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Highlighting unique function of immobilized superoxide on TiO2 for selective photocatalytic degradation
Separation and Purification Technology xxx (xxxx) xxxx
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Highlighting unique function of immobilized superoxide on TiO2 for selective photocatalytic degradation Zhiling Maa, Qunpeng Jiaa, Chang Taoa, Bing Hana,b, a b
⁎
College of Chemistry and Environmental Science, Hebei University. Baoding 071002, PR China Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, VIC 3010, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar energy Dye degradation Reactive oxygen species Wastewater treatment Steric effect
Selective photocatalytic degradation reactions hold great promise for environmental control by utilizing clean and inexhaustible solar energy, but the selectivity and tunability due to uncontrollable oxidation process remains a challenge. Given the photogenerated reactive oxygen species (ROS) as major oxidants in selective degradation systems, we propose that desirable selectivity for organic pollutant degradation can be achieved by adjusting the corresponding photocatalytic radical production processes. Using the anatase-rich and rutile-rich titanium dioxide (TiO2) with methyl orange-methylene blue (MO-MB) dye aqueous mixture as a model system, we investigate tunability and mechanism details for selective degradation via photocatalytic evaluation and trapping experiments. Benefiting from the selective generation of ROS on rutile-rich TiO2 and unique properties of immobilized superoxide, the photocatalysts display outstanding tunability and selectivity. This work provides insights into the actual function of immobilized superoxide on selective photocatalytic degradation reactions by discussing a plausible rational reaction process.
1. Introduction By harvesting and utilizing solar energy, the most economical and environmentally friendly resource on earth, photocatalysis can help minimize the negative impact of fossil fuel combustion on greenhouse gas emission and eco-environmental deterioration. Recently, the selective photocatalysis attracted enormous interest and extended to a variety of applications such as selective synthesis, selective CO2 reduction to fuels, and selective degradation of organic molecules [1]. Those technologies have shown tremendous advantage over traditional thermal processes which are subjected to high energy consumption and undesired hazardous waste production leading to secondary environmental pollution [1,2]. For example, selective photocatalytic degradation can remove specific type of undesirable organic pollutant that is toxic, non-biodegradable and potentially carcinogenic in textile effluents under ambient conditions [3,4]. To date, many photocatalytic systems have been developed for selective pollutant degradation. However, their industrial applications were greatly hindered by dissatisfactory selectivity and tunability of photo-oxidation process [1]. Superoxide radical (O2%−) as one of the most important reactive oxygen species (ROS) was widely known as a key player for organic pollutant degradation and selective photocatalytic reactions [5,6]. By contrast, the generation of valence band holes (h+) and hydroxyl ⁎
radicals (%OH) with strong oxidization power leads to simultaneous and indistinguishable degradation of organic species, which could not only waste harvested energy but also decrease the toxin removal efficiency [7,8]. Unfortunately, after the photoexcited electron migrating to photocatalyst surface to reduce O2, the formed O2%− can be further transformed into %OH via different pathways [9]. For example, intracellular superoxide dismutase (SOD) can catalyze disproportionation reactions of O2%− to generate hydrogen peroxide (H2O2) which can further transformed into more chemical reactive and nonselective %OH [10]. In this context, adjusting the corresponding photocatalytic radical production processes towards preferential generation of O2%− while minimizing %OH production would be crucial to a controllable degradation reaction. So far, engineering functional inorganic and metallic nanocomposite to enhance O2%− production capacity is among the most studied strategies [6,11,12]. Especially, nontoxic and stable properties of TiO2 make it an excellent material for mechanism study despite the shortcoming of not absorbing visible light [13]. Generally, desirable selectivity and tunability of photocatalytic degradation reaction requires understanding the completed relationship between catalyst surface structure and reactivity of ROS. Unfortunately, although the production of different types of ROS such as immobilized O2%− on TiO2 surface has been demonstrated [14–16], investigation on their physiochemical
Corresponding author at: College of Chemistry and Environmental Science, Hebei University. Baoding 071002, PR China. E-mail address: [email protected] (B. Han).
https://doi.org/10.1016/j.seppur.2019.116402 Received 15 August 2019; Received in revised form 2 December 2019; Accepted 5 December 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhiling Ma, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116402
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Fig. 1. Characterization of TiO2 samples: (a) TEM images of original TiO2 samples and (b) Raman spectra, (c) XRD pattern and (d) UV–vis diffuse reflectance spectra for rutile-rich (blue) and anatase-rich (red) TiO2 nanoparticles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Experimental section
Table 1 Characteristics of anatase-rich and rutile-rich TiO2 samples prepared under different conditions. Catalysts
Anatase (%)*
Rutile (%)*
Eg (eV)**
SBET (m2 g−1)
500 °C – 5 h 700 °C – 9 h
92 17
8 83
3.22 3.04
46.97 11.26
2.1. Materials Commercial TiO2 nanoparticles (P25, Degussa), Methylene blue (MB, Tianjin East China Reagent company), methyl orange (MO, Tianjin Medical Company), p-benzoquinone (BQ, Aladdin Reaggent Co. Ltd), triethanolamine (TEOA, Aladdin Reaggent Co. Ltd) and isopropyl alcohol (IPA, Aladdin Reagent Co. Ltd) were used as received without further purification.
* Anatase/rutile proportions were calculated from Spurr and Myers’ equation:
WR WA
= 1.22
( ) − 0.025 based on the X-ray diffraction patterns [24]. IR IA
WR WA
stands for the ratio of rutile and anatase phases. IR refers to the intensity of the rutile (1 1 0) diffraction line, and IA refers to the intensity of anatase (1 0 1) diffraction line. ** Obtained by extrapolation method.
2.2. Preparation of anatase- and rutile-rich TiO2 The TiO2 materials with controllable phase compositions were prepared by a facile crystalline engineering method. In a typical experimental procedure, the P25 TiO2 powder was heated in a ceramic tube reactor (diameter of 3 cm) to 500 °C with ramp rate of 15 °C min−1 and remaining the temperature for 5 h in air. Consequently, anataserich TiO2 materials were generated. Similarly, rutile-rich TiO2 was obtained by heating to 700 °C for 9 h in air.
properties in context of organic pollution removal is still lacking, and in-depth scenario depicting their actual functions in selective photocatalytic reactions is unclear [17]. In this work, we attempt to elucidate the effect of crystalline engineering on selectivity and tunability of photocatalytic degradation using methyl orange-methylene blue (MO-MB) aqueous mixture as model system. In addition, the contribution of generation of ROS, especially the immobilized O2%− on TiO2, is explored via trapping experiment and a possible mechanism pathway was illustrated based on crystal structure and thermodynamics analysis. In context of interfacial photocatalytic reaction, we advance our understanding of photo-oxidation catalytic system by rationalizing the main experimental findings.
2.3. Characterization The crystal phases were determined by a D8 Advance (Bruker, Germany) X-ray diffractometer with Cu Kα radiation (λ = 1.542 Å) at a scan rate (2θ) of 0.02 s−1. The surface compositions were analyzed by a LabRAM Xplo RA (Horiba Jobin Yvon, France) Raman spectrophotometer with a laser wavelength of 532 nm, and a UV-3600 (Shimadzu, Japan) UV–vis diffuse reflectance spectroscopy (DRS) was 2
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Fig. 2. A comparison of photocatalytic degradation of methyl orange (MO) and methylene blue (MB) by TiO2 nanoparticles: (a) anatase-rich TiO2 samples; (b) rutilerich TiO2 samples; (c) rutile-rich TiO2 samples with BQ as O2%− quencher. (d) The selective photocatalytic capability is defined as the ratio (r) of the apparent rate constants (k): r = kMB/kMO. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 2 Apparent rate constants (k) and selective photocatalytic capability (r) and for various photocatalytic situations in the MO-MB mixed solution. Sample
Dye and quencher
k
R2
r = kMB/kMO
Anatase-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2
MB MB-BQ MB-IPA MB-TEOA MB MB-BQ MB-IPA MB-TEOA MO MO-BQ MO-IPA MO-TEOA MO MO-BQ MO-IPA MO-TEOA
0.03231 0.02239 0.01657 0.01947 0.0077 0.00559 0.0057 0.00737 0.05002 0.04 0.04928 0.04685 0.00625 0.0022 0.00534 0.00676
0.99777 0.98833 0.99407 0.99842 0.99196 0.97691 0.99496 0.99729 0.99298 0.94296 0.99688 0.99843 0.99491 0.97676 0.99571 0.99828
0.641457 0.55975 0.336242 0.415582 1.232 2.540909 1.067416 1.090237 0.641457 0.55975 0.336242 0.415582 1.232 2.540909 1.067416 1.090237
Fig. 3. A comparison of 90 min adsorption of methyl orange (MO) and methyl blue (MB) on anatase-rich and rutile-rich TiO2 nanoparticles.
applied to measure the light absorption properties using BaSO4 as a reference material for baseline collection. The Brunauer-Emmett-Teller (BET) specific surface areas of the samples were obtained via nitrogen (N2) adsorption/desorption using a V-sorb2800P analyzer (Gold APP, China) apparatus and surface area was calculated with multi-point method. Particle morphologies were examined using a Tecnai G2F20 STWIN (FEI, US) transmission electron microscope (TEM). Room temperature Photoluminescence (PL) measurements were performed using a F7000 Fluorescence spectrophotometer (Hitachi, Japan) with Xenon lamp at λ = 365 nm after introducing N2 or O2 for 30 min.
