Improved Visible-Light Activities of Rutile Nanorod ... - ACS Publications

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Improved visible-light activities of rutile nanorod by co-modifying highly-dispersed SPR Au nanoparticles and HF groups for aerobic selective alcohol oxidation Hongwei Lu, Yang Qu, Liqun Sun, Xiaoyu Chu, Jiadong Li, Yanduo Liu, Linlu Bai, and Liqiang Jing ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03222 • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018

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ACS Sustainable Chemistry & Engineering

Improved visible-light activities of rutile nanorod by co-modifying

highly-dispersed

SPR

Au

nanoparticles and HF groups for aerobic selective alcohol oxidation Hongwei Lu,

†

Yang Qu, † Liqun Sun, † Xiaoyu Chu, †‡ Jiadong Li,† Yanduo Liu, † Linlu Bai,*†‡

Liqiang Jing*† †

Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry

of Education, School of Chemistry and Materials Science, International Joint Research Center for Catalytic Technology, Harbin 150080, PR China. ‡

School of Chemical and Environmental Engineering, Harbin University of Science and

Technology, Harbin 150080, PR China. Email address: [email protected] (Linlu Bai); [email protected] (Liqiang Jing).

ABSTRACT: In this work, rutile nanorod co-modified by highly-dispersed surface plasmon resonance (SPR) Au nanoparticles (NPs) and HF groups exhibits remarkable visible-light activities for the benzyl alcohol oxidation conversion of 90% compared to that (25%) of unmodified one and the high selectivity (>99%) at room temperature with O2 as the oxidant.

ACS Paragon Plus Environment

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Based on the time-resolved surface photovoltage responses and O2-temperature programmed desorption curves, it is confirmed that the outstanding activity of co-modified rutile nanorod is synergistically attributed to the large surface area of and the favored charge separation of assynthesized nanorod, to the extended visible-light response and the promoted charge separation of deposited SPR Au NPs, and then to the promoted O2 adsorption of modified HF groups. Moreover, it is suggested that the produced superoxide-type species be the main active oxygen species to dominate the conversion of benzyl alcohol (BA). Especially, the co-modified rutile nanorod is more active for the visible-light oxidation of alcohol with the electron-donating group like methyl. This work has provided feasible design strategy for highly effective visiblelight photocatalysts for the selective aerobic organic oxidation and pollutant degradation.

KEYWORDS: Rutile nanorod; SPR Au; O2 adsorption; Charge separation; Visible-light photocatalysis; Selective alcohol oxidation

INTRODUCTION Selective oxidation of alcohol to produce ketone or aldehyde is a vital chemical process in industry.1-3 It is highly desired to develop green catalytic process for alcohol oxidation using O2 as the oxidant.4,5 Photocatalytic selective oxidation of alcohol, by using O2 as oxidant, especially under the visible-light irradiation, is obviously advantageous compared with traditional chemical processes.6-9 The key of realizing this conversion is to develop highly active and selective visible-light photocatalysts. Anatase titanium oxide (TiO2) is inexpensive, earth abundant and nontoxic material, which is the most traditional and effective photocatalyst as applied in various photocatalytic conversions including the


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aerobic selective alcohol oxidation.10-12 Nevertheless, due to the limited light absorption of anatase TiO2 in the ultraviolet range, to realize the visible-light alcohol oxidation always needs the addition of organic dyes or complex as the photosensitizer to extend the visible-light absorption.13 The complexity of the photocatalytic system using anatase TiO2 and limited photoactivities urge the development of facile and effective visible-light responsive photocatalytic systems. Compared with anatase, rutile TiO2 shows significant advantages, such as certain extent of visible-light absorption, high chemical stability and hardness, etc.14,15 Therefore, rutile TiO2 might become a promising candidate as the visible-light photocatalyst for the aerobic selective alcohol oxidation. However, traditionally fabricated rutile through the crystal-phase change from anatase by the heat treatment normally has low surface area, which explicitly restricts its application as active photocatalysts. Accordingly, it’s desired to develop an alternative fabrication method to obtain large-surface-area rutile nanomaterial, then to further develop series of rutile-based photocatalysts for highly effective visible-light alcohol oxidation, while which has seldom been studied so far. Besides, the insufficient visible-light absorption, unfavorable charge separation and low affinity with O2 also influence the photoactivity of rutile greatly.16 Gold NPs could strongly absorb visible light because of the SPR derived from the collective oscillation of conductive electrons.17,18 As reported, Au on anatase TiO2 with the heterojunction between metal and the p-type semiconductor could effectively catalyze the visible-light aerobic alcohol oxidation, and the introduction of Au could extend the visible-light response.19,20 Generally accepted, the excited hot electrons of Au NPs under the visible-light irradiation would transfer to the conduction band (CB) of TiO2, leading


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to the enhanced charge separation. Compared with anatase TiO2, the p-type semiconductor rutile TiO2 has lower CB potential rendering more facile electron transfer between SPR Au and TiO2. Noteworthily, since rutile has certain visible-light absorption, so there exists another interfacial electron transfer mode. When rutile is excited, electrons would transfer from the CB of rutile to Au as the electron shuttle, resulting in the enhanced charge separation. Therefore, deposited Au on rutile TiO2 might become more effective photocatalyst for the visible-light aerobic alcohol oxidation. Regretfully, relevant photocatalysts have seldom been investigated, and the specific charge separation mechanism for SPR Au/rutile TiO2, even for well-studied SPR Au/anatase TiO2, is still ambiguous. Moreover, the adsorption and activation of O2 is vital for the photocatalytic aerobic alcohol oxidation. The reduction of O2 consumes the photogenerated electrons to enhance the charge separation, meanwhile produces oxygen species to attend the oxidation reaction. However, the weak affinity of rutile surface with O2 limits the effective activation of O2. Therefore, to effectively promote the O2 adsorption of rutile TiO2 surface might significantly improve the photoactivity by favoring the adsorbed O2 capturing electrons, while which has been neglected as a feasible design strategy to fabricate the TiO2-based photocatalysts for the aerobic alcohol oxidation. It’s reported in our previous work that HF residues on 001 facet-exposed TiO2 surface with the Ti-F coordination could significantly promote the O2 adsorption so as to facilitate the charge separation.21 Therefore, it’s feasible to modify rutile TiO2 with HF groups to further improve the photoactivity by promoting the O2 adsorption.


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Based on above, in this work we have distinctly fabricated the rutile TiO2 nanorod (noted as R for short) with large surface area and favored charge separation, then comodified R with highly-dispersed SPR Au NPs and HF groups. It is clearly demonstrated that the visible-light absorption and charge separation of R is obviously enhanced with SPR Au NPs as the electron shuttle to accept the photoexcited electrons from R, meanwhile the O2 adsorption of the photocatalysts is promoted by HF modification. Consequently, the visible-light activity for selective alcohol oxidation at room temperature with O2 as the oxidant of R is obviously improved through co-modifying highly-dispersed SPR Au NPs and HF groups. Moreover, the specific charge transfer mechanism of the photocatalyst has been clarified to explain the high visible-light activity of R co-modified with SPR Au and HF groups. In addition, it is also confirmed that the produced ·O2- be the main active oxygen species to dominate the conversion of BA. EXPERIMETNAL SECTION Materials and methods All substances used in this study were of analytical grade and used without further purification. Deionized (DI) water was used in all experiments. Rutile nanorod was synthesized by a low-temperature hydrothermal process in the presence of 2.0 M hydrochloric acid (HCl). Tetrabutyl titanate was used as the main starting material. Initially, this reagent was dropwise added to a desired concentrated HCl solution, maintained below 10 oC by an ice-water bath. Then, the mixture was heated in a water bath for 4 h at 80 oC so as to produce white suspension. Subsequently, the suspension was placed in Teflon-lined hydrothermal reactors and heated at 160 oC for 6 h. After that, a white precipitate was collected and washed repeatedly with isopropanol and


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distilled water. The obtained white precipitate was calcined at 450 oC for 2 h and marked as R. For comparison, anatase has been synthesized by traditional method. 5 mL tetrabutyl titanate and 5 mL ethylalcohol were mixed and then dropwise added to a mixture which contains 20 mL ethylalcohol, 5 mL water and 1 mL HNO3. The mixture was placed in Teflon-lined hydrothermal reactors and heated at 160 oC for 6 h. After that, a white precipitate was collected and washed repeatedly with isopropanol and distilled water. The obtained white precipitate was calcined at 450 oC for 2 h and remarked as A. Sample A was calcined at 700 oC for 2 h to prepare rutile TiO2 and marked as R-700. xAuy/R

was

synthesized

by

the

deposition-precipitation

(DP)

method.

Au

nanoparticles were loaded on R by the DP method using HAuCl4·3H2O as a starting material. The obtained photocatalysts were marked as xAuy/R, where x is the amount (in wt %) of Au loaded [wt % = Au/(Au + R)×100%] and y is the calcination temperature (in y oC). Powder R was dispersed into the water containing HAuCl4·3H2O. The mixed solution was adjust to PH=7 value by 1 mM NaOH solution, kept 80 oC by water bathing and stirred for 3h. Finally, the mixed solution were centrifuged and washed. The obtained precipitate was dried at 80 oC and calcined at y oC for 1 h. R-zHF was synthesized by a simple wet-chemical process. R was dispersed into 50 mL 1 M HF acid solution (z is the molar ratio between F and Ti) by vigorously stirring for 1 h. Subsequently, the suspension was dried at 80 oC and then calcined at 300 oC for 10 min. 2Au300/R-zHF was synthesized by a simple wet-chemical process. 2Au300/R was dispersed into HF acid solution (z is the molar ratio between F and Ti) by vigorously


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stirring for 1 h. Subsequently, the suspension was dried at 80 oC and then calcined at 300 o

C for 1 h.

Characterization The as-prepared samples were characterized by various techniques. The crystal structure was determined with the help of X-ray diffraction (XRD) which was performed on a Bruker D8 Advance diffractometer by using CuK radiation (1.5406 Å). The instrument was operated at an accelerating voltage of 30 kV and the emission current was fixed at 20 mA. The chemical composition and elemental states were examined by XPS using a Kratos Axis Ultra DLD apparatus with an Al (mono) X-ray source. During measurement, the binding energies of the samples were calibrated with respect to the signal for adventitious carbon with binding energy equal to 284.55 eV. TEM and HRTEM observation was performed on a JEOL JEM-1400, operated at 100 kV. The samples were suspended in DI water and dried on holey carbon-coated Cu grids. UV-Vis diffuse reflectance spectra (DRS) and UV-Vis absorption spectra were recorded with a Model Shimadzu UV-2550 spectrophotometer, using BaSO4 as reference. Temperatureprogrammed desorption (TPD) for desorption of O2 was performed in a conventional apparatus monitored by a chemisorption analyzer, tp 5080 Chemisorb, which was equipped with a thermal conductivity detector (TCD). About 50 mg of the samples were preheated to 300 oC for 1 h to remove any moisture and then were cooled to room temperature under an ultra-high-purity He stream with a flow rate of 30 mL·min-1. Then, highly pure O2 gas was introduced at a constant temperature of 30 oC and a flow rate 30 mL·min-1 for 60 min. The excess weakly adsorbed O2 was removed by exposure to ultrahigh pure He at 30 oC for 60 min. Then the temperature was increased to 700 oC with a


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heating rate of 10 oC· min-1 under pure He. The surface photovoltage spectroscopy (SPS) measurements were carried out with a home-built apparatus equipped with a lock-in amplifer (SR830) and synchronized with a light chopper (SR540). The powder sample was sandwiched between two indium-tin-oxide (ITO) glass electrodes and the electrodes were kept in an atmosphere-controlled sealed container. Radiations from a 500 W xenon lamp (CHF XQ500W, Global xenon lamp power) were passed through a double prism monochromator (SBP300) to get a monochromatic light. Time-resolved surface photovoltage (TR-SPV) measurements of the samples were taken. The sample chamber connected an indium-tin oxide (ITO) glass as the top electrode and a steel substrate as the bottom electrode, and a 10 µm thick mica spacer was placed in between the ITO glass and the sample to decrease the space charge region at the ITO-sample interface. The samples were then excited by radiation pulses at 355 nm and 532 nm with 10 ns width from a second harmonic Nd: YAG laser (Lab-130-10H, Newport, Co.). The intensity of the pulse was measured by a high energy pyroelectric sensor (PE50BF-DIF-C, Ophir Photonics Group). The signals were amplified with a preamplifier and then registered by a 1 GHz digital phosphor oscilloscope (DPO 4104B, Tektronix). The TR-SPV measurements were performed in an air atmosphere and at room temperature. The analysis of hydroxyl radicals was performed as follow. Each sample (0.05 g) was dispersed in 40 mL of 0.001 M coumarin solution contained in a beaker. Before irradiation, the mixture was magnetically stirred for 10 min to achieve adsorption-desorption equilibrium. Each sample was irradiated under visible-light irradiation and at the wavelength of 405, 470, 520, 590, 660 nm, respectively, for 1 h. After centrifugation, certain amount of each sample was taken in a Pyrex glass cell for fluorescence measurement of 7-


