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TiO2 composite nanoparticles synthesized by electron beam irradiation for preferential CO oxidation
Materials Research Bulletin 48 (2013) 1347–1351
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Pt/TiO2 composite nanoparticles synthesized by electron beam irradiation for preferential CO oxidation Satoru Kageyama a,*, Yoshitsune Sugano b, Yukihiro Hamaguchi a, Junichiro Kugai a, Yuji Ohkubo a, Satoshi Seino a, Takashi Nakagawa a, Satoshi Ichikawa c, Takao A. Yamamoto a a b c
Grad. Sch. Eng., Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan Research Center for Solar Energy Chemistry and Grad. Sch. Eng. Sci., Osaka University, Machikaneyama 1-3, Toyonaka, Osaka 560-8531, Japan Institute for NanoScience Design, Osaka University, Machikaneyama 1-3, Toyonaka, Osaka 560-8531, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 April 2012 Received in revised form 13 September 2012 Accepted 6 November 2012 Available online 14 December 2012
This paper describes a novel synthesis method of stabilizer-free Pt/TiO2 composite nanoparticles using electron beam irradiation. The chemical compositions were analyzed by inductively coupled plasmaatomic emission spectroscopy. The microstructures of the samples were observed by using transmission electron microscope. Pt nanoparticles with the sizes of 2–4 nm were deposited on TiO2 without any use of stabilizers. The concentrations of Pt ions and 2-propanol notably affected the size and shape of Pt nanoparticles. Their reactions of preferential CO oxidation were measured in temperature region from 60 to 140 8C. The Pt/TiO2 catalyst with spherical Pt nanoparticles exhibited a 67% of CO conversion rate and 100% of selectivity at a low temperature of 60 8C. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Composites C. Electron microscopy D. Catalytic properties
1. Introduction Pt nanoparticles supported on various oxide supports have been widely studied owing to their interesting chemical property. In particular Pt nanoparticles supported on TiO2 (Pt/TiO2) have been attracting much attention, because they are effectively applicable to catalysts for CO oxidation [1], water–gas shift reaction [2], and various photocatalysis [3]. Many researchers study on various synthesis methods of Pt/TiO2 catalysts: impregnation [4], sol–gel [5], sonochemical reduction [1] and photodeposition method [6]. To control the size of Pt nanoparticles, these methods need to use stabilizers, such as polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP) and gelatin. However, residual stabilizers over Pt surfaces cause a loss of catalytic surfaces. Thus, it is important to develop a synthesis method for stabilizer-free nanoparticles. The synthesis method using g-ray irradiation has been established for synthesis of monodispersed metal nanoparticles [7]. Water radiolysis induced by irradiation is effective to obtain a uniform reduction of metal ions in the synthetic solution because much solvated electrons, which have a strong reducing ability, are generated by the radiolysis [8]. Some researchers have reported pioneering work in the synthesis of practical nanocomposites using g-ray; for instance, Kapoor et al. have investigated the CO
* Corresponding author. Tel.: +81 6 6879 7886; fax: +81 6 6879 7886. E-mail address: [email protected] (S. Kageyama). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.11.028
oxidation property of Pt/SiO2 catalyst [9]. However, due to the low dose rate, these methods using g-ray need to use stabilizers. To overcome this problem, we have investigated the application of electron beam irradiation to synthesis of stabilizer-free nanocomposites [10–13]. This investigation revealed that the high dose rate of about 180 kGy/min achieves the synthesis of monodispersed metal nanoparticles without any use of stabilizers. In this process, the reduction reactions of metal ions only take a few seconds. Thus, this method using electron beam has a high potential to synthesis stabilizer-free nanocomposites. Using the electron beam irradiation, we expect to obtain novel catalysts with interesting property. In particular, it is interesting to investigate Pt nanoparticles supported on various oxide supports because of the active interaction between Pt particles and the oxide supports. Recently Pt-based nanoparticles supported on g-Fe2O3 or carbon nanoparticles have been reported [11–13], but a study on TiO2 support has never been reported so far. In this paper, we report a stabilizer-free synthesis using electron beam as a novel synthesis method of Pt/TiO2. To obtain Pt/TiO2 composites, our radiolytic synthesis needed no heat treatment (e.g., thermal reduction treatments in impregnation methods [4], refluxing in sol–gel methods [5]). In addition, our method took remarkably little time for Pt reduction reaction, 6.7 s (e.g., 90 min in sonochemical reduction [1], 30 min in photodeposition method [6]). Transmission electron microscope (TEM) observation revealed that the synthetic condition affects the size and shape of Pt nanoparticles. Their reactions of
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preferential CO oxidation (PROX), which is a reaction applied to fuel cell systems [1], were measured. The sample with spherical Pt particles showed a highest CO conversion and 100% of selectivity CO oxidation at a low temperature of 60 8C. We discuss the effect of synthetic condition and relationship between the structure of Pt/TiO2 and their PROX reaction.
