its good low-temperature reducibility, high oxygen migration rate, and generation of reactive oxygen species (ROS). Co³⁺ and oxygen vacancies are the active sites for formaldehyde chemical adsorption and O₂ activation, respectively. As a representative non-layered material, 2D Co₃O₄ nanosheets exhibit the characteristics of both the bulk and 2D structure. Ultra-thin 2D Co₃O₄ nanosheets have been employed for catalytic oxidation of gaseous pollutants [7,20,21]. However, considering that most reported Co₃O₄-based catalysts completely degrade HCHO at temperatures above room temperature, achieving effective catalytic oxidation of HCHO on Co₃O₄ remains challenging [6,22].

Nickel foam (NF) possesses a unique porous structure that is suitable for loading other metal oxides or active functional groups, making it widely studied in the fields of electrocatalysis, photocatalysis, and more [23]. Additionally, NF exhibits excellent thermal shock resistance, ensuring sufficient stability as a catalyst support. Moreover, the good thermal conductivity of NF is advantageous for many thermos-catalytic reaction processes. Therefore, using NF as a carrier for photo-thermal catalysis is highly suitable [9,24].

In this study, LDO catalysts were successfully prepared on nickel foam via a well-designed hydrothermal method and alkaline etching. The influence of etching time on the catalytic oxidation of formaldehyde (HCHO) and the degradation efficiency was investigated. The crystal structure and morphology of LDO were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Brunauer–Emmett–Teller (BET) analysis, hydrogen temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The samples exhibited a unique two-dimensional nanoarray structure and abundant large-scale porous morphology. The experimental results indicated that catalysts containing metal cation vacancies exhibited superior catalytic performance in formaldehyde oxidation compared to conventional hydrotalcite-derived composite oxides. H₂-TPR confirmed a significant enhancement in the participation of lattice oxygen in the catalytic oxidation reaction; it was found that the proportion of surface lattice oxygen consumption by the E₅-LDO catalyst (30.2%) is higher than that of the LDO catalyst (23.4%), and the proportion of surface lattice oxygen consumption by the E₁-LDO/NF catalyst (27.5%) is higher than that of the LDO/NF catalyst (14.6%), suggesting that cation vacancies can activate the surface lattice oxygen of the material, thereby facilitating improved catalytic activity. The excellent HCHO purification performance of the prepared material demonstrated that the rational selection of the preparation process and alkali etching modification to create oxygen vacancies are effective strategies for enhancing the material’s HCHO purification capability. The experimental results indicated that catalysts containing metal cation vacancies exhibited superior catalytic performance in formaldehyde oxidation compared to conventional hydrotalcite-derived composite oxides.

2. Experimental Section

2.1. Materials

All of the reagents employed in this study were of analytical reagent grade and without purification. Cobaltous nitrate hexahydrate [Co(NO₃)₂·6H₂O], aluminum nitrate nonahydrate [Al(NO₃)₃·9H₂O], urea [CO(NH₂)₂], sodium NaOH, C₃H₆O, NaOH, HCl, and C₂H₅OH were commercially available from China National Pharmaceutical Reagent Co., Ltd. Foam nickel (purity: 99.9%, thickness: 1.5 mm, porosity: 97 ± 2%) was purchased from Taiyuan Lizhiyuan Technology Co., Ltd. (Shanxi, China).

2.2. Catalyst Preparation

2.2.1. Pretreatment of Nickel Foam

First, foam nickel was cut into rectangular blocks measuring 2.5 cm × 6 cm. Subsequently, the cut foam nickel blocks were placed in separate beakers and immersed in acetone solution for 10 min under ultrasonic treatment to remove surface-adsorbed organic compounds and impurities. After ultrasonic treatment, the foam nickel was rinsed with distilled water for 5 min to remove residual acetone. Then, the foam nickel blocks were soaked in 2 mol/L HCl for 10 min to remove the surface oxide layer. Finally, the pre-treated foam nickel was washed three times with deionized water and dried in a vacuum drying oven at 80 °C to obtain the prepared foam nickel.

2.2.2. Synthesis of LDO and LDO on Ni Foam

Briefly, 9 mmol of cobalt nitrate and 3 mmol of aluminum nitrate were separately weighed into a beaker and dissolved in an appropriate amount of deionized water, resulting in solution A. Then, 40 mmol of urea was weighed and dissolved in an appropriate amount of deionized water, resulting in solution B. Solution A and solution B were mixed together, and 2 mL of ethylene glycol solution was added to prepare a 70 mL mixed solution. The mixed solution was subjected to ultrasonic treatment for 30 min and then transferred to a reaction vessel. The pre-treated foam nickel was placed into the solution, and the reaction was conducted at 100 °C for 9 h. After the 9 h reaction at 100 °C, the foam nickel samples were washed three times with deionized water and anhydrous ethanol, respectively. Meanwhile, the residual solution in the reaction vessel was left undisturbed for 1 h, and the supernatant was removed. The obtained precipitate was filtered using a suction filtration apparatus until neutral, resulting in the CoAl-LDH sample. The obtained foam nickel and CoAl-LDH were dried in an air drying oven at 60 °C for 6 h and then calcined in a muffle furnace at 350 °C for 3 h to obtain CoAl-LDO supported on foam nickel (CoAl-LDO/NF) and powdered CoAl-LDO.