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hydroxycoumarin at 390 nm excitation and 470 nm emission wavelengths through a spectroﬂuorometer (Perkin-Elmer LS55). In-situ FTIR experiments were performed on a Bruker Vector FTIR 6700 spectrometer at a spectral resolution of 4 cm-1. Prior to the insitu FTIR test, the photocatalyst 2Au300/R-1.5HF was dried at 80 oC over night. A 300 W Xenon arc lamp with a 420 CUT filter was used as the light source. Photocatalytic reactions Standard photocatalytic reaction: the photocatalyst (100 mg) and the reactant benzyl alcohol (BA, 200 µmol) was added to toluene (5 mL) within a three-neck flask (25 mL), and the flask was sealed with rubber septum caps. The photocatalyst was dispersed well by ultrasonication for 5 min, and O2 was bubbled through the solution for 20 min. The flask with the reflux of cooling water was immersed in a temperature-controlled water bath (30Error! Reference source not found.2 oC) and photoirradiated at λ> 400 nm with magnetic stirring using a 300 W Xe lamp. After reaction, the same moles of 1, 2diclorobezene with BA was added in the mixture as the external standard. The mixture was then diluted with ethyl acetate to 25 mL. The photocatalyst powder was separated from the liquid by filtration. The liquid mixture was analyzed using an Agilent gas chromatograph 6890 equipped with a HP-5 capillary column (30 m long and 0.32 mm in diameter, packed with silica-based supel cosil) and flame ionization detector (FID). The injector temperature was 250 oC and the spilt is 0.1 µl. The column head pressure of the carrier gas (helium) during the analysis was maintained at 22.57 psi. Temperature program: 50 oC to 180 oC; 20 · min-1; hold for 4 min. The by-products were identified by a gas chromatography-mass spectrometry (GC-MS, Agilent, GC 6890N, MS 5973 inert). Recycle test: photocatalyst was recovered by centrifugation and washed with 50 mL


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acetone and dried at 60 oC overnight. Upon drying, the same amount of the recovered photocatalyst was reused in a standard photocatalytic reaction as described above. Photocatalytic aerobic oxidation of BA with scavengers: tiny amounts of isopropanol, triethanolamine and benzoquinone were added in the photocatalytic oxidation system, respectively, to find out the active species in the conversion process of BA. RESULTS AND DISCUSSION Structural Characterization The rutile TiO2 was fabricated by the direct hydrothermal method with HCl as the additive to control the crystalline phase, which was identified by the XRD (Fig. S1a).22 One can see that the crystal phase of R does not change after modifying with Au NPs and HF groups (Fig. S1b, Fig. S1c, Fig. S1d and Fig. S1e). As shown in the TEM and HRTEM image of 2Au300/R-1.5HF, Au NPs with an average diameter of ca. 3 nm are highly dispersed on the R surface (Fig. 1). Figure 1 In Fig.1 b the lattice fringes of (110) plane with interplanar distance of 0.32 nm and (111) planes with interplanar distance of 0.24 nm are attributed to R and Au NPs, respectively. Heterogeneous junctions of Au-R are formed from TEM and HRTEM. This is further confirmed by XPS. A small red shift of Ti2p after Au loading as shown in Fig. S2a and 2b, revealing the feasibility of the electron transfer between the Au and TiO2. Additionally, the binding energies at 83.1 eV in Fig. S2c are attributed to metallic gold Au0 in 2Au300/R and 2Au300/R-1.5HF samples. Moreover, compared with 2Au300/R, a


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new peak at 684.5 eV can be observed in Fig. S2d which is ascribed to the introduced surface fluoride coordinated with titanium after the post modification by HF groups21. Visible-light activities of R-based samples for the aerobic selective oxidation of benzyl alcohol The aerobic oxidation of BA under the visible-light irradiation (λ>400 nm) was chosen as the benchmarked reaction. The rutile nanorod exhibits favourable visible-light activity with the BA conversion of 24.9% and selectivity towards benzyl aldehyde (BAD) of 91.8%, respectively (Fig. S3a). For the contrast, anatase TiO2 and bulk rutile TiO2 (transferred from anatase by calcination at 700 oC) show rather limited activities. Based on above, the fabricated rutile nanorod could act as effective photocatalyst for the highly selective aerobic alcohol oxidation without any sacrificial agents, which is closely related to its large surface area (ca. 33 cm2/g) compared with bulk rutile and to the extended visible-light absorption till ca. 450 nm according to its UV-Vis spectrum compared with anatase (Fig. S3b). To further improve the photoactivity, Au nanoparticles were loaded on the surface of rutile nanorod by the deposition-precipitation (DP) method. As shown in Fig. 2a with the calcination temperature at 300 oC, increasing the loading amount of Au, the photoactivity of Au modified R increases, and 2Au300/R reaches the maximum conversion of 76.9%. While further increase of Au amount leads to the decrease of the photoactivity. Besides, by adjusting the calcination temperature in the range, it is found 300 oC is the optimum temperature giving the best activity (Fig. 2b). Figure 2


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All the Au modified R samples exhibit an additional broad band from 420 to 650 nm, centering at ca. 550 nm (In Fig. S4a, S4b and S4c), which originates from the SPR effect. The extended visible-light absorption by Au NPs would definitely contribute to the improved photoactivity. Notably with the post-modification of HF, the photoactivity of 2Au300/R is further improved. When increasing the modification amount of HF, the best activity is obtained for 2Au300/R-1.5HF with the BA conversion of 90.0% (Fig. 2c). In addition, the single-component modification of rutile nanorod by HF groups didn’t change the crystalline structure or light absorption of rutile nanorod (Fig. S1e and S5a). Using the same amount of HF to modify the rutile nanorod, the obtained R-1.5HF exhibits the improved photoactivity with the BA conversion of 31.9% compared with bare rutile nanorod (Fig. S5b). Besides, for all R-based samples the selectivity of BAD as the main product maintains above 90% (Table S1). Especially the BAD selectivity of Aucontaining samples all reaches the value beyond 99%, which could provide high yield of product combining with the high BA conversion. Moreover, the photoactivity of 2Au300/R-1.5HF is compared with that of reported TiO2 supported with Au photocatalysts for the aerobic selective oxidation of benzyl alcohol. Noteworthily, under analogous reaction conditions, 2Au300/R-1.5HF has shown significant superiority in terms of both visible-light activity and selectivity among Ausupported photocatalysts for the aerobic oxidation of benzyl alcohol.23-26 In addition, the stability is another vital evaluation index for the heterogeneous photocatalyst. With the same reaction duration after running for 5 cycles, 2Au300/R-1.5HF showed a reasonable stability during the visible-light BA oxidation process, nearly without activity lost (Fig. 2d).


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Photogenerated Charge Separation To further explain the excellent photoactivities of R-based samples, the charge separation measurements were carefully performed. The surface photovoltage spectroscopy (SSSPS) signal could reflect the charge separation situation.27 For xAu300/R samples, as the loading amount of Au increases, the intensity of SS-SPS response increases, till the maximum for 2Au300/R under the visible-light irradiation (Fig. S6a). While for the 2Auy/R samples, the largest SS-SPS response is observed for 2Au300/R (Fig. S6b). For the 2Au300/R-zHF samples, 2Au300/R-1.5HF shows the largest response (Fig. S6c). Moreover, R-1.5HF shows larger response than R (Fig. S6d). In Fig. 3a, the intensities of the SS-SPS signals under visible-light irradiation (λ>400 nm) obeys the sequence that R

Improved Visible-Light Activities of Rutile Nanorod ... - ACS Publications

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