Table 1 Synthetic conditions of Pt/TiO2 catalysts. Sample
2-Propanol concentration (vol.%)
Pt ion concentration (mM)
1 2 3
1 1 80
0.010 0.025 0.500
2. Experimental 2.1. Synthesis of Pt/TiO2
3. Results and discussion
Raw materials used were metal precursors of H2PtCl66H2O (99.9%, Wako), ultrapure water (18 MV cm), rutile TiO2 supports (JRC-TIO-6 supplied by the Catalysis Society of Japan) and 2propanol (Wako). 50 ml of ultrapure water, 1 vol.% of 2propanol, Pt metal precursors and TiO2 supports were charged in a 100-ml glass vial. The vials were kept in the dark to prevent photochemical reductions. To disperse the TiO2 supports, the synthetic solution was sonicated for 1 min by an ultrasonic bath before irradiation. The synthetic solution in vial was irradiated with an electron beam at room temperature (20 kGy for 6.7 s, 4.8 MeV, 10 mA, at Japan Electron Beam Irradiation Service Co., Ltd.). The beam was vertically emitted and scanned horizontally over 1-m width where carts carrying many vials traverse at ordinary temperatures and pressures. The cart speed was controlled to adjust the total dose 20 kGy, so that irradiation of each vial takes only 6.7 s. The net dose was measured by the radiochromic dosimeter pasted on the vial wall. In this process, vacuum equipment is not needed because the electron beam has a very high energy, 4.8 MeV. This energy is enough to penetrate the vial wall (1.2 mm) and to give rise to radiochemical reduction in the reactant solution homogeneously [12]. The radiation-induced radicals reduced the precursors to form Pt metal nanoparticles stabilized on the TiO2 supports. All the samples were washed and dried to obtain catalyst powders after the irradiation. The concentrations of Pt ions (0.01–0.50 mM) and 2propanol (1 or 80 vol.%) in the synthetic solution were varied (Table 1). The TiO2 was charged to adjust the total Pt loading to 1 wt.%.
3.1. Structural characterization
2.2. Characterization The chemical composition was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES; ICPE9000, SHIMADZU). The microstructure of the samples was investigated using TEM (TECNAI G2 20 S-TWIN, 200 kV). The PROX reaction was evaluated by monitoring O2 and CO concentration in a glass tube with 4-mm-diameter reactor. The catalyst powder (50 mg) in the reactor was exposed to reactant gas mixture of 1% CO, 0.5% O2, 67.2% H2 and N2 as balance (gas flow rate: 25 ml/min). The samples were protected from light during reaction. The temperature dependence was investigated every 20 8C by stepwise changing the reactor temperature from 60 8C to 140 8C. The inner gas was analyzed with a gas chromatograph, Varian 490 Micro-GC equipped with two columns (MS-5A, Plot Q) and TCD detectors. O2 consumption, CO conversion and selectivity were defined as follows: O2 consumption ¼ ð1 P O2 =PO0 2 Þ 100 0 Þ 100 CO conversion ¼ ð1 PCO =PCO
Table 2 lists the Pt loadings and reduction rates of Pt ions determined by using ICP-AES. The reduction rates of Pt ion were derived from the ratio of Pt loading quantity to initial Pt ion quantity. The Pt loadings of samples 1–3 are 0.8, 0.9 and 0.8 wt.%, respectively. No significant difference of loading was found among them, but the reduction rate of Pt ions for sample 3 was significantly low. It seems that a too high concentration of 2propanol caused the low reduction rate. Increasing the Pt ion concentration did not result in increasing Pt loaded weight on TiO2 surface, because the reduction rates and TiO2 amounts were different. Fig. 1(a)–(c) shows typical TEM images of the samples 1, 2 and 3, respectively. Pt nanoparticles with diameters of about 2–5 nm were highly dispersed on TiO2. Fig. 2(a)–(c) shows the histograms of the size distributions of the present samples, obtained by counting more than 100 particles. The sizes of Pt nanoparticles of all the samples slightly differ according to synthesis conditions. The modes of the samples 1–3 are 2 (64%), 3 (63%) and 4 nm (55%), respectively. To investigate their morphology of Pt nanoparticles, TEM observations with higher magnification were performed. Fig. 3(a)– (c) shows typical images of Pt nanoparticles with various aspect ratios from all the samples. The lattice spacings of 0.23 nm correspond to Pt (1 1 1). TEM observation revealed that larger particles tend to have larger values of aspect ratio. Pt nanoparticles with the sizes of about 3, 4 and 5 nm have aspect ratios of 1.0, 1.2 and 1.6, respectively. In addition, it seems that their contact angles between Pt nanoparticles and TiO2 surfaces decrease with increasing the aspect ratio. The size and shape of Pt particles were notably affected by the concentrations of Pt ions and 2propanol. 3.2. Particle growth process We discuss the effect of solution condition on the size and shape of Pt nanoparticles. Electron beam irradiation could cause surface defects of TiO2. However, the water solutions preserved TiO2 supports from direct irradiation of electron beam in our synthesis. Therefore, we suppose that the irradiation effect on TiO2 supports was negligible. The Pt nanoparticle in Fig. 3(a) has a spherical shape, but one in Fig. 3(c) has a hemispherical shape. These results imply that smaller particles were formed by homogeneous growth and larger particles were formed by heterogeneous growth at TiO2 surfaces. In general hemispherical shapes like the particle in
Table 2 Results of composition analysis of Pt/TiO2 catalysts. Sample
Pt loading on TiO2 (wt.%)
Reduction rate of Pt ion (%)
1 2 3
0.8 0.9 0.8
83 89 40
Selectivity ¼ ðCO conversionÞ=ðO2 consumptionÞ 100
where P O2 and PCO are partial pressures of O2 and CO in outlet. PO0 2 0 and PCO are those in outlet.
S. Kageyama et al. / Materials Research Bulletin 48 (2013) 1347–1351
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Fig. 1. TEM images of samples (a) 1, (b) 2 and (c) 3.
Fig. 2. Histograms of size distribution of samples (a) 1, (b) 2 and (c) 3.
Fig. 3(c) often observed in various synthesis methods [14], because this shape contributes the relaxation of surface energy with the high contact area. On the other hand, one in Fig. 3(a) has a spherical shape and the contact area between Pt and TiO2 is very lower. Sample 2 and 3 were obtained with higher concentrations of Pt
ions and 2-propanol as compared with sample 1 (Table 1). We infer that this concentration of ion and 2-propanol affected the growth process of Pt nanoparticles. In the radiolytic method, the water radiolysis with electron beam irradiation generates some radicals
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of 2-propanol in order to investigate the effect of solvated electron yields. Especially, solvated electron has a highest reducing ability (2.9 V [15]). The reduction of Pt ions by solvated electrons easily occurs because the standard redox potential of Pt is lower, E0(Pt4+/ Pt) = 1.04 V. The solvated electrons reduce metal ions M+ to zerovalent state M0 as follows: 0 Mþ þ e aq ! M
(2)
Then, nuclei consisting of a few reduced atoms are formed and particle growths occur on these nuclei. Therefore, the yield of solvated electron is of importance for the particle formation. For samples 2 and 3, we consider that relatively low yields of solvated electron caused the increasing their sizes and aspect ratios of Pt nanoparticles because the concentration of Pt ions for samples 2 and 3 was 2.5 and 50 times higher than sample 1, respectively. In addition, the solvent of high 2-propanol concentration [16,17], yields of solvated electron are very lower than that in pure water solvent. The particle growth mainly occurs in two ways: the coalescence of nuclei (Eqs. (3) and (4)), and the bonding between zero-valent nuclei and unreduced ions (Eqs. (5) and (6)) [8]. M 0 þ M0 ! M 2
(3)
Mm þ M p ! Mn
(4)
M 0 þ Mþ ! Mþ 2
(5)
yþ zþ Mxþ mþx þ M pþy ! Mnþz
(6)
In the condition with low concentrations of ion and 2-propanol, homogeneous growths of particles occur and then they are stabilized on TiO2 surfaces. After this stabilization of spherical particles, further significant growth cannot occur because almost all the Pt ions were already reduced by solvated electrons at this point. However, in the condition with high concentration of ion or 2-propanol, further growth can occur easily because residual Pt ions can contribute the process of at least (6). This further growth at the TiO2 surface occurs heterogeneously, and consequently a hemispherical shape is formed. Nuclei consisting of a few reduced atoms are mainly formed in the homogeneous solution [8]. In addition, it is reasonable to suppose that their growth to nanoparticles occurred both at the homogeneous solution and heterogeneous solution/TiO2 interfaces.
Fig. 3. Pt nanoparticles supported on TiO2 with various aspect ratios: (a) aspect ratio = 1, (b) aspect ratio = 1.2 and (c) aspect ratio = 1.6.
as follows [8]: hv
H2 O !H3 Oþ ; H ; OH ; H2 ; H2 O2 ; HO2 ; e aq ;
(1)
where e aq is solvated electron. OH is oxidative radical, but 2propanol H3C–HCOH–CH3 scavenge OH and convert into reductive radical, H3C–COH–CH3. While 2-propanol assists the reduction of Pt ions through scavenging oxidative radicals OH, high concentrations of 2-propanol cause lower yields of solvated electron; the yield of solvated electron in pure 2-propanol is very lower than that in pure water [17]. Therefore, we used 1 or 80 vol.%
Fig. 4. O2 consumption rates of the present samples.
S. Kageyama et al. / Materials Research Bulletin 48 (2013) 1347–1351
Fig. 5. CO conversion rates of the present samples.
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68 and 56%, respectively. Sample 1 exhibited its significantly highest conversion and 100% of selectivity at a low temperature of 60 8C Only sample 1 showed significantly higher conversion and selectivity despite its similar average size and Pt loading to the other samples. Sample 1 (average size: 2.4 nm, Pt loading: 0.8 wt.%) is very similar to sample 2 (average size: 3.2 nm, Pt loading: 0.9 wt.%) and sample 3 (average size: 3.9 nm, Pt loading: 0.8 wt.%) in average size and Pt loading (Table 2 and Fig. 2). For Pt/ TiO2 system, the structure of Pt particles is main factor in determining PROX behaviors, because Haruta and coworkers demonstrated that the reactions that occur near nano-hetero interfaces between Pt and TiO2 are negligible [18]. The particle size must have influenced the exposed face. In addition, the electronic structure probably affected by geometric structure [19] and charge transfer between Pt and TiO2 [20] according to particle’s shape formed by homogeneous or heterogeneous growth. To elucidate the mechanism of high performance of sample 1, we will study simultaneous control of size and shape, varying total dose, dose rate and condition of solvent in our future work.
For sample 3, we used the quite different condition from the other two conditions in order to adjust Pt loading to approximately 1 wt.%. In the present study, this synthesis condition was unavoidable for precise investigation of the PROX property. We did not investigate the system excluding 2-propanol because of insufficient amounts of Pt loading; we infer that the shapes of Pt particles on TiO2 supports significantly affected by dose rates of electron beam and concentrations of Pt ion in the system. For more general discussion, it is necessary to investigate not only aspect ratios but also contact angles, precise shapes, particle sizes, and so on. In addition, the shapes of loaded Pt particles are probably affected by dose rates of electron beam. We would like to consider them for our future work.
We have successfully synthesized Pt/TiO2 catalysts by the radiolytic process employing electron beam irradiation without any use of stabilizers. TEM observation revealed that the supported Pt nanoparticles have particle sizes of 2, 3 and 5 nm. Pt nanoparticles with 2-nm size exhibited highest conversion and 100% of selectivity of CO oxidation at a low temperature of 60 8C. We suggested that relative yields of solvated electron affect the growth process. The present study provides a novel synthesis of stabilizer-free Pt/TiO2 and the detailed characterization of their structures.
3.3. PROX reaction
Acknowledgments
Figs. 4–6, respectively, show results of O2 consumption, CO conversion and selectivity of the present samples. At 60 8C, which is a actual operating temperature, sample 1 exhibited 66% of O2 consumption, whereas the other samples showed only consumption rates of less than 15% (Fig. 4). Sample 1 showed a significantly high conversion of CO oxidation (Fig. 5). Sample 1 had a 8.4 times higher conversion at 60 8C; the CO conversion rates of sample 1 reached 67%, but the conversion of samples 2 and 3 was only 8%. Furthermore, sample 1 achieved a high selectivity, 100% at 60 8C (Fig. 6). In contrast, the selectivities of samples 2 and 3 were only
The authors thank K. Ueno (EBIS, Japan) for providing us beam time of the electron accelerator. This research was partially supported by a Grant-in-Aid for Scientific Research (A) (No. 22241023).
Fig. 6. Selectivities of CO oxidation of the present samples.
4. Conclusions
References [1] M. Sivakumar, A. Towata, K. Yasui, T. Tuziuti, T. Kozuka, M. Tsujimoto, Z.Y. Zhong, Y. Iida, Ultrason. Sonochem. 17 (2010) 213–218. [2] H. Iida, A. Igarashi, Appl. Catal. A: Gen. 298 (2006) 152–160. [3] J.L. Falconer, K.A. Magrini-Bair, J. Catal. 179 (1998) 171–178. [4] V. Keller, P. Bernhardt, F. Garin, J. Catal. 215 (2003) 129–138. [5] E. Sanchez, T. Lopez, R. Gomez, A. Morales, O. Novaro, J. Solid State Chem. 122 (1996) 309–314. [6] S. Sato, J. Catal. 92 (1985) 11–16. [7] A.D. Belapurkar, S. Kapoor, S.K. Kulshreshtha, J.P. Mittal, Mater. Res. Bull. 36 (2001) 145–151. [8] J. Belloni, Catal. Today (2006) 141–156. [9] S. Kapoor, A.D. Belapurkar, J.P. Mittal, T. Mukherjee, Mater. Res. Bull. 40 (2005) 1654–1661. [10] S. Seino, T. Kinoshita, T. Nakagawa, T. Kojima, R. Taniguci, S. Okuda, T.A. Yamamoto, J. Nanopart. Res. 10 (2008) 1071–1076. [11] S. Kageyama, S. Seino, T. Nakagawa, H. Nitani, K. Ueno, H. Daimon, T.A. Yamamoto, J. Nanopart. Res. 13 (2011) 5275–5287. [12] T.A. Yamamoto, T. Nakagawa, S. Seino, H. Nitani, Appl. Catal. A: Gen. 387 (2010) 195–202. [13] J. Kugai, R. Kitagawa, S. Seino, T. Nakagawa, Y. Ohkubo, H. Nitani, H. Daimon, T.A. Yamamoto, Appl. Catal. A: Gen. 406 (2011) 43–50. [14] S.C. Chan, M.A. Barteau, Langmuir 21 (2005) 5588–5595. [15] A. Henglein, J. Phys. Chem. B 104 (2000) 1206–1211. [16] J. Cygler, G.R. Freeman, Can. J. Chem. Rev. Can. Chim. 62 (1984) 1265–1270. [17] Y. Muroya, M.Z. Lin, Z.H. Han, Y. Kumagai, A. Sakumi, T. Ueda, Y. Katsumura, Radiat. Phys. Chem. 77 (2008) 1176–1182. [18] G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, Catal. Lett. 44 (1997) 83–87. [19] L.Y. Wang, A. Roudgar, M. Eikerling, J. Phys. Chem. C 113 (2009) 17989–17996. [20] K. Okazaki, Y. Morikawa, S. Tanaka, K. Tanaka, M. Kohyama, J. Mater. Sci. 40 (2005) 3075–3080.
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Pt/TiO2 composite nanoparticles synthesized by electron beam irradiation for preferential CO oxidation Satoru Kageyama a,*, Yoshitsune Sugano b, Yukihiro Hamaguchi a, Junichiro Kugai a, Yuji Ohkubo a, Satoshi Seino a, Takashi Nakagawa a, Satoshi Ichikawa c, Takao A. Yamamoto a a b c
Grad. Sch. Eng., Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan Research Center for Solar Energy Chemistry and Grad. Sch. Eng. Sci., Osaka University, Machikaneyama 1-3, Toyonaka, Osaka 560-8531, Japan Institute for NanoScience Design, Osaka University, Machikaneyama 1-3, Toyonaka, Osaka 560-8531, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 April 2012 Received in revised form 13 September 2012 Accepted 6 November 2012 Available online 14 December 2012
This paper describes a novel synthesis method of stabilizer-free Pt/TiO2 composite nanoparticles using electron beam irradiation. The chemical compositions were analyzed by inductively coupled plasmaatomic emission spectroscopy. The microstructures of the samples were observed by using transmission electron microscope. Pt nanoparticles with the sizes of 2–4 nm were deposited on TiO2 without any use of stabilizers. The concentrations of Pt ions and 2-propanol notably affected the size and shape of Pt nanoparticles. Their reactions of preferential CO oxidation were measured in temperature region from 60 to 140 8C. The Pt/TiO2 catalyst with spherical Pt nanoparticles exhibited a 67% of CO conversion rate and 100% of selectivity at a low temperature of 60 8C. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Composites C. Electron microscopy D. Catalytic properties
1. Introduction Pt nanoparticles supported on various oxide supports have been widely studied owing to their interesting chemical property. In particular Pt nanoparticles supported on TiO2 (Pt/TiO2) have been attracting much attention, because they are effectively applicable to catalysts for CO oxidation [1], water–gas shift reaction [2], and various photocatalysis [3]. Many researchers study on various synthesis methods of Pt/TiO2 catalysts: impregnation [4], sol–gel [5], sonochemical reduction [1] and photodeposition method [6]. To control the size of Pt nanoparticles, these methods need to use stabilizers, such as polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP) and gelatin. However, residual stabilizers over Pt surfaces cause a loss of catalytic surfaces. Thus, it is important to develop a synthesis method for stabilizer-free nanoparticles. The synthesis method using g-ray irradiation has been established for synthesis of monodispersed metal nanoparticles [7]. Water radiolysis induced by irradiation is effective to obtain a uniform reduction of metal ions in the synthetic solution because much solvated electrons, which have a strong reducing ability, are generated by the radiolysis [8]. Some researchers have reported pioneering work in the synthesis of practical nanocomposites using g-ray; for instance, Kapoor et al. have investigated the CO
* Corresponding author. Tel.: +81 6 6879 7886; fax: +81 6 6879 7886. E-mail address: [email protected] (S. Kageyama). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.11.028
oxidation property of Pt/SiO2 catalyst [9]. However, due to the low dose rate, these methods using g-ray need to use stabilizers. To overcome this problem, we have investigated the application of electron beam irradiation to synthesis of stabilizer-free nanocomposites [10–13]. This investigation revealed that the high dose rate of about 180 kGy/min achieves the synthesis of monodispersed metal nanoparticles without any use of stabilizers. In this process, the reduction reactions of metal ions only take a few seconds. Thus, this method using electron beam has a high potential to synthesis stabilizer-free nanocomposites. Using the electron beam irradiation, we expect to obtain novel catalysts with interesting property. In particular, it is interesting to investigate Pt nanoparticles supported on various oxide supports because of the active interaction between Pt particles and the oxide supports. Recently Pt-based nanoparticles supported on g-Fe2O3 or carbon nanoparticles have been reported [11–13], but a study on TiO2 support has never been reported so far. In this paper, we report a stabilizer-free synthesis using electron beam as a novel synthesis method of Pt/TiO2. To obtain Pt/TiO2 composites, our radiolytic synthesis needed no heat treatment (e.g., thermal reduction treatments in impregnation methods [4], refluxing in sol–gel methods [5]). In addition, our method took remarkably little time for Pt reduction reaction, 6.7 s (e.g., 90 min in sonochemical reduction [1], 30 min in photodeposition method [6]). Transmission electron microscope (TEM) observation revealed that the synthetic condition affects the size and shape of Pt nanoparticles. Their reactions of
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preferential CO oxidation (PROX), which is a reaction applied to fuel cell systems [1], were measured. The sample with spherical Pt particles showed a highest CO conversion and 100% of selectivity CO oxidation at a low temperature of 60 8C. We discuss the effect of synthetic condition and relationship between the structure of Pt/TiO2 and their PROX reaction.
Table 1 Synthetic conditions of Pt/TiO2 catalysts. Sample
2-Propanol concentration (vol.%)
Pt ion concentration (mM)
1 2 3
1 1 80
0.010 0.025 0.500
2. Experimental 2.1. Synthesis of Pt/TiO2
3. Results and discussion
Raw materials used were metal precursors of H2PtCl66H2O (99.9%, Wako), ultrapure water (18 MV cm), rutile TiO2 supports (JRC-TIO-6 supplied by the Catalysis Society of Japan) and 2propanol (Wako). 50 ml of ultrapure water, 1 vol.% of 2propanol, Pt metal precursors and TiO2 supports were charged in a 100-ml glass vial. The vials were kept in the dark to prevent photochemical reductions. To disperse the TiO2 supports, the synthetic solution was sonicated for 1 min by an ultrasonic bath before irradiation. The synthetic solution in vial was irradiated with an electron beam at room temperature (20 kGy for 6.7 s, 4.8 MeV, 10 mA, at Japan Electron Beam Irradiation Service Co., Ltd.). The beam was vertically emitted and scanned horizontally over 1-m width where carts carrying many vials traverse at ordinary temperatures and pressures. The cart speed was controlled to adjust the total dose 20 kGy, so that irradiation of each vial takes only 6.7 s. The net dose was measured by the radiochromic dosimeter pasted on the vial wall. In this process, vacuum equipment is not needed because the electron beam has a very high energy, 4.8 MeV. This energy is enough to penetrate the vial wall (1.2 mm) and to give rise to radiochemical reduction in the reactant solution homogeneously [12]. The radiation-induced radicals reduced the precursors to form Pt metal nanoparticles stabilized on the TiO2 supports. All the samples were washed and dried to obtain catalyst powders after the irradiation. The concentrations of Pt ions (0.01–0.50 mM) and 2propanol (1 or 80 vol.%) in the synthetic solution were varied (Table 1). The TiO2 was charged to adjust the total Pt loading to 1 wt.%.
3.1. Structural characterization
2.2. Characterization The chemical composition was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES; ICPE9000, SHIMADZU). The microstructure of the samples was investigated using TEM (TECNAI G2 20 S-TWIN, 200 kV). The PROX reaction was evaluated by monitoring O2 and CO concentration in a glass tube with 4-mm-diameter reactor. The catalyst powder (50 mg) in the reactor was exposed to reactant gas mixture of 1% CO, 0.5% O2, 67.2% H2 and N2 as balance (gas flow rate: 25 ml/min). The samples were protected from light during reaction. The temperature dependence was investigated every 20 8C by stepwise changing the reactor temperature from 60 8C to 140 8C. The inner gas was analyzed with a gas chromatograph, Varian 490 Micro-GC equipped with two columns (MS-5A, Plot Q) and TCD detectors. O2 consumption, CO conversion and selectivity were defined as follows: O2 consumption ¼ ð1 P O2 =PO0 2 Þ 100 0 Þ 100 CO conversion ¼ ð1 PCO =PCO
Table 2 lists the Pt loadings and reduction rates of Pt ions determined by using ICP-AES. The reduction rates of Pt ion were derived from the ratio of Pt loading quantity to initial Pt ion quantity. The Pt loadings of samples 1–3 are 0.8, 0.9 and 0.8 wt.%, respectively. No significant difference of loading was found among them, but the reduction rate of Pt ions for sample 3 was significantly low. It seems that a too high concentration of 2propanol caused the low reduction rate. Increasing the Pt ion concentration did not result in increasing Pt loaded weight on TiO2 surface, because the reduction rates and TiO2 amounts were different. Fig. 1(a)–(c) shows typical TEM images of the samples 1, 2 and 3, respectively. Pt nanoparticles with diameters of about 2–5 nm were highly dispersed on TiO2. Fig. 2(a)–(c) shows the histograms of the size distributions of the present samples, obtained by counting more than 100 particles. The sizes of Pt nanoparticles of all the samples slightly differ according to synthesis conditions. The modes of the samples 1–3 are 2 (64%), 3 (63%) and 4 nm (55%), respectively. To investigate their morphology of Pt nanoparticles, TEM observations with higher magnification were performed. Fig. 3(a)– (c) shows typical images of Pt nanoparticles with various aspect ratios from all the samples. The lattice spacings of 0.23 nm correspond to Pt (1 1 1). TEM observation revealed that larger particles tend to have larger values of aspect ratio. Pt nanoparticles with the sizes of about 3, 4 and 5 nm have aspect ratios of 1.0, 1.2 and 1.6, respectively. In addition, it seems that their contact angles between Pt nanoparticles and TiO2 surfaces decrease with increasing the aspect ratio. The size and shape of Pt particles were notably affected by the concentrations of Pt ions and 2propanol. 3.2. Particle growth process We discuss the effect of solution condition on the size and shape of Pt nanoparticles. Electron beam irradiation could cause surface defects of TiO2. However, the water solutions preserved TiO2 supports from direct irradiation of electron beam in our synthesis. Therefore, we suppose that the irradiation effect on TiO2 supports was negligible. The Pt nanoparticle in Fig. 3(a) has a spherical shape, but one in Fig. 3(c) has a hemispherical shape. These results imply that smaller particles were formed by homogeneous growth and larger particles were formed by heterogeneous growth at TiO2 surfaces. In general hemispherical shapes like the particle in
Table 2 Results of composition analysis of Pt/TiO2 catalysts. Sample
Pt loading on TiO2 (wt.%)
Reduction rate of Pt ion (%)
1 2 3
0.8 0.9 0.8
83 89 40
Selectivity ¼ ðCO conversionÞ=ðO2 consumptionÞ 100
where P O2 and PCO are partial pressures of O2 and CO in outlet. PO0 2 0 and PCO are those in outlet.
S. Kageyama et al. / Materials Research Bulletin 48 (2013) 1347–1351
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Fig. 1. TEM images of samples (a) 1, (b) 2 and (c) 3.
Fig. 2. Histograms of size distribution of samples (a) 1, (b) 2 and (c) 3.
Fig. 3(c) often observed in various synthesis methods [14], because this shape contributes the relaxation of surface energy with the high contact area. On the other hand, one in Fig. 3(a) has a spherical shape and the contact area between Pt and TiO2 is very lower. Sample 2 and 3 were obtained with higher concentrations of Pt
ions and 2-propanol as compared with sample 1 (Table 1). We infer that this concentration of ion and 2-propanol affected the growth process of Pt nanoparticles. In the radiolytic method, the water radiolysis with electron beam irradiation generates some radicals
S. Kageyama et al. / Materials Research Bulletin 48 (2013) 1347–1351
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of 2-propanol in order to investigate the effect of solvated electron yields. Especially, solvated electron has a highest reducing ability (2.9 V [15]). The reduction of Pt ions by solvated electrons easily occurs because the standard redox potential of Pt is lower, E0(Pt4+/ Pt) = 1.04 V. The solvated electrons reduce metal ions M+ to zerovalent state M0 as follows: 0 Mþ þ e aq ! M
(2)
Then, nuclei consisting of a few reduced atoms are formed and particle growths occur on these nuclei. Therefore, the yield of solvated electron is of importance for the particle formation. For samples 2 and 3, we consider that relatively low yields of solvated electron caused the increasing their sizes and aspect ratios of Pt nanoparticles because the concentration of Pt ions for samples 2 and 3 was 2.5 and 50 times higher than sample 1, respectively. In addition, the solvent of high 2-propanol concentration [16,17], yields of solvated electron are very lower than that in pure water solvent. The particle growth mainly occurs in two ways: the coalescence of nuclei (Eqs. (3) and (4)), and the bonding between zero-valent nuclei and unreduced ions (Eqs. (5) and (6)) [8]. M 0 þ M0 ! M 2
(3)
Mm þ M p ! Mn
(4)
M 0 þ Mþ ! Mþ 2
(5)
yþ zþ Mxþ mþx þ M pþy ! Mnþz
(6)
In the condition with low concentrations of ion and 2-propanol, homogeneous growths of particles occur and then they are stabilized on TiO2 surfaces. After this stabilization of spherical particles, further significant growth cannot occur because almost all the Pt ions were already reduced by solvated electrons at this point. However, in the condition with high concentration of ion or 2-propanol, further growth can occur easily because residual Pt ions can contribute the process of at least (6). This further growth at the TiO2 surface occurs heterogeneously, and consequently a hemispherical shape is formed. Nuclei consisting of a few reduced atoms are mainly formed in the homogeneous solution [8]. In addition, it is reasonable to suppose that their growth to nanoparticles occurred both at the homogeneous solution and heterogeneous solution/TiO2 interfaces.
Fig. 3. Pt nanoparticles supported on TiO2 with various aspect ratios: (a) aspect ratio = 1, (b) aspect ratio = 1.2 and (c) aspect ratio = 1.6.
as follows [8]: hv
H2 O !H3 Oþ ; H ; OH ; H2 ; H2 O2 ; HO2 ; e aq ;
(1)
where e aq is solvated electron. OH is oxidative radical, but 2propanol H3C–HCOH–CH3 scavenge OH and convert into reductive radical, H3C–COH–CH3. While 2-propanol assists the reduction of Pt ions through scavenging oxidative radicals OH, high concentrations of 2-propanol cause lower yields of solvated electron; the yield of solvated electron in pure 2-propanol is very lower than that in pure water [17]. Therefore, we used 1 or 80 vol.%
Fig. 4. O2 consumption rates of the present samples.
S. Kageyama et al. / Materials Research Bulletin 48 (2013) 1347–1351
Fig. 5. CO conversion rates of the present samples.
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68 and 56%, respectively. Sample 1 exhibited its significantly highest conversion and 100% of selectivity at a low temperature of 60 8C Only sample 1 showed significantly higher conversion and selectivity despite its similar average size and Pt loading to the other samples. Sample 1 (average size: 2.4 nm, Pt loading: 0.8 wt.%) is very similar to sample 2 (average size: 3.2 nm, Pt loading: 0.9 wt.%) and sample 3 (average size: 3.9 nm, Pt loading: 0.8 wt.%) in average size and Pt loading (Table 2 and Fig. 2). For Pt/ TiO2 system, the structure of Pt particles is main factor in determining PROX behaviors, because Haruta and coworkers demonstrated that the reactions that occur near nano-hetero interfaces between Pt and TiO2 are negligible [18]. The particle size must have influenced the exposed face. In addition, the electronic structure probably affected by geometric structure [19] and charge transfer between Pt and TiO2 [20] according to particle’s shape formed by homogeneous or heterogeneous growth. To elucidate the mechanism of high performance of sample 1, we will study simultaneous control of size and shape, varying total dose, dose rate and condition of solvent in our future work.
For sample 3, we used the quite different condition from the other two conditions in order to adjust Pt loading to approximately 1 wt.%. In the present study, this synthesis condition was unavoidable for precise investigation of the PROX property. We did not investigate the system excluding 2-propanol because of insufficient amounts of Pt loading; we infer that the shapes of Pt particles on TiO2 supports significantly affected by dose rates of electron beam and concentrations of Pt ion in the system. For more general discussion, it is necessary to investigate not only aspect ratios but also contact angles, precise shapes, particle sizes, and so on. In addition, the shapes of loaded Pt particles are probably affected by dose rates of electron beam. We would like to consider them for our future work.
We have successfully synthesized Pt/TiO2 catalysts by the radiolytic process employing electron beam irradiation without any use of stabilizers. TEM observation revealed that the supported Pt nanoparticles have particle sizes of 2, 3 and 5 nm. Pt nanoparticles with 2-nm size exhibited highest conversion and 100% of selectivity of CO oxidation at a low temperature of 60 8C. We suggested that relative yields of solvated electron affect the growth process. The present study provides a novel synthesis of stabilizer-free Pt/TiO2 and the detailed characterization of their structures.
3.3. PROX reaction
Acknowledgments
Figs. 4–6, respectively, show results of O2 consumption, CO conversion and selectivity of the present samples. At 60 8C, which is a actual operating temperature, sample 1 exhibited 66% of O2 consumption, whereas the other samples showed only consumption rates of less than 15% (Fig. 4). Sample 1 showed a significantly high conversion of CO oxidation (Fig. 5). Sample 1 had a 8.4 times higher conversion at 60 8C; the CO conversion rates of sample 1 reached 67%, but the conversion of samples 2 and 3 was only 8%. Furthermore, sample 1 achieved a high selectivity, 100% at 60 8C (Fig. 6). In contrast, the selectivities of samples 2 and 3 were only
The authors thank K. Ueno (EBIS, Japan) for providing us beam time of the electron accelerator. This research was partially supported by a Grant-in-Aid for Scientific Research (A) (No. 22241023).
Fig. 6. Selectivities of CO oxidation of the present samples.
4. Conclusions
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