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TiO2 catalysts for toluene oxidation under visible light irradiation
Accepted Manuscript Full Length Article Facile synthesis of NaOH-promoted Pt/TiO2 catalysts for toluene oxidation under visible light irradiation Yuanyuan Zhang, Zhichun Si, Jian Gao, Yuxiang Liu, Liping Liu, Xiaodong Wu, Rui Ran, Duan Weng PII: DOI: Reference:
S0169-4332(18)33128-3 https://doi.org/10.1016/j.apsusc.2018.11.060 APSUSC 40905
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
27 August 2018 22 October 2018 7 November 2018
Please cite this article as: Y. Zhang, Z. Si, J. Gao, Y. Liu, L. Liu, X. Wu, R. Ran, D. Weng, Facile synthesis of NaOH-promoted Pt/TiO2 catalysts for toluene oxidation under visible light irradiation, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.11.060
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Facile synthesis of NaOH-promoted Pt/TiO2 catalysts for toluene oxidation under visible light irradiation Yuanyuan Zhang1, Zhichun Si*1, Jian Gao1, Yuxiang Liu1, Liping Liu2, Xiaodong Wu2, Rui Ran2, Duan Weng2 1
Graduate School at Shenzhen, Tsinghua University, Shenzhen City, 518055, China
2
The Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science
and Engineering, Tsinghua University, Beijing City, 10084, China
*
Corresponding author: [email protected]
Abstract NaOH-promoted Pt/TiO2 catalysts are successfully prepared by a facile impregnation method assisted by NaOH and tested for the toluene oxidation reaction under visible light irradiation. Keeping molar ratio of Pt:Na =1:8, the 0.1Pt-Na/TiO2 catalyst (0.1 mol% Pt) shows excellent catalytic performance with more than 60% of toluene conversion at 45 oC and achieving total toluene removal at 280 oC, which is significantly better than 0.1Pt/TiO2 catalyst. The structures and properties of catalysts are characterized by XRD, SEM, N2 adsorption, XPS and UV-vis spectra. The positively charged Pt species stabilized by NaOH are the most active sites for photo-thermal catalyzing toluene oxidation. The influence of Pt loading and the effect of supports (active TiO2 and inert Al2O3) are also discussed. Key words: Pt; NaOH; TiO2; photo-thermal catalysis; toluene oxidation
1. Introduction Volatile organic compounds (VOCs) are ubiquitous in indoor air and atmosphere, and are harmful to human health and the environment. Inhalation of VOCs may cause irritation, headache and fatigue, and damage the central nervous system, even is
carcinogenic in severe cases [1, 2]. On the other hand, VOCs will contribute to the formation of photochemical smog which has a remarkable impact to the air quality [3]. It’s urgent to find feasible ways to remove VOCs in mild conditions, therefore many techniques such as adsorption [4-7], ozonation [8-10], and catalysis [11-14] have been developed. Catalysis is a promising method because of the high VOCs removing efficiency. Some small molecules like formaldehyde can be thermal catalytically decomposed at room temperature, but many VOCs containing aromatic rings must be degraded at a relatively high temperature, which needs more energy consumption and special reactors [15]. Thus, photo-thermal synergistic degradation of such pollutants has received a lot of attentions due to its effective utilization of renewable light energy and much higher VOCs removal efficiency [16].
Many semiconductors have been employed in heterogeneous photocatalysis among which titanium dioxide (TiO2) is the most investigated for degradation of gaseous VOCs due to its nontoxicity, cheapness, excellent catalytic performance and high chemical stability [17-20]. When the energy of incident light is larger than the band gap energy of TiO2 (3.2 eV for anatase and 3.0 eV for rutile), the electrons will be excited from the valence band to the conduction band and leaving holes in the valence band, and then the hot electrons and holes can react with oxygen, water or hydroxyls on the surface of catalysts to generate radicals with strong oxidizing properties which can remove the VOCs efficiently [1]. However, the fast recombination of photogenerated electron-hole pairs reduce the overall quantum efficiency and inhibit the practical applications of pure TiO2 [21].
Deposition of noble metals like Pt, Pd and Au on the semiconductors is one of the desirable ways to improve the charge separation efficiency [22-25]. Noble metals can act as an electron trap by promoting interfacial charge transfer, and be the active components in thermal catalysis as well [21]. Generally, in such systems noble metals are highly dispersed on the surface of supports in order to utilize of catalytically active sites efficiently. Downsizing the nanoparticles to clusters or even single atoms
not only obtains more low-coordination and unsaturated atoms as active sites, but also reduces the demand for noble metals, which is a promising research direction [26, 27]. However, when the size of noble metals decreases to cluster level, its specific surface area becomes very large, which leads to a sharp increase of surface free energy, so agglomeration is likely to occur during the preparing and using courses, resulting in deactivation of catalysts. According to the published studies, there are some strategies usually applied in the synthesis of cluster or even single-atom catalysts, such as co-precipitation [26, 28], impregnation [29, 30], atomic layer deposition [31, 32], successive reduction [33] and so on. Over the past several years, a facile alkali-assisted impregnation method to prepare highly dispersed noble-metal based catalysts has been reported. In Zugic’s work [34], the coimpregnation of Pt and Na onto the surface of multi-walled carbon nanotubes contributed to the formation of Pt-Nax-Oy-(OH)z clusters and these subnanometer-sized active species significantly improved the water gas shift (WGS) reaction activity. In Yang’s work [35], tetraammine platinum nitrate and NaOH were successively impregnated on three kinds of supports, including TiO2, KLTL zeolites and MCM-41 zeolites. The active sites for WGS were Pt-O(OH)x- species stabilized by Na and were independent with the supports in this system.
In our present work, we used the facile impregnation method to load Pt and Na onto the surface of TiO2 P25. The precursors employed in the synthesis process were H2Pt(OH)6 and NaOH, in order to avoid introducing other anions which might cause some negative impacts. Four catalysts, denoted as 0.1Pt-Na/TiO2, 0.5Pt-Na/TiO2, 0.1Pt/TiO2 and 0.1Pt-Na/Al2O3, were synthesized to study the effect of Pt loading, Na addition and characters of supports. The toluene oxidation was used as the probe reaction to evaluate the catalytic performance of these samples. The structures and properties of catalysts were characterized by XRD, SEM, BET, XPS and UV-vis spectra. Among them the 0.1Pt-Na/TiO2 catalyst exhibits the best performance for toluene conversion under visible light irradiation. This paper offers a promising direction for preparing highly active photo-thermal catalysts which needs only a small
amount of noble metals for oxidation of toluene or other VOCs.
2. Experimental 2.1 Catalyst preparation Pt-Na/TiO2 catalysts were prepared by a simple NaOH-assisted impregnation method. In a typical synthesis, desired amount of dihydrogen hexahydroxyplatinate (H2Pt(OH)6, Alfa aesar) was dissolved in 5mL NaOH (Aladdin) aqueous solution (molar ratio Pt:Na=1:8), then 0.6g TiO2 (Degussa P25) was added into the solution and stirred for 12h. The resulting samples were dried at 105 oC overnight. The amount of Pt loading was 0.1mol% and 0.5mol%, which were denoted as 0.1Pt-Na/TiO2 and 0.5Pt-Na/TiO2 respectively. In order to study the effect of support, 0.1Pt-Na/Al2O3 was synthesized by a similar process. In this control sample, the support TiO2 was replaced by γ-Al2O3 (Aladdin, 10nm). 0.1Pt/TiO2 was prepared by impregnating TiO2 with H2Pt(OH)6 /HNO3 aqueous solution without alkali additives. All the chemicals were AR grade and were used without further purification. Ultra-pure water was from Milli-Q water purification system.
2.2 Characterization Crystalline structure of catalysts were characterized in the range of 20-80° (2θ) through Bruker D8 Advance X-ray diffractometer (XRD) using Cu Kα (λ=1.5406 Å) radiation. The microstructure and morphology were observed using field emission scanning electron microscopy (FESEM, ZEISS SUPRA 55) and transmission electron microscopy (TEM, FEI TECNAI G2 F30) equipped with an energy dispersive spectroscopy (EDS) accessory and a high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) accessory. The BET and BJH method were employed to analyze specific surface area and pore structure of catalysts through ASAP 2020M+C surface area analyzer by N2 adsorption. The UV-vis absorption spectra was measured by SHIMADZU UV-2450 UV-Visible near-infrared spectrophotometer, in which BaSO4 was used as an internal standard. Chemical states
of these samples were characterized by X-ray photoelectron spectra (XPS, PHI-5000 Versaprobe II) using monochromatic Al Kα radiation. The actual Pt loading of different samples were obtained by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Thermo IRIS Intrepid II).
2.3 Catalytic activity measurements The experimental apparatus used to measure the activities of catalysts for toluene oxidation was a fixed-bed flow reactor with inner diameter of 15 mm which has a quartz window to allow visible light irradiation. The Xeon lamp (100-300 W, CEAULIGHT CEL-HXF300) with a 400-800 nm cutoff filter was used as visible light source. 0.1g catalyst was dispersed on fine quartz wool and was put into the reactor for each cycle of measurement. The inlet gas was ~500 ppm toluene mixed with 20% O2 balanced by N2. The total flow rate was kept at 38.9 mL/min (weight hourly space velocity (WHSV) = 23340 mL/(g•h)). The outlet concentrations of toluene were obtained by a gas chromatograph (FULI GC9790Plus) equipped with a flame ionization detector (FID). The schematic diagram of the experimental setup is presented in Fig.1.
Fig. 1. The schematic diagram of the experimental setup.
We proved the activities of catalysts by toluene conversion. The definition of toluene
conversion is
where [toluene]in represents the inlet concentration of toluene, [toluene]out represents the outlet concentration of toluene and T is the reaction temperature.
3. Results and discussion 3.1. Characterizations
Fig. 2. The XRD patterns of (a) 0.1Pt-Na/TiO2, 0.5Pt-Na/TiO 2, 0.1Pt/TiO2 and (b) 0.1Pt-Na/Al2O3.
The XRD patterns of the catalysts are shown in Fig.2. All P25 supported catalysts present the XRD peaks of P25 consisting of anatase phase (JCPDS 86-0148) and rutile phase (JCPDS 84-1285) (Fig.2(a)). The five relatively strong diffraction peaks at 2θ =25.4o, 37.8o, 48.0o, 53.9o and 55.1o are assigned to anatase phase and the peaks at 2θ =27.5o, 36.1o and 41.2o are ascribed to rutile phase [36]. The reference catalyst in Fig.2(b) displays the XRD pattern of γ-Al2O3 (JCPDS 10-425). No diffractive signals of Pt and Na species appears in all the XRD patterns because of the low loading and high dispersion of these species. The SEM images of support TiO2 and 0.1Pt-Na/TiO2 sample are shown in Fig.3. The nanoparticles in both samples have an average diameter of 25 nm. It means that the experimental processes didn’t change the apparent shape and particle size of the support. TEM-EDS mapping and HAADF-STEM images are presented in Fig.4 in order to characterize the dispersion of Pt species. The EDS mapping of 0.1Pt-Na/TiO2 derived from the red rectangle section in Fig.4(a) suggests that Pt and Na elements are
highly dispersed on TiO2, which is corresponding to the results of XRD. The HAADF-STEM image in Fig.4(c) clearly indicates Pt clusters with an average diameter of 7 Å dispersed on the support in 0.1Pt-Na/TiO2 catalyst. As shown in the particle size distribution statistics (Fig.4(d) ), all the Pt clusters are less than 1nm, which further proves the high dispersion of Pt species.
Fig. 3. The SEM images of (a) support P25 and (b) 0.1Pt-Na/TiO2 catalyst.
Fig.4. (a) The TEM dark field image of 0.1Pt-Na/TiO2, (b) EDS mapping derived from the red rectangle section in Fig.4(a), (c) the HAADF-STEM image of 0.1Pt-Na/TiO2, the brighter spots are Pt clusters and (d) the particle size distribution statistics derived from Fig.4(c).
The nitrogen adsorption-desorption isotherms and the pore size distribution curves are
shown in Fig.5 and the related parameters are listed in Table 1. All the catalysts have the type IV isotherm characteristic and possess mesoporous structure. The P25supported samples have similar specific surface area, but the mean pore diameter and total pore volume of 0.1Pt/TiO2 are slightly lower than others, which may be caused by the poor dispersion of Pt species without alkali addition in 0.1Pt/TiO2 and leads to mesopore blocking. Combined with the next catalytic activity tests, it suggests that the specific surface area and pore structure of catalysts have little effect on their catalytic activities in our experiments.
Fig.5. (a) The nitrogen adsorption−desorption isotherms and (b) the pore size distribution curves. Table 1 Specific surface area (SBET), mean pore diameter (dV) and total pore volume (Vp) of catalysts. Samples
SBET (m2/g)
dV (nm)
Vp (cm3/g)
0.1Pt-Na/TiO2 0.5Pt-Na/TiO2 0.1Pt/TiO2 0.1Pt-Na/Al2O3
58.34 55.84 57.68 237.97
37.02 38.06 30.09 14.10
0.5375 0.5277 0.4296 0.8279
The XPS spectra calibrated with C 1s at 284.8 eV are shown in Fig.6. We do not show the Pt 4f of 0.1Pt-Na/Al2O3 because the Pt 4f and Al 2p peaks overlaps. The high-resolution Pt 4f spectra of P25-supported catalysts in Fig.6(a) show two peaks at 72.4 and 75.6 eV in which can be assigned to Pt 4f7/2 and Pt 4f5/2 of Pt in PtOx [37-40]. On 0.1Pt/TiO2, the Pt 4f signal is too weak to recognize the peak position, which may be ascribed to the sites selected to detect containing less Pt or the little number of signal acquisitions. For a number of WGS reactions, reverse water gas shift (RWGS)
reactions and HCHO catalytic oxidation reactions, Ptδ+ species act as active sites in the form of Pt-OHx-Na species [34, 41-44]. The Ti 2p spectra are shown in Fig.6(b) and there are two peaks at 458.7 and 464.4 eV which can be assigned to Ti 2p3/2 and Ti 2p1/2 respectively on 0.1Pt/TiO2. A negative shift of Ti 2p is observed on 0.1Pt-Na/TiO2 and 0.5Pt-Na/TiO2 catalysts and can be ascribed to that Ti4+ is partially reduced into Ti3+ because of the disturbance of Na [37, 45, 46]. As shown in Fig.6(c), each O 1s spectra can be resolved into two peaks which are corresponding to two kinds of oxygen species except 0.1Pt-Na/Al2O3. The peaks located at 531.4 eV and 529.6-529.8 eV are ascribed to surface oxygen species and lattice oxygen of bulk TiO2, respectively [37, 39, 40]. While there is only one peak located at 531.0 eV and no obvious shoulder peak is observed in 0.1Pt-Na/Al2O3. The peak ascribed to lattice oxygen is also negatively shift in the two NaOH-promoted catalysts compared with 0.1Pt/TiO2, because oxygen vacancies are generated at the metal-support interface during the Ti4+→Ti3+ reduction process and oxygen from the gas phase will dissociatively adsorbed on these vacancies, leading to the decrease of lattice oxygen’s binding energies [37, 45, 46]. It further proves that the addition of Na has significant influence on the electronic structure of the TiO2-supported catalysts.
Fig.6. The XPS spectra of (a) Pt 4f, (b) Ti 2p and (c) O 1s of catalysts.
As known, the optical property of semiconductor materials is an important factor of their photocatalytic performance. The UV-vis absorption spectra of all the catalysts are shown in Fig.7. The pure P25 sample is used as a reference. The P25 has an intense absorption band in the UV region (200-400 nm) which is ascribed to electron transition from the valence band to the conduction band of TiO 2. But P25 has almost no absorption in the visible-light region (400-800 nm) due to its relative high energy of band gap [47, 48]. As for the three P25-supported Pt catalysts, their absorption intensities are comparable with pure TiO2 in the UV region but are stronger than pure TiO2 in the visible-light region, which means that loading the NaOH promoted PtOx species on TiO2 did not change the band structure of P25. The 0.1Pt-Na/TiO2 catalyst has the strongest absorption in the wavelength range of 400-800 nm, which should be ascribed to the surface oxygen deficiencies [49-51]. It means that 0.1Pt-Na/TiO2 catalyst can utilize the visible light more efficiently in our photo-thermal catalysis experiments. As an inert material, γ-Al2O3 has a band gap of 5.8 eV and has no
light-absorption in the experimental region (400-800 nm), indicating that thermal catalysis dominates when 0.1Pt-Na/Al2O3 is employed in toluene oxidation catalysis.
Fig.7. The UV-vis absorption spectra of pure TiO2 P25 and three TiO2-supported catalysts.
The actual loading amount of Pt and Na atoms measured by ICP-OES are listed in Table 2 and are almost same with nominal loadings. The actual loadings are used to calculate the TOF (turnover frequency) value at a specific temperature.
3.2. Catalytic performance and discussion
Fig.8. (a) The performance of catalytic oxidation of toluene under visible light irradiation and (b) the comparison of catalytic performance of 0.1Pt-Na/TiO2 under visible light irradiation and dark conditions. Table 2 Actual Pt and Na loading, T50 and TOF of catalysts. Samples
Actual loading (mol%) a Pt
Na
T50 (oC) b
TOF (s-1) c at 100 oC
at 200 oC
light
light
dark
dark
0.1Pt-Na/TiO2 0.5Pt-Na/TiO2 0.1Pt/TiO2 0.1Pt-Na/Al2O3 TiO2
0.113 0.453 0.101 0.106 ——
0.826 3.124 —— 0.576 ——
<44 137 72 209 170
182.4 26.3 165.9 10.6 ——
20.3
208.4 51.0 202.1 122.7 ——
199.3
a
obtained by ICP-OES. defined as the temperature of 50% toluene conversion. c reaction conditions: catalyst mass = 0.1g, toluene concentration =500 ppm, O2 = 20%, N2 balance and total flow rate = 38.9 mL/min. b
The toluene oxidation under visible light irradiation at various temperatures are characterized to evaluate the catalytic performances of catalysts, and the results are shown in Fig.8(a). The T50 of the five catalysts are in the sequence of 0.1Pt-Na/TiO2 < 0.1Pt/TiO2 < 0.5Pt-Na/TiO2 < TiO2 < 0.1Pt-Na/Al2 O3. The performances of TiO2-supported catalysts improve significantly after Pt precipitations, indicating the catalytic effect of Pt species. The 0.1Pt-Na/TiO2 catalyst shows the highest photo-thermal activity in toluene oxidation. While increasing the loading amount of Pt to 0.5 mol% (0.5Pt-Na/TiO2), the catalyst show a decrease in toluene oxidation. The T50 and TOF (normalized to Pt atoms) at 100 oC and 200 oC are listed in Table 2 which reflects the atom efficiency of Pt. The 0.1Pt-Na/TiO2 catalyst exhibits the best catalytic activity with a toluene conversion of 60% at 45 oC. The comparison of the 0.1Pt-Na/TiO2 catalyst under visible light irradiation and dark conditions is presented in Fig.8(b) and Table 2. The TOF of 0.1Pt-Na/TiO2 under visible light (182.4 s-1) is about 9 times that of under dark condition (20.3 s-1) at 100 oC and is slightly higher than dark conditions at 200 oC. The fact indicates that visible light irradiation significantly improves the toluene oxidation performance of 0.1Pt-Na/TiO2 at low temperature but contributes little at relatively high temperature (>200 oC).
The effect of Pt loading amount can be analyzed by comparing the performance of 0.1Pt-Na/TiO2 and 0.5Pt-Na/TiO2. It is shown that these two catalysts exhibit comparable activities in the temperature range of 170-285 oC for toluene conversion but the activity of 0.5Pt-Na/TiO2 drops remarkably at temperatures lower than 170 oC. The possible reason is the ratio of Pt clusters to Pt nanoparticles decreases when the
loading amount of Pt increases from 0.1 mol% to 0.5 mol%, suggesting that Pt clusters may be the most active sites when the photocatalysis predominates while nanoparticles are less active at relatively low temperature. On the other hand, the relative high proportion of Pt nanoparticles may hinder the diffusion and transfer of the photo-induced active intermediates on catalysts’ surface which results in lower activity. With temperature ramping to 170 oC, the thermal catalysis gradually predominates. The effect of Pt loading amount is weakened and the temperature becomes the dominant factor determining the catalytic performance. From all discussed above, we can conclude that 0.1Pt-Na/TiO2 is a better catalyst compared with 0.5Pt-Na/TiO2, not only because of its more excellent performance for toluene oxidation but also for less consumption of noble metals, which is meaningful to obtain high atom utilization efficiency.
The performance comparison of catalysts with (0.1Pt-Na/TiO2) and without addition of Na (0.1Pt/TiO2) during the preparation are shown in Fig.8(a). The toluene conversion-temperature curves of these two catalysts have the similar shape but the conversion percentage of 0.1Pt-Na/TiO2 at specific temperature is higher than 0.1Pt/TiO2. This result suggests that the difference in the reaction rates between the 0.1Pt-Na/TiO2 and 0.1Pt/TiO2 catalysts is attributed to their different number of active sites available to the reactants. In previous works [26, 34, 38, 41-43], it was revealed that an oxidized platinum state can be stabilized by the presence of alkali atoms. When a platinum atom and several alkali atoms formed Pt(alkali)n clusters, alkali atoms had the trend of donating electrons to Pt. Hence, to create the sub-oxidation state of Ptδ+, electron-withdrawing groups like O or OH were added to the Pt(alkali)n clusters to form Pt-alkali-Ox(OH)y species wich were active in WGS reactions [41]. The oxygen-containing species (surface oxygen and hydroxyl groups) also play an important role in toluene oxidation [52]. In our work, these species can be activated by visible light-induced electrons and holes to generate highly active O• and OH• species in toluene reactions. It can explain why the catalytic activity of 0.1Pt-Na/TiO2 is higher than 0.1Pt/TiO2. From the above, the promoting effect of Na on Pt catalysts
for toluene conversion reaction can be concluded: 1) dispersing and stabilizing the oxygen-containing species around Pt atoms; 2) inducing highly active O• and OH• species under visible light irradiation.
TiO2 and Al2O3 are employed as active and inert supports respectively in our experiments to study the effect of supports. TiO2 is reducible and possess photocatalytic activity while Al2O3 is a non-reducible oxide and has no response to the visible light [35]. From Fig.8(a), we can see that the catalytic activity of 0.1Pt-Na/Al2O3 deteriorates significantly compared with 0.1Pt-Na/TiO2. The possible reason is that there are strong metal-support interactions between Pt species and TiO2 support in 0.1Pt-Na/TiO2 [15, 39, 53]. The electron transfer from TiO2 to Pt clusters makes Pt clusters have a tendency to be negatively charged. However, Pt are at oxidation state proved by XPS, indicating that Pt(alkali) n clusters will attract O and/or OH groups to keep Pt atoms in oxidation state. It also can explain why there is no shoulder peak ascribed to surface oxygen species can be seen in O 1s spectra of 0.1Pt-Na/Al2O3. Moreover, a negative shift of the Ti 2p peak position of 0.1Pt-Na/TiO2 indicates that a fraction of Ti4+ in the support is reduced into Ti3+. The Ti3+ defects on TiO2 can act as the active sites for O2 adsorption and dissociation to active O• species under visible light irradiation [54]. This is one of the reasons why 0.1Pt-Na/TiO2 sample performs significantly better than 0.1Pt-Na/Al2O3 for toluene oxidation.
4. Conclusion We successfully prepared 0.1Pt-Na/TiO2 catalyst via a simple NaOH-assisted impregnation method. The 0.1Pt-Na/TiO2 catalyst shows excellent activity in photo-thermal catalytic oxidation of toluene, showing more than 60% toluene conversion at 45 oC and achieving total removal at around 280 oC under visible light irradiation. The addition of alkali metals contributes to the stabilization of Pt clusters which increases utilization efficiency of noble metals, and the Pt-Nan clusters attract
oxygen-containing groups like O and OH to form Pt-Ox(OH)y-Nan species which are considered as the active sites for photocatalyzing toluene oxidation. The highlight of the catalyst is that the loading amount of noble metals is low. Such systems may be applied to other VOCs and can be widely used in the field of photo-thermal catalysis.
Acknowledgements The authors would like to acknowledge the National Key Research and Development Program of China for financial support of Project 2016YFC0700902-3. Moreover, we would also thank the financial support from the Strategic Emerging Industry Development Funds of Shenzhen (JCYJ20160301150948119).
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Highlights
The loading amount (0.1 mol% Pt) of noble metals is low, which is meaningful to obtain high atom utilization efficiency.
The catalyst shows excellent catalytic performance with more than 60% of toluene conversion at 45 oC and achieving total toluene removal at 280 oC under visible light irradiation.
Pt-Nan clusters will attract O and/or OH groups to keep Pt atoms in oxidation state, these species can be activated by visible light-induced electrons and holes to generate highly active O• and OH• species in toluene reactions.
S0169-4332(18)33128-3 https://doi.org/10.1016/j.apsusc.2018.11.060 APSUSC 40905
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
27 August 2018 22 October 2018 7 November 2018
Please cite this article as: Y. Zhang, Z. Si, J. Gao, Y. Liu, L. Liu, X. Wu, R. Ran, D. Weng, Facile synthesis of NaOH-promoted Pt/TiO2 catalysts for toluene oxidation under visible light irradiation, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.11.060
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Facile synthesis of NaOH-promoted Pt/TiO2 catalysts for toluene oxidation under visible light irradiation Yuanyuan Zhang1, Zhichun Si*1, Jian Gao1, Yuxiang Liu1, Liping Liu2, Xiaodong Wu2, Rui Ran2, Duan Weng2 1
Graduate School at Shenzhen, Tsinghua University, Shenzhen City, 518055, China
2
The Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science
and Engineering, Tsinghua University, Beijing City, 10084, China
*
Corresponding author: [email protected]
Abstract NaOH-promoted Pt/TiO2 catalysts are successfully prepared by a facile impregnation method assisted by NaOH and tested for the toluene oxidation reaction under visible light irradiation. Keeping molar ratio of Pt:Na =1:8, the 0.1Pt-Na/TiO2 catalyst (0.1 mol% Pt) shows excellent catalytic performance with more than 60% of toluene conversion at 45 oC and achieving total toluene removal at 280 oC, which is significantly better than 0.1Pt/TiO2 catalyst. The structures and properties of catalysts are characterized by XRD, SEM, N2 adsorption, XPS and UV-vis spectra. The positively charged Pt species stabilized by NaOH are the most active sites for photo-thermal catalyzing toluene oxidation. The influence of Pt loading and the effect of supports (active TiO2 and inert Al2O3) are also discussed. Key words: Pt; NaOH; TiO2; photo-thermal catalysis; toluene oxidation
1. Introduction Volatile organic compounds (VOCs) are ubiquitous in indoor air and atmosphere, and are harmful to human health and the environment. Inhalation of VOCs may cause irritation, headache and fatigue, and damage the central nervous system, even is
carcinogenic in severe cases [1, 2]. On the other hand, VOCs will contribute to the formation of photochemical smog which has a remarkable impact to the air quality [3]. It’s urgent to find feasible ways to remove VOCs in mild conditions, therefore many techniques such as adsorption [4-7], ozonation [8-10], and catalysis [11-14] have been developed. Catalysis is a promising method because of the high VOCs removing efficiency. Some small molecules like formaldehyde can be thermal catalytically decomposed at room temperature, but many VOCs containing aromatic rings must be degraded at a relatively high temperature, which needs more energy consumption and special reactors [15]. Thus, photo-thermal synergistic degradation of such pollutants has received a lot of attentions due to its effective utilization of renewable light energy and much higher VOCs removal efficiency [16].
Many semiconductors have been employed in heterogeneous photocatalysis among which titanium dioxide (TiO2) is the most investigated for degradation of gaseous VOCs due to its nontoxicity, cheapness, excellent catalytic performance and high chemical stability [17-20]. When the energy of incident light is larger than the band gap energy of TiO2 (3.2 eV for anatase and 3.0 eV for rutile), the electrons will be excited from the valence band to the conduction band and leaving holes in the valence band, and then the hot electrons and holes can react with oxygen, water or hydroxyls on the surface of catalysts to generate radicals with strong oxidizing properties which can remove the VOCs efficiently [1]. However, the fast recombination of photogenerated electron-hole pairs reduce the overall quantum efficiency and inhibit the practical applications of pure TiO2 [21].
Deposition of noble metals like Pt, Pd and Au on the semiconductors is one of the desirable ways to improve the charge separation efficiency [22-25]. Noble metals can act as an electron trap by promoting interfacial charge transfer, and be the active components in thermal catalysis as well [21]. Generally, in such systems noble metals are highly dispersed on the surface of supports in order to utilize of catalytically active sites efficiently. Downsizing the nanoparticles to clusters or even single atoms
not only obtains more low-coordination and unsaturated atoms as active sites, but also reduces the demand for noble metals, which is a promising research direction [26, 27]. However, when the size of noble metals decreases to cluster level, its specific surface area becomes very large, which leads to a sharp increase of surface free energy, so agglomeration is likely to occur during the preparing and using courses, resulting in deactivation of catalysts. According to the published studies, there are some strategies usually applied in the synthesis of cluster or even single-atom catalysts, such as co-precipitation [26, 28], impregnation [29, 30], atomic layer deposition [31, 32], successive reduction [33] and so on. Over the past several years, a facile alkali-assisted impregnation method to prepare highly dispersed noble-metal based catalysts has been reported. In Zugic’s work [34], the coimpregnation of Pt and Na onto the surface of multi-walled carbon nanotubes contributed to the formation of Pt-Nax-Oy-(OH)z clusters and these subnanometer-sized active species significantly improved the water gas shift (WGS) reaction activity. In Yang’s work [35], tetraammine platinum nitrate and NaOH were successively impregnated on three kinds of supports, including TiO2, KLTL zeolites and MCM-41 zeolites. The active sites for WGS were Pt-O(OH)x- species stabilized by Na and were independent with the supports in this system.
In our present work, we used the facile impregnation method to load Pt and Na onto the surface of TiO2 P25. The precursors employed in the synthesis process were H2Pt(OH)6 and NaOH, in order to avoid introducing other anions which might cause some negative impacts. Four catalysts, denoted as 0.1Pt-Na/TiO2, 0.5Pt-Na/TiO2, 0.1Pt/TiO2 and 0.1Pt-Na/Al2O3, were synthesized to study the effect of Pt loading, Na addition and characters of supports. The toluene oxidation was used as the probe reaction to evaluate the catalytic performance of these samples. The structures and properties of catalysts were characterized by XRD, SEM, BET, XPS and UV-vis spectra. Among them the 0.1Pt-Na/TiO2 catalyst exhibits the best performance for toluene conversion under visible light irradiation. This paper offers a promising direction for preparing highly active photo-thermal catalysts which needs only a small
amount of noble metals for oxidation of toluene or other VOCs.
2. Experimental 2.1 Catalyst preparation Pt-Na/TiO2 catalysts were prepared by a simple NaOH-assisted impregnation method. In a typical synthesis, desired amount of dihydrogen hexahydroxyplatinate (H2Pt(OH)6, Alfa aesar) was dissolved in 5mL NaOH (Aladdin) aqueous solution (molar ratio Pt:Na=1:8), then 0.6g TiO2 (Degussa P25) was added into the solution and stirred for 12h. The resulting samples were dried at 105 oC overnight. The amount of Pt loading was 0.1mol% and 0.5mol%, which were denoted as 0.1Pt-Na/TiO2 and 0.5Pt-Na/TiO2 respectively. In order to study the effect of support, 0.1Pt-Na/Al2O3 was synthesized by a similar process. In this control sample, the support TiO2 was replaced by γ-Al2O3 (Aladdin, 10nm). 0.1Pt/TiO2 was prepared by impregnating TiO2 with H2Pt(OH)6 /HNO3 aqueous solution without alkali additives. All the chemicals were AR grade and were used without further purification. Ultra-pure water was from Milli-Q water purification system.
2.2 Characterization Crystalline structure of catalysts were characterized in the range of 20-80° (2θ) through Bruker D8 Advance X-ray diffractometer (XRD) using Cu Kα (λ=1.5406 Å) radiation. The microstructure and morphology were observed using field emission scanning electron microscopy (FESEM, ZEISS SUPRA 55) and transmission electron microscopy (TEM, FEI TECNAI G2 F30) equipped with an energy dispersive spectroscopy (EDS) accessory and a high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) accessory. The BET and BJH method were employed to analyze specific surface area and pore structure of catalysts through ASAP 2020M+C surface area analyzer by N2 adsorption. The UV-vis absorption spectra was measured by SHIMADZU UV-2450 UV-Visible near-infrared spectrophotometer, in which BaSO4 was used as an internal standard. Chemical states
of these samples were characterized by X-ray photoelectron spectra (XPS, PHI-5000 Versaprobe II) using monochromatic Al Kα radiation. The actual Pt loading of different samples were obtained by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Thermo IRIS Intrepid II).
2.3 Catalytic activity measurements The experimental apparatus used to measure the activities of catalysts for toluene oxidation was a fixed-bed flow reactor with inner diameter of 15 mm which has a quartz window to allow visible light irradiation. The Xeon lamp (100-300 W, CEAULIGHT CEL-HXF300) with a 400-800 nm cutoff filter was used as visible light source. 0.1g catalyst was dispersed on fine quartz wool and was put into the reactor for each cycle of measurement. The inlet gas was ~500 ppm toluene mixed with 20% O2 balanced by N2. The total flow rate was kept at 38.9 mL/min (weight hourly space velocity (WHSV) = 23340 mL/(g•h)). The outlet concentrations of toluene were obtained by a gas chromatograph (FULI GC9790Plus) equipped with a flame ionization detector (FID). The schematic diagram of the experimental setup is presented in Fig.1.
Fig. 1. The schematic diagram of the experimental setup.
We proved the activities of catalysts by toluene conversion. The definition of toluene
conversion is
where [toluene]in represents the inlet concentration of toluene, [toluene]out represents the outlet concentration of toluene and T is the reaction temperature.
3. Results and discussion 3.1. Characterizations
Fig. 2. The XRD patterns of (a) 0.1Pt-Na/TiO2, 0.5Pt-Na/TiO 2, 0.1Pt/TiO2 and (b) 0.1Pt-Na/Al2O3.
The XRD patterns of the catalysts are shown in Fig.2. All P25 supported catalysts present the XRD peaks of P25 consisting of anatase phase (JCPDS 86-0148) and rutile phase (JCPDS 84-1285) (Fig.2(a)). The five relatively strong diffraction peaks at 2θ =25.4o, 37.8o, 48.0o, 53.9o and 55.1o are assigned to anatase phase and the peaks at 2θ =27.5o, 36.1o and 41.2o are ascribed to rutile phase [36]. The reference catalyst in Fig.2(b) displays the XRD pattern of γ-Al2O3 (JCPDS 10-425). No diffractive signals of Pt and Na species appears in all the XRD patterns because of the low loading and high dispersion of these species. The SEM images of support TiO2 and 0.1Pt-Na/TiO2 sample are shown in Fig.3. The nanoparticles in both samples have an average diameter of 25 nm. It means that the experimental processes didn’t change the apparent shape and particle size of the support. TEM-EDS mapping and HAADF-STEM images are presented in Fig.4 in order to characterize the dispersion of Pt species. The EDS mapping of 0.1Pt-Na/TiO2 derived from the red rectangle section in Fig.4(a) suggests that Pt and Na elements are
highly dispersed on TiO2, which is corresponding to the results of XRD. The HAADF-STEM image in Fig.4(c) clearly indicates Pt clusters with an average diameter of 7 Å dispersed on the support in 0.1Pt-Na/TiO2 catalyst. As shown in the particle size distribution statistics (Fig.4(d) ), all the Pt clusters are less than 1nm, which further proves the high dispersion of Pt species.
Fig. 3. The SEM images of (a) support P25 and (b) 0.1Pt-Na/TiO2 catalyst.
Fig.4. (a) The TEM dark field image of 0.1Pt-Na/TiO2, (b) EDS mapping derived from the red rectangle section in Fig.4(a), (c) the HAADF-STEM image of 0.1Pt-Na/TiO2, the brighter spots are Pt clusters and (d) the particle size distribution statistics derived from Fig.4(c).
The nitrogen adsorption-desorption isotherms and the pore size distribution curves are
shown in Fig.5 and the related parameters are listed in Table 1. All the catalysts have the type IV isotherm characteristic and possess mesoporous structure. The P25supported samples have similar specific surface area, but the mean pore diameter and total pore volume of 0.1Pt/TiO2 are slightly lower than others, which may be caused by the poor dispersion of Pt species without alkali addition in 0.1Pt/TiO2 and leads to mesopore blocking. Combined with the next catalytic activity tests, it suggests that the specific surface area and pore structure of catalysts have little effect on their catalytic activities in our experiments.
Fig.5. (a) The nitrogen adsorption−desorption isotherms and (b) the pore size distribution curves. Table 1 Specific surface area (SBET), mean pore diameter (dV) and total pore volume (Vp) of catalysts. Samples
SBET (m2/g)
dV (nm)
Vp (cm3/g)
0.1Pt-Na/TiO2 0.5Pt-Na/TiO2 0.1Pt/TiO2 0.1Pt-Na/Al2O3
58.34 55.84 57.68 237.97
37.02 38.06 30.09 14.10
0.5375 0.5277 0.4296 0.8279
The XPS spectra calibrated with C 1s at 284.8 eV are shown in Fig.6. We do not show the Pt 4f of 0.1Pt-Na/Al2O3 because the Pt 4f and Al 2p peaks overlaps. The high-resolution Pt 4f spectra of P25-supported catalysts in Fig.6(a) show two peaks at 72.4 and 75.6 eV in which can be assigned to Pt 4f7/2 and Pt 4f5/2 of Pt in PtOx [37-40]. On 0.1Pt/TiO2, the Pt 4f signal is too weak to recognize the peak position, which may be ascribed to the sites selected to detect containing less Pt or the little number of signal acquisitions. For a number of WGS reactions, reverse water gas shift (RWGS)
reactions and HCHO catalytic oxidation reactions, Ptδ+ species act as active sites in the form of Pt-OHx-Na species [34, 41-44]. The Ti 2p spectra are shown in Fig.6(b) and there are two peaks at 458.7 and 464.4 eV which can be assigned to Ti 2p3/2 and Ti 2p1/2 respectively on 0.1Pt/TiO2. A negative shift of Ti 2p is observed on 0.1Pt-Na/TiO2 and 0.5Pt-Na/TiO2 catalysts and can be ascribed to that Ti4+ is partially reduced into Ti3+ because of the disturbance of Na [37, 45, 46]. As shown in Fig.6(c), each O 1s spectra can be resolved into two peaks which are corresponding to two kinds of oxygen species except 0.1Pt-Na/Al2O3. The peaks located at 531.4 eV and 529.6-529.8 eV are ascribed to surface oxygen species and lattice oxygen of bulk TiO2, respectively [37, 39, 40]. While there is only one peak located at 531.0 eV and no obvious shoulder peak is observed in 0.1Pt-Na/Al2O3. The peak ascribed to lattice oxygen is also negatively shift in the two NaOH-promoted catalysts compared with 0.1Pt/TiO2, because oxygen vacancies are generated at the metal-support interface during the Ti4+→Ti3+ reduction process and oxygen from the gas phase will dissociatively adsorbed on these vacancies, leading to the decrease of lattice oxygen’s binding energies [37, 45, 46]. It further proves that the addition of Na has significant influence on the electronic structure of the TiO2-supported catalysts.
Fig.6. The XPS spectra of (a) Pt 4f, (b) Ti 2p and (c) O 1s of catalysts.
As known, the optical property of semiconductor materials is an important factor of their photocatalytic performance. The UV-vis absorption spectra of all the catalysts are shown in Fig.7. The pure P25 sample is used as a reference. The P25 has an intense absorption band in the UV region (200-400 nm) which is ascribed to electron transition from the valence band to the conduction band of TiO 2. But P25 has almost no absorption in the visible-light region (400-800 nm) due to its relative high energy of band gap [47, 48]. As for the three P25-supported Pt catalysts, their absorption intensities are comparable with pure TiO2 in the UV region but are stronger than pure TiO2 in the visible-light region, which means that loading the NaOH promoted PtOx species on TiO2 did not change the band structure of P25. The 0.1Pt-Na/TiO2 catalyst has the strongest absorption in the wavelength range of 400-800 nm, which should be ascribed to the surface oxygen deficiencies [49-51]. It means that 0.1Pt-Na/TiO2 catalyst can utilize the visible light more efficiently in our photo-thermal catalysis experiments. As an inert material, γ-Al2O3 has a band gap of 5.8 eV and has no
light-absorption in the experimental region (400-800 nm), indicating that thermal catalysis dominates when 0.1Pt-Na/Al2O3 is employed in toluene oxidation catalysis.
Fig.7. The UV-vis absorption spectra of pure TiO2 P25 and three TiO2-supported catalysts.
The actual loading amount of Pt and Na atoms measured by ICP-OES are listed in Table 2 and are almost same with nominal loadings. The actual loadings are used to calculate the TOF (turnover frequency) value at a specific temperature.
3.2. Catalytic performance and discussion
Fig.8. (a) The performance of catalytic oxidation of toluene under visible light irradiation and (b) the comparison of catalytic performance of 0.1Pt-Na/TiO2 under visible light irradiation and dark conditions. Table 2 Actual Pt and Na loading, T50 and TOF of catalysts. Samples
Actual loading (mol%) a Pt
Na
T50 (oC) b
TOF (s-1) c at 100 oC
at 200 oC
light
light
dark
dark
0.1Pt-Na/TiO2 0.5Pt-Na/TiO2 0.1Pt/TiO2 0.1Pt-Na/Al2O3 TiO2
0.113 0.453 0.101 0.106 ——
0.826 3.124 —— 0.576 ——
<44 137 72 209 170
182.4 26.3 165.9 10.6 ——
20.3
208.4 51.0 202.1 122.7 ——
199.3
a
obtained by ICP-OES. defined as the temperature of 50% toluene conversion. c reaction conditions: catalyst mass = 0.1g, toluene concentration =500 ppm, O2 = 20%, N2 balance and total flow rate = 38.9 mL/min. b
The toluene oxidation under visible light irradiation at various temperatures are characterized to evaluate the catalytic performances of catalysts, and the results are shown in Fig.8(a). The T50 of the five catalysts are in the sequence of 0.1Pt-Na/TiO2 < 0.1Pt/TiO2 < 0.5Pt-Na/TiO2 < TiO2 < 0.1Pt-Na/Al2 O3. The performances of TiO2-supported catalysts improve significantly after Pt precipitations, indicating the catalytic effect of Pt species. The 0.1Pt-Na/TiO2 catalyst shows the highest photo-thermal activity in toluene oxidation. While increasing the loading amount of Pt to 0.5 mol% (0.5Pt-Na/TiO2), the catalyst show a decrease in toluene oxidation. The T50 and TOF (normalized to Pt atoms) at 100 oC and 200 oC are listed in Table 2 which reflects the atom efficiency of Pt. The 0.1Pt-Na/TiO2 catalyst exhibits the best catalytic activity with a toluene conversion of 60% at 45 oC. The comparison of the 0.1Pt-Na/TiO2 catalyst under visible light irradiation and dark conditions is presented in Fig.8(b) and Table 2. The TOF of 0.1Pt-Na/TiO2 under visible light (182.4 s-1) is about 9 times that of under dark condition (20.3 s-1) at 100 oC and is slightly higher than dark conditions at 200 oC. The fact indicates that visible light irradiation significantly improves the toluene oxidation performance of 0.1Pt-Na/TiO2 at low temperature but contributes little at relatively high temperature (>200 oC).
The effect of Pt loading amount can be analyzed by comparing the performance of 0.1Pt-Na/TiO2 and 0.5Pt-Na/TiO2. It is shown that these two catalysts exhibit comparable activities in the temperature range of 170-285 oC for toluene conversion but the activity of 0.5Pt-Na/TiO2 drops remarkably at temperatures lower than 170 oC. The possible reason is the ratio of Pt clusters to Pt nanoparticles decreases when the
loading amount of Pt increases from 0.1 mol% to 0.5 mol%, suggesting that Pt clusters may be the most active sites when the photocatalysis predominates while nanoparticles are less active at relatively low temperature. On the other hand, the relative high proportion of Pt nanoparticles may hinder the diffusion and transfer of the photo-induced active intermediates on catalysts’ surface which results in lower activity. With temperature ramping to 170 oC, the thermal catalysis gradually predominates. The effect of Pt loading amount is weakened and the temperature becomes the dominant factor determining the catalytic performance. From all discussed above, we can conclude that 0.1Pt-Na/TiO2 is a better catalyst compared with 0.5Pt-Na/TiO2, not only because of its more excellent performance for toluene oxidation but also for less consumption of noble metals, which is meaningful to obtain high atom utilization efficiency.
The performance comparison of catalysts with (0.1Pt-Na/TiO2) and without addition of Na (0.1Pt/TiO2) during the preparation are shown in Fig.8(a). The toluene conversion-temperature curves of these two catalysts have the similar shape but the conversion percentage of 0.1Pt-Na/TiO2 at specific temperature is higher than 0.1Pt/TiO2. This result suggests that the difference in the reaction rates between the 0.1Pt-Na/TiO2 and 0.1Pt/TiO2 catalysts is attributed to their different number of active sites available to the reactants. In previous works [26, 34, 38, 41-43], it was revealed that an oxidized platinum state can be stabilized by the presence of alkali atoms. When a platinum atom and several alkali atoms formed Pt(alkali)n clusters, alkali atoms had the trend of donating electrons to Pt. Hence, to create the sub-oxidation state of Ptδ+, electron-withdrawing groups like O or OH were added to the Pt(alkali)n clusters to form Pt-alkali-Ox(OH)y species wich were active in WGS reactions [41]. The oxygen-containing species (surface oxygen and hydroxyl groups) also play an important role in toluene oxidation [52]. In our work, these species can be activated by visible light-induced electrons and holes to generate highly active O• and OH• species in toluene reactions. It can explain why the catalytic activity of 0.1Pt-Na/TiO2 is higher than 0.1Pt/TiO2. From the above, the promoting effect of Na on Pt catalysts
for toluene conversion reaction can be concluded: 1) dispersing and stabilizing the oxygen-containing species around Pt atoms; 2) inducing highly active O• and OH• species under visible light irradiation.
TiO2 and Al2O3 are employed as active and inert supports respectively in our experiments to study the effect of supports. TiO2 is reducible and possess photocatalytic activity while Al2O3 is a non-reducible oxide and has no response to the visible light [35]. From Fig.8(a), we can see that the catalytic activity of 0.1Pt-Na/Al2O3 deteriorates significantly compared with 0.1Pt-Na/TiO2. The possible reason is that there are strong metal-support interactions between Pt species and TiO2 support in 0.1Pt-Na/TiO2 [15, 39, 53]. The electron transfer from TiO2 to Pt clusters makes Pt clusters have a tendency to be negatively charged. However, Pt are at oxidation state proved by XPS, indicating that Pt(alkali) n clusters will attract O and/or OH groups to keep Pt atoms in oxidation state. It also can explain why there is no shoulder peak ascribed to surface oxygen species can be seen in O 1s spectra of 0.1Pt-Na/Al2O3. Moreover, a negative shift of the Ti 2p peak position of 0.1Pt-Na/TiO2 indicates that a fraction of Ti4+ in the support is reduced into Ti3+. The Ti3+ defects on TiO2 can act as the active sites for O2 adsorption and dissociation to active O• species under visible light irradiation [54]. This is one of the reasons why 0.1Pt-Na/TiO2 sample performs significantly better than 0.1Pt-Na/Al2O3 for toluene oxidation.
4. Conclusion We successfully prepared 0.1Pt-Na/TiO2 catalyst via a simple NaOH-assisted impregnation method. The 0.1Pt-Na/TiO2 catalyst shows excellent activity in photo-thermal catalytic oxidation of toluene, showing more than 60% toluene conversion at 45 oC and achieving total removal at around 280 oC under visible light irradiation. The addition of alkali metals contributes to the stabilization of Pt clusters which increases utilization efficiency of noble metals, and the Pt-Nan clusters attract
oxygen-containing groups like O and OH to form Pt-Ox(OH)y-Nan species which are considered as the active sites for photocatalyzing toluene oxidation. The highlight of the catalyst is that the loading amount of noble metals is low. Such systems may be applied to other VOCs and can be widely used in the field of photo-thermal catalysis.
Acknowledgements The authors would like to acknowledge the National Key Research and Development Program of China for financial support of Project 2016YFC0700902-3. Moreover, we would also thank the financial support from the Strategic Emerging Industry Development Funds of Shenzhen (JCYJ20160301150948119).
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Highlights
The loading amount (0.1 mol% Pt) of noble metals is low, which is meaningful to obtain high atom utilization efficiency.
The catalyst shows excellent catalytic performance with more than 60% of toluene conversion at 45 oC and achieving total toluene removal at 280 oC under visible light irradiation.
Pt-Nan clusters will attract O and/or OH groups to keep Pt atoms in oxidation state, these species can be activated by visible light-induced electrons and holes to generate highly active O• and OH• species in toluene reactions.