1. Introduction
The consumption of considerable quantities of fossil fuels in China has resulted in significant annual NO
x emissions. Many measures have been adopted to reduce the amount of NO
x (NO, NO
2, N
2O, N
2O
5, etc.) released into the atmosphere; however, the proportion of this gas in the air remains high. Developing technologies capable of NO
x removal, even at the parts per billion (ppb) level, is an important objective because NO
x is highly toxic, very mobile, and reactive [
1], [
2]. In this context, photocatalysis is regarded as a promising technology for NO
x removal given its green, effective, and sustainable features [
3]. Many studies have shown that photocatalysis readily oxidizes NO
x to nitrate, which is then transferred to the water phase for further treatment [
4].
Catalysts are the core of photocatalytic technology [
5], [
6], [
7], [
8]. The catalyst surface absorbs light and converts it into energetic charge carriers that facilitate the direct oxidation of NO
x or trigger the formation of more oxidative free radicals, such as O
2。− and OH, which oxidize NO
x [
9], [
10]. Layered double hydroxides (LDHs) are a large family of two-dimensional inorganic layered host materials with the general formula [M
1− x 2+M
x 3+ (OH)
2]
z +(A
n −)
z / n ·
mH
2O (
x,
n,
z, and
m were constants), where M and A correspond to cations and interlayer anions, respectively. LDHs are effective photocatalysts owing to their layered structures, tunable chemical and band structures, and abundant surface OH
− moieties [
11], [
12], [
13]. Many research groups have shown that the OH
− on an LDH can be converted into oxidative
。OH radicals when trapped by photoexcited holes [
14], [
15], [
16]. This feature enables LDHs to deliver robust and steady photocatalytic performance, even in a water-deficient environment. Consequently, LDHs are considered ideal photocatalysts for abating air pollution. Despite these advantages, however, LDH photocatalysts generally demonstrate poor performance under visible-light illumination, possibly because of their low energetic charge-carrier efficiency and the limited exposure of their surface active sites [
12], [
14].
Vacancy engineering is an effective method for activating or promoting the performance of a photocatalyst, and vacancy introduction has been documented to improve the band structure of photocatalysts. Vacancies serve as trapping centers for charge carriers, promoting light absorbance and charge-carrier separation [
17]. For example, Miao et al. [
18] developed a polymeric carbon nitride (PCN) photocatalyst rich in three coordinate nitrogen (N
3C) vacancies that demonstrated enhanced visible-light absorption and photoexcited charge-carrier separation. The defective PCN produced H
2O
2 4.5 times more rapidly than pristine PCN. In addition, the vacancies in some systems are actual active sites that efficiently adsorb and activate reactants. For instance, Wu et al. [
19] recently reported significant enhancements in the oxygen evolution reaction (OER) on oxygen- and cation-vacancy-bearing NiFe-LDHs. Both these groups revealed that the observed enhancements in activity originated from the excellent adsorption of OER intermediates at vacancies. Many research groups have recently reported various strategies for constructing cationic, anionic, or dual vacancies on LDHs, and successfully developed photocatalytic systems for various reactions, including nitrogen reduction [
20], CO
2 reduction [
21], formaldehyde decomposition [
22], and NO
x oxidation [
10], [
23], [
24], [
25]. However, the construction of vacancies, particularly cationic/anionic dual vacancies, on an LDH usually involves a complex or time/energy-intensive procedure, which hinders the scaled use of LDHs in photocatalytic applications. To date, an efficient and cost-effective method for constructing dual vacancies on LDHs demonstrating excellent performance for the photocatalytic removal of NO
x has not yet been reported.
Herein, we report the development of a facile N2H4-driven etching strategy to introduce Ni2+ and OH− dual vacancies into NiFe-LDH under mild conditions. The etching process was completed within 10 min, and the number of vacancies could be tuned by adjusting the N2H4 concentration. We investigated the optical properties and charge separation and transfer efficiencies of the defective NiFe-LDHs, and assessed how defects affected their photocatalytic performance for the oxidative removal of NO under visible-light illumination. The reactive oxygen species (ROS) generated during photocatalysis and reaction pathway for NO oxidation were also examined. Finally, the roles played by the dual vacancies in the NiFe-LDHs were discussed based on a combined spectroscopic and density functional theory (DFT) study.
2. Experimental section
2.1. Materials
All chemicals used in this study were of analytical grade. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and formamide were purchased from Chengdu Kelong Chemical Co., Ltd., China. Ferric nitrate nonahydrate (Fe(NO3)2·9H2O), terephthalic acid (TA), and ethanol were purchased from Shanghai Titan Scientific Co., Ltd., China. Hydrazine hydrate (N2H4·H2O), silver nitrate (AgNO3), and acetonitrile were provided by Shanghai Aladdin Bio-Chem Technology Co., Ltd., China.
2.2. Photocatalyst synthesis
NiFe-LDH was prepared following a previously reported coprecipitation method [
17]. In a typical procedure, Ni(NO
3)
2·6H
2O (7.5 mmol) and Fe(NO
3)
3·9H
2O (2.5 mmol) were dissolved in a mixture of deionized water (20 mL) and formamide solution (20 mL) with magnetic stirring for 10 min. The mixture was then heated to 80 °C, and the pH of the solution was adjusted to about 10 using 2.5 mol·L
−1 aqueous NaOH. After 10 min, NiFe-LDH was collected by centrifugation, washed with deionized water, and then freeze-dried for 12 h. NiFe-LDH (∼0.6 g) and a specific amount of N
2H
4·H
2O were dispersed in deionized water (30 mL) to construct vacancies on the LDH. The mixture was then heated to 70 °C and maintained at this temperature for 10 min. The product was subsequently collected by centrifugation, washed with water/ethanol, and freeze-dried for 12 h. The N
2H
4 concentration was adjusted to 0.96, 1.50, and 2.64 mmol·L
−1 to gain insights into the effects of the N
2H
4 dose on the number of vacancies formed on the LDH.
2.3. Characterization
The crystal phase of the catalysts was examined by X-ray diffractometry (XRD) with Cu Kα radiation (D/max RA, Rigaku Co., Japan). The metal content in the catalysts was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES; Agilent 5110, Agilent Technologies, USA). Scanning transmission electron microscopy (TEM) images were collected using a JEM-2010 microscope (JEOL, Japan), and photoluminescence (PL) spectra were obtained on an F-7000 spectrophotometer (HITACHI, Japan). The elemental electronic states at the catalyst surface were examined by X-ray photoelectron spectroscopy (XPS; Thermo ESCALAB 250, Thermo Fisher Scientific, USA). Raman spectra was measured on Thermo Scientific DXR (Thermo Fisher Scientific). The surface structure of the material was obtained by atomic force microscopy (AFM; Bruker Dimension ICON, Bruker, Germany). Ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy (DRS) was performed on a UV-2450 spectrometer (Shimadzu, Japan). Photocurrent testing and electrochemical impedance spectroscopy (EIS) were conducted using a CHI 660E electrochemical workstation (Chenhua, China) [
26]. Electron paramagnetic resonance (EPR) and electron spin resonance (ESR) spectra were acquired on an EMX PLUS spectrometer (Bruker).
In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was performed on a Vertex 70v spectrometer (Bruker) [
27]. NO temperature-programmed-desorption-mass spectrometry (TPD-MS) was conducted using an AutoChem II 2920 instrument (Micromeritics, USA) [
28].
2.4. Photocatalytic performance evaluation
The photocatalytic performance of various LDH samples for NO removal was evaluated under visible light in a sealed rectangular reactor (30 cm × 15 cm × 10 cm); a schematic of the apparatus is shown in Fig. S1 in Appendix A. The catalyst (0.2 g) was dispersed in ethanol (10 mL) to form a catalyst suspension prior to testing. This suspension was then evenly coated onto the surfaces of two culture dishes measuring 12 cm in diameter. The catalyst-loaded dishes were placed in the reactor, sealed, and then charged with flowing air containing about 500 ppb NO at a relative humidity of 45%-55%. The catalyst was illuminated with visible light using a 150 W commercial tungsten halogen lamp; here, the ultraviolet region was filtered out using an ultraviolet cutoff filter (λ ≥ 420 nm). The concentrations of NO and NO2 in the outlet flow were tracked using a gas analyzer (42i-TL; Thermo Fisher Scientific). The NO removal ratio (η) of each catalyst was calculated using the formula η = (1 − Ct/C 0) × 100%, where Ct and C 0 are the concentrations of NO at reaction times t and 0, respectively.
3. Results and discussion
3.1. Photocatalyst synthesis and characterization
Fig. 1(a) shows the N
2H
4-driven etching strategy for producing defective NiFe-LDH (hereafter referred to as NiFe-LDH-et). In this strategy, N
2H
4 molecules attack the [Ni
2+(OH)
6] moieties in NiFe-LDH to form soluble Ni(N
2H
4)
x 2+ species through displacement chemistry, leaching away Ni
2+ and OH
− to form dual vacancies in the LDH. The XRD patterns in
Fig. 1(b) reveal that both pristine and defective Ni
76Fe
24-LDH possess the same hydrotalcite structure (Powder diffraction file (PDF): 40-0215), which implies that N
2H
4 etching does not destroy the LDH crystal structure. The photograph and TEM images in
Fig. 1(c) and the AFM image in Appendix A, Fig. S2 show that pristine Ni
76Fe
24-LDH is light brown and has a nanosheet morphology that is approximately 2.04-2.98 nm thick, with a lateral dimension of 20-30 nm. Furthermore, Ni, Fe, and O are distributed evenly over the LDH sheet (
Fig. 1(c)). By comparison, NiFe-LDH-et exhibits the same nanosheet morphology as pristine Ni
76Fe
24-LDH but is dark brown (
Fig. 1(d)). In addition, a considerable number of nanopores (highlighted by yellow arrows) are observed in the sheet, indicating vacancy formation. The amounts of metals leached from Ni
76Fe
24-LDH and the metal content in Ni
76Fe
24-LDH-et were examined using ICP-AES.
Fig. 1(e) reveals that 55.9 mg·g
−1 of Ni
2+ and 0.50 mg·g
−1 of Fe
3+ are leached from Ni
76Fe
24-LDH after treatment with 1.50 mmol·L
−1 aqueous N
2H
4.
Fig. 1(f) shows that the Ni/(Ni + Fe) molar ratio decreases during etching from the original value of 76% observed for the pristine LDH to 72% in the etched LDH. These results demonstrate that N
2H
4 primarily etches away [Ni
2+(OH)
x ] moieties (0 <
x ≤ 6) rather than [Fe
3+(OH)
x ] moieties.
The defective structure of Ni
76Fe
24-LDH-et was further investigated using a combination of Raman and EPR spectroscopy, along with XPS. The Raman spectra in
Fig. 2(a) show characteristic bands at 458, 526, and 705 cm
−1, which are ascribed to the Fe/Ni-OH bond vibrations in Ni
76Fe
24-LDH; these bands are weakened during etching with N
2H
4 [
29], [
30], [
31]. In the spectrum of Ni
76Fe
24-LDH-et, a new band that could be assigned to OH
− vibrations in close proximity to cationic vacancies is observed at approximately 590 cm
−1 [
32], [
33]. These results reveal that N
2H
4 etching induces the formation of cationic vacancies.
EPR spectroscopy and XPS were performed to provide evidence for the formation of OH
− vacancies (OH
v). Both Ni
76Fe
24-LDH-et and Ni
76Fe
24-LDH display characteristic Lorentzian OH
v signals at
g = 2.0048 in their EPR spectra (
Fig. 2(b)). The greater intensity of the signal observed for Ni
76Fe
24-LDH-et than that for Ni
76Fe
24-LDH suggests a higher number of OH
v moieties following N
2H
4 etching, which is also supported by XPS. The O 1s spectra shown in
Fig. 2(c) reveal that Ni
76Fe
24-LDH-et has an OH
v/lattice-OH
− molar ratio (i.e., O
v/O
L, O
V: assigned to OH
− vacancy, O
L: assigned to OH
−) of 3.54, which is considerably higher than that determined for Ni
76Fe
24-LDH (2.12) [
34], [
35], [
36].
The schematic in
Fig. 2(d) shows that the structural evolution of the LDH during N
2H
4 etching can form OH
v sites that lie between Ni
2+ and Ni
2+ (Site I) or between Ni
2+ and Fe
3+ (Site I). Accordingly, we analyzed the Ni 2p and Fe 2p XPS profiles of Ni
76Fe
24-LDH
-et to determine the OH
v position, the results of which are shown in Appendix A, Fig. S3. The figure reveals that the Ni
2+ in Ni
76Fe
24-LDH
-et maintains its +2 valence, whereas Fe
3+ becomes electron-rich relative to that in Ni
76Fe
24-LDH. This enrichment is ascribable to the loss of OH
−, which provides additional electrons to the coordination-unsaturated Fe
3+. These results imply that OH
v is mainly formed at Site I.
Gibbs free energy changes (Δ
G) associated with the formation of Ni
v and OH
v at Sites I and II were calculated using DFT to rationalize the aforementioned results; the details of these calculations are provided in Supplementary Text S1 in Appendix A. The removal of Ni
2+ from the LDH appears to be a spontaneous reaction, with a Δ
G of −1.37 eV (1 eV = 1.602 × 10
−19 J,
Fig. 2(d)), while the subsequent removal of OH
− at both sites requires additional energy. OH
v is more favorably formed at Site I than at Site II from a thermodynamic perspective (4.14 vs 4.37 eV), which is consistent with the XPS data showing that OH
− positioned between Ni
2+ and Fe
3+ is more easily removed than that positioned between Ni
2+ and Ni
2+.
Given that N2H4 effectively forms dual vacancies, Ni76Fe24-LDH was treated with solutions containing different concentrations of N2H4 (i.e., 0.96, 1.50, or 2.64 mmol·L−1) to afford Ni76Fe24-LDH-et-x (x = 0.96, 1.50, or 2.64). The photographs in Fig. S4 in Appendix A show that the color of NiFe-LDH-et darkens with increasing N2H4 concentration. The TEM images in Fig. S4 reveal that the LDH sheets contain fewer pores when 0.96 mmol·L−1 N2H4 is used and are nearly fragmented following treatment with 2.64 mmol·L−1 N2H4. The XRD patterns in Appendix A, Fig. S5 confirm that the LDH phase is conserved in all three systems. ICP-AES data (Appendix A, Table S1) show that increasing amounts of Ni2+ (from 6.14 to 55.9 and 88.1 mg·g−1) are leached from the LDH as the N2H4 concentration is increased from 0.96 to 1.50 and 2.64 mmol·L−1; as expected, only small amounts of Fe are lost (0.04-0.67 mg·g−1) under the same conditions.
3.2. Photocatalytic performance for the oxidative removal of NO
The photocatalytic performance of Ni
76Fe
24-LDH-et-
x and Ni
76Fe
24-LDH for the oxidative removal of NO was evaluated in terms of their ability to remove NO from continuously flowing air (2.4 L·min
−1, 45%-55% humidity) containing about 500 ppb NO under visible-light irradiation; a schematic of the testing procedure is provided in
Fig. 3(a).
Fig. 3(b) shows the ratio of the outlet NO concentration (
C) to the feed concentration (
C 0) as a function of the reaction time for the various catalysts. Pristine Ni
76Fe
24-LDH is inert when irradiated with visible light, as
C similar with
C 0. By contrast, all Ni
76Fe
24-LDH
-et
-x samples exhibit catalytic activity under visible light, as
C far less than
C 0 for all three systems. The NO removal efficiency of the samples was calculated from the
C/
C 0 values presented in
Fig. 3(b) using the formula (1 −
C/
C 0) × 100%; the results are shown in
Fig. 3(c). Among the samples, Ni
76Fe
24-LDH-et-1.50 clearly delivers the highest NO removal efficiency (32.8%), followed by Ni
76Fe
24-LDH-et-0.96 and Ni
76Fe
24-LDH-et-2.64. Notably,
Fig. 3(d) reveals that none of the Ni
76Fe
24-LDH-et catalysts produce notable amounts of NO
2, which is the most toxic molecule in the NO
x family and has an atmospheric threshold concentration of 40 ppb (Level I in GB 3095-2012) [
37]; these low NO
2 yields suggest that Ni
76Fe
24-LDH-et can be used in very reliable and green photocatalytic NO cleanup procedures [
38]. The above results show that while defects confer NiFe-LDH with visible-light photocatalytic activity, an appropriate defect content is required to maximize its photocatalytic performance.
