Noble Metal Decoration and Presulfation on TiO<sub>2</sub>: Increased Photocatalytic Activity and Efficient Esterification of<i> n</i>-Butanol with Citric Acid

Niu, Yu; Huang, Pan; Li, Fuying; Yang, Kai; Yang, Jinbei; Wang, Renzhang; Lin, Cheng; Qiu, Ting

doi:https://doi.org/10.1155/2016/4618924

International Journal of Photoenergy

On this page

Abstract Introduction Experimental Results and Discussion Conclusion Acknowledgments Supplementary Materials References Copyright Related Articles

Special Issue

Heterogeneous Photochemistry: Solar Energy Conversion and Environmental Remediation

View this Special Issue

Research Article | Open Access

Volume 2016 | Article ID 4618924 | https://doi.org/10.1155/2016/4618924

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

Yu Niu,^1,2Pan Huang,^1,3Fuying Li,^2,3Kai Yang,³Jinbei Yang,¹Renzhang Wang,²Cheng Lin,¹and Ting Qiu¹

Academic Editor: Andrea Speltini

Received05 Jan 2016

Accepted11 Feb 2016

Published02 Mar 2016

Abstract

TiO₂ has been widely used as a key catalyst in photocatalytic reactions; it also shows good catalytic activity for esterification reactions. Different sulfated M-TiO₂ 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 TiO₂ surface, slightly affected the crystal phase and particle size of TiO₂. Among all the catalysts, /Au-TiO₂ exhibited the best catalytic activity in the esterification reaction for the synthesis of citric acid n-butyl acetate and in the decomposition of methyl 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.

1. Introduction

TiO₂ has received much attention because of its superior photocatalytic activity, chemical stability, low cost, and nontoxicity [1–3]. However, pure TiO₂ is not effective for solar-driven applications. This can be achieved by the modification of semiconductors [4, 5], addition of transition metals [6], nonmetal doping [7–10], and use of coupled semiconductors. Several studies have been reported using noble metals as the cocatalysts for the decomposition of dyes [11–13]. The role of noble metals in the acceleration of dye degradation is twofold: (i) the injected electrons from TiO₂ are easily trapped by noble metals, thus achieving more efficient charge separation; (ii) noble metals are excellent catalysts for the reduction of O₂ 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 O₂, 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 [15–18]. modified TiO₂ solid acid nanoparticles exhibited better catalytic activity than pure TiO₂ 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 /TiO₂ 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 TiO₂ nanotubes as the catalyst for the esterification of acetic acid with cyclohexanol. It showed a high catalytic activity for acetic acid conversion, whereas anatase TiO₂ showed no catalytic activity. Muggli and Ding [12] reported that sulfated TiO₂ was more active and deactivated more slowly than 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 /M-TiO₂ 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 TiO₂ solid acid composite by a photodeposition method. The synthesized noble metal-modified /M-TiO₂ (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 /Au-TiO₂ 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 /Au-TiO₂ 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- and DMPO- adducts in the ESR spectrum. The /Au-TiO₂ catalyst was simultaneously used to esterification reactions and decomposition of organic dyes for industrial applications.

2. Experimental

2.1. Catalyst Preparation

All the chemicals including TiO₂ (pure anatase phase, 99% content, 15 nm, 170 m²/g, Alfa Aesar) were of reagent grade and were used as received without any further purification.

The noble metal-loaded TiO₂ samples were prepared by the photodeposition method. The TiO₂ 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 TiO₂ was prepared by the ultrasonic method. In a typical synthesis, 1 g of TiO₂ powder was dissolved in 15 mL of 1 M H₂SO₄, 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 /M-TiO₂ (M = Ag, Au, Rh, and Pt) was the same as that for sulfated TiO₂; only TiO₂ was changed to M-TiO₂.

2.2. 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 X-ray diffractometer (PANalytical, Netherlands) using Cu Kα radiation (λ = 0.15406 nm) at a scan rate of 2°/min from 20° to 90° (2θ). 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 N₂ 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 TiO₂ 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 BaSO₄ as the reference sample.

