Degradation of 4-Tert-Butylphenol in Water Using Mono-Doped (M1: Mo, W) and Co-Doped (M2-M1: Cu, Co, Zn) Titania Catalysts

Mono-doped (Mo-TiO2 and W-TiO2) and co-doped TiO2 (Co-Mo-TiO2, Co-W-TiO2, Cu-Mo-TiO2, Cu-W-TiO2, Zn-Mo-TiO2, and Zn-W-TiO2) catalysts were synthesized by simple impregnation methods and tested for the photocatalytic degradation of 4-tert-butylphenol in water under UV (365 nm) light irradiation. The catalysts were characterized with various analytical methods. X-ray diffraction (XRD), Raman, Diffuse reflectance (DR) spectroscopies, Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), and Energy dispersive spectroscopy (EDS) were applied to investigate the structure, optical properties, morphology, and elemental composition of the prepared catalysts. The XRD patterns revealed the presence of peaks corresponding to the WO3 in W-TiO2, Co-W-TiO2, Cu-W-TiO2, and Zn-W-TiO2. The co-doping of Cu and Mo to the TiO2 lattice was evidenced by the shift of XRD planes towards higher 2θ values, confirming the lattice distortion. Elemental mapping images confirmed the successful impregnation and uniform distribution of metal particles on the TiO2 surface. Compared to undoped TiO2, Mo-TiO2 and W-TiO2 exhibited a lower energy gap. Further incorporation of Mo-TiO2 with Co or Cu introduced slight changes in energy gap and light absorption characteristics, particularly visible light absorption. In addition, photoluminescence (PL) showed that Cu-Mo-TiO2 has a weaker PL intensity than undoped TiO2. Thus, Cu-Mo-TiO2 showed better catalytic activity than pure TiO2, achieving complete degradation of 4-tert-butylphenol under UV light irradiation after 60 min. The application of Cu-Mo-TiO2 under solar light conditions was also tested, and 70% of 4-tert-butylphenol degradation was achieved within 150 min.


Introduction
Industrialization on a large scale, along with urbanization and population growth, results in the development of vast volumes of wastewater with different pollutants (inorganic and organic). Numerous organic pollutants found in wastewater are hazardous and may pose a threat to the aquatic environment and living beings [1]. These pollutants include endocrine disrupting chemicals (EDC), pharmaceuticals, and personal care products (PPCPs) [2]. The presence of EDCs in water sources has become one of the major environmental issues [3]. Even at a low exposure level, they may cause the disruption of endocrine and reproductive systems [4]. 4-tert-Butylphenol (4-t-BP) is a synthetic EDC that nium heptamolybdate ((NH 4 ) 6   , used for the deposition of the dopant metal; and NH 4 OH solution (28-30% ACS reagent, CAS Number 1336-21-6) were purchased from Sigma Aldrich (Saint Louis, MO, USA). The ultrapure water used in all experiments was obtained by means of a Direct-Q 3UV (Millipore, Darmstadt, Germany) water purification system. All chemical reagents were used without further purification.

Preparation of Photocatalysts
In this work, two base photocatalysts, namely, Mo/TiO 2 and W/TiO 2 , were prepared using commercial TiO 2 (Degussa P25, Saint Louis, MO, USA) as a support with the wet impregnation method, and the active species were W(VI) or Mo(VI) oxoanions using ammonium metatungstate and ammonium heptamolybdate, respectively. A proper amount of TiO 2 (3.0 g) was suspended in the solution containing the required amount of Mo or W oxo species (2.5 × 10 -4 moles Mo or W) for coverage with Mo or W of 1 at/nm 2 . The pH of the suspension was around 5 for the Mo solution, while, for the W solution, it was raised to 10 using the 28% NH 4 OH solution in order to depolymerize the polytungstate species and increase solubility. The suspension was place in a rotary evaporator and left under rotation for 90 min at 45 • C in order to maximize the amount of adsorption. Afterwards, a vacuum was applied and the water evaporated, followed by drying at 105 • C for 2 h and calcination at 400 • C for 5 h. The two base catalysts were then used to prepare ternary systems by dry impregnation. The third cation deposited was either Co(II), Cu(II), or Zn(II), using the corresponding nitrate salts, with the surface concentration of the M(II) ion set to 0.5 at/nm 2 (4.15 × 10 -5 moles M(II) ions for 1 g of M1/TiO 2 ). After impregnation, the samples were dried and calcined under the same conditions as for the base catalysts.