Table S2 in Appendix A shows that Ni
76Fe
24-LDH-et-1.50 outperforms most LDH catalysts reported in the literature in terms of NO removal efficiency under closed-reaction conditions. As an important catalyst descriptor for practical applications, the durability of Ni
76Fe
24-LDH-et-1.50 for photocatalytic NO removal was evaluated through repeated testing under the same reaction conditions.
Figs. 3(e) and (f) show
C/
C 0 profiles that are similar in shape over five cycles of testing, with low NO
2 yields. Furthermore, the XRD data presented in Fig. S6 in Appendix A show that the LDH crystal structure of Ni
76Fe
24-LDH-et-1.50 is preserved during repeated testing. These results collectively imply that Ni
76Fe
24-LDH-et-1.50 exhibits high long-term photocatalytic performance.
3.3. Origin of the enhanced photocatalytic activity of NiFe-LDH-et
The photocatalyst activity is generally associated with its ability to absorb light and separate/transfer charges, and the number of reactive centers and their activities [
39]. The light-absorption efficiencies of Ni
76Fe
24-LDH and Ni
76Fe
24-LDH-et-
x were first examined using UV-vis DRS. The spectra presented in
Fig. 4(a) reveal that Ni
76Fe
24-LDH and Ni
76Fe
24-LDH-et-
x absorb substantial amounts of visible light in the 400-800 nm wavelength range. However, the NiFe-LDH-et-
x absorption edge gradually redshifts with increasing
x, significantly enhancing light adsorption in the 500-800 nm wavelength region. These results indicate that the introduction of vacancies into the LDH improves its ability to absorb visible light. Because NiFe-LDH-et-2.64 performs most efficiently in the visible-light region but does not deliver peak NO removal efficiency, we conclude that improved light adsorption is not principally responsible for the superior photocatalytic performance of NiFe-LDH-et.
The charge-separation efficiencies of Ni
76Fe
24-LDH and Ni
76Fe
24-LDH
-et-
x were investigated using PL spectroscopy. The PL spectra in
Fig. 4(b) reveal that all Ni
76Fe
24-LDH-et-
x samples exhibit lower PL intensities than Ni
76Fe
24-LDH owing to their enhanced charge separation, which is ascribable to the presence of vacancies in the LDH [
3], [
40]. Furthermore, the PL intensity of the LDH exhibits a volcano-like relationship with
x. Among the etched samples, Ni
76Fe
24-LDH-et-1.50 displays the lowest PL intensity and, consequently, the highest photoexcited charge-carrier separation efficiency. The charge-transfer kinetics of Ni
76Fe
24-LDH and Ni
76Fe
24-LDH-et-
x were evaluated by transient photocurrent response spectroscopy and EIS [
41], [
42].
Fig. 4(c) shows that Ni
76Fe
24-LDH-et-1.50 delivers the highest photocurrents, followed by Ni
76Fe
24-LDH
-et-2.64, Ni
76Fe
24-LDH
-et-0.96, and Ni
76Fe
24-LDH. Hence, the charge-transfer rate follows the order Ni
76Fe
24-LDH-et-1.50 > Ni
76Fe
24-LDH
-et
-2.64 > Ni
76Fe
24-LDH-et-0.96 > Ni
76Fe
24-LDH; the same order was also determined by EIS.
Fig. 4(d) shows that Ni
76Fe
24-LDH-et-1.50 exhibits the smallest EIS semicircle, followed by Ni
76Fe
24-LDH-et-2.64, Ni
76Fe
24-LDH-et-0.96, and Ni
76Fe
24-LDH. Taken together, the data reveal that both the charge-separation and -transfer efficiencies of the catalysts follow the order Ni
76Fe
24-LDH-et-1.50 > Ni
76Fe
24-LDH-et-0.96 > Ni
76Fe
24-LDH-et-2.64 > Ni
76Fe
24-LDH. This order is almost consistent with that observed for the photocatalytic NO removal efficiency of the catalysts (except for the Ni
76Fe
24-LDH-et-2.64 and Ni
76Fe
24-LDH-et-0.96), suggesting that enhanced charge separation and transfer in defective LDH is the critical factor but not the sole one responsible for its excellent photocatalytic performance. Given that Ni
76Fe
24-LDH-et-2.64 contains the largest number of vacancies among the etched samples but does not exhibit optimal charge-carrier separation and transfer, we infer that an excessive number of vacancies is detrimental to its performance [
43]. Cao et al. [
44] also reported similar observations.
