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Highly effective UV–Vis-IR and IR photothermocatalytic CO abatement on Zn doped OMS-2 nanorods
Applied Surface Science 483 (2019) 827–834
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Highly effective UV–Vis-IR and IR photothermocatalytic CO abatement on Zn doped OMS-2 nanorods
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Zhongkai Jiang, Yan Ma, Yuanzhi Li , Huihui Liu State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalytic Photothermocatalytic Photoactivation Zn doped OMS-2
The catalysts of Zn doped OMS-2 nanorods with the Zn/Mn atomic ratios of 0.052, 0.056, and 0.073 were synthesized through the reaction among Zn(NO3)2, KMnO4, and Mn(NO3)2 at 75 °C via changing the molar ratios of Zn(NO3)2/KMnO4/Mn(NO3)2 in reactants. The Zn doped OMS-2 catalysts were characterized with ICP, XRD, TEM, BET, XPS, UV–Vis-IR, and CO-TPR. The catalysts demonstrate excellent photothermocatalytic activity for CO abatement with UV–Vis-IR irradiation. The Zn doping substantially promotes the photothermocatalytic activity of OMS-2. In comparison with TiO2(P25), the CO2 production rate of the optimum catalyst is raised by 64.5 times. The optimum catalyst demonstrates very good photothermocatalytic activity with Vis-IR irradiation and even with λ > 830 nm IR irradiation. The excellent photothermocatalytic activity of the Zn doped OMS-2 catalysts is mainly ascribed to their effective light-driven thermocatalytic properties. The substantial elevation in the photothermocatalytic activity of OMS-2 through the Zn doping is attributed to the thermocatalytic activity of the Zn doped OMS-2 catalysts being far superior to pure OMS-2 because the Zn doping promotes the reducibility of OMS-2. A new photoactivation substantially promotes the light-driven thermocatalytic activity of Zn doped OMS-2: UV–Vis-IR irradiation obviously improves the reducibility of Zn doped OMS-2, thus substantially promoting its catalytic activity.
1. Introduction
been reported to have photocatalytic activity for air pollutant abatement. Recently, a very effective strategy of photothermocatalysis or lightdriven thermocatalysis for air pollutant abatement has been developed on the basis of a number of photothermocatalysts such as manganese oxides [41–44], metal ion doped manganese oxide [45–47], CuO [48,49], Co3O4 [50,51], Pt/LaVO4/TiO2 [52], MnOx/TiO2 [53], Ce1−xBixO2−δ [54], ABO3-type perovskites [55], etc. The photothermocatalysis strategy, combining the good thermocatalytic activity of the photothermocatalysts and their efficient photothermal conversion owing to their intense absorption in wide solar spectrum region even up to IR, is able to efficiently utilize UV–Vis light and even IR light for air pollutant abatement. Among the photothermocatalysts, cryptomelane manganese oxide with a structure of octahedral molecular sieve (KMn8O16, OMS-2) [56,57] is very attractive because of its earthabundance, low-cost, and high photothermocatalytic activity with UV–Vis-IR irradiation [41]. It was reported that doping OMS-2 with metal ions such as Mg [47], Ag [58], and Ce [59] improved the catalytic activity of OMS-2. Finding effective strategy of further substantially promoting the photothermocatalytic activity of OMS-2 with UV–Vis-IR irradiation is still highly desirable and great challenging.
Using inexhaustible solar energy for abating water and air pollutants and producing fuels based on numerous photocatalysts has attracted extensive interests for several decades [1–12]. The majority of the documented photocatalysts such as TiO2 [5–10], TiO2 nanocomposites [5,11,12], and so on are merely able to use UV and a fraction of visible light. Persistent efforts have been devoted to find strategies of designing effective photocatalysts that are able to use IR light, accounting for approximately 50% of solar energy [13–15]. The reported: 1) designing photocatalysts with small band gap to be activated by near-IR light such as Bi2WO6-based hybrid photocatalyst [16], WS2 [17], Cu2(OH)PO4 [18], Bi2MO6 (M = W, Mo) [19], Ag2X [20], BiErWO6 [21], 2) designing photocatalysts sensitized by near-IR responsive compounds [22,23], 3) designing plasmonic photocatalysts composed of precious metal nanoparticles that are able to be activated by near-IR light [24–28], 4) designing upconversion photocatalysts of which upconversion material converts near-IR photons to UV or visible photons that can excite photocatalyst [29–40]. Almost all of near-IR photocatalysts only show photocatalytic activity for the abatement of easily degradable water pollutants. Few of near-IR photocatalysts have
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Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.apsusc.2019.04.022 Received 23 January 2019; Received in revised form 25 March 2019; Accepted 2 April 2019 Available online 03 April 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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with UV–Vis-IR irradiation for a long time, a 10.7 g m−3 CO stream, acquired through merging a 5.0 vol% CO/He stream and an air stream, continuously flowed at 20.0 mL min−1 in the reactor. The photocatalytic CO abatement on Zn-OMS-2II with UV–Vis-IR irradiation was conducted at near room temperature on the same reactor as the photothermocatalytic CO abatement according to the procedure detailed in the previous publications [41,45].
In this work, we prepared the catalysts of Zn doped OMS-2 nanorods with several Zn/Mn atomic ratios by a facile approach. It is found that the Zn doping substantially promotes the photothermocatalytic activity of OMS-2 for CO abatement. The optimum catalyst demonstrates excellent photothermocatalytic activity with UV–Vis-IR irradiation and even with λ > 830 nm IR irradiation due to the light-driven thermocatalytic CO oxidation promoted by a new photoactivation. The origin of both the substantial catalytic elevation of OMS-2 by the Zn doping and the new photoactivation is revealed on the basis of the experimental evidences. To the best of our knowledge, the excellent UV–VisIR and IR photothermocatalytic activity of Zn doped OMS-2 for air pollutant abatement have not been reported.
2.4. Thermocatalytic activity A quartz tubular reactor was used to conduct the thermocatalytic CO abatement on the Zn doped OMS-2 catalysts in the dark on a WFS2015 apparatus. 0.0500 g of the catalyst powder supported by quartz wool was placed in the reactor. A thermocouple, which was placed inside the reactor, was used to monitor the reaction temperature. A 12.2 g m−3 CO stream, acquired through merging a 5.0 vol% CO/He stream and an air stream, continuously flowed at 40.0 mL min−1 in the reactor. A quartz tubular reactor linked with a quartz window was used to conduct catalytic CO abatement on Zn-OMS-2II at the different temperatures in the dark and with the irradiation. The catalyst amount is 0.0050 g. A 12.7 g m−3 CO stream, acquired through merging a 5.0 vol % CO/He stream and an air stream, continuously flowed at 20.0 mL min−1in the reactor. The detailed procedure was reported in previous publications [45,46].
2. Experimental section 2.1. Preparation The Zn doped OMS-2 catalyst was synthesized in accordance to the procedure as follows. 3.5790 g of 50 wt% Mn(NO3)2 and 0.4767 g Zn (NO3)2·6H2O were dissolved in 100 mL distilled water under magnetic stirring, respectively. 3.1608 g KMnO4 was dissolved in the aqueous solution of Mn(NO3)2 and Zn(NO3)2 under magnetic stirring. The acquired mixture in a beaker was put in an oven, heated to 75 °C, and kept at 75 °C for 72 h. The resultant precipitate was filtered, thoroughly washed, and dried at 75 °C overnight. The acquired catalyst of Zn doped OMS-2 is labeled as Zn-OMS-2I. The catalysts of Zn-OMS-2II, Zn-OMS-2III, and pure OMS-2 were synthesized by the same procedure as that of Zn-OMS-2I except for using 0.7150, 0.9533, and 0.0 g of Zn(NO3)2·6H2O, respectively.
3. Results and discussion 3.1. Characterization Pure OMS-2 was synthesized through the reaction between KMnO4 and Mn(NO3)2 at 75 °C with KMnO4/Mn(NO3)2 molar ratio of 2.0/1.0. The Zn doped OMS-2 catalysts with different Zn/Mn atomic ratios were synthesized through the reaction among Zn(NO3)2, KMnO4, and Mn (NO3)2 at 75 °C with different Zn(NO3)2/KMnO4/Mn(NO3)2 molar ratios of 0.12/2.0/1.0, 0.24/2.0/1.0, and 0.36/2.0/1.0, respectively. The corresponding Zn doped OMS-2 catalysts are labeled as Zn-OMS-2I, ZnOMS-2II, and Zn-OMS-2III, respectively. ICP analysis shows that the K/ Mn atomic ratio of pure OMS-2 is 0.103, approximating 0.125 of OMS-2 (KMn8O16). When the Zn(NO3)2/KMnO4/Mn(NO3)2 molar ratio in reactants is 0.12/2.0/1.0, the Zn/Mn atomic ratio of Zn-OMS-2I is 0.052, while its K/Mn atomic ratio decreases to 0.075. This suggests the substitution of K+ ions by Zn2+ ions in Zn doped OMS-2. When the Zn (NO3)2/KMnO4/Mn(NO3)2 molar ratio increases to 0.24/2.0/1.0, the Zn/Mn atomic ratio of Zn-OMS-2II increases to 0.056, while its K/Mn atomic ratio decreases to 0.063. When the Zn(NO3)2/KMnO4/Mn(NO3)2 molar ratio further increases to 0.36/2.0/1.0, the Zn/Mn atomic ratio of Zn-OMS-2III increases to 0.073. However, the K/Mn atomic ratio of ZnOMS-2III (0.065) does not have a further decrease in comparison with that of Zn-OMS-2II. XRD characterization shows that Zn-OMS-2I, Zn-OMS-2II, and ZnOMS-2III have pure tetragonal cryptomelane structure, the same as that of pure OMS-2 (JCPDF-29-1020) (Fig. 1). No crystalline ZnO is observed for Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III. The result indicates that the tetragonal cryptomelane structure of OMS-2 is not altered by the Zn doping. The morphologies of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III were observed by TEM. Zn-OMS-2I, ZnOMS-2II, and Zn-OMS-2III have morphology of nanorods, the same as that of pure OMS-2 (Fig. 2). The observation suggests that the nanorod morphology of OMS-2 is not altered by the Zn doping. The Zn doped OMS-2 catalysts were further characterized by EDX mapping. As shown in Fig. 3 (Zn-OMS-2I), Fig. S1 (Zn-OMS-2II), and Fig. S2 (Zn-OMS-2III), the elements of Zn, K, Mn, and O are well distributed on nanorods. The observation further confirms OMS-2 doped by Zn. The surface areas of the catalysts were measured by N2 adsorption.
2.2. Characterization The compositions of the Zn doped OMS-2 catalysts were analyzed on ICP-OES spectrometer (Prodigy 7). XRD patterns were acquired on a X-ray diffractometer (Rigaku Dmax) with low scan speed (0.50 min−1). TEM images were acquired on an electron microscope (JEM-100CX). Surface area was measured through N2 adsorption on ASAP2020. XPS spectra were acquired on a X-ray photoelectron spectrometer (VG Multilab 2000). UV–Vis-IR spectra were acquired on a spectrophotometer (Lambda 750). A multifunctional adsorption equipment (TP-5080) was used to conduct CO temperature-programmed reduction of the Zn doped OMS2 catalysts (CO-TPR) in the dark and with the irradiation. The used reactor and the procedures are detailed in the previous works [45,46]. 2.3. Photothermocatalytic and photocatalytic activity A homemade cylindrical stainless-steel reactor (447 mL) with a quartz window (11 cm in diameter) was utilized to conduct the photothermocatalytic CO abatement on the catalysts with the irradiation from a 500 W Xe lamp. 0.1000 g of the catalyst powder was dispersed on a thermal insulation slice (4 × 4 cm) of aluminum silicate fiber, which was put on the bottom of the reactor. The thickness of the catalyst powder is about 0.5 mm. A thermocouple was closely placed on the catalyst to measure the temperature under the irradiation. A 15.5 g m−3 CO stream, acquired through merging a 5.0 vol% CO/He stream and an air stream, continuously flowed at 40.0 mL min−1 in the reactor. The CO and CO2 concentrations were determined on a gas chromatograph (GC, GC9560). The experimental procedure and GC analysis conditions are reported in the previous works [41,45]. The UV–Vis-IR irradiation intensity is 436.1 mW cm−2. For conducting the photothermocatalytic CO abatement on Zn-OMS-2II with Vis-IR or IR irradiation, corresponding long wave pass filters were utilized. The λ > 420, 560, and 830 nm irradiation intensities are 365.9, 317.2, and 231.8 mW cm−2, respectively. To conduct photothermocatalytic CO abatement on Zn-OMS-2II 828
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2III, thus decreasing the BET surface area. The Zn valence states of Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III were determined through analyzing their Zn 2p XPS spectra. All the Zn doped OMS-2 catalysts have two peaks around 1021.0 and 1043.8 eV (Fig. 4), which are attributed to Zn 2p3/2 and Zn 2p1/2 of Zn2+ [60]. The Mn valence state of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and ZnOMS-2III were determined through analyzing their Mn 2p XPS spectra (Fig. 3). The peaks around 641.8 and 653.4 eV are ascribed to Mn 2p3/ 2 and Mn 2p1/2 of Mn3+, respectively. The peaks around 643.6 and 654.6 eV are ascribed to Mn 2p3/2 and Mn 2p1/2 of Mn4+, respectively [43,44]. The Mn3+/Mn4+ atomic ratios for pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III are 2.08, 2.30, 2.43, and 3.83, respectively. This suggests that the Zn doping increases the Mn3+ amount. This is attributed to the substitution of K+ ions by Zn2+ ions in Zn doped OMS-2, resulting in the increase in the Mn3+/Mn4+ atomic ratio to keep charge balance.
d c b a
Fig. 1. XRD patterns of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and ZnOMS-2III (d).
Pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III have BET surface areas of 98.7, 173.0, 202.7, and 127.6 m2 g−1, respectively (Fig. S3–6). This suggests that the Zn doping substantially causes an increase in the surface area. The possible reason is as follows: K+ ions are located in the channel of OMS-2 [56,57]. The substitution of K+ ions with larger radius (1.33 Å) by Zn2+ ions with smaller radius (0.74 Å) in Zn doped OMS-2 as discussed above is favorable to the diffusion of more N2 molecules into the channel of OMS-2, thus resulting in the increase in surface area. The surface area of Zn-OMS-2III less than that of ZnOMS-2II suggests that further increasing the Zn/Mn atomic ratio causes surface area decrease. The possible reason is as follows: As discussed above, compared to Zn-OMS-2II, the increase of the Zn/Mn atomic ratio from 0.056 to 0.073 for Zn-OMS-2III does not result in the substitution of more K+ ions by Zn2+ ions. This means that more Zn2+ ions probably lead to the partially blocking of the channel of OMS-2 for Zn-OMS-
3.2. Photothermocatalytic performance The photothermocatalytic CO abatement on pure OMS-2, Zn-OMS2I, Zn-OMS-2II, and Zn-OMS-2III with the UV–Vis-IR irradiation was conducted to evaluate their photothermocatalytic activity (see Experimental section). With the UV–Vis-IR irradiation, pure OMS-2 has good photothermocatalytic activity (Fig. 5A). Its CO2 production rate (rCO2) is 97.7 μmol g−1 min−1. The photothermocatalytic activity of OMS-2 is substantially raised through the Zn doping. In comparison with that of pure OMS-2, the rCO2 values of Zn-OMS-2I and Zn-OMS-2II substantially increase to 138.8 and 166.2 μmol g−1 min−1, respectively. Further elevating Zn/Mn atomic ratio does not cause an increase in the photothermocatalytic activity, evidenced by the fact that the rCO2 value of Zn-OMS-2III (165.8 μmol g−1 min−1) are similar to that of Zn-OMS2II. TiO2(P25) is a well-known photocatalyst that is extensively utilized
Fig. 2. TEM images of pure OMS-2 (A), Zn-OMS-2I (B), Zn-OMS-2II (C), and Zn-OMS-2III (D). 829
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Fig. 3. EDX mappings of Zn, K, Mn, and O in Zn-OMS-2I.
for the abatement of various pollutants. For comparison, the photothermocatalytic CO abatement on TiO2(P25) under the conditions identical to those of the Zn doped OMS-2 catalysts was conducted. The photothermocatalytic activity of TiO2(P25) is quite low, evidenced by its very low rCO2 value (2.58 μmol g−1 min−1). In comparison with TiO2(P25), the rCO2 values of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III are raised by as high as 37.9, 53.9, 64.5, and 64.4 times, respectively. Long-term photothermocatalytic CO abatement on the optimum catalyst (Zn-OMS-2II) with the UV–Vis-IR irradiation was studied. The CO conversion of Zn-OMS-2II is 85.6% at the initial 1 h. After 24 h reaction, Zn-OMS-2II still has high CO conversion (87.3%) (Fig. 5B). This result indicates that Zn-OMS-2II has excellent photothermocatalytic durability. To demonstrate if the Zn doped OMS-2 catalysts have Vis-IR or IR photothermocatalytic activity, photothermocatalytic CO abatement on Zn-OMS-2II and TiO2(P25) with the Vis-IR and IR irradiation was studied. With the λ > 420 and 560 nm Vis-IR irradiation, Zn-OMS-2II has excellent photothermocatalytic activity (Fig. 6), evidenced by its high rCO2 values (158.0 and 137.5 μmol g−1 min−1, respectively). In striking contrast, with the λ > 420 nm Vis-IR irradiation, TiO2(P25) nearly loses photocatalytic activity because of to its large band gap (3.0 eV for rutile, 3.2 eV for anatase). Remarkably, even with the λ > 830 nm IR irradiation, Zn-OMS-2II still has very good photothermocatalytic activity. The rCO2 value of Zn-OMS-2II is still as high as 125.5 μmol g−1 min−1.
d c b a
3.3. Light-driven thermocatalytic CO oxidation
Fig. 4. Zn 2p and Mn 2p spectra of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and Zn-OMS-2III (d).
To clarify the origin of the excellent photothermocatalytic property of the Zn doped OMS-2 catalysts with the UV–Vis-IR irradiation, the 830
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Fig. 5. The CO2 production rates of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), Zn-OMS-2III (d), and TiO2(P25) (e) for photothermocatalytic CO abatement with the UV–Vis-IR irradiation (A). Photothermocatalytic durability of Zn-OMS2II for CO abatement with the UV–Vis-IR irradiation (B).
Fig. 7. Diffusive reflectance UV–Vis-IR absorption spectra (A) of the catalysts (R: reflectance). Time course of the CO2 production rate of Zn-OMS-2II for photocatalytic CO oxidation with the UV–Vis-IR irradiation at near ambient temperature (B). The equilibrium temperatures of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and Zn-OMS-2III (d) with the UV–Vis-IR irradiation (C).
(3.31 μmol g−1 min−1). The result indicates that it is light-driven thermocatalytic CO abatement on Zn-OMS-2II rather than photocatalysis to contribute to the very high CO2 production rate of Zn-OMS2II with UV–Vis-IR irradiation as shown in Fig. 5: The intense absorption of Zn-OMS-2II upon the irradiation causes its surface temperature increase. When the temperature is raised above the light-off temperature (Tlight-off) of thermocatalytic CO abatement on Zn-OMS-2II, the thermocatalytic CO abatement is trigged. To confirm whether it is true, we recorded the temperature of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III with the UV–Vis-IR irradiation when we evaluated their photothermocatalytic activity (Fig. 5). Upon the irradiation, the photothermal conversion due to their intense absorption causes their surface temperature to quickly increase to equilibrium temperatures. With the UV–Vis-IR irradiation, pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III have equilibrium temperatures of 214, 204, 204, and 205 °C, respectively. The equilibrium temperatures of Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III
Fig. 6. The production rates of Zn-OMS-2II and TiO2(P25) for photothermocatalytic CO abatement with the Vis-IR and IR irradiation.
optical adsorption of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and ZnOMS-2III were measured. Pure OMS-2 and all Zn doped OMS-2 catalysts have intense absorption in the region of 240 to 2400 nm (Fig. 7A). In comparison with pure OMS-2, the absorption above ~1380 nm of ZnOMS-2I, Zn-OMS-2II, and Zn-OMS-2III is slightly attenuated. To clarify if the intense absorption of the Zn doped OMS-2 catalysts is able to trigger photocatalytic CO abatement, photocatalytic CO abatement on Zn-OMS-2II with UV–Vis-IR irradiation at near ambient temperature was studied. As can be seen from Fig. 7B, Zn-OMS-2II has very low photocatalytic activity, evidenced by its very low rCO2 value
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different BET surface area as discussed in Section 3.1, we compared their turn over frequencies, defined as molecule numbers of the produced CO2 per hour per catalytic active site. The active sites of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III are their active lattice oxygen or Mn binding to the active lattice oxygen (discussed later in Section 3.4). Their quantities are determined by their CO consumption quantities based on the CO-TPR profiles of pure OMS-2, Zn-OMS2I, Zn-OMS-2II, and Zn-OMS-2III, calibrated through the CO-TPR profile of pure CuO with a known quantity. As can be seen from Fig. 8B, the TOF values of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III at 160 °C is 0.97, 2.53, 3.35, and 3.04 h−1, respectively. In comparison to that of pure OMS-2, the TOF values of Zn-OMS-2I, Zn-OMS-2II, and ZnOMS-2III are raised by 2.6, 3.5, and 3.1 times. The result clearly shows that the Zn doping substantially raises the intrinsic thermocatalytic property of OMS-2, thus substantially raising photothermocatalytic property of OMS-2 (Fig. 5) in accordance to the light-driven thermocatalysis mechanism.
3.4. Origin of the catalytic enhancement by Zn doping It is extensively accepted that the catalytic oxidation on OMS-2 abides by Mars–van Krevelen mechanism [57,61]: OMS-2 is reduced by CO molecules adsorbed. Subsequently, the resultant oxygen vacancies are refilled by O2. As the reduction of OMS-2 is sluggish in comparison with the re-oxidization of the reduced OMS-2, the catalytic activity of OMS-2 primarily rests with its lattice oxygen activity or reducibility [57,61]. To clarify the origin of substantial thermocatalytic elevation through the Zn doping, the reducibility of Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III was studied by CO-TPR. Pure OMS-2 has two major CO consumption processes (Fig. 9). The first CO consumption process occurring below 345 °C arises from the reduction of KMn8O16 (OMS-2) to Mn3O4 [57,61]. This CO consumption process is composed of several CO-TPR peaks with the maximum peaks around 271 °C, indicating that several types of lattice oxygen in pure OMS-2 react with CO. The second CO consumption process occurring around 402 °C arises from the reduction of Mn3O4 to MnO [57,61]. In comparison with pure OMS-2, the maximum CO consumption peak of KMn8O16 to Mn3O4 for Zn-OMS-2I (Zn/Mn ratio = 0.052) shifts from 271 °C to 239 °C. The result indicates that the Zn doping substantially promotes the reducibility of OMS-2, thus substantially promoting its thermocatalytic activity as shown in Fig. 8. Increasing the Zn/Mn atomic ratio promotes the reducibility of OMS-2. Zn-OMS-2II (Zn/Mn ratio = 0.056) has the highest reducibility, evidenced by the fact that its maximum CO consumption peak of KMn8O16 to Mn3O4 shifts to the lowest temperature (167 °C). The highest reducibility of Zn-OMS-2II results in its highest thermocatalytic activity as shown in Fig. 8. Further elevating the Zn/Mn atomic ratio causes a decrease in the reducibility of OMS-2. The maximum CO
Fig. 8. CO conversions vs the temperatures for thermocatalytic CO abatement in the dark on the catalysts (A) and TOF values of the catalysts at 160 °C (B): pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and Zn-OMS-2III (d).
slightly less than that of pure OMS-2 is attributed to their attenuated absorption in λ > ~1380 nm IR region (Fig. 7A). We also recorded the equilibrium temperatures of Zn-OMS-2II with the Vis-IR and IR irradiation when we measured its Vis-IR and IR photothermocatalytic activity (Fig. 6). With the Vis-IR and IR irradiation of λ > 420, 560, and 830 nm, Zn-OMS-2II has equilibrium temperatures of 203,193, and 181 °C, respectively (Fig. S7). To verify if the temperature increase upon the irradiation could trigger the thermocatalytic CO abatement on the catalysts, thermocatalytic CO abatement on pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III in the dark was studied. As shown in Fig. 8, when the temperature is elevated above 100 °C, thermocatalytic CO abatement on pure OMS-2 takes place (Tlight-off = ~100 °C). For Zn-OMS-2I, ZnOMS-2II, and Zn-OMS-2III, when the temperature is elevated above 80 °C, thermocatalytic CO abatement takes place (Tlight-off = ~80 °C). As all the equilibrium temperatures of pure OMS-2, Zn-OMS-2I, ZnOMS-2II, and Zn-OMS-2III with the UV–Vis-IR irradiation far outstrip their corresponding Tlight-off values, effective UV–Vis-IR light-driven thermocatalytic CO abatement takes places on the catalysts. As all the equilibrium temperatures of Zn-OMS-2II with the Vis-IR and IR irradiation far outstrip its Tlight-off value, effective Vis-IR and IR light-driven thermocatalytic CO abatement takes places on Zn-OMS-2II. Pure OMS-2 has good thermocatalytic activity for CO oxidation (Fig. 8). At the temperatures of 140, 160, and 180 °C, its CO conversions are 11.0%, 23.1%, and 45.9%, respectively. In comparison with pure OMS-2, the Zn doping substantially raises the thermocatalytic activity of OMS-2. For Zn-OMS-2I with the lowest Zn/Mn ratio of 0.052, at 140, 160, and 180 °C, its CO conversions increase to 32.2%, 50.9%, and 66.1%, respectively. Zn-OMS-2II (Zn/Mn ratio = 0.056) has the maximum thermocatalytic activity. The CO conversions of Zn-OMS-2II at 140, 160, and 180 °C increase to 61.3%, 79.3%, and 93.9%, respectively. Further elevating Zn/Mn atomic ratio causes a reduction in the thermocatalytic activity. The CO conversions of Zn-OMS-2III (Zn/Mn ratio = 0.073) at 140, 160, and 180 °C slightly decrease to 31.8%, 65.5%, and 90.1%, respectively. As pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III have
Fig. 9. CO-TPR profiles of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and Zn-OMS-2III (d). 832
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substantially promoting its photothermocatalytic activity. 4. Conclusion In conclusion, the photothermocatalytic activity of OMS-2 is substantially raised through the Zn doping. This is attributed to the thermocatalytic activity of Zn doped OMS-2 being far superior to pure OMS2 because the Zn doping substantially promotes the reducibility of Zn doped OMS-2. The excellent photothermocatalytic activity of Zn doped OMS-2 is primarily ascribed to the light-driven thermocatalysis. A new photoactivation was discovered to promote the light-driven thermocatalytic activity of Zn doped OMS-2: UV–Vis-IR irradiation obviously promotes the reducibility of Zn doped OMS-2, thus substantially promoting its catalytic activity. The work provides an approach of substantially promoting the catalytic activity of OMS-2 for air pollutant abatement utilizing inexhaustible solar energy, which is applicable to other manganese oxides.
Fig. 10. CO2 production rates of Zn-OMS-2II at the different temperatures for CO abatement in the dark and with the UV–Vis-IR irradiation.
Acknowledgment This work was supported by National Natural Science Foundation of China (21673168, 21473127). Appendix A. Supplementary data EDX mappings, N2 adsorption/desorption isotherms, BJH adsorption pore size distribution, and the equilibrium temperatures with the Vis-IR and IR irradiation. Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2019.04.022. Fig. 11. CO-TPR profiles of Zn-OMS-2II in the dark and with the UV–Vis-IR irradiation.
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consumption peak (KMn8O16 to Mn3O4) of Zn-OMS-2III (Zn/Mn ratio = 0.073) shifts to a larger temperature (193 °C) in comparison with that of Zn-OMS-2II (167 °C). This accounts for the thermocatalytic activity of Zn-OMS-2III being lower than that of Zn-OMS-2II. 3.5. Photoactivation The issue is if the excellent photothermocatalytic activity of ZnOMS-2II (Fig. 5) is merely ascribed to the light-driven thermocatalysis. To resolve the issue, the catalytic CO abatement on Zn-OMS-2II at different temperatures with the UV–Vis-IR irradiation and in the dark was studied. As shown in Fig. 10, at the same temperature above 60 °C, the UV–Vis-IR irradiation substantially promotes the CO2 production rate of Zn-OMS-2II in comparison with that in the dark. For example, at 120 and 140 °C, the rCO2 values of Zn-OMS-2II in the dark are 68.5 and 170.7 μmol g−1 min−1, respectively. With the UV–Vis-IR irradiation, its corresponding rCO2 values at 120 and 140 °C substantially increase to 212.1 and 384.5 μmol g−1 min−1. In comparison with the values at 120 and 140 °C in the dark, the rCO2 values with the UV–Vis-IR irradiation are raised by 3.1 and 2.3 times, respectively. As Zn-OMS-2II shows a quite low photocatalytic activity (Fig. 7B), the substantial elevation in the CO2 production rates induced by the UV–Vis-IR irradiation clearly shows the presence of new photoactivation that is completely different from the photoactivation on the semiconductor photocatalyts such as TiO2. As discussed on the Section 3.4, the catalytic activity of OMS-2 primarily rests with its reducibility. Therefore, CO-TPR of Zn-OMS-2II with the UV–Vis-IR irradiation and in the dark was studied in order to deeply reveal the origin of the new photoactivation. As shown in Fig. 11, with the UV–Vis-IR irradiation, the CO consumption peaks of Zn-OMS-2II obviously move to lower temperatures in comparison with the corresponding peaks in the dark. The result reveals that the UV–VisIR irradiation obviously promotes the reducibility of Zn-OMS-2II, thus 833
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Highly effective UV–Vis-IR and IR photothermocatalytic CO abatement on Zn doped OMS-2 nanorods
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Zhongkai Jiang, Yan Ma, Yuanzhi Li , Huihui Liu State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalytic Photothermocatalytic Photoactivation Zn doped OMS-2
The catalysts of Zn doped OMS-2 nanorods with the Zn/Mn atomic ratios of 0.052, 0.056, and 0.073 were synthesized through the reaction among Zn(NO3)2, KMnO4, and Mn(NO3)2 at 75 °C via changing the molar ratios of Zn(NO3)2/KMnO4/Mn(NO3)2 in reactants. The Zn doped OMS-2 catalysts were characterized with ICP, XRD, TEM, BET, XPS, UV–Vis-IR, and CO-TPR. The catalysts demonstrate excellent photothermocatalytic activity for CO abatement with UV–Vis-IR irradiation. The Zn doping substantially promotes the photothermocatalytic activity of OMS-2. In comparison with TiO2(P25), the CO2 production rate of the optimum catalyst is raised by 64.5 times. The optimum catalyst demonstrates very good photothermocatalytic activity with Vis-IR irradiation and even with λ > 830 nm IR irradiation. The excellent photothermocatalytic activity of the Zn doped OMS-2 catalysts is mainly ascribed to their effective light-driven thermocatalytic properties. The substantial elevation in the photothermocatalytic activity of OMS-2 through the Zn doping is attributed to the thermocatalytic activity of the Zn doped OMS-2 catalysts being far superior to pure OMS-2 because the Zn doping promotes the reducibility of OMS-2. A new photoactivation substantially promotes the light-driven thermocatalytic activity of Zn doped OMS-2: UV–Vis-IR irradiation obviously improves the reducibility of Zn doped OMS-2, thus substantially promoting its catalytic activity.
1. Introduction
been reported to have photocatalytic activity for air pollutant abatement. Recently, a very effective strategy of photothermocatalysis or lightdriven thermocatalysis for air pollutant abatement has been developed on the basis of a number of photothermocatalysts such as manganese oxides [41–44], metal ion doped manganese oxide [45–47], CuO [48,49], Co3O4 [50,51], Pt/LaVO4/TiO2 [52], MnOx/TiO2 [53], Ce1−xBixO2−δ [54], ABO3-type perovskites [55], etc. The photothermocatalysis strategy, combining the good thermocatalytic activity of the photothermocatalysts and their efficient photothermal conversion owing to their intense absorption in wide solar spectrum region even up to IR, is able to efficiently utilize UV–Vis light and even IR light for air pollutant abatement. Among the photothermocatalysts, cryptomelane manganese oxide with a structure of octahedral molecular sieve (KMn8O16, OMS-2) [56,57] is very attractive because of its earthabundance, low-cost, and high photothermocatalytic activity with UV–Vis-IR irradiation [41]. It was reported that doping OMS-2 with metal ions such as Mg [47], Ag [58], and Ce [59] improved the catalytic activity of OMS-2. Finding effective strategy of further substantially promoting the photothermocatalytic activity of OMS-2 with UV–Vis-IR irradiation is still highly desirable and great challenging.
Using inexhaustible solar energy for abating water and air pollutants and producing fuels based on numerous photocatalysts has attracted extensive interests for several decades [1–12]. The majority of the documented photocatalysts such as TiO2 [5–10], TiO2 nanocomposites [5,11,12], and so on are merely able to use UV and a fraction of visible light. Persistent efforts have been devoted to find strategies of designing effective photocatalysts that are able to use IR light, accounting for approximately 50% of solar energy [13–15]. The reported: 1) designing photocatalysts with small band gap to be activated by near-IR light such as Bi2WO6-based hybrid photocatalyst [16], WS2 [17], Cu2(OH)PO4 [18], Bi2MO6 (M = W, Mo) [19], Ag2X [20], BiErWO6 [21], 2) designing photocatalysts sensitized by near-IR responsive compounds [22,23], 3) designing plasmonic photocatalysts composed of precious metal nanoparticles that are able to be activated by near-IR light [24–28], 4) designing upconversion photocatalysts of which upconversion material converts near-IR photons to UV or visible photons that can excite photocatalyst [29–40]. Almost all of near-IR photocatalysts only show photocatalytic activity for the abatement of easily degradable water pollutants. Few of near-IR photocatalysts have
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Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.apsusc.2019.04.022 Received 23 January 2019; Received in revised form 25 March 2019; Accepted 2 April 2019 Available online 03 April 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 483 (2019) 827–834
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with UV–Vis-IR irradiation for a long time, a 10.7 g m−3 CO stream, acquired through merging a 5.0 vol% CO/He stream and an air stream, continuously flowed at 20.0 mL min−1 in the reactor. The photocatalytic CO abatement on Zn-OMS-2II with UV–Vis-IR irradiation was conducted at near room temperature on the same reactor as the photothermocatalytic CO abatement according to the procedure detailed in the previous publications [41,45].
In this work, we prepared the catalysts of Zn doped OMS-2 nanorods with several Zn/Mn atomic ratios by a facile approach. It is found that the Zn doping substantially promotes the photothermocatalytic activity of OMS-2 for CO abatement. The optimum catalyst demonstrates excellent photothermocatalytic activity with UV–Vis-IR irradiation and even with λ > 830 nm IR irradiation due to the light-driven thermocatalytic CO oxidation promoted by a new photoactivation. The origin of both the substantial catalytic elevation of OMS-2 by the Zn doping and the new photoactivation is revealed on the basis of the experimental evidences. To the best of our knowledge, the excellent UV–VisIR and IR photothermocatalytic activity of Zn doped OMS-2 for air pollutant abatement have not been reported.
2.4. Thermocatalytic activity A quartz tubular reactor was used to conduct the thermocatalytic CO abatement on the Zn doped OMS-2 catalysts in the dark on a WFS2015 apparatus. 0.0500 g of the catalyst powder supported by quartz wool was placed in the reactor. A thermocouple, which was placed inside the reactor, was used to monitor the reaction temperature. A 12.2 g m−3 CO stream, acquired through merging a 5.0 vol% CO/He stream and an air stream, continuously flowed at 40.0 mL min−1 in the reactor. A quartz tubular reactor linked with a quartz window was used to conduct catalytic CO abatement on Zn-OMS-2II at the different temperatures in the dark and with the irradiation. The catalyst amount is 0.0050 g. A 12.7 g m−3 CO stream, acquired through merging a 5.0 vol % CO/He stream and an air stream, continuously flowed at 20.0 mL min−1in the reactor. The detailed procedure was reported in previous publications [45,46].
2. Experimental section 2.1. Preparation The Zn doped OMS-2 catalyst was synthesized in accordance to the procedure as follows. 3.5790 g of 50 wt% Mn(NO3)2 and 0.4767 g Zn (NO3)2·6H2O were dissolved in 100 mL distilled water under magnetic stirring, respectively. 3.1608 g KMnO4 was dissolved in the aqueous solution of Mn(NO3)2 and Zn(NO3)2 under magnetic stirring. The acquired mixture in a beaker was put in an oven, heated to 75 °C, and kept at 75 °C for 72 h. The resultant precipitate was filtered, thoroughly washed, and dried at 75 °C overnight. The acquired catalyst of Zn doped OMS-2 is labeled as Zn-OMS-2I. The catalysts of Zn-OMS-2II, Zn-OMS-2III, and pure OMS-2 were synthesized by the same procedure as that of Zn-OMS-2I except for using 0.7150, 0.9533, and 0.0 g of Zn(NO3)2·6H2O, respectively.
3. Results and discussion 3.1. Characterization Pure OMS-2 was synthesized through the reaction between KMnO4 and Mn(NO3)2 at 75 °C with KMnO4/Mn(NO3)2 molar ratio of 2.0/1.0. The Zn doped OMS-2 catalysts with different Zn/Mn atomic ratios were synthesized through the reaction among Zn(NO3)2, KMnO4, and Mn (NO3)2 at 75 °C with different Zn(NO3)2/KMnO4/Mn(NO3)2 molar ratios of 0.12/2.0/1.0, 0.24/2.0/1.0, and 0.36/2.0/1.0, respectively. The corresponding Zn doped OMS-2 catalysts are labeled as Zn-OMS-2I, ZnOMS-2II, and Zn-OMS-2III, respectively. ICP analysis shows that the K/ Mn atomic ratio of pure OMS-2 is 0.103, approximating 0.125 of OMS-2 (KMn8O16). When the Zn(NO3)2/KMnO4/Mn(NO3)2 molar ratio in reactants is 0.12/2.0/1.0, the Zn/Mn atomic ratio of Zn-OMS-2I is 0.052, while its K/Mn atomic ratio decreases to 0.075. This suggests the substitution of K+ ions by Zn2+ ions in Zn doped OMS-2. When the Zn (NO3)2/KMnO4/Mn(NO3)2 molar ratio increases to 0.24/2.0/1.0, the Zn/Mn atomic ratio of Zn-OMS-2II increases to 0.056, while its K/Mn atomic ratio decreases to 0.063. When the Zn(NO3)2/KMnO4/Mn(NO3)2 molar ratio further increases to 0.36/2.0/1.0, the Zn/Mn atomic ratio of Zn-OMS-2III increases to 0.073. However, the K/Mn atomic ratio of ZnOMS-2III (0.065) does not have a further decrease in comparison with that of Zn-OMS-2II. XRD characterization shows that Zn-OMS-2I, Zn-OMS-2II, and ZnOMS-2III have pure tetragonal cryptomelane structure, the same as that of pure OMS-2 (JCPDF-29-1020) (Fig. 1). No crystalline ZnO is observed for Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III. The result indicates that the tetragonal cryptomelane structure of OMS-2 is not altered by the Zn doping. The morphologies of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III were observed by TEM. Zn-OMS-2I, ZnOMS-2II, and Zn-OMS-2III have morphology of nanorods, the same as that of pure OMS-2 (Fig. 2). The observation suggests that the nanorod morphology of OMS-2 is not altered by the Zn doping. The Zn doped OMS-2 catalysts were further characterized by EDX mapping. As shown in Fig. 3 (Zn-OMS-2I), Fig. S1 (Zn-OMS-2II), and Fig. S2 (Zn-OMS-2III), the elements of Zn, K, Mn, and O are well distributed on nanorods. The observation further confirms OMS-2 doped by Zn. The surface areas of the catalysts were measured by N2 adsorption.
2.2. Characterization The compositions of the Zn doped OMS-2 catalysts were analyzed on ICP-OES spectrometer (Prodigy 7). XRD patterns were acquired on a X-ray diffractometer (Rigaku Dmax) with low scan speed (0.50 min−1). TEM images were acquired on an electron microscope (JEM-100CX). Surface area was measured through N2 adsorption on ASAP2020. XPS spectra were acquired on a X-ray photoelectron spectrometer (VG Multilab 2000). UV–Vis-IR spectra were acquired on a spectrophotometer (Lambda 750). A multifunctional adsorption equipment (TP-5080) was used to conduct CO temperature-programmed reduction of the Zn doped OMS2 catalysts (CO-TPR) in the dark and with the irradiation. The used reactor and the procedures are detailed in the previous works [45,46]. 2.3. Photothermocatalytic and photocatalytic activity A homemade cylindrical stainless-steel reactor (447 mL) with a quartz window (11 cm in diameter) was utilized to conduct the photothermocatalytic CO abatement on the catalysts with the irradiation from a 500 W Xe lamp. 0.1000 g of the catalyst powder was dispersed on a thermal insulation slice (4 × 4 cm) of aluminum silicate fiber, which was put on the bottom of the reactor. The thickness of the catalyst powder is about 0.5 mm. A thermocouple was closely placed on the catalyst to measure the temperature under the irradiation. A 15.5 g m−3 CO stream, acquired through merging a 5.0 vol% CO/He stream and an air stream, continuously flowed at 40.0 mL min−1 in the reactor. The CO and CO2 concentrations were determined on a gas chromatograph (GC, GC9560). The experimental procedure and GC analysis conditions are reported in the previous works [41,45]. The UV–Vis-IR irradiation intensity is 436.1 mW cm−2. For conducting the photothermocatalytic CO abatement on Zn-OMS-2II with Vis-IR or IR irradiation, corresponding long wave pass filters were utilized. The λ > 420, 560, and 830 nm irradiation intensities are 365.9, 317.2, and 231.8 mW cm−2, respectively. To conduct photothermocatalytic CO abatement on Zn-OMS-2II 828
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2III, thus decreasing the BET surface area. The Zn valence states of Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III were determined through analyzing their Zn 2p XPS spectra. All the Zn doped OMS-2 catalysts have two peaks around 1021.0 and 1043.8 eV (Fig. 4), which are attributed to Zn 2p3/2 and Zn 2p1/2 of Zn2+ [60]. The Mn valence state of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and ZnOMS-2III were determined through analyzing their Mn 2p XPS spectra (Fig. 3). The peaks around 641.8 and 653.4 eV are ascribed to Mn 2p3/ 2 and Mn 2p1/2 of Mn3+, respectively. The peaks around 643.6 and 654.6 eV are ascribed to Mn 2p3/2 and Mn 2p1/2 of Mn4+, respectively [43,44]. The Mn3+/Mn4+ atomic ratios for pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III are 2.08, 2.30, 2.43, and 3.83, respectively. This suggests that the Zn doping increases the Mn3+ amount. This is attributed to the substitution of K+ ions by Zn2+ ions in Zn doped OMS-2, resulting in the increase in the Mn3+/Mn4+ atomic ratio to keep charge balance.
d c b a
Fig. 1. XRD patterns of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and ZnOMS-2III (d).
Pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III have BET surface areas of 98.7, 173.0, 202.7, and 127.6 m2 g−1, respectively (Fig. S3–6). This suggests that the Zn doping substantially causes an increase in the surface area. The possible reason is as follows: K+ ions are located in the channel of OMS-2 [56,57]. The substitution of K+ ions with larger radius (1.33 Å) by Zn2+ ions with smaller radius (0.74 Å) in Zn doped OMS-2 as discussed above is favorable to the diffusion of more N2 molecules into the channel of OMS-2, thus resulting in the increase in surface area. The surface area of Zn-OMS-2III less than that of ZnOMS-2II suggests that further increasing the Zn/Mn atomic ratio causes surface area decrease. The possible reason is as follows: As discussed above, compared to Zn-OMS-2II, the increase of the Zn/Mn atomic ratio from 0.056 to 0.073 for Zn-OMS-2III does not result in the substitution of more K+ ions by Zn2+ ions. This means that more Zn2+ ions probably lead to the partially blocking of the channel of OMS-2 for Zn-OMS-
3.2. Photothermocatalytic performance The photothermocatalytic CO abatement on pure OMS-2, Zn-OMS2I, Zn-OMS-2II, and Zn-OMS-2III with the UV–Vis-IR irradiation was conducted to evaluate their photothermocatalytic activity (see Experimental section). With the UV–Vis-IR irradiation, pure OMS-2 has good photothermocatalytic activity (Fig. 5A). Its CO2 production rate (rCO2) is 97.7 μmol g−1 min−1. The photothermocatalytic activity of OMS-2 is substantially raised through the Zn doping. In comparison with that of pure OMS-2, the rCO2 values of Zn-OMS-2I and Zn-OMS-2II substantially increase to 138.8 and 166.2 μmol g−1 min−1, respectively. Further elevating Zn/Mn atomic ratio does not cause an increase in the photothermocatalytic activity, evidenced by the fact that the rCO2 value of Zn-OMS-2III (165.8 μmol g−1 min−1) are similar to that of Zn-OMS2II. TiO2(P25) is a well-known photocatalyst that is extensively utilized
Fig. 2. TEM images of pure OMS-2 (A), Zn-OMS-2I (B), Zn-OMS-2II (C), and Zn-OMS-2III (D). 829
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Fig. 3. EDX mappings of Zn, K, Mn, and O in Zn-OMS-2I.
for the abatement of various pollutants. For comparison, the photothermocatalytic CO abatement on TiO2(P25) under the conditions identical to those of the Zn doped OMS-2 catalysts was conducted. The photothermocatalytic activity of TiO2(P25) is quite low, evidenced by its very low rCO2 value (2.58 μmol g−1 min−1). In comparison with TiO2(P25), the rCO2 values of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III are raised by as high as 37.9, 53.9, 64.5, and 64.4 times, respectively. Long-term photothermocatalytic CO abatement on the optimum catalyst (Zn-OMS-2II) with the UV–Vis-IR irradiation was studied. The CO conversion of Zn-OMS-2II is 85.6% at the initial 1 h. After 24 h reaction, Zn-OMS-2II still has high CO conversion (87.3%) (Fig. 5B). This result indicates that Zn-OMS-2II has excellent photothermocatalytic durability. To demonstrate if the Zn doped OMS-2 catalysts have Vis-IR or IR photothermocatalytic activity, photothermocatalytic CO abatement on Zn-OMS-2II and TiO2(P25) with the Vis-IR and IR irradiation was studied. With the λ > 420 and 560 nm Vis-IR irradiation, Zn-OMS-2II has excellent photothermocatalytic activity (Fig. 6), evidenced by its high rCO2 values (158.0 and 137.5 μmol g−1 min−1, respectively). In striking contrast, with the λ > 420 nm Vis-IR irradiation, TiO2(P25) nearly loses photocatalytic activity because of to its large band gap (3.0 eV for rutile, 3.2 eV for anatase). Remarkably, even with the λ > 830 nm IR irradiation, Zn-OMS-2II still has very good photothermocatalytic activity. The rCO2 value of Zn-OMS-2II is still as high as 125.5 μmol g−1 min−1.
d c b a
3.3. Light-driven thermocatalytic CO oxidation
Fig. 4. Zn 2p and Mn 2p spectra of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and Zn-OMS-2III (d).
To clarify the origin of the excellent photothermocatalytic property of the Zn doped OMS-2 catalysts with the UV–Vis-IR irradiation, the 830
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Fig. 5. The CO2 production rates of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), Zn-OMS-2III (d), and TiO2(P25) (e) for photothermocatalytic CO abatement with the UV–Vis-IR irradiation (A). Photothermocatalytic durability of Zn-OMS2II for CO abatement with the UV–Vis-IR irradiation (B).
Fig. 7. Diffusive reflectance UV–Vis-IR absorption spectra (A) of the catalysts (R: reflectance). Time course of the CO2 production rate of Zn-OMS-2II for photocatalytic CO oxidation with the UV–Vis-IR irradiation at near ambient temperature (B). The equilibrium temperatures of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and Zn-OMS-2III (d) with the UV–Vis-IR irradiation (C).
(3.31 μmol g−1 min−1). The result indicates that it is light-driven thermocatalytic CO abatement on Zn-OMS-2II rather than photocatalysis to contribute to the very high CO2 production rate of Zn-OMS2II with UV–Vis-IR irradiation as shown in Fig. 5: The intense absorption of Zn-OMS-2II upon the irradiation causes its surface temperature increase. When the temperature is raised above the light-off temperature (Tlight-off) of thermocatalytic CO abatement on Zn-OMS-2II, the thermocatalytic CO abatement is trigged. To confirm whether it is true, we recorded the temperature of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III with the UV–Vis-IR irradiation when we evaluated their photothermocatalytic activity (Fig. 5). Upon the irradiation, the photothermal conversion due to their intense absorption causes their surface temperature to quickly increase to equilibrium temperatures. With the UV–Vis-IR irradiation, pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III have equilibrium temperatures of 214, 204, 204, and 205 °C, respectively. The equilibrium temperatures of Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III
Fig. 6. The production rates of Zn-OMS-2II and TiO2(P25) for photothermocatalytic CO abatement with the Vis-IR and IR irradiation.
optical adsorption of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and ZnOMS-2III were measured. Pure OMS-2 and all Zn doped OMS-2 catalysts have intense absorption in the region of 240 to 2400 nm (Fig. 7A). In comparison with pure OMS-2, the absorption above ~1380 nm of ZnOMS-2I, Zn-OMS-2II, and Zn-OMS-2III is slightly attenuated. To clarify if the intense absorption of the Zn doped OMS-2 catalysts is able to trigger photocatalytic CO abatement, photocatalytic CO abatement on Zn-OMS-2II with UV–Vis-IR irradiation at near ambient temperature was studied. As can be seen from Fig. 7B, Zn-OMS-2II has very low photocatalytic activity, evidenced by its very low rCO2 value
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different BET surface area as discussed in Section 3.1, we compared their turn over frequencies, defined as molecule numbers of the produced CO2 per hour per catalytic active site. The active sites of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III are their active lattice oxygen or Mn binding to the active lattice oxygen (discussed later in Section 3.4). Their quantities are determined by their CO consumption quantities based on the CO-TPR profiles of pure OMS-2, Zn-OMS2I, Zn-OMS-2II, and Zn-OMS-2III, calibrated through the CO-TPR profile of pure CuO with a known quantity. As can be seen from Fig. 8B, the TOF values of pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III at 160 °C is 0.97, 2.53, 3.35, and 3.04 h−1, respectively. In comparison to that of pure OMS-2, the TOF values of Zn-OMS-2I, Zn-OMS-2II, and ZnOMS-2III are raised by 2.6, 3.5, and 3.1 times. The result clearly shows that the Zn doping substantially raises the intrinsic thermocatalytic property of OMS-2, thus substantially raising photothermocatalytic property of OMS-2 (Fig. 5) in accordance to the light-driven thermocatalysis mechanism.
3.4. Origin of the catalytic enhancement by Zn doping It is extensively accepted that the catalytic oxidation on OMS-2 abides by Mars–van Krevelen mechanism [57,61]: OMS-2 is reduced by CO molecules adsorbed. Subsequently, the resultant oxygen vacancies are refilled by O2. As the reduction of OMS-2 is sluggish in comparison with the re-oxidization of the reduced OMS-2, the catalytic activity of OMS-2 primarily rests with its lattice oxygen activity or reducibility [57,61]. To clarify the origin of substantial thermocatalytic elevation through the Zn doping, the reducibility of Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III was studied by CO-TPR. Pure OMS-2 has two major CO consumption processes (Fig. 9). The first CO consumption process occurring below 345 °C arises from the reduction of KMn8O16 (OMS-2) to Mn3O4 [57,61]. This CO consumption process is composed of several CO-TPR peaks with the maximum peaks around 271 °C, indicating that several types of lattice oxygen in pure OMS-2 react with CO. The second CO consumption process occurring around 402 °C arises from the reduction of Mn3O4 to MnO [57,61]. In comparison with pure OMS-2, the maximum CO consumption peak of KMn8O16 to Mn3O4 for Zn-OMS-2I (Zn/Mn ratio = 0.052) shifts from 271 °C to 239 °C. The result indicates that the Zn doping substantially promotes the reducibility of OMS-2, thus substantially promoting its thermocatalytic activity as shown in Fig. 8. Increasing the Zn/Mn atomic ratio promotes the reducibility of OMS-2. Zn-OMS-2II (Zn/Mn ratio = 0.056) has the highest reducibility, evidenced by the fact that its maximum CO consumption peak of KMn8O16 to Mn3O4 shifts to the lowest temperature (167 °C). The highest reducibility of Zn-OMS-2II results in its highest thermocatalytic activity as shown in Fig. 8. Further elevating the Zn/Mn atomic ratio causes a decrease in the reducibility of OMS-2. The maximum CO
Fig. 8. CO conversions vs the temperatures for thermocatalytic CO abatement in the dark on the catalysts (A) and TOF values of the catalysts at 160 °C (B): pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and Zn-OMS-2III (d).
slightly less than that of pure OMS-2 is attributed to their attenuated absorption in λ > ~1380 nm IR region (Fig. 7A). We also recorded the equilibrium temperatures of Zn-OMS-2II with the Vis-IR and IR irradiation when we measured its Vis-IR and IR photothermocatalytic activity (Fig. 6). With the Vis-IR and IR irradiation of λ > 420, 560, and 830 nm, Zn-OMS-2II has equilibrium temperatures of 203,193, and 181 °C, respectively (Fig. S7). To verify if the temperature increase upon the irradiation could trigger the thermocatalytic CO abatement on the catalysts, thermocatalytic CO abatement on pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III in the dark was studied. As shown in Fig. 8, when the temperature is elevated above 100 °C, thermocatalytic CO abatement on pure OMS-2 takes place (Tlight-off = ~100 °C). For Zn-OMS-2I, ZnOMS-2II, and Zn-OMS-2III, when the temperature is elevated above 80 °C, thermocatalytic CO abatement takes place (Tlight-off = ~80 °C). As all the equilibrium temperatures of pure OMS-2, Zn-OMS-2I, ZnOMS-2II, and Zn-OMS-2III with the UV–Vis-IR irradiation far outstrip their corresponding Tlight-off values, effective UV–Vis-IR light-driven thermocatalytic CO abatement takes places on the catalysts. As all the equilibrium temperatures of Zn-OMS-2II with the Vis-IR and IR irradiation far outstrip its Tlight-off value, effective Vis-IR and IR light-driven thermocatalytic CO abatement takes places on Zn-OMS-2II. Pure OMS-2 has good thermocatalytic activity for CO oxidation (Fig. 8). At the temperatures of 140, 160, and 180 °C, its CO conversions are 11.0%, 23.1%, and 45.9%, respectively. In comparison with pure OMS-2, the Zn doping substantially raises the thermocatalytic activity of OMS-2. For Zn-OMS-2I with the lowest Zn/Mn ratio of 0.052, at 140, 160, and 180 °C, its CO conversions increase to 32.2%, 50.9%, and 66.1%, respectively. Zn-OMS-2II (Zn/Mn ratio = 0.056) has the maximum thermocatalytic activity. The CO conversions of Zn-OMS-2II at 140, 160, and 180 °C increase to 61.3%, 79.3%, and 93.9%, respectively. Further elevating Zn/Mn atomic ratio causes a reduction in the thermocatalytic activity. The CO conversions of Zn-OMS-2III (Zn/Mn ratio = 0.073) at 140, 160, and 180 °C slightly decrease to 31.8%, 65.5%, and 90.1%, respectively. As pure OMS-2, Zn-OMS-2I, Zn-OMS-2II, and Zn-OMS-2III have
Fig. 9. CO-TPR profiles of pure OMS-2 (a), Zn-OMS-2I (b), Zn-OMS-2II (c), and Zn-OMS-2III (d). 832
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substantially promoting its photothermocatalytic activity. 4. Conclusion In conclusion, the photothermocatalytic activity of OMS-2 is substantially raised through the Zn doping. This is attributed to the thermocatalytic activity of Zn doped OMS-2 being far superior to pure OMS2 because the Zn doping substantially promotes the reducibility of Zn doped OMS-2. The excellent photothermocatalytic activity of Zn doped OMS-2 is primarily ascribed to the light-driven thermocatalysis. A new photoactivation was discovered to promote the light-driven thermocatalytic activity of Zn doped OMS-2: UV–Vis-IR irradiation obviously promotes the reducibility of Zn doped OMS-2, thus substantially promoting its catalytic activity. The work provides an approach of substantially promoting the catalytic activity of OMS-2 for air pollutant abatement utilizing inexhaustible solar energy, which is applicable to other manganese oxides.
Fig. 10. CO2 production rates of Zn-OMS-2II at the different temperatures for CO abatement in the dark and with the UV–Vis-IR irradiation.
Acknowledgment This work was supported by National Natural Science Foundation of China (21673168, 21473127). Appendix A. Supplementary data EDX mappings, N2 adsorption/desorption isotherms, BJH adsorption pore size distribution, and the equilibrium temperatures with the Vis-IR and IR irradiation. Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2019.04.022. Fig. 11. CO-TPR profiles of Zn-OMS-2II in the dark and with the UV–Vis-IR irradiation.
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consumption peak (KMn8O16 to Mn3O4) of Zn-OMS-2III (Zn/Mn ratio = 0.073) shifts to a larger temperature (193 °C) in comparison with that of Zn-OMS-2II (167 °C). This accounts for the thermocatalytic activity of Zn-OMS-2III being lower than that of Zn-OMS-2II. 3.5. Photoactivation The issue is if the excellent photothermocatalytic activity of ZnOMS-2II (Fig. 5) is merely ascribed to the light-driven thermocatalysis. To resolve the issue, the catalytic CO abatement on Zn-OMS-2II at different temperatures with the UV–Vis-IR irradiation and in the dark was studied. As shown in Fig. 10, at the same temperature above 60 °C, the UV–Vis-IR irradiation substantially promotes the CO2 production rate of Zn-OMS-2II in comparison with that in the dark. For example, at 120 and 140 °C, the rCO2 values of Zn-OMS-2II in the dark are 68.5 and 170.7 μmol g−1 min−1, respectively. With the UV–Vis-IR irradiation, its corresponding rCO2 values at 120 and 140 °C substantially increase to 212.1 and 384.5 μmol g−1 min−1. In comparison with the values at 120 and 140 °C in the dark, the rCO2 values with the UV–Vis-IR irradiation are raised by 3.1 and 2.3 times, respectively. As Zn-OMS-2II shows a quite low photocatalytic activity (Fig. 7B), the substantial elevation in the CO2 production rates induced by the UV–Vis-IR irradiation clearly shows the presence of new photoactivation that is completely different from the photoactivation on the semiconductor photocatalyts such as TiO2. As discussed on the Section 3.4, the catalytic activity of OMS-2 primarily rests with its reducibility. Therefore, CO-TPR of Zn-OMS-2II with the UV–Vis-IR irradiation and in the dark was studied in order to deeply reveal the origin of the new photoactivation. As shown in Fig. 11, with the UV–Vis-IR irradiation, the CO consumption peaks of Zn-OMS-2II obviously move to lower temperatures in comparison with the corresponding peaks in the dark. The result reveals that the UV–VisIR irradiation obviously promotes the reducibility of Zn-OMS-2II, thus 833
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