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Article

Visible-Light-Driven Selective Oxidation of Toluene to Benzaldehyde over CeO2@NiFe-LDH Heterostructure

by
Fang Fang
*,
Dongping Sun
and
Xinhua Peng
*
School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
*
Authors to whom correspondence should be addressed.
Catalysts 2026, 16(9), 757; https://doi.org/10.3390/catal16090757
Submission received: 14 July 2026 / Revised: 14 August 2026 / Accepted: 18 August 2026 / Published: 23 August 2026
(This article belongs to the Section Catalysis for Sustainable Energy)
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Abstract

The transformation of toluene to benzaldehyde via green and sustainable routes is of great significance in the fine chemical industry. However, hard activation of benzylic C(sp3)-H bonds and facile overoxidation of the generated benzaldehyde collectively render the selective oxidation of toluene extremely challenging. In this study, we constructed a core–shell heterostructure photocatalyst, CeO2@NiFe-LDH, employing molecular oxygen as the oxidant. Under mild conditions of room temperature and visible-light illumination, the catalyst achieves a toluene conversion rate of 1.936 mmol·g−1·h−1 with an excellent benzaldehyde selectivity of 81.0%, and its catalytic performance is significantly superior to that of the individual single-phase materials and the simple physical mixture. Optical and electrochemical measurements confirm enhanced visible-light absorption and utilization, as well as greatly improved separation and migration efficiency of photogenerated charge carriers. Furthermore, the CeO2@NiFe-LDH heterostructure features staggered band alignment, promoting S-scheme charge transfer across the heterointerface, thereby substantially boosting the redox capacity of the composite catalyst. Consequently, the photogenerated carriers with high reactivity are fully engaged in catalytic reactions, enabling efficient carrier utilization and ultimately leading to a significantly enhanced photocatalytic performance. This study not only demonstrates the outstanding application potential of CeO2@NiFe-LDH for the visible-light-driven selective oxidation of toluene to benzaldehyde, but also offers a novel strategy for enhancing the photocatalytic performance of LDH-based materials.

Graphical Abstract

1. Introduction

Selective oxidation of toluene to benzaldehyde represents a crucial synthetic route for producing high-value-added oxygenated fine chemicals and organic synthesis intermediates, which has been widely applied in industrial fields such as fragrances, pharmaceuticals, agrochemicals, and functional materials [1,2]. However, the benzylic C(sp3)-H bond of toluene possesses high bond dissociation energy, rendering its activation difficult. Moreover, benzaldehyde exhibits significantly higher oxidation reactivity than toluene, making it highly susceptible to consecutive over-oxidation to deep oxidation products such as benzoic acid, which imposes a severe challenge on selectivity control [3,4].
Traditional thermal catalytic selective oxidation of toluene to benzaldehyde generally requires harsh reaction conditions. For the solvent-free thermal oxidation system catalyzed by CoCrOx, a high-pressure oxygen atmosphere is required to promote toluene conversion [5]. Gas-phase selective oxidation of toluene with air represents a promising route for producing high-purity benzaldehyde. For example, thermal gas-phase oxidation over VMoNb/CeO2 demands an elevated temperature of 500 °C [6]; likewise, the V-Ag-Ce/TiO2 catalytic system also operates at high temperatures [7]. In contrast, green liquid-phase selective oxidation processes have been extensively investigated owing to their mild reaction conditions, environmental friendliness, and chlorine-free products. Considerable research efforts have been devoted to various transition-metal-based catalytic materials, including spinel-type CuCr2O4 nanocatalysts [8], manganese tungstate (MnWO4) nanorods [9], and Ag/WO3 nanostructured materials [10]; all of them exhibit excellent catalytic performance in the liquid-phase selective oxidation of toluene to benzaldehyde. However, these systems commonly employ hydrogen peroxide as the oxidant, which suffers from relatively high cost and insufficient stability, thereby imposing practical application limitations.
In contrast to conventional thermal catalysis, which demands harsh reaction conditions and costly oxidants, solar-driven photocatalytic technology offers a mild and green alternative route for the selective oxidation of toluene. Notably, visible-light-driven systems, which account for approximately 43% of the solar spectrum, exhibit great energy-saving potential with promising industrial application prospects [11,12]. Studies on photocatalysts for the selective oxidation of toluene chiefly center on narrow-bandgap semiconductors [13], doped semiconductors [14,15], single-atom catalysts [16] and heterostructure catalysts [17]. As a universal modification method to elevate photocatalytic activity, heterostructure fabrication ranks among the most effective strategies [18]. When two semiconductors with well-matched band structures are coupled, a built-in electric field forms at the heterostructure interface. This field drives photogenerated electrons and holes to migrate in opposite directions, thereby achieving spatial charge separation and extending carrier lifetimes. Consequently, more active sites are available for surface redox reactions.
Layered double hydroxides (LDHs) are high-performance semiconductor photocatalysts with bandgap energies ranging from 1.5 to 3.0 eV [19]. However, pristine LDHs suffer from several intrinsic drawbacks that severely limit their photocatalytic efficiency, including weak visible-light response due to wide bandgaps, rapid recombination of photogenerated charge carriers, low carrier mobility, poor electrical conductivity, limited specific surface area, insufficient exposure of active sites, and unsatisfactory cycling stability [20]. Constructing heterostructures by coupling LDHs with other functional semiconductors has thus become an important strategy to overcome these issues and enhance their photocatalytic performance. Furthermore, owing to the compositional tunability of LDHs, the introduction of specific metal elements such as Cr, Co, Ni, Cu, Ti, and Fe into the layers or interlayers can narrow the bandgap, facilitating the excitation of valence electrons to the conduction band under light irradiation and thus achieving more efficient production of electron-hole pairs [20,21]. As an inexpensive and earth-abundant rare-earth material, cerium dioxide(CeO2) features superior redox capacity and plentiful oxygen vacancies owing to the reversible Ce3+/Ce4+ redox cycle [22]. More importantly, as a typical n-type semiconductor, CeO2 has been found to possess a band structure that is particularly suitable for constructing Z-scheme heterostructures [23].
Based on the above analysis, we designed and prepared a CeO2@NiFe-LDH heterostructure photocatalyst. The morphology and structure of the as-synthesized materials were characterized by various techniques including XRD, FT-IR, SEM, TEM, and XPS. Their optical and electrochemical properties were investigated by UV-vis DRS, photoluminescence (PL) spectroscopy, transient photocurrent response, Mott-Schottky measurements, and ultraviolet photoelectron spectroscopy (UPS). Using molecular oxygen as a green oxidant, the photocatalytic performance of the catalyst for the selective oxidation of toluene to benzaldehyde was systematically evaluated under visible-light irradiation at room temperature, and its structure-activity relationship was explored. Meanwhile, the substrate scope and reusability of the catalyst were also examined. To further reveal the reaction mechanism, the band structure and charge transfer pathway were analyzed, and the reactive oxygen species generated during the photoreaction were identified by radical trapping experiments combined with electron paramagnetic resonance (EPR) spin-trapping technique. This work aims to develop an efficient photocatalytic system for the selective oxidation of toluene, expand the application of LDH-based materials in solar-driven organic synthesis, and provide new insights and experimental foundations for the performance optimization of LDH photocatalysts.

