Networks of AuxPd1-x model catalysts obtained by the di-block copolymer inverse micelle method: insights on nanoparticle formation and network stability at high temperature under gas pressure within the environmental TEM

MICROSCOPIE+MEME+FCA:EEH:LBU:TEPInternational audienceSynthesis of nanoparticles (NPs) with well controlled characteristics is crucial to better understand the behaviour of the active phase during reaction and to better correlate it with its catalytic performance. Among other self-assembly methods, the di-block copolymer approach is well suited to obtain monodisperse supported NPs which can serve as supported metallic catalysts on both flat (model catalysts) and powder (realistic catalysts) supports. In this approach, an amphiphilic di-block copolymer dissolved in toluene yields a system of inverse micelles of which the core can be charged with metallic precursors to ultimately yield networks with well-controlled characteristics (size, composition, inter-distance). Important questions must be addressed, in particular in the case of bimetallic systems, concerning the formation of unique NPs at the cores from the initially pre-formed bimetallic seeds or the stability of the NPs networks with increasing temperature in presence of reactive gases. We have chosen to work with PS-b-P2VP di-block copolymer micelles that were functionalized with chloro-auric acid and palladium di-acetate precursors. The precursors selectively bind with N-functions in the P2VP cores to form (bi-)metallic seeds within the micelles’ cores as it was previously established in the case of PdAg alloys. We have used an Environmental TEM (Titan ETEM G2, FEI/TFS) operated at 300kV under gas pressures up to 20 mbar to gain insight on their evolution/stability during heating under oxygen and CO (for CO oxidation catalysis). Temperature variation was achieved with a WildFire sample holder with dedicated heating MEMS-microchips (DENS Solutions) capable of reaching temperatures up to 1300°C. Monolayers of controlled AuxPd1-x core-metallized PS-b-P2VP micellar solutions were deposited by spin-coating (2000/4000 rpm) on the microchips. Special care was taken to minimize/prevent any influence of the electron beam on the observed events.The Au-rich system (Au80Pd20) showed that, under oxygen, the (bi-)metallic seeds within the P2VP cores begin to sinter around 350° and unique NPs are eventually formed around 500°C which is consistent with the decomposition temperature range of the PS-b-P2VP copolymer associated with metallic nuclei. Further heating up shows that the NPs network is stable up to 900°C. Approaching the melting temperature of gold (1064°C) the NPs begin to shrink; concomitantly, smaller NPs (that grow even at 1100°C) form around the shrinking NPs. These smaller NPs are formed with Pd atoms from the initial bimetallic NPs.In the case of the Pd-rich system (Au20Pd80), under oxygen, the seeds begin to sinter around 300°C and at 450°C demixing of the NPs is observed leading to Janus NPs (Au-rich/PdO, cf. Figure); the latter are readily reduced and homogeneous NPs reform upon CO exposure (cf. Figure).The stability of the bimetallic NPs seems to be driven by the intrinsic properties of the metals composing them.Thanks are due to the METSA network for supporting this study and to CLYM for the access to the ETEM