OH
v sites have been documented to play critical roles in various catalytic systems [
45], with studies demonstrating that OH
v-exposed Lewis acidic sites strongly adsorb and activate reactants. Fe
3+ and Ni
2+, which potentially serve as reactive centers that adsorb both NO and O
2, are exposed in Ni
76Fe
24-LDH-et-
x owing to the formation of OH
v. To confirm this speculation, we examined the adsorption of NO on Ni
76Fe
24-LDH and Ni
76Fe
24-LDH-et
-1.50 using TPD-MS; we also compared the adsorption energies of NO and O
2 on defective LDH using DFT calculations.
Fig. 5(a) reveals that Ni
76Fe
24-LDH-et-1.50 exhibits a considerably larger desorption peak in the 450-700 °C temperature range than Ni
76Fe
24-LDH; MS confirms that this peak corresponds to NO desorption, suggesting that NO is more efficiently adsorbed by defective LDH than by pristine LDH. The DFT results displayed in
Fig. 5(b) reveal that the OH
v site has a greater affinity for both NO and O
2 than the Ni
v site. Furthermore, the OH
v in Site I, which exposes Fe
3+ and Ni
2+, binds more strongly with NO and O
2 than the OH
v in Site II, which only exposes Ni
2+. Such results suggest that the exposed Fe
3+ can promote NO and O
2 adsorption, which was similar with the observations reported by Ref. [
46].
Electron-hole pairs are formed in a photocatalyst under light irradiation; these species can directly oxidize pollutants or trigger the formation of ROS, such as
。OH or O
2。−, which mineralize pollutants [
47]. We first used ESR spectroscopy to identify the ROS formed on Ni
76Fe
24-LDH and Ni
76Fe
24-LDH-et-1.50, the results of which are shown in
Fig. 5(c). Both Ni
76Fe
24-LDH and Ni
76Fe
24-LDH-et-1.05 produce O
2。− via the e
− + O
2 →O
2。− reaction under visible light; however, the latter produces more O
2。− than the former.
Fig. 5(d) shows that only Ni
76Fe
24-LDH-et-1.50 produces
。OH in aqueous solution via the known photogenerated hole h
+ + H
2O →
。OH + H
+ reaction under visible light. These EPR results reveal that a larger number of electron-hole pairs are generated in Ni
76Fe
24-LDH-et-1.50 than in the unetched sample. More importantly, the holes formed in Ni
76Fe
24-LDH-et-1.50 are sufficiently oxidative to transform H
2O into
。OH. The OH
− in LDH reportedly replaces H
2O as the
。OH precursor; consequently, LDH-based photocatalysts usually perform well in an H
2O-deficient atmospheric environment. To confirm whether the OH
− in Ni
76Fe
24-LDH-et-1.50 is available for
。OH production, we illuminated the photocatalyst in an H
2O-free acetonitrile solution containing AgNO
3 and TA to trap photoexcited electrons and the generated
。OH (TA reacts with
。OH to form TAOH), respectively.
Fig. 5(e) displays the AgNO
3-content-dependent fluorescence spectra of the solution following visible-light illumination for 0.5 h. The addition of AgNO
3 and TA leads to two characteristic fluorescence peaks at 410 and 431 nm, which are assigned to TAOH. Furthermore, these peaks become more intense as the AgNO
3 content is increased. The results confirm that the OH
− in Ni
76Fe
24-LDH-et-1.50 is available for
。OH production. This feature is significant because it contributes to photocatalytic NO oxidation even in an H
2O-deficient gaseous atmosphere. Taken together, the above results demonstrate that ROS species that photocatalytically oxidize NO, including O
2。− and
。OH, are generated from Ni
76Fe
24-LDH-et-1.50.
