CO Oxidation on Supported Single Pt Atoms: Experimental and ab Initio Density Functional Studies of CO Interaction with Pt Atom on θ‑Al 2 O 3 (010) Surface Melanie Moses-DeBusk, † Mina Yoon, ‡ Lawrence F. Allard, † David R. Mullins, § Zili Wu, ‡,§ Xiaofan Yang, † Gabriel Veith, † G. Malcolm Stocks, † and Chaitanya K. Narula* ,† † Materials Science & Technology Division, ‡ Center for Nanophase Materials Sciences, and § Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6133, United States * S Supporting Information ABSTRACT: Although there are only a few known examples of supported single-atom catalysts, they are unique because they bridge the gap between homogeneous and heterogeneous catalysis. Here, we report the CO oxidation activity of monodisperse single Pt atoms supported on an inert substrate, θ-alumina (Al 2 O 3 ), in the presence of stoichiometric oxygen. Since CO oxidation on single Pt atoms cannot occur via a conventional Langmuir-Hinshelwood scheme (L-H scheme) which requires at least one Pt-Pt bond, we carried out a first- principles density functional theoretical study of a proposed pathway which is a variation on the conventional L-H scheme and inspired by the organometallic chemistry of platinum. We find that a single supported Pt atom prefers to bond to O 2 over CO. CO then bonds with the oxygenated Pt atom and forms a carbonate which dissociates to liberate CO 2 , leaving an oxygen atom on Pt. Subsequent reaction with another CO molecule regenerates the single-atom catalyst. The energetics of the proposed mechanism suggests that the single Pt atoms will get covered with CO 3 unless the temperature is raised to eliminate CO 2 . We find evidence for CO 3 coverage at room temperature supporting the proposed mechanism in an in situ diffuse reflectance infrared study of CO adsorption on the catalyst’s supported single atoms. Thus, our results clearly show that supported Pt single atoms are catalytically active and that this catalytic activity can occur without involving the substrate. Characterization by electron microscopy and X-ray absorption studies of the monodisperse Pt/θ- Al 2 O 3 are also presented. 1. INTRODUCTION The success of emission treatments can be attributed to heterogeneous catalysis where noble metals supported on high surface area oxides, Pt, Rh, and/or Pd supported on γ-alumina and ceria-zirconia, convert toxic gases to inert ones. 1 These noble metal catalysts oxidize CO and hydrocarbons and reduce nitrogen oxides in engine emissions. The widely accepted mechanism of CO oxidation on transition metals, the Langmuir-Hinshelwood scheme (L-H), 2 has been extensively studied by both theoretical 3,4 and experimental methods. 5 The size of the metal particles is considered an important factor in defining the performance of the catalyst. 6-11 A recent detailed study of CO oxidation on supported platinum nanoparticles, by Iglesia et al., shows that CO oxidation can proceed via the L-H mechanism or a CO-assisted, O 2 dissociation step leading to a O-O-CO intermediate that decomposes to form free CO 2 and chemisorbed O. 12 The smallest size supported particles studied are single atoms supported on metal oxides. 7,11,13-15 Supported single atoms can be isoelectronic with organo- metallic compounds and might exhibit catalytic activity similar to that of organometallic species in homogeneous environ- ments. For example, density functional theoretical modeling of a palladium single atom supported on magnesium oxide 16 has shown it to be a d 10 palladium species which would be isoelectronic with a 14-electron bis(triphenylphosphine)- palladium(0), believed to be an intermediate in the catalytic Heck arylation 17 and cyclization reactions. 18 Samples of supported single Pd atoms have indeed been synthesized and shown to be catalytically active for CO oxidation and acetylene cyclotrimerization. 7,19 Another important example of single Pd atom catalysis is heterogeneous hydrogenation by Pd atoms isolated on the Cu surface. 20 Support participation during CO oxidation has been proposed for the Pd/MgO catalyst. Platinum single atoms supported on iron oxide (Pt/Fe 2 O 3 ) 11 have also been synthesized by wet chemical methods and found to be active for catalytic oxidation of CO. The proposed mechanism of CO oxidation on Pt/Fe 2 O 3 is a modified L-H scheme where CO absorbs on Pt atoms while oxygen for CO oxidation is provided by the Fe 2 O 3 support. Thus, both Pd/ MgO and Pt/Fe 2 O 3 require support participation for CO oxidation. This raises a question about the catalytic activity of single atoms supported on inert substrates which cannot participate in the catalytic process. To our knowledge, indirect Received: February 20, 2013 Article pubs.acs.org/JACS © XXXX American Chemical Society A dx.doi.org/10.1021/ja401847c | J. Am. Chem. Soc. XXXX, XXX, XXX-XXX