No abstract

We show that coverage fluctuations on catalyst particles can drastically alter their macroscopic catalytic behavior. Scrutinizing the occurrence of kinetic bistabilities, it is demonstrated by molecular beam experiments on model catalysts that macroscopically observable bistabilities vanish completely with decreasing particle size, as previously predicted by theory. The effect is attributed to fluctuation-induced transitions between two kinetic reaction regimes, with a transition rate controlled by both particle size and surface defects. These results suggest that fluctuation-induced effects represent a general phenomenon affecting the reaction kinetics on nanostructured surfaces.

We present the microfabrication of silicon nitride membrane supports (‘windows’) for transmission electron microscopy (TEM) and illustrate their usefulness for direct preparation and studies of nanostructures. These membrane ‘windows’ enable TEM to be incorporated as an affordable diagnostic tool in nanostructure fabrication and applications thereof, in an iterative fashion, both during and after preparation as well as subsequent experimental steps with the nanostructures, and even for real time in situ TEM observations. Several examples are shown, including protein adsorption and nanofabrication for applications in heterogeneous catalysis.

No abstract

The relationship between metal particle structure and the reactivity and selectivity of oxide−supported metal catalysts has been explored here with well-defined Pt particles on the Zn-terminated ZnO(0001)−Zn surface, using X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), and temperature-programmed desorption (TPD). These are model catalysts for the steam reforming and selective dehydrogenation of methanol. Vapor-deposited Pt on ZnO(0001) grows in two-dimensional (2D) islands up to ∼0.7ML. The effect of the 2D island size and 3D island height are studied. Even 0.01 ML of Pt suppresses the dissociative adsorption of methanol to methoxy on Zn sites by preferential decoration of defects. Tiny amounts of Pt also affect the selectivity of methanol conversion at the Zn sites:  the dehydration pathway of its Zn-bound formate intermediate is suppressed and its dehydrogenation pathway to formaldehyde is eliminated. Even Pt sites on 2D Pt islands catalyze the partial oxidation of methanol to CO and H2, characteristic of low-index Pt facets. When the Pt islands are 2−3 layers thick, methanol decomposition is similar to that on low-index Pt faces.

No abstract

No abstract

The adsorption of Cs on a TiO 2 (110) rutile surface was investigated at 130-800 K using x-ray photoelectron spectroscopy, x-ray excited Auger electron spectroscopy, temperature-programmed desorption, work-function, and band-bending measurements. Below room temperature, the Cs displays a Stranski-Krastanov growth mode, with the completion of a uniform monolayer ͑ML͒ containing (6Ϯ2)ϫ10 14 Cs adatoms per cm 2 , followed by the growth of three-dimensional clusters of Cs that cover only a small fraction of the surface. The Cs in the first ϳ 1 2 ML is very cationic, donating electron density to the TiO 2 . Most of this charge is localized near the topmost atomic layers, with Ti 4ϩ ions being reduced to Ti 3ϩ . This gives rise to a local dipole moment of the adsorbate-substrate complex of ϳ6D at ϳ0.1 ML. However, a small part of the charge transferred to the substrate also goes much deeper into the solid, giving rise to downward band bending of ϳ0.2-0.3 eV. This band bending nearly saturates at ϳ0.05 ML. The local dipole moment of the alkali-metal-substrate complex decreases smoothly with coverage in the first ML, due to dipole-dipole repulsions and their consequent mutual depolarization, similar to transition-metal surfaces. This gives rise to a rapid and smooth decrease in the heat of adsorption with coverage from Ͼ208kJ/mol down to ϳ78kJ/mol.