Chinese Chemical Letters  2018, Vol. 29 Issue (6): 787-790   PDF    
Pt nanocrystallines/TiO2 with thickness-controlled carbon layers: Preparation and activities in CO oxidation
Man Zhoua,b, Muhong Lia, Chujun Houb, Zhongyu Lib, Yongzheng Wanga, Kun Xianga, Xuefeng Guoa    
a Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China;
b School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China
Abstract: In this work, a series of Pt nanocrystallines (Pt NCs) supported on TiO2 substrate with controlled thickness of carbon layers (C-Pt/TiO2) were synthesized. Well-dispersed Pt NCs were facilely synthesized at room temperature by a photo-reduction process in lytropic liquid crystal (LCs). Surface tuning of the carbon layers on Pt/TiO2 catalysts was achieved by varying the calcination atmospheres (in argon, air, and oxygen) and characterized by XPS and HRTEM. The influence of the coated carbon layers on the catalytic activity of catalysts is investigated by CO oxidation reaction which presented the following ranks:C-Pt/TiO2-O2 > C-Pt/TiO2-Air > C-Pt/TiO2-Ar. It is found that the carbon layer coating can stabilize the Pt NCs and enable them anti-sintering at high temperature. This finding provides new insight into understanding the C-Pt/TiO2 ternary system for tuning their catalytic performance.
Key words: Photoreduction     Pt nanocrystallines     Carbon layers     CO oxidation     Anti-sintering    

Carbon monoxide (CO), recognized as a toxic pollutant on blood and nervous system, caused lots of harmful effects on the environment and human health. Catalytic oxidation of CO into carbon dioxide (CO2) is one of the most effective method to remove this toxic pollutant [1, 2]. Meanwhile, CO oxidation was generally used as a prototypical reaction to fundamentally investigate the heterogeneous catalytic mechanism and performance of the catalyst [3-6]. Many efforts have been devoted to the synthesis of supported noble metal (e.g., Au, Ag, Ru, and Pt) nanostructures. Among various nanocatalysts, TiO2 supported Pt nanostructures including Pt nanoparticles, Pt nanoclusters, and single Pt atoms attract increasing attention because of their outstanding catalytic activities. Over the past decades, different methods of preparing Pt/TiO2 catalysts have been reported including sol-gel [7], impregnation [8], ultrasonic chemical method [9], and microwave assisted sol-gel route [10]. Due to high-energy facets and strong metal-support interaction (SMSI), supported Pt nanocatalysts cover lots of applications including catalytic oxidations, hydrogenations, and hydrocarbon rearrangement reactions [11]. Practical applications always require precise designing and synthesis of Pt nanoparticles with tunable size, shape, and surface state [12-15]. And more importantly, the highly active Pt nanoparticles need to be stable during various catalytic reactions. However, the highly active Pt nanoparticles on the supports are easy to agglomerate into larger particles via surface diffusion and decrease the catalytic activity in relatively high temperature conditions. Thus, researchers developed several methods to protect the surface of Pt active sites to avoid the agglomeration of the active nanoparticles during the catalytic reaction process. For example, to obtain a sinter-resistant catalytic system, Xia et al. reported a ternary catalytic system fabricated by SiO2 deposition onto the TiO2 surface and the Pt nanoparticles surface [16]. Almana et al. designed Pt@SiO2 core-shell nanoparticles with controlled thickness of SiO2 from 1 nm to 15 nm which has apparent influence on CO oxidation [17]. So far, the fabrication of controlled Pt/TiO2 with protection structures still remains challenging.

Recently, inducing non-metal elements (e.g., C, N and S) onto TiO2 is another common modification to adjust the surface conditions. Typically, modification by carbonaceous materials such as carbon dots (CDs) [18], carbon nanotubes (CNTs) [19], and graphene nanosheets (GR) [20] has attracted great attention due to their high electrical conductivity and thermal stability. As for noble metal-TiO2 system, only a few papers have been published on the constructing of protecting carbon layers on the noble metal-TiO2 system due to lacking of effective methods [21]. Therefore, the development of a general synthetic strategy capable of controlling the surrounding environment of noble metal active sites on TiO2 is highly desirable.

In this communication, we present a novel and thicknesscontrolled route for the synthesis of carbon protected Pt nanocrystallines (NCs) supported on TiO2 (C-Pt/TiO2) with lyotropic liquid crystals (LCs). Herein, soft LCs system is extended for the photochemical synthesis of three different kinds of catalysts: Pt NCs with thick carbon layers (C-Pt/TiO2-Ar), Pt NCss with thin carbon layers (C-Pt/TiO2-Air), and Pt NCs with relatively clean surface (C-Pt/TiO2-O2). The roles of carbon layers upon the Pt/TiO2 nanocatalyst were investigated through CO oxidation reactions. It is found that the appropriate carbon layer can stabilize the highly active Pt NCs resisting grain growth.

Recent advances in colloid chemistry have enabled the design of Pt nanoparticles with tunable size, shape, and metal composition. However, for the wet chemical reducing methods, producing highly dispersed colloidal Pt nanoclusters was still a complex and time-consuming process. Herein, via a facile "photo-reduction" process (Scheme 1a), ultrathin Pt nanocrystallines were prepared within LCs. Firstly, LCs system consist of lamellar bilayer membranes was formed by water, Tween 40 (polyoxyethylene sorbitan monopalmitate), and CSA (camphorsulfonic acid) (Scheme 1b). More details can be checked in Supporting information. As reported by us earlier [22, 23], the organic LCs acted as both protective agent and photo-reducing agent for Pt NCs. After being irradiated by UV light (250 W), an appropriate amount of H·/OH· radicals were emerged in water layers and then attacked the –CH2OH groups of Tween 40 to produce –CH·OH radicals which had strong reducing properties. Then, [PtCl6]2- precursors were reduced to Pt00 nanoclusters which were immediately protected by the surrounding LCs. Indeed, Fig. 1a shows a typical transmission electron microscopy (TEM) image at low magnification of the Pt@LCs after being irradiated under UV light for 2 h, in which plenty of ultrathin Pt NCs were obtained. It should be noted that almost all the nanocrystallines are monodispersed due to the capping effect of LCs as shown in an enlarged TEM image (Fig. 1b). High resolution transmission electron microscopy (HRTEM) image of Pt@LCs reveals the lattice spacings of 0.227 nm and 0.198 nm according to the (111) and (200) lattice planes of metallic Pt (Fig. 1c). The size distribution of Pt NCs derived from large amount of TEM measurements was shown in Fig. 1d, the average size of Pt NCs was about 2 nm with a narrow size distribution. Curve fitting of size distribution histograms is calculated by Gauss model. In this curve, average size of Pt NCs is 2.004 nm and the correlation coefficient is 0.9992 which means size distribution histograms approximate a standard normal distribution. (Fig. 1d)

Download:
Scheme 1. (a) Schematic diagram of photochemical synthesis of three catalysts with different thickness of carbon layers: C-Pt/TiO2-Ar, C-Pt/TiO2-Air, and C-Pt/TiO2-O2 (b) Structural formulas of Tween 40 and CSA within LCs templates.

Download:
Fig. 1. Typical TEM images of Pt NCs@LCs at low magnification, scale bar:100 nm (a) and high magnification, scale bar: 10 nm (b); (c) HRTEM image of individual Pt NCs; (d) size distribution histograms of Pt NCs calculated from corresponding TEM images, curve fitting of size distribution histograms by Gauss model.

Pt NCs@LCs was then mixed with commercial Degussa P25 TiO2 (particle size, ~25 nm; Brunauer-Emmett-Teller surface area, 48.5 m2/g; anatase/utile phase ratio, 4/1) in ethanol solution (~20 wt%) at room temperature (Figs. 2ac). LCs was employed as the in-situ carbon precursor on the surface of Pt/TiO2, followed by carbonization under Ar atmosphere at 773 K to yield 3 ~ 5 nm thick carbon layers (remarked as red area in Fig. 2d). During the calcination in Ar, the Pt NCs slightly grew up from ~2 nm to ~3 nm. The as-synthesized hybrid material C-Pt/TiO2-Ar was further calcined at 773 K under atmospheres of static air for one hour to obtain C-Pt/TiO2-Air. The thickness of outside carbon layers was reduced to ~1 nm via limited oxidation (thin carbon layers, remarked as green area in Fig. 2e). Compared the typical TEM images of C-Pt/TiO2-Ar and C-Pt/TiO2-Air, however, it is clear that the Pt NCs are almost the same size of ~3 nm, indicating no further growth of Pt NCs during the calcination of C-Pt/TiO2-Ar in air. When the calcination atmosphere of C-Pt/TiO2-Ar changed from air to oxygen, the carbon layers upon the surface of Pt/TiO2 were almost removed via oxidation in O2 for 6 h, the clean Pt nanocrystallines were obtained (Fig. 2f). Surprisingly, the size of Pt NCs in the obtained C-Pt/TiO2-O2 was still almost the same as those in C-Pt/TiO2-Ar and C-Pt/TiO2-Air, showing excellent antisintering properties. In contrast to the inevitable growth of Pt NCs in liquid crystalsduring the first calcination in Ar, the critical roleof solid carbon layers in stabilizing Pt NCs during the further calcination processes was confirmed by the typical HRTEM images (Fig. 2). No obvious aggregation or sintering of Pt nanoparticles was observed for all the three samples, and the average size of Pt NCs remained ~3nm in the three samples.

Download:
Fig. 2. (a-c) Representative TEM images and (d-f) HRTEM images of C-Pt/TiO2-Ar, CPt/TiO2-Air and C-Pt/TiO2-O2 catalysts; red area denotes thick carbon layers, green area denotes thin carbon layers; (h-j) schematic diagram of three catalysts with different thickness of carbon layers.

Figs. 3ac show a series of C 1s spectra for the Pt/C-TiO2-Ar, Pt/ C-TiO2-Air and Pt/C-TiO2-O2 catalysts by X-ray photoelectron spectroscopy (XPS). In all three samples, the peak at 284.6eV corresponds to C-C, and the peaks at 285.4eV and 288.4eV were attributed to the carbon species contained in C-OR and C=O, respectively. The C-OR groups mainly derived from the liquid crystals precursor of Tween 40. The peak at 288.4eV corresponding to C=O emerged in in Fig. 3b indicated that some carbon species contained in C-OR were oxidized to C=O during the calcination in static air atmosphere. After being calcined in O2 for 6h, most of the carbon species were removed, based on the largely reduced C 1s peaks shown in Fig. 3c. A few carbon species corresponding to C-C and C-OR, however, still existed after the longtime calcination in O2 for 6h. Based on the results of TEM/ HRTEM and XPS analysis, structure schematic diagrams for the three catalysts were shown in Figs. 2hj, respectively. As mentioned above, the hard carbon layers covered on the surface of Pt/TiO2 play pivotal role in the stabilization and anti-sintering of Pt NCs. The Pt/C-TiO2-O2 show extraordinary stability even after longtime/high-temperature calcination in O2 at 773K for 6h. Thus, a strong interaction between Pt NCs, residual carbon species and TiO2 may contribute to the extraordinary anti-sintering ability. To confirm it, a further calcination of Pt/C-TiO2-O2 in O2 at 773K for 6h was carried out. The corresponding catalysts were denoted as C-Pt/TiO2-O2-1 and C-Pt/TiO2-O2-2. The typical HRTEM image of the obtained Pt/C-TiO2-O2-2 was shown in Fig. 4b. Compared to the size of Pt NCs in Pt/C-TiO2-O2-1 (HRTEM image shown in Fig. 4a), the Pt NCs in Pt/C-TiO2-O2-2 remained unchanged, without aggregation or sintering, showing outstanding stability and anti-sintering ability.

Download:
Fig. 3. C 1s XPS spectra of C-Pt/TiO2-Ar (a), C-Pt/TiO2-Air (b) and C-Pt/TiO2-O2 (c) catalysts.

Download:
Fig. 4. HRTEM images of Pt/C-TiO2-O2 catalysts treated after once calcination (in O2 for 6h at 773K) and twice calcination.

The activities of these catalysts were tested in a fixed-bed reactor system (more details can be seen in Supporing information). The temperature of CO complete oxidation (TOC) on C-Pt/ TiO2-Ar was 160 ℃ as shown in Fig. 5. After being calcined in air atmosphere, C-Pt/TiO2-Air with thin carbon layers exhibited much higher activity (with TOC of 120 ℃) than that C-Pt/TiO2-Ar with thick carbon layers, which was denoted as arrow (a). Obviously, it indicated that more active sites exposed to the feed gas of CO and O2, which leads to bettercatalytic performance. When changed the atmosphere from Air into O2, the TOC further decreased from 120 ℃ to 90 ℃, which was denoted as arrow (b). This result suggested that calcination in O2 provided a cleaner surface of Pt NCs than that treated in static air which was in well accordance with XPS and HRTEM results. Interestingly, calcination in O2 once and twice exhibited different results. As the orange line (C-Pt/TiO2-O2-2) in Fig. 5 shown, CO completely oxidized at 80 ℃ which was the lowest TOC in all four catalysts, as arrow (c) showed. The highest activity of C-Pt/TiO2-O2-2 further confirmed that the obtained catalyst possesses both clean surface of Pt NCs and extraordinary anti-sintering ability of ~3nm Pt NCs.

Download:
Fig. 5. CO conversion on C-Pt/TiO2-Ar, C-Pt/TiO2-Air and C-Pt/TiO2-O2-1, C-Pt/TiO2-O2-2 catalysts.

In conclusion, we have demonstrated a facile method for in situ synthesis of ultrathin and well-dispersed Pt NCs in the lyotropic liquid crystals through UV light photoreduction. Pt NCs/ TiO2 with controlled thickness of carbon coating layers were synthesized via different atmospheres and periods of calcination. C-Pt/TiO2-O2-1 obtained by calcination in O2 possesses ~3 nm Pt NCs with clean surface and outstanding anti-sintering ability, which showed the highest catalytic activity for CO oxidation. Strong interaction between Pt NCs, residual carbon species and TiO2 may contribute to the extraordinary anti-sintering ability of Pt NCs. This work highlight the importance of carbon layers in fabrication of highly active nanoparticles with excellent antisintering ability.

Acknowledgments

This work was financially supported by the National Key Technology R & D Program of China (No. 2017YFB0310704), the National Natural Science Foundation of China (Nos. 21773112, 21173119 and 21303083), Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (No. 17KJB150001), the Natural Science Foundation of Jiangsu Province (No. BK20130563), and the Fundamental Research Funds for the Central Universities.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2018.03.010.

References
[1]
W.C. Zhan, J.L. Wang, H.F. Wang, et al., J. Am. Chem. Soc. 193 (2017) 8846-8854.
[2]
K. Taira, K. Nakao, K. Suzuki, H. Einaga, Environ. Sci. Technol. 50 (2016) 9773-9780. DOI:10.1021/acs.est.6b01652
[3]
H. Jeong, J. Bae, J.W. Han, H. Lee, ACS Catal. 7 (2017) 7097-7105. DOI:10.1021/acscatal.7b01810
[4]
R.B. Zhang, K. Lu, L.J. Zong, et al., Appl. Surf. Sci. 416 (2017) 183-190. DOI:10.1016/j.apsusc.2017.04.158
[5]
L. Wang, C.H. Pu, Y.F. Cai, et al., Fuel Process. Technol. 160 (2017) 152-157. DOI:10.1016/j.fuproc.2017.02.037
[6]
L.P. Zeng, K.Z. Li, H. Wang, et al., J. Phys. Chem. C 121 (2017) 12696-12710. DOI:10.1021/acs.jpcc.7b01363
[7]
H.B. Zhang, C.H. Ma, Y. Li, et al., Appl. Catal. A:Gen. 503 (2015) 209-217. DOI:10.1016/j.apcata.2015.07.006
[8]
H.B. Huang, D.Y.C. Leung, D.Q. Ye, J. Mater. Chem. 21 (2011) 9647-9652. DOI:10.1039/c1jm10413f
[9]
M. Sivakumar, A. Towata, K. Yasui, et al., Ultrason. Sonochem. 17 (2010) 213-218. DOI:10.1016/j.ultsonch.2009.06.019
[10]
R. Hernández, S.M. Duróntorres, K. Esquivel, et al., Mol. Ecol. Resour. 5 (2017) 90-92.
[11]
A. Chen, P. Holt-Hindle, Chem. Rev. 110 (2010) 3767-3804. DOI:10.1021/cr9003902
[12]
P. Panagiotopoulou, A. christodoulakis, D.I. Kondarides, et al., J. Catal. 240 (2006) 114-125. DOI:10.1016/j.jcat.2006.03.012
[13]
P.K. Dahlstrom, D.A. Harrington, F. Seland, Electrochim. Acta 82 (2012) 550-557. DOI:10.1016/j.electacta.2012.04.150
[14]
E.I. Vovk, A.V. Kalinkin, M.Y. Smirnov, et al., J. Phys. Chem. C 121 (2017) 17297-17304. DOI:10.1021/acs.jpcc.7b04569
[15]
K. An, S. Alayoglu, N. Musselwhite, et al., J. Am. Chem. Soc. 135 (2013) 16689-16696. DOI:10.1021/ja4088743
[16]
P. Lu, C.T. Campbell, Y.N. Xia, Nano Lett. 13 (2013) 4957-4962. DOI:10.1021/nl4029973
[17]
N. Almana, S.P. Phivilay, P. Laveille, et al., J. Catal. 340 (2016) 368-375. DOI:10.1016/j.jcat.2016.06.002
[18]
Y.M. Dong, J.J. Zhang, P.P. Jiang, et al., New J. Chem. 39 (2015) 4141-4146. DOI:10.1039/C5NJ00012B
[19]
D.J. Guo, X.P. Qiu, L.Q. Chen, et al., Carbon 14 (2009) 1680-1685.
[20]
B.Y. Xia, B. Wang, H.B. Wu, et al., J. Mater. Chem. 22 (2012) 16499-16505. DOI:10.1039/c2jm32816j
[21]
W.C. Zhan, Q. He, X.F. Liu, et al., J. Am. Chem. Soc. 138 (2016) 16130-16139. DOI:10.1021/jacs.6b10472
[22]
M. Zhou, M. Lin, L. Chen, et al., Chem. Commun. 51 (2015) 5116-5119. DOI:10.1039/C4CC10040A
[23]
M. Zhou, M. Lin, Y.Z. Wang, et al., Chem. Commun. 51 (2015) 11841-11843. DOI:10.1039/C5CC03974F