Waste-free scale up synthesis of nanocrystalline hexaaluminate: properties in oxygen transfer and oxidation reactions Said Laassiri, ab Nicolas Bion, b Fabien Can, b Xavier Courtois, b Daniel Duprez, b Se ´bastien Royer* ab and Houshang Alamdari* a Received 11th May 2012, Accepted 3rd July 2012 DOI: 10.1039/c2ce25737h Synthesis of nanocrystalline hexaaluminate is reported using an original activated reactive synthesis process. Starting from a classical ceramic solid, exhibiting low surface area and a micrometric crystal size, a two-step grinding process allows reduction of the crystal size down to a few nanometers and development of high surface areas. The synthetic process was then used to produce transition metal- and noble metal-doped structures. The effects of (i) morphological and structural properties and (ii) substitution on oxygen transfer properties and catalytic properties in CO and CH 4 oxidation reactions were studied. Crystal size was shown to be a key parameter in controlling the bulk oxygen transfer. Study of the catalytic properties in low and high temperature oxidation reactions also shows the crucial effect of the morphological parameters. Highest activities were achieved over nanocrystalline high surface compositions. Finally, even if less active than classical palladium supported solids, these new structures exhibited extremely high thermal stability. 1. Introduction With the growing demand for energy all around the world, energy conversion processes including natural gas combustion, steam reforming and partial oxidation of hydrocarbons will play key roles in the near future as promising economically and environmentally viable technologies for energy production. These catalytic processes often involve the use of supported noble metal based active materials. It is recognized that such noble metal phases are, until now, the best existing compromises between activity and long term stability. 1,2 However, at high operating temperatures, the unavoidable loss of active species, combined with the sintering process, lead to an irreversible deactivation of the catalyst. 3 Considering the severe operating conditions required in energy conversion processes and the high performance needed in industrial application, the design of active materials allowing a reduction of operating temperatures and being thermally resistant remains a real challenge. In order to meet these industrial requirements, in terms of activity and thermal stability, and to progressively reduce noble metal loading in active materials, mixed oxide structures were widely investigated in heterogeneous catalysis since the begin- ning of the 1970s. 4,5 Indeed, the wide variety of mixed structures including perovskite, spinel and hexaaluminate, having different surface and chemical compositions, 6 offers the possibility to fine tune the solid properties in line with the application. Consequently, some of them have been shown to be highly active for a variety of redox reactions in their oxide form or after reduction to generate a dispersed metal active phase. 7,8 Among the mixed oxide families, only hexaaluminate (AAl 12 O 19 , where A is an alkali, alkaline earth, or rare earth cation) has shown sufficient thermal stability for catalytic application at high temperature, e.g. catalytic combustion. 9 Some perovskite-based structures (ABO 3 where A x+ and B y+ are respectively lanthanides and transition metal cations), also exhibit high activity. 10 These materials however suffer from a dramatic surface area decrease at high temperature, making them less attractive than hexaaluminates for high temperature reactions. 11 Indeed, the ability of hexaaluminate structures to maintain phase stability, high surface area and resistance to sintering at high temperature is related to their particular layered crystalline structure. 12 The hexaaluminate structure consists of alternate Al 2 O 3 spinel blocks intercalated by mirror planes in which large A-cations are located. Besides thermal stability, hexaaluminate structures can host different redox cations M n+ /M (n2x)+ through Al 3+ site substitution (AM x Al 122x O 192d where M refers to the doping cation including transition or noble metal active phases, e.g. Mn 3+ /Mn 2+ , Fe 3+ /Fe 2+ , Pd n+ /Pd 2+ …). 13,14 Such substitutions induce surface functionalization, which confers activity to the material. Several studies reveal the role of A and M on the redox behaviour of hexaaluminates and their catalytic activity. 15–17 The catalytic activity of a mixed oxide, provided by the redox cycle of the M cation, can be further enhanced by reducing the particle size for a given composition. Indeed, with the decrease of crystal size at a Department of Mining, Metallurgical and Materials Engineering, Universite ´ Laval, Que ´bec, Canada, G1V 0A6. E-mail: houshang.alamdari@gmn.ulaval.ca; Tel: +1-418 656-7666 b Universite ´ de Poitiers, CNRS UMR 7285, IC2MP, 4 Rue Michel Brunet, 86022 Poitiers Cedex - France. E-mail: sebastien.royer@univ-poitiers.fr; Tel: +33-5-49-45-34-79 CrystEngComm Dynamic Article Links Cite this: CrystEngComm, 2012, 14, 7733–7743 www.rsc.org/crystengcomm PAPER This journal is ß The Royal Society of Chemistry 2012 CrystEngComm, 2012, 14, 7733–7743 | 7733