Heterogeneous Catalysis DOI: 10.1002/ange.200903918 Water-Gas Shift Reaction on a Highly Active Inverse CeO x /Cu(111) Catalyst: Unique Role of Ceria Nanoparticles** JosØA. Rodriguez,* Jesffls Graciani, Jaime Evans, JoonB. Park, Fan Yang, Dario Stacchiola, Sanjaya D. Senanayake, Shuguo Ma, Manuel PØrez, Ping Liu, Javier Fdez. Sanz, and Jan Hrbek The water-gas shift (WGS, CO + H 2 O!CO 2 + H 2 ) is an important reaction frequently used in the chemical industry for the production of clean H 2 . [1] Oxide-supported copper catalysts show significant water-gas shift activity but their performance is not fully understood and is highly dependent on the synthesis conditions or the nature of the oxide support. [1–4] Either metallic Cu or Cu + cations have been proposed as active sites for the WGS. [1–8] In addition, the oxide support may not be a simple spectator and may play a direct role in the catalytic process. [1, 4, 9] Extended surfaces and nanoparticles of metallic copper are able to catalyze the WGS [3, 5, 6, 10, 11] and Cu(111) has become a benchmark surface for studying the WGS. [3, 5, 11–14] Specific rates, activation energies, and reaction orders are consistent with data reported for ZnO-supported Cu catalysts. [5, 13] The rate-limit- ing step for the WGS on Cu(111) seems to be the dissociation of water, [11–13] and it is affected by the presence of surface modifiers such as S, O, or Cs. [13] Herein, we investigate the WGS reaction on Cu(111) surfaces partially covered with ceria nanoparticles. Bulk ceria is a well known oxide support for WGS catalysts. [1, 4, 8, 9] The CeO x /Cu(111) system allows us to study in detail the role of the oxide in the catalytic process and, furthermore, catalysts with an inverse oxide/metal configuration have some advan- tages for practical applications. [15] Figure 1 shows images of scanning tunneling microscopy (STM) acquired after dosing Ce to Cu(111) at 650 K under an atmosphere of O 2 (p  5 10 7 Torr). The reaction of O 2 with the Cu(111) substrate leads to formation of a layer of copper oxide which exhibits domains of different reconstructions of a Cu 2 O(111) sur- face. [16] On top of the layer of copper oxide, there are two types of ceria features. These features were not seen in blank experiments for the adsorption of O 2 on Cu(111). In Figure 1, small islands of ceria (2–5 nm in size, CeO x -I) appear on the terraces of the copper substrate, whereas large islands of ceria (30–50 nm in size, triangular shape, CeO x -II) are embedded in the substrate step edges. Inside the large ceria islands, a moirØ pattern can be seen with a separation of approximately 5 nm in between the depressions. The large ceria islands have a morphology different from that seen for the two most stable surfaces of bulk ceria : CeO 2 (111) and CeO 2 (110). An analysis of the structure of these islands reveals that they essentially have a height of 0.31 nm, which is consistent with a single layer of cerium sandwiched in between two layers of oxygen. [17] An O-Ce-O-Cu(interface) stacking is likely. The oxide nanoparticles in CeO x /Cu(111) display a morphology quite different from that found before for the growth of ceria nanoparticles on a Au(111) substrate using the same depo- sition methodology. [17] In the CeO x /Au(111) systems there were only large oxide islands at the step edges and these were exposing the (111) face of bulk ceria. [17] The interactions of Figure 1. STM images recorded after dosing Ce to Cu(111) at 650 K under an atmosphere of O 2 (p  510 7 Torr). The two differentiated images at the top were taken with V t = 3.1 V and I t = 0.03 nA. The height image at the bottom right, showing the inside of a ceria island, was taken at imaging conditions of 2.7 V, 0.05 nA. The scheme (bot- tom left) was composed using the line profile indicated by the green line shown near the middle of the top right image. [*] Dr. J.A. Rodriguez, Dr. J. Graciani, Dr. J.B. Park, Dr. F. Yang, Dr. D. Stacchiola, Dr. S.D. Senanayake, Dr. S. Ma, Dr. P. Liu, Dr. J. Hrbek Chemistry Department, Brookhaven National Laboratory Upton, NY 11973 (USA) Fax: (+ 1) 631-344-5815 E-mail: rodrigez@bnl.gov Prof. J. Evans, Prof. M. PØrez Facultad de Ciencias, Universidad Central de Venezuela Caracas 1020A (Venezuela) Prof. J. F. Sanz Departamento de Química Física, Universidad de Sevilla 41012-Seville (Spain) [**] The work performed at BNL was supported by the US Department of Energy, Office of Basic Energy Sciences, under contract DE-AC02- 98CH10886. J.E. and M.P. are grateful to INTEVEP for partial support of the work carried out at the UCV. The work done at Seville was funded by MICINN, grant no MAT2008-04918 and the Barcelona Supercomputing Center—Centro Nacional de Super- computación (Spain). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200903918. Angewandte Chemie 8191 Angew. Chem. 2009, 121, 8191 –8194  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim