Size control of Pt clusters on CeO 2 nanoparticles via an incorpora- tion-segregation mechanism and study of segregation kinetics. Frank Pilger 1,2 , Andrea Testino 1,* , Agnese Carino 1,2 , Christian Proff 1,3 , Antonio Cervellino 3 , and Christian Ludwig 1,2 1 Paul Scherrer Institut, General Energy Research Department, Villigen PSI, CH-5232 Switzerland 2 École Polytechnique Fédérale de Lausanne, School of Architecture, Civil and Environmental Engineering, EPFL ENAC, CH-1015 Lausanne, Switzerland 3 Paul Scherrer Institut, Synchrotron Radiation and Nanotechnology Research Department, Villigen PSI, CH-5232 Switzerland KEYWORDS: Pt/CeO 2 ; segregation kinetics; ionic dispersion; polyol synthesis; in situ XRD ABSTRACT: Recent literature on Pt/CeO 2 catalysts reveal that ionic Pt species could be incorporated into the CeO 2 lat- tice. Upon thermal treatment, sub-nanometric Pt clusters maybe segregated on the host lattice surface. Here we present a detailed study of Pt segregation in air and under a reducing atmosphere carried out on thermally treated samples by high resolution XRD and XAS. We further perform a kinetic study by in-situ XRD measurements with the aim to estimate the activation energy for Pt clusters segregation and coalescence in air, which was found to be about 3.88 eV. This high acti- vation energy indicates that Pt clusters are strongly anchored on the CeO 2 surface and that their mobility is activated only at T>900 °C. We demonstrate that an unprecedented control over the size of anchored metallic Pt clusters is achieved using our wet chemical synthesis method combined with an appropriate thermal treatment. 1. Introduction Despite the fact that the Rare Earth Elements (REE), mainly in the form of oxides, are a worldwide issue due to limited resources and – under the present economic situ- ation – only few countries are controlling the market, their demand – and consequently costs –, have risen dra- matically over the past decade. 1 Among REE, cerium-dioxide-based materials have at- tracted considerable attention in recent years because of their superior performance in several technological fields. 2-11 The high cost of the raw materials is justified by the performance and stability of the derived product ma- terials. In catalysis CeO 2 is used as a support for noble metals, such as Pt or Pd, which act as a catalytic center for a series of reactions, such as CO oxidation. Nowadays it is largely accepted that CeO 2 is not merely a support for the noble metals but it is actively involved in the catalytic process itself. Its action is associated with remarkable redox properties. 12-13 CeO 2 is able to accommodate a large number of oxygen vacancies, compensated by Ce 3+ spe- cies, resulting in a material with significant oxygen mobil- ity and oxygen storage capacity (OSC). 14-15 As a conse- quence, the catalytic activity is localized to areas where the noble metal, CeO 2 , and the gaseous reactants are sim- ultaneously present (three phase points). In order to in- crease the number of these catalytic centers the noble metal should be dispersed on the metal oxide surface in entities that are as small as possible. Moreover, such enti- ties should not diffuse on the metal oxide surface in oper- ando otherwise they rapidly coalesce into larger particles with a rapid activity loss. The diffusion of the noble met- als entities can be prevented if they are strongly bonded on the CeO 2 surface. Pt and Pd present both a rather sta- ble oxidation state IV and an ionic size compatible with the CeO 2 host lattice, meaning that Pt ions can be ac- commodated in the Ce positions (Pt Ce ) without the need for compensation defects. Thus, Ce – O – Pt (or Pd) chemical bonds are possible both on the metal oxide sur- face and in form of dispersed ions in the fluorite-like CeO 2 lattice. The presence of such surface chemical inter- actions as well as the ionic dispersion of Pt and Pd into the metal oxide lattice, has been theoretically predicted 16- 17 and experimentally proven. 18-19 In parallel, several au- thors have reported interesting correlations between the type of interaction and the catalytic activity. 20-42 The applied procedure to prepare catalysts and their processing is highly relevant. Traditionally, catalysts are prepared by impregnation or deposition-precipitation methods followed by an appropriate thermal treatment to reduce the deposited metal salts to metal particles. As a result, the active metal particles are poorly anchored to the support and control over the size of the metal particle is questionable. An innovative alternative approach is to follow a segre- gative preparation method: the bi-phasic solid nanophase is obtained by promoting phase exsolution from a parent compound. Neagu et al. 43 presented a study on non- stoichiometric perovskites where, upon thermal treat- Page 1 of 14 ACS Paragon Plus Environment ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 This document is the accepted manuscript version of the following article: Pilger, F., Testino, A., Carino, A., Proff, C., Kambolis, A., Cervellino, A., & Ludwig, C. (2016). Size Control of Pt Clusters on CeO2 Nanoparticles via an Incorporation-Segregation Mechanism and Study of Segregation Kinetics. ACS Catalysis, 6(6), 3688-3699. https://doi.org/10.1021/acscatal.6b00934