2.2.3. Synthesis of Etched LDO and LDO on Ni Foam

Alkaline etching of prepared CoAl-LDO/NF and CoAl-LDO: The prepared CoAl-LDO/NF and CoAl-LDO samples were individually subjected to reaction in a 6 mol/L NaOH solution in a reaction vessel for 1 h and 5 h, respectively. After removing the samples from the solution, they were washed repeatedly with deionized water and ethanol, followed by drying at 60 °C for 8 h. The obtained samples were labeled as LDO/NF, E₁-LDO/NF, E₅-LDO/NF and LDO, E₁-LDO, E₅-LDO.

2.3. Catalyst Characterization

The powder X-ray diffraction (XRD) measurements were carried out on a desktop X-ray diffractometer (Bruker D8 Advance, German) in reflection mode with Cu Kα radiation (λ = 1.54060 Å). Diffraction patterns were recorded within the 2θ range of 10–80° with a scanning rate of 2° min⁻¹.

The morphology and microstructure of the samples were tested using a scanning electron microscope (SEM, ZEISS Gemini 300) and elemental mapping was performed by EDX spectroscopy using an OXFORD XPLORE30 detector.

The Brunauer-Emmett-Teller (BET) surface area and pore size distributions of catalysts were measured by N₂ adsorption and desorption isotherms at 77.35 K using an Autosorb iQ Station 2.

The redox capability of the catalyst was analyzed using H₂ temperature-programmed reduction (H₂-TPR). The H₂-TPR experiment was conducted on a chemisorption instrument equipped with a thermal conductivity detector (TCD). Firstly, 20 mg of the catalyst (in the case of bulk catalyst, it was cut into small pieces) was weighed and placed into a quartz reactor. Then, the catalyst was pretreated under N₂ atmosphere at 100 °C for 1 h. After the catalyst reached room temperature, the reduction experiment was carried out under a 5% H₂/N₂ atmosphere, and the temperature was increased from room temperature to 800 °C at a heating rate of 10 °C/min.

X-ray photoelectron spectroscopy (XPS) is a method used to measure the energy distribution of emitted photoelectrons and Auger electrons from the surface of a sample when irradiated with X-ray photons, using an electron spectrometer. XPS is employed for qualitative and quantitative analysis of the elemental composition and chemical states of the surface species based on their unique binding energies. XPS measurements were performed on a Thermo ESCALAB 250Xi electron spectrometer.

2.4. Catalytic Activity Tests

Catalyst performance evaluation was carried out in a micro-fixed-bed reactor, with 50 mg of catalyst loaded into a reaction tube with an inner diameter of 6 mm. A diffusion bottle containing 150 g of paraformaldehyde was heated using a water bath, causing the decomposition of paraformaldehyde and generating a gas stream containing formaldehyde. The inlet gas composition was 100 mL/min of 21% O₂/N₂, and nitrogen (N₂) as carrier gas was introduced through the paraformaldehyde solid powder immersed in the water bath to generate formaldehyde gas, with a concentration of 22 ppm and a water bath temperature of 40 °C. The reaction temperature ranged from 50 to 200 °C, and the temperature ramp was controlled by a temperature controller with a heating rate of 5 °C/min. Prior to heating, the catalyst was purged with a flow of gas containing formaldehyde (100 ppm) at 30 °C for 3 h to test the formaldehyde adsorption performance of the catalyst and eliminate the interference of adsorption on subsequent reaction evaluations. The reaction gas flow passed through the catalyst in the quartz reaction tube, where formaldehyde was oxidized to carbon dioxide (CO₂) and H₂O. The gas flow rate was controlled using a mass flow controller, and the temperature of the catalyst bed was maintained. The reaction product, CO₂, was detected and analyzed using a gas chromatograph equipped with a carbon dioxide detector. Before recording data, the corresponding temperature was maintained for 40 min to obtain stable values. The concentration of CO₂ was recorded every 11 min during the test. Three samples were taken at each temperature point, and then the temperature was increased for data collection at the next temperature point. No other carbon-containing compounds were detected in the reaction products besides CO₂. Therefore, the concentration of CO₂ reflects the activity of the catalyst, i.e., the conversion rate of formaldehyde. The formula is as follows:

$\mathrm{HCHO} \mathrm{conversion} \%=\frac{[\mathsf{\Delta }\mathrm{C}{\mathrm{O}}_2]}{[\mathrm{HCHO}{]}_{\mathrm{in}}}\text{×}100\%$

Here, [HCHO]_in represents the concentration of formaldehyde (ppm) in the gas stream before the reaction, and [∆CO₂] represents the difference in CO₂ concentration (ppm) in the gas stream before and after the reaction.

