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Ce0.7Zr0.3O2 by in situ Raman spectroscopy
Accepted Manuscript Insight of Pt-support interaction in S-Pt/Ce0.7Zr0.3O2 by in situ Raman spectroscopy
Quanwen Wu, Jingwen Ba, Xiayan Yan, Jinchun Bao, Zhiyong Huang, Sanping Dou, Danling Dai, Tao Tang, Wenhua Luo, Daqiao Meng PII: DOI: Reference:
S1566-7367(17)30179-6 doi: 10.1016/j.catcom.2017.04.045 CATCOM 5030
To appear in:
Catalysis Communications
Received date: Revised date: Accepted date:
16 January 2017 24 April 2017 25 April 2017
Please cite this article as: Quanwen Wu, Jingwen Ba, Xiayan Yan, Jinchun Bao, Zhiyong Huang, Sanping Dou, Danling Dai, Tao Tang, Wenhua Luo, Daqiao Meng , Insight of Pt-support interaction in S-Pt/Ce0.7Zr0.3O2 by in situ Raman spectroscopy. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.04.045
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Insight of Pt-support interaction in S-Pt/Ce0.7Zr0.3O2 by in situ Raman spectroscopy
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Quanwen Wu, Jingwen Ba, Xiayan Yan, Jinchun Bao, Zhiyong Huang, Sanping Dou, Danling Dai, Tao Tang, Wenhua Luo*, Daqiao Meng* China Academy of Engineering Physics, Mianyang, Sichuan, 621900, China * Corresponding author: E-mail address: [email protected] Telephone: 0816-3625304
ACCEPTED MANUSCRIPT Abstract
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Strong Metal-Support Interactions (SMSI) are important in Pt supported catalysts for their catalytic performance. SMSI in Pt/CeO2, Pt/Ce0.7Zr0.3O2 and single atom S-Pt/Ce0.7Zr0.3O2 catalysts were studied by in situ Raman spectroscopy and H2 Temperature-Programmed Reduction. Evidence of SMSI in Pt/CeO2 and Pt/Ce0.7Zr0.3O2 that improve the reduction properties of supports other than H2 dissociation and spillover is obtained. It was also found that Pt-O and Pt-O-Ce bonding in the said catalysts play an important role in SMSI. The Pt bonding could be ascribed to Pt oxides species (Pt1+ and Pt2+) with different Pt adsorption sites on CeO2 surface and which act as intermediates in reduction process. This gives a new insight into the SMSI and explains the observed Pt/CeO2 reduction characteristics.
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Pt/CeO2, Pt/Ce0.7Zr0.3O2, Strong Metal-Support Interaction, Single atom catalyst, Raman Spectroscopy
ACCEPTED MANUSCRIPT 1. Introduction
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Ceria-based metal oxides are widely used as promoters and supports in three-way catalysts (TWCs) for automobile exhaust purification because of their excellent performances towards oxygen storing and release and stabilization of noble metal particles against sintering [1,2]. It has been proposed that CeO2-based metal oxides prevent Pt particles from sintering because of the strong Pt-support interactions developed [1-6]. Nagai et al. [1] reported that the strong interaction between oxidized Pt and CeO2-based supports stabilize Pt atoms and the Pt-O-Ce bond acts as the driving force for re-dispersing sintered Pt particles [6]. The Pt nanostructure that forms Pt-O-Ce bonds has been determined using XPS and high-resolution TEM [2]. The strength of Pt-O-support and Pt-support interactions on Pt/Al2O3, Pt/SiO2 and Pt/CeO2, respectively, were investigated with in situ Raman spectroscopy [7]. However, the redox behaviour of Pt-O and Pt-O-Ce bonding during reduction-oxidation process has not been yet clearly understood. Recently, Jones and Xiong [8] reported a new process of thermally stable single-atom platinum on ceria catalysts which can be employed for the investigation of the Pt-support interactions. In the present study, the redox behaviour of the Pt–O and Pt-O-Ce bonding in the S-Pt/Ce0.7Zr0.3O2 catalyst were studied by in situ Raman spectroscopy combined with H2 Temperature-Programmed Reduction (H2-TPR) give insights into the Pt-support interactions.
2.1. Sample preparation
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CeO2 and Ce0.7Zr0.3O2 catalyst supports were prepared by the sol-gel method. Ce(NO3)3.6H2O and ZrO(NO3)2.H2O (used for Ce0.7Zr0.3O2) were dissolved (molar ratio Ce:Zr=7:3) in deionized water and citric acid was added dropwise and the resulting solution was stirred at 65℃ for 2 hours. The solution was then kept at 80℃ in water bath until a gel was formed. After dried (overnight), the gel was calcined at 500℃ for 3 hours in a muffle furnace. Pt/Al2O3, Pt/CeO2 and Pt/Ce0.7Zr0.3O2 catalysts were prepared by the wet-impregnation method. The supports of Al2O3, CeO2 and Ce0.7Zr0.3O2 were immersed into an appropriate volume of H2PtCl6.6H2O solution so as to provide 1wt% Pt nominal loading and stirred at 40℃ for 2 hours. After evaporation at 80℃, the samples were dried overnight and calcined at 500℃ for 3 hours. For the Pt single atom catalyst preparation, Pt/Al2O3 was first pressed into pellets. Then Ce0.7Zr0.3O2 powder was aged with Pt/Al2O3 pellets in air at 650℃ for 12 hours. Finally, Ce0.7Zr0.3O2 powder was separated from Pt/Al2O3 pellets.
2.2. Characterization of catalysts H2-TPR profiles were obtained with Autosorb-1-C Physic/Chemisorptions Analyzer. The samples were loaded on top of quartz wool in a U-shaped quartz tube (i.d. =10 mm), and then pretreated with a helium flow at 300℃ for 1h to remove adsorbed
ACCEPTED MANUSCRIPT gases. After cooling to room temperature, samples were reduced using 5vol % H2/Ar gas mixture (30 ml/min) with a heating rate of 10℃/min. Hydrogen consumption was monitored with a thermal conductivity detector (TCD).
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Raman spectra were collected with a back scattering Raman spectrometer (LabRAM HR800, form Horiba JY) using a 457 nm Argon ion laser (Stabilite 2017, from Spectra Physics). The laser power was set relatively low (3mW), with collection time of 50s. A 20× objective was employed for both focusing the laser beam on the sample and collecting the scattered photons. In situ Raman spectra were collected using a sample cell under 5vol %H2/Ar and 5vol %O2/Ar with a flow rate of 50ml/min and temperature in the 25- 300℃ range.
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3. Results and discussion
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3.1 Redox performance of catalysts
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H2-TPR traces of supports and supported Pt catalysts are shown in Fig. 1A (left and right graphs). After doping CeO2 with Zr4+ a significant improvement in the reduction of bulk Ce4+ occurred, which is in agreement with previous studies [9, 10]. For the supported Pt catalysts, reduction temperatures were much lower. The main reduction peak at about 200℃ is ascribed to the reduction of surface Ce4+ promoted by Pt, and the peak at about 350℃ is related to the reduction of Pt oxide species. The reduction temperatures of Pt oxide species in Pt/CeO2 and Pt/Ce0.7Zr0.3O2 are much higher than that in Pt/Al2O3 (about 100℃)，revealing the stronger SMSI of Pt-CeO2 (Ce0.7Zr0.3O2) than Pt-Al2O3. The removed O estimated from integrated area of reduction peaks in Fig. 1A is 1218 μmol/g for Pt/Ce0.7Zr0.3O2 below 350℃, which exceeds the amount of oxygen bound with Pt cations, revealing the reduction of support. The reduction process of Pt supported catalyst will be further discussed in Section 3.3. Usually, promotion of reduction by the presence of Pt is ascribed to H2 dissociation and spillover processes [9]. Hydrogen is first dissociated on Pt and then spilt-over H-atoms greatly accelerate the reduction kinetics. However, the interaction between Pt and support might be another important reason. The interaction could promote reduction of support by forming oxygen vacancies, which is verified by some theoretical studies [11, 12]. Here, CO-TPR tests were carried out, where dissociation of CO on Pt was minimized. Curves a and b in Fig. 1A (right) show that Pt indeed promoted the reduction of surface Ce4+ by shifting the reduction peak toward lower temperature. The H2-TPR trace of S-Pt/Ce0.7Zr0.3O2 (curve c, Fig. 1A right graph) is similar to that of CO-TPR trace of Pt/Ce0.7Zr0.3O2 (curve b, Fig. 1A right graph), suggesting that H2 dissociation was almost absent and SMSI dominated. This could be further explained by TEM (Fig. 1B) and XPS (Fig. 1C) results. In the TEM image of S-Pt/Ce0.7Zr0.3O2, no Pt particle/cluster could be observed, where the contrast analysis indicated that some sites on CeO2 lattice with high contrast might be related to the presence of Pt atoms. However, in the Pt/Ce0.7Zr0.3O2 solid, many particles with 1-2 nm in size were obviously detected. Pt single atom dispersed on S-Pt/Ce0.7Zr0.3O2 should be regarded as Pt2+ state. The latter species cannot dissociate hydrogen, thus H2 dissociation and spillover of H were not observed during reduction of
ACCEPTED MANUSCRIPT S-Pt/Ce0.7Zr0.3O2. The reduction of S-Pt/Ce0.7Zr0.3O2 and Pt/Al2O3 mixed sample (curve d, Fig. 1A right graph) is different from that of S-Pt/Ce0.7Zr0.3O2. The first reduction peak sharpened and shifted toward lower temperature, because Pt/Al2O3 brought in the influence of H2 dissociation. Thus, SMSI and H2 dissociation-spillover both improved the reduction of support in Pt/Ce0.7Zr0.3O2 system. In the following Section, the mechanism of SMSI on improving the reduction of support is elaborated.
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Figure 1: (A) H2-TPR profiles of CeO2, Ce0.7Zr0.3O2 and supported catalysts. CO-TPR profiles of Ce0.7Zr0.3O2 and Pt/Ce0.7Zr0.3O2 (curve a b, right graph). H2-TPR of S-Pt/Ce0.7Zr0.3O2 (curve c) as well as Pt/Al2O3 and Pt/Ce0.7Zr0.3O2 mixed sample (curve d). (B) TEM images revealing the presence of Pt single atom in S-Pt/Ce0.7Zr0.3O2 and Pt particles (1-2 nm in size) in Pt/Ce0.7Zr0.3O2. (C) XP spectrum of S-Pt/Ce0.7Zr0.3O2 indicating the presence of Pt2+ species.
3.2. In situ Raman spectroscopy In situ Raman spectra of CeO2, Ce0.7Zr0.3O2, Pt/CeO2, Pt/Ce0.7Zr0.3O2 and S-Pt/Ce0.7Zr0.3O2 recorded under reducing/oxidizing gas atmosphere are provided in Fig. 2. Raman bands at about 455, 554 and 651cm-1 were detected for Pt/CeO2 and 455, 560 and 640cm-1 for Pt/Ce0.7Zr0.3O2. The strong 455cm-1 vibration was associated with the F2g symmetry of the crystalline CeO2 [13], while Raman features at 554 and 651cm-1 were respectively assigned to Pt-O-Ce vibration and Pt-O vibration [7, 14-16]. Raman bands for Pt/Ce0.7Zr0.3O2 shifted to the middle mainly because of the band at 603 cm-1 for Ce0.7Zr0.3O2 in Fig. 2A left graph. Pyrochlore-like Ce2Zr2O7 structure formed when Ce0.5Zr0.5O2 was reduced at high temperature
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(1000℃) also exhibited a Raman band at about 550cm-1 [17]. However, the latter band was not found in present Ce0.7Zr0.3O2 with relatively low reduction temperature. Tetragonal lattice of Ce0.5Zr0.5O2 was reported to give a Raman band at 630cm-1 [18], where the corresponding band for the Ce0.7Zr0.3O2 support was 603cm-1 (Fig. 2A). Hence, the two Raman bands in this study are mainly due to the two Pt oxide species on the surface of supports. During reduction, Pt-O-Ce and Pt-O disappeared with increasing temperature, and complete reduction occurred at 200℃, 150℃ and 250℃ for Pt/CeO2, Pt/Ce0.7Zr0.3O2 and S-Pt/Ce0.7Zr0.3O2, respectively. The highest reduction temperature observed in the S-Pt/Ce0.7Zr0.3O2 presented the strongest SMSI. The lowest reduction temperature observed in the Pt/Ce0.7Zr0.3O2 is probably due to the enhanced mobility of lattice O, which was induced by Zr4+ present in the ceria lattice, since O atoms in Pt-O and Pt-O-Ce involve lattice oxygen [7, 19]. In addition, it is found that Pt-O-Ce is easier to be reduced than Pt-O, especially in Pt/Ce0.7Zr0.3O2. A Similar result was reported in the case of Pt/CexZryYzO2 by other researcher [1], where Pt-O-Ce disappeared while Pt-O just decreased after reduction at 400℃. During re-oxidation, Pt-O-Ce and Pt-O appeared at 100℃, 50℃ and 50℃ in the case of Pt/CeO2, Pt/Ce0.7Zr0.3O2 and S-Pt/Ce0.7Zr0.3O2, respectively. The Pt-O-Ce signal was stronger than that of Pt-O at low temperature, while the opposite was observed at high temperature, indicating that the re-oxidation of Pt-O-Ce was easier than that of Pt-O.
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Figure 2 In situ Raman spectra of CeO2, Ce0.7Zr0.3O2 and Pt/CeO2 (A), Pt/Ce0.7Zr0.3O2 and S-Pt/Ce0.7Zr0.3O2 (B).
ACCEPTED MANUSCRIPT 3.3. Discussion
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The XP spectrum shown in Fig. 1C indicates that Pt deposited on CeO2 appears in the Pt1+ and Pt2+ oxidation states, while Pt-O and Pt-O-Ce bonding is evidenced by Raman (Fig. 2). It is reasonable to believe that Pt-O-Ce can be ascribed to Pt2+ since it is easier to be reduced. However, this is contradictory to the observed fact that Pt-O-Ce is easier to be re-oxidized. A model is established in this work (Fig. 3) based on DFT (density functional theory) [20], which could be used to properly explain the redox behaviours observed. In Case 1, Pt is bonded on the top site of surface O, and only one electron is transferred from Pt (resulting to the formation of Pt1+) to 3 Ce4+ cations via oxygen atom. In Case 2, Pt is bonded on the bridge site of surface O and two electrons are transferred to 6 Ce4+ cations (resulting in the formation of Pt2+). O-bridge site is the preferential bond site for Pt compared to the O-top site. As a result, Pt-O-Ce appeared preferentially at lower temperature during re-oxidation. Given the bond lengths of Pt-O (0.193 nm), Pt-Ce (0.316 nm) in Case 1 and Pt-O (0.214 nm), Pt-Ce (0.26 nm) in Case 2, it is indicated that Pt-O bonding in Case 1 is stronger than that in Case 2. Therefore, Pt-O-Ce bonding is considered more weak and thus this species is reduce more easily, although Pt is in higher oxidation state. This proposed model is consistent to other studies [1, 2]. Pt-O signal decreased but Pt-O-Ce signal increased when oxidation occurred at temperature higher than 600℃ [2], indicating the higher oxidation degree for Pt in the Pt-O-Ce than Pt-O bonding state. However, it should be pointed out that only part of Pt could bound to the lattice O on CeO2 surface in Pt/CeO2. It was reported that Pt segregated to CeO2 surface both as isolated ions and neutral cluster [21]. In addition, Pt95 and Pt122 clusters (～1.5nm size) on CeO2 (111) were reported to be stable, and only few Pt atoms on the interface interacted with the support [22]. Pt particles in the 1-2 nm range were also observed in Pt/Ce0.7Zr0.3O2 (Fig. 1B). However, in the case of S-Pt/Ce0.7Zr0.3O2 all Pt atoms strongly interacted with the supports and only Pt2+ was found in XP spectra (Fig. 1C). The reduction of S-Pt/Ce0.7Zr0.3O2 is also described in Fig. 3, according to the discussion offered above. The reduction could be divided into 3 steps. First, Pt oxide species is reduced by H2, meanwhile O vacancies are formed at the surface. Second, O vacancies are compensated by lattice O, away from Pt via O atom diffusion. Third, Pt is bound to the active O atoms to form Pt oxide species. In this process, Pt oxide species are continuously reduced and oxidized until all active lattices O in the support is depleted. Thus, Pt oxide species played important roles as intermediates during reduction. And the reduction temperature of residual Pt oxide species (about 350℃, shown in Fig. 1A left graph) was higher than that of surface Ce4+ (about 200℃). That was the mechanism of SMSI on promoting reduction. Doping of CeO2 with Zr4+ enhances mobility of lattice O as discussed above, thus it significantly improved the second step. As a result, the quantity of reducible O increased. For Pt/Ce0.7Zr0.3O2, H2 dissociation effect further improved reduction because of the existence of Pt neutral cluster.
