Photocatalytic oxidation of small molecule hydrocarbons over Pt/TiO₂ nanocatalysts

Abstract

Highly active Pt/TiO₂ catalysts were prepared by a simple photo-reduction method and used for catalytic oxidation of alkanes (C₂H₆ and C₃H₈) and alkenes (C₂H₄ and C₃H₆). It was found that significantly improved photo-activity can be reached even by loading a very small amount of Pt (0.2–0.5 wt%). Moreover, the Pt loading resulted in unexpected visible light activity for the oxidation of small molecule hydrocarbons. Further investigation using photoluminescence spectra indicate that the Pt loading helps reduce the charge carrier recombination within the TiO₂ nanoparticles. Electron Paramagnetic Resonance (EPR) spectra reveal both oxygen molecules and lattice oxygen participate in the hydrocarbons' photooxidation. The transfer and reaction mechanisms of charge carriers during the photo-oxidation process are discussed in detail.

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Introduction

The environmental impact of atmospheric hydrocarbons (HC) that are released from human activities is receiving increasing attention.^1–4 On one hand, the presence of HC in the environment has an adverse effect on human health. Prolonged exposure can cause damage to the central nervous system. In certain conditions it can generate irritating photochemical smog as well as other more noxious compounds. On the other hand, HC are greenhouse gases and methane has been recognized to be a major contributor to global warming. Therefore, it is imperative to develop techniques to treat HC pollutants in the atmosphere.

Taking into account that effluent HC released from human activities are usually of low concentration and high flow volume, thermal catalytic oxidation of the HC species is costly.^5,6 Adsorption is a cheaper alternative,⁷ however, it is less active for small molecule hydrocarbons. In general, small molecule hydrocarbons such as methane, ethane, propane, etc., are one of the least reactive of all volatile organic compounds (VOCs) owing to their high C–H bond energy and weak molecule polarity, thus making their oxidation a highly energy intensive process. With these aspect in view, heterogeneous photocatalysis using semiconductors is a potentially important strategy in the removal of low level HC pollutants.

For photocatalytic oxidation of HC species, the generation of the active oxygen species O₂⁻ and ˙OH radicals is crucial step. Semiconductors with a conduction band minimum higher than the potential of O₂/O₂⁻ (−0.16 V vs. NHE⁸) and valence band maximum lower than the ˙OH/OH⁻ (+2.59 V vs. NHE⁹) potential is needed for organic pollutant degradation. TiO₂, who's conduction band and valence band located at −0.29 eV and 2.91 eV respectively, was widely used in the process of environmental cleanup.^10–15 For example, Izumi, I. et al. reported the photocatalytic activities of Pt/P25 for benzene decomposition in aqueous solution.¹⁶ F. B. Li et al. investigated the photocatalytic oxidation of methylene blue (MB) and methyl orange (MO) in aqueous solutions using the Pt–TiO₂ catalyst.¹⁷ Wang et al. obtained highly dispersed platinum (Pt) nanoparticles embedded in a cubic mesoporous nanocrystalline anatase (meso-ncTiO₂) thin film and tested its catalytic activity in oxidation of CO.¹⁸ Yu et al. fabricated Pt/TiO₂ nanosheets with exposed (001) facets and investigated its activity in photocatalytic water splitting.¹⁹ Wang, C. C. et al. prepared uniformly dispersed Pt nanoparticles on TiO₂-based nanowires and investigated its activity in degradation of rhodamine B and hydrogen evolution.²⁰ Brigden, C. T. et al.²¹ studied the UV-induced photo-oxidation of propene, propane, ethene, ethane, n-butane and n-hexane over a TiO₂ photo-catalyst at 150 °C. Finger, M. et al.²² reviewed previous reports and discussed the kinetics and mechanisms of photocatalyzed total oxidation reaction of HC species with TiO₂ in the gas phase. van der Meulen, T. et al.²³ studied the photocatalytic oxidation of propane on anatase, rutile, and mixed-phase anatase-rutile TiO₂ nanoparticles. However, photo-oxidation of small molecule alkanes and alkenes using Pt/TiO₂ catalyst is still lacking. Besides, the transfer mechanism of electrons and holes in bulk of Pt/TiO₂ during photo-oxidation of HC remain uncertain.

In light the fact that TiO₂ is a stable, inexpensive and harmless semiconductor, and, depositing noble metals on the surface of TiO₂ have been proved to be an effective method in improving performance. In this paper, we report photocatalytic activity and photooxidative mechanism of small molecule hydrocarbons over Pt/TiO₂ nanocatalysts. It was found for the first time that, significantly improved photo-activity can be reached even by loading a very small amount of Pt (0.2–0.5 wt%). Moreover, the Pt loading resulted in unexpectedly visible light activity for the oxidation of small molecule hydrocarbons.

Experimental

Material

Degussa P25 (BET surface area about 50 m² g⁻¹ and particle size about 20–30 nm) was purchased from Degussa Corporation, Germany. Methanol (CH₃OH) and chloroplatinic acid (H₂PtCl₆) were all purchased from Sinopharm Chemical Reagent Co., Ltd. All these chemicals were used without further purification and all the water used in this paper was deionized water.

Synthesis of Pt/P25 photocatalysts

The Pt/P25 powder photocatalysts were prepared by the photo-reduction method, according to the following procedure: first, 1.0 g of P25 was dispersed in 100 ml water and methanol mixture solution (the volume ratio of H₂O to CH₃OH equals 7 : 3). Then, H₂PtCl₆ solution (0.154 M) with controlled Pt loading amount (0–0.7 wt%) was added into the suspension and magnetically stirred in dark for 3 hours. After that, the suspension was illuminated for 3 hours using a 300 W Xe lamp. The obtained products were filtered, washed and then dried. Finally, the Pt/P25 powders were annealed at 350 °C for 2 hours in vacuum and then cooled naturally to room temperature.

Characterization

The powder X-ray diffraction (XRD) patterns of the samples were collected on a Miniflex 600 diffractometer with Cu k_α radiation (λ = 0.154178 nm). The morphology and microstructure of the samples were examined by both transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The UV-Visible absorption spectra of the as-synthesized samples were measured on a UV-Vis-NIR spectrophotometer (Lambda 950), during which BaSO₄ was employed as the internal reflectance standard. A TriStar II3020-BET/BJH surface area analyzer was used to measure the surface area of the samples. Electron Paramagnetic Resonance (EPR) spectra were collected on an ELEXSYS E500 spectrometer equipped with a nitrogen cryostat. All the spectra were collected at 100 K. The photoluminescence (PL) spectra of the photocatalysts were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 325 nm.

Measurements of photocatalytic activity

The photocatalytic oxidation of hydrocarbons (HC) over the obtained catalysts were evaluated by measuring the decomposition rate of low molecular weight alkanes and alkenes (C₂H₄, C₃H₆, C₂H₆, and C₃H₈) in a sealed quartz reactor of 435 ml capacity (ESI, Fig. S1†) and a homemade flow-bed pyrex reactor (ESI, Fig. S2†), respectively. All the experiments were conducted at atmospheric pressure and room temperature.

In a typical fixed-bed reaction: first, 0.2 g of the obtained catalysts were uniformly dispersed on the bottom of a circular glass dish (diameter 60 mm). Then the glass dish was placed in the bottom of the reactor. After that, the reactor was sealed. The initial gas atmosphere was dry air and followed by injection of 200 μL HC gas. Simulated solar light (ESI, Fig. S3†) was provided by a 300 W Xe lamp. The cut-off filters were employed when visible light (λ > 420 nm, ESI, Fig. S4†) illumination was in demand. At a certain time interval, 4 ml of reactor gas was sampled and analyzed on a gas chromatograph (GC9720 Fuli) equipped with a HP-Plot/U capillary column, a molecular sieve 13X column, a flame ionization detector (FID) and a thermal conductivity detector (TCD).

The continuous flow photocatalytic tests of the samples were carried out in a flow reactor (Fig. S2†): typically, the catalysts were first experienced pelleting and sieving to obtain particle size in the range of 0.15–0.2 mm. Then 0.5 g of the particles were filled into a quartz reactor (28 mm × 18 mm × 1 mm) and flowing N₂ was used to expel CO₂ and other species that adsorbed on the surface of the catalysts. After that, the mixed gas consisted of 78.9% N₂, 21.1% O₂ and small quantity of HC was flowed through the samples and analyzed directly by the gas chromatography (GC9720 Fuli). During the reaction, a 300 W Xe lamp was used to provide simulated solar light.

Results and discussion

The powder XRD patterns of P25 and the as-prepared Pt/TiO₂ samples are shown in Fig. 1. All the diffraction peaks for all samples can be assigned to either anatase (JCPDS, no. 21-1272) or rutile (JCPDS, no. 21-1276) phase of TiO₂. The fact that no diffraction peaks of Pt was detected is ascribed to its low loading amount (<0.7 wt%) as well as its good dispersity.^24–26 As shows in Fig. 2, TEM and HRTEM observation revealed small Pt nanoparticles (∼5 nm) deposited on the surface of TiO₂ particles (∼30 nm) with a high dispersion. The lattice fringes of 0.223, 0.352 and 0.190 nm in the HRTEM image (Fig. 2b) are assigned to the (111) plane of Pt,¹⁶ the (101) and (200) planes of anatase-TiO₂, respectively.

Fig. 1 The powder XRD patterns of the Pt loaded P25 samples with different weight percentage.

Fig. 2 TEM (a) and HRTEM (b) images of the Pt (0.5 wt%)/P25 sample.

Fig. 3a shows the images and UV-Vis absorbance spectra of P25 and the Pt/P25 powders. For all the samples, a particularly strong absorption at wavelengths shorter than 400 nm is attributed to the intrinsic bandgap absorption of anatase TiO₂ (∼3.2 eV). The band gap energy (E_g) of the samples was estimated from the intercept of the tangents to the plots of (Ahν)² vs. hν. As shown in Fig. 3b, with the increase of Pt, the band gap slightly decreases from 3.25 eV to 3.16 eV. Besides the narrow of intrinsic bandgap, note that the Pt loading significantly increased the visible light absorption of P25. Moreover, the absorption intensity increases with increasing the mass fraction of Pt. Accompanying these changes, the colour of the samples changes gradually from white to grey and to black for the 0.0%, 0.05% and 0.7% Pt loaded P25 samples, respectively. The obvious visible light absorption is well corresponding to the excellent activity under irradiation of visible light.

Fig. 3 The images and UV-Vis absorbance spectra (a) as well as the plots of (Ahν)² vs. hν (b) of the Pt loaded P25 samples with different weight percentage.

Fig. 4 shows the photocatalytic oxidation of C₂H₄, C₂H₆, C₃H₆ and C₃H₈ over P25 and the as prepared Pt/P25 catalysts under simulated solar light irradiation. The initial concentration of HC in every experiment was ∼450 ppm. The results indicated that the activity of P25 could be significantly enhanced by loading appropriate Pt in oxidizing both alkenes and alkanes. For the oxidation of alkenes (Fig. 4a and c), 0.5 wt% Pt loading exhibited the highest performance. 450 ppm C₂H₄ and C₃H₆ could be completely oxidized within 5 and 6 minutes, respectively, under the simulated solar light irradiation. However, for the oxidation of alkanes (Fig. 4b and d), the samples with Pt content of 0.2%, 0.5% and 0.7% showed less distinction. The activity of these samples was enhanced about 4 and 2 times for oxidizing C₂H₆ and C₃H₈, respectively, in comparison with P25. The further analysis of the photo-oxidation reaction of HC indicated that all the reactions follow first order reaction kinetics. The rate constant k in photo-oxidation reaction of HC was listed in Table 1. The lower k values for alkanes (C₂H₆ and C₃H₈) than that for alkenes (C₂H₄ and C₃H₆) is consistent with the understanding that alkanes are more difficult to decompose. That the k values for the 0.05% Pt samples is lower than that of pure P25 can be understood as follows: the fabrication of the Pt/P25 samples involves processing P25 in water and methanol mixture solution, which may cause the decrease of active sites from P25. When the Pt loading fraction is very low (0.05 wt%), the increased active sites from Pt deposition can't offset the decrease of active sites from P25.

Fig. 4 Photocatalytic oxidation of some small molecule hydrocarbons over P25 and the as-prepared Pt/P25 catalysts under simulated solar light irradiation: (a) C₂H₄; (b) C₂H₆; (c) C₃H₆ and (d) C₃H₈.

Table 1 The BET surface area and rate constant k in photo-oxidation reaction of P25 and the Pt/P25 samples

Sample	BET (m^2 g^−1)	Rate constant k (min^−1)
		C2H4	C2H6	C3H6	C3H8
P25	45.6	0.28	0.06	0.69	0.26
Pt(0.05%)/P25	48.2	0.17	0.07	0.47	0.24
Pt(0.2%)/P25	48.9	0.43	0.26	0.64	0.45
Pt(0.5%)/P25	50.5	1.09	0.32	0.91	0.47
Pt(0.7%)/P25	45.8	0.67	0.34	0.75	0.41

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Because loading Pt nanoparticles on the surface of P25 could significantly increase visible light absorption (Fig. 3), the photo-oxidation activity of Pt/P25 under visible light was evaluated as well. Fig. 5 shows photooxidation of C₂H₄, C₂H₆, C₃H₆ and C₃H₈ over P25 and the Pt (0.2%)/P25 sample under the visible light irradiation. The results indicated that P25 exhibits very weak visible light activity under the function of its bulk and surface defects. However, for the Pt (0.2%)/P25 sample, significantly increased visible light activity was realized. The 450 ppm of alkenes (C₂H₄ and C₃H₆) was completely degraded within 1 hour (Fig. 5a and c) and the 450 ppm of C₂H₆ and C₃H₈ were degraded more than 60% and 80% (Fig. 5b and d), respectively within 4 hours irradiation.

Fig. 5 Time course of HC oxidation upon P25 and the Pt(0.2%)/P25 powders under the irradiation of visible light: (a) C₂H₄; (b) C₂H₆; (c) C₃H₆ and (d) C₃H₈.

Fig. 6 shows durability test of the hydrocarbons photooxidation upon the Pt (0.2%)/P25 catalysts under simulated solar light illumination. For all the HC gases, within seven cycle reactions, the very stable performance indicated that the as-prepared Pt/P25 catalysts are very stable in the processing of HC photo-oxidation.

Fig. 6 Durability tests of the hydrocarbons photo-oxidation upon the Pt (0.2%)/P25 catalysts under the simulated solar light irradiation: (a) C₂H₄, (b) C₂H₆, (c) C₃H₆ and (d) C₃H₈.

To examine the mineralization rate, we carried out a flow mode (Fig. S2†) test on the Pt (0.2%)/P25 catalysts. Before illumination, CO₂ in the reaction system was removed by flowing carrier gas. After that, the reaction gas consisted of 78.9% N₂, 21.1% O₂ and a small quantity of HC was flowed through the samples and analyzed directly by the gas chromatography (GC9720 Fuli). During the reaction, a 300 W Xe lamp was used to provide simulated solar light with light density of ∼200 mW cm⁻². Fig. 7 shows the time courses of C₂H₆ and C₃H₈ photooxidation upon the as-synthesized Pt (0.2%)/P25 catalysts under simulated sunlight illumination in the flow mode experiment. Before light was turned on, the concentration of HC was 70 and 205 ppm for C₂H₆ and C₃H₈ respectively and no CO₂ was detected. When the lamp is turned on, the amount of ethane and propane decreases rapidly to 0–5 ppm. Simultaneously, the concentration of CO₂ increases promptly to ∼140 and ∼610 ppm, respectively. The generation of CO₂ was basically in line with the degradation of HC and no other carbon-containing species were detected. When the light is turned off, the concentration of CO₂ rapidly decreases to zero; moreover, the amount of ethane and propane comes back to a constant value. These results confirmed that the HCs oxidation is truly driven by a photodriven process, besides, under the presence of plenty oxygen, HC tends to totally convert to carbon dioxide. What's more, the activities of the sample have no decrease after 20 hours irradiation, which evidence again high stability of the Pt/P25 catalysts.

Fig. 7 Time courses of the HC photooxidation upon the as-synthesized Pt (0.2%)/P25 samples under simulated sunlight illumination in the flow mode (mass of sample: 0.5 g; size of sample particle: 0.15–0.2 mm; one 300 W Xe lamp; reactor size: 28 mm × 18 mm × 1 mm).