3. Results and Discussion

3.1. Morphology and Structure

In this study, the microstructure of synthesized material samples was characterized using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) (Figure 1). Nickel foam (Figure 1g) exhibited an interconnected framework structure with large voids, making it an ideal template for the growth of other active materials and providing a higher specific surface area for anchoring CoAl-LDH. After loading with LDO, the originally smooth surface of the nickel foam was covered with a uniform layer of E₁-LDO (Figure 1a,c,e), which showed a cross-linked structure, significantly enhancing the adhesion of the active metal. Meanwhile, the thickness of the active substance loaded on the surfaces of E₁-LDO/NF and E₅-LDO/NF was thinner than that of LDO/NF, which may be attributed to the etching of Al³⁺. Figure 1a,b displayed the growth of LDO on nickel foam, showing a uniform layered structure that was consistent with the morphology reported in the literature [16]. Figure 1c,d were SEM images of E₁-LDO/NF thin films on nickel foam, which exhibited a thinner and well-separated structure compared to LDO/NF, featuring a unique porous morphology and higher specific surface area. Figure 1e,f were SEM images of E₅-LDO/NF thin films on nickel foam, and the sample etched for 5 h exhibited more porous structures than the one etched for 1 h, maximizing the contact efficiency between gas molecules and active sites. The SEM image of LDO (Figure 2b) revealed that the original regular hexagonal flakes were disrupted and became disordered, increasing the specific surface area of LDO, which indicated a higher exposure of active sites compared to LDH for subsequent experiments [25]. However, it can be observed that the regular hexagonal flakes on the surface of E₅-LDO/NF remained intact, possibly due to insufficient calcination. The structure of the E₅-LDO/NF sample and E₅-LDO were analyzed via energy-dispersive spectroscopy (EDS) (Figure 1i,j), and it was observed that Co and Al elements were uniformly distributed on the surface of the nickel foam, indicating the successful preparation of LDO material on the nickel foam. Furthermore, with an increase in etching time, the ratio of Co to Al gradually increased (Table 1). However, the presence of residual Al indicated that E_X-LDO/NF was only partially etched by NaOH, and not completely etched.

In this work, X-ray diffraction (XRD) patterns were used to investigate the composition and structural characteristics of the materials. Figure 2 shows the XRD spectra of CoAl-LDH, LDO and E_X-LDO (X = 1, 5). Figure 2a presents a comparison of XRD patterns between CoAl-LDH and LDO prepared under different etching times. It can be observed that the diffraction peaks of CoAl-LDH at 11.5°, 23.5°, 34.8°, 39.4°, 46.6°, 56.2°, and 61.5° correspond to the (003), (006), (009), (015), (018), (110), and (113) planes, respectively [26,27,28]. These peaks indicate the typical layered double hydroxide (LDH) structure of the sample (JCPDS 51-0045). The XRD diffraction peaks of the precursor material, CoAl-LDH are consistent with the literature, confirming the successful synthesis of the precursor and the target product. The sharpness of the (003) and (006) diffraction peaks suggests the high crystallinity of the prepared CoAl-LDH [29,30]. By comparing the XRD patterns of the precursor CoAl-LDH and LDO and E_X-LDO (X = 1, 5), significant changes in the crystal plane characteristics of CoAl-LDH can be observed. After calcination at 350 °C and etching treatment with NaOH solution, the XRD characteristic peaks of CoAl-LDH disappear, and the characteristic peaks of LDO are observed at 19.2°, 31.2°, 36.6°, 44.8°, 59.4°, and 65.1°, corresponding to the (011), (220), (311), (400), (511), and (440) planes of CoAl₂O₄ (JCPDS no. 38e0814), respectively. This indicates a significant phase transformation of CoAl-LDH during high-temperature treatment. The layered structure of CoAl-LDH is destroyed after calcination, resulting in a highly dispersed mixture of metal oxide. Figure 2b shows the XRD spectra of LDO/NF and E_X-LDO/NF (X = 1, 5). For the bulk catalyst LDO/NF, the characteristic peak at 36.7° (311) can be attributed to NiO (JCPDS card No. 47-1049), while the peaks at 44.45° (111), 51.8° (200), and 76.3° (220) correspond to Ni [23,31]. No peaks from other phases were detected in both modes, indicating that the loaded LDO structure is not prominent. Additionally, in the XRD spectrum of E₅-LDO/NF, the Ni characteristic peaks are stronger and have larger integrated areas compared to E₅-LDO, indicating that extending the etching time exposes more active sites and can improve the crystallinity of other metal oxides on nickel foam.

The N₂ adsorption–desorption isotherms and pore size distributions of LDO and E₁-LDO are shown in Figure 3, according to the standard of scientific papers. The isotherms are consistent with type IV isotherms and exhibit an H₃ hysteresis loop (P/P₀ > 0.4), indicating the formation of mesopores with a three-dimensional interconnected porous structure, which is beneficial for rapid mass transfer [28,32,33]. The specific surface areas of both catalysts can be calculated from the N₂ desorption isotherms using the BET method. The specific surface area of E₁-LDO (136 m² g⁻¹) is significantly larger than that of LDO (69 m² g⁻¹). E₁-LDO possesses more active sites than LDO, suggesting that the etching effect of NaOH results in a higher specific surface area for E₁-LDO. The pore size distribution reveals that E₁-LDO with a diameter of 3–7 nm has a narrower pore size distribution compared to LDO with a diameter of 3–10 nm. The larger specific surface area and uniform porous structure provide more active sites and facilitate the adsorption of reactants, which is advantageous for excellent catalytic performance. However, the specific surface area of E₅-LDO (71 m² g⁻¹) is not significantly different from that of LDO (69 m² g⁻¹). However, the catalytic activity of E₅-LDO is much higher than that of LDO, indicating that the specific surface area is not the primary factor affecting the catalytic performance of the catalyst for formaldehyde oxidation.

3.2. Activation Performance of Prepared Material

The variation in formaldehyde conversion rates with temperature (50–175 °C) for the LDO, E_X-LDO (X = 1, 5) and LDO/NF, E_X-LDO/NF (X = 1, 5) catalysts with different etching times are shown in Figure 4. It can be observed that etching has a significant effect on the catalytic activity of both catalysts towards formaldehyde, and the etching time greatly influences the catalytic performance of the catalysts. It can be observed that at the highest temperature of 175 °C, the HCHO conversion rate of LDO catalyst is less than 50%, indicating low catalytic activity. Compared to LDO, the etched catalysts E₁-LDO and E₅-LDO exhibit improved catalytic activity over the entire temperature range. E₁-LDO achieves a 50% HCHO conversion rate at 81 °C, while E₅-LDO reaches a 90% HCHO conversion rate at 110 °C. E₅-LDO shows higher HCHO conversion rates than E₁-LDO across the entire temperature range, which may be attributed to the longer etching time, resulting in a higher concentration of active oxygen species in E₅-LDO, thereby promoting the activity towards HCHO. Comparing the powder-type CoAl-LDO_X catalysts to the supported catalysts, the former exhibit higher HCHO conversion rates, which could be attributed to the lower loading of active components on the surface of LDO/NF and E_X-LDO/NF (X = 1, 5).

3.3. Surface Chemistry

The surface elemental composition and oxidation states of surface materials were investigated using X-ray photoelectron spectroscopy (XPS). All binding energies were calibrated using the C1s peak at 284.6 eV. The Co 2p and O 1s XPS spectra of the catalyst are shown in Figure 5, and the quantitative analysis of the different oxidation states of Co and O is presented in Table 2, based on the XPS spectra. As shown in Figure 5a, the Co 2p peak exhibits spin–orbit splitting into doublets. The spin–orbit peaks can be fitted with two pairs of peaks. One pair of peaks located at 780.6 and 796.2 eV is assigned to Co 2p3/2, while the other pair at 779.4 and 794.7 eV is assigned to Co 2p1/2. The peaks at 790.1 and 805.2 eV are attributed to satellite peaks. These values are in good agreement with the reference data for Co₃O₄ [34,35]. Furthermore, the coexistence and transformation of Co²⁺ and Co³⁺ in the Co-Al system provide possible catalytic mechanisms [36,37]. The ratio of surface Co²⁺/Co³⁺ in the catalyst was calculated based on the peak areas in the XPS spectra, as shown in Table 2. The results indicate that the Co²⁺ content on the catalyst surface is higher after etching compared to before, and the Co²⁺ content increases with longer etching time. Generally, a higher surface Co²⁺ content indicates a higher concentration of surface oxygen vacancies, which is considered a prerequisite for achieving high catalytic activity [38,39]. In Figure 5c,d, it can be seen that the O1s spectrum is deconvoluted into three characteristic peaks with binding energies of 532.47~533.2, 531.3~531.4, and 529.5~529.8 eV. The black peaks (529.5~529.8 eV) are attributed to lattice oxygen (O_latt: O²⁻) in a coordinated saturated environment. The red peaks (531.3~531.4 eV) are associated with surface-adsorbed oxygen (O_ads: O₂²⁻, O₂⁻ or O⁻) and oxygen defects in an unsaturated coordination mode. The blue peaks (532.47~533.2 eV) are related to the OH groups of water molecules adsorbed on the sample surface. A higher density of surface oxygen vacancies (O_V) is generally associated with the easier activation of gaseous oxygen to form electrophilic O_ads species, which plays a crucial role in the deep oxidation of VOCs [40]. According to Table 2, the O_ads/O_latt ratio of the catalyst increases after etching compared to before, indicating a higher amount of surface adsorbed oxygen. Therefore, enhanced surface oxygen properties can promote the oxidation of formaldehyde. Consistent with the results of the activity experiments, it can be concluded that the abundance of Co²⁺ and surface oxygen is an important factor contributing to the excellent catalytic activity of the catalyst.

The oxidation of formaldehyde is closely related to the redox performance of the catalysts, wherein the reduction temperature and hydrogen consumption collectively reflect the catalyst’s reduction ability. H₂-TPR (temperature-programmed reduction) was conducted, as shown in Figure 6, to determine the reduction behavior of Co and Al species. Previous studies suggest that the reduction of Co₃O₄ typically proceeds in the sequence of Co³⁺→Co²⁺→Co⁰, with T_max at 300 °C and 400 °C [41,42]. The catalyst exhibits reduction in three major temperature ranges: the low-temperature reduction (200–300 °C) represents the transition from Co³⁺ to Co²⁺, the mid-temperature peak (300–500 °C) corresponds to the reduction of Co²⁺ to Co⁰, and the high-temperature peak (500–900 °C) can be attributed to the reduction of CoAl₂O₄ [41,42,43]. E₁-LDO and E₅-LDO show more pronounced reducible peaks compared to LDO, indicating the presence of more active oxygen in E₁-LDO and E₅-LDO. In the CoAl₂O₄ compound, the polarization of Co-O bonds by Al³⁺ ions leads to an increase in the reduction temperature of Co²⁺, providing a plausible explanation for the presence of the third TPR peak [44] in the Figure 6a. For LDO, E₁-LDO, and E₅-LDO, the reduction of Co³⁺ to metallic Co⁰ gives rise to three peaks in the temperature range, which are associated with oxygen adsorbed on surface vacancies, surface lattice oxygen, and bulk lattice oxygen, respectively. Compared to LDO/NF, the low-temperature peak of E₁-LDO/NF and E₅-LDO/NF is enhanced, and the peaks of the catalyst shift towards the lower temperature region after etching. A larger H₂ consumption area indicates that alkaline etching significantly promotes the activation of surface lattice oxygen, suggesting the stronger redox capability of the latter. The etched catalyst facilitates the completion of redox cycles. By calculating the ratio of the amount of hydrogen consumed by the surface lattice oxygen of the catalyst to the total amount of hydrogen consumed, it is found that the proportion of surface lattice oxygen consumption by the E₅-LDO catalyst (30.2%) is higher than that of the LDO catalyst (23.4%), and the proportion of surface lattice oxygen consumption by the E₁-LDO/NF catalyst (27.5%) is higher than that of the LDO/NF catalyst (14.6%). These results show that cationic vacancy is formed on the surface of the material by alkali etching, which can activate the lattice oxygen of the oxide surface, and promote the catalytic oxidation of cobalt aluminum composite metal oxides to formaldehyde (Figure 7).

