Applied Catalysis B: Environmental 136–137 (2013) 351–360 Contents lists available at SciVerse ScienceDirect Applied Catalysis B: Environmental jo ur n al homepage: www.elsevier.com/locate/apcatb Synthesis of Pt-based hollow nanoparticles using carbon-supported Co@Pt and Ni@Pt core–shell structures as templates: Electrocatalytic activity for the oxygen reduction reaction D.A. Cantane a , F.E.R. Oliveira a , S.F. Santos b , F.H.B. Lima a,∗ a Instituto de Química de São Carlos, Universidade de São Paulo – USP, CP 780, 13560-970 São Carlos – SP, Brazil b Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas – UFABC, Rua Santa, Adélia 166, Santo André – SP 09210-170, Brazil a r t i c l e i n f o Article history: Received 30 October 2012 Received in revised form 24 January 2013 Accepted 25 January 2013 Available online 14 February 2013 Keywords: Pt hollow electrocatalysts Core–shell nanoparticles Oxygen reduction reaction a b s t r a c t Pt-based hollow nanoparticles were investigated as electrocatalysts for the oxygen reduction reaction (ORR) in acid electrolyte. The electrocatalysts were synthesized via Ni or Co diffusion/dissolution, induced by the vacancy-mediated Kirkendall effect, during electrochemical potential cycling of Ni@Pt and Co@Pt core–shell nanoparticles in acid media. The nanoparticles were characterized by high resolution trans- mission electron microscopy, in situ X-ray absorption spectroscopy and X-ray diffraction measurements. The results show substantial differences in nanoparticle structure/composition and in the activity for the ORR, depending on the nature of the non-noble metal in the nanoparticle core. The Pt hollow nanostruc- tures showed higher specific catalytic activity than that of the state-of-the-art Pt/C electrocatalyst. This was attributed to three main effects: (i) hollow-induced lattice contraction in the multilayer Pt shells, (ii) mismatch-induced lattice contraction of the thick Pt shell by the remaining Ni or Co atoms and (iii) a ligand effect, due to the electronic interaction of Pt with the remaining Ni or Co atoms in the Pt multilayers of the hollow structure. These three effects caused a Pt d-band center down-shift, which decreased the adsorption strength of oxygenated reaction intermediates and spectators, thus increasing the ORR rate. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Platinum is the best single metal catalyst for the oxygen reduction reaction (ORR) for low temperature polymer electrolyte membrane fuel cells (PEM), in both acid and alkaline electrolytes [1–5]. Aiming at increasing the reaction rate, several bimetallic electrocatalysts composed of Pt and 3d-transition metals (Co, Ni, Fe) have been investigated [6–9]. These electrocatalysts exhibit improved catalytic performance for the ORR when compared to Pt alone, both in terms of mass activity (per g of Pt) or specific activ- ity (per real cm 2 of Pt) [10–12]. Previous works have shown that the ORR activity of these bimetallic electrocatalysts can be further improved by enriching the surface of the catalyst with Pt (sur- face segregation), induced by annealing a Pt 3 M surface at elevated temperatures [13–15] and/or induced by dissolution of the 3d- transition metal in an acidic environment [16,7]. The enhancement in ORR specific activity was achieved by using the mismatch- induced lattice contraction and/or the ligand effect [17–21]. In this type of electrocatalyst, Pt atoms suffer a significant d-band center ∗ Corresponding author at: Av. Trabalhador Saocarlense, 400, CP 780, CEP 13560- 960, São Carlos, SP, Brazil. Tel.: +55 16 3373 8681; fax: +55 16 3373 9952. E-mail addresses: fabiohbl@iqsc.usp.br, fabiohbl@gmail.com (F.H.B. Lima). down-shift and, therefore, chemisorb oxygenated and spectator species more weakly than pure Pt. Another strategy to enhance the Pt electrocatalytic activity that has been explored is the so-called de-alloying method [22]. In this procedure, the formation of a Pt “skeleton” surface in a bimetallic particle is achieved via dissolution of the alloying elements, such as Cu, Ni and Co in an acid electrolyte. The increased activity for the ORR has been ascribed to variations in the number and kind of neighbors surrounding the Pt atoms, as well compression or expan- sion in the Pt–Pt bond distance, which induces a structural change that directly affects the Pt electronic density of occupied states in the Pt 5d-band, weakening the adsorption strength of reaction intermediates and spectators. Furthermore, in order to enhance the Pt catalytic activity and dramatically decrease the total mass of Pt in the electrocatalyst, some works have focused on the study of core–shell nanostruc- tures, in which an ultra-thin platinum shell is either deposited on a noble [23–25] or non-noble/noble metal nanoparticle core [19,26,27]. Therefore, the total mass of Pt is reduced, while the metal core plays an important role in modifying the Pt electronic structure, decreasing the Pt d-band center. The use of core–shell and bi-metallic alloys with non-noble metal in their composition or in their cores (such as Co, Fe, Ni or Cu atoms) as electrocatalysts would be an important way for 0926-3373/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcatb.2013.01.060