Characterization
The phase composition of all of the prepared catalysts was evaluated by X-ray diffraction (XRD, Rigaku SmartLab automated multifunctional X-ray Diffractometer, Tokyo, Japan) using Cu Kα radiation in the scanning range of 10-80 • . The Scherrer equation (1) [39] was used to determine the average size of the TiO 2 nanoparticles: where D is the crystallite size of the catalyst, λ is the X-ray wavelength (1.54060 Å), β is the full width at half maximum of the diffraction peak, and θ is the diffraction angle. Raman spectra were recorded using a Raman spectrometer (Horiba, LabRam HR evolution, Kyoto, Japan) with a 532 nm laser excitation. A transmission electron microscope (TEM, JEM-2100 from Jeol Ltd., Japan) and a scanning electron microscope (SEM, Carl Zeiss Auriga Cross Beam 540) equipped with an energy-dispersive spectroscopy (EDS) system were applied to perform surface morphology measurements and to analyze the elemental composition of the catalysts. The optical properties of the mono-and co-doped TiO 2 nanoparticles were investigated by means of diffuse reflectance spectroscopy (DRS, Varian Cary 3, Palo Alto, CA, USA). The recombination behaviors of charge carriers for Cu-Mo-TiO 2 were obtained via photoluminescence (PL) emission analysis performed on a fluorescence spectrophotometer (F-7000, Hitachi, Tokyo, Japan). The specific surface area (SSA) of the catalysts was determined from N 2 adsorption isotherms in liquid N 2 temperature in a Tristar 3000 porosimeter (Micromeritics, Norcross, GA 30093-2901, USA) with the BET method.

Experimental Procedure
The photodegradation of 4-t-BP was carried out using a photochemical reactor operated in batch mode (Lanphan industry, Zhengzhou City, Henan Province, China) under UV irradiation (365 nm). In a typical experiment, 10 mg of photocatalyst was added to 100 mL of 15 ppm 4-t-BP solution. Prior to irradiation, the solution was stirred for 90 min in the dark to achieve an adsorption-desorption equilibrium between catalyst and pollutant. At a regular time interval (every 30 min), the sample was taken out and filtered through a 0.22 µm Millex syringe filter to remove the photocatalyst for further analysis.
The same procedure was applied to test the photocatalytic activity of Cu-Mo-TiO 2 under simulated solar light irradiation (LCS-100 solar simulator, Oriel, Newport, Darmstadt, Germany) within 150 min.
The concentration of 4-t-BP was measured by a high-performance liquid chromatography instrument (HPLC, Agilent 1290 Infinity II, Santa Clara, CA, USA) equipped with a SB-C8 column (2.1 mm × 100 mm, 1.8 µm). The mobile phase composition was methanol and ultrapure water (50:50, v/v), which were mixed to compose the mobile phase.

Characterization of Photocatalysts
The SSA of the pure TiO 2 (P25) was measured to be equal to 54 m 2 g −1 . The Mo-TiO 2 catalysts maintained the SSA value (53 m 2 g −1 ) after the deposition of Mo species, while a second impregnation with either Co, Zn, or Cu resulted in an almost unchanged SSA (52 m 2 g −1 ) for all of the ternary Mo-TiO 2 systems. This was expected, since the loading of the second metal ion is low, while the deposition of Mo species occurs mainly with adsorption or interfacial deposition [40].
On the other hand, the deposition of W oxo species decreased the SSA value to 47 m 2 g −1 . The decrease in SSA value can be due to the lower contribution of adsorption in the deposition of W species in contrast with the Mo deposition. This is caused by the higher solution pH, which does not favor adsorption [41]. The deposition of the second metal ion had no influence on the SSA value (45 m 2 g −1 ).
XRD analysis was performed to investigate the crystal structures of the mono-and co-doped TiO 2 catalysts, and the results are shown in Since the ionic radii of doped transition metals (Mo 6+ , W 6+ , Cu 2+ , Co 2+ , and Zn 2+ ) are close to that of Ti 4+ [42][43][44], minimum changes occurred in the original structure of TiO 2 .
The structure of the catalyst did not change drastically after the deposition of metals. No typical peaks were detected, which verified the impregnation of Mo-dopant into TiO 2 ( Figure 1) as well as the subsequent introduction of Cu, Co, and Zn particles into Mo-TiO 2 ( Figure 2). This finding could be attributed to the high dispersion of metal particles on the surface of TiO 2 [45]. Interestingly, a slight shift of the intense TiO 2 (101) peak towards a higher angle (from 24.2 • to 24.6 • ) was observed only in Mo-Cu-TiO 2 , suggesting the existence of some disorders in the anatase crystal lattice [46,47].
Unlike Mo-doping, the introduction of W-into TiO 2 formed new peaks at 2θ = 22.65 • and 32.75 • assigned to the WO 3 phase (Figure 3). This is in accordance with the low decrease in SSA value for the W-TiO 2 catalyst. The deposition of the second metal ion did not significantly alter the XRD pattern.
The average crystallite size of all prepared catalysts was calculated using Scherrer's equation, and the results are listed in Table 1. The values confirmed the well-dispersed Mo phase and the existence of the nanoparticles on the prepared catalysts.       The structure of the catalyst did not change drastically after the deposition of metals. No typical peaks were detected, which verified the impregnation of Mo-dopant into TiO2 ( Figure 1) as well as the subsequent introduction of Cu, Co, and Zn particles into Mo-TiO2 ( Figure 2). This finding could be attributed to the high dispersion of metal particles on the surface of TiO2 [45]. Interestingly, a slight shift of the intense TiO2 (101) peak towards a  Raman spectroscopy was used to obtain more information about the mono-and codoped TiO 2 nanoparticles, and the spectra in the range of 100-800 cm −1 are depicted in Figures 4 and 5. Accordingly, the peaks located at 142 cm −1 , 192 cm −1 , 394 cm −1 , 513 cm −1 , and 634 cm −1 matched well with the anatase phase, , while 268 cm −1 and 803 cm −1 confirm the presence of rutile phase. Any peaks corresponding to the doped transition metals could not be detected, although the shift of the main peak at 142 cm −1 towards a greater wavelength was observed for Cu-Mo-TiO 2 and W-doped catalysts (W-TiO 2 , Cu-W-TiO 2 , Co-W-TiO 2 , and Zn-W-TiO 2 ). The results of Raman spectroscopy related to the alternation in structure are in good agreement with experimental X-ray findings [48]. Co-W-TiO2, and Zn-W-TiO2). The results of Raman spectroscopy related to the alternation in structure are in good agreement with experimental X-ray findings [48].    The surface morphology of the obtained catalysts was studied by both SEM and TEM analysis, depicted in Figures 6-9. As can be seen from SEM images ( Figures 6 and 7), all prepared catalysts were found to be relatively spherical in shape, with particle sizes between 23 nm and 35 nm, like pure TiO 2 . These observations confirm the fact that the introduction of metals did not significantly affect the morphology of TiO 2 . The surface morphology of the obtained catalysts was studied by both SEM and TEM analysis, depicted in Figures 6-9. As can be seen from SEM images ( Figures 6 and 7), all prepared catalysts were found to be relatively spherical in shape, with particle sizes between 23 nm and 35 nm, like pure TiO2. These observations confirm the fact that the introduction of metals did not significantly affect the morphology of TiO2.     In addition, EDS analysis was employed to investigate the elemental composition of the prepared catalysts. Although the doped metals were not visible as separate particles in TEM micrographs, EDS mappings ( Figure 10-12) revealed the presence and homogeneous allocation of impregnated metals throughout the surface of TiO2. Notably, close-up TEM images (Figures 8 and 9) reveal lattice spacing values of 0.37-0.41 nm that correspond to the [101] plane of TiO 2 anatase. Overall, the results obtained from SEM and TEM characterizations (average particle size, crystal structure) are in good agreement with XRD and Raman findings.
In addition, EDS analysis was employed to investigate the elemental composition of the prepared catalysts. Although the doped metals were not visible as separate particles in TEM micrographs, EDS mappings ( Figures 10-12) revealed the presence and homogeneous allocation of impregnated metals throughout the surface of TiO 2 .          The origin of these interactions is the formation of M1-O-Ti bonds (M1: Mo or W) and the charge transfer from Ti to Mo. These charge transfer phenomena are common in systems where an oxidic support is covered by a transition metal oxide, as in our case [40,41,[52][53][54]. about 3.09 eV of undoped TiO2, the energy gap decreased to 2.92 eV and 2.87 eV after doping with Mo-and W-. A possible reason is the interaction between Mo or W with TiO2 [49][50][51]. The origin of these interactions is the formation of M1-O-Ti bonds (M1: Mo or W) and the charge transfer from Ti to Mo. These charge transfer phenomena are common in systems where an oxidic support is covered by a transition metal oxide, as in our case [40,41,[52][53][54].       As can be seen, these interactions were rather higher in the case of Mo-TiO2, since the F(R), an analogue to absorption, was more intense for this sample, while the surface coverage seemed to be a little smaller in the case of W-TiO2, as the F(R) was higher in the UV region. This is in accordance with the XRD results, where crystallites of WO3 were de- As can be seen, these interactions were rather higher in the case of Mo-TiO 2 , since the F(R), an analogue to absorption, was more intense for this sample, while the surface coverage seemed to be a little smaller in the case of W-TiO 2 , as the F(R) was higher in the UV region. This is in accordance with the XRD results, where crystallites of WO 3 were detected. Both binary systems absorb less in the UV region than bare TiO 2 .
Concerning the M2-Mo-TiO 2 samples, no significant differences could be observed ( Figure 13). The coverage of TiO 2 was higher, while, in the case of Co-Mo-TiO 2 , the adsorption in the near-UV region was higher, suggesting more intense interactions with the Co phase. Absorption in the visible region was small for the samples Co-Mo-TiO 2 and Cu-Mo-TiO 2 , although the black color of the corresponding bulk oxides was due to the small quantity of the Co and Cu phases. This may suggest that the above oxides were rather well-dispersed on the surface of the catalyst.
The M2-W-TiO 2 samples had similar behavior. Only the Cu-W-TiO 2 sample had smaller absorption in the UV region, suggesting that the coverage of TiO 2 was higher in this case.
As was discussed, the doping of Mo-TiO 2 and W-TiO 2 with Co, Cu, or Zn caused the formation of a more intense peak centered at about 400 nm (Figures 14 and 15) and a slight reduction in the energy gap. The energy gap ( Figure 16) for Zn-Mo-TiO 2 , Cu-Mo-TiO 2 , and Co-Mo-TiO 2 were estimated to be 2.85 eV, 2.82 eV, and 2.72 eV, respectively. Additionally, as for the Co-W-TiO 2 , Zn-W-TiO 2 , and Cu-W-TiO 2 , the Eg values were 2.87 eV, 2.86 eV, and 2.85 eV, respectively.

Adsorption and Photocatalytic Degradation of 4-t-BP
The adsorption and photocatalytic degradation of 4-t-BP using mono-and co-doped TiO2 catalysts were evaluated under dark conditions for 90 min and UV/solar light irradiation, respectively. The adsorption performance of each catalyst was identified through the determination of adsorption capacity q (mg/g) by Equation (2): where C0 and Ce represent the initial and equilibrium concentrations (mg/L) of 4-t-BP in the solution, V (L) is the volume of the 4-t-BP solution, and m catalyst is the mass of the catalyst. As shown in Figure 17, the amount of 4-t-BP adsorbed increased more than two-fold after doping TiO2 with Mo-or W-. At the equilibria, the adsorption capacities of Mo-TiO2 and W-TiO2 were found to be 63 mg/g for both catalysts. The enhanced adsorption capacities could be ascribed to changes in strong electrical aspects between the 4-t-BP and the doped catalyst [55]. At this point, it should be noted that the deposition of W or Mo phase increases the acidity of the surface. In a recent paper [56] about the W-TiO2 system, it was found that the addition of W oxo species resulted in lower point of zero values, although it was less acidic than the correspondence value for mixed oxides and changed the acidbase properties. The electron transfer between well-dispersed W phase and the TiO2 sur-

Adsorption and Photocatalytic Degradation of 4-t-BP
The adsorption and photocatalytic degradation of 4-t-BP using mono-and co-doped TiO 2 catalysts were evaluated under dark conditions for 90 min and UV/solar light irradiation, respectively. The adsorption performance of each catalyst was identified through the determination of adsorption capacity q (mg/g) by Equation (2): where C 0 and C e represent the initial and equilibrium concentrations (mg/L) of 4-t-BP in the solution, V (L) is the volume of the 4-t-BP solution, and m catalyst is the mass of the catalyst. As shown in Figure 17, the amount of 4-t-BP adsorbed increased more than two-fold after doping TiO 2 with Mo-or W-. At the equilibria, the adsorption capacities of Mo-TiO 2 and W-TiO 2 were found to be 63 mg/g for both catalysts. The enhanced adsorption capacities could be ascribed to changes in strong electrical aspects between the 4-t-BP and the doped catalyst [55]. At this point, it should be noted that the deposition of W or Mo phase increases the acidity of the surface. In a recent paper [56] about the W-TiO 2 system, it was found that the addition of W oxo species resulted in lower point of zero values, although it was less acidic than the correspondence value for mixed oxides and changed the acid-base properties. The electron transfer between well-dispersed W phase and the TiO 2 surface increases the surface electron density, which enhances the surface basicity of TiO 2 . Further addition of Co to Mo-TiO2 had a negative impact on the adsorption performance, while the incorporation of Cu or Co metal ions slightly improved the adsorption of 4-t-BP ( Figure 18). Similar results were obtained for doped W-TiO2 ( Figure 19). Among all of the synthesized catalysts, Zn-doped materials exhibited the highest 4-t-BP adsorption capacity, while doping with Co had a detrimental effect on the adsorption capacity of both binary systems. Doping with Co increases the interactions between Co and Mo or W and, as a result, decreases the interactions of Mo and W oxo species with the titania surface.  Further addition of Co to Mo-TiO 2 had a negative impact on the adsorption performance, while the incorporation of Cu or Co metal ions slightly improved the adsorption of 4-t-BP ( Figure 18). Similar results were obtained for doped W-TiO 2 ( Figure 19). Among all of the synthesized catalysts, Zn-doped materials exhibited the highest 4-t-BP adsorption capacity, while doping with Co had a detrimental effect on the adsorption capacity of both binary systems. Doping with Co increases the interactions between Co and Mo or W and, as a result, decreases the interactions of Mo and W oxo species with the titania surface. Further addition of Co to Mo-TiO2 had a negative impact on the adsorption performance, while the incorporation of Cu or Co metal ions slightly improved the adsorption of 4-t-BP ( Figure 18). Similar results were obtained for doped W-TiO2 ( Figure 19). Among all of the synthesized catalysts, Zn-doped materials exhibited the highest 4-t-BP adsorption capacity, while doping with Co had a detrimental effect on the adsorption capacity of both binary systems. Doping with Co increases the interactions between Co and Mo or W and, as a result, decreases the interactions of Mo and W oxo species with the titania surface.   It was reported [57] that surface hydroxyl groups play an important role in the surface properties of a material. These groups often have Brønsted acidity, and, therefore, they play an important role in adsorption or in photocatalytic reactions. For the Mo-TiO2 system, the interactions between Mo phase and TiO2 generate hydroxyl groups. These groups can interact with the second metal ion and immobilize it on the binary system surface.
The deposition of Co 2+ ions on either Mo-TiO2 or W-TiO2 shifts the absorption to higher wavelengths, evidence that the Co species can be adsorbed onto the surface -OH groups, diminishing the adsorption sites for 4-t-BP. This is expected, since it is well-known that CoMo catalysts are very stable and active, especially in hydrotreatment. On the other hand, this can significantly alter the photocatalytic properties of the ternary system.
In the presence of UV light, only 50% of 4-t-BP can be photodegraded in 120 min without the application of any catalyst ( Figure 20). All prepared catalysts exhibited significant photodegradation when the light was on. Although the impregnation of TiO2 with W and Mo metals led to an improved adsorption of 4-t-BP, the photocatalytic activity of pure TiO2 was higher. This may be due to the higher absorption of TiO2 in the UV region, as was determined by DRS measurements. The results are in agreement with previous reports. For example, it was reported that the presence of various transition metals was not beneficial for the oxidation ability of the solid [58] and that only W-TiO2 had a positive effect on its activity. Generally, the doped catalysts exhibit recombination rates significantly higher than that of the support, which results in lower oxidation ability. Additionally, Mo deposition can have some positive effects on the activity if the deposition is not surface but subsurface [59]. In all cases, the loading of the oxoanion is crucial for the performance of the photocatalyst. Higher loadings result in lower degradation activity. It was reported [57] that surface hydroxyl groups play an important role in the surface properties of a material. These groups often have Brønsted acidity, and, therefore, they play an important role in adsorption or in photocatalytic reactions. For the Mo-TiO 2 system, the interactions between Mo phase and TiO 2 generate hydroxyl groups. These groups can interact with the second metal ion and immobilize it on the binary system surface.
The deposition of Co 2+ ions on either Mo-TiO 2 or W-TiO 2 shifts the absorption to higher wavelengths, evidence that the Co species can be adsorbed onto the surface -OH groups, diminishing the adsorption sites for 4-t-BP. This is expected, since it is well-known that CoMo catalysts are very stable and active, especially in hydrotreatment. On the other hand, this can significantly alter the photocatalytic properties of the ternary system.
In the presence of UV light, only 50% of 4-t-BP can be photodegraded in 120 min without the application of any catalyst ( Figure 20). All prepared catalysts exhibited significant photodegradation when the light was on. Although the impregnation of TiO 2 with W and Mo metals led to an improved adsorption of 4-t-BP, the photocatalytic activity of pure TiO 2 was higher. This may be due to the higher absorption of TiO 2 in the UV region, as was determined by DRS measurements. The results are in agreement with previous reports. For example, it was reported that the presence of various transition metals was not beneficial for the oxidation ability of the solid [58] and that only W-TiO 2 had a positive effect on its activity. Generally, the doped catalysts exhibit recombination rates significantly higher than that of the support, which results in lower oxidation ability. Additionally, Mo deposition can have some positive effects on the activity if the deposition is not surface but subsurface [59]. In all cases, the loading of the oxoanion is crucial for the performance of the photocatalyst. Higher loadings result in lower degradation activity.
The introduction of Cu, Zn, and Co metals into Mo-TiO 2 and W-TiO 2 resulted in different photocatalytic performances of the catalyst. The improved adsorption properties of Mo-TiO 2 and W-TiO 2 after doping with Cu and Zn facilitated a faster degradation of 4-t-BP (Figures 21 and 22). More specifically, the incorporation of Cu into both Mo-TiO 2 and W-TiO 2 was favorable, where a slight 4-t-BP degradation increase was observed for Cu-Mo-TiO 2 compared with pure TiO 2 . This is likely due to structural changes induced by the presence of Mo and Cu, which was evidenced by the high adsorption capacity and reduced energy gap coupled with the extended light absorption in the visible region. The introduction of Cu, Zn, and Co metals into Mo-TiO2 and W-TiO2 resulted in different photocatalytic performances of the catalyst. The improved adsorption properties of Mo-TiO2 and W-TiO2 after doping with Cu and Zn facilitated a faster degradation of 4-t-BP (Figures 21 and 22). More specifically, the incorporation of Cu into both Mo-TiO2 and W-TiO2 was favorable, where a slight 4-t-BP degradation increase was observed for Cu-Mo-TiO2 compared with pure TiO2. This is likely due to structural changes induced by the presence of Mo and Cu, which was evidenced by the high adsorption capacity and reduced energy gap coupled with the extended light absorption in the visible region.   The introduction of Cu, Zn, and Co metals into Mo-TiO2 and W-TiO2 resulted in different photocatalytic performances of the catalyst. The improved adsorption properties of Mo-TiO2 and W-TiO2 after doping with Cu and Zn facilitated a faster degradation of 4-t-BP (Figures 21 and 22). More specifically, the incorporation of Cu into both Mo-TiO2 and W-TiO2 was favorable, where a slight 4-t-BP degradation increase was observed for Cu-Mo-TiO2 compared with pure TiO2. This is likely due to structural changes induced by the presence of Mo and Cu, which was evidenced by the high adsorption capacity and reduced energy gap coupled with the extended light absorption in the visible region.  On the other hand, Cu-Mo-TiO 2 showed a relatively lower PL intensity than that of pure TiO 2 ( Figure 23). This observation indicates a better charge separation, which could promote the photocatalytic performance of the catalyst.
Accordingly, the photocatalytic activity of Cu-Mo-TiO 2 was investigated towards 4-t-BP degradation under solar light irradiation, and its performance was compared with that of mono-doped Mo-TiO 2 and W-TiO 2 catalysts ( Figure 24). It was observed that the application of solar light required more time to achieve decent degradation for all tested catalysts. In 150 min of solar light exposure, about 70% of 4-t-BP could be degraded using the Cu-Mo-TiO 2 /solar system. Although the Cu-Mo-TiO 2 catalyst exhibited better degradation efficiency, the difference was negligible in comparison to the Mo-TiO 2 and W-TiO 2 catalysts. On the other hand, Cu-Mo-TiO2 showed a relatively lower PL intensity than that of pure TiO2 (Figure 23). This observation indicates a better charge separation, which could promote the photocatalytic performance of the catalyst. Accordingly, the photocatalytic activity of Cu-Mo-TiO2 was investigated towards 4t-BP degradation under solar light irradiation, and its performance was compared with that of mono-doped Mo-TiO2 and W-TiO2 catalysts ( Figure 24). It was observed that the application of solar light required more time to achieve decent degradation for all tested catalysts. In 150 min of solar light exposure, about 70% of 4-t-BP could be degraded using the Cu-Mo-TiO2/solar system. Although the Cu-Mo-TiO2 catalyst exhibited better On the other hand, Cu-Mo-TiO2 showed a relatively lower PL intensity than that of pure TiO2 (Figure 23). This observation indicates a better charge separation, which could promote the photocatalytic performance of the catalyst. Accordingly, the photocatalytic activity of Cu-Mo-TiO2 was investigated towards 4t-BP degradation under solar light irradiation, and its performance was compared with that of mono-doped Mo-TiO2 and W-TiO2 catalysts ( Figure 24). It was observed that the application of solar light required more time to achieve decent degradation for all tested catalysts. In 150 min of solar light exposure, about 70% of 4-t-BP could be degraded using the Cu-Mo-TiO2/solar system. Although the Cu-Mo-TiO2 catalyst exhibited better  In Table 2, the results of this work are compared with previously reported ones.  In Table 2, the results of this work are compared with previously reported ones.

Conclusions
Mono-and co-doped TiO 2 nanoparticles with similar morphology were synthesized by simple preparation methods. The catalyst characterization evidenced that the incorporation of transition metals (Mo, W, Cu, Co, and Zn) led to homogeneous distribution of metal particles over the TiO 2 surface and reduced the energy gap, which led to optical properties different from those of TiO 2 . Specifically, impregnation of Cu into Mo-TiO 2 led to an increase in light absorption, particularly visible light. The catalysts were further investigated for the adsorption and photocatalytic degradation of 4-t-BP by means of UV (365 nm). Doping with transition metals increased the adsorption capacity of TiO 2 . The prepared Cu-Mo-TiO 2 exhibited higher catalytic activity towards degradation of 4-t-BP than that of pure TiO 2 , probably due to the synergistic effect of visible light absorption, improved adsorption capacity, and suppressed electron-hole pair recombination. Complete and about 70% 4-t-BP degradation could be achieved within 60 min and 150 min using UV (365 nm) and solar light exposure, respectively.