Photocatalytic Degradation of Gaseous Benzene Using Cu/Fe-Doped TiO2 Nanocatalysts under Visible Light

Visible-light-enhanced TiO2 nanocatalysts doped with Cu and Fe were synthesized using the sol–gel method to investigate their performance in degrading gaseous benzene. The structure and morphology of mono- and co-doped TiO2 (i.e., Cu/Fe-TiO2, Cu-Fe-TiO2) were characterized using SEM, EDS, XRD, BET, Raman, UV-vis-DRS, and XPS techniques. The results indicated that the presence of Cu/Fe mono- and co-doped TiO2 leads to the formation of an anatase phase similar to pure TiO2. Furthermore, the introduction of Cu/Fe enhanced the presence of lattice defects and increased the specific surface area of TiO2. This enhancement can be attributed to the increase in oxygen vacancies, especially in the case of Cu-Fe-TiO2. Additionally, Cu-Fe-TiO2 showed a higher concentration of surface-bound hydroxyl groups/chemically adsorbed oxygen and a narrower bandgap than pure TiO2. Consequently, Cu-Fe-TiO2 exhibited the highest photocatalytic performance of 658.33 μgC6H6/(g·h), achieving a benzene degradation rate of 88.87%, surpassing that of pure TiO2 (5.09%), Cu-TiO2 (66.92%), and Fe-TiO2 (59.99%). Reusability tests demonstrated that Cu-Fe-TiO2 maintained a high benzene degradation efficiency of 71.4%, even after five experimental cycles, highlighting its exceptional stability and reusability. In summary, the addition of Cu/Fe to TiO2 enhances its ability to degrade gaseous benzene by prolonging the catalyst’s lifespan and expanding its photoresponse range to include visible light.


Introduction
Gaseous benzene is a typical volatile organic compound (VOC) that is widely generated in various industrial sectors, such as petroleum refining, chemical manufacturing, printing operations, and transportation [1].It produces by-products that contribute to the formation of O 3 , which has detrimental effects on the environment, human health, and ecosystems.Several technologies have been employed for the degradation of gaseous benzene, including physical and chemical adsorption, biodegradation, plasma technology, membrane separation, and catalytic oxidation [2,3].In contrast, semiconductor photocatalysis has gained significant attention in research due to its ability to effectively mineralize organic pollutants under mild reaction conditions.The key aspect of this technology lies in the development of efficient and stable photocatalysts [4].TiO 2 is widely used as a photocatalyst due to its favorable chemical properties, non-toxic nature, and resistance to photo-corrosion [5].However, its photocatalytic efficiency is limited to UV light and does not respond to visible light [6,7].Moreover, the performance of TiO 2 is impeded by the recombination of electrons and holes generated during the photocatalytic process [8].Therefore, current research focuses on reducing the band gap and electron-hole recombination rate of TiO 2 to enhance its responsiveness to a wider spectrum of light and prolong the lifespan of photocatalysis.Recently, there has been significant interest in developing visible-light-induced photocatalysts using modified TiO 2 .These modified photocatalysts have shown promising results in effectively degrading pollutants [9,10].One approach to achieve this is by establishing p-n-n heterojunctions or prolonging the lifespan of charges, thereby enhancing the structural properties of the photocatalysts.Consequently, enhancing the visible light response of TiO 2 -based materials with exceptional photocatalytic ability has become a key research focus in the field of photocatalysis [11].
To enhance the performance of TiO 2 under visible light, researchers have focused on various technologies including ion doping, surface modification, and semiconductor coupling [12,13].Among these, ion doping is considered the most effective method for altering the TiO 2 surface.Researchers have investigated the use of non-metal-doped TiO 2 to enhance its photocatalytic performance in degrading gaseous benzene.However, the improvements achieved through non-metal doping are not significant due to differences in valence state and ionic radius.For instance, the benzene degradation rate of N-doped TiO 2 under visible light is only 24% [14].On the other hand, incorporating the S element into the S-TiO 2 system has shown promising results, with the photocatalytic degradation efficiency of benzene ranging from 34% to 41%, surpassing that of N-doped TiO 2 [15].Nevertheless, further improvements in catalytic efficiency are still required to meet practical demands.
Transition metal ion doping is a convenient and effective technology for modifying the surface of TiO 2 [16,17].This process involves overlapping the ns 2 or empty d orbital of the metal ion with the 2p orbital of oxygen in TiO 2 , resulting in the creation of a new energy band within the wide bandgap of the semiconductor.Consequently, the band gap width of the semiconductor decreases [18].Naghibiet found that Fe-doped TiO 2 had the smallest crystal size and the narrowest forbidden band compared to Ce/Cd/Cudoped TiO 2 , indicating that Fe-doped TiO 2 could enhance photocatalytic performance [19].Furthermore, Cu has shown potential as a photocatalyst for absorbing visible light [20][21][22].When copper oxide is combined with TiO 2 , it exhibits excellent photocatalytic properties on various substrates or materials.The synergistic effect of this combination further enhances the photocatalytic activity [23,24].This combination expands the range of light absorption to include the abundant visible wavelengths of solar radiation.
Unfortunately, mono-doping a metal into a crystal can disturb the distribution of charge within the crystal.This interference is primarily caused by changes in valence state and ionic radius, resulting in an increased recombination rate of photogenerated carriers and a decrease in the photocatalyst's performance [25].However, research has shown that incorporating different metal elements during doping can effectively regulate charge by introducing electron donors and acceptors.These elements interact with each other's electronic structure without adversely affecting the charge density in the crystal lattice.This technique shows promise in minimizing the formation of unnecessary defects [26,27].In recent years, researchers have extensively studied co-doping systems such as Cu-Co [28], Cu-Mo [29], and Cu-Mn [30] to enhance material properties.However, these co-doping systems have shown limited success.Hence, it is crucial to explore alternative co-doping systems that can improve the desired properties.Iron, with its multiple valence states, facilitates electron transfer between catalysts and reactants, thereby enhancing photochemical reactions [31].Furthermore, doping TiO 2 with Cu and Fe can results in distinct valence states, each possessing unique photocatalytic properties.However, there is still limited research on the doping of copper and iron in TiO 2 .Therefore, it holds great significance to synthesize Cu-and Fe-doped TiO 2 photocatalysts and investigate their effectiveness in degrading gaseous benzene under visible light.
In this study, Cu/Fe mono-and co-doped TiO 2 nanoparticles were synthesized using the sol-gel method.The synthesized catalysts were characterized using SEM, EDS, XRD, BET, Raman, UV-Vis, and XPS techniques to comprehensively analyze the effects of mono-and co-doping on the photocatalytic properties of TiO 2 .Furthermore, the photocatalytic performance of various catalysts was evaluated by measuring the degradation of gaseous benzene.

Photocatalysis Experimental Setups
The photocatalytic reaction system consists of three main parts: gas transportation, reactor, and exhaust treatment (Figure 1).The gas delivery system includes a nitrogen gas purging system and an air pump to generate ambient air.Before introducing benzene and air into the reactor in the gas phase, they are thoroughly mixed using a gas mixing device.The reactor, made of quartz, is divided into three main parts: an optical condensing unit, a photocatalytic reactor, and a volatile organic compound tester (model RAE 3000) that measures concentrations in parts per billion (ppb).The light source condensing unit filters the UV rays emitted by the tungsten iodine lamps to ensure that only the desired wavelengths of UV light are used in the photocatalytic reaction.A condensation liquid containing a sodium nitrite solution with a concentration of 2 mol/L is employed to selectively utilize the specific wavelengths of UV light necessary for the photocatalytic reaction.Nickel foam is used as a catalyst carrier.To prepare the nickel foam, it is immersed in a 0.1 mol/L HNO 3 solution for 15 min and then calcinated at 300 • C for 1 h.Subsequently, the nickel foam is pretreated in a 1:1 solution of titanate coupling agent and anhydrous ethanol for 15 min.Finally, the powdered catalyst is evenly applied onto the nickel foam.During testing, a black box is used to separate the reactor from outside sunlight.Additionally, activated carbon is used to adsorb any residual waste gas that remains undegraded during photocatalytic experiments.

Photocatalysis Experimental Setups
The photocatalytic reaction system consists of three main parts: gas transportation, reactor, and exhaust treatment (Figure 10).The gas delivery system includes a nitrogen gas purging system and an air pump to generate ambient air.Before introducing benzene and air into the reactor in the gas phase, they are thoroughly mixed using a gas mixing device.The reactor, made of quartz, is divided into three main parts: an optical condensing unit, a photocatalytic reactor, and a volatile organic compound tester (model RAE 3000) that measures concentrations in parts per billion (ppb).The light source condensing unit filters the UV rays emitted by the tungsten iodine lamps to ensure that only the desired wavelengths of UV light are used in the photocatalytic reaction.A condensation liquid containing a sodium nitrite solution with a concentration of 2 mol/L is employed to selectively utilize the specific wavelengths of UV light necessary for the photocatalytic reaction.Nickel foam is used as a catalyst carrier.To prepare the nickel foam, it is immersed in a 0.1 mol/L HNO3 solution for 15 min and then calcinated at 300 °C for 1 h.Subsequently, the nickel foam is pretreated in a 1:1 solution of titanate coupling agent and anhydrous ethanol for 15 min.Finally, the powdered catalyst is evenly applied onto the nickel foam.During testing, a black box is used to separate the reactor from outside sunlight.Additionally, activated carbon is used to adsorb any residual waste gas that remains undegraded during photocatalytic experiments.

Synthesis of Photocatalysts
Pure TiO2 was synthesized using the sol-gel method as previously reported [44].Solution A was prepared by combining 25 mL of tetrabutyl titanate, 35 mL of ethanol, and

Synthesis of Photocatalysts
Pure TiO 2 was synthesized using the sol-gel method as previously reported [32].Solution A was prepared by combining 25 mL of tetrabutyl titanate, 35 mL of ethanol, and 1.2 mL of acetylacetone.Similarly, solution B was prepared by combining 35 mL of ethanol, 1.2 mL of acetylacetone, and deionized water.Solution A was then gradually added to solution B at a rate of 1 drop/s while stirring.This process lasted 3 h to obtain a mixed solution.The mixed solution was then heated to 30 • C to promote the formation of a gel with high moisture content.Before gel formation, the pH value of the mixed solution was adjusted to 4.0 using nitric acid.The wet gel was subsequently centrifuged and dried at 100 • C to obtain dehydrated gel.Finally, the dried gel was calcined at 400 • C for 2 h to obtain pure TiO 2 .
Mono-doped TiO 2 (i.e., Cu-TiO 2 , Fe-TiO 2 ) was synthesized by adding a specific amount of Cu(NO 3 ) 2 •3H 2 O or Fe(NO 3 ) 3 •9H 2 O to liquid A. The remaining steps in the synthesis process are identical to those for pure TiO 2 .Co-doped TiO 2 was synthesized by adding a certain amount of Cu(NO 3 ) 2 •3H 2 O to liquid A and a specific quantity of Fe(NO 3 ) 3 •9H 2 O to liquid B. The remaining steps in the synthesis process are also identical to those for pure TiO 2 .

Characterization Instrumentation
Scanning electron microscopy (SEM) (model: Zeiss Sigma 300+ Oxford Spectroscopy, Oberkochen, Germany) was used to examine the catalyst's surface morphology and reduce its size from the micron level to the nanoscale.Energy Dispersive X-ray Spectroscopy (EDS) (model: Zeiss Sigma 300+ Oxford Spectroscopy, Oberkochen, Germany), and EDS Elemental Mapping was used to perform qualitative and quantitative analysis of elemental distributions.X-ray diffractometry (XRD) (model: D8 ADVANCE, Bruker, Germany) was used to analyze the crystalline phase composition and degree of crystallization of the catalysts.A confocal Raman spectrometer (model: Horiba scientific LabRAM, Osaka, Japan) and BET-specific surface area (model: Quantachrome NOVA 4200e, Rochester, NY, USA) were used to calculate specific surface areas and analyze micro-and macro-porosity.A UV-Vis diffuse reflection spectrometer was performed using a UV-2600 with ISR-2600 Plus (model: Shimazu, Japan) to estimate the catalyst's optical properties.X-ray photoelectron spectroscopy (XPS) (model: */Escalab 250Xi, Waltham, MA, USA) with monochromatized aluminum anodic targets and an X-ray beam spot size of 650 µm was used to analyze the surface components and chemical status of the catalyst.

Assessment of Photocatalytic Benzene Degradation
The catalysts were quantitatively evaluated by measuring the degradation of benzene under photocatalytic conditions.The initial concentration of benzene was set at 90 ± 5 mg/m 3 , and the exposure time was 80 s.The experiment maintained a consistent relative humidity level of 40%.A catalyst mass of 5 g was used.The photocatalytic reaction was initiated by turning on the tungsten iodine light when the benzene concentrations in the gas phase before and after the reactor reached adsorption equilibrium.The tests were conducted for 80 min under each reaction condition until a steady state was reached.Real-time measurements of gaseous benzene concentrations were taken before and after the reactor.The benzene elimination (t) was calculated using the following formula: The benzene degradation rate is represented by η, where C 0 indicates the import concentration of gaseous benzene and C t indicates the export concentration.To assess the efficiency of the catalyst in the degradation process of benzene, we utilize the unit capacity metric.The unit capacity is calculated using the following formula: where κ indicates the unit capacity, µg C 6 H 6 /(g•h); Q indicates the flow of total gas, (L/h); m indicates the mass of the catalyst (g).

SEM and EDS Analysis
SEM analysis was conducted to examine the morphology and structure of the pure and doped TiO 2 catalysts.Figure 2a showed the morphology of the TiO 2 catalyst, indicating that the TiO 2 nanoparticles possess an irregular shape, with a few spherical particles dispersed throughout.These particles are evenly distributed on the surface, although some clustering can be observed.Figure 2b showed that the Fe-TiO 2 nanoparticles are spherical in shape and uniformly distributed in an aggregated state.These nanoparticles partially overlap with the TiO 2 catalyst.Additionally, the catalyst surface exhibited a multi-layer structure, which enhanced the penetration of gaseous benzene into the catalyst's interior.This, in turn, increased the specific surface area and improved the efficiency of the reaction.Figure 2c showed the clear aggregation and uniform crystallization of Cu-TiO 2 , resulting in a dense and uniform surface.The aggregation phenomenon can be attributed to the hydrophilic nature of TiO 2 nanoparticles, which causes them to aggregate through van der Waals forces.However, the introduction of Cu into TiO 2 leads to significant aggregation, reducing the specific surface area and hindering the formation of active sites on the catalyst [33,34].Figure 2d demonstrated the presence of numerous spherical nanoparticles on the surface of Cu-Fe-TiO 2 , indicating that the addition of Fe to Cu-TiO 2 increases the specific surface area.This doping further enhances the catalyst's efficiency in benzene degradation.

SEM and EDS Analysis
SEM analysis was conducted to examine the morphology and structure of the pure and doped TiO2 catalysts.Figure 1a showed the morphology of the TiO2 catalyst, indicating that the TiO2 nanoparticles possess an irregular shape, with a few spherical particles dispersed throughout.These particles are evenly distributed on the surface, although some clustering can be observed.Figure 1b showed that the Fe-TiO2 nanoparticles are spherical in shape and uniformly distributed in an aggregated state.These nanoparticles partially overlap with the TiO2 catalyst.Additionally, the catalyst surface exhibited a multi-layer structure, which enhanced the penetration of gaseous benzene into the catalyst's interior.This, in turn, increased the specific surface area and improved the efficiency of the reaction.Figure 1c showed the clear aggregation and uniform crystallization of Cu-TiO2, resulting in a dense and uniform surface.The aggregation phenomenon can be attributed to the hydrophilic nature of TiO2 nanoparticles, which causes them to aggregate through van der Waals forces.However, the introduction of Cu into TiO2 leads to significant aggregation, reducing the specific surface area and hindering the formation of active sites on the catalyst [32,33].Figure 1d demonstrated the presence of numerous spherical nanoparticles on the surface of Cu-Fe-TiO2, indicating that the addition of Fe to Cu-TiO2 increases the specific surface area.This doping further enhances the catalyst's efficiency in benzene degradation.

XRD and BET Analysis
XRD analysis was conducted to investigate the crystal phase and structure of the synthesized catalyst powder, as shown in Figure 2. Results indicated that the pure, mono-, and co-doped TiO2 all exhibited a high-purity anatase phase, suggesting that the employed doping techniques did not cause a transformation to a tetragonal structure [34].Furthermore, the catalyst did not exhibit the characteristic peaks associated with Cu/Fe, which could be attributed to their low concentration.Table 1 presented the anatase crystal size, lattice distortion, and specific surface area of the catalysts.The results indicated that both mono-and co-doped TiO2 have smaller particle sizes compared to pure TiO2, indicating that the presence of Cu or Fe ions hindered the crystallization process of TiO2.Among the four catalysts, the Cu-Fe-TiO2 catalyst exhibited the smallest crystal size of 16.4 nm.Different dopants cause varying lattice distortions within the catalyst structure.The introduction of a small amount of Fe 3+ (less than 0.1% mole percentage) significantly deformed the crystal lattice and reduced the particle size.This could be attributed to the smaller ionic radius of Fe 3+ (0.064 nm) compared to Cu 2+ (0.073 nm) and Ti 4+ (0.068 nm), suggesting that Fe 3+ can replace Cu 2+ in the TiO2 lattice structure.The BET analysis revealed that Cu-Fe-TiO2 had the highest specific surface area, approximately 140.71 m 2 /g.Therefore, co-doping of Cu and Fe effectively reduces particle size, enhances lattice distortion, and increases the specific surface area.

XRD and BET Analysis
XRD analysis was conducted to investigate the crystal phase and structure of the synthesized catalyst powder, as shown in Figure 3. Results indicated that the pure, mono-, and co-doped TiO 2 all exhibited a high-purity anatase phase, suggesting that the employed doping techniques did not cause a transformation to a tetragonal structure [35].Furthermore, the catalyst did not exhibit the characteristic peaks associated with Cu/Fe, which could be attributed to their low concentration.Table 1 presented the anatase crystal size, lattice distortion, and specific surface area of the catalysts.The results indicated that both monoand co-doped TiO 2 have smaller particle sizes compared to pure TiO 2 , indicating that the presence of Cu or Fe ions hindered the crystallization process of TiO 2 .Among the four catalysts, the Cu-Fe-TiO 2 catalyst exhibited the smallest crystal size of 16.4 nm.Different dopants cause varying lattice distortions within the catalyst structure.The introduction of a small amount of Fe 3+ (less than 0.1% mole percentage) significantly deformed the crystal lattice and reduced the particle size.This could be attributed to the smaller ionic radius of Fe 3+ (0.064 nm) compared to Cu 2+ (0.073 nm) and Ti 4+ (0.068 nm), suggesting that Fe 3+ can replace Cu 2+ in the TiO 2 lattice structure.The BET analysis revealed that Cu-Fe-TiO 2 had the highest specific surface area, approximately 140.71 m 2 /g.Therefore, co-doping of Cu and Fe effectively reduces particle size, enhances lattice distortion, and increases the specific surface area.

Raman Analysis
The Raman spectra of the synthesized catalysts is shown in Figure 3.The spectra of the pure, mono-, and co-doped catalysts exhibit five distinct bands associated with the anatase crystal phase.These bands are observed at approximately 147, 198, 398, 515, and 640 cm −1 and correspond to the E1g, E2g, B1g, A1g, and E3g inactivity modes, respectively [35].No Cu or Fe peaks were detected in the Raman spectra, which aligns with the findings of the XRD analysis.Figure 3 also demonstrates a significant increase in the halfmaximum width of the distinctive peaks after the doping procedure.This increase can be attributed to the reduction in particle size and alterations in specific lattice constants.Notably, the spectral band near 147 cm −1 of the doped catalyst exhibited changes, with the E1g peak of pure TiO2 distributed in different bands, namely 144.32 cm −1 , 149.71 cm −1 , 148.71 cm −1 , and 150.99 cm −1 , corresponding to Cu-TiO2, Fe-TiO2, and Cu-Fe-TiO2, respectively.In comparison to pure TiO2, a slight upward shift in the Venturi number of the E1g peak was observed, which can be attributed to oxygen vacancies, molecular distortions, and non-cooperative effects of molecular force fields [36].

Raman Analysis
The Raman spectra of the synthesized catalysts is shown in Figure 4.The spectra of the pure, mono-, and co-doped catalysts exhibit five distinct bands associated with the anatase crystal phase.These bands are observed at approximately 147, 198, 398, 515, and 640 cm −1 and correspond to the E1g, E2g, B1g, A1g, and E3g inactivity modes, respectively [36].No Cu or Fe peaks were detected in the Raman spectra, which aligns with the findings of the XRD analysis.Figure 4 also demonstrates a significant increase in the half-maximum width of the distinctive peaks after the doping procedure.This increase can be attributed to the reduction in particle size and alterations in specific lattice constants.Notably, the spectral band near 147 cm −1 of the doped catalyst exhibited changes, with the E1g peak of pure TiO 2 distributed in different bands, namely 144.32 cm −1 , 149.71 cm −1 , 148.71 cm −1 , and 150.99 cm −1 , corresponding to Cu-TiO 2 , Fe-TiO 2 , and Cu-Fe-TiO 2 , respectively.In comparison to pure TiO 2 , a slight upward shift in the Venturi number of the E1g peak was observed, which can be attributed to oxygen vacancies, molecular distortions, and non-cooperative effects of molecular force fields [37].

UV-Vis-DRS Analysis
UV-Vis-DRS technology was utilized to examine the optical properties of the synthesized catalysts.Figure 4a shows the UV-visible spectra of the catalysts, indicating significant absorption across the UV to visible light range for all synthesized catalysts.The addition of copper and iron elements to the catalysts significantly broadened the absorption spectrum, thereby enhancing their ability to absorb visible light.Figure 4b was used to determine the band gap energy of the sample using the Kubelka-Munk formula.This involved plotting the relationship between (ahv) 2 and photon energy (hv) [37].The band gap (Eg) of pure TiO2 was evaluated to be 3.2 (±0.06) eV.However, our observations indicated that the doping process affected the band gap of the catalyst.The band gap energy

UV-Vis-DRS Analysis
UV-Vis-DRS technology was utilized to examine the optical properties of the synthesized catalysts.Figure 5a shows the UV-visible spectra of the catalysts, indicating significant absorption across the UV to visible light range for all synthesized catalysts.The addition of copper and iron elements to the catalysts significantly broadened the absorption spectrum, thereby enhancing their ability to absorb visible light.Figure 5b was used to determine the band gap energy of the sample using the Kubelka-Munk formula.This involved plotting the relationship between (ahv) 2 and photon energy (hv) [38].The band gap (Eg) of pure TiO 2 was evaluated to be 3.2 (±0.06) eV.However, our observations indicated that the doping process affected the band gap of the catalyst.The band gap energy of Cu-TiO 2 , Fe-TiO 2 , and Cu-Fe-TiO 2 decreased from 3.27 eV to 2.97 eV, indicating that the addition of Cu and Fe to TiO 2 enhances the stability of the band gap structure and improves the efficiency of visible light absorption.This finding suggests that these improvements have the potential to advance the photocatalytic mechanism [39].

UV-Vis-DRS Analysis
UV-Vis-DRS technology was utilized to examine the optical properties of the synthesized catalysts.Figure 4a shows the UV-visible spectra of the catalysts, indicating significant absorption across the UV to visible light range for all synthesized catalysts.The addition of copper and iron elements to the catalysts significantly broadened the absorption spectrum, thereby enhancing their ability to absorb visible light.Figure 4b was used to determine the band gap energy of the sample using the Kubelka-Munk formula.This involved plotting the relationship between (ahv) 2 and photon energy (hv) [37].The band gap (Eg) of pure TiO2 was evaluated to be 3.2 (±0.06) eV.However, our observations indicated that the doping process affected the band gap of the catalyst.The band gap energy of Cu-TiO2, Fe-TiO2, and Cu-Fe-TiO2 decreased from 3.27 eV to 2.97 eV, indicating that the addition of Cu and Fe to TiO2 enhances the stability of the band gap structure and improves the efficiency of visible light absorption.This finding suggests that these improvements have the potential to advance the photocatalytic mechanism [38].

XPS Analysis
XPS analysis was conducted to comprehensively examine the chemical composition of the catalysts.As shown in Figure 5, the catalysts surface mainly consists of Ti, C, and O.The presence of carbon in the XPS instrument can be attributed to the use of oily carbon for spectral calibration.

