Shape-controlled synthesis and catalytic application of ceria nanomaterials

Because of their excellent properties and extensive applications, ceria nanomaterials have attracted much attention in recent years. This perspective provides a comprehensive review of current research activities that focus on the shape-controlled synthesis methods of ceria nanostructures. We elaborate on the synthesis strategies in the following four sections: (i) oriented growth directed by the crystallographic structure of cerium-based materials; (ii) oriented growth directed by the use of an appropriate capping reagent; (iii) growth confined or dictated by various templates; (iv) other potential methods for generating CeO(2) nanomaterials. In this perspective, we also discuss the catalytic applications of ceria nanostructures. They are often used as active components or supports in many catalytic reactions and their catalytic activities show morphology dependence. We review the morphology dependence of their catalytic performances in carbon monoxide oxidation, water-gas shift, nitric oxide reduction, and reforming reactions. At the end of this review, we give a personal perspective on the probable challenges and developments of the controllable synthesis of CeO(2) nanomaterials and their catalytic applications.

Theoretical tools for studying gold nanoparticles as catalysts for oxidation and hydrogenation reactions

In this contribution, the ability of small isolated gold NP to dissociate O2 and generate a reactive surface oxide layer, the nature of the new gold active sites generated, and their implication in the mechanism of alcohol oxidation to aldehydes has been analyzed from a theoretical point of view. The nature of the active sites involved in H2 dissociation and the possible ways in which Au/TiO2 catalysts can be modified in order to increase their activity toward hydrogenation of nitroaromatics without modifying their high chemoselectivity is also explored.

Atomically dispersed Au-(OH)x species bound on titania catalyze the low-temperature water-gas shift reaction

We report a new method for stabilizing appreciable loadings (~1 wt %) of isolated gold atoms on titania and show that these catalyze the low-temperature water-gas shift reaction. The method combines a typical gold deposition/precipitation method with UV irradiation of the titania support suspended in ethanol. Dissociation of H2O on the thus-created Au-O-TiO(x) sites is facile. At higher gold loadings, nanoparticles are formed, but they were shown to add no further activity to the atomically bound gold on titania. Removal of this "excess" gold by sodium cyanide leaching leaves the activity intact and the atomically dispersed gold still bound on titania. The new materials may catalyze a number of other reactions that require oxidized active metal sites.

ZnO is a CO(2)-selective steam reforming catalyst

ZnO was tested as possible methanol and - since formaldehyde is one of the key intermediates in methanol conversion reactions - also as formaldehyde steam reforming catalyst. Catalytic experiments in a batch as well as a flow reactor resulted in highly selective steam reforming, though at low specific activities, of formaldehyde and methanol over ZnO toward CO(2) (selectivity of 95-99.6%). Comparison of the behavior of ZnPd near-surface intermetallic phases, unsupported intermetallic ZnPd and supported ZnPd/ZnO catalysts reveals that formaldehyde is formed from methanol in parallel with CO(2) on the former, while on unsupported intermetallic ZnPd and ZnO-supported ZnPd, it is efficiently reacted toward CO(2), thus, a beneficial role of ZnO in oxidizing formaldehyde-derived intermediates toward CO(2) is evident.

Surface-enhanced Raman spectroscopy to probe photoreaction pathways and kinetics of isolated reactants on surfaces: flat versus curved substrates

We identify and control the photoreaction paths of self-assembled monolayers (SAMs) of thiolate-linked anthracene phenylethynyl molecules on Au substrate surfaces, and study the effects of nanoscale morphology of substrates on regioselective photoreactions. Two types of morphologies, atomically flat and curved, are produced on Au surfaces by controlling substrate structure and metal deposition. We employ surface-enhanced Raman spectroscopy (SERS), combined with Raman mode analyses using density functional theory, to identify the different photoreaction paths and to track the photoreaction kinetics and efficiencies of molecules in monolayers. The SAMs on curved surfaces exhibit dramatically lower regioselective photoreaction kinetics and efficiencies than those on atomically flat surfaces. This result is attributed to the increased intermolecular distances and variable orientations on the curved surfaces. Better understanding of the morphological effects of substrates will enable control of nanoparticle functionalization in ligand exchange in targeted delivery of therapeutics and theranostics and in catalysis.

Atomically dispersed supported metal catalysts

Our aim in this review is to assess key recent findings that point to atomically dispersed noble metals as catalytic sites on solid supports, which may be viewed as ligands bonded to the metal. Both zeolites and open metal oxide supports are considered; the former offer the advantages of uniform, crystalline structures to facilitate fundamental understanding, and the latter offer numerous advantages in applications. The notion of strong interactions between metals and supports has resurfaced in the recent literature to explain how subnanometer clusters and even atoms of noble metals such as platinum and gold survive under often harsh reaction conditions on some supports, such as ceria and perovskites. Individual cations of platinum, palladium, rhodium, or other metals anchored to supports through M-O bonds can be formed on these supports in configurations that are stable and catalytically active for several reactions illustrated here, notably, oxidation and reduction. The development of effective synthesis methods and the identification of suitable stabilizers and promoters are expected to lead to the increasing application of atomically dispersed noble metal catalysts for practical processes characterized by efficient resource utilization and cost savings.