Noble Metal Decoration and Presulfation on TiO2: Increased Photocatalytic Activity and Efficient Esterification of n-Butanol with Citric Acid

TiO 2 has been widely used as a key catalyst in photocatalytic reactions; it also shows good catalytic activity for esterification reactions. Different sulfated M-TiO 2 nanoparticles (M = Ag, Au, Rh, and Pt) were prepared by photodeposition and ultrasonic methods. The results show that the noble metal nanoparticles, which were loaded onto a TiO 2 surface, slightly affected the crystal phase and particle size of TiO 2 . Among all the catalysts, SO 4 /Au-TiO 2 exhibited the best catalytic activity in the esterification reaction for the synthesis of citric acid n-butyl acetate and in the decomposition ofmethyl orange, as confirmed by a high conversion rate of up to 98.2% and 100% degradation rate, respectively. This can be attributed to an increase in the Lewis acidity of the catalyst and increased separation efficiency of electron-hole pairs. This superior catalyst has great potential applications in esterification reactions and wastewater treatments.


Introduction
TiO 2 has received much attention because of its superior photocatalytic activity, chemical stability, low cost, and nontoxicity [1][2][3].However, pure TiO 2 is not effective for solardriven applications.This can be achieved by the modification of semiconductors [4,5], addition of transition metals [6], nonmetal doping [7][8][9][10], and use of coupled semiconductors.Several studies have been reported using noble metals as the cocatalysts for the decomposition of dyes [11][12][13].The role of noble metals in the acceleration of dye degradation is twofold: (i) the injected electrons from TiO 2 are easily trapped by noble metals, thus achieving more efficient charge separation; (ii) noble metals are excellent catalysts for the reduction of O 2 because of easy activation and adsorption; they can absorb more low-energy visible light.The confined electrons on the surface of noble metal nanoparticles can be rapidly scavenged by O 2 , thus avoiding the accumulation of electrons on noble metals and also increasing the formation of superoxide radical anions [14].
Furthermore, acid-modified titania has great potential as a catalyst [15][16][17][18].SO 4 2− modified TiO 2 solid acid nanoparticles exhibited better catalytic activity than pure TiO 2 nanomaterials, because of the coexistence of both Brønsted and Lewis acid sites and many other unique properties [19,20].In recent years, various studies have reported the extensive use of SO 4 2− /TiO 2 solid acid catalyst, an environmentally friendly catalyst, in many catalytic reactions such as esterification and also in the efficient degradation of organic dyes [11,12,21].For example, Wind et al. [22] used sulfated TiO 2 nanotubes as the catalyst for the esterification of acetic acid with cyclohexanol.It showed a high catalytic activity for acetic acid conversion, whereas anatase TiO 2 showed no catalytic activity.Muggli and Ding [12] reported that sulfated TiO 2 was more active and deactivated more slowly than 2 International Journal of Photoenergy P-25 during the photocatalytic oxidation of acetaldehyde above 373 K. Sulfate doping has been shown to strengthen visible absorption, thus accelerating the degradation reactions [23].
Although esterification and degradation of organic dyes are seemingly very different, similar catalysts are used in both the reactions and widely used in various industries.Therefore, the utility and versatility of the SO 4 2− /M-TiO 2 catalysts can be verified by using these two very different types of reaction systems; relevant common factors can also be identified to help in the design and preparation of catalysts.
Based on the above considerations, in this study, Ag, Au, Rh, and Pt were selected as the model noble metals to modify a sulfated TiO 2 solid acid composite by a photodeposition method.The synthesized noble metal-modified SO 4 2− /M-TiO 2 (M = Ag, Au, Rh, and Pt) solid-acid catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), electron spin resonance (ESR), and electrochemical impedance spectroscopy (EIS) analyses.The SO 4 2− /Au-TiO 2 catalyst showed the best catalytic activity for the synthesis of citric acid n-butyl acetate (CABu) in the esterification reaction of citric acid (CA) with n-butyl alcohol (nBuOH) and in the decomposition of methyl orange (MO).Among all the samples, the SO

Experimental
2.1.Catalyst Preparation.All the chemicals including TiO 2 (pure anatase phase, 99% content, 15 nm, 170 m 2 /g, Alfa Aesar) were of reagent grade and were used as received without any further purification.
The noble metal-loaded TiO 2 samples were prepared by the photodeposition method.The TiO 2 powder and a certain amount of noble metal salt were mixed in deionized water; a 300 W Xenon arc lamp (PLS-SXE 300, Beijing Perfectlight) was used as the light source.After irradiating for 5 h, the mixture was filtered, washed, and dried.Then, the resulting solid was filtered and washed twice with deionized water.
The sulfated TiO 2 was prepared by the ultrasonic method.In a typical synthesis, 1 g of TiO 2 powder was dissolved in 15 mL of 1 M H 2 SO 4 , and the mixture was sonicated for 1 h.The solid products were collected after filtration, and then dried in an oven at 60 ∘ C for 12 h, and finally calcined in air at 500 ∘ C for 3 h.
The procedure for the preparation of sulfated SO 4 2− /M-TiO 2 (M = Ag, Au, Rh, and Pt) was the same as that for sulfated TiO 2 ; only TiO 2 was changed to M-TiO 2 .

Characterization of Catalysts.
The phase composition of the samples and crystallite size were determined from their XRD patterns, which were obtained using an X'Pert Xray diffractometer (PANalytical, Netherlands) using Cu K radiation ( = 0.15406 nm) at a scan rate of 2 ∘ /min from 20 ∘ to 90 ∘ (2).The accelerating voltage and applied current were 40 kV and 40 mA, respectively.The crystallite size was calculated from the X-ray line-broadening analysis using the Scherrer formula.
The BET surface areas of the samples were obtained from the N 2 adsorption/desorption isotherms determined at liquid nitrogen temperature (77 K) using an automatic analyzer ASAP 2010 (Micromeritics, China).The samples were degassed for 2 h under vacuum at 350 ∘ C prior to adsorption.The equilibrium time for each point of the BET was 30 min.
The microstructures of the samples were determined by TEM and high-resolution TEM (HRTEM) images at an accelerating voltage of 200 kV using an EM 2010 EX instrument (Jeol, Japan).The samples were deposited on TiO 2 nanoparticles supported by copper grids from the ultrasonically processed ethanol solutions of the products.
The UV-vis diffuse reflection spectra (DRS) were recorded using a Varian Cary 500 Scan UV-vis-NIR spectrometer with BaSO 4 as the reference sample.
The XPS spectra were recorded using a VG ESCALAB 250 XPS system (Thermo Fisher Scientific, USA) using a monochromated Al K X-ray source (15 kV, 200 W, 500 mm pass energy = 20 eV) and a charge neutralizer.All the binding energies were referenced to the C 1s peak at 284.6 eV of surface adventitious carbon.
The FT-IR spectra were recorded using a Nicolet 670 FT-IR spectrometer (Nicolet, USA).The samples were pressed using a KBr disk (18 mm diameter, 25-30 mg) preparation apparatus.The samples were dried at 250 ∘ C for 2 h prior to pressing.The FT-IR spectra were recorded using a Nicolet 670 FT-IR spectrometer using a deuterated triglycine sulfate (DTGS) detector at a resolution of 4 cm −1 and for 32 scans.
The light/dark short-circuit photocurrent response was recorded using the Epsilon electrochemical workstation (BAS, USA) equipped with a Pt foil counter electrode and saturated calomel electrode (SCE) as the reference electrode.A size of 0.5 × 0.5 cm 2 sample was coated on a fluorine-doped FTO glass as the working electrode.The three electrodes were immersed in a Na 2 SO 4 electrolyte solution (0.5 M), and the working electrode was irradiated by a 365 nm light irradiation (Hamamatsu, LC8).The properties of the catalysts under AC polarization were evaluated by EIS experiments using a potentiostat (Zahner-IM6, Zahner, Germany).
The electron paramagnetic resonance (EPR) spectra were recorded using a Bruker A-300-EPR X-band spectrometer.The • OH radicals were detected in a dimethyl pyridine N-oxide (DMPO)/catalyst/water suspension, and the • O 2 − radicals in a DMPO/catalyst/methanol suspension (DMPO = 0.05 M, mass of samples = 3 mg, volume of solvent = 0.5 mL, and wavelength of excitation = 365 nm). a Calculated from the XRD results.

