Highly Selective Photocatalytic Oxidation Biomass Valorization Over Nb2O5/g‐C3N4 Heterojunction

The selective biomass conversion of 5‐hydroxymethylfurfural (HMF) to 2,5‐diformylfurane (DFF) through photocatalysis is of immense importance and has attracted much interest recently. However, its selectivity and conversion are still limited at present due to the lack of efficient photocatalysts to controllably activate the alcohol hydroxyl groups. Herein, Nb2O5/g‐C3N4 type‐II heterojunction photocatalysts to restrain the over‐oxidation are developed, which successfully adjust the active oxygen species and enhance the separation and transfer of photoexcited carriers. The system achieves selective oxidation of HMF to DFF with high selectivity (>99%) and conversion (53%) via the inhibition of hydroxyl radicals (·OH) production arising from the decomposition of H2O2. Moreover, h+ is the main active species for photocatalytic selective oxidation of HMF, and ·O2− is the second for that. Herein, it offers an effective strategy to effectively improve the high‐selective conversion of biomass derivatives into high‐value‐added products.

DOI: 10.1002/aesr.202200116 The selective biomass conversion of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfurane (DFF) through photocatalysis is of immense importance and has attracted much interest recently. However, its selectivity and conversion are still limited at present due to the lack of efficient photocatalysts to controllably activate the alcohol hydroxyl groups. Herein, Nb 2 O 5 /g-C 3 N 4 type-II heterojunction photocatalysts to restrain the over-oxidation are developed, which successfully adjust the active oxygen species and enhance the separation and transfer of photoexcited carriers. The system achieves selective oxidation of HMF to DFF with high selectivity (>99%) and conversion (53%) via the inhibition of hydroxyl radicals (·OH) production arising from the decomposition of H 2 O 2 . Moreover, h þ is the main active species for photocatalytic selective oxidation of HMF, and ·O 2 À is the second for that. Herein, it offers an effective strategy to effectively improve the high-selective conversion of biomass derivatives into high-value-added products.
which greatly reduces the photocatalytic efficiency. As reported in previous studies, constructing matched heterojunctions has become a more competitive approach for enhanced performance in photocatalytic selective oxidation. [35,36] Therefore, it is imperative to introduce an appropriate component to modify g-C 3 N 4 with decreased H 2 O 2 evolution and improved separation of charge carriers. Niobium pentoxide (Nb 2 O 5 ) is an n-type transition metal oxide semiconductor for photocatalysis. Different from g-C 3 N 4 , nearly no H 2 O 2 is generated on Nb 2 O 5 through photocatalysis. [37] Besides, Nb 2 O 5 is suitable for selective oxidation with high selectivity. [38] In addition, the bandgap edge of Nb 2 O 5 can match well with g-C 3 N 4 . In this case, the electrons from g-C 3 N 4 can transfer to the mild conduction band position through the type-II heterojunction. [25] Thereby, the introduction of Nb 2 O 5 would provide an outstanding strategy for improving oxidation properties of g-C 3 N 4 .
Herein, an Nb 2 O 5 /g-C 3 N 4 type-II heterojunction is exquisitely designed for highly selective and efficient photocatalytic HMF oxidation to DFF. Nb 2 O 5 /g-C 3 N 4 heterojunction successfully modulates the oxidative active species and exhibits effective photoexcited carries separation and transfer to improve DFF selectivity and oxidative capacity of g-C 3 N 4 . The system achieves an outstanding DFF selectivity of >99% and an HMF conversion (53%) approximately 3 times that of pure g-C 3 N 4 under simulated sunlight. Mechanistic investigation reveals that the key to the effective inhibition of ·OH formation is to hamper the H 2 O 2 production because of the electrons of g-C 3 N 4 transfer to Nb 2 O 5 resulting in the reduction of H 2 O 2 active site. The degree of inhibition is about 7.7-fold, thus further inhibiting the formation of ·OH, which finally effectively enhances the selectivity of DFF. In addition, the introduction of Nb 2 O 5 promotes the generation of ·O 2 À and photoexcited holes to enhance the conversion activity. It is worthy to note that the selectivity of DFF after 5 cycle times is almost unchanged with >99%. This work provides an effective strategy for the highly selective conversion of HMF to DFF.

