Unraveling the Dynamic Nature of a CuO/CeO 2 Catalyst for CO Oxidation in Operando: A Combined Study of XANES (Fluorescence) and DRIFTS Siyu Yao, †,∇ Kumudu Mudiyanselage, † Wenqian Xu, † Aaron C. Johnston-Peck, ∥ Jonathan C. Hanson, † Tianpin Wu, ‡ Dario Stacchiola, † Jose ́ A. Rodriguez, † Haiyan Zhao, ‡ Kevin A. Beyer, ‡ Karena W. Chapman, ‡ Peter J. Chupas, ‡ Arturo Martínez-Arias, § Rui Si, † Trudy B. Bolin, ‡ Wenjian Liu, ∇ and Sanjaya D. Senanayake* ,† † Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States ‡ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States § Instituto de Cata ́ lisis y Petroleoquímica, Consejo Superior de Investigaciones Científicas (ICP-CSIC), Madrid E-28049, Spain ∇ Center for Computational Science & Engineering, and PKU Green Chemistry Center, Peking University, Beijing 100871, People’s Republic of China ∥ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States * S Supporting Information ABSTRACT: The redox chemistry and CO oxidation (2CO + O 2 → 2CO 2 ) activity of catalysts generated by the dispersion of CuO on CeO 2 nanorods were investigated using a multitechnique approach. Combined measurements of time-resolved X-ray absorption near-edge spectroscopy (XANES) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in one setup were made possible with the development of a novel reaction cell in which fluorescence mode detection was applied to collect the XANES spectra. This is the first reported example using XANES in a similar technique combination. With the assistance of parallel time-resolved X-ray diffraction (XRD) measurements under operando conditions, we successfully probed the redox behavior of CuO/CeO 2 under CO reduction, constant-flow (steady-state) CO oxidation and during CO/O 2 cycling reactions. A strong copper ↔ ceria synergistic effect was observed in the CuO/CeO 2 catalyst. Surface Cu(I) species were found to exhibit a strong correlation with the catalyst activity for the CO oxidation reaction. By analysis of phase transformations as well as changes in oxidation state during the nonsteady states in the CO/O 2 cycling reaction, we collected information on the relative transformation rates of key species. Elementary steps in the mechanism for the CO oxidation reaction are proposed based on the understandings gained from the XANES/DRIFTS operando studies. KEYWORDS: Operando catalysis, XANES, DRIFTS, CO oxidation, copper, ceria 1. INTRODUCTION Catalysis offers a versatile ability to meet the demands for energy, 1 synthesis of fuels for the future 2,3 and the remediation of environmental pollutants. 4 However, the fundamental understanding of even simple catalytic processes remains difficult, because of the complex nature of processes that occur dynamically between reactants and polycrystalline catalyst materials. The development of in situ characterization techniques (scattering, imaging, and other spectroscopy techniques) has made it possible for researchers to identify and monitor reaction intermediates and catalyst structures that evolve under reaction conditions. 5−7 The combination of Received: February 4, 2014 Revised: April 11, 2014 Research Article pubs.acs.org/acscatalysis © XXXX American Chemical Society 1650 dx.doi.org/10.1021/cs500148e | ACS Catal. 2014, 4, 1650−1661