The XPS spectra were recorded using a VG ESCALAB 250 XPS system (Thermo Fisher Scientific, USA) using a monochromated Al Kα 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 cm² sample was coated on a fluorine-doped FTO glass as the working electrode. The three electrodes were immersed in a Na₂SO₄ 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 radicals were detected in a dimethyl pyridine N-oxide (DMPO)/catalyst/water suspension, and the 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).

2.3. 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 well-stirred 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: % (: initial absorbance; : final absorbance), by measuring its absorbance at 664 nm using a BK UV-1600 UV-vis spectrometer (Biobase, China).

3. Results and Discussion

3.1. Crystalline Phases and Texture of Samples

The crystalline phases and texture of the samples were characterized by their XRD and N₂ adsorption. Figure 1(a) shows the XRD patterns of TiO₂ and /M-TiO₂ (M = Ag, Au, Rh, and Pt) nanoparticles. For all the samples, the peaks at 2θ = 25.1°, 37.6°, 48.0°, 53.8°, 55.0°, and 62.7° can be attributed to the typical anatase phase of TiO₂ (JCPDS: 21-1272) [24]. The peaks at 2θ = 27.4°, 36.1°, and 54.3° with a lower intensity can be attributed to the rutile crystal structure. The XRD patterns show that the loading of noble metal nanoparticles did not change the crystal structure of TiO₂. For Au-TiO₂, four additional peaks at 2θ = 38.185°, 44.381°, 64.571°, and 77.566° were observed; they can be attributed to Au (JCPDS: 04-0784). This indicates that a small amount of Au was formed during the synthesis. Similarly, the corresponding peaks for noble metal-modified nanoparticles were observed corresponding to Ag (JCPDS: 65-2871), Rh (JCPDS: 05-0685), and Pt (JCPDS: 65-2868) in the Ag-TiO₂, Rh-TiO₂, and Pt-TiO₂ powders, respectively (Figure 1(b)).

(a)

(b)

The change in the physic-chemical properties of the samples was studied by N₂ adsorption-desorption at 77 K. Figure 2 shows the N₂-sorption isotherms (Figure 2(a)) and the corresponding pore-size distribution curves (Figure 2(b)) of TiO₂, /TiO₂, and /M-TiO₂ (M = Ag, Au, Rh and Pt); their pore sizes are shown in Table 1. It can be seen obviously that they exhibited the typical type-IV adsorption curves with an hysteresis loop between the partial pressure , suggesting that TiO₂, /TiO₂ and /M-TiO₂ maintained the mesoporous structure of the TiO₂ support. The BET surface areas of pure TiO₂, /TiO₂, and /M-TiO₂ samples were 168.69 m²/g, 190.03 m²/g, and 193.26–198.85 m²/g, respectively. Although the pore sizes increased, the /TiO₂ and /M-TiO₂ samples still exhibited much higher BET surface areas than TiO₂, 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 TiO₂ support, and noble metal nanoparticles were well dispersed on the surface of TiO₂.

(a)

(b)

3.2. 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 TiO₂, /TiO₂ and /M-TiO₂ (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 TiO₂ 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 TiO₂ (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 TiO₂ among all the samples.

3.3. UV-Vis Diffuse Reflection Spectra (DRS)

Figure 4 shows the DRS of TiO₂, /TiO₂, and /M-TiO₂ (M = Ag, Au, Rh, and Pt) samples. An optical absorption threshold was observed at 383 nm, corresponding to the band gap of TiO₂ at 3.2 eV. This value is consistent with the reported value of anatase TiO₂ [28]. Compared to pure TiO₂ and /TiO₂, the broad absorption bands of /M-TiO₂ (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 /Au-TiO₂, corresponding to the located surface plasmon resonance of Au nanoparticles [29]. Therefore, the loading of noble metal nanoparticles into anatase /TiO₂ may have contributed to the increased activities.

3.4. Surface Chemical States of the Samples

The chemical states of the samples were investigated by XPS. The C 1s peak of the aliphatic carbon with a binding energy (BE) of 284.6 eV was used as the reference. The spectra show no difference in the BEs of O and Ti atoms before and after the noble metal nanoparticles were dispersed homogeneously (Figure 5(a)). The O 1s, Ti 2p, and S 2p peaks of /M-TiO₂ (M = Ag, Au, Rh, and Pt) are shown in Figure 5(b), confirming sulfation. The XPS spectra of Ag 3d, Au 4f, Rh 3d, and Pt 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 Ag⁰ cations in /Ag-TiO₂; 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 Au⁰ cations in /Au-TiO₂ [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 Rh⁰ cations in /Rh-TiO₂; 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 Rh³⁺ 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 Pt⁰ cations in /Pt-TiO₂; 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 Pt²⁺ valence state, respectively [33].