4. Conclusions

In this study, CoAl-LDO_X/NF catalysts were successfully prepared on nickel foam via a well-designed hydrothermal method and alkaline etching. The influence of etching time on the catalytic oxidation of formaldehyde (HCHO) and the degradation efficiency was investigated. XRD confirmed the successful synthesis of the precursor CoAl-LDH and CoAl-LDO. SEM demonstrated that CoAl-LDO was successfully loaded onto the surface of nickel foam, and increasing the etching time resulted in more porous structures in the samples. EDS analysis indicated a decrease in the aluminum content after alkaline etching. XPS revealed that the presence of abundant Co²⁺ and surface oxygen is an important factor contributing to the catalyst’s excellent catalytic activity. The experimental results showed that catalysts containing metal cation vacancies exhibited superior catalytic performance in formaldehyde oxidation compared to conventional hydrotalcite-derived composite oxides. H₂-TPR confirmed a significant enhancement in the involvement of lattice oxygen in the catalytic oxidation reaction, indicating that cation vacancies can activate the lattice oxygen on the material’s surface, thereby promoting improved catalytic activity, By calculating the ratio of the amount of hydrogen consumed by the surface lattice oxygen of the catalyst to the total amount of hydrogen consumed, it is found that the proportion of surface lattice oxygen consumption by the E₅-LDO catalyst (30.2%) is higher than that of the LDO catalyst (23.4%), and the proportion of surface lattice oxygen consumption by the E₁-LDO/NF catalyst (27.5%) is higher than that of the LDO/NF catalyst (14.6%). This study not only elucidates the crucial role of surface lattice oxygen in catalytic oxidation activity, but also contributes to the development of novel catalysts for the efficient catalytic oxidation of formaldehyde at room temperature. Furthermore, it demonstrates the possibility of developing high-performance catalysts through surface modification.

Author Contributions

All authors contributed to the study conception and design. Y.C. and S.Z. put forward the review ideas and organized the first draft. H.Y., X.T. and S.Z. refined and finally approved the whole content. The Sections of Results and Discussion were arranged by F.G., Q.Y. and Y.Z. All authors commented on previous versions of the manuscript. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets used and/or analyzed during the current study are available in the MDPI journals. Publicly available datasets were analyzed in this study. Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. SEM images of (a,b) LDO/NF, (c,d) E1-LDO/NF, (e,f) E5-LDO/NF, (g) Ni foam, (h) EDX pattern of E5-LDO/NF, (i) EDS mapping of the E5-LDO/NF, and (j) EDS mapping of the E5-LDO.

Figure 2. XRD patterns of (a) CoAl-LDH, LDO and EX-LDO (X = 1, 5) and (b) LDO/NF and EX-LDO/NF (X = 1, 5).

Figure 3. N2 adsorption and desorption isotherms and pore size distribution (inset) of LDO and EX−LDO (X = 1, 5) catalysts prepared by calcination at 500 °C.

Figure 4. HCHO conversion of (a) CoAl-LDOX and (b) EX-LDO/NF catalysts (X = 1, 5). Reaction conditions: [HCHO] = 20 ppm, [O2] = 21 vol%, GHSV = 30,000 h−1 and N2 as balance gas.

Figure 5. XPS spectra of the (a) Co 2p core level of LDO and EX-LDO (X = 1, 5); (b) Co 2p core level of LDO/NF and EX-LDO/NF (X = 1, 5); (c) O1s core level of LDO and EX-LDO (X = 1, 5); and (d) O1s core level of LDO/NF and EX-LDO/NF (X = 1, 5).

Figure 6. H2-TPR patterns of (a) LDO and EX-LDO (X = 1, 5) and (b) LDO/NF and EX-LDO/NF (X = 1, 5).

Figure 7. The cation vacancy activated the surface lattice oxygen and promoted the oxidation of formaldehyde.

Table 1

Distribution of element content on catalyst surface, obtained via EDS surface scanning.

wt%	LDO/NF	E1-LDO/NF	E5-LDO/NF
Co	19	17	18
Al	6.	4	4
C	17	19	19
O	43	44	43
Ni	15	16	16

Table 2

The surface compositions of the different samples.

	LDO	E1-LDO	E5-LDO	LDO/NF	E1-DO/NF	E5-LDO/NF
Co2+/Co3+	97.0394	105.9704	122.922	100.0167	110.9602	120.4848
Oads/Olatt	90.33875	93.54537	104.5528	45.57956	125.4108	127.7475

References

1. Wang, W.; Niu, Q.; Zeng, G.; Zhang, C.; Huang, D.; Shao, B.; Zhou, C.; Yang, Y.; Liu, Y.; Guo, H. et al. 1D porous tubular g-C₃N₄ capture black phosphorus quantum dots as 1D/0D metal-free photocatalysts for oxytetracycline hydrochloride degradation and hexavalent chromium reduction. Appl. Catal. B Environ.; 2020; 273, 119051. [DOI: https://dx.doi.org/10.1016/j.apcatb.2020.119051]

2. Xu, Q.; Zhang, Y.; Mo, J.; Li, X. Indoor Formaldehyde Removal by Thermal Catalyst: Kinetic Characteristics, Key Parameters, and Temperature Influence. Environ. Sci. Technol.; 2011; 45, pp. 5754-5760. [DOI: https://dx.doi.org/10.1021/es2009902]

3. Ye, J.; Yu, Y.; Fan, J.; Cheng, B.; Yu, J.; Ho, W. Room-temperature formaldehyde catalytic decomposition. Environ. Sci. Nano; 2020; 7, pp. 3655-3709. [DOI: https://dx.doi.org/10.1039/D0EN00831A]

4. Robert, B.; Nallathambi, G. Indoor formaldehyde removal by catalytic oxidation, adsorption and nanofibrous membranes: A review. Environ. Chem. Lett.; 2021; 19, pp. 2551-2579. [DOI: https://dx.doi.org/10.1007/s10311-020-01168-6]

5. Zhou, H.; Zeng, Y.; Low, Z.; Zhang, F.; Zhong, Z.; Xing, W. Core-dual-shell structure MnO₂@Co–C@SiO₂ nanofiber membrane for efficient indoor air cleaning. J. Membr. Sci.; 2023; 677, 121644. [DOI: https://dx.doi.org/10.1016/j.memsci.2023.121644]

6. Zhang, H.; Guo, G.; Wang, Z.; He, Q.; He, X.; Ji, H. Superior performance of formaldehyde complete oxidation at ambient temperature over Co single-atom catalysts. Appl. Catal. B Environ.; 2023; 333, 122774. [DOI: https://dx.doi.org/10.1016/j.apcatb.2023.122774]

7. Li, R.; Shi, X.; Huang, Y.; Chen, M.; Zhu, D.; Ho, W.; Cao, J.; Lee, S. Catalytic oxidation of formaldehyde on ultrathin Co₃O₄ nanosheets at room temperature: Effect of enhanced active sites exposure on reaction path. Appl. Catal. B Environ.; 2022; 319, 121902. [DOI: https://dx.doi.org/10.1016/j.apcatb.2022.121902]

8. Du, X.; Li, C.; Zhang, J.; Zhao, L.; Li, S.; Lyu, Y.; Zhang, Y.; Zhu, Y.; Huang, L. Highly efficient simultaneous removal of HCHO and elemental mercury over Mn-Co oxides promoted Zr-AC samples. J. Hazard. Mater.; 2021; 408, 124830. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2020.124830]

9. Wang, D.; Cuo, Z.; Du, Y.; Yang, W.; Zhang, M.; Chen, Y. Hierarchical NiCo₂O₄-MnO_x-NF monolithic catalyst synthesized by in-situ alternating anode and cathode electro-deposition strategy: Strong interfacial anchoring force promote catalytic performance. Appl. Surf. Sci.; 2020; 532, 147485. [DOI: https://dx.doi.org/10.1016/j.apsusc.2020.147485]

10. Zhang, S.; Zhang, L.; Liu, L.; Wang, X.; Pan, J.; Pan, X.; Yu, H.; Song, S. NiCo-LDH@MnO₂ nanocages as advanced catalysts for efficient formaldehyde elimination. Colloids Surf. A Physicochem. Eng. Asp.; 2022; 650, 129619. [DOI: https://dx.doi.org/10.1016/j.colsurfa.2022.129619]

11. Sun, L.; Liu, Y.; Yan, M.; Liu, W.; Liu, X.; Shi, W. ZIFs derived multiphase CoSe₂ nanoboxes induced and fixed on CoAl-LDH nanoflowers for high-performance hybrid supercapacitor. Chem. Eng. Sci.; 2022; 252, 117241. [DOI: https://dx.doi.org/10.1016/j.ces.2021.117241]

12. Gong, J.; Wang, X.; Dong, X.; Li, J.; Yang, F.; Yuan, A.; Ji, H. MnCo-Layered double hydroxides nanosheets supported Pd nanoparticles for complete catalytic oxidation of formaldehyde at room temperature. Appl. Surf. Sci.; 2022; 606, 154702. [DOI: https://dx.doi.org/10.1016/j.apsusc.2022.154702]

13. Trafela, Š.; Šturm, S.; Žužek Rožman, K. Surface modification for the enhanced electrocatalytic HCHO oxidation performance of Ni-thin-film-based catalysts. Appl. Surf. Sci.; 2021; 537, 147822. [DOI: https://dx.doi.org/10.1016/j.apsusc.2020.147822]

14. Zhang, G.; Zhang, X.; Meng, Y.; Pan, G.; Ni, Z.; Xia, S. Layered double hydroxides-based photocatalysts and visible-light driven photodegradation of organic pollutants: A review. Chem. Eng. J.; 2020; 392, 123684. [DOI: https://dx.doi.org/10.1016/j.cej.2019.123684]

15. Li, S.; Ezugwu, C.I.; Zhang, S.; Xiong, Y.; Liu, S. Co-doped MgAl-LDHs nanosheets supported Au nanoparticles for complete catalytic oxidation of HCHO at room temperature. Appl. Surf. Sci.; 2019; 487, pp. 260-271. [DOI: https://dx.doi.org/10.1016/j.apsusc.2019.05.083]

16. Jing, C.; Zhu, Y.; Liu, X.; Ma, X.; Dong, F.; Dong, B.; Li, S.; Li, N.; Lan, T.; Zhang, Y. Morphology and crystallinity-controlled synthesis of etched CoAl LDO/MnO₂ hybrid nanoarrays towards high performance supercapacitors. J. Alloys Compd.; 2019; 806, pp. 917-925. [DOI: https://dx.doi.org/10.1016/j.jallcom.2019.07.304]

17. Zeng, J.; Lu, K.; Zhang, J.; Sun, Y.; Chang, Z.; Li, J.; Dai, B.; Yu, F.; Li, J.; Liu, J. Solution plasma-assisted preparation of highly dispersed NiMnAl-LDO catalyst to enhance low-temperature activity of CO₂ methanation. Int. J. Hydrogen Energy; 2022; 47, pp. 2234-2244. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2021.10.183]

18. Xie, Z.-H.; He, C.-S.; Pei, D.-N.; Zheng, Y.-Z.; Wu, X.-Y.; Xiong, Z.; Du, Y.; Pan, Z.-C.; Yao, G.; Lai, B. Efficient degradation of micropollutants in CoCaAl-LDO/peracetic acid (PAA) system: An organic radical dominant degradation process. J. Hazard. Mater.; 2023; 452, 131286. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2023.131286]

19. Chen, S.; Vasiliades, M.A.; Yan, Q.; Yang, G.; Du, X.; Zhang, C.; Li, Y.; Zhu, T.; Wang, Q.; Efstathiou, A.M. Remarkable N₂-selectivity enhancement of practical NH₃-SCR over Co_0.5Mn₁Fe_0.25Al_0.75O_x-LDO: The role of Co investigated by transient kinetic and DFT mechanistic studies. Appl. Catal. B Environ.; 2020; 277, 119186. [DOI: https://dx.doi.org/10.1016/j.apcatb.2020.119186]

20. Chen, Y.; Fan, Z.; Zhang, Z.; Niu, W.; Li, C.; Yang, N.; Chen, B.; Zhang, H. Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications. Chem. Rev.; 2018; 118, pp. 6409-6455. [DOI: https://dx.doi.org/10.1021/acs.chemrev.7b00727]

21. Zhou, N.; Yang, R.; Zhai, T. Two-dimensional non-layered materials. Mater. Today Nano; 2019; 8, 100051. [DOI: https://dx.doi.org/10.1016/j.mtnano.2019.100051]

22. Huang, M.; Chen, J.; Tang, H.; Jiao, Y.; Zhang, J.; Wang, G.; Wang, R. Improved oxygen activation over metal–organic-frameworks derived and zinc-modulated Co@NC catalyst for boosting indoor gaseous formaldehyde oxidation at room temperature. J. Colloid Interface Sci.; 2021; 601, pp. 833-842. [DOI: https://dx.doi.org/10.1016/j.jcis.2021.05.173] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/34116471]

23. Li, Y.; Sun, P.; Liu, T.; Cheng, L.; Chen, R.; Bi, X.; Dong, X. Efficient photothermal conversion for oxidation removal of formaldehyde using an rGO-CeO₂ modified nickel foam monolithic catalyst. Sep. Purif. Technol.; 2023; 311, 123236. [DOI: https://dx.doi.org/10.1016/j.seppur.2023.123236]

24. Wang, D.; Li, S.; Du, Y.; Wu, X.; Chen, Y. Self-Templating Synthesis of 3D Hierarchical NiCo₂O₄@NiO Nanocage from Hydrotalcites for Toluene Oxidation. Catalysts; 2019; 9, 352. [DOI: https://dx.doi.org/10.3390/catal9040352]

25. Zeng, H.; Zhu, H.; Deng, J.; Shi, Z.; Zhang, H.; Li, X.; Deng, L. New insight into peroxymonosulfate activation by CoAl-LDH derived CoOOH: Oxygen vacancies rather than Co species redox pairs induced process. Chem. Eng. J.; 2022; 442, 136251. [DOI: https://dx.doi.org/10.1016/j.cej.2022.136251]

26. Qiu, H.; Sun, X.; An, S.; Lan, D.; Cui, J.; Zhang, Y.; He, W. Microwave synthesis of histidine-functionalized graphene quantum dots/Ni-Co LDH with flower ball structure for supercapacitor. J. Colloid Interface Sci.; 2020; 567, pp. 264-273. [DOI: https://dx.doi.org/10.1016/j.jcis.2020.02.018]

27. Zhao, L.-X.; Li, M.-H.; Jiang, H.-L.; Xie, M.; Zhao, R.-S.; Lin, J.-M. Activation of peroxymonosulfate by a stable Co-Mg-Al LDO heterogeneous catalyst for the efficient degradation of ofloxacin. Sep. Purif. Technol.; 2022; 294, 121231. [DOI: https://dx.doi.org/10.1016/j.seppur.2022.121231]

28. Wang, H.E.; Chen, W.; Jin, W.; Liu, Y.L. Mn mixed oxide catalysts supported on Sn-doped CoAl-LDO for low-temperature NH₃-SCR. Catal. Sci. Technol.; 2023; 13, pp. 3147-3157. [DOI: https://dx.doi.org/10.1039/D3CY00272A]

29. Cheng, J.Y.; Shen, B.; Yu, F.X. Precipitation in a Cu–Cr–Zr–Mg alloy during aging. Mater. Charact.; 2013; 81, pp. 68-75. [DOI: https://dx.doi.org/10.1016/j.matchar.2013.04.008]

30. Diao, Z.P.; Zhang, Y.X.; Hao, X.D.; Wen, Z.Q. Facile synthesis of CoAl-LDH/MnO₂ hierarchical nanocomposites for high-performance supercapacitors. Ceram. Int.; 2014; 40 Pt B, pp. 2115-2120. [DOI: https://dx.doi.org/10.1016/j.ceramint.2013.07.127]

31. Mo, S.P.; Li, S.D.; Xiao, H.L.; He, H.; Xue, Y.D.; Zhang, M.Y.; Ren, Q.M.; Chen, B.X.; Chen, Y.F.; Ye, D.Q. Low-temperature CO oxidation over integrated penthorum chinense-like MnCo₂O₄ arrays anchored on three-dimensional Ni foam with enhanced moisture resistance. Catal. Sci. Technol.; 2018; 8, pp. 1663-1676. [DOI: https://dx.doi.org/10.1039/C7CY02474F]

32. Sing, K.S.W. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). Pure Appl. Chem.; 1985; 57, pp. 603-619. [DOI: https://dx.doi.org/10.1351/pac198557040603]

33. Liu, Q.; Gao, J.; Gu, F.; Lu, X.; Liu, Y.; Li, H.; Zhong, Z.; Liu, B.; Xu, G.; Su, F. One-pot synthesis of ordered mesoporous Ni–V–Al catalysts for CO methanation. J. Catal.; 2015; 326, pp. 127-138. [DOI: https://dx.doi.org/10.1016/j.jcat.2015.04.003]

34. Chen, R.; Fang, X.; Li, J.; Zhang, Y.; Liu, Z. Mechanistic investigation of the enhanced SO₂ resistance of Co-modified MnO_x catalyst for the selective catalytic reduction of NO_x by NH₃. Chem. Eng. J.; 2023; 452, 139207. [DOI: https://dx.doi.org/10.1016/j.cej.2022.139207]

35. Wang, Z.; Lan, J.; Haneda, M.; Liu, Z. Selective catalytic reduction of NO_x with NH₃ over a novel Co-Ce-Ti catalyst. Catal. Today; 2021; 376, pp. 222-228. [DOI: https://dx.doi.org/10.1016/j.cattod.2020.05.040]

36. Feng, Y.; Liu, J.; Wu, D.; Zhou, Z.; Deng, Y.; Zhang, T.; Shih, K. Efficient degradation of sulfamethazine with CuCo₂O₄ spinel nanocatalysts for peroxymonosulfate activation. Chem. Eng. J.; 2015; 280, pp. 514-524. [DOI: https://dx.doi.org/10.1016/j.cej.2015.05.121]

37. Zeng, H.; Zhang, W.; Deng, L.; Luo, J.; Zhou, S.; Liu, X.; Pei, Y.; Shi, Z.; Crittenden, J. Degradation of dyes by peroxymonosulfate activated by ternary CoFeNi-layered double hydroxide: Catalytic performance, mechanism and kinetic modeling. J. Colloid Interface Sci.; 2018; 515, pp. 92-100. [DOI: https://dx.doi.org/10.1016/j.jcis.2018.01.016]

38. Li, J.-R.; Wang, F.-K.; He, C.; Huang, C.; Xiao, H. Catalytic total oxidation of toluene over carbon-supported CuCo oxide catalysts derived from Cu-based metal organic framework. Powder Technol.; 2020; 363, pp. 95-106. [DOI: https://dx.doi.org/10.1016/j.powtec.2019.12.060]

39. Zhang, W.; Díez-Ramírez, J.; Anguita, P.; Descorme, C.; Valverde, J.L.; Giroir-Fendler, A. Nanocrystalline Co₃O₄ catalysts for toluene and propane oxidation: Effect of the precipitation agent. Appl. Catal. B Environ.; 2020; 273, 118894. [DOI: https://dx.doi.org/10.1016/j.apcatb.2020.118894]

40. Guo, H.; Niu, H.-Y.; Wang, W.-J.; Wu, Y.; Xiong, T.; Chen, Y.-R.; Su, C.-Q.; Niu, C.-G. Schottky barrier height mediated Ti₃C₂ MXene based heterostructure for rapid photocatalytic water disinfection: Antibacterial efficiency and reaction mechanism. Sep. Purif. Technol.; 2023; 312, 123412. [DOI: https://dx.doi.org/10.1016/j.seppur.2023.123412]

41. Sexton, B.A.; Hughes, A.E.; Turney, T.W. An XPS and TPR study of the reduction of promoted cobalt-kieselguhr Fischer-Tropsch catalysts. J. Catal.; 1986; 97, pp. 390-406. [DOI: https://dx.doi.org/10.1016/0021-9517(86)90011-4]

42. Jiang, S.; Song, S. Enhancing the performance of Co₃O₄/CNTs for the catalytic combustion of toluene by tuning the surface structures of CNTs. Appl. Catal. B Environ.; 2013; 140–141, pp. 1-8. [DOI: https://dx.doi.org/10.1016/j.apcatb.2013.03.040]

43. Lamonier, J.-F.; Boutoundou, A.-B.; Gennequin, C.; Pérez-Zurita, M.J.; Siffert, S.; Aboukais, A. Catalytic Removal of Toluene in Air over Co–Mn–Al Nano-oxides Synthesized by Hydrotalcite Route. Catal. Lett.; 2007; 118, pp. 165-172. [DOI: https://dx.doi.org/10.1007/s10562-007-9196-4]

44. An, S.; Yang, H.; Wang, J. Revealing Recurrent Urban Congestion Evolution Patterns with Taxi Trajectories. ISPRS Int. J. Geo-Inf.; 2018; 7, 128. [DOI: https://dx.doi.org/10.3390/ijgi7040128]