2. Results and Discussion

2.1. Morphological and Structural Characterizations of CeO2@NiFe-LDH Heterostructure

As displayed in Figure 1, through X-ray powder diffraction (XRD) analysis, the composition and structure of CeO2, NiFe-LDH, and the CeO2@NiFe-LDH heterostructure were explored. The characteristic diffraction peaks at 2θ = 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6 and 79.0 were corresponded to (111), (200), (220), (311), (222), (400), (331) and (420) planes of CeO2 (JCPDS No. 34-0394), respectively. The diffraction peaks at 2θ = 11.5, 23.3, 34.5, 39.0, 46.4, 60.0 and 61.3 were attributed to (003), (006), (012), (015), (018), (110) and (113) planes of NiFe-LDH (JCPDS No. 40-0215) [24]. The XRD pattern of CeO2@NiFe-LDH retains the characteristic diffraction peaks of both NiFe-LDH and CeO2. Moreover, compared with pristine CeO2 and NiFe-LDH, no new diffraction peaks appear in the composite material, which demonstrates the successful fabrication of the nanocomposite structure.
The FT-IR spectra of CeO2, NiFe-LDH, and CeO2@NiFe-LDH heterostructure were shown in Figure 2. The as-synthesized pristine NiFe-LDH displays an obvious wide and high-intensity absorption band spanning 3440–3460 cm−1, which stems from stretching vibrations of O-H bonds. Such hydroxyl sources cover two parts: hydroxyl groups exposed on the outermost surface of layered NiFe-LDH and crystalline water trapped between adjacent LDH layers. A weak-to-medium absorption signal appearing at approximately 1640 cm−1 matches the bending vibration mode of H-O-H bonds from interlayer water. In the wavenumber range of 1350–1360 cm−1, the detectable sharp absorption band serves as the diagnostic peak for interlayer carbonate anions (CO32−). In the low-wavenumber region of 500–1000 cm−1, a series of characteristic peaks can be ascribed to lattice vibrations generated by metal-oxygen (M-O) bonds in the layered skeleton of NiFe-LDH. For pure CeO2, the broad peak at around 3450 cm−1 and the peak at 1630 cm−1 were attributed to the O-H stretching vibrations of adsorbed water molecules. The peak at approximately 1070 cm−1 could be attributed to the C-OH stretching vibration. The peak below 1000 cm−1 could be coincident with the stretching vibration of Ce-O-Ce bond, which is the characteristic peak of CeO2 [25]. All the characteristic peaks of CeO2 and NiFe-LDH were observed in the resulting CeO2@NiFe-LDH composites. In addition, the peak intensity of CeO2@NiFe-LDH at around 418 cm−1 was sharply lower than that of bare CeO2, since the in situ formed NiFe-LDH layer encapsulates CeO2 nanoparticles, which verifies the formation of the CeO2@NiFe-LDH heterostructure [26].
Figure 3 shows the SEM pictures of CeO2 (a), NiFe-LDH (b) and CeO2@NiFe-LDH-3 (c and d). SEM characterization results reveal that CeO2 consists predominantly of irregular polygonal sheet-like particles sized 100–300 nm with noticeable agglomeration. Such morphology arises from differences in precursor decomposition, grain nucleation, and grain growth rates under calcination. While the particles are generally irregular polygons, well-defined cubic crystallites originating from the crystal habit of the cubic fluorite structure can be seen in local regions, indicating that crystal growth is regulated by its inherent crystal structure and confirming the successful fabrication of phase-pure CeO2 [27]. The SEM image of NiFe-LDH clearly showed the plate-like structure with regular and orderly stacking. These layers were relatively thick and exhibited a uniform size distribution with the lateral dimension of approximately 80–150 nm. SEM images of the CeO2@NiFe-LDH-3 nanocomposite clearly demonstrate that abundant NiFe-LDH nanosheets grow in situ on the surface of CeO2 and form a loose and porous shell layer, thereby constructing a core–shell heterostructure successfully [24].
To identify CeO2@NiFe-LDH-3 nanostructures more clearly, the composite material was further characterized by transmission electron microscopy. As can be seen in Figure 4, it is worth noting that the heterostructure interface of CeO2@NiFe-LDH-3 was shown in the HRTEM image (Figure 4a), proving the formation of the CeO2@NiFe-LDH-3 heterostructure. The interplanar spacing can be indexed to the (220) and (222) crystal planes of CeO2, and the (018) crystal plane of NiFe-LDH. Furthermore, the EDX full spectrum and elemental mapping images of CeO2@NiFe-LDH-3 (Figure 4b) reveal that Ce, Ni, Fe and O elements are homogeneously distributed throughout the composite. The above characterization results fully confirm the successful fabrication of core–shell heterostructures [28].
The XPS analysis of CeO2, NiFe-LDH, and CeO2@NiFe-LDH-3 nanohybrid was performed to further investigate the elemental compositions and chemical states. The results are shown in Figure 5.
As shown in Figure 5a, it can be clearly seen that the survey spectrum of the CeO2@NiFe-LDH-3 composite was similar to that of NiFe-LDH, with indistinct characteristic peaks of Ce. This phenomenon may be due to the encapsulation of CeO2 by NiFe-LDH nanosheets. As can be seen in Figure 5b, the high-resolution Fe 2p XPS spectrum of NiFe-LDH displays two characteristic peaks at 711.9 eV and 725.1 eV, which are assigned to Fe 2p3/2 and Fe 2p1/2, respectively. In comparison, positive shifts in Fe 2p3/2 (712.3 eV) and Fe 2p1/2 (725.5 eV) are observed for CeO2@NiFe-LDH-3, which is probably attributed to electron transfer induced by the construction of heterostructures [29]. The Fe 2p spectrum of NiFe-LDH can be deconvoluted into eight peaks (Figure S1a), the peaks at around 712.07 and 725.21 eV were attributed to Fe3+ 2p3/2 and Fe3+ 2p1/2, along with the satellite peaks at around 718.31 and 733.01 eV; the peaks at about 709.06 and 721.86 eV belong to Fe2+ 2p3/2 and Fe2+ 2p1/2, along with the satellite peaks at around 714.80 and 728.09 eV, indicating the coexistence of Fe3+ and Fe2+ [30]. The same peak-fitting method is also applicable to the CeO2@NiFe-LDH-3 composite. In the high-resolution Ni 2p spectra (Figure 5c), the peaks of pure NiFe-LDH at 873.3 eV and 855.5 eV are assigned to Ni 2p1/2 and Ni 2p3/2, respectively. Similarly, positive shifts in the binding energies of Ni 2p1/2 and Ni 2p3/2 are observed for the CeO2@NiFe-LDH-3 composite, revealing that electrons transfer from NiFe-LDH to CeO2 during the formation of the heterostructure [31]. The high-resolution Ce 3d spectrum of CeO2 can be deconvoluted into ten peaks (Figure S1b), which are divided into two sets of signals assigned to Ce 3d3/2 and Ce 3d5/2. Six peaks located at 916.36 eV, 907.25 eV, 900.62 eV, 898.02 eV, 888.63 eV and 882.06 eV were ascribed to Ce4+, while the remaining four peaks at 879.9 eV, 897.5 eV, 903.16 eV and 884.64 eV are attributed to Ce3+. These results verify the coexistence of Ce4+ and Ce3+ in CeO2, with Ce4+ as the dominant cerium species [32].
Furthermore, as can be seen in Figure 5d, the peak position of Ce 3d for the CeO2@NiFe-LDH-3 composites shifted to lower binding energy, and the intensity of Ce 3d diffraction peaks was markedly weaker than that of pure CeO2. This phenomenon can be ascribed to the encapsulation of CeO2 cores by NiFe-LDH shells. Nevertheless, distinct Ce 3d signals can still be clearly detected in the composite system, which indicates that the NiFe-LDH shell is thin and porous, further verifying the successful construction of core–shell heterostructures. The unexpected peak observed for CeO2@NiFe-LDH-3 within the binding energy range of 870–880 eV originates from orbital overlap between Ce 3d and Ni 2p.
It should be noted that the contribution of oxygen vacancies has not been experimentally evaluated via high-resolution O 1s XPS in the present work. Although oxygen vacancies are not the primary focus of this study, we cannot rule out their potential influences on charge transfer and catalytic reactivity, which deserves further exploration in future investigations.

2.2. Optical and Electrochemical Properties

UV-visible diffuse reflectance spectra (DRS) were utilized to elucidate the optical properties of the prepared samples. The UV-Vis absorption spectra of CeO2, NiFe-LDH, CeO2@NiFe-LDH-2, CeO2@NiFe-LDH-3 and CeO2@NiFe-LDH-4 are presented in Figure 6a. As can be seen from the spectra, all the samples exhibited absorption peaks in the ultraviolet-visible range. Pure CeO2 exhibits weak visible-light absorption with low overall absorbance in the visible region, whereas NiFe-LDH displays prominent visible-light responsiveness with intense absorption bands in the range of 400–600 nm. Furthermore, two characteristic broad absorption bands centered at 650 nm and 760 nm are observed, corresponding to d-d transitions of octahedral Ni2+ and Fe3+ in the LDH layers, respectively. Compared with pure CeO2, the CeO2@NiFe-LDH composites display a red shift in absorption edges, and the absorption edge extends to most of the visible light region that overlaps with that of NiFe-LDH. The results suggest that the composite of CeO2 and NiFe-LDH possesses remarkably enhanced visible light utilization efficiency, which favors visible light absorption and confirms strong interactions between CeO2 and NiFe-LDH [28].
The band gap of all samples is calculated via the Kubelka–Munk formula: (αhν)2 = K (hν-Eg). Where α, h, v, K and Eg are the absorption coefficient, Planck’s constant (J s), light frequency (s−1), proportionality coefficient and band gap energy (eV). The band gaps of the samples can be calculated by plotting (αhν)2 versus hν based on UV-Vis spectral data. As seen from the Tauc plot in Figure 6b, the band gap energies of CeO2, NiFe-LDH, CeO2@NiFe-LDH-2, CeO2@NiFe-LDH-3, and CeO2@NiFe-LDH-4 were computed and determined as 2.91 eV, 2.17 eV, 2.67 eV, 2.55 eV and 2.58 eV, respectively. The heterostructures have band gap energies between those of CeO2 and NiFe-LDH, implying that the construction of heterostructures via coupling CeO2 with NiFe-LDH narrows the band gap, which further demonstrates that heterostructures possess higher visible light utilization efficiency than pure CeO2 [33]. Among all samples, CeO2@NiFe-LDH-3 exhibits the narrowest band gap and the strongest visible light response capability.
The separation and transfer efficiency of photogenerated charge carriers is another critical factor governing the photocatalytic activity of heterostructures. Apart from light absorption and the generation of electron-hole pairs, the separation efficiency directly determines the quantity of active species participating in surface reactions. Photoluminescence (PL) spectroscopy was applied to investigate the recombination rate of photogenerated electron-hole pairs over the prepared samples. As displayed in Figure 7a, both CeO2 and CeO2@NiFe-LDH-DH-3 exhibit photoluminescence (PL) emission peaks at approximately 440 nm under an excitation wavelength of 355 nm. The PL signal intensity of the CeO2@NiFe-LDH-3 heterostructure is remarkably quenched compared with that of the single component. Such quenching effect originates from the synergistic interaction between NiFe-LDH and CeO2, revealing that the heterostructure composite possesses a lower recombination rate of photogenerated charge carriers.
The enhanced charge transfer efficiency was also confirmed by the transient photocurrent response analyses. Figure 7b showed the transient photocurrent response of the synthesized CeO2, NiFe-LDH and CeO2@NiFe-LDH-3 photocatalysts. The results showed that the immediate responses when the light is switched on and off can be observed for all samples. The photocurrent intensity of the CeO2@NiFe-LDH-3 heterostructure was noticeably stronger than that of pristine CeO2 and NiFe-LDH. As is well known, a higher photocurrent generally corresponds to more efficient separation and transfer of photogenerated charge carriers. This result is consistent with the photoluminescence (PL) analysis, collectively verifying the highly efficient separation and migration of photogenerated carriers within the heterostructure, with efficiency far superior to that of single components. It further confirms that intimate contact interfaces form between CeO2 and NiFe-LDH, rather than a simple physical mixture [34].

2.3. Photocatalytic Performance of Catalysts in the Selective Oxidation of Toluene

Having confirmed successful construction of CeO2@NiFe-LDH-3 heterostructure, we then turned our attention to evaluate its photocatalytic performance using toluene as a model substrate and molecular oxygen as a green oxidant under visible light. The corresponding results are presented in Figure 8a. Meanwhile, as shown in Figure S2, benzaldehyde was the major product over all tested catalysts, while the byproducts benzyl alcohol and benzoic acid were also detected.
As displayed in Figure 8a, pristine NiFe-LDH exhibits better catalytic performance than bare CeO2. The toluene conversion and product selectivity reach 3.6% and 24.4% for NiFe-LDH, while those of CeO2 are 2.5% and 17.5%, respectively. This originates from the band gap of NiFe-LDH (2.17 eV), which is considerably narrower than that of CeO2 (2.91 eV) and enables more efficient visible light utilization. However, despite its strong visible-light absorption capability, NiFe-LDH suffers from a high recombination rate of charge carriers, which leads to unsatisfactory oxidation efficiency. Although oxygen vacancies in CeO2 can promote the separation of photogenerated charge carriers, the recombination of electron-hole pairs still occurs rapidly in single-phase CeO2. As a result, both materials show low toluene conversion and benzaldehyde selectivity.
The construction of the CeO2@NiFe-LDH heterostructure achieves a prominent increase in toluene conversion and greatly boosted catalytic activity. The toluene conversion over CeO2@NiFe-LDH-2 reaches 15.7%, which is considerably higher than those of pristine CeO2 and pristine NiFe-LDH. Notably, all CeO2@NiFe-LDH photocatalysts exhibit a benzaldehyde selectivity of over 70%. Compared with CeO2@NiFe-LDH-2 (15.7% conversion and 70.1% selectivity) and CeO2@NiFe-LDH-4 (20.1% conversion and 76.5% selectivity), CeO2@NiFe-LDH-3 exhibits the optimal photocatalytic performance, with a toluene conversion of 24.2% and benzaldehyde selectivity of 81%.
With the core–shell structure, CeO2@NiFe-LDH-3 achieves optimal synergy throughout the multi-step reaction process. First, it possesses the narrowest band gap. Theoretically, this enables absorption of visible light across a broader wavelength range and thus generates more photogenerated electrons and holes. Second, the formed CeO2@NiFe-LDH heterostructure creates a built-in electric field that drives efficient separation and transfer of photogenerated carriers, suppresses carrier recombination, and produces abundant active species for surface reactions. Third, its shell has moderate thickness, which guarantees rapid carrier migration while exposing sufficient active sites for catalytic reactions. CeO2@NiFe-LDH-4 exhibits the thickest shell layer. The migration path of charge carriers from the inner CeO2 core to the surface NiFe-LDH shell becomes excessively long, leading to severe recombination of numerous carriers during transportation. This reduces the quantity of reactive species that can actually participate in catalytic reactions and thus impairs the photocatalytic performance. Compared with CeO2@NiFe-LDH-3, the band gap of CeO2@NiFe-LDH-4 slightly increases to 2.58 eV, further verifying that the electronic structure may be altered by the excessively thick shell.
The physically mixed sample of CeO2 and NiFe-LDH with a molar ratio of 1:3 delivers a toluene conversion of 6.8% and benzaldehyde selectivity of 60.7%, whose catalytic activity and selectivity are far inferior to those of the CeO2@NiFe-LDH-3 heterostructure. This result further demonstrates that the chemical interaction between CeO2 and NiFe-LDH serves as the critical factor for boosting photocatalytic performance. Blank control experiments were carried out under three separate conditions: without visible light irradiation, without oxygen, and without photocatalyst. Hardly any target product could be detected in any of the blank tests, which confirms that the reaction proceeds via an aerobic photocatalytic pathway. All catalytic activity originates from photocatalysis, indicating that the photocatalyst, visible light and oxidant are all indispensable for the reaction system.
Using CeO2@NiFe-LDH-3 with optimal catalytic performance as the photocatalyst, the effects of catalyst dosage and irradiation time on the toluene oxidation performance were systematically investigated. As displayed in Figure 8b, the toluene conversion increases from 11.6% to 24.2% as the catalyst dosage rises from 20 mg to 50 mg. At this stage, the number of active sites acted as the dominant rate-limiting factor for the reaction; the toluene conversion rapidly increased with the growing catalyst dosage. At a catalyst dosage of 50 mg, abundant active sites were available, and a balance was achieved among the generation and migration of photogenerated charges, reactant adsorption, and product desorption, corresponding to the maximum conversion and optimal selectivity. Further increasing the catalyst dosage to 60 mg led to a decline in photocatalytic activity, accompanied by a drop in toluene conversion to 23.7%. Excessive catalyst particles induced a light-shielding effect, which prevented the underlying catalyst from receiving sufficient light irradiation. Meanwhile, particle agglomeration reduced the effective reactive surface area, and overoxidation of the benzaldehyde product was likely aggravated. Collectively, these factors contributed to the simultaneous reduction in conversion and selectivity. As illustrated in Figure 8c, the toluene conversion continuously improved as the irradiation time extended from 2 h to 5 h, while the benzaldehyde selectivity remained steadily above 70%. When the irradiation time was prolonged to 6 h, the benzaldehyde selectivity decreased, implying the occurrence of benzaldehyde overoxidation.
A comparison between the as-synthesized catalyst and previously reported heterogeneous photocatalysts (Table S1) shows that bismuth-based catalysts have a well-established research system for toluene oxidation. Such catalysts generally achieve high benzaldehyde selectivity exceeding 84%, while most of them are limited by low toluene conversion. Composite photocatalysts based on metal sulfides and carbon nitride materials have been extensively investigated, both of which serve as high-performance photocatalysts. In this work, the CeO2@NiFe-LDH catalyst exhibits outstanding photocatalytic activity for toluene conversion. It realizes a favorable trade-off between toluene conversion and benzaldehyde selectivity, and its overall catalytic performance surpasses most photocatalysts reported to date.
Under the optimal reaction conditions, the catalytic oxidation performance of CeO2@NiFe-LDH-3 toward various toluene-derived aromatic substrates was further evaluated to explore its substrate compatibility, and the corresponding results are summarized in Table 1. This photocatalyst also exhibits outstanding catalytic activity toward toluene-based aromatic substrates. Nevertheless, o-chlorotoluene and o-xylene display lower reaction rates compared with p-chlorotoluene and p-xylene; the conversion efficiency of chlorotoluene is lower than that of toluene. This phenomenon arises because chlorine atoms act as electron-withdrawing groups and hinder the oxidation of methyl groups. P-Nitrotoluene bearing a strong electron-withdrawing nitro group, shows remarkably suppressed oxidation efficiency. In contrast, electron-donating substituents (p-methyl and p-methoxy groups) distinctly boost the oxidation efficiency of toluene derivatives. These observations reveal that the photocatalytic oxidation reaction is governed by electronic effects and steric hindrance effects [17,35]. Notably, no CO2 production was detected throughout all catalytic tests, demonstrating that deep oxidation of the substrates does not occur over the CeO2@NiFe-LDH-3 photocatalyst.
Considering the significance of catalyst cost for practical industrial applications, an ideal photocatalyst should possess favorable recyclability. Therefore, after each photocatalytic reaction, the CeO2@NiFe-LDH-3 catalyst was recovered by centrifugation, washed with ethanol and deionized water, and then subjected to cyclic stability tests under identical reaction conditions to evaluate its stability and recyclability. As shown in Figure 8d, nearly no obvious decline in toluene conversion and benzaldehyde selectivity was observed over CeO2@NiFe-LDH-3 after five consecutive cycles. Fresh and spent catalysts were characterized by SEM and XRD, and the results (Figure S3) demonstrate that no remarkable alterations occurred in their morphology and crystal phase structure, which verifies the excellent stability of CeO2@NiFe-LDH-3 for visible-light-driven toluene oxidation.

2.4. Energy Band Structure and Plausible Mechanism

Mott-Schottky (M-S) curves were further used to estimate the carrier density, flat band potential, and conduction band potential of the catalysts. As displayed in Figure 9a, positive slopes of M-S curves implied that both NiFe-LDH and CeO2 were typical n-type semiconductors [24].
Meanwhile, it is widely reported that the converted potential from the flat band (vs. Ag/AgCl) to the normal hydrogen electrode (vs. NHE) is about 0.197 V. Thus, the Mott-Schottky (M-S) curves were used for estimating the conduction band potential of the n-type semiconductors by extending the linear part of the curve to intercept the X-axis at a value equal to Efb. According to the intercept on the X-axis, the flat band positions of CeO2 and NiFe-LDH were −0.735 and −0.976 V vs. Ag/AgCl, corresponding to −0.538 and −0.779 V vs. the normal hydrogen electrode (NHE), respectively. Theoretically, the flat-band potential of n-type semiconductors is 0.2 V more positive than the conduction band position. Therefore, the conduction band positions of CeO2 and NiFe-LDH were −0.738 and −0.979 V vs. NHE, respectively [34]. Furthermore, the valence band potentials of CeO2 and NiFe-LDH were calculated to be 2.172 and 1.191 V vs. NHE by combining the obtained band gaps from the transformed Kubelka–Munk function.
Finally, the energy band structure of CeO2 and NiFe-LDH was shown in Figure 9b. From the energy band alignment of CeO2 and NiFe-LDH, it could be seen that the conduction band and valence band potentials of NiFe-LDH were more negative than those of CeO2. Furthermore, the energy band structure of CeO2@NiFe-LDH is staggered. Such energy band configurations enable accelerated migration and spatial segregation of charge carriers in the material [28,35].
Ultraviolet photoelectron spectroscopy (UPS) equipped with a He I UV excitation lamp was utilized to identify the heterostructure type of CeO2@NiFe-LDH composites, which enables quantitative calculation of the work function (Φ) of materials via the formula Φ = hν − Ecut-off. In this formula, 21.2 eV is the incoming photon energy hν supplied by He I irradiation, while Ecut-off denotes the secondary electron cut-off energy [36]. By analyzing the secondary electron cut-off edges illustrated in Figure 10a, the work functions of individual CeO2 and NiFe-LDH samples are calculated to be 2.40 eV and 2.10 eV, respectively. Generally speaking, interfacial electrons migrate from semiconductors with lower work functions to those with higher work functions. Thus, when CeO2 and NiFe-LDH were combined to form a heterostructure, the work function difference between CeO2 and NiFe-LDH promotes the electron transfer between them until the Fermi energy level tends to be the same. As a result, electrons transfer from NiFe-LDH to CeO2, which is consistent with the XPS results [37]. Furthermore, the electron density of NiFe-LDH decreases, and the electron density of CeO2 increases; thus, the built-in electric field is then formed at the interface (the field strength direction is NiFe-LDH→CeO2). The formation of the built-in electric field (BIEF) is conducive to the acceleration of electron transfer, and the direction of electron flow is opposite to the direction of the field strength; that is, transfer from CeO2 to NiFe-LDH. Upon illumination, photogenerated electrons on the conduction band of CeO2 recombine with photogenerated holes from the valence band of NiFe-LDH. Such interfacial charge migration matches the characteristic S-scheme heterostructure mechanism, which effectively restrains the recombination of charge carriers inside pure CeO2 and NiFe-LDH. As a result, powerful photogenerated holes and electrons that remain separated at the VB of CeO2 and CB of NiFe-LDH can serve as active species to drive the target catalytic reaction [38].
Dioxygen (O2) activation is important in photocatalytic oxidation processes when O2 is used as the sole oxidant. It is apparent that the reactive oxygen species (ROS) originate from O2. To illustrate the reaction mechanisms and how the ROS contributed to the conversion of toluene over the CeO2@NiFe-LDH photocatalyst, a series of radical trapping tests were performed in the presence of scavengers of the photogenerated charge carriers and different radicals.
As shown in Figure 10b, the conversion of toluene to benzaldehyde was 24.2% after 5 h photoreaction with 50 mg CeO2@NiFe-LDH as the catalyst (Blank). The reaction was completely inhibited when TEMPO (for all radicals) was introduced to the reaction system, which was attributed to the complete consumption of all radicals. That is to say, toluene was not directly oxidized by the photogenerated holes. When ammonium oxalate (AO) was introduced to trap holes or benzoquinone (BQ) was used to trap ·O2−, the conversion of the photoreaction was also considerably reduced, and the effect of ·O2− was greater, indicating that ·O2− rather than the other oxidative species is the primary photogenerated reactive oxygen species and both photogenerated holes and ·O2− played important roles in catalyzing the oxidation of toluene. In addition, the conversion of toluene was also inhibited after the addition of K2S2O8 to trap the photogenerated electrons, and butylated hydroxytoluene (BHT) to trap carbon-centered radicals, signifying that e− and carbon-centered radicals were also involved in the reaction. It should be noted that benzaldehyde conversion was almost unchanged in the presence of TBA (scavenger for·OH), suggesting that ·OH might not participate in the reaction process [36,39]. The active radicals generated in the photocatalytic toluene oxidation by the CeO2@NiFe-LDH photocatalyst were further detected with EPR spectra during visible light irradiation. As seen in Figure 11, the signal of free radicals cannot be observed under dark conditions, but after illumination, the typical EPR signals for the DMPO-·O2− adducts can be detected, and the signal intensities increased with increasing irradiation time; this indicated that the synthetic CeO2@NiFe-LDH catalyst can effectively activate O2 to generate ·O2− successfully. These findings demonstrate that photogenerated holes, electrons, superoxide radicals, and carbon-centered radicals serve as core active species for converting toluene to oxygen-containing products. The radical-mediated reaction pathway, with benzyl radicals as critical intermediates, is consistent with mechanistic findings from previous literature [39].
The conduction band potentials of CeO2 and NiFe-LDH are −0.738 and −0.979 V (vs. NHE), respectively. Both of them have more negative CB potentials than the O2/·O2− couple (E0 = −0.33 V vs. NHE), implying that they are capable of producing ·O2− in the presence of O2 and light. Moreover, there is no obvious generation of ·OH because the photocatalysts possess more negative potentials (2.172 and 1.191 V (vs. NHE) for CeO2 and NiFe-LDH, respectively) for the OH−/·OH reaction; that is, with an oxidation potential of 2.31 V (vs. NHE) [40,41]. Based on the above results and earlier reports, a probable reaction route for the photocatalytic oxidation of toluene to benzaldehyde can be hypothesized. As shown in Figure 12, the band gaps of CeO2 and LDH are 2.91 eV and 2.17 eV, respectively. Both of them can be excited under visible-light to generate electrons and holes. Due to the energy band induced to construct an S-scheme system, photogenerated electrons and holes are effectively separated and enriched in the conduction band of NiFe-LDH and the valence band of CeO2, respectively. Photogenerated electrons with negative potentials (−0.979 V vs. NHE) in the CB of NiFe-LDH activate molecular oxygen to form superoxide radical (·O2−); the photogenerated holes (h+) at the VB of CeO2, which stay in the initial energy bands, oxidize the C(sp3)-H bond in toluene to the carbon-centered (benzyl) radicals, respectively. The generated ·O2− oxidized the benzyl radicals to the peroxy radicals; meanwhile, a portion of benzyl radicals reacted with O2 molecules to generate benzyl alcohol. The formed peroxy radicals and benzyl alcohol were transformed into the product (benzaldehyde) via dehydration and oxidative dehydrogenation, respectively.

3. Conclusions

In this paper, CeO2@NiFe-LDH heterostructure photocatalysts were synthesized via a facile two-step hydrothermal strategy, where NiFe-LDH nanosheets were in situ grown on CeO2 substrates. Utilizing molecular oxygen as the oxidant under visible light illumination, their catalytic activity toward the selective oxidation of toluene to benzaldehyde was comprehensively evaluated. Morphological and structural characterizations confirm the core–shell configuration of the as-fabricated heterostructure. Optical and electrochemical measurements confirm that the integration of CeO2 with NiFe-LDH enhanced visible-light absorption and utilization, as well as greatly improved separation and migration efficiency of photogenerated charge carriers. Under mild conditions of room temperature and visible-light illumination, the CeO2@NiFe-LDH-3 catalyst achieves a toluene conversion rate of 1.936 mmol·g−1·h−1 with an excellent benzaldehyde selectivity of 81.0%, and its catalytic performance is significantly superior to that of the individual single-phase materials and the simple physical mixture. A suitable shell thickness plays a critical role: it facilitates efficient charge carrier transport while exposing abundant active sites. Furthermore, the heterostructure photocatalyst exhibits outstanding cycling stability and broad substrate applicability. Band structure and reaction mechanism investigations demonstrate that this heterostructure follows an S-scheme charge transfer pathway. Such a mechanism effectively suppresses the recombination of electron-hole pairs within individual components, preserving the highly oxidizing photogenerated holes in the valence band of CeO2 and strongly reductive photogenerated electrons in the conduction band of NiFe-LDH, thereby drastically boosting the redox capacity of the photocatalyst. Radical trapping experiments combined with EPR spin-trapping characterizations reveal that the reaction proceeds via a radical-initiated pathway.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal16090757/s1: Figure S1: Peak fitting results of (a) Fe 2p in NiFe-LDH and (b) Ce 3d in CeO2; Figure S2: Selectivity of each product; Figure S3: (a) XRD patterns of CeO2@NiFe-LDH-3 before and after recycling; (b) SEM image after recycling; Table S1: Comparison of the performance with different photocatalysts [42,43,44,45,46].

Author Contributions

F.F.: Conceptualization, Methodology, Writing—Original draft, Data curation, Writing—Reviewing and Editing, Investigation, Formal analysis, Software, Visualization. D.S.: Validation, Resources. X.P.: Supervision, Project administration, Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 16 00757 g001
Figure 1. XRD patterns of the prepared samples.
Figure 1. XRD patterns of the prepared samples.
Catalysts 16 00757 g001
Catalysts 16 00757 g002
Figure 2. FT-IR spectra of the prepared catalysts.
Figure 2. FT-IR spectra of the prepared catalysts.
Catalysts 16 00757 g002
Catalysts 16 00757 g003
Figure 3. SEM images of (a) CeO2, (b) NiFe-LDH, and (c,d) CeO2@NiFe-LDH-3 observed at different magnifications.
Figure 3. SEM images of (a) CeO2, (b) NiFe-LDH, and (c,d) CeO2@NiFe-LDH-3 observed at different magnifications.
Catalysts 16 00757 g003
Catalysts 16 00757 g004
Figure 4. (a) HRTEM image of CeO2@NiFe-LDH-3; (b) HAADF-STEM image, corresponding elemental mapping images of Ce, Ni, Fe, O and EDX full spectrum of CeO2@NiFe-LDH-3.
Figure 4. (a) HRTEM image of CeO2@NiFe-LDH-3; (b) HAADF-STEM image, corresponding elemental mapping images of Ce, Ni, Fe, O and EDX full spectrum of CeO2@NiFe-LDH-3.
Catalysts 16 00757 g004
Catalysts 16 00757 g005
Figure 5. (a) XPS survey spectrum and high-resolution spectra of (b) Fe 2p, (c) Ni 2p, and (d) Ce 3d for the catalyst.
Figure 5. (a) XPS survey spectrum and high-resolution spectra of (b) Fe 2p, (c) Ni 2p, and (d) Ce 3d for the catalyst.
Catalysts 16 00757 g005
Catalysts 16 00757 g006
Figure 6. (a) UV-vis diffuse reflectance spectra of the as-prepared photocatalysts; (b) Tauc plots: linear transformation plots of hν versus (αhν)2 from UV-vis diffuse reflectance spectra.
Figure 6. (a) UV-vis diffuse reflectance spectra of the as-prepared photocatalysts; (b) Tauc plots: linear transformation plots of hν versus (αhν)2 from UV-vis diffuse reflectance spectra.
Catalysts 16 00757 g006
Catalysts 16 00757 g007
Figure 7. (a) Photoluminescence (PL) emission spectra; (b) transient photocurrent response curves.
Figure 7. (a) Photoluminescence (PL) emission spectra; (b) transient photocurrent response curves.
Catalysts 16 00757 g007
Catalysts 16 00757 g008
Figure 8. (a) Catalytic performance of different catalysts for toluene selective oxidation; (b–d) Photocatalytic performance of CeO2@NiFe-LDH-3, including catalyst dosage (b), reaction time (c) and recycling stability.
Figure 8. (a) Catalytic performance of different catalysts for toluene selective oxidation; (b–d) Photocatalytic performance of CeO2@NiFe-LDH-3, including catalyst dosage (b), reaction time (c) and recycling stability.
Catalysts 16 00757 g008
Catalysts 16 00757 g009
Figure 9. (a) Mott-Schottky plots of CeO2 and NiFe-LDH; (b) Schematic diagram of the band structure of CeO2@NiFe-LDH-3.
Figure 9. (a) Mott-Schottky plots of CeO2 and NiFe-LDH; (b) Schematic diagram of the band structure of CeO2@NiFe-LDH-3.
Catalysts 16 00757 g009
Catalysts 16 00757 g010
Figure 10. (a) Secondary electron cutoff edges of CeO2 and NiFe-LDH; (b) Photocatalytic activity in the presence of scavengers.
Figure 10. (a) Secondary electron cutoff edges of CeO2 and NiFe-LDH; (b) Photocatalytic activity in the presence of scavengers.
Catalysts 16 00757 g010
Catalysts 16 00757 g011
Figure 11. EPR spectra of DMPO-·O2− adducts formed under different irradiation times.
Figure 11. EPR spectra of DMPO-·O2− adducts formed under different irradiation times.
Catalysts 16 00757 g011
Catalysts 16 00757 g012
Figure 12. Reaction pathway for toluene oxidation catalyzed by CeO2@NiFe-LDH-3.
Figure 12. Reaction pathway for toluene oxidation catalyzed by CeO2@NiFe-LDH-3.
Catalysts 16 00757 g012
Table 1. Results of substrate scope investigation.
Table 1. Results of substrate scope investigation.
EntrySubstrateProductTimeConv. (%) aSel. (%) a
1Catalysts 16 00757 i001Catalysts 16 00757 i002524.281.0
2Catalysts 16 00757 i003Catalysts 16 00757 i004421.581.0
3Catalysts 16 00757 i005Catalysts 16 00757 i006519.178.0
4Catalysts 16 00757 i007Catalysts 16 00757 i008623.677.0
5Catalysts 16 00757 i009Catalysts 16 00757 i010617.875.0
6Catalysts 16 00757 i011Catalysts 16 00757 i012711.974.8
7Catalysts 16 00757 i013Catalysts 16 00757 i014421.179.7
a Dodecane was adopted as the internal standard, and quantitative analysis was performed via GC-MS. Reaction conditions: substrate 2 mmol, catalyst 50 mg, solvent 3 mL, room temperature.
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Fang, F.; Sun, D.; Peng, X. Visible-Light-Driven Selective Oxidation of Toluene to Benzaldehyde over CeO2@NiFe-LDH Heterostructure. Catalysts 2026, 16, 757. https://doi.org/10.3390/catal16090757

AMA Style

Fang F, Sun D, Peng X. Visible-Light-Driven Selective Oxidation of Toluene to Benzaldehyde over CeO2@NiFe-LDH Heterostructure. Catalysts. 2026; 16(9):757. https://doi.org/10.3390/catal16090757

Chicago/Turabian Style

Fang, Fang, Dongping Sun, and Xinhua Peng. 2026. "Visible-Light-Driven Selective Oxidation of Toluene to Benzaldehyde over CeO2@NiFe-LDH Heterostructure" Catalysts 16, no. 9: 757. https://doi.org/10.3390/catal16090757

APA Style

Fang, F., Sun, D., & Peng, X. (2026). Visible-Light-Driven Selective Oxidation of Toluene to Benzaldehyde over CeO2@NiFe-LDH Heterostructure. Catalysts, 16(9), 757. https://doi.org/10.3390/catal16090757

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