Letter pubs.acs.org/NanoLett
Dynamical Observation and Detailed Description of Catalysts under Strong Metal−Support Interaction Shuyi Zhang,†,‡ Philipp N. Plessow,§,∥ Joshua J. Willis,∥ Sheng Dai,†,‡ Mingjie Xu,†,‡ George W. Graham,†,‡ Matteo Cargnello,∥ Frank Abild-Pedersen,§ and Xiaoqing Pan*,†,⊥ †
Department of Chemical Engineering and Materials Science, University of CaliforniaIrvine, Irvine, California 92697, United States Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States § SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ∥ Department of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, California 94305, United States ⊥ Department of Physics and Astronomy, University of CaliforniaIrvine, Irvine, California 92697, United States ‡
S Supporting Information *
ABSTRACT: Understanding the structures of catalysts under realistic conditions with atomic precision is crucial to design better materials for challenging transformations. Under reducing conditions, certain reducible supports migrate onto supported metallic particles and create strong metal−support states that drastically change the reactivity of the systems. The details of this process are still unclear and preclude its thorough exploitation. Here, we report an atomic description of a palladium/titania (Pd/ TiO2) system by combining state-of-the-art in situ transmission electron microscopy and density functional theory (DFT) calculations with structurally defined materials, in which we visualize the formation of the overlayers at the atomic scale under atmospheric pressure and high temperature. We show that an amorphous reduced titania layer is formed at low temperatures, and that crystallization of the layer into either mono- or bilayer structures is dictated by the reaction environment and predicted by theory. Furthermore, it occurs in combination with a dramatic reshaping of the metallic surface facets. KEYWORDS: in situ microcopy, SMSI, Pd nanoparticles, TiO2
T
of reducible oxides on catalytic reactions has only been inferred from indirect experimental evidence or theoretical studies. In the present work, we apply in situ transmission electron microscopy (TEM), performed at atmospheric pressure, to directly observe the dynamical process of NP encapsulation by oxide support in one of the classical SMSI systems, TiO2supported Pd. We are thereby able to reveal, in unprecedented detail, the transient stages and final structures that occur in this process.
ransition-metal nanoparticles (NPs) in oxide-supported metal catalysts play the leading role in promoting heterogeneous catalytic reactions, but the oxide often plays a critical supporting role, affecting both activity and selectivity.1−3 In one extreme, reducible oxide supports can severely suppress activity by what is known as the strong metal support interaction (SMSI), which occurs when the partially reduced oxide migrates onto the metal NPs, effectively blocking access of gas-phase molecules to the metal surface.4−7 In many other cases, however, oxides, including reducible oxides, may beneficially participate in reactions by activating one or more of the reactants and presenting them to the metal NP at the three-phase (gas-metal−support) boundary, thereby accelerating the reaction or significantly altering the reaction pathway.3,8−11 Much of what is known about such competing effects © 2016 American Chemical Society
Received: May 1, 2016 Revised: June 7, 2016 Published: June 9, 2016 4528
DOI: 10.1021/acs.nanolett.6b01769 Nano Lett. 2016, 16, 4528−4534
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Nano Letters
Figure 1. Reversible formation of the TiOx overlayer on Pd nanocrystal in Pd/TiO2. Sequential in situ observations, first under reducing conditions (H2 (5 vol %)/Ar at 1 atm) of the Pd/TiO2 sample at 250 °C (A), then 500 °C for 10 min (B); next, under oxidizing conditions (150 Torr O2) at 250 °C for 8 min (C), then 15 min (D), and then at a final stable state at 500 °C (E); and finally, under reducing conditions again (H2 (5 vol %)/Ar at 1 atm) at 500 °C for 5 min (F). (G, H) are higher magnification ABF and HAADF images, respectively, of a section of part (B) showing the TiOx double layer. (I, J) EELS spectra extracted from a line scan of another particle, shown in (I), under H2 (5 vol %)/Ar at 1 atm and 500 °C, and Ti3+ and Ti4+ reference spectra acquired from LaTiO3 and TiO2.