Catalytic Performance.
The catalytic performance of the solid-acid samples was investigated by evaluating their catalytic activity in the esterification reaction of CA and nBuOH to synthesize CABu.The reaction was carried out in a wellstirred oil batch reactor and conducted in a liquid phase.
A predetermined amount (12.6 g CA, 27.5 mL nBuOH, and 0.25 g catalyst) of the reagents was loaded into the reactor and heated to 400 K for 6 h.The liquid products were analyzed by gas chromatography (GC 7900, Techcomp, China) after each 30 min of the reaction.
The photocatalytic performances of the samples were evaluated from their activities in the decomposition of MO in an aqueous solution; a halogen lamp was used as the light source.Each of these catalysts (100 mg) was added to 100 mL of an aqueous MO solution (20 ppm) at room temperature.To achieve the adsorption-desorption equilibrium, the solution was continuously stirred for 30 min in the dark before the light was turned on.At a specific time interval (15 min) of irradiation, 2 mL aliquots were withdrawn and centrifuged to separate the catalysts.The degradation rate () of MO was calculated using the equation:  = ( 0 − )/ 0 × 100% ( 0 : initial absorbance; : final absorbance), by measuring its absorbance at 664 nm using a BK UV-1600 UV-vis spectrometer (Biobase, China).
The change in the physic-chemical properties of the samples was studied by N 2 adsorption-desorption at 77 K. Figure 2 shows the N 2 -sorption isotherms (Figure 2(a)) and the corresponding pore-size distribution curves (Figure 2 Although the pore sizes increased, the SO 4 2− /TiO 2 and SO 4 2− /M-TiO 2 samples still exhibited much higher BET surface areas than TiO 2 , possibly because of the reservation of the porous structure inside the particles under supercritical conditions [25].Even the S-modification further increased the crystal sizes, because the O atom in the O-Ti-O network was replaced with the S atom with a relatively large atomic radius [26].The results show that all the samples modified with noble metals maintained the mesoporous structure of the TiO 2 support, and noble metal nanoparticles were well dispersed on the surface of TiO 2 .

Morphologies of Samples.
The TEM and HRTEM analyses showed the morphologies and distributions of noble metal-modified nanoparticles in the solid samples.The TEM and HRTEM images of TiO 2 , SO 4 2− /TiO 2 and SO 4 2− /M-TiO 2 (M = Ag, Au, Rh, and Pt) are shown in Figures 3(a1)-3(f1) and 3(a2)-3(f2), respectively.Small noble metal nanoparticles homogeneously dispersed on the surface of a larger TiO 2 phase [27].No obvious difference was observed in the morphology of these nanoparticles, indicating that the particle dispersion of noble metals cannot change the lattice spacing of TiO 2 (101).The lattice spacing at 0.234 nm, 0.232 nm, 0.232 nm, and 0.209 nm can be attributed to Ag (111), Au (111), Rh (111), and Pt (200), respectively.Au nanoparticles showed the best dispersion on the surface of TiO 2 among all the samples.anatase TiO 2 [28].Compared to pure TiO 2 and SO 4 2− /TiO 2 , the broad absorption bands of SO 4 2− /M-TiO 2 (M = Ag, Au, Rh, and Pt) can be attributed to the type of loaded noble metal nanoparticles.Clearly, a special strong absorption band was present in the range 450-650 nm for SO 4 2− /Au-TiO 2 , corresponding to the located surface plasmon resonance of Au nanoparticles [29].Therefore, the loading of noble metal nanoparticles into anatase SO 4 2− /TiO 2 may have contributed to the increased activities.4f are shown in Figures 5(c)-5(f), respectively.The observed peaks at ca. 368 eV and 374 eV (Figure 5(c)) correspond to the 3d 5/2 and 3d 3/2 core levels of the Ag 0 cations in SO 4 2− /Ag-TiO 2 ; however, Ag + cations could not be identified because of the small differences in Bes [30].The observed peaks at ca. 83.64 eV and 86.86 eV (Figure 5(d)) correspond to the 4f 7/2 and 4f 5/2 core levels of Au 0 cations in SO 4 2− /Au-TiO 2 [31].The observed peaks at ca. 307.5 eV and 312.5 eV (Figure 5(e)) correspond to the 3d 5/2 and 3d 3/2 core levels of Rh 0 cations in SO 4 2− /Rh-TiO 2 ; the second pair of peaks with BEs of ∼309.45 eV and 314.2 eV can be assigned to the 3d 5/2 and 3d 3/2 core levels of Rh 3+ valence state, respectively [32].The peaks at ca. 70.7 eV and 74.0 eV (Figure 5(f)) can be attributed to the 4f 7/2 and 4f 5/2 core levels of Pt 0 cations in SO 4 2− /Pt-TiO 2 ; the second pair of peaks with BEs of ∼72.6 eV and 75.9 eV can be assigned to the 4f 7/2 and 4f 5/2 core levels of Pt 2+ valence state, respectively [33].

UV-Vis Diffuse Reflection Spectra (DRS)
Although the Au nanoparticle-loaded sulfated TiO 2 shows only the presence of Au 0 valence state, less than the BE of pure metallic Au, this is probably responsible for the improved catalytic activity.This indicates that the nanoparticles with Au 0 valence states are responsible for the catalytic activity in the esterification reaction to synthesize CABu and decomposition of MO. , respectively [34].Based on the reported S-O stretching frequencies [35,36], the two new absorption bands at 1133 cm −1 and 1062 cm −1 for SO 4 2− /M-TiO 2 and SO 4 2− /TiO 2 were the characteristics of chelating bidentate SO 4 2− group.Herein, the zero valence state of Au 0 may have changed the surface properties of the catalyst, consistent with the XPS data.The strongest intensity of both the 1133 cm −1 and 1062 cm −1 bands of the SO 4 2− /Au-TiO 2 can be attributed to the high concentration of surfaceadsorbed sulfate groups.

Photoelectrochemical Properties and ESR Measurements of Samples.
The photoelectronic chemical and ESR measurements of the samples were conducted.Figure 7 shows that the transient photocurrent responses of SO 4 2− /M-TiO 2 (M = Ag, Au, Rh, and Pt) were higher than those of SO 4 2− /TiO 2 and TiO 2 under 365 nm light irradiation (Hamamatsu Co., LC8).This is because the modification with noble metals can significantly enhance the photocurrent.This indicates a more efficient separation of the photoexcited charge carriers on catalysts under their radiation.SO 4 2− /Au-TiO 2 showed the highest transient photocurrent response (Figure 7(a)) and the smallest EIS radius (Figure 7(b)) in all the samples.This indicates that Au nanoparticles may have the best ability to decrease the impedance of electron transfer and increase the charge mobility.This is because of the optimized electronic band structure and interface/surface properties induced by the modification [37][38][39].In the esterification reaction for CABu and decomposition of MO, the electron transfer efficiency and formation of active species induced by Au modification also play an important role.
• OH and The presence of the active species can be attributed to the reaction of photoinduced holes (h + ) with H 2 O molecules [40,41].The signal intensities of the DMPO-• OH and DMPO-• O 2 − adducts in the ESR spectra of SO 4 2− /Au-TiO 2 were the strongest, indicating that this catalyst has the highest photocatalytic activity in the decomposition of MO.Thus, SO 4 2− /Au-TiO 2 catalyst may have increased the surface acidity for ester synthesis and dye decomposition, great practical value for industrial applications.

Acid Density Test.
The total acid densities of all these catalysts were determined as follows: the samples (0.1 g) were placed in an Erlenmeyer flask and mixed with 15 mL of 2 mol/L NaCl solution.As H + ions were present in the SO 3 H group of the sulfonated catalyst, they exchanged with Na + ions by ultrasonication for 60 min.After the filtration, a 0.02 mol/L NaOH solution was used to titrate the filtrate  using phenolphthalein as the indicator.When the color of the filtrate changed from colorless to slightly red, the endpoint of the titration reached [35].The accurate acid quantity was calculated as follows: where (H + ) is the acid quantity of the sulfated samples; (OH − ) is the concentration of the NaOH solution; Δ is the volume of the NaOH solution consumed in titration; and  is the quality of the samples used in ultrasonication.
The results of acid density are shown in Table 2.The amount of sulfate groups on TiO 2 was 201 mol/g or ∼0.71 SO 4 2− groups per nm 2 on the surface of TiO 2 .The surface coverage of SO 4 2− was calculated using the BET surface area.However, the amounts of SO 4 2− groups on M-TiO 2 (M = Ag, Au, Rh, and Pt) were 296 mol/g, 804 mol/g, 542 mol/g, 664 mol/g, or about 1.32, 2.71, 1.87, 2.05 SO 4 2− groups per nm 2 on the surface of TiO 2 , respectively.The surface sulfate groups played an important role in catalysis by offering active acid sites; evidently, the SO 4 2− /Au-TiO 2 had the most abundant Lewis acid sites with the highest acid density, increasing the catalytic activities.CABu can be widely used as a solvent in medical products, food packaging materials, and resin plasticizers [42].Therefore, TiO 2 , SO 4 2− /TiO 2 and SO 4 2− /M-TiO 2 were used to test the performance of the catalysts in the esterification reaction of CA and nBuOH to synthesize CABu.The results are shown in Figure 9.
Previously, 97.67% CABu conversion was attained at 393 K, with a reaction time of >270 min via the phosphotungstic acid loading of an epoxy resin catalyst [43].In this study, a conversion rate of >98.2% was achieved at 400 K with SO 4 2− /Au-TiO 2 catalysts; the reaction time was almost the same as that of the previous report.Figure 9 shows the amount of CABu conversion in the esterification reaction using TiO 2 , SO 4 2− /TiO 2 and SO 4 2− /M-TiO 2 as the catalysts.The results show that all the sulfated samples had a better stability over 270 min, consistent with the reported esterification reactions as shown in Table 3.After the reactions reached equilibrium, SO 4 2− /Au-TiO 2 showed the highest catalytic activity for CABu at 400 K, whereas a blank experiment (with TiO 2 catalyst) showed that the conversion of CABu was <63%.Notably, the sulfated TiO 2 catalysts clearly increased the catalytic activity for the esterification reaction to synthesize CABu; the loading of noble metals further increased the catalytic activity.Particularly, SO 4 2− /Au-TiO 2 exhibited the highest conversion rate; this can be attributed to the stronger acidity and the strong interactions between Au and SO 4 2− /TiO 2 .
3.9.Photocatalytic Decomposition of MO.The photocatalytic activities of TiO 2 , SO 4 2− /TiO 2 , and SO 4 2− /M-TiO 2 (M = Ag, Au, Rh, and Pt) samples were evaluated by using the photodecomposition of MO under halogen lamp irradiation.The results are shown in Figure 10.Noble metals had a high work function and formed a Schottky barrier between the semiconductor and metal; this trapped the injected electrons of the conduction band in TiO 2 and suppressed the recombination of photoelectrons and holes [44].Moreover, SO 4 2− /Au-TiO 2 exhibited a much higher activity for the degradation rate than other noble metal-modified TiO 2 samples in the decomposition of MO.This is because Au nanoparticles not only increased the rate of electron-hole pair formation but also acted as a photosensitizer to harvest visible photons and converted them to electrical energy [45].

Conclusion
Noble metal-modified SO 4 2− /M-TiO 2 (M = Ag, Au, Rh, and Pt) catalysts were synthesized using the photodeposition and ultrasonic methods.The results show that SO Figure 4  shows the DRS of TiO 2 , SO 4 2− /TiO 2 , and SO 4 2− /M-TiO 2 (M = Ag, Au, Rh, and Pt) samples.An optical absorption threshold was observed at 383 nm, corresponding to the band gap of TiO 2 at 3.2 eV.This value is consistent with the reported value of International Journal of Photoenergy

O 2 − 2 −
radicals were detected by the DMPO spin-trapping EPR technique, contributing to a better understanding of the photoinduced hole and electrontransfer and photoredox processes as shown in Figures8(a) and 8(b), respectively.Under the irradiation of  = 365 nm, four characteristic peaks of the ESR signal of DMPO-• OH adduct were detected in all the aqueous suspensions of the samples (Figure 8(a)).Six characteristic peaks of the DMPO-• O adduct were clearly observed in the ESR spectra (Figure 8(b)).

3. 8 .
Esterification Reactions for CABu on Samples.The catalytic activities of the SO 4 2− /TiO 2 and SO 4 2− /M-TiO 2 (M = Ag, Au, Rh, and Pt) samples were investigated using the CABu esterification reaction shown in Figure 11 as the model reaction, where the mass fraction of the catalyst was 2% of CA.

Figure 11 and • O 2 −
Figure 11 4 2− /Au-TiO 2 catalyst showed the largest BET specific surface area, strongest intensity of surface-adsorbed sulfate groups in the FT-IR spectrum, highest transient photocurrent response, and smallest EIS radius in the photoelectric chemical test, and strongest signal intensities of the DMPO-• OH and DMPO- • O 2 − adducts in the ESR spectrum.The SO 4 2− /Au-TiO 2 catalyst was simultaneously used to esterification reactions and decomposition of organic dyes for industrial applications.

Table 1 :
Physicochemical characteristics of as-prepared samples.

Table 2 :
Amount of SO 4 2− groups on prepared catalysts.