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Article

Carbonaceous Material Modified MoO2 Nanospheres with Oxygen Vacancies for Enhanced Visible-Light Photocatalytic Oxidative Coupling of Benzylamine

by
Yuhong Chang
,
Yanxia Zhang
,
Tianjun Hu
,
Wenwen Chen
,
Tao Tang
,
Ergui Luo
and
Jianfeng Jia
*
Key Laboratory of Magnetic Molecules and Magnetic Information Materials of Ministry of Education, School of Chemistry and Materials Science of Shanxi Normal University, Taiyuan 030032, China
*
Author to whom correspondence should be addressed.
Molecules 2023, 28(12), 4739; https://doi.org/10.3390/molecules28124739
Submission received: 22 May 2023 / Revised: 10 June 2023 / Accepted: 12 June 2023 / Published: 13 June 2023
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Versions Notes

Abstract

:
Surface oxygen vacancy (OV) plays a pivotal role in the activation of molecular oxygen and separation of electrons and holes in photocatalysis. Herein, carbonaceous materials-modified MoO2 nanospheres with abundant surface OVs (MoO2/C-OV) were successfully synthesized via glucose hydrothermal processes. In situ introduction of carbonaceous materials triggered a reconstruction of the MoO2 surface, which introduced abundant surface OVs on the MoO2/C composites. The surface oxygen vacancies on the obtained MoO2/C-OV were confirmed via electron spin resonance spectroscopy (ESR) and X-ray photoelectron spectroscopy (XPS). The surface OVs and carbonaceous materials boosted the activation of molecular oxygen to singlet oxygen (1O2) and superoxide anion radical (•O2−) in selectively photocatalytic oxidation of benzylamine to imine. The conversion of benzylamine was 10 times that of pristine MoO2 nanospheres with a high selectivity under visible light irradiation at 1 atm air pressure. These results open an avenue to modify Mo-based materials for visible light-driven photocatalysis.

Graphical Abstract

1. Introduction

Photocatalytic oxidative coupling of amines to imines with a green, efficient and economical process has attracted great attention owing to the essential intermediates of imines for the production of chemical and pharmaceutical agents [1,2,3]. Researchers have made enormous efforts to develop high-performance photocatalysts for an innovative imines synthetic strategy [4,5,6]. Many kinds of photocatalysts have been developed for oxidation coupling of amines, such as plasmonic metals (Au, Ag, Cu, etc.) [7,8], semiconductor oxides (TiO2, WO3, BiOCl, etc.) [9,10], carbon materials [11,12], polymers [13] and so on. Semiconductor oxides possess environmental friendliness, cost-effectiveness, and excellent capability to harvest sunlight [14,15]. Among them, molybdenum dioxide is a promising candidate for semiconductor photocatalyst due to its cheapness, chemical stability and green synthesis [16]. MoO2 is the rutile-type transition metallic oxide and has been widely studied due to its visible light absorption capacity and specific metallic properties, which differ from those of other oxides [17]. The existence of a large amount of free electrons makes MoO2 trap electrons and enhance charge separation, which is profitable for improving the catalytic performance [18]. Additionally, MoO2 is widely utilized in the various fields owing to acid and alkali resistance and thermal stability [19,20]. In order to enhance the photocatalytic performance of MoO2, the surficial state of semiconductor oxides could be adjusted by post-treatment to construct a surface defect structure, improving charge separation and the interaction between photocatalyst and reactant molecules [21].
Oxygen vacancies (OVs) have been confirmed to act as electron donors and adsorption sites and photocatalysis active sites for O2 adsorption, which is highly desirable for enhancing the photocatalytic performance [22,23,24]. It has been accepted that the bulk defects are the recombination centers of photo-induced electrons and holes, which is unfavorable for photocatalytic reaction [25,26]. In comparison, surface OVs play an important role not only in the extension of the photo response, but also in increasing the charge separation efficiency [27,28,29]. Semiconductor photocatalysts with rich surface OVs, such as TiO2, BiO2, MoO3, WO3, ZnO and so on, have been extensively reported [16,30,31,32,33]. BiOCl nanosheets with surface OVs resulted in abundant surface low-coordinated Bi atoms, contributing to the assembly of O2 molecules on the surface of photocatalyst [34]. The appropriate concentration of surface OVs on TiO2 nanosheets promotes the activation of O2 in photocatalytic oxidative of benzylamine to imine [35]. The visible-light response ability of these photocatalysts still needs to be further improved. The common strategies for introducing surface OVs into the semiconductors include thermal treatment, chemical reduction, ultraviolet irradiation and so on [25,36,37]. Generally, these methods always involved high temperature or production of hydrogen [36]. Therefore, it makes sense to develop a simple and effective method to incorporate surface OVs into photocatalysts [27,38]. In situ introduction of carbonaceous precursors can trigger a reconstruction of the oxide surface and the formation of nanocomposites with interfacial disordered regions. [39]
In this paper, carbonaceous materials modified MoO2 nanospheres with abundant surface OVs (MoO2/C-OV) were synthesized by glucose hydrothermal processes aimed at the oxidative coupling of benzylamine. The surface OVs were constructed by the reconstruction of the MoO2 surface caused by the in situ introduction of carbonaceous precursors after glucose hydrothermal treatment. The presence of surface OVs could effectively improve the photocatalytic transformation of benzylamine to its corresponding imines under visible light irradiation at 1 atm pressure. The morphology, surface chemical states and electronic structures of the MoO2/C-OV were further elucidated by a series of characterization techniques. The unsaturated sites caused by surface OVs sites strongly interacted with molecular oxygen, which could establish a photoexcited electron transport channel. The adsorption and activation of O2 molecules produced active 1O2 and •O2− species with the assistance of the surface OVs and carbonaceous materials.

2. Results and Discussion

2.1. Synthesis and Characterization of the Samples

The phase, morphology and crystal structure of the samples were characterized using an X-ray powder diffractometer (XRD), a scanning electron microscope (SEM), a transmission electron microscope (TEM), a high-resolution transmission electron microscope (HRTEM), selected area electron diffraction (SAED) and an X-ray energy spectrometer (EDS). The morphology of the prepared samples was analyzed via SEM and TEM. As shown in Figure S1a, MoO2 nanospheres with a diameter of about 500 nm consisted of a large number of tightly connected irregular nanoparticles. As shown in Figure 1a, there was no obvious change in the morphology of MoO2 after hydrothermal treatment with glucose. High-magnification SEM images (Figure 1b) further exhibited rough surface and sharp projections of MoO2 nanospheres, which would provide more surface catalytic active sites. As shown in Figure 1c,d, the self-assembled nanospheres with a rough surface are composed of many small nanoparticles approximately several tens of nanometers in size. These nanospheres are not loose aggregations of the small nanoparticles. As shown in Figure S2, the structure and morphology of MoO2 nanoparticles remained stable after hydrothermal and high-temperature calcination. Figure 1e displayed that the MoO2 nanoparticle was tightly connected with carbonaceous materials. A clear disordered area was constructed at the interface between the MoO2 nanosphere and carbonaceous material, revealing more active Mo atoms and providing active sites. The inset in Figure 1e is the corresponding High resolution-TEM (HRTEM), it could be confirmed that the MoO2 nanospheres owned clear lattice fringes. The spacing with 0.28 nm of the adjacent lattice planes was consistent with the (−102) plane of MoO2. As shown in Figure 1f, the characteristic polycrystalline SAED pattern showed the (011), (020), (012), (220) plane, in accordance with the XRD diffraction pattern. The diffraction ring of carbon is not clearly observed in the pattern, indicating that the carbonaceous material was in an amorphous state. As shown in the elemental mapping images (Figure 1g), a certain amount of C was uniformly distributed on the whole MoO2 nanosphere, expressing the successful fabrication of the MoO2/C-OV catalyst. The elemental mapping images of MoO2 and MoO2/C were shown in Figures S3 and S4. The results revealed that a unique interface microstructure was formed after glucose hydrothermal treatment.
The crystal structures of the prepared catalysts were characterized using XRD patterns. As shown in Figure 2a, the XRD pattern of MoO2 nanospheres revealed that the obtained sample was well indexed as the monoclinic-phase MoO2 (JCPDS No. 78-1069), the space group was P21/c and the cell parameters were a = 5.609 Å, b = 4.86 Å, c = 5.628 Å. Among the three common types, monoclinic MoO2 is the most stable structure with a deformed rutile structure. The XRD pattern exhibited the characteristic diffraction peaks at 26.1°, 37.0°, 41.5°, 49.5°, 53.5°, 60.5°, 66.7°, which are associated with (011), (020), (012), (−302), (220), (013) and (131) crystal planes. There was no change in the crystal structure of MoO2 after glucose hydrothermal and high-temperature calcination treatment, which shows that the oxygen vacancies did not disrupt the crystal structure. All the prepared products owned strong diffraction peaks, and no other peaks existed in the XRD pattern, which confirmed the formation of samples with high crystallinity and phase purity. The MoO2/C-OV and MoO2/C composites were synthesized successfully via a hydrothermal and calcination method using glucose as the carbon source and MoO2(acac)2 as the Mo source.
The oxygen vacancy and chemical states of the surface atoms on MoO2, MoO2/C-OV and MoO2/C nanospheres were further characterized via X-ray photoelectron spectroscopy (XPS). As shown in Figure 2b, the full-survey XPS spectra of the three samples displayed the copresence of Mo, O and C elements. The contents of the Mo, O, and C elements according to the XPS spectra were shown in Table S1. The contents ratios of C and Mo elements for MoO2/C-OV and MoO2/C were higher than those of MoO2 nanospheres, indicating that carbonaceous matter was introduced into the surface of MoO2 nanospheres after the treatment via a glucose hydrothermal process. As shown in Figure 2c, the deconvoluted high-resolution Mo 3d spectrum of MoO2 exhibited four characteristic peaks at 235.0, 232.4, 231.7 and 229.2 eV. Two peaks at 232.4 eV and 229.2 eV could be assigned to Mo 3d3/2 and Mo 3d5/2 of the Mo4+ state. Two weaker peaks at 235.0 eV and 231.7 eV could be assigned to Mo6+, illustrating slight surface oxidization due to exposure in air. For MoO2/C-OV sample, the binding energy of 228.9 and 232.1 eV is assigned to the low valence Mo4+ 3d5/2 and Mo4+ 3d3/2 in MoO2. In addition, the peaks at 231,1.0 and 234.4 eV are ascribed to the high-valence Mo6+ 3d5/2 and Mo6+ 3d3/2. After glucose hydrothermal treatment, the Mo 3d core levels presented significant shifting to lower binding energy, indicating that more electrons were transferred from carbonaceous material and a more reduced state of Mo compared to the pristine MoO2 nanosphere. Compared to the MoO2 and MoO2/C-OV samples, the chemical state of MoO2/C composite differed significantly. Two peaks of the Mo6+ species were observed at 235.7 and 232.7 eV; this was ascribed to the Mo 3d3/2 and Mo 3d5/2 states. Two peaks of the Mo4+ species were observed at 231.0 and 229.4 eV, which was ascribed to the Mo 3d3/2 and Mo 3d5/2 states [40,41,42]. This revealed the remarkable surface oxidation of MoO2 after high-temperature calcination treatment. The stability of oxygen vacancies is due to the adsorption of oxygen species, which is a typical feature of defect-rich oxides. As shown in Figure 2d, the deconvoluted O 1s spectrum of MoO2/C-OV exhibited four peaks at 529.8, 531.6, 532.8, 533.4, which could be assigned to the lattice oxygen, the surface-adsorbed oxygen or oxygen vacancies, the adsorption of H2O on the surface and the oxygen of C=O-C. At the same time, the characteristic peak area of the lattice oxygen is relatively small, further demonstrating that the lattice oxygen atoms have reduced and oxygen vacancies have been introduced in the MoO2/C-OV catalyst [43]. As listed in Table S2, the atomic percentages of each oxygen species (at%) are calculated from the XPS data of the MoO2, MoO2/C-OV and MoO2/C nanospheres, respectively. The results showed that the proportion of the surface-adsorbed oxygen or oxygen vacancies on the surface of MoO2/C-OV was higher than that of MoO2 and MoO2/C. In addition, the ratio of the lattice oxygen on the surface of MoO2/C-OV was about 5.4%, which was less than that of MoO2 and MoO2/C (63.4% and 52.7%). In addition, the O 1s spectrum showed the existence of C=O-C on the surface of MoO2/C-OV and MoO2/C, which further demonstrated the introduction of carbonaceous matter onto the surface of MoO2/C-OV and MoO2/C. The proportion of the oxygen species adsorbed at oxygen vacancies for MoO2/C-OV was about 55.3%, which was higher than that in MoO2 (18.7%) and MoO2/C (15.9%). It was further testified that many more oxygen vacancies were formed on the surface of MoO2/C-OV. As shown in Figure 2e, the deconvoluted C 1s spectrum showed three characteristic peaks, located at 284.5, 285.8 and 288.4 eV, belong to C−C, C−O−Mo, C=O−O [38,44,45]. It is inferred that a carbonaceous complex layer may be constructed around MoO2 nanospheres and combined with the MoO2 surface via C−O−Mo bonds after glucose hydrothermal treatment. The deconvoluted C 1s spectra of MoO2 and MoO2/C-OV were shown in Figures S4 and S5. The above results revealed that the introduction of carbonaceous precursors in situ could trigger the reconstruction and fabricate surface oxygen vacancies on the surface of MoO2 via a hydrothermal process.
The presence of oxygen vacancies was further confirmed via room-temperature EPR. As shown in Figure 2f, no signal was observed in the EPR spectrum of the pristine MoO2 nanospheres. In comparison, the obvious oxygen vacancies signal with a g value of 2.003 was exhibited for the MoO2/C-OV, which could be identified as unpaired electrons trapped in the oxygen vacancy [34,46]. More oxygen vacancies were generated due to the introduction of carbon atoms. The results were consistent with the XPS spectra. This demonstrated that the MoO2/C-OV possesses more oxygen vacancies in the surface after the glucose hydrothermal process.
The photo-response of catalysts is an important indicator of photocatalytic efficiency. The optical properties of the samples with different structures were investigated via UV-vis diffuse reflectance spectroscopy (DRS) measurement. As shown in Figure 3a, the visible-light absorption intensity of MoO2, especially in the near-infrared region, was significantly enhanced after the glucose hydrothermal process. MoO2/C-OV exhibited a powerful absorption intensity from the UV to the NIR region. The band gap energy (Eg) of the photocatalysts plays an important role in evaluating the physical and optical properties, which could be calculated according to the following Formula [40].
α h v = A h v − E g n / 2
where α, hν, A and Eg represent the absorption coefficient, Planck constant, light frequency, proportionality and band gap energy, respectively. As shown in Figure 3b, MoO2, MoO2/C-OV and MoO2/C possessed a corresponding Eg of 1.4 eV, 1.8 eV and 1.6 eV. The energy band structures of the samples were evaluated using Mott–Schottky plots, as shown in Figure 3c and Figure S7. The slope of the Mott–Schottky curve of the catalysts was positive, indicating that all samples were n-type semiconductors with electrons as the majority carriers. The indicates the flat-band potential of MoO2 and MoO2/C-OV was −0.75 V and −1.1 V vs. NHE. These results indicated that the conduction band energy of MoO2 and MoO2/C-OV was more negative than that of the O2/•O2− (−0.33 V vs. NHE). O2 molecules adsorbed on the surface of the samples were reduced to active ∙O2− via the photogenerated electrons, which were beneficial as they accelerated the oxidation of BA. By considering the band gap energies, the valence band energy levels (VB) of MoO2 and MoO2/C-OV were calculated to be 0.65 V and 0.7 V, respectively.
Transient photocurrent measurements were further carried out to verify the charge generation and separation efficiency, as shown in Figure 3d. Compared with the MoO2 and MoO2/C, MoO2/C-OV exhibited a higher photocurrent response, which suggested that the MoO2/C-OV would produce more photo-electrons and holes than MoO2 and MoO2/C in the same timeframe. Moreover, the separation and transfer of these generated photo-electrons and holes occurred more efficiently in the MoO2/C-OV than MoO2 and MoO2/C. The results demonstrated that the surface OVs would hasten the photo-electrons and holes separation to further induce the photo-electron transfer from the inside to the surface of the MoO2/C-OV. Therefore, it revealed that surface OVs promote the generation of the photo-electrons and accelerate the charges transfer on the interface to improve the transformation of the benzylamine molecules on the MoO2/C-OV.

2.2. Photocatalytic Activity for Oxidation of Amines to Imines

The photocatalytic performance of the MoO2 samples was evaluated via selective oxidative coupling of amines to imines under white LED light irradiation at air pressure. The oxidative coupling of BA to N-benzylidenebenzylamine was chosen as the type of reaction to explore the activation of molecular oxygen. The control experiment was run in the absence of catalyst, and there was no product under visible light irradiation for 3.5 h. As shown in Figure 4a, the conversion was only 25% with >99% selectivity when air was replaced with N2, demonstrating the indispensable role of molecular oxygen for the photocatalytic oxidative coupling of BA. To confirm the effect of illuminant on the oxidation reaction, the dark reaction was carried out with other things being equal. The control reaction was testified to be very slow, and the conversion was situated at 9% in the presence of MoO2/C-OV under the dark condition. In the presence of visible light under the same conditions, the MoO2/C-OV exhibited much higher conversion (96%) and selectivity (>99%) for the formation of N-benzylidenebenzylamine. The concentration of oxygen vacancy sites of MoO2/C sample was less than that of MoO2/C-OV. As the comparison, the conversion of MoO2/C was only 50% with >99% selectivity of N-benzylidenebenzylamine under identical conditions. The results confirmed that the presence of surface OVs was the critical requirement for the activation of molecular oxygen. The conversion of MoO2 (7%) was much lower than that of MoO2/C-OV, which indicated that the synergistic effect of carbonaceous material and oxygen vacancies played an important role in the photocatalytic reaction.
The main photo-generated active species during the photocatalytic BA oxidation were verified through radical trapping experiments with a series of scavengers over the MoO2/C-OV sample. As shown in Figure 4b, the conversion of BA dramatically decreased with the addition of KI (h+ scavenger), AgNO3 (e− scavenger), p-BQ (•O2− scavenger) or NaN3 (1O2 scavenger), while IPA (•OH scavenger) exhibited a negligible quenching effect [47,48,49]. The BA conversion significantly reduced with the addition of trapping agents: 34.5% (with KI), 32.4% (NaN3), 18.7% (p-BQ) and 51.9% (AgNO3). The results indicated that h+, e−, •O2− and 1O2 played a key role in the reaction process for the MoO2/C-OV sample, where •O2− and 1O2 are the main active species in the photocatalytic reaction. The results proved that both photo-generated holes and electrons were the initial driving forces for the photocatalytic coupling reaction. The conversions of MoO2/C with the addition of trapping agents were lower than that of MoO2/C-OV. As shown in Figure 4c, the MoO2/C sample has the same changing tendency as the results of the MoO2/C-OV sample, which implied that a similar photocatalytic process was involved. The results further revealed that the oxygen vacancy played an important role in visible light photocatalytic selective oxidation of BA. The formation of H2O2 was detected via the iodometry method in the photocatalytic oxidation of BA. The result illustrated that the H2O2, produced by the reaction of •O2− and protons, was an intermediate in the photocatalytic aerobic oxidative coupling reaction of BA. Hence, the radical experiments indicated that h+, e−, •O2− and 1O2 species played a key role in the photocatalytic oxidation of BA.
In order to prove the versatility of the as-prepared photocatalyst, the selective oxidation of amine derivatives was considered. The MoO2/C-OV sample showed a high efficiency for the visible light-driven photocatalytic oxidation of amines at 1 atm air pressure. As listed in Table 1, the arylamines with electron-withdrawing groups would be more challenging to transform into the corresponding imines than that of arylamines with electron-donating groups. It is reported that the electron-induced effect of an electron-withdrawing group could enhance the oxidative coupling of the C-N bond via the α-carbon activation. The electron inductive effect of the -Cl group is relatively weaker than that of the -F group. For Entry 2 and 3, the reactant substrate with the -F group showed high conversion under the same conditions. Entries 6 and 7, the reactant substrates with an electron-donating group, exhibited a high yield to produce their corresponding imines with high selectivity under the same reaction conditions, owing to the conjugative effect of the substituent groups. In addition, for Entries 3–5 and 7–9, the para and meta substitution of the reactant substrates was beneficial, offering greater activation of α-carbon than that of ortho substitution due to the steric effect.

2.3. Proposed Mechanism of Photocatalytic BA Coupling to N-benzylidenebenzylamine

A reaction mechanism is proposed for the selective transformation of BA to N-benzylidenebenzylamine over MoO2/C-OV, as shown in Figure 5, based on the above results and discussions. The surface OVs sites of MoO2/C-OV would collect abundant O2 molecules from the air. Under the visible light irradiation, electrons and holes were generated in the MoO2/C-OV photocatalyst. The photogenerated electrons rapidly transferred to the positively charged surface OVs of the MoO2/C-OV, which played an important role in the further reduction of adsorbed O2 molecules to produce active •O2− anion radicals. The holes generated transferred from bulk to surface and oxidized most of the•O2− into 1O2. Moreover, the adsorbed BA underwent oxidation via photogenerated holes to form amine radical cations, and subsequent activation by 1O2 and •O2− radicals promoted formation of imine intermediate (RCH=NH) and H2O2. As shown in Figure S9, the H2O2 intermediate in the process of reaction was detected via the iodometry method.

3. Materials and Methods

3.1. Chemicals and Reagents

Molybdenyl acetylacetonate (MoO2(acac)2, 97%) was purchased from Sigma-Aldrich Co., Ltd. (Shanghai, China) Ethanol. D-(+)-glucose (AR) and isopropanol (IPA) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the reagents were analytical grade and used without further purifications. Ultrapure water (18.25 MΩ·cm) was used throughout the experiments.

3.2. Preparation of MoO2 Nanospheres

MoO2 nanospheres were synthesized via a facile one-pot solvothermal process. Typically, 600 mg of MoO2(acac)2 was dissolved into 12 mL of water, 12 mL of ethanol and 36 mL of isopropyl alcohol via magnetic stirring. After 2 h stirring, the mixed solution was transferred into a 100 mL Teflon-lined stainless-teel autoclave at 200 °C for 10 h. After cooling to room temperature, the obtained black and blue precipitates were washed with absolute ethanol and distilled water, then dried at 60 °C overnight. Finally, the MoO2 nanospheres were obtained.

3.3. Preparation of Defective MoO2/C Nanospheres

In a typical synthesis process, 80 mg of MoO2 nanospheres and 1200 mg of glucose were added into 40 mL of ultrapure water, then stirred to obtain a homogeneous solution. The mixture was poured into a 100 mL Teflon-lined stainless-teel autoclave at 180 °C for 3 h. The precipitates were gathered via centrifugation and washed with absolute ethanol and water. Finally, the samples were marked MoO2/C-OV. The as-prepared MoO2/C-OV sample was calcined in an air flow at 300 °C to obtain MoO2/C.

3.4. Electrochemistry Measurements

Electrochemical measurements were characterized on a CH660D electrochemical workstation in a three-electrode model, using the sample films as the working electrode, saturated calomel electrode (SCE) as the reference electrode and Pt wire as the counter electrode. The working electrode was prepared as follows: 5 mg of catalyst was dispersed in 0.5 mL of ethanol and 25 μL of Nafion solution, and ultrasonically treated until a homogeneous mixture was obtained. The Mott–Schottky, electrochemical impedance spectroscopy (EIS) and photocurrent response measurements were performed at the electrochemical workstation with the working electrodes immersed in 0.1 M Na2SO4 aqueous solution.

3.5. Photocatalytic Oxidative Coupling of Benzylamine

The photocatalytic reactions were conducted under an air atmosphere at 45 °C for 3.5 h. The reaction system was composed of a catalyst (6 mg) and benzylamine (0.077 mmol) in acetonitrile (3 mL) using white LED light as the light source. The optical power density at the liquid surface was about 300 mW cm−2. The reaction products were analyzed via gas chromatography (GC, Agilent 7890B, Agilent, Santa Clara, CA, USA). The reactive oxygen species (ROS) quenching experiments Radical trapping experiments were carried out under the above conditions, except for the addition of scavengers. Isopropanol (IPA), KI, p-benzoquinone (pBQ) and NaN3 were used as the scavengers for hydroxyl radicals (•OH), holes (h+), superoxide radicals (•O2−) and singlet oxygen (1O2), respectively.

3.6. Detection of H2O2

The iodometry method was selected to detect the H2O2 produced in the photocatalytic reaction process. The reaction equation is as follows: H 2 O 2 + 3 I − + 2 H + → I 3 − + 2 H 2 O The H2O2 reacted with I− under acidic conditions to form I3−, which shows strong absorption at about 350 nm. The solution A: KI (249 mg) was dissolved in ultrapure water (15 mL). Solution B: the potassium biphthalate (1.225 g) was added to 15 mL of ultrapure water. The detection process is as follows: after mixing 2 mL of solution A and 2 mL of solution B, 0.1 mL reaction solution and 0.9 mL ultrapure water were added and sonicated for 1 min. The absorption spectrum was measured using a UV-vis spectrophotometer.

4. Conclusions

In summary, defective MoO2/C nanospheres were successfully prepared via the one-pot glucose hydrothermal method. The MoO2 nanospheres combined closely with carbonaceous materials, forming a clear disordered region and a unique interface microstructure. In situ introduction of carbonaceous materials triggered a reconstruction of the MoO2 surface, which introduced abundant surface OVs on the MoO2/C composites. The surface OVs and carbonaceous materials played a synergistic role in the activation of molecular oxygen into 1O2 and •O2− species. Furthermore, the surface OVs structure led to the effective separation of the photogenerated carriers. The MoO2/C-OV sample displayed superior photocatalytic oxidation of benzylamine under visible light irradiation at 1 atm air pressure, and the performance was 10 times that of pristine MoO2 nanospheres with high selectivity (>99%). This work can provide an avenue for the design of photocatalysts used for photocatalytic transformation of amines to imines by introducing oxygen vacancy of Mo-based metal oxides.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28124739/s1, characterizations and measurement details, Figures S1–S9, Tables S1 and S2. Figure S1. SEM images of (a) MoO2 samples, (b) MoO2/C samples; Figure S2. TEM images of (a) MoO2 and (b) MoO2/C; Figure S3. The corresponding TEM image and EDX elemental mapping of Mo and O for the MoO2; Figure S4. The corresponding TEM image and EDX elemental mapping of C, Mo and O for the MoO2/C; Figure S5. XPS spectra of C 1s in the as-prepared MoO2; Figure S6. XPS spectra of C 1s in the as-prepared MoO2/C; Figure S7. Mott–Schottky plots of MoO2; Figure S8. EIS Nyquist plots of MoO2, MoO2/C-OV, MoO2/C; Figure S9. UV-vis absorption spectra of the solution after photocatalytic reaction for detection H2O2; Table S1. The contents of the Mo, O, and C elements according to the XPS spectra; Table S2. The spectrum parameters of the fitted O 1s peaks.

Author Contributions

Conceptualization, Y.C. and J.J.; experiment, Y.C.; writing—original draft preparation, Y.C.; writing—review and editing, Y.C., T.H., T.T. and J.J.; data curation, Y.C. and Y.Z.; supervision, T.H., W.C. and E.L.; funding acquisition, Y.C., W.C. and E.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 22209102) and Natural Science Foundation of Shanxi Province (No. 202203021221134, 202203021222233).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Not applicable.

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Molecules 28 04739 g001 550
Figure 1. (a,b) High-magnification SEM image of the as-prepared MoO2/C-OV. (c) TEM image, (d) high-magnification TEM image, (e) HRTEM image and (f) SAED profiles of the as-prepared MoO2/C-OV. (g) The corresponding TEM image and EDX elemental mapping of C, Mo and O for the MoO2/C-OV.
Figure 1. (a,b) High-magnification SEM image of the as-prepared MoO2/C-OV. (c) TEM image, (d) high-magnification TEM image, (e) HRTEM image and (f) SAED profiles of the as-prepared MoO2/C-OV. (g) The corresponding TEM image and EDX elemental mapping of C, Mo and O for the MoO2/C-OV.
Molecules 28 04739 g001
Molecules 28 04739 g002 550
Figure 2. (a) XRD pattern, (b) XPS survey spectra and XPS spectra of (c) Mo 3d and (d) O 1s of MoO2, MoO2/C-OV and MoO2/C. (e) XPS spectra of C 1s in the as-prepared MoO2/C-OV. (f) Room-temperature ESR spectra of MoO2, MoO2/C-OV and MoO2/C.
Figure 2. (a) XRD pattern, (b) XPS survey spectra and XPS spectra of (c) Mo 3d and (d) O 1s of MoO2, MoO2/C-OV and MoO2/C. (e) XPS spectra of C 1s in the as-prepared MoO2/C-OV. (f) Room-temperature ESR spectra of MoO2, MoO2/C-OV and MoO2/C.
Molecules 28 04739 g002
Molecules 28 04739 g003 550
Figure 3. (a) UV-vis DRS of MoO2, MoO2/C-OV and MoO2/C. (b) The Tauc plot of MoO2, MoO2/C-OV and MoO2/C. (c) Mott–Schottky plots of MoO2/C-OV. (d) Transient photocurrent responses under visible light illumination of the samples.
Figure 3. (a) UV-vis DRS of MoO2, MoO2/C-OV and MoO2/C. (b) The Tauc plot of MoO2, MoO2/C-OV and MoO2/C. (c) Mott–Schottky plots of MoO2/C-OV. (d) Transient photocurrent responses under visible light illumination of the samples.
Molecules 28 04739 g003
Molecules 28 04739 g004 550
Figure 4. (a) Control experiments for photocatalytic oxidation of benzylamine to N-benzylidenebenzylamine under visible light irradiation. Effect of scavengers on the photocatalytic oxidative coupling of (b) MoO2/C-OV and (c) MoO2/C.
Figure 4. (a) Control experiments for photocatalytic oxidation of benzylamine to N-benzylidenebenzylamine under visible light irradiation. Effect of scavengers on the photocatalytic oxidative coupling of (b) MoO2/C-OV and (c) MoO2/C.
Molecules 28 04739 g004
Molecules 28 04739 g005 550
Figure 5. Proposed mechanism for the photocatalytic selective oxidative coupling of BA on MoO2/C-OV catalyst under visible light irradiation.
Figure 5. Proposed mechanism for the photocatalytic selective oxidative coupling of BA on MoO2/C-OV catalyst under visible light irradiation.
Molecules 28 04739 g005
Table
Table 1. Aerobic photocatalytic oxidation of benzylamine substitutes over MoO2/C-OV.
Table 1. Aerobic photocatalytic oxidation of benzylamine substitutes over MoO2/C-OV.
Molecules 28 04739 i001
EntrySubstrateProductConv. (%)Sel. (%)
1Molecules 28 04739 i002Molecules 28 04739 i00395>99
2Molecules 28 04739 i004Molecules 28 04739 i00589>99
3Molecules 28 04739 i006Molecules 28 04739 i0078398
4Molecules 28 04739 i008Molecules 28 04739 i00977>99
5Molecules 28 04739 i010Molecules 28 04739 i01173>99
6Molecules 28 04739 i012Molecules 28 04739 i01395>99
7Molecules 28 04739 i014Molecules 28 04739 i01599>99
8Molecules 28 04739 i016Molecules 28 04739 i01799>99
9Molecules 28 04739 i018Molecules 28 04739 i0199697
Reaction conditions: Benzylamine (0.077 mmol), catalyst (6 mg), acetonitrile (3 mL), air (1 atm), white LED light (300 mW cm−2), irradiation time (3.5 h).
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Chang, Y.; Zhang, Y.; Hu, T.; Chen, W.; Tang, T.; Luo, E.; Jia, J. Carbonaceous Material Modified MoO2 Nanospheres with Oxygen Vacancies for Enhanced Visible-Light Photocatalytic Oxidative Coupling of Benzylamine. Molecules 2023, 28, 4739. https://doi.org/10.3390/molecules28124739

AMA Style

Chang Y, Zhang Y, Hu T, Chen W, Tang T, Luo E, Jia J. Carbonaceous Material Modified MoO2 Nanospheres with Oxygen Vacancies for Enhanced Visible-Light Photocatalytic Oxidative Coupling of Benzylamine. Molecules. 2023; 28(12):4739. https://doi.org/10.3390/molecules28124739

Chicago/Turabian Style

Chang, Yuhong, Yanxia Zhang, Tianjun Hu, Wenwen Chen, Tao Tang, Ergui Luo, and Jianfeng Jia. 2023. "Carbonaceous Material Modified MoO2 Nanospheres with Oxygen Vacancies for Enhanced Visible-Light Photocatalytic Oxidative Coupling of Benzylamine" Molecules 28, no. 12: 4739. https://doi.org/10.3390/molecules28124739

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