Effect of Micropore Filling on Chemisorption by Large Surface Area Materials. Adsorption of O2 and N2O by CuZnO Catalysts

In irreversible chemisorption by large surface area catalysts, the partially covered surface takes up additional amounts of adsorbate only if a given threshold pressure is attained. A model is suggested in which micropore filling acts as a precursor state for other gas-solid interactions. The model is applied to the adsorption of oxygen by CuZnO catalysts with various copper contents, and containing particles and interparticle voids of various sizes

Single Metal Atoms on Oxide Surfaces: Assessing the Chemical Bond through 17O Electron Paramagnetic Resonance

[Image: see text] Even in the gas phase single atoms possess catalytic properties, which can be crucially enhanced and modulated by the chemical interaction with a solid support. This effect, known as electronic metal–support interaction, encompasses charge transfer, orbital overlap, coordination structure, etc., in other words, all the crucial features of the chemical bond. These very features are the object of this Account, with specific reference to open-shell (paramagnetic) single metal atoms or ions on oxide supports. Such atomically dispersed species are part of the emerging class of heterogeneous catalysts known as single-atom catalysts (SACs). In these materials, atomic dispersion ensures maximum atom utilization and uniform active sites, whereby the nature of the chemical interaction between the metal and the oxide surface modulates the catalytic activity of the metal active site by tuning the energy of the frontier orbitals. A comprehensive set of examples includes fourth period metal atoms and ions in zeolites on insulating (e.g., MgO) or reducible (e.g., TiO(2)) oxides and are among the most relevant catalysts for a wealth of key processes of industrial and environmental relevance, from the abatement of NO(x) to the selective oxidation of hydrocarbons and the conversion of methane to methanol. There exist several spectroscopic techniques able to inform on the geometric and electronic structure of isolated single metal ion sites, but either they yield information averaged over the bulk or they lack description of the intimate features of chemical bonding, which include covalency, ionicity, electron and spin delocalization. All of these can be recovered at once by measuring the magnetic interactions between open-shell metals and the surrounding nuclei with Electron Paramagnetic Resonance (EPR) spectroscopy. In the case of oxides, this entails the synthesis of (17)O isotopically enriched materials. We have established (17)O EPR as a unique source of information about the local binding environment around oxygen of magnetic atoms or ions on different oxidic supports to rationalize structure–property relationships. Here, we will describe strategies for (17)O surface enrichments and approaches to monitor the state of charge and spin delocalization of atoms or ions from K to Zn dispersed on oxide surfaces characterized by different chemical properties (i.e., basicity or reducibility). Emphasis is placed on chemical insight at the atomic-scale level achieved by (17)O EPR, which is a crucial step in understanding the structure–property relationships of single metal atom catalysts and in enabling efficient design of future materials for a range of end uses

The interaction of oxygen with the surface of CeO2–TiO2 mixed systems: an example of fully reversible surface-to-molecule electron transfer

The interaction of oxygen with the surface of CeO2-TiO2 mixed oxides prepared via sol gel was investigated by means of electron paramagnetic resonance (EPR). Upon admission of molecular oxygen onto the surface of the as prepared materials (which underwent final oxidative calcination) the formation of superoxide O-2(-) ions is observed without the need for preliminary annealing in a vacuum and consequent oxygen depletion. The superoxide species is symmetrically adsorbed ("side-on" structure) on the top of a Ce4+ ion. Surprisingly the electron transfer is fully reversible at room temperature having the typical behavior shown by molecular oxygen carriers, which, however, link to oxygen in a completely different manner ("end-on" structure). We suggest that the active sites are Ce3+ ions present in the stoichiometric cerium titanate which forms during the synthesis. The features of these Ce3+ ions must be different from those of the same ions formed in CeO2 by reductive treatments, which show a different reactivity to O-2. The observation reported here opens up innovative perspectives in the field of heterogeneous catalysis and in that of sensors as the total reversibility of the electron transfer is observed in a significant range of oxygen pressure