Next, we examined the evolution of intermediate products during the NO oxidation reaction over Ni
76Fe
24-LDH and Ni
76Fe
24-LDH-et-1.50 by
in situ DRIFTS to gain further insights into the photocatalytic process on defective LDH. The spectra presented in
Figs. 6(a) and (b) show that NO is more intensely adsorbed by Ni
76Fe
24-LDH-et than by Ni
76Fe
24-LDH. More intriguingly, NO molecules are primarily adsorbed via hydrogen bonding to the OH
− moieties on Ni
76Fe
24-LDH, whereas they are adsorbed at Lewis acidic sites (i.e., Fe
3+ and Ni
2+ at OH
v sites) on Ni
76Fe
24-LDH-et-1.50. These findings are consistent with the TPD-MS and DFT results shown in
Fig. 5. Importantly, the NO adsorbed at Lewis acidic sites is converted into NO
+, which is reportedly oxidized to NO
3 − without the production of NO
2 or N
2O as intermediates [
34], [
48]. Therefore, we believe that the Lewis acidic sites help alleviate NO
2 production on defective LDH.
Figs. 6(a) and (b) also show that while both NO
3 − and NO
2 species are formed under light illumination, only NO
3 − accumulates with increasing reaction time. In addition, Ni
76Fe
24-LDH-et-1.50 exhibits a higher NO
3 − yield than Ni
76Fe
24-LDH, consistent with its higher photocatalytic activity for NO oxidation.
A clearer picture of the origin of the enhanced photocatalytic ability of defective NiFe-LDH toward NO oxidation is provided by the discussion above.
Fig. 6(c) shows that the N
2H
4-driven etching process triggers the formation of Ni
v and OH
v on the LDH; these dual vacancies promote both light adsorption and charge-carrier separation, leading to the enhanced production of ROS, including O
2。− and
。OH. Notably, the OH
− on LDH is an effective replacement for H
2O during the production of
。OH, which endows the LDH with stable photocatalytic performance in H
2O-deficient atmospheric environments. Lewis acidic sites, particularly exposed Fe
3+ at OH
v sites, play important roles in the photocatalytic NO oxidation process; these sites have a strong affinity for NO and contribute to the accumulation of NO on the LDH. More importantly, the adsorbed NO is converted into NO
+, which is subsequently oxidized to NO
3 − without the notable production of NO
2, thereby alleviating any risks associated with its production and emission.
4. Conclusions
We developed a facile N2H4-driven etching approach to prepare dual Niv- and OHv-containing NiFe-LDH-et photocatalysts. In contrast to the inertness of pristine LDH, NiFe-LDH-et actively removed NO under visible-light illumination. Ni76Fe24-LDH-et etched with 1.50 mmol·L−1 N2H4 solution removed 32.8% of the NO in continuously flowing air under visible light. The mechanistic study revealed that the introduction of dual vacancies improved charge separation and transfer within the LDH, leading to the intensified production of ROS (O2。− and 。OH); these vacancies also triggered the formation of Lewis acidic sites (Fe3+ and Ni2+ exposed at OHv), which have high NO and O2 affinities. In situ spectroscopic studies verified that NO was preferably adsorbed at Lewis acidic sites, particularly exposed Fe3+. Notably, NO transformed into NO+, which was directly oxidized to NO3 − without the notable production of the more toxic intermediate NO2. This study provides new perspectives for the development of efficient and durable visible-light-responsive LDH photocatalysts for the abatement of air pollution.
Acknowledgment
This study was financially supported by the Natural Science Foundation of Chongqing Science & Technology Commission (CSTB2023NSCQ-LZX0020), the National Natural Science Foundation of China (51978110), and the Special Project for Performance Incentive and Guidance of Research Institutions in Chongqing (CSTB2023JXJL-YFX0030).
Compliance with ethics guidelines
Xiaoyu Li, Xiaoshu Lv, Jian Pan, Peng Chen, Huihui Peng, Yan Jiang, Haifeng Gong, Guangming Jiang, and Li’an Hou declare that they have no conflicts of interest or financial conflicts to disclose.
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