Article
Nanoscale architecture of ceria-based model catalysts: Pt–Co nanostructures on well-ordered CeO2(111) thin films

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Abstract

We have prepared and characterized atomically well-defined model systems for ceria-supported Pt–Co core–shell catalysts. Pt@Co and Co@Pt core–shell nanostructures were grown on well-ordered CeO2(111) films on Cu(111) by physical vapour deposition of Pt and Co metals in ultrahigh vacuum and investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The deposition of Co onto CeO2(111) yields Co–CeO2(111) solid solution at low Co coverage (0.5 ML), followed by the growth of metallic Co nanoparticles at higher Co coverages. Both Pt@Co and Co@Pt model structures are stable against sintering in the temperature range between 300 and 500 K. After annealing at 500 K, the Pt@Co nanostructure contains nearly pure Co-shell while the Pt-shell in the Co@Pt is partially covered by metallic Co. Above 550 K, the re-ordering in the near surface regions yields a subsurface Pt–Co alloy and Pt-rich shells in both Pt@Co and Co@Pt nanostructures. In the case of Co@Pt nanoparticles, the chemical ordering in the near surface region depends on the initial thickness of the deposited Pt-shell. Annealing of the Co@Pt nanostructures in the presence of O2 triggers the decomposition of Pt–Co alloy along with the oxidation of Co, regardless of the thickness of the initial Pt-shell. Progressive oxidation of Co coupled with adsorbate-induced Co segregation leads to the formation of thick CoO layers on the surfaces of the supported Co@Pt nanostructures. This process is accompanied by the disintegration of the CeO2(111) film and encapsulation of oxidized Co@Pt nanostructures by CeO2 upon annealing in O2 above 550 K. Notably, during oxidation and reduction cycles with O2 and H2 at different temperatures, the changes in the structure and chemical composition of supported Co@Pt nanostructures were driven mainly by oxidation while reduction treatments had little effect regardless of the initial thickness of the Pt-shell.

Graphical Abstract

The chemical composition and atomic ordering in the model Co@Pt core–shell nanostructure and in the CeO2(111) support are functions of temperature. The most important changes are associated with the formation of subsurface Pt–Co alloy.

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Introduction

The development of active and durable catalysts with reduced Pt content is of paramount importance for commercialization of proton exchange membrane fuel cells (PEMFCs) [1, 2, 3]. Recently, significant progress was made by using atomically dispersed Pt as an anode catalyst for the direct hydrogen PEMFCs [4]. However, due to the slow kinetics of the oxygen reduction reaction (ORR) at the cathode, the cathode catalysts contain much higher quantities of Pt. Recent studies suggested that alloying Pt with a second, less expensive metal leads to synergetic improvements of the catalyst performance while significantly reducing the Pt loading [5, 6, 7, 8, 9]. The major effect is achieved by the formation of core–shell nanostructures, where Pt forms a thin shell over the core of an inexpensive metal [7, 8]. Among the studied bimetallic catalysts, Pt–Co systems demonstrated the highest activity for ORR [5]. It was suggested that the improvements in ORR activity are associated with strain effects, i.e. the changes in Pt–Pt bond distances, and the electronic effects arising from bimetallic interactions between the Pt and the Co metals [9, 10, 11, 12, 13, 14]. The corresponding effects have been investigated as a function of Pt–Co alloy stoichiometry, the structure, and the composition of core–shell nanostructures supported on carbon [15, 16, 17, 18, 19]. However, the analysis of the electrochemically treated Pt–Co systems revealed structural instabilities associated with Ostwald ripening, aggregation of supported nanostructures, and leaching of Co [6, 7, 17, 20]. It was found that the most active and stable Pt–Co systems consisted of Pt3Co@Pt or Co@Pt core–shell nanostructures with 2–3 atomic layers thick Pt-shell [5, 6, 21].

With regard to the agglomeration, several strategies were suggested including encapsulation of core–shell nanostructures by sandwich-like carbon sheets [22] and the anchoring of supported nanostructures on functional oxides [23, 24]. In particular, the use of reducible oxides as supports, e.g. CeO2, improves the stability of supported noble nanoparticles against sintering [25, 26]. The intrinsic property of CeO2-based materials is their oxygen storage capacity associated with the redox conversion between the oxidations state Ce4+ and Ce3+ during release and uptake of oxygen [27, 28]. In the presence of supported noble metal nanoparticles, e.g. Pt, electronic metal–support interactions (EMSI) give rise to charge transfer between the supported nanoparticle and the support [29]. The magnitude of the charge transfer depends on the size of the supported nanoparticles and the stoichiometry of the support. Therefore, EMSI represents a tunable parameter controlling the oxidation state and, as a result, the reactivity of supported nanoparticles [29]. Additionally, the CeO2-based materials participate in chemical reactions by providing active oxygen species to the surface of supported nanoparticles by means of reverse oxygen spillover [30].

In the present paper, we employ well-ordered CeO2(111) films as a functional support for model Co@Pt and Pt@Co core–shell nanostructures. The thermal stability of supported nanostructures is discussed with respect to segregation phenomena and sintering of the nanoparticles under different experimental conditions. The work provides an insight into the thermodynamically driven restructuring phenomena in supported Pt–Co nanostructures.

Section snippets

Experimental

High-resolution synchrotron radiation photoelectron spectroscopy (SRPES) and resonant photoemission spectroscopy (RPES) experiments were performed at the Materials Science Beamline (MSB), Elettra synchrotron light facility in Trieste, Italy. The MSB, with a bending magnet source provided synchrotron light in the energy range of 21–1000 eV. The UHV end-station (base pressure 2 × 10−10 mbar) was equipped with a multichannel electron energy analyzer (Specs Phoibos 150), a rear view low energy

Co nanoparticles on CeO2(111)

The evolution of the Co 2p spectra upon stepwise deposition of Co onto the well-ordered CeO2(111)/Cu(111) film at 300 K in UHV is shown in Fig. 1a. At the limit of low Co coverage, three doublet peaks were resolved in the Co 2p spectra at 780.8 (Co 2p3/2), 782.9 (Co 2p3/2), and 787.0 eV (Co 2p3/2). In line with the work of Biesinger et al. [35], we associate all three peaks with the formation of Co2+ ions. The complex structure of the Co 2p spectra results from the multiplet splitting and

Conclusions

Pt@Co and Co@Pt core–shell nanostructures and Co nanoparticles were assembled on well-ordered CeO2(111) films grown on Cu(111) by means of physical vapor deposition of Co and Pt in UHV. The systems were investigated by means of SRPES and RPES. Below we summarize the most important findings.

  • (1)

    Co/CeO2. The deposition of 0.5 ML of Co onto a CeO2(111) film at 300 K in UHV leads to the formation of atomically dispersed Co2+ ions, coupled with the reduction of Ce4+ to Ce3+. The corresponding process is

Acknowledgments

This work was funded by the European Community (FP7-NMP.2012.1.1-1 project chipCAT, Reference No. 310191), by the Deutsche Forschungsgemeinschaft (DFG) within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. Additional support by the DFG is acknowledged through the Priority Program SPP 1708 and the Research Unit FOR 1878. The project was supported by structural funds under project CZ.02.1.01/0.0/0.0/16_025/0007414 and by the Czech

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    Published 5 June 2020

    Present address: Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 18221 Prague, Czech Republic

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