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.

The surface atomic arrangement of metal oxides determines their physical and chemical properties, and the ability to control and optimize structural parameters is of crucial importance for many applications, in particular in heterogeneous catalysis and photocatalysis. Whereas the structures of macroscopic single crystals can be determined with established methods, for nanoparticles (NPs), this is a challenging task. Herein, we describe the use of CO as a probe molecule to determine the structure of the surfaces exposed by rod-shaped ceria NPs. After calibrating the CO stretching frequencies using results obtained for different ceria single-crystal surfaces, we found that the rod-shaped NPs actually restructure and expose {111} nanofacets. This finding has important consequences for understanding the controversial surface chemistry of these catalytically highly active ceria NPs and paves the way for the predictive, rational design of catalytic materials at the nanoscale.

An atomic-level understanding of dioxygen activation on metal oxides remains one of the major challenges in heterogeneous catalysis. By performing a thorough surface-science study of all three low-index single-crystal surfaces of ceria, probably the most important redox catalysts, we provide a direct spectroscopic characterization of reactive dioxygen species at defect sites on the reduced ceria (110) and (100) surfaces. Surprisingly, neither of these superoxo and peroxo species was found on ceria (111), the thermodynamically most stable surface of this oxide. Applying density functional theory, we could relate these apparently inconsistent findings to a sub-surface diffusion of O vacancies on (111) substrates, but not on the less-closely packed surfaces. These observations resolve a long standing debate concerning the location of O vacancies on ceria surfaces and the activation of O on ceria powders.

Polymer/graphene-analogous nanosheet composites have great potential for improving their physical and mechanical properties during the past few years. Herein, ultrathin molybdenum disulfide (MoS 2 ) nanosheets were simultaneously exfoliated and non-covalently modified by ultrasonication in an aqueous solution of chitosan. The chitosan-modified MoS 2 (CS-MoS 2 ) nanosheets were then transferred from the aqueous solution to tetrahydrofuran by a simple solvent-exchange method for the fabrication of epoxy (EP) nanocomposites. Transmission electron microscopy and scanning electron microscopy were performed to display the homogeneous dispersion of CS-MoS 2 in an EP matrix. On incorporating 2 wt% CS-MoS 2 into an EP matrix, EP nanocomposites exhibited reductions of up to 43.3% and 14.6% in peak heatrelease rate and total heat release derived from cone calorimeters compared to those of neat EP, respectively. Moreover, toxic volatiles, such as hydrocarbons, aromatic compounds and CO, that escaped from the flaming EP nanocomposites were decreased compared to that of neat EP, demonstrating the higher smoke safety. Combined with the analyses of char residues and thermal stability of EP nanocomposites, the reduced fire hazards of EP nanocomposites could be attributed to the nano-barrier effects of MoS 2 , which could effectively inhibit the release of combustible gas to support burning and restrain the effusion of volatile toxic substances that cause the majority of deaths in fires.