2.4. Photocatalytic activity measurement The examination of photocatalytic selectivity of mixed MO and MB dyes was conducted in pH = 3 aqueous solution adjusted by hydrochloride acid at room temperature (RT) to simulate acidic wastewater. Typically, 20 mg photocatalyst was dispersed into 50 mL of mixed solution of 20 mg L−1 MO and MB in a quartz reaction vessel. Before irradiation, the mixed suspension was placed in dark for 30 min to reach an adsorption-desorption equilibrium, and then exposed to a 500 W high-pressure Hg lamp (7*104 LUX). At certain intervals, ~5 mL 3
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3. Results and discussion 3.1. Catalyst Characterization The morphology of original TiO2 samples was revealed by TEM images, which showed irregular shape with a particle size of approximately 20–30 nm (Fig. 1a). Anatase- and rutile-rich TiO2 samples were prepared by a crystalline engineering method. The structural composition of both TiO2 samples was first monitored by Raman spectroscopy (Fig. 1b). It is well known that anatase and rutile polymorphs of TiO2 belong to I41/amd and P42/mnm space groups respectively, which show distinctive Raman features [21–23]. The TiO2 treated at 500 °C for 5 h mainly depicts anatase vibrational modes, with frequencies at 145, 198, 398, 514, 518 (superimposed with the 514 cm−1 band) and 638 cm−1, respectively. But new bands at 230, 445 and 610 cm−1 emerge for rutile-rich TiO2 after treatment at 700 °C for 9 h. It is noted no brookite characteristic peaks were observed in this Raman measurement for all samples. Fig. 1c contains XRD patterns of the resulting samples that match anatase phase of TiO2 (JCPDS NO. 21-1272) and rutile phase (JCPDS NO. 89-4920). Strong XRD signals also indicate that TiO2 nanoparticles were highly crystallized. No other characteristic peak was found which consistent with Raman measurement. We further quantified phase compositions of the two TiO2 samples with Spurr and Myers’ equation as shown in Table 1 [24]. According to the result, the anataserich TiO2 mainly contains 92% anatase phase, whereas the rutile phase weight percent increased to 83% in rutile-rich TiO2. UV–vis spectroscopy was used to examine the optical absorption characteristics of prepared TiO2 samples (Fig. 1d). Both samples show intrinsic absorption in the ultraviolet region attributed to the inter band transition of TiO2 but show no response for visible light. Furthermore, anatase-rich TiO2 shows strong absorption band edge at 385 nm (Eg = 3.22 eV), while rutile-rich TiO2 show a slight red shift in the absorption edge to 3.04 eV due to phase transformation.
Fig. 4. PL intensity of rutile-rich TiO2 monitored after 30 min continuous gas flow for pure O2 (black) and N2 (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
of the reaction solution was centrifuged to measure the concentration of the dyes using UV–Vis spectrometer according to reported methods [18]. The photocatalytic reaction was fitted with a pseudo-first-order relationship and its kinetics was expressed as ln(ct/c0) = −kt according to Langmuir Hinshelwood model [19], where k is the apparent rate constant, and c0 and ct are the dye concentrations at initial state and after irradiation for t min, respectively. The selective photocatalytic capability was quantified as r = kMB/kMO. Catalyst-free photolysis was also performed under the same conditions which shows negligible activity and exclude dye sensitization effect.
2.5. Radical trapping experiments
3.2. Photocatalytic activity
To highlight the effect of ROS on photocatalytic reaction selectivity, 1.0 mM isopropyl alcohol (IPA, a quencher of %OH), p-benzoquinone (BQ, a quencher of O2%−), and triethanolamine (TEOA, a quencher of h+) were applied to testing, respectively [20]. The rest of the procedure was identical to photocatalytic activity measurement.
The photocatalytic selectivity of anatase-rich TiO2 sample towards decolorization process of MO-MB aqueous solution was illustrated in Fig. 2a. The reaction constant k can be obtained through fitting the pseudo-first-order reaction kinetics equation according to Langmuir Hinshelwood model [19]. As shown in Table 2, the determined rate constant is 0.04694 min−1 with MO over anatase-rich TiO2, which is higher than that of MB (0.03011 min−1), confirming a preferential degradation of MO over MB. This is because the chemical structure of MO is more susceptible to photodegradation compared with MB [25]. Based on above results, the selective photocatalytic capability r Fig. 5. Proposed mechanisms of selective photocatalytic degradation for MO-MB aqueous mixtures: (a) Preferential degradation of MB by immobilized O2%− on rutile-rich TiO2 that can easily access thiophene-S; (b) Degradation of MO was hindered on rutile-rich TiO2 due to steric effect where rotating benzene rings can protect amino-N; (c) Illustration of the energy shift of immobilized O2 on rutile-rich TiO2 in comparison with free O2 due to electrostatic interaction between Ti cation and O2.
4
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photo-generated electrons migrate to the rutile-rich TiO2 surface to generate immobilized O2%− radicals at Ov through electron transfer. The immobilized O2%− can selectively decompose MB due to steric effects, which is obvious because the sp2 hybridized thiophene-S in MB can be easily accessed by immobilized O2%− whereas sp3 hybridized amino-N in MO was blocked by the rotating benzene rings (Fig. 5a and 5b). This rule still holds even taking into account resonance effect in MO and MB because two methyl groups bonded to N also show prominent steric hinderance. By contrast, free O2%− can transform into strong oxidant %OH via H2O2 formation in acidic aqueous environment and cause non-selective degradation [14]. Such unique property of immobilized O2%− is further supported by the thermodynamic analysis that explains why immobilized O2%− remains in presence of BQ. As shown in Fig. 5c, the 2π state of immobilized O2%− at TiO2 surface Ov can shift downwards significantly (> 1.0 eV) in energy due to the electrostatic interaction with Ti cation [29,30], which makes electron transfer from immobilized O2%− to BQ thermodynamically unfavorable and suppresses the quenching reaction. Specifically, the E0(O2/ O2%−) of free O2 (−0.33 V vs. SHE) is higher than E0(BQ/HQ) (0.050 V vs. SHE) at pH = 3 [17,31], but E0(O2/O2%−) of immobilized O2 is estimated to be lower than E0(BQ/HQ). These thermodynamic analyses, together with our experimental photocatalysis results, indicate the possibility to control the photocatalytic selectivity by manipulating generation of immobilized O2%− on rutile-rich TiO2. This conclusion would be critically important for noncomplete or selective photocatalytic oxidation reactions using organic molecules as feedstock.