Turnover number (TON) of the HC photo-oxidation was obtained by oxidizing a larger amount of HC gases (10 ml) upon the Pt (0.2%)/P25 catalysts (Fig. 8). The calculated TON (ESI S5†) for the HC photo-oxidization was 2.18 for C₂H₄, 2.55 for C₂H₆, 3.23 for C₃H₆ and 3.64 for C₃H₈, respectively, which indicated that the photo-oxidation reaction was truly driven by a catalytic process.

Fig. 8 Photo-oxidation of 10 ml HC upon the Pt (0.2%)/P25 (catalyst: 0.2 g, reactor: 435 ml, reaction gas: 10 ml HC and 425 ml air).

The BET surface areas of P25 and the Pt/P25 samples were summarized in Table 1. All the Pt/P25 samples have similar BET surface area with that of P25. Therefore, the increased HC photooxidation activity is obviously correlated with the loading of Pt. One of the benefit is loading of noble metal nanoparticles on the surface of semiconductor could significantly enhance the separation rate of photo-generated electrons and holes,^27,28 which can be examined from the changes of the intensity of photoluminescence (PL) spectra. Lower intensity of PL spectra corresponds to lower recombination rate of photogenerated electron–hole pairs, thus corresponding to higher activity of catalyst.²⁹ Fig. 9 shows the room-temperature PL spectra of P25 and the Pt (0.2%)/P25 catalysts under excitation wavelength of 325 nm. The PL peak at about 400 nm is attributed to the emission close to bandgap transition with the energy of light approximately equal to or larger than the bandgap energy of anatase (3.2 eV) and rutile (3.0 eV) whereas the broad extension to the visible light range might be correlate with the surface and bulk defects of TiO₂,¹⁹ which will be discussed later. After Pt nanoparticles decorated on the surface of P25, the fluorescence intensity was obviously decreased compare to pure P25. This result indicates that Pt nanoparticles on P25 surface are able to extract electrons from the conduction band of TiO₂ and thus reduce charge carrier recombination within TiO₂ nanoparticles.

Fig. 9 The room-temperature PL spectra of P25 and the Pt (0.2%)/P25 catalysts under excitation wavelength of 325 nm.

In order to study the mechanism of electron transfer in the HC photo-oxidation process, we performed the EPR measurements at 100 K over the Pt (0.2%)/P25 catalysts (Fig. 10). Under the dark and air atmosphere, the sample shows two signals with g = 2.004 and g = 1.979 (Fig. 10a). The signal of g = 2.004 has been assigned to the characteristic of single-electron-trapped oxygen vacancies.^30–33 We suppose that these oxygen vacancies might be created during the process of photo-depositing Pt nanoparticles. The signal with g = 1.979 is attributable to the lattice electron trapping sites (Ti³⁺) in the bulk of TiO₂.^34–36 After irradiated by visible light (λ > 420 nm, Fig. 10b), the signal of oxygen vacancy and Ti³⁺ were obviously increased. In addition, two new signals with g = 2.028 and g = 2.009 emerged, which are assigned to a trapped hole at the surface oxygen of the TiO₂ as Ti⁴⁺-O⁻.^35,36 These results indicate that there were more electrons and holes generated and then the electrons were trapped by oxygen vacancy and Ti⁴⁺, whereas the holes were trapped by subsurface lattice oxygen. We should note that visible light have no enough energy to excite electrons from valence band to the conduction band of TiO₂.

Fig. 10 EPR spectra collected upon the Pt/P25 sample at 100 K under various conditions: (a) in dark and air atmosphere; (b) irradiated in air atmosphere for 10 min under visible light; (c) irradiated in air atmosphere for 10 min under ultraviolet light; (d) subsequently inject C₂H₄ immediately after (c); (e) continue lighting for 10 min under simulated sunlight after the injection of C₂H₄.

However, it was reported that oxygen vacancies generated during synthesis could enhance the visible light activity.^30,37 Oxygen vacancies should involve in the transfer process of photo-generated electron–hole pairs. According to the earlier reports,^38–40 there should be a two-step electron excitation through the defect level. It had been reported that oxygen vacancy locates at 0.75–1.18 eV below the conduction band minimum.⁴¹ Therefore, with the irradiation of visible light, electrons in the valance band of TiO₂ can be excited to the local level of oxygen vacancy and then from there to the conduction band. Note the possibility of electrons transfer directly from the defect level to the adsorbed hydrocarbons cannot be ruled out herein. When irradiated under ultraviolet light (Fig. 10c), the signal of Ti⁴⁺-O⁻ increased significantly; at the same time, the signal of single-electron-trapped oxygen vacancy decreased compared to visible light irradiation. It can be explained as follows: after absorbing ultraviolet light, a lot of electrons and holes were generated. Then, more holes trapped at surface oxygen of the TiO₂ lead to the increased signal of Ti⁴⁺-O⁻; meanwhile, electrons trapped at V_O⁺ lead to the decreases of its intensity. When C₂H₄ was injected into the reactor, as shown in Fig. 10d, the signal of Ti⁴⁺-O⁻ completely disappeared; besides, the signal of V_O⁺ continue decreases compared to Fig. 10c. This result is due to C₂H₄ gives one electron to the surface O⁻ through a still unknown process after it touch with catalyst, result in the disappear of Ti⁴⁺-O⁻; besides, the decrease of V_O⁺ is owing to electrons continue trapping at this sites and form V_O. In order to examine this speculation, the quartz glass tube containing air and C₂H₄ was irradiated under simulated sun light for 10 minutes (Fig. 10e), it is not surprise that the signal of Ti⁴⁺-O⁻ emerge again.

According to the above discussion, the transfer of charge carriers during the HC photooxidization upon the Pt/P25 catalyst is described in Fig. 11. During the irradiation of simulated solar light, the absorption of UV photons led to the generation of electron–hole pairs, and, the visible light irradiation will result in two-step electron excitation through the defect levels.^38,42,43

UV: TiO₂ + hν → TiO₂(e_CB⁻ + h_VB⁺) (1)

Vis: V_O²⁺ + e_VB⁻ + hν(vis) → V_O⁺ + h_VB⁺ (2)

V_O⁺ + hν(vis) → V_O²⁺ + e_CB⁻ (3)

Fig. 11 Suggested mechanism of visible and ultraviolet light induced reaction on the Pt/P25 photocatalyst.

Then, in the bulk of TiO₂, these charge carriers can be trapped at several centers.^42,43 In addition, part of the electrons will transfer to the loaded Pt nanoparticles.

V_o⁺ + e_CB⁻ → V_O (5)

Finally, the trapped electrons and holes transfer to electron acceptor O₂ and electron donor C_xH_y. The generated O₂⁻ and lattice O⁻ reacted with C_xH_y to produce carbon dioxide and water (formula 8, 9), then the V_O⁺ could be recovered to O²⁻ by absorbing oxygen in air (formula 10).

Conclusions

High active and stable Pt/TiO₂ catalysts were obtained by a facile photo-reduction method and the activity of oxidizing alkanes and alkenes on these catalysts was examined. The results indicate that the photocatalytic activity of P25 could be significantly enhanced by deposition of Pt and the best mass fraction of Pt on P25 is about 0.2–0.5 wt%. The transfer mechanism of charge carriers during photo-oxidation process was also investigated and discussed. Photo-generated electrons are trapped by Ti⁴⁺ and then transfer to O₂ in air to form O₂⁻; lattice O²⁻ traps one hole to from O⁻ and then recover to O²⁻ by obtaining one electron from the HC electron donator. The Pt loading as well as the defect such as oxygen vacancy in bulk and surface of TiO₂ should be the main factor responsible for the photo-response under visible light.

Acknowledgements

This work was financially supported by the National Key Project on Basic Research (Grant No. 2013CB933203), the Natural Science Foundation of China (Grant No. 21373224, 21577143 and 51502289), the Natural Science Foundation of Fujian Province (Grant No. 2014H0054 and 2015J05044), and the One Hundred Talents Program of the Chinese Academy of Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: (a) The sketch of photoreaction in a sealed quartz reactor; (b) the schematic diagram of continues flow photocatalytic test; (c) the spectrum of simulated solar light; (d) the visible light spectrum of simulated solar light; (e) the turnover number (TON) calculations. See DOI: 10.1039/c5ra22459d

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