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Unravelling the Formation of Pt–Ga Alloyed Nanoparticles on Calcined Ga-Modified Hydrotalcites by in Situ XAS

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journal contribution
posted on 28.10.2014, 00:00 by Matthias Filez, Evgeniy A. Redekop, Hilde Poelman, Vladimir V. Galvita, Ranjith K. Ramachandran, Jolien Dendooven, Christophe Detavernier, Guy B. Marin
The chemical transformations taking place during the formation of catalytic Pt–Ga alloyed nanoparticles supported on calcined Ga-modified hydrotalcite Mg­(Ga)­(Al)­Ox are investigated. The starting point is a Pt­(acac)2 precursor impregnated onto a Mg­(Ga)­(Al)­Ox support. An oxidative treatment first yields Pt nanoparticles, while subsequent reduction efficiently delivers Ga from the support framework to Pt, forming Pt–Ga alloyed clusters. Different steps are discerned in this process based on in situ XAS analysis. During oxidative heating to 350 °C, the initially adsorbed Pt­(acac)2 precursor molecules decompose and form atomically dispersed Pt4+ species with 5-/6-fold oxygen coordination. A fraction of the formed Pt–O bonds consists of strong anchoring points between Pt4+ species and support oxygen, decreasing the Pt mobility induced by the basic support. Further calcination to 650 °C leads to scission of these Pt–O support bonds, allowing more mobile Pt species to form 3–11 atom Pt fcc nanoparticles with an oxidized external surface. These subnanometer clusters are proposed to be bound to the Mg­(Ga)­(Al)­Ox support by extended 2.5 Å Pt0–(OH) induced dipole-ion interfacial bonds. Cooling down and subsequent heating in H2 up to 450 °C causes sintering and reduction of these nanoparticles. In the course of this reduction, the proposed 2.5 Å Pt0–(OH) interfacial bonds are replaced by common 2.0 Å Pt–O bonds. Further heating in H2 to 650 °C causes reduction of framework Ga, allowing Ga transport from the support to the Pt clusters to form 1.5 nm Pt–Ga alloyed nanoparticles. This activated Pt–Ga/Mg­(Ga)­(Al)­Ox catalyst is subjected to one O2/H2 redox cycle at 650 °C to mimick the catalyst regeneration procedure. The redox cycle induces an alloy restructuring, leading to a decrease in the degree of Pt–Ga alloying. Wavelet transformed XAScomplemented by XANES and EXAFSis shown to be a key tool to disclose the mechanistic details occurring during Pt–Ga formation.