(r = kMB/kMO) is calculated to be 0.65 for anatase-rich TiO2. By contrast, when we use rutile-rich TiO2 for selective photocatalysis under same condition, the obtained R is 1.23, which is about 2 folds higher than that of anatase-rich TiO2. In other words, the enrichment of rutile phase TiO2 exhibits a positive influence on the photocatalytic selectivity towards MB degradation. It also worth mentioning that leuco MB is unlikely to form in large quantity in this aerobic condition. The above observations proves the feasibility of controlling the photocatalytic degradation selectivity by altering the TiO2 phase composition in photocatalysts. To understand the shift of selective photocatalytic degradation, we conducted 90 min adsorption experiment with MO and MB solution over both anatase-rich and rutile-rich TiO2 photocatalysts (Fig. 3). The anatase-rich TiO2 shows negligible MO adsorption (< 1%) and moderate MB adsorption (24.9%), while rutile-rich TiO2 shows enhanced adsorption of MO adsorption (3.6%) and decreased MB adsorption (21.4%). However, the selective photocatalytic capability r of rutilerich TiO2 is larger than that on anatase-rich TiO2 (Fig. 2d and Table 2). In other words, rutile-rich TiO2 has poor photocatalytic selectivity towards MO even though it shows better adsorption for MO than that of anatase-rich TiO2. These results together with above photocatalytic testing indicate that the change in preferential photocatalytic degradation in this work is not due to the substrate adsorption selectivity [18]. Therefore, other explanation must be thought. After careful consideration, we propose the change of photocatalytic selectivity can be explained by preferential generation of O2%− radicals, which is strongly related to the surface structures of TiO2 photocatalysts. The most available and frequent surface of anatase and rutile TiO2 particles is thermodynamically stable (1 0 1) and (1 1 0) facets, respectively, due to their lower surface free energy [26]. Oxygen vacancies (Ov) in rutile (1 1 0) facet is favored by producing two more stable 5-fold coordinated Ti atoms (Ti5c), whereas Ov at anatase (1 0 1) facet is thermodynamically unfavorable due to stiffness of the surface Ti5c-O2c bond [26,27]. As a result, the density of surface Ov in rutile is much higher than that in anatase, which was also experimentally proved by UHV-FTIRS study [15]. Those surface Ov tends to interact strongly with molecular O2 because many stable adsorption sites (e.g. Ti5c) exist around Ov with high binding energy [14,16], whereas O2 activation is sluggish on fully oxidized anatase surface. Therefore we anticipate that the rutile-rich TiO2 sample can highly activate O2 reduction process. Realizing the key role of reactive species in the photocatalytic process, we further carried out trapping experiments for both anatase and rutile-rich TiO2 for mechanism study. Among all different trapping agents added into the photoreaction system, only quencher of O2%− (BQ) has significant effects on photocatalytic activity and selectivity (Fig. 2 and Table 2). Interestingly, addition of 1 mM BQ increased selective photocatalytic capability r further from 1.29 to 2.54, which means MB was selectively decomposed in comparison with MO. By contrast, applying IPA (a quencher of %OH) or TEOA (a quencher of h+) results in r of 1.07 and 1.09, respectively, which is even slightly smaller than that of rutile-rich TiO2 alone. Those results indicate that %OH and h+ show little contribution to the selective degradation of MB on rutilerich TiO2. Bearing this in mind, we also realize that adding BQ as a quencher of O2%− only partially suppressed the reactions, e.g. r = 0.0077 decreasing to r = 0.00559 for MB, which suggests at least one more stable O2%− -like ROS still exists in the catalytic system. Furthermore, the enhancement of PL under O2 atmosphere on rutilerich TiO2 indicates the formation of immobilized O2%− which cause an upward band bending and accumulation of holes in the subsurface region leading to increased radiative recombination (Fig. 4) [28]. Therefore, we propose the immobilized O2%− on TiO2 can survive even in presence of BQ and plays a key role in this selective photocatalysis system, which has been ignored in most of previous studies. A possible mechanism pathway for selective photocatalytic degradation by immobilized O2%− is proposed and shown in Fig. 5. The
4. Conclusion In summary, excellent tunability and selectivity of photocatalytic degradation was successfully achieved by modulating surface crystallinity of TiO2 and the unique property of immobilized O2%− radical. Mechanism study indicates that selective production of O2%− radical plays a dominant role for preferential degradation of MB, which is due to steric effect rather than substrate adsorption selectivity. Importantly, we propose immobilized O2%− on TiO2 surface have shifted 2π energy level, which stabilize O2%− from BQ quenching and prevent subsequent production of nonselective hydroxyl radicals. Furthermore, this process was expected to be much more environmentally benign compared with conventional catalytic degradation reaction which is conducted in harsh conditions. The in depth understanding of specific role of immobilized O2%− as well as established approach for preferential generation of such radicals would provide a novel pathway for the design of advanced oxidation technologies with desirable photocatalytic selectivity.
CRediT authorship contribution statement Zhiling Ma: Conceptualization, Supervision. Qunpeng Jia: Data curation, Investigation. Chang Tao: . Bing Han: Funding acquisition, writing - review & editing.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This work was supported by Natural Science Foundation of Hebei (B2019201064) and Advanced Talents Incubation Program of the Hebei University (1081/801260201284). 5
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Appendix A. Supplementary material
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Highlighting unique function of immobilized superoxide on TiO2 for selective photocatalytic degradation Zhiling Maa, Qunpeng Jiaa, Chang Taoa, Bing Hana,b, a b
⁎
College of Chemistry and Environmental Science, Hebei University. Baoding 071002, PR China Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, VIC 3010, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar energy Dye degradation Reactive oxygen species Wastewater treatment Steric effect
Selective photocatalytic degradation reactions hold great promise for environmental control by utilizing clean and inexhaustible solar energy, but the selectivity and tunability due to uncontrollable oxidation process remains a challenge. Given the photogenerated reactive oxygen species (ROS) as major oxidants in selective degradation systems, we propose that desirable selectivity for organic pollutant degradation can be achieved by adjusting the corresponding photocatalytic radical production processes. Using the anatase-rich and rutile-rich titanium dioxide (TiO2) with methyl orange-methylene blue (MO-MB) dye aqueous mixture as a model system, we investigate tunability and mechanism details for selective degradation via photocatalytic evaluation and trapping experiments. Benefiting from the selective generation of ROS on rutile-rich TiO2 and unique properties of immobilized superoxide, the photocatalysts display outstanding tunability and selectivity. This work provides insights into the actual function of immobilized superoxide on selective photocatalytic degradation reactions by discussing a plausible rational reaction process.
1. Introduction By harvesting and utilizing solar energy, the most economical and environmentally friendly resource on earth, photocatalysis can help minimize the negative impact of fossil fuel combustion on greenhouse gas emission and eco-environmental deterioration. Recently, the selective photocatalysis attracted enormous interest and extended to a variety of applications such as selective synthesis, selective CO2 reduction to fuels, and selective degradation of organic molecules [1]. Those technologies have shown tremendous advantage over traditional thermal processes which are subjected to high energy consumption and undesired hazardous waste production leading to secondary environmental pollution [1,2]. For example, selective photocatalytic degradation can remove specific type of undesirable organic pollutant that is toxic, non-biodegradable and potentially carcinogenic in textile effluents under ambient conditions [3,4]. To date, many photocatalytic systems have been developed for selective pollutant degradation. However, their industrial applications were greatly hindered by dissatisfactory selectivity and tunability of photo-oxidation process [1]. Superoxide radical (O2%−) as one of the most important reactive oxygen species (ROS) was widely known as a key player for organic pollutant degradation and selective photocatalytic reactions [5,6]. By contrast, the generation of valence band holes (h+) and hydroxyl ⁎
radicals (%OH) with strong oxidization power leads to simultaneous and indistinguishable degradation of organic species, which could not only waste harvested energy but also decrease the toxin removal efficiency [7,8]. Unfortunately, after the photoexcited electron migrating to photocatalyst surface to reduce O2, the formed O2%− can be further transformed into %OH via different pathways [9]. For example, intracellular superoxide dismutase (SOD) can catalyze disproportionation reactions of O2%− to generate hydrogen peroxide (H2O2) which can further transformed into more chemical reactive and nonselective %OH [10]. In this context, adjusting the corresponding photocatalytic radical production processes towards preferential generation of O2%− while minimizing %OH production would be crucial to a controllable degradation reaction. So far, engineering functional inorganic and metallic nanocomposite to enhance O2%− production capacity is among the most studied strategies [6,11,12]. Especially, nontoxic and stable properties of TiO2 make it an excellent material for mechanism study despite the shortcoming of not absorbing visible light [13]. Generally, desirable selectivity and tunability of photocatalytic degradation reaction requires understanding the completed relationship between catalyst surface structure and reactivity of ROS. Unfortunately, although the production of different types of ROS such as immobilized O2%− on TiO2 surface has been demonstrated [14–16], investigation on their physiochemical
Corresponding author at: College of Chemistry and Environmental Science, Hebei University. Baoding 071002, PR China. E-mail address: [email protected] (B. Han).
https://doi.org/10.1016/j.seppur.2019.116402 Received 15 August 2019; Received in revised form 2 December 2019; Accepted 5 December 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhiling Ma, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116402
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Fig. 1. Characterization of TiO2 samples: (a) TEM images of original TiO2 samples and (b) Raman spectra, (c) XRD pattern and (d) UV–vis diffuse reflectance spectra for rutile-rich (blue) and anatase-rich (red) TiO2 nanoparticles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Experimental section
Table 1 Characteristics of anatase-rich and rutile-rich TiO2 samples prepared under different conditions. Catalysts
Anatase (%)*
Rutile (%)*
Eg (eV)**
SBET (m2 g−1)
500 °C – 5 h 700 °C – 9 h
92 17
8 83
3.22 3.04
46.97 11.26
2.1. Materials Commercial TiO2 nanoparticles (P25, Degussa), Methylene blue (MB, Tianjin East China Reagent company), methyl orange (MO, Tianjin Medical Company), p-benzoquinone (BQ, Aladdin Reaggent Co. Ltd), triethanolamine (TEOA, Aladdin Reaggent Co. Ltd) and isopropyl alcohol (IPA, Aladdin Reagent Co. Ltd) were used as received without further purification.
* Anatase/rutile proportions were calculated from Spurr and Myers’ equation:
WR WA
= 1.22
( ) − 0.025 based on the X-ray diffraction patterns [24]. IR IA
WR WA
stands for the ratio of rutile and anatase phases. IR refers to the intensity of the rutile (1 1 0) diffraction line, and IA refers to the intensity of anatase (1 0 1) diffraction line. ** Obtained by extrapolation method.
2.2. Preparation of anatase- and rutile-rich TiO2 The TiO2 materials with controllable phase compositions were prepared by a facile crystalline engineering method. In a typical experimental procedure, the P25 TiO2 powder was heated in a ceramic tube reactor (diameter of 3 cm) to 500 °C with ramp rate of 15 °C min−1 and remaining the temperature for 5 h in air. Consequently, anataserich TiO2 materials were generated. Similarly, rutile-rich TiO2 was obtained by heating to 700 °C for 9 h in air.
properties in context of organic pollution removal is still lacking, and in-depth scenario depicting their actual functions in selective photocatalytic reactions is unclear [17]. In this work, we attempt to elucidate the effect of crystalline engineering on selectivity and tunability of photocatalytic degradation using methyl orange-methylene blue (MO-MB) aqueous mixture as model system. In addition, the contribution of generation of ROS, especially the immobilized O2%− on TiO2, is explored via trapping experiment and a possible mechanism pathway was illustrated based on crystal structure and thermodynamics analysis. In context of interfacial photocatalytic reaction, we advance our understanding of photo-oxidation catalytic system by rationalizing the main experimental findings.
2.3. Characterization The crystal phases were determined by a D8 Advance (Bruker, Germany) X-ray diffractometer with Cu Kα radiation (λ = 1.542 Å) at a scan rate (2θ) of 0.02 s−1. The surface compositions were analyzed by a LabRAM Xplo RA (Horiba Jobin Yvon, France) Raman spectrophotometer with a laser wavelength of 532 nm, and a UV-3600 (Shimadzu, Japan) UV–vis diffuse reflectance spectroscopy (DRS) was 2
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Fig. 2. A comparison of photocatalytic degradation of methyl orange (MO) and methylene blue (MB) by TiO2 nanoparticles: (a) anatase-rich TiO2 samples; (b) rutilerich TiO2 samples; (c) rutile-rich TiO2 samples with BQ as O2%− quencher. (d) The selective photocatalytic capability is defined as the ratio (r) of the apparent rate constants (k): r = kMB/kMO. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 2 Apparent rate constants (k) and selective photocatalytic capability (r) and for various photocatalytic situations in the MO-MB mixed solution. Sample
Dye and quencher
k
R2
r = kMB/kMO
Anatase-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Anatase-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2 Rutile-rich TiO2
MB MB-BQ MB-IPA MB-TEOA MB MB-BQ MB-IPA MB-TEOA MO MO-BQ MO-IPA MO-TEOA MO MO-BQ MO-IPA MO-TEOA
0.03231 0.02239 0.01657 0.01947 0.0077 0.00559 0.0057 0.00737 0.05002 0.04 0.04928 0.04685 0.00625 0.0022 0.00534 0.00676
0.99777 0.98833 0.99407 0.99842 0.99196 0.97691 0.99496 0.99729 0.99298 0.94296 0.99688 0.99843 0.99491 0.97676 0.99571 0.99828
0.641457 0.55975 0.336242 0.415582 1.232 2.540909 1.067416 1.090237 0.641457 0.55975 0.336242 0.415582 1.232 2.540909 1.067416 1.090237
Fig. 3. A comparison of 90 min adsorption of methyl orange (MO) and methyl blue (MB) on anatase-rich and rutile-rich TiO2 nanoparticles.
applied to measure the light absorption properties using BaSO4 as a reference material for baseline collection. The Brunauer-Emmett-Teller (BET) specific surface areas of the samples were obtained via nitrogen (N2) adsorption/desorption using a V-sorb2800P analyzer (Gold APP, China) apparatus and surface area was calculated with multi-point method. Particle morphologies were examined using a Tecnai G2F20 STWIN (FEI, US) transmission electron microscope (TEM). Room temperature Photoluminescence (PL) measurements were performed using a F7000 Fluorescence spectrophotometer (Hitachi, Japan) with Xenon lamp at λ = 365 nm after introducing N2 or O2 for 30 min.
2.4. Photocatalytic activity measurement The examination of photocatalytic selectivity of mixed MO and MB dyes was conducted in pH = 3 aqueous solution adjusted by hydrochloride acid at room temperature (RT) to simulate acidic wastewater. Typically, 20 mg photocatalyst was dispersed into 50 mL of mixed solution of 20 mg L−1 MO and MB in a quartz reaction vessel. Before irradiation, the mixed suspension was placed in dark for 30 min to reach an adsorption-desorption equilibrium, and then exposed to a 500 W high-pressure Hg lamp (7*104 LUX). At certain intervals, ~5 mL 3
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3. Results and discussion 3.1. Catalyst Characterization The morphology of original TiO2 samples was revealed by TEM images, which showed irregular shape with a particle size of approximately 20–30 nm (Fig. 1a). Anatase- and rutile-rich TiO2 samples were prepared by a crystalline engineering method. The structural composition of both TiO2 samples was first monitored by Raman spectroscopy (Fig. 1b). It is well known that anatase and rutile polymorphs of TiO2 belong to I41/amd and P42/mnm space groups respectively, which show distinctive Raman features [21–23]. The TiO2 treated at 500 °C for 5 h mainly depicts anatase vibrational modes, with frequencies at 145, 198, 398, 514, 518 (superimposed with the 514 cm−1 band) and 638 cm−1, respectively. But new bands at 230, 445 and 610 cm−1 emerge for rutile-rich TiO2 after treatment at 700 °C for 9 h. It is noted no brookite characteristic peaks were observed in this Raman measurement for all samples. Fig. 1c contains XRD patterns of the resulting samples that match anatase phase of TiO2 (JCPDS NO. 21-1272) and rutile phase (JCPDS NO. 89-4920). Strong XRD signals also indicate that TiO2 nanoparticles were highly crystallized. No other characteristic peak was found which consistent with Raman measurement. We further quantified phase compositions of the two TiO2 samples with Spurr and Myers’ equation as shown in Table 1 [24]. According to the result, the anataserich TiO2 mainly contains 92% anatase phase, whereas the rutile phase weight percent increased to 83% in rutile-rich TiO2. UV–vis spectroscopy was used to examine the optical absorption characteristics of prepared TiO2 samples (Fig. 1d). Both samples show intrinsic absorption in the ultraviolet region attributed to the inter band transition of TiO2 but show no response for visible light. Furthermore, anatase-rich TiO2 shows strong absorption band edge at 385 nm (Eg = 3.22 eV), while rutile-rich TiO2 show a slight red shift in the absorption edge to 3.04 eV due to phase transformation.
Fig. 4. PL intensity of rutile-rich TiO2 monitored after 30 min continuous gas flow for pure O2 (black) and N2 (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
of the reaction solution was centrifuged to measure the concentration of the dyes using UV–Vis spectrometer according to reported methods [18]. The photocatalytic reaction was fitted with a pseudo-first-order relationship and its kinetics was expressed as ln(ct/c0) = −kt according to Langmuir Hinshelwood model [19], where k is the apparent rate constant, and c0 and ct are the dye concentrations at initial state and after irradiation for t min, respectively. The selective photocatalytic capability was quantified as r = kMB/kMO. Catalyst-free photolysis was also performed under the same conditions which shows negligible activity and exclude dye sensitization effect.
2.5. Radical trapping experiments
3.2. Photocatalytic activity
To highlight the effect of ROS on photocatalytic reaction selectivity, 1.0 mM isopropyl alcohol (IPA, a quencher of %OH), p-benzoquinone (BQ, a quencher of O2%−), and triethanolamine (TEOA, a quencher of h+) were applied to testing, respectively [20]. The rest of the procedure was identical to photocatalytic activity measurement.
The photocatalytic selectivity of anatase-rich TiO2 sample towards decolorization process of MO-MB aqueous solution was illustrated in Fig. 2a. The reaction constant k can be obtained through fitting the pseudo-first-order reaction kinetics equation according to Langmuir Hinshelwood model [19]. As shown in Table 2, the determined rate constant is 0.04694 min−1 with MO over anatase-rich TiO2, which is higher than that of MB (0.03011 min−1), confirming a preferential degradation of MO over MB. This is because the chemical structure of MO is more susceptible to photodegradation compared with MB [25]. Based on above results, the selective photocatalytic capability r Fig. 5. Proposed mechanisms of selective photocatalytic degradation for MO-MB aqueous mixtures: (a) Preferential degradation of MB by immobilized O2%− on rutile-rich TiO2 that can easily access thiophene-S; (b) Degradation of MO was hindered on rutile-rich TiO2 due to steric effect where rotating benzene rings can protect amino-N; (c) Illustration of the energy shift of immobilized O2 on rutile-rich TiO2 in comparison with free O2 due to electrostatic interaction between Ti cation and O2.
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photo-generated electrons migrate to the rutile-rich TiO2 surface to generate immobilized O2%− radicals at Ov through electron transfer. The immobilized O2%− can selectively decompose MB due to steric effects, which is obvious because the sp2 hybridized thiophene-S in MB can be easily accessed by immobilized O2%− whereas sp3 hybridized amino-N in MO was blocked by the rotating benzene rings (Fig. 5a and 5b). This rule still holds even taking into account resonance effect in MO and MB because two methyl groups bonded to N also show prominent steric hinderance. By contrast, free O2%− can transform into strong oxidant %OH via H2O2 formation in acidic aqueous environment and cause non-selective degradation [14]. Such unique property of immobilized O2%− is further supported by the thermodynamic analysis that explains why immobilized O2%− remains in presence of BQ. As shown in Fig. 5c, the 2π state of immobilized O2%− at TiO2 surface Ov can shift downwards significantly (> 1.0 eV) in energy due to the electrostatic interaction with Ti cation [29,30], which makes electron transfer from immobilized O2%− to BQ thermodynamically unfavorable and suppresses the quenching reaction. Specifically, the E0(O2/ O2%−) of free O2 (−0.33 V vs. SHE) is higher than E0(BQ/HQ) (0.050 V vs. SHE) at pH = 3 [17,31], but E0(O2/O2%−) of immobilized O2 is estimated to be lower than E0(BQ/HQ). These thermodynamic analyses, together with our experimental photocatalysis results, indicate the possibility to control the photocatalytic selectivity by manipulating generation of immobilized O2%− on rutile-rich TiO2. This conclusion would be critically important for noncomplete or selective photocatalytic oxidation reactions using organic molecules as feedstock.
