Systems of self-organized nanoparticles (NPs) are useful in many domains such as, for instance, optics, nanostructure growth and, the domain of most interest in our case, catalysis. It is indeed a way to control the classic characteristics of catalysts (size, dispersion, chemical phase, …) and add a new tunable parameter, often poorly controlled, which is the long-distance ordering and, thus, the inter-nanoparticle distance. Well-controlled physicochemical properties of the catalysts lead to better correlation between these properties, their possible evolution, and their catalytic performance (activity, selectivity, stability). Among self-assembly techniques, the use of di-block copolymer method, where an amphiphilic di-block copolymer dissolved in toluene yields a system of inverse micelles, is a rather simple method to obtain well controlled supported NPs which can be used as supported metallic catalysts on both flat (model catalysts) and powder (realistic catalysts) supports [1]. After formation of the micellar solution, we can indeed charge the core with metal salts that can then be deposited on flat surfaces by dip or spin coating for instance. Our main interest deals with applications of bimetallic catalysts that we study in the form of both realistic catalysts [2] and extended catalytic surfaces [3]; self-organized NPs on flat surfaces is an intermediate configuration between these two approaches which is useful to better extrapolate results between model and realistic systems. With this in mind we have extended the di-block copolymer method to the synthesis of bimetallic catalysts [4]. For these systems composed of metals with different characteristics, several questions remain unanswered such as the evolution of the pre-formed bimetallic seeds (Fig. 1b-d) with temperature, the temperature formation of unique NPs at the micellar core or the stability of the network of bimetallic NPs (Fig. 1a) at high temperature in presence of reactive gases. An objective lens aberration-corrected Environmental TEM (Titan ETEM G2 from FEI/ThermoFisher Scientific), operated at 300kV and capable of gas pressure up to 20 mbar, is a well-adapted tool to study those phenomena. An Au-rich AuPd core-metallized PS-b-P2VP micellar solution was synthesized and deposited by spin-coating on dedicated heating microchips containing a silicon nitride electron transparent (discrete) film capable of reaching temperatures up to 1300°C within a WildFire support holder (DENS solutions). Special care was taken to minimize/prevent any influence of the electron beam on the observed events. In order to observe both the formation of the individual NPs, together with the elimination of the copolymer, and the high-temperature stability of the network we have worked under oxygen pressure (up to 2 mbar). We observed that the seeds within the micellar cores (Fig. 2a-left) begin to sinter around 350°C (Fig. 2a-center) which is consistent with the temperature onset at which the copolymer (associated to a metal) begins to decompose [5]; unique NPs are formed around 500°C (Fig. 2a-right), when the copolymer is almost completely gasified [5]. The network is remarkably stable up to 900°C (Fig. 2b); finally, around 1000°C, close to the melting temperature of Au (1064°C), the NPs begin to decompose (Fig. 2b). At 1100° C, unique Au-rich NPs (Fig. 3a) begin to shrink and yield smaller satellite NPs (Fig. 3b); then the facetted NP (Fig. 3c) becomes spherical (Fig. 3d) and finally disappears to leave only the satellite NPs that grow on the surface (Fig. 3e). The volume ratio between the satellite NPs and the initial Au-rich NP indicate that only the Au within the NP has disappeared and that the remaining satellite NPs are pure Pd. The stability of the network seems to depend only on the intrinsic properties of the chosen metals [6]. References: [1] B Roldan Cuenya, Accounts of Chemical Research 46 (2013) 1682. [2] B Pongthawornsakun et al, Applied Catalysis A: General 549 (2018) 1. [3] MC Saint-Lager et al, ACS Catalysis 9 (2019) 4448. [4] E Ehret et al, Nanoscale 7 (2015) 13239. [5] T Orhan Lekesiz et al, Journal of Analytical and Applied Pyrolysis 106 (2014) 81. [6] The authors acknowledge the French Microscopy and Atom probe network (METSA) and the Consortium Lyon – St-Etienne de Microscopie (CLYM) for supporting this work.