Characterization of Photocatalysts
The Nb 2 O 5 /g-C 3 N 4 type-II heterojunction was manufactured through calcination under a nitrogen atmosphere (Scheme 1). The morphology and microstructure of photocatalysts were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) technologies. The SEM images of g-C 3 N 4 , Nb 2 O 5 , and 4.5 wt% Nb/CN are shown in Figure S1a-c, Supporting Information. Evidently, Nb 2 O 5 nanosheets are uniformly deposited on the surface of g-C 3 N 4 . From the typical TEM images (Figure 1a and S1d, Supporting Information), it is found that both pure g-C 3 N 4 and Nb 2 O 5 have similar layered nanosheet structures, in which g-C 3 N 4 shows a 2D irregular flaky shape, whereas the high crystallinity is observed for Nb 2 O 5 due to the treatment by HF and calcination ( Figure S2, Supporting Information). As shown in Figure 1b, it is clearly observed that g-C 3 N 4 nanosheets are combined tightly with Nb 2 O 5 nanosheets, and Nb 2 O 5 nanosheets are dispersed at the edges of g-C 3 N 4 nanosheets, indicating the formation of 2D/2D Nb 2 O 5 /g-C 3 N 4 type-II heterojunction. In addition, the high-resolution TEM (HRTEM) image illustrated that a clear boundary exists between g-C 3 N 4 and Nb 2 O 5 by enlarging the yellow frame area ( Figure 1c). Furthermore, the crystal lattice fringes of 0.389 nm are attributed to the (001) crystal plane of Nb 2 O 5 .
To confirm the crystal structure and functional groups of Nb 2 O 5 /g-C 3 N 4 heterojunction, X-ray diffraction (XRD) and Fourier transform-infrared (FT-IR) measurements were performed. The phase and crystal structures of samples are provided in Figure 1d. Typical XRD pattern of the pure g-C 3 N 4 exhibits two diffraction peaks at 13.1°and 27.4°corresponding to the (100) plane of the characteristic interplanar staking peak of aromatic systems and (002) plane of the interlayer structural packing, respectively. [39,40] Notably, the (002) plane of g-C 3 N 4 nanosheets is sharp and strong, indicating the high crystallinity. The XRD pattern of pure Nb 2 O 5 is in consistent with pseudohexagonal phase (JCPDS card: 28-0317), [41] and this phase hardly change and nitrided after calcination at 520°C under a nitrogen atmosphere. [42] The loading of Nb 2 O 5 on g-C 3 N 4 weakens the characteristic peak of g-C 3 N 4 at 13.1°, suggesting that much-lowered order of the structural packing of g-C 3 N 4 . [43] Simultaneously, the XRD patterns of Nb/CN samples exhibit a combination of characteristic peaks of g-C 3 N 4 and Nb 2 O 5 without any impurity phases, indicating that the samples are well crystallized. Moreover, the crystallinity of Nb 2 O 5 becomes higher by using HF treatment. The growth of Nb 2 O 5 nanosheets follows their (001) direction, which is conducive to improving the photocatalytic performance ( Figure S3, Supporting Information). [44] In Figure 1e, the FT-IR spectra of the Nb/CN composites show that stretching vibrations at %1200-1600 and 3000-3300 cm À1 , which can be assigned to the stretching vibrations of C-N heterocycles and amino groups present in the g-C 3 N 4 structure with some contribution of adsorbed OH groups. [45] A band at %1400 cm À1 corresponds to the stretching vibration Nb-O. [46] All these results certificate the successful construction of Nb 2 O 5 /g-C 3 N 4 type-II heterojunction.
X-ray photoelectron spectroscopy (XPS) was performed to investigate the elemental composition, chemical states, and valence states of 4.5 wt% Nb/CN. Figure S4, Supporting Information, shows that the corresponding elements of samples can all be observed in the survey spectrum. Figure 2a shows the Scheme 1. Schematic illustration for the fabrication of Nb 2 O 5 /g-C 3 N 4 heterojunction and its application in photocatalytic HMF oxidation.
www.advancedsciencenews.com www.advenergysustres.com   high-resolution XPS spectra of Nb 3d, and the binding energies of Nb 3d 5/2 and Nb 3d 3/2 for 4.5 wt% Nb/CN are 207.9 and 205.2 eV, suggesting the major Nb 5þ species on composites. [47,48] The C 1s spectra (Figure 2b) of both g-C 3 N 4 and 4.5 wt% Nb/CN can be deconvoluted into two peaks at 288.2 and 284.8 eV, which can be assigned to N-C═N and amorphous carbon in the environment. For 4.5 wt% Nb/CN, the C 1s binding energies are almost unchanged compared to pure g-C 3 N 4 , indicating the combination of Nb 2 O 5 does not change the structure of g-C 3 N 4 . [49,50] While the N 1s binding energies (Figure 2c) of 4.5 wt% Nb/CN shift a little toward high binding energy compared with g-C 3 N 4 , [51] illustrating that the electron transfer exists at the interface between g-C 3 N 4 and Nb 2 O 5 . In the O 1s high-resolution XPS spectra of 4.5 wt% Nb/CN (Figure 2d), the binding energies of 528.3, 530.2, and 535.7 eV correspond to Nb-O, Nb-O-H, and C-O, respectively. [52] The results of XPS spectra prove the interaction at the heterogeneous interface between g-C 3 N 4 and Nb 2 O 5 .
To analyze the specific surface area and pore structure, N 2 adsorption-desorption isotherms measurements were carried out ( Figure S5 and Table S1, Supporting Information). As shown in Figure S5, Supporting Information, the modification of Nb 2 O 5 can increase the specific surface area to 54.8504 m 2 g À1 (4.5 wt% Nb/CN), which confirms that Nb/CN heterojunction can expose more photocatalytic activity and adsorption sites.

Evaluation of Photocatalytic Performance
The photocatalytic selective HMF oxidation of different samples was evaluated using atmospheric O 2 as an oxidant under simulated sunlight irradiation (AM 1.5G), in which acetonitrile (ACN) and benzotrifluoride (PhCF 3 ) were used as the solvent. The loading amount of Nb 2 O 5 is first optimized in Figure 3a. As expected, pure g-C 3 N 4 exhibits low photocatalytic activity with 17.3% conversion of HMF and a poor DFF selectivity at 55.6% during the reaction time of 6 h. With the introduction of Nb 2 O 5 , an enhanced performance is observed. When the loading amount of Nb 2 O 5 increased to be 4.5 wt%, the best performance is achieved for 4.5 wt% Nb/CN with a DFF selectivity of >99% and an HMF conversion of 53%, which is also much higher than the literature reports (Table S2, Supporting Information). When the content of Nb 2 O 5 is over 4.5 wt%, the HMF conversion and DFF selectivity are decreased, which is probably due to the reduced interfacial contact of the heterojunctions. [53] Besides, we investigated the effect of reaction time (Figure 3b). Apparently, the HMF conversion increases with the prolongation of reaction time; meanwhile, the DFF selectivity retains over 90%. Figure 3c showed the performance under different atmospheric conditions, and it is confirmed that O 2 is an essential oxidant for the selective oxidation of HMF to DFF. [54] As observed in Figure S6a,b, Supporting Information, the increment of ACN content promotes the DFF selectivity, while the increment of PhCF 3 amount improves HMF conversion. Under the optimal ACN/PhCF 3 ratio of 2:3, the best photocatalytic conversion and selectivity are attained over 4.5 wt% Nb/CN heterostructure. In addition, the effect of solvent and photocatalyst dosage is analyzed ( Figure S6c, Supporting Information). The recycle stability of 4.5 wt% Nb/CN is further studied. The performance of 4.5 wt% Nb/CN for the photocatalytic selective www.advancedsciencenews.com www.advenergysustres.com oxidation of HMF to DFF is almost unchanged after five cycles (Figure 3d). In order to furtherly analyze the reasons for the highly selective DFF generation by 4.5 wt% Nb/CN, the H 2 O 2 was first quantified based on an spectrophotometry method by measuring the absorbance at 400 nm. [55] The yield of hydrogen peroxide was consistent with its selectivity of DFF ( Figure S7, Supporting Information). As shown in Figure 3e, 4.5 wt% Nb/CN exhibits a lower H 2 O 2 yield than that of g-C 3 N 4 , benefiting from the addition of Nb 2 O 5 . The degree of inhibition of H 2 O 2 production is around 7.7-fold under O 2 atmosphere, since pure Nb 2 O 5 does not produce H 2 O 2 . This change is rationally ascribed to rapid electrons of g-C 3 N 4 to Nb 2 O 5 via the formed type-II pathway at the interface, thus effectively inhibiting the formation of ·OH radicals. Moreover, the change in the amount of ·OH radicals was also detected using the coumarin fluorescence probe techniques. After the simulated sunlight irradiation for 6 h, the fluorescence peak intensity of 4.5 wt% Nb/CN significantly decreased in comparison with g-C 3 N 4 (Figure 3f ), demonstrating that the less·OH radicals are generated for g-C 3 N 4 when Nb 2 O 5 is loaded on the surface. According to these results, we believe that the decrease in the production of H 2 O 2 , accompanied by the reduction in the generation of ·OH radicals, is the primary reason for the improved selectivity of 4.5 wt% Nb/CN.

