15856 Phys. Chem. Chem. Phys., 2013, 15, 15856--15862 This journal is c the Owner Societies 2013 Cite this: Phys. Chem. Chem. Phys., 2013, 15, 15856 Probing adsorption sites for CO on ceria Kumudu Mudiyanselage, a Hyun You Kim, b Sanjaya D. Senanayake, a Ashleigh E. Baber, a Ping Liu b and Dario Stacchiola* a Ceria based catalysts show remarkable activity for CO conversion reactions such as CO oxidation and the water-gas shift reaction. The identification of adsorption sites on the catalyst surfaces is essential to understand the reaction mechanisms of these reactions, but the complexity of heterogeneous powder catalysts and the propensity of ceria to easily change oxidation states in the presence of small concentrations of either oxidizing or reducing agents make the process difficult. In this study, the adsorption of CO on CuO x /Cu(111) and CeO x /Cu(111) systems has been studied using infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. IR peaks for the adsorbed CO on O/Cu(111) with only chemisorbed oxygen, well-ordered Cu 2 O/Cu(111) and disordered copper oxide [CuO x /Cu(111)] were observed at 2070–2072, 2097–2098 and 2101–2111 cm À1 , respectively. On CeO x /Cu(111) systems CO chemisorbs at 90 K only on Cu sites under ultra-high vacuum (UHV) conditions, whereas at elevated CO pressures and low temperatures adsorption of CO on Ce 3+ is observed, with a corresponding IR peak at 2162 cm À1 . These experimental results are further supported by DFT calculations, and help to unequivocally distinguish the presence of Ce 3+ cations on catalyst samples by using CO as a probe molecule. 1. Introduction CO is widely used as a probe molecule to characterize powder samples due to its very intense IR signal and its sensitivity to changes in local environments and adsorption sites. The complexity and heterogeneity of powder systems, including the various degrees of oxidation or reduction state of the catalysts, can in some cases cause difficulties in the assignment of vibrational features to specific sites. Well-defined model catalysts can be used to help in the clear identification of critical catalyst structures and adsorption sites, as well as for fundamental mechanistic studies of catalytic reactions. Ceria–copper based catalysts show remarkable activity for CO conversion 1 reactions such as CO oxidation 2–6 and the water-gas shift. 7–9 One of the main properties of ceria catalysts is their facile oxidation and reduction in various environments, which makes the homogeneous preparation of a particular oxidation state extremely difficult since small amounts of oxygen or CO immediately interact with their surface sites. On powder cerium oxide systems exposed to CO at 300 K, a peak at 2150–2175 cm À1 was assigned to adsorbed CO on Ce 4+ , 10–12 and a peak at 2120–2127 cm À1 was assigned to CO adsorbed on Ce 3+ for samples heavily reduced. 11 The assigned frequency of the CO peak on Ce 3+ coincides with a feature due to the forbidden 2 F 5/2 - 2 F 7/2 electronic transition of Ce 3+ , 11,13 which can also be generated by reduction without the use of CO, by exposure of ceria powders to hydrogen. 14 In another study, peaks observed at 2120 and 2180 cm À1 for adsorbed CO on reduced samples of Cu/CeO 2 were attributed to purely electronic transitions in donor (Ce 3+ ) centers. 6 These peaks were not observed on heavily oxidized samples. An IR peak observed at 2109–2117 cm À1 on a ceria–copper powder system was assigned to CO adsorbed on Cu + . 10 Even though there have been studies on the adsorption of CO on single crystal based CeO x systems, a clear identification of adsorbed CO on Ce 3+ / Ce 4+ has not been achieved. 15–18 Recently, an IR peak at 2120 cm À1 has been assigned to CO adsorbed on Ce 3+ sites from Pt/CeO 2 /Cu(111), 16 based on the conflicting previous assignment of powder systems, 11 which coincides with the above mentioned electronic transition on Ce 3+ cations. In addition, CO adsorbed on oxidized Cu(111) also shows a peak at 2117 cm À1 . 19 Therefore, a clear identification of CO on ceria sites using well-defined model systems is required since it is essential for further mechanistic understanding of the CO conversion reactions on ceria based catalysts. In this study we present a series of different Cu–O systems where the adsorption of CO is followed by IRRAS. Oxidized and reduced CeO x /Cu(111) systems are used to study the adsorption of CO under ultra- high vacuum (UHV) and elevated pressure conditions. This approach allows us to unequivocally identify the vibrational peak for adsorbed CO on Ce 3+ at 2162 cm À1 . Results presented a Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA. E-mail: djs@bnl.gov b Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA Received 31st May 2013, Accepted 14th July 2013 DOI: 10.1039/c3cp52295d www.rsc.org/pccp PCCP PAPER Published on 16 July 2013. Downloaded by BNL Research Library on 19/09/2013 15:50:38. View Article Online View Journal | View Issue