The dry reforming of methane was systematically studied over a series (2-30 wt%) of Co (~5nm in size) loaded CeO2 catalysts, with an effort to elucidate the behavior of Co and ceria in the catalytic process using in-situ methods. For the systems under study, the reaction activity scaled with increasing Co loading, and a 10 wt% Co-CeO2 catalyst exhibiting the best catalytic activity and good stability at 500 °C with little evidence for carbon accumulation. The phase transitions and the nature of active components in the catalyst were investigated during pretreatment and under reaction conditions by ex-situ/in-situ techniques including X-ray diffraction (XRD) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). These studies showed a dynamical evolution in the chemical composition of the catalysts under reaction conditions. A clear transition of Co3O4 → CoO → Co, and Ce 4+ to Ce 3+ , was observed during the temperature programmed reduction under H2 and CH4. However, introduction of CO2, led to partial re-oxidation of all components at low temperatures, followed by reduction at high temperatures. Under optimum CO and H2 producing conditions both XRD and AP-XPS indicated that the active phase involved a majority of metallic Co with a small amount of CoO both supported on a partially reduced ceria (Ce 3+ /Ce 4+). We identified the importance of dispersing Co, anchoring it onto ceria surface sites, and then utilizing the redox properties of ceria for activating and then oxidatively converting methane while inhibiting coke formation. Furthermore, a synergistic effect between cobalt and ceria and the interfacial site are essential to successfully close the catalytic cycle.

The confinement of noble gases on nanostructured surfaces, in contrast to bulk materials, at non-cryogenic temperatures represents a formidable challenge. In this work, individual Ar atoms are trapped at 300 K in nano-cages consisting of (alumino)silicate hexagonal prisms forming a two-dimensional array on a planar surface. The trapping of Ar atoms is detected in situ using synchrotron-based ambient pressure X-ray photoelectron spectroscopy. The atoms remain in the cages upon heating to 400 K. The trapping and release of Ar is studied combining surface science methods and density functional theory calculations. While the frameworks stay intact with the inclusion of Ar atoms, the permeability of gasses (for example, CO) through them is significantly affected, making these structures also interesting candidates for tunable atomic and molecular sieves. These findings enable the study of individually confined noble gas atoms using surface science methods, opening up new opportunities for fundamental research.

Subnanoscale spaces at the interface between weakly coupled thin films and their metal supports offer exciting opportunities for studying chemical reactions under confinement. Here, we investigated the kinetics of water formation (from hydrogen and chemisorbed oxygen) in the confined space at the interface between bilayer (BL) silica and a Ru(0001) support, compared to the reaction on the bare Ru(0001) surface. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) experiments were carried out at different temperatures at elevated pressures of H 2 to follow the reaction kinetics. The apparent activation energy at the BL-silica/Ru(0001) interface was found to be 0.38 eV lower than that on bare Ru( 0001), consistent with a recent report by Prieto et al. (Angew. Chem., Int. Ed. 2018, 57(28), 8749−8753) carried out at lower H 2 pressures using low-energy electron microscopy. Density functional theory calculations revealed that the ratelimiting step in the direct hydrogenation pathway on the Ru(0001) surface is the first hydrogen addition step (*H + *O ↔ *OH). The confinement at the BL-silica/Ru(0001) interface only marginally affects the energy barrier of the first hydrogen addition. Instead, it activates an alternative disproportionation reaction pathway (*H 2 O + *O ↔ 2*OH). On the bare Ru( 0001) surface, the disproportionation pathway can only occur at cryogenic temperatures or under high water vapor pressures. However, the presence of the BL-silica increases the desorption barrier for water molecules at the interface. The increased residence time allows trapped water molecules to react with chemisorbed oxygen to produce two *OH per H 2 O with an activation energy 0.25 eV lower than that of the first hydrogen addition step. This work reveals the origin of the observed accelerated water formation reaction at the BL-silica/ Ru(0001) interface in the low-temperature regime (T < 350 K) and points to a route to engineer chemical reaction pathways by leveraging subnanoscale confined spaces at metal−oxide interfaces.