SrTiO3 Heterojunction for Visible-Light Induced

Jul 5, 2019 - Z-scheme heterojunctions are an emerging kind of photocatalysts for environmental and energy applications due to their unique charges ...
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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Z‑scheme Ag3PO4/Ag/SrTiO3 Heterojunction for Visible-Light Induced Photothermal Synergistic VOCs Degradation with Enhanced Performance Weikang Ji,† Zebao Rui,*,† and Hongbing Ji*,‡,§

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†

School of Chemical Engineering and Technology, Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Sun Yat-sen University, Zhuhai 519082, P.R. China ‡ Fine Chemical Industry Research Institute, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P.R. China § School of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, P.R. China S Supporting Information *

ABSTRACT: Z-scheme heterojunctions are an emerging kind of photocatalysts for environmental and energy applications due to their unique charges separation and transfer route as well as strong redox ability. Considering that the electron transport media used in Z-scheme, such as noble metal particles, are also potential thermal catalysts, herein, we propose the construction of Z-scheme photothermal synergistic catalysts and their applications in VOCs degradation. Z-scheme heterojunction Ag3PO4/Ag/SrTiO3 (or AgPO/Ag/STO) was constructed and showed remarkably strong visible light response and improved redox ability for oxidative species (•OH and •O2−) production in comparison with the single AgPO and STO. A cooperative effect between visible-light induced photocatalysis and thermocatalysis was proved in the degradation of VOCs over AgPO/Ag/STO, leading to an enhanced performance. Such a Zscheme photothermocatalyst concept provides a new catalyst design route for future environmental application.

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INTRODUCTION Volatile organic compounds (or VOCs) are typical harmful air pollutants.1,2 Photocatalytic oxidation, which can mineralize VOCs under mild reaction conditions using light irradiation,2,3 is an efficient method for VOCs degradation. However, the practical application of traditional photocatalysts in VOCs removal is usually limited by low photocatalytic efficiency, mainly due to narrow photo response range, weak redox ability of charge carriers, and easy recombination of photogenerated electron−hole pairs.2,4−6 Many strategies, such as anion surface modification,4 noble metal loading,7−9 morphology control,10 and heterojunction construction,11,12 have been used to enhance their photocatalytic performance. Among these methods, the heterojunction construction has attracted notable attention because it can effectively inhibit the photogenerated charges recombination and broaden the light response range.2,11,12 Kong et al.11 constructed p-n heterojunction CuxO/SrTiO3 for improving visible-light response and photoinduced charges separation, and a satisfactory performance of visible-light photocatalytic toluene oxidation was obtained. Gong et al.12 designed Ag3PO4/BiVO4 heterojunction for photocatalytic methylene blue degradation under visible light and attributed the enhanced performance to the exposed active (040) facets and interconnected heterojunction. However, the conventional type-I or type-II heterojunctions formation is harmful to the photocatalytic redox activity of the photo© XXXX American Chemical Society

generated electrons/holes in comparison with the single components due to the shift in conduction band (CB) toward positive and valence band (VB) toward negative.13,14 Z-scheme heterojunctions, in which the photogenerated charges follow a Z scheme transfer route, have attracted much attention because they can ensure a wide range of light response and improve the oxidation and reduction ability of semiconductors.13,15−22 Li et al.23 demonstrated the Z-scheme structured Cu2O/Au/BiPO4 could effectively inhibit the recombination of photoinduced carriers. Zhang et al.24 showed the Z-scheme charges transport in 3DOM-SrTiO3/Ag/Ag3PO4 upon exposure to UV−vis light, which was the main reason for its enhanced activity and stability in photodegradation of RhB in comparison with the single components. In contrast, failure formation of Z scheme heterojunction in 3DOM-SrTiO3/Ag/ Ag3PO4 caused the photo corrosion and poor catalytic stability in their cases.24 An important characteristic of typical Zscheme photocatalysts is the existence of redox intermediates (or electron transfer media) enabling the effective transfer and separation of photoinduced charges.13,19−21 Notably, the electron transfer media used in Z-scheme photocatalysts, Received: Revised: Accepted: Published: A

April 23, 2019 June 27, 2019 July 5, 2019 July 5, 2019 DOI: 10.1021/acs.iecr.9b02176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. TEM images and EDS Ag element mapping of AgPO/Ag/STO.

AgPO/Ag/STO was obtained by separation, washing with C2H5OH and purified water, and drying overnight at 120 °C. The reference samples Ag/STO and Ag/AgPO were synthesized by the same quantitative agents and procedure without addition of Na2HPO4 or STO, respectively. The reference sample AgPO/STO was obtained using the similar procedure without the light irradiation step. Catalysts Characterization and Evaluation. X-ray photoelectron spectroscopy (XPS) was tested by an ESCALAB 250 spectrometer with vacuum of ∼2 × 10−7 Pa, and C 1s peak set at 284.8 eV was taken for checking the charging effects. Xray diffraction patterns (XRD) were studied on a RIGAKU equipment (Cu Kα, scanning rate = 6°/min, step size = 0.02°). Ultraviolet−visible (UV−vis) diffuse reflectance spectra were measured by UV2450. High-resolution transmission electron microscopy (HRTEM) measurements were studied by a spectrometer (JEM2010-HR) instrument. Electron paramagnetic resonance (EPR) was measured on a JES-FA200 after 10 min of visible-light exposure (λ = 420−780 nm) at room temperature. The samples consisted of 5 mg of sample, 20 μL of 50 mM 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) and 5 mL of CH3OH for DMPO-•O2− or H2O for DMPO-•OH. A standard three-electrode system in an electrochemical setup (CHI660E) equipped with a xenon lamp (100 mW/cm2) was used to study photocurrent spectra under 0.2 V, composing of the Pt wire (counter electrode), about 2.0 cm2 area of working electrodes with prepared samples, and Ag/AgCl (reference electrode, saturated KCl) with the electrolyte of 0.05 mol/L Na2SO4. Fourier transform infrared (FTIR) was tested by an EQUINOX 55 instrument. In situ DRIFTS (diffuse reflectance infrared fourier transformed spectroscopy) spectra of toluene + He + O2 gaseous mixture reaction over AgPO/Ag/STO under 405 nm light irradiation at 30 °C, at 90 °C in dark and under 405 nm light irradiation at 90 °C were obtained respectively on Bruker EQVINOX-55 FFT spectroscope apparatus with a mercury cadmium telluride (MCT) detector and a UV LED spot light source (UVSP8 V1344, λ = 405 nm, optical power density is 80 mW cm−2). About 10 mg sample was packed in each test, which was heated in a flowing He at 120 °C for 1 h to remove the adsorbed water and other species prior the test. The spectra under specific conditions were recorded with a resolution of 4 cm−1 after 64 scans. Toluene was bubbled into the chamber by He. O2 was fed into the chamber as the reactant with a final molar ratio of He/O2 = 4. The total flow rate was 100 mL/min. Photoluminescence (PL) was performed by a FLSP920 under an excitation wavelength of

such as noble metal particles, are also potential thermal catalysts,25−27 which are overlooked in advanced Z-scheme based photocatalytic oxidation systems due to the weak oxidation ability of the transfer media at a low temperature. Considering the positive effect of heat on photogenerated charges transfer27 and the thermal catalytic activity of noble metal particles,27 herein, we propose the construction of photothermal synergistic Z-scheme catalysts and their application in VOCs degradation. Strontium titanate (SrTiO3 or STO), a perovskite structured semiconductor with suitable band gap structure and good thermal stability,2,4 was chosen to cooperate with silver phosphate (Ag3PO4 or AgPO) by a Ag nanoparticles (NP) bridge to construct heterojunction AgPO/Ag/STO. AgPO was reported to have an efficient visible-light response, but suffered from the stability problem (e.g., light corrosion),24,28 which could be alleviated through the heterojunction construction.12,24 The superior electron trapping ability27,29 and surface plasma resonance (SPR) effect27,30 of Ag NP were frequently reported, which could contribute to the charges transfer and separation as well as the production of oxygen species. The visible light induced Z-scheme charges transfer route is demonstrated in the as-developed AgPO/Ag/STO, which shows a strong visible light response and high redox ability to produce oxidative species (•OH and •O2−). Z-scheme AgPO/ Ag/STO holds a unique photothermocatalytic synergistic and stable performance toward typical VOCs (toluene, xylene, and benzene) degradation upon visible light irradiation and mild heating (not higher than 90 °C). These results indicate that Zscheme photocatalysts are promising candidates for robust photothermal synergistic reactions.

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EXPERIMENTAL PROCEDURES Catalysts Preparation. SrTiO3 (STO) was prepared by a hydrothermal method at 200 °C based on our previous work.27 AgPO/Ag/STO composites were synthesized by a depositionprecipitation method. A certain quantity of STO was dispersed in 30 mL purified water under magnetic stirring, and then 50 mL of 0.1 mol/L AgNO3 aqueous solution was added. Then, Na2HPO4 aqueous solution was used as a precipitant, and dropped into the mixture suspension with a molar ratio of AgNO3/Na2HPO4 = 3. After stirring for 30 min, the mixture suspension was irradiated under a 300 W Xe lamp modified by a cutter (λ > 420 nm) for 10 min under constant stirring. Then, the suspension was stationary for delamination and the B

DOI: 10.1021/acs.iecr.9b02176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 380 nm with a filter (λ > 420 nm). Photothermocatalytic tests were performed using a batch reactor. About 0.2 g sample distributed on a 7 cm2 of quartz holder and ∼800 ppm of vaporous toluene, xylene, or benzene in a N2:O2 = 3:1 gaseous mixture were used for each test. The conversions were calculated based on CO2 yield and carbon balance. The details of catalytic test procedures are described in our previous work.27 The action spectra of AgPO/Ag/STO for photocatalytic oxidation of toluene (∼350 ppm) were tested with homogeneous light illumination for 2 h in a multichannel photochemical reaction system.11 The Ag element content in AgPO/Ag/STO was measured by inductively coupled plasma (ICP, IRIS (HR)), giving a value of ∼25.6 wt %.

peaks at 372.61 and 366.35 eV, of the Ag+ peaks of AgPO, which are related to the effect of phosphate on the chemical state of Ag. The atom ratio of Ag0/Ag+ in AgPO/Ag/STO is 0.21. O 1s spectra (Figure 3b) show two peaks at 531.5 and 529.5 eV, which are ascribed to the surface hydroxyl species (OII) and lattice oxygen (OI).30 The semiquantitative ratio of OII/OI in AgPO/Ag/STO is 0.24, which is larger than the value of 0.21 in STO and 0.11 in AgPO. The strong P 2p peak at 133.3 eV in AgPO indicates the state of PO42− (Figure 3c). Due to the coincidence of Sr 3d and P 2p orbits,31 P 2s peak around 190.6 eV was measured in AgPO/Ag/STO to demonstrate the presence of PO42− (Figure 3c). Ti 2P spectra of AgPO/Ag/STO and STO in Figure 3d present two main peaks of Ti 2p3/2 and Ti 2p1/2 locating at 457.8 and 463.7 eV, attributing to the Ti4+ state in the samples.27 Photoelectric Characterization. Figure 4a compares the UV−vis absorption spectra of AgPO, STO, Ag/AgPO, Ag/ STO, AgPO/STO, and AgPO/Ag/STO. As presented, STO holds good ultraviolet absorption but negligible visible-light absorption. Both AgPO/Ag/STO and Ag/STO hold obviously stronger visible light response because of the surface plasmon response (SPR) effect of Ag NP,27 especially at the wavelength larger than 500 nm. The references Ag/AgPO, AgPO/STO, and AgPO have better absorption between 350 and 500 nm. The bandgaps of pristine semiconductors STO (∼3.21 eV) and AgPO (∼2.44 eV) were calculated based on these absorption data (SI Figure S2).27,31 The valence band maximum values of STO (ca. 2.73 eV) and AgPO (ca. 2.81 eV) were evaluated by their XPS valence band data (SI Figure S3). Then, the conduction band minimum values of STO (ca. −0.48 eV) and AgPO (ca. 0.37 eV) were calculated, respectively. The room temperature photoluminescence spectra (PL) of Ag/STO, Ag/AgPO, AgPO/STO, and AgPO/Ag/STO are presented in Figure 4b. These samples are excited at 380 nm and the near band emission (NBE) around 580 nm has an intensity order of AgPO/STO> AgPO/ Ag/STO > Ag/AgPO > Ag/STO. Considering the electrons trapping function of Ag nanoparticles,29,30 the improved charges separation or PL intensity of Ag/STO and Ag/AgPO lower than that of AgPO/STO can be expected. In addition, the lower PL intensity of AgPO/Ag/STO than that of AgPO/ STO indicates the structure of AgPO/Ag/STO is different from that of AgPO/STO. Moreover, the stronger intensity in AgPO/Ag/STO than those in Ag/STO and Ag/AgPO shows that more recombination of electron−hole pairs happens in AgPO/Ag/STO. Figure 4c shows a transient photocurrent order of AgPO/STO (∼2.04 μA·cm−2) < Ag/AgPO (∼2.68 μA·cm−2) < Ag/STO (∼2.86 μA·cm−2) < AgPO/Ag/STO (∼3.73 μA·cm−2) under simulated sunlight irradiation. The photocurrent intensity of AgPO/Ag/STO is the highest among these samples, indicating the unique charges transfer in AgPO/ Ag/STO. Figure 5 compares the production ability of O2− and OH radicals over different samples upon visible-light irradiation. As shown, EPR signals of DMPO-•OH and DMPO-•O2− over these samples are not responding in darkness. However, after 10 min of visible-light exposure, the signals of both DMPO•OH and DMPO-•O2− are detected over the samples with a same intensity order of AgPO/STO < Ag/AgPO < Ag/STO < AgPO/Ag/STO as photocurrent intensity (Figure 4c). Apparently, AgPO/Ag/STO holds the highest •O2− and •OH production ability, which proves the strong redox ability of Z-scheme structure. Although the pristine STO shows no

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RESULTS Textural Structure. TEM image and EDS analysis of AgPO/Ag/STO show a uniform Ag elemental distribution (Figure 1a,b), ∼5.3 nm of Ag nanoparticle size (Supporting Information (SI) Figure S1n) and a surface Ag amount of ∼1.66 at. % and P amount of ∼2.82 at. % in AgPO/Ag/STO (SI Table S1, Supporting Information). The local enlarged images of AgPO/Ag/STO in Figure 1c−e show a tricompositions of STO with a (110) lattice distance of 0.273 nm, AgPO with a 0.247 nm lattice spacing of (211), and the Ag nanoparticles existing in the intermediate part between STO and AgPO confirmed by the EDS analysis. Thus, the heterojunction formation with Ag NP anchored between AgPO and STO is demonstrated. Figure 2 presents XRD patterns of Ag/AgPO, Ag/STO, AgPO/STO, and AgPO/Ag/STO. No peaks attributed to

Figure 2. XRD patterns of Ag/AgPO, Ag/STO, AgPO/STO, and AgPO/Ag/STO

metallic Ag are found in these samples because of its good dispersion and low amount. Ag/AgPO shows characteristic peaks of Ag3PO4 (JCPDS no. 06−0505) and Ag/STO presents characteristic peaks of tausonite structured SrTiO3 (JCPDS no. 35-0734). Besides, both AgPO/STO and AgPO/Ag/STO have characteristic peaks of Ag3PO4 and SrTiO3. The chemical state and surface element composition of the samples were analyzed by XPS characterization and presented in Figure 3 and SI Table S2. Ag 3d spectra in Figure 3a show the copresence of Ag+ and Ag0 in AgPO/Ag/STO. The strong peaks at 367.5 and 373.5 eV are assigned to the Ag+ in AgPO, and the weak signals at 368.5 and 374.8 eV are attributed to Ag0 in AgPO/Ag/STO.22,24 There are two additional minor C

DOI: 10.1021/acs.iecr.9b02176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. XPS spectra of STO, AgPO and AgPO/Ag/STO: (a) Ag 3d, (b) O 1s, (c) P 2s or 2p, and (d) Ti 2p.

the photocatalytic oxidation of toluene over AgPO/Ag/STO is driven by incident light, and the light wavelength and light absorption control the photocatalytic activity of AgPO/Ag/ STO. For further enhancing the toluene oxidation rate of AgPO/ Ag/STO, heat was introduced into the reaction system. Figure 6b and SI Figure S4 present the toluene degradation rates by photocatalytic oxidation (PCO) under visible light at room temperature (RT), thermocatalytic oxidation (TCO) at various temperatures and photothermocatalytic oxidation (PTO) induced by visible light at different temperatures over AgPO/Ag/STO for 3h. As compared, PCO gives a toluene conversion of 59% at room temperature, and the conversion of toluene only rises from 3% to 14% with raising the temperature from 45 to 90 °C for the TCO process. In PTO, the toluene conversion rises from 65% to 94% over AgPO/Ag/STO with increasing the temperature from 45 to 90 °C for 3h. The TCO and PTO of toluene over AgPO/Ag/STO also comply with first-order oxidation kinetics (SI Figure S5 and Table S4. The fitted rate constant of PTO is 15.1 × 10−3 min−1 at 90 °C and it is 3 and 10 times higher than that of PCO (5.05 × 10−3 min−1) and TCO at 90 °C (1.52 × 10−3 min−1). In addition, the PTO rate constant of AgPO/Ag/STO at 90 °C is also larger than those of reported PCO for toluene (0.1−13.0 × 10−3 min−1) (SI Table S5). The apparent activation energies (Ea) of toluene degradation over AgPO/Ag/STO were calculated (SI Figure S6 and Table S6), which are 22 kJ mol−1 for PTO and 61 kJ mol−1 for TCO, respectively. Clearly, the contribution of PTO is greater than the sum of PCO and TCO over AgPO/Ag/STO and the visible-light irradiation could significantly reduce the Ea of TCO over AgPO/Ag/STO.

visible light response, the SPR effect of Ag NP causes strong visible-light absorption (Figure 4a) and fairly good •O2− and •OH productivity of Ag/STO. Similar Ag SPR effects were also demonstrated in the literatures.11,30,32 Similarly, since the conduction band of AgPO (ca. 0.37 eV) is much more positive than the electrochemical potential of O2/•O2− (−0.33 eV), AgPO/STO exhibits poor •O2− signal intensity. However, the photoinduced electrons in AgPO could be conducted to Ag NP because of the SPR effect of Ag NP in Ag/AgPO and AgPO/Ag/STO, and thus promote the formation of •O2−.32 Photothermocatalytic Oxidation Performance. Figure 6a shows the visible-light photocatalytic degradation activity of toluene over AgPO/STO, Ag/AgPO, Ag/STO, and AgPO/ Ag/STO (λ ≥ 420 nm, 150 mW/cm2) at room temperature. As presented, AgPO/Ag/STO shows a conversion of (>) 90% after 6 h reaction, which holds better photocatalytic activity than the other samples. Notably, AgPO/Ag/STO shows negligible toluene adsorption and conversion in dark at room temperature (SI Figure S4b). For quantitative comparison, the apparent rate constants (k) of these catalysts were calculated by pseudo-first order oxidation kinetics and the ln(C0/C) versus t plots (SI Figure S5a and Table S3). The k values of 1.18 × 10−3, 1.71 × 10−3, 2.96 × 10−3, and 5.05 × 10−3 min−1 for AgPO/STO, Ag/AgPO, Ag/STO, and AgPO/Ag/STO are obtained, respectively. Apparently, the bridge of Ag NP in AgPO/STO significantly improves the photocatalytic activity. The action spectra of AgPO/Ag/STO for photocatalytic oxidation of toluene were tested with a multichannel photochemical reaction system to estimate its monochromatic light activity (SI Figure S6). As found, the toluene conversion decreases with increasing the cutoff wavelength, indicating that D

DOI: 10.1021/acs.iecr.9b02176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. (a) UV−vis absorption spectra, (b) Photoluminescence spectra, and (c) photoelectrochemical current under simulated sunlight irradiation.

Figure 5. EPR spectra of (a) DMPO-•O2− and (b) DMPO-•OH formed in methanol or aqueous solution of Ag/STO, Ag/AgPO, AgPO/STO, and AgPO/Ag/STO

light irradiation at 90 °C) conditions, respectively. Noting that the light source used here is different with those employed in the performance tests due to the limitation of setup condition. As shown, there is CO2, H2O and intermediate species accumulation on AgPO/Ag/STO upon exposure to the reactants. The band around 3250 cm−1 is attributed to H2O adsorbed on catalysts,33 the bands between 2364 and 2329 cm−1 belong to CO2,34,35 and those at 1748−1719 cm−1 are assigned to aliphatic C = O groups and formic acid.36,37 Some crowded bands at 1560−1542 and 1418 cm−1 are attributed to asymmetric and symmetric of carboxylate groups.34 Besides, the aldehydic species signals around 1653−1650 cm−1,38 and anhydrides signals at 1868−1840 cm−138,39 are also detected. As compared, PTO of toluene results in strong CO2 signals but

Cyclic PTO and PCO stability of AgPO/Ag/STO is compared in Figure 6c. Apparently, PTO leads to a better cyclic stability of AgPO/Ag/STO than PCO. Only 5% conversion loss is observed in PTO after a five cycle reaction, whereas it is 12% loss in PCO. The application potential of the as-developed PTO process over AgPO/Ag/STO for VOCs removal was further checked in the degradation of xylene and benzene. As shown in Figure 6d, the conversions of toluene, xylene and benzene all exceed 85% after PTO for 4 h at 90 °C, indicating a good application potential. Figure 7 shows in situ DRIFTS spectra of toluene + He + O2 gaseous mixture reaction over AgPO/Ag/STO as a function of exposure time under PCO (under 405 nm light irradiation at 30 °C), TCO (at 90 °C in dark) and PTO (under 405 nm E

DOI: 10.1021/acs.iecr.9b02176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. (a) photocatalytic oxidation (PCO) of toluene under visible light at room temperature for 6 h; (b) thermocatalytic oxidation (TCO), photocatalytic oxidation (PCO), and photothermocatalytic oxidation (PTO) of toluene over AgPO/Ag/STO for 3 h (RT refers to room temperature); (c) stability tests of PCO at room temperature and PTO at 90 °C with a cyclic time of 4 h over AgPO/Ag/STO; and (d) PTO of xylene, toluene or benzene over AgPO/Ag/STO at 90 °C

weaker H2O and intermediate species accumulation with increasing reaction time in comparison with TCO and PCO processes, demonstrating the superior and photothermocatalytic synergistic performance of AgPO/Ag/STO. Figure 8a shows the FTIR spectra of AgPO/Ag/STO after cyclic PTO and PCO stability tests. The band around 3108 cm−1 assigned to adsorbed water,33 2920 and 2851 cm−1 ascribed to the symmetric and asymmetric υC−H of the methyl group,38,39 1654 and 1547 cm−1 assigned to C−OH or carboxylate groups,38−40 1384, 1016, and 554 cm−1 attributed to P−O vibration were detected on AgPO/Ag/STO.41 As compared, AgPO/Ag/STO after PTO exhibits stronger vibration bands of the catalyst, whereas the sample after PCO shows slightly higher vibration bands of the adsorbed species, indicating that less reaction intermediates accumulation and surface water after five PTO cycles, which could occupy or cover surface active sites and thus decrease catalytic activity.25,37 In other words, the photothermocatalytic synergistic effect can improve surface species decomposition over AgPO/Ag/STO and achieve a stable catalytic performance, which is consistent with in situ DRIFTS analysis. Figure 8b shows Ag 3d of XPS after PTO stability tests over AgPO/ Ag/STO, determining a surface Ag0/Ag+ atom ratio of 0.25 for the used AgPO/Ag/STO, which is close to the value of the fresh AgPO/Ag/STO (∼0.21), proving the good performance stability of Z-scheme AgPO/Ag/STO in PTO process. Moreover, the TEM and SAED images of AgPO/Ag/STO after cyclic stability test were measured (SI Figure S8), which showed no obvious textural difference with the fresh sample,

further demonstrating the stability of the Z-scheme AgPO/Ag/ STO.

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DISCUSSION Based on the estimated band edge values of STO and AgPO as well as the SPR effect of Ag NP, the charge transfer process in Ag/STO, Ag/AgPO, AgPO/STO, and AgPO/Ag/STO induced by visible-light was proposed in Figure 9. Due to the small bandgap of AgPO (∼2.44 eV) and SPR effect of Ag NP,29 the introduction of AgPO and Ag NP results in the excitation of catalysts upon the visible-light absorption (Figure 4a). In the cases of Ag/STO and Ag/AgPO (Figure 9a,b), the Ag NP trap the photogenerated electrons and enhance the charges separation due to their SPR effect and lower Fermi level,29,30,32,42 which leads better charges separation and more •O2− and •OH production than the conventional heterojunction AgPO/STO in Figure 9c. For AgPO/Ag/STO, according to the observed formation (Figure 1) and their bands positions, the unique Z-scheme structure, in which photogenerated electrons from CB of AgPO and holes from VB of STO recombine through Ag NP, was proposed in Figure 9d. As a result, measured recombination of the photogenerated charges is higher in AgPO/Ag/STO than that in Ag/AgPO and Ag/STO (Figure 4b). However, the unique Z-scheme charges transfer process can inhibit the recombination of photogenerated charges in AgPO or STO and guarantees the strong production ability of the oxidative species (•OH and •O2−). Such a photogenerated charges recombination route protects the oxidative ability of photogenerated holes from AgPO and promotes the production of •OH in AgPO/Ag/ F

DOI: 10.1021/acs.iecr.9b02176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 7. In situ DRIFTS spectra of toluene + He + O2 gaseous mixture reaction over AgPO/Ag/STO: (a) under 405 nm light irradiation at 30 °C, (b) at 90 °C in dark and (c) under 405 nm light irradiation at 90 °C.

Figure 8. (a) FTIR spectra of AgPO/Ag/STO after cyclic PCO and PTO stability tests and (b) Ag 3d of XPS over AgPO/Ag/STO after cyclic PTO stability tests for toluene degradation.

while TCO at 90 °C is only one-third the degradation rate of PCO over AgPO/Ag/STO. It was reported that the complete toluene degradation temperature by TCO with Ag based catalysts is usually higher than 300 °C because of the difficulty in oxygen activation.8,43 This work shows that visible-light irradiation greatly promotes the catalytic activity of thermocatalysis and reduces the activation energy from 61 to 22 kJ mol−1 (SI Table S6). In PTO, the SPR effects of Ag NP between AgPO and STO induced by visible light may generate hot electrons,8,30 promote oxygen activation and benefit oxidation rate.27 In the other way, increasing the reaction temperature can accelerate the transfer of photogenerated

STO (Figure 5b). In the same way, the reduction ability of the photoinduced electrons from STO and the generation of active •O2− can be promoted (Figure 5a). Accordingly, the transient photocurrent (Figure 4c), the active groups (Figure 5) and photocatalytic activity of AgPO/Ag/STO (Figure 6a) are the largest among the samples. Moreover, the selective recombination of the photogenerated electrons from AgPO and holes from STO on Ag NP protects AgPO against light corrosion from Ag+ to Ag0 (Figure 8b) and thus enhance photocatalytic stability of AgPO/Ag/STO (Figure 6c). Aforementioned toluene degradation evaluation shows that the PCO rate can be greatly enhanced by heating at 90 °C, G

DOI: 10.1021/acs.iecr.9b02176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 9. Charge transfer process in the photocatalytic oxidation process over (a) Ag/STO, (b) Ag/AgPO, (c) AgPO/STO and (d) AgPO/Ag/ STO.

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electrons44 and then promote the separation of hole−electron pairs over AgPO and STO, respectively. In addition, since both TCO and PTO process hold positive activation energies, heating in PCO is also beneficial for oxygen activation and the oxidation reaction, which in turn can promote the removal of surface accumulated H2O and intermediate products and the performance stability of AgPO/Ag/STO (Figure 6c and Figure 8). As a result, corporative effect between thermocatalysis and photocatalysis is observed in oxidation of VOCs over AgPO/ Ag/STO. Both the presence of Ag NP and Z-scheme structure are crucial for PTO synergistic process through their functions in enhancing the separation and transfer of charges and their redox ability. Shortly, we report a robust Z-scheme structure AgPO/Ag/STO for efficient photothermocatalytic degradation of VOCs. The corresponding PTO mechanism for VOCs degradation over AgPO/Ag/STO is described in Figure 10.

CONCLUSIONS Semiconductors SrTiO3 (STO) and Ag3PO4 (AgPO) were bridged by Ag nanoparticles to construct AgPO/Ag/STO with a Z-scheme photoinduced charges transfer route. Z-scheme AgPO/Ag/STO ensured a strong visible light response and improved the reduction and oxidation ability of the semiconductors for the production of oxidative species (•O2− and •OH). The Ag nanoparticles in AgPO/Ag/STO not only performed as electron transfer media enabling the Z-scheme formation and effective charges production, transfer and separation, but also facilitated the oxygen activation upon heating. Accordingly, Z-scheme AgPO/Ag/STO held a unique photothermocatalytic synergistic performance toward typical VOCs toluene degradation with a higher contribution than the sum of thermocatalytic and photocatalytic oxidation. The conversions of benzene, toluene and xylene all exceeded 85% over AgPO/Ag/STO by photothermocatalytic oxidation with visible light irradiation for 4 h at 90 °C. In short, this work demonstrates the Z-scheme structure with a thermal catalytic active connector is a good choice for photothermocatalytic application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02176. EDS, XPS data, TEM images, and action spectra of the samples; photothermocatalytic and thermocatalytic oxidation of toluene over the samples; regression of rate constants and activation energies; performance comparison with the catalysts used in the literature (PDF)

Figure 10. Schematics of photothermocatalytic VOCs oxidation over AgPO/Ag/STO. H

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AUTHOR INFORMATION

Corresponding Authors

*(Z.B.R.) E-mail: [email protected]. *(H.B.J.) E-mail: [email protected]. ORCID

Weikang Ji: 0000-0001-6419-0796 Zebao Rui: 0000-0003-4251-0298 Hongbing Ji: 0000-0003-1684-9925 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Finantional support by Natural Science Foundation of China (Nos. 21776322, 21576298, U1663220, and 21425627), Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (No. 2017BT01C102), and Science and Technology Program of Guangzhou (No. 201804010154) is acknowledged.

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DOI: 10.1021/acs.iecr.9b02176 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX