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First Direct Visualization of Spillover Species Emitted from Pt Nanoparticles† Satoru Takakusagi,‡ Ken-ichi Fukui,§ Ryugo Tero, Kiyotaka Asakura,*,‡ and Yasuhiro Iwasawa*,^ ‡
Catalysis Research Center, Hokkaido University, Kita-ku N21W10, Sapporo, Hokkaido 001-0021, Japan, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan, Division of Biomolecular Sensing, Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan, and ^Department of Engineering Science, The University of Electro-Communications,1-5-1 Tyouhugaok, Tyouhu, Tokyo 182-8585, Japan )
§
Received May 19, 2010. Revised Manuscript Received July 4, 2010 We studied the methanol adsorption behavior of Pt nanoparticles that were vacuum-deposited on a TiO2(110) surface at room temperature by using an ultrahigh vacuum (UHV) scanning tunneling microscope (STM). A large number of bright spots were observed on fivefold-coordinated Ti (Ti5c) rows of the TiO2(110) surface after exposure of the Pt/ TiO2(110) to methanol vapor. We assigned the bright spots to methoxy species. These were mobile and were found to hop along the Ti5c rows. In situ time-resolved STM observations of the formation and migration of the bright spots on the Pt/TiO2(110) were carried out in the presence of methanol. The bright spots were produced at the periphery of the Pt nanoparticles and migrated to the substrate Ti5c rows. We discuss the spillover process and behavior of the methoxy species on the Pt/TiO2(110).
1. Introduction Many practical catalysts are used in the form of supported catalysts wherein metal nanoparticles are highly dispersed on powder oxide surfaces with large surface areas. The oxides not only physically stabilize the nanoparticles but also electronically and/or chemically modify the nanoparticles. Thus, the shape, structure, and electronic state of the nanoparticles strongly depend on properties and structures of the oxide surfaces.1 The metal-support interaction is in itself an old concept but is still important in catalysis science. The metal-support interactions can be categorized into three effects.2 (1) Electronic effects, characterized by electron flow from or to the support. (2) Geometric effects, which refer to the support effects on the shape and size of the metal species. These include decoration and migration of support species onto the metal particles. (3) Boundary effects, wherein a special active phase is created at the boundary between the metal and oxide support. There is another kind of dynamic metal-support interaction through the diffusion of adsorbates. “Spillover” is the term used for a system wherein adsorbed species on the metal particle diffuse to the oxide surface.3-5 For example, dissociatively adsorbed † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. (K.A.) Telephone & Fax: þ81-11-706-9113. E-mail:
[email protected]. (Y.I.) Telephone & Fax: þ81-42-443-5952. E-mail:
[email protected].
(1) Ertl, G.; Kn€ozinger, H.; Sch€uth, F.; Weitkamp, J. Handbook of Heterogeneous Catalysis, 2nd ed.; Weinheim: Wiley-VCH, 2008; Vol. 2. (2) Ruppert, A. M.; Weckhuysen, B. M. Active Phase-Support Interactions. In Handbook of Heterogeneous Catalysis; Ertl, G., Kn€ozinger, H., Sch€uth, F., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 2008; pp 1178. (3) Pajonk, G. M. Appl. Catal., A 2000, 202, 157. (4) Delmon, B.; Froment, G. F. Catal. Rev. - Sci. Eng. 1996, 38, 69. (5) Conner, W. C.; Falconer, J. L. Chem. Rev. 1995, 95, 759.
16392 DOI: 10.1021/la102013a
hydrogen on Pt diffuses to oxide surfaces and modifies the support properties, whereas hydrogen cannot normally be dissociatively adsorbed on oxide supports such as Al2O3, SiO2 or TiO2 at room temperature (the exception is late transition metal oxides such as ZnO and NiO).6 The spillover hydrogen modifies the acidity and hydrogenation properties of the supports. The “capture effect” of the support surface has also been reported. CO molecules physisorbed on the oxide support diffuse to metal particles where they are stably chemisorbed.7,8 The adsorption rate of methanol on Pd particles increases when they are supported on an Al2O3 film.9 Thus, interaction through diffusion plays an important role in the understanding of catalytic activities. Direct observation of the spillover process at an atomic level is still difficult due to limited characterization techniques available. Rotermund and Ertl reported the diffusion of adsorbate from one active phase to another inactive phase by photoemission electron microscopy (PEEM); however, the spatial resolution was on the order of micrometers.10-15 Bennett et al. found, using STM, that Pd nanoparticles on a reduced TiO2(110) surface dissociatively adsorb O2 at 673 K and the oxygen “spills over” onto the support, although the spillover oxygen was not visualized in their STM (6) Sermon, P. A.; Bond, G. C. Catal. Rev. 1974, 8, 211. (7) Henry, C. R.; Chapton, C.; Duriez, C. J. Chem. Phys. 1991, 95, 700. (8) Bowker, M.; Stone, P.; Bennett, R.; Perkins, N. Catal. Lett. 2002, 497, 155. (9) Hoffmann, J.; Schauermann, S.; Johanek, V.; Hartmann, J.; Libuda, J. J. Catal. 2003, 213, 176. (10) Lauterbach, J.; Asakura, K.; Rasmussen, P. B.; Rotermund, H. H.; B€aer, M.; Graham, M. D.; Kevrekidis, I. G. Physica D 1998, 123, 493. (11) Asakura, K.; Lauterbach, J.; Rotermund, H. H.; Ertl, G. Surf. Sci. 1997, 374, 125. (12) Asakura, K.; Lauterbach, J.; Rotermund, H. H.; Ertl, G. J. Chem. Phys. 1995, 102, 8175. (13) Asakura, K.; Lauterbach, J.; Rotermund, H. H.; Ertl, G. Phys. Rev. B 1994, 50, 8043. (14) Gorodetskii, V.; Lauterbach, J.; Rotermund, H. H.; Block, J. H.; Ertl, G. Nature 1994, 370, 276. (15) Lauterbach, J.; Haas, G.; Rotermund, H. H.; Ertl, G. Surf. Sci. 1993, 294, 116.
Published on Web 07/23/2010
Langmuir 2010, 26(21), 16392–16396
Takakusagi et al.
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observations.16 They observed preferential layer growth of TiO2 around the Pd particles by the spillover oxygen. Pt is one of the most important catalyst metals, widely used for hydrogenation, reforming, NOx reduction, and other reactions. It also displays the spillover phenomenon in the case of Pt/ Al2O36,17,18 and Pt in zeolite.19,20 The adsorption and reaction properties of methanol (CH3OH) on Pt particles are very important from the standpoint of fuel cell applications, particularly in direct methanol fuel cells (DMFCs). In this study, Pt nanoparticles were prepared on a TiO2(110) surface by metal vapor deposition (MVD), and the adsorption properties of methanol on Pt/TiO2(110) model catalyst surfaces were studied by ultrahigh vacuum (UHV) scanning tunneling microscope (STM). Surprisingly, we found a large number of bright spots assignable to the methoxy species on fivefold-coordinated Ti (Ti5c) rows after exposure of the Pt/TiO2(110) to methanol vapor at room temperature. Such bright spots were not observed when a bare TiO2(110) surface was exposed to methanol vapor at room temperature. Methanol can be adsorbed dissociatively only at oxygen defect sites.21 In situ STM measurements of the methanol adsorption process on these Pt/TiO2(110) surfaces allowed for direct observations of the spillover process of each methoxy group from the Pt nanoparticles to the substrate TiO2(110) surface. To the best of the authors’ knowledge, this is the first atomic/molecular level visualization of the spillover phenomenon of an adsorbate. We discuss the assignment of the adsorbed species to methoxy and their spillover process observed in the Pt/TiO2(110) system.
2. Experimental Section The experiments were performed in a UHV system (JEOL JSTM4500VT), consisting of three separate UHV chambers: a preparation chamber (base pressure < 2 10-8 Pa), an STM chamber (base pressure < 2 10-8 Pa), and a loadlock chamber (base pressure < 1 10-6 Pa). The first chamber is equipped with a sputter gun (VG), LEED-AES optics (OMICRON), a mass spectrometer (Balzers), and a UHV platinum evaporator (Focus, OMICRON). In the evaporator, platinum vapor was generated through electron bombardment of a Pt rod by applying a high voltage (typically 800 V) between the filament and the Pt rod. Pt vapor flux at the sample was estimated form the calibration curve of the emission current. A polished rutile-TiO2(110) wafer (Earth Chemicals), with a size of 6.5 1 0.25 mm3, was calcined in air at 1100 K for 1 h. Nickel film was deposited on the backside of the wafer for resistive heating. Cycles of Arþ sputtering (3 keV, 3 min) and vacuum annealing (900 K, 60 s) yielded a sharp (1 1) LEED pattern. The MVD process was carried out on the TiO2(110) surface at room temperature. The methanol used (Suprapure reagent grade, Wako Pure Chemicals) was purified by several freeze-pump-thaw cycles using liquid nitrogen before introduction onto the sample surfaces and was dosed through a variable leak valve by backfilling. One monolayer (1 ML) was defined as the density of the (1 1) units of the TiO2(110) surface, that is 5.2 1018 m-2. STM images were recorded in constant current mode using an electrochemically etched W tip at room temperature. Positive sample bias voltages (Vs) in the range of 0.9-1.2 V and tunnel(16) Bennett, R. A.; Stone, P.; Bowker, M. Catal. Lett. 1999, 59, 99. (17) Ahmed, F.; Alam, Md. K.; Suzuki, A.; Koyama, M.; Tsuboi, H.; Hatakeyama, N.; Endou, A.; Takaba, H.; Del Carpio, C. A.; Kubo, M.; Miyamoto, A. J. Phys. Chem. C 2009, 113, 15676. (18) Miller, J. T.; Meyers, B. L.; Modica, F. S.; Lane, G. S.; Vaarkamp, M.; Koningsberger, D. C. J. Catal. 1993, 143, 395. (19) Miller, J. T.; Pei, S. Appl. Catal., A 1998, 168, 1. (20) Zhang, A.; Nakamura, I.; Fujimoto, K. J. Catal. 1997, 168, 328. (21) Zhang, Z.; Bondarchuk, O.; White, J. M.; Kay, B. D.; Dohnalek, Z. J. Am. Chem. Soc. 2006, 128, 4198.
Langmuir 2010, 26(21), 16392–16396
Figure 1. (a) STM image of Pt particles on TiO2(110) prepared by exposure of a clean TiO2(110) surface to Pt metal vapor at a rate of 5.8 10-3 ML 3 s-1 for 60 s (0.35 ML, 1.8 1018 m-2). 29.6 29.6 nm2; Vs, þ1.10 V; It, 0.52 nA. The histogram of the apparent diameter of the Pt particles is also shown. A region of 50 50 nm2 including the imaged area was considered for the histogram. (b) Apparent height (measured from Ti5c row) versus diameter of the Pt particles. The same 50 50 nm2 region as that for the above histogram was used for the plot. ing currents (It) of 0.1-0.6 nA were employed for the STM observations.
3. Results Figure 1a shows an STM image of a TiO2(110) surface after exposure to Pt metal vapor at an evaporation rate of 5.8 10-3 ML 3 s-1 for 60 s (0.35 ML, 1.8 1018 m-2) at room temperature. The MVD particles were uniformly distributed on terraces without selective decoration of the step edges.22-24 The density of the Pt particles was 1.1 1017 m-2. A histogram of the apparent diameter of the Pt particles is also shown in Figure 1a. Pt particles smaller than 2 nm in diameter were formed on the TiO2(110) surface under the applied deposition condition. The average diameter of the Pt particles was estimated to be 1.20 nm. Figure 1b shows a plot of the apparent height (measured from the Ti5c row) as a function of the diameter of the Pt particles. The height of the Pt particles increased linearly with increasing diameter. The minimum height observed was around 0.14 nm, which could be regarded as the monatomic Pt height, although the height was measured from the Ti row by STM and its value may not accurately reflect the geometric height due to convolution of geometric and electronic effects. If we assume that about 0.14 nm corresponds to the monatomic Pt height, the data indicate that less than a few atomic layers of Pt particles are formed on the TiO2(110) surface. The ratio of the height to the diameter, which was about 0.2 as determined from the plot in Figure 1b, suggested the formation of flat-shaped particles on the TiO2(110) surface and the presence of rather strong interaction between the nanoparticles (22) Sasahara, A.; Pang, C. L.; Onishi, H. J. Phys. Chem. B 2006, 110, 17584. (23) Uetsuka, H.; Pang, C.; Sasahara, A.; Onishi, H. Langmuir 2005, 21, 11802. (24) Gan, S.; Liang, Y.; Baer, D. R.; Grant, A. W. Surf. Sci. 2001, 475, 159.
DOI: 10.1021/la102013a
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Figure 2. (a) STM image of the Pt/TiO2(110) surface after exposure to 3 L of methanol vapor at room temperature and evacuation of the methanol vapor. 29.6 29.6 nm2; Vs, þ1.15 V; It, 0.15 nA. An enlarged STM image of the region indicated by the white square is shown in the upper left. (b) Another wide-scan image. 50 50 nm2; Vs, þ1.15 V; It, 0.15 nA.
Figure 3. Successive STM images showing hopping behavior of the bright spots on Ti5c rows of the Pt/TiO2(110) surface under UHV conditions. (a) STM image of the Pt/TiO2(110) surface after exposure to 3 L of methanol vapor at room temperature and evacuation of the methanol vapor. 24.6 10.5 nm2; Vs, þ0.95 V; It, 0.15 nA. (b) The same region as image (a) acquired 90 s after image (a) was captured. 24.6 10.5 nm2; Vs, þ0.95 V; It, 0.15 nA.
and support. It has been reported that the Pt thin film on the TiO2(110) shows low wettability.25 The nanosizing effect of Pt may enhance the interaction between the oxide and the particles. Figure 2a shows an STM image of the Pt/TiO2(110) surface after exposure to 3 L of methanol vapor at room temperature and evacuation of the methanol vapor. A large number of additional bright spots were observed on the Ti5c rows of the substrate TiO2(110) surface (see the magnified image in the upper left of Figure 2a). Interestingly, the bright spots had an asymmetric shape, as can be seen in the magnified image. The surface coverage was 0.12 ML. The apparent height and lateral average size of the spots were 0.15 and 0.4 nm, respectively. They could be distinguished from the Pt particles by their shape, height, and location. Another wide-scan area is illustrated in Figure 2b. As shown in the regions surrounded by the white dotted lines, the bright spots were observed not only on the terraces but also at step edges running across the [001] direction. Figure 3 shows sequential STM images under UHV conditions (