Photoelectrochemical Behaviors and Photocatalytic Hydrogen Peroxide Inhibition
To find the reasons for the improved performance of Nb/CN heterojunction, the relevant energy levels of g-C 3 N 4 and Nb 2 O 5 were investigated. According to UV-vis diffuse reflectance spectra (DRS), pure g-C 3 N 4 exhibits an absorption edge at about 460 nm. Compared with pure g-C 3 N 4 , pure Nb 2 O 5 displays a smaller absorption edge at %417 nm, indicating the weak absorption in the visible light region. As for Nb/CN heterojunction, its absorption presents a slight red shift in contrast to g-C 3 N 4 . [56] Consequently, the construction of the Nb/CN heterojunctions possesses a better photoresponse ability than g-C 3 N 4 ( Figure 4a). As shown from the Tauc plots in the insert of Figure 4a, the bandgap energies (E g ) of Nb 2 O 5 and g-C 3 N 4 are calculated to be 3.07 and 2.83 eV, respectively. The VB-XPS spectra ( Figure S8, Supporting Information) display that the valance band maximum of Nb 2 O 5 and g-C 3 N 4 are located at about 2.70 and 1.86 eV, below the Fermi level, respectively. In addition, based on the Mott-Schottky measurements (Figure 4b,c), [57] both Nb 2 O 5 and g-C 3 N 4 are determined to be n-type semiconductors, and their flat band potentials are calculated to be À0.26 (Nb 2 O 5 ) and À0.73 V (g-C 3 N 4 ) versus Ag/AgCl. According to these results, the energy-level alignment of the Nb/CN composite is illustrated in Figure 4d. The electrons in the CB of g-C 3 N 4 are www.advancedsciencenews.com www.advenergysustres.com easily transferred to the CB of Nb 2 O 5 , and the holes in the VB of Nb 2 O 5 are transferred to that of g-C 3 N 4 . Thus, we speculate that type-II heterojunction is formed between g-C 3 N 4 and Nb 2 O 5 , which accelerates the separation of electron-hole pairs and promotes the photocatalytic performance. In this case, the photogenerated electrons are transferred from g-C 3 N 4 to Nb 2 O 5 , giving rise to the reduced active sites for H 2 O 2 generation on g-C 3 N 4 . Furthermore, the separation and transfer of photogenerated charges are studied. Photoluminescence (PL) spectroscopy and the time-resolved photoluminescence (TRPL) decay spectra were first applied to assess the separation efficiency of electron-hole pairs. [58] As illustrated in Figure 5a, the PL emission peak of g-C 3 N 4 exhibits strong at about 437 nm. In comparison, the peak intensity of 4.5 wt% Nb/CN is much lower, implying the effective charge separation and transfer of g-C 3 N 4 when Nb 2 O 5 is loaded on the surface. According to the results of TRPL decay spectra (Figure 5b), it is found that the average fluorescence decay lifetime of 4.5 wt% Nb/CN (4.06 ns) is much longer than that of g-C 3 N 4 (2.17 ns). Based on this, it is also confirmed that 4.5 wt% Nb/CN heterojunction showed the best photocatalytic selective oxidation of HMF activity owing to the efficient transfer and delayed lifetime of photogenerated e À and h þ . In addition, the transient photocurrent response (TPC) and electrochemical impendence spectroscopy (EIS) plots of 4.5 wt% Nb/CN, Nb 2 O 5 , and g-C 3 N 4 were further used to study the separation and migration of electrons and holes. As given in Figure 5c, the 4.5 wt% Nb/CN heterojunction shows the highest photocurrent responsive intensity, which implies that the modification of Nb 2 O 5 effectively improve photogenerated electron-hole separation of g-C 3 N 4 . Moreover, their EIS Nyquist plots (Figure 5d) exhibit the smallest arc radius at the Nb/CN heterojunction interface, indicating the smallest interfacial resistance. All these results demonstrate that Nb/CN heterostructure possesses accelerated charge separation and transfer for enhanced selective oxidation of HMF.
In addition, the inhibition mechanism of H 2 O 2 production over the 4.5 wt% Nb/CN-type II heterojunction was explored by analyzing the formation of H 2 O 2 . As shown in Figure 6a, the average number of electrons (n) involved in the overall O 2 reduction was measured in terms of the reduction current of O 2 (I d ) and the oxidation current of the produced H 2 O 2 (I r ). As seen from Figure 6b, the average number of electrons (n) of g-C 3 N 4 and 4.5 wt% Nb/CN was calculated to be 2.26 and 2.60 by the obtained current intensity (at À0.45 V vs Ag/AgCl). These results imply that the formation of H 2 O 2 over g-C 3 N 4 is mainly via 2e À ORR pathway, whereas another competitive 4e À ORR process takes place in the photocatalytic H 2 O 2 generation over 4.5 wt% Nb/CN. In other words, in comparison with g-C 3 N 4 , the selectivity of photocatalytic H 2 O 2 generation is disturbed when Nb 2 O 5 is loaded. The decrease of H 2 O 2 generation selectivity is also the reason for the inhibition of photocatalytic H 2 O 2 production over 4.5 wt% Nb/CN.

