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Article

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

by
Yimeng Chen
1,†,
Shunzheng Zhao
1,2,†,
Fengyu Gao
1,2,
Qingjun Yu
1,2,
Yuansong Zhou
1,2,
Xiaolong Tang
1,2 and
Honghong Yi
1,2,*
1
School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China
2
Beijing Key Laboratory of Resource-Oriented Treatment of Industrial Pollutants, Beijing 100083, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Processes 2023, 11(7), 2154; https://doi.org/10.3390/pr11072154
Submission received: 26 June 2023 / Revised: 16 July 2023 / Accepted: 17 July 2023 / Published: 19 July 2023
(This article belongs to the Special Issue Environmental Catalysis and Air Pollution Control)
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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 (EX-LDO and EX-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 [M1−x2+Mx3+(OH)2]x+•Ax/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 Mn1Fe0.25Al0.75Ox-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:
HCHO   conversion   % = [ Δ C O 2 ] [ HCHO ] in × 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 EX-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 EX-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 EX-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 EX-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, EX-LDO (X = 1, 5) and LDO/NF, EX-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-LDOX 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 EX-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 (Olatt: O2−) in a coordinated saturated environment. The red peaks (531.3~531.4 eV) are associated with surface-adsorbed oxygen (Oads: 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 (OV) is generally associated with the easier activation of gaseous oxygen to form electrophilic Oads species, which plays a crucial role in the deep oxidation of VOCs [40]. According to Table 2, the Oads/Olatt 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 Tmax 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-LDOX/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.

Funding

This work was financially supported by the National Natural Science Foundation of China (Nos. 21677010 and 51808037), the National Key R&D Program of China (2021YFB3500702), and the Special Fund of Beijing Key Laboratory of Indoor Air Quality Evaluation and Control (No. BZ0344KF21-04).

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.

References

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Processes 11 02154 g001 550
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 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.
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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 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).
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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 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.
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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 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.
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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 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).
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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 6. H2-TPR patterns of (a) LDO and EX-LDO (X = 1, 5) and (b) LDO/NF and EX-LDO/NF (X = 1, 5).
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Figure 7. The cation vacancy activated the surface lattice oxygen and promoted the oxidation of formaldehyde.
Figure 7. The cation vacancy activated the surface lattice oxygen and promoted the oxidation of formaldehyde.
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Table
Table 1. Distribution of element content on catalyst surface, obtained via EDS surface scanning.
Table 1. Distribution of element content on catalyst surface, obtained via EDS surface scanning.
wt%LDO/NFE1-LDO/NFE5-LDO/NF
Co191718
Al6.44
C171919
O434443
Ni151616
Table
Table 2. The surface compositions of the different samples.
Table 2. The surface compositions of the different samples.
LDOE1-LDOE5-LDOLDO/NFE1-DO/NFE5-LDO/NF
Co2+/Co3+97.0394105.9704122.922100.0167110.9602120.4848
Oads/Olatt90.3387593.54537104.552845.57956125.4108127.7475
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Chen, Y.; Zhao, S.; Gao, F.; Yu, Q.; Zhou, Y.; Tang, X.; Yi, H. The Promoting Effect of Metal Vacancy on CoAl Hydrotalcite-Derived Oxides for the Catalytic Oxidation of Formaldehyde. Processes 2023, 11, 2154. https://doi.org/10.3390/pr11072154

AMA Style

Chen Y, Zhao S, Gao F, Yu Q, Zhou Y, Tang X, Yi H. The Promoting Effect of Metal Vacancy on CoAl Hydrotalcite-Derived Oxides for the Catalytic Oxidation of Formaldehyde. Processes. 2023; 11(7):2154. https://doi.org/10.3390/pr11072154

Chicago/Turabian Style

Chen, Yimeng, Shunzheng Zhao, Fengyu Gao, Qingjun Yu, Yuansong Zhou, Xiaolong Tang, and Honghong Yi. 2023. "The Promoting Effect of Metal Vacancy on CoAl Hydrotalcite-Derived Oxides for the Catalytic Oxidation of Formaldehyde" Processes 11, no. 7: 2154. https://doi.org/10.3390/pr11072154

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