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7. Atomic-Level Analysis and Catalytic Activity of Size-Selected Pt Clusters Deposited on Tio 2

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7. Atomic-Level Analysis and Catalytic Activity of Size-Selected Pt Clusters Deposited on Tio 2 R&D Review of Toyota CRDL, Vol.42 No.1 (2011) 51-62 51 Special Feature: Automotive Exhaust Catalyst Research Report Atomic-level Analysis and Catalytic Activity of Size-selected Pt Clusters Deposited on TiO2(110) Yoshihide Watanabe, Noritake Isomura, Hirohito Hirata and Xingyang Wu Report received on Jan. 20, 2011 A new experimental setup to study catalytic and electronic properties of size-selected clusters on metal oxide substrates for developing heterogeneous catalysts, such as automotive exhaust catalysts, has been developed. The apparatus consists of a size-selected cluster source, a photoemission spectrometer, a scanning tunneling microscope (STM), and a high-pressure reaction cell. The high-pressure reaction cell measurements provide information on catalytic properties under conditions close to those in n practical uses. We investigated size-selected Ptn ( = 4, 7-10, 15) clusters deposited on TiO2(110) under soft- landing conditions. The catalytic activity measurements showed that the catalytic activities have a cluster size dependence. We obtained atomic-resolution images of size-selected clusters on surfaces, enabling the identification of atomic alignment in the clusters. Clusters smaller than Pt7 lay flat on the surface with a n planar structure, and a planar-to-three-dimensional (3D) transition occurred at = 8 for Ptn clusters on TiO2. f n The binding energies of Pt 4 7/2 decreased steeply with increasing cluster size up to = 7 for Ptn, and decreased gradually for n ≥ 8. This inflection point (n = 8) agrees well with the cluster size at the planar-to- 3D transition. It was found that the core-level shifts of size-selected Pt clusters on TiO2 are closely correlated with cluster geometries. Cluster, Mass-selected, Size-selected, Scanning tunneling microscopy (STM), Reaction, TiO2(110), Platinum, CO, Oxidation, Deposition 1. Introduction well-defined uni-sized clusters on a surface for comparison with those deposited using a cluster beam Heterogeneous catalysts, such as those used in method. Some studies have been carried out on automotive exhaust systems, consist of precious metal systems wherein size-selected clusters have apparently particles supported on oxide surfaces. Currently, there been deposited successfully without fragmentation or is a real and urgent need to reduce precious metal aggregation. usage. It is speculated that the catalytic activity of Heiz et al. developed the first instrument that metal clusters has a strong size-dependence. Clusters allowed for detailed study of the chemical properties on a surface would provide further specificity because of size-selected deposited clusters using the of the interaction between the clusters and the surface. temperature programmed desorption (TPD) method(1); To study the catalytic and electronic properties of size- they also reported that the catalytic activity of size- selected clusters on metal oxide substrates from the selected Ptn (n = 5-20) clusters on MgO(100) films in view point of cluster-support interaction, as well as to the oxidation of carbon monoxide increased abruptly (2,3) devise a method for controlling catalytic activity and during the transition from Pt8 to Pt15. A similar n thermal stability, an experimental setup has been investigation of Aun ( = 1-7) clusters on TiO2(110) by developed. Cluster size and cluster-support interactions Lee et al.(4) showed that catalytic activity increased are key parameters that control catalytic activity and substantially for Au6 and Au7. Although these two thermal stability of a cluster catalyst. studies show strong dependence of catalytic activity There have been a number of studies on the catalytic on the deposited cluster size, the geometries of the properties of free size-selected clusters; however, few metal clusters on the surfaces were not measured studies have been conducted on the catalytic properties directly in either case. In the case of single crystal of surface-deposited size-selected clusters. This dearth metal surfaces, the catalytic activity was different on of studies is mainly due to the difficulty in preparing different crystal faces of the same metal(5-7) as well as © Toyota Central R&D Labs., Inc. 2011 http://www.tytlabs.co.jp/review/ 52 R&D Review of Toyota CRDL, Vol.42 No.1 (2011) 51-62 on different sites of the same crystal face.(8) This estimated from other observations. Recently, the suggests that the catalytic activity may depend on the relation between cluster size and core-level shifts have alignment of the atoms in the clusters. Tong et al.(9) been studied using size-selected metal clusters (28-31) investigated the shapes and sizes of size-selected Aun deposited on surfaces. Although these studies n ( = 1-8) clusters on TiO2(110) using scanning show strong size dependence of the core-level shifts, tunneling microscopy (STM). The alignment of gold the geometries of the clusters on the surfaces were not atoms in the clusters was not identified clearly in the measured directly. This makes it difficult to discuss the reported STM images. On the other hand, Piednoir et origin of the strong size dependence. al.(10) showed atomic-resolution images of palladium In this paper, we report the geometries and size clusters on surfaces but the clusters were not size- dependence of size-selected Ptn (n = 4, 7-10, 15) preselected. In both cases, it is difficult to discuss the clusters deposited on TiO2(110)–(1×1) surfaces at origin of the strong size dependence of catalytic room temperature in ultrahigh vacuum (UHV). activity of clusters on surfaces. Alignments of the Pt atoms in the clusters are imaged We present a new experimental setup to study the at atomic resolution using the CNT STM tip. Size specificity of size-selected clusters on a surface. The distributions of the clusters on the surfaces are setup combines a mass-selective cluster deposition measured as a function of the size of the deposited source with an X-ray photoelectron spectrometer, clusters. Further study by XPS makes it possible to ultraviolet photoelectron spectrometer, STM, and a distinguish the correlation between cluster geometries high-pressure reaction cell. The high-pressure reaction and core-level shifts. cell measurements provided information on catalytic properties under conditions close to those used in 2. Experimental practical applications. We chose TiO2(110) as the initial support for the study because it is one of most Figure 1 shows an overview of the experimental popular metal oxides and it is conductive after a typical setup used in this study, and Fig. 2 shows a schematic cleaning procedure, which allows it to be deposited of the setup. The apparatus combines a cluster source, without charge built-up and to be studied by STM. ion optics, quadrupole mass filter, deposition stage, The surface structures and electronic properties of reaction cell, and surface analysis systems. All the TiO2(110) surfaces were studied by various surface- chambers are connected to each other, and the sample sensitive techniques, including X-ray photoelectron was transferred under UHV conditions to investigate spectroscopy (XPS),(11,12) ultraviolet photoelectron as-deposited clusters on the surface. The deposition, spectroscopy (UPS),(13,14) and STM.(15,16) We have STM observation, and transfer between the deposition studied Mass-selected Pt clusters deposited on chamber and STM chamber can be performed under (17,18) TiO2(110) surfaces in recent years. cryogenic temperatures to freeze the movement of the We investigated the geometries of size-selected Pt deposited clusters. We deposited size-selected Pt clusters deposited on TiO2(110) surfaces. In the STM clusters on TiO2(110) surfaces prepared by the typical measurements described here, a carbon nanotube procedure by using a new UHV cluster deposition (CNT) STM tip that can provide atomic-resolution(19) apparatus with a magnetron-sputter ion source. images was used to image the alignment of the Pt The deposited Pt clusters and TiO2 surfaces were atoms in the clusters. The high aspect ratio of the CNT observed by STM. This system was also equipped with tip enables imaging at a high lateral resolution, low-energy electron diffraction (LEED)/Auger electron especially on surfaces with steep changes in height, spectroscopy (AES), XPS, and UPS (see Fig. 2). e.g., clusters on a surface. The catalysts were The experimental details will be elaborated in each characterized by XPS, and core-level shifts of the section. supported nanoclusters were often observed.(20,21) The core-level shift is of great interest because it reflects 2. 1 Sample preparation cluster-surface interactions that affect catalytic (22) activities. Most previous studies on supported metal We used 10 mm × 10 mm TiO2(110) single crystals nanoclusters by photoelectron spectroscopy were for the deposition. carried out on evaporated or sputtered thin films,(23-27) The samples for the experiments were prepared by + where the average cluster size distribution was Ptn deposition on rutile TiO2(110) single crystals (10 © Toyota Central R&D Labs., Inc. 2011 http://www.tytlabs.co.jp/review/ R&D Review of Toyota CRDL, Vol.42 No.1 (2011) 51-62 53 × 2 10 mm , Shinkosha Co., Ltd). Prior to the deposition, 2. 2 Cluster source and deposition the TiO2(110) surfaces were cleaned by repeated cycles of Ar+ sputtering (1 keV, 10 min) and annealed at 980 Figure 3 shows a schematic view of the cluster K in vacuum, until a well-defined (1×1) LEED pattern source. Metal clusters were produced by magnetron was observed and no impurities were detected by AES. sputtering and gas condensation.(32) Energetic metal This treatment also creates bulk oxygen vacancies, atoms sputtered from the target were cooled by He gas, making the sample surfaces sufficiently conductive for leading to nucleation of clusters. After expansion ion deposition with minimal charging. through a nozzle, the ionized clusters were accelerated and focused by an ion funnel. The cluster beam was then mass-selected by an Extrel quadrupole mass filter (Extrel MEXM-4000, mass range of 1- 4000 amu) and finally deflected to the substrate or to another quadrupole mass analyzer.
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