J. Phys. Chem. C 2010, 114, 11853–11860
11853
Growth of Pt Nanoparticles on Reducible CeO2(111) Thin Films: Effect of Nanostructures and Redox Properties of Ceria Yinghui Zhou,† Justin M. Perket,‡ and Jing Zhou*,† Department of Chemistry, UniVersity of Wyoming, Laramie, Wyoming 82071, and Department of Chemistry, UniVersity of WisconsinsSteVens Point, SteVens Point, Wisconsin 54481 ReceiVed: January 25, 2010; ReVised Manuscript ReceiVed: June 5, 2010
Pt nanoparticles grown on fully oxidized and partially reduced CeOx(111) thin films have been studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy to understand the effect of redox properties and nanostructures of ceria supports on the growth of Pt. Deposition of 0.2 ML of Pt on CeO2 at 300 K produces two atomic layer high nanoparticles, while on reduced ceria films Pt favors the growth of smaller particles of one-two layer thick with a larger particle density. With the increase of Pt coverage, Pt particles on CeO2 grow in size while the Pt particle density significantly increases on the reduced ceria. Heating the surface to higher temperatures causes the Pt particle agglomeration, but Pt particles sinter less on the reduced ceria compared to those on the fully oxidized ceria. New particle structures are formed on reduced ceria as a result of heating which are suggested due to the encapsulation of Pt particles by ceria. In addition to the structural changes of the Pt particles, modifications of electronic properties of both ceria and Pt were observed upon Pt deposition as well as after heating. Our combined scanning tunneling microscopy and X-ray photoelectron spectroscopy studies suggest a complex growth behavior of Pt on ceria and a strong interaction between the Pt and the ceria support. 1. Introduction Ceria-supported Pt catalysts, widely used in automobile catalytic converter, have attracted great attention nowadays. This is because they can exhibit interesting reactivity in many important catalytic processes including the low-temperature CO oxidation, NO reduction, hydrocarbon oxidation, and the watergas shift reactions.1–6 Recent studies have indicated that the catalytic reactivity of these ceria-supported Pt nanoparticles can be influenced by the redox properties of ceria as well as the synergistic effect between the two.7–9 To elucidate the nature of chemistry of Pt/ceria catalysts, it is of significance and practical importance to gain a fundamental understanding of the Pt growth on ceria and have a thorough examination of the effect of the nanostructures and redox properties associated with oxygen vacancies on ceria on the electronic and geometric structures of Pt nanoparticles. Despite the increasing interest in the Pt/ceria catalytic systems, a fundamental-level investigation of the structure and morphology of ceria-supported Pt nanoparticles is still lacking in the literature, which motivates the current study. In the paper, well-ordered CeOx(111) (1.5 < x < 2) thin films with controlled Ce oxidation states were prepared on Ru(0001) and used as model supports for Pt. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) studies suggest a complex growth behavior of Pt on ceria. The nanostructures and oxidation states of Ce in ceria can influence not only the size and structure of deposited Pt nanoparticles but also their electronic properties. Furthermore, deposition of Pt can also affect the electronic properties of ceria supports, indicating a strong interaction between the two. The growth of * To whom correspondence should be addressed: phone (307) 766-4335; e-mail
[email protected]. † University of Wyoming. ‡ University of WisconsinsStevens Point.
Pt was further compared on CeOx(111) thin films with respect to Pt deposition temperatures, postdeposition annealing temperatures, and Pt coverage. 2. Experimental Methods The experiments were performed in a multitechnique surface analysis ultrahigh-vacuum system manufactured by Omicron Nanotechnology with a base pressure below 5 × 10-11 Torr. This system is equipped with a variable-temperature scanning tunneling microscope (VT STM XA650), an X-ray source (DAR 400), a hemispherical energy analyzer (EA 125), low-energy electron diffraction (LEED) optics (SPECTA), a sputter gun (ISE 5), a liquid N2-cooled sample manipulator, and a fast-entry load lock. Additionally, a quadrupole mass spectrometer (Hiden HAL/3F PIC), homemade metal evaporation sources, and gas handling lines were installed onto the system. A Ru(0001) single crystal with a diameter of 10 mm and a thickness of 2 mm (Princeton Scientific Corp., one side polishing, roughness
