There is a critical need for improved methane-oxidation catalysts to both reduce emissions of methane, a greenhouse gas, and improve the performance of gas turbines. However, materials that are currently available either have low activity below 400°C or are unstable at higher temperatures. Here, we describe a supramolecular approach in which single units composed of a palladium (Pd) core and a ceria (CeO(2)) shell are preorganized in solution and then homogeneously deposited onto a modified hydrophobic alumina. Electron microscopy and other structural methods revealed that the Pd cores remained isolated even after heating the catalyst to 850°C. Enhanced metal-support interactions led to exceptionally high methane oxidation, with complete conversion below 400°C and outstanding thermal stability under demanding conditions.

It is generally accepted that good hydrogenation noble and nonnoble metal catalysts such as Pt, Ru, or Ni are not chemoselective for hydrogenation of nitro groups in substituted aromatic molecules. We have found that it is possible to transform nonchemoselective into highly chemoselective metal catalysts by controlling the coordination of metal surface atoms while introducing a cooperative effect between the metal and a properly selected support. Thus, highly chemoselective and general hydrogenation Pt, Ru, and Ni catalysts can be prepared by generating nanosized crystals of the metals on the surface of a TiO 2 support and decorating the exposed (111) and (100) crystal faces by means of a simple catalyst activation procedure. By doing this, it has been possible to change the relative rate for hydrogenating competitive groups present in the molecule by almost 2 orders of magnitude, increasing the chemoselectivity from less than 1% to more than 95%.

Subnanometric metal species (single atoms and clusters) have been demonstrated to be unique compared to their nanoparticulate counterparts. However, the poor stabilization of subnanometric metal species towards sintering at high temperature (>500 o C) under oxidative or reductive reaction conditions limits their catalytic application. Zeolites can serve as an ideal support to stabilize subnanometric metal catalysts, but it is challenging to localize subnanometric metal species on specific sites and modulate their reactivity. We have achieved a very high preference for localization of highly stable subnanometric Pt and Pt-Sn clusters in the sinusoidal channels of purely siliceous MFI zeolite, as revealed by atomically resolved electron microscopy combining high-angle annular dark-field and integrated differential phase contrast imaging techniques. These catalysts show very high stability, selectivity, and activity for the industrially important dehydrogenation of propane to form propylene. This stabilization strategy could be extended to other crystalline porous materials.

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The relationship existing between aging conditions, redox behavior, and surface/bulk structural properties of two thermally aged ceria-zirconia mixed oxides, CZ-MO and CZ-SO, is analyzed. The samples were prepared by applying to a fresh Ce 0.62 Zr 0.38 O 2 mixed oxide two alternative aging routines consisting of a reduction with H 2 at 1223 K (5 h), followed by either a mild, CZ-MO, or severe, CZ-SO, re-oxidation treatment. By combining high-resolution electron microscopy and a number of chemical charcterization techniques, it is shown that the nanostructure of the aged oxides, specifically the total amount and surface presence of the phase exhibiting an ordered cationic sublattice (κ-like phase), is a key factor in determining their redox response. In the low-temperature reduction range (T redn e 773 K), the enhanced reducibility of the CZ-MO sample is proposed to be kinetically controlled by its surface structure mainly consisting of the κ-like phase. In accordance with the reported results, the surface activation of the H 2 molecule, much faster on the CZ-MO sample, is proposed to be the rate controlling step of the overall reduction process. This proposal was further confirmed by the dramatic downward shift observed in the temperatureprogrammed reduction diagrams recorded for the corresponding oxide-supported rhodium samples. By contrast, in the high-temperature reduction range (T redn g 973 K), the observed difference of reducibility, higher in the case of the CZ-MO sample, is interpreted as due to thermodynamic factors related to the nature of the predominant cationic sublattice structure, ordered for CZ-MO and disordered in the case of the CZ-SO sample.