(a)

(b)

(c)

(d)

(e)

(f)

Although the Au nanoparticle-loaded sulfated TiO₂ shows only the presence of Au⁰ 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 Au⁰ valence states are responsible for the catalytic activity in the esterification reaction to synthesize CABu and decomposition of MO.

3.5. FT-IR Spectra of Samples

The FT-IR spectra of TiO₂, /TiO₂, and /M-TiO₂ (M = Ag, Au, Rh, and Pt) are shown in Figure 6 band was observed at 1105 cm⁻¹, corresponding to free ; that is, free groups is present in /M-TiO₂. /TiO₂ and /M-TiO₂ showed two bands at 3422 cm⁻¹ and 1637 cm⁻¹, in which one can be attributed to adsorbed water and the other can be attributed to the surface hydroxyl group of TiO₂. The S–O stretching frequencies of /TiO₂ and /M-TiO₂ were found at 1137–1128 cm⁻¹ and 1068–1051 cm⁻¹, 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 /M-TiO₂ and /TiO₂ were the characteristics of chelating bidentate group. Herein, the zero valence state of Au⁰ 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 /Au-TiO₂ can be attributed to the high concentration of surface-adsorbed sulfate groups.

3.6. 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 /M-TiO₂ (M = Ag, Au, Rh, and Pt) were higher than those of /TiO₂ and TiO₂ 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. /Au-TiO₂ 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 [37–39]. 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.

(a)

(b)

and 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 Figures 8(a) and 8(b), respectively. Under the irradiation of nm, four characteristic peaks of the ESR signal of DMPO- adduct were detected in all the aqueous suspensions of the samples (Figure 8(a)). Six characteristic peaks of the DMPO- adduct were clearly observed in the ESR spectra (Figure 8(b)). The presence of the active species can be attributed to the reaction of photoinduced holes (h⁺) with H₂O molecules [40, 41]. The signal intensities of the DMPO- and DMPO- adducts in the ESR spectra of /Au-TiO₂ were the strongest, indicating that this catalyst has the highest photocatalytic activity in the decomposition of MO. Thus, /Au-TiO₂ catalyst may have increased the surface acidity for ester synthesis and dye decomposition, great practical value for industrial applications.

(a)

(b)

3.7. 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 SO₃H 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; 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 TiO₂ was 201 μmol/g or ~0.71 groups per nm² on the surface of TiO₂. The surface coverage of was calculated using the BET surface area. However, the amounts of groups on M-TiO₂ (M = Ag, Au, Rh, and Pt) were 296 μmol/g, 804 μmol/g, 542 μmol/g, 664 μmol/g, or about 1.32, 2.71, 1.87, 2.05 groups per nm² on the surface of TiO₂, respectively. The surface sulfate groups played an important role in catalysis by offering active acid sites; evidently, the /Au-TiO₂ had the most abundant Lewis acid sites with the highest acid density, increasing the catalytic activities.

3.8. Esterification Reactions for CABu on Samples

The catalytic activities of the /TiO₂ and /M-TiO₂ (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.

CABu can be widely used as a solvent in medical products, food packaging materials, and resin plasticizers [42]. Therefore, TiO₂, /TiO₂ and /M-TiO₂ 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 /Au-TiO₂ 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 TiO₂, /TiO₂ and /M-TiO₂ 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, /Au-TiO₂ showed the highest catalytic activity for CABu at 400 K, whereas a blank experiment (with TiO₂ catalyst) showed that the conversion of CABu was <63%. Notably, the sulfated TiO₂ catalysts clearly increased the catalytic activity for the esterification reaction to synthesize CABu; the loading of noble metals further increased the catalytic activity. Particularly, /Au-TiO₂ exhibited the highest conversion rate; this can be attributed to the stronger acidity and the strong interactions between Au and /TiO₂.

3.9. Photocatalytic Decomposition of MO

The photocatalytic activities of TiO₂, /TiO₂, and /M-TiO₂ (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 TiO₂ and suppressed the recombination of photoelectrons and holes [44]. Moreover, /Au-TiO₂ exhibited a much higher activity for the degradation rate than other noble metal-modified TiO₂ 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].

4. Conclusion

Noble metal-modified /M-TiO₂ (M = Ag, Au, Rh, and Pt) catalysts were synthesized using the photodeposition and ultrasonic methods. The results show that /Au-TiO₂ had the best catalytic activity in the esterification reactions of CA and nBuOH for the synthesis of CABu, and also in the decomposition of MO under halogen lamp irradiation. The enhanced activity can be attributed to stronger acidic sites, larger specific surface areas, and the composition or valence states of noble metal nanoparticles. Moreover, the photoelectrochemical and ESR measurements confirmed the highest electron transfer efficiency and formation of ^•OH and active species in /Au-TiO₂ sample. This study indicates a great potential of the solid-acid catalysts for esterification reactions and wastewater treatments.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was financially supported by the Collaborative Innovation Center of Clean Coal Gasification Technology (XK1403, XK1401), Natural Fund of Fujian Province (2015J01601), and Youth Fund of Fujian Province (JA14290, JA15475).

Supplementary Materials

Sulfated SO₄^2-/Au-TiO₂ nanoparticles exhibited 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), which has great potential applications in esterification reactions and wastewater treatments.

Supplementary Material

References

J. Zhang, Q. Xu, Z. Feng, M. Li, and C. Li, “Importance of the relationship between surface phases and photocatalytic activity of TiO₂,” Angewandte Chemie—International Edition, vol. 47, no. 9, pp. 1766–1769, 2008.
View at: Publisher Site | Google Scholar
C. Yu, D. Cai, K. Yang, J. C. Yu, Y. Zhou, and C. Fan, “Sol–gel derived S, I-codoped mesoporous TiO₂ photocatalyst with high visible-light photocatalytic activity,” Journal of Physics and Chemistry of Solids, vol. 71, no. 9, pp. 1337–1343, 2010.
View at: Publisher Site | Google Scholar
A. Fujishima, X. Zhang, and D. A. Tryk, “TiO₂ photocatalysis and related surface phenomena,” Surface Science Reports, vol. 63, no. 12, pp. 515–582, 2008.
View at: Publisher Site | Google Scholar
M. Zhang, C. Chen, W. Ma, and J. Zhao, “Visible-light-induced aerobic oxidation of alcohols in a coupled photocatalytic system of dye-sensitized TiO₂ and TEMPO,” Angewandte Chemie—International Edition, vol. 47, no. 50, pp. 9730–9733, 2008.
View at: Publisher Site | Google Scholar
M. Grätzel, “Dye-sensitized solar cells,” Journal of Photochemistry and Photobiology C: Photochemistry Reviews, vol. 4, no. 2, pp. 145–153, 2003.
View at: Publisher Site | Google Scholar
X.-Q. Gong, A. Selloni, O. Dulub, P. Jacobson, and U. Diebold, “Small Au and Pt clusters at the anatase TiO₂(101) surface: behavior at terraces, steps, and surface oxygen vacancies,” Journal of the American Chemical Society, vol. 130, no. 1, pp. 370–381, 2008.
View at: Publisher Site | Google Scholar
G. Liu, Y. Zhao, C. Sun, F. Li, G. Q. Lu, and H.-M. Cheng, “Synergistic effects of B/N doping on the visible-light photocatalytic activity of mesoporous TiO₂,” Angewandte Chemie—International Edition, vol. 47, no. 24, pp. 4516–4520, 2008.
View at: Publisher Site | Google Scholar
C. Yu and C. Y. Jimmy, “A simple way to prepare C–N–codoped TiO₂ photocatalyst with visible-light activity,” Catalysis Letters, vol. 129, no. 3-4, pp. 462–470, 2009.
View at: Publisher Site | Google Scholar
X. Chen, X. Wang, Y. Hou, J. Huang, L. Wu, and X. Fu, “The effect of postnitridation annealing on the surface property and photocatalytic performance of N-doped TiO₂ under visible light irradiation,” Journal of Catalysis, vol. 255, no. 1, pp. 59–67, 2008.
View at: Publisher Site | Google Scholar
C. Yu, J. C. Yu, and M. Chan, “Sonochemical fabrication of fluorinated mesoporous titanium dioxide microspheres,” Journal of Solid State Chemistry, vol. 182, no. 5, pp. 1061–1069, 2009.
View at: Publisher Site | Google Scholar
P. Periyat, S. C. Pillai, D. E. McCormack, J. Colreavy, and S. J. Hinder, “Improved high-temperature stability and sun-light-driven photocatalytic activity of sulfur-doped anatase TiO₂,” Journal of Physical Chemistry C, vol. 112, no. 20, pp. 7644–7652, 2008.
View at: Publisher Site | Google Scholar
D. S. Muggli and L. Ding, “Photocatalytic performance of sulfated TiO₂ and Degussa P-25 TiO₂ during oxidation of organics,” Applied Catalysis B: Environmental, vol. 32, no. 3, pp. 181–194, 2001.
View at: Publisher Site | Google Scholar
S. Liu, J.-H. Yang, and J.-H. Choy, “Microporous SiO₂–TiO₂ nanosols pillared montmorillonite for photocatalytic decomposition of methyl orange,” Journal of Photochemistry and Photobiology A: Chemistry, vol. 179, no. 1-2, pp. 75–80, 2006.
View at: Publisher Site | Google Scholar
C. Chen, W. Ma, and J. Zhao, “Semiconductor-mediated photodegradation of pollutants under visible-light irradiation,” Chemical Society Reviews, vol. 39, no. 11, pp. 4206–4219, 2010.
View at: Publisher Site | Google Scholar
B. Krishnakumar and M. Swaminathan, “A recyclable and highly effective sulfated TiO₂-P25 for the synthesis of quinoxaline and dipyridophenazine derivatives at room temperature,” Journal of Organometallic Chemistry, vol. 695, no. 24, pp. 2572–2577, 2010.
View at: Publisher Site | Google Scholar
J. L. Ropero-Vega, A. Aldana-Pérez, R. Gómez, and M. E. Niño-Gómez, “Sulfated titania [TiO₂/SO₄²⁻]: a very active solid acid catalyst for the esterification of free fatty acids with ethanol,” Applied Catalysis A: General, vol. 379, no. 1-2, pp. 24–29, 2010.
View at: Publisher Site | Google Scholar
D. Yang, J. Li, M. Wen, and C. Song, “Selective catalytic reduction of NO_x with CH₄ over the in/sulfated TiO₂ catalyst,” Catalysis Letters, vol. 122, no. 1-2, pp. 138–143, 2008.
View at: Publisher Site | Google Scholar
B. Wang, X. Cui, H. Ma, and J. Li, “Photocatalytic activity of SO₄²⁻/ SnO₂–TiO₂ catalysts: direct oxidation of n-heptane to ester under mild conditions,” Energy & Fuels, vol. 21, no. 6, pp. 3748–3749, 2007.
View at: Publisher Site | Google Scholar
S. Han, G. Zhang, H. Xi, D. Xu, X. Fu, and X. Wang, “Sulfated TiO₂ decontaminate 2-CEES and DMMP in vapor phase,” Catalysis Letters, vol. 122, no. 1-2, pp. 106–110, 2008.
View at: Publisher Site | Google Scholar
P. Chen, M. Du, H. Lei et al., “ ${SO}_{4}^{2 -}$ /ZrO₂-titania nanotubes as efficient solid superacid catalysts for selective mononitration of toluene,” Catalysis Communications, vol. 18, pp. 47–50, 2012.
View at: Google Scholar
L. Liu, G. Zhang, L. Wang, T. Huang, and L. Qin, “Highly active S-modified ZnFe₂O₄ heterogeneous catalyst and its photo-Fenton behavior under UV-visible irradiation,” Industrial and Engineering Chemistry Research, vol. 50, no. 12, pp. 7219–7227, 2011.
View at: Publisher Site | Google Scholar
S. J. Wind, J. Appenzeller, R. Martel, V. Derycke, and P. Avouris, “Vertical scaling of carbon nanotube field-effect transistors using top gate electrodes,” Applied Physics Letters, vol. 80, no. 20, pp. 3817–3819, 2002.
View at: Publisher Site | Google Scholar
H. Nishikiori, M. Hayashibe, and T. Fujii, “Visible light-photocatalytic activity of sulfate-doped titanium dioxide prepared by the sol−gel method,” Catalysts, vol. 3, no. 2, pp. 363–377, 2013.
View at: Publisher Site | Google Scholar
S. M. Jung and P. Grange, “Characterization and reactivity of pure TiO₂–SO₄²⁻ SCR catalyst: influence of SO₄²⁻ content,” Catalysis Today, vol. 59, no. 3, pp. 305–312, 2000.
View at: Publisher Site | Google Scholar
X. P. Lin, D. M. Song, X. Q. Gu, Y. L. Zhao, and Y. H. Qiang, “Synthesis of hollow spherical TiO₂ for dye-sensitized solar cells with enhanced performance,” Applied Surface Science, vol. 263, pp. 816–820, 2012.
View at: Publisher Site | Google Scholar
H. Li, X. Zhang, Y. Huo, and J. Zhu, “Supercritical preparation of a highly active S-doped TiO₂ photocatalyst for methylene blue mineralization,” Environmental Science & Technology, vol. 41, no. 12, pp. 4410–4414, 2007.
View at: Publisher Site | Google Scholar
I. Lee, J. B. Joo, Y. Yin, and F. Zaera, “A yolk@shell nanoarchitecture for Au/TiO₂ catalysts,” Angewandte Chemie—International Edition, vol. 50, no. 43, pp. 10208–10211, 2011.
View at: Publisher Site | Google Scholar
J. R. Sohn and D. G. Lee, “Characterization of zirconium sulfate supported on TiO₂ and activity for acid catalysis,” Korean Journal of Chemical Engineering, vol. 20, no. 6, pp. 1030–1036, 2003.
View at: Publisher Site | Google Scholar
K. Yang, K. Huang, Z. He, X. Chen, X. Fu, and W. Dai, “Promoted effect of PANI as electron transfer promoter on CO oxidation over Au/TiO₂,” Applied Catalysis B: Environmental, vol. 158-159, pp. 250–257, 2014.
View at: Publisher Site | Google Scholar
C. Yu, L. Wei, X. Li, J. Chen, Q. Fan, and J. C. Yu, “Synthesis and characterization of Ag/TiO₂-B nanosquares with high photocatalytic activity under visible light irradiation,” Materials Science and Engineering B: Solid-State Materials for Advanced Technology, vol. 178, no. 6, pp. 344–348, 2013.
View at: Publisher Site | Google Scholar
N. Kruse and S. Chenakin, “XPS characterization of Au/TiO₂ catalysts: binding energy assessment and irradiation effects,” Applied Catalysis A: General, vol. 391, no. 1, pp. 367–376, 2011.
View at: Publisher Site | Google Scholar
Y. V. Larichev, O. V. Netskina, O. V. Komova, and V. I. Simagina, “Comparative XPS study of Rh/Al₂O₃ and Rh/TiO₂ as catalysts for NaBH₄ hydrolysis,” International Journal of Hydrogen Energy, vol. 35, no. 13, pp. 6501–6507, 2010.
View at: Publisher Site | Google Scholar
T. Huizinga, H. v. T. Blik, J. Vis et al., “XPS investigations of Pt and Rh supported on γ-Al₂O₃ and TiO₂,” Surface Science, vol. 135, no. 1, pp. 580–596, 1983.
View at: Google Scholar
Y.-H. Xu, L.-Y. Wang, Q. Zhang, S.-J. Zheng, X.-J. Li, and C. Huang, “Correlation between photoreactivity and photophysics of sulfated TiO₂ photocatalyst,” Materials Chemistry and Physics, vol. 92, no. 2-3, pp. 470–474, 2005.
View at: Publisher Site | Google Scholar
T. Yamaguchi, “Recent progress in solid superacid,” Applied Catalysis, vol. 61, no. 1, pp. 1–25, 1990.
View at: Publisher Site | Google Scholar
Y. M. Wang, C. Z. Chen, J. H. Luo et al., “Studies on the acidity, structures and crystalline phases of SO₄^2-/TiO₂, PO₄^3-/TiO₂, BO₃^3-/TiO₂ solid acids,” Chinese Journal of Structunal Chemistry, vol. 18, no. 3, pp. 175–181, 1999.
View at: Google Scholar
G. Zhang, M. Zhang, X. Ye, X. Qiu, S. Lin, and X. Wang, “Iodine modified carbon nitride semiconductors as visible light photocatalysts for hydrogen evolution,” Advanced Materials, vol. 26, no. 5, pp. 805–809, 2014.
View at: Publisher Site | Google Scholar
N. Zhang, M.-Q. Yang, Z.-R. Tang, and Y.-J. Xu, “CdS-graphene nanocomposites as visible light photocatalyst for redox reactions in water: a green route for selective transformation and environmental remediation,” Journal of Catalysis, vol. 303, pp. 60–69, 2013.
View at: Publisher Site | Google Scholar
G. Zhang and X. Wang, “A facile synthesis of covalent carbon nitride photocatalysts by Co-polymerization of urea and phenylurea for hydrogen evolution,” Journal of Catalysis, vol. 307, pp. 246–253, 2013.
View at: Publisher Site | Google Scholar
W. Wu, L. Wen, L. Shen, R. Liang, R. Yuan, and L. Wu, “A new insight into the photocatalytic reduction of 4-nitroaniline to p-phenylenediamine in the presence of alcohols,” Applied Catalysis B: Environmental, vol. 130-131, pp. 163–167, 2013.
View at: Publisher Site | Google Scholar
W. Adam, M. A. Arnold, M. Grüne, W. M. Nau, U. Pischel, and C. R. Saha-Möller, “Spiroiminodihydantoin is a major product in the photooxidation of 2′-deoxyguanosine by the triplet states and oxyl radicals generated from hydroxyacetophenone photolysis and dioxetane thermolysis,” Organic Letters, vol. 4, no. 4, pp. 537–540, 2002.
View at: Publisher Site | Google Scholar
J. Xu, J. Jiang, L. V. Wei, and Y. Gao, “Synthesis of tributyl citrate using solid acid as a catalyst,” Chemical Engineering Communications, vol. 198, no. 4, pp. 474–482, 2010.
View at: Publisher Site | Google Scholar
Y. Huang, J. Guo, N. Yu et al., “Preparation and catalytic performance of PW₁₂/E-12 over esterification reaction of citric acid with n-ButylL alcohol,” Speciality Petrochemicals, vol. 32, no. 4, pp. 6–10, 2015.
View at: Google Scholar
V. Subramanian, E. E. Wolf, and P. V. Kamat, “Catalysis with TiO₂/gold nanocomposites. Effect of metal particle size on the Fermi level equilibration,” Journal of the American Chemical Society, vol. 126, no. 15, pp. 4943–4950, 2004.
View at: Publisher Site | Google Scholar
J. Long, H. Chang, Q. Gu et al., “Gold-plasmon enhanced solar-to-hydrogen conversion on the {001} facets of anatase TiO₂ nanosheets,” Energy & Environmental Science, vol. 7, no. 3, pp. 973–977, 2014.
View at: Publisher Site | Google Scholar

Copyright

Copyright © 2016 Yu Niu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

PDF Download Citation

Download other formats

Order printed copies

Views

1778

Downloads

1380

Citations

International Journal of Photoenergy

Heterogeneous Photochemistry: Solar Energy Conversion and Environmental Remediation

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

Abstract

1. Introduction

2. Experimental

2.1. Catalyst Preparation

2.2. Characterization of Catalysts

2.3. Catalytic Performance

3. Results and Discussion

3.1. Crystalline Phases and Texture of Samples

3.2. Morphologies of Samples

3.3. UV-Vis Diffuse Reflection Spectra (DRS)

3.4. Surface Chemical States of the Samples

3.5. FT-IR Spectra of Samples

3.6. Photoelectrochemical Properties and ESR Measurements of Samples

3.7. Acid Density Test

3.8. Esterification Reactions for CABu on Samples

3.9. Photocatalytic Decomposition of MO

4. Conclusion

Conflict of Interests

Acknowledgments

Supplementary Materials

References

Copyright

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