1 Unravelling the Formation of Pt-Ga Alloyed Nanoparticles on 2 Calcined Ga-Modified Hydrotalcites by in Situ XAS 3 Matthias Filez, † Evgeniy A. Redekop, † Hilde Poelman,* ,† Vladimir V. Galvita, † Ranjith K. Ramachandran, ‡ 4 Jolien Dendooven, ‡ Christophe Detavernier, ‡ and Guy B. Marin † 5 † Laboratory for Chemical Technology, Ghent University, Technologiepark 914, B-9052 Zwijnaarde, Belgium 6 ‡ Department of Solid State Sciences, COCOON, Ghent University, Krijgslaan 281/S1, B-9000 Ghent, Belgium 7 * S Supporting Information 8 ABSTRACT: The chemical transformations taking place 9 during the formation of catalytic Pt-Ga alloyed nanoparticles 10 supported on calcined Ga-modified hydrotalcite Mg(Ga)(Al)- 11 O x are investigated. The starting point is a Pt(acac) 2 precursor 12 impregnated onto a Mg(Ga)(Al)O x support. An oxidative 13 treatment first yields Pt nanoparticles, while subsequent 14 reduction efficiently delivers Ga from the support framework 15 to Pt, forming Pt-Ga alloyed clusters. Different steps are 16 discerned in this process based on in situ XAS analysis. During 17 oxidative heating to 350 °C, the initially adsorbed Pt(acac) 2 18 precursor molecules decompose and form atomically dispersed 19 Pt 4+ species with 5-/6-fold oxygen coordination. A fraction of the formed Pt-O bonds consists of strong anchoring points 20 between Pt 4+ species and support oxygen, decreasing the Pt mobility induced by the basic support. Further calcination to 650 °C 21 leads to scission of these Pt-O support bonds, allowing more mobile Pt species to form 3-11 atom Pt fcc nanoparticles with an 22 oxidized external surface. These subnanometer clusters are proposed to be bound to the Mg(Ga)(Al)O x support by extended 2.5 23 Å Pt 0 -(OH) - induced dipole-ion interfacial bonds. Cooling down and subsequent heating in H 2 up to 450 °C causes sintering 24 and reduction of these nanoparticles. In the course of this reduction, the proposed 2.5 Å Pt 0 -(OH) - interfacial bonds are 25 replaced by common 2.0 Å Pt-O bonds. Further heating in H 2 to 650 °C causes reduction of framework Ga, allowing Ga 26 transport from the support to the Pt clusters to form 1.5 nm Pt-Ga alloyed nanoparticles. This activated Pt-Ga/Mg(Ga)(Al)O x 27 catalyst is subjected to one O 2 /H 2 redox cycle at 650 °C to mimick the catalyst regeneration procedure. The redox cycle induces 28 an alloy restructuring, leading to a decrease in the degree of Pt-Ga alloying. Wavelet transformed XAScomplemented by 29 XANES and EXAFSis shown to be a key tool to disclose the mechanistic details occurring during Pt-Ga formation. 1. INTRODUCTION 30 The rational design of functional materials, such as heteroge- 31 neous catalysts, aims at manipulating the material’s structure by 32 steering the solid-phase chemistry in order to achieve the 33 desired properties and improve a material’s performance. 1-6 An 34 in-depth understanding of materials chemistry is therefore 35 crucial for the development of better catalysts and enabling the 36 chemical industry to shift from nonselective thermal to selective 37 catalytic processes. Alloying of metal catalysts with promoter 38 elements presents a particularly relevant example of better 39 catalyst design through manipulation of materials chemistry. 40 Even moderate gains in catalytic performance of industrial 41 metal catalysts can result in significant improvements of 42 economics and environmental impact of large scale hydro- 43 carbon processing. 44 At present, ethene, propene, and butene, the building blocks 45 of many products in the chemical industry, are mainly produced 46 via steam cracking of oil fractions. This nonselective high 47 temperature process is energy intensive and is accompanied by 48 significant methane and coke formation. In contrast, catalytic 49 dehydrogenation of light alkanes offers an attractive alternative 50 for alkene production, since the catalyst can be tuned to be 51 selective and durable 7-9 and stoichiometric amounts of H 2 are 52 produced. Hydrogen is a valuable product in itself and is widely 53 used for hydrocracking and heteroatom removal. 7,10 54 Bimetallic Pt-Ga nanoparticles supported on a calcined 55 hydrotalcite, i.e. Pt-Ga/Mg(Al)O x , have shown to be efficient 56 dehydrogenation catalysts. 11 Upon introduction of Ga into Pt 57 nanoparticles, the degree of coke formation decreases during 58 reaction. In part, this is due to the basic nature of the Mg(Al)O x 59 support, which prevents rapid coke formation during reaction 60 and stabilizes the Pt-Ga clusters. 9,12 However, Pt-Ga 61 interactions seem to play the leading role in coking prevention. 62 Also, an alkene product selectivity close to 100% is observed on 63 these catalysts in addition to an increased activity. 64 Recently, a novel method to efficiently produce Pt-Ga 65 bimetallic catalysts has been developed. 11,13 For this purpose, 66 Ga is first incorporated into the hydrotalcite framework. This Received: July 19, 2014 Revised: September 11, 2014 Article pubs.acs.org/cm © XXXX American Chemical Society A dx.doi.org/10.1021/cm502658d | Chem. Mater. XXXX, XXX, XXX-XXX jmb00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i5 HF03:4230 | 2.0 alpha 39) 2014/07/15 09:23:00 | PROD-JCA1 | rq_2877750 | 10/02/2014 11:49:06 | 14 | JCA-DEFAULT