(r = kMB/kMO) is calculated to be 0.65 for anatase-rich TiO2. By contrast, when we use rutile-rich TiO2 for selective photocatalysis under same condition, the obtained R is 1.23, which is about 2 folds higher than that of anatase-rich TiO2. In other words, the enrichment of rutile phase TiO2 exhibits a positive influence on the photocatalytic selectivity towards MB degradation. It also worth mentioning that leuco MB is unlikely to form in large quantity in this aerobic condition. The above observations proves the feasibility of controlling the photocatalytic degradation selectivity by altering the TiO2 phase composition in photocatalysts. To understand the shift of selective photocatalytic degradation, we conducted 90 min adsorption experiment with MO and MB solution over both anatase-rich and rutile-rich TiO2 photocatalysts (Fig. 3). The anatase-rich TiO2 shows negligible MO adsorption (< 1%) and moderate MB adsorption (24.9%), while rutile-rich TiO2 shows enhanced adsorption of MO adsorption (3.6%) and decreased MB adsorption (21.4%). However, the selective photocatalytic capability r of rutilerich TiO2 is larger than that on anatase-rich TiO2 (Fig. 2d and Table 2). In other words, rutile-rich TiO2 has poor photocatalytic selectivity towards MO even though it shows better adsorption for MO than that of anatase-rich TiO2. These results together with above photocatalytic testing indicate that the change in preferential photocatalytic degradation in this work is not due to the substrate adsorption selectivity [18]. Therefore, other explanation must be thought. After careful consideration, we propose the change of photocatalytic selectivity can be explained by preferential generation of O2%− radicals, which is strongly related to the surface structures of TiO2 photocatalysts. The most available and frequent surface of anatase and rutile TiO2 particles is thermodynamically stable (1 0 1) and (1 1 0) facets, respectively, due to their lower surface free energy [26]. Oxygen vacancies (Ov) in rutile (1 1 0) facet is favored by producing two more stable 5-fold coordinated Ti atoms (Ti5c), whereas Ov at anatase (1 0 1) facet is thermodynamically unfavorable due to stiffness of the surface Ti5c-O2c bond [26,27]. As a result, the density of surface Ov in rutile is much higher than that in anatase, which was also experimentally proved by UHV-FTIRS study [15]. Those surface Ov tends to interact strongly with molecular O2 because many stable adsorption sites (e.g. Ti5c) exist around Ov with high binding energy [14,16], whereas O2 activation is sluggish on fully oxidized anatase surface. Therefore we anticipate that the rutile-rich TiO2 sample can highly activate O2 reduction process. Realizing the key role of reactive species in the photocatalytic process, we further carried out trapping experiments for both anatase and rutile-rich TiO2 for mechanism study. Among all different trapping agents added into the photoreaction system, only quencher of O2%− (BQ) has significant effects on photocatalytic activity and selectivity (Fig. 2 and Table 2). Interestingly, addition of 1 mM BQ increased selective photocatalytic capability r further from 1.29 to 2.54, which means MB was selectively decomposed in comparison with MO. By contrast, applying IPA (a quencher of %OH) or TEOA (a quencher of h+) results in r of 1.07 and 1.09, respectively, which is even slightly smaller than that of rutile-rich TiO2 alone. Those results indicate that %OH and h+ show little contribution to the selective degradation of MB on rutilerich TiO2. Bearing this in mind, we also realize that adding BQ as a quencher of O2%− only partially suppressed the reactions, e.g. r = 0.0077 decreasing to r = 0.00559 for MB, which suggests at least one more stable O2%− -like ROS still exists in the catalytic system. Furthermore, the enhancement of PL under O2 atmosphere on rutilerich TiO2 indicates the formation of immobilized O2%− which cause an upward band bending and accumulation of holes in the subsurface region leading to increased radiative recombination (Fig. 4) [28]. Therefore, we propose the immobilized O2%− on TiO2 can survive even in presence of BQ and plays a key role in this selective photocatalysis system, which has been ignored in most of previous studies. A possible mechanism pathway for selective photocatalytic degradation by immobilized O2%− is proposed and shown in Fig. 5. The
4. Conclusion In summary, excellent tunability and selectivity of photocatalytic degradation was successfully achieved by modulating surface crystallinity of TiO2 and the unique property of immobilized O2%− radical. Mechanism study indicates that selective production of O2%− radical plays a dominant role for preferential degradation of MB, which is due to steric effect rather than substrate adsorption selectivity. Importantly, we propose immobilized O2%− on TiO2 surface have shifted 2π energy level, which stabilize O2%− from BQ quenching and prevent subsequent production of nonselective hydroxyl radicals. Furthermore, this process was expected to be much more environmentally benign compared with conventional catalytic degradation reaction which is conducted in harsh conditions. The in depth understanding of specific role of immobilized O2%− as well as established approach for preferential generation of such radicals would provide a novel pathway for the design of advanced oxidation technologies with desirable photocatalytic selectivity.
CRediT authorship contribution statement Zhiling Ma: Conceptualization, Supervision. Qunpeng Jia: Data curation, Investigation. Chang Tao: . Bing Han: Funding acquisition, writing - review & editing.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This work was supported by Natural Science Foundation of Hebei (B2019201064) and Advanced Talents Incubation Program of the Hebei University (1081/801260201284). 5
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Appendix A. Supplementary material
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