Confined Catalysts DOI: 10.1002/ange.200805273 An Efficient Strategy to Drive Nanoparticles into Carbon Nanotubes and the Remarkable Effect of Confinement on Their Catalytic Performance** Eva Castillejos, Pierre-Jean Debouttire, Lucian Roiban, Abderrahim Solhy, Victor Martinez, Yolande Kihn, Ovidiu Ersen, Karine Philippot, Bruno Chaudret, and Philippe Serp* The possibility of using the inner cavity of a carbon nanotube (CNT) as a nanoreactor of a few nanometers in diameter and a few micrometers in length is an exciting challenge, which led only a few years after the discovery of CNTs to the first studies concerning their opening, filling, capillarity, and wetting. [1] The complete filling of CNTs, which allows for the production of metallic nanowires [2] for electronic or magnetic devices, has been mastered and well-documented. [3] However, the selective confinement of discrete nanoparticles (NPs) in the CNT cavity is still a synthetic challenge. Success in this endeavor could pave the way to interesting perspec- tives for drug delivery or for performing chemistry in a confined space while exploiting the unique CNT properties. Owing to synthetic difficulties, few studies provide results on confinement effects in CNTs. For example, the reactivity of iron oxide NPs is increased (easy reduction), while that of metallic iron NPs is decreased (difficult reoxidation) when confined inside CNTs. [4] Dimensionally confined phase tran- sitions have been reported for water [5] and ionic liquids [6] encapsulated within CNTs. Heptene confined in CNTs shows a reduced reactivity towards atomic hydrogen, [7] and rhodium NPs confined in CNTs are one order of magnitude more active for ethanol production from syngas than their counter- parts deposited on the convex CNT surface. [8] Furthermore, the ultraefficient transport of water and gas through the hydrophobic cavity of CNTs makes CNT membranes prom- ising for many applications. [9] Currently, three main routes are used to introduce NPs inside CNTs: 1) Incipient wetness impregnation techniques rely on the filling of oxidized CNTs by capillarity with a solution containing a metal salt [10–15] or preformed NPs. [16] X- ray photoelectron spectroscopy (XPS) [11] and 3D TEM [12] studies have indicated that the fraction of metal present in the inner cavity of CNTs is between 15 and 50 %. More selective filling of very large diameter (300 nm or larger) carbon filaments by preformed NPs has been achieved. 2) Sublimation of a metal precursor can be used to introduce it into the CNT cavity. [17] For these two methods, an improve- ment in the confinement selectivity can be obtained if an additional selective washing step is performed to eliminate NPs deposited on the external surface. [18] The main drawback of these methods is that a significant amount of precursor is lost during the synthesis. 3) Another multistep approach consists of producing CNTs inside anodic aluminum oxide membranes, filling the resulting template with a solution of NPs, and dissolving the alumina membrane. [19] This latter method, however, suffers from scale-up difficulties. Herein we report 1) a simple and efficient method for the selective confinement of NPs in the inner cavity of CNTs, and 2) the effect of spatial confinement of bimetallic PtRu NPs on their catalytic performance for the selective hydrogenation of cinnamaldehyde. The strategy we have developed to selec- tively confine NPs inside CNTs is based on surface chemistry (Figure 1). Figure 1. Strategy adopted to drive NPs into CNTs. PtRu NPs pink, N red, O blue, C gray and green. [*] Dr. E. Castillejos, Dr. A. Solhy, Dr. P. Serp CNRS; LCC (Laboratoire de Chimie de Coordination) composante ENSIACET 118, route de Narbonne, 31077 Toulouse (France) and UniversitØ de Toulouse; UPS, INP; LCC 31077 Toulouse (France) Fax: (+ 33) 5-6288-5600 E-mail: philippe.serp@ensiacet.fr Homepage: http://www.lcc-toulouse.fr/ Dr. P.-J. Debouttire, Dr. V. Martinez, Dr. K. Philippot, Dr. B. Chaudret CNRS; LCC (Laboratoire de Chimie de Coordination) 205, route de Narbonne, 31077 Toulouse (France) and UniversitØ de Toulouse; UPS, INP; LCC 31077 Toulouse (France) Dr. Y. Kihn CEMES, CNRS UPR 8001 29 Rue Jeanne Marvig, 31055 Toulouse (France) L. Roiban, Dr. O. Ersen IPCMS—Groupe Surfaces et Interfaces, CNRS—ULP UMR 7504 23 Rue du Loess BP 43, 67034 Strasbourg (France) [**] This work was performed under ANR PNano 2005 ANR-05-NANO- 030-01. Vincent Collire is acknowledged for technical assistance in TEM imaging, and Dr. D. Gonbeau (CNRS, Pau, France) for her help in the XPS experiments. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200805273. Angewandte Chemie 2567 Angew. Chem. 2009, 121, 2567 –2571  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim