CO Oxidation at the Au − Cu Interface of Bimetallic Nanoclusters

DFT+U calculations of the structure of CeO2(111)-supported Aubased bimetallic nanoclusters (NCs) show that a strong support−metal interaction induces a preferential segregation of the more reactive element to the NC−CeO2 perimeter, generating an interface with the Au component. We studied several Au -based bimetallic NCs (Au-X, X: Ag, Cu, Pd, Pt, Rh, and Ru) and found that (Au− Cu)/CeO2 is optimal for catalyzing CO oxidation via a bifunctional mechanism. O2 preferentially binds to the Cu-rich sites, whereas CO binds to the Au-rich sites. Engineering a two-component system in which the reactants do not compete for binding sites is the key to the high catalytic activity at the interface between the components. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis T critical role of the interface between a supporting oxide and supported metal nanoparticles (NPs)/nanoclusters (NCs) has been highlighted by many experimental and theoretical studies. Moreover, recent studies are suggesting that interfaces in nanocatalysts can be designed on the atomic scale for specific purposes. The Rodriguez and Adzic groups, in particular, have reported various kinds of tunable interfaces, metal and oxide, metal and carbide, and oxide and oxide, and highlighted the important role of these interfaces for various heterogeneous catalytic reactions. Since Haruta’s pioneering finding on the excellent catalytic activity of oxide-supported Au NPs, the oxidation chemistry of oxide-supported Au NPs or NCs has been studied extensively, with a focus on a determination of the active site. Of particular interest is how the system is able to activate the oxygen molecule to give the high catalytic activity observed experimentally. Theoretical studies of O2 activation by supported or unsupported Au NPs/NCs have reported low O2 binding energies and high O2 dissociation barriers. 21 In our previous study of CO oxidation by CeO2-supported Au NCs (Au/CeO2), we found a relatively strong CO binding as compared with O2 on the Au NC of Au/CeO2(111), leading to CO poisoning and a low oxidation rate. These results suggest that a different reaction mechanism is available that involves another source of oxygen. In this regard, the oxygen spillover mechanism, the Mars−van Krevelen (M-vK) mechanism of CO oxidation, and O2 binding at the Au−support interface are considered as better alternatives to explain the rich chemistry of CO oxidation by oxide-supported Au catalysts that is observed experimentally. We have previously reported that the low-coordinated interfacial oxygen atoms oxidize CO bound to Au NCs (Au-CO*) by the M-vK mechanism of CO oxidation, emphasizing the role of the NC−CeO2 interface. 14 We suggest a strategy to improve the catalytic activity of Au NPs/NCs by more intensive interface engineering, utilizing the strong metal−support interaction. We study a set of CeO2(111) supported Au-based bimetallic NCs composed of 10 atoms (Au7-X3, where X is Ag, Cu, Pd, Pt, Rh, or Ru) and find that a strong oxygen affinity of the alloying elements, X, drives their preferential segregation to the NC-CeO2(111) perimeter. Segregation of the metal components results in three interfaces between Au, CeO2, and the alloying element. CO oxidation at these interfacial sites is examined using density functional theory (DFT). The different alloying elements change the reaction energetics; Cu is found to produce a particularly active CO oxidation mechanism at the interface with Au. A 4 × 4 CeO2(111) slab model with six atomic layers and 20 Å of vacuum was prepared to describe the CeO2 support. Sensitivity tests on the model parameters (energy cutoff, kpoint sampling, and system size) showed that our calculation parameters sufficiently describe the energetics of the oxidation catalysis by CeO2-supported Au NP/NCs. 1,2,14 A highly symmetric hexagonal two-layered Au NC composed of 10 atoms was supported on the CeO2(111) surface (Figure 1a). The entire Au/CeO2 system was fully optimized prior to catalysis studies. To study the effect of the CeO2(111) support on the structure of supported Au-X bimetallic NPs, we replaced three Au atoms in the top layer of the Au10 NC with one of the following alloying elements: Ag, Cu, Pd, Pt, Rh, or Ru (Figure 1b). The preferred geometry for the alloying elements in the clusters was determined using two metrics. First, the exchange Received: July 18, 2013 Accepted: August 10, 2013 Letter