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...