Cu/CeO2 catalysts are highly active for the low-temperature water-gas shift-a core reaction in syngas chemistry for tuning H2/CO/CO2 proportions in feed-streams-but direct identification and a quantitative description of the active sites remains challenging. Here, we report that the active copper clusters consist of a bottom layer of mainly Cu + atoms bonded on the oxygen vacancies of ceria, in a form of Cu +-Ov-Ce 3+ , and a top layer of Cu 0 atoms coordinated with the underlying Cu + atoms. This atomic structure model is based on directly observing copper clusters dispersed on ceria by a combination of scanning transmission electron microscopy and electron energy loss spectroscopy, in situ probing the interfacial copper-ceria bonding environment by infrared spectroscopy, and rationalization by density functional theory calculations. These results, together with reaction kinetics, reveal that the reaction occurs at the copper-ceria interfacial perimeter via a site cooperation mechanism: the Cu + site chemically adsorbs CO while the neighboring-Ov-Ce 3+ site dissociatively activates H2O. Copper nanoparticles, dispersed on ceria, constitute a highly efficient catalyst system for reactions in syngas (a mixture of H2, CO, and CO2) chemistry, such as the low-temperature water-gas shift (WGS) reaction 1-7 and CO/CO2 hydrogenation yielding methanol 8-13. In these technologically highly relevant Cu/CeO2 catalysts, copper is commonly viewed as the active component, while the ceria support, with a prominent redox behavior, tunes the dispersion and chemical state of the copper nanoparticles via strong metal-support interactions 14-16. In the case of the low-temperature WGS, a crucial reaction for regulating the H2/CO/CO2 proportions in feed gases for the downstream industrial applications, the active sites have been presumably proposed to locate at the copper-ceria interface. This hypothesis is based on intensive experimental studies on both real Cu/CeO2 catalysts 2-6 and model CeO2/Cu systems 17,18 as well as theoretical simulations of copper-ceria interactions 19-23. A direct experimental verification of the geometric and electronic structures of the copper-ceria interface at atomic scale, however, together with a quantitative description of the active sites for the activation of CO and H2O molecules during the low-temperature WGS reaction on the Cu/CeO2 catalysts, has not yet been obtained.

Density functional theory calculations that account for the on-site Coulomb interaction via a Hubbard term (DFT+U) reveal the mechanisms for the oxidation of CO catalyzed by isolated Au atoms as well as small clusters in Au/CeO(2) catalysts. Ceria (111) surfaces containing positively charged Au ions, either as supported Au(+) adatoms or as substitutional Au(3+) ions, are shown to activate molecular CO and to catalyze its oxidation to CO(2). In the case of supported single Au(+) adatoms, the limiting rate for the CO oxidation is determined by the adsorbate spillover from the adatom to the oxide support. The reaction then proceeds with the CO oxidation via lattice oxygen and O vacancy formation. These vacancies are shown to readily attract the supported Au(+) adatoms and to turn them into negatively charged Au(delta-) adspecies that deactivate the catalyst, preventing further CO adsorption. Au(3+) ions dispersed into the ceria lattice as substitutional point defects can instead sustain a full catalytic cycle consisting of three individual steps maintaining their activity along the reaction process: Au cations in Au(x)Ce(1-x)O(2) systems promote multiple oxidations of CO without any activation energy via formation of surface O vacancies. Molecular oxygen adsorbs at these vacancies and forms O adspecies that then catalyze the oxidation of molecular CO, closing the catalytic cycle and recovering the stoichiometric Au(x)Ce(1-x)O(2) system. The interplay between the reversible Ce(4+)/Ce(3+) and Au(3+)/Au(+) reductions underpins the high catalytic activity of dispersed Au atoms into the ceria substrate. It is shown that the positive oxidation state of the substitutional Au ions is retained along the catalytic cycle, thus preventing the deactivation of Au(x)Ce(1-x)O(2) catalysts in operation conditions. Finally, although a single Au(+) adatom bound to an O vacancy is shown to deactivate during CO oxidation, the calculations predict that the reactivity of gold nanoparticles nucleated at O vacancies can be recovered for cluster sizes as small as Au(2).

Challenges in energy and the environment call for the development of highly active catalysts, allowing for a more efficient and cleaner use of energy supplies.[1] Catalytic combustion of methane is a leading technology in emission prevention and cleanup.[2] Its main advantage over traditional flame combustion is to stabilize complete oxidation of fuel at low temperature while simultaneously controlling NO x emissions. Catalysts yielding the highest activity at low temperatures consist of noble metals dispersed on high-surface-area oxide supports. PdO particles dispersed on oxide carriers are the most active methane combustion catalysts, but they still suffer from inadequate activity at low temperature (below 673 K) and deactivation at high temperature (above 973 K) owing to formation of metallic Pd from PdO particles.[3] This transformation is regulated by a complex dynamic of formation and decomposition of PdO to Pd under reaction conditions, which is affected by the temperature and the reaction mixture.[4] One possibility for avoiding this transformation is to disperse Pd already in the ionic form over an oxide support. Stabilization of precious metals as ionic moieties over reducible supports such as ceria (CeO 2 ) has been shown to be effective for several reactions, such as the water-gas shift reaction and total oxidation, [5] and the ability of ceria to stabilize Pd in a highly dispersed state is wellrecognized.[6] Insertion of the precious metal into the metal oxide lattice would lead to the highest degree of dispersion for a given metal loading, with important consequences in several catalytic applications. Isolated encapsulated Pd metal in ceria as a result of a strong metal-support interaction was reported in early studies of noble-metal / ceria systems. [6,7] Solid solutions based on PdO/CeO 2 of composition Ce 0.99 Pd 0.01 O 2Àd or Ce 0.76 Zr 0.19 Pd 0.05 O 2Àd were reported more recently and found to be active in CO/NO reaction and methane combustion; [8] this finding is also corroborated by recent density functional theory (DFT) calculations suggesting that insertion of Pd into CeO 2 surfaces provides a lower energy barrier for dissociative adsorption of methane.[9] However, stabilization of Pdsubstituted ceria is difficult, and Pd segregation out of the oxide to form PdO or metallic Pd crystallites is commonly observed at high temperatures.[8]Herein we report an ordered and stable Pd-O-Ce surface superstructure as revealed by DFT calculations on the basis of high-resolution (HR) TEM data. It results from a complex reconstruction of the (110) CeO 2 surface and leads to the opening of wide surface channels exposing highly undercoordinated oxygen atoms.We have prepared two Pd/CeO 2 catalysts by one-step solution combustion synthesis (SCS). The new catalysts contain between 1 and 1.71 wt % Pd and are denoted SCS1 and SCS2 (Table 1). We also prepared samples of conventional Pd/CeO 2 catalysts by incipient wetness impregnation (IWI). These catalysts were prepared from two different samples of commerc...

Single-atom catalysts maximize the utilization of supported precious metals by exposing every single metal atom to reactants. To avoid sintering and deactivation at realistic reaction conditions, single metal atoms are stabilized by specific adsorption sites on catalyst substrates. Here we show by combining photoelectron spectroscopy, scanning tunnelling microscopy and density functional theory calculations that Pt single atoms on ceria are stabilized by the most ubiquitous defects on solid surfaces—monoatomic step edges. Pt segregation at steps leads to stable dispersions of single Pt2+ ions in planar PtO4 moieties incorporating excess O atoms and contributing to oxygen storage capacity of ceria. We experimentally control the step density on our samples, to maximize the coverage of monodispersed Pt2+ and demonstrate that step engineering and step decoration represent effective strategies for understanding and design of new single-atom catalysts.

The dynamics of an F-center created by an oxygen vacancy on the TiO2(110) rutile surface has been investigated using ab initio molecular dynamics. These simulations uncover a truly complex, time-dependent behavior of fluctuating electron localization topologies in the vicinity of the oxygen vacancy. Although the two excess electrons are found to populate preferentially the second subsurface layer, they occasionally visit surface sites and also the third subsurface layer. This dynamical behavior of the excess charge explains hitherto conflicting interpretations of both theoretical findings and experimental data.PACS numbers: 71.15. Pd, 73.20.At, 82.65.+r, 73.20.Jc Titanium dioxide (TiO 2 ) is one of the most thoroughly investigated metal oxides, due to its broad range of uses in several key technologies including heterogeneous catalysis, pigment materials, photocatalysis, and energy production, to name but a few [1][2][3]. It is well known that bulk and surface defects govern the properties of titania, and are thus of fundamental importance in virtually all its applications [4][5][6]. The most common point defects on the TiO 2 (110) rutile surface are oxygen vacancies (O v ) in the two-fold coordinated O rows and Ti interstitials [7,8]. In particular, removal of an O atom gives rise to two excess electrons and the appearance of new electronic states in the band gap at about 0.7-0.9 eV below the conduction band edge creating an F-center [9][10][11]. Although the two excess electrons can in principle be localized on any Ti atom, they are believed to preferentially occupy specific Ti-3d orbitals, thus formally creating Ti 3+ sites [10,12]. In stark contrast, recent experiments [13] suggest a qualitatively different viewpoint: charge localization is found to be more disperse, with the excess electrons being shared by several surface and subsurface Ti ions. Furthermore, STM/STS experiments have revealed charge delocalization involving more than ten Ti sites [14].Unfortunately, different computational methods yield conflicting results [11]. Local/semilocal density functionals (LDA/GGA) predict a rather delocalized defect level for O vacancies on TiO 2 (110) with an energy right at the bottom of the conduction band [11]. However, it is well known that such functionals bias against localization on strongly correlated d-states, and hence alternative methodologies are welcome. Recent studies of defective TiO 2 surfaces [15-21] have focused on "pragmatic and practical" correction schemes using hybrid functionals or a Hubbard correction. Although both schemes yield the expected gap states, they each predict vastly different localization topologies of the excess charge.Using B3LYP on a c(4×2) slab with an O vacancy, the defect charge is found to be localized on d -orbitals of two surface Ti atoms [15]. In particular, one unpaired electron is found on the under-coordinated Ti(11) site, while the other moves to an adjacent five-fold coordinated Ti 5c atom, such as Ti (7); see Fig. 1 for our site labeling scheme. By contrast,...

Density functional theory (DFT) calculations are used to identify correlations among reactivity, structural stability, cohesion, size, and morphology of small Au clusters supported on stoichiometric and defective CeO 2 (111) surfaces. Molecular adsorption significantly affects the cluster morphology and in some cases induces cluster dissociation into smaller particles and deactivation. We present a thermodynamic rationalization of these effects and identify Au 3 as the smallest stable nanoparticle that can sustain catalytic cycles for CO oxidation without incurring structural/morphological changes that jeopardize its reactivity. The proposed Mars van Krevelen reaction pathway displays a low activation energy, which we explain in terms of the cluster fluxionality and of labile CO 2 intermediates at the Au/ceria interface. These findings shed light on the importance of cluster dynamics during reaction and provide key guidelines for engineering more efficient metal−oxide interfaces in catalysis.

The reactivity of atomically dispersed Pt(2+) species on the surface of nanostructured CeO2 films and the mechanism of H2 activation on these sites have been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. Isolated Pt(2+) sites are found to be inactive towards H2 dissociation due to high activation energy required for H-H bond scission. Trace amounts of metallic Pt are necessary to initiate H2 dissociation on Pt-CeO2 films. H2 dissociation triggers the reduction of Ce(4+) cations which, in turn, is coupled with the reduction of Pt(2+) species. The mechanism of Pt(2+) reduction involves reverse oxygen spillover and formation of oxygen vacancies on Pt-CeO2 films. Our calculations suggest the existence of a threshold concentration of oxygen vacancies associated with the onset of Pt(2+) reduction.

Wet conditions in heterogeneous catalysis can substantially improve the rate of surface reactions by assisting the diffusion of reaction intermediates between surface reaction sites. The atomistic mechanisms underpinning this accelerated mass transfer are, however, concealed by the complexity of the dynamic water/solid interface. Here we employ ab initio molecular dynamics simulations to disclose the fast diffusion of protons and hydroxide species along the interface between water and ceria, a catalytically important, highly reducible oxide. Up to 20% of the interfacial water molecules are shown to dissociate at room temperature via proton transfer to surface O atoms, leading to partial surface hydroxylation and to a local increase of hydroxide species in the surface solvation layer. A water-mediated Grotthus-like mechanism is shown to activate the fast and long-range proton diffusion at the water/oxide interface. We demonstrate the catalytic importance of this dynamic process for water dissociation at ceria-supported Pt nanoparticles, where the solvent accelerates the spillover of ad-species between oxide and metal sites.