Photocatalytic Mechanism
To furtherly elucidate photocatalytic HMF oxidized mechanism of Nb 2 O 5 /g-C 3 N 4 heterojunction with high selectivity under simulated sunlight irradiation, we investigated the reactive oxygen species through the radical-trapping experiment. Triethanolamine (TEOA), tertiary butanol (TBA), and AgNO 3 were added to quench h þ , ·OH, and e À during the photocatalytic reaction, respectively. As shown in Table 1, for pure g-C 3 N 4 , when TBA was added to quench ·OH, the conversion rate was almost unchanged, while the selectivity of DFF increased from 56% to >99% (Table 1, entry 7). The addition of TEOA brings a lower HMF conversion and a higher DFF selectivity ( Since the photogenerated electrons are the key substance for forming the ·O 2 À radicals, it is concluded that the electrons transfer occurred at the interface of Nb 2 O 5 /g-C 3 N 4 type-II heterojunction and ·OH would not react with HMF molecules directly, thereby leading to the high selectivity. Simultaneously, the results also demonstrated the important roles of h þ and ·O 2 À in photocatalytic selective oxidation of HMF to DFF. Electron spin-resonance (ESR) spectroscopy was also carried out to confirm the above results, and the results are illustrated in Figure 7. As shown in Figure 7a, there are no ·O 2 À radical signals were detected in dark. Under light irradiation, pure g-C 3 N 4 shows only a pretty weak DMPO-·O 2 À signal intensity. In contrast, the intensity of 4.5 wt% Nb/CN is significantly higher than that of pure g-C 3 N 4 , corresponding to their photocatalytic activities. The carbon-centered radicals of 4.5 wt% Nb/CN were further detected using the in situ EPR technique, as shown in Figure 7b. Furthermore, the typical characteristic signals of ·OH and ·O 2 À could be detected. It is noted that the signal intensity of ·OH is quietly weak. These results further demonstrated that h þ plays a major role, and ·O 2 À plays a synergistic role in the photocatalytic conversion of HMF to DFF with 4.5 wt% Nb/CN type-II heterojunction.
Based on the above-mentioned analyses, we concluded that the solar light-driven oxidation of HMF to DFF is achieved through combined contribution from holes (h þ ), photogenerated electrons (e À ), ·O 2 À , and O 2 . Consequently, we proposed a possible photocatalytic HMF oxidation mechanism of Nb 2 O 5 /g-C 3 N 4 type-II heterojunction (Scheme 2). First, the electrons (e À ) and holes (h þ ) are generated on the surface of Nb 2 O 5 and g-C 3 N 4 under  via the two-electron oxygen reduction reaction (2e À ORR) would suffer from the competing reaction of the 4e À pathway, thus favoring the inhibition of ·OH and over-oxidation. Therefore, Nb 2 O 5 /g-C 3 N 4 type-II heterojunction not only enhances the oxidation capacity but also improves the selectivity of DFF.

Conclusion
In summary, we have developed a highly selective Nb 2 O 5 /g-C 3 N 4 type-II heterojunction for photocatalytic HMF conversion under mild conditions. Under the catalytic induction by Nb 2 O 5 /g-C 3 N 4 , the production of active species is successfully adjusted. The key to achieving high selectivity is that our designed type-II heterojunction can effectively reduce H 2 O 2 production and directly inhibit the production of nonselective ·OH. This is because the electrons are transferred from the conduction band of g-C 3 N 4 , when Nb 2 O 5 is loaded, which thus reduces the effective active site for H 2 O 2 production. At the same time, the O 2 undergoes a four-electron reduction competition reaction at the surface of Nb 2 O 5 /g-C 3 N 4 , thus effectively reducing the transformation to generate nonselectively oxidized ·OH from H 2 O 2 . Besides, This catalyst promotes the ·O 2 À generation and behaves with high photoexcited carrier separation and superior stability. This work provides an effective strategy to effectively improve the highselective conversion of biomass derivatives into high-value-added products.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author. www.advancedsciencenews.com www.advenergysustres.com