understand the phenomenon,12 but a series of early surface
The term SMSI originated with Tauster and co-workers, who used it to describe the dramatic suppression of H2 and CO chemisorption by transition metal NPs supported on TiO2 following a H2-reduction treatment at around 500 °C.4 Initially, both electronic and steric effects were considered in attempts to
science and ex situ TEM studies eventually demonstrated that a thin TiOx layer forms on the metal NPs, physically blocking the interaction of gas molecules with their surfaces.13−16 4529
DOI: 10.1021/acs.nanolett.6b01769 Nano Lett. 2016, 16, 4528−4534
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Nano Letters
Figure 2. Computational and experimental characterization of TiOx overlayers under different experimental conditions. (A) Theoretical calculations of free energy G for different TiOx phases on Pd(111) or Pd(100) surfaces as a function of oxygen chemical potential μO. (B) TiOx/Pd double layer found under H2 (5 vol %)/Ar at 1 atm total pressure and 500 °C. (C) TiOx/Pd single layer found under H2 (4.9 vol %)/O2 (2 vol %)/Ar at 1 atm total pressure and 500 °C. (D) No layer was observed under H2 (4.7 vol %)/O2 (5.7 vol %)/Ar at 1 atm total pressure and 500 °C. The top-down views of the corresponding structures of the experimentally observed surface layers are shown above the TEM images. Pd is in dark green, Ti in gray, and oxygen in red.
permeable, but the overlayer formed by HTR is very effective in blocking access of gas molecules to the underlying metal surface.4,5 We demonstrate that the reduced oxide is one or two atomic layers thick, depending on the reactive gas atmosphere, which we adjust by controlling H2 and O2 partial pressures in a MEMS-based closed TEM gas cell. We also show that crystallization of the overlayers is accompanied by a reconstruction of the underlying Pd nanocrystal facets as a result of surface energy minimization. Our Pd/TiO2 catalyst was prepared by deposition of monodisperse Pd nanocrystals on commercial P25 TiO2 powder.11 The use of preformed, monodisperse metal nanocrystals and a high-surface-area support provides the opportunity to explore a system that has a well-defined structure and that can be used under realistic reaction conditions, as opposed to UHV. In situ TEM observation of the sample at elevated temperatures and pressures of different gaseous environments was accomplished using Protochips Atmosphere system.21 Representative sequential annular bright field (ABF) images of an individual Pd nanocrystal supported on TiO2 under reducing and oxidizing atmospheres are reported in Figure 1A− F. The Pd particles were examined ex situ at room temperature before performing in situ experiments, and they all exhibited clean surfaces, as shown in Figure S1. Under reducing conditions (H2 (5 vol %)/Ar at 1 atm) at 250 °C shown in
More recently, scanning tunneling microscopy (STM) has been employed to study a variety of model catalyst systems, where either metal NPs deposited on single crystalline oxide substrates17,18 or thin oxide layers deposited on single crystalline metal substrates19,20 were examined under UHV conditions. Various TiOx structures, mostly single TiOx layers with large and complex unit cells (∼100 Å2), have been characterized on (111) fcc metals and identified as possible SMSI-like analogues. It must be recognized, however, that UHV conditions are far from those used to create the SMSI state in real catalysts. In order to actually capture the dynamic formation process of the SMSI overlayer and truly understand its structure, we employ in situ ambient pressure scanning TEM (STEM), supported by density functional theory (DFT), on high-surfacearea Pd/TiO2 catalysts, thus providing unprecedented detail at the atomic scale. We demonstrate that an amorphous TiOx layer is initially formed at low temperature (∼300 °C) in a reducing atmosphere, followed by crystallization into an ordered layer epitaxial with the Pd(111) surface at ∼500 °C. With reference to well-known properties of the SMSI, the amorphous layer corresponds to the state produced by lowtemperature reduction (LTR), whereas the crystalline layer corresponds to the state produced by high-temperature reduction (HTR): the overlayer formed by LTR is gas 4530
DOI: 10.1021/acs.nanolett.6b01769 Nano Lett. 2016, 16, 4528−4534
Letter
Nano Letters
of a given structure depends only on temperature and oxygen partial pressure. Because the amount of Ti supplied by the supporting titania is sufficient to fully cover the particles and because the surface area does not change significantly, the most stable surface is that which minimizes the free energy per surface area and not per formula unit of reaction shown in eq 1 or 2. T and p(O2) can be further related to the chemical potential of oxygen, μO, as the only variable. Plotting the free energy of formation of different structures against μO therefore allows us to study the relative stability of different thin surface structures under varying conditions. According to our calculations summarized in Figure 2, a double layer resulting from stacking two hexagonal k-phase Ti2O3 layers22 in the sequence Pd−Ti−O−Ti−O is the thermodynamically most stable phase at μO < −3.7 eV, whereas a single layer k-phase is expected to be stable between −3.7 eV< μO < −2.8 eV. At μO > −2.8 eV, the Pd particles are expected to be clean. Although single and double layer structures have the same stoichiometry (TiO1.5), the transition is predicted to occur because the double layer has a higher density per area. Therefore, the oxygen deficiency per area is higher and so is the slope of the free energy per area with respect to μO. The oxygen chemical potential at which the double layer has been found experimentally, as shown in Figure 2B, μO ≤ −3.7 eV (this value representing an estimated upper limit), is in agreement with the range expected theoretically (
