Water Chemisorption and Reconstruction of the MgO Surface

The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab-initio total energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with Delta E = -90.2kJ/mol. We conclude that protonation stabilizes the otherwise unstable (111) surface and that this, not the bare (001), is the most stable surface of MgO under ambient conditions.Comment: RevTeX, 4 pages, 1 Encapsulated Postscript Figur

Effect of mill type on the size reduction and phase transformation of gamma alumina

The influence of stress modes and comminution conditions on the effectiveness of particle size reduction of a common catalyst support; γ-Alumina is examined through a comparative assessment of three different mill types. Air jet milling is found to be the most effective in reducing particle size from a d90 of 37 µm to 2.9 µm compared to planetary ball milling (30.2 µm) and single ball milling (10.5 µm). XRD and TEM studies confirm that the planetary ball mill causes phase transformation to the less desired α-Alumina resulting in a notable decrease in surface area from 136.6 m2/g to 82.5 m2/g as measured by the BET method. This is consistent with the large shear stresses under high shear rates prevailing in the planetary ball mill when compared to the other mill types. These observations are consistent with a shear-induced phase transformation mechanism brought about by slip on alternate close packed oxygen layers from a cubic close packed to a hexagonal close packed structure

CO hydrogenation catalyzed by alumina-supported osmium: Particle size effects

Alumina-supported catalysts were prepared by conventional aqueous impregnation with [H2OsCl6] and by reaction of organoosmium clusters {[Os3(CO)12], [H4Os4(CO)12], and [Os6(CO)18]} with the support. The catalysts were tested for CO hydrogenation at 250-325 [deg]C and 10 atm, the products being Schulz-Flory distributions of hydrocarbons with small yields of dimethyl ether. The fresh and used catalysts were characterized by infrared spectroscopy and high-resolution transmission electron microscopy. The catalyst prepared from [H2OsCl6] had larger particles of Os (~70 A). The cluster-derived catalysts initially consisted of molecular clusters on the support; the used catalysts contained small Os aggregates (typically 10-20 A in diameter). The catalytic activity for hydrocarbon formation increased with increasing Os aggregate size, but the activity for dimethyl ether formation was almost independent of aggregate size. The hydrocarbon synthesis was evidently catalyzed by the Os aggregates, and the ether synthesis was perhaps catalyzed by mononuclear Os Complexes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25555/1/0000097.pd

Structure and Morphology of Vanadia-Promoted Rh/SiO₂: A Transmission Electron Microscopy Study

Vanadia-promoted Rh/SiO2-catalysts have been prepared by impregnation followed by calcination at 573, 773, 973, and 1173 K. The structure and morphology of these materials in the oxidized state, and after low (523 K) and high (773 K) temperature reduction, were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), microdiffraction, X-ray diffraction (XRD), and CO chemisorption. During calcination at temperatures ≥ 973 K, a RhVO4 phase is formed, which consists of well-crystallized rod-like particles after calcination at 1173 K. After reduction in H2, the catalysts consist of highly dispersed Rh° particles as judged from the electron micrographs. This high dispersion is presumably stabilized by interaction of the zerovalent Rh° with the promoter oxide V2O3. The well-crystallized sample (calcination at 1173 K) cannot be reduced at 523 K, but at 773 K the rhodium, originally present as RhVO4, is quantitatively reduced to small metal particles in contact with V2O3. In contrast to the high dispersion derived from TEM, CO chemisorption gave unexpectedly low CO/Rh ratios, which were even zero for the catalyst calcined at 1173 K. The CO/Rh ratios decreased with increasing calcination temperature (RhVO4 formation) at constant reduction temperature and with increasing reduction temperature at a given calcination temperature. It is suggested that the surface of the highly dispersed Rh° particles is decorated and blocked by VOx species. This effect, though more pronounced at higher reduction temperature, already occurs after reduction at 523 K. Highly dispersed Rh° particles are produced in the materials studied, particularly when a RhVO4 precursor phase was present, these particles being in intimate contact with a V2O3 promoter oxide. In extreme cases (calcination at 1173 K), the encapsulation of Rh° particles seems to be complete, so that no metal surface is exposed