Critical to understanding heterogeneous catalysis is the identification of the active sites involved in reactive processes. Recently, researchers from the University of Pennsylvania have developed an approach to grow ordered, mesoscale assemblies of nanocrystals of two types, a metal (Au) and an oxide (FeOx). These binary nanocrystal superlattices (BNSLs) form with specific crystal structures and have a specific number density of metal/oxide contacts. The BNSLs were used to catalyze CO oxidation, and quantitative measurement of the reaction rates were correlated to the number of contacts, leading to a linear scaling. This confirms that the active sites for CO oxidation are at the metal/oxide contacts. This approach—the formation of specific BNSL structures to determine active sites—is general, providing a systematic method to explore this important question. Interestingly, limiting direct contact between metal nanocrystals through the formation of appropriate superlattice structures was found to have enhanced the thermal stability of these systems. Myriad nanocrystal superlattice structures and compositions are available; therefore, this approach facilitates the exploration, the understanding, and the improvement of catalytic processes, bridging the gap between traditional, highly idealized single crystal experiments and less controlled, supported catalyst studies.
Binary NCs form different crystal structures depending on the relative sizes of the constituents (referred to as A and B, corresponding to Au and FeOx, respectively). TEM images of a) AB, b) AB2, and c) AB13 BNSLs; STEM-HAADF images of d) AB, e) AB2, and f) AB13 BNSLs; g) STEM-HAADF image of AB Au-FeOx BNSL, illustrating that one Au NC is in contact with four adjacent FeOx NCs. The activity (reaction rate of CO oxidation expressed by the rate of CO2 production) normalized to h) the mass of Au and i) the mass of Fe as a function of the number of the Au-FeOx contacts h) per Au NC and i) per FeOx NC. Scale bars: a, b, d, f) 20 nm, c) 50nm, e) 10 nm, g) 2 nm.
Binary nanocrystal superlattices (BNSL) are ordered, mesoscale assemblies of two different nanocrystal (NC) components. Specific structures can be formed that control the number density of contacts between metal (Au) NCs and support (FeOx) NCs. By correlating measurements of the activity of BNSLs for carbon monoxide (CO) oxidation with the number density of catalytic contacts, conclusive identification of the catalytically active site can be made.
1. Established a systematic methodology for determining catalytically active sites using BNSLs.
2. The structure of the superlattices spatially confines the active metals, thereby enhancing their thermal stability during reactions.
3. This approach is generalizable, making BNSLs effective model systems to explore, to understand, and to improve catalytic processes.
Engineering Catalytic Contacts and Thermal Stability: Gold/Iron Oxide Binary Nanocrystal Superlattices for CO Oxidation, Yijin Kang, Xingchen Ye, Jun Chen, Liang Qi, Rosa E. Diaz, Vicky Doan-Nguyen, Guozhong Xing, Cherie R. Kagan, Ju Li, Raymond J. Gorte, Eric A. Stach, and Christopher B. Murray, J. Am. Chem. Soc. 135, 1499-1505, 2013.
C.B.M. and Y.K. acknowledge the partial support from the National Science Foundation MRSEC DMR11-20901, and the U.S. Army Research Oﬃce (ARO) under Award MURI W911NF-08-1-0364. X.Y. acknowledges the support from the Oﬃce of Naval Research (ONR) Multidisciplinary University Research Initiative (MURI) on Optical Metamaterials through award N00014-10-1-0942. J.C. and V.D.-N. acknowledge the DOE Oﬃce of ARPA-E for support under Award DEAR0000123. G.X. and C.R.K. acknowledge support from the U.S. Department of Energy, Oﬃce of Basic Energy Sciences, Division of Materials Science and Engineering, under Award No. DE-SC0002158. L.Q. and J.L. acknowledge support by NSF DMR-1120901 and AFOSR FA9550-08-1-0325. C.B.M. thanks the Richard Perry University Professorship for the support of his supervisor role. Research carried out in part at the Center for Functional Nanomaterials (CFN), Brookhaven National Laboratory (BNL), which is supported by the U.S. Department of Energy, Oﬃce of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.
2013-3880 INT/EXT | Media & Communications Office