Core/shell catalysts DOI: 10.1002/smll.200800895 Highly Active and Sinter-Resistant Pd-Nanoparticle Catalysts Encapsulated in Silica** Jung-Nam Park, Arnold J. Forman, Wei Tang, Jihong Cheng, Yong-Sheng Hu, Hongfei Lin, * and Eric W. McFarland* Pd-based heterogeneous catalysts have been used in a wide range of important chemical processes including CO oxida- tion, [1,2] aerobic alcohol oxidation, [3] NO x reduction, [4] con- versions of volatile organic compounds, [5,6] hydrogenation, [7–9] and dehydrogenation. [10] Recent work has suggested that when the dimensions of the Pd particles are reduced to less than 10 nm the catalysts are extraordinarily active and may exhibit unique properties. [11–14] Unfortunately, Pd as well as other metal nanoparticulate catalysts are often unstable with respect to sintering on oxide supports at elevated reaction temperatures and their activity rapidly decays. [15–19] To stabilize nanoparticulate metals, unique structures consisting of metal cores surrounded by oxide, carbon, or other metal shells (core/shell) have been created. [20–28] Recently, approximately 15-nm Au cores in spherical ZrO 2 shells (Au@ZrO 2 ) showed high activity for CO oxidation at 240 8C without sintering. [20] Pd particles were also recently synthesized in silica shells [21] but no investigations of their catalytic properties or sintering resistance have been reported. We have investigated the catalytic properties of Pd nanoparticle catalysts encapsulated in silica, specifically addressing the following questions: 1) Can Pd nanoparticles be synthesized in spherical SiO 2 shells (Pd@SiO 2 ) that are sufficiently porous to allow unhindered mass transfer? 2) How does the activity of the encapsulated catalysts compare to traditional supported Pd catalysts for CO oxidation and acetylene hydrogenation? 3) Do the shell structures provide stability with respect to sintering at high temperatures? Nanoscale core/shell Pd@SiO 2 spherical particles were prepared following the method described by Bae et al. [21] with the modification that a mixture of tetraethyl orthosilicate (TEOS) and n-octadecyl trimethoxysilane (C 18 -TMS) was used as the SiO 2 precursor. The C 18 -TMS was employed as a porogen to increase the porosity of the SiO 2 shell. Control samples of Pd supported on silica spheres (Pd/SiO 2 ) were prepared by impregnation. The loading of Pd for both Pd@SiO 2 and Pd/SiO 2 was 6.12 wt%. The morphology of the catalysts was characterized by transmission electron microscopy (TEM). The micrographs in Figure 1 show that, as synthesized, both the Pd@SiO 2 and Pd/ SiO 2 catalysts contain well dispersed Pd nanoparticles. For the Pd@SiO 2 nanoparticles, the Pd cores appeared uniform in shape and monodisperse in size (diameter ¼ 4.2  2.0 nm; Figs. S1 and S2). The SiO 2 shells are approximately 10 nm in thickness. The micrographs of the control sample, Pd/SiO 2 , exhibit a more broad distribution of Pd sizes with predomi- nately 2–4 nm dimensions and scattered evidence of larger particles (Fig. S1). The catalysts were subjected to calcination at 700 8C for 6 h in air to examine their stability. The core/shell particles showed minimal changes in the Pd core size (4.2 to 4.6  2.0 nm; Fig. 1b), but there was no evidence of breakdown in the core/shell structure. The Pd/SiO 2 control catalysts showed noticeable agglomeration of Pd nanoparticles after identical calcination, Figure 1d. X-ray diffraction (XRD) of the Pd nanoparticle catalysts identified metal oxide phases in both as-synthesized samples (Fig. 2). Comparison of the diffraction patterns for the as- synthesized Pd@SiO 2 sample (Fig. 2a) and that after calcination at 700 8C in air (Fig. 2b) shows only the presence of PdO (this finding was also corroborated by XPS and Raman spectra as shown in Figures S3, S4, and Table S1 of the Supporting Information), with essentially no change in crystallite size as calculated by Scherrer broadening of the diffraction peaks. The control samples, Pd/SiO 2 (both as- synthesized and after calcination at 700 8C), were also observed to be PdO (Fig. 2[c and d]). However, after calcination the calculated crystallite size increased significantly from 26.7 nm to 73.0 nm, due to sintering. The catalytic activities of Pd@SiO 2 and Pd/SiO 2 were measured in a plug flow reactor loaded with 30 mg of catalyst. The activities were determined for CO oxidation and acetylene hydrogenation as a function of temperature at space times of 0.07 and 0.05 seconds, respectively. Both Pd catalysts exhibited little or no CO oxidation activity below 140 8C (Fig. 3a), which is in agreement with previous work. [1,2,29] The as-synthesized control catalyst, Pd/SiO 2 , was more active at lower temperature (apparent activation energy ¼ 103.0  3.7 kJ mol 1 ) than the as-synthesized Pd@SiO 2 catalyst (apparent activation energy ¼ 129.0  5.3 kJ mol 1 ), as shown in Figure S6 (a plot of ln[conversion] vs. 1/T). The modest difference in activation energies may be due to a communications [  ] Dr. H. Lin, Dr. J. Cheng Gas Reaction Technologies Inc Santa Barbara, CA 93111 (USA) E-mail: Hongfei.lin@gmail.com Prof. E. W. McFarland, Dr. J.-N. Park, Dr. Y.-S. Hu Department of Chemical Engineering University of California Santa Barbara Santa Barbara, CA 93106 (USA) E-mail: mcfar@engineering.ucsb.edu A. J. Forman, W. Tang Department of Chemistry and Biochemistry University of California Santa Barbara Santa Barbara, CA, 93106 (USA) [  ] This work was supported by the U.S. Department of Energy, (Basic Energy Sciences grant DE-FG02-89ER14048) and a Korea Research Foundation Grant funded by the Korean Government (KRF-2006- 352-D00044). UCSB Materials Research Laboratory Central Facili- ties were funded by National Science Foundation Award # DMR00- 80034. A.J.F. thanks the United States Environmental Protection Agency (EPA) under the Science to Achieve Results (STAR) Gradu- ate Fellowship Program for three years of generous support. EPA has not officially endorsed this publication and the views expressed herein may not reflect the views of the EPA. : Supporting Information is available on the WWW under http:// www.small-journal.com or from the author. 1694 ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2008, 4, No. 10, 1694–1697