Eric I. Altman

CATALYTIC COMBUSTION OF METHANE OVER PALLADIUM-BASED CATALYSTS

Palladium-based catalysts are widely applied in exhaust catalytic converter and catalytic combustion systems. The mechanism for methane oxidation on a Pd-based catalyst is complex. Catalyst activity is influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatment under reducing or oxidizing atmospheres. In this paper, we review the literature on supported Pd catalysts for combustion of methane. The mechanisms involved are discussed taking into consideration the oxidation/reduction mechanisms for supported palladium, poisoning, restructuring, the form of oxygen on the surface, methane activation over Pd and PdO phases, and transient behavior. Our review helps explain the array of experimental results reported in the literature.

The oxidation of Pd(111)

The oxidation of Pd(111) was characterized using scanning tunneling microscopy (STM ), temperature-programmed desorption (TPD), and low energy electron diVraction (LEED). Initial exposure of Pd(111) to O 2 at temperatures between 300 and 575 K resulted in a (2◊2) structure observable by both LEED and STM. The maximum coverage achieved by O 2 exposure at 300 K was 0.25 ML. By increasing the temperature above 500 K, the oxygen coverage could be increased to 0.37 ML. To increase the oxygen coverage further, NO 2 was used as the oxidant. On Pd(111), NO 2 dissociatively adsorbs, with NO going to the gas phase below 500 K, leaving oxygen on the surface. Above 500 K, initial exposure of NO 2 to Pd(111) also produced the (2◊2) structure. Increasing the oxygen coverage to between 1.0 and 2.2 ML resulted in a complicated LEED pattern. This pattern could be explained as the superposition of three equivalent domains of two surface structures: one with a square surface lattice rotated 15° with respect to Pd(110), the other with a rectangular surface lattice with the short sides of the rectangles parallel to Pd(110). In STM movies, ad-islands and peninsulas were observed to nucleate and grow as the oxygen coverage reached this regime. The rectangular structure was observed on the original Pd(111) terraces, and the square structure on the islands and peninsulas. This suggested that when the oxygen coverage exceeds 0.25 ML, oxygen atoms penetrate the surface creating a rectangular structure with a lower Pd atom density than the clean surface; the liberated Pd atoms, along with oxygen, then form the islands and peninsulas. The lattice constants obtained from the STM images were 0.679±0.012 nm for the square structure and 0.394±0.008 nm and 0.638±0.022 nm for the rectangular structure, both consistent with LEED observations. Neither of these structures can be simply related to any crystal orientation of Pd or PdO, indicating that there are states intermediate between Pd and PdO. After further increasing the oxygen coverage, the complicated LEED patterns became faint and a low temperature shoulder attributed to PdO decomposition developed in TPD traces. The results indicate that Pd(111) oxidation proceeds through three stages involving four distinct surface states.

Three-dimensional imaging of short-range chemical forces with picometre resolution

Chemical forces on surfaces have a central role in numerous scientific and technological fields, including catalysis, thin film growth and tribology. Many applications require knowledge of the strength of these forces as a function of position in three dimensions, but until now such information has only been available from theory. Here, we demonstrate an approach based on atomic force microscopy that can obtain this data, and we use this approach to image the three-dimensional surface force field of graphite. We show force maps with picometre and piconewton resolution that allow a detailed characterization of the interaction between the surface and the tip of the microscope in three dimensions. In these maps, the positions of all atoms are identified, and differences between atoms at inequivalent sites are quantified. The results suggest that the excellent lubrication properties of graphite may be due to a significant localization of the lateral forces.

Chemistry of ferroelectric surfaces.

It has been recognized since the 1950s that the polar and switchable nature of ferroelectric surfaces can potentially lead to polarization direction-dependent surface chemistry. Recent theoretical studies and advances in growing high quality epitaxial ferroelectric thin films have motivated a flurry of experimental studies aimed at creating surfaces with switchable adsorption and catalytic properties, as well as films whose polarization direction switches depending on the gas phase environment. This research news article briefly reviews the key findings of these studies. These include observations that the adsorption strengths, and in certain cases the activation energies for reactions, of polar molecules on the surfaces of ferroelectric materials are sensitive to the polarization direction. For bare ferroelectric surfaces, the magnitudes of these differences are not large, but are still comparable to the energy barrier required to switch the polarization of approximately 10 nm thick films. Highlights of a recent study where chemical switching of a thin film ferroelectric was demonstrated are presented. Attempts to use the ferroelectric polarization to influence the behavior of supported catalytic metals will also be described. It will be shown that the tendency of the metals to cluster into particles makes it difficult to alter the chemical properties of the metal surface, since it is separated from the ferroelectric by several layers of metal atoms. An alternate approach to increasing the reactivity of ferroelectric surfaces is suggested that involves modifying the surface with atoms that bind strongly to the surface and thus remain atomically dispersed.

Experimental study of the interfacial cobalt oxide in Co 3 O 4 / α − Al 2 O 3 ( 0001 ) epitaxial films

A detailed spectroscopic and structural characterization of ultrathin cobalt oxide films grown by O-assisted molecular-beam epitaxy on

Three-Dimensional Atomic Force Microscopy – Taking Surface Imaging to the Next Level

\ensuremath{\alpha}\ensuremath{-}{\text{Al}}_{2}{\text{O}}_{3}(0001)

Combined low-temperature scanning tunneling/atomic force microscope for atomic resolution imaging and site-specific force spectroscopy.

single crystals is reported. The experimental results show that the cobalt oxide films become progressively more disordered with increasing thickness, starting from the early stages of deposition. Low-energy electron-diffraction patterns suggest that the unit cell remains similar to that of

A high-speed variable-temperature ultrahigh vacuum scanning tunneling microscope

\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}(0001)

Development of procedures for obtaining clean, low-defect-density Ge(100) surfaces

up to a thickness of

Anisotropic magnetoresistance in colossal magnetoresistive La1-xSrxMnO3 thin films

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