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TiO2 catalysts for toluene total oxidation
Chemical Physics Letters 730 (2019) 95–99
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
The effect of preparation method on oxygen activation over Pt/TiO2 catalysts for toluene total oxidation ⁎
Yiqing Zenga, Yanan Wanga, Yahan Menga, Fujiao Songb,c, Shule Zhanga, , Qin Zhonga,
T
⁎
a
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China School of Environmental Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, PR China c Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, PR China b
H I GH L IG H T S
supported Pt catalysts were prepared by various preparation methods. • TiO method affected the catalytic activity in toluene total oxidation. • Preparation • Oxygen activation was a key factor to determine the catalytic activity. 2
A R T I C LE I N FO
A B S T R A C T
Keywords: PtTi catalyst Preparation method Oxygen activation Toluene oxidation
Herein, we compared the catalyst formation process of three Pt/TiO2 catalysts prepared by impregnation (PtTi–I), deposition precipitation (PtTi–D) and photo-deposition (PtTi–P) to investigate its effect on oxgyen activation and toluene oxidation activity. The encapsulation of TiO2 on Pt particles induced by high temperature strengthened the metal–support interaction (MSI) but inhibited the activation of oxygen over PtTi–I catalyst, thus it showed the poorest activity in toluene oxidation. Since the selective deposition of Pt on certain face of TiO2 during photo-deposition process, PtTi–P exhibited stronger MSI and higher activity although it possessed much larger Pt particles than PtTi–D.
1. Introduction Most volatile organic compounds (VOCs) can lead to the decomposition of tropospheric ozone, photochemical smog in atmosphere and cause various diseases threat to human life and health, thus, developing effective VOCs elimination techniques is of great significance and urgent [1–3]. Nowadays, recovery methods (adsorption, condensation, absorption and membrane) and destruction techniques (incineration, catalytic oxidation, plasma catalysis and biodegradable) have been applied for abatement VOCs [1]. Among them, catalytic oxidation is known as one of the most effective method due to the features of low cost and minimized secondary pollutions. Supported noble metals (Pt, Rh, Pd) catalysts, as the most effective catalyst, are widely used in total oxidation of VOCs [2–5]. Among them, Pt supported on TiO2 (PtTi) is one of the most utilized and studied catalyst. The oxidation of VOCs over noble metals over Pt based catalyst may process via Mars–van Krevelen (MVK) mechanism [6,7], Langemuir–Hinshelwood (L–H) mechanism [8,9] (reaction with chemisorbed reactant) or Eley–Rideal E–R mechanism (reaction with gaseous ⁎
reactant) [10]. The validity of each mechanism strongly depends on the catalyst (noble metal and nature of the support) as well as on the character of the VOC molecule and it is difficult to be generalized [2]. Even though, it has been proved that the total oxidation of toluene over Pt based catalyst strongly relies on the chemisorbed oxygen on the Pt particles [10–12]. Hence, many methods have been developed for enhancing the oxygen activation ability of PtTi. The strong metal support interaction (SMSI) is first discovered by Tauster in 1978 based on the fact that hydrogen reduced noble metal supported on TiO2 catalysts lose it H2 and CO adsorption ability [13]. But it can promote the transfer of electrons from metal oxide supports to Pt nanoparticles and affect the unfilled d band vacancies of Pt, so that the adsorption, dissociation and desorption of oxygen species can be facilitated [14,15]. Zhang et al. proved that higher temperature can induce higher degree SMSI in PtTi, while excessive temperature will decrease the performance in oxygen activation because of encapsulation effect [16]. Therefore, mild methods are developed to synthesize PtTi catalyst. Rui et al. found that the PtTi obtained by mild solution reduction process exhibits higher degree SMSI and better toluene oxidation activity than that prepared by
Corresponding authors. E-mail addresses: [email protected] (S. Zhang), [email protected] (Q. Zhong).
https://doi.org/10.1016/j.cplett.2019.05.048 Received 12 April 2019; Received in revised form 9 May 2019; Accepted 28 May 2019 Available online 29 May 2019 0009-2614/ © 2019 Published by Elsevier B.V.
Chemical Physics Letters 730 (2019) 95–99
Y. Zeng, et al.
60 mL·min−1 and 0.3 g sample was used. The TDP experimental began with pre-treating sample with 3 vol%O2/N2 at 50 °C for 60 min, and then the gas was switched to He for 30 min. Subsequently, O2–TPD was performed by ramping the temperature at 10 °C·min−1 to 800 °C in He flow.
H2 reduction at high temperature [17]. Wong and his co–workers pointed that photo–deposition method can also induce high degree SMSI in PtTi catalyst and lead to the high performance in oxygen reduction reaction in acid medium [18]. The most used preparation method of PtTi mainly included wet impregnation with H2 reduction, deposition–precipitation with reducing agent (such as NaBH4, N2H4·H2O and HCHO) reduction and photo–deposition. The comparation study of two preparation method had been widely reported. As our best known, except for the study [19] reported by Leung groups that investigating the effect of three reduction processes (NaBH4, H2 and photo-deposition reduction) on impregnation of PtTi catalyst for HCHO oxidation at room temperature, little study had been done to analyse the effect of these three preparation method on the catalytic activity of PtTi catalyst. In this study, we compared the PtTi (PtTi–I, PtTi–D and PtTi–P) catalysts prepared by impregnation (PtTi–I), deposition precipitation (PtTi–D) and photo–deposition (PtTi–P) from the catalyst formation process and catalytic activity in toluene oxidation. Combined with the results of TEM, BET, XRD, XPS, UV–vis, O2–TPD and EPR, it was revealed that preparation method naturally affected the structural characteristics, surface properties of PtTi catalysts. These features dominated the oxygen activation process, which plays a key role in toluene total oxidation. We expected this study could provide some inspiration in the design of noble metal based catalysts.
2.3. Catalytic activity test Total oxidation of toluene was carried out at 130–190 °C in a quartz fixed-bed reactor (i,d, 5 mm) using 0.3 g catalyst with 40–80 mesh under atmospheric pressure. The typical reactant gas composition was as follows: 500 ppm toluene, 20 vol% O2 and N2 as the balance gas. Total flow rate was 250 mL·min−1, corresponding to a space velocity of about 40,000 h−1. The concentration of toluene in the inlet and outlet gas was measured by on-line gas chromatography equipped with a TCD and two FID (one for analysis of organics and another for determine of CO2 and CO via methanation reactor) detectors. CO2 and H2O could be detected as the only products, so the toluene conversion was calculated by follow equation:
(
Toluene conversion (%) = n outlet
CO2
− nintlet
)/7ntoluene × 100%
CO2
where nCO2 is the molar flow of CO2 at the outlet and ntoluene is the molar flow of toluene at the inlet.
2. Experimental
3. Results and discussion
2.1. Catalyst preparation
The TEM images and particles size distribution of PtTi sample are presented in Fig. 1. The size of 100 TiO2 nanoparticles and 50 Pt nanoparticles were counted for obtaining the histogram of particle size distribution for each sample. It can be noted that the TiO2 particles size in PtTi–I was greater than that in PtTi–D and PtTi–P, inferring that TiO2 support was prone to sintering during hydrogen reduction process in comparison with photodeposition and mild solution reduction process. For Pt particles over PtTi–I, their size located at a small range of 1.6–2.4 nm. And it was worth noting that Pt nanoparticles could only be found on a portion of the TiO2 particles, which suggested that Pt nanoparticles were tended to agglomeration at high temperature hydrogen reduction process. Moreover, from the HRTEM images shown in Fig. S1, the lattice line of TiO2 could be observed next to Pt nanoparticles. This well consistent with the reported phenomenon that high temperature reducing atmosphere can lead to the transfer of TiO2 onto the surface of Pt0 particles and then encapsulate them [16,20]. For PtTi–D sample, there no obvious Pt particles could be seen. The Pt content obtained from ICP-MS for PtTi–I, PtTi–D and PtTi–P respectively were 1.02 wt%, 0.95 wt% and 0.92 wt%, which close to the theoretical value. Thus, the Pt particle on PtTi–D was too small to detect. As to PtTi–P, its displayed the largest Pt particles with a size of 2.8–5.7 nm. Fig. 2 displays the XRD patterns of catalysts. All catalysts displayed a typical P25 TiO2 diffraction peaks, which composed with anatase (PDF#21–1272) and few rutile (PDF#21–1276). No diffraction peaks of Pt could be observed, inferring that Pt species were well dispersed on TiO2 or the Pt species were too small to be detected by XRD. Notably, all peaks of PtTi catalysts presented the same intensity, which were lower than that of TiO2 supported. This result showed that the interaction between Pt nanoparticles and TiO2 affected the near surface structure of TiO2. To investigate the influence of Pt loading on the specific surface area, the BET was carried out. As listed in Table 1, PtTi–I had the smallest specific surface area, while TiO2, PtTi–D and PtTi–P exhibited a specific surface area of 54 ± 1 m2·g−1, indicating that Pt loading had no obvious effect on the specific surface area via mild preparation method. The smallest specific surface area of PtTi–I could be ascribed to the agglomeration of TiO2 support, which was consistent with the results observed in TEM images. Fig. 3a shows the Pt 4f XPS spectra. Clearly, the binding energy of Pt
TiO2 was purchased from Degussa (P25) and other analytical pure chemicals were purchased from Aladdin and used without any treatment. 1 wt%Pt/TiO2 catalysts were prepared by impregnation, deposition precipitation and photo-deposition methods, respectively. In a typical impregnation process, 2 g TiO2 (P25, Degussa) was added into 60 mL stoichiometric H2PtCl6 solution under stirring, and then the solvent was removed by evaporation at 100 °C. Finally, PtTi–I (impregnation) was obtained after drying at 80 °C and reducing at 450 °C (To avoid phase transformation from anatase TiO2 into rutile TiO2 [15]) for 2 h in 10 vol%H2/Ar atmosphere. For light deposition reduction process, 2 M NaOH solution was dropped into the mixture of 2 g TiO2 and H2PtCl6 solution under vigorous stirring until the pH = 12, then 10 mL 50 wt% hydrated hydrazine solution was added. After reduction for 2 h, the solid was washed by filtration for three times, and then PtTi–D (deposition precipitation) catalyst was obtained after dried at 80 °C for 6 h in vacuum oven. As to photo-deposition process, it was performed in a quartz glass reactor under N2 atmosphere. Generally, 2 g TiO2 and 20 mL methanol was added into 180 mL stoichiometric H2PtCl6 solution under stirring, then mixture was irradiated by UV light (CEL-HXUV 300, wavelength < 300 nm) for 2 h. Finally, PtTi–P (photo–deposition) was got via filtration, washing for three times and dried at 80 °C for 6 h in vacuum oven. 2.2. Characterization The catalysts were characterized by X–ray diffraction measurements (XRD Purkinje General Instrument Cu, Ltd, China, XD–3), Inductively coupled plasma (ICP, Varian ICP 720), Brunauer–Emmett–Teller (BET Quadrasorb-S1, Quantachrome, USA), High–resolution transmission electron microscope (HRTEM, JEOL JEM2100, 100 kV), UV–vis spectrophotometer (UV–2550 Shimadzu), X–ray photoelectron spectroscopy (XPS, PHI–5000C, ESCA) and Electron paramagnetic resonance (EPR EMX–10/12–type, Bruker). Oxygen temperature-programmed desorption (O2–TPD) experiment was performed on an automated chemisorption analyzer (Quantachrome Instruments), equipped with thermal conductivity detector (TCD). During experiment, all gaseous flow rates were 96
Chemical Physics Letters 730 (2019) 95–99
Y. Zeng, et al.
Fig. 1. The TEM images of PtTi–I (a and d), PtTi–D (b and e) and PtTi–P (c and f).
was used and the results are presented in Fig. 3b. Obviously, almost no absorbance could be seen in 400–700 nm for TiO2 because the standard band gap of anatase and rutile are 3.2 eV and 3.0 eV, respectively [23]. After loading with Pt nanoparticles, the absorbance at the range of visible light was significantly enhanced and followed the order of PtTi–P > PtTi–I > PtTi–D. These results indicated that the transfer of electron from TiO2 to Pt nanoparticles on PtTi–P was more effective than that on PtTi–I and PtTi–D. To investigate the oxygen activation over PtTi catalysts, Hence, O2–TPD and EPR was performed. As can be seen from Fig. 4a, all prepared samples displayed significant oxygen desorption peaks above 200 °C. Among these, the oxygen desorption peak below 500 °C could be attributed to the desorption of surface chemisorbed oxygen [24]. Notably, the desorption peak area of PtTi–P below 500 °C followed the PtTi–P > PtTi–D > PtTi–I order. These indicated that the surface chemisorbed oxygen capacity of the three samples is in an order of PtTi–P > PtTi–D > PtTi–I. Oxygen molecular adsorbed on oxygen vacancies can be reduced to superoxide radical (active oxygen) by capture one electron, which can be detected by EPR [25]. Hence, to further explore the ability of samples in activating oxygen, the samples were pre-treated at 100 mL/min of air at 160 °C for 1 h and their EPR was measured. As displayed in Fig. 4b, the signal at g = 2.003 due to superoxide radical was observed in the EPR spectrum of each samples [26]. The peak intensity followed PtTi–P > PtTi–D > PtTi–I, which indicated that the ability of the three catalysts in oxygen activation is: PtTi–P > PtTi–D > PtTi–I. The catalytic activity of prepared samples for toluene was tested, and the results are shown in Fig. 5. All PtTi catalysts displayed a high catalytic activity in total oxidation of toluene. The toluene conversion of sample at the range of 140–170 °C decreased with a sequence of PtTi–P > PtTi–D > PtTi–I, and the lowest temperature of 100% toluene conversion for them were 160, 170 and 180 °C, respectively. From the results of XPS, UV–vis, O2-TPD and EPR, the ability of the three catalysts in oxygen activation was positively related with their
Fig. 2. XRD patterns of catalysts. Table 1 BET data for samples. Samples
Surface area (m2 g−1)
pore volume (cm3 g−1)
pore size (nm)
TiO2 PtTi–I PtTi–D PtTi–P
54.5 48.6 53.6 55.1
0.262 0.287 0.261 0.264
17.0 21.6 16.8 17.0
4f for catalysts increased in the order of PtTi–D, PtTi–P and PtTi–I. It is well established that lower binding energy corresponds to higher electronic cloud density [21]. Therefore, the electronic cloud density of PtTi catalysts followed the order of PtTi–D > PtTi–P > PtTi–I. In the design of photocatalyst, noble metals are used as electron mediator of TiO2 and UV–vis spectrum is an effective way to display the electron transfer ability from TiO2 to noble metal [22]. Thus, UV–vis spectrum 97
Chemical Physics Letters 730 (2019) 95–99
Y. Zeng, et al.
Fig. 3. The Pt 4f XPS (a) and UV–vis (b) spectra of catalysts.
catalytic activity in toluene oxidation. Therefore, the adsorption and activation of oxygen molecular on Pt based catalyst were the controlling step for total oxidation of toluene and this result well consistent with previous studies [10–12]. Based on above analysis and previous studies [10–12], the proposed reaction process and formation mechanism of catalysts is presented in Scheme 1. For PtTi–I catalyst, the Pt4+ ions were reduced to Pt by H2 at high temperature on TiO2. At the same time, high temperature induced the encapsulation of TiO2 on Pt particles. This encapsulation enhanced the MSI between Pt and TiO2 in PtTi–I due to the high contact area, but inhibited the adsorption and activation of oxygen over Pt particles, which suppressed the activity of PtTi–I in total oxidation of toluene. During the formation process of PtTi-D catalyst, Pt4+ ions was deposited on TiO2 in the form of Pt(OH)4, then it was reduced to Pt by the follow added hydrazine. The size of obtained Pt particle was small and it could not be coated by TiO2 because of mild solution reduction process. Since the deposition position of Pt was unselective, thus, it is impossible for all Pt particles to form effective MSI (that is, TiO2 can effectively supply elections to Pt particles) with TiO2 support (shown in UV–vis). However, the formed effective MSI could effectively activate oxygen and reacted with toluene, therefore, PtTi–D catalyst exhibited better catalytic performance than PtTi–I. As to PtTi–P catalyst, Pt4+ ions captured the photogenerated electrons and then were reduced to Pt particles. Hence, the Pt particles would be selectively deposited on electro-rich surface of TiO2. Since the Pt deposited first was electro–richer than TiO2, Pt4+ in the solution would capture the electron on Pt and deposited on it. Therefore, the size of Pt particles on PtTi-P was larger than that on PtTi–I and PtTi–D. It must be emphasized that all
Fig. 5. Toluene conversion for prepared PtTi catalysts.
formed MSI in PtTi–P is effective in oxygen activation. Therefore, it exhibited a much higher toluene total oxidation activity than PtTi–I and PtTi–D. 4. Conclusions In this study, three PtTi catalysts were prepared by impregnation, deposition precipitation and photo-deposition to compare the effect of preparation method on the toluene oxidation activity of PtTi catalysts.
Fig. 4. O2-TPD patterns (a) and EPR spectra (b) for samples. 98
Chemical Physics Letters 730 (2019) 95–99
Y. Zeng, et al.
Scheme 1. Schematic illustration of the proposed formation mechanism of catalysts and reaction process of toluene oxidation.
Appendix A. Supplementary material
The Pt particles on PtTi–I prepared by impregnation possessed a narrow size distribution of 1.6–2.4 nm. While high reducing temperature resulted in the sintering of TiO2 and the agglomeration of Pt particles. Moreover, it also induced the encapsulation of TiO2 on Pt particles. This encapsulation enhanced the MSI between Pt and TiO2 in PtTi–I due to the high contact area, but it also inhibited the adsorption and activation of oxygen over Pt particles, which suppressed the activity of PtTi–I in total oxidation of toluene. The mild deposition precipitation prepared PtTi–D exhibited a very small Pt particle, but the existence of ineffective interaction between Pt and TiO2 suppressed its activity. As to PtTi–P catalyst, the Pt particles would be selectively deposited on electro–rich surface of TiO2, thus all Pt particles on PtTi–P are effective in oxygen activation. The catalytic activity test revealed that the activity of PtTi catalyst was positively correlated with the ability in oxygen activation.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.05.048. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Declaration of Competing Interest
[10]
There are no conflicts to declare.
[11] [12] [13] [14]
Acknowledgements This work was financially supported by the Key Project of Chinese National Programs for Research and Development (2016YFC0203800), the National Natural Science Foundation of China (51578288), Industry-Academia Cooperation Innovation Fund Projects of Jiangsu Province (BY2016004–09), Jiangsu Province Scientific and Technological Achievements into a Special Fund Project (BA2015062, BA2016055 and BA2017095), Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_0445), Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Key Laboratory under Construction for Volatile Organic Compounds Controlling of Jiangsu Province.
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
The effect of preparation method on oxygen activation over Pt/TiO2 catalysts for toluene total oxidation ⁎
Yiqing Zenga, Yanan Wanga, Yahan Menga, Fujiao Songb,c, Shule Zhanga, , Qin Zhonga,
T
⁎
a
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China School of Environmental Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, PR China c Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, PR China b
H I GH L IG H T S
supported Pt catalysts were prepared by various preparation methods. • TiO method affected the catalytic activity in toluene total oxidation. • Preparation • Oxygen activation was a key factor to determine the catalytic activity. 2
A R T I C LE I N FO
A B S T R A C T
Keywords: PtTi catalyst Preparation method Oxygen activation Toluene oxidation
Herein, we compared the catalyst formation process of three Pt/TiO2 catalysts prepared by impregnation (PtTi–I), deposition precipitation (PtTi–D) and photo-deposition (PtTi–P) to investigate its effect on oxgyen activation and toluene oxidation activity. The encapsulation of TiO2 on Pt particles induced by high temperature strengthened the metal–support interaction (MSI) but inhibited the activation of oxygen over PtTi–I catalyst, thus it showed the poorest activity in toluene oxidation. Since the selective deposition of Pt on certain face of TiO2 during photo-deposition process, PtTi–P exhibited stronger MSI and higher activity although it possessed much larger Pt particles than PtTi–D.
1. Introduction Most volatile organic compounds (VOCs) can lead to the decomposition of tropospheric ozone, photochemical smog in atmosphere and cause various diseases threat to human life and health, thus, developing effective VOCs elimination techniques is of great significance and urgent [1–3]. Nowadays, recovery methods (adsorption, condensation, absorption and membrane) and destruction techniques (incineration, catalytic oxidation, plasma catalysis and biodegradable) have been applied for abatement VOCs [1]. Among them, catalytic oxidation is known as one of the most effective method due to the features of low cost and minimized secondary pollutions. Supported noble metals (Pt, Rh, Pd) catalysts, as the most effective catalyst, are widely used in total oxidation of VOCs [2–5]. Among them, Pt supported on TiO2 (PtTi) is one of the most utilized and studied catalyst. The oxidation of VOCs over noble metals over Pt based catalyst may process via Mars–van Krevelen (MVK) mechanism [6,7], Langemuir–Hinshelwood (L–H) mechanism [8,9] (reaction with chemisorbed reactant) or Eley–Rideal E–R mechanism (reaction with gaseous ⁎
reactant) [10]. The validity of each mechanism strongly depends on the catalyst (noble metal and nature of the support) as well as on the character of the VOC molecule and it is difficult to be generalized [2]. Even though, it has been proved that the total oxidation of toluene over Pt based catalyst strongly relies on the chemisorbed oxygen on the Pt particles [10–12]. Hence, many methods have been developed for enhancing the oxygen activation ability of PtTi. The strong metal support interaction (SMSI) is first discovered by Tauster in 1978 based on the fact that hydrogen reduced noble metal supported on TiO2 catalysts lose it H2 and CO adsorption ability [13]. But it can promote the transfer of electrons from metal oxide supports to Pt nanoparticles and affect the unfilled d band vacancies of Pt, so that the adsorption, dissociation and desorption of oxygen species can be facilitated [14,15]. Zhang et al. proved that higher temperature can induce higher degree SMSI in PtTi, while excessive temperature will decrease the performance in oxygen activation because of encapsulation effect [16]. Therefore, mild methods are developed to synthesize PtTi catalyst. Rui et al. found that the PtTi obtained by mild solution reduction process exhibits higher degree SMSI and better toluene oxidation activity than that prepared by
Corresponding authors. E-mail addresses: [email protected] (S. Zhang), [email protected] (Q. Zhong).
https://doi.org/10.1016/j.cplett.2019.05.048 Received 12 April 2019; Received in revised form 9 May 2019; Accepted 28 May 2019 Available online 29 May 2019 0009-2614/ © 2019 Published by Elsevier B.V.
Chemical Physics Letters 730 (2019) 95–99
Y. Zeng, et al.
60 mL·min−1 and 0.3 g sample was used. The TDP experimental began with pre-treating sample with 3 vol%O2/N2 at 50 °C for 60 min, and then the gas was switched to He for 30 min. Subsequently, O2–TPD was performed by ramping the temperature at 10 °C·min−1 to 800 °C in He flow.
H2 reduction at high temperature [17]. Wong and his co–workers pointed that photo–deposition method can also induce high degree SMSI in PtTi catalyst and lead to the high performance in oxygen reduction reaction in acid medium [18]. The most used preparation method of PtTi mainly included wet impregnation with H2 reduction, deposition–precipitation with reducing agent (such as NaBH4, N2H4·H2O and HCHO) reduction and photo–deposition. The comparation study of two preparation method had been widely reported. As our best known, except for the study [19] reported by Leung groups that investigating the effect of three reduction processes (NaBH4, H2 and photo-deposition reduction) on impregnation of PtTi catalyst for HCHO oxidation at room temperature, little study had been done to analyse the effect of these three preparation method on the catalytic activity of PtTi catalyst. In this study, we compared the PtTi (PtTi–I, PtTi–D and PtTi–P) catalysts prepared by impregnation (PtTi–I), deposition precipitation (PtTi–D) and photo–deposition (PtTi–P) from the catalyst formation process and catalytic activity in toluene oxidation. Combined with the results of TEM, BET, XRD, XPS, UV–vis, O2–TPD and EPR, it was revealed that preparation method naturally affected the structural characteristics, surface properties of PtTi catalysts. These features dominated the oxygen activation process, which plays a key role in toluene total oxidation. We expected this study could provide some inspiration in the design of noble metal based catalysts.
2.3. Catalytic activity test Total oxidation of toluene was carried out at 130–190 °C in a quartz fixed-bed reactor (i,d, 5 mm) using 0.3 g catalyst with 40–80 mesh under atmospheric pressure. The typical reactant gas composition was as follows: 500 ppm toluene, 20 vol% O2 and N2 as the balance gas. Total flow rate was 250 mL·min−1, corresponding to a space velocity of about 40,000 h−1. The concentration of toluene in the inlet and outlet gas was measured by on-line gas chromatography equipped with a TCD and two FID (one for analysis of organics and another for determine of CO2 and CO via methanation reactor) detectors. CO2 and H2O could be detected as the only products, so the toluene conversion was calculated by follow equation:
(
Toluene conversion (%) = n outlet
CO2
− nintlet
)/7ntoluene × 100%
CO2
where nCO2 is the molar flow of CO2 at the outlet and ntoluene is the molar flow of toluene at the inlet.
2. Experimental
3. Results and discussion
2.1. Catalyst preparation
The TEM images and particles size distribution of PtTi sample are presented in Fig. 1. The size of 100 TiO2 nanoparticles and 50 Pt nanoparticles were counted for obtaining the histogram of particle size distribution for each sample. It can be noted that the TiO2 particles size in PtTi–I was greater than that in PtTi–D and PtTi–P, inferring that TiO2 support was prone to sintering during hydrogen reduction process in comparison with photodeposition and mild solution reduction process. For Pt particles over PtTi–I, their size located at a small range of 1.6–2.4 nm. And it was worth noting that Pt nanoparticles could only be found on a portion of the TiO2 particles, which suggested that Pt nanoparticles were tended to agglomeration at high temperature hydrogen reduction process. Moreover, from the HRTEM images shown in Fig. S1, the lattice line of TiO2 could be observed next to Pt nanoparticles. This well consistent with the reported phenomenon that high temperature reducing atmosphere can lead to the transfer of TiO2 onto the surface of Pt0 particles and then encapsulate them [16,20]. For PtTi–D sample, there no obvious Pt particles could be seen. The Pt content obtained from ICP-MS for PtTi–I, PtTi–D and PtTi–P respectively were 1.02 wt%, 0.95 wt% and 0.92 wt%, which close to the theoretical value. Thus, the Pt particle on PtTi–D was too small to detect. As to PtTi–P, its displayed the largest Pt particles with a size of 2.8–5.7 nm. Fig. 2 displays the XRD patterns of catalysts. All catalysts displayed a typical P25 TiO2 diffraction peaks, which composed with anatase (PDF#21–1272) and few rutile (PDF#21–1276). No diffraction peaks of Pt could be observed, inferring that Pt species were well dispersed on TiO2 or the Pt species were too small to be detected by XRD. Notably, all peaks of PtTi catalysts presented the same intensity, which were lower than that of TiO2 supported. This result showed that the interaction between Pt nanoparticles and TiO2 affected the near surface structure of TiO2. To investigate the influence of Pt loading on the specific surface area, the BET was carried out. As listed in Table 1, PtTi–I had the smallest specific surface area, while TiO2, PtTi–D and PtTi–P exhibited a specific surface area of 54 ± 1 m2·g−1, indicating that Pt loading had no obvious effect on the specific surface area via mild preparation method. The smallest specific surface area of PtTi–I could be ascribed to the agglomeration of TiO2 support, which was consistent with the results observed in TEM images. Fig. 3a shows the Pt 4f XPS spectra. Clearly, the binding energy of Pt
TiO2 was purchased from Degussa (P25) and other analytical pure chemicals were purchased from Aladdin and used without any treatment. 1 wt%Pt/TiO2 catalysts were prepared by impregnation, deposition precipitation and photo-deposition methods, respectively. In a typical impregnation process, 2 g TiO2 (P25, Degussa) was added into 60 mL stoichiometric H2PtCl6 solution under stirring, and then the solvent was removed by evaporation at 100 °C. Finally, PtTi–I (impregnation) was obtained after drying at 80 °C and reducing at 450 °C (To avoid phase transformation from anatase TiO2 into rutile TiO2 [15]) for 2 h in 10 vol%H2/Ar atmosphere. For light deposition reduction process, 2 M NaOH solution was dropped into the mixture of 2 g TiO2 and H2PtCl6 solution under vigorous stirring until the pH = 12, then 10 mL 50 wt% hydrated hydrazine solution was added. After reduction for 2 h, the solid was washed by filtration for three times, and then PtTi–D (deposition precipitation) catalyst was obtained after dried at 80 °C for 6 h in vacuum oven. As to photo-deposition process, it was performed in a quartz glass reactor under N2 atmosphere. Generally, 2 g TiO2 and 20 mL methanol was added into 180 mL stoichiometric H2PtCl6 solution under stirring, then mixture was irradiated by UV light (CEL-HXUV 300, wavelength < 300 nm) for 2 h. Finally, PtTi–P (photo–deposition) was got via filtration, washing for three times and dried at 80 °C for 6 h in vacuum oven. 2.2. Characterization The catalysts were characterized by X–ray diffraction measurements (XRD Purkinje General Instrument Cu, Ltd, China, XD–3), Inductively coupled plasma (ICP, Varian ICP 720), Brunauer–Emmett–Teller (BET Quadrasorb-S1, Quantachrome, USA), High–resolution transmission electron microscope (HRTEM, JEOL JEM2100, 100 kV), UV–vis spectrophotometer (UV–2550 Shimadzu), X–ray photoelectron spectroscopy (XPS, PHI–5000C, ESCA) and Electron paramagnetic resonance (EPR EMX–10/12–type, Bruker). Oxygen temperature-programmed desorption (O2–TPD) experiment was performed on an automated chemisorption analyzer (Quantachrome Instruments), equipped with thermal conductivity detector (TCD). During experiment, all gaseous flow rates were 96
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Fig. 1. The TEM images of PtTi–I (a and d), PtTi–D (b and e) and PtTi–P (c and f).
was used and the results are presented in Fig. 3b. Obviously, almost no absorbance could be seen in 400–700 nm for TiO2 because the standard band gap of anatase and rutile are 3.2 eV and 3.0 eV, respectively [23]. After loading with Pt nanoparticles, the absorbance at the range of visible light was significantly enhanced and followed the order of PtTi–P > PtTi–I > PtTi–D. These results indicated that the transfer of electron from TiO2 to Pt nanoparticles on PtTi–P was more effective than that on PtTi–I and PtTi–D. To investigate the oxygen activation over PtTi catalysts, Hence, O2–TPD and EPR was performed. As can be seen from Fig. 4a, all prepared samples displayed significant oxygen desorption peaks above 200 °C. Among these, the oxygen desorption peak below 500 °C could be attributed to the desorption of surface chemisorbed oxygen [24]. Notably, the desorption peak area of PtTi–P below 500 °C followed the PtTi–P > PtTi–D > PtTi–I order. These indicated that the surface chemisorbed oxygen capacity of the three samples is in an order of PtTi–P > PtTi–D > PtTi–I. Oxygen molecular adsorbed on oxygen vacancies can be reduced to superoxide radical (active oxygen) by capture one electron, which can be detected by EPR [25]. Hence, to further explore the ability of samples in activating oxygen, the samples were pre-treated at 100 mL/min of air at 160 °C for 1 h and their EPR was measured. As displayed in Fig. 4b, the signal at g = 2.003 due to superoxide radical was observed in the EPR spectrum of each samples [26]. The peak intensity followed PtTi–P > PtTi–D > PtTi–I, which indicated that the ability of the three catalysts in oxygen activation is: PtTi–P > PtTi–D > PtTi–I. The catalytic activity of prepared samples for toluene was tested, and the results are shown in Fig. 5. All PtTi catalysts displayed a high catalytic activity in total oxidation of toluene. The toluene conversion of sample at the range of 140–170 °C decreased with a sequence of PtTi–P > PtTi–D > PtTi–I, and the lowest temperature of 100% toluene conversion for them were 160, 170 and 180 °C, respectively. From the results of XPS, UV–vis, O2-TPD and EPR, the ability of the three catalysts in oxygen activation was positively related with their
Fig. 2. XRD patterns of catalysts. Table 1 BET data for samples. Samples
Surface area (m2 g−1)
pore volume (cm3 g−1)
pore size (nm)
TiO2 PtTi–I PtTi–D PtTi–P
54.5 48.6 53.6 55.1
0.262 0.287 0.261 0.264
17.0 21.6 16.8 17.0
4f for catalysts increased in the order of PtTi–D, PtTi–P and PtTi–I. It is well established that lower binding energy corresponds to higher electronic cloud density [21]. Therefore, the electronic cloud density of PtTi catalysts followed the order of PtTi–D > PtTi–P > PtTi–I. In the design of photocatalyst, noble metals are used as electron mediator of TiO2 and UV–vis spectrum is an effective way to display the electron transfer ability from TiO2 to noble metal [22]. Thus, UV–vis spectrum 97
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Fig. 3. The Pt 4f XPS (a) and UV–vis (b) spectra of catalysts.
catalytic activity in toluene oxidation. Therefore, the adsorption and activation of oxygen molecular on Pt based catalyst were the controlling step for total oxidation of toluene and this result well consistent with previous studies [10–12]. Based on above analysis and previous studies [10–12], the proposed reaction process and formation mechanism of catalysts is presented in Scheme 1. For PtTi–I catalyst, the Pt4+ ions were reduced to Pt by H2 at high temperature on TiO2. At the same time, high temperature induced the encapsulation of TiO2 on Pt particles. This encapsulation enhanced the MSI between Pt and TiO2 in PtTi–I due to the high contact area, but inhibited the adsorption and activation of oxygen over Pt particles, which suppressed the activity of PtTi–I in total oxidation of toluene. During the formation process of PtTi-D catalyst, Pt4+ ions was deposited on TiO2 in the form of Pt(OH)4, then it was reduced to Pt by the follow added hydrazine. The size of obtained Pt particle was small and it could not be coated by TiO2 because of mild solution reduction process. Since the deposition position of Pt was unselective, thus, it is impossible for all Pt particles to form effective MSI (that is, TiO2 can effectively supply elections to Pt particles) with TiO2 support (shown in UV–vis). However, the formed effective MSI could effectively activate oxygen and reacted with toluene, therefore, PtTi–D catalyst exhibited better catalytic performance than PtTi–I. As to PtTi–P catalyst, Pt4+ ions captured the photogenerated electrons and then were reduced to Pt particles. Hence, the Pt particles would be selectively deposited on electro-rich surface of TiO2. Since the Pt deposited first was electro–richer than TiO2, Pt4+ in the solution would capture the electron on Pt and deposited on it. Therefore, the size of Pt particles on PtTi-P was larger than that on PtTi–I and PtTi–D. It must be emphasized that all
Fig. 5. Toluene conversion for prepared PtTi catalysts.
formed MSI in PtTi–P is effective in oxygen activation. Therefore, it exhibited a much higher toluene total oxidation activity than PtTi–I and PtTi–D. 4. Conclusions In this study, three PtTi catalysts were prepared by impregnation, deposition precipitation and photo-deposition to compare the effect of preparation method on the toluene oxidation activity of PtTi catalysts.
Fig. 4. O2-TPD patterns (a) and EPR spectra (b) for samples. 98
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Scheme 1. Schematic illustration of the proposed formation mechanism of catalysts and reaction process of toluene oxidation.
Appendix A. Supplementary material
The Pt particles on PtTi–I prepared by impregnation possessed a narrow size distribution of 1.6–2.4 nm. While high reducing temperature resulted in the sintering of TiO2 and the agglomeration of Pt particles. Moreover, it also induced the encapsulation of TiO2 on Pt particles. This encapsulation enhanced the MSI between Pt and TiO2 in PtTi–I due to the high contact area, but it also inhibited the adsorption and activation of oxygen over Pt particles, which suppressed the activity of PtTi–I in total oxidation of toluene. The mild deposition precipitation prepared PtTi–D exhibited a very small Pt particle, but the existence of ineffective interaction between Pt and TiO2 suppressed its activity. As to PtTi–P catalyst, the Pt particles would be selectively deposited on electro–rich surface of TiO2, thus all Pt particles on PtTi–P are effective in oxygen activation. The catalytic activity test revealed that the activity of PtTi catalyst was positively correlated with the ability in oxygen activation.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.05.048. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Declaration of Competing Interest
[10]
There are no conflicts to declare.
[11] [12] [13] [14]
Acknowledgements This work was financially supported by the Key Project of Chinese National Programs for Research and Development (2016YFC0203800), the National Natural Science Foundation of China (51578288), Industry-Academia Cooperation Innovation Fund Projects of Jiangsu Province (BY2016004–09), Jiangsu Province Scientific and Technological Achievements into a Special Fund Project (BA2015062, BA2016055 and BA2017095), Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_0445), Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Key Laboratory under Construction for Volatile Organic Compounds Controlling of Jiangsu Province.
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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