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PtRu Alloy Nanoparticles I. Physicochemical Characterizations of Structures Formed as a Function of the Type of Deposition and Their Evolutions on Annealing R. Bavand,† A. Korinek,‡ G. A. Botton,‡ A. Yelon,† and E. Sacher*,† Department of Engineering Physics, École Polytechnique, Montréal, Québec H3C 3A7, Canada Department of Materials Science and Engineering, and Canadian Centre for Electron Microscopy, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada
† ‡
S Supporting Information *
ABSTRACT: We have physicochemically characterized the formation of PtRu nanoparticles, deposited by evaporation onto highly oriented pyrolytic graphite, using in situ X-ray photoelectron spectroscopy, ex situ high-angle annular darkfield/scanning transmission electron microscopy, electron energy loss spectroscopy, and time-of-flight secondary ion mass spectrometry. We used three different orders of metal deposition: Pt evaporated onto Ru, Ru evaporated onto Pt and both metals evaporated simultaneously, and then followed the evolutions of the alloys as a function of annealing temperature. The C 1s, O 1s, Ru 3d, and Pt 4f core level spectra were employed to describe the alloying interactions between the metals. For all deposition methods, Ru diffuses to the NP surface through the Pt, and not the reverse. Each of the preparation methods produces surface and volume structures that differ from those of the others, even after prolonged annealing at temperatures over 700 °C, indicating the source of confusion in the literature concerning the physicochemical characterization of PtRu nanoparticles. dehydrogenation occurs more readily on Pt.15,16 Thus, Ru can provide preferential sites for OH adsorption by dehydrogenating water.17 These OH species would then lead to the complete removal of CO by oxidizing it to CO2. The compositions, morphologies, and structures of PtRu alloy nanoparticle (NP) systems are of great importance for their surface catalytic activity.18 In order to understand their role in electro-catalysis and to improve performance, one must determine and understand the PtRu NP surface structure. Unfortunately, this structure remains an issue. Some authors maintain that Pt diffuses into Ru,19−21 while others claim the reverse.18,22,23 There is also disagreement as to whether Pt19−21 or Ru18,22 is found at alloy NP surfaces, and as to which of them form surface oxides.18,19,24 This inconsistency has presented a challenge to the determination of the optimum structure of PtRu electro-catalysts. Despite this, most theoretical papers contain calculations based on the assumptions that there are no surface contaminants and that Pt is always at the surface.25 As we show here, both are incorrect. Many experimental papers also assume the same, probably following the theoretical assumptions. Because of this, positively
1. INTRODUCTION Pt is the transition metal catalyst most commonly used in hydrogen- and methanol-based fuel cells, because of its high activity toward the electro-oxidation of hydrogen and methanol fuels.1,2 High loadings of this costly metal are required in order to achieve acceptable current densities at reasonable overpotentials.3 Therefore, reducing the Pt loading, and the cost, is of great importance in the commercialization of fuel cell technologies.4−6 Furthermore, significant quantities of CO are produced in the reforming of hydrocarbons, poisoning the Pt catalyst,7,8 so that catalysts must also remove CO from hydrogen through selective oxidation.7 Thus, although Pt is known to be the most effective electrocatalyst, as both the anode and the cathode of hydrogen, and of direct methanol fuel cells, an alternate electrocatalyst is needed to reduce both cost and Pt poisoning, while maintaining similar efficiency. PtRu alloys are considered to be among the most promising catalysts in such fuel cells, due to their lower cost, high electrocatalytic activity, high tolerance for CO, superior performance in decomposing methanol, and high activity in oxidizing CO to form CO2.9−13 It has been suggested that the increased ability of the PtRu bimetallic surface to take on electrons results in the enhancement of the oxidation process.14 Moreover, water dehydrogenation occurs more readily on Ru, while methanol © XXXX American Chemical Society
Received: May 9, 2017 Revised: August 31, 2017 Published: September 7, 2017 A
DOI: 10.1021/acs.jpcc.7b04434 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
High-resolution transmission electron microscopy (HRTEM) was performed on each deposit; confirming the formation of NPs at the HOPG surface, even after annealing, as expected. The low deposition rate (0.13 nm/min) of the ebeam evaporator gives sufficient time for the condensation energy to dissipate, minimizing the extensive diffusion, as was found for pure Ru.33 2.2. XPS Measurements, Annealing, and XPS Data Analysis. After deposition at room temperature, the samples were transferred to the analysis chamber of the instrument, without exposure to atmosphere. In-situ XPS was performed in this chamber, at a base pressure of