XPS Analysis
XPS analysis was conducted to comprehensively examine the chemical composition of the catalysts.As shown in Figure 6, the catalysts surface mainly consists of Ti, C, and O.The presence of carbon in the XPS instrument can be attributed to the use of oily carbon for spectral calibration.The high-resolution XPS spectrum of the Ti2p is shown in Figure 6a.The measured binding energies of Ti2p3/2 and Ti2p1/2 range from 457.47 eV to 458.57eV and from 463.21 eV to 464.32 eV, respectively.These results indicate that Ti in Ti-O clusters predominantly exists in the +4 valence state.Furthermore, the valence state of Ti remains unchanged even in the presence of Cu and Fe ions.However, the Ti2p binding energy of the composite is slightly lower compared to pure TiO2.Specifically, the measured binding energy of Ti2p3/2 is 459.3 eV, while the binding energy of Ti2p1/2 is 465.0 eV.Consequently, the electron cloud density around titanium is reduced, affecting the binding energy of Ti2p orbitals in the TiO2 lattice defects and causing a decrease in binding energy.The high-resolution XPS spectrum of the Ti2p is shown in Figure 7a.The measured binding energies of Ti2p3/2 and Ti2p1/2 range from 457.47 eV to 458.57eV and from 463.21 eV to 464.32 eV, respectively.These results indicate that Ti in Ti-O clusters predominantly exists in the +4 valence state.Furthermore, the valence state of Ti remains unchanged even in the presence of Cu and Fe ions.However, the Ti2p binding energy of the composite is slightly lower compared to pure TiO 2 .Specifically, the measured binding energy of Ti2p3/2 is 459.3 eV, while the binding energy of Ti2p1/2 is 465.0 eV.Consequently, the electron cloud density around titanium is reduced, affecting the binding energy of Ti2p orbitals in the TiO 2 lattice defects and causing a decrease in binding energy.The high-resolution XPS spectrum of the Ti2p is shown in Figure 6a.The measured binding energies of Ti2p3/2 and Ti2p1/2 range from 457.47 eV to 458.57eV and from 463.21 eV to 464.32 eV, respectively.These results indicate that Ti in Ti-O clusters predominantly exists in the +4 valence state.Furthermore, the valence state of Ti remains unchanged even in the presence of Cu and Fe ions.However, the Ti2p binding energy of the composite is slightly lower compared to pure TiO2.Specifically, the measured binding energy of Ti2p3/2 is 459.3 eV, while the binding energy of Ti2p1/2 is 465.0 eV.Consequently, the electron cloud density around titanium is reduced, affecting the binding energy of Ti2p orbitals in the TiO2 lattice defects and causing a decrease in binding energy.The high-resolution XPS spectra of the O1s are shown in Figure 6b.Each sample exhibited two distinct peaks corresponding to O1s.The energy levels measured at 528.72 eV, 529.01 eV, and 529.86 eV indicated the presence of lattice oxygen in Fe-TiO2, Cu-Fe-TiO2, and Cu-TiO2, respectively.The increased binding energy in mono-and co-doped TiO2, The high-resolution XPS spectra of the O1s are shown in Figure 7b.Each sample exhibited two distinct peaks corresponding to O1s.The energy levels measured at 528.72 eV, 529.01 eV, and 529.86 eV indicated the presence of lattice oxygen in Fe-TiO 2 , Cu-Fe-TiO 2 , and Cu-TiO 2 , respectively.The increased binding energy in mono-and co-doped TiO 2 , compared to pure TiO 2 , suggests a partial substitution of the oxide state [40].The Cu-Fe-TiO 2 compound, resulting from the redox reaction between Cu and Fe, exhibited a higher binding energy than TiO 2 doped with either Cu or Fe.The energy values for Cu-Fe-TiO 2 , Cu-TiO 2 , and Fe-TiO 2 were measured as 531.03 eV, 531.97 eV, and 531.85 eV, respectively.These results indicated the presence of hydroxyl groups attached to the surface or chemisorbed oxygen in the Cu-Fe-TiO 2 , Cu-TiO 2 , and Fe-TiO 2 systems.The experimental findings demonstrated that the Cu-Fe-TiO 2 catalyst has a higher fraction of adsorbed oxygen compared to other catalysts.
The Cu2p scan of Cu-TiO 2 and Cu-Fe-TiO 2 is shown in Figure 7c.The binding energies of Cu-TiO 2 at Cu2p1/2 and Cu2p3/2 were measured to be 951.99 eV and 932.34 eV, respectively.These values indicate the presence of both CuO and Cu 3 O 2 states, suggesting complete and incomplete oxidation of Cu, respectively [41].Only one passive band was observed in the spectrum of Cu-Fe-TiO 2 , indicating the reaction of Cu 2+ with TiO 2 .However, it is important to note that XPS technology has a detection limit of 5 nm, which means that this reaction cannot be detected.
The Fe2p scan of Fe-TiO 2 and Cu-Fe-TiO 2 is shown in Figure 7d.The absence of a distinct peak shape suggests a low concentration of doped Fe 3+ .In contrast, Cu-Fe-TiO 2 exhibited a clear dual structure in the iron-related region, with Fe2p1/2 and Fe2p3/2 peaks at binding energies of 722.72 eV and 710.32 eV, respectively.These observations indicate the presence of Fe 3+ .Interestingly, no iron oxide is detected in the Cu-Fe-TiO 2 composite, possibly due to the redox interaction between Fe 3+ and Cu 2+ during the catalyst synthesis process.Furthermore, the decrease in the intensity of Fe2p peaks in both catalysts suggests that the addition of Fe 3+ leads to a more uniform distribution within the TiO 2 framework, rather than being concentrated solely on the surface layer of the nanospheres.

Catalytic Activity
The degradation curve of benzene catalysts under visible light is shown in Figure 8a.The results demonstrated that mono-and co-doped TiO 2 catalysts are more effective in degrading benzene under visible light compared to pure TiO 2 .This can be attributed to the broadened light absorption spectrum, which enhances the photocatalytic performance.Among the four catalysts, Cu-Fe-TiO 2 exhibited the highest rate of benzene degradation, achieving a remarkable rate of 88.87%.During photocatalysis, the presence of Cu in various valence states significantly reduces the recombination rate between photogenerated electrons and holes.Furthermore, the distribution of Fe on the titanium base helps reduce oxide formation, which can act as a recombination center for photogenerated carriers.The Cu-Fe-TiO 2 catalyst surface has a higher concentration of hydroxyl groups or chemisorbed oxygen, thereby improving the efficiency of benzene degradation under visible light.Cu-TiO 2 , Fe-TiO 2 , and Cu-Fe-TiO 2 were deactivated after approximately 18, 35, and 60 min, respectively.This decrease in the rate of benzene degradation can be attributed to the interaction between Cu 2+ and Fe 3+ in the mono-and co-doped systems.Specifically, Cu 2+ transfers to the catalyst surface and reacts with oxygen to form copper oxide, which acts as a complex center.This leads to an increase in the number of electron-hole pairs at the available sites, thereby enhancing the complexing rate.However, the presence of reaction intermediates or by-products can deactivate or block the active sites on the catalyst surface, limiting the availability of active sites for the adsorption and degradation of benzene molecules and thus reducing the degradation rate [42].Moreover, the reaction process can induce changes in the structure of Cu-Fe-TiO 2 , leading to the redistribution or migration of metal ions within the catalyst.This modification impacts the active sites and electronic properties of the catalyst, thereby affecting its capacity to adsorb and degrade benzene molecules.Furthermore, as the reaction progresses, the recombination rate between photogenerated electrons and holes could potentially rise, causing a decrease in the efficiency of the photocatalytic system and subsequently reducing the degradation rate.The processing capability of the photocatalysts is illustrated in Figure 7b.Among the four tested photocatalysts, Cu-Fe-TiO2 demonstrated the highest processing capacity, reaching 658.33 µgC6H6/(g•h).By substituting Ti 4+ with Cu 2+ in the TiO2 crystal structure, lattice distortion and imperfections are introduced.This leads to the formation of an oxide layer on the catalyst surface, which traps electrons and hinders the generation of electronhole pairs.On the other hand, Fe 3+ can substitute TiO2 but is unable to capture electrons.However, when both Cu 2+ and Fe 3+ are incorporated into the catalyst, its valence state is altered.These metal ions efficiently capture electrons, preventing the formation of complexes with photogenerated carriers and ultimately enhancing the photocatalytic performance of the catalyst.The processing capability of the photocatalysts is illustrated in Figure 8b.Among the four tested photocatalysts, Cu-Fe-TiO 2 demonstrated the highest processing capacity, reaching 658.33 µgC 6 H 6 /(g•h).By substituting Ti 4+ with Cu 2+ in the TiO 2 crystal structure, lattice distortion and imperfections are introduced.This leads to the formation of an oxide layer on the catalyst surface, which traps electrons and hinders the generation of electron-hole pairs.On the other hand, Fe 3+ can substitute TiO 2 but is unable to capture electrons.However, when both Cu 2+ and Fe 3+ are incorporated into the catalyst, its valence state is altered.These metal ions efficiently capture electrons, preventing the formation of complexes with photogenerated carriers and ultimately enhancing the photocatalytic performance of the catalyst.

Reusability Tests
Gaseous benzene degradation tests require stable Cu-Fe-TiO 2 photocatalysts for practical applications.To assess the stability of the photocatalyst, we conducted five consecutive degradation experiments.Prior to the photocatalytic degradation process, the Cu-Fe-TiO 2 photocatalyst underwent centrifugation, purification, filtration, and drying.As shown in Figure 9, the results demonstrated that the photocatalytic degradation rates of Cu-Fe-TiO 2 were 88.87%, 84.35%, 80.63%, 77.16%, and 71.74% for each consecutive experiment.Although there was a slight decrease in the degradation rate, the photocatalytic stability of the catalyst remained relatively high.This decrease can be attributed to the depletion of the material with repeated use and the partial deactivation of its surface photocatalytically active sites.

Reaction Mechanism
As shown in Figure 9, this study presents a mechanism to explain the enhanced photocatalytic effectiveness of Cu 2+ and Fe 3+ co−doped TiO2 catalysts in the degradation of gaseous benzene.Our findings indicate that the substitution of Ti 4+ with Cu 2+ and Fe 3+ introduces a new energy level associated with impurities.This energy level is located between the valence band and the conduction band in the crystal structure of TiO2.Electrons from higher energy levels readily transfer to the conduction band, while electrons from the valence band can move to another impurity energy level introduced by the iron dopant.The addition of Cu 2+ /Fe 3+ dopants to TiO2 photocatalysts reduces the band gap, expanding their absorption range in the visible light spectrum compared to pure TiO2.As a result, released electrons can migrate to the catalyst's surface and react with adsorbed O2 and H2O molecules, ultimately leading to the degradation of gaseous benzene [42,43].Furthermore, Cu 2+ and Fe 3+ can act as carrier traps, impeding carrier recombination and prolonging the time taken for carriers to reach the catalyst surface, thereby enhancing photocatalytic efficiency.During the photocatalytic degradation of gaseous benzene using TiO2, intermediates such as six-membered ring alcohols may form.The catalyst absorbs O2 and H2O from the surrounding air during the reaction.When activated by light sources, various free radicals like −O2and −OH are generated.These radicals then react with water to produce H2O2 and C6H6(OH)2.Subsequently, H2O2 reacts with −O2-to form the −OH radical.Finally, C6H6(OH)2 interacts with various free radicals to produce CO2 and H2O.

Reaction Mechanism
As shown in Figure 10, this study presents a mechanism to explain the enhanced photocatalytic effectiveness of Cu 2+ and Fe 3+ co−doped TiO 2 catalysts in the degradation of gaseous benzene.Our findings indicate that the substitution of Ti 4+ with Cu 2+ and Fe 3+ introduces a new energy level associated with impurities.This energy level is located between the valence band and the conduction band in the crystal structure of TiO 2 .Electrons from higher energy levels readily transfer to the conduction band, while electrons from the valence band can move to another impurity energy level introduced by the iron dopant.The addition of Cu 2+ /Fe 3+ dopants to TiO 2 photocatalysts reduces the band gap, expanding their absorption range in the visible light spectrum compared to pure TiO 2 .As a result, released electrons can migrate to the catalyst's surface and react with adsorbed O 2 and H 2 O molecules, ultimately leading to the degradation of gaseous benzene [43,44].Furthermore, Cu 2+ and Fe 3+ can act as carrier traps, impeding carrier recombination and prolonging the time taken for carriers to reach the catalyst surface, thereby enhancing photocatalytic efficiency.During the photocatalytic degradation of gaseous benzene using TiO catalytic efficiency.During the photocatalytic degradation of gaseous benzene using TiO2, intermediates such as six-membered ring alcohols may form.The catalyst absorbs O2 and H2O from the surrounding air during the reaction.When activated by light sources, various free radicals like −O2and −OH are generated.These radicals then react with water to produce H2O2 and C6H6(OH)2.Subsequently, H2O2 reacts with −O2-to form the −OH radical.Finally, C6H6(OH)2 interacts with various free radicals to produce CO2 and H2O.

Conclusions
In this study, mono-and co-doped TiO 2 catalysts (i.e., Cu/Fe-TiO 2 and Cu-Fe-TiO 2 ) were synthesized using the sol-gel method.The degradation rate of gaseous benzene was investigated using Cu-TiO 2 , Fe-TiO 2 , and Cu-Fe-TiO 2 as photocatalysts.The synthesized catalysts were characterized using various techniques including SEM, EDS, XRD, BET, Raman, UV-vis-DRS, and XPS.Results indicated that both mono-and co-doped TiO 2 consisted mainly of the anatase phase.The introduction of Cu/Fe dopants into TiO 2 resulted in a noticeable reduction in particle size and band gap, which led to an increase in lattice defects and specific surface area, particularly in co-doped TiO 2 .Cu-Fe-TiO 2 demonstrated significant photocatalytic activity, achieving an efficiency of 88.87% in benzene degradation and a benzene treatment capacity of 658.33 µgC 6 H 6 /(g•h).This excellent performance can be attributed to the reduction in band gap resulting from the co-doping of Cu and Fe, which enhances its sensitivity to visible light.Additionally, the presence of Cu and Fe in different valence states on the titanium substrate effectively captures photogenerated electrons, improves the separation of electron-hole pairs, and extends their lifespan.Furthermore, the Cu-Fe-TiO 2 catalyst demonstrated remarkable stability and reusability, even after five degradation cycles.Overall, this study highlights the potential of metal-doped TiO 2 catalysts for effectively degrading gaseous benzene.

Figure 1 .
Figure 1.SEM micrographs of TiO2 catalysts under different loading metal: (a1-a3) TiO2, (b1-b3) Fe-TiO2, (c1-c3) Cu-TiO2, (d1-d3) Cu-Fe-TiO2, (e) elemental mapping of Cu-Fe-TiO2, EDS (f).Element mapping of the Cu-Fe-TiO2 nanocatalyst is shown in Figure 1e.These images clearly demonstrate the uniform distribution of Fe, Cu, Ti, and O elements, indicating the absence of copper and iron particle accumulation.The EDS analysis of Cu-Fe-TiO2 in Figure 1f reveals the presence of Fe, Cu, Ti, and O elements.The quantitative results from EDS indicate that the mass percentages (W %) of Fe, Cu, Ti, and O are 0.69, 3.11, 45.51, and 50.69, respectively.The presence of Fe and Cu elements confirms the titanium shell doping and the infiltration of Fe and Cu into the TiO2 lattice.

Figure 2 .
Figure 2. SEM micrographs of TiO 2 catalysts under different loading metal: (a1-a3) TiO 2 , (b1-b3) Fe-TiO 2 , (c1-c3) Cu-TiO 2 , (d1-d3) Cu-Fe-TiO 2 , (e) elemental mapping of Cu-Fe-TiO 2 , EDS (f).Element mapping of the Cu-Fe-TiO 2 nanocatalyst is shown in Figure 2e.These images clearly demonstrate the uniform distribution of Fe, Cu, Ti, and O elements, indicating the absence of copper and iron particle accumulation.The EDS analysis of Cu-Fe-TiO 2 in Figure 2f reveals the presence of Fe, Cu, Ti, and O elements.The quantitative results from EDS indicate that the mass percentages (W %) of Fe, Cu, Ti, and O are 0.69, 3.11, 45.51, and 50.69, respectively.The presence of Fe and Cu elements confirms the titanium shell doping and the infiltration of Fe and Cu into the TiO 2 lattice.

Figure 2 .
Figure 2. The X-ray diffraction patterns of the pure, mono-, and co-doped TiO2.

Figure 3 .
Figure 3.The X-ray diffraction patterns of the pure, mono-, and co-doped TiO 2 .

Figure 7 .
Figure 7. (a) The unit capacity of different catalysts to treat benzene under visible light; (b) photocatalytic performance of the obtained catalytic under visible light.

Figure 8 .
Figure 8.(a) The unit capacity of different catalysts to treat benzene under visible light; (b) photocatalytic performance of the obtained catalytic under visible light.
2 , intermediates such as six-membered ring alcohols may form.The catalyst absorbs O 2 and H 2 O from the surrounding air during the reaction.When activated by light sources, various free radicals like −O 2 -and −OH are generated.These radicals then react with water to produce H 2 O 2 and C 6 H 6 (OH) 2 .Subsequently, H 2 O 2 reacts with −O 2 -to form the −OH radical.Finally, C 6 H 6 (OH) 2 interacts with various free radicals to produce CO 2 and H 2 O.

Table 1 .
XRD crystallite size and BET surface area of the pure, mono-, and co-doped TiO 2 .

Table 1 .
XRD crystallite size and BET surface area of the pure, mono-, and co-doped TiO2.