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Figure 3. Pt oxide species supported on CeO2 (111) surface and the mechanism of S-Pt/Ce0.7Zr0.3O2 reduction.
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4. Conclusions
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Combined in situ Raman spectroscopy and H2-TPR were used to investigate the redox behaviour of the Pt-O-Ce and Pt-O in S-Pt/Ce0.7Zr0.3O2 catalyst. Evidence for the existence of SMSI in Pt/CeO2 and Pt/Ce0.7Zr0.3O2 that improves reduction properties of support, in addition to H2 dissociation and spillover, was obtained. Pt-O-Ce and Pt-O species play important roles in SMSI attributed to Pt2+ and Pt1+ species, respectively. Furthermore, Pt-O-Ce and Pt-O species act as intermediates in the mechanism of reduction process which involves SMSI between Pt and the reducible support.
Acknowledgements
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References
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The authors would like to thank Weidong Liu, Haicheng Ding, Pengfei Yang and Tiezhong Liu for supporting the experiments. Also thank to Huaqin Kou and Guikai Zhang for useful advices on writing. This research was supported by China Academy of Engineering Physics (Grant No. TCGH0711).
[1] Y. Nagai, T. Hirabayashi, K. Dohmae, N. Takagi, T. Minami, H. Shinjoh, S. Matsumoto, J. Catal. 242 (2006) 103-109. [2] M. Hatanaka, N. Takahashi, N. Takahashi, T. Tanabe, Y. Nagai, A. Suda, H. Shinjoh, J. Catal. 266 (2009) 182-190. [3] A.F. Diwell, R.R. Rajaram, H.A. Shaw, T.J. Truex, Stud. Surf. Sci. Catal. 71 (1991) 139-152. [4] L.L. Murrell, S.J. Tauster, D.R. Anderson, Stud. Surf. Sci. Catal. 71 (1991) 275-289. [5] A. Morikawa, T. Suzuki, T. Kanazawa, K. Kikuta, A. Suda, H. Shinjoh, Appl. Catal. B 78 (2008) 210-221. [6] Y. Nagai, K. Dohmae, Y. Ikeda, N.I. Takagi, T. Tanabe, N. Hara, G. Guilera,
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S.Pascarelli, M.A. Newton, O. Kuno, H. Jiang, H. Shinjoh, S. Matsumoto, Angew.Chem. 120 (2008) 9443-9446. [7] W.Y. Lin, A.A. Herzing, C.J. Kiely, I.E. Wachs, J. Phys. Chem. C 112 (2008) 5942-5951. [8] J. Jones, H.F. Xiong, A.T. DeLaRiva, E.J. Peterson, H. Pham, S.R. Challa, G.S. Qi, S. Oh, M.H. Wiebenga, X.I.P. Hernandez, Y. Wang, A.K. Date, Science 353(2016) 150-154. [9] G.R. Ranga, Bull. Mater. Sci. 22 (1999) 89. [10] L.J. Meng, L.C. Liu, X.H. Zi, H.X. Dai, Z. Zhao, X.P. Wang, H. He, Front. Environ. Sci. Engin. China 4(2) (2010) 164-171. [11] Z.X. Yang, Z.S. Lu, G.X. Luo, K. Hermansson, Phys. Lett. A 369 (2007) 132-139. [12] P. Tereshchuk, R.L.H. Freire, C.G. Ungureanu, Y. Seminovshi, A. Kiejna, J.L.F.D. Silva, Phys. Chem. Chem. Phys. 17 (2015) 13520-13530. [13] H.B. Li, P.C. Zhang, G. Li, J.B. Lu, Q.W. Wu, Y.J. Gu, J. Alloy Compd. 682 (2016) 132-137. [14] M. Daniel, S. Loridant, J. Raman Spectrosc. 43 (2012) 1312-1319. [15] L.L. Murrell, S.J. Tauster, D.R. Anderson, Stud. Surf. Sci. Catal. 71 (1991) 275-289. [16] M.S. Brogan, T.J. Dines, J.A. Cairns, J. Chem. Soc. Faraday Trans. 90 (1994) 1461-1466. [17] I. Alessandri, M.A. Banares, L.E. Depero, M. Ferroni, P. Fornasiero, F.C. Gennari, N. Hickey, M.V. Martinez-Huerta, T. Montini, Top. Catal. 41 (2006) 35-42. [18] S. Urban, P. Dolcet, M. Moller, L. Chen, P.J. Klar, Appl. Catal. B. 197 (2016) 23-34. [19] M.Y. Smirnov, G.W. Graham, Catal. Lett. 72 (2001) 39-44. [20] Z.S. Lu, G.X. Luo, Z.X. Yang, J. Henan University. 36 (2008) 40-43. [21] T.X.T. Sayle, S.C. Parker, C.D.A. Catlow, J. Phys. Chem. 98 (1994) 13625-13630. [22] S.M. Kozlov, K.M. Neyman, J. Catal. 344 (2016) 507-514.
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Graphical Abstract
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Pt bonded to lattice O on CeO2 (111) surface, forming Pt oxide species (represented as Pt1+ and Pt2+ or Pt-O and Pt-O-Ce). This effect was the essence of Strong Metal-Support Interaction (SMSI) in single atom catalyst (S-Pt/Ce0.7Zr0.3O2). The reduction of S-Pt/Ce0.7Zr0.3O2 was significantly improved by SMSI via the Pt oxide species as intermediates in reduction.
ACCEPTED MANUSCRIPT Highlights Pt single atom catalyst was employed for SMSI study. Pt-O and Pt-O-Ce were ascribed to Pt1+ and Pt2+, with different Pt adsorption sites on CeO2.
Pt oxides act as intermediates in reduction, which is the SMSI improving mechanism.
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Quanwen Wu, Jingwen Ba, Xiayan Yan, Jinchun Bao, Zhiyong Huang, Sanping Dou, Danling Dai, Tao Tang, Wenhua Luo, Daqiao Meng PII: DOI: Reference:
S1566-7367(17)30179-6 doi: 10.1016/j.catcom.2017.04.045 CATCOM 5030
To appear in:
Catalysis Communications
Received date: Revised date: Accepted date:
16 January 2017 24 April 2017 25 April 2017
Please cite this article as: Quanwen Wu, Jingwen Ba, Xiayan Yan, Jinchun Bao, Zhiyong Huang, Sanping Dou, Danling Dai, Tao Tang, Wenhua Luo, Daqiao Meng , Insight of Pt-support interaction in S-Pt/Ce0.7Zr0.3O2 by in situ Raman spectroscopy. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.04.045
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Insight of Pt-support interaction in S-Pt/Ce0.7Zr0.3O2 by in situ Raman spectroscopy
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Quanwen Wu, Jingwen Ba, Xiayan Yan, Jinchun Bao, Zhiyong Huang, Sanping Dou, Danling Dai, Tao Tang, Wenhua Luo*, Daqiao Meng* China Academy of Engineering Physics, Mianyang, Sichuan, 621900, China * Corresponding author: E-mail address: [email protected] Telephone: 0816-3625304
ACCEPTED MANUSCRIPT Abstract
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Strong Metal-Support Interactions (SMSI) are important in Pt supported catalysts for their catalytic performance. SMSI in Pt/CeO2, Pt/Ce0.7Zr0.3O2 and single atom S-Pt/Ce0.7Zr0.3O2 catalysts were studied by in situ Raman spectroscopy and H2 Temperature-Programmed Reduction. Evidence of SMSI in Pt/CeO2 and Pt/Ce0.7Zr0.3O2 that improve the reduction properties of supports other than H2 dissociation and spillover is obtained. It was also found that Pt-O and Pt-O-Ce bonding in the said catalysts play an important role in SMSI. The Pt bonding could be ascribed to Pt oxides species (Pt1+ and Pt2+) with different Pt adsorption sites on CeO2 surface and which act as intermediates in reduction process. This gives a new insight into the SMSI and explains the observed Pt/CeO2 reduction characteristics.
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Keywords
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Pt/CeO2, Pt/Ce0.7Zr0.3O2, Strong Metal-Support Interaction, Single atom catalyst, Raman Spectroscopy
ACCEPTED MANUSCRIPT 1. Introduction
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Ceria-based metal oxides are widely used as promoters and supports in three-way catalysts (TWCs) for automobile exhaust purification because of their excellent performances towards oxygen storing and release and stabilization of noble metal particles against sintering [1,2]. It has been proposed that CeO2-based metal oxides prevent Pt particles from sintering because of the strong Pt-support interactions developed [1-6]. Nagai et al. [1] reported that the strong interaction between oxidized Pt and CeO2-based supports stabilize Pt atoms and the Pt-O-Ce bond acts as the driving force for re-dispersing sintered Pt particles [6]. The Pt nanostructure that forms Pt-O-Ce bonds has been determined using XPS and high-resolution TEM [2]. The strength of Pt-O-support and Pt-support interactions on Pt/Al2O3, Pt/SiO2 and Pt/CeO2, respectively, were investigated with in situ Raman spectroscopy [7]. However, the redox behaviour of Pt-O and Pt-O-Ce bonding during reduction-oxidation process has not been yet clearly understood. Recently, Jones and Xiong [8] reported a new process of thermally stable single-atom platinum on ceria catalysts which can be employed for the investigation of the Pt-support interactions. In the present study, the redox behaviour of the Pt–O and Pt-O-Ce bonding in the S-Pt/Ce0.7Zr0.3O2 catalyst were studied by in situ Raman spectroscopy combined with H2 Temperature-Programmed Reduction (H2-TPR) give insights into the Pt-support interactions.
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CeO2 and Ce0.7Zr0.3O2 catalyst supports were prepared by the sol-gel method. Ce(NO3)3.6H2O and ZrO(NO3)2.H2O (used for Ce0.7Zr0.3O2) were dissolved (molar ratio Ce:Zr=7:3) in deionized water and citric acid was added dropwise and the resulting solution was stirred at 65℃ for 2 hours. The solution was then kept at 80℃ in water bath until a gel was formed. After dried (overnight), the gel was calcined at 500℃ for 3 hours in a muffle furnace. Pt/Al2O3, Pt/CeO2 and Pt/Ce0.7Zr0.3O2 catalysts were prepared by the wet-impregnation method. The supports of Al2O3, CeO2 and Ce0.7Zr0.3O2 were immersed into an appropriate volume of H2PtCl6.6H2O solution so as to provide 1wt% Pt nominal loading and stirred at 40℃ for 2 hours. After evaporation at 80℃, the samples were dried overnight and calcined at 500℃ for 3 hours. For the Pt single atom catalyst preparation, Pt/Al2O3 was first pressed into pellets. Then Ce0.7Zr0.3O2 powder was aged with Pt/Al2O3 pellets in air at 650℃ for 12 hours. Finally, Ce0.7Zr0.3O2 powder was separated from Pt/Al2O3 pellets.
2.2. Characterization of catalysts H2-TPR profiles were obtained with Autosorb-1-C Physic/Chemisorptions Analyzer. The samples were loaded on top of quartz wool in a U-shaped quartz tube (i.d. =10 mm), and then pretreated with a helium flow at 300℃ for 1h to remove adsorbed
ACCEPTED MANUSCRIPT gases. After cooling to room temperature, samples were reduced using 5vol % H2/Ar gas mixture (30 ml/min) with a heating rate of 10℃/min. Hydrogen consumption was monitored with a thermal conductivity detector (TCD).
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Raman spectra were collected with a back scattering Raman spectrometer (LabRAM HR800, form Horiba JY) using a 457 nm Argon ion laser (Stabilite 2017, from Spectra Physics). The laser power was set relatively low (3mW), with collection time of 50s. A 20× objective was employed for both focusing the laser beam on the sample and collecting the scattered photons. In situ Raman spectra were collected using a sample cell under 5vol %H2/Ar and 5vol %O2/Ar with a flow rate of 50ml/min and temperature in the 25- 300℃ range.
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3. Results and discussion
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3.1 Redox performance of catalysts
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H2-TPR traces of supports and supported Pt catalysts are shown in Fig. 1A (left and right graphs). After doping CeO2 with Zr4+ a significant improvement in the reduction of bulk Ce4+ occurred, which is in agreement with previous studies [9, 10]. For the supported Pt catalysts, reduction temperatures were much lower. The main reduction peak at about 200℃ is ascribed to the reduction of surface Ce4+ promoted by Pt, and the peak at about 350℃ is related to the reduction of Pt oxide species. The reduction temperatures of Pt oxide species in Pt/CeO2 and Pt/Ce0.7Zr0.3O2 are much higher than that in Pt/Al2O3 (about 100℃)，revealing the stronger SMSI of Pt-CeO2 (Ce0.7Zr0.3O2) than Pt-Al2O3. The removed O estimated from integrated area of reduction peaks in Fig. 1A is 1218 μmol/g for Pt/Ce0.7Zr0.3O2 below 350℃, which exceeds the amount of oxygen bound with Pt cations, revealing the reduction of support. The reduction process of Pt supported catalyst will be further discussed in Section 3.3. Usually, promotion of reduction by the presence of Pt is ascribed to H2 dissociation and spillover processes [9]. Hydrogen is first dissociated on Pt and then spilt-over H-atoms greatly accelerate the reduction kinetics. However, the interaction between Pt and support might be another important reason. The interaction could promote reduction of support by forming oxygen vacancies, which is verified by some theoretical studies [11, 12]. Here, CO-TPR tests were carried out, where dissociation of CO on Pt was minimized. Curves a and b in Fig. 1A (right) show that Pt indeed promoted the reduction of surface Ce4+ by shifting the reduction peak toward lower temperature. The H2-TPR trace of S-Pt/Ce0.7Zr0.3O2 (curve c, Fig. 1A right graph) is similar to that of CO-TPR trace of Pt/Ce0.7Zr0.3O2 (curve b, Fig. 1A right graph), suggesting that H2 dissociation was almost absent and SMSI dominated. This could be further explained by TEM (Fig. 1B) and XPS (Fig. 1C) results. In the TEM image of S-Pt/Ce0.7Zr0.3O2, no Pt particle/cluster could be observed, where the contrast analysis indicated that some sites on CeO2 lattice with high contrast might be related to the presence of Pt atoms. However, in the Pt/Ce0.7Zr0.3O2 solid, many particles with 1-2 nm in size were obviously detected. Pt single atom dispersed on S-Pt/Ce0.7Zr0.3O2 should be regarded as Pt2+ state. The latter species cannot dissociate hydrogen, thus H2 dissociation and spillover of H were not observed during reduction of
ACCEPTED MANUSCRIPT S-Pt/Ce0.7Zr0.3O2. The reduction of S-Pt/Ce0.7Zr0.3O2 and Pt/Al2O3 mixed sample (curve d, Fig. 1A right graph) is different from that of S-Pt/Ce0.7Zr0.3O2. The first reduction peak sharpened and shifted toward lower temperature, because Pt/Al2O3 brought in the influence of H2 dissociation. Thus, SMSI and H2 dissociation-spillover both improved the reduction of support in Pt/Ce0.7Zr0.3O2 system. In the following Section, the mechanism of SMSI on improving the reduction of support is elaborated.
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Figure 1: (A) H2-TPR profiles of CeO2, Ce0.7Zr0.3O2 and supported catalysts. CO-TPR profiles of Ce0.7Zr0.3O2 and Pt/Ce0.7Zr0.3O2 (curve a b, right graph). H2-TPR of S-Pt/Ce0.7Zr0.3O2 (curve c) as well as Pt/Al2O3 and Pt/Ce0.7Zr0.3O2 mixed sample (curve d). (B) TEM images revealing the presence of Pt single atom in S-Pt/Ce0.7Zr0.3O2 and Pt particles (1-2 nm in size) in Pt/Ce0.7Zr0.3O2. (C) XP spectrum of S-Pt/Ce0.7Zr0.3O2 indicating the presence of Pt2+ species.
3.2. In situ Raman spectroscopy In situ Raman spectra of CeO2, Ce0.7Zr0.3O2, Pt/CeO2, Pt/Ce0.7Zr0.3O2 and S-Pt/Ce0.7Zr0.3O2 recorded under reducing/oxidizing gas atmosphere are provided in Fig. 2. Raman bands at about 455, 554 and 651cm-1 were detected for Pt/CeO2 and 455, 560 and 640cm-1 for Pt/Ce0.7Zr0.3O2. The strong 455cm-1 vibration was associated with the F2g symmetry of the crystalline CeO2 [13], while Raman features at 554 and 651cm-1 were respectively assigned to Pt-O-Ce vibration and Pt-O vibration [7, 14-16]. Raman bands for Pt/Ce0.7Zr0.3O2 shifted to the middle mainly because of the band at 603 cm-1 for Ce0.7Zr0.3O2 in Fig. 2A left graph. Pyrochlore-like Ce2Zr2O7 structure formed when Ce0.5Zr0.5O2 was reduced at high temperature
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(1000℃) also exhibited a Raman band at about 550cm-1 [17]. However, the latter band was not found in present Ce0.7Zr0.3O2 with relatively low reduction temperature. Tetragonal lattice of Ce0.5Zr0.5O2 was reported to give a Raman band at 630cm-1 [18], where the corresponding band for the Ce0.7Zr0.3O2 support was 603cm-1 (Fig. 2A). Hence, the two Raman bands in this study are mainly due to the two Pt oxide species on the surface of supports. During reduction, Pt-O-Ce and Pt-O disappeared with increasing temperature, and complete reduction occurred at 200℃, 150℃ and 250℃ for Pt/CeO2, Pt/Ce0.7Zr0.3O2 and S-Pt/Ce0.7Zr0.3O2, respectively. The highest reduction temperature observed in the S-Pt/Ce0.7Zr0.3O2 presented the strongest SMSI. The lowest reduction temperature observed in the Pt/Ce0.7Zr0.3O2 is probably due to the enhanced mobility of lattice O, which was induced by Zr4+ present in the ceria lattice, since O atoms in Pt-O and Pt-O-Ce involve lattice oxygen [7, 19]. In addition, it is found that Pt-O-Ce is easier to be reduced than Pt-O, especially in Pt/Ce0.7Zr0.3O2. A Similar result was reported in the case of Pt/CexZryYzO2 by other researcher [1], where Pt-O-Ce disappeared while Pt-O just decreased after reduction at 400℃. During re-oxidation, Pt-O-Ce and Pt-O appeared at 100℃, 50℃ and 50℃ in the case of Pt/CeO2, Pt/Ce0.7Zr0.3O2 and S-Pt/Ce0.7Zr0.3O2, respectively. The Pt-O-Ce signal was stronger than that of Pt-O at low temperature, while the opposite was observed at high temperature, indicating that the re-oxidation of Pt-O-Ce was easier than that of Pt-O.
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Figure 2 In situ Raman spectra of CeO2, Ce0.7Zr0.3O2 and Pt/CeO2 (A), Pt/Ce0.7Zr0.3O2 and S-Pt/Ce0.7Zr0.3O2 (B).
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The XP spectrum shown in Fig. 1C indicates that Pt deposited on CeO2 appears in the Pt1+ and Pt2+ oxidation states, while Pt-O and Pt-O-Ce bonding is evidenced by Raman (Fig. 2). It is reasonable to believe that Pt-O-Ce can be ascribed to Pt2+ since it is easier to be reduced. However, this is contradictory to the observed fact that Pt-O-Ce is easier to be re-oxidized. A model is established in this work (Fig. 3) based on DFT (density functional theory) [20], which could be used to properly explain the redox behaviours observed. In Case 1, Pt is bonded on the top site of surface O, and only one electron is transferred from Pt (resulting to the formation of Pt1+) to 3 Ce4+ cations via oxygen atom. In Case 2, Pt is bonded on the bridge site of surface O and two electrons are transferred to 6 Ce4+ cations (resulting in the formation of Pt2+). O-bridge site is the preferential bond site for Pt compared to the O-top site. As a result, Pt-O-Ce appeared preferentially at lower temperature during re-oxidation. Given the bond lengths of Pt-O (0.193 nm), Pt-Ce (0.316 nm) in Case 1 and Pt-O (0.214 nm), Pt-Ce (0.26 nm) in Case 2, it is indicated that Pt-O bonding in Case 1 is stronger than that in Case 2. Therefore, Pt-O-Ce bonding is considered more weak and thus this species is reduce more easily, although Pt is in higher oxidation state. This proposed model is consistent to other studies [1, 2]. Pt-O signal decreased but Pt-O-Ce signal increased when oxidation occurred at temperature higher than 600℃ [2], indicating the higher oxidation degree for Pt in the Pt-O-Ce than Pt-O bonding state. However, it should be pointed out that only part of Pt could bound to the lattice O on CeO2 surface in Pt/CeO2. It was reported that Pt segregated to CeO2 surface both as isolated ions and neutral cluster [21]. In addition, Pt95 and Pt122 clusters (～1.5nm size) on CeO2 (111) were reported to be stable, and only few Pt atoms on the interface interacted with the support [22]. Pt particles in the 1-2 nm range were also observed in Pt/Ce0.7Zr0.3O2 (Fig. 1B). However, in the case of S-Pt/Ce0.7Zr0.3O2 all Pt atoms strongly interacted with the supports and only Pt2+ was found in XP spectra (Fig. 1C). The reduction of S-Pt/Ce0.7Zr0.3O2 is also described in Fig. 3, according to the discussion offered above. The reduction could be divided into 3 steps. First, Pt oxide species is reduced by H2, meanwhile O vacancies are formed at the surface. Second, O vacancies are compensated by lattice O, away from Pt via O atom diffusion. Third, Pt is bound to the active O atoms to form Pt oxide species. In this process, Pt oxide species are continuously reduced and oxidized until all active lattices O in the support is depleted. Thus, Pt oxide species played important roles as intermediates during reduction. And the reduction temperature of residual Pt oxide species (about 350℃, shown in Fig. 1A left graph) was higher than that of surface Ce4+ (about 200℃). That was the mechanism of SMSI on promoting reduction. Doping of CeO2 with Zr4+ enhances mobility of lattice O as discussed above, thus it significantly improved the second step. As a result, the quantity of reducible O increased. For Pt/Ce0.7Zr0.3O2, H2 dissociation effect further improved reduction because of the existence of Pt neutral cluster.
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Figure 3. Pt oxide species supported on CeO2 (111) surface and the mechanism of S-Pt/Ce0.7Zr0.3O2 reduction.
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4. Conclusions
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Combined in situ Raman spectroscopy and H2-TPR were used to investigate the redox behaviour of the Pt-O-Ce and Pt-O in S-Pt/Ce0.7Zr0.3O2 catalyst. Evidence for the existence of SMSI in Pt/CeO2 and Pt/Ce0.7Zr0.3O2 that improves reduction properties of support, in addition to H2 dissociation and spillover, was obtained. Pt-O-Ce and Pt-O species play important roles in SMSI attributed to Pt2+ and Pt1+ species, respectively. Furthermore, Pt-O-Ce and Pt-O species act as intermediates in the mechanism of reduction process which involves SMSI between Pt and the reducible support.
Acknowledgements
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References
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The authors would like to thank Weidong Liu, Haicheng Ding, Pengfei Yang and Tiezhong Liu for supporting the experiments. Also thank to Huaqin Kou and Guikai Zhang for useful advices on writing. This research was supported by China Academy of Engineering Physics (Grant No. TCGH0711).
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Graphical Abstract
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Pt bonded to lattice O on CeO2 (111) surface, forming Pt oxide species (represented as Pt1+ and Pt2+ or Pt-O and Pt-O-Ce). This effect was the essence of Strong Metal-Support Interaction (SMSI) in single atom catalyst (S-Pt/Ce0.7Zr0.3O2). The reduction of S-Pt/Ce0.7Zr0.3O2 was significantly improved by SMSI via the Pt oxide species as intermediates in reduction.
ACCEPTED MANUSCRIPT Highlights Pt single atom catalyst was employed for SMSI study. Pt-O and Pt-O-Ce were ascribed to Pt1+ and Pt2+, with different Pt adsorption sites on CeO2.
Pt oxides act as intermediates in reduction, which is the SMSI improving mechanism.
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