Heterogeneous Catalysis DOI: 10.1002/anie.201210077 Importance of the Metal–Oxide Interface in Catalysis: In Situ Studies of the Water–Gas Shift Reaction by Ambient-Pressure X-ray Photoelectron Spectroscopy** Kumudu Mudiyanselage, Sanjaya D. Senanayake, Leticia Feria, Shankhamala Kundu, Ashleigh E. Baber, Jesffls Graciani, AlbaB. Vidal, Stefano Agnoli, Jaime Evans, Rui Chang, Stephanus Axnanda, Zhi Liu, JavierF. Sanz, Ping Liu, JosØ A. Rodriguez, and Darío J. Stacchiola* The traditional approach to the optimization of metal/oxide catalysts has focused on the properties of the metal and the selection of the proper oxide for its dispersion. The impor- tance of metal–oxide interfaces has long been recognized, [1] but the molecular determination of their properties and role is only now emerging. [2] Atoms with properties ranging from metallic to ionic are available at the interface and create unique reaction sites. We show herein how sites associated with a metal–ceria interface can dramatically change the reaction mechanism of the water–gas shift reaction (WGSR; CO + H 2 O!H 2 + CO 2 ). The WGSR is critical in the produc- tion of hydrogen. Multiple reaction mechanisms have been proposed. [3] In the redox mechanism, CO reacts with oxygen derived from the dissociation of H 2 O. In the associative process, the formation of a carbonaceous CO x H y intermediate must precede the production of H 2 and CO 2 . In situ studies are essential for the detection of surface species and active phases only present under the reaction conditions. [4] We present a combination of near-ambient-pressure X-ray photo- electron spectroscopy (NAP XPS), infrared reflection absorption spectroscopy (IRRAS), and density functional theory (DFT) calculations used to study the WGSR on CeO x nanoparticles deposited on Cu(111) and Au(111). Under WGSR conditions, adsorbed bent carboxylate (CO 2 dÀ ) spe- cies were identified over both CeO x /Cu(111) and CeO x / Au(111), with the ceria in a highly reduced state. By combining in situ experimental results with calculations, we show that the precursor for the formation of CO 2 dÀ is a carboxy (HOCO) intermediate on the metal–oxide inter- face. The WGSR has been investigated at the molecular level over single crystals. [5] To establish the importance of the morphology and electronic structure of oxide nanoparticle– metal interfaces, inverse (oxide/metal) model catalysts have been introduced. [6] Au(111) is inactive for the WGSR but can be activated in the presence of CeO x nanoparticles. [6a] A large enhancement in activity was also found on CeO x /Cu(111); [6b] Cu(111) is a typical benchmark for WGSR studies. [6] The rate of H 2 production over these catalysts is compared in Figure 1. Why are the CeO x /Cu(111) and CeO x /Au(111) catalysts highly active? The dissociation of H 2 O is considered to be the rate- determining step in the WGSR. [3c] Water does not dissociate Figure 1. Arrhenius plots of the WGSR rate (CO: 20 Torr, H 2 O: 10 Torr) on clean Cu(111), CeO x /Cu(111), and CeO x /Au(111). [*] Dr. K. Mudiyanselage, Dr. S.D. Senanayake, Dr. S. Kundu, Dr. A.E. Baber, Dr. J. Graciani, Dr. A.B. Vidal, Dr. S. Agnoli, Dr. P. Liu, Dr. J. A. Rodriguez, Dr. D. J. Stacchiola Chemistry Department, Brookhaven National Laboratory Upton, NY 11973 (USA) E-mail: djs@bnl.gov Dr. L. Feria, Dr. J. Graciani, Prof. J. F. Sanz Departamento de Química Física, Universidad de Sevilla 41012 Seville (Spain) Prof. J. Evans Facultad de Ciencias, Universidad Central de Venezuela Caracas 1020A (Venezuela) Dr. R. Chang Shanghai Institute of Microsystem and Information Technology Shanghai 200050 (China) Dr. S. Axnanda, Dr. Z. Liu The Advanced Light Source, Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) [**] Research carried at BNL was financed by the US DOE, Office of BES (Grant No. DE-AC02-98CH10086). Some of the calculations were performed at the Center for Functional Nanomaterials at BNL. The main theoretical part was carried out by the group of J.F.S. and funded by the Ministerio de Economía y Competitividad (Spain, grants MAT2012-31526 and CSD2008-0023). Computational resources were provided by the Barcelona Centro Nacional de Supercomputación (Spain). J.E. thanks INTEVEP and IDB for grants used for the research in Venezuela. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201210077. A ngewandte Chemi e 5101 Angew. Chem. Int. Ed. 2